Paul-Peter Tak-New Therapeutic Targets in Rheumatoid Arthritis (Progress in Inflammation Research) - Birkhäuser Basel (2009)
Paul-Peter Tak-New Therapeutic Targets in Rheumatoid Arthritis (Progress in Inflammation Research) - Birkhäuser Basel (2009)
Paul-Peter Tak-New Therapeutic Targets in Rheumatoid Arthritis (Progress in Inflammation Research) - Birkhäuser Basel (2009)
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Preface .............................................................................. xi
Wim B. van den Berg, Leo A. B. Joosten and Fons A. J. van de Loo
Role of IL-1 in erosive arthritis, lessons from animal models . . . . . . . . . . . . . . . . . . . . . 59
Charles A. Dinarello
Role of IL-18 in inflammatory diseases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
Edward C. Keystone
Perspectives in targeted therapy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207
vi
List of contributors
vii
List of contributors
Alisa E. Koch, Veterans’ Administration, Ann Arbor Healthcare System, Ann Arbor,
MI, USA and University of Michigan Health System, Department of Internal Medi-
cine, Division of Rheumatology, Ann Arbor, MI, USA
Jagtar Nijar Singh, Centre for Rheumatic Diseases, Division of Immunology, Infec-
tion and Inflammation, University of Glasgow, 10 Alexandra Parade, Glasgow
Royal Infirmary, Glasgow G312ER, UK
viii
List of contributors
Fons A.J. van de Loo, Rheumatology Research & Advanced Therapeutics, Depart-
ment of Rheumatology, Radboud University Nijmegen Medical Centre, Geert
Grooteplein 28, 6525 GA, Nijmegen, The Netherlands
Wim B. van den Berg, Rheumatology Research & Advanced Therapeutics, Depart-
ment of Rheumatology, Radboud University Nijmegen Medical Centre, Geert
Grooteplein 28, 6525 GA, Nijmegen, The Netherlands;
e-mail: w.vandenberg@reuma.umcn.nl
ix
Preface
During the past decades important breakthroughs have been made in the treatment
of rheumatoid arthritis (RA). First, the implementation of low-dose methotrexate
and other conventional disease-modifying anti-rheumatic drugs was introduced
as an effective treatment. Second, it was recognized that early immunomodula-
tory treatment is crucial for controlling the disease and its long-term destructive
effects more effectively. Parallel advances in research on the pathogenesis of RA
and cytokine biology converged in identifying tumor necrosis factor (TNF) as a
key factor in inflammation and matrix destruction. The concept arose that elevated
TNF concentrations at the sites of inflammation were driving disease pathology,
and the removal of excess TNF from sites of inflammation became a therapeutic
goal. Clearly, TNF blockade has revolutionized the treatment of RA, as well as
other immune-mediated inflammatory diseases. Anti-TNF treatment results in clini-
cal benefit in a significant proportion of the patients, and it has provided proof of
concept for the principle of targeted therapy.
Despite the impressive disease-modifying effects of the TNF blockers, not all
patients respond, and patients who exhibited an initial response may lose response
due to the development of anti-drug antibodies (human anti-chimeric antibodies
and human anti-human antibodies, respectively) and perhaps as a result of escape
mechanisms related to the disease process. In fact, the majority of the patients still
have disease activity in at least one or two actively inflamed joints. In addition,
there have been numerous reports of moderate to severe adverse events associated
with their continuous use. There is also still some uncertainty as to how long the
available anti-rheumatic biologicals can be continuously employed as RA therapies.
Thus, there is still a huge unmet need in the current management of RA.
New therapeutic options would include new targeted therapies, perhaps com-
binations of biological therapies, targeting of synovial fibroblasts, individualized
therapies determined by personal profiles of clinical features and biomarkers, and
local treatment for persistent joint disease.
The aim of this book is to highlight advances regarding new therapeutic targets
in RA. Obviously, not all therapeutic targets that are currently under investigation
xi
can be discussed in one book. Therefore, a choice has been made that will allow
insight into targeted therapies with novel mechanisms of action that have recently
entered the market. In addition, we present examples of new therapeutic strategies
that are in preclinical or clinical development. The first three chapters are devoted
to new treatments interfering with B cells, T lymphocyte costimulatory pathways
and the interleukin (IL)-6 receptor. These chapters summarize the clinical effects
and mechanism of action of new anti-rheumatic treatments that have been shown
to be effective (rituximab, abatacept, and tocilizumab) and discuss new related
approaches. Rituximab, abatacept, and tocilizumab have become useful additions
to the available treatments for RA and, together with the TNF blockers, they have
also paved the way for the development of new targeted therapeutic strategies.
These biological treatments have raised the bar considerably for the efficacy of new
treatments, but the fact that roughly one third of the patients does not respond to
currently available treatments leaves a major challenge in RA research.
The next six chapters review data on new approaches that have, in part, been
tested in patients with RA, but where there is still uncertainty about the clinical
effects, or where the results of clinical trials have not yet been published: e.g., tar-
geting IL-1, IL-15, IL-17, IL-18, chemokines, and signaling pathways. Chapter 10
describes oncostatin M, a pleiotropic cytokine with potential utility as a treatment
for inflammatory arthritis; clinical trials aimed at blocking this cytokine may be
expected in the near future. Chapter 11 is dedicated to a fundamentally different
approach: targeting the epigenetic modifications of synovial cells, which is still in
the preclinical phase. Finally, chapter 12 provides a more general perspective of the
lessons learned from the use of targeted therapies with regard to the utility of animal
models of RA, clinical trial design, pharmacodynamics, immunobiology and key
pathogenic elements of disease.
I am grateful to all contributors to this volume for sharing their expertise from
basic science to clinical trials and vice versa; all of them are key opinion leaders who
work at the cutting edge of the development of innovative therapies for RA. I would
also like to thank Hans Detlef Klüber of Birkhäuser Verlag AG for his patience and
support. I hope all readers will be as thrilled as I am with the exciting developments
in this field.
Abstract
B cells are critical to the pathogenesis of rheumatoid arthritis (RA). There is substantial evidence
of the efficacy of depletion of B cells in many patients with RA using the first licensed agent,
rituximab. Recent research has focused on enhancing efficacy using other targets to inhibit B cell
function, including other B cell-depleting antibodies and cytokines critical to B cell function. The
rationale for new B cell targets is discussed, as well as clinical data.
Introduction
Figure 1
Expression of surface markers and BLyS/APRIL receptors during B cell development.
2
B cell targets in rheumatoid arthritis
Immune complexes in RA
Rheumatoid factor (RF) is present in more than 80% of patients with RA, and
may be detectable in the synovium (but not the blood) of some “seronegative RA”
patients [8]. Low-affinity “physiological” RF is produced transiently in infectious
disease, in which it may facilitate clearance of immune complexes by causing aggre-
gation into larger complexes [9, 10]. In addition to this effect, RF-producing B cells
may bind immune complexes containing foreign antigen and present it efficiently to
T cells, thus receiving T cell help without needing the presence of an autoreactive,
Fc-specific T cell [11]. High-affinity RF-producing B cells that occur in RA may
survive and proliferate by a similar mechanism [12].
An early observation in RA was the correlation of the titre of IgM RF with
synovial complement consumption [2, 3]. Although IgM RF is most frequent, class-
switched RFs are also found, especially in patients with higher RF titres. IgG RFs
are able to self-associate, particularly in the joints where there is a relative paucity of
normal immunoglobulin (Ig) but large numbers of RF-producing plasma cells [13].
Self-associated IgG RF has also been hypothesized to contribute to pathogenicity of
immune complexes in RA by forming smaller, dimeric complexes that evade clear-
ance by complement in blood and access the joint more easily [14].
3
Edward M. Vital, Shouvik Dass and Paul Emery
4
B cell targets in rheumatoid arthritis
5
Edward M. Vital, Shouvik Dass and Paul Emery
6
B cell targets in rheumatoid arthritis
to anti-CD20. Thus, type I mAbs induce translocation into Triton X-100 (Tx-100)-
insoluble lipid rafts and are much more potent at activating lytic complement than
type II mAbs, which do not induce such lateral translocation of antibody–antigen
complexes [62] (lipid rafts are highly ordered membrane domains abundant in gly-
cosphingolipids, cholesterol and signalling proteins such as Src kinases).
Anti-CD20 mAbs can also trigger signals leading to cell death or cell arrest [58–
62, 64]. In patients treated with rituximab for B cell chronic lymphocytic leukaemia
(B-CLL), better clinical responses and more extensive depletion of CD20-positive
cells correlated with apoptosis as detected by caspase-3 and -9 processing [80]. The
redistribution of CD20 into lipid rafts may also be of importance. Changes in the
regulation and activity of Src-like tyrosine kinases, which are present in high levels
in that environment, may be important in CD20-induced apoptosis [59]. Other
methods of inducing cell death not related to lipid rafts may also include a certain
degree of CD20 cross-linking, which triggers apoptotic pathways, including caspase
activation [81].
CD20 is an attractive target antigen for a number of reasons. The range of its B
cell expression means that depletion of CD20-positive cells leaves the population of
stem cells and plasma cells intact, ensuring eventual repletion of B cells as well as
IgG levels, at least in the early stages of therapy. The lack of modulation of CD20
also makes it attractive for therapies that may work via CDC and ADCC. These
mechanisms are also aided by the high expression of CD20 (100 000–200 000 copies
on most B cell lines).
Future research into CD20 as a therapeutic target, particularly in autoimmune
disease, may centre on the antigen itself as well as targeting antibodies. With regard
to the former, questions have been raised about the the presence of a circulating
soluble CD20 (cCD20), which may block the binding of rituximab to target cells.
Such cCD20 has been detect in patients with non-Hodgkin’s lymphoma (NHL) and
CLL, and to a lesser extent in normal controls [82, 83]. This could also affect the
measurement of rituximab concentrations in pharmocokinetic studies. Many of the
factors influencing the pharmacokinetics of rituximab are incompletely understood.
The initial choices of doses in haematology, which influenced to a degree those
eventually selected in rheumatology, were mostly based on industrial considerations
[84]. Relationships have been reported between rituximab exposure and tumour
response [85] and progression-free survival [86]. In systemic lupus erythematosus
(SLE), the degree of lymphocyte depletion correlated inversely with rituximab con-
centration 2 months after treatment [71]. In both haematological and autoimmune
disease, rituximab pharmacokinetics can be described by a two-compartment model
[84]. This suggests that the expression of CD20 (presumably higher in patients with
tumours than in autoimmune disease) does not affect rituximab pharmacokinetics
and exposure. Thus, the variation in exposure to rituximab is not yet fully explained
but may relate to factors influencing the avidity of rituximab binding, e.g. FcR sub-
types and polymorphisms therein, as described earlier.
7
Edward M. Vital, Shouvik Dass and Paul Emery
With regard to anti-CD20 mAbs as a group, which antibody is optimal for the
different disease processes it is not yet clear. Rituximab may work mainly via CDC
mechanisms. Complement processes may be most effective in situations where the
target cell is blood borne, allowing good access, i.e. type I anti-CD20 mAbs may be
most suitable. Whether this applies to autoimmune rheumatic diseases is also not
entirely clear and indeed in different diseases, such as RA and SLE, different mAbs
may be useful. In the former, it remains to be seen how well rituximab affects B
cells in compartments other than blood (e.g. synovium) and whether this is relevant
to clinical outcomes; in the latter, which is often associated with complement defi-
ciency or defects, mAbs that act by non-complement-dependent mechanisms may
be more useful. Understanding the cell death pathway more completely will clearly
be of key importance and recent advances demonstrating the possibility that CD20
may be a calcium channel may be of great relevance here. Parallel to this lies further
work in developing different CD20 mAbs and studying the true differences in the
mechanisms by which different mAbs induce cell death.
The molecules of the B lymphocyte stimulator (BLyS) system seem to have a role at
most, if not all, stages of B cell development. Two related TNF family ligands and
three TNFR family receptors have been described that have complex and sometimes
overlapping binding and function in B cell homeostasis (Fig. 2). The ligands are
BLyS (also commonly called BAFF and TALL-1) [87–90] and ‘a proliferation-induc-
ing ligand’ (APRIL, TALL-2) [91]. These interact with the receptors transmembrane
activator and calcium modulator and cyclophylin ligand interactor (TACI, CD267),
BLyS receptor 3 (BR3, BAFF-R, CD268) and B cell maturation antigen (BCMA,
CD269). BLyS binds strongly to BR3, and more weakly to BCMA. APRIL binds
strongly to BCMA. Both ligands bind TACI equally. The function of individual
components has been elucidated in mouse and human studies.
The ligands BLyS and APRIL are closely homologous and, like other members
of the TNF family, form active soluble homo- and heterotrimers [92–94]. Although
BLyS is predominantly produced by monocytes, macrophages and dendritic cells
[95], there is also evidence of production by neutrophils [96], and in some situations
B cells [97, 98]. The release of BLyS by myeloid cells is stimulated by pro-inflamma-
tory cytokines including interferon (IFN)-G, TNF-A and IL-10 [95, 99].
Functions of BLyS
When mice are treated with BLyS, splenic B cell numbers are increased [87] due to
increased B cell survival [100]. This is mediated by modifying the ratio of expression
8
B cell targets in rheumatoid arthritis
Figure 2
Binding between ligands and receptors of the BLyS/APRIL system. Note that BLyS and APRIL
can also form homo- or heterotrimers that are biologically active (solid lines indicate stronger
binding than the dashed line).
of pro- and anti-apoptotic molecules [101]. BLyS transgenic mice have expansion
of mature B cells in peripheral blood, lymph nodes and spleen. They also develop
hypergammaglobulinaemia, autoantibodies characteristic of human rheumatic dis-
eases including RF, antibodies to nuclear antigens and cryoglobulins, and also renal
immune complex deposition [102, 103]. In contrast, BLyS knockout mice have T1,
but no T2, marginal zone or follicular B cells in the spleen or lymph nodes and no
B2 in the peritoneum and markedly reduced Ig levels [104]. In contrast to mice,
neither transitional nor plasma cells proliferate in response to BLyS in humans, but
mature B cells and plasmablasts do [105]. As well as its role in B cell development
and homeostasis, BLyS is important for normal B cell function in response to anti-
gen. The production of BLyS by dendritic cells under stimulation by CD40L and
IFN-G has been shown to be crucial to induction of class-switch recombination fol-
lowing BCR engagement [106].
Functions of APRIL
The function of APRIL is less well understood. APRIL does not have equivalent
in vitro B cell stimulatory properties [107, 108]. APRIL transgenic animals do
9
Edward M. Vital, Shouvik Dass and Paul Emery
Human transitional, naïve, germinal centre and memory B cells in the spleen,
peripheral blood and bone marrow all bind BLyS, but pro-, pre- and immature B
cells do not. Human subsets of B cells differ in their expression of receptor for BLyS
and APRIL, which determines their sensitivity to these ligands (Fig. 1).
In transitional and naïve B cells, only BR3 is expressed. Other BLyS receptors are
only expressed on later subsets. Memory B cells bind BLyS by either BR3 or TACI,
but not BCMA [110]. BLyS enhances their survival but not proliferation. After dif-
ferentiating into plasmablasts, BR3 is down-regulated, and TACI and BCMA are
expressed [111]; BLyS also enhances survival of these cells. Terminally differentiated
plasma cells, however, do not express any BLyS receptor nor respond functionally
[110].
BLyS system in RA
Serum levels of BLyS and APRIL are elevated in many rheumatic diseases [112] and
in RA higher levels in synovium confirm that there is local production here [34].
The production and function of BLyS system molecules differs in synovia with dif-
ferent lymphoid architecture [113]. While macrophages produce BLyS in all types of
rheumatoid synovitis, APRIL production (by dendritic cells) is greatest when there
is a germinal centre-like pattern, lower in aggregate synovitis and lowest in diffuse
synovitis. Similarly, although BR3 and TACI are expressed equally in all tissues,
the relatively APRIL-specific receptor BCMA is expressed more strongly in germi-
nal centre or aggregate synovitis than in diffuse synovitis. Treatment of a human
synovium-SCID mouse chimera with TACI-Fc inhibits Ig and IFN-G production
in germinal centre synovitis; it also destroys the germinal centres themselves and
reduces B, T and follicular dendritic cell numbers. In diffuse and aggregate synovi-
tis however, Ig production is not affected by treatment with TACI-Fc, and IFN-G is
actually increased. Thus, the roles of BLyS and APRIL in the rheumatoid synovium
are complex and may vary according to disease subtype.
10
B cell targets in rheumatoid arthritis
Rituximab
Data regarding the clinical efficacy and safety of rituximab are largely derived from
three double-blind, randomised, placebo-controlled studies, each of which demon-
strated superiority to methotrexate in control of disease activity [114–116].
In RA, rituximab has been given as a fixed dose (i.e. not determined by surface
area) as two infusions, separated by 14 days. Each infusion comprised 500 mg or
1 g. The first Phase IIa study [115] indicated that the combination of two infusions
of 1 g rituximab with methotrexate (10–25 mg weekly) was significantly better than
methotrexate alone in allowing patients to achieve American College of Rheuma-
tology (ACR) 50 response 6 months after therapy (43% vs 13%, p = 0.005). The
REFLEX study [114], which investigated patients who had inadequate response to
anti-TNF therapy, also demonstrated that use of two infusions of 1 g rituximab with
methotrexate was superior to methotrexate alone in achieving ACR20 response
after 6 months (51% vs 18% respectively, p < 0.0001).
The Phase IIa study [115] demonstrated that the combination of rituximab with
either methotrexate or cyclophosphamide was significantly better than methotrex-
ate monotherapy in achieving ACR50 response at 24 weeks and this was main-
tained at 48 weeks. However, the combination of rituximab and methotrexate was
the only one to demonstrate significantly higher frequency of ACR70 response at
24 weeks compared to the control group and has become the standard combination
in subsequent studies. Although rituximab monotherapy was significantly better
than methotrexate monotherapy at 24 weeks (p < 0.002), this improvement was no
longer statistically significant at week 48. How methotrexate prolongs the benefits
of rituximab therapy is not yet fully understood. The concomitant use of methotrex-
ate does not appear to affect the duration of B cell depletion [117]. There are case
series that suggest that other disease-modifying anti-rheumatic drugs (DMARDs)
may be safe and effective in combination with rituximab [118], although further
studies are required with such agents.
The initial use of rituximab in RA drew considerably on experience from hae-
matology and so rituximab infusions were preceded by intravenous steroids and
a course of oral steroids was prescribed between infusions. The DANCER study
showed that intravenous steroids significantly reduced the incidence and severity of
acute infusion reactions but neither intravenous nor oral steroids appear to have any
significant effect on efficacy at 24 weeks [116].
11
Edward M. Vital, Shouvik Dass and Paul Emery
Conventional flow cytometry analysis suggests that there is nearly complete deple-
tion of B cells from the peripheral blood following rituximab therapy in all patients.
This being so, there would seem to be no correlation between B cell depletion and
clinical response. However, highly sensitive flow cytometric techniques, developed
to assess disease response in haematological malignancies and known as minimal
residual disease flow cytometry (MRD Flow), has given early indication that B cell
depletion may not be as complete as initially thought [121]. In terms of B cell recon-
stitution, data from the initial Phase IIa study are extensive, with patients now being
followed for up to 2 years. The data suggest that B cells begin to rise between 6 and
12 months after treatment in most patients. Clinical deterioration appears to occur
between 6 and 12 months after the initial infusion [122]. The relationship between
the return of B cells and that of disease activity is not clearly defined. In individual
patients, the phenomenon of B cell counts rising with concurrent disease relapse
is well observed. However, some patients have demonstrated return of B cells in
12
B cell targets in rheumatoid arthritis
peripheral blood without relapse. In an open label study, in patients in whom return
of disease was associated with return of B cells, there was a higher proportion of
memory B cells compared to those who underwent B cell reconstitution without
relapse [123].
In the clinical trial setting, patients with relapse of disease following initial response
were identified (on clinical grounds: numbers of swollen and tender joint counts) and
entered into retreatment protocols. Most patients who received retreatment did so
when their B cell levels, although rising, were lower than before their first exposure
to rituximab. Retreatment has generally been well tolerated and effective [124, 125].
The median time between the first and second course of rituximab was 30.9 weeks
for patients who had received prior anti-TNF therapy and 36.7 weeks in those who
had not had prior anti-TNF therapy. The median time between second and third
courses was 30.1 weeks for the former and 43.0 for the latter [126]. Patients gener-
ally received retreatment at levels of disease activity that were lower than at baseline,
hence ACR responses (from original baseline) following the second course were bet-
ter than those seen after the first course and similar when compared with the course
baseline. When EULAR responses are considered, more patients had good responses
and entered low disease states or remission after the second course than the first.
With regards to the safety of repeat courses of rituximab, data are available from
570 patients who have had a second course and 191 who have had a third [124,
125]. For patients who had received anti-TNF previously, the median time to retreat-
ment with a second course was 30.9 weeks and between second and third courses
was 30.1 weeks. These times for patients who had not previously received anti-TNF
were 36.7 and 40.3 weeks, respectively [126]. The rate of infections (including those
deemed serious) did not change significantly with repeated therapy. The incidence of
infusion reactions fell with the second and subsequent courses of therapy. Ig levels fell
with repeated therapy, and after the third course, 23.5% of patients had lower levels of
IgM than normal. However, the rate of serious infections before and after detection of
low IgM did not change significantly (5.1 vs 5.9 per 100 patient years, respectively).
A number of patients from the initial study (45% of those receiving one cycle
of rituximab with continuing methotrexate) did not have worsening of disease that
warranted additional therapy, and improvements in physical function were present
at 2 years after therapy [127, 128]. In the small number of patients who had pro-
longed B cell depletion for up to 2 years after therapy, there was no increase in the
rates of infection. The median duration of B cell depletion in patients who experi-
enced infections was similar to that in patients who did not have infections.
Ocrelizumab
13
Edward M. Vital, Shouvik Dass and Paul Emery
Clinical responses were observed across the doses that were improved over placebo.
Further dose-finding studies are being conducted [130].
Ofatumumab
Belimumab
Atacicept
14
B cell targets in rheumatoid arthritis
BR3-Fc
Conclusions
15
Edward M. Vital, Shouvik Dass and Paul Emery
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26
Co-stimulatory pathways in the therapy of rheumatoid arthritis
Abstract
Although T lymphocytes are widely recognized as important effector cells in the immunopatho-
genesis of rheumatoid arthritis (RA), therapies targeting T cell populations have not been clinically
successful, largely due to the toxicity associated with nonspecific T cell depletion. An alternative
approach involves targeting T cell activation, a process that requires two distinct signals. In addi-
tion to the cognate interaction between the T cell receptor on T cells and antigen bound to the
major histocompatibility complex on the antigen-presenting cell (APC), a second, co-stimulatory,
signal is required for T cell activation. Therapies targeting co-stimulatory pathways, aimed at mod-
ifying the activation of T cells, rather than reducing their absolute numbers, may be an effective
alternative to T cell depletion in RA and other rheumatic diseases. One such treatment, abatacept
(CTLA4Ig), a fusion protein combining cytotoxic T lymphocyte antigen 4 (CTLA4) and a portion
of the Fc domain of human IgG1, has been approved in the United States and the European Union
for the treatment of RA. Abatacept may modulate the T cell or the APC to produce several differ-
ent outcomes within the joint, including down-regulation of T cell activation, stimulation of T cell
apoptosis, or possibly modulation of T regulatory cell activity. In large, controlled trials in patients
with RA with an inadequate response to either methotrexate or TNF antagonists, abatacept effec-
tively reduced disease activity. In the methotrexate-inadequate responder population, radiographic
progression was slowed when compared to continued treatment with methotrexate alone. The
safety profile of this therapy is similar to that of other biological response modifiers, with infection
being the most concerning treatment-emergent adverse event. Additional co-stimulatory pathways
may offer attractive targets in RA and other immune-mediated diseases, although, to date, none
has had the clinical success of abatacept and the related CTLA4Ig fusion protein, belatacept.
Introduction
T cell-targeted therapy has been attempted in rheumatoid arthritis (RA) for many
years. These efforts have included lymphatic duct drainage, total lymphoid irradia-
tion, inhibition of T cell trafficking and T cell depletion with antibodies targeting T
lymphocyte surface markers. While some of these therapies initially showed prom-
ising results, none turned out to be clinically effective and all have been hampered
CD28-CD80/86 co-stimulation
One of the most prominent T cell co-stimulatory signals is mediated through the
interaction of the cell surface protein CD28 on T cells and its ligands on APCs
(reviewed in [1]). CD28 is constitutively expressed on most T cells, and it binds
to both CD80 (B7-1) and CD86 (B7-2), which are expressed on APCs, including
dendritic cells, B cells, and macrophages (Fig. 1A). CD80 and CD86 are increased
during dendritic cell maturation, which enhances the ability of mature dendritic cells
to activate T cells [2]. Macrophages from the joints of patients with RA express
increased levels of CD80 and CD86 compared to RA peripheral blood monocytes
[3]. CD80 and CD86 are not only expressed on APCs within the RA joint, they are
also expressed on activated T cells, suggesting a potential self-sustaining mechanism
for T cell activation [4]. Binding of CD28 to CD80 or CD86 provides the second
signal required for maximal T cell activation; the absence of such a signal may result
in anergy and apoptotic cell death.
Cytotoxic T lymphocyte antigen 4 (CTLA4 or CD152) is up-regulated following
T cell activation, and it also interacts with CD80 and CD86, providing an impor-
tant control for regulating T cell function [5, 6]. CTLA4 has a higher affinity for
CD80 and CD86 and can displace CD28 from its interaction with these molecules,
interrupting the activation signal [7] (Fig. 1B). Since the level of CTLA4 expression
on activated T cells is proportional to the strength of T cell receptor signal, this
pathway provides a mechanism to down-regulate T cell activation [8]. CTLA4 is
28
Co-stimulatory pathways in the therapy of rheumatoid arthritis
Figure 1
Abatacept (CTLA4Ig) interrupts co-stimulatory signals. T cell activation requires presenta-
tion of antigen presented by MHC molecules on antigen presenting cells (APC) together
with co-stimulation through CD28 and CD80/86 (A). Following T cell activation CTLA4 is
up-regulated and its affinity for CD80/86 is greater than that of CD28, which interrupts co-
stimulation, suppressing T cell activation (B). CTLA4Ig is capable of interrupting signaling
through CD28 and potentially through CTLA4 (C).
expressed on T cells from the RA joint [4], supporting the potential importance of
this pathway in regulating T cell activation in this disease.
In addition to interfering with activation, CTLA4-CD80/86 interaction may
also provide negative signals that lead to long-term tolerance. The binding of cell
surface CTLA4 (or with CTLA4Ig) to CD80/86 on dendritic cells may induce the
production of indoleamine 2,3-dioxygenase (IDO) by the dendritic cell [9]. IDO is
an enzyme that promotes local tryptophan depletion, resulting in the inhibition of
T cell proliferation and the induction of apoptosis [10].
CTLA4 signaling may also play a role in the development of an important class
of T cells, CD4+CD25+ T regulatory cells. The development of T regulatory cells
is dependent upon the transcription factor forkhead-box protein 3 (FoxP3), since
deficiency of FoxP3 results in the lack of T regulatory cells and autoimmune disease
[11]. Interactions between CD28 and CD80/86 are necessary for the generation of
T regulatory cells [12]. Increased numbers of CD4+CD25+ T regulatory cells are
present in the RA joint, and they are highly active at suppressing the activation
of other T cells ex vivo [13]. However, in the presence of tumor necrosis factor
29
Eric M. Ruderman and Richard M. Pope
(TNF)-A, which is present in the RA joint, the local T regulatory cells demonstrate
a diminished capacity to suppress T cell activation [3]. Together, these observations
support the potential role of CTLA4 expressed in the RA joint in the development
of T regulatory cells.
Two recombinant molecules (CTLA4Ig) have recently been developed that com-
bine the extracellular domain of human CTLA4 with a portion of the Fc domain
of IgG1. One of these fusion proteins, abatacept, binds CD80 more avidly than
30
Co-stimulatory pathways in the therapy of rheumatoid arthritis
The first large clinical trial to examine the efficacy of co-stimulatory blockade in
RA was a placebo-controlled, dose-ranging study of abatacept and belatacept, a
second-generation modification of abatacept, in patients with refractory RA [21]
(Tab. 1). Two hundred and fourteen patients, all of whom had failed treatment with
at least one disease-modifying anti-rheumatic drug (DMARD), were randomized
to receive four infusions of placebo, abatacept, or belatacept over 2 months. Prior
therapy with etanercept, the only biological available at the time of the trial, was
allowed, although the published manuscript does not state how many had actually
received this treatment. Three doses of each of the two compounds were tested
(0.5, 2, or 10 mg/kg), and these were given at baseline, 2, 4, and 8 weeks. This
dosing strategy was selected to provide a loading dose that could lead to a more
rapid response, to be followed by less frequent maintenance dosing. Abatacept was
given as monotherapy in this trial; all background DMARD therapy was stopped.
Both compounds proved effective at generating the primary endpoint, an ACR20
response, and both had a clear dose response. Abatacept appeared somewhat more
effective at producing higher levels of response (ACR50 and ACR70), although the
trial was not powered to show statistical differences between the two compounds.
No further RA trials have studied belatacept, and this compound is currently being
pursued for transplant indications, as discussed below [24]. No significant safety
signals were seen in this trial, even at the higher dose levels.
31
Table 1 - Abatacept efficacy in blinded, controlled clinical trials.a
32
Trial n Trial duration Study Background ACR20 on ACR20 on Remission on
population therapy abatacept placebo abatacept (DAS28
< 2.6)
Moreland 122 12 weeks Active RA None 53% 31% N/A
et al. [21]
Kremer et 339 6 months Methotrexate Methotrexate 60.0% 35.3% 26.1%
al. [27] inadequate responders
Eric M. Ruderman and Richard M. Pope
Following the dose-ranging trial, abatacept has been studied in two published
trials in RA patients showing an inadequate response to methotrexate [25–27]
(Tab. 1). Both trials had an initial placebo-controlled period, followed by an open
label extension treatment period. Both used the dosing schedule established in the
pilot study: three loading doses given 2 weeks apart, followed by dosing every
4 weeks thereafter. The first (Phase II) study included two doses, 2 and 10 mg/kg,
compared with placebo, with blinded treatment continued for 6 months [25, 27].
The second, pivotal trial compared placebo with a fixed dose that was determined
by patient weight range, approximating 10 mg/kg (i.e., 500 mg for patients under
60 kg, 750 mg for those 60–100 kg, and 1000 mg for those over 100 kg), with
blinded therapy continued for a year [26]. This latter dosing strategy has become
the commercially approved dose.
33
Eric M. Ruderman and Richard M. Pope
study drug. It should be noted that subjects in this trial were permitted to adjust
their methotrexate and steroid doses, or to add additional non-biological DMARDs,
during the second 6 months of therapy; the number who actually did so was not
reported.
Twelve months of therapy with abatacept in the phase II trial produced signifi-
cant improvements in function and health-related quality of life (HRQOL) measure-
ments relative to placebo [26, 28]. The health assessment questionnaire (HAQ),
which showed only modest functional impairment at baseline (mean score 1.0), was
nevertheless significantly improved in the 10 mg/kg group relative to placebo (42%
vs 10%). Compared with placebo, there were statistically significant improvements
in all eight scales of the Short Form-36 (SF-36) as well as the summary scores of
the physical component and mental component. Improvements were greater than
one-half standard deviation, which has been described as clinically significant [29],
and were of a magnitude similar to that seen with other effective disease modifying
therapies in RA. The greatest effect sizes were seen for the physical function, bodily
pain, and vitality scales. Improvement in HRQOL was only marginal for the 2 mg/
kg dose and was related to the clinical response for the 10 mg/kg dose.
Perhaps not surprisingly, biomarkers of inflammation also showed improvement
with abatacept therapy in the Phase II study [30]. Interleukin (IL)-6, soluble IL-2
receptor (sIL-2R), C-reactive protein (CRP), soluble E-selectin, and soluble intercel-
lular adhesion molecule-1 were all significantly reduced in the 10 mg/kg treatment
group compared with placebo. The only one of these molecules produced by the
activated T cells directly impacted by abatacept is sIL2-R, so the implication of this
observation is that abatacept has downstream effects on other immunologically
active cells. No radiographic data were collected in this trial.
34
Co-stimulatory pathways in the therapy of rheumatoid arthritis
73%, 48%, and 29%, compared with 40%, 18%, and 6% for placebo; for ACR50
and ACR70, the increased proportion of responders was statistically significant.
In addition, the percentage of abatacept-treated patients achieving remission by
DAS28 criteria increased from 15% at 6 months to 24% at 1 year. As in the Phase
II trial, methotrexate dose adjustments and the addition of a second non-biological
DMARD were allowed during the second 6 months of the study, although subjects
and investigators remained blinded to treatment assignment. Additional therapy
was begun in just 8% of the abatacept-treated patients compared with 14% of the
placebo group, suggesting that the increasing likelihood of higher level response
seen with longer-term therapy was not attributable to modification of medications
other than abatacept.
Both function and HRQOL were improved by abatacept in this study [26, 31].
Baseline function was much more impaired than in the group studied in the Phase
II trial, with a baseline HAQ disability index of 1.7 for both treatment groups. The
mean improvement in HAQ score, as well as the proportion of patients with a clini-
cally meaningful improvement, was statistically greater for the abatacept group at
both 6 and 12 months. Significant improvements were also seen in both the physical
and mental component summaries of the SF-36 at both time points.
The AIM study was the first to demonstrate structural benefit with abatacept.
At 1 year, radiographic progression, measured using the Genant modification of
the Sharp scoring system, was reduced by approximately 50% in the abatacept-
treated group compared with placebo [26]. Although the radiographic data set was
not complete (only 92% of study subjects had both baseline and at least one post-
treatment radiograph), sensitivity analysis suggested that this effect was real.
Overall, adverse events were generally similar for abatacept and placebo in this
study. The two most commonly reported adverse events, headache and nasopharyn-
gitis, were more frequent in the abatacept group, and there were also more discon-
tinuations due to adverse events in this group (4.2% vs 1.8%). Serious infections, in
particular pneumonia, were numerically more common with abatacept therapy. Six
abatacept-treated patients (1.4%) were classified as having pneumonia or broncho-
pneumonia, compared with a single placebo patient (0.5%). One case of tuberculo-
sis was reported in each group, although neither was confirmed by culture.
The effectiveness of abatacept has been confirmed in a second Phase III study in a
different population. It is now well-recognized that TNF-A antagonists, while very
effective for the signs and symptoms of RA, fail to achieve an adequate response,
fail to maintain that response, or are poorly tolerated in a subset of patients. Abata-
cept has been evaluated in this population, where other approaches to controlling
disease are obviously required. In the ATTAIN study, a group of 391 patients with
35
Eric M. Ruderman and Richard M. Pope
long-standing active disease (mean just under 12 years) who had had an inadequate
response or were intolerant to a TNF antagonist received blinded treatment with
abatacept or placebo while continuing background DMARDs other than the TNF
antagonist [32] (Tab. 1). The same fixed dose, approximating 10 mg/kg, was used,
along with the standard dosing schedule.
In this trial, 50% of the abatacept-treated patients achieved an ACR20, the
primary endpoint, compared with 20% of the placebo-treated patients. Abatacept-
treated patients also did significantly better at higher levels of response, including
ACR50 (20% vs 4%), ACR70 (4% vs 2%), and remission by DAS28 criteria
(10% vs 1%). Patient-centered outcomes also improved in this study. Statistically
significant improvements relative to placebo were seen in HAQ scores, fatigue, and
both the physical and mental components of scores of the SF-36 [33]. Differences
were generally seen within 2–3 months; in the case of fatigue, a difference could
be seen after just 4 weeks of therapy (two doses of abatacept). These were notable
improvements in patients who had previously had an inadequate response to TNF
antagonists, the best previously available therapy. No additional safety signals were
seen in the ATTAIN trial beyond those previously seen in the trials with methotrex-
ate inadequate responders [32].
A second trial also looked at abatacept treatment in patients failing a TNF
antagonist, although it was designed to evaluate combination therapy, rather than
a switch to abatacept, in the hopes that targeting two distinct pathways in the
pathophysiology of RA would have a greater effect [34] (Tab. 1). In this trial, 121
patients with persistently active disease despite receiving etanercept 25 mg twice
weekly (the standard dose at the time the study was undertaken) were randomized
2:1 to receive abatacept 2 mg/kg or placebo, in addition to etanercept, for 1 year.
For the primary endpoint in this study, the proportion of ACR20 responders at
6 months, there was no demonstrable benefit with abatacept (48% vs 30%, differ-
ence not statistically significant). ACR50 responses at 6 months also did not differ,
although there was a statistically significant difference in ACR70 responses (11%
vs 0%, p = 0.042). Safety, however, was compromised by combination therapy in
this trial. The incidence of overall serious adverse events (16.5% vs 2.8%), serious
adverse events judged related to study drug (5.9% vs 0%), and serious infections
(3.5% vs 0%) during 1 year of blinded therapy were all greater with the combina-
tion of abatacept and etanercept.
Ultimately, 2 mg/kg, as discussed above, was found to be a sub-therapeutic
dose of abatacept. Subjects in this combination trial were allowed to enroll into an
open-label extension study, during which they received abatacept at 10 mg/kg. The
pattern of increased adverse events seen in the 80 subjects who entered this 2-year
extension was similar, with 32.5% reporting serious adverse events, 3.8% reporting
serious adverse events judged related to therapy, and 1 patient (1.3%) developing
a serious infection (septic arthritis). Three patients developed malignancies during
this extension phase.
36
Co-stimulatory pathways in the therapy of rheumatoid arthritis
An additional, larger trial has expanded the evaluation of the safety of abatacept
in combination with other background therapies [35]. In this 1-year, blinded trial,
patients with active disease despite treatment with at least one biological or non-
biological DMARD, were randomized 2:1 to receive abatacept at the fixed dose
approximating 10 mg/kg or placebo, in addition to their existing therapy. While
the clinical outcomes captured in this study were limited, it did confirm that the
addition of abatacept leads to significant improvements in patient pain assessments,
patient and physician global assessments, and HAQ scores over 1 year.
With respect to safety outcomes, the primary goal of this study, there were
important signals seen with the addition of abatacept to background therapy. Over-
all incidence of adverse events, serious adverse events, and discontinuations due to
adverse events did not show any differences between the groups receiving abatacept
or placebo. Serious infections, however, were numerically more frequent in the
abatacept-treated group (2.9% vs 1.9%). Perhaps more importantly, in the subset
of patients receiving therapy with a background biological DMARD (primarily TNF
antagonists), the incidence of serious adverse events (22.3% vs 12.5%) and serious
infections (5.8% vs 1.6%) was markedly elevated in the abatacept-treated group,
confirming the additional risk seen with combination biological therapy seen in the
earlier study.
Two additional controlled trials of abatacept, both recently published, are worth
noting. In a trial of abatacept for juvenile inflammatory arthritis, 122 of 190 sub-
jects met criteria for improvement after 4 months of open-label therapy with abata-
cept and were randomized to receive blinded therapy with abatacept or placebo for
an additional 6 months [37] (Tab. 1). During the blinded portion of the trial, 53%
of the placebo-treated patients flared, compared with just 20% of those treated
37
Eric M. Ruderman and Richard M. Pope
with abatacept (p = 0.0003). No patients in either group withdrew from the study
due to an adverse event. The overall incidence of adverse events was similar during
abatacept and placebo treatment, and there were no serious adverse events in the
abatacept group during the double-blind period.
In another trial, the first blinded comparison of two biological therapies in 431
patients with RA, treatment with abatacept ~10 mg/kg every 4 weeks was compared
with treatment with infliximab 3 mg/kg every 8 weeks over 1 year [38]. During the
first 6 months, a third group received placebo infusions every 4 weeks; these sub-
jects were switched to abatacept therapy during the last 6 months of the trial. There
was no statistical difference between abatacept and infliximab for the primary end-
point of reduction in DAS28 at 6 months; both groups showed statistically greater
clinical improvement than placebo. At 1 year the improvement of the DAS28 in
the abatacept-treated patients (2.88 DAS28 units) was modestly, but statistically,
greater than the improvement in the infliximab group (2.25 DAS28 units). Safety
outcomes in this study favored abatacept, with 18.2% of the infliximab-treated sub-
jects reporting serious adverse events over 1 year of double blind therapy, compared
with 9.6% of the abatacept-treated subjects. Serious infections were reported in
8.5% of infliximab treated vs 1.9% of abatacept-treated patients during this same
time period. The two reported cases of tuberculosis both occurred in infliximab-
treated patients.
One of the more interesting findings of the trials of abatacept in the methotrexate
and TNF inadequate responders is that the drug worked so well in patients with
many years of disease. From its purported mechanism, to affect the induction of the
immune response, one might assume that this treatment would be most effective
early in the disease course, before the arthritogenic T cell clones have become fully
activated. The demonstration of abatacept’s effectiveness later in the disease course
has a number of potential explanations. The most obvious is that T cell activation
is an ongoing process in RA, either through the cumulative addition of new popula-
tions of activated cells or the continued stimulation of the existing cells, and that
abatacept is able to interrupt this ongoing process. Another possibility is that the
clinical effect of abatacept relies on mechanisms other than simply interrupting the
CD28-CD80/86 axis, such as inducing production of IDO via its ligation to APCs,
which in turn may promote T cell tolerance. It is also possible that CD28 promotes
activation of APCs, which is interrupted by CTLA4Ig, or that CTLA4Ig signaling
38
Co-stimulatory pathways in the therapy of rheumatoid arthritis
Belatacept
Belatacept, the modified version of abatacept used in several arms of the original
dose-ranging study, has not been studied further in RA. This compound is currently
being developed as an immunomodulatory treatment in organ transplantation.
Belatacept compared favorably with cyclosporine at preventing acute rejection after
renal transplantation [24]. Subjects treated with belatacept in this trial had higher
mean glomerular filtration rates and a lower incidence of chronic allograft neph-
ropathy at 12 months than those treated with cyclosporine.
39
Eric M. Ruderman and Richard M. Pope
antibody was administered to human volunteers, cytokine storm was induced result-
ing from the rapid induction of cytokines [47]. The patients developed pulmonary
infiltrates, acute renal failure and intravascular coagulation which was life threat-
ening [47]. Therefore, not all mechanisms of interfering with the CD28-CD80/86
pathway are safe.
Modulation of co-stimulatory pathways represents a novel approach to the
treatment of immune-mediated diseases that seeks to normalize host immune
responses, rather than to disrupt intercellular signaling or to reduce specific popula-
tions of immunoactive cells. Interruption of signaling at the CD80/86-CD28 axis
has proven to be an effective therapy for RA; disruption of the CD40-CD40L axis
has been less successful in SLE. Future approaches to this type of modulation may
involve the targeting of additional co-stimulatory pathways, interference with the
expression of co-stimulatory molecules, or disruption of the intracellular signaling
pathways triggered by cell-surface molecule interaction.
References
40
Co-stimulatory pathways in the therapy of rheumatoid arthritis
41
Eric M. Ruderman and Richard M. Pope
42
Co-stimulatory pathways in the therapy of rheumatoid arthritis
Efficacy and safety of abatacept or infliximab vs placebo in ATTET: a phase III, multi-
centre, randomised, double-blind, placebo-controlled study in patients with rheumatoid
arthritis and an inadequate response to methotrexate. Ann Rheum Dis 67: 1096–1103
39 Nadkarni S, Mauri C, Ehrenstein MR (2007) Anti-TNF-alpha therapy induces a distinct
regulatory T cell population in patients with rheumatoid arthritis via TGF-beta. J Exp
Med 204: 33–9
40 Ehrenstein MR, Evans JG, Singh A, Moore S, Warnes G, Isenberg DA et al (2004)
Compromised function of regulatory T cells in rheumatoid arthritis and reversal by
anti-TNFalpha therapy. J Exp Med 200: 277–85
41 Merill JT, Burgos-Vargas R, Westhovens R, Chalmers A, D’Cruz D, Wallace D et al
(2008) The efficacy and safety of abatacept in SLE: Result of a 12-month exploratory
study. American College of Rheumatology Annual Meeting, Abstract L 15
42 Daoussis D, Andonopoulos AP, Liossis SN (2004) Targeting CD40L: A promising thera-
peutic approach. Clin Diagn Lab Immunol 11: 635–41
43 Toubi E, Shoenfeld Y (2004) The role of CD40-CD154 interactions in autoimmunity
and the benefit of disrupting this pathway. Autoimmunity 37: 457–64
44 Boumpas DT, Furie R, Manzi S, Illei GG, Wallace DJ, Balow JE et al (2003) A short
course of BG9588 (anti-CD40 ligand antibody) improves serologic activity and decreas-
es hematuria in patients with proliferative lupus glomerulonephritis. Arthritis Rheum
48: 719–27
45 Kalunian KC, Davis JC Jr, Merrill JT, Totoritis MC, Wofsy D (2002) Treatment of
systemic lupus erythematosus by inhibition of T cell costimulation with anti-CD154: A
randomized, double-blind, placebo-controlled trial. Arthritis Rheum 46: 3251–8
46 Rodriguez-Palmero M, Franch A, Castell M, Pelegri C, Perez-Cano FJ, Kleinschnitz C
et al. (2006) Effective treatment of adjuvant arthritis with a stimulatory CD28-specific
monoclonal antibody. J Rheumatol 33: 110–8
47 Suntharalingam G, Perry MR, Ward S, Brett SJ, Castello-Cortes A, Brunner MD et
al (2006) Cytokine storm in a phase 1 trial of the anti-CD28 monoclonal antibody
TGN1412. N Engl J Med 355: 1018–28
43
Immunobiology of IL-6 – Tocilizumab (humanised anti-IL-6
receptor antibody) for the treatment of rheumatoid arthritis
1
Chugai Pharmaceutical Co. Ltd., Tokyo 103-8324, Japan
2
Graduate School of Frontier Bioscience, Osaka University, Osaka 565-0871, Japan
Abstract
The cloning of IL-6 cDNA in 1986 revealed that IL-6 is a multifunctional cytokine that plays
important roles in the immunopathogenesis of rheumatoid arthritis (RA). A close relation-
ship was observed between IL-6 levels in the synovial compartment and disease activity in RA
patients, and overproduction of IL-6 could readily explain the abnormal laboratory findings and
clinical symptoms seen in these patients. IL-6 therefore appeared to be a worthwhile and attrac-
tive therapeutic target for RA. In practice, blockage of IL-6 signalling by a humanised anti-IL-6
receptor antibody [tocilizumab (TCZ); also known as MRA] has been found to be very effective
in the treatment of patients with RA. In recent Japanese Phase III clinical studies in RA patients,
TCZ clearly prevented radiographic progression of joint destruction and greatly improved signs
and symptoms. Very interestingly and importantly, this therapy has also proved quite effective at
improving fever, fatigue and anaemia. No serious adverse events have been reported. At present,
several international clinical studies of TCZ are ongoing in more than 4000 patients with active
RA in 41 countries. The results are continuing to confirm the efficacy and safety of TCZ in the
treatment of patients with RA.
Introduction
IL-6 was originally identified as a T cell-derived soluble factor that causes differen-
tiation of B cells into antibody-producing plasma cells [1]. When the gene coding
for IL-6 was cloned in 1986, it became apparent that IL-6 had been studied under
several different names in various laboratories. It has since been clarified that IL-6
does indeed have various biological functions in addition to B cell activation, and it
is now well known that IL-6 plays important roles in immunity, inflammation and
haematopoiesis. More importantly, evidence has accumulated that deregulation of
IL-6 results in the development of various autoimmune diseases. In fact, the multiple
biological activities of IL-6 provide explanations for various clinical symptoms of
rheumatoid arthritis (RA) [2–4].
These findings suggest that IL-6 is a worthwhile and attractive therapeutic target
molecule for RA. In this review, we discuss the immunopathological roles of IL-6
in RA, and the clinical usefulness of tocilizumab (TCZ), a humanised anti-IL-6R
antibody that blocks IL-6 signalling, in the treatment of RA.
IL-6 acts on various cell types and has a variety of biological functions, e.g. IL-6 acts
as a hepatocyte-stimulating factor (HSF) [5, 6]. Acute inflammation is accompanied
by changes in the plasma concentration of many proteins, such as a decrease in
albumin and increases in many “acute-phase proteins”, including C-reactive pro-
tein (CRP), fibrinogen, serum amyloid A protein and haptoglobin. Inflammation,
injury and cancer all induce the expression of IL-6, resulting in increased synthesis
of acute-phase proteins in the liver [5, 6]. Moreover, in IL-6 knockout mice, it has
been shown that IL-6 is essential for antiviral antibody response, as well as for the
induction of acute-phase reaction [7]. We have also reported that the injection of
recombinant human IL-6 into cynomolgus monkeys increased the serum CRP level
and the platelet count in the peripheral blood [8].
One of the recent advances regarding IL-6 is the discovery that IL-6 plays a
critical role in the development of chronic anaemia. It has been shown that IL-6
induces hepcidin, which is an iron regulatory peptide produced in the liver. Hepcidin
regulates the recycling of iron by macrophages and the absorption of iron from the
intestine. Thus, excessive IL-6 causes hypoferraemia, which leads to “anaemia of
chronic inflammation” (also known as “anaemia of chronic disease”).
Another important activity of IL-6 is the induction of osteoclast differentiation,
which may contribute to joint destruction in patients with RA. IL-6 also stimulates
the expression of vascular endothelial growth factor (VEGF), which is an essential
factor for neo-vascularisation. Moreover, IL-6 also enhances the function of leptin,
an anti-appetite hormone, resulting in anorexia in patients with chronic inflamma-
tory diseases. In addition, it has been reported that injection of IL-6 into cancer
patients caused fever.
We succeeded in isolating the cDNA for IL-6 and IL-6 receptor in 1986 and 1988,
respectively [9, 10]. We found that the receptor has an Ig-like domain at the N ter-
minus but no unique sequences in any other regions. It also has a very short intra-
cytoplasmic portion and no kinase domains. These features make it unlike what is
considered an “authentic receptor”. Another protein, a 130 kDa cell-surface glyco-
46
Immunobiology of IL-6 – Tocilizumab (humanised anti-IL-6 receptor antibody) for the treatment of RA
protein that we named gp130, is necessary for IL-6 signal transduction. We isolated
a cDNA encoding gp130. It was eventually concluded that the full IL-6 receptor
consists of two polypeptide chains of 80 and 130 kDa, and that IL-6 stimulation
triggers association of these two chains leading to IL-6 signalling [11–14]. A recent
crystal structure study has demonstrated that two of each molecule associate to
form a hexamer complex [15, 16].
Importantly, gp130 is expressed ubiquitously in all tissues, even in cells that lack
detectable expression of the 80-kDa IL-6 receptor [12]. This suggested that gp130
is not merely a component of IL-6 receptor, and that it might function as a common
signal transducer for various cytokines. In fact, many different cytokines do share
the same receptor component, and this can explain the redundant activity of several
cytokines.
We and others have reported that ciliary neurotropic factor (CNTF), leukaemia
inhibitory factor (LIF), oncostatin M (OM), IL-11 and cardiotropin-1 (CT-1) all use
gp130 as a component of their receptors [17–20]. This explains why these cytokines
have very similar activities.
More importantly, soluble IL-6 receptor (sIL-6R) that lacks transmembrane and
cytoplasmic domains is present in serum and synovial fluids. Once sIL-6R binds
to its ligand, the complex becomes capable of associating with gp130 to transduce
the IL-6 signal into cells. This means that the IL-6 signalling pathway functions, by
means of gp130, even for cells that do not express IL-6R on their surface. This is
called trans-signalling (Fig. 1A).
IL-6 and RA
It became evident that IL-6 is involved in various diseases, including chronic inflam-
mation. While trying to isolate the cDNA for IL-6, we noticed that the same activity
was observed in cardiac myxoma cells [21, 22]. Cardiac myxoma is a benign heart
tumour that arises from the atrium. Patients with cardiac myxoma exhibit a wide
variety of autoimmune and inflammatory symptoms, including autoantibodies,
fever, joint pains and anaemia. All these symptoms disappear after surgical removal
of the tumour.
We found that cardiac myxoma cells produce a large amount of IL-6. This result
suggested that IL-6 might contribute to the pathology of autoimmune diseases and
play an important role, not only in B cell immunology, but also in a variety of dis-
ease symptoms.
We also found an abnormal overproduction of IL-6 in patients with Castleman’s
disease [23]. Affected lymph node cells overproduce IL-6, which explains symptoms
such as high fever, anaemia, fatigue, anorexia, acute-phase reactions, hypergam-
maglobulinaemia, secondary amyloidosis and massive plasma cell infiltration into
47
Yoshiyuki Ohsugi and Tadamitsu Kishimoto
Figure 1
IL-6 signalling pathway and blockade of the signalling by tocilizumab (modified from: Bio-
technology J. Yodosha (2006) 7–8: 517–520).
48
Immunobiology of IL-6 – Tocilizumab (humanised anti-IL-6 receptor antibody) for the treatment of RA
On the basis of the above experimental and clinical results, we set out to develop
an anti-IL-6 receptor blockade therapy (Fig. 1B). In collaboration with the MRC
Collaborative Centre in London, mouse monoclonal antibody-binding human IL-6
receptor was humanised by means of complementarity-determining region (CDR)
grafting technology [26].
To investigate the direct role of IL-6 in the development of RA, IL-6-deficient mice
were backcrossed into C57BL/6 mice for eight generations, and the histological
manifestations following the induction of antigen-induced arthritis in IL-6-deficient
mice and wild-type littermates were compared [29]. Wild-type mice developed severe
arthritis, whereas IL-6-deficient mice displayed little or no arthritis. The expression
of TNF mRNA in synovial tissues in IL-6-deficient mice was comparable to that
in wild-type mice, even though no arthritis was observed in the former. Recently,
S. Sakaguchi and colleagues reported that deleting the IL-6 gene in the SKG mice
(which develop RA owing to a mutation on the T cell signalling pathway) gave
complete protection from development of RA, whereas 20% of TNF-A-deficient
SKG mice developed the disease [30, 31]. All of these basic studies encouraged us
to apply anti-IL-6 receptor therapy to patients with RA.
49
Yoshiyuki Ohsugi and Tadamitsu Kishimoto
The safety and efficacy of TCZ treatment were evaluated in multi-centre, double-
blind, randomised, placebo-controlled Phase II trials in RA patients in Japan [34]
and Europe [35] that were completed in 2001 and 2002, respectively. In Japan,
164 patients with refractory RA received 4 or 8 mg/kg TCZ, or placebo, i.v. every
4 weeks for a total of 12 weeks, and the clinical response was evaluated using the
American College of Rheumatology (ACR) criteria. As reported by Nishimoto et al.
[34], TCZ treatment significantly improved all measures of disease activity in the
ACR core set, and the results were comparable to or better than those obtained with
anti-TNF antibody or soluble TNF receptor therapy. The incidence of at least 20%
improvement in disease activity according to the ACR criteria (ACR20) was 78%
in the higher dose group. This was higher than in the lower-dose group (57%), and
significantly higher than in the control group (11%) (p < 0.001). The incidences of
50% and 70% improvements in disease activity (ACR50 and ACR70) were 40%
and 16%, respectively, in the higher dose group, and both of these incidences were
significantly higher than in the placebo group. Efficacy was also evaluated using the
Disease Activity Score 28-joint count (DAS28) categories; the incidence of good or
moderate response was 91% in the 8 mg/kg group, 72% (p = 0.012) in the 4 mg/kg
group and 19% (p < 0.001) in the placebo group. TCZ treatment also improved lab-
oratory findings such as haemoglobin levels, platelet counts, and the serum levels of
CRP, fibrinogen, SAA, albumin and rheumatoid factors. In addition, TCZ treatment
50
Immunobiology of IL-6 – Tocilizumab (humanised anti-IL-6 receptor antibody) for the treatment of RA
significantly improved bone metabolism, suggesting that IL-6 blockade may prevent
the osteoporosis seen in RA patients. In long-term trials (more than 15 months),
ACR20, 50 and 70 reached 88%, 67% and 42%, respectively. During long-term
administration, the serum IL-6 level gradually decreased, becoming undetectable in
some patients. This suggests that anti-IL-6 receptor therapy may go beyond simple
anti-inflammatory therapy to affect fundamental aspects of the immune system.
In relation to safety, the overall incidences of adverse events were 56%, 59% and
51% in the placebo, 4 and 8 mg/kg groups, respectively, so there was no dose depen-
dency in adverse events. One patient died because of reactivation of chronic active
Epstein-Barr virus (EBV) infection and consequent haemophagocytosis syndrome
after receiving a single dose of 8 mg/kg TCZ. Retrospectively, it was found that she
had Hodgkin’s disease with increased EBV DNA in plasma before enrolment in the
study, but had not been excluded [36].
Among laboratory findings, an increase in total cholesterol level was reported
frequently (44% of patients) in the TCZ groups. However, mean total cholesterol
levels did not continue to increase with repeated dosing, and stabilised close to the
upper limit of the normal range. High-density lipoprotein (HDL) cholesterol levels
also increased, so the atherogenic index [(total cholesterol – HDL cholesterol)/HDL
cholesterol] did not change throughout the study period. No cardiovascular compli-
cations associated with increased total cholesterol were observed.
Mild to moderate increases in liver function test values were also observed in
14 (12.8%) of 109 patients in the TCZ groups. The above data indicate that TCZ
treatment is generally well tolerated and shows clinical benefits.
In a Phase II study conducted in Europe (the CHARISMA study) [35], 359
patients with active RA and an inadequate response to methotrexate (MTX) therapy
(q 10 mg/week MTX for at least 6 months) were administered TCZ or TCZ placebo
together with 10–25 mg/week of MTX or MTX placebo every 4 weeks for a total
of 12 weeks.
The patients were randomised to receive 2, 4 or 8 mg/kg TCZ, either as mono-
therapy or in combination with MTX, or MTX monotherapy. As evaluated by
change in DAS28 from baseline, 8 mg/kg TCZ monotherapy and 8 mg/kg TCZ
plus MTX both yielded significantly higher responses than MTX alone. However,
there was no significant difference between 8 mg/kg TCZ monotherapy and 8 mg/
kg TCZ plus methotrexate.
Large-scale Phase III trials have been completed in Japan and other countries, includ-
ing Europe and the United States, and the results have recently been published.
In Japan, a Phase III, randomised, controlled trial was performed to investigate
the efficacy and safely of TCZ treatment in 306 patients with active RA [37].
51
Yoshiyuki Ohsugi and Tadamitsu Kishimoto
Patients with disease duration of less than 5 years were randomised to receive either
TCZ monotherapy (8 mg/kg i.v. once every 4 weeks) or conventional disease modi-
fying anti-rheumatic drugs (DMARDs) for 52 weeks.
As measured by change in Total Sharp Score, patients in the TCZ group showed
a significant delay in the radiographic progression of joint destruction compared
with those receiving conventional DMARDs (mean values: 2.3 vs 6.1) (p < 0.01).
Compared with the placebo group, TCZ also significantly decreased erosion and
joint space narrowing (p < 0.001 and p < 0.018, respectively). In relation to signs and
symptoms, ACR20, 50, and 70 response was achieved by 89%, 70% and 47%,
respectively, of patients in the TCZ group, which was significantly better than the
35%, 14% and 6% who achieved these responses in the conventional DMARD
group (p < 0.001) (Fig. 2).
The overall incidences of adverse events were 89% and 82% in the TCZ and
control groups, respectively. This trial showed that TCZ monotherapy is more
efficacious than conventional DMARDs at delaying and stopping radiographic pro-
gression of joint destruction, and at improving signs and symptoms [37].
Figure 2
Results of the Japanese Phase III trial in RA patients: Improvements in signs and symptoms
[37]. Patients received tocilizumab at a dose of 8 mg/kg or placebo every 4 weeks. ACR
response rate was compared at 52 weeks. Statistical difference was analysed by paired
t-test.
52
Immunobiology of IL-6 – Tocilizumab (humanised anti-IL-6 receptor antibody) for the treatment of RA
The results of the first two of five multi-national Phase III studies have provided
further evidence that IL-6 receptor inhibition is likely to play a significant role in
the treatment of RA. In the first multi-national Phase III study (the OPTION study,
a double-blind, randomised, controlled study), 623 patients with moderate to severe
active RA, refractory to MTX, were allocated to receive 8 or 4 mg/kg TCZ, or
placebo, i.v. every 4 weeks. All three groups also received MTX. The results show
that the proportion of patients that achieved ACR20 response at 24 weeks was
significantly higher in the 8 and 4 mg/kg TCZ groups than in the placebo group
(p < 0.0001). Reduction in disease activity score (DAS-28) was observed in both
TCZ groups. The proportion of patients who achieved a good or moderate EULAR
response at 24 weeks was also significantly higher in the TCZ groups than in the
placebo group (p < 0.0001). These data demonstrate that TCZ is highly effective and
has a good safety and tolerability profile [38].
The second multi-national Phase III study [the TOWARD (tocilizumab in com-
bination with traditional DMARD therapy) study] was conducted in 1216 patients
with moderate to severe active RA and inadequate response to DMARDs. The study
was conducted at 130 study sites in 18 countries, including the United States. In
this two-arm, randomised, double-blind study, patients received either 8 mg/kg TCZ
or placebo i.v. every 4 weeks in combination with stable anti-rheumatic therapy,
including traditional DMARDs but excluding biologicals. Compared to patients
treated with traditional DMARDs alone, a greater proportion of patients treated
with TCZ plus traditional DMARDs achieved significant improvement in disease
signs and symptoms at 24 weeks. This study also explored the pharmacokinetics
and pharmacodynamic parameters of TCZ, as well as immune response to TCZ, in
this patient population. Patient symptoms were measured using the standard ACR
score assessment method. The TOWARD trial data further document the efficacy
and safety of TCZ and the value of its IL-6 receptor inhibiting activity (Press release
by Roche). The results were presented at ACR meetings in 2007.
The above clinical studies have clearly indicated the benefits of using TCZ to
block IL-6 signalling in the treatment of patients with RA.
The fact that the effects of TCZ therapy in RA patients have been so dramatic sug-
gests that the overproduction of IL-6 is deeply involved in the pathogenesis and
progression of RA. As mentioned above, excessive production of IL-6 can readily
explain almost all of the symptoms seen in RA patients.
For example, blockade of IL-6 signalling by TCZ causes dramatic improvement
of anaemia, which is highly beneficial for maintaining and improving the quality
of life of RA patients. This effect could be the result of the inhibition of the pro-
duction of hepcidin (an iron regulatory peptide secreted by the liver cells) [39–42]
53
Yoshiyuki Ohsugi and Tadamitsu Kishimoto
or the recovery of signal transduction via the erythropoietin (EPO) receptor. The
EPO receptor and IL-6R share the Janus kinase-signal transducer and activator of
transcription (JAK-STAT) signalling pathway [43]. Since excessive IL-6 signalling
induces expression of suppressors of cytokine signalling (SOCS), which are intra-
cellular negative feedback factors that inhibit the JAK-STAT pathway, TCZ may
down-regulate these factors, resulting in increased EPO signalling over time.
Another mechanism of the activity of TCZ may be that blockade of IL-6 signal-
ling causes a decrease in serum VEGF, which inhibits angiogenesis in the synovial
tissues, which inhibits hyperplasia of the synovium [44].
Yet another possible mechanism is that interference with activation/differen-
tiation of osteoclasts may contribute to the prevention of joint destruction [45].
Finally, one of the most notable recent advances is related to the discovery that a
newly identified type of T helper cell, the Th17 cell (which produces IL-17), may be
deeply involved in the pathogenesis of autoimmune diseases, including RA [46–48].
Very excitingly, IL-6, together with TGF-B, is involved in the differentiation of this
particularly pathogenic T cell lineage, and it has already been found that blockade
of IL-6 signalling results in suppression of the development of Th17 cells in mice
[49]. Thus, it seems that TCZ is far more than just an anti-inflammatory agent, and
it inhibits the pathogenesis of RA by its effects on the underlying aetiology of the
disease.
Further comparisons of the immune system and gene expression before and after
TCZ treatment in RA patients may provide important insights into the pathogenesis
of RA. If so, the transition “from laboratory to clinic” will lead to a further transi-
tion “from clinic to basic studies”.
References
54
Immunobiology of IL-6 – Tocilizumab (humanised anti-IL-6 receptor antibody) for the treatment of RA
55
Yoshiyuki Ohsugi and Tadamitsu Kishimoto
56
Immunobiology of IL-6 – Tocilizumab (humanised anti-IL-6 receptor antibody) for the treatment of RA
57
Role of IL-1 in erosive arthritis, lessons from animal models
Wim B. van den Berg, Leo A. B. Joosten and Fons A. J. van de Loo
Abstract
Tumor necrosis factor (TNF), interleukin-1 (IL-1) and IL-6 are considered master cytokines in
chronic destructive arthritis. IL-1 drives chronic erosive arthritis and its blockade has been shown
to ameliorate joint destruction in many animal models of arthritis. This ranges from a dominant
role of IL-1 in immune complex arthritis, to a key role in development of T cell-dependent arthritis
and TNF transgenic arthritis. This makes IL-1 an attractive therapeutic target, in addition to TNF
and IL-6. However, IL-1 dependency can be lost under conditions of T cell IL-17 abundance as
well as the presence of Toll-like receptor ligands. The latter may underlie the variable responsive-
ness of rheumatoid arthritis patients to anti-cytokine therapy and warrants combination therapy
for optimal control.
Introduction
Studies in well-defined animal models of arthritis make it clear that tumor necro-
sis factor (TNF) is involved in early joint swelling and cell influx. However, TNF
alone is poorly arthritogenic and hardly destructive, and exerts its full arthritogenic
potential through induction of IL-1. Intriguingly, TNF-independent IL-1 production
is found in many model situations, including pathways driven by macrophages, T
cells and immune complexes. Its relevance is underlined by the great efficacy of anti-
IL-1 therapy and a profound lack of erosive arthritis in IL-1B-deficient mice. IL-1
is a prominent inducer of RANKL and RANKL-mediated activation of osteoclasts.
TNF, in synergy with T cell-derived IL-17 also up-regulates RANKL and induces
bone erosion. Cartilage destruction is heavily dependent on IL-1. IL-1 is a strong
activator of chondrocytes, induces cartilage breakdown through up-regulation of
metalloproteinases and causes profound suppression of cartilage matrix synthesis.
This catabolic activity, combined with impaired anabolic activity, results in marked
cartilage loss. Collagen damage and therefore irreversible cartilage erosion is greatly
amplified by the presence of immune complexes in the joint, through FcG receptor-
mediated activation of IL-1-induced latent metalloproteinases [1]. Cartilage destruc-
Figure 1
The vicious circle in chronic joint inflammation. Schematic presentation of pathways of syno-
vitis and concomitant cartilage and bone destruction. Note the amplifying elements through
endogenous TLR ligands, T cell activation and generation of autoantibodies. The latter will
trigger macrophages after immune-complex (IC) formation, through FcG receptors. IL-1 plays
an important role in enhancing T cell activation through induction of CD40L and OX40.
Thus, excess IL-1 signaling may activate these pathways, leading to the development of T
cell-mediated autoimmune diseases. TLR, Toll-like receptor; Ch, chondrocyte.
tion induced by IL-1 can itself further amplify and perpetuate joint inflammation
in two ways. Breakdown fragments such as biglycan provide endogenous activators
for Toll-like receptors (TLRs), in particular TLR4, on synovial macrophages and
fibroblasts, leading to inflammatory mediator production [2]. In addition, these
fragments may form autoimmune stimuli and may sustain arthritis when tolerance
is lost. IL-1, together with IL-6, is probably a major driver of Th17 generation
against autologous antigens. Some elements of the pathways discussed are depicted
in Figure 1.
60
Role of IL-1 in erosive arthritis, lessons from animal models
The IL-1 family consists of ten members, including IL-1A, IL-1B, IL-1 receptor
antagonist (IL-1Ra) and IL-18 [3–5]. IL-1A and IL-1B are produced from two
different genes located on chromosome 2 and synthesized as 31-kDa precursors.
Pro-IL-1A is a cell-bound cytokine and is activated by proteases called calpains.
There is growing evidence that IL-1A is involved in intracellular signaling [6]. The
production of IL-1B is via non-classical pathways of protein secretion. TLR agonists
such as endotoxins initiate the synthesis of the inactive IL-1B precursor. The IL-1B
precursor co-localizes with procaspase-1 followed by the conversion of the inactive
procaspase-1 to active caspase-1 by a complex of proteins termed the “IL-1B inflam-
masome” (Fig. 2) [6, 7]. In resting cells procaspase-1 is bound to a large inhibitor
molecule, which prevents its activation. During initiation of IL-1B synthesis, there
is activation of caspase-1, which then processes the IL-1B precursor into a mature
form ready for secretion. Autocatalytic activation of pro-caspase-1 occurs via efflux
of potassium ions as a result of triggering the P2X7 receptor by ATP [6]. Recently,
it was found that a small peptide LL37 released from activated neutrophils and
epithelial cells can stimulate the secretion of mature IL-1B via the P2X7 receptor
[8]. There is evidence that under inflammatory conditions IL-1B processing can be
caspase-1 independent, hampering caspase inhibition as a therapeutic control. Using
caspase-1 gene-deficient mice it was demonstrated that IL-1B was still produced in
both acute and chronic joint inflammation. Several proteases, such as proteinase 3
and granzyme A, have been suggested to be involved in the caspase-1-independent
cleavage of pro-IL-1B [9].
IL-1 binds to the IL-1R complex that consists of IL-1 receptor type I (IL-1R)
and IL-1 receptor accessory protein (IL-1RacP). IL-1Ra is the natural inhibitor that
is able to block IL-1R interaction. After binding of IL-1 to the IL-1R, IL-1RacP is
recruited and a functional high-affinity complex is formed. Signaling occurs via
MyD88 adaptor protein that binds to the TIR domain in the intracellular part of
the receptor complex. The presence of the death domain in MyD88 allows recruit-
ment of the IL-1 receptor-activated kinases (IRAK1-4). Thereafter, another adaptor
protein is bound (TRAF-6), which leads to activation of several protein kinases [10],
including JNK, ERKs and IKK (Fig. 3). Finally, this results in activation of transcrip-
tion factors (NF-KB and AP-1) involved in regulation of inflammation-related gene
expression, such as cytokines and chemokines.
IL-1 inhibition
61
Wim B. van den Berg, Leo A. B. Joosten and Fons A. J. van de Loo
Figure 2
IL-1B synthesis, processing and secretion. (A) Gene expression and protein synthesis of
IL-1B precursor is induced by TLR ligands such as endotoxin. The precursor remains in the
cytosol of the cells together with inactive pro-caspase-1 that is bound to the IL-1B inflam-
masome complex. The inflammasome complex contains products of the NALP-3 gene and
is in an inactive state due to binding to a putative inhibitor. (B) TLR signaling activates the
inflammasome by uncoupling of the inhibitor and the NAPL-3 gene products from the pro-
caspase-1.
identified, which has a natural regulatory function by binding IL-1 and consuming
IL-1RacP. Soluble forms have been found of both the type II receptor and the IL-
1RacP. An engineered form of the latter has been shown to be effective in collagen-
induced arthritis (CIA), when applied with gene therapy [11].
The most studied and therapeutically applied inhibitor in rheumatoid arthritis
(RA) patients is the natural receptor antagonist IL-1Ra. To fully prevent IL-1 signal-
ing, excessive levels (> 1000-fold) of IL-1Ra are needed, which impairs therapeutic
potential. Stabilized forms with better pharmacokinetics have been prepared and
have shown efficacy in reduction of erosion in RA [12]. However, whether sufficient
IL-1 blocking was reached is still a matter of debate. Neutralizing antibodies to IL-1
62
Role of IL-1 in erosive arthritis, lessons from animal models
Figure 3
Intracellular IL-1 pathway.
63
Wim B. van den Berg, Leo A. B. Joosten and Fons A. J. van de Loo
are the most powerful tools for identifying the role of IL-1 in arthritis and have been
used extensively in animal models. Therapeutic application is under study in recent
trials in RA patients.
Arthritogenicity of IL-1
It is generally accepted that arthritis can be induced in mice by IL-1B. This was con-
vincingly demonstrated by local injection of IL-1B or intra-articular overexpression
by local gene transfer. One single injection of IL-1B in knee joints of mice results
in disturbance of cartilage proteoglycan synthesis and influx of inflammatory cells
[13]. Prolonged IL-1B exposure of rabbit or murine knee joints, using IL-1B gene
transfer technology results in chronic destructive arthritis that resembles most fea-
tures of RA. When compared with TNF-A, IL-1B is much more potent in inducing
cartilage destruction in vivo. Tiny amounts of IL-1 are already sufficient to cause
chondrocyte proteoglycan synthesis inhibition, whereas roughly a 100–1000-fold
higher dose of TNF-A is needed to obtain the same effect [13]. IL-1B is the domi-
nant form in most arthritis models (see below), but the potency of both isoforms is
similar. Transgenic overexpression of human IL-1A results in florid arthritis, with a
major role of membrane-bound IL-1 [14, 15].
A strong argument for the dominant role of IL-1 in erosive arthritis has emerged
from studies in TNF transgenic mice. The group of George Kollias have already
shown that arthritis was arrested when these mice were treated with antibodies
against the IL-1R [16]. More recent work further clarified separate roles of TNF
and IL-1 in inflammation and erosion. In TNF transgenic mice (hTNFtg) that were
crossed with IL-1A,B-deficient mice, the synovial inflammation was almost unaf-
fected. However, bone erosion was highly reduced and cartilage damage was absent
[17]. TNF levels were still high, which implies that TNF alone is hardly erosive
[18]. This has led to the conclusion that TNF-induced structural joint damage is
mediated by IL-1.
Table 1 shows arguments for a leading role of IL-1 in destructive arthritis. Apart
from a clear role in single mediator systems, the exact impact of IL-1 has been iden-
tified in models of arthritis using antibodies, IL-1Ra or IL-1 gene-deficient mice. It
appears that considerable IL-1 production occurs in many models, independent of
TNF. This is in line with greater efficacy of anti-IL-1 treatment as compared to anti-
TNF treatment. TNF blockade was effective when started before or shortly after
onset of CIA, whereas anti-IL-1 treatment was more efficient and also suppressed
64
Role of IL-1 in erosive arthritis, lessons from animal models
advanced erosive arthritis [19]. Studies in mice deficient for the TNF receptor or
TNF itself showed reduced incidence and severity of CIA. However, once joints
were affected, full progression to erosive damage was seen in an TNF-independent
fashion [20, 21]. Similar examples are seen in other models and are described here
under the respective headings.
First clinical trials showed major protective effects of anti-TNF treatment in RA
patients. Although the initial experimental findings were in favor of IL-1 as com-
pared to TNF, clinical studies in RA patients were unfortunately disappointing. The
soluble IL-1 type I receptor used had a high affinity for IL-1Ra, thus scavenging
the endogenous IL-1 inhibitor. Later studies with IL-1Ra as a therapeutic modality
showed significant reduction of joint erosion, although the effects on joint inflam-
mation were limited. Recently, trials were started with a solid neutralizing antibody
to IL-1B, and have shown efficacy in a subgroup of RA [22].
Further insight into relative roles of TNF, IL-1 and IL-17 has emerged from detailed
studies in a range of experimental arthritis models and findings are summarized
in Table 2. The model systems include innate, nonimmune triggering of synovial
cells, as well as different mixtures of arthritogenic pathways driven by T cells and
immune complexes. Crucial findings are based on blocking studies with specific
inhibitors as well as cytokine-deficient mice. Comparative studies with neutralizing
antibodies are potentially flawed by the efficacy of the various antibodies to fully
neutralize a particular cytokine. On the other hand, observations from knockout
mice predominantly provide insight into a role at the onset of arthritis; however,
studies with arthritis models in conditional cytokine knockouts are scant.
65
Wim B. van den Berg, Leo A. B. Joosten and Fons A. J. van de Loo
Table 2 - Involvement of IL-1 and other features of various murine arthritis models.
CIA and antigen-induced arthritis (AIA) are models based on preimmunization with
a cartilage-specific or an exogenous protein, with generation of T cell reactivity and
antibodies. The onset of arthritis is a mixture of pathways driven by immune com-
plexes and T cells. TNF is important at onset of CIA, but IL-1 blocking is highly
efficacious both in acute and advanced stage [19]. The latter is probably linked to
IL-1-mediated generation of cartilage-derived autoantigens, epitope spreading and
a role of IL-1 in generation of T cell autoreactivity at the site. IL-1B is the dominant
isotype, and IL-1Ra-deficient DBA mice show enhanced susceptibility [23]. IL-17
blockade was effective in established arthritis and mainly prevented erosions [24].
The onset of AIA is vigorous and only partly dependent on TNF and IL-1. Car-
tilage erosion and propagation of inflammation are dependent on IL-1 [25, 26].
Intriguingly, when smoldering chronic arthritis is exacerbated with a small dose of
antigen, T cell-mediated flares were strongly IL-17 dependent [27], underlining that
processes can become relatively TNF/IL-1 independent, when sufficient Th17 cells
are generated at the site.
66
Role of IL-1 in erosive arthritis, lessons from animal models
T cell-driven arthritis
The classic model of adjuvant arthritis (AA) in rats is a pure T cell model. Syn-
ergistic effects were noted of combined TNF/IL-1 blocking [31]. More recently,
novel transgenic mouse models have been developed, which provide further
insight into a role of IL-1 in generation of arthritogenic T cells. Mice deficient
in IL-1Ra display uncontrolled IL-1 activity, and develop spontaneous T cell-
dependent autoimmune arthritis in a defined genetic background [32]. The model
is impaired in TNF-, IL-6- and IL-17-deficient mice [33, 34]. It argues that exces-
sive IL-1, together with IL-6, generates autoreactive Th17 cells. When neutralizing
antibodies are given after onset of arthritis, anti-TNF was ineffective, anti-IL-17
halted further progression, but anti-IL-1 reduced the arthritis (personal unpub-
lished observations).
Other examples of manipulated T cell function leading to autoimmune arthritis
are the SKG and GP 130 arthritis models [35–37]. In the SKG arthritis, aberrant T
cell receptor function allows positive selection of autoimmune T cells, whereas in
the GP 130 model a mutation in the IL-6 receptor induced enhanced signaling and
identified excessive IL-6 signaling as being able to drive T cell-dependent autoim-
mune arthritis. SKG arthritis was strikingly impaired when the mice were crossed
with IL-1-deficient mice.
67
Wim B. van den Berg, Leo A. B. Joosten and Fons A. J. van de Loo
Figure 4
Pattern of arthritis induced by a single or consecutive repeat intra-articular injections of SCW
fragments. Note the increasing chronicity. The histology shows a picture at day 7 after the
fourth SCW rechallenge in control mice (left) and IL-1B-deficient mice (right). Inflammation
and erosion is markedly reduced in the latter .
68
Role of IL-1 in erosive arthritis, lessons from animal models
Figure 5
Cytokine patterns may change dependent on the driving elements. Shift in total TNF depen-
dence of IL-1 production to an almost independent role, ultimately culminating in autonomous
IL-17 production, where sufficient Th17 cells are generated with IL-1, IL-6 and IL-23 help.
As mentioned above, IL-17 is a cytokine mainly derived from the recently identi-
fied T cell subset Th17. It shares many properties with IL-1, including its effect on
articular chondrocytes to drive cartilage destruction and its potential to up-regulate
RANKL and to mediate bone erosion. Although less potent than IL-1, it shows
strong synergy with TNF and can greatly exaggerate arthritis driven by other stim-
uli, as identified in passive GPI arthritis [43]. Local IL-17 overexpression strongly
exacerbates CIA, but also overrules the IL-1 dependency of this model [24].
69
Wim B. van den Berg, Leo A. B. Joosten and Fons A. J. van de Loo
Another intriguing finding was the capacity of the TLR4 agonist LPS to circum-
vent IL-1 dependency of passive KRN arthritis. When serum of arthritic KRN mice,
containing anti-GPI antibodies, is passively transferred to normal recipients, these
animals develop florid arthritis, which is absent in IL-1-deficient mice. However,
when LPS is co-administered GPI arthritis develops undisturbed in IL-1–/– mice
[44]. The TLR4 pathway shares many of the signaling elements of the IL-1 pathway
and this might explain IL-1 redundancy. It argues that cytokine dependency of an
arthritic process may shift when environmental TLR ligands such as bacteria and
viruses become involved.
It has been long recognized that reduction of IL-1 is a powerful therapeutic approach
to prevent chronic erosive joint inflammation in various murine arthritis models. If
elements of the models apply to the clinical situation in RA patients, IL-1 directed
therapy makes sense. There is growing evidence that autoantibodies contribute to
severity and erosive character of RA. Efficacy of anti-B cell therapy and blocking
of activated T cells (CTLA4) underline immune involvement in RA patients and
would suggest that IL-1 is a crucial player. Nevertheless, clinical trials with IL-1Ra
(anakinra) have been disappointing so far. Joint erosion was markedly suppressed
but effects on joint inflammation were, at best, moderate. It has long been argued
that IL-1Ra treatment might have been suboptimal, due to poor pharmacokinetics.
This was also the impression of experimental arthritis work, where IL-1Ra continu-
ously supplied at high dose with Alzet minipumps was efficacious in CIA, whereas
daily dosing was insufficient [19]. However, the great efficacy of IL-1Ra treatment
in adult-onset Still’s disease [45], several autoinflammatory disorders linked to
mutations of proteins controlling IL-1B secretion, and more recently gout, fueled
the disbelief of a dominant role of IL-1 in RA. Novel anti-IL-1 therapies with high
quality neutralizing anti-IL-1B antibodies [22] are under investigation at present,
and will provide definite insight into the role of IL-1 in RA. It is warranted to pay
proper attention to impact on cartilage erosion, which is a hallmark of IL-1 activity.
Most trials just score joint space narrowing on X-rays, which is a poor read out of
focal cartilage damage. Hopefully, improved MRI technology will become available
to offer greater sensitivity.
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Role of IL-1 in erosive arthritis, lessons from animal models
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74
Interleukin-15
Abstract
Interleukin-15 (IL-15) is a cytokine of the four-A-helix superfamily that mediates pleiotropic effects
in regulating components of both the innate and adaptive immune system. It binds to a heterotri-
meric receptor consisting of the common G-chain receptor, IL-15/IL-2 receptor B-chain and unique
IL-15 receptor A-chain. IL-15 is expressed at the mRNA level in a variety of cell lineages and is
expressed as protein as part of the rapid early inflammatory response. It mediates activation of
NK cells, T cells, neutrophils and macrophages and as such is considered a broad immune activat-
ing moiety. IL-15 expression has been described in a variety of inflammatory diseases, including
particularly rheumatoid arthritis, psoriatic arthritis and reactive arthritis. Within synovial tissues in
particular it has been ascribed an inflammatory role by virtue of its capacity to activate T cells, NK
cells macrophages and neutrophils. Moreover, in vivo model studies suggest that IL-15 neutrali-
sation leads to reduction in articular inflammation and damage. Early clinical trials have shown
promise in that IL-15 blockade using a monoclonal antibody in rheumatoid arthritis patients lead
to some trends to improvement, providing biological proof of concept.
Introduction
Cytokines such as TNF-A and IL-1 are established therapeutic targets with strong
basic preclinical rationale taken through to clinical trials and clinical practice across
a rage of disorders. IL-15 is a cytokine with structural similarities to IL-2 [1, 2] that
has been implicated in both the innate and adaptive arms of the immune response.
Furthermore, it is likely to be involved in autoimmune and inflammatory conditions
and, therefore, is under evaluation for its therapeutic potential. This chapter reviews
the biological structure of IL-15, summarise its expression in normal tissues and its
implication in disease processes and finally reviews recent findings implicating IL-15
as a therapeutic target.
In most leucocytes (except perhaps mast cells) IL-15R signals through JAK1/3- and
STAT3/5-dependent pathways [3, 4]. Further signals may be transduced through src-
related tyrosine kinases and Ras/Raf/MAPK to fos/jun activation. Other implicated
pathways include the Bcl-related proteins and may account for the role of IL-15 in
apoptosis. In fibroblasts, IL-15RA may act as a co-stimulator with other receptor
superfamilies including the receptor tyrosine kinase Axl, which signals through
PI3K, Akt and finally Bcl-2/Bcl-XL [14]. IL-15 and IL-15RA may interact with the
B/G-chain on adjacent cells facilitating trans signalling [15]. This is due to IL-15 hav-
ing two sites at which IL-15RA can bind, therefore giving the possibility that one
cytokine molecule could bind two receptors allowing the potential for bi-directional
signalling [4]. This could be of particular importance in the expansion and control
of CD8 T cell subsets and raises important issues for potential therapeutic targeting.
The IL-15 molecule itself may be involved by reverse signalling whereby IL-15 is
expressed as an integral membrane protein via the long signalling peptide. Ligation
of such IL-15 leads to serine phosphorylation and activation of mitogen-activated
76
Interleukin-15
protein kinases (ERK1/2 and p38). A further pathway involving small Rho-GTPase
Rac3 may be activated in a similar fashion – both of these pathways may be impli-
cated in monocyte activation [16, 17].
IL-15 bioactivity
IL-15 has many effects commensurate with broad receptor expression (Tab. 1). These
effects, however, are often characterised on the basis of addition of exogenous IL-15 and
confirmation of a role for endogenous molecule is rather sparse – this is important given
the limited extracellular expression of the cytokine that is described in most systems.
77
Jagtar Nijar Singh and Iain B. McInnes
T cells
T cells up-regulate IL-15RA as a feature of early activation. IL-15 induces the prolif-
eration of both CD4 and CD8 T cells thereby driving clonal expansion, both antigen
specific and polyclonal. IL-2 release is induced and cytotoxicity may be enhanced in
relevant cellular subsets [3, 18, 19]. Various membrane activation markers such as
CD69 or FasL have been shown to be up-regulated [20, 21] mainly on CD45RO+
but not CD45RA+ cells [20]. IL-15 also promotes T cell chemokinesis and adhe-
sion molecule redistribution [22–24]. IL-15 in turn up-regulates both chemokine
expression and chemokine receptor levels to further enhance cellular migration
and recruitment to tissues as required. IL-15 has generally been shown to favour
development of type 1 responses. For example, synovial T cells are induced to
release high concentrations of IFN-G in vitro by IL-15, and T cells from HIV-infected
patients produce more IFN-G in the presence of high dose IL-15 [25]. However,
other studies have shown that IL-15 induces IL-5 production from allergen-specific
T cell clones implying evidence for a role in type 2 responses [26]. IL-15 is now
established as having a critical role in maintenance of T cell memory in both the
CD8 [27] and CD4 T cell compartments. In particular, studies in IL-15 transgenic
mice infected with Listeria monocytogenes support a role for specific memory in the
CD8+ compartment [28] with further reports suggesting the observations extending
into the CD4+ T cells [29, 30].
Macrophages
IL-15 may act as an autocrine regulator of macrophages with low levels suppressing
activity and high levels inducing both pro-inflammatory cytokine and chemokine
production [31]. Human macrophages also constitutively express membrane-bound
IL-15 and this may be of importance in their early activation. Both LPS and GM-
CSF induce translocation of cytoplasmic stores of IL-15 to the cell surface where it is
able to sustain T cell proliferation [32]. This activity may comprise a major pathway
for IL-15 effector function in early innate responses.
Dendritic cells
IL-15 along with GM-CSF has been show to mature monocytes into DCs (CD1a+,
DR+, CD14–). DCs could be further matured using LPS, TNF-A or CD40L into
CD83+, DC-LAMP+ cells [33]. Some of these cells express Langerhans cell mark-
ers such as E-cadherin and CCR6. Further studies suggest that IL-15 is involved in
promoting IL-12 and NO release from myeloid DC (mDC) and also perhaps IL-2
secretion [34, 35]. Moreover, IL-15 is broadly expressed in mDC and plasmacytoid
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Interleukin-15
Neutrophils
Neutrophils express the IL-15 receptor and IL-15 can induce activation and rear-
rangement of the cytoskeleton [36, 37]. It also enhances phagocytosis, increases both
mRNA transcription and translation of a variety of cytokines and chemokines as
well as reducing apoptosis. The latter may be mediated by decreased levels of caspase
1 and 3 thereby modifying Bax expression [4, 38]. The functional importance of
these has been confirmed in vitro using Candida albicans but the significance in vivo
in autoimmune inflammatory and host defence states needs to be further evaluated.
Fibroblasts
IL-15 is expressed on fibroblasts grown from a variety of tissues and may be impor-
tant given the theoretical ability of these cells to modify the immune response.
Membrane IL-15 on fibroblasts is thought to be able to activate both NK and T
cells [43, 44] and may, through an Akt/PI3K pathway and Bcl-2, be able to sustain
fibroblast survival [14].
79
Jagtar Nijar Singh and Iain B. McInnes
80
Interleukin-15
Several of the approaches mentioned above have been tested in relevant disease mod-
els. We have used full-length soluble IL-15RA administration to manipulate IL-15
bioactivity in vivo. When sIL-15RA is injected daily following antigen challenge
the development of collagen induced arthritis (CIA) is suppressed, associated with
delayed development of anti-collagen-specific antibodies (IgG2a) and with reduced
antigen-specific IFN-G and TNF-A production in vitro [57]. On discontinuation of
sIL-15RA administration, CIA developed to levels comparable with controls, sug-
gesting that anti-inflammatory effects are transient. In subsequent studies we have
generated targeted mutants of IL-15RA and identified the sushi domain as essential
for functional cytokine neutralisation [58]. Selected deletion of cysteine residues
similarly disrupted folding to abrogate binding and function. Studies are ongoing to
determine whether small molecule derivatives of sIL-15RA are of therapeutic utility
in the CIA model. This also provides opportunities to investigate the potential for
dual targeting of synergistic cytokine activities, e.g. IL-15 and IL-18.
An alternate approach has been to generate mutant IL-15 forms that can spe-
cifically modify IL-15 activities. An IL-15/FcG2a fusion protein that antagonises the
activities of IL-15 in vitro and lyses receptor-bearing cells, suppresses the onset of
delayed-type hypersensitivity responses in vivo, associated with reduction in CD4+
T cell infiltration [59]. This fusion protein has also proven effective in vivo in pre-
venting rejection of murine islet cell allografts in combination with CTLA4/Fc [60].
Studies in CIA indicate that this fusion protein is effective in treating not only devel-
oping CIA but also established disease, and that after treatment disease recurrence is
81
Jagtar Nijar Singh and Iain B. McInnes
Clinical studies in humans have been performed using the neutralising antibody,
AMG714, and a monoclonal antibody targeting IL-2/15R B-chain, MIKB2. The
optimal approach in clinical trials has not yet been established. The fully human
IgG1 monoclonal anti-IL-15 antibody AMG714 binds and neutralises the activity of
soluble and membrane-bound IL-15 in vitro. AMG714 has been tested in two clini-
cal trials in RA. In a 12-week, dose-ascending, placebo-controlled study, RA patients
(n = 30) that had failed several previous DMARDs received a randomised, controlled,
single dose of AMG714 (0.5–8 mg/kg) followed by open label weekly doses for
4 weeks. IL-15 neutralisation was well tolerated [63]. This study was not placebo-
controlled throughout; however, encouraging signs of efficacy were obtained. Around
60% of patients achieved an ACR20 response with some 25% achieving an ACR70
improvement. In parallel studies, AMG714 was shown to inhibit endogenous RA
synovial T cell activation and to suppress IL-15-induced cytokine release [63]. A
subsequent dose-finding study has now been performed [64] in which RA patients
received increasing fixed does (up to 280 mg per injection) of anti-IL-15 antibody
every 2 weeks by subcutaneous injection for 3 months. An interim analysis indi-
cated satisfactory tolerance compared with placebo and ACR20 improvements were
observed in approximately 60% of recipients receiving higher doses of AMG714. No
significant alterations in the levels of circulating leucocyte subsets, including NK cells
and CD8+ memory T cells, were observed. Extension of this study was performed
to compare the highest dose of AMG714 (n = 121) with placebo (n = 58). Significant
improvements in ARC20 responses occurred in AMG714 recipients compared to pla-
cebo at weeks 12 and 16 of follow-up. Of note, however, ACR20 responses were not
significantly different from placebo at week 14 (reflecting a higher placebo response
at this time point), the pre-designated primary outcome time for this study. Clear and
significant improvements in acute-phase reactants occurred in AMG714 recipients
compared with placebo. Thus, although there is clear evidence of biological activity
and biological proof of concept, larger confirmatory studies are now required to
facilitate proper interpretation of these data and at this stage IL-15 should not as yet
be considered a validated therapeutic target in RA.
Several outstanding issues remain in this clinical area. The relative role of IL-15
as a target compared with TNF and IL-6 is unclear. Its role in early T cell/DC inter-
actions suggests that it may have some role in tolerance induction and therefore
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Interleukin-15
manipulation of IL-15 may offer potential in early disease beyond its capability in
later RA, the only subjects thus far treated. IL-15 mediates effects on epithelial cells
of the gut, keratinocytes, myocytes, hepatocytes and several CNS subsets indicating
broad tissue effector function in host defence [65–70]. Elevated levels are detected
in a variety of inflammatory diseases and there is momentum currently to explore
its therapeutic role across a range of disorders. In particular psoriasis offers attrac-
tive potential based on expression patterns in disease tissue, the beneficial effects
of IL-15 blockade in relevant models and the potential for interruption of IL-15
function in remitting-relapsing inflammatory disease typical in some psoriatic dis-
ease patterns. Finally, it will now be necessary to extend the range of modalities of
blockade. Pre-clinical studies are underway using IL-15 mutant proteins and addi-
tional anti-IL-15 monoclonal antibodies are under consideration. A Phase I trial has
been performed in which IL-15 was blocked using MikB1 monoclonal antibody in
patients with large granular lymphocyte leukaemia [71] – this reagent is now being
tested in a variety of inflammatory conditions. In particular, there is interest in
utilising signal molecule inhibitors, e.g. JAK inhibitors, which are in ongoing clini-
cal trials in RA with encouraging early results. These do not yet however facilitate
specific cytokine targeting. This may not be a deficit in their strategic importance as
focussing on a given pathological signalling pathways may offer some advantages
over pan cytokine inhibition.
Conclusion
IL-15 and its receptor are expressed in a wide range of cell types. It contributes to
a pathway involved in the early activation of the immune system and enhances NK,
polymorphonuclear and T cell responses. It has been implicated in several of the
inflammatory arthropathies and in vivo clinical trials suggest a role in attenuating
the aberrant immune response. However, as ever, further trials are required with
larger numbers of patients to further elucidate its effect, in particular the interaction
with TNF-A and other pro-inflammatory cytokines.
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Jagtar Nijar Singh and Iain B. McInnes
88
IL-17 and Th17 cells, key players in arthritis
Abstract
IL-17 was identified in 1995/96 as a T cell-derived cytokine with effects on inflammation and neu-
trophil activation. Rheumatoid arthritis (RA) has emerged as the best-studied situation to justify
the selection of IL-17 as a therapeutic target. By interacting with other proinflammatory cytokines,
IL-17 was found to induce bone and cartilage destruction. In 2006, the precise cell source of IL-17
was identified in the mouse. These cells were named Th17, and a key role for these cells was dem-
onstrated in various situations associated with inflammation. These new findings confirmed and
extended the results previously obtained following the identification of IL-17 as a T cell-derived
cytokine. At the same time, additional information was obtained on the other members of the
IL-17 family and on the structure of the IL-17 receptor complex. Such knowledge has further
extended the choice of possible modalities to control IL-17.
Introduction
Interest in IL-17 increased further recently, when in 2006, the precise cell source of
IL-17 was identified in the mouse. These cells were named Th17, and a key role for
these cells was demonstrated in various situations associated with inflammation.
These new findings confirmed and extended the results previously obtained follow-
ing the identification of IL-17 as a T cell-derived cytokine.
Demonstration of the role of IL-17 in many inflammatory conditions further
supported the concept of IL-17 targeting for treatment. We review these new find-
ings in light of the previous knowledge [1]. We focus on rheumatoid arthritis (RA),
which has emerged as the best studied situation to justify the selection of IL-17 as
a therapeutic target.
Identification of IL-17
Figure 1
Effects of IL-17 on interactions between cells and cytokines associated with inflammation,
cartilage, and bone destruction in rheumatoid arthritis.
cells/fibroblasts was able to increase IL-6 and other proinflammatory cytokine pro-
duction, indicating immediately its role in inflammation [2, 3]. At the same time,
IL-17 was shown to be able to induce neutrophil induction and maturation, an
indication of its role in the acute mechanisms in host defense (Fig. 1).
Following the discovery of the molecule and of its key properties, a number of
both human and mouse, spontaneous as well as induced, diseases were shown to be
associated with IL-17 overexpression and production. For instance, IL-17 was on
the list of genes, obtained through extensive gene array studies, found to be overex-
pressed in the brain of patients with multiple sclerosis. Using different approaches,
similar conclusions were reached for Crohn’s disease and psoriasis. Although the
list of diseases will certainly increase, RA emerged as the best-studied situation
making IL-17 a therapeutic target. Although we focus here mainly on this condi-
tion, it should be kept in mind that most of the results can probably be transferred
to the long list of conditions where chronic inflammation is associated with matrix
destruction, with the examples of myelin or bone and cartilage, respectively.
Concordant results showed that IL-17 was involved in the proinflammatory
patterns associated with joint inflammation and RA using mouse and human mod-
90
IL-17 and Th17 cells, key players in arthritis
els (Fig. 1) [4]. In the mouse, an injection of IL-17 alone into a normal knee was
sufficient to induce cartilage damage [5]. The continuous administration of IL-17
by gene overexpression induced massive damage with extensive inflammatory cell
migration, bone erosions, and cartilage degradation [6]. Conversely, inhibition
with specific inhibitors including blocking antibodies and soluble receptor, or with
IL-4 acting more broadly on other cytokines, provided protection from inflamma-
tion and destruction [7]. The studies showed an increased production of functional
IL-17 by RA synovium but also by bone explants, indicating the role of T cells
in juxta-articular bone destruction [8, 9]. As expected, this effect was associated
with RANK ligand expression by these T cells, interacting with RANK-expressing
cells, certainly osteoclasts but also mature dendritic cells [9, 10]. To clarify the role
of IL-17 in chronicity, it was recently shown that in the mouse collagen arthritis
model, IL-17 effect was dependent on the presence of TNF at the early phase,
whereas at a later stage the disease was mostly IL-17 driven, and no longer TNF
dependent [11].
TNF is now considered as the key cytokine in RA. To consider the role of IL-17, it
was necessary, to some extent, to take into account the role of T cells, or at least
of some T cells. A key property for IL-17 was the synergistic interactions with
TNF and to a lesser extent with IL-1 [12]. This critical property indicated that T
cells could directly contribute to the inflammatory response. Furthermore, IL-17
increased the production of TNF and IL-1 by monocytes [13]. Opposite results were
observed with IL-4, the prototype of a Th2 cytokine. This was another indication
that IL-17-producing cells were a particular subset of T cells.
The sequential analysis indicated that synergy was observed only when cells
such as synoviocytes were first exposed to IL-17 and then to TNF. Synergy was not
observed when cells were exposed first to TNF then to IL-17. This is in line with the
role of these Th17 cells in the amplification of the initial response associated with
TNF and IL-1 secretion.
The most critical and recent step was the identification in 2006 of the cell source of
IL-17 [14]. IL-17 was first described as a T cell product [2]. IFN-G is characteristic
of Th1 cells and IL-4 of Th2 cells. The source of IL-17 was found to be different and
these cells were named ThIL-17 or Th17 for short (Fig. 2). In the mouse, this new
subset was identified by the demonstration of the inhibitory effect of IFN-G on IL-17
production in mouse models of autoimmune diseases [15–17]. Induction of IFN-G
91
Pierre Miossec, Ling Toh and Saloua Zrioual
Figure 2
Th17: A new subset of Th cells.
was shown to be sensitive to the synergistic interaction between IL-12 and IL-18,
two classical monocyte products [18]. The new finding was the inhibition of IL-17
by IFN-G. The next step was the discovery of IL-23, another monocyte product,
shown to be a key cytokine in the induction of brain inflammation in experimental
models of encephalomyelitis [19].
IL-12 and IL-23 are two cytokines of the same family, both composed of dimers
[20]. IL-12 is a heterodimer composed of the p35 and p40 subunits, whereas IL-23
is composed of the p19 and p40 subunits. The first antibodies against IL-12 were in
fact against the p40 protein, which is common to IL-12 and IL-23. The protective
effects first thought to be due to the inhibition of IL-12 through p40, could also
result from IL-23 inhibition. This was shown by the specific inhibition of p19, the
IL-23-specific subunit, whereas no effect was shown with the inhibition of p35 [21].
Conclusive demonstration came from the use of mice deficient for these IL-12- and
IL-23-specific subunits. Finally, the enhancing effect of IL-23 on IL-17 production
was demonstrated in various models of autoimmune diseases. We do not know yet
how these results on IL-23 can be applied to human RA.
Transcription factors have been identified as markers of the Th subsets with
T-bet for Th1, and GATA-3 for Th2 cells. The transcription factor retinoic acid-
related orphan receptor G t (RORGt) was found to be associated with Th17. Indeed,
mice with RORGt-deficient T cells have attenuated autoimmune disease, and do not
92
IL-17 and Th17 cells, key players in arthritis
have tissue-infiltrating Th17 cells [22]. Conversely, T-bet inhibits IL-17 production
in vivo [23].
Cytokine receptors have been employed the same way using the IL-12-specific
IL-12R B2 chain, as a marker of Th1 cells, and the IL-23-specific IL-23R-specific
chain, as a marker of Th17 cells. The IL-12R B1 chain is common to both the IL-12
and IL-23 receptors. The inhibitory effect of IL-17 on the Th1 pathway results from
the IL-17 induced inhibition of IL-12R B2 expression, making cells not responsive
to IL-12 effects on IFN-G production.
IL-17 can induce IL-1 and TNF production by monocytes [13]. This effect on
the amplification of inflammation was further expanded by showing that IL-6 and
IL-1, two key proinflammatory cytokines, could in turn increase IL-17 production
through the induction of IL-23, leading then to an increase in IL-17 production
[24].
These results have been obtained in the mouse and as such should be considered
with caution when applied to the human situation. Even in the mouse, more recent
results indicated the frequent co-expression of IFN-G and IL-17. Contribution of
one of the two cytokines could lead to different pathogenic pathways, leading to
the same clinical presentation as observed in mouse models of autoimmunity [25,
26]. Our results with RA T cells clones indicated that IL-17 was often produced in
association with IFN-G but not with IL-4 [27]. In situ immunostaining of the RA
synovium showed two isolated populations of T cells producing either IFN-G or
IL-17. Double-positive cells were rarely seen. It thus remains to be demonstrated
whether these secreting patterns are still dynamic or fixed. Of interest in this context
was the demonstration that cytokine-secreting T cells have a particular morphology
with a plasma cell appearance, a pattern that can be induced in vitro and is associ-
ated with the loss of TCR and CD3 but not of CD4 [28]. As for B cells, the plasma
cell morphology of the IFN-G and IL-17-producing cells strongly suggests that this
is a fixed pattern related to a final stage of differentiation.
A key issue is the interaction between Th17 and regulatory T cells. Regulatory T
cells are in charge of the control of the immune response. At baseline, in the absence
of any particular stress or aggression, regulatory T cells are active and limit the
intensity of the baseline response. In the context of stress, as seen during infection,
this control is turned off to let the defense mechanisms be expressed according to
the stimulation.
The first line of defense following exogenous stress results from the early nonspe-
cific stimulation of monocytes, followed by a second wave of inflammatory signals
from T cells. The related inflammation through the production of proinflammatory
cytokines such IL-6 and IL-1 activates the Th17 pathway. At the same time the regu-
93
Pierre Miossec, Ling Toh and Saloua Zrioual
The published results mainly refer to IL-17A, the founding member of the IL-17
family, which includes IL-17A–F. IL-17F has been given more attention because of
its 50% sequence homology with IL-17A. When IL-17F is used alone, it appears
to have similar effects to IL-17A but to a lower extent [34]. Sometimes IL-17F has
minimal or even no effect when used alone. However, when combined with TNF,
a synergistic effect is observed, almost as potent as with IL-17A. In contrast to the
proinflammatory effects of IL-17A and F, IL-17E (also named IL-25) acts as a Th2
cytokine with anti-inflammatory properties [35, 36].
Th17 cells can produce several proinflammatory cytokines including IL-17A,
IL-17F, IL-22, TNF, and IL-6. IL-22 is a member of the IL-10 family, and synergizes
with IL-17A or IL-17F to regulate genes associated with skin innate immunity [37].
Recently, IL-22 was shown to mediate IL-23-induced dermal inflammation [38].
In addition, Th17 cells are involved in cell interactions through the expression of
RANKL. Such RANK-RANKL interaction is the final bridge whereby osteoblasts
activate osteoclasts leading to bone destruction. Similar interactions are found
between synoviocytes and dendritic cells.
94
IL-17 and Th17 cells, key players in arthritis
Figure 3
IL-17 A and IL-17F interactions with IL-17 receptors.
Not only the IL-17 family but also the IL-17 receptor (IL-17R) family have been
characterized (Fig. 3). Some limited knowledge on the IL-17R was apparent from
the first results demonstrating a rather low affinity for the IL-17R, suggesting the
presence of additional chains [39, 40]. Sequence screening showed proteins with
a partial homology with the IL-17R. In the mouse, at least two members have to
be taken into account [41]. The first is the original IL-17R renamed IL-17RA. The
second is IL-17RC. The physical association of the two receptors has been shown,
although it is still unclear if these are two chains of a single receptor or two different
receptors. It was previously proposed that IL-17A could be the receptor for IL-17A
and IL-17RC the receptor for IL-17F. In the human situation, our impression is that
the two receptors can bind either IL-17A or IL-17F, possibly with different affinities.
These findings result from small interference RNA studies, leading to the inhibition
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Pierre Miossec, Ling Toh and Saloua Zrioual
of the cell surface expression of one of the two receptors. Inhibition of expression of
one of the two receptors is sufficient to inhibit response to IL-17 alone in synovio-
cytes. However, the inhibition of the two receptors is needed to reduce the response
to the combination of IL-17 with TNF [34]. As indicated previously, the effect of
IL-17 used first increases that of TNF used in a second phase.
The efficacy of cytokine targeting has first shown with TNF inhibitors. Similarly, the
first available tools to inhibit IL-17 were a mouse soluble IL-17R receptor and an
anti-IL-17 mouse monoclonal antibody. These tools were the equivalent of Enbrel
and Remicade, developed to block TNF.
Today, the list of targets and tools has increased. The two major options are the
targeting of the ligand or the receptor [42] (Fig. 4). Regarding the ligand, the pos-
sible choice is between IL-17A or IL-17F or both, and for the receptor, IL-17RA
or IL-17RC. In addition, administration of IL-25, the member of the IL-17 family
with opposite effects, may be of interest because of its Th2 properties to inhibit,
at the same time, IL-17, IL-1, TNF, and IL-6. These results show similarities with
the effects of IL-4. In addition, as for the other proinflammatory cytokines, active
research is looking for small molecules able to control the intracellular signaling
pathways.
Targeting cytokines may interfere with immune defense. Blocking TNF was asso-
ciated with an increased risk of tuberculosis reactivation. The mechanism implies an
effect on the Th1 pathway and on the induction of IFN-G by IL-12 and IL-18 [18].
For IL-17, a link with neutrophils was apparent from the first results. This implies
that inhibition of IL-17 may have consequences on the acute defense mechanisms
involving neutrophils. Indeed, in the mouse, inhibition of the IL-17 system has
been associated with increased mortality from bacterial lung infections [43]. IL-17
appears to be critical for neutrophil activation and migration [44]. IL-17 is a strong
inducer of IL-8, a key chemokine for neutrophils. Conversely, IL-17 appears to have
inhibitory effect on the production of other chemokines involved in the migration of
mononuclear cells. In addition, inhibition of the IL-23 pathway has been associated
with defects in the cell-mediated immunity including increased severity of mycobac-
terial infections.
The position of IL-17 inhibition in the treatment of RA and other inflammatory
conditions remains to be defined. Coming back to the synergistic interactions, an
enhanced inhibitory activity was observed with the combination of TNF and IL-17
inhibitors using ex vivo samples of RA synovium and bone [45]. Thus, primary or
secondary lack of response to TNF inhibitors may represent a useful addition. It is
possible that the combined inhibition of TNF and IL-17 may have the advantage of
targeting two different cell types, monocytes and T cells. This would also control
96
IL-17 and Th17 cells, key players in arthritis
Figure 4
Modalities to inhibit IL-17 action.
Conclusion
The story of IL-17 started 10 years ago and this is the time it took to become a
cytokine in fashion. The identification of the Th17 subset indicates that some T
cells are involved in and amplify the link between chronic inflammation and extra-
97
Pierre Miossec, Ling Toh and Saloua Zrioual
cellular matrix destruction. Similar concepts apply to other complex diseases with
inflammation-induced destruction, such as multiple sclerosis and Crohn’s disease
where the contribution of IL-17 has already been identified. Tools are now almost
ready to verify whether these concepts, already 10 years old, are indeed correct.
Acknowledgements
I would like to thank all contributors to the IL-17 studies from our group over
the years: Martine Chabaud, Masanori Kawashima, Corinne Granet, Guillaume
Chevrel, Guillaume Page, Yuan Zhou, and today, Ling Toh and Saloua Zrioual.
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101
Role of IL-18 in inflammatory diseases
Charles A. Dinarello
Abstract
IL-18 is a member of the interleukin (IL)-1 family. IL-1B and IL-18 are closely related, and both
require the intracellular cysteine protease caspase-1 for biological activity. Several autoimmune
diseases are thought to be mediated, in part, by IL-18. Many are those with associated elevated
interferon-G (IFN-G) levels, such as systemic lupus erythematosus, macrophage activation syn-
drome, rheumatoid arthritis, Crohn’s disease, psoriasis and graft versus host disease. In addition,
ischemia, including acute renal failure in humans, appears to involve IL-18. Animal studies also
support the concept that IL-18 is a key player in models of lupus erythematosus, atherosclerosis,
graft versus host disease and hepatitis. Unexpectedly, IL-18 plays a role in appetite control and
the development of obesity. The IL-18-binding protein, a naturally occurring, specific inhibitor
of IL-18, neutralizes IL-18 activities and has been shown to be safe in patients. Other options
for reducing IL-18 activities are inhibitors of capsase-1, human monoclonal antibodies to IL-18,
soluble IL-18 receptors and anti-IL-18 receptor monoclonal antibodies.
Introduction
IL-18 is a member of the IL-1 family of cytokines and is structurally related to IL-1B
[1]. Recently, a new member of the IL-1 family, IL-33, has been reported; structural-
ly IL-33 is closely related to IL-18 [2]. However, unlike IL-18, IL-33 binds to its own
receptor, ST2, a long-time orphan receptor in the IL-1 family of cytokines [2]. The
IL-1B and the IL-18 precursors require caspase-1 for cleavage, activity and release
[3–5]. Therefore, anti-proteases that inhibit capase-1 reduce both the processing
and release of IL-1B and IL-18. Now, IL-33 can be added to the list of members of
the IL-1 family that require caspase-1 for processing and release [2]. However, it is
important to note that IL-18 is not a recapitulation of the biology or clinical sig-
nificance of IL-1, or its activity similar to the biologically activity of IL-33; in fact,
IL-18 is a unique cytokine exhibiting inflammatory as well as immunoregulatory
processes distinct from IL-1B or IL-33. For example, IL-1B is not required for IFN-G
production, whereas IL-18 is [6]. Initially thought of as primarily a Th1-polarizing
cytokine, IL-18 is also relevant to Th2 diseases [7]. As discussed in this review, ani-
mal models reveal that targeting IL-18 holds promise for the treatment of autoim-
mune and inflammatory diseases.
104
Role of IL-18 in inflammatory diseases
Because IL-18 can increase IFN-G production, blocking IL-18 activity in autoim-
mune diseases is potentially an attractive therapeutic target. However, anti-IL-12 has
been shown to reduce the severity of Crohn’s Disease as well as psoriasis. Therefore,
IL-12 can induce IFN-G in the absence of IL-18. However, there are many models of
IL-18 activity independent of IFN-G. For example, we have recently reported a new
cytokine, IL-32, which was discovered in the total absence of IL-12 or IFN-G [14].
Furthermore, models of inhibition of proteoglycan synthesis are IL-18 dependent
but IFN-G independent [15]. In addition, IL-18-dependent melanoma metastasis to
the liver is IFN-G independent [16]. The results of preclinical studies and the target-
ing of IL-18 to treat autoimmune and inflammatory diseases are discussed in this
article.
The strategies for reducing IL-18 activity include neutralizing anti-IL-18 monoclo-
nal antibodies, caspase-1 inhibitors and blocking antibodies to the IL-18 receptor
(IL-18R) chains. Caspase-1 inhibitors are oral agents and are presently in clinical
trials in rheumatoid arthritis; a reduction in the signs and symptoms of the disease
has been observed. Caspase-1 inhibitors prevent the release of active IL-1B and
IL-18 and, therefore, may derived clinical benefit by reducing the activities of both
cytokines [3, 4, 17]. A naturally occurring IL-18-binding protein (IL-18BP) was dis-
covered in 1999; IL-18BP is effective in neutralizing IL-18 activity [18]. IL-18BP is
not a soluble form of either chain of the IL-18R but rather a constitutively secreted,
high-affinity and specific inhibitor of IL-18 [19, 20]. IL-18BP is currently in clinical
trials for the treatment of rheumatoid arthritis and severe psoriasis. The pharma-
cokinetics of IL-18BP have been reported and IL-18BP is safe even at the highest
doses in over 6 weeks of treatment [21].
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Charles A. Dinarello
The non-caspase-1 enzyme associated with processing both the IL-1 and
IL-18 precursors is proteinase-3 (PR-3) [25]. Agonistic autoantibodies to PR-3 are
pathological in Wegener’s granulomatosis and may contribute to the non-caspase-1
cleavage of the IL-18 precursor and IFN-G production in this disease. Epithelial
cells stimulated with PR-3 in the presence of endotoxin release active IL-18 into
the supernatant [26]. Since lactate dehydrogenase activity is not released, the
appearance of active IL-18 is not due to cell leakage or death. Injecting mice with
recombinant FasL results in hepatic damage that is IL-18 dependent [27]. However,
FasL-mediated cell death is IL-18 dependent and caspase-1 independent [27], but
ischemia-reperfusion injury results is cell death is via an IL-18- as well as a caspase-
1-dependent pathway [28, 29].
The P2X7 receptor is involved in the secretion of IL-1B as well as IL-18 [30–32].
Stimulation of this receptor by ATP is a well-described event in the release of IL-1B
and IL-18. A tyrosine derivative named KN-62 exhibits selective P2X7 receptor-
blocking properties [33]. In a study of small molecule inhibitors of this receptor,
analogues of KN-62-related compounds were characterized for their ability to
affect the human P2X7 receptor on monocyte-derived human macrophages [33].
Although several analogues inhibited the secretion of IL-1B, no data exist on the
effect of these inhibitors on IL-18 secretion [33]. Unlike IL-1B, the secretion of IL-18
is mostly studied in vivo in mice that have been treated with Cryptosporidium par-
vum [4], rather than in vitro. In vitro, the release of IL-18 requires the presence of
activated T cells [34, 35].
Antibodies to either chain of the IL-18R complex are attractive options for treating
IL-18-mediated diseases. The IL-18R chains (IL-18RA and IL-18RB) are members of
the IL-1 receptor family. The binding sites for IL-18 to the IL-18RA chain are similar
to those for IL-1 binding to the IL-1 receptor type I [36–38]. Two sites bind to the
ligand binding chain (IL-18RA) and a third site binds to the IL-18RB chain, also
called the signal transducing chain. The intracellular chains of the IL-18Rs contain
the Toll domains, which are essential for initiating signal transduction (see Fig. 1).
The Toll domains of the IL-18Rs are similar to the same domains of the Toll-like
receptors, which recognize various microbial products, viruses and nucleic acids.
As a therapeutic option, however, commercial antibodies generated to the IL-18RA
and B chains are 100-fold less effective in neutralizing IL-18 activity compared to
the IL-18BP [39]. Nevertheless, the development of blocking antibodies to IL-18R
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Role of IL-18 in inflammatory diseases
Figure 1
IL-18 activation of cell signaling. Mature IL-18 binds to the IL-18RA chain and recruits the
IL-18RB chain, resulting in the formation of a heterodimeric complex. As a result of the
formation of the extracellular complex, the intracellular chains also form a complex, which
brings the Toll domains of each receptor chain into close proximity. Although poorly under-
stood, the close proximity of the Toll domains recruits the intracellular protein MyD88 to
the receptor chains. MyD88 is common to cells activated by IL-1, IL-18 and TLR-4 ligands
(endotoxins). Following MyD88 recruitment, there is a rapid phosphorylation of the IL-1
receptor-activating kinases (IRAK). There are four IRAK proteins. Depending on the cell type,
other kinases have been reported to undergo phosphorylation. These are the TNF recep-
tor activating factor (TRAF)-6 and inhibitory kappa B kinases (IKK) A and B (not shown).
Phosphorylation of IKK results in the phosphorylation of IKB and translocation of NF-KB to
the nucleus. However, this is not observed uniformly in all cell types and there are distinct
differences in NF-KB activation in different cells stimulated with IL-18 [13]. In addition,
IL-18-activated cells phosphorylate mitogen-activated protein kinase (MAPK) p38. In IL-18-
activated cells, new genes are expressed and translated. Those shown in the figure represent
the pro-inflammatory genes. The presence of IL-18BP prevents IL-18-induced cellular activa-
tion. IL-18BP is present in the extracellular milieu as a constitutively expressed protein where
it can bind and neutralize IL-18, thus preventing activation of the cell surface receptors.
In addition, formation of inactive complexes of IL-18BP with IL-18 and the IL-18RB chain
deprives the cell of the participation of IL-18RB chain in activating the cell.
107
Charles A. Dinarello
chains remains a viable therapeutic option since an antibody to the type I IL-1 recep-
tor chain is in clinical trials in rheumatoid arthritis.
Unless converted into a fusion protein in somewhat the same manner as that for
other soluble cytokine receptors, it is unlikely that the soluble form of the mono-
meric form of the IL-18RA is a candidate therapeutic agent due to its low affinity.
Another member of the IL-1 family (IL-1F), IL-1F7 [40], may be the naturally
occurring receptor antagonist of IL-18. IL-1F7 binds to the IL-18RA chain with a
high affinity but this binding does not recruit the IL-18RB chain. The occupancy of
the IL-18RA without formation of the heterodimer with the IL-18RB is the same
mechanism by which the IL-1 receptor antagonist prevents the activity of IL-1.
However, IL-1F7 does not affect the activity of IL-18 [41, 42] and the biological
significance of IL-1F7 binding to the IL-18RA remains unclear. However, in the
presence of low concentrations of IL-18BP, IL-1F7 has been shown to reduce the
activity of IL-18 [43].
IL-18BP
The discovery of the IL-18BP occurred during the search for the extracellular (solu-
ble) receptors for IL-18 in human urine. Nearly all the soluble cytokine receptors are
found in human urine [44]. For example, the TNF p75 soluble receptor, used widely
for the treatment of rheumatoid arthritis, ankylosing spondylitis and psoriasis, was
initially purified and sequenced using ligand-specific affinity chromatography [45].
In searching for soluble IL-18 receptors, IL-18 was covalently bound to a matrix
and highly concentrated human urine, donated by Italian nuns, was passed over the
matrix and eluted with acid to disrupt the ligand (in this case IL-18) for its soluble
receptors. Unexpectedly, instead of the elution of soluble forms of the cell surface
IL-18Rs, the IL-18BP was discovered [18]. This was due to the higher affinity of the
IL-18BP for the ligand compared to the soluble receptors.
The IL-18BP is a constitutively secreted protein, with a high affinity (400 pM)
binding to IL-18. There is very limited amino acid sequence homology between
IL-18BP and the cell surface IL-18Rs; IL-18BP lacks a transmembrane domain and
contains only one Ig-like domain [20, 46]. IL-18BP shares many characteristics
with the soluble form of the IL-1 type II receptor in that both function as decoys
to prevent the binding of their respective ligands to the signaling receptor chains
[47]. The fact that there is limited amino acid homology between IL-18BP and the
IL-1 receptor type II suggests a common ancestor. In humans, IL-18BP is highly
expressed in spleen and the intestinal tract, both immunologically active tissues
[18]. Alternate mRNA splicing of IL-18BP results in four isoforms [18, 20]. Of
considerable importance is that the prominent ‘a’ isoform is present in the serum
of healthy humans at a 20-fold molar excess compared to IL-18 [19]. This level of
108
Role of IL-18 in inflammatory diseases
Viral IL-18BP
The most convincing evidence that IL-18 is a major player in inflammatory con-
ditions and that IL-18BP is functional in combating inflammation comes from a
natural experiment in humans. Molluscum contagiosum is a common viral infec-
tion of the skin often seen in children and individuals with HIV-1 infection. The
infection is characterized by raised but bland eruptions; there are large numbers
of viral particles in the epithelial cells of the skin but histologically there are few
inflammatory or immunologically active cells in or near the lesions. Clearly, the
virus fails to elicit an inflammatory or immunological response. A close amino acid
similarity exists between human IL-18BP and a gene found in various members of
the Poxviruses. The greatest homology is with in M. contagiosum [18, 52, 53]. The
viral genes encoding for viral IL-18BP have been expressed and the recombinant
proteins neutralize mammalian IL-18 activity [52, 53]. The ability of viral IL-18BP
to reduce the activity of mammalian IL-18 likely explains the lack of inflammatory
and immune cells in the infected skin and the blandness of the lesions. One can
conclude from this natural experiment of M. contagiosum infection that blocking
IL-18 reduces immune and inflammatory processes such as the function of dendritic
and inflammatory cells.
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Charles A. Dinarello
increased stimulation of macrophages and natural killer (NK) cells. Most impor-
tantly, concentrations of IL-18BP were only moderately elevated, resulting in a high
level of biologically active free IL-18 [19] in MAS (4.6-fold increase compared with
controls (p < 0.001). Others have reported marked expression of IL-18 in fatal MAS
[54]. The concentrations of free IL-18 but not IL-12 significantly correlated with
clinical status and the biological markers of MAS such as anemia, hypertriglyceri-
demia and hyperferritinemia, and also with markers of Th1 lymphocyte or mac-
rophage activation such as elevated concentrations of IFN-G, soluble IL-2 and TNF
receptors. Therefore, treatment of life-threatening MAS with IL-18BP is a logical
therapeutic intervention to correct the severe IL-18:IL-18BP imbalance resulting in
Th1 lymphocyte and macrophage activation.
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Role of IL-18 in inflammatory diseases
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Charles A. Dinarello
Table 1 (continued)
As with any cytokine, the role of IL-18 in a particular disease process is best assessed
employing specific neutralization of the cytokine in a complex disease model.
Although mice deficient in IL-18 have been generated and tested for the develop-
ment of autoimmune diseases [57], any reduction in severity may be due to a reduc-
tion in the immune response such as to antigens or the sensitization processes itself
and does not address the effect of IL-18 on established disease. IL-18 neutralization
in wild-type mice is effective in reducing collagen-induced arthritis [58] as well as
inflammatory arthritis [15]. Inflammatory arthritis is of particular relevance since
this is a model of cartilage loss due to decreased proteoglycan synthesis and is inde-
pendent of IFN-G. IL-18 contributes to the lupus-like disease in mice [59] via IFN-G
production. Caspase-1-deficient mice provide useful models for disease [6, 29] but
here the effect may be on IL-1B, IL-18 or both.
Most investigations initially focused on IL-18 in Th1-mediated diseases in
which IFN-G plays a prominent role. However, it soon became clear that block-
ing IL-18 resulted in reduction of disease severity in models where IFN-G has no
significant role or in mice deficient in IFN-G. For example, IL-18-mediated loss
of cartilage synthesis in arthritis models is IFN-G independent [15]. Prevention
of melanoma metastases is IL-18 dependent but IFN-G independent [16], and
similar findings exist for ischemia-reperfusion injury in the heart, kidney and
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Role of IL-18 in inflammatory diseases
liver. Table 1 lists various animal models of Th1-, Th2- as well as non-immune-
mediated disease where the effect of reducing endogenous IL-18 activities has
been reported.
In driving the Th1 response, IL-18 appears to act in association with IL-12 or IL-15,
as IL-18 alone does not induce IFN-G. The effect of IL-12 is, in part, to increase the
expression of IL-18Rs on T lymphocytes, thymocytes and NK cells [7, 60, 61]. It
appears that the role of IL-18 in the polarization of the Th1 response is dependent
on IFN-G and IL-12 receptor B2-chain expression. The production of IFN-G by the
combination of IL-18 plus IL-12 is an example of true synergism in cytokine biology,
similar to the synergism of IL-1 and TNF-A in models of inflammation. Since IFN-G
is the “signature” cytokine of CD4+ and CD8+ T cells as well as NK cells, a great
deal of the biology of IL-18 is considered due to IFN-G production. Dendritic cells
deficient in the IFN-G transcription factor T-bet exhibit impaired IFN-G production
after stimulation with IL-18 plus IL-12 [62]. IL-18 is constitutively present in mono-
cytes and monocyte-derived dendritic cells type 1 cells. Thus, IFN-G induced by the
combination of IL-12 plus IL-18 appears to be via the T-bet transcription factor.
IFN-G plays a major pathological role in this disease due to its Th1-inducing prop-
erties and the generation of cytotoxic T cells. Using a cohort of 157 patients who
received unrelated donor bone marrow transplantation and developed graft versus
host disease, a polymorphism in the IL-18 promoter (G137C, C607A, G656T) was
identified and associated with statistically significant decreased risk death [63]. At
100 days after the transplant, the mortality in patients with this polymorphism was
23% compared to 48% in those patients without the polymorphism and after 1 year
the mortality was 36% versus 65%, respectively. The probability of the survival was
twofold in patients with this haplotype [63]. In the case of graft versus host disease
in mice, paradoxical effects of IL-18 have been reported depending on whether the
disease is CD4+ or CD8+ T cell mediated. In humans, T cells are responsible for
the disease following allogeneic bone marrow transplantation. Administration of
IL-18 to recipient mice increased survival in CD4+-mediated disease but resulted in
worsening in the CD8+-mediated disease [64]. Neutralizing anti-IL-18 monoclonal
antibodies significantly reduced CD8+-mediated mortality [64]. Administration of
IL-18 reduces the severity of the disease by inducing the production of Th2 cytok-
ines [65].
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Charles A. Dinarello
The combination of IL-18 plus IL-12 suppresses IgE synthesis via IFN-G production
and suggests a role for IL-18 in Th2 polarization. For example, in models of allergic
asthma, injecting both IL-12 plus IL-18 suppresses IgE synthesis, eosinophila and
airway hyperresponsiveness (reviewed in [7]). In contrast, the administration of IL-18
alone enhanced basophil production of IL-4 and histamine and increased serum IgE
levels in wild-type and IL-4-deficient mice [66]. Overexpression of mature IL-18 in
the skin results is worsening of allergic and non-allergic cutaneous inflammation via
Th2 cytokines [67]. Mice overexpressing IL-18 or overexpressing caspase-1 develop an
atopic-like dermatitis with mastocytosis and the presence of Th2 cytokines; also pres-
ent in these mice was elevated serum IgE [68]. Although IL-18 remains a Th1 cytokine,
there are increasing reports showing a role for IL-18 in promoting Th2-mediated dis-
eases [69]. Upon neutralization of IL-18 in co-cultures of dendritic cells type 1 cells
with allogeneic naive T lymphocytes, the Th1/Th2 phenotype was not affected, where-
as anti-IL-12 down-regulated the Th1 response [70]. In fact, IL-18Rs were expressed
on dendritic cells of the type-2 lineage, suggesting a Th2 response [70].
Insulin-producing islet B-cells secrete IL-18 and supernatants from stimulated islets
induce IFN-G in T cells in an IL-18-dependent manner [71]. Inside islets, however,
the expression of IL-18R is limited to resident non-B-cells [72]. Islet-derived IL-18
can therefore function by engaging the IL-18R expressed on islet stromal cells,
i.e., macrophages, T cells, fibroblasts and endothelial cells [71]. For this reason,
the effects of B-cell-derived IL-18 on B-cell responses is observed in intact islets, or
in islets surrounded by neighboring cells. Indeed, evidence suggests an association
between local IL-18 levels and B-cell damage. Islets isolated from the non-obese
diabetic mouse strain exhibit IL-18 expression prior to T cell invasion [71] and
exogenous administration of IL-18 worsens diabetes in these mice [73]. IL-18 also
contributes to the injury of streptozotocin (STZ)-induced diabetes [74] and IL-18
blockade with IL-18BP delays the development of diabetes in the non-obese diabetic
mouse [75]. Similarly, mice deficient in IL-18 exhibit delayed STZ-induced hyperg-
lycemia [76]. In humans, the gene for IL-18 maps to an interval on chromosome 9,
where a diabetes susceptibility locus, Idd2, resides [77].
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Role of IL-18 in inflammatory diseases
force of the heart. It appears that the role of IL-18 in myocardial dysfunction is
independent of IFN-G but likely related to the induction of Fas ligand. Human heart
tissue contains preformed IL-18 in macrophages and endothelial cells [28]. Upon
reducing IL-18 activity with either IL-18BP or a caspase-1 inhibitor, the functional
impairment of the ischemia reperfusion injury was reduced [28]. A neutralizing
anti-IL-18 polyclonal antibody resulted in near prevention of endotoxin-induced
myocardial suppression in mice, and myocardial IL-1B levels were also reduced [78].
Using caspase-1-deficient mice subjected to ligation of the left anterior descending
coronary artery as a model for myocardial infarction, significantly lower mortality
was observed in the deficient mice compared to the wild-type mice [79]. Caspase-
1-deficient mice also had lower levels of IL-18, metalloproteinase-3 activity and
myocyte apoptosis following the injury. In humans, myocardial tissue steady-state
levels of IL-18, IL-18RA chain and IL-18BP mRNA and their respective protein
levels were measured in patients with end-stage heart failure. Circulating plasma
and myocardial tissue levels of IL-18 were increased in the patients compared to
age-match healthy subjects [80]. However, mRNA levels of IL-18 BP were decreased
in the failing myocardium. In fact, plasma IL-18 levels were significantly higher in
patients who died compared to levels in survivors [80].
There is increasing evidence that IL-18 contributes to atherosclerosis. Unlike
the IFN-G-independent role of IL-18 in ischemic heart disease, the atherosclerotic
process involves infiltration of the arterial wall by macrophages and T cells and
IFN-G has been identified in the plaque and considered essential for the disease
[81]. Human atherosclerotic plaques from the coronary arteries exhibit increased
IL-18 and IL-18Rs compared to non-diseased segments of the same artery [82]. The
post-caspase-1 cleavage IL-18 was found to co-localize with macrophages, whereas
IL-18Rs were expressed on endothelial and smooth muscle cells. The localization of
IL-18 and IL-18Rs in smooth muscle cells is an unexpected but important finding
for the pathogenesis of atherosclerosis [81, 82].
Atherosclerotic arterial lesions with infiltrating, lipid-laden macrophages as well
as T cells develop spontaneously in male apolipoprotein E (apoE)-deficient mice
fed a normal diet. When injected for 30 days with IL-18, these mice exhibited a
doubling of the lesion size without a change in serum cholesterol [81]. There was
also a fourfold increase in infiltrating T cells. However, when apoE-deficient mice
were backcrossed into IFN-G-deficient mice, the IL-18-induced increase in lesion size
was not observed [81]. Although exogenous administration of IL-18 worsened the
disease, such an experimental design can be related to the dose of IL-18. Therefore,
reduction of natural levels of IL-18 in the apoE-deficient mice is a more rigorous
assessment for a role for IL-18 in atherosclerosis. Using apoE-deficient mice and
overexpression of IL-18BP by transfection with an IL-18BP-containing plasmid,
reduced numbers of infiltrating macrophages and T cells as well as decreases in cell
death, and lipid content of the plaques were found [83]. In addition, increases in
smooth muscle cells and collagen content suggested a stable plaque phenotype with
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Charles A. Dinarello
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Role of IL-18 in inflammatory diseases
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Charles A. Dinarello
IL-18 in these mice. These findings indicate a new role of IL-18 in the homeostasis
of energy intake and insulin sensitivity.
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Role of IL-18 in inflammatory diseases
would provide a basis for approval since preventing acute renal failure is an unmet
medical need.
The third clinical setting for testing agents that reduce IL-18 activity is acute
lupus nephritis and vasculitis. Here the role of IL-18 in the production of IFN-G may
be of paramount importance. Animal models of lupus indicate a pathological role
for IL-18 in the kidney. However, the effect of IL-18 on the vasculature may also
be part of the lupus vasculitis. Some early trials have blocked TNF-A in acute lupus
nephritis using monoclonal anti-TNF-A antibodies. In this case, these antibodies
reduce both TNF-A and IFN-G. Taken together, these preclinical observations pro-
vide a rationale for an intervention study in lupus nephritis. Testing the concept with
oral caspase-1 inhibitors or IL-18BP administered subcutaneously asks whether the
vessel wall inflammation can be reduced over that presently achieved by heparin,
corticosteroids and aspirin.
Acknowledgements
These studies are supported by Supported by NIH Grants AI-15614 and HL-68743
and the Colorado Cancer Center.
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Chemokines and chemokine receptors
1
Department of Rheumatology, Institute of Medicine, University of Debrecen Medical and
Health Science Center, Debrecen, 4004, Hungary
2
Veterans’ Administration, Ann Arbor Healthcare System, Ann Arbor, MI, USA
3
University of Michigan Health System, Department of Internal Medicine, Division of Rheu-
matology, Ann Arbor, MI, USA
Abstract
There is a structural and a functional classification of chemokines. The former includes four
groups: CXC, CC, C and CX3C chemokines. There is a redundancy and binding promiscuity
between chemokine receptors and their ligands. Recently, a functional classification distinguishing
between inflammatory and homeostatic chemokines has been introduced. However, numerous
effects of these chemokines overlap. For example, numerous homeostatic chemokines, which are
involved in lymphocyte recruitment and lymphoid tissue organization, may also play a role in B cell
migration underlying germinal center formation within the inflamed synovium. Anti-chemokine
and anti-chemokine receptor targeting may be therapeutically used in future biological therapy
of arthritis. In addition to the clear clinical benefit, we can learn a lot from these trials about the
actions of the targeted chemokines and their receptors. Today, most data in this field are obtained
from experimental models of arthritis; however, results of some human trials have also become
available. Thus, it is possible that a number of specific chemokine and chemokine receptor antago-
nists will be administered to arthritis patients in the near future. Hopefully, some of these potential
treatment modalities will be used to control inflammation, prevent joint destruction and thus will
benefit our patients.
Introduction
tion. These functions often overlap: for example, as described below, some homeo-
static chemokines have been implicated in the pathogenesis of RA. Many of these
chemokines are also angiogenic or angiostatic (for reviews see [5, 12, 13]).
First we give a brief overview of the chemokine and chemokine receptor fami-
lies. The inflammatory, angiogenic/angiostatic and homeostatic chemokines and
chemokine receptors that are involved in the pathogenesis of RA and thus may
become targets for anti-chemokine therapy are discussed in more detail. We also
summarize recent targeting data obtained in RA trials and in animal models of
arthritis. It is very likely that several anti-chemokine and anti-chemokine receptor
trials will be conducted in RA during the next decade.
Chemokines in RA
In RA, chemokines drive inflammatory leukocytes into the inflamed synovial tissue
(ST) (for reviews see [1–8, 14, 15]). As mentioned above, chemokines have been
classified into four distinct supergene families designated as CXC, CC, C and CX3C
chemokines (Tab. 1). The respective receptor types of these chemokine subsets are
CXCR, CCR, CR and CX3CR [1–3, 6–8, 16] (Tab. 1). There are about 50 known
chemokines and 19 chemokine receptors (for reviews see [1–9]) (Tab. 1). There are
two nomenclatures for chemokines: apart from their unique classical name (see
later) they are also considered as chemokine ligands and they have been assigned
a designation of CXCL(1–16), CCL(1–28), XCL(1, 2) or CX3CL1 (1, 4, 8, 16)
(Tab. 1). In this review, both designations are used.
CXC chemokines
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Chemokines and chemokine receptors
131
Zoltán Szekanecz and Alisa E. Koch
132
Chemokines and chemokine receptors
[1, 9, 10]. However, this chemokine is also expressed on follicular dendritic cells,
as well as EC and fibroblasts in the RA ST [43]. Thus, BCA-1/CXCL13 has been
implicated in inflammatory lymphoid tissue organization and aggregate formation
in the RA ST [43].
CXCL16, the single specific ligand for CXCR6, may also be considered a
homeostatic chemokine as it mediates lymphocyte recruitment to lymph nodes.
Large amounts of CXCL16 were detected in RA SF and ST [23, 24]. Synovial
macrophages and fibroblasts release this chemokine [23, 24]. CXCL16 recruits
mononuclear cells to the RA ST [23]. CXCL16-mediated cell recruitment to the ST
is dependent upon the MAP kinase pathway [23].
CC chemokines
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Zoltán Szekanecz and Alisa E. Koch
Members of these two chemokine subsets exert a special position of C residues [2,
58]. The C family contains two members, lymphotactin/XCL1 and single C motif 1B
(SCM-1B)/XCL2. The CX3C subset contains a single member, fractalkine/CX3CL1
[2, 3, 58, 59].
Lymphotactin/XCL1 is primarily involved in T cell migration to inflammatory
sites [1, 60]. This chemokine has been detected on CD8+ and CD4+/CD28– T cells
in RA [60]. Lymphotactin/XCL1 augments T cell ingress into the RA joint [60].
Fractalkine/CX3CL1 is chemotactic for mononuclear cells and also mediates cell
adhesion [58, 61]. This chemokine has been detected in RA SF and ST [61, 62].
Synovial macrophages, fibroblasts, EC and dendritic cells produce this chemokine
in RA [61, 62]. Fractalkine/CX3CL1 also enhances the adhesion of CD4+ T cells to
134
Chemokines and chemokine receptors
Chemokine receptors in RA
135
Zoltán Szekanecz and Alisa E. Koch
kocyte ingress into the RA joint [72]. There is an increasing body of evidence for the
role of the truncated $32-CCR5 non-functional receptor allele in RA. This polymor-
phism of CCR5 may be protective against the development of RA [73]. CCR6, the
single receptor for MIP-3A/CCL20 has been detected on RA ST leukocytes [49]. A
putative chemokine receptor, CCR-like receptor 2 (CCRL2) has been identified on
RA SF neutrophils and macrophages [74].
Regarding the C and CX3C chemokine receptors, XCR1 is expressed on RA ST
lymphocytes, macrophages and fibroblasts [1, 2]. CX3CR1 has been detected on
macrophages and dendritic cells in the RA ST [59].
The receptor for the Duffy blood group antigen, DARC, also recognizes some
CXC and CC chemokines (Tab. 1) [75]. DARC is expressed on RA ST EC [75].
136
Chemokines and chemokine receptors
137
Zoltán Szekanecz and Alisa E. Koch
138
Chemokines and chemokine receptors
Table 2 (continued)
from RA patients. Decrease in cell surface CCR2 density was associated with lower
disease activity [82].
Among anti-TNF-A agents, infliximab inhibited the expression of IL-8/CXCL8,
RANTES/CCL5 and MCP-1/CCL2 in RA sera and ST [83, 84]. Infliximab also
suppressed serum levels of groA/CXCL1 and CXCL16 in RA [85, 86]. In another
study, infliximab reduced the expression of CCR3 and CCR5 on RA T cells [87].
Treatment of RA patients with infliximab or etanercept resulted in the clearance
of CXCR3+ T cells from the ST [88]. Anti-TNF therapy also attenuated CXCL16
expression on synovial macrophages [89]. Chemokine inhibition may also have
relevance for safety of anti-TNF therapy as infliximab reduced the secretion of
IL-8/CXCL8, MIP-1A/CCL3 and MCP-1/CCL2 in response to mycobacteria. These
results suggest that the increased incidence of tuberculosis in infliximab-treated RA
patients may be related, in part, to the inhibition of TNF-dependent chemokine
gradients and abnormal leukocyte migration [90].
139
Zoltán Szekanecz and Alisa E. Koch
140
Chemokines and chemokine receptors
141
Zoltán Szekanecz and Alisa E. Koch
Numerous CCR1 and CCR2 antagonists have been developed and preclinical
studies with chemokine receptor-deficient mice have been initiated during the last
decade [120–125]. J-113863, a small molecule CCR1 antagonist improved paw
inflammation and joint destruction in murine CIA [125]. Preventatively or thera-
peutically administered Met-RANTES, a CCR1/CCR5 antagonist, inhibited murine
CIA and rat AIA. Met-RANTES also suppressed CCR1 and CCR5 expression in
the joint [126, 127]. A nonpeptide CCR5 antagonist preventatively inhibited mouse
CIA [128]. Some anti-chemokine receptor effects may be dose-dependent. For
example, while low doses of the MC-21 anti-CCR2 monoclonal antibody markedly
improved murine CIA, high doses of this antibody rather had pro-inflammatory
effects [129].
Studies with gene-deficient versus antagonist-treated animals resulted in
somewhat conflicting results. For example, while a non-peptide CCR1 antago-
nist improved murine CIA, CCR1-deficient mice produced more TNF-A than
controls [125]. In one study, CCR5–/– mice developed CIA to the same extent as
wild-type animals [130]. However, in another study, CCR5 gene-deficient animals
exerted a significant reduction in the incidence of CIA, which was associated
with an increased IL-10 production by spleen cells [131]. Similar controversy was
observed when comparing results obtained using anti-CCR2 antibodies or CCR2
gene-deficient animals. Anti-CCR2 antibody administered during the initiation
of murine CIA markedly improved the clinical symptoms, while blockade during
the later stages of the disease aggravated arthritis [123]. CCR2–/– mice developed
more severe CIA than did wild-type controls [123, 124, 130]. These data suggest
that data obtained from chemokine receptor antagonist studies in comparison to
those obtained from studies using gene-deficient animals may not be comparable.
Yet, CCR blockade using antibodies or synthetic inhibitors may be promising for
future therapies.
Regarding the limited number of human RA trials with chemokine receptor
antagonists, a CCR1 antagonist has been tried in a 2-week Phase Ib study. This
inhibitor decreased the number of ST macrophages [122]. One-third of the patients
also fulfilled the ACR20% criteria for clinical improvement [122]. CP-481,715,
a selective CCR1 antagonist inhibited monocyte chemotactic activity present in
human RA SF samples [121]. This compound has been evaluated in Phase I for
pharmacokinetics and safety [132].
Some CCR2 inhibitors have also entered clinical trials [123]. In a recent Phase
IIa clinical trial, MLN1202, a human CCR2 blocking antibody was administered
to RA patients. Treatment with this blocking antibody reduced the levels of free
CCR2 on CD14+ monocytes by 57–94% demonstrating the biological activity of
this agent. However, no clinical improvement was observed, suggesting that CCR2
blockade itself may not be sufficient to control synovitis in RA [133].
Among CCR5 inhibitors, maraviroc has been introduced to Phase II-III trials in
HIV infection and AIDS, as well as to Phase II trial in RA [134].
142
Chemokines and chemokine receptors
Conclusions
In this chapter, we have discussed the putative role of chemokines and their recep-
tors in RA. We also presented some examples for recent chemokine and chemokine
receptor targeting strategies. There is a structural and a functional classification of
chemokines. The former includes four groups: CXC, CC, C and CX3C chemok-
ines. Chemokines may also be distinguished as being inflammatory or homeostatic;
however, some chemokines exert overlapping functions. Anti-chemokine and anti-
chemokine receptor targeting may be therapeutically used in the future biological
therapy of arthritis. In addition to the clear clinical benefit, we can learn a lot from
these trials about the actions of the targeted chemokines and their receptors. Until
now, most data in this field have been obtained from animal models of arthritis
as only very few human RA trials have been completed. However, it is very likely
that numerous specific chemokine and chemokine receptor antagonists will be
developed, administered to RA patients and some caveats discussed above will be
clarified in the near future.
Acknowledgements
This work was supported by NIH grants AR-048267 and AI-40987 (A.E.K.), the
William D. Robinson and Frederick Huetwell Endowed Professorship (A.E.K.),
Funds from the Veterans’ Administration (A.E.K.); and grant No T048541 from the
National Scientific Research Fund (OTKA) (Z.S.).
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Signaling pathways in rheumatoid arthritis
1
University of California, San Diego, Department of Medicine, La Jolla, CA, 92093-0656, USA
2
University of Geneva, Centre médical universitaire, 1 rue Michel Servet, 1211 Geneva,
Switzerland
Abstract
Signaling pathways orchestrate the inflammatory response by regulating various cellular functions
such as programmed cell death, cell differentiation and proliferation or secretion of signaling
molecules. They are classically activated by ligand engagement of surface receptors but increas-
ing evidence suggests that intracellular proteins can also detect danger signals. Protection from
pathogens, chemical or physical injury, or neoplasia relies on a tightly regulated activation of these
mechanisms. The same signaling cascades sometimes escape from normal controls and increase the
production of cytokines, proteases, growth factors and chemokines up to harmful levels, leading to
an autodestructive process as seen in rheumatoid arthritis. Mapping the hierarchy of these path-
ways identifies which specific targets can be inhibited to safely reduce the levels of inflammatory
molecules and reset the homeostasis of the organism.
Introduction
Intracellular signaling pathways provide cells with the ability to respond to extracel-
lular signals in their environment. Depending on the nature of the stimulus and the
particular cell type involved, the signaling pathways regulate various cellular func-
tions such as programmed cell death, cell growth, differentiation and proliferation.
Protection from pathogens, chemical or physical injury, or neoplasia also involves
intracellular pathways that regulate the expression of cytokines, chemokines and
other secreted or membrane-bound molecules that contribute to host defense. When
signaling cascades escape from normal controls, they can lead to a pathological or
destructive process as seen in diseases like systemic lupus erythematosus (SLE) or
rheumatoid arthritis (RA).
Many extra- and intracellular mediators are released into the environment
during inflammation. These molecules arise from endogenous sources (hormones,
cytokines, free radicals, products of cell metabolism or apoptosis) or from exog-
enous microorganisms. The most common methods for activating intracellular sig-
naling cascades include ligand engagement of surface receptors, although increasing
evidence suggests that intracellular receptors play a key role in detecting danger
The key cytokines interleukin-1 (IL-1) and tumor necrosis factor (TNF) are rapidly
released by cells of the innate immune system as a first line of defense against exog-
enous pathogens. During chronic autoimmune diseases such as RA, cytokines orches-
trate tissue injury and are proposed to maintain an autoinflammatory loop. The clini-
cal success of biological agents in RA underlines the clinical importance of TNF and,
to a lesser extent IL-1. However, the available anti-cytokine therapies induce a true
remission in only a minority of patients, which is probably due to the redundancy of
the signaling network in RA and the heterogeneous nature of the disease.
IL-1 signaling
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Signaling pathways in rheumatoid arthritis
Figure 1
Signaling pathways for IL-1 and Toll-like receptors (TLRs). Activation of IL-1 receptors or
TLRs, such as TLR4 by lipopolysaccharide, converge on MyD88 to activate NF-K B and the
mitogen-activated protein kinases (MAPKs) like p38 and JNK. These lead to transcription of
genes involved in the inflammatory response. The inflammasome is also activated by envi-
ronmental danger signals, thereby activating IL-1 cleavage and NF-K B.
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Jean-Marc Waldburger and Gary S. Firestein
naling pathways. The IL-1R complex comprises IL-1RI and IL-1RAcP. The sole
known function of IL-1RII is to quench the activity of IL-1 [6]. Both the membrane-
bound and the soluble form of IL-1RII are able to capture IL-1. The cytoplasmic
tail of IL-1 RII lacks a TIR domain and cannot transduce intracellular signaling
[7]. Moreover, IL-1RII can form a dominant negative complex with IL-1RAcP. IL-
1RA acts as a classical receptor antagonist because of its higher affinity for IL-1RI
compared with IL-1 itself. Because IL-1RII binds IL-1RA much less efficiently than
IL-1RI, the two anti-IL-1 molecules synergize.
The IL-1R–IL-1 complex is recognized by IL-1RAcP. The result of this associa-
tion is the formation of a heterodimeric transmembrane receptor complex where
both IL-1R and IL-1RAcP are needed to initiate signal transduction. The close
spatial association of IL-1R and IL-1RAcP in the complex presumably allows homo-
typic protein-protein interactions of their respective TIR domains. Conformational
changes result that enable recruitment of the cytosolic TIR domain-containing adap-
tor protein MyD88. MyD88 then interacts with IL-1R-associated kinases (IRAKs),
leading to the activation and ubiquitinylation of TNF receptor-associated factor
6 (TRAF6). Downstream of TRAF6, TAK1, a mitogen-activated protein (MAP)
3 kinase, activates the IKB kinase (IKK) complex and thus NF-KB, leading to the
expression of inflammatory cytokine genes. In addition, TAK1 with other MAP3Ks
launches MAPK activation, which further increase cytokine levels.
Endogenous regulatory molecules have been described along the IL-1R intracel-
lular pathway. MyD88s, a splice variant of MyD88, lacks the interaction domain
involved in IRAK4 recruitment and acts as a natural dominant negative molecule
that inhibits NF-KB activation [8]. IRAK-M prevents the dissociation of IRAK1 and
IRAK4 from MyD88 [9].
To minimize potential infectious side effects, small molecules that specifically
target IL-1R signaling could be designed to leave other TLR responses needed for
host defense intact. In RA, however, it might be desirable to block some of the
other TLR pathways as well, since many might be involved in the pathogenesis of
synovial inflammation (see below). Direct targeting of most signaling components
downstream of IL-1R will affect other TLRs because they share common kinases
and adaptors (except TLR3, which does not use MyD88). Another strategy relies on
disrupting TIR-TIR domain homotypic contacts. The interaction between MyD88
and IL-1RI can be disrupted by a small molecule [10]. This compound showed
no inhibitory effect on the association of TLR4 with Myd88 and was thus able to
interfere with IL-1 signaling but left LPS responses intact.
TNF
In RA, the efficacy of TNF inhibitors currently approved for clinical use correlates
with animal models proving TNF as a major regulator of joint inflammation and
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Signaling pathways in rheumatoid arthritis
Figure 2
Signaling pathways for TNF. The TNF receptor assembles a signaling complex that can
activate the MAPKs, NF-K B, and caspases. This process can induce transcription of pro-
inflammatory genes and influence cell survival.
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Signaling pathways in rheumatoid arthritis
The different TLRs initiate both shared and distinct signaling pathways by
recruiting four different TIR domain-containing adaptor molecules (see Fig. 1):
MyD88, TIRAP (MAL), TRIF (TICAM1), and TRAM [15]. MyD88 is used by all
TLRs (and IL-1R, see above) except TLR3; TIRAP is downstream of TLR2 and
TLR4; TRIF is recruited by TLR3 and TLR4; and TRAM is used only by TLR4.
These signaling pathways raise inflammatory cytokine production via the transcrip-
tion factors NF-KB, activator protein-1 (AP-1) and MAPKs such as JNKs and p38.
TLRs 3, 4, 7, 8, and 9 also activate interferon (IFN) regulatory factor 3 (IRF3) and/
or IRF7, leading to the production of type I IFNs such as IFN-B and IFN-A.
TLR4 is expressed in RA joints and its ligand LPS can trigger arthritis in sev-
eral rodent models [16]. The serum and synovial fluid of RA patients contain an
unknown TLR4 ligand [17]. TLR4 sets up both MyD88-dependent and TRIF-
dependent pathways. The MyD88-dependent pathway is shared with IL-1 and
induces the expression of inflammatory cytokines such as IL-6 in fibroblasts and
IL-12 and TNF-A in myeloid cells. The MyD88-independent pathway involves the
consecutive recruitment of TRAM, TRIF then TRAF6, which links to TAK1 and
other MAP3Ks activation. Downstream of TRIF, RIP1 also connects to NF-KB
and to TRAF3, which evokes IRF3-dependent antiviral responses. The TRIF and
MyD88 pathways synergize to maximize the expression of inflammatory cytokines
by providing optimal MAPK and NF-KB activation.
RNA released from necrotic synovial fluid cells can activate rheumatoid fibro-
blast to produce high levels of IFN-B, CXCL10, CCL5, and IL-6 protein by binding
to TLR3 [18]. TLR3 shares the TRIF-TRAF3 pathway of TLR4, which eventually
triggers the IRF3-dependent antiviral program via TBK1 and IKK. TLR3 also sig-
nals to NF-KB through TRAF6 and TAK1, which might account for induction of
IL-6 in synovial fibroblasts exposed to RNA or poly(I:C). TLR3 ligands increase
IRF-3 phosphorylation and production of IFN and RANTES in cultured synovio-
cytes, via IKKE [19]. Activation of both IRF3 and c-Jun by IKKE also increases
matrix metalloproteases (MMP) expression in synovial tissue [20]. The dual role of
IKKE in the synovial inflammatory response suggests that this pathway is a potential
therapeutic target in arthritis.
In RA, TLR2 is chiefly expressed in fibroblast-like synoviocytes (FLS) and infil-
trating lymphocytes [16]. Stimulation of FLS with the TLR2 ligand peptidoglycan
but not the TLR9 ligand CpG up-regulates the expression of IL-6, IL-8, chemokines
and MMPs 1, 3, and 13 [21]. TLR2 can broaden the repertoire of its ligands and the
outcome of its signaling by collaborating with other TLRs or non-TLR PRRs. This
is relevant to the clinical setting where the response to PRR is caused by complex
ligands as opposed to agonists chemically purified in vitro.
TLR9 and TLR7 are thought to link innate recognition of endogenous DNA and
RNA, respectively, to pathological B and dendritic cell activation in autoimmune
diseases. Immune complexes of IgG and self DNA induce RF production by activat-
ing autoreactive B cells [22]. The signal triggered by TLR9 synergizes with the RF B
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cell receptor, which is activated by the Fc portion of IgGs in the immune complex.
TLR9/7 stimulation also leads to type I IFN production. Antiviral gene products
have been shown to be involved in the pathology of SLE but might be protective
in arthritis models. TLR9/7 stimulates the synthesis of inflammatory cytokines and
intra-articular injection of bacterial DNA containing unmethylated CpG motifs
causes arthritis [23]. The inflammatory pathway downstream of TLR9/7 is similar
to TLR4 (MyD88-IRAK4-TRAF6-TAK1-MAPK/IKK-AP-1/NF-KB). On the other
hand, type I IFN production differs from TLR3 and TLR4 signaling since TRIF
and IRF3 are dispensable [24]. Instead, a multiprotein complex comprising at least
MyD88, IRAK4, IRAK1, TRAF6, and TRAF3 activates the antiviral transcription
factor IRF7.
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Signaling pathways in rheumatoid arthritis
dependent potassium loss. How NLR family members sense their agonists in the
cytosol remains poorly understood. The demonstration of a direct physical interac-
tion between NLRs and microbial components or endogenous danger molecules is
still lacking and might involve another set of unknown adaptors.
NLR PRRs can also signal to NF-KB. NOD1 and NOD2 ligands are the pepti-
doglycan (PGN)-derived peptides G-D-glutamyl-meso-diaminopimelic acid (iE-DAP)
and MDP, respectively. NOD1 and NOD2 can form a complex with RIP2 (receptor-
interacting protein 2), which leads to activation of NF-KB. Caspase-1 can activate
Mal (TIRAP) to link NF-KB and p38 MAP kinase pathways [28]. Mutations in both
Card4 (NOD1) and Card15 (NOD2) can cause inflammatory diseases. Variants
linked to Crohn’s disease (CD) are mapped to the LRR region of NOD2, whereas
those encoded in the NACHT domain are found in Blau syndrome and early onset
sarcoidosis patients. Many of these polymorphisms confer a gain of function to
NOD2 and increase NF-KB activation [29].
Beside the many regulatory mechanisms of IL-1 signaling, endogenous inhibi-
tors of caspase-1 and therefore IL-1 release have been described. Caspase-12 has a
dominant negative effect on caspase-1 [30]. CARD-containing molecules have been
shown to bind to the CARD domain of pro-casapase-1 and to inhibit caspase-1
[31]. Pyrin, the protein mutated in familial Mediterranean fever patients (FMF),
regulates caspase-1 activation by interacting with the adaptor ASC to inhibit cas-
pase-1 [32].
Inhibition of the inflammasome is a new therapeutic alternative in IL-1-mediated
diseases, which now also comprises crystal-induced arthropathies. P2X7 antago-
nists, which are under development for the treatment of multiple sclerosis, can
inhibit the ATP pathway. Blocking Pannexin-1 could have a broader anti-IL-1 activ-
ity assuming a general role of this molecule in mediating cytosolic entry of extracel-
lular NALP ligands. Pannexin-1 inhibitory peptides show efficacy in vitro [27]. The
mechanism by which IL-1 induces itself is thought to be a major pathogenic loop in
several inflammatory diseases and represents an unresolved issue [33]. Recombinant
IL-1 antagonist is an effective therapy in cryopyrin-associated periodic syndromes
(CAPS) [34]. This contrasts with the preventing action of colchicine in most but
not all FMF variants [35]. Caspase-1 inhibitors, which are in clinical development,
have the ability to block IL-1 and IL-18 processing regardless of the specificity of
the NLR involved and can also block NF-KB activation by caspase-1 [36].
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Figure 3
Signaling pathways for the T cell receptor (TCR). The TCR is activated by antigen in the con-
text of MHC and engagement of costimulatory signals. ZAP70 leads to a cascade of signaling
events, including NF-K B translocation and stimulation of the MAPK pathway. Depending on
the intensity of the signal and the cytokine milieu, T cells can be directed along specific dif-
ferentiation and activation pathways.
164
Signaling pathways in rheumatoid arthritis
these pathways during antigenic stimulation can produce two main opposing out-
comes: differentiation to effector T cells or anergy. Full activation of T cells depends
on the engagement of the TCR and CD3 by the peptide-loaded MHC on the APC
(signal I) and the costimulatory receptors CD28 and B7 (signal II). Cytokine pro-
duction, survival and proliferation of the T cell need both signals, whereas engage-
ment of the TCR without signal II induces anergy resulting in antigen tolerance even
after rechallenge with proper costimulation.
The A and B chains of the TCR that recognize the antigenic peptide lack enzy-
matic activity and have a short cytoplasmic tail [39]. They are associated with the
CD3 complex, the most upstream substrate of intracellular signaling kinases. On
TCR triggering the src-family kinases Lck and Fyn phosphorylate two tyrosine
residues within the immunoreceptor tyrosine-based activation motifs (ITAMs)
of the CD3-Z, -G, -D and -E subunits. Phosphorylated ITAMs recruits ZAP-70
(Z-chain-associated protein of 70 kDa). After it is attached to the ITAM motifs,
Lck activates ZAP-70, the function of which is further increased by dephosphoryla-
tion of inhibitory sites. Higher affinity TCR-MHC contacts correlate with greater
number of phosphorylated ITAM motifs up to the maximum of ten ITAMs present
in each TCR complex. Downstream signaling events include the activation of Ras
and Rho-family GTPases, MAPK cascades, phosphatidylinositol 3 kinase (PI3K),
PKCQ, and the NF-KB pathway. The transcription of biologically important genes
is also modulated by intracellular calcium flux triggered by phospholipase C
(PLC)-G.
Phosphorylation of the transmembrane adaptor molecule LAT and the cytosolic
adaptor protein SLP-76 by ZAP-70 orchestrate these downstream events. Phospho-
rylated LAT and SLP-76 then serve as docking surfaces for other adaptors such as
growth factor receptor-bound protein 2 (Grb2), GADS, the p85 regulatory subunit
of PI3K and PLC-G. Through these molecules, LAT effectively controls calcium flux,
Ras/extracellular signal-regulated kinase (ERK), NFAT/AP-1, and PI3K activation.
The activity of nuclear factor of activated T cells (NFAT), which promotes IL-2 tran-
scription, is controlled by the sustained intracellular calcium flux initiated by PLC-G.
Ca2+ activates the serine phosphatase calcineurin, promoting the dephosphorylation
and nuclear translocation of NFAT. Calcineurin is targeted by cyclosporine A and
FK506 [39]. Binding of the adaptor Grb2 on LAT further recruits the GDP/GTP
exchange factor Sos that activates the Ras/Raf/ERK pathway.
Once activated, the ERKs play an essential role in the expression of the AP-1
transcription factor c-Fos, as well as c-myc. c-Fos contributes to the transcriptional
regulation of AP-1 response elements in the IL-2 promoter. Vav is recruited both by
SLP-76 and Grb2 and is involved in NF-KB, NFAT, AP-1, and JNK activation and
for sustained Ca2+ signaling. Vav and Lck are required for the membrane relocaliza-
tion of PKCQ into lipid rafts where various other molecules, such as Lck, ZAP-70,
LAT have been shown to accumulate [40]. The presence of PKCQ in these structures
is necessary for NF-KB activation.
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Signaling pathways in rheumatoid arthritis
Existing anti-cytokines therapies block the interaction of a single ligand with its spe-
cific receptor. A significant percentage of RA patients do not respond to such thera-
pies. Moreover, disease control or remission usually requires the concomitant use of
broader immunosuppressive drugs such as methotrexate and/or steroids, exposing
patients to significant side effects. This is not surprising given the redundancy of
pro-inflammatory factors in the rheumatoid joint. However, these ligands rely on
restricted set of core signaling pathways. Mapping the hierarchy of these pathways
can identify specific targets that control the production of the most harmful set of
inflammatory proteins and minimize potential toxicity.
NF-KB
NF-KB affects multiple aspects of the inflammatory and destructive processes in RA.
In the RA synovium, activated NF-KB is detected in both macrophages and FLS, and
induces the transcription of cytokines like IL-1, TNF and IL-6 [48–50]. NF-KB also
plays an important role in leukocyte adhesion and transmigration by controlling
the transcription of chemokines and cell adhesion molecules. In animal models of
RA, NF-KB activation in the joint heralds the appearance of clinical disease. The
expression of MMPs, but not their tissue inhibitor, is under the control of NF-KB, as
well as the differentiation of T and B cells. Bone destruction is enhanced by RANK
activation (receptor activator of NF-KB) in osteoclasts.
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NF-KB is a family of transcription factors that increase the mRNA levels of its
target genes by binding to specific DNA sequences called KB or Rel sites [51]. These
DNA motifs are usually found in the promoter, a regulatory region of the gene
close to the transcription start site. The five NF-KB members are classified in two
subfamilies called NF-KB and Rel. The C-terminal part of NF-KB p105 and p100
proteins contain IKB-like inhibitory domains characterized by multiple copies of
the ankyrin repeat (ANK). These precursors are processed by proteasome-mediated
partial proteolysis to their mature DNA-binding forms p50 and p52, respectively.
c-Rel, RelA and RelB are not processed. NF-KB transcription complexes are homo-
or heterodimers of any of the subunits p50, p52, c-Rel, RelA (p65) and RelB.
The primary regulation of the NF-KB pathway is through the association of
NF-KB complexes with IKB inhibitors that comprise IKBA, IKBB, and IKBE. IKB bind
to the Rel domain of NF-KB dimers and block the nuclear localization function of
Rel. These inactive IKB-NF-KB multimeric complexes are thus maintained within
the cytoplasm. The two main pathways leading to NF-KB activation are called the
classical and alternative pathways [52]. Both are activated by an IKB kinase complex
consisting of catalytic subunits (IKKA and/or IKKB, also called IKK1 and IKK2). In
addition, the classical IKK complex comprises the scaffold protein NF-KB essential
modulator (NEMO) or IKKG.
Innate immunity and inflammation relies strongly on the classical pathway
[49]. Adaptors such as TRAF2 (downstream of TNFRI) and TRAF6 (downstream
of IL-1R/TLR4) link to MAP/ERK kinase kinase 3 (MEKK3) and TAK1. This in
turn activates IKKs, which then phosphorylate IKB inhibitors, targeting them for
ubiquitination and degradation by the 26S proteasome. The main latent classical
complex is a p50-RelA-IKBA trimer. The p50-RelA dimer drives most of the KB
transcriptional activity.
Other receptors preferentially induce the alternative pathway, such as BAFF-R
that controls B cell survival and RANK that increases osteoclastogenesis and bone
destruction [52]. However, although IKKA is required for RANK ligand-induced
osteoclast formation in vitro, it was dispensable in vivo in a mouse model [53].The
alternative IKK signalosome consists of IKKA subunits only, which are activated
by NIK (NF-KB inducing kinase). The latent complex comprises RelB and p100.
The slow degradation of p100 into the active p52/RelB dimers results in a delayed
kinetic of onset (hours versus minutes for the classical pathway).
Different classes of compounds can inhibit the NF-KB pathway including pep-
tides, oligonucleotides, microbial and viral proteins, small molecules or engineered
dominant negative or constitutively active polypeptides [54]. Some inhibit NF-KB
globally, while others target specific steps of the classical or the alternative pathway.
Several immunosuppressive drugs are also able to modulate NF-KB activity. Glu-
cocorticoids inhibit DNA binding of Rel or IKK activity depending on the cellular
context. Cyclosporine interferes with the proteasome to decrease IKB degradation.
FK506 blocks c-Rel nuclear translocation but not p50/RelA. Many other drugs
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Jean-Marc Waldburger and Gary S. Firestein
MAPKs
MAPKs are a family of serine/threonine protein kinases that control an array of cel-
lular responses to external stress signals. In RA, they regulate both cytokine produc-
tion and cytokine action [70]. MAPK cascades are organized into three individual
subfamilies: ERK, JNK and p38. The hierarchy of signaling events within each
family is based on a similar linear scheme: MAPK kinases (MAPKK or MAP2K)
activate p38, JNK or ERK, while MAPKK are themselves activated by MAPKK
kinases (MAPKKK or MAP3K). However, complex parallel and crossover signaling
occurs. For instance, the MAP3K MEKK3 can activate both MKK3 (a p38-specific
MAP2K) and MKK4 (a JNK MAP2K). Thus, many extracellular signals activate
more than one MAPK. For instance, cytokine receptors and growth factor recep-
tors lead mainly to ERK activation, whereas TLRs, proinflammatory cytokines,
and osmotic shock activate p38. Ultraviolet light, protein synthesis inhibitors, and
cytokines such as IL-1 and TNF stimulate the JNK pathway.
170
Signaling pathways in rheumatoid arthritis
All MAPK regulate the expression of several genes that participate in synovial
inflammation, including TNF and IL-1, but also MMPs that are involved in articu-
lar destruction. JNK works mainly by inducing the phosphorylation of c-Jun, which
stimulates the transcription of genes controlled by the transcription factor AP-1.
p38 acts at several levels including increasing the transcription, stability or transla-
tion of the mRNA, depending on the cell type and the stimulus. The ERKs 1 and 2
are widely expressed and regulate cellular proliferation and differentiation. Thus,
inhibition of the ERK pathway is considered to be mainly relevant to the treatment
of malignancies.
171
Jean-Marc Waldburger and Gary S. Firestein
bowel disease. It is not clear why some components of the inflammatory response,
such as acute phase reactants, appear to escape from p38 inhibition.
Among the dose-limiting side effects that contributed to the limited efficacy of
p38 inhibitors, liver tissues could be targeted since structurally distinct compounds
displayed hepatoxicity. However, this is not certain and greater selectivity for p38
over other kinases and p38 isoforms might improve the tolerability. Allosteric
inhibitors might also help reduce off target toxicities. Another alternative would be
to target upstream regulators or downstream substrates of p38. MKK3 deficiency
suppresses inflammation and articular cytokine production in a passive murine
arthritis model but does not affect IL-6 release after LPS injection. Thus, target-
ing this upstream kinase could protect from synovial inflammation, while leaving
host defense responses intact [77]. MAPKAP kinase 2 (MK2) is a substrate of p38
involved in the post-transcriptional regulation of TNF biosynthesis. Deletion of the
MK2 gene protects mice from collagen-induced arthritis [78]. However, this strategy
decreases cytokine release after LPS challenge in vivo and might lower the resistance
to microbial infection.
JNK pathway
Both JNK1 and JNK2 are phosphorylated in RA but not osteoarthritis synovium
[79]. JNK3 expression is restricted to neurological tissue. Activated forms of MKK4
and MKK7, the upstream activators of JNK, are both expressed in rheumatoid syn-
ovium with its major substrate c-Jun [80]. JNK controls local inflammation in the
joint by regulating cytokine and MMPs expression. Studies in JNK-deficient mice
indicate that this MAPK also regulates T cell differentiation into the T helper 1
subset, which has been incriminated in the pathology of RA [81]. Thus, JNK inhibi-
tion could diminish adaptive autoimmune responses in RA in addition to blocking
MMP production by FLS.
Peptide-based approaches that can target JNK signaling have been reported. Spe-
cific fragments of JNK-interacting protein 1 (JIP1), a scaffold protein, inhibit JNK
activity in various cell types. Short JIP1-derived peptides are selective and appear
to inhibit only JNK and its upstream activators, MKK4 and MKK7 [82]. Systemic
administration of a cell-penetrating and protease-resistant peptide inhibitor of JIP1
can prevent neuronal degeneration in different in vivo models [82].
SP600125 is a reversible ATP competitive inhibitor for all three JNK isoforms.
Administration of this compound during rat adjuvant arthritis had a modest anti-
inflammatory effect. In contrast, radiographic analysis showed a dramatic decrease
in bone and cartilage damage [83]. This was likely due to suppression of effector
mechanisms such as MMP production in the inflamed joints because the treatment
was started after initiation of T cell-dependent autoimmune responses. Data from
mice deficient in either JNK1 or JNK2 suggests that both JNK isoforms need to be
inhibited. Indeed, JNK2 deficiency offered very little protection in passive collagen
172
Signaling pathways in rheumatoid arthritis
arthritis, indicating that JNK1 can compensate [84]. Conversely, JNK1 knockout
does not protect from synovial inflammation in a TNF transgenic model [85].
Another molecule, spleen tyrosine kinase (Syk), plays a key role in Fc receptor and
ITAM signaling. Additional data suggest that it can also influence JNK function in
cultured synoviocytes [86]. The Syk inhibitor R406/R788 has demonstrated efficacy
in a Phase II study in RA patients concomitantly treated with methotrexate [87].
Long-term exposure to JNK inhibitors in chronic diseases could cause neurologi-
cal toxicity. Mice lacking both JNK1 and JNK2 isoforms present late embryonic
lethality and neural tube closure defects, while single deficiency in JNK1 elicit an
age-dependent alteration of axons and dendrites associated with pronounced loss of
microtubules [88, 89]. In cultured FLS, MKK7 but not MKK4 is required for JNK
activation after cytokine stimulation [90]. This suggests that this upstream kinase
might be an efficient and safer target than JNK in arthritis.
Conclusion
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180
Targeting oncostatin M in the treatment of rheumatoid arthritis
Abstract
Oncostatin M (OSM) is a pleiotropic cytokine with potential utility as a treatment for inflam-
matory arthritis. This pro-inflammatory cytokine is increased in the rheumatoid but not in the
osteoarthritic joint. Strategies to block the actions of OSM for use in inflammatory arthritis are
being developed and these show significant promise in murine models of disease. Targeting OSM
may have a beneficial effect by inhibiting some of the mechanisms of joint destruction in rheuma-
toid arthritis, which may limit long-term disability in patients. The challenge now is to convert this
potential into firm compounds that can be tested in the clinic.
Introduction
Figure 1
OSM binds to a gp130 receptor and signals through the JAK/STAT and MAPK pathways to
alter gene expression (reproduced with permission from Heinrich et al (2003) Principles of
interleukin (IL)-6-type cytokine signalling and its regulation. Biochem J 374: 1–20. © The
Biochemical Society.
182
Targeting oncostatin M in the treatment of rheumatoid arthritis
Table 1. OSM has pleiotropic actions on many different tissues. Outlined here are the main
established actions.
a relevant therapeutic target for RA and proof of concept studies are under way in
humans to demonstrate whether this may be beneficial to patients with this disease.
Various studies have demonstrated that OSM is elevated in the rheumatoid synovi-
um and synovial fluid. A variety of cells have been shown to produce OSM, includ-
ing activated T cells and macrophages [5, 6] especially within the synovium [7].
183
Theresa C. Barnes and Robert J. Moots
Neutrophils, however, are the most abundant inflammatory cell found in the
rheumatoid joint. These cells are not only found in synovial effusions, but also in the
synovial pannus of patients with RA, particularly at the site of bony erosions, where
they appear to have degranulated [8]. We have shown that neutrophils are capable
of expressing and releasing significant amounts of OSM in response to stimulation
with granulocyte/macrophage colony-stimulating factor (GM-CSF) [2]. Further-
more, we have found that neutrophils isolated from the synovial fluid of patients
with RA are unresponsive, in terms of expressing and secreting OSM, to GM-CSF
in vitro. This observation was in contrast to neutrophils isolated from the periph-
eral blood of the same patients, which expressed and secreted OSM on stimulation
with GM-CSF as normal. This may represent in vivo stimulation of neutrophils to
express and release OSM rendering them insensitive to further in vitro stimulation.
Given the vast abundance of neutrophils in RA synovial fluid, these cells appear to
be the major contributor of OSM in this compartment [2].
MMPs are protease enzymes that can degrade all elements of the extracellular
matrix. Their activity is strictly controlled at several points, including synthesis,
secretion, processing of the inactive proform and binding to specific inhibitors
(TIMPs). MMPs have been found in increased concentrations in the serum, synovial
184
Targeting oncostatin M in the treatment of rheumatoid arthritis
fluid and synovial tissue of RA patients, where levels correlate with disease activity
and structural damage [12–16]. The production of MMPs with potent collagenase
activity (MMP-1 and MMP-13) is increased in RA synovial fibroblasts (RASFs) by
the cytokines IL-1 and TNF-A [17]. OSM and retinoic acid both result in an increase
in bovine and porcine cartilage degradation, as assessed by collagen and proteogly-
can release. Together they act synergistically to produce cartilage degradation and
increase expression of MMPs. Neither agent alone or in combination was able to
lead to human cartilage degradation in vitro [18], reinforcing the importance of
considering species differences in response to this cytokine. OSM and IL-1 synergis-
tically increase collagen and proteoglycan release from porcine, bovine and human
cartilage in the presence of increased MMP activity. Although OSM alone increases
TIMP activity, IL-1 and OSM in combination lead to a net decrease in TIMP activ-
ity. This combination of OSM with IL-1 is, so far, the only combination of cytokines
shown to promote collagenolysis in human cartilage [7].
The effects of OSM and IL-1 alone, and in combination on human dermal micro-
vascular endothelial cells (HDMECs) and RASFs, have been studied in vitro. RASF
and HDMEC proliferation increase in response to OSM and IL-1 alone with a syn-
ergistic effect when these cytokines are combined. OSM and IL-1, whether alone or
in combination, increase the percentage of HDMECs and RASFs expressing ICAM-
1, an important adhesion molecule for leucocytes enhancing migration into sites of
inflammation.
Angiogenesis is an essential part of the development of inflammatory RA pan-
nus. In vitro, OSM increases the formation of endothelial cell tubules and increases
HDMEC migration, indicating that it provides an angiogenic stimulus. OSM
and IL-1 together result in a modest but consistent increase in vascular endothe-
lial growth factor (VEGF) in cartilage. The fact that VEGF is a potent angiogenic
stimulus further supports the evidence that OSM may be a regulator of angiogenesis
[19].
Co-culture of RASFs with chondrocytes dictates the response of each cell type
to cytokines and leads to higher baseline MMP production compared to each cell
type alone. In addition, co-culture results in higher levels of MMP production
when stimulated with OSM and IL-1, than either cell type alone. When cartilage is
incubated alone with OSM and IL-1 there appears to be a decrease in TIMP pro-
duction. However, in co-cultures of RASF with cartilage, OSM and IL-1 result in a
dramatic shift in the MMP:TIMP ratio in favour of the MMPs. Thus, the effects of
the microenvironment, including the different cells types and expressed cytokines,
need to be taken into account during in vitro experiments to be able to make a
sensible prediction as to what is truly happening in vivo [19]. These experiments
185
Theresa C. Barnes and Robert J. Moots
indicate that, although OSM can increase TIMP expression, within the context of
the rheumatoid joint, TIMP is decreased – suggesting that the net effect of OSM is
proinflammatory.
The injection of human recombinant OSM into synovial joints of goats has been found
to produce clinical features of joint inflammation, with an influx of leucocytes into
the synovial fluid [20]. In this model, there was a significant reduction in cartilage
proteoglycan content, indicating proteoglycan release, coupled with a decrease in ex
vivo proteoglycan production. Some of these features were abrogated by the co-admin-
istration of the IL-1 decoy receptor IL-1Ra, which blocks the action of IL-1. However,
even in the presence of IL-1Ra there was significant joint inflammation and reduction
of cartilage proteoglycan content in response to intra-articular OSM, indicating that
not all of the effects of OSM could be mediated by an increase in IL-1 [20].
Hui et al. [21] overexpressed murine OSM in combination with either IL-1 or
TNF in the joints of a murine host using adenoviral gene transfer. Each cytokine on
its own induced moderate synovial hyperplasia and bone erosions. However, both
combinations resulted in a significant increase in synovial hyperplasia, inflamma-
tion and bone damage. In addition, the combinations induced synergistic osteoclast
formation and activation and an increase in the expression of RANK and RANKL
in inflammatory cells, inflamed synovium and articular cartilage. RANKL is a
member of the TNF superfamily. It signals through its receptor, RANK and is a key
factor in the differentiation and activation of osteoclasts, the main cells responsible
for bone resorption. Other studies from this group have shown that overexpres-
sion of these combinations of cytokines in mice led to a synergistic increase in joint
inflammation with bone destruction and proteoglycan and collagen release [22]. In
addition, there was a synergistic increase in MMP expression in the cartilage and
synovium. While TIMP expression was increased by OSM alone, this effect was
abrogated when OSM and IL-1 were co-expressed [23].
The ability of synovial fibroblasts from patients with RA to proliferate and
form colonies in anchorage-independent conditions is thought to reflect their ability
to contribute to aggressive pannus formation in vivo. Langdon et al. [24] overex-
pressed murine OSM in mouse synovial fibroblasts by adenoviral gene transfer and
found that OSM significantly increased the ability of mouse synovial fibroblasts to
form colonies in anchorage-independent conditions. Like other groups, they found
that intra-articular administration of the murine OSM via an adenoviral gene trans-
fer technique resulted in pronounced joint inflammation and cartilage erosion. They
also noted that the effects outlasted the predicted effect of the adenoviral vector,
indicating a sustained change in phenotype towards one of chronic inflammation
[24].
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Targeting oncostatin M in the treatment of rheumatoid arthritis
Conclusions
OSM is a pleiotropic cytokine whose net actions in the rheumatoid joint appear to
be pro-inflammatory. One of the most important actions of OSM is to act alone
or synergistically with other proinflammatory cytokines (IL-1, TNF-A) to increase
the ratio of MMPs:TIMPs. This results in catabolism of cartilage and bone, which
is crucial for joint erosion and destruction. In addition, OSM appears to have an
important role in angiogenesis. Angiogenesis within the rheumatoid pannus is not
only essential to support the proliferation of the pannus but also promotes inflam-
187
Theresa C. Barnes and Robert J. Moots
matory cell infiltration of the joint in conjunction with endothelial cell activation.
Neutrophils, macrophages and T cells once recruited to the rheumatoid joint act
as a source of further OSM, potentially causing a positive feedback loop that may
result in the chronic inflammation characteristic of RA.
The actions of OSM vary subtly, depending both on the species and the microen-
vironment. Like other cytokines, its actions are modulated by other cytokines that
are inevitably co-expressed within the rheumatoid joint and its effects should be
understood within the context of both species and microenvironment. The effects of
OSM in RA and on inflamed rheumatoid joints suggest that this cytokine might be
a suitable candidate for development as a therapeutic target for the future.
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191
Targeting the epigenetic modifications of synovial cells
Abstract
Rheumatoid arthritis (RA) is a systemic inflammatory disease that mainly affects the synovial tis-
sues of joints. As in other autoimmune-related disorders, neither the etiology nor the pathogenesis
of RA has as yet been completely unraveled. It is generally accepted, however, that autoimmune
disorders develop through a combination of the individual genetic susceptibility, environmental
factors, and dysregulated immune responses. Genetic predisposition has been described in RA, in
particular as “shared epitope”, a distinct sequence of amino acids within the antigen-presenting
peptide groove of the major histocompatibility complex. Imbalanced immunity is reflected by the
production of autoantibodies and the accumulation of reactive helper T cells within the rheuma-
toid synovium. In addition, environmental factors have been postulated as disease-modulating
agents, including smoking, nutrition and infectious agents. So far, these factors have been studied
almost exclusively as separate agents. However, gene transcription might be affected by ageing
and environmental effects (such as nutrition and infections) – without changes in the nucleotide
sequence of the underlying DNA. These patterns of alterations in the gene expression profiles are
called “epigenetics”. The term epigenetics is used to refer to molecular processes that regulate
gene expression patterns but without changing the DNA nucleotide sequence. These epigenetic
changes comprise the post-synthetic methylation of DNA and post-transcriptional modifications
of histones, including methylation, phosphorylation, ubiquitination, sumoylation, biotinlyation
and, most importantly, deacetylation and acetylation. With respect to the complex pathogenesis
of rheumatic diseases, the “epigenome” is an emerging concept that integrates different etiologies
and, thus, offers the opportunity for novel therapeutic strategies. Based on the fact that current
therapies have not resulted in an ACR70 above 60% and have never been targeting the activated
synovial fibroblast, novel therapeutic strategies should target the epigenetic pathways of synovial
activation in RA.
Introduction
Less than a decade ago, the number of genes encoded within the nucleus of a single
human cell was estimated to at least 100 000 genes. Much hope for our understand-
ing of pathogenesis and treatment of diseases was put on the successful accom-
plishment of the human genome project. It was, therefore, most surprising that the
number of genes finally detected was quite low. Most of the 25 000 genes identified
encode biological functions that remain undiscovered so far, and the functional
characterization of these genes in normal physiology as well as in the pathogenesis
of diseases remains the main issue for biomedical research in the coming years [1].
However, this approach through “functional genomics” might be biased by post-
replicational, post-transcriptional as well as post-translational modifications – and
proteome diversity due to alternative splicing of mRNA transcripts and other bio-
chemical alterations somehow limits the utility of genomic information [2, 3]. The
question how the genome integrates intrinsic and environmental factors thus might
be answered by the emerging concept of the “epigenotype”. The term “epigenetics”
comprises stable alterations of the genetic information that are heritable but do not
involve mutations of the DNA sequence itself. Epigenetic regulations are mediated
by several biochemical phenomena, most importantly, however, by “histone modi-
fications” and “DNA methylation”. Epimutations and epigenetics are required for
development and differentiation of cells within a multicellular organism. Moreover,
they allow a cell to respond to environmental stimuli throughout adult life by means
of stable expression or repression of genes in specific cell types. Currently, no epige-
netic information is systematically analyzed and epigenetic modifications have not
been assessed within the human genome project. Since epigenetics might play the
linking role between environmental factors and genetics in determining a certain
phenotype, the investigation of epigenetic alterations along the lines of chromo-
some-wide and promoter-specific arrays will represent an fascinating area of future
research. Already, pilot studies for the human epigenome project have been under-
taken [4]. In this regard, the epigenome could provide a readout of an individual’s
environmental history [5]. The conventional method of studying human diseases by
molecular genetic approaches and additional environmental factors could soon be
extended to a novel field of “epigenetic epidemiology”.
Within this chapter, we focus on the emerging concept of epigenetics and its
implications for potential treatment strategies in rheumatoid arthritis (RA).
Definitions
The term epigenetics is used to refer to molecular processes that regulate gene
expression patterns, without however changing the DNA nucleotide sequence. The
epigenome of a cell is defined by two major groups of biochemical alterations: post-
transcriptional methylation of DNA and modifications of histones that package
DNA and, thus, modulate the accessibility for transcription factors to information
present on nucleic acid. These modifications are mitotically heritable and can be
transmitted during cell division from one generation of cells to the next. Both physi-
ological and pathological responses to environmental stimuli are probably mediated
194
Targeting the epigenetic modifications of synovial cells
Figure 1
Interaction between genes (genome), epigenetics (epigenome) and phenotype (adapted and
modified after [49]). Post-synthetic modifications of DNA are inherited (as epimutations)
or establish during development and cell differentiation. Physiological and pathological
responses to environmental stimuli (such as nutrition, age, and infections) are also governed
by epigenetic mechanisms. Even in the absence of such environmental factors, the epigenom-
ic profile is reversible and highly variable, probably due to stochastic events in the somatic
inheritance and in maintenance of epigenetic profiles.
DNA methylation
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Lars C. Huber, Astrid Jüngel and Steffen Gay
DNMT3a and 3b for example are de novo methyltransferases and introduce cyto-
sine methylation at CpG sites that were previously unmethylated. DNMT1 acts dur-
ing replication and cell division by copying existing methyltransferase pattern onto
newly synthesized DNA strands. For the process of DNA methylation, DNMTs use
S-adenosylmethionine, whose generation is mainly modulated by the availability of
different methyl donors. Briefly, methyltetrahydrofolate governs the conversion of
homocysteine to methionine, which is further metabolized to S-adenosylmethionine.
Deficiencies in the enzymes involved in these processes result in hypomethylation
of DNA. Several nutrients play key roles within this metabolism. The major dietary
sources of methyl groups include folate, choline and vitamin B12 [10–13].
The insertion of a methyl group at position five of the cytosine ring leads
to structural changes of chromatin and is associated with gene repression. This
silencing function on the level of gene expression can be achieved by different
mechanisms. Structural modifications of the DNA might block the proper docking
of DNA-binding factors to their fitting recognition sites, thus inhibiting gene tran-
scription, whereas methyl-CpG-binding proteins (MBPs, such as MeCP2, MBD1–4)
function redundantly as transcriptional co-repressors. Moreover, MBPs have been
shown to interact with enzymes regulating histone modifications. This interaction
could provide a link between different epigenetic mechanisms.
Histone modifications
Apart from DNA methylation, local chromatin architecture and, thus, transcrip-
tional regulation of gene expression is strongly influenced by covalent biochemical
modifications subsumed under the term “histone code”. These epigenetic changes
include post-transcriptional modifications of histones, including methylation, phos-
phorylation, ubiquitination, sumoylation, biotinlyation and, most importantly,
deacetylation and acetylation [14–16]. “Nucleosomes” are the fundamental build-
ing blocks of the heterochromatin consisting of an octamer of four core histones
and DNA. This octameric structure is made out of an H3-H4 tetramer and two
H2A-H2B dimers. Histone H1 has a linker function between DNA and protein
and governs the path of the DNA as it exits from the nucleosome. With respect to
histone modification, the DNA of physiologically resting cells is wrapped tightly
around the core histones, thus preventing the binding of basal transcription factors
(e.g., the TATA box binding protein) and RNA polymerase II [17]. Gene transcrip-
tion is initiated when histones are modified to create an open, accessible form of
chromatin. Probably the most investigated modification of the histone code is the
(de)acetylation of core histones. Histone acetylation is performed by histone acetyl-
transferases (short: histone acetylases, HATs) that neutralize positive charges at the
E amino groups of lysine residues at the N termini. Hyperacetylation of histones is
generally associated with enhanced rates of gene transcription. Conversely, the space
196
Targeting the epigenetic modifications of synovial cells
between histones and surrounding DNA is reduced by de- and hypoacetylation, and
transcription factors are sterically hindered from binding, leading to gene silencing.
Taken together, the gene transcription rate is regulated by the balance between his-
tone acetylation and histone deacetylation. The targeted deacetylation of histones
is performed by several multisubunit enzyme complexes, i.e., histone deacetylases
(HDACs) [16, 18]. Figure 2 shows the dynamic interplay of epigenetic mechanisms
between states of silent and transcriptionally active chromatin.
Eukaryotic members of the HDAC family can be divided into three major
groups, of which class I and class II HDACs so far comprise the best characterized
classes with respect to function. The class I comprises HDAC1–3 and 8, and localize
almost exclusively within the nucleus to exert their function. HDACs 4–7, 9, and
10 are subsumed under the term class II HDACs, which are mainly found in the
Figure 2
The dynamic balance between silent and transcriptionally active chromatin (modified after
[23]). Histone deacetylases (HDACs), DNA methyltransferases (DNMTs) and methyl-CpG-
binding proteins (MBPs) provide gene repression, whereas transcription factors (TFs), histone
acetyltransferases (HATs) and HDAC inhibitors (HDACi) lead to enhanced gene transcription
rates. Disturbances and changes in one or more of these components shift the balance to any
side of gene expression.
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Lars C. Huber, Astrid Jüngel and Steffen Gay
cytoplasm and are shuttled into the nucleus when needed. Whereas class I HDACs
are expressed in most cell types, the expression pattern of class II HDACs appears
to be tissue-specific, indicating a possible role in cellular differentiation and develop-
ment. HDACs remove the acetyl group from the nucleosomal core histones using
a sophisticated charge-relay system using Zn2+ ions as prosthetic group. HDAC
inhibitors such as Trichostatin A (TSA) fit into the active catalytic pocket of the
enzyme, exchange the zinc ion and, thus, make the system dysfunctional [14, 19].
HATs, on the other hand, comprise a family of proteins that catalyze the acety-
lation of lysine residues of one of the core histone proteins [20–22]. Traditionally,
HATs are categorized in two groups based on their subcellular localization: type A,
located in the nucleus and type B, located in the cytoplasm. Nuclear type A HATs
acetylate nucleosomal histones within chromatin in the nucleus, and thus type A
HATs are related to transcriptional regulation processes. On the other hand, type B
HATs acetylate newly synthesized free histones in the cytoplasm. Since recent data
indicate that HAT activity can also be induced in multiple protein complexes that
are related to transcriptional processes, this historical categorization is no longer
used. Amino acid sequence analyses of all HAT proteins revealed the important fea-
ture that HAT proteins fall into distinct families that share relatively poor sequence
similarity. All three superfamilies (GNAT, MYST and p300/CBP-HATs), however,
have a highly conserved acetyl-CoA binding site in common.
The best-understood family of HATs is the GNAT [general control non-dere-
pressilble-5 (Gcn5)-related N-acetyltransferase] superfamily. Humans express two
Gcn5-like acetyltransferases: Gcn5 and PCAF (p300/CBP-associated factor). Both
of these proteins can interact with another HAT protein complex, i.e., p300/CBP.
P300/CBP is a ubiquitously expressed, global transcriptional co-activator that
regulates cell cycle, differentiation and apoptosis. The HAT activities of p300/CBP
enables the transactivation of DNA binding transcription factors (p53, E2F, myb,
GATA1, Rb) as well as the acetylation of all four histone proteins. Mutations in the
HAT active site inhibit their transcriptional activating function.
The MYST family of HATs is particularly interesting as these proteins show
similarity with other acetyltransferases exclusively within the acetyl-coenzyme A
binding motif. The members of the MYST family in this HAT group are: MOZ
(monocytic leukemia zinc finger protein), Ybf2/sas3, Sas2, and Tip60. MOZ is
involved in the chromosome translocations associated with acute myeloid leukemia.
MOZ acts as a transcriptional coactivator for AML1, which is essential for estab-
lishment of definitive hematopoiesis. An overview of the different HAT families is
provided in Table 1.
Finally, DNA methylation and histone modifications have been considered as
two distinct mechanisms, which influence the level of gene expression in an indepen-
dent manner. It was shown, however, that HDACs are correlated to DNA methyla-
tion, either through direct interaction of HDACS with DNMTs or by the function
of MBPs. Another line of evidence suggesting that MBPs interact with HDAC1 and
198
Targeting the epigenetic modifications of synovial cells
HDAC2 to recruit the Sin3 corepressor protein further supports a link between
DNA methylation and histone modifications [23]. Thus, DNMTs appear to be
forms of dual function proteins, which are recruited by transcriptional repressors.
On the other hand, they have a non-enzymatic function interacting with histone
methyltransferases and HDACs, hence leading to chromatin remodeling [24–26].
Epigenetic modifications in RA
RA is a systemic disease mainly affecting the synovial tissues of joints. The process
of ongoing inflammation and erosion of articular cartilage and subchondral bone
causes severe pain, functional impairment and ultimately disability [27]. As in other
autoimmune-related disorders, neither the etiology nor the pathogenesis of RA has
yet been completely unraveled. It is generally accepted, however, that autoimmune
disorders develop through a combination of the individual genetic susceptibility,
environmental factors, and dysregulated immune responses [28, 29].
Genetic predisposition has been described in RA, in particular as “shared
epitope”, a distinct sequence of amino acids within the antigen-presenting peptide
groove of the major histocompatibility complex. Imbalanced immunity is reflected
by the production of autoantibodies such as rheumatoid factors and cyclic citrul-
linated peptides as well as by the accumulation of reactive helper T cells within the
199
Lars C. Huber, Astrid Jüngel and Steffen Gay
200
Targeting the epigenetic modifications of synovial cells
201
Lars C. Huber, Astrid Jüngel and Steffen Gay
Perspectives
202
Targeting the epigenetic modifications of synovial cells
In addition, the combination of agents that block both DNA methylation and
histone modifications might be of therapeutical interest. TSA, which has long been
considered as a specific histone deacetylase inhibitor leading to histone hyperacety-
lation and activation of unmethylated gene sequences, has been shown to induce
systemic and replication-independent demethylation of DNA [46]. The results from
this study should be taken into account when novel, TSA-related epigenetic drugs
are designed.
Finally, dietary recommendations for putative epigenetic drugs as found in green
tea, garlic, broccoli and other phytochemical compounds [47, 48] might be an addi-
tional option apart from pharmaceutical interventions.
Until then, however, novel biologicals that reverse the epigenetic pattern have to
be designed. Moreover, hurdles facing in vivo efficacy and toxic side effects have to
be overcome. The epigenotype has to be investigated, along the line of chromosome-
wide and promoter-specific arrays, especially with respect to the activated RA SFs.
This will open the scope for future therapies and might push epigenetic inhibitors as
potent agents to treat or prevent disease on an individual basis.
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44 Huber LC, Distler O, Tarner I, Gay RE, Gay S, Pap T (2006) Synovial fibroblasts: Key
players in rheumatoid arthritis. Rheumatology (Oxford) 45: 669–75
205
Lars C. Huber, Astrid Jüngel and Steffen Gay
206
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Edward C. Keystone
Abstract
Targeted therapeutic agents have changed the landscape of therapy in rheumatoid arthritis (RA).
They have also provided valuable insights into the utility of animal models for development of
targeted therapies, clinical trial design, pharmacodynamics, immunobiology and key pathogenic
elements of disease. Studies of chimeric anti-CD4 monoclonal antibodies in RA demonstrated the
need for pre-clinical studies to more closely approximate the human therapeutic paradigm as well
as the importance of synovium as an appropriate pharmacodynamic window to predict efficacy
and adverse side effects of the agents. Targeted therapies have been instructive in discerning the
importance of TNF, IL-1, IL-6, IL-15 and RANKL in the pathological process themselves, such as
the uncoupling of inflammation and structural damage. Current trends in the use of targeted thera-
peutics include aggressive earlier use, combination with methotrexate, use in moderate rather than
severe disease, tight control as well as induration and maintenance regimes. Despite therapeutic
advances with target therapies a number of unmet needs exist, including a low remission rate, cost
and inadequate access as well as the lack of biomarkers to predict response and safety concerns.
Despite this, target therapies have revolutionized the treatment of RA. In addition to having a
substantial effect on clinical outcomes, a number of valuable lessons have been learned.
Introduction
Insights
208
Perspectives in targeted therapy
[11]. Although both antibodies were generated in Chinese hamster ovary cell lines,
differences in the level of aggregation and a nonglycosylated heavy chain were
thought to account for the results. The data demonstrate how subtle differences
in biological agents can result in substantial differences in pharmacodynamics and
clinical effectiveness.
The lessons learned in CD4+ T cell depletion in RA have been particularly
helpful in allaying concerns about B cell depletion. Despite profound depletion of
peripheral B cells with rituximab, an anti-CD20 mAb, the incidence of infection
appears to be no greater than that observed with tumor necrosis factor inhibitors
(TNFi) [12]. Additional insight into the relationship between circulating immune
cell depletion and clinical efficacy was shown by the lack of correlation between
loss of efficacy after a course of rituximab and repletion of circulating B cells [13].
However, when a high-throughput FACS analysis was utilized, a significant correla-
tion was observed between B repletion and RA flare. The presence of B cells within
the synovium despite marked depletion of circulating B cells again emphasizes the
concept of the synovium as an important pharmacodynamic window [14]. An
analysis of synovium after rituximab treatment demonstrated a significant positive
correlation between reduction of clinical disease and initial macrophages and plas-
ma cells [15]. The results suggest that treatment with rituximab causes an indirect
decrease in inflammatory cells other than B cells, thus providing insight into the role
of B cells orchestrating synovial inflammation. The correlation between the clinical
response and synovial plasma cells suggests that rituximab exerts its effect, at least
in part, through an effect on autoreactive plasma cells associated with autoantibody
production. Other important insights have been gained from evaluation of synovial
biomarkers in clinical trials of targeted therapies [16]. One of the most instructive
lessons was derived from synovial biopsies following TNF blockade. Although the
rapid reduction in cellularity at the site of synovial inflammation with TNF block-
ade was hypothesized to result from apoptosis of TNF-reactive cells, the clinical
efficacy of certolizumab (a PEGylated Fab fragment of a humanized anti-TNF anti-
body that is unable to induce apoptosis) in RA suggests that induction of apoptosis
might not be a requirement for efficacy of TNF inhibitors in RA [17]. More recent
synovial biopsy data suggest that rapid reduction in cellularity and inflammation
in the rheumatoid synovium after TNF blockade is a result of dampening of TNF-
driven cytokine and chemokine cascades associated with a reduction in cellular
recruitment and retention of inflammatory cells [18].
Synovial biopsy studies have also provided a rationale for new therapeutic tar-
gets in RA. Thus, the accumulation of CCR1 in the synovium of RA patients pro-
vided a rationale for CCR1 blockade. A small proof of concept study with an oral
CCR1 antagonist demonstrated a marked decrease in the number of macrophages
and CCR1-positive cells in actively treated patients associated with a trend towards
clinical improvement even after short-term treatment [19]. To date, it is unclear
whether blocking a single chemokine receptor is significant to ameliorate RA.
209
Edward C. Keystone
Taken together, the data suggest that analysis of synovial biomarkers can be used
for screening purposes during early drug development.
Targeted therapies have also provided insight into the effect of immunomodula-
tion on different immune compartments. Several combinations of biological agents
have shown to be ineffective in modulating synovitis at the local tissue level [20,
21]. Yet, substantial effects have been observed on the systemic immune system as
indicated by a significant increase in serious infectious and in some cases malig-
nancy [20, 21]. These observations point out the sensitivity of the systemic immune
system to immunodulation compared with the local immunoreactive site such as the
inflamed joint. The results suggest significant caution is necessary when considering
a strategy using a combination of targeted therapies for human disease.
As with efficacy, some adverse events may have been predicted by animal mod-
els. Pre-clinical data might have predicted susceptibility of TNFi-treated patients to
intracellular infections such as tuberculosis (Tb). Several studies of TNFi in a latent
Tb model in mice clearly demonstrated susceptibility for Tb to disseminate [22].
This raises the issue of the diligence required by industry to determine risks prior to
treatment of human disease. In addition, more pre-clinical studies involving chal-
lenge with infectious agents known to be controlled by the targeted immune element
are needed.
Selective targeting of TNF has also provided preliminary evidence for the role of
TNF in tumor surveillance. Despite a substantial body of data demonstrating that
immune modulation is not generally associated with non-cutaneous solid tumors,
studies with TNFis have challenged this notion. The increase in solid malignancies
observed in association with the combination of the subtle TNF receptor (etaner-
cept) and cyclophosphamide as well as high-dose infliximab has been sobering [23].
The results of the metanalysis of anti-TNF mAbs by Bongartz, although controver-
sial, have enhanced our index of suspicion for the possibility that TNFi may increase
the risk of solid tumors [24].
Some adverse events of targeted therapies may not be predicted in the animal
models of disease. Recent reports of multifocal leukoencephalopathy with anti-
VCAM-1 (natalizumab) in patients with multiple sclerosis [25] and with rituximab
in patients with systemic lupus erythematosus [26] highlight this concept. They
emphasize the need for long-term safety surveillance, to detect rare adverse events.
Although numerous long-term databases of biologicals have been established
across the globe, differences in criteria to initiate a biological, the surveillance
data collected, validation and monitoring techniques, and particularly the nature
of the non-biological controls make comparisons and/or metanalyses difficult.
While propensity score have improved the comparability of control populations,
more attention must be paid to disease severity (i.e., risk of damage over time) and
disease activity over time. More precise data with respect to the reason for failure
of a DMARD or biological, i.e., efficacy versus safety, and the adequacy of a treat-
ment course must be taken into consideration. Data on progression of structural
210
Perspectives in targeted therapy
damage, genomics and biomarkers in conjunction with high quality clinical data
are needed.
A recent review of the lessons learned from therapies with biologicala has
emphasized the role of FcGR receptors in predicting differences in the outcome of
therapies having the same target (reviewed in [27]). The authors note that differ-
ences in efficacy and adverse event profiles of mAbs can be predicted based on
FcGR polymorphisms. Additionally, they point out the association of disease and
infectious susceptibility with specific FcGR polymorphisms. Although target antigen
specificity is the major influence on the benefit risk profile of therapies, the conse-
quences of mAb binding are influenced Fc design. The authors conclude that disease
indication should determine Fc design and that collecting information in clinical
trials of FcDR genotype would be useful.
One of the most valuable attributes of targeted therapies is the dissection of key
pathogenic elements of disease. Therapeutic studies have clearly delineated some of
the key cytokine mediators in the pathogenesis of RA, including TNF [28–30], IL-6
[31], and RANKL [32]. Disorders such as psoriasis, psoriatic arthritis, colitis and
sarcoidosis have also benefited substantially by selective targeting. The role of T
and B cells in the pathogenesis of RA has been clarified through the clinical benefit
derived from CTLA4 Ig [33] and anti-CD20 mAb [34], respectively.
When a particular molecular entity is addressed by a targeted therapeutic agent,
significant insights have been provided into the pharmacodynamic/pharmacoki-
netic properties of the agents as well as disease pathogenesis. Thus, differential
clinical benefit has been observed with agents targeting the same molecular entity.
While anti-TNF mAbs, infliximab and adalimumab as well as etanercept, a soluble
TNFR-Fc fusion protein demonstrated clinical benefit in RA, only the anti-TNFm
Abs demonstrated benefit in Crohn’s disease, sarcoidosis, and Wegener’s granulo-
matosis [35]. Studies are currently underway to address mechanisms accounting for
these clinical differences to gain a better understanding of the pathogenesis of these
disorders. The efficacy of a PEGylated humanized Fab’ fragment targeting TNF in
Crohn’s disease provides additional insights into the mechanism of action of TNFi
as well as disease pathogenesis [36]. A striking example of the insight provided
by targeted therapy on the pathogenetics of disease is the amelioration, in part, of
multiple sclerosis by rituximab – a B cell-depleting agent [37]. While the rationale
for the use of a B cell-depleting agent in patients with multiple sclerosis (previously
considered a T cell driven disease) is unclear, the result suggests that classifying
diseases as either T cell or B cell directed may be all too naïve. Together these data
emphasize the utility of targeted therapeutic approaches to dissecting the pathogenic
elements of disease.
Immunogenicity of targeted biological therapies was expected but has still gen-
erated a few surprises. The relatively low immunogenicity of chimeric mAbs (i.e.,
infliximab) particularly with methotrexate (MTX) has been particularly rewarding
[38]. Over time and with an increased frequency of infusion, reactions particularly
211
Edward C. Keystone
with monotherapy or after a time gap between infusions have been observed [39].
The reason why one TNFi, infliximab, reduces a sustained efficacy while another,
adalimumab, reduces initial efficacy is still not clear. A reduction in human anti-
chimeric antibodies has been accomplished though the use of high doses of TNFi
and regular periodicity of infusions. It is of interest that despite IgG Fc components
of soluble receptors, such as TNFR-Fc (etanercept) or CTLA4 Ig (abatacept), little
effect of immunogenicity on efficacy or safety has been reported. More surprising
was the generation of neutralizing antibodies against the “fully human” anti-TNF
mAb, adalimumab. These antibodies against adalimumab are associated with lower
serum adalimumab concentrations and a lack of response to adalimumab treatment
[40]. Taken together, the data provide a clear signal that the efficacy of any mAb,
regardless of the degree of humanization, is likely to be influenced by immunoge-
nicity.
The development of biologicals has had a significant effect on clinical trial
design. Anti-CD4 mAb therapy was one of the first unique approaches to RA treat-
ment. Although eight open label studies demonstrated promising results in 60–75%
of patients, randomized placebo-controlled trials of both murine and chimeric anti-
CD4 mAbs demonstrated no clinical efficacy (reviewed in [41]). The results likely
reflected an expectation bias on the part of the investigator and patients [42]. As a
consequence of these observations, Phase I trials were blinded thereafter to avoid
this pitfall.
Targeted therapies have been instructive not only in discerning the elements
that are pathogenic in RA but have also in defining the mechanisms involved in
the pathological processes themselves. One striking example of the latter was the
discovery of a significant uncoupling of inflammation (as determined by joint swell-
ing) and structural damage (assessed radiographically) particularly with biologicals.
Numerous studies have demonstrated that, despite a similar degree of joint swell-
ing in patients treated with MTX or a TNFi, striking differences in radiographic
progression are seen [43–45]. Moreover, TNFis have demonstrated superior inhi-
bition of radiographic progression at every level of response and disease activity
state achieved. The minimal radiographic progression that occurs in TNFi-treated
patients in a low disease activity state, in contrast to MTX where progression
continues, suggests the possibility that a low disease activity state in patients that
are comfortable with their symptoms may be an acceptable therapeutic target. In
patients receiving MTX alone, remission would still be the only acceptable target.
Future studies evaluating a low disease activity state in terms of progressive defor-
mity, disability and cardiovascular outcomes are needed. The data suggest that the
pathogenic elements leading to inflammation are not identical to those leading to
structural damage in the joints. It is conceivable based on these data that TNF is a
critical cytokine in causing structural damage, while cytokines in addition to TNF
play a significant role in generating inflammation. Further studies utilizing more
sensitive imaging techniques, such as ultrasound and MRI, may provide further
212
Perspectives in targeted therapy
insight into the observed dissociation. The capability of selective targeting of the
elements leading to structural damage in RA has also shown us that healing of
damage is possible.
Recent studies of early use of targeted biological agents suggest the possibility of
a window of opportunity in RA to change the course of disease. Thus, virtually all
patients achieving low disease activity with the initiation of combination of MTX
and infliximab were able to continue on low-dose MTX after discontinuation of inf-
liximab for up to 3 years [46]. A significant caveat to the concept of induction and
maintenance is the recent demonstration of an inability to discontinue infliximab in
patients with baseline characteristics suggesting a poor prognosis [47]. These find-
ings suggest that only patients who had a good prognosis and were likely to respond
well to MTX were able to discontinue infliximab. More importantly, 26% of
patients achieving low disease activity were able to discontinue all DMARDs. This
contrasts with the inability of patients initiating MTX only to discontinue the drug
even when progression from undifferentiated arthritis to RA is prevented [48].
TNFis have dramatically changed the therapeutic paradigms in RA. They have
resulted in markedly improved clinical outcomes with a substantial reduction in
irreversible structural damage. As a consequence of TNFi, the therapeutic goal in
RA of complete remission including clinical, laboratory and imaging outcomes is
now achievable. The success of TNFi has led to their earlier and increased use, esti-
mated in the USA to constitute as much as 35% of DMARD-treated RA patients.
Longer term experience with efficacy and safety of TNFi will undoubtedly increase
their use further. Currently, the therapeutic paradigm in the treatment of RA is the
initiation of MTX to a dose of 15–25 mg for 3–6 months followed by initiation of
a TNFi in MTX inadequate responders. If an adequate response is not achieved, the
trend is to switch to another TNFi, before the use of a newly approved biological
such as abatacept or rituximab is recommended.
A significant trend in clinical practice where TNFis are readily accessible is their
use in patients who have more moderate disease activity unlike those in clinical tri-
als where the majority of patients had severe disease activity. Retrospective analyses
of clinical trial patients as well as those in surveillance databases of clinical practice
with moderate RA have shown marked improvement in the outcomes of patients
moderate RA patients, with a considerably larger proportion of patients achieving
a low disease activity or remission state compared with patients who initially had
severe disease activity [49]. Of note, the methodology used to determine response to
therapy, i.e. ACR responses, significantly underestimates the proportion of patients
achieving a low disease activity state. Thus, ACR20 non-responders may have
substantial reductions in their ACR core set measures such as tender and swollen
213
Edward C. Keystone
joints, ESR etc. This likely accounts for some of the dissociation between clinical
and radiological outcomes in RA. Taken together, the results suggest that patients in
clinical practice who are treated with TNFis have a much better outcome than those
evaluated in clinical trials. The data suggest then that the outcomes in clinical trials
do not reflect those in clinical practice. These results have important implications
for the field. The data suggest that pharmacoeconomic analyses of TNFi should be
performed on patients initiating therapy with moderate RA to more closely approxi-
mate the situation in clinical practice. Such analyses will likely show a more marked
pharmacoeconomic benefit than has been observed with clinical trial population.
This could substantially enhance the access to TNFis by the payers. The data also
suggest that the unmet need for new therapies in the context of TNFi availability
may not be as great as previously thought. A concept that is likely to drive biologi-
cal use is that of an imaging remission in addition to clinical remission. Improved
imaging technology (MRI and ultrasound) has resulted in better detection of syno-
vitis compared with the clinical exam. Increased use of these techniques for detect-
ing synovitis coupled with a greater expectation on the part of rheumatologists to
achieve a remission will increase the tendency to aggressively treat patients even in
a low disease activity state.
Given their substantial clinical and radiographic efficacy, current TNFis will
remain the first line targeted therapy. They have raised the bar for more agents
currently in development. This concept is supported by the second line use of both
abatacept and rituximab: after TNFis-abatacept because of a more modest radio-
graphic outcome, and rituximab because of safety issues associated with retreatment
and use of another biological in the case of rituximab failures while B cells remain
depleted.
With the realization that rapid progression of structural damage occurs in early
RA, a number of novel therapeutic strategies have been developed to reduce such
progression. The utility of initiating a TNFi and MTX early in disease has demon-
strated unequivocally the superiority of combination therapy over monotherapy.
Unfortunately, the pharmacoeconomic data to support its use in very early RA are
still inadequate. The use of tight control strategies has escalated in recent years. Its
beneficial affect has been demonstrated but issues such as time to optimal response
and target endpoints require further clarification. Although substantial data support
TNFi switching, a number of confounders mar their interpretation. These include
the small sample size, short duration of studies, poorly defined outcomes, the single
center studies and most particularly, the lack of controlled studies. The issues above
coupled with inadequate information concerning the optimization of dose and dura-
tion of prior TNFi therapy confounds the interpretation of the studies. Controlled
trials are clearly warranted.
The future of targeted therapies in RA over the next 10 years is difficult to pre-
dict. However, a number of factors will drive utilization. A key driver will continue
214
Perspectives in targeted therapy
to be cost and hence access to expensive biologicals. The increasing co-payment for
agents funded by private payers, coupled with rising costs will dominate any discus-
sion of utilization. Over the next several years, approval is expected of novel agents
interfering with new targets such as anti-IL-6 receptor mAb (tocilizimab) and anti-
RANKL mAb (denosumab). The results of several novel B cell-depleting agents are
encouraging. Whether low-cost TNFis can be generated with small molecule agents
or a PEGylated anti-TNF-directed Fab fragment, i.e., certolizumab, by cheaper tech-
nology remains unclear. The failure to develop small molecule inhibitors of signal
transduction (i.e., p38) has been discouraging; however, recent preliminary data
from studies of the small molecule inhibitor of JAK3 have generated excitement in
the field. Other targets in development that are currently being evaluated include
IL-15, IL-17 and vascular endothelial growth factor, and B cell-stimulating factors,
i.e., BLYSS and APRIL.
Despite therapeutic advances with targeted therapies, there are a number of
unmet needs. Although many patients now achieve a low disease activity state with
the new therapeutic regimens, fewer achieve complete remission. Sensitive imaging
techniques suggest that these patients still have subclinical synovitis with the poten-
tial for further structural damage. Further research is need to define the imaging
thresholds for progressive structural damage and loss of function. Cost remains a
barrier to accessing current targeted therapies, and is a key concern; what is even
more worrisome is the reluctance of the majority of rheumatologists to embrace the
use of targeted therapies despite their excellent risk/benefit profile. Even ~10 years
after approval by regulatory authorities, 80% of biologics are prescribed by ~20%
of rheumatologists even where access is not a barrier to use. Whether this imbalance
in prescribing habits reflects over aggressive therapy by early adopters or under-
utilization by slow adopters needs to be clarified. A critical unmet need with respect
to agents where cost and access is an issue is the need for biomarkers to predict
appropriate utilization in the patient based on efficacy and safety concerns. The
imprecision of physician-driven outcomes and modest correlation with long-term
structural damage and disability supports the need for surrogate markers for both
disease activity and severity. While many biomarkers are available, few surrogates
exist.
In summary, targeted therapies have made an enormous impact in the field
of rheumatology, particularly for RA. They have substantially reduced signs and
symptoms of disease, improved function and quality of life, and prevented structural
damage and hence disability. They have been hugely instructive in the immunology
of disease and normal human biology. With the evolution of small molecule inhibi-
tors, further advances in biotechnology and a better understanding of the intricacies
of the pathogenic processes in RA, there should be a time when cost is no longer
the driver for these novel therapies and access is no longer the barrier to complete
remission.
215
Edward C. Keystone
Acknowledgement
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220
Index
221
Index
222
Index
223
Index
224
Index
ubiquitination 168
ZAP-70 165
225
The PIR-Series
Progress in Inflammation Research
Homepage: www.birkhauser.ch
Up-to-date information on the latest developments in the pathology, mechanisms and the-
rapy of inflammatory disease are provided in this monograph series. Areas covered include
vascular responses, skin inflammation, pain, neuroinflammation, arthritis cartilage and bone,
airways inflammation and asthma, allergy, cytokines and inflammatory mediators, cell signal-
ling, and recent advances in drug therapy. Each volume is edited by acknowledged experts
providing succinct overviews on specific topics intended to inform and explain. The series is
of interest to academic and industrial biomedical researchers, drug development personnel
and rheumatologists, allergists, pathologists, dermatologists and other clinicians requiring
regular scientific updates.
Available volumes:
T Cells in Arthritis, P. Miossec, W. van den Berg, G. Firestein (Editors), 1998
Medicinal Fatty Acids, J. Kremer (Editor), 1998
Cytokines in Severe Sepsis and Septic Shock, H. Redl, G. Schlag (Editors), 1999
Cytokines and Pain, L. Watkins, S. Maier (Editors), 1999
Pain and Neurogenic Inflammation, S.D. Brain, P. Moore (Editors), 1999
Apoptosis and Inflammation, J.D. Winkler (Editor), 1999
Novel Inhibitors of Leukotrienes, G. Folco, B. Samuelsson, R.C. Murphy (Editors), 1999
Metalloproteinases as Targets for Anti-Inflammatory Drugs,
K.M.K. Bottomley, D. Bradshaw, J.S. Nixon (Editors), 1999
Gene Therapy in Inflammatory Diseases, C.H. Evans, P. Robbins (Editors), 2000
Cellular Mechanisms in Airways Inflammation, C. Page, K. Banner, D. Spina (Editors), 2000
Inflammatory and Infectious Basis of Atherosclerosis, J.L. Mehta (Editor), 2001
Neuroinflammatory Mechanisms in Alzheimer’s Disease. Basic and Clinical Research,
J. Rogers (Editor), 2001
Inflammation and Stroke, G.Z. Feuerstein (Editor), 2001
NMDA Antagonists as Potential Analgesic Drugs,
D.J.S. Sirinathsinghji, R.G. Hill (Editors), 2002
Mechanisms and Mediators of Neuropathic pain, A.B. Malmberg, S.R. Chaplan (Editors), 2002
Bone Morphogenetic Proteins. From Laboratory to Clinical Practice,
S. Vukicevic, K.T. Sampath (Editors), 2002
The Hereditary Basis of Allergic Diseases, J. Holloway, S. Holgate (Editors), 2002
Inflammation and Cardiac Diseases, G.Z. Feuerstein, P. Libby, D.L. Mann (Editors), 2003
Mind over Matter – Regulation of Peripheral Inflammation by the CNS,
M. Schäfer, C. Stein (Editors), 2003
Heat Shock Proteins and Inflammation, W. van Eden (Editor), 2003
Pharmacotherapy of Gastrointestinal Inflammation, A. Guglietta (Editor), 2004
Arachidonate Remodeling and Inflammation, A.N. Fonteh, R.L. Wykle (Editors), 2004
Recent Advances in Pathophysiology of COPD, P.J. Barnes, T.T. Hansel (Editors), 2004
Cytokines and Joint Injury, W.B. van den Berg, P. Miossec (Editors), 2004
Cancer and Inflammation, D.W. Morgan, U. Forssmann, M.T. Nakada (Editors), 2004
Bone Morphogenetic Proteins: Bone Regeneration and Beyond, S. Vukicevic, K.T. Sampath
(Editors), 2004
Antibiotics as Anti-Inflammatory and Immunomodulatory Agents, B.K. Rubin, J. Tamaoki
(Editors), 2005
Antirheumatic Therapy: Actions and Outcomes, R.O. Day, D.E. Furst, P.L.C.M. van Riel,
B. Bresnihan (Editors), 2005
Regulatory T-Cells in Inflammation, L. Taams, A.N. Akbar, M.H.M Wauben (Editors), 2005
Sodium Channels, Pain, and Analgesia, K. Coward, M. Baker (Editors), 2005
Turning up the Heat on Pain: TRPV1 Receptors in Pain and Inflammation, A.B Malmberg,
K.R. Bley (Editors), 2005
The NPY Family of Peptides in Immune Disorders, Inflammation, Angiogenesis and Cancer,
Z. Zukowska, G.Z. Feuerstein (Editors), 2005
Toll-like Receptors in Inflammation, L.A.J. O’Neill, E. Brint (Editors), 2005
Complement and Kidney Disease, P.F. Zipfel (Editor), 2006
Chemokine Biology – Basic Research and Clinical Application, Volume 1: Immunobiology
of Chemokines, B. Moser, G.L. Letts, K. Neote (Editors), 2006
The Hereditary Basis of Rheumatic Diseases, R. Holmdahl (Editor), 2006
Lymphocyte Trafficking in Health and Disease, R. Badolato, S. Sozzani (Editors), 2006
In Vivo Models of Inflammation, 2nd Edition, Volume I, C.S. Stevenson, L.A. Marshall,
D.W. Morgan (Editors), 2006
In Vivo Models of Inflammation, 2nd Edition, Volume II, C.S. Stevenson, L.A. Marshall,
D.W. Morgan (Editors), 2006
Chemokine Biology – Basic Research and Clinical Application. Volume II: Pathophysiology
of Chemokines, K. Neote, G.L. Letts, B. Moser (Editors), 2007
Adhesion Molecules: Function and Inhibition, K. Ley (Editor), 2007
The Immune Synapse as a Novel Target for Therapy, L. Graca (Editor), 2008
The Resolution of Inflammation, A.G. Rossi, D.A. Sawatzky (Editors), 2008
Bone Morphogenetic Proteins: From Local to Systemic Therapeutics, S. Vukicevic,
K.T. Sampath (Editors), 2008
Angiogenesis in Inflammation: Mechanisms and Clinical Correlates, M.P. Seed, D.A. Walsh
(Editors), 2008
Matrix Metalloproteinases in Tissue Remodelling and Inflammation, V. Lagente, E. Boichot
(Editors), 2008
Microarrays in Inflammation, A. Bosio, B. Gerstmayer (Editors), 2009
Th 17 Cells: Role in Inflammation and Autoimmune Disease, B. Ryffel, F. Di Padova (Edi-
tors), 2009