CLINICAL SCIENCES
Clinical Investigations
Chronic Fatigue Syndrome: Exercise
Performance Related to Immune Dysfunction
JO NIJS1,2, MIRA MEEUS1,2, NEIL R. MCGREGOR3, ROMAIN MEEUSEN1, GUY DE SCHUTTER1,
ELKE VAN HOOF1, and KENNY DE MEIRLEIR1
1
Department of Human Physiology, Faculty of Physical Education and Physiotherapy, Vrije Universiteit Brussel,
Brussels, BELGIUM; 2Division of Musculoskeletal Physiotherapy, Department of Health Sciences, Hogeschool
Antwerpen, Antwerp, BELGIUM; and 3Bio21, Institute of Biomedical Research, University of Melbourne, Parksville,
Victoria, AUSTRALIA
ABSTRACT
NIJS, J., M. MEEUS, N. R. MCGREGOR, R. MEEUSEN, G. DE SCHUTTER, E. VAN HOOF, and K. DE MEIRLEIR. Chronic
Fatigue Syndrome: Exercise Performance Related to Immune Dysfunction. Med. Sci. Sports Exerc., Vol. 37, No. 10, pp. 1647–1654,
2005. Purpose: To date, the exact cause of abnormal exercise response in chronic fatigue syndrome (CFS) remains to be revealed, but
evidence addressing intracellular immune deregulation in CFS is growing. Therefore, the aim of this cross-sectional study was to
examine the interactions between several intracellular immune variables and exercise performance in CFS patients. Methods: After
venous blood sampling, subjects (16 CFS patients) performed a maximal exercise stress test on a bicycle ergometer with continuous
monitoring of cardiorespiratory variables. The following immune variables were assessed: the ratio of 37 kDa Ribonuclease (RNase)
L to the 83 kDa native RNase L (using a radiolabeled ligand/receptor assay), RNase L enzymatic activity (enzymatic assay), protein
kinase R activity assay (comparison Western blot), elastase activity (enzymatic– colorimetric assay), the percent of monocytes, and
nitric oxide determination (for monocytes and lymphocytes; flow cytometry, live cell assay). Results: Forward stepwise multiple
regression analysis revealed 1) that elastase activity was the only factor related to the reduction in oxygen uptake at a respiratory
exchange ratio (RER) of 1.0 (regression model: R2 ⫽ 0.53, F (1,14) ⫽ 15.5, P ⬍ 0.002; elastase activity P ⬍ 0.002); 2) that the protein
kinase R activity was the principle factor related to the reduction in workload at RER ⫽ 1.0; and 3) that elastase activity was the
principle factor related to the reduction in percent of target heart rate achieved. Conclusion: These data provide evidence for an
association between intracellular immune deregulation and exercise performance in patients with CFS. To establish a causal
relationship, further study of these interactions using a prospective longitudinal design is required. Key Words: EXERCISE
PHYSIOLOGY, IMMUNITY, ELASTASE, PROTEIN KINASE R
P
revious research has shown that patients with chronic
fatigue syndrome (CFS) present with an abnormal
exercise response and exacerbation of symptoms after
physical activity. Some of the main findings were a reduction in peak oxygen uptake (2,10), reduction in peak heart
rate (10), earlier exhaustion (10), and accelerated glycolysis
with increased lactate production (30). Contrary to these
findings, others found that the aerobic capacity of CFS
patients lies within the low normal range (23). The highly
heterogeneous nature of the CFS population and the lack of
uniformity in both the utilized diagnostic criteria and exercise testing protocols preclude pooling of data and hence to
draw firm conclusions. Still, we conclude that at least a
subgroup of CFS patients present with an abnormal response to exercise. In addition, because several exercise
capacity variables (e.g., functional aerobic impairment,
body weight–adjusted peak oxygen uptake, exercise duration) correlated with activity limitations/participation restrictions (20), evidence supporting the clinical importance
of impairments in exercise performance fitness in CFS patients was provided (i.e., a poor exercise performance was
associated with more severe activity limitations/participation restrictions). Importantly, the exacerbation of symptoms after exercise is seen only in the CFS population, and
not in fatigue-associated disorders such as depression, rheumatoid arthritis, systemic lupus erythematosus, or multiple
sclerosis (26). To date, the exact cause of the abnormal
exercise performance in CFS remains to be elucidated. Earlier attempts revealed that in CFS patients kinesiophobia
Address for correspondence: Dr Jo Nijs, SPORT KRO-1 V.U.B., Laarbeeklaan 101, B-1090 Brussels, Belgium; E-mail: Jo.Nijs@vub.ac.be.
Submitted for publication January 2005.
Accepted for publication May 2005.
0195-9131/05/3710-1647/0
MEDICINE & SCIENCE IN SPORTS & EXERCISE®
Copyright © 2005 by the American College of Sports Medicine
DOI: 10.1249/01.mss.0000181680.35503.ce
1647
(irrational fear of movement) is not related to exercise
performance (18), and that an exercise challenge further
enhances complement activation (26).
Type I interferons trigger the 2=,5=-oligoadenylate (2-5A)
synthetase/Ribonuclease (RNase) L activation and induce
the expression of the double-stranded RNA dependent protein kinase R (PKR). The PKR enzyme and 2-5A synthetase/RNase L system are termed the “cellular doublestranded RNA-detecting systems” that are responsible for
the translational inhibition in response to (viral) infection
(11). The deregulation of the 2-5A synthetase/RNase L
pathway in subsets of CFS patients has been reported at
length in the scientific literature (3,27,28). Both elastases
and calpain are capable of initiating high molecular weight
RNase L (83 kDa) proteolysis, generating two major fragments with molecular masses of 37 (a truncated low molecular weight RNase L) and 30 kDa, respectively (5).
Experimental data point to an activation of the PKR enzyme, parallel to the 83 kDa RNase L proteolysis, in subsets
of CFS (8). PKR activation leads to phosphorylation of the
inhibitor of NF(nuclear factor)-kB (IkB) and consequent
NF-kB activation, which in turn causes inducible nitric
oxide synthetase (iNOS) expression. iNOS generates increased production of NO by monocytes/macrophages. NO
mediates important vital physiological functions such as
neurotransmission, cell-mediated immune responses (strong
antimicrobial and antitumour activities), and vasodilatation.
Excessive and/or persistent production of NO, however, is
detrimental to the body’s functions (21). Elevated NO has
been documented in CFS patients (14). Elevated NO levels
and consequent vasodilatation might limit CFS patients to
increase blood flow during exercise, and might even cause
and enhance postexercise hypotension (19). It is hypothesized that PKR activation and consequent elevated NO
levels are related to poor exercise performance in CFS
patients.
Snell and colleagues (25) showed that CFS patients with
evidence of a deregulated 2=,5=-oligoadenylate (2-5A) synthetase/RNase L pathway have a lower peak oxygen uptake
than CFS patients without the intracellular immune deregulation, suggesting a link between immunopathology and
exercise performance in CFS. As outlined previously (19),
the deregulation of the 2-5A synthetase/RNase L pathway
may be related to a channelopathy, capable of initiating both
intracellular hypomagnesaemia in skeletal muscles and transient hypoglycemia. This might explain muscle weakness
and the reduced peak oxygen uptake seen in CFS patients.
Thus, it is hypothesized that various components of the
2-5A synthetase/RNase L pathway (i.e., the ratio of 37 kDa
RNase L to the 83 kDa native RNase L for the assessment
of 83 kDa RNase L proteolysis, RNase L enzymatic activity,
and elastase activity) are related to exercise performance in
CFS patients. Summarizing the research questions, this
study aims at 1) examining whether PKR activation and
consequent elevated NO levels predict poor exercise performance in CFS patients, and 2) examining whether exercise performance in CFS is associated with deregulation of
the 2-5A synthetase/RNase L pathway (i.e., 83 kDa RNase
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Official Journal of the American College of Sports Medicine
L proteolysis, RNase L activity, and elastase activity). It is
hypothesized that in CFS patients, 1) both PRK activation
and consequent elevated NO levels predict poor exercise
performance during a graded exercise cycle test, and 2)
deregulation of the 2-5A synthetase/RNase L pathway is
associated with poor exercise performance during a graded
exercise cycle test.
METHODS
Patient recruitment and research design. Sixteen
randomly allocated untrained patients with CFS were enrolled. Patients were randomly allocated from consecutive
referrals to our chronic fatigue clinic. To be included into
the study, patients had to fulfill the U.S. Centers for Disease
Control and Prevention criteria for CFS (9). Therefore, all
patients underwent an extensive medical evaluation before
study participation (see below). All patients had Dutch as
their native language, and were within the age range of
18 – 65 yr. The study sample consisted of eight female and
eight male CFS patients. The mean age was 38 ⫾ 10 yr
(range 19 –59), and the mean illness duration was 31 ⫾ 11
months (range 12– 48). An information leaflet was handed
out to all patients, and they were instructed to read it
carefully and, if applicable, to ask for additional clarification. The study protocol was approved by the local ethics
committee (Academical Hospital Vrije Universiteit Brussel;
O.G. 016). Patients provided written informed consent and
underwent venous blood sampling (arm vein; 40 mL; lying
supine). The following immunological variables were assessed: the ratio of 37 kDa RNase L to the 83 kDa native
RNase L, RNase L enzymatic activity, protein kinase R
activity assay, elastase activity, the percent of monocytes,
and nitric oxide determination. Patients were instructed not
to take any medication during the 24 h before study participation, and not to smoke, or to drink coffee or tea on the
testing day. Afterwards, all patients performed a maximal
exercise test on a bicycle ergometer with continuous monitoring of cardiorespiratory variables.
Diagnosis of CFS. All patients were diagnosed as CFS
cases by the same physician (the final author). To fulfill the
diagnostic criteria for CFS, clinically evaluated, unexplained, persistent, or relapsing chronic fatigue that is of
new or definite onset, should result in a substantial reduction
in previous levels of occupational, educational, social, or
personal activities (9). Furthermore, at least four of the
following symptoms must have persisted or recurred during
six or more consecutive months and must not have predated
the fatigue: impairment in short-term memory or concentration, tender cervical or axillary lymph nodes, muscle
pain, multijoint pain, headache, unrefreshing sleep, and postexertional malaise for more than 24 h (9). Any active
medical condition that may explain the presence of chronic
fatigue prohibits the diagnosis of CFS (diabetes, cancer,
AIDS, etc.). Hence, all patients underwent an extensive
medical evaluation, consisting of a standard physical examination, medical history, exercise capacity test, and routine
laboratory tests. The laboratory tests included a complete
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blood cell count, determination of the erythrocyte sedimentation rate, serum electrolyte panel, measures of renal, hepatic, and thyroid function, and rheumatic and viral screens.
If a patient’s medical history did not exclude a psychiatric
problem at the time of disease onset, then a structured
psychiatric interview was performed. In a number of cases
further neurological, gynecological, endocrine, cardiac,
and/or gastrointestinal evaluations were performed. The
medical records were also reviewed to determine whether
patients suffered from organic or psychiatric illness that
could explain their symptoms. If any of the laboratory/
additional analyzes revealed any active medical condition
that may explain the presence of the patient’s symptoms, the
patient was excluded from the sample.
Exercise testing. The exercise tests were performed at
a humidity of ⫾60% and at a room temperature of ⫾20°C
(Klimakamer, Jaeger, Germany). The patients performed a
bicycle ergometric test against a graded increase in workload until exhaustion was reached (2). The patients were
asked to take a sitting position on the electromagnetically
braked ergometer (Lode B.V., Excalibur, Groningen, the
Netherlands), after 3–5 min of adjustment the test was
started. Heart rate was monitored continuously at rest and
during exercise. There was continuous recording of the
12-lead electrocardiogram using an electrocardiograph
(ECG Esaote Biomedica S.P.A., Firenze, Italy). To collect
pulmonary data during the test, an open-circuit spirometer
(Metamax Cortex, Biophysics, Germany) with automatic
printout every 30 s was used. Automatically averages were
attained for V̇O2PEAK (peak oxygen uptake) and maximal
carbon dioxide production during every 30-s interval for the
duration of each stage of the exercise. A two-way breathing
valve attached to a mask, which covered the patients’ nose
and mouth, was used to collect the expired air. The air was
analyzed continuously for ventilatory and metabolic variables. Before each test, the spirometer was calibrated for
environmental conditions. For the assessment of blood lactate concentration during the exercise stress test, blood was
drawn every 2 min from an anticubital vein using natriumheparinized capillaries (EKF Diagnostics, Germany). Twenty-microliter blood samples only for lactate determination
were taken at the hyperaemized earlobe and assayed by
ESAT 6660 lactate (Medingen GmbH, Germany). All patients started the test at 10 W, with an increase of 10
W䡠min⫺1 (2). To avoid early onset of fatigue in the lower
extremities due to inadequate physical fitness, the duration of the exercise was kept below 15 min. Patients were
instructed to bicycle at a constant speed of 70 rpm. The
following variables were measured: heart rate at rest
(HRREST), peak heart rate (HRPEAK), exercise duration,
maximal work capacity attained, work capacity attained
at a respiratory exchange ratio (RER) of 1.0, V̇O2PEAK per
kilogram of body weight, body weight–adjusted oxygen
uptake at RER ⫽ 1.0, resting and peak RER (RERPEAK), the
percentage of target heart rate achieved, and both the resting
and peak lactate concentrations. The age-predicted HRPEAK
was calculated as 220 minus the patients’ age in years. The
metabolic data analyzed were the means of the last 30 s from
EXERCISE IMMUNOLOGY IN CFS
the final stage of exercise or the highest value attained if a
decline in V̇O2 occurred at the final workload (2). For
estimating the peak workload, the following equation was
used: (workload of the highest fully completed stage) ⫹
(number of seconds achieved during the final stage/60 ⫻
10). Exercise performance testing is widely used for the
assessment of patients with CFS, and it appears to be both
reproducible and valid (15). The exercise testing protocol
used in the present study was able to distinguish between
female CFS patients and healthy sedentary females (2), and
the exercise performance data obtained with this protocol
correlated with activity limitations/participation restrictions
in CFS patients (20).
RNase L-ratio determination. The assay is performed by 1) preparation of a cytoplasmic extract of the
patient’s peripheral mononuclear blood cells, 2) combination of this extract with a labeled probe that binds specifically to 2=-5= A binding proteins such as RNase L and the
low molecular weight species, 3) sodium dodecylsulfate
polyacrylamide gel electrophoresis, and 4) densitometry to
determine the relative quantities of 2=-5= A binding proteins.
The RNase L-ratio was counted using the following equation: RNase L-ratio ⫽ [low molecular weight RNase
L]/[high molecular weight RNase L] ⫻ 10. In detail, peripheral mononuclear blood cells (PBMC) were separated
from heparinized blood (30 mL) by Ficoll–Hypaque density
gradient centrifugation within 4 h of phlebotomy. In addition, PBMC were stored at ⫺70°C until cytoplasmic extraction preparation. The latter was performed in the presence of
protease inhibitors elastase inhibitor III (Calbiochem, San
Diego, CA), aprotinin, leupeptin, pefabloc-SC, and EDTA
(Roche Biochemicals, Mannheim, Germany). Protease inhibitors are required for preventing proteolytic cleavage.
Standard laboratory procedures were used to separate serum
from coagulated blood, and to store it at ⫺70°C until analysis. A modified Bradford assay method (Bio-Rad Laboratories, Hercules, CA) was used for quantification of total
proteins in the patients’ cell extracts. The probe specifically
attaches to 2=-5=A binding proteins like 80 kDa RNase L and
37 kDa RNase L. Two hundred milligrams of PBMC extract
was incubated with 2=-5= A trimer radiolabeled at the 3= end
with 32P-pCp, at 2– 4°C for 15 min. In addition, it was
covalently attached to the binding proteins by the addition of
cyanoborohydride (20 mM in 100 mM of phosphate buffer,
pH 8.0). This reduction reaction was allowed to progress for
20 min at 2– 4°C. A tracking dye were added to the samples,
and incubated at 95°C for 5 min followed by separation
using standard SDS-PAGE with a 4% stacking and a 10%
separating gel. The gel was dried and autoradiography was
used to detect the radioactivity of the marked probe (BioRad Laboratories Molecular Imager® Fx, Hercules, CA).
Densitometric analysis of the autoradiographs was followed
by quantification of any present 2=-5= A binding proteins
(using specializes software: Quantity One® Software, BioRad Laboratories, Hercules, CA). For ratio of the 37 kDa
RNase L isoform over the 83 kDa RNase L, a threshold
value of 0.4 for the diagnosis of CFS was found to have a
sensitivity of 91% and a specificity of 71% (29). Using a
Medicine & Science in Sports & Exercise姞
1649
threshold value of 0.5, the RNase L-ratio was able to distinguish between CFS patients (abnormal in 41 of 57 subjects or 72%), healthy controls (3/28 or 11%), and patients
with Fibromyalgia (0/11) and Depression (0/14) (3). In
another study the amount of 37 kDa RNase L was able to
distinguish between CFS patients (N ⫽ 53) and healthy
controls (N ⫽ 26; P ⫽ 0.007) (27).
The RNase L enzymatic activity (enzymatic assay) was
assessed as described previously (24). In several studies, the
RNase L activity was able to distinguish between CFS
patients and healthy controls, with higher activity in the
patients’ group (27,28). For the PKR activity measurement,
PAGE-separated proteins were transferred to a membrane
and visualized by immunodetection. In detail, the membranes were prepared by cutting out membranes of appropriate size (15 ⫻ 10 cm) and the membranes were prewet in
Methanol 100%, shaken gently until soaked, and then the
liquid was poured off. The membranes were immersed in
Towbin buffer, and were shaken gently (for a minimum of
2–3 min) until soaked. To remove the gel, the braces
from the glass plates were removed from the slab gel
unit from the electrophoresis system, the spacers were removed from between the glass plates using a spatula and
opened up, the upper glass plate was lifted up, leaving the
gel on the bottom glass plate, and the gel was adhered to the
membrane. Next, the Mylar mask was placed on the bottom
of the Electroblot instrument. For each gel/membrane, 2 ⫻
4 pieces of Whatmann were cut out and soaked in Towbin
buffer. To make a “sandwich,” four layers of Whatmann,
PVDF- or nitrocellulose membrane, Poly Acrylamide Gel,
and four layers of Whatmann were placed on top of the
Mylar mask. Air bubbles were avoided, and equal contact
was ascertained by rolling smoothly over the surfaces with
a plastic pipette.
For the electroblot, the lid was put on the electroblot
instrument, the electrodes were connected to the power
supply, a weight was put on the lid (less than 1 kg to avoid
buffer being squeezed out of the sandwich), and a current of
0.8 mA 䡠 cm–2 was applied for 2 h (i.e., 240 mA for two
membranes of 150 cm2 each). Expose the blotted membranes to air at room temperature (RT) for at least 1 h to
make sure the blotted membrane(s) is (are) dry. To visualize
the proteins, the total surface of blotted membranes (regular
membrane ⫽ 10 ⫻ 15 cm ⫽ 150 cm2) was calculated and
the membranes were prewet by soaking them in Methanol
70% (approximately 12 mL per blot), shaking until they
were completely wet, and removing the Methanol by gently
pouring it off. The membranes were washed two times with
phosphate-buffered saline (PBS)-Tween® (5 min each at
room temperature while gently shaking ⫺0.25 mL·cm⫺2 ⫽
37.5 mL for regular membrane). Next, the membranes were
blocked with 5% NF milk in PBS-Tween® for 1 h at room
temperature while gently shaking or overnight at 4°C, and
the tray was covered with a lid or with aluminum foil (0.25
mL·cm⫺2). For incubation with the primary antibody, a
dilution (dependent on the manufacturer’s instructions) was
made in PBS-Tween® (0.1 mL·cm⫺2 ⫽ 15 mL for regular
membrane), incubated for 2 h at room temperature while
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Official Journal of the American College of Sports Medicine
gently shaking or overnight at 4°C, and the tray was covered
with a lid or with aluminum foil. The membranes were
washed two times with PBS-Tween® (5 min each at room
temperature while gently shaking ⫺0.25 mL·cm⫺2) and
incubated with secondary antibody; a dilution (dependent on
the manufacturer’s instructions) was made in PBS-Tween®
(0.1 mL·cm⫺2), incubated for 30 min at room temperature,
shaken gently, and the tray was covered with a lid or with
aluminum foil. The membranes were washed two times
(colorimetric) or four times (chemiluminescence) with PBSTween®, 5 min each at room temperature while gently
shaking: 0.25 mL·cm⫺2. The colorimetric analysis was performed using Opti4CN® (Bio-Rad) by mixing Elix-H2O
9/10, Opti4CN®-diluent 1/10, and Opti4CN®-substrate
0.2/10 (10 mL per blot), incubating while gently shaking
until color develops, washing for 10 min with Elix-H2O, and
drying by blotting the membranes on Whatmann paper. The
specific protein bands were quantified by density scanning.
Elastase activity in PBMC was measured using an enzymatic– colorimetric assay: EnzChek® Elastase Assay Kit
E-12056 (Molecular Probes). The EnzChek kit contains
DQTM elastin–soluble bovine neck ligament elastin that has
been labeled with BODIPY®FL dye such that the conjugate
can be digested by elastase or other proteases to yield highly
fluorescent fragments. The resulting increase in fluorescence was monitored with a fluorescence microplate reader.
First, a PBMC pellet and a PBMC pellet extract (in absence
of elastase inhibitor III) were prepared, the protein concentration was measured, and the samples (patient proteins–
extracts) were placed on ice and let thawed for 20 –30 min.
DQ Elastin Substrate (BODIPY®FL): the new vial was
reconstituted by adding 1 mL of dH2O (final concentration
1.0 mg·mL⫺1) and by mixing thoroughly to dissolve. Five
milliliters of the stock buffer (10X) were diluted and 45 mL
of dH2O was added to obtain the reaction buffer. To obtain
a positive control (porcine pancreatic elastase), the new vial
was reconstituted by adding 0.5 mL of dH2O up to a final
concentration of 100 U·mL⫺1. Then, a standard dilution
curve was constructed from the positive control (porcine
pancreatic elastase with a starting concentration of 100
U·mL⫺1) with concentration of 5.0, 1.0, 0.5, 0.1, 0.05, and
0.01 U·mL⫺1. The controls were pipetted in triplicate into
the microplate. Samples of 100 g of proteins extract (X
L) were used. The total reaction volume was set at 200 L
(200 L – X L ⫽ Y L buffer 1X). Y L of buffer 1X was
added into the samples, and 50 L of sample dilution/well
was added. The samples were pipetted in triplicate into the
microplate. For the substrate, 5 L of DQ elastin/sample
was added to 145 L of reaction buffer/sample. Next, 150
L of the substrate/well sample was added. Protected from
light, everything was incubated for 2 h at 37°C. Afterwards,
the fluorescence intensity was measured in a fluorescence
microplate reader (Molecular phospho-imager®FX BioRad
and external laser Molecular ImagerâFX BioRad). The values were extrapolated from the equation of the curve (fluorescence vs elastase concentration) and multiplied by 200
(U䡠mg⫺1). For each sample, the value derived from the
no-enzyme control was subtracted to correct for background
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TABLE 1. The descriptive statistics of the exercise performance variables (N ⫽ 16).
Variable
Mean ⴞ SD*
Range
Exercise duration (min)
HRREST† (bpm)
HRPEAK (bpm)
% target heart rate achieved
Workload (W)
Workload per body weight (W䡠kg⫺1)
Workload at RER‡ ⫽ 1.0 (W)
V̇O2PEAK¶ (mL䡠kg⫺1䡠min⫺1)
V̇O2 at RER ⫽ 1.0 (mL䡠kg⫺1䡠min⫺1)
RERREST
RERPEAK
Lactate concentration at rest (mmol䡠L⫺1)
Peak lactate concentration (mmol䡠L⫺1)
14.8 ⫾ 6.5
78.6 ⫾ 14.7
158.6 ⫾ 22.3
87.3 ⫾ 13.4
148.3 ⫾ 65.0
2.0 ⫾ 0.7
121.8 ⫾ 52.6
31.0 ⫾ 9.7
26.0 ⫾ 7.2
0.68 ⫾ 0.10
1.1 ⫾ 0.2
0.9 ⫾ 0.4
8.5 ⫾ 4.9
[7–26]
[47–110]
[125–197]
[62–105]
[70–260]
[0.9–3.2]
[72–215]
[16.2–50.1]
[15.7–40.2]
[0.42–0.91]
[0.9–1.6]
[0.5–1.8]
[1.3–21.2]
* SD, standard deviation; † HRREST, resting heart rate; ‡ RER, respiratory exchange ratio; ¶ V̇O2PEAK, peak oxygen uptake.
fluorescence. According to the company supplying the assay, the elastase activity assay had been thoroughly tested
before it was brought on the market, but reliability and
validity data are proprietary and unpublished (personal communication).
The measurement of nitric oxide level in isolated PBMC
was performed using a live cell assay (12,13). The cells or
PBMC were washed once with 500 L PBS, and spun for
2.5 min at 2500 ⫻ g. A 15-mM DAF-FM solution (4-amino5-methylamino 2=,7=-difluorofluorescein diacetate, Molecular Probes D-23844: 3 mL of 5-mM stock in 1 mL of PBS)
was prepared, and the cells were resuspended in 100 mL of
this solution, left untouched for 45 min at room temperature
in a dark environment, and spun for 2.5 min at 2500 ⫻ g. A
solution of the CD14 staining (Becton Dickinson,
BD345785) was prepared (6 L CD14 in 60 L PBS), and
the cells were resuspended in 66 mL of this solution, left
untouched for 25 min at room temperature in a dark environment, spun for 2.5 min at 2500 ⫻ g, and resuspended in
500 mL of PBS. Analysis was performed with a flow cytometer; the monocyte population was gated and the mean
fluorescence (525-nm band pass) of this population was
measured. Cells were analyzed quickly, and kept in the dark
until processed. For a more detailed prescription of the
assay, the reader is referred to references (12) and (13).
Statistical analysis. Data were analyzed using Statistica version 5.1 (Statsoft, Tulsa, OK). Appropriate descriptive statistics were used: frequencies and percentages for the
gender distribution; mean, standard deviation (SD), and
range for illness duration; age; the exercise performance
variables; and the immunological variables. To examine the
associations between exercise performance and the immune
variables, Pearson correlation analyzes were used. A one
sample Kolmogorov–Smirnov (K–S) goodness-of-fit test
was used to examine whether the variables entering a Pear-
son correlation analysis were normally distributed. If a variable was not normally distributed, then the nonparametric
Spearman correlation analysis was used. For interpreting
correlation coefficients, they were squared to obtain the
coefficient of determination. For the correlation analysis,
the significance level was set at 0.01 to help protect against
potential type I errors. In case of a “trend” towards a
significant association (0.01 ⬍ P ⬍0.05), a power analysis
was performed (22). A power of 80% was considered fair.
Finally, the interactions between the exercise performance
variables and the immunological variables were further assessed using forward stepwise multiple regression analysis.
For the regression analysis, the significance level was set at
0.05.
RESULTS
The descriptive statistics of the exercise performance
variables are displayed in Table 1, the descriptives of the
immune variables in Table 2. The mean percentage of
monocytes was 18.9 ⫾ 6.9 (range [9.1–32.2]). All subjects
displayed abnormal responses for both the elastase activity
and RNase L-ratio, and RNase L activity was abnormal in
15/16 CFS patients. However, the Protein Kinase R activity
was within the normal range in the majority of the study
sample (11/16), and approximately 50% of the subjects
presented with normal NO levels in both monocytes and
lymphocytes.
Apart from the peak RER (K-S z ⫽ 1.49; P ⫽ 0.02), all
exercise performance and immune variables were normally
distributed. Thus, a Pearson correlation analysis was used
for analyzing the majority of associations. For the examination of the associations between RERPEAK and the immune variables, a nonparametric Spearman correlation analysis was used. The outcome of the correlation analysis,
TABLE 2. The descriptive statistics of the immunological variables (N ⫽ 16).
Variable
Mean ⴞ SD*
Range
Normal
Range†
Abnormal in N
(%) patients
Elastase activity (U䡠mg⫺1 extract)
Protein kinase R activity
RNase L activity
RNase L ratio
[NO] in monocytes
[NO] in lymphocytes
388.0 ⫾ 469.5
1.9 ⫾ 1.1
160.1 ⫾ 111.1
1.8 ⫾ 1.7
38.0 ⫾ 18.8
20.2 ⫾ 8.2
[112–2080]
[0.6–5.4]
[46–537]
[0.5–7.7]
[13.2–72.9]
[10.4–36.1]
⬍70
⬍2.0
⬍50
⬍.5
[29.6–40.9]
[11.2–18.5]
16 (100.0)
5 (31.3)
15 (94.8)
16 (100.0)
8 (50.0)
9 (56.3)
* SD, standard deviation; † normal range based on laboratory data from 60 healthy volunteers (unpublished data provided by the laboratory).
EXERCISE IMMUNOLOGY IN CFS
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TABLE 3. Intracellular immunity vs exercise performance in 16 CFS subjects.
Variable
Elastase
Activity
r* (p†)
Exercise duration (min)
HRREST (bpm)
HRPEAK (bpm)
% target heart rate achieved
Workload (W)
Workload per body weight (W䡠kg⫺1)
Workload at RER ⫽ 1.0 (W)
V̇O2PEAK (mL䡠kg⫺1䡠min⫺1)
V̇O2 at RER ⫽ 1.0 (mL䡠kg⫺1䡠min⫺1)
RERREST
RERPEAK
Lactate concentration at rest (mmol䡠L⫺1)
Peak lactate concentration (mmol䡠L⫺1)
Age
⫺0.39 (0.14)
0.10 (0.72)
⫺0.46 (0.07)
⫺0.58 (0.02)
⫺0.39 (0.14)
⫺0.38 (0.15)
⫺0.59 (0.02)
⫺0.44 (0.08)
⫺0.73 (0.001)
⫺0.67 (0.004)
0.05‡ (0.87)
⫺0.16 (0.56)
⫺0.41 (0.11)
⫺0.51 (0.04)
PKR Activity
r (p)
⫺0.45 (0.08)
0.27 (0.31)
⫺0.38 (0.14)
⫺0.50 (0.05)
⫺0.45 (0.08)
⫺0.41 (0.11)
⫺0.63 (0.009)
⫺0.46 (0.07)
⫺0.70 (0.002)
⫺0.69 (0.003)
0.02‡ (0.95)
⫺0.04 (0.88)
⫺0.37 (0.16)
⫺0.49 (0.06)
RNase L
Activity
r (p)
⫺0.42 (0.11)
0.26 (0.33)
⫺0.43 (0.10)
⫺0.51 (0.04)
⫺0.42 (0.11)
⫺0.39 (0.14)
⫺0.57 (0.02)
⫺0.48 (0.06)
⫺0.70 (0.003)
⫺0.65 (0.007)
⫺0.11‡ (0.69)
⫺0.09 (0.75)
⫺0.46 (0.08)
⫺0.42 (0.11)
RNase L
Ratio
r (p)
⫺0.31 (0.25)
0.29 (0.29)
⫺0.25 (0.35)
⫺0.36 (0.17)
⫺0.31 (0.25)
⫺0.31 (0.24)
⫺0.50 (0.05)
⫺0.37 (0.15)
⫺0.66 (0.006)
⫺0.70 (0.002)
0.27‡ (0.31)
⫺0.07 (0.81)
⫺0.33 (0.21)
⫺0.46 (0.07)
NOmonocyte
r (p)
NOlymphocyte
r (p)
0.40 (0.13)
⫺0.14 (0.61)
0.11 (0.68)
0.04 (0.87)
0.40 (0.13)
0.16 (0.57)
0.26 (0.34)
0.04 (0.88)
0.26 (0.34)
0.03 (0.93)
⫺0.00‡ (0.99)
⫺0.20 (0.45)
⫺0.16 (0.56)
⫺0.17 (0.54)
0.48 (0.06)
⫺0.20 (0.45)
0.20 (0.47)
0.14 (0.61)
0.48 (0.06)
0.27 (0.31)
0.32 (0.22)
0.17 (0.53)
0.32 (0.22)
0.18 (0.51)
⫺0.07‡ (0.81)
⫺0.28 (0.30)
⫺0.09 (0.73)
⫺0.12 (0.65)
* Pearson correlation coefficient; † level of significance was set at 0.01 to help protect against potential type I errors; ‡ nonparametric Spearman’s rho because RERPEAK was not
normally distributed (Kolmogorov–Smirnov test).
examining the associations between the immune and exercise performance variables, is displayed in Table 3. Strong
correlations (r ranged between 0.65 and 0.73) were observed
between four intracellular immune variables (i.e., elastase
activity, PKR activity, RNase L activity, and proteolysis)
and both the resting RER and the oxygen uptake at RER ⫽
1.0. The achieved workload at RER ⫽ 1.0 correlated with
the PKR activity, and displayed at trend towards a statistical
significant association (0.01⬍P ⬍ 0.05) with elastase activity (power ⫽ 81%), RNase L activity (power ⫽ 81%),
and the RNase L-ratio (power ⫽ 63%). Likewise, elastase
activity (power ⫽ 81%) and RNase L activity (power ⫽
63%) displayed a trend towards a correlation with the percentage of target heart rate achieved. Neither the NO concentration in monocytes nor the lymphocytes’ NO concentration correlated with any of the exercise performance
variables.
Forward stepwise multiple regression analysis revealed 1)
that elastase activity was the only factor related to the
reduction in oxygen uptake at a RER of 1.0 (regression
model: R2 ⫽ 0.53, F (1,14) ⫽ 15.5, P ⬍ 0.002; elastase
activity P ⬍ 0.002); 2) that the PKR activity was the
principle factor related to the reduction in workload at
RER ⫽ 1.0 (regression model: R2 ⫽ 0.77, F (6,9) ⫽ 5.2,
P ⬍ 0.01; PKR activity P ⬍ 0.009; monocyte NO P ⬍ 0.13;
% monocytes P ⬍ 0.17; RNase L-ratio P ⬍ 0.23; elastase
activity P ⬍ 0.051; RNase L activity P ⬍ 0.32); 3) that the
elastase activity was the principle factor related to the reduction in % of target heart rate achieved (regression model:
R2 ⫽ 0.89, F (7,8) ⫽ 9.5, P ⬍ 0.002; elastase activity P ⬍
0.02; RNase L-ratio P ⬍ 0.02; PKR activity P ⬍ 0.02); and
4) that the level of elastase was the only factor inversely
related to the increase in age (regression model: R2 ⫽ 0.52,
F (1,14) ⫽ 5.3, P ⬍ 0.03; elastase activity P ⬍ 0.04).
DISCUSSION
These data add to the body of literature showing impairments of intracellular immunity in patients with CFS. The
results provide evidence for an association between intracellular immune deregulation and exercise performance in
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Official Journal of the American College of Sports Medicine
patients with CFS. Elastase activity and PKR activity were
identified as determinants of the reduction in oxygen uptake
at RER ⫽ 1.0, the reduction in workload at RER ⫽ 1.0, and
the reduction in percent of target heart rate achieved. RNase
L activity and proteolysis correlated strongly with both the
resting RER and the oxygen uptake at RER ⫽ 1.0, whereas
resting NO levels were not related to any of the exercise
performance variables.
The results are in accordance with an earlier report, providing preliminary evidence of an association between
RNase L proteolysis (as assessed using the RNase L-ratio)
and exercise performance in CFS patients (25). Still, the
present study is the first to study numerous intracellular
immune variables together with exercise performance in
CFS patients. The role of elastase might be related to impairments of lung diffusion and impairments of oxygen
delivery in tissues. Indeed, the exercise performance variables that displayed an association with the immune variables, including elastase activity, were mainly related to
respiration (oxygen uptake and RER). On the other hand,
intracellular elastase activity in peripheral monocytes/lymphocytes was assessed. Peripheral blood characteristics may
not correspond to the alterations in the capillaries surrounding lung alveoli. In addition, it seems unlikely that intracellular elastase in peripheral blood is capable of causing lung
tissue damage, as seen in other diseases like cystic fibrosis
(7). In an animal model of cystic fibrosis, elastase degraded
several alveolar surfactant proteins important for alveolar
tension reduction and innate immune function (1). Apart
from one study showing an association between immune
activation and bronchial hyperresponsiveness (17), we are
unaware of experimental data providing evidence for impairments in lung tissue in CFS patients. An increased
number of cytotoxic T-cells, accompanied by a decreased
amount of naı̈ve T-cells, was observed in CFS patients with
bronchial hyperresponsiveness compared with CFS patients
without bronchial hyperresponsiveness (17). T-cells release
elastase to establish their cytotoxicity, linking the current
with our previous observations.
RNase L activity and proteolysis correlated strongly with
both the resting RER and the oxygen uptake at RER ⫽ 1.0.
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These observations may be related to the increased elastase activity: elastase has been identified as one of the
proteolytic enzymes responsible for RNase L cleavage
(5). Whether RNase L proteolysis triggers a channelopathy and consequent intracellular hypomagnesaemia in
skeletal muscles and transient hypoglycemia, as suggested in the introduction, requires further investigation.
Furthermore, it was hypothesized that PKR activation and
consequent elevated NO levels might limit exercise performance in CFS patients. The current observations do
not support this hypothesis; although PKR activation
appeared to be a determinant of exercise performance, no
associations between NO levels and exercise performance
were observed. However, further studying of the hypothesis assessing extracellular NO levels during the exercise
challenge rather than resting NO levels in peripheral
monocytes/lymphocytes is warranted.
Addressing the study limitations, the cross-sectional
nature of the study should be acknowledged. To establish
a causal relationship, further study of these interactions in
a larger study sample, using a prospective longitudinal
design, is required. In addition, given that a number of
correlations were significant at the 0.05, but not at the
0.01 level, one can argue that the sample size lacked
strength. Depending on the parameter of interest, the
power of the study varied between 63 and 81%, with an
associated probability of Type II error of 37 and 19%
respectively. Because a power of 80% was considered
fair, increasing the sample size would only be appropriate
to reveal statistically significant correlations between
RNase L activity and the percentage of age-predicted
target heart rate achieved, and between the RNase L-ratio
and the workload at RER ⫽ 1.0. Increasing the sample
size to approximately 22 subjects might have revealed
significant correlations between these parameters. A
power analysis should have determined the sample size
before the study started, but our budget did not allow us
to include more than 16 subjects. It has been concluded
that the main findings of the present study were not
biased by the sample size. Finally, the study sample may
not be representative of the CFS population in general;
the study participants were randomly selected from patients visiting a specialized chronic fatigue clinic, and the
gender distribution (50% females) is not in accordance
with the epidemiology of CFS. Women appear more
likely to develop the disease than men and children (6).
Because it is well established that women have a lower
maximal oxygen uptake compared with men (16), a distinct gender distribution and even pooling of gender data
may bias the results (23). In two earlier studies (18,20),
however, reanalyzing the exercise performance data without pooling gender data did not change the outcome.
With respect to the study limitations, it has been concluded that new evidence supportive of an interaction between intracellular immune dysfunction and exercise performance in CFS patients was provided. These findings may
aid a variety of health care workers (physiotherapists, physicians, rehabilitation specialists, occupational therapists) in
understanding exercise physiology in patients with CFS.
Further study of these interactions is warranted.
The authors would like to thank Freya Vaeyens (MSc) and Marc
Frémont (PhD) (both working at RED Laboratories, Zellik, Belgium)
for editing the methods’ description of the immunological parameters.
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