HIV

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This article is about the virus. For the infection caused by the virus, see HIV/AIDS. For other uses,
see HIV (disambiguation).
"AIDS virus" redirects here. For the computer virus, see AIDS (computer virus).

Human immunodeficiency virus

Scanning electron micrograph of HIV-1 (in

green) budding from cultured lymphocyte.

Multiple round bumps on cell surface represent

sites of assembly and budding of virions.

Virus classification

Group: Group VI (ssRNA-RT)

Order: Unassigned

Family: Retroviridae

Subfamily: Orthoretrovirinae

Genus: Lentivirus

Species

 Human immunodeficiency virus 1

  Human immunodeficiency virus 2

The human immunodeficiency virus (HIV) is a lentivirus (a subgroup of retrovirus) that causes HIV
infection and over timeacquired immunodeficiency syndrome (AIDS).[1][2] AIDS is a condition in
humans in which progressive failure of the immune system allows life-threatening opportunistic
infections and cancers to thrive. Without treatment, average survival time after infection with HIV is
estimated to be 9 to 11 years, depending on the HIV subtype.[3] In most cases, HIV is a sexually
transmitted infection and occurs by contact with or transfer of blood, pre-ejaculate, semen,
and vaginal fluids. Non-sexual transmission can occur from an infected mother to her infant
through breast milk.[4][5][6] An HIV-positive mother can transmit HIV to her baby both during pregnancy
and childbirth due to exposure to her blood or vaginal fluid.[7] Within these bodily fluids, HIV is
present as both free virus particles and virus within infected immune cells.
HIV infects vital cells in the human immune system such as helper T cells (specifically CD4+ T
cells), macrophages, anddendritic cells.[8] HIV infection leads to low levels of CD4+ T cells through a
number of mechanisms, including pyroptosis of abortively infected T cells,[9] apoptosis of uninfected
bystander cells,[10] direct viral killing of infected cells, and killing of infected CD4+ T cells
by CD8+ cytotoxic lymphocytes that recognize infected cells.[11] When CD4+ T cell numbers decline
below a critical level, cell-mediated immunity is lost, and the body becomes progressively more
susceptible to opportunistic infections, leading to the development of AIDS.

Contents

 1Virology
o 1.1Classification
o 1.2Structure and genome
o 1.3Tropism
o 1.4Replication cycle
o 1.5Spread within the body
o 1.6Genetic variability
 2Diagnosis
 3Research
 4Treatment
 5History
o 5.1Discovery
o 5.2Origins
 6See also
 7References
 8Further reading
 9External links

Virology
Classification
See also: Subtypes of HIV

Comparison of HIV species

 Species Virulence Infectivity Prevalence Inferred origin

HIV-1 High High Global Common chimpanzee

HIV-2 Lower Low West Africa Sooty mangabey

HIV is a member of the genus Lentivirus,[12] part of the family Retroviridae.[13] Lentiviruses have
many morphologies and biological properties in common. Many species are infected by lentiviruses,
which are characteristically responsible for long-duration illnesses with a long incubation
period.[14]Lentiviruses are transmitted as single-stranded, positive-sense, enveloped RNA viruses.
Upon entry into the target cell, the viral RNA genome is converted (reverse transcribed) into double-
strandedDNA by a virally encoded enzyme, reverse transcriptase, that is transported along with the
viral genome in the virus particle. The resulting viral DNA is then imported into the cell nucleus and
integrated into the cellular DNA by a virally encoded enzyme,integrase, and host co-factors.[15] Once
integrated, the virus may become latent, allowing the virus and its host cell to avoid detection by the
immune system, for an indiscriminate amount of time.[16] The HIV virus can remain dormant in the
human body for up to ten years after primary infection; during this period the virus does not cause
symptoms. Alternatively, the integrated viral DNA may be transcribed, producing new RNA genomes
and viral proteins, using host cell resources, that are packaged and released from the cell as new
virus particles that will begin the replication cycle anew.
Two types of HIV have been characterized: HIV-1 and HIV-2. HIV-1 is the virus that was initially
discovered and termed both LAV (Lymphadenopathy Associated Virus) and HTLV-III (Human T cell
Lymphotropic Virus III). HIV-1 is more virulent and more infective than HIV-2,[17] and is the cause of
the majority of HIV infections globally. The lower infectivity of HIV-2 compared to HIV-1 implies that
fewer of those exposed to HIV-2 will be infected per exposure. Due to its relatively poor capacity for
transmission, HIV-2 is largely confined to West Africa.[18]
Structure and genome
Main article: Structure and genome of HIV

Diagram of the HIV virion

HIV is different in structure from other retroviruses. It is roughly spherical[19] with a diameter of about
120 nm, around 60 times smaller than a red blood cell.[20] It is composed of two copies of positive-
sense single-strandedRNA that codes for the virus's nine genes enclosed by a
conical capsid composed of 2,000 copies of the viral protein p24.[21] The single-stranded RNA is
tightly bound to nucleocapsid proteins, p7, and enzymes needed for the development of the virion
such as reverse transcriptase, proteases, ribonuclease and integrase. A matrix composed of the
viral protein p17 surrounds the capsid ensuring the integrity of the virion particle.[21]
This is, in turn, surrounded by the viral envelope, that is composed of the lipid bilayer taken from the
membrane of a human host cell when the newly formed virus particle buds from the cell. The viral
envelope contains proteins from the host cell and relatively few copies of the HIV Envelope
protein,[21] which consists of a cap made of three molecules known as glycoprotein (gp) 120, and a
stem consisting of three gp41 molecules that anchor the structure into the viral envelope.[22][23] The
Envelope protein, encoded by the HIV env gene, allows the virus to attach to target cells and fuse
the viral envelope with the target cell's membrane releasing the viral contents into the cell and
initiating the infectious cycle.[22]
As the sole viral protein on the surface of the virus, the Envelope protein is a major target for HIV
vaccine efforts.[24] Over half of the mass of the trimeric envelope spike is N-linked glycans. The
density is high as the glycans shield the underlying viral protein from neutralisation by antibodies.
This is one of the most densely glycosylated molecules known and the density is sufficiently high to
prevent the normal maturation process of glycans during biogenesis in the endoplasmic and Golgi
apparatus.[25][26] The majority of the glycans are therefore stalled as immature 'high-mannose' glycans
not normally present on human glycoproteins that are secreted or present on a cell surface.[27] The
unusual processing and high density means that almost all broadly neutralising antibodies that have
so far been identified (from a subset of patients that have been infected for many months to years)
bind to or, are adapted to cope with, these envelope glycans.[28]
The molecular structure of the viral spike has now been determined by X-ray crystallography[29] and
cryo-electron microscopy.[30] These advances in structural biology were made possible due to the
development of stable recombinant forms of the viral spike by the introduction of an intersubunit
disulphide bond and an isoleucine to proline mutation in gp41.[31] The so-called SOSIP trimers not
only reproduce the antigenic properties of the native viral spike but also display the same degree of
immature glycans as presented on the native virus.[32] Recombinant trimeric viral spikes are
promising vaccine candidates as they display less non-neutralising epitopes than recombinant
monomeric gp120, which act to suppress the immune response to target epitopes.[33]

Structure of the RNA genome of HIV-1

The RNA genome consists of at least seven structural landmarks (LTR, TAR, RRE, PE, SLIP, CRS,
and INS), and nine genes (gag, pol, and env, tat, rev, nef, vif, vpr, vpu, and sometimes a tenth tev,
which is a fusion of tat, env and rev), encoding 19 proteins. Three of these genes, gag, pol, and env,
contain information needed to make the structural proteins for new virus particles.[21] For
example, env codes for a protein called gp160 that is cut in two by a cellular protease to form gp120
and gp41. The six remaining genes, tat, rev, nef, vif, vpr, and vpu (or vpx in the case of HIV-2), are
regulatory genes for proteins that control the ability of HIV to infect cells, produce new copies of virus
(replicate), or cause disease.[21]
The two Tat proteins (p16 and p14) are transcriptional transactivators for the LTR promoter acting by
binding the TAR RNA element. The TAR may also be processed into microRNAs that regulate
the apoptosis genes ERCC1 and IER3.[34][35] The Rev protein (p19) is involved in shuttling RNAs from
the nucleus and the cytoplasm by binding to the RRE RNA element. The Vif protein (p23) prevents
the action of APOBEC3G (a cellular protein that deaminates Cytidine to Uridine in the single
stranded viral DNA and/or interferes with reverse transcription[36]). The Vpr protein (p14) arrests cell
division at G2/M. The Nef protein (p27) down-regulatesCD4 (the major viral receptor), as well as
the MHC class I and class II molecules.[37][38][39]
Nef also interacts with SH3 domains. The Vpu protein (p16) influences the release of new virus
particles from infected cells.[21] The ends of each strand of HIV RNA contain an RNA sequence called
the long terminal repeat (LTR). Regions in the LTR act as switches to control production of new
viruses and can be triggered by proteins from either HIV or the host cell. The Psi element is involved
in viral genome packaging and recognized by Gag and Rev proteins. The SLIP element (TTTTTT) is
involved in the frameshift in the Gag-Pol reading frame required to make functional Pol.[21]
Tropism
Main article: HIV tropism

Diagram of the immature and mature forms of HIV

The term viral tropism refers to the cell types a virus infects. HIV can infect a variety of immune cells
such as CD4+ T cells,macrophages, and microglial cells. HIV-1 entry to macrophages and CD4+ T
cells is mediated through interaction of the virion envelope glycoproteins (gp120) with the CD4
molecule on the target cells' membrane and also with chemokine co-receptors.[22][40]
Macrophage-tropic (M-tropic) strains of HIV-1, or non-syncytia-inducing strains (NSI; now called R5
viruses[41]) use the β-chemokine receptor CCR5 for entry and are, thus, able to replicate in both
macrophages and CD4+ T cells.[42] This CCR5 co-receptor is used by almost all primary HIV-1
isolates regardless of viral genetic subtype. Indeed, macrophages play a key role in several critical
aspects of HIV infection. They appear to be the first cells infected by HIV and perhaps the source of
HIV production when CD4+ cells become depleted in the patient. Macrophages and microglial cells
are the cells infected by HIV in the central nervous system. In tonsils and adenoids of HIV-infected
patients, macrophages fuse into multinucleated giant cells that produce huge amounts of virus.
T-tropic strains of HIV-1, or syncytia-inducing (SI; now called X4 viruses[41]) strains replicate in
primary CD4+ T cells as well as in macrophages and use the α-chemokine receptor, CXCR4, for
entry.[42][43][44]
Dual-tropic HIV-1 strains are thought to be transitional strains of HIV-1 and thus are able to use both
CCR5 and CXCR4 asco-receptors for viral entry.
The α-chemokine SDF-1, a ligand for CXCR4, suppresses replication of T-tropic HIV-1 isolates. It
does this by down-regulating the expression of CXCR4 on the surface of HIV target cells. M-tropic
HIV-1 isolates that use only the CCR5 receptor are termed R5; those that use only CXCR4 are
termed X4, and those that use both, X4R5. However, the use of co-receptor alone does not explain
viral tropism, as not all R5 viruses are able to use CCR5 on macrophages for a productive
infection[42] and HIV can also infect a subtype of myeloid dendritic cells,[45]which probably constitute a
reservoir that maintains infection when CD4+ T cell numbers have declined to extremely low levels.
Some people are resistant to certain strains of HIV.[46] For example, people with the CCR5-
Δ32 mutation are resistant to infection by the R5 virus, as the mutation leaves HIV unable to bind to
this co-receptor, reducing its ability to infect target cells.
Sexual intercourse is the major mode of HIV transmission. Both X4 and R5 HIV are present in
the seminal fluid, which enables the virus to be transmitted from a male to his sexual partner. The
virions can then infect numerous cellular targets and disseminate into the whole organism. However,
a selection process leads to a predominant transmission of the R5 virus through this
pathway.[47][48][49] In patients infected with subtype B HIV-1, there is often a co-receptor switch in late-
stage disease and T-tropic variants that can infect a variety of T cells through CXCR4.[50] These
variants then replicate more aggressively with heightened virulence that causes rapid T cell
depletion, immune system collapse, and opportunistic infections that mark the advent of
AIDS.[51] Thus, during the course of infection, viral adaptation to the use of CXCR4 instead of CCR5
may be a key step in the progression to AIDS. A number of studies with subtype B-infected
individuals have determined that between 40 and 50 percent of AIDS patients can harbour viruses of
the SI and, it is presumed, the X4 phenotypes.[52][53]
HIV-2 is much less pathogenic than HIV-1 and is restricted in its worldwide distribution to West
Africa. The adoption of "accessory genes" by HIV-2 and its more promiscuous pattern of co-receptor
usage (including CD4-independence) may assist the virus in its adaptation to avoid innate restriction
factors present in host cells. Adaptation to use normal cellular machinery to enable transmission and
productive infection has also aided the establishment of HIV-2 replication in humans. A survival
strategy for any infectious agent is not to kill its host but ultimately become a commensal organism.
Having achieved a low pathogenicity, over time, variants that are more successful at transmission
will be selected.[54]
Replication cycle
The HIV replication cycle

Entry to the cell

Mechanism of viral entry: 1. Initial interaction between gp120 and CD4. 2. Conformational change in gp120 allows for
secondary interaction with CCR5. 3. The distal tips of gp41 are inserted into the cellular membrane. 4. gp41 undergoes
significant conformational change; folding in half and forming coiled-coils. This process pulls the viral and cellular
membranes together, fusing them.

The HIV virion enters macrophages and CD4+ T cells by the adsorption of glycoproteins on its
surface to receptors on the target cell followed by fusion of the viral envelope with the target cell
membrane and the release of the HIV capsid into the cell.[55][56]
Entry to the cell begins through interaction of the trimeric envelope complex (gp160 spike) on the
HIV viral envelope and both CD4 and a chemokine co-receptor (generally either CCR5 orCXCR4,
but others are known to interact) on the target cell surface.[55][56] Gp120 binds
tointegrin α4β7 activating LFA-1, the central integrin involved in the establishment of virological
synapses, which facilitate efficient cell-to-cell spreading of HIV-1.[57] The gp160 spike contains
binding domains for both CD4 and chemokine receptors.[55][56]
The first step in fusion involves the high-affinity attachment of the CD4 binding domains ofgp120 to
CD4. Once gp120 is bound with the CD4 protein, the envelope complex undergoes a structural
change, exposing the chemokine receptor binding domains of gp120 and allowing them to interact
with the target chemokine receptor.[55][56] This allows for a more stable two-pronged attachment, which
allows the N-terminal fusion peptide gp41 to penetrate the cell membrane.[55][56] Repeat sequences in
gp41, HR1, and HR2 then interact, causing the collapse of the extracellular portion of gp41 into a
hairpin. This loop structure brings the virus and cell membranes close together, allowing fusion of
the membranes and subsequent entry of the viral capsid.[55][56]
After HIV has bound to the target cell, the HIV RNA and various enzymes, including reverse
transcriptase, integrase, ribonuclease, and protease, are injected into the cell.[55][not in citation given] During the
microtubule-based transport to the nucleus, the viral single-strand RNA genome is transcribed into
double-strand DNA, which is then integrated into a host chromosome.
HIV can infect dendritic cells (DCs) by this CD4-CCR5 route, but another route using mannose-
specific C-type lectin receptors such as DC-SIGN can also be used.[58] DCs are one of the first cells
encountered by the virus during sexual transmission. They are currently thought to play an important
role by transmitting HIV to T-cells when the virus is captured in the mucosa by DCs.[58] The presence
of FEZ-1, which occurs naturally in neurons, is believed to prevent the infection of cells by HIV.[59]
Clathrin-dependent endocytosis

HIV-1 entry, as well as entry of many other retroviruses, has long been believed to occur exclusively
at the plasma membrane. More recently, however, productive infection by pH-independent, clathrin-
dependent endocytosis of HIV-1 has also been reported and was recently suggested to constitute
the only route of productive entry.[60][61][62][63][64]
Replication and transcription

Reverse transcription of the HIV genome into double-stranded DNA

Shortly after the viral capsid enters the cell, an enzyme called reverse transcriptaseliberates the
positive-sense single-stranded RNA genome from the attached viral proteins and copies it into
a complementary DNA (cDNA) molecule.[65] The process of reverse transcription is extremely error-
prone, and the resulting mutations may causedrug resistance or allow the virus to evade the body's
immune system. The reverse transcriptase also has ribonuclease activity that degrades the viral
RNA during the synthesis of cDNA, as well as DNA-dependent DNA polymerase activity that creates
a sense DNA from the antisense cDNA.[66]Together, the cDNA and its complement form a double-
stranded viral DNA that is then transported into the cell nucleus. The integration of the viral DNA into
the host cell's genome is carried out by another viral enzyme called integrase.[65]
The integrated viral DNA may then lie dormant, in the latent stage of HIV infection.[65] To actively
produce the virus, certain cellular transcription factors need to be present, the most important of
which is NF-κB (nuclear factor kappa B), which is upregulated when T cells become activated.[67] This
means that those cells most likely to be targeted, entered and subsequently killed by HIV are those
currently fighting infection.
During viral replication, the integrated DNA provirus is transcribed into RNA, some of which then
undergo RNA splicing to produce mature mRNAs. These mRNAs are exported from the nucleus into
the cytoplasm, where they are translated into the regulatory proteins Tat (which encourages new
virus production) and Rev. As the newly produced Rev protein is produced it moves to the nucleus,
where it binds to full-length, unspliced copies of virus RNAs and allows them to leave the
nucleus.[68] Some of these full-length RNAs function as new copies of the virus genome, while others
function as mRNAs that are translated to produce the structural proteins Gag and Env. Gag proteins
bind to copies of the virus RNA genome to package them into new virus particles.[69]
HIV-1 and HIV-2 appear to package their RNA differently.[70][citation needed] HIV-1 will bind to any
appropriate RNA.[citation needed] HIV-2 will preferentially bind to the mRNA that was used to create the Gag
protein itself.[71]
Recombination
Two RNA genomes are encapsidated in each HIV-1 particle (see Structure and genome of HIV).
Upon infection and replication catalyzed by reverse transcriptase, recombination between the two
genomes can occur.[72][73] Recombination occurs as the single-strand (+)RNA genomes are reverse
transcribed to form DNA. During reverse transcription, the nascent DNA can switch multiple times
between the two copies of the viral RNA. This form of recombination is known as copy-choice.
Recombination events may occur throughout the genome. Anywhere from two to 20 recombination
events per genome may occur at each replication cycle, and these events can rapidly shuffle the
genetic information that is transmitted from parental to progeny genomes.[73]
Viral recombination produces genetic variation that likely contributes to the evolution of resistance
to anti-retroviral therapy.[74] Recombination may also contribute, in principle, to overcoming the
immune defenses of the host. Yet, for the adaptive advantages of genetic variation to be realized,
the two viral genomes packaged in individual infecting virus particles need to have arisen from
separate progenitor parental viruses of differing genetic constitution. It is unknown how often such
mixed packaging occurs under natural conditions.[75]
Bonhoeffer et al.[76] suggested that template switching by reverse transcriptase acts as a repair
process to deal with breaks in the single-stranded RNA genome. In addition, Hu and
Temin[72] suggested that recombination is an adaptation for repair of damage in the RNA genomes.
Strand switching (copy-choice recombination) by reverse transcriptase could generate an
undamaged copy of genomic DNA from two damaged single-stranded RNA genome copies. This
view of the adaptive benefit of recombination in HIV could explain why each HIV particle contains
two complete genomes, rather than one. Furthermore, the view that recombination is a repair
process implies that the benefit of repair can occur at each replication cycle, and that this benefit can
be realized whether or not the two genomes differ genetically. On the view that recombination in HIV
is a repair process, the generation of recombinational variation would be a consequence, but not the
cause of, the evolution of template switching.[76]
HIV-1 infection causes chronic ongoing inflammation and production of reactive oxygen
species.[77] Thus, the HIV genome may be vulnerable to oxidative damages, including breaks in the
single-stranded RNA. For HIV, as well as for viruses generally, successful infection depends on
overcoming host defensive strategies that often include production of genome-damaging reactive
oxygen. Thus, Michod et al.[78] suggested that recombination by viruses is an adaptation for repair of
genome damages, and that recombinational variation is a byproduct that may provide a separate
benefit.
Assembly and release

HIV assembling on the surface of an infected macrophage. The HIV virions have been marked with a greenfluorescent
tag and then viewed under a fluorescent microscope.

The final step of the viral cycle, assembly of new HIV-1 virions, begins at the plasma membrane of
the host cell. The Env polyprotein (gp160) goes through the endoplasmic reticulum and is
transported to the Golgi complex where it is cleaved byfurin resulting in the two HIV envelope
glycoproteins, gp41 and gp120.[79] These are transported to the plasma membrane of the host cell
where gp41 anchors gp120 to the membrane of the infected cell. The Gag (p55) and Gag-Pol (p160)
polyproteins also associate with the inner surface of the plasma membrane along with the HIV
genomic RNA as the forming virion begins to bud from the host cell. The budded virion is still
immature as the gag polyproteins still need to be cleaved into the actual matrix, capsid and
nucleocapsid proteins. This cleavage is mediated by the packaged viral protease and can be
inhibited by antiretroviral drugs of the protease inhibitor class. The various structural components
then assemble to produce a mature HIV virion.[80] Only mature virions are then able to infect another
cell.
Spread within the body

Animation demonstrating cell-free spread of HIV.

The classical process of infection of a cell by a virion can be called "cell-free spread" to distinguish it
from a more recently recognized process called "cell-to-cell spread".[81] In cell-free spread (see
figure), virus particles bud from an infected T cell, enter the blood or extracellular fluid and then
infect another T cell following a chance encounter.[81] HIV can also disseminate by direct transmission
from one cell to another by a process of cell-to-cell spread, for which two pathways have been
described. Firstly, an infected T cell can transmit virus directly to a target T cell via a virological
synapse.[57][82] Secondly, an antigen-presenting cell (APC), such as a macrophage or dendritic cell,
can transmit HIV to T cells by a process that either involves productive infection (in the case of
macrophages) or capture and transfer of virions in trans (in the case of dendritic cells).[83] Whichever
pathway is used, infection by cell-to-cell transfer is reported to be much more efficient than cell-free
virus spread.[84] A number of factors contribute to this increased efficiency, including polarised virus
budding towards the site of cell-to-cell contact, close apposition of cells, which minimizes fluid-phase
diffusion of virions, and clustering of HIV entry receptors on the target cell to the contact
zone.[82] Cell-to-cell spread is thought to be particularly important in lymphoid tissues where CD4+ T
cells are densely packed and likely to interact frequently.[81] Intravital imaging studies have supported
the concept of the HIV virological synapse in vivo.[85] The hybrid spreading mechanisms of HIV
contribute to the virus' ongoing replication in spite of anti-retroviral therapies.[81][86]
Genetic variability
Further information: Subtypes of HIV
The phylogenetic tree of the SIV and HIV

HIV differs from many viruses in that it has very high genetic variability. This diversity is a result of its
fast replication cycle, with the generation of about 1010 virions every day, coupled with a
high mutation rate of approximately 3 x 10−5 per nucleotide base per cycle of replication
and recombinogenic properties of reverse transcriptase.[87][88][89]
This complex scenario leads to the generation of many variants of HIV in a single infected patient in
the course of one day.[87]This variability is compounded when a single cell is simultaneously infected
by two or more different strains of HIV. When simultaneous infection occurs, the genome of progeny
virions may be composed of RNA strands from two different strains. This hybrid virion then infects a
new cell where it undergoes replication. As this happens, the reverse transcriptase, by jumping back
and forth between the two different RNA templates, will generate a newly synthesized retroviral DNA
sequencethat is a recombinant between the two parental genomes.[87] This recombination is most
obvious when it occurs between subtypes.[87]
The closely related simian immunodeficiency virus (SIV) has evolved into many strains, classified by
the natural host species. SIV strains of the African green monkey (SIVagm) and sooty
mangabey (SIVsmm) are thought to have a long evolutionary history with their hosts. These hosts
have adapted to the presence of the virus,[90] which is present at high levels in the host's blood, but
evokes only a mild immune response,[91] does not cause the development of simian AIDS,[92] and
does not undergo the extensive mutation and recombination typical of HIV infection in humans.[93]
In contrast, when these strains infect species that have not adapted to SIV ("heterologous" or similar
hosts such as rhesus or cynomologus macaques), the animals develop AIDS and the virus
generates genetic diversity similar to what is seen in human HIV infection.[94] Chimpanzee SIV
(SIVcpz), the closest genetic relative of HIV-1, is associated with increased mortality and AIDS-like
symptoms in its natural host.[95] SIVcpz appears to have been transmitted relatively recently to
chimpanzee and human populations, so their hosts have not yet adapted to the virus.[90] This virus
has also lost a function of the Nef gene that is present in most SIVs. For non-pathogenic SIV
variants, Nef suppresses T cell activation through the CD3 marker. Nef's function in non-pathogenic
forms of SIV is to downregulate expression of inflammatory cytokines, MHC-1, and signals that
affect T cell trafficking. In HIV-1 and SIVcpz, Nef does not inhibit T-cell activation and it has lost this
function. Without this function, T cell depletion is more likely, leading to immunodeficiency.[95][96]
Three groups of HIV-1 have been identified on the basis of differences in the envelope (env) region:
M, N, and O.[97] Group M is the most prevalent and is subdivided into eight subtypes (or clades),
based on the whole genome, which are geographically distinct.[98] The most prevalent are subtypes B
(found mainly in North America and Europe), A and D (found mainly in Africa), and C (found mainly
in Africa and Asia); these subtypes form branches in the phylogenetic tree representing the lineage
of the M group of HIV-1. Co-infection with distinct subtypes gives rise to circulating recombinant
forms (CRFs). In 2000, the last year in which an analysis of global subtype prevalence was made,
47.2% of infections worldwide were of subtype C, 26.7% were of subtype A/CRF02_AG, 12.3% were
of subtype B, 5.3% were of subtype D, 3.2% were of CRF_AE, and the remaining 5.3% were
composed of other subtypes and CRFs.[99] Most HIV-1 research is focused on subtype B; few
laboratories focus on the other subtypes.[100] The existence of a fourth group, "P", has been
hypothesised based on a virus isolated in 2009.[101] The strain is apparently derived from gorilla SIV
(SIVgor), first isolated from western lowland gorillas in 2006.[101]
HIV-2's closest relative is SIVsm, a strain of SIV found in sooty mangabees. Since HIV-1 is derived
from SIVcpz, and HIV-2 from SIVsm, the genetic sequence of HIV-2 is only partially homologous to
HIV-1 and more closely resembles that of SIVsm.[citation needed][102]

Diagnosis
Main article: Diagnosis of HIV/AIDS

A generalized graph of the relationship between HIV copies (viral load) and CD4 counts over the average course of
untreated HIV infection; any particular individual's disease course may vary considerably.
CD4+ T cell count (cells per µL)
HIV RNA copies per mL of plasma

Many HIV-positive people are unaware that they are infected with the virus.[103] For example, in 2001
less than 1% of the sexually active urban population in Africa had been tested, and this proportion is
even lower in rural populations.[103] Furthermore, in 2001 only 0.5% of pregnant women attending
urban health facilities were counselled, tested or receive their test results.[103] Again, this proportion is
even lower in rural health facilities.[103] Since donors may therefore be unaware of their
infection, donor blood and blood products used in medicine and medical research are routinely
screened for HIV.[104]
HIV-1 testing is initially done using an enzyme-linked immunosorbent assay (ELISA) to detect
antibodies to HIV-1. Specimens with a non-reactive result from the initial ELISA are considered HIV-
negative unless new exposure to an infected partner or partner of unknown HIV status has occurred.
Specimens with a reactive ELISA result are retested in duplicate.[105] If the result of either duplicate
test is reactive, the specimen is reported as repeatedly reactive and undergoes confirmatory testing
with a more specific supplemental test (e.g., a polymerase chain reaction (PCR), western blot or,
less commonly, an immunofluorescence assay (IFA)). Only specimens that are repeatedly reactive
by ELISA and positive by IFA or PCR or reactive by western blot are considered HIV-positive and
indicative of HIV infection. Specimens that are repeatedly ELISA-reactive occasionally provide an
indeterminate western blot result, which may be either an incomplete antibody response to HIV in an
infected person or nonspecific reactions in an uninfected person.[106]

HIV deaths (other than U.S.) in 2014.[107]

Nigeria (15.76%)
 South Africa (12.51%)
India (11.50%)
Tanzania (4.169%)
Mozambique (4.061%)
Zimbabwe (3.49%)
Cameroon (3.09%)
Indonesia (3.04%)
Kenya (2.98%)
Uganda (2.97%)
Malawi (2.94%)
DR Congo (2.17%)
Ethiopia (2.11%)
Other (29.21%)

Although IFA can be used to confirm infection in these ambiguous cases, this assay is not widely
used. In general, a second specimen should be collected more than a month later and retested for
persons with indeterminate western blot results. Although much less commonly available, nucleic
acid testing (e.g., viral RNA or proviral DNA amplification method) can also help diagnosis in certain
situations.[105] In addition, a few tested specimens might provide inconclusive results because of a low
quantity specimen. In these situations, a second specimen is collected and tested for HIV infection.
Modern HIV testing is extremely accurate. A single screening test is correct more than 99% of the
time.[108][needs update] The chance of a false-positive result in standard two-step testing protocol is
estimated to be about 1 in 250,000 in a low risk population.[108] Testing post-exposure is
recommended immediately and then at six weeks, three months, and six months.[109]
The latest recommendations of the CDC show that HIV testing must start with an immunoassay
combination test for HIV-1 and HIV-2 antibodies and p24 antigen. A negative result rules out HIV
exposure, while a positive one must be followed by an HIV-1/2 antibody differentiation immunoassay
to detect which antibodies are present. This gives rise to four possible scenarios:

 1. HIV-1 (+) & HIV-2 (−): HIV-1 antibodies detected

 2. HIV-1 (−) & HIV-2 (+): HIV-2 antibodies detected
 3. HIV-1 (+) & HIV-2 (+): both HIV-1 and HIV-2 antibodies detected
 4. HIV-1 (−) or indeterminate & HIV-2 (−): Nucleic acid test must be carried out to detect the
acute infection of HIV-1 or its absence.[110]
An updated algorithm published by the CDC in June 2014 recommends that diagnosis starts with the
p24 antigen test. A negative result rules out infection, while a positive one must be followed by an
HIV-1/2 antibody differentiation immunoassay. A positive differentiation test confirms diagnosis,
while a negative or indeterminate result must be followed by nucleic acid test (NAT). A positive NAT
result confirms HIV-1 infection whereas a negative result rules out infection (false positive p24).[111]

Research
Main article: HIV/AIDS research
HIV/AIDS research includes all medical research that attempts to prevent, treat, or cure HIV/AIDS,
as well as fundamental research about the nature of HIV as an infectious agent and AIDS as the
disease caused by HIV.
Many governments and research institutions participate in HIV/AIDS research. This research
includes behavioral health interventions, such as research into sex education, and drug
development, such as research into microbicides for sexually transmitted diseases, HIV vaccines,
and anti-retroviral drugs.[112] Other medical research areas include the topics of pre-exposure
prophylaxis, post-exposure prophylaxis, circumcision and HIV, and accelerated aging effects.
After many years of research, an untested HIV vaccine has been created.[113] Bi-specific antibodies,
that target both the surface of T-cells and viral epitopes, can prevent entry of the virus into human
cells.[114] Another group has utilised the same technology to develop a bi-specific antibody that
neutralises viral particles by cross-linking of envelope glycoproteins.[115]

Treatment
Main article: Management of HIV/AIDS
HIV latency, and the consequent viral reservoir in CD4+ T cells, dendritic cells, as well as
macrophages, is the main barrier to eradication of the virus.[16]
It is important to note that although HIV is highly virulent, transmission is greatly reduced when an
HIV-infected person has a suppressed or undetectable viral load(<50 copies/ml) due to prolonged
and successful anti-retroviral treatment. Hence, it can be said to be almost impossible (but still non-
zero) for an HIV-infected person who has an undetectable viral load to transmit the virus, even
during unprotected sexual intercourse, as there would be a negligible amount of HIV present in the
seminal fluid, vaginal secretions or blood, for transmission to occur.[116][117] This does not mean
however, that prolonged anti-retroviral treatment will result in a suppressed viral load. An
undetectable viral load, generally agreed as less than 50 copies per milliliter of blood, can only be
proven by a polymerase chain reaction (PCR) test.[118]
At the same time, it is important to recognise that reaching an undetectable viral load is determined
by many factors, including treatment adherence, HIV resistance to certain anti-retroviral drugs,
stigma, and inadequate health systems.[119]

History
Main article: History of HIV/AIDS
Discovery
AIDS was first clinically observed in 1981 in the United States.[120] The initial cases were a cluster of
injection drug users and gay men with no known cause of impaired immunity who showed symptoms
of Pneumocystis carinii pneumonia (PCP), a rare opportunistic infection that was known to occur in
people with very compromised immune systems.[121] Soon thereafter, additional gay men developed a
previously rare skin cancer called Kaposi's sarcoma (KS).[122][123] Many more cases of PCP and KS
emerged, alerting U.S. Centers for Disease Control and Prevention (CDC) and a CDC task force
was formed to monitor the outbreak.[124] The earliest retrospectively described case of AIDS is
believed to have been in Norway beginning in 1966.[125]
In the beginning, the CDC did not have an official name for the disease, often referring to it by way of
the diseases that were associated with it, for example,lymphadenopathy, the disease after which the
discoverers of HIV originally named the virus.[126][127] They also used Kaposi's Sarcoma and
Opportunistic Infections, the name by which a task force had been set up in 1981.[128] In the general
press, the term GRID, which stood for gay-related immune deficiency, had been coined.[129] The
CDC, in search of a name, and looking at the infected communities coined "the 4H disease", as it
seemed to single out homosexuals, heroin users,hemophiliacs, and Haitians.[130][131] However, after
determining that AIDS was not isolated to the gay community,[128] it was realized that the term GRID
was misleading and AIDS was introduced at a meeting in July 1982.[132] By September 1982 the CDC
started using the name AIDS.[133]
Françoise Barré-Sinoussi, co-discoverer of HIV

In 1983, two separate research groups led by American Robert Gallo and French
investigators Françoise Barré-Sinoussi andLuc Montagnier independently declared that a novel
retrovirus may have been infecting AIDS patients, and published their findings in the same issue of
the journal Science.[134][135][136] Gallo claimed that a virus his group had isolated from a person with
AIDS was strikingly similar in shape to other human T-lymphotropic viruses (HTLVs) his group had
been the first to isolate. Gallo's group called their newly isolated virus HTLV-III. At the same time,
Montagnier's group isolated a virus from a patient presenting with swelling of the lymph nodes of the
neck and physical weakness, two classic symptoms of primary HIV infection. Contradicting the
report from Gallo's group, Montagnier and his colleagues showed that core proteins of this virus
were immunologically different from those of HTLV-I. Montagnier's group named their isolated virus
lymphadenopathy-associated virus (LAV).[124] As these two viruses turned out to be the same, in 1986
LAV and HTLV-III were renamed HIV.[137]
Another group working contemporaneously with the Montagnier and Gallo groups was that of Dr. Jay
Levy at the University of California, San Francisco. He independently discovered the AIDS virus in
1983 and named it the AIDS associated retrovirus (ARV).[138] This virus was very different from the
virus reported by the Montagnier and Gallo groups. The ARV strains indicated, for the first time, the
heterogeneity of HIV isolates and several of these remain classic examples of the AIDS virus found
in the United States.[139]
Origins
Both HIV-1 and HIV-2 are believed to have originated in non-human primates in West-central Africa,
and are believed to have transferred to humans (a process known as zoonosis) in the early 20th
century.[140][141]
HIV-1 appears to have originated in southern Cameroon through the evolution of SIV(cpz), a simian
immunodeficiency virus (SIV) that infects wild chimpanzees (HIV-1 descends from the SIV(cpz)
endemic in the chimpanzee subspecies Pan troglodytes troglodytes).[142][143] The closest relative of
HIV-2 is SIV (smm), a virus of thesooty mangabey (Cercocebus atys atys), an Old World
monkey living in littoral West Africa (from southern Senegal to western Côte d'Ivoire).[18] New World
monkeyssuch as the owl monkey are resistant to HIV-1 infection, possibly because of a genomic
fusion of two viral resistance genes.[144] HIV-1 is thought to have jumped the species barrier on at
least three separate occasions, giving rise to the three groups of the virus, M, N, and O.[145]
Left to right: the African green monkey source of SIV, the sooty mangabey source of HIV-2, and the chimpanzee source
of HIV-1

There is evidence that humans who participate in bushmeat activities, either as hunters or as
bushmeat vendors, commonly acquire SIV.[146] However, SIV is a weak virus, and it is typically
suppressed by the human immune system within weeks of infection. It is thought that several
transmissions of the virus from individual to individual in quick succession are necessary to allow it
enough time to mutate into HIV.[147] Furthermore, due to its relatively low person-to-person
transmission rate, it can only spread throughout the population in the presence of one or more high-
risk transmission channels, which are thought to have been absent in Africa prior to the 20th century.
Specific proposed high-risk transmission channels, allowing the virus to adapt to humans and spread
throughout the society, depend on the proposed timing of the animal-to-human crossing. Genetic
studies of the virus suggest that the most recent common ancestor of the HIV-1 M group dates back
to circa 1910.[148] Proponents of this dating link the HIV epidemic with the emergence
of colonialism and growth of large colonial African cities, leading to social changes, including
different patterns of sexual contact (especially multiple, concurrent partnerships), the spread
of prostitution, and the concomitant high frequency of genital ulcer diseases (such as syphilis) in
nascent colonial cities.[149] While transmission rates of HIV during vaginal intercourse are typically
low, they are increased manyfold if one of the partners suffers from a sexually transmitted
infectionresulting in genital ulcers. Early 1900s colonial cities were notable due to their high
prevalence of prostitution and genital ulcers to the degree that as of 1928 as many as 45% of female
residents of eastern Leopoldville were thought to have been prostitutes and as of 1933 around 15%
of all residents of the same city were infected by one of the forms of syphilis.[149]
An alternative view—unsupported by evidence—holds that unsafe medical practices in Africa during
years following World War II, such as unsterile reuse of single-use syringes during mass vaccination,
antibiotic, and anti-malaria treatment campaigns, were the initial vector that allowed the virus to
adapt to humans and spread.[147][150][151]
The earliest, well-documented case of HIV in a human dates back to 1959 in the Belgian
Congo.[152] The virus may have been present in the United States as early as the mid-to-late 1950s,
as a sixteen-year-old male presented with symptoms in 1966 and died in 1969.[153