Viruses target specific cells in the host animal or plant they infect. The host cells for HIV are those carrying CD4 molecules: macrophages and CD4 T-cells.

HIV uses proteins on its surface called gp120 and gp41 to attach to the CD4 molecule on cells. Specific antibodies to gp120 can block its attachment to CD4 molecules. Antibodies which do this are called neutralising antibodies because they block the action of gp120 binding to CD4, thus neutralising that particular virus.

In addition to the CD4 receptor, HIV must also bind to other receptors on the cell surface called 'co-receptors'. The most common co-receptors are CCR5 and CXCR4.

CCR5

In 1996 it was discovered that HIV binds to a second protein on the surface of human cells, called chemokine receptor 5 or CCR5, as part of the process of infecting a cell. Also known as CKR5 or CC-CKR5, CCR5 is HIV's principal co-receptor.

HIV which uses CCR5 is sometimes referred to as the macrophage or M-tropic strain or as the non-syncytium-inducing (NSI) strain. Although not as virulent a virus as the CXCR4 strain, most people who die of AIDS have the CCR5 strain. People who develop the CXCR4 using strain tend to have higher viral loads and much lower CD4 cell counts. However viral load and CD4 cell count tests do not predict which type of virus a person has. Therefore a special test is required to see which virus a person has, if it is felt relevant to clinical care (Moyle 2005).

The genetic instructions for producing the CCR5 protein are contained in a gene called the CCR5 gene. Everyone has two copies of this gene, but a significant proportion of the population have a mutation in one or both of these genes which interferes in the production of the protein. This mutation is called CCR5-Δ32. About one in seven caucasians and one in 59 African Americans in the United States has this mutation in one or both copies of the CCR5 gene.

People who have this mutation in both their copies of the CCR5 gene may be partially protected against infection with NSI strains of HIV. However, they are not completely protected against HIV infection. HIV may also enter cells using the CXCR4 co-receptor. There have now been four separate papers describing cases in which people with two mutant CCR5 genes have become infected with HIV. In these cases, CD4 decline is rapid but viral load is not high and disease progression is not rapid.

Some researchers have expressed concern that this model of infection may predict what happens when people infected with the CCR5 strain are treated with CCR5 inhibitors. However, there is evidence that a reduction in CCR5 availability does not increase the emergence of the more virulent CXCR4 tropic virus, especially since this more virulent virus is thought to be at a disadvantage compared to the less virulent CCR5 tropic virus (Michael 1999).

Non-progressors have also been found to have higher levels of certain variations in parts of the DNA that control the amount of the CCR5 receptor that the cells produce. These 'promoter polymorphisms', such as CCR5 59353-C, may also delay disease progression (Easterbrook 1999).

Studies have found that among HIV-positive people who were infected sexually, those who have one mutant CCR5 gene experience slightly slower HIV disease progression than people with none. No reduction in disease progression was seen among people infected by other routes, such as blood products or drug injecting, possibly because they were probably exposed to larger amounts of HIV when they became infected.

There is also evidence that the mutant CCR5 gene may offer some protection against HIV-related diseases, such as HIV-associated dementia and toxoplasmosis (Meyer 1999; van Rij 1999). It may also be protective against mother-to-baby transmission of HIV (Philpott 1999).

While the CCR5-Δ32 mutation is mostly seen in caucasian people, significant numbers of non-caucasian people in Asia and Africa are known to resist HIV infection despite repeated exposure. One study in Vietnam identified at least five additional CCR5 mutations in as many as 1% of Vietnamese and Cambodian individuals. Two of these mutations, which were only found in HIV-negative people, altered the way CCR5 functions, suggesting that these other mutations in CCR5 may play a role in protection from HIV infection or disease progression in non-caucasian people (Capoulade-M鴡y 2004).

CCR5 and chemokines

The CCR5 protein is believed normally to act as a receptor for three chemokines: RANTES, MIP-1 alpha and MIP-1 beta. Studies have shown that HIV-positive people who do not progress, and HIV-negative people who have been repeatedly exposed to the virus through unprotected sex yet do not become infected, often have unusually high levels of these chemokines. Increased chemokine production correlates with a greater proliferation of HIV-specific T-cells (Garzino-Demo 1999).

There are several theories about how chemokines prevent HIV infection of cells. They may inhibit HIV by binding to the CCR5 protein and 'blocking' it, their presence may cause the shape of the CCR5 protein to change, or they may cause the cell to display fewer CCR5 proteins on its surface. In any event, chemokines prevent HIV from using the receptor and so reduce the chance that the cells will be infected. It is speculated that levels of these chemokines may fall with age, which might explain why older people tend to experience faster HIV disease progression than younger people. In contrast, however,, a retrospective study of blood samples from 21 people by Italian researchers found that high levels of RANTES were associated with rapid progression to AIDS (Polo 1999).

Interleukin-15 (IL-15) may also play a part in disease progression. Research suggests that it induces the CCR5 chemokine receptor, and that it facilitates the transition to the more virulent, syncytia-forming CXCR4 tropic HIV.

CXCR4

Another co-receptor, also discovered in 1996, is called CXCR4 of 'fusin'. This co-receptor is used by 'syncytium-inducing' (SI) strains of HIV to enter cells. The strains of HIV that are usually transmitted from person to person are CCR5-tropic strains, and these do not seem to rely on CXCR4 to infect cells.

CXCR4-tropic HIV strains tend to emerge in the body during the course of HIV infection. As with CCR5, a proportion of the population have a genetic mutation that means that their cells do not produce CXCR4, and so they may be less susceptible to CXCR4-tropic strains of HIV. Around 1% of caucasians do not produce this co-receptor.

Researchers are puzzled about why it takes CXCR4-tropic HIV so long to emerge in an infected individual. It has been postulated that there is an unknown selective pressure against the emergence of CXCR4 during early infection. One theory is that the degradation of lymphoid tissue disrupts the natural ligand of CXCR4.

One group of researchers has studied the way in which HIV slowly diversifies within the human body. At the time of peak diversification prior to progression, CXCR4-tropic HIV is dominant. However, this dominance is transient, possibly due to the destruction of CD4 T-cells with the CXCR4 co-receptor. The destruction of this pool of CD4 T-cells may trigger disease progression. This study highlights the needs for treatments which interfere with the destruction of the CD4 T-cells with the CXCR4 co-receptor (Mullins 1998).

Researchers from Johns Hopkins University have reported that virus that attaches to human cells via the CXCR4 co-receptor can affect the orientation or movement of both CD4 and CD8 T-cells. It seems that CXCR4-tropic virus can act independently of the CD4 receptor (Iyengar 1999).

Dual tropic HIV

Some strains of HIV are dual tropic - that is, they can enter both T cells and macrophages. There are two mechanisms of dual tropism: HIV can either utilise both the CCR5 and CXCR4 co-receptors (called dual tropic R5X4 HIV) or it can use the CXCR4 on both macrophages and T cells (called dual-tropic X4 HIV; Yanjie 1999).

CCR2

In August 1997, researchers identified another gene mutation that affects disease progression. This mutation affects the gene that encodes the CCR2 receptor on the outside of cells, and is more common than the CCR5 mutation: between 10 and 25% of the population are believed to have at least one mutant CCR2 gene. In a study of over 3000 HIV-positive people, those who had one mutant CCR2 gene developed AIDS two to four years later than people who had two normal copies of the CCR2 gene.

The results were even more striking when the data on the effects of CCR2 and CCR5 mutations were combined. About 30% of long-term survivors who had been infected with HIV for at least 16 years or more without developing AIDS had at least one CCR2 or CCR5 mutant gene (Smith 1997). The protective effect of CCR2B-64I has also been demonstrated by other studies (Easterbrook 1999).

However, as with the CCR5-Δ32 mutation, the CCR2b mutation did not affect disease progression among injecting drug users (Schinkel 1999).

CX₃CR1

CX₃CR1 is a minor co-receptor for HIV, which normally acts as the receptor for the chemokine fractaline. There have been conflicting reports of the role of this co-receptor in progression of HIV disease in adults. However, one study in children has shown that the I/I249 genotype is associated with more rapid disease progression, and that this is unaffected by the presence of CCR5-Δ32 (Singh 2004).

SDF-1-3A

A mutation in the gene for the chemokine stromal derived factor 1 (SDF-1), termed SDF-1-3A, may also be associated with a reduced risk of disease progression. A study of 2419 people found that people with the SDF-1-3A mutation were twice as likely to be alive after twelve years as people with either a CCR2 or CCR5 mutation. The protective effect of having both SDF-1-3A mutation and either the CCR5 or CCR2 mutation was even greater (Winkler 1998).

However, a British study of 132 people found that the SDF-1 mutation was not protective against disease progression, even in the latter stages of HIV disease. Easterbrook (1999) found that CCR5-Δ32 and CCR2B-64I were protective, and the authors were at a loss to explain why SDF-1-3A was not protective in their group. Another study of over 1000 people found that SDF-1-3A was not protective against disease progression and in fact found a trend towards more rapid progression among individuals with the SDF-1-3A mutation (Mummidi 1998).

US28

HIV also uses a protein produced by cytomegalovirus (CMV) to enter some cells. US28 is produced by cells which have already been infected by CMV. When this protein is expressed on the surface of a cell in the absence of the CCR5 receptor, HIV variants which would normally need to use the CCR5 receptor can gain entry to the cell. This may broaden the range of infectable cells.

Whilst CMV infects CD4 T-cells rarely, it can infect brain and retinal cells which are not accessible to HIV. Some researchers believe that this allows HIV to gain access to some types of cells in the nervous system and retina more easily. It has also been suggested that CMV may transfer US28 to cells without actually infecting them (Pleskoff 1997).

Further research is likely to elucidate additional co-receptors that are required for gp120 to bind to the host cell, permitting viral entry.

Compartmentalisation

Genotypes of HIV develop in particular compartments in the body, which may have an impact on co-receptor usage. One group of researchers has reported that HIV in the lungs uses CCR5 but not CXCR4 (Singh 1999). A different study found that the there were subtle differences in the receptors used by HIV in the blood and the cerebrospinal fluid in 10% of the patients they assessed (Spudich 2005). This may have significant implications for treatment strategies that target co-receptors, as detection of sensitivity of HIV in the blood to new CCR5 inhibitors may give a false sense of security if HIV in other compartments uses different receptors, such as CXCR4.

Impact of co-receptors on disease progression

Despite the growing body of research which links particular co-receptor mutations with delayed progression, it seems that these genetic factors can only explain a small proportion of non-progressors. For example, while 40% of non-progressors carry the CCR5-Δ32 mutation, 60% of non-progressors carry the normal CCR5 gene. Similarly, 80% of non-progressors carry the CCR2 wild type gene. This suggests that a number of host and viral factors may combine to determine disease progression in any individual (Easterbrook 1999).

An international meta-analysis of data from 19 cohorts was published in 2001. The analysis covered four outcomes:

• Time from seroconversion to AIDS (1987 definition).

• Time from seroconversion to death.

• Time from AIDS diagnosis to death.

• First HIV-1 viral load measurement after enrollment.

Data after January 1 1996 were censored to minimise any bias attributable to the effects of highly active antiretroviral therapy. Analysis of CCR5-Δ32 alleles was restricted to patients of European descent because this pattern is very rare in individuals of African descent. Cohorts that comprised individuals with a known seroconversion date were analysed separately from cohorts which enrolled HIV-positive patients with variable periods of prior infection.

CCR5-Δ32 heterozygosity was associated with a slower rate of progression to AIDS in seroconverter and seroprevalent cohorts, and time to death compared to those who were homozygous for CCR5-Δ32. There was no clear relationship between heterozygosity for this allele and lengthened survival after an AIDS diagnosis.

Individuals with the one or two copies of the CCR2-64I polymorphism had a reduced risk of progression to AIDS or death in the seroconverter cohort when compared with those who had normal version of both CCR5 and CCR2, but there was no effect on time to death in the seroprevalent cohort. HIV-1 viral load after seroconversion was also significantly lower. Furthermore, homozygosity for CCR2-64I was highly protective in the absence of the CCR5-Δ32 allele.

A second meta-analysis involving 1850 people from ten cohort studies found that CCR5-Δ32 protects against progression to AIDS by 31% over the course of HIV infection, and protects against death by 39%. In contrast, CCR2-64I provides the most substantial protection against progression during the early years of HIV infection, but this protection appears to decline after four years (Mulherin 2003).

Supporting this limited effect, a Greek study found that CCR2-64I and CCR5-Δ32 are protective against progression in HIV-infected children only during the early years of life (Ioannidis 2003). Another study found that CCR5-Δ32 did not protect against progression in HIV-infected children, although this effect was observed in adults (Iversen 2003).

SDF-1 3`A has shown a protective effect in other studies, but homozygosity was not associated with a reduced risk of disease progression in this meta-analysis.

Mutation 280 in the CX3CR1 receptor has also been linked to faster HIV disease progression (Faure 2003).

For further information, see Non-infectious co-factors in The immune system and HIV: Factors affecting disease progression.

Treatment implications

While it might seem attractive to try to use chemokines as treatments, researchers have warned that chemokines may actually stimulate HIV replication in macrophages. Researchers are developing artificial molecules designed to block specific HIV co-receptors such as CCR5. Tests for the type of virus a person has may be advisable before starting treatment with any of the new CCR5 inhibitors. Even a very low amount of CXCR4 tropic virus present when a person starts CCR5 inhibitor therapy could cause viral breakthrough (Lalezari 2004).

For more information, see Preventing viral attachment or fusion in Anti-HIV therapy: Ways of attacking HIV.

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