Attachment of HIV to a CD4+ T-helper cell: 1) the gp120 viral protein attaches to CD4. 2) gp120 variable loop attaches to a coreceptor, either CCR5 or CXCR4. 3) HIV enters the cell.
C-C chemokine receptor type 5, also known as CCR5 or CD195, is a protein on the surface of white blood cells that is involved in the immune system as it acts as a receptor for chemokines. This is the process by which T cells are attracted to specific tissue and organ targets. Many forms of HIV, the virus that causes AIDS, initially use CCR5 to enter and infect host cells. A few individuals carry a mutation known as CCR5-Δ32 in the CCR5 gene, protecting them against these strains of HIV.
The natural chemokine ligands that bind to this receptor are RANTES (a chemotacticcytokine protein also known as CCL5) and macrophage inflammatory protein (MIP) 1α and 1β (also known as CCL3 and CCL4, respectively). A computationally derived structure of CCL5 (RANTES) in complex with CCR5 is reported in. The CCL5 : CCR5 structure is in excellent agreement with experimental findings and clarifies the functional role of CCL5 and CCR5 residues which are associated with binding and signaling. It also interacts with CCL3L1.
HIV most commonly uses CCR5 and/or CXCR4 as a co-receptor to enter its target cells. Several chemokine receptors can function as viral coreceptors, but CCR5 is likely the most physiologically important coreceptor during natural infection. Molecular Dynamics simulations delineated the binding of a specific dual-tropic HIV-1 gp120 V3 loop to CCR5; the computationally derived complex structure exhibits exceptional agreement with previous experimental findings and sheds light into the functional role of HIV-1 gp120 V3 loop and CCR5 residues associated with the HIV-1 coreceptor activity. The normal ligands for this receptor, RANTES, MIP-1β, and MIP-1α, are able to suppress HIV-1 infection in vitro. The blocking mechanism of HIV-1 by RANTES is reported in. A comparison between the CCL5 (RANTES) : CCR5  and the HIV-1 gp120 V3 loop : CCR5  complex structures depicts that both RANTES and the virus primarily interact with the same CCR5 residues. In individuals infected with HIV, CCR5-using viruses are the predominant species isolated during the early stages of viral infection, suggesting that these viruses may have a selective advantage during transmission or the acute phase of disease. Moreover, at least half of all infected individuals harbor only CCR5-using viruses throughout the course of infection.
A number of new experimental HIV drugs, called CCR5 receptor antagonists, have been designed to interfere with the interaction between CCR5 and HIV, including PRO140 (Progenics), Vicriviroc(Phase III trials were cancelled in July 2010) (Schering Plough), Aplaviroc (GW-873140) (GlaxoSmithKline) and Maraviroc (UK-427857) (Pfizer). The blocking mechanism of HIV-1 by Maraviroc was proposed in. A problem of this approach is that, while CCR5 is the major co-receptor by which HIV infects cells, it is not the only such co-receptor. It is possible that under selective pressure HIV will evolve to use another co-receptor. However, examination of viral resistance to AD101, molecular antagonist of CCR5, indicated that resistant viruses did not switch to another coreceptor (CXCR4) but persisted in using CCR5, either through binding to alternative domains of CCR5, or by binding to the receptor at a higher affinity.
CCR5-Δ32 is a deletion mutation of a gene that has a specific impact on the function of T cells. The deleted portion of the CCR5 gene consists of thirty-two base pairs that correpsond to the second extracellular loop of the receptor; the mutated receptor is non-functional and does not allow M-tropic HIV-1 virus entry, thus resulting in infection resistance. One study found the frequency of the CCR5-Δ32 allele among the Caucasian population in the United States to be 0.10. Another study found the allele frequency to be 0.01 for those of Western European descent; the CCR5-Δ32 allele frequency was much lower in a sampling from Venezuela. A third found the frequency of the mutant allele in Caucasians of European descent to be 0.092. The same study examined DNA samples from several Western and Central African countries as well as Japan; no mutant alleles were found. At least one copy of CCR5-Δ32 is found in about 4–16% of people of European descent. It has been speculated that this allele was favored by natural selection during the Black Death for Northern Europeans, but further research has revealed that the gene did not protect against the Black Death. The current hypothesis is of protection vs smallpox throughout Europe, especially in the major trade cities and in isolated islands and archipelagos, such as Iceland and the Azores.
In the ancient world in areas such as Corinth in Ancient Greece, prostitution may have led to infection, since a virus similar to HIV existed which had flu-like symptoms and later continued to weaken the immune system of those infected. It was at the time not known how it was spread but the Plague of Athens and many later diseases in the Balkans may have also influenced the genetic mutations. This coalescence date is contradicted by supported evidence of CCR5-Δ32 in Bronze Age samples, at levels comparable to the modern European population.Smallpox may be another candidate for the high level of the mutation in the European population. The highest frequency of the mutation exists in Ashkenazi Jews, with the overall frequency of the CCR5-Delta32 allele is elevated 13.7% on average.
The allele has a negative effect upon T cell function, but appears to protect against smallpox and HIV. It has been shown in many studies that professional sex workers exposed frequently to HIV-1 are resistant to infection. Many showed reduced CD4 T cell activation which is associated with lower susceptibility to HIV-1 infection; CD4 T cells act as regulatory cells that, when activated, suppress immune response. In those particular cases reduced T cell function resulted in the beneficial effect of protection against HIV infection. Yersinia pestis (the bubonic plague bacterium) was demonstrated in the laboratory not to associate with CCR5. Individuals with the Δ32 allele of CCR5 are healthy, suggesting that CCR5 is largely dispensable. However, CCR5 apparently plays a role in mediating resistance to West Nile virus infection in humans, as CCR5-Δ32 individuals have shown to be disproportionately at higher risk of West Nile virus in studies, indicating that not all of the functions of CCR5 may be compensated by other receptors.
While CCR5 has multiple variants in its coding region, the deletion of a 32-bp segment results in a nonfunctional receptor, thus preventing HIV R5 entry; two copies of this allele provide strong protection against HIV infection. This allele is found in 5–14% of Europeans but is rare in Africans and Asians. CCR5-Δ32 decreases the number of CCR5 proteins on the outside of the CD4 cell, which can have a large effect on the HIV disease progression rates. Multiple studies of HIV-infected persons have shown that presence of one copy of this allele delays progression to the condition of AIDS by about two years. Immune activation and T cell function also significantly effect the development of AIDS. Studies done on primates have shown that species lacking immune activation did not develop AIDS while another species with strong T-cell activation did. It is possible that a person with the CCR5-Δ32 receptor allele will not be infected with HIV R5 strains. One study found that homozygotes for the mutated allele were strongly resistant to HIV-1 infection, and heterozygotes showed showed some level of resistance. One study found that several commercial testing companies offer tests for CCR5-Δ32.
A genetic approach involving intrabodies that block CCR5 expression has been proposed as a treatment for HIV-1 infected individuals. When T-cells modified so they no longer express CCR5 were mixed with unmodified T-cells expressing CCR5 and then challenged by infection with HIV-1, the modified T-cells that do not express CCR5 eventually take over the culture, as HIV-1 kills the non-modified T-cells. This same method might be used in vivo to establish a virus resistant cell pool in infected individuals.
This hypothesis was tested in an AIDS patient who had also developed myeloid leukemia, and was treated with chemotherapy to suppress the cancer. A bone marrow transplant containing stem cells from a matched donor was then used to restore the immune system. However, the transplant was performed from a donor with 2 copies of CCR5-Δ32 mutation gene. After 600 days, the patient was healthy and had undetectable levels of HIV in the blood and in examined brain and rectal tissues. Before the transplant, low levels of HIV X4, which does not use the CCR5 receptor, were also detected. Following the transplant, however, this type of HIV was not detected either, further baffling doctors. However, this is consistent with the observation that cells expressing the CCR5-Δ32 variant protein lack both the CCR5 and CXCR4 receptors on their surfaces, thereby conferring resistance to a broad range of HIV variants including HIV X4. After over six years, the patient has maintained the resistance to HIV and has been pronounced cured of the HIV infection.
Enrollment of HIV-positive patients in a clinical trial was started in 2009 in which the patients' cells were genetically modified with a zinc finger nuclease to carry the CCR5-Δ32 trait and then reintroduced into the body as a potential HIV treatment. Results reported in 2014 were promising.
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