|Chemokine (C-C motif) receptor 5 (gene/pseudogene)|
PDB rendering based on 4mbs (yellow) in cell membrane (grey).
|Symbols||; CC-CKR-5; CCCKR5; CCR-5; CD195; CKR-5; CKR5; CMKBR5; IDDM22|
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.
In humans, the CCR5 gene that encodes the CCR5 protein is located on the short (p) arm at position 21 on chromosome 3. Certain populations have inherited the Delta 32 mutation resulting in the genetic deletion of a portion of the CCR5 gene. Homozygous carriers of this mutation are resistant to M-tropic strains of HIV-1 infection.
The CCR5 protein belongs to the beta chemokine receptors family of integral membrane proteins. It is a G protein–coupled receptor which functions as a chemokine receptor in the CC chemokine group.
The natural chemokine ligands that bind to this receptor are RANTES (a chemotactic cytokine protein also known as CCL5) and macrophage inflammatory protein (MIP) 1α and 1β (also known as CCL3 and CCL4, respectively). It also interacts with CCL3L1.
CCR5 is predominantly expressed on T cells, macrophages, dendritic cells, eosinophils and microglia. It is likely that CCR5 plays a role in inflammatory responses to infection, though its exact role in normal immune function is unclear. Regions of this protein are also crucial for chemokine ligand binding, functional response of the receptor, and HIV co-receptor activity.
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. Chemokine receptors are located on the surface of cells acting as a doorway by providing entry for HIV to enter the cell. It is also called a co-receptor because it works with CD4 for this process to occur. Gp120 is only one of the two protein products of the HIV env gene that was first translated as gp160 before being cleaved into gp 120 and gp41. Both of these glycoproteins conciliate viral binding and membrane fusion with the cells. Because binding to CD4 alone can sometimes result in gp120 shedding, gp120 must next bind to co-receptor CCR5 in order for fusion to proceed. The tyrosine sulfated amino terminus of this co-receptor is the "essential determinant" of binding to the gp120 glycoprotein. Co-receptor recognition also include the V1-V2 region of gp120, and the bridging sheet (an antiparallel, 4-stranded β sheet that connects the inner and outer domains of gp120). The V1-V2 stem can influence "co-receptor usage through its peptide composition as well as by the degree of N-linked glycosylation." Unlike V1-V2 however, the V3 loop is highly variable and thus is the most important determinant of co-receptor specificity. The normal ligands for this receptor, RANTES, MIP-1β, and MIP-1α, are able to suppress HIV-1 infection in vitro. 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.
CCR5 is the primary co-receptor used by gp120 sequentially with CD4. This bind results in gp41, the other protein product of gp160, to be released from its metastable conformation and insert itself into the membrane of the host cell. Although it hasn't been finalized as a proven theory yet, binding of gp120-CCR5 involves two crucial steps: 1) The tyrosine sulfated amino terminus of this co-receptor is an "essential determinant" of binding to gp120 (as stated previously) 2) Following step 1., there must be reciprocal action (synergy, intercommunication) between gp120 and the CCR5 transmembrane domains 
CCR5 is essential for the spread of HIV-1 throughout the entire transmission process. Therefore, wanting to downplay its significance resulted in the search to block CCR5 function. A number of new experimental HIV drugs, called CCR5 receptor antagonists, have been designed to interfere with the interaction between CCR5 and HIV (specifically to its envelope glycoprotein gp120), including PRO140 (Progenics), Vicriviroc(Phase III trials were cancelled in July 2010) (Schering Plough), Aplaviroc (GW-873140) (GlaxoSmithKline) and Maraviroc (UK-427857) (Pfizer). Maraviroc was approved for use by the FDA in August 2007. It is the only one thus far approved by the FDA for clinical use, thus becoming the first CCR5 inhibitor. 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. However, because there is still another co-receptor available, this indicates that lacking the CCR5 gene doesn't make one immune to the virus; it simply implies that it would be more challenging for the individual to contract it. Also, the virus still has access to the CD4. Unlike CCR5, which the body apparently doesn't really need due to those still living healthy lives even with the lack of/or absence of the gene (as a result of the delta 32 mutation), CD4 is critical in the bodies defense system (fighting against infection). According to one study, even without the availability of either co-receptors (even CCR5), the virus can still invade cells if gp41 were to go through an alteration (including it's cytoplasmic tail), resulting in the independence of CD4 without the need of CCR5 and/or CXCR4 as a doorway.
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 correspond 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. Two copies of the gene are found in 1% of the Caucasian population. To receive both copies, both parents would have to carry the gene. 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 a few 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 affect 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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