The chemokine receptor 5 (CCR5) serves as a co-receptor for the human immunodeficiency virus-1 (HIV-1) that facilitates viral entry into cells. Individuals who are homozygous for a 32 bp deletion of the CCR5 gene are resistant to infection, while for heterozygote carriers their HIV disease progression is delayed. To understand the variable HIV-1 disease course that still exists among rare carriers of this deletion, studies have focused on polymorphisms of the CCR5 gene. Nine different CCR5 human haplotypes (HH) were defined as HHA, HHB, HHC, HHD, HHE, (HHF*1 and HHF*2), and (HHG*1 and HHG*2). Less is known about the effect of these polymorphisms during combined antiretroviral treatment (cART) as the influence of the CCR5 haplotypes on the treatment outcome was not studied extensively. We have reported four novel indels and two new polymorphisms from one ethnic adult Omani population in the 5′UTR of the CCR5 gene. Genetic diversity of the CCR5 haplotypes among acquired immunodeficiency syndrome (AIDS) patients receiving cART and healthy people is discussed. Our results demonstrated the importance of genetic diversity in the CCR5 gene and clearly indicate the importance of understanding the pattern of genetic diversity and its implications for better understanding AIDS.
- antiretroviral treatment
Human immunodeficiency virus (HIV) is the virus that causes acquired immunodeficiency syndrome (AIDS) . Two types of HIV exist: HIV-1 which is the most common virus worldwide and HIV-2 that is less common and mainly restricted to East Africa. HIV targets and destructs the CD4+ T-helper helper cells, one of the most important cells of the immune system. AIDS is a major global health emergency. Not all individuals infected with HIV-1 will progress at the same rate toward the terminal stages of the disease. It is well known that HIV-1-infected patients on combined antiretroviral therapy (cART) do not respond equally to treatment, despite proper compliance to therapy.
Accumulating evidence, over the years, strongly suggests the involvement of host genetic factors in the HIV-1 infection and rate of disease progression [2–4]. Therefore, intensive research efforts were directed toward defining the host genetic factors that may influence HIV-1 infection, virus transmission, and disease progression. Studies were focused on exploring the interaction between host genetic factors and the immunological and virological parameters [4, 5]. This will allow a better understanding of the immunopathogenesis of HIV-1 infection and disease outcome.
Candidate gene analysis approach was initially used to test for association of genetic variants with HIV-1 acquisition and disease rate. These variants were already known or suspected to influence HIV-1 infection and rate of disease progression. Studies suggested several important immune response genes to be associated with HIV-1 pathogenesis [6–8]. This includes genes that regulate HIV-1 recognition and entry such as chemokine co-receptors, human leukocyte antigens (HLA) type, T-cell receptors (TCRs), killer cell immunoglobulin-like receptors (KIRs), and toll like receptors (TLRs). These also include genes that are involved in immune cell trafficking such as chemokine ligands and adhesion molecules, genes implicated in signaling pathways and cytokines . An important host factor found to be strongly associated with HIV-1 disease rate was the chemokine receptor 5 (CCR5), the major HIV-1 co-receptor. A deletion of 32 bases from the coding region of the
A number of observational studies have found that immunological non-responders are at higher risk of disease progression and non-AIDS-related mortality than patients who had a complete response, raising a concern about the long-term effect of suboptimal immune response as previously reviewed in Ref. . As a result, it is essential to investigate critically the factors that contribute to the increased risk of disease progression and predict immunologic non-response during long-term cART. A number of host factors were found to be associated with immunological failure (Figure 1). Figure 1 shows a list of the most common factors known to affect immune recovery (i.e., CD4+ T-helper cell recovery).
2. The chemokine receptors
Chemokine receptors are transmembrane (TM) proteins that belong to a G protein-coupled receptors (GPCRs), which are classified into four distinct families—CXC, CC, CX3C, and XC—according to the structure of their natural ligands (Figure 2a and b) .
All GPCR share a common molecular structure, in which they have seven TM helices and transmit signals from extracellular ligands to intracellular biological pathways via heterotrimeric G proteins . The structural regions required for ligand binding and receptor activation are defined for several chemokines. For example, in C-C chemokine receptor type 2 (CCR2) which is a protein expressed on human cells such as monocytes that is encoded by the
Structurally, the GPCR can be divided into three main parts: (1) the extracellular region, consisting of the N-terminus and three extracellular loops (ECL1–ECL3); (2) the transmembrane (TM) region, composed of seven α-helices (TM1–TM7); and (3) the intracellular region, consisting of three intracellular loops (ICL1–ICL3), an intracellular amphipathic helix (H8), and the cellular C-terminus domain. The GPCRs possess a structural motif, which are critical for ligand-dependent signaling, desensitization, and receptor trafficking .
Another common feature of the GPCRs is the presence of aspartic acid, arginine and tyrosine (DRY) conserved motif at the cytoplasmic end of the third transmembrane domain, which has shown to be critical for signaling. The C-terminus of the GPCR receptor contains serine and threonine residues, which can be phosphorylated to induce recruitment of arrestin proteins. Beta (β) arrestin proteins have shown to be critical for receptor internalization and signal termination and to play an important role in the formation of a G protein-independent signaling complex pathway. A redundant role was played by the chemokine and their receptors, for instance, the selective activation of G protein versus β-arrestin-dependent signaling pathways may result in differential cellular responses. Moreover, the chemokine receptors can be regulated at the level of phosphorylation by the recruitment of different GPCR kinases, thus modulating the signaling complex cascade. For example, the CCR7 binds both CCL19 and CCL21 leading to different cellular responses .
2.1. Cell signaling and chemokine receptor functions
The network signal transduction through chemokine receptors is complex and leads to different functional consequences including homeostatic distribution of leukocytes and their mobilization from the bone marrow. In addition, chemokine receptors were found to be critical during inflammation, autoimmunity, lymphoid organogenesis, angiogenesis and immune regulation . Growing evidence suggests synergism between chemokines in leukocyte migration through chemokine and/or receptor heterodimerization and further contributes to chemokine signaling.
This leads to enhancing the local leukocyte influx, thereby amplifying the outcome of inflammatory responses . There is little information known about the signaling pathways for chemokine receptors because the activated pathways depend primarily on the specific type of the ligand and the receptor involved. A two-step model was proposed for chemokine receptor binding and activation, in which the core domain of the ligand is closely associated with the N-terminus and extracellular loops of the chemokine receptor. In the next step, the N-terminus domain of the ligand penetrates the helical bundle of the receptor .
2.2. The CCR5 receptor
The CCR5 is a pro-inflammatory receptor that binds to chemokine (C-C motif) ligand 3 (CCL3), CCL4, and CCL5, while CCL4 is known to be specific for CCR5 [16, 21]. CCR5 receptor was first identified in 1996  using molecular cloning and localized the gene to chromosome 3p21 in which they suggested a potential role of this receptor in the granulocyte proliferation and differentiation. The CCR5 receptor is expressed on resting (memory/effector) T lymphocytes, monocytes, macrophages, and immature dendritic cells . This receptor was the focus for an intensive research after being identified as a co-receptor for the R5 strain of HIV-1 [24, 25]. HIV strain that uses the CC
CCR5gene and HIV/AIDS
Researchers had a particular interest to some individuals who were able to remain uninfected with HIV-1 despite repeated exposure to the virus. Researchers were able to explain the cause of such phenomenon by the presence of molecular variants in the
Since the role of CCR5 receptor is vital for the primary R5 HIV-1 strains that are present early during HIV-1 infection, this makes the
3.1. Polymorphisms in the
CCR5gene and rate of AIDS progression
To understand the variable HIV-1 disease course that still exists among rare carriers of the
We have previously reported the existence of four novel indels and two new polymorphisms from the Omani adult individuals . The four new indels were detected out of 32 variable positions, −2973A/–, −2894A/–, −2827TA/–, and −2769T/–, and all were located in the 5′UTR of the
We also performed a linkage disequilibrium (LD) pattern to classify polymorphisms on the
Moreover, we have also investigated the possible correlation of the
Our results showed that the common HHC haplotype was significantly associated with higher mean CD4+ T-cell counts among the Omani HIV-1-infected patients (
Furthermore, we have studied the association of the average CD4+ T-cell count with immune recovery. The results showed that the immune responder (IR) group has a significantly higher level of average CD4+ T-cell count than the immune non-responder (INR) group during cART and before reaching undetectable viral load level (
It was noted that among the factors that could attribute to the observed phenomenon is the size of the thymus. It was found that HIV-1-infected patients with large thymus were able to better restore their CD4+ T-cell levels and have a broader immunological repertoire than patients with smaller thymus. In addition, the positive association observed between the CD4+ T-cell counts and the immune recovery strengthens the classification method used for grouping the IR and INR in our studies.
To explain the variable HIV-1 disease course that still exists among rare carriers of the
Several single nucleotide polymorphisms (SNPs) have been defined in the
A group of researchers identified single nucleotide polymorphisms (SNPs) in the
To identify all the polymorphic sites that exist within the
The influence of HHA, HHC, HHD, and HHE haplotypes on CCR5 receptor expression on B and T lymphocytes, NK cells, and monocytes among populations with either African or Caucasian ancestry in South Africa has been investigated . In South African-Caucasians (SACs), the carriers of the HHC expressed CCR5 receptors at significantly higher density on CD56+ NK cells and CD16+ CD56+ NK cell subset than South African-Africans (SAA) who do not carry this haplotype . This may partly explain the protective role played by the HHC in Caucasians but not in Africans . Likewise, such impact on the density of CCR5 receptor expression may be the explanation of the protective role suggested in Omani patients carrying HHC. However, the experimental design of the current study was different from those of other studies in that the effects of the haplotypes were examined during the cART, whereas other studies were in the absence of the cART.
The results of our studies have shown that the HHC may play a protective role among Omani HIV-infected patients because of its association with a high average CD4+ T-cell count. Therefore, the mechanism of protection to understand the involvement of
4. Summary and future directions
Since HIV/AIDS is a major global concern, many governments implemented strategies and plans to control the spread of the disease and provide optimum antiretroviral treatment to AIDS patients. In this context, the literature review revealed that the genetic makeup of an individual is an important factor that may influence the acquisition of the HIV and the rate of HIV/AIDS progression. Several genes were found to be associated with HIV/AIDS, among which is the
Since the beginning of the AIDS pandemic and in the 1980s of the last century, scientists and clinicians were working with a disease that was not treatable at all, and the majority of patients died relatively quickly. In the 1990s of the last century and due to better recognition of how HIV spreads and to new drugs developed for treatments, AIDS became more treatable. The goal is now to go for a complete cure, in other words “complete viral eradication,” having the ability to remove the virus from the body or to suppress it to the point where it can no longer cause any damage to the host. This goal is reachable as the progress we are making against HIV/AIDS is much larger than in other comparable serious viral infections such as polio and smallpox [48, 49]. Currently, medications are much better than ever at treating AIDS patients, and our knowledge of how the immune system works against HIV/AIDS is improving.
Despite the heavy research to uncover the pathogenesis of HIV/AIDS, the whole picture of AIDS pathogenesis is not yet completely clear. During the infection with HIV, there is a known period of four to six weeks called the “window period” that we do not know much about. How exactly our immune system works during this critical “window period” is not completely understood. This was referred to as the “window of vulnerability.” This can be considered now as a “window of opportunity,” because in that very short period of time, there is a release of a huge amount of viruses and establishment of a latent HIV reservoir. It is in that short period of time our success or failure with vaccines as well as curing HIV/AIDS will rest. HIV reservoir is established literally within few days of acute infection. It has been shown that not only an HIV reservoir is formed early but also byproducts of CD4+ T-cell deaths increase significantly within days, and these cells are able to suppress the immune response to the virus. In fact, we have an interesting situation, an HIV reservoir that is almost immediately formed and products of cell death suppressing the immune response that is expected to prevent the formation of that reservoir. Future research should aim at reversing these processes. But we must take into consideration the importance of the existence of genetic diversity in the
This work was supported by a grant from the Research Council of Oman (Grant no. RC/MED/MICR/11/10/01) and the Sultan Qaboos University (SQU), Sultanate of Oman. We thank all our colleagues and students who contributed to this work.
Barré-Sinoussi F, Chermann JC, Rey F, Nugeyre MT, Chamaret S, Gruest J, et al. Isolation of a T-lymphotropic retrovirus from a patient at risk for acquired immune deficiency syndrome (AIDS). Science. 1983; 220(4599):868-871.
Al-Jabri AA. Mechanisms of host resistance against HIV infection and progression to AIDS. Sultan Qaboos Univ Med J. 2007; 7:82-96.
Al-Jabri AA, McCloskey D, Addawee M, Bottazzo FG, Sachs J, Oxford JS. In vitro correlation between human leukocyte antigen class I and II phenotype and HIV infectivity of activated peripheral blood mononuclear cell cultures. AIDS. 1998; 12:217-218.
Sharma G, Kaur G, Mehra N. Genetic correlates influencing immunopathogenesis of HIV infection. Indian J Med Res. 2011; 134:749-768.
Song H, Pavlicek JW, Cai F, Bhattacharya T, Li H, Iyer SS, et al. Impact of immune escape mutations on HIV-1 fitness in the context of the cognate transmitted/founder genome. Retrovirology. 2012; 9:89.
Fellay J, Shianna KV, Telenti A, Goldstein DB. Host genetics and HIV-1: the final phase? PLoS Pathog. 2010; 6:e1001033.
Malhotra R, Hu L, Song W, Brill I, Mulenga J, Allen S, et al. Association of chemokine receptor gene (CCR2-CCR5) haplotypes with acquisition and control of HIV-1 infection in Zambians. Retrovirology. 2011; 8:22.
Al Jabri AA. HLA and in vitro susceptibility to HIV infection. Mol Immunol. 2002; 38:959-967.
Kaur G, Mehra N. Genetic determinants of HIV-1 infection and progression to AIDS: immune response genes. Tissue Antigens. 2009; 74:373-385.
Dean M, Carrington M, Winkler C, Huttley GA, Smith MW, Allikmets R, et al. Genetic restriction of HIV-1 infection and progression to AIDS by a deletion allele of the CKR5 structural gene. Hemophilia Growth and Development Study, Multicenter AIDS Cohort Study, Multicenter Hemophilia Cohort Study, San Francisco City Cohort, ALIVE Study. Science. 1996; 273:1856-1862.
Rappaport J, Cho Y-Y, Hendel H, Schwartz EJ, Schachter F, Zagury J-F. 32 bp CCR-5 gene deletion and resistance to fast progression in HIV-1 infected heterozygotes. Lancet. 1997; 349:922-923.
van Manen D, van't Wout AB, Schuitemaker H. Genome-wide association studies on HIV susceptibility, pathogenesis and pharmacogenomics. Retrovirology. 2012; 9:70.
Gazzola L, Tincati C, Bellistré GM, d'Arminio Monforte A, Marchetti G. The absence of CD4+ T cell count recovery despite receipt of virologically suppressive highly active antiretroviral therapy: clinical risk, immunological gaps, and therapeutic options. Clin Infect Dis. 2009; 48:328-337.
Pinzone MR, Di Rosa M, Cacopardo B, Nunnari G. HIV RNA suppression and immune restoration: can we do better? Clin Develop Immunol. 2012; 2012:12.
Maghazachi AA, Al-Aoukaty A. Chemokines activate natural killer cells through heterotrimeric G- proteins: implications for the treatment of AIDS and cancer. FASEB J. 1998; 12:913-924.
Choi W-T, An J. Biology and clinical relevance of chemokines and chemokine receptors CXCR4 and CCR5 in human diseases. Exp Biol Med. 2011; 236:637-647.
White GE, Iqbal AJ, Greaves DR. CC chemokine receptors and chronic inflammation-therapeutic opportunities and pharmacological challenges. Pharmacol Rev. 2013; 65:47-89.
Venkatakrishnan AJ, Deupi X, Lebon G, Tate CG, Schertler GF, Babu MM. Molecular signatures of G-protein-coupled receptors. Nature. 2013; 494:185-194.
Lira SA, Furtado GC. The biology of chemokines and their receptors. Immunol Res. 2012; 54:111-120.
Mantovani A, Locati M, Uguccioni M. Chemokines in immunopathology: from dark sides to clinical translation. Immunol Lett. 2012; 145:1.
Mueller A, Strange PG. The chemokine receptor, CCR5. Int J Biochem Cell Biol. 2004; 36:35-38.
Samson M, Labbe O, Mollereau C, Vassart G, Parmentier M. Molecular cloning and functional expression of a new human CC-chemokine receptor gene. Biochemistry. 1996; 35:3362-3367.
Oppermann M. Chemokine receptor CCR5: insights into structure, function, and regulation. Cell Signal. 2004; 16:1201-1210.
Deng H, Liu R, Ellmeier W, Choe S, Unutmaz D, Burkhart M, et al. Identification of a major co-receptor for primary isolates of HIV-1. Nature. 1996; 381:661-666.
Dragic, T, Litwin V, Allaway GP, Martin SR, Huang Y, Nagashima KA, et al. HIV-1 entry into CD4+ cells is mediated by the chemokine receptor CC-CKR-5. Nature. 1996; 381:667-673.
Liu R, Paxton WA, Choe S, Ceradini D, Martin SR, Horuk R, et al. Homozygous defect in HIV-1 coreceptor accounts for resistance of some multiply-exposed individuals to HIV-1 infection. Cell. 1996; 86:367-377.
Blanpain C, Lee B, Tackoen M, Puffer B, Boom A, Libert F, et al. Multiple nonfunctional alleles of CCR5 are frequent in various human populations. Blood. 2000; 96:1638-1645.
Martinson JJ, Chapman NH, Rees DC, Liu YT, Clegg JB. Global distribution of the CCR5 gene 32-basepair deletion. Nat Genet. 1997; 16:100-103.
Novembre J, Galvani AP, Slatkin M. The geographic spread of the CCR5 Δ32 HIV-resistance allele. PLoS Biol. 2005; 3:e339.
McDermott DH, Zimmerman PA, Guignard F, Kleeberger CA, Leitman SF, Murphy PM. CCR5 promoter polymorphism and HIV-1 disease progression. Multicenter AIDS Cohort Study (MACS). Lancet. 1998; 352:866-870.
Mummidi S, Ahuja SS, Gonzalez E, Anderson SA, Santiago EN, Stephan KT, et al. Genealogy of the CCR5 locus and chemokine system gene variants associated with altered rates of HIV-1 disease progression. Nature Med. 1998; 4:786-793.
Gonzalez E, Bamshad M, Sato N, Mummidi S, Dhanda R, Catano G, Cabrera S, et al. Race-specific HIV-1 disease-modifying effects associated with CCR5 haplotypes. Proc Natl Acad Sci U S A. 1999; 96:12004-12009.
Tang J, Shelton B, Makhatadze NJ, Zhang Y, Schaen M, Louie LG, et al. Distribution of chemokine receptor CCR2 and CCR5 genotypes and their relative contribution to human immunodeficiency virus type 1 (HIV-1) seroconversion, early HIV-1 RNA concentration in plasma, and later disease progression. J Virol. 2002; 76:662-672.
Al-Mahruqi SH, Zadjali F, Koh CY, Balkhair A, Said EA, Al-Balushi MS, et al. New genetic variants in the CCR5 gene and the distribution of known polymorphisms in Omani population. Int J Immunogenet. 2014; 41:20-28.
Al-Mahruqi SH, Zadjali F, Beja-Pereira A, Koh CY, Balkhair A, Al-Jabri AA. Genetic diversity and prevalence of CCR2-CCR5 gene polymorphisms in the Omani population. Genet Mol Biol. 2014; 37:7-14.
Nguyen L, Li M, Chaowanachan T, Hu DJ, Vanichseni S, Mock PA, et al. CCR5 promoter human haplogroups associated with HIV-1 disease progression in Thai injection drug users. AIDS. 2004; 18:1327-1333.
Said EA, Al-Yafei F, Zadjali F, Al-Balushi M, Hasson SS, Al-Mahruqi S. et al. Frequency of TLR4 (1063A/G and 1363C/T) polymorphisms in healthy and HIV-infected Omani individuals and their relationship to viral load and T cell count. Genet Mol Res. 2016; 15:(2). doi: 10.4238/gmr.15027671.
Said EA, Al-Yafei F, Zadjali F, Hasson SS, Al-Balushi MS, Al-Mahruqi S, et al. Association of single-nucleotide polymorphisms in TLR7 (Gln11Leu) and TLR9 (1635A/G) with a higher CD4T cell count during HIV infection. Immunol Lett. 2014; 160:58-64.
Kageyama S, Mimaya J, Yamada K, Kurimura T, Shiraki, K. 2001. Polymorphism of CCR5 affecting HIV disease progression in the Japanese population. AIDS Res Hum Retroviruses. 17; 991-995.
Kaur G, Singh P, Rapthap CC, Kumar N, Vajpayee M, Sharma SK, et al. Polymorphism in the CCR5 gene promoter and HIV-1 infection in North Indians. Human Immunol. 2007; 68:454-461.
Xu L, Qiao Y, Zhang X, Sun H, Wang J, Sun D, et al. A haplotype in the CCR5 gene promoter was associated with the susceptibility to HIV-1 infection in a northern Chinese population. Mol Biol Rep. 2011; 38:327-332.
Dronda F, Moreno S, Moreno A, José LC, Pérez-Elías MJ, Antela A. 2002. Long-term outcomes among antiretroviral-naive human immunodeficiency virus–infected patients with small increases in CD4+ Cell counts after successful virologic suppression. Clin Infect Dis. 2002; 35:1005-1009.
McKinnon LR, Kimani M, Wachihi C, Nagelkerke NJ, Muriuki FK, Kariri A, et al. Effect of baseline HIV disease parameters on CD4+ T cell recovery after antiretroviral therapy initiation in Kenyan women. PLoS One. 2010; 5:e11434.
Kostrikis LG, Huang Y, Moore JP, Wolinsky SM, Zhang L, Guo Y, et al. A chemokine receptor CCR2 allele delays HIV-1 disease progression and is associated with a CCR5 promoter mutation. Nature Med. 1998; 4(3):350-353.
Picton ACP, Shalekoff S, Paximadis M, Tiemessen CT. Marked differences in CCR5 expression and activation levels in two South African populations. Immunol. 2012; 136:397-407
Picton AC, Paximadis M, Tiemessen CT. CCR5 promoter haplotypes differentially influence CCR5 expression on natural killer and T cell subsets in ethnically divergent HIV-1 uninfected South African populations. Immunogenetics. 2012; 64:795-806.
Picton AP, Paximadis M, Tiemessen C. CCR5 promoter haplotypes differentially influence CCR5 expression on natural killer and T cell subsets in ethnically divergent HIV-1 uninfected South African populations. Immunogenetics. 2012; 64:795-806.
Al-Jabri AA. Forcing the enemy to come out and surrender: a strategy for an AIDS cure. AIDS Res Hum Retroviruses. 2015; 31(1):45-46.
Al-Jabri AA, Alenzi FQ. Vaccines, virucides and drugs against HIV/AIDS: hopes and optimisms for the future. Open AIDS J. 2009; 3:1-3.
Barmania F, Pepper MS. C-C chemokine receptor type five (CCR5): an emerging target for the control of HIV infection. Appl Trasns Genom. 2013; 2:3-16.