Acquired immune deficiency syndrome (AIDS) was first defined in the early 1980s as a new infectious disease caused by HIV, which leads to a progressive depletion of CD4+ T cells as its hallmark (1, 2). Within a short time, it quickly became clear that CD4, a TCR coreceptor, was also the major receptor for entry of HIV into CD4+ T cells (3). However, it was puzzling that expression of a CD4 transgene rendered human cells, but not mouse cells, permissive for HIV infection. This key finding suggested the presence of one or more human cofactor(s) required for HIV cell entry. Furthermore, it was shown that two HIV isolates had different tropisms for the infection of target cells in vitro. HIV strains that were able to infect primary macrophages and T cells, but not CD4+ human immortalized T cell lines, were present during the long asymptomatic phase of the infection and were referred to as macrophage-tropic (M-tropic) virus strains. Other strains that arise in vivo during progression to AIDS, called T cell tropic, could infect only primary human T cells and T cell lines, but not macrophages. It was also clear that the determinant for this tropism was the viral envelope protein (Env), which was also the ligand for CD4 Ag. These foundational studies sparked years of arduous searching for the coreceptors that were responsible for the distinct tropisms.
Almost a decade after the discovery of CD4 as the primary HIV receptor, Feng et al. (4) identified the first additional coreceptor responsible for T cell–tropic HIV strains using an elegant Env-mediated cell fusion assay. Specifically, they used a CD4+ mouse cell line and a reporter mouse cell line expressing HIV Env and a HeLa cell line transfected with a cDNA plasmid library. On screening they found one of these would fuse to the mouse cell expressing HIV Env cell lines, thus uncovering a G protein–coupled receptor of unknown function. The seven-transmembrane molecule exhibited a high degree of homology to the receptor for IL-8, which was one of the few chemokines identified at the time, suggesting that the candidate also might be a chemokine receptor. Feng et al. (4) originally named this cofactor, fusin, based on its capacity to fuse HIV Env and CD4-expressing cells, but it was renamed CXCR4 after identification of its ligand, the chemokine called SDF1 or CXCL12.
A key point, however, was that CXCR4 enabled entry of HIV Env only from T cell–tropic HIV. Thus, M-tropic HIV strains must use a different cofactor, which was hotly pursued by a number of laboratories throughout the world. A key clue came from Cocchi et al. (5) in late 1995, showing that three β-chemokines produced by CD8+ T cells—RANTES (CCL5), MIP-1α (CCL3), and MIP-1β (CCL4)—could inhibit M-tropic HIV infection in vitro. Given that β chemokines had blocked the entry of HIV-1, it was then reasonable to hypothesize that these strains used a β-chemokine receptor as a cofactor, similar to CXCR4 for T-tropic strains. Indeed, Deng et al. (6) used this hypothesis to rapidly clone several β chemokine receptors known at that time. They then tested these for interaction with M-tropic HIV-Env by transfecting murine cells that also expressed human CD4 but were still resistant to infection by HIV (6). This approach rapidly led to discovery of CCR5 (originally named CKR5) as the coreceptor for M-tropic HIV strains, because mouse cells expressing both CCR5 and CD4 now fused with other cells that expressed M-tropic HIV Env (6). Further, when both CD4 and CCR5 were expressed in several human cell lines devoid of these receptors, M-tropic strains of HIV-1 very efficiently infected and replicated in these cells (6). Simultaneous to this study, four other research teams also identified CCR5 as the key receptor that rendered human cells susceptible to both entry and replication by M-tropic, but not T-tropic, HIV strains; thus, five separate laboratories conclusively demonstrated that CCR5 was the key receptor for these HIV strains (6–10). The speed and fortuitous nature of this discovery was even more remarkable given that CCR5 was cloned just a few months earlier, in March 1996, by Samson et al. (11).
In very short order, another landmark discovery showed that some individuals who can resist HIV infection, despite repeated exposures, contained a 32-bp deletion in the coding region of the CCR5 gene (12). It was later confirmed that people who had homozygous deletions of the CCR5 gene were highly resistant to HIV infection, and that heterozygotes seemed to exhibit a delayed progression to AIDS (13). The finding that individuals who lacked CCR5 appeared to have normal immune function propelled efforts to pharmacologically or genetically target CCR5 to block the entry of HIV into cells. This was later accomplished in an HIV-infected individual from Berlin (known famously as the Berlin patient) who had been given stem cells from a donor, as a result of his leukemia treatment, who was homozygous for the CCR5 delta32 mutation, and this patient became the first HIV patient to be cured completely of the infection (14). This remarkable finding provided clear evidence that CCR5 was key to controlling HIV infection in vivo. Indeed, in 2007, a highly effective CCR5 antagonist, called Maraviroc, became the first Food and Drug Administration–approved drug for HIV treatment, blocking HIV entry through CCR5.
The discovery of CCR5 as the main HIV coreceptor also provided important clues into the immunopathogenesis of HIV infection. It was found that CCR5 expression was restricted to a subset of primary human memory T cells, mostly defined as effector memory T cells, whereas CXCR4 was expressed on all primary human T cells, including those with the naive phenotype (15). These findings provided a mechanistic framework in which the virus first infects T cells with a memory phenotype, which predominate mucosal regions, and for the massive decline of effector T cells resident at gut mucosa, most of which also expressed CCR5. Another insight from these seminal articles was understanding the rapid decline in CD4+ T cells when the virus switched to X4-tropic (those that use CXCR4) around the time AIDS develops in most individuals (15).
In conclusion, the landmark discovery of HIV coreceptors CCR5 and CXCR4 paved the path to understanding the complex immunopathogenesis and dynamics of HIV infection in vivo, to the development of therapeutics targeting host factors, and to the remarkable demonstration that targeting CCR5 was sufficient to cure HIV infection.
The authors have no financial conflicts of interest.