CHEMOKINE RECEPTORS AND HUMAN IMMUNODEFICIENCY VIRUS INFECTION

Paul D. Bieniasz and Bryan R. Cullen

Department of Genetics and Howard Hughes Medical Institute, Duke University Medical Center, Durham, NC 27710

Received 12/15/97 Accepted 12/19/97

4. CHEMOKINE RECEPTORS: ENTRY COFACTORS FOR HIV AND SIV

A large body of recent work has established that several chemokine receptors are capable of mediating cell fusion and infection by HIV and SIV strains. The crucial advance was made by Feng et al. (20) who identified Lestr/fusin, a member of the 7 transmembrane (7TM) spanning, G-protein coupled receptor family, as a fusion cofactor for T-tropic HIV-1 strains. Thus, expression of CD4 and fusin on the surface of murine cells was sufficient to render them susceptible infection by T-tropic HIV-1 strains. Fusin is expressed on a wide range of human cell lines, those which do not express fusin are not infected by T- tropic HIV-1. Importantly, cells expressing both CD4 and fusin remained refractory to M-tropic HIV envelope mediated fusion, consistent with the earlier prediction that fusion cofactors for T- and M-tropic HIV-1 strains would be distinct moieties. Furthermore, a CXC chemokine (SDF-1), has subsequently been identified as a ligand which both binds fusin and specifically blocks infection by T-tropic but not M-tropic HIV-1 strains (21,22). Based on the identification of its chemokine ligand, fusin was renamed CXCR-4, the term used for the remainder of this article.

4.1 Additional Coreceptors for HIV-1, HIV-2 and SIV strains

The discovery that CXCR-4, whose closest known homolog is the IL-8 receptor (CXCR-2), constitutes a functional coreceptor for T-tropic HIV-1 strains gave added significance to the earlier finding that the C-C chemokines MIP-1-alpha, MIP-1-beta and RANTES are able to block infection of CD4+ human T-cells by M-tropic but not T-tropic HIV-1 (23). Thus, since certain chemokines are known to bind several 7TM receptors it became likely that discovery of a chemokine receptor that is recognised by all three of these chemokines would also result in the identification of an M-tropic HIV-1 specific coreceptor.

Thus, very soon after the identification of CCR-5 as a receptor for MIP-1-alpha, MIP-1-beta and RANTES a number of groups independently showed that CCR-5 served as a coreceptor for a wide range of M-tropic strains (24-28). In fact, to date all M-tropic HIV-1 strains (which includes isolates of very diverse genotypes (12, 24, 29)) that have been analysed for coreceptor usage have been shown to be capable of utilising CCR-5 (table 1 and refs therein). Expression of this coreceptor is largely confined to primary T-cells and macrophages, consistent with its role as a coreceptor for M-tropic HIV-1 strains. Nevertheless, it soon became clear that a more restricted subset of M-tropic HIV-1 strains could also recognise a related chemokine receptor, CCR-3 (25), and one particularly promiscuous strain (89.6) could make use of no less than 4 distinct coreceptors; CCR-5, CXCR-4, CCR-3 and CCR-2b (27). In addition, other primate lentiviruses can exploit these and other cell surface molecules during virus entry (30-37, table 1). HIV-2 strains are capable of using a number of of the same coreceptors as HIV-1 including CCR5, CCR-3 and CXCR-4 as well as other recently identified coreceptors (33, 37, table 2). A number of SIV strains are capable of utilising CCR-5, but usually not CXCR-4, as a coreceptor (30-35). However, since the same SIV strains are fully capable of replication in a number of human T-cell lines which do not express significant levels of CCR-5, it was immediately apparent that additional SIV specific coreceptors should exist (32). The recent identification of three additional coreceptors, Bonzo/STRL33, BOB/GPR15 and GPR1 which support infection by SIV strains was thus predictable (32-35). The susceptibility of CCR-5 negative cell lines to SIV infection can, in at least some instances, be explained by the presence of Bonzo/STRL33, BOB/GPR15 and/or GPR1. Interestingly, Bonzo/STRL33 and BOB/GPR15 are also functional for some HIV-1 and HIV-2 isolates (33, 35, 36). While both are expressed on primary T-cells, and Bonzo/STRL33 is expressed on monocytes, what contribution these coreceptors make to the establishment and maintenance of HIV-1, HIV-2 or SIV infection in vivo is not yet known. The resistance of CCR-5 negative primary T-cells and macrophages cells to M-tropic HIV-1 infection (see below) suggests that these receptors play a relatively minor, if any role in HIV-1 infection in vivo (38, 39). However the ability of some dual-tropic HIV-1 strains to infect macrophages when they are apparantly incapable of infecting transfected cells where CCR-5 is the only expressed coreceptor (11), potentially indicates a role for Bonzo/STRL33, or another as yet unidentified coreceptor.

4.2 CCR-5 is the major coreceptor utilized by HIV-1 in vivo

A long observed, but poorly understood phenomenon associated with the HIV-1 epidemic is the existence of individuals who have been repeatedly exposed to virus but remain uninfected. In some of these cases, a substantial in vitro infection resistance (specifically to CCR-5 utilizing, M-tropic strains) of PBL and macrophages is evident (38, 39). In addition, it is well known that there is large variation in the rate at which disease progresses in individuals who do become infected. Analysis of CCR-5 genes has revealed the existence of a defective CCR-5 allele which may contribute to each of these observations (40-42). Specifically, approximately 10% of the CCR-5 alleles in Caucasian populations contain a 32 base pair deletion which results in a frame shift and premature truncation of the receptor. These truncated receptors are not expressed on the cell surface and therefore do not function as HIV coreceptors. About 20% and 1% of Caucasians are heterozygous and homozygous, respectively, for the CCR-5 delta32 allele and, importantly, homozygousity for the CCR-5 delta32 allele is associated with a significant degree of infection resistance (41, 42). In fact only four individuals (out of several thousand examined) have been shown to be both HIV infected and homozygous for CCR-5 delta32 (43-46), whereas the expected frequency of homozygotes would be approximately 10 per 1000 HIV-1 infected individuals, given the frequency of this genotype in the general population. Individuals who are heterozygous for the two major CCR-5 alleles do not manifest a high degree of infection resistance (41, 42). However, once infected, the progression of disease in heterozygotes appears to be somewhat retarded. This phenotype is associated with a measurably reduced virus load post seroconversion, and a decrease in frequency of symptomatic primary infection (47). Taken together, these observations strongly imply that the major route of HIV transmission both between individuals and between cells within an individual (at least during the early stages of infection) is mediated by the CCR-5 coreceptor.

CCR-3 is only expressed in a restricted sub-population of T-cells (48) and is, therefore, unlikely to play a major role in HIV infection of these cells. However, it is relatively abundant on microglial cells, the major targets of HIV-1 in the brain (49). Eotaxin, the physiological ligand for CCR-3, and a CCR-3 reactive monoclonal antibody have both been reported to posses infection inhibiting properties in primary brain cultures. M-tropic strains that are unable to use CCR-3 can also infect brain cultures, suggesting that both CCR-3 and CCR-5 play a role in infection of these cells.

An interesting, but as yet unexplained, observation is association of a CCR-2b polymorphism with retarded disease progression (50). This is unexpected given that the great majority of HIV-1 strains are not able to use CCR2b as a coreceptor. The mutant CCR-2b allele, which encodes a receptor with a single valine to isoleucine change, is invariably associated with an intact CCR-5 allele. Since the two genes are very closely linked, it is quite likely that the mutant CCR-2b gene is associated with some, as yet unidentified, defect in CCR-5 expression, although this has not yet been thoroughly investigated. Wide variation of CCR-5 expression levels among individuals homozygous for intact reading frames has been documented, as has a measurably lower expression level in CCR-5 delta32 heterozygotes (51). It might well be that coreceptor expression levels contribute to the very large variation in rates of disease progression, particularly since the amount of CCR-5 expressed on PBL from different donors correlates with their ability to support the replication of M-tropic strains in vitro (51).