Genetically divergent strains of human immunodeficiency virus type 2 use multiple coreceptors for viral entry

V3: HIV's switch-hitter

The third variable region, V3, of the gp120 surface envelope glycoprotein is an approximately 35-residue-long, frequently glycosylated, highly variable, disulfide-bonded structure that has a major influence on HIV-1 tropism. Thus the sequence of V3, directly or indirectly, can determine which coreceptor (CCR5 or CXCR4) is used to trigger the fusion potential of the Env complex, and hence which cells the virus can infect. V3 also influences HIV-1's sensitivity to, and ability to escape from, entry inhibitors that are being developed as antiviral drugs. For some strains, V3 is a prominent target for HIV-1 neutralizing antibodies (NAbs); indeed, for many years it was considered to be the "principal neutralization determinant" (PND). Some efforts to use V3 as a vaccine target continue to this day, despite disappointing progress over more than a decade. Recent findings on the structure, function, antigenicity, and immunogenicity of V3 cast new doubts on the value of this vaccine approach. Here, we review recent advances in the understanding of V3 as a determinant of viral tropism, and discuss how this new knowledge may inform the development of HIV-1 drugs and vaccines.

Apoptosis of uninfected cells induced by HIV envelope glycoproteins

Apoptosis, or programmed cell death, is a key event in biologic homeostasis but is also involved in the pathogenesis of many human diseases including human immunodeficiency virus (HIV) infection. Although multiple mechanisms contribute to the gradual T cell decline that occurs in HIV-infected patients, programmed cell death of uninfected bystander T lymphocytes, including CD4+ and CD8+ T cells, is an important event leading to immunodeficiency. The HIV envelope glycoproteins (Env) play a crucial role in transducing this apoptotic signal after binding to its receptors, the CD4 molecule and a coreceptor, essentially CCR5 and CXCR4. Depending on Env presentation, the receptor involved and the complexity of target cell contact, apoptosis induction is related to death receptor and/or mitochondria-dependent pathways. This review summarizes current knowledge of Env-mediated cell death leading to T cell depletion and clinical complications and covers the sometimes conflicting studies that address the possible mechanisms of T cell death.

Identification of cell surface targets for HIV-1 therapeutics using genetic screens

Human immunodeficiency virus (HIV) drugs designed to interfere with obligatory utilization of certain host cell factors by virus are less likely to encounter development of resistant strains than drugs directed against viral components. Several cellular genes required for productive infection by HIV were identified by the use of genetic suppressor element (GSE) technology as potential targets for anti-HIV drug development. Fragmented cDNA libraries from various pools of human peripheral blood mononuclear cells (PBMC) were expressed in vitro in human immunodeficiency virus type 1 (HIV-1)-susceptible cell lines and subjected to genetic screens to identify GSEs that interfered with viral replication. After three rounds of selection, more than 15000 GSEs were sequenced, and the cognate genes were identified. The GSEs that inhibited the virus were derived from a diverse set of genes including cell surface receptors, cytokines, signaling proteins, transcription factors, as well as genes with unknown function. Approximately 2.5% of the identified genes were previously shown to play a role in the HIV-1 life cycle; this finding supports the biological relevance of the assay. GSEs were derived from the following 12 cell surface proteins: CXCR4, CCR4, CCR7, CD11C, CD44, CD47, CD68, CD69, CD74, CSF3R, GABBR1, and TNFR2. Requirement of some of these genes for viral infection was also investigated by using RNA interference (RNAi) technology; accordingly, 10 genes were implicated in early events of the viral life cycle, before viral DNA synthesis. Thus, these cell surface proteins represent novel targets for the development of therapeutics against HIV-1 infection and AIDS.

Primate models for human immunodeficiency virus infection. Evolution of receptor use during pathogenesis

Animal models greatly facilitate understanding of transmission, pathogenesis and immune responses in HIV and SIV infection and provide models for studies on the effect of candidate drugs or vaccines. However, there are several aspects that one should consider when drawing conclusions from results obtained from animal models. First, the genetic relationship of primate lentiviruses cannot be disregarded because it is known that HIV-1 is more closely related to SIV of chimpanzee origin (SIVcpz) than to SIV from sooty mangabey (SIVsm) origin. Nevertheless, SIVsm and SIVmac are the ones most often used as model systems. Second, there are differences in the biological properties, like CXCR4 use and CD4-independent coreceptor use, of HIV and SIV. These differences might be relevant in virus transmission, pathogenesis and in evoking immune responses. Third, in vivo and in vitro selection may influence the results. Neutralizing antibodies may play a role in selection of variant viruses since neutralization sensitive, CD4-independent SIVsm variants seemed to be suppressed in animals that mounted a neutralizing antibody response. It is tempting to speculate that neutralizing antibodies shape the SIV/HIV infection by selecting variants with a more "closed" envelope conformation with consequences for both receptor binding and neutralization sensitivity. The SIV/monkey model, although it has important advantages, may not answer all questions asked about HIV-1 infection in human.

Amino acid 324 in the simian immunodeficiency virus SIVmac V3 loop can confer CD4 independence and modulate the interaction with CCR5 and alternative coreceptors

The V3 loop of the simian immunodeficiency virus (SIV) envelope protein (Env) largely determines interactions with viral coreceptors. To define amino acids in V3 that are critical for coreceptor engagement, we functionally characterized Env variants with amino acid substitutions at position 324 in V3, which has previously been shown to impact SIV cell tropism. These changes modulated CCR5 engagement and, in some cases, allowed the efficient usage of CCR5 in the absence of CD4. The tested amino acid substitutions had highly differential effects on viral infectivity. Eleven of sixteen substitutions disrupted entry via CCR5 or the alternative coreceptor GPR15. Nevertheless, most of these variants replicated in the macaque T-cell line 221-89 and some also replicated in rhesus macaque peripheral blood monocytes, suggesting that efficient usage of CCR5 and GPR15 on cell lines is not a prerequisite for SIV replication in primary cells. Four variants showed enhanced entry into the macaque sMagi reporter cell line. However, sMagi cells did not express appreciable amounts of CCR5 and GPR15 mRNA, and entry into these cells was not efficiently blocked by a small-molecule CCR5 antagonist, suggesting that sMagi cells express as-yet-unidentified entry cofactors. In summary, we found that a single amino acid at position 324 in the SIV Env V3 loop can modulate both the efficiency and the types of coreceptors engaged by Env and allow for CD4-independent fusion in some cases.

Evolution of cell recognition by viruses: a source of biological novelty with medical implications

The picture beginning to form from genome analyses of viruses, unicellular organisms, and multicellular organisms is that viruses have shared functional modules with cells. A process of coevolution has probably involved exchanges of genetic information between cells and viruses for long evolutionary periods. From this point of view present-day viruses show flexibility in receptor usage and a capacity to alter through mutation their receptor recognition specificity. It is possible that for the complex DNA viruses, due to a likely limited tolerance to generalized high mutation rates, modifications in receptor specificity will be less frequent than for RNA viruses, albeit with similar biological consequences once they occur. It is found that different receptors, or allelic forms of one receptor, may be used with different efficiency and receptor affinities are probably modified by mutation and selection. Receptor abundance and its affinity for a virus may modulate not only the efficiency of infection, but also the capacity of the virus to diffuse toward other sites of the organism. The chapter concludes that receptors may be shared by different, unrelated viruses and that one virus may use several receptors and may expand its receptor specificity in ways that, at present, are largely unpredictable.

Improved Breadth and Potency of an HIV-1-Neutralizing Human Single-chain Antibody by Random Mutagenesis and Sequential Antigen Panning

Several human monoclonal antibodies can neutralize a range of human immunodeficiency virus type 1 (HIV-1) primary isolates but their potency and related ability to suppress generation of HIV-1 escape mutants is significantly lower than the activity of antiretroviral drugs currently in clinical use. Recently, a human Fab, X5, was identified and found to neutralize primary isolates from different clades. Further improvement of the potency and breadth of HIV-1 neutralization by this antibody could be critical for its potential use in the treatment of HIV-1-infected patients. However, increasing potency of an antibody by selection from libraries may lead to a decrease in the breadth of neutralization. In an attempt to solve this problem, we subjected a random mutagenesis library of the scFv X5 to sequential rounds of selection on non-homologous HIV-1 envelope glycoproteins (Envs) dubbed sequential antigen panning (SAP). By using SAP, we identified two scFv antibodies, m6 and m9, that were tested with a panel of 33 diverse primary HIV-1 infectious isolates in an assay based on a reporter cell-line expressing high levels of CD4, CCR5 and CXCR4. The IC(50) was less than 50 microg/ml for 21 (m6) and 19 (m9) out of 29 isolates from group M (subtypes A-C, F, G and CRF-01AE) and one isolate from group N; three isolates from group O were not significantly inhibited at 50 microg/ml. The average IC(50) values for the two antibodies were significantly (p<0.001, n=29) lower compared to scFv X5. Their inhibitory activity does not appear to be related to the HIV-1 subtype, coreceptor usage or the disease stage. m9 inhibited infection of peripheral blood mononuclear cells by the primary isolates JRCSF, 89.6 and BR020 with IC(90) of 4, 6 and 25 microg/ml, respectively; for a single-round infection by pseudovirus, the IC(90) for JRSCF, 89.6, YU2 and HXBc2 was 15, 5, 15 and 5 microg/ml, respectively. In these two assays the IC(90) for m9 was, on average, two- to threefold lower than for scFv X5. These results demonstrate that both the potency and the breadth of HIV-1 neutralization of one of the few known potent broadly cross-reactive human monoclonal antibodies, scFv X5, could be improved significantly. However, only experiments in animal models and clinical trials in humans will show whether these new scFvs and the approach for their identification have potential in the development of prophylactics and therapeutics for HIV-1 infections.

Identification and characterization of HIV-2 strains obtained from asymptomatic patients that do not use CCR5 or CXCR4 coreceptors

In vivo, human immunodeficiency virus type 2 (HIV-2) infection reveals several unique characteristics when compared to HIV-1 infection, the most remarkable of which is the extraordinarily long asymptomatic period. Here we describe two HIV-2 primary isolates, obtained from asymptomatic individuals, which do not infect any coreceptor-expressing cell lines tested. In those cells, we show that the absence of replication is directly related to cell entry events. Furthermore, productive infection observed in peripheral blood mononuclear cells (PBMC) was not inhibited by natural ligands and monoclonal antibodies directed to CCR5 and CXCR4. Finally, viral entry efficiency and viral progeny production of these viruses are markedly impaired in PBMC, indicating a reduced replicative fitness of both viruses. In conclusion, our data suggest that in some HIV-2 asymptomatic individuals, the circulating viruses are unable to use the major coreceptors to infect PBMC. This fact should have important implications in HIV-2 pathogenesis and transmission.

Characterization of entry mechanisms of human herpesvirus 8 by using an Rta-dependent reporter cell line

To analyze the mechanisms of entry of human herpesvirus 8 (HHV-8), we established a reporter cell line T1H6 that contains the lacZ gene under the control of the polyadenylated nuclear RNA promoter, known to be strongly activated by a viral transactivator, Rta. We found that infection with cell-free virus, as well as cocultivation with HHV-8-positive primary effusion lymphoma cell lines, activated the lacZ gene of T1H6 in a sensitive and dose-dependent manner. Addition of Polybrene and centrifugation enhanced, but polysulfonate compounds inhibited, the HHV-8 infectivity. RGD-motif-containing polypeptides and integrins did not decrease the infectivity, suggesting the presence of an additional cellular receptor other than the reported one. The entry was dependent on pH acidification but not on the clathrin pathway. Although conditioned media obtained from human immunodeficiency virus (HIV)-infected cells did not have any effect on the early steps of HHV-8 infection, intracellular expression of a proviral HIV type 1, but not of Tat alone, increased the HHV-8-dependent reporter activation slightly, suggesting a potential of HIV-mediated enhancement of an early step of HHV-8 infection.

Susceptibility of diverse primary HIV isolates with varying co-receptor specificity's to CXCR4 antagonistic compounds

The chemokine receptors CCR5 and CXCR4 are an obvious target for HIV therapies. Two compounds, T-22 and AMD-3100, have been shown to inhibit infection of CXCR4-using HIV-1 isolates. The specificity of T-22 and AMD-3100 was further confirmed by their ability to block entry of HIV-1 in GHOST-CXCR4 transfected cells with no effect on viral entry in the GHOST-CCR5 cells. The ability of T-22 to block replication of diverse HIV-1 isolates (group M, subtypes A, B, D, E, and F as well as group O) and HIV-2 primary isolates with varying coreceptor specificities ranging from exclusive CCR5 usage to multiple coreceptor usage was examined in detail. T-22 was found to be highly effective (>90%) at blocking infection of diverse HIV-1 (subtypes A-F, and group O) and HIV-2 isolates that use multiple coreceptors in human PBMCs homozygous for a 32-bp deletion in CCR5 (CCR5-/-), but less effective in CCR5 +/+ PBMCs. Additionally, sequential primary HIV-1 isolates obtained from a longitudinal cohort who had switched from single coreceptor usage to a broad range of multiple receptors could be blocked effectively by both T-22 and AMD-3100 in CCR5-/- PBMCs. Our data suggest that CXCR4 antagonistic compounds are highly effective in blocking the entry of X4-tropic HIV-1, and that these compounds could be a useful additive to current anti-retroviral therapy for clinical management of HIV disease.

Coreceptor Switch of [MLV(SIVagm)] pseudotype vectors by V3-loop exchange

Retroviral vectors derived from murine leukemia virus (MLV) have been pseudotyped with a variant of the envelope glycoprotein (Env) of nonpathogenic simian immunodeficiency virus from African green monkeys (SIVagm) to result in [MLV(SIVagm-wt)] vector particles. The variant env gene encodes a full-length surface envelope glycoprotein (SU) and a C-terminally truncated transmembrane protein (TM). To change the coreceptor usage of this vector from CCR5 to CXCR4, which is predominant on human CD4-positive lymphocytes, the putative V3-loop of SIVagm SU was replaced by that of the T cell tropic HIV-1 variant BH10. The resulting [MLV(SIVagm-X4)] vectors were shown to specifically transduce CD4/CXCR4-positive cell lines, demonstrating the equivalent function in cell entry and choice of coreceptor usage of the V3-loops of SIVagm and HIV-1. These modified vectors were able to transduce primary human lymphocytes and were resistant to neutralization by sera from HIV-1-infected individuals. The [MLV(SIVagm-X4)] pseudotype vector generated is thus a promising candidate vector, e.g., for in vivo gene therapy of HIV-1 infection.

Detection of simian immunodeficiency virus in diverse species and of human immunodeficiency virus Type 2 by using consensus primers within the pol region

Human immunodeficiency virus type 2 (HIV-2) is the result of cross-species transmission of simian immunodeficiency virus (SIV) from sooty mangabey monkeys to humans. Primer pairs (intHIV-2/SIV) based on a region of integrase that has considerable homology across HIV-2 and SIV lineages were designed to develop a broadly cross-reactive molecular assay to detect lentivirus infection in primates. The intHIV-2/SIV primers detect HIV-2 and simian viruses SIVcpz, SIVsmm, SIVsyk, SIVagm, and SIVmnd. The primers are also capable of amplifying some HIV-1 strains. Additionally, sequences from the integrase amplicons were of sufficient genetic diversity to permit not only phylogenetic clustering of all simian viruses to their respective lineages but also HIV type and group classification. Thus, the primers described here provide a method to detect primate lentiviruses from diverse species of nonhuman primates, as well as from persons infected with HIV-1 and HIV-2.

CCR5 or CXCR4 is required for efficient infection of peripheral blood mononuclear cells by promiscuous human immunodeficiency virus type 2 primary isolates

Primary human immunodeficiency virus type 2 (HIV-2) isolates are characterized by their ability to use a broad range of coreceptors, including CCR5, CXCR4, and several alternative coreceptors. However, the in vivo relevance of this in vitro promiscuity in coreceptor usage remains unclear. We set out to evaluate the relative importance of CCR5 and CXCR4 for infection of activated peripheral blood mononuclear cells (PBMC). PBMC from donors homozygous for wild-type CCR5 (CCR5(+/+) or CCR5Delta32 (CCR5(-/-)) were tested for their susceptibility to infection with 10 primary HIV-2 isolates with known coreceptor usage by parallel 50% tissue culture infectious dose (TCID50) titrations. Although all isolates, except one, were able to establish productive infection in CCR5(-/-) PBMC, the infection of these cells was inefficient for all isolates that were unable to use CXCR4. For CXCR4-using isolates there were only minor differences in TCID50 between CCR5(+/+) and CCR5(-/-) PBMC. When we compared the replication kinetics in PBMC from donors of the two genotypes we observed an average delay in replication onset of 9 days in the CCR5(-/-) PBMC. This study shows that HIV-2 can use alternative coreceptors for infection of PBMC, but that this infection is much less efficient than infection mediated by CCR5 or CXCR4. Thus, CCR5 and CXCR4 appear to be the major coreceptors for HIV-2 infection of PBMC.

Conserved CXCR4 usage and enhanced replicative capacity of HIV-2/287, an isolate highly pathogenic in Macaca nemestrina

OBJECTIVE

To investigate viral properties that contribute to the pathogenic potential of HIV-2 in macaques.

DESIGN

We compared HIV-2/287, a virus highly pathogenic in Macaca nemestrina, with its non-pathogenic progenitor HIV-2 EHO, for coreceptor usage and ability to infect human and macaque peripheral blood mononuclear cells (PBMC).

METHODS

Coreceptor usage was determined in GHOST cells expressing known coreceptors, and in PBMC with coreceptor-specific inhibitors. Infectivity in PBMC was determined by virus titration and p27 antigen production. Early and late products of reverse transcription were measured by PCR with primers specific for the long terminal repeat (LTR), or the gag region, respectively.

RESULTS

Both viruses preferentially infect HOS-CD4 cells expressing CXCR4. Inhibition by CXCR4-specific peptide TW70 and monoclonal antibody 12G5 indicated that both viruses use predominantly CXCR4 to infect macaque and human PBMC. HIV-2/287 showed greater infectivity than HIV-2 EHO in macaque cells, but the situation was reversed in human cells. Kinetic analysis of reverse transcription products revealed no restriction in reverse transcription following HIV-2 EHO infection of macaque PBMC. However, comparison of the level of newly initiated HIV-2 EHO DNA in macaque and human PBMC indicated that there is an early restriction, prior to the initiation of reverse transcription.

CONCLUSIONS

Results indicate that the adaptation of HIV-2 EHO in M. nemestrina to a highly pathogenic virus HIV-2/287 is not correlated with a shift in or an expansion of coreceptor usage, but with the acquisition of an ability to overcome restrictions for growth in macaque PBMC.

Basic amino acid residues in the V3 loop of simian immunodeficiency virus envelope alter viral coreceptor tropism and infectivity but do not allow efficient utilization of CXCR4 as entry cofactor

In contrast to human immunodeficiency viruses type 1 and type 2 (HIV-1 and HIV-2, respectively), simian immunodeficiency virus (SIVmac) rarely uses CXCR4 (X4) for efficient entry into target cells. Basic amino acid residues in the V3 loop of HIV Env allow efficient coreceptor utilization of X4. Therefore, we investigated if similar changes in the SIVmac Env protein also mediate a coreceptor switch from CCR5 (R5) to X4. Functional analysis revealed that none of eight SIVmac variants, containing V3 regions with an overall charge between +4 and +10, efficiently utilized X4 as entry cofactor. Nonetheless, these alterations had differential effects on SIV coreceptor tropism and on Env expression levels. A single amino acid substitution of L328R, located near the tip of the V3 loop, resulted in grossly reduced Env expression levels and impaired viral infectivity. Notably, additional basic residues restored efficient Env expression and virion incorporation but not infectivity. In comparison to the L328R mutation, changes of P334K and D337K had little disruptive effects on SIVmac entry and replication. Interestingly, mutation of L320K and P321R disrupted coreceptor usage of GPR15 but not R5. These changes also impaired SIVmac replication in peripheral blood mononuclear cells (PBMC) derived from a Delta32/Delta32 donor but not in R5-expressing human or simian PBMC. Our results show that positively charged amino acid residues in the V3 loop affect SIVmac coreceptor tropism and infectivity but do not allow efficient utilization of X4.

Cyclophilin A regulates HIV-1 infectivity, as demonstrated by gene targeting in human T cells

The human immunodeficiency virus type 1 (HIV-1) Gag polyprotein binds most members of the cyclophilin family of peptidyl-prolyl isomerases. Of 15 known human cyclophilins, cyclophilin A (CypA) has been the focus of investigation because it was detected in HIV-1 virions. To determine whether CypA promotes HIV-1 replication, we deleted the gene encoding CypA (PPIA) in human CD4(+) T cells by homologous recombination. HIV-1 replication in PPIA(-/-) cells was decreased and not inhibited further by cyclosporin or gag mutations that disrupt Gag's interaction with cyclophilins, indicating that no other cyclophilin family members promote HIV-1 replication. The defective replication phenotype was specific for wild-type HIV-1 since HIV-2/SIV isolates, as well as HIV-1 bearing a gag mutation that confers cyclosporin resistance, replicated the same in PPIA(+/+) and PPIA(-/-) cells. Stable re-expression of CypA in PPIA(-/-) cells restored HIV-1 replication to an extent that correlated with steady-state levels of CypA. Finally, virions from PPIA(-/-) cells possessed no obvious biochemical abnormalities but were less infectious than virions from wild-type cells. These data formally demonstrate that CypA regulates the infectivity of HIV-1 virions.

HIV type 1 molecular clones able to use the Bonzo/STRL-33 coreceptor for virus entry

We describe the cloning of env genes from the mother-infant HIV-1 isolate pair P6-v3 and M6-v3. These viruses are unusual in that they can use the coreceptor Bonzo/STRL33 as well as CCR5 and, in the case of M6, CXCR4, to enter transfected cell lines in vitro. The phenotype of the parental isolates is generally reflected by the properties of the cloned env genes, when these are used in an Env-complementation assay of virus entry. Chimeric viruses were also made that contain the env genes of P6-v3 and M6-v3 inserted into the background of the infectious molecular clone, HIV-1 NL4-3. Some of the chimeric viruses derived from HIV1 P6-v3 were able to use Bonzo for entry into transfected cell lines, albeit to a lesser extent than they could use CCR5. There are some indications that one of these chimeric viruses, P6-v3-22-1, can use a coreceptor other than CCR5, perhaps Bonzo, to enter mitogen-stimulated PBMC, although only weakly. However, formal proof that this virus can use Bonzo in primary cells has not been obtained. The P6-v3-22-1 chimeric virus was unable to infect CD4-negative, placental cell lines, in the presence or absence of soluble CD4. Env sequence analysis revealed several differences among viruses with different tropisms, most notably a four amino acid deletion in the central region of the V3 loop that distinguishes the R5 virus P6-v3-25-4 from the R5, Bonzo virus P6-v3-22-1.

Cytopathicity of human immunodeficiency virus type 2 (HIV-2) in human lymphoid tissue is coreceptor dependent and comparable to that of HIV-1

Epidemiological studies have shown that human immunodeficiency virus type 2 (HIV-2) is markedly less pathogenic than HIV-1 in vivo. Individuals infected with HIV-2 exhibit a remarkably slow rate of disease development, and these clinical properties have been attributed presumptively to an "attenuated" phenotype of HIV-2 itself. Here, we investigated the impact of coreceptor usage on the cytopathicity of HIV-2 and compared its pathogenic potential with that of HIV-1 in a unique human lymphoid histoculture model. We found that HIV-2 strains, as well as closely related simian immunodeficiency viruses (SIV), displayed mildly or highly aggressive cytopathic phenotypes depending on their abilities to use the coreceptor CCR5 or CXCR4, respectively. A side-by-side comparison of primary X4 HIV-1 and HIV-2 strains revealed similar, high degrees of cytopathicity induced by both HIV types. Furthermore, we found that HIV-2 coreceptor specificity for CCR5 and CXCR4 determined the target cell population for T-cell depletion in lymphoid tissue. Finally, utilization of the alternate coreceptors BOB and Bonzo did not significantly increase the cytopathic properties of HIV-2. These findings demonstrate that coreceptor preference is a key regulator of target cell specificity and the cytopathic potential of HIV-2, with indistinguishable rules compared with HIV-1. Moreover, HIV-2 strains are not characterized by an intrinsically lower cytopathicity than HIV-1 strains. Therefore, direct cytopathic potential per se does not explain the unique behavior of HIV-2 in people, highlighting that other unknown factors need to be elucidated as the basis for their lesser virulence in vivo.

Use of inhibitors to evaluate coreceptor usage by simian and simian/human immunodeficiency viruses and human immunodeficiency virus type 2 in primary cells

We have used coreceptor-targeted inhibitors to investigate which coreceptors are used by human immunodeficiency virus type 1 (HIV-1), simian immunodeficiency viruses (SIV), and human immunodeficiency virus type 2 (HIV-2) to enter peripheral blood mononuclear cells (PBMC). The inhibitors are TAK-779, which is specific for CCR5 and CCR2, aminooxypentane-RANTES, which blocks entry via CCR5 and CCR3, and AMD3100, which targets CXCR4. We found that for all the HIV-1 isolates and all but one of the HIV-2 isolates tested, the only relevant coreceptors were CCR5 and CXCR4. However, one HIV-2 isolate replicated in human PBMC even in the presence of TAK-779 and AMD3100, suggesting that it might use an undefined, alternative coreceptor that is expressed in the cells of some individuals. SIV(mac)239 and SIV(mac)251 (from macaques) were also able to use an alternative coreceptor to enter PBMC from some, but not all, human and macaque donors. The replication in human PBMC of SIV(rcm) (from a red-capped mangabey), a virus which uses CCR2 but not CCR5 for entry, was blocked by TAK-779, suggesting that CCR2 is indeed the paramount coreceptor for this virus in primary cells.

Simian immunodeficiency virus utilizes human and sooty mangabey but not rhesus macaque STRL33 for efficient entry

It has been established that many simian immunodeficiency virus (SIV) isolates utilize the orphan receptors GPR15 and STRL33 about as efficiently as the chemokine receptor CCR5 for entry into target cells. Most studies were performed, however, with coreceptors of human origin. We found that SIV from captive rhesus macaques (SIVmac) can utilize both human and simian CCR5 and GPR15 with comparable efficiencies. Strikingly, however, only human STRL33 (huSTRL33), not rhesus macaque STRL33 (rhSTRL33), functioned efficiently as an entry cofactor for a variety of isolates of SIVmac and SIV from sooty mangabeys. A single amino acid substitution of S30R in huSTRL33 impaired coreceptor activity, and the reverse change in rhSTRL33 greatly increased coreceptor activity. In comparison, species-specific sequence variations in N-terminal tyrosines in STRL33 had only moderate effects on SIV entry. These results show that a serine residue located just outside of the cellular membrane in the N terminus of STRL33 is critical for SIV coreceptor function. Interestingly, STRL33 derived from sooty mangabeys, a natural host of SIV, also contained a serine at the corresponding position and was used efficiently as an entry cofactor. These results suggest that STRL33 is not a relevant coreceptor in the SIV/macaque model but may play a role in SIV replication and transmission in naturally infected sooty mangabeys.

Simian immunodeficiency viruses of diverse origin can use CXCR4 as a coreceptor for entry into human cells

Primary simian immunodeficiency virus (SIV) isolated from sooty mangabey (SIVsm [n = 6]), stumptail (SIVstm [n = 1]), mandrill (SIVmnd [n = 1]), and African green (SIVagm [n = 1]) primates were examined for their ability to infect human cells and for their coreceptor requirements. All isolates infected human peripheral blood mononuclear cells (PBMCs) from a CCR5(+/+) donor, and seven of eight isolates tested also infected CCR5(-/-) PBMCs. Analysis of coreceptor utilization using GHOST and U87 cell lines revealed that all of the isolates tested used CCR5 and the orphan receptors STRL33 and GPR15. Coreceptors such as CCR2b, CCR3, CCR8, and CX3CR1 were also utilized by some primary SIV isolates. More importantly, we found that CXCR4 was used as a coreceptor by the SIVstm, the SIVagm, and four of the SIVsm isolates in GHOST and U87 cells. These data suggest that primary SIV isolates from diverse primate species can utilize CXCR4 for viral entry, similar to what has been described for human immunodeficiency viruses.

Plasma RNA viral load predicts the rate of CD4 T cell decline and death in HIV-2-infected patients in West Africa

OBJECTIVE

To examine whether the levels of plasma RNA and DNA provirus predict the rate of CD4 cell decline and patient death.

DESIGN

Retrospective analysis of HIV-2 cohort subjects.

METHODS

Fifty-two subjects were recruited between January 1991 and December 1992. HIV-2 RNA levels in plasma and DNA levels in peripheral blood mononuclear cells (PBMC) were measured using in-house quantitative PCR assays. The annual rate of CD4 cell decline was calculated using the least-squares method. The survival data on 31 December 1997 were used.

RESULTS

The mean percentage of CD4 cells at baseline was 30.7 (SD, 9.5). In a linear regression model, the annual rate of CD4 cell decline was 1.76 CD4% faster for every increase in one log10 RNA copies/ml [95% confidence interval (CI), 0.81-2.7; P = 0.0006; r = 0.46; n = 52] and 1.76 CD4% faster for every increase in log10 DNA copies/10(5) PBMC (95% CI 0.46-3.1; P = 0.01; r = 0.33; n = 42). In a multiple linear regression model, RNA load was related to CD4 decline independently of DNA load (P = 0.02). The overall mortality rate was 7.29/100 person-years. In a Cox regression model, the hazard rate increased by 2.12 for each log10 increase in RNA load (95% CI, 1.3-3.5; P = 0.0023) but only by 1.09 for each log10 increase in DNA load (95% CI, 0.64-1.87; P = 0.8).

CONCLUSION

This longitudinal study shows for the first time that a baseline HIV-2 RNA load predicts the rate of disease progression. HIV-2-infected patients with a high viral load may need to be treated as vigorously as HIV-1 patients.

Partial resistance to infection by R5X4 primary HIV type 1 isolates in an exposed-uninfected individual homozygous for CCR5 32-base pair deletion

It is known that certain individuals remain persistently seronegative despite repeated exposure to HIV-1. Studies have shown that some exposed uninfected (EU) individuals who are homozygous for a 32-bp deletion in the CCR5 gene are resistant to infection with non-syncytium-inducing (R5) viruses. In the present investigation, we provide evidence that a highly exposed-uninfected individual with the CCR5 32-bp deletion (EUdelta32-1) also has partial resistance to syncytium-inducing (R5X4) HIV-1 viruses, when compared with unexposed-uninfected individuals with (UUdelta32-1 and UUdelta32-2) and without (UU-1 and UU-2) the 32-bp deletion. The partial resistance of EU cells was due neither to altered coreceptor expression, nor to specific mutation or deletion in the coding region of chemokine coreceptors CXCR4 and CCR3. While SDF-1, the ligand for CXCR4, blocked entry of R5X4 viruses to a similar extent in EUdelta32 and UUdelta32, there was a differential production of soluble factors by EUdelta32. Both CD4+ and CD8+ cells from EUdelta32-1 produced soluble factors that efficiently suppressed infection by HIV-1 R5X4 viruses when compared with supernatant from UUdelta32. These data provide evidence that additional soluble factors are involved in resistance to infection with R5X4 viruses.

Primary human immunodeficiency virus type 2 (HIV-2) isolates infect CD4-negative cells via CCR5 and CXCR4: comparison with HIV-1 and simian immunodeficiency virus and relevance to cell tropism in vivo

Cell surface receptors exploited by human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) for infection are major determinants of tropism. HIV-1 usually requires two receptors to infect cells. Gp120 on HIV-1 virions binds CD4 on the cell surface, triggering conformational rearrangements that create or expose a binding site for a seven-transmembrane (7TM) coreceptor. Although HIV-2 and SIV strains also use CD4, several laboratory-adapted HIV-2 strains infect cells without CD4, via an interaction with the coreceptor CXCR4. Moreover, the envelope glycoproteins of SIV of macaques (SIV(MAC)) can bind to and initiate infection of CD4(-) cells via CCR5. Here, we show that most primary HIV-2 isolates can infect either CCR5(+) or CXCR4(+) cells without CD4. The efficiency of CD4-independent infection by HIV-2 was comparable to that of SIV, but markedly higher than that of HIV-1. CD4-independent HIV-2 strains that could use both CCR5 and CXCR4 to infect CD4(+) cells were only able to use one of these receptors in the absence of CD4. Our observations therefore indicate (i) that HIV-2 and SIV envelope glycoproteins form a distinct conformation that enables contact with a 7TM receptor without CD4, and (ii) the use of CD4 enables a wider range of 7TM receptors to be exploited for infection and may assist adaptation or switching to new coreceptors in vivo. Primary CD4(-) fetal astrocyte cultures expressed CXCR4 and supported replication by the T-cell-line-adapted ROD/B strain. Productive infection by primary X4 strains was only triggered upon treatment of virus with soluble CD4. Thus, many primary HIV-2 strains infect CCR5(+) or CXCR4(+) cell lines without CD4 in vitro. CD4(-) cells that express these coreceptors in vivo, however, may still resist HIV-2 entry due to insufficient coreceptor concentration on the cell surface to trigger fusion or their expression in a conformation nonfunctional as a coreceptor. Our study, however, emphasizes that primary HIV-2 strains carry the potential to infect CD4(-) cells expressing CCR5 or CXCR4 in vivo.

Detection of phylogenetically diverse human immunodeficiency virus type 1 groups M and O from plasma by using highly sensitive and specific generic primers

The high degree of genetic diversity within human immunodeficiency virus type 1 (HIV-1), which includes two major groups, M (major) and O (outlier), and various env subtypes within group M (subtypes A to J), has made designing assays that will detect all known HIV-1 strains difficult. We have developed a generic primer set based on the conserved immunodominant region of transmembrane protein gp41 that can reliably amplify as few as 10 copies/PCR of viral DNA from near-full-length clones representing group M subtypes A to H (subtypes I and J were not available). The assay is highly sensitive in detecting plasma viral RNA from HIV-1 strains of diverse geographic origins representing different subtypes of HIV-1 group M as well as HIV-1 group O. Of the 253 group M plasma specimens (subtypes A, 68 specimens; B, 71; C, 19; D, 27; E, 23; F, 33; and G, 12), 250 (98.8%) were amplified by using the gp41 M/O primer set. More importantly, all 32 (100%) group O plasma samples were also amplified with these primers. In vitro spiking experiments further revealed that the assay could reliably detect as few as 25 copies/ml of viral RNA and gave positive signals in HIV-1-seropositive specimens with plasma copy numbers below the limits of detection by all commercially available viral load assays. In addition, analysis of five seroconversion panels indicated that the assay is highly sensitive for early detection of plasma viremia during the "window period." Thus, the highly sensitive assay will be useful for early detection of HIV-1 in clinical specimens from all known HIV-1 infections, regardless of their genotypes and geographic origins.

Coreceptor requirements of primary HIV type 1 group O isolates from Cameroon

HIV-1 group O has its epicenter in Cameroon and neighboring countries and is responsible for 3 to 5% of all HIV infections in this region. It is believed that HIV-1 group O was introduced into the human population by a separate cross-species transmission, occurring independently of the HIV-1 (group M and group N) and HIV-2 transmissions. We have studied the coreceptor requirements of 12 primary HIV-1 O-type isolates from individuals with different clinical symptoms. Only 2 of these 12 viruses showed a syncytium-inducing phenotype after infection of primary peripheral blood mononuclear cells (PBMCs) and were infectious for the T cell line C8166. These isolates used CXCR4 as a coreceptor for entry, whereas the remaining isolates used only CCR5 efficiently. One isolate was able to use BOB and CCR8 as coreceptors in addition to CXCR4. All group O isolates tested were efficiently inhibited by SDF-1 or RANTES, the natural ligands of CXCR4 and CCR5, respectively. These results indicate that CXCR4 and CCR5 are the principal coreceptors for HIV-1 O-type viruses. Most of the HIV-1 group O isolates studied were derived from patients at later stages of the disease. Although HIV-1 group O and group M infections do not differ in their pathogenesis, the studied isolates did not evolve to use a broad range of coreceptors as described for HIV-1 group M and HIV-2.

Establishment of a new system for determination of coreceptor usages of HIV based on the human glioma NP-2 cell line

CD4 and one of the G-protein-coupled receptors (GPCRs) on the cell surface function as a receptor and a coreceptor, respectively, in infection of cells with human and simian immunodeficiency viruses (HIV/SIV). To determine which GPCRs can be coreceptors for HIV (HIV-1 and HIV-2) or SIV infection, several cell lines, including human osteosarcoma HOS-T4 cells and human glioma U87/CD4 cells, have been used. However, these cells often show susceptibilities to some HIV or SIV strains before transduction of GPCRs. The results of this study showed that a CD4-transduced human glioma cell line, NP-2/CD4, a human erythroleukemia cell line, K562/CD4, and a human ovarian cancer cell line, TYK/CD4, were completely resistant to the HIV-1 and HIV-2 strains tested. After transduction of several GPCRs into NP-2/CD4, K562/CD4, or TYK/CD4 cells, NP-2/CD4 cells but not K562/CD4 or TYK/CD4 cells mostly showed expected susceptibilities to several HIV strains. Therefore, an NP-2 cell system would be useful to determine the coreceptor usage of HIV isolates, to find a new coreceptor for HIV/SIV, and to analyze the early stages of HIV/SIV infection.

V3 recombinants indicate a central role for CCR5 as a coreceptor in tissue infection by human immunodeficiency virus type 1

Binding of the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein gp120 to both CD4 and one of several chemokine receptors (coreceptors) permits entry of virus into target cells. Infection of tissues may establish latent viral reservoirs as well as cause direct pathologic effects that manifest as clinical disease such as HIV-associated dementia. We sought to identify the critical coreceptors recognized by HIV-1 tissue-derived strains as well as to correlate these coreceptor preferences with site of infection and dementia diagnosis. To reconstitute coreceptor use, we cloned HIV-1 envelope V3 sequences encoding the primary determinants of coreceptor specificity from 13 brain-derived and 6 colon-derived viruses into an isogenic (NL4-3) viral background. All V3 recombinants utilized the chemokine receptor CCR5 uniformly and efficiently as a coreceptor but not CXCR4, BOB/GPR15, or Bonzo/STRL33. Other receptors such as CCR3, CCR8, and US28 were inefficiently and variably used as coreceptors by various envelopes. CCR5 without CD4 present did not allow for detectable infection by any of the tested recombinants. In contrast to the pathogenic switch in coreceptor specificity frequently observed in comparisons of blood-derived viruses early after HIV-1 seroconversion and after onset of AIDS, the characteristics of these V3 recombinants suggest that CCR5 is a primary coreceptor for brain- and colon-derived viruses regardless of tissue source or diagnosis of dementia. Therefore, tissue infection may not depend significantly on viral envelope quasispeciation to broaden coreceptor range but rather selects for CCR5 use throughout disease progression.

HIV coreceptors, cell tropism and inhibition by chemokine receptor ligands

HIV is a persistent virus that survives and replicates despite an onslaught by the host's immune system. A strategy for cell entry, requiring the use of two receptors, has evolved that may help evade neutralizing antibodies. HIV and SIV usually require both CD4 and a seven transmembrane (7TM) coreceptor for infection. At least eleven different 7TM coreceptors have been identified that confer HIV and/or SIV entry. For HIV-1, the major coreceptors are CCR5 and CXCR4, while the role of other coreceptors for replication and cell tropism in vivo is currently unclear. Polymorphisms in the CCR5 gene that reduce CCR5 expression levels, protect against disease progression, suggesting that drugs targeted to CCR5 could be effective. Such therapies however will not work if HIV simply adapts to use alternative coreceptors. In the light of these themes, this review will discuss the following topics: (i) the coreceptors used by primary HIV-1 and HIV-2 viruses, (ii) the properties and coreceptors of HIV-2 strains that infect cells without CD4, (iii) the role of coreceptors in HIV cell tropism and particularly macrophage infection and (iv) the properties of chemokine receptor ligands that block HIV infection.

Use of coreceptors other than CCR5 by non-syncytium-inducing adult and pediatric isolates of human immunodeficiency virus type 1 is rare in vitro

We have tested a panel of pediatric and adult human immunodeficiency virus type 1 (HIV-1) primary isolates for the ability to employ the following proteins as coreceptors during viral entry: CCR1, CCR2b, CCR3, CCR4, CCR5, CCR8, CXCR4, Bonzo, BOB, GPR1, V28, US28, and APJ. Most non-syncytium-inducing isolates could utilize only CCR5. All syncytium-inducing viruses used CXCR4, some also employed V28, and one (DH123) used CCR8 and APJ as well. A longitudinal series of HIV-1 subtype B isolates from an infected infant and its mother utilized Bonzo efficiently, as well as CCR5. The maternal isolates, which were syncytium inducing, also used CXCR4, CCR8, V28, and APJ.

Chemokine receptors and their crucial role in human immunodeficiency virus infection: major breakthroughs in HIV research

Within the last three years, major progress in the understanding of acquired immune deficiency syndrome pathogenesis has been achieved. The discovery that human immunodeficiency virus (HIV), in addition to the CD4 receptor, requires the presence of a coreceptor in order to infect cells has led to a series of breakthroughs in HIV research and knowledge. These include an increased understanding of viral entry, a connection of viral phenotype to specific coreceptor use, and an unequivocal linkage of a single human gene to host susceptibility. All in all these achievements provide a number of promising new strategies for combating HIV.

Adaptation to promiscuous usage of CC and CXC-chemokine coreceptors in vivo correlates with HIV-1 disease progression

OBJECTIVE

To study coreceptor usage of sequential primary HIV-1 isolates in a longitudinal follow-up cohort of HIV-1-infected men to understand its contribution to pathogenesis of HIV disease.

DESIGN

Viral coreceptor usage of sequential primary isolates from HIV-1-infected individuals was examined at various timepoints and data was compared with CD4 cell counts, rates of disease progression and beta-chemokine production.

METHODS

Fifty-eight sequential primary isolates were obtained from four rapid progressors, six late progressors, and three long-term nonprogressors (LTNP) and their coreceptor usage was examined by infection of peripheral blood mononuclear cells (PBMC) from donors with wild-type or non-functional CC-chemokine receptor (CCR)-5, and by infection of GHOST4 cells expressing CD4 and various chemokine receptors [CCR-1-CCR-5, CXC-chemokine receptor (CXCR)-4, BOB/GPR15, BONZO/STRL33]. Production of RANTES and macrophage inflammatory protein (MIP)-1beta was examined using unstimulated or phytohemagglutinin (PHA)-stimulated PBMC isolated from these individuals at multiple timepoints during infection.

RESULTS

A switch from single CCR-5 coreceptor usage to multiple coreceptor usage occurred in all four rapid progressors and three out of six late progressors. In addition to the commonly used coreceptors CXCR-4, CCR-5, and CCR-3, some of the viruses isolated from patients in the terminal stage of infection also used CCR-1, CCR-2b, CCR-4, and BOB as coreceptors. The emergence of viral variants capable of utilizing multiple coreceptors generally preceded CD4 cell decline to < 200 x 10(6)/l and correlated with the onset of AIDS. In contrast, three LTNP maintained exclusive usage of CCR-5 over a period of 7-12 years post-infection. Endogenous production of RANTES and MIP-1beta by PBMC from LTNP was not significantly different from rapid and late progressors. However, PHA-driven production of both chemokines was significantly higher in LTNP, suggesting that in vivo activating stimuli might curtail HIV replication by inducing these chemokines.

CONCLUSIONS

Viral variants capable of utilizing a broad range of coreceptors correlated with HIV-1 disease progression. In contrast, LTNP maintain exclusive usage of CCR-5 and produce higher levels of beta-chemokines. Thus, both viral and host determinants leading to the emergence of viral variants capable of using an expanded range of coreceptors may be likely determinants of disease progression.