V3-independent determinants of macrophage tropism in a primary human immunodeficiency virus type 1 isolate

T cell-tropic HIV efficiently infects alveolar macrophages through contact with infected CD4+ T cells

Alveolar macrophages (AMs) are critical for defense against airborne pathogens and AM dysfunction is thought to contribute to the increased burden of pulmonary infections observed in individuals living with HIV-1 (HIV). While HIV nucleic acids have been detected in AMs early in infection, circulating HIV during acute and chronic infection is usually CCR5 T cell-tropic (T-tropic) and enters macrophages inefficiently in vitro. The mechanism by which T-tropic viruses infect AMs remains unknown. We collected AMs by bronchoscopy performed in HIV-infected, antiretroviral therapy (ART)-naive and uninfected subjects. We found that viral constructs made with primary HIV envelope sequences isolated from both AMs and plasma were T-tropic and inefficiently infected macrophages. However, these isolates productively infected macrophages when co-cultured with HIV-infected CD4+ T cells. In addition, we provide evidence that T-tropic HIV is transmitted from infected CD4+ T cells to the AM cytosol. We conclude that AM-derived HIV isolates are T-tropic and can enter macrophages through contact with an infected CD4+ T cell, which results in productive infection of AMs. CD4+ T cell-dependent entry of HIV into AMs helps explain the presence of HIV in AMs despite inefficient cell-free infection, and may contribute to AM dysfunction in people living with HIV.

Differentiation into an Effector Memory Phenotype Potentiates HIV-1 Latency Reversal in CD4+ T Cells

By performing phenotypic analysis of latency reversal in CD4⁺ T cells from virally suppressed individuals, we identify the T_EM subset as the largest contributor to the inducible HIV reservoir. Differential responses of memory CD4⁺ T cell subsets to latency-reversing agents (LRAs) demonstrate that HIV gene expression is associated with heightened expression of transcriptional pathways associated with differentiation, acquisition of effector function, and cell cycle entry. In vitro modeling of the latent HIV reservoir in memory CD4⁺ T cell subsets identify LRAs that reverse latency with ranges of efficiency and specificity. We found that therapeutic induction of latency reversal is associated with upregulation of identical sets of T_EM-associated genes and cell surface markers shown to be associated with latency reversal in our ex vivo and in vitro models. Together, these data support the idea that the effector memory phenotype supports HIV latency reversal in CD4⁺ T cells.

During antiretroviral therapy (ART), human immunodeficiency virus type 1 (HIV-1) persists as a latent reservoir in CD4⁺ T cell subsets in central memory (T_CM), transitional memory (T_TM), and effector memory (T_EM) CD4⁺ T cells. We have identified differences in mechanisms underlying latency and responses to latency-reversing agents (LRAs) in ex vivo CD4⁺ memory T cells from virally suppressed HIV-infected individuals and in an in vitro primary cell model of HIV-1 latency. Our ex vivo and in vitro results demonstrate the association of transcriptional pathways of T cell differentiation, acquisition of effector function, and cell cycle entry in response to LRAs. Analyses of memory cell subsets showed that effector memory pathways and cell surface markers of activation and proliferation in the T_EM subset are predictive of higher frequencies of cells carrying an inducible reservoir. Transcriptional profiling also demonstrated that the epigenetic machinery (known to control latency and reactivation) in the T_EM subset is associated with frequencies of cells with HIV-integrated DNA and inducible HIV multispliced RNA. T_CM cells were triggered to differentiate into T_EM cells when they were exposed to LRAs, and this increase of T_EM subset frequencies upon LRA stimulation was positively associated with higher numbers of p24⁺ cells. Together, these data highlight differences in underlying biological latency control in different memory CD4⁺ T cell subsets which harbor latent HIV in vivo and support a role for differentiation into a T_EM phenotype in facilitating latency reversal.

IMPORTANCE By performing phenotypic analysis of latency reversal in CD4⁺ T cells from virally suppressed individuals, we identify the T_EM subset as the largest contributor to the inducible HIV reservoir. Differential responses of memory CD4⁺ T cell subsets to latency-reversing agents (LRAs) demonstrate that HIV gene expression is associated with heightened expression of transcriptional pathways associated with differentiation, acquisition of effector function, and cell cycle entry. In vitro modeling of the latent HIV reservoir in memory CD4⁺ T cell subsets identify LRAs that reverse latency with ranges of efficiency and specificity. We found that therapeutic induction of latency reversal is associated with upregulation of identical sets of T_EM-associated genes and cell surface markers shown to be associated with latency reversal in our ex vivo and in vitro models. Together, these data support the idea that the effector memory phenotype supports HIV latency reversal in CD4⁺ T cells.

eCD4-Ig promotes ADCC activity of sera from HIV-1-infected patients

Antibody-dependent cell-mediated cytotoxity (ADCC) can eliminate HIV-1 infected cells, and may help reduce the reservoir of latent virus in infected patients. Sera of HIV-1 positive individuals include a number of antibodies that recognize epitopes usually occluded on HIV-1 envelope glycoprotein (Env) trimers. We have recently described eCD4-Ig, a potent and exceptionally broad inhibitor of HIV-1 entry that can be used to protect rhesus macaques from multiple high-dose challenges with simian-human immunodeficiency virus AD8 (SHIV-AD8). Here we show that eCD4-Ig bearing an IgG1 Fc domain (eCD4-IgG1) can mediate efficient ADCC activity against HIV-1 isolates with differing tropisms, and that it does so at least 10-fold more efficiently than CD4-Ig, even when more CD4-Ig molecules bound cell surface-expressed Env. An ADCC-inactive IgG2 form of eCD4-Ig (eCD4-IgG2) exposes V3-loop and CD4-induced epitopes on cell-expressed trimers, and renders HIV-1-infected cells susceptible to ADCC mediated by antibodies of these classes. Moreover, eCD4-IgG2, but not IgG2 forms of the broadly neutralizing antibodies VRC01 and 10-1074, enhances the ADCC activities of serum antibodies from patients by 100-fold, and significantly enhanced killing of two latently infected T-cell lines reactivated by vorinostat or TNFα. Thus eCD4-Ig is qualitatively different from CD4-Ig or neutralizing antibodies in its ability to mediate ADCC, and it may be uniquely useful in treating HIV-1 infection or reducing the reservoir of latently infected cells.

High-Sequence Diversity and Rapid Virus Turnover Contribute to Higher Rates of Coreceptor Switching in Treatment-Experienced Subjects with HIV-1 Viremia

Coreceptor switching from CCR5 to CXCR4 is common during chronic HIV-1 infection, but is even more common in individuals who have failed antiretroviral therapy (ART). Prior studies have suggested rapid mutation and/or recombination of HIV-1 envelope (env) genes during coreceptor switching. We compared the functional and genotypic changes in env of viruses from viremic subjects who had failed ART just before and after coreceptor switching and compared those to viruses from matched subjects without coreceptor switching. Analysis of multiple unique functional env clones from each subject revealed extensive diversity at both sample time points and rapid diversification of sequences during the 4-month interval in viruses from both 9 subjects with coreceptor switching and 15 control subjects. Only two subjects had envs with evidence of recombination. Three findings distinguished env clones from subjects with coreceptor switching from controls: (1) lower entry efficiency via CCR5; (2) longer V1/V2 regions; and (3), lower nadir CD4 T cell counts during prior years of infection. Most of these subjects harbored virus with lower replicative capacity associated with protease (PR) and/or reverse transcriptase inhibitor resistance mutations, and the extensive diversification tended to lead either to improved entry efficiency via CCR5 or the gain of entry function via CXCR4. These results suggest that R5X4 or X4 variants emerge from a diverse, low-fitness landscape shaped by chronic infection, multiple ART resistance mutations, the availability of target cells, and reduced entry efficiency via CCR5.

HIV-1 tropism testing and clinical management of CCR5 antagonists: Quebec review and recommendations

HIV-1 tropism assays play a crucial role in determining the response to CCR5 receptor antagonists. Initially, phenotypic tests were used, but limited access to these tests prompted the development of alternative strategies. Recently, genotyping tropism has been validated using a Canadian technology in clinical trials investigating the use of maraviroc in both experienced and treatment-naive patients. The present guidelines review the evidence supporting the use of genotypic assays and provide recommendations regarding tropism testing in daily clinical management.

Comparison of in vivo and in vitro evolution of CCR5 to CXCR4 coreceptor use of primary human immunodeficiency virus type 1 variants

During the course of at least 50% of HIV-1 subtype B infections, CCR5-using (R5) viruses evolve towards a CXCR4-using phenotype. To gain insight in the transition from CCR5 to CXCR4 coreceptor use, we investigated whether acquisition of CXCR4 use in vitro of R5 viruses from four patients resembled this process in vivo. R5 variants from only one patient acquired CXCR4 use in vitro. These variants had envelopes with higher V3 charge and higher number of potential N-linked glycosylation sites when compared to R5 variants that failed to gain CXCR4 use in vitro. In this patient, acquisition of CXCR4 use in vitro and in vivo was associated with multiple mutational patterns not necessarily involving the V3 region. However, changes at specific V3 positions were prerequisite for persistence of CXCR4-using variants in vivo, suggesting that positive selection targeting the V3 loop is required for emergence of CXCR4-using variants during natural disease course.

The antiviral spectra of TRIM5α orthologues and human TRIM family proteins against lentiviral production

Background

Rhesus monkey TRIM5α (TRIM5αrh) recognizes the incoming HIV-1 core through its C-terminal B30.2(PRYSPRY) domain and promotes its premature disassembly or degradation before reverse transcription. Previously, we have shown that TRIM5αrh blocks HIV-1 production through the N-terminal RBCC domain by the recognition of Gag polyproteins. Although all TRIM family proteins have RBCC domains, it remains elusive whether they possess similar late-restriction activities.

Methodology/Principal Findings

We examined the antiviral spectra of TRIM5α orthologues and human TRIM family members which have a genetic locus proximal to human TRIM5α (TRIM5αhu), against primate lentiviral production. When HIV-1 virus-like particles (VLPs) were generated in the presence of TRIM5α proteins, rhesus, African green and cynomolgus monkey TRIM5α (TRIM5αag and TRIM5αcy), but not TRIM5αhu, were efficiently incorporated into VLPs, suggesting an interaction between HIV-1 Gag and TRIM5α proteins. TRIM5αrh potently restricted the viral production of HIV-1 groups M and O and HIV-2, but not simian lentiviruses including SIV_MAC1A11, SIV_AGMTan-1 or SIV_AGMSAB-1. TRIM5αhu did not show notable late restriction activities against these lentiviruses. TRIM5αag and TRIM5αcy showed intermediate restriction phenotypes against HIV-1 and HIV-2, but showed no restriction activity against SIV production. A series of chimeric TRIM5α constructs indicated that the N-terminal region of TRIM5αag and TRIM5αcy are essential for the late restriction activity, while the C-terminal region of TRIM5αcy negatively regulates the late restriction activity against HIV-1. When select human TRIM family proteins were examined, TRIM21 and 22 were efficiently incorporated into HIV-1 VLPs, while only TRIM22 reduced HIV-1 titers up to 5-fold. The antiviral activities and encapsidation efficiencies did not correlate with their relative expression levels in the producer cells.

Conclusions/Significance

Our results demonstrated the variations in the late restriction activities among closely related TRIM5α orthologues and a subset of human TRIM family proteins, providing further insights into the late restriction activities of TRIM proteins.

Genetic variability of gag and env regions of HIV type 1 strains circulating in Slovenia

The aim of this study was to investigate the genetic diversity of HIV-1 strains circulating in Slovenia. Proviral DNA isolated from peripheral blood mononuclear cells (PBMCs) of 20 randomly selected HIV-1-infected individuals was classified into subtypes by sequence-based phylogenetic analysis of the env (C2V3) and gag (p24) regions of the viral genome. The phylogenetic tree based on env C2V3 sequences showed that 15 of the 20 samples were subtype B, two A1, one F1, one CRF01_AE, and one CRF02_AG. The phylogenetic analysis of the gag gene yielded identical results expect for one sample that had a discordant subtype; it was identified as subtype A1 in the env and AE in the gag region. Our study confirmed that although subtype B predominates, other subtypes and circulating recombinant forms (CRFs) are also present in Slovenia. The high intrasubtype genetic diversity of subtype B sequences suggests a multiple introduction of subtype B strains into Slovenia.

Comparative study on the structure and cytopathogenic activity of HIV Vpr/Vpx proteins

The three-dimensional (3-D) structure of human immunodeficiency virus type 2 (HIV-2) Vpr/Vpx was predicted by homology modeling based on the NMR structure of human immunodeficiency virus type 1 (HIV-1) Vpr. The three proteins similarly have three major amphipathic alpha-helices. In contrast to HIV-1 Vpr, Vpr/Vpx of HIV-2 have a long N-terminal loop and clustered prolines in the second half of the C-terminal loop. HIV-2 Vpx uniquely contains a long region between the second and third major helices, and bears several glycines in the first half of the C-terminal loop. Instead of the glycines, there is a group of hydrophilic amino acids and arginines in the corresponding regions of the two Vprs. To compare the cytopathogenic potentials of HIV-1 Vpr and HIV-2 Vpr/Vpx, we examined the production of luciferase as a marker of cell damage. We further analyzed the characteristics of cells transduced with vpr/vpx genes driven by an inducible promoter. The results obtained clearly show that structurally similar, but distinct, HIV Vpr/Vpx proteins are detrimental to target cells.

Effect of partial and complete variable loop deletions of the human immunodeficiency virus type 1 envelope glycoprotein on the breadth of gp160-specific immune responses

Induction of cross-reactive cellular and humoral responses to the HIV-1 envelope (env) glycoprotein was examined after DNA immunization of BALB/c mice with gp140(89.6)-derived constructs exhibiting partial or complete deletions of the V1, V2, and V3 domains. It was demonstrated that specific modification of the V3 loop (mV3) in combination with the V2-modified (mV2) or V1/V2-deleted (DeltaV1/V2) region elicited increased levels of cross-reactive CD8(+) T cell responses. Mice immunized with the mV2/mV3 or DeltaV1/V2/mV3 gp140(89.6) plasmid DNA were greater than 50-fold more resistant to challenge with recombinant vaccinia virus (rVV) expressing heterologous env gene products than animals immunized with the wild-type (WT) counterpart. Sera from mV2/mV3- and DeltaV1/V2/mV3-immunized mice exhibited the highest cross-neutralizing activity and displayed intermediate antibody avidity values which were further enhanced by challenge with rVV expressing the homologous gp160 glycoprotein. In contrast, complete deletion of the variable regions had little or no effect on the cross-reactive antibody responses. The results of these experiments indicate that the breadth of antibody responses to the HIV-1 env glycoprotein may not be increased by removal of the variable domains. Instead, partial deletions within these regions may redirect specific responses toward conserved epitopes and facilitate approaches for boosting cross-reactive cellular and antibody responses to the env glycoprotein.

Differential prevalence of HIV type 1 subtype B and CRF01_AE among different sexual transmission groups in Tokyo, Japan, as revealed by subtype-specific PCR

We determined the subtype of HIV-1 in 89 infected individuals attending three reference hospitals located in the Tokyo metropolitan area of Japan. Subtyping was performed with subtype-specific polymerase chain re-action (PCR) distinguishing subtype A, B, C, and CRF01_AE and/or phylogenetic analysis of HIV-1 env C2V3C3 sequences. Subtype-specific PCR provided unequivocal results in 97% of samples. Sixty-five subjects were infected with subtype B, 16 with CRFO1_AE, 4 with subtype A, 1 with CRF02_AG, and 3 with subtype C. Among 31 Japanese individuals infected through heterosexual contact, 13 were infected with subtype Band 12 with CRFO1_AE. All of the 41 Japanese men infected through homosexual contact harbored subtype B. These results indicate that subtype B is exclusively predominant in a homosexually transmitted group, whereas subtype B and CRFO1_AE are evenly predominant in a heterosexually transmitted group.

Phylogenetic and phenotypic analysis of HIV type 1 env gp120 in cases of subtype C mother-to-child transmission

Human immunodeficiency virus type 1 (HIV-1) subtype C constitutes a majority of the newly transmitted cases of HIV-1 in many developing countries. There is growing evidence suggesting that subtype C viruses may display characteristics that make them distinct from subtype B and other subtypes, and such differences may affect its transmission and pathogenesis in infected individuals. In this study, HIV-1 sequences from the C2-V4 region of the envelope gene were analyzed from four infected mother-infant pairs (MIPs) after perinatal transmission in Zambia. We found that all viral isolates from four Zambian MIPs were of the subtype C HIV-1 subgroup. All tested isolates were macrophage tropic and did not infect any tested T cell lines or form syncytia in vitro. In addition, the isolates appear to use exclusively the CCR5 coreceptor. Phylogenetic analyses clearly indicated two contrasting phylogenetic transmission patterns for the four MIPs analyzed in the present study. Three cases displayed a pattern of selective transmission of a single or a few variants, and one case was found to transmit multiple maternal HIV-1 variants to her infant. The differences in the mode and timing of transmission may account for the observed transmission pattern among four MIPs.

Molecular epidemiology of HIV type 1 infection in Portugal: high prevalence of non-B subtypes

In this study, we have investigated the diversity of current HIV-1 strains circulating in the metropolitan area of Lisbon, Portugal. A total of 217 HIV-1-positive blood samples, collected between October 1998 and December 2000, was genetically characterized in the gp120 C2V3C3 region (n = 205) or part of the gp41 N-terminal segment (n = 12) by heteroduplex mobility assay (HMA) and/or DNA sequencing. The HMA subtyping efficiency (number of samples unambiguously subtyped by HMA divided by the total number of samples subtyped) was 65.9% (143 of 217), with indeterminate migration patterns of subtype A and G strains contributing significantly to this value. On the overall, subtype B was the most prevalent (50.2%), followed by subtypes G (21.7%), A (17.5%), and F (5.5%), whereas subtypes C, D, H, and J accounted altogether for 5.1% of the infections. Non-B subtypes were responsible for 77.4 and 33.1% of the infections among African immigrants and Portuguese subjects, respectively. Angolan individuals (n = 25) were the only ones infected with all the HIV-1 subtypes documented, probably reflecting a high degree of viral genetic diversification in their country of origin. Phylogenetic analysis showed a predominance of IbNG-like viruses among subtype A sequences and two new major subclusters within subtype G (G(P) and G(P)'). The majority of the Portuguese G sequences described formed a well-defined subcluster (G(P)), supported by bootstrap values >90%, phylogenetically distant from clade G sequences in databases. gag (p24/p7) sequence analysis of these variants confirmed the maintenance of the subtype G subclusters. The multiple subclustering observed for the major clades A, B, D, and G, as well as the variety of subtypes found, indicate a high diversity of HIV-1 variants circulating in Portugal and suggest a need for continuous epidemiologic surveillance.

Rhabdovirus-based vectors with human immunodeficiency virus type 1 (HIV-1) envelopes display HIV-1-like tropism and target human dendritic cells

We describe replication-competent, vaccine strain-based rabies viruses (RVs) that lack their own single glycoprotein and express, instead, a chimeric RV-human immunodeficiency virus type 1 (HIV-1) envelope protein composed of the ectodomain and transmembrane domains of HIV-1 gp160 and the cytoplasmic domain of RV G. The envelope proteins from both X4 (NL4-3)- and R5X4 (89.6)-tropic HIV-1 strains were utilized. These recombinant viruses very closely mimicked an HIV-1- like tropism, as indicated by blocking experiments. Infection was inhibited by SDF-1 on cells expressing CD4 and CXCR4 for both viruses, whereas RANTES abolished infection of cells expressing CCR5 in addition to CD4 in studies of the RV expressing HIV-1(89.6) Env. In addition, preincubation with soluble CD4 or monoclonal antibodies directed against HIV-1 gp160 blocked the infectivity of both G-deficient viruses but did not affect the G-containing RVs. Our results also indicated that the G-deficient viruses expressing HIV-1 envelope protein, in contrast to wild-type RV but similar to HIV-1, enter cells by a pH-independent pathway. As observed for HIV-1, the surrogate viruses were able to target human peripheral blood mononuclear cells, macrophages, and immature and mature human dendritic cells (DC). Moreover, G-containing RV-based vectors also infected mature human DC, indicating that infection of these cells is also supported by RV G. The ability of RV-based vectors to infect professional antigen-presenting cells efficiently further emphasizes the potential use of recombinant RVs as vaccines.

HIV-1 LTR C/EBP binding site sequence configurations preferentially encountered in brain lead to enhanced C/EBP factor binding and increased LTR-specific activity

Recent studies have shown that two CAAT/enhancer binding protein (C/EBP) sites are critically important for efficient human immunodeficiency virus (HIV) type 1 (HIV-1) replication within cells of the monocyte/macrophage lineage, a primary cell type infected by HIV-1 and a potentially important vehicle for transport of virus to the central nervous system (CNS). Given the relevance of HIV-1 LTR sequence variation with respect to HIV-1 replication within monocyte populations and the important role that monocyte tropism likely plays in HIV-1 infection of the brain, C/EBP site sequence variation was examined within peripheral blood- and brain-derived LTR populations. Brain-derived LTRs commonly possessed a C/EBP site I configuration (6G, comprised of a thymidine to guanosine substitution with respect to the clade B consensus sequence at position 6 of C/EBP site I) that leads to enhanced binding of C/EBP proteins over that observed with the HIV-1 clade B consensus sequence at this site. In contrast, the 6GC/EBP site I configuration appeared infrequently within sequenced peripheral blood-derived LTRs. In addition, C/EBP site II was even more highly conserved in brain-derived HIV-1 LTR populations than site I. This was not the case with peripheral blood-derived LTR C/EBP site II sequences. The high degree of C/EBP site II conservation in brain-derived LTRs was likely important in LTR regulation since the clade B consensus sequence conserved at C/EBP site II recruited high amounts of C/EBP family members. Transient transfection analyses indicated that conservation of the strong C/EBP site II in brain-derived LTRs was likely due to important interactions with Tat. Overall, brain-derived HIV-1 LTRs preferentially contained two highly reactive C/EBP binding sites, which may suggest that these sites play important roles in LTR-directed transcription during invasion and maintenance of HIV-1 in the central nervous system.

Emergence of new HIV-1 subtypes other than Subtype C among antenatal women in Lusaka, Zambia

Molecular epidemiology of HIV-1 in Zambia was investigated by direct sequencing of PCR products from samples collected from antenatal attendees in Lusaka, Zambia. One hundred and forty samples were initially screened for HIV, using antibody assays. Thirty-three (23.6%) samples were HIV-1 positive. Sequences of the HIV-1 env gp120 region were obtained from 28 of 33 (85%) HIV-1-positive samples. Twenty-six of the 28 sequences were HIV-1 env subtype C-like as previously reported. However, one HIV-1 env subtype D-like virus and one HIV-1 env subtype G-like virus were identified. This is the first time that these two HIV-1 env subtype viruses have been identified in Zambia, suggesting that more subtypes could be in existence.

Evolution of the human immunodeficiency virus type 1 envelope during infection reveals molecular corollaries of specificity for coreceptor utilization and AIDS pathogenesis

The evolution of human immunodeficiency virus type 1 infection is associated with a shift in the target cell population, driven by variability in coreceptor utilization resulting from diversity in env. To elucidate the potential consequences of these changes for Env-mediated fusion over the course of AIDS, we examined the biological properties of serial viral isolates and determined coreceptor utilization by the products of env cloned from two individuals, followed from the detection of seroconversion throughout the course of their infection. One had a typical course, and the other had an accelerated progression. Early isolates were non-syncytium inducing, and the corresponding Env exclusively utilized CCR5, whereas Env from late phases of infection showed restricted utilization of CXCR4 in both patients. Env from subject SC24, who had a standard progression, demonstrated multitropism, manifested by utilization of CCR3, CXCR4, and CCR5 in the intervening period. In contrast, Env from patient SC51, who experienced early conversion to the syncytium-inducing phenotype, developed dualtropic coreceptor utilization of CCR5 and CXCR4. Genetic analysis of env from each isolate revealed that those with an X4 phenotype formed a distinct subcluster within each subject. Analysis of chimeras constructed from R5 and multispecific env from patient SC24 demonstrated that while the V3 domain played a dominant role in determining coreceptor utilization, sequences in the V4-V5 region also contributed to the latter phenotype. Immunoprecipitation experiments confirmed that the hybrid Env proteins were expressed at similar levels. These experiments demonstrate that progression from the R5 to X4 phenotype may occur through a multi- or dual-tropic intermediate and that multiple domains contribute to this process.

Heterogeneous spectrum of coreceptor usage among variants within a dualtropic human immunodeficiency virus type 1 primary-isolate quasispecies

Human immunodeficiency virus type 1 (HIV-1) variants that use the coreceptor CCR5 for entry (R5; macrophage tropic) predominate in early infection, while variants that use CXCR4 emerge during disease progression. Some late-stage variants use CXCR4 alone (X4; T-cell tropic), while others use both CXCR4 and CCR5 (R5X4; dualtropic). It has been proposed that dualtropic R5X4 strains are intermediates in the evolution from R5 to X4, and we hypothesized that a dualtropic primary-isolate quasispecies might contain variants that represent the spectrum of coreceptor use in vivo. We generated a panel of 35 functional full-length env clones from the primary-isolate quasispecies of a dualtropic prototype strain, HIV-1 89.6(PI). Thirty of the functional env clones (86%) were R5X4, four (11%) were R5, and one (3%) was predominantly X4. V3 to V5 sequences did not reveal clustering by coreceptor usage, and no specific sequence motif or V3 charge pattern corresponded to coreceptor utilization. Complete sequencing of seven functionally divergent Env proteins revealed > or =98.7% homology and conservation of structurally important domains. Chimeras between the R5X4 89.6 prototype and an R5 variant indicated that multiple regions contributed to the use of CXCR4, while chimeras with the X4 variant implicated a single residue in V4 in CCR5 use. These results confirm, at the molecular level, both that dualtropic variants are a predominant component of late-stage syncytium-inducing isolates and that variants restricted to each coreceptor coexist with dualtropic species in vivo. Coreceptor-restricted minority variants may reflect residual R5 species from earlier in disease as well as emerging X4 variants.

Analysis of the critical domain in the V3 loop of human immunodeficiency virus type 1 gp120 involved in CCR5 utilization

Human immunodeficiency virus type 1 (HIV-1) infection of CD4(+) lymphocytes and macrophages involves interaction of the surface subunit of the envelope protein (gp120) with coreceptors. Isolates have been found with specific tropism for macrophages and/or T-cell lines, through the utilization of chemokine receptor CCR5 (R5) or CXCR4 (X4). The third hypervariable loop (V3 loop) of gp120 is the major determinant of tropism. Using chimeric envelopes between HXB2 (X4) and ADA (R5), we found that the C-terminal half of the V3 loop was sufficient to confer on HXB2 the ability to infect CCR5-expressing cells. A sequence motif was identified at positions 289 to 292 allowing 30% of wild-type levels of infection, whereas full activity was achieved with the conversion of Lys to Glu at position 287 in addition to the above motif. Moreover, V3 loops from either SF2 (X4R5) or SF162 (R5) also allowed infection of CCR5-expressing cells, supporting the importance of V3 loops in influencing CCR5 utilization. The effects of amino acid changes at position 287 on the level of infection via CCR5 showed that negatively charged residues (Glu and Asp) were optimal for efficient interaction whereas only bulky hydrophobic residues drastically reduced infection. In addition, sequences at the N terminus of the V3 loop independently modulated the level of infection via CCR5. This study also examined the susceptibility of chimeric envelopes to neutralization by anticoreceptor antibodies and suggested the presence of differential interaction between the chimeric envelopes and CCR5. These findings highlight the critical residues in the V3 loop that mediate HIV-1 infection.

Expanded tropism of primary human immunodeficiency virus type 1 R5 strains to CD4(+) T-cell lines determined by the capacity to exploit low concentrations of CCR5

Human immunodeficiency virus type 1 (HIV-1) non-syncytium-inducing (NSI) strains predominantly use the chemokine receptor CCR5, while syncytium-inducing (SI) strains use CXCR4. In vitro, SI isolates infect and replicate in a range of CD4(+) CXCR4(+) T-cell lines, whereas NSI isolates usually do not. Here we describe three NSI strains that are able to infect two CD4(+) T-cell lines, Molt4 and SupT1. For one strain, a variant of JRCSF selected in vitro, replication on Molt4 was previously shown to be conferred by a single amino-acid change in the V1 loop (M.T. Boyd et al., J. Virol. 67:3649-3652, 1993). On CD4(+) cell lines expressing different coreceptors, these strains use CCR5 predominantly and do not replicate in CCR5-negative peripheral blood mononuclear cells derived from individuals homozygous for Delta32 CCR5. Furthermore, infection of Molt4 and SupT1 by each of these three strains is potently inhibited by ligands for CCR5, including 2D7, a monoclonal antibody specific for CCR5. CCR5 mRNA was present in both Molt4 and SupT1 by reverse transcription-PCR, although CCR5 protein could not be detected either on the cell surface or in intracellular vesicles. The expanded tropism of the three strains shown here is therefore not due to adaptation to a new coreceptor but due to the capacity to exploit extremely low levels of CCR5 on Molt4 and SupT1 cells. This novel tropism observed for a subset of primary HIV-1 isolates may represent an extended tropism to new CD4(+) cell types in vivo.

Role of the HIV type 1 glycoprotein 120 V3 loop in determining coreceptor usage

Macrophage (M)-tropic HIV-1 isolates use the beta-chemokine receptor CCR5 as a coreceptor for entry, while T cell line-adapted (TCLA) strains use CXCR4 and dual-tropic strains can use either CCR5 or CXCR4. To investigate the viral determinants involved in choice of coreceptor, we used a fusion assay based on the infection of CD4+ HeLa cells that express one or both coreceptors with Semliki Forest virus (SFV) recombinants expressing the native HIV-1 gp160 of a primary M-tropic isolate (HIV-1BX08), a TCLA isolate (HIV-1LAI), or a dual-tropic strain (HIV-1MN). We examined whether the V3 region of these glycoproteins interacts directly with the corresponding coreceptors by assaying coreceptor-dependent cell-to-cell fusion mediated by the different recombinants in the presence of various synthetic linear peptides. Synthetic peptides corresponding to different V3 loop sequences blocked syncytium formation in a coreceptor-specific manner. Synthetic V2 peptides were also inhibitory for syncytium formation, but showed no apparent coreceptor specificity. A BX08 V3 peptide with a D320 --> R substitution retained no inhibitory capacity for BX08 Env-mediated cell-to-cell fusion, but inhibited LAI Env-mediated fusion as efficiently as the homologous LAI V3 peptide. The same mutation engineered in the BX08 env gene rendered it able to form syncytia on CD4+CXCR4+CCR5-HeLa cells and susceptible to inhibition by SDF-1alpha and MIP-1beta. Other substitutions tested (D320 --> Q/D324 --> N or S306 --> R) exhibited intermediate effects on coreceptor usage. These results underscore the importance of the V3 loop in modulating coreceptor choice and show that single amino acid modifications in V3 can dramatically modify coreceptor usage. Moreover, they provide evidence that linear V3 loop peptides can compete with intact cell surface-expressed gp120/gp41 for CCR5 or CXCR4 interaction.

Protection of Macaques against pathogenic simian/human immunodeficiency virus 89.6PD by passive transfer of neutralizing antibodies

The role of antibody in protection against human immunodeficiency virus (HIV-1) has been difficult to study in animal models because most primary HIV-1 strains do not infect nonhuman primates. Using a chimeric simian/human immunodeficiency virus (SHIV) based on the envelope of a primary isolate (HIV-89.6), we performed passive-transfer experiments in rhesus macaques to study the role of anti-envelope antibodies in protection. Based on prior in vitro data showing neutralization synergy by antibody combinations, we evaluated HIV immune globulin (HIVIG), and human monoclonal antibodies (MAbs) 2F5 and 2G12 given alone, compared with the double combination 2F5/2G12 and the triple combination HIVIG/2F5/2G12. Antibodies were administered 24 h prior to intravenous challenge with the pathogenic SHIV-89.6PD. Six control monkeys displayed high plasma viremia, rapid CD4(+)-cell decline, and clinical AIDS within 14 weeks. Of six animals given HIVIG/2F5/2G12, three were completely protected; the remaining three animals became SHIV infected but displayed reduced plasma viremia and near normal CD4(+)-cell counts. One of three monkeys given 2F5/2G12 exhibited only transient evidence of infection; the other two had marked reductions in viral load. All monkeys that received HIVIG, 2F5, or 2G12 alone became infected and developed high-level plasma viremia. However, compared to controls, monkeys that received HIVIG or MAb 2G12 displayed a less profound drop in CD4(+) T cells and a more benign clinical course. These data indicate a general correlation between in vitro neutralization and protection and suggest that a vaccine that elicits neutralizing antibody should have a protective effect against HIV-1 infection or disease.

HIV-1 envelope determinants for cell tropism and chemokine receptor use

Isolates of human immunodeficiency virus type-1 (HIV-1) display marked differences in their ability to replicate in macrophages and transformed T-cell lines in vitro, a property that has important implications for disease pathogenesis. The restriction in replication between these two CD4-positive cell types is largely at the level of viral entry and is regulated by the viral envelope (env) gene. The envelope protein (Env) is responsible for fusion of the viral and host membranes, and a particular region of Env called the V3-loop has been implicated in regulating viral tropism. However, other regions of Env, such as the V1- and V2-loops, have been shown to modulate the effects of the V3-loop. The discovery that Env initially binds the CD4 molecule on the target cell surface and then makes subsequent interactions with one of several members of the chemokine receptor family has greatly enhanced the molecular understanding of HIV-1 entry. The differential use of chemokine receptors by different viral isolates and their expression in different cell types largely explains viral tropism. The same regions in Env responsible for virus tropism have also been shown to play an important role in mediating chemokine receptor use. The recent crystallization of HIV-1 Env in complex with CD4 illuminates the architecture of the components involved in mediating fusion between the viral and host membranes. The spatial relationship between variable structures of Env previously implicated in tropism and chemokine receptor use and conserved Env structures potentially involved in chemokine receptor binding are discussed.

T-tropic human immunodeficiency virus type 1 (HIV-1)-derived V3 loop peptides directly bind to CXCR-4 and inhibit T-tropic HIV-1 infection

Certain types of chemokine receptors have been identified as coreceptors for HIV-1 infection. The process of viral entry is initiated by the interaction between an envelope protein gp120 of HIV-1, CD4, and one of the relevant coreceptors. To understand the precise mechanism of the Env-mediated fusion and entry of HIV-1, we examined whether the V3 region of gp120 of T-cell line tropic (T-tropic) virus directly interacts with the coreceptor, CXCR-4, by using five synthetic V3 peptides: two cyclized V3 peptides (V3-BH10 and V3-ELI) which correspond to the V3 regions of the T-tropic HIV-1 IIIB and HIV-1 ELI strains, respectively, a linear V3 peptide (CTR36) corresponding to that of HIV-1 IIIB strain; and cyclized V3 peptides corresponding to that of the macrophage-tropic (M-tropic) HIV-1 ADA strain (V3-ADA) or the dualtropic HIV-1 89.6 strain (V3-89. 6). FACScan analysis with a CXCR-4(+) human B-cell line, JY, showed that V3-BH10, V3-ELI, and V3-89.6 but not CTR36 or V3-ADA blocked the binding of IVR7, an anti-CXCR-4 monoclonal antibody (MAb), to CXCR-4 with different magnitudes in a dose-dependent manner, while none of the V3 peptides influenced binding of an anti-CD19 MAb at all. Next, the effects of the V3 peptides on SDF-1beta-induced transient increases in intracellular Ca2+ were investigated. Three V3 peptides (V3-BH10, V3-ELI, and V3-89.6) prevented Ca2+ mobilization. Furthermore, the three peptides inhibited infection by T-tropic HIV-1 in a dose-dependent manner as revealed by an MTT assay and a reverse transcriptase assay, while the other peptides had no effects. These results present direct evidence that the V3 loop of gp120 of T-tropic HIV-1 can interact with its coreceptor CXCR-4 independently of the V1/V2 regions of gp120 or cellular CD4.

Rhesus macaques infected with macrophage-tropic simian immunodeficiency virus (SIVmacR71/17E) exhibit extensive focal segmental and global glomerulosclerosis

We previously showed that inoculation of rhesus macaques with molecularly cloned lymphocytetropic simian immunodeficiency virus (SIVmac239) results in SIV-associated nephropathy (SIVAN) and that the glomerulosclerotic lesions were associated with the selection of macrophagetropic (M-tropic) variants (V. H. Gattone et al., AIDS Res. Hum. Retroviruses 14:1163-1180, 1998). In the present study, seven rhesus macaques were inoculated with M-tropic SIVmacR71/17E, and the renal pathology was examined at necropsy. All SIVmacR71/17E-infected macaques developed AIDS, and most developed other systemic complications, including SIV-induced encephalitis and lentivirus interstitial pneumonia. There was no correlation between the length of infection (42 to 97 days), circulating CD4(+) T-cell counts, and renal disease. Of the seven macaques inoculated with SIVmacR71/17E, five developed significant mesangial hyperplasia and expansion of matrix and four were clearly azotemic (serum urea nitrogen concentration of 40 to 112 mg/dl). These same five macaques developed focal segmental to global glomerulosclerotic lesions. Increased numbers of glomerular CD68(+) cells (monocytes/macrophages) were found in glomeruli but not the tubulointerstitium of the macaques inoculated with SIVmacR71/17E. All macaques had glomerular deposits of immunoglobulin G (IgG), IgM, and tubuloreticular inclusions, and six of seven had IgA deposition. However, there was no correlation between the presence of circulating anti-SIVmac antibodies, immunoglobulin deposition, and glomerular disease. Tubulointerstitial infiltrates were mild, with little or no correlation to azotemia, while microcystic tubules were evident in those with glomerulosclerosis or azotemia. The four most severely affected macaques were positive for diffuse glomerular immunostaining for viral core p27 antigen, and there was intense staining in the glomeruli of the two macaques with the most severe glomerulosclerosis. Viral sequences were isolated from glomerular and tubulointerstitial fractions from macaques with severe glomerulosclerosis but only from the tubulointerstitial compartment of those that did not develop glomerulosclerosis. Interviral recombinant viruses generated with env sequences isolated from glomeruli confirmed the M-tropic nature of the virus found in the glomeruli. The correlation between the increased number of CD68(+) cells (monocytes/macrophages) in the glomeruli, the localization of p27 antigen in the glomeruli, and the glomerular pathology confirms and extends our previous observations of an association between glomerular infection and infiltration by M-tropic virus and SIVAN.

Determinants of entry cofactor utilization and tropism in a dualtropic human immunodeficiency virus type 1 primary isolate

Human immunodeficiency virus type 1 strain 89.6 is a dualtropic isolate that replicates in macrophages and transformed T cells, and its envelope mediates CD4-dependent fusion and entry with CCR5, CXCR-4, and CCR3. To map determinants of cofactor utilization by 89.6 and determine the relationship between cofactor use and tropism, we analyzed recombinants generated between 89.6 and T-cell-tropic (HXB) or macrophage-tropic (JRFL) strains. These chimeras showed that regions of 89.6 env outside V3 through V5 determine CXCR-4 utilization and T-cell line tropism as well as CCR5 utilization and macrophage tropism. However, the 89.6 env V3 domain also conferred on HXB the ability to use CCR5 for fusion and entry but not the ability to establish productive macrophage infection. CCR3 use was conferred on HXB by 89.6 env V3 or V3 through V5 sequences. While replacement of the 89.6 V3 through V5 region with HXB sequences abrogated CCR3 utilization, replacement of V3 or V4 through V5 separately did not. Thus, CCR3 use is determined by sequences within V3 through V5 and most likely can be conferred by either the V3 or the V4 through V5 domains. These results indicate that cofactor utilization and tropism in this dualtropic isolate are determined by complex interactions among multiple env segments, that distinct regions of the Env glycoprotein may be important for utilization of different chemokine receptors, and that determinants in addition to cofactor usage participate in postentry stages in the virus replication cycle that contribute to target cell tropism.

Viral determinants of HIV-1 sufficient to extend tropism to macrophages are distinct from the determinants that control the cytopathic phenotype in HL-60 cells

DESIGN

Infection of the human promyelocytic cell line HL-60 with NL4-3, a molecularly cloned HIV-1 strain that productively infects T cells, results in adaptation of the virus and production of a variant, NL4-3(M). Unlike NL4-3, NL4-3(M) has a rapid cytopathic effect in HL-60 and other myeloid cell lines.

OBJECTIVE

To demonstrate that the tropism of NL4-3(M) is extended to primary monocyte-derived macrophages (MDM), and to determine whether the envelope gene, env, of NL4-3(M) is responsible for cytopathicity in HL-60 cells and replication in MDM.

METHODS

A chimeric virus (NL4-3envA) containing the majority of env of NL4-3(M) was generated, and tested for virus replication and cytopathic effect in H9 and HL-60 cells, as well as for virus replication in primary MDM. To assess virus replication, the cultures were analyzed for expression of viral envelope glycoproteins on the infected cells and production of extracellular HIV-1 p24 antigen. Cytopathic effect on HL-60 cells was evaluated by monitoring the viabilities of the cultures. In addition, the majority of env of NL4-3envA was sequenced.

RESULTS

The biological phenotypes of NL4-3, NL4-3(M), and NL4-3envA are distinctly different. Although both NL4-3(M) and NL4-3envA replicate in MDM, only NL4-3(M) is rapidly cytopathic in HL-60 cells. Nine amino-acid changes were identified within the envelope glycoproteins of NL4-3envA compared with NL4-3.

CONCLUSIONS

The viral determinants of NL4-3(M) sufficient to extend the tropism of this virus to MDM reside, in part, in env. These genetic determinants are distinct from the viral determinants that control the cytopathic phenotype of this virus in HL-60 cells.

Host-cell-specific glycosylation of HIV-2 envelope glycoprotein

Neutral complex-type N-glycans of the envelope glycoprotein 120 of HIV-2, propagated in different host cells, display cell-type specific variations. In order to identify typical structural elements, glycans were analysed by gel filtration, by enzymic sequencing and, in part, by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. The characteristic substituents of di- tri- and tetraantennary carbohydrate units thus observed include N-acetyllactosamine repeats, bisecting N-acetylglucosamine and fucose linked to the chitobiose core as well as to N-acetyllactosamine antennae. Each glycoprotein preparation displayed a characteristic set of glycoforms.

Molecular characterization of HIV viruses generated after in vivo ligation

During the course of infection, human immunodeficiency virus type 1 (HIV-1) displays wide genotypic and phenotypic differences. Construction of chimeric viruses is useful to determine the genotypic basis that underlies phenotypic variations, but the procedure is time-consuming. Previously, it has been shown that co-transfection of truncated hemi-genomic HIV-1 proviral DNA can lead to generation of full-length infectious virus. In the study of HIV phenotypes, using this technique, it is important to determine whether recombination between the two hemigenomes occurs without mutations. After co-transfection, progeny recombinant viruses replicated at the same rate as the control. We purified progeny viruses from culture supernatants and determined mutations at the recombination site. It appeared that correct in vivo ligation depended on the purity of DNA and the restriction site used. It also appeared that some of the mutations observed affect replication, as progeny viruses bearing one of these mutations disappeared during in vitro cultures, whereas other mutants did not. Although this technique is widely applied to generate chimeric viruses, the results should be evaluated with care, since mutations influencing the phenotype of the progeny viruses may have been introduced.

A monoclonal antibody (12G5) directed against CXCR-4 inhibits infection with the dual-tropic human immunodeficiency virus type 1 isolate HIV-1(89.6) but not the T-tropic isolate HIV-1(HxB)

We used a monoclonal antibody (12G5) directed against an extracellular domain of CXCR-4 to investigate the role of this receptor in infection of immortalized lymphoid cell lines, peripheral blood mononuclear cells (PBMCs), and primary brain microglia with a dual-tropic strain of human immunodeficiency virus (HIV-1(89.6)) and a T-tropic strain (HIV-1(IIIB)). Addition of antibody 12G5 to cells prior to and during infection with HIV-1(89.6) inhibited p24 production 100- to 10,000-fold in CEMx174 and 174-CD4 cells and about 10-fold in PBMC cultures but had no activity against infection of either monocyte-derived macrophages or brain microglia. In contrast, 12G5 had little or no effect on infection of CEMx174 cells with HIV-1(IIIB) or HIV-1(HxB). To identify the region of the HIV-1(89.6) envelope that confers sensitivity to 12G5, we used chimeric molecular clones. Chimeras containing the V3 loop region of HIV-1(89.6) were inhibited by 12G5 to the same degree as wild-type HIV-1(89.6) whereas replication of those viruses containing the V3 loop of HIV-1(HxB) was not inhibited by the antibody. A similar pattern was seen in infections of a U87 glioblastoma line that coexpresses CD4 and CXCR-4. Antibody 12G5 was also able to block fusion between HeLa-CD4 cells and CEMx174 cells chronically infected with HIV-1(89.6) but had no effect on fusion mediated by cells chronically infected with HIV-1(IIIB). Taken together, these results suggest that different strains of HIV-1 may interact with different sites on CXCR-4 or may have different binding affinities for the coreceptor.

Abortive infection in HeLaCD4 cells by a primary HIV type 1 isolate: implications for differential host cell tropism

The emergence of T cell-tropic, syncytium-inducing (T-tropic/SI) HIV-1 variants from the background of macrophage-tropic, non-syncytium-inducing (M-tropic/NSI) strains is associated with disease progression in infected individuals. HIV89.6 is a primary isolate with a transitional phenotype: like M-tropic strains it replicates in primary macrophages and lymphocytes but not in most transformed cells, yet it is also syncytium inducing. We have shown that HIV89.6 can utilize both the M-tropic and T-tropic cofactors CCR-5 and CXCR-4, respectively, in conjunction with CD4 for fusion and entry into otherwise nonpermissive nonhuman cells. To better understand the nature of restricted HIV89.6 infection of transformed cells, we analyzed its interaction with CD4-expressing transformed human HeLaCD4-LTR/beta-Gal cells, which contain the beta-galactosidase gene linked to the HIV-1 LTR. Here we show that HIV89.6 enters these cells and undergoes reverse transcription and integration. Furthermore, HIV89.6 induces LTR-driven beta-galactosidase expression, indicating Tat-dependent trans-activation, in a similar number of cells as the permissive T-tropic/SI isolate HIV(HXB). Acute infection with HIV89.6, however, produces markedly lower levels of p24 antigen and infectious virus per trans-activation-positive cell than HIV(HXB). In contrast, transfection results in high levels of expression for both viruses but HIV89.6 still fails to establish spreading infection. HIV89.6 is also blocked after entry in two other nonpermissive cell lines, SUP-T1 and U937. HIV89.6 arrest in HeLaCD4-LTR/beta-Gal cells at a stage subsequent to entry, reverse transcription, integration, and Tat expression is a novel level at which HIV-1 strain- and cell-specific restrictions define host cell tropism. These studies emphasize that complex patterns of tropism are determined by the interplay of permissive or restricted virus-cell interactions at multiple steps in the replication cycle.

A cell-free stock of simian-human immunodeficiency virus that causes AIDS in pig-tailed macaques has a limited number of amino acid substitutions in both SIVmac and HIV-1 regions of the genome and has offered cytotropism

We have examined both the sequence changes in the LTR, gag, vif, vpr, vpx, tat, rev, vpu, env, and nef genes and the cell tropism of a cell-free stock of chimeric simian-human immunodeficiency virus (SHIV) isolated from the cerebrospinal fluid of a pig-tailed macaque (PNb) that developed AIDS. This virus (SHIVKU-1) is highly pathogenic when inoculated into other macaques. DNA sequence analysis of PCR-amplified products revealed a total of 5 nucleotide changes in the LTR while vif had 2 consensus amino acid changes. The gag, vif, and vpx had no consensus amino acid substitutions, whereas vpr had 1 consensus substitution. The tat and rev genes of the HXB2 region of SHIVKU-1 had 2 and 1 consensus amino acid changes, respectively. The vpu gene of the HXB2 region of SHIV, which originally had an ACG at the beginning of the gene, reverted to an initiation ATG codon and in addition contained a consensus amino acid substitution at position 69 of this protein. As expected, the majority of the nucleotide substitutions were found in the env and nef genes. Thirteen and 5 amino acid changes were predicted for the corresponding Env and Nef proteins, respectively. In addition, one-third of the env gene clones isolated from the SHIVKU-1 stock had a 5-amino-acid deletion in the V4 region. Using three independent assays, we determined that the changes in the SHIVKU-1 were associated with an increase in the efficiency of replication in macrophages. The strikingly few consensus changes in the virus suggest that conversion of this virus to one capable of causing AIDS in pig-tailed macaques was associated with relatively few changes in the viral envelope and/or accessory genes. These results will provide the basis for the development of a pathogenic, molecular clone of SHIV capable of causing AIDS in pig-tailed macaques.

Role of CCR5 in infection of primary macrophages and lymphocytes by macrophage-tropic strains of human immunodeficiency virus: resistance to patient-derived and prototype isolates resulting from the delta ccr5 mutation

The alpha-chemokine receptor fusin (CXCR-4) and beta-chemokine receptor CCR5 serve as entry cofactors for T-cell (T)-tropic and macrophage (M)-tropic human immunodeficiency virus type 1 (HIV-1) strains, respectively, when expressed with CD4 in otherwise nonpermissive cells. Some M-tropic and dual-tropic strains can also utilize other beta-chemokine receptors, such as CCR2b and CCR3. A mutation of CCR5 (delta ccr5) was recently found to be common in certain populations and appears to confer protection against HIV-1 in vivo. Here, we show that this mutation results in a protein that is expressed intracellularly but not on the cell surface. Primary CD4 T cells from delta ccr5 homozygous individuals were highly resistant to infection with prototype M-tropic HIV-1 strains, including an isolate (YU-2) that uses CCR5 and CCR3, but were permissive for both a T-tropic strain (3B) and a dual-tropic variant (89.6) that uses CXCR-4, CCR5, CCR3, or CCR2b. These cells were also resistant to M-tropic patient isolates but were readily infected by T-tropic patient isolates. Primary macrophages from delta ccr5 homozygous individuals were also resistant to infection with M-tropic strains, including YU-2, but the dual-tropic strain 89.6 was able to replicate in them even though macrophages are highly resistant to CXCR-4-dependent T-tropic isolates. These data show that CCR5 is the essential cofactor for infection of both primary macrophages and T lymphocytes by most M-tropic strains of HIV-1. They also suggest that CCR3 does not function for HIV-1 entry in primary lymphocytes or macrophages, but that a molecule(s) other than CCR5 can support entry into macrophages by certain virus isolates. These studies further define the cellular basis for the resistance to HIV-1 infection of individuals lacking functional CCR5.

In vivo and in vitro infection with two different molecular clones of visna virus

The behavior of two genetically different molecular clones of visna virus KV1772-kv72/67 and LV1-1KS1 was compared in vivo and in vitro. On intracerebral inoculation, clone KV1772-kv72/67 induced a similar response in five sheep as has already been reported with neurovirulent derivates of visna virus. Virus was frequently isolated from blood, cerebrospinal fluid (CSF), and lymphoid organs and induced characteristic central nervous system (CNS) lesions. A strong humoral immune response was detected by ELISA, immunoblotting, and neutralization. Six sheep infected with clone LV1-1KS1 showed a completely different picture. No virus could be isolated from blood or CSF during 6 months of infection. At sacrifice all organs were virus-negative except the CNS of one sheep. None of the six sheep developed significant neutralizing antibodies and only low titer antibodies were detected by ELISA and immunoblotting. Minimal CNS lesions were present in one sheep. The molecular clones were also tested in sheep choroid plexus cells (SCP) and macrophages. In macrophages LV1-1KS1 replicated to a significantly lower titer but induced much more cell fusion than KV1772-kv72/67. The clones replicated equally well in SCP cells. Thus, these molecular clones of visna virus, which differ only by 1% in nucleotide sequence, showed a profound difference in replication and pathogenicity both in vitro and in vivo. These results can be used to map viral genetic determinants important for host-lentivirus interactions.

Chemokine receptors as fusion cofactors for human immunodeficiency virus type 1 (HIV-1)

CD4 is the primary cellular receptor for human immunodeficiency virus type 1 (HIV-1), but is not sufficient for entry of HIV-1 into cells. After a decade-long search, the cellular coreceptors that HIV-1 requires in conjunction with CD4 have been identified as members of the chemokine receptor family of seven-transmembrane G-protein coupled receptors. The discovery of distinct chemokine receptors that support entry of T-cell tropic (CXCR-4) and macrophage tropic HIV-1 strains (CCR-5) explains the differences in cell tropism between viral strains, the inability of HIV-1 to infect most nonprimate cells, and the resistance of a small percentage of the population to HIV-1 infection. Further understanding of the role of chemokine receptors in viral entry may also help explain the evolution of more pathogenic forms of the virus, viral transmission, and HIV-induced pathogenesis. These recent discoveries will aid the development of strategies for combating HIV-1 transmission and spread, the understanding of HIV-1 fusion mechanisms, and the possible development of small animal models for HIV-1 drug and vaccine testing.

Mapping of independent V3 envelope determinants of human immunodeficiency virus type 1 macrophage tropism and syncytium formation in lymphocytes

The V3 region of the human immunodeficiency virus type 1 (HIV-1) envelope protein is known to have a major influence on macrophage tropism as well as the ability to cause syncytium formation or fusion in CD4-positive lymphocyte cultures. Using infectious molecular HIV-1 clones, a series of mutant clones was created which allowed detailed mapping of V3 amino acid positions involved in these properties. In these experiments the non-syncytium-inducing phenotype in T cells did not always correlate with macrophage tropism. Macrophage tropism appeared to depend on the presence of certain combinations of amino acids at five specific positions within and just outside of the V3 loop itself, whereas syncytium formation in lymphocytes was influenced by substitution of particular residues at two to four positions within V3. In most cases, different V3 amino acid positions were found to independently influence macrophage tropism and syncytium formation in T cells and position 13 was the only V3 location which appeared to simultaneously influence both macrophage tropism and syncytium formation in lymphocytes.

Twin studies demonstrate a host cell genetic effect on productive human immunodeficiency virus infection of human monocytes and macrophages in vitro

Biological and genetic variability is a prominent feature of human immunodeficiency virus (HIV) strains, especially in tropism, syncytium formation, and replicative capacity. To determine whether there were variable host cell effects on HIV replication in monocytes, three different strains of low-passage-number monocytotropic blood isolates of HIV and the laboratory-adapted strain Ba-L were inoculated into panels of adherent monocytes drawn from 44 different donors, and peak extracellular HIV p24 antigen titers were compared. The clinical HIV strains showed patterns of either moderate or low-level replication in most donor monocytes (20 to 4,000 pg/ml). However, within this range there was marked variation in peak titers in most donors. HIV type 1 Ba-L replicated in all donor monocytes to much higher levels with less variability (30 to 40 ng/ml). Furthermore, replication of 21 clinical blood-derived strains of HIV in blood monocytes and monocyte-derived macrophages (MDM) from pairs of identical twins and age-matched unrelated donors (URD) of the same sex were compared. In all of the seven pairs of identical twins, the kinetics of replication (measured by extracellular HIV p24 antigen) of panels of four clinical HIV type 1 isolates in monocytes were similar within pairs. However, marked and significant differences in kinetics of HIV production occurred within 10 of the 12 unrelated donor pairs (P = 0.0007). The remaining two URD pairs showed similar kinetic patterns, but only one pair had the same HLA-DR genotype. Similar results were observed with monocytes/MDMs obtained from a second bleed of the same donor. Hence, discordant patterns of HIV replication kinetics between URD monocyte pairs contrasted with concordant patterns in identical twin monocytes. These data strongly suggest a host cell genetic effect on productive viral replication in monocytes and MDMs. So far, no consistent genetic linkage of HIV replication pattern with HLA-DR genotype has been observed.

Infection of SK-N-MC cells, a CD4-negative neuroblastoma cell line, with primary human immunodeficiency virus type 1 isolates

Most studies looking at CD4-independent infection have used laboratory strains or their respective molecular clones. To determine whether primary human immunodeficiency virus type 1 isolates could infect CD4-negative cells, we obtained a panel of 23 clinical isolates and characterized the early steps of the viral life cycle in SK-N-MC cells, a CD4-negative, galactosylceramide-positive neuroblastoma cell line. Eight of 23 isolates established a nonproductive infection; entry and postentry restrictions were noted in the others. We were unable to correlate the infectivity for SK-N-MC cells with established biological phenotypes, such as syncytium induction, or with genetic classifications, suggesting that pantropism is an independent biological variable.

A dual-tropic primary HIV-1 isolate that uses fusin and the beta-chemokine receptors CKR-5, CKR-3, and CKR-2b as fusion cofactors

Here, we show that the beta-chemokine receptor CKR-5 serves as a cofactor for M-tropic HIV viruses. Expression of CKR-5 with CD4 enables nonpermissive cells to form syncytia with cells expressing M-tropic, but not T-tropic, HIV-1 env proteins. Expression of CKR-5 and CD4 enables entry of a M-tropic, but not a T-tropic, virus strain. A dual-tropic primary HIV-1 isolate (89.6) utilizes both Fusin and CKR-5 as entry cofactors. Cells expressing the 89.6 env protein form syncytia with QT6 cells expressing CD4 and either Fusin or CKR-5. The beta-chemokine receptors CKR-3 and CKR-2b support HIV-1 89.6 env-mediated syncytia formation but do not support fusion by any of the T-tropic or M-tropic strains tested. Our results suggest that the T-tropic viruses characteristic of disease progression may evolve from purely M-tropic viruses prevalent early in virus infection through changes in the env protein that enable the virus to use multiple entry cofactors.

An env gene derived from a primary human immunodeficiency virus type 1 isolate confers high in vivo replicative capacity to a chimeric simian/human immunodeficiency virus in rhesus monkeys

To explore the roles played by specific human immunodeficiency virus type 1 (HIV-1) genes in determining the in vivo replicative capacity of AIDS viruses, we have examined the replication kinetics and virus-specific immune responses in rhesus monkeys following infection with two chimeric simian/human immunodeficiency viruses (SHIVs). These viruses were composed of simian immunodeficiency virus SIVmac239 expressing HIV-1 env and the associated auxiliary HIV-1 genes tat, vpu, and rep. Virus replication was assessed during primary infection of rhesus monkeys by measuring plasma SIVmac p27 levels and by quantifying virus replication in lymph nodes using in situ hybridization. SHIV-HXBc2, which expresses the HIV-1 env of a T-cell-tropic, laboratory-adapted strain of HIV-1 (HXBc2), replicated well in rhesus monkey peripheral blood leukocytes (PBL) in vitro but replicated only to low levels when inoculated in rhesus monkeys. In contrast, SHIV-89.6 was constructed with the HIV-1 env gene of a T-cell- and macrophage-tropic clone of a patient isolate of HIV-1 (89.6). This virus replicated to a lower level in monkey PBL in vitro but replicated to a higher degree in monkeys during primary infection. Moreover, monkeys infected with SHIV-89.6 developed an inversion in the PBL CD4/CD8 ratio coincident with the clearance of primary viremia. The differences in the in vivo consequences of infection by these two SHIVs could not be explained by differences in the immune responses elicited by these viruses, since infected animals had comparable type-specific neutralizing antibody titers, proliferative responses to recombinant HIV-1 gp120, and virus-specific cytolytic effector T-cell responses. With the demonstration that a chimeric SHIV can replicate to high levels during primary infection in rhesus monkeys, this model can now be used to define genetic determinants of HIV-1 pathogenicity.

Human immunodeficiency virus type 1 infection of SK-N-MC cells: domains of gp120 involved in entry into a CD4-negative, galactosyl ceramide/3' sulfo-galactosyl ceramide-positive cell line

The primary receptor for human immunodeficiency virus (HIV) is the CD4 molecule; however, in vitro evidence suggests that a neutral glycolipid, galactosyl ceramide (GalCer) or a derivative molecule, 3' sulfogalactosyl ceramide (GalS), may serve as an alternative receptor for HIV type 1 (HIV-1) in cells of neural and colonic origin. Biochemical studies have demonstrated that recombinant gp120 envelope protein binds to GalCer/GalS in both solid-phase enzyme-linked immunosorbent assay and high-performance thin-layer chromatography overlays. We have used the SK-N-MC cell line, a CD4-negative, GalCer/GalS-positive cell line previously characterized as susceptible to HIV-1 infection, to identify virus isolates with either a positive infection phenotype, HIVHxB2, or a negative infection phenotype, HIV-1(89.6). Using a solid-phase virus binding assay, we determined the level of restriction in HIV-1(89.6) infection to be at the level of virus-glycolipid binding. Furthermore, using HIV-1HxB2-HIV-1(89.6) chimeras, we have identified a 193-amino-acid fragment from the envelope region of HIV-1HxB2 containing the V3, V4, and V5 regions which confers a positive infection phenotype on the HIV-1(89.6) background. Recombinant viruses which separate this 193-amino-acid fragment into two distinct chimeras are each able to confer a positive infection phenotype on the background of HIV89.6, suggesting that a stable GalCer/GalS-envelope interaction is dependent on the conformation of the envelope protein in the context of the viral membrane. Alternatively, the GalCer/GalS-gp120 bond may involve multiple sites on the oligomeric envelope protein.