Genetically divergent strains of simian immunodeficiency virus use CCR5 as a coreceptor for entry

Determinants of human immunodeficiency virus type 1 envelope glycoprotein activation by soluble CD4 and monoclonal antibodies

Infection by some human immunodeficiency virus type 1 (HIV-1) isolates is enhanced by the binding of subneutralizing concentrations of soluble receptor, soluble CD4 (sCD4), or monoclonal antibodies directed against the viral envelope glycoproteins. In this work, we studied the abilities of different antibodies to mediate activation of the envelope glycoproteins of a primary HIV-1 isolate, YU2, and identified the regions of gp120 envelope glycoprotein contributing to activation. Binding of antibodies to a variety of epitopes on gp120, including the CD4 binding site, the third variable (V3) loop, and CD4-induced epitopes, enhanced the entry of viruses containing YU2 envelope glycoproteins. Fab fragments of antibodies directed against either the CD4 binding site or V3 loop also activated YU2 virus infection. The activation phenotype was conferred on the envelope glycoproteins of a laboratory-adapted HIV-1 isolate (HXBc2) by replacing the gp120 V3 loop or V1/V2 and V3 loops with those of the YU2 virus. Infection by the YU2 virus in the presence of activating antibodies remained inhibitable by macrophage inhibitory protein 1beta, indicating dependence on the CCR5 coreceptor on the target cells. Thus, antibody enhancement of YU2 entry involves neither Fc receptor binding nor envelope glycoprotein cross-linking, is determined by the same variable loops that dictate enhancement by sCD4, and probably proceeds by a process fundamentally similar to the receptor-activated virus entry pathway.

Neutralizing antibodies in sera from macaques immunized with attenuated simian immunodeficiency virus

Infection with attenuated simian immunodeficiency virus (SIV) in rhesus macaques has been shown to raise antibodies capable of neutralizing an animal challenge stock of primary SIVmac251 in CEMx174 cells that correlate with resistance to infection after experimental challenge with this virulent virus (M. S. Wyand, K. H. Manson, M. Garcia-Moll, D. C. Montefiori, and R. C. Desrosiers, J. Virol. 70:3724-3733, 1996). Here we show that these neutralizing antibodies are not detected in human and rhesus peripheral blood mononuclear cells (PBMC). In addition, neutralization of primary SIVmac251 in human and rhesus PBMC was rarely detected with plasma samples from a similar group of animals that had been infected either with SIVmac239Deltanef for 1.5 years or with SIVmac239Delta3 for 3.2 years, although low-level neutralization was detected in CEMx174 cells. Potent neutralization was detected in CEMx174 cells when the latter plasma samples were assessed with laboratory-adapted SIVmac251. In contrast to primary SIVmac251, laboratory-adapted SIVmac251 did not replicate in human and rhesus PBMC despite its ability to utilize CCR5, Bonzo/STRL33, and BOB/gpr15 as coreceptors for virus entry. These results illustrate the importance of virus passage history and the choice of indicator cells for making assessments of neutralizing antibodies to lentiviruses such as SIV. They also demonstrate that primary SIVmac251 is less sensitive to neutralization in human and rhesus PBMC than it is in established cell lines. Results obtained in PBMC did not support a role for neutralizing antibodies as a mechanism of protection in animals immunized with attenuated SIV and challenged with primary SIVmac251.

Parallel evolution of CCR5-null phenotypes in humans and in a natural host of simian immunodeficiency virus

The C-C chemokine receptor CCR5 in humans and rhesus macaques (Macaca mulatta) serves as the primary coreceptor for cellular entry by macrophagetropic strains of human immunodeficiency virus type 1 (HIV-1) and all reported strains of simian immunodeficiency virus (SIV) [1-6]. Humans homozygous for a 32 bp deletion allele of CCR5, resulting in a null phenotype, are highly resistant to infection by HIV-1 [7-9], prompting development of therapies and vaccines targeting CCR5. We now report a novel deletion allele of CCR5, with an allele frequency of 0.04, in sooty mangabey monkeys (Cercocebus torquatus atys), a natural host of SIV (SIVsmm) [10]. The mutant protein was not expressed at the cell surface and accordingly did not function as a viral coreceptor. Primary activated lymphocytes from mangabeys heterozygous for the deletion allele expressed significantly less CCR5 on the cell surface. Moreover, SIV seroprevalence and viremia were comparable among CCR5 heterozygotes and wild-type animals. Parallel evolution of CCR5-null alleles in humans and sooty mangabeys suggests that similar negative selection pressures have acted against CCR5, as would occur during epidemics of infectious agents that require CCR5 for pathogenesis. Sooty mangabeys bred to homozygosity for the deletion allele will be useful for experimental studies on the context-dependent role of CCR5 in host defense and microbial pathogenesis.

Evolution of neutralizing antibody response against HIV type 1 virions and pseudovirions in multicenter AIDS cohort study participants

Changes in neutralizing antibody (NA) titers in stored sera collected over 5 years from 10 participants in the Multicenter AIDS Cohort Study (MACS) were evaluated. The participants were HIV-1 infected on enrollment in the MACS, and remained AIDS free during the 5-year study interval. Seven viruses derived from molecular clones were used in NA assays; five of the viruses were T tropic (NL4-3, ALA1, NY5, SF2, and Z2Z6) and two were M tropic [AD8 and NL(SF162)]. In addition, pseudoviruses (PVs) were constructed that expressed envelope genes from NL4-3, ALA1, AD8, and SF162 and from primary viruses from two MACS participants (PV-9 and PV-10). There was significant correlation between NA titers obtained in four of five virus/PV comparisons, while the SF162 PV was more sensitive to NA than the corresponding virus. Comparable changes in NA titers were detected using viruses and PVs. Fourfold or greater increases in NA titers were noted in each of the participants, involving recognition of one to five of the nine strains tested. In some patients these NA titer changes appeared as discrete episodes of immune responses, while in others there may have been either multiple episodes or continuous evolution of the NA responses. The data indicate that changes in NA specificity occur during HIV-1 infection, which may result from the occurrence of neutralization escape mutation. The use of PVs for the study of phenotypic characteristics of envelope glycoproteins should facilitate the study of neutralization escape mutation in HIV-1 infection.

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

Several members of the seven-transmembrane chemokine receptor family have been shown to serve, with CD4, as coreceptors for entry by human immunodeficiency virus type 1 (HIV-1). While coreceptor usage by HIV-1 primary isolates has been studied by several groups, there is only limited information available concerning coreceptor usage by primary HIV-2 isolates. In this study, we have analyzed coreceptor usage of 15 primary HIV-2 isolates, using lymphocytes from a donor with nonfunctional CCR5 (CCR5 -/-; homozygous 32-bp deletion). Based on the infections of PBMCs, seven of these primary isolates had an absolute requirement for CCR5 expression, whereas the remaining eight exhibited a broader coreceptor usage. All CCR5-requiring isolates were non-syncytium inducing, whereas isolates utilizing multiple coreceptors were syncytium inducing. Blocking experiments using known ligands for chemokine receptors provided indirect evidence for additional coreceptor utilization by primary HIV-2 isolates. Analysis of GHOST4 cell lines expressing various chemokine receptors (CCR1, CCR2b, CCR3, CCR4, CCR5, CXCR4, BONZO, and BOB) further defined specific coreceptor usage of primary HIV-2 isolates. The receptors used included CXCR4, CCR1-5, and the recently described receptors BONZO and BOB. However, the efficiency at which the coreceptors were utilized varied greatly among the various isolates. Analysis of V3 envelope sequences revealed no specific motif that correlated with coreceptor usage. Our data demonstrate that primary HIV-2 isolates are capable of using a broad range of coreceptors for productive infection in vitro. Additionally, our data suggest that expanded coreceptor usage by HIV-2 may correlate with disease progression.

Genomic organization and expression of the CXCR4 gene in mouse and man: absence of a splice variant corresponding to mouse CXCR4-B in human tissues

The human CXC chemokine receptor CXCR4 is activated by stromal cell-derived factor 1. The receptor is present in many cell types and regulates a variety of cellular functions, including chemotaxis, adhesion, hematopoiesis, and organogenesis. Human CXCR4 also serves as a cofactor for cell entry by certain strains of HIV-1 and HIV-2. In the mouse, alternative RNA splicing produces two transcripts encoding two CXCR4 isoforms, mCXCR4-A and mCXCR4-B, differing by the presence of two amino acids in the amino terminal portion of the longer protein, mCXCR4-B. Only one CXCR4 transcript, encoding the human counterpart of mCXCR4-A, is known in man. The involvement of the aminoterminal-most portion of CXCR4 in both ligand and HIV envelope protein recognition led us to determine whether a CXCR4 variant corresponding to mCXCR4-B is present in human tissues. To this end, the genomic organization and expression of the human CXCR4 gene was examined. Both the human and the mouse CXCR4 gene consist of two exons separated by an approximately 2.1 kbp intron between codons five and six and carry splice donor sites at the 5' end of their introns. These similarities notwithstanding, single nucleotide primer extension, reverse transcriptase PCR amplification, and sequencing of CXCR4 cDNA clones show that a splice variant of CXCR4 corresponding to mCXCR4-B is absent in man.

Primary SIVsm isolates use the CCR5 coreceptor from sooty mangabeys naturally infected in west Africa: a comparison of coreceptor usage of primary SIVsm, HIV-2, and SIVmac

Genetically divergent strains of simian immunodeficiency virus (SIV) from macaques (mac), chimpanzees, and sooty mangabeys (SM) efficiently used rhesus and human CCR5 (R5), but not CXCR4 (xR4), for cell entry. Thus far, however, no studies have characterized primary SIVsm strains for their use of coreceptors derived from their own natural host. Coreceptor usage of two primary, blood-derived SIVsm isolates, SIVsmSL92b and SIVsmFNS from naturally infected sooty mangabeys, was determined. Primary SIVsm efficiently used SM-CCR5 expressed on HOS.CD4 and U87.CD4 cells. Sequence polymorphisms in CCR5 found in four sooty mangabeys did not alter viral entry. Unlike primary rhesus blood-derived R5-tropic SIVmac251, primary SM blood-derived R5-tropic SIVsm was strongly CD4 dependent. The SM-CXCR4 gene was fully functional for xR4-tropic primate lentiviruses, but was not used by primary SIVsm. Therefore, the lack of xR4 tropism among naturally occurring SIVsm strains was not due to CxCR4 gene defects in the natural host. SIVmac derived from four macaques with AIDS also did not use macaque- or SM-derived CXCR4, showing that xR4 tropism did not develop during progression to disease as for humans infected with HIV-1. Three of four primary HIV-2 strains used CCR5 from human, sooty mangabey, and macaque. The fourth, HIV-27924A, obtained from a patient with AIDS, was xR4-tropic. Because SIVmac is most closely related to HIV-2, SIVmac might be expected to rnimic tropisms of HIV-2 infections. However, the correlation between xR4 tropism and AIDS may be a species-specific phenomenon limited to humans. The R5-tropic primary SIVsm and HIV-2 strains grew in CCR5-negative human PBMC, consistent with their use of non-CCR5 coreceptors. However, primary SIVsmSL92b did not use non-CCR5 coreceptors efficiently. The two primary SIVsm isolates replicated poorly in CEMx174 cells, which do not express CCR5, compared to CCR5-positive PM1 cells. SIVmac grew equally well in both cell lines. The findings show that SM-chemokine receptors are fully functional for virus entry and that multicoreceptor tropism is a common property of primary lentiviruses within the SIVsm/HIV-2 subfamily.

Characterization of the cutaneous exanthem in macaques infected with a Nef gene variant of SIVmac239

The molecularly cloned viruses known as SIVmac239/R17Y and SIVmac239/YEnef cause extensive lymphocyte activation and induce an acute disease syndrome in macaque monkeys. One manifestation of this syndrome is a severe diffuse cutaneous maculopapular exanthem that is similar to the exanthem associated with HIV-1 infection. To examine the pathogenesis of this exanthem, biopsies obtained throughout the course of clinically evident rash were examined for the presence of virus by in situ hybridization and immunohistochemistry, and the cellular infiltrate was characterized with respect to cellular immunophenotype and chemokine receptor expression. The onset of rash was associated with abundant simian immunodeficiency virus nucleic acid and protein within perivascular dermal infiltrates and occasionally within intraepithelial cells. Analysis of cellular infiltrates showed that biopsies, obtained on the day of rash onset, were composed of equal numbers of CD4+ and CD8+ lymphocytes and abundant alphaEbeta7 positive cells surrounding vessels with upregulated endothelial E-selectin. Moreover, by examining virus expression in sequential skin biopsies from the same animal, the clearance of virus and the resolution of rash were associated with an increase in the percentage of cells expressing CD8, the chemokine receptor CXCR3, and GMP-17, a marker of cytotoxic granules. These results suggest that activated cytotoxic T cells are trafficking to sites of inflammation in the skin and directly or indirectly affect levels of viral replication at these sites.

CCR5 coreceptor utilization involves a highly conserved arginine residue of HIV type 1 gp120

The seven-transmembrane CCR5 was recently found to double as a coreceptor for a genetically diverse family of human and nonhuman primate lentiviruses. Paradoxically, the main region of the envelope protein believed to be involved in CCR5 utilization was mapped to hypervariable region 3, or V3, of the envelope glycoprotein gp120. In this study, we addressed the question of whether functional convergence in CCR5 utilization is mediated by certain V3 residues that are highly conserved among HIV type 1 (HIV-1), HIV type 2, and simian immunodeficiency virus. Site-directed mutagenesis carried out on three such V3 residues revealed that the Arg-298 of HIV-1 gp120 has an important role in CCR5 utilization. In contrast, no effect was observed for the other residues we tested. The inability of Arg-298 mutants to use CCR5 was not attributed to global alteration of gp120 conformation. Neither the expression, processing, and incorporation of mutant envelope proteins into virions, nor CD4 binding were significantly affected by the mutations. This interpretation is further supported by the finding that alanine substitutions of five residues immediately adjacent to the arginine residue had no effect on CCR5 utilization. Taken together, our data strongly suggests that the highly conserved Arg-298 residue identified in the V3 of HIV-1 has a significant role in CCR5 utilization, and may represent an unusually conserved target for future anti-viral designs.

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.

A broad range of chemokine receptors are used by primary isolates of human immunodeficiency virus type 2 as coreceptors with CD4

Like human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus (SIV), HIV-2 requires a coreceptor in addition to CD4 for entry into cells. HIV and SIV coreceptor molecules belong to a family of seven-transmembrane-domain G-protein-coupled receptors. Here we show that primary HIV-2 isolates can use a broad range of coreceptor molecules, including CCR1, CCR2b, CCR3, CCR4, CCR5, and CXCR4. Despite broad coreceptor use, the chemokine ligand SDF-1 substantially blocked HIV-2 infectivity of peripheral blood mononuclear cells, indicating that its receptor, CXCR4, was the predominant coreceptor for infection of these cells. However, expression of CXCR4 together with CD4 on some cell types did not confer susceptibility to infection by all CXCR4-using virus isolates. These data therefore indicate that another factor(s) influences the ability of HIV-2 to replicate in human cell types that express the appropriate receptors for virus entry.

An intricate Web: chemokine receptors, HIV-1 and hematopoiesis

Cellular infection by the human immunodeficiency virus type 1 (HIV-1) requires interaction of the viral envelope protein with CD4 and at least one additional cell surface molecule, termed a "cofactor" or "coreceptor." Recent discoveries have determined that macrophage-tropic strains of HIV-1 which are largely responsible for sexual transmission require the beta-chemokine receptor CCR5 in addition to CD4, while the T cell tropic viruses that emerge later after infection use the alpha-chemokine receptor CXCR4. Thus, both CD4 and the appropriate chemokine receptor must be expressed on the cell surface in order for HIV-1 to enter the cell and establish an infection. The in vivo importance of CCR5 for HIV-1 is demonstrated by the finding that individuals homozygous for a 32 bp deletion (delta 32) in the CCR5 gene that renders them effectively CCR5-negative are highly resistant to virus infection. In this review, the structure-function correlates of the chemokine receptors that serve as major coreceptors for HIV-1 and simian immunodeficiency virus entry will be reviewed. Since certain chemokines have been implicated as stem cell inhibitory factors, the biological consequences of chemokine receptor expression as it relates to HIV-1-associated hematodyspoiesis will also be discussed.

Generation of CD8 suppressor factor and beta chemokines, induced by xenogeneic immunization, in the prevention of simian immunodeficiency virus infection in macaques

Previous xenogeneic immunization experiments in rhesus macaques with simian immunodeficiency virus (SIV) grown in human CD4(+) T cells consistently elicited protection from challenge with live SIV. However, the mechanism of protection has not been established. We present evidence that xenogeneic immunization induced significant CD8 suppressor factor, RANTES (regulated upon activation, normal T cell expressed and secreted), macrophage inflammatory protein (MIP) 1alpha, and MIP-1beta (P < 0.001 - P < 0.02). The concentrations of these increased significantly in protected as compared with infected macaques (P < 0.001). Xenogeneic stimulation in vitro also up-regulated CD8 suppressor factors (SF; P < 0.001) and the beta chemokines which were neutralized by antibodies to the 3 beta chemokines. Recombinant human RANTES, MIP-1alpha and MIP-1beta which bind to simian CCR5, suppressed SIV replication in a dose-dependent manner, with RANTES being more effective than the other two chemokines. The results suggest that immunization with SIV grown in human CD4(+) T cells induces CD8-suppressor factor, RANTES, MIP-1alpha and MIP-1beta which may block CCR5 receptors and prevent the virus from binding and fusion to CD4(+) cells.

Expression and in vitro evaluation of rhesus macaque wild type (wt) and modified CC chemokines

Several human CC chemokines have been shown to inhibit HIV/ SIV infection in vitro, providing the rationale for their potential use in vivo. However, because of their inherent physiological effect, such chemokines are reasoned to be of limited therapeutic value due to potential side effects. The knowledge that amino terminus modified or deleted human RANTES retains its receptor binding properties but loses its signaling properties has provided a means to use such modified chemokines in vivo for possible therapeutic benefits. In efforts to test the efficacy of such modified chemokines, our laboratory has cloned, sequenced, and prepared recombinant forms of wild-type (wt) and amino-terminus modified rhesus macaque chemokines MIP-1alpha, MIP-1beta, and RANTES. These sets of chemokines were tested for their potential to inhibit SIV infection and induce signaling. The data showed that whereas wt chemokines retained both virus inhibitory and signaling functions, corresponding amino-terminus modified chemokines only showed virus inhibitory effects without detectable signaling effects. Such reagents will be valuable for evaluation of their therapeutic potential in vivo, either alone or as adjuncts to other chemotherapeutic drugs.

Large quantity production with extreme convenience of human SDF-1alpha and SDF-1beta by a Sendai virus vector

We describe a robust expression of human stromal cell-derived factor-1alpha (SDF-1alpha) and SDF-1beta, the members of CXC-chemokine family, with a novel vector system based upon Sendai virus, a non-segmented negative strand RNA virus. Recombinant SDF-1alpha and SDF-1beta were detected as a major protein species in culture supernatants, reached as high as 10 microg/ ml. This remarkable enrichment of the products allowed us to use even the crude supernatants as the source for biological and antiviral assays without further concentration nor purification and will thus greatly facilitate to screen their genetically engineered derivatives.

Chemokine receptor expression on resident and inflammatory cells in the brain of macaques with simian immunodeficiency virus encephalitis

Although the mechanisms of human immunodeficiency virus (HIV) neuroinvasion, neuronal injury, and subsequent development of HIV-1-associated AIDS dementia complex are not fully understood, a correlation between monocyte/macrophage infiltrates in the brain and neurological disease exists. In light of the many potential roles that chemokines and chemokine receptors may play in HIV neuropathogenesis, we sought to describe their pattern of expression in the SIV-infected rhesus macaque model of HIV encephalitis. We previously demonstrated elevated expression of the chemokines macrophage inflammatory protein (MIP)-1alpha, MIP-1beta, RANTES, and interferon-inducible protein (IP)-10 in brain of macaque monkeys with SIV encephalitis. In this study, we demonstrate that the corresponding chemokine receptors CCR3, CCR5, CXCR3, and CXCR4 are expressed in perivascular infiltrates in these same tissues. In addition, we detected CCR3, CCR5, and CXCR4 on subpopulations of large hippocampal and neocortical pyramidal neurons and on glial cells in both normal and encephalitic brain. These findings suggest that multiple chemokines and their receptors contribute to monocyte and lymphocyte recruitment to the brain in SIV encephalitis. Furthermore, the expression of known HIV/SIV co-receptors on neurons suggests a possible mechanism whereby HIV or SIV can directly interact with these cells, disrupting their normal physiological function and contributing to the pathogenesis of AIDS dementia complex.

Evidence that antibody-mediated neutralization of human immunodeficiency virus type 1 by sera from infected individuals is independent of coreceptor usage

Human immunodeficiency virus type 1 (HIV-1) uses a variety of chemokine receptors as coreceptors for virus entry, and the ability of the virus to be neutralized by antibody may depend on which coreceptors are used. In particular, laboratory-adapted variants of the virus that use CXCR4 as a coreceptor are highly sensitive to neutralization by sera from HIV-1-infected individuals, whereas primary isolates that use CCR5 instead of, or in addition to, CXCR4 are neutralized poorly. To determine whether this dichotomy in neutralization sensitivity could be explained by differential coreceptor usage, virus neutralization by serum samples from HIV-1-infected individuals was assessed in MT-2 cells, which express CXCR4 but not CCR5, and in mitogen-stimulated human peripheral blood mononuclear cells (PBMC), where multiple coreceptors including CXCR4 and CCR5 are available for use. Our results showed that three of four primary isolates with a syncytium-inducing (SI) phenotype and that use CXCR4 and CCR5 were neutralized poorly in both MT-2 cells and PBMC. The fourth isolate, designated 89.6, was more sensitive to neutralization in MT-2 cells than in PBMC. We showed that the neutralization of 89.6 in PBMC was not improved when CCR5 was blocked by having RANTES, MIP-1alpha, and MIP-1beta in the culture medium, indicating that CCR5 usage was not responsible for the decreased sensitivity to neutralization in PBMC. Consistent with this finding, a laboratory-adapted strain of virus (IIIB) was significantly more sensitive to neutralization in CCR5-deficient PBMC (homozygous delta32-CCR5 allele) than were two of two SI primary isolates tested. The results indicate that the ability of HIV-1 to be neutralized by sera from infected individuals depends on factors other than coreceptor usage.

Modulation of feline immunodeficiency virus infection by stromal cell-derived factor

The alpha-chemokine receptor CXCR4 has recently been shown to support syncytium formation mediated by strains of feline immunodeficiency virus (FIV) that have been selected for growth in the Crandell feline kidney cell line (CrFK-tropic virus). Given that both human and feline CXCR4 support syncytium formation mediated by FIV, we investigated whether human stromal cell-derived factor (SDF-1) would inhibit infection with FIV. Human SDF-1alpha and SDF-1beta bound with a high affinity (K(D)s of 12.0 and 10.4 nM, respectively) to human cells stably expressing feline CXCR4, and treatment of CrFK cells with human SDF-1alpha resulted in a dose-dependent inhibition of infection by FIV(PET). No inhibitory activity was detected when the interleukin-2 (IL-2)-dependent feline T-cell line Mya-1 was used in place of CrFK cells, suggesting the existence of a CXCR4-independent mechanism of infection. Furthermore, neither the human beta-chemokines RANTES, MIP-1alpha, MIP-1beta, and MCP-1 nor the alpha-chemokine IL-8 had an effect on infection of either CrFK or Mya-1 cells with CrFK-tropic virus. Envelope glycoprotein purified from CrFK-tropic virus competed specifically for binding of SDF-1alpha to feline CXCR4 and CXCR4 expression was reduced in FIV-infected cells, suggesting that the inhibitory activity of SDF-1alpha in CrFK cells may be the result of steric hindrance of the virus-receptor interaction following the interaction between SDF and CXCR4. Prolonged incubation of CrFK cells with SDF-1alpha led to an enhancement rather than an inhibition of infection. Flow cytometric analysis revealed that this effect may be due largely to up-regulation of CXCR4 expression by SDF-1alpha on CrFK cells, an effect mimicked by treatment of the cells with phorbol myristate acetate. The data suggest that infection of feline cells with FIV can be mediated by CXCR4 and that, depending on the assay conditions, infection can be either inhibited or enhanced by SDF-1alpha. Infection with FIV may therefore prove a valuable model in which to study the development of novel therapeutic interventions for the treatment of AIDS.

Sequence comparisons of non-human primate HIV-1 coreceptor homologues

Infection of non-human primate peripheral blood mononuclear cells (PBMCs) in vitro with primary human immunodeficiency virus type 1 (HIV-1) isolates is extremely inefficient and often unattainable. The mechanism of resistance to infection by primary HIV-1 isolates in chimpanzee and baboon PBMCs is unknown. In this study, two HIV-1 coreceptors, CCR5 and CXCR4, were sequenced from chimpanzee and baboon PBMCs to determine if any sequence variations or mutations in these genes could be responsible for resistance to HIV infection. Primers were designed from the human coreceptor sequences and were able to amplify the CCR5 and CXCR4 genes from these non-human primate cells. No 32 base pair deletion (delta32) mutations were found in any of the non-human primate samples tested. CXCR4 sequence analysis showed chimpanzee and baboon share 99.7 and 98% nucleotide sequence homology and 100 and 98.9% amino acid sequence homology, respectively, compared to the human sequence. CCR5 sequence analysis demonstrated that chimpanzee and baboon share 99.6 and 98% nucleotide homology and 100 and 98% amino acid homology, respectively, with the human sequence. These data indicate that no variations in these coreceptor gene sequences exist that can explain the lack of susceptibility to infection with primary HIV-1 isolates in non-human primate PBMCs.

Neither Human Immunodeficiency Virus-1 (HIV-1) nor HIV-2 Infects Most-Primitive Human Hematopoietic Stem Cells as Assessed in Long-Term Bone Marrow Cultures

Attempts to clarify the pathophysiology of human immunodeficiency virus (HIV)-mediated bone marrow (BM) dysfunction have yielded inconsistent results regarding the susceptibility of BM progenitors to the viral infection. To specifically address this question, we exposed highly purified subpopulations of human BM progenitor cells to various HIV-1 and HIV-2 strains and assessed (pro)viral gene presence and expression in more-committed (CD34+CD38+) as well as most-primitive (CD34+CD38−) cells in long-term BM cultures. Quantitative analysis of long-term culture-initiating cells (LTCIC) failed to demonstrate adverse effects of exposing hematopoietic stem cells to HIV. Our results show that HIV-2, similar to HIV-1, does not infect hematopoietic stem cells in vitro with any significant frequency and infected cells are not present within LTCICs. Cytofluorometric analysis of CD34+ cells for surface molecules that facilitate HIV entry was consistent with the functional assay in that expression of virus receptors was predominantly on the more-committed subsets of BM progenitors. The failure to detect productive or latent HIV in the most-primitive human BM progenitor and stem cells has important implications for future therapeutic strategies, including those dealing with transduction of these cells with protective genes as a treatment modality for AIDS.

Neither Human Immunodeficiency Virus-1 (HIV-1) nor HIV-2 Infects Most-Primitive Human Hematopoietic Stem Cells as Assessed in Long-Term Bone Marrow Cultures

Attempts to clarify the pathophysiology of human immunodeficiency virus (HIV)-mediated bone marrow (BM) dysfunction have yielded inconsistent results regarding the susceptibility of BM progenitors to the viral infection. To specifically address this question, we exposed highly purified subpopulations of human BM progenitor cells to various HIV-1 and HIV-2 strains and assessed (pro)viral gene presence and expression in more-committed (CD34+CD38+) as well as most-primitive (CD34+CD38−) cells in long-term BM cultures. Quantitative analysis of long-term culture-initiating cells (LTCIC) failed to demonstrate adverse effects of exposing hematopoietic stem cells to HIV. Our results show that HIV-2, similar to HIV-1, does not infect hematopoietic stem cells in vitro with any significant frequency and infected cells are not present within LTCICs. Cytofluorometric analysis of CD34+ cells for surface molecules that facilitate HIV entry was consistent with the functional assay in that expression of virus receptors was predominantly on the more-committed subsets of BM progenitors. The failure to detect productive or latent HIV in the most-primitive human BM progenitor and stem cells has important implications for future therapeutic strategies, including those dealing with transduction of these cells with protective genes as a treatment modality for AIDS.

Interferon treatment inhibits the replication of simian immunodeficiency virus at an early stage: evidence for a block between attachment and reverse transcription

Interferon (IFN) dramatically reduces both SIV and HIV-1 replication in vitro. However, we previously found that whereas IFN treatment of SIV-infected cells results in a decrease in the level of viral RNA, IFN treatment of HIV-1-infected cells has no effect on viral RNA expression but rather leads to a decrease in viral protein stability and a deregulation of polyprotein processing (M. B. Agy, R. L. Acker, C. H. Sherbert, and M.G. Katze, Virology 214, 379-386, 1995). To more closely define the stage of SIV replication adversely affected by IFN, we used several approaches, including PCR amplification, to examine the effects of IFN on viral DNA synthesis and integration in MT4 and 174 x CEM cells synchronously infected with SIV. Unexpectedly, we found that IFN blocked the synthesis of viral DNA in SIV-infected cells but appeared to have no effect on HIV-1 DNA synthesis. Using a p27 ELISA, we demonstrated that IFN had no effect on the attachment of SIV to MT4 cells. Thus, our results indicate that IFN blocks an early stage of SIV replication, at a step between attachment and reverse transcription. To our knowledge this is the first report to examine the effects of IFN on discrete stages of the SIV life cycle.

The rhesus macaque CCR3 chemokine receptor is a cell entry cofactor for HIV-2, but not for HIV-1

The eotaxin receptor (CCR3) is a CD4-associated coreceptor for human immunodeficiency virus type 1 (HIV-1) and type 2 (HIV-2). By comparison with other chemokine receptors, such as CCR5 and CXCR4, the primary sequences of human CCR3 and its rhesus macaque homolog were markedly different in their extracellular domains. Human CD4+ cells expressing CCR3 from either human or macaque origin could be infected by HIV-2, with apparently similar efficiency, but only cells expressing human CCR3 could be infected by HIV-1. It suggests that HIV-1 and HIV-2 envelope proteins interact differently with the CCR3 coreceptor HIV-1 could infect cells expressing chimeric human/macaque CCR3 bearing either the first and second, or the third and fourth extracellular domains of human CCR3. As previously observed for CCR5, there seems to be a certain functional redundancy between domains supporting the coreceptor activity of CCR3. In spite of their close genetic relationship to HIV-2, two macaque simian immunodeficiency virus strains were apparently unable to use the CCR3 coreceptor from either human or simian origin.

A genetic and viral load analysis of the simian immunodeficiency virus during the acute phase in macaques inoculated by the vaginal route

A comparative genetic analysis of SIV-infected female macaques during the first 120 days postinfection was undertaken. The same dose of a macaque-passaged SIVmac239(nef open) was administered to three macaques intravenously (i.v.) and to three macaques intravaginally (i.VAG). Clinical outcomes observed ranged from rapid to nonprogression, while two of the i.v.-infected macaques developed an uncommon hindleg paresis. Analysis of viral load (bDNA assay) determined that both i.v.- and i.VAG-infected macaques had comparable high viral loads at the observed viral peak of 14 days postinfection. A study of viral quasispecies diversity by the heteroduplex mobility assay indicated that (1) the i.v.-infected macaques had a highly heterogeneous quasispecies population similar to the infecting viral stock; and (2) in two of three i.VAG-infected macaques multiple viral genotypes (minimum, three or four) were observed in blood and lymph tissues at early times postinfection, which indicated that limited numbers of viral variants crossed the vaginal mucosa and established infection. Therefore, the route of infection can clearly influence early viral selection and diversity. In addition, a third i.VAG-infected macaque, which was a rapid progressor, did not seroconvert and progressed to AIDS in 120 days. This macaque exhibited a high viral load and heterogeneous quasispecies. These data demonstrate differences in the quasispecies complexity associated with route of infection and rate of disease progression.

Changes in the extracellular envelope glycoprotein of variants that evolve during the course of simian immunodeficiency virus SIVMne infection affect neutralizing antibody recognition, syncytium formation, and macrophage tropism but not replication, cytopathicity, or CCR-5 coreceptor recognition

Simian immunodeficiency virus SIVMne, like human immunodeficiency virus, evolves from a macrophage-tropic, non-syncytium-inducing virus at early times in infection to a T-cell-tropic, syncytium-inducing, cytopathic virus population over the course of progression to AIDS. Because the viruses isolated late in SIVMne infection of macaques include a complex mixture of variants, the viral determinants of such phenotypic changes have not been defined. To identify genetic changes that are important to virus evolution in the host, we constructed chimeric viruses by introducing variant envelope genes representative of proviruses throughout the course of infection and disease into the SIVMne parental clone (SIVMneCL8) that infected the macaque. The chimeric viruses expressed sequences encoding the surface unit of the envelope glycoprotein (Env-SU) of variants cloned between 35 and 170 weeks postinfection. The chimera with Env-SU from 35 weeks postinfection encoded only four changes in V1 compared to SIVMneCL8, whereas the chimeras encoding Env-SU from variants isolated later in infection encoded progressively more mutations both in V1 and elsewhere. Like SIVMneCL8, the chimeras were infectious for CEMx174 cells and macaque peripheral blood mononuclear cells. However, in contrast to SIVMneCL8, the chimeric viruses did not infect macaque macrophages, although each retained the ability to recognize the CCR-5 coreceptor. Thus, these data provide direct evidence that changes which evolve in Env-SU during the course of SIVMne infection do not alter CCR-5 interactions. Viruses encoding Env-SU from the latest times in infection (121 to 170 weeks postinfection), after disease was apparent, were syncytium inducing. However, these viruses were not highly cytopathic, suggesting that additional viral determinants may be required for the rapidly replicating, cytopathic phenotype of the uncloned mixed variant population. Changes in Env-SU did allow the virus to escape serum neutralizing antibodies that recognized the SIVMneCL8 parent. Moreover, the chimera encoding the Env-SU of a virus from 35 weeks postinfection, which differed from SIVMneCL8 only in V1, was not sensitive to neutralization by infected macaque sera, suggesting that V1 may define a portion of the principal neutralizing determinant for SIVMne. Together, these data suggest that SIV variants with changes in the Env-SU may be selected primarily by virtue of their ability to escape neutralizing antibody recognition.

Natural infection of a household pet red-capped mangabey (Cercocebus torquatus torquatus) with a new simian immunodeficiency virus

A seroprevalence survey was conducted for simian immunodeficiency virus (SIV) antibody in household pet monkeys in Gabon. Twenty-nine monkeys representing seven species were analyzed. By using human immunodeficiency virus type 2 (HIV-2)/SIVsm, SIVmnd, and SIVagm antigens, one red-capped mangabey (RCM) (Cercocebus torquatus torquatus) was identified as harboring SIV-cross-reactive antibodies. A virus isolate, termed SIVrcm, was subsequently established from this seropositive RCM by cocultivation of its peripheral blood mononuclear cells (PBMC) with PBMC from seronegative humans or RCMs. SIVrcm was also isolated by cocultivation of CD8-depleted RCM PBMC with Molt 4 clone 8 cells but not with CEMx174 cells. The lack of growth in CEMx174 cells distinguished this new SIV from all previously reported sooty mangabey-derived viruses (SIVsm), which grow well in this cell line. SIVrcm was also successfully transmitted (cell free) to human and rhesus PBMC as well as to Molt 4 clone 8 cells. To determine the evolutionary origins of this newly identified virus, subgenomic pol (475 bp) and gag (954 bp) gene fragments were amplified from infected cell culture DNA and sequenced. The position of SIVrcm relative to those of members of the other primate lentivirus lineages was then examined in evolutionary trees constructed from deduced protein sequences. This analysis revealed significantly discordant phylogenetic positions of SIVrcm in the two genomic regions. In trees derived from partial gag sequences, SIVrcm clustered independently from all other HIV and SIV strains, consistent with a new primate lentivirus lineage. However, in trees derived from pol sequences, SIVrcm grouped with the HIV-1/SIVcpz lineage. These findings suggest that the SIVrcm genome is mosaic and possibly is the result of a recombination event involving divergent lentiviruses in the distant past. Further analysis of this and other SIVrcm isolates may shed new light on the origin of HIV-1.

Characterization of a molecular clone of HIV type 2 infectious for Macaca nemestrina

A lambda phage clone containing a full-length HIV-2 provirus, designated HIV-2KR, was obtained from the genomic DNA of Molt4 clone 8 (Molt4/8) lymphoblastic cells infected with the HIV-2PEI2 strain. HIV-2KR is genetically distinct from known HIV-2 isolates, possessing both a unique deletion in the LTR promoter region, and a long rev reading frame. It is replication competent in vitro after transfection into Molt4/8 cells, replicates in a variety of established human T lymphoblastic (Molt-3, Molt4/8, SupT1, H9, C8166) and myelomonocytic (U937) cell lines, and displays prominent cytopathic effects on infection of Molt4/8 cells, reflecting usage of both CCR5 and CXCR4 coreceptors. In addition, HIV-2KR was found to be infectious for human and Macaca nemestrina peripheral blood lymphocytes, and primary human monocyte-macrophage cultures. Intravenous inoculation of cell-free virus into M. nemestrina resulted in infection characterized by transient, low-level viremia and modest temporary decline in CD4 lymphocyte numbers, making HIV-2KR the first HIV-2 molecular clone reported to be infectious for this primate species.

Tracking members of the simian immunodeficiency virus deltaB670 quasispecies population in vivo at single-cell resolution

Genetically distinct lentiviruses constitute a quasispecies population that can evolve in response to selective forces. To move beyond characterization of the population as a whole to the behavior of individual members, we devised an in situ hybridization approach that uses genotype-specific probes. We used probes that detect simian immunodeficiency viruses (SIV) that differ in sequence in the V1 region of the surface envelope glycoprotein (env) gene to investigate the replication and cellular tropisms of four viral variants in the tissues of infected rhesus macaques. We found that the V1 genotypic variants replicated in spatially defined patterns and to different extents at each anatomic site. The two variants that replicated most extensively in animals with AIDS were detected in both macrophages and T lymphocytes in tissues. By extension of this approach, it will be possible to investigate the role of individual lentiviruses in a quasispecies in pathogenesis and to evaluate the effects of antiviral or immunotherapeutic treatment on select members of a quasispecies.

CD4-independent, CCR5-dependent infection of brain capillary endothelial cells by a neurovirulent simian immunodeficiency virus strain

Brain capillary endothelial cells (BCECs) are targets of CD4-independent infection by HIV-1 and simian immunodeficiency virus (SIV) strains in vitro and in vivo. Infection of BCECs may provide a portal of entry for the virus into the central nervous system and could disrupt blood-brain barrier function, contributing to the development of AIDS dementia. We found that rhesus macaque BCECs express chemokine receptors involved in HIV and SIV entry including CCR5, CCR3, CXCR4, and STRL33, but not CCR2b, GPR1, or GPR15. Infection of BCECs by the neurovirulent strain SIV/17E-Fr was completely inhibited by aminooxypentane regulation upon activation, normal T cell expression and secretion in the presence or absence of ligands, but not by eotaxin or antibodies to CD4. We found that the envelope (env) proteins from SIV/17E-Fr and several additional SIV strains mediated cell-cell fusion and virus infection with CD4-negative, CCR5-positive cells. In contrast, fusion with cells expressing the coreceptors STRL33, GPR1, and GPR15 was CD4-dependent. These results show that CCR5 can serve as a primary receptor for SIV in BCECs and suggest a possible CD4-independent mechanism for blood-brain barrier disruption and viral entry into the central nervous system.

Chimpanzee CXCR4 and CCR5 act as coreceptors for HIV type 1

Chemokine receptors are molecules involved in the fusion of immunodeficiency viruses after their attachment. As chimpanzees are the animal model for infection by HIV-1, we cloned and sequenced chimpanzee CXCR4 and CCR5 from PBMCs. Chimpanzee CXCR4 was found to be identical to human CXCR4, which provides an explanation for the sensitivity of chimpanzees to lymphotropic isolates of HIV-1. Chimpanzee CCR5 showed two substitutions with respect to human CCR5. However, we show that the macrophage-tropic isolate HIV-1-Ba-L can use chimpanzee CCR5 as a fusion receptor. Therefore, the resistance of chimpanzee PBMCs to infection by macrophage-tropic isolates of HIV-1 is unlikely to be due to substitutions in CCR5.

Utilization of chemokine receptors, orphan receptors, and herpesvirus-encoded receptors by diverse human and simian immunodeficiency viruses

Human immunodeficiency virus type 1 (HIV-1) requires both CD4 and a coreceptor to infect cells. Macrophage-tropic (M-tropic) HIV-1 strains utilize the chemokine receptor CCR5 in conjunction with CD4 to infect cells, while T-cell-tropic (T-tropic) strains generally utilize CXCR4 as a coreceptor. Some viruses can use both CCR5 and CXCR4 for virus entry (i.e., are dual-tropic), while other chemokine receptors can be used by a subset of virus strains. Due to the genetic diversity of HIV-1, HIV-2, and simian immunodeficiency virus (SIV) and the potential for chemokine receptors other than CCR5 or CXCR4 to influence viral pathogenesis, we tested a panel of 28 HIV-1, HIV-2, and SIV envelope (Env) proteins for the ability to utilize chemokine receptors, orphan receptors, and herpesvirus-encoded chemokine receptor homologs by membrane fusion and virus infection assays. While all Env proteins used either CCR5 or CXCR4 or both, several also used CCR3. Use of CCR3 was strongly dependent on its surface expression levels, with a larger number of viral Env proteins being able to utilize this coreceptor at the higher levels of surface expression. ChemR1, an orphan receptor recently shown to bind the CC chemokine I309 (and therefore renamed CCR8), was expressed in monocyte and lymphocyte cell populations and functioned as a coreceptor for diverse HIV-1, HIV-2, and SIV Env proteins. Use of ChemR1/CCR8 by SIV strains was dependent in part on V3 loop sequences. The orphan receptor V28 supported Env-mediated cell-cell fusion by four T- or dual-tropic HIV-1 and HIV-2 strains. Three additional orphan receptors failed to function for any of the 28 Env proteins tested. Likewise, five of six seven-transmembrane-domain receptors encoded by herpesviruses did not support Env-mediated membrane fusion. However, the chemokine receptor US28, encoded by cytomegalovirus, did support inefficient infection by two HIV-1 strains. These findings indicate that additional chemokine receptors can function as HIV and SIV coreceptors and that surface expression levels can strongly influence coreceptor use.

CD4-independent binding of SIV gp120 to rhesus CCR5

CCR5 and CD4 are coreceptors for immunodeficiency virus entry into target cells. The gp120 envelope glycoprotein from human immunodeficiency virus strain HIV-1(YU2) bound human CCR5 (CCR5hu) or rhesus macaque CCR5 (CCR5rh) only in the presence of CD4. The gp120 from simian immunodeficiency virus strain SIVmac239 bound CCR5rh without CD4, but CCR5hu remained CD4-dependent. The CD4-independent binding of SIVmac239 gp120 depended on a single amino acid, Asp13, in the CCR5rh amino-terminus. Thus, CCR5-binding moieties on the immunodeficiency virus envelope glycoprotein can be generated by interaction with CD4 or by direct interaction with the CCR5 amino-terminus. These results may have implications for the evolution of receptor use among lentiviruses as well as utility in the development of effective intervention.

Cellular localization of the chemokine receptor CCR5. Correlation to cellular targets of HIV-1 infection

The chemokine receptor CCR5 has recently been described as a co-receptor for macrophage-tropic strains of human immunodeficiency virus (HIV)-1. In this study, using a panel of monoclonal antibodies specific for human CCR5, we show by immunohistochemistry and flow cytometry that CCR5 is expressed by bone-marrow-derived cells known to be targets for HIV-1 infection, including a subpopulation of lymphocytes and monocyte/macrophages in blood, primary and secondary lymphoid organs, and noninflamed tissues. In the central nervous system, CCR5 is expressed on neurons, astrocytes, and microglia. In other tissues, CCR5 is expressed on epithelium, endothelium, vascular smooth muscle, and fibroblasts. Chronically inflamed tissues contain an increased number of CCR5+ mononuclear cells, and the number of immunoreactive cells is directly associated with a histopathological correlate of inflammatory severity. Collectively, these results suggest that CCR5+ cells are recruited to inflammatory sites and, as such, may facilitate transmission of macrophage-tropic strains of HIV-1.

In vivo evolution of HIV-1 co-receptor usage and sensitivity to chemokine-mediated suppression

Following the identification of the C-C chemokines RANTES, MIP-1alpha and MIP-1beta as major human immunodeficiency virus (HIV)-suppressive factors produced by CD8+ T cells, several chemokine receptors were found to serve as membrane co-receptors for primate immunodeficiency lentiretroviruses. The two most widely used co-receptors thus far recognized, CCR5 and CXCR4, are expressed by both activated T lymphocytes and mononuclear phagocytes. CCR5, a specific RANTES, MIP-1alpha and MIP-1 receptor, is used preferentially by non-MT2-tropic HIV-1 and HIV-2 strains and by simian immunodeficiency virus (SIV), whereas CXCR4, a receptor for the C-X-C chemokine SDF-1, is used by MT2-tropic HIV-1 and HIV-2, but not by SIV. Other receptors with a more restricted cellular distribution, such as CCR2b, CCR3 and STRL33, can also function as co-receptors for selected viral isolates. The third variable region (V3) of the gp120 envelope glycoprotein of HIV-1 has been fingered as a critical determinant of the co-receptor choice. Here, we document a consistent pattern of evolution of viral co-receptor usage and sensitivity to chemokine-mediated suppression in a longitudinal follow-up of children with progressive HIV-1 infection. Viral isolates obtained during the asymptomatic stages generally used only CCR5 as a co-receptor and were inhibited by RANTES, MIP-1alpha and MIP-1beta, but not by SDF-1. By contrast, the majority of the isolates derived after the progression of the disease were resistant to C-C chemokines, having acquired the ability to use CXCR4 and, in some cases, CCR3, while gradually losing CCR5 usage. Surprisingly, most of these isolates were also insensitive to SDF-1, even when used in combination with RANTES. An early acquisition of CXCR4 usage predicted a poor prognosis. In children who progressed to AIDS without a shift to CXCR4 usage, all the sequential isolates were CCR5-dependent but showed a reduced sensitivity to C-C chemokines. Discrete changes in the V3 domain of gp120 were associated with the loss of sensitivity to C-C chemokines and the shift in co-receptor usage. These results suggest an adaptive evolution of HIV-1 in vivo, leading to escape from the control of the antiviral C-C chemokines.

Infection of Macaca nemestrina neonates with HIV-1 via different routes of inoculation

OBJECTIVES

Receptive anal intercourse but not orogenital sex has been identified as a major risk factor for transmission of HIV-1. Recent studies using simian immunodeficiency virus (SIV) in rhesus macaques have demonstrated relatively efficient infection following oral administration, indicating that modes of transmission may vary between HIV-1 and SIV. Here, we investigate whether HIV-1 infection of macaques via the oral route is more efficient than via the rectal route.

DESIGN

Eleven Macaca nemestrina neonates were exposed to HIV-1 via different routes (four oral, two intravenous, and five rectal). One animal was orally inoculated with a sham inoculum and two control animals were not exposed.

METHODS

All animals were followed for virological signs of infection, and for pathogenesis associated with HIV-1 infection by general physical examinations, complete blood cell counts and lymphocyte subset analysis, and full necropsies.

RESULTS

Three out of five rectally exposed macaques and both of the intravenously inoculated animals became infected with HIV-1, whereas none of the orally exposed animals showed evidence of HIV-1 infection. Clinical observations following exposure included failure to thrive in the orally inoculated animals and low CD4/CD8 ratios in the rectally exposed macaques.

CONCLUSIONS

The finding that, contrary to what has been reported for SIV, transmission of HIV-1 via the oral route is not more efficient than via the rectal route, indicates important biological differences between HIV-1 and SIV, with direct implications for the spread of HIV and associated AIDS, and for development of anti-HIV-1 vaccines.

Usage of the coreceptors CCR-5, CCR-3, and CXCR-4 by primary and cell line-adapted human immunodeficiency virus type 2

The chemokine receptors CCR-5 and CXCR-4, and possibly CCR-3, are the principal human immunodeficiency virus type 1 (HIV-1) coreceptors, apparently interacting with HIV-1 envelope, in association with CD4. Cell lines coexpressing CD4 and these chemokine receptors were infected with a panel of seven primary HIV-2 isolates passaged in peripheral blood mononuclear cells (PBMC) and three laboratory HIV-2 strains passaged in T-cell lines. The CCR-5, CCR-3, and CXCR-4 coreceptors could all be used by HIV-2. The ability to use CXCR-4 represents a major difference between HIV-2 and the closely related simian immunodeficiency viruses. Most HIV-2 strains using CCR-5 could also use CCR-3, sometimes with similar efficiencies. As observed for HIV-1, the usage of CCR-5 or CCR-3 was observed principally for HIV-2 strains derived from asymptomatic individuals, while HIV-2 strains derived from AIDS patients used CXCR-4. However, there were several exceptions, and the patterns of coreceptor usage seemed more complex for HIV-2 than for HIV-1. The two T-tropic HIV-2 strains tested used CXCR-4 and not CCR-5, while T-tropic HIV-1 can generally use both. Moreover, among five primary HIV-2 strains all unable to use CXCR-4, three could replicate in CCR-5-negative PBMC, which has not been reported for HIV-1. These observations suggest that the CCR-5 coreceptor is less important for HIV-2 than for HIV-1 and indicate that HIV-2 can use other cell entry pathways and probably other coreceptors. One HIV-2 isolate replicating in normal or CCR-5-negative PBMC failed to infect CXCR-4+ cells or the U87MG-CD4 and sMAGI cell lines, which are permissive to infection by HIV-2 but not by HIV-1. This suggests the existence of several HIV-2-specific coreceptors, which are differentially expressed in cell lines and PBMC.

Macrophage-tropic HIV and SIV envelope proteins induce a signal through the CCR5 chemokine receptor

Human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) enter target cells by forming a complex between the viral envelope protein and two cell-surface membrane receptors: CD4 and a 7-span transmembrane chemokine receptor. Isolates of HIV that differ in cellular tropism use different subsets of chemokine receptors as entry cofactors: macrophage-tropic HIVs primarily use CCR5, whereas T-cell-tropic and dual-tropic isolates use CXCR4 receptors. HIV-mediated signal transduction through CCR5 is not required for efficient fusion and entry of HIV in vitro. Here we show that recombinant envelope proteins from macrophage-tropic HIV and SIV induce a signal through CCR5 on CD4+ T cells and that envelope-mediated signal transduction through CCR5 induces chemotaxis of T cells. This chemotactic response may contribute to the pathogenesis of HIV in vivo by chemo-attracting activated CD4+ cells to sites of viral replication. HIV-mediated signalling through CCR5 may also enhance viral replication in vivo by increasing the activation state of target cells. Alternatively, envelope-mediated CCR5 signal transduction may influence viral-associated cytopathicity or apoptosis.

Two orphan seven-transmembrane segment receptors which are expressed in CD4-positive cells support simian immunodeficiency virus infection

Clinical isolates of primate immunodeficiency viruses, including human immunodeficiency virus type 1 (HIV-1), enter target cells by sequential binding to CD4 and the chemokine receptor CCR5, a member of the seven-transmembrane receptor family. HIV-1 variants which use additional chemokine receptors are present in the central nervous system or emerge during the course of infection. Simian immunodeficiency viruses (SIV) have been shown to use CCR5 as a coreceptor, but no other receptors for these viruses have been identified. Here we show that two orphan seven-transmembrane segment receptors, gpr1 and gpr15, serve as coreceptors for SIV, and are expressed in human alveolar macrophages. The more efficient of these, gpr15, is also expressed in human CD4(+) T lymphocytes and activated rhesus macaque peripheral blood mononuclear cells. The gpr15 and gpr1 proteins lack several hallmarks of chemokine receptors, but share with CCR5 an amino-terminal motif rich in tyrosine residues. These results underscore the potential diversity of seven-transmembrane segment receptors used as entry cofactors by primate immunodeficiency viruses, and may contribute to an understanding of viral variation and pathogenesis.

Co-receptors for HIV-1 entry

HIV-1 enters its target cells by fusion at the plasma membrane. The primary cellular receptor for HIV is CD4, but this molecule is insufficient to permit viral fusion. During 1996, the necessary entry co-factors (co-receptors or second receptors) were identified as being members of the seven-transmembrane-spanning receptor family fusin: CXCR4 for T-tropic strains and CCR5, principally, for M-tropic strains. The co-receptor functions of these proteins are inhibited by their natural alpha- and beta-chemokine ligands.

Discovery of cyanovirin-N, a novel human immunodeficiency virus-inactivating protein that binds viral surface envelope glycoprotein gp120: potential applications to microbicide development

We have isolated and sequenced a novel 11-kDa virucidal protein, named cyanovirin-N (CV-N), from cultures of the cyanobacterium (blue-green alga) Nostoc ellipsosporum. We also have produced CV-N recombinantly by expression of a corresponding DNA sequence in Escherichia coli. Low nanomolar concentrations of either natural or recombinant CV-N irreversibly inactivate diverse laboratory strains and primary isolates of human immunodeficiency virus (HIV) type 1 as well as strains of HIV type 2 and simian immunodeficiency virus. In addition, CV-N aborts cell-to-cell fusion and transmission of HIV-1 infection. Continuous, 2-day exposures of uninfected CEM-SS cells or peripheral blood lymphocytes to high concentrations (e.g., 9,000 nM) of CV-N were not lethal to these representative host cell types. The antiviral activity of CV-N is due, at least in part, to unique, high-affinity interactions of CV-N with the viral surface envelope glycoprotein gp120. The biological activity of CV-N is highly resistant to physicochemical denaturation, further enhancing its potential as an anti-HIV microbicide.

Multiple extracellular domains of CCR-5 contribute to human immunodeficiency virus type 1 entry and fusion

Human immunodeficiency virus type 1 (HIV-1) entry is governed by the interaction of the viral envelope glycoprotein (Env) with its receptor. The HIV-1 receptor is composed of two molecules, the CD4 binding receptor and a coreceptor. The seven-membrane-spanning chemokine receptor CCR-5 is one of the coreceptors used by primary isolates of HIV-1. We demonstrate that the mouse homolog of CCR-5 (mCCR-5) does not function as an HIV-1 coreceptor. A set of chimeras of human CCR-5 and mCCR-5 was studied for Env-induced cell fusion and HIV-1 infection. Using the HIV-1ADA envelope glycoprotein in a syncytium formation assay, we show that replacement of any fragment containing extracellular domains of mCCR-5 by its human counterparts is sufficient to allow Env-induced fusion. Conversely, replacement of any fragment containing human extracellular domains by its murine counterpart did not lead to coreceptor function loss. These results show that several domains of CCR-5 participate in coreceptor function. In addition, using a panel of primary nonsyncytium-inducing and syncytium-inducing isolates that use CCR-5 or both CXCR-4 and CCR-5 as coreceptors, we show that the latter dual-tropic isolates are less tolerant to changes in CCR-5 than strains with a more restricted coreceptor use. Thus, different strains are likely to have different ways of interacting with the CCR-5 coreceptor.

Identification of a chemokine receptor encoded by human cytomegalovirus as a cofactor for HIV-1 entry

The human cytomegalovirus encodes a beta-chemokine receptor (US28) that is distantly related to the human chemokine receptors CCR5 and CXCR4, which also serve as cofactors for the entry into cells of human immunodeficiency virus-type 1 (HIV-1). Like CCR5, US28 allowed infection of CD4-positive human cell lines by primary isolates of HIV-1 and HIV-2, as well as fusion of these cell lines with cells expressing the viral envelope proteins. In addition, US28 mediated infection by cell line-adapted HIV-1 for which CXCR4 was an entry cofactor.