In contrast to simian immunodeficiency viruses (SIVs), which induce immunodeficiency over a 1- to 3-year period, highly pathogenic simian-human immunodeficiency viruses (SHIVs) cause a complete, irreversible, and systemic depletion of CD4⁺T lymphocytes in rhesus monkeys within weeks of infection. By using small-molecule competitors specific for CCR5 and CXCR4 inex vivoassays, we found that highly pathogenic SHIV_DH12Rexclusively uses CXCR4 for infection of rhesus peripheral blood mononuclear cells, whereas SIV_mac239and SIV_smE543use CCR5 for entry into the same cells. During the period of peak virus production in SHIV_DH12R- or SHIV_89.6P-infected rhesus monkeys, massive elimination of CXCR4⁺naïve CD4⁺T cells occurred. In contrast, circulating CCR5⁺memory CD4⁺T cells were selectively depleted in rapidly progressing SIV-infected monkeys. At the time of their death, two SIV rapid progressors had experienced a nearly complete loss of the memory CD4⁺T cell subset from the blood and mesenteric lymph nodes. Thus, pathogenic SHIVs and SIVs target different subsets of CD4⁺T cellsin vivo, with the pattern of CD4⁺T lymphocyte depletion being inextricably linked to chemokine receptor use. In the context of developing an effective prophylactic vaccine, which must potently control virus replication during the primary infection, regimens that suppress SHIVs might not protect monkeys against SIV or humans against HIV-1.

The narrow host range of human immunodeficiency virus type 1 (HIV-1) is caused in part by innate cellular factors such as apolipoprotein B mRNA-editing enzyme-catalytic polypeptide-like 3G (APOBEC3G) and TRIM5α, which restrict virus replication in monkey cells. Variant HIV-1 molecular clones containing both a 21-nucleotide simian immunodeficiency virus (SIV) Gag CA element, corresponding to the HIV-1 cyclophilin A-binding site, and the entire SIV vif gene were constructed. Long-term passage in a cynomolgus monkey lymphoid cell line resulted in the acquisition of two nonsynonymous changes in env , which conferred improved replication properties. A proviral molecular clone, derived from infected cells and designated NL-DT5R, was used to generate virus stocks capable of establishing spreading infections in the cynomolgus monkey T cell line and CD8-depleted peripheral blood mononuclear cells from five of five pig-tailed macaques and one of three rhesus monkeys. NL-DT5R, which genetically is >93% HIV-1, provides the opportunity, not possible with currently available SIV/HIV chimeric viruses, to analyze the function of multiple HIV-1 genes in a broad range of nonhuman primate species.

It is widely believed that the induction of a broadly neutralizing antibody (bNAb) response will be a critical component of a successful vaccine against HIV. A significant fraction of HIV-infected individuals mount bNAb responses, providing support for the notion that such responses could be elicited through vaccination. Infection of macaques with simian immunodeficiency virus (SIV) or SIV/HIV chimeric virus (SHIV) has been widely used to model aspects of HIV infection, but to date, only limited bNAb responses have been described. Here, we screened plasma from 14 R5-tropic SHIV-infected macaques for broadly neutralizing activity and identified a macaque with highly potent cross-clade plasma NAb response. Longitudinal studies showed that the development of broad and autologous NAb responses occurred coincidentally in this animal. Serum-mapping studies, using pseudovirus point mutants and antigen adsorption assays, indicated that the plasma bNAbs are specific for epitopes that include carbohydrates and are critically dependent on the glycan at position 332 of Env gp120. The results described herein provide insight into the development and evolution of a broad response, suggest that certain bNAb specificities may be more rapidly induced by immunization than others, and provide a potential model for the facile study of the development of bNAb responses.

The induction of broadly reacting neutralizing antibodies has been a major goal of HIV vaccine research. Characterization of a pathogenic CCR5 (R5)-tropic SIV/HIV chimeric virus (SHIV) molecular clone (SHIV _AD8-EO ) revealed that eight of eight infected animals developed cross-reactive neutralizing antibodies (NAbs) directed against an envelope glycoprotein derived from the heterologous HIV-1 _DH12 strain. A panel of plasmas, collected from monkeys inoculated with either molecularly cloned or uncloned SHIV _AD8 stocks, exhibited cross-neutralization against multiple tier 1 and tier 2 HIV-1 clade B isolates. One SHIV _AD8 -infected animal also developed NAbs against clades A and C HIV-1 strains. In this particular infected macaque, the cross-reacting anti–HIV-1 NAbs produced between weeks 7 and 13 were directed against a neutralization-sensitive virus strain, whereas neutralizing activities emerging at weeks 41–51 targeted more neutralization-resistant HIV-1 isolates. These results indicate that the SHIV _AD8 macaque model represents a potentially valuable experimental system for investigating B-cell maturation and the induction of cross-reactive NAbs directed against multiple HIV-1 strains.

Sequential immunization elicits antibodies against the V3-glycan patch of HIV-1 Env in rhesus macaques and also induces off-target antibody responses.

A highly pathogenic simian/human immunodeficiency virus (SHIV), SHIV _DH12R , isolated from a rhesus macaque that had been treated with anti-human CD8 monoclonal antibody at the time of primary infection with the nonpathogenic, molecularly cloned SHIV _DH12 , induced marked and rapid CD4 ⁺ T cell loss in all rhesus macaques intravenously inoculated with 1.0 50% tissue culture infective dose (TCID ₅₀ ) to 4.1 × 10 ⁵ TCID ₅₀ s of virus. Animals inoculated with 650 TCID ₅₀ s of SHIV _DH12R or more experienced irreversible CD4 ⁺ T lymphocyte depletion and developed clinical disease requiring euthanasia between weeks 12 and 23 postinfection. In contrast, the CD4 ⁺ T-cell numbers in four of five monkeys receiving 25 TCID ₅₀ s of SHIV _DH12R or less stabilized at low levels, and these surviving animals produced antibodies capable of neutralizing SHIV _DH12R . In the fifth monkey, no recovery from the CD4 ⁺ T cell decline occurred, and the animal had to be euthanized. Viral RNA levels, subsequent to the initial peak of infection but not at peak viremia, correlated with the virus inoculum size and the eventual clinical course. Both initial infection rate constants, k , and decay constants, d , were determined, but only the latter were statistically correlated to clinical outcome. The attenuating effects of reduced inoculum size were also observed when virus was inoculated by the mucosal route. Because the uncloned SHIV _DH12R stock possessed the genetic properties of a lentivirus quasispecies, we were able to assess the evolution of the input virus swarm in animals surviving the acute infection by monitoring the emergence of neutralization escape viral variants.