The basis for the switch from CCR5 to CXCR4 coreceptor usage seen in ϳ50% of human immunodeficiency virus type 1 (HIV-1) subtype B-infected individuals as disease advances is not well understood. Among the reasons proposed are target cell limitation and better immune recognition of the CXCR4 (X4)-tropic compared to the CCR5 (R5)-tropic virus. We document here X4 virus emergence in a rhesus macaque (RM) infected with R5-tropic simian/human immunodeficiency virus, demonstrating that coreceptor switch can happen in a nonhuman primate model of HIV/AIDS. The switch to CXCR4 usage in RM requires envelope sequence changes in the V3 loop that are similar to those found in humans, suggesting that the R5-to-X4 evolution pathways in the two hosts overlap. Interestingly, compared to the inoculating R5 virus, the emerging CXCR4-using virus is highly neutralization sensitive. This finding, coupled with the observation of X4 evolution and appearance in an animal with undetectable circulating virus-specific antibody and low cellular immune responses, lends further support to an inhibitory role of antiviral immunity in HIV-1 coreceptor switch.The human immunodeficiency virus (HIV) enters target cells via interaction of the viral glycoprotein with the cellular receptor CD4 and two principal coreceptors, CCR5 (R5 viruses) and CXCR4 (X4 viruses) (2). Most HIV type 1 (HIV-1) transmission results in a predominantly R5 virus infection. With time, X4 variants arise and coexist with R5 virus variants in ϳ50% of subtype B-infected individuals, and this event is associated with rapid CD4 ϩ T-cell loss and disease progression (22, 37). The determinant(s) of phenotypic change from R5 to X4 maps largely to the V3 loop of the envelope gp120 (6, 18, 39) and can be inferred by analysis of the amino acid sequence of this region (11). Although the underlying basis for virus coreceptor switch late in infection remains ill defined, several hypotheses that include changes in target cell populations during the course of infection and/or differential immune recognition of X4 and R5 viruses have been proposed (31,34). Furthermore, it is unclear whether X4 viruses evolve during the course of infection or are transmitted but selected against early in infection.We have used infection of rhesus macaques (RM) with simian/human immunodeficiency viruses (SHIV) expressing the envelopes of X4 and R5 HIV-1 isolates to study the impact of coreceptor usage in virus transmission and pathogenesis. We previously reported that both X4 and R5 SHIVs can be transmitted intravenously or intravaginally but showed that the basis for the immunodeficiencies caused by these viruses is different. Whereas primary infection with X4 SHIV caused severe and sustained peripheral blood and secondary lymphoid tissue CD4ϩ T-cell loss, infection with R5 SHIV resulted in transient loss of CD4 ϩ T cells at these sites (15, 17). Thus, infection with SHIVs of different coreceptor usage recapitulates the different stages of HIV infection in humans: R5 SHIV provides a model of early infection ...

Environmental stressors impact physiology and behavior in many species of animals. These effects are partly mediated through changing concentrations of glucocorticoids, which also vary with reproductive state and social conditions. Prior research has focused largely on seasonal breeders, but the close temporal linkage between season and reproductive state in these species hinders ability to disentangle environmental effects from those of the animal's reproductive status. Here we assessed the effects of environmental challenges on the fecal glucocorticoid (fGC) levels of non-seasonal breeders, female baboons (Papio cynocephalus) of Amboseli, Kenya. Amboseli is characterized by a long dry season, during which food and water become scarce, and by extreme temperatures above 40°C in the shade during some months of the year. We found that after accounting for female reproductive status and individual variability, females exhibited higher fGC levels during the dry season than during the wet season. Further, during the wet season, fGC levels were higher in months of high average daily maximum temperatures. During the dry season, fGC levels were elevated both in hotter months and in months during which the baboons spent a relatively high proportion of time feeding. In spite of these stressors, female baboons reproduce during all months of the year in Amboseli, unlike most other mammals in this environment. This may be attributable to their extreme adaptability, specifically their diversified diet, and their ability to modify their behavior, including their activity profiles.

We used experimental infection of rhesus macaques with envelope gp120 V3 loop isogenic simian-human immunodeficiency virus (SHIV) molecular clones to more clearly define the impact of human immunodeficiency virus type 1 coreceptor usage in target cell selectivity and the rates of CD4؉ -T-cell depletion. Functional assays demonstrate that substitution of the V3 loop of the pathogenic CXCR4-tropic (X4) SHIV SF33A2 molecular clone with the corresponding sequences from the CCR5-tropic (R5) SHIV SF162P3 isolate resulted in a switch of coreceptor usage from CXCR4 to CCR5. The resultant R5 clone, designated SHIV SF33A2(V3) , is replication competent in vivo, infecting two of two macaques by intravenous inoculation with peak viremia that is comparable to that seen in monkeys infected with X4-SHIV SF33A2 . But while primary infection with the X4 clone was accompanied by rapid and significant loss of peripheral and secondary lymphoid CD4 ؉ T lymphocytes, infection with R5-SHIV SF33A2(V3) led to only a modest and transient loss. However, substantial depletion of intestinal CD4 ؉ T cells was observed in R5-SHIV SF33A2(V3) -infected macaques. Moreover, naïve T cells that expressed high levels of CXCR4 were rapidly depleted in X4-SHIV SF33A2 -infected macaques, whereas R5-SHIV SF33A2(V3) infection mainly affected memory T cells that expressed CCR5. These findings in a unique isogenic system illustrate that coreceptor usage is the principal determinant of tissue and target cell specificity of the virus in vivo and dictates the dynamics of CD4؉ -T-cell depletion during SHIV infection.