Naturally SIV-infected sooty mangabeys (SMs) do not progress to AIDS despite high-level virus replication. We previously showed that the fraction of CD4+CCR5+ T-cells is lower in SMs compared to humans and macaques. Here we found that, after in vitro stimulation, SM CD4+ T-cells fail to up-regulate CCR5, and that this phenomenon is more pronounced in CD4+ central-memory T-cells (TCM). CD4+ T-cell activation was similarly uncoupled from CCR5 expression in SMs in vivo during (i) acute SIV infection and (ii) following antibody-mediated CD4+ T-cell depletion. Remarkably, CD4+ TCM of SMs that express low levels of CCR5 demonstrated reduced susceptibility to SIV infection both in vivo and in vitro when compared to CD4+ TCM of RMs. These data suggest that low CCR5 expression on SM CD4+ T-cells favors the preservation of CD4+ T-cell homeostasis and promotes an AIDS-free status by protecting CD4+ TCM from direct virus infection.

In contrast to HIV infection in humans and SIV in macaques, SIV infection of natural hosts including sooty mangabeys (SM) is non-pathogenic despite robust virus replication. We identified a novel SM CCR5 allele containing a two base pair deletion (Δ2) encoding a truncated molecule that is not expressed on the cell surface and does not support SIV entry in vitro. The allele was present at a 26% frequency in a large SM colony, along with 3% for a CCR5Δ24 deletion allele that also abrogates surface expression. Overall, 8% of animals were homozygous for defective CCR5 alleles and 41% were heterozygous. The mutant allele was also present in wild SM in West Africa. CD8+ and CD4+ T cells displayed a gradient of CCR5 expression across genotype groups, which was highly significant for CD8+ cells. Remarkably, the prevalence of natural SIVsmm infection was not significantly different in animals lacking functional CCR5 compared to heterozygous and homozygous wild-type animals. Furthermore, animals lacking functional CCR5 had robust plasma viral loads, which were only modestly lower than wild-type animals. SIVsmm primary isolates infected both homozygous mutant and wild-type PBMC in a CCR5-independent manner in vitro, and Envs from both CCR5-null and wild-type infected animals used CXCR6, GPR15 and GPR1 in addition to CCR5 in transfected cells. These data clearly indicate that SIVsmm relies on CCR5-independent entry pathways in SM that are homozygous for defective CCR5 alleles and, while the extent of alternative coreceptor use in SM with CCR5 wild type alleles is uncertain, strongly suggest that SIVsmm tropism and host cell targeting in vivo is defined by the distribution and use of alternative entry pathways in addition to CCR5. SIVsmm entry through alternative pathways in vivo raises the possibility of novel CCR5-negative target cells that may be more expendable than CCR5+ cells and enable the virus to replicate efficiently without causing disease in the face of extremely restricted CCR5 expression seen in SM and several other natural host species.

Increased mortality in antiretroviral (ARV)-treated, HIV-infected individuals has been attributed to persistent immune dysfunction, in part due to abnormalities at the gastrointestinal barrier. In particular, the poor reconstitution of gastrointestinal TH17 cells correlates with residual translocation of dysbiotic, immunostimulatory microflora across a compromised intestinal epithelial barrier. We have previously demonstrated that oral probiotics promote increased intestinal CD4+ T-cell reconstitution during ARV treatment in a non-human primate model of HIV infection; however, essential mucosal T-cell subsets, such as TH17 cells, had limited recovery. Here, we sought to promote TH17 cell recovery by administering IL-21 to a limited number of ARV-treated, probiotic-supplemented, SIV-infected pigtailed macaques. We demonstrate that probiotic and IL-21 supplementation of ARVs is associated with enhanced polyfunctional TH17 expansion and reduced markers of microbial translocation and dysbiosis as compared to infected controls receiving ARVs alone. Importantly, treatment resulted in fewer morbidities compared to controls, and was independent of increased immune activation or loss of viral suppression. We propose that combining ARVs with therapeutics aimed at restoring intestinal stasis may significantly improve disease prognosis of ARV-treated, HIV-infected, individuals.

Natural host sooty mangabeys (SM) infected with simian immunodeficiency virus SIVsmm do not develop AIDS despite high viremia. SM and other natural hosts express very low levels of CCR5 on CD4 ؉ T cells, and we recently showed that SIVsmm infection and robust replication occur in vivo in SM genetically lacking CCR5, indicating the use of additional entry pathways. SIVsmm uses several alternative coreceptors of human origin in vitro, but which molecules of SM origin support entry is unknown. We cloned a panel of putative coreceptors from SM and tested their ability to mediate infection, in conjunction with smCD4, by pseudotypes carrying Envs from multiple SIVsmm subtypes. smCXCR6 supported efficient infection by all SIVsmm isolates with entry levels comparable to those for smCCR5, and smGPR15 enabled entry by all isolates at modest levels. smGPR1 and smAPJ supported low and variable entry, whereas smCCR2b, smCCR3, smCCR4, smCCR8, and smCXCR4 were not used by most isolates. In contrast, SIVsmm from rare infected SM with profound CD4 ؉ T cell loss, previously reported to have expanded use of human coreceptors, including CXCR4, used smCXCR4, smCXCR6, and smCCR5 efficiently and also exhibited robust entry through smCCR3, smCCR8, smGPR1, smGPR15, and smAPJ. Entry was similar with both known alleles of smCD4. These alternative coreceptors, particularly smCXCR6 and smGPR15, may support virus replication in SM that have restricted CCR5 expression as well as SM genetically lacking CCR5. Defining expression of these molecules on SM CD4 ؉ subsets may delineate distinct natural host target cell populations capable of supporting SIVsmm replication without CD4 ؉ T cell loss. HIV-1 infection of humans and simian immunodeficiency virus SIVmac infection of non-natural host rhesus macaques (RM) result in high viral loads, peripheral CD4 ϩ T cell loss, and progression to AIDS. In contrast, SIVsmm infection of natural host sooty mangabeys (SM) rarely leads to peripheral CD4 ϩ T cell loss or disease despite viral loads comparable to those measured in non-natural hosts. Identifying the mechanisms responsible for the different outcomes of infection in natural hosts compared with non-natural simian or recent human hosts has become a central focus of efforts to understand AIDS pathogenesis. Although many models of natural host infection exist, SIVsmm is unique because cross-species transmission of SIVsmm from SM to humans and RM gave rise to pathogenic HIV-2 and SIVmac, respectively (2, 28). Similarly, experimental transmission of SIVsmm from infected SM hosts results in AIDS in non-natural host RM (40). While the factors regulating pathogenic versus nonpathogenic outcomes of infection are complex and not fully understood (9), comparison of infected SM with RM recently revealed differential targeting of CD4 ϩ T cell subsets (45). Thus, an understanding of SIVsmm cellular tropism may identify cells in natural host SM that maintain viral replication without leading to CD4 ϩ T cell depletion or AIDS.Classically, SIVsmm was thought to exclus...

Natural-host sooty mangabeys (SM) infected with simian immunodeficiency virus (SIV) exhibit high viral loads but do not develop disease, whereas infection of rhesus macaques (RM) causes CD4؉ T cell loss and AIDS. Several mechanisms have been proposed to explain these divergent outcomes, including differences in cell targeting, which have been linked to low expression of the canonical SIV entry receptor CCR5 on CD4 ؉ T cells of SM and other natural hosts. We previously showed that infection and high-level viremia occur even in a subset of SM that genetically lack functional CCR5, which indicates that alternative entry coreceptors are used by SIV in vivo in these animals. We also showed that SM CXCR6 is a robust coreceptor for SIV smm in vitro. Here we identify CXCR6 as a principal entry pathway for SIV in SM primary lymphocytes. We show that ex vivo SIV infection of lymphocytes from CCR5 wild-type SM is mediated by both CXCR6 and CCR5. In contrast, infection of RM lymphocytes is fully dependent on CCR5. These data raise the possibility that CXCR6-directed tropism in CCR5-low natural hosts may alter CD4 ؉ T cell subset targeting compared with that in nonnatural hosts, enabling SIV to maintain high-level replication without leading to widespread CD4 ؉ T cell loss.