Since its inception, the Immunopathogenesis Section has defined host factors involved in the pathogenesis of HIV disease, including the impact of the immune system on the propagation and/or control of virus replication. Conversely, the LIR has demonstrated the the effects of high levels of circulating HIV on the function of numerous components of the immune system; however, little is known regarding the potential role of endogenous suppression of HIV-specific immune responses in HIV disease. This project was designed to evaluate the potential role of immunosuppressive CD25+CD4+ regulatory T (Treg) cells in the dysfunction of HIV-specific responses in chronically HIV-infected subjects. In addition, the direct and indirect effects of CD25+ Treg cells on HIV replication were assessed. CD25+ CD4+ T cells isolated from the peripheral blood of HIV-infected donors included a subpopulation that expressed the cell surface and nuclear antigen phenotype characteristic of CD25+hi regulatory CD4+ T cells (Treg). The CD25+ CD4+ T cell subset was found to reduce, but not completely suppress, both CD4+ and CD8+ HIV-specific T cell proliferation and effector cytokine production in the majority of healthy asymptomatic HIV-infected subjects tested. As previously reported, CD25+hi Treg-mediated suppression was not significantly overcome by neutralization of IL-10 or TGF-beta. Unexpectedly, among individuals possessing HIV-specific CD4+ T helper responses, those who did not exhibit strong in vitro HIV-specific CD25+CD4+ Treg suppressor activity had a less favorable clinical status than those in whom this activity was detected. CD25+CD4+ Treg activity directed against allogeneic antigens was detected in most HIV-infected individuals regardless of the presence or absence of HIV-specific Treg cells. These data suggest that CD25+CD4+ Treg-mediated immunosuppression may play a role in the diminution of HIV-specific CD4+ and CD8+ T cell responses early in disease, but that other factors associated with HIV infection obscure or reduce this activity in individuals with more advanced disease. Furthermore, the association between the presence of functional HIV-specific Treg cells in the peripheral blood and a favorable clinical status suggests that host-mediated suppressive mechanisms may have potential benefits in the context of chronic immune activation such as that observed in HIV disease. The direct and indirect effects of CD25+CD4+ regulatory T cells on HIV replication were also investigated. CD25+CD4+ T cells isolated from HIV-infected subjects were tested for their effects on the HIV suppressive activities of autologous CD8+ T cells. Short term exposure to CD25+, but not CD25-, CD4+ T cells during polyclonal activation of CD8+ T cells resulted in a significant (up to 99 %) reduction in both soluble and cell contact-mediated suppression of endogenous HIV replication by CD8+ T cells. These effects required cell contact between CD25+CD4+ and CD8+ T cells. Supernatants derived from CD8+ T cells exposed to CD25+ cells were found to contain lower levels of RANTES and MIP-1beta, potent inhibitors of R5 HIV entry, than control supernatants. Loss of CD8+ T cell contact-mediated HIV suppressive activity resulting from short-term exposure to CD25+ CD4+ T cells was associated with significant reductions in the expression levels of surface activation markers and proliferation (CFSE) of CD8+ T cells over the period of virus isolation. The direct effects of CD25+CD4+ Treg-like cells on HIV replication were assessed using cellular subsets obtained from HIV uninfected subjects. CD25+CD4+ Treg-like cells that were generated in vitro expressed surface and nuclear markers consistent with fresh CD25+ Treg cells. Autologous CD4+ T cells pre- exposed to HIV produced significantly less virus following co-culture with uninfected CD25+ Treg-like cells as compared to control cells. In addition, CD25+ Treg-like cells were found to inhibit viral gene expression in HIV-exposed CD4+ T cells using single round replication competent HIV. Furthermore, direct infection of Treg-like cells resulted in very poor HIV production. These results have been confirmed using freshly isolated CD25+ Treg cells. Taken together, these data suggest that the potential impact of CD25+Treg cells on HIV immunopathogenesis is extremely complex and may include both detrimental and beneficial effects. A second project was designed to investigate the role of host factors in the generation and/or maintenance of cellular reservoirs of HIV. We have demonstrated that the lymphoid tissue microenvironment is very supportive of productive HIV infection in resting CD4+ T cells, a well described cellular reservoir of HIV. Using an in vitro human lymphoid tissue (tonsil) culture model, we have found that purified resting CD4+ T cells produced HIV mRNA, p24 protein and infectious virus while maintaining a resting, non-dividing T cell phenotype when re-introduced into the microenvironment of autologous lymphoid tissue, but not when cultured alone. HIV production from this cellular population was driven, in part, by pro-inflammatory cytokines and additional signals generated by the tissue matrix. These data repudiate the paradigm that HIV production in T cells requires classical T cell activation and demonstrate that the lymphoid tissue is an ideal microenvironment for suboptimal cellular activation leading to HIV replication in phenotypically resting CD4+ T cells.
