This project was designed to investigate the cellular and molecular pathways involved in the regulation of HIv replication by host and viral factors. We had previously demonstrated that the level of HIV replication in peripheral blood white cells in vitro reflects a complex balance of inductive and suppressive effects of numerous host factors, some of which exert their effects in an HIV strain-specific manner. Our studies have been expanded to focus on understanding the potentially unique properties of the lymphoid tissue (LT) microenvironment in regard to HIV replication in CD4+ T cell subsets. Results from experiments employing purified factors suggest that LT-associated factors, such as the chemokines and extracellular matrixes, may stimulate HIV life cycle events, including entry and proviral integration. We have expanded these investigations using an in vitro culture model to investigate productive HIV infection of various CD4+ T cell subsets in human LT (tonsil) . We have found in vitro HIV infection of intact LT fragments results in viral production from both activated and resting [cells lacking markers of classic T cell activation or proliferation] CD4+ T cells. Overall, the majority of CD4+ T cells producing HIV protein were found to be memory cells. X4 HIV strains were more effective than R5 HIV strains in productively infecting both resting and naive CD4+ T cells. The ability of resting CD4+ T cells to support HIV production was dependent on components of the tissue microenvironment; viral production did not occur in isolated HIV- exposed resting tonsillar CD4+ T cells. The potent immunosuppressive agent, mycophenoloic acid, was found to inhibit HIV production from activated, but not resting, CD4+ T cells; whereas inflammatory cytokine antagonists/antibodies appeared to effectively suppress viral production from resting CD4+ T cells. HIV envelope protein (gp160) has been demonstrated to induce a variety of cellular activities through interactions with surface CD4 and chemokine HIV co-receptors, including the induction of HIV expression from CD4+ PB T cells. We investigated whether gp160-mediated signaling was sufficient to induce HIV expression from the long-lived resting CD4+ T viral reservoir and cellular activation/proliferation or apoptosis. HIV gp160 was found to induce infectious HIV from resting CD4+ T cells isolated from infected individuals and this effect was not associated with expression of cellular activation markers, progression through the cell cycle or apoptosis. These data suggest that the latently HIV-infected resting CD4+ T cell reservoir may serve as a source of infectious HIV while maintaining its long-lived resting T cell phenotype. Finally, the relevance of CCR7, a chemokine receptor that plays an important role in homing of cells to T cell areas of LT and in mounting an appropriate immune response, in HIV pathogenesis is being investigated. We have found a reduced frequency of CCR7+ cells in both CD4+ and CD8+ memory and in CD8+ naive T cell compartments in viremic (> 50 copies of HIV RNA per ml) HIV-infected individuals. In addition, decreases in CCR7+ expression on CD8+ T cells precede the appearance of plasma viremia and many other markers of immune activation in individuals who ultimately experience viral rebound following cessation of effective HAART. These data demonstrate that CCR7 is a sensitive marker for early immune activation associated with low/undetectable HIV replication. Finally, we have found a 5-20 fold lower frequency of CCR5 expression in the CCR7+ versus the CCR7-subset of memory CD4+ T cells and this is associated with a considerable reduction in susceptibility of the CCR7+ subset to in vitro infection with R5, but not X4, HIV strains.
