For the past several years, we have been investigating the role of latently infected, resting CD4+ T cells and persistent viral replication in the pathogenesis of HIV infection and the impact of this reservoir on the treatment of HIV-infected individuals. We previously demonstrated that the latent viral reservoir in the resting CD4+ T cell compartment persists in virtually all infected individuals receiving effective antiviral therapy. Consequently, this viral reservoir is a major impediment to the eradication of HIV in vivo. In addition, we realized that HIV continually replicates at low levels in chronically infected individuals who are receiving effective antiviral therapy that renders them consistently aviremic for prolonged periods of time. Over the past year, we have focused our research on: 1) delineating the mechanisms by which HIV persists in infected individuals receiving effective antiretroviral therapy for extended periods of time and 2) examining the effect of intensifying antiretroviral therapy on the decay and size of viral reservoirs in infected individuals who had been on clinically successful drug regimens. First, we investigated the presence and status of residual HIV in individuals who had received effective antiretroviral therapy for varying periods of time and we examined the underlying mechanisms by which HIV persists in resting CD4+ T cells of such individuals. It has been proposed that the HIV present in infected resting CD4+ T cells is virologically quiescent. We demonstrated that a subset of resting CD4+ T cells that express the programmed death (PD)-1 gene carry substantially higher levels of HIV proviral DNA compared to PD-1-negative resting CD4+ T cells in the blood of infected individuals receiving effective antiretroviral therapy. The majority of PD-1+ resting CD4+ T cells express CXCR3, a tissue homing receptor, which suggests that these cells may have recently migrated out of various tissue sites into the peripheral blood compartment. PD-1+ resting CD4+ T cells from the majority of patients on clinically successful antiretroviral therapy spontaneously released HIV in the absence of any activating stimuli. This suggests that these cells are physiologically distinct from unfractionated latently infected, resting CD4+ T cells. Taken together, our data suggest that PD-1+ resting CD4+ T cells represent a unique population of infected cells that persists during antiretroviral therapy in infected individuals and that may require a novel therapeutic approach in order to achieve complete clearance of virus in vivo. Second, over the past year, we initiated a clinical trial to examine how antiretroviral therapy intensification affects the decay of viral reservoirs in infected individuals who had been on clinically successful drug regimens for greater than 3 years. Despite the success of therapeutic strategies, it has not been possible to eradicate HIV in infected individuals receiving effective antiretroviral therapy mainly due to the persistence of various viral reservoirs. We have recently demonstrated the persistence of HIV in activated as well as resting CD4+ T cells and provided evidence for cross-infection between these cellular compartments in aviremic patients receiving up to 9.1 years of effective antiretroviral therapy. This suggests ongoing reactivation of latently infected, resting CD4+ T cells and spread of virus in vivo. In order to decrease levels of residual viral replication and to design effective antiretroviral strategies aimed at eradicating virus, we are examining the effect of co-administering a newly developed antiretroviral drug, the integrase inhibitor raltegravir, on the decay of the viral reservoir in peripheral blood and gut-associated lymphoid tissues of infected individuals who are receiving effective antiretroviral therapy for some time. We are near the completion of the above clinical trial (96 weeks) and will conduct analyses of various immunologic and virologic parameters in the coming months.