CDId-restricted natural killer T (NKT) cells are innate immune cells with key immunoregulatory functions. They directly recognize and respond to lipid antigens of bacterial origin and take active part in immune responses against such pathogens. Our studies have shown that the NKT cell compartment is severely compromised by HIV-1 infection, but can be partially restored by treatment with interleukin-2 (IL-2) therapy. In the first specific we will ascertain whether treatment of HIV-1 infected subjects with antiretroviral therapy in combination with IL-2 can induce sustained increases in circulating NKT cell frequency and functions. We aim to determine the circulating NKT cell frequencies, surface phenotypes, and ex vivo effector functions longitudinally in subjects treated with ART alone or ART in combination with IL-2. In specific aim 2 we will determine the mechanisms and consequences of CD1d down-regulation in HIV-1 infected dendritic cells. We aim to determine the mechanism of HIV-1 interference with CD1-mediated antigen presentation, and to investigate the functional consequences of this for NKT cell recognition of glycolipid antigens, and the downstream activation of both the DC and NK cells. In specific aim 3 we will investigate the relationship between NKT cell loss in HIV-1 infected subjects and the emergence of mycobacterial infections. Our preliminary data suggests that control of Mycobacterium tuberculosis is impaired in HIV-1-uninfected subjects with low levels of circulating NKT cells. We will compare NKT cell frequencies in HIV-1 infected subjects with MTB with age and CD4 T cell count matched controls to determine whether MTB is associated with lower NKT cell frequencies. We will also test whether treatment of MTB is associated with increases in NKT cells to assess whether lower NKT cell counts may be an underlying risk factor for MTB or may represent a sequelae of untreated MTB. We believe that the continued studies we propose will contribute considerably to our understanding of how NKT cells function in HIV-1 disease, how the virus attempts to escape NKT cell activation, and how they may contribute to the innate defense against HIV-1 infection and AIDS-defining opportunistic infections.