Mature Langerhans cells (mLC), the ex vivo correlates of interdigitating dendritic cells (IDC), are susceptible to infection with HIV-1. As IDC are important activators of T helper (Th) cells in vivo, we examined the interaction of HIV-1-infected mLC with CD4+ T lymphocytes. HIV-1-infected mLC readily formed clusters with the T cells and efficiently transmitted HIV-1 to the CD4+ Th cells. Formation of syncytia between mLC and T cells was initiated by HIV-1-infected mLC. In the clusters of HIV-1-infected mLC and activated T cells a massive HIV-1 production was observed leading to the subsequent elimination of the activated and infected T helper cells. Examination of the cytokine pattern produced during interaction of infected mLC with CD4+ T cells revealed an enhanced production of IFN-gamma and IL-10 in the cocultures. These results suggest that during antigen presentation-driven T cell activation by IDC in the lymphoid tissues, HIV-1-infected IDC might efficiently transmit the virus to Th cells, leading to altered Th cell responses.