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Dendritic cells (DC) have been implicated in the initial selection for macrophage-tropic HIV-1 during transmission and in the generation of high-level virus replication during interactions with CD4 T cells. The role of DC as viral reservoirs and the extent of productive infection is unclear, but the ability to generate large numbers of DC from blood monocytes has produced a tractable model for study of DC-HIV-1 interactions. When cultured in granulocyte-macrophage colony stimulating factor and IL-4, sorted CD14+ monocytes rapidly lost phagocytic function for both 93 nm and 977 nm latex particles and developed the surface markers and function of DC. After 7 days, when returned to medium containing human serum without cytokines, some monocyte-derived dendritic cells (MDDC) became adherent, but retained the costimulatory markers CD80 and CD86 and continued to express CD83 and CD40. The MDDC stimulated allogeneic CD4 T cells, did not express new macrophage markers and remained non-phagocytic. With or without TNF-alpha, MDDC generated in cytokines were infected by macrophage and T cell-tropic virus and produced higher reverse transcriptase levels than did the autologous monocyte-derived macrophages (MDM). When added to T cells, the infected MDDC were able to infect T cells with a wider range of viral isolates than were MDM.