Recent experiments (11-13) have shown that antigen-specific, CD8+, CD4- T lymphocytes can be induced to proliferate and become killer cells in the absence of a second population of "helper" CD8-, CD4+ cells. We have studied early events in the activation of CD4+ and CD8+ T cell subsets in the primary mixed leukocyte reaction. Dendritic cells are a major if not essential accessory cell for the activation of both subpopulations. Antigen-bearing macrophages fail to stimulate unprimed CD8+ cells, but act as targets for the sensitized cytolytic lymphocytes that are induced by dendritic cells. The initial proliferative response is comparable for CD4+ and CD8+ lymphocyte subsets. For both subpopulations, dendritic cells efficiently cluster the responding lymphocytes on the first day and induce the release of IL-2. The data indicate that CD4+ and CD8+ lymphocytes can be activated by a similar mechanism, and illustrate the special role of dendritic cells in the sensitization stage of cell-mediated immunity.

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