Although alloantigen-specific suppressor T cells are generated in MLR, the cellular signals that lead to activation of suppressor T cells as opposed to cytotoxic T cells are unknown. The current study was undertaken to characterize interactions among T cell subsets involved in the generation of suppressor T cells in MLR. Human peripheral blood Leu-2+ (suppressor/cytotoxic) and Leu-3+ (helper/inducer) T cell subsets were activated with allogeneic non-T cells and then examined for their inductive effects on fresh autologous T cells. Fresh Leu-2+ cells proliferated in response to alloantigen-primed Leu-3+ cells and subsequently suppressed the response of fresh autologous Leu-3+ cells to the original, but not third party, allogeneic stimulator non-T cells. Moreover, only Leu-2+ cells that lacked the 9.3 marker, an antigen present on the majority of T cells including precursors of cytotoxic T cells, differentiated into suppressor cells. The alloantigen-specific suppressive effect of Leu-2+,9.3-cells was not mediated by cytolysis of allogeneic stimulator cells, nor could it be explained by alteration of MLR kinetics. Suppression was observed only when activated Leu-2+ cells were added to fresh MLRs within 24 h of initiation of cultures, suggesting that these cells block an early phase of the activation of Leu-3+ cells in MLR. These results indicate that alloantigen-primed inducer T cells can activate alloantigen-specific suppressor T cells in the absence of allogeneic stimulator cells.

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