The suppressor macrophages generated by in vitro culture of spleen cells are shown to be derived from nonadherent splenic precursors. The induction of suppressor macrophage generation requires the presence of both plastic-adherent and plastic-nonadherent spleen cells during the first 24 to 48 hr of culture. After this induction period, the primed nonadherent cells can continue to generate suppressor macrophages through several serial transfers. The primed nonadherent spleen cell population is predominantly comprised of cFcR+ cells (40 to 50%) and Thy-1+ cells (40%), many of which appear to be undergoing blastogenesis. The generation of suppressor macrophages appears to proceed in two stages. The first stage is radiosensitive and has a minimum duration of 1 to 2 days. Although this stage initially coincides with the inductive period, it is also required for the continuous propagation of the suppressor macrophages upon serial transfer of the nonadherent spleen cells. The second stage is radioresistant and has a maximum duration of 2 to 3 days. The in vitro events thus bear similarities to events occurring during in vivo differentiation and activation of effector macrophages. It is suggested that the in vitro generation of suppressor macrophages may be one component of syngeneic mixed lymphocyte responses when spleen cells are used as the responding population, and that the suppressors represent part of a regulatory system controlling lymphocyte and macrophage differentiation and activation.

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