DNA synthesis of normal rat spleen cells in response to endotoxin increases markedly if adherent cells are first removed from the cell suspension. Addition of small numbers of purified macrophages to the cultures restores the response to a low level. T-deprived cells show these effects in very much lesser degree. Large numbers of macrophages completely suppress the response of both normal and T-deprived spleen. We conclude that two mechanisms of suppression are at work: a direct effect of macrophages and a macrophage-dependent "suppressor T cell" effect.

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