The effects of nonspecific phytomitogens on primary plaque-forming cell (PFC) responses of mouse spleen cells to heterologous erythrocytes in vitro were studied. Spleen cell cultures treated with concanavalin A or phytohemagglutinin in vitro or established with spleen cells derived from mice injected with concanavalin A 24 h previously were similarly affected. In both cases, submitogenic doses resulted in substantial enhancement of PFC responses, whereas 10-fold larger doses were profoundly inhibitory. In contrast to the suppressive effects of mitogenic doses of phytomitogens added at culture initiation, addition of these same doses to cultures 48 h later resulted in increased PFC responses. This enhancement could be observed within 1 h after treatment and consequently could not be ascribed only to mitotic expansion of the antibody-synthesizing clone. Activation of spleen cells with specific antigen before mitogen treatment was not required for expression of the enhancing or suppressing effects on PFC responses. IgM and IgG PFC responses were similarly affected. Studies of cell interactions revealed that as few as 105 spleen cells obtained from mice treated with concanavalin A in vivo synergistically enhanced the PFC responses of 107 normal spleen cells. This enhancement was mediated by mitogen-activated T lymphocytes which were resistant to 2000 R irradiation 24 h after activation. The relevance of these observations to emerging concepts of helper and suppressor T cell activity is discussed.

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