Augmentation of mitogen-induced thymocyte proliferation by bacterial products is mediated by an Mr 36,000 monokine

The addition of picogram quantities of bacterial products to rat thymic cells in culture produced a doubling of the proliferative response to suboptimal levels of Concanavalin-A (Con-A). This effect was prevented by the depletion of adherent cells which comprised less than 0.1% of the total population. The response was restored by the addition of supernatants from peritoneal macrophages which had been stimulated 2 h previously with lipopolysaccharide. Treatment of these supernatants with phenylglyoxal, an inhibitor of interleukin-1 (IL-1), did not prevent the stimulatory effect. Augmentation of the thymocyte proliferative response could also be achieved by the addition of a partially purified monokine of relative molecular weight (Mr) 36,000 which is biochemically and functionally distinct from IL-1 and by a monoclonal antibody which binds to a common determinant on the Ia molecule. Fractionation of the thymic cells on a density gradient yielded a buoyant population which accounted for the majority of the proliferative activity and a dense fraction which was poorly responsive to the mitogen. The addition of the monokine to this latter fraction produced a significant increase in proliferation in response to Con-A. It is proposed that in the thymus, bacterial products stimulate thymic macrophages to release the Mr 36,000 monokine which in turn stimulates the thymic epithelial cells to release products which promote the survival and maturation of immature thymocytes. This work has implications for the regulation of thymocyte maturation.