Thymic maturation in vitro by a secretory product from macrophages

Immature thymocytes can be induced by macrophage culture fluid (MCF) to differentiate in vitro. This maturation occurs during several days of culture but does not require DNA synthesis. It is accompanied by an augmentation of surface H-2 which precedes a loss of susceptibility to cytolysis with anti-TL and complement. There is no physical loss of thymus leukemia antigen within the thymus. Thymocytes cultured with MCF also acquire the capacity to respond in an MLC. Differentiation was shown not to be due to interferon nor could it be reproduced by 2-mercaptoethanol. The stimulating molecule is distinct from the principal mitogenic protein found in MCF.