Thymocyte maturation induced by a thymus-dependent serum factor.
By means of direct measurement of intracellular cyclic AMP in thymocytes in vitro we demonstrated the existence in human serum of a thymus-dependent factor (SF). This activity of SF appeared to be due to a very low M.W. (less than 500) peptide distinct from other circulating thymic peptides. SF was found to act selectively on immunologically immature, hydrocortisone (HC)-sensitive peanut lectin-agglutinating thymocytes. In a period of 240 min SF sequentially induces in thymocyte synthesis of cyclic AMP, protein synthesis and synthesis of phosphorylated non-histone chromatin proteins of high molecular weight. Because SF does not induce a change in DNA synthesis, it seems likely that the events induced by SF are associated with a process requiring DNA translation, but not DNA replication. The biochemical events induced by SF are accompanied by the acquisition of immunological maturation by thymocytes. This has been shown by the acquisition of HC resistance, by the expression of FC receptors for IgM, by the decrease of terminal transferase activity and by the induction of the capacity to elicit a graft-versus-host reaction. We conclude that, most likely, SF acts on immature thymocytes inducing a part of them to acquire some properties of mature T cells.