Intracellular events involved in the induction of immune competence in lymphoid cells by a thymus humoral factor.

A humoral factor extracted from calf thymus (THF) restores the immunocompetence of spleen cells from neonatally thymectomized (NTx) mice to induce an in vitro graft-vs-host (GVH) response. This acquistion of immunocompetence consists of a series of biochemical events, the first of which involves an obligatory rapid increase in adenyl cyclase acitivity and in intracellular levels of cyclic AMP. Protein synthesis occurs as a further step in the events leading to induction of immunocompetence by THF and could be blocked by cycloheximide with the consequent abolishment of the immunocompetence of the lymphoid cells tested. The induction of competence by THF is accompanied by a simultaneous reduction in DNA synthesis resulting from the increase in intracellular cyclic AMP levels. These steps have been studied in the absence of antigenic stimulation which is not required for the induction of competence by THF. Spleen extracts prepared by a similar procedure as THF were found to be devoid of the properties described above.