Some effects thymocyte-stimulating factor.

Thymocyte-stimulating factor (TSF) produced in supernatants of murine spleen cells stimulated with mitogens or with allogeneic cells confers to thymocytes the ability to respond to concanavalin A (Con A) with the dose-response characteristic of mature, immunocompetent T cells. No enhancement of the responsiveness of bone marrow cells to lipopolysaccharide (LPS) was found. Thymocytes from mice of different strains acquire, after treatment with TSF, a responsiveness to phytohemagglutinin (PHA) and Con A proportional to that shown by spleen or lymph node cells from mice of the same strain. It was also shown that murine thymocytes treated with TSF cause a graft-vs-host reaction when injected into an appropriate hybrid. All these activities are thermolabile and disappear from supernatants at the same rate, thus showing that they are, very probably, due to the same substance. Spleen cells of mice bearing tumors causing splenomegaly (C3HBA and H2712 adenocarcinomas) show a decreased production of TSF if judged on the basis of TSF produced per million spleen cells. Rat spleen cells produce a substance (rat TSF) which stimulates the PHA and Con A responsiveness of murine thymocytes. Rat TSF has a molecular weight similar or identical to that of murine TSF. However, on the basis of the different rates of thermodenaturation, it appears that rat TSF and murine TSF are two different molecules.