Induction of postthymic T-cell maturation by thymic humoral factor(s) derived from a tumor cell of thymic epithelial origin.

The presence of desmosomes and microfilaments, together with a variety of cytoplasmic organelles in tumor cell lumps isolated from the peritoneal cavity and tumor cells grown in monolayers, indicated the epithelial nature of the thymic tumor cell and was suggestive of its secretory activity. This is further confirmed by analysing the effects of tumor cell supernatants. The effect of supernatants from rat thymic epithelial tumor cell cultures on the mitogenic response of T-lymphocytes was studied in vitro and in vivo. An in vivo dose-effect curve could be established, revealing a significant increase in response to PHA and Con A of rat thymocytes using highly diluted supernatant (1:2000). Repeated intraperitoneal injection of purified thymic epithelial cell supernatant in mice and rats resulted in an increased mitogenic response of peripheral lymphocytes. No species specificity of the thymic humoral factor(s) was observed. The discrepancy in peripheral lymphocyte response between athymic nude rats and their heterozygous littermates points to the presence of target cells among the postthymic T cells and consequently to the induction of post-thymic T cell maturation. The data presented in this study show a tumor cell of thymic epithelial origin which synthesizes and secretes one or more factors displaying thymic hormone activity. Therefore, this tumor cell system might be used as a new source of thymic humoral factors(s).