Induction of T-Cell Differentiation In Vii Polypeptide Hormone of the Thymus

Thymin, a purified polypeptide isolated from bovine thymus, was shown to induce the expression of differentiation antigens characteristic of thymocytes (TL and Thy-I (0)) when incubated in vitro with mouse bone marrow or spleen cells. This induction occurred in 5-10% of the cells from bone marrow after a 2-hr incuba- tion with subnanogram concentrations of thymin. The induced cells expressed more TL and Thy-I (0) antigens than average normal thymocytes. The thymus is a compound organ in which the epithelial elements provide an appropriate micro-environment for the differentiation of lymphoid stem cells. These cells arise in hematopoietic tissues, undergo maturation and proliferation within the thymus, and are then distributed to peripheral tissues as immunocompetent "thymus-derived" lymphocytes (T-cells). A humoral secretion of the thymic epithelial cells