Separation and characterization of rat thymocyte subpopulations.

Using a supernatant of a thymic epithelial cell line (STEL) as a differentiation inducer, we characterized target cells in the thymus for the differentiation induction by STEL. Major efforts were made; (1) to distinguish inducible cells from others, and (2) to compare their phenotypes based upon the concept that "immature' cells should be differentiation inducible. From the results obtained in this study, four differentiation inducible populations and one non inducible fraction were separated: (1) The most immature thymocytes might be fraction 1 rosette forming cells (Fr. 1-FRC) characterized by low buoyant density, ability to form rosette, high spontaneously proliferating activity, low response to Con A, and the differentiation inducibility. (2) The most mature thymocytes may be Fr.1-non RFC, judging from their high response to Con A, their relatively low expression of Thy 1 and the lack of rosette forming ability. (3) There may exist two differentiation inducible populations of Fr.2 cells between the most immature Fr.1-RFC and the most nature Fr.1-non RFC. (4) Fr.3 cells are regarded as dying cells. They did not respond to Con A, did not incorporate 3H-thymidine, could not be induced to differentiate, and died very early in vitro. Several controversial points on the thymocyte differentiation are discussed, and our working model for the differentiation of rat thymocytes is presented.