Phenotypic properties, interleukin 2 production, and developmental origin of a "mature" subpopulation of Lyt-2- L3T4- mouse thymocytes.

Two-color flow microfluorometry using monoclonal antibodies to cell surface determinants has shown that a subpopulation of mouse Lyt-2- L3T4- thymocytes, comprising 18% of Lyt-2- L3T4- cells in adult C57BL/6 mice, appears in the thymus late during fetal development. These Lyt-2- L3T4- cells are characterized by lack of expression of determinants recognized by monoclonal antibody J11d, a phenotype characteristic of more "mature" functional T cells. This J11d- subpopulation of Lyt-2- L3T4- thymocytes has now been shown to produce significant quantities of interleukin 2 following mitogen stimulation and to express T-cell receptor molecules recognized by monoclonal antibodies KJ-16 and F23.1. Furthermore, culturing of fetal thymus lobes has shown that precursors of this subpopulation of Lyt-2- L3T4- thymocytes are already present in thymus at 14 days of embryonic development and are thus derived from an intrathymic precursor cell. So, within the mouse thymus, phenotypic changes and acquisition of mature T-cell characteristics occur within a subpopulation of cells originally thought of as exclusively "immature."