Differentiation in vitro of Lyt 2⁺ thymocytes from embryonic Lyt 2^- precursors

In mice, the thymus is regarded as being the primary anatomical site for the generation of immunologically competent T lymphocytes¹. Such cells comprise approximately 20% of the cells in the thymus and share with T lymphocytes from peripheral lymphoid tissues certain phenotypic properties defined by anti-Lyt antibodies^2,3. Thus, most immunocompetent T cells are either Lyt 1⁺2⁺ or Lyt 1⁺2^- with the former cells being restricted to recognizing antigen in association with class I (H-2K, D) major histocompatability complex (MHC) products and the latter to class II (H-2I) MHC products⁴. Although evidence suggests that Lyt 1⁺2⁺ cells are generated from Lyt 1 ⁺2^- precursor^5,6 the independent development of two separate Lyt-defined lineages of thymocytes could not be ruled out ^5,7. Here, the acquisition of Lyt2 antigen by Lyt2^- cells from late embryonic and early postnatal thymuses is directly demonstrated. Furthermore, by combining cell cycle and Lyt phenotype analysis on a flow microfluorometer, the role of cell division in this differentiation process has been investigated.