A role for T3+4-6-8- transitional thymocytes in the differentiation of mature and functional T cells from human prothymocytes.

In vivo, immunocompetent T lymphocytes are only detected late in ontogeny, among mature thymocytes expressing either T4 (L3T4 in mouse) or T8 (Lyt-2) surface glycoproteins. We have previously shown, however, that there are functional precursors among T3+4-6-8- human thymocytes in vivo. Here we report on the in vitro differentiation of prothymocytes into T3+4-6-8- and mature T cells. T11+3-4-6-8- prothymocytes (0.5% of total thymocytes, greater than 98% pure) were obtained after treatment of thymocytes with OKT3 (T3), OKT4A (T4), Na1/34 (T6), and B9.4 (T8) monoclonal antibodies plus complement. During culture, the prothymocyte precursors acquire first T3 and then either T4 or T8, but not T6. The largest subpopulation in the thymus, T4+6+8+ cells, are not detected among the in vitro T-cell precursors. During culture, the precursors acquire cytolytic activity as soon as they express either the T3+4-6-8- or the mature (T3+4+8- or T3+4-8+) phenotypes. We suggest that T3+4-6-8- cells are a productive, transitional stage in T-lymphocyte development.