Phenotypic and functional properties of murine thymocytes. I. Precursors of cytolytic T lymphocytes and interleukin 2-producing cells are all contained within a subpopulation of 'mature' thymocytes as analyzed by monoclonal antibodies and flow microfluorometry

The correlation between surface phenotype and function in subpopulations of murine thymocytes has been investigated using flow microfluorometry (FMF). C57BL/6 thymocytes stained with monoclonal antibodies directed against Lyt-2, H-2K(b), and Thy-1.2 and passed on an FACS II flow cytometer could be resolved into at least four distinct subpopulations on the basis of fluorescence and forward light scatter (FLS) measurements. (a) Medium-sized Lyt-2⁺ cells that stained strongly with H-2K(b) and weakly with Thy-1.2 (5% of total cells); (b) medium-sized Lyt-2^- cells with other properties as in (a) (10%); (c) small Lyt-2⁺ cells that stained weakly with H-2K(b) and strongly with Thy-1.2 (60%); and (d) large Lyt-2⁺ cells that stained weakly with H-2K(b) and very strongly with Thy-1.2 (23%). Cortisone-resistant thymocytes (CRT) were found to correspond phenotypically to populations (a) and (b). The distribution of cytolytic T lymphocyte precursors (CTL-P) directed against H-2(d) alloantigens in subpopulations of C57BL/6 thymocytes that had been sorted according to the phenotypic criteria described above was then investigated. CTL-P in sorted and control populations were quantitated by limiting dilution analysis of mixed leukocyte microcultures established in an excess of interleukin 2 (IL-2). These studies established that all thymus CTL-P could be quantitatively recovered in a subpopulation of cells that was cortisone-resistant, medium-sized, Lyt-2⁺, H-2K(b+), and weakly stained with Thy-1.2. In parallel studies, the production of IL-2 by subpopulations of C57BL/6 thymocytes was quantitatively assessed using a recently developed sensitive microassay system. Graded numbers of sorted or control thymocytes were stimulated with irradiated T cell-depleted allogeneic cells and assayed for their ability to support the growth of an IL-2-dependent cytolytic T lymphocyte clone. Using this method, IL-2 production was found to reside entirely in a subpopulation of cortisone-resistant, medium-sized Lyt-2^- thymocytes. Further phenotypic analysis of this subpopulation of cells indicated that it was homogeneously H-2K(b+) and weakly staining with Thy-1.2. Taken together with the CTL-P results, these data directly demonstrate that a subpopulation of thymocytes with a mature phenotype (i.e., cortisone-resistant, medium-sized, H-2K(b+), and weakly staining with Thy-1.2) accounts for all the functional activity in the thymus. Reasons for the apparent discrepancy between these results and other recent studies will be discussed.