Murine Thymocytes with Ability to Inhibit IL-2 Production II. Characterization of a Subpopulation with Regulatory Function in the Thymus

Studies were performed to characterize the thymocyte subset responsible for the efficient inhibition of spleen cell interleukin-2 (IL-2) production. By different cell separation techniques, C-mediated cytotoxicity, immunoabsorbance, and cell sorting by flow cytometry, we have identified two phenotypically distinct subpopulations of thymocytes. One subset, belonging to the minor population (3-5%) of the CD4-CD8- , i.e., double-negative thymocytes, is defined as the subset from which the suppressive thymocytes are generated. After 28 hr of Con A stimulation, these cells undergo a phenotypical change in vitro and generate a population exerting the inhibitory effect. This latter subset inhibits 95-99% of the IL-2 produced by spleen cells and is characterized by expressing the CD8 antigen, high levels of HSA, low levels of CD3, and being IL-2R positive (HSA+CD4-CD8+CD3lowIL-2R+). Based on the experiments where stimulated CD4+CD8+, i.e., double-positive thymocytes, failed to suppress IL-2 production, we conclude that the CD8+ immature single-positive thymocytes are generated directly from the DN subset as an intermediate stage to the DP cells. When CD8+-stimulated thymocytes were enriched, the suppression was efficient even at thymocyte:spleen cell ratio of 0.01:1. It is suggested that this subpopulation of thymocytes may serve as a regulatory set of cells during critical stages of thymic maturation.