Subpopulations of fetal thymocytes defined by expression of T cell receptor/CD3 and IL-2 receptor. CD3 and IL-2 receptor alpha-chain are expressed on reciprocal cell populations

The expression of the TCR/CD3 complex and the IL-2R alpha chain (p55) on fetal thymocytes has been analyzed by flow cytometry (FCM). Two-parameter immunofluorescence identified three subpopulations which were respectively IL-2R alpha-/CD3+, IL-2R alpha+/CD3-, or IL-2R alpha-/CD3-; no detectable population of IL-2R alpha+/CD3+ cells was found in unstimulated fetal thymocytes. Fractionation by "panning" and by sterile flow cytometric separation was used to characterize the functional responsiveness of these three subpopulations to a variety of stimuli. All three populations proliferated in response to PMA + ionomycin + rIL-2. In contrast, stimulation with anti-CD3 + IL-2 induced proliferation in IL-2R alpha-/CD3+ and IL-2R alpha-/CD3- but not in IL-2R alpha+/CD3- thymocytes. IL-2R alpha- cells, including sorted IL-2R alpha-/CD3- thymocytes, underwent a phenotypic change in response to in vitro stimulation with anti-CD3 + IL-2, resulting in the appearance of an IL-2R alpha+/CD3+ population that was not detected in freshly isolated thymocytes. The ability of fractionated fetal thymocytes to produce lymphokine in response to PMA + ionomycin was also evaluated. Only the IL-2R alpha-/CD3- fraction generated detectable IL-2. These findings demonstrate for the first time that CD3 and IL-2R alpha are expressed in a mutually exclusive fashion in fetal thymocytes and define three subpopulations of thymocytes that differ significantly in their proliferative and differentiative responses to TCR-mediated, IL-2R-mediated, and pharmacologic stimulation.