Functional and phenotypic delineation of two subsets of CD4 single positive cells in the thymus

CD4 single positive thymocytes are the traction of mature thymocytes that contains precursors of MHC class II restricted T cells. In the experiments presented here, we demonstrate phenotyplc and functional heterogeneity amongst CD4 single positive thymocytes from adult mice. Approximately 70% of these cells adhere to antl-CD8 antibody-coated dishes and therefore express low levels of CD8 molecules. They are referred to here as CD8^loCD4^hl. The remaining 30% are CD8-CD4^hl. The CD8^loCD4^hl subset also expresses 3-fold higher surface levels of heat-stable antigen (HSA) than CD8-CD4^hl thymocytes. Both CD4^hl subsets express high levels of the αβTCR/CD3 complex on the cell surface, and can proliferate in response to ailogeneic cells expressing MHC class II differences but not to cells expressing only class I disparate molecules. However, CD8-CD4^hl thymocytes are self-sufficient in such a proliferative response, whereas CD8^loCD4^hl thymocytes require exogenous IL-2 for optimal proliferation. The results suggest that CD8^loCD4^hl thymocytes are not completely mature. Their phenotype suggests that they might be descendants from CD8^loCD4^hl double positive thymocytes, and that they have begun to down-regulate gradually CD8 and HSA molecules. The relationship between the two CD4^hl single positive subsets is discussed.