Effects of cytokines and glucocorticoids on endogenous class II MHC antigen expression by activated rat CD4+ T cells. Mature CD4+ CD8- thymocytes are phenotypically heterogeneous on activation

By the use of mixed leukocyte cultures it was shown that a population of allogeneically activated rat T cells synthesize and express class II MHC antigens, in confirmation of other studies. Compatible with the finding that the MHC molecules detected on these cells were of T cell origin rather than passively acquired, it was found that mRNA for class II transactivator could readily be detected in the T cells stimulated in these cultures. In contrast there was no evidence that mouse T cells synthesized class II MHC antigens. The size of the population of activated rat T cells expressing class II MHC antigens was affected by the presence of IL-4 and glucocorticoids in the activating cultures. However, whereas IL-4 increased the frequency of thymocytes and peripheral T cells expressing class II antigens in culture, glucocorticoids diminished this frequency. The expression of class II MHC antigens by allogeneically activated thymocytes demonstrated a novel heterogeneity amongst mature CD4+ CD8- thymocytes that could not readily be accounted for in terms of differences in maturity of the cells, in the affinity of the TCR for the stimulating ligands or in the stage in the cell cycle. The data suggest that CD4+ single-positive thymocytes do not constitute a homogeneous population differing only in TCR clonotypes.