Th1/Th2 cell differentiation of developing CD4 single‐positive thymocytes

In this study we investigate the stage at which developing T cells in the thymus acquire the ability to differentiate into Th1 and Th2 cells. We addressed this question by using sorted heat-stable antigen (HSA)+ and HSA‐ CD4 single-positive (SP) thymocytes prepared from ovalbumin-specific TCRab transgenic mice and an in vitro Th1/Th2 differentiation culture system. HSA‐ CD4 SP thymocytes show nearly full functional capacity to differentiate into either Th 1o r T h2 cells. A dramatic difference was observed, however, between HSA+ and HSA‐ CD4 SP thymocytes in the efficiency for Th1 cell differentiation. TCR function of HSA + CD4 SP thymocytes appeared to be fully developed because antigen-induced proliferation and IL-2 production were essentially equivalent to that of HSA ‐ CD4 SP thymocytes. However, the levels in IL-12 receptor (IL-12R) b2 chain expression following anti-TCR stimulation were dramatically low in the HSA + CD4 SP thymocytes. Decreased IL-12-induced STAT4 phosphorylation was also observed. Moreover, IL-12-dependent transcriptional up-regulation of T-bet and STAT4 was deficient in the HSA + CD4 SP thymocytes. Thus, the poor capacity of HSA + CD4 SP thymocytes to proceed to Th1 cell differentiation appears to be at least partly due to underdeveloped capacity in IL-12R expression and function.