T_h1/T_h2 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 T_h1 and T_h2 cells. We addressed this question by using sorted heat-stable antigen (HSA)⁺ and HSA^- CD4 single-positive (SP) thymocytes prepared from ovalbumin-specific TCRαβ transgenic mice and an in vitro T_h1/T_h2 differentiation culture system. HSA^- CD4 SP thymocytes show nearly full functional capacity to differentiate into either T_h1 or T_h2 cells. A dramatic difference was observed, however, between HSA⁺ and HSA^- CD4 SP thymocytes in the efficiency for T_h1 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) β2 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 T_h1 cell differentiation appears to be at least partly due to underdeveloped capacity in IL-12R expression and function.