p38α^Y323F T cells polarize into Th1 cells in vivo but produce less IFN-γ upon TCR stimulation. (A) T-bet and GATA-3 expressions were analyzed in whole cell lysates of WT and p38α^Y323F T cells stimulated with plate-bound anti-CD3/CD28 Abs and harvested on the indicated days. (B) WT and p38α^Y323F mice were immunized with 20 μg of STAg and, 12 days later, splenocytes were restimulated with either anti-CD3 or PMA/Ion. IL-4 and IFN-γ in CD4⁺ T cells were analyzed by intracellular staining. The numbers indicate the frequency of positive cells in each quadrant. The data are representative for 1 of 2 independent experiments. (C) The expression of IFN-γ per responding cell (MFI) and (D) the percentage of CD4⁺ IFN-γ–positive T cells from individual immunized animals from 2 independent experiments are summarized (bars indicate the mean ± SEM; P value was calculated using the Student t test).