Role for STAT4 in cytokine- or antigen receptor-driven CD8 T-cell proliferation. The intrinsic proliferative responsiveness of WT and STAT4-deficient CD8 T cells were examined ex vivo by stimulation with IL-2 or anti-CD3. Highly purified CD8 T cells were labeled with CFSE and cultured for 5 days with IL-2 (A) or anti-CD3 (B). Proliferation was evaluated by dilution of the dye. Numbers given in histograms represent percentages. Sensitivity to type 1 IFN-mediated inhibition was evaluated by titrating in the indicated concentrations of IFNα or IFNβ in the presence of IL-2 (A) or anti-CD3 (B). The bar graphs show proportions under the different conditions. The highest dose of IFNα was not tested. The results are representative of 2 or more independent experiments. (C) The effects of stimulation on STAT4 expression and the consequences for STAT1 induction were evaluated with CD8 T cells purified from WT and STAT4-deficient mice. The cells were cultured for 2 days with IL-2 or anti-CD3 with or without the addition of 10 000 U of IFNα. The proteins were then extracted. Western blot analyses of STAT4, STAT1, and β-actin were performed as indicated in “Western blot analysis.” The results are representative of 3 independent experiments.