p38 MAPK regulates IL-17 production by in vitro–generated Th17 cells. FACS-sorted CD4 T cells from WT B6 mice in vitro differentiated into Th17 cells in the presence of different concentrations of SB203580 (A) or BIRB796 (B) for 72 hours. IL-17 production by Th17-polarized FACS-sorted CD4 T cells from WT B6 or MKK3^−/−MKK6^+/− mice (C) or total CD4 T cells from WT B10.BR and dn-p38-Tg (D) or MKK6 Tg (E) differentiated into Th17 cells. IL-17 levels in the supernatants were assessed by ELISA. The significance of differences observed in panels A and B was determined by linear regression analysis (A, P < .0001; B, P = .0001). The significance of the differences observed in panels C-E was determined using the Student t test (*** ≤ 0.001).