IRF8 is required for Treg cell-mediated suppression of Th1 cells. In vitro-differentiated CD45.1+ Th1 cells were mixed with WT- or IRF8-deficient CD45.2+ iTreg cells at the ratios of 10:0, 9:1, 7:3, and 5:5 and cultured under Th1-polarizing conditions for two days. CD45.1+ cells were sorted, and expression of Ifng was examined by qRT-PCR. The error bar shows standard deviation (n = 3). Statistical difference was analyzed by Student's t-test. *p < 0.05, ***p < 0.001.