IL21 and Crohn disease: FOXP3 negatively regulates RORγt, inducing Treg cell differentiation, and down-regulates Th17 cells through STAT6.157 However, the role of IL-21 in Th17 cell differentiation remains controversial, as inconsistent effects of IL-21 on Th17 cells have been found in autoimmune encephalomyelitis and Crohn’s disease.158,159 We suggest that this is probably because Th17 cell differentiation is regulated by multiple cytokines.