(A) P4 effects on Th17 cell induction. Treg-depleted naïve CD4+ T cells, isolated from lymph nodes and spleen, were cultured for 6–7 days in a Th17 cell condition (anti-IFNγ, anti-IL4, IL6, TGFβ1, and concanavalin A) in the presence or absence of P4 (2 μg/ml or 6.4 μM). (B) PR is not required for P4-mediated suppression of Th17 cells. Naïve CD4+ T cells isolated from WT and PR(−/−) mice were cultured as in panel A and examined for induction of Th17 cells. Representative and combined data from 4–8 independent experiments are shown. *Significant differences between control and P4 groups.