Treg-mediated Th17 differentiation requires TCR-reactivation. (A) Naïve CD45.1+ OT-II cells from Rag1−/− mice were cultured with IL-6 (10ng/ml), dendritic cells, and pre-activated CD4+Foxp3+ T-cells in the presence of soluble anti-CD3 (left), or OVA peptide (323-339) for 4 days. (B) Naïve CD45.1+ Marilyn cells from Rag2−/− mice were cultured with dendritic cells with OVA and HY peptides either with IL-6 alone (left panel) or with IL-6 and pre-activated OT-II CD4+Foxp3+ Tregs (center panels). The right panel was cultured with IL-6, dendritic cells, soluble anti-CD3, and pre-activated OT-II CD4+Foxp3+ Tregs.