Activator protein 1 (AP-1) transcription factors are dimers of Jun, Fos, MAF and activating transcription factor (ATF) family proteins characterized by basic region and leucine zipper domains1. Many AP-1 proteins contain defined transcriptional activation domains (TADs), but Batf and the closely related Batf3 (refs 2, 3) contain only a basic region and leucine zipper and have been considered inhibitors of AP-1 activity3–8. Here we show that Batf is required for the differentiation of IL-17-producing T helper (TH17) cells9. TH17 cells comprise a CD4+ T cell subset that coordinates inflammatory responses in host defense but is pathogenic in autoimmunity10–13.Batf −/−mice have normal TH1 and TH2 differentiation, but show a defect in TH17 differentiation, and are resistant to experimental autoimmune encephalomyelitis (EAE).Batf −/−T cells fail to induce known factors required for TH17 differentiation, such as RORγt11 and the cytokine IL-21 (refs 14–17). Neither addition of IL-21 nor overexpression of RORγt fully restores IL-17 production in Batf−/− T cells. The IL-17 promoter is Batf-responsive, and upon TH17 differentiation, Batf binds conserved intergenic elements in the IL-17A/F locus and to the IL-17, IL-21 and IL-22 (ref 18) promoters. These results demonstrate that the AP-1 protein Batf plays a critical role in TH17 differentiation.

Phenotypic plasticity of T helper 17 (Th17) cells suggests instability of chromatin structure of key genes of this lineage. Here we identify epigenetic modifications across the clustered Il17a and Il17f, and Ifng loci before and after differential IL-12 or TGFβ signaling, which induce divergent fates of Th17 cell precursors. We find that Th17 precursors have substantial remodeling of the Ifng locus but undergo critical additional modifications to enable high-level expression when stimulated by IL-12. Permissive modifications across the Il17a-Il17f locus are amplified by TGFβ signaling in Th17 cells, but are rapidly reversed downstream of IL-12–induced, STAT4– and T-bet–mediated silencing of the Rorc gene. These findings reveal substantial chromatin instability of key transcription factor and cytokine genes of Th17 cells and support a model of Th17 lineage plasticity in which cell-extrinsic factors modulate Th17 cell fates through differential effects on the epigenetic status of Th17 lineage factors.

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