Oxysterols are agonist ligands of RORγt and drive Th17 cell differentiation

The RAR-related orphan receptor gamma t (ROR gamma t) is a nuclear receptor required for generating IL-17-producing CD4(+) Th17 T cells, which are essential in host defense and may play key pathogenic roles in autoimmune diseases. Oxysterols elicit profound effects on immune and inflammatory responses as well as on cholesterol and lipid metabolism. Here, we describe the identification of several naturally occurring oxysterols as ROR gamma t agonists. The most potent and selective activator for ROR gamma t is 7 beta, 27-dihydroxycholesterol (7 beta, 27-OHC). We show that these oxysterols reverse the inhibitory effect of an ROR gamma t antagonist, ursolic acid, in ROR gamma- or ROR gamma t-dependent cell-based reporter assays. These ligands bind directly to recombinant ROR gamma ligand binding domain (LBD), promote recruitment of a coactivator peptide, and reduce binding of a corepressor peptide to ROR gamma LBD. In primary cells, 7 beta, 27-OHC and 7 alpha, 27-OHC enhance the differentiation of murine and human IL-17-producing Th17 cells in an ROR gamma t-dependent manner. Importantly, we showed that Th17, but not Th1 cells, preferentially produce these two oxysterols. In vivo, administration of 7 beta, 27-OHC in mice enhanced IL-17 production. Mice deficient in CYP27A1, a key enzyme in generating these oxysterols, showed significant reduction of IL-17-producing cells, including CD4(+) and gamma delta(+) T cells, similar to the deficiency observed in ROR gamma t knockout mice. Our results reveal a previously unknown mechanism for selected oxysterols as immune modulators and a direct role for CYP27A1 in generating these ROR gamma t agonist ligands, which we propose as ROR gamma t endogenous ligands, driving both innate and adaptive IL-17-dependent immune responses.