Human gut bacterial metabolism drives Th17 activation and colitis

Bacterial activation of T helper 17 (Th17) cells exacerbates mouse models of autoimmunity, but how human-associated bacteria impact Th17-driven disease remains elusive. We show that human gut Actinobacterium Eggerthella lenta induces intestinal Th17 activation by lifting inhibition of the Th17 transcription factor Rorγt through cell- and antigen-independent mechanisms. E. lenta is enriched in inflammatory bowel disease (IBD) patients and worsens colitis in a Rorc-dependent manner in mice. Th17 activation varies across E. lenta strains, which is attributable to the cardiac glycoside reductase 2 (Cgr2) enzyme. Cgr2 is sufficient to induce interleukin (IL)-17a, a major Th17 cytokine. cgr2+ E. lenta deplete putative steroidal glycosides in pure culture; related compounds are negatively associated with human IBD severity. Finally, leveraging the sensitivity of Cgr2 to dietary arginine, we prevented E. lenta-induced intestinal inflammation in mice. Together, these results support a role for human gut bacterial metabolism in driving Th17-dependent autoimmunity. [Display omitted] •The prevalent gut Actinobacterium Eggerthella lenta activates Th17 cells•E. lenta is associated with human disease and exacerbates colitis in mice•A strain-specific enzyme Cgr2 induces IL-17a via the metabolism of Rorγt inhibitors•Dietary arginine blocks E. lenta-induced intestinal inflammation Alexander et al. show an autoimmune-associated microbe, Eggerthella lenta, activates Th17 cells and worsens mouse models of colitis. Using strain-level variation, comparative genomics, and bacterial genetics they demonstrate that the cardiac glycoside reductase 2 (Cgr2) enzyme is sufficient for Th17 activation and that elevated dietary arginine blocks E. lenta-induced colitis.