Having excluded a direct effect for butyrate in activating AhR, as butyrate did not upregulate the marker of AhR-activation Cyp1a1 compared to vehicle-treated B cells in vitro (Figure S7D), we investigated whether the endogenous microbiota or their metabolites are important in the butyrate-mediated suppression of arthritis and Breg maintenance. Here, CYP1A1 is linked to arthritic joint disease.