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Intestinal health relies on the immunosuppressive activity of CD4⁺ regulatory T (T_reg) cells. Expression of the transcription factor Foxp3 defines this lineage, and can be induced extrathymically by dietary or commensal-derived antigens in a process assisted by a Foxp3 enhancer known as conserved non-coding sequence 1 (CNS1)^{, –}. Products of microbial fermentation including butyrate facilitate the generation of peripherally induced T_reg (pT_reg) cells^{, –}, indicating that metabolites shape the composition of the colonic immune cell population. In addition to dietary components, bacteria modify host-derived molecules, generating a number of biologically active substances. This is epitomized by the bacterial transformation of bile acids, which creates a complex pool of steroids with a range of physiological functions. Here we screened the major species of deconjugated bile acids for their ability to potentiate the …