Bile acids (BAs) regulate lipid uptake, epithelial integrity, and immune responses in the gut. Hepatocytes synthesize primary BAs, which microbiota metabolize into secondary metabolites. Together, these species form a composite pool that circulates enterohepatically between the liver and intestines. Here, we show that immunoregulatory outputs of the enterohepatic BA pools involve competition between multiple BA species for individual nuclear receptors and are dependent on intestinal health. In healthy mice, the primary BA, tauro {beta} muricholic acid (t{beta}MCA) antagonized ROR{gamma}t-mediated Th17 function in the presence of two secondary BAs (tDCA, tLCA) which activated ROR{gamma}t. Conversely, ileitis in Crohns disease patients and Tnf{delta}ARE mice depleted primary BAs by reducing the number and function of BA transporting enterocytes. In mice, ileitis driven t{beta}MCA depletion enhanced tDCA and tLCA mediated ROR{gamma}t activation and supported Th17 cell function. Replenishing t{beta}MCA in Tnf{delta}ARE mice reestablished BA dependent Th17 cell regulation. Thus, intra pool competition underpins BA immunoregulatory functions and reveals new opportunities for precision Crohns disease therapy.
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