Emerging roles of bile acids in mucosal immunity and inflammation.

Bile acids are cholesterol-derived surfactants that circulate actively between the liver and ileum and that are classically recognized for emulsifying dietary lipids to facilitate absorption. More recent studies, however, have revealed new functions of bile acids; as pleotropic signaling metabolites that regulate diverse metabolic and inflammatory pathways in multiple cell types and tissues through dynamic interactions with both germline-encoded host receptors and the microbiota. Accordingly, perturbed bile acid circulation and/or metabolism is now implicated in the pathogenesis of cholestatic liver diseases, metabolic syndrome, colon cancer, and inflammatory bowel diseases (IBDs). Here, we discuss the three-dimensional interplay between bile acids, the microbiota, and the mucosal immune system, focusing on the mechanisms that regulate intestinal homeostasis and inflammation. Although the functions of bile acids in mucosal immune regulation are only beginning to be appreciated, targeting bile acids and their cellular receptors has already proven an important area of new drug discovery.