The biliary tree: A new frontier in mucosal immunology?

Mucosal immunology traditionally applies to mucosal surfaces encountering microorganisms and environmental antigens, while the biliary tree is mostly regarded as a duct connecting the liver to the intestines. On the one hand, the biliary epithelium is considered a mucosal surface; on the other, it is not recognized as a mucosal immune epithelium. Nonetheless, emerging studies have robustly demonstrated its intricate roles in liver immunology and bile homeostasis, with direct consequences for the initiation and progression of liver diseases. Interestingly, emerging evidence suggests that bile ducts, and biliary epithelial cells in particular, display broad sensing capabilities and marked plasticity in their proteome and secretome, paralleling functions classically associated to mucosal immune epithelia. In this review, we discuss whether the bile ducts should be considered as part of the mucosal immunology system.