The immunobiology of bile and biliary epithelium

Long thought to be just a simple pipe involved in the delivery of bile from hepatocytes to the gallbladder and intestine, bile ducts are now regarded as highly dynamic structures consisting of cell populations involved in formation, transport and modification of bile by both secretory and absorptive processes. In fact, both bile and biliary epithelium appear to have active immunologic roles in both innate and adaptive immune responses. These roles are becoming increasingly clear as techniques have been developed allowing for the study of bile and biliary epithelial cells (BECs) in mucosal immunity. Bile is actively involved in the transport of immunoglobulin to the intestine, while BECs secrete chemokines and cytokines and serve to localize the immune response by expressing critical cell adhesion molecules. Evidence suggests that BECs may also function as professional antigen-presenting cells (APC) and, in the process, contribute to the modulation of inflammatory reactions. Bile ducts and, in particular, BECs, are the primary site of damage in several immunologically mediated liver diseases. Progress in these important areas has been rapid and forms the basis of this review.