Epithelial barrier: an interface for the cross-communication between gut flora and immune system.

Large numbers of environmental antigens, including commensal bacteria and food-derived antigens, constitutively interact with the epithelial layer of the gastrointestinal (GI) tract. Commensal bacteria peacefully cohabit with the host GI tract and exert multiple beneficial or destructive effects on their host. Intestinal epithelial cells (IECs) constitute the first physical and immunological protective wall against invasive pathogens and a cohabitation niche for commensal bacteria. As the physiological homeostasis of IECs is maintained by multiple biological processes such as apoptosis, autophagy, and the handling of endoplasmic reticulum stress, the aberrant kinetics of these biological events, which have genetic and environmental causes, leads to the development of host intestinal pathogenesis such as inflammatory bowel disease. In addition, IECs recognize and interact with commensal bacteria and give instructions to mucosal immune cells to initiate an immunological balance between active and quiescent conditions, eventually establishing intestinal homeostasis. The mucosal immune system regulates the homeostasis of gut microbiota by producing immunological molecules such as secretory immunoglobulin A, the production of which is mediated by IECs. IECs therefore play a central role in the creation and maintenance of a physiologically and immunologically stable intestinal environment.