ABSTRACT
A healthy gut consists of commensal flora,epithelial layer and gut-associated lymphoid tissue (GALT). GALT is hyporesponsive to commensal flora and dietary antigens,but can recognize and at-tack pathogens. Accumulating evidence suggests that dendritic cells (DCs) play a crucial role in managing this paradoxical situation and maintaining the complex homeostasis in gut. Influenced by intestinal epithelial cells (IECs) and commensal flora,intestinal DCs possess unique properties that enable them to regulate T-helper 2 (Th2) cells,regulatory T cells (Tregs) and immunoglobulin A (IgA)-producing cells in a steady state. During infection,intestinal DCs are involved in the induction of effector lymphocytes, although they are also responsible for initiating pathogenic responses in inflammatory bowel diseases (IBDs). Therefore, intestinal DCs are associated with not only the maintenance of immune tolerance to commensal flora,but also the induction of protective immune responses against pathogens. This review outlines the roles of commensal flora, epithelial layer, and GALT in mucosal homeostasis and inflammation and summarizes recent progress in DCs-mediated intestinal immune homeostasis.