Oral tolerance and regulation of mucosal immunity

Regulated mechanisms sustain the ability of the gut immune system to discriminate harmless food antigens (Ag) and commensal bacteria from pathogenic microorganisms, resulting in tolerance versus protective immunity, respectively. Antigens of the gut commensals are not simply ignored, but rather trigger an active immunosuppressive process, more commonly known as oral tolerance, which prevents the outcome of immunopathology. Both intrinsic properties of the gut microenvironment and cellular actors, as well as peripheral events induced by systemic dissemination of oral Ag, promote the induction of regulatory mechanisms that ensure maintenance of gut homeostasis. The aim of this review is to provide a synthetic update on the mechanisms of oral tolerance, with particular emphasis on the complex interplay between regulatory CD4(+) T cells, dendritic cells and the gut microenvironment.