THE INNATE AND ADAPTIVE IMMUNE SYSTEM OF THE INTESTINAL EPITHELIUM PER BRANDTZAEG Laboratory for Immunohistochemistry and Immunopathology (LIIPAT),

It is increasingly being appreciated that innate immune mechanisms play a key role for the tuning of adaptive immunity in the gut and maintenance of mucosal homeostasis. Innate responses use preformed and rapidly synthesised effectors and sensors that lead to spontaneous activation or modulation of cellular functions, aiming at elimination of microbial challenges and return of the local tissue to a basal state with minimal pathology. Efficient adaptive immunity depends on postnatal ‘education’ of specific immune cells and induction of immunological memory in processes that are significantly modulated by innate immune responses. The intestinal immune system has through evolution developed two layers of adaptive non-inflammatory defence: Immune exclusion provided by secretory antibodies to limit epithelial penetration and host invasion of microorganisms as well as other potentially dangerous antigens; and immunosuppressive mechanisms to inhibit overreaction against innocuous luminal antigens – often referred to collectively as ‘oral tolerance’. Both these strategies depend on co-operation of adaptive immune mechanisms with the innate defence system, including crosstalk between the intestinal epithelium and various lamina propria cells. This review focuses on immunological mechanisms taking place at the epithelial level.