Involvement of intestinal dendritic cells in oral tolerance, immunity to pathogens, and inflammatory bowel disease.

Dendritic cells (DCs) are composed of a family of cells, now recognized to be essential for innate and acquired immunity. DCs at mucosal surfaces have a particular capacity to induce the differentiation of regulatory T cells producing interleukin-10 (IL-10) and transforming growth factor-beta (TGF-beta) in the steady state (non-infected, non-immunized), yet they retain the capacity to induce effector T cells in response to invasive pathogens. This decision between the induction of active immunity and tolerance will depend on the subpopulation of DC involved and the surface receptors engaged during DC activation and T-cell priming. The local microenvironment will likely play an important role both in defining the DC phenotype and in providing direct signals to responding T cells. Furthermore, DCs in organized mucosal lymphoid tissues preferentially induce the expression of CCR9 and alpha4beta7 on T cells, which results in T-cell homing to the intestinal lamina propria. Finally, DCs may play an important role in the maintenance of abnormal intestinal inflammation either by driving pathogenic T-cell responses in mesenteric lymph nodes or by acting to expand or maintain pathogenic T cells locally at sites of inflammation. In this review, a brief discussion of general issues of DC biology that are pertinent to mucosal immunity is followed by a more in-depth discussion of the phenotype and function of DC populations in the intestine.