Generation of gut-homing T cells and their localization to the small intestinal mucosa.

The intestinal mucosa represents the largest body surface toward the external environment and harbors numerous T lymphocytes that take up resident within the intestinal epithelium or in the underlying lamina propria (LP). The intraepithelial lymphocytes include subsets of 'unconventional' T cells with unclear ontogeny and reactivity that localize to this site independently of antigen-specific activation in secondary lymphoid organs. In contrast, the majority of the 'conventional' gut T cells are recruited into the intestinal mucosa subsequent to their activation in intestinal inductive sites, including Peyer's patches (PPs) and mesenteric lymph nodes (MLNs). T cells homing to the small intestine express a distinct pattern of homing molecules, allowing them to interact with and transmigrate across intestinal postcapillary endothelium. At least some of these homing molecules, including the integrin alpha(4)beta(7) and the chemokine receptor CCR9, are induced on T cells during their activation in PPs or MLNs. Mucosal dendritic cells (DCs) play a key role in this process, but not all intestinal DCs possess the ability to confer a gut-homing capacity to T cells. Instead, functionally specialized CD103(+) DCs derived from the small intestinal LP appear to selectively regulate T-cell homing to the small intestine.