Lymphocyte homing to the gut: attraction, adhesion, and commitment.

Lymphocytes continuously migrate from the blood into the intestine. Naive lymphocytes leave the blood through high endothelial venules in Peyer's patches. During the multistep extravasation cascade, they sequentially roll on, firmly adhere to, and transmigrate through the endothelial layer using multiple adhesion molecules and chemotactic signals. In the organized lymphoid tissues of the gut, lymphocytes can become activated, if they meet their cognate antigens transported to Peyer's patches through the gut epithelium. During activation and proliferation, the lymphocytes become imprinted by the local dendritic cells, so that after returning to systemic circulation via the efferent lymphatic vasculature, they preferentially home to lamina propria of the gut to execute their effector functions. In inflammation, the recirculation routes of lymphocytes are altered, and these may explain the pathogenesis of certain extra-intestinal manifestations of gut infections and inflammatory bowel diseases. The increased knowledge on the mechanisms that regulate lymphocyte homing and imprinting has clear applicability in designing more effective vaccination regimens. A detailed understanding of the mucosal homing has recently led to the development of the first successful anti-adhesive therapeutics in human.