Lymphocyte migration into the lamina propria of the gut is mediated by specialized HEV-like blood vessels.

Migration of lymphocytes into the lamina propria of the small intestines was studied in mice using short-term in vivo migration experiments in combination with immunocytochemistry and autoradiography. The results show that, shortly after intravenous injection, most of the lymphocytes present in the lamina propria are actually located within the capillary network of the villi. Furthermore, it was shown that lymphocytes leave the blood stream and enter the lamina propria via small blood vessels at the base of the villi. These blood vessels can be discriminated by their positive staining with MECA-325, a monoclonal antibody that is specific for high endothelial venules (HEV) in lymphoid organs. From the results it is concluded that the gut contains specialized venules at specific sites, involved in the emigration of lymphocytes, comparable to HEV in lymphoid organs. The flatness of the endothelium of these MECA-325-positive intestinal blood vessels, which is in contrast to the situation in lymphoid organs, could not be changed by inducing an intestinal inflammation. This flatness may be directly correlated to the less efficient transmigration of lymphocytes, as demonstrated in our experiments.