Lymphocyte attachment to high endothelial venules during recirculation: a possible role for carbohydrates as recognition determinants.

During the course of their recirculation through the body, blood-borne lymphocytes specifically adhere to high endothelial venules (HEV) within secondary lymphoid organs such as peripheral lymph nodes (PN) and gut-associated Peyer's patches (PP). This adherence event, which initiates the extravasation of the lymphocyte, is highly specific in terms of the class of lymphocyte and the anatomic location of the HEV. We review evidence that the lymphocyte adhesive molecule ('homing receptor') involved in attachment to PN HEV is a carbohydrate-binding receptor (lectin-like) with specificity for mannose-6-phosphate (M6P)-like ligands. We describe the use of a novel cytochemical probe for the detection and characterization of cell surface carbohydrate-binding receptors. Using a M6P-based probe, we show that the carbohydrate-binding receptor on lymphocytes is closely-related or identical to the MEL-14 antigen, a putative homing receptor identified by a monoclonal antibody. Evidence is presented that the lymphocyte attachment sites on both PN and PP HEV are inactivated by mild periodate oxidation and hence are probably carbohydrate in nature. Yet, the sites are biochemically distinguishable in that one class (PN) requires sialidase-sensitive structures whereas the other (PP) does not. We raise the possibility that diversity in the carbohydrate-based recognition determinants on HEV may underlie the adhesive specificities in this system.