CHARACTERIZATION OF A NOVEL BOVINE LEUKOCYTE PROTEIN INVOLVED IN CELL-CELL ADHESION

Preliminary characterization of an apparently novel bovine leukocyte a dhesion protein is described. Two IgG(1) monoclonal antibodies, UC-Cl and UC-H5, raised against established cultures of IL-2-dependent bovin e peripheral blood lymphocytes (PBL) were found to react with an antig en expressed by the majority of bovine peripheral blood leukocytes. Im munoprecipitation and polyacrylamide gel electrophoresis of the antige n produced a distinct protein band of molecular weight 160 000, and ad ditional diffuse protein bands of approximate molecular weight 180 000 , 175 000 and 150 000. Two-color flow cytometric analyses showed that the antigen was expressed at low density on a small proportion of circ ulating B lymphocytes, but was highly expressed on all circulating T l ymphocytes. The majority of monocytes and all granulocytes expressed t he antigen at a density lower than that of T lymphocytes. Peripheral b lood lymphocytes stimulated with concanavalin A had an approximately 3 -fold increased expression of the antigen, which was apparent within 1 8 h and remained stable in long-term cultures. Expression of the antig en in thymus, analyzed by the immunoperoxidase technique, was predomin antly restricted to thymocytes in the immediate subcapsular cortex and medulla; expression in lymph nodes and spleen was predominantly confi ned to lymphocytes in T-cell areas. Flow-cytometric analysis demonstra ted that thymocytes and the majority of peripheral and mesenteric lymp h node-derived T cells had relatively low surface density of antigen c ompared to circulating T cells. Binding of UC-Cl or UC-H5 to the antig en on lymphocytes induced homotypic aggregation. UC-Cl completely bloc ked binding of FITC-conjugated UC-H5 to blood mononuclear cells, sugge sting that the antibodies recognize the same epitope or proximal epito pes.