Two closely related antigens expressed on granulocytes, macrophages and some reticular elements in rat lymphoid tissues: characterization by monoclonal antibodies.

Two distinct novel antigen systems preferentially expressed in rat granulocytes and macrophages were detected using two different monoclonal antibodies (R2-1A6 and R2-2B1). These two antibodies reacted with approximately 50% of rat bone marrow cells, most granulocytes, blood monocytes, alveolar macrophages and peptone-elicited peritoneal macrophages, but not with red blood cells, platelets, thymocytes and T lymphocytes. In addition, R2-2B1 but not R2-1A6 antibody cross-reacted weakly with rat B cells. These two monoclonals also reacted with some reticular elements in rat lymphoid organs including epithelial reticular cells in the thymic medulla and follicular dendritic cells in the lymphoid germinal centre, as well as with the specialized endothelium in the marginal sinuses of the spleen and the post-capillary venules of the lymph node, where lymphocyte recirculation takes place. These antibodies, however, did not label so-called 'dendritic cells' bearing Ia antigens on their cell surfaces, which were found to be located in the thymic medulla, thymus-dependent areas of rat lymphoid tissues and the interstitium of various non-lymphoid organs, suggesting that these dendritic cells, presumably ascribed to those associated with accesory cell function, are separable from the mononuclear phagocyte system in rats by their different reactivities with R2-1A6 and R2-2B1 antibodies.