Identification of Two Distinct Antigens on Rat Granulocytes and Macrophages with Special Reference to Their Expression on Hemopoietic Stem Cells

Two distinct antigen systems expressed on rat hemopoietic cells differentiated into granulocytes and macrophages were detected using two different monoclonal antibodies (R2-1A6 and R2-2B1). These two antibodies reacted with approximately 40% of rat bone marrow cells, granulocytes, blood monocytes, alveolar macrophages and peptone-elicited peritoneal macrophages, but not with red blood cells, platelets, thymocytes nor T lymphocytes. In addition, R2-2B1 antibody but not R2-1A6 antibody weakly reacted with B cells. These two monoclonal antibodies also reacted with some reticular elements in the thymic medulla and follicular dendritic cells of the lymphoid germinal center, as well as with the specialized endothelium of 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 the 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 den-dritic cells, presumably with accessory cell function, are separable from the mononuclear phagocyte system in rats by their different reactivities with R2-1A6 and R2-2B1 antibodies. Furthermore, the expression of these two antigens on hemopoietic stem cells were studied using in vitro colony forming assay, colony forming unit-E (CFU-E) and colony forming unit-C (CFU-C). Treatment of bone marrow cells with either R2-1A6 or R2-2B1 antibody plus complement almost completely abolished colony formation in CFU-C. While R2-1A6 treatment strongly inhibited CFU-E, R2-2B1 only weak-ly inhibited CFU-E. These antibodies are useful for analyzing rat hemopoietic stem cells