Abstract Text: Senescent erythrocytes are phagocytosed by the macrophages that make up the reticuloendothelial system, a network of myeloid cell rich tissues responsible for filtering circulating cells. The principal effector tissue of the reticuloendothelial system is the splenic red pulp, whose principal effector cells are resident red pulp macrophages. During their lifespans, erythrocytes are protected from non-specific destruction by red pulp macrophages by their expression of CD47, a ‘don’t eat me’ signal that binds to macrophage expressed SIRPa to inhibit phagocytosis. However, as monoclonal antibodies have been developed to block the CD47-SIRPa interaction for cancer immunotherapy, it has been necessary to overcome the toxicity associated with the off-target destruction of erythrocytes. It was found that a small loading dose of CD47 antibody confers to erythrocytes protection against much greater subsequent dosing. Here we identify the mechanism of this protection in mice. We find that a loading dose of CD47 antibody protects erythrocytes by impairing antibody binding and phagocytosis by red pulp macrophages. We begin by demonstrating that CD47 antibody induces FcgR-mediated pruning of erythroid CD47, with global concomitant FcgR loss by reticuloendothelial myeloid populations. We then show that CD47 antibody therapy impairs red pulp macrophage phagocytosis of erythrocytes. Finally, we show that CD47 antibody therapy protects erythrocytes and platelets from antibody-mediated destruction and thus may have therapeutic potential for autoimmune hemolytic anemia (AIHA) and immune thrombocytopenia (ITP), disorders caused by autoantibody-mediated destruction of erythrocytes and platelets, respectively.
