Reduced expression of CD47 during murine red blood cell (RBC) senescence and its role in RBC clearance from the circulation

Abstract

Almost 2 percent of murine blood red blood cells (RBCs) are destroyed each day and are replaced by fresh RBCs generated through the process of erythropoiesis. RBCs to be destroyed are phagocytosed by macrophages in the reticuloendothelial system, especially in the spleen. CD47 molecules on RBCs may regulate the susceptibility of RBC to destruction by phagocytosis because its recognition by inhibitory receptor (signal regulatory protein alpha) on macrophages sends a negative signal, which if sufficiently strong, may abort the phagocytic response altogether. The aim of this study was to investigate whether age-dependent changes in CD47 expression on circulating RBCs have a role in destruction of senescent RBCs by macrophages. A two-step in vivo biotinylation method for labeling mouse RBCs in vivo was used to track the CD47 expression levels as well as the turnover of circulating RBCs of defined age groups. Our results indicate that CD47 expression levels decrease on circulating RBCs throughout their life span in circulation. The oldest RBCs in circulation have 30 percent lower mean expression of CD47 than the youngest RBCs. Depletion of macrophages by administration of clodronate-loaded liposomes resulted in a significant decrease in the mean expression of CD47 on RBCs of all age groups and a significant accumulation of senescent RBCs in blood and spleen. A decrease in mean expression of CD47 and accumulation of senescent RBCs in macrophage-depleted mice were significantly higher for spleen RBCs compared to blood RBCs. Our results provide supportive evidence for a role of decreasing CD47 expression on aging circulating RBCs in their destruction by macrophages.