Protein S Enhances the Phagocytosis of Phosphatidylserine-Exposing Erythrocytes: Implications in Sickle Cell Disease.

The major anticoagulant Protein S (PROS1) also contributes to the phagocytosis of apoptotic cells by bridging exposed phosphatidylserine (PtdSer) to the MerTK receptor on macrophages (efferocytosis). Whether PROS1 is involved in the splenic clearance of PtdSer-positive senescent and altered erythrocytes such as erythrocyte ghosts (eryghosts) is unknown. Here, we investigate the contribution of PROS1 and MerTK to the phagocytosis of intact RBC and eryghosts in healthy subjects and patients with sickle cell disease (SCD). We show that PROS1 enhances the phagocytosis of ionomycin-treated PtdSer-positive erythrocytes and of eryghosts generated in vitro. We confirm that eryghosts circulate in patients with SCD at higher levels than in healthy subjects and observe increased hemolysis and decreased levels of plasmatic PROS1 in patients with the highest concentration of eryghosts. The proportion of circulating eryghosts is correlated with the intensity of hyposplenism, and eryghosts are less frequently observed in sections of SCD compared to control spleens. We demonstrate that circulating eryghosts are procoagulant and adhere to endothelial cells. In SCD, PROS1 enhances their phagocytosis in a MerTK-dependent manner but has no such effect on intact erythrocytes. PROS1 is therefore involved in erythrophagocytosis, a physiological process insufficient in patients with SCD due to intense intravascular hemolysis and hyposplenism, leading to PROS1 consumption and the abnormal persistence of eryghosts in circulation. PROS1 deficiency may in turn initiate a pathogenic loop, enhancing unregulated activation of coagulation and defective clearance of procoagulant and adherent eryghosts. This deeper understanding of physiological and pathological erythrophagocytosis opens new therapeutic approaches targeting PROS1 in SCD.