PHOSPHATIDYLSERINE VESICLES INHIBIT PHAGOCYTOSIS OF ERYTHROCYTES WITH A SYMMETRICAL TRANSBILAYER DISTRIBUTION OF PHOSPHOLIPIDS

In the plasma membrane of erythrocytes, the phospholipids are asymmetrically distributed between the two leaflets of the bilayer. Erythrocytes that have lost this asymmetric distribution are more readily phagocytosed in vitro by macrophages. Surface exposure of phosphatidylserine (PS), normally restricted to the inner leaflet of the bilayer, has been suggested as a signal by which macrophages recognize erythrocytes. To test whether lipid-symmetric erythrocytes are recognized by this mechanism, artificial lipid vesicles made of PS were tested for their ability to inhibit phagocytosis. J774 mouse macrophages, which preferentially phagocytose PS vesicles, were incubated with a mixture of lipid-symmetric erythrocytes and vesicles. PS vesicles reduced phagocytosis of lipid-symmetric erythrocytes to the level of lipid-asymmetric erythrocytes, whereas phosphatidylcholine vesicles had only a limited effect. These results support a role for PS in recognition of erythrocytes and lend credence to the more general hypothesis that PS is involved in the recognition of all blood cells by the reticuloendothelial system.