Surface expression of phosphatidylserine on macrophages is required for phagocytosis of apoptotic thymocytes

Cells generally maintain an asymmetric distribution of phospholipids across the plasma membrane bilayer, restricting the phospholipid, phosphatidylserine (PS), to the inner leaflet of the plasma membrane. When cells undergo apoptosis, this asymmetric transbilayer distribution is lost, bringing PS to the surface where it acts as a signal for engulfment by phagocytes. The fluorescent dye merocyanine 540 specifically stains the plasma membrane of apoptotic cells which have lost their asymmetric distribution of phospholipids. However, it also stains non-apoptotic macrophages, suggesting that phospholipid asymmetry may not be maintained in these cells, and thus that they may express PS on their surface. Here, the PS-binding protein, annexin V, was used to show that in fact normal macrophages do express PS on their surface. Furthermore, pre-treating macrophages with annexin V was found to inhibit phagocytosis of apoptotic thymocytes and thymocytes on which PS expression was artificially induced, but did not inhibit phagocytosis of latex beads or Fc receptor-mediated phagocytosis of opsonized erythrocytes. These results indicate that PS is constitutively expressed on the surface of macrophages and is functionally significant for the phagocytosis of PS-expressing target cells.