Mechanisms for recognition and phagocytosis of apoptotic lymphocytes by macrophages.
Erythrocytes have an asymmetric distribution of phospholipids across the bilayer of their plasma membranes, maintained by an ATP-dependent aminophospholipid translocase, and dissipated by activation of a non-specific lipid flipsite. Loss of asymmetry provokes recognition by the reticuloendothelial system. In vitro, enhanced phagocytosis of erythrocytes with a symmetric bilayer can be inhibited by artificial lipid vesicles made of phosphatidylserine (PS), indicating that macrophages recognize the PS that appears on the erythrocyte surface upon loss of asymmetry. It is becoming increasingly clear that these same fundamental membrane structure/function relationships established in the erythrocyte paradigm also apply to lymphocytes. All evidence suggests that lymphocytes maintain an asymmetric transbilayer distribution of phospholipids in their plasma membranes, maintained by an aminophospholipid translocase. Asymmetry is lost as part of the program of cell death, by down-regulation of the translocase and activation of the non-specific lipid flipsite, exposing PS on the cell surface. That PS exposure has functional consequences is demonstrated by the ability of artificial lipid vesicles containing PS to inhibit enhanced phagocytosis of apoptotic lymphocytes by macrophages. However, other signals besides PS are also involved in recognition of apoptotic lymphocytes. Studies with other inhibitors indicate that macrophages also utilize integrin-mediated and lectin-like recognition systems, although each is restricted to either unactivated or activated macrophages. These results indicate that although many fundamental features of recognition by the reticuloendothelial system may be analogous in erythrocytes and lymphocytes, the signals for recognition of apoptotic lymphocytes ae more complex and involve multiple recognition systems.