Oxidative stress inhibits the phagocytosis of apoptotic cells that have externalized phosphatidylserine

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Abstract
The efficient phagocytosis of apoptotic cells by macrophages reduces the potential for an inflammatory response by ensuring that the dying cells are cleared before their intracellular contents are released. Early apoptotic cells are targeted for phagocytosis through the translocation of phosphatidylserine (PS) from the inner to the outer leaflet of the plasma membrane. In this report, we show that the oxidant H2O2 inhibits phagocytosis of apoptotic cells even though the cells express functional PS on their surface. Thus, B lymphoma cells induced to undergo apoptosis by the chemotherapy drug etoposide are efficiently phagocytosed by macrophages in a process that is mediated by PS (inhibitable by PS liposomes). Exposure of the apoptotic cells to H2O2 inhibits phagocytosis even though the cells still express functional PS on their surface. In addition, Jurkat cells and thymocytes induced to undergo apoptosis by H2O2 alone are poorly phagocytosed. Inhibition of phagocytosis by H2O2 cannot be attributed to oxidative inactivation or redistribution of PS on the cell surface. The results indicate that PS externalization is necessary but is not sufficient to target apoptotic cells for phagocytosis. Another phagocytosis recognition factor must therefore exist to facilitate uptake of apoptotic cells, and this factor is sensitive to modification by H2O2.