Phagocytosis of necrotic cells by macrophages is phosphatidylserine dependent and does not induce inflammatory cytokine production

Apoptotic cells are cleared by phagocytosis during development, homeostasis, and pathology. However, it is still unclear how necrotic cells are removed. We compared the phagocytic uptake by macrophages of variants of L929sA murine fibrosarcoma cells induced to die by tumor necrosis factor-induced necrosis or by Fas-mediated apoptosis. We show that apoptotic and necrotic cells are recognized and phagocytosed by macrophages, whereas living cells are not. In both cases, phagocytosis occurred through a phosphatidylserine-dependent mechanism, suggesting that externalization of phosphatidylserine is a general trigger for clearance by macrophages. However, uptake of apoptotic cells was more efficient both quantitatively and kinetically than phagocytosis of necrotic cells. Electron microscopy showed clear morphological differences in the mechanisms used by macrophages to engulf necrotic and apoptotic cells. Apoptotic cells were taken up as condensed membrane-bound particles of various sizes rather than as whole cells, whereas necrotic cells were internalized only as small cellular particles after loss of membrane integrity. Uptake of neither apoptotic nor necrotic L929 cells by macrophages modulated the expression of proinflammatory cytokines by the phagocytes.