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.

Mechanisms of internalization of apoptotic and necrotic L929 cells by a macrophage cell line studied by electron microscopy.

Rapid and efficient phagocytic removal of dying cells is a key feature of apoptosis. In necrotic caspase-independent modes of death, the role and extent of phagocytosis is not well documented. To address this issue, we studied at the ultrastructural level the phagocytic response to dying cells in an in vitro phagocytosis assay with a mouse macrophage cell line (Mf4/4). As target cells, murine L929sAhFas cells were induced to die by TNFR1-mediated necrosis or by Fas-mediated apoptosis. Apoptotic L929sAhFas cells are taken up by complete engulfment of apoptotic bodies as single entities forming a tight-fitting phagosome, thus resembling the "zipper"-like mechanism of internalization. In contrast, primary and secondary necrotic cells were internalized by a macropinocytotic mechanism with formation of multiple ruffles by the ingesting macrophage. Ingestion of necrotic cellular material was invariably taking place after the integrity of the cell membrane was lost and did not occur as discrete particles, in contrast to apoptotic material that is surrounded by an intact membrane. Although nuclei of necrotic cells have been observed in the vicinity of macrophages, no uptake of necrotic nuclei was observed. The present report provides a basis for future studies aimed at discovering molecular pathways that precede these diverse mechanisms of uptake.

More than one way to die: methods to determine TNF-induced apoptosis and necrosis.

In most cellular systems tumor necrosis factor (TNF) induces apoptotic cell death. However, in some particular cell lines, such as the L929sA fibrosarcoma, TNF induces necrotic cell death. This effect is not the result of an inability to die apoptotically, because triggering of Fas in L929sAhFas cells leads to apoptosis. Moreover, TNFR-1-induced necrosis can be reverted to apoptosis when cells are pretreated with geldanamycin, an Hsp90 inhibitor. In contrast, addition of caspase-inhibitors (zVAD-fmk) prevents Fas-induced apoptosis and switches it to necrosis. These results demonstrate that depending on the cellular context, the same stimulus can induce either apoptosis or necrosis. Apoptosis and necrosis are clearly distinguished by their morphology, although in the absence of phagocytosis, the late stage of apoptosis is associated with secondary necrotic cell death, which is hard to distinguish from necrotic cell death. Necrosis is described mostly in negative terms as cell death that is characterized by the absence of apoptotic parameters, such as caspase activation, cytochrome c release, and DNA fragmentation. Here we describe a selection of techniques used to distinguish both modes of TNFR-1-induced cell death, namely apoptotic or necrotic cell death.

Differential signaling to apoptotic and necrotic cell death by Fas-associated death domain protein FADD.

Two general pathways for cell death have been defined, apoptosis and necrosis. Previous studies in Jurkat cells have demonstrated that the Fas-associated death domain (FADD) is required for Fas-mediated signaling to apoptosis and necrosis. Here we developed L929rTA cell lines that allow Tet-on inducible expression and FK506-binding protein (FKBP)-mediated dimerization of FADD, FADD-death effector domain (FADD-DED), or FADD-death domain (FADD-DD). We show that expression and dimerization of FADD leads to necrosis. However, pretreatment of the cells with the Hsp90 inhibitor geldanamycin, which leads to proteasome-mediated degradation of receptor interacting protein 1 (RIP1), reverts FKBP-FADD-induced necrosis to apoptosis. Expression and dimerization of FADD-DD mediates necrotic cell death. We found that FADD-DD is able to bind RIP1, another protein necessary for Fas-mediated necrosis. Expression and dimerization of FADD-DED initiates apoptosis. Remarkably, in the presence of caspase inhibitors, FADD-DED mediates necrotic cell death. Coimmunoprecipitation studies revealed that FADD-DED in the absence procaspase-8 C/A is also capable of recruiting RIP1. However, when procaspase-8 C/A and RIP1 are expressed simultaneously, FADD-DED preferentially recruits procaspase-8 C/A.