Apoptotic Cells, at All Stages of the Death Process, Trigger Characteristic Signaling Events That Are Divergent from and Dominant over Those Triggered by Necrotic Cells

Abstract

Current models of autoimmunity suggest that delayed clearance of apoptotic cells leads to the presentation of apoptotic antigens in the context of inflammatory signals, with resultant autoimmunity. These models implicitly assume that, in contrast to early apoptotic cells (that retain membrane integrity), late apoptotic cells (with compromised membranes) act like necrotic cells (which also lack intact membranes), possibly because of the release of proinflammatory intracellular contents. We showed previously that early apoptotic and necrotic cells induce distinct mitogen-activated protein kinase modules in macrophages with which they interact. Exposure to apoptotic cells led to nearly complete inhibition of both basal and macrophage colony-stimulating factor-induced ERK1/2 by macrophages. In contrast, necrotic cells induced ERK1/2. We show here that apoptotic cells also strongly induced both c-Jun N-terminal kinase and p38, whereas necrotic cells had no detectable effect on c-Jun N-terminal kinase and p38. We also compared the signaling events induced in macrophages by exposure to early apoptotic cells, late apoptotic cells, and necrotic cells. The signaling events induced by late apoptotic cells were identical to and just as potent as those induced by early apoptotic cells. Thus, apoptotic cells are functionally equivalent throughout the cell death process, irrespective of membrane integrity. Moreover, the effects of both early and late apoptotic cells on signaling were dominant over those of necrotic cells. These data show that apoptotic cells do not become proinflammatory upon the loss of membrane integrity and are inconsistent with the notion that delayed clearance alone can lead to autoimmunity.Apoptosis is an active energy-dependent process that generally occurs without inflammation or injury to surrounding tissues (1). Apoptotic cells express surface markers that permit their rapid recognition and ingestion by phagocytes (2, 3). Moreover, the cell membrane of cells undergoing apoptosis remains intact until relatively late (1). Thus, the vast majority of cells dying by apoptosis are cleared by phagocytes while their cell membranes are still intact and before they can release their potentially inflammatory intracellular contents.In this view, the noninflammatory behavior of apoptotic cells is essentially passive in that inflammation is avoided by rapid and efficient clearance of apoptotic cells. In fact, apoptotic cells are also actively antiinflammatory (4, 5). For example, the uptake of apoptotic cells actively inhibits the release of proinflammatory mediators such as interleukin 1 and tumor necrosis factor-␣ by macrophages (m) 2 (4 -6). This contrasts with the effect of necrotic cell uptake, which may lead to m activation and the release of proinflammatory cytokines (7). Based on this differential response to apoptotic versus necrotic cells, antigens derived from cells dying by these two distinct processes are thought to have opposite effects on the activation of T cells (8). The proinflammatory effects of...

Clearance of dying autophagic cells of different origin by professional and non-professional phagocytes

“…In the case of anoikic autophagy this may be explained by the presence of late apoptotic cells, which have been shown to be proinflammatory, 37 in high proportion at day 6 of anoikis (see percent of annexin-V þ PI þ cells in Figure 2a). Regarding cells dying through autophagy, they may expose autophagy-related pro-inflammatory signals on their surface; this may have significance when autophagic death occurs in tumor cells and the inflammatory response can promote tumor growth.…”

From regulation of dying cell engulfment to development of anti-cancer therapy

“…74 Apoptotic and necrotic cells, upon recognition by macrophages, have different effects on MAPK signaling. 71 Early apoptotic cells were found to be as effective as late apoptotic cells (secondary necrotic) in inhibiting extracellular signalregulated kinase 1/2 activity and stimulating c-Jun N-terminal kinases 1/2 (JNK1/2) and p38. By contrast, necrotic cells had no detectable effect on JNK and p38.…”

“…By contrast, necrotic cells had no detectable effect on JNK and p38. 71 Moreover, it was recently proposed that TNF release by LPS-stimulated macrophages is suppressed by TGF-b only if the macrophages have first contacted apoptotic cells; thus, bystander macrophages appear to be refractory to TGF-b released by engulfing macrophages. 68 This observation may explain how macrophages balance pro-and anti-inflammatory responses in an in vivo situation where apoptotic cells are associated with inflammation and infection.…”

“…82 Ucker and colleagues elaborated further the differences between apoptotic and necrotic cell clearance with respect to inflammatory responses. 67,71 They showed that the inhibitory effect of apoptotic targets, at all stages of apoptotic cells death, irrespective of cell membrane integrity, is dominant over the stimulatory effect of necrotic targets. 67,71 It is conceivable that differential responses of macrophages to apoptotic and necrotic cells are triggered by distinct receptor-mediated recognition, while similar responses are triggered by the shared machinery of phagocytosis.…”

“…67,71 They showed that the inhibitory effect of apoptotic targets, at all stages of apoptotic cells death, irrespective of cell membrane integrity, is dominant over the stimulatory effect of necrotic targets. 67,71 It is conceivable that differential responses of macrophages to apoptotic and necrotic cells are triggered by distinct receptor-mediated recognition, while similar responses are triggered by the shared machinery of phagocytosis. In that respect, we demonstrated that in both apoptotic and necrotic cell death in the same cell type, clearance occurs in a PS-dependent manner, and in both cases does not activate NF-kB (DV Krysko and P Vandenabeele, unpublished data) or cytokine induction.…”