Identification of a factor that links apoptotic cells to phagocytes

Abstract

Apoptotic cells are rapidly engulfed by phagocytes to prevent the release of potentially noxious or immunogenic intracellular materials from the dying cells, thereby preserving the integrity and function of the surrounding tissue. Phagocytes engulf apoptotic but not healthy cells, indicating that the apoptotic cells present a signal to the phagocytes, and the phagocytes recognize the signal using a specific receptor. Here, we report a factor that links apoptotic cells to phagocytes. We found that milk fat globule-EGF-factor 8 (MFG-E8), a secreted glycoprotein, was produced by thioglycollate-elicited macrophages. MFG-E8 specifically bound to apoptotic cells by recognizing aminophospholipids such as phosphatidylserine. MFG-E8, when engaged by phospholipids, bound to cells via its RGD (arginine-glycine-aspartate) motif--it bound particularly strongly to cells expressing alpha(v)beta(3) integrin. The NIH3T3 cell transformants that expressed a high level of alpha(v)beta(3) integrin were found to engulf apoptotic cells when MFG-E8 was added. MFG-E8 carrying a point mutation in the RGD motif behaved as a dominant-negative form, and inhibited the phagocytosis of apoptotic cells by peritoneal macrophages in vitro and in vivo. These results indicate that MFG-E8 secreted from activated macrophages binds to apoptotic cells, and brings them to phagocytes for engulfment.

The phospholipid code: a key component of dying cell recognition, tumor progression and host–microbe interactions

“…For example, PS can be recognized by cell surface receptors including TIM-4 (Kobayashi et al 13 and Miyanishi et al 14 ) and BAI-1. 15 PS can also be recognized by soluble opsonins such as MFG-E8 and Gas6, that are in turn detected by cell surface molecules α V β 3 and TAM receptors, respectively [16][17][18] (Figure 2a). The recognition of PS by phagocytes through various phospholipid detectors is an important molecular step to trigger the engulfment of apoptotic cells, 3 and exemplifies how modification of the phospholipid code on the surface of apoptotic cells can promote their clearance.…”

p63(TP63) elicits strong trans-activation of the MFG-E8/lactadherin/BA46 gene through interactions between the TA and ΔN isoforms

“…In contrast, this study dealt with the functional p63 proteins localized to undifferentiated keratinocytes in the basal layer of stratified epithelia and in carcinomas derived from the stage. MFG-E8-L, the mouse-specific long isoform, having a Pro/Thr-enriched domain, facilitates phagocytosis of apoptotic cells by macrophages as demonstrated by recent studies with MFGE8-deficient mice (Hanayama et al, 2002;Hanayama and Nagata, 2005). However, the other isoform, MFG-E8-S, lacking the phagocytosisfacilitating activity is more ubiquitously, abundantly expressed (Watanabe et al, 2005) and expected to facilitate various types of cell-cell interactions.…”

“…Mouse Mfge8 produces MFG-E8-L and MFG-E8-S, in which the former is mousespecific, and the latter conserved among humans and animals (Watanabe et al, 2005). While the L form containing a Pro/Thr-enriched domain facilitates phagocytosis of apoptotic cells by macrophages (Hanayama et al, 2002(Hanayama et al, , 2004, the S form was reported to support the gamete interaction upon fertilization (Ensslin and Shur, 2003). Very importantly, recent studies determined peptide sequences derived from human MFG-E8 as the potential targets for tumor immunotherapy (Carmon et al, 2002;Liu et al, 2005).…”

Anti-idiotype-mediated epitope spreading and diminished phagocytosis by a human monoclonal antibody recognizing late-stage apoptotic cells

“…Many molecules on the phagocyte have been implicated; 24 the recognition of PS on apoptotic cells, with or without the conjunction of intermediary molecules, appears to be important. 20,25,26 The notion that defects in the clearance of apoptotic debris can give rise to a lupus-like pathology has been supported by a number of knockout mouse models; C1q À/À , 27 c-mer À/À9 and most recently MFG-E8 À/À 28 mice. Injection of mutant MFG-E8 (engineered to mask PS and reduce apoptotic cell uptake) also elicits autoimmune responses, particularly when administered along with apoptotic cells.…”