Rôle du microenvironnement apoptotique tissulaire et du MFG-E8 dans la modulation de la réponse inflammatoire

Inflammation is an important component of the « response to injury » process, allowing host integrity. In response to injury, released inflammatory mediators from damaged cells play a crucial role in the modification of the inflammatory microenvironment, which can lead to more cellular and tissue damages. Macrophages can be activated by those danger signals, leading to a spectrum of cytokines and chemokines secretion and modulating the tissular microenvironment. Tissue injuries can lead to cell death such as apoptosis. Mediators released during apoptosis contribute to the nature of the microenvironment, by their influence on macrophage amongst others. We have identified that Milk Fat Globule-Epidermal growth factor 8 (MFG-E8), an important actor in inflammation resolution, is specifically released by apoptotic cells. We hypothesized that tissular apoptotic microenvironment, through MFG-E8 release, modulates macrophage phenotype, resulting in the modification of   microenvironment, inflammatory response and tissu injury outcome. Thus, our objectives were to 1) characterize this tissular apoptotic microenvironment by studying MFG-E8 release kinetic by apoptotic cells, to 2) evaluate its role in macrophage phenotype modulation and to 3) study, in vivo, its influence on inflammatory environment and tissu damage outcome.   In the first study, we demonstrated that MFG-E8 is released by apoptotic endothelial cells in a caspase-3-dependent manner. When macrophages were exposed to conditioned media from apoptotic endothelial cells, they adopt a high anti-inflammatory, low pro-inflammatory cytokine/chemokine secreting phenotype that is lost if apoptosis is inhibited or if MFG-E8 is absent from the media. Furthermore, MFG-E8 potential to anti-inflammatory macrophage reprogramming has been demonstrated in the experimental peritonitis model. This MFG-E8-mediated reprogramming of macrophages occurs through increased phosphorylation of STAT-3. As apoptotic endothelial cells, apoptotic epithelial cells also release MFG-E8 in an apoptotic-dependent manner. Thus, we investiguated more exhaustively an in vivo epithelial apoptosis rich model, the unilateral ureteral obstruction. In this second study, we report the positive impact of MFG-E8 in this renal obstructive model. MFG-E8 administration reduced kidney damage and fibrosis compared to the control, whereas its absence in MFG-E8 KO mice was associated with more severe disease. Moreover, we demonstrated that  the protective role of MFG-E8 is mediated through inflammasome activation modulation in the kidney. Furthermore, our results showed the importance of the anti-inflammatory macrophage  phenotype that results in decreased inflammasome activation, preventing severe tissue damage.  This thesis presents the first description of apoptosis-dependent release of MFG-E8 by apoptotic cells. It also demonstrate the importance of MFG-E8 in inflammatory apoptotic microenvironment, leading to pro-inflammatory macrophage phenotype attenuation. Moreover, we demonstrated MFG-E8 protective role in an aortic transplantation and a tissue repair models, as well as in a chronic kidney disease model where we showed that this MFG-E8 confered protection is mediated by negative regulation of tissular inflammasome activation. These data provide valuable insight for identifying MFG-E8 as a novel target in the modulation of inflammatory diseases and could be considerate as an inflammatory switch.

Keyphrases: Inflammation, MFG-E8, Macrophage, Inflammasome, Cytokines, Apoptosis, Phenotype, Chemokines, Monocyte