Phagocytosis of cells dying through autophagy induces inflammasome activation and IL-1β release in human macrophages

Phagocytosis of naturally dying cells usually blocks inflammatory reactions in host cells. We have recently observed that clearance of cells dying through autophagy leads to a pro-inflammatory response in human macrophages. Investigating this response further, we found that during engulfment of MCF-7 or 293T cells undergoing autophagic death, but not apoptotic or anoikic ones, caspase-1 was activated and IL-1 beta was processed, then secreted in an MyD88-independent manner. Autophagic dying cells were capable of preventing some LPS-induced pro-inflammatory responses, such as TNF alpha, IL-6 and IL-8 induction, but synergized with LPS for IL-1 beta production. Caspase-1 inhibition prevented macrophage IL-1 beta release triggered by the dying cells and also other pro-inflammatory cytokines which were not formed in the presence of IL-1 receptor antagonist anakinra either. IL-1 beta secretion was also observed using calreticulin knockdown or necrostatin-treated autophagic MCF-7 cells and it required phagocytosis of the dying cells which led to ATP secretion from macrophages. Blocking K+ efflux during phagocytosis, the presence of apyrase, adding an antagonist of the P2X(7) receptor or silencing the nod-like receptor protein NALP3 inhibited IL-1 beta secretion. These data suggest that during phagocytosis of autophagic dying cells ATP, acting through its receptor, initiates K+ efflux, inflammasome activation and secretion of IL-1 beta, which initiates further pro-inflammatory events. Thus, autophagic death of malignant cells and their clearance may lead to immunogenic response.