Lipid droplet-mediated danger signaling balances Trem2+ macrophage induction and NLRP3 inflammasome activation in NASH

Macrophages are exquisitely capable of sensing danger-associated molecular patterns (DAMPs) and orchestrating inflammatory response during tissue injury. Nonalcoholic steatohepatitis (NASH) represents an advanced stage of metabolic fatty liver disease that increases the risk for cirrhosis and liver cancer. Pathogenic mechanisms of NASH center on hepatocyte injury and the ensuing immune response within the liver microenvironment. However, the nature of DAMPs released by injured hepatocytes and how they shape the liver immune milieu remain largely unknown. Here we show that lipid droplets (LDs) released by injured fatty hepatocytes provide a potent signal that triggers monocyte infiltration and maturation into Trem2+ macrophages, recently described as NASH-associated macrophages (NAMs) or lipid- associated macrophages. LD treatment exacerbated liver injury in mice with diet-induced NASH. We identified Membrane spanning 4-domains a7 (Ms4a7) as a NAM-specific pathogenic factor that was strongly induced in mouse and human NASH. Ms4a7 inactivation ameliorated key aspects of diet-induced NASH pathologies in mice. At the mechanistic level, Ms4a7 physically interacts with NLR family pyrin domain containing 3 (NLRP3) and is required for endosomal NLRP3 inflammasome activation. These findings illustrate a crucial role of Ms4a7 in driving pro-inflammatory signaling in NASH liver. Surprisingly, LD treatments attenuated Ms4a7 expression and NLRP3 inflammasome activation in cultured macrophages. As such, LDs serve as a DAMP signal that balances the induction of Trem2+ macrophages and NLRP3 inflammasome activation in NASH liver