The complement membrane attack complex triggers intracellular Ca2+ fluxes leading to NLRP3 inflammasome activation

The membrane attack complex of complement (MAC), apart from its classical role of lysing cells, can also trigger a range of non-lethal effects on cells, acting as a drive to inflammation. In the present study, we chose to investigate these non-lethal effects on inflammasome activation. We found that, following sublytic MAC attack, there is increased cytosolic Ca2+ concentration, at least partly through Ca2+ release from the endoplasmic reticulum lumen via the inositol 1,4,5-triphosphate receptor (IP3R) and ryanodine receptor (RyR) channels. This increase in intracellular Ca2+ concentration leads to Ca2+ accumulation in the mitochondrial matrix via the 'mitochondrial calcium uniporter' (MCU), and loss of mitochondrial transmembrane potential, triggering NLRP3 inflammasome activation and IL-1 beta release. NLRP3 co-localises with the mitochondria, probably sensing the increase in calcium and the resultant mitochondrial dysfunction, leading to caspase activation and apoptosis. This is the first study that links non-lethal effects of sublytic MAC attack with inflammasome activation and provides a mechanism by which sublytic MAC can drive inflammation and apoptosis.