Macrophages utilize mitochondrial fission to enhance mROS production during responses to Streptococcus pneumoniae

Abstract

Immunometabolism and regulation of mitochondrial reactive oxygen species (mROS) control the immune effector phenotype of differentiated macrophages. Mitochondrial function requires dynamic fission and fusion, but whether effector function is coupled to altered dynamics during bacterial responses is unknown. We show that macrophage mitochondria undergo fission after 12 h of progressive ingestion of live Streptococcus pneumoniae (pneumococci), without evidence of Drp-1 phosphorylation at S616. Fission is associated with progressive reduction in oxidative phosphorylation but increased mROS generation. Fission is enhanced by mROS production, PI3Kγ signaling and by cathepsin B, but is independent of inflammasome activation or IL-1β generation. Inhibition of fission reduces bacterial killing. Fission is associated with Parkin recruitment to mitochondria, but not mitophagy. Fission occurs upstream of apoptosis induction and independently of caspase activation. During macrophage innate responses to bacteria mitochondria shift from oxidative phosphorylation and ATP generation to mROS production for microbicidal responses by undergoing fission. Author summary Changes in metabolism regulate function in immune cells, including macrophages which are key cells in pathogen clearance. Mitochondria are cellular organelles that generate energy during metabolism but also mitochondrial reactive oxygen species (mROS) that contribute to bacterial killing. Mitochondria are dynamic organelles that form complex networks with varying degrees of fragmentation or fusion, but the functional consequences of these processes on macrophage function during bacterial infection are unknown. We show that sustained ingestion of live bacteria triggers mitochondrial fragmentation, reducing metabolism but enhancing mROS generation. Mitochondrial fragmentation is not part of a clearance pathway for damaged mitochondria and is initiated before signs of cell death. Macrophage signalling pathways activated during infection, and mROS generation, enhance mitochondrial fragmentation, and inhibition of pathways promoting fragmentation reduces bacterial killing. Overall, these findings suggest that responses to ingested bacteria trigger mitochondrial fragmentation, allowing mitochondria to switch from energy generation during metabolism to organelles facilitating bacterial killing.