NADPH oxidase activation regulates apoptotic neutrophil clearance by murine macrophages

The phagocyte NADPH oxidase generates superoxide, the precursor to reactive oxygen species (ROS) that have both antimicrobial and immunoregulatory functions. Inactivating mutations in NADPH oxidase alleles cause chronic granulomatous disease (CGD), characterized by enhanced susceptibility to life-threatening microbial infections and inflammatory disorders, and hypomorphic NADPH oxidase alleles are associated with autoimmunity. Impaired apoptotic cell (AC) clearance is implicated as an important contributing factor in chronic inflammation and autoimmunity, but the role of NADPH oxidase-derived ROS in this process is incompletely understood. Here, we demonstrate that phagocytosis of AC (efferocytosis) potently activated NADPH oxidase in mouse peritoneal exudate macrophages (PEM). ROS generation was dependent on macrophage CD11b, TLR2, TLR4, and MyD88, and was also regulated by phosphotidylinositol 3-phosphate binding to the p40 phox oxidase subunit. Maturation of efferosomes containing apoptotic neutrophils was significantly delayed in CGD PEM, including acidification and acquisition of proteolytic activity, and associated with slower digestion of apoptotic neutrophil proteins. Treatment of WT macrophages with the vacuolar-type H+ ATPase inhibitor bafilomycin also delayed proteolysis within efferosomes, showing that luminal acidification was essential for efficient digestion of efferosome proteins. Finally, cross-presentation of AC-associated antigens by CGD PEM to CD8 T cells was increased. These studies unravel a key role for the NADPH oxidase in the disposal of apoptotic cells by inflammatory macrophages. The oxidants generated promote efferosome maturation and acidification that facilitate the degradation of ingested apoptotic cells.