MPO and vasculitis: In anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis, pathogenic myeloperoxidase (MPO)-ANCA binds to MPO expressed on tumor necrosis factor α-primed neutrophils, and the Fc region of ANCA crosslinks with the Fcγ receptor coupled with Mac-1 on neutrophils to activate spleen tyrosine kinase signaling and ROS production, which results in peptidylarginine deiminase 4 activation and NET formation (30–32).