Myeloperoxidase: A new player in autoimmunity

•Myeloperoxidase diminishes dendritic cell activation.•In addition to its bactericidal activity myeloperoxidase can damage tissues during inflammation.•Vascular permeability induced by myeloperoxidase is a contributing factor to inflammatory diseases.•Myeloperoxidase is a therapeutic target in a variety of autoimmune diseases. Myeloperoxidase (MPO) is the most toxic enzyme found in the azurophilic granules of neutrophils. MPO utilizes H2O2 to generate hypochlorous acid (HClO) and other reactive moieties, which kill pathogens during infections. In contrast, in the setting of sterile inflammation, MPO and MPO-derived oxidants are thought to be pathogenic, promoting inflammation and causing tissue damage. In contrast, evidence also exists that MPO can limit the extent of immune responses. Elevated MPO levels and activity are observed in a number of autoimmune diseases including in the central nervous system (CNS) of multiple sclerosis (MS) and the joints of rheumatoid arthritis (RA) patients. A pathogenic role for MPO in driving autoimmune inflammation was demonstrated using mouse models. Mechanisms whereby MPO is thought to contribute to disease pathogenesis include tuning of adaptive immune responses and/or the induction of vascular permeability.