MPO and rheumatoid arthritis: NET formation was first described as a defense mechanism to fight severe bacterial infections,12 but has now also been connected to other infectious diseases, as well as conditions characterized by sterile inflammation, such as rheumatoid arthritis, diabetes and cancer.13 NETs are formed by activated neutrophils expelling decondensed chromatin, decorated with granular proteases such as neutrophil elastase (NE) and myeloperoxidase (MPO), resulting in a web-like structure.