MPO and pulmonary arterial hypertension: Although NET markers (DNA, myeloperoxidase, and citrullinated histone H3) were previously identified in PAH lung tissue (42), and although NE on NETs is shielded from natural circulating large-molecule inhibitors such as alpha-1 antitrypsin (41), our studies now provide evidence that small protein inhibitors such as elafin can block NE activity and NET formation.