MPO and anti-neutrophil cytoplasmic antibody-associated vasculitis: Anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) is a group of systemic autoimmune diseases, characterized by pauci-immune necrotizing small-vessel vasculitis and circulating autoantibodies against neutrophil cytoplasmic constituents, especially proteinase 3 (PR3) and myeloperoxidase (MPO).