MPO and autoimmune disease: Antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) is a group of complex autoimmune diseases characterized by the vasculitis, endothelial injury, and tissue damage as well as the presence of serum autoantibodies targeting myeloperoxidase (MPO-ANCA) or proteinase 3 (PR3-ANCA) [1, 2].