MPO and granulomatosis with polyangiitis: MPA, GPA (formerly known as Wegener granulomatosis, WG), and EGPA (formerly known as Churg–Strauss syndrome, CSS) are three major kinds of small-vessel vasculitis marked by significant loss of immune tolerance to primary zymoproteins in neutrophil plasma, most commonly myeloperoxidase (MPO) and proteinase 3 (PR3) to produce MPO-ANCA and PR3-ANCA, respectively (Kitching et al., 2020).