MPO and microscopic polyangiitis: Several animal experiments have demonstrated that MPO-ANCA might be a directly pathogenic autoantibody involved in MPA pathogenesis (Xiao et al., 2002; Little et al., 2005; Wang Q. et al., 2017), thus indicating that MPO-ANCA could be sufficient to cause pulmonary capillaritis and glomerulonephritis in certain biological environments (Figure 1).