MPO and granulomatosis with polyangiitis: However, the etiopathogenesis has been proposed to be due to anti-neutrophilic cytoplasmic antibodies (ANCA), presumed to be the cause of inflammation in GPA. Cytoplasmic ANCA (c-ANCA), where autoantibodies are directed against proteinase 3 antibodies, is seen in up to 90% of GPA cases, whereas perinuclear ANCA (p-ANCA), directed against myeloperoxidase antibodies, is seen in only a handful of cases.