MPO and microscopic polyangiitis: Moreover, a close linkage between pulmonary fibrosis and microscopic polyangiitis (MPA), a type of systemic necrotizing small vasculitis characterized by both pulmonary and renal involvement and associated with circulating antineutrophil cytoplasm antibodies (ANCAs) against myeloperoxidase (MPO), has been recently identified in both clinical [15] and experimental setting [16].