MPO and cystic fibrosis: Prior observations reported the presence of anti-neutrophil cytoplasmic antibodies (ANCA) in CF that target cytoplasmic components of neutrophils such as MPO (MPO-ANCA (19, 20)), bacterial permeability increasing protein (BPI-ANCA (21–23)) and proteinase 3 (PR3-ANCA (19, 20)).