Identification of MPO-positive capillaries of the pleura by immunohistochemistry in MPO-ANCA associated vasculitis.

We present a case of a middle-aged woman with myeloperoxidase anti-neutrophil cytoplasmic autoantibody (MPO-ANCA)-associated vasculitis that demonstrated immunohistochemically positive MPO capillaries of the pleura. The patient initially presented with proteinuria and microscopic hematuria at the age of 38. Acute progressive glomerulonephritis and pulmonary hemorrhage occurred 4 years later, and a high serum titer of MPO-ANCA was detected therefore a diagnosis of microscopic polyangiitis was made. Steroid-pulse therapy was performed and the pulmonary shadow improved, but the renal failure did not improve, thus, hemodialysis was initiated. Thereafter, an 18-year asymptomatic phase followed, but high serum levels of MPO-ANCA persisted during this period. Chronic pulmonary hemorrhage was discovered at the age of 60, and video-assisted thoracoscopic surgery was performed. Resected tissue revealed diffuse aloveolar hemorrhage accompanied by marked hemosiderin deposition, whereas MPO-immunopositive capillaries were identified only in the pleura. To our knowledge, this is the first report demonstrating MPO-positive capillaries in a disease other than glomerulonephritis. Judging from this unique case, MPO-positive endothelial cells may appear only during the hyperacute stage before hemorrhage, and may diminish thereafter, thus, may be associated with the trigger of microscopic polyangiitis.