Histopathologic Assessment of Pulmonary Capillaritis Using Combined Immunohistochemical and Special Stains: An Autopsy Case Double-Positive for MPO-ANCA and Anti-GBM Antibodies.

Anti-glomerular basement membrane (anti-GBM) disease may coexist with antineutrophil cytoplasmic antibodies (ANCA), a clinicopathologic entity termed double-positive disease, in which diffuse alveolar hemorrhage (DAH) is common but histologic confirmation of pulmonary capillaritis remains difficult. A 93-year-old woman presented with rapidly progressive renal failure, and serologic evaluation revealed markedly elevated myeloperoxidase (MPO)-ANCA and anti-GBM antibody levels. Despite corticosteroid therapy, the patient died of progressive renal failure. At autopsy, the kidneys demonstrated diffuse crescentic glomerulonephritis with linear IgG deposition along the glomerular basement membrane, consistent with anti-GBM nephritis, together with multiple cortical scars composed of clusters of globally sclerotic glomeruli and interstitial fibrosis. The lungs exhibited DAH with focal neutrophilic infiltration along the alveolar septa, accompanied by focal destruction of capillary walls with subtle leukocytoclasia. Immunohistochemical analysis using epithelial marker (cytokeratin AE1/AE3), endothelial markers (CD31, CD34, and ERG), and platelet markers (CD61 and CD42b) highlighted alveolar epithelial injury, loss of endothelial continuity, and extravascular platelet deposition, respectively, in areas where capillaritis was suspected. Spatial concordance of these findings supported the presence of structural injury associated with pulmonary capillaritis. This case highlights the diagnostic importance of rigorous histopathologic assessment complemented by combined immunohistochemistry in patients with DAH.