Three-dimensional characterization of pathologic lesions in pulmonary langerhans cell histiocytosis.

The characteristic lesions of pulmonary Langerhans cell histiocytosis (LCH) associate destructive granulomas containing large numbers of Langerhans cells and cysts. The lesions are usually considered to develop around small airways, and cysts are thought to result from destruction of the bronchiolar wall by the granulomatous reaction. However, the extent to which the granulomatous reaction is truly bronchocentric remains unknown, and the mode of formation of the cysts has not been defined. By using serial sections, this study aimed to explore further the relationships between pulmonary LCH lesions and distal airways, and the development of cysts. The results demonstrated that the granulomatous process of pulmonary LCH affected exclusively small airways, in an acinar distribution. The lesions extended without interruption along the bronchiolar axis, forming a continuous sheath around distal airways. The granulomatous reaction seemed to progress along the bronchiolar axis over time, extending the abnormalities in both the proximal and distal directions. Cystic lesions resulted from the destruction of the bronchiolar wall and progressive dilatation of the lumen, subsequently circumscribed by fibrous tissue. Because pulmonary LCH lesions affect and progressively destroy distal airways, it may be proper to consider the disease a bronchiolitis rather than an interstitial lung disorder.