Obliteration of the small airways is a largely unresolved challenge in pulmonary medicine. It represents either the irreversible cause of functional impairment or a morphologic disorder of limited importance in a multitude of diseases. Bronchiolitis obliterans is a key complication of lung transplantation. No predictive markers for the onset of obliterative remodeling are currently available. To further elucidate the molecular mechanisms of airway remodeling, compartment-specific expression patterns were analyzed in patients. For this purpose, remodeled and nonremodeled bronchioli were isolated from transplanted and nontransplanted lung explants using laser-assisted microdissection (n ‫؍‬ 24). The histologic term "bronchiolitis obliterans" describes obliterative changes in the airways that commonly occur in a variety of pulmonary diseases. These changes demonstrate divergent histologic and radiologic findings, various potencies to progress to additional compartments of the lung, and different clinical outcomes.1,2 At histologic analysis, two major variants can be distinguished. The first is varying degrees of discontinuous submucosal collagenous deposits that narrow the lumen of the bronchioli up to the point of total obliteration, referred to as "constrictive bronchiolitis." This type is most commonly observed after hematopoietic stem cell transplantation or allogeneic lung transplantation, in which it significantly limits long-term survival and is considered the hallmark of chronic graft dysfunction.3 The second variant is bronchiolitis obliterans with intraluminal polyps, in which mesenchymal protrusions bulge into the lumen of the airways and the adjacent alveoli.

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