Zonal distribution of mast cells in bronchiolitis obliterans organizing pneumonia and constrictive bronchiolitis as compared with interstitial pulmonary fibrosis

Mast cells are known to play an important role in the pathogenesis of inter stitial pulmonary fibrosis, but their participation in the pathogenesis of bronchiolitis obliterans organizing pneumonia is less well known. We examin ed the zonal distribution of mast cells in 22 patients with bronchiolitis o bliterans organizing pneumonia by using a monoclonal antibody targeting mas t cell tryptase. Cases of interstitial pulmonary fibrosis (n = 5) and norma l lung tissue (n = 12) were used for comparative analysis. The 22 patients (nine men, 13 women) included seven patients with constrictive bronchioliti s after lung transplantation. Mast cells were counted by two observers in t hree pulmonary zones. Zone 1, the most peripheral area, was defined as an a rea of 10 high-power fields immediately adjacent to zone 2, which encompass ed a circular area located within one high-power field from the bronchial b asement membrane. Zone 3 was located within the intraluminal fibrous plugs. The study showed a trizonal distribution of mast cells in bronchiolitis ob literans organizing pneumonia and bizonal mast cell distribution in constri ctive bronchiolitis. This difference is most probably related to the absenc e of intraluminal plugs in constrictive bronchiolitis and is likely to be n onsignificant. Thus bronchiolitis obliterans organizing pneumonia and const rictive bronchiolitis demonstrated an essentially similar peribronchiolar m ast cell distribution. However, interstitial pulmonary fibrosis demonstrate d a greater increase of mast cells as compared with either bronchiolitis ob literans organizing pneumonia or constrictive bronchiolitis. This suggests that mast cells play a role in the fibrogenesis of both airway and nonairwa y fibrosis, However, mast cell counts alone did not appear to explain the w ell-known morphologic differences between bronchiolitis obliterans organizi ng pneumonia and constrictive bronchiolitis.