Intraluminal plugs in idiopathic and secondary organizing pneumonia: repair or remodelling?

Aims:  Organizing pneumonia (OP) is an inflammatory lung disease characterized histologically by intraluminal plugs involving alveolar ducts and alveoli. The aim was to examine extracellular matrix repair and remodelling in 12 cases of idiopathic OP and compare these with 11 cases of secondary OP. Methods and results:  Collagen/elastic fibre density, myofibroblast proliferation, microvascular density and endothelial activity in the intraluminal plugs were evaluated by histochemistry, immunohistochemistry and morphometry. The density of the collagen system fibres was greater in plugs of idiopathic OP when compared with secondary OP (P < 0.001). Quantification of myofibroblastic cells confirmed differences observed in patterns of immunoexpression; when compared with secondary OP, idiopathic OP contained fewer myofibroblastic cells in intraluminal plugs (P = 0.01). Microvascular density (CD34) and endothelial activity (vascular cell adhesion molecule-1 and E-selectin) were significantly greater in secondary OP (P < 0.05). Conclusions:  Idiopathic and secondary OP show significant variation in morphological features that may represent different responses to injury. Increased collagen synthesis, low myofibroblast proliferation, poor microvascularization and minimal endothelial activity found in idiopathic OP may be more of a remodelling process than secondary OP repair.