Abstract

Idiopathic usual interstitial pneumonia (UIP) and bronchiolitis obliterans organizing pneumonia (BOOP) are fibrous pulmonary disorders in both of which there is newly formed connective tissue in distal air spaces. UIP is a progressive and usually fatal lung disease without any efficient treatment, while the prognosis of BOOP is good. In both diseases, an injury of the alveolar epithelium and its basement membrane (BM) leads to migration of fibroblasts and myofibroblasts into air spaces and production of extracellular matrix by these cells. In UIP, the newly formed intraluminal connective tissue lesions cause fusion of alveolar structures and interstitial remodeling, while in BOOP the newly formed connective tissue may resolve completely.

One of the major aims of the research on pulmonary fibrosis is to define the mechanisms that lead to persistence of the newly formed connective tissue and thus to irreversible fibrosis in UIP. The aim of the present study was to compare the extent of capillarization, apoptotic activity and re-epithelialization of the newly formed connective tissue in BOOP and UIP. The number of capillaries per tissue surface area was measured. Furthermore, the expression of angiogenic growth factors vascular endothelial growth factor-A (VEGF-A) and basic fibroblast growth factor (bFGF) was evaluated in the same areas, in addition to the expression of Flt-1 and Flk-1, which serve as receptors for VEGF. Apoptotic activity was analyzed using TUNEL-method, and the immunohistochemical expression of apoptosis regulating proteins bcl-2, mcl-1, and bax was studied. Finally, the extent of re-epithelialization was studied with the immunohistochemical and ultrastructural localization of laminin-5 γ2 chain, and the sites of synthesis of laminin-5 γ2 chain mRNA.

In BOOP, an efficient repair process with good capillarization along with high expression of VEGF and bFGF, and orderly re-epithelialization of the newly formed connective tissue takes place after lung injury. The apoptotic activity of the newly formed connective tissue is also high, presumably leading to resolution of the intraluminal connective tissue in BOOP. In UIP, the newly formed connective tissue showed poor capillarization, inadequate re-epithelialization and low apoptotic activity. The results suggest disturbed or delayed repair process in UIP, contributing to irreversible interstitial fibrosis and remodeling.