Apoptotic activity is increased in the newly formed fibromyxoid connectivetissue in bronchiolitis obliterans organizing pneumonia

Bronchiolitis obliterans organizing pneumonia (BOOP) and usual interstitial pneumonia (UIP) are clinically and histologically distinguishable intersti tial lung diseases both sharing the presence of the newly formed fibromyxoi d connective tissue as a histologic feature. In BOOP the newly formed conne ctive tissue is susceptible to even complete reversal, but in UIP it partic ipates in the remodeling of the pulmonary tissue. In this study we have tes ted the hypothesis that the apoptotic activity in the fibromyxoid lesions i s higher in BOOP than in UTP. Apoptotic activity in cells of the newly form ed connective tissue in BOOP and UIP was visualized by TUNEL. Apoptosis-reg ulating proteins bcl-2, mcl-1, and bar were visualized by immunohistochemis try. The number of the apoptotic events was given as an apoptotic index giv ing the percentage of the apoptotic cells and bodies of the total cell numb er. Our results show that the apoptotic activity is clearly higher in the f ibromyxoid lesions of BOOP (mean, 0.7; S.D., +/-0.51) compared with those o f UIP (mean, 0.13; S.D,, +/-0.14, p < 0.003). The results thus suggest that apoptosis has an important role in the resolution process of the newly for med connective tissue in BOOP. The low level of apoptosis in UIP could, on the other hand, suggest that a decreased apoptosis in cells of the fibromyx oid stroma might pathogenetically relate to a decreased resolution of these lesions in UIP, No significant difference was found in the expression of b cl-2, mcl-1, and bar in BOOP and in UIP, suggesting that regulation of apop tosis might partly bypass the influence of these proteins.