Imbalance between cysteine proteases and inhibitors in a baboon model of bronchopulmonary dysplasia

Rationale: Bronchopulmonary dysplasia (BPD) continues to be a major morbidity in preterm infants. The lung pathology in BIRD is characterized by impaired alveolar and capillary development. An imbalance between proteases and protease inhibitors in association with changes in lung elastic fibers has been implicated in the pathogenesis of BIRD. Objective: To investigate the expression and activity levels of papain-like lysosomal cysteine proteases, cathepsins B, H, K, L, S, and their inhibitors, cystatins B and C, in a baboon model of BPD. Methods: Real-time reverse transcriptase-polymerase chain reaction, immunohistochemistry, immunoblotting, active site labeling of cysteine proteases, and in situ hybridization were performed. Measurements and Main Results: The steady-state mRNA and protein levels of all cathepsins were significantly increased in the lung tissue of baboons with BPD. In contrast, the steady-state mRNA and protein levels of two major cysteine protease inhibitors, cystatin B and C, were unchanged. Correlating with these alterations, the activity of cysteine proteases in lung tissue homogenates and bronchoalveolar lavage fluid was significantly higher in the BPD group. The levels of cathepsin B, H, and S increased and cathepsin K decreased with advancing gestation. All cathepsins, except for cat K, were immunolocalized to macrophages in BIRD. In addition, cathepsin H and cystatin B were colocalized in type 2 alveolar epithelial cells. Cathepsin L was detected in some bronchial epithelial, endothelial, and interstitial cells. Cathepsin K was localized to some perivascular cells by in situ hybridization. Conclusions: Cumulatively, these findings demonstrate an imbalance between cysteine proteases and their inhibitors in BIRD.