Cathepsin B, Airway Pathogens, and Inflammation in the Lower Airways of Children With Cystic Fibrosis.

Dysregulated protease activity contributes to airway inflammation and tissue remodeling in cystic fibrosis (CF); however, the role of the lysosomal cysteine protease Cathepsin B (CTSB) remains incompletely defined. This cross-sectional study investigates relationships between pro-CTSB and mature CTSB activity with CF-specific pathogens and airway inflammation in children with and without CF. Bronchoalveolar lavage fluid (BALF) was collected from clinically indicated bronchoscopies in children (N = 52 CF, N = 161 non-CF). CTSB was interrogated using ELISA, fluorogenic activity assay, and Western blot analysis to distinguish pro- and mature CTSB. Total bacterial and total fungal load (TFL) were quantified by quantitative polymerase chain reaction, and community composition was determined by 16S bacterial and 18S fungal sequencing. Concentrations of proinflammatory cytokines and neutrophil elastase (NE) were measured via Luminex multiplatform and a spectrophotometric assay, respectively. Analyses included Spearman's rank correlations and Wilcoxon rank-based tests. Pro-CTSB and CTSB activity were significantly (p < 0.01) elevated in CF BALF and in samples with a positive Staphylococcus aureus airway culture. Pro-CTSB concentrations correlated with staphylococcal relative abundance (RA, ρ = 0.25, p < 0.02) and reduced bacterial diversity ( ρ  = -0.41, p < 0.01). Mature CTSB activity correlated with TFL ( ρ  = 0.50, p < 0.05) and Aspergillus spp. RA ( ρ  = 0.36, p < 0.04). Western blot analysis confirmed pro-CTSB expression and mature CTSB in BALF with measurable activity. Both CTSB measures correlated strongly with NE and proinflammatory cytokines ( ρ  ≥ 0.47, p < 0.001). Pro-CTSB concentrations negatively correlated with FEV1/FVC measurements in CF ( ρ  = -0.32, p = 0.05). BALF CTSB concentration may serve as a CF-specific biomarker of infection-related inflammation and obstructive lung disease driven by specific pathogen interactions.