Plasma Cathepsin S and Cathepsin S/Cystatin C Ratios Are Potential Biomarkers for COPD

Purpose. This study aimed to examine whether plasma levels of cathepsin S or its inhibitor, cystatin C, may serve as biomarkers for COPD. Patients and Methods. We measured anthropometrics and performed pulmonary function tests and chest CT scans on 94 patients with COPD and 31 subjects with productive cough but no airflow obstruction ("at risk"; AR). In these subjects and in 52 healthy nonsmokers (NS) and 66 healthy smokers (HS) we measured plasma concentrations of cathepsin S and cystatin C using an ELISA. Data were analyzed using simple and logistic regression and receiver operating characteristic analyses. Results. Cathepsin S and cystatin C plasma levels were significantly higher in the COPD and AR groups than in the NS and HS groups (p < 0.01). Among the COPD patients and AR subjects, plasma cathepsin S levels and cathepsin S/cystatin C ratios, but not cystatin C levels, were negatively related to severe airflow limitation (% FEV₁ predicted < 50%; p = 0.005) and severe emphysema as assessed by low attenuation area (LAA) score on chest CT scans (LAA ≥ 8.0; p = 0.001). Conclusion. Plasma cathepsin S and cathepsin S/cystatin C ratios may serve as potential biomarkers for COPD.