Decreased serum cathepsin S levels in patients with systemic sclerosis‐associated interstitial lung disease

Cathepsin S (CTSS) is a lysosomal proteolytic enzyme regulating intracellular and extracellular biological activities, including immunity/inflammation and remodeling of vasculature and extracellular matrix, which are the three cardinal pathological events associated with systemic sclerosis (SSc). To elucidate the potential role of CTSS in the development of SSc, we investigated the clinical correlation of serum CTSS levels. Because serum CTSS levels were inversely correlated with estimated glomerular filtration rate (eGFR) in SSc patients with renal dysfunction (eGFR, <60 min/mL per 1.73 m2 ), SSc patients with normal renal function (eGFR, ≥60 min/mL per 1.73 m2 ) were analyzed. Serum CTSS levels were significantly decreased in diffuse cutaneous SSc patients compared with limited cutaneous SSc patients and healthy controls. Among vascular and fibrotic clinical manifestations, Raynaud's phenomenon and interstitial lung disease (ILD) were relevant to a significant decrease in serum CTSS levels. Importantly, serum CTSS levels negatively correlated with serum levels of Krebs von den Lungen-6 and surfactant protein D in total SSc patients, while not correlating with modified Rodnan total skin thickness score and the percentage of predicted diffusion lung capacity for carbon monoxide and showing a positive trend with the percentage of predicted vital capacity. These results suggest a potential contribution of decreased CTSS expression to the development of ILD in patients with SSc.