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Background: Rheumatoid arthritis (RA) is characterized by chronic synovitis and articular cartilage destruction. Increased activities of cathepsin S and cathepsin L, two potent cysteine proteases, are thought to play a role in the pathogenesis of the irreversible articular cartilage destruction. Nevertheless, data regarding the potential importance of the cathepsins as circulating biomarkers in RA patients are limited.

Method: Subjects enrolled in this study are part of a larger study where patients from the three northern counties of Sweden diagnosed with early RA are followed in an ongoing prospective study. In total, 71 patients were included, along with 44 age- and sex-matched control subjects. Plasma levels of cathepsin S and L were analysed. Disease severity was assessed using the 28-joint count Disease Activity Score (DAS28).

Results: Plasma levels of cathepsin S and L were significantly increased in patients with RA compared to healthy controls (p < 0.05 for both). However, in the patients with RA, no association between the cathepsins and the severity of the disease, as characterized by DAS28, was observed (p > 0.51).

Conclusions: Although circulating levels of cathepsin S and L were significantly increased in patients with recently diagnosed RA, our data do not support the notion that circulating levels of cathepsins are relevant biomarkers for disease severity.