Serum and urine free light chains measurements in patients with systemic sclerosis: novel biomarkers for disease activity

Circulating free light chains (FLCs), considered biomarkers of B cell activity, are frequently elevated in patients affected by systemic inflammatory autoimmune diseases. As the systemic sclerosis (SSc) clinical course can be variable, this study is aimed at evaluating FLCs levels in affected individuals as biomarkers of disease activity. We assessed FLC levels in serum and urine of 72 SSc patients and 30 healthy controls (HC). Results were analyzed in comparison with overall clinical and laboratory findings, disease activity index (DAI) and disease severity scale (DSS). SSc patients displayed increased levels of kappa and lambda FLC in serum significantly higher than HC (p = 0.0001) alongside the mean values of free kappa/lambda ratio and kappa + lambda sum (p = 0.0001). SSc patients showed increased free kappa in urine with a kappa/lambda higher than HC (p = 0.0001). SSc patients with increased kappa + lambda in serum showed that erythro-sedimentation rate (p = 0.034), C-reactive protein (p = 0.003), DAI (p = 0.024) and DSS (p = 0.015) were higher if compared to SSc patients with normal levels of FLC. A positive linear correlation was found between serum levels of free kappa and DAI (r = 0.29, p = 0.014). In addition, SSc patients with increased free kappa in urine had higher DAI (p = 0.048) than SSc patients with normal kappa levels. Our results strengthen the role of serum FLC as useful biomarker in clinical practice to early diagnosis and monitor disease activity, showing for the first time that also urine FLC levels correlated with disease activity in SSc patients.