Urinary free light chain is a potential biomarker for ISN/RPS class III/IV lupus nephritis

Objectives. To evaluate the use of urinary free light chains (FLCs) as a biomarker for proliferative LN and the potential association between the intensity of plasma cell infiltration of the kidney and urinary FLC levels in LN. Methods. Forty-three SLE patients were consecutively enrolled in the study. These patients were divided into an International Society of Nephrology and Renal Pathology Society (ISN/RPS) class III/IV LN subset (n = 18) and an ISN/RPS class I/II/V (class non-III/IV) LN subset (n = 25). The expression of kappa-LCs, lambda-LCs, CD19 and CD138 in kidney specimens was also evaluated with immunohistochemical staining. To measure FLC levels before and after treatment, an additional six patients with class III/IV LN were consecutively enrolled. Results. Urinary FLCs were significantly higher in the class III/IV LN subset than in the class non-III/IV LN subset. Urinary lambda-FLC levels were significantly correlated with the urinary protein-creatinine ratio in the class III/IV LN subset (r(s) = 0.67, P < 0.01). Moreover, the LC-secreting CD19(-)/CD138(+) cell counts in the kidney specimens were higher in the class III/IV LN subset than in the class non-III/IV LN subset. Total urinary FLC levels were correlated with the numbers of CD138(+) cells in the kidney (r = 0.71, P = 0.03). Following treatment, urinary lambda-FLCs could not be detected in any of the patients. Conclusion. The intensity of plasma cell infiltration of the kidney is associated with urinary FLC levels. Urinary FLCs are potentially useful biomarkers in ISN/RPS class III/IV LN or proliferative LN.