The role of osteopontin as a candidate biomarker of renal involvement in systemic lupus erythematosus.

Kidney biopsy is the gold standard for the diagnosis of lupus nephritis (LN). Conventional biomarkers of disease activity or renal function, such as complement levels, anti-dsDNA, serum creatinine, urinary sediment and proteinuria, do not have a sensitive diagnostic and prognostic value, therefore new biomarkers are needed to help predict or monitor LN. Osteopontin (OPN) is a pro-inflammatory molecule detectable in serum and renal tissue. The aim of this study was to evaluate OPN as a biomarker of renal involvement in patients with systemic lupus erythematosus (SLE) and correlate its levels with disease activity and laboratory features. OPN was measured in the serum and urine of SLE patients with active LN (n=14), LN in remission (n=20), SLE without kidney involvement (n=22) and age- and sex-matched healthy controls (HC, n=20). OPN levels were significantly higher in urine than in serum in both groups of patients and controls (p<0.001). Serum OPN levels were higher in the LN patients than in HC and in SLE patients without renal involvement (p<0.0001 and 0.0032, respectively), regardless of the phase of renal activity. SLE patients without renal involvement and controls showed similar serum levels. We detected a direct correlation between low complement levels and OPN serum levels in patients with LN (p=0.014; R=0.438). Moreover, a higher percentage of patients with LN, compared to SLE without LN and HC, showed abnormal serum OPN. Our data suggest that serum OPN could be considered a biomarker of renal involvement, without differentiating between active and remission LN.