Anti-C1q antibody is a valuable biological marker for prediction of renal pathological characteristics in lupus nephritis.

The aim of this study is to investigate the association of serum anti-C1q antibody levels with renal pathological characteristics in SLE patients with lupus nephritis. Fifty-two patients with biopsy-proven lupus nephritis were enrolled. Serum anti-C1q levels (units per milliliter) were measured before renal biopsy using ELISA kit. A cross-sectional study analyzed the association of anti-C1q antibody levels with SLE global activity, nephritic activity, and renal histopathology. Thirty-nine of 52 patients (75 %) were positive for anti-C1q antibody. Anti-C1q antibody levels were positively correlated with the values for the SLE Disease Activity Index (r = 0.628, p < 0.001), anti-nucleosome (r = 0.591, p < 0.001), and anti-dsDNA antibody (r = 0.507, p < 0.001), but negatively correlated to serum C3 (r = -0.626, p < 0.001) and C4 (r = -0.57, p < 0.001). The prevalence of anti-C1q in patients with proliferative LN (class III + class IV) was higher than those with mesangial LN (class II), but there is no statistical significance. Among different ISN/RPS classification, serum anti-C1q antibody level was highest in patients with class IV, followed by class III. The concentration of anti-C1q in patients with class IV was higher than those in class II [50.00 (23.00, 97.70) vs 20.25 (7.16, 54.78) U/ml, p<0.05]. The levels of anti-C1q antibody were positively correlated with renal active indices (r = 0.59, p < 0.001) while negatively correlated with chronic indices (r = -0.326, p < 0.05). Moreover, anti-C1q antibody was found to be positively associated with glomerular C1q deposition. These findings indicate that serum anti-C1q antibody is a valuable noninvasive biological marker for prediction of renal histopathology in lupus nephritis. Low or negative anti-C1q antibody titers might influence therapeutic decisions in SLE.