Anti-C1q antibodies and IgG subclass distribution in sera from Chinese patients with lupus nephritis

Objective. Anti-C1q antibodies are common in sera from patients with lupus nephritis (LN) and are associated with disease activity. The current study aimed to further investigate the prevalence of serum IgG anti-C1q antibody, its subclass distribution and their clinical and pathological association in patients with LN.

Sera were collected from 150 patients with renal biopsy-proven LN, diagnosed from 2000 to 2006 in our hospital, 30 patients with systemic lupus erythematosus (SLE) without clinical evidence of renal involvement (non-renal SLE, NR-SLE) and 63 healthy donors. ELISA was used to detect serum IgG anti-C1q antibody and its subclass. Their clinical and pathological associations were further analysed.

The prevalence of IgG anti-C1q antibody in LN (84/150, 56%) was significantly higher than that in NR-SLE (6/30, 20%) and healthy controls (3/63, 4.8%) (P < 0.005, P < 0.001, respectively). The prevalence of anti-C1q antibody in patients with diffuse proliferative renal lesions (class IV) (59/82, 71.95%) was significantly higher than that in those with non-diffuse proliferative renal lesions (class II + III) (12/26, 46.15%, P = 0.016) and class V (13/42, 30.95%, P < 0.001). The prevalence of IgG2 (60/135, 44.44%) was significantly higher than that of IgG1 (37/135, 27.41%) and IgG3 (25/135, 18.52%) (P < 0.005, P < 0.001, respectively). IgG2 was associated with the occurrence of arthritis (P < 0.05), higher serum creatinine (P < 0.05) and lower serum C3 (P < 0.05). Of the 38 LN patients with sera both in active phase and in remission, 17 were anti-C1q antibody-positive in active phase and the antibody levels decreased in all and turned to negative in 9 (52.94%) in remission. Meanwhile, the ratio of turning negative of IgG1, IgG2 and IgG3 anti-C1q was 33%(2/6), 53.85% (7/13) and 100% (7/7), respectively.

Anti-C1q antibodies are prevalent in LN and are closely associated with diffuse proliferative lesions. IgG2 anti-C1q might be pathogenic and IgG3 anti-C1q might be a more specific biomarker for monitoring disease activity.