Anti‐C1q antibodies are associated with systemic lupus erythematosus global activity but not specifically with nephritis: A controlled study of 126 consecutive patients

OBJECTIVE: Several studies have shown that anti-C1q antibodies correlate with the occurrence and activity of nephritis in systemic lupus erythematosus (SLE). However, the significance of anti-C1q antibodies in SLE has not been fully characterized. The aim of this study was to investigate associations between anti-C1q antibodies and clinical and serologic parameters of SLE. METHODS: An enzyme-linked immunosorbent assay kit was used to measure anti-C1q antibodies in the sera of 126 consecutive patients with active SLE who were admitted to our university hospital from 2007 through 2009. Sera obtained from patients with high titers of anti-C1q antibodies at the initial evaluation (n = 20) were reevaluated following treatment. Control sera were obtained from patients with other autoimmune diseases and from normal healthy control subjects (n = 20 in each group). Associations between anti-C1q antibodies and clinical and serologic parameters of SLE were statistically analyzed. RESULTS: Anti-C1q antibodies were detected in the sera of 79 of 126 patients with SLE. The prevalence and titers of anti-C1q antibodies were significantly (P < 0.0001) higher in SLE patients than in patients with rheumatoid arthritis, patients with systemic sclerosis, and normal healthy control subjects. The prevalence and titers of anti-C1q antibodies were not significantly associated with active lupus nephritis (P = 0.462 and P = 0.366, respectively). Anti-C1q antibody titers were significantly correlated with SLE Disease Activity Index 2000 scores and the levels of anti-double-stranded DNA antibodies, C3, C4, CH50, and C1q (P < 0.0001 for all comparisons). Moreover, anti-C1q antibody titers significantly decreased as clinical disease was ameliorated following treatment (P = 0.00097). CONCLUSION: These findings indicate that anti-C1q antibodies are associated with SLE global activity but not specifically with active lupus nephritis.