Anti-chromatin and anti-C1q antibodies in systemic lupus erythematosus compared to other systemic autoimmune diseases

Objective: To evaluate the prevalence, sensitivity, and specificity of anti-chromatin and anti-C1q antibodies in systemic lupus erythematosus (SLE) and lupus nephritis compared to small vessel vasculitis and other connective tissue diseases. To provide long-term follow-up data for anti-chromatin antibodies in lupus nephritis. Methods: We determined the significance of anti-nuclear antibodies (ANA), anti-double-stranded DNA (anti-dsDNA), anti-chromatin, and anti-C1q antibodies, as well as complement factors C3 and C4, in relation to disease activity in SLE patients with (n=47; long- term follow- up data for 33 patients) and without (n=31) biopsy- confirmed lupus nephritis, microscopic polyangiitis (n=37), Wegener's granulomatosis (n=66), primary Sjogren's syndrome (n=17), limited scleroderma (CREST syndrome) (n=6), and progressive systemic scleroderma ( PSS) ( n=11). Results: Anti-chromatin antibodies were more specific and sensitive than anti-C1q antibodies in distinguishing SLE patients from those with other systemic autoimmune diseases [anti-chromatin: sensitivity 64.1%, specificity 99.2%, odds ratio (OR) 219.6; anti-C1q: sensitivity 50%, specificity 72.6%, OR 2.65]. Anti-C1q antibodies were present in 75% of patients with Sjogren's syndrome and 35.1% of patients with microscopic polyangiitis. Antichromatin antibodies could identify SLE in patients with positive ANA but negative anti-dsDNA antibodies. Persisting anti-chromatin antibodies indicated SLE disease activity, even if anti-dsDNA antibodies had become negative. In long-term follow-up, those SLE patients with negative anti- dsDNA antibodies but persisting ANA and anti- chromatin antibodies relapsed if immunosuppression had been tapered. Anti-chromatin antibodies correlated with the SLE disease activity index (SLEDAI) as a marker of disease activity. Conclusions: The measurement of anti-chromatin, but not anti-C1q, antibodies in patients with systemic autoimmune diseases increases diagnostic sensitivity and specificity for SLE and assists in treatment decisions in anti- dsDNA-negative patients.