Antiphospholipid Antibodies Are Associated with Low Levels of Complement C3 and C4 in Patients with Systemic Lupus Erythematosus

Complement activation and low complement levels are common in systemic lupus erythematosus (SLE). Antiphospholipid antibodies (aPL) are found in about 30-40% of patients with SLE. This study aimed to investigate the association between aPL and complement levels in patients with SLE. Serum samples were collected from 269 patients with SLE enrolled in the Norwegian Systemic Connective Tissue and Vasculitis Registry (NOSVAR) during 2003-2009, and from 353 controls. All samples were analysed for anti-β2 glycoprotein 1 (anti-β2GP1) and anticardiolipin antibodies (aCL), C-reactive protein (CRP) and complement components C3 and C4. Median CRP level was significantly higher in cases than in controls (2.06 versus 0.90 mg/l; P < 0.0001). No significant difference in CRP was found between SLE patients with or without aPL (2.09 versus 1.89; P = 0.665). Median C3 levels were similar in cases (1.03 g/l) and controls (1.00 g/l), whereas median C4 levels were 0.16 g/l in cases versus. 0.19 in controls (P < 0.0001). However, aPL-positive SLE patients had significantly lower median C3 levels (0.92 versus. 1.07 g/l; P = 0.001) and C4 levels (0.11 versus 0.16 g/l; P < 0.0001) compared to aPL-negative patients. Lower C3 and C4 values in aPL-positive SLE patients may reflect a higher consumption of C3 and C4 due to more pronounced complement activation in aPL-positive SLE patients compared to SLE patients without aPL.