Antiphospholipid antibody carriers and patients with quiescent antiphospholipid syndrome show persistent subclinical complement activation.

OBJECTIVES: Complement activation has been advocated as one mechanism by which antiphospholipid antibodies (aPLs) can induce thrombosis. In patients with catastrophic aPL syndrome or re-thrombosis, enhanced complement activation was shown, even in quiescent phase of the disease. We aimed to assess complement activation and to investigate its association to clinical variables in aPL positive patients with a favorable disease course. METHODS: Subjects with at least two consecutive positive aPL antibody results obtained ≥12 weeks apart were enrolled. They were subjects without history of thrombosis or pregnancy morbidity (aPL carriers), patients with pregnancy morbidity alone (OAPS), and/or with arterial, venous, or small-vessel thrombosis (TAPS); all patients should have been free of symptoms for ≥2 years. Patients affected with systemic autoimmune diseases were excluded. Healthy age and sex-matched subjects were included as controls. Plasma C5a and C5b-9 levels were assessed by commercially available ELISA assays. Non-parametric Mann-Whitney test and Spearman's correlation were applied. RESULTS: Thirty-seven OAPS, 38 TAPS, 42 aPL carriers, and 30 healthy subjects were enrolled. Median C5a and C5b-9 levels were significantly higher in quiescent aPL positive patients (OAPS, TAPS, aPL carriers) compared with controls: C5a ng/ml 10.61 (IQR 6.87-15.46) vs 4.06 (2.66-7.35), p< 0.001; C5b-9 ng/ml 283.95 (175.8-439.40) vs 165.90 (124.23-236.8), p< 0.001. Similar C5a and C5b-9 levels were observed in OAPS and TAPS patients and aPL carriers. A positive correlation between C5b-9 median levels and the number of aPL positive tests was found (p= 0.002). CONCLUSIONS: The persistence of aPL antibodies is associated to a persistent subclinical activation of the complement cascade.