LAUSR

The antiphospholipid syndrome (APS) is characterized by thrombosis and/or pregnancy morbidity in the presence of antiphospholipid antibodies including anti-beta-2-glycoprotein-I (anti-β2GPI) that are considered central to APS pathogenesis. Based on animal studies showing a role of complement in APS-related clinical events, we used the modified Ham assay (complement-dependent cell killing) and cell-surface deposition of C5b-9 to test the hypothesis that complement activation is associated with thrombotic events in APS. A positive modified Ham (mHam) (and corresponding C5b-9 deposition) were present in 85.7% of CAPS, 35.6% of APS (68.5% within one year of thrombosis), and only 6.8% of SLE sera. A positive mHam assay was associated with triple-positivity (positive for lupus anticoagulant, anticardiolipin and anti-β2GPI antibodies) and recurrent thrombosis. Patient-derived anti-β2GPI antibodies also induced C5b-9 deposition, which was blocked completely by an anti-C5 monoclonal antibody, but not by a factor D inhibitor indicating that complement activation by anti-β2GPI antibodies occurs primarily through the classical pathway of complement. Finally, patients with catastrophic APS have high rates of rare germline variants in complement regulatory genes (60%) versus patients with APS (21.8%), SLE (28.6%), or normal controls (23.3%), and similar to the rate of mutations in aHUS (51.5%). Taken together, our data suggest that anti-β2GPI antibodies activate complement and contribute to thrombosis in APS, while patients with catastrophic APS have underlying mutations in complement regulatory genes that serve as a 'second-hit' leading to uncontrolled complement activation and a more severe thrombotic phenotype.