OBJECTIVE: The release of neutrophil extracellular traps (NETs) by hyperactive neutrophils has recently been recognized to play an important role in antiphospholipid syndrome (APS). Here, we aimed to evaluate autoantibodies targeting NETs in patients with primary APS, and to determine their potential functions and clinical associations. METHOD: We measured global anti-NET activity in 76 patients with primary APS, 23 patients with systemic lupus erythematosus without antiphospholipid antibodies (aPL), 11 patients with history of unprovoked venous thrombosis without aPL, and 44 healthy controls. The ability of APS sera to degrade NETs was also assessed. RESULT: We found markedly elevated levels of anti-NET IgG and IgM in patients with primary APS as compared with healthy controls (IgG mean optical density 0.55 ± 0.34 vs. 0.33 ± 0.17; IgM mean optical density 0.76 ± 0.51 vs. 0.26 ± 0.23). This anti-NET activity did not correlate with levels of traditional aPL and was relatively stable over time. Mechanistically, anti-NET antibodies (especially of the IgG isotype) impaired the ability of patient sera to degrade NETs (r=0.4, p=0.003). Levels of anti-NET IgM inversely correlated with complement C4 (r=0.4, p=0.019). Clinically, anti-NET antibodies associated with certain APS clinical manifestations, and in particular recurrent venous thrombosis (OR=4.3, p=0.002). Interestingly, anti-NET antibodies also appeared to associate with unprovoked venous thrombosis in the general population (IgM mean optical density 0.67 ± 0.34 vs. 0.26 ± 0.23). CONCLUSION: These data reveal high levels of anti-NET antibodies in patients with primary APS, which may impair NET clearance and activate the complement cascade. These findings may ultimately enable more effective risk stratification.
Anti-NET antibodies and impaired NET degradation in antiphospholipid syndrome