Anti‐protein C antibodies are associated with resistance to endogenous protein C activation and a severe thrombotic phenotype in antiphospholipid syndrome

Summary Background Antiphospholipid antibodies may interfere with the anticoagulant activity of activated protein C (APC) to induce acquired APC resistance (APCr). Aims To investigate the frequency and characteristics of APCr by using recombinant human APC (rhAPC) and endogenous protein C activation in antiphospholipid syndrome (APS). Methods APCr was assessed in APS and non‐APS venous thromboembolism (VTE) patients on warfarin and normal controls with rhAPC or Protac by thrombin generation. IgG anti‐protein C and anti‐protein S antibodies and avidity were assessed by ELISA. Results APS patients showed greater resistance to both rhAPC and Protac than non‐APS patients and normal controls (median normalized endogenous thrombin potential inhibition): APS patients with rhAPC, 81.3% (95% confidence interval [CI] 75.2–88.3%; non‐APS patients with rhAPC, 97.7% (95% CI 93.6–101.8%; APS patients with Protac, 66.0% (95% CI 59.5–72.6%); and non‐APS patients with Protac, 80.7 (95% CI 74.2–87.2%). APS patients also had a higher frequency and higher levels of anti‐protein C antibodies, with 60% (15/25) high‐avidity antibodies. High‐avidity anti‐protein C antibodies were associated with greater APCr and with a severe thrombotic phenotype (defined as the development of recurrent VTE while patients were receiving therapeutic anticoagulation or both venous and arterial thrombosis). Twelve of 15 (80%) patients with high‐avidity anti‐protein C antibodies were classified as APS category I. Conclusion Thrombotic APS patients showed greater APCr to both rhAPC and activation of endogenous protein C by Protac. High‐avidity anti‐protein C antibodies, associated with greater APCr, may provide a marker for a severe thrombotic phenotype in APS. However, in patients with category I APS, it remains to be established whether anti‐protein C or anti‐β2‐glycoprotein I antibodies are responsible for APCr.