Anti-ß2-glycoprotein I and anti-phosphatidylserine/prothrombin antibodies interfere with cleavage of factor V(a) by activated protein C.

BACKGROUND: The acquired thrombotic risk factor known as lupus anticoagulant (LA) interferes with laboratory clotting assays and can be caused by autoantibodies against ß2-glycoprotein I (ß2GPI) and prothrombin. LA is associated with activated protein C (APC) resistance, which might contribute to thrombotic risk in patients with antiphospholipid syndrome. How antibodies against ß2GPI and prothrombin cause APC resistance is currently unclear. OBJECTIVES: To investigate how anti-ß2GPI and antiphosphatidylserine/prothrombin (PS/PT) antibodies induce APC resistance. METHODS: The effects of anti-ß2GPI and anti-PS/PT antibodies on APC resistance were studied in plasma (of patients with antiphospholipid syndrome) and with purified coagulation factors and antibodies. RESULTS: APC resistance was observed in LA-positive patients with anti-ß2GPI or anti-PS/PT antibodies and in normal plasma spiked with monoclonal anti-ß2GPI or anti-PS/PT antibodies with LA activity. Analysis of factor (F)V cleavage patterns after APC incubation indicated that anti-ß2GPI antibodies attenuated APC-mediated FV cleavage at R506 and R306. APC-mediated cleavage at R506 is required for FV cofactor activity during inactivation of FVIIIa. Assays with purified coagulation factors confirmed that anti-ß2GPI antibodies interfered with the cofactor function of FV during FVIIIa inactivation but not with FVa inactivation. Anti-PS/PT antibodies attenuated APC-mediated FVa and FVIIIa inactivation. Analysis of FV(a) cleavage patterns after APC incubation indicated that anti-PS/PT antibodies interfere with APC-mediated cleavage of FV at positions R506 and R306. CONCLUSION: Anti-ß2GPI antibodies with LA activity contribute to a procoagulant state by causing APC resistance via interference with the cofactor function of FV during FVIIIa inactivation. LA-causing anti-PS/PT antibodies interfere with the anticoagulant function of APC by preventing FV(a) cleavage.

Interference in point-of-care international normalized ratio monitoring in patients with lupus anticoagulant is correlated with anti-ß2-glycoprotein I antibody titers.

Background: Patients with antiphospholipid syndrome (APS) receive anticoagulant therapy with vitamin K antagonists (VKAs) to prevent recurrent thrombosis. VKA treatment requires strict monitoring with an international normalized ratio (INR). It is known that lupus anticoagulants (LAs) can lead to elevated INR results with point-of-care-testing (POCT) devices, which could result in inadequate adaptation of anticoagulant therapy. Objective: To determine discrepancies between POCT-INR and laboratory-INR in patients who are LA-positive on VKA therapy. Methods: Paired INR testing was performed with 1 POCT device (CoaguChek XS) and 2 laboratory assays (Owren and Quick method) in 33 patients with LA-positive APS on VKA in a single-center cross-sectional study. Patients were tested for anti-ß2-glycoprotein I, anticardiolipin, and antiphosphatidylserine/prothrombin immunoglobulin (Ig) G and IgM antibodies. Agreement between assays was evaluated with Spearman's correlation, Lin's correlation coefficient, and Bland-Altman plots. Agreement limits were considered satisfactory if differences were ≤20% as determined by the Clinical and Laboratory Standards Institute. Results: We found poor agreement between POCT-INR and laboratory-INR based on Lin's concordance correlation coefficient (ρc) of 0.42 (95% CI, 0.26-0.55) between POCT-INR and Owren-INR, a ρc of 0.64 (95% CI, 0.47-0.76) between POCT-INR and Quick-INR, and a ρc of 0.77 (95% CI, 0.64-0.85) between Quick-INR and Owren-INR. High anti-ß2-glycoprotein I IgG antibody titers correlated with INR disagreement between POCT-INR and laboratory-INR. Conclusion: There is a disagreement between INR values measured with the CoaguChek XS and laboratory-INR in a proportion of patients with LA. Consequently, laboratory-INR monitoring should be preferred over POCT-INR monitoring in patients with LA-positive APS, especially in patients with high anti-ß2-glycoprotein IgG antibody titers.

Lupus anticoagulant associates with thrombosis in patients with COVID-19 admitted to intensive care units: A retrospective cohort study.

Background: Thrombosis is a frequent and severe complication in patients with coronavirus disease 2019 (COVID-19) admitted to the intensive care unit (ICU). Lupus anticoagulant (LA) is a strong acquired risk factor for thrombosis in various diseases and is frequently observed in patients with COVID-19. Whether LA is associated with thrombosis in patients with severe COVID-19 is currently unclear. Objective: To investigate if LA is associated with thrombosis in critically ill patients with COVID-19. Patients/Methods: The presence of LA and other antiphospholipid antibodies was assessed in patients with COVID-19 admitted to the ICU. LA was determined with dilute Russell's viper venom time (dRVVT) and LA-sensitive activated partial thromboplastin time (aPTT) reagents. Results: Of 169 patients with COVID-19, 116 (69%) tested positive for at least one antiphospholipid antibody upon admission to the ICU. Forty (24%) patients tested positive for LA; of whom 29 (17%) tested positive with a dRVVT, 19 (11%) tested positive with an LA-sensitive aPTT, and 8 (5%) tested positive on both tests. Fifty-eight (34%) patients developed thrombosis after ICU admission. The odds ratio (OR) for thrombosis in patients with LA based on a dRVVT was 2.5 (95% confidence interval [CI], 1.1-5.7), which increased to 4.5 (95% CI, 1.4-14.3) in patients at or below the median age in this study (64 years). LA positivity based on a dRVVT or LA-sensitive aPTT was only associated with thrombosis in patients aged less than 65 years (OR, 3.8; 95% CI, 1.3-11.4) and disappeared after adjustment for C-reactive protein. Conclusion: Lupus anticoagulant on admission is strongly associated with thrombosis in critically ill patients with COVID-19, especially in patients aged less than 65 years.

Anti-ß2-glycoprotein I and anti-prothrombin antibodies cause lupus anticoagulant through different mechanisms of action.

BACKGROUND: The presence of lupus anticoagulant (LA) is an independent risk factor for thrombosis. This laboratory phenomenon is detected as a phospholipid-dependent prolongation of the clotting time and is caused by autoantibodies against beta2-glycoprotein I (ß2GPI) or prothrombin. How these autoantibodies cause LA is unclear. OBJECTIVE: To elucidate how anti-ß2GPI and anti-prothrombin antibodies cause the LA phenomenon. METHODS: The effects of monoclonal anti-ß2GPI and anti-prothrombin antibodies on coagulation were analyzed in plasma and with purified coagulation factors. RESULTS: Detection of LA caused by anti-ß2GPI or anti-prothrombin antibodies required the presence of the procofactor factor V (FV) in plasma. LA effect disappeared when FV was replaced by activated FV (FVa), both in a model system and in patient plasma, although differences between anti-ß2GPI and anti-prothrombin antibodies were observed. Further exploration of the effects of the antibodies on coagulation showed that the anti-ß2GPI antibody attenuated FV activation by activated faxtor X (FXa), whereas the anti-prothrombin antibody did not. Binding studies showed that ß2GPI--antibody complexes directly interacted with FV with high affinity. Anti-prothrombin complexes caused the LA phenomenon through competition for phospholipid binding sites with coagulation factors as reduced FXa binding to lipospheres was observed with flow cytometry in the presence of these antibodies. CONCLUSION: Anti-ß2GPI and anti-prothrombin antibodies cause LA through different mechanisms of action: While anti-ß2GPI antibodies interfere with FV activation by FXa through a direct interaction with FV, anti-prothrombin antibodies compete with FXa for phospholipid binding sites. These data provide leads for understanding the paradoxical association between thrombosis and a prolonged clotting time in the antiphospholipid syndrome.