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.

The Lupus Anticoagulant Paradox.

Lupus anticoagulant (LA) represents the most enigmatic antibody population in patients with antiphospholipid syndrome and represents a paradox that is still unsolved. This class of antiphospholipid antibody causes a phospholipid-dependent prolongation of the clotting time but is associated with an increased risk of thrombosis and pregnancy morbidity. In this review, we will provide an overview of the different antibodies that have been associated with LA activity, their importance based on clinical studies, and address the question why this prolongation of the clotting time is associated with thrombosis rather than a bleeding tendency.

Networks of enzymatically oxidized membrane lipids support calcium-dependent coagulation factor binding to maintain hemostasis.

Blood coagulation functions as part of the innate immune system by preventing bacterial invasion, and it is critical to stopping blood loss (hemostasis). Coagulation involves the external membrane surface of activated platelets and leukocytes. Using lipidomic, genetic, biochemical, and mathematical modeling approaches, we found that enzymatically oxidized phospholipids (eoxPLs) generated by the activity of leukocyte or platelet lipoxygenases (LOXs) were required for normal hemostasis and promoted coagulation factor activities in a Ca2+- and phosphatidylserine (PS)-dependent manner. In wild-type mice, hydroxyeicosatetraenoic acid-phospholipids (HETE-PLs) enhanced coagulation and restored normal hemostasis in clotting-deficient animals genetically lacking p12-LOX or 12/15-LOX activity. Murine platelets generated 22 eoxPL species, all of which were missing in the absence of p12-LOX. Humans with the thrombotic disorder antiphospholipid syndrome (APS) had statistically significantly increased HETE-PLs in platelets and leukocytes, as well as greater HETE-PL immunoreactivity, than healthy controls. HETE-PLs enhanced membrane binding of the serum protein ß2GP1 (ß2-glycoprotein 1), an event considered central to the autoimmune reactivity responsible for APS symptoms. Correlation network analysis of 47 platelet eoxPL species in platelets from APS and control subjects identified their enzymatic origin and revealed a complex network of regulation, with the abundance of 31 p12-LOX-derived eoxPL molecules substantially increased in APS. In summary, circulating blood cells generate networks of eoxPL molecules, including HETE-PLs, which change membrane properties to enhance blood coagulation and contribute to the excessive clotting and immunoreactivity of patients with APS.

Antiphospholipid Antibodies and the Risk of Stroke in Urban and Rural Tanzania: A Community-Based Case-Control Study.

BACKGROUND AND PURPOSE: The burden of stroke is high in sub-Saharan Africa, and improved knowledge of risk factors is needed. Antiphospholipid antibodies are a common acquired stroke risk factor in young individuals. Antiphospholipid antibodies may be induced by infectious diseases. Sub-Saharan Africa has a high infectious burden, and we analyzed the contribution of antiphospholipid antibodies to the risk of stroke in an incident population from rural and urban Tanzania. METHODS: Stroke cases and age- and sex-matched community-acquired controls from the rural Hai district and urban Dar-es-Salaam areas of Tanzania were recruited in a wider study of stroke incidence between June 2003 and June 2006. Lupus anticoagulant, anticardiolipin, anti-ß2-glycoprotein I, and antiphosphatidylserine/prothrombin antibodies were determined in stored plasma, as well as IgG antibodies against Treponema pallidum. RESULTS: Data from 158 stroke cases and 369 controls were analyzed. Thirty cases (19%) and 4 controls (1%) had a lupus anticoagulant (odds ratio, 20.8; 95% confidence interval, 7.2-60.5). Anticardiolipin IgG was the only other antiphospholipid antibody subtype associated with increased stroke risk (odds ratio, 2.1; 95% confidence interval, 1.0-4.3), but this association disappeared when corrected for IgG antibodies against Treponema pallidum results. The prevalence of anti-ß2-glycoprotein I IgG antibodies in the Tanzanian healthy population was high when Dutch cutoff values were applied (67%), whereas presence of anti-ß2-glycoprotein I IgM was associated with a reduced stroke risk (odds ratio 0.3; 95% confidence interval, 0.1-1.1). CONCLUSIONS: The presence of lupus anticoagulant is a strong, and to date unrecognized, risk factor for stroke in Tanzania, especially in young and middle-aged individuals.