β2-glycoprotein I–dependent lupus anticoagulant highly correlates with thrombosis in the antiphospholipid syndrome

Diagnostic biologique du syndrome des antiphospholipides

PURPOSE

The antiphospholipid syndrome is a bioclinical entity defined by the occurrence of thromboses, and/or obstetrical complications in the persistent presence of antiphospholipid antibodies, i.e. lupus anticoagulant and/or anticardiolipin antibodies. This review focuses on the methods for antiphospholipid antibodies detection and their clinical usefulness.

CURRENT KNOWLEDGE AND KEY POINTS

Lupus anticoagulant is the strongest risk factor for thrombosis in antiphospholipid syndrome. Twenty years after its description, anticardiolipin ELISA, despite a still improvable standardization and its lack of specificity, is still required for sensitive diagnosis of antiphospholipid syndrome.

FUTURE PROSPECTS AND PROJECTS

A better knowledge of the beta-2-glycoprotein-I role in the pathophysiology of antiphospholipid syndrome might lead to the development of new markers of thrombotic risk.

International consensus statement on an update of the classification criteria for definite antiphospholipid syndrome (APS)

New clinical, laboratory and experimental insights, since the 1999 publication of the Sapporo preliminary classification criteria for antiphospholipid syndrome (APS), had been addressed at a workshop in Sydney, Australia, before the Eleventh International Congress on antiphospholipid antibodies. In this document, we appraise the existing evidence on clinical and laboratory features of APS addressed during the forum. Based on this, we propose amendments to the Sapporo criteria. We also provide definitions on features of APS that were not included in the updated criteria.

Annexin A5 polymorphism (-1C-->T) and the presence of anti-annexin A5 antibodies in the antiphospholipid syndrome

BACKGROUND

Annexin A5 is thought to have a role in the pathophysiology of the antiphospholipid syndrome (APS)-a syndrome characterised by recurrent thrombosis and pregnancy morbidity.

OBJECTIVE

To investigate whether anti-annexin A5 immunoglobulin (Ig)M or IgG antibodies, or the -1C-->T polymorphism of annexin A5, is a risk factor for thrombosis or miscarriage, and whether the -1C-->T polymorphism is correlated with APS.

METHODS

A cohort study was carried out with a population of 198 patients with primary APS, systemic lupus erythematosus or lupus-like disease. For the detection of anti-annexin A5 antibodies and the measurement of annexin A5 plasma levels, ELISA-type methods were used. The annexin A5 -1C-->T mutation was detected by restriction fragment length polymorphism.

RESULTS

71 patients were positive for annexin A5 IgM or IgG antibodies, of whom 53 patients were positive for anti-annexin A5 IgG antibodies and 27 of 198 patients were positive for anti-annexin A5 IgM antibodies. The prevalence of IgM or IgG anti-annexin A5 antibodies was not significantly associated with thrombosis or miscarriage on multivariate analysis. The prevalence of the -1C-->T mutation in the annexin A5 gene (46/198 patients) was significantly associated with miscarriage (odds ratio 2.7, 95% confidence interval 1.1 to 6.7, independent risk factor).

CONCLUSION

The detection of anti-annexin A5 antibodies does not seem relevant for estimating the risk for thrombosis or miscarriage in APS. The -1C-->T mutation was an independent risk factor for miscarriage, which is independent of APS.

Pathogenic anti-beta2-glycoprotein I antibodies recognize domain I of beta2-glycoprotein I only after a conformational change

Recently, we published the existence of 2 populations of anti-beta2-glycoprotein I (beta2-GPI) IgG antibodies. Type A antibodies recognize epitope G40-R43 in domain I of beta2-GPI and are strongly associated with thrombosis. Type B antibodies recognize other parts of beta2-GPI and are not associated with thrombosis. In this study we demonstrate that type A antibodies only recognize plasma-purified beta2-GPI when coated onto a negatively charged surface and not when coated onto a neutrally charged surface. The affinity of type B antibodies toward plasma-purified beta2-GPI was independent of the charge of the surface to which beta2-GPI was coated. Type A antibodies did not recognize plasma-purified beta2-GPI in solution, whereas they did recognize recombinant beta2-GPI both in solution and coated onto a neutrally charged plate. When the carbohydrate chains were removed from plasma-purified beta2-GPI, we found that type A antibodies did recognize the protein in solution. This supports the hypothesis that the difference in recognition of plasma-purified and recombinant beta2-GPI is caused by the difference in glycosylation and that epitope G40-R43 of plasma-purified beta2-GPI is covered by a carbohydrate chain. Type A anti-beta2-GPI antibodies can only recognize this epitope when this carbohydrate chain is displaced as a result of a conformational change. This finding has major implications both for the detection of pathogenic anti-beta2-GPI antibodies and the comprehension of the pathophysiology of the antiphospholipid syndrome.

Assessment of Hemostatic Risk Factors in Predicting Arterial Thrombotic Events

Arterial thrombosis results from endovascular injury and, to a lesser extent, alterations in hemostatic equilibrium. Although multiple hereditary and acquired hemostatic risk factors have been described in the pathophysiology of venous thrombosis, the degree and type of abnormalities that contribute to arterial thrombosis are less well understood. Endothelial cell injury with the elaboration of proinflammatory mediators stimulates the process of arterial thrombosis. Although this is most often the result of endovascular injury attributable to atherosclerotic disease, other disease states can elicit a similar response as well. Similarly, once thrombosis has been initiated, variations in the activity of coagulation proteins and endogenous anticoagulants, as well as the kinetics of platelet aggregation, may alter the effectiveness of thrombus formation. Epidemiological studies have identified several acquired or inherited states that may result in endothelial damage or altered hemostatic equilibrium, thereby predisposing patients to arterial thrombosis. These include hyperhomocysteinemia, elevated C-reactive protein, antiphospholipid antibodies, elevated fibrinogen, Factor VII, plasminogen activator inhibitor-1 (PAI-1), hereditary thrombophilias, and platelet hyper-reactivity. This review explores our present understanding of these risk factors in the development of arterial thrombotic events. At present, the literature supports a role for hyperhomocysteinemia, elevated C-reactive protein, and elevated fibrinogen as risk factors for arterial thrombosis. Similarly, the literature suggests that lupus anticoagulants and, to a lesser extent, elevated titers of cardiolipin IgG antibodies predispose to arterial vascular events. In certain subsets of patients, including those with concomitant cardiac risk factors, <55 years of age, and women, hereditary thrombophilias such as carriership of the factor V Leiden and the prothrombin G20210A mutations may confer a higher risk of arterial thrombosis. However, the data on Factor VII, PAI-1, and platelet receptor polymorphisms are contradictory or lacking.

Pathophysiology of the antiphospholipid syndrome

Antiphospholipid syndrome is a distinct disorder with the clinical features of recurrent thrombosis in the venous or arterial circulation and fetal losses. Its serological marker is the presence of antiphospholipid antibodies in the blood of these patients. The relation between the presence of antibodies against anionic phospholipids and thromboembolic complications is well established over the last 25 years but the pathophysiology of the syndrome is largely unclear. Even after all these years, there is a persisting debate about the specificity and sensitivity of the assays for the detection of antiphospholipid antibodies. We now accept that antibodies to beta2-glycoprotein I rather than to anionic phospholipids are the major pathological antibodies, although there is no clear consensus on how the presence of these antibodies correlates with the different clinical manifestations of the syndrome. In this review, we discuss the current methods of detection of the antibodies and our insight into the pathobiology of the syndrome. We propose a mechanism for describing how the presence of anti-beta2-glycoprotein I antibodies relates to the different clinical manifestations observed.

The antiphospholipid syndrome: clinical characteristics, laboratory features and pathogenesis

PURPOSE OF REVIEW

The antiphospholipid syndrome is characterized by the paradoxical linkage between thrombosis in vivo and prolongation of clotting times in vitro. Recent reports that are discussed in this overview have taught us that this inconsistency is not so incomprehensible after all.

RECENT FINDINGS

Only a subpopulation of the heterogeneous population of antiphospholipid antibodies is pathogenic. New hypotheses have been proposed to explain the thrombotic tendency in this syndrome. In contrast to what is generally thought, it seems that the increased risk of thrombotic complication is independent of chronic vascular disease.

SUMMARY

The latest developments could make it possible to define patients with the syndrome unambiguously. This is absolutely necessary to facilitate studies designed to test the current hypotheses on the pathophysiology of antiphospholipid syndrome.

A prospective study of antibodies to beta2-glycoprotein I and prothrombin, and risk of thrombosis

Antiphospholipid syndrome (APS) is a clinical autoimmune disorder characterized by thrombosis/pregnancy morbidity associated with the persistence of lupus anticoagulant (LA) and/or anticardiolipin (aCL) antibodies. We assessed the contribution of antibodies to beta2-glycoprotein I (anti-beta2GPI) and prothrombin (anti-PT) to the thrombotic risk in a cohort of 194 consecutive patients with persistent LA and/or aCL. Median follow-up was 45 months. A total of 39 patients (20.1%) had one documented episode of thrombosis during follow-up. Eleven of these patients had no previous thrombosis before enrollment in the study and 28 had recurrences of thrombosis. There were 21 venous and 18 arterial thrombotic events and the overall incidence of thrombosis was 5.6% per patient-year. After multivariate analysis, the male sex (P = 0.025), a previous thrombosis (P < 0.01), the presence of anti-beta2GPI (P = 0.001), and the presence of anti-PT (P = 0.03) remained as independent risk factors for recurrent thrombosis. Only IgG anti-beta2GPI and anti-PT were associated with an increased risk of thrombosis (P < 0.01 and P = 0.005). Patients testing positive for anti-beta2GPI had a higher rate of thrombosis than did antiphospholipid patients without anti-beta2GPI (8.0% vs. 3.1% per patient-year). Similarly, a higher rate of thrombosis was found in patients with positive anti-PT compared with patients without anti-PT (8.6% vs. 3.5% per patient-year). Considering only the group of 142 LA positive patients, the highest incidence of thrombosis was found in LA patients positive for both anti-beta2GPI and anti-PT (8.4% per patient-year). In conclusion, the presence of IgG anti-beta2GPI and anti-PT in patients with LA and/or aCL and mainly in those with LA predicts a higher risk of thromboembolic events.

Inhibition of APC anticoagulant activity on oxidized phospholipid by anti-{beta}2-glycoprotein I monoclonal antibodies

Activated protein C (APC) anticoagulant activity and the ability to be inhibited by auto-antibodies associated with thrombosis are strongly augmented by the presence of phosphatidylethanolamine (PE) and phospholipid oxidation. beta(2)-glycoprotein I (beta(2)-GPI) is a major antigen for antiphospholipid antibodies present in patients with the antiphospholipid syndrome. We therefore investigated whether anti-beta(2)-GPI monoclonal antibodies (mAbs) could inhibit APC with similar membrane specificity. Five mouse mAbs that reacted with different epitopes on beta(2)-GPI were examined. Each inhibited the PE-, phospholipid oxidation-dependent enhancement of APC anticoagulant activity and required antibody divalency. A chimeric APC that retains anticoagulant activity but is relatively unaffected by protein S, PE, or oxidation was not inhibited by the antibodies. In purified systems, anti-beta(2)-GPI mAb inhibition of factor Va inactivation was greater in the presence of protein S and required beta(2)-GPI. Surprisingly, although the mAbs did increase beta(2)-GPI affinity for membranes, PE and oxidation had little influence on the affinity of the beta(2)-GPI antibody complex for the membrane vesicles. We conclude that antibodies to beta(2)-GPI inhibit APC function specifically and contribute to a hypercoaguable state by disrupting specific protein-protein interactions induced by oxidation of PE-containing membranes.

Beta2-glycoprotein I and LDL-receptor family members

The presence of antiphospholipid antibodies in plasma is a risk factor for thrombo-embolic complications. In vitro, however, the same antibodies can prolong clotting times in coagulation assays, a classic marker for a bleeding tendency. For years this contradiction puzzles many scientists.We now know that the term antiphospholipid antibodies comprises a heterogeneous population of antibodies and there is growing evidence that only subpopulations of antiphospholipid antibodies are relevant for the clinical complication. In combination with new information on the complex interaction between antiphospholipid antibodies, the protein beta2-Glycoprotein I, and cellular surfaces have opened new avenues for the understanding of the pathology of this syndrome.

IgG antibodies that recognize epitope Gly40-Arg43 in domain I of beta 2-glycoprotein I cause LAC, and their presence correlates strongly with thrombosis

Anti-beta(2)-glycoprotein I antibodies are known to have a heterogeneous reactivity against beta(2)-glycoprotein I. We performed this study to characterize the epitope on beta(2)-glycoprotein I to which pathologic anti-beta(2)-glycoprotein I antibodies are directed. Plasma samples from 198 patients with various systemic autoimmune diseases were tested for the presence of lupus anticoagulant and anti-beta(2)-glycoprotein I immunoglobulin G (IgG) antibodies. The reactivity of the anti-beta(2)-glycoprotein I-positive samples was further tested by coating recombinant full-length beta(2)-glycoprotein I and 8 deletion mutants of beta(2)-glycoprotein I onto hydrophilic and hydrophobic enzyme-linked immunosorbent assay (ELISA) plates. Full-length beta(2)-glycoprotein I with point mutations in domain I at positions 8, 40, and 43 were used in inhibition experiments. Fifty-two patients with anti-beta(2)-glycoprotein I IgG antibodies could be divided into 2 patterns. Type A antibodies only recognize domain I when coated onto hydrophobic plates; they do not recognize domain I coated onto hydrophilic plates. Type B antibodies have heterogeneous reactivity for all domains. Type A antibodies recognize the epitope around amino acids Gly40-Arg43 and cause lupus anticoagulant activity. In contrast to type B antibodies, those of type A strongly correlated with thrombosis. In conclusion, antibodies directed at domain I (epitope comprising Gly40 and Arg43) have lupus anticoagulant activity and strongly associate with thrombosis.