Complexes of anti-prothrombin antibodies and prothrombin cause lupus anticoagulant activity by competing with the binding of clotting factors for catalytic phospholipid surfaces

Lipid-binding antiphospholipid antibodies: significance for pathophysiology and diagnosis of the antiphospholipid syndrome

The antiphospholipid syndrome (APS) is an autoimmune disease characterized by the presence of pathogenic antiphospholipid antibodies (aPL). Since approximately 30 years ago, lipid-binding aPL, which do not require a protein cofactor, have been regarded as irrelevant for APS pathogenesis even though anticardiolipin are a diagnostic criterion of APS. In this review, we will summarize the available evidence from in vitro studies, animal models, and epidemiologic studies, which suggest that this concept is no longer tenable. Accordingly, we will only briefly touch on the role of other aPL in APS. This topic has been amply reviewed in detail elsewhere. We will discuss the consequences for laboratory diagnostics and future research required to resolve open questions related to the pathogenic role of different aPL specificities.

Cryo-EM structure and functional basis of prothrombin recognition by a type I antiprothrombin antiphospholipid antibody

ABSTRACT

Antiprothrombin antibodies are found in antiphospholipid patients, but how they interact with prothrombin remains elusive. Prothrombin adopts closed and open forms. We recently discovered type I and type II antibodies and proposed that type I recognizes the open form. In this study, we report the discovery and structural and functional characterization in human plasma of a type I antibody, POmAb (prothrombin open monoclonal antibody). Using surface plasmon resonance and single-molecule spectroscopy, we show that POmAb interacts with kringle-1 of prothrombin, shifting the equilibrium toward the open form. Using single-particle cryogenic electron microscopy (cryo-EM), we establish that the epitope targeted by POmAb is in kringle-1, comprising an extended binding interface centered at residues R90-Y93. The 3.2-Å cryo-EM structure of the complex reveals that the epitope overlaps with the position occupied by the protease domain of prothrombin in the closed state, explaining the exclusive binding of POmAb to the open form. In human plasma, POmAb prolongs phospholipid-initiated and diluted Russell's viper venom clotting time, which could be partly rescued by excess phospholipids, indicating POmAb is an anticoagulant but exerts a weak lupus anticoagulant effect. These studies reveal the structural basis of prothrombin recognition by a type I antiphospholipid antibody and uncover an exciting new strategy to achieve anticoagulation in human plasma.

Efficacy of the Hepcon system in reducing hemorrhagic and thrombotic complications in antiphospholipid syndrome patients undergoing cardiac surgery

INTRODUCTION

Patients with Antiphospholipid Syndrome (APS) undergoing cardiopulmonary bypass (CPB) surgery are at increased risk for thrombotic and hemorrhagic complications. Anticoagulation during CPB is typically monitored with activated clotting time (ACT) which may be falsely prolonged in patients with APS. The Hepcon Hemostasis Management System quantitatively determines the whole blood heparin concentration through heparin/protamine titration.

METHODS

This was a retrospective study of APS patients who underwent cardiac surgery requiring CPB at the Cleveland Clinic between April 2013, and July 2020. The primary endpoint was the composite rate of hemorrhagic or thromboembolic complications per surgical case in patients monitored by Hepcon versus patients monitored by ACT. Secondary endpoints were median volume of chest tube output and packed red blood cell (PRBC) transfusion within the first three post-operative days.

RESULTS

43 patients were included. 20 (47%) patients were monitored using Hepcon while 23 (53%) were monitored using ACT. For the primary endpoint of rate of thromboembolic or hemorrhagic complications per surgical case, there was no statistically significant difference between the Hepcon and ACT groups (HMS, 6/20 [30%]; ACT, 7/23 [30%]; p = >0.99). For the secondary endpoints, there was no statistically significant difference in median post-operative chest tube output (780 mL vs. 850 mL; p = 0.88) and median post-operative PRBC transfusion (1 unit vs. 0 unit; p = 0.28) between the Hepcon and ACT groups, respectively.

CONCLUSION

There was no difference in the composite outcome of thrombotic or hemorrhagic complications in patients monitored by Hepcon versus those monitored by ACT.

Testing for Lupus Anticoagulants

Lupus anticoagulant (LA) is one of the three criteria antiphospholipid antibodies (aPLs) employed in classification, and by default diagnosis, of antiphospholipid syndrome (APS). Detection of LA is not via calibrated assays but is based on functional behavior of the antibodies in a medley of coagulation assays. A prolonged clotting time in a screening test is followed by demonstration of phospholipid dependence and inhibitory properties in confirmatory and mixing tests, respectively, which are modifications of the parent screening test. Complications arise because no single screening test is sensitive to every LA, and no test is specific for LA, because they are prone to interference by other causes of elevated clotting times. Several screening tests are available but the pairing of dilute Russell's viper venom time (dRVVT) with LA-sensitive activated partial thromboplastin time (aPTT) is widely used and recommended because it is proven to have good detection rates. Nonetheless, judicious use of other assays can improve diagnostic performance, such as dilute prothrombin time to find LA unreactive with dRVVT and aPTT, and the recently validated Taipan snake venom time with ecarin time confirmatory test that are unaffected by vitamin K antagonist and direct factor Xa inhibitor anticoagulation. Expert body guidelines and their updates have improved harmonization of laboratory practices, although some issues continue to attract debate, such as the place of mixing tests in the medley hierarchy, and areas of data manipulation such as assay cut-offs and ratio generation. This article reviews current practices and challenges in the laboratory detection of LA.

Lupus anticoagulant single positivity at acute phase is not associated with venous thromboembolism or in-hospital mortality in COVID-19

Objective

The clinical relevance of antiphospholipid antibodies (aPLs) in COVID‐19 is controversial. This study was undertaken to investigate the prevalence and prognostic value of conventional and nonconventional aPLs in patients with COVID‐19.

Methods

This was a multicenter, prospective observational study in a French cohort of patients hospitalized with suspected COVID‐19.

Results

Two hundred forty‐nine patients were hospitalized with suspected COVID‐19, in whom COVID‐19 was confirmed in 154 and not confirmed in 95. We found a significant increase in lupus anticoagulant (LAC) positivity among patients with COVID‐19 compared to patients without COVID‐19 (60.9% versus 23.7%; P < 0.001), while prevalence of conventional aPLs (IgG and IgM anti–β₂‐glycoprotein I and IgG and IgM anticardiolipin isotypes) and nonconventional aPLs (IgA isotype of anticardiolipin, IgA isotype of anti‐β₂‐glycoprotein I, IgG and IgM isotypes of anti–phosphatidylserine/prothrombin, and IgG and IgM isotypes of antiprothrombin) was low in both groups. Patients with COVID‐19 who were positive for LAC, as compared to patients with COVID‐19 who were negative for LAC, had higher levels of fibrinogen (median 6.0 gm/liter [interquartile range 5.0–7.0] versus 5.3 gm/liter [interquartile range 4.3–6.4]; P = 0.028) and C‐reactive protein (CRP) (median 115.5 mg/liter [interquartile range 66.0–204.8] versus 91.8 mg/liter [interquartile range 27.0–155.1]; P = 0.019). Univariate analysis did not show any association between LAC positivity and higher risks of venous thromboembolism (VTE) (odds ratio 1.02 [95% confidence interval 0.44–2.43], P = 0.95) or in‐hospital mortality (odds ratio 1.80 [95% confidence interval 0.70–5.05], P = 0.24). With and without adjustment for CRP level, age, and sex, Kaplan‐Meier survival curves according to LAC positivity confirmed the absence of an association with VTE or in‐hospital mortality (unadjusted P = 0.64 and P = 0.26, respectively; adjusted hazard ratio 1.13 [95% confidence interval 0.48–2.60] and 1.80 [95% confidence interval 0.67–5.01], respectively).

Conclusion

Patients with COVID‐19 have an increased prevalence of LAC positivity associated with biologic markers of inflammation. However, LAC positivity at the time of hospital admission is not associated with VTE risk and/or in‐hospital mortality.

Anti-β2-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 (β2GPI) or prothrombin. How these autoantibodies cause LA is unclear.

OBJECTIVE

To elucidate how anti-β2GPI and anti-prothrombin antibodies cause the LA phenomenon.

METHODS

The effects of monoclonal anti-β2GPI and anti-prothrombin antibodies on coagulation were analyzed in plasma and with purified coagulation factors.

RESULTS

Detection of LA caused by anti-β2GPI 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-β2GPI and anti-prothrombin antibodies were observed. Further exploration of the effects of the antibodies on coagulation showed that the anti-β2GPI antibody attenuated FV activation by activated faxtor X (FXa), whereas the anti-prothrombin antibody did not. Binding studies showed that β2GPI--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-β2GPI and anti-prothrombin antibodies cause LA through different mechanisms of action: While anti-β2GPI 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.

B cells in primary antiphospholipid syndrome: Review and remaining challenges

It is now widely accepted that antiphospholipid antibodies (aPL) have direct pathogenic effects and that B cells, notably through aPL production, play a key role in the development of antiphospholipid syndrome (APS). Recent findings strengthened the implication of B cells with the description of specific B cell phenotype abnormalities and inborn errors of immunity involving B cell signaling in APS patients. In addition, it has been shown in preclinical models that cross-reactivity between APS autoantigens and mimotopes expressed by human gut commensals can lead to B cell tolerance breakdown and are sufficient for APS development. However, B cell targeting therapies are surprisingly not as effective as expected in APS compared to other autoimmune diseases. Elucidation of the B cell tolerance breakdown mechanisms in APS patients may help to develop and guide the use of novel therapeutic agents that target B cells or specific immune pathway.

Lupus anticoagulant identifies two distinct groups of patients with different antibody patterns

BACKGROUND

Whether antibodies directed to β2-Glycoprotein I (aβ2GPI) are responsible for LA activity is not well defined. However, in the absence of such antibodies the molecule responsible for LA phenomenon is unknown.

OBJECTIVE

The aim of this study was the biochemical identification of the target antigen epitope of aPL responsible of LA activity in the absence of aβ2GPI antibodies together with the biological and clinical characteristics of these patients in comparison with classical triple positive patients.

PATIENTS/METHODS

A comparison of patients with LA without (LA+/aβ2GPI-) and those with (LA+/aβ2GPI+) associated aβ2GPI antibodies was performed. Size exclusion chromatography and analytical chromatography were used to identify the molecule with LA activity in patients LA+/aβ2GPI-.

RESULTS AND CONCLUSIONS

Analytical size-exclusion chromatography revealed a peak of 996Kd with LA activity perfectly overlapping that of IgM anti phosphatidylserine/prothrombin (aPS/PT) antibodies. Similarly, all the 25 LA+/aβ2GPI- patients were positive for aPS/PT antibodies. LA+/aβ2GPI- compared to 33 LA+/aβ2GPI+ patients turned out to be significantly older, with a lower rate of previous thromboembolic events and a weaker LA activity. Search for aPS/PT and aβ2GPI antibodies in patients with LA is useful to identify two subgroups of LA at different risk of thromboembolic events.

Clinical Risk Assessment in the Antiphospholipid Syndrome: Current Landscape and Emerging Biomarkers

PURPOSE OF REVIEW

Laboratory criteria for the classification of antiphospholipid syndrome include the detection of a lupus anticoagulant and/or anticardiolipin and anti-β2-glycoprotein I antibodies. However, the majority of patients who test positive in these assays do not have thrombosis. Current risk-stratification tools are largely limited to the antiphospholipid antibody profile and traditional thrombotic risk factors.

RECENT FINDINGS

Novel biomarkers that correlate with disease activity and potentially provide insight into future clinical events include domain 1 specific anti-β₂GPI antibodies, antibodies to other phospholipids or phospholipid/protein antigens (such as anti-PS/PT), and functional/biological assays such as thrombin generation, complement activation, levels of circulating microparticles, and annexin A5 resistance. Clinical risk scores may also have value in predicting clinical events. Biomarkers that predict thrombosis risk in patients with antiphospholipid antibodies have been long sought, and several biomarkers have been proposed. Ultimately, integration of biomarkers with established assays and clinical characteristics may offer the best chance of identifying patients at highest risk of APS-related complications.

Antibodies to phosphatidylserine/prothrombin (aPS/PT) enhanced the diagnostic performance in Chinese patients with antiphospholipid syndrome

Background:

Increasing evidence has highlighted the role of non-criteria antiphospholipid antibodies (aPLs) as important supplements to the current criteria aPLs for the diagnosis of antiphospholipid syndrome (APS). In this retrospective study, we evaluated the clinical relevance of antibodies to phosphatidylserine/prothrombin (aPS/PT) in Chinese patients with APS.

Methods:

A total of 441 subjects were tested, including 101 patients with primary APS (PAPS), 140 patients with secondary APS (SAPS), 161 disease controls (DCs) and 39 healthy controls (HCs). Serum IgG/IgM aPS/PT was determined by ELISA.

Results:

The levels of IgG/IgM aPS/PT were significantly increased in patients with APS compared with DCs and HCs. IgG and IgM aPS/PT were present in 29.7% and 54.5% of PAPS, and 42.1% and 53.6% of SAPS, respectively. For diagnosis of APS, IgG aCL exhibited the highest positive likelihood ratio (LR+) of 21.60, followed by LA (13.84), IgG aβ2GP1 (9.19) and IgG aPS/PT (8.49). aPS/PT was detected in 13.3% of seronegative PAPS patients and 31.3% of seronegative SAPS patients. LA exhibited the highest OR of 3.64 in identifying patients with thrombosis, followed by IgG aCL (OR, 2.63), IgG aPS/PT (OR, 2.55) and IgG aβ2GP1 (OR, 2.33). LA and IgG aCL were correlated with both arterial and venous thrombosis, whereas IgG aPS/PT and IgG aβ2GP1 correlated with venous or arterial thrombosis, respectively.

Conclusions:

Our findings suggest that the inclusion of IgG/IgM aPS/PT may enhance the diagnostic performance for APS, especially in those in whom APS is highly suspected, but conventional aPLs are repeatedly negative. In addition, IgG aPS/PT may contribute to identify patients at risk of thrombosis.

Hyperhomocysteinemia Relates to the Subtype of Antiphospholipid Antibodies in Non-SLE Patients

Abnormal increases of antiphospholipid antibody and plasma homocysteine levels are recently emerging as nonlipidic risk factors for cerebral atherogenesis and thrombosis. Both antiphospholipid antibody and homocysteine share many similar bioeffects in hemostasis, but their interaction is still inconsistent. In this study, we examined the relation between the plasma homocysteine level and lupus anticoagulant, anticardiolipin antibody, and anti-β2-glycoprotein I antibody in patients with noncardiac cerebral ischemia. Systemic lupus erythrematosus patients were excluded. The results showed a higher frequency of moderate hyperhomocysteinemia in patients with an abnormal increase of lupus anticoagulant only. Neither the serum folate and cobalamin levels nor methylenetetrahydrofolate reductase allele mutation contributes to this result. Accordingly, homocysteine interacts with lupus anticoagulant to promote cerebral atherosclerosis and ischemia. The role of vasculopathic or prothrombotic autoantibody generation in response to specific pathological change such as hyperhomocysteinemia warrants further investigation.

The challenges of lupus anticoagulants

The term "lupus anticoagulant" (LA) refers to a heterogeneous group of immunoglobulins behaving as acquired in vitro inhibitors of coagulation. These antibodies, namely anti-β2GPI and anti-prothrombin antibodies, induce the in vitro elongation of clotting time interfering with phospholipid-dependent coagulation cofactors. Positive LA is associated with thrombosis and pregnancy complications, providing one of the three laboratory criteria for the classification of the anti-phospholipid syndrome. LA is the strongest predictor of clinical events, especially when associated with other anti-phospholipid antibodies. Much more controversial is the risk conveyed by isolated and weak LA. LA detection is technically laborious, envisaging screening, mixing and confirming tests. Hopefully critical issues in LA detection, such as the interference of anticoagulants, will be overcome, in the next future.

Antiphospholipid syndrome: 30 years and our contribution

In 1983, Graham Hughes first described the concept of antiphospholipid syndrome (APS). In 1984, we described the enzyme-linked immunosorbent assay (ELISA) system which directly detected circulating aCL in patients with systemic lupus erythematosus (SLE) who revealed biological false positive serological test for syphilis. In 1990, three groups, including our group, independently reported the necessity of a cofactor for the binding of autoimmune anticardiolipin antibodies (aCL) to the solid phase phospholipids. β2-glycoprotein I (β2GPI) was identified as this cofactor. In 1994,the epitope for aCL was shown to develop when β2GPI is adsorbed on polyoxygenated polystyrene plates. In 2000, we described antiprothrombin antibodies bind to prothrombin exposed to immobilized phosphatidylserine and established a phosphatidylserine dependent monoclonal antiprothrombin antibody. In 2004, a novel role of nicked β2GPI was identified in the negative feedback pathway of extrinsic fibrinolysis. Nicked β2GPI was found to bind angiostatin 4.5 and to attenuate its antiangiogenic property. In 2004, we demonstrated that the p38 MAPK pathway mediates induction of the TF gene in stimulated with human monoclonal anti- β2GPI antibodies. Very recently, β2GPI was identified as a complement regulator. The cross-link between complement activation and prothrombotic status in patients with APS has been drawn much attention. Genetic factors are hypothesized to play a role in the susceptibility to APS based on several family studies in patients with antiphospholipid antibodies (aPL) and/or clinical manifestations of APS. The genetics of β2GPI has been extensively studied. 247 Val/Leu polymorphism can affect the conformational change of β2-GPI and the exposure of the epitopes for aCL. We found that 247 Val was correlated with anti-β2-GPI production in patients with primary APS, and 247 Val may be important for β2-GPI antigenicity. STAT4 SNP in Japanese patients with SLE and/or APS. T allele frequencies in SLE and APS were significantly elevated compared with that in healthy controls. When analyzed only in primary APS patients, T allele frequency was further higher. BANK1, BLK and SNP in 1q25.1 region were associated with not only SLE but also APS in Japanese population. These results suggest that APS and SLE, in part, share a common genetic background.

Lupus anticoagulant-hypoprothrombinemia syndrome: report of two cases and review of the literature

Lupus anticoagulant-hypoprothrombinemia syndrome (LA-HPS) is a rare acquired disorder caused by prothrombin antibodies. The disease is most common in the pediatric age group (<16 years), and more prevalent in women. There are well-established clinical diseases associated with LA-HPS, most notably systemic lupus erythematosus (SLE) and viral infections. The clinical manifestation of LA-HPS varies greatly in severity and it may cause severe life-threatening bleeding diathesis. LA-HPS is to be suspected when a patient presents with bleeding and a prolonged activated partial thromboplastin and prothrombin time, in combination with a lupus anticoagulant. The diagnosis is confirmed in the laboratory by identification of reduced prothrombin levels. There are no standardized recommendations for treatment of the hemorrhage associated with the syndrome; corticosteroids are used as first-line treatment. This review summarizes what is currently known about the pathogenesis, clinical features, diagnosis, treatment and prognosis of LA-HPS, and presents two case reports.

Antiprothrombin antibody: why do we need more assays?

Anticardiolipin antibodies (aCL), anti-beta(2)-glycoprotein I (beta(2)GPI) antibodies and lupus anticoagulant (LA) are the only laboratory tests considered within the revised criteria for the classification of the antiphospholipid syndrome (APS). Recently, the significance to assay the antibodies against phosphatidylserine-prothrombin complex (aPS/PT) has been discussed, and these antibodies, rather than antibodies against prothrombin alone, are closely associated with APS and LA. The sensitivity and specificity of aPS/PT for the diagnosis of APS were assessed in a population of patients with a variety of autoimmune disorders. The aCL and aPS/PT have similar diagnostic value for APS, and most of APS patients with aPS/PT had positive LA. Therefore, aPS/PT should be further explored, not only for research purposes, but also as a candidate for one of the enzyme-linked immunosorbent assay (ELISA)-based confirmatory test for APS associated LA.

Prevalence and significance of anti-prothrombin (aPT) antibodies in patients with Lupus Anticoagulant (LA)

OBJECTIVE

Anti-prothrombin (aPT) antibodies have been found in Lupus Anticoagulant (LA) positive patients. Their prevalence and relative contribution to thromboembolic risk in LA-positive patients is not well defined. The aim of this study was to determine their presence and association with thromboembolic events in a large series of patients with confirmed LA.

METHODS

Plasma from LA-positive patients was collected at Thrombosis Centers and sent to a reference central laboratory for confirmation. Positive plasma was tested using home-made ELISA for the presence of aPT and anti-beta(2)GPI antibodies.

RESULTS

LA was confirmed in 231 patients. Sixty-one of 231 (26%, 95%CI 22-33) LA positive subjects were positive for IgG aPT and 62 (27%, 95% CI 21-33) were positive for IgM aPT antibodies. Clinical features of Antiphospholipid Syndrome (APS) were not associated with the presence of IgG aPT [43 APS in 61 (70%) positive and 109 APS in 170 (64%) negative IgG aPT subjects, p=ns] or IgM aPT. Rate of positivity of IgG and IgM a beta(2)GPI was significantly higher than that of IgG and IgM aPT. Clinical events accounting for APS occurred in 97 of 130 (75%) IgG a beta(2)GPI positive and in 55 of 101 (54%) IgG a beta(2)GPI negative patients (OR 2.4, 95% CI 1.4 to 4.3, p=0.002). No significant association with clinical events in patients positive for both IgG aPT and IgG a beta(2)GPI as compared to those positive for one or another test was found. When patients negative for both IgG aPT and IgG a beta(2)GPI (LA positive only) were compared with remaining patients, a significantly lower association with clinical events was found (OR=0.4, 95% CI: 0.2 to 0.7, p=0.004).

CONCLUSIONS

As compared to IgG a beta(2)GPI, the prevalence of IgG aPT in patients with LA is significantly lower and not associated with the clinical features of APS.

Pathogenetic potential of antiphospholipid antibodies

Antiphospholipid antibodies are autoantibodies that recognize phospholipid-binding proteins such as beta2 glycoprotein (beta2GP)-I, prothrombin or annexins. These antibodies have been associated with arterial or venous thrombotic events and pregnancy morbidity. The molecular mechanisms responsible for the pathogenetic potential of these antibodies include: resistance to activated protein C, acquired Factor XII deficiency resulting in suppression of intrinsic fibrinolytic activity, activation of endothelial cells through the nuclear factor kappaB pathway leading to tissue factor upregulation, adhesion molecule and cytokine expression and activation of platelets. Opposite effects, such as the potentiation of the inhibitory action of beta2GPI on the activation of Factor XI, make the dynamics of the interaction of these antibodies with the coagulation system rather complex. Many of the above functions can be mediated by signaling through molecules of the tumor necrosis factor receptor family, such as CD40, which is recognized by purified anti-beta2GPI antibodies.

Mechanisms of antiphospholipid-induced thrombosis: effects on the protein C system

An acquired resistance to activated protein C (APC) has been demonstrated in patients with antiphospholipid antibodies (aPL). Recent studies report interactions between beta2 glycoprotein I (beta2GPI) and prothrombin-binding antibodies and the protein C system. Some aPL in patients recognize one or more conformational epitopes shared by beta2GPI and catalytic domains of APC. Both beta2GPI and anti-prothrombin antibodies are associated with APC resistance. Several clinical studies have focused on an association between aPL and APC resistance, determined by classic activated partial thromboplastin time-based tests. It has been shown in most studies that APC resistance was associated with lupus anticoagulants. APC resistance is also associated with thrombosis, especially venous thromboembolism. Several recent studies have reported a novel integrated approach of coagulation using calibrated automated thrombography. This technique allows an approach of APC sensitivity without interference with lupus anticoagulants. Clinical associations between APC resistance and thromboembolic events have been demonstrated.

Study of phosphatidylserine-dependent anti-prothrombin antibody in cerebral infarction

PURPOSE

This study was carried out to clarify the relationship of IgG phosphatidylserine-dependent anti-prothrombin antibody (aPS/PT), IgG beta 2 glycoprotein I-dependent anticardiolipin antibody (beta 2-GPI aCL), and lupus anticoagulant (LA) to cerebral infarction, using data from 93 patients who visited our hospitals.

MATERIALS AND METHODS

We computed the positive rates for each of IgG aPS/PT, beta 2-GPI aCL, and LA in the 93 patients with cerebral infarction, and carried out logistic regression analysis with IgG aPS/PT as the outcome variable and with beta 2-GPI aCL, LA, and each risk factor as predictor variables in order to assess the relationship of IgG aPS/PT with each factor.

RESULTS

IgG aPS/PT was more highly correlated with LA than beta 2-GPI aCL in IgG aPS/PT-positive patients with cerebral infarction. IgG aPS/PT itself appears to have high specificity as a marker for antiphospholipid syndrome (APS), because there were patients who were IgG aPS/PT-positive but both beta 2-GPI aCL- and LA-negative.

CONCLUSION

In IgG aPS/PT-positive patients with cerebral infarction, IgG aPS/PT is more highly correlated with LA than beta 2-GPI aCL. It is also strongly associated with APS. Measurement of IgG aPS/PT in patients with cerebral infarction could be of diagnostic value.

The Activated Seven Lupus Anticoagulant Assay Detects Clinically Significant Antibodies

Lupus anticoagulants are a heterogeneous group of autoantibodies detected by their effects on phospholipid-dependent coagulation assays. Persistent lupus anticoagulants are associated with thrombotic disease, but not all are clinically significant. Antibody heterogeneity and reagent and test variability dictate that at least 2 tests, of different types, should be used to screen lupus anticoagulants. The objective of this study was to investigate whether the activated seven lupus anticoagulant assay detects clinically significant antibodies. Eighty-two patients with antiphospholipid syndrome (APS) and 32 with systemic lupus erythematosus + positive for activated seven lupus anticoagulant and who were without thrombosis, who were positive by activated seven lupus anticoagulant assay, were investigated for lupus anticoagulants by dilute Russell's viper venom time, dilute activated partial thromboplastin time, and Taipan snake venom time, and for anticardiolipin antibodies. Fifty-seven of the APS patients were positive for lupus anticoagulants in multiple assays, 25 in activated seven lupus anticoagulant alone. Fourteen of the latter group were previously positive in other antiphospholipid antibodies assays, and 11 had only been positive for lupus anticoagulants by activated seven lupus anticoagulant. Twenty-eight had elevated anticardiolipin antibodies, 6 of whom were from the group that was positive in activated seven lupus anticoagulant only. Eight of the systemic lupus erythematosus + lupus anticoagulants (without thrombosis) patients were positive for lupus anticoagulant by activated seven lupus anticoagulant alone and had only been positive in activated seven lupus anticoagulant previously, and none had elevated anticardiolipin antibodies. The remaining 24 patients were lupus-anticoagulant positive in multiple assays, and 9 had elevated anticardiolipin antibodies. Dilute Russell's viper venom time and Dilute activated partial thromboplastin time are widely used to detect lupus anticoagulants and are considered to detect clinically significant antibodies. Activated seven lupus anticoagulant detected antibodies in APS patients who were positive by these assays and also lupus anticoagulants undetectable by the dilute Russell's viper venom time/dilute activated partial thromboplastin time reagents used, demonstrating its utility as a first-line or second-line assay.

Antiphospholipid antibodies: laboratory and pathogenetic aspects

Antiphospholipid antibodies (aPL) constitute a heterogeneous group of autoantibodies that share the ability to bind phospholipids (PL) alone, protein-PL complexes, or PL-binding proteins. They have been detected in isolation, in association with autoimmune diseases such as systemic lupus erythematosus (SLE), and during the course of different infections. aPL have been associated with an array of clinical manifestations in virtually every organ, although deep vein and arterial thrombosis as well as pregnancy morbidity are predominant. The co-occurrence of these clinical findings with aPL constitutes the so-called antiphospholipid syndrome (APS). aPL can be detected by immunological methods [e.g., anticardiolipin antibodies (aCL)] or by functional methods that exploit the effect of aPL on blood coagulation [lupus anticoagulant (LA)]. Since aPL are heterogeneous, numerous immunological and coagulation assays have been developed. These assays have not been fully standardized, and, therefore, problems such as high interlaboratory variation are relatively frequent. Recently, recommendations have been published regarding LA and aCL testing. Not all aPL are pathogenic. However, when they are not associated with infections, they have a role in the pathogenesis of APS. Clinical and experimental data have shown that aPL exert their pathogenic activity by interfering with the function of coagulation factors, such as thrombin and factors X, XI and XII, and with the function of anticoagulant proteins of the protein C system. In addition, aPL interaction with platelets and endothelial cells induces a pro-adhesive activated phenotype.

β2-Glycoprotein I inhibits von Willebrand factor–dependent platelet adhesion and aggregation

Patients with antiphospholipid syndrome are characterized by the association of thrombosis or pregnancy morbidity and the presence of antiphospholipid autoantibodies. Particularly, anti-β2-glycoprotein (β2 GPI) autoantibodies correlate with thrombosis, suggesting an antibody-induced gain of prothrombotic function and/or an antibody-induced loss of antithrombotic function of β2 GPI. In the search for potential antithrombotic properties of β2 GPI, we found that β2 GPI inhibits von Willebrand factor (VWF)–induced platelet aggregation. In addition, platelet adhesion to a VWF-coated surface was decreased by 50% in the presence of β2 GPI (P < .03). β2 GPI binds to the A1 domain of VWF but preferably when the A1 domain is in its active glycoprotein Ibα-binding conformation. Anti-β2 GPI antibodies isolated from a subset of antiphospholipid syndrome patients neutralized the β2 GPI-VWF interactions and thus the inhibitory activity of β2 GPI. In comparison to healthy individuals, the amounts of active VWF in circulation were increased 1.5-fold (P < .001) in patients positive for lupus anticoagulant (LAC) due to anti-β2 GPI antibodies. Thus, β2 GPI is a biologically relevant inhibitor of VWF function by interfering with VWF-dependent platelet adhesion. Anti-β2 GPI autoantibodies neutralize this inhibitory function and are associated with increased levels of active VWF. This mode of action could contribute to the thrombosis and consumptive thrombocytopenia observed in patients with anti-β2 GPI antibodies.

Does the anti-prothrombin antibodies measurement provide additional information in patients with thrombosis?

The aim of this study is to get new insight into the relevance of IgG anti-prothrombin antibodies in patients with thrombosis and to determine whether human prothrombin alone (aPT) or complexed to phosphatidylserine (aPS/PT) should be preferentially used for measuring these antibodies by enzyme-linked immunosorbent assay (ELISA). To this end, prevalence of anti-prothrombin antibodies, their characteristics in terms of avidity and heterogeneity, and their relationship with anti-beta2 glycoprotein I antibodies (abeta2GPI) were studied in 152 patients with thrombosis. Patients were divided into two groups according to the presence or absence of antiphospholipid antibodies (aPL), called aPL+ or aPL-, respectively. In the aPL- group (n=90), the prevalence of anti-prothrombin antibodies was substantial (10%) but not significantly different from that of control (5%). In the aPL+ group (n=62), lupus anticoagulant (LA) or anticardiolipin antibodies (aCL) positive, 61% were positive for anti-prothrombin antibodies with no statistical difference between aPT and aPS/PT prevalence (42% vs. 55%, respectively). In the whole thrombotic population, 19% were only aPT and 34% only aPS/PT suggesting the presence of different antibodies. Absorption experiments confirmed the heterogeneity of aPT and aPS/PT. No difference in their avidity was demonstrated. From the aPL+ group, 60 were LA positive. Among them, 18% were negative for abeta2GPI and anti-prothrombin antibodies showing that the detection of these antibodies could not substitute for LA determination. In conclusion, our data show that the screening of the different anti-prothrombin antibodies is not warranted in the aPL+ group since these antibodies do not provide additional information compared to aCL, LA and/or abeta2GPI measurement. Nevertheless, the substantial prevalence of anti-prothrombin antibodies in the aPL- group should be further explored in a large prospective study.

Clinical significance of different antiphospholipid antibodies in the WAPS (warfarin in the antiphospholipid syndrome) study

To assess the clinical significance of lupus anticoagulants (LAs) and antiphospholipid antibodies (aPLs) toward thrombosis and abortions, we measured them in 112 patients whose samples were available at enrollment in the warfarin in the antiphospholipid syndrome (WAPS) study. Enzyme-linked immunosorbent assay (ELISA) and coagulation test values in the highest and lowest tertiles were compared. When considered separately, IgG antibodies to β2-glycoprotein I (aβ2GPI) and prothrombin (aPT) were associated with anamnestic arterial and venous thrombosis, respectively, and those to annexin AV (aAnAV) with abortions. IgM antibodies to protein S and the lupus ratio of the dilute prothrombin time were associated with prospective thrombosis. No other association for IgM antibodies was seen. LA-positive patients who carried aβ2GPI antibodies were at risk of anamnestic arterial and total thrombosis and aPT antibodies to that of anamnestic venous and total thrombosis. LA-positive patients who carried IgG aβ2GPI and aAnAV antibodies were at risk for both anamnestic abortion and prospective thrombosis. Overall, these data support the inclusion of aβ2GPI antibodies in and suggest the removal of anticardiolipin antibodies from the laboratory criteria of the antiphospholipid syndrome. They also suggest that the measurement of aPT and aAnAV antibodies is useful in some selected situations and that there is little role for IgM antibody detection.

Recurrent pregnancy loss: the key potential mechanisms

Recurrent pregnancy loss (RPL), which occurs in 0.5%-1% of total pregnancies, is usually defined as three or more consecutive spontaneous abortions before 20 weeks of gestation. Although an immunology-based etiology underlying unexplained RPL has been demonstrated, the exact molecular mechanisms are still poorly understood. Recent studies using transcriptomics and proteomics have implicated several factors. Here, we review the key possible mechanisms, in particular immunological abnormalities and thrombophilic aberration, that might underlie RPL. Taken together, these findings provide insight into the development of effective therapeutics for RPL.

High incidence of autoantibodies in Fabry disease patients

Fabry disease (FD) is an X-linked disorder of glycosphingolipid catabolism that results from a deficiency of the lysosomal enzyme alpha-galactosidase A. This defect leads to the accumulation of its substrates, mainly globotriaosylceramide, in lysosomes of cells of different tissues. Different studies have shown the involvement of immunopathologies in different sphingolipidoses. The coexistence of FD and immune disorders such as systemic lupus erythematosus, rheumatoid arthritis and IgA nephropathy, has been described in the literature. The aim of this study was to evaluate the prevalence of a group of autoantibodies in a series of Argentine FD patients. Autoantibodies against extractable nuclear antigens (ENAs), double-stranded DNA, anticardiolipin and phosphatidylserine were assayed by ELISA. Lupus anticoagulants were also tested. Fifty-seven per cent of the samples showed reactivity with at least one autoantigen. Such reactivities were more frequent among males than among females. Antiphospholipid autoantibodies were detected in 45% of our patients. The high rate of thrombosis associated with FD could be related, at least in part, to the presence of antiphospholipid autoantibodies in Fabry patients. We found the presence of ENAs, which are a characteristic finding of rheumatological diseases, previous a frequent misdiagnosis of FD, in around 39% of the cases. The detection of a high level of autoantibodies must be correlated clinically to determine the existence of an underlying autoimmune disease. With the recent development of therapy, the life expectancy in FD will increase and autoimmune diseases might play an important role in the morbidity of FD.

Anti-prothrombin antibodies predict thrombosis in patients with systemic lupus erythematosus: a 15-year longitudinal study

OBJECTIVE

To evaluate the role of anti-prothrombin (anti-PT) antibodies in predicting thrombosis in patients with systemic lupus erythematosus (SLE).

METHODS

An inception cohort of 101 SLE patients (12 males, 89 females; mean age 30 +/- 8 years), was considered. Clinical and laboratory evaluations were regularly performed during a 15-year follow-up (median 108 months) with a special focus on thromboembolic events. Serum samples were collected at time of diagnosis and at least once a year thereafter. IgG and IgM anti-PT, anti-cardiolipin (aCL) and anti-beta(2)glycoprotein I (beta(2)GPI) antibodies were measured by enzyme-linked immunosorbent assay (ELISA); lupus anticoagulant (LAC) was assayed by the dilute Russell's viper venom time and activated partial thromboplastin time tests. The analytical specificity of anti-PT ELISA was investigated. The timing of thrombosis occurrence was calculated using the Kaplan-Meier method.

RESULTS

In the 15-year follow-up, thrombosis occurred in 14 out of the 101 patients: venous thrombosis in nine cases and arterial thrombosis in five. IgG and/or IgM anti-PT, anti-beta(2)GPI and aCL antibodies, and LAC activity were detected in ten, nine, seven, and nine cases, with sensitivity for thrombosis of 71.4%, 64.3%, 50% and 64.3%, respectively. Thrombosis-free survival was 90% at 5 years and 85.8% at 10 and 15 years, respectively. Thrombosis was predicted by anti-PT (P = 0.001), anti-beta(2)GPI antibodies (P = 0.002) and LAC activity (P = 0.001). Moreover, the risk of thrombosis progressively increased with the number of positive antiphospholipid antibody tests. The presence of four positive antibody tests was associated with a risk of thrombosis thirtyfold higher than in their absence.

CONCLUSIONS

This longitudinal study shows that IgG anti-PT antibodies are predictors of thrombosis in SLE patients.

The clinical significance of anti-prothrombin antibodies for risk assessment of thromboembolism in patients with lupus anticoagulant

INTRODUCTION

Thromboembolism is a common manifestation of lupus anticoagulant (LA), however only a subgroup of LA-patients is affected by thrombosis. Study objective was to investigate whether anti-prothrombin antibodies can identify LA-patients at increased risk for thrombosis.

MATERIALS AND METHODS

In total 79 patients, 50 with (42 men/8 women) and 29 without thrombosis (21 men/8 women), were investigated for their presence of anti-prothrombin IgG and IgM antibodies using assays from two different manufacturers (Aeskulisa=assay I, CoaChrom=assay II).

RESULTS

The prevalence of elevated levels of anti-prothrombin IgG, IgM as well as IgG and/or IgM antibodies was 66% [assayI] (36% [assayII]), 38% (24%) and 72% (50%) in patients with thrombosis and 55% (24%), 28% (28%) and 66% (41%) in patients without thrombosis, respectively. Neither anti-prothrombin IgG or IgM nor IgG and/or IgM antibodies were found to indicate an increased risk for thrombosis. In the subgroup of patients with arterial or venous thrombosis there was also no association between anti-prothrombin antibodies and thrombosis. The comparison of median levels of IgG and IgM anti-prothrombin antibodies between patients with and without thrombosis yielded a borderline statistically significant difference only for anti-prothrombin IgG antibodies by using assay II (p=0.033), all other comparisons were not statistically significant.

CONCLUSIONS

In conclusion, presence of anti-prothrombin antibodies was not associated with thromboembolism in LA-patients.

Detection of acquired resistance to activated protein C associated with antiphospholipid antibodies using a novel clotting assay

Antiphospholipid antibodies (aPA) frequently interfere with the protein C pathway. This manifests as acquired activated protein C (APC) resistance in the absence of factor V Leiden and has been proposed as a putative mechanism for the pathogenesis of the antiphospholipid syndrome (APS). We have developed a Russell's viper venom test, performed with and without activation of endogenous protein C, which is sensitive to aPA-associated APC resistance. Results were reported as the endogenous APC ratio (EAPCr); the ratio of the two clotting times normalized against pooled normal plasma. Forty-four patients with aPA, anticardiolipin and/or lupus anticoagulant, including 34 with a history of thrombosis or pregnancy morbidity; a control group of aPA-negative patients; and 26 healthy normals were studied. EAPCr (mean, SD) was significantly higher in APS patients (1.94, 0.58) than normals (0.98, 0.12) or controls (1.14, 0.19; P < 0.00001). Elevated EAPCr (> 1.22) occurred in 91% of aPA-positive patients, predominantly due to resistance to APC (87%) rather than prolonged basal clotting times alone (15%). Significant correlation was observed between the EAPCr value and dilute Russell's viper venom time (rs = 0.44, P = 0.003), IgG anticardiolipin (rs = 0.54, P = 0.002), protein S (r = -0.46, P = 0.01) and activated partial thromboplastin time-based APC resistance (r = -0.61, P = 0.001). There was no significant relationship between EAPCr and protein C concentration, anti-beta2-glycoprotein-I (anti-beta2GPI) or IgM anticardiolipin. Purified aPA IgG caused a dose-dependent increase in APC resistance when added to normal plasma. We conclude that aPA-associated acquired APC resistance is a common feature of APS and may be independent of anti-beta2GPI.

Antiphospholipid syndrome: An evolving story

Antiphospholipid Syndrome (APS) is a form of immune mediated thrombophilia, presenting as recurrent thrombotic events and pregnancy morbidity, in association with positive laboratory tests for antiphospholipid antibodies (APA) in the form of Lupus Anticoagulant (LA) or anticardiolipin antibodies(ACA). Insights into the pathophysiology of the condition suggest that some antibodies are prothrombotic in vivo, and that the mechanism of thrombosis is likely to be multifactorial. APS has a broad spectrum of clinical presentations, and the laboratory diagnosis can be difficult due to heterogeneity of APAs and poor standardisation of laboratory tests. Anticoagulation is the mainstay of the management of the thrombotic and obstetric complications of APS. The risk of recurrent thrombosis appears to be high, and the duration and intensity of therapy remains controversial. Randomised controlled trials have shown that standard intensity anticoagulation is adequate in most cases of venous thrombosis. Further trials are required to establish whether high intensity coagulation is of benefit in recurrent or arterial thrombosis. The optimal management of recurrent fetal loss is debated and large studies are required to establish a clear benefit of heparin and aspirin over aspirin alone or supportive care.

Tissue factor upregulation drives a thrombosis-inflammation circuit in relation to cardiovascular complications

The extrinsic coagulation is recognized as an 'inducible' signalling cascade resulting from tissue factor (TF) upregulation by exposure to clotting zymogen FVII upon inflammation or tissue injury. Following the substantial initiation, an array of proteolytic activation generates mediating signals (active serine proteases: FVIIa, FXa and FIIa) that lead to hypercoagulation with fibrin overproduction manifesting thrombosis. In addition, TF upregulation plays a central role in driving a thrombosis-inflammation circuit. Coagulant mediators (FVIIa, FXa and FIIa) and endproduct (fibrin) are proinflammatory, eliciting tissue necrosis factor, interleukins, adhesion molecules and many other intracellular signals in different cell types. Such resulting inflammation could ensure 'fibrin' thrombosis via feedback upregulation of TF. Alternatively, the resulting inflammation triggers platelet/leukocyte/polymononuclear cell activation thus contributing to 'cellular' thrombosis. TF is very vulnerable to upregulation resulting in hypercoagulability and subsequent thrombosis and inflammation, either of which presents cardiovascular risks. The prevention and intervention of TF hypercoagulability are of importance in cardioprotection. Blockade of inflammation reception and its intracellular signalling prevents TF expression from upregulation. Natural (activated protein C, tissue factor pathway inhibitor, or antithrombin III) or pharmacological anticoagulants readily offset the extrinsic hypercoagulation mainly through FVIIa, FXa or FIIa inhibition. Therefore, anticoagulants turn off the thrombosis-inflammation circuit, offering not only antithrombotic but anti-inflammatory significance in the prevention of cardiovascular complications.

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.

Anti-phospholipid antibodies (aPL) and apoptosis: prothrombin-dependent aPL as a paradigm for phospholipid-dependent interactions with apoptotic cells

The natural targets of anti-phospholipid antibodies (aPL) and the stimuli that induce them remain unknown. Apoptotic cells have been proposed as both potential targets and immunogens for anti-phospholipid antibodies. Demonstration of selective recognition by anti-phospholipid antibodies provides support for apoptotic cells as antigenic targets. Here, we summarize data showing that prothrombin (PT) binds to apoptotic, but not viable, cells, and that apoptotic-cell bound prothrombin provides a target for human polyclonal and murine monoclonal lupus anticoagulant (LA) antibodies. We discuss findings for two monoclonal lupus anticoagulant antibodies that have high (antibody 29J3-62) or low (antibody 29I4-24) affinity, respectively, for soluble prothrombin. Despite their very different affinities for soluble prothrombin, both monoclonal antibodies reacted similarly with prothrombin bound to phospholipid or apoptotic cells. Furthermore, both antibodies enhanced the binding of prothrombin to apoptotic cells. We propose that the recognition of apoptotic cells by these prothrombin-dependent monoclonal antibodies provides a paradigm for other anti-phospholipid autoantibodies. 29I4-24 is prototypical of phospholipid-dependent anti-phospholipid antibodies, while 29J3-62 represents a prototype for phospholipid-independent anti-phospholipid antibodies. Proteins such as prothrombin and beta2-glycoprotein I (beta2GPI) bind to apoptotic cells, thereby enhancing the recognition of apoptotic cells by anti-phospholipid antibodies. Furthermore, anti-phospholipid antibodies potentiate the interaction of these proteins with apoptotic cells. While it is unclear whether apoptotic cells are the inducing stimuli in patients with anti-phospholipid antibodies or even whether anti-phospholipid antibodies interact with apoptotic cells in vivo, it is nonetheless clear that anti-phospholipid antibodies have the potential to affect both the procoagulant activity and the uptake and clearance of apoptotic cells.

Acquired Activated Protein C Resistance Associated with IgG Antibodies against β2-Glycoprotein I and Prothrombin as a Strong Risk Factor for Venous Thromboembolism

Background: Venous thromboembolic events such as deep vein thrombosis and pulmonary embolism are common manifestations of antiphospholipid syndrome. Our aim was to clarify the roles of anti-phospholipid (aPL) antibodies in the pathogenesis of venous thromboembolism (VTE) in patients with systemic lupus erythematosus (SLE).

Methods and Results: We examined anti-cardiolipin/β2-glycoprotein I (anti-CL/β2-GPI) antibody concentrations, anti-phosphatidylserine/prothrombin (anti-PS/PT) antibody concentrations, and lupus anticoagulant (LA) activity in 87 patients with SLE (21 with VTE and 66 without thrombosis). Both anti-CL/β2-GPI and anti-PS/PT antibodies strongly correlated with LA activity. Multivariate logistic analysis confirmed that both anti-CL/β2-GPI and anti-PS/PT antibodies were significant independent risk factors for VTE (odds ratios = 4.98 and 7.54, respectively; 95% confidence intervals, 1.51–16.4 and 2.30–24.7, respectively). We therefore studied the in vitro effects of IgG fractions containing anti-CL/β2-GPI or anti-PS/PT antibodies on the anticoagulant activity of activated protein C (APC) and found that purified IgG containing anti-CL/β2-GPI or anti-PS/PT antibodies significantly hampered the anticoagulant activity of APC. We also studied the ability of IgG fractions to impede the anticoagulant activity of APC before and after complete removal of anti-CL/β2-GPI or anti-PS/PT antibodies by adsorption. Removal of anti-CL/β2-GPI or anti-PS/PT antibodies from all positive IgG samples clearly decreased the inhibitory effect of those samples on APC anticoagulant activity.

Conclusions: Anti-CL/β2-GPI and anti-PS/PT antibodies independently cause APC resistance, which may contribute to risk of VTE in patients with SLE.

Prevalence and Clinical Correlation of Anti-Phospholipid–Binding Protein Antibodies in Anticardiolipin-Negative Patients With Systemic Lupus Erythematosus and Women With Unexplained Recurrent Miscarriages

Context.—Anti-phospholipid antibodies (aPL) are a heterogeneous group of autoantibodies, the presence of which is associated with thrombotic events and miscarriage.

Objective.—To establish whether antibodies directed against phospholipid-binding plasma proteins such as β2-glycoprotein I (β2GPI), prothrombin (PT), and annexin V (Anx V) constitute a risk factor for thromboembolism in patients with systemic lupus erythematosus (SLE) and for miscarriage in women with recurrent pregnancy loss (RPL), independently of the presence of the classic anticardiolipin (aCL) antibodies, and whether their determination together with that of aCL would help to increase the diagnostic sensitivity of aPL tests.

Design.—The prevalence of various antibodies directed toward phospholipids (CL and other anionic phospholipids [APL]) and phospholipid-binding proteins (β2GPI, PT, and Anx V) was determined by immunoenzymatic methods in 311 serum samples.

Patients.—Twenty-five patients with aCL-positive primary anti-phospholipid syndrome (pAPS); 89 patients with SLE, 23 of whom had thrombotic complications (SLE/APS) and 66 of whom had no thrombosis; and 77 women with unexplained recurrent pregnancy loss comprised our study group. One hundred twenty healthy subjects matched for age and sex were studied as the control group.

Results.—Immunoglobulin (Ig) G and/or IgM aAPL, anti-β2GPI, anti-PT, and IgG anti-Anx V antibodies were detected in 25 (100%), 20 (80%), 15 (60%), and 6 (24%), respectively, of the 25 aCL-positive pAPS patients; IgG and/or IgM aCL, aAPL, anti-β2GPI, anti-PT, and IgG anti-Anx V antibodies were detected in 33 (37%), 42 (47%), 31 (35%), 40 (45%), and 12 (13%) of the 89 SLE patients, respectively. Of the 56 SLE patients who proved to be aCL negative, anti-β2GPI was present in 3 patients (5%), anti-PT in 13 (23%) patients, and anti-Anx V in 5 (9%) patients. In the subset of 23 SLE/APS patients, IgG anti-PT prevalence was higher than that of the other autoantibodies (87% vs 70% aCL, 66% aAPL, 57% anti-β2GPI, and 4% anti-Anx V), and in 26% of cases, IgG anti-PT was the only antibody present. Anti-PT had a slightly lower specificity than aCL (46% vs 49%); however, the occurrence of both antibodies brought the specificity to 92.4%. The highest risk for thrombosis in SLE patients was associated with the presence of IgG anti-PT antibody (odds ratio [OR] 15.3, P &lt; .001, vs 6.5 aCL, 3.5 aAPL, 3.4 anti-β2GPI, 0.2 anti-Anx V). Fifty-one of the 77 women with recurrent pregnancy loss were negative for all antibodies investigated; the prevalence of IgG and/or IgM aCL, aAPL, anti-β2GPI, anti-PT, and IgG anti-Anx V antibodies was 6% (5), 12% (9), 6% (5), 16% (12), and 17% (13), respectively. Of the 67 aCL-negative women, none had anti-β2GPI antibodies, 7 (11%) were anti-PT positive, and 13 (19%) were anti-Anx V positive. In the subgroup of 26 recurrent pregnancy loss patients who had at least one antibody, anti-Anx V was present in 50% of cases (in 42% as the sole antibody) and was the only antibody significantly associated with miscarriage (P = .02).

Conclusions.—The results of this study indicate that it is useful to measure anti-PT antibodies in addition to the more widely used aCL and anti-β2GPI antibodies in the prognostic evaluation of SLE patients for the risk of thrombosis, and the results also confirm that anti-Anx V antibodies may play an important role in recurrent pregnancy loss.

Antiprothombin antibodies and the diagnosis of antiphospholipid syndrome

The preliminary classification criteria for definite antiphospholipid syndrome (APS) include the presence of anticardiolipin antibodies (aCL) and/or lupus anticoagulant (LA) as laboratory criteria. However, antiphospholipid antibodies (aPL) are a heterogeneous group of antibodies comprising also antibodies against phospholipid-binding proteins or their complexes with phospholipids. Prothrombin is one of the antigen recognized by aPL. In the last decade, there has been increasing interest in antibodies against prothrombin alone and those against phosphatidylserine-prothrombin complex. The latter, phosphatidylserine-dependent antiprothrombin antibodies (aPT), have been closely associated with APS and LA. In this paper, we review the properties of antiprothrombin antibodies.

The effect of phospholipids on the formation of immune complexes between autoantibodies and β2-glycoprotein I or prothrombin

In the last decennium, it became clear that antiphospholipid antibodies found in patients with antiphospholipid syndrome (APS) are in fact antibodies against lipid-bound plasma proteins. The most frequently occurring antigens are beta2-glycoprotein I and prothrombin, although several other lipid-bound plasma proteins have been reported as antigen for antiphospholipid antibodies. Both proteins bind to anionic phospholipids, mainly phosphatidylserine, which becomes exposed at the surface of activated platelets, apoptotic cells, or cell-derived microparticles. The binding of beta2-glycoprotein I and prothrombin to these cell surfaces or to artificial lipid vesicles with comparable amounts of anionic phospholipids is rather weak. Antiphospholipid antibodies from patients are predominantly of low affinity regarding their interaction with beta2-glycoprotein I or prothrombin in solution. In the presence of a suitable phospholipid surface, however, this interaction is strongly enhanced. There is now strong evidence that formation of bivalent, trimolecular immune complexes at the lipid membrane essentially contributes to the binding of these intrinsically low affinity patient antibodies. Depending on the affinity, the epitope specificity, and the polyclonality of a particular IgG preparation, multimeric structures of lipid-bound immune complexes may form a lattice with multiple interactions on the lipid (cell) surface. It is hypothesized that the functional activity, that is, the ability of antibodies to interfere with lipid-dependent reactions, not only depends on their affinity for the antigen, but also on their ability to form multiple interconnected bivalent trimolecular complexes at the lipid (or cell) surface. It is further proposed that the rate of desorption of immune complexes may present a better indicator for the functional properties of the antibodies than the amount of adsorbed immune complexes.

Antiphospholipid antibodies: update on detection, pathophysiology, and treatment

PURPOSE OF REVIEW

The antiphospholipid syndrome is a noninflammatory disease characterized by the presence of antiphospholipid antibodies in the plasma of patients with venous or arterial thrombosis or obstetric complications. Understanding of the pathology of the syndrome is only rudimentary and no physician can pretend that treatment is adequate. It is not possible to identify patients with the syndrome for certain. This paper reviews the substantial and promising headway made in the past year.

RECENT FINDINGS

It has become clear that lupus anticoagulant is the assay of choice to detect the syndrome and that antibodies directed towards beta2Glycoprotein 1 are the clinical relevant autoantibodies. These antibodies are able to activate a number of cells involved in the regulation of hemostasis.

SUMMARY

The latest developments will make it soon possible to define patient with the syndrome unambiguously. This is absolutely necessary to understand why the presence of antiphospholipid antibodies cause an increased thrombotic risk.

Autoimmune disease as a cause of reproductive failure

High-risk pregnancy is the most common clinical association with antiphospholipid antibodies; the principal manifestations are pregnancy loss and early preeclampsia. Membership in this family of antibodies is continually growing and includes antibodies against a variety of phospholipids, phospholipid-protein complexes, and phospholipid-binding proteins. The current information in the literature is inadequate to clearly implicate a subgroup of antiphospholipid antibodies or a particular pathophysiologic mechanism as being responsible for poor pregnancy outcomes. It is clear, however, that prevalent diagnostic tests for LA and aCL are extremely useful to identify many of these patients, but are inadequate for diagnosis of all patients with autoimmune pregnancy loss or to elucidate the pathophysiology. Many patients who present clinically with autoimmune-like pregnancy complications currently are negative in tests for LA or aCL, but have antibodies against annexin V, phosphatidylserine, or other relevant antigens. The greatest risk for a complicated pregnancy is conveyed by a subgroup of antibodies that affect the normal function of placental trophoblast. As clinical laboratory tests designed to detect more members of the antiphospholipid antibody family become available, understanding of this complicated disease (APS) will increase.

Current status and implications of autoimmune antiphospholipid antibodies in relation to thrombotic disease

This review briefly describes the development of the concepts of antiphospholipid antibody and of antiphospholipid syndrome. It focuses on the two main antigenic targets, beta2 glycoprotein I and prothrombin. An excessive production of natural antibodies rather than an immune response to exogenous antigen is proposed as pathogenetic for the development of these antibodies. The review attempts to explain how some of these antibodies are anticoagulant in vitro yet prothrombotic in vivo. The final section discusses when to test for such antibodies, how to test and how to consider treatment of patients with the antiphospholipid syndrome.

A simple method to discriminate between beta2-glycoprotein I- and prothrombin-dependent lupus anticoagulants

Lupus anticoagulants (LAC) are a heterogeneous group of autoantibodies that prolong phospholipid-dependent clotting assays. The autoantibodies that cause LAC activity are predominantly directed against beta2-glycoprotein I (beta2GPI) or prothrombin. In the present study, we describe a method to differentiate between LAC caused by antibodies directed against beta2GPI or prothrombin. Monoclonal antibodies, affinity purified patient antibodies, and selected patient samples were used to show that in an aPTT-based clotting assay (PTT-LA; Diagnostica Stago), the use of cardiolipin vesicles in the neutralization procedure discriminates between beta2GPI- or prothrombin-dependent LAC activities. Addition of cardiolipin vesicles shortened the prolonged clotting time caused by anti-beta2GPI antibodies with LAC activity, whereas this procedure further prolonged clotting times caused by antiprothrombin antibodies with LAC activity. In contrast, addition of phosphatidylcholine/phosphatidylserine vesicles corrected prolonged clotting times caused by either anti-beta2GPI or antiprothrombin antibodies with LAC activity. The effects of cardiolipin (CL) on beta2GPI-induced LAC activity were specific for contact activation mediated clotting assays. Possible explanations for these findings are the relatively high affinity of beta2GPI for cardiolipin, as determined by surface plasmon resonance analysis, and inhibition by anti-beta2GPI antibodies of the CL-induced prolongation of the PTT-LA.

Prothrombin binds to the surface of apoptotic, but not viable, cells and serves as a target of lupus anticoagulant autoantibodies

Anti-phospholipid Ab (aPL) are a heterogeneous group of autoantibodies directed against various combinations of phospholipids (PL) and PL-binding proteins. Lupus anticoagulant (LA) Ab, a subset of aPL, exhibit anticoagulant properties in vitro, but are procoagulant in vivo. Most LA Ab are specific for either beta(2)-glycoprotein I (beta(2)GPI) or prothrombin (PT), two PL-binding proteins. We have previously shown that beta(2)GPI and beta(2)GPI-dependent aPL bind specifically to apoptotic, but not viable, thymocytes. In this study, we demonstrate that PT, like beta(2)GPI, binds selectively to the surface of apoptotic, but not viable, Jurkat cells. Furthermore, PT supports the binding of systemic lupus erythematosus-derived polyclonal and murine monoclonal LA Ab to apoptotic cells. Two LA mAb, which differed dramatically in their relative affinities for PT, were studied. Although one mAb (29J3-62) had a high affinity for PT alone, the other (29I4-24) showed minimal reactivity with PT alone and required PL for elevated binding. Monovalent fragments of 29I4-24 reacted with PL-bound PT with high affinity, suggesting that this mAb recognizes a PL-dependent epitope. Despite these differences, PT-dependent binding of both mAb to apoptotic cells was 30-fold greater than that to viable cells. Moreover, binding of PT to apoptotic cells was, itself, increased in the presence of bivalent, but not monovalent, forms of either mAb. In summary, our data demonstrate the following: 1) specific binding of PT to apoptotic cells, an effect enhanced by PT-dependent LA Ab; 2) heterogeneity of PT-dependent LA Ab; and 3) potential pathogenicity of Ab of either low or high affinity for PT.

Kinetics of prothrombin-mediated binding of lupus anticoagulant antibodies to phosphatidylserine-containing phospholipid membranes: an ellipsometric study

Antiphospholipid antibodies interact with phospholipid membranes via lipid binding plasma proteins, mostly, prothrombin and beta(2)-glycoprotein I. Using ellipsometry, we characterized prothrombin-mediated binding of lupus anticoagulant (LA) positive IgG, isolated from patients with antiphospholipid syndrome, to phosphatidylserine (PS)-containing membranes. LA IgG did not bind to membranes in the absence of prothrombin, but addition of prothrombin resulted in high-affinity binding of prothrombin-LA IgG complexes; half-maximal binding was attained at IgG and prothrombin concentrations of 10 microg/mL and 4 nM, respectively. Adsorption to membranes containing 10-40 mol % PS revealed that membrane-bound rather than solution-phase prothrombin determines the adsorption kinetics. Depletion of prothrombin and LA IgG from the solution results in rapid desorption which is strongly inhibited by addition of prothrombin but not of LA IgG. Prothrombin-mediated adsorption of monovalent Fab1 fragments prepared from patient LA IgG was negligible, indicating that monovalent interaction between prothrombin and LA IgG is weak. The kinetics of adsorption and desorption indicate that divalent binding of LA IgG to prothrombin at the lipid membrane occurs.