Lupus antibody bivalency is required to enhance prothrombin binding to phospholipid

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.

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.

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.

Discovery and characterization of 2 novel subpopulations of aPS/PT antibodies in patients at high risk of thrombosis

Anti-phosphatidylserine/prothrombin (aPS/PT) antibodies are often detected in patients with antiphospholipid syndrome (APS), but how aPS/PT engage prothrombin at the molecular level remains unknown. Here, the antigenic determinants of immunoglobulin G aPS/PT were investigated in 24 triple-positive APS patients at high risk of thrombosis by using prothrombin mutants biochemically trapped in closed and open conformations, and relevant fragments spanning the entire length of prothrombin. Two novel unexpected findings emerged from these studies. First, we discovered that some aPS/PT are unique among other anti-prothrombin antibodies insofar as they efficiently recognize prothrombin in solution after a conformational change requiring exposure of fragment-1 to the solvent. Second, we identified and characterized 2 previously unknown subpopulations of aPS/PT, namely type I and type II, which engage fragment-1 of prothrombin at different epitopes and with different mechanisms. Type I target a discontinuous density-dependent epitope, whereas type II engage the C-terminal portion of the Gla-domain, which remains available for binding even when prothrombin is bound to the phospholipids. Based on these findings, APS patients positive for aPS/PT were classified into 2 groups, group A and group B, according to their autoantibody profile. Group A contains mostly type I antibodies whereas group B contains both type I and type II antibodies. In conclusion, this study offers a first encouraging step toward unveiling the heterogeneity of anti-prothrombin antibodies in correlation with thrombosis, shedding new light on the mechanisms of antigen-autoantibody recognition in APS.

Human monoclonal antibodies against the complex of phosphatidylserine and prothrombin from patients with the antiphospholipid antibodies

The presence of antibodies against the complex of prothrombin and phosphatidylserine (aPS/PT) more significantly correlates with manifestations of antiphospholipid syndrome (APS) and with the presence of lupus anticoagulants (LA) than antibodies against prothrombin bound to oxygenated polystyrene (aPT-A). To investigate immunological specificities and functional activities of aPS/PT, four monoclonal aPS/PT, designated as HG-4, KE-6, KF-5 and KF-6, from two patients with antiphospholipid antibodies (aPL) were established and characterized. Three of these antibodies (HG-4, KF-5 and KF-6) recognized the complex of phosphatidylserine and prothrombin, but did not react to prothrombin directly coated on oxygenated plates. KE-6 bound not only to the complex of phosphatidylserine and prothrombin but also to prothrombin on oxygenated plates. None of them showed the binding activity to prothrombin directly coated on non-oxygenated plates. HG-4, KE-6 and KF-5 had LA-like activity. The findings support the hypothesis that autoimmune aPS/PT recognize the cryptic epitopes or neoepitopes exposed upon interaction between prothrombin and phosphatidylserine, and that aPS/PT are, at least in part, responsible for LA activity. Lupus (2007) 16, 509—516.

The influence of anti-endothelial/antiphospholipid antibodies on fibrin formation and lysis on endothelial cells

The prothrombotic mechanisms associated with antiphospholipid antibodies remain incompletely defined. Antibody binding to endothelial cells in vitro is a feature of antiphospholipid antibody-positive sera. We hypothesised that impairment of endothelium-dependent fibrinolysis by antiphospholipid/anti-endothelial antibodies is a contributory factor in the pathogenesis of thrombosis. We also aimed to confirm the displacement of annexin-V from endothelial cells and enhanced fibrin formation. Binding of immunoglobulin (Ig) from antiphospholipid antibody-positive sera to endothelial cells was examined using a cell-based enzyme-linked immunosorbent assay. Effects on fibrin formation and lysis were examined on cultured endothelial cell monolayers. Plasminogen activator inhibitor-1 (PAI-1) was assayed in supernatants. We confirmed antibody binding to endothelial cells. With four of 14 antiphospholipid antibody-positive sera there was some prolongation of fibrin clot lysis time, consistent with impairment of endothelial fibrinolytic activity. Secretion of PAI-1 was significantly correlated with clot lysis time on endothelial cell monolayers incubated with antiphospholipid/anti-endothelial antibody-positive sera, but not with control sera. IgG from antiphospholipid antibody-positive sera had little effect on endothelial cell surface annexin-V expression. We conclude that impaired endothelial fibrinolysis is a potential prothrombotic mechanism in subjects with antiphospholipid antibodies. We were unable to confirm enhanced displacement of annexin-V from endothelium by antiphospholipid antibodies.

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.

Anti-prothrombin IgG from patients with anti-phospholipid antibodies inhibits the inactivation of factor Va by activated protein C

Interference of anti-phospholipid antibodies with the protein C pathway has been suggested to play a role in the development of thrombosis in the anti-phospholipid syndrome. We studied the effect of IgG preparations containing anti-prothrombin antibodies of 17 lupus anticoagulant-positive patients and 12 controls on the inactivation of factor Va (FVa) by activated protein C (APC) in a system with purified coagulation factors. Test IgG was incubated with human prothrombin, phospholipid vesicles and CaCl(2). Protein S, FVa and APC were added and the APC-dependent loss of FVa activity was monitored over time. The residual amount of FVa remaining after 10 min was 14 +/- 4% (mean +/- SD) when 1.5 mg/ml normal IgG was present and ranged between 17% and 82% with 1.5 mg/ml patient IgG. Twelve patients IgG gave values of residual FVa >22% (i.e. 2 SD above the mean of controls), indicating that APC-mediated inactivation of FVa was significantly inhibited. The inhibition was strictly dependent on the presence of prothrombin, proportional to the concentration of IgG and strongly diminished at a 20-fold higher phospholipid concentration. Most, although not all, IgG containing anti-prothrombin antibodies inhibit the APC-catalysed FVa inactivation, which may contribute to the increased risk of thrombosis in patients with the anti-phospholipid syndrome.

Antiphospholipid syndrome: association between laboratory tests and clinical practice

Antiphospholipid antibodies are a wide and heterogeneous group of immunoglobulins, whose presence inpatients with arterial and venous thrombosis, and obstetrical complications defines the antiphospholipid syndrome. We systematically reviewed published articles on this syndrome to investigate the association between thrombosis and the most common antiphospholipid antibodies. Lupus anticoagulants were a clear risk factor for thrombosis, irrespective of the site and type of thrombosis, the presence of systemic lupus erythematosus, and the methods used to detect them. Anticardiolipin and anti 2-glycoprotein I antibodies were possible risk factors of thrombosis, at least in some selected situations. Conversely, the measurement of antiprothrombin antibodies was not helpful to define the patient's risk of thrombosis. These results are mainly due to the still far from optimal standardization of the methods to detect the various antiphospholipid 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.

The antiphospholipid syndrome

The antiphospholipid (aPL) antibody syndrome is an autoimmune condition in which vascular thrombosis and/or recurrent pregnancy losses occur in patients with laboratory evidence for antibodies that bind to phospholipids. There have been significant advances in the recognition of the role of phospholipid-binding cofactors, primarily beta2GPI, as the true immunologic targets of the antibodies. Recent evidence suggests that the antibodies disrupt phospholipid-dependent anticoagulant mechanisms and/or that aPL antibodies induce the expression of procoagulant and proadhesive molecules on endothelial cells. Current diagnosis is based on clinical findings and empirically derived tests, such as assays for antibodies that bind to phospholipids or putative cofactors and coagulation assays that detect inhibition of phospholipid-dependent coagulation reactions. Current treatment relies primarily on anticoagulant therapy. Research advances are expected to bring mechanistically based diagnostic tests and improved therapy that target the roots of the disease process.

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.

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.

Thrombogenicity of beta 2-glycoprotein I-dependent antiphospholipid antibodies in a photochemically induced thrombosis model in the hamster

We previously showed that beta(2)-glycoprotein I (beta(2)GPI)-dependent lupus anticoagulants (LAs) form bivalent antigen-antibody complexes with high affinity for phospholipids; these complexes are responsible for their in vitro anticoagulant effect. We now studied the role of these bivalent complexes in arterial thrombosis in the hamster. Three monoclonal antibodies (mAbs) raised against human beta(2)GPI were selected on the basis of their cross-reactivity with hamster beta(2)GPI. Two of these, one with LA activity, 5H2, and one with only anticardiolipin properties, 11E8, were infused at 0 to 10 mg/kg prior to photochemically induced vessel damage. 5H2 promoted thrombus formation dose dependently, raising the thrombus size from 6.0 arbitrary units (AU) in controls (n = 9) to 65.0 AU in the high-dose group (10 mg/kg, n = 6, P =.007). The LA(-) mAb 11E8 and mAb 27A8, reactive with human beta(2)GPI exclusively, did not significantly promote thrombus formation. In a second set of experiments, intact mAb 5H2 was compared to its fragments. Intact mAb 5H2 at 3.3 mg/kg and the equimolar dose of F(ab')(2) fragments (2.2 mg/kg) promoted thrombus formation equally well (55.8 AU, n = 8 and 62.5 AU, n = 7, respectively); mAb 5H2-derived Fab' fragments were inactive. Immunohistochemical analysis showed platelet-rich thrombi, with 5H2 or its F(ab')(2) fragments mainly bound to individual platelets. Our results indicate that bivalent immune complex formation plays an important role in the genesis of arterial thrombosis by certain antiphospholipid antibodies. Cellular activation via the Fc portion of these immune complexes, however, is not essential, because F(ab')(2) fragments of 5H2 still promote thrombus formation.

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.

Survey of the year 2001 commercial optical biosensor literature

We have assembled references of 700 articles published in 2001 that describe work performed using commercially available optical biosensors. To illustrate the technology's diversity, the citation list is divided into reviews, methods and specific applications, as well as instrument type. We noted marked improvements in the utilization of biosensors and the presentation of kinetic data over previous years. These advances reflect a maturing of the technology, which has become a standard method for characterizing biomolecular interactions.

Bivalency of plasminogen monoclonal antibodies is required for plasminogen bridging to fibrin and enhanced plasmin formation

Binding of plasminogen to fibrin and cell surfaces is essential for fibrinolysis and pericellular proteolysis. We used surface plasmon resonance and enzyme kinetic analyses to study the effect of two mAbs (A10.2, CPL15) on plasminogen binding and activation at fibrin surfaces. A10.2 is directed against the lysine-binding site (LBS) of kringle 4, whereas CPL15 recognises a region in kringle 1 outside the LBS. In the presence of CPL15 and A10.2 mAbs, binding of plasminogen (K(d)=1.16+/-0.22 micromol/l) to fibrin was characterised by a mAb concentration-dependent bell-shaped isotherm. A progressive increase in the concentration of mAbs at the surface was also detected, and reached a plateau corresponding to the maximum of plasminogen bound. These data indicated that at low mAb concentration, bivalent plasminogen-mAb-plasminogen ternary complexes are formed, whereas at high mAb concentration, a progressive shift to monovalent plasminogen-mAb binary complexes is observed. Plasmin formation in the presence of mAbs followed a similar bell-shaped profile. Monovalent Fab fragments of mAb A10.2 showed no effect on the binding of plasminogen, confirming the notion that a bivalent mAb interaction is essential to increase plasminogen binding and activation at the surface of fibrin.