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

[Contribution of antiphosphatidylserine/prothrombin (anti-PS/PT) antibody detection in the diagnosis and management of antiphospholipid syndrome (APS)]

Antiphospholipid syndrome (APS) is an autoimmune disease and one of the most common causes of acquired thrombophilia. It is characterised by the occurrence of thrombotic or obstetric events associated with the presence of persistent antiphospholipid antibodies. The diagnosis can be challenging, particularly because some biological tests can be disturbed by anticoagulant treatment or inflammation. In the recent years, new antiphospholipid antibodies, including anti-phosphatidylserine/prothrombin antibodies (anti-PS/PT), have emerged but their clinical significance and causality remain uncertain. Biologically, several studies have found a strong correlation between the presence of lupus anticoagulant (LA) and anti-PS/PT antibodies. Clinically, the presence of anti-PS/PT antibodies is associated with an increased risk of thrombosis and obstetric complications. There is also an association with thrombocytopenia, suggesting that the presence of anti-PS/PT antibodies may be associated with more severe clinical APS. Among seronegative APS patients, 6-17% of patients are positive for anti-PS/PT antibodies. This might influence the therapeutic management of patients. This article aims to provide an update on contribution of anti-PS/PT antibodies detection for the diagnosis and management of APS.

Interaction between Antiphospholipid Antibodies and Protein C Anticoagulant Pathway: A Narrative Review

Thrombotic antiphospholipid syndrome (APS) is a condition in which thrombosis in venous, arterial, and/or small vessels is ascribed to the presence of antiphospholipid antibodies (aPL). Among the various proposed pathogenic theories to explain thrombotic APS, those involving the interaction between aPL and the protein C system have gained much consensus. Indeed, robust data show an acquired activated protein C resistance (APC-R) in these patients. The role of aPL in this impairment is clear, but the mechanism of action is uncertain, as the type of aPL and to what extent aPL are involved remains a gray area. Lupus anticoagulant (LA) is often associated with APC-R, but antibodies generating LA comprise those directed to β2-glycoprotein I and antiphosphatidylserine/prothrombin. Moreover, the induction of APC-R by aPL requires the presence of phospholipids and is suppressed by the presence of an excess of phospholipids. How phospholipids exposed on the cell membranes work in the system in vivo is unknown. Interestingly, acquired APC-R due to aPL might explain the clinical phenotypes of thrombotic APS. Indeed, the literature reports cases of both venous and arterial thromboembolism as well as skin necrosis, the latter observed in the severe form of protein C deficiency and in catastrophic APS.

Neuropsychiatric Manifestations of Antiphospholipid Syndrome—A Narrative Review

Antiphospholipid syndrome (APS) is a common autoimmune pro-thrombotic condition characterised by thrombosis and pregnancy morbidity. There are a broad range of neuropsychiatric manifestations associated with APS, from focal symptoms to more global dysfunction. Patients commonly present with transient ischaemic attacks and ischaemic strokes, with identifiable lesions on brain imaging. However, the underlying pathogenesis remains uncertain in other manifestations, such as cognitive dysfunction, seizures, headache and chorea. The aim is to provide a comprehensive review of the various neuropsychiatric manifestations associated with APS. A detailed literature search was applied to PubMed, including citations from 1983 to December 2021.

Laboratory Approaches to Test the Function of Antiphospholipid Antibodies

Antiphospholipid syndrome (APS) is a systemic autoimmune disorder caused by the presence of aPLs (antiphospholipid antibodies, i.e., anti-β₂-glycoprotein I and anti-cardiolipin). Everyday practice in terms of laboratory diagnostics of APS includes determination of aPLs and well-known functional assays assessing for lupus anticoagulant (LA), in turn using various tests. According to recent guidelines, the recommended method for LA identification or exclusion is based on the Russell Viper Venom test and a sensitive activated partial thromboplastin time assay. Despite the fact that LA can be quantified in laboratory practice in this way, LA is still used as a binary parameter that is just one of the risk factors of thrombosis in APS. As of today, there are no other functional assays to routinely assess the risk of thrombosis in APS. It is well-known that APS patients display a wide range of clinical outcomes although they may express very similar laboratory findings. One way to solve this dilemma, could be if antibodies could be further delineated using more advanced functional tests. Therefore, we review the diagnostic approaches to test the function of aPLs. We further discuss how thrombin generation assays, and rotational thromboelastometry tests can be influenced by LA, and how experimental methods, such as flow cytometric platelet activation, surface plasmon resonance, or nano differential scanning fluorimetry can bring us closer to the puzzling interaction of aPLs with platelets as well as with their soluble protein ligand. These novel approaches may eventually enable better characterization of aPL, and also provide a better linkage to APS pathophysiology.

Anti-β2-glycoprotein I autoantibodies influence thrombin generation parameters via various mechanisms

INTRODUCTION

Antiphospholipid syndrome (APS) is a systemic autoimmune disease characterised by recurrent thrombotic events, pregnancy loss and thrombocytopenia and the presence of antiphospholipid antibodies (APL). The exact pathomechanism of APS is still unknown, thus we investigated the effect of anti-β₂-glycoprotein I (anti-β₂GPI) on thrombin generation in different plasma samples.

METHODS

For the separation of anti-β₂GPI IgG, overall 12 APS patients were selected. The criteria were the existence of lupus anticoagulant, and the presence of anti-CL and anti-β₂GPI, the latter exceeding at least 25 times the upper reference limit. We purified anti-β₂GPI IgG antibodies from APS patients by affinity chromatography and added the antibodies to normal pooled, and heterozygous forms of inherited thrombophilia plasma samples (prothrombin G20210A, factor V Leiden). To further specify the mechanism of the effect, we also used factor deficient plasmas in the thrombin generation assay.

RESULTS

In normal pooled plasma, the anti-β₂GPI significantly prolonged Lag Time according to the lupus anticoagulant effect, in contrast, it also elevated Peak Thrombin significantly, which suggests a procoagulant effect. The antibody was also able to exert this multi-faceted effect both in FV_Leiden heterozygous plasma and prothrombin G20210A heterozygous polymorphism, however, the prolonging effect was more remarkable in the latter. By using factor deficient plasmas, it was found that FVII is required for the prolongation, while intrinsic factors are needed for the elevation of the Peak Thrombin.

CONCLUSION

The anti-β₂GPI autoantibodies exert their effect in both normal and thrombophilic plasmas via various mechanisms.

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.

Mechanisms of Antiphospholipid Syndrome Induction: Role of NKT Cells

The review discusses the mechanisms of participation of natural killer T cells (NKT cells) in the induction of antiphospholipid antibodies (APA) that play a major pathogenetic role in the formation of antiphospholipid syndrome (APS), summarizes the data on APS pathogenesis, and presents modern concepts on the antibody formation involving follicular helper type II NK cells.

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.

Diagnosis and management of the antiphospholipid syndrome

Antiphospholipid syndrome (APS) is characterized by thrombosis and/or pregnancy complications in the presence of persistent antiphospholipid antibodies (APLA). Laboratory diagnosis of APLA depends upon the detection of a lupus anticoagulant, which prolongs phospholipid-dependent anticoagulation tests, and/or anticardiolipin (aCL) and anti-β2-glycoprotein-1 (β2GPI) antibodies. APLA are primarily directed toward phospholipid binding proteins. Pathophysiologic mechanisms underlying thrombosis and pregnancy loss in APS include APLA induced cellular activation, inhibition of natural anticoagulant and fibrinolytic systems, and complement activation, among others. There is a high rate of recurrent thrombosis in APS, especially in triple positive patients (patients with lupus anticoagulant, aCL and anti-β2GPI antibodies), and indefinite anticoagulation with a vitamin K antagonist is the standard of care for thrombotic APS. There is currently insufficient evidence to recommend the routine use of direct oral anticoagulants (DOAC) in thrombotic APS. Aspirin with low molecular weight or unfractionated heparin may reduce the incidence of pregnancy loss in obstetric APS. Recent insights into the pathogenesis of APS have led to the identification of new potential therapeutic interventions, including anti-inflammatory and immunomodulatory therapies. Additional research is needed to better understand the effects of APLA on activation of signaling pathways in vascular cells, to identify more predictive biomarkers that define patients at greatest risk for a first or recurrent APLA-related clinical event, and to determine the safety and efficacy of DOACs and novel anti-inflammatory and immune-modulatory therapies for refractory APS.

Recent advances in understanding antiphospholipid syndrome

Antiphospholipid syndrome (APS), also known as Hughes Syndrome, is a systemic autoimmune disease characterized by thrombosis and/or pregnancy morbidity in the presence of persistently positive antiphospholipid antibodies. A patient with APS must meet at least one of two clinical criteria (vascular thrombosis or complications of pregnancy) and at least one of two laboratory criteria including the persistent presence of lupus anticoagulant (LA), anticardiolipin antibodies (aCL), and/or anti-b2 glycoprotein I (anti-b2GPI) antibodies of IgG or IgM isotype at medium to high titres in patient’s plasma. However, several other autoantibodies targeting other coagulation cascade proteins (i.e. prothrombin) or their complex with phospholipids (i.e. phosphatidylserine/prothrombin complex), or to some domains of β2GPI, have been proposed to be also relevant to APS. In fact, the value of testing for new aPL specificities in the identification of APS in thrombosis and/or pregnancy morbidity patients is currently being investigated.

Treatment of the antiphospholipid syndrome

The antiphospholipid syndrome is characterized by a combination of laboratory findings (i.e., the presence of at least one antiphospholipid antibody) and clinical manifestations (arterial and/or venous thrombosis, obstetrical complications). Long-term oral anticoagulant is recommended to prevent recurrence of both arterial and venous thrombosis, whereas (low molecular weight) heparin plus aspirin is the treatment of choice to prevent further obstetrical complications. In the rare case of catastrophic antiphospholipid syndrome, heparin plus high-dose corticosteroids plus plasma exchange is associated with the highest recovery rate. Some new, non-antithrombotic-based treatments of antiphospholipid syndrome with rituximab, autologous stem cell transplantation, or hydroxychloroquine are also reviewed.

Lupus anticoagulants in two children--bleeding due to nonphospholipid-dependent antiprothrombin antibodies

We describe two children with significant bleeding: one with multiple ecchymoses and the other with scrotal bleeding. In both patients, the activated partial thromboplastin time (APTT) was prolonged, with positivity for lupus anticoagulants (LA). However, the Owren prothrombin time (PT), usually insensitive for LA, was also prolonged. The presence of LA is associated with diverse clinical manifestations, with most patients being asymptomatic while others present venous or arterial thrombosis. Bleeding in conjunction with LA is rare and it is unusual to see prolongation of the Owren PT assay due to LA. An extended laboratory investigation of one of the patient's plasma revealed not only LA but also a specific nonphospholipid-dependent antiprothrombin antibody causing an acquired hypoprothrombinemia.

CONCLUSION

It is likely that the low prothrombin activity and not the LA caused the bleeding. The bleeding signs and symptoms in both patients subsided when the PT was normalized, although the prolonged APTT and the LA remained.

Antiphospholipid syndrome: a challenging hypercoagulable state with systemic manifestations

Antiphospholipid syndrome (APS) is a systemic disease that causes venous and arterial thrombosis in virtually any organ and is responsible for fetal losses and pregnancy disorders. Previously, APS was thought to be present mainly in patients with systemic lupus erythematosus. The spectrum of clinical manifestations is wide, because the thrombotic process may involve arterial and venous vessels of any size in any organ. At present, there is no evidence to support or refute specific treatment strategies for primary prophylaxis of thrombosis.

A novel mechanism of thrombosis in antiphospholipid antibody syndrome

Antiphospholipid antibody syndrome (APS) is an autoimmune thrombophilia mediated by autoantibodies directed against phospholipid-binding plasma proteins, mainly β2 Glycoprotein I (β2GPI)-a plasma apolipoprotein and prothrombin (PT). A subgroup of these antibodies termed "Lupus Anticoagulant" (LA) elongate in vitro the clotting times, this elongation not corrected by adding normal plasma in the detection system. The exact mechanism by which these autoantibodies induce thrombosis is not well understood. Resistance to natural anticoagulants such as protein C, impaired fibrinolysis, activation of endothelial cells to a pro-coagulant phenotype and activation of platelets, are among the mechanisms partially supported by experimental evidence. Artificially dimerized β2GPI binds tightly to platelet membrane activating them. We search for mechanisms of natural dimerization of β2GPI by proteins of the platelet membranes and found that platelet factor 4 (PF4) assembled in homotetramers binds two molecules of β2GPI and this complex is recognized by anti-β2GPI antibodies, the whole complexes being thrombogenic in terms of activating platelets as confirmed by p38MAP kinase phosphorylation and thromboxane B2 production. Of note PF4/heparin complexes are also immunogenic triggering the production of anti-PF4/heparin antibodies which activate also platelets (the so-called "heparin-induced thrombocytopenia and thrombosis syndrome", HITT). The anti-β2GPI antibodies activate platelets by their F(ab)2, while the anti-PF4/heparin by their Fc fragments. Thus PF4 is a common denominator in the pathogenesis of APS and HITT which share also clinical characteristics such as thrombocytopenia and thrombosis.

Antiphospholipid antibodies and the protein C pathway

Haemostasis is a delicate balance between procoagulant and anticoagulant processes. In the human body usually anticoagulant mechanisms prevail over procoagulant mechanisms, thereby preventing a prothrombotic state. The antiphospholipid syndrome is an example in which this balance is shifted to a more prothrombotic state due to the presence of antiphospholipid antibodies. One of the most extensively proposed pathogenic mechanisms within the antiphospholipid syndrome is the inhibition of protein C by antiphospholipid antibodies. Antiphospholipid antibodies have been described to have different actions on the protein C pathway, for example decreasing protein C and/or S plasma levels, inducing increased resistance against activated protein C and lowering thrombin levels (resulting in an impaired protein C activation). This review briefly discusses the actions of protein C in human body but mainly focuses on the effects of antiphospholipid antibodies on the protein C pathway that have been described in literature.

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.

The role of antiphospholipid antibodies toward the protein C/protein S system in venous thromboembolic disease

The association between venous thromboembolism (VTE) and antibodies anti-Protein C (PC)/Protein S (PS) is still uncertain. We performed a case-control study to determine the risk of VTE related to the presence of these auto-antibodies considered independently of the presence of lupus anticoagulant (LAC) or anti-cardiolipin antibodies (ACA). One hundred thirty-five patients with idiopathic VTE and 164 healthy subjects were enrolled. Anti-PC and anti-PS antibodies (both IgG and IgM) were assessed using commercially available ELISA kits. Among cases there was a higher prevalence of elevated anti-PC IgM antibodies than in controls (OR 2.44, 95%CI 1.00-5.94). The presence of anti-PC IgG and anti-PS IgG and IgM antibodies was also higher in cases than in controls, but the difference was not statistically significant. Only five patients had both anti-PC or anti-PS antibodies and LAC or ACA. We performed a stepwise multivariate logistic regression analysis showing that anti-PC IgM>958 percentile was a significant predictor of VTE after adjustment for LAC or ACA (OR 2.52, 95%CI 1.01-6.24)). Larger prospective studies are needed to confirm this finding.

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.

Autoantibody explosion in antiphospholipid syndrome

Antiphospholipid syndrome (APS) is characterized by thrombosis and pregnancy loss in the presence of antiphospholipid antibodies (aPL), mainly anticardiolipin, anti-beta2-glycoprotein I and lupus anticoagulant. However, similar to systemic lupus erythematosus, APS is also characterized by multiple other autoantibodies including 'non-classical' aPL, as well as other antibodies. Herein we describe the autoantigen properties, prevalence and clinical importance of 30 different antibodies in APS. Among the other antibodies characterizing APS are autoantibodies directed to platelets, glycoproteins, various coagulation factors, lamins, mitochondrial antigens and cell surface markers. Few of these autoantibodies are correlated with the presence of other antibodies, and some may have an additive role in the pro-thrombotic tendency of the syndrome. This autoantibody explosion might be important in early identification of the syndrome and its manifestations.

Activated protein C resistance determined with a thrombin generation-based test is associated with thrombotic events in patients with lupus anticoagulants

BACKGROUND

Several studies suggest that antiphospholipid antibodies interfere with the activity of activated protein C (APC). This acquired form of APC resistance has been proposed as a possible pathogenic mechanism underlying hypercoagulability associated with the antiphospholipid syndrome (APS).

OBJECTIVES

We wanted to investigate the inhibitory effect of recombinant APC (rAPC) on ex vivo thrombin generation in plasma and the modification of this effect by the presence of lupus anticoagulants (LA).

PATIENTS/METHODS

We analyzed plasmas from 81 patients with LA (52 patients fulfilling the criteria for the APS) and 91 controls. Percent inhibition of the endogenous thrombin potential (ETP) as a parameter of APC sensitivity was determined in plasmas using a thrombin generation-based APC resistance test probed with rAPC. All results were normalized using pooled normal plasma (PNP) as a reference.

RESULTS

Normalized percent inhibition of ETP by APC was lower in patients with LA [61.4%, 95% confidence interval (CI) 45.8-74.5%] compared to controls (107.8%, 95% CI: 107.1-109.3%). In patients with LA and APS, median inhibition was lower than in patients with LA without APS (44.6%, 95% CI: 30.1-55.7% vs. 78.8%, 95% CI: 73.9-95.8%). This difference also persisted when patients on warfarin therapy were excluded from the APS subgroup.

CONCLUSIONS

APC resistance can be demonstrated with a thrombin generation-based test in a majority of patients with the LA laboratory phenotype. A history of thrombotic events in patients with LA is associated with a stronger resistance to the anticoagulant effect of APC.

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.

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.

Activation of endothelial cells by antiphospholipid antibodies--a possible mechanism triggering thrombosis in patients with antiphospholipid syndrome

Antiphospholipid syndrome (APS) is an antibody-mediated hypercoagulable state characterized by recurrent venous and arterial thromboembolic events. The presence of serum antibodies are collectively termed as antiphospholipid antibodies (aPL) and is the hallmark of the disease. Interest in the pathogenesis has mostly been focused on the blood coagulation factor. However, endothelial cells might play an important role. When stimulated, cell membrane would flip to expose negatively charged phospholipids and activation markers such as adhesive molecules may appear. We consider that these changes may play an important role in the initiation of the thrombotic process when endothelial cells encounter a PL. In this study, we incubated human umbilical vein endothelial cells (HUVECs) with IgG isolated from patients with APS and found that the HUVECs were activated by the expression of negatively charged phospholipids, as shown by high annexin V binding and negative propidium iodide staining and by an increase in the level of intracellular cell adhesion molecule-1 on the cell surface. The above findings indicate that endothelial cells can be activated on exposure to aPL and trigger the thrombotic event.

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.

Current concepts on the pathogenesis of the antiphospholipid syndrome

The antiphospholipid syndrome (APS) is an important cause of acquired thrombophilia. It is characterized by the core clinical manifestations of thrombosis, either venous or arterial, and in women it can also be associated with recurrent fetal loss. The detection of persistently elevated levels of antiphospholipid antibodies (aPL Abs) is a requisite laboratory feature for the diagnosis to be made. The dominant antigenic targets in APS are beta 2-glycoprotein I (β2-GPI) and prothrombin. There is an accumulating body of experimental evidence that suggests that specific subgroups of aPL Abs may directly contribute to disease pathogenesis. This review critically examines the experimental evidence underlying the various propositions made to explain how these antibodies may predispose to disease in humans. Furthermore, it also examines the evidence relating to the immunologic mechanisms that may contribute to the breakage of peripheral tolerance in this disorder. Delineating the strengths and limitations of the experimental evidence accumulated thus far will hopefully stimulate further experimentation toward achieving the ultimate goal of precisely defining the dominant pathogenic mechanisms operational in APS. This may pave the way for the development of improved therapies.