Impaired thrombin generation in beta 2-glycoprotein I null mice

Antiphospholipid syndrome in pregnancy

PURPOSE OF REVIEW

Antiphospholipid syndrome is widely recognized as a risk factor for numerous obstetric complications including miscarriage, intrauterine growth restriction, preeclampsia, fetal death and preterm labour. The many recent changes in concept regarding this syndrome, the role of the relevant antibodies, mechanism of action, diagnosis and treatment are assessed in this review.

RECENT FINDINGS

In recent years, our understanding of antiphospholipid syndrome has grown. The antigen has become better defined and is now thought to be beta2 glycoprotein 1. The 'classical' antibodies, lupus anticoagulant and anticardiolipin antibody are known to be pathogenic even when passively transferred to animal hosts. It seems, however, that the pathogenic antibodies are those directed towards beta2 glycoprotein 1, and that those which are directed to phospholipids without binding to beta2 glycoprotein 1 may not be pathogenic, but merely epiphenomena. The treatment of this condition has also been changed due to the influence of randomized trials in which heparin or low molecular weight heparin has replaced the use of steroids.

SUMMARY

There are numerous pitfalls in managing this condition. As beta2 glycoprotein 1 antibodies are not usually tested, the condition may be over diagnosed or misdiagnosed. Similarly, the results of treatment are not usually corrected for confounding factors such as fetal chromosomal aberrations. In the absence of other confounding factors low molecular weight heparins are probably the treatment of choice.

Beta 2-Glycoprotein I binds factor XI and inhibits its activation by thrombin and factor XIIa: loss of inhibition by clipped beta 2-glycoprotein I

Activation of factor XI (FXI) by thrombin in vivo plays a role in coagulation by providing an important positive feedback mechanism for additional thrombin generation. FXI is activated in vitro by thrombin, or FXIIa in the presence of dextran sulfate. In this report, we investigated the effect of beta(2)-glycoprotein I (beta(2)GPI) on the activation of FXI. beta(2)GPI bound FXI in vitro and inhibited its activation to FXIa by thrombin and FXIIa. The affinity of the interaction between beta(2)GPI and FXI was equivalent to the interaction between FXI and high molecular weight kininogen. Inhibition of FXI activation occurred with lower concentrations of beta(2)GPI than found in human plasma. Proteolytic clipping of beta(2)GPI by plasmin abolished its inhibition of FXI activation. The results suggest a mechanism of regulation whereby physiological concentrations of beta(2)GPI may attenuate thrombin generation in vivo by inhibition of FXI activation. Plasmin cleavage of beta(2)GPI provides a negative feedback that counteracts its inhibition of FXI activation.

Beta 2 glycoprotein I-function in health and disease

Beta-2 glycoprotein I (beta2GPI) is the principal target of autoantibodies in the antiphospholipid syndrome (APS). It is abundant in human plasma and shares high homology between different mammalian species. Although the exact physiological function of beta2GPI has not been fully elucidated, several interactions have been described with other proteins and with negatively charged surfaces, such as anionic phospholipids, dextran and heparin. beta2GPI is involved in the coagulation pathway, exerting both procoagulant and anticoagulant activities. Plasma from beta2GPI-deficient mice exhibits impaired thrombin generation in vitro. Recently, it has been demonstrated that beta2GPI binds factor (F) XI in vitro at concentrations lower than those of the protein in human plasma, and this binding inhibits FXI activation to FXIa by thrombin and FXIIa. Proteolytic cleavage of the fifth domain of beta2GPI abolishes its inhibition of FXI activation and results in reduced ability of the cleaved beta2GPI to bind phospholipids. It retains its ability to bind FXI. In vivo activation of FXI by thrombin is thought to be an important mechanism by which coagulation is accelerated via components of the contact activation pathway. Thus beta2GPI may attenuate the contact activation pathway by inhibiting activation of FXI by thrombin. Moreover, because beta2GPI is the dominant autoantigen in patients with APS, dysregulation of this pathway by autoantibodies may be an important mechanism for thrombosis in patients with APS.

Anti-beta(2)-glycoprotein I antibodies and the antiphospholipid syndrome

The mechanism of antiphospholipid syndrome (APS) development is still not completely understood. Accumulating evidence indicates that beta(2)-glycoprotein I (beta(2)GPI), is the major target antigen for antiphospholipid (aPL) antibodies, which play a crucial role in the pathogenesis of this autoimmune condition. Knowledge about the molecular structure, biological characteristics and function of beta(2)GPI has been expanding in recent years. In this review, we have focused on some recent important findings on beta(2)GPI and anti-beta(2)GPI antibodies in patients and animal models with APS.

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.

Platelet-endothelial interactions: sepsis, HIT, and antiphospholipid syndrome

Acquired abnormalities in platelets, endothelium, and their interaction occur in sepsis, immune heparin-induced thrombocytopenia (HIT), and the antiphospholipid syndrome. Although of distinct pathogeneses, these three disorders have several clinical features in common, including thrombocytopenia and the potential for life- and limb-threatening thrombotic events, ranging from microvascular (sepsis > antiphospholipid > HIT) to macrovascular (HIT > antiphospholipid > sepsis) thrombosis, both venous and arterial. In Section I, Dr. William Aird reviews basic aspects of endothelial-platelet interactions as a springboard to considering the common problem of thrombocytopenia (and its mechanism) in sepsis. The relationship between thrombocytopenia and other aspects of the host response in sepsis, including activation of coagulation/inflammation pathways and the development of organ dysfunction, is discussed. Practical issues of platelet count triggers and targeted use of activated protein C concentrates are reviewed. In Section II, Dr. Theodore Warkentin describes HIT as a clinicopathologic syndrome, i.e., the diagnosis should be based on the concurrence of an appropriate clinical picture together with detection of platelet-activating and/or platelet factor 4-dependent antibodies (usually in high levels). HIT is a profound prothrombotic state (odds ratio for thrombosis, 20-40), and the risk for thrombosis persists for a time even when heparin is stopped. Thus, pharmacologic control of thrombin (or its generation), and postponing oral anticoagulation pending substantial resolution of thrombocytopenia, is appropriate. Indeed, coumarin-associated protein C depletion during uncontrolled thrombin generation of HIT can explain limb loss (coumarin-associated venous limb gangrene) or skin necrosis syndromes in some patients. In Section III, Dr. Jacob Rand presents the most recent concepts on the mechanisms of thrombosis in the antiphospholipid syndrome, and focuses on the role of beta(2)-glycoprotein I as a major antigenic target in this condition. Diagnosis of the syndrome is often complicated because the clinical laboratory tests to identify this condition have been empirically derived. Dr. Rand addresses the practical aspects of current testing for the syndrome and current recommendations for treating patients with thrombosis and with spontaneous pregnancy losses.

A functional polymorphism at the transcriptional initiation site in beta2-glycoprotein I (apolipoprotein H) associated with reduced gene expression and lower plasma levels of beta2-glycoprotein I

Human beta2-glycoprotein I (beta2GPI), also known as apolipoprotein H, has been implicated in haemostasis and the production of anti-phospholipid antibodies. There is a wide range of interindividual variation in beta2GPI plasma levels that is thought to be under genetic control, but its molecular basis remains unknown. To understand the genetic basis of beta2GPI variation, we analyzed the 5' flanking region of the beta2GPI gene for mutation detection by DHPLC and identified a point mutation at the transcriptional initiation site (-1C-->A) with a carrier frequency of 12.1%. The mutation was associated with significantly lower beta2GPI plasma levels (P < 0.0001) and low occurrence of anti-phospholipid antibodies in lupus patients (4.8% antibody-positive group vs. 16.6% in the antibody-negative group; P = 0.019). Northern blot analysis confirmed that the -1C-->A mutation was associated with lower mRNA levels and it reduced the reporter (luciferase) gene expression by twofold. Electrophoretic gel mobility shift assay (EMSA) revealed that the -1C-->A mutation disrupts the binding for crude hepatic nuclear extracts and purified TFIID. These results suggest that the substitution of C with A at the beta2GPI transcriptional initiation site is a causative mutation that affects its gene expression at the transcriptional level and ultimately beta2GPI plasma levels and the occurrence of anti-phospholipid antibodies.

Antiphospholipid antibodies: biological basis and prospects for treatment

Antiphospholipid antibodies (aPL) are autoantibodies that are associated with recurrent reproductive failure and thrombotic disease. There are two well-characterised aPL, lupus anticoagulant and anticardiolipin antibodies. aPL were originally thought to bind to negatively-charged phospholipids but it is now clear that the title aPL is a misnomer and that the antigens for these autoantibodies are actually phospholipid-binding proteins. Chief amongst these phospholipid-binding proteins are prothrombin and beta(2) glycoprotein I. This review concentrates on the role of beta(2) glycoprotein I in the reproductive failure caused by aPL. Exactly how aPL cause reproductive failure remains unknown but there is emerging evidence that the antibodies may have several different adverse effects on trophoblasts. There is also evidence questioning the traditional hypothesis that fetal demise is secondary to thrombosis of the utero-placental circulation. Heparin is commonly used to treat pregnant women with aPL but if these antibodies do not cause fetal demise primarily by a thrombotic mechanism a question must be raised over the role of heparin. However, heparin binds to many proteins including beta(2) glycoprotein I and it is possible that the reported beneficial effects of heparin in aPL-affected pregnancies may be due to the ability of heparin to prevent the interaction of aPL and beta(2) glycoprotein I.

Bacterial induction of autoantibodies to beta2-glycoprotein-I accounts for the infectious etiology of antiphospholipid syndrome

The antiphospholipid syndrome (APS) is characterized by the presence of pathogenic autoantibodies against beta2-glycoprotein-I (beta2GPI). The factors causing production of anti-beta2GPI remain unidentified, but an association with infectious agents has been reported. Recently, we identified a hexapeptide (TLRVYK) that is recognized specifically by a pathogenic anti-beta2GPI mAb. In the present study we evaluated the APS-related pathogenic potential of microbial pathogens carrying sequences related to this hexapeptide. Mice immunized with a panel of microbial preparations were studied for the development of anti-beta2GPI autoantibodies. IgG specific to the TLRVYK peptide were affinity purified from the immunized mice and passively infused intravenously into naive mice at day 0 of pregnancy. APS parameters were evaluated in the infused mice on day 15 of pregnancy. Following immunization, high titers of antipeptide [TLRVYK] anti-beta2GPI Ab's were observed in mice immunized with Haemophilus influenzae, Neisseria gonorrhoeae, or tetanus toxoid. The specificity of binding to the corresponding target molecules was confirmed by competition and immunoblot assays. Naive mice infused with the affinity-purified antipeptide Ab's had significant thrombocytopenia, prolonged activated partial thromboplastin time and elevated percentage of fetal loss, similar to a control group of mice immunized with a pathogenic anti-beta2GPI mAb. Our study establishes a mechanism of molecular mimicry in experimental APS, demonstrating that bacterial peptides homologous with beta2GPI induce pathogenic anti-beta2GPI Ab's along with APS manifestations.

Bacterial induction of autoantibodies to beta2-glycoprotein-I accounts for the infectious etiology of antiphospholipid syndrome

The antiphospholipid syndrome (APS) is characterized by the presence of pathogenic autoantibodies against beta2-glycoprotein-I (beta2GPI). The factors causing production of anti-beta2GPI remain unidentified, but an association with infectious agents has been reported. Recently, we identified a hexapeptide (TLRVYK) that is recognized specifically by a pathogenic anti-beta2GPI mAb. In the present study we evaluated the APS-related pathogenic potential of microbial pathogens carrying sequences related to this hexapeptide. Mice immunized with a panel of microbial preparations were studied for the development of anti-beta2GPI autoantibodies. IgG specific to the TLRVYK peptide were affinity purified from the immunized mice and passively infused intravenously into naive mice at day 0 of pregnancy. APS parameters were evaluated in the infused mice on day 15 of pregnancy. Following immunization, high titers of antipeptide [TLRVYK] anti-beta2GPI Ab's were observed in mice immunized with Haemophilus influenzae, Neisseria gonorrhoeae, or tetanus toxoid. The specificity of binding to the corresponding target molecules was confirmed by competition and immunoblot assays. Naive mice infused with the affinity-purified antipeptide Ab's had significant thrombocytopenia, prolonged activated partial thromboplastin time and elevated percentage of fetal loss, similar to a control group of mice immunized with a pathogenic anti-beta2GPI mAb. Our study establishes a mechanism of molecular mimicry in experimental APS, demonstrating that bacterial peptides homologous with beta2GPI induce pathogenic anti-beta2GPI Ab's along with APS manifestations.

Molecular pathogenesis of the antiphospholipid syndrome

The antiphospholipid (aPL) syndrome is an acquired autoimmune disorder of unknown etiology in which patients present with thrombosis together with laboratory evidence for antibodies in blood that recognize anionic phospholipid-protein complexes. The main antigenic target for the aPL antibodies has been identified to be beta(2) glycoprotein I (beta(2)GPI), a phospholipid-binding protein. The high affinity of aPL antibody-beta(2)GPI complex for phospholipid membranes seems to be a critical step in the mechanism of this disease. This review focuses on some of the major mechanisms that have been proposed to explain this disorder.

Beta2-glycoprotein I and atherosclerosis

Appreciation of the multifactorial nature of atherosclerosis requires a broad understanding of the mechanisms that underlie its pathogenesis. Autoimmune factors have recently been shown to be associated with the initiation and progression of atherosclerosis. In this context, modified lipoproteins were explored because of their de-novo occurrence within the vessel wall, and heat shock proteins are also being reported by several authors as triggers of autoimmune-like reactions that associate with atherosclerosis. Antiphospholipid antibodies in general and anti-beta2-glycoprotein I (beta2GPI) antibodies in particular have been shown to confer a procoagulant tendency in humans, either in the presence or the absence of the antiphospholipid syndrome. These findings and the ability of antibodies to beta2GPI to activate monocytes and endothelial cells led us to consider whether they are proatherogenic. In a series of studies it was shown that inducing an immune response to beta2GPI in atherosclerosis-prone mice accelerated atherosclerosis. We also demonstrated the abundance of beta2GPI in the atheroma, in conjunction with immunopotent cells. Moreover, when beta2GPI-reactive lymph node and spleen cells were transferred to LDL-receptor-deficient mice they promoted fatty streak formation, proving a direct proatherogenic role for beta2GPI-specific lymphocytes. Perhaps the most important implications of the existence of antigen-specific immune reactions within the atheroma is the ability to exploit them for the purpose of selective immunomodulation. Indeed, we have found that inducing immunological tolerance to beta2GPI by prior oral feeding with the antigen resulted in a significant reduction in the extent of atherosclerotic lesions. Thus, beta2GPI is a candidate player in the atherosclerotic plaque, and can possibly be employed as an immunomodulator of plaque progression.