Evolutionary conservation of the lipopolysaccharide binding site of β2-glycoprotein I

Antiphospholipid antibody testing

Antiphospholipid antibodies (aPL) are a family of autoantibodies targeting phospholipid-binding proteins and are associated with several clinical settings, and most notably define the antiphospholipid syndrome (APS). These antibodies can be identified using a variety of laboratory tests, which include both solid-phase immunological assays and functional clotting assays that detect lupus anticoagulants (LA). aPLs are linked to a range of adverse medical conditions, such as thrombosis and complications affecting the placenta and fetus, potentially leading to morbidity and mortality. The specific aPL identified, along with the pattern of reactivity, correlates with the severity of these conditions. Therefore, laboratory testing for aPL is crucial for evaluating the risk of complications and for fulfilling certain classification criteria for APS, which are also applied as diagnostic markers in medical practice. This review provides an overview of the available laboratory tests currently for measuring aPL and discusses their clinical implications.

Interaction of the antiphospholipid syndrome autoantigen beta-2 glycoprotein I with DNA and neutrophil extracellular traps

Beta-2 glycoprotein I (β2GPI) is a phospholipid-binding plasma protein and prominent autoantigen in antiphospholipid syndrome (APS). Here, we tested the hypothesis that β2GPI might bind to not only phospholipids, but also cell-free DNA and neutrophil extracellular traps (NETs). We report that β2GPI interacts with cell-free DNA from different species, as well as NETs, in a dose-dependent manner, retarding their migration in an agarose-gel electrophoretic mobility shift assay. We confirm the direct binding interaction of β2GPI with DNA and NETs by ELISA. We also demonstrate that β2GPI colocalizes with NET strands by immunofluorescence microscopy. Finally, we provide evidence that β2GPI-DNA complexes can be detected in the plasma of APS patients, where they positively correlate with an established biomarker of NET remnants. Taken together, our findings indicate that β2GPI interacts with DNA and NETs, and suggest that this interaction may play a role as a perpetuator and/or instigator of autoimmunity in APS.

The pathogenesis of obstetric APS: a 2023 update

The antiphospholipid syndrome (APS) is an autoimmune disease characterized by the persistent presence of antibodies directed against phospholipids and phospholipid-binding proteins that are associated with thrombosis and pregnancy-related morbidity. The latter includes fetal deaths, premature birth and maternal complications. In the early 1990s, a distinct set of autoantibodies, termed collectively antiphospholipid antibodies (aPL), were identified as the causative agents of this disorder. Subsequently histological analyses of the placenta from APS pregnancies revealed various abnormalities, including inflammation at maternal-fetal interface and poor placentation manifested by reduced trophoblast invasion and limited uterine spiral artery remodeling. Further preclinical investigations identified the molecular targets of aPL and the downstream intracellular pathways of key placental cell types. While these discoveries suggest potential therapeutics for this disorder, definitive clinical trials have not been completed. This concise review focuses on the recent developments in the field of basic and translational research pursuing novel mechanisms underlying obstetric APS.

From Co-Infections to Autoimmune Disease via Hyperactivated Innate Immunity: COVID-19 Autoimmune Coagulopathies, Autoimmune Myocarditis and Multisystem Inflammatory Syndrome in Children

Neutrophilia and the production of neutrophil extracellular traps (NETs) are two of many measures of increased inflammation in severe COVID-19 that also accompany its autoimmune complications, including coagulopathies, myocarditis and multisystem inflammatory syndrome in children (MIS-C). This paper integrates currently disparate measures of innate hyperactivation in severe COVID-19 and its autoimmune complications, and relates these to SARS-CoV-2 activation of innate immunity. Aggregated data include activation of Toll-like receptors (TLRs), nucleotide-binding oligomerization domain (NOD) receptors, NOD leucine-rich repeat and pyrin-domain-containing receptors (NLRPs), retinoic acid-inducible gene I (RIG-I) and melanoma-differentiation-associated gene 5 (MDA-5). SARS-CoV-2 mainly activates the virus-associated innate receptors TLR3, TLR7, TLR8, NLRP3, RIG-1 and MDA-5. Severe COVID-19, however, is characterized by additional activation of TLR1, TLR2, TLR4, TLR5, TLR6, NOD1 and NOD2, which are primarily responsive to bacterial antigens. The innate activation patterns in autoimmune coagulopathies, myocarditis and Kawasaki disease, or MIS-C, mimic those of severe COVID-19 rather than SARS-CoV-2 alone suggesting that autoimmunity follows combined SARS-CoV-2-bacterial infections. Viral and bacterial receptors are known to synergize to produce the increased inflammation required to support autoimmune disease pathology. Additional studies demonstrate that anti-bacterial antibodies are also required to account for known autoantigen targets in COVID-19 autoimmune complications.

Anti-phosphatidylserine/prothrombin complex antibodies (aPS/PT) increase the risk for thrombosis based on lupus anticoagulant positivity

BACKGROUND

Lupus anticoagulants (LA) increase the risk of thrombotic and obstetric events in patients with antiphospholipid syndrome than in those with other antiphospholipid antibodies. Anti-phosphatidylserine/prothrombin (aPS/PT) complex antibodies are thought to cause LA positivity. Therefore, we aimed to explore whether aPS/PT antibodies could prolong phospholipid (PL)-dependent clotting time and increase the risk of thrombosis or pregnancy complications based on LA positivity.

METHODS

We recruited 222 patients with positive LA and estimated their aPS/PT, anticardiolipin (aCL), anti-β2-glycoprotein I (aβ2GPI), and anti-β2GPI domain I (anti-D1) antibody (IgM and IgG) titers and PL-dependent clotting time.

RESULTS

PT was longer in aPS/PT IgG-positive patients than in aPS/PT IgM-negative patients (P < 0.001), while there was no significant difference between aPS/PT IgM-positive and IgM-negative patients (P = 0.100). Both SCT-S and dRVVT-S were prolonged in aPS/PT (IgG and IgM)-positive patients compared to aPS/PT-negative patients (P < 0.001, P = 0.010, P < 0.001, P = 0.002, respectively). Similarly, the associations between aPS/PT IgG or IgM antibody titers and SCT-S or dRVVT-S were significant. SCT-C and dRVVT-C did not show any significant differences. The incidence of thrombosis in the aPS/PT IgG-positive group was much higher than that in the IgG-negative group (P = 0.012). Likewise, the incidence of thrombosis was higher in the anti-D1- and aPS/PT IgG-positive patients than in the negative controls (40 % vs 14.3 %, χ² = 3.934, P = 0.047). Furthermore, the aPS/PT IgG-positive group showed the strongest association with thrombosis [OR 2.584, 95 % CI (1.213, 5.505)].

CONCLUSION

The aPS/PT antibodies prolonged PL-dependent clotting time, especially SCT-S and dRVVT-S. In addition, the presence of aPS/PT IgG antibodies increased the risk of thrombosis in LA positivity.

Effects of anti-beta 2-glycoprotein 1 antibodies and its association with pregnancy-related morbidity in antiphospholipid syndrome

Antiphospholipid syndrome (APS) is an autoimmune disease characterized by venous, arterial, or small-vessel thrombosis and/or pregnancy-related morbidity, associated with persistent positivity of antiphospholipid antibodies (aPL). Pregnancy-related morbidity in APS patients is characterized by unexplained fetal deaths, premature birth of morphologically normal newborns, and/or consecutive pregnancy losses before the 10^th week of gestation. Beta 2-glycoprotein 1 (ß2GP1) is the main antigen recognized by aPL and plays an essential role in the pathogenesis of APS. Antibodies against ß2GP1 (aß2GP1) are involved in damage-generating mechanisms in APS due to their interaction with trophoblasts, decidua, and endothelial cells. aß2GP1 might be used as a prognostic tool for obstetric risk stratification and ß2GP1 could be a target for molecular-targeted treatment to prevent pregnancy morbidity in APS. This review describes these aspects of aß2GP1, including effects on different cellular targets, its association with the severity of obstetric manifestations and the potential of ß2GP1-targeted therapies for APS.

Quantitation of Total and Free Thiol β2-Glycoprotein I Levels for Diagnostic and Prognostic Purposes in the Antiphospholipid Syndrome

β₂-Glycoprotein I is the major autoantigen in the antiphospholipid syndrome (APS), a prothrombotic disorder characterized by the occurrence of either venous or arterial thrombosis. In women it is also associated with an increased risk of obstetric complications such as recurrent miscarriages. We have identified that the plasma protein β₂-glycoprotein I in healthy individuals exists in an optimal ratio between two distinct forms, an oxidized and free thiol, reduced form. This ratio is disrupted in pathophysiological conditions associated with increased oxidative stress such as the APS, but also in the setting of age-related macular degeneration and gram-negative sepsis. We have developed assays that quantify plasma/serum levels of total and free thiol β₂-glycoprotein I which can potentially be used for risk stratification and prognostic purposes in the early stages of the aforementioned conditions.

Protective Effects of Reduced Beta 2 Glycoprotein I on Liver Injury in Streptozotocin (STZ)-Diabetic Rats by Activation of AMP-Activated Protein Kinase

Background

Protective effects of reduced beta 2 glycoprotein I (Rβ2GPI) against vascular injury of diabetes mellitus have been extensively investigated. However, the effects of Rβ2GPI on liver injury in diabetic animals have not been reported.

Material/Methods

A diabetic rat model of was produced by systemic injection of streptozotocin (STZ). Rats were divided into a normal control group, a model group, and an Rβ2GPI treatment group (N=6 in each group). After treatments, blood serum and liver tissue were collected to test the protection of Rβ2GPI. AMP-activated protein kinase (AMPK) was detected by immunohistochemistry and Western blotting.

Results

Our results revealed that Rβ2GPI reduced blood glucose, serum creatinine, and urea nitrogen levels, as well as serum inflammation cytokines, including interleukin (IL)-6, tumor necrosis factor (TNF)-a and C-reactive protein in the diabetic rats. Importantly, Rβ2GPI prevented liver injury in the diabetic rats as confirmed by hematoxylin-eosin (H&E) staining, alanine transaminase, aspartate transaminase, and gamma-glutamyl transferase. Reactive oxygen species (ROS) were promoted by diabetic modeling and were attenuated by Rβ2GPI administration. Moreover, Rβ2GPI significantly reduced liver catalase, malondialdehyde, and superoxide dismutase levels in the diabetic rats. Rβ2GPI reduced liver glycolipid storage in STZ diabetic rats. Both immunohistochemistry and Western blotting demonstrated that Rβ2GPI promoted AMPK phosphorylation in the diabetic rats.

Conclusions

Our data proved that Rβ2GPI prevented liver injury in diabetic rats, likely through activating the AMPK signaling pathway.

Antiphospholipid syndrome

Antiphospholipid syndrome (APS) is an autoimmune disease characterized by the presence of antiphospholipid antibodies, such as lupus anticoagulant, anticardiolipin antibodies and anti-β2-glycoprotein 1 antibodies. APS can present with a variety of clinical phenotypes, including thrombosis in the veins, arteries and microvasculature as well as obstetrical complications. The pathophysiological hallmark is thrombosis, but other factors such as complement activation might be important. Prevention of thrombotic manifestations associated with APS includes lifestyle changes and, in individuals at high risk, low-dose aspirin. Prevention and treatment of thrombotic events are dependent mainly on the use of vitamin K antagonists. Immunosuppression and anticomplement therapy have been used anecdotally but have not been adequately tested. Pregnancy morbidity includes unexplained recurrent early miscarriage, fetal death and late obstetrical manifestation such as pre-eclampsia, premature birth or fetal growth restriction associated with placental insufficiency. Current treatment to prevent obstetrical morbidity is based on low-dose aspirin and/or low-molecular-weight heparin and has improved pregnancy outcomes to achieve successful live birth in >70% of pregnancies. Although hydroxychloroquine and pravastatin might further improve pregnancy outcomes, prospective clinical trials are required to confirm these findings.

Mechanisms of thrombosis in systemic lupus erythematosus and antiphospholipid syndrome

The presence of antiphospholipid antibodies is one of the most common acquired risk factors for thrombosis. Antiphospholipid antibodies is a collective term for a set of autoantibodies with closely related but different specificity. Experiments in which isolated patient antibodies were injected into mice have shown that a specific subset of autoantibodies, those directed against the first domain of plasma protein β₂-glycoprotein I, can explain the increased risk of thrombosis. Experiments performed with these mice have shown that autoantibodies against β₂-glycoprotein I bind to and activate cells such as endothelial cells, monocytes, and platelets. Activation of these cells, all involved in the regulation of hemostasis, results in a shift towards a prothrombotic state. How this process is regulated, whether this is the only mechanism involved, and whether this is the only subpopulation responsible for the increased thrombotic risk is unknown. In this review, we will critically discuss what is known and what is debatable on the pathophysiology of antiphospholipid syndrome.

Effects of reduced β2 glycoprotein I on high glucose‑induced cell death in HUVECs

Reduced β2 glycoprotein I (β2GPI) has been demonstrated to exhibit a beneficial effect in diabetic atherosclerosis and retinal neovascularization. However, the effect of reduced β2GPI on vascular disorders in diabetic mellitus (DM) remains to be elucidated. The present study established a high glucose‑induced injury model using human umbilical cords veins (HUVECs) and evaluated the protective effects of reduced β2GPI against the injury. The data demonstrated that a low concentration of reduced β2GPI (0.5 µM) mitigated high glucose‑induced cell loss, decreased nitric oxide (NO) production and resulted in calcium overloading. Mechanically, reduced β2GPI additionally reversed high glucose‑induced phosphatase and tensin homolog (PTEN) accumulation, decrease of protein kinase B phosphorylation and nitric oxide synthase activity, and increase of cyclooxygenase‑2 activity. It was further confirmed that PTEN inhibitor‑bpV (1 µM) exhibited similar effects to those resulting from reduced β2GPI. Overall, the data revealed that reduced β2GPI exerts protective effects from glucose‑induced injury in HUVECs, potentially via decreasing PTEN levels. The present study suggests reduced β2GPI may act as a novel therapeutic strategy for the treatment of vascular disorders in DM.

β2GP1, Anti-β2GP1 Antibodies and Platelets: Key Players in the Antiphospholipid Syndrome

Anti-beta 2 glycoprotein 1 (anti-β2GP1) antibodies are commonly found in patients with autoimmune diseases such as the antiphospholipid syndrome (APS) and systemic lupus erythematosus (SLE). Their presence is highly associated with increased risk of vascular thrombosis and/or recurrent pregnancy-related complications. Although they are a subtype of anti-phospholipid (APL) antibody, anti-β2GP1 antibodies form complexes with β2GP1 before binding to different receptors associated with anionic phospholipids on structures such as platelets and endothelial cells. β2GP1 consists of five short consensus repeat termed "sushi" domains. It has three interchangeable conformations with a cryptic epitope at domain 1 within the molecule. Anti-β2GP1 antibodies against this cryptic epitope are referred to as 'type A' antibodies, and have been suggested to be more strongly associated with both vascular and obstetric complications. In contrast, 'type B' antibodies, directed against other domains of β2GP1, are more likely to be benign antibodies found in asymptomatic patients and healthy individuals. Although the interactions between anti-β2GP1 antibodies, β2GP1, and platelets have been investigated, the actual targeted metabolic pathway(s) and/or receptor(s) involved remain to be clearly elucidated. This review will discuss the current understanding of the interaction between anti-β2GP1 antibodies and β2GP1, with platelet receptors and associated signalling pathways.

The Fifth Domain of Beta 2 Glycoprotein I Protects from Natural IgM Mediated Cardiac Ischaemia Reperfusion Injury

Reperfusion after a period of ischemia results in reperfusion injury (IRI) which involves activation of the inflammatory cascade. In cardiac IRI, IgM natural antibodies (NAb) play a prominent role through binding to altered neoepitopes expressed on damaged cells. Beta 2 Glycoprotein I (β2GPI) is a plasma protein that binds to neoepitopes on damaged cells including anionic phospholipids through its highly conserved Domain V. Domain I of β2GPI binds circulating IgM NAbs and may provide a link between the innate immune system, IgM NAb binding and cardiac IRI. This study was undertaken to investigate the role of Β2GPI and its Domain V in cardiac IRI using wild-type (WT), Rag-1 ^-/- and β2GPI deficient mice. Compared with control, treatment with Domain V prior to cardiac IRI prevented binding of endogenous β2GPI to post-ischemic myocardium and resulted in smaller myocardial infarction size in both WT and β2GPI deficient mice. Domain V treatment in WT mice also resulted in less neutrophil infiltration, less apoptosis and improved ejection fraction at 24 h. Rag-1 -/- antibody deficient mice reconstituted with IgM NAbs confirmed that Domain V prevented IgM NAb induced cardiac IRI. Domain V remained equally effective when delivered at the time of reperfusion which has therapeutic clinical relevance.Based upon this study Domain V may function as a universal inhibitor of IgM NAb binding in the setting of cardiac IRI, which offers promise as a new therapeutic strategy in the treatment of cardiac IRI.

Fortifying the barrier: the impact of lipid A remodelling on bacterial pathogenesis

Gram-negative bacteria decorate their outermost surface structure, lipopolysaccharide, with elaborate chemical moieties, which effectively disguises them from immune surveillance and protects them from the onslaught of host defences. Many of these changes occur on the lipid A moiety of lipopolysaccharide, a component that is crucial for host recognition of Gram-negative infection. In this Review, we describe the regulatory mechanisms controlling lipid A modification and discuss the impact of modifications on pathogenesis, bacterial physiology and bacterial interactions with the host immune system.

Recent advances in the diagnosis and treatment of heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is a drug-mediated, prothrombotic disorder caused by immunization against platelet factor 4 (PF4) after complex formation with heparin or other polyanions. After their binding to PF4/heparin complexes on the platelet surface, HIT antibodies are capable of intravascular platelet activation by cross-linking Fcγ receptor IIA leading to a platelet count decrease and/or thrombosis. Diagnosis of HIT is often difficult. This, and the low specificity of the commercially available immunoassays, leads currently to substantial overdiagnosis of HIT. Timing of onset, the moderate nature of thrombocytopenia, and the common concurrence of thrombosis are very important factors, which help to differentiate HIT from other potential causes of thrombocytopenia. A combination of a clinical pretest scoring system and laboratory investigation is usually necessary to diagnose HIT. Although HIT is considered to be a rare complication of heparin treatment, the very high number of hospital inpatients, and increasingly also hospital outpatients receiving heparin, still result in a considerable number of patients developing HIT. If HIT occurs, potentially devastating complications such as life-threatening thrombosis make it one of the most serious adverse drug reactions. If HIT is strongly suspected, all heparin must be stopped and an alternative nonheparin anticoagulant started at a therapeutic dose to prevent thromboembolic complications. However, the nonheparin alternative anticoagulants bear a considerable bleeding risk, especially if given to patients with thrombocytopenia due to other reasons than HIT. While established drugs for HIT are disappearing from the market (lepirudin, danaparoid), bivalirudin, fondaparinux and potentially the new anticoagulants such as dabigatran, rivaroxaban and apixaban provide new treatment options.

Antiphospholipid antibody syndrome

PURPOSE OF REVIEW

Antiphospholipid syndrome is a rare cause of ocular vaso-occlusive disease, but is associated with significant systemic morbidity and mortality. Thus, early diagnosis and treatment is essential.

RECENT FINDINGS

Although the pathophysiology of antiphospholipid syndrome continues to be poorly understood, there has been continued progress with regard to the relationship between antiphospholipid antibody and its target, β-2-glycoprotein I. Due to numerous limitations with standard serologic evaluation, new approaches to the evaluation of patient serum are being considered. New guidelines for the treatment and management of antiphospholipid antibody syndrome have been established by the 13th International Committee on Antiphospholipid Antibodies.

SUMMARY

A better understanding of the pathophysiology behind antiphospholipid antibody syndrome has led to novel approaches in the diagnosis and treatment of this disease.

Genetic determinants of plasma β₂-glycoprotein I levels: a genome-wide association study in extended pedigrees from Spain

BACKGROUND

β2 -Glycoprotein I (β2 -GPI), also designated apolipoprotein H, is a 50-kDa protein that circulates in blood at high concentrations, playing important roles in autoimmune diseases, hemostasis, atherogenesis, and angiogenesis, as well as in host defense against bacteria and in protein/cellular waste removal. Plasma β2 -GPI levels have a significant genetic component (heritability of ~ 80%).

OBJECTIVES

To present the results of a genome-wide association study for plasma β2 -GPI levels in a set of extended pedigrees from the Genetic Analysis of Idiopathic Thrombophilia (GAIT) Project.

PATIENTS/METHODS

A total of 306 individuals for whom β2 -GPI plasma measurements were available were typed for 307,984 single-nucleotide polymorphisms (SNPs) with the Infinium 317k Beadchip (Illumina). Association with the β2 -GPI phenotype was investigated through variance component analysis, and the most significant results were followed up for association with coronary artery disease (CAD) in an independent in silico analysis involving 5765 CAD cases from the PROCARDIS Project and 7264 controls from the PROCARDIS Project and the Wellcome Trust Case Control Consortium (WTCCC) collection.

RESULTS

After correction for multiple testing, three SNPs located in/around two genes (ELF5 and SCUBE2) reached genome-wide significance. Moreover, an SNP in the APOH gene showed suggestive association with the β2 -GPI phenotype. Some of the identified genes are plausible biological candidates, as they are actually or potentially involved in inflammatory processes.

CONCLUSIONS

Our results represent a first step towards identifying common variants reflecting the genetic architecture influencing plasma β2 -GPI levels, and warrant further validation by functional experiments, as the functions of some of the discovered loci are still unknown.

Pathophysiology of thrombotic APS: where do we stand?

The antiphospholipid syndrome (APS) is diagnosed when patients with thrombotic complications or foetal losses have elevated levels of antiphospholipid antibodies in their plasmas. The term APS is confusing, because the pathogenic auto-antibodies are not directed against phospholipids but towards a plasma protein, β(2)-glycoprotein I. For many years the reason why auto-antibodies against β(2)-glycoprotein I were pro-thrombotic was unclear, because man and mice deficient in β(2)-glycoprotein I do not express a clear phenotype. Animal models in which passive transfer of patient antibodies into mice resulted in an increased thrombotic response have provided novel insights in the importance of this protein in the pathology of APS.

Conformation of beta2glycoprotein I and its effect on coagulation

β(2)glycoprotein I (β(2)GPI) is the major antigen in the antiphospholipid syndrome. It has been shown that β(2)GPI can adapt to different conformations, a circular, a S-shaped and a J-shaped conformation. In literature anticoagulant properties of β(2)GPI have been indicated, though there is no consensus on how β(2)GPI exerts a certain action. This article will first review existing data on the conformation of β(2)GPI. In addition, we will investigate whether the conformation of β(2)GPI plays a role in in the proposed anticoagulant activity of β(2)GPI. We investigated the effect of native β(2)GPI and phospholipid-bound β(2)GPI on thrombin generation (TG). Native β(2)GPI was found to have no significant effect on the TG regardless of the concentration of tissue factor. On the contrary, β(2)GPI preincubated with phospholipids significantly inhibited TG triggered with low TF concentration, suggesting an effect on the intrinsic pathway. This indicates that native β(2)GPI in circulation obtains its anticoagulant activity in the presence of anionic phospholipids such as activated blood cells thereby serving as an inhibitory modulator in hemostasis.

Human β2-glycoprotein I attenuates mouse intestinal ischemia/reperfusion induced injury and inflammation

Intestinal ischemia-reperfusion (IR)-induced injury results from a complex cascade of inflammatory components. In the mouse model of intestinal IR, the serum protein, β2-glycoprotein I (β2-GPI) binds to the cell surface early in the cascade. The bound β2-GPI undergoes a conformational change which exposes a neoantigen recognized by naturally occurring antibodies and initiates the complement cascade. We hypothesized that providing additional antigen with exogenous β2-GPI would alter IR-induced tissue injury. Administration of human but not mouse β2-GPI attenuated IR-induced tissue damage and prostaglandin E(2) production indicating a physiological difference between β2-GPI isolated from the two species. To investigate whether structural features were responsible for this physiological difference, we compared the chemical, physical and biochemical properties of the two proteins. Despite possessing 76% amino acid identity and 86% sequence homology, we found that mouse β2-GPI differs from the human protein in size, carbohydrate chain location, heterogeneity and secondary structural content. These data suggest that the structural differences result in mouse Ab recognition of soluble human but not mouse β2-GPI and attenuated IR-induced injury. We conclude that caution should be exercised in interpreting results obtained by using human β2-GPI in a mouse model.

The significance of autoantibodies against β2-glycoprotein I

The antiphospholipid syndrome (APS) is defined by the persistent presence of antiphospholipid antibodies in patients with a history of thrombosis and/or pregnancy morbidity, including fetal loss. APS is an autoimmune disease with a confusing name because the pathologic auto-antibodies are shown to be directed against the plasma protein β(2)-glycoprotein I and not against phospholipids. In fact, auto-antibodies that recognize phospholipids themselves are not associated with thrombosis but with infectious diseases. One of the intriguing questions is why autoantibodies against β(2)-glycoprotein I are so commonly found in both patients and the healthy. Several potential mechanisms have been suggested to explain the increased thrombotic risk in patients with these autoantibodies. In this overview, we will summarize our knowledge on the etiology of the autoantibodies, and we will discuss the evidence that identify autoantibodies against β(2)-glycoprotein I as the culprit of APS.

Recent developments in our understanding of the antiphospholipid syndrome

The antiphospholipid syndrome is an autoimmune disease that manifests clinically as recurrent thrombotic complications or foetal losses and serologically with elevated levels of antiphospholipid antibodies in the plasmas of these patients. The term 'antiphospholipid syndrome' is confusing, because the auto-antibodies are not directed against phospholipids but towards a plasma protein, β(2) -glycoprotein I. For many years, the reason why auto-antibodies against β(2) -glycoprotein I were pro-thrombotic was unclear, because β(2) -glycoprotein I seems to be an obsolete protein in our circulation. Human and mice deficient in this protein do not express a clear phenotype. Recent studies on the structure and function of β(2) -glycoprotein I have provided novel insights into the importance of this protein in physiology and its role in the pathology of the antiphospholipid syndrome.