Neutrophil activation by anti-beta 2 glycoprotein I monoclonal antibodies via Fc gamma receptor II

A Comprehensive Review of Risk Factors and Thrombophilia Evaluation in Venous Thromboembolism

Venous thromboembolism (VTE), which encompasses deep vein thrombosis (DVT) and pulmonary embolism (PE), is a significant cause of morbidity and mortality worldwide. There are many factors, both acquired and inherited, known to increase the risk of VTE. Most of these result in increased risk via several common mechanisms including circulatory stasis, endothelial damage, or increased hypercoagulability. Overall, a risk factor can be identified in the majority of patients with VTE; however, not all risk factors carry the same predictive value. It is important for clinicians to understand the potency of each individual risk factor when managing patients who have a VTE or are at risk of developing VTE. With this, many providers consider performing a thrombophilia evaluation to further define a patient's risk. However, guidance on who to test and when to test is controversial and not always clear. This comprehensive review attempts to address these aspects/concerns by providing an overview of the multifaceted risk factors associated with VTE as well as examining the role of performing a thrombophilia evaluation, including the indications and timing of performing such an evaluation.

Antiphospholipid syndrome: advances in diagnosis, pathogenesis, and management

Antiphospholipid syndrome (APS) is a thrombo-inflammatory disease propelled by circulating autoantibodies that recognize cell surface phospholipids and phospholipid binding proteins. The result is an increased risk of thrombotic events, pregnancy morbidity, and various other autoimmune and inflammatory complications. Although antiphospholipid syndrome was first recognized in patients with lupus, the stand alone presentation of antiphospholipid syndrome is at least equally common. Overall, the diagnosis appears to affect at least one in 2000 people. Studies of antiphospholipid syndrome pathogenesis have long focused on logical candidates such as coagulation factors, endothelial cells, and platelets. Recent work has shed light on additional potential therapeutic targets within the innate immune system, including the complement system and neutrophil extracellular traps. Vitamin K antagonists remain the mainstay of treatment for most patients with thrombotic antiphospholipid syndrome and, based on current data, appear superior to the more targeted direct oral anticoagulants. The potential role of immunomodulatory treatments in antiphospholipid syndrome management is receiving increased attention. As for many systemic autoimmune diseases, the most important future direction is to more precisely identify mechanistic drivers of disease heterogeneity in pursuit of unlocking personalized and proactive treatments for patients.

Mechanisms of immunothrombosis and vasculopathy in antiphospholipid syndrome

Antiphospholipid syndrome (APS) is an autoimmune thrombophilia propelled by circulating antiphospholipid antibodies that herald vascular thrombosis and obstetrical complications. Antiphospholipid antibodies recognize phospholipids and phospholipid-binding proteins and are not only markers of disease but also key drivers of APS pathophysiology. Thrombotic events in APS can be attributed to various conspirators including activated endothelial cells, platelets, and myeloid-lineage cells, as well as derangements in coagulation and fibrinolytic systems. Furthermore, recent work has especially highlighted the role of neutrophil extracellular traps (NETs) and the complement system in APS thrombosis. Beyond acute thrombosis, patients with APS can also develop an occlusive vasculopathy, a long-term consequence of APS characterized by cell proliferation and infiltration that progressively expands the intima and leads to organ damage. This review will highlight known pathogenic factors in APS and will also briefly discuss similarities between APS and the thrombophilic coagulopathy of COVID-19.

Fc receptors gone wrong: A comprehensive review of their roles in autoimmune and inflammatory diseases

Systemic autoimmune and inflammatory diseases have a complex and only partially known pathophysiology with various abnormalities involving all the components of the immune system. Among these components, antibodies, and especially autoantibodies are key elements contributing to autoimmunity. The interaction of antibody fragment crystallisable (Fc) and several distinct receptors, namely Fc receptors (FcRs), have gained much attention during the recent years, with possible major therapeutic perspectives for the future. The aim of this review is to comprehensively describe the known roles for FcRs (activating and inhibitory FcγRs, neonatal FcR [FcRn], FcαRI, FcεRs, Ro52/tripartite motif containing 21 [Ro52/TRIM21], FcδR, and the novel Fc receptor-like [FcRL] family) in systemic autoimmune and inflammatory disorders, namely rheumatoid arthritis, Sjögren's syndrome, systemic lupus erythematosus, systemic sclerosis, idiopathic inflammatory myopathies, mixed connective tissue disease, Crohn's disease, ulcerative colitis, immunoglobulin (Ig) A vasculitis, Behçet's disease, Kawasaki disease, IgG4-related disease, immune thrombocytopenia, autoimmune hemolytic anemia, antiphospholipid syndrome and heparin-induced thrombocytopenia.

NETs in APS: Current Knowledge and Future Perspectives

PURPOSE OF REVIEW

Antiphospholipid syndrome (APS) is a thrombo-inflammatory disease that is primarily treated with anticoagulation. Better understanding the inflammatory aspects of APS could lead to safer, more effective, and more personalized therapeutic options. To this end, we sought to understand recent literature related to the role of neutrophils and, in particular, neutrophil extracellular traps (NETs) in APS.

RECENT FINDINGS

Expression of genes associated with type I interferons, endothelial adhesion, and pregnancy regulation are increased in APS neutrophils. APS neutrophils have a reduced threshold for NET release, which likely potentiates thrombotic events and perhaps especially large-vein thrombosis. Neutrophil-derived reactive oxygen species also appear to play a role in APS pathogenesis. There are new approaches for preventing and disrupting NETs that could potentially be leveraged to reduce the risk of APS-associated thrombosis. Neutrophils and NETs contribute to APS pathophysiology. More precisely understanding their roles at a mechanistic level should help identify new therapeutic targets for inhibiting NET formation, enhancing NET dissolution, and altering neutrophil adhesion. Such approaches may ultimately lead to better clinical management of APS patients and thereby reduce the chronic burden of this disease.

Cryptic conspirators: a conversation about thrombocytopenia and antiphospholipid syndrome

Purpose of review

Although antiphospholipid syndrome (APS) is best known for conveying increased risk of thrombotic events and pregnancy morbidity, thrombocytopenia is also recognized as a common association. In this review, we will explore the relationship between thrombocytopenia and APS, highlighting our evolving understanding – and persistent knowledge gaps – through clinically oriented questions and answers.

Recent findings

A history of thrombocytopenia likely portends a more severe APS phenotype (including increased risk of thrombosis). Although the pathophysiology underlying thrombocytopenia in APS has yet to be definitively revealed, mechanisms that play a role (at least in subsets of patients) include: immune thrombocytopenic purpura/ITP-like autoantibodies against platelet glycoproteins; antiphospholipid antibody (aPL)-mediated platelet activation and consumption; and potentially life threatening thrombotic microangiopathy. Although thrombocytopenia is often ‘mild’ in APS (and therefore, may not require specific therapy), there are causes of acute-onset thrombocytopenia that mandate emergent work-up and treatment. When APS-related thrombocytopenia does require therapy, the approach must be individualized (requiring an understanding of pathophysiology in the particular APS patient). For patients with ITP-like disease, rituximab is emerging as a popular approach to treatment; in contrast, there are hints that thrombopoietin mimetics may be associated with elevated thrombotic risk.

Summary

Thrombocytopenia is common in APS, and is likely associated with more severe disease. Improved understanding of thrombocytopenia in APS has the potential to improve risk stratification, reveal novel aspects of APS pathophysiology, and lead to treatments that are more individualized and holistic.

Pathogenesis of antiphospholipid syndrome: recent insights and emerging concepts

Introduction: Even though our understanding of the antiphospholipid syndrome (APS) has improved tremendously over the last decades, we are still not in a position to replace symptomatic anticoagulation by pathogenesis based causal treatments. Areas covered: Recent years have provided further insights into pathogenetically relevant mechanisms. These include a differentiation of pathogenic subtypes of antiphospholipid antibodies (aPL), novel mechanisms modulating disease activity, for example, extracellular vesicles and microRNA, and novel players in pathogenesis, for example, neutrophils and neutrophil extracellular traps (NETs). Expert commentary: It is evident that aPL induce a proinflammatory and procoagulant state and recent data suggest that different aPL species activate different signaling pathways which sometimes converge into a common cellular response. This implies that presence of more than one aPL species may disproportionally increase the risk for the major manifestations of APS, that is, thrombosis and fetal loss. Further delineation of the pathogenic mechanisms will hopefully provide clues to causal rather than symptomatic treatments of APS.

Neutrophils and redox stress in the pathogenesis of autoimmune disease

Polymorphonuclear leukocytes, or neutrophils, are specialist phagocytic cells of the innate immune system. Their primary role is host defence against micro-organisms, which they kill via phagocytosis, followed by release of reactive oxygen species (ROS) and proteolytic enzymes within the phagosome. ROS are generated via the action of the NADPH oxidase (also known as NOX2), in a process termed the 'Respiratory Burst'. This process consumes large amounts of oxygen, which is converted into the highly-reactive superoxide radical O₂^- and H₂O₂. Subsequent activation of myeloperoxidase (MPO) generates secondary oxidants and chloroamines that are highly microbiocidal in nature, which together with proteases such as elastase and gelatinase provide a toxic intra-phagosomal environment able to kill a broad range of micro-organisms. However, under certain circumstances such as during an auto-immune response, neutrophils can be triggered to release ROS and proteases extracellularly causing damage to host tissues, modification of host proteins, lipids and DNA and dysregulation of oxidative homeostasis. This review describes the range of ROS species produced by human neutrophils with a focus on the implications of neutrophil redox products in autoimmune inflammation.

Networks of enzymatically oxidized membrane lipids support calcium-dependent coagulation factor binding to maintain hemostasis

Blood coagulation functions as part of the innate immune system by preventing bacterial invasion, and it is critical to stopping blood loss (hemostasis). Coagulation involves the external membrane surface of activated platelets and leukocytes. Using lipidomic, genetic, biochemical, and mathematical modeling approaches, we found that enzymatically oxidized phospholipids (eoxPLs) generated by the activity of leukocyte or platelet lipoxygenases (LOXs) were required for normal hemostasis and promoted coagulation factor activities in a Ca²⁺- and phosphatidylserine (PS)-dependent manner. In wild-type mice, hydroxyeicosatetraenoic acid-phospholipids (HETE-PLs) enhanced coagulation and restored normal hemostasis in clotting-deficient animals genetically lacking p12-LOX or 12/15-LOX activity. Murine platelets generated 22 eoxPL species, all of which were missing in the absence of p12-LOX. Humans with the thrombotic disorder antiphospholipid syndrome (APS) had statistically significantly increased HETE-PLs in platelets and leukocytes, as well as greater HETE-PL immunoreactivity, than healthy controls. HETE-PLs enhanced membrane binding of the serum protein β2GP1 (β2-glycoprotein 1), an event considered central to the autoimmune reactivity responsible for APS symptoms. Correlation network analysis of 47 platelet eoxPL species in platelets from APS and control subjects identified their enzymatic origin and revealed a complex network of regulation, with the abundance of 31 p12-LOX-derived eoxPL molecules substantially increased in APS. In summary, circulating blood cells generate networks of eoxPL molecules, including HETE-PLs, which change membrane properties to enhance blood coagulation and contribute to the excessive clotting and immunoreactivity of patients with APS.

Release of neutrophil extracellular traps by neutrophils stimulated with antiphospholipid antibodies: a newly identified mechanism of thrombosis in the antiphospholipid syndrome

OBJECTIVE

Antiphospholipid antibodies (aPL), especially those targeting β2 -glycoprotein I (β2 GPI), are well known to activate endothelial cells, monocytes, and platelets, with prothrombotic implications. In contrast, the interaction of aPL with neutrophils has not been extensively studied. Neutrophil extracellular traps (NETs) have recently been recognized as an important activator of the coagulation cascade, as well as an integral component of arterial and venous thrombi. This study was undertaken to determine whether aPL activate neutrophils to release NETs, thereby predisposing to the arterial and venous thrombosis inherent in the antiphospholipid syndrome (APS).

METHODS

Neutrophils, sera, and plasma were prepared from patients with primary APS (n = 52) or from healthy volunteers and characterized. No patient had concomitant systemic lupus erythematosus.

RESULTS

Sera and plasma from patients with primary APS had elevated levels of both cell-free DNA and NETs, as compared to healthy volunteers. Freshly isolated neutrophils from patients with APS were predisposed to high levels of spontaneous NET release. Further, APS patient sera, as well as IgG purified from APS patients, stimulated NET release from control neutrophils. Human aPL monoclonal antibodies, especially those targeting β2 GPI, also enhanced NET release. The induction of APS NETs was abrogated with inhibitors of reactive oxygen species formation and Toll-like receptor 4 signaling. Highlighting the potential clinical relevance of these findings, APS NETs promoted thrombin generation.

CONCLUSION

Our findings indicate that NET release warrants further investigation as a novel therapeutic target in APS.

Delineating the deranged immune system in the antiphospholipid syndrome

The antiphospholipid syndrome (APS) is a systemic autoimmune disease that is characterized serologically by the presence of antiphospholipid antibodies (aPL) and clinically by vascular thrombosis and obstetric complications. The protein β2 glycoprotein I (β2GPI) is identified as the most important autoantigen in this syndrome. Activation of endothelial cells, thrombocytes and placental tissue by anti-β2GPI antibodies relates to the clinical manifestations of APS. This review describes genetic and environmental factors in relation to APS and summarizes the current knowledge on abnormalities in components of both the innate and adaptive immune system in APS. The role of dendritic cells, T-cells, B-cells, monocytes, neutrophils and NK-cells as well as the complement system in APS are discussed. Several gaps in our knowledge on the pathophysiology of APS are identified and a plea is made for future extensive immune cell profiling by a systems medicine approach in order to better unravel the pathogenesis of APS, to gain more insight in the role of the immune system in APS as well as having the potential to reveal biomarkers or novel therapeutic targets.

Neutrophils in the pathogenesis and manifestations of SLE

Systemic lupus erythematosus (SLE) is an autoimmune disease of unclear etiology that affects mostly women of childbearing age. Profound abnormalities in both innate and adaptive immunity triggered by genetic and environmental factors are well documented to play an important part in the pathogenesis of SLE. Nonetheless, the role of neutrophils--the most abundant immune cell type--in the pathology of this disease has been unclear. Over the past decade, compelling evidence has emerged that implicates neutrophils in the initiation and perpetuation of SLE and also in the resultant organ damage frequently observed in patients with this disease. SLE-derived low-density granulocytes (LDGs) induce vascular damage and synthesize increased amounts of type I interferons and, as such, could play a prominent part in the pathogenesis of SLE. Furthermore, increased cell death and enhanced extracellular trap formation observed in SLE-derived neutrophils might have key roles in the induction of autoimmunity and the development of organ damage in patients with SLE. Together, these events could have significant deleterious effects and promote aberrant immune responses in this disease. This Review highlights the role of neutrophils in the pathogenesis of SLE, with a particular focus on the putative deleterious effects of LDGs and neutrophil extracellular trap formation.

Antiphospholipid antibodies promote leukocyte-endothelial cell adhesion and thrombosis in mice by antagonizing eNOS via β2GPI and apoER2

In antiphospholipid syndrome (APS), antiphospholipid antibodies (aPL) binding to β2 glycoprotein I (β2GPI) induce endothelial cell-leukocyte adhesion and thrombus formation via unknown mechanisms. Here we show that in mice both of these processes are caused by the inhibition of eNOS. In studies of cultured human, bovine, and mouse endothelial cells, the promotion of monocyte adhesion by aPL entailed decreased bioavailable NO, and aPL fully antagonized eNOS activation by diverse agonists. Similarly, NO-dependent, acetylcholine-induced increases in carotid vascular conductance were impaired in aPL-treated mice. The inhibition of eNOS was caused by antibody recognition of domain I of β2GPI and β2GPI dimerization, and it was due to attenuated eNOS S1179 phosphorylation mediated by protein phosphatase 2A (PP2A). Furthermore, LDL receptor family member antagonism with receptor-associated protein (RAP) prevented aPL inhibition of eNOS in cell culture, and ApoER2-/- mice were protected from aPL inhibition of eNOS in vivo. Moreover, both aPL-induced increases in leukocyte-endothelial cell adhesion and thrombus formation were absent in eNOS-/- and in ApoER2-/- mice. Thus, aPL-induced leukocyte-endothelial cell adhesion and thrombosis are caused by eNOS antagonism, which is due to impaired S1179 phosphorylation mediated by β2GPI, apoER2, and PP2A. Our results suggest that novel therapies for APS can now be developed targeting these mechanisms.

Role of the Fcgamma receptor IIA polymorphism in the antiphospholipid syndrome: an international meta-analysis

OBJECTIVE

To assess the impact of the FcgammaRIIA-R/H131 polymorphism on the risk for antiphospholipid syndrome (APS), both primary and secondary to systemic lupus erythematosus (SLE).

METHODS

This international meta-analysis combined data from 9 research teams. FcgammaRIIA-R/H131 genotypes were determined in 481 APS cases (206 with primary APS), 1,420 SLE controls, and 1,655 disease-free controls. Data were combined using fixed-effects and random-effects models.

RESULTS

Compared with disease-free controls, the RR genotype was enriched in the entire group of APS cases (odds ratio [OR] 1.65, 95% confidence interval [95% CI] 1.28-2.14); this was driven mostly by patients with secondary APS (OR 1.95, 95% CI 1.45-2.63). The excess of RR homozygotes but not heterozygotes among APS patients suggested a recessive mode of inheritance, rather than the additive model seen for SLE susceptibility, where RR conferred greatest risk, and RH intermediate risk, for SLE. This probably reflected the additional influence of another opposing genetic effect of HH homozygosity on APS predisposition (OR 0.72 for RH versus HH, 95% CI 0.55-0.96). Among SLE patients, those with APS were more frequently HH homozygotes than heterozygotes (OR 0.56 for RH versus HH, 95% CI 0.39-0.81). HH homozygosity also tended to predominate in primary APS compared with secondary APS (OR 0.50 for RR versus HH, 95% CI 0.25-0.99 by fixed-effects model). There was no significant between-study heterogeneity for any of these effects.

CONCLUSION

The FcgammaRIIA-R/H131 polymorphism is an important determinant of predisposition to APS, with different influences on SLE and APS susceptibility per se.

Which antiphospholipid antibody tests are most useful?

Despite an active international effort to improve diagnosis and treatment of the antiphospholipid syndrome (Hughes syndrome), there remain problems of lack of standardization and lack of prospective and multivariate epidemiologic analysis which restrict the diagnostic and predictive ability of commercially available tests. Nevertheless, current published series provide some data from which strategic approaches can be used to maximize the efficiency and usefulness of available tests. For further updates on new research and developments of interest to physicians and patients with this syndrome, the following web sites may prove helpful: www.slrapls.org, www.hematology.org, www.acforum.org, www.americanheart.org, www.rarediseases.org, www.aarda.org, and www.lupus.org.

Monovalent binding of autoantibodies to beta2-glycoprotein I, detected using surface plasmon resonance at low antigen density

The precise mechanism of interaction between autoantibodies and beta2-glycoprotein I (beta2GPI) and the experimental conditions to be used for their detection are still under debate. Until now, these interactions have been studied under static conditions. We have investigated the interactions of purified IgG from 25 lupus anticoagulant-positive patients with immobilized beta2GPI under flow conditions by real-time analysis based on surface plasmon resonance technology. Sensor chips were coated with purified human beta2GPI coupled to dextran via amino groups at low densities (1.4, 1.8 or 2. 4 ng beta2GPI/mm2). Four patients' IgG displayed efficient binding and had the highest so-called antiphospholipid IgG levels by enzyme-linked immunosorbent assay (ELISA) and the highest absorbance values in an anti- beta2GPI ELISA at a beta2GPI density reported to be around 12 ng/mm2. Binding of antibodies to the beta2GPI sensor chips proved to be dependent upon the IgG concentration and beta2GPI density and was inhibited by a rabbit antibody against beta2GPI. Similar association and dissociation profiles were observed for the four efficient binders. The fast rate of dissociation limited the binding of autoantibodies to beta2GPI and was highly suggestive of a monovalent association, confirmed by binding of Fab fragments under similar experimental conditions. In conclusion, monovalent binding of low-affinity antibodies to beta2GPI immobilized at a density as low as 1.8 ng/mm2 could be detected using surface plasmon resonance.

Both kinetic data and epitope mapping provide clues for understanding the anti-coagulant effect of five murine monoclonal antibodies to human beta2-glycoprotein I

The interaction between five murine monoclonal antibodies (mAb) and beta2-glycoprotein I (beta2GPI) in the absence of phospholipids was studied using surface plasmon resonance-based biosensor technology. Two separate epitope regions were confirmed for the five mAb but epitopes of two mAb were shown to be overlapping but not identical. The characteristics of binding on both immobilized beta2GPI, using different chemistries of coupling to a dextran matrix and antibody surfaces prepared by two strategies of immobilization, were compared. Binding was strongly influenced by the orientation of the immobilized partner, and the five mAb showed heterogeneity in their binding to immobilized and soluble beta2GPI. The observed stoichiometries of mAb-beta2GPI complexes and the detailed analysis of the kinetics of the association and dissociation phases of the interactions with soluble and immobilized beta2GPI revealed differences in the dissociation rate constants, resulting in a 10-fold higher affinity for immobilized beta2GPI compared to soluble beta2GPI for four out of five mAb. This suggests bivalent binding of these mAb to immobilized beta2GPI. In addition, the kinetic data helped explain the differing anti-coagulant properties of these mAb.

Lupus anticoagulants and antiphospholipid antibodies

In recent years, clinical syndromes involving lupus anticoagulants and antiphospholipid antibodies have come into increasing clinical prominence. Since the discovery that most antiphospholipid antibodies require the presence of anionic phospholipid-binding proteins such as B2-glycoprotein I and prothrombin, a large number of studies have attempted to delineate the specificity of these antibodies. Several mechanisms have been proposed to explain the hypercoagulable state associated with these antibodies. This review attempts to summarize these data and the challenges that confront efforts to delineate the pathogenesis of the prothrombotic state associated with the presence of these antibodies.

Mechanism of action of beta2-glycoprotein I-dependent lupus anticoagulants

There is accumulating evidence showing that lupus anticoagulants (LA) are more strongly associated with thrombosis than anticardiolipin antibodies. In addition, indirect evidence has been presented indicating that beta2GPI-dependent LA are more strongly associated with thrombosis than prothrombin-dependent LA. From this, one may assume that anti-beta2GPI antibodies with LA activity are more pathogenic than anti-beta2GPI antibodies without LA activity. Therefore, it is of the utmost importance to understand the molecular basis on which some anti-beta2GPI antibodies behave as LA. In this presentation, the current knowledge on the interaction of beta2GPI with phospholipids and with anti-beta2GPI antibodies is reviewed and an integrated model for the anti-beta2GPI-dependent LA activity is proposed with implications for a pathogenic role of these particular antibodies.

Anticardiolipin IgG subclasses: association of IgG2 with arterial and/or venous thrombosis

OBJECTIVE

To determine whether the presence of anticardiolipin antibodies (aCL) of a specific IgG subclass is associated with clinical complications of the antiphospholipid antibody syndrome (APS) and whether polymorphisms of Fc receptors for IgG (FcgammaR) with differential binding preferences contribute to an increased risk of thrombotic complications.

METHODS

In 60 patients with IgG aCL, we assessed clinical complications of the APS, measured the level of antibody activity, and determined the IgG subclass distribution of aCL by a modified enzyme-linked immunosorbent assay (ELISA) with murine anti-human IgG subclass monoclonal antibodies. Selective IgG subclass adsorption studies were performed to determine the relative contribution of specific IgG subclasses to overall aCL activity. Fcgamma receptor IIA (FcgammaRIIA) genotypes of aCL patients with thrombosis and of non-systemic lupus erythematosus controls were determined by polymerase chain reaction amplification of genomic DNA and allele-specific probes.

RESULTS

IgG2 aCL, detected in 75% of the patients, was the major subclass of aCL. Selective adsorption studies demonstrated that IgG2, in contrast to IgG1, was the predominant subclass responsible for aCL reactivity. IgG2 aCL was the only subclass associated with clinical complications, specifically, arterial and/or venous thrombosis (P < 0.04). The presence of FcgammaRIIA-H131, a receptor expressed on platelets, monocytes, and endothelial cells and the only human FcgammaR which efficiently recognizes IgG2, was associated with thrombosis in aCL patients. Among 45 high-titer (>40 GPL [IgG phospholipid] units) aCL patients with thrombosis, 40% were homozygous for FcgammaRIIA-H131, compared with 25% of disease-free controls (P = 0.042).

CONCLUSION

While all 4 IgG subclasses are found in autoimmune aCL, only the presence of IgG2 is significantly associated with thrombotic complications. Reactivity in aCL ELISA is largely due to the presence of IgG2 in high-titer patients. The presence of IgG2 aCL, particularly in association with FcgammaRIIA-H131, may be a useful clinical predictor of increased thrombotic risk in patients with autoimmune IgG aCL. Allelic variants of FcgammaRIIA with distinct capacities to interact with IgG subclasses provide a mechanism for genetic susceptibility to an autoantibody-induced prothrombotic state.