Characteristics and pathogenic role of anti-beta2-glycoprotein I single-chain Fv domains: induction of experimental antiphospholipid syndrome

16th International congress on antiphospholipid antibodies task force report on antiphospholipid syndrome laboratory diagnostics and trends

Classification criteria for antiphospholipid syndrome (APS) require IgG or IgM isotypes of the anticardiolipin (aCL) antibodies, anti-β2 glycoprotein I (anti-β2GPI) antibodies, and/or the lupus anticoagulant (LA) to satisfy the laboratory disease definition. Over the past 20 years, non-criteria antiphospholipid antibodies (aPL) directed to other proteins of the coagulation cascade (i.e. prothrombin and/or phosphatidylserine-prothrombin complex) or to some domains of β2GPI have been proposed. This task force concentrated and reviewed the literature on data including aPS/PT, antibodies to domain 4/5 of β2GPI and the newly described antibodies to protein/HLA-DR complex. In addition, we discussed testing of LA in the 'new' oral anticoagulants' era and the value of triple positivity in the risk assessment of aPL. The conclusions were presented at a special session during the 16th International Congress on aPL, Manchester, UK, September 2019.

Oral administration of Domain-I of beta-2glycoprotein-I induces immunological tolerance in experimental murine antiphospholipid syndrome

It is well established that the humoral immunity in antiphospholipid syndrome (APS) is presented by circulating pathogenic anti-β2GPI autoantibodies targeting mainly domain I of the β2GPI protein, playing a major role in the disease pathogenesis. Previously, we have demonstrated that treatment of experimental APS mice with tolerogenic dendritic cells loaded with domain-I was more efficient in tolerance induction than with the whole molecule or domain-V. In the current study we had orally administered a domain-I derivative of the β2GPI molecule, as a new therapeutic approach to induce oral tolerance in this mouse model of APS. BALB/c mice immunized with β2GPI, were fed with either domain-I, domain-V derivative or the complete β2GPI protein. β2GPI immunized mice developed experimental APS which were fed with domain-I significantly had decreased fetal loss (p < 0.004), a lower size of thrombi (p < 0.001) and lower circulating anti-β2GPI Abs in comparison to mice fed with domain-V or PBS (p < 0.002). Likewise, Domain-I fed mice had a lowered inflammatory response, exhibited by decreased expression of inflammatory cytokines (IFNγ, IL-6, IL-17) and elevated production of IL-10 anti-inflammatory cytokine by splenocytes. Moreover, the anti-inflammatory response in the domain-I fed APS mice was associated with increased circulating miRNA variations (155, 146, 182, 98) by RT-PCR, which are associated with immunomodulation of the immune network. We propose that oral tolerance with domain-I can be a novel therapy for patients with APS.

Autoantibodies associated with reproductive failure

Autoimmune factors are involved in some of the cases of reproductive failure. These factors entail several autoantibodies, especially in patients having systemic lupus erythematosus (SLE) or the antiphospholipid syndrome (APS). These autoantibodies include mainly antibodies directed to phospholipid such as cardiolipin, phosphatidylserine, phosphatidylethanolamine or phospholipids binding glycoproteins such as b2glycoprotein-I, annexin V, prothrombin and protein-Z. There are also some other autoantibodies directed to laminin-I, thromboplastin, mitochondrial antibodies of the M5 type, corpus luteum, prolactin, poly (ADP-ribose), thyroglobulin and more, which were also found in SLE or APS patients with reproductive failure. Moreover, the presence of additional autoantibodies directed to actin, enolase, cubilin and others, needs further investigation to support a firm association to reproductive failure in women. Future studies are likely to help to determine and expand the number of autoantibodies screened in these patients, as well as by the use of proteomics technology, to determine peptides resembling the epitope specificities associated with the specific clinical manifestations.

Tolerogenic dendritic cells specific for β2-glycoprotein-I Domain-I, attenuate experimental antiphospholipid syndrome

Tolerogenic dendritic cells (tDCs) have the potential to control the outcome of autoimmunity by modulating the immune response. The aim of this study was to uncover the tolerance efficacy attributed to beta-2-glycoprotein-I (β2GPI) tDCs or β2GPI domain-I (D-I) and domain-V (D-V)-tDCs in mice with antiphospholipid syndrome (APS). tDCs were pulsed with β2GPI or D-I or D-V derivatives. Our results revealed that β2GPI related tDCs phenotype includes CD80(high), CD86(high) CD40(high) MHC class II(high). The miRNA profiling encompass miRNA 23b(high), miRNA 142-3p(low) and miRNA 221(low). In addition the β2GPI related tDCs showed reduced secretion of IL-1β, IL-12 and IL-23. D-I tDCs treatment was more efficient than β2GPI tDCs in inducing of tolerance in APS mice, manifested by lowered titers of anti- β2GPI antibodies (Abs) and reduced percentage of fetal loss. Tolerance induction was accompanied by poor T cell response to β2GPI, high numbers of CD4 + CD25 + FOXP3 + T-regulatory cells (Treg), reduced levels of IFNγ, IL-17 and increased expression of IL-10 and TGFβ. Tolerance was successfully transferred by Treg cells from the tolerized mice to β2GPI immunized mice. We conclude that predominantly D-I-tDCs and β2GPI tDCs have the potential to attenuate experimental APS by induction of Treg cells, reduction of anti- β2GPI Abs titers and increased expression of anti-inflammatory cytokines. We suggest that β2-GPI-D-I-tDCs may offer a novel approach for developing therapy for APS patients.

The avidity of anti-β2-glycoprotein I antibodies in patients with or without antiphospholipid syndrome: a collaborative study in the frame of the European forum on antiphospholipid antibodies

Objective. The objective of this study was to extend the findings of the preliminary study by measuring the avidity of IgG anti-β2-glycoprotein I antibodies (anti-β2-GPI) on a larger group of patients with primary or secondary antiphospholipid syndrome (APS) and anti-β2-GPI positive patients without APS in the frame of the European Forum on antiphospholipid antibodies (aPL).

Methods. Serum from 137 patients with primary APS, APS associated with autoimmune diseases, and patients with autoimmune diseases other than APS from five EU rheumatology centres were tested for anti-β2-GPI antibodies. The 109 patients who were sera positive for anti-β2-GPI by the in-house anti-β2-GPI enzyme-linked immunosorbent assay (ELISA) at the Immunology Laboratory, UMC Ljubljana were selected for further testing on avidity with chaotropic anti-β2-GPI ELISA.

Results. High, low and heterogeneous avidity IgG anti-β2-GPI was found in 32/109, 17/109 and 60/109 patients respectively. Significantly more patients with APS were in the high avidity than in the low avidity anti-β2-GPI group, while the opposite was observed for non-APS (both p < 0.001). The most common clinical feature among patients with high avidity anti-β2-GPI was thrombosis, mainly due to venous thrombosis ( p < 0.01 and p < 0.001, versus low avidity anti-β2-GPI group).

Conclusion. Patients with or without APS had anti-β2-GPI of high, low or heterogeneous avidity. High avidity anti-β2-GPI was associated with thrombosis and APS, while in the low avidity anti-β2-GPI group non-APS (predominantly SLE) patients prevailed. Determination of anti-β2-GPI avidity should be considered in the analytical strategies for further differentiation of patients with anti-β2-GPI antibodies.

Patient-derived monoclonal antibodies directed towards beta2 glycoprotein-1 display lupus anticoagulant activity

BACKGROUND

Patients with antiphospholipid syndrome (APS) display a heterogeneous population of antibodies with beta(2) glycoprotein-1 (β(2)GP1) as the major antigen.

OBJECTIVES

We isolated and characterized human mAbs directed against β(2)GP1 from the immune repertoire of APS patients.

METHODS

Variable heavy chain repertoires from B cells from two APS patients with anti-β(2)GP1 antibodies were cloned into the pHEN1-VLrep vector. Constructed full-length IgG antibodies were tested for lupus anticoagulant (LAC) activity and binding to β(2)GP1 and its domains.

RESULTS

Two clones of each patient were selected on the basis of the reactivity of single chain Fv (scFv) fragments displayed on phages towards full-length β(2)GP1 and its isolated domain I. The affinity of selected antibodies for β(2)GP1 was lost when transforming from phages to monovalent scFvs, and was regained when antibodies were constructed as complete IgG, indicating a role for bivalency in binding to β(2)GP1. Both selected clones from patient 2 recognized domain I of β(2)GP1, and for both clones selected from patient 1, binding required the presence of both domain I and domain II. All mAbs displayed LAC activity in both activated partial thromboplastin time-based and dilute Russell's viper venom test-based clotting assays and in thrombin generation.

CONCLUSIONS

In this study, we show successful cloning of patient-derived mAbs that require domain I of β(2)GP1 for binding, and that display LAC activity that is dependent on their affinity for β(2)GP1. These antibodies can help us to gain more insights into the pathogenesis of APS, and may facilitate standardization of APS diagnosis.

Antiphospholipid antibody-mediated reproductive failure in antiphospholipid syndrome

The association of elevated titers of circulating antiphospholipid (anti-PL) Abs in antiphospholipid syndrome (APS) and reproductive failure is well established in the literature. The clinical features include recurrent abortions at various stages, including implantation, placentation in the first trimester, miscarriages in the second and third trimesters, intrauterine growth retardation, preeclampsia with placental insufficiency and growth restrictions, arterial and venous thrombosis, and possibly also infertility. APS-mediated recurrent pregnancy loss and other features of reproductive failure might result from diverse autoimmune factors, inflammation, involving different mechanisms, which encompass pathogenic anti-PL Abs. Herein, we discuss the association of anti-PL Abs with reproductive failure with special emphasis on antiphospholipid autoantibodies characterizing APS. This association is evident from either human studies or murine models.

Anti-beta(2)GP-I and anti-prothrombin antibodies generated by phage display

This chapter describes the construction and screening of a library of single-chain variable fragments (svFv) derived from patients with autoimmune disease. The methods cover the isolation of mononuclear cells from peripheral blood, preparation of RNA, and recovery of immunoglobulin-coding sequences by polymerase chain reaction (PCR). Cloning into a phage display vector and screening of the scFv display library by a simple panning procedure are described. These methods are applicable to library construction from any patient group or (with alternative primer sets) any mammalian species.

Autoantibodies against galectin-8: their specificity, association with lymphopenia in systemic lupus erythematosus and detection in rheumatoid arthritis and acute inflammation

The role of autoantibodies in the pathogenesis of systemic lupus erythematosus (SLE) has not been completely defined. From more than a hundred autoantibodies described in SLE, relatively few have been associated with clinical manifestations. The glycan-binding proteins of the galectin family can modulate the immune system. Anti-galectin autoantibodies thus could have functional and/or pathogenic implications in inflammatory processes and autoimmunity. We previously reported function-blocking autoantibodies against galectin-8 (Gal-8) in SLE. Here we tested these autoantibodies against a series of other human galectins and demonstrated their specificity for Gal-8, being detectable in 23% of 78 SLE patients. Remarkably, they associated with lymphopenia (50% of 18 anti-Gal-8-positive versus 18% of 60 anti-Gal-8-negative cases, Fisher’s Exact test two-tailed: P < 0.012). Lymphopenia is a common clinical manifestation in SLE, yet of unknown mechanism. In addition, six of eight patients with both lymphopenia and malar rash had anti-Gal-8 in their sera. Occurrence of these autoantibodies was not confined to SLE as we also found them in sera of patients with rheumatoid arthritis (16%) and septicemia (20%). This study thus establishes occurrence of specific anti-Gal-8 autoantibodies in autoimmune rheumatic diseases and in acute inflammation, with an apparent association to a clinical subset in SLE.

Possible therapeutic applications of single-chain antibodies in systemic autoimmune diseases

B cells participate in the induction and maintenance of systemic autoimmune diseases, such as rheumatoid arthritis and systemic lupus erythematosus, via production of pathogenic autoantibodies, contributing to the formation of immune complexes. Immune complex deposition in the kidney and joints causes inflammation and organ destruction, and chemokine production enhances T cell activation and tissue damage. The development of the disorder depends on several factors, for example, genetic susceptibility, environmental factors or immune dysregulation. Traditional therapies, which aimed at the alleviation of symptoms, are giving way to biological therapies with the potential of disrupting disease progression. This article focuses on antibody therapies, especially on the applications of single-chain antibodies, as new biological agents for the treatment of systemic autoimmune disorders.

Avidity of anti-beta-2-glycoprotein I antibodies

The terms affinity and avidity are often used indiscriminately, despite clearly differing. Since affinity refers to monovalent binding of antibodies to a monovalent epitope, the majority of data on the binding of anti-beta2-glycoprotein I antibodies (anti-beta2-GPI) characterized their avidity rather than affinity. Anti-beta2-GPI were generally believed to be of low avidity, but heterogeneous avidity of patients' IgG anti-beta2-GPI has been demonstrated. High avidity anti-beta2-GPI monoclonals were reported to possess higher pathogenicity than low avidity anti-beta2-GPI. Polyclonal high avidity anti-beta2-GPI were found to be more common in patients with antiphospholipid syndrome (APS) and associated with thrombosis. Some conformational changes of beta2-GPI are required for the binding of polyclonal anti-beta2-GPI to the antigen: neither high density of the antigen nor high avidity of the anti-beta2-GPI alone is sufficient for the recognition. Avidity of anti-beta2-GPI should be considered in any attempt of inter-laboratory standardisation and/or evaluation of anti-beta2-GPI enzyme-linked immunosorbent assay (ELISA).

Anti-β2-glycoprotein-I antibodies in scFv format

Phage display was introduced almost 20 years ago. It has been used to produce large amounts of diverse proteins, to analyze protein-ligand interactions, to improve the affinity of proteins for their binding receptors, and to characterize antibody binding sites. The recombinant version of the antibody Fv is termed single-chain variable fragment (scFv). Many large phage libraries have been developed that have yielded antibodies to several hundred antigens, but only 5 human anti-beta2-glycoprotein-I and three anti-prothrombin antibodies in scFV have been so far characterized. Antibodies to beta2GP-I thus generated show 92-94% homology with their nearest germ line genes. Their mutations frequently appear to be independent of antigen. Two anti-prothrombin antibodies show strong crossreactivity with beta2GP-I. Four mouse anti-beta2GP-I scFV show less binding properties than their original counterparts, but had the same capacity of inducing experimental antiphospholipid syndrome. This pathogenicity appears to reside in the V(H)DJ(H)C(H) region of the scFv since the V(H)DJ(H)C(H) regions of pathogenic scFV combined with irrelevant V(L) J(L)C(L) regions retained their pathogenicity while the opposite failed to do so.

Avidity of Anti-β2-Glycoprotein I and Thrombosis or Pregnancy Loss in Patients with Antiphospholipid Syndrome

We aimed to evaluate avidity of anti-beta(2)-glycoprotein I antibodies (anti-beta(2)-GPIs) in patients with antiphospholipid syndrome (APS) at the time of acute thrombotic events or pregnancy loss as compared with clinical event-free periods. To do so, 69 sera samples from 16 patients (6 with primary APS and 10 with APS secondary to systemic lupus erythematosus ) were selected on the basis of anti-beta(2)-GPI positivity. Avidity of IgG anti-beta(2)-GPIs was determined by chaotropic enzyme-linked immunosorbent assay (ELISA), using increased NaCl concentration during antibody binding. High, heterogeneous, and low-avidity anti-beta(2)-GPIs were measured in APS patients, with no clear pattern regarding the time of thrombotic events or pregnancy failure. In general, anti-beta(2)-GPI avidity did not change substantially during disease course. We concluded that avidity of anti-beta(2)-GPIs appears to be a rather stable parameter in an individual APS patient. Considering the previously shown association of high-avidity anti-beta(2)-GPIs with venous thrombosis, avidity of anti-beta(2)-GPIs may be a better predictor of predisposition to thrombosis and unsuccessful pregnancy than levels of antiphospholipid antibodies, which may fluctuate over time owing to several factors.

The presence of IgG antibodies against beta2-glycoprotein I predicts the risk of thrombosis in patients with the lupus anticoagulant

BACKGROUND

Lupus anticoagulant (LA) is a strong risk factor of thrombosis. However, a subgroup of patients positive for LA is unaffected by thrombosis and currently no predictive markers are available to identify patients positive for LA at increased risk for thrombosis.

OBJECTIVE

The aim of the study was to investigate whether anti-beta-2-glycoprotein I (anti-beta2GPI) or anticardiolipin antibodies (ACA) are associated with an increased risk of thrombosis in patients persistently positive for LA.

PATIENTS AND METHODS

A cohort of 87 consecutive patients persistently positive for LA was investigated, 55 with and 32 without a history of thrombosis. Immunoglobulin G (IgG) and M (IgM) antibodies against beta2GPI and cardiolipin were determined by enzyme-linked immunoassay.

RESULTS

Patients positive for LA with thrombosis had significantly higher levels of anti-beta2GPI IgG (median 16.7 standard units, interquartile range 3.0-75.2, P = 0.002) and of ACA IgG (41.1 IgG phospholipid units per mL, 8.9-109.0, P = 0.002) than those without thrombosis (2.6, 1.4-7.9 and 9.7, 4.6-22.1, respectively). Levels of anti-beta2GPI IgM and ACA IgM did not differ significantly between LA patients with and without thrombosis (P = 0.25 and 0.12, respectively). Elevated anti-beta2GPI IgG was associated with an increased risk for thrombosis (OR = 4.0, 95% CI 1.2-13.1), especially for venous thromboembolism (OR = 5.2, 95% CI 1.5-18.0).

CONCLUSIONS

Increased levels of anti-beta2GPI IgG were associated with thrombosis. We conclude that anti-beta2GPI IgG levels above normal predict an increased risk of thrombosis in patients persistently positive for LA.

β2-glycoprotein I–dependent lupus anticoagulant highly correlates with thrombosis in the antiphospholipid syndrome

The antiphospholipid syndrome is characterized by the presence of antiphospholipid antibodies in plasma of patients with thromboembolic complications. A major problem in defining the syndrome is that serologic assays to detect antiphospholipid antibodies have a low specificity. We recently published a method that specifically detects lupus anticoagulant (LAC) caused by anti–β2-glycoprotein I antibodies. Here, we studied the clinical relevance of detecting β2-glycoprotein I–dependent LAC. Plasma samples were collected from 198 patients with autoimmune diseases. In those samples with a positive partial thromboplastin time–lupus anticoagulant (PTT-LA), a modified activated partial thromboplastin time (aPTT)–based LAC test was performed with cardiolipin as confirming agent. Twenty-five of 58 patients with an aPTT-based LAC were dependent on the presence of anti–β2-glycoprotein I antibodies. Presence of β2-glycoprotein I–dependent LAC was almost completely associated with a history of thromboembolic complications (odds ratio, 42.3; 95% confidence interval, 194.3-9.9). An increased frequency of thrombosis was not found in 33 patients with LAC independent of anti–β2-glycoprotein I antibodies (odds ratio, 1.6; 95% confidence interval, 3.9-0.8). The use of an LAC assay with cardiolipin as confirming agent strongly improves the detection of patients at risk of thrombosis. Our findings suggest that anti–β2-glycoprotein I antibodies with LAC activity are antibodies that are responsible for the thromboembolic complications in the antiphospholipid syndrome.

The critical role of arginine residues in the binding of human monoclonal antibodies to cardiolipin

Previously we reported that the variable heavy chain region (V_H) of a human beta₂ glycoprotein I-dependent monoclonal antiphospholipid antibody (IS4) was dominant in conferring the ability to bind cardiolipin (CL). In contrast, the identity of the paired variable light chain region (V_L) determined the strength of CL binding. In the present study, we examine the importance of specific arginine residues in IS4V_H and paired V_L in CL binding. The distribution of arginine residues in complementarity determining regions (CDRs) of V_H and V_L sequences was altered by site-directed mutagenesis or by CDR exchange. Ten different 2a2 germline gene-derived V_L sequences were expressed with IS4V_H and the V_H of an anti-dsDNA antibody, B3. Six variants of IS4V_H, containing different patterns of arginine residues in CDR3, were paired with B3V_L and IS4V_L. The ability of the 32 expressed heavy chain/light chain combinations to bind CL was determined by ELISA. Of four arginine residues in IS4V_H CDR3 substituted to serines, two residues at positions 100 and 100 g had a major influence on the strength of CL binding while the two residues at positions 96 and 97 had no effect. In CDR exchange studies, V_L containing B3V_L CDR1 were associated with elevated CL binding, which was reduced significantly by substitution of a CDR1 arginine residue at position 27a with serine. In contrast, arginine residues in V_L CDR2 or V_L CDR3 did not enhance CL binding, and in one case may have contributed to inhibition of this binding. Subsets of arginine residues at specific locations in the CDRs of heavy chains and light chains of pathogenic antiphospholipid antibodies are important in determining their ability to bind CL.

Beta-2-glycoprotein-I, infections, antiphospholipid syndrome and therapeutic considerations

Evidence supports the association between infectious agents, antiphospholipid syndrome (APS), and the presence of antiphospholipid antibodies and anti-beta2-glycoprotein-I (beta2GPI) antibodies. Several mechanisms have been proposed to explain the role of bacteria/viruses in induction of an autoimmune condition, such as molecular mimicry between structures of a pathogen and self antigen and bystander activation or bacterial/viral superantigens. Protein databases reveal high homologies between the beta2GPI-related synthetic peptides and infectious agents. Studies employing experimental APS models proved molecular mimicry between beta2GPI-related synthetic peptides, which serve as target epitopes for anti-beta2GPI Abs, and structures within bacteria, viruses (e.g., CMV), and tetanus toxoid. Any explanation of how microbial infections might induce APS must take into account the genetic predisposition. In this paper, we discuss the association of antiphospholipid antibodies, infectious states, and molecular mimicry as a proposed mechanism for development of APS.

Molecular and genetic characterizations of five pathogenic and two non-pathogenic monoclonal antiphospholipid antibodies

Antiphospholipid syndrome (APS) is an autoimmune disease that is characterized by thrombosis, recurrent fetal loss and thrombocytopenia. Antiphospholipid antibodies, detected by enzyme-linked immunoabsorbent assays (aCL) and/or in vitro blood clotting assays (LAC) are strongly associated with APS. Both the molecular structures used by pathogenic antiphospholipid antibodies and the genetic mechanisms leading to their production are unknown. We describe here the variable region genes of seven IgG antiphospholipid antibodies derived from two APS patients. Of these, five are pathogenic as defined in a mouse model of thrombosis and two are not. Analyses of the expressed variable region genes show no preferential V gene usage. However, similar to anti-DNA antibodies, pathogenic antiphospholipid antibodies contain an increased number of arginine residues in the third complimentarity-determining region (CDR3) of their H chains. The increased accumulation of arginine residues in the V(H) CDR3 may act to enhance antigen binding, promote disease and point to the importance of the H chain in the pathogenic potential of certain antiphospholipid antibodies.

Antibodies and vascular involvement in inflammatory joint disease: clinical relevance

The vascular endothelium is a common target of inflammatory joint disease. Autoimmune diseases including rheumatoid arthritis, systemic lupus erythematosus, and Sjögren's syndrome can be responsible for a spectrum of vascular disorders that encompasses vasculitis, thrombosis and/or atheroma associated with the antiphospholipid syndrome, and vascular damage caused by cryoglobulin deposition. These mechanisms can coexist, particularly in lupus patients. Joint disease is sometimes the presenting manifestation in primary vasculitis. Autoantibodies are detectable in most patients with vascular involvement and inflammatory joint disease. They are not merely markers for vascular involvement: in vitro and in vivo data suggest that some autoantibodies may contribute to the genesis of endothelial lesions, together with other factors. For instance, evidence of pathogenic effects has been found for antineutrophil cytoplasmic antibody (ANCA), most notably with antimyeloperoxidase or antiproteinase-3 specificity, in small-vessel vasculitides (Wegener's granulomatosis, Churg-Strauss syndrome, and microscopic polyangiitis); for immune complexes, particularly those containing cryoglobulins, in vasculitides secondary to CTDs; and for circulating anticoagulant and anticardiolipin antibodies, above all anti-beta2-glycoprotein I, in antiphospholipid syndrome. Antibodies to annexin V, modified lipoproteins, and endothelial cells may be of interest; their clinical relevance is unclear, however, and no standardized assays are available, so thatthese antibodies are not looked for in everyday practice. When deciding which antibody tests should be performed in a given patient, the circumstances surrounding the onset of the vasculopathy should be borne in mind. In patients with previous CTD, the tests are selected based on the diagnosis. In contrast, in a patient with no previous diagnosis, a vasculopathy can be either primary or secondary to undiagnosed CTD or to antiphospholipid syndrome: consequently, a broader array of tests is needed in this situation.