Pathogenic antiphospholipid antibody: an antigen-selected needle in a haystack

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.

B cells in primary antiphospholipid syndrome: Review and remaining challenges

It is now widely accepted that antiphospholipid antibodies (aPL) have direct pathogenic effects and that B cells, notably through aPL production, play a key role in the development of antiphospholipid syndrome (APS). Recent findings strengthened the implication of B cells with the description of specific B cell phenotype abnormalities and inborn errors of immunity involving B cell signaling in APS patients. In addition, it has been shown in preclinical models that cross-reactivity between APS autoantigens and mimotopes expressed by human gut commensals can lead to B cell tolerance breakdown and are sufficient for APS development. However, B cell targeting therapies are surprisingly not as effective as expected in APS compared to other autoimmune diseases. Elucidation of the B cell tolerance breakdown mechanisms in APS patients may help to develop and guide the use of novel therapeutic agents that target B cells or specific immune pathway.

Methodology of generation and purification of anti-beta 2 glycoprotein I antibodies

In this Method Article we are showing the methodology for generation and purification of Anti-Beta 2 Glycoprotein I (β₂GPI) antibodies. First β₂GPI was purified from human plasma, and recognized by Western Blot and anti-β₂GPI antibodies of serum from patients with antiphospholipid syndrome (APS). The C57BL/6 mice were immunized intraperitonealy with 150 μg of protein in adjuvant (β₂GPI or bovine serum albumin) on days 1, 8 and 14. Then the anti-β₂GPI antibodies were purified by affinity chromatography (Affi-Gel protein A sepharose) and affinity column using human β₂GPI coupled to CNBr-activated Sepharose 4B. Titles of anti-β₂GPI antibodies were determined by ELISA assays.

•

We purified β₂GPI with great efficacy and that is recognized antigenically by serum from patients with SAP or an anti-β₂gpi antibody.

•

We found that our purified antibody had 13 fold increased activity in ELISA test compared with the control and in Western Blot recognized with β₂GPI (reference and purified).

Antiphospholipid Antibodies Overlapping in Isolated Neurological Syndrome and Multiple Sclerosis: Neurobiological Insights and Diagnostic Challenges

Antiphospholipid syndrome (APS) is characterized by arterial and venous thrombosis, pregnancy morbidity and fetal loss caused by pathogenic autoantibodies directed against phospholipids (PL) and PL-cofactors. Isolated neurological APS may represent a significant diagnostic challenge, as epidemiological, clinical and neuroimaging features may overlap with those of multiple sclerosis (MS). In an open view, MS could be considered as an organ-specific anti-lipid (phospholipid and glycosphingolipid associated proteins) disease, in which autoreactive B cells and CD8+ T cells play a dominant role in its pathophysiology. In MS, diverse autoantibodies against the lipid-protein cofactors of the myelin sheath have been described, whose pathophysiologic role has not been fully elucidated. We carried out a review to select clinical studies addressing the prevalence of antiphospholipid (aPL) autoantibodies in the so-called MS-like syndrome. The reported prevalence ranged between 2% and 88%, particularly aCL and aβ2GPI, with predominant IgM isotype and suggesting worse MS prognosis. Secondarily, an updated summary of current knowledge on the pathophysiological mechanisms and events responsible for these conditions is presented. We draw attention to the clinical relevance of diagnosing isolated neurological APS. Prompt and accurate diagnosis and antiaggregant and anticoagulant treatment of APS could be vital to prevent or at least reduce APS-related morbidity and mortality.

Antiphospholipid Antibodies: Their Origin and Development

Antiphospholipid antibodies (aPL) are a hallmark of the antiphospholipid syndrome (APS), which is the most commonly acquired thrombophilia. To date there is consensus that aPL cause the clinical manifestations of this potentially devastating disorder. However, there is good evidence that not all aPL are pathogenic. For instance, aPL associated with syphilis show no association with the manifestations of APS. While there has been intensive research on the pathogenetic role of aPL, comparably little is known about the origin and development of aPL. This review will summarize the current knowledge and understanding of the origin and development of aPL derived from animal and human studies.

Lepidopteran cells, an alternative for the production of recombinant antibodies?

Monoclonal antibodies are used with great success in many different therapeutic domains. In order to satisfy the growing demand and to lower the production cost of these molecules, many alternative systems have been explored. Among them, the baculovirus/insect cells system is a good candidate. This system is very safe, given that the baculoviruses have a highly restricted host range and they are not pathogenic to vertebrates or plants. But the major asset is the speed with which it is possible to obtain very stable recombinant viruses capable of producing fully active proteins whose glycosylation pattern can be modulated to make it similar to the human one. These features could ultimately make the difference by enabling the production of antibodies with very low costs. However, efforts are still needed, in particular to increase production rates and thus make this system commercially viable for the production of these therapeutic agents.

Control of B cells expressing naturally occurring autoantibodies

Naturally occurring autoantibodies (NAbs) are typically polyreactive, bind with low affinity to a discrete set of autoantigens and are encoded by variable region genes in germline configuration. They differ from disease-associated autoantibodies (autoAb), which are mostly monoreactive, somatically mutated and of high affinities. Structure-function studies have shown that polyreactivity of NAbs relies on the somatically generated complementarity determining region, CDR3, of the heavy chain. This finding suggested that NAb-producing B cells were positively selected from the pre-immune B-cell repertoire. The biological significance of this selection remains, however, unclear. Data originating mainly from transgenic mice have shown that mature NAb-producing B cells are frequently ignorant toward their antigen, possibly due to their low affinity, though active tolerance mechanisms are not excluded. An important issue is whether NAb-producing B cells constitute the pool from which pathologic auto Ab emerge after autoantigen-driven maturation. We summarize results obtained in mouse models, showing that some infectious agents are able to induce an autoantigen-driven activation of certain NAb-producing B cells. However direct proof that selection by autoantigen may lead to somatic hypermutation are still lacking. Other data tend to suggest that pathologic auto Abs may derive from non-autoimmune B cells that have diversified by somatic hypermutation of their variable region genes.

[Immunological aspects of the antiphospholipid syndrome]

Antiphospholipid antibodies constitute a group of heterogeneous antibodies, which mainly recognize complexes made of proteins and anionic phospholipids. The nature of these complexes is currently better defined, as well as known, the structure of the antiphospholipid antibodies owing to the analysis of the monoclonal forms of these antibodies which were also studied both in terms of their precise specificities and cross-reactivity. However, the origin of these autoantibodies is not clearly understood, as well as the possible link between antiphospholipid antibodies present in healthy individuals, and those observed in autoimmune diseases. Only a fraction of antiphospholipid antibodies are pathogenic and directly responsible for the clinical manifestations of the antiphospholipid syndrome, but there is, to date, no biological test able to accurately detect pathogenic antiphospholipid antibodies. The diverse mechanisms which link these autoantibodies to the occurrence of symptoms, mainly during obstetrical complications, are better understood, and suggest new therapeutic avenues.

Evidence for heterogeneity of the obstetric antiphospholipid syndrome: thrombosis can be critical for antiphospholipid-induced pregnancy loss

BACKGROUND

Antiphospholipid antibodies are associated with thrombosis and repeated pregnancy losses during the antiphospholipid syndrome. Several experimental findings indicate that purified antiphospholipid antibodies are directly responsible for inflammation-induced pregnancy losses, or for disruption of the annexin A5 shield at the trophoblastic interface. We previously showed that passive transfer of CIC15, a monoclonal antiphospholipid antibody binding to cardiolipin and annexin A5 that was isolated from a patient with primary antiphospholipid syndrome, induces fetal resorption in pregnant mice.

OBJECTIVES

To investigate the mechanisms of CIC15-induced pregnancy loss.

METHODS/RESULTS

We show that CIC15 induces fetal loss through a new mechanism that is probably related to procoagulant activity. The time course is different from those of previously described models, and histologic analysis shows that the placentas are devoid of any sign of inflammation but display some signs of thrombotic events. Despite these differences, the CIC15 and 'inflammatory' models share some similarities: lack of FcγRI/III dependency, and the efficacy of heparin in preventing fetal losses. However, this latter observation is here mostly attributable to anticoagulation rather than complement inhibition, because fondaparinux sodium and hirudin show similar efficiency. In vitro, CIC15 enhances cardiolipin-induced thrombin generation. Finally, using a combination of surface-sensitive methods, we show that, although it binds complexes of cardiolipin-annexin A5, CIC15 is not able to disrupt the two-dimensional ordered arrays of annexin A5.

CONCLUSIONS

This human monoclonal antibody is responsible for pregnancy loss through a new mechanism involving thrombosis. This mechanism adds to the heterogeneity of the obstetric antiphospholipid syndrome.

The use of atomic force microscopy to study the pathologic effects of anti-annexin autoantibodies

Patients with recurrent pregnancy loss and a history of thrombotic events have often been noted to have autoantibodies directed at annexin A5. However, the relationship of these autoantibodies to immunopathology is still unknown, although it has been proposed that they have a direct effect on the function of annexin A5. Annexin A5 may be a significant immunological target with pathologic implications. Essentially, annexin A5 is an anticoagulant protein that crystallizes over negatively charged phospholipid surfaces and thereby blocks them from availability for coagulation reactions. To address this issue, we have taken advantage of our expertise with atomic force microscopy and studied anti-annexin A5 autoantibodies isolated from patients and focused on the ability of these antibodies to influence annexin A5 crystallization on planar mica-supported phospholipid bilayers. We report herein that such antibodies from patients, but not controls, produced a significant disruption of incomplete annexin A5 crystalline shield on phospholipid bilayer. In addition, the IgG fraction isolated from such patients significantly decreased the velocity of annexin A5 crystallization. Atomic force microscopy is a powerful tool to study the pathologic mechanisms of autoantibodies and the data herein reflect the potential of anti-annexin A5 antibodies that produce pathology in a number of varied but overlapping clinical conditions, including autoimmune thrombosis and antiphospholipid syndrome.

Use of monoclonal antibodies to dissect specificity and pathogenesis of antiphospholipid antibodies

Monoclonal antiphospholipid antibodies (aPL) have been utilized to dissect the relationship between their sequence, structure, binding and biological properties relevant to the pathogenesis of the antiphospholipid syndrome. In particular, sequence analysis of aPL has highlighted the clustering of certain amino acid residues in the antigen contact sites of their heavy and light chains. Therefore, these sequence motifs are likely to be important in determining aPL binding properties and their pathogenic effects. Experiments, however, using monoclonal aPL engineered to contain specific point mutations in their sequence which alter their ability to bind relevant antigens have shown that these alterations in binding are not directly mirrored by their pathogenic effects. In this review we focus on work carried out by others and ourselves using monoclonal antibodies with specific binding properties to extend our knowledge of the non-linear structure-binding-function relationship of aPL.

Natural autoantibodies: immune homeostasis and therapeutic intervention

The importance of natural autoantibodies reacting with self-antigens has long been neglected, as tolerance to self was believed to be primarily dependent on the deletion of autoreactive clones during ontogeny. However, it is now well established that autoreactive antibodies and B- and T-cells are present in healthy individuals. Research into the properties of natural autoantibodies and their role in immune homeostasis has been extensively investigated for a number of years. This information should lead towards the therapeutic exploitation of natural autoantibodies in the immunotherapy of autoimmune and inflammatory disorders.

Observational study of pregnant women with a previous spontaneous abortion before the 10th gestation week with and without antiphospholipid antibodies

BACKGROUND

A clinical subtype of purely obstetrical antiphospholipid antibody (aPL-Ab) syndrome (APS) requires three or more unexplained consecutive embryonic losses before the 10th week of gestation associated with persistently positive lupus anticoagulant (LAC), and/or anticardiolipin IgG or IgM, and/or anti-beta2-glycoprotein I (abeta2GpI) IgG or IgM. Although this diagnostic classification of APS appeared to be the most sensitive, the APS-associated serological criteria are still debated.

PATIENTS/METHODS

We prospectively observed the second pregnancy of 284 women with a previous embryonic loss, both with and without aPL-Ab.

RESULTS

aPL-Ab-positive women were more prone to pregnancy loss, embryonic loss, pre-eclampsia, placental abruption and intrauterine fetal growth restriction. Type IIa aPL-Ab positivity (LAC present alone) was associated with the highest risk of recurrent embryonic loss and intrauterine growth restriction. Type I aPL-Ab positivity (combinations of aPL-Ab type positivity) was associated with the strongest risks of late complications, pre-eclampsia and placental abruption. Finally, abeta2GpI-M positivities were not clinically relevant in these women.

CONCLUSION

Patients with a first unexplained pregnancy loss before the 10th week of gestation who are also positive for aPL-Abs have a higher risk of various complications in their second pregnancy. In this study, measurement of abeta2GpI-M had a questionable prognostic value.

Maternal anti-protein Z antibodies in pregnancies complicated by pre-eclampsia, SGA and fetal death

OBJECTIVE

Low maternal plasma protein Z (PZ) concentrations were reported in patients with pre-eclampsia (PE), a small for gestational age (SGA) neonate, and a fetal demise (FD). Anti-protein Z antibodies (APZ-AB) have been proposed as a possible underlying mechanism leading to low plasma PZ concentrations. The objective of this study was to determine the maternal plasma concentration of APZ-AB in women with a normal pregnancy, and patients with PE, an SGA neonate or a FD.

STUDY DESIGN

A cross-sectional study included women in the following groups: (1) non-pregnant women (n = 45); and pregnant women with: (2) normal pregnancies (n = 70); (3) PE (n = 123); (4) SGA neonates (n = 51); and (5) a FD (n = 51). Plasma concentrations of anti-protein Z IgM and IgG antibodies were measured by ELISA. Elevated APZ-AB was defined as >75th, 90th and 95th percentile of the normal pregnancy group. Non-parametric statistics were used for analyses.

RESULTS

(1) Patients with an SGA neonate had a higher median maternal plasma IgG APZ-AB concentration than women with normal pregnancies (p < 0.001), and patients with PE (p < 0.001) or with a FD (p = 0.001). (2) The proportion of patients with a maternal plasma IgM APZ-AB concentration >90th percentile was higher in the SGA group than in the PE group (p = 0.01). (3) Patients with PE maternal plasma IgM APZ-AB concentration >90th percentile had a higher rate of villous thrombosis (p = 0.03) and persistent muscularization of basal plate arteries (p = 0.01) than those with IgM APZ-AB concentration <90th percentile; and (5) Patients with FD and maternal plasma IgM APZ-AB concentration >90th percentile had a higher rate of umbilical phlebitis and arteritis than those with IgM APZ-AB concentration <90th percentile (p = 0.003).

CONCLUSIONS

(1) Patients with SGA neonates have a higher median plasma concentration of IgG APZ-AB than normal pregnant women, or patients with PE or FD; and (2) maternal plasma IgM APZ-AB concentration >90th percentile was associated with vascular placental lesions in patients with PE, but not in those with an SGA neonate, suggesting that in a subset of patients, these antibodies can be associated with abnormal placentation and pregnancy complications.

Examining the non-linear relationship between monoclonal antiphospholipid antibody sequence, structure and function

In the antiphospholipid syndrome (APS), pathogenic antiphospholipid antibodies (aPL) that cause thrombosis or pregnancy morbidity are characterized by binding to anionic phospholipids (PL) and β2-glycoprotein I (β2GPI). Sequence analysis of human monoclonal aPL has shown that high affinity for these antigens is associated with the presence of three particular amino acids: arginine (Arg), asparagine and lysine in the complementarity determining regions (CDRs) of their heavy and light chains. In vitro expression systems have been used to create variants of the antibodies in which these amino acids have been altered. In general, removal of Arg residues reduces affinity for anionic PL and β2GPI. Arg at different positions in the sequence, however, have different effects on binding affinity and effects on binding are not always mirrored by effects on pathogenicity. This review will focus upon the sequence motifs that have been found to distinguish pathogenic from non-pathogenic aPL, and whether these or other properties may help to identify distinct pathogenic subsets of aPL. In particular, we will focus on our recent work in which we are trying to develop a better understanding of the molecular mechanisms involved in activation of target cells by pathogenic aPL. These studies, together with molecular models of antigen/antibody complexes, help us to understand exactly how pathogenic antibodies interact with antigens. Ultimately, this understanding may aid the design of more powerful diagnostic/prognostic assays and targeted therapeutic agents to block the pathogenic effects of these antibodies.

Physiopathologie du syndrome des antiphospholipides

The antiphospholipid syndrome is characterized by the association of clinical events (thrombosis and/or obstetrical complications) and heterogeneous autoantibodies reacting with complexes of proteins and anionic phospholipids. Most of these recognized proteins can bind to anionic phospholipids and play a role in natural regulation of coagulation. Inhibition by these autoantibodies of the natural regulators of excessive coagulation is probably responsible for the prothrombotic state that characterizes this disease. Animal models have helped explain the mechanisms of obstetrical complications. Human antiphospholipid antibodies passively transferred to pregnant mice directly cause fetal resorption. This effect is mediated by complement fractions and neutrophil activation and is inhibited by heparin. The origin of these autoantibodies is still debated. Physiologically, the cause may be associated with exposure to anionic phospholipids on the surface of apoptotic cells. The affinity maturation process, which leads to the acquisition of somatic mutations, can then generate highly pathogenic antiphospholipid antibodies.

Arginine residues are important in determining the binding of human monoclonal antiphospholipid antibodies to clinically relevant antigens

In the antiphospholipid syndrome (APS), antiphospholipid Abs (aPL) bind to anionic phospholipids (PL) and various associated proteins, especially beta(2)-glycoprotein I (beta2GPI) and prothrombin. In the present study, we show that altering specific Arg residues in the H chain of a human pathogenic beta2GPI-dependent aPL, IS4, has major effects on its ability to bind these clinically important Ags. We expressed whole human IgG in vitro by stable transfection of Chinese hamster ovary cells with expression plasmids containing different V(H) and V(L) sequences. V(H) sequences were derived from IS4 by altering the number of Arg residues in CDR3. V(L) sequences were those of IS4, B3 (anti-nucleosome Ab), and UK4 (beta2GPI-independent aPL). Binding of the expressed H/L chain combinations to a range of anionic, neutral, and zwitterionic PL, as well as prothrombin, beta2GPI, dsDNA, and chicken OVA, was determined by ELISA. Of four Arg residues in IS4VH CDR3 substituted to Ser, two at positions 100 and 100g, reduced binding to all Ags, while two at positions 96 and 97 reduced binding to beta2GPI but increased or decreased binding to different PL. Eleven of 14 H/L chain combinations displayed weak binding to OVA with Arg to Ser replacements of all four Arg residues enhancing binding to this Ag. Only one H/L chain combination bound neutral PL and none bound dsDNA; hence, these effects are particularly relevant to Ags important in antiphospholipid syndrome. We hypothesize that these four Arg residues have developed as a result of somatic mutations driven by an Ag containing both PL and beta2GPI.

Do antiphospholipid antibodies develop for a purpose?

There is a growing body of literature suggesting that antiphospholipid antibodies develop for a purpose, that they play several key roles in the innate immune response, and are only rendered pathologic in susceptible people under adverse intravascular conditions. This paper will review evidence that the autoantibodies associated with the antiphospholipid syndrome develop from natural autoantibodies for purposes that are beneficial to host defense, and are only rendered pathologic as a result of adverse intravascular events.

Pathophysiology of the antiphospholipid syndrome

Antiphospholipid syndrome is a distinct disorder with the clinical features of recurrent thrombosis in the venous or arterial circulation and fetal losses. Its serological marker is the presence of antiphospholipid antibodies in the blood of these patients. The relation between the presence of antibodies against anionic phospholipids and thromboembolic complications is well established over the last 25 years but the pathophysiology of the syndrome is largely unclear. Even after all these years, there is a persisting debate about the specificity and sensitivity of the assays for the detection of antiphospholipid antibodies. We now accept that antibodies to beta2-glycoprotein I rather than to anionic phospholipids are the major pathological antibodies, although there is no clear consensus on how the presence of these antibodies correlates with the different clinical manifestations of the syndrome. In this review, we discuss the current methods of detection of the antibodies and our insight into the pathobiology of the syndrome. We propose a mechanism for describing how the presence of anti-beta2-glycoprotein I antibodies relates to the different clinical manifestations observed.

Defective B cell tolerance checkpoints in systemic lupus erythematosus

A cardinal feature of systemic lupus erythematosus (SLE) is the development of autoantibodies. The first autoantibodies described in patients with SLE were those specific for nuclei and DNA, but subsequent work has shown that individuals with this disease produce a panoply of different autoantibodies. Thus, one of the constant features of SLE is a profound breakdown in tolerance in the antibody system. The appearance of self-reactive antibodies in SLE precedes clinical disease, but where in the B cell pathway tolerance is first broken has not been defined. In healthy humans, autoantibodies are removed from the B cell repertoire in two discrete early checkpoints in B cell development. We found these checkpoints to be defective in three adolescent patients with SLE. 25–50% of the mature naive B cells in SLE patients produce self-reactive antibodies even before they participate in immune responses as compared with 5–20% in controls. We conclude that SLE is associated with abnormal early B cell tolerance.

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.