Characterization of monoclonal anti-β2-glycoprotein-I and anti-prothrombin antibody fragments generated by phage display from a patient with primary antiphospholipid syndrome

Defective germinal center selection results in persistence of self-reactive B cells from the primary to the secondary repertoire in Primary Antiphospholipid Syndrome

Primary antiphospholipid syndrome (PAPS) is a life-threatening clotting disorder mediated by pathogenic autoantibodies. Here we dissect the origin of self-reactive B cells in human PAPS using peripheral blood and bone marrow of patients with triple-positive PAPS via combined single-cell RNA sequencing, B cell receptors (BCR) repertoire profiling, CITEseq analysis and single cell immortalization. We find that antiphospholipid (aPL)-specific B cells are present in the naive compartment, polyreactive, and derived from the natural repertoire. Furthermore, B cells with aPL specificities are not eliminated in patients with PAPS, persist until the memory and long-lived plasma cell stages, likely after defective germinal center selection, while becoming less polyreactive. Lastly, compared with the non-PAPS cells, PAPS B cells exhibit distinct IFN and APRIL signature as well as dysregulated mTORC1 and MYC pathways. Our findings may thus elucidate the survival mechanisms of these autoreactive B cells and suggest potential therapeutic targets for the treatment of PAPS.

Effect of hydroxychloroquine on antiphospholipid antibodies-inhibited endometrial angiogenesis

BACKGROUND

Antiphospholipid syndrome (APS) is an autoimmune disease characterized by thrombotic events and/or pregnancy morbidity (≥3 recurrent early miscarriage or fetal death or a prematurity <34 weeks of gestation) with persistently positive antiphospholipid antibodies (aPLs). It is reported that aPLs damage the placental tissue by binding to β2-glycoprotein I (β2GPI) on the surface of trophoblast and endothelial cells. Hydroxychloroquine (HCQ) is considered to be beneficial in the treatment of obstetrical APS and shown to restore the aPL-inhibited invasion and differentiation of trophoblast. However, not enough evidence exists regarding the effect of HCQ on endometrial angiogenesis. The aim of our study was to assess whether HCQ has an effect on aPL-inhibited endothelial angiogenesis.

METHODS

In this research, to explore the effect of HCQ for angiogenesis, we investigated: (1) Human umbilical vein endothelial cells (HUVECs) viability by CCK-8; (2) HUVECs migration by wound healing; (3) HUVEC angiogenesis by Matrigel assay in vitro; (4) mRNA expression of MMP-2 and VEGF by real-time quantitative Polymerase Chain Reaction (RT-PCR); (5) protein expression of VEGF, MMP-2 by western blot.

RESULTS

We found that HCQ treatment significantly restored the expression of aPL-inhibited VEGF and MMP-2. HCQ restored aPL-inhibited HUVEC proliferation, migration, and angiogenesis in vitro.

CONCLUSION

In conclusion, aPLs inhibit HUVECs angiogenesis, however, HCQ can restore the effect of aPL-inhibited HUVECs migration and angiogenesis in vitro, demonstrating its beneficial therapeutic role in obstetrical APS.

Anti-phosphatidylserine/prothrombin antibodies: an additional diagnostic marker for APS?

Among the diagnostic assays for anti-phospholipid syndrome (APS), lupus anticoagulant (LA) is the strongest predictor of thrombosis; however, it presents several limitations as interference with anticoagulant therapy and poor inter-laboratory agreement. Two-thirds of LA activity is apparently due to antibodies against prothrombin (PT), usually detectable by ELISA. Binding of PT to phosphatidylserine (PS) has been shown to enhance solid-phase anti-PT assay sensitivity. To determine the prevalence of antibodies against PS/PT (aPS/PT) in APS, we tested the semiquantitative QUANTA Lite(®) aPS/PT ELISA in a cohort of 80 APS patients. The prevalence of aPS/PT was 81.3%, rising to 87.6% when considering LA-positive subjects only. We observed a strong correlation between aPS/PT and LA (p = 0.006). To note, APS patients with thrombotic manifestations displayed significantly higher IgG aPS/PT titers compared to 20 aPL asymptomatic carriers (p = 0.012). To rule out a possible cross-reactivity of anti-β2 glycoprotein I antibodies (aβ2GPI) with PS/PT complex, we tested two monoclonal aβ2GPI antibodies and an affinity-purified (AP) polyclonal aβ2GPI IgG obtained from the serum of a patient reacting against both β2GPI and PS/PT. The two monoclonal antibodies did not show any reactivity against PS/PT complex, similarly the AP IgGs did not react toward PS/PT antigen while preserved their aβ2GPI activity. Our findings suggest that aPS/PT are a definite antibody population in APS. Moreover, the good correlation between aPS/PT ELISA and LA may support its use as a surrogate test for LA, particularly useful to overcome the technical limitations of the functional assay.

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.

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.

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.

Predictive, protective, orphan autoantibodies: the example of anti-phospholipid antibodies

Anti-phospholipid antibodies (aPL) are one of the most recent examples of autoantibodies that can appear even long time before any clinical manifestation can be associated with them. There is a general agreement that they may represent a strong risk factor for recurrent thrombosis and/or fetal losses. Anti-phospholipid antibodies represent a necessary but not sufficient factor (first hit) for thrombosis, and require additional triggering factors (second hit) to disclose the thrombogenic activity. Several factors may affect the predictive value of aPL, including titre, immunoglobulin isotype, fine antigenic specificity and affinity binding activity. Their careful evaluation is suggested in order to characterize the true predictive value of aPL.

APS--more systemic disease than SLE

The antiphospholipid syndrome is a systemic autoimmune disease that can have serious consequences for patients. Importantly, there is a wide range of clinical presentations. In this issue we have attempted to provide an overview of these features and place it in the context of autoimmunity.

Pathogenesis of the antiphospholipid syndrome: an additional example of the mosaic of autoimmunity

The antiphospholipid syndrome (APS) is a systemic autoimmune disease characterized by an adaptive immune response against self-PL-binding proteins ending in the production of specific autoantibodies. Antiphospholipid antibodies (aPL; and in particular anti-beta2 glycoprotein I antibodies) are formal diagnostic markers and pathogenic antibodies. Although APS may be considered as an autoantibody-mediated disease, there is now evidence that aPL are necessary but not sufficient to trigger some of the clinical manifestations of the syndrome. For example, additional factors, such as mediators of the innate immunity are now recognized to play a key role as second hits able to induce the thrombotic events in the presence of the autoantibodies. The APS scenario is also supplemented by the influence of genetically determined factors. Finally, environmental agents - in particular infectious ones - were reported to act as triggers for the production of autoantibodies cross-reacting with PL-binding proteins as well as inflammatory stimuli that potentiate the aPL thrombogenic effect. Altogether these findings do support the concept of a mosaic of factors that participate to the pathogenesis of the syndrome at different levels.

Autoantibody explosion in antiphospholipid syndrome

Antiphospholipid syndrome (APS) is characterized by thrombosis and pregnancy loss in the presence of antiphospholipid antibodies (aPL), mainly anticardiolipin, anti-beta2-glycoprotein I and lupus anticoagulant. However, similar to systemic lupus erythematosus, APS is also characterized by multiple other autoantibodies including 'non-classical' aPL, as well as other antibodies. Herein we describe the autoantigen properties, prevalence and clinical importance of 30 different antibodies in APS. Among the other antibodies characterizing APS are autoantibodies directed to platelets, glycoproteins, various coagulation factors, lamins, mitochondrial antigens and cell surface markers. Few of these autoantibodies are correlated with the presence of other antibodies, and some may have an additive role in the pro-thrombotic tendency of the syndrome. This autoantibody explosion might be important in early identification of the syndrome and its manifestations.

Intracellular trafficking of beta2-glycoprotein I complexes with lipid vesicles in macrophages: implications on the development of antiphospholipid syndrome

Beta(2)-glycoprotein I (beta(2)GPI) is known as a major autoantigen for antiphospholipid antibodies. Our recent data show that binding of beta(2)GPI to oxidized low-density lipoprotein (oxLDL) or to liposomes containing anionic phospholipid(s) may facilitate the presentation of beta(2)GPI's epitope by macrophages/dendritic cells to autoreactive T cells. In the present study, we investigated intracellular trafficking of beta(2)GPI and its complexes with oxLDL or liposomes containing phosphatidylserine (PS-liposomes) in mouse macrophage-like J774 cells. A relatively small amount of non-complexed beta(2)GPI was taken up and stagnated in the late endosome after incubating for 16h. In contrast, beta(2)GPI complexes with oxLDL or PS-liposomes were transported into the lysosome. In the presence of the IgG anti-beta(2)GPI autoantibody, WB-CAL-1, beta(2)GPI/oxLDL complexes were rapidly incorporated into intracellular space and were finally localized in the lysosome. Interestingly, in vitro pulses by beta(2)GPI/oxLDL complexes together with WB-CAL-1 led to the expression of membranous CD36 as well as Fcgamma type I receptors (FcgammaRI). These observations suggest that IgG immune complexes of beta(2)GPI/oxLDL provide not only FcgammaRI- but also scavenger receptor-mediated uptake of beta(2)GPI/oxLDL complexes by macrophages. Thus, beta(2)GPI/oxLDL complexes as a major atherogenic autoantigen and IgG anti-beta(2)GPI autoantibodies may facilitate antigen presentation and foam cell formation in antiphospholipid syndrome.

Insights into atherosclerosis therapy in antiphospholipid syndrome

The presence of early atheroma in antiphospholipid syndrome (APS) underscores the necessity of new therapies for this disorder. Inflammation plays an essential role in the pathogenesis of both APS secondary to systemic lupus erythematosus and atheroma. Several drugs currently used in APS have also anti-atherogenic properties via their anti-inflammatory effect. Furthermore, in addition to screening and treatment of traditional cardiovascular risk factors, it has been suggested that the introduction of some new medication such as statins in APS patients could be useful. This review assesses the current knowledge in this field and the justification for this new therapeutical approach. Furthermore, these data underscore the necessity for the use of other medication for atherosclerosis in APS patients, such as immunomodulatory drugs, which would be complementary to the anti-thrombotic and anti-platelet therapy.

Some antiphospholipid antibodies recognize conformational epitopes shared by beta2-glycoprotein I and the homologous catalytic domains of several serine proteases

OBJECTIVE

To test the hypothesis that some antiphospholipid antibodies (aPL) in patients with the antiphospholipid syndrome (APS) recognize a conformational epitope shared by beta2-glycoprotein I (beta2GPI; the major autoantigen for the antiphospholipid antibodies) and the homologous catalytic domains of several serine proteases (such as thrombin, activated protein C [APC], and plasmin) involved in hemostasis.

METHODS

We generated 4 new IgG monoclonal aPL (2 screened against beta2GPI, 1 against thrombin, and 1 against protein C) from 2 APS patients. The monoclonal antibodies (mAb) were analyzed for binding to beta2GPI, thrombin, APC, and plasmin, as well as for anticardiolipin antibody (aCL) activity. To demonstrate a shared epitope between beta2GPI and a serine protease, 1 mAb was studied by cross-inhibition analysis.

RESULTS

Both of the IgG anti-beta2GPI mAb bound to thrombin, APC, and plasmin. On the other hand, the 1 anti-thrombin mAb and the 1 anti-protein C mAb also bound to beta2GPI. Moreover, the binding of 1 cross-reactive mAb to beta2GPI was inhibited by alpha-thrombin (which contains only the catalytic domain of thrombin). All 4 mAb displayed aCL activity.

CONCLUSION

Taken together with the findings that some aCL bind to several serine proteases that participate in hemostasis and share homologous catalytic domains, these data demonstrate that some aCL in APS patients recognize one or more conformational epitopes shared by beta2GPI and the catalytic domains of disease-relevant serine proteases.

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.