Distinct and overlapping effects of β2-glycoprotein I conformational variants in ligand interactions and functional assays

Structural analyses of β2-glycoprotein I: is there a circular conformation?

BACKGROUND

Antiphospholipid antibodies targeting β2-glycoprotein I (β2GPI) cause thrombosis and pregnancy morbidity in antiphospholipid syndrome (APS) patients. How these antibodies recognize β2GPI remains controversial.

OBJECTIVES

This study aimed to elucidate the structure of β2GPI and evaluate how pathogenic anti-domain I (DI) antibodies recognize it in human plasma.

METHODS

β2GPI was made recombinant and purified from human plasma using different protocols. Structural and functional analyses were conducted using orthogonal techniques, namely, electron microscopy, size-exclusion chromatography, single-molecule Förster resonance energy transfer, and microfluidic diffusional sizing.

RESULTS

Electron microscopy and size-exclusion chromatography showed that the structure of β2GPI produced recombinantly and purified from plasma is elongated, even when subjected to conditions previously reported to favor circularization. Single-molecule Förster resonance energy transfer analyses of β2GPI labeled at positions 88 in DII and 278 in DV showed that these residues are located >90 Å apart, consistent with an elongated form. They also documented that the distance between these 2 residues did not change when the protein was reconstituted in human plasma. Microfluidic diffusional sizing documented that β2GPI binds with moderate affinity to a prototypical anti-DI antibody targeting the epitope G40-R43 despite being elongated.

CONCLUSION

Circulating β2GPI is elongated and, therefore, fully capable of binding to anti-DI antibodies. Binding of β2GPI to negatively charged phospholipids drives autoantibody recognition by increasing the local concentration of the antigen and not by dramatically changing its conformation. These findings clarify the structural properties of β2GPI, which have important implications for understanding APS pathogenesis and the development of APS diagnostics and therapeutics.

Managing Antiphospholipid Syndrome in Children and Adolescents: Current and Future Prospects

Pediatric antiphospholipid syndrome (APS) is a rare acquired multisystem autoimmune thromboinflammatory condition characterized by thrombotic and non-thrombotic clinical manifestations. APS in children and adolescents typically presents with large-vessel thrombosis, thrombotic microangiopathy, and, rarely, obstetric morbidity. Non-thrombotic clinical manifestations are frequently seen in pediatric APS and may be present even before the vascular thrombotic events occur. We review insights into the pathogenesis of APS and discuss potential targets for therapy. The identification of multiple immunologic abnormalities in patients with APS reveals molecular targets for current or future treatment. Management strategies, especially for APS in adolescents, require screening for additional prothrombotic risk factors and consideration of counseling regarding contraceptive strategies, lifestyle recommendations, treatment adherence, and mental health issues associated with this autoimmune thrombophilia. The main goal of therapy in pediatric APS is the prevention of thrombosis. The management of acute thrombosis events in children and adolescents is the same as for primary APS, which involves isolated occurrences, and secondary APS, which is seen in association with another autoimmune disease, e.g., systemic lupus erythematosus. A pediatric hematologist should be consulted so other differential thrombophilic conditions can be eliminated. Therapy includes unfractionated heparin or low-molecular-weight heparin followed by vitamin K antagonists. Treatment of catastrophic APS involves triple therapy (anticoagulation, intravenous corticosteroid pulse therapy, and plasma exchange) and may include intravenous immunoglobulin for children and adolescents with this condition. New drugs such as eculizumab and sirolimus seem to be promising drugs for APS.

Laboratory Approaches to Test the Function of Antiphospholipid Antibodies

Antiphospholipid syndrome (APS) is a systemic autoimmune disorder caused by the presence of aPLs (antiphospholipid antibodies, i.e., anti-β₂-glycoprotein I and anti-cardiolipin). Everyday practice in terms of laboratory diagnostics of APS includes determination of aPLs and well-known functional assays assessing for lupus anticoagulant (LA), in turn using various tests. According to recent guidelines, the recommended method for LA identification or exclusion is based on the Russell Viper Venom test and a sensitive activated partial thromboplastin time assay. Despite the fact that LA can be quantified in laboratory practice in this way, LA is still used as a binary parameter that is just one of the risk factors of thrombosis in APS. As of today, there are no other functional assays to routinely assess the risk of thrombosis in APS. It is well-known that APS patients display a wide range of clinical outcomes although they may express very similar laboratory findings. One way to solve this dilemma, could be if antibodies could be further delineated using more advanced functional tests. Therefore, we review the diagnostic approaches to test the function of aPLs. We further discuss how thrombin generation assays, and rotational thromboelastometry tests can be influenced by LA, and how experimental methods, such as flow cytometric platelet activation, surface plasmon resonance, or nano differential scanning fluorimetry can bring us closer to the puzzling interaction of aPLs with platelets as well as with their soluble protein ligand. These novel approaches may eventually enable better characterization of aPL, and also provide a better linkage to APS pathophysiology.

Specific domain V reduction of beta-2-glycoprotein I induces protein flexibility and alters pathogenic antibody binding

Beta-2-glycoprotein I (β2GPI) is a blood protein and the major antigen in the autoimmune disorder antiphospholipid syndrome (APS). β2GPI exists mainly in closed or open conformations and comprises of 11 disulfides distributed across five domains. The terminal Cys288/Cys326 disulfide bond at domain V has been associated with different cysteine redox states. The role of this disulfide bond in conformational dynamics of this protein has not been investigated so far. Here, we report on the enzymatic driven reduction by thioredoxin-1 (recycled by Tris(2-carboxyethyl)phosphine; TCEP) of β2GPI. Specific reduction was demonstrated by Western blot and mass spectrometry analyses confirming majority targeting to the fifth domain of β2GPI. Atomic force microscopy images suggested that reduced β2GPI shows a slightly higher proportion of open conformation and is more flexible compared to the untreated protein as confirmed by modelling studies. We have determined a strong increase in the binding of pathogenic APS autoantibodies to reduced β2GPI as demonstrated by ELISA. Our study is relevant for understanding the effect of β2GPI reduction on the protein structure and its implications for antibody binding in APS patients.