β2-Glycoprotein I can exist in 2 conformations: implications for our understanding of the antiphospholipid syndrome

Abstract

The antiphospholipid syndrome is defined by the presence of antiphospholipid antibodies in blood of patients with thrombosis or fetal loss. There is ample evidence that ␤ 2 -glycoprotein I (␤ 2 GPI) is the major antigen for antiphospholipid antibodies. The autoantibodies recognize ␤ 2 GPI when bound to anionic surfaces and not in solution. We showed that ␤ 2 GPI can exist in at least 2 different conformations: a circular plasma conformation and an "activated" open conformation. We also showed that the closed, circular conformation is maintained by interaction between the first and fifth domain of ␤ 2 GPI. By changing pH and salt concentration, we were able to convert the conformation of ␤ IntroductionThe antiphospholipid syndrome (APS) is defined as the presence of antiphospholipid antibodies in the blood of patients with thrombosis or fetal loss. The APS is one of the most common causes of acquired thrombophilia, 1 especially at younger age. In 1990, it was shown that the so-called antiphospholipid antibodies do not recognize phospholipids directly but they interact with phospholipids via the plasma protein ␤ 2 -glycoprotein I (␤ 2 GPI). [2][3][4] However, the discovery of ␤ 2 GPI as target for the autoantibodies did not provide a more in-depth knowledge on the underlying cause of the syndrome. It was unclear which metabolic pathway was disturbed by the autoantibodies because no physiologic function has convincingly been ascribed to ␤ 2 GPI to date. Nevertheless, as antibodies against ␤ 2 GPI can induce thrombosis in animal models, 5-7 the protein ␤ 2 GPI must hold an important functional clue to our understanding of the syndrome.␤ 2 GPI is a highly abundant 43-kDa protein that circulates at a concentration of approximately 200 g/mL. ␤ 2 GPI consists of 326 amino acids arranged in 5 short consensus repeat domains. 8,9 The first 4 domains contain 60 amino acids each, whereas the fifth domain has a 6-residue insertion and an additional 19-amino acid C-terminal extension. The extra amino acids are responsible for the formation of a large positively charged patch within the fifth domain of ␤ 2 GPI 10 that forms the binding site for anionic phospholipids. The anti-␤ 2 GPI antibodies that recognize an epitope located in the first domain correlate better with the thrombotic complications than antibodies directed against other domains. 11-13 Antibodies directed against ␤ 2 GPI have become one of the serologic markers characterizing the APS. 14 After binding to anionic surfaces, ␤ 2 GPI exposes a cryptic epitope that is recognized by the autoantibodies present in the APS. 11,12,15,16 These antibodies only recognize ␤ 2 GPI when it is bound to a surface and do not recognize ␤ 2 GPI in solution. Moreover, no circulating immune complexes between antibodies and ␤ 2 GPI have been detected in patient plasmas. 17 This seems not to be the result of clearance of these complexes from plasma because plasma levels of ␤ 2 GPI in antiphospholipid patients are the same as plasma levels of ␤ 2 GPI in healthy persons. 18 The crystal st...

Receptors involved in cell activation by antiphospholipid antibodies

“…Domain V mediates the binding of the molecule to anionic phospholipids while domain I seems to be the main target of antibodies associated with an increased risk of thrombosis [24,25]. At least two different conformations are known for β2GP1: a circular plasma conformation in which domain I interacts with domain V and an "activated" fishhook-like conformation [26]. The fishhook-like conformation is obtained after binding of positively charged patch of domain V to anionic phospholipids.…”

“…The fishhook-like conformation is obtained after binding of positively charged patch of domain V to anionic phospholipids. The dissociation of domain I and domain V leads to the exposure of an epitope containing the amino acids Arg39 and Arg43 that are critical for binding of pathogenic aPLA [26]. While the contribution of anti-β2GP1 antibodies in APS is well described, the physiological function of β2GP1 remains to be established.…”

Differential assay reactivity of immunglobulin A anti-ß2 glycoprotein I antibodies: implications for the clinical interpretation of antiphospholipid antibody testing

“…We investigated the nature of the IgA anti-β 2 GPI antibody discrepancy on selected clinically and serologically well-characterized SLE and/ or APS samples by isolating IgA antibodies to analyze the reactivity of the fractions on two discrepant assays. One hypothesis to explain the discrepancy was that coated β 2 GPI of one assay displayed the open (reactive) β 2 GPI configuration, while the other assay had the closed (non-reactive) configuration (9).…”

The role of TLR4 in pathophysiology of antiphospholipid syndrome‐associated thrombosis and pregnancy morbidity

“…b2GPI exists in at least 2 different conformations: a circular plasma conformation and an "activated" open conformation (Agar et al, 2010). In circular b2GPI, the epitopes for the autoantibodies interacts with domain V of the same molecule.…”