Apolipoprotein E receptor 2 is involved in the thrombotic complications in a murine model of the antiphospholipid syndrome

Abstract

Antiphospholipid (aPL)/anti-␤ 2 glycoprotein I (anti-␤ 2 GPI) antibodies stimulates tissue factor (TF) expression within vasculature and in blood cells, thereby leading to increased thrombosis. Several cellular receptors have been proposed to mediate these effects, but no convincing evidence for the involvement of a specific one has been provided. We investigated the role of Apolipoprotein E receptor 2 (ApoER2) on the pathogenic effects of a patient-derived polyclonal aPL IgG preparation (IgG-APS), a murine anti-␤ 2 GPI monoclonal antibody (E7) and of a constructed dimeric ␤ 2 GPI I (dimer), which in vitro mimics ␤ 2 GPI-antibody immune complexes, using an animal model of thrombosis, and ApoER2-deficient (؊/؊) mice. In wild type mice, IgG-APS, E7 and the dimer increased thrombus formation, carotid artery TF activity as well as peritoneal macrophage TF activity/expression. Those pathogenic effects were significantly reduced in ApoER2 (؊/؊) mice. In addition, those effects induced by the IgG-APS, by E7 and by the dimer were inhibited by treatment of wild-type mice with soluble binding domain 1 of ApoER2 (sBD1). Altogether these data show that ApoER2 is involved in pathogenesis of antiphospholipids antibodies. (Blood. 2011;117(4):1408-1414) IntroductionThe association between persistently present antiphospholipid (aPL) antibodies and the clinical manifestations of thrombosis or pregnancy morbidity is known as the antiphospholipid syndrome (APS). 1 aPL antibodies are heterogeneous and recognize a wide variety of plasma proteins with phospholipid-binding properties, such as prothrombin 2 and ␤ 2 glycoprotein I (␤ 2 GPI). 3,4 aPL antibodies directed against ␤ 2 GPI, a plasma protein without known physiologic function, are considered the most pathologically relevant antibodies.There is strong experimental evidence that anti-␤ 2 GPI antibodies have thrombogenic properties. In studies on endothelial cell activation, 5-8 authors have shown the induction of a prothrombotic and proinflammatory phenotype upon exposure to anti-␤ 2 GPI antibodies, indicated by expression of tissue factor (TF) and increased surface expression of adhesion molecules, such as intercellular adhesion molecule-1, vascular-cell adhesion molecule-1, and E-selectin. Activation of monocytes by anti-␤ 2 GPI antibodies leads to TF expression as well. 9 Furthermore, anti-␤ 2 GPI antibodies, or recombinant dimers of ␤ 2 GPI that mimic ␤ 2 GPI-antibody immune complexes, increase platelet deposition to extracellular matrix components in in vitro flow models. 10 Injection of anti-␤ 2 GPI antibodies in murine 11 or hamster 12 thrombosis models leads to increased thrombus formation.Several receptors were postulated to mediate the prothrombotic cellular effects of anti-␤ 2 GPI antibodies. The interaction between annexin A2 and ␤ 2 GPI-antibody immune complexes has been reported to lead to endothelial cell activation. 13 It seems unlikely, however, that annexin A2 is able to convey activation signals across the cell membrane because this phospholipid-binding protei...

Receptors involved in cell activation by antiphospholipid antibodies

“…The interaction of dimerized β2GP1 with either ApoER2' or GPIbα was found to lower the threshold of platelet activation by collagen or fibronectin surfaces [31,65]. While EC and monocytes do not express ApoER2', other members of LDL-receptor family were shown to bind the dimerized form of β2GP1 [64,66].…”

“…Several candidate receptors have been proposed for aPLA: in vivo studies have shown that deficiencies of TLR4 [29], Annexin A2 [30], ApoER2 [31] and several complement factors [20] reduced the pathogenic effects of aPLA. Moreover, ex vivo studies suggest a role for TLR2 [32][33][34], CD14 [34], GPIbα [35] and TLR8 [36] in aPLA-activated monocytes and EC.…”

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

“…Additionally, a recombinant-binding domain I of ApoER2 has been shown to act as an inhibitor of increased thrombus formation in different murine models of APS. Moreover, aPL-induced increases in leucocyte adhesion to endothelium, peritoneal macrophage TF activity/expression and thrombus formation were significantly reduced in ApoER2 (À/À) mice (Urbanus et al, 2008b;Romay-Penabad et al, 2011). Interestingly, ApoER2′ needs a receptor cooperation for its optimal function and specificity.…”

“…However, in vitro and in vivo experimental models highlight that TLR4 only partially contributes to the thrombogenic effect of aPL, suggesting that more than one receptor may be involved in b2GPI cellular binding. The main candidate receptors for b2GPI on cell membranes have been identified as TLR2, megaline, apolipoprotein Ereceptor2′ (ApoER2′), glycoprotein Iba (GPIba) and platelet factor 4 (PF4) (Kobayashi Review ª 2013John Wiley & Sons Ltd et al, 2001Shi et al, 2006;Alard et al, 2010;Sikara et al, 2010;Romay-Penabad et al, 2011).…”

Pathogenic Mechanisms of Thrombosis in Antiphospholipid Syndrome (APS)

“…The role of apoER2' was recently investigated in an animal model of thrombosis. Passive immunization of apoER2'-/-mice with aPL purified from patients or monoclonal anti-2GPI or 2GPI chimeric dimers, caused significantly reduced clots and reduced TF expression of vascular cells and macrophages, compared with immunization of control mice [ 86 ]. The role of GPIba as a 2GPI-receptor and potential mediator of platelet activation has been studied by in vitro binding assays and selective inhibition using specific antibodies [ 87 ].…”