In vivo distribution of β2 glycoprotein I under various pathophysiologic conditions

Abstract

In vitro studies have documented ␤2 glycoprotein I (␤2GPI) binding to endothelial cells (ECs) and trophoblast using antiphospholipid antibodies. The in vivo binding of ␤2GPI to these cells and the conditions that favor their interaction have not been investigated. We analyzed the in vivo distribution of cyanine 5.5-labeled ␤2GPI in mice and evaluated the effect of pregnancy and circulating antibodies on its tissue localization. The signal was detected in the liver by whole body scan and ex vivo analysis. The ␤2GPI failed to bind to the vascular endothelium and reacted only with the ECs of uterine vessels. In pregnant mice the protein was localized on ECs and trophoblast at the embryo implantation sites. Immunized mice showed a similar ␤2GPI biodistribution to naive mice but the immunized pregnant animals exhibited a significant increase in fetal loss associated with C3 and C9 deposition at the implantation sites. Treatment of mice with LPS after ␤2GPI-Cy5.5 injection promoted protein localization on gut and brain ECs associated with IgG, C1q, and C9 deposition in immunized mice. These findings indicate that ␤2GPI binding to EC requires priming with pro-inflammatory factors which is not needed for uterine and placental localization probably dependent on hormonal changes. (Blood. 2011;118(15): 4231-4238) IntroductionAntiphospholipid syndrome (APS) is characterized by vascular thrombosis and adverse pregnancy outcome associated with circulating antiphospholipid antibodies (aPL) which are believed to play an important pathogenic role in the development of the clinical manifestations of the syndrome. 1,2 Human ␤2-glycoprotein I (␤2GPI) has been recognized as the major antigenic target for antiphospholipid antibodies and in vivo models have shown that antibodies directed against this molecule are able to mediate thrombus formation. 3,4 Beta2GPI is a heavily glycosylated glycoprotein that circulates in blood at a concentration of 150-300 g/mL 5 and is synthesized mainly in the liver, although expression of ␤2GPI mRNA has also been detected in endothelial cells (ECs), central nervous system, astrocytes, and placenta. The physiologic function of this protein is still unclear, but the apparently healthy life of humans and mice deficient in ␤2GPI suggests that its role is not all that critical. [6][7][8][9] Most of the information now available on ␤2GPI has been collected following the observation that this protein is the main target of antiphospholipid (aPL) antibodies. 10,11 Patients with circulating antibodies to ␤2GPI are at increased risk of venous and arterial thrombosis as well as of pregnancy complications including miscarriage, preeclampsia and retarded fetal growth. 1,12 For this reason antibodies with this specificity have been included among the criteria for the diagnosis of aPL syndrome (APS). 13,14 The induction of fetal loss and promotion of thrombosis in animal models as a result of immunization with ␤2GPI or passive transfer of antibodies further support the pathogenic role of these antibodies. [15][16][17...

Receptors involved in cell activation by antiphospholipid antibodies

“…In addition, LPS or other bacterial PAMPs may induce the conformational change of β2GP1 into the fishhook-like structure, which facilitates the interaction of aPLA with β2GP1, thereby increasing their pathogenic effects. In case of pregnancy morbidity, instead of infection, hormonal changes could play a role in aPLA pathogenicity [61,62].…”

A non–complement-fixing antibody to β2 glycoprotein I as a novel therapy for antiphospholipid syndrome

“…8 The domain specificity of the anti-b2GPI scFv-Fc was investigated by ELISA using recombinant domains kindly provided by Dr P. G. De Groot (Amsterdam, The Netherlands). 22 To displace b2GPI-bound patient antibodies, immobilized human b2GPI (10 mg/mL) was first incubated with patient serum (1:200) for 30 minutes at room temperature and then with MBB2DCH2 (2 mg/100 mL) for an additional 30 minutes at room temperature.…”

“…[4][5][6][7] We recently reported a significant increase in fetal loss in pregnant mice immunized with human b2GPI following the injection of a fluorescein-labeled purified protein that binds selectively to decidual endothelial cells and trophoblasts. 8 We also determined that the removal of anti-b2GPI antibodies from patient aPL-IgG using affinity chromatography significantly reduced the thrombotic effect in the rat mesenteric microcirculation. 9 Although aPL antibodies exhibit a modest reactivity with the soluble target molecules, it is widely accepted that the antibodies interact preferentially with b2GPI bound to endothelial cells, platelets, monocytes, and trophoblasts, all of which contribute to the pathogenesis of APS.…”

“…9,12 Although most of these observations were made using aPL of undefined specificity, the role of complement in inducing aPL-mediated blood clots and fetal resorption has been confirmed using specific antibodies to b2GPI. 8,9 In recent years, major efforts have been made to prevent aPLmediated thrombus formation and miscarriages. Currently, anticoagulants, such as heparin or warfarin, are used to prevent vascular thrombi, and a combination of low-dose aspirin and low-molecularweight heparin represents the first-line treatment of the obstetric complications of APS.…”

Obstetric Antiphospholipid Syndrome: Lobsters Only? Or Should We Also Look for Selected Red Herrings?

“…However, we still believe that obstetric APS does represent a distinct biological entity in comparison with the vascular variant. For example, β 2 -GPI is present only on the endothelial cells of uterine vessels but not on other vascular districts in resting animals; and aPL-positive IgG fractions from pure obstetric patients trigger different cell signaling in comparison with those from thrombotic APS sera 15,16 .…”