Apolipoprotein E Receptor 2′ Mediates Pathogenic Effects of Dimeric β2glycoprotein I and of Anti- β2glycoprotein I Antibodies in Vivo

Antiphospholipid antibodies (aPL) recognize β2Glycoprotein (β2GPI)-bound to receptor (s) in target cells and trigger a pro-coagulant/pro-inflammatory phenotype [i e.:expression of tissue factor (TF), vascular cell adhesion molecule-1 (VCAM-1)] that lead to thrombosis. The interaction of β2GPI with target cells may involve more than one protein. Investigators have shown that dimeric β2GPI binds to apolipoprotein E receptor 2′ (apoER2′) in platelets, in the absence of anti-β2GPI antibodies, increases their activation and induces enhanced thrombosis and TF activity in mice. However, the role of apoER2′ in vivo in Antiphospholipid Syndrome (APS) is not completely understood. Here, we examined the in vivo effects of dimeric β2GPI and of anti-β2GPI antibodies (IgG-APS) in apoER2′ deficient (−/−) mice and in normal mice pre-treated with recombinant soluble domain 1 of apoER2′ (BD1). In vivo, dynamics of thrombus formation (thrombus sizes), TF activities in carotid artery homogenates and in peritoneal macrophages and ex vivo expression of VCAM-1 in aortas and of TF activity in peritoneal macrophages were examined in the various types of mice after two i.p. injections with 40 μg of recombinant dimeric β2GPI – or with the corresponding monomer control – or with 500 μg IgG-APS (isolated from a patient with APS by protein G Sepharose) or with control IgG (IgG-NHS). Mice injected with IgG-APS had significant titers of anticardiolipin (aCL) and anti-β2GPI antibodies in their sera. In vivo, IgG-APS increased significantly the size of the induced thrombi as well as the TF activities in carotid arteries and in peritoneal macrophages in C57BL/6J (wild type) mice when compared to same type of mice treated with IgG-NHS. Similarly, ex vivo expression of VCAM-1 in mouse aortas and of TF in peritoneal macrophages, detected by two photon excitation laser scanning microscopy were increased in normal mice treated with IgG-APS when compared to control mice. The pre-treatment with 40 μg of BD1 i.p., significantly reduced those effects. Importantly, dimeric β2GPI (in the absence of anti-β2GPI antibodies) or IgGAPS did not increase significantly thrombus size, TF activities in homogenates of carotid arteries or in peritoneal macrophages, or ex vivo expression of VCAM-1 and TF in mice lacking apoER2′. Conclusions: Altogether these data show that dimers of β2GPI mimic pathogenic effects of anti-β2GPI antibodies in mice. Most importantly, apoER2′ is a mediator of those effects in vivo. These findings may provide insights not only for a better understanding of the pathophysiology of APS but may be important in the development of new targeted therapies, by means of interfering with the binding of β2GPI-aPL complexes with their receptor(s) in target cells in vivo.