Effects of Soluble Domain 1 of Apolipoprotein ER2′ (BD1 apoER2′) on Pathogenic Effects of a Dimer of β2glycoprotein I (β2GPI) In Vivo.

Background: It has been shown that endothelial cell (EC) activation and tissue factor (TF) upregulation in EC and monocytes and platelet activation by anti-β2glycoprotein I (aβ2GPI) antibodies leads to a prothrombotic state. However, the receptor recognized by aβ₂GPI Abs in target cells is not known. It has been demonstrated that dimers of β2GPI mimic the effects of β2GPI-aβ2GPI complexes in that they increased platelet adhesion to collagen and that these dimers bind to apolipoprotein ER2′ (apoER2′) in platelets. Here, we examined the effects of a dimeric form of β2GPI on thrombosis, on platelet activation and TF activity in vivo in mice. The effects of soluble recombinant binding domain 1 (BD1) of apoER2′ were also studied.

Methods: We treated CD1 mice with 50 μg/mL (i.p) recombinant dimeric β2GPI or with recombinant monomeric β2GPI twice at 0 and 48 hours later. Some mice were infused i.p. with 40 μg of BD1. The size of an induced thrombus in the femoral vein of the mice was examined as described elsewhere 72 hours after the first injection. TF activity was determined using a chromogenic assay in homogenates of carotid arteries, and in peritoneal cells of mice. Platelet aggregation was determined by aggregometry in mouse platelet rich plasma.

Results: In vivo, dimers of β2GP1 increased significantly the size of the induced thrombi, TF activity in carotid artery homogenates, in peritoneal macrophages and platelet aggregation, when compared to mice treated with control protein. (Table). These effects were significantly diminished by pre-treatment of the mice with BD1apoER2′.

Conclusions: Thus, when β2GPI is dimerised, it can induce thrombosis, TF and platelet activation in mice independently of the presence of antibodies. The data also indicate that the binding of dimerised β2GPI to target cells may involve apoER2′ since the effects observed were ameliorated by BD1 apoER2′. These findings may be important in understanding the pathogenic mechanisms of thrombosis in Antiphospholipid Syndrome (APS) and in designing new modalities to prevent/ameliorate clinical manifestations of APS.