b 2-glycoprotein I : Its role in the antiphospholipid syndrome
Promotoren: Prof. Dr. J.W.J. Bijlsma, Prof. Dr. Ph.G. de Groot
Co-promotor: Dr. R.H.W.M. Derksen
Abstract: The association between the development of thrombosis in patients and the persistent presence of antiphospholipid antibodies in their plasma is called the antiphospholipid syndrome. These antiphospholipid antibodies are not directed against phospholipids directly, but against phospholipid-bound proteins. One of the most important proteins against which these antibodies are directed is b 2-glycoproteine I (b 2GPI). The antibodies can form a complex with b 2GPI by two mechanisms: (1) one antibody binds one molecule of b 2GPI, resulting in a conformational change in b 2GPI and (2) one antibody binds two molecules of b 2GPI, resulting in the formation of a bivalent complex. Both mechanisms result in an enormously increased affinity of b 2GPI for negatively charged phospholipids. This is used for the detection of these antibodies: by the enormous increase in affinity for negatively charged phospholipids, the b 2GPI-antibody complex competes with clotting factors to bind to these phospholipids in a coagulation assay. Therefore, the phospholipids, needed for coagulation, are not available. This results in a prolongation of the clotting time, which is named lupus anticoagulant (LAC) activity.
In this study, fusion proteins between b 2GPI and the dimerisation domain of factor XI were made. This dimeric b 2GPI, which mimics the formation of a bivalent complex, binds with an enormously increased affinity to negatively charged phospholipids and possesses LAC activity. Thus, the contructs made mimic the in vitro effects of b 2GPI-antibody complexes.
The association between the thrombosis in patients and the LAC activity in vitro is paradoxal. Therefore, the effects of dimeric b 2GPI on platelet adhesion in an in vitro thrombosis model were investigated. In this model, blood flows with a defined over a thrombogenic surface (e.g. collagen), resulting in the adhesion of platelets to the surface. This surface coverage is analysed afterwards. In this study, the flow rate of the blood resembles the flow rate in arteries. With this model, it was shown that in the presence of dimeric b 2GPI or anti-b 2GPI antibodies platelet deposition increased. Furthermore, it was shown that the increased adhesion disappeared in the presence of inhibitors of thromboxane formation. The increased platelet adhesion, observed in the presence of anti-b 2GPI antibodies, was not affected when the Fc-receptor on platelets was blocked. An anti-b 2GPI antibody, that did not increase platelet deposition, inhibited the increase in platelet adhesion observed in the presence of dimeric b 2GPI, suggesting a role for a receptor specific for b 2GPI.
Taken these observations together, a model by which anti-b 2GPI antibodies can cause arterial thrombosis was postulated. The b 2GPI-antibody complex binds to negatively charged phospholipids exposed on the platelet surface. The exposition of these phospholipids is a result of the activation of the platelets by for example collagen. This is followed by the binding of the b 2GPI-antibody complex to a receptor specific for b 2GPI, which results in the activation of this receptor. Then, further activation of the platelets takes place, in which the formation of thromboxane is involved. Finally, this leads to sensitisation of the platelets, which are thereby more prone to form a thrombus. The described findings may lead to the development of new diagnostic tests and therapeutics in the future.
The complete thesis can be found using the link below:
URL: B.C.H. Lutters