Abstract
This thesis provides a better understanding of the role of platelets in atherothrombosis. Platelets play an important role in the initiation and progression of atherosclerosis via their capturing of monocytes. We first identified a new mechanism of monocyte adhesion to the vessel wall via platelet derived flow-induced protrusions. Adherent and
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activated platelets form long membrane elongations that can reach lengths up to 250 mm. These structures can form microparticles and support monocyte rolling. This study introduces novel insights in the capacity of platelets to capture circulating monocytes, possibly playing a role in inflammation. Furthermore, we measured the responsiveness of platelets towards ADP in patients undergoing carotid endarterectomy. From these patients, the atherosclerotic plaque is removed from the carotid artery and analyzed for the presence of macrophages. Patients with a high responsiveness towards ADP have more macrophages in their carotid plaque, independent of medication use. Both of these studies show that platelets are very important for the infiltration of monocytes into the atherosclerotic plaque. Platelets furthermore play an important role in thrombosis and microthrombi formation. We found that plasmin is able to cleave platelet-von Willebrand Factor (VWF) complexes (microthrombi) in vitro. In addition, patients with acute thrombotic thrombocytopenic purpura (TTP) episodes have elevated levels of activated plasmin which inversely correlate with the degree of thrombocytopenia. We propose that during reduced or absent activity of VWF cleaving enzyme ADAMTS13 (as with TTP), plasmin can act as a natural backup for the degradation of obstructive platelet-VWF complexes. Besides the description of these novel mechanisms, we have translated our knowledge into the improvement of current therapies. We first optimized aCD34 stent coatings to improve reendothelialization and decrease restenosis, important determinants of clinical outcome. Platelets are very important for the outgrowth of CD34+ endothelial progenitor cells. We first compared the effects of biological stent coatings, fibronectin, fibrinogen and tropoelastin, on endothelial cell and smooth muscle cell characteristics, and their capacity to adhere platelets. Fibronectin/fibrinogen/tropoelastin inhibits VSMCs while compensating the inflammatory and procoagulant effects, and supporting the formation of a platelet monolayer. We suggested that coating a mixture of fibronectin/fibrinogen/tropoelastin on a stent may promote reendothelialization, while keeping unfavorable processes such as restenosis and procoagulant activity limited. This was tested in vivo by placing coated stents in the iliac arteries of rabbits. The proteins of our choice were unfortunately not able to improve reendothelialization or decrease neointima formation. Finally, we’ve searched for new biomarkers to predict the risk for restenosis. We therefore developed a cell assay that measures endothelial and smooth muscle cell migration in the presence of plasma samples of individual atherosclerotic patients and healthy donors. Unfortunately, cell migration did not correlate to any clinical characteristics, use of medication and follow-up data.Larger studies on the effects of the microenvironment on cell migration should show the consequences of this effect for vascular diseases.
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