Background and Purpose-Rupture of atherosclerotic plaques is one of the main causes of ischemic strokes. The aim of this study was to investigate carotid plaque vulnerability markers in relation to blood flow direction and the mechanisms leading to plaque rupture at the upstream side of carotid stenoses. Methods-Frequency and location of rupture, endothelial erosion, neovascularization, and hemorrhage were determined in longitudinal sections of 80 human carotid specimens. Plaques were immunohistochemically analyzed for markers of vulnerability. Plaque geometry was measured to reconstruct shape profiles of ruptured versus stable plaques and to perform computational fluid dynamics analyses. Results-In 86% of ruptured plaques, rupture was observed upstream. In this region, neovascularization and hemorrhage were increased, along with increased immunoreactivity of vascular endothelial and connective tissue growth factor, whereas endothelial erosion was more frequent downstream. Proteolytic enzymes, mast cell chymase and cathepsin L, and the proapoptotic protein Bax showed significantly higher expression upstream as compared with the downstream shoulder of atherosclerotic lesions. Comparison of geometric profiles for ruptured and stable plaques showed increased longitudinal asymmetry of fibrous cap and lipid core thickness in ruptured plaques. The specific geometry of plaques ruptured upstream induced increased levels of shear stress and increased pressure drop between the upstream and the downstream plaque shoulders. Conclusions-Vulnerability of the upstream plaque region is associated with enhanced neovascularization, hemorrhage, and cap thinning induced by proteolytic and proapoptotic mechanisms. These processes are reflected in structural plaque characteristics, analyses of which could improve the efficacy of vascular diagnostics and prevention.

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Background: Atherosclerotic plaques develop at arterial regions subjected to non-uniform shear stress, and are initiated by increased leukocyte-endothelial interactions. In this study, we investigated the effects of distinct shear stress patterns on endothelial recruitment of monocytic cells.Methods: Human umbilical vein endothelial cells (ECs) were exposed to laminar or non-uniform shear stress in bifurcating flow-through slides, followed by 2 h stimulation with TNF-␣. To study cell adhesion, ECs were perfused with medium containing THP-1 monocytic cells for 1 h. Endothelial protein expression was determined by immunofluorescence.Results: Exposure to non-uniform shear stress and TNF-␣ lead to progressive induction of adhesion molecules and increase in monocytic cell adhesion observed over 0.5-3 h. To investigate the relative role of the shear stress patterns in monocytic cell recruitment, ECs were exposed to reduced levels of shear stress, resulting in a reduced gradient steepness in the non-uniform shear stress regions. Lowering the shear stress from 10 to 5 and 2 dyne/cm 2 resulted in increased monocytic cell adhesion under laminar shear stress. However, in these conditions, adherent monocytic cells under non-uniform shear stress were strongly reduced. Moreover, in the region exposed to shear stress gradient parallel to flow direction, monocytic cell adhesion was significantly lower than in the region of non-uniform shear stress, characterized by transversal gradient.Conclusion: Exposure to non-uniform shear stress results in progressive induction of adhesion molecules and monocytic cell recruitment in response to circulating TNF-␣. Enhanced monocytic cell recruitment at bifurcations is affected not only by the magnitude and steepness of shear stress gradient, but also by its direction in relation to the flow.