From intraplaque haemorrhages to plaque vulnerability: biological consequences of intraplaque haemorrhages

Intraplaque hemorrhages are mainly related to inward neoangiogenesis, initiated from the adventitia by lipid-dependent outwardly convected signals, and by the immaturity of these neovessels, allowing leaks and hemorrhages. Repeated intraplaque hemorrhages play a major role in the evolution of thrombotic occlusive disease, similar to the role of intraluminal thrombus in the progression of abdominal aortic aneurysm toward rupture. Red blood cells (RBCs) are an important source of unesterified cholesterol, because their membranes are particularly cholesterol rich. This unesterified cholesterol is rapidly organized in cholesterol crystals, highly toxic for cell and membranes. Oxidized cholesterol and LDL provoke the irreversible covalent aggregation of proteins, including hemoglobin, forming ceroids, which are also highly toxic. Hemoglobin play a major role of prooxydant molecules in this context, by its ability to release heme and iron, the main catalyser of oxidative reaction. In the context of type 2 diabetes, the oxidative potential of intraplaque-free hemoglobin play a predominant role in the progression of atherothrombotic disease toward clinical expression. Associated to RBC, intraplaque hemorrhages convey leukocyte, mainly neutrophils in human, and plasma zymogen that are the main source of proteases, including coagulation proteases, activation of the fibrinolytique system, release of leukocyte serine proteases and cathepsins and activation of MMPs. These proteases concentrate in the hemorrhagic/necrotic core rendered plaque highly vulnerable. An adaptive immune response takes place in the adventitia, in regard of hemorrhagic plaques, in relation to outwardly convected oxidized or proteolyzed neoantigens, and chemokinic signals. Finally, intraplaque hemorrhages and thrombi are the site of weak pathogen entrapment, which promote all these oxydative and proteolytic phenomenons.