Murine Abdominal Aortic Aneurysm Intraluminal Thrombus Composition and Structure

An abdominal aortic aneurysm (AAA) is a dilation of the aortic wall in the abdomen. Many AAA patients develop intraluminal thrombus (ILT), but the role of ILT in AAA progression and rupture is not well understood. To evaluate ILT in AAAs, we induced AAAs in male C57Bl6/J mice (n=25) via surgical application of topical elastase (5 µL of 5 or 10 mg/mL) to the abdominal aorta below the renal arteries and administration of β-aminopropionitrile (BAPN, 0.2%) drinking water. We collected weekly/biweekly ultrasound images over 56 days. Mice were euthanized and histology images were collected. We semi-quantitatively assessed elastin degradation and inflammation from Movat’s pentachrome and H&E-stained samples, respectively. Mice with ILT had more significant expansion over the length of the study (beginning at day 14, p<0.05). From histology, ILT samples showed more elastin disorganization and greater inflammation. From scanning electron microscopy, we were able to confirm the presence of layered sheets of fibrin and abnormally shaped red blood cells (polyhedrocytes) within the ILT deposits. In this model, elastase causes aortic injury by degrading elastin fibrils in the aortic wall, reducing the ability of the aorta to contract during high-pressure blood flow. Further damage to the extracellular matrix is likely driven by subsequent inflammation. Here we observed tissue samples with greater acute-on-chronic inflammation were correlated with more elastin damage, and therefore greater aortic expansion. Further, larger aortic expansions were correlated with slower blood flow, likely due to increased cross-sectional area. Thus, increased aortic expansion and damage to the aortic wall may be more likely to create hemodynamic conditions that are conducive to the initiation of ILT deposition: endothelial damage and reduced blood flow. Understanding the relationship between ILT formation, aortic wall degradation, and inflammation could help refine therapeutic strategies for treating AAAs.