Characterization of a large animal model of abdominal aortic aneurysm (AAA) and its comparison with human and murine AAA

Objective:  Abdominal aortic aneurysm (AAA) is a prevalent disease with high morbidity generated in affected patients. To better understand the pathomechanisms involved in AAA formation animal models with translational relevance to human conditions are essential. AAA disease has been extensively studied in mouse models of AAA, such as the porcine pancreatic elastase (PPE) model. In recent years, porcine AAA models have been established and promise greater translational potential due to closer proximity to the human cardiovascular system, morphology and haemodynamic parameters. This study refines a porcine AAA model using a triple-hit strategy and compares it to murine PPE model and human AAA, assessing advantages and limitations of both small and large animal models.

Methods:  Human aortic specimens were obtained during surgery. Porcine AAA was induced in pigs via intraluminal delivery of PPE and collagenase, combined with balloon dilation and pre- and postoperative oral administration of the lysyl oxidase inhibitor β-Aminopropionitrile (BAPN). In mice, AAA was induced according to the established PPE model. Aortic diameters were measured by MRI (pigs) and ultrasound (both species) at specific intervals until euthanasia at 28 days. 

After euthanasia and harvesting, histological stains including hematoxylin-eosin, Movat-pentachrome, Elastica-Van-Gieson and Kossa-Silver stain were performed to evaluate AAA characteristics including aortic wall-layer thickness, elastic fragmentation and destruction and calcification. Immunohistochemical staining for αSMA visualized smooth muscle cell degradation in the media. To study the inflammatory response, cytocine expression was assessed using a cytocine array for common cytocines including IL-1β, IL-6, GM-CSF and TNFα. 

Results:  Successful AAA induction, defined by a 150% increase in aortic diameter was achieved in four juvenile domestic pigs, measured 14 and 28 days after surgery with a maximum increase of mean aortic diameter to 1,76 ± 0,11 cm. Histological analysis showed significant increases in intima (p<0.001), media (p=0.033) and adventitia (p=0.0378) wall layers in porcine AAA. Altered wall layer ratios, which in human pathology are characterized by a relative increase in tunica intima compared to tunica media and adventitia could only be detected in porcine aortas. The number and length of elastin fibers were not altered in the murine model, whereas both parameters were reduced in porcine aortas (p<0.0001 and p=0.0002). Only porcine AAA exhibited aortic calcification comparable to human AAA(p=0.0017). A decrease in αSMA-positive cells was observed in mice (p=0.0449) and pigs (p=0.1229). Comparison of cytokine profiles of all three species with AAA showed a greater similarity between humans and pigs, particularly for IL-1β, GM-CSF and TNFα (p=0.0001, p=0.003). 

Conclusions: At the time point analyzed, the refined large animal model shows greater similarity to human AAA regarding vessel-wall-ratios, elastic degradation, calcification, media destruction and inflammation. Although not all human AAA features are replicated, these results highlight the relevance of large animal models for translational AAA research, facilitating the development of therapeutic strategies. Further in-depth studies are warranted to enhance model accuracy and applicability.