Endovascular needle injection of cholesteryl linoleate into the arterial wall produces complex vascular lesions identifiable by intravascular ultrasound: early development in a porcine model of vulnerable plaque

Mouse models of atherosclerosis and their suitability for the study of myocardial infarction

Atherosclerotic plaques impair vascular function and can lead to arterial obstruction and tissue ischaemia. Rupture of an atherosclerotic plaque within a coronary artery can result in an acute myocardial infarction, which is responsible for significant morbidity and mortality worldwide. Prompt reperfusion can salvage some of the ischaemic territory, but ischaemia and reperfusion (IR) still causes substantial injury and is, therefore, a therapeutic target for further infarct limitation. Numerous cardioprotective strategies have been identified that can limit IR injury in animal models, but none have yet been translated effectively to patients. This disconnect prompts an urgent re-examination of the experimental models used to study IR. Since coronary atherosclerosis is the most prevalent morbidity in this patient population, and impairs coronary vessel function, it is potentially a major confounder in cardioprotective studies. Surprisingly, most studies suggest that atherosclerosis does not have a major impact on cardioprotection in mouse models. However, a major limitation of atherosclerotic animal models is that the plaques usually manifest in the aorta and proximal great vessels, and rarely in the coronary vessels. In this review, we examine the commonly used mouse models of atherosclerosis and their effect on coronary artery function and infarct size. We conclude that none of the commonly used strains of mice are ideal for this purpose; however, more recently developed mouse models of atherosclerosis fulfil the requirement for coronary artery lesions, plaque rupture and lipoprotein patterns resembling the human profile, and may enable the identification of therapeutic interventions more applicable in the clinical setting.

Accelerated miniature swine models of advanced atherosclerosis: A review based on morphology

In order to accelerate development of atherosclerosis(AS) in miniature swine models, varieties of strategies and methods have been explored. In addition to traditional methods such as high cholesterol feeding and balloon injury, new methods such as familial hypercholesterolemia induced by gene editing and intramural injection have been applied in recent years. Although it has been claimed that these methods have successfully aggravated lesion areas and stenosis, lesion features induced by different strategies have shown heterogeneity in morphology. In addition, time consumption, high cost, and unavailability are problems that restrict application of these AS models. Here, we summarize strategies and methods to accelerate AS models and further analyze their values, advantages, and shortcomings.

Local intravascular delivery of low-density-lipoprotein cholesterol corresponds with increased intimal thickening in a healthy porcine coronary model. A prelude to development of a model of atherosclerosis

Introduction

Preclinical, vascular response studies are limited due to lack of underlying disease. The available cholesterol-diet-based and genetic atherosclerotic models are not satisfactory due to long breeding, unpredictable lesion formation, low plaque volume and degree of stenosis.

Aim

To evaluate the vascular response to local, intramural delivery of human, highly atherogenic lipids into healthy domestic swine (DS) coronary arteries.

Material and methods

A total of 24 coronary artery segments of 10 DS were enrolled. Following balloon injury (plain old balloon angioplasty – POBA), segments were assigned to local delivery of 2 ml of human LDL from apheresis (400 mg/dl, n = 9), 0.9% NaCl (control, n = 7) or to POBA alone. The solutions were infused with a modified, triple micro-needle catheter into the vessel wall. After 28 days, optical coherence tomography (OCT), virtual histology IVUS (VH-IVUS) and near-infra-red spectroscopy (NIRS) were performed. Following euthanasia, vessel segments were harvested for pathological evaluation.

Results

At 28 days the % area stenosis in OCT was highest in the LDL group (23.6 ±13 vs. 10.8 ±7 vs. 8.1 ±7%; p = 0.02). The presence of necrotic core (LDL: 55.5%, control: 37.5% and POBA: 42.8%; p = 0.77) and dense calcium (LDL: 33.3%, control: 28.5%, POBA: 37.5%; p = 0.94) in VH-IVUS were comparable between groups. The lipid core burden index in NIRS was negative in all cases. In pathology, the injury was comparable between groups (LDL: 1.6 ±0.4, control: 1.7 ±0.8, POBA: 1.7; p = 0.8) and specimens showed no signs of necrotic or lipid core. The tissue consisted of smooth muscle cells (SMC)/proteoglycan-rich lesions and inflammatory cells.

Conclusions

Local delivery of saturated human LDL into the coronary artery wall was feasible and resulted in a higher degree of stenosis caused by intimal thickening. A discrepancy between histopathological findings and virtual histology intravascular ultrasound (VH-IVUS) was also noted.

Animal models of atherosclerosis

Atherosclerosis is a significant cause of morbidity and mortality globally. Many animal models have been developed to study atherosclerosis, and permit experimental conditions, diet and environmental risk factors to be carefully controlled. Pathophysiological changes can be produced using genetic or pharmacological means to study the harmful consequences of different interventions. Experiments using such models have elucidated its molecular and pathophysiological mechanisms, and provided platforms for pharmacological development. Different models have their own advantages and disadvantages, and can be used to answer different research questions. In the present review article, different species of atherosclerosis models are outlined, with discussions on the practicality of their use for experimentation.

Development of Accelerated Coronary Atherosclerosis Model Using Low Density Lipoprotein Receptor Knock-Out Swine with Balloon Injury

Background

Several animal models have facilitated the evaluation and pathological understanding of atherosclerosis, but a definitive animal model of coronary atherosclerosis is not available. We therefore developed low density lipoprotein receptor knockout (LDLR-KO) pigs with hypercholesterolemia, a model which rapidly developed coronary atherosclerosis following balloon injury.

Methods and Results

We deleted LDLR exon regions from cultured porcine fetal fibroblasts and cloned LDLR knockout (LDLR-KO) embryos microinjecting fetal fibroblast nuclei into enucleated oocytes. Twelve LDLR-KO pigs were fed a 2.0% cholesterol and 20% fat diet. Baseline serum LDL cholesterol level was 510.0±86.1 mg/dL. Balloon injury was created in 46 coronary segments and necropsy were obtained 2, 4, 8 and 12 weeks later. Coronary artery sections were reviewed to evaluate lesion progression. We found lipid accumulation with foam cells and inflammatory cells beginning four weeks after balloon injury. The mean ratio of macrophages to plaque area was significantly higher in the four- weeks and eight-week animals compared with those at 2-weeks (8.79% ± 5.98% and 17.00% ± 10.38% vs. 1.14% ± 1.88%, P < 0.0001). At 12 weeks the ratio decreased toward the level at 2 week level (4.00% ± 4.56%, P = 0.66 vs. baseline). Advanced coronary atherosclerotic lesions contained lipid pools at eight-weeks with fibrous components beginning at 12 weeks.

Conclusions

We developed a model of rapid coronary atherosclerosis using LDLR KO pigs with balloon injury. This model may be useful for preclinical evaluation of medication or devices, and may also help investigate mechanisms of plaque progression.

Development of Advanced Atherosclerotic Plaque by Injection of Inflammatory Proteins in a Rabbit Iliac Artery Model

PURPOSE

Appropriate animal models of atherosclerotic plaque are crucial to investigating the pathophysiology of atherosclerosis, as well as for the evaluation of the efficacy and safety of vascular devices. We aimed to develop a novel animal model that would be suitable for the study of advanced atherosclerotic lesions in vivo.

MATERIALS AND METHODS

Atherosclerotic plaque was induced in 24 iliac arteries from 12 rabbits by combining a high cholesterol diet, endothelial denudation, and injection into the vessel wall with either saline (n=5), olive oil (n=6), or inflammatory proteins [n=13, high-mobility group protein B1 (HMGB1) n=8 and tumor necrosis factor (TNF)-α n=5] using a Cricket™ Micro-infusion catheter. Optical coherence tomography (OCT) was performed to detect plaque characteristics after 4 weeks, and all tissues were harvested for histological evaluation.

RESULTS

Advanced plaque was more frequently observed in the group injected with inflammatory proteins. Macrophage infiltration was present to a higher degree in the HMGB1 and TNF-α groups, compared to the oil or saline group (82.1±5.1% and 94.6±2.2% compared to 49.6±14.0% and 46.5±9.6%, p-value<0.001), using RAM11 antibody staining. On OCT, lipid rich plaques were more frequently detected in the inflammatory protein group [saline group: 2/5 (40%), oil group: 3/5 (50%), HMGB1 group: 6/8 (75%), and TNF-α group: 5/5 (100%)].

CONCLUSION

These data indicate that this rabbit model of atherosclerotic lesion formation via direct injection of pro-inflammatory proteins into the vessel wall is useful for in vivo studies investigating atherosclerosis.

Animal models of atherosclerosis and magnetic resonance imaging for monitoring plaque progression

Atherosclerosis, the main cause of heart attack and stroke, is the leading cause of death in most modern countries. Preventing clinical events depends on a better understanding of the mechanism of atherosclerotic plaque destabilization. Our knowledge on the characteristics of vulnerable plaques in humans has grown past decades. Histological studies have provided a precise definition of high-risk lesions and novel imaging methods for human atherosclerotic plaque characterization have made significant progress. However the pathological mechanisms leading from stable lesions to the formation of vulnerable plaques remain uncertain and the related clinical events are unpredictable. An animal model mimicking human plaque destablization is required as well as an in vivo imaging method to assess and monitor atherosclerosis progression. Magnetic resonance imaging (MRI) is increasingly used for in vivo assessment of atherosclerotic plaques in the human carotids. MRI provides well-characterized morphological and functional features of human atherosclerotic plaque which can be also assessed in animal models. This review summarizes the most common species used as animal models for experimental atherosclerosis, the techniques to induce atherosclerosis and to obtain vulnerable plaques, together with the role of MRI for monitoring atherosclerotic plaques in animals.

Animal models of atherosclerosis

Cardiovascular disease is currently the predominant cause of mortality worldwide and its incidence is expected to increase significantly during the next decades owing to the unhealthy effects of modern lifestyle habits (e.g., obesity and lack of physical exercise). Cardiovascular death is frequently associated with acute myocardial infarction or stroke, which are generally the ultimate consequence of an underlying atherosclerotic process. Small and big animal models are valuable tools to understand the molecular mechanisms underlying atherosclerotic plaque formation and progression, as well as the occurrence of associated ischemic events. Moreover, animal models of atherosclerosis are pivotal for testing mechanistic hypothesis and for translational research, including the assessment of dietary and/or pharmacological interventions and the development of imaging technologies and interventional devices. In this chapter, we will describe the most widely used animal models that have permitted major advances in atherosclerosis research and significant improvements in the treatment and diagnosis of atherosclerotic disease.

Intramural coronary lipid injection induces atheromatous lesions expressing proinflammatory chemokines: implications for the development of a porcine model of atherosclerosis

BACKGROUND

Intramural delivery of lipids into the coronaries of pigs fed high-cholesterol diet results in the formation of localized atherosclerotic-like lesions within 12 weeks. These lesions are located in positively remodeled vessels and are associated to the development of abundant adventitial vasa vasorum and mononuclear cell infiltrate. In this study, we aimed to analyze the degree of expression of various inflammatory chemokines within the developed lesions compared with control segments injected with saline.

METHODS

Balloon injury was performed in 15 coronary arteries of pigs fed high-cholesterol diet for 12 weeks. Two weeks after procedure, 60 coronary segments were randomized to either intramural injections of complex lipids (n=30) or normal saline (n=30). Neovessel density in the lesions was analyzed by lectin stain. Segments were processed for RNA expression of inflammatory chemokines such as monocyte chemoattractant protein-1 and vascular endothelial growth factor.

RESULTS

At 12 weeks, the percentage area of stenosis seen in histological sections was modest in both groups (lipids: 17.3±15 vs. saline: 32.4±22.8, P=.017). The lipid group showed higher vasa vasorum (VV) quantity (saline: 18.2±14.9 VV/section vs. lipids: 30.6±21.6 VV/section, P<.05) and vasa vasorum density (saline: 7.3±4.6 VV/mm(2) vs. lipids: 16.5±9 VV/mm(2), P<.001). In addition, monocyte chemoattractant protein-1 expression was higher in the lipid group (1.5±1.12) compared with saline control group (0.83±0.34, P<.01). Vascular endothelial growth factor expression was also higher in the lipid group (1.36±0.9) compared with saline group (0.87±0.33, P<.05).

CONCLUSION

The intramural injection of complex lipids into the coronary arteries of pigs maintained in a high-cholesterol diet results in focal lesions located in positively remodeled vessels that have a high neovessel count and express proinflammatory chemokines.

The representative porcine model for human cardiovascular disease

To improve human health, scientific discoveries must be translated into practical applications. Inherent in the development of these technologies is the role of preclinical testing using animal models. Although significant insight into the molecular and cellular basis has come from small animal models, significant differences exist with regard to cardiovascular characteristics between these models and humans. Therefore, large animal models are essential to develop the discoveries from murine models into clinical therapies and interventions. This paper will provide an overview of the more frequently used large animal models, especially porcine models for preclinical studies.

The pre-clinical animal model in the translational research of interventional cardiology

Scientific discoveries for improvement of human health must be translated into practical applications. Such discoveries typically begin at "the bench" with basic research, then progress to the clinical level. In particular, in the field of interventional cardiology, percutaneous cardiovascular intervention has rapidly evolved from an experimental procedure to a therapeutic clinical setting. Pre-clinical studies using animal models play a very important role in the evaluation of efficacy and safety of new medical devices before their use in human clinical studies. This review provides an overview of the emerging role, results of pre-clinical studies and development, and evaluation of animal models for percutaneous cardiovascular intervention technologies for patients with symptomatic cardiovascular disease.

Pathological mechanisms and dose dependency of erythrocyte-induced vulnerability of atherosclerotic plaques

To test our hypothesis that erythrocytes may induce plaque vulnerability and investigate the mechanism involved, we established a novel model of intraplaque hemorrhage in 56 New Zealand white rabbits with established plaques. Three distinct abdominal aortic plaques with similar thickness were identified in each rabbit with use of intravascular ultrasound (IVUS) imaging. Rabbits were equally divided into 4 groups depending on dosage of treatment; with the guidance of IVUS, one of the three plaques from each rabbit was injected from adventitia with autologous erythrocytes (RBC) or cholesterol (CH) for the following groups: RBC, 50 microL or 100 microL, and CH, 50 microL or 100 microL. One of the other two plaques in each rabbit received an equal volume of normal saline (NS) and one received no injection. Plaques in the 100 microL RBC group had a higher plaque rupture rate than its respective NS or blank controls plaques (57.1% vs. 14.3% or 14.3%, P<0.05). Plaques from the RBC or cholesterol groups showed, dose-dependently, more macrophage infiltration, more superoxide and lipid content, thinner plaque fibrous cap, higher mRNA level of MCP-1, IL-1 or IFN-gamma and higher vulnerability index than controls, especially in the RBC group. Thus, erythrocyte treatment can dose-dependently induce the vulnerability of plaques. Accumulation of lipid content and augmentation of oxidative stress and inflammation in the plaques are the probable pathological mechanisms involved.

In vivo plaque characterization using intravascular ultrasound-virtual histology in a porcine model of complex coronary lesions

OBJECTIVE

To determine the accuracy of detection of different tissue types of intravascular ultrasound-virtual histology (IVUS-VH) in a porcine model of complex coronary lesions.

METHODS AND RESULTS

Coronary lesions were induced by injecting liposomes containing human oxidized low-density lipoprotein into the adventitia of the arteries. IVUS-VH imaging was performed in vivo at 8.2+/-1.6 weeks after injection. A total of 60 vascular lesions were analyzed and compared with their correspondent IVUS-VH images. Correlation analysis was performed using linear regression models. Compared with histology, IVUS-VH correctly identified the presence of fibrous, fibro-fatty, and necrotic tissue in 58.33%, 38.33%, and 38.33% of lesions, respectively. The sensitivity of IVUS-VH for the detection of fibrous, fibro-fatty, and necrotic core tissue was 76.1%, 46%, and 41.1% respectively. A linear regression analysis performed for each individual plaque component did not show strong correlation that would allow significant prediction of individual values.

CONCLUSIONS

In a porcine model of complex coronary lesions, IVUS-VH was not accurate in detecting the relative amount of specific plaque components within each individual corresponding histological specimen.