Co-localization of Disturbed Flow Patterns and Occlusive Cardiac Allograft Vasculopathy Lesion Formation in Heart Transplant Patients

The Biological Parallels Between Atherosclerosis and Cardiac Allograft Vasculopathy: Implications for Solid Organ Chronic Rejection

Atherosclerosis and solid organ chronic rejection are pervasive chronic disease states that account for significant morbidity and mortality in developed countries. Recently, a series of shared molecular pathways have emerged, revealing biological parallels from early stages of development up to the advanced forms of pathology. These shared mechanistic processes are inflammatory in nature, reflecting the importance of inflammation in both disorders. Vascular inflammation triggers endothelial dysfunction and disease initiation through aberrant vasomotor control and shared patterns of endothelial activation. Endothelial dysfunction leads to the recruitment of immune cells and the perpetuation of the inflammatory response. This drives lesion formation through the release of key cytokines such as IFN-y, TNF-alpha, and IL-2. Continued interplay between the adaptive and innate immune response (represented by T lymphocytes and macrophages, respectively) promotes lesion instability and thrombotic complications; hallmarks of advanced disease in both atherosclerosis and solid organ chronic rejection. The aim of this study is to identify areas of overlap between atherosclerosis and chronic rejection. We then discuss new approaches to improve current understanding of the pathophysiology of both disorders, and eventually design novel therapeutics.

An agent-based model of cardiac allograft vasculopathy: toward a better understanding of chronic rejection dynamics

Cardiac allograft vasculopathy (CAV) is a coronary artery disease affecting 50% of heart transplant (HTx) recipients, and it is the major cause of graft loss. CAV is driven by the interplay of immunological and non-immunological factors, setting off a cascade of events promoting endothelial damage and vascular dysfunction. The etiology and evolution of tissue pathology are largely unknown, making disease management challenging. So far, in vivo models, mostly mouse-based, have been widely used to study CAV, but they are resource-consuming, pose many ethical issues, and allow limited investigation of time points and important biomechanical measurements. Recently, agent-based models (ABMs) proved to be valid computational tools for deciphering mechanobiological mechanisms driving vascular adaptation processes at the cell/tissue level, augmenting cost-effective in vivo lab-based experiments, at the same time guaranteeing richness in observation time points and low consumption of resources. We hypothesize that integrating ABMs with lab-based experiments can aid in vivo research by overcoming those limitations. Accordingly, this work proposes a bidimensional ABM of CAV in a mouse coronary artery cross-section, simulating the arterial wall response to two distinct stimuli: inflammation and hemodynamic disturbances, the latter considered in terms of low wall shear stress (WSS). These stimuli trigger i) inflammatory cell activation and ii) exacerbated vascular cell activities. Moreover, an extensive analysis was performed to investigate the ABM sensitivity to the driving parameters and inputs and gain insights into the ABM working mechanisms. The ABM was able to effectively replicate a 4-week CAV initiation and progression, characterized by lumen area decrease due to progressive intimal thickening in regions exposed to high inflammation and low WSS. Moreover, the parameter and input sensitivity analysis highlighted that the inflammatory-related events rather than the WSS predominantly drive CAV, corroborating the inflammatory nature of the vasculopathy. The proof-of-concept model proposed herein demonstrated its potential in deepening the pathology knowledge and supporting the in vivo analysis of CAV.

Circulating Progenitor Cells Are Associated With Plaque Progression And Long-Term Outcomes In Heart Transplant Patients

AIMS

Circulating progenitor cells (CPCs) play a role in vascular repair and plaque stability, while osteocalcin (OC) expressing CPCs have been linked to unstable plaque and adverse cardiovascular outcomes. However, their role in cardiac allograft vasculopathy (CAV) has not been elucidated. This cohort study aimed to investigate the contribution of CPCs on CAV progression and cardiovascular events after heart transplantation.

METHODS AND RESULTS

A total of 80 heart transplant patients (mean age 55 ± 14 years, 72% male) undergoing annual intravascular ultrasound (IVUS) had fresh CPCs marked by CD34, CD133, and OC counted in peripheral blood using flow cytometry, on the same day as baseline IVUS. CAV progression was assessed by IVUS as the change (Δ) in plaque volume divided by segment length (PV/SL), adjusted for the time between IVUS measurements (median 3.0, interquartile range (IQR) [2.8, 3.1] years), and was defined as ΔPV/SL that is above the median ΔPV/SL of study population. Major adverse cardiac events (MACE) was defined as any incident of revascularization, myocardial infarction, heart failure admission, re-transplantation, stroke and death. Patients with higher CD34+CD133+ CPCs had a decreased risk of CAV progression (odds ratio 0.58, 95% confidence interval [CI] [0.37, 0.92], p = 0.01) and MACE (hazard ratio [HR] 0.79, 95% CI [0.66, 0.99], p = 0.05) during a median (IQR) follow up of 8.0 years (7.2, 8.3). Contrarily, higher OC+ cell counts were associated with an increased risk of MACE (HR 1.26, 95% CI [1.03, 1.57], p = 0.02).

CONCLUSIONS

Lower levels of CD34+CD133+ CPCs are associated with plaque progression and adverse long-term outcomes in patients who underwent allograft heart transplantation. In contrast, higher circulating OC+ levels are associated with adverse long term outcomes. Thus, CPCs might play a role in amelioration of transplant vasculopathy, while OC expression by these cells might play a role in progression.

TRANSLATIONAL PERSPECTIVE

The results of the current study suggest lower levels of circulating CD34+CD133+ cell levels are associated with cardiac allograft vasculopathy progression and future adverse cardiovascular events, while higher OC+ cell levels are associated with a greater risk of future cardiovascular events. Further studies confirming our findings might elucidate the role of circulating progenitor cells in the pathophysiology of CAV. Moreover, our findings might support the use of circulating progenitors as biomarkers, as well as the notion of cell therapy as potential treatment option for CAV, a disease with severe burden and limited treatment options.

Oscillatory wall shear stress is a dominant flow characteristic affecting lesion progression patterns and plaque vulnerability in patients with coronary artery disease

Although experimental studies suggest that low and oscillatory wall shear stress (WSS) promotes plaque transformation to a more vulnerable phenotype, this relationship has not been examined in human atherosclerosis progression. Thus, the aim of this investigation was to examine the association between oscillatory WSS, in combination with WSS magnitude, and coronary atherosclerosis progression. We hypothesized that regions of low and oscillatory WSS will demonstrate progression towards more vulnerable lesions, while regions exposed to low and non-oscillatory WSS will exhibit progression towards more stable lesions. Patients (n = 20) with non-flow-limiting coronary artery disease (CAD) underwent baseline and six-month follow-up angiography, Doppler velocity and radiofrequency intravascular ultrasound (VH-IVUS) acquisition. Computational fluid dynamics models were constructed to compute time-averaged WSS magnitude and oscillatory WSS. Changes in VH-IVUS-defined total plaque and constituent areas were quantified in focal regions (i.e. sectors; n = 14 235) and compared across haemodynamic categories. Compared with sectors exposed to low WSS magnitude, high WSS sectors demonstrated regression of total plaque area (p < 0.001) and fibrous tissue (p < 0.001), and similar progression of necrotic core. Sectors subjected to low and oscillatory WSS exhibited total plaque area regression, while low and non-oscillatory WSS sectors demonstrated total plaque progression (p < 0.001). Furthermore, compared with low and non-oscillatory WSS areas, sectors exposed to low and oscillatory WSS demonstrated regression of fibrous (p < 0.001) and fibrofatty (p < 0.001) tissue and similar progression of necrotic core (p = 0.82) and dense calcium (p = 0.40). Herein, we demonstrate that, in patients with non-obstructive CAD, sectors subjected to low and oscillatory WSS demonstrated regression of total plaque, fibrous and fibrofatty tissue, and progression of necrotic core and dense calcium, which suggest a transformation to a more vulnerable phenotype.

Influence of shear stress magnitude and direction on atherosclerotic plaque composition

The precise flow characteristics that promote different atherosclerotic plaque types remain unclear. We previously developed a blood flow-modifying cuff for ApoE^-/- mice that induces the development of advanced plaques with vulnerable and stable features upstream and downstream of the cuff, respectively. Herein, we sought to test the hypothesis that changes in flow magnitude promote formation of the upstream (vulnerable) plaque, whereas altered flow direction is important for development of the downstream (stable) plaque. We instrumented ApoE^-/- mice (n = 7) with a cuff around the left carotid artery and imaged them with micro-CT (39.6 µm resolution) eight to nine weeks after cuff placement. Computational fluid dynamics was then performed to compute six metrics that describe different aspects of atherogenic flow in terms of wall shear stress magnitude and/or direction. In a subset of four imaged animals, we performed histology to confirm the presence of advanced plaques and measure plaque length in each segment. Relative to the control artery, the region upstream of the cuff exhibited changes in shear stress magnitude only (p < 0.05), whereas the region downstream of the cuff exhibited changes in shear stress magnitude and direction (p < 0.05). These data suggest that shear stress magnitude contributes to the formation of advanced plaques with a vulnerable phenotype, whereas variations in both magnitude and direction promote the formation of plaques with stable features.

Comparison of angiographic and IVUS derived coronary geometric reconstructions for evaluation of the association of hemodynamics with coronary artery disease progression

Wall shear stress (WSS) has been investigated as a prognostic marker for the prospective identification of rapidly progressing coronary artery disease (CAD) and atherosclerotic lesions likely to gain high-risk (vulnerable) characteristics. The goal of this study was to compare biplane angiographic vs. intravascular ultrasound (IVUS) derived reconstructed coronary geometries to evaluate agreement in geometry, computed WSS, and association of WSS and CAD progression. Baseline and 6-month follow-up angiographic and IVUS imaging data were collected in patients with non-obstructive CAD (n = 5). Three-dimensional (3D) reconstructions of the coronary arteries were generated with each technique, and patient-specific computational fluid dynamics models were constructed to compute baseline WSS values. Geometric comparisons were evaluated in arterial segments (n = 9), and hemodynamic data were evaluated in circumferential sections (n = 468). CAD progression was quantified from serial IVUS imaging data (n = 277), and included virtual-histology IVUS (VH-IVUS) derived changes in plaque composition. There was no significant difference in reconstructed coronary segment lengths and cross-sectional areas (CSA), however, IVUS derived geometries exhibited a significantly larger left main CSA than the angiographic reconstructions. Computed absolute time-averaged WSS (TAWSS_ABS) values were significantly greater in the IVUS derived geometries, however, evaluations of relative TAWSS (TAWSS_REL) values revealed improved agreement and differences within defined zones of equivalence. Associations between VH-IVUS defined CAD progression and angiographic or IVUS derived WSS exhibited poor agreement when examining TAWSS_ABS data, but improved when evaluating the association with TAWSS_REL data. We present data from a small cohort of patients highlighting strong agreement between angiographic and IVUS derived coronary geometries, however, limited agreement is observed between computed WSS values and associations of WSS with CAD progression.