Neointimal Inflammation and Adventitial Angiogenesis Correlate With Severity of Cardiac Allograft Vasculopathy in Pediatric Recipients

Recurrent Mild Acute Rejections and Donor-specific Antibodies as Risk Factors for Cardiac Allograft Vasculopathy in a National Pediatric Heart Transplant Cohort

Background

Immune-mediated factors such as acute cellular rejections and donor-specific antibodies (DSAs) are risk factors for cardiac allograft vasculopathy (CAV). We studied a national cohort with a unified setting and thorough protocol endomyocardial biopsy (EMB) data for an association between cellular rejections, especially when mild and recurrent, and DSAs with CAV in pediatric heart transplant (HTx) patients.

Methods

This is a retrospective, national cohort study of 94 pediatric HTxs performed between 1991 and 2019 and followed until December 31, 2020. Diagnosis of CAV was based on reevaluation of angiographies. Protocol and indication EMB findings with other patient data were collected from medical records. Associations between nonimmune and immune-mediated factors and CAV were analyzed with univariable and multivariable Cox regression analyses.

Results

Angiographies performed on 76 patients revealed CAV in 23 patients (30%). Altogether 1138 EMBs (92% protocol biopsies) were performed on 78 patients (83%). During the first posttransplant year, grade 1 rejection (G1R) appeared in 45 patients (58%), and recurrent (≥2) G1R findings in 14 patients (18%). Pretransplant DSAs occurred in 13 patients (17%) and posttransplant DSAs in 37 patients (39%). In univariable analysis, pretransplant DSAs, appearance and recurrence of G1R findings, and total rejection score during the first posttransplant year, as well as recurrent G1R during follow-up, were all associated with CAV. In multivariable analysis, pretransplant DSAs and recurrent G1R during the first posttransplant year were found to be associated with CAV.

Conclusions

Our results indicate that pretransplant DSA and recurrent G1R findings, especially during the first posttransplant year, are associated with CAV after pediatric HTx.

Acute and chronic phagocyte determinants of cardiac allograft vasculopathy

Post-transplant immunosuppression has reduced the incidence of T cell-mediated acute rejection, yet long-term cardiac graft survival rates remain a challenge. An important determinant of chronic solid organ allograft complication is accelerated vascular disease of the transplanted graft. In the case of cardiac allograft vasculopathy (CAV), the precise cellular etiology remains inadequately understood; however, histologic evidence hints at the accumulation and activation of innate phagocytes as a causal contributing factor. This includes monocytes, macrophages, and immature dendritic cell subsets. In addition to crosstalk with adaptive T and B immune cells, myeloid phagocytes secrete paracrine signals that directly activate fibroblasts and vascular smooth muscle cells, both of which contribute to fibrous intimal thickening. Though maladaptive phagocyte functions may promote CAV, directed modulation of myeloid cell function, at the molecular level, holds promise for tolerance and prolonged cardiac graft function.

Association of periarterial neovascularization with progression of cardiac allograft vasculopathy and long-term clinical outcomes in heart transplant recipients

BACKGROUND

This study investigated the relationship between periarterial neovascularization, development of cardiac allograft vasculopathy (CAV), and long-term clinical outcomes after heart transplantation. Proliferation of the vasa vasorum is associated with arterial inflammation. The contribution of angiogenesis to the development of CAV has been suggested.

METHODS

Serial (baseline and 1-year post-transplant) intravascular ultrasound was performed in 102 heart transplant recipients. Periarterial small vessels (PSV) were defined as echolucent luminal structures <1 mm in diameter, located ≤2 mm outside of the external elastic membrane. The signal void structures were excluded when they connected to the coronary lumen (considered as side branches) or could not be followed in ≥3 contiguous frames. The number of PSV was counted at 1-mm intervals throughout the first 50 mm of the left anterior descending artery, and the PSV score was calculated as the sum of cross-sectional values. Patients with a PSV score increase of ≥ 4 between baseline and 1-year post-transplant were classified as the "proliferative" group. Maximum intimal thickness was measured for the entire analysis segment.

RESULTS

During the first year post-transplant, the proliferative group showed a greater increase in maximum intimal thickness (0.33 ± 0.36 mm vs 0.10 ± 0.28 mm, p < 0.001) and had a higher incidence of acute cellular rejection (50.0% vs 23.9%, p = 0.025) than the non-proliferative group. On Kaplan-Meier analysis, cardiac death-free survival rate over a median of 4.7 years was significantly lower in the proliferative group than in the non-proliferative group (hazard ratio, 3.10; p = 0.036).

CONCLUSIONS

The increase in PSV, potentially representing an angioproliferative response around the coronary arteries, was associated with early CAV progression and reduced survival after heart transplantation.

Evaluation of coronary adventitial vasa vasorum using 3D optical coherence tomography--animal and human studies

Objectives

This study sought to evaluate adventitial vasa vasorum (VV) in vivo with novel imaging technique of optical coherence tomography (OCT).

Methods

To verify OCT methods for quantification of VV, we first studied 2 swine carotid arteries in a model of focal angiogenesis by autologous blood injection, and compared microchannel volume (MCV) by OCT and VV by m-CT, and counts of those. In OCT images, adventitial MC was identified as signal-voiding areas which were located within 1 mm from the lumen-intima border. After manually tracing microchannel areas and the boundaries of lumen-intima and media-adventitial in all slices, we reconstructed 3D images. Moreover, we performed with OCT imaging in 8 recipients referred for evaluation of cardiac allograft vasculopathy at 1 year after heart transplantation. MCV and plaque volume (PV) were assessed with 3D images in each 10-mm-segment.

Results

In the animal study, among the 16 corresponding 1-mm-segments, there were significant correlations of count and volume between both the modalities (count r²=0.80, P<0.01; volume r² =0.50, P<0.01) and a good agreement with a systemic bias toward underestimation with m-CT. In the human study, there was a significant positive correlation between MCV and PV (segment number=24, r² =0.63, P<0.01).

Conclusion

Our results suggest that evaluation of MCV with 3D OCT imaging might be a novel method to estimate the amount of adventitial VV in vivo, and further has the potential to provide a pathophysiological insight into a role of the VV in allograft vasculopathy.

Interacting mechanisms in the pathogenesis of cardiac allograft vasculopathy

Cardiac allograft vasculopathy is the major cause of late graft loss in heart transplant recipients. Histological studies of characteristic end-stage lesions reveal arterial changes consisting of a diffuse, confluent, and concentric intimal expansion containing graft-derived cells expressing smooth muscle markers, extracellular matrix, penetrating microvessels, and a host mononuclear cell infiltrate concentrated subjacent to an intact graft-derived luminal endothelial cell lining with little evidence of acute injury. This intimal expansion combined with inadequate compensatory outward remodeling produces severe generalized stenosis extending throughout the epicardial and intramyocardial arterial tree that causes ischemic graft failure. Cardiac allograft vasculopathy lesions affect ≥50% of transplant recipients and are both progressive and refractory to treatment, resulting in ≈5% graft loss per year through the first 10 years after transplant. Lesions typically stop at the suture line, implicating alloimmunity as the primary driver, but pathogenesis may be multifactorial. Here, we will discuss 6 potential contributors to lesion formation (1) conventional risk factors of atherosclerosis; (2) pre- or peritransplant injuries; (3) infection; (4) innate immunity; (5) T-cell-mediated immunity; and (6) B-cell-mediated immunity through production of donor-specific antibody. Finally, we will consider how these various mechanisms may interact with each other.

Recombinant adenoviral gene transfer does not affect cardiac allograft vasculopathy

BACKGROUND

Adenovirus serotype 5 has remained the pre-eminent vector in pre-clinical gene therapy applications in cardiac transplantation. Concerns over the potential effects of adenoviral vectors on the later development of cardiac allograft vasculopathy (CAV) are addressed in this study.

METHODS

Hearts (n = 22) harvested from Brown Norway rats were perfused ex vivo with either University of Wisconsin (UW) solution with no virus, Ad-CMV-LacZ or Ad-CMV-Null. Donor hearts were transplanted heterotopically into the abdomen of Lewis rats. All recipients received cyclosporine for the duration of the experiment. Transplanted hearts were recovered for analysis at 120 days. Sections of the heart were stained with elastic-van Gieson stain for morphometric analysis of the vessels to ascertain the degree of vascular luminal occlusion. Hematoxylin-eosin staining facilitated diagnosis of chronic rejection.

RESULTS

Seventy-seven percent of transplanted hearts showed signs of chronic rejection with no difference in the proportion of animals between groups (p = 0.797). No difference was noted in the degree of vascular luminal occlusion between the Ad-Null (0.57 +/- 0.22), Ad-LacZ (0.62 +/- 0.19) and UW (0.47 +/- 0.29) groups (p = 0.653).

CONCLUSIONS

Vascularized cardiac allografts transplanted from Brown Norway to Lewis rats demonstrated cardiac allograft vasculopathy CAV at 120 days. Adenoviral perfusion of the donor heart ex vivo did not affect the development of CAV.

Angiogenesis: a potentially critical part of remodelling in chronic airway diseases?

Angiogenesis is a prominent feature of the structural tissue remodelling that occurs in the chronic airway diseases of asthma, Bronchiolitis Obliterans Syndrome (BOS, post-lung transplantation), and in smoking-related Chronic Obstructive Pulmonary Disease (COPD). For each, we have explored the relationship between angiogenesis and underlying chronic inflammatory processes--are the hypervascular changes secondary to inflammation, or do they occur in parallel? What are the likely growth factors which stimulate the angiogenic process? We discuss the relationships that have been studied between angiogenesis and the physiological impairment of airflow obstruction. The pattern that emerges is complex and variable. In asthma, there is strong evidence to suggest that Vascular Endothelial Growth Factor (VEGF) and its receptor system is upregulated in the airway. Local production of VEGF has also been implicated as a major driver of angiogenesis in the airway component of COPD, though paradoxically emphysema seems to be due to lack of VEGF in the lung parenchyma. In BOS, the evidence suggests that VEGF is lacking in the airway: other mediators and especially C-X-C chemokines such as Interleukin (IL)-8, are likely to be more important in angiogenesis. The physiological consequences of angiogenesis are likely to be important in asthma (especially during acute episodes of deterioration), and probably also in COPD, although data is equivocal. In BOS, increased airway vascularity appears to occur early, but is not progressive. In terms of therapy, evidence for anti-angiogenic effectiveness is strongest for Inhaled Corticosteroid (ICS) and Long Acting Beta-Agonists (LABA) in asthma.

Pathophysiology and diagnosis of allograft rejection in pediatric heart transplantation

PURPOSE OF REVIEW

Attempts at improving diagnostic monitoring of allograft rejection continue at a steady pace. The main issue remains whether these methods will replace the standard method of tissue histology from endomyocardial biopsy.

RECENT FINDINGS

The aim in the development of novel techniques to diagnose rejection is the application of noninvasive methods. These range from echocardiography, biomarkers, and genomic profiling to more sensitive antibody detection systems. No single method has the accuracy to be a stand-alone test. Methods of assessing graft dysfunction alone may not be accurate enough in this population. Nonetheless, these and other clinical descriptive studies help us better understand the rejection process in pediatric recipients.

SUMMARY

Solid organ transplantation creates the ideal medium where basic science meets clinical science. Clinical cardiology continues to improve on ways to assess organ dysfunction, but to correlate these methods to early graft rejection, immunobiologic techniques will probably need to be incorporated.

Cardiovascular risk reduction in high-risk pediatric patients: a scientific statement from the American Heart Association Expert Panel on Population and Prevention Science; the Councils on Cardiovascular Disease in the Young, Epidemiology and Prevention, Nutrition, Physical Activity and Metabolism, High Blood Pressure Research, Cardiovascular Nursing, and the Kidney in Heart Disease; and the Interdisciplinary Working Group on Quality of Care and Outcomes Research

Although for most children the process of atherosclerosis is subclinical, dramatically accelerated atherosclerosis occurs in some pediatric disease states, with clinical coronary events occurring in childhood and very early adult life. As with most scientific statements about children and the future risk for cardiovascular disease, there are no randomized trials documenting the effects of risk reduction on hard clinical outcomes. A growing body of literature, however, identifies the importance of premature cardiovascular disease in the course of certain pediatric diagnoses and addresses the response to risk factor reduction. For this scientific statement, a panel of experts reviewed what is known about very premature cardiovascular disease in 8 high-risk pediatric diagnoses and, from the science base, developed practical recommendations for management of cardiovascular risk.

Cardiac allograft vasculopathy: real or a normal morphologic variant?

BACKGROUND

Naive coronary vessels may appear to have intimal thickening histologically characteristic of cardiac allograft vasculopathy (CAV). This study appraises the experimental and clinical impact of this observation.

METHODS

Tissue sections from 12 naive hearts of miniature swine, 13 native porcine hearts of recipients of heterotopic cardiac allografts, 3 native human hearts and 3 human hearts with CAV were compared with light microscopy and morphometric analysis. Results were also compared with morphometric data previously gathered from 3 grafts in a standard experimental model of CAV (rejectors) and 3 grafts harvested from swine rendered tolerant to their donor hearts (chimeras).

RESULTS

In the naive and native porcine hearts, the prevalence of CAV "mimics" was 0% to 6.94% (mean +/- SD: 1.99 +/- 1.97%) and 0% to 7.57% (2.97 +/- 2.20%), respectively (p = 0.12). The prevalence of CAV in the grafts of porcine rejectors and chimeras was 9.9% to 14.8% (12.4 +/- 2.5%) and 0.6% to 4.5% (2.6 +/- 2.0%), respectively (p < 0.05). CAV in the chimeras was similar in prevalence to that of the naive and native hearts. In native human hearts and human grafts, the prevalence was 1.86% to 2.00% (1.95 +/- 0.08%) and 9.09% to 17.50% (12.80 +/- 4.29%), respectively (p = 0.01).

CONCLUSIONS

Smooth muscle bundles inside the internal elastic laminae are similarly prevalent in human and porcine coronary vasculature. Their histologic similarity to intimal thickening of CAV could lead to an inaccurate distinction between graft tolerance and CAV in both clinical and experimental studies of heart transplantation.

VEGFR-1 and -2 Regulate Inflammation, Myocardial Angiogenesis, and Arteriosclerosis in Chronically Rejecting Cardiac Allografts

OBJECTIVE

Interplay between inflammation and angiogenesis is important in pathological reparative processes such as arteriosclerosis. We investigated how the two vascular endothelial growth factor receptors VEGFR-1 and -2 regulate these events in chronically rejecting cardiac allografts.

METHODS AND RESULTS

Chronic rejection in mouse cardiac allografts induced primitive myocardial, adventitial, and intimal angiogenesis with endothelial expression of CD31, stem cell marker c-kit, and VEGFR-2. Experiments using marker gene mice or rats as cardiac allograft recipients revealed that replacement of cardiac allograft endothelial cells with recipient bone marrow- or non-bone marrow-derived cells was rare and restricted only to sites with severe injury. Targeting VEGFR-1 with neutralizing antibodies in mice reduced allograft CD11b+ myelomonocyte infiltration and allograft arteriosclerosis. VEGFR-2 inhibition prevented myocardial c-kit+ and CD31+ angiogenesis in the allograft, and decreased allograft inflammation and arteriosclerosis.

CONCLUSIONS

These results suggest interplay of inflammation, primitive donor-derived myocardial angiogenesis, and arteriosclerosis in transplanted hearts, and that targeting VEGFR-1 and -2 differentially regulate these pathological reparative processes.

Cardiovascular Risk Reduction in High-Risk Pediatric Patients

Although for most children the process of atherosclerosis is subclinical, dramatically accelerated atherosclerosis occurs in some pediatric disease states, with clinical coronary events occurring in childhood and very early adult life. As with most scientific statements about children and the future risk for cardiovascular disease, there are no randomized trials documenting the effects of risk reduction on hard clinical outcomes. A growing body of literature, however, identifies the importance of premature cardiovascular disease in the course of certain pediatric diagnoses and addresses the response to risk factor reduction. For this scientific statement, a panel of experts reviewed what is known about very premature cardiovascular disease in 8 high-risk pediatric diagnoses and, from the science base, developed practical recommendations for management of cardiovascular risk.

Heart and lung transplantation in children

During the past two decades, several advances have resulted in marked improvement in medium-term survival for infants and children undergoing heart transplantation. Unfortunately, progress has been less dramatic in the field of lung and heart-lung transplantation, where there is little evidence of improved outcomes. The procedures remain palliative and all transplant recipients are at risk for the adverse effects of non-specific immunosuppression, including infections, lymphoproliferative disorders, and non-lymphoid malignancies. In addition, current immunosuppressive agents have narrow therapeutic windows and exhibit a wide array of organ toxicities, posing special challenges for the young patient who must endure life-long immunosuppression. New immunosuppressive regimens have lowered the rates of acute rejection but appear to have had relatively little impact on the incidence of chronic rejection, the principal cause of late graft loss. The ultimate goal is to induce a state of donor-specific tolerance, wherein the recipient will accept the allograft indefinitely without the need for long-term immunosuppression. This quest is currently being realised in animal models of solid organ transplantation, and offers great hope for children undergoing heart and lung transplantation in the future.