Bioptic Study of Left and Right Atrial Interstitium in Cardiac Patients with and without Atrial Fibrillation: Interatrial but Not Rhythm-Based Differences

Quantification of right atrial fibrosis by cardiac magnetic resonance: verification of the method to standardize thresholds

INTRODUCTION AND OBJECTIVES

Late gadolinium-enhanced cardiac magnetic resonance (LGE-CMR) allows noninvasive detection of left atrial fibrosis in patients with atrial fibrillation (AF). However, whether the same methodology can be used in the right atrium (RA) remains unknown. Our aim was to define a standardized threshold to characterize RA fibrosis in LGE-CMR.

METHODS

A 3 Tesla LGE-CMR was performed in 53 individuals; the RA was segmented, and the image intensity ratio (IIR) calculated for the RA wall using 1 557 767 IIR pixels (40 994±10 693 per patient). The upper limit of normality of the IIR (mean IIR+2 standard deviations) was estimated in healthy volunteers (n=9), and patients who had undergone previous typical atrial flutter ablation (n=9) were used to establish the dense scar threshold. Paroxysmal and persistent AF patients (n=10 each) were used for validation. IIR values were correlated with a high-density bipolar voltage map in 15 patients undergoing AF ablation.

RESULTS

The upper normality limit (total fibrosis threshold) in healthy volunteers was set at an IIR = 1.21. In the postablation group, 60% of the maximum IIR pixel (dense fibrosis threshold) was calculated as IIR = 1.29. Endocardial bipolar voltage showed a weak but significant correlation with IIR. The overall accuracy between the electroanatomical map and LGE-CMR to characterize fibrosis was 56%.

CONCLUSIONS

An IIR > 1.21 was determined to be the threshold for the detection of right atrial fibrosis, while an IIR > 1.29 differentiates interstitial fibrosis from dense scar. Despite differences between the left and right atria, fibrosis could be assessed with LGE-CMR using similar thresholds in both chambers.

Progressive and Simultaneous Right and Left Atrial Remodeling Uncovered by a Comprehensive Magnetic Resonance Assessment in Atrial Fibrillation

Background Left atrial structural remodeling contributes to the arrhythmogenic substrate of atrial fibrillation (AF), but the role of the right atrium (RA) remains unknown. Our aims were to comprehensively characterize right atrial structural remodeling in AF and identify right atrial parameters predicting recurrences after ablation. Methods and Results A 3.0 T late gadolinium enhanced-cardiac magnetic resonance was obtained in 109 individuals (9 healthy volunteers, 100 patients with AF undergoing ablation). Right and left atrial volume, surface, and sphericity were quantified. Right atrial global and regional fibrosis burden was assessed with validated thresholds. Patients with AF were systematically followed after ablation for recurrences. Progressive right atrial dilation and an increase in sphericity were observed from healthy volunteers to patients with paroxysmal and persistent AF; fibrosis was similar among the groups. The correlation between parameters recapitulating right atrial remodeling was mild. Subsequently, remodeling in both atria was compared. The RA was larger than the left atrium (LA) in all groups. Fibrosis burden was higher in the LA than in the RA of patients with AF, whereas sphericity was higher in the LA of patients with persistent AF only. Fibrosis, volume, and surface of the RA and LA, but not sphericity, were strongly correlated. Tricuspid regurgitation predicted right atrial volume and shape, whereas diabetes was associated with right atrial fibrosis burden; sex and persistent AF also predicted right atrial volume. Fibrosis in the RA was mostly located in the inferior vena cava-RA junction. Only right atrial sphericity is significantly associated with AF recurrences after ablation (hazard ratio, 1.12 [95% CI, 1.01-1.25]). Conclusions AF progression associates with right atrial remodeling in parallel with the LA. Right atrial sphericity yields prognostic significance after ablation.

Autoimmunity in human CE: Correlative with the fertility status of the CE cyst

Cystic echinococcosis is speculated to exert several immune-evasion strategies involving autoimmune-phenomena. We evaluated the hypothesizes that the prevalence of autoantibodies increases in the sera of CE patients that may evidence the association between the parasite and autoimmune diseases. Sera from 63 subjects at distinct types of CE cyst fertility were investigated for antinuclear antibodies (ANA), and anti-CCP antibodies. Plasma levels and cellular production of IL-17A cytokine were specifically defined as being assumed to prime for autoimmunity. Healthy-controls were age and gender-matched to test sera. ANA expressions inside the surgically removed metacestode and adventitial layer were also assayed. Out of 63 patients, 35 % had fertile highly viable cysts (group-1), 41 % had fertile low viable cysts (group-2) and 24 % had non-fertile cysts (group-3). A four-fold increase in ANA sera-levels was detected in group-1 compared with their controls (p-value 0.001) while anti-CCP levels were of insignificant differences. In group-2 and group-3, no significant differences were detected between ANA and anti-CCP sera-levels in CE patients and their controls. IL-17A sera-levels in group-1 and group- 2 were significantly higher than their healthy-controls while being of insignificant differences in group-3, p-value= 0.300. No association was detected between sera-levels of IL-17A and ANA as well as anti-CCP antibodies. Interestingly, relative IL-17A cellular expression associated positive ANA deposition in the parasite cells and adventitial layer. Collectively, based on the parasite fertility, IL-17A and ANA seemed to be involved in the host immune defenses against CE. There is no association between CE and anti-CCP antibodies.

Elastin Structure, Synthesis, Regulatory Mechanism and Relationship With Cardiovascular Diseases

As the primary component of elastic fibers, elastin plays an important role in maintaining the elasticity and tensile ability of cardiovascular, pulmonary and many other tissues and organs. Studies have shown that elastin expression is regulated by a variety of molecules that have positive and negative regulatory effects. However, the specific mechanism is unclear. Moreover, elastin is reportedly involved in the development and progression of many cardiovascular diseases through changes in its expression and structural modifications once deposited in the extracellular matrix. This review article summarizes the role of elastin in myocardial ischemia-reperfusion, atherosclerosis, and atrial fibrillation, with emphasis on the potential molecular regulatory mechanisms.

Regional Diversities in Fibrogenesis Weighed as a Key Determinant for Atrial Arrhythmogenesis

Atrial fibrosis plays a key role in atrial myopathy, resulting in the genesis of atrial fibrillation (AF). The abnormal distribution of fibrotic tissue, electrical coupling, paracrine interactions, and biomechanical–electrical interactions have all been suggested as causes of fibrosis-related arrhythmogenesis. Moreover, the regional difference in fibrogenesis, specifically the left atrium (LA) exhibiting a higher arrhythmogenesis and level of fibrosis than the right atrium (RA) in AF, is a key contributor to atrial arrhythmogenesis. LA fibroblasts have greater profibrotic cellular activities than RA fibroblasts, but knowledge about the regional diversity of atrial regional fibrogenesis remains limited. This article provides a comprehensive review of research findings on the association between fibrogenesis and arrhythmogenesis from laboratory to clinical evidence and updates the current understanding of the potential mechanism underlying the difference in fibrogenesis between the LA and RA.

Radiation-Induced Changes in Ventricular Myocardium After Stereotactic Body Radiotherapy for Recurrent Ventricular Tachycardia

Stereotactic body radiotherapy (SBRT) has been suggested as a promising therapeutic alternative in cases of failed catheter ablation for recurrent ventricular tachycardias (VTs) in patients with structural heart disease. This case series is the first postmortem immunohistochemical analysis of morphologic changes in the myocardium early and late after SBRT. The present findings are in line with experimental observations on apoptosis followed by fibrosis. This may explain why the effect of SBRT on VT is not predominantly immediate. Together with observation of early recurrences after SBRT for VT, these data suggest that this strategy may have rather delayed antiarrhythmic effects.

Extracellular Matrix Protein Ratios in the Human Heart and Vessels: How to Distinguish Pathological From Physiological Changes?

Cardiovascular pathology is often accompanied by changes in relative content and/or ratios of structural extracellular matrix (ECM) proteins within the heart and elastic vessels. Three of these proteins, collagen-I, collagen-III, and elastin, make up the bulk of the ECM proteins in these tissues, forming a microenvironment that strongly dictates the tissue biomechanical properties and effectiveness of cardiac and vascular function. In this review, we aim to elucidate how the ratios of collagen-I to collagen-III and elastin to collagen are altered in cardiovascular diseases and the aged individuum. We elaborate on these major cardiovascular ECM proteins in terms of structure, tissue localization, turnover, and physiological function and address how their ratios change in aging, dilated cardiomyopathy, coronary artery disease with myocardial infarction, atrial fibrillation, aortic aneurysms, atherosclerosis, and hypertension. To the end of guiding in vitro modeling approaches, we focus our review on the human heart and aorta, discuss limitations in ECM protein quantification methodology, examine comparability between studies, and highlight potential in vitro applications. In summary, we found collagen-I relative concentration to increase or stay the same in cardiovascular disease, resulting in a tendency for increased collagen-I/collagen-III and decreased elastin/collagen ratios. These ratios were found to fall on a continuous scale with ranges defining distinct pathological states as well as a significant difference between the human heart and aortic ECM protein ratios.

Vascular endothelial growth factor promotes atrial arrhythmias by inducing acute intercalated disk remodeling

Atrial fibrillation (AF) is the most common arrhythmia and is associated with inflammation. AF patients have elevated levels of inflammatory cytokines known to promote vascular leak, such as vascular endothelial growth factor A (VEGF). However, the contribution of vascular leak and consequent cardiac edema to the genesis of atrial arrhythmias remains unknown. Previous work suggests that interstitial edema in the heart can acutely promote ventricular arrhythmias by disrupting ventricular myocyte intercalated disk (ID) nanodomains rich in cardiac sodium channels (Na_V1.5) and slowing cardiac conduction. Interestingly, similar disruption of ID nanodomains has been identified in atrial samples from AF patients. Therefore, we tested the hypothesis that VEGF-induced vascular leak can acutely increase atrial arrhythmia susceptibility by disrupting ID nanodomains and slowing atrial conduction. Treatment of murine hearts with VEGF (30–60 min, at clinically relevant levels) prolonged the electrocardiographic P wave and increased susceptibility to burst pacing-induced atrial arrhythmias. Optical voltage mapping revealed slower atrial conduction following VEGF treatment (10 ± 0.4 cm/s vs. 21 ± 1 cm/s at baseline, p < 0.05). Transmission electron microscopy revealed increased intermembrane spacing at ID sites adjacent to gap junctions (GJs; 64 ± 9 nm versus 17 ± 1 nm in controls, p < 0.05), as well as sites next to mechanical junctions (MJs; 63 ± 4 nm versus 27 ± 2 nm in controls, p < 0.05) in VEGF–treated hearts relative to controls. Importantly, super-resolution microscopy and quantitative image analysis revealed reorganization of Na_V1.5 away from dense clusters localized near GJs and MJs to a more diffuse distribution throughout the ID. Taken together, these data suggest that VEGF can acutely predispose otherwise normal hearts to atrial arrhythmias by dynamically disrupting Na_V1.5-rich ID nanodomains and slowing atrial conduction. These data highlight inflammation-induced vascular leak as a potential factor in the development and progression of AF.

Native cardiac environment and its impact on engineering cardiac tissue

Human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) generally have an immature fetal-like phenotype when directly compared to isolated CMs from human hearts, despite significant advance in differentiation of human pluripotent stem cells (hPSCs) to multiple cardiac lineages. Therefore, hPSC-CMs may not accurately mimic all facets of healthy and diseased human adult CMs. During embryonic development, the cardiac extracellular matrix (ECM) experiences a gradual assembly of matrix proteins that transits along the maturation of CMs. Mimicking these dynamic stages may contribute to hPSC-CMs maturation in vitro. Thus, in this review, we describe the progressive build-up of the cardiac ECM during embryonic development, the ECM of the adult human heart and the application of natural and synthetic biomaterials for cardiac tissue engineering with hPSC-CMs.

Cardiac tissue engineering therapeutic products to enhance myocardial contractility

Researchers continue to develop therapeutic products for the repair and replacement of myocardial tissue that demonstrates contractility equivalent to normal physiologic states. As clinical trials focused on pure adult stem cell populations undergo meta-analysis for preclinical through clinical design, the field of tissue engineering is emerging as a new clinical frontier to repair the myocardium and improve cardiac output. This review will first discuss the three primary tissue engineering product themes that are advancing in preclinical to clinical models: (1) cell-free scaffolds, (2) scaffold-free cellular, and (3) hybrid cell and scaffold products. The review will then focus on the products that have advanced from preclinical models to clinical trials. In advancing the cardiac regenerative medicine field, long-term gains towards discovering an optimal product to generate functional myocardial tissue and eliminate heart failure may be achieved.

Fibrosis independent atrial fibrillation in older patients is driven by substrate leukocyte infiltration: diagnostic and prognostic implications to patients undergoing cardiac surgery

Background

The objectives of the study were to characterize and quantify cellular inflammation and structural remodeling of human atria and correlate findings with molecular markers of inflammation and patient surrogate outcome.

Methods

Voluntary participants undergoing heart surgery were enrolled in the study and blood samples were collected prior to surgery, and right atrium samples were harvested intraoperatively. Blood samples were analyzed by flow cytometry and complete blood counts. Atrial samples were divided for fixed fibrosis analysis, homogenized for cytokine analysis and digested for single cell suspension flow cytometry.

Results

A total of 18 patients were enrolled and samples assessed. Isolated cells from the atria revealed a CD45+ population of ~ 20%, confirming a large number of leukocytes. Further characterization revealed this population as 57% lymphocytes and 26% monocyte/macrophages (MoΦ), with the majority of the latter cells being classical (CD14++/CD16−). Interstitial fibrosis was present in 87% of samples and correlated significantly with patient age. Older patients (> 65) had significantly more atrial fibrosis and cellular inflammation. AFib patients had no distinguishing feature of atrial fibrosis and had significantly greater CD45+ MoΦ, increased expression of MMP9 and presented with a significant correlation in length of stay to CCL-2/MCP-1 and NLR (neutrophil-to-lymphocyte ratio).

Conclusion

Atrial fibrosis is correlated with age and not determinate to AFib. However, severity of atrial leukocyte infiltration and markers of matrix degradation are determinant to AFib. This also correlated with CCL2 (or MCP-1) and NLR-indicative of marked inflammation. These data show the potential importance of diagnostic and prognostic assessments that could inform clinical decision making in regard to the intensity of AFib patient management.

Lymphocyte Populations in the Adventitial Layer of Hydatid Cysts in Cattle: Relationship With Cyst Fertility Status and Fasciola Hepatica Co-Infection

Cystic echinococcosis is a worldwide zoonosis caused by the cestode Echinococcus granulosus. Two types of hydatid cysts occur in intermediate hosts: fertile cysts that generate protoscoleces from the germinal layer of the cyst, and infertile cysts that do not produce protoscoleces and are unable to continue the life cycle of the parasite. The adventitial layer, a host-derived fibrous capsule surrounding the hydatid cyst, is suggested to play an important role in local immune regulation during infection and in fertility of the cysts. Fasciola hepatica, another important parasite of cattle, induces a characteristic Th2-like immune response that could modulate the immune response against E. granulosus. Natural co-infection of both parasites is common in cattle, but no reports describe the local immune response against E. granulosus with F. hepatica infection in the same host. This study analyzed the number and distribution of T and B cells in the adventitial layer of liver and lung cysts and the relationship with cyst fertility and F. hepatica co-infection. T lymphocytes were the predominant cell type in the adventitial layer of infertile hydatid cysts and were more numerous in infertile hydatid cysts. B lymphocyte numbers were not associated with hydatid cyst fertility. Mast cells were infrequent in the adventitial layer. The number of T and B cells was not associated with F. hepatica co-infection. The present study contributes to the understanding of local immune responses in bovine cystic echinococcosis.

Macro- and micromechanical remodelling in the fish atrium is associated with regulation of collagen 1 alpha 3 chain expression

Numerous pathologies lead to remodelling of the mammalian ventricle, often associated with fibrosis. Recent work in fish has shown that fibrotic remodelling of the ventricle is 'reversible', changing seasonally as temperature-induced changes in blood viscosity alter haemodynamic load on the heart. The atrial response to varying haemodynamic load is less understood in mammals and completely unexplored in non-mammalian vertebrates. To investigate atrial remodelling, rainbow trout were chronically cooled (from 10 ± 1 to 5 ± 1 °C) and chronically warmed (from 10 ± 1 to 18 ± 1 °C) for a minimum of 8 weeks. We assessed the functional effects on compliance using ex vivo heart preparations and atomic force microscopy nano-indentation and found chronic cold increased passive stiffness of the whole atrium and micromechanical stiffness of tissue sections. We then performed histological, biochemical and molecular assays to probe the mechanisms underlying functional remodelling of the atrial tissue. We found cooling resulted in collagen deposition which was associated with an upregulation of collagen-promoting genes, including the fish-specific collagen I alpha 3 chain, and a reduction in gelatinase activity of collagen-degrading matrix metalloproteinases (MMPs). Finally, we found that cooling reduced mRNA expression of cardiac growth factors and hypertrophic markers. Following long-term warming, there was an opposing response to that seen with cooling; however, these changes were more moderate. Our findings suggest that chronic cooling causes atrial dilation and increased myocardial stiffness in trout atria analogous to pathological states defined by changes in preload or afterload of the mammalian atria. The reversal of this phenotype following chronic warming is particularly interesting as it suggests that typically pathological features of mammalian atrial remodelling may oscillate seasonally in the fish, revealing a more dynamic and plastic atrial remodelling response.

Analysis of immune cell populations in atrial myocardium of patients with atrial fibrillation or sinus rhythm

Background

Atrial fibrillation (AF) is the most common arrhythmia and despite obvious clinical importance remains its pathogenesis only partially explained. A relation between inflammation and AF has been suggested by findings of increased inflammatory markers in AF patients.

Objective

The goal of this study was to characterize morphologically and functionally CD45-positive inflammatory cell populations in atrial myocardium of patients with AF as compared to sinus rhythm (SR).

Methods

We examined 46 subjects (19 with AF, and 27 in SR) undergoing coronary bypass or valve surgery. Peroperative bioptic samples of the left and the right atrial tissue were examined using immunohistochemistry.

Results

The number of CD3+ T-lymphocytes and CD68-KP1+ cells were elevated in the left atrial myocardium of patients with AF compared to those in SR. Immune cell infiltration of LA was related to the rhythm, but not to age, body size, LA size, mitral regurgitation grade, type of surgery, systemic markers of inflammation or presence of diabetes or hypertension. Most of CD68-KP1+ cells corresponded to dendritic cell population based on their morphology and immunoreactivity for DC-SIGN. The numbers of mast cells and CD20+ B-lymphocytes did not differ between AF and SR patients. No foci of inflammation were detected in any sample.

Conclusions

An immunohistochemical analysis of samples from patients undergoing open heart surgery showed moderate and site-specific increase of inflammatory cells in the atrial myocardium of patients with AF compared to those in SR, with prevailing population of monocyte-macrophage lineage. These cells and their cytokine products may play a role in atrial remodeling and AF persistence.

[Morphological features of the myocardium of the atrial appendages in patients with different forms of atrial fibrillation]

AIM

to analyze the morphological features of the myocardium of the atrial appendages in patients with different forms of atrial fibrillation (AF) and to compare the findings with the clinical parameters of patients.

MATERIAL AND METHODS

Light and electron microscopies were used to examine the myocardium of the atrial appendages in adult patients with paroxysmal (PAF), persistent (PrAF), or long-standing persistent atrial fibrillation (LPAF) and in comparison group patients with sinus rhythm without history of AF. A morphometric method was employed to evaluate myocardial fibrosis and to measure the diameter of cardiomyocytes (CMCs); the degree of lipomatosis and amyloidosis was semiquantitatively determined; and the content of CMC myofibrils was estimated. Atrial natriuretic peptide content in the myocytes was measured by immunoconfocal microscopy.

RESULTS

In all groups, the patients with AF were found to have signs of atrial structural remodeling: fibrosis, lipomatosis, isolated atrial amyloidosis, CMC hypertrophy with the phenomena of a partial loss of myofibrils without significant differences between these parameters in different groups. In PAF patients, atrial remodeling was accompanied by hypertrophy of a number of CMCs with their higher myofibrilar mass; the increased CMC size in the left atrial appendage prevented left atrial enlargement; the degree of amyloidosis negatively correlated with the CMC myofibrillar loss that was recorded in the minor CMCs; the degree of CMC myolysis positively correlated with mitral valve insufficiency and left atrial enlargement. In contrast to the clinical and morphological changes that are typical of PAF, in LPAF the increase in CMC sizes was positively correlated with left atrial enlargement and mitral annular dilatation; while the myofibrillar loss phenomenon was noted in the most hypertrophied CMCs; the degree of amyloidosis was positively correlated with CMC myofibrillar loss. In the patients with PrAF, the size of CMCs did not correlate with their myofibril content.

CONCLUSION

The patients with PAF were ascertained to have opposite changes in the ratio of CMC hypertrophy to left atrial enlargement, amyloidosis, and CMC myofibrillar loss, hypertrophy of CMCs and their myofibril content in comparison with these indicators in LPAF.

Early left atrial tissue features in patients with chronic mitral regurgitation and sinus rhythm: Alterations of not remodeled left atria

OBJECTIVE

Left atrial (LA) enlargement, a compensatory mechanism in chronic mitral regurgitation (MR) increasing the risk of atrial fibrillation (AF) and predictive of cardiac events, involves structural alterations. We characterized LA features in patients in sinus rhythm with severe degree of MR, similar degrees of left ventricular remodeling but divergent LA size.

METHODS

Among a consecutive series of 163 patients in stable sinus rhythm undergoing isolated mitral valve surgery for severe non-rheumatic MR, two groups were arbitrarily selected according to their LA size (antero-posterior): NRLA group (non-remodeled LA) included 8 patients with LA≤40mm, RLA group (remodeled LA) included 8 patients with LA>55mm. LA biopsies were processed for paraffin inclusion and sectioning. Fibrosis, cardiomyocytes morphology, capillaries density, cytochrome c and F-actin expression were evaluated by microscopy.

RESULTS

Histology and immunohistochemistry demonstrated alteration of moderate entity: higher amounts of endomysial fibrosis (not of collagen type III) and of hypertrophic cardiomyocytes in RLA than in NRLA. Confocal microscopy displayed focally disorganized F-actin and no nuclear fragmentation in both groups, but more intra-cytoplasm cytochrome c in RLA vs. NRLA, possibly indicative of more successful escape to apoptosis by NRLA cardiomyocytes.

CONCLUSIONS

Our study shows the presence of early cellular and interstitial alterations in LA tissue in patients with chronic MR and sinus rhythm. These features were analogous to those of patients with AF, and suggest that macroscopic remodeling LA in the settings of MR is preceded by structural changes, paving the way to further investigation on the preventive role of early mitral valve repair.