The Dynamic Role of Cardiac Macrophages in Aging and Disease

The molecular mechanisms of cardiac development and related diseases

Cardiac development is a complex and intricate process involving numerous molecular signals and pathways. Researchers have explored cardiac development through a long journey, starting with early studies observing morphological changes and progressing to the exploration of molecular mechanisms using various molecular biology methods. Currently, advancements in stem cell technology and sequencing technology, such as the generation of human pluripotent stem cells and cardiac organoids, multi-omics sequencing, and artificial intelligence (AI) technology, have enabled researchers to understand the molecular mechanisms of cardiac development better. Many molecular signals regulate cardiac development, including various growth and transcription factors and signaling pathways, such as WNT signaling, retinoic acid signaling, and Notch signaling pathways. In addition, cilia, the extracellular matrix, epigenetic modifications, and hypoxia conditions also play important roles in cardiac development. These factors play crucial roles at one or even multiple stages of cardiac development. Recent studies have also identified roles for autophagy, metabolic transition, and macrophages in cardiac development. Deficiencies or abnormal expression of these factors can lead to various types of cardiac development abnormalities. Nowadays, congenital heart disease (CHD) management requires lifelong care, primarily involving surgical and pharmacological treatments. Advances in surgical techniques and the development of clinical genetic testing have enabled earlier diagnosis and treatment of CHD. However, these technologies still have significant limitations. The development of new technologies, such as sequencing and AI technologies, will help us better understand the molecular mechanisms of cardiac development and promote earlier prevention and treatment of CHD in the future.

Melatonin alleviates aging-related heart failure through melatonin receptor 1A/B knockout in mice

Age-related cardiovascular diseases continue to be important issues that contribute to the societal burden. Unveiling the molecular mechanisms underlying age-related cardiovascular diseases provides novel opportunities to delay aging and facilitate early disease diagnosis and treatment. This study utilized knockout mice lacking melatonin receptors type 1A (MT1) and 1B (MT2). Ultrasonography, pathological staining, and transcriptomics were used to investigate the role of MT1/2 in the hearts of aging mice. Knockout of both receptors decreased ejection fraction and exacerbated fibrosis, inflammation, oxidative stress, and apoptosis levels in aging mice. Our findings indicated that the cardiac function of MT1 knockout mice was more severely affected than that of MT2 knockout mice. Additionally, we observed that intraperitoneal administration of melatonin (20 mg/kg/day for 90 days) ameliorated abnormal cardiac function in aging mice. However, the absence of MT1/2 resulted in the inability of melatonin to improve cardiac function. Our study, utilizing an aging polymerase chain reaction assay and cell experiments, revealed that melatonin receptors potentially influence cardiac function in aging mice through their effects on leukocyte differentiation antigen 14 (CD14) expression. Consequently, melatonin receptors, particularly MT1, are key contributors to cardiac aging, and therapeutic interventions targeting this receptor are promising for delaying the progression of cardiac aging.

The protective role of GATA6+ pericardial macrophages in pericardial inflammation

Prior research has suggested that GATA6+ pericardial macrophages may traffic to the myocardium to prevent interstitial fibrosis after myocardial infarction (MI), while subsequent literature claims that they do not. We demonstrate that GATA6+ pericardial macrophages are critical for preventing IL-33 induced pericarditis and attenuate trafficking of inflammatory monocytes and granulocytes to the pericardial cavity after MI. However, absence of GATA6+ macrophages did not affect myocardial inflammation due to MI or coxsackievirus-B3 induced myocarditis, or late-stage cardiac fibrosis and cardiac function post MI. GATA6+ macrophages are significantly less transcriptionally active following stimulation in vitro compared to bone marrow-derived macrophages and do not induce upregulation of inflammatory markers in fibroblasts. This suggests that GATA6+ pericardial macrophages attenuate inflammation through their interactions with surrounding cells. We therefore conclude that GATA6+ pericardial macrophages are critical in modulating pericardial inflammation, but do not play a significant role in controlling myocardial inflammation or fibrosis.

Autoimmune Myocarditis, Old Dogs and New Tricks

Autoimmunity significantly contributes to the pathogenesis of myocarditis, underscored by its increased frequency in autoimmune diseases such as systemic lupus erythematosus and polymyositis. Even in cases of myocarditis caused by viral infections, dysregulated immune responses contribute to pathogenesis. However, whether triggered by existing autoimmune conditions or viral infections, the precise antigens and immunologic pathways driving myocarditis remain incompletely understood. The emergence of myocarditis associated with immune checkpoint inhibitor therapy, commonly used for treating cancer, has afforded an opportunity to understand autoimmune mechanisms in myocarditis, with autoreactive T cells specific for cardiac myosin playing a pivotal role. Despite their self-antigen recognition, cardiac myosin-specific T cells can be present in healthy individuals due to bypassing the thymic selection stage. In recent studies, novel modalities in suppressing the activity of pathogenic T cells including cardiac myosin-specific T cells have proven effective in treating autoimmune myocarditis. This review offers an overview of the current understanding of heart antigens, autoantibodies, and immune cells as the autoimmune mechanisms underlying various forms of myocarditis, along with the latest updates on clinical management and prospects for future research.

Role of immune cells in the pathogenesis of myocarditis

Myocarditis is an inflammatory heart disease that mostly affects young people. Myocarditis involves a complex immune network; however, its detailed pathogenesis is currently unclear. The diversity and plasticity of immune cells, either in the peripheral blood or in the heart, have been partially revealed in a number of previous studies involving patients and several kinds of animal models with myocarditis. It is the complexity of immune cells, rather than one cell type that is the culprit. Thus, recognizing the individual intricacies within immune cells in the context of myocarditis pathogenesis and finding the key intersection of the immune network may help in the diagnosis and treatment of this condition. With the vast amount of cell data gained on myocarditis and the recent application of single-cell sequencing, we summarize the multiple functions of currently recognized key immune cells in the pathogenesis of myocarditis to provide an immune background for subsequent investigations.

Myocardial Immune Cells: The Basis of Cardiac Immunology

The mammalian heart is characterized by the presence of striated myocytes, which allow continuous rhythmic contraction from early embryonic development until the last moments of life. However, the myocardium contains a significant contingent of leukocytes from every major class. This leukocyte pool includes both resident and nonresident immune cells. Over recent decades, it has become increasingly apparent that the heart is intimately sensitive to immune signaling and that myocardial leukocytes exhibit an array of critical functions, both in homeostasis and in the context of cardiac adaptation to injury. Here, we systematically review current knowledge of all major leukocyte classes in the heart, discussing their functions in health and disease. We also highlight the connection between the myocardium, immune cells, lymphoid organs, and both local and systemic immune responses.

Clinicopathological classification of immune checkpoint inhibitor-associated myocarditis: possible refinement by measuring macrophage abundance

BACKGROUND

Immune checkpoint inhibitor (ICI) myocarditis is associated with high morbidity and mortality. While endomyocardial biopsy (EMB) is considered a gold standard for diagnosis, the sensitivity of EMB is not well defined. Additionally, the pathological features that correlate with the clinical diagnosis of ICI-associated myocarditis remain incompletely understood.

METHODS

We retrospectively identified and reviewed the clinicopathological features of 26 patients with suspected ICI-associated myocarditis based on institutional major and minor criteria. Seventeen of these patients underwent EMB, and the histopathological features were assessed by routine hematoxylin and eosin (H&E) staining and immunohistochemical (IHC) staining for CD68, a macrophage marker.

RESULTS

Only 2/17 EMBs obtained from patients with suspected ICI myocarditis satisfied the Dallas criteria. Supplemental IHC staining and quantification of CD68⁺ macrophages identified an additional 7 patients with pathological features of myocardial inflammation (> 50 CD68⁺ cells/HPF). Macrophage abundance positively correlated with serum Troponin I (P = 0.010) and NT-proBNP (N-terminal pro-brain natriuretic peptide, P = 0.047) concentration. Inclusion of CD68 IHC could have potentially changed the certainty of the diagnosis of ICI-associated myocarditis to definite in 6/17 cases.

CONCLUSIONS

While the Dallas criteria can identify a subset of ICI-associated myocarditis patients, quantification of macrophage abundance may expand the diagnostic role of EMB. Failure to meet the traditional Dallas Criteria should not exclude the diagnosis of myocarditis.

Single-cell transcriptome sequencing of macrophages in common cardiovascular diseases

Macrophages are strategically located throughout the body at key sites in the immune system. A key feature in atherosclerosis is the uptake and accumulation of lipoproteins by arterial macrophages, leading to the formation of foam cells. After myocardial infarction, macrophages derived from monocytes infiltrate the infarcted heart. Macrophages are also closely related to adverse remodeling after heart failure. An in-depth understanding of the functions and characteristics of macrophages is required to study heart health and pathophysiological processes; however, the heterogeneity and plasticity explained by the classic M1/M2 macrophage paradigm are too limited. Single-cell sequencing is a high-throughput sequencing technique that enables the sequencing of the genome or transcriptome of a single cell. It effectively complements the heterogeneity of gene expression in a single cell that is ignored by conventional sequencing and can give valuable insights into the development of complex diseases. In the present review, we summarize the available research on the application of single-cell transcriptome sequencing to study the changes in macrophages during common cardiovascular diseases, such as atherosclerosis, myocardial infarction, and heart failure. This article also discusses the contribution of this knowledge to understanding the pathogenesis, development, diagnosis, and treatment of heart diseases.