Embryonic and Adult-Derived Resident Cardiac Macrophages Are Maintained through Distinct Mechanisms at Steady State and during Inflammation

Self-renewal of peripheral nerve resident macrophage: does it represent a unique activation status?

Resident cardiac macrophages have been demonstrated to facilitate the electrical conduction in the heart. The physiologic heart rhythm is initiated by electrical impulses generated in sinoatrial node (SAN) and then conducted to ventricles via atrioventricular node (AVN). To further study the role of resident macrophages in cardiac conduction system, a proper isolation of resident macrophages from SAN and AVN is necessary, but it remains challenging. Here, we provide a protocol for the reliable microdissection of the SAN and AVN in murine hearts followed by the isolation and culture of resident macrophages. Both, SAN which is located at the junction of the crista terminalis with the superior vena cava, and AVN which is located at the apex of the triangle of Koch, are identified and microdissected. Correct location is confirmed by histologic analysis of the tissue performed with Masson's trichrome stain and by anti-HCN4. Microdissected tissues are then enzymatically digested to obtain single cell suspensions followed by the incubation with a specific panel of antibodies directed against cell-type specific surface markers. This allows to identify, count, or isolate different cell populations by fluorescent activated cell sorting. To differentiate cardiac resident macrophages from other immune cells in the myocardium, especially recruited monocyte-derived macrophages, a delicate devised gating strategy is needed. First, lymphoid lineage cells are detected and excluded from further analysis. Then, myeloid cells are identified with resident macrophages being determined by high expression of both CD45 and CD11b, and low expression of Ly6C. With cell sorting, isolated cardiac macrophages can then be cultivated in vitro over several days for further investigation. We, therefore, describe a protocol to isolate cardiac resident macrophages located within the cardiac conduction system. We discuss pitfalls in microdissecting and digesting SAN and AVN, and provide a gating strategy to reliably identify, count and sort cardiac macrophages by fluorescence-activated cell sorting.

Doxorubicin-Induced Ascension of Resident Cardiac Macrophages.

Objective: Adaptive cardiomyocyte growth is an essential compensatory response to hypertension. While hypertension activates cardiac immune cells, their role in adaptation is unclear. Here, we define the transcriptional heterogeneity and functional role of cardiac resident macrophages (RMs) in vivo in hypertensive heart disease. Methods: We performed flow cytometry, immunofluorescence, and single-cell transcriptomics on fate-mapped cardiac RMs isolated from mice with angiotensin II infusion in acute (day 4) and chronic hypertensive stress (day 28). Following inducible depletion of RMs or specific genetic deletion of Igf1 in RMs during hypertension in mice, functional outcomes were tested with echocardiography and immunohistochemistry. Lastly, we also performed single-cell RNA sequencing on human cardiac macrophages from healthy and diseased samples. Results: Cardiac RMs possess numerous transcriptionally diverse cell states with a core repertoire of reparative gene programs that includes high expression of Igf1 in normotensive animals. Individual cell states were differentially responsive during hypertension while all maintained their original transcriptional identity. Hypertension drove selective in situ proliferation and numerical expansion of some cardiac RMs, directly correlating with increased cardiomyocyte size. Inducible ablation of RMs, or selective deletion of RM-derived IGF-1 caused complete absence of adaptive cardiomyocyte growth and development of cardiac dysfunction. Single-cell transcriptomics further identified a conserved IGF1 -expressing macrophage subpopulation in human cardiomyopathy. Conclusions: Here, we defined the absolute requirement of cardiac RM-produced IGF-1 in adaptive cardiomyocyte growth during hypertension, identifying a novel and essential pathway of RM-directed cardiac adaptation to disease.

Introduction: A proinflammatory state plays a central role in heart failure with preserved ejection fraction (HFpEF) but the role of specific immune cells remains unclear. Transcutaneous vagus nerve stimulation (tVNS) modulates immune responses via the cholinergic anti-inflammatory pathway, which involves acetylcholine-dependent activation of α7 nicotinic acetylcholine receptors (α7nAchR) on macrophages. Hypothesis: tVNS reverses the HFpEF phenotype by reducing resident macrophage number and pro-inflammatory cytokine production in an acetylcholine-α7nAchR-dependent manner. Methods: We induced HFpEF in male C57BL/6 CCR2-RFP mice with nitric oxide synthase blocker (L-NAME) and a high-fat diet for 5 weeks. C57BL/6 CCR2-RFP mice fed with standard chow diet served as controls (n=7). We randomized the mice to sham, or tVNS with or without the α7nAchR inhibitor methyllycaconitine (MLA) (n=6 in each group) for 4 weeks, followed by euthanasia. Cardiac macrophages were analyzed using flow cytometry. Resident macrophages were further subdivided based on MHC2 and CCR2 expression. Results: HFpEF mice exposed to sham developed hypertension, diastolic dysfunction, increased heart weight normalized to tibia length, and left ventricular fibrosis compared to the control mice, while tVNS restored these parameters towards the control values. MLA partially reversed the protective effect of tVNS in these parameters. Flow cytometry data revealed a significant increase in resident macrophages in HFpEF sham compared to control hearts, mainly driven by the MHC-/CCR2- subtype (F). Notably, tVNS restored the numbers of resident macrophages to the control levels, while MLA partially attenuated this effect (Figure). Conclusions: tVNS improves the cardiac phenotype in HFpEF mice by modulating cardiac resident macrophages, in an acetylcholine-α7nAchR-dependent manner.

Tissue resident macrophages have vital homeostatic roles, often acting as professional scavengers, phagocytosing apoptotic cells and debris. Well characterised resident populations exist in liver, brain, as well as peritoneal cavity...

Many facets of tissue engineered models aim at understanding cellular mechanisms to recapitulate in vivo behavior, to study and mimic diseases for drug interventions, and to provide a better understanding toward improving regenerative medicine. Recent and rapid advances in stem cell biology, material science and engineering, have made the generation of complex engineered tissues much more attainable. One such tissue, human myocardium, is extremely intricate, with a number of different cell types. Recent studies have unraveled cardiac resident macrophages as a critical mediator for normal cardiac function. Macrophages within the heart exert phagocytosis and efferocytosis, facilitate electrical conduction, promote regeneration, and remove cardiac exophers to maintain homeostasis. These findings underpin the rationale of introducing macrophages to engineered heart tissue (EHT), to more aptly capitulate in vivo physiology. Despite the lack of studies using cardiac macrophages in vitro, there is enough evidence to accept that they will be key to making EHTs more physiologically relevant. In this review, we explore the rationale and feasibility of using macrophages as an additional cell source in engineered cardiac tissues.Impact statementMacrophages play a critical role in cardiac homeostasis and in disease. Over the past decade, we have come to understand the many vital roles played by cardiac resident macrophages in the heart, including immunosurveillance, regeneration, electrical conduction, and elimination of exophers. There is a need to improve our understanding of the resident macrophage population in the heart in vitro, to better recapitulate the myocardium through tissue engineered models. However, obtaining them in vitro remains a challenge. Here, we discuss the importance of cardiac resident macrophages and potential ways to obtain cardiac resident macrophages in vitro. Finally, we critically discuss their potential in realizing impactful in vitro models of cardiac tissue and their impact in the field.

The initial hypertrophy response to cardiac pressure overload is considered compensatory, but with sustained stress, it eventually leads to heart failure. Recently, a role for recruited macrophages in determining the transition from compensated to decompensated hypertrophy has been established. However, whether cardiac resident immune cells influence the early phase of hypertrophy development has not been established. To assess the role of cardiac immune cells in the early hypertrophy response to cardiac pressure overload induced by transverse aortic constriction (TAC). We performed cytometry by time-of-flight to determine the identity and abundance of immune cells in the heart at 1 and 4 weeks after TAC. We observed a substantial increase in cardiac macrophages 1 week after TAC. We then conducted Cite-Seq single-cell RNA sequencing of cardiac immune cells isolated from 4 sham and 6 TAC hearts. We identified 12 clusters of monocytes and macrophages, categorized as either resident or recruited macrophages, that showed remarkable changes in their abundance between sham and TAC conditions. To determine the role of cardiac resident macrophages early in the response to a hypertrophic stimulus, we used a blocking antibody against macrophage colony-stimulating factor 1 receptor (CD115). As blocking CD115 initially depletes all macrophages, we allowed the replenishment of recruited macrophages by monocytes before performing TAC. This preferential depletion of resident macrophages resulted in enhanced fibrosis and a blunted angiogenesis response to TAC. Macrophage depletion in CCR2 (C-C chemokine receptor type 2) knockout mice showed that aggravated fibrosis was primarily caused by the recruitment of monocyte-derived macrophages. Finally, 6 weeks after TAC these early events lead to depressed cardiac function and enhanced fibrosis, despite complete restoration of cardiac immune cells. Cardiac resident macrophages are a heterogeneous population of immune cells with key roles in stimulating angiogenesis and inhibiting fibrosis in response to cardiac pressure overload.

Macrophages promote both injury and repair following myocardial infarction, but discriminating functions within mixed populations remains challenging. Here we used fate mapping and single-cell transcriptomics to demonstrate that at steady state, TIMD4+LYVE1+MHC-IIloCCR2− resident cardiac macrophages self-renew with negligible blood monocyte input. Monocytes partially replaced resident TIMD4−LYVE1−MHC-IIhiCCR2− macrophages and fully replaced TIMD4−LYVE1−MHC-IIhiCCR2+ macrophages, revealing a hierarchy of monocyte contribution to functionally distinct macrophage subsets. Ischemic injury reduced TIMD4+ and TIMD4− resident macrophage abundance within infarcted tissue while recruited, CCR2+ monocyte-derived macrophages adopted multiple cell fates, including those nearly indistinguishable from resident macrophages. Despite this similarity, inducible depletion of resident macrophages using a Cx3cr1-based system led to impaired cardiac function and promoted adverse remodeling primarily within the peri-infarct zone, highlighting a non-redundant, cardioprotective role of resident cardiac macrophages. Lastly, we demonstrate the ability of TIMD4 to be used as a durable lineage marker of a subset of resident cardiac macrophages.

Resident macrophages of the peripheral nervous system have recently been shown to respond rapidly to Wallerian degeneration before the influx of blood-derived macrophages. Because resident endoneurial macrophages are slowly but incompletely exchanged from the blood within 3 months, they could potentially comprise a heterogenous cell population consisting of long-term resident cells and more mobile cells undergoing turnover. We used bone marrow chimeric mice created by transplanting bone marrow from green fluorescent protein-transgenic mice into irradiated wildtype recipients to selectively analyse the response of these two resident macrophage populations to Wallerian degeneration in sciatic nerve explant cultures. In such nerves, recently immigrated macrophages exhibit green fluorescence whereas long-term resident macrophages do not. Studies in cultures from wildtype controls revealed rapid morphological changes of resident macrophages towards a bloated phenotype, a proliferative response resulting in a 3.7-fold increase of macrophage numbers over 2 weeks, and phagocytosis of myelin basic protein-immunoreactive myelin debris. When chimeric mice were analysed, both populations of resident endoneurial macrophages participated in morphological transformation, proliferation and phagocytosis. Quantitative studies revealed a stronger proliferative and phagocytic response in long-term resident endoneurial macrophages compared with recently immigrated macrophages. Our results point towards subtle, but not principal, differences between the two macrophage populations, which might indicate different stages of macrophage differentiation rather than the existence of entirely distinct endoneurial macrophage populations. The results further underline the versatility of resident endoneurial macrophages following peripheral nerve injury, which is reminiscent of the lesion response of microglial cells within the brain.

Doxorubicin-induced cardiomyopathy (DiCM) is a primary cause of heart failure and mortality in cancer patients, in which macrophage-orchestrated inflammation serves as an essential pathological mechanism. However, the specific roles of tissue-resident and monocyte-derived macrophages in DiCM remain poorly understood. Uncovering the origins, phenotypes, and functions of proliferative cardiac resident macrophages and mechanistic insights into the self-maintenance of cardiac macrophage during DiCM progression. Mice were administrated with doxorubicin to induce cardiomyopathy. Dynamic changes of resident and monocyte-derived macrophages were examined by lineage tracing, parabiosis, and bone marrow transplantation. We found that the monocyte-derived macrophages primarily exhibited a proinflammatory phenotype that dominated the whole DiCM pathological process and impaired cardiac function. In contrast, cardiac resident macrophages were vulnerable to doxorubicin insult. The survived resident macrophages exhibited enhanced proliferation and conferred a reparative role. Global or myeloid specifically ablation of SR-A1 (class A1 scavenger receptor) inhibited proliferation of cardiac resident reparative macrophages and, therefore, exacerbated cardiomyopathy in DiCM mice. Importantly, the detrimental effect of macrophage SR-A1 deficiency was confirmed by transplantation of bone marrow. At the mechanistic level, we show that c-Myc (Avian myelocytomatosis virus oncogene cellular homolog), a key transcriptional factor for the SR-A1-P38-SIRT1 (Sirtuin 1) pathway, mediated the effect of SR-A1 in reparative macrophage proliferation in DiCM. The SR-A1-c-Myc axis may represent a promising target to treat DiCM through augmentation of cardiac resident reparative macrophage proliferation.

Cardiac fibrosis is a key pathological link of various cardiovascular diseases to heart failure. It is of great significance to deeply understand the development process of cardiac fibrosis and the cellular and molecular mechanisms involved. Macrophages play a special role in promoting heart development, maintaining myocardial cell homeostasis and heart function. They are involved in the whole process from inflammatory to cardiac fibrosis. This article summarizes the relationship between inflammation and fibrosis, discusses the bidirectional regulation of cardiac fibrosis by macrophages and analyses the functional heterogeneity of macrophages from different sources. It is believed that CCR2– cardiac resident macrophages can promote cardiac function, but the recruitment and infiltration of CCR2+ cardiac non-resident macrophages aggravate cardiac dysfunction and heart remodeling. After heart injury, damage associated molecular patterns (DAMPs) are released in large quantities, and the inflammatory signal mediated by macrophage chemoattractant protein-1 (MCP-1) promotes the infiltration of CCR2+ monocytes and transforms into macrophages in the heart. These CCR2+ non-resident macrophages not only replace part of the CCR2– resident macrophage subpopulation in the heart, but also cause cardiac homeostasis and hypofunction, and release a large number of mediators that promote fibroblast activation to cause cardiac fibrosis. This article reveals the cell biology mechanism of resident and non-resident macrophages in regulating cardiac fibrosis. It is believed that inhibiting the infiltration of cardiac non-resident macrophages and promoting the proliferation and activation of cardiac resident macrophages are the key to improving cardiac fibrosis and improving cardiac function.

Sectm1a Facilitates Protection against Inflammation-Induced Organ Damage through Promoting TRM Self-Renewal

Recruited macrophages incorporate into the resident cardiac macrophage pool and converge into a common phenotype in the healing infarct, but not in the failing heart exposed to pressure overload

Bromodomain and extraterminal domain (BET) proteins, including BRD4, bind acetylated chromatin and coactivate gene transcription. A BET inhibitor, JQ1, prevents and reverses pathological cardiac remodeling in preclinical models of heart failure. However, the underlying cellular mechanisms by which JQ1 improves cardiac structure and function remain poorly defined. Here, we demonstrate that BRD4 knockdown reduced expression of genes encoding CC chemokines in cardiac fibroblasts, suggesting a role for this epigenetic reader in controlling fibroblast-immune cell cross talk. Consistent with this, JQ1 dramatically suppressed recruitment of monocytes to the heart in response to stress. Normal mouse hearts were found to have approximately equivalent numbers of major histocompatibility complex (MHC-II)high and MHC-IIlow resident macrophages, whereas MHC-IIlow macrophages predominated following JQ1 treatment. Single-cell RNA-seq data confirmed that JQ1 treatment or BRD4 knockout in CX3CR1+ cells reduced MHC-II gene expression in cardiac macrophages, and studies with cultured macrophages further illustrated a cell autonomous role for BET proteins in controlling the MHC-II axis. Bulk RNA-seq analysis demonstrated that JQ1 blocked pro-inflammatory macrophage gene expression through a mechanism that likely involves repression of NF-κB signaling. JQ1 treatment reduced cardiac infarct size in mice subjected to ischemia/reperfusion. Our findings illustrate that BET inhibition affords a powerful pharmacological approach to manipulate monocyte-derived and resident macrophages in the heart. Such an approach has the potential to enhance the reparative phenotype of macrophages to promote wound healing and limit infarct expansion following myocardial ischemia.NEW & NOTEWORTHY BRD4 inhibition blocks stress-induced recruitment of pro-inflammatory monocytes to the heart. BRD4 inhibition reprograms resident cardiac macrophages toward a reparative phenotype marked by reduced NF-κB signaling and diminished MHC-II expression. BRD4 inhibition reduces infarct size in an acute model of ischemia/reperfusion injury in mice.