The role of cardiac resident macrophage in cardiac aging

Abstract Advancements in longevity research have provided insights into the impact of cardiac aging on the structural and functional aspects of the heart. Notable changes include the gradual remodeling of the myocardium, the occurrence of left ventricular hypertrophy, and the decline in both systolic and diastolic functions. Macrophages, a type of immune cell, play a pivotal role in innate immunity by serving as vigilant agents against pathogens, facilitating wound healing, and orchestrating the development of targeted acquired immune responses. Distinct subsets of macrophages are present within the cardiac tissue and demonstrate varied functions in response to myocardial injury. The differentiation of cardiac macrophages according to their developmental origin has proven to be a valuable strategy in identifying reparative macrophage populations, which originate from embryonic cells and reside within the tissue, as well as inflammatory macrophages, which are derived from monocytes and recruited to the heart. These subsets of macrophages possess unique characteristics and perform distinct functions. This review aims to summarize the current understanding of the roles and phenotypes of cardiac macrophages in various conditions, including the steady state, aging, and other pathological conditions. Additionally, it will highlight areas that require further investigation to expand our knowledge in this field.


| INTRODUC TI ON
Based on data provided by the Chinese government, the demographic group comprising individuals aged 65 and above has attained a proportion of 11.9%, signifying a 0.5% rise compared to the preceding year.Scholars anticipate that advancements in health care and enhancements in living standards may lead to a potential increase of 4 years in life expectancy for the global population by the year 2040.Notably, certain nations, including Japan, Singapore, Spain, and Switzerland, exhibit an average life expectancy surpassing 85 years.These projections underscore the substantial potential for notable advancements in life expectancy and the consequent aging of populations in the forthcoming decades (Foreman et al., 2018).The aging of the population plays a substantial role in the heightened occurrence of age-related ailments, specifically CVD (Donato et al., 2018).By the year 2030, approximately 20% of the American population will surpass the age of 65, and nearly half of them will be afflicted with cardiovascular disease (Benjamin et al., 2017).Globally, CVD persists as the primary cause of mortality, with cardiac aging recognized as a significant autonomous risk element (Ren & Zhang, 2018).
Consequently, it becomes imperative to explore the underlying mechanisms of cardiac aging and discover strategies to alleviate or avert its progression.
The investigation into the involvement of the immune system in maintaining cardiac equilibrium, the aging process, and pathological conditions is an expanding area of study.The cardiac structure encompasses a diverse range of cell types, namely cardiomyocytes, fibroblasts, endothelial cells, smooth muscle cells, pericytes, endocardial cells, epicardial cells, as well as various immune cells including monocytes, macrophages, dendritic cells, T cells, natural killer (NK) cells, mast cells, and B cells (Koenig et al., 2022;Pinto et al., 2016;Tucker et al., 2020).Among these immune populations, macrophages are the predominant cellular component.The immune landscape within the heart is dynamic, exhibiting significant shifts in composition and phenotype based on the specific disease context (e.g., infection, sterile injury, hemodynamic changes, aging, myocarditis).In this review, we will focus on discussing the inflammatory response in the heart following acute myocardial injury.
Through the examination of the immune response in cardiac injury, valuable insights can be obtained regarding the underlying mechanisms of inflammation, thereby potentially enabling the identification of therapeutic targets to mitigate the adverse consequences of acute myocardial injury.It is imperative to comprehend the dynamic behavior of macrophages and their interactions with other immune and stromal cells within the cardiac context, as this knowledge is pivotal for elucidating the mechanisms that drive cardiac inflammation and pathology.By unraveling the intricate immune responses, the identification of novel therapeutic targets and the development of interventions to modulate the immune landscape become feasible prospects.

| C ARDIAC-RE S IDENT MACROPHAG E S
The immune milieu within the cardiac tissue experiences dynamic alterations as it progresses from a state of equilibrium and vigilance to an inflammatory condition.Among the immune cells present, tissue-resident macrophages prevail as the predominant cellular entity (Ramos et al., 2017).As constituents of the innate immune system, macrophages possess specialized phagocytic abilities that enable them to identify and eradicate apoptotic cells and pathogens.
Additionally, they assume a pivotal role in intercellular communication with neighboring stromal and immune cells.
The traditional belief that macrophages originate exclusively from bone marrow monocytes and spleen macrophages has been the subject of controversy (van Furth & Cohn, 1968).Recent studies have presented evidence indicating that tissue-resident macrophages, including those found in various organs such as the brain, spleen, liver, lung, bone marrow, kidney, pancreas, peritoneum, and heart, are established during prenatal development, persist throughout an individual's lifespan, and possess the ability to selfrenew locally (Ginhoux & Jung, 2014;Nahrendorf & Swirski, 2016).
In the steady state, resident macrophages in the heart make up approximately 5%-10% of nonmyocytes (Heidt et al., 2014;Pinto et al., 2016).The spindle-shaped cardiac macrophages exhibit close interactions with myocytes, endothelial cells, and fibroblasts.Research utilizing genetic fate mapping and lineage-tracing techniques has revealed that the majority of these resident cardiac macrophages derive from embryonic yolk sacs and fetal livers (Epelman et al., 2014).Furthermore, these resident macrophages maintain their population by undergoing proliferation approximately once per month (Epelman et al., 2014;Heidt et al., 2014).
The elucidation of tissue-resident cardiac macrophages' origins, characteristics, and maintenance offers significant contributions to our comprehension.By delineating the heterogeneity of subpopulations within cardiac macrophages, we can delve deeper into their unique roles and functions concerning cardiac homeostasis, aging, and disease.Several studies have elucidated the distinct and tissue-specific roles of CRMs, similar to other macrophage populations residing in tissues such as microglia, Kupffer cells, and alveolar macrophages (Ginhoux et al., 2010;Gomez Perdiguero et al., 2015;Guilliams et al., 2013;Yona et al., 2013).Throughout development and maintenance of equilibrium, CRMs actively participate in diverse processes, including the removal of apoptotic cells, stimulation of coronary growth and organization, elimination of impaired mitochondria, and facilitation of electrical conduction (Dick et al., 2019;Epelman et al., 2014;Hulsmans et al., 2017;Nicolás-Ávila et al., 2020).Moreover, they inhibit monocyte and neutrophil recruitment, possibly by releasing IL-10 and/or TGFβ (Hilgendorf et al., 2014).
The regulatory molecules belonging to the CRM family have a significant impact on the coordination of repair and regeneration processes in the neonatal heart.They achieve this by controlling myocardial proliferation and angiogenesis, as evidenced by studies (Aurora et al., 2014;Lavine et al., 2014;Wang et al., 2019).In cases of chronic heart failure, CRMs respond to mechanical stretch through the TRPV4 pathway, triggering adaptations essential for sustaining cardiac output.These adaptations involve the reorganization of myocardial tissue and the development of coronary arteries (Revelo et al., 2021;Wong et al., 2021;Zaman et al., 2021).Despite the expression of high levels of MHC-II and possession of antigenpresenting capabilities by certain CRMs, the functional implications of this characteristic remain uncertain [8].It is customary for CRMs to coexist with CCR2+ MHC-IIhi macrophages, which originate from monocytes.CCR2+ MHC-IIhi macrophages colonize the heart postnatally, approximately 2 weeks after birth.These macrophages exhibit potent inflammatory potential, secreting chemokines and facilitating the recruitment of neutrophils and monocytes through monocyte recruitment (Nahrendorf et al., 2007).
In addition, the heart harbors a limited number of Ly6C hiCCR2+ monocytes, which can be differentiated from cardiac macrophages based on their expression of MertK (Gautier et al., 2012).Moreover, immunosurveillance in the heart involves the participation of dendritic cells, mast cells, memory T cells, regulatory T cells, and intravascular B cells.Dendritic cells are believed to serve as guardians of self-tolerance (Hart & Fabre, 1981;Satpathy et al., 2012).Nevertheless, the specific functions of these cellular populations within the heart remain incompletely understood.
To summarize, CRMs exhibit unique functionalities within the myocardium, engaging in diverse physiological processes, modulating inflammatory responses, and facilitating tissue healing and regeneration.Gaining a comprehensive understanding of the intricate interplay among distinct immune cell subsets within the cardiac milieu will enhance our comprehension of cardiac immunology and potentially pave the way for innovative therapeutic approaches targeting cardiovascular disorders.

| C ARD IAC AG ING
The phenomenon of aging, characterized by the gradual decline in physiological function in the majority of living organisms, has historically captivated human curiosity and imagination.Presently, with the advancement of knowledge regarding the molecular and The distribution and prominent characteristics of different subsets of macrophages in the heart are of academic interest.In the steady state, during neonatal heart regeneration, and in adoptive remodeling, the heart predominantly harbors tissue-resident macrophages lacking the chemokine receptor-2 (CCR2−).However, in cases of sterile injury, myocarditis, chronic heart failure, and aging, there is a notable transition toward an expansion of CCR2+ macrophages.cellular mechanisms governing life and disease, aging has emerged as a prominent area of scientific inquiry.
In comparison to individuals aged 70-99, centenarians exhibit a reduced occurrence of cardiovascular diseases, including hypertension, myocardial infarction, angina, and diabetes (Galioto et al., 2008;Selim et al., 2005).This observation suggests that centenarians and their offspring possess a genetic or epigenetic makeup that confers protection against cardiovascular-related mortality, thereby contributing to their longevity (Perls & Terry, 2003) (Figure 2).
Longitudinal studies, such as the Framingham Heart Study and the Baltimore Longitudinal Study on aging, have observed an age-related increase in left ventricular wall thickness in apparently healthy adults.Additionally, the E/A ratio, a Doppler measurement indicating the ratio between early (E) and late (A) diastolic left ventricular filling, decreases with advancing age (Dai et al., 2009;Dai & Rabinovitch, 2009).This decline implies that a larger proportion of left ventricular filling takes place during late diastole rather than early diastole, which is clinically referred to as diastolic dysfunction or HFpEF.Age-related left ventricular hypertrophy and diastolic dysfunction significantly increase (Dai et al., 2009).These changes may occur even in seemingly healthy individuals who do not have hypertension, indicating intrinsic cardiac aging.
Despite the relatively preserved systolic function, as indicated by ejection fraction, in the elderly population, there is a significant decline in exercise capacity and cardiovascular reserve with advancing age (Correia et al., 2002).The decrease in ejection fraction following maximal exercise and the decline in maximal heart rate contribute to the reduced exercise capacity observed in the elderly, which can be attributed to the effects of aging.
The augmented dependence on atrial contraction for left ventricular filling in diastolic dysfunction results in heightened atrial pressure, thereby exacerbating atrial hypertrophy, dilatation, and an escalated susceptibility to atrial fibrillation.This aligns with the notable rise in atrial fibrillation incidence among older individuals (Lakatta, 2003;Lakatta & Levy, 2003a, 2003b).In the geriatric cohort, atrial fibrillation detrimentally affects exercise capacity and predisposes individuals to HFpEF.HFpEF constitutes more than 50% of all heart failure instances in individuals aged 75 years and above, particularly without structural or ischemic heart disease.
Valvular changes associated with aging involve myxomatous degeneration, collagen deposition, and calcium deposition, resulting in valve sclerosis.Aortic valve sclerosis is present in 30%-80% of elderly individuals (Karavidas et al., 2010;Nassimiha et al., 2001;Stewart et al., 1997), detected by echocardiography as calcification of the annulus and leaflets of the aortic valve (Freeman & Otto, 2005;Otto et al., 1999).In the geriatric population, fibrosis and valvular calcification are prominent etiological factors in the pathogenesis of aortic stenosis, characterized by the progressive narrowing of the aortic valve orifice due to leaflet stiffening and calcification (Olsen et al., 2005).Consequently, this impedes efficient blood flow through the aortic valve, leading to the establishment of a pressure gradient between the aorta and the left ventricle.In order to sustain sufficient systolic function, the left ventricular walls undergo myocardial hypertrophy.
The cardiac aging-related alterations in ventricular and valvular dynamics compromise the reserve capacity of the heart and reduce the threshold for functional impairment (Correia et al., 2002).becomes more susceptible to stress and disease-related challenges, resulting in a higher incidence of heart failure and cardiovascular mortality.Inflammaging, which is characterized by heightened basal inflammatory responses associated with aging (Franceschi & Campisi, 2014), leads to alterations in the immune composition of the cardiac tissue.Notably, there is a reduction in the replication of CRMs and a concomitant increase in the presence of proinflammatory CCR2+ MHC-IIhi macrophages (Molawi et al., 2014).Furthermore, aging is accompanied by an elevation in the levels of circulating proinflammatory cytokines and CCL2, indicating dysregulation of the innate immune system (Bruunsgaard et al., 2000;Chiao et al., 2011).
The connection between inflammaging and the proliferation of mutations in hematopoietic stem cells, leading to the presence of mutated pro-inflammatory myeloid cells in the cardiac immune system, has been emphasized in a recent study on clonal hematopoiesis (Pardali et al., 2020).In a mouse model of clonal hematopoiesis with Tet2 deficiency, there was an observed expansion of CCR2+ MHC-IIhi macrophages in the cardiac region, but not CRMs, which exhibited an inflammatory reaction (Wang et al., 2020).

| EMERG ING ROLE S OF MACROPHAG E S IN C ARD IAC DE VELOPMENT, C ARD IAC AG ING , AND OTHER REL ATED C ARDIOVA SCUL AR DIS E A S E S MACROPHAG E S IN C ARD IAC DE VELOPMENT
During the process of heart development, the epicardium, which serves as the outer mesothelial layer, assumes a critical function in the recruitment of primitive yolk sac-derived macrophages to the subepicardial space through signaling pathways (Stevens et al., 2016).This recruitment of embryonic macrophages aligns with notable morphological transformations in the developing heart, including chamber septation, valve remodeling, myocardial growth, electrical conduction system development, and the formation of coronary and lymphatic vasculature.
Tissue-resident macrophages have been implicated in these processes through their involvement in engulfing dying cells, releasing cytokines, interacting with or recruiting progenitor cells, and undergoing trans-differentiation into alternative cell types.Previous studies have demonstrated the functional significance of cardiac macrophages in valvular remodeling, normal conduction, and coronary development (Hulsmans et al., 2017;Leid et al., 2016;Shigeta et al., 2019).The development of the heart relies on the epicardium's secretion of paracrine signals, which stimulate the proliferation and maturation of cardiomyocytes, as well as support the expansion of the cardiac vasculature (Simões & Riley, 2018).

Tissue-resident macrophages commonly coexist with coronary
and lymphatic endothelium in the subepicardial space.Research has shown that macrophages play a crucial role in the growth and remodeling of cardiac lymphatic vessels.Macrophages colonize the embryonic heart prior to the initiation of lymphatic expansion and establish close associations and interactions with the adventitial surface and leading edges of lymphatic vessels.Their presence facilitates the growth and fusion of lymphatic vessels, ensuring sufficient coverage over the subepicardial surface (Cahill et al., 2021).Moreover, macrophages derived from the yolk sac are crucial for coronary maturation as they promote remodeling of the primitive coronary plexus, selectively expanding the perfused coronary vasculature (Leid et al., 2016).
Moreover, cardiac macrophages facilitate electrical conduction through the distal atrioventricular node, where conducting cells are interspersed with elongated macrophages expressing connexin 43.
Through connexin 43-containing gap junctions, these macrophages are coupled to spontaneously beating cardiomyocytes.Macrophages in the heart have a negative resting membrane potential and depolarize in synchrony with cardiomyocytes.According to computational modeling, macrophages increase the resting membrane potential of cardiomyocytes, thus accelerating their repolarization (Hulsmans et al., 2017).
In brief, the involvement of the epicardium and tissue-resident macrophages is crucial in various aspects of heart development, including valvular remodeling, coronary and lymphatic vessel development, and electrical conduction.The epicardium functions by releasing paracrine signals that stimulate the proliferation of cardiomyocytes and aid in the expansion of the vasculature.On the other hand, macrophages contribute to the growth and remodeling of lymphatic vessels, promote the maturation of coronary vessels, and facilitate electrical conduction in specific regions of the heart.

| ROLE OF MACROPHAG E S IN C ARD IAC AG IN G
Numerous age-related alterations in cardiac function, encompassing myocardial sarcopenia, hypertrophy, vascular hyperpermeability, inflammation, fibrosis, and slight impairment of cardiac physiology, contribute to the development of cardiac morbidity and mortality (Lin et al., 2008;Lindsey et al., 2005;Yabluchanskiy et al., 2014).
Prior investigations have demonstrated an age-dependent increase in the population of cardiac macrophages in mice, exhibiting a positive correlation with age-associated factors like MMPs and CCL2, which are implicated in cardiomyocyte hypertrophy and ventricular enlargement (Lindsey et al., 2005;Ma et al., 2015Ma et al., , 2012;;Ma, Yuan, et al., 2018).
Based on flow cytometric analysis, it can be inferred that there is a noticeable increase in CD206+ cardiac resident macrophages and a corresponding decrease in CD206-CRMs as individuals age.
This phenomenon appears to be influenced by MMP-9 (Mouton et al., 2018;Toba et al., 2017).Furthermore, the process of aging has been associated with a rise in Ly6C+CCR2+ monocyte-derived cardiac resident macrophages, commonly referred to as "inflammaging."In addition, studies have shown that TLR2 signaling plays a role in ameliorating age-related adverse cardiac remodeling and dysfunction (Meschiari et al., 2017;Spurthi et al., 2018;Wagner & Dimmeler, 2020).While there have been reports suggesting the classification of macrophages into activated and alternatively activated subsets, recent evidence indicates that further division of macrophage subsets is possible based on in vitro stimuli.For example, M1 macrophages can be categorized as M1a when stimulated with tolllike receptors or as M1b subsets when stimulated with high-mobility group protein B1 (Ben-Mordechai et al., 2015).Subsets within macrophages exhibit distinct cell physiology.For instance, M1b demonstrates lower phagocytic activity compared to M1a.M2 macrophages can be further classified into M2a when stimulated with IL-4 or IL-13, M2b when stimulated with immune complexes in conjunction with IL-1β, and M2c when stimulated with IL-10, TGFβ, or glucocorticoids (Gombozhapova et al., 2017;Martinez et al., 2008).Additionally, it is possible for different phenotypes to undergo reciprocal conversion under in vitro conditions.For instance, M1 macrophages can transition into the M2 phenotype upon stimulation with pro-M2 factors, and vice versa (Pelegrin & Surprenant, 2009).Further investigation is required to elucidate the contribution of distinct subsets of CRMs and their mechanisms of renewal in cardiac aging, including the roles of self-renewal and differentiation of blood monocytes (Figure 3).
Aging is characterized by the augmentation of the innate immune system's fundamental inflammatory response, commonly referred to as inflammaging, as well as the decline of the immune system in relation to cardiac function, known as cardiac immunosenescence.Inflammaging is a consequence of prolonged physiological stimulation of the innate immune system, which may incur damage during the aging process.Numerous factors contribute to the occurrence of inflammaging.
The first contributing factor to inflammaging involves the accumulation of damaged macromolecules and cells (self-debris) over time, resulting from heightened production and insufficient elimination.Additionally, chronic inflammation can arise from the presence of harmful substances originating from microbial constituents, such as those found in the oral or gut microbiota.Lastly, inflammaging may be attributed to cellular senescence, which is a cellular reaction to damage and stress (Franceschi & Campisi, 2014).The concept of immunosenescence was initially introduced by Roy Walford, encompassing the process of immune system deterioration and restructuring.This phenomenon leads to suboptimal vaccination results and heightened vulnerability to infections (Lian et al., 2020).Immunosenescence is marked by the incapacity to generate proficient humoral and cellular immune reactions against pathogens or vaccines, alongside a persistent low-level inflammatory state known as "inflammaging."This inflammatory state contributes to the dysregulation of various elements within the innate and adaptive immune systems (Franceschi et al., 2000;Xia et al., 2016).
F I G U R E 3 Macrophage involvement in age-related changes to heart.
| 7 of 18 Macrophages in the aging heart demonstrate compromised phagocytic capability and modified polarization, while innate immune cells exhibit reduced expression and functionality of TLRs (Khare et al., 1996;Mahbub et al., 2012;Spurthi et al., 2018;Wang & Shah, 2015).These processes, in conjunction with clonal hematopoiesis, contribute to the heightened susceptibility to cardiovascular diseases linked to the aging process.Nevertheless, the specific cellular states associated with aging and their correlation with acute or chronic inflammation remain incompletely comprehended, underscoring the significance of single-cell omics studies as a valuable avenue for further investigation.
In summary, cardiac aging involves multiple changes in macrophage populations, inflammation, and immune function.Understanding these processes at the cellular level will provide valuable insights into the mechanisms underlying age-related cardiovascular diseases.

| ROLE OF MACROPHAG E S IN MI
Following a myocardial infarction, the heart undergoes cardiac remodeling as a result of functional or structural stress on the cardiac system.This phenomenon holds significant importance in the progression of the disease.Initially, inflammatory cells, namely Ly-6Chigh monocytes and M1 macrophages, are present (Figure 3).However, over time, these inflammatory cells are gradually substituted by anti-inflammatory cells, specifically Ly6Clow monocytes and M2 macrophages.Following the resolution of the inflammatory phase, the macrophages assume responsibility for wound healing by producing various factors, including IL-10 (anti-inflammatory), VEGF, PDGF, and IGF-1 (pro-angiogenic), as well as TGF-β1 and fibronectin (pro-fibrotic).These factors aid in the reconstruction of the vascular supply and the repair of necrotic tissue (Atri et al., 2018;Lafuse et al., 2020).While this remodeling initially enhances cardiac performance, it can have detrimental consequences, such as cardiomyocyte death, compromised ventricular wall integrity, impaired ventricular function, and cardiac fibrosis, ultimately contributing to the development of heart failure.Despite the abundance of macrophages in infarcted hearts and their significant role in inflammation, the precise mechanism by which these receptors interact to suppress inflammatory signals and resolve leukocyte infiltration remains unclear.In the context of post-myocardial infarction inflamed heart tissue, distinct functional subpopulations of macrophages, such as regulatory and reparative macrophages, have been identified.However, the temporal dynamics of macrophage populations following infarction are not well understood (Khare et al., 1996).It is uncertain whether new subpopulations emerge to replace the original macrophages or if the original cells undergo transformation into different phenotypes.Recent research has focused on the utilization of macrophages as a therapeutic approach for addressing the progressive adverse remodeling of the heart following myocardial infarction.Specifically, the enhancement of cardiac function and mitigation of detrimental ventricular remodeling have been observed through the promotion of survival in Ly6Clow macrophages and their CCR2-MHCII low subsets via BDNF-mediated pathways, facilitated by environmental eustress (Mahbub et al., 2012).A separate study has provided evidence that the absence of EDIL3 enhances the healing process of the heart by decreasing the polarization of M1 macrophages.Furthermore, the removal of TAZ encourages the inhibition of IL-6 and the expression of Arg1, resulting in a decrease in ventricular hypertrophy and fibrosis, as well as an increase in angiogenesis (Renshaw et al., 2002).In addition, it has been discovered that macrophages contribute to the growth of lymphatic vessels in the myocardium and aid in the reduction of cardiac damage following a myocardial infarction by releasing VEGFC (Atri et al., 2018;D'Amore & Alcaide, 2022).Understanding the regulation of macrophage polarization and their effects on cardiac function post-MI can provide new insights into potential therapeutic strategies.While transitioning from a quiescent to an activated state, macrophages undergo a global rewiring of metabolic pathways (Lafuse et al., 2020).To meet the bioenergetic and biosynthetic demands of the inflammation process, pro-inflammatory macrophages switch from oxidative metabolism to glycolysis (Mosser & Edwards, 2008).Activated macrophages rapidly generate adenosine triphosphate (ATP), leading to the production of NADPH for bactericidal activity through the pentose phosphate pathway (Bai et al., 2022).In pro-inflammatory macrophages, mitochondrial oxidative metabolism is suppressed, and there is a disruption and rewiring of tricarboxylic acid cycle flux (Mia et al., 2020;Wei et al., 2022).In the context of myocardial infarction, infiltrating monocytes/macrophages mount a similar inflammatory response.This results in activated macrophages exhibiting glycolytic polarization due to the release of damage-associated molecular patterns by dead cells and subsequent activation of tolllike receptors (Liu et al., 2019).Macrophages MI exhibit a significant reprogramming of mitochondrial genes, which is indicative of their active involvement in tissue repair during the transition from injury to the infarct zone.This finding underscores the pivotal role played by mitochondrial function in shaping macrophage phenotype and facilitating cardiac remodeling following MI.

The clearance of apoptotic cells by immune cells plays a crucial role
in resolving inflammation and promoting tissue repair after myocardial infarction (Gordon, 2016;Serhan & Savill, 2005;Wan et al., 2013).
Phagocytic macrophages involved in the process of efferocytosis undergo a metabolic shift characterized by heightened oxygen consumption, upregulated FAO and OXPHOS, and reduced glycolysis (Park et al., 2011).These metabolic alterations coincide with the decrease in cytokines and damage-associated molecular patterns (DAMPs) in the microenvironment as inflammation subsides.The involvement of mitochondrial function in cardiac repair and remodeling has been evidenced by the impaired wound healing and heightened cardiac rupture observed in mice with myeloid-specific deficiency of Complex III in the electron transport chain(B.Zhang et al., 2018).Nevertheless, additional investigation is required to comprehend the underlying mechanisms and therapeutic implications of mitochondrial metabolism in macrophage phenotype and cardiac remodeling.
In the context of myocardial infarction, the process of cardiac remodeling encompasses intricate interplays among macrophages, inflammation, and metabolic pathways.By focusing on macrophages and manipulating their polarization and metabolism, there exists the possibility of implementing strategies aimed at ameliorating adverse ventricular remodeling and enhancing cardiac function subsequent to MI. including Hippo-Yap, Jak1-STAT3, and Notch (Bassat et al., 2017;Han et al., 2015;Xin et al., 2013).Yan et al. discovered that macrophages possess the ability to release oncostatin M, thereby exerting regulatory control over cardiac myocyte proliferation and cardiac regeneration via the activation of the gp130/Src/Yap-Notch and Yap-ctgf/ Areg signaling pathways (Li et al., 2020) (Figure 4).and inhibits matrix degeneration, ultimately facilitating tissue repair (Weber et al., 2013).A recent study has provided evidence that Igr4, a G protein-coupled receptor containing leucine-rich repeats, has the ability to enhance AP-1 activation within inflammatory macrophages through pathways mediated by CREB (Huang et al., 2020).In recent years, a multitude of scholarly investigations have been conducted to examine the significance of macrophage-derived exosomes in the context of fibrosis.It has been observed that M2-like macrophages possess the ability to stimulate the circUbe3a/miR-138-5p/Rhoc signaling axis, thereby facilitating the progression of cardiac fibrosis subsequent to myocardial infarction (Y.Wang et al., 2021).Abe et al. discovered that following myocardial infarction, Ly-6Chigh macrophages release hypoxia-inducible factor 1 (HIF-1) in order to modulate the OSM gene, consequently impeding the activation of cardiac fibroblasts through the ERK1/2-SMAD2-TGFβ1 axis (Abe et al., 2019).Furthermore, the involvement of Gata6 + pericardial macrophages (GPCMs) in the progression of cardiac fibrosis subsequent to myocardial infarction (MI) was observed.Deniset et al. discovered a gradual recruitment of GPCMs to the infarcted site following MI, accompanied by alterations in their phenotypic characteristics.Notably, Gata6-knockout mice exhibited an augmented presence of detrimental fibrosis within the infarcted region (Deniset et al., 2019) (Figure 4).

| CROSS TALK B E T WEEN MACROPHAG E S AND T LYMPHO C Y TE S
T lymphocyte populations can be broadly categorized into two subsets: helper CD4+ T lymphocytes and cytotoxic CD8+ T lymphocytes.The former subset plays a role in myocardial infarction through communication with macrophages.CD4+ T cells after MI can also be classified into two subsets based on Foxp3 expression: "effector" (Tef; Foxp3-) and "regulatory" (Treg; Foxp3+).Treg cells (secreting IL-10 and TGFβ) promote anti-inflammatory macrophage phenotypes, thus influencing the process of healing and scar formation (Ramos et al., 2016)

| ROLE OF MACROPHAG E S IN IN C ARD IAC FIB ROS IS
The accumulation of extracellular matrix (ECM) proteins within the cardiac interstitium is a defining characteristic of cardiac fibrosis, a condition that arises from persistent injury to the myocardium.This pathological process gives rise to myocardial wall thickening, as well as systolic and diastolic dysfunction, ultimately impairing overall cardiac performance.Under normal circumstances, the ECM serves to provide structural support to cardiac cells, ensuring the integrity and functionality of the heart (Fan et al., 2012).Additionally, the ECM facilitates the transmission of electrical conduction and contractile force throughout the cardiomyocytes within the entire organ.Furthermore, the ECM serves as a reservoir for latent growth factors (Kong et al., 2014).Myocardial injury can be induced by either sterile or non-sterile inflammation resulting from an infection.The resolution of inflammation necessitates an anti-inflammatory response, subsequently activating profibrotic signaling (Frangogiannis, 2008).
Furthermore, interstitial, and perivascular fibrosis can arise from pathophysiological stimuli, including pressure overload, volume overload, metabolic dysfunction, and aging, even in the absence of myocardial inflammation (Figure 3).
Both resident embryonic macrophages and recruited macrophages are responsible for removing debris and dead cardiomyocytes during the initial inflammatory phase, while matrix metalloproteinases break down the extracellular matrix.Once enough clearance is accomplished, a proliferative phase is initiated, resulting in the transformation of fibroblasts into myofibroblasts.Myofibroblasts have a significant role in producing collagen and other matrix proteins to compensate for cellular loss in different heart failure-related conditions (Kong et al., 2014;Murphy et al., 2015).During the proliferative phase, both immune and nonimmune cells in the injured myocardium release mediators that have anti-inflammatory and pro-fibrotic properties.These mediators include lactoferrin, annexin A1, TGF-β1, IL-10, lipoxins, resolvins, and MMPs.They play a crucial role in resolving inflammation and initiating remodeling processes (DeLeon-Pennell et al., 2017;Frangogiannis, 2012;Huynh et al., 2002;Soehnlein & Lindbom, 2010).TGF-β1, which is typically bound to the ECM, becomes activated and facilitates myofi-

| ROLE OF MACROPHAG E S IN C ARDIAC REMODELING IN HYPERTENS ION AND DIAB E TIC C ARDIOMYOPATHY
Diabetes and hypertension, frequently coexisting, are significant risk factors for cardiovascular diseases, encompassing myocardial infarction and heart failure.These conditions are distinguished by persistent, mild inflammation, which contributes to adverse alterations in the structure and function of the heart.Inflammation assumes a pivotal role in the heart's reaction to injury and adaptive remodeling (DeBerge et al., 2019) (Figure 3).Nonetheless, inflammation can impede the adaptive response and lead to cardiac impairment.The activation of macrophages by diabetes and hypertension can lead to chronic inflammation, prompting them to adopt an inflammatory phenotype.While macrophages play a vital role in cardiac remodeling, an imbalance between pro-inflammatory and anti-inflammatory phenotypes can lead to excessive inflammation and subsequent cardiac damage (Mouton et al., 2020).Under normal circumstances, a healthy heart primarily utilizes fatty acids as its main energy source through mitochondrial oxidative phosphorylation.Conversely, in cases of decompensated heart failure, the heart predominantly relies on glycolysis (Berthiaume et al., 2012;Dodd et al., 2012).In the context of heart failure, the activation of the HIF-1α transcription factor in this particular state results in the transcription of genes that are linked to glycolysis and inflammation, thereby contributing to adverse remodeling (Boutens et al., 2018;Lauterbach & Wunderlich, 2017).Additionally, HIF-1α can be induced by nonhypoxic mechanisms associated with obesity and hypertension, including inflammatory cytokines, hyperglycemia, activation of toll-like receptor 4 by saturated fatty acids, and oxidized low-density lipoprotein (Groh et al., 2018;Lancaster et al., 2018;Tannahill et al., 2013;Wu et al., 2014).Furthermore, individuals with diabetes exhibit increased vulnerability to bacterial infections and impaired wound healing.The impaired pro-reparative/anti-inflammatory functions of macrophages and the heightened expression of long-chain acyl-CoA synthetase (Kanter et al., 2012;Pavlou et al., 2018;Wicks et al., 2014), may account for this phenomenon.Additionally, macrophages are susceptible to protein glycation and the formation of advanced glycation end products (AGEs) in response to elevated glucose levels.This activation of the NF-κB pathway subsequently induces the production of inflammatory cytokines (Jin et al., 2015;Mishra et al., 2017).Furthermore, it has been observed that macrophages derived from individuals with coronary artery disease, which is frequently associated with obesity and hypertension, display notably increased levels of IL-1β and TNFα compared to those with inflammatory vascular diseases (Watanabe et al., 2018).Consequently, in the presence of hyperglycemia, macrophages seem to enhance their uptake and utilization of glucose, resulting in heightened production of inflammatory cytokines and the induction of an inflammatory phenotype (Watanabe et al., 2018).Moreover, the presence of free fatty acids, lipid mediators, and adipokines can also contribute to the promotion of this inflammatory phenotype (Francisco et al., 2018;Kain et al., 2018;Namgaladze & Brüne, 2016).
Cardiac macrophages have a significant impact on the regulation of cardiac remodeling in cases of obesity and hypertension.Normally, the healthy heart harbors a limited number of M2-like macrophages, derived from embryonic cells, which are deemed to have a protective function (Bajpai et al., 2019;Ben-Mordechai et al., 2013;Ma, Mouton, & Lindsey, 2018).Nevertheless, in the absence of injury, CCR2+ monocyte-derived macrophages progressively replace these M2-like macrophages as individuals age (Bajpai et al., 2019).In a mouse model of pressure overload, resident M2-like macrophages demonstrate their ability to facilitate adaptive myocardial remodeling in response to mechanical stress.However, the infiltration of CCR2+ monocytes has been found to contribute to maladaptive remodeling during the transition to decompensated heart failure (Liao et al., 2018).EAT is known to contain various immune cells such as macrophages, neutrophils, and lymphocytes, which may play a role in protecting against heart infections (Horckmans et al., 2018).
Notably, during exercise, myokines released by the heart muscles have been shown to influence the phenotype of EAT macrophages, promoting a protective M2-like phenotype (Aldiss et al., 2017).Conversely, in the context of cardiac injury, EAT can serve as a significant source of inflammatory macrophages due to hypoxia-induced activation, leading to their easy infiltration into the myocardium (Guzzardi & Iozzo, 2019;Vianello et al., 2019).In mice, the role of EAT in inflammation following myocardial infarction has been found to be significant (Vianello et al., 2019).Moreover, in patients with MI, an augmented thickness of EAT has been linked to visceral obesity and cardiac fibrosis (Gruzdeva et al., 2018).Furthermore, in cases of severe cardiac injury, necrotic myocytes have the ability to recruit and activate macrophages through the release of damage-associated molecular patterns (Ma, Mouton, & Lindsey, 2018;Mouton et al., 2018).Additionally, myocytes with impaired mitochondria can release damage-associated molecular patterns, which subsequently enhance the secretion of pro-inflammatory chemokines and cytokines (Zhang et al., 2010).The occurrence of endothelial cell injury, induced by pressure-induced shear stress and mechanical stretch, is a significant factor in the development of cardiac inflammation in individuals with hypertension.This injury prompts an augmented production of ROS and hampers NO signaling, thereby potentially attracting neighboring macrophages (Savoia et al., 2011).
Extensive research has been conducted on the involvement of macrophages in cardiac remodeling subsequent to myocardial infarction, as evidenced by numerous studies (Andreadou et al., 2019;Ben-Mordechai et al., 2013;Chen & Frangogiannis, 2018;Dick et al., 2019).
However, the exploration of macrophage metabolism in this context remains constrained.Notably, a mouse model of myocardial infarction demonstrates a significant upregulation of glycolytic genes, including GAPDH, in macrophages within the infarcted region on the first day.Conversely, mitochondrial genes, such as succinate dehydrogenase, exhibit an increase on the third day following myocardial infarction (Mouton et al., 2018), suggesting that the cardiac microenvironment is hypoxic on the first day but becomes reoxygenated by Day 3 due to vasculogenesis (Mouton et al., 2018(Mouton et al., , 2019)).In a rat model of myocardial infarction, the utilization of a glycolysis inhibitor resulted in a reduction of glycolysis and inflammation in cardiac macrophages, ultimately leading to an improvement in left ventricular function (Lewis et al., 2018).The underlying mechanism involves the process of efferocytosis, which enhances the intracellular supply of fatty acids, thereby promoting mitochondrial fatty acid oxidation and inducing a shift in macrophage polarization toward M2 phenotypes (Zhang et al., 2019).This M2-like polarization has been found to contribute to cardiac fibrosis in mice with hypertension and diastolic dysfunction, but its initiation is dependent on the initial infiltration and expansion of inflammatory macrophages (Hulsmans et al., 2018).
Furthermore, it has been previously discussed that resident macrophages in the mouse heart, which do not express CCR2, can be categorized into two groups based on their MHCII expression (Epelman et al., 2014).Additionally, CCR2+ cardiac monocyte-derived macrophages can originate from infiltrating Ly6Chi (M1-like) or Ly6Clow (M2-like) monocytes (Liao et al., 2018;Nahrendorf et al., 2007).This suggests that there might be variations in metabolic profiles not only between the M1 and M2 paradigms, but also between resident macrophages and infiltrating monocytes.However, the field of immunometabolism is still in its nascent stage and predominantly relies on Despite the considerable progress made in modern medicine, heart failure continues to have a substantial impact on morbidity and mortality rates.The management of HF is contingent upon identifying its root cause, as failure to address it promptly can lead to rapid disease progression and a decline in the patient's quality of life (McDonagh et al., 2022).Stem cell therapy, an emerging technology, has demonstrated potential as a therapeutic strategy for both prevention and treatment of cardiovascular diseases (Goradel et al., 2018).In this context, macrophages play a crucial role in the implementation of cardiac stem cell therapy.Numerous studies have provided evidence that, subsequent to ischemia-reperfusion injury, the cardiac function in mice undergoes enhancement via an acute sterile immune response, which entails the activation of CCR2+ and CX3CR1+ macrophages, rather than the generation of new cardiomyocyte (Vagnozzi et al., 2020).This specific macrophage response leads to improved activity of cardiac fibroblasts, decreased extracellular matrix content, and augmented mechanical properties within the ischemic region, thereby presenting a novel mechanism for cell-mediated cardiac protection.To comprehensively comprehend the therapeutic implications of these findings, further research is imperative.The reciprocal regulation between macrophages and stem cells is observed in the context of cardiac regeneration in the injured heart.Previous research has demonstrated that the interaction between BMMSCs and immune cells, specifically macrophages, leads to a shift in macrophage polarization toward an antiinflammatory state (Cho et al., 2014;Hare et al., 2012).This effect is further enhanced when macrophages are primed with BMMSCs, as evidenced by Lim et al. (2018) in their study utilizing a rat model of myocardial infarction.The administration of a coculture of bone marrow-derived mesenchymal stem cells and bone marrow-derived macrophages (BMDMs) via injection led to a notable augmentation in the prevalence of M2 macrophages, thereby yielding enhanced cardiac functionality, improved angiogenesis, and diminished cardiac fibrosis.These findings suggest that macrophages possess considerable therapeutic capabilities, and BMDMs that have been primed with BMMSCs serve as an efficacious adjunctive therapy for cardiac restoration.

| CON CLUS ION
As a result of these factors, the aging heart in geriatric individuals F I G U R E 2 Overview of the age-related changes to heart and aging-related pathological processes.| 5 of 18 LI et al.

Following
myocardial infarction (MI), the process of efferocytosis performed by macrophages assumes significance in the effective removal of deceased and apoptotic cardiac myocytes (CMs).Numerous investigations have successfully identified distinct efferocytosis receptors present on macrophages.Notably, several studies have demonstrated the capacity of macrophages to identify CMs through the utilization of the MerTK receptor (Cai et al., 2017; Zagórska et al., 2014).The impairment of cardiac metabolic function occurs when either macrophages or MerTK are ablated (Nicolás-Ávila et al., 2020).Furthermore, the proteins CD47 and CD72, which are associated with integrin, exert an influence on the intercommunication between macrophages and cardiac myocytes after myocardial infarction (MI).The upregulation of CD47 expression hampers the process of efferocytosis through the CD47-SIRPα axis (Nicolás-Ávila et al., 2020).Additionally, it has been observed that macrophages play a significant role in facilitating cardiac muscle (CM) proliferation and regeneration following myocardial infarction.Notably, the hearts of neonatal mice have demonstrated the ability to fully regenerate post-MI through the activation of several downstream signaling pathways, During the wound-healing phase subsequent to myocardial infarction, macrophages that are recruited have the capability to generate angiotensinogen II, thereby activating the classical renin-angiotensinaldosterone system and inducing an upregulation of TGF-β1.This regulatory process effectively governs the deposition of matrix proteins F I G U R E 4 Crosstalk between macrophages and other types of cells, including cardiomyocytes, fibroblasts, and Treg cells.

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Jia et al. observed that following myocardial infarction, regulatory T cells (Treg cells) exhibit an increased secretion of interleukin-35 compared to normal physiological conditions.This heightened IL-35 production stimulates the transcription of CX3CR1 (C-X3-C motif chemokine receptor 1) and transforming growth factor-beta 1 in macrophages.This activation is mediated through the GP130 and IL12Rβ2 signaling pathways, as well as the phosphorylation of STAT1 and STAT4.Consequently, the enhanced expression of CX3CR1 and TGF-β1 promotes the survival of Ly-6Clow macrophages and facilitates the deposition of extracellular matrix (Jia et al., 2019) (Figure 4).
broblast trans-differentiation through signaling pathways involving Smad3 and the expression of smooth muscle actin (DeLeon-Pennell et al., 2017).To ensure the preservation of structural integrity and mitigate the occurrence of wall rupture and myocardial dysfunction, activated myofibroblasts exhibit heightened secretion of collagens and other extracellular matrix proteins.Nevertheless, an excessive deposition of ECM results in ventricular stiffness, impaired contractile function, and an augmented susceptibility to arrhythmogenesis and mortality.Additionally, inflammation and fibrosis in perivascular regions contribute significantly to pathological remodeling.During the maturation stage, a scar is formed with a cross-linked extracellular matrix subsequent to the proliferative stage.Apoptosis occurs in myofibroblasts, temporary disintegration of microvasculature takes place, and tissue-resident macrophages play a role in facilitating inflammation resolution and tissue repair.The suppression of monocyte recruitment to the injured adult heart is observed due to reduced inflammation and accelerated repair, while the embryonic macrophage population remains unaffected.In conclusion, the intricate interaction among inflammatory cells, myofibroblasts, extracellular matrix remodeling, and macrophages plays a pivotal role in the advancement and resolution of cardiac fibrosis.
the classification of M1/M2 phenotypes, with future investigations focusing on exploring different subsets of cardiac macrophages in the context of cardiac remodeling.12 | MACROPHAG E-TARG E TED THER APIE S Extensive research has been conducted on the subject of macrophagetargeted therapies in heart failure; nevertheless, the development of effective clinical treatments based on this knowledge remains elusive.Macrophages assume a crucial role in the mediation of tissue damage and the formation of fibrotic scars in heart failure.Leveraging macrophages as therapeutic targets holds the potential to mitigate the deleterious impacts of the innate immune system, while concurrently preserving its indispensable functions.The implementation of macrophage-targeted therapy necessitates the design of nanoparticles and nano-based drug delivery systems due to the phagocytic capabilities exhibited by macrophages.The regulation of inflammatory monocyte migration, which gives rise to classically activated macrophages and contributes to inflammatory diseases, is governed by the CCR2 marker.Leuschner et al. (2011) devised siRNA nanomolecules with a specific affinity for monocytes to effectively suppress CCR2 mRNA expression in inflammatory monocytes, thereby selectively impeding their migration.These nano-molecules were successfully internalized by monocytes and exhibited accumulation in the spleen and bone marrow of mice upon systemic administration.Notably, the degradation of CCR2 mRNA in monocytes resulted in diminished accumulation at inflammatory sites.This therapeutic approach demonstrated a reduction in infarct size following coronary artery occlusion, a decrease in atherosclerotic plaques, and a decline in tumor-associated macrophages in mouse models.Viral infection and autoimmune myocarditis are substantial contributors to heart failure in adolescents, thus necessitating the implementation of macrophage-targeted therapy.It has been documented that individuals with myocarditis exhibit heightened levels of CSF-1 expression.CSF-1, which is synthesized by cells of the mononuclear phagocyte system, exerts influence over the genesis and progression of monocytes/macrophages via the CSF-1R.Meyer et al. (2018) employed nanoparticle-encapsulated siRNA to suppress the CSF-1 axis, resulting in the alleviation of acute inflammatory heart injury and the mitigation of long-term repercussions associated with acute myocardial damage.This therapeutic strategy demonstrated efficacy in the treatment of viral and autoimmune myocarditis.Due to their participation in inflammatory heart tissue and phenotypic plasticity, macrophages have been recognized as a distinct target for the treatment of cardiovascular diseases.Potential therapies targeting macrophages in heart failure may utilize macrophage markers to deliver treatment specific to the affected tissues.Furthermore, recent advancements in comprehending the epigenetic programming of fibrosis and HF, including macrophage polarization, present encouraging prospects for intervening in macrophage epigenetics through genetic or pharmacological means (Davis & Gallagher, 2019; Liu & Tang, 2019).The CRISPR technology, which enables enduring genetic modifications in cells, could potentially be utilized to modify macrophage polarization and functional phenotype (Stoppe et al., 2018).The potential protective function of macrophage migration inhibitory factor has been documented following cardiac surgery.Despite the limited research on macrophages and cardiac surgery at present, this field exhibits promising prospects for future exploration.
With the growing proportion of elderly individuals in the global population, it becomes imperative to comprehend the alterations in cardiac structure and function that occur as part of the natural aging process.Cardiac tissue-resident macrophages represent the predominant immune cell population within the heart.Therefore, gaining insight into the mechanistic underpinnings of CRM involvement during the aging process holds significant importance for the development of novel therapeutic interventions aimed at enhancing patient outcomes.