Healing after myocardial infarction (MI) involves the biphasic accumulation of inflammatory Ly-6Chigh and reparative Ly-6Clow monocytes/macrophages (Mo/MΦ). According to one model, Mo/MΦ heterogeneity in the heart originates in the blood and involves the sequential recruitment of distinct monocyte subsets that differentiate to distinct macrophages. Alternatively, heterogeneity may arise in tissue from one circulating subset via local macrophage differentiation and polarization. The orphan nuclear hormone receptor, Nr4a1, is essential to Ly-6Clow monocyte production but dispensable to Ly-6Clow macrophage differentiation; dependence on Nr4a1 can thus discriminate between systemic and local origins of macrophage heterogeneity.
This study tested the role of Nr4a1 in MI in the context of the two Mo/MΦ accumulation scenarios.
Methods and Results
We show that Ly-6Chigh monocytes infiltrate the infarcted myocardium and, unlike Ly-6Clow monocytes, differentiate to cardiac macrophages. In the early, inflammatory phase of acute myocardial ischemic injury, Ly-6Chigh monocytes accrue in response to a brief Ccl2 burst. In the second, reparative phase, accumulated Ly-6Chigh monocytes give rise to reparative Ly-6Clow F4/80high macrophages that proliferate locally. In the absence of Nr4a1, Ly-6Chigh monocytes express heightened levels of Ccr2 on their surface, avidly infiltrate the myocardium, and differentiate to abnormally inflammatory macrophages, which results in defective healing and compromised heart function.
Ly-6Chigh monocytes orchestrate both inflammatory and reparative phases during MI and depend on Nr4a1 to limit their influx and inflammatory cytokine expression.
Monocyte; macrophage; myocardial infarction; nuclear hormone receptor; healing
Atherosclerotic lesions grow via the accumulation of leukocytes and oxidized lipoproteins in the vessel wall. Leukocytes can attenuate or augment atherosclerosis through the release of cytokines, chemokines, and other mediators. Deciphering how leukocytes develop, oppose and complement each other’s function, and shape the course of disease, can illuminate understanding of atherosclerosis. Innate response activator (IRA) B cells are a recently described population of GM-CSF-secreting cells of hitherto unknown function in atherosclerosis.
Methods and Results
Here we show that IRA B cells arise during atherosclerosis in mice and humans. In response to high cholesterol diet, IRA B cell numbers increase preferentially in secondary lymphoid organs via Myd88-dependent signaling. Mixed chimeric mice lacking B cell-derived GM-CSF develop smaller lesions with fewer macrophages and effector T cells. Mechanistically, IRA B cells promote the expansion of classical dendritic cells, which then generate IFNγ-producing TH1 cells. This IRA B cell-dependent TH1 skewing manifests in an IgG1 to IgG2c isotype switch in the immunoglobulin response against oxidized lipoproteins.
GM-CSF-producing IRA B cells alter adaptive immune processes and shift the leukocyte response toward a TH1-associated mileu that aggravates atherosclerosis.
atherosclerosis; immunology; B cells; Dendritic cells; T cells; Granulocyte macrophage colony-stimulating factor
Monocytes are mononuclear circulating phagocytes that originate in the bone marrow and give rise to macrophages in peripheral tissue. For decades, our understanding of monocyte lineage was bound to a stepwise model that favored an inverse relationship between cellular proliferation and differentiation. Sophisticated molecular and surgical cell tracking tools have transformed our thinking about monocyte topo-ontogeny and function. Here, we discuss how recent studies focusing on progenitor proliferation and differentiation, monocyte mobilization and recruitment, and macrophage differentiation and proliferation are reshaping knowledge of monocyte lineage in steady state and disease.
monocyte; hematopoiesis; macrophage; proliferation; atherosclerosis
During progression of atherosclerosis, myeloid cells destabilize lipid-rich plaque in the arterial wall and cause its rupture, thus triggering myocardial infarction and stroke. Survivors of acute coronary syndromes have a high risk of recurrent events for unknown reasons. Here we show that the systemic response to ischemic injury aggravates chronic atherosclerosis. After myocardial infarction or stroke, apoE−/− mice developed larger atherosclerotic lesions with a more advanced morphology. This disease acceleration persisted over many weeks and was associated with markedly increased monocyte recruitment. When seeking the source of surplus monocytes in plaque, we found that myocardial infarction liberated hematopoietic stem and progenitor cells from bone marrow niches via sympathetic nervous system signaling. The progenitors then seeded the spleen yielding a sustained boost in monocyte production. These observations provide new mechanistic insight into atherogenesis and provide a novel therapeutic opportunity to mitigate disease progression.
Atherosclerotic lesions are believed to grow via the recruitment of bone marrow-derived monocytes. Among the known murine monocyte subsets, Ly-6Chigh monocytes are inflammatory, accumulate in lesions preferentially, and differentiate. Here we hypothesized that the bone marrow outsources the production of Ly-6Chigh monocytes during atherosclerosis.
Methods and Results
Using murine models of atherosclerosis and fate-mapping approaches, we show that hematopoietic stem and progenitor cells (HSPC) progressively relocate from the bone marrow to the splenic red pulp where they encounter GM-CSF and IL-3, clonally expand, and differentiate to Ly-6Chigh monocytes. Monocytes born in such extramedullary niches intravasate, circulate, and accumulate abundantly in atheromata. Upon lesional infiltration, Ly-6Chigh monocytes secrete inflammatory cytokines, reactive oxygen species, and proteases. Eventually, they ingest lipids and become foam cells.
Our findings indicate that extramedullary sites supplement the bone marrow’s hematopoietic function by producing circulating inflammatory cells that infiltrate atherosclerotic lesions.
Atherosclerosis; Imaging; Immune System; Immunology; Macrophage
editorials; atherosclerosis; folate; folate receptor; immunotoxins; macrophages
Healing of myocardial infarction (MI) requires monocytes/macrophages. These mononuclear phagocytes likely degrade released macromolecules and aid in scavenging of dead cardiomyocytes, while mediating aspects of granulation tissue formation and remodeling. The mechanisms that orchestrate such divergent functions remain unknown. In view of the heightened appreciation of the heterogeneity of circulating monocytes, we investigated whether distinct monocyte subsets contribute in specific ways to myocardial ischemic injury in mouse MI. We identify two distinct phases of monocyte participation after MI and propose a model that reconciles the divergent properties of these cells in healing. Infarcted hearts modulate their chemokine expression profile over time, and they sequentially and actively recruit Ly-6Chi and -6Clo monocytes via CCR2 and CX3CR1, respectively. Ly-6Chi monocytes dominate early (phase I) and exhibit phagocytic, proteolytic, and inflammatory functions. Ly-6Clo monocytes dominate later (phase II), have attenuated inflammatory properties, and express vascular–endothelial growth factor. Consequently, Ly-6Chi monocytes digest damaged tissue, whereas Ly-6Clo monocytes promote healing via myofibroblast accumulation, angiogenesis, and deposition of collagen. MI in atherosclerotic mice with chronic Ly-6Chi monocytosis results in impaired healing, underscoring the need for a balanced and coordinated response. These observations provide novel mechanistic insights into the cellular and molecular events that regulate the response to ischemic injury and identify new therapeutic targets that can influence healing and ventricular remodeling after MI.
Macrophages populate the steady-state myocardium. Previously, all macrophages were thought to arise from monocytes; however, it emerged that in several organs tissue-resident macrophages may self-maintain through local proliferation.
To study the contribution of monocytes to cardiac resident macrophages in steady-state, after macrophage depletion in CD11bDTR/+ mice and in myocardial infarction.
Methods and Results
Using in vivo fate mapping and flow cytometry, we estimated that during steady-state the heart macrophage population turns over in about one month. To explore the source of cardiac resident macrophages, we joined the circulation of mice using parabiosis. After 6 weeks, we observed blood monocyte chimerism of 35.3±3.4% while heart macrophages showed a much lower chimerism of 2.7±0.5% (p<0.01). Macrophages self renewed locally through proliferation: 2.1±0.3% incorporated BrdU 2 hours after a single injection and 13.7±1.4% heart macrophages stained positive for the cell cycle marker Ki67. The cells likely participate in defense against infection, as we found them to ingest fluorescently labeled bacteria. In ischemic myocardium, we observed that tissue resident macrophages died locally while some also migrated to hematopoietic organs. If the steady-state was perturbed by coronary ligation or diphtheria toxin-induced macrophage depletion in CD11bDTR/+ mice, blood monocytes replenished heart macrophages. However, in the chronic phase after myocardial infarction, macrophages residing in the infarct were again independent from the blood monocyte pool, returning to the steady-state situation.
In this study we show differential contribution of monocytes to heart macrophages during steady-state, after macrophage depletion or in the acute and chronic phase after myocardial infarction. We found that macrophages participate in the immunosurveillance of myocardial tissue. These data correspond with previous studies on tissue-resident macrophages and raise important questions on the fate and function of macrophages during the development of heart failure.
Macrophage; monocyte; heart; myocardial infarction; myocardial
myocardial infarction; monocyte; macrophage; lymphocyte; healing
Tissue macrophages function to maintain homeostasis and regulate immune responses. While tissue macrophages derive from one of a small number of progenitor programs, the transcriptional requirements for site-specific macrophage subset development are more complex. We have identified a new tissue macrophage subset in the thymus and have discovered that its development is dependent on transcription factor NR4A1. Functionally, we find that NR4A1-dependent macrophages are critically important for clearance of apoptotic thymocytes. These macrophages are largely reduced or absent in mice lacking NR4A1, and Nr4a1-deficient mice have impaired thymocyte engulfment and clearance. Thus, NR4A1 functions as a master transcription factor for the development of this novel thymus-specific macrophage subset.
Essential protectors against infection and injury, macrophages can also contribute to many common and fatal diseases. Here we discuss the mechanisms that control different types of macrophage activities in mice. We follow the cells’ maturational pathways over time and space, and elaborate on events that influence the type of macrophage eventually settling a particular destination. The nature of the precursor cells, developmental niches, tissues, environmental cues, and other connecting processes appear to contribute to the identity of macrophage type. Together, the spatial and developmental relationships of macrophages comprise a topo-ontogenic map that can guide our understanding of their biology.
macrophage; monocyte; hematopoesis
The aim of the study was to test wether silencing of the transcription factor Interferon Regulatory Factor 5 (IRF5) in cardiac macrophages improves infarct healing and attenuates post-MI remodeling.
In healing wounds, M1➛M2 macrophage phenotype transition supports resolution of inflammation and tissue repair. Persistence of inflammatory M1 macrophages may derail healing and compromise organ functions. The transcription factor IRF5 promotes genes associated with M1 macrophages.
Here we used nanoparticle-delivered siRNA to silence the transcription factor IRF5 in macrophages residing in myocardial infarcts (MI) and in surgically induced skin wounds in mice.
Infarct macrophages expressed high levels of IRF5 during the early inflammatory wound healing stages (day 4 after coronary ligation) whereas expression of the transcription factor decreased during the resolution of inflammation (day 8). Following in vitro screening, we identified an siRNA sequence that, when delivered by nanoparticles to wound macrophages, efficiently suppressed expression of IRF5 in vivo. Reduction of IRF5 expression, a factor that regulates macrophage polarization, reduced inflammatory M1 macrophage markers, supported resolution of inflammation, accelerated cutaneous and infarct healing and attenuated development of post-MI heart failure after coronary ligation as measured by protease targeted FMT-CT imaging and cardiac MRI (p<0.05 respectively).
This work identifies a new therapeutic avenue to augment resolution of inflammation in healing infarcts by macrophage phenotype manipulation. This therapeutic concept may be used to attenuate post-MI remodeling and heart failure.
Eliminating inflammatory monocytes using microparticles that bind to the MARCO receptor represents a promising strategy to reduce inflammation and injury (Getts et al., this issue).
atherosclerosis; imaging; intravital microscopy; monocyte; neutrophil; platelet
Atherosclerosis is characterized by the progressive accumulation of lipids and leukocytes in the arterial wall. Leukocytes such as macrophages accumulate oxidized lipoproteins in the growing atheromata and give rise to foam cells, which can then contribute to a lesion’s necrotic core. Lipids and leukocytes interact also in other important ways. In experimental models, systemic hypercholesterolemia is associated with severe neutrophilia and monocytosis. Recent evidence indicates that cholesterol sensing pathways control the proliferation of hematopoietic stem cell progenitors. Here we review some of the studies that are forging this particular link between metabolism and inflammation and propose several strategies that could target this axis for the treatment of cardiovascular disease.
hypercholesterolemia; atherosclerosis; hematopoietic stem and progenitor cells; monocytes; neutrophils
Exposure to psychosocial stress is a risk factor for many diseases, including atherosclerosis1,2. While incompletely understood, interaction between the psyche and the immune system provides one potential mechanism linking stress and disease inception and progression. Known crosstalk between the brain and immune system includes the hypothalamic–pituitary–adrenal axis, which centrally drives glucocorticoid production in the adrenal cortex, and the sympathetic–adrenal–medullary axis, which controls stress–induced catecholamine release in support of the fight–or–flight reflex3,4. It remains unknown however if chronic stress changes hematopoietic stem cell activity. Here we show that stress increases proliferation of these most primitive progenitors, giving rise to higher levels of disease–promoting inflammatory leukocytes. We found that chronic stress induced monocytosis and neutrophilia in humans. While investigating the source of leukocytosis in mice, we discovered that stress activates upstream hematopoietic stem cells. Sympathetic nerve fibers release surplus noradrenaline, which uses the β3 adrenergic receptor to signal bone marrow niche cells to decrease CXCL12 levels. Consequently, elevated hematopoietic stem cell proliferation increases output of neutrophils and inflammatory monocytes. When atherosclerosis–prone ApoE−/− mice encounter chronic stress, accelerated hematopoiesis promotes plaque features associated with vulnerable lesions that cause myocardial infarction and stroke in humans.
In response to lung infection, pleural innate response activator B cells produce GM-CSF–dependent IgM and ensure a frontline defense against bacterial invasion.
Pneumonia is a major cause of mortality worldwide and a serious problem in critical care medicine, but the immunophysiological processes that confer either protection or morbidity are not completely understood. We show that in response to lung infection, B1a B cells migrate from the pleural space to the lung parenchyma to secrete polyreactive emergency immunoglobulin M (IgM). The process requires innate response activator (IRA) B cells, a transitional B1a-derived inflammatory subset which controls IgM production via autocrine granulocyte/macrophage colony-stimulating factor (GM-CSF) signaling. The strategic location of these cells, coupled with the capacity to produce GM-CSF–dependent IgM, ensures effective early frontline defense against bacteria invading the lungs. The study describes a previously unrecognized GM-CSF-IgM axis and positions IRA B cells as orchestrators of protective IgM immunity.
Only a few decades ago, students of the pathophysiology of cardiovascular disease paid little heed to the involvement of inflammation and immunity. Multiple lines of evidence now point to the participation of innate and adaptive immunity and inflammatory signaling in a variety of cardiovascular conditions. Hence, interest has burgeoned in this intersection. This review will focus on the contribution of innate immunity to both acute injury to the heart muscle itself, notably myocardial infarction, and to chronic inflammation in the artery wall, namely atherosclerosis, the cause of most myocardial infarctions. Our discussion of the operation of innate immunity in cardiovascular diseases will focus on functions of the mononuclear phagocytes with special attention to emerging data regarding the participation of different functional subsets of these cells in cardiovascular pathophysiology.
myocardial infarction; atherosclerosis; acute coronary syndromes; inflammation; mononuclear phagocytes; innate immunity
Monocytes and macrophages are innate immune cells that reside and accumulate in the healthy and injured heart. The cells and their subsets pursue distinct functions in steady state and disease, and their tenure may range between hours to months. Some subsets are highly inflammatory, while others support tissue repair. This review discusses current concepts of lineage relationships and systems’ cross talk, highlights open questions, and describes tools for studying monocyte and macrophage subsets in the murine and human heart.
heart failure; myocardial infarction; healing; monocyte; macrophage
During the inflammatory response that drives atherogenesis, macrophages accumulate progressively in the expanding arterial wall1,2. The observation that circulating monocytes give rise to lesional macrophages3–9 has reinforced the concept that monocyte infiltration dictates macrophage build-up. Recent work indicates, however, that macrophages do not depend on monocytes in some inflammatory contexts10. We therefore revisited the mechanism of macrophage accumulation in atherosclerosis. We show that murine atherosclerotic lesions experience a surprisingly rapid, 4-week, cell turnover. Replenishment of macrophages in these experimental atheromata depends predominantly on local macrophage proliferation rather than monocyte influx. The microenvironment orchestrates macrophage proliferation via the involvement of scavenger receptor (SR)-A. Our study reveals macrophage proliferation as a key event in atherosclerosis and identifies macrophage self-renewal as a therapeutic target for cardiovascular disease.
Macrophages frequently infiltrate tumors and can enhance cancer growth, yet the origins of the macrophage response are not well understood. Here we address molecular mechanisms of macrophage production in a conditional mouse model of lung adenocarcinoma. We report that over-production of the peptide hormone Angiotensin II (AngII) in tumor-bearing mice amplifies self-renewing hematopoietic stem cells (HSCs) and macrophage progenitors. The process occurred in the spleen but not the bone marrow, and was independent of hemodynamic changes. The effects of AngII required direct hormone ligation on HSCs, depended on S1P1 signaling, and allowed the extramedullary tissue to supply new tumor-associated macrophages throughout cancer progression. Conversely, blocking AngII production prevented cancer-induced HSC and macrophage progenitor amplification and thus restrained the macrophage response at its source. These findings indicate that AngII acts upstream of a potent macrophage amplification program and that tumors can remotely exploit the hormone’s pathway to stimulate cancer-promoting immunity.
Cardiovascular diseases claim more lives worldwide than any other. Etiologically, the dominant trajectory involves atherosclerosis, a chronic inflammatory process of lipid-rich lesion growth in the vascular wall that can cause life-threatening myocardial infarction (MI). Those who survive MI can develop congestive heart failure, a chronic condition of inadequate pump activity that is frequently fatal. Leukocytes – white blood cells – are important participants at the various stages of cardiovascular disease progression and complication. This review will discuss leukocyte function in atherosclerosis, myocardial infarction, and heart failure.
Macrophages (MΦ) predominate among the inflammatory cells in rejecting allografts. These innate immune cells, in addition to allospecific T cells, can damage cardiomyocytes directly.
Methods and Results
We explored if sensitive PET/CT imaging of MΦ-avid nanoparticles detects rejection of heart allografts in mice. In addition, we employed the imaging method to follow the immunomodulatory impact of angiotensin converting enzyme inhibititor therapy (ACEi) on myeloid cells in allografts. Dextran nanoparticles were derivatized with the PET isotope copper-64 and imaged seven days after transplantation. C57/BL6 recipients of BALB/c allografts displayed robust PET signal (standard uptake value allograft, 2.8±0.3; isograft control, 1.7±0.2; p<0.05). Autoradiography and scintillation counting confirmed the in vivo findings. We then imaged the effects of ACEi (5mg/kg Enalapril). ACEi significantly decreased nanoparticle signal (p<0.05). Histology and flow cytometry showed a reduced number of myeloid cells in the graft, blood and lymph nodes, as well as diminished antigen presentation (p<0.05 versus untreated allografts). ACEi also significantly prolonged allograft survival (12 versus 7 days, p<0.0001).
Nanoparticle MΦ PET-CT detects heart transplant rejection and predicts organ survival by reporting on myeloid cells.
heart transplantation; macrophages; imaging; PET/CT
Macrophages are central regulators of disease progression in both atherosclerosis and myocardial infarction. In atherosclerosis, macrophages are the dominant leukocyte population that influences lesional development. In myocardial infarction, which is caused by atherosclerosis, macrophages accumulate readily and play important roles in inflammation and healing. Molecular imaging has grown considerably as a field and can reveal biological process at the molecular, cellular, and tissue levels. Here we explore how various imaging modalities, from intravital microscopy in mice to organ-level imaging in patients, are contributing to our understanding of macrophages and their progenitors in cardiovascular disease.
From many perspectives, cardiovascular diseases and cancers are fundamentally different. On the one hand, atherosclerosis is a disease of lipid accumulation driven by diet and lifestyle, whereas cancer is an attack “from within” driven by mutations. Nevertheless, studies over the last 20 years have forced us to re-evaluate such a view. We are learning that, among other factors, the immune system is indispensable to the development and progression of both diseases. Its components are not only reactive but can also orchestrate both tumor and atherosclerotic lesion growth. In this Viewpoint, we explore how monocytes, which are key constituents of the immune system, forge links between cardiovascular diseases and cancers.