CD169⁺ macrophages provide a niche promoting erythropoiesis under homeostasis and stress

Energy restriction impairs dendritic cell development in C57BL/6J mice

Dendritic cells (DC) are antigen-presenting cells known for stimulating naïve T lymphocytes. The sequential stages of DC development from common myeloid progenitors have been elucidated in murine bone marrow. Energy-restriction (ER) is a pro-longevity dietary intervention with mixed immunological outcomes. The objective of this study was to examine the development of DC in adult C57Bl6J mice fed a 40% ER diet. We observed increased myeloid progenitors, but decreased common DC progenitors, precursor conventional DC and plasmacytoid DC. Furthermore, we observed increased macrophages and cells expressing CD169 in the bone marrow of ER mice. There was no significant difference in DC subsets from unfractionated ER and ad libitum-fed murine bone marrow samples cultured in GM-CSF-supplemented media or Flt3L-supplemented media. Examining rates of proliferation with 6h BrdU incorporation and Ki-67 staining showed these DC progenitor populations have different proliferation rates in ER compared with AL mice. We show here, for the first time, ER results in altered myelopoiesis resulting in reduced DC development but enhanced monocyte/macrophage development in steady-state C57Bl6J mice. In conclusion, these data may partially explain prior observations of impaired early innate immune responses to primary infection such as influenza in ER mice.

Macrophages and regulation of erythropoiesis

PURPOSE OF REVIEW

The nature and function of macrophages at the center of erythroblastic islands is not fully understood. This review discusses novel findings on the phenotypic and molecular characterization of erythroblastic island macrophages, and their role in regulating normal and pathological erythropoiesis.

RECENT FINDINGS

The phenotype to prospectively isolate erythroblastic island macrophages from mouse bone marrow has been identified. In-vivo depletion of erythroblastic island macrophages causes blockade of erythroblast maturation and delays erythropoietic recovery following chemical insults. The cytokine granulocyte colony-stimulating factor arrests medullary erythropoiesis by depleting erythroblastic island macrophages from the bone marrow. In-vivo ablation of macrophages improves anemia associated with β-thalassemia and reduces red blood cell counts in the mouse model of polycythemia vera. The role of cell adhesion molecules regulating interactions between erythroblastic island macrophages and erythroblasts has been clarified, and mechanisms of pyrenocyte engulfment by erythroblastic island macrophages have been demonstrated to involve Mer tyrosine kinase receptor.

SUMMARY

Prospective isolation of mouse erythroblastic island macrophages together with new genetic mouse models to specifically target erythroblastic island macrophages will enable molecular studies to better define their role in controlling erythroblast maturation. These studies have revealed the key role of erythroblastic island macrophages in regulating normal erythropoiesis and could be interesting targets to treat β-thalassemia or polycythemia vera.

Macrophages: An Inflammatory Link Between Angiogenesis and Lymphangiogenesis

Angiogenesis and lymphangiogenesis often occur in response to tissue injury or in the presence of pathology (e.g., cancer), and it is these types of environments in which macrophages are activated and increased in number. Moreover, the blood vascular microcirculation and the lymphatic circulation serve as the conduits for entry and exit for monocyte-derived macrophages in nearly every tissue and organ. Macrophages both affect and are affected by the vessels through which they travel. Therefore, it is not surprising that examination of macrophage behaviors in both angiogenesis and lymphangiogenesis has yielded interesting observations that suggest macrophages may be key regulators of these complex growth and remodeling processes. In this review, we will take a closer look at macrophages through the lens of angiogenesis and lymphangiogenesis, examining how their dynamic behaviors may regulate vessel sprouting and function. We present macrophages as a cellular link that spatially and temporally connects angiogenesis with lymphangiogenesis, in both physiological growth and in pathological adaptations, such as tumorigenesis. As such, attempts to therapeutically target macrophages in order to affect these processes may be particularly effective, and studying macrophages in both settings will accelerate the field's understanding of this important cell type in health and disease.

Macrophages: Key regulators of steady-state and demand-adapted hematopoiesis

Hematopoietic stem cell (HSC) function is required for balanced blood production throughout life; it is thus essential to understand the mechanisms regulating this highly dynamic process. Bone marrow-resident macrophages (Mϕs) have recently emerged as an important component of the HSC niche, where they contribute to regulating HSC and progenitor cell (HSPC) mobilization and function. Here we review the role of macrophages (Mϕs) on immune cell production, HSPC pool size, and mobilization at steady state and under inflammatory conditions. Inflammation induces marked changes in hematopoiesis to restrict or promote generation of specific cell lineages, and this often has a negative impact on HSC function. Cytokines and growth factors induced during inflammation influence hematopoiesis by acting directly on HSPCs and/or by modulating niche cell function. We focus particular attention on the opposing effects of two key inflammatory proteins, interferon-γ and granulocyte-colony stimulating factor, in regulating bone marrow-resident macrophages (Mϕs) and HSPCs. Macrophages (Mϕs) are essential for tissue homeostasis, and here we highlight their emerging role as a central regulator of both steady-state and demand-adapted hematopoiesis.

Targeting of the pulmonary capillary vascular niche promotes lung alveolar repair and ameliorates fibrosis

Although the lung can undergo self-repair after injury, fibrosis in chronically injured or diseased lungs can occur at the expense of regeneration. Here we study how a hematopoietic-vascular niche regulates alveolar repair and lung fibrosis. Using intratracheal injection of bleomycin or hydrochloric acid in mice, we show that repetitive lung injury activates pulmonary capillary endothelial cells (PCECs) and perivascular macrophages, impeding alveolar repair and promoting fibrosis. Whereas the chemokine receptor CXCR7, expressed on PCECs, acts to prevent epithelial damage and ameliorate fibrosis after a single round of treatment with bleomycin or hydrochloric acid, repeated injury leads to suppression of CXCR7 expression and recruitment of vascular endothelial growth factor receptor 1 (VEGFR1)-expressing perivascular macrophages. This recruitment stimulates Wnt/β-catenin-dependent persistent upregulation of the Notch ligand Jagged1 (encoded by Jag1) in PCECs, which in turn stimulates exuberant Notch signaling in perivascular fibroblasts and enhances fibrosis. Administration of a CXCR7 agonist or PCEC-targeted Jag1 shRNA after lung injury promotes alveolar repair and reduces fibrosis. Thus, targeting of a maladapted hematopoietic-vascular niche, in which macrophages, PCECs and perivascular fibroblasts interact, may help to develop therapy to spur lung regeneration and alleviate fibrosis.

[Relationship between macrophages and erythropoiesis]

Macrophages have two major roles in regulating the dynamic equilibrium in erythropoiesis, promoting the differentiation and maturation of nucleated red blood cells into reticulocytes and removing old red blood cells. A recent mouse study has demonstrated that the phenotype of macrophages in erythroblastic islands is CD169+ VCAM-1+ ER-HR3+ CD11b+ F4/80+ Ly-6G+. Molecular connections between erythroid progenitor cells and central macrophages help to maintain the function and integrity of erythroblastic islands. New research advances in Kruppel-like factor 1 (KLF1) provide new evidence for the important role of macrophages in erythroblastic islands. Macrophages play an important role in erythropoiesis both in sickness and in health, and provide a potential targeted therapy for diseases such as polycythemia vera and beta-thalassemia in the future.

Ontogeny of Tumor-associated Macrophages and Its Implication in Cancer Regulation

Macrophages are innate immune cells with evolutionarily conserved functions in tissue maintenance and host defense. As such, macrophages are among the first hematopoietic cells that seed developing tissues, and respond to inflammatory insults by in situ proliferation or de novo differentiation from monocytes. Recent studies have revealed that monocyte-derived tumor-induced macrophages represent a major tumor-associated macrophage population, which can further expand following their differentiation in tumors. Compared to tissue-resident tumor-associated macrophages, these newly differentiated cells are phenotypically distinct, and likely play a unique role in tissue dysregulation and immune modulation in cancer. These findings imply that tumor growth elicits a specific innate immune response. In this review, we explore the different routes of macrophage seeding and maintenance in tissues during steady state and inflammation and how these principles underlie the responses observed during tumor development. In addition, we highlight the relationship between the origin and function of macrophages in different settings and how this knowledge may be used to create new opportunities for cancer immunotherapy.

[Relationship between macrophages and erythropoiesis]

Macrophages have two major roles in regulating the dynamic equilibrium in erythropoiesis, promoting the differentiation and maturation of nucleated red blood cells into reticulocytes and removing old red blood cells. A recent mouse study has demonstrated that the phenotype of macrophages in erythroblastic islands is CD169+ VCAM-1+ ER-HR3+ CD11b+ F4/80+ Ly-6G+. Molecular connections between erythroid progenitor cells and central macrophages help to maintain the function and integrity of erythroblastic islands. New research advances in Kruppel-like factor 1 (KLF1) provide new evidence for the important role of macrophages in erythroblastic islands. Macrophages play an important role in erythropoiesis both in sickness and in health, and provide a potential targeted therapy for diseases such as polycythemia vera and beta-thalassemia in the future.

Hematopoietic niches, erythropoiesis and anemia of chronic infection

Anemia is a significant co-morbidity of chronic infections, as well as other inflammatory diseases. Anemia of chronic infection results from defective bone marrow erythropoiesis. Although the limitation of iron availability has been considered a key factor, the exact mechanisms underlying blockade in erythroid generation during infection are not fully understood. Erythropoiesis is a tightly regulated process that is very sensitive to environmental changes. During the last decade, the importance of the bone marrow hematopoietic niche has been progressively acknowledged. Several bone marrow cell types (such as macrophages, mesenchymal stem cells, and progenitor cells) and molecular mediators (such as CXCL12) have been identified as fundamental for both the maintenance of hematopoietic stem cell pluripotency and their most adequate differentiation into each hematopoietic cell lineage. Importantly, both niche-supporting cells and hematopoietic progenitors were found to be able to sense local and systemic cues to adapt the hematopoietic output to needs of the organism. Here, we review how hematopoietic progenitors and niche-supporting cells sense and respond to stress cues and suggest a potential role for the hematopoietic niche in the development of anemia of chronic infection.

A perisinusoidal niche for extramedullary haematopoiesis in the spleen

Haematopoietic stresses mobilize haematopoietic stem cells (HSCs) from the bone marrow to the spleen and induce extramedullary haematopoiesis (EMH). However, the cellular nature of the EMH niche is unknown. Here we assessed the sources of the key niche factors, SCF (also known as KITL) and CXCL12, in the mouse spleen after EMH induction by myeloablation, blood loss, or pregnancy. In each case, Scf was expressed by endothelial cells and Tcf21(+) stromal cells, primarily around sinusoids in the red pulp, while Cxcl12 was expressed by a subset of Tcf21(+) stromal cells. EMH induction markedly expanded the Scf-expressing endothelial cells and stromal cells by inducing proliferation. Most splenic HSCs were adjacent to Tcf21(+) stromal cells in red pulp. Conditional deletion of Scf from spleen endothelial cells, or of Scf or Cxcl12 from Tcf21+ stromal cells, severely reduced spleen EMH and reduced blood cell counts without affecting bone marrow haematopoiesis. Endothelial cells and Tcf21(+) stromal cells thus create a perisinusoidal EMH niche in the spleen, which is necessary for the physiological response to diverse haematopoietic stresses.

Phenotypic and functional characterization of macrophages with therapeutic potential generated from human cirrhotic monocytes in a cohort study

BACKGROUND AIMS

Macrophages have complex roles in the liver. The aim of this study was to compare profiles of human monocyte-derived macrophages between controls and cirrhotic patients, to determine whether chronic inflammation affects precursor number or the phenotype, with the eventual aim to develop a cell therapy for cirrhosis.

METHODS

Infusion of human macrophages in a murine liver fibrosis model demonstrated a decrease in markers of liver injury (alanine transaminase, bilirubin, aspartate transaminase) and fibrosis (transforming growth factor-β, α-smooth muscle actin, phosphatidylserine receptor) and an increase in markers of liver regeneration (matrix metalloproteinases [MMP]-9, MMP-12 and TNF-related weak inducer of apoptosis). CD14+ monocytes were then isolated from controls. Monocytes were matured into macrophages for 7 days using a Good Manufacturing Practice-compatible technique.

RESULTS

There was no significant difference between the mean number of CD14+ monocytes isolated from cirrhotic patients (n = 9) and controls (n = 10); 2.8 ± SEM 0.54 × 10(8) and 2.5 ± 0.56 × 10(8), respectively. The mean yield of mature macrophages cultured was also not significantly different between cirrhotic patients and controls (0.9 × 10(8) ± 0.38 × 10(8), with more than 90% viability and 0.65 × 10(8) ± 0.16 × 10(8), respectively. Maturation to macrophages resulted in up-regulation of a number of genes (MMP-9, CCL2, interleukin [IL]-10 and TNF-related weak inducer of apoptosis). A cytokine and chemokine polymerase chain reaction array, comparing the control and cirrhotic macrophages, revealed no statistically significant differences.

CONCLUSIONS

Macrophages can be differentiated from cirrhotic patients' apheresis-derived CD14 monocytes and develop the same pro-resolution phenotype as control macrophages, indicating their suitability for clinical therapy.

Inflammation as a Driver of Clonal Evolution in Myeloproliferative Neoplasm

Our understanding of inflammation's role in the pathogenesis of myeloproliferative neoplasm (MPN) is evolving. The impact of chronic inflammation, a characteristic feature of MPN, likely goes far beyond its role as a driver of constitutional symptoms. An inflammatory response to the neoplastic clone may be responsible for some pathologic aspects of MPN. Moreover, JAK2V617F mutated hematopoietic stem and progenitor cells are resistant to inflammation, and this gives the neoplastic clone a selective advantage allowing for its clonal expansion. Because inflammation plays a central role in MPN inflammation is a logical therapeutic target in MPN.

Mesenchymal stem cells use extracellular vesicles to outsource mitophagy and shuttle microRNAs

Mesenchymal stem cells (MSCs) and macrophages are fundamental components of the stem cell niche and function coordinately to regulate haematopoietic stem cell self-renewal and mobilization. Recent studies indicate that mitophagy and healthy mitochondrial function are critical to the survival of stem cells, but how these processes are regulated in MSCs is unknown. Here we show that MSCs manage intracellular oxidative stress by targeting depolarized mitochondria to the plasma membrane via arrestin domain-containing protein 1-mediated microvesicles. The vesicles are then engulfed and re-utilized via a process involving fusion by macrophages, resulting in enhanced bioenergetics. Furthermore, we show that MSCs simultaneously shed micro RNA-containing exosomes that inhibit macrophage activation by suppressing Toll-like receptor signalling, thereby de-sensitizing macrophages to the ingested mitochondria. Collectively, these studies mechanistically link mitophagy and MSC survival with macrophage function, thereby providing a physiologically relevant context for the innate immunomodulatory activity of MSCs.

Stress-associated erythropoiesis initiation is regulated by type 1 conventional dendritic cells

Erythropoiesis is an important response to certain types of stress, including hypoxia, hemorrhage, bone marrow suppression, and anemia, that result in inadequate tissue oxygenation. This stress-induced erythropoiesis is distinct from basal red blood cell generation; however, neither the cellular nor the molecular factors that regulate this process are fully understood. Here, we report that type 1 conventional dendritic cells (cDC1s), which are defined by expression of CD8α in the mouse and XCR1 and CLEC9 in humans, are critical for induction of erythropoiesis in response to stress. Specifically, using murine models, we determined that engagement of a stress sensor, CD24, on cDC1s upregulates expression of the Kit ligand stem cell factor on these cells. The increased expression of stem cell factor resulted in Kit-mediated proliferative expansion of early erythroid progenitors and, ultimately, transient reticulocytosis in the circulation. Moreover, this stress response was triggered in part by alarmin recognition and was blunted in CD24 sensor- and CD8α+ DC-deficient animals. The contribution of the cDC1 subset to the initiation of stress erythropoiesis was distinct from the well-recognized role of macrophages in supporting late erythroid maturation. Together, these findings offer insight into the mechanism of stress erythropoiesis and into disorders of erythrocyte generation associated with stress.

β-thalassemias: paradigmatic diseases for scientific discoveries and development of innovative therapies

β-thalassemias are monogenic disorders characterized by defective synthesis of the β-globin chain, one of the major components of adult hemoglobin. A large number of mutations in the β-globin gene or its regulatory elements have been associated with β-thalassemias. Due to the complexity of the regulation of the β-globin gene and the role of red cells in many physiological processes, patients can manifest a large spectrum of phenotypes, and clinical requirements vary from patient to patient. It is important to consider the major differences in the light of potential novel therapeutics. This review summarizes the main discoveries and mechanisms associated with the synthesis of β-globin and abnormal erythropoiesis, as well as current and novel therapies.

Mouse Siglec-1 Mediates trans-Infection of Surface-bound Murine Leukemia Virus in a Sialic Acid N-Acyl Side Chain-dependent Manner

Siglec-1 (sialoadhesin, CD169) is a surface receptor on human cells that mediates trans-enhancement of HIV-1 infection through recognition of sialic acid moieties in virus membrane gangliosides. Here, we demonstrate that mouse Siglec-1, expressed on the surface of primary macrophages in an interferon-α-responsive manner, captures murine leukemia virus (MLV) particles and mediates their transfer to proliferating lymphocytes. The MLV infection of primary B-cells was markedly more efficient than that of primary T-cells. The major structural protein of MLV particles, Gag, frequently co-localized with Siglec-1, and trans-infection, primarily of surface-bound MLV particles, efficiently occurred. To explore the role of sialic acid for MLV trans-infection at a submolecular level, we analyzed the potential of six sialic acid precursor analogs to modulate the sialylated ganglioside-dependent interaction of MLV particles with Siglec-1. Biosynthetically engineered sialic acids were detected in both the glycolipid and glycoprotein fractions of MLV producer cells. MLV released from cells carrying N-acyl-modified sialic acids displayed strikingly different capacities for Siglec-1-mediated capture and trans-infection; N-butanoyl, N-isobutanoyl, N-glycolyl, or N-pentanoyl side chain modifications resulted in up to 92 and 80% reduction of virus particle capture and trans-infection, respectively, whereas N-propanoyl or N-cyclopropylcarbamyl side chains had no effect. In agreement with these functional analyses, molecular modeling indicated reduced binding affinities for non-functional N-acyl modifications. Thus, Siglec-1 is a key receptor for macrophage/lymphocyte trans-infection of surface-bound virions, and the N-acyl side chain of sialic acid is a critical determinant for the Siglec-1/MLV interaction.

CD14+ cells from peripheral blood positively regulate hematopoietic stem and progenitor cell survival resulting in increased erythroid yield

Expansion of erythroblasts from human peripheral blood mononuclear cells is 4- to 15-fold more efficient than that of CD34(+) cells purified from peripheral blood mononuclear cells. In addition, purified CD34(+) and CD34(-) populations from blood do not reconstitute this erythroid yield, suggesting a role for feeder cells present in blood mononuclear cells that increase hematopoietic output. Immunodepleting peripheral blood mononuclear cells for CD14(+) cells reduced hematopoietic stem and progenitor cell expansion. Conversely, the yield was increased upon co-culture of CD34(+) cells with CD14(+) cells (full contact or transwell assays) or CD34(+) cells re-constituted in conditioned medium from CD14(+) cells. In particular, CD14(++)CD16(+) intermediate monocytes/macrophages enhanced erythroblast outgrowth from CD34(+) cells. No effect of CD14(+) cells on erythroblasts themselves was observed. However, 2 days of co-culturing CD34(+) and CD14(+) cells increased CD34(+) cell numbers and colony-forming units 5-fold. Proliferation assays suggested that CD14(+) cells sustain CD34(+) cell survival but not proliferation. These data identify previously unrecognized erythroid and non-erythroid CD34(-) and CD34(+) populations in blood that contribute to the erythroid yield. A flow cytometry panel containing CD34/CD36 can be used to follow specific stages during CD34(+) differentiation to erythroblasts. We have shown modulation of hematopoietic stem and progenitor cell survival by CD14(+) cells present in peripheral blood mononuclear cells which can also be found near specific hematopoietic niches in the bone marrow.

Macrophages: gatekeepers of tissue integrity

Macrophages form a heterogeneous group of hematopoietic cells that reside in tissues, where they are required to maintain organ integrity. Tissue macrophages contribute to tissue formation, metabolism, homeostasis, and repair. They have a unique ability to sense and respond to tissue damage. They serve as the first line of defense during infection and help promote immune tolerance in the steady state. Although most tissue macrophages share a high phagocytic and degradative potential, they are heterogeneous in origin, as well as in homeostatic function and response to insults. Here, we will discuss recent developments in our understanding of the origin of tissue macrophages and their functional specialization in tissues.

Bone Marrow Macrophages Contribute to Diabetic Stem Cell Mobilopathy by Producing Oncostatin M

Diabetes affects bone marrow (BM) structure and impairs mobilization of stem cells (SCs) into peripheral blood (PB). This amplifies multiorgan complications because BMSCs promote vascular repair. Because diabetes skews macrophage phenotypes and BM macrophages (BMMΦ) prevent SC mobilization, we hypothesized that excess BMMΦ contribute to diabetic SC mobilopathy. We show that patients with diabetes have increased M1 macrophages, whereas diabetic mice have increased CD169(+) BMMΦ with SC-retaining activity. Depletion of BMMΦ restored SC mobilization in diabetic mice. We found that CD169 labels M1 macrophages and that conditioned medium (CM) from M1 macrophages, but not from M0 and M2 macrophages, induced chemokine (C-X-C motif) ligand 12 (CXCL12) expression by mesenchymal stem/stromal cells. In silico data mining and in vitro validation identified oncostatin M (OSM) as the soluble mediator contained in M1 CM that induces CXCL12 expression via a mitogen-activated protein kinase kinase-p38-signal transducer and activator of a transcription 3-dependent pathway. In diabetic mice, OSM neutralization prevented CXCL12 induction and improved granulocyte-colony stimulating factor and ischemia-induced mobilization, SC homing to ischemic muscles, and vascular recovery. In patients with diabetes, BM plasma OSM levels were higher and correlated with the BM-to-PB SC ratio. In conclusion, BMMΦ prevent SC mobilization by OSM secretion, and OSM antagonism is a strategy to restore BM function in diabetes, which can translate into protection mediated by BMSCs.

Origin, development, and homeostasis of tissue-resident macrophages

Macrophages are versatile cells of the hematopoietic system that display remarkable functional diversity encompassing innate immune responses, tissue development, and tissue homeostasis. Macrophages are present in almost all tissues of the body and display distinct location-specific phenotypes and gene expression profiles. Recent studies also demonstrate distinct origins of tissue-resident macrophages. This emerging picture of ontological, functional, and phenotypic heterogeneity within tissue macrophages has altered our understanding of these cells, which play important roles in many human diseases. In this review, we discuss the different origins of tissue macrophages, the transcription factors regulating their development, and the mechanisms underlying their homeostasis at steady state.

Atheroprotective role of C5ar2 deficiency in apolipoprotein E-deficient mice

Atherogenic processes and vascular remodelling after arterial injury are controlled and driven by a plethora of factors amongst which the activation of the complement system is pivotal. Recently, we reported a clear correlation between high expressions of the second receptor for complement anaphylatoxin C5a, the C5a receptor-like 2 (C5L2, C5aR2), with high pro-inflammatory cytokine expression in advanced human atherosclerotic plaques. This prompted us to speculate that C5aR2 might have a functional role in atherosclerosis. We, therefore, investigated the role of C5aR2 in atherosclerosis and vascular remodelling. Here, we demonstrate that C5ar2 deletion, in atherosclerosis-prone mice, attenuates atherosclerotic as well as neointimal plaque formation, reduces macrophages and CD3+ T cells and induces features of plaque stability, as analysed by histomorphometry and quantitative immunohistochemistry. As a possible underlying mechanism, C5ar2-deficient plaques showed significantly reduced expression of C5a receptor (C5ar1), Tnf-α as well as Vcam-1, as determined by qPCR and quantitative immunohistochemistry. In addition, in vitro mechanistic studies revealed a reduction of these pro-inflammatory and pro-atherosclerotic mediators in C5ar2-deficient macrophages. Finally, blocking C5ar1 with antagonist JPE1375, in C5ar2(-/-)/Apoe(-/-) mice, led to a further reduction in neointimal plaque formation with reduced inflammation. In conclusion, C5ar2 deficiency attenuates atherosclerosis and neointimal plaque formation after arterial injury. This identifies C5aR2 as a promising target to reduce atherosclerosis and restenosis after vascular interventions.

Making Blood: The Haematopoietic Niche throughout Ontogeny

Approximately one-quarter of all cells in the adult human body are blood cells. The haematopoietic system is therefore massive in scale and requires exquisite regulation to be maintained under homeostatic conditions. It must also be able to respond when needed, such as during infection or following blood loss, to produce more blood cells. Supporting cells serve to maintain haematopoietic stem and progenitor cells during homeostatic and pathological conditions. This coalition of supportive cell types, organised in specific tissues, is termed the haematopoietic niche. Haematopoietic stem and progenitor cells are generated in a number of distinct locations during mammalian embryogenesis. These stem and progenitor cells migrate to a variety of anatomical locations through the conceptus until finally homing to the bone marrow shortly before birth. Under stress, extramedullary haematopoiesis can take place in regions that are typically lacking in blood-producing activity. Our aim in this review is to examine blood production throughout the embryo and adult, under normal and pathological conditions, to identify commonalities and distinctions between each niche. A clearer understanding of the mechanism underlying each haematopoietic niche can be applied to improving ex vivo cultures of haematopoietic stem cells and potentially lead to new directions for transplantation medicine.

Macrophage-Lineage Cells Negatively Regulate the Hematopoietic Stem Cell Pool in Response to Interferon Gamma at Steady State and During Infection

Bone marrow (BM) resident macrophages (Mϕs) regulate hematopoietic stem cell (HSC) mobilization; however, their impact on HSC function has not been investigated. We demonstrate that depletion of BM resident Mϕs increases HSC proliferation as well as the pool of quiescent HSCs. At the same time, during bacterial infection where BM resident Mϕs are selectively increased we observe a decrease in HSC numbers. Moreover, strategies that deplete or reduce Mϕs during infection prevent HSC loss and rescue HSC function. We previously found that the transient loss of HSCs during infection is interferon-gamma (IFNγ)-dependent. We now demonstrate that IFNγ signaling specifically in Mϕs is critical for both the diminished HSC pool and maintenance of BM resident Mϕs during infection. In addition to the IFNγ-dependent loss of BM HSC and progenitor cells (HSPCs) during infection, IFNγ reduced circulating HSPC numbers. Importantly, under infection conditions AMD3100 or G-CSF-induced stem cell mobilization was impaired. Taken together, our data show that IFNγ acts on Mϕs, which are a negative regulator of the HSC pool, to drive the loss in BM and peripheral HSCs during infection. Our findings demonstrate that modulating BM resident Mϕ numbers can impact HSC function in vivo, which may be therapeutically useful for hematologic conditions and refinement of HSC transplantation protocols.

Cancer stem cells and tumor-associated macrophages: a roadmap for multitargeting strategies

The idea that tumor initiation and progression are driven by a subset of cells endowed with stem-like properties was first described by Rudolf Virchow in 1855. 'Cancer stem cells', as they were termed more than a century later, represent a subset of tumor cells that are able to generate all tumorigenic and nontumorigenic cell types within the malignancy. Although their existence was hypothesized >150 years ago, it was only recently that stem-like cells started to be isolated from different neoplastic malignancies. Interestingly, Virchow, in suggesting a correlation between cancer and the inflammatory microenvironment, also paved the way for the 'Seed and Soil' theory proposed by Paget a few years later. Despite the time that has passed since these two important concepts were suggested, the relationships between Virchow's 'stem-like cells' and Paget's 'soil' are far from being fully understood. One emerging topic is the importance of a stem-like niche in modulating the biological properties of stem-like cancer cells and thus in affecting the response of the tumor to drugs. This review aims to summarize the recent molecular data concerning the multilayered relationship between cancer stem cells and tumor-associated macrophages that form a key component of the tumor microenvironment. We also discuss the therapeutic implications of targeting this synergistic interplay.

Immune modulation of stem cells and regeneration

The immune system, best known as the first line of defense against invading pathogens, is integral to tissue development, homeostasis, and wound repair. In recent years, there has been a growing appreciation that cellular and humoral components of the immune system also contribute to regeneration of damaged tissues, including limbs, skeletal muscle, heart, and the nervous system. Here, we discuss key findings that implicate inflammatory cells and their secreted factors in tissue replacement after injury via stem cells and other reparative mechanisms. We highlight clinical conditions that are amenable to immune-mediated regeneration and suggest immune targeting strategies for tissue regeneration.

Macrophages and iron trafficking at the birth and death of red cells

Macrophages play a critical role in iron homeostasis via their intimate association with developing and dying red cells. Central nurse macrophages promote erythropoiesis in the erythroblastic island niche. These macrophages make physical contact with erythroblasts, enabling signaling and the transfer of growth factors and possibly nutrients to the cells in their care. Human mature red cells have a lifespan of 120 days before they become senescent and again come into contact with macrophages. Phagocytosis of red blood cells is the main source of iron flux in the body, because heme must be recycled from approximately 270 billion hemoglobin molecules in each red cell, and roughly 2 million senescent red cells are recycled each second. Here we will review pathways for iron trafficking found at the macrophage-erythroid axis, with a focus on possible roles for the transport of heme in toto.

Macrophages support splenic erythropoiesis in 4T1 tumor-bearing mice

Anemia is a common complication of cancer; a role of spleen in tumor-stress erythropoiesis has been suggested. However, the molecular mechanisms involved in the splenic erythropoiesis following tumor maintenance remain poorly understood. Here we show that tumor development blocks medullar erythropoiesis by granulocyte colony-stimulating factor (G-CSF) and then causes anemia in murine 4T1 breast tumor-bearing mice. Meanwhile, tumor-stress promotes splenic erythropoiesis. Splenectomy worsened tumor-induced anemia, and reduced tumor volume and tumor weight, indicating the essential role of spleen in tumor-stress erythropoiesis and tumor growth. Tumor progression of these mice led to increased amounts of bone morphogenetic protein 4 (BMP4) in spleen. The in vivo role of macrophages in splenic erythropoiesis under tumor-stress conditions was investigated. Macrophage depletion by injecting liposomal clodronate decreased the expression of BMP4, inhibited splenic erythropoiesis, aggravated the tumor-induced anemia and suppressed tumor growth. Our results provide insight that macrophages and BMP4 are positive regulators of splenic erythropoiesis in tumor pathological situations. These findings reveal that during the tumor-stress period, the microenvironment of the spleen is undergoing changes, which contributes to adopt a stress erythropoietic fate and supports the expansion and differentiation of stress erythroid progenitors, thereby replenishing red blood cells and promoting tumor growth.

Macrophages and iron trafficking at the birth and death of red cells

Macrophages play a critical role in iron homeostasis via their intimate association with developing and dying red cells. Central nurse macrophages promote erythropoiesis in the erythroblastic island niche. These macrophages make physical contact with erythroblasts, enabling signaling and the transfer of growth factors and possibly nutrients to the cells in their care. Human mature red cells have a lifespan of 120 days before they become senescent and again come into contact with macrophages. Phagocytosis of red blood cells is the main source of iron flux in the body, because heme must be recycled from approximately 270 billion hemoglobin molecules in each red cell, and roughly 2 million senescent red cells are recycled each second. Here we will review pathways for iron trafficking found at the macrophage-erythroid axis, with a focus on possible roles for the transport of heme in toto.

Making sense of hematopoietic stem cell niches

The hematopoietic stem cell (HSC) niche commonly refers to the pairing of hematopoietic and mesenchymal cell populations that regulate HSC self-renewal, differentiation, and proliferation. Anatomic localization of the niche is a dynamic unit from the developmental stage that allows proliferating HSCs to expand before they reach the bone marrow where they adopt a quiescent phenotype that protects their integrity and functions. Recent studies have sought to clarify the complexity behind the HSC niche by assessing the contributions of specific cell populations to HSC maintenance. In particular, perivascular microenvironments in the bone marrow confer distinct vascular niches that regulate HSC quiescence and the supply of lineage-committed progenitors. Here, we review recent data on the cellular constituents and molecular mechanisms involved in the communication between HSCs and putative niches.

Regulation of leucocyte homeostasis in the circulation

The functions of blood cells extend well beyond the immune functions of leucocytes or the respiratory and hemostatic functions of erythrocytes and platelets. Seen as a whole, the bloodstream is in charge of nurturing and protecting all organs by carrying a mixture of cell populations in transit from one organ to another. To optimize these functions, evolution has provided blood and the vascular system that carries it with various mechanisms that ensure the appropriate influx and egress of cells into and from the circulation where and when needed. How this homeostatic control of blood is achieved has been the object of study for over a century, and although the major mechanisms that govern it are now fairly well understood, several new concepts and mediators have recently emerged that emphasize the dynamism of this liquid tissue. Here we review old and new concepts that relate to the maintenance and regulation of leucocyte homeostasis in blood and briefly discuss the mechanisms for platelets and red blood cells.

Tissue-resident macrophage enhancer landscapes are shaped by the local microenvironment

Macrophages are critical for innate immune defense and also control organ homeostasis in a tissue-specific manner. They provide a fitting model to study the impact of ontogeny and microenvironment on chromatin state and whether chromatin modifications contribute to macrophage identity. Here, we profile the dynamics of four histone modifications across seven tissue-resident macrophage populations. We identify 12,743 macrophage-specific enhancers and establish that tissue-resident macrophages have distinct enhancer landscapes beyond what can be explained by developmental origin. Combining our enhancer catalog with gene expression profiles and open chromatin regions, we show that a combination of tissue- and lineage-specific transcription factors form the regulatory networks controlling chromatin specification in tissue-resident macrophages. The environment is capable of shaping the chromatin landscape of transplanted bone marrow precursors, and even differentiated macrophages can be reprogrammed when transferred into a new microenvironment. These results provide a comprehensive view of macrophage regulatory landscape and highlight the importance of the microenvironment, along with pioneer factors in orchestrating identity and plasticity.

Ontogeny and polarization of macrophages in inflammation: blood monocytes versus tissue macrophages

The explosion of new information in recent years on the origin of macrophages in the steady-state and in the context of inflammation has opened up numerous new avenues of investigation and possibilities for therapeutic intervention. In contrast to the classical model of macrophage development, it is clear that tissue-resident macrophages can develop from yolk sac-derived erythro-myeloid progenitors, fetal liver progenitors, and bone marrow-derived monocytes. Under both homeostatic conditions and in response to pathophysiological insult, the contribution of these distinct sources of macrophages varies significantly between tissues. Furthermore, while all of these populations of macrophages appear to be capable of adopting the polarized M1/M2 phenotypes, their respective contribution to inflammation, resolution of inflammation, and tissue repair remains poorly understood and is likely to be tissue- and disease-dependent. A better understanding of the ontology and polarization capacity of macrophages in homeostasis and disease will be essential for the development of novel therapies that target the inherent plasticity of macrophages in the treatment of acute and chronic inflammatory disease.

In vitro culture of stress erythroid progenitors identifies distinct progenitor populations and analogous human progenitors

Tissue hypoxia induces a systemic response designed to increase oxygen delivery to tissues. One component of this response is increased erythropoiesis. Steady-state erythropoiesis is primarily homeostatic, producing new erythrocytes to replace old erythrocytes removed from circulation by the spleen. In response to anemia, the situation is different. New erythrocytes must be rapidly made to increase hemoglobin levels. At these times, stress erythropoiesis predominates. Stress erythropoiesis is best characterized in the mouse, where it is extramedullary and utilizes progenitors and signals that are distinct from steady-state erythropoiesis. In this report, we use an in vitro culture system that recapitulates the in vivo development of stress erythroid progenitors. We identify cell-surface markers that delineate a series of stress erythroid progenitors with increasing maturity. In addition, we use this in vitro culture system to expand human stress erythroid progenitor cells that express analogous cell-surface markers. Consistent with previous suggestions that human stress erythropoiesis is similar to fetal erythropoiesis, we demonstrate that human stress erythroid progenitors express fetal hemoglobin upon differentiation. These data demonstrate that similar to murine bone marrow, human bone marrow contains cells that can generate BMP4-dependent stress erythroid burst-forming units when cultured under stress erythropoiesis conditions.

Red blood cell generation by three-dimensional aggregate cultivation of late erythroblasts

Stem cell-derived erythroid cells hold great potential for the treatment of blood-loss anemia and for erythropoiesis research; however, cultures using conventional flat plates or bioreactors have failed to show promising results. By mimicking the in vivo bone marrow (BM) environment in which most erythroid cells are physically aggregated, we show that a three-dimensional (3D) aggregate culture system facilitates erythroid cell maturation and red blood cell (RBC) production more effectively than two-dimensional high-density cell cultivation. Late erythroblasts (polychromatic or orthochromatic erythroblasts) were differentiated from cord blood CD34(+) cells over 15 days and then allowed to form tight aggregates at a minimum density of 1×10(7) cells/mL for 2-3 days. To scale up the cell culture and to make the media supply efficient throughout the cell aggregates, several macroporous microcarriers and porous scaffolds were applied to the 3D culture system. In comparison to control culture conditions, erythroid cells in 3D aggregates were significantly more differentiated toward RBCs with significantly reduced nuclear dysplasia. When 3D culture was performed inside macroporous microcarriers, the cell culture scale was increased and cells exhibited enhanced differentiation and enucleation. Microcarriers with a pore diameter of approximately 400 μm produced more mature cells than those with a smaller pore diameter. In addition, this aggregate culture method minimized the culture space and media volume required. In conclusion, a 3D aggregate culture system can be used to generate transfusable human erythrocytes at the terminal maturation stage, mimicking the in vivo BM microenvironment. Porous structures can efficiently maximize the culture scale, enabling large-scale production of RBCs. These results enhance our understanding of the importance of physical contact among late erythroblasts for their final maturation into RBCs.

Role of the clathrin adaptor PICALM in normal hematopoiesis and polycythemia vera pathophysiology

Clathrin-dependent endocytosis is an essential cellular process shared by all cell types. Despite this, precisely how endocytosis is regulated in a cell-type-specific manner and how this key pathway functions physiologically or pathophysiologically remain largely unknown. PICALM, which encodes the clathrin adaptor protein PICALM, was originally identified as a component of the CALM/AF10 leukemia oncogene. Here we show, by employing a series of conditional Picalm knockout mice, that PICALM critically regulates transferrin uptake in erythroid cells by functioning as a cell-type-specific regulator of transferrin receptor endocytosis. While transferrin receptor is essential for the development of all hematopoietic lineages, Picalm was dispensable for myeloid and B-lymphoid development. Furthermore, global Picalm inactivation in adult mice did not cause gross defects in mouse fitness, except for anemia and a coat color change. Freeze-etch electron microscopy of primary erythroblasts and live-cell imaging of murine embryonic fibroblasts revealed that Picalm function is required for efficient clathrin coat maturation. We showed that the PICALM PIP2 binding domain is necessary for transferrin receptor endocytosis in erythroblasts and absolutely essential for erythroid development from mouse hematopoietic stem/progenitor cells in an erythroid culture system. We further showed that Picalm deletion entirely abrogated the disease phenotype in a Jak2(V617F) knock-in murine model of polycythemia vera. Our findings provide new insights into the regulation of cell-type-specific transferrin receptor endocytosis in vivo. They also suggest a new strategy to block cellular uptake of transferrin-bound iron, with therapeutic potential for disorders characterized by inappropriate red blood cell production, such as polycythemia vera.

Dexamethasone targeted directly to macrophages induces macrophage niches that promote erythroid expansion

Cultures of human CD34(pos) cells stimulated with erythroid growth factors plus dexamethasone, a model for stress erythropoiesis, generate numerous erythroid cells plus a few macrophages (approx. 3%; 3:1 positive and negative for CD169). Interactions occurring between erythroblasts and macrophages in these cultures and the biological effects associated with these interactions were documented by live phase-contrast videomicroscopy. Macrophages expressed high motility interacting with hundreds/thousands of erythroblasts per hour. CD169(pos) macrophages established multiple rapid 'loose' interactions with proerythroblasts leading to formation of transient erythroblastic island-like structures. By contrast, CD169(neg) macrophages established 'tight' interactions with mature erythroblasts and phagocytosed these cells. 'Loose' interactions of CD169(pos) macrophages were associated with proerythroblast cytokinesis (the M phase of the cell cycle) suggesting that these interactions may promote proerythroblast duplication. This hypothesis was tested by experiments that showed that as few as 103 macrophages significantly increased levels of 3-(4,5-dimethylthiazolyl-2)-2,5-diphenyltetrazolium bromide incorporation frequency in S/G2/M and cytokinesis expressed by proerythroblasts over 24 h of culture. These effects were observed also when macrophages were co-cultured with dexamethasone directly conjugated to a macrophage-specific CD163 antibody. In conclusion, in addition to promoting proerythroblast proliferation directly, dexamethasone stimulates expansion of these cells indirectly by stimulating maturation and cytokinesis supporting activity of macrophages.

Crosstalk between muscularis macrophages and enteric neurons regulates gastrointestinal motility

Intestinal peristalsis is a dynamic physiologic process influenced by dietary and microbial changes. It is tightly regulated by complex cellular interactions; however, our understanding of these controls is incomplete. A distinct population of macrophages is distributed in the intestinal muscularis externa. We demonstrate that, in the steady state, muscularis macrophages regulate peristaltic activity of the colon. They change the pattern of smooth muscle contractions by secreting bone morphogenetic protein 2 (BMP2), which activates BMP receptor (BMPR) expressed by enteric neurons. Enteric neurons, in turn, secrete colony stimulatory factor 1 (CSF1), a growth factor required for macrophage development. Finally, stimuli from microbial commensals regulate BMP2 expression by macrophages and CSF1 expression by enteric neurons. Our findings identify a plastic, microbiota-driven crosstalk between muscularis macrophages and enteric neurons that controls gastrointestinal motility. PAPERFLICK:

Megakaryocytes maintain homeostatic quiescence and promote post-injury regeneration of hematopoietic stem cells

Multiple bone marrow stromal cell types have been identified as hematopoietic stem cell (HSC)-regulating niche cells. However, whether HSC progeny can serve directly as HSC niche cells has not previously been shown. Here we report a dichotomous role of megakaryocytes (MKs) in both maintaining HSC quiescence during homeostasis and promoting HSC regeneration after chemotherapeutic stress. We show that MKs are physically associated with HSCs in the bone marrow of mice and that MK ablation led to activation of quiescent HSCs and increased HSC proliferation. RNA sequencing (RNA-seq) analysis revealed that transforming growth factor β1 (encoded by Tgfb1) is expressed at higher levels in MKs as compared to other stromal niche cells. MK ablation led to reduced levels of biologically active TGF-β1 protein in the bone marrow and nuclear-localized phosphorylated SMAD2/3 (pSMAD2/3) in HSCs, suggesting that MKs maintain HSC quiescence through TGF-β-SMAD signaling. Indeed, TGF-β1 injection into mice in which MKs had been ablated restored HSC quiescence, and conditional deletion of Tgfb1 in MKs increased HSC activation and proliferation. These data demonstrate that TGF-β1 is a dominant signal emanating from MKs that maintains HSC quiescence. However, under conditions of chemotherapeutic challenge, MK ablation resulted in a severe defect in HSC expansion. In response to stress, fibroblast growth factor 1 (FGF1) signaling from MKs transiently dominates over TGF-β inhibitory signaling to stimulate HSC expansion. Overall, these observations demonstrate that MKs serve as HSC-derived niche cells to dynamically regulate HSC function.

Hematopoietic stem cell niche maintenance during homeostasis and regeneration

The bone marrow niche has mystified scientists for many years, leading to widespread investigation to shed light into its molecular and cellular composition. Considerable efforts have been devoted toward uncovering the regulatory mechanisms of hematopoietic stem cell (HSC) niche maintenance. Recent advances in imaging and genetic manipulation of mouse models have allowed the identification of distinct vascular niches that have been shown to orchestrate the balance between quiescence, proliferation and regeneration of the bone marrow after injury. Here we highlight the recently discovered intrinsic mechanisms, microenvironmental interactions and communication with surrounding cells involved in HSC regulation, during homeostasis and in regeneration after injury and discuss their implications for regenerative therapy.

Vascular-resident CD169-positive monocytes and macrophages control neutrophil accumulation in the kidney with ischemia-reperfusion injury

Monocytes and kidney-resident macrophages are considered to be involved in the pathogenesis of renal ischemia-reperfusion injury (IRI). Several subsets of monocytes and macrophages are localized in the injured tissue, but the pathologic roles of these cells are not fully understood. Here, we show that CD169(+) monocytes and macrophages have a critical role in preventing excessive inflammation in IRI by downregulating intercellular adhesion molecule-1 (ICAM-1) expression on vascular endothelial cells. Mice depleted of CD169(+) cells showed enhanced endothelial ICAM-1 expression and developed irreversible renal damage associated with infiltration of a large number of neutrophils. The perivascular localization of CD169(+) monocytes and macrophages indicated direct interaction with blood vessels, and coculture experiments showed that the direct interaction of CD169(+) cell-depleted peripheral blood leukocytes augments the expression levels of ICAM-1 on endothelial cells. Notably, the transfer of Ly6C(lo) monocytes into CD169(+) cell-depleted mice rescued the mice from lethal renal injury and normalized renal ICAM-1 expression levels, indicating that the Ly6C(lo) subset of CD169(+) monocytes has a major role in the regulation of inflammation. Our findings highlight the previously unknown role of CD169(+) monocytes and macrophages in the maintenance of vascular homeostasis and provide new approaches to the treatment of renal IRI.

'Ride on the ferrous wheel'--the cycle of iron in macrophages in health and disease

Iron homeostasis and macrophage biology are closely interconnected. On the one hand, iron exerts multiple effects on macrophage polarization and functionality. On the other hand, macrophages are central for mammalian iron homeostasis. The phagocytosis of senescent erythrocytes and their degradation by macrophages enable efficient recycling of iron and the maintenance of systemic iron balance. Macrophages express multiple molecules and proteins for the acquisition and utilization of iron and many of these pathways are affected by inflammatory signals. Of note, iron availability within macrophages has significant effects on immune effector functions and metabolic pathways within these cells. This review summarizes the physiological and pathophysiological aspects of macrophage iron metabolism and highlights its relevant consequences on immune function and in common diseases such as infection and atherosclerosis.

The Instructive Role of the Vasculature in Stem Cell Niches

An important hallmark of many adult stem cell niches is their proximity to the vasculature in vivo, a feature common to neural stem cells (NSCs), mesenchymal stem cells (MSCs) from bone marrow, adipose, and other tissues, hematopoietic stem cells (HSCs), and many tumor stem cells. This review summarizes key studies supporting the vasculature's instructive role in adult stem cell niches, and the putative underlying molecular mechanisms by which blood vessels in these niches exert control over progenitor cell fates. The importance of the perivascular niche for pathology, notably tumor metastasis and dormancy, is also highlighted. Finally, the implications of the perivascular regulation of stem and progenitor cells on biomaterial design and the impact on future research directions are discussed.

Extrinsic and intrinsic control by EKLF (KLF1) within a specialized erythroid niche

The erythroblastic island provides an important nutritional and survival support niche for efficient erythropoietic differentiation. Island integrity is reliant on adhesive interactions between erythroid and macrophage cells. We show that erythroblastic islands can be formed from single progenitor cells present in differentiating embryoid bodies, and that these correspond to erythro-myeloid progenitors (EMPs) that first appear in the yolk sac of the early developing embryo. Erythroid Krüppel-like factor (EKLF; KLF1), a crucial zinc finger transcription factor, is expressed in the EMPs, and plays an extrinsic role in erythroid maturation by being expressed in the supportive macrophage of the erythroblastic island and regulating relevant genes important for island integrity within these cells. Together with its well-established intrinsic contributions to erythropoiesis, EKLF thus plays a coordinating role between two different cell types whose interaction provides the optimal environment to generate a mature red blood cell.

Regulation of hematopoietic and leukemic stem cells by the immune system

Hematopoietic stem cells (HSCs) are rare, multipotent cells that generate via progenitor and precursor cells of all blood lineages. Similar to normal hematopoiesis, leukemia is also hierarchically organized and a subpopulation of leukemic cells, the leukemic stem cells (LSCs), is responsible for disease initiation and maintenance and gives rise to more differentiated malignant cells. Although genetically abnormal, LSCs share many characteristics with normal HSCs, including quiescence, multipotency and self-renewal. Normal HSCs reside in a specialized microenvironment in the bone marrow (BM), the so-called HSC niche that crucially regulates HSC survival and function. Many cell types including osteoblastic, perivascular, endothelial and mesenchymal cells contribute to the HSC niche. In addition, the BM functions as primary and secondary lymphoid organ and hosts various mature immune cell types, including T and B cells, dendritic cells and macrophages that contribute to the HSC niche. Signals derived from the HSC niche are necessary to regulate demand-adapted responses of HSCs and progenitor cells after BM stress or during infection. LSCs occupy similar niches and depend on signals from the BM microenvironment. However, in addition to the cell types that constitute the HSC niche during homeostasis, in leukemia the BM is infiltrated by activated leukemia-specific immune cells. Leukemic cells express different antigens that are able to activate CD4⁺ and CD8⁺ T cells. It is well documented that activated T cells can contribute to the control of leukemic cells and it was hoped that these cells may be able to target and eliminate the therapy-resistant LSCs. However, the actual interaction of leukemia-specific T cells with LSCs remains ill-defined. Paradoxically, many immune mechanisms that evolved to activate emergency hematopoiesis during infection may actually contribute to the expansion and differentiation of LSCs, promoting leukemia progression. In this review, we summarize mechanisms by which the immune system regulates HSCs and LSCs.

CSF-1R signaling in health and disease: a focus on the mammary gland

Colony-stimulating factor-1 (CSF-1), also known as macrophage-colony stimulating factor (M-CSF), is the primary growth factor regulating survival, proliferation and differentiation of macrophages. It is also a potent chemokine for macrophages and monocytes. Signaling via the CSF-1 receptor (CSF-1R) is necessary for the production of almost all tissue resident macrophage populations and these macrophages participate, via trophic mechanisms, in the normal development and homeostasis of tissues and organs in which they reside, including the mammary gland. The drawback of this close interaction between macrophages and parenchymal cells is that dysregulation of macrophage trophic functions assists in the development and progression of many cancers, including breast cancer. Furthermore, tumour cells secrete CSF-1 to attract more macrophages to the tumour microenvironment where CSF-1R signaling frequently drives the behaviour of these tumour-associated macrophages (TAMs) to promote tumour progression and metastasis. Evidence is mounting that treated tumours secrete more CSF-1 and the increased recruitment of TAMs limits treatment efficacy. Thus, therapeutic targeting of the CSF-1R to inhibit TAM function is likely to enhance tumour response and improve patient outcomes in the treatment of cancer, including breast cancer.