On-demand erythrocyte disposal and iron recycling requires transient macrophages in the liver

Macrophage colony-stimulating factor increases hepatic macrophage content, liver growth, and lipid accumulation in neonatal rats

Signaling via the colony-stimulating factor 1 receptor (CSF1R) controls the survival, differentiation, and proliferation of macrophages. Mutations in CSF1 or CSF1R in mice and rats have pleiotropic effects on postnatal somatic growth. We tested the possible application of pig CSF1-Fc fusion protein as a therapy for low birth weight (LBW) at term, using a model based on maternal dexamethasone treatment in rats. Neonatal CSF1-Fc treatment did not alter somatic growth and did not increase the blood monocyte count. Instead, there was a substantial increase in the size of liver in both control and LBW rats, and the treatment greatly exacerbated lipid droplet accumulation seen in the dexamethasone LBW model. These effects were reversed upon cessation of treatment. Transcriptional profiling of the livers supported histochemical evidence of a large increase in macrophages with a resident Kupffer cell phenotype and revealed increased expression of many genes implicated in lipid droplet formation. There was no further increase in hepatocyte proliferation over the already high rates in neonatal liver. In conclusion, treatment of neonatal rats with CSF1-Fc caused an increase in liver size and hepatic lipid accumulation, due to Kupffer cell expansion and/or activation rather than hepatocyte proliferation. Increased liver macrophage numbers and expression of endocytic receptors could mitigate defective clearance functions in neonates. NEW & NOTEWORTHY This study is based on extensive studies in mice and pigs of the role of CSF1/CSF1R in macrophage development and postnatal growth. We extended the study to neonatal rats as a possible therapy for low birth weight. Unlike our previous studies in mice and pigs, there was no increase in hepatocyte proliferation and no increase in monocyte numbers. Instead, neonatal rats treated with CSF1 displayed reversible hepatic steatosis and Kupffer cell expansion.

Investigating zoonotic infection barriers to ape Plasmodium parasites using faecal DNA analysis

African apes are endemically infected with numerous Plasmodium spp. including close relatives of human Plasmodium falciparum, Plasmodium vivax, Plasmodium ovale, and Plasmodium malariae. Although these ape parasites are not believed to pose a zoonotic threat, their ability to colonise humans has not been fully explored. In particular, it remains unknown whether ape parasites are able to initiate exo-erythrocytic replication in human hepatocytes following the bite of an infective mosquito. Since animal studies have shown that liver stage infection can result in the excretion of parasite nucleic acids into the bile, we screened faecal samples from 504 rural Cameroonians for Plasmodium DNA. Using pan-Laverania as well as P. malariae- and P. vivax-specific primer sets, we amplified human P. falciparum (n = 14), P. malariae (n = 1), and P. ovale wallikeri (n = 1) mitochondrial sequences from faecal DNA of 15 individuals. However, despite using an intensified PCR screening approach we failed to detect ape Laverania, ape P. vivax or ape P. malariae parasites in these same subjects. One faecal sample from a hunter-gatherer contained a sequence closely related to the porcupine parasite Plasmodium atheruri. Since this same faecal sample also contained porcupine mitochondrial DNA, but a matching blood sample was Plasmodium-negative, it is likely that this hunter-gatherer consumed Plasmodium-infected bushmeat. Faecal Plasmodium detection was not secondary to intestinal bleeding and/or infection with gastrointestinal parasites, but indicative of blood parasitaemia. Quantitative PCR identified 26-fold more parasite DNA in the blood of faecal Plasmodium-positive than faecal Plasmodium-negative individuals (P = 0.01). However, among blood-positive individuals only 10% - 20% had detectable Plasmodium sequences in their stool. Thus, faecal screening of rural Cameroonians failed to uncover abortive ape Plasmodium infections, but detected infection with human parasites, albeit with reduced sensitivity compared with blood analysis.

African Trypanosomiasis-Associated Anemia: The Contribution of the Interplay between Parasites and the Mononuclear Phagocyte System

African trypanosomosis (AT) is a chronically debilitating parasitic disease of medical and economic importance for the development of sub-Saharan Africa. The trypanosomes that cause this disease are extracellular protozoan parasites that have developed efficient immune escape mechanisms to manipulate the entire host immune response to allow parasite survival and transmission. During the early stage of infection, a profound pro-inflammatory type 1 activation of the mononuclear phagocyte system (MPS), involving classically activated macrophages (i.e., M1), is required for initial parasite control. Yet, the persistence of this M1-type MPS activation in trypanosusceptible animals causes immunopathology with anemia as the most prominent pathological feature. By contrast, in trypanotolerant animals, there is an induction of IL-10 that promotes the induction of alternatively activated macrophages (M2) and collectively dampens tissue damage. A comparative gene expression analysis between M1 and M2 cells identified galectin-3 (Gal-3) and macrophage migration inhibitory factor (MIF) as novel M1-promoting factors, possibly acting synergistically and in concert with TNF-α during anemia development. While Gal-3 enhances erythrophagocytosis, MIF promotes both myeloid cell recruitment and iron retention within the MPS, thereby depriving iron for erythropoiesis. Hence, the enhanced erythrophagocytosis and suppressed erythropoiesis lead to anemia. Moreover, a thorough investigation using MIF-deficient mice revealed that the underlying mechanisms in AT-associated anemia development in trypanosusceptible and tolerant animals are quite distinct. In trypanosusceptible animals, anemia resembles anemia of inflammation, while in trypanotolerant animals’ hemodilution, mainly caused by hepatosplenomegaly, is an additional factor contributing to anemia. In this review, we give an overview of how trypanosome- and host-derived factors can contribute to trypanosomosis-associated anemia development with a focus on the MPS system. Finally, we will discuss potential intervention strategies to alleviate AT-associated anemia that might also have therapeutic potential.

Anemia at older age: etiologies, clinical implications, and management

Anemia is quite frequently diagnosed in older individuals and is a key indicator of various reactive and clonal conditions. Many underlying diseases, like myelodysplastic syndrome (MDS), develop preferentially in elderly individuals. The prevalence of anemia at older age is increasing, and this is mainly attributable to more frequently applied diagnostics and demographic changes in our societies. The etiology of anemia at older age is complex and ranges from bone marrow failure syndromes to chronic kidney disease, and from nutritional deficiencies to inflammatory processes including inflammaging in immunosenescence. In a smaller number of cases, no clear-cut etiology is identified. These patients are referred to as unexplained anemia or idiopathic cytopenia of unknown significance. In others, somatic mutations in leukocytes are found, but diagnostic criteria for MDS or other hematologic diseases are not fulfilled, a condition termed clonal cytopenia of undetermined significance. Management of anemias at older age depends on (1) the severity of the anemia, (2) underlying condition(s), and (3) patient-related factors, including comorbidities. Even a mild anemia may substantially affect physical and cognitive capacities and quality of life. An underestimated aspect is that because of age-related changes, organ function such as erythropoietin production in the kidney may become suboptimal. Management and treatment of anemia in older patients often require a multidisciplinary approach and detailed investigations of organ function. In this article, we review current concepts around anemias at older age, with special emphasis on etiologies, clinical implications, and innovative concepts in the management of these patients.

Cardiac macrophage biology in the steady-state heart, the aging heart, and following myocardial infarction

Macrophages play critical roles in homeostatic maintenance of the myocardium under normal conditions and in tissue repair after injury. In the steady-state heart, resident cardiac macrophages remove senescent and dying cells and facilitate electrical conduction. In the aging heart, the shift in macrophage phenotype to a proinflammatory subtype leads to inflammaging. Following myocardial infarction (MI), macrophages recruited to the infarct produce both proinflammatory and anti-inflammatory mediators (cytokines, chemokines, matrix metalloproteinases, and growth factors), phagocytize dead cells, and promote angiogenesis and scar formation. These diverse properties are attributed to distinct macrophage subtypes and polarization status. Infarct macrophages exhibit a proinflammatory M1 phenotype early and become polarized toward an anti-inflammatory M2 phenotype later post-MI. Although this classification system is oversimplified and needs to be refined to accommodate the multiple different macrophage subtypes that have been recently identified, general concepts on macrophage roles are independent of subtype classification. This review summarizes current knowledge about cardiac macrophage origins, roles, and phenotypes in the steady state, with aging, and after MI, as well as highlights outstanding areas of investigation.

Cre Driver Mice Targeting Macrophages

The Cre/loxP system is a widely applied technology for site-specific genetic manipulation in mice. This system allows for deletion of the genes of interest in specific cells, tissues, and whole organism to generate a diversity of conditional knockout mouse strains. Additionally, the Cre/loxP system is useful for development of cell- and tissue-specific reporter mice for lineage tracing, and cell-specific conditional depletion models in mice. Recently, the Cre/loxP technique was extensively adopted to characterize the monocyte/macrophage biology in mouse models. Compared to other relatively homogenous immune cell types such as neutrophils, mast cells, and basophils, monocytes/macrophages represent a highly heterogeneous population which lack specific markers or transcriptional factors. Though great efforts have been made toward establishing macrophage-specific Cre driver mice in the past decade, all of the current available strains are not perfect with regard to their depletion efficiency and targeting specificity for endogenous macrophages. Here we overview the commonly used Cre driver mouse strains targeting macrophages and discuss their major applications and limitations.

Glucocorticoids induce differentiation of monocytes towards macrophages that share functional and phenotypical aspects with erythroblastic island macrophages

The classical central macrophage found in erythroblastic islands plays an important role in erythroblast differentiation, proliferation and enucleation in the bone marrow. Convenient human in vitro models to facilitate the study of erythroid-macrophage interactions are desired. Recently, we demonstrated that cultured monocytes/macrophages enhance in vitro erythropoiesis by supporting hematopoietic stem and progenitor cell survival. Herein, we describe that these specific macrophages also support erythropoiesis. Human monocytes cultured in serum-free media supplemented with stem cell factor, erythropoietin, lipids and dexamethasone differentiate towards macrophages expressing CD16, CD163, CD169, CD206, CXCR4 and the phagocytic TAM-receptor family. Phenotypically, they resemble both human bone marrow and fetal liver resident macrophages. This differentiation is dependent on glucocorticoid receptor activation. Proteomic studies confirm that glucocorticoid receptor activation differentiates monocytes to anti-inflammatory tissue macrophages with a M2 phenotype, termed GC-macrophages. Proteins involved in migration, tissue residence and signal transduction/receptor activity are upregulated whilst lysosome and hydrolase activity GO-categories are downregulated. Functionally, we demonstrate that GC-macrophages are highly mobile and can interact to form clusters with erythroid cells of all differentiation stages and phagocytose the expelled nuclei, recapitulating aspects of erythroblastic islands. In conclusion, glucocorticoid-directed monocyte differentiation to macrophages represents a convenient model system to study erythroid-macrophage interactions.

Developmental control of macrophage function

The combination between novel fate-mapping tools and single-cell RNA-sequencing technology has revealed the presence of multiple macrophage progenitors. This raises the fascinating possibility that what was once perceived as immense functional plasticity of macrophages could in fact come down to separate macrophage subsets performing distinct functions because of their differential cellular origin. The question of macrophage plasticity versus macrophage heterogeneity is broader than the difference between macrophages of embryonic or adult hematopoietic origin and is particularly relevant in the context of inflammation. In this manuscript, we review the potential impact of cellular origin on the function of macrophages. We also highlight the need for novel 'functional fate-mapping' tools that would reveal the history of the functional state of macrophages, rather than their cellular origin, in order to finally study their true plasticity in vivo.

Dopamine promotes cellular iron accumulation and oxidative stress responses in macrophages

Iron is essential for many biological functions including neurotransmitter synthesis, where the metal is a co-factor of tyrosine hydroxylase, which converts tyrosine to dopamine and further to norepinephrine. As the shared chemical structure, called catechol, may potentially bind iron we questioned whether tyrosine derived hormones would impact on cellular iron homeostasis in macrophages, which are central for the maintenance of body iron homeostasis. Using murine bone marrow-derived macrophages (BMDMs), we investigated the effect of catecholamines and found that only dopamine but neither tyrosine, nor norepinephrine, affected cellular iron homeostasis. Exposure of macrophages to dopamine increased the uptake of non-transferrin bound iron into cells. The expansion of intracellular iron upon dopamine treatment resulted in oxidative stress responses as evidenced by increased expression of nuclear factor erythroid 2-related factor (Nrf2) and hypoxia inducible factor-1α. As a consequence, the transcriptional expression of stress response genes such as heme oxygenase-1 and the iron export protein ferroportin1 were significantly increased. Genetic deletion of Nrf2 abolished these effects of dopamine. Dopamine directly affects cellular iron homeostasis by increasing iron incorporation into macrophages and subsequently promoting intracellular oxidative stress responses. Our observations are of interest for disorders involving dopamine and iron dyshomeostasis such as Parkinson's disease and restless legs syndrome, partly enlightening the underlying pathology or the therapeutic efficacy of dopamine agonists to overcome neuronal iron deficiency.

Inhibition of heme oxygenase ameliorates anemia and reduces iron overload in a β-thalassemia mouse model

Thalassemias are a heterogeneous group of red blood cell disorders, considered a major cause of morbidity and mortality among genetic diseases. However, there is still no universally available cure for thalassemias. The underlying basis of thalassemia pathology is the premature apoptotic destruction of erythroblasts causing ineffective erythropoiesis. In β-thalassemia, β-globin synthesis is reduced causing α-globin accumulation. Unpaired globin chains, with heme attached to them, accumulate in thalassemic erythroblasts causing oxidative stress and the premature cell death. We hypothesize that in β-thalassemia heme oxygenase (HO) 1 could play a pathogenic role in the development of anemia and ineffective erythropoiesis. To test this hypothesis, we exploited a mouse model of β-thalassemia intermedia, Th3/⁺ We observed that HO inhibition using tin protoporphyrin IX (SnPP) decreased heme-iron recycling in the liver and ameliorated anemia in the Th3/⁺ mice. SnPP administration led to a decrease in erythropoietin and increase in hepcidin serum levels, changes that were accompanied by an alleviation of ineffective erythropoiesis in Th3/⁺ mice. Additionally, the bone marrow from Th3/⁺ mice treated with SnPP exhibited decreased heme catabolism and diminished iron release as well as reduced apoptosis. Our results indicate that the iron released from heme because of HO activity contributes to the pathophysiology of thalassemia. Therefore, new therapies that suppress heme catabolism may be beneficial in ameliorating the anemia and ineffective erythropoiesis in thalassemias.

Iron Induces Anti-tumor Activity in Tumor-Associated Macrophages

Tumor-associated macrophages (TAMs) frequently help to sustain tumor growth and mediate immune suppression in the tumor microenvironment (TME). Here, we identified a subset of iron-loaded, pro-inflammatory TAMs localized in hemorrhagic areas of the TME. The occurrence of iron-loaded TAMs (iTAMs) correlated with reduced tumor size in patients with non-small cell lung cancer. Ex vivo experiments established that TAMs exposed to hemolytic red blood cells (RBCs) were converted into pro-inflammatory macrophages capable of directly killing tumor cells. This anti-tumor effect could also be elicited via iron oxide nanoparticles. When tested in vivo, tumors injected with such iron oxide nanoparticles led to significantly smaller tumor sizes compared to controls. These results identify hemolytic RBCs and iron as novel players in the TME that repolarize TAMs to exert direct anti-tumor effector function. Thus, the delivery of iron to TAMs emerges as a simple adjuvant therapeutic strategy to promote anti-cancer immune responses.

Barrier-tissue macrophages: functional adaptation to environmental challenges

Macrophages are found throughout the body, where they have crucial roles in tissue development, homeostasis and remodeling, as well as being sentinels of the innate immune system that can contribute to protective immunity and inflammation. Barrier tissues, such as the intestine, lung, skin and liver, are exposed constantly to the outside world, which places special demands on resident cell populations such as macrophages. Here we review the mounting evidence that although macrophages in different barrier tissues may be derived from distinct progenitors, their highly specific properties are shaped by the local environment, which allows them to adapt precisely to the needs of their anatomical niche. We discuss the properties of macrophages in steady-state barrier tissues, outline the factors that shape their differentiation and behavior and describe how macrophages change during protective immunity and inflammation.

The infarcted myocardium solicits GM-CSF for the detrimental oversupply of inflammatory leukocytes

Myocardial infarction elicits massive recruitment of monocytes and neutrophils to the myocardium, but the mechanisms that control these processes are not fully understood. Here, Anzai et al. show that GM-CSF is a powerful orchestrator contributing to monocyte and neutrophil production, recruitment, and function.

Myocardial infarction (MI) elicits massive inflammatory leukocyte recruitment to the heart. Here, we hypothesized that excessive leukocyte invasion leads to heart failure and death during acute myocardial ischemia. We found that shortly and transiently after onset of ischemia, human and mouse cardiac fibroblasts produce granulocyte/macrophage colony-stimulating factor (GM-CSF) that acts locally and distally to generate and recruit inflammatory and proteolytic cells. In the heart, fibroblast-derived GM-CSF alerts its neighboring myeloid cells to attract neutrophils and monocytes. The growth factor also reaches the bone marrow, where it stimulates a distinct myeloid-biased progenitor subset. Consequently, hearts of mice deficient in either GM-CSF or its receptor recruit fewer leukocytes and function relatively well, whereas mice producing GM-CSF can succumb from left ventricular rupture, a complication mitigated by anti–GM-CSF therapy. These results identify GM-CSF as both a key contributor to the pathogenesis of MI and a potential therapeutic target, bolstering the idea that GM-CSF is a major orchestrator of the leukocyte supply chain during inflammation.

Exploring evidence of positive selection signatures in cattle breeds selected for different traits

Since domestication, the genome landscape of cattle has been changing due to natural and artificial selection forces resulting in several general and specialized cattle breeds of the world. Identifying genomic regions affected due to these forces in livestock gives an insight into the history of selection for economically important traits and genetic adaptation to specific environments of the populations under consideration. This study explores the genes/genomic regions under selection in relation to the phenotypes of Holstein, Hanwoo, and N'Dama cattle breeds using Tajima's D, XP-CLR, and XP-EHH population statistical methods. The whole genomes of 10 Holstein (South Korea), 11 Hanwoo (South Korea), and 10 N'Dama (West Africa-Guinea) cattle breeds re-sequenced to ~11x coverage and retained 37 million SNPs were used for the study. Selection signature analysis revealed 441, 512, and 461 genes under selection from Holstein, Hanwoo, and N'Dama cattle breeds, respectively. Among all these, seven genes including ARFGAP3, SNORA70, and other RNA genes were common between the breeds. From each of the gene lists, significant functional annotation cluster terms including milk protein and thyroid hormone signaling pathway (Holstein), histone acetyltransferase activity (Hanwoo), and renin secretion (N'Dama) were enriched. Genes that are related to the phenotypes of the respective breeds were also identified. Moreover, significant breed-specific missense variants were identified in CSN3, PAPPA2 (Holstein), C1orf116 (Hanwoo), and COMMD1 (N'Dama) genes. The genes identified from this study provide an insight into the biological mechanisms and pathways that are important in cattle breeds selected for different traits of economic significance.

Erythropoietin enhances Kupffer cell number and activity in the challenged liver

Erythropoietin (EPO) is the main hormone driving mammalian erythropoiesis, with activity mediated via the surface receptor, EPO-R, on erythroid progenitor cells. Recombinant human EPO is currently used clinically for the treatment of anemia in patients with end-stage renal disease, and in certain cancer patients suffering from anemia induced either by the tumor itself or by chemotherapy. EPO-R expression is also detected in non-erythroid cells, including macrophages present in the peritoneum, spleen, and bone marrow (BM). Here we demonstrate that Kupffer cells (KCs) - the liver-resident macrophages - are EPO targets. We show that, in vitro, EPO initiated intracellular signalling and enhanced phagocytosis in a rat KC line (RKC-2) and in sorted KCs. Moreover, continuous EPO administration in mice, resulted in an increased number of KCs, up-regulation of liver EPO-R expression and elevated production of the monocyte chemoattractant CCL2, with corresponding egress of Ly6C^hi monocytes from the BM. In a model of acute acetaminophen-induced liver injury, EPO administration increased the recruitment of Ly6C^hi monocytes and neutrophils to the liver. Taken together, our results reveal a new role for EPO in stimulating KC proliferation and phagocytosis, and in recruiting Ly6C^hi monocytes in response to liver injury.

Maturation of phagosomes containing different erythrophagocytic particles in primary macrophages

Erythrophagocytosis is a physiological process that aims to remove damaged red blood cells from the circulation in order to avoid hemolysis and uncontrolled liberation of iron. Many efforts have been made to understand heme trafficking inside macrophages, but little is known about the maturation of phagosomes containing different types of erythrophagocytic particles with different signals at their surfaces. Therefore, we performed a comparative study on the maturation of phagosomes containing three different models of red blood cells (RBC): aged/senescent, complement-opsonized, and IgG-opsonized. We also used two types of professional phagocytes: bone marrow-derived and peritoneal macrophages. By comparing markers from different stages of phagosomal maturation, we found that phagosomes carrying aged RBC reach lysosomes with a delay compared to those containing IgG- or complement-opsonized RBC, in both types of macrophages. These findings contribute to understanding the importance of the different signals at the RBC surface in phagolysosome biogenesis, as well as in the dynamics of RBC removal.

Nrf2 induces cisplatin resistance via suppressing the iron export related gene SLC40A1 in ovarian cancer cells

Induction of Nuclear factor erythroid 2 (NF-E2)-related factor 2 (Nrf2) has been demonstrated to be involved in cisplatin resistance in ovarian cancer. Solute carrier family 40 member 1 (SLC40A1) is an iron exporter, which possesses many putative Nrf2 binding sites. Here we hypothesize that it may be a possible downstream gene of Nrf2. Elevated level of Nrf2 and reduced level of SLC40A1 were found in cisplatin–resistant ovarian cancer cells as compared with cisplatin-sensitive ovarian cancer cells. Exogenous knockdown of Nrf2 leaded to increased expression of SLC40A1. While overexpression of Nrf2 resulted in decreased expression of SLC40A1. Chromatin immunoprecipitation (ChIP) and dual-luciferase reporter assay revealed that Nrf2 inhibited the transcription of SLC40A1. Overexpression of SLC40A1 was able to reverse cisplatin resistance induced by Nrf2, while knockdown of SLC40A1 restored cisplatin resistance and increased iron concentration. Desferal, an iron chelator, was found to overcome cisplatin resistance through iron deprivation. Its function was boosted when combined with brusatol, an Nrf2 inhibitor. Taken together, this study first demonstrated that Nrf2 could transcriptionally suppress the expression of SLC40A1. Iron overload induced by SLC40A1 resulted in cisplatin resistance in ovarian cancer. Targeting iron metabolism may be a new therapeutic strategy to reverse drug resistance in ovarian cancer treatment.

Macrophages Facilitate Electrical Conduction in the Heart

Organ-specific functions of tissue-resident macrophages in the steady-state heart are unknown. Here, we show that cardiac macrophages facilitate electrical conduction through the distal atrioventricular node, where conducting cells densely intersperse with elongated macrophages expressing connexin 43. When coupled to spontaneously beating cardiomyocytes via connexin-43-containing gap junctions, cardiac macrophages have a negative resting membrane potential and depolarize in synchrony with cardiomyocytes. Conversely, macrophages render the resting membrane potential of cardiomyocytes more positive and, according to computational modeling, accelerate their repolarization. Photostimulation of channelrhodopsin-2-expressing macrophages improves atrioventricular conduction, whereas conditional deletion of connexin 43 in macrophages and congenital lack of macrophages delay atrioventricular conduction. In the Cd11b^DTR mouse, macrophage ablation induces progressive atrioventricular block. These observations implicate macrophages in normal and aberrant cardiac conduction.

Eryptosis in health and disease: A paradigm shift towards understanding the (patho)physiological implications of programmed cell death of erythrocytes

During the course of their natural ageing and upon injury, anucleate erythrocytes can undergo an unconventional apoptosis-like cell death, termed eryptosis. Eryptotic erythrocytes display a plethora of morphological alterations including volume reduction, membrane blebbing and breakdown of the membrane phospholipid asymmetry resulting in phosphatidylserine externalization which, in turn, mediates their phagocytic recognition and clearance from the circulation. Overall, the eryptosis machinery is tightly orchestrated by a wide array of endogenous mediators, ion channels, membrane receptors, and a host of intracellular signaling proteins. Enhanced eryptosis shortens the lifespan of circulating erythrocytes and confers a procoagulant phenotype; this phenomenon has been tangibly implicated in the pathogenesis of anemia, deranged microcirculation, and increased prothrombotic risk associated with a multitude of clinical conditions. Herein, we reviewed the molecular mechanisms dictating eryptosis and erythrophagocytosis and critically analyzed the current evidence leading to the pathophysiological ramifications of eryptotic cell death in the context of human disease.

Iron homeostasis: An anthropocentric perspective

The regulation of iron metabolism in biological systems centers on providing adequate iron for cellular function while limiting iron toxicity. Because mammals cannot excrete iron, mechanisms have evolved to control iron acquisition, storage, and distribution at both systemic and cellular levels. Hepcidin, the master regulator of iron homeostasis, controls iron flows into plasma through inhibition of the only known mammalian cellular iron exporter ferroportin. Hepcidin is feedback-regulated by iron status and strongly modulated by inflammation and erythropoietic demand. This review highlights recent advances that have changed our understanding of iron metabolism and its regulation.

Cell Removal: Efferocytosis

In metazoans, removal of cells in situ is involved in larval maturation, metamorphosis, and embryonic development. In adults, such cell removal plays a role in the homeostatic maintenance of cell numbers and tissue integrity as well as in the response to cell injury and damage. This removal involves uptake of the whole or fragmented target cells into phagocytes. Depending on the organism, these latter may be near-neighbor tissue cells and/or professional phagocytes such as, in vertebrates, members of the myeloid family of cells, especially macrophages. The uptake processes appear to involve specialized and highly conserved recognition ligands and receptors, intracellular signaling in the phagocytes, and mechanisms for ingestion. The recognition of cells destined for this form of removal is critical and, significantly, is distinguished for the most part from the recognition of foreign materials and organisms by the innate and adaptive immune systems. In keeping with the key role of cell removal in maintaining tissue homeostasis, constant cell removal is normally silent, i.e., does not initiate a local tissue reaction. This article discusses these complex and wide-ranging processes in general terms as well as the implications when these processes are disrupted in inflammation, immunity, and disease.

Targeting hepatic macrophages to treat liver diseases

Our view on liver macrophages in the context of health and disease has been reformed by the recognition of a remarkable heterogeneity of phagocytes in the liver. Liver macrophages consist of ontogenically distinct populations termed Kupffer cells and monocyte-derived macrophages. Kupffer cells are self-renewing, resident and principally non-migratory phagocytes, serving as sentinels for liver homeostasis. Liver injury triggers Kupffer cell activation, leading to inflammatory cytokine and chemokine release. This fosters the infiltration of monocytes into the liver, which give rise to large numbers of inflammatory monocyte-derived macrophages. Liver macrophages are very plastic and adapt their phenotype according to signals derived from the hepatic microenvironment (e.g. danger signals, fatty acids, phagocytosis of cellular debris), which explains their manifold and even opposing functions during disease. These central functions include the perpetuation of inflammation and hepatocyte injury, activation of hepatic stellate cells with subsequent fibrogenesis, and support of tumor development by angiogenesis and T cell suppression. If liver injury ceases, specific molecular signals trigger hepatic macrophages to switch their phenotype towards reparative phagocytes that promote tissue repair and regression of fibrosis. Novel strategies to treat liver disease aim at targeting macrophages. These interventions modulate Kupffer cell activation (e.g. via gut-liver axis or inflammasome formation), monocyte recruitment (e.g. via inhibiting chemokine pathways like CCR2 or CCL2) or macrophage polarization and differentiation (e.g. by nanoparticles). Evidence from mouse models and early clinical studies in patients with non-alcoholic steatohepatitis and fibrosis support the notion that pathogenic macrophage subsets can be successfully translated into novel treatment options for patients with liver disease.

LAY SUMMARY

Macrophages (Greek for "big eaters") are a frequent non-parenchymal cell type of the liver that ensures homeostasis, antimicrobial defense and proper metabolism. However, liver macrophages consist of different subtypes regarding their ontogeny (developmental origin), differentiation and function. Understanding this heterogeneity and the critical regulation of inflammation, fibrosis and cancer by macrophage subsets opens promising new options for treating liver diseases.

A Red Carpet for Iron Metabolism

200 billion red blood cells (RBCs) are produced every day, requiring more than 2 × 10¹⁵ iron atoms every second to maintain adequate erythropoiesis. These numbers translate into 20 mL of blood being produced each day, containing 6 g of hemoglobin and 20 mg of iron. These impressive numbers illustrate why the making and breaking of RBCs is at the heart of iron physiology, providing an ideal context to discuss recent progress in understanding the systemic and cellular mechanisms that underlie the regulation of iron homeostasis and its disorders.

Diversity and Versatility of Phagocytosis: Roles in Innate Immunity, Tissue Remodeling, and Homeostasis

Phagocytosis, a critical early event in the microbicidal response of neutrophils, is now appreciated to serve multiple functions in a variety of cell types. Professional phagocytes play a central role in innate immunity by eliminating pathogenic bacteria, fungi and malignant cells, and contribute to adaptive immunity by presenting antigens to lymphocytes. In addition, phagocytes play a part in tissue remodeling and maintain overall homeostasis by disposing of apoptotic cells, a task shared by non-professional phagocytes, often of epithelial origin. This functional versatility is supported by a vast array of receptors capable of recognizing a striking variety of foreign and endogenous ligands. Here we present an abbreviated overview of the different types of phagocytes, their varied modes of signaling and particle engulfment, and the multiple physiological roles of phagocytosis.

Does niche competition determine the origin of tissue-resident macrophages?

Most tissue-resident macrophages are derived from embryonic precursors but, under certain circumstances, circulating monocytes can differentiate into self-maintaining tissue-resident macrophages that resemble their embryonic counterparts. In this Opinion article, we propose that distinct macrophage precursors have an almost identical potential to develop into resident macrophages but they compete for a restricted number of niches. The tight regulation of the niche ensures that monocytes do not differentiate into macrophages when the niche is full but that these cells can differentiate efficiently into macrophages when the niche is available. Imprinting by the niche would be the dominant factor conferring macrophage identity and self-maintenance capacity, rather than origin as was previously proposed.

Genetic and Dietary Iron Overload Differentially Affect the Course of Salmonella Typhimurium Infection

Genetic and dietary forms of iron overload have distinctive clinical and pathophysiological features. HFE-associated hereditary hemochromatosis is characterized by overwhelming intestinal iron absorption, parenchymal iron deposition, and macrophage iron depletion. In contrast, excessive dietary iron intake results in iron deposition in macrophages. However, the functional consequences of genetic and dietary iron overload for the control of microbes are incompletely understood. Using Hfe^+/+ and Hfe^-/- mice in combination with oral iron overload in a model of Salmonella enterica serovar Typhimurium infection, we found animals of either genotype to induce hepcidin antimicrobial peptide expression and hypoferremia following systemic infection in an Hfe-independent manner. As predicted, Hfe^-/- mice, a model of hereditary hemochromatosis, displayed reduced spleen iron content, which translated into improved control of Salmonella replication. Salmonella adapted to the iron-poor microenvironment in the spleens of Hfe^-/- mice by inducing the expression of its siderophore iron-uptake machinery. Dietary iron loading resulted in higher bacterial numbers in both WT and Hfe^-/- mice, although Hfe deficiency still resulted in better pathogen control and improved survival. This suggests that Hfe deficiency may exert protective effects in addition to the control of iron availability for intracellular bacteria. Our data show that a dynamic adaptation of iron metabolism in both immune cells and microbes shapes the host-pathogen interaction in the setting of systemic Salmonella infection. Moreover, Hfe-associated iron overload and dietary iron excess result in different outcomes in infection, indicating that tissue and cellular iron distribution determines the susceptibility to infection with specific pathogens.

Liver macrophages in tissue homeostasis and disease

Macrophages represent a key cellular component of the liver, and are essential for maintaining tissue homeostasis and ensuring rapid responses to hepatic injury. Our understanding of liver macrophages has been revolutionized by the delineation of heterogeneous subsets of these cells. Kupffer cells are a self-sustaining, liver-resident population of macrophages and can be distinguished from the monocyte-derived macrophages that rapidly accumulate in the injured liver. Specific environmental signals further determine the polarization and function of hepatic macrophages. These cells promote the restoration of tissue integrity following liver injury or infection, but they can also contribute to the progression of liver diseases, including hepatitis, fibrosis and cancer. In this Review, we highlight novel findings regarding the origin, classification and function of hepatic macrophages, and we discuss their divergent roles in the healthy and diseased liver.

Protective vaccination and blood-stage malaria modify DNA methylation of gene promoters in the liver of Balb/c mice

Epigenetic mechanisms such as DNA methylation are increasingly recognized to be critical for vaccination efficacy and outcome of different infectious diseases, but corresponding information is scarcely available for host defense against malaria. In the experimental blood-stage malaria Plasmodium chabaudi, we investigate the possible effects of a blood-stage vaccine on DNA methylation of gene promoters in the liver, known as effector against blood-stage malaria, using DNA methylation microarrays. Naturally susceptible Balb/c mice acquire, by protective vaccination, the potency to survive P. chabaudi malaria and, concomitantly, modifications of constitutive DNA methylation of promoters of numerous genes in the liver; specifically, promoters of 256 genes are hyper(=up)- and 345 genes are hypo(=down)-methylated (p < 0.05). Protective vaccination also leads to changes in promoter DNA methylation upon challenge with P. chabaudi at peak parasitemia on day 8 post infection (p.i.), when 571 and 1013 gene promoters are up- and down-methylated, respectively, in relation to constitutive DNA methylation (p < 0.05). Gene set enrichment analyses reveal that both vaccination and P. chabaudi infections mainly modify promoters of those genes which are most statistically enriched with functions relating to regulation of transcription. Genes with down-methylated promoters encompass those encoding CX3CL1, GP130, and GATA2, known to be involved in monocyte recruitment, IL-6 trans-signaling, and onset of erythropoiesis, respectively. Our data suggest that vaccination may epigenetically improve parts of several effector functions of the liver against blood-stage malaria, as, e.g., recruitment of monocyte/macrophage to the liver accelerated liver regeneration and extramedullary hepatic erythropoiesis, thus leading to self-healing of otherwise lethal P. chabaudi blood-stage malaria.

"Pumping iron"-how macrophages handle iron at the systemic, microenvironmental, and cellular levels

Macrophages reside in virtually every organ. First arising during embryogenesis, macrophages replenish themselves in the adult through a combination of self-renewal and influx of bone marrow-derived monocytes. As large phagocytic cells, macrophages participate in innate immunity while contributing to tissue-specific homeostatic functions. Among the key metabolic tasks are senescent red blood cell recycling, free heme detoxification, and provision of iron for de novo hemoglobin synthesis. While this systemic mechanism involves the shuttling of iron between spleen, liver, and bone marrow through the concerted function of defined macrophage populations, similar circuits appear to exist within the microenvironment of other organs. The high turnover of iron is the prerequisite for continuous erythropoiesis and tissue integrity but challenges macrophages’ ability to maintain cellular iron homeostasis and immune function.

This review provides a brief overview of systemic, microenvironmental, and cellular aspects of macrophage iron handling with a focus on exciting and unresolved questions in the field.

New insights into iron deficiency and iron deficiency anemia

Recent advances in iron metabolism have stimulated new interest in iron deficiency (ID) and its anemia (IDA), common conditions worldwide. Absolute ID/IDA, i.e. the decrease of total body iron, is easily diagnosed based on decreased levels of serum ferritin and transferrin saturation. Relative lack of iron in specific organs/tissues, and IDA in the context of inflammatory disorders, are diagnosed based on arbitrary cut offs of ferritin and transferrin saturation and/or marker combination (as the soluble transferrin receptor/ferritin index) in an appropriate clinical context. Most ID patients are candidate to traditional treatment with oral iron salts, while high hepcidin levels block their absorption in inflammatory disorders. New iron preparations and new treatment modalities are available: high-dose intravenous iron compounds are becoming popular and indications to their use are increasing, although long-term side effects remain to be evaluated.

From the Cradle to the Grave: The Role of Macrophages in Erythropoiesis and Erythrophagocytosis

Erythropoiesis is a highly regulated process where sequential events ensure the proper differentiation of hematopoietic stem cells into, ultimately, red blood cells (RBCs). Macrophages in the bone marrow play an important role in hematopoiesis by providing signals that induce differentiation and proliferation of the earliest committed erythroid progenitors. Subsequent differentiation toward the erythroblast stage is accompanied by the formation of so-called erythroblastic islands where a central macrophage provides further cues to induce erythroblast differentiation, expansion, and hemoglobinization. Finally, erythroblasts extrude their nuclei that are phagocytosed by macrophages whereas the reticulocytes are released into the circulation. While in circulation, RBCs slowly accumulate damage that is repaired by macrophages of the spleen. Finally, after 120 days of circulation, senescent RBCs are removed from the circulation by splenic and liver macrophages. Macrophages are thus important for RBCs throughout their lifespan. Finally, in a range of diseases, the delicate interplay between macrophages and both developing and mature RBCs is disturbed. Here, we review the current knowledge on the contribution of macrophages to erythropoiesis and erythrophagocytosis in health and disease.

Differential miRNA Expression in the Liver of Balb/c Mice Protected by Vaccination during Crisis of Plasmodium chabaudi Blood-Stage Malaria

MicroRNAs are increasingly recognized as epigenetic regulators for outcome of diverse infectious diseases and vaccination efficacy, but little information referring to this exists for malaria. This study investigates possible effects of both protective vaccination and P. chabaudi malaria on the miRNome of the liver as an effector against blood-stage malaria using miRNA microarrays and quantitative PCR. Plasmodium chabaudi blood-stage malaria takes a lethal outcome in female Balb/c mice, but a self-healing course after immunization with a non-infectious blood-stage vaccine. The liver robustly expresses 71 miRNA species at varying levels, among which 65 miRNA species respond to malaria evidenced as steadily increasing or decreasing expressions reaching highest or lowest levels toward the end of the crisis phase on day 11 p.i. in lethal malaria. Protective vaccination does not affect constitutive miRNA expression, but leads to significant (p < 0.05) changes in the expression of 41 miRNA species, however evidenced only during crisis. In vaccination-induced self-healing infections, 18 miRNA-species are up- and 14 miRNA-species are down-regulated by more than 50% during crisis in relation to non-vaccinated mice. Vaccination-induced self-healing and survival of otherwise lethal infections of P. chabaudi activate epigenetic miRNA-regulated remodeling processes in the liver manifesting themselves during crisis. Especially, liver regeneration is accelerated as suggested by upregulation of let-7a-5p, let-7b-5p, let-7c-5p, let-7d-5p, let-7f-5p, let-7g-5p, let-7i-5p, miR-26a, miR-122-5p, miR30a, miR27a, and mir-29a, whereas the up-regulated expression of miR-142-3p by more than 100% is compatible with the view of enhanced hepatic erythropoiesis, possibly at expense of megakaryopoiesis, during crisis of P. chabaudi blood-stage malaria.

Stored blood: how old is too old?

Transfusion of rbc is a routine, often lifesaving procedure that depends on a stored supply of blood. In the US, 42 days is the maximum duration allowed for rbc storage; however, several lines of evidence indicate that patients that receive blood at the upper end of this storage limit are at a higher risk of morbidity and mortality. In this issue of the JCI, Rapido and colleagues evaluated the effects of transfusing one unit of blood close to the storage limit into healthy adults. Compared to those that received rbc stored for five weeks or less, those that received blood stored for six weeks showed several effects associated with increased harm, including disruption in iron handling, increased extravascular hemolysis, and the formation of circulating non-transferrin-bound iron. Together, the results of this study suggest that current maximum storage durations should be carefully reevaluated.

Monocyte Conversion During Inflammation and Injury

Monocytes are circulating leukocytes important in both innate and adaptive immunity, primarily functioning in immune defense, inflammation, and tissue remodeling. There are 2 subsets of monocytes in mice (3 subsets in humans) that are mobilized from the bone marrow and recruited to sites of inflammation, where they carry out their respective functions in promoting inflammation or facilitating tissue repair. Our understanding of the fate of these monocyte subsets at the site of inflammation is constantly evolving. This brief review highlights the plasticity of monocyte subsets and their conversion during inflammation and injury.

Characterization of ferucarbotran-loaded RBCs as long circulating magnetic contrast agents

AIM

The biomedical application of contrast agents based on superparamagnetic iron oxide nanoparticles is still limited because of their short intravascular half-life. The potential of red blood cells (RBCs) loaded with new ferucarbotran nanoparticles as magnetic contrast agents with longer blood retention time has been investigated.

MATERIALS & METHODS

Ferucarbotran was loaded into RBCs by a procedure of hypotonic dialysis and isotonic resealing. Ferucarbotran amounts encapsulated in RBCs were determined by NMR. The survival of ferucarbotran-loaded RBCs and bulk ferucarbotran was evaluated in the mouse bloodstream.

RESULTS

Blood retention time of these RBC constructs is longer (∼14 days) than the bulk ferucarbotran (∼1 h) with a slower Fe clearance from liver and spleen.

CONCLUSION

Ferucarbotran-loaded RBCs could be used as potential contrasting agents for diagnostic applications in MRI/magnetic particle imaging.