Pathways for the regulation of interferon-gamma-inducible genes by iron in human monocytic cells

Iron metabolism in rheumatic diseases

Iron is a crucial element for living organism in terms of oxygen transport, hematopoiesis, enzymatic activity, mitochondrial respiratory chain function and also immune system function. The human being has evolved a mechanism to regulate body iron. In some rheumatic diseases such as rheumatoid arthritis (RA), systemic lupus erythematous (SLE), systemic sclerosis (SSc), ankylosing spondylitis (AS), and gout, this balanced iron regulation is impaired. Altered iron homeostasis can contribute to disease progression through ROS production, fibrosis, inflammation, abnormal bone homeostasis, NETosis and cell senescence. In this review, we have focused on the iron metabolism in rheumatic disease and its role in disease progression.

Absolute and functional iron deficiency: Biomarkers, impact on immune system, and therapy

Iron is essential for numerous physiological processes and its deficiency often leads to anemia. Iron deficiency (ID) is a global problem, primarily affecting reproductive-age women and children, especially in developing countries. Diagnosis uses classical biomarkers like ferritin or transferrin saturation. Recent advancements include using soluble transferrin receptor (sTfR) or hepcidin for improved detection and classification of absolute and functional iron deficiencies, though mostly used in research. ID without anemia may present symptoms like asthenia and fatigue, even without relevant clinical consequences. ID impacts not only red-blood cells but also immune system cells, highlighting its importance in global health and immune-related comorbidities. Managing ID, requires addressing its cause and selecting appropriate iron supplementation. Various improved oral and intravenous products are available, but further research is needed to refine treatment strategies. This review updates on absolute and functional iron deficiencies, their relationships with the immune system and advancements in diagnosis and therapies.

Understanding How Minerals Contribute to Optimal Immune Function

Sufficient mineral supply is vital not only for the innate immune system but also for the components of the adaptive immune defense, which encompass defense mechanisms against pathogens and the delicate balance of pro- and anti-inflammatory regulation in the long term. Generally, a well-balanced diet is capable of providing the necessary minerals to support the immune system. Nevertheless, specific vulnerable populations should be cautious about obtaining adequate amounts of minerals such as magnesium, zinc, copper, iron, and selenium. Inadequate levels of these minerals can temporarily impair immune competence and disrupt the long-term regulation of systemic inflammation. Therefore, comprehending the mechanisms and sources of these minerals is crucial. In exceptional circumstances, mineral deficiencies may necessitate supplementation; however, excessive intake of supplements can have adverse effects on the immune system and should be avoided. Consequently, any supplementation should be approved by medical professionals and administered in recommended doses. This review emphasizes the crucial significance of minerals in promoting optimal functioning of the immune system. It investigates the indispensable minerals required for immune system function and the regulation of inflammation. Moreover, it delves into the significance of maintaining an optimized intake of minerals from a nutritional standpoint.

Severe anaemia, iron deficiency, and susceptibility to invasive bacterial infections

Severe anaemia and invasive bacterial infections remain important causes of hospitalization and death among young African children. The emergence and spread of antimicrobial resistance demand better understanding of bacteraemia risk factors to inform prevention strategies. Epidemiological studies have reported an association between severe anaemia and bacteraemia. In this review, we explore evidence that severe anaemia is associated with increased risk of invasive bacterial infections in young children. We describe mechanisms of iron dysregulation in severe anaemia that might contribute to increased risk and pathogenesis of invasive bacteria, recent advances in knowledge of how iron deficiency and severe anaemia impair immune responses to bacterial infections and vaccines, and the gaps in our understanding of mechanisms underlying severe anaemia, iron deficiency, and the risk of invasive bacterial infections.

Comparative analysis of oral and intravenous iron therapy in rat models of inflammatory anemia and iron deficiency

Anemia is a major health issue and associated with increased morbidity. Iron deficiency anemia (IDA) is the most prevalent, followed by anemia of chronic disease (ACD). IDA and ACD often co-exist, challenging diagnosis and treatment. While iron supplementation is the first-line therapy for IDA, its optimal route of administration and the efficacy of different repletion strategies in ACD are elusive. Female Lewis rats were injected with group A streptococcal peptidoglycan-polysaccharide (PG-APS) to induce inflammatory arthritis with associated ACD and/or repeatedly phlebotomized and fed with a low iron diet to induce IDA, or a combination thereof (ACD/IDA). Iron was either supplemented by daily oral gavage of ferric maltol or by weekly intravenous (i.v.) injection of ferric carboxymaltose for up to 4 weeks. While both strategies reversed IDA, they remained ineffective to improve hemoglobin (Hb) levels in ACD, although oral iron showed slight amelioration of various erythropoiesis-associated parameters. In contrast, both iron treatments significantly increased Hb in ACD/IDA. In ACD and ACD/IDA animals, i.v. iron administration resulted in iron trapping in liver and splenic macrophages, induction of ferritin expression and increased circulating levels of the iron hormone hepcidin and the inflammatory cytokine interleukin-6, while oral iron supplementation reduced interleukin-6 levels. Thus, oral and i.v. iron resulted in divergent effects on systemic and tissue iron homeostasis and inflammation. Our results indicate that both iron supplements improve Hb in ACD/IDA, but are ineffective in ACD with pronounced inflammation, and that under the latter condition, i.v. iron is trapped in macrophages and may enhance inflammation.

A Bioinformatics-Assisted Review on Iron Metabolism and Immune System to Identify Potential Biomarkers of Exercise Stress-Induced Immunosuppression

The immune function is closely related to iron (Fe) homeostasis and allostasis. The aim of this bioinformatics-assisted review was twofold; (i) to update the current knowledge of Fe metabolism and its relationship to the immune system, and (ii) to perform a prediction analysis of regulatory network hubs that might serve as potential biomarkers during stress-induced immunosuppression. Several literature and bioinformatics databases/repositories were utilized to review Fe metabolism and complement the molecular description of prioritized proteins. The Search Tool for the Retrieval of Interacting Genes (STRING) was used to build a protein-protein interactions network for subsequent network topology analysis. Importantly, Fe is a sensitive double-edged sword where two extremes of its nutritional status may have harmful effects on innate and adaptive immunity. We identified clearly connected important hubs that belong to two clusters: (i) presentation of peptide antigens to the immune system with the involvement of redox reactions of Fe, heme, and Fe trafficking/transport; and (ii) ubiquitination, endocytosis, and degradation processes of proteins related to Fe metabolism in immune cells (e.g., macrophages). The identified potential biomarkers were in agreement with the current experimental evidence, are included in several immunological/biomarkers databases, and/or are emerging genetic markers for different stressful conditions. Although further validation is warranted, this hybrid method (human-machine collaboration) to extract meaningful biological applications using available data in literature and bioinformatics tools should be highlighted.

Fermented goat's milk modulates immune response during iron deficiency anemia recovery

BACKGROUND

Iron deficiency and iron overload can affect the normal functioning of the innate and adaptive immune responses. Fermented milk products may enhance immune functions, but little is known about the effect of fermented milks on modulation of the immune response during iron deficiency anemia and recovery with normal or high dietary iron intake. Eighty male Wistar rats were randomly assigned to a control group fed a standard diet or to an anemic group fed a diet deficit in iron. Control and anemic groups were fed for 30 days with diets based on a fermented goat's or cow's milk product, with normal iron content or iron overload.

RESULTS

In general, during anemia recovery lectin and alternative complement pathway activity and lactoferrin decreased, because it improves iron homeostasis, which is critically important in immune system functions. Fermented goat's milk diet enhanced immune function during iron deficiency recovery, suppressed oxidant-induced eotaxin and fractalkine expression due to the concurrent reduction of free radical production and pro-inflammatory cytokines, and decreased monocyte chemoattractant protein-1 and monocyte migration and adhesion. The increase in interferon-γ can confer immunological colonization of gut microbiota and downregulate inflammation.

CONCLUSION

Fermented goat's milk consumption enhances immune function, modifying complement pathway activity and decreasing pro-inflammatory cytokines as well as lactoferrin concentration, due to the improvement of iron homeostasis, which is critically important in the normal function of the immune system. © 2021 The Authors. Journal of The Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Host glyceraldehyde-3-phosphate dehydrogenase-mediated iron acquisition is hijacked by intraphagosomal Mycobacterium tuberculosis

Availability of iron is a key factor in the survival and multiplication of Mycobacterium tuberculosis (M.tb) within host macrophage phagosomes. Despite host cell iron regulatory machineries attempts to deny supply of this essential micronutrient, intraphagosomal M.tb continues to access extracellular iron. In the current study, we report that intracellular M.tb exploits mammalian secreted Glyceraldehyde 3-phosphate dehydrogenase (sGAPDH) for the delivery of host iron carrier proteins lactoferrin (Lf) and transferrin (Tf). Studying the trafficking of iron carriers in infected cells we observed that sGAPDH along with the iron carrier proteins are preferentially internalized into infected cells and trafficked to M.tb containing phagosomes where they are internalized by resident mycobacteria resulting in iron delivery. Collectively our findings provide a new mechanism of iron acquisition by M.tb involving the hijack of host sGAPDH. This may contribute to its successful pathogenesis and provide an option for targeted therapeutic intervention.

The impact of metal availability on immune function during infection

Nutrient transition metals are required cofactors for many proteins to perform functions necessary for life. As such, the concentration of nutrient metals is carefully maintained to retain critical biological processes while limiting toxicity. During infection, invading bacterial pathogens must acquire essential metals, such as zinc, manganese, iron, and copper, from the host to colonize and cause disease. To combat this, the host exploits the essentiality and toxicity of nutrient metals by producing factors that limit metal availability, thereby starving pathogens or accumulating metals in excess to intoxicate the pathogen in a process termed 'nutritional immunity'. As a result of inflammation, a heterogeneous environment containing both metal-replete and -deplete niches is created, in which nutrient metal availability may have an underappreciated role in regulating immune cell function during infection. How the host manipulates nutrient metal availability during infection, and the downstream effects that nutrient metals and metal-sequestering proteins have on immune cell function, are discussed in this review.

Nifedipine Potentiates Susceptibility of Salmonella Typhimurium to Different Classes of Antibiotics

The calcium channel blocker nifedipine induces cellular iron export, thereby limiting the availability of the essential nutrient iron for intracellular pathogens, resulting in bacteriostatic activity. To study if nifedipine may exert a synergistic anti-microbial activity when combined with antibiotics, we used the mouse macrophage cell line RAW267.4, infected with the intracellular bacterium Salmonella Typhimurium, and exposed the cells to varying concentrations of nifedipine and/or ampicillin, azithromycin and ceftriaxone. We observed a significant additive effect of nifedipine in combination with various antibiotics, which was not observed when using Salmonella, with defects in iron uptake. Of interest, increasing intracellular iron levels increased the bacterial resistance to treatment with antibiotics or nifedipine or their combination. We further showed that nifedipine increases the expression of the siderophore-binding peptide lipocalin-2 and promotes iron storage within ferritin, where the metal is less accessible for bacteria. Our data provide evidence for an additive effect of nifedipine with conventional antibiotics against Salmonella, which is partly linked to reduced bacterial access to iron.

Baseline iron status and presence of anaemia determine the course of systemic Salmonella infection following oral iron supplementation in mice

BACKGROUND

Iron deficiency anaemia (IDA) is a major health concern. However, preventive iron supplementation in regions with high burden of infectious diseases resulted in an increase of infection related morbidity and mortality.

METHODS

We fed male C57BL/6N mice with either an iron deficient or an iron adequate diet. Next, they received oral iron supplementation or placebo followed by intraperitoneal infection with Salmonella Typhimurium (S.Tm).

FINDINGS

We found that mice with IDA had a poorer clinical outcome than mice on an iron adequate diet. Interestingly, iron supplementation of IDA mice resulted in higher bacterial burden in organs and shortened survival. Increased transferrin saturation and non-transferrin bound iron in the circulation together with low expression of ferroportin facilitated the access of the pathogen to iron and promoted bacterial growth. Anaemia, independent of iron supplementation, was correlated with reduced neutrophil counts and cytotoxic T cells. With iron supplementation, anaemia additionally correlated with increased splenic levels of the cytokine IL-10, which is suggestive for a weakened immune control to S.Tm infection.

INTERPRETATION

Supplementing iron to anaemic mice worsens the clinical course of bacterial infection. This can be traced back to increased iron delivery to bacteria along with an impaired anti-microbial immune response. Our findings may have important implications for iron supplementation strategies in areas with high endemic burden of infections, putting those individuals, who potentially profit most from iron supplementation for anaemia, at the highest risk for infections.

FUNDING

Financial support by the Christian Doppler Laboratory for Iron Metabolism and Anemia Research.

Distinct iron homeostasis in C57BL/6 and Balb/c mouse strains

C57BL/6 (BL6) and Balb/c mice exhibit prototypical Th1- and Th2-dominant immune predispositions, respectively. Iron is a proinflammatory metal ion; however, limited information is documented on the differences in iron homeostasis between BL6 and Balb/c strains. The objective of this study was to investigate the extent to which strain-level differences in these mice dictates the regulation of iron homeostasis during physiologic and inflammatory conditions. At basal levels, Balb/c mice displayed significantly higher levels of iron in systemic circulation and tissue compared to BL6 mice. Moreover, Balb/c mice had greater iron absorption as indicated by higher gene expressions of duodenal DcytB, DMT1, Fpn, SFT, and Heph. Similarly, hepatic Tf, TfR1, TfR2, and DMT1 expressions were augmented in Balb/c mice. Interestingly, there was no change in hepatic Hamp expression between the two strains, suggesting that the disparity in their maintenance of iron is independent of hepcidin. Additionally, the basal levels of intracellular labile iron pool in Balb/c intestinal epithelial cells, and bone marrow-derived macrophages and neutrophils, were higher compared to BL6 mice. When mice were challenged with lipopolysaccharide, the acute inflammatory response in BL6 mice was more pronounced than in Balb/c mice, as indicated by the more rapid development of hypoferremia and upregulation of serum IL-6 and TNF-α levels in BL6 mice. In conclusion, this study underscores that iron homeostasis is distinct between BL6 and Balb/c strains under both physiologic and inflammatory conditions.

The Iron Curtain: Macrophages at the Interface of Systemic and Microenvironmental Iron Metabolism and Immune Response in Cancer

Macrophages fulfill central functions in systemic iron metabolism and immune response. Infiltration and polarization of macrophages in the tumor microenvironment is associated with differential cancer prognosis. Distinct metabolic iron and immune phenotypes in tumor associated macrophages have been observed in most cancers. While this prompts the hypothesis that macroenvironmental manifestations of dysfunctional iron metabolism have direct associations with microenvironmental tumor immune response, these functional connections are still emerging. We review our current understanding of the role of macrophages in systemic and microenvironmental immune response and iron metabolism and discuss these functions in the context of cancer and immunometabolic precision therapy approaches. Accumulation of tumor associated macrophages with distinct iron pathologies at the invasive tumor front suggests an "Iron Curtain" presenting as an innate functional interface between systemic and microenvironmental iron metabolism and immune response that can be harnessed therapeutically to further our goal of treating and eliminating cancer.

TAM-ing the CIA-Tumor-Associated Macrophages and Their Potential Role in Unintended Side Effects of Therapeutics for Cancer-Induced Anemia

Cancer-induced anemia (CIA) is a common consequence of neoplasia and has a multifactorial pathophysiology. The immune response and tumor treatment, both intended to primarily target malignant cells, also affect erythropoiesis in the bone marrow. In parallel, immune activation inevitably induces the iron-regulatory hormone hepcidin to direct iron fluxes away from erythroid progenitors and into compartments of the mononuclear phagocyte system. Moreover, many inflammatory mediators inhibit the synthesis of erythropoietin, which is essential for stimulation and differentiation of erythroid progenitor cells to mature cells ready for release into the blood stream. These pathophysiological hallmarks of CIA imply that the bone marrow is not only deprived of iron as nutrient but also of erythropoietin as central growth factor for erythropoiesis. Tumor-associated macrophages (TAM) are present in the tumor microenvironment and display altered immune and iron phenotypes. On the one hand, their functions are altered by adjacent tumor cells so that they promote rather than inhibit the growth of malignant cells. As consequences, TAM may deliver iron to tumor cells and produce reduced amounts of cytotoxic mediators. Furthermore, their ability to stimulate adaptive anti-tumor immune responses is severely compromised. On the other hand, TAM are potential off-targets of therapeutic interventions against CIA. Red blood cell transfusions, intravenous iron preparations, erythropoiesis-stimulating agents and novel treatment options for CIA may interfere with TAM function and thus exhibit secondary effects on the underlying malignancy. In this Hypothesis and Theory, we summarize the pathophysiological hallmarks, clinical implications and treatment strategies for CIA. Focusing on TAM, we speculate on the potential intended and unintended effects that therapeutic options for CIA may have on the innate immune response and, consequently, on the course of the underlying malignancy.

Croix C, Tyurina YY, Kagan VE, Mallampalli RK, Ray A, Ray P, Lee JS. Stressed erythrophagocytosis induces immunosuppression during sepsis through heme-mediated STAT1 dysregulation

Macrophages are main effectors of heme metabolism, increasing transiently in the liver during heightened disposal of damaged or senescent RBCs (sRBCs). Macrophages are also essential in defense against microbial threats, but pathological states of heme excess may be immunosuppressive. Herein, we uncovered a mechanism whereby an acute rise in sRBC disposal by macrophages led to an immunosuppressive phenotype after intrapulmonary Klebsiella pneumoniae infection characterized by increased extrapulmonary bacterial proliferation and reduced survival from sepsis in mice. The impaired immunity to K. pneumoniae during heightened sRBC disposal was independent of iron acquisition by bacterial siderophores, in that K. pneumoniae mutants lacking siderophore function recapitulated the findings observed with the WT strain. Rather, sRBC disposal induced a liver transcriptomic profile notable for suppression of Stat1 and IFN-related responses during K. pneumoniae sepsis. Excess heme handling by macrophages recapitulated STAT1 suppression during infection that required synergistic NRF1 and NRF2 activation but was independent of heme oxygenase-1 induction. Whereas iron was dispensable, the porphyrin moiety of heme was sufficient to mediate suppression of STAT1-dependent responses in human and mouse macrophages and promoted liver dissemination of K. pneumoniae in vivo. Thus, cellular heme metabolism dysfunction negatively regulated the STAT1 pathway, with implications in severe infection.

Iron in immune cell function and host defense

The control over iron availability is crucial under homeostatic conditions and even more in the case of an infection. This results from diverse properties of iron: first, iron is an important trace element for the host as well as for the pathogen for various cellular and metabolic processes, second, free iron catalyzes Fenton reaction and is therefore producing reactive oxygen species as a part of the host defense machinery, third, iron exhibits important effects on immune cell function and differentiation and fourth almost every immune activation in turn impacts on iron metabolism and spatio-temporal iron distribution. The central importance of iron in the host and microbe interplay and thus for the course of infections led to diverse strategies to restrict iron for invading pathogens. In this review, we focus on how iron restriction to the pathogen is a powerful innate immune defense mechanism of the host called "nutritional immunity". Important proteins in the iron-host-pathogen interplay will be discussed as well as the influence of iron on the efficacy of innate and adaptive immunity. Recently described processes like ferritinophagy and ferroptosis are further covered in respect to their impact on inflammation and infection control and how they impact on our understanding of the interaction of host and pathogen.

Iron Supplementation Interferes With Immune Therapy of Murine Mammary Carcinoma by Inhibiting Anti-Tumor T Cell Function

Iron is both, an essential compound for many metabolic processes, and iron deficiency can impact on the proliferation of cells including lymphocytes but also tumor cells. On the other hand, excess iron-catalyzed radical formation can induce cellular toxicity which has been previously demonstrated for T cells in hereditary iron overload. Despite these interconnections, little is known on the effects of clinically approved intravenous iron supplements for curing cancer-related anemia, on T cell differentiation, tumor proliferation, anti-tumor T cell responses and, of clinical importance, on efficacy of cancer immunotherapies. Herein, we analyzed the effects of intravenous iron supplementation on T cell function and on the effectiveness of anti-cancer chemotherapy with IL-2/doxorubicin or immunotherapy with checkpoint-inhibitor anti-PD-L1 in C57Bl/6N female mice with implanted E0771 mammary carcinomas. We found that iron application resulted to an increased availability of iron in the tumor microenvironment and stimulation of tumor growth. In parallel, iron application inhibited the activation, expansion and survival of cytotoxic CD8⁺ T cells and of CD4⁺ T helper cells type 1 and significantly reduced the efficacy of the investigated anti-cancer treatments. Our results indicate that iron administration has a tumor growth promoting effect and impairs anti-cancer responses of tumor infiltrating T lymphocytes along with a reduced efficacy of anti-cancer therapies. Iron supplementation in cancer patients, especially in those treated with immunotherapies in a curative setting, may be thus used cautiously and prospective studies have to clarify the impact of such intervention on the outcome of patients.

Impact of bacterial infections on erythropoiesis

INTRODUCTION

The importance of iron is highlighted by the many complex metabolic pathways in which it is involved. A sufficient supply is essential for the effective production of 200 billion erythrocytes daily, a process called erythropoiesis.

AREAS COVERED

During infection, the human body can withhold iron from pathogens, mechanism termed nutritional immunity. The subsequent disturbances in iron homeostasis not only impact on immune function and infection control, but also negatively affect erythropoiesis. The complex interplay between iron, immunity, erythropoiesis and infection control on the molecular and clinical level are highlighted in this review. Diagnostic algorithms for correct interpretation and diagnosis of the iron status in the setting of infection are presented. Therapeutic concepts are discussed regarding effects on anemia correction, but also toward their role on the course of infection.

EXPERT OPINION

In the setting of infection, anemia is often neglected and its impact on the course of diseases is incompletely understood. Clinical expertise can be improved in correct diagnosing of anemia and disturbances of iron homeostasis. Systemic studies are needed to evaluate the impact of specific therapeutic interventions on anemia correction on the course of infection, but also on patients' cardiovascular performance and quality of life.

Iron in infection and immunity

Iron is an essential micronutrient for virtually all living cells. In infectious diseases, both invading pathogens and mammalian cells including those of the immune system require iron to sustain their function, metabolism and proliferation. On the one hand, microbial iron uptake is linked to the virulence of most human pathogens. On the other hand, the sequestration of iron from bacteria and other microorganisms is an efficient strategy of host defense in line with the principles of 'nutritional immunity'. In an acute infection, host-driven iron withdrawal inhibits the growth of pathogens. Chronic immune activation due to persistent infection, autoimmune disease or malignancy however, sequesters iron not only from infectious agents, autoreactive lymphocytes and neoplastic cells but also from erythroid progenitors. This is one of the key mechanisms which collectively result in the anemia of chronic inflammation. In this review, we highlight the most important interconnections between iron metabolism and immunity, focusing on host defense against relevant infections and on the clinical consequences of anemia of inflammation.

How Severe Anaemia Might Influence the Risk of Invasive Bacterial Infections in African Children

Severe anaemia and invasive bacterial infections are common causes of childhood sickness and death in sub-Saharan Africa. Accumulating evidence suggests that severely anaemic African children may have a higher risk of invasive bacterial infections. However, the mechanisms underlying this association remain poorly described. Severe anaemia is characterized by increased haemolysis, erythropoietic drive, gut permeability, and disruption of immune regulatory systems. These pathways are associated with dysregulation of iron homeostasis, including the downregulation of the hepatic hormone hepcidin. Increased haemolysis and low hepcidin levels potentially increase plasma, tissue and intracellular iron levels. Pathogenic bacteria require iron and/or haem to proliferate and have evolved numerous strategies to acquire labile and protein-bound iron/haem. In this review, we discuss how severe anaemia may mediate the risk of invasive bacterial infections through dysregulation of hepcidin and/or iron homeostasis, and potential studies that could be conducted to test this hypothesis.

Cytokines and metabolic regulation: A framework of bidirectional influences affecting Leishmania infection

Leishmania, a protozoan parasite inflicting the complex of diseases called Leishmaniases, resides and replicates as amastigotes within mammalian macrophages. As macrophages are metabolically highly active and can generate free radicals that can destroy this parasite, Leishmania also devise strategies to modulate the host cell metabolism. However, the metabolic changes can also be influenced by the anti-leishmanial immune response mediated by cytokines. This bidirectional, dynamic and complex metabolic coupling established between Leishmania and its host is the result of a long co-evolutionary process. Due to the continuous alterations imposed by the host microenvironment, such metabolic coupling continues to be dynamically regulated. The constant pursuit and competition for nutrients in the host-Leishmania duet alter the host metabolic pathways with major consequences for its nutritional reserves, eventually affecting the phenotype and functionality of the host cell. Altered phenotype and functions of macrophages are particularly relevant to immune cells, as perturbed metabolic fluxes can crucially affect the activation, differentiation, and functions of host immune cells. All these changes can deterministically direct the outcome of an infection. Cytokines and metabolic fluxes can bidirectionally influence each other through molecular sensors and regulators to dictate the final infection outcome. Our studies along with those from others have now identified the metabolic nodes that can be targeted for therapy.

Neutrophils from hereditary hemochromatosis patients are protected from iron excess and are primed

Iron is required for the oxidative response of neutrophils to allow the production of reactive oxygen species (ROS). However, neutrophil function may be severely altered in conditions of iron overload, as observed in chronically transfused patients. Therefore, a tight regulation of neutrophil iron homeostasis seems to be critical for avoiding iron toxicity. Hepcidin is the key iron regulator in organisms; however, no studies have investigated its role in maintaining neutrophil iron homeostasis or characterized neutrophil function in patients with hereditary hemochromatosis (HH), a common iron overload genetic disorder that results from a defect in hepcidin production. To explore these issues, we studied 2 mouse models of iron overload: an experimentally induced iron overload model (EIO), in which hepcidin is increased, and a genetic HH model of iron overload with a deletion of hepatic hepcidin. We found that iron-dependent increase of hepatic hepcidin results in neutrophil intracellular iron trapping and consecutive defects in oxidative burst activity. In contrast, in both HH mouse models and HH patients, the lack of hepcidin expression protects neutrophils from toxic iron accumulation. Moreover, systemic iron overload correlated with a surprising neutrophil priming and resulted in a more powerful oxidative burst. Indeed, important factors in neutrophil priming and activation, such as tumor necrosis factor α (TNF-α), VCAM-1, and ICAM-1 are increased in the plasma of HH patients and are associated with an increase in HH neutrophil phagocytosis capacity and a decrease in L-selectin surface expression. This is the first study to characterize neutrophil iron homeostasis and associated functions in patients with HH.

A Glance on the Role of Bacterial Siderophore from the Perspectives of Medical and Biotechnological Approaches

Iron, which is described as the most basic component found in nature, is hard to be assimilated by microorganisms. It has become increasingly complicated to obtain iron from nature as iron (II) in the presence of oxygen oxidized to press (III) oxide and hydroxide, becoming unsolvable at neutral pH. Microorganisms appeared to produce organic molecules known as siderophores in order to overcome this condition. Siderophore's essential function is to connect with iron (II) and make it dissolvable and enable cell absorption. These siderophores, apart from iron particles, have the ability to chelate various other metal particles that have collocated away to focus the use of siderophores on wound care items. There is a severe clash between the host and the bacterial pathogens during infection. By producing siderophores, small ferric iron-binding molecules, microorganisms obtain iron. In response, host immune cells produce lipocalin 2 to prevent bacterial reuptake of siderophores loaded with iron. Some bacteria are thought to produce lipocalin 2-resistant siderophores to counter this risk. The aim of this article is to discuss the recently described roles and applications of bacterial siderophore.

The Effect of Abnormal Iron Metabolism on Osteoporosis

Iron is one of the important trace elements in life activities. Abnormal iron metabolism increases the incidence of many skeletal diseases, especially for osteoporosis. Iron metabolism plays a key role in the bone homeostasis. Disturbance of iron metabolism not only promotes osteoclast differentiation and apoptosis of osteoblasts but also inhibits proliferation and differentiation of osteoblasts, which eventually destroys the balance of bone remodeling. The strength and density of bone can be weakened by the disordered iron metabolism, which increases the incidence of osteoporosis. Clinically, compounds or drugs that regulate iron metabolism are used for the treatment of osteoporosis. The goal of this review summarizes the new progress on the effect of iron overload or deficiency on osteoporosis and the mechanism of disordered iron metabolism on osteoporosis. Explaining the relationship of iron metabolism with osteoporosis may provide ideas for clinical treatment and development of new drugs.

The Role of Iron Regulation in Immunometabolism and Immune-Related Disease

Immunometabolism explores how the intracellular metabolic pathways in immune cells can regulate their function under different micro-environmental and (patho-)-physiological conditions (Pearce, 2010; Buck et al., 2015; O'Neill and Pearce, 2016). In the last decade great advances have been made in studying and manipulating metabolic programs in immune cells. Immunometabolism has primarily focused on glycolysis, the TCA cycle and oxidative phosphorylation (OXPHOS) as well as free fatty acid synthesis and oxidation. These pathways are important for providing the energy needs of cell growth, membrane rigidity, cytokine production and proliferation. In this review, we will however, highlight the specific role of iron metabolism at the cellular and organismal level, as well as how the bioavailability of this metal orchestrates complex metabolic programs in immune cell homeostasis and inflammation. We will also discuss how dysregulation of iron metabolism contributes to alterations in the immune system and how these novel insights into iron regulation can be targeted to metabolically manipulate immune cell function under pathophysiological conditions, providing new therapeutic opportunities for autoimmunity and cancer.

Associations between the presence of specific antibodies to the West Nile Virus infection and candidate genes in Romanian horses from the Danube delta

The West Nile virus (WNV) is a mosquito-borne flavivirus causing meningoencephalitis in humans and animals. Due to their particular susceptibility to WNV infection, horses serve as a sentinel species. In a population of Romanian semi-feral horses living in the Danube delta region, we have analyzed the distribution of candidate polymorphic genetic markers between anti WNV-IgG seropositive and seronegative horses. Thirty-six SNPs located in 28 immunity-related genes and 26 microsatellites located in the MHC and LY49 complex genomic regions were genotyped in 57 seropositive and 32 seronegative horses. The most significant association (p_corr < 0.0002) was found for genotypes composed of markers of the SLC11A1 and TLR4 genes. Markers of five other candidate genes (ADAM17, CXCR3, IL12A, MAVS, TNFA), along with 5 MHC class I and LY49-linked microsatellites were also associated with the WNV antibody status in this model horse population. The OAS1 gene, previously associated with WNV-induced clinical disease, was not associated with the presence of anti-WNV antibodies.

Genetic susceptibility to West Nile virus infection in Camargue horses

West Nile virus (WNV) is a mosquito-borne zoonotic neurotropic virus capable to cause lethal meningoencephalitis (WNE) in infected hosts such as birds, horses, and humans. Due to their sensitivity, horses serve as sentinel species in areas at risk. We studied a population of Camargue horses living in Southern France in two zones with endemic WNV circulation where WNV outbreaks were recorded in 2000 and 2003-4. Two sets of microsatellite markers located in MHC and Ly49 genomic regions were genotyped as well as multiple SNPs in ten immunity-related candidate gene regions. Associations between genetic polymorphisms and resistance/susceptibility to WNE were tested. While single marker associations were weak, compound two-gene genotypes of SNPs located within the MAVS, NCR2 and IL-10 genes and microsatellites HMS082 and CZM013 were associated with susceptibility to WNE. Combinations of microsatellite markers CZM009, ABGe17402 and ABGe9019 were associated with simple seroconversion without clinical signs of WNE (resistance). In addition, a distribution of polymorphic markers between WNV-IgG seropositive horses and a control group of WNV-IgG seronegative horses was tested. One SNP in the OAS1 gene (NC_009151.3:g.21961328A>G) was significantly associated with the seropositive phenotype (p_corr = 0.023; OR = 40.5 CI (4.28; 383.26); RR = 8.18 CI (1.27; 52.89) in the Camargue breed. In compound genotypes, SNP markers for SLC11A1, MAVS, OAS1, TLR4, ADAM17 and NCR2 genes and ten microsatellites showed non-random distribution between seropositive and seronegative groups of horses. Further analysis of associated markers could contribute to our understanding of anti-WNV defense mechanisms in horses.

Established and Emerging Concepts to Treat Imbalances of Iron Homeostasis in Inflammatory Diseases

Inflammation, being a hallmark of many chronic diseases, including cancer, inflammatory bowel disease, rheumatoid arthritis, and chronic kidney disease, negatively affects iron homeostasis, leading to iron retention in macrophages of the mononuclear phagocyte system. Functional iron deficiency is the consequence, leading to anemia of inflammation (AI). Iron deficiency, regardless of anemia, has a detrimental impact on quality of life so that treatment is warranted. Therapeutic strategies include (1) resolution of the underlying disease, (2) iron supplementation, and (3) iron redistribution strategies. Deeper insights into the pathophysiology of AI has led to the development of new therapeutics targeting inflammatory cytokines and the introduction of new iron formulations. Moreover, the discovery that the hormone, hepcidin, plays a key regulatory role in AI has stimulated the development of several therapeutic approaches targeting the function of this peptide. Hence, inflammation-driven hepcidin elevation causes iron retention in cells and tissues. Besides pathophysiological concepts and diagnostic approaches for AI, this review discusses current guidelines for iron replacement therapies with special emphasis on benefits, limitations, and unresolved questions concerning oral versus parenteral iron supplementation in chronic inflammatory diseases. Furthermore, the review explores how therapies aiming at curing the disease underlying AI can also affect anemia and discusses emerging hepcidin antagonizing drugs, which are currently under preclinical or clinical investigation.

Hepcidin mediated iron homoeostasis as immune regulator in visceral leishmaniasis patients

AIM

Iron is key ingredient for immunosurveillance and host-pathogen interaction. Intracellular pathogen steals the iron from the host, but how parasite orchestrates iron acquisition and affects immune responses remains controversial. We aimed to study the iron homoeostasis in visceral leishmaniasis (VL) and its influence on immune machinery.

METHODS AND RESULTS

This study was performed on purified monocytes and T cells, peripheral blood mononuclear cells and splenic aspirates for transcriptional analyses of iron homoeostasis (hepcidin, DMT1, transferrin receptor, ferroportin) and immune modulations (IFN-γ, HLA-DR, IL-10, iNOS, IL-6). Serum/plasma was used for determination of iron, total/transferrin iron-binding capacity and anti-leishmania antibody titres in cases. We report that VL-induced perturbation in iron homoeostasis may cause immune dysfunctions. VL cases had decreased iron uptake by transferrin-dependent and transferrin-independent routes while elevated hepcidin, degraded sole iron exporter ferroportin. Therefore, it appears that perturbation in iron homoeostasis has essential role in HLA-DR mediated antigen presentation and innate armoury by downregulating iNOS as well as altering IFN-γ, IL-6 and IL-10 profiles.

CONCLUSION

The iron homoeostasis by hepcidin can serve as one of the crucial determinants for regulating immune cell signalling; therefore, targeting iron metabolism, specifically hepcidin alone or in combination with agonists, can serve to clear infection.

Iron and innate antimicrobial immunity-Depriving the pathogen, defending the host

The acute-phase response is triggered by the presence of infectious agents and danger signals which indicate hazards for the integrity of the mammalian body. One central feature of this response is the sequestration of iron into storage compartments including macrophages. This limits the availability of this essential nutrient for circulating pathogens, a host defence strategy known as 'nutritional immunity'. Iron metabolism and the immune response are intimately linked. In infections, the availability of iron affects both the efficacy of antimicrobial immune pathways and pathogen proliferation. However, host strategies to withhold iron from microbes vary according to the localization of pathogens: Infections with extracellular bacteria such as Staphylococcus aureus, Streptococcus, Klebsiella or Yersinia stimulate the expression of the iron-regulatory hormone hepcidin which targets the cellular iron-exporter ferroportin-1 causing its internalization and blockade of iron egress from absorptive enterocytes in the duodenum and iron-recycling macrophages. This mechanism disrupts both routes of iron delivery to the circulation, contributes to iron sequestration in the mononuclear phagocyte system and mediates the hypoferraemia of the acute phase response subsequently resulting in the development of anaemia of inflammation. When intracellular microbes are present, other strategies of microbial iron withdrawal are needed. For instance, in macrophages harbouring intracellular pathogens such as Chlamydia, Mycobacterium tuberculosis, Listeria monocytogenes or Salmonella Typhimurium, ferroportin-1-mediated iron export is turned on for the removal of iron from infected cells. This also leads to reduced iron availability for intra-macrophage pathogens which inhibits their growth and in parallel strengthens anti-microbial effector pathways of macrophages including the formation of inducible nitric oxide synthase and tumour necrosis factor. Iron plays a key role in infectious diseases both as modulator of the innate immune response and as nutrient for microbes. We need to gain a more comprehensive understanding of how the body can differentially respond to infection by extra- or intracellular pathogens. This knowledge may allow us to modulate mammalian iron homeostasis pharmaceutically and to target iron-acquisition systems of pathogens, thus enabling us to treat infections with novel strategies that act independent of established antimicrobials.

Serum Iron Protects from Renal Postischemic Injury

Renal transplants remain a medical challenge, because the parameters governing allograft outcome are incompletely identified. Here, we investigated the role of serum iron in the sterile inflammation that follows kidney ischemia-reperfusion injury. In a retrospective cohort study of renal allograft recipients (n=169), increased baseline levels of serum ferritin reliably predicted a positive outcome for allografts, particularly in elderly patients. In mice, systemic iron overload protected against renal ischemia-reperfusion injury-associated sterile inflammation. Furthermore, chronic iron injection in mice prevented macrophage recruitment after inflammatory stimuli. Macrophages cultured in high-iron conditions had reduced responses to Toll-like receptor-2, -3, and -4 agonists, which associated with decreased reactive oxygen species production, increased nuclear localization of the NRF2 transcription factor, increased expression of the NRF2-related antioxidant response genes, and limited NF-κB and proinflammatory signaling. In macrophage-depleted animals, the infusion of macrophages cultured in high-iron conditions did not reconstitute AKI after ischemia-reperfusion, whereas macrophages cultured in physiologic iron conditions did. These findings identify serum iron as a critical protective factor in renal allograft outcome. Increasing serum iron levels in patients may thus improve prognosis of renal transplants.

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.

"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.

Iron Reduces M1 Macrophage Polarization in RAW264.7 Macrophages Associated with Inhibition of STAT1

Iron metabolism in inflammation has been mostly characterized in macrophages exposed to pathogens or inflammatory conditions. The aim of this study is to investigate the cross-regulatory interactions between M1 macrophage polarization and iron metabolism. Firstly, we characterized the transcription of genes related to iron homeostasis in M1 RAW264.7 macrophages stimulated by IFN-γ. The molecular signature of M1 macrophages showed high levels of iron storage (ferritin), a low level of iron export (ferroportin), and changes of iron regulators (hepcidin and transferrin receptors), which favour iron sequestration in the reticuloendothelial system and are benefit for inflammatory disorders. Then, we evaluated the effect of iron on M1 macrophage polarization. Iron significantly reduced mRNA levels of IL-6, IL-1β, TNF-α, and iNOS produced by IFN-γ-polarized M1 macrophages. Immunofluorescence analysis showed that iron also reduced iNOS production. However, iron did not compromise but enhanced the ability of M1-polarized macrophages to phagocytose FITC-dextran. Moreover, we demonstrated that STAT1 inhibition was required for reduction of iNOS and M1-related cytokines production by the present of iron. Together, these findings indicated that iron decreased polarization of M1 macrophages and inhibited the production of the proinflammatory cytokines. The results expanded our knowledge about the role of iron in macrophage polarization.

Siderophores in Iron Metabolism: From Mechanism to Therapy Potential

Iron is an essential nutrient for life. During infection, a fierce battle of iron acquisition occurs between the host and bacterial pathogens. Bacteria acquire iron by secreting siderophores, small ferric iron-binding molecules. In response, host immune cells secrete lipocalin 2 (also known as siderocalin), a siderophore-binding protein, to prevent bacterial reuptake of iron-loaded siderophores. To counter this threat, some bacteria can produce lipocalin 2-resistant siderophores. This review discusses the recently described molecular mechanisms of siderophore iron trafficking between host and bacteria, highlighting the therapeutic potential of exploiting pathogen siderophore machinery for the treatment of antibiotic-resistant bacterial infections. Because the latter reflect a persistent problem in hospital settings, siderophore-targeting or siderophore-based compounds represent a promising avenue to combat such infections.

Iron acquisition in Leishmania and its crucial role in infection

Iron is an essential cofactor for many basic metabolic pathways in pathogenic microbes and their hosts. It is also dangerous as it can catalyse the production of reactive free radicals. This dual character makes the host can either limit iron availability to invading microbes or exploit iron to induce toxicity to pathogens. Successful pathogens, including Leishmania species, must possess mechanisms to circumvent host's iron limitation and iron-induced toxicity in order to survive. In this review, we discuss the regulation of iron metabolism in the setting of infection and delineate the iron acquisition strategies used by Leishmania parasites and their subversions to host iron metabolism to overcome host's iron-related defences.

Impact of individual intravenous iron preparations on the differentiation of monocytes towards macrophages and dendritic cells

Background

Treatment of iron deficiency with intravenous (i.v.) iron is a first-line strategy to improve anaemia of chronic kidney disease. Previous in vitro experiments demonstrated that different i.v. iron preparations inhibit differentiation of haematopoietic stem cells to monocytes, but their effect on monocyte differentiation to macrophages and mature dendritic cells (mDCs) has not been assessed. We investigated substance-specific effects of iron sucrose (IS), sodium ferric gluconate (SFG), ferric carboxymaltose (FCM) and iron isomaltoside 1000 (IIM) on monocytic differentiation to M1/M2 macrophages and mDCs.

Methods

Via flow cytometry and microRNA (miRNA) expression analysis, we morphologically and functionally characterized monocyte differentiation to M1/M2 macrophages and mDCs after monocyte stimulation with IS, SFG, FCM and IIM (0.133, 0.266 and 0.533 mg/mL, respectively). To assess potential clinical implications, we compared monocytic phagocytosis capacity in dialysis patients who received either 500 mg IS or IIM.

Results

Phenotypically, IS and SFG dysregulated the expression of macrophage (e.g. CD40, CD163) and mDC (e.g. CD1c, CD141) surface markers. Functionally, IS and SFG impaired macrophage phagocytosis capacity. Phenotypic and functional alterations were less pronounced with FCM, and virtually absent with IIM. In miRNA expression analysis of mDCs, IS dysregulated miRNAs such as miR-146b-5p and miR-155-5p, which are linked to Toll-like receptor and mitogen-activated protein kinase signalling pathways. In vivo, IS reduced monocytic phagocytosis capacity within 1 h after infusion, while IIM did not.

Conclusions

This study demonstrates that less stable i.v. iron preparations specifically affect monocyte differentiation towards macrophages and mDCs.

Heme oxygenase 1 controls early innate immune response of macrophages to Salmonella Typhimurium infection

Macrophages are central for the immune control of intracellular microbes. Heme oxygenase 1 (HO-1, hmox) is the first and rate limiting enzyme in the breakdown of heme originating from degraded senescent erythrocytes and heme-proteins, yielding equal amounts of iron, carbon monoxide and biliverdin. HO-1 is strongly up-regulated in macrophages in response to inflammatory signals, including bacterial endotoxin. In view of the essential role of iron for the growth and proliferation of intracellular bacteria along with known effects of the metal on innate immune function, we examined whether HO-1 plays a role in the control of infection with the intracellular bacterium Salmonella Typhimurium. We studied the course of infection in stably-transfected murine macrophages (RAW264.7) bearing a tetracycline-inducible plasmid producing hmox shRNA and in primary HO-1 knockout macrophages. While uptake of bacteria into macrophages was not affected, a significantly reduced survival of intracellular Salmonella was observed upon hmox knockdown or pharmacological hmox inhibition, which was independent of Nramp1 functionality. This could be traced to limitation of iron availability for intramacrophage bacteria along with enhanced stimulation of innate immune effector pathways, including the formation of reactive oxygen and nitrogen species and increased TNF-α expression. Mechanistically, these latter effects result from intracellular iron limitation with subsequent activation of NF-κB and further inos, tnfa and p47phox transcription along with reduced formation of the anti-inflammatory and radical scavenging molecules, CO and biliverdin as a consequence of HO-1 silencing. Taken together our data provide novel evidence that the infection-driven induction of HO-1 exerts detrimental effects in the early control of Salmonella infection, whereas hmox inhibition can favourably modulate anti-bacterial immune effector pathways of macrophages and promote bacterial elimination.

Contrasting regulation of macrophage iron homeostasis in response to infection with Listeria monocytogenes depending on localization of bacteria

Due to its multiple roles for the proliferation and pathogenicity of many microbes on the one hand and via modulation of immune effector functions on the other hand the control over iron homeostasis is thought to play a decisive role in the course of infections. Diversion of cellular iron traffic is considered as an important defense mechanism of macrophages to reduce metal availability for intracellular bacteria residing in the phagosome. However, evidence is lacking whether such alterations of iron homeostasis also become evident upon infection with bacteria gaining access to the cytosol like Listeria monocytogenes. Here we show that infection of macrophages with L. monocytogenes triggers the expression of the major cellular iron exporter ferroportin1 and induces cellular iron egress. As the growth of Listeria within macrophages is promoted by iron, stimulation of ferroportin1 functionality limits the availability of the metal for Listeria residing in the cytoplasm, whereas ferroportin1 degradation upon hepcidin treatment increases intracellular bacterial growth. In parallel to an increase of ferroportin1 expression, infected macrophages induce anti-microbial immune effector mechanisms such as TNFα formation or NO expression which are aggravated upon iron deficiency. These adaptive changes of iron homeostasis and immune response pathways are only found in macrophages infected with Listeria which express listeriolysin O and are therefore able to escape from the phagosome to the cytoplasm. Listeriolysin O deficient Listeria which are restricted to the phagosome are even killed by excess iron which may be based on "iron intoxification" via macrophage radical formation, because iron supplementation in that setting is paralleled by increased ROS formation. Our results indicate that ferroportin1 mediated iron export is a nutritional immune effector pathway to control infection with Listeria residing in the cytoplasm, whereas a different strategy is observed in mutant Listeria restricted to the phagosome, where iron remains in the macrophages likewise contributing to ROS mediated intoxification of bacteria.

The Iron age of host-microbe interactions

Microbes exert a major impact on human health and disease by either promoting or disrupting homeostasis, in the latter instance leading to the development of infectious diseases. Such disparate outcomes are driven by the ever-evolving genetic diversity of microbes and the countervailing host responses that minimize their pathogenic impact. Host defense strategies that limit microbial pathogenicity include resistance mechanisms that exert a negative impact on microbes, and disease tolerance mechanisms that sustain host homeostasis without interfering directly with microbes. While genetically distinct, these host defense strategies are functionally integrated, via mechanisms that remain incompletely defined. Here, we explore the general principles via which host adaptive responses regulating iron (Fe) metabolism impact on resistance and disease tolerance to infection.

Hepcidin-induced hypoferremia is a critical host defense mechanism against the siderophilic bacterium Vibrio vulnificus

Hereditary hemochromatosis, an iron overload disease caused by a deficiency in the iron-regulatory hormone hepcidin, is associated with lethal infections by siderophilic bacteria. To elucidate the mechanisms of this susceptibility, we infected wild-type and hepcidin-deficient mice with the siderophilic bacterium Vibrio vulnificus and found that hepcidin deficiency results in increased bacteremia and decreased survival of infected mice, which can be partially ameliorated by dietary iron depletion. Additionally, timely administration of hepcidin agonists to hepcidin-deficient mice induces hypoferremia that decreases bacterial loads and rescues these mice from death, regardless of initial iron levels. Studies of Vibrio vulnificus growth ex vivo show that high iron sera from hepcidin-deficient mice support extraordinarily rapid bacterial growth and that this is inhibited in hypoferremic sera. Our findings demonstrate that hepcidin-mediated hypoferremia is a host defense mechanism against siderophilic pathogens and suggest that hepcidin agonists may improve infection outcomes in patients with hereditary hemochromatosis or thalassemia.

Iron Deficiency Impairs Intra-Hepatic Lymphocyte Mediated Immune Response

Hepatic expression of iron homeostasis genes and serum iron parameters predict the success of immunosuppression withdrawal following clinical liver transplantation, a phenomenon known as spontaneous operational tolerance. In experimental animal models, spontaneous liver allograft tolerance is established through a process that requires intra-hepatic lymphocyte activation and deletion. Our aim was to determine if changes in systemic iron status regulate intra-hepatic lymphocyte responses. We used a murine model of lymphocyte-mediated acute liver inflammation induced by Concanavalin A (ConA) injection employing mice fed with an iron-deficient (IrDef) or an iron-balanced diet (IrRepl). While the mild iron deficiency induced by the IrDef diet did not significantly modify the steady state immune cell repertoire and systemic cytokine levels, it significantly dampened inflammatory liver damage after ConA challenge. These findings were associated with a marked decrease in T cell and NKT cell activation following ConA injection in IrDef mice. The decreased liver injury observed in IrDef mice was independent from changes in the gut microflora, and was replicated employing an iron specific chelator that did not modify intra-hepatic hepcidin secretion. Furthermore, low-dose iron chelation markedly impaired the activation of isolated T cells in vitro. All together, these results suggest that small changes in iron homeostasis can have a major effect in the regulation of intra-hepatic lymphocyte mediated responses.

Macrophage defense mechanisms against intracellular bacteria

Macrophages and neutrophils play a decisive role in host responses to intracellular bacteria including the agent of tuberculosis (TB), Mycobacterium tuberculosis as they represent the forefront of innate immune defense against bacterial invaders. At the same time, these phagocytes are also primary targets of intracellular bacteria to be abused as host cells. Their efficacy to contain and eliminate intracellular M. tuberculosis decides whether a patient initially becomes infected or not. However, when the infection becomes chronic or even latent (as in the case of TB) despite development of specific immune activation, phagocytes have also important effector functions. Macrophages have evolved a myriad of defense strategies to combat infection with intracellular bacteria such as M. tuberculosis. These include induction of toxic anti-microbial effectors such as nitric oxide and reactive oxygen intermediates, the stimulation of microbe intoxication mechanisms via acidification or metal accumulation in the phagolysosome, the restriction of the microbe's access to essential nutrients such as iron, fatty acids, or amino acids, the production of anti-microbial peptides and cytokines, along with induction of autophagy and efferocytosis to eliminate the pathogen. On the other hand, M. tuberculosis, as a prime example of a well-adapted facultative intracellular bacterium, has learned during evolution to counter-balance the host's immune defense strategies to secure survival or multiplication within this otherwise hostile environment. This review provides an overview of innate immune defense of macrophages directed against intracellular bacteria with a focus on M. tuberculosis. Gaining more insights and knowledge into this complex network of host-pathogen interaction will identify novel target sites of intervention to successfully clear infection at a time of rapidly emerging multi-resistance of M. tuberculosis against conventional antibiotics.

Estrogen contributes to regulating iron metabolism through governing ferroportin signaling via an estrogen response element

Ferroportin (FPN) is the only known iron exporter in mammalian cells, and is universally expressed in most types of cells. FPN signaling plays a crucial role in maintaining iron homeostasis through governing the level of intracellular iron. Serum iron storage is conversely related with the estrogen level in the female bodies, and women in post-menopause are possibly subjected to iron retention. However, the potential effects of estrogen on iron metabolism are not clearly understood. Here, FPN mRNA transcription in all selected estrogen receptor positive (ER+) cells was significantly reduced upon 17β-estradiol (E2) treatment; and this inhibitory effect could be attenuated by ER antagonist tamoxifen. Likewise, in murine bone marrow-derived macrophages (BMDMs), FPN reduction with elevated intracellular iron (reflected by increased ferritin) was observed in response to E2; however, ferritin level barely responded to E2 in FPN-null BMDMs. The observation of inhibition of FPN mRNA expression was not replicated in ER(-) cells upon E2. A functional estrogen response element (ERE) was identified within the promoter of FPN, and this ERE was responsible for the suppressive effect of E2 on FPN expression. Moreover, ovariectomized (OVX) and sham-operated (SHAM) mice were used to further confirm the in vitro finding. The expression of hepatic FPN was induced in OVX mice, compared to that in the SHAM mice. Taken together, our results demonstrated that estrogen is involved in regulating FPN expression through a functional ERE on its promoter, providing additional insights into a vital role of estrogen in iron metabolism.

Iron supplementation and mortality in incident dialysis patients: an observational study

BACKGROUND

Studies on the association between iron supplementation and mortality in dialysis patients are rare and conflicting.

METHODS

In our observational single-center cohort study (INVOR study) we prospectively studied 235 incident dialysis patients. Time-dependent Cox proportional hazards models using all measured laboratory values for up to 7.6 years were applied to study the association between iron supplementation and all-cause mortality, cardiovascular and sepsis-related mortality. Furthermore, the time-dependent association of ferritin levels with mortality in patients with normal C-reactive protein (CRP) levels (<0.5 mg/dL) and elevated CRP levels (≧0.5 mg/dL) was evaluated by using non-linear P-splines to allow flexible modeling of the association.

RESULTS

One hundred and ninety-one (81.3%) patients received intravenous iron, 13 (5.5%) patients oral iron, whereas 31 (13.2%) patients were never supplemented with iron throughout the observation period. Eighty-two (35%) patients died during a median follow-up of 34 months, 38 patients due to cardiovascular events and 21 patients from sepsis. Baseline CRP levels were not different between patients with and without iron supplementation. However, baseline serum ferritin levels were lower in patients receiving iron during follow up (median 93 vs 251 ng/mL, p<0.001). Iron supplementation was associated with a significantly reduced all-cause mortality [HR (95%CI): 0.22 (0.08-0.58); p = 0.002] and a reduced cardiovascular and sepsis-related mortality [HR (95%CI): 0.31 (0.09-1.04); p = 0.06]. Increasing ferritin concentrations in patients with normal CRP were associated with a decreasing mortality, whereas in patients with elevated CRP values ferritin levels>800 ng/mL were linked with increased mortality.

CONCLUSIONS

Iron supplementation is associated with reduced all-cause mortality in incident dialysis patients. While serum ferritin levels up to 800 ng/mL appear to be safe, higher ferritin levels are associated with increased mortality in the setting of concomitant inflammation.

'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.

Iron at the interface of immunity and infection

Both, mammalian cells and microbes have an essential need for iron, which is required for many metabolic processes and for microbial pathogenicity. In addition, cross-regulatory interactions between iron homeostasis and immune function are evident. Cytokines and the acute phase protein hepcidin affect iron homeostasis leading to the retention of the metal within macrophages and hypoferremia. This is considered to result from a defense mechanism of the body to limit the availability of iron for extracellular pathogens while on the other hand the reduction of circulating iron results in the development of anemia of inflammation. Opposite, iron and the erythropoiesis inducing hormone erythropoietin affect innate immune responses by influencing interferon-gamma (IFN-γ) mediated (iron) or NF-kB inducible (erythropoietin) immune effector pathways in macrophages. Thus, macrophages loaded with iron lose their ability to kill intracellular pathogens via IFN-γ mediated effector pathways such as nitric oxide (NO) formation. Accordingly, macrophages invaded by the intracellular bacterium Salmonella enterica serovar Typhimurium increase the expression of the iron export protein ferroportin thereby reducing the availability of iron for intramacrophage bacteria while on the other side strengthening anti-microbial macrophage effector pathways via increased formation of NO or TNF-α. In addition, certain innate resistance genes such as natural resistance associated macrophage protein function (Nramp1) or lipocalin-2 exert part of their antimicrobial activity by controlling host and/or microbial iron homeostasis. Consequently, pharmacological or dietary modification of cellular iron trafficking enhances host resistance to intracellular pathogens but may increase susceptibility to microbes in the extracellular compartment and vice versa. Thus, the control over iron homeostasis is a central battlefield in host–pathogen interplay influencing the course of an infectious disease in favor of either the mammalian host or the pathogenic invader.

Innate defects of the IL-12/IFN-γ axis in susceptibility to infections by mycobacteria and salmonella

Since 1996, several studies characterizing the association between primary immunodeficiencies and susceptibility to infections with environmental and non-pathogenic mycobacteria such as the Bacillus Calmette-Guérin (Mycobacterium bovis Bacillus of Calmette Guérin strain) as well as disseminated infections by Salmonella spp. have been conducted. These conditions, grouped in the so-called Mendelian susceptibility to mycobacterial diseases, include a primary immunodeficiency caused by mutations in 7 autosomal genes (IFNGR1, IFNGR2, IL12B, IL12BR1, STAT1, ISG15, and IRF8) and an X-linked gene (NEMO). This syndrome presents a high degree of allelic heterogeneity and variable penetrance. This review focuses on the analysis of the first reported cases of these diseases, as well as on the molecular findings involved in each of them.

The complex interplay of iron metabolism, reactive oxygen species, and reactive nitrogen species: insights into the potential of various iron therapies to induce oxidative and nitrosative stress

Production of minute concentrations of superoxide (O2(*-)) and nitrogen monoxide (nitric oxide, NO*) plays important roles in several aspects of cellular signaling and metabolic regulation. However, in an inflammatory environment, the concentrations of these radicals can drastically increase and the antioxidant defenses may become overwhelmed. Thus, biological damage may occur owing to redox imbalance-a condition called oxidative and/or nitrosative stress. A complex interplay exists between iron metabolism, O2(*-), hydrogen peroxide (H2O2), and NO*. Iron is involved in both the formation and the scavenging of these species. Iron deficiency (anemia) (ID(A)) is associated with oxidative stress, but its role in the induction of nitrosative stress is largely unclear. Moreover, oral as well as intravenous (iv) iron preparations used for the treatment of ID(A) may also induce oxidative and/or nitrosative stress. Oral administration of ferrous salts may lead to high transferrin saturation levels and, thus, formation of non-transferrin-bound iron, a potentially toxic form of iron with a propensity to induce oxidative stress. One of the factors that determine the likelihood of oxidative and nitrosative stress induced upon administration of an iv iron complex is the amount of labile (or weakly-bound) iron present in the complex. Stable dextran-based iron complexes used for iv therapy, although they contain only negligible amounts of labile iron, can induce oxidative and/or nitrosative stress through so far unknown mechanisms. In this review, after summarizing the main features of iron metabolism and its complex interplay with O2(*-), H2O2, NO*, and other more reactive compounds derived from these species, the potential of various iron therapies to induce oxidative and nitrosative stress is discussed and possible underlying mechanisms are proposed. Understanding the mechanisms, by which various iron formulations may induce oxidative and nitrosative stress, will help us develop better tolerated and more efficient therapies for various dysfunctions of iron metabolism.

Epidemiological and nonclinical studies investigating effects of iron in carcinogenesis--a critical review

The efficacy and tolerability of intravenous (i.v.) iron in managing cancer-related anemia and iron deficiency has been clinically evaluated and reviewed recently. However, long-term data in cancer patients are not available; yet, long-term i.v. iron treatment in hemodialysis patients is not associated with increased cancer risk. This review summarizes epidemiological and nonclinical data on the role of iron in carcinogenesis. In humans, epidemiological data suggest correlations between certain cancers and increased iron exposure or iron overload. Nonclinical models that investigated whether iron can enhance carcinogenesis provide only limited evidence relevant for cancer patients since they were typically based on high iron doses as well as injection routes and iron formulations which are not used in the clinical setting. Nevertheless, in the absence of long-term outcome data from prospectively defined trials in i.v. iron-treated cancer patients, iron supplementation should be limited to periods of concomitant anti-tumor treatment.