Iron, anaemia, and inflammatory bowel diseases

Iron deficiency anaemia is one of the most common disorders in the world. Also, one third of inflammatory bowel disease (IBD) patients suffer from recurrent anaemia. Anaemia has significant impact on the quality of life of affected patients. Chronic fatigue, a frequent IBD symptom itself, is commonly caused by anaemia and may debilitate patients as much as abdominal pain or diarrhoea. Common therapeutic targets are the mechanisms behind anaemia of chronic disease and iron deficiency. It is our experience that virtually all patients with IBD associated anaemia can be successfully treated with a combination of iron sucrose and erythropoietin, which then may positively affect the misled immune response in IBD.

Mechanisms of nutrient modulation of the immune response

Lack of adequate macronutrients or selected micronutrients, especially zinc, selenium, iron, and the antioxidant vitamins, can lead to clinically significant immune deficiency and infections in children. Undernutrition in critical periods of gestation and neonatal maturation and during weaning impairs the development and differentiation of a normal immune system. Infections are both more frequent and more often become chronic in the malnourished child. Recent identification of genetic mechanisms is revealing critical pathways in the gastrointestinal immune response. New studies show that the development of tolerance, control of inflammation, and response to normal mucosal flora are interrelated and linked to specific immune mechanisms. Nutrients act as antioxidants and as cofactors at the level of cytokine regulation. Protein calorie malnutrition and zinc deficiency activate the hypothalamic-pituitary-adrenal axis. Increased circulating levels of glucocorticoids cause thymic atrophy and affect hematopoiesis. Chronic undernutrition and micronutrient deficiency compromise cytokine response and affect immune cell trafficking. The combination of chronic undernutrition and infection further weakens the immune response, leading to altered immune cell populations and a generalized increase in inflammatory mediators. Obesity caused by excess nutrition or excess storage of fats relative to energy expenditure is a form of malnutrition that is increasingly seen in children. Leptin is emerging as a cytokine-like immune regulator that has complex effects in both overnutrition and in the inflammatory response in malnutrition. Because the immune system is immature at birth, malnutrition in childhood might have long-term effects on health.

Iron in innate immunity: starve the invaders

Iron is essential for nearly all living organisms. Innate immunity effectively restricts iron availability to microbial invaders. Some microbes have evolved effective countermeasures that blunt the effect of iron restriction. Recent epidemiologic studies have highlighted the potentiating effect of iron on microbial infections. Laboratory studies have focused on specific immune mechanisms that mediate iron withholding from microbes constitutively and in response to infections. Specialized inflammation-regulated proteins chelate iron, trap siderophores, and transport iron or modulate its transport to alter its tissue distribution during infections.

Iron and immunity: a double-edged sword

Iron is a crucial element for many central metabolic pathways of the body. Lack of iron leads to growth arrest and anaemia while increased accumulation of this metal, as it occurs in highly frequent inherited diseases such as hereditary haemochromatosis and thalassaemia, is associated with toxic radical formation and progressive tissue damage. As shown by several groups, iron also modulates immune effector mechanisms, such as cytokine activities (IFN-gamma effector pathways towards macrophages), nitric oxide (NO) formation or immune cell proliferation, and thus host immune surveillance. Therefore, gaining control over iron homeostasis is one of the central battlefields in deciding the fate of an infection with intracellular pathogens or a malignant disease. Thus, the reticulo-endothelial system has evoked sophisticated strategies to control iron metabolism in general and especially the handling of the metal within immune cells.

Expression and localization of hepcidin in macrophages: a role in host defense against tuberculosis

Hepcidin is an antimicrobial peptide produced by the liver in response to inflammatory stimuli and iron overload. Hepcidin regulates iron homeostasis by mediating the degradation of the iron export protein ferroportin 1, thereby inhibiting iron absorption from the small intestine and release of iron from macrophages. Here, we examined the expression of hepcidin in macrophages infected with the intracellular pathogens Mycobacterium avium and Mycobacterium tuberculosis. Stimulation of the mouse RAW264.7 macrophage cell line and mouse bone marrow-derived macrophages with mycobacteria and IFN-gamma synergistically induced high levels of hepcidin mRNA and protein. Similar results were obtained using the human THP-1 monocytic cell line. Stimulation of macrophages with the inflammatory cytokines IL-6 and IL-beta did not induce hepcidin mRNA expression. Iron loading inhibited hepcidin mRNA expression induced by IFN-gamma and M. avium, and iron chelation increased hepcidin mRNA expression. Intracellular protein levels and secretion of hepcidin were determined by a competitive chemiluminescence ELISA. Stimulation of RAW264.7 cells with IFN-gamma and M. tuberculosis induced intracellular expression and secretion of hepcidin. Furthermore, confocal microscopy analyses showed that hepcidin localized to the mycobacteria-containing phagosomes. As hepcidin has been shown to possess direct antimicrobial activity, we investigated its activity against M. tuberculosis. We found that hepcidin inhibited M. tuberculosis growth in vitro and caused structural damage to the mycobacteria. In summary, our data show for the first time that hepcidin localizes to the phagosome of infected, IFN-gamma-activated cells and has antimycobacterial activity.

Iron and Immunity

Iron is an essential nutrient for most life on Earth because it functions as a crucial redox catalyst in many cellular processes. However, when present in excess iron can lead to the formation of harmful hydroxyl radicals. Hence, the cellular iron balance must be tightly controlled. Perturbation of iron homeostasis is a major strategy in host-pathogen interactions. Plants use iron-withholding strategies to reduce pathogen virulence or to locally increase iron levels to activate a toxic oxidative burst. Some plant pathogens counteract such defenses by secreting iron-scavenging siderophores that promote iron uptake and alleviate iron-regulated host immune responses. Mutualistic root microbiota can also influence plant disease via iron. They compete for iron with soil-borne pathogens or induce a systemic resistance that shares early signaling components with the root iron-uptake machinery. This review describes the progress in our understanding of the role of iron homeostasis in both pathogenic and beneficial plant-microbe interactions.

Alternative activation deprives macrophages of a coordinated defense program toMycobacterium tuberculosis

A potent Th1 immune response is critical to the control of tuberculosis. The impact of an additive Th2 response on the course of disease has so far been insufficiently characterized, despite increased morbidity after co-infection with Mycobacterium tuberculosis and Th2-eliciting helminths and possible involvement of Th2 polarization in reactivation of latent tuberculosis. Here, we describe the gene expression profile of murine bone marrow-derived macrophages alternatively activated by IL-4 in response to infection with M. tuberculosis. Comparison of transcriptional profiles of infected IL-4- and IFN-gamma-activated macrophages revealed delayed and partially diminished responses to intracellular bacteria in alternatively activated macrophages, characterized by reduced exposure to nitrosative stress and increased iron availability, respectively. Alternative activation of host macrophages correlated with elevated expression of the M. tuberculosis iron storage protein bacterioferritin as well as reduced expression of the mycobactin synthesis genes mbtI and mbtJ. The extracellular matrix-remodeling enzyme matrix metalloproteinase (MMP)-12 was induced in alternatively activated macrophages in vitro, and MMP-12-expressing macrophages were abundant at late, but not early, stages of tuberculosis in murine lungs. Our findings emphasize that alternative activation deprives macrophages of control mechanisms that limit mycobacterial growth in vivo, thus supporting intracellular persistence of M. tuberculosis.

Transition Metal Sequestration by the Host-Defense Protein Calprotectin

In response to microbial infection, the human host deploys metal-sequestering host-defense proteins, which reduce nutrient availability and thereby inhibit microbial growth and virulence. Calprotectin (CP) is an abundant antimicrobial protein released from neutrophils and epithelial cells at sites of infection. CP sequesters divalent first-row transition metal ions to limit the availability of essential metal nutrients in the extracellular space. While functional and clinical studies of CP have been pursued for decades, advances in our understanding of its biological coordination chemistry, which is central to its role in the host-microbe interaction, have been made in more recent years. In this review, we focus on the coordination chemistry of CP and highlight studies of its metal-binding properties and contributions to the metal-withholding innate immune response. Taken together, these recent studies inform our current model of how CP participates in metal homeostasis and immunity, and they provide a foundation for further investigations of a remarkable metal-chelating protein at the host-microbe interface and beyond.

Multiple nutritional factors and thyroid disease, with particular reference to autoimmune thyroid disease

Hashimoto's thyroiditis (HT) and Graves' disease (GD) are examples of autoimmune thyroid disease (AITD), the commonest autoimmune condition. Antibodies to thyroid peroxidase (TPO), the enzyme that catalyses thyroid-hormone production and antibodies to the receptor for the thyroid-stimulating hormone, are characteristic of HT and GD, respectively. It is presently accepted that genetic susceptibility, environmental factors, including nutritional factors and immune disorders contribute to the development of AITD. Aiming to investigate the effect of iodine, iron and selenium in the risk, pathogenesis and treatment of thyroid disease, PubMed and the Cochrane Library were searched for relevant publications to provide a narrative review. Iodine: chronic exposure to excess iodine intake induces autoimmune thyroiditis, partly because highly-iodinated thyroglobulin (Tg) is more immunogenic. The recent introduction of universal salt iodisation can have a similar, although transient, effect. Iron: iron deficiency impairs thyroid metabolism. TPO is a haem enzyme that becomes active only after binding haem. AITD patients are frequently iron-deficient since autoimmune gastritis, which reduces iron absorption and coeliac disease which causes iron loss, are frequent co-morbidities. In two-thirds of women with persistent symptoms of hypothyroidism despite appropriate levothyroxine therapy, restoration of serum ferritin above 100 µg/l ameliorated symptoms. Selenium: selenoproteins are essential to thyroid action. In particular, the glutathione peroxidases remove excessive hydrogen peroxide produced there for the iodination of Tg to form thyroid hormones. There is evidence from observational studies and randomised controlled trials that selenium, probably as selenoproteins, can reduce TPO-antibody concentration, hypothyroidism and postpartum thyroiditis. Appropriate status of iodine, iron and selenium is crucial to thyroid health.

The role of iron in the immune response to bacterial infection

My laboratory has been interested for some time in the influence of iron, a nutrient that is essential for both microbial pathogens and their mammalian hosts, on the course of infectious disease. Our studies indicate that alterations in the expression of host molecules that sequester or transport iron can have direct effects on pathogen growth and can also have an impact on the ability to mount normal immune responses. We have elucidated the mechanistic basis for some of these observations, and have started to apply our findings in strategies to control abnormalities of inflammation and iron metabolism. I will review here what we have learned about the interactions between iron and immunity and discuss the implications of the information that we have acquired.

Iron and immunity: immunological consequences of iron deficiency and overload

The influence of iron on immune function has been long appreciated. However, the molecular basis for this interaction is less well understood. Recently, there have been several important advances that have shed light on the mechanisms that regulate mammalian iron metabolism. The new insights provide a conceptual framework for understanding and manipulating the cross-talk between iron homeostasis and the immune system. This article will review what is currently known about how disturbances of iron metabolism can affect immunity and how activation of the immune system can lead to alterations in iron balance.

Endogenous hepcidin and its agonist mediate resistance to selected infections by clearing non-transferrin-bound iron

The iron-regulatory hormone hepcidin is induced early in infection, causing iron sequestration in macrophages and decreased plasma iron; this is proposed to limit the replication of extracellular microbes, but could also promote infection with macrophage-tropic pathogens. The mechanisms by which hepcidin and hypoferremia modulate host defense, and the spectrum of microbes affected, are poorly understood. Using mouse models, we show that hepcidin was selectively protective against siderophilic extracellular pathogens (Yersinia enterocolitica O9) by controlling non-transferrin-bound iron (NTBI) rather than iron-transferrin concentration. NTBI promoted the rapid growth of siderophilic but not nonsiderophilic bacteria in mice with either genetic or iatrogenic iron overload and in human plasma. Hepcidin or iron loading did not affect other key components of innate immunity, did not indiscriminately promote intracellular infections (Mycobacterium tuberculosis), and had no effect on extracellular nonsiderophilic Y enterocolitica O8 or Staphylococcus aureus Hepcidin analogs may be useful for treatment of siderophilic infections.

Iron sequestration by transferrin 1 mediates nutritional immunity in Drosophila melanogaster

Iron sequestration is a recognized innate immune mechanism against invading pathogens mediated by iron-binding proteins called transferrins. Despite many studies on antimicrobial activity of transferrins in vitro, their specific in vivo functions are poorly understood. Here we use Drosophila melanogaster as an in vivo model to investigate the role of transferrins in host defense. We find that systemic infections with a variety of pathogens trigger a hypoferremic response in flies, namely, iron withdrawal from the hemolymph and accumulation in the fat body. Notably, this hypoferremia to infection requires Drosophila nuclear factor κB (NF-κB) immune pathways, Toll and Imd, revealing that these pathways also mediate nutritional immunity in flies. Next, we show that the iron transporter Tsf1 is induced by infections downstream of the Toll and Imd pathways and is necessary for iron relocation from the hemolymph to the fat body. Consistent with elevated iron levels in the hemolymph, Tsf1 mutants exhibited increased susceptibility to Pseudomonas bacteria and Mucorales fungi, which could be rescued by chemical chelation of iron. Furthermore, using siderophore-deficient Pseudomonas aeruginosa, we discover that the siderophore pyoverdine is necessary for pathogenesis in wild-type flies, but it becomes dispensable in Tsf1 mutants due to excessive iron present in the hemolymph of these flies. As such, our study reveals that, similar to mammals, Drosophila uses iron limitation as an immune defense mechanism mediated by conserved iron-transporting proteins transferrins. Our in vivo work, together with accumulating in vitro studies, supports the immune role of insect transferrins against infections via an iron withholding strategy.

Antibody-conjugated magnetoliposomes for targeting cancer cells and their application in hyperthermia

Magnetoliposomes for hyperthermia treatment of cancer were prepared by coating phospholipid on to magnetite particles. The optimum phospholipid composition for dispersibility was a phosphatidylcholine/phosphatidylethanolamine ratio of 2:1. The average size of the magnetoliposomes, which were aggregates of 10 nm core magnetite particles, was about 80 nm. The magnetoliposomes were coated with hydrazide pullulan to stabilize the phospholipid capsules and provide an anchor for the immobilization of antibodies. By this method, 90-180 molecules of a monoclonal antibody were immobilized on to a magnetoliposome particle. When the antibody-conjugated magnetoliposomes were incubated with cancer cells, they were adsorbed on to the cell surface and incorporated by the cells about 12 times more effectively than the control after 4 h. The amount of incorporated magnetite was 0.61-3.6 pg/cell, depending on the antigen-antibody system. The heating properties of the magnetoliposomes were also measured and found to vary with the size of the core magnetite.

Iron, anemia, and infection

The data on the relationship between iron deficiency and infection are conflicting. Some researchers conclude that mild iron deficiency is beneficial for immunity, whereas others contend that any deficit is not good for immunity. Additionally, infection or inflammation generate anemia and profound changes in iron metabolism mediated by cytokines. These changes are important confounders to consider in assessments of iron status.

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.

The Role of Nutrition in COVID-19 Susceptibility and Severity of Disease: A Systematic Review

BACKGROUND

Many nutrients have powerful immunomodulatory actions with the potential to alter susceptibility to coronavirus disease 2019 (COVID-19) infection, progression to symptoms, likelihood of severe disease, and survival.

OBJECTIVE

The aim was to review the latest evidence on how malnutrition across all its forms (under- and overnutrition and micronutrient status) may influence both susceptibility to, and progression of, COVID-19.

METHODS

We synthesized information on 13 nutrition-related components and their potential interactions with COVID-19: overweight, obesity, and diabetes; protein-energy malnutrition; anemia; vitamins A, C, D, and E; PUFAs; iron; selenium; zinc; antioxidants; and nutritional support. For each section we provide: 1) a landscape review of pertinent material; 2) a systematic search of the literature in PubMed and EMBASE databases, including a wide range of preprint servers; and 3) a screen of 6 clinical trial registries. All original research was considered, without restriction to study design, and included if it covered: 1) severe acute respiratory syndrome coronavirus (CoV) 2 (SARS-CoV-2), Middle East respiratory syndrome CoV (MERS-CoV), or SARS-CoV viruses and 2) disease susceptibility or 3) disease progression, and 4) the nutritional component of interest. Searches took place between 16 May and 11 August 2020.

RESULTS

Across the 13 searches, 2732 articles from PubMed and EMBASE, 4164 articles from the preprint servers, and 433 trials were returned. In the final narrative synthesis, we include 22 published articles, 38 preprint articles, and 79 trials.

CONCLUSIONS

Currently there is limited evidence that high-dose supplements of micronutrients will either prevent severe disease or speed up recovery. However, results of clinical trials are eagerly awaited. Given the known impacts of all forms of malnutrition on the immune system, public health strategies to reduce micronutrient deficiencies and undernutrition remain of critical importance. Furthermore, there is strong evidence that prevention of obesity and type 2 diabetes will reduce the risk of serious COVID-19 outcomes. This review is registered at PROSPERO as CRD42020186194.

Effect of daily iron supplementation on iron status, cell-mediated immunity, and incidence of infections in 6-36 month old Togolese children

OBJECTIVE

To assess the impact of a daily oral iron supplementation on hematological status, cell-mediated immunity and susceptibility to infections in children living in an environment where iron deficiency, malaria and other infections are frequent.

DESIGN

Randomized, double-blind iron supplementation including a placebo group.

SETTING

A village in Togo, West Africa.

SUBJECTS

Of the 229 6-36-month-old children of both sexes recruited, 197 with hemoglobin concentration >/=80 g/l were included and 163 completed the study.

INTERVENTION

Children received daily a placebo (n=79) or a dose of 2-3 mg of elemental iron per kg of body weight (n=84) for 3 months. Hematological, nutritional and immune status were assessed at the beginning and at the end of the supplementation period, and 6 months later. Morbidity was recorded throughout the study.

RESULTS

Iron supplementation had a significant and positive effect on iron status of children and no impact on the incidence of infections, especially malaria. Its probable effect on immune status was masked by interference of infections and their treatment, which contributed to improve hematological and immune status in both groups.

CONCLUSION

According to the negative consequences of anemia and iron deficiency on global child development, control of iron deficiency by oral iron supplementation in young children has to be conducted, associated with prophylaxis and treatment of malaria and repeated deworming.

SPONSORSHIP

Program supported by IRD. European Journal of Clinical Nutrition (2000) 54, 29-35

Immune function is impaired in iron-deficient, homebound, older women

BACKGROUND

Aging is often associated with a dysregulation of immune function. Iron deficiency may further impair immunity in older adults. Published reports on iron deficiency and immune response in humans are inconsistent. Most studies are focused on young children in developing countries and are often confounded by comorbid conditions, infections, and nutrient deficiencies.

OBJECTIVE

Our objective was to determine the relation of iron status with immune function in homebound older women, who often have impairments in both iron status and immune response. The subjects were selected according to rigorous exclusion criteria for disease, infection, and deficiencies in key nutrients known to affect immunocompetence.

DESIGN

Seventy-two homebound elderly women provided blood for comprehensive evaluation of iron status and cell-mediated and innate immunity. Women were classified as iron-deficient or iron-sufficient on the basis of multiple abnormal iron status test results. Groups were compared with respect to lymphocyte subsets, phagocytosis, oxidative burst capacity, and T cell proliferation upon stimulation with mitogens.

RESULTS

In iron-deficient women, T cell proliferation upon stimulation with concanavalin A and phytohemagglutinin A was only 40-50% of that in iron-sufficient women. Phagocytosis did not differ significantly between the 2 groups, but respiratory burst was significantly less (by 28%) in iron-deficient women than in iron-sufficient women.

CONCLUSIONS

Iron deficiency is associated with impairments in cell-mediated and innate immunity and may render older adults more vulnerable to infections. Further prospective studies using similar exclusion criteria for disease, infection, and concomitant nutrient deficiencies are needed for simultaneous examination of the effects of iron deficiency on immune response and morbidity.

The two channel catfish intelectin genes exhibit highly differential patterns of tissue expression and regulation after infection with Edwardsiella ictaluri

Intelectins (IntL) are Ca(2+)-dependent secretory glycoproteins that play a role in the innate immune response. The mammalian IntL is also known as lactoferrin receptor (LfR) that is involved in iron metabolism. The objective of this study was to characterize the intelectin genes in both channel catfish and blue catfish, to determine their genomic organization and copy numbers, to determine their patterns of tissue expression, and to establish if they are involved in defense responses of catfish after bacterial infection. Two types of IntL genes have been identified from catfish, and IntL2 was completely sequenced. The genomic structure and organization of IntL2 were similar to those of the mammalian species and of zebrafish and grass carp, but orthologies cannot be established with mammalian IntL genes. The IntL genes are highly conserved through evolution. Sequence analysis also indicated the presence of the fibrinogen-related domain in the catfish IntL genes, suggesting their structural conservations. Phylogenetic analysis suggested the presence of at least two prototypes of IntL genes in teleosts, but only one in mammals. The catfish IntL genes exhibited drastically different patterns of expression as compared to those of the mammalian species, or even with the grass carp gene. The catfish IntL1 gene is widely expressed in various tissues, whereas the channel catfish IntL2 gene was mainly expressed in the liver. While the catfish IntL1 is constitutively expressed, the catfish IntL2 was drastically induced by intraperitoneal injection of Edwardsiella ictaluri and/or iron dextran. Such induction was most dramatic when the fish were treated with both the bacteria and iron dextran. While IntL1 was expressed in all leukocyte cell lines, no expression of IntL2 was detected in any of the leukocyte cell lines, suggesting that the up-regulated channel catfish IntL2 expression after bacterial infection may be a consequence of the initial immune response, and/or a downstream immune response rather than a part of the primary immune responses.

Micronutrients and immune function: some recent developments

Micronutrient deficiencies probably have few direct effects on the functioning of immune cells. The main effect appears to be a reduction in cell mass that may indirectly affect immune cell function, particularly where T helper cell numbers are reduced. Results of many human studies are contradictory. Some of this contradiction may be accounted for by the fact that disease may lower concentrations of micronutrients in plasma that may be misinterpreted as deficiency. Low plasma vitamin A concentrations however appear to impair immune responsiveness and have deleterious effects on membrane integrity and mucosal function. Zinc may have similar effects on gut integrity and appears to be particularly useful in the treatment of acute diarrhoea. Low concentrations of other nutrients such as ascorbate and iron, may not necessarily impair immune function. Low plasma ascorbate may assist the removal of iron from plasma and low iron concentrations appear to increase the cytotoxicity of macrophages.

Ferrous Ions and Reactive Oxygen Species Increase Antigen-binding and Anti-inflammatory Activities of Immunoglobulin G

Polyspecific antibodies represent a first line of defense against infection and regulate inflammation, properties hypothesized to rely on their ability to interact with multiple antigens. We demonstrated that IgG exposure to pro-oxidative ferrous ions or to reactive oxygen species enhances paratope flexibility and hydrophobicity, leading to expansion of the spectrum of recognized antigens, regulation of cell proliferation, and protection in experimental sepsis. We propose that ferrous ions, released from transferrin and ferritin at sites of inflammation, synergize with reactive oxygen species to modify the immunoglobulins present in the surrounding microenvironment, thus quenching pro-inflammatory signals, while facilitating neutralization of pathogens.

Ferritin H Deficiency in Myeloid Compartments Dysregulates Host Energy Metabolism and Increases Susceptibility to Mycobacterium tuberculosis Infection

Iron is an essential factor for the growth and virulence of Mycobacterium tuberculosis (Mtb). However, little is known about the mechanisms by which the host controls iron availability during infection. Since ferritin heavy chain (FtH) is a major intracellular source of reserve iron in the host, we hypothesized that the lack of FtH would cause dysregulated iron homeostasis to exacerbate TB disease. Therefore, we used knockout mice lacking FtH in myeloid-derived cell populations to study Mtb disease progression. We found that FtH plays a critical role in protecting mice against Mtb, as evidenced by increased organ burden, extrapulmonary dissemination, and decreased survival in Fth-/- mice. Flow cytometry analysis showed that reduced levels of FtH contribute to an excessive inflammatory response to exacerbate disease. Extracellular flux analysis showed that FtH is essential for maintaining bioenergetic homeostasis through oxidative phosphorylation. In support of these findings, RNAseq and mass spectrometry analyses demonstrated an essential role for FtH in mitochondrial function and maintenance of central intermediary metabolism in vivo. Further, we show that FtH deficiency leads to iron dysregulation through the hepcidin-ferroportin axis during infection. To assess the clinical significance of our animal studies, we performed a clinicopathological analysis of iron distribution within human TB lung tissue and showed that Mtb severely disrupts iron homeostasis in distinct microanatomic locations of the human lung. We identified hemorrhage as a major source of metabolically inert iron deposition. Importantly, we observed increased iron levels in human TB lung tissue compared to healthy tissue. Overall, these findings advance our understanding of the link between iron-dependent energy metabolism and immunity and provide new insight into iron distribution within the spectrum of human pulmonary TB. These metabolic mechanisms could serve as the foundation for novel host-directed strategies.

Nonhematological benefits of iron

Iron deficiency anemia is common in people with chronic kidney disease (CKD) and its importance in supporting erythropoiesis is unquestioned especially in those patients treated with erythropoietin. Clinical symptomatology such as fatigability, cold intolerance, failure to concentrate and poor effort intolerance is often attributed to anemia or uremia. That iron deficiency, per se, can cause these symptoms is poorly recognized. Clinical and animal studies that support the benefits of iron supplementation, independent of increasing hemoglobin, such as those on immune function, physical performance, thermoregulation, cognition, and restless leg syndrome and aluminum absorption is the subject of this narrative review.

Mechanisms of disease: The role of hepcidin in iron homeostasis--implications for hemochromatosis and other disorders

The defensin-like circulatory peptide hepcidin is the iron-regulatory hormone that links innate immunity and iron metabolism. In response to inflammatory stimuli, the liver produces hepcidin: this antimicrobial peptide then limits the iron that is vital to invading pathogens, by decreasing iron release/transfer from enterocytes and macrophages and causing secondary hypoferremia. This may lead, however, to reduced iron availability for erythropoiesis and therefore to anemia (and anemia of chronic disease). When iron is scarce, the rate at which it is released into the bloodstream must be enhanced: indeed, iron starvation and hypoxia readily abrogate hepcidin expression. Conversely, if excess iron enters the circulation, hepcidin transcription is turned on and iron release from the intestine and macrophages abrogated. Circumstantial evidence indicates that the effect of circulatory iron on hepcidin requires functional HFE and hemojuvelin, two proteins of unknown function that have recently been linked to human hereditary hemochromatosis. In this disease it is likely that inadequate levels of circulating hepcidin lead to the uncontrolled release of iron from the intestine and macrophages, followed by tissue iron overload and organ damage. Given its role as the iron-regulatory hormone, the modulation of hepcidin activity using agonists or antagonists might offer new treatment opportunities in different human iron-dependent disorders.