Hepcidin-mediated iron sequestration protects against bacterial dissemination during pneumonia

Integrative Physiology of Pneumonia

Pneumonia is a type of acute lower respiratory infection that is common and severe. The outcome of lower respiratory infection is determined by the degrees to which immunity is protective and inflammation is damaging. Intercellular and interorgan signaling networks coordinate these actions to fight infection and protect the tissue. Cells residing in the lung initiate and steer these responses, with additional immunity effectors recruited from the bloodstream. Responses of extrapulmonary tissues, including the liver, bone marrow, and others, are essential to resistance and resilience. Responses in the lung and extrapulmonary organs can also be counterproductive and drive acute and chronic comorbidities after respiratory infection. This review discusses cell-specific and organ-specific roles in the integrated physiological response to acute lung infection, and the mechanisms by which intercellular and interorgan signaling contribute to host defense and healthy respiratory physiology or to acute lung injury, chronic pulmonary disease, and adverse extrapulmonary sequelae. Pneumonia should no longer be perceived as simply an acute infection of the lung. Pneumonia susceptibility reflects ongoing and poorly understood chronic conditions, and pneumonia results in diverse and often persistent deleterious consequences for multiple physiological systems.

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 Therapeutics

Hepcidin is a key hormonal regulator of systemic iron homeostasis and its expression is induced by iron or inflammatory stimuli. Genetic defects in iron signaling to hepcidin lead to “hepcidinopathies” ranging from hereditary hemochromatosis to iron-refractory iron deficiency anemia, which are disorders caused by hepcidin deficiency or excess, respectively. Moreover, dysregulation of hepcidin is a pathogenic cofactor in iron-loading anemias with ineffective erythropoiesis and in anemia of inflammation. Experiments with preclinical animal models provided evidence that restoration of appropriate hepcidin levels can be used for the treatment of these conditions. This fueled the rapidly growing field of hepcidin therapeutics. Several hepcidin agonists and antagonists, as well as inducers and inhibitors of hepcidin expression have been identified to date. Some of them were further developed and are currently being evaluated in clinical trials. This review summarizes the state of the art.

Ionic Modulation of Bacterial Virulence and Its Role in Surgical Infection

Background: Bacterial virulence is a dynamic property of pathogens that is expressed in a context-dependent manner. For a bacterial pathogen, the expression of virulence is a tradeoff, as there is an energy cost that can disturb other functions. As a result, virulence is activated only when bacteria sense the need for it. Methods: Recent work from our laboratory has identified many of the local cues in the environmental context that activate bacterial virulence during surgical injury, resulting in bacterial invasion, tissue inflammation, and, in some cases, lethal sepsis. Results: After surgical injury, cytokines, opioids, and end-products of ischemia can activate bacterial virulence circuits, such as the quorum-sensing signaling system, directly. However, when key ions are present, such as phosphate and iron, certain pathogenic bacteria become insensitive to these incoming host cues. Conclusion: In this review, we provide molecular insight into the process by which certain surgical infections may be prevented by ionic modulation of the local microenvironment.

Modulation of Iron Metabolism in Response to Infection: Twists for All Tastes

Iron is an essential nutrient for almost all living organisms, but is not easily made available. Hosts and pathogens engage in a fight for the metal during an infection, leading to major alterations in the host’s iron metabolism. Important pathological consequences can emerge from the mentioned interaction, including anemia. Several recent reports have highlighted the alterations in iron metabolism caused by different types of infection, and several possible therapeutic strategies emerge, based on the targeting of the host’s iron metabolism. Here, we review the most recent literature on iron metabolism alterations that are induced by infection, the consequent development of anemia, and the potential therapeutic approaches to modulate iron metabolism in order to correct iron-related pathologies and control the ongoing 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.

Hepcidin Protects against Lethal Escherichia coli Sepsis in Mice Inoculated with Isolates from Septic Patients

Iron is an essential micronutrient for most microbes and their hosts. Mammalian hosts respond to infection by inducing the iron-regulatory hormone hepcidin, which causes iron sequestration and a rapid decrease in the plasma and extracellular iron concentration (hypoferremia). Previous studies showed that hepcidin regulation of iron is essential for protection from infection-associated mortality with the siderophilic pathogens Yersinia enterocolitica and Vibrio vulnificus However, the evolutionary conservation of the hypoferremic response to infection suggests that not only rare siderophilic bacteria but also common pathogens may be targeted by this mechanism. We tested 10 clinical isolates of Escherichia coli from children with sepsis and found that both genetic iron overload (by hepcidin-1 knockout [HKO]) and iatrogenic iron overload (by intravenous iron) potentiated infection with 8 out of the 10 studied isolates: after peritoneal injection of E. coli, iron-loaded mice developed sepsis with 60% to 100% mortality within 24 h, while control wild-type mice suffered 0% mortality. Using one strain for more detailed study, we show that iron overload allows rapid bacterial multiplication and dissemination. We further found that the presence of non-transferrin-bound iron (NTBI) in the circulation is more important than total plasma or tissue iron in rendering mice susceptible to infection and mortality. Postinfection treatment of HKO mice with just two doses of the hepcidin agonist PR73 abolished NTBI and completely prevented sepsis-associated mortality. We demonstrate that the siderophilic phenotype extends to clinically common pathogens. The use of hepcidin agonists promises to be an effective early intervention in patients with infections and dysregulated iron metabolism.

Low hepcidin in liver fibrosis and cirrhosis; a tale of progressive disorder and a case for a new biochemical marker

Liver fibrosis is a precursor of liver cirrhosis, which is associated with increased mortality. Though liver biopsy remains the gold standard for the diagnosis of fibrosis, noninvasive biochemical methods are cost-effective, practical and are not linked with major risks of complications. In this respect, serum hepcidin, has emerged as a new marker of fibrosis and cirrhosis. In this review the discussion uncovers molecular links between hepcidin disturbance and liver fibrosis/cirrhosis. The discussion also expands on clinical studies that suggest that hepcidin can potentially be used as a biochemical parameter of fibrosis/cirrhosis and target of therapeutic strategies to treat liver diseases. The debatable issues such as the complicated nature of hepcidin disturbance in non-alcoholic liver disease, serum levels of hepcidin in acute hepatitis C virus infection, cause of hepcidin disturbance in autoimmune hepatitis and hepatic insulin resistance are discussed, with potential solutions unveiled in order to be studied by future research.

Hepcidin agonists as therapeutic tools

Hepcidin agonists are a new class of compounds that regulate blood iron levels, limit iron absorption, and could improve the treatment of hemochromatosis, β-thalassemia, polycythemia vera, and other disorders in which disrupted iron homeostasis causes or contributes to disease. Hepcidin agonists also have the potential to prevent severe complications of siderophilic infections in patients with iron overload or chronic liver disease. This review highlights the preclinical studies that support the development of hepcidin agonists for the treatment of these disorders.

For when bacterial infections persist: Toll-like receptor-inducible direct antimicrobial pathways in macrophages

Macrophages are linchpins of innate immunity, responding to invading microorganisms by initiating coordinated inflammatory and antimicrobial programs. Immediate antimicrobial responses, such as NADPH-dependent reactive oxygen species (ROS), are triggered upon phagocytic receptor engagement. Macrophages also detect and respond to microbial products through pattern recognition receptors (PRRs), such as TLRs. TLR signaling influences multiple biological processes including antigen presentation, cell survival, inflammation, and direct antimicrobial responses. The latter enables macrophages to combat infectious agents that persist within the intracellular environment. In this review, we summarize our current understanding of TLR-inducible direct antimicrobial responses that macrophages employ against bacterial pathogens, with a focus on emerging evidence linking TLR signaling to reprogramming of mitochondrial functions to enable the production of direct antimicrobial agents such as ROS and itaconic acid. In addition, we describe other TLR-inducible antimicrobial pathways, including autophagy/mitophagy, modulation of nutrient availability, metal ion toxicity, reactive nitrogen species, immune GTPases (immunity-related GTPases and guanylate-binding proteins), and antimicrobial peptides. We also describe examples of mechanisms of evasion of such pathways by professional intramacrophage pathogens, with a focus on Salmonella, Mycobacteria, and Listeria. An understanding of how TLR-inducible direct antimicrobial responses are regulated, as well as how bacterial pathogens subvert such pathways, may provide new opportunities for manipulating host defence to combat infectious diseases.

Iron-related markers are associated with infection after liver transplantation

Though serum iron has been known to be associated with an increased risk of infection, hepcidin, the major regulator of iron metabolism, has never been systematically explored in this setting. Finding early biomarkers of infection, such as hepcidin, could help identify patients in whom early empiric antimicrobial therapy would be beneficial. We prospectively enrolled consecutive patients (n = 128) undergoing first-time, single-organ orthotopic liver transplantation (OLT) without known iron overload disorders at 2 academic hospitals in Boston from August 2009 to November 2012. Cox regression compared the associations between different iron markers and the development of first infection at least 1 week after OLT; 47 (37%) patients developed a primary outcome of infection at least 1 week after OLT and 1 patient died. After adjusting for perioperative bleeding complications, number of hospital days, and hepatic artery thrombosis, changes in iron markers were associated with the development of infection post-OLT including increasing ferritin (hazard ratio [HR], 1.51; 95% confidence interval [CI], 1.12-2.05), rising ferritin slope (HR, 1.10; 95% CI, 1.03-1.17), and increasing hepcidin (HR, 1.43; 95% CI, 1.05-1.93). A decreasing iron (HR, 1.76; 95% CI, 1.20-2.57) and a decreasing iron slope (HR, 4.21; 95% CI, 2.51-7.06) were also associated with subsequent infections. In conclusion, hepcidin and other serum iron markers and their slope patterns or their combination are associated with infection in vulnerable patient populations. Liver Transplantation 23 1541-1552 2017 AASLD.

Pulmonary Iron Homeostasis in Hepcidin Knockout Mice

Pulmonary iron excess is deleterious and contributes to a range of chronic and acute inflammatory diseases. Optimal lung iron concentration is maintained through dynamic regulation of iron transport and storage proteins. The iron-regulatory hormone hepcidin is also expressed in the lung. In order to better understand the interactions between iron-associated molecules and the hepcidin-ferroportin axis in lung iron balance, we examined lung physiology and inflammatory responses in two murine models of systemic iron-loading, either hepcidin knock-out (Hepc KO) or liver-specific hepcidin KO mice (Hepc KOliv), which do (Hepc KOliv) or do not (Hepc KO) express lung hepcidin. We have found that increased plasma iron in Hepc KO mice is associated with increased pulmonary iron levels, consistent with increased cellular iron uptake by pulmonary epithelial cells, together with an increase at the apical membrane of the cells of the iron exporter ferroportin, consistent with increased iron export in the alveoli. Subsequently, alveolar macrophages (AM) accumulate iron in a non-toxic form and this is associated with elevated production of ferritin. The accumulation of iron in the lung macrophages of hepcidin KO mice contrasts with splenic and hepatic macrophages which contain low iron levels as we have previously reported. Hepc KOliv mice with liver-specific hepcidin deficiency demonstrated same pulmonary iron overload profile as the Hepc KO mice, suggesting that pulmonary hepcidin is not critical in maintaining local iron homeostasis. In addition, the high iron load in the lung of Hepc KO mice does not appear to enhance acute lung inflammation or injury. Lastly, we have shown that intraperitoneal LPS injection is not associated with pulmonary hepcidin induction, despite high levels of inflammatory cytokines. However, intranasal LPS injection stimulates a hepcidin response, likely derived from AM, and alters pulmonary iron content in Hepc KO mice.

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.