Infection with Mycobacterium avium differentially regulates the expression of iron transport protein mRNA in murine peritoneal macrophages

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.

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.

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.

Genome-wide whole blood microRNAome and transcriptome analyses reveal miRNA-mRNA regulated host response to foodborne pathogen Salmonella infection in swine

To understand the role of miRNAs in regulating genes involved in host response to bacterial infection and shedding of foodborne pathogens, a systematic profiling of miRNAs and mRNAs from the whole blood of pigs upon Salmonella challenge was performed. A total of 62 miRNAs were differentially expressed post infection (false discovery rate <0.1). An integrative analysis of both the differentially expressed miRNAs and mRNAs using sequence-based miRNA target prediction and negative correlation of miRNA-mRNA profiles helped identify miRNA-mRNA networks that may potentially regulate host response to Salmonella infection. From these networks, miR-214 and miR-331-3p were identified as new candidates potentially associated with Salmonella infection. An miRNA seed sequence analysis suggested that these miRNAs regulate several critical immune-related genes including SLC11A1, PIGE-108A11.3 and VAV2. We showed that challenged pigs had reduced miR-214 expression and increased miR-331-3p expression in the whole blood. Furthermore, the expression of the proposed targets of miR-214 (SLC11A1 and PIGE-108A11.3) increased while that of the proposed target of miR-331-3p (VAV2) decreased following challenge (expression changes confirmed by in vitro assays). Based on these observations, we propose potential roles for miR-214 and miR-331-3p in regulation of immune responses to Salmonella infection.

Mycobacterium avium Complex Infection in a Patient with Sickle Cell Disease and Severe Iron Overload

A 34-year-old female with sickle cell anemia (hemoglobin SS disease) and severe iron overload presented to our institution with the subacute presentation of recurrent pain crisis, fever of unknown origin, pancytopenia, and weight loss. A CT scan demonstrated both lung and liver nodules concerning for granulomatous disease. Subsequent biopsies of the liver and bone marrow confirmed the presence of noncaseating granulomas and blood cultures isolated Mycobacterium avium complex MAC. Disseminated MAC is considered an opportunistic infection typically diagnosed in the immunocompromised and rarely in immunocompetent patients. An appreciable number of mycobacterial infection cases have been reported in sickle cell disease patients without immune dysfunction. It has been reported that iron overload is known to increase the risk for mycobacterial infection in vitro and in vivo studies. While iron overload is primarily known to cause end organ dysfunction, the clinical relationship with sickle cell disease and disseminated MAC infection has not been reported. Clinical iron overload is a common condition diagnosed in the sub-Saharan African population. High dietary iron, genetic defects in iron trafficking, as well as hemoglobinopathy are believed to be the etiologies for iron overload in this region. Patients with iron overload in this region were 17-fold more likely to die from Mycobacterium tuberculosis. Both experimental and clinical evidence suggest a possible link to iron overload and mycobacterial infections; however larger observational studies are necessary to determine true causality.

'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.

Natural history of SLC11 genes in vertebrates: tales from the fish world

BACKGROUND

The SLC11A1/Nramp1 and SLC11A2/Nramp2 genes belong to the SLC11/Nramp family of transmembrane divalent metal transporters, with SLC11A1 being associated with resistance to pathogens and SLC11A2 involved in intestinal iron uptake and transferrin-bound iron transport. Both members of the SLC11 gene family have been clearly identified in tetrapods; however SLC11A1 has never been documented in teleost fish and is believed to have been lost in this lineage during early vertebrate evolution. In the present work we characterized the SLC11 genes in teleosts and evaluated if the roles attributed to mammalian SLC11 genes are assured by other fish specific SLC11 gene members.

RESULTS

Two different SLC11 genes were isolated in the European sea bass (Dicentrarchus. labrax), and named slc11a2-α and slc11a2-β, since both were found to be evolutionary closer to tetrapods SLC11A2, through phylogenetic analysis and comparative genomics. Induction of slc11a2-α and slc11a2-β in sea bass, upon iron modulation or exposure to Photobacterium damselae spp. piscicida, was evaluated in in vivo or in vitro experimental models. Overall, slc11a2-α was found to respond only to iron deficiency in the intestine, whereas slc11a2-β was found to respond to iron overload and bacterial infection in several tissues and also in the leukocytes.

CONCLUSIONS

Our data suggests that despite the absence of slc11a1, its functions have been undertaken by one of the slc11a2 duplicated paralogs in teleost fish in a case of synfunctionalization, being involved in both iron metabolism and response to bacterial infection. This study provides, to our knowledge, the first example of this type of sub-functionalization in iron metabolism genes, illustrating how conserving the various functions of the SLC11 gene family is of crucial evolutionary importance.

Leprosy pathogenetic background: a review and lessons from other mycobacterial diseases

Leprosy is a disease caused by Mycobacterium leprae that initially affects the peripheral nervous system with patients exhibiting contrasting clinical, immunological, and pathological manifestations despite minimal genetic variation among bacilli isolates. Its clinical manifestations are related to M. leprae survival, innate and acquired immune responses, and interactions between host and bacterial proteins, preventing their invasion and infection, or promoting their development and pathogenesis. The complex molecular interactions in affected individuals influenced by the pathogenetic background will be explored in this review. However, the great genetic diversity imposes difficulty for understanding disease development, and it is likely that many factors and metabolic pathways regulating the immense and contrasting symptomatology will yet be revealed. Four pathways may play a central role in leprosy, including the TLR/LIR-7, VDR, TNF-alpha, and TGF-beta1 for which a large amount of gene polymorphisms have been described that could potentially affect the clinical outcome. Cross-talk pathways may significantly change the course of the disease, depending on the specific disequilibrium of genic homeostasis, which is highly dependent on the environment, antigens that are presented to the host cell, and specific polymorphisms that interact with other genes, external factors, and pathogen survival, culminating in leprosy occurrence. Currently, the microarray-based genomic survey of gene polymorphisms, multiple gene expression analyses, and proteomic technologies, such as mass spectrometry and phage display applied in the discovery of antigens, represent a great potential for evaluating individual responses of leprosy patients and contacts to predict the outcome and progression of the disease. At present, none of the genes is good prognostic marker; however, in the near future we may use multiple targets to predict infection and leprosy development.

The iron export protein ferroportin 1 is differentially expressed in mouse macrophage populations and is present in the mycobacterial-containing phagosome

Intracellular pathogens, including Mycobacterium tuberculosis, obtain iron from the host for their survival. Ferroportin 1 (FPN1; SLC40A1) is the sole iron exporter from mammalian cells and is expressed in the duodenum and macrophages. In the present study, we show that FPN1 mRNA levels in the mouse macrophage cell line RAW264.7 are synergistically induced by treatment with live or gamma-irradiated M. tuberculosis and IFN-gamma. FPN1 mRNA levels were also induced by Mycobacterium avium and IFN-gamma in RAW264.7 cells and the mouse alveolar macrophage cell line AMJ2-C8. Treatment of mouse resident peritoneal macrophages with M. tuberculosis and IFN-gamma resulted in a sixfold increase in FPN1 mRNA expression. In contrast, M. tuberculosis and IFN-gamma inhibited FPN1 mRNA expression in bone marrow-derived macrophages and lung macrophages, which have high basal levels of FPN1 mRNA expression. Using confocal microscopy, FPN1 protein localized rapidly to M. tuberculosis phagosomes after infection in RAW264.7 macrophages. In RAW264.7 cells expressing wild-type natural resistance-associated macrophage protein 1 (Nramp1(Gly169)), FPN1 and Nramp1 partially colocalized in late endosomes/lysosomes prior to infection. After 2 h of infection, Nramp1 and FPN1 were present in M. tuberculosis phagosomes. Our studies provide evidence for transcriptional regulation of FPN1 by pathogenic mycobacteria and IFN-gamma, which is dependent on the macrophage type. The trafficking of FPN1 to the M. tuberculosis phagosome suggests that it is involved in regulating iron availability to the mycobacteria in this locale.

Iron overload and immunity

Progress in the characterization of genes involved in the control of iron homeostasis in humans and in mice has improved the definition of iron overload and of the cells affected by it. The cell involved in iron overload with the greatest effect on immunity is the macrophage. Intriguing evidence has emerged, however, in the last 12 years indicating that parenchymal iron overload is linked to genes classically associated with the immune system. This review offers an update of the genes and proteins relevant to iron metabolism expressed in cells of the innate immune system, and addresses the question of how this system is affected in clinical situations of iron overload. The relationship between iron and the major cells of adaptive immunity, the T lymphocytes, will also be reviewed. Most studies addressing this last question in humans were performed in the clinical model of Hereditary Hemochromatosis. Data will also be reviewed demonstrating how the disruption of molecules essentially involved in adaptive immune responses result in the spontaneous development of iron overload and how they act as modifiers of iron overload.

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.

Hereditary hemochromatosis results in decreased iron acquisition and growth by Mycobacterium tuberculosis within human macrophages

Iron (Fe) acquisition is essential for the growth of intracellular Mycobacterium tuberculosis (M.tb). How this occurs is poorly understood. Hereditary hemochromatosis is an inherited disease in which most cells become overloaded with Fe. However, hereditary hemochromatosis macrophages have lower than normal levels of intracellular Fe. This suggests M.tb growth should be slower in those cells if macrophage intracellular Fe is used by M.tb. Therefore, we compared trafficking and acquisition of transferrin (Tf)- and lactoferrin (Lf)-chelated Fe by M.tb within the phagosome of monocyte-derived macrophages (MDM) from healthy controls and subjects with hereditary hemochromatosis. M.tb in both sets of macrophages acquired more Fe from Lf than Tf. Fe acquisition by M.tb within hereditary hemochromatosis macrophages was decreased by 84% from Tf and 92% from Lf relative to that in healthy control macrophages. There was no difference in Fe acquired from Tf and Lf by the two macrophage phenotypes. Both acquired 3 times more Fe from Lf than Tf. M.tb infection and incubation with interferon gamma (IFN-gamma) reduced macrophage Fe acquisition by 20% and 50%, respectively. Both Tf and Lf colocalized with M.tb phagosomes to a similar extent, independent of macrophage phenotype. M.tb growth was 50% less in hereditary hemochromatosis macrophages. M.tb growing within macrophages from subjects with hereditary hemochromatosis acquire less Fe compared with healthy controls. This is associated with reduced growth of M.tb. These data support a role for macrophage intracellular Fe as a source for M.tb growth.

Function and mechanism of action of Dictyostelium Nramp1 (Slc11a1) in bacterial infection

Dictyostelium amoebae are professional phagocytes, which ingest bacteria as the principal source of food. We have cloned the Dictyostelium homologue of human natural resistance-associated membrane protein 1 (Nramp1) [solute carrier family 11 member 1 (Slc11a1)], an endo-lysosomal membrane protein that confers on macrophages resistance to infection by a variety of intracellular bacteria and protozoa. The Dictyostelium Nramp1 gene encodes a protein of 53 kDa with 11 putative transmembrane domains. The Nramp1 gene is transcribed during the growth-phase and downregulated to barely detectable levels upon starvation. To gain insights into their intracellular localization, we fused Nramp1 or the vatB subunit of the V-H(+)ATPase with green fluorescent protein and expressed in cells. Green fluorescent protein-vatB was inserted in membranes of all acidic compartments and the contractile vacuole network and decorated macropinosomes and phagosomes. Green fluorescent protein-Nramp1 decorated macropinosomes and phagosomes, in addition to intracellular vesicular compartments positive for endosomal SNARE protein Vti1 or vacuolin, a marker of the exocytic pathway. Nramp1 disruption generated mutants that were more permissive hosts than wild-type cells for intracellular growth of Legionella pneumophila and Micobacterium avium. Nramp1 overexpression protected cells from L. pneumophila infection. Evidence is provided that Nramp1 transports metal cations out of the phagolysosome in an ATP-dependent process and that L. pneumophila and M. avium use different mechanisms to neutralize Nramp1 activity.

Modulation of Iron Availability at the Host-Pathogen Interface in Phagocytic Cells

This review summarizes recent data on iron metabolism in macrophages, with a special emphasis on possible bacteriostatic and bactericidal consequences for intracellular pathogens. It includes the role of biological chelators and transporters in normal macrophage physiology and antimicrobial defense. Iron is an essential metal cofactor for many biochemical pathways in mammals. However, excess iron promotes the formation of cytotoxic oxygen derivatives so that systemic iron levels must be tightly regulated. The mechanism of iron recycling by macrophages including iron efflux from erythrocyte-containing phagosomes, iron release from macrophages, and entry into the transferrin (Tf) cycle remain poorly understood. Ferroportin expression in the liver, spleen, and bone marrow cells appears to be restricted to macrophages. Mutant mice bearing a conditional deletion of the ferroportin gene in macrophages show retention of iron by hepatic Kupffer cells and splenic macrophages. Hepcidin is induced by lipopolysaccharide (LPS) in mouse spleens and splenic macrophage in vitro and appears to mediate the LPS-induced down-regulation of ferroportin in the intestine and in splenic macrophages, suggesting that inflammatory agents may regulate iron metabolism through modulation of ferroportin expression. The host transporter Nramp1 may compete directly with bacterial divalent-metal transport systems for the acquisition of divalent metals within the phagosomal space. The ultimate outcome of these competing interactions influences the ability of pathogens to survive and replicate intracellularly. This seems particularly relevant to the Salmonella, Leishmania, and Mycobacterium spp., in which inactivating mutations in Nramp1 abrogate the natural resistance of macrophages to these pathogens.

Iron modulates the replication of virulent Mycobacterium bovis in resting and activated bovine and possum macrophages

Bovine and possum macrophages were infected in vitro with a virulent strain of Mycobacterium bovis, and mycobacterial replication was measured in the infected macrophages cultured under a variety of conditions. Virulent M. bovis replicated substantially in alveolar possum macrophages as well as in bovine blood monocyte-derived macrophages. Addition of recombinant bovine interferon-gamma (IFN-gamma) with low concentrations of lipopolysaccharide (LPS) rendered bovine macrophages significantly more resistant to M. bovis replication. Disruption of iron levels in infected macrophages by addition of apotransferrin or bovine lactoferrin blocked replication of M. bovis in both bovine and possum macrophages. On the other hand, addition of exogenous iron, either in the form of iron citrate or iron-saturated transferrin, rendered macrophages of both species much more permissive for the replication of M. bovis. The impact of iron deprivation/loading on the mycobacteriostatic activity of cells was independent of nitric-oxide release, as well as independent of the generation of oxygen radical species in both possum and bovine macrophages. Exogenous iron was shown to reverse the ability of IFN-gamma/LPS pulsed bovine macrophages to restrict M. bovis replication. When autologous possum lymphocytes from animals vaccinated with M. bovis strain BCG were added to infected macrophages, they rendered the macrophages less permissive for virulent M. bovis replication. Loading the cells with iron prior to this macrophage-lymphocyte interaction, reversed this immune effect induced by sensitized cells. We conclude that, in two important animal species, intracellular iron level plays an important role in M. bovis replication in macrophages, irrespective of their activation status.

Changes of the phagosomal elemental concentrations by Mycobacterium tuberculosis Mramp

Pathogenic mycobacteria survive within phagosomes which are thought to represent a nutrient-restricted environment. Divalent cation transporters of the Nramp family in phagosomes and mycobacteria (Mramp) may compete for metals that are crucial for bacterial survival. The elemental concentrations in phagosomes of macrophages infected with wild-type Mycobacterium tuberculosis (M. tuberculosis strain H37Rv) and a M. tuberculosis Mramp knockout mutant (Mramp-KO), derived from a clinical isolate isogenic to the strain MT103, were compared. Time points of 1 and 24 h after infection of mouse peritoneal macrophages (bcg(S)) were compared in both cases. Increased concentrations of P, Ni and Zn and reduced Cl concentration in Mramp-KO after 1 h of infection were observed, compared to M. tuberculosis vacuoles. After 24 h of infection, significant differences in the P, Cl and Zn concentrations were still present. The Mramp-KO phagosome showed a significant increase of P, Ca, Mn, Fe and Zn concentrations between 1 and 24 h after infection, while the concentrations of K and Ni decreased. In the M. tuberculosis vacuole, the Fe concentration showed a similar increase, while the Cl concentration decreased. The fact that the concentration of several divalent cations increased in the Mramp-KO strain suggests that Mramp may have no impact on the import of these divalent cations into the mycobacterium, but may function as a cation efflux pump. The concordant increase of Fe concentrations within M. tuberculosis, as well as within the Mramp-KO vacuoles, implies that Mramp, in contrast to siderophores, might not be important for the attraction of Fe and its retention in phagosomes of unstimulated macrophages.

Elemental Analysis ofMycobacterium avium-,Mycobacterium tuberculosis-, andMycobacterium smegmatis-Containing Phagosomes Indicates Pathogen-Induced Microenvironments within the Host Cell’s Endosomal System

Mycobacterium avium and Mycobacterium tuberculosis are human pathogens that infect and replicate within macrophages. Both organisms live in phagosomes that fail to fuse with lysosomes and have adapted their lifestyle to accommodate the changing environment within the endosomal system. Among the many environmental factors that could influence expression of bacterial genes are the concentrations of single elements within the phagosomes. We used a novel hard x-ray microprobe with suboptical spatial resolution to analyze characteristic x-ray fluorescence of 10 single elements inside phagosomes of macrophages infected with M. tuberculosis and M. avium or with avirulent M. smegmatis. The iron concentration decreased over time in phagosomes of macrophages infected with Mycobacterium smegmatis but increased in those infected with pathogenic mycobacteria. Autoradiography of infected macrophages incubated with (59)Fe-loaded transferrin demonstrated that the bacteria could acquire iron delivered via the endocytic route, confirming the results obtained in the x-ray microscopy. In addition, the concentrations of chlorine, calcium, potassium, manganese, copper, and zinc were shown to differ between the vacuole of pathogenic mycobacteria and M. smegmatis. Differences in the concentration of several elements between M. avium and M. tuberculosis vacuoles were also observed. Activation of macrophages with recombinant IFN-gamma or TNF-alpha before infection altered the concentrations of elements in the phagosome, which was not observed in cells activated following infection. Siderophore knockout M. tuberculosis vacuoles exhibited retarded acquisition of iron compared with phagosomes with wild-type M. tuberculosis. This is a unique approach to define the environmental conditions within the pathogen-containing compartment.

In vitro response of the striped bass natural resistance-associated macrophage protein, Nramp, to LPS and Mycobacterium marinum exposure

Mycobacteriosis in Chesapeake Bay (USA) striped bass Morone saxatilis is an ongoing disease problem with important economic implications for a large commercial and recreational fishery. Additionally, striped bass serve as a reservoir of potential mycobacterial zoonoses. Recently, we described a striped bass gene homolog of the natural resistance-associated macrophage protein family (MsNramp), which is responsible for resistance to mycobacterial infections in mice. Striped bass MsNramp is strongly induced in peritoneal exudate cells (PE) in vivo after intraperitoneal injection with Mycobacterium spp. The purpose of the present study was to investigate short-term in vitro MsNramp expression and reactive oxygen intermediate (ROI) production in primary cultures of adherent PE after exposure to bacterial lipopolysaccharide (LPS), or live- or heat-killed (HK) Mycobacterium marinum. PE expressed significantly higher levels of MsNramp at 4 and 24 h post-treatment with live and HK M. marinum. MsNramp response to LPS was dose-dependent in these cells, with maximum expression at 4 h and 20 microg/ml LPS. Treatment of PE with LPS resulted in increased intracellular superoxide anion levels, whereas treatment with live M. marinum caused a significant depression. This study is the first report of induction of a teleost Nramp in vitro by mycobacteria, and supports findings of teleost Nramp induction by LPS.

The nature of extracellular iron influences iron acquisition by Mycobacterium tuberculosis residing within human macrophages

We have reported that Mycobacterium tuberculosis residing within the phagosomes of human monocyte-derived macrophages (MDM) can acquire Fe from extracellular transferrin (TF) and sources within the MDM. In the lung, Fe is also bound to lactoferrin (LF) and low-molecular-weight chelates. We therefore investigated the ability of intraphagosomal M. tuberculosis to acquire Fe from these sources. M. tuberculosis acquired 30-fold and 3-fold more Fe from LF and citrate, respectively, compared to TF, in spite of similar MDM-associated Fe. M. tuberculosis infection decreased MDM-associated Fe relative to uninfected MDM as follows: TF (38.7%), citrate (21.1%), and LF (15.3%). M. tuberculosis Fe acquisition from extracellular chelates (exogenous source) and from endogenous MDM Fe initially acquired from the three chelates (endogenous source) was compared. M. tuberculosis Fe acquisition was similar from exogenous and endogenous sources supplied as Fe-TF. In contrast, there was much greater intracellular M. tuberculosis Fe uptake from LF and citrate from the exogenous than endogenous source. Gamma interferon (IFN-gamma) reduced MDM Fe uptake from each chelate by approximately 50% and augmented the M. tuberculosis-induced decrease in MDM Fe uptake from exogenous TF, but not from LF or citrate. IFN-gamma minimally decreased intracellular M. tuberculosis Fe acquisition from exogenous Fe-TF but significantly increased Fe uptake from LF and citrate. Intraphagosomal M. tuberculosis Fe acquisition from both exogenous and endogenous MDM sources, and the effect of IFN-gamma on this process, is influenced by the nature of the extracellular Fe chelate. M. tuberculosis has developed efficient mechanisms of acquiring Fe from a variety of Fe chelates that it likely encounters within the human lung.

SLC11 family of H+-coupled metal-ion transporters NRAMP1 and DMT1

NRAMP1 (natural resistance-associated macrophage protein-1) and DMT1 (divalent metal-ion transporter-1) make up the SLC11 gene family of metal-ion transporters that are energized by the H(+) electrochemical gradient. Long known to confer resistance to bacterial infection, NRAMP1 functions at the phagolysosomal membrane of macrophages and neutrophils. NRAMP1 most likely contributes to macrophage antimicrobial function by extruding essential metal ions (including Mn(2+)) from the phagolysosome via H(+)/metal-ion cotransport. An alternative hypothesis in the literature proposes that NRAMP1 concentrate Fe(2+) within the phagolysosome by means of H(+)/Fe(2+) antiport, resulting in the generation of toxic free radicals. DMT1 is expressed widely and accepts as substrates a broad range of transition metal ions, among which Fe(2+) is transported with high affinity ( K(0.5) approximately 2 microM). DMT1 accounts both for the intestinal absorption of free Fe(2+) and for transferrin-associated endosomal Fe(2+) transport in erythroid precursors and many other cell types. DMT1 is up-regulated dramatically in the intestine by dietary iron restriction and, despite high serum iron levels, is not appropriately down-regulated in hereditary hemochromatosis.

Photoperiodic adjustments in immune function protect Siberian hamsters from lethal endotoxemia

Seasonal changes in day length enhance or suppress components of immune function in individuals of several mammalian species. Siberian hamsters (Phodopus sungorus) exhibit multiple changes in neuroendocrine, reproductive, and immune function after exposure to short days. The manner in which these changes are integrated into the host response to pathogens is not well understood. The present experiments tested the hypothesis that short-day changes in immune function alter the pathogenesis of septic shock and survival after challenge with endotoxin. Male and female Siberian hamsters raised in long-day photoperiods were transferred as adults to short days or remained in their natal photoperiod. Six to 8 weeks later, hamsters were injected i.p. with 0, 1, 2.5, 10, 25, or 50 mg/kg bacterial lipopolysaccharide (LPS) (the biologically active constituent of endotoxin), and survival was monitored for 96 h. Short days significantly improved survival of male hamsters treated with 10 or 25 mg/kg LPS and improved survival in females treated with 50 mg/kg LPS. Transfer from long to short days shifted the LD50 in males by approximately 90%, from 5.3 to 9.9 mg/kg, and in females from 11.1 to 15.0 mg/kg (+35%). Long-day females were more resistant than were males to lethal endotoxemia. In vitro production of the proinflammatory cytokine TNFalpha in response to LPS stimulation was significantly lower in macrophages extracted from short-day relative to long-day hamsters, as were circulating concentrations of TNFalpha in vivo after i.p. administration of LPS, suggesting that diminished cytokine responses to LPS in short days may mitigate the lethality of endotoxemia. Adaptation to short days induces changes in immune parameters that affect survival in the face of immune challenges.

Intraphagosomal Mycobacterium tuberculosis acquires iron from both extracellular transferrin and intracellular iron pools. Impact of interferon-gamma and hemochromatosis

Mycobacterium tuberculosis multiplies within the macrophage phagosome and requires iron for growth. We examined the route(s) by which intracellular M. tuberculosis acquires iron. During intracellular growth of the virulent Erdman M. tuberculosis strain in human monocyte-derived macrophages (MDM), M. tuberculosis acquisition of (59)Fe from transferrin (TF) provided extracellularly (exogenous source) was compared with acquisition when MDM were loaded with (59)Fe from TF prior to M. tuberculosis infection (endogenous sources). M. tuberculosis (59)Fe acquisition required viable bacteria and was similar from exogenous and endogenous sources at 24 h and greater from exogenous iron at 48 h. Interferon-gamma treatment of MDM reduced (59)Fe uptake from TF 51% and TF receptor expression by 34%. Despite this, intraphagosomal M. tuberculosis iron acquisition in IFN-gamma-treated cells was decreased by only 30%. Macrophages from hereditary hemochromatosis patients have altered iron metabolism. Intracellular M. tuberculosis acquired markedly less iron in MDM from these individuals than in MDM from healthy donors, regardless of the iron source (exogenous and endogenous): 36 +/- 3.8% and 17 +/- 9.6% of control, respectively. Thus, intraphagosomal M. tuberculosis can acquire iron from both extracellular TF and endogenous macrophage sources. Acquisition of iron from macrophage cytoplasmic iron pools may be critical for the intracellular growth of M. tuberculosis. This acquisition is altered by IFN-gamma treatment to a small extent, but is markedly reduced in macrophages from hemochromatosis patients.

Role of the ferroportin iron-responsive element in iron and nitric oxide dependent gene regulation

The newly described iron transporter, ferroportin (MTP1, IREG1), is expressed in a variety of tissues including the duodenum and cells of the mononuclear phagocyte system (MPS). In the MPS, ferroportin is hypothesized to be a major exporter of iron scavenged from senescent erythrocytes. Changes in iron metabolism, including the sequestration of iron in the MPS, are characteristic of both acute and chronic inflammation and these conditions induce changes in ferroportin expression. In a mouse model of acute inflammation, LPS administration is associated with reduced MPS ferroportin protein and mRNA expression. In addition, the ferroportin 5' UTR also has an iron-responsive element that binds to the iron-response proteins, but whether there is a role for this IRE in inflammation induced regulation of ferroportin has been unclear. A luciferase reporter gene under the control of the mouse ferroportin promoter and 5' UTR was used to determine if this 5' UTR conferred IRE-dependent regulation on this reporter gene. Stimulation of reporter gene transfected RAW 264.7 cells (a mouse macrophage cell line) with LPS resulted in IRE-dependent inhibition of luciferase production. Inhibitors of nitric oxide synthase abrogated the IRE-dependent effect of LPS. In addition, direct treatment of RAW 264.7 and with NO donor S-nitroso-N-acetylpenicillamine resulted in IRE-dependent down-regulation of luciferase expression. The effect of NO was consistent with IRP1/IRE mediated translation block. There are most likely both inflammation-mediated transcriptional and post-transcriptional (IRE-dependent) mechanisms for inhibiting ferroportin expression in MPS cells.

Multivalent metal-induced iron acquisition from transferrin and lactoferrin by myeloid cells

We previously described a unique, high-capacity, ATP-independent mechanism through which myeloid cells acquire Fe from low-m.w. chelates. The rate of this Fe acquisition is markedly increased by cellular exposure to multivalent metal cations. Because most Fe in vivo is bound to transferrin or lactoferrin, we examined whether this mechanism also contributes to myeloid cell acquisition of Fe from transferrin and/or lactoferrin. Using HL-60 cells as a model system, we show cellular acquisition of (59)Fe from both lactoferrin and transferrin that was unaffected by conditions that depleted the cells of ATP or disrupted their cytoskeleton. Fe acquisition was dramatically increased by cell exposure to various metals including Ga(3+), Gd(3+), Al(3+), Fe(3+), La(3+), Zr(4+), Sn(4+), Cu(2+), and Zn(2+) by a process that was reversible. Exposure to these same metals also increased binding of both transferrin and lactoferrin to the cell surface by a process that does not appear to involve the well-described plasma membrane receptor for transferrin. Approximately 60% of the Fe acquired by the cells from transferrin and lactoferrin remained cell associated 18 h later. HL-60 cells possess a high-capacity multivalent metal-inducible mechanism for Fe acquisition from transferrin and lactoferrin that bears many similarities to the process previously described that allows these and other cell types to acquire Fe from low-m.w. Fe chelates. The biologic importance of this mechanism may relate to its high Fe acquisition capacity and the speed with which it is able to rapidly adapt to the level of extracellular Fe.