Functional expression of Nramp1 in vitro in the murine macrophage line RAW264.7

Molecular Mechanism of Nramp-Family Transition Metal Transport

The Natural resistance-associated macrophage protein (Nramp) family of transition metal transporters enables uptake and trafficking of essential micronutrients that all organisms must acquire to survive. Two decades after Nramps were identified as proton-driven, voltage-dependent secondary transporters, multiple Nramp crystal structures have begun to illustrate the fine details of the transport process and provide a new framework for understanding a wealth of preexisting biochemical data. Here we review the relevant literature pertaining to Nramps' biological roles and especially their conserved molecular mechanism, including our updated understanding of conformational change, metal binding and transport, substrate selectivity, proton transport, proton-metal coupling, and voltage dependence. We ultimately describe how the Nramp family has adapted the LeuT fold common to many secondary transporters to provide selective transition-metal transport with a mechanism that deviates from the canonical model of symport.

Genetics of Immune Disease in the Horse

Host defenses against infection by viruses, bacteria, fungi, and parasites are critical to survival. It has been estimated that upwards of 7% of the coding genes of mammals function in immunity and inflammation. This high level of genomic investment in defense has resulted in an immune system characterized by extraordinary complexity and many levels of redundancy. Because so many genes are involved with immunity, there are many opportunities for mutations to arise that have negative effects. However, redundancy in the mammalian defense system and the adaptive nature of key immune mechanisms buffer the untoward outcomes of many such deleterious mutations.

Transmembrane helix 6b links proton and metal release pathways and drives conformational change in an Nramp-family transition metal transporter

The natural resistance-associated macrophage protein (Nramp) family encompasses transition metal and proton cotransporters that are present in many organisms from bacteria to humans. Recent structures of Deinococcus radiodurans Nramp (DraNramp) in multiple conformations revealed the intramolecular rearrangements required for alternating access of the metal-binding site to the external or cytosolic environment. Here, using recombinant proteins and metal transport and cysteine accessibility assays, we demonstrate that two parallel cytoplasm-accessible networks of conserved hydrophilic residues in DraNramp, one lining the wide intracellular vestibule for metal release and the other forming a narrow proton transport pathway, are essential for metal transport. We further show that mutagenic or posttranslational modifications of transmembrane helix (TM) 6b, which structurally links these two pathways, impede normal conformational cycling and metal transport. TM6b contains two highly conserved histidines, His²³² and His²³⁷ We found that different mutagenic perturbations of His²³², just below the metal-binding site along the proton exit route, differentially affect DraNramp's conformational state, suggesting that His²³² serves as a pivot point for conformational changes. In contrast, any replacement of His²³⁷, lining the metal exit route, locked the transporter in a transport-inactive outward-closed state. We conclude that these two histidines, and TM6b more broadly, help trigger the bulk rearrangement of DraNramp to the inward-open state upon metal binding and facilitate return of the empty transporter to an outward-open state upon metal release.

Human Macrophages Clear the Biovar Microtus Strain of Yersinia pestis More Efficiently Than Murine Macrophages

Yersinia pestis is the etiological agent of the notorious plague that has claimed millions of deaths in history. Of the four known Y. pestis biovars (Antiqua, Medievalis, Orientalis, and Microtus), Microtus strains are unique for being highly virulent in mice but avirulent in humans. Here, human peripheral lymphocytes were infected with the fully virulent 141 strain or the Microtus strain 201, and their transcriptomes were determined and compared. The most notable finding was that robust responses in the pathways for cytokine-cytokine receptor interaction, chemokine signaling pathway, Toll-like receptor signaling and Jak-STAT signaling were induced at 2 h post infection (hpi) in the 201- but not the 141-infected lymphocytes, suggesting that human lymphocytes might be able to constrain infections caused by strain 201 but not 141. Consistent with the transcriptome results, much higher IFN-γ and IL-1β were present in the supernatants from the 201-infected lymphocytes, while inflammatory inhibitory IL-10 levels were higher in the 141-infected lymphocytes. The expressions of CSTD and SLC11A1, both of which are functional components of the lysosome, increased in the 201-infected human macrophage-like U937 cells. Further assessment of the survival rate of the 201 bacilli in the U937 cells and murine macrophage RAW 264.7 cells revealed no viable bacteria in the U937 cells at 32 hpi.; however, about 5–10% of the bacteria were still alive in the RAW264.7 cells. Our results indicate that human macrophages can clear the intracellular Y. pestis 201 bacilli more efficiently than murine macrophages, probably by interfering with critical host immune responses, and this could partially account for the host-specific pathogenicity of Y. pestis Microtus strains.

DMT1 and iron transport

Many past and recent advances in the field of iron metabolism have relied upon the discovery of divalent metal transporter 1, DMT1 in 1997. DMT1 is the major iron transporter and contributes non-heme iron uptake in most types of cell. Each DMT1 isoform exhibits different expression patterns in cell-type specificity and distinct subcellular distribution, which enables cells to uptake both transferrin-bound and non-transferrin-bound irons efficiently. DMT1 expression is regulated by iron through the translational and degradation pathways to ensure iron homeostasis. It is considered that mammalian iron transporters including DMT1 cannot transport ferric iron but ferrous iron. Being reduced to ferrous state is likely to damage cells and tissues through the production of reactive oxygen species. Recently, iron chaperones have been identified, which can provide an answer to how ferrous iron is transported safely in cytosol. We summarize DMT1 expression depending on the types of cell or tissue and the function and mechanism of one of the iron chaperones, PCBP2.

c-Src kinase is involved in the tyrosine phosphorylation and activity of SLC11A1 in differentiating macrophages

Studies have demonstrated that the solute carrier family 11 member 1 (SLC11A1) is heavily glycosylated and phosphorylated in macrophages. However, the mechanisms of SLC11A1 phosphorylation, and the effects of phosphorylation on SLC11A1 activity remain largely unknown. Here, the tyrosine phosphorylation of SLC11A1 is observed in SLC11A1-expressing U937 cells when differentiated into macrophages by phorbol myristate acetate (PMA). The phosphorylation of SLC11A1 is almost completely blocked by treatment with PP2, a selective inhibitor of Src family kinases. Furthermore, we found that SLC11A1 is a direct substrate for active c-Src kinase and siRNA-mediated knockdown of cellular Src (c-Src) expression results in a significant decrease in tyrosine phosphorylation. We found that PMA induces the interaction of SLC11A1 with c-Src kinase. We demonstrated that SLC11A1 is phosphorylated by Src family kinases at tyrosine 15 and this type of phosphorylation is required for SLC11A1-mediated modulation of NF-κB activation and nitric oxide (NO) production induced by LPS. Our results demonstrate important roles for c-Src tyrosine kinase in phosphorylation and activation of SLC11A1 in macrophages.

The Use of CRISPR/Cas9 Gene Editing to Confirm Congenic Contaminations in Host-Pathogen Interaction Studies

Murine models of Salmonella enterica serovar Typhimurium infection are one of the commonest tools to study host-pathogen interactions during bacterial infections. Critically, the outcome of S. Typhimurium infection is impacted by the genetic background of the mouse strain used, with macrophages from C57BL/6 and BALB/c mice lacking the capacity to control intracellular bacterial replication. For this reason, the use of congenic strains, which mix the genetic backgrounds of naturally protected mouse strains with those of susceptible strains, has the capacity to significantly alter results and interpretation of S. Typhimurium infection studies. Here, we describe how macrophage knockout cell lines generated by CRISPR/Cas9 gene editing can help determine the contribution of background contaminations in the phenotypes of primary macrophages from congenic mice, on the outcome of S. Typhimurium infection studies. Our own experience illustrates how the CRISPR/Cas9 technology can be used to complement pre-existing knockout models, and shows that there is great merit in performing concurrent studies with both genetic models, to exclude unanticipated side-effects on host-pathogen interactions.

High-Throughput Quantification of Bacterial-Cell Interactions Using Virtual Colony Counts

The quantification of bacteria in cell culture infection models is of paramount importance for the characterization of host-pathogen interactions and pathogenicity factors involved. The standard to enumerate bacteria in these assays is plating of a dilution series on solid agar and counting of the resulting colony forming units (CFU). In contrast, the virtual colony count (VCC) method is a high-throughput compatible alternative with minimized manual input. Based on the recording of quantitative growth kinetics, VCC relates the time to reach a given absorbance threshold to the initial cell count using a series of calibration curves. Here, we adapted the VCC method using the model organism Salmonella enterica sv. Typhimurium (S. Typhimurium) in combination with established cell culture-based infection models. For HeLa infections, a direct side-by-side comparison showed a good correlation of VCC with CFU counting after plating. For MDCK cells and RAW macrophages we found that VCC reproduced the expected phenotypes of different S. Typhimurium mutants. Furthermore, we demonstrated the use of VCC to test the inhibition of Salmonella invasion by the probiotic E. coli strain Nissle 1917. Taken together, VCC provides a flexible, label-free, automation-compatible methodology to quantify bacteria in in vitro infection assays.

Nramp1 and NrampB Contribute to Resistance against Francisella in Dictyostelium

The Francisella genus comprises highly pathogenic bacteria that can cause fatal disease in their vertebrate and invertebrate hosts including humans. In general, Francisella growth depends on iron availability, hence, iron homeostasis must be tightly regulated during Francisella infection. We used the system of the professional phagocyte Dictyostelium and the fish pathogen F. noatunensis subsp. noatunensis (F.n.n.) to investigate the role of the host cell iron transporters Nramp (natural resistance associated macrophage proteins) during Francisella infection. Like its mammalian ortholog, Dictyostelium Nramp1 transports iron from the phagosome into the cytosol, whereas the paralog NrampB is located on the contractile vacuole and controls, together with Nramp1, the cellular iron homeostasis. In Dictyostelium, Nramp1 localized to the F.n.n.-phagosome but disappeared from the compartment dependent on the presence of IglC, an established Francisella virulence factor. In the absence of Nramp transporters the bacteria translocated more efficiently from the phagosome into the host cell cytosol, its replicative niche. Increased escape rates coincided with increased proteolytic activity in bead-containing phagosomes indicating a role of the Nramp transporters for phagosomal maturation. In the nramp mutants, a higher bacterial load was observed in the replicative phase compared to wild-type host cells. Upon bacterial access to the cytosol of wt cells, mRNA levels of bacterial iron uptake factors were transiently upregulated. Decreased iron levels in the nramp mutants were compensated by a prolonged upregulation of the iron scavenging system. These results show that Nramps contribute to host cell immunity against Francisella infection by influencing the translocation efficiency from the phagosome to the cytosol but not by restricting access to nutritional iron in the cytosol.

Robust growth of avirulent phase II Coxiella burnetii in bone marrow-derived murine macrophages

Published data show that murine bone marrow-derived macrophages (BMDM) restrict growth of avirulent phase II, but not virulent phase I, Coxiella burnetii. Growth restriction of phase II bacteria is thought to result from potentiated recognition of pathogen-associated molecular patterns, which leads to production of inhibitory effector molecules. Past studies have used conditioned medium from L-929 murine fibroblasts as a source of macrophage-colony stimulating factor (M-CSF) to promote differentiation of bone marrow-derived myeloid precursors into macrophages. However, uncharacterized components of conditioned medium, such as variable amounts of type I interferons, can affect macrophage activation status and their permissiveness for infection. In the current study, we show that the C. burnetii Nine Mile phase II (NMII) strain grows robustly in primary macrophages from C57BL/6J mice when bone marrow cells are differentiated with recombinant murine M-CSF (rmM-CSF). Bacteria were readily internalized by BMDM, and replicated within degradative, LAMP1-positive vacuoles to achieve roughly 3 logs of growth over 6 days. Uninfected BMDM did not appreciably express CD38 or Egr2, markers of classically (M1) and alternatively (M2) activated macrophages, respectively, nor did infection change the lack of polarization. In accordance with an M0 phenotype, infected BMDM produced moderate amounts of TNF and nitric oxide. Similar NMII growth results were obtained using C57BL/6J myeloid progenitors immortalized with an estrogen-regulated Hoxb8 (ER-Hoxb8) oncogene. To demonstrate the utility of the ER-Hoxb8 system, myeloid progenitors from natural resistance-associated macrophage protein 1 (Nramp1) C57BL/6J knock-in mice were transduced with ER-Hoxb8, and macrophages were derived from immortalized progenitors using rmM-CSF and infected with NMII. No difference in growth was observed when compared to macrophages from wild type mice, indicating depletion of metal ions by the Nramp1 transporter does not negatively impact NMII growth. Results with NMII were recapitulated in primary macrophages where C57BL/6J Nramp1+ BMDM efficiently killed Salmonella enterica serovar Typhimurium. M-CSF differentiated murine macrophages from bone marrow and conditional ER-Hoxb8 myeloid progenitors will be useful ex vivo models for studying Coxiella-macrophage interactions.

RNA-seq Brings New Insights to the Intra-Macrophage Transcriptome of Salmonella Typhimurium

Salmonella enterica serovar Typhimurium is arguably the world’s best-understood bacterial pathogen. However, crucial details about the genetic programs used by the bacterium to survive and replicate in macrophages have remained obscure because of the challenge of studying gene expression of intracellular pathogens during infection. Here, we report the use of deep sequencing (RNA-seq) to reveal the transcriptional architecture and gene activity of Salmonella during infection of murine macrophages, providing new insights into the strategies used by the pathogen to survive in a bactericidal immune cell. We characterized 3583 transcriptional start sites that are active within macrophages, and highlight 11 of these as candidates for the delivery of heterologous antigens from Salmonella vaccine strains. A majority (88%) of the 280 S. Typhimurium sRNAs were expressed inside macrophages, and SPI13 and SPI2 were the most highly expressed pathogenicity islands. We identified 31 S. Typhimurium genes that were strongly up-regulated inside macrophages but expressed at very low levels during in vitro growth. The SalComMac online resource allows the visualisation of every transcript expressed during bacterial replication within mammalian cells. This primary transcriptome of intra-macrophage S.-Typhimurium describes the transcriptional start sites and the transcripts responsible for virulence traits, and catalogues the sRNAs that may play a role in the regulation of gene expression during infection.

The burden of Salmonellosis remains unacceptably high throughout the world and control measures have had limited success. Because Salmonella bacteria can be transmitted from the wider environment to animals and humans, the bacteria encounter diverse environments that include food, water, plant surfaces and the extracellular and intracellular phases of infection of eukaryotic hosts. An intricate transcriptional network has evolved to respond to a variety of environmental signals and control the “right time/ right place” expression of virulence genes. To understand how transcription is rewired during intracellular infection, we determined the primary transcriptome of Salmonella enterica serovar Typhimurium within murine macrophages. We report the coding genes, sRNAs and transcriptional start sites that are expressed within macrophages at 8 hours after infection, and use these to infer gene function. We identified gene promoters that are specifically expressed within macrophages and could drive the intracellular delivery of antigens by S. Typhimurium vaccine strains. These data contribute to our understanding of the mechanisms used by Salmonella to regulate virulence gene expression whilst replicating inside mammalian cells.

Salmonella enterica infection stimulates macrophages to hemophagocytose

Hemophagocytes are cells of the monocyte lineage that have engulfed erythrocytes and leukocytes. Hemophagocytes frequently accumulate in patients with severe acute bacterial infections, such as those caused by Salmonella enterica, Brucella abortus, and Mycobacterium tuberculosis. The relationship between hemophagocytosis and infection is not well understood. In the murine liver, S. enterica serovar Typhimurium resides within hemophagocytic macrophages containing leukocytes. Here we show that S. Typhimurium also resides within hemophagocytes containing erythrocytes. In cell culture, S. Typhimurium benefits from residence within hemophagocytes by accessing iron, but why macrophages hemophagocytose is unknown. We show that treatment of macrophages with a cocktail of the proinflammatory cytokine interferon gamma (IFN-γ) and lipopolysaccharide (LPS) stimulates engulfment of nonsenescent erythrocytes. Exposure of resting or IFN-γ-treated macrophages to live, but not to heat-killed, S. Typhimurium cells also stimulates erythrocyte engulfment. Single-cell analyses show that S. Typhimurium-infected macrophages are more likely to erythrophagocytose and that infected macrophages engulf more erythrocytes than uninfected macrophages within the same culture well. In addition, macrophages containing erythrocytes harbor more bacteria. However, S. Typhimurium does not promote macrophage engulfment of polystyrene beads, suggesting a role for a ligand on the target cell. Finally, neither of the two S. Typhimurium type 3 secretion systems, T3SS1 or T3SS2, is fully required for hemophagocytosis. These results indicate that infection of macrophages with live S. Typhimurium cells stimulates hemophagocytosis.

Macrophages are white blood cells (leukocytes) that engulf and destroy pathogens. Hemophagocytes, a subset of macrophages, are characteristic of severe acute infection in patients with, for instance, typhoid fever, brucellosis, tuberculosis, and leishmaniasis. Each of these diseases has the potential to become chronic. Hemophagocytes (blood-eating cells) engulf and degrade red and white blood cells for unknown reasons. The bacterial pathogen Salmonella acquires the essential nutrient iron from murine hemophagocytes. We report that Salmonella stimulates macrophages to engulf blood cells, indicating that cells of this bacterium actively promote the formation of a specialized cellular niche in which they can acquire nutrients, evade killing by the host immune system, and potentially transition to chronic infection.

Effect of Ocimum basilicum L. on cyclo-oxygenase isoforms and prostaglandins involved in thrombosis

ETHNOPHARMACOLOGICAL RELEVANCE

Ocimum basilicum L. (OBL) is a plant used in traditional Uyghur medicine for the treatment and prevention of cardiovascular disease. In previous studies we had found an antihypertensive and antithrombotic effect suggestive of an effect on prostaglandins, which we attempt to document here.

MATERIALS AND METHODS

6-keto-PGF1α, the metabolite of prostacyclin, and PGE2 were measured in the supernatant of human umbilical vein endothelial cells (HUVEC) and basal or LPS-stimulated mouse coeliac macrophage cultures exposed to OBL ethanol (OBL-E) extracts and petroleum ether, chloroform, ethylacetate and butanol (PE, C, EA, B) fractions. In addition, 6-keto-PGF1α and thromboxane B2 (TXB2) were measured in a rat model of thromboangiitis obliterans exposed or not to OBL.

RESULTS

Short-term exposure to OBL-E dose-dependently increased 6-keto-PGF1α from HUVEC, and long-term (24h) exposure decreased it. OBL-C and OBL-B increased 6-keto-PGF1α, whereas the other fractions tended to decrease it after 24h exposure. The extract and all fractions decreased basal and stimulated PGE2 production, but only OBL-EA and OBL-B reduced PGE2 in stimulated cultures to concentrations below the unstimulated values (P<0.05). In vivo OBL increased 6-keto-PGF1α and decreased TXB2.

CONCLUSION

OBL and its extracts increased 6-keto-PGF1α and reduced PGE2 and TXB2 production in a dose and time-related manner. This could indicate simultaneous inhibition of COX-2 and stimulation of endothelial COX-1. The butanol fraction seemed most promising in this respect.

Functional analysis of bovine Nramp1 and production of transgenic cloned embryos in vitro

Natural resistance-associated macrophage protein 1 (Nramp1) plays an important role in restraining the growth of intracellular pathogens within macrophages. In this study, Nramp1 cDNA was cloned from Qinchuan cattle and its anti-bacterial activity was demonstrated as being able to significantly inhibit the growth of Salmonella abortusovis and Brucella abortus in macrophages. Calf fibroblasts stably transfected with pSP-NRAMP1-HA vector were used to reconstruct bovine embryos by somatic cell nuclear transfer (SCNT). Reconstructed embryos were maturated in vitro and the blastocyst formation rate (14.0%) was similar to that of control embryos (14.5%). Transgenic blastocysts were transplanted into 43 recipient cattle, of which 14 recipients became pregnant as evidenced by non-return estrus and by rectal palpation. One fetus was aborted after 6½ months of pregnancy and transgene integration was confirmed by semi-quantitative polymerase chain reaction. Together, this study showed that bovine Nramp1 retains biological function against the growth of intracellular bacteria and can be used to reconstruct embryos and produce Nramp1 transgenic cattle, which may benefit the animal and enhance their ability to prevent attack by intracellular pathogens.

Salmonella typhimurium's transthyretin-like protein is a host-specific factor important in fecal survival in chickens

The transthyretin-like protein (TLP) from Salmonella enterica subspecies I is a periplasmic protein with high level structural similarity to a protein found in mammals and fish. In humans, the protein homologue, transthyretin, binds and carries retinol and thyroxine, and a series of other, unrelated aromatic compounds. Here we show that the amino acid sequence of the TLP from different species, subspecies and serovars of the Salmonella genus is highly conserved and demonstrate that the TLP gene is constitutively expressed in S. Typhimurium and that copper and other divalent metal ions severely inhibit enzyme activity of the TLP, a cyclic amidohydrolase that hydrolyses 5-hydroxyisourate (5-HIU). In order to determine the in vivo role of the S. Typhimurium TLP, we constructed a strain of mouse-virulent S. Typhimurium SL1344 bearing a mutation in the TLP gene (SL1344 ΔyedX). We assessed the virulence of this strain via oral inoculation of mice and chickens. Whilst SL1344 ΔyedX induced a systemic infection in both organisms, the bacterial load detected in the faeces of infected chickens was significantly reduced when compared to the load of S. Typhimurium SL1344. These data demonstrate that the TLP gene is required for survival of S. Typhimurium in a high uric acid environment such as chicken faeces, and that metabolic traits of Salmonellae in natural and contrived hosts may be fundamentally different. Our data also highlight the importance of using appropriate animal models for the study of bacterial pathogenesis especially where host-specific virulence factors or traits are the subject of the study.

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.

β 1-4 mannobiose enhances Salmonella-killing activity and activates innate immune responses in chicken macrophages

Salmonella spp. is one of the major causes of food-borne illness in humans, and Salmonella enteritidis (SE) infection in commercial poultry is a world-wide problem. Here we have investigated the in vitro immune-modulating effects of β 1-4 mannobiose (MNB), which was previously found to prevent SE infection in vivo in chickens, using chicken macrophage (MQ-MCSU) cells. Treatment of MQ-NCSU cells with MNB dose-dependently increased both phagocytic activity and Salmonella-killing activity of macrophages, with the highest reduction in SE viability observed at a concentration of 40 μg/ml at 48 h post-infection. Likewise, both hydrogen peroxide (H(2)O(2)) and nitric oxide (NO) production were increased in a dose-dependent manner by MNB. Gene expression analysis of MNB-treated macrophages revealed significant increases in the expression of iNOS, NOX-1, IFN-γ, NRAMP1, and LITAF, genes critical for host defense and antimicrobial activity, when compared to untreated cells. This data confirms that MNB possesses potent innate immune-modulating activities and can up-regulate antibacterial defenses in chicken macrophages.

Ribonucleotide reductases of Salmonella typhimurium: transcriptional regulation and differential role in pathogenesis

Ribonucleotide reductases (RNRs) are essential enzymes that carry out the de novo synthesis of deoxyribonucleotides by reducing ribonucleotides. There are three different classes of RNRs (I, II and III), all having different oxygen dependency and biochemical characteristics. Salmonella enterica serovar Typhimurium (S. Typhimurium) harbors class Ia, class Ib and class III RNRs in its genome. We have studied the transcriptional regulation of these three RNR classes in S. Typhimurium as well as their differential function during infection of macrophage and epithelial cells. Deletion of both NrdR and Fur, two main transcriptional regulators, indicates that Fur specifically represses the class Ib enzyme and that NrdR acts as a global repressor of all three classes. A Fur recognition sequence within the nrdHIEF promoter has also been described and confirmed by electrophoretic mobility shift assays (EMSA). In order to elucidate the role of each RNR class during infection, S. Typhimurium single and double RNR mutants (as well as Fur and NrdR mutants) were used in infection assays with macrophage and epithelial cell lines. Our results indicate class Ia to be mainly responsible for deoxyribonucleotide production during invasion and proliferation inside macrophages and epithelial cells. Neither class Ib nor class III seem to be essential for growth under these conditions. However, class Ib is able to maintain certain growth in an nrdAB mutant during the first hours of macrophage infection. Our results suggest that, during the early stages of macrophage infection, class Ib may contribute to deoxyribonucleotide synthesis by means of both an NrdR and a Fur-dependent derepression of nrdHIEF due to hydrogen peroxide production and DNA damage associated with the oxidative burst, thus helping to overcome the host defenses.

Functional assessment of Nramp-like metal transporters and manganese in Caenorhabditis elegans

Nramp1 (natural resistance-associated macrophage protein-1) is a functionally conserved iron-manganese transporter in macrophages. Manganese (Mn), a superoxide scavenger, is required in trace amounts and functions as a cofactor for most antioxidants. Three Nramp homologs, smf-1, smf-2, and smf-3, have been identified thus far in the nematode Caenorhabditis elegans. A GFP promoter assay revealed largely intestinal expression of the smf genes from early embryonic through adult stages. In addition, smf deletion mutants showed increased sensitivity to excess Mn and mild sensitivity to EDTA. Interestingly, these smf deletion mutants demonstrated hypersensitivity to the pathogen Staphylococcus aureus, an effect that was rescued by Mn feeding or knockdown of the Golgi calcium/manganese ATPase, pmr-1, indicating that Mn uptake is essential for the innate immune system. This reversal of pathogen sensitivity by Mn feeding suggests a protective and therapeutic role of Mn in pathogen evasion systems. We propose that the C. elegans intestinal lumen may mimic the mammalian macrophage phagosome and thus could be a simple model for studying Mn-mediated innate immunity.

Use of high-throughput mass spectrometry to elucidate host-pathogen interactions in Salmonella

Capabilities in mass spectrometry are evolving rapidly, with recent improvements in sensitivity, data analysis and, most important from the standpoint of this review, much higher throughput, allowing analysis of many samples in a single day. This short review describes how these improvements in mass spectrometry can be used to dissect host-pathogen interactions using Salmonella as a model system. This approach has enabled direct identification of the majority of annotated Salmonella proteins, quantitation of expression changes under various in vitro growth conditions and new insights into virulence and expression of Salmonella proteins within host cells. One of the most significant findings is that a relatively high percentage of all the annotated genes (>20%) in Salmonella are regulated post-transcriptionally. In addition, new and unexpected interactions have been identified for several Salmonella virulence regulators that involve protein-protein interactions, suggesting additional functions of these regulators in coordinating virulence expression. Overall high-throughput mass spectrometry provides a new view of host-pathogen interactions, emphasizing the protein products and defining how protein interactions determine the outcome of infection.

Hemophagocytic macrophages harbor Salmonella enterica during persistent infection

Salmonella enterica subspecies can establish persistent, systemic infections in mammals, including human typhoid fever. Persistent S. enterica disease is characterized by an initial acute infection that develops into an asymptomatic chronic infection. During both the acute and persistent stages, the bacteria generally reside within professional phagocytes, usually macrophages. It is unclear how salmonellae can survive within macrophages, cells that evolved, in part, to destroy pathogens. Evidence is presented that during the establishment of persistent murine infection, macrophages that contain S. enterica serotype Typhimurium are hemophagocytic. Hemophagocytic macrophages are characterized by the ingestion of non-apoptotic cells of the hematopoietic lineage and are a clinical marker of typhoid fever as well as certain other infectious and genetic diseases. Cell culture assays were developed to evaluate bacterial survival in hemophagocytic macrophages. S. Typhimurium preferentially replicated in macrophages that pre-phagocytosed viable cells, but the bacteria were killed in macrophages that pre-phagocytosed beads or dead cells. These data suggest that during persistent infection hemophagocytic macrophages may provide S. Typhimurium with a survival niche.

Nramp1 expression by dendritic cells modulates inflammatory responses during Salmonella Typhimurium infection

Host resistance against Salmonella enterica serovar Typhimurium (S. Typhimurium) is mediated by natural resistance-associated macrophage protein 1 (Nramp1/Slc11a1). Nramp1 is critical to host defence, as mice lacking Nramp1 fail to control bacterial replication and succumb to low doses of S. Typhimurium. Despite this crucial role, the mechanisms underlying Nramp1's protective effects are unclear. Dendritic cells (DCs) that sample the intestinal lumen are among the first cells encountered by S. Typhimurium following oral infection and act as a conduit for S. Typhimurium to cross the intestinal epithelial barrier. We report that DCs, including intestinal, splenic and bone marrow-derived DCs (BMDCs), express Nramp1 protein. In the small intestine, Nramp1 expression is greater in a subset of DCs (CD11c(+)CD103(-)) characterized by the elevated expression of pro-inflammatory cytokines in response to bacterial products. While Nramp1 expression did not affect S. Typhimurium replication in BMDCs, infected Nramp1+/+ BMDCs and intestinal CD11c(+)CD103(-) DCs secreted more inflammatory cytokines (IL-6, IL-12 and TNF-alpha) than Nramp1-/-, suggesting that Nramp1 expression may promote a more rapid inflammatory response following infection. Collectively, these findings reveal a new role for DCs and Nramp1 in modulating the host inflammatory response to S. Typhimurium.

Aminoglycosides affect intracellular Salmonella enterica serovars typhimurium and virchow

The high antibacterial activity and selectivity of aminoglycosides and their low activity against intracellular bacteria associated with eukaryotic cells make them the antibiotics of choice for the elimination of extracellular bacteria during intracellular studies. Given the evidence that aminoglycosides can penetrate the eukaryotic cell membrane, the goal of this study was to examine the influence of aminoglycosides on macrophage-associated Salmonella. Herein, we show that gentamicin, kanamycin, and tobramycin at concentrations between 15 to 150 microg ml(-1) do not kill intracellular Salmonella but have other effects on the bacterial physiology. By using Salmonella enterica serovars Typhimurium and Virchow harboring luciferase reporter plasmid, we observed that the light produced by intracellular Salmonella declined immediately upon exposure to aminoglycosides, indicating that the bacteria were under stress. The extent of this effect was dependent on the macrophage host, on the identity of the aminoglycoside and its concentration, on the exposure time, and on the Salmonella serovar. Salmonella associated with Nramp1-negative macrophages, in which the phagosomal pH is higher, were more susceptible to aminoglycosides than Salmonella associated with Nramp1-expressing macrophages. These results verify that aminoglycosides affect intracellular bacteria and that the extent of this effect is dependent on the acidity level within the phagosome, suggesting that for the study of intracellular bacteria, the aminoglycoside concentration should be limited to two to five times the MIC for the bacterial strain studied. This precaution should guarantee the complete execution of extracellular bacteria with minimal effects on the intracellular bacteria and the host cells.

Genetic analysis of resistance to infections in mice: A/J meets C57BL/6J

Susceptibility to infectious diseases has long been known to have a genetic component in human populations. This genetic effect is often complex and difficult to study as it is further modified by environmental factors including the disease-causing pathogen itself. The laboratory mouse has proved a useful alternative to implement a genetic approach to study host defenses against infections. Our laboratory has used genetic analysis and positional cloning to characterize single and multi-gene effects regulating inter-strain differences in the susceptibility of A/J and C57BL/6J mice to infection with several bacterial and parasitic pathogens. This has led to the identification of several proteins including Nrampl (Slc11a1), Birc1e, Icsbp, C5a, and others that play critical roles in the antimicrobial defenses of macrophages against intracellular pathogens. The use of AcB/BcA recombinant congenic strains has further facilitated the characterization of single gene effects in complex traits such as susceptibility to malaria. The genetic identification of erythrocyte pyruvate kinase (Pklr) and myeloid pantetheinase enzymes (Vnn1/3) as key regulators of blood-stage parasitemia has suggested that cellular redox potential may be a key biochemical determinant of Plasmodium parasite replication. Expanding these types of studies to additional inbred strains and to emerging stocks of mutagenized mice will undoubtedly continue to unravel the molecular basis of host defense against infections.

Innate immunity to intraphagosomal pathogens is mediated by interferon regulatory factor 8 (IRF-8) that stimulates the expression of macrophage-specific Nramp1 through antagonizing repression by c-Myc

Macrophages are a central arm of innate immune defense against intracellular pathogens. They internalize microbes into phagosomes where the invaders are being killed by oxygen and nitrogen reactive species. Despite this battery of antimicrobial molecules, some are able to thrive within the phagosome thus termed intraphagosomal pathogens among which are Salmonella, Leishmania, and Mycobacteria. In mice, a single dominant gene termed Nramp1/Slc11a1 controls innate resistance to such pathogens. This gene is expressed exclusively in myeloid cells. Previously, we have shown that the restricted expression of Nramp1 is regulated by a myeloid cell-specific transcription factor termed IRF-8/ICSBP. It is demonstrated here that the induction of Nramp1 expression in activated macrophages is accompanied by a promoter shift from a repression state elicited by c-Myc to an activation state elicited by the induction of IRF-8 in activated macrophages. This transition from repression to activation is facilitated by a competitive protein-protein interaction with the transcription factor Miz-1. To show that IRF-8 is directly involved in the elimination of intraphagosomal pathogens through the regulation of Nramp1 gene expression, we bred wild type as well as IRF-8 and Nramp1 null mouse strains and examined macrophages derived from bone marrow and peritoneum. Our results clearly show that the absence of IRF-8 and Nramp1 leads to the same phenotype; defective killing of intraphagosomal Salmonella enterica serovar typhimurium and Mycobacterium bovis. Thus, interplay between repression and activation state of the Nramp1 promoter mediated by IRF-8 provides the molecular basis by which macrophages resist intraphagosomal pathogens at early stage after infection.

Pyruvate kinase deficiency confers susceptibility to Salmonella typhimurium infection in mice

The mouse response to acute Salmonella typhimurium infection is complex, and it is under the influence of several genes, as well as environmental factors. In a previous study, we identified two novel Salmonella susceptibility loci, Ity4 and Ity5, in a (AcB61 × 129S6)F2 cross. The peak logarithm of odds score associated with Ity4 maps to the region of the liver and red blood cell (RBC)–specific pyruvate kinase (Pklr) gene, which was previously shown to be mutated in AcB61. During Plasmodium chabaudi infection, the Pklr mutation protects the mice against this parasite, as indicated by improved survival and lower peak parasitemia. Given that RBC defects have previously been associated with resistance to malaria and susceptibility to Salmonella, we hypothesized that Pklr is the gene underlying Ity4 and that it confers susceptibility to acute S. typhimurium infection in mice. Using a fine mapping approach combined with complementation studies, comparative studies, and functional analysis, we show that Pklr is the gene underlying Ity4 and that it confers susceptibility to acute S. typhimurium infection in mice through its effect on the RBC turnover and iron metabolism.

Evidence that the p38 MAP kinase pathway is dysregulated in HLA-B27-expressing human monocytic cells: correlation with HLA-B27 misfolding

OBJECTIVE

To investigate the cause of the enhanced intracellular replication of Salmonella enteritidis in HLA-B27-transfected U937 human monocytic cells and the contribution of HLA-B27 heavy chain (HC) misfolding.

METHODS

U937 monocytic cell transfectants stably expressing pSV2neo resistant vector (mock), wild-type HLA-B27, or mutated HLA-B27 HCs with amino acid substitutions in the B pocket were differentiated, infected with S enteritidis, and treated with signaling pathway inhibitors or specific p38 small interfering RNA (siRNA). The numbers of living intracellular bacteria were determined with the colony-forming unit method. To visualize S enteritidis, the bacteria were transformed with green fluorescent protein, and studied by microscopy.

RESULTS

Treatment with the p38 MAPK inhibitors or with p38 siRNA enhanced the replication of S enteritidis in U937 transfectants, whereas the other inhibitors had no effect. In mock-transfected cells and in cells expressing the mutated B27 HCs in which the misfolding had been corrected, p38 inhibitors impaired their ability to resist the replication of bacteria (mock, B27.A2B, B27.E45M, and B27.C67A). In contrast, the number of intracellular bacteria was not significantly increased in p38 inhibitor-treated cells expressing misfolded B27 HCs (B27g, B27cDNA, and B27.H9F).

CONCLUSION

Our results show that p38 activity plays a crucial role in controlling intracellular S enteritidis in U937 cells. Enhanced replication of bacteria in B27-expressing cells requires that the HCs contain glutamic acid at position 45 and cysteine at position 67. Furthermore, in transfectants expressing misfolded B27 HCs, p38 inhibition had no significant effect on bacterial replication, suggesting that in these cells, the p38 pathway may not function properly.

Molecular cloning, characterization and expression analysis of natural resistance associated macrophage protein (Nramp) cDNA from turbot (Scophthalmus maximus)

Nramp (natural resistance associated macrophage protein) has been identified as one of the major candidate genes for controlling natural resistance and/or susceptibility to intracellular pathogens in vertebrates. However, few reports are available about the structure and function of Nramp in teleost. We have recently isolated the cDNA encoding Nramp from turbot (Scophthalmus maximus). The full-length cDNA of the Nramp is 2584 bp in length, including 69 bp 5' terminal UTR, 850 bp 3' terminal UTR and 1665 bp open reading frame for a protein with 554 amino acid residues (Genbank accession number: DQ263240). Comparison of amino acid sequence indicated that turbot Nramp consists of 12 transmembrane regions (TM) domains. A consensus transport motif (CTM) containing 20 residues was observed between transmembrane domains 8 and 9. The deduced amino acid sequence of turbot Nramp exhibited between 60 and 92% homology with 13 other vertebrate Nramp sequences. Nramp transcripts were found to be highly abundant in head kidney, kidney and spleen, abundant in intestine and gill, less abundant in liver, brain, heart and gonad, least in muscle and skin. The level of Nramp mRNA in embryos gradually increases during embryogenesis from blastula stage to fry stage. Challenge of turbot with pathogenic bacteria, Vibrio anguillarum, elevated Nramp mRNA levels in liver and spleen. The Nramp transcripts were detected in turbot embryonic cell line (TEC). Challenge of the TEC cell cultures with pathogenic bacteria, V. anguillarum, significantly elevated Nramp mRNA levels in TEC cell cultures.

Host genetics of mycobacterial diseases in mice and men: forward genetic studies of BCG-osis and tuberculosis

In humans, genetic factors have long been suspected to contribute to the onset and outcome of tuberculosis. Such effects are difficult to identify owing to their complex inheritance, and to the confounding impact of environmental factors, notably pathogen-associated virulence determinants. Recently, forward genetic approaches in mouse models and in human populations have been used to elucidate a molecular basis for predisposition to mycobacterial diseases. The genetic dissection of host predisposition to infection with Mycobacterium bovis BCG and M. tuberculosis will help to define the key molecules involved in host antituberculous immunity and should provide new insights into this important infectious disease.

The mgtC gene of Burkholderia cenocepacia is required for growth under magnesium limitation conditions and intracellular survival in macrophages

Burkholderia cenocepacia, a bacterium commonly found in the environment, is an important opportunistic pathogen in patients with cystic fibrosis (CF). Very little is known about the mechanisms by which B. cenocepacia causes disease, but chronic infection of the airways in CF patients may be associated, at least in part, with the ability of this bacterium to survive within epithelial cells and macrophages. Survival in macrophages occurs in a membrane-bound compartment that is distinct from the lysosome, suggesting that B. cenocepacia prevents phagolysosomal fusion. In a previous study, we employed signature-tagged mutagenesis and an agar bead model of chronic pulmonary infection in rats to identify B. cenocepacia genes that are required for bacterial survival in vivo. One of the most significantly attenuated mutants had an insertion in the mgtC gene. Here, we show that mgtC is also needed for growth of B. cenocepacia in magnesium-depleted medium and for bacterial survival within murine macrophages. Using fluorescence microscopy, we demonstrated that B. cenocepacia mgtC mutants, unlike the parental isolate, colocalize with the fluorescent acidotropic probe LysoTracker Red. At 4 h postinfection, mgtC mutants expressing monomeric red fluorescent protein cannot retain this protein within the bacterial cytoplasm. Together, these results demonstrate that, unlike the parental strain, an mgtC mutant does not induce a delay in phagolysosomal fusion and the bacterium-containing vacuoles are rapidly targeted to the lysosome, where bacteria are destroyed.

Identification of a tyrosine-based motif (YGSI) in the amino terminus of Nramp1 (Slc11a1) that is important for lysosomal targeting

In macrophages, Nramp1 (Slc11a1) is expressed in lysosomes and restricts replication of intracellular pathogens by removing divalent metals (Mn2+ and Fe2+) from the phagolysosome. Nramp2 (DMT1, Slc11a2) is expressed both at the duodenal brush border where it mediates uptake of dietary iron and ubiquitously at the plasma membrane/recycling endosomes of many cell types where it transports transferrin-associated iron across the endosomal membrane. In Nramp2, a carboxyl-terminal cytoplasmic motif ((555)YLLNT(559)) is critical for internalization and recycling of the transporter from the plasma membrane. Here we studied the subcellular trafficking properties of Nramp1 and investigated the cis-acting sequences responsible for targeting to lysosomes. For this, we constructed and studied Nramp1/Nramp2 chimeric proteins where homologous domains of each protein were exchanged. Chimeras exchanging the amino-(upstream TM1) and carboxyl-terminal (downstream TM12) cytoplasmic segments of both transporters were stably expressed in porcine LLC-PK1 kidney cells and were studied with respect to expression, maturation, stability, cell surface targeting, transport activity, and subcellular localization. An Nramp2 isoform II chimera bearing the amino terminus of Nramp1 was not expressed at the cell surface but was targeted to lysosomes. This lysosomal targeting was abolished by single alanine substitutions at Tyr15 and Ile18 of a (15)YGSI(18) motif present in the amino terminus of Nramp1. These results identify YGSI as a tyrosine-based sorting signal responsible for lysosomal targeting of Nramp1.

Proteomic analysis of Salmonella enterica serovar typhimurium isolated from RAW 264.7 macrophages: identification of a novel protein that contributes to the replication of serovar typhimurium inside macrophages

To evade host resistance mechanisms, Salmonella enterica serovar Typhimurium (STM), a facultative intracellular pathogen, must alter its proteome following macrophage infection. To identify new colonization and virulence factors that mediate STM pathogenesis, we have isolated STM cells from RAW 264.7 macrophages at various time points following infection and used a liquid chromatography-mass spectrometry-based proteomic approach to detect the changes in STM protein abundance. Because host resistance to STM infection is strongly modulated by the expression of a functional host-resistant regulator, i.e. natural resistance-associated macrophage protein 1 (Nramp1, also called Slc11a1), we have also examined the effects of Nramp1 activity on the changes of STM protein abundances. A total of 315 STM proteins have been identified from isolated STM cells, which are largely housekeeping proteins whose abundances remain relatively constant during the time course of infection. However, 39 STM proteins are strongly induced after infection, suggesting their involvement in modulating colonization and infection. Of the 39 induced proteins, 6 proteins are specifically modulated by Nramp1 activity, including STM3117, as well as STM3118-3119 whose time-dependent abundance changes were confirmed using Western blot analysis. Deletion of the gene encoding STM3117 resulted in a dramatic reduction in the ability of STM to colonize wild-type RAW 264.7 macrophages, demonstrating a critical involvement of STM3117 in promoting the replication of STM inside macrophages. The predicted function common for STM3117-3119 is biosynthesis and modification of the peptidoglycan layer of the STM cell wall.

The iron efflux protein ferroportin regulates the intracellular growth of Salmonella enterica

We investigated the influence of the macrophage iron exporter ferroportin and its ligand hepcidin on intracellular Salmonella growth. Elevated ferroportin expression inhibited bacterial multiplication; hepcidin-induced ferroportin down-regulation enhanced it. Expression analysis of iron-responsive Salmonella genes indicated ferroportin-mediated iron deprivation. These results demonstrate a role for ferroportin in antimicrobial resistance.

Genome-wide screen for Salmonella genes required for long-term systemic infection of the mouse

A microarray-based negative selection screen was performed to identify Salmonella enterica serovar Typhimurium (serovar Typhimurium) genes that contribute to long-term systemic infection in 129X1/SvJ (Nramp1^r) mice. A high-complexity transposon-mutagenized library was used to infect mice intraperitoneally, and the selective disappearance of mutants was monitored after 7, 14, 21, and 28 d postinfection. One hundred and eighteen genes were identified to contribute to serovar Typhimurium infection of the spleens of mice by 28 d postinfection. The negatively selected mutants represent many known aspects of Salmonella physiology and pathogenesis, although the majority of the identified genes are of putative or unknown function. Approximately 30% of the negatively selected genes correspond to horizontally acquired regions such as those within Salmonella pathogenicity islands (SPI 1–5), prophages (Gifsy-1 and −2 and remnant), and the pSLT virulence plasmid. In addition, mutations in genes responsible for outer membrane structure and remodeling, such as LPS- and PhoP-regulated and fimbrial genes, were also selected against. Competitive index experiments demonstrated that the secreted SPI2 effectors SseK2 and SseJ as well as the SPI4 locus are attenuated relative to wild-type bacteria during systemic infection. Interestingly, several SPI1-encoded type III secretion system effectors/translocases are required by serovar Typhimurium to establish and, unexpectedly, to persist systemically, challenging the present description of Salmonella pathogenesis. Moreover, we observed a progressive selection against serovar Typhimurium mutants based upon the duration of the infection, suggesting that different classes of genes may be required at distinct stages of infection. Overall, these data indicate that Salmonella long-term systemic infection in the mouse requires a diverse repertoire of virulence factors. This diversity of genes presumably reflects the fact that bacteria sequentially encounter a variety of host environments and that Salmonella has evolved to respond to these selective forces in a way that permits both the bacteria and the host to survive.

Bacteria belonging to the genus Salmonella are capable of establishing a long-term systemic infection in a variety of hosts, including humans, rodents, fowl, and cattle. The ability of Salmonella to subvert the active immune response of the host represents millions of years of co-evolution and is the result of specialized virulence factors that promote long-term infection. This study describes a microarray-based genome-wide screen designed to identify genes required by Salmonella enterica serovar Typhimurium (serovar Typhimurium) to persist and replicate in the spleen and liver of mice for up to 28 days. The results demonstrate that serovar Typhimurium utilizes a diverse repertoire of virulence factors, including both known and novel virulence genes, to establish infection and to persist in the host. The authors' data further established a previously unappreciated role for Salmonella pathogenicity island 1 in maintaining a persistent systemic infection. In addition, a progressive selection against serovar Typhimurium mutants based upon the duration of the infection was observed, suggesting that certain classes of genes are required at specific times during infection and providing a foundation to further dissect Salmonella pathogenesis into distinct temporal phases.

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.

Comparative studies of duodenal and macrophage ferroportin proteins

Intestinal epithelial cells and reticuloendothelial macrophages are, respectively, involved in diet iron absorption and heme iron recycling from senescent erythrocytes, two critical processes of iron homeostasis. These cells appear to use the same transporter, ferroportin (Slc40a1), to export iron. The aim of this study was to compare the localization, expression, and regulation of ferroportin in both duodenal and macrophage cells. Using a high-affinity purified polyclonal antibody, we analyzed the localization and expression of ferroportin protein in the spleen, liver, and duodenum isolated from normal mice as well as from well-characterized mouse models of altered iron homeostasis. Ferroportin was found to be predominantly expressed in enterocytes of the duodenum, in splenic macrophages, and in liver Kupffer cells. Interestingly, the protein species detected in these cells migrated differently on SDS-PAGE. These differences in apparent molecular masses were partly explained by posttranslational complex N-linked glycosylations. In addition, in enterocytes, the transporter was mostly expressed at the basolateral membrane, whereas in bone marrow-derived macrophages, ferroportin was found predominantly localized in the intracellular vesicular compartment. However, some microdomains positive for ferroportin were also detected at the plasma membrane of macrophages. Despite these differences, we observed a parallel upregulation of ferroportin expression in tissue macrophages and enterocytes in response to iron-restricted erythropoiesis, suggesting that iron homeostasis is likely maintained through coordinate expression of the iron exporter in both intestinal and phagocytic cells. Our data also confirm a predominant regulation of ferroportin through systemic regulator(s) likely including hepcidin.

New concepts in Salmonella virulence: the importance of reducing the intracellular growth rate in the host

The literature refers to Salmonella enterica as an intracellular bacterial pathogen that proliferates within vacuoles of mammalian cells. However, recent in vivo studies have revealed that the vast majority of infected cells contain very few intracellular bacteria (three to four organisms). Salmonella intracellular growth is also limited in cultured dendritic cells and fibroblasts, two cell types abundant in tissues located underneath the intestinal epithelium. Recently, a Salmonella factor previously known for its role as a negative regulator of intracellular growth has been shown to tightly repress certain pathogen functions upon host colonization and to be critical for virulence. The connection between virulence and the negative control of intracellular growth is further sustained by the fact that some attenuated mutants overgrow in non-phagocytic cells located in the intestinal lamina propria. These findings are changing our classical view of Salmonella as a fast growing intracellular pathogen and suggest that this pathogen may trigger responses directed to reduce the growth rate within the infected cell. These responses could play a critical role in modulating the delicate balance between disease and persistence.

Visualization of vacuolar acidification-induced transcription of genes of pathogens inside macrophages

The objective of these studies was to analyze the role of the ionic environment of phagosomal vacuoles in the control of pathogens by macrophages. Digital imaging and flow cytometry were used to follow the induction of the phoP promoter of Salmonella enterica Typhimurium within live macrophages. Manipulating the Mg2+ concentration within the Salmonella-containing vacuole (SCV) was without effect on the early induction of PhoPQ. Moreover, direct measurement of [Mg2+] within the SCV using nanosensor particles showed that, during this initial period of phoP activation, the concentration of the divalent cation is rapidly regulated and stabilizes around 1 mm. Extrusion of other divalent cations via the Nramp1 efflux pump was similarly ruled out as an important contributor to the activation of the regulon. By contrast, induction of PhoP was greatly attenuated when the pH gradient across the SCV membrane was dissipated. A second, more modest pH-independent component of PhoP induction was unmasked by inhibition of the vacuolar proton pump. This second component was eliminated by pretreatment of cells with IFNgamma, even though the cytokine augmented the overall PhoP response. These findings demonstrate the existence of at least three separate activators of phoP transcription: resting and IFNgamma-stimulated pH-sensitive components, plus a pH-independent component.

NRAMP1 is not associated with asthma, atopy, and serum immunoglobulin E levels in the French Canadian population

Reduced infection by mycobacteria, including Mycobacterium tuberculosis, may be partly responsible for increased prevalence of allergic and autoimmune diseases in developed countries. In a murine model of innate resistance to mycobacteria, the Nramp1 gene has been shown to affect asthma susceptibility. From this observation, it was proposed that human NRAMP1 may be a modulator of asthma risk in human populations. To experimentally test the candidacy of NRAMP1 in asthma susceptibility, we characterized five genetic variants of NRAMP1 (5'CAn, 274C>T, 469+14G>C, D543N, and 1729+del4) in an asthma family-based cohort from northeastern Quebec. We did not observe any significant association between NRAMP1 variants (either allele or haplotype specific) with asthma, atopy, or serum immunoglobulin E levels. These results demonstrate that, in spite of direct involvement of Nramp1 in a murine asthma model, in human populations NRAMP1 is not likely to be a major contributor to the genetic etiology of asthma and asthma-related phenotypes.

Single gene effects in mouse models of host: pathogen interactions

Inbred mouse strains have been known for many years to vary in their degree of susceptibility to different types of infectious diseases. The genetic basis of these interstrain differences is sometimes simple but often complex. In a few cases, positional cloning has been used successfully to identify single gene effects. The natural resistance-associated macrophage protein 1 (Nramp1) gene (Slc11a1) codes for a metal transporter active at the phagosomal membrane of macrophages, and Nramp1 mutations cause susceptibility to Mycobacterium, Salmonella, and Leishmania. Furthermore, recent advances in gene transfer technologies in transgenic mice have enabled the functional dissection of gene effects mapping to complex, repeated parts of the genome, such as the Lgn1 locus, causing susceptibility to Legionella pneumophila in macrophages. Finally, complex traits such as the genetically determined susceptibility to malaria can sometimes be broken down into multiple single gene effects. One such example is the case of pyruvate kinase, where a loss-of-function mutation was recently shown by our group to be protective against blood-stage infection with Plasmodium chabaudi. In all three cases reviewed, the characterization of the noted gene effect(s) has shed considerable light on the pathophysiology of the infection, including host response mechanisms.

The Salmonella enterica serovar typhimurium divalent cation transport systems MntH and SitABCD are essential for virulence in an Nramp1G169 murine typhoid model

Nramp1 is a transporter that pumps divalent cations from the vacuoles of phagocytic cells and is associated with the innate resistance of mice to diverse intracellular pathogens. We demonstrate that sitA and mntH, genes encoding high-affinity metal ion uptake systems in Salmonella enterica serovar Typhimurium, are upregulated when Salmonella is internalized by Nramp1-expressing macrophages and play an essential role in systemic infection of congenic Nramp1-expressing mice.

Enhanced intracellular replication of Salmonella enteritidis in HLA-B27-expressing human monocytic cells: dependency on glutamic acid at position 45 in the B pocket of HLA-B27

OBJECTIVE

To reveal the cause of the impaired elimination of Salmonella enteritidis in HLA-B27-transfected human monocytic cells and to study whether the B pocket of HLA-B27 contributes to these modulatory effects.

METHODS

Stable U937 cell transfectants expressing HLA-A2, B27, or different forms of B27 with amino acid substitutions in the B pocket were prepared. Mock-transfected cells were prepared using the antibiotic resistance vector (pSV2neo) alone. Cells were differentiated, infected with S enteritidis, and the number of live intracellular S enteritidis organisms was determined using the colony-forming unit method. To visualize intracellular S enteritidis, the bacteria were transformed with green fluorescent protein (GFP), and studied by confocal microscopy.

RESULTS

Cells expressing wild-type HLA-B27 were more permissive of intracellular replication of S enteritidis compared with mock-transfected or A2-transfected controls. Cells expressing B27 with an altered B pocket composition having either 6 amino acid substitutions (B27.A2B; substitutions H9F, T24A, E45M, I66K, C67V, and K70H) or a single substitution (B27.E45M) were no longer permissive of S enteritidis replication. In contrast, cells expressing B27 with the single substitution of F for H at position 9 (B27.H9F) retained their permissiveness. Studies using GFP-transformed S enteritidis confirmed that the increase in the amount of intracellular bacteria in B27-expressing cells was due to replication of the bacteria.

CONCLUSION

Our data indicate that HLA-B27 expression modulates the host-microbe interaction that results in an impaired capacity of monocytes to resist intracellular replication of S enteritidis. The phenotype is dependent on glutamic acid at position 45 in the B pocket and, thus, may be due to properties of the B27 heavy chain that are related to this residue. The ability of HLA-B27 to confer susceptibility to Salmonella-triggered reactive arthritis may occur, at least in part, through these modulatory effects.

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.