SLC11A1 (formerly NRAMP1) and disease resistance

The immunopathology of experimental visceral leishmaniasis

Experimental murine infection with the parasites that cause human visceral leishmaniasis (VL) results in the establishment of infection in the liver, spleen, and bone marrow. In most strains of mice, parasites are eventually cleared from the liver, and hepatic resistance to infection results from a coordinated host response involving a broad range of effector and regulatory pathways targeted within defined tissue structures called granulomas. In contrast, parasites persist in the spleen and bone marrow by mechanisms that are less well understood. Parasite persistence is accompanied by the failure of granuloma formation and by a variety of pathologic changes, including splenomegaly, disruption of lymphoid tissue microarchitecture, and enhanced hematopoietic activity. Here, we review the salient features of these distinct tissue responses and highlight the varied roles that cytokines of the tumor necrosis factor family play in immunity to this infection. In addition, we also discuss recent studies aimed at understanding how splenomegaly affects the survival and function of memory cells specific for heterologous antigens, an issue of considerable importance for our understanding of the disease-associated increase in secondary infections characteristic of human VL.

Slc11a1-mediated resistance to Salmonella enterica serovar Typhimurium and Leishmania donovani infections does not require functional inducible nitric oxide synthase or phagocyte oxidase activity

Solute carrier family 11a member 1 (Slc11a1; formerly natural resistance-associated macrophage protein 1) encodes a late endosomal/lysosomal protein/divalent cation transporter, which regulates iron homeostasis in macrophages. During macrophage activation, Slc11a1 exerts pleiotropic effects on gene regulation and function, including generation of nitric oxide (NO) via inducible NO synthase (iNOS; encoded by Nos2A) and of reactive oxygen intermediates (ROI) via the phagocyte oxidase complex. As NO and ROI have potent antimicrobial activity in macrophages, it was assumed that their activities would contribute to Slc11a1-regulated innate resistance to Salmonella enterica serovar Typhimurium and Leishmania donovani. By intercrossing mice with gene disruptions at Nos2A and Cybb (encoding gp91phox, the heavy chain subunit of cytochrome b-245 and an essential component of phagocyte NADPH oxidase) onto equivalent Slc11a1 wild-type and mutant genetic backgrounds, we demonstrate that neither iNOS nor gp91phox activity is required for Slc11a1-mediated innate resistance to either infection. Functional gp91phox and iNOS are required to control S. enterica serovar Typhimurium in non-Slc11a1-regulated phases of infection. For L. donovani, an organ-specific requirement for iNOS to clear parasites from the spleen was observed at 50 days post-infection, but neither iNOS nor gp91phox influenced late-phase infection in the liver. This contrasted with Leishmania major infection, which caused rapid lesion growth and death in iNOS knockout mice and some exacerbation of disease with gp91phox deficiency. This highlights the adaptive differences in tissue and cellular tropisms between L. donovani and L. major and the different genes and mechanisms that regulate visceral versus cutaneous forms of the disease.

GENE ORGANIZATION AND EXPRESSION OF THE DIVALENT CATION TRANSPORTER NRAMP IN THE PROTISTAN PARASITE PERKINSUS MARINUS

Trophozoites of the protistan parasite Perkinsus marinus reside and proliferate inside phagosomelike structures of hemocytes from the host, the eastern oyster Crassostrea virginica. In a murine model, it has been proposed that the outcome of intracellular parasite-host interactions is determined, at least in part, by the activity of the host's divalent cation transporter natural resistance-associated macrophage protein 1 (Nramp1). Although nucleotide sequences from members of the Nramp family in protozoan parasites have recently become available in public databases, little is known about their molecular, structural, and functional aspects that may relate to the parasite's survival of intracellular killing by the host. The complementary DNA (cDNA) sequence of the Nramp from P. marinus (PmNramp) was obtained by polymerase chain reaction amplification with degenerated primers, followed by rapid amplification of cDNA ends. The 2,082-bp cDNA sequence encoded a predicted protein of 558 amino acids. PmNramp is a single-copy gene composed of 7 exons and 6 short introns (44-61 bp) with the canonical splicing signal (GT/AG). A phylogenetic analysis indicates that P. marinus and apicomplexan Nramp genes derive from a common "archetype" Nramp ancestor. However, the apicomplexan Nramps are highly divergent from the P. marinus sequence and the rest of the archetype Nramp group. Preliminary studies suggest that expression of PmNramp in in vitro-cultured P. marinus trophozoites is modulated by metals and by exogenous oxidative stress.

Exploring the genetic basis of disease using RNA interference

Cancer and autoimmunity are polygenic diseases. In order to better understand the mechanisms of disease development and progression it is essential to uncover which genes are involved. Much has been learned from population studies in human patients by searching for polymorphic genetic loci associated with disease. In addition, animal models of tumor development, as well as models for various autoimmune conditions such as multiple sclerosis and Type I diabetes, have helped determine genetic loci that contribute to disease susceptibility. However, characterization of the exact genes involved is often difficult and requires lengthy and technically demanding genetic manipulations. The generation of knockout animals is the method of choice to probe single genes. However, this is not possible in all species or even in all inbred strains within a species. The recent discovery of a new post-transcriptional gene silencing pathway termed RNA interference, which is mediated by short fragments of double-stranded RNA (short-interfering RNA), has opened up new avenues for genetic manipulation of experimental animals. This review will consider how silencing genes by RNA interference within the context of experimental disease models promises to become a powerful new tool for the genetic analysis of cancer and autoimmunity. Advances in RNA interference technology now permit the relatively rapid generation of transgenic animals in a wide range of species with complex genetic backgrounds that were previously inaccessible to genetic manipulation. This novel approach should help refine the characterization of disease-associated genes, either by silencing specific candidates or even by screening a larger number of genes in vivo within a comparatively short time frame.

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.

Genetic resistance to Salmonella infection in domestic animals

Genetic resistance to Salmonella infection in experimental animal models is well described. However, genetic resistance in domestic animals, which has potentially great value in terms of controlling Salmonella in the food chain, has been relatively poorly described. Recent advances in genetics and immunology have identified several factors that influence resistance in chickens and pigs in particular. Resistance to systemic salmonellosis in the chicken is encoded by a number of factors including Nramp1 (now termed Slc11a1) and a novel gene, SAL1 that leads to increased macrophage activity against Salmonella. Studies in outbred, and in particular, inbred chickens have revealed considerable differences in levels of colonization of the gastrointestinal tract and responses to vaccination. Factors influencing this appear to include innate immune function, MHC and Nramp. In pigs several immune factors, including polymorphonuclear cell activity, have been shown to influence resistance.

SLC11A1 (formerly NRAMP1) and susceptibility to visceral leishmaniasis in The Sudan

Genetic susceptibility to visceral leishmaniasis (VL) is indicated by differences in incidence and clinical phenotypes between ethnic groups in Sudan. In mice, innate susceptibility to Leishmania donovani, the etiological agent of VL, is controlled by Slc11a1 (formerly Nramp1). We therefore examined polymorphisms at SLC11A1 in 59 multicase families of VL from the high-incidence Masalit tribe in Sudan. Multipoint nonparametric analysis in ALLEGRO shows a significant linkage across SLC11A1 (Zlr scores 2.38-2.55; 0.008< or =P< or =0.012; information content 0.88). The extended transmission disequilibrium test shows biased transmission of alleles at 5' polymorphisms in the promoter (P=0.0145), exon 3 (P=0.0037) and intron 4 (P=0.0049), and haplotypes formed by them (P=0.0089), but not for 3' polymorphisms at exon 15 or the 3'UTR. Stepwise logistic regression analysis using a case/pseudo-control data set derived from the 59 families was consistent with main effects contributed by the intron 4 469+14G/C polymorphism. Although the two alleles for 469+14G/C lie on haplotypes carrying different alleles for the functional promoter GTn polymorphism, the latter did not itself contribute separate main effects. Sequence analysis of 36 individuals failed to identify new putative functional polymorphisms in the coding region, intron 1, intron/exon boundaries, intron 4/exon 4a, or in the 3'UTR. One novel promoter polymorphism (-86G/A) was located within a putative nuclear factor kappa B binding site that could be functional. Further work will determine whether additional polymorphisms occur upstream in the promoter, which could be in linkage disequilibrium with the intron 4 polymorphism. These studies contribute to knowledge of the role of SLC11A1 in infectious disease.

Comparative analysis of two slc11 (Nramp) loci in Takifugu rubripes

To study the evolution of the solute carrier family 11 (slc11; formerly Nramp) protein, we isolated and characterized two paralogs from the pufferfish Takifugu rubripes (Fugu). These teleost genes, designated Fugu slc11a-a and Fugu slc11a-b, comprise open reading frames of 1743 nucleotides (581 amino acids) and 1662 nt (554 aa), respectively. The proteins are 81% similar, and both exhibit signature features of the slc11 family of proteins including 12 transmembrane domains, a conserved transport motif and a glycosylated loop. Both Fugu paralogs are more Slc11a2-like based on sequence homology and phylogenetic studies. Analysis of gene environment placed both in the proximity of multiple loci syntenic to human chromosome 12q13, that is, within a SLC11A2 gene environment. However, Fugu slc11a-a also gave one match with chromosome 2q35, where human SLC11A1 resides. Functional diversification was suggested by differences in tissue distribution and subcellular localization. Fugu slc11a-a exhibits a restricted expression profile and a complex subcellular localization, including LAMP1 positive late endosomes/lysosomes in transiently transfected mouse macrophages. Fugu slc11a-b is expressed ubiquitously and localizes solely to late endosomes/lysosomes. This comparative analysis extends our understanding of the evolution and function of this important family of divalent cation transporters. [Sequence data from this article have been deposited with the EMBL/GenBank Data Libraries under accession nos. AJ496547/8/9 and AJ496550.]

Selected lipids activate phagosome actin assembly and maturation resulting in killing of pathogenic mycobacteria

Pathogenic mycobacteria such as Mycobacterium tuberculosis and Mycobacterium avium facilitate disease by surviving intracellularly within a potentially hostile environment: the macrophage phagosome. They inhibit phagosome maturation processes, including fusion with lysosomes, acidification and, as shown here, membrane actin assembly. An in vitro assay developed for latex bead phagosomes (LBPs) provided insights into membrane signalling events that regulate phagosome actin assembly, a process linked to membrane fusion. Different lipids were found to stimulate or inhibit actin assembly by LBPs and mycobacterial phagosomes in vitro. In addition, selected lipids activated actin assembly and phagosome maturation in infected macrophages, resulting in a significant killing of M. tuberculosis and M. avium. In contrast, the polyunsaturated sigma-3 lipids behaved differently and stimulated pathogen growth. Thus, lipids can be involved in both stimulatory and inhibitory signalling networks in the phagosomal membrane.

The Slc11a1 (Nramp1) gene controls efficacy of mycobacterial treatment of allergic asthma

Genes controlling antibacterial resistance may be important in the hygiene hypothesis, which states that lack of bacterial infections during childhood would favor development of allergic disease. We, therefore, studied whether Nramp1 (Slc11a1) alleles, which determine susceptibility (Nramp1(s)) or resistance (Nramp1(r)) to intracellular bacteria, affect the efficacy of heat-killed Mycobacterium vaccae in the treatment of allergic asthma in a mouse model. Treatment of OVA-sensitized Nramp1(s) mice with M. vaccae suppressed airway hyperresponsiveness, airway eosinophilia, Ag-specific IgE, and IL-4 and IL-5 production after OVA aerosol challenge. In contrast, M. vaccae hardly affected these parameters in Nramp1(r) mice. In addition, The Nramp1 gene affected both T cell-mediated responses to M. vaccae in vivo and the level of macrophage activation after stimulation with M. vaccae in vitro. In conclusion, the efficacy of M. vaccae in preventing allergic and asthmatic manifestations in a mouse model is strongly affected by Nramp1 alleles. These findings could have important implications for the future use of mycobacteria and their components in the prevention or treatment of allergic asthma. A new link is described between genes, the environment, and the development of allergy, in which the Nramp1 gene fine tunes the hygiene hypothesis.

Molecular call-and-response: how Salmonella learns the gospel from its host

Host-microbe interactions are often portrayed as a game of molecular hide-and-seek or tug-of-war where one partner seeks to establish an upper-hand over the other. Perhaps a more useful analogy is the traditional call-and-response preaching method used so effectively in churches of the southern USA to encourage participation by the assembled parishioners. The preacher calls out a line of a gospel or hymn and the congregation responds as one to the cue. A recent paper identifies Nramp as a potential molecular preacher, and Salmonella, and probably other pathogenic bacteria, are singing back full-throated.

Genetic control of susceptibility to bacterial infections in mouse models

Historically, the laboratory mouse (Mus musculus) has been the experimental model of choice to study pathophysiology of infection with bacterial pathogens, including natural and acquired host defence mechanisms. Inbred mouse strains differ significantly in their degree of susceptibility to infection with various human pathogens such as Mycobacterium, Salmonella, Legionella and many others. Segregation analyses and linkage studies have indicated that some of these differences are under simple genetic control whereas others behave as complex traits. Major advances in genome technologies have greatly facilitated positional cloning of single gene effects. Thus, a number of genes playing a key role in initial susceptibility, progression and outcome of infection have been uncovered and the functional characterization of the encoded proteins has provided new insight into the molecular basis of antimicrobial defences of polymorphonuclear leukocytes, macrophages, as well as T and B lymphocytes. The multigenic control of susceptibility to infection with certain human pathogens is beginning to be characterized by quantitative trait locus mapping in genome wide scans. This review summarizes recent progress on the mapping, cloning and characterization of genes and proteins that affect susceptibility to infection with major intracellular bacterial pathogens.

Genetic control of visceral leishmaniasis in a Sudanese population: candidate gene testing indicates a linkage to the NRAMP1 region

There is some evidence showing that genetic factors are involved in human susceptibility to parasitic diseases such as schistosomiasis and malaria. Studies have shown that the Nramp1 and H-2 genes are implicated in the control of Leishmania donovani infection in mice. We sought genetic loci involved in the control of susceptibility to visceral disease caused by L. donovani in humans. We studied 37 families with at least two affected sibs living in a village in eastern Sudan, where an outbreak of visceral leishmaniasis occurred between 1995 and 2000. The genetic markers located in five chromosomal regions containing candidate genes were typed: 2q35 (NRAMP1), 5q31-q33 (Th2 cytokine cluster), 6p21 (HLA/TNF-alpha), 6q23 (INFGRI) and 12q15 (INF-gamma). Linkage (multipoint lod-score=1.08; P=0.01) was observed for the 5'(CA) repeat polymorphism in the NRAMP1 promoter. This suggests that genetic variations of this gene affect susceptibility to visceral leishmaniasis in this population.

Mice, microbes and models of infection

We urgently need animal models to study infectious disease. Mice are susceptible to a similar range of microbial infections as humans. Marked differences between inbred strains of mice in their response to pathogen infection can be exploited to analyse the genetic basis of infections. In addition, the genetic tools that are available in the laboratory mouse, and new techniques to monitor the expression of bacterial genes in vivo, make it the principal experimental animal model for studying mechanisms of infection and immunity.

Nramp1 is not a major determinant in the control of Brucella melitensis infection in mice

Brucella, the causative agent of brucellosis in animals and humans, can survive and proliferate within macrophages. Macrophages mediate mouse resistance to various pathogens through the expression of the Nramp1 gene. The role of this gene in the control of Brucella infection was investigated. When BALB/c mice (Nramp1(s)) and C.CB congenic mice (Nramp1(r)) were infected with Brucella melitensis, the number of Brucella organisms per spleen was significantly larger in the C.CB mice than in the BALB/c mice during the first week postinfection (p.i.). This Nramp1-linked susceptibility to Brucella was temporary, since similar numbers of Brucella were recovered from the two strains of mice 2 weeks p.i. The effect of Nramp1 expression occurred within splenocytes intracellularly infected by BRUCELLA: However, there was no difference between in vitro replication rates of Brucella in macrophages isolated from the two strains of mice infected in vivo or in Nramp1 RAW264 transfectants. In mice, infection with Brucella induced an inflammatory response, resulting in splenomegaly and recruitment of phagocytes in the spleen, which was amplified in C.CB mice. Reverse transcription-PCR (RT-PCR), performed 5 days p.i., showed that inducible nitric oxide synthase, tumor necrosis factor alpha (TNF-alpha), interleukin-12 p40 (IL-12p40), gamma interferon (IFN-gamma), and IL-10 mRNAs were similarly induced in spleens of the two strains. In contrast, the mRNA of KC, a C-X-C chemokine, was induced only in infected C.CB mice at this time. This pattern of mRNA expression was maintained at 14 days p.i., with IFN-gamma and IL-12p40 mRNAs being more intensively induced in the infected C.CB mice, but TNF-alpha mRNA was no longer induced. The higher recruitment of neutrophils observed in the spleens of infected C.CB mice could explain the temporary susceptibility of C.CB mice to B. melitensis infection. In contrast to infections with Salmonella, Leishmania, and Mycobacterium, the expression of the Nramp1 gene appears to be of limited importance for the natural resistance of mice to Brucella.

Divalent cation transport and susceptibility to infectious and autoimmune disease: continuation of the Ity/Lsh/Bcg/Nramp1/Slc11a1 gene story

Solute carrier family 11 member a1 (Slc11a1), formerly known as Nramp1/Ity/Lsh/Bcg, is a proton/divalent cation antiporter that regulates susceptibility to infectious and autoimmune disease. Here we review recent studies on (1) the role of Slc11a1 in iron metabolism and iron recycling in macrophages; (2) the use of mouse breeding and introgression of knockouts onto Slc11a1 congenic backgrounds for genes encoding the multiple pleiotropic functions associated with Slc11a1; and (3) associations/linkages of SLC11A1 with human disease and how these relate to functional promoter region polymorphisms.

Host-pathogen interactions: Host resistance factor Nramp1 up-regulates the expression of Salmonella pathogenicity island-2 virulence genes

Nramp1 (Natural resistance-associated macrophage protein-1; also known as Slc11a1) is a host resistance gene that provides protection against several intracellular pathogens, including Salmonella enterica serovar Typhimurium. Little is known about the dynamic interplay that occurs between mammalian host resistance determinants such as Nramp1 and pathogens during infection. To explore these interactions, we examined the effect of Nramp1 on expression of Salmonella typhimurium (STM) virulence factors. We demonstrate that Salmonella pathogenicity island 2 (SPI2) is essential for replication of STM in spleens of infected Nramp1(+/+) mice. Furthermore, the presence of Nramp1 in transfected cell lines and congenic knockout mice resulted in the up-regulation of STM SPI2-associated virulence genes critical for intramacrophage survival. This Nramp1-dependent up-regulation of SPI2 was mimicked in vitro by chelation of iron, demonstrating the iron-responsive nature of expression of STM SPI2-associated virulence genes. We propose that acquisition of SPI2 by S. enterica not only enabled this bacterium to become an effective intracellular pathogen but also allowed the bacterium to withstand the effects of macrophage defense mechanisms such as Nramp1 early in the evolution of its pathogenic character. These dynamic Nramp1-pathogen interactions may be essential for regulating the course of an infection. This study demonstrates the presence of a previously undescribed direct influence of a mammalian innate host resistance locus on a pathogen at the genetic level.

Identification of quantitative trait loci controlling acute virulence in Toxoplasma gondii

Strains of Toxoplasma gondii can be grouped into three predominant clonal lineages with members of the type I group being uniformly lethal in mice. To elucidate the basis of this extreme virulence, a genetic cross was performed between a highly virulent type I strain (GT-1) and a less-virulent type III strain (CTG), and the phenotypes of resulting progeny were analyzed by genetic linkage mapping. Analysis of independent recombinant progeny identified several quantitative trait loci that contributed to acute virulence. A major quantitative trait locus located on chromosome VII accounted for approximately 50% of the virulence phenotype, whereas a minor locus on chromosome IV, linked to the ROP1 gene, accounted for approximately 10%. These loci are conserved in other type I strains, indicating that acute virulence is controlled by discrete genes common to the type I lineage.

Solute carrier 11a1 (Slc11a1; formerly Nramp1) regulates metabolism and release of iron acquired by phagocytic, but not transferrin-receptor-mediated, iron uptake

Solute carrier 11a1 (Slc11a1; formerly Nramp1; where Nramp stands for natural-resistance-associated macrophage protein) is a proton/bivalent cation antiporter that localizes to late endosomes/lysosomes and controls resistance to pathogens. In the present study the role of Slc11a1 in iron turnover is examined in macrophages transfected with Slc11a1(Gly169) (wild-type) or Slc11a1(Asp169) (mutant=functional null) alleles. Following direct acquisition of transferrin (Tf)-bound iron via the Tf receptor, iron uptake and release was equivalent in wild-type and mutant macrophages and was not influenced by interferon-gamma/lipopolysaccharide activation. Following phagocytosis of [(59)Fe]Tf-anti-Tf immune complexes, iron uptake was equivalent and up-regulated similarly with activation, but intracellular distribution was markedly different. In wild-type macrophages most iron was in the soluble (60%) rather than insoluble (12%) fraction, with 28% ferritin (Ft)-bound. With activation, the soluble component increased to 82% at the expense of Ft-bound iron (<5%). In mutant macrophages, 40-50% of iron was in insoluble form, 50-60% was soluble and <5% was Ft-bound. Western-blot analysis confirmed failure of mutant macrophages to degrade complexes 24 h after phagocytic uptake. Confocal microscopy showed that complexes were within lysosome-associated membrane protein 1-positive vesicles in wild-type and mutant macrophages at 30 min and 24 h, implying failure in the degradative process in mature phagosomes in mutant macrophages. NO-mediated iron release was 2.4-fold higher in activated wild-type macrophages compared with mutant macrophages. Overall, our data suggest that iron acquired by phagocytosis and degradation is retained within the phagosomal compartment in wild-type macrophages, and that NO triggers iron release by direct secretion of phagosomal contents rather than via the cytoplasm.