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

Role of solute carrier transporters in regulating dendritic cell maturation and function

Dendritic cells (DCs) are specialized antigen-presenting cells that initiate and regulate innate and adaptive immune responses. Solute carrier (SLC) transporters mediate diverse physiological functions and maintain cellular metabolite homeostasis. Recent studies have highlighted the significance of SLCs in immune processes. Notably, upon activation, immune cells undergo rapid and robust metabolic reprogramming, largely dependent on SLCs to modulate diverse immunological responses. In this review, we explore the central roles of SLC proteins and their transported substrates in shaping DC functions. We provide a comprehensive overview of recent studies on amino acid transporters, metal ion transporters, and glucose transporters, emphasizing their essential contributions to DC homeostasis under varying pathological conditions. Finally, we propose potential strategies for targeting SLCs in DCs to bolster immunotherapy for a spectrum of human diseases.

Roles of natural resistance-associated macrophage protein-1 in modulating bacterial distribution and immune responses during Salmonella enterica serovar Typhimurium infection in murine pregnancy

PROBLEM

Salmonella enterica serovar Typhimurium (S.Tm) infection in Nramp1+/+ mice during pregnancy can lead to profound bacterial growth in the feto-placental unit and adverse pregnancy outcomes, including fetal loss, maternal illness and death. The kinetics and mechanisms by which S.Tm gains entry within individual feto-placental unit, and disseminates through tissues leading to placental resorption and fetal demise remain unclear.

METHOD OF STUDY

Mice were systemically infected with S.Tm. Bacterial burden within spleen and individual placentas, and placental/fetal resorptions were quantified. Flow cytometric analysis of immune cell types in the spleen and individual placentas was performed. Cytokine expression in maternal serum was determined through cytometric bead array.

RESULTS

Systemic infection with S.Tm resulted in preferential bacterial proliferation in placentas compared to the spleen in Nramp1+/+ mice. At 24 h post-infection, the mean infection rate of individual placentas per mouse was ∼50%, increasing to >75% by 72 h post-infection, suggesting that initial infection in few sites progresses to rapid spread of infection through the uterine milieu. This correlated with a steady increase in placental/fetal resorption rates. Placental infection was associated with local increased neutrophil percentages, whereas numbers and percentages in the spleen remained unchanged, suggesting dichotomous modulation of inflammation between the systemic compartment and the feto-maternal interface. Reduced survival rates of pregnant mice during infection correlated with decreased serum IFN-γ but increased IL-10 levels relative to non-pregnant controls.

CONCLUSION

Pregnancy compromises host resistance conferred by Nramp1 against S.Tm through compartment-specific regulation of maternal and placental cellular responses, and modulation of systemic cytokine expression.

Microbiota manipulation to increase macrophage IL-10 improves colitis and limits colitis-associated colorectal cancer

Inflammatory bowel disease (IBD) is a chronic life-long inflammatory disease affecting almost 2 million Americans. Although new biologic therapies have been developed, the standard medical treatment fails to selectively control the dysregulated immune pathways involved in chronic colonic inflammation. Further, IBD patients with uncontrolled colonic inflammation are at a higher risk for developing colorectal cancer (CRC). Intestinal microbes can impact many immune functions, and here we asked if they could be used to improve intestinal inflammation. By utilizing an intestinal adherent E. coli that we find increases IL-10 producing macrophages, we were able to limit intestinal inflammation and restrict tumor formation. Macrophage IL-10 along with IL-10 signaling to the intestinal epithelium were required for protection in both inflammation and tumor development. Our work highlights that administration of immune modulating microbes can improve intestinal outcomes by altering tissue inflammation.

Mouse Model to Study Salmonella-Induced Colitis

Salmonella efficiently colonizes the cecum and proximal colon of mice where it induces inflammation resulting in colitis. To study intestinal infection of non-typhoidal Salmonella enterica serovars in mice, the colonization resistance of the intestine is overcome by transiently reducing the gut microbiota by an oral antibiotic treatment 1 day prior to infection with Salmonella. The in vivo colitis model is crucial for understanding the role of mucosal host defenses, analysis of histopathological changes, and the identification of host and bacterial factors leading to acute infections or facilitating bacterial persistence.

Cytokine-Mediated Regulation of ARG1 in Macrophages and Its Impact on the Control of Salmonella enterica Serovar Typhimurium Infection

Arginase 1 (ARG1) is a cytosolic enzyme that cleaves L-arginine, the substrate of inducible nitric oxide synthase (iNOS), and thereby impairs the control of various intracellular pathogens. Herein, we investigated the role of ARG1 during infection with Salmonella enterica serovar Typhimurium (S.tm). To study the impact of ARG1 on Salmonella infections in vitro, bone marrow-derived macrophages (BMDM) from C57BL/6N wild-type, ARG1-deficient Tie2Cre+/-ARG1fl/fl and NRAMPG169 C57BL/6N mice were infected with S.tm. In wild-type BMDM, ARG1 was induced by S.tm and further upregulated by the addition of interleukin (IL)-4, whereas interferon-γ had an inhibitory effect. Deletion of ARG1 did not result in a reduction in bacterial numbers. In vivo, Arg1 mRNA was upregulated in the spleen, but not in the liver of C57BL/6N mice following intraperitoneal S.tm infection. The genetic deletion of ARG1 (Tie2Cre+/-ARG1fl/fl) or its pharmacological inhibition with CB-1158 neither affected the numbers of S.tm in spleen, liver and blood nor the expression of host response genes such as iNOS, IL-6 or tumour necrosis factor (TNF). Furthermore, ARG1 was dispensable for pathogen control irrespective of the presence or absence of the phagolysosomal natural resistance-associated macrophage protein 1 (NRAMP1). Thus, unlike the detrimental function of ARG1 seen during infections with other intraphagosomal microorganisms, ARG1 did not support bacterial survival in systemic salmonellosis, indicating differential roles of arginine metabolism for host immune response and microbe persistence depending on the type of pathogen.

Thymic development of gut-microbiota-specific T cells

Humans and their microbiota have coevolved a mutually beneficial relationship in which the human host provides a hospitable environment for the microorganisms and the microbiota provides many advantages for the host, including nutritional benefits and protection from pathogen infection¹. Maintaining this relationship requires a careful immune balance to contain commensal microorganisms within the lumen while limiting inflammatory anti-commensal responses^1,2. Antigen-specific recognition of intestinal microorganisms by T cells has previously been described^3,4. Although the local environment shapes the differentiation of effector cells^3-5 it is unclear how microbiota-specific T cells are educated in the thymus. Here we show that intestinal colonization in early life leads to the trafficking of microbial antigens from the intestine to the thymus by intestinal dendritic cells, which then induce the expansion of microbiota-specific T cells. Once in the periphery, microbiota-specific T cells have pathogenic potential or can protect against related pathogens. In this way, the developing microbiota shapes and expands the thymic and peripheral T cell repertoire, allowing for enhanced recognition of intestinal microorganisms and pathogens.

IFN-alpha receptor deficiency enhances host resistance to oral Salmonella enterica serovar Typhimurium infection during murine pregnancy

PROBLEM

Maternal tolerance during pregnancy increases the risk of infection with certain intracellular pathogens. Systemic Salmonella enterica serovar Typhimurium (S.Tm) infection during pregnancy in normally resistant 129X1/SvJ mice leads to severe placental infection, as well as fetal and maternal deaths. However, the effect of oral infection with S.Tm in pregnant mice and the roles of infection-induced inflammation and cell death pathways in contributing to susceptibility to infection are unclear.

METHOD OF STUDY

Non-pregnant and pregnant C57BL/6J wild-type (WT) and cell death pathway-altered mice (IFNAR1^-/- , Caspase-1, 11^-/- , RIP3^-/- ) were infected orally with S.Tm. Host survival and fetal resorption were determined. Bacterial burden in mesenteric lymph nodes (MLNs), spleen, liver, and placentas was enumerated at various time points post-infection. Serum cytokine expression was measured through cytometric bead array.

RESULTS

Oral infection of WT mice with S.Tm on days 9-10 of gestation resulted in systemic dissemination of the bacteria, substantial placental colonization, and fetal loss 5 days post-infection. Histopathological examination of the placentas indicated that infection-induced widespread focal necrosis and neutrophil infiltration throughout the spongiotrophoblast (SpT) layer. In the non-pregnant state, IFNAR1^-/- mice exhibited increased survival following oral S.Tm infection relative to Caspase-1, 11^-/- , RIP3^-/- , and WT mice. The increased resistance to S.Tm infection in IFNAR1^-/- mice was seen during pregnancy as well, with decreased bacterial burden within MLNs, spleen, and placenta, which correlated with the decreased resorptions relative to WT and Caspase-1, 11^-/- mice.

CONCLUSION

Oral S.Tm exposure leads to placental infection, inflammation, and resorption, whereas IFNAR1 deficiency enhances host resistance both in the non-pregnant and pregnant states.

Genetic variation in the MacAB-TolC efflux pump influences pathogenesis of invasive Salmonella isolates from Africa

The various sub-species of Salmonella enterica cause a range of disease in human hosts. The human-adapted Salmonella enterica serovar Typhi enters the gastrointestinal tract and invades systemic sites to cause enteric (typhoid) fever. In contrast, most non-typhoidal serovars of Salmonella are primarily restricted to gut tissues. Across Africa, invasive non-typhoidal Salmonella (iNTS) have emerged with an ability to spread beyond the gastrointestinal tract and cause systemic bloodstream infections with increased morbidity and mortality. To investigate this evolution in pathogenesis, we compared the genomes of African iNTS isolates with other Salmonella enterica serovar Typhimurium and identified several macA and macB gene variants unique to African iNTS. MacAB forms a tripartite efflux pump with TolC and is implicated in Salmonella pathogenesis. We show that macAB transcription is upregulated during macrophage infection and after antimicrobial peptide exposure, with macAB transcription being supported by the PhoP/Q two-component system. Constitutive expression of macAB improves survival of Salmonella in the presence of the antimicrobial peptide C18G. Furthermore, these macAB variants affect replication in macrophages and influence fitness during colonization of the murine gastrointestinal tract. Importantly, the infection outcome resulting from these macAB variants depends upon both the Salmonella Typhimurium genetic background and the host gene Nramp1, an important determinant of innate resistance to intracellular bacterial infection. The variations we have identified in the MacAB-TolC efflux pump in African iNTS may reflect evolution within human host populations that are compromised in their ability to clear intracellular Salmonella infections.

IL17A Regulates Tumor Latency and Metastasis in Lung Adeno and Squamous SQ.2b and AD.1 Cancer

Somatic mutations can promote malignant transformation of airway epithelial cells and induce inflammatory responses directed against resultant tumors. Tumor-infiltrating T lymphocytes (TIL) in early-stage non-small cell lung cancer (NSCLC) secrete distinct proinflammatory cytokines, but the contribution of these TILs to tumor development and metastasis remains unknown. We show here that TILs in early-stage NSCLC are biased toward IL17A expression (Th17) when compared with adjacent tumor-free tissue, whereas Th17 cells are decreased in tumor infiltrating locoregional lymph nodes in advanced NSCLC. Mice in which Pten and Smad4 (Pts4d/d ) are deleted from airway epithelial cells develop spontaneous tumors, that share genetic signatures with squamous- (SQ.2b), and adeno- (AD.1) subtypes of human NSCLC. Pts4d/d mice globally lacking in IL17a (Pts4d/dIl17a-/- ) showed decreased tumor latency and increased metastasis. Th17 cells were required for recruitment of CD103+ dendritic cells, and adoptive transfer of IL17a-sufficient CD4+ T cells reversed early tumor development and metastasis in Pts4d/dIl17a-/- mice. Together, these findings support a key role for Th17 cells in TILs associated with the Pts4d/d model of NSCLC and suggest therapeutic and biomarker strategies for human SQ2b and AD1 lung cancer. Cancer Immunol Res; 6(6); 645-57. ©2018 AACR.

Burkholderia pseudomallei Evades Nramp1 (Slc11a1)- and NADPH Oxidase-Mediated Killing in Macrophages and Exhibits Nramp1-Dependent Virulence Gene Expression

Bacterial survival in macrophages can be affected by the natural resistance-associated macrophage protein 1 (Nramp1; also known as solute carrier family 11 member a1 or Slc11a1) which localizes to phagosome membranes and transports divalent cations, including iron. Little is known about the role of Nramp1 in Burkholderia infection, in particular whether this differs for pathogenic species like Burkholderia pseudomallei causing melioidosis or non-pathogenic species like Burkholderia thailandensis. Here we show that transfected macrophages stably expressing wild-type Nramp1 (Nramp1⁺) control the net replication of B. thailandensis, but not B. pseudomallei. Control of B. thailandensis was associated with increased cytokine responses, and could be abrogated by blocking NADPH oxidase-mediated production of reactive oxygen species but not by blocking generation of reactive nitrogen species. The inability of Nramp1⁺ macrophages to control B. pseudomallei was associated with rapid escape of bacteria from phagosomes, as indicated by decreased co-localization with LAMP1 compared to B. thailandensis. A B. pseudomallei bipB mutant impaired in escape from phagosomes was controlled to a greater extent than the parent strain in Nramp1⁺ macrophages, but was also attenuated in Nramp1⁻ cells. Consistent with reduced escape from phagosomes, B. thailandensis formed fewer multinucleated giant cells in Nramp1⁺ macrophages at later time points compared to B. pseudomallei. B. pseudomallei exhibited elevated transcription of virulence-associated genes of Type VI Secretion System cluster 1 (T6SS-1), the Bsa Type III Secretion System (T3SS-3) and the bimA gene required for actin-based motility in Nramp1⁺ macrophages. Nramp1⁺ macrophages were found to contain decreased iron levels that may impact on expression of such genes. Our data show that B. pseudomallei is able to evade Nramp1- and NADPH oxidase-mediated killing in macrophages and that expression of virulence-associated genes by pathogenic B pseudomallei is enhanced in macrophages expressing wild-type compared to non-functional Nramp1. B. thailandensis has been proposed as surrogate for B. pseudomallei in the study of melioidosis however our study highlights important differences in the interaction of these bacteria with macrophages.

Solute carrier protein family 11 member 1 (Slc11a1) activation efficiently inhibits Leishmania donovani survival in host macrophages

Visceral leishmaniasis (kala-azar), a life threatening disease caused by L. donovani, is a latent threat to more than 147 million people living in disease endemic South East Asia region of the Indian subcontinent. The therapeutic option to control leishmanial infections are very limited, and at present comprise only two drugs, an antifungal amphotericin B and an antitumor miltefosine, which are also highly vulnerable for parasitic resistance. Therefore, identification and development of alternate control measures is an exigent requirement to control leishmanial infections. In this study, we report that functionally induced expression of solute carrier protein family 11 member 1 (Slc11a1), a transmembrane divalent cationic transporter recruited on the surface of phagolysosomes after phagocytosis of parasites, effectively inhibits Leishmania donovani growth in host macrophages. Further, the increased Slc11a1 functionality also resulted in increased production of NOx, TNF-α and IL-12 by activated macrophages. The findings of this study signify the importance of interplay between Slc11a1 expression and macrophages activation that can be effectively used to control of Leishmania growth and survival.

Genetic determinants in the development of sensitization to environmental allergens in early childhood

Sensitization to environmental allergens remains one of the strongest risk factors for asthma, and there is likely a genetic basis. We sought to identify genetic determinants for the development of allergic sensitization to environmental allergens, particularly cockroach allergen, in early childhood. A total of 631 children with the information about genotypic data on 895 single nucleotide polymorphisms (SNPs) in 179 candidate genes were selected from an existing dataset (Boston Birth Cohort). Genetic analysis was performed for allergic sensitizations among all subjects and sub-population, Black/African, respectively. Eight SNPs in seven genes showed significant association with allergic sensitization with P < 0.05, including two top SNPs, rs7851969 in JAK2 (P = 0.003) and rs11739089 in CNOT6 (P = 0.008). When analyses were specifically performed for cockroach sensitization, 16 SNPs in 13 genes showed P < 0.05, including five genes with SNPs at P < 0.01 (JAK1, JAK3, IL5RA, FCER1A, and ADAM33). Particularly, haplotype analyses demonstrated that multiple-haplotypes in FCER1A were significantly associated with cockroach sensitization with the strongest association for a 2-marker haplotype (rs6665683T-rs12136904T, P = 0.001). Furthermore, SNP rs6665683 was marginally associated with the levels of cockroach allergen specific IgE. When a similar analysis was performed for house dust mite, four SNPs in three genes (JAK2, MAML1, and NOD1) had P < 0.01. Of these, JAK2 appeared to be an only gene showing association across the sensitizations we analyzed. Some of findings were further validated when analysis was limited to black population. Our study identified several loci that may confer the susceptibility to allergic sensitization, and suggested that sensitization to allergens may depend on their unique loci.

Salmonellae PhoPQ regulation of the outer membrane to resist innate immunity

Salmonellae sense host cues to regulate properties important for bacterial survival and replication within host tissues. The PhoPQ two-component regulatory system senses phagosome acidification and cationic antimicrobial peptides (CAMP) to regulate the protein and lipid contents of the bacterial envelope that comprises an inner and outer membrane. PhoPQ-regulated lipid components of the outer membrane include lipopolysaccharides and glycerophospholipids. Envelope proteins regulated by PhoPQ, include: components of virulence associated secretion systems, the flagellar apparatus, membrane transport systems, and proteins that are likely structural components of the outer membrane. PhoPQ alteration of the bacterial surface results in increased bacterial resistance to CAMP and decreased detection by the innate immune system. This review details the molecular complexity of the bacterial cell envelope and highlights the outer membrane lipid bilayer as an environmentally regulated bacterial organelle.

Active modification of host inflammation by Salmonella

The dampening of host immune responses is a critical aspect of pathogenesis for the enteropathogen Salmonella enterica. Our laboratory has recently described a role for the secreted effector GogB in disruption of NFκB activation and limitation of the host inflammatory response to infection. GogB alters the NFκB pathway by preventing IκB degradation by the host SCF E3 ubiquitin ligase, through an interaction with Skp1 and FBXO22. The prevention of NFκB activation through this interaction dampens the host inflammatory response in the gut, which in turn limits the damage to host tissues during chronic infection. In this addendum, we summarize these recent findings and discuss their implications and impact in the area of host-pathogen interactions.

Salmonella enterica causes more severe inflammatory disease in C57/BL6 Nramp1G169 mice than Sv129S6 mice

Salmonella enterica serovar Typhimurium (S. Typhimurium) causes systemic inflammatory disease in mice by colonizing cells of the mononuclear leukocyte lineage. Mouse strains resistant to S. Typhimurium, including Sv129S6, have an intact Nramp1 (Slc11a1) allele and survive acute infection, whereas C57/BL6 mice, homozygous for a mutant Nramp1 allele, Nramp1(G169D) , develop lethal infections. Restoration of Nramp1 (C57/BL6 Nramp1(G169) ) reestablishes resistance to S. Typhimurium; mice survive at least 3 to 4 weeks postinfection. Since many transgenic mouse strains are on a C57/BL6 genetic background, C57/BL6 Nramp1(G169) mice provide a model to examine host genetic determinants of resistance to infection. To further evaluate host immune response to S. Typhimurium, we performed comparative analyses of Sv129S6 and C57/BL6 Nramp1(G169) mice 3 weeks following oral S. Typhimurium infection. C57/BL6 Nramp1(G169) mice developed more severe inflammatory disease with splenic bacterial counts 1000-fold higher than Sv129S6 mice and relatively greater splenomegaly and blood neutrophil and monocyte counts. Infected C57/BL6 Nramp1(G169) mice developed higher proinflammatory serum cytokine and chemokine responses (interferon-γ, tumor necrosis factor-α, interleukin [IL]-1β, and IL-2 and monocyte chemotactic protein-1 and chemokine [C-X-C motif] ligand 1, respectively) and marked decreases in anti-inflammatory serum cytokine concentrations (IL-10, IL-4) compared with Sv129S6 mice postinfection. Splenic dendritic cells and macrophages in infected compared with control mice increased to a greater extent in C57/BL6 Nramp1(G169) mice than in Sv129S6 mice. Overall, data show that despite the Nramp1 gene present in both strains, C57/BL6 Nramp1(G169) mice develop more severe, Th1-skewed, acute inflammatory responses to S. Typhimurium infection compared with Sv129S6 mice. Both strains are suitable model systems for studying inflammation in the context of adaptive immunity.

Microbiota restricts trafficking of bacteria to mesenteric lymph nodes by CX(3)CR1(hi) cells

The intestinal microbiota have critical roles in immune system and metabolic homeostasis, but they must be tolerated by the host to avoid inflammatory responses that can damage the epithelial barrier separating the host from the luminal contents^1-6. Breakdown of this regulation and the resulting inappropriate immune response to commensals are thought to lead to the development of inflammatory bowel diseases (IBDs) such as Crohn's disease and ulcerative colitis⁷. We hypothesized that the intestinal immune system is instructed by the microbiota to limit responses to luminal antigens. We demonstrate that, at steady state, the microbiota inhibit the transport of both commensal and pathogenic bacteria from the lumen to a key immune inductive site, the mesenteric lymph node (MLN). However, in the absence of Myd88 or under conditions of antibiotic-induced dysbiosis, non-invasive bacteria trafficked to the MLN in a CCR7-dependent manner and induced both T cell responses and IgA production. Trafficking was carried out by CX₃CR1^hi mononuclear phagocytes, an intestinal cell population previously reported to be non-migratory⁸. These findings define a central role for commensals in regulating the migration to the MLN of CX₃CR1^hi mononuclear phagocytes endowed with the ability to capture luminal bacteria, thereby compartmentalizing the intestinal immune response to avoid inflammation.

Identification and functional characterization of Leishmania donovani secretory peroxidase: delineating its role in NRAMP1 regulation

Leishmania silently evades host immune system and establish in the hostile environment of host macrophage phagolysosomes. For differentiation, growth and division parasite acquires divalent cations especially iron from the host nutritive pool. Natural resistance associated with macrophage protein1 (NRAMP1), a cation transporter that effluxes out divalent cations specifically iron from phagosomal milieu to the cytosol, to create ions deprived status for pathogenic microorganisms. The mechanisms of NRAMP1 regulation are largely unknown in leishmanial infections. In the present study, we identified a secretory Leishmania donovani peroxidase (Prx) that showed peroxidoxin like peroxidase activity and significantly reduced H₂O₂, O₂.⁻ and NO levels in LPS activated macrophages. Further, we also observed down regulated Nramp1 expression and concomitantly declined labile iron pool in activated macrophages treated with identified peroxidase. Prx also decreased levels of TNF-α, IFN-γ and IL-12 in LPS activated macrophages. These observations indicate a bifunctional protective role of secretory Prx; first it reduces redox activation of macrophages, and secondly it allows iron access to Leishmania by down regulating NRAMP1 expression.

Exploring the immune response of porcine mesenteric lymph nodes to Salmonella enterica serovar Typhimurium: an analysis of transcriptional changes, morphological alterations and pathogen burden

Infections caused by Salmonella enterica serovar Typhimurium (S. typhimurium) cause important economic problems in the swine industry and threaten the integrity of a safe and healthy food supply. Controlling the prevalence of Salmonella in pig production requires a thorough knowledge of the response processes that occurs in the gut associated immune tissues. To explore the in vivo porcine response to S. typhimurium, MLN samples from four control pigs and twelve infected animals at 1, 2 and 6 days post infection (dpi) were collected to quantify the mRNA expression of gene coding for 42 innate immune-related molecules. In addition, the presence of S. typhimurium in MLN was examined and its effect on tissue micro-anatomy. Higher S. typhimurium loads were observed at 2dpi, triggering an innate immune response, marked by a substantial infiltration of phagocytes and up-regulation of pro-inflammatory genes. Such response resulted in a significant decrease in pathogen burden in MLN at 6dpi, although Salmonella could not be completely eliminated from tissue. Furthermore, our results suggest that in porcine infections, S. typhimurium might interferes with dendritic cell-T cell interactions and this strategy could be involved in the conversion of Salmonella infected pigs to a carrier state.

The streptomycin mouse model for Salmonella diarrhea: functional analysis of the microbiota, the pathogen's virulence factors, and the host's mucosal immune response

The mammalian intestine is colonized by a dense microbial community, the microbiota. Homeostatic and symbiotic interactions facilitate the peaceful co-existence between the microbiota and the host, and inhibit colonization by most incoming pathogens ('colonization resistance'). However, if pathogenic intruders overcome colonization resistance, a fierce, innate inflammatory defense can be mounted within hours, the adaptive arm of the immune system is initiated, and the pathogen is fought back. The molecular nature of the homeostatic interactions, the pathogen's ability to overcome colonization resistance, and the triggering of native and adaptive mucosal immune responses are still poorly understood. To study these mechanisms, the streptomycin mouse model for Salmonella diarrhea is of great value. Here, we review how S. Typhimurium triggers mucosal immune responses by active (virulence factor elicited) and passive (MyD88-dependent) mechanisms and introduce the S. Typhimurium mutants available for focusing on either response. Interestingly, mucosal defense turns out to be a double-edged sword, limiting pathogen burdens in the gut tissue but enhancing pathogen growth in the gut lumen. This model allows not only studying the molecular pathogenesis of Salmonella diarrhea but also is ideally suited for analyzing innate defenses, microbe handling by mucosal phagocytes, adaptive secretory immunoglobulin A responses, probing microbiota function, and homeostatic microbiota-host interactions. Finally, we discuss the general need for defined assay conditions when using animal models for enteric infections and the central importance of littermate controls.

The aryl hydrocarbon receptor regulates gut immunity through modulation of innate lymphoid cells

Innate lymphoid cells (ILCs) expressing the nuclear receptor RORγt are essential for gut immunity presumably through production of interleukin-22 (IL-22). The molecular mechanism underlying the development of RORγt(+) ILCs is poorly understood. Here, we have shown that the aryl hydrocarbon receptor (Ahr) plays an essential role in RORγt(+) ILC maintenance and function. Expression of Ahr in the hematopoietic compartment was important for accumulation of adult but not fetal intestinal RORγt(+) ILCs. Without Ahr, RORγt(+) ILCs had increased apoptosis and less production of IL-22. RORγt interacted with Ahr and promoted Ahr binding at the Il22 locus. Upon IL-23 stimulation, Ahr-deficient RORγt(+) ILCs had reduced IL-22 expression, consistent with downregulation of IL-23R in those cells. Ahr-deficient mice succumbed to Citrobacter rodentium infection, whereas ectopic expression of IL-22 protected animals from early mortality. Our data uncover a previously unrecognized physiological role for Ahr in promoting innate gut immunity by regulating RORγt(+) ILCs.

Temporal expression of bacterial proteins instructs host CD4 T cell expansion and Th17 development

Pathogens can substantially alter gene expression within an infected host depending on metabolic or virulence requirements in different tissues, however, the effect of these alterations on host immunity are unclear. Here we visualized multiple CD4 T cell responses to temporally expressed proteins in Salmonella-infected mice. Flagellin-specific CD4 T cells expanded and contracted early, differentiated into Th1 and Th17 lineages, and were enriched in mucosal tissues after oral infection. In contrast, CD4 T cells responding to Salmonella Type-III Secretion System (TTSS) effectors steadily accumulated until bacterial clearance was achieved, primarily differentiated into Th1 cells, and were predominantly detected in systemic tissues. Thus, pathogen regulation of antigen expression plays a major role in orchestrating the expansion, differentiation, and location of antigen-specific CD4 T cells in vivo.

Cellular aspects of immunity to intracellular Salmonella enterica

Salmonella enterica is a frequent gastrointestinal pathogen with ability to cause diseases ranging from local gastrointestinal inflammation and diarrhea to life-threatening typhoid fever. Salmonella is an invasive, facultative intracellular pathogen that infects various cell types of the host and can survive and proliferate in different populations of immune cells. During pathogenesis, Salmonella is confronted with various lines of immune defense. To successfully colonize host organisms, the pathogen deploys a set of sophisticated mechanisms of immune evasion and direct manipulation of immune cell functions. In addition to resistance against innate immune mechanisms, including the ability to avoid killing by macrophages and dendritic cells (DCs), Salmonella interferes with antigen presentation by DCs and the formation of an efficient adaptive immune response. In this review, we describe the current understanding of Salmonella virulence factors during intracellular life and focus on the recent advances in the understanding of interference of intracellular Salmonella with cellular functions of immune cells.

The intestinal microbiota plays a role in Salmonella-induced colitis independent of pathogen colonization

The intestinal microbiota is composed of hundreds of species of bacteria, fungi and protozoa and is critical for numerous biological processes, such as nutrient acquisition, vitamin production, and colonization resistance against bacterial pathogens. We studied the role of the intestinal microbiota on host resistance to Salmonella enterica serovar Typhimurium-induced colitis. Using multiple antibiotic treatments in 129S1/SvImJ mice, we showed that disruption of the intestinal microbiota alters host susceptibility to infection. Although all antibiotic treatments caused similar increases in pathogen colonization, the development of enterocolitis was seen only when streptomycin or vancomycin was used; no significant pathology was observed with the use of metronidazole. Interestingly, metronidazole-treated and infected C57BL/6 mice developed severe pathology. We hypothesized that the intestinal microbiota confers resistance to infectious colitis without affecting the ability of S. Typhimurium to colonize the intestine. Indeed, different antibiotic treatments caused distinct shifts in the intestinal microbiota prior to infection. Through fluorescence in situ hybridization, terminal restriction fragment length polymorphism, and real-time PCR, we showed that there is a strong correlation between the intestinal microbiota composition before infection and susceptibility to Salmonella-induced colitis. Members of the Bacteroidetes phylum were present at significantly higher levels in mice resistant to colitis. Further analysis revealed that Porphyromonadaceae levels were also increased in these mice. Conversely, there was a positive correlation between the abundance of Lactobacillus sp. and predisposition to colitis. Our data suggests that different members of the microbiota might be associated with S. Typhimurium colonization and colitis. Dissecting the mechanisms involved in resistance to infection and inflammation will be critical for the development of therapeutic and preventative measures against enteric pathogens.

TLR signaling is required for Salmonella typhimurium virulence

Toll-like receptors (TLRs) contribute to host resistance to microbial pathogens and can drive the evolution of virulence mechanisms. We have examined the relationship between host resistance and pathogen virulence using mice with a functional allele of the nramp-1 gene and lacking combinations of TLRs. Mice deficient in both TLR2 and TLR4 were highly susceptible to the intracellular bacterial pathogen Salmonella typhimurium, consistent with reduced innate immune function. However, mice lacking additional TLRs involved in S. typhimurium recognition were less susceptible to infection. In these TLR-deficient cells, bacteria failed to upregulate Salmonella pathogenicity island 2 (SPI-2) genes and did not form a replicative compartment. We demonstrate that TLR signaling enhances the rate of acidification of the Salmonella-containing phagosome, and inhibition of this acidification prevents SPI-2 induction. Our results indicate that S. typhimurium requires cues from the innate immune system to regulate virulence genes necessary for intracellular survival, growth, and systemic infection.

Quantitative PCR-based competitive index for high-throughput screening of Salmonella virulence factors

Salmonella enterica serovar Typhimurium is an intracellular pathogen and a main cause of food-borne illness. In this study, a quantitative PCR (qPCR)-based competitive index (CI) method was developed to simultaneously compare the growth of multiple Salmonella strains. This method was applied to a mixture of 17 Salmonella mutants lacking regulator genes, and their survival ratios were compared based on expression of natural resistance-associated macrophage protein 1 (Nramp1). Nramp1, as a major host innate immune component, controls the intracellular replication of pathogens. Deletion strains containing unique DNA barcodes in place of regulator genes were mixed with the parental control, and the bacteria were inoculated into congenic mice differing only at Nramp1. Most of the deletion strains were outcompeted by wild-type bacteria in either mouse strain, and the lack of Nramp1 didn't increase the tested strain/parent control replication ratios. When the same collection of mutants was tested in congenic mouse-derived primary macrophages, a major Nramp1-expressing cell type, six strains (ΔhimD, ΔphoP/phoQ, ΔrpoE, ΔrpoS, ΔompR/envZ, and Δhfq strains) grew better in Nramp1(-/-) than in Nramp1(+/+) macrophages, suggesting that these six regulators may play roles in overcoming Nramp1-mediated bactericidal activity in primary macrophages. The discrepancy in survival of macrophages and that of mice suggests either that there are differences in macrophage populations or that other cell types expressing Nramp1 control Salmonella proliferation in the host. The method described allows competitive infection analysis to be carried out on complex mixtures of bacteria and provides high reproducibility from independent biological replicates.

Chronic Salmonella infected mouse model

The bacterial infected mouse model is a powerful model system for studying areas such as infection, inflammation, immunology, signal transduction, and tumorigenesis. Many researchers have taken advantage of the colitis induced by Salmonella typhimurium for the studies on the early phase of inflammation and infection. However, only few reports are on the chronic infection in vivo. Mice with Salmonella persistent existence in the gastrointestinal tract allow us to explore the long-term host-bacterial interaction, signal transduction, and tumorigenesis. We have established a chronic bacterial infected mouse model with Salmonella typhimurium colonization in the mouse intestine over 6 months. To use this system, it is necessary for the researcher to learn how to prepare the bacterial culture and gavage the animals. We detail a methodology for prepare bacterial culture and gavage mice. We also show how to detect the Salmonella persistence in the gastrointestinal tract. Overall, this protocol will aid researchers using the bacterial infected mouse model to address fundamentally important biological and microbiological questions.

Chronic effects of a Salmonella type III secretion effector protein AvrA in vivo

BACKGROUND

Salmonella infection is a common public health problem that can become chronic and increase the risk of inflammatory bowel diseases and cancer. AvrA is a Salmonella bacterial type III secretion effector protein. Increasing evidence demonstrates that AvrA is a multi-functional enzyme with critical roles in inhibiting inflammation, regulating apoptosis, and enhancing proliferation. However, the chronic effects of Salmonella and effector AvrA in vivo are still unknown. Moreover, alive, mutated, non-invasive Salmonella is used as a vector to specifically target cancer cells. However, studies are lacking on chronic infection with non-pathogenic or mutated Salmonella in the host.

METHODS/PRINCIPAL FINDINGS

We infected mice with Salmonella Typhimurium for 27 weeks and investigated the physiological effects as well as the role of AvrA in intestinal inflammation. We found altered body weight, intestinal pathology, and bacterial translocation in spleen, liver, and gallbladder in chronically Salmonella-infected mice. Moreover, AvrA suppressed intestinal inflammation and inhibited the secretion of cytokines IL-12, IFN-gamma, and TNF-alpha. AvrA expression in Salmonella enhanced its invasion ability. Liver abscess and Salmonella translocation in the gallbladder were observed and may be associated with AvrA expression in Salmonella.

CONCLUSION/SIGNIFICANCE

We created a mouse model with persistent Salmonella infection in vivo. Our study further emphasizes the importance of the Salmonella effector protein AvrA in intestinal inflammation, bacterial translocation, and chronic infection in vivo.

Molecular mechanisms of Salmonella virulence and host resistance

Salmonella species can cause typhoid fever and gastroenteritis in humans and pose a global threat to human health. In order to establish a successful infection, Salmonella utilize a large number of genes encoding a variety of virulence factors. Different animal models of infection have been used to better understand the mechanisms underlying each disease including cattle, rodents, and nematodes. To date, a number of different bacterial virulence factors have been identified using such animal models, most of which are secreted by two type three secretion systems (T3SS) encoded within Salmonella pathogenicity islands (SPI) 1 and 2. These proteins alter various host cell pathways, facilitating the invasion of epithelial cells during infection, as well as the survival and replication of Salmonella inside phagocytic cells. On the other hand, host genetics and resistance also play a role in the susceptibility to Salmonella infection. The natural resistance-associated macrophage protein 1 (Nramp1), for example, is critical for host defense, since mice lacking Nramp1 fail to control bacterial replication and succumb to low doses of S. Typhimurium. In this chapter, we analyze the different pathogen and host factors that play a role in the dynamic interaction between Salmonella and its host and their impact on disease.

Salmonella SPI-1-mediated neutrophil recruitment during enteric colitis is associated with reduction and alteration in intestinal microbiota

Gastrointestinal infections involve an interactive tripartite relationship between the invading pathogen, the host, and the host's resident intestinal microbiota. To characterize the host inflammatory response and microbiota alterations during enteric salmonellosis, C57BL/6 mice were pre-treated with a low dose of streptomycin (LD model) and then infected with S. typhimurium strains, including mutants in the two Type III secretion systems, SPI-1 and SPI-2 (invAmut and ssaRmut, respectively). Cecal colonization and inflammation in the LD model were evaluated to assess infection success and progression, and compared to the traditional high dose (HD) model. Perturbations to the microbial community in the LD model were assessed via evaluation of total microbial numbers, the proportion of intestinal γ-Proteobacteria and tRFLP analysis. In the LD model, consistently high colonization by the parental strain (WT) and invAmut S. typhimurium was associated with significant intestinal pathology. However, microbial community profiles were more similar both in numbers and composition between mice infected with the mutant strains, than with the WT strain. Consequently, significant infection-induced inflammation did not always produce similar microbiota perturbations. Large numbers of luminal neutrophils were observed in the ceca of WT-infected, but not in invAmut or ssaRmut infected mice. Neutrophils were thus implicated as a potential mediator of microbiota perturbations during WT enteric salmonellosis. These studies offer a new model of S. typhimurium-induced intestinal disease that retains the three participants of the disease process and further defines the role of virulence factors, the host microbiota, and inflammation in S. typhimurium-induced intestinal disease.

Mesenteric lymph nodes confine dendritic cell-mediated dissemination of Salmonella enterica serovar Typhimurium and limit systemic disease in mice

In humans with typhoid fever or in mouse strains susceptible to Salmonella enterica serovar Typhimurium (S. Typhimurium) infection, bacteria gain access to extraintestinal tissues, causing severe systemic disease. Here we show that in the gut-draining mesenteric lymph nodes (MLN), the majority of S. Typhimurium-carrying cells show dendritic-cell (DC) morphology and express the DC marker CD11c, indicating that S. Typhimurium bacteria are transported to the MLN by migratory DCs. In vivo FLT-3L-induced expansion of DCs, as well as stimulation of DC migration by Toll-like receptor agonists, results in increased numbers of S. Typhimurium bacteria reaching the MLN. Conversely, genetically impaired DC migration in chemokine receptor CCR7-deficient mice reduces the number of S. Typhimurium bacteria reaching the MLN. This indicates that transport of S. Typhimurium from the intestine into the MLN is limited by the number of migratory DCs carrying S. Typhimurium bacteria. In contrast, modulation of DC migration does not affect the number of S. Typhimurium bacteria reaching systemic tissues, indicating that DC-bound transport of S. Typhimurium does not substantially contribute to systemic S. Typhimurium infection. Surgical removal of the MLN results in increased numbers of S. Typhimurium bacteria reaching systemic sites early after infection, thereby rendering otherwise resistant mice susceptible to fatal systemic disease development. This suggests that the MLN provide a vital barrier shielding systemic compartments from DC-mediated dissemination of S. Typhimurium. Thus, confinement of S. Typhimurium in gut-associated lymphoid tissue and MLN delays massive extraintestinal dissemination and at the same time allows for the establishment of protective adaptive immune responses.

Slc11a1 limits intracellular growth of Salmonella enterica sv. Typhimurium by promoting macrophage immune effector functions and impairing bacterial iron acquisition

The natural resistance-associated macrophage protein 1, Slc11a1, is a phagolysosomal transporter for protons and divalent ions including iron that confers host protection against diverse intracellular pathogens including Salmonella. We investigated and compared the regulation of iron homeostasis and immune function in RAW264.7 murine phagocytes stably transfected with non-functional Slc11a1 and functional Slc11a1 controls in response to an infection with Salmonella enterica serovar Typhimurium. We report that macrophages lacking functional Slc11a1 displayed an increased expression of transferrin receptor 1, resulting in enhanced acquisition of transferrin-bound iron. In contrast, cellular iron release mediated via ferroportin 1 was significantly lower in Salmonella-infected Slc11a1-negative macrophages in comparison with phagocytes bearing Slc11a1. Lack of Slc11a1 led to intracellular persistence of S. enterica serovar Typhimurium within macrophages, which was paralleled by a reduced formation of nitric oxide, tumour necrosis factor-alpha and interleukin-6 in Slc11a1-negative macrophages following Salmonella infection, whereas interleukin-10 production was increased. Moreover, Slc11a1-negative phagocytes exhibited higher cellular iron content, resulting in increased iron acquisition by intracellular Salmonella. Our observations indicate a bifunctional role for Slc11a1 within phagocytes. Slc11a restricts iron availability, which first augments pro-inflammatory macrophage effector functions and second concomitantly limits microbial iron access.

Nramp1 drives an accelerated inflammatory response during Salmonella-induced colitis in mice

A recently developed model for enterocolitis in mice involves pre-treatment with the antibiotic streptomycin prior to infection with Salmonella enterica serovar Typhimurium (S. Typhimurium). The contribution of Nramp1/Slc11a1 protein, a critical host defence mechanism against S. Typhimurium, to the development of inflammation in this model has not been studied. Here, we analysed the impact of Nramp1 expression on the early development of colitis using isogenic Nramp1(+/+) and Nramp1(-/-) mice. We hypothesized that Nramp1 acts by rapidly inducing an inflammatory response in the gut mucosa creating an antibacterial environment and limiting spread of S. Typhimurium to systemic sites. We observed that Nramp1(+/+) mice showed lower numbers of S. Typhimurium in the caecum compared with Nramp1(-/-) mice at all times analysed. Acute inflammation was much more pronounced in Nramp1(+/+) mice 1 day after infection. The effect of Nramp1 on development of colitis was characterized by higher secretion of the pro-inflammatory cytokines IFN-gamma, TNF-alpha and MIP-1alpha and a massive infiltration of neutrophils and macrophages, compared with Nramp1(-/-) animals. These data show that an early and rapid inflammatory response results in protection against pathological effects of S. Typhimurium infection in Nramp1(+/+) mice.

A murine intraperitoneal infection model reveals that host resistance to Campylobacter jejuni is Nramp1 dependent

We tested the hypothesis that host resistance to Campylobacter jejuni is Nramp1 dependent. Following intraperitoneal (IP) inoculation of Nramp1+/+ and isogenic Nramp1-deficient (Nramp1-/-) mice C. jejuni primarily associated with mac1-positive cells in liver tissue. A significant reduction of C. jejuni was observed in Nramp1+/+ mice 4 days post-infection (PI) (liver) and 8 days PI cecum-colon. In contrast, Nramp1-/- mice showed no significant reduction of C. jejuni and instead had a chronic inflammatory response and significant histopathological lesions 30 days PI. Differential cytokine profiles were observed in C. jejuni infected Nramp1+/+ and Nramp1-/- primary dendritic cells. Taken together these data indicate that Nramp1 is critical for host resistance to C. jejuni.