A role for natural killer cells in intestinal inflammation caused by infection with Salmonella enterica serovar Typhimurium

Distinct developmental pathways generate functionally distinct populations of natural killer cells

Natural killer (NK) cells function by eliminating virus-infected or tumor cells. Here we identified an NK-lineage-biased progenitor population, referred to as early NK progenitors (ENKPs), which developed into NK cells independently of common precursors for innate lymphoid cells (ILCPs). ENKP-derived NK cells (ENKP_NK cells) and ILCP-derived NK cells (ILCP_NK cells) were transcriptionally different. We devised combinations of surface markers that identified highly enriched ENKP_NK and ILCP_NK cell populations in wild-type mice. Furthermore, Ly49H+ NK cells that responded to mouse cytomegalovirus infection primarily developed from ENKPs, whereas ILCP_NK cells were better IFNγ producers after infection with Salmonella and herpes simplex virus. Human CD56dim and CD56bright NK cells were transcriptionally similar to ENKP_NK cells and ILCP_NK cells, respectively. Our findings establish the existence of two pathways of NK cell development that generate functionally distinct NK cell subsets in mice and further suggest these pathways may be conserved in humans.

Acute Ascaris infection impairs the effector functions of natural killer cells in single and Salmonella co-infected pigs

Natural killer (NK) cells play a key role in defense against Salmonella infections during the early phase of infection. Our previous work showed that the excretory/secretory products of Ascaris suum repressed NK activity in vitro. Here, we asked if NK cell functionality was influenced in domestic pigs during coinfection with Ascaris and Salmonella enterica serotype Typhimurium. Ascaris coinfection completely abolished the IL-12 and IL-18 driven elevation of IFN-γ production seen in CD16 + CD8α + perforin + NK cells of Salmonella single-infected pigs. Furthermore, Ascaris coinfection prohibited the Salmonella-driven rise in NK perforin levels and CD107a surface expression. In line with impaired effector functions, NK cells from Ascaris-single and coinfected pigs displayed elevated expression of the inhibitory KLRA1 and NKG2A receptors genes, contrasting with the higher expression of the activating NKp46 and NKp30 receptors in NK cells during Salmonella single infection. These differences were accompanied by the highly significant upregulation of T-bet protein expression in NK cells from Ascaris-single and Ascaris/Salmonella coinfected pigs. Together, our data strongly indicate a profound repression of NK functionality by an Ascaris infection which may hinder infected individuals from adequately responding to a concurrent bacterial infection.

Innate Lymphoid Cells and Natural Killer Cells in Bacterial Infections: Function, Dysregulation, and Therapeutic Targets

Infectious diseases represent one of the largest medical challenges worldwide. Bacterial infections, in particular, remain a pertinent health challenge and burden. Moreover, such infections increase over time due to the continuous use of various antibiotics without medical need, thus leading to several side effects and bacterial resistance. Our innate immune system represents our first line of defense against any foreign pathogens. This system comprises the innate lymphoid cells (ILCs), including natural killer (NK) cells that are critical players in establishing homeostasis and immunity against infections. ILCs are a group of functionally heterogenous but potent innate immune effector cells that constitute tissue-resident sentinels against intracellular and extracellular bacterial infections. Being a nascent subset of innate lymphocytes, their role in bacterial infections is not clearly understood. Furthermore, these pathogens have developed methods to evade the host immune system, and hence permit infection spread and tissue damage. In this review, we highlight the role of the different ILC populations in various bacterial infections and the possible ways of immune evasion. Additionally, potential immunotherapies to manipulate ILC responses will be briefly discussed.

A detailed analysis of innate and adaptive immune responsiveness upon infection with Salmonella enterica serotype Enteritidis in young broiler chickens

Salmonella enterica serotype Enteritidis (SE) is a zoonotic pathogen which causes foodborne diseases in humans as well as severe disease symptoms in young chickens. More insight in innate and adaptive immune responses of chickens to SE infection is needed to understand elimination of SE. Seven-day-old broiler chickens were experimentally challenged with SE and numbers and responsiveness of innate and adaptive immune cells as well as antibody titers were assessed. SE was observed in the ileum and spleen of SE-infected chickens at 7 days post-infection (dpi). At 1 dpi numbers of intraepithelial cytotoxic CD8⁺ T cells were significantly increased alongside numerically increased intraepithelial IL-2Rα⁺ and 20E5⁺ natural killer (NK) cells at 1 and 3 dpi. At both time points, activation of intraepithelial and splenic NK cells was significantly enhanced. At 7 dpi in the spleen, presence of macrophages and expression of activation markers on dendritic cells were significantly increased. At 21 dpi, SE-induced proliferation of splenic CD4⁺ and CD8⁺ T cells was observed and SE-specific antibodies were detected in sera of all SE-infected chickens. In conclusion, SE results in enhanced numbers and activation of innate cells and we hypothesized that in concert with subsequent specific T cell and antibody responses, reduction of SE is achieved. A better understanding of innate and adaptive immune responses important in the elimination of SE will aid in developing immune-modulation strategies, which may increase resistance to SE in young broiler chickens.

Curcumin and Intestinal Inflammatory Diseases: Molecular Mechanisms of Protection

Intestinal inflammatory diseases, such as Crohn’s disease, ulcerative colitis, and necrotizing enterocolitis, are becoming increasingly prevalent. While knowledge of the pathogenesis of these related diseases is currently incomplete, each of these conditions is thought to involve a dysfunctional, or overstated, host immunological response to both bacteria and dietary antigens, resulting in unchecked intestinal inflammation and, often, alterations in the intestinal microbiome. This inflammation can result in an impaired intestinal barrier allowing for bacterial translocation, potentially resulting in systemic inflammation and, in severe cases, sepsis. Chronic inflammation of this nature, in the case of inflammatory bowel disease, can even spur cancer growth in the longer-term. Recent research has indicated certain natural products with anti-inflammatory properties, such as curcumin, can help tame the inflammation involved in intestinal inflammatory diseases, thus improving intestinal barrier function, and potentially, clinical outcomes. In this review, we explore the potential therapeutic properties of curcumin on intestinal inflammatory diseases, including its antimicrobial and immunomodulatory properties, as well as its potential to alter the intestinal microbiome. Curcumin may play a significant role in intestinal inflammatory disease treatment in the future, particularly as an adjuvant therapy.

Aspergillus fumigatus responds to natural killer (NK) cells with upregulation of stress related genes and inhibits the immunoregulatory function of NK cells

Natural Killer (NK) cells are active against Aspergillus fumigatus, which in turn is able to impair the host defense. Unfortunately, little is known on the mutual interaction of NK cells and A. fumigatus. We coincubated human NK cells with A. fumigatus hyphae and assessed the gene expression and protein concentration of selected molecules. We found that A. fumigatus up-regulates the gene expression of pro-inflammatory molecules in NK cells, but inhibited the release of these molecules resulting in intracellular accumulation and limited extracellular availability. A. fumigatus down-regulatedmRNA levels of perforin in NK cells, but increased its intra- and extracellular protein concentration. The gene expression of stress related molecules of A. fumigatus such as heat shock protein hsp90 was up-regulated by human NK cells. Our data characterize for the first time the immunosuppressive effect of A. fumigatus on NK cells and may help to develop new therapeutic antifungal strategies.

p40 phox -Deficient Mice Exhibit Impaired Bacterial Clearance and Enhanced Pro-inflammatory Responses during Salmonella enterica serovar Typhimurium Infection

Salmonella enterica serovar Typhimurium (S. Typhimurium) is a major cause of acute gastroenteritis in humans. During infection, reactive oxygen species (ROS), generated from NADPH oxidase (a multisubunit enzyme complex), are required for pathogen killing upon phagocytosis and for regulating pro-inflammatory signaling in phagocytic cells. Mutations in subunits forming the NADPH complex may lead to enhanced susceptibility to infection and inflammatory disease. Compared to other NADPH oxidase subunits, the function of p40^phox is relatively understudied, particularly in the context of intestinal bacterial infection. In this study, we utilized genetically engineered mice to determine the role of p40^phox in the response to S. Typhimurium infection. We show that mice lacking p40^phox are more susceptible to oral infection with S. Typhimurium, as demonstrated by significantly enhanced bacterial dissemination to spleen and liver, and development of exacerbated bacterial colitis. Moreover, we demonstrate that the increased infection and disease severity are correlated with markedly increased F4/80⁺ macrophage and Ly6G⁺ neutrophil infiltration in the infected tissues, coincident with significantly elevated pro-inflammatory cytokines (IL-1β and TNF-α) and chemoattractant molecules in the infected tissues. Functional analysis of macrophages and neutrophils further shows that p40^phox deficiency impairs bacteria- or PMA-induced intracellular ROS production as well as intracellular killing of Salmonella. These observations indicate that the p40^phox subunit of NADPH oxidase plays an essential role in suppressing intracellular multiplication of Salmonella in macrophages and in the regulation of both systemic and mucosal inflammatory responses to bacterial infection.

Salmonella Typhimurium Diarrhea Reveals Basic Principles of Enteropathogen Infection and Disease-Promoted DNA Exchange

Despite decades of research, efficient therapies for most enteropathogenic bacteria are still lacking. In this review, we focus on Salmonella enterica Typhimurium (S. Typhimurium), a frequent cause of acute, self-limiting food-borne diarrhea and a model that has revealed key principles of enteropathogen infection. We review the steps of gut infection and the mucosal innate-immune defenses limiting pathogen burdens, and we discuss how inflammation boosts gut luminal S. Typhimurium growth. We also discuss how S. Typhimurium-induced inflammation accelerates the transfer of plasmids and phages, which may promote the transmission of antibiotic resistance and facilitate emergence of pathobionts and pathogens with enhanced virulence. The targeted manipulation of the microbiota and vaccination might offer strategies to prevent this evolution. As gut luminal microbes impact various aspects of the host's physiology, improved strategies for preventing enteropathogen infection and disease-inflicted DNA exchange may be of broad interest well beyond the acute infection.

Colonization and effector functions of innate lymphoid cells in mucosal tissues

Innate lymphoid cells (ILCs) protect mucosal barrier tissues to fight infection and maintain tissue integrity. ILCs and their progenitors are developmentally programmed to migrate, differentiate and populate various mucosal tissues and associated lymphoid tissues. Functionally mature ILC subsets respond to diverse pathogens such as bacteria, viruses, fungi and parasites in subset-specific manners. In this review, we will discuss how ILCs populate mucosal tissues and regulate immune responses to distinct pathogens to protect the host and maintain tissue integrity.

An NK Cell Perforin Response Elicited via IL-18 Controls Mucosal Inflammation Kinetics during Salmonella Gut Infection

Salmonella Typhimurium (S.Tm) is a common cause of self-limiting diarrhea. The mucosal inflammation is thought to arise from a standoff between the pathogen's virulence factors and the host's mucosal innate immune defenses, particularly the mucosal NAIP/NLRC4 inflammasome. However, it had remained unclear how this switches the gut from homeostasis to inflammation. This was studied using the streptomycin mouse model. S.Tm infections in knockout mice, cytokine inhibition and –injection experiments revealed that caspase-1 (not -11) dependent IL-18 is pivotal for inducing acute inflammation. IL-18 boosted NK cell chemoattractants and enhanced the NK cells' migratory capacity, thus promoting mucosal accumulation of mature, activated NK cells. NK cell depletion and Prf^-/- ablation (but not granulocyte-depletion or T-cell deficiency) delayed tissue inflammation. Our data suggest an NK cell perforin response as one limiting factor in mounting gut mucosal inflammation. Thus, IL-18-elicited NK cell perforin responses seem to be critical for coordinating mucosal inflammation during early infection, when S.Tm strongly relies on virulence factors detectable by the inflammasome. This may have broad relevance for mucosal defense against microbial pathogens.

Salmonella Typhimurium is a common cause of foodborne diarrhea. The disease symptoms arise already a few hours after infection. However, it had remained unclear how the immune system can mount the responses eliciting the disease symptoms so quickly. Earlier work in a mouse model had shown that the gut epithelium expresses a sensor, called NAIP/NLRC4/caspase-1 inflammasome that can detect the pathogen and mount a defense by 12-18h p.i. However, it has remained uncharacterized how inflammasome sensing drives the initial gut inflammation. Here, we found that the caspase-1 inflammasome triggers the production of IL-18, a pro-inflammatory cytokine that appears essential for the early onset of inflammation. IL-18 is driving the accumulation of NK cells into the infected mucosa, via the upregulation of NK cell chemoattractants and by the stimulation of their migratory capacity. Mature NK cells seem to induce mucosal inflammation via a perforin-mediated cytotoxic response. These data suggest that the inflammasome/IL-18/NK cell axis is a driver of early mucosal inflammation via a perforin-dependent cytotoxic NK cell response. Future work will have to address, if this mechanism is equally potent in the human gut and may contribute to ramping up the host's response during the first hours of infection. This may have implications for other gut infections and might provide leads for developing therapies.

Influenza Virus Affects Intestinal Microbiota and Secondary Salmonella Infection in the Gut through Type I Interferons

Human influenza viruses replicate almost exclusively in the respiratory tract, yet infected individuals may also develop gastrointestinal symptoms, such as vomiting and diarrhea. However, the molecular mechanisms remain incompletely defined. Using an influenza mouse model, we found that influenza pulmonary infection can significantly alter the intestinal microbiota profile through a mechanism dependent on type I interferons (IFN-Is). Notably, influenza-induced IFN-Is produced in the lungs promote the depletion of obligate anaerobic bacteria and the enrichment of Proteobacteria in the gut, leading to a “dysbiotic” microenvironment. Additionally, we provide evidence that IFN-Is induced in the lungs during influenza pulmonary infection inhibit the antimicrobial and inflammatory responses in the gut during Salmonella-induced colitis, further enhancing Salmonella intestinal colonization and systemic dissemination. Thus, our studies demonstrate a systemic role for IFN-Is in regulating the host immune response in the gut during Salmonella-induced colitis and in altering the intestinal microbial balance after influenza infection.

Influenza is a respiratory illness. Symptoms of flu include fever, headache, extreme tiredness, dry cough, sore throat, runny or stuffy nose, and muscle aches. Some people, especially children, can have additional gastrointestinal symptoms, such as nausea, vomiting, and diarrhea. In humans, there is no evidence that the influenza virus replicates in the intestine. Using an influenza mouse model, we found that influenza infection alters the intestinal microbial community through a mechanism dependent on type I interferons induced in the pulmonary tract. Futhermore, we demonstrate that influenza-induced type I interferons increase the host susceptibility to Salmonella intestinal colonization and dissemination during secondary Salmonella-induced colitis through suppression of host intestinal immunity.

Lack of the programmed death-1 receptor renders host susceptible to enteric microbial infection through impairing the production of the mucosal natural killer cell effector molecules

The programmed death-1 receptor is expressed on a wide range of immune effector cells, including T cells, natural killer T cells, dendritic cells, macrophages, and natural killer cells. In malignancies and chronic viral infections, increased expression of programmed death-1 by T cells is generally associated with a poor prognosis. However, its role in early host microbial defense at the intestinal mucosa is not well understood. We report that programmed death-1 expression is increased on conventional natural killer cells but not on CD4(+), CD8(+) or natural killer T cells, or CD11b(+) or CD11c(+) macrophages or dendritic cells after infection with the mouse pathogen Citrobacter rodentium. Mice genetically deficient in programmed death-1 or treated with anti-programmed death-1 antibody were more susceptible to acute enteric and systemic infection with Citrobacter rodentium. Wild-type but not programmed death-1-deficient mice infected with Citrobacter rodentium showed significantly increased expression of the conventional mucosal NK cell effector molecules granzyme B and perforin. In contrast, natural killer cells from programmed death-1-deficient mice had impaired expression of those mediators. Consistent with programmed death-1 being important for intracellular expression of natural killer cell effector molecules, mice depleted of natural killer cells and perforin-deficient mice manifested increased susceptibility to acute enteric infection with Citrobacter rodentium. Our findings suggest that increased programmed death-1 signaling pathway expression by conventional natural killer cells promotes host protection at the intestinal mucosa during acute infection with a bacterial gut pathogen by enhancing the expression and production of important effectors of natural killer cell function.

Coinfection with an intestinal helminth impairs host innate immunity against Salmonella enterica serovar Typhimurium and exacerbates intestinal inflammation in mice

Salmonella enterica serovar Typhimurium is a Gram-negative food-borne pathogen that is a major cause of acute gastroenteritis in humans. The ability of the host to control such bacterial pathogens may be influenced by host immune status and by concurrent infections. Helminth parasites are of particular interest in this context because of their ability to modulate host immune responses and because their geographic distribution coincides with those parts of the world where infectious gastroenteritis is most problematic. To test the hypothesis that helminth infection may negatively regulate host mucosal innate immunity against bacterial enteropathogens, a murine coinfection model was established by using the intestinal nematode Heligmosomoides polygyrus and S. Typhimurium. We found that mice coinfected with S. Typhimurium and H. polygyrus developed more severe intestinal inflammation than animals infected with S. Typhimurium alone. The enhanced susceptibility to Salmonella-induced intestinal injury in coinfected mice was found to be associated with diminished neutrophil recruitment to the site of bacterial infection that correlated with decreased expression of the chemoattractants CXCL2/macrophage inflammatory protein 2 (MIP-2) and CXCL1/keratinocyte-derived chemokine (KC), poor control of bacterial replication, and exacerbated intestinal inflammation. The mechanism of helminth-induced inhibition of MIP-2 and KC expression involved interleukin-10 (IL-10) and, to a lesser extent, IL-4 and IL-13. Ly6G antibody-mediated depletion of neutrophils reproduced the adverse effects of H. polygyrus on Salmonella infection. Our results suggest that impaired neutrophil recruitment is an important contributor to the enhanced severity of Salmonella enterocolitis associated with helminth coinfection.

Neutrophils are a source of gamma interferon during acute Salmonella enterica serovar Typhimurium colitis

Gamma interferon (IFN-γ) is an important driver of intestinal inflammation during colitis caused by Salmonella enterica serovar Typhimurium. Here we used the mouse colitis model to investigate the cellular sources of IFN-γ in the cecal mucosa during the acute phase of an S. Typhimurium infection. While IFN-γ staining was detected in T cells, NK cells, and inflammatory monocytes at 2 days after infection, the majority of IFN-γ-positive cells in the cecal mucosa were neutrophils. Furthermore, neutrophil depletion blunted mucosal Ifng expression and reduced the severity of intestinal lesions during S. Typhimurium infection. We conclude that neutrophils are a prominent cellular source of IFN-γ during the innate phase of S. Typhimurium-induced colitis.

Altered IFN-γ-mediated immunity and transcriptional expression patterns in N-Ethyl-N-nitrosourea-induced STAT4 mutants confer susceptibility to acute typhoid-like disease

Salmonella enterica is a ubiquitous Gram-negative intracellular bacterium that continues to pose a global challenge to human health. The etiology of Salmonella pathogenesis is complex and controlled by pathogen, environmental, and host genetic factors. In fact, patients immunodeficient in genes in the IL-12, IL-23/IFN-γ pathway are predisposed to invasive nontyphoidal Salmonella infection. Using a forward genomics approach by N-ethyl-N-nitrosourea (ENU) germline mutagenesis in mice, we identified the Ity14 (Immunity to Typhimurium locus 14) pedigree exhibiting increased susceptibility following in vivo Salmonella challenge. A DNA-binding domain mutation (p.G418_E445) in Stat4 (Signal Transducer and Activator of Transcription Factor 4) was the causative mutation. STAT4 signals downstream of IL-12 to mediate transcriptional regulation of inflammatory immune responses. In mutant Ity14 mice, the increased splenic and hepatic bacterial load resulted from an intrinsic defect in innate cell function, IFN-γ-mediated immunity, and disorganized granuloma formation. We further show that NK and NKT cells play an important role in mediating control of Salmonella in Stat4(Ity14/Ity14) mice. Stat4(Ity14/Ity14) mice had increased expression of genes involved in cell-cell interactions and communication, as well as increased CD11b expression on a subset of splenic myeloid dendritic cells, resulting in compromised recruitment of inflammatory cells to the spleen during Salmonella infection. Stat4(Ity14/Ity14) presented upregulated compensatory mechanisms, although inefficient and ultimately Stat4(Ity14/Ity14) mice develop fatal bacteremia. The following study further elucidates the pathophysiological impact of STAT4 during Salmonella infection.

NADPH oxidase deficient mice develop colitis and bacteremia upon infection with normally avirulent, TTSS-1- and TTSS-2-deficient Salmonella Typhimurium

Infections, microbe sampling and occasional leakage of commensal microbiota and their products across the intestinal epithelial cell layer represent a permanent challenge to the intestinal immune system. The production of reactive oxygen species by NADPH oxidase is thought to be a key element of defense. Patients suffering from chronic granulomatous disease are deficient in one of the subunits of NADPH oxidase. They display a high incidence of Crohn’s disease-like intestinal inflammation and are hyper-susceptible to infection with fungi and bacteria, including a 10-fold increased risk of Salmonellosis. It is not completely understood which steps of the infection process are affected by the NADPH oxidase deficiency. We employed a mouse model for Salmonella diarrhea to study how NADPH oxidase deficiency (Cybb^−/−) affects microbe handling by the large intestinal mucosa. In this animal model, wild type S. Typhimurium causes pronounced enteropathy in wild type mice. In contrast, an avirulent S. Typhimurium mutant (S.Tm^avir; invGsseD), which lacks virulence factors boosting trans-epithelial penetration and growth in the lamina propria, cannot cause enteropathy in wild type mice. We found that Cybb^−/− mice are efficiently infected by S.Tm^avir and develop enteropathy by day 4 post infection. Cell depletion experiments and infections in Cybb^−/−Myd88^−/− mice indicated that the S.Tm^avir-inflicted disease in Cybb^−/− mice hinges on CD11c⁺CX₃CR1⁺ monocytic phagocytes mediating colonization of the cecal lamina propria and on Myd88-dependent proinflammatory immune responses. Interestingly, in mixed bone marrow chimeras a partial reconstitution of Cybb-proficiency in the bone marrow derived compartment was sufficient to ameliorate disease severity. Our data indicate that NADPH oxidase expression is of key importance for restricting the growth of S.Tm^avir in the mucosal lamina propria. This provides important insights into microbe handling by the large intestinal mucosa and the role of NADPH oxidase in maintaining microbe-host mutualism at this exposed body surface.

Innate lymphoid cells in the defense against infections

Barrier surfaces are under constant attack by potentially dangerous microbes. Interestingly, mucosal tissues contain a large number of innate lymphocytes now collectively referred to as innate lymphoid cells (ILCs). Different groups of ILCs are being distinguished, each of which produce an array of cytokines strikingly resembling the profile of the various T helper cell effector subsets. Over the last couple of years, evidence has been emerging that the various ILC subsets play important roles in immune defense against mucosal infections. In this review, I will introduce the various groups of ILCs and then focus on their roles for immunity to mucosal infections.

Contribution of Thy1+ NK cells to protective IFN-γ production during Salmonella typhimurium infections

IFN-γ is critical for immunity against infections with intracellular pathogens, such as Salmonella enterica. However, which of the many cell types capable of producing IFN-γ controls Salmonella infections remains unclear. Using a mouse model of systemic Salmonella infection, we observed that only a lack of all lymphocytes or CD90 (Thy1)(+) cells, but not the absence of T cells, Retinoic acid-related orphan receptor (ROR)-γt-dependent lymphocytes, (NK)1.1(+) cells, natural killer T (NKT), and/or B cells alone, replicated the highly susceptible phenotype of IFN-γ-deficient mice to Salmonella infection. A combination of antibody depletions and adoptive transfer experiments revealed that early protective IFN-γ was provided by Thy1-expressing natural killer (NK) cells and that these cells improved antibacterial immunity through the provision of IFN-γ. Further analysis of NK cells producing IFN-γ in response to Salmonella indicated that less mature NK cells were more efficient at mediating antibacterial effector function than terminally differentiated NK cells. Inspired by recent reports of Thy1(+) NK cells contributing to immune memory, we analyzed their role in secondary protection against otherwise lethal WT Salmonella infections. Notably, we observed that a newly generated Salmonella vaccine strain not only conferred superior protection compared with conventional regimens but that this enhanced efficiency of recall immunity was afforded by incorporating CD4(-)CD8(-)Thy1(+) cells into the secondary response. Taken together, these findings demonstrate that Thy1-expressing NK cells play an important role in antibacterial immunity.

Microbial amyloids induce interleukin 17A (IL-17A) and IL-22 responses via Toll-like receptor 2 activation in the intestinal mucosa

The Toll-like receptor 2 (TLR2)/TLR1 receptor complex responds to amyloid fibrils, a common component of biofilm material produced by members of the phyla Firmicutes, Bacteroidetes, and Proteobacteria. To determine whether this TLR2/TLR1 ligand stimulates inflammatory responses when bacteria enter intestinal tissue, we investigated whether expression of curli amyloid fibrils by the invasive enteric pathogen Salmonella enterica serotype Typhimurium contributes to T helper 1 and T helper 17 responses by measuring cytokine production in the mouse colitis model. A csgBA mutant, deficient in curli production, elicited decreased expression of interleukin 17A (IL-17A) and IL-22 in the cecal mucosa compared to the S. Typhimurium wild type. In TLR2-deficient mice, IL-17A and IL-22 expression was blunted during S. Typhimurium infection, suggesting that activation of the TLR2 signaling pathway contributes to the expression of these cytokines. T cells incubated with supernatants from bone marrow-derived dendritic cells (BMDCs) treated with curli fibrils released IL-17A in a TLR2-dependent manner in vitro. Lower levels of IL-6 and IL-23 production were detected in the supernatants of the TLR2-deficient BMDCs treated with curli fibrils. Consistent with this, three distinct T-cell populations-CD4(+) T helper cells, cytotoxic CD8(+) T cells, and γδ T cells-produced IL-17A in response to curli fibrils in the intestinal mucosa during S. Typhimurium infection. Notably, decreased IL-6 expression by the dendritic cells and decreased IL-23 expression by the dendritic cells and macrophages were observed in the cecal mucosa of mice infected with the curli mutant. We conclude that TLR2 recognition of bacterial amyloid fibrils in the intestinal mucosa represents a novel mechanism of immunoregulation, which contributes to the generation of inflammatory responses, including production of IL-17A and IL-22, in response to bacterial entry into the intestinal mucosa.

The role of innate immunity in the host defense against intestinal bacterial pathogens

Eradication of infectious disease is our global health challenge. After encountering intestinal infection with a bacterial pathogen, the host defense program is initiated by local antigen-presenting cells (APCs) that eliminate invading pathogens by phagocytosis and establish localized inflammation by secreting cytokines and chemokines. These pathogen-experienced APCs migrate to the mesenteric lymph nodes, where host immune responses are precisely orchestrated. Initiation and regulation of this defense program appear to be largely dependent on innate immunity which is antigen non-specific and provides a rapid defense against broader targets. On the other hand, many bacterial enteropathogens have evoked abilities to modify the host defense program to their advantage. Therefore, better understanding of the host-pathogen interactions is essential to establish effective eradication strategies for enteric infectious diseases. In this review, we will discuss the current understanding of innate immune regulation of the host defense mechanisms against intestinal infection by bacterial pathogens.

Homeostatic regulation of Salmonella-induced mucosal inflammation and injury by IL-23

IL-12 and IL-23 regulate innate and adaptive immunity to microbial pathogens through influencing the expression of IFN-γ, IL-17, and IL-22. Herein we define the roles of IL-12 and IL-23 in regulating host resistance and intestinal inflammation during acute Salmonella infection. We find that IL-23 alone is dispensable for protection against systemic spread of bacteria, but synergizes with IL-12 for optimal protection. IL-12 promotes the production of IFN-γ by NK cells, which is required for resistance against Salmonella and also for induction of intestinal inflammation and epithelial injury. In contrast, IL-23 controls the severity of inflammation by inhibiting IL-12A expression, reducing IFN-γ and preventing excessive mucosal injury. Our studies demonstrate that IL-23 is a homeostatic regulator of IL-12-dependent, IFN-γ-mediated intestinal inflammation.

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.

Phenotypic variation in IgG receptors by nonclassical FCGR2C alleles

The balance between activating and inhibitory signals from the different FcγRs for IgG ensures homeostasis of many inflammatory responses. FCGR2C is the product of an unequal crossover of the FCGR2A and FCGR2B genes encoding the activating FcγRIIa (CD32a) and inhibitory FcγRIIb (CD32b), respectively. A single nucleotide polymorphism (SNP) in exon 3 of FCGR2C results in either expression of the activating FcγRIIc (CD32c) (FCGR2C-open reading frame [ORF]) or its absence because of a stop codon (FCGR2C-Stop). Two additional variations in FcγRIIb/c expression on leukocytes have now been identified. In case of "nonclassical" FCGR2C-ORF alleles, FcγRIIc expression was unexpectedly absent, because of novel splice site mutations near exon 7 leading to another stop codon. In some individuals with FCGR2C-Stop alleles FcγRIIb was detected on NK cells, which normally are devoid of this protein. Individuals with these nonclassical FCGR2C-Stop alleles carried a deletion of FCGR2C-FCGR3B that extends into the promoter region of the adjacent FCGR2B gene and probably deletes a negative regulatory element in the FCGR2B promoter in NK cells. FcγRIIb expression on NK cells effectively inhibited killing mediated by FcγRIIIa (CD16a) in Ab-dependent cytotoxicity tests. Our findings demonstrate a more extensive and previously unnoticed variation in FcγR expression with relevance to immunity and inflammation.

Stromal IFN-γR-signaling modulates goblet cell function during Salmonella Typhimurium infection

Enteropathogenic bacteria are a frequent cause of diarrhea worldwide. The mucosal defenses against infection are not completely understood. We have used the streptomycin mouse model for Salmonella Typhimurium diarrhea to analyze the role of interferon gamma receptor (IFN-γR)-signaling in mucosal defense. IFN-γ is known to contribute to acute S. Typhimurium diarrhea. We have compared the acute mucosal inflammation in IFN-γR(-/-) mice and wild type animals. IFN-γR(-/-) mice harbored increased pathogen loads in the mucosal epithelium and the lamina propria. Surprisingly, the epithelium of the IFN-γR(-/-) mice did not show the dramatic "loss" of mucus-filled goblet cell vacuoles, a hallmark of the wild type mucosal infection. Using bone marrow chimeric mice we established that IFN-γR-signaling in stromal cells (e.g. goblet cells, enterocytes) controlled mucus excretion/vacuole loss by goblet cells. In contrast, IFN-γR-signaling in bone marrow-derived cells (e.g. macrophages, DCs, PMNs) was required for restricting pathogen growth in the gut tissue. Thus IFN-γR-signaling influences different mucosal responses to infection, including not only pathogen restriction in the lamina propria, but, as shown here, also goblet cell function.

The IL-23 axis in Salmonella gastroenteritis

Non-typhoidal Salmonella (NTS) serotypes cause a localized gastroenteritis in immunocompetent individuals. In contrast, primary immunodeficiencies that impair interleukin-23 (IL-23)-dependent pathways are associated in humans with disseminated NTS bloodstream infections (bacteraemia). The recent use of animal models has helped to define the role the IL-23 axis plays during NTS gastroenteritis, but additional work is needed to elucidate how this host defence pathway prevents NTS bacteraemia.

Early interferon-γ production in human lymphocyte subsets in response to nontyphoidal Salmonella demonstrates inherent capacity in innate cells

Background

Nontyphoidal Salmonellae frequently cause life-threatening bacteremia in sub-Saharan Africa. Young children and HIV-infected adults are particularly susceptible. High case-fatality rates and increasing antibiotic resistance require new approaches to the management of this disease. Impaired cellular immunity caused by defects in the T helper 1 pathway lead to intracellular disease with Salmonella that can be countered by IFNγ administration. This report identifies the lymphocyte subsets that produce IFNγ early in Salmonella infection.

Methodology

Intracellular cytokine staining was used to identify IFNγ production in blood lymphocyte subsets of ten healthy adults with antibodies to Salmonella (as evidence of immunity to Salmonella), in response to stimulation with live and heat-killed preparations of the D23580 invasive African isolate of Salmonella Typhimurium. The absolute number of IFNγ-producing cells in innate, innate-like and adaptive lymphocyte subpopulations was determined.

Principal Findings

Early IFNγ production was found in the innate/innate-like lymphocyte subsets: γδ-T cells, NK cells and NK-like T cells. Significantly higher percentages of such cells produced IFNγ compared to adaptive αβ-T cells (Student's t test, P<0.001 and ≤0.02 for each innate subset compared, respectively, with CD4⁺- and CD8⁺-T cells). The absolute numbers of IFNγ-producing cells showed similar differences. The proportion of IFNγ-producing γδ-T cells, but not other lymphocytes, was significantly higher when stimulated with live compared with heat-killed bacteria (P<0.0001).

Conclusion/Significance

Our findings indicate an inherent capacity of innate/innate-like lymphocyte subsets to produce IFNγ early in the response to Salmonella infection. This may serve to control intracellular infection and reduce the threat of extracellular spread of disease with bacteremia which becomes life-threatening in the absence of protective antibody. These innate cells may also help mitigate against the effect on IFNγ production of depletion of Salmonella-specific CD4⁺-T lymphocytes in HIV infection.

Dietary fish oil impairs induction of gamma-interferon and delayed-type hypersensitivity during a systemic Salmonella enteritidis infection in rats

Fish oil that is rich in n-3 polyunsaturated fatty acids markedly modulates immunological responses. Literature data indicate that the fish oil reduces cellular immunity and therefore impairs resistance to infections. We have investigated how dietary fish oil affects the immune response against a facultative intracellular pathogen, Salmonella enteritidis. Wistar rats were fed a diet containing 16% (w/w) of either fish oil or corn oil. After a 4-week adaptation period, rats were intraperitoneally challenged with 4 x 10(5) cfu of S. enteritidis. During the 14-day infection period, urine was collected on a daily basis. At days 2 and 14, eight rats per group were sacrificed. Urinary nitrate, used as a marker for NO production, was lower on a fish oil diet during days 3-8. At day 2, serum gamma-interferon was 48 +/- 7 pg/mL in the fish oil-fed rats compared with 162 +/- 52 pg/mL in the corn oil-fed rats. No effects were found on living salmonella in liver and spleen. At day 14, as markers of an impaired T-helper 1 (Th-1) response, a 38% lower delayed-type hypersensitivity responses and a lower salmonella-specific IgG2b were observed in the fish oil-fed rats. Although here dietary fish oil has affected only immune parameters, this impairment of the innate and Th-1-mediated immune response may have implications for the host resistance against other intracellular pathogens.

Influence of 5 major Salmonella pathogenicity islands on NK cell depletion in mice infected with Salmonella enterica serovar Enteritidis

Background

In this study we were interested in the colonisation and early immune response of Balb/C mice to infection with Salmonella Enteritidis and isogenic pathogenicity island free mutants.

Results

The virulence of S. Enteritidis for Balb/C mice was exclusively dependent on intact SPI-2. Infections with any of the mutants harbouring SPI-2 (including the mutant in which we left only SPI-2 but removed SPI-1, SPI-3, SPI-4 and SPI-5) resulted in fatalities, liver injures and NK cell depletion from the spleen. The infection was of minimal influence on counts of splenic CD4 CD8 T lymphocytes and γδ T-lymphocytes although a reduced ability of splenic lymphocytes to respond to non-specific mitogens indicated general immunosuppression in mice infected with SPI-2 positive S. Enteritidis mutants. Further investigations showed that NK cells were depleted also in blood but not in the caecal lamina propria. However, NK cell depletion was not directly associated with the presence of SPI-2 and was rather an indicator of virulence or avirulence of a particular mutant because the depletion was not observed in mice infected with other attenuated mutants such as lon and rfaL.

Conclusions

The virulence of S. Enteritidis for Balb/C mice is exclusively dependent on the presence of SPI-2 in its genome, and a major hallmark of the infection in terms of early changes in lymphocyte populations is the depletion of NK cells in spleen and blood. The decrease of NK cells in circulation can be used as a marker of attenuation of S. Enteritidis mutants for Balb/C mice.

Interferon-gamma secretion is induced in IL-12 stimulated human NK cells by recognition of Helicobacter pylori or TLR2 ligands

Helicobacter pylori induce a chronic inflammation in the human gastric mucosa characterized by increased production of interferon-gamma (IFN-γ). The presence of natural killer (NK) cells in the human gastric mucosa and the ability of NK cells to produce IFN-γ suggest an important role of NK cells in the immune response directed towards H. pylori infection. Since NK cells previously have been shown to respond to bacterial components with IFN-γ production, we investigated the mechanisms for the recognition of H. pylori. We found that inhibition of MyD88 homodimerization resulted in decreased production of IFN-γ and that inhibition of the p38 MAPK decreased the production as well as the secretion of IFN-γ. Further studies indicated an involvement of Toll-like receptors (TLRs), in particular TLR2. Finally, we showed that the H. pylori specific membrane bound lipoprotein HpaA induced IFN-γ production from NK cells through recognition by TLR2. In conclusion, we suggest an involvement of TLR2 in the recognition of H. pylori by human NK cells and that HpaA is a TLR2 ligand important for recognition.

Interactions between human NK cells and macrophages in response to Salmonella infection

NK cells play a key role in host resistance to a range of pathogenic microorganisms, particularly during the initial stages of infection. NK cell interactions with cells infected with viruses and parasites have been studied extensively, but human bacterial infections have not been given the same attention. We studied crosstalk between human NK cells and macrophages infected with intracellular Salmonella. These macrophages activated NK cells, resulting in secretion of IFN-gamma and degranulation. Reciprocally, NK cell activation led to a dramatic reduction in numbers of intramacrophagic live bacteria. We identified three elements in the interaction of NK cells with infected macrophages. First, communication between NK cells and infected macrophages was contact-dependent. The second requirement was IL-2- and/or IL-15-dependent priming of NK cells to produce IFN-gamma. The third was activation of NK cells by IL-12 and IL-18, which were secreted by the Salmonella-infected macrophages. Adhesion molecules and IL-12Rbeta2 were enriched in the contact zone between NK cells and macrophages, consistent with contact- and IL-12/IL-18-dependent NK activation. Our results suggest that, in humans, bacterial clearance is consistent with a model invoking a "ménage à trois" involving NK cells, IL-2/IL-15-secreting cells, and infected macrophages.

Influence of the transcription factor RORgammat on the development of NKp46+ cell populations in gut and skin

NKp46+CD3- natural killer lymphocytes isolated from blood, lymphoid organs, lung, liver and uterus can produce granule-dependent cytotoxicity and interferon-gamma. Here we identify in dermis, gut lamina propria and cryptopatches distinct populations of NKp46+CD3- cells with a diminished capacity to degranulate and produce interferon-gamma. In the gut, expression of the transcription factor RORgammat, which is involved in the development of lymphoid tissue-inducer cells, defined a previously unknown subset of NKp46+CD3- lymphocytes. Unlike RORgammat- lamina propria and dermis natural killer cells, gut RORgammat+NKp46+ cells produced interleukin 22. Our data show that lymphoid tissue-inducer cells and natural killer cells shared unanticipated similarities and emphasize the heterogeneity of NKp46+CD3- cells in innate immunity, lymphoid organization and local tissue repair.

Interleukin-15 and NK1.1+ cells provide innate protection against acute Salmonella enterica serovar Typhimurium infection in the gut and in systemic tissues

Control of bacterial colonization at mucosal surfaces depends on rapid activation of the innate immune system. Interleukin-15 (IL-15) directs the development, maturation, and function of a population of cells positive for NK1.1, such as natural killer (NK) cells, which are critical components of the innate immune defense against several viral and bacterial pathogens. Using IL-15-deficient mice, in vivo depletion of NK1.1(+) cells from wild-type mice, and in vivo overexpression of IL-15 from a recombinant adenovirus, we tested the role of IL-15 and NK1.1(+) cells in innate protection of the murine gut and reticuloendothelial system from Salmonella enterica serovar Typhimurium infection. IL-15 and the NK1.1(+) cell population provided innate protection from serovar Typhimurium in mice at the enteric mucosae and in the reticuloendothelial system during murine typhoid. Interestingly, serovar Typhimurium extensively colonized the gut of IL-15(-/-) mice and wild-type C57BL/6 mice depleted of NK1.1(+) cells prior to infection, even though the animals were not pretreated with antibiotics to reduce colonization resistance and there was an absence of overt inflammation in the colon and cecum. Enhanced dissemination of Salmonella from the gut of mice depleted of NK1.1(+) cells correlated with a localized disruption of IL-17 in the colon. These data suggest a relationship between the gut ecosystem and the innate mucosal immune system, which may be linked via IL-15 and NK1.1(+) cells.

Attenuated inflammatory responses in hemochromatosis reveal a role for iron in the regulation of macrophage cytokine translation

Disturbances of iron homeostasis are associated with altered susceptibility to infectious disease, but the underlying molecular mechanisms are poorly understood. To study this phenomenon, we examined innate immunity to oral Salmonella infection in Hfe knockout (Hfe(-/-)) mice, a model of the human inherited disorder of iron metabolism type I hemochromatosis. Salmonella- and LPS-induced inflammatory responses were attenuated in the mutant animals, with less severe enterocolitis observed in vivo and reduced macrophage TNF-alpha and IL-6 secretion measured in vitro. The macrophage iron exporter ferroportin (FPN) was up-regulated in the Hfe(-/-) mice, and correspondingly, intramacrophage iron levels were lowered. Consistent with the functional importance of these changes, the abnormal cytokine production of the mutant macrophages could be reproduced in wild-type cells by iron chelation, and in a macrophage cell line by overexpression of FPN. The results of analyzing specific steps in the biosynthesis of TNF-alpha and IL-6, including intracellular concentrations, posttranslational stability and transcript levels, were consistent with reduced translation of cytokine mRNAs in Hfe(-/-) macrophages. Polyribosome profile analysis confirmed that elevated macrophage FPN expression and low intracellular iron impaired the translation of specific inflammatory cytokine transcripts. Our results provide molecular insight into immune function in type I hemochromatosis and other disorders of iron homeostasis, and reveal a novel role for iron in the regulation of the inflammatory response.

NK but not CD1-restricted NKT cells facilitate systemic inflammation during polymicrobial intra-abdominal sepsis

Evidence suggests that NK and NKT cells contribute to inflammation and mortality during septic shock caused by cecal ligation and puncture (CLP). However, the specific contributions of these cell types to the pathogenesis of CLP-induced septic shock have not been fully defined. The goal of the present study was to determine the mechanisms by which NK and NKT cells mediate the host response to CLP. Control, NK cell-deficient, and NKT cell-deficient mice underwent CLP. Survival, cytokine production, and bacterial clearance were measured. NK cell trafficking and interaction with myeloid cells was also studied. Results show that mice treated with anti-asialoGM1 (NK cell deficient) or anti-NK1.1 (NK/NKT cell deficient) show less systemic inflammation and have improved survival compared with IgG-treated controls. CD1 knockout mice (NKT cell deficient) did not demonstrate decreased cytokine production or improved survival compared with wild type mice. Trafficking studies show migration of NK cells from blood and spleen into the inflamed peritoneal cavity where they appear to facilitate the activation of peritoneal macrophages (F4-80(+)GR-1(-)) and F4-80(+)Gr-1(+) myeloid cells. These findings indicate that NK but not CD1-restricted NKT cells contribute to acute CLP-induced inflammation. NK cells appear to mediate their proinflammatory functions during septic shock, in part, by migration into the peritoneal cavity and amplification of the proinflammatory activities of specific myeloid cell populations. These findings provide new insights into the mechanisms used by NK cells to facilitate acute inflammation during septic shock.

A role for interleukin-12/23 in the maturation of human natural killer and CD56+ T cells in vivo

Natural killer (NK) cells have been originally defined by their "naturally occurring" effector function. However, only a fraction of human NK cells is reactive toward a panel of prototypical tumor cell targets in vitro, both for the production of interferon-gamma (IFN-gamma) and for their cytotoxic response. In patients with IL12RB1 mutations that lead to a complete IL-12Rbeta1 deficiency, the size of this naturally reactive NK cell subset is diminished, in particular for the IFN-gamma production. Similar data were obtained from a patient with a complete deficit in IL-12p40. In addition, the size of the subset of effector memory T cells expressing CD56 was severely decreased in IL-12Rbeta1- and IL-12p40-deficient patients. Human NK cells thus require in vivo priming with IL-12/23 to acquire their full spectrum of functional reactivity, while T cells are dependent upon IL-12/23 signals for the differentiation and/or the maintenance of CD56(+) effector memory T cells. The susceptibility of IL-12/23 axis-deficient patients to Mycobacterium and Salmonella infections in combination with the absence of mycobacteriosis or salmonellosis in the rare cases of human NK cell deficiencies point to a role for CD56(+) T cells in the control of these infections in humans.