Salmonella enterica serovar Typhimurium infection induces cyclooxygenase 2 expression in macrophages: involvement of Salmonella pathogenicity island 2

The Role of Egg Yolk in Modulating the Virulence of Salmonella Enterica Serovar Enteritidis

Contribution of food vehicles to pathogenicity of disease-causing microorganisms is an important but overlooked research field. The current study was initiated to reveal the relationship between virulence of Salmonella enterica serovar Enteritidis and egg yolk as a hosting medium. Mice were orally challenged with Salmonella Enteritidis cultured in egg yolk or tryptic soy broth (TSB). Additionally, mice were challenged with Salmonella Enteritidis cultured in TSB, followed by administration of sterile egg yolk, to discern the difference between pre-growth of the pathogen and its mere presence in egg yolk during infection. The pathogen’s Lethal dose 50 (LD₅₀) was the lowest when grown in yolk (2.8×10² CFU), compared to 1.1×10³ CFU in TSB, and 4.6×10³ CFU in TSB followed by administration of sterile yolk. Additionally, mice that orally received Salmonella Enteritidis grown in egg yolk expressed a high death rate. These findings were supported by transcriptional analysis results. Expression of promoters of virulence-related genes (sopB and sseA) in genetically modified Salmonella Enteritidis reporter strains was significantly higher (p < 0.05) when the bacterium was grown in the yolk, compared to that grown in TSB. Sequencing of RNA (RNA-seq) revealed 204 differentially transcribed genes in Salmonella Enteritidis grown in yolk vs. TSB. Yolk-grown Salmonella Enteritidis exhibited upregulated virulence pathways, including type III secretion systems, epithelial cell invasion, and infection processes; these observations were confirmed by RT-qPCR results. The transcriptomic analysis suggested that upregulation of virulence machinery of Salmonella Enteritidis grown in egg yolk was related to increased iron uptake, biotin utilization, flagellar biosynthesis, and export of virulence proteins encoded on Salmonella pathogenicity island 1, 2, 4, and 5. These biological responses may have acted in concert to increase the virulence of Salmonella infection in mice. In conclusion, growth in egg yolk enhanced Salmonella Enteritidis virulence, indicating the significance of this food vehicle to the risk assessment of salmonellosis.

Lipid larceny: channelizing host lipids for establishing successful pathogenesis by bacteria

Lipids are complex organic compounds made up of carbon, oxygen, and hydrogen. These play a diverse and intricate role in cellular processes like membrane trafficking, protein sorting, signal transduction, and bacterial infections. Both Gram-positive bacteria (Staphylococcus sp., Listeria monocytogenes, etc.) and Gram-negative bacteria (Chlamydia sp., Salmonella sp., E. coli, etc.) can hijack the various host-lipids and utilize them structurally as well as functionally to mount a successful infection. The pathogens can deploy with various arsenals to exploit host membrane lipids and lipid-associated receptors as an attachment for toxins' landing or facilitate their entry into the host cellular niche. Bacterial species like Mycobacterium sp. can also modulate the host lipid metabolism to fetch its carbon source from the host. The sequential conversion of host membrane lipids into arachidonic acid and prostaglandin E2 due to increased activity of cPLA-2 and COX-2 upon bacterial infection creates immunosuppressive conditions and facilitates the intracellular growth and proliferation of bacteria. However, lipids' more debatable role is that they can also be a blessing in disguise. Certain host-lipids, especially sphingolipids, have been shown to play a crucial antibacterial role and help the host in combating the infections. This review shed light on the detailed role of host lipids in bacterial infections and the current understanding of the lipid in therapeutics. We have also discussed potential prospects and the need of the hour to help us cope in this race against deadly pathogens and their rapidly evolving stealthy virulence strategies.

Salmonella fimbrial protein StcD induces cyclooxygenase-2 expression via Toll-like receptor 4

INTRODUCTION

The genome of Salmonella enterica serovar Typhimurium contains 13 operons with homology to fimbrial genes.

METHODS

To investigate the involvement of these fimbrial gene clusters in the expression of cyclooxygenase-2 (COX-2), which is an inducible enzyme involved in the synthesis of prostanoids, in J774 macrophages infected with S. enterica serovar Typhimurium, we constructed strains carrying a mutation in genes encoding the putative subunit proteins in 12 fimbrial operons.

RESULTS

The level of COX-2 expression was lower in macrophages infected with fimA or stcA mutant Salmonella than in those infected with wild-type Salmonella. Therefore, we focused on putative subunit protein StcA and adhesive like protein StcD encoded in the stc operon. Treatment of macrophages with purified recombinant StcD protein, but not StcA, resulted in the activation of the mitogen-activated protein kinase and nuclear factor kappa B signaling pathways, leading to the expression of not only COX-2 but also of pro-inflammatory cytokines such as interleukin (IL)-1β, IL-6, and tumor necrosis factor alpha. The expression of StcD-induced COX-2 was inhibited by treatment with the Toll-like receptor 4 (TLR4) inhibitor TAK-242, but not by treatment with the lipopolysaccharide (LPS) antagonist polymyxin B. Furthermore, StcD treatment stimulated HEK293 cells expressing TLR4 in the presence of CD14 and MD-2.

CONCLUSION

StcD is a pathogen-associated molecular pattern of S. enterica serovar Typhimurium that is recognized by TLR4 and plays a significant role in the induction of COX-2 expression in macrophages.

Yersinia pseudotuberculosis YopJ Limits Macrophage Response by Downregulating COX-2-Mediated Biosynthesis of PGE2 in a MAPK/ERK-Dependent Manner

Prostaglandin E2 (PGE2) is an essential immunomodulatory lipid released by cells in response to infection with many bacteria, yet its function in macrophage-mediated bacterial clearance is poorly understood. Yersinia overall inhibits the inflammatory circuit, but its effect on PGE2 production is unknown. We hypothesized that one of the Yersinia effector proteins is responsible for the inhibition of PGE2 biosynthesis. We identified that yopB-deficient Y. enterocolitica and Y. pseudotuberculosis deficient in the secretion of virulence proteins via a type 3 secretion system (T3SS) failed to inhibit PGE2 biosynthesis in macrophages. Consistently, COX-2-mediated PGE2 biosynthesis is upregulated in cells treated with heat-killed or T3SS-deficient Y. pseudotuberculosis but diminished in the presence of a MAPK/ERK inhibitor. Mutants expressing catalytically inactive YopJ induce similar levels of PGE2 as heat-killed or ΔyopB Y. pseudotuberculosis, reversed by YopJ complementation. Shotgun proteomics discovered host pathways regulated in a YopJ-mediated manner, including pathways regulating PGE2 synthesis and oxidative phosphorylation. Consequently, this study identified that YopJ-mediated inhibition of MAPK signal transduction serves as a mechanism targeting PGE2, an alternative means of inflammasome inhibition by Yersinia. Finally, we showed that EP4 signaling supports macrophage function in clearing intracellular bacteria. In summary, our unique contribution was to determine a bacterial virulence factor that targets COX-2 transcription, thereby enhancing the intracellular survival of yersiniae. Future studies should investigate whether PGE2 or its stable synthetic derivatives could serve as a potential therapeutic molecule to improve the outcomes of specific bacterial infections. Since other pathogens encode YopJ homologs, this mechanism is expected to be present in other infections. IMPORTANCE PGE2 is a critical immunomodulatory lipid, but its role in bacterial infection and pathogen clearance is poorly understood. We previously demonstrated that PGE2 leads to macrophage polarization toward the M1 phenotype and stimulates inflammasome activation in infected macrophages. Finally, we also discovered that PGE2 improved the clearance of Y. enterocolitica. The fact that Y. enterocolitica hampers PGE2 secretion in a type 3 secretion system (T3SS)-dependent manner and because PGE2 appears to assist macrophage in the clearance of this bacterium indicates that targeting of the eicosanoid pathway by Yersinia might be an adaption used to counteract host defenses. Our study identified a mechanism used by Yersinia that obstructs PGE2 biosynthesis in human macrophages. We showed that Y. pseudotuberculosis interferes with PGE2 biosynthesis by using one of its T3SS effectors, YopJ. Specifically, YopJ targets the host COX-2 enzyme responsible for PGE2 biosynthesis, which happens in a MAPK/ER-dependent manner. Moreover, in a shotgun proteomics study, we also discovered other pathways that catalytically active YopJ targets in the infected macrophages. YopJ was revealed to play a role in limiting host LPS responses, including repression of EGR1 and JUN proteins, which control transcriptional activation of proinflammatory cytokine production such as interleukin-1β. Since YopJ has homologs in other bacterial species, there are likely other pathogens that target and inhibit PGE2 biosynthesis. In summary, our study's unique contribution was to determine a bacterial virulence factor that targets COX-2 transcription. Future studies should investigate whether PGE2 or its stable synthetic derivatives could serve as a potential therapeutic target.

Roles of Eicosanoids in Regulating Inflammation and Neutrophil Migration as an Innate Host Response to Bacterial Infections

Eicosanoids are lipid-based signaling molecules that play a unique role in innate immune responses. The multiple types of eicosanoids, such as prostaglandins (PGs) and leukotrienes (LTs), allow the innate immune cells to respond rapidly to bacterial invaders. Bacterial pathogens alter cyclooxygenase (COX)-derived prostaglandins (PGs) in macrophages, such as PGE2 15d-PGJ₂, and lipoxygenase (LOX)-derived leukotriene LTB_4, which has chemotactic functions. The PG synthesis and secretion are regulated by substrate availability of arachidonic acid and by the COX-2 enzyme, and the expression of this protein is regulated at multiple levels, both transcriptionally and posttranscriptionally. Bacterial pathogens use virulence strategies such as type three secretion systems (T3SSs) to deliver virulence factors altering the expression of eicosanoid-specific biosynthetic enzymes, thereby modulating the host response to bacterial lipopolysaccharides (LPS). Recent advances have identified a novel role of eicosanoids in inflammasome activation during intracellular infection with bacterial pathogens. Specifically, PGE₂ was found to enhance inflammasome activation, driving the formation of pore-induced intracellular traps (PITs), thus trapping bacteria from escaping the dying cell. Finally, eicosanoids and IL-1β released from macrophages are implicated in the efferocytosis of neighboring neutrophils. Neutrophils play an essential role in phagocytosing and degrading PITs and associated bacteria to restore homeostasis. This review focuses on the novel functions of host-derived eicosanoids in the host-pathogen interactions.

Identification of single-nucleotide variants associated with susceptibility to Salmonella in pigs using a genome-wide association approach

BACKGROUND

Salmonella enterica serovars are a major cause of foodborne illness and have a substantial impact on global human health. In Canada, Salmonella is commonly found on swine farms and the increasing concern about drug use and antimicrobial resistance associated with Salmonella has promoted research into alternative control methods, including selecting for pig genotypes associated with resistance to Salmonella. The objective of this study was to identify single-nucleotide variants in the pig genome associated with Salmonella susceptibility using a genome-wide association approach. Repeated blood and fecal samples were collected from 809 pigs in 14 groups on farms and tonsils and lymph nodes were collected at slaughter. Sera were analyzed for Salmonella IgG antibodies by ELISA and feces and tissues were cultured for Salmonella. Pig DNA was genotyped using a custom 54 K single-nucleotide variant oligo array and logistic mixed-models used to identify SNVs associated with IgG seropositivity, shedding, and tissue colonization.

RESULTS

Variants in/near PTPRJ (p = 0.0000066), ST6GALNAC3 (p = 0.0000099), and DCDC2C (n = 3, p < 0.0000086) were associated with susceptibility to Salmonella, while variants near AKAP12 (n = 3, p < 0.0000358) and in RALGAPA2 (p = 0.0000760) may be associated with susceptibility.

CONCLUSIONS

Further study of the variants and genes identified may improve our understanding of neutrophil recruitment, intracellular killing of bacteria, and/or susceptibility to Salmonella and may help future efforts to reduce Salmonella on-farm through genetic approaches.

Cyclooxygenase-2 Inhibition Reduces Autophagy of Macrophages Enhancing Extraintestinal Pathogenic Escherichia coli Infection

Extraintestinal pathogenic Escherichia coli (ExPEC) is one of the top pathogens responsible for bloodstream infection and severe, often fatal, sepsis. Although the virulence factors and host immune responses to ExPEC infection have been investigated, the responses to a particular ExPEC strain could be very different. In this study, we investigated the mechanisms of Cyclooxygenase-2 (COX-2) up-regulation in influencing the host defenses against infection of ExPEC XM O2:K1:H7. Our results demonstrated that ExPEC XM O2:K1:H7 infection in mouse and RAW264.7 macrophages leads to COX-2 up-regulation, and COX-2 inhibition significantly enhances ExPEC infection. The up-regulation of COX-2 in macrophages was mediated by Toll-like receptor 4 (TLR4) through the activation of p38 and extracellular signal-regulated kinase/Mitogen-activated protein kinase (ERK/MAPK) pathways. Further studies showed that COX-2 inhibition significantly decreased autophagy in macrophages during ExPEC XM O2:K1:H7 infection. Autophagy inhibition significantly enhanced, while induction reduced ExPEC XM O2:K1:H7 survival in macrophages. In addition, COX-2 inhibition significantly increased macrophage cell death during ExPEC XM O2:K1:H7 infection and increased the expression of anti-inflammatory cytokine interleukin-10 (IL-10). Our results indicate that COX-2 up-regulation benefits host defense against ExPEC XM O2:K1:H7 infection by increasing autophagy in macrophages and by reducing IL-10 expression and macrophage cell death during ExPEC infection.

Controversy Surrounding the Function of SpiC Protein in Salmonella: An Overview

Salmonella is an important pathogenic microorganism that can infect humans and animals and has been studied globally as a model microorganism for its pathogenesis. The SpiC protein of T3SS2 is a significant factor that has been studied for almost 20 years, but to date, the function/effect of SpiC in the pathogenesis of Salmonella has not been completely understood. There is controversy over the functions of SpiC protein in the literature. Thus, an overview of the literature on SpiC protein is provided here which highlights expression features of SpiC protein and its various functions and effect.

Salmonella Fimbrial Protein FimH Is Involved in Expression of Proinflammatory Cytokines in a Toll-Like Receptor 4-Dependent Manner

Type 1 fimbriae are proteinaceous filamentous structures present on bacterial surfaces and are mainly composed of the major fimbrial protein subunit FimA and the adhesive protein FimH, which is located at the tip of the fimbrial shaft. Here, we investigated the involvement of type 1 fimbriae in the expression of proinflammatory cytokines in macrophages infected with Salmonella enterica serovar Typhimurium. The level of interleukin-1β (IL-1β) mRNA was lower in macrophages infected with fimA or fimH mutant strains than in those infected with wild-type Salmonella Treatment of macrophages with purified recombinant FimH protein, but not FimA, resulted in the activation of the mitogen-activated protein kinase and nuclear factor κB signaling pathways, leading to the expression of not only IL-1β but also other proinflammatory cytokines, such as IL-6 and tumor necrosis factor alpha. However, FimH carrying an N-terminal region deletion or heat-treated FimH did not show such effects. The expression of FimH-induced IL-1β was inhibited by treatment with the Toll-like receptor 4 (TLR4) inhibitor TAK-242 but not by treatment with polymyxin B, a lipopolysaccharide antagonist. Furthermore, FimH treatment stimulated HEK293 cells expressing TLR4 and MD-2/CD14 but did not stimulate HEK293 cells expressing only TLR4. Collectively, FimH is a pathogen-associated molecular pattern of S. enterica serovar Typhimurium that is recognized by TLR4 in the presence of MD-2 and CD14 and plays a significant role in the expression of proinflammatory cytokines in Salmonella-infected macrophages.

PGE2 Augments Inflammasome Activation and M1 Polarization in Macrophages Infected With Salmonella Typhimurium and Yersinia enterocolitica

Eicosanoids are cellular metabolites, which shape the immune response, including inflammatory processes in macrophages. The effects of these lipid mediators on inflammation and bacterial pathogenesis are not clearly understood. Certain eicosanoids are suspected to act as molecular sensors for the recruitment of neutrophils, while others regulate bacterial uptake. In this study, gene expression analyses indicated that genes involved in eicosanoid biosynthesis including COX-1, COX-2, DAGL, and PLA-2 are differentially regulated in THP-1 human macrophages infected with Salmonella enterica Typhimurium or Yersinia enterocolitica. By using targeted metabolomics approach, we found that the eicosanoid precursor, arachidonic acid (AA) as well as its derivatives, including prostaglandins (PGs) PGF2α or PGE2/PGD2, and thromboxane TxB2, are rapidly secreted from macrophages infected with these Gram-negative pathogenic bacteria. The magnitude of eicosanoid biosynthesis in infected host cells depends on the presence of virulence factors of Y. enterocolitica and S. Typhimurium strains, albeit in an opposite way in Y. enterocolitica compared to S. Typhimurium infection. Trials with combinations of EP2/EP4 PGE2 receptor agonists and antagonists suggest that PGE2 signaling in these infection models works primarily through the EP4 receptor. Downstream of EP4 activation, PGE2 enhances inflammasome activation and represses M2 macrophage polarization while inducing key M1-type markers. PGE2 also led to a decreased numbers of Y. enterocolitica within macrophages. To summarize, PGE2 is a potent autocrine/paracrine activator of inflammation during infection in Gram-negative bacteria, and it affects macrophage polarization, likely controlling bacterial clearance by macrophages.

Comparative Genomic Studies of Salmonella Heidelberg Isolated From Chicken- and Turkey-Associated Farm Environmental Samples

Salmonella is one of the leading causes of human foodborne gastroenteritis in the United States. In addition, Salmonella contributes to morbidity and mortality in livestock. The control of Salmonella is an increasing problematic issue in livestock production due to lack of effective control methods and the constant adaptation of Salmonella to new management practices, which is often related to horizontal acquisition of virulence or antibiotic resistance genes. Salmonella enterica serotype Heidelberg is one of the most commonly isolated serotypes in all poultry production systems in North America. Emergence and persistence of multi-drug resistant Salmonella Heidelberg isolates further impact the poultry production and public health. We hypothesized that distinct poultry production environments affect Salmonella genomic content, and by consequence its survival and virulence abilities. This study compared the genomic composition of S. Heidelberg isolated from environmental samples (19 chicken and 12 turkey isolates) of different breeder farms (16 chicken and 8 turkey farms) in the Midwest, United States. Whole genome comparison of 31 genomes using RAST and SEED identified differences in specific sub-systems in isolates between the chicken- and turkey-associated farm environmental samples. Genes associated with the type IV secretion system (n = 12) and conjugative transfer (n = 3) were absent in turkey farm isolates compared to the chicken ones (p-value < 0.01); Further, turkey farm isolates were enriched in prophage proteins (n = 53; p-value < 0.01). Complementary studies using PHASTER showed that prophages were all Caudovirales phages and were more represented in turkey environmental isolates than the chicken isolates. This study corroborates that isolates from distinct farm environment show differences in S. Heidelberg genome content related to horizontal transfer between bacteria or through viral infections. Complementary microbiome studies of these samples would provide critical insights on sources of these variations. Overall, our findings enhance the understanding of Salmonella genome plasticity and may aid in the development of future effective management practices to control Salmonella.

Prostaglandin E2 as a Regulator of Immunity to Pathogens

The body is exposed to foreign pathogens every day, but remarkably, most pathogens are effectively cleared by the innate immune system without the need to invoke the adaptive immune response. Key cellular components of the innate immune system include macrophages and neutrophils and the recruitment and function of these cells are tightly regulated by chemokines and cytokines in the tissue space. Innate immune responses are also known to regulate development of adaptive immune responses often via the secretion of various cytokines. In addition to these protein regulators, numerous lipid mediators can also influence innate and adaptive immune functions. In this review, we cover one particular lipid regulator, prostaglandin E2 (PGE2) and describe its synthesis and signaling and what is known about the ability of this lipid to regulate immunity and host defense against viral, fungal and bacterial pathogens.

Macrophage-Bacteria Interactions-A Lipid-Centric Relationship

Macrophages are professional phagocytes at the front line of immune defenses against foreign bodies and microbial pathogens. Various bacteria, which are responsible for deadly diseases including tuberculosis and salmonellosis, are capable of hijacking this important immune cell type and thrive intracellularly, either in the cytoplasm or in specialized vacuoles. Tight regulation of cellular metabolism is critical in shaping the macrophage polarization states and immune functions. Lipids, besides being the bulk component of biological membranes, serve as energy sources as well as signaling molecules during infection and inflammation. With the advent of systems-scale analyses of genes, transcripts, proteins, and metabolites, in combination with classical biology, it is increasingly evident that macrophages undergo extensive lipid remodeling during activation and infection. Each bacterium species has evolved its own tactics to manipulate host metabolism toward its own advantage. Furthermore, modulation of host lipid metabolism affects disease susceptibility and outcome of infections, highlighting the critical roles of lipids in infectious diseases. Here, we will review the emerging roles of lipids in the complex host-pathogen relationship and discuss recent methodologies employed to probe these versatile metabolites during the infection process. An improved understanding of the lipid-centric nature of infections can lead to the identification of the Achilles' heel of the pathogens and host-directed targets for therapeutic interventions. Currently, lipid-moderating drugs are clinically available for a range of non-communicable diseases, which we anticipate can potentially be tapped into for various infections.

Role of yqiC in the Pathogenicity of Salmonella and Innate Immune Responses of Human Intestinal Epithelium

The yqiC gene of Salmonella enterica serovar Typhimurium (S. Typhimurium) regulates bacterial growth at different temperatures and mice survival after infection. However, the role of yqiC in bacterial colonization and host immunity remains unknown. We infected human LS174T, Caco-2, HeLa, and THP-1 cells with S. Typhimurium wild-type SL1344, its yqiC mutant, and its complemented strain. Bacterial colonization and internalization in the four cell lines significantly reduced on yqiC depletion. Post-infection production of interleukin-8 and human β-defensin-3 in LS174T cells significantly reduced because of yqiC deleted in S. Typhimurium. The phenotype of yqiC mutant exhibited few and short flagella, fimbriae on the cell surface, enhanced biofilm formation, upregulated type-1 fimbriae expression, and reduced bacterial motility. Type-1 fimbriae, flagella, SPI-1, and SPI-2 gene expression was quantified using real-time PCR. The data show that deletion of yqiC upregulated fimA and fimZ expression and downregulated flhD, fliZ, invA, and sseB expression. Furthermore, thin-layer chromatography and high-performance liquid chromatography revealed the absence of menaquinone in the yqiC mutant, thus validating the importance of yqiC in the bacterial electron transport chain. Therefore, YqiC can negatively regulate FimZ for type-1 fimbriae expression and manipulate the functions of its downstream virulence factors including flagella, SPI-1, and SPI-2 effectors.

Metabolic Adaptations of Intracellullar Bacterial Pathogens and their Mammalian Host Cells during Infection ("Pathometabolism")

Several bacterial pathogens that cause severe infections in warm-blooded animals, including humans, have the potential to actively invade host cells and to efficiently replicate either in the cytosol or in specialized vacuoles of the mammalian cells. The interaction between these intracellular bacterial pathogens and the host cells always leads to multiple physiological changes in both interacting partners, including complex metabolic adaptation reactions aimed to promote proliferation of the pathogen within different compartments of the host cells. In this chapter, we discuss the necessary nutrients and metabolic pathways used by some selected cytosolic and vacuolar intracellular pathogens and--when available--the links between the intracellular bacterial metabolism and the expression of the virulence genes required for the intracellular bacterial replication cycle. Furthermore, we address the growing evidence that pathogen-specific factors may also trigger metabolic responses of the infected mammalian cells affecting the carbon and nitrogen metabolism as well as defense reactions. We also point out that many studies on the metabolic host cell responses induced by the pathogens have to be scrutinized due to the use of established cell lines as model host cells, as these cells are (in the majority) cancer cells that exhibit a dysregulated primary carbon metabolism. As the exact knowledge of the metabolic host cell responses may also provide new concepts for antibacterial therapies, there is undoubtedly an urgent need for host cell models that more closely reflect the in vivo infection conditions.

Distinct innate responses are induced by attenuated Salmonella enterica serovar Typhimurium mutants

Upon bacterial infection the host cells generate a wide variety of cytokines. Genetic attenuation of bacterial physiological pathogens can be accomplished not only by disruption of normal bacterial processes, but also by the loss of the ability to redirect the host immune system. We examined nine attenuated Salmonella Typhimurium mutants for their ability to replicate as well as the cytokines produced after infection of Bone Marrow Derived Macrophages (BMDM). Infection of BMDM with attenuated Salmonella mutants led to host cytokine patterns distinct from those that followed WT infection. Surprisingly, each bacterial mutant had a unique cytokine signature. Because some of the mutants induced an IL-10 response not seen in WT, we examined the role of IL-10 on Salmonella replication. Surprisingly, addition of IL-10 before or concurrent with infection restricted growth of WT Salmonella in BMDM. Bacterial attenuation is not a single process and results in attenuated host responses, which result in unique patterns for each attenuated mutants.

Salmonella enterica serovar enteritidis modulates intestinal epithelial miR-128 levels to decrease macrophage recruitment via macrophage colony-stimulating factor

BACKGROUND

The mechanism underlying the ability of virulent Salmonella organisms to escape clearance by macrophages is incompletely understood. Here, we report a novel mechanism by which Salmonella escapes macrophages.

METHODS

Microarray and quantitative real-time polymerase chain reaction analyses were used to screen key microRNAs regulating Salmonella-host cell interactions. Target gene was tested using luciferase reporter and Western blot assays. The role of microRNA 128 (miR-128) was assayed using intestinal epithelial cells and a mouse infection model.

RESULTS

The miR-128 level in human intestinal epithelial HT29 cells was strongly increased by infection with strain SE2472, and the elevation in miR-128 levels in mouse intestine and colon tissues correlated with the level of Salmonella infection in mice. Macrophage colony-stimulating factor (M-CSF) was identified as a target of miR-128, and increased miR-128 levels in epithelial cells due to infection with strain SE2472 significantly decreased the level of cell-secreted M-CSF, leading to impaired M-CSF-mediated macrophage recruitment. The secreted proteins from Salmonella were identified as possible effectors to induce miR-128 expression via the p53 signaling pathway. Moreover, intragastric delivery of anti-miR-128 antagomir into mice significantly increased M-CSF-mediated macrophage recruitment and suppressed Salmonella infection.

CONCLUSIONS

Salmonella can upregulate intestinal epithelial miR-128 expression, which, in turn, decreases levels of epithelial cell-secreted M-CSF and M-CSF-induced macrophage recruitment.

PGE(2) suppression of innate immunity during mucosal bacterial infection

Prostaglandin E2 (PGE2) is an important lipid mediator in inflammatory and immune responses during acute and chronic infections. Upon stimulation by various proinflammatory stimuli such as lipopolysaccharide (LPS), interleukin (IL)-1β, and tumor necrosis factor (TNF)-α, PGE2 synthesis is upregulated by the expression of cyclooxygenases. Biologically active PGE2 is then able to signal through four primary receptors to elicit a response. PGE2 is a critical molecule that regulates the activation, maturation, migration, and cytokine secretion of several immune cells, particularly those involved in innate immunity such as macrophages, neutrophils, natural killer cells, and dendritic cells. Both Gram-negative and Gram-positive bacteria can induce PGE2 synthesis to regulate immune responses during bacterial pathogenesis. This review will focus on PGE2 in innate immunity and how bacterial pathogens influence PGE2 production during enteric and pulmonary infections. The conserved ability of many bacterial pathogens to promote PGE2 responses during infection suggests a common signaling mechanism to deter protective pro-inflammatory immune responses. Inhibition of PGE2 production and signaling during infection may represent a therapeutic alternative to treat bacterial infections. Further study of the immunosuppressive effects of PGE2 on innate immunity will lead to a better understanding of potential therapeutic targets within the PGE2 pathway.

Metabolic host responses to infection by intracellular bacterial pathogens

The interaction of bacterial pathogens with mammalian hosts leads to a variety of physiological responses of the interacting partners aimed at an adaptation to the new situation. These responses include multiple metabolic changes in the affected host cells which are most obvious when the pathogen replicates within host cells as in case of intracellular bacterial pathogens. While the pathogen tries to deprive nutrients from the host cell, the host cell in return takes various metabolic countermeasures against the nutrient theft. During this conflicting interaction, the pathogen triggers metabolic host cell responses by means of common cell envelope components and specific virulence-associated factors. These host reactions generally promote replication of the pathogen. There is growing evidence that pathogen-specific factors may interfere in different ways with the complex regulatory network that controls the carbon and nitrogen metabolism of mammalian cells. The host cell defense answers include general metabolic reactions, like the generation of oxygen- and/or nitrogen-reactive species, and more specific measures aimed to prevent access to essential nutrients for the respective pathogen. Accurate results on metabolic host cell responses are often hampered by the use of cancer cell lines that already exhibit various de-regulated reactions in the primary carbon metabolism. Hence, there is an urgent need for cellular models that more closely reflect the in vivo infection conditions. The exact knowledge of the metabolic host cell responses may provide new interesting concepts for antibacterial therapies.

The Salmonella kinase SteC targets the MAP kinase MEK to regulate the host actin cytoskeleton

After host cell entry, Salmonella replicate in membrane-bound compartments, which accumulate a dense meshwork of F-actin through the kinase activity of the Salmonella SPI-2 type III secretion effector SteC. We find that SteC promotes actin cytoskeleton reorganization by activating a signaling pathway involving the MAP kinases MEK and ERK, myosin light chain kinase (MLCK) and Myosin IIB. Specifically, SteC phosphorylates MEK directly on serine 200 (S200), a previously unstudied phosphorylation site. S200 phosphorylation is predicted to displace a negative regulatory helix causing autophosphorylation on the known MEK activatory residues, S218 and S222. In support of this, substitution of S200 with alanine prevented phosphorylation on S218 and S222, and phosphomimetic mutations of S200 stimulated phosphorylation of these residues. Both steC-null and kinase-deficient mutant strains displayed enhanced replication in infected cells, suggesting that SteC manipulates the actin cytoskeleton to restrain bacterial growth, thereby regulating virulence.

Attenuated Salmonella Typhimurium lacking the pathogenicity island-2 type 3 secretion system grow to high bacterial numbers inside phagocytes in mice

Intracellular replication within specialized vacuoles and cell-to-cell spread in the tissue are essential for the virulence of Salmonella enterica. By observing infection dynamics at the single-cell level in vivo, we have discovered that the Salmonella pathogenicity island 2 (SPI-2) type 3 secretory system (T3SS) is dispensable for growth to high intracellular densities. This challenges the concept that intracellular replication absolutely requires proteins delivered by SPI-2 T3SS, which has been derived largely by inference from in vitro cell experiments and from unrefined measurement of net growth in mouse organs. Furthermore, we infer from our data that the SPI-2 T3SS mediates exit from infected cells, with consequent formation of new infection foci resulting in bacterial spread in the tissues. This suggests a new role for SPI-2 in vivo as a mediator of bacterial spread in the body. In addition, we demonstrate that very similar net growth rates of attenuated salmonellae in organs can be derived from very different underlying intracellular growth dynamics.

Interactions of Salmonella enterica with dendritic cells

Dendritic cells (DCs) form an important link between innate and adaptive immunity. However, DCs are also deployed as vehicles for systemic spread of pathogens. Salmonella is an important gastrointestinal pathogen causing diseases ranging from gastroenteritis to typhoid fever. DCs play an important role in the immunity against Salmonella infection, but this pathogen has also evolved efficient mechanisms to persist after phagocytosis by DCs, to spread using DCs as vehicles and to interfere with the central function of DCs, the processing of antigens and presentation of antigen-derived peptides to T cells for the stimulation of adaptive immune responses. Here we review the routes used by Salmonella to breach intestinal barriers, the intracellular habitat of Salmonella in DCs, molecular mechanisms of Salmonella virulence factors for intracellular life and intracellular activities in DCs resulting in manipulation of DC functions.

Defense Mechanisms of Hepatocytes Against Burkholderia pseudomallei

The Gram-negative facultative intracellular rod Burkholderia pseudomallei causes melioidosis, an infectious disease with a wide range of clinical presentations. Among the observed visceral abscesses, the liver is commonly affected. However, neither this organotropism of B. pseudomallei nor local hepatic defense mechanisms have been thoroughly investigated so far. Own previous studies using electron microscopy of the murine liver after systemic infection of mice indicated that hepatocytes might be capable of killing B. pseudomallei. Therefore, the aim of this study was to further elucidate the interaction of B. pseudomallei with these cells and to analyze the role of hepatocytes in anti-B. pseudomallei host defense. In vitro studies using the human hepatocyte cell line HepG2 revealed that B. pseudomallei can invade these cells. Subsequently, B. pseudomallei is able to escape from the vacuole, to replicate within the cytosol of HepG2 cells involving its type 3 and type 6 secretion systems, and to induce actin tail formation. Furthermore, stimulation of HepG2 cells showed that IFNγ can restrict growth of B. pseudomallei in the early and late phase of infection whereas the combination of IFNγ, IL-1β, and TNFα is required for the maximal antibacterial activity. This anti-B. pseudomallei defense of HepG2 cells did not seem to be mediated by inducible nitric oxide synthase-derived nitric oxide or NADPH oxidase-derived superoxide. In summary, this is the first study describing B. pseudomallei intracellular life cycle characteristics in hepatocytes and showing that IFNγ-mediated, but nitric oxide- and reactive oxygen species-independent, effector mechanisms are important in anti-B. pseudomallei host defense of hepatocytes.

IL-10 suppresses bactericidal response of macrophages against Salmonella Typhimurium

We report, herein, an attempt to determine whether an IL-10-induced immunological state affects the response of macrophages against Salmonella Typhimurium (ST). Pretreatment with mrIL-10 induced the intracellular invasion of ST into macrophages in a dose-dependent manner. It also activated AKT phosphorylation, cyclin D1, Bcl-X(L), and COX-2 upon ST infection, which may correlate with Salmonella's survival within the macrophages. However, I-κB phosphorylation was shown to be inhibited, along with the expression of TNF-α and MIP-2α mRNA. Therefore, IL-10 not only suppresses the bactericidal response of macrophages against ST, but also ultimately causes infected macrophages to function as hosts for ST replication.

Effect of the Salmonella pathogenicity island 2 type III secretion system on Salmonella survival in activated chicken macrophage-like HD11 cells

In order to better identify the role of the Salmonella pathogenicity island 2 (SPI-2) type III secretion system (T3SS) in chickens, we used the well-known gentamicin protection assay with activated HD11 cells. HD11 cells are a macrophage-like chicken cell line that can be stimulated with phorbol 12-myristate 13-acetate (PMA) to exhibit more macrophage-like morphology and greater production of reactive oxygen species (ROS). Activated HD11 cells were infected with a wild-type Salmonella enterica subspecies enterica serovar Typhimurium (S. Typhimurium) strain, a SPI-2 mutant S. Typhimurium strain, a wild-type Salmonella enterica subspecies enterica serovar Enteritidis (S. Enteritidis) strain, a SPI-2 mutant S. Enteritidis strain, or a non-pathogenic Escherichia coli (E. coli) strain. SPI-2 mutant strains were found to survive as well as their parent strain at all time points post-uptake (PU) by the HD11 cells, up to 24 h PU, while the E. coli strain was no longer recoverable by 3 h PU. We can conclude from these observations that the SPI-2 T3SS of S. Typhimurium and S. Enteritidis is not important for survival of Salmonella in the activated macrophage-like HD11 cell line, and that Salmonella must employ other mechanisms for survival in this environment, as E. coli is effectively eliminated.

The WxxxE effector EspT triggers expression of immune mediators in an Erk/JNK and NF-κB-dependent manner

Enteropathogenic Escherichia coli (EPEC), enterohaemorrhagic E. coli (EHEC) and Citrobacter rodentium colonize their respective hosts while forming attaching and effacing lesions. Their infection strategy relies on translocation of a battery of type III secretion system effectors, including Map, EspM and EspT, which belong to the WxxxE/SopE family of guanine nucleotide exchange factors. Using the C. rodentium mouse model we found that EspT triggers expression of KC and TNFα in vivo. Indeed, a growing body of evidence suggests that, in addition to subversion of actin dynamics, the SopE and the WxxxE effectors activate signalling pathways involved in immune responses. In this study we found that EspT induces expression of the pro-inflammatory mediators cyclooxygenase-2 (COX-2) an enzyme involved in production of prostaglandin E(2) (PGE2), interleukin (Il)-8 and Il-1β in U937 human macrophages by activating the nuclear factor kappa-B (NF-κB), the extracellular signal-regulated kinases 1 and 2 (Erk1/2) and c-Jun N-terminal kinase (JNK) pathways. Since EspT modulates the activation of Cdc42 and Rac1, which mediates bacterial invasion into epithelial cells, we investigated the involvement of these Rho GTPases and bacterial invasion on pro-inflammatory responses and found that (i) Rac1, but not Cdc42, is involved in EspT-induced Il-8 and Il-1β secretion and (ii) cytochalasin D inhibits EspT-induced EPEC invasion into U937 but not Il-8 or Il-1β secretion. These results suggest that while EPEC translocates a number of effectors (i.e. NleC, NleD, NleE, NleH) that inhibit inflammation, a subset of strains, which encode EspT, employ an infection strategy that also involves upregulation of immune mediators.

Impact of salmonella infection on host hormone metabolism revealed by metabolomics

The interplay between pathogens and their hosts has been studied for decades using targeted approaches, such as the analysis of mutants and host immunological responses. Although much has been learned from such studies, they focus on individual pathways and fail to reveal the global effects of infection on the host. To alleviate this issue, high-throughput methods, such as transcriptomics and proteomics, have been used to study host-pathogen interactions. Recently, metabolomics was established as a new method to study changes in the biochemical composition of host tissues. We report a metabolomic study of Salmonella enterica serovar Typhimurium infection. Our results revealed that dozens of host metabolic pathways are affected by Salmonella in a murine infection model. In particular, multiple host hormone pathways are disrupted. Our results identify unappreciated effects of infection on host metabolism and shed light on mechanisms used by Salmonella to cause disease and by the host to counter infection.

Involvement of SPI-2-encoded SpiC in flagellum synthesis in Salmonella enterica serovar Typhimurium

BACKGROUND

SpiC encoded within Salmonella pathogenicity island 2 on the Salmonella enterica serovar Typhimurium chromosome is required for survival within macrophages and systemic infection in mice. Additionally, SpiC contributes to Salmonella-induced activation of the signal transduction pathways in macrophages by affecting the expression of FliC, a component of flagella filaments. Here, we show the contribution of SpiC in flagellum synthesis.

RESULTS

Quantitative RT-PCR shows that the expression levels of the class 3 fliD and motA genes that encode for the flagella cap and motor torque proteins, respectively, were lower for a spiC mutant strain than for the wild-type Salmonella. Further, this mutant had lower expression levels of the class 2 genes including the fliA gene encoding the flagellar-specific alternative sigma factor. We also found differences in flagella assembly between the wild-type strain and the spiC mutant. Many flagella filaments were observed on the bacterial surface of the wild-type strain, whereas the spiC mutant had only few flagella. The absence of spiC led to reduced expression of the FlhD protein, which functions as the master regulator in flagella gene expression, although no significant difference at the transcription level of the flhDC operon was observed between the wild-type strain and the spiC mutant.

CONCLUSION

The data show that SpiC is involved in flagella assembly by affecting the post-transcription expression of flhDC.

Proteomic investigation of the time course responses of RAW 264.7 macrophages to infection with Salmonella enterica

To investigate the extent to which macrophages respond to Salmonella infection, we infected RAW 264.7 macrophages with Salmonella enterica serotype Typhimurium and analyzed macrophage proteins at various time points following infection by using a global proteomic approach. A total of 1,006 macrophage and 115 Salmonella proteins were identified with high confidence. Most of the Salmonella proteins were observed in the late stage of the infection time course, which is consistent with the fact that the bacterial cells proliferate inside RAW 264.7 macrophages. The peptide abundances of most of the identified macrophage proteins remained relatively constant over the time course of infection. Compared to those of the control, the peptide abundances of 244 macrophage proteins (i.e., 24% of the total identified macrophage proteins) changed significantly after infection. The functions of these Salmonella-affected macrophage proteins were diverse, including production of antibacterial nitric oxide (i.e., inducible nitric oxide synthase), production of prostaglandin H(2) (i.e., cyclooxygenase 2), and regulation of intracellular traffic (e.g., sorting nexin 5 [SNX5], SNX6, and SNX9). Diverse functions of the Salmonella-affected macrophage proteins demonstrate a global macrophage response to Salmonella infection. Western blot analysis not only confirmed the proteomic results for a selected set of proteins but also revealed that (i) the protein abundance of mitochondrial superoxide dismutase increased following macrophage infection, indicating an infection-induced oxidative stress in mitochondria, and (ii) in contrast to infection of macrophages by wild-type Salmonella, infection by the sopB deletion mutant had no negative impact on the abundance of SNX6, suggesting a role for SopB in regulating the abundance of SNX6.

Effect of long chain fatty acids on Salmonella killing, superoxide and nitric oxide production by chicken macrophages

The objective of this study was to investigate the effect of uptake of different commonly consumed long chain fatty acids on superoxide (O(2)(-)), nitric oxide (NO) production, and ability to kill Salmonella enterica serotype typhimurium (S. typhimurium) by chicken macrophages (HD11 cells). All the fatty acids were taken up by HD11 cells with stearic acid uptake higher than polyunsaturated fatty acids. Uptake of green fluorescent protein-labeled bacteria and the viability of HD11 cells (measured by flow cytometry) was not affected by any of the fatty acids tested. Bacterial clearance (measured by the plating of sorted viable infected cells) was significantly higher with n-3 fatty acids alpha-linolenic acid (ALA) and docosahexanoic acid (DHA). However, stearic acid (SA) and the n-6 fatty acid, arachidonic acid (ARA) did not influence S. typhimurium killing by HD11 cells. The improved S. typhimurium clearance by ALA and DHA was not associated with increased NO or O(2)(-) production by HD11 cells. These results suggest a role for n-3 polyunsaturated fatty acids in Salmonella clearance by chicken macrophages however in vivo studies are essential to confirm their efficacy in controlling Salmonella infection in chickens and contamination in shell eggs.

Salmonella enterica serovar Enteritidis dam mutant induces low NOS-2 and COX-2 expression in macrophages via attenuation of MAPK and NF-kappaB pathways

Although dam mutants of Salmonella have been proposed as live vaccines, their capacity to trigger cell inflammatory cascades has not been fully elucidated. We investigated in detail the ability of Salmonella enterica dam mutant to activate the signalling pathways of the inflammatory response in RAW 264.7 cells. Apoptosis in macrophages treated with Salmonella dam mutant was low. Similarly, the expression of both NOS-2 and COX-2 and subsequently the production of NO and PGE(2) was significantly reduced. Also, Salmonella dam mutant induced an attenuated activation of the inflammatory signalling pathway as indicated by the reduced degradation of IkappaBalpha and IkappaBbeta and the low IkappaBalpha phosphorylation found. In addition, translocation of p65 to the nucleus was notably impaired and the amount of phosphorylated p44, p42 and p38 MAPKs was clearly reduced in extracts from dam-infected macrophages. These results indicate that the lack of ERK and p38 phosphorylation at the proper time in dam-infected cells notably reduces the engagement of subsequent signalling pathways involved in the full activation of NF-kappaB in response to infection. Taken together, these results suggest that Salmonella activation of both signalling cascades in the inflammatory response is a mechanism requiring Dam protein participation.

Mannose-capped lipoarabinomannan- and prostaglandin E2-dependent expansion of regulatory T cells in human Mycobacterium tuberculosis infection

We evaluated the role of regulatory T cells (CD4(+) CD25(+) Foxp3(+) cells, Tregs) in human Mycobacterium tuberculosis infection. Tregs were expanded in response to M. tuberculosis in healthy tuberculin reactors, but not in tuberculin-negative individuals. The M. tuberculosis mannose-capped lipoarabinomannan (ManLAM) resulted in regulatory T cell expansion, whereas the M. tuberculosis 19-kDa protein and heat shock protein 65 had no effect. Anti-IL-10 and anti-TGF-beta alone or in combination, did not reduce expansion of Tregs. In contrast, the cyclooxygenase enzyme-2 inhibitor NS398 significantly inhibited expansion of Tregs, indicating that prostaglandin E2 (PGE2) contributes to Treg expansion. Monocytes produced PGE2 upon culturing with heat-killed M. tuberculosis or ManLAM, and T cells from healthy tuberculin reactors enhanced PGE2 production by monocytes. Expanded Tregs produced significant amounts of TGF-beta and IL-10 and depletion of Tregs from PBMC of these individuals increased the frequency of M. tuberculosis-responsive CD4(+) IFN-gamma cells. Culturing M. tuberculosis-expanded Tregs with autologous CD8(+) cells decreased the frequency of IFN-gamma(+)cells. Freshly isolated PBMC from tuberculosis patients had increased percentages of Tregs, compared to healthy tuberculin reactors. These findings demonstrate that Tregs expand in response to M. tuberculosis through mechanisms that depend on ManLAM and PGE2.

Intracellular bacterial growth is controlled by a kinase network around PKB/AKT1

With the emergence of multidrug resistant (MDR) bacteria, it is imperative to develop new intervention strategies. Current antibiotics typically target pathogen rather than host-specific biochemical pathways. Here we have developed kinase inhibitors that prevent intracellular growth of unrelated pathogens such as Salmonella typhimurium and Mycobacterium tuberculosis. An RNA interference screen of the human kinome using automated microscopy revealed several host kinases capable of inhibiting intracellular growth of S. typhimurium. The kinases identified clustered in one network around AKT1 (also known as PKB). Inhibitors of AKT1 prevent intracellular growth of various bacteria including MDR-M. tuberculosis. AKT1 is activated by the S. typhimurium effector SopB, which promotes intracellular survival by controlling actin dynamics through PAK4, and phagosome-lysosome fusion through the AS160 (also known as TBC1D4)-RAB14 pathway. AKT1 inhibitors counteract the bacterial manipulation of host signalling processes, thus controlling intracellular growth of bacteria. By using a reciprocal chemical genetics approach, we identified kinase inhibitors with antibiotic properties and their host targets, and we determined host signalling networks that are activated by intracellular bacteria for survival.

Catalytically inactive cyclooxygenase 2 and absence of prostaglandin E2 biosynthesis in murine peritoneal macrophages following in vivo phagocytosis of heat-killed Mycobacterium bovis bacillus Calmette-Guérin

Over 25 years ago, it was observed that peritoneal macrophages (Mphi) isolated from mice given heat-killed Mycobacterium bovis bacillus Calmette-Guérin (HK-BCG) i.p. did not release PGE(2). However, when peritoneal Mphi from untreated mice are treated with HK-BCG in vitro, cyclooxygenase 2 (COX-2), a rate-limiting enzyme for PGE(2) biosynthesis, is expressed and the release of PGE(2) is increased. The present study of peritoneal Mphi obtained from C57BL/6 mice and treated either in vitro or in vivo with HK-BCG was undertaken to further characterize the cellular responses that result in suppression of PGE(2) release. The results indicate that Mphi treated with HK-BCG in vivo express constitutive COX-1 and inducible COX-2 that are catalytically inactive, are localized subcellularly in the cytoplasm, and are not associated with the nuclear envelope (NE). In contrast, Mphi treated in vitro express catalytically active COX-1 and COX-2 that are localized in the NE and diffusely in the cytoplasm. Thus, for local Mphi activated in vivo by HK-BCG, the results indicate that COX-1 and COX-2 dissociated from the NE are catalytically inactive, which accounts for the lack of PGE(2) production by local Mphi activated in vivo with HK-BCG. Our studies further indicate that the formation of catalytically inactive COX-2 is associated with in vivo phagocytosis of HK-BCG, and is not dependent on extracellular mediators produced by in vivo HK-BCG treatment. This attenuation of PGE(2) production may enhance Mphi-mediated innate and Th1-acquired immune responses against intracellular infections which are suppressed by PGE(2).

SopD acts cooperatively with SopB during Salmonella enterica serovar Typhimurium invasion

The intracellular bacterial pathogen, Salmonella enterica serovar Typhimurium (S. typhimurium), causes disease in a variety of hosts. To invade and replicate in host cells, these bacteria subvert host molecular machinery using bacterial proteins, called effectors, which they translocate into host cells using specialized protein delivery systems. One of these effectors, SopD, contributes to gastroenteritis, systemic virulence and persistence of S. typhimurium in animal models of infection. Recently, SopD has been implicated in invasion of polarized epithelial cells and here we investigate the features of SopD-mediated invasion. We show that SopD plays a role in membrane fission and macropinosome formation during S. typhimurium invasion, events previously shown to be mediated by the SopB effector. We further demonstrate that SopD acts cooperatively with SopB to promote these events during invasion. Using live cell imaging we show that a SopD-GFP fusion does not localize to HeLa cell cytosol as previously described, but instead is membrane associated. Upon S. typhimurium infection of these cells, SopD-GFP is recruited to the invasion site, and this recruitment required the phosphatase activity of SopB. Our findings demonstrate a role for SopD in manipulation of host-cell membrane during S. typhimurium invasion and reveal the nature of its cooperative action with SopB.

Synthetic prostacyclin analogs differentially regulate macrophage function via distinct analog-receptor binding specificities

PGI(2) (prostacyclin) is a lipid mediator with vasodilatory and antithrombotic effects used in the treatment of vasoconstrictive/ischemic diseases including pulmonary artery hypertension. However, emerging research supports a role for PGs, including PGI(2), in the regulation of both innate and acquired immunity. As PGI(2) is unstable, we sought to define the effects of various PGI(2) analogs on resident alveolar macrophage (AM) and peritoneal macrophage (PM) innate immune functions. The effects of iloprost, carbaprostacyclin, and treprostinil on the regulation of phagocytosis, bacterial killing, and inflammatory mediator production were determined in both macrophage populations from rats. Iloprost failed to suppress AM functions to the same degree that it did in PMs, a characteristic shared by carbaprostacyclin. This difference reflected greater expression of the G(alphas) protein-coupled I prostanoid receptor and greater cAMP generation in PMs than AMs. Treprostinil inhibited phagocytosis, bacterial killing, and cytokine generation in AMs to a much greater degree than the other PGI(2) analogs and more closely resembled the effects of PGE(2). Studies with the E prostanoid (EP) 2 receptor antagonist AH-6809 and EP2-null macrophages indicated that this was due in part to the previously unknown ability of treprostinil to stimulate the EP2 receptor. The present investigation for the first time identifies differences in immunoregulatory properties of clinically administered PGI(2) analogs. These studies are the first to explore the capacity of PGI(2) to regulate bacterial killing and phagocytosis in macrophages, and our findings may hold important consequences regarding the risk of infection for patients receiving such agents.

Francisella tularensis-infected macrophages release prostaglandin E2 that blocks T cell proliferation and promotes a Th2-like response

Francisella tularensis is a highly infectious bacterial pathogen, and is likely to have evolved strategies to evade and subvert the host immune response. In this study, we show that F. tularensis infection of macrophages alters T cell responses in vitro, by blocking T cell proliferation and promoting a Th2-like response. We demonstrate that a soluble mediator is responsible for this effect and identify it as PGE(2). Supernatants from F. tularensis-infected macrophages inhibited IL-2 secretion from both MHC class I and MHC class II-restricted T cell hybridomas, as well as enhanced a Th2-like response by inducing increased production of IL-5. Furthermore, the soluble mediator blocked proliferation of naive MHC class I-restricted T cells when stimulated with cognate tetramer. Indomethacin treatment partially restored T cell proliferation and lowered IL-5 production to wild-type levels. Macrophages produced PGE(2) when infected with F. tularensis, and treatment of infected macrophages with indomethacin, a cyclooxygenase-1/cyclooxygenase-2 inhibitor, blocked PGE(2) production. To further demonstrate that PGE(2) was responsible for skewing of T cell responses, we infected macrophages from membrane PGE synthase 1 knockout mice (mPGES1(-/-)) that cannot produce PGE(2). Supernatants from F. tularensis-infected membrane PGE synthase 1(-/-) macrophages did not inhibit T cell proliferation. Furthermore, treatment of T cells with PGE(2) recreated the effects seen with infected supernatant. From these data, we conclude that F. tularensis can alter host T cell responses by causing macrophages to produce PGE(2). This study defines a previously unknown mechanism used by F. tularensis to modulate adaptive immunity.

Salmonella, the host and disease: a brief review

Salmonella species cause substantial morbidity, mortality and burden of disease globally. Infections with Salmonella species cause multiple clinical syndromes. Central to the pathophysiology of all human salmonelloses is the induction of a strong host innate immune/inflammatory response. Whether this ultimately reflects an adaptive advantage to the host or pathogen is not clear. However, it is evident that both the host and pathogen have evolved mechanisms of triggering host responses that are detrimental to the other. In this review, we explore some of the host and pathogenic mechanisms mobilized in the two predominant clinical syndromes associated with infection with Salmonella enterica species: enterocolitis and typhoid.

Infection of human endothelial cells with spotted Fever group rickettsiae stimulates cyclooxygenase 2 expression and release of vasoactive prostaglandins

Rickettsiae, a diverse group of obligately intracellular gram-negative bacteria, include etiologic agents of the spotted fever and typhus groups of diseases. Rocky Mountain spotted fever and boutonneuse fever, due to Rickettsia rickettsii and R. conorii, respectively, are characterized by widespread infection of the vascular endothelium, microvascular injury, and vasculitis. Cultured human endothelial cells (EC) are highly susceptible to infection and respond by altering the expression of adhesion molecules, regulatory cytokines, and the antioxidant enzyme heme oxygenase (HO). In the vasculature, HO regulates the cyclooxygenase (COX) enzymes, among which the inducible isozyme COX-2 facilitates the synthesis of prostaglandins (PGs). Using in vitro and ex vivo models of infection, we demonstrate here that R. rickettsii infection of human EC causes robust induction of COX-2 mRNA and protein expression but has no apparent effect on the constitutive COX-1 isoform. Cells infected with viable rickettsiae consistently displayed significantly increased secretion of 6-keto-PGF(1alpha) and PGE(2). R. rickettsii-induced COX-2 was sensitive to inhibitors of de novo transcription and the pyridinylimidazole-based compound SB 203580, suggesting that this transcriptional host cell response involves signaling through p38 mitogen-activated protein kinase. PG production by infected cells was abrogated by NS 398 (a selective COX-2 inhibitor) and indomethacin (a pan-COX inhibitor). Immunohistochemical staining of sections of infected umbilical cords and corresponding uninfected controls revealed comparatively more intense and abundant staining for COX-2 in infected endothelia. Induction of the endothelial COX-2 system and the resultant enhanced release of vasoactive PGs may contribute to the regulation of inflammatory responses and vascular permeability changes during spotted fever rickettsioses.

Salmonella pathogenicity island 2-dependent expression of suppressor of cytokine signaling 3 in macrophages

Salmonella pathogenicity island 2 (SPI-2), which is located at centisome 30.7 on the chromosome of Salmonella enterica serovar Typhimurium, is required for growth within macrophages and systemic infection in mice. We recently reported that the infection of macrophages with Salmonella induces the expression of cyclooxygenase-2 in a manner dependent on SPI-2. In the present study, gene expression analysis using a cDNA array further showed the involvement of SPI-2 in the expression of suppressor of cytokine signaling 3 (SOCS-3), which is involved in the inhibition of cytokine signaling via the Janus kinase/signal transducer and activator of transcription (JAK/STAT) signaling pathway. A high level of SOCS-3 expression was induced in J774 macrophages infected with wild-type Salmonella compared to that in macrophages infected with a strain carrying a mutation in the spiC gene within SPI-2. Other members of the SOCS family were not detected in Salmonella-infected macrophages. The SPI-2-induced up-regulation of SOCS-3 expression was dependent on activation of the extracellular signal-regulated kinase 1/2 (ERK1/2) signaling pathway. Furthermore, the inhibition of gamma-interferon-induced STAT-1 and interleukin-6-induced STAT-3 tyrosine phosphorylation correlated with the expression of SOCS-3. Taken together, these results indicate that Salmonella causes SPI-2-dependent activation of ERK1/2, leading to SOCS-3 expression, which in turn inhibits cytokine signaling via the JAK/STAT pathway.

COX-2 inhibition affects growth rate of Chlamydia muridarum within epithelial cells

Chlamydiae alter apoptosis of host target cells, which regulates their growth. Cyclooxygenase-2 (COX-2), the rate-limiting enzyme for prostaglandin E2 (PGE2) production, modulates epithelial cell survival. We addressed whether endogenous PGE2 alters chlamydial growth or apoptosis of epithelial cells infected with Chlamydia muridarum. PGE2 is secreted by infected host cells in the genital tract (GT). Using immunohistochemical techniques, we found that COX-2 enzyme was localized to epithelial cells in the GT in vivo. Pellets of the COX-2 enzyme inhibitor, NS-398, and placebo were implanted in mice subcutaneously and released a constant amount of these chemicals throughout the infection. NS-398-treated mice were found to exhibit 10-fold lower bacterial load than the placebo group on day 3 post infection, suggesting disruption of the chlamydial developmental cycle. To prove this, the human lung adenocarcinoma cell line A549 was then infected with different MOIs of C. muridarum in the presence of multiple concentrations of NS-398 in vitro. There was no difference in inclusion forming units (IFUs) between NS-389-treated and untreated cells. We also found no alterations in C. muridarum IFUs in A549 cells transfected with a 2.0 kb cDNA fragment of human COX-2 cloned in the sense (S) or anti-sense (AS) orientation. However, the inclusion size was reduced and the number of EB was significantly diminished during reinfection in AS-transfected cells. In addition, the absence of COX-2 did not significantly modify apoptosis in infected cells. In total, COX-2 deficiency reduces the infectious burden in vivo and may modulate transmission of the organism.

Depletion of immature B cells during Trypanosoma cruzi infection: involvement of myeloid cells and the cyclooxygenase pathway

The ability of a microorganism to elicit or evade B cell responses represents a determinant factor for the final outcome of an infection. Although pathogens may subvert humoral responses at different stages of B cell development, most studies addressing the impact of an infection on the B cell compartment have focused on mature B cells within peripheral lymphoid organs. Herein, we report that a protozoan infection, i.e. a Trypanosoma cruzi infection, induces a marked loss of immature B cells in the BM, which also compromises recently emigrated B cells in the periphery. The depletion of BM immature B cells is associated with an increased rate of apoptosis mediated by a parasite-indirect mechanism in a Fas/FasL-independent fashion. Finally, we demonstrated that myeloid cells play an important role in B cell depletion, since CD11b(+) BM cells from infected mice secrete a product of the cyclooxygenase pathway that eliminates immature B cells. These results highlight a previously unrecognized maneuver used by a protozoan parasite to disable B cell generation, limiting host defense and favoring its chronic establishment.