Shigella flexneri invasion of HeLa cells induces NF-kappa B DNA-binding activity

Development of a visual Adhesion/Invasion Inhibition Assay to assess the functionality of Shigella-specific antibodies

Introduction

Shigella is the etiologic agent of a bacillary dysentery known as shigellosis, which causes millions of infections and thousands of deaths worldwide each year due to Shigella's unique lifestyle within intestinal epithelial cells. Cell adhesion/invasion assays have been extensively used not only to identify targets mediating host-pathogen interaction, but also to evaluate the ability of Shigella-specific antibodies to reduce virulence. However, these assays are time-consuming and labor-intensive and fail to assess differences at the single-cell level.

Objectives and methods

Here, we developed a simple, fast and high-content method named visual Adhesion/Invasion Inhibition Assay (vAIA) to measure the ability of anti-Shigellaantibodies to inhibit bacterial adhesion to and invasion of epithelial cells by using the confocal microscope Opera Phenix.

Results

We showed that vAIA performed well with a pooled human serum from subjects challenged with S. sonnei and that a specific anti-IpaD monoclonal antibody effectively reduced bacterial virulence in a dose-dependent manner.

Discussion

vAIA can therefore inform on the functionality of polyclonal and monoclonal responses thereby supporting the discovery of pathogenicity mechanisms and the development of candidate vaccines and immunotherapies. Lastly, this assay is very versatile and may be easily applied to other Shigella species or serotypes and to different pathogens.

PRELP Enhances Host Innate Immunity against the Respiratory Tract Pathogen Moraxella catarrhalis

Respiratory tract infections are one of the leading causes of mortality worldwide urging better understanding of interactions between pathogens causing these infections and the host. Here we report that an extracellular matrix component proline/arginine-rich end leucine-rich repeat protein (PRELP) is a novel antibacterial component of innate immunity. We detected the presence of PRELP in human bronchoalveolar lavage fluid and showed that PRELP can be found in alveolar fluid, resident macrophages/monocytes, myofibroblasts, and the adventitia of blood vessels in lung tissue. PRELP specifically binds respiratory tract pathogens Moraxella catarrhalis, Haemophilus influenzae, and Streptococcus pneumoniae, but not other bacterial pathogens tested. We focused our study on M. catarrhalis and found that PRELP binds the majority of clinical isolates of M. catarrhalis (n = 49) through interaction with the ubiquitous surface protein A2/A2H. M. catarrhalis usually resists complement-mediated serum killing by recruiting to its surface a complement inhibitor C4b-binding protein, which is also a ligand for PRELP. We found that PRELP competitively inhibits binding of C4b-binding protein to bacteria, which enhances membrane attack complex formation on M. catarrhalis and thus leads to increased serum sensitivity. Furthermore, PRELP enhances phagocytic killing of serum-opsonized M. catarrhalis by human neutrophils in vitro. Moreover, PRELP reduces Moraxella adherence to and invasion of human lung epithelial A549 cells. Taken together, PRELP enhances host innate immunity against M. catarrhalis through increasing complement-mediated attack, improving phagocytic killing activity of neutrophils, and preventing bacterial adherence to lung epithelial cells.

Attachment of Staphylococcus aureus is required for activation of nuclear factor kappa B in human osteoblasts

Nuclear factor kappa B (NF-κB) plays a prominent role in the pathogenesis of infectious diseases. Staphylococcus aureus (S. aureus), which can attach to and invade human osteoblasts, is the most common causative agent of osteomyelitis. To determine whether S. aureus can activate NF-κB in human osteoblasts and explore the possible factors of activation in response to infection, we used flow cytometry, enzyme-linked immunosorbent assay, immunoblots, and electrophoretic mobility shift assays to quantify the invasion of bacteria, to measure the interleukin-6 (IL-6) of culture supernatants, and to investigate the IκBα degradation and NF-κB activation in human osteoblasts. Moreover, we explored the possible factors responsible for the activation of NF-κB by preventing S. aureus from physically touching human osteoblasts or inhibiting the invasion of S. aureus into human osteoblasts under co-culture conditions, by incubating proteinase K-treated or ultraviolet-killed S. aureus with human osteoblasts and by treating human osteoblasts with peptidoglycan (PGN) or lipoteichoic acid (LTA). We found that S. aureus induced the IκBα degradation and NF-κB activation, which could regulate IL-6 secretion in the culture supernatants of human osteoblasts in response to infection. In addition, the maximal IκBα degradation and NF-κB activation in human osteoblasts occurred prior to the maximal invasion of S. aureus. It was the attachment not invasion or the secreted soluble factor(s), PGN, LTA of S. aureus, that could induce the IκBα degradation and NF-κB activation in human osteoblasts. These results indicated that S. aureus can activate NF-κB in human osteoblasts and that the attachment of S. aureus is required for this activation in response to infection.

Campylobacter-induced interleukin-8 secretion in polarized human intestinal epithelial cells requires Campylobacter-secreted cytolethal distending toxin- and Toll-like receptor-mediated activation of NF-kappaB

Campylobacter jejuni and Campylobacter coli colonize and infect the intestinal epithelium and cause acute inflammatory diarrhea. The intestinal epithelium serves as a physical barrier to, and a sensor of, bacterial infection by secreting proinflammatory cytokines. This study examined the mechanisms for Campylobacter-induced secretion of the proinflammatory chemokine interleukin-8 (IL-8) by using polarized T84 human colonic epithelial cells as a model. C. jejuni increased the secretion of both IL-8 and tumor necrosis factor alpha (TNF-alpha) in polarized epithelial cells. However, the increase in IL-8 secretion was independent of Campylobacter-stimulated TNF-alpha secretion. Polarized T84 cells secreted IL-8 predominantly to the basolateral medium independently of the inoculation direction. While there was a significant correlation between the levels of IL-8 secretion and Campylobacter invasion, all 11 strains tested increased IL-8 secretion by polarized T84 cells despite their differences in adherence, invasion, and transcytosis efficiencies. Cell-free supernatants of Campylobacter-T84-cell culture increased IL-8 secretion to levels similar to those induced by live bacterial inoculation. The ability of the supernatant to induce IL-8 secretion was reduced by flagellum and cytolethal distending toxin (CDT) gene mutants, treatment of the supernatant with protease K or heat, or treatment of T84 cells with the Toll-like receptor (TLR) inhibitor MyD88 inhibitory peptide or chloroquine. NF-kappaB inhibitors or cdtB mutation plus MyD88 inhibitor, but not flaA cdtB double mutations, abolished the ability of the supernatant to induce IL-8 secretion. Taken together, our results demonstrate that Campylobacter-induced IL-8 secretion requires functional flagella and CDT and depends on the activation of NF-kappaB through TLR signaling and CDT in human intestinal epithelial cells.

Thioaptamer decoy targeting of AP-1 proteins influences cytokine expression and the outcome of arenavirus infections

Viral haemorrhagic fever (VHF) is caused by a number of viruses, including arenaviruses. The pathogenesis is believed to involve dysregulation of cytokine production. The arenaviruses Lassa virus and Pichinde virus have a tropism for macrophages and other reticuloendothelial cells and both appear to suppress the normal macrophage response to virus infection. A decoy thioaptamer, XBY-S2, was developed and was found to bind to AP-1 transcription factor proteins. The P388D1 macrophage-like cell line contains members of the AP-1 family which may act as negative regulators of AP-1-controlled transcription. XBY-S2 was found to bind to Fra-2 and JunB, and enhance the induction of cytokines IL-6, IL-8 and TNF-alpha, while reducing the binding to AP-1 promoter elements. Administration of XBY-S2 to Pichinde virus-infected guinea pigs resulted in a significant reduction in Pichinde virus-induced mortality and enhanced the expression of cytokines from primary guinea pig macrophages, which may contribute to its ability to increase survival of Pichinde virus-infected guinea pigs. These data demonstrate a proof of concept that thioaptamers can be used to modulate the outcome of in vivo viral infections by arenaviruses by the manipulation of transcription factors involved in the regulation of the immune response.

Role of Toll-like receptors in health and diseases of gastrointestinal tract

The human gastrointestinal (GI) tract is colonized by non-pathogenic commensal microflora and frequently exposed to many pathogenic organisms. For the maintenance of GI homeostasis, the host must discriminate between pathogenic and non-pathogenic organisms and initiate effective and appropriate immune and inflammatory responses. Mammalian toll-like receptors (TLRs) are members of the pattern-recognition receptor (PRR) family that plays a central role in the initiation of innate cellular immune responses and the subsequent adaptive immune responses to microbial pathogens. Recent studies have shown that gastrointestinal epithelial cells express almost all TLR subtypes characterized to date and that the expression and activation of TLRs in the GI tract are tightly and coordinately regulated. This review summarizes the current understanding of the crucial dual roles of TLRs in the development of host innate and adaptive immune responses to GI infections and the maintenance of the immune tolerance to commensal bacteria through down-regulation of surface expression of TLRs in intestinal epithelial cells.

Streptococcus pyogenes induces epithelial inflammatory responses through NF-kappaB/MAPK signaling pathways

Innate immunity involves a cascade of inflammatory events, resulting in the secretion of chemokines and cytokines to recruit mediator cells in adaptive immunity. To study epithelial inflammatory responses initiated by Streptococcus pyogenes infection, we investigated chemotaxis ability in the supernatant of infected human respiratory epithelial HEp-2 cells. Our results showed that these supernatants showed significantly increased ability to attract monocytes, implying the release of inflammatory chemoattractants into the medium. Expression of interleukin (IL)-8 and IL-6 in HEp-2 cells was significantly increased at both the mRNA and protein levels after infection with S. pyogenes. Electrophoretic mobility shift and reporter-gene assays demonstrated that the transcription factors NF-kappaB and AP-1, regulated by mitogen-activated protein (MAP) kinase, were activated after streptococcal infection. The increases in mRNAs for IL-8 and IL-6 were abrogated by addition of NF-kappaB and MAP kinase inhibitors, suggesting that the upregulation of IL-8 and IL-6 is mediated through NF-kappaB and MAP kinase signaling pathways. Taken together, our results indicate that S. pyogenes infection of epithelial cells induces the secretion of pro-inflammatory chemokines/cytokines through activation of NF-kappaB and MAP kinase signaling pathways. These early innate responses initiated by S. pyogenes-infected respiratory epithelial cells may recruit immune cells to the airway and induce inflammation.

Invasive phenotype of Candida albicans affects the host proinflammatory response to infection

Candida albicans is a major opportunistic pathogen in immunocompromised patients. Production of proinflammatory cytokines by host cells in response to C. albicans plays a critical role in the activation of immune cells and final clearance of the organism. Invasion of host cells and tissues is considered one of the virulence attributes of this organism. The purpose of this study was to investigate whether the ability of C. albicans to invade host cells and tissues affects the proinflammatory cytokine responses by epithelial and endothelial cells. In this study we used the invasion-deficient RIM101 gene knockout strain DAY25, the highly invasive strain SC5314, and highly invasive RIM101-complemented strain DAY44 to compare the proinflammatory cytokine responses by oral epithelial or endothelial cells. Using a high-throughput approach, we found both qualitative and quantitative differences in the overall inflammatory responses to C. albicans strains with different invasive potentials. Overall, the highly invasive strains triggered higher levels of proinflammatory cytokines in host cells than the invasion-deficient mutant triggered. Significant differences compared to the attenuated mutant were noted in interleukin-1alpha (IL-1alpha), IL-6, IL-8, and tumor necrosis factor alpha in epithelial cells and in IL-6, growth-related oncogene, IL-8, monocyte chemoattractant protein 1 (MCP-1), MCP-2, and granulocyte colony-stimulating factor in endothelial cells. Our results indicate that invasion of host cells and tissues by C. albicans enhances the host proinflammatory response to infection.

Leishmania donovani activates nuclear transcription factor-kappaB in macrophages through reactive oxygen intermediates

Interaction of Leishmania donovani with macrophages antagonizes host defense mechanisms by interfering with a cascade of cell signaling processes in the macrophages. An early intracellular signaling event that follows receptor engagement is the activation of transcription factor NF-kappaB. It has been reported earlier that NF-kappaB-dependent signaling pathway regulates proinflammatory cytokine release. We therefore investigated the effect of L. donovani infectivity on this nuclear transcription factor in macrophage cell line J774A.1. Both L. donovani and its surface molecule lipophosphoglycan (LPG) resulted in a dose- and time-dependent activation of NF-kappaB-DNA binding activity in an electrophoretic mobility shift assay. We also report the involvement of IkappaB-alpha and IkappaB-beta in the persistent activation of NF-kappaB by L. donovani. We demonstrate that the NF-kappaB activation was independent of viability of the parasite. Electrophoretic mobility supershift assay indicated that the NF-kappaB complex consists of p65 and c-rel subunits. The interaction of parasite with the macrophages and not the cellular uptake was important for NF-kappaB activation. Both p38 and ERK mitogen activated protein kinase (MAP) activation appears to be necessary for NF-kappaB activation by LPG. Preincubation of cells with antioxidants resulted in inhibition of L. donovani induced NF-kappaB activation, thereby suggesting a potential role of reactive oxygen species in L. donovani induced intracellular signaling. The present data indicate that antioxidants could play an important role in working out various therapeutic modalities to control leishmaniasis.

Interleukin-8 production by THP-1 cells stimulated by Salmonella enterica serovar Typhimurium porins is mediated by AP-1, NF-κB and MAPK pathways

Interleukin-8 (IL-8) is released in response to inflammatory stimuli, such as bacterial products. Either porins or lipopolysaccharide (LPS) stimulated THP-1 cells to release IL-8 after 24 h. We have previously reported that stimulation of monocytic cells with Salmonella enterica serovar Typhimurium porins led to the activation of mitogen-activated protein kinase cascades and of protein tyrosine kinases (PTKs). In this report, we demonstrate, using two potent and selective inhibitors of MEK activation by Raf-1 (PD-098059) and p38 (SB-203580), that both ERK1/2 and p38 pathways play a key role in the production of IL-8 by porins and LPS. Porin-stimulated expression of activating protein 1 (AP-1) and correlated IL-8 release is also inhibited by PD-098059 or SB-203580 indicating that the Raf-1/MEK1-MEK2/MAPK cascade is required for their activation. Also PTKs modulate the pathway that control IL-8 gene expression, in fact its expression is abolished by tyrphostin. By using N-acetyl-leucinyl-leucinyl-norleucinal-H (ALLN) an inhibitor of nuclear factor-kappaB (NF-kappaB) activity, we also observed IL-8 release modulation. Our results elucidate some of the molecular mechanisms by which AP-1 and NF-kappaB regulate IL-8 release and open new strategies for the design of specific molecules that will modulate IL-8 effects in various infectious diseases.

Activation of NF-kappaB in intestinal epithelial cells by E. coli strains isolated from the colonic mucosa of IBD patients

BACKGROUND AND AIMS

The involvement of bacteria in the pathogenesis of inflammatory bowel disease has been discussed for several years. In this study we evaluated the ability of E. coli isolates from inflamed and noninflamed colonic mucosa to activate NF-kappaB.

MATERIALS AND METHODS

Fifteen bacterial strains from inflamed and six from noninflamed colonic tissues from IBD patients. Their ability to induce NF-kappaB activation was examined in vitro by gel-shift assays. The activation of the TNF-alpha promoter was determined by reporter gene assays. Bacterial isolates were characterized by invasion assays, electron microscopy, and PCR.

RESULTS

Four of 15 E. coli bacterial isolates from inflamed IBD tissues induced NF-kappaB activity in intestinal epithelial cells as determined by gel-shift assays. NF-kappaB activation was only seen with living bacteria but not with heat-inactivated cells. Isolates from noninflamed tissues and a wild-type E. coli control strain induced a weaker or no activation. Reporter gene assays with a construct comprising a luciferase gene driven by the TNF-alpha promoter revealed that isolates from Crohn's disease patients induced a stronger activation of the TNF-alpha gene than isolates from ulcerative colitis patients. The isolated bacteria invaded HT-29 cells, although typical virulence genes for enteropathogenic, enterhemorrhagic, or enteroinvasive E. coli, i.e., eae, tir, EspA, Per (A-C), ipaC, were not detected in these cells. Bacterial invasion was additionally confirmed by electron microscopy examination.

CONCLUSION

Our results indicate that E. coli strains can be found in the mucosa of some IBD patients which are able to activate NF-kappaB similar to known pathogenic strains. The absence of several virulence genes in these cells suggests that they are members of the luminal flora which acquire as yet unidentified virulence determinants and are therefore involved in the pathophysiology of IBD.

Anti-inflammatory role for intracellular dimeric immunoglobulin a by neutralization of lipopolysaccharide in epithelial cells

Intestinal epithelial cells (IEC) play a central role in innate and acquired mucosal immunity. They ensure early signaling to trigger an inflammatory response against pathogens. Moreover, IEC mediate transcytosis of dimeric IgA (dIgA), through the polymeric-immunoglobulin receptor (pIgR), to provide secretory IgA, the major protective Ig in mucosal secretions. Using an in vitro model of polarized IEC, we describe an additional anti-inflammatory mechanism of dIgA-mediated protection against intracellular bacterial components involved in the proinflammatory activation of IEC. Specific dIgA colocalizes to lipopolysaccharide (LPS) in the apical recycling endosome compartment, preventing LPS-induced NF-kappaB translocation and subsequent proinflammatory response. Thus, intracellular neutralization by dIgA limits the acute local inflammation induced by proinflammatory pathogen-associated molecular patterns such as LPS.

JlpA of Campylobacter jejuni interacts with surface-exposed heat shock protein 90alpha and triggers signalling pathways leading to the activation of NF-kappaB and p38 MAP kinase in epithelial cells

Campylobacter jejuni is a leading cause of acute bacterial gastroenteritis in humans. The mechanism by which C. jejuni interacts with host cells, however, is still poorly understood. Our previous study has shown that the C. jejuni surface lipoprotein JlpA mediates adherence of the bacterium to epithelial cells. In this report, we demonstrated that JlpA interacts with HEp-2 cell surface heat shock protein (Hsp) 90alpha and initiates signalling pathways leading to activation of NF-kappaB and p38 MAP kinase. Gel overlay and GST pull down assays showed that JlpA interacts with Hsp90alpha. Geldanamycin, a specific inhibitor of Hsp90, and anti-human Hsp90alpha antibody significantly blocked the interaction between JlpA and Hsp90alpha, suggesting a direct interaction between JlpA and HEp-2 cell surface-exposed Hsp90alpha. The treatment of HEp-2 cells with GST-JlpA initiated two signalling pathways: one leading to the phosphorylation and degradation of IkappaB and nuclear translocation of NF-kappaB; and another one to the phosphorylation of p38 MAP kinase. The activation of NF-kappaB and p38 MAP kinase in HEp-2 cells suggest that JlpA triggers inflammatory/immune responses in host cells following C. jejuni infection.

Francisella tularensis inhibits Toll-like receptor-mediated activation of intracellular signalling and secretion of TNF-alpha and IL-1 from murine macrophages

Microbial ligands, including lipopolysaccharide (LPS) and bacterial lipoproteins, activate Toll-like receptors (TLR) of mononuclear phagocytes, thereby inducing proinflammatory cytokines and antimicrobial activity. We show that Francisella tularensis, an intracellular pathogen, is capable of inhibiting this macrophage response. Infection with the live vaccine strain F. tularensis LVS rendered cells of the murine macrophage-like cell line J774A.1 incapable of secreting TNF-alpha or IL-1beta and mobilizing an antimicrobial activity in response to bacterial lipopeptide or Escherichia coli-derived LPS. Inhibition of TNF-alpha secretion occurred also when J774 cells were infected with F. tularensis LVS in the presence of chloramphenicol, but not when they were infected with a mutant of F. tularensis LVS defective in expression of a 23 kDa protein that is upregulated during intracellular infection. Purified F. tularensis LPS did not show an agonistic or antagonistic effect on the E. coli LPS-induced activation of the J774 cells. Francisella tularensis LVS suppressed the capability of the cells to respond to LPS or bacterial lipopeptide (BLP) with activation of nuclear factor kappa B (NF-kappaB), and degradation of the in-hibitor of NF-kappaB, IkappaB, was blocked during the infection. Also the LPS- or BLP-induced phosphorylation of the mitogen-activated protein kinase p38 and the transcription factor c-Jun was inhibited by F. tularensis LVS but not by the 23 kDa protein mutant. In conclusion, F. tularensis appears capable of abrogating the TNF-alpha and IL-1 responses of macrophages induced by E. coli LPS or BLP via a mechanism that involves suppression of several intracellular pathways and is dependent on expression of a bacterial 23 kDa protein.

Induction of NF-kappaB nuclear translocation in human respiratory epithelial cells by group A streptococci

The activation of the transcription factor NF-kappaB and the production of inflammatory mediators play an essential role in the host response to pathogenic organisms. The objective of this study was to investigate the ability of group A streptococci (GAS) to stimulate the nuclear translocation of NF-kappaB in cultured human epithelial (HEp-2) cells. Infection of HEp-2 cells with a strain of Streptococcus pyogenes capable to efficiently internalize HEp-2 cells (strain A40) resulted in translocation of NF-kappaB during the first 15 min of infection, reaching a peak after 30 min that persisted at slightly lower levels 1h thereafter. Inhibition of bacterial internalization by cytochalasin D resulted in lower levels of nuclear NF-kappaB at 30 min and 1h of infection, however, it did not affect the initial nuclear translocation of NF-kappaB observed at 15 min postinfection. These results suggest that adhesion of S. pyogenes alone might be sufficient to stimulate nuclear translocation of NF-kappaB, however, bacterial internalization is required for a sustained nuclear translocation of this transcriptional factor.

Helicobacter pylori-induced activation of human endothelial cells

Helicobacter pylori infection causes active chronic inflammation with a continuous recruitment of neutrophils to the inflamed gastric mucosa. To evaluate the role of endothelial cells in this process, we have examined adhesion molecule expression and chemokine and cytokine production from human umbilical vein endothelial cells stimulated with well-characterized H. pylori strains as well as purified proteins. Our results indicate that endothelial cells actively contribute to neutrophil recruitment, since stimulation with H. pylori bacteria induced upregulation of the adhesion molecules VCAM-1, ICAM-1, and E-selectin as well as the chemokines interleukin 8 (IL-8) and growth-related oncogene alpha (GRO-alpha) and the cytokine IL-6. However, there were large variations in the ability of the different H. pylori strains to stimulate endothelial cells. These interstrain variations were seen irrespective of whether the strains had been isolated from patients with duodenal ulcer disease or asymptomatic carriers and were not solely related to the expression of known virulence factors, such as the cytotoxin-associated gene pathogenicity island, vacuolating toxin A, and Lewis blood group antigens. In addition, one or several unidentified proteins which act via NF-kappaB activation seem to induce endothelial cell activation. In conclusion, human endothelial cells produce neutrophil-recruiting factors and show increased adhesion molecule expression after stimulation with certain H. pylori strains. These effects probably contribute to the continuous recruitment of neutrophils to H. pylori-infected gastric mucosa and may also contribute to tissue damage and ulcer formation.

Effects of cadmium and vanadium ions on antigen-induced signaling in CD4(+) T cells

Heavy metal environmental pollutants modulate antigen-directed responses by T lymphocytes, but the molecular mechanisms by which certain metal ions exert their effects are only poorly understood. We tested the hypothesis that cadmium and vanadium ions alter antigen-induced T cell signal transduction pathways in CD4(+) T helper cells. We used CD4(+) primary T lymphocytes and splenic T cells from DO.11.10 T cell receptor transgenic mice. We determined the effects of cadmium chloride and sodium orthovanadate at concentrations that did not induce apoptotic cell death, but affected cytokine or proliferation responses to antigenic stimulation. We used electrophoretic mobility shift assays to measure effects of cadmium and vanadium ions on antigen-induced activation of the nuclear transcriptional regulator proteins, nuclear factor-kappaB, cyclic AMP response element binding protein, nuclear factor of activated T cells, and activator protein-1. Different signaling pathways lead to activation of these transcription factors. Our results suggest that the two heavy metal ions differentially affect signaling pathways. This knowledge will help in the development of molecular epidemiological assays.

Galanin-1 receptor up-regulation mediates the excess colonic fluid production caused by infection with enteric pathogens

Galanin is widely distributed in enteric nerve terminals lining the gastrointestinal tract. We previously showed that pathogenic Escherichia coli, but not normal commensal organisms, increase galanin-1 receptor expression by epithelial cells lining the colon (i.e., colonocytes). When present, galanin-1 receptor activation by ligand causes colonocyte Cl- secretion. We herein demonstrate that disparate pathogens including Salmonella typhimurium and Shigella flexerii also increase colonocyte galanin-1 receptor expression, whose activation is responsible for a principal component of the increased colonic fluid secretion observed. Although eliminating the GAL1R gene by homologous recombination does not alter basal colonic fluid secretion, removal of one or both alleles completely attenuates the increase in fluid secretion due to infection with enteric pathogens. Galanin-1 receptor up-regulation therefore represents a novel mechanism accounting for the increased colonic fluid secretion observed in infectious diarrhea due to several different pathogens.

Pathogen-induced apoptosis of macrophages: a common end for different pathogenic strategies

Microbe-macrophage interactions play a central role in the pathogenesis of many infections. Several bacterial pathogens induce apoptosis specifically in macrophages, but the mechanisms by which it occurs differ, and the resulting pathology can take different courses. Macrophage death caused by Shigella flexneri and Salmonella spp. has been shown to result in the release of pro-inflammatory cytokines. Conversely, Yersinia spp. induce apoptosis by suppressing the signalling pathways that lead to the production of tumour necrosis factor (TNF)-alpha, a cytokine essential for the control of this infection. It is likely that there are a variety of reasons why macrophages are particularly susceptible to pathogen-induced apoptosis. One reason may be the expression of surface receptors that recognize highly conserved bacterial components, such as lipopolysaccharide (LPS) and bacterial lipoproteins (BLPs). These receptors have recently been shown to activate pro-apoptotic signalling pathways. The roles of macrophage apoptosis in different disease processes are discussed.

Dextran sulfate sodium-induced murine colitis activates NF-kappaB and increases galanin-1 receptor expression

Galanin is widely distributed in enteric nerve terminals and acts to modulate intestinal motility by altering smooth muscle contraction. This ligand causes Cl(-) secretion when colonic epithelial cells express the galanin-1 receptor (Gal1-R) subtype. Because Gal1-R expression by colonic epithelia is upregulated by the transcription factor nuclear factor-kappaB (NF-kappaB), increasingly appreciated as critical in the pathophysiology of inflammatory bowel disease, we wondered whether the diarrhea associated with this condition could be due to NF-kappaB-mediated increases in Gal1-R expression. To test this hypothesis, we provided oral dextran sulfate sodium (DSS) to C57BL/6J mice. Although Gal1-R are not normally expressed by epithelial cells lining the mouse colon, DSS treatment resulted in increased NF-kappaB activation and Gal1-R expression. Whereas galanin had no effect on murine colonic tissues studied ex vivo, it progressively increased short-circuit current and colonic fluid secretion in DSS-treated mice. Concomitant parenteral administration of the NF-kappaB inhibitor dexamethasone attenuated the activation of this transcription factor by DSS, inhibiting the increase in Gal1-R expression. Although Gal1-R-specific antagonists do not exist, intracolonic administration of commercially available galanin antibody diminished the DSS-induced increase in colonic fluid accumulation. Overall, these data demonstrate that a significant component of the excessive fluid secretion observed in DSS-treated mice is due to increased Gal1-R expression.

A novel function of InIB from Listeria monocytogenes: activation of NF-kappaB in J774 macrophages

Listeria monocytogenes causes a pro-inflammatory response on adhesion to macrophages. Upregulation of inflammation genes involves the transcription factor NF-kappaB. Several components of L. monocytogenes, including lipoteichoic acid (LTA), phospholipases and listeriolysin O (LLO), have since been shown to mediate NF-kappaB activation. Here, we report that purified recombinant InlB, but not internalin (InlA), is a potent activator of NF-kappaB in the mouse macrophage-like cell line J774. Expression of InlB in Listeria innocua enhances its ability to activate NF-kappaB, while deletion of InlB from L. monocytogenes marginally decreases its effect on NF-kappaB, possibly because of the presence of NF-kappaB activators such as LTA and LLO. The effect correlates with the rapid degradation of IkappaBalpha, a sustained degradation of IkappaBbeta and increases in tumour necrosis factor alpha (TNF-alpha) and interleukin (IL) 6 production, two cytokines controlled by NF-kappaB. Using a series of anti-InlB monoclonal antibodies and domains of InlB, NF-kappaB activation was shown to be dependent upon the N-terminal 213-amino-acid leucine-rich repeat (LRR) domain of InlB, recently demonstrated to be responsible for InlB-mediated L. monocytogenes invasion and phosphoinositide-3 (PI-3) kinase activation. The effect of InlB was blocked by PI-3 kinase inhibitors, indicating the involvement of PI-3 kinase in this response. This report thus illustrates that InlB not only promotes invasion, but also contributes to the macrophage pro-inflammatory response.

Involvement of protein kinase C in Rickettsia rickettsii-induced transcriptional activation of the host endothelial cell

Our laboratory has reported on a biphasic pattern of nuclear factor kappaB (NF-kappaB) activation in cultured human umbilical vein endothelial cells during infection with Rickettsia rickettsii, an obligate, intracellular bacterium, and the etiologic agent of Rocky Mountain spotted fever. Transcriptional activation of the tissue factor (TF) gene during this infection has been shown to involve NF-kappaB. To further understand the signal transduction events underlying these phenomena, we studied the role of protein kinase C (PKC), a ubiquitous family of phospholipid-dependent enzymes implicated in the regulation of a variety of cell signaling pathways. Two inhibitors of PKC, namely, bisindolylmaleimide I hydrochloride (BM-1) and calphostin C, which exhibit different inhibitory properties towards various isozymes of PKC, were used. Infection of cells with R. rickettsii in the presence of BM-1 (50 nM) did not significantly affect NF-kappaB, whereas calphostin C (2.5 microM) completely blocked the early phase of NF-kappaB activation. The late, sustained phase also was not affected by treatment with BM-1. Downregulation of phorbol ester-sensitive PKCs by long-term treatment with phorbol 12-myristate 13-acetate (PMA) did not inhibit NF-kappaB activation. Likewise, this downregulation had no effect on induction of TF activity. The activity of TF was, however, sensitive to BM-1 and calphostin C, whereas expression of TF mRNA was inhibited only by calphostin C. Overall, these results suggest the lack of involvement of classical PKC pathways in R. rickettsii-induced NF-kappaB activation but the possible involvement of a non-PMA-responsive PKC isoform in the posttranscriptional control of TF expression.

Activators and target genes of Rel/NF-kappaB transcription factors

The vertebrate transcription factor NF-kappaB is induced by over 150 different stimuli. Active NF-kappaB, in turn, participates in the control of transcription of over 150 target genes. Because a large variety of bacteria and viruses activate NF-kappaB and because the transcription factor regulates the expression of inflammatory cytokines, chemokines, immunoreceptors, and cell adhesion molecules, NF-kappaB has often been termed a 'central mediator of the human immune response'. This article contains a complete listing of all NF-kappaB inducers and target genes described to date. The collected data argue that NF-kappaB functions more generally as a central regulator of stress responses. In addition, NF-kappaB activation blocks apoptosis in several cell types. Coupling stress responsiveness and anti-apoptotic pathways through the use of a common transcription factor may result in increased cell survival following stress insults.

Activators and target genes of Rel/NF-kappaB transcription factors

The vertebrate transcription factor NF-kappaB is induced by over 150 different stimuli. Active NF-kappaB, in turn, participates in the control of transcription of over 150 target genes. Because a large variety of bacteria and viruses activate NF-kappaB and because the transcription factor regulates the expression of inflammatory cytokines, chemokines, immunoreceptors, and cell adhesion molecules, NF-kappaB has often been termed a 'central mediator of the human immune response'. This article contains a complete listing of all NF-kappaB inducers and target genes described to date. The collected data argue that NF-kappaB functions more generally as a central regulator of stress responses. In addition, NF-kappaB activation blocks apoptosis in several cell types. Coupling stress responsiveness and anti-apoptotic pathways through the use of a common transcription factor may result in increased cell survival following stress insults.

Infection of endothelial cells with Trypanosoma cruzi activates NF-kappaB and induces vascular adhesion molecule expression

Transcriptional activation of vascular adhesion molecule expression, a major component of an inflammatory response, is regulated, in part, by the nuclear factor-kappaB/Rel (NF-kappaB) family of transcription factors. We therefore determined whether Trypanosoma cruzi infection of endothelial cells resulted in the activation of NF-kappaB and the induction or increased expression of adhesion molecules. Human umbilical vein endothelial cells (HUVEC) were infected with trypomastigotes of the Tulahuen strain of T. cruzi. Electrophoretic mobility shift assays with an NF-kappaB-specific oligonucleotide and nuclear extracts from T. cruzi-infected HUVEC (6 to 48 h postinfection) detected two major shifted complexes. Pretreatment with 50x cold NF-kappaB consensus sequence abolished both gel-shifted complexes while excess SP-1 consensus sequence had no effect. These data indicate that nuclear extracts from T. cruzi-infected HUVEC specifically bound to the NF-kappaB consensus DNA sequence. Supershift analysis revealed that the gel-shifted complexes were comprised of p65 (RelA) and p50 (NF-kappaB1). Northern blot analyses demonstrated both the induction of vascular cell adhesion molecule 1 and E-selectin and the upregulation of intercellular adhesion molecule 1 mRNA in HUVEC infected with T. cruzi. Immunocytochemical staining confirmed adhesion molecule expression in response to T. cruzi infection. These findings are consistent with the hypothesis that the activation of the NF-kappaB pathway in endothelial cells associated with T. cruzi infection may be an important factor in the inflammatory response and subsequent vascular injury and endothelial dysfunction that lead to chronic cardiomyopathy.

Listeria monocytogenes infection of Caco-2 human epithelial cells induces activation of transcription factor NF-kappa B/Rel-like DNA binding activities

The effect of Listeria monocytogenes infection on the cellular level of transcription factor NF-kappa B in the human epithelia-like cell line Caco-2 was investigated. Infection with L. monocytogenes or treatment with lipoteichoic acid induced the formation of three NF-kappa B-like DNA-protein complexes C1, C2, and C3, which were identified as containing either, RelA and p50, RelB and p50, or p50, respectively. NF-kappa B activation in L. monocytogenes-infected Caco-2 cells was distinct from NF-kappa B activation in infected P388D1 macrophages concerning the NF-kappa B complexes formed and the kinetics of the induction.

Innate mechanisms of epithelial host defense: spotlight on intestine

The single layer of epithelial cells lining the intestinal tract is charged with a most difficult task: protecting the underlying biological compartments from both the normal commensal flora that reside within the intestinal lumen as well as the uninvited pathogens. To such an end, the intestinal epithelial cells are equipped with a panoply of defense mechanisms, both constitutive and inducible. This review focuses only on those defense mechanisms that are initiated and executed by the intestinal epithelial cell. Fitting these strict criteria are three major categories of epithelial host defense: enhanced salt and water secretion, expression of antimicrobial proteins and peptides, and production of intestinal mucins. Each of these areas is discussed in this review.

NF-κB Is a Central Regulator of the Intestinal Epithelial Cell Innate Immune Response Induced by Infection with Enteroinvasive Bacteria

Human intestinal epithelial cells up-regulate the expression of an inflammatory gene program in response to infection with a spectrum of different strains of enteroinvasive bacteria. The conserved nature of this program suggested that diverse signals, which are activated by enteroinvasive bacteria, can be integrated into a common signaling pathway that activates a set of proinflammatory genes in infected host cells. Human intestinal epithelial cell lines, HT-29, Caco-2, and T84, were infected with invasive bacteria that use different strategies to induce their uptake and have different intracellular localizations (i.e., Salmonella dublin, enteroinvasive Escherichia coli, or Yersinia enterocolitica). Infection with each of these bacteria resulted in the activation of TNF receptor associated factors, two recently described serine kinases, IκB kinase (IKK) α and IKKβ, and increased NF-κB DNA binding activity. This was paralleled by partial degradation of IκBα and IκBε in bacteria-infected Caco-2 cells. Mutant proteins that act as superrepressors of IKKβ and IκBα inhibited the up-regulated transcription and expression of downstream targets genes of NF-κB that are key components of the epithelial inflammatory gene program (i.e., IL-8, growth-related oncogene-α, monocyte chemoattractant protein-1, TNF-α, cyclooxygenase-2, nitric oxide synthase-2, ICAM-1) activated by those enteroinvasive bacteria. These studies position NF-κB as a central regulator of the epithelial cell innate immune response to infection with enteroinvasive bacteria.

Pathogenic Escherichia coli increase Cl- secretion from intestinal epithelia by upregulating galanin-1 receptor expression

Galanin is widely distributed in enteric nerve terminals lining the human gastrointestinal (GI) tract. We have shown previously that galanin-1 receptors (Gal1-R) are expressed by epithelial cells lining the human GI tract, and upon activation cause Cl- secretion. Because expression of this receptor is transcriptionally regulated by nuclear factor-kappa B (NF-kappa B), which is activated by enteric pathogens as a part of the host epithelial response to infection, we investigated whether such bacterial pathogens could directly increase Gal1-R expression in the T84-cell model system. Pathogenic Escherichia coli, but not nonpathogenic E. coli, activate a p50/p65 NF-kappa B complex that binds to oligonucleotides corresponding to a recognition site located within the 5' flanking region of the human GAL1R gene. Pathogenic E. coli, but not normal commensal organisms, increase Gal1-R mRNA synthesis and [(125)I]galanin binding sites. Whereas galanin increases short-circuit current (Isc) approximately 5-fold in uninfected T84 cells, exposure to pathogenic, but not nonpathogenic, E. coli results in galanin increasing Isc approximately 20-fold. To confirm the validity of these in vitro observations, we also studied C57BL/6J mice infected with enterohemorrhagic E. coli (EHEC) by gavage. Infection caused a progressive increase in both NF-kappa B activation and Gal1-R expression, with maximal levels of both observed 3 days after gavage. Ussing chamber studies revealed that colons infected with EHEC, but not those exposed to normal colonic flora, markedly increased Isc in response to galanin. These data indicate that pathogen-induced increases in Gal1-R expression by epithelial cells lining the colon may represent a novel unifying pathway responsible for at least a portion of the excessive fluid secretion observed during infectious diarrhea.

Interference of eukaryotic signalling pathways by the bacteria Yersinia outer protein YopJ

Upon contact with bacteria, eukaryotic cells activates a slurry of defence mechanisms via distinct signalling transduction pathways. However, some bacteria have evolved strategies to escape or inhibit these host defence systems. We have recently shown that the bacteria Yersinia pseudotuberculosis, which encodes the Yersinia outer protein (YopJ) appears to inhibit the activation of NF-kappaB by preventing the phosphorylation of IkappaB. In a subsequent series of experiments it has also been shown that YopJ blocks the phosphorylation of the p38 MAP kinase. Here the regulatory function of YopJ on eukaryotic signal transduction is discussed.

Listeriolysin O-dependent activation of endothelial cells during infection with Listeria monocytogenes: activation of NF-kappa B and upregulation of adhesion molecules and chemokines

The facultative intracellular bacterium Listeria monocytogenes is an invasive pathogen that crosses the vascular endothelium and disseminates to the placenta and the central nervous system. Its interaction with endothelial cells is crucial for the pathogenesis of listeriosis. By infecting in vitro human umbilical vein endothelial cells (HUVEC) with L. monocytogenes, we found that wild-type bacteria induced the expression of the adhesion molecules (ICAM-1 and E-selectin), chemokine secretion (IL-8 and monocyte chemotactic protein-1) and NF-kappa B nuclear translocation. The activation of HUVEC required viable bacteria and was abolished in prfA-deficient mutants of L. monocytogenes, suggesting that virulence genes are associated with endothelial cell activation. Using a genetic approach with mutants of virulence genes, we found that listeriolysin O (LLO)-deficient mutants inactivated in the hly gene did not induce HUVEC activation, as opposed to mutants inactivated in the other virulence genes. Adhesion molecule expression, chemokine secretion and NF-kappa B activation were fully restored by a strain of Listeria innocua transformed with the hly gene encoding LLO. The relevance in vivo of endothelial cell activation for listerial pathogenesis was investigated in transgenic mice carrying an NF-kappa B-responsive lacZ reporter gene. NF-kappa B activation was visualized by a strong lacZ expression in endothelial cells of capillaries of mice infected with a virulent haemolytic strain, but was not seen in those infected with a non-haemolytic isogenic mutant. Direct evidence that LLO is involved in NF-kappa B activation in transgenic mice was provided by injecting intravenously purified LLO, thus inducing stimulation of NF-kappa B in endothelial cells of blood capillaries. Our results demonstrate that functional listeriolysin O secreted by bacteria contributes as a potent inflammatory stimulus to inducing endothelial cell activation during the infectious process.

Bacterial invasion is not required for activation of NF-kappaB in enterocytes

Pathogenic enteric microorganisms induce the NF-kappaB-dependent expression of proinflammatory genes in intestinal epithelial cells. The purpose of the present study was to clarify the contribution of microbial invasion to the degradation of the regulatory protein Ikappa Balpha and the subsequent activation of NF-kappaB in cultured intestinal epithelial cells. Caco-2BBe cells were incubated with Salmonella dublin, Salmonella typhimurium, or a weakly invasive strain of E. coli. S. dublin and S. typhimurium (10(7) organisms/ml) induced equivalent concentration-dependent gel mobility shifts of an NF-kappaB consensus sequence that was preceded by Ikappa Balpha degradation. E. coli (10(7) organisms/ml) did not induce Ikappa Balpha degradation or NF-kappaB translocation. Pretreatment with cytochalasin D blocked invasion of all three strains but had no effect on Ikappa Balpha degradation or NF-kappaB activation. S. dublin and S. typhimurium adhered to Caco-2BBe cells 3- to 10-fold more than E. coli. NF-kappaB activation was prevented by physical separation of S. dublin from Caco-2BBe cells by a 0. 4-micrometers-pore-size filter. Our results imply that bacterial adhesion, rather than invasion or release of a secreted factor, is sufficient to induce IkappaBalpha degradation and NF-kappaB activation in intestinal epithelial cells. Our data suggest that strategies to reduce enteric inflammation should be directed to the reduction of bacterial enterocyte adhesion.

Mycobacterium avium complex activates nuclear factor kappaB via induction of inflammatory cytokines

A variety of microorganisms has been reported to directly induce NF-kappaB, a critical step in the regulation of genes involved in the cellular immune response. In this study, we demonstrate that proinflammatory cytokines such as tumor necrosis factor alpha (TNFalpha) produced upon activation by the Mycobacterium avium complex (MAC) preceed NF-kappaB activity in U937, a human monocytoid cell line. MAC induction of TNFalpha mRNA expression was detected within 15 min after MAC infection, whereas enhanced NF-kappaB binding activity was not detected until 90 to 120 min postinfection. Supershift analysis revealed increased p50 in the MAC-induced NF-kappaB binding complexes. Consistent with an autocrine mechanism, anti-TNFalpha antibody and dexamethasone, a known cytokine inhibitor, both completely suppressed the effect of MAC on the induction of NF-kappaB. Taken together, these findings suggest that exposure of monocyte cell membranes to MAC induces endogenous TNFalpha, which in turn enhances NF-kappaB binding activity. The rapid induction of TNFalpha may be important in the initial host response to MAC infection.

Galanin causes Cl- secretion in the human colon. Potential significance of inflammation-associated NF-kappa B activation on galanin-1 receptor expression and function

Galanin is widely distributed in enteric nerves and nerve terminals throughout the gastrointestinal (GI) tract. Within the GI tract galanin is best known for its ability to alter smooth muscle contractility and regulate intestinal motility. However, recent studies also indicate that galanin can modulate epithelial ion transport. We previously showed that epithelial cells lining the human GI tract, including those of colonic origin, express Gal1 galanin receptors (Gal1-R). We herein demonstrate that epithelial cells lining the human colon only express Gal1-R receptors and do not express other galanin receptor subtypes. We previously showed that Gal1-R expression was transcriptionally regulated by the transcription factor NF-kappa B. Consistent with this transcription factor being activated in a number of inflammatory conditions, we show increased colonic Gal1-R expression in patients with colitis due to a variety of causes. To further evaluate the physiology of Gal1-R activation, we studied this receptor expressed by the human colon epithelial cell line T84. Gal1-R activation resulted in a dose-dependent increase in Cl- secretion; whereas infection of T84 cells with pathogens known to activate NF-kappa B augmented Gal1-R expression and Cl- secretion. Thus, galanin acts as a secretagogue in epithelial cells lining the human colon, with alterations in Gal1-R expression possibly playing an important role in the diarrhea associated with various inflammatory processes affecting the GI tract.

Epithelial cell-initiated inflammation plays a crucial role in early tissue damage in amebic infection of human intestine

BACKGROUND & AIMS

Entamoeba histolytica infection of the intestine can induce severe gut inflammation. The aims of this study were to assess the role of the host inflammatory response in the tissue damage observed with amebiasis and the role of the intestinal epithelial cell in initiating that response.

METHODS

E. histolytica infection was established in human intestinal xenografts in severe combined immunodeficient (SCID-HU-INT) mice. Human intestinal epithelial cell inflammatory responses to amebic infection were inhibited by the intraluminal administration of an antisense oligonucleotide to the human p65 subunit of nuclear factor kappaB, and the role of neutrophils in tissue damage observed with amebiasis was studied by depleting neutrophils from SCID-HU-INT mice.

RESULTS

Administration of the antisense oligonucleotide blocked the production of human interleukin 1beta and interleukin 8 by intestinal epithelial cells and inhibited neutrophil influx into the E. histolytica-infected intestinal xenografts. Inhibition of the gut inflammatory response by the antisense oligonucleotide or the depletion of neutrophils from SCID-HU-INT mice blocked the increase in intestinal permeability observed with amebic infection.

CONCLUSIONS

Intestinal epithelial cells initiate an inflammatory response with resulting neutrophil-mediated tissue damage in response to E. histolytica infection; this inflammatory cascade can be blocked by inhibiting the transcription of genes regulated by nuclear factor kappaB.

Apoptosis of human intestinal epithelial cells after bacterial invasion

Epithelial cells that line the human intestinal mucosa are the initial site of host invasion by bacterial pathogens. The studies herein define apoptosis as a new category of intestinal epithelial cell response to bacterial infection. Human colon epithelial cells are shown to undergo apoptosis following infection with invasive enteric pathogens, such as Salmonella or enteroinvasive Escherichia coli. In contrast to the rapid onset of apoptosis seen after bacterial infection of mouse monocyte-macrophage cell lines, the commitment of human intestinal epithelial cell lines to undergo apoptosis is delayed for at least 6 h after bacterial infection, requires bacterial entry and replication, and the ensuing phenotypic expression of apoptosis is delayed for 12-18 h after bacterial entry. TNF-alpha and nitric oxide, which are produced as components of the intestinal epithelial cell proinflammatory program in the early period after bacterial invasion, play an important role in the later induction and regulation of the epithelial cell apoptotic program. Apoptosis in response to bacterial infection may function to delete infected and damaged epithelial cells and restore epithelial cell growth regulation and epithelial integrity that are altered during the course of enteric infection. The delay in onset of epithelial cell apoptosis after bacterial infection may be important both to the host and the invading pathogen since it provides sufficient time for epithelial cells to generate signals important for the activation of mucosal inflammation and concurrently allows invading bacteria time to adapt to the intracellular environment before invading deeper mucosal layers.

Mycobacterium avium-intracellulare complex Activates Nuclear Transcription Factor-κB in Different Cell Types Through Reactive Oxygen Intermediates

Mycobacterium avium-intracellulare complex (MAC) is one of the most common opportunistic pathogens in HIV-infected patients. Their synergistic interaction leads to a rapid deterioration of the host defense. In vivo, MAC manifests as a disseminated granulomatous disease that produces a massive inflammatory tissue response perhaps through its activation of inflammatory cytokines. The intracellular signaling following interaction of the mycobacterium with host cells is incompletely understood. Because the response is dependent, in part, on the activation of NF-κB, we investigated the effect of MAC on this nuclear transcription factor in cells of macrophage and nonmacrophage lineage. We demonstrate that both high and low virulence strains of MAC potently and rapidly activated NF-κB. In supershift assays, using specific Abs against the NF-κB subunits, we identified a p50/p65 heterodimer that was formed within 5 min after incubation with the bacterium too rapidly for cytokines to be involved in the activation. This activation was instead mediated through the generation of reactive oxygen intermediates, inasmuch as preincubation of cells with a variety of antioxidants inhibited NF-κB activation. Likewise, the transfection of cells with Mn-superoxide dismutase blocked the NF-κB activation induced by the bacterium. These data suggest that NF-κB activation is a consequence of interaction of host cells with the bacterium and that the interaction may play a pivotal role in the pathogenesis of the disease.

Alteration of HLA-B27 peptide presentation after infection of transfected murine L cells by Shigella flexneri

Shigella flexneri is a triggering agent for reactive arthritis in HLA-B27-susceptible individuals. Considering the intracellular multiplication of bacteria, it seems likely that bacterial peptides may be presented by the major histocompatibility complex (MHC) class I pathway. To examine this hypothesis, we infected HLA-B*2705- and/or human beta2-microglobulin-transfected murine L-cell lines with M90T, an invasive strain of S. flexneri. Bacterial infection induced no detectable modifications in the biosynthesis and expression level of HLA-B27, as assessed by immunoprecipitation, Northern blot analysis, and flow cytometry. Using confocal microscopy, we observed that bacterial infection induced a clustering of HLA-B27 molecules during macropinocytosis and before bacterial dissemination from cell to cell. Peptides naturally bound to HLA-B27 molecules were acid eluted from infected cells and separated by high-performance liquid chromatography. Major differences were observed in high-performance liquid chromatography profiles and in the nature of peptides presented following bacterial infection. Although most of the antigens presented were not accessed by Edman degradation, we obtained two sequences partially homologous to bacterial proteins. These peptides lacked the major HLA-B27 peptide anchor (Arg) at position 2, and one had an unusual length of 14 amino acids. These data suggest that alterations in the peptide presentation by HLA-B27 occur during infection, which could be relevant to the pathogenesis of HLA-B27-related arthritis.

Shiga Toxin Type 1 Activates Tumor Necrosis Factor-α Gene Transcription and Nuclear Translocation of the Transcriptional Activators Nuclear Factor-κB and Activator Protein-1

Shiga toxins (Stxs) produced by Shigella dysenteriae 1 andEscherichia coli have been implicated in the pathogenesis of bloody diarrhea, acute renal failure, and neurologic abnormalities. The pathologic hallmark of Stx-mediated tissue damage is the development of vascular lesions in which endothelial cells are swollen and detached from underlying basement membranes. However, in vitro studies using human vascular endothelial cells demonstrated minimal Stx-induced cytopathic effects, unless the target cells were also incubated with the proinflammatory cytokines tumor necrosis factor-α (TNF-α) or interleukin-1β (IL-1β). These cytokines have been shown to upregulate the expression of the Stx-binding membrane glycolipid globotriaosylceramide (Gb3). We show here that purified Stx1 induces TNF secretion by a human monocytic cell line, THP-1, in a dose- and time-dependent manner. Treatment of cells with both lipopolysaccharides (LPS) and Stx1 results in augmented TNF production. Treatment with the nontoxic Gb3-binding subunit of Stx1 or with an anti-Gb3 monoclonal antibody did not trigger TNF production. Northern blot analyses show that Stx1 causes increased TNF-α production through transcriptional activation. Increased levels of TNF-α mRNA are preceded by the nuclear translocation of the transcriptional activators NF-κB and AP-1 and the loss of cytoplasmic IκB-α. These data are the first to show that, in addition to direct cytotoxicity, Stxs possess cellular signaling capabilities sufficient to induce the synthesis of cytokines that may be necessary for target cell sensitization and the development of vascular lesions.

Shiga Toxin Type 1 Activates Tumor Necrosis Factor-α Gene Transcription and Nuclear Translocation of the Transcriptional Activators Nuclear Factor-κB and Activator Protein-1

Shiga toxins (Stxs) produced by Shigella dysenteriae 1 andEscherichia coli have been implicated in the pathogenesis of bloody diarrhea, acute renal failure, and neurologic abnormalities. The pathologic hallmark of Stx-mediated tissue damage is the development of vascular lesions in which endothelial cells are swollen and detached from underlying basement membranes. However, in vitro studies using human vascular endothelial cells demonstrated minimal Stx-induced cytopathic effects, unless the target cells were also incubated with the proinflammatory cytokines tumor necrosis factor-α (TNF-α) or interleukin-1β (IL-1β). These cytokines have been shown to upregulate the expression of the Stx-binding membrane glycolipid globotriaosylceramide (Gb3). We show here that purified Stx1 induces TNF secretion by a human monocytic cell line, THP-1, in a dose- and time-dependent manner. Treatment of cells with both lipopolysaccharides (LPS) and Stx1 results in augmented TNF production. Treatment with the nontoxic Gb3-binding subunit of Stx1 or with an anti-Gb3 monoclonal antibody did not trigger TNF production. Northern blot analyses show that Stx1 causes increased TNF-α production through transcriptional activation. Increased levels of TNF-α mRNA are preceded by the nuclear translocation of the transcriptional activators NF-κB and AP-1 and the loss of cytoplasmic IκB-α. These data are the first to show that, in addition to direct cytotoxicity, Stxs possess cellular signaling capabilities sufficient to induce the synthesis of cytokines that may be necessary for target cell sensitization and the development of vascular lesions.

The yopJ locus is required for Yersinia-mediated inhibition of NF-kappaB activation and cytokine expression: YopJ contains a eukaryotic SH2-like domain that is essential for its repressive activity

Upon exposure to bacteria, eukaryotic cells activate signalling pathways that result in the increased expression of several defence-related genes. Here, we report that the yopJ locus of the enteropathogen Yersinia pseudotuberculosis encodes a protein that inhibits the activation of NF-kappaB transcription factors by a mechanism(s), which prevents the phosphorylation and subsequent degradation of the inhibitor protein IkappaB. Consequently, eukaryotic cells infected with YopJ-expressing Yersinia become impaired in NF-kappaB-dependent cytokine expression. In addition, the blockage of inducible cytokine production coincides with yopJ-dependent induction of apoptosis. Interestingly, the YopJ protein contains a region that resembles a src homology domain 2 (SH2), and we show that a wild-type version of this motif is required for YopJ activity in suppressing cytokine expression and inducing apoptosis. As SH2 domains are found in several eukaryotic signalling proteins, we propose that YopJ, which we show is delivered into the cytoplasm of infected cells, interacts directly with signalling proteins involved in inductive cytokine expression. The repressive activity of YopJ on the expression of inflammatory mediators may account for the lack of an inflammatory host response observed in experimental yersiniosis. YopJ-like activity may also be a common feature of commensal bacteria that, like Yersinia, do not provoke a host inflammatory response.

NF-kappa B-dependent inhibition of apoptosis is essential for host cellsurvival during Rickettsia rickettsii infection

The possibility that bacteria may have evolved strategies to overcome host cell apoptosis was explored by using Rickettsia rickettsii, an obligate intracellular Gram-negative bacteria that is the etiologic agent of Rocky Mountain spotted fever. The vascular endothelial cell, the primary target cell during in vivo infection, exhibits no evidence of apoptosis during natural infection and is maintained for a sufficient time to allow replication and cell-to-cell spread prior to eventual death due to necrotic damage. Prior work in our laboratory demonstrated that R. rickettsii infection activates the transcription factor NF-kappa B and alters expression of several genes under its control. However, when R. rickettsii-induced activation of NF-kappa B was inhibited, apoptosis of infected but not uninfected endothelial cells rapidly ensued. In addition, human embryonic fibroblasts stably transfected with a superrepressor mutant inhibitory subunit Ikappa B that rendered NF-kappa B inactivatable also underwent apoptosis when infected, whereas infected wild-type human embryonic fibroblasts survived. R. rickettsii, therefore, appeared to inhibit host cell apoptosis via a mechanism dependent on NF-kappa B activation. Apoptotic nuclear changes correlated with presence of intracellular organisms and thus this previously unrecognized proapoptotic signal, masked by concomitant NF-kappa B activation, likely required intracellular infection. Our studies demonstrate that a bacterial organism can exert an antiapoptotic effect, thus modulating the host cell's apoptotic response to its own advantage by potentially allowing the host cell to remain as a site of infection.

Activation of IL-8 Gene Expression by Helicobacter pylori Is Regulated by Transcription Factor Nuclear Factor-κB in Gastric Epithelial Cells

In vivo, gastric infection with Helicobacter pylori leads to substantial production of the inflammatory cytokines IL-1, IL-6, TNF-α, and IL-8. H. pylori strains that contain the cag pathogenicity island (cag+) and are associated with ulceration and gastric carcinoma induce greater cytokine production than cag− strains. Expression of these cytokines is often regulated by the transcription factor complex, nuclear factor-κB (NF-κB) through κB-binding elements in the enhancer/promoter regions of their genes. We report that more virulent cag+ H. pylori strains induce increased NF-κB-DNA binding activity, which elevates IL-8 expression in AGS gastric epithelial cells. The cag+ H. pylori strains induce significant stimulation of IL-8 promoter-driven reporter activity, while cag− strains do not. Furthermore, mutation of specific genes within the cag island (picA1 and picB) ablates enhanced NF-κB activation and IL-8 transcription. Increased IL-8 expression is inhibited by mutation in either the NF-κB or NF-IL-6 binding element. The cag+ strains, compared with the cag− strains, induce enhanced nuclear localization of a RelA-containing NF-κB binding complex, but no increase in NF-IL-6 binding activity. These studies demonstrate that the ability of different types of H. pylori strains to activate NF-κB correlates with their ability to induce IL-8 transcription and indicate a mechanism for the heightened inflammatory response seen in subjects infected with cag+ H. pylori strains.

Bidirectional signaling between Yersinia and its target cell

Preventing the early host immune defense allows pathogenic Yersinia to proliferate in lymphatic tissue. This ability depends on signaling that occurs between the bacteria and the host cells. Following intimate contact with the target cell a signal is generated within the bacterium that results in increased expression of virulence-associated proteins that are subsequently delivered into the infected cell. These proteins, designated Yops, interfere with the host-cell signaling pathways that are normally activated to eliminate infectious agents.

Bacterial induction of inducible nitric oxide synthase in cultured human intestinal epithelial cells

BACKGROUND & AIMS

Enterocytes play a major role in the mucosa as a source of proinflammatory cytokines and cytotoxins. We tested the hypothesis that bacteria induce expression of the inducible nitric oxide synthase (iNOS) in cultured human enterocytes.

METHODS

DLD-1 and Caco-2BBe cell monolayers exposed to Salmonella dublin were analyzed for iNOS up-regulation and nitric oxide production (NOx) in the presence of various proinflammatory cytokines.

RESULTS

S. dublin augmented NOx in interferon gamma (IFN-gamma)-primed cells but had no independent effect on iNOS expression. S. dublin-induced NOx was not mediated by endotoxin and was augmented by an enteroinvasive phenotype. In DLD-1 cells, S. dublin-mediated NOx was blocked by inhibitors of nuclear factor kappa B (NF-kappa B) and tyrosine kinase activation and was steroid resistant. Cis-acting elements in the human iNOS promoter responsive to endotoxin and S. dublin stimulation of IFN-gamma-treated DLD-1 cells were identified between 10.9 and 8.7 kilobases upstream of the transcription initiation site.

CONCLUSIONS

S. dublin alters the regulation of iNOS messenger RNA in IFN-gamma-treated intestinal epithelial cells via a steroid-resistant pathway involving NF-kappa B and tyrosine kinase activity. Because bacterial interaction with cytokine-primed epithelial cells induces the synthesis of NO, an endogenous antimicrobial agent, these findings may have implications for the regulation of mucosal immunity.

Uptake of pathogenic intracellular bacteria into human and murine macrophages downregulates the eukaryotic 26S protease complex ATPase gene

A differential PCR technique detected the transcriptional downregulation of the mss1 (mammalian suppressor of svg1) gene in murine J774A.1 macrophages following uptake of Salmonella typhimurium. This downregulation was also noted after entry of virulent strains of Listeria monocytogenes and Shigella flexneri, two other facultative intracellular bacterial species. In contrast, uptake of nonpathogenic Escherichia coli HB101, an aroA mutant of S. typhimurium, an invasion plasmid antigen B (ipaB) mutant of S. flexneri, hemolysin (hly) and positive-regulatory factor (prfA) mutants of L. monocytogenes, or latex beads produced mss1 expression levels similar to that of uninfected macrophages. Transcriptional downregulation of mss1 was also shown to occur in S. typhimurium-infected human U937 cells, albeit to an extent less than that in murine J774A.1 cells. In addition to a lower abundance of mss1 transcripts, we also demonstrate for the first time that less MSS1 protein was detected in intracellular-bacterium-infected cells (beginning about 1 h after entry of the pathogenic intracellular bacteria) than in noninfected cells. Some strains with specific mutations in characterized genes, such as an ipaB mutant strain of S. flexneri and an hly mutant strain of L. monocytogenes, did not elicit this lower level of expression of MSS1 protein. The decrease in MSS1 within infected macrophages resulted in an accumulation of ubiquitinated proteins, substrates for MSS1. Since MSS1 comprises the ATPase part of the 26S protease that degrades ubiquitinated proteins, we hypothesize that downregulation of the mss1 gene by intracellular bacterial entry may help subvert the host cell's normal defensive response to internalized bacteria, allowing the intracellular bacteria to survive.

Listeria monocytogenes infection of P388D1 macrophages results in a biphasic NF-kappaB (RelA/p50) activation induced by lipoteichoic acid and bacterial phospholipases and mediated by IkappaBalpha and IkappaBbeta degradation

As previously reported, Listeria monocytogenes infection of P388D1 macrophages results in a rapid induction of NF-kappaB DNA-binding activity. Here we show that this induction of NF-kappaB activity occurs in a biphasic mode: first, a transient, IkappaBalpha degradation-dependent phase of activity, also induced by the nonvirulent species Listeria innocua, which is mediated by binding of the bacteria to the macrophage, or by adding Listeria-derived lipoteichoic acid to the macrophage; the second persistent phase of activation is only markedly induced when the bacteria enter the cytoplasm of the host cell and express the virulence genes plcA and plcB, encoding two phospholipases. We suggest that products of the enzymatic activity of phospholipases directly interfere with host cell signal transduction pathways, thus leading to persistent NF-kappaB activation via persistent IkappaBbeta degradation.

Responses by murine macrophages infected with Listeria monocytogenes crucial for the development of immunity to this pathogen

Macrophages and other mammalian cells respond to infection with Listeria monocytogenes (L. monocytogenes) by the transient or persistent activation of host cell signal transduction pathways. In addition, L. monocytogenes infection influences expression of various host cell genes, such as stress genes, genes from the MHC I and II complex, cytokine genes, and cytokine receptor genes. The possible influences of the different host cell responses on the outcome of an L. monocytogenes infection in vitro as well as for the development of immunity are discussed.

Neisseria gonorrhoeae epithelial cell interaction leads to the activation of the transcription factors nuclear factor kappaB and activator protein 1 and the induction of inflammatory cytokines

We have studied the effect of human bacterial pathogen Neisseria gonorrhoeae (Ngo) on the activation of nuclear factor (NF)-kappaB and the transcriptional activation of inflammatory cytokine genes upon infection of epithelial cells. During the course of infection, Ngo, the etiologic agent of gonorrhea, adheres to and penetrates mucosal epithelial cells. In vivo, localized gonococcal infections are often associated with a massive inflammatory response. We observed upregulation of several inflammatory cytokine messenger RNAs (mRNAs) and the release of the proteins in Ngo-infected epithelial cells. Moreover, infection with Ngo induced the formation of a NF-kappaB DNA-protein complex and, with a delay in time, the activation of activator protein 1, whereas basic leucine zipper transcription factors binding to the cAMP-responsive element or CAAT/enhancer-binding protein DNA-binding sites were not activated. In supershift assays using NF-kappaB-specific antibodies, we identified a NF-kappaB p50/p65 heterodimer. The NF-kappaB complex was formed within 10 min after infection and decreased 90 min after infection. Synthesis of tumor necrosis factor alpha and interluekin (IL)-1beta occurred at later times and therefore did not account for NF-kappaB activation. An analysis of transiently transfected IL-6 promoter deletion constructs suggests that NF-kappaB plays a crucial role for the transcriptional activation of the IL-6 promoter upon Ngo infection. Inactivation of NF-kappaB conferred by the protease inhibitor N-tosyl--phenylalanine chloromethyl ketone inhibited mRNA upregulation of most, but not all, studied cyctokine genes. Activation of NF-kappaB and cytokine mRNA upregulation also occur in Ngo-infected epithelial cells that were treated with cytochalasin D, indicating an extracellular signaling induced before invasion.

Role of intestinal epithelial cells in the host secretory response to infection by invasive bacteria. Bacterial entry induces epithelial prostaglandin h synthase-2 expression and prostaglandin E2 and F2alpha production

Increased intestinal fluid secretion is a protective host response after enteric infection with invasive bacteria that is initiated within hours after infection, and is mediated by prostaglandin H synthase (PGHS) products in animal models of infection. Intestinal epithelial cells are the first host cells to become infected with invasive bacteria, which enter and pass through these cells to initiate mucosal, and ultimately systemic, infection. The present studies characterized the role of intestinal epithelial cells in the host secretory response after infection with invasive bacteria. Infection of cultured human intestinal epithelial cell lines with invasive bacteria, but not noninvasive bacteria, is shown to induce the expression of one of the rate-limiting enzymes for prostaglandin formation, PGHS-2, and the production of PGE2 and PGF2alpha. Furthermore, increased PGHS-2 expression was observed in intestinal epithelial cells in vivo after infection with invasive bacteria, using a human intestinal xenograft model in SCID mice. In support of the physiologic importance of epithelial PGHS-2 expression, supernatants from bacteria-infected intestinal epithelial cells were shown to increase chloride secretion in an in vitro model using polarized epithelial cells, and this activity was accounted for by PGE2. These studies define a novel autocrine/paracrine function of mediators produced by intestinal epithelial cells in the rapid induction of increased fluid secretion in response to intestinal infection with invasive bacteria.