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

Oral Epithelial Cell Culture Model for Studying the Pathogenesis of Chronic Inflammatory Disease

The interactions between bacteria, epithelium, and neutrophilic polymorphonuclear leukocytes (neutrophils) are the key to the initiation and progression of many chronic inflammatory-immune diseases. In addition, all can be influenced by external factors, such as micronutrients, thereby providing potentially novel approaches to therapy. This chapter will therefore provide detailed methods for core techniques involved in studying cellular and molecular epithelial responses to a bacterial challenge in relation to chronic inflammatory disease pathogenesis and therapy.

Oral Epithelial Cell Culture Model for Studying the Pathogenesis of Chronic Inflammatory Disease

The interactions between bacteria, epithelium, and neutrophilic polymorphonuclear leukocytes (neutrophils) are the key to the initiation and progression of many chronic inflammatory-immune diseases. In addition, all can be influenced by external factors, such as micronutrients, thereby providing potentially novel approaches to therapy. This chapter will therefore provide detailed methods for core techniques involved in studying cellular and molecular epithelial responses to a bacterial challenge in relation to chronic inflammatory disease pathogenesis and therapy.

Pathogenesis of human enterovirulent bacteria: lessons from cultured, fully differentiated human colon cancer cell lines

Hosts are protected from attack by potentially harmful enteric microorganisms, viruses, and parasites by the polarized fully differentiated epithelial cells that make up the epithelium, providing a physical and functional barrier. Enterovirulent bacteria interact with the epithelial polarized cells lining the intestinal barrier, and some invade the cells. A better understanding of the cross talk between enterovirulent bacteria and the polarized intestinal cells has resulted in the identification of essential enterovirulent bacterial structures and virulence gene products playing pivotal roles in pathogenesis. Cultured animal cell lines and cultured human nonintestinal, undifferentiated epithelial cells have been extensively used for understanding the mechanisms by which some human enterovirulent bacteria induce intestinal disorders. Human colon carcinoma cell lines which are able to express in culture the functional and structural characteristics of mature enterocytes and goblet cells have been established, mimicking structurally and functionally an intestinal epithelial barrier. Moreover, Caco-2-derived M-like cells have been established, mimicking the bacterial capture property of M cells of Peyer's patches. This review intends to analyze the cellular and molecular mechanisms of pathogenesis of human enterovirulent bacteria observed in infected cultured human colon carcinoma enterocyte-like HT-29 subpopulations, enterocyte-like Caco-2 and clone cells, the colonic T84 cell line, HT-29 mucus-secreting cell subpopulations, and Caco-2-derived M-like cells, including cell association, cell entry, intracellular lifestyle, structural lesions at the brush border, functional lesions in enterocytes and goblet cells, functional and structural lesions at the junctional domain, and host cellular defense responses.

Comparative secretome analysis of four isogenic Bacillus clausii probiotic strains

Background

The spore-bearing alkaliphilic Bacillus species constitute a large, heterogeneous group of microorganisms, important for their ability to produce enzymes, antibodies and metabolites of potential medical use. Some Bacillus species are currently being used for manufacturing probiotic products consisting of bacterial spores, exhibiting specific features (colonization, immune-stimulation and antimicrobial activity) that can account for their claimed probiotic properties. In the present work a comparative proteomic study was performed aimed at characterizing the secretome of four closely related isogenic O/C, SIN, N/R and T B. clausii strains, already marketed in a pharmaceutical mixture as probiotics.

Results

Proteomic analyses revealed a high degree of concordance among the four secretomes, although some proteins exhibited considerable variations in their expression level in the four strains. Among these, some proteins with documented activity in the interaction with host cells were identified, such as the glycolytic enzyme enolase, with a putative plasminogen-binding activity, GroEL, a molecular chaperone shown to be able to bind to mucin, and flagellin protein, a structural flagella protein and a putative immunomodulation agent.

Conclusion

This study shows, for the first time, differences in the secretome of the OC, SIN, NR and T B. clausii strains. These differences indicate that specific secretome features characterize each of the four strains despite their genotypic similarity. This could confer to the B. clausii strains specific probiotic functions associated with the differentially expressed proteins and indicate that they can cooperate as probiotics as the secretome components of each strain could contribute to the overall activity of a mixed probiotic preparation.

Interplay between the QseC and QseE bacterial adrenergic sensor kinases in Salmonella enterica serovar Typhimurium pathogenesis

The bacterial adrenergic sensor kinases QseC and QseE respond to epinephrine and/or norepinephrine to initiate a complex phosphorelay regulatory cascade that modulates virulence gene expression in several pathogens. We have previously shown that QseC activates virulence gene expression in Salmonella enterica serovar Typhimurium. Here we report the role of QseE in S. Typhimurium pathogenesis as well as the interplay between these two histidine sensor kinases in gene regulation. An S. Typhimurium qseE mutant is hampered in the invasion of epithelial cells and intramacrophage replication. The ΔqseC strain is highly attenuated for intramacrophage survival but has only a minor defect in invasion. However, the ΔqseEC strain has only a slight attenuation in invasion, mirroring the ΔqseC strain, and has an intermediary intramacrophage replication defect in comparison to the ΔqseE and ΔqseC strains. The expressions of the sipA and sopB genes, involved in the invasion of epithelial cells, are activated by epinephrine via QseE. The expression levels of these genes are still decreased in the ΔqseEC double mutant, albeit to a lesser extent, congruent with the invasion phenotype of this mutant. The expression level of the sifA gene, important for intramacrophage replication, is decreased in the qseE mutant and the ΔqseEC double mutant grown in vitro. However, as previously reported by us, the epinephrine-dependent activation of this gene occurs via QseC. In the systemic model of S. Typhimurium infection of BALB/c mice, the qseC and qseE mutants are highly attenuated, while the double mutant has an intermediary phenotype. Altogether, these data suggest that both adrenergic sensors play an important role in modulating several aspects of S. Typhimurium pathogenesis.

The antimicrobial peptide pheromone Plantaricin A increases antioxidant defenses of human keratinocytes and modulates the expression of filaggrin, involucrin, β-defensin 2 and tumor necrosis factor-α genes

Plantaricin A (PlnA) is a peptide with antimicrobial and pheromone activities. PlnA was synthesized chemically and used as a pure peptide or synthesized biologically using Lactobacillus plantarum DC400 co-cultured with Lactobacillus sanfranciscensis DPPMA174. Cell-free supernatant (CFS) was used as a crude PlnA preparation. As estimated using the 3-(4,5-dimethyl-2-yl)-2,5-diphenyltetrazolium bromide and the 2',7'-dichlorofluorescein diacetate assays, both PlnA preparations increased the antioxidant defenses of human NCTC 2544 keratinocytes. PlnA (10 μg/ml) had a higher activity than hyaluronic acid or 125 μg/ml α-tocopherol. Effects on the transcriptional regulation of filaggrin (FLG), involucrin (IVL), hyaluronan synthase (HAS2), human β-defensin-2 (HBD-2) and tumor necrosis factor-alpha (TNF-α) genes were assayed. Compared with the control, expression of the FLG gene in NCTC 2544 cells increased in cells treated with hyaluronic acid, 1 or 10 μg/ml PlnA. Compared with the control, the level of IVL gene expression increased in NCTC 2544 cells treated with 10 μg/ml PlnA. No significant difference was found between the level of the HAS2 gene expressed by control cells and cells treated with PlnA. Compared with chemically synthesized PlnA, the up-regulation of the HBD-2 gene by CFS was higher. Compared with the control, expression of TNF-α decreased in NCTC 2544 cells after treatment with 1 or 10 μg/ml of chemically synthesized PlnA. In contrast, the level of TNF-α was highest in the presence of 10 μg/ml CFS-PlnA. These findings suggest that the PlnA was positively sensed by human keratinocytes, promoting antioxidant defenses, barrier functions and antimicrobial activity of the skin.

Plantaricin A synthesized by Lactobacillus plantarum induces in vitro proliferation and migration of human keratinocytes and increases the expression of TGF-β1, FGF7, VEGF-A and IL-8 genes

This work showed the effect of pheromone plantaricin A (PlnA) on the proliferation and migration of the human keratinocytes NCTC 2544. PlnA was chemically synthesized and used as pure peptide or biologically synthesized during co-cultivation of Lactobacillus plantarum DC400 and Lactobacillus sanfranciscensis DPPMA174. The cell-free supernatant (CFS) was used as the crude preparation containing PlnA. The inductive effect of PlnA on the proliferation of NCTC 2544 cells was higher than that found for hyaluronic acid, a well known skin protective compound. As shown by scratch assay and image analyses, PlnA enhanced the migration of NCTC 2544 cells. Compared to the basal serum free medium (control), the highest inductive effect was found using 10μg/ml of chemically synthesized PlnA. Similar results (P>0.05) were found for CFS. In agreement, the percentage of the starting scratch area was decreased after treatment (24h) with PlnA. The expression of transforming growth factor-β1 (TGF-β1), keratinocyte growth factor 7 (FGF7), vascular endothelial growth factor (VEGF-A), and interleukin-8 (IL-8) genes was affected by PlnA. Compared to control, TGF-β1 gene was under expressed in the first 4h of treatments and up-regulated after 8-24h. On the contrary, FGF7 gene was strongly up-regulated in the first 4h of treatments. Compared to control, VEGF-A and IL-8 genes were always up-regulated during the 4-24h from scratching. Since capable of promoting the proliferation and migration of the human keratinocytes and of stimulating IL-8 cytokine, the use of PlnA for dermatological purposes should be considered.

Transcriptional profiling of cecal gene expression in probiotic- and Salmonella-challenged neonatal chicks

Probiotics are currently used to improve health and reduce enteric pathogens in poultry. However, the mechanisms by which they reduce or prevent disease are not known. Salmonella are intracellular pathogens that cause acute gastroenteritis in humans, and infections by nontyphoid species of Salmonella also can result in diarrhea, dehydration, and depression in poultry. Frequently, however, no clinical signs of infection are apparent in poultry flocks. In this study, day-of-hatch chicks were challenged with Salmonella enterica serovar Enteritidis (SE) and treated 1 h later with a poultry-derived, Lactobacillus-based probiotic culture (FloraMax-B11, Pacific Vet Group USA Inc., Fayetteville, AR). Cecae were collected 12 and 24 h posttreatment for Salmonella detection and RNA isolation for microarray analysis of gene expression. At both 12 and 24 h, SE was significantly reduced in chicks treated with the probiotic as compared with the birds challenged with only SE (P < 0.05). Microarray analysis revealed gene expression differences among all treatment groups. At 12 h, 170 genes were expressed at significantly different levels (P < 0.05), with a minimum difference in expression of 1.2-fold. At 24 h, the number of differentially regulated genes with a minimum 1.2-fold change was 201. Pathway analysis revealed that at both time points, genes associated with the nuclear factor kappa B complex, as well as genes involved in apoptosis, were significantly regulated. Based on this analysis, probiotic-induced differential regulation of the genes growth arrest-specific 2 (GAS2) and cysteine-rich, angiogenic inducer, 61 (CYR61) may result in increased apoptosis in the cecae of chicks. Because Salmonella is an intracellular pathogen, we suggest that increased apoptosis may be a mechanism by which the probiotic culture reduces Salmonella infection.

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.

Quorum sensing in sourdough Lactobacillus plantarum DC400: induction of plantaricin A (PlnA) under co-cultivation with other lactic acid bacteria and effect of PlnA on bacterial and Caco-2 cells

This work aimed at showing the effect of pheromone plantaricin A (PlnA) by Lactobacillus plantarum DC400 towards other sourdough lactic acid bacteria and the potential of PlnA to protect the function of the human intestinal barrier. Growth and survival of sourdough lactic acid bacteria were differently affected by co-cultivation with L. plantarum DC400. Compared to mono-cultures, Lactobacillus sanfranciscensis DPPMA174 and Pediococcus pentosaceus 2XA3 showed growth inhibition and decreased viability when co-cultured with L. plantarum DC400. L. sanfranciscensis DPPMA174 induced the highest synthesis of PlnA. Survival of strain DPPMA174 only slightly varied by comparing the addition of PlnA to the culture medium and the co-cultivation with L. plantarum DC400. Compared to mono-culture, the proteome of L. sanfranciscensis DPPMA174 grown in co-culture with L. plantarum DC400 showed the variation of expression of 58 proteins (47 over expressed and 11 repressed). Thirty-four of them were also over expressed or repressed during growth of DPPMA174 with PlnA. Fifty-one of the above 58 proteins were identified. They had a central role in stress response, amino acid, energy and nucleotide metabolisms, membrane transport, regulation of transcription, and cell redox homeostasis. PlnA markedly increased the viability of human Caco-2/TC7 cells and the transepithelial electrical resistance.

Molecular adaptation of sourdough Lactobacillus plantarum DC400 under co-cultivation with other lactobacilli

This work was aimed at investigating the molecular mechanisms of Quorum Sensing (QS) in Lactobacillus plantarum DC400 when co-cultured with other sourdough lactobacilli. The growth and survival of L. plantarum DC400 was not affected when co-cultivated with Lactobacillus sanfranciscensis DPPMA174 or Lactobacillus rossiae A7. Nevertheless, 2-DE analysis showed that the level of protein expression of L. plantarum DC400 increased under co-culture conditions. Although several proteins were commonly induced in both co-cultures, the highest induction was found in co-culture with L. rossiae A7. Overexpressed proteins, related to QS and stress response mechanisms, were identified: DnaK, GroEL, 30S ribosomal protein S1 and S6, ATP synthase subunit beta, adenosylmethionine synthetase (MetK), phosphopyruvate hydratase, phosphoglycerate kinase, elongation factor Tu, putative manganese-dependent inorganic pyrophosphatase, d-lactate dehydrogenase, triosephosphate isomerase, fructose-bisphosphate aldolase and nucleoside-diphosphate kinase. As shown by real-time PCR, expression of the luxS gene of L. plantarum DC400 was also affected during co-cultivation. According to overexpression of MetK and luxS during co-cultivation, synthesis of AI-2-like substances was also influenced by the type of microbial co-cultures. This study showed that expression of some genes/proteins, also QS-related, in L. plantarum was influenced by co-cultivation of other sourdough lactobacilli.

Activation of NF-kappaB-dependent gene expression by Salmonella flagellins FliC and FljB

Bacterial flagellin is recognized by Toll-like receptor (TLR5) and activates NF-kappaB which leads to the induction of proinflammatory gene expression. Salmonella expresses two flagellin proteins, FliC and FljB. We purified FliC and FljB and examined the ability of the Salmonella flagellins to activate the NF-kappaB transcription factor in human embryonic kidney cells. We found that FliC and FljB as purified proteins possessed a comparable specific activity for activation of NF-kappaB-dependent gene expression in HEK293 cells. We also determined the ability of UV-inactivated bacteria, both wild-type and fliC and fljB mutant strains, to activate NF-kappaB. Wild-type fliC(+)/fljB(+)Salmonella and the fliC(+)/fljB(-) mutant strain were robust activators, whereas the fliC(-)/fljB(+) and flhC(-) mutant strains were very poor activators. The NF-kappaB activation capacity of bacterial strains correlated with their flagellin expression level. Finally, Salmonella cell wall-associated polymeric flagellin displayed greatly reduced ability to activate NF-kappaB compared to purified monomeric flagellin.

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.

Induction of interleukin-8 production via nuclear factor-kappaB activation in human intestinal epithelial cells infected with Vibrio vulnificus

Vibrio vulnificus, a Gram-negative estuarine bacterium, is a causative agent of food-borne diseases, such as life-threatening septicaemia and wound infection disease. V. vulnificus penetrating into the epithelial barrier stimulates an inflammatory response in the adjacent mucosa. Therefore, interaction between V. vulnificus and epithelial cells is important for understanding of both the immunology of mucosal surfaces and V. vulnificus. In this study, we investigated the effect and action mechanism of V. vulnificus infection on production of interleukin (IL)-8, a proinflammatory cytokine, in human intestinal epithelial INT-407 cells. V. vulnificus infection significantly induced IL-8 production in a time- and multiplicity of infection (MOI)-dependent manner, as determined by human IL-8 enzyme-linked immunosorbent assay (ELISA). In addition, V. vulnificus infection significantly increased IL-8 mRNA levels in INT-407 cells, indicating that the increased IL-8 production by V. vulnificus occurred at the transcriptional level. V. vulnificus infection also enhanced IL-8 gene promoter activity in INT-407 cells transiently transfected with IL-8 promoter constructs, but this effect was impaired in INT-407 cells transfected with IL-8 promoter constructs deleted or mutated of a kappaB site. V. vulnificus infection increased the nuclear factor-kappaB (NF-kappaB) binding activity to a kappaB site and the degradation of IkappaB-alpha protein in a time- and a MOI-dependent manner. Furthermore, BAY11-7082, an inhibitor of NF-kappaB activation, significantly reduced the IL-8 production, NF-kappaB binding activity and IkappaB-alpha degradation induced by V. vulnificus infection. Taken together, these results indicate clearly that V. vulnificus infection significantly induces IL-8 production in human intestinal epithelial cells via NF-kappaB activation.

Interferon gamma induces translocation of commensal Escherichia coli across gut epithelial cells via a lipid raft-mediated process

BACKGROUND & AIMS

The "leaky gut" hypothesis proposes that leakage of enteric bacteria into the body resulting from disruption of the epithelial barrier is a critical step in the pathophysiology of various disorders such as inflammatory bowel disease and sepsis. However, the pathways and underlying mechanisms by which commensal bacteria cross the epithelial barrier in inflammatory conditions remain unclear. This study investigated the mechanisms of interferon gamma-mediated bacterial translocation across human colonic epithelial monolayers.

METHODS

Caco-2 and T84 monolayers were exposed to interferon gamma. Barrier function was assessed by transepithelial electrical resistance and lucifer yellow permeability. Internalization and translocation of Escherichia coli strain C25 were measured by quantitative bacterial culture. Expression and distribution of junctional proteins were assessed by immunoblotting and confocal imaging.

RESULTS

Minimal apical to basolateral translocation of C25 was observed in untreated T84 and Caco-2 monolayers. Interferon gamma caused a dramatic, dose-dependent increase in C25 translocation, which was uncoupled from cytokine-induced increases in paracellular permeability and disruption of tight junction proteins at low interferon gamma concentrations. These effects were associated with increased internalization of viable bacteria into, but not adherence to, Caco-2 cells. Interferon gamma-mediated bacterial translocation was abolished by pretreatment with the cholesterol-disrupting drugs filipin and methyl-beta-cyclodextrin, whereas these agents had no effect on infection of Caco-2 by the enteric pathogen Shigella sonnei.

CONCLUSIONS

Normally poorly invasive enteric bacteria may, in situations of inflammatory stress, exploit lipid raft-mediated transcytotic pathways to cross the intestinal epithelium, and these effects may precede cytokine-induced disruption of tight junctions.

Factors contributing to infectious diarrhea-associated pancreatic enzyme alterations

OBJECTIVES

Several pathogens have been involved as etiologic agents of acute pancreatitis. We studied 59 patients presenting acute infectious diarrhea in order to determine the incidence as well as to identify factors which may contribute to the occurrence of pancreatic enzyme alteration or true acute pancreatitis.

METHODS

Patients were evaluated for serum lipase and amylase, and 24-hours urinary amylase. Clinical and biological parameters were noted. Abdominal sonography and rectosigmoidoscopy were performed.

RESULTS

Pancreatic enzyme alteration was found in 24% of patients. Twelve had salmonellosis and 2 Campylobacter jejuni infection. They had more prolonged diarrhea, more frequent renal impairment and increased triglyceridemia. Triglyceridemia was correlated to blood amylase, inflammatory syndrome and renal impairment. Serum amylase was linked to serum urea and creatinine and to biological markers of inflammation. Three patients had true acute pancreatitis.

CONCLUSION

Patients presenting dysentery-like infectious diarrhea and upper abdominal pain should be investigated for concomitant pancreatic reaction or acute pancreatitis which seems more frequent in patients with enterocolitis due to enteroinvasive microbes, mostly non-typhoidal Salmonella. Pancreatic disturbances are related to the severity of these infections. However, overt infectious diarrhea-associated pancreatitis is a rare event.

Cellular mechanisms of bacterial internalization counteracted by Yersinia

Upon host-cell contact, human pathogenic Yersinia species inject Yop virulence effectors into the host through a Type III secretion-and-translocation system. These virulence effectors cause a block in phagocytosis (YopE, YopT, YpkA, and YopH) and suppression of inflammatory mediators (YopJ). The Yops that block phagocytosis either interfere with the host cell actin regulation of Rho GTPases (YopE, YopT, and YpkA) or specifically and rapidly inactivate host proteins involved in signaling from the receptor to actin (YopH). The block in uptake has been shown to be activated following binding to Fc, Complement, and beta1-integrin receptors in virtually any kind of host cell. Thus, the use of Yersinia as a model system to study Yersinia-host cell interactions provides a good tool to explore signaling pathways involved in phagocytosis.

Flagellin acting via TLR5 is the major activator of key signaling pathways leading to NF-kappa B and proinflammatory gene program activation in intestinal epithelial cells

Background

Infection of intestinal epithelial cells by pathogenic Salmonella leads to activation of signaling cascades that ultimately initiate the proinflammatory gene program. The transcription factor NF-κB is a key regulator/activator of this gene program and is potently activated. We explored the mechanism by which Salmonella activates NF-κB during infection of cultured intestinal epithelial cells and found that flagellin produced by the bacteria and contained on them leads to NF-κB activation in all the cells; invasion of cells by the bacteria is not required to activate NF-κB.

Results

Purified flagellin activated the mitogen activated protein kinase (MAPK), stress-activated protein kinase (SAPK) and Ikappa B kinase (IKK) signaling pathways that lead to expression of the proinflammatory gene program in a temporal fashion nearly identical to that of infection of intestinal epithelial cells by Salmonella. Flagellin expression was required for Salmonella invasion of host cells and it activated NF-κB via toll-like receptor 5 (TLR5). Surprisingly, a number of cell lines found to be unresponsive to flagellin express TLR5 and expression of exogenous TLR5 in these cells induces NF-κB activity in response to flagellin challenge although not robustly. Conversely, overexpression of dominant-negative TLR5 alleles only partially blocks NF-κB activation by flagellin. These observations are consistent with the possibility of either a very stable TLR5 signaling complex, the existence of a low abundance flagellin co-receptor or required adapter, or both.

Conclusion

These collective results provide the evidence that flagellin acts as the main determinant of Salmonella mediated NF-κB and proinflammatory signaling and gene activation by this flagellated pathogen. In addition, expression of the fli C gene appears to play an important role in the proper functioning of the TTSS since mutants that fail to express fli C are defective in expressing a subset of Sip proteins and fail to invade host cells. Flagellin added in trans cannot restore the ability of the fli C mutant bacteria to invade intestinal epithelial cells. Lastly, TLR5 expression in weak and non-responding cells indicates that additional factors may be required for efficient signal propagation in response to flagellin recognition.

Activation of NF-kappaB and IL-8 by Yersinia enterocolitica invasin protein is conferred by engagement of Rac1 and MAP kinase cascades

Yersinia enterocolitica triggers activation of the nuclear factor (NF)-kappaB and production of the proinflammatory chemokine interleukin (IL)-8 in intestinal epithelial cells. This activation is due to adhesion of the bacteria via their outer membrane protein invasin to the host cells. Using Clostridium difficile toxins that specifically inactivate small GTPases, and transfection of inhibitory proteins of the Rho-GTPases, we demonstrate that Rac1, but not Cdc42 or Rho, is required for activation of NF-kappaB by invasin. Invasin activated the mitogen activated protein kinases (MAPK) p38 and c-Jun N-terminal protein kinase (JNK) but not extracellular signal regulated kinase (ERK). The functional relevance of these pathways for invasin-mediated IL-8 expression was assessed by protein kinase inhibitors and dominant-negative kinase mutants. While NF-kappaB and JNK contribute to IL-8 transcription, p38 MAPK also acts through stabilization of IL-8 mRNA, as confirmed by quantitative RT-PCR and electrophoretic mobility shift assays. Transfection experiments with I-kappaB kinase (IKK)1 and IKK2 mutants indicate that the release of NF-kappaB from its cytoplasmic inhibitor I-kappaB and its translocation into the nucleus is mediated by these kinases. Our data identify Rac1 as a key intermediate in invasin-triggered IL-8 synthesis and demonstrate that maximum IL-8 induction involves several MAP kinase cascades.

Membrane ruffling and invasion of human and avian cell lines is reduced for aflagellate mutants of Salmonella enterica serotype Enteritidis

Independent studies have demonstrated that flagella are associated with the invasive process of Salmonella enterica serotypes, and aflagellate derivatives of Salmonella enterica serotype Enteritidis are attenuated in murine and avian models of infection. One widely held view is that the motility afforded by flagella, probably aided by chemotactic responses, mediates the initial interaction between bacterium and host cell. The adherence and invasion properties of two S. Enteritidis wild-type strains and isogenic aflagellate mutants were assessed on HEp-2 and Div-1 cells that are of human and avian epithelial origin, respectively. Both aflagellate derivatives showed a significant reduction of invasion compared with wild type over the three hours of the assays. Complementation of the defective fliC allele recovered partially the wild-type phenotype. Examination of the bacterium-host cell interaction by electron and confocal microscopy approaches showed that wild-type bacteria induced ruffle formation and significant cytoskeletal rearrangements on HEp-2 cells within 5 minutes of contact. The aflagellate derivatives induced fewer ruffles than wild type. Ruffle formation on the Div-1 cell line was less pronounced than for HEp-2 cells for wild-type S. Enteritidis. Collectively, these data support the hypothesis that flagella play an active role in the early events of the invasive process.

Pseudomonas aeruginosa slime glycolipoprotein is a potent stimulant of tumor necrosis factor alpha gene expression and activation of transcription activators nuclear factor kappa B and activator protein 1 in human monocytes

Pseudomonas aeruginosa, an opportunistic pathogen, causes infections associated with a high incidence of morbidity and mortality in immunocompromised hosts. Production of tumor necrosis factor alpha (TNF-alpha), primarily by cells of monocytic lineage, is a crucial event in the course of these infections. During in vivo infections with P. aeruginosa, both lipopolysaccharide (LPS) and extracellular slime glycolipoprotein (GLP) produced by mucoid and nonmucoid strains are released. In the present study, we sought to explore the relative contributions of these two bacterial products to TNF-alpha production by human monocytes. To this end, fresh human monocytes and THP-1 human monocytic cells were stimulated with P. aeruginosa LPS or GLP. GLP was found to be a more potent stimulus for TNF-alpha production (threefold higher) by human monocytes than LPS. Moreover, its effect was comparable to that of viable bacteria. Quantitative mRNA analysis revealed predominantly transcriptional regulation. Electrophoretic mobility shift assays and transfection assays demonstrated activation of NF-kappa B and activator protein 1 (AP-1). NF-kappa B activation by GLP was rapid and followed the same time course as that by viable bacteria, suggesting that bacteria could directly activate NF-kappa B through GLP. Moreover P. aeruginosa GLP induced the formation of AP-1 complex with delayed kinetics compared with NF-kappa B but much more efficiently than the homologous LPS. These results identify GLP as the most important stimulant for TNF-alpha production by human monocytes. Activation of NF-kappa B and AP-1 by P. aeruginosa GLP may be involved not only in TNF-alpha induction but also in many of the inflammatory responses triggered in the course of infection with P. aeruginosa.

Interaction of Yersinia enterocolitica with epithelial cells: invasin beyond invasion

The chromosomally encoded inv gene product is an outer membrane protein that is functionally expressed in the enteropathogenic Yersinia species Yersinia enterocolitica and Yersinia pseudotuberculosis. Invasin protein is a high-affinity ligand for beta1 integrins and especially important in the early phase of intestinal infection for efficient translocation through the M cells located in the follicle-associated epithelium overlying the Peyer's patches. In addition to bacterial internalization, Yersinia invasin mediates proinflammatory epithelial cell reactions. Epithelial cells exhibit immunological functions including production of cytokines thereby signaling to the immune system the presence of invasive or pathogenic bacteria. Several other enteropathogenic bacteria also induce cytokine production in epithelial cells. However, the signaling pathways by which this reaction is accomplished differ for various pathogens. Binding of invasin-expressing Yersinia to beta1 integrin receptors of epithelial cells induces activation of a signal cascade involving Rac1, MAP kinases, activation of the transcription factor NF-kappaB, and the subsequent production of chemotactic cytokines. The Yersinia invasin-triggered inflammatory epithelial cell reaction may lead to the recruitment of phagocytes followed by tissue disruption which may be part of the strategy of the pathogen to promote its dissemination in the host tissue.

Salmonella infection induces a hypersecretory phenotype in human intestinal xenografts by inducing cyclooxygenase 2

Enteric Salmonella infection is accompanied by inflammation and diarrhea, and yet little is known about its effects on intestinal epithelial physiology. Since species differences limit the utility of animal tissues and cell lines lack relevant cell-cell interactions, we have used a human model of fetal intestine grown as xenografts in SCID mice. We investigated here the effects of Salmonella enterica serovar Typhimurium SL1344 on xenograft ion transport. Harvested xenografts were stripped of seromuscular layers by blunt dissection, infected with Salmonella, and mounted in Ussing chambers. Salmonella infection for 1 h increased baseline ion transport without altering tissue conductance or morphology. The increased transport was blocked by the cyclooxygenase inhibitor, indomethacin, or the specific Cox-2 inhibitor, NS-398. Further, xenografts infected for 2 h showed increased secretory responses to the calcium-dependent agonist, carbachol, and the cyclic AMP-dependent agonists prostaglandin E(2) (PGE(2)) and forskolin, which were blocked by indomethacin. Western blot experiments revealed that infection was accompanied by increased cyclooxygenase 2 (Cox-2) expression, with no change in Cox-1 levels. Immunoassay demonstrated basolateral PGE(2) release, which was inhibited by indomethacin. Histological examination of infected xenografts illustrated that upregulated Cox-2 expression was restricted to the epithelium and that little or no invasion of the tissue by Salmonella occurred for up to 2 h. In summary, Salmonella infection rapidly increases Cox-2 expression in human intestinal tissue, accounting for increased epithelial ion transport characteristic of infectious diarrhea.

Intestinal epithelial responses to enteric pathogens: effects on the tight junction barrier, ion transport, and inflammation

The effects of pathogenic organisms on host intestinal epithelial cells are vast. Innumerable signalling pathways are triggered leading ultimately to drastic changes in physiological functions. Here, the ways in which enteric bacterial pathogens utilise and impact on the three major physiological functions of the intestinal epithelium are discussed: alterations in the structure and function of the tight junction barrier, induction of fluid and electrolyte secretion, and activation of the inflammatory cascade. This field of investigation, which was virtually non-existent a decade ago, has now exploded, thus rapidly expanding our understanding of bacterial pathogenesis. Through increased delineation of the ways in which microbes alter host physiology, we simultaneous gain insight into the normal regulatory mechanisms of the intestinal epithelium.

Toll-like receptor-5 and the innate immune response to bacterial flagellin

The innate immune system identifies the presence of infection by detecting structures that are unique to microbes and that are not expressed in the host. The bacterial flagellum (Latin, a whip) confers motility, on a wide range of bacterial species. Vertebrates, plants, and invertebrates all have evolved flagellar recognition systems that are activated by flagellin, the major component of the bacterial flagellar filament. In mammals, flagellin is recognized by Toll-like receptor-5 and activates defense responses both systemically and at epithelial surfaces. Here, we review the role for Toll-like receptor-5 in mediating the mammalian innate immune response to flagellin, and how this provides for defense against infections caused by many different species of flagellated bacteria.

Role of flagellin in the pathogenesis of shock and acute respiratory distress syndrome: therapeutic opportunities

OBJECTIVE

To provide an overview of the role of flagellin as an immunostimulatory and proinflammatory factor.

DESIGN

A systematic overview of the literature on this subject.

SETTING

An office equipped with a computer and Internet access to PubMed.

SUBJECTS

MEDLINE citations between 1960 and 2002.

MAIN RESULTS

Flagellin, a protein of 40-60 kD, is the principal constituent of the flagellum, a prominent surface structure found in motile bacteria. Recent work reveals that monomeric flagellin, a protein component of flagellated bacteria, can act as a soluble immunostimulatory and proinflammatory factor, activating the immune/inflammatory axis via the toll-like receptor 5-nuclear factor-kappaB axis. Monocytes, macrophages, and intestinal and pulmonary epithelial cells respond to monomeric flagellin at low concentrations. Monomeric flagellin can induce prominent local and systemic immune/inflammatory responses and.

CONCLUSIONS

Recognition of the flagellin-toll-like receptor 5 pathway offers novel opportunities for the experimental therapy of various forms of shock, sepsis, and acute respiratory distress syndrome.

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.

Activation of mucin exocytosis and upregulation of MUC genes in polarized human intestinal mucin-secreting cells by the thiol-activated exotoxin listeriolysin O

The secreted thiol-activated cytolysin listeriolysin O (LLO) was responsible for L. monocytogenes-induced high-molecular glycoproteins (HMGs) exocytosis in cultured human mucosecreting HT29-MTX cells. By biochemical analysis we demonstrate that the majority of secreted HMGs in LLO-stimulated cells are of mucin origin. In parallel, analysis of the expression of MUCs genes showed that the transcription of the MUC3, MUC4 and MUC12 genes encoding for membrane-bound mucins was increased in LLO-stimulated cells. Upregulation of the MUC3 gene correlates with an increased expression of the membrane-bound MUC3 mucin. In contrast, increase in secretion of the gel-forming MUC5AC mucin develops without upregulation of the MUC5AC gene. Finally, results showed that NF-kappaB and AP-1 transcription factors were not involved in LLO-induced upregulation of MUCs genes in HT29-MTX cells, whereas L. monocytogenes infection was able to promote the degradation of IkappaB proteins in the cells.

Endotoxin, but not platelet-activating factor, activates nuclear factor-kappaB and increases IkappaBalpha and IkappaBbeta turnover in enterocytes

Bacterial endotoxin (lipopolysaccharide; LPS) and platelet-activating factor (PAF) are important triggers of bowel inflammation and injury. We have previously shown that LPS activates the transcription factor nuclear factor (NF)-kappaB in the intestine, which up-regulates many pro-inflammatory genes. This effect partly depends on neutrophils and endogenous PAF. However, whether LPS and PAF directly activate NF-kappaB in enterocytes remains controversial. In this study, we first investigated the effect of LPS and PAF on NF-kappaB activation in IEC-6 (a non-transformed rat small intestinal crypt cell line) cells, by electrophoresis mobility shift assay and supershift, and found that LPS, but not PAF, activates NF-kappaB mostly as p50-p65 heterodimers. The effect was slower than tumour necrosis factor (TNF). Both LPS and TNF induce the expression of the NF-kappaB-dependent gene inducible nitric oxide synthase (iNOS), which occurs subsequent to NF-kappaB activation. We then examined the effect of LPS and TNF on the inhibitory molecules IkappaBalpha and IkappaBbeta. We found that TNF causes rapid degradation of IkappaBalpha and IkappaBbeta. In contrast, LPS did not change the levels of IkappaBalpha and IkappaBbeta up to 4 hr (by Western blot). However, in the presence of cycloheximide, there was a slow reduction of IkappaBalpha and IkappaBbeta, which disappeared almost completely at 4 hr. These observations suggest that LPS causes slow degradation and synthesis of IkappaBalpha and IkappaBbeta and therefore activates NF-kappaBeta via at least two mechanisms: initially, through an IkappaB-independent mechanism, and later, via an increased turnover of the inhibitor IkappaB. NF-kappaBeta activation precedes the gene expression of iNOS (assayed by reverse transcription-polymerase chain reaction), suggesting that LPS up-regulates iNOS via this transcription factor.

Na+/H+ exchanger blockade inhibits enterocyte inflammatory response and protects against colitis

Na+/H+ exchangers (NHEs) are integral transmembrane proteins found in all mammalian cells. There is substantial evidence indicating that NHEs regulate inflammatory processes. Because intestinal epithelial cells express a variety of NHEs, we tested the possibility that NHEs are also involved in regulation of the epithelial cell inflammatory response. In addition, since the epithelial inflammatory response is an important contributor to mucosal inflammation in inflammatory bowel disease (IBD), we examined the role of NHEs in the modulation of disease activity in a mouse model of IBD. In human gut epithelial cells, NHE inhibition using a variety of agents, including amiloride, 5-(N-methyl-N-isobutyl)amiloride, 5-(N-ethyl-N-isopropyl)- amiloride, harmaline, clonidine, and cimetidine, suppressed interleukin-8 (IL-8) production. The inhibitory effect of NHE inhibition on IL-8 was associated with a decrease in IL-8 mRNA accumulation. NHE inhibition suppressed both activation of the p42/p44 mitogen-activated protein kinase and nuclear factor-kappaB. Finally, NHE inhibition ameliorated the course of IBD in dextran sulfate-treated mice. Our data demonstrate that inhibition of NHEs may be an approach worthy of pursuing for the treatment of IBD.

Effect of probiotic strains on interleukin 8 production by HT29/19A cells

OBJECTIVES

Promising results from clinical studies on the effect of probiotics as maintenance therapy in inflammatory bowel disease and in the prevention of onset of pouchitis ask for studies to unravel the still poorly understood mechanism of action of probiotics.

METHODS

To evaluate whether the probiotic bacteria that were used in the clinical studies (VSL#3, Escherichia coli Nissle 1917, and Lactobacillus GG) are able to induce chemokine production in epithelial cells, HT29/19A monolayers were incubated with cell debris and cell extract fractions of single strains of the probiotic bacteria in doses ranging from 10(3) to 10(9) colony-forming units/ml for 32 h. Supernatants were measured for interleukin 8 by ELISA.

RESULTS

Lactobacilli and bifidobacteria strains from VSL#3 and Lactobacillus GG did not induce interleukin 8, whereas both cell debris and cell extracts from E. coli Nissle 1917 induced interleukin 8 production in a dose-dependent way. Cell extracts from streptococcal strains induced interleukin 8 when applied at high concentrations.

CONCLUSIONS

Probiotic Gram-positive bacteria did not induce interleukin 8, whereas the nonpathogenic, Gram-negative E. coli Nissle 1917 strain induced interleukin 8 in a dose-dependent way in this culture model. These results suggest that probiotic Gram-positive bacteria and E. coli Nissle 1917 may exert their beneficial effects on the host by a different mechanism of action.

Salmonella flagellin-dependent proinflammatory responses are localized to the conserved amino and carboxyl regions of the protein

Flagellin, the monomeric subunit of flagella, is an inducer of proinflammatory mediators. Bacterial flagellin genes have conserved domains (D1 and D2) at the N terminus and C terminus and a middle hypervariable domain (D3). To identify which domains induced proinflammatory activity, r6-histidine (6HIS)-tagged fusion constructs were generated from the Salmonella dublin (SD) fliC flagellin gene. A full-length r6HIS SD flagellin (6HIS flag) induced IkappaBalpha loss poststimulation and NF-kappaB activation in Caco-2BBe cells and was as potent as native-purified SD flagellin. IFN-gamma-primed DLD-1 cells stimulated with 1 microg/ml of 6HIS flag induced high levels of NO (60 +/- 0.95 microM) comparable to the combination of IL-1beta and IFN-gamma (77 +/- 1.2) or purified native SD flag (66.3 +/- 0.98). Selected rSD flagellin proteins representing the D1, D2, or D3 domains alone or in combination were tested for proinflammatory properties. Fusion proteins representing the D3, amino, or carboxyl regions alone did not induce proinflammatory mediators. The results with a recombinant protein containing the amino D1 and D2 and carboxyl D1 and D2 separated by an Escherichia coli hinge (ND1-2/ECH/CD2) indicated that D1 and D2 were bioactive when coupled to an ECH element to allow protein folding. This chimera, but not the hinge alone, induced IkappaBalpha degradation, NF-kappaB activation, and NO and IL-8 production in two intestinal epithelial cell lines. ND1-2/ECH/CD2-1 also induced high levels of TNF-alpha (900 pg/ml) in human monocytes comparable to native SD flagellin (991.5 pg/ml) and 6HIS flag (987 pg/ml). The potent proinflammatory activity of flagellin, therefore, resides in the highly conserved N and C D1 and D2 regions.

GTPases of the Rho subfamily are required for Brucella abortus internalization in nonprofessional phagocytes: direct activation of Cdc42

Members of the genus Brucella are intracellular alpha-Proteobacteria responsible for brucellosis, a chronic disease of humans and animals. Little is known about Brucella virulence mechanisms, but the abilities of these bacteria to invade and to survive within cells are decisive factors for causing disease. Transmission electron and fluorescence microscopy of infected nonprofessional phagocytic HeLa cells revealed minor membrane changes accompanied by discrete recruitment of F-actin at the site of Brucella abortus entry. Cell uptake of B. abortus was negatively affected to various degrees by actin, actin-myosin, and microtubule chemical inhibitors. Modulators of MAPKs and protein-tyrosine kinases hampered Brucella cell internalization. Inactivation of Rho small GTPases using clostridial toxins TcdB-10463, TcdB-1470, TcsL-1522, and TcdA significantly reduced the uptake of B. abortus by HeLa cells. In contrast, cytotoxic necrotizing factor from Escherichia coli, known to activate Rho, Rac, and Cdc42 small GTPases, increased the internalization of both virulent and non-virulent B. abortus. Expression of dominant-positive Rho, Rac, and Cdc42 forms in HeLa cells promoted the uptake of B. abortus, whereas expression of dominant-negative forms of these GTPases in HeLa cells hampered Brucella uptake. Cdc42 was activated upon cell contact by virulent B. abortus, but not by a noninvasive isogenic strain, as proven by affinity precipitation of active Rho, Rac, and Cdc42. The polyphasic approach used to discern the molecular events leading to Brucella internalization provides new alternatives for exploring the complexity of the signals required by intracellular pathogens for cell invasion.

Cutting edge: bacterial flagellin activates basolaterally expressed TLR5 to induce epithelial proinflammatory gene expression

Flagellin, the structural component of bacterial flagella, is secreted by pathogenic and commensal bacteria. Flagellin activates proinflammatory gene expression in intestinal epithelia. However, only flagellin that contacts basolateral epithelial surfaces is proinflammatory; apical flagellin has no effect. Pathogenic Salmonella, but not commensal Escherichia coli, translocate flagellin across epithelia, thus activating epithelial proinflammatory gene expression. Investigating how epithelia detect flagellin revealed that cell surface expression of Toll-like receptor 5 (TLR5) conferred NF-kappaB gene expression in response to flagellin. The response depended on both extracellular leucine-rich repeats and intracellular Toll/IL-1R homology region of TLR5 as well as the adaptor protein MyD88. Furthermore, immunolocalization and cell surface-selective biotinylation revealed that TLR5 is expressed exclusively on the basolateral surface of intestinal epithelia, thus providing a molecular basis for the polarity of this innate immune response. Thus, detection of flagellin by basolateral TLR5 mediates epithelial-driven inflammatory responses to Salmonella.

Salmonella enteritidis FliC (flagella filament protein) induces human beta-defensin-2 mRNA production by Caco-2 cells

Antimicrobial peptides are crucial for host defense at mucosal surfaces. Bacterial factors responsible for induction of human beta-defensin-2 (hBD-2) mRNA expression in Caco-2 human carcinoma cells were determined. Salmonella enteritidis, Salmonella typhimurium, Salmonella typhi, Salmonella dublin, and culture supernatants of these strains induced hBD-2 mRNA expression in Caco-2 human carcinoma cells. Using luciferase as a reporter gene for a approximately 2.1-kilobase pair hBD-2 promoter, the hBD-2-inducing factor in culture supernatant of S. enteritidis was isolated. The supernatant factor was heat-stable and proteinase-sensitive. After purification by anion exchange and gel filtration chromatography, the hBD-2-inducing factor was identified as a 53-kDa monomeric protein with the amino-terminal sequence AQVINTNSLSLLTQNNLNK, which is identical to that of the flagella filament structural protein (FliC) of S. enteritidis. Consistent with this finding, the 53-kDa protein reacted with anti-FliC antibody, which prevented its induction of hBD-2 mRNA in Caco-2 cells. In agreement, the hBD-2-inducing activity in culture supernatant was completely neutralized by anti-FliC antibody. In gel retardation analyses, FliC increased binding of NF-kappaB (p65 homodimer) to hBD-2 gene promoter sequences. We conclude that S. enteritidis FliC induces hBD-2 expression in Caco-2 cells via NF-kappaB activation and thus plays an important role in up-regulation of the innate immune response.

Flagellin, a novel mediator of Salmonella-induced epithelial activation and systemic inflammation: I kappa B alpha degradation, induction of nitric oxide synthase, induction of proinflammatory mediators, and cardiovascular dysfunction

Gram-negative sepsis is mediated by the actions of proinflammatory genes induced in response to microbes and their products. We report that flagellin, the monomeric subunit of flagella, is a potent proinflammatory species released by Salmonella. Flagellin (1 microgram/ml) induces IkappaBalpha degradation, NF-kappaB nuclear translocation, and inducible NO synthase expression in cultured intestinal epithelial cells (IEC). Aflagellic Salmonella mutants do not induce NF-kappaB activation or NO production by cultured IEC. Antiserum to flagellin blocks NO production in IEC induced by medium conditioned by a variety of motile Gram-negative enteric pathogens (Escherichia coli, Salmonella muenchen, Serratia marcescens, Proteus mirabilis, and Proteus vulgaris). Flagellin, when injected systemically (approximately 10 microgram/mouse), induces systemic inflammation characterized by the systemic expression of a range of proinflammatory cytokines and chemokines and of inducible NO synthase. At higher doses (approximately 300 microgram/mouse), flagellin induces shock, characterized by hypotension, reduced vascular contractility in mice, and death. The effects of flagellin do not diminish in C3H/HeJ LPS-resistant mice, indicating that the Toll-like receptor-4 receptor is not involved in flagellin's actions. In LPS-resistant mice, i.p. injection of S. dublin flagellin or medium conditioned by wild-type S. dublin induces serum IFN-gamma and TNF-alpha, whereas medium conditioned by aflagellic mutants has no effect. Flagellin can be detected in the blood of rats with septic shock induced by live bacteria at approximately 1 microg/ml. We propose that flagellin released by Gram-negative pathogens may contribute to the inflammatory response by an LPS- and Toll-like receptor-4-independent pathway.

Salmonella typhimurium translocates flagellin across intestinal epithelia, inducing a proinflammatory response

This study investigated whether soluble paracrine factors mediated Salmonella-induced IL-8 expression in polarized model intestinal epithelia. We found that the basolateral media of model epithelia that had been apically infected with Salmonella typhimurium for a short period (10 minutes) could activate IL-8 secretion in virgin model epithelia, demonstrating that a proinflammatory factor (PIF) was indeed present. Initial characterization found that PIF was a heat-stable protein with a molecular mass of about 50 kDa that acts on the basolateral, but not apical, surface of model intestinal epithelia to elicit IL-8 secretion. PIF was not present in the media of model epithelia stimulated with other inducers of IL-8 secretion (TNF-alpha or carbachol) but was present in S. typhimurium supernatants, indicating PIF is of bacterial origin. PIF was purified from bacterial culture supernatants by anion/cation exchange chromatography and SDS-PAGE and found by using microsequencing to be the protein flagellin. In support of this finding, flagellin-deficient S. typhimurium mutants did not secrete detectable levels of PIF (i.e., a bioactivity that induced IL-8 secretion when placed basolaterally on model epithelia). Furthermore, viable flagellin-deficient mutant organisms (fliC/fljB and flhD) failed to elicit IL-8 secretion when added apically to model intestinal epithelia. These findings indicate that translocation of flagellin across epithelia, subsequent to apical epithelial-S. typhimurium interaction, is likely a major means of activating a mucosal inflammatory response.

Activation of Akt/protein kinase B in epithelial cells by the Salmonella typhimurium effector sigD

The serine-threonine kinase Akt is a protooncogene involved in the regulation of cell proliferation and survival. Activation of Akt is initiated by binding to the phospholipid products of phosphoinositide 3-kinase at the inner leaflet of the plasma membranes followed by phosphorylation at Ser(473) and Thr(308). We have found that Akt is activated by Salmonella enterica serovar Typhimurium in epithelial cells. A bacterial effector protein, SigD, which is translocated into host cells via the specialized type III secretion system, is essential for Akt activation. In HeLa cells, wild type S. typhimurium induced translocation of Akt to membrane ruffles and phosphorylation at residues Thr(308) and Ser(473) and increased kinase activity. In contrast, infection with a SigD deletion mutant did not induce phosphorylation or activity although Akt was translocated to membrane ruffles. Complementation of the SigD deletion strain with a mutant containing a single Cys to Ser mutation (C462S), did not restore the Akt activation phenotype. This residue has previously been shown to be essential for inositol phosphatase activity of the SigD homologue, SopB. Our data indicate a novel mechanism of Akt activation in which the endogenous cellular pathway does not convert membrane-associated Akt into its active form. SigD is also the first bacterial effector to be identified as an activator of Akt.

Salmonella enterica serovar typhimurium-dependent regulation of inducible nitric oxide synthase expression in macrophages by invasins SipB, SipC, and SipD and effector SopE2

When Salmonella enterica invades mammalian cells, it activates signals leading to increased expression of inflammatory mediators. One such mediator is nitric oxide (NO), which is produced under control of the enzyme inducible NO synthase (iNOS). Induction of iNOS in response to Salmonella infection has been demonstrated, but the bacterial effector molecules that regulate expression of the enzyme have not been identified. In the study reported here, an analysis of Salmonella-dependent iNOS expression in macrophages was carried out. Wild-type Salmonella strains increased the levels of both iNOS protein and mRNA in murine macrophage cell lines in an invasion-independent fashion. Mutant strains lacking a functional pathogenicity island 1-encoded type III secretion system, as well as strains lacking the invasins SipB, SipC, and SipD, were impaired in iNOS induction. Complementation experiments indicated that all three of the invasins were required for induction of iNOS expression. These results suggested that an effector protein, translocated into macrophages via the type III secretion system in a SipB-, SipC-, and SipD-dependent manner, might be the ultimate mediator of iNOS induction. In keeping with this idea, a mutant strain deficient in SopE2, a recently described homolog of SopE, was found to be impaired in the induction of iNOS expression. These observations suggest that iNOS expression is regulated by signals activated by SopE2 (and possibly SopE) and that the role of SipB, SipC, and SipD in this process is to facilitate translocation of the relevant effector.

Lactobacilli and Streptococci activate NF-kappa B and STAT signaling pathways in human macrophages

Gram-positive bacteria induce the production of several cytokines in human leukocytes. The molecular mechanisms involved in Gram-positive bacteria-induced cytokine production have been poorly characterized. In this work we demonstrate that both nonpathogenic Lactobacillus rhamnosus GG and pathogenic Streptococcus pyogenes (group A streptococci) induce NF-kappa B and STAT DNA-binding activity in human primary macrophages as analyzed by EMSA. NF-kappa B activation was rapid and was not inhibited by a protein synthesis inhibitor cycloheximide, suggesting that these bacteria could directly activate NF-kappa B. STAT1, STAT3, and IFN regulatory factor-1 DNA binding was induced by both bacteria with delayed kinetics compared with NF-kappa B. In addition, streptococci induced the formation of IFN-alpha-specific transcription factor complex and IFN-stimulated gene factor-3 (ISGF3). STAT1 and STAT3 activation and ISGF3 complex formation were inhibited by cycloheximide or by neutralization with IFN-alpha/beta-specific Abs. Streptococci were more potent than lactobacilli in inducing STAT1, ISGF3, and IFN regulatory factor-1 DNA binding. Accordingly, only streptococci induced IFN-alpha production. The activation of the IFN-alpha signaling pathway by streptococci could play a role in the pathogenesis of these bacteria. These results indicate that extracellular Gram-positive bacteria activate transcription factors involved in cytokine signaling by two mechanisms: directly, leading to NF-kappa B activation, and indirectly via cytokines, leading to STAT activation.

The I kappa B/NF-kappa B system: a key determinant of mucosalinflammation and protection

The ubiquitous transcription factor NF-kappa B is a central regulator of the transcriptional activation of a number of genes involved in cell adhesion, immune and proinflammatory responses, apoptosis, differentiation, and growth. Induction of these genes in intestinal epithelial cells (IECs) by activated NF-kappa B profoundly influences mucosal inflammation and repair. NF-kappa B activation requires the removal of I kappa B from NF-kappa B by inducible proteolysis, which liberates this transcription factor for migration to the nucleus, where it binds to kappa B-regulatory elements and induces transcription. I kappa B alpha degradation is incomplete and delayed in IECs, resulting in buffered responses to luminal stimuli. The stimulatory environment partially determines whether the effect of NF-kappa B is protective or deleterious for the host. kappa B-dependent proinflammatory gene expression, particularly chemokines, major histocompatibility complex class II antigens, and adhesion molecules may be extremely important in early protective responses to mucosal pathogens but, when dysregulated, could lead to the development of chronic inflammation, as seen in inflammatory bowel diseases. The key role of NF-kappa B in regulating expression of a number of proinflammatory genes makes this protein an attractive target for selective therapeutic intervention.

Salmonella typhimurium induces epithelial IL-8 expression via Ca(2+)-mediated activation of the NF-kappaB pathway

Interactions between the enteric pathogen Salmonella typhimurium and the luminal surface of the intestine provoke an acute inflammatory response, mediated in part by epithelial cell secretion of the chemokine IL-8 and other proinflammatory molecules. This study investigated the mechanism by which this pathogen induces IL-8 secretion in physiologically polarized model intestinal epithelia. IL-8 secretion induced by both the prototypical proinflammatory cytokine TNF-alpha and S. typhimurium was NF-kappaB dependent. However, NF-kappaB activation and IL-8 secretion induced by S. typhimurium, but not by TNF-alpha, was preceded by and required an increase in intracellular [Ca(2+)]. Additionally, agonists that increased intracellular [Ca(2+)] by receptor-dependent (carbachol) or independent (thapsigargin, ionomycin) means also induced IL-8 secretion. Furthermore, the ability of S. typhimurium mutants to induce IkappaB-alpha degradation, NF-kappaB translocation, and IL-8 transcription and secretion correlated precisely with their ability to induce an intracellular [Ca(2+)] increase in model intestinal epithelia, but not with their ability to invade these cells. Finally, S. typhimurium, but not TNF-alpha, induced a Ca(2+)-dependent phosphorylation of IkappaB-alpha. These results indicate that S. typhimurium-induced activation of NF-kappaB-dependent epithelial inflammatory responses proceeds by a Ca(2+)-mediated activation of an IkappaB-alpha kinase. These observations raise the possibility that pharmacologic intervention of the acute inflammatory response can be selectively matched to the specific class of initiating event.

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.

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.