Gangliosides inhibit flagellin signaling in the absence of an effect on flagellin binding to toll-like receptor 5

Naïve CD4+ T Cell Activation in the Nasal-Associated Lymphoid Tissue following Intranasal Immunization with a Flagellin-Based Subunit Vaccine

The nasal-associated lymphoid tissues (NALT) are generally accepted as an immune induction site, but the activation of naïve T-cells in that compartment has not been well-characterized. I wanted to determine if early events in naïve CD4⁺ T cell activation and the extent of antigen specific cell division are similar in NALT to that observed in other secondary lymphoid compartments. I performed antigen tracking experiments and analyzed the activation of naïve antigen-specific CD4⁺ T cells in the nasal-associated lymphoid tissues (NALT). I directly observed transepithelial transport of fluorescently labeled antigen from the lumen of the airway to the interior of the NALT two hours following immunization. One day following intranasal (i.n.) immunization with antigen and adjuvant, antigen-specific CD4⁺ T cells in the NALT associated as clusters, while antigen-specific CD4⁺ T cells in control mice immunized with adjuvant only remained dispersed. The antigen-specific CD4⁺ populations in the NALT and cranial deep cervical lymph nodes of immunized mice expanded significantly by day three following immunization. These findings are consistent with initial activation of naïve CD4⁺ T cells in the NALT and offer insight into adjuvant mechanism of flagellin in the upper respiratory compartment.

GM1-Binding Conjugates To Improve Intestinal Permeability

Drugs and proteins with poor intestinal permeability have a limited oral bioavailability. To remediate this problem, a receptor-mediated endocytosis and transcytosis approach was explored. Indeed, the nontoxic β subunit of cholera toxin (CTB) can cross the intestinal barrier by binding to receptor GM1. In this study, we explored the use of GM1-binding peptides and CTB as potential covalent carriers of poorly permeable molecules. GM1-binding peptides (G23, P3) and CTB were conjugated to poorly permeable fluorescent probes such as fluorescein isothiocyanate (FITC) and albumin-FITC using triethylene glycol spacers and click chemistry. The affinity of the peptide conjugates with receptor GM1 was confirmed by isothermal titration calorimetry or microscale thermophoresis, and the results suggested the involvement of nonspecific interactions. Conjugating the model drugs to G23 and P3 improved the internalization into Caco-2 and T84 cells, although the process was not dependent on the amount of GM1 receptor. However, conjugation of bovine serum albumin FITC to CTB increased the internalization in the same cells in a GM1-dependent pathway. Peptide conjugates demonstrated a limited permeability through a Caco-2 monolayer, whereas G23 and CTB conjugates slightly enhanced permeability through a T84 cell monolayer compared to model drugs alone. Since CTB can improve the permeability of large macromolecules such as albumin, it is an interesting carrier for the improvement of oral bioavailability of various other macromolecules such as heparins, proteins, and siRNAs.

Sequence determinants of specific pattern-recognition of bacterial ligands by the NAIP-NLRC4 inflammasome

The NLR apoptosis inhibitory proteins (NAIPs) function as specific cytosolic receptors for bacterial ligands to form the NAIP-NLRC4 inflammasome for anti-bacterial defenses. In mice, NAIP5/6 and NAIP2 recognize bacteria flagellin and the rod protein of the type III secretion system (T3SS), respectively. However, molecular mechanism for specific ligand pattern-recognition by the NAIPs is largely unknown. Here, through extensive domain swapping and truncation analyses, three structural domains, the pre-BIR, BIR1, and HD1, in NAIP2 and NAIP5 are identified, that are important for specific recognition of their respective ligand(s). The three domains are sufficient to confer the ligand specificity for NAIP2. Asp-18, Arg-108, and Arg-667, respectively, in the pre-BIR, BIR1 and HD1 of NAIP2 are further identified, each of which is essential for efficient binding to the rod protein. To our surprise, we find that the C-terminal leucine-rich repeat domain is dispensable for NAIP2 recognition of the T3SS rod protein, but is required for NAIP5 binding to flagellin. At the ligand side, we discover that the C-terminal 35 residues in flagellin are crucial for binding to NAIP5. Among the 35 residues, three critical residues are identified, which determine flagellin recognition by NAIP5 and subsequent inflammasome activation. The differences in the three amino-acid residues among flagellins from various pathogenic and commensal bacterial species correlate well with whether they are susceptible to NAIP5-mediated immune detection. Taken together, our studies identify critical sequence and amino-acid determinants in both NAIP receptors and the bacterial ligand flagellin that are important for the specificity of the pattern-recognition.

Second-generation Flagellin-rPAc Fusion Protein, KFD2-rPAc, Shows High Protective Efficacy against Dental Caries with Low Potential Side Effects

Dental caries is one of the most common global chronic diseases affecting all ages of the population; thus a vaccine against caries is urgently needed. Our previous studies demonstrated that a fusion protein, KF-rPAc, in which rPAc of S. mutans is directly fused to the C-terminal of E. coli-derived flagellin (KF), could confer high prophylactic and therapeutic efficiency against caries. However, possible side effects, including the high antigenicity of flagellin and possible inflammatory injury induced by flagellin, may restrict its clinical usage. Here, we produced a second-generation flagellin-rPAc fusion protein, KFD2-rPAc, by replacing the main antigenicity region domains D2 and D3 of KF with rPAc. Compared with KF-rPAc, KFD2-rPAc has lower TLR5 agonist efficacy and induces fewer systemic inflammatory responses in mice. After intranasal immunization, KFD2-rPAc induces significantly lower flagellin-specific antibody responses but a comparable level of rPAc-specific antibody responses in mice. More importantly, in rat challenge models, KFD2-rPAc induces a robust rPAc-specific IgA response, and confers efficient prophylactic and therapeutic efficiency against caries as does KF-rPAc, while the flagellin-specific antibody responses are highly reduced. In conclusion, low side effects and high protective efficiency against caries makes the second-generation flagellin-rPAc fusion protein, KFD2-rPAc, a promising vaccine candidate against caries.

Utilizing a TLR5-Adjuvanted Cytomegalovirus as a Lentiviral Vaccine in the Nonhuman Primate Model for AIDS

Despite tremendous progress in our understanding of human immunodeficiency virus (HIV) natural history and advances in HIV treatment, there is neither an approved vaccine nor a cure for infection. Here, we describe the development and characterization of a novel replicating vaccine vector utilizing Cytomegalovirus (CMV) and a TLR5 adjuvant. After partial truncation of the central, immunodominant hypervariable domain, flagellin (fliC) from Salmonella was cloned downstream of a codon optimized gag gene from simian immunodeficiency virus (SIV) and transiently expressed in telomerized rhesus fibroblast (TeloRF) cells in culture. Lysates generated from these transfected cells induced the tumor necrosis factor alpha (TNF-α), in a mouse macrophage cell line, in a TLR5-dependent manner. The Gag/FliC expression construct was cloned into a bacterial artificial chromosome encoding the rhesus CMV (RhCMV) genome, and infectious RhCMV was generated following transfection of TeloRF cells. This virus stably expressed an SIV Gag/FliC fusion protein through four serial passages. Lysates generated from infected cells induced TNF-α in a TLR5-dependent manner. Western blot analysis of infected cell lysates verified expression of a Gag/FliC fusion protein using a SIV p27 capsid monoclonal antibody. Lastly, rhesus macaques inoculated with this novel RhCMV virus demonstrated increased inflammatory responses at the site of inoculation seven days post-infection when compared to the parental RhCMV. These results demonstrate that an artificially constructed replicating RhCMV expressing an SIV Gag/FliC fusion protein is capable of activating TLR5 in a macrophage cell line in vitro and induction of an altered inflammatory response in vivo. Ongoing animals studies are aimed at determining vaccine efficacy, including subsequent challenge with pathogenic SIV.

Flagellins of Salmonella Typhi and nonpathogenic Escherichia coli are differentially recognized through the NLRC4 pathway in macrophages

Flagellin is recognized by both Toll-like receptor (TLR)5 and NAIP5/NLRC4 inflammasome receptors. We hypothesized that the flagellins derived from different bacteria might differentially activate TLR5 and/or NAIP5/NLRC4 signal pathways. To test this, the immune recognition of recombinant flagellins derived from pathogenic Salmonella Typhi (SF) and the nonpathogenic Escherichia coli K12 strain MG1655 (KF) were examined by the activation of TLR5 and NLRC4 pathways in various cell types. While flagellins SF and KF were not distinguishable in activating the TLR5 pathway, KF induced significantly less interleukin-1β production and pyroptotic cell death in peritoneal macrophages than SF, and showed markedly lower efficiency in activating caspase-1 through the NLRC4 pathway than SF. Macrophages may differentially recognize flagellins by intracellular sensors and thereby initiate the immune response to invading pathogenic bacteria. Our findings suggest an active role of flagellin as an important determinant in host differential immune recognition and for the control of bacteria infection.

Antigen replacement of domains D2 and D3 in flagellin promotes mucosal IgA production and attenuates flagellin-induced inflammatory response after intranasal immunization

Targeting early infection in mucosal sites is one of the primary goals for mucosal vaccines so as to prevent pathogen mucosal transmission and infection. The TLR5 agonist flagellin was deemed to be a mucosal adjuvant candidate for clinical usage. However, the high antigenicity of flagellin and the possible inflammatory injury induced by flagellin might restrict its clinical usage. Here HIV-1 p24 protein was selected as an antigen model and we replaced the main antigenicity region domains D2 and D3 of non-pathogenic E.coli-derived flagellin (KF). The derived soluble protein KFD-p24 3D was then compared with KF-p24, which fused p24 directly to the C-terminal of KF. In vitro and ex vivo experiments showed that KFD-p24 3D has lower TLR5 agonist efficacy and less immunocyte-activating efficacy. Interestingly, the production of KF- specific antibody was highly reduced, and KFD-p24 3D induced IgA-biased antibody responses in mucosal sites. Moreover, KFD-p24 3D induced far fewer systemic inflammatory responses and abrogated detectable inflammatory side effects on mice, even at the high dose. The properties of enhanced IgA generation and attenuated inflammatory responses broaden the safe-dose range of KFD-p24 3D flagellin, creating a potentially promising mucosal adjuvant.

A mouse model of Salmonella typhi infection

Salmonella spp. are gram-negative flagellated bacteria that can cause food- and waterborne gastroenteritis and typhoid fever in humans. We now report that flagellin from Salmonella spp. is recognized in mouse intestine by Toll-like receptor 11 (TLR11). Absence of TLR11 renders mice more susceptible to infection by S. Typhimurium, with increased dissemination of the bacteria and enhanced lethality. Unlike S. Typhimurium, S. Typhi, a human obligatory pathogen that causes typhoid fever, is normally unable to infect mice. TLR11 is expressed in mice, but not in humans, and remarkably, we find that tlr11(-/-) mice are efficiently infected with orally administered S. Typhi. We also find that tlr11(-/-) mice can be immunized against S. Typhi. Therefore, tlr11(-/-) mice represent a small-animal model for the study of the immune response to S. Typhi and for the development of vaccines against this important human pathogen.

Role of lipid rafts and flagellin in invasion of colonic epithelial cells by Shiga-toxigenic Escherichia coli O113:H21

Shiga-toxigenic Escherichia coli (STEC) O113:H21 strains that lack the locus of enterocyte effacement (LEE) efficiently invade eukaryotic cells in vitro, unlike LEE-positive O157:H7 strains. We used a fliC deletion mutant of the O113:H21 STEC strain 98NK2 (98NK2ΔfliC) to show that invasion of colonic epithelial (HCT-8) cells is heavily dependent on production of flagellin, even though adherence to the cells was actually enhanced in the mutant. Flagellin binds and signals through Toll-like receptor 5 (TLR5), but there was no evidence that either TLR5, the adaptor protein myeloid differentiation primary response gene 88 (MyD88), or the serine kinase interleukin-1 receptor-associated kinase (IRAK) were required for invasion of HCT-8 cells by strain 98NK2, as judged by transfection, RNA knockdown, or inhibitor studies. However, pretreatment of cells with anti-asialo-GM1 significantly decreased 98NK2 invasion (by 40.8%), while neuraminidase treatment (which cleaves terminal sialic acid residues, thus converting GM1 into asialo-GM1) significantly increased invasion (by 70.7%). Pretreatment of HCT-8 cells with either the cholesterol-depleting agent methyl-β-cyclodextrin (MβCD) or the tyrosine kinase inhibitor genistein significantly decreased invasion by 98NK2, indicating a potential role for lipid rafts in the invasion mechanism. Confocal microscopy also showed invading 98NK2 colocalized with lipid raft markers caveolin-1 and GM1. Interestingly, anti-asialo-GM1, neuraminidase, MβCD, and genistein have similar effects on the vestigial level of STEC invasion seen for STEC strain 98NK2ΔfliC, indicating that lipid rafts mediate a common step in flagellin-dependent and flagellin-independent cellular invasion.

Flagellin as an adjuvant: cellular mechanisms and potential

Flagellin is a potent activator of a broad range of cell types involved in innate and adaptive immunity. An increasing number of studies have demonstrated the effectiveness of flagellin as an adjuvant, as well as its ability to promote cytokine production by a range of innate cell types, trigger a generalized recruitment of T and B lymphocytes to secondary lymphoid sites, and activate TLR5(+)CD11c(+) cells and T lymphocytes in a manner that is distinct from cognate Ag recognition. The plasticity of flagellin has allowed for the generation of a range of flagellin-Ag fusion proteins that have proven to be effective vaccines in animal models. This review summarizes the state of our current understanding of the adjuvant effect of flagellin and addresses important areas of current and future research interest.

A recombinant flagellin-poxvirus fusion protein vaccine elicits complement-dependent protection against respiratory challenge with vaccinia virus in mice

Bacterial flagellin is a potent adjuvant that enhances adaptive immune responses to a variety of protein antigens. The vaccinia virus antigens L1R and B5R are highly immunogenic in the context of the parent virus, but recombinant forms of the proteins are only weakly immunogenic. Therefore we evaluated the humoral response to these antigens in mice when flagellin was used as an adjuvant. Flagellin-L1R and flagellin-B5R fusion proteins were more potent than flagellin, L1R, and B5R as separate proteins. At least three immunizations with flagellin-L1R and flagellin-B5R fusion proteins were required to confer protection in mice against challenge with vaccinia virus. Immune mice exhibited only limited signs of disease following challenge. Additionally, virus neutralization titers correlated with protection. Depletion of complement using cobra venom factor resulted in a marked decrease in the survival of immunized mice after challenge with vaccinia virus. Our results are consistent with the conclusion that flagellin-L1R and flagellin-B5R fusion proteins are effective in eliciting protective immunity against vaccinia virus that is dependent, in large part, on complement.

Naturally produced outer membrane vesicles from Pseudomonas aeruginosa elicit a potent innate immune response via combined sensing of both lipopolysaccharide and protein components

Pseudomonas aeruginosa is a prevalent opportunistic human pathogen that, like other Gram-negative pathogens, secretes outer membrane vesicles. Vesicles are complex entities composed of a subset of envelope lipid and protein components that have been observed to interact with and be internalized by host cells. This study characterized the inflammatory responses to naturally produced P. aeruginosa vesicles and determined the contribution of vesicle Toll-like receptor (TLR) ligands and vesicle proteins to that response. Analysis of macrophage responses to purified vesicles by real-time PCR and enzyme-linked immunosorbent assay identified proinflammatory cytokines upregulated by vesicles. Intact vesicles were shown to elicit a profoundly greater inflammatory response than the response to purified lipopolysaccharide (LPS). Both TLR ligands LPS and flagellin contributed to specific vesicle cytokine responses, whereas the CpG DNA content of vesicles did not. Neutralization of LPS sensing demonstrated that macrophage responses to the protein composition of vesicles required the adjuvantlike activity of LPS to elicit strain specific responses. Protease treatment to remove proteins from the vesicle surface resulted in decreased interleukin-6 and tumor necrosis factor alpha production, indicating that the production of these specific cytokines may be linked to macrophage recognition of vesicle proteins. Confocal microscopy of vesicle uptake by macrophages revealed that vesicle LPS allows for binding to macrophage surfaces, whereas vesicle protein content is required for internalization. These data demonstrate that macrophage sensing of both LPS and protein components of outer membrane vesicles combine to produce a bacterial strain-specific response that is distinct from those triggered by individual, purified vesicle components.

MUC1 expression by human airway epithelial cells mediates Pseudomonas aeruginosa adhesion

Human MUC1 (Muc1 in animals) is an extensively O-glycosylated membrane-tethered mucin expressed on the surface of epithelial cells and some cells of the hematopoietic system. Recently, we showed that the hamster Muc1 on Chinese hamster ovary (CHO) cells served as a binding site for Pseudomonas aeruginosa (PA) through interaction between bacterial flagellin and the Muc1 ectodomain. Because CHO cells are known to produce an atypical pattern of protein glycosylation, we determined whether or not PA interacted with MUC1 endogenously expressed on human airway epithelial cells. Knock down of MUC1 expression in bronchial (NuLi-1) or alveolar (A549) epithelial cells by RNA interference significantly reduced PA binding to the cells. Conversely, over-expression of MUC1 in HEK293 cells increased bacterial adherence. By confocal microscopy, PA and MUC1 were colocalized on the surface of NuLi-1 cells. Taken together, these results confirm our previous observations in CHO cells and suggest that MUC1 serves as a binding site for PA on the surface of airway epithelial cells, which may have important consequences in the pathogenesis of PA lung infections.

Induction of IL-8 expression by bacterial flagellin is mediated through lipid raft formation and intracellular TLR5 activation in A549 cells

We investigated the mechanism for the induction of a chemokine, IL-8, by bacterial flagellins in the human alveolar type II epithelial cell line, A549. Bacterial flagellin induced expression of IL-8 mRNA and protein in dose- and time-dependent manners. IL-8 expression was inhibited by nystatin (a lipid rafts inhibitor) but not by chlorpromazine (a clathrin-coated pits inhibitor). Interestingly, Toll-like receptor 5 (TLR5) recognizing flagellins was found in the intracellular compartment of A549 but rarely on the cell surface. Flagellin-induced IL-8 expression appears to be mediated through TLR5 as determined by in vitro transient transfection experiment in HEK-293 cells expressing TLR5 using a reporter gene construct containing IL-8 promoter. IL-8 expression was attenuated by inhibitors for protein kinase C (PKC) and mitogen-activated protein (MAP) kinases. Furthermore, NF-kappaB and NF-IL6 transcription factors played an important role in the flagellin-induced IL-8 gene expression in A549 cells. Collectively, these results suggest that flagellin-induced IL-8 expression requires formation of lipid rafts, intracellular TLR activation, and subsequent activation of PKC and MAP kinases leading to the activation of the transcription factors NF-kappaB and NF-IL6 in human alveolar type II epithelial cells.

Direct stimulation of tlr5+/+ CD11c+ cells is necessary for the adjuvant activity of flagellin

Flagellin is a highly effective adjuvant, but the cellular mechanism underlying this activity remains uncertain. More specifically, no consensus exists as to whether flagellin activates dendritic cells (DC) directly or indirectly. Intramuscular immunization with flagellin-OVA fusion protein resulted in enhanced in vivo T cell clustering in draining lymph nodes and IL-2 production by OVA-specific CD4(+) T cells. Immunization with flagellin-OVA also triggered greater levels of Ag-specific CD4(+) T cell proliferation than immunization with flagellin and OVA as separate proteins. To determine whether flagellin, in the context of a fusion protein with OVA, was acting directly on DC, we used a combination of CD4(+) T cell adoptive transfers and bone marrow chimera mice in which the presence or absence of potential tlr5(+/+) CD11c(+) cells was controlled by injection of diphtheria toxin. The Ag-specific CD4(+) T cell response in mice with CD11c(+) cells from a tlr5(-/-) background and mixed populations of all other hematopoietic cells was dramatically reduced in comparison to mice that had DC from tlr5(-/-) and wild-type backgrounds. Immunization of MyD88(-/-)tlr5(+/+) mice revealed that the enhanced response following immunization with flagellin-OVA is dependent on signaling via the TLR5-MyD88 pathway as well as enhanced Ag uptake and processing resulting from Ag targeting via TLR5. In summary, our data are consistent with the conclusion that direct stimulation of tlr5(+/+) CD11c(+) cells is necessary for the adjuvant activity of a flagellin fusion protein and that this adjuvant effect requires signaling through TLR5.

Flagellin-F1-V fusion protein is an effective plague vaccine in mice and two species of nonhuman primates

A number of studies have clearly demonstrated that flagellin is a potent adjuvant that promotes robust immune responses when it is given with a protein antigen. In view of the potential biological and practical benefits of a recombinant protein vaccine composed of a single fusion protein containing flagellin and antigen, we have evaluated the efficacy of a fusion protein composed of flagellin and two protective antigens of Yersinia pestis (F1 and V) in eliciting protection against respiratory challenge with Y. pestis. Flagellin-F1-V was produced and purified in high yield under good manufacturing practices conditions. The fusion protein retains full Toll-like receptor 5-stimulating activity in vitro. Using a prime-boost immunization protocol, we found that flagellin-F1-V elicits robust antigen-specific humoral immunity in mice and two species of nonhuman primates. Immune mice were fully protected against intranasal challenge with 150 mean tolerated doses of Y. pestis CO92. In immune mice, the bacteria were completely cleared within 3 days after challenge. Flagellin-F1-V exhibited full stability for at least 297 days at 4 degrees C and at least 168 days at 25 degrees C. At between 29 and 84 days at 37 degrees C, the protein exhibited a loss of biological activity that appeared to be associated with a substantial change in protein diameter, possibly due to oligomerization. On the basis of our results, we believe that flagellin-F1-V is an outstanding candidate for evaluation in studies with humans.

Engineered expression of the TLR5 ligand flagellin enhances paramyxovirus activation of human dendritic cell function

The paramyxovirus simian virus 5 (SV5) is a poor activator of human dendritic cell (DC) maturation pathways in vitro, and infected DC do not upregulate cell surface costimulatory proteins or secretion of immunomodulatory cytokines. We evaluated the hypothesis that activation of SV5-infected DC would be enhanced by engineering SV5 to express a Toll-like-receptor (TLR) ligand. To test this hypothesis, a novel virus was engineered such that the gene encoding an intracellular form of the TLR5 ligand flagellin was expressed from the genome of wild-type (WT) SV5 (SV5-flagellin). Cells infected in vitro with the flagellin-expressing virus released low levels of biologically active flagellin, which was capable of stimulating TLR5 signaling. Infection of human peripheral blood mononuclear cell-derived immature DC with SV5-flagellin resulted in enhanced levels of interleukin-6 (IL-6) and IL-12 compared to infection with DC with the parental virus, WT SV5. In contrast to cytokine induction, the flagellin-expressing virus did not appreciably increase DC surface expression of the costimulatory molecule CD80 or CD86 above the level seen with WT SV5 alone. In mixed-culture assays, DC infected with the flagellin-expressing virus were more effective at activating gamma interferon secretion from both CD8(+) and CD4(+) allogeneic T cells than DC infected with WT SV5. Our results with SV5-directed intracellular expression of flagellin may be applicable to other vectors or pathogenic viruses where overcoming impairment of DC activation could contribute to the development of safer and more effective vaccines.

Effects of flagellin on innate and adaptive immunity

Flagella are locomotive organelles present on a wide range of bacteria and are important for the pathogenesis of many species. Cells of the innate immune system lack memory per se, but recognize conserved pathogen-associated molecular patterns (PAMPs) through a family of type I membrane receptors known as Toll-like receptors (TLRs). Flagellin, the major structural component of flagella, is a highly conserved protein recognized in hosts by TLR5. Signaling of flagellin via TLR5/TLR4 heteromeric complexes enhances the diversity of the response, likely by engaging MyD88-independent adaptors to activate the interferon pathway. Flagellin is a potent immune activator, stimulating diverse biologic effects that mediate both innate inflammatory responses as well as the development of adaptive immunity. Binding of flagellin to the extracellular domain of TLR5 rapidly induces a signal cascade that culminates in the production of proinflammatory mediators such as cytokines, chemokines, and costimulatory molecules. This review focuses on the mechanisms of action of flagellin and its effects on both innate and adaptive immunity.

Human corneal epithelial cells respond to ocular-pathogenic, but not to nonpathogenic-flagellin

In this study, we investigated the expression of TLR5 in human corneal epithelial cells (CEC), and the functional outcome of TLR5 triggering by flagellins of pathogenic- and nonpathogenic bacteria. Flagellins derived from Pseudomonas aeruginosa, Salmonella typhimurium, Serratia marcescense or Bacillus subtilis were used. The TLR5 protein and TLR5 specific mRNA expression was evident on human CEC. In human corneal epithelium tissues, TLR5 protein was detected at the basal and wing cells of the tissues. Ocular pathogenic bacteria, namely P. aeruginosa and S. marcescense, derived flagellin induced the significantly increased level of gene activation and IL-6 and IL-8 production. In contrast, ocular nonpathogenic S. typhimurium- and B. subtilis-derived flagellin induced neither the gene activation nor the increased production of IL-6 and IL-8 in human CEC. Human CEC would respond only to flagellin derived of ocular pathogenic bacteria, but not to those derived of ocular nonpathogenic bacteria, to generate pro-inflammatory cytokines.

Role of Shiga toxin versus H7 flagellin in enterohaemorrhagic Escherichia coli signalling of human colon epithelium in vivo

Enterohaemorrhagic Escherichia coli O157:H7 (EHEC) is a clinically important foodborne pathogen that colonizes human colon epithelium and induces acute colonic inflammation, but does not invade the epithelial cells. Whereas Shiga toxin (Stx) and bacterial flagellin have been studied for their ability to upregulate the production of proinflammatory chemokines by cultured human colon cancer cell lines, the relevance of studies in colon cancer cell lines to the production of proinflammatory signals by normal epithelial cells in EHEC-infected human colon is not known. We show herein that Stx does not bind to human colon epithelium in vivo. Moreover, globotriaosylceramide (Gb3/CD77) synthase, the enzyme required for synthesis of the Gb3/CD77 receptor for Stx, was not expressed by normal or inflamed human colon epithelium in vivo. In contrast, Toll-like receptor (TLR) 5, the receptor for bacterial flagellin, was expressed by normal human colon epithelium and by colon epithelium in human intestinal xenografts. EHEC H7 flagellin instilled in the lumen of human colon xenografts that contain an intact human epithelium upregulated the expression of epithelial cell proinflammatory chemokines, which was accompanied by a subepithelial influx of neutrophils. Isogenic mutants of EHEC that lacked flagellin did not significantly upregulate prototypic neutrophil and dendritic cell chemoattractants by model human colon epithelia, irrespective of Stx production. We conclude that EHEC H7 flagellin and not Stx is the major EHEC factor that directly upregulates proinflammatory chemokine production by human colon epithelium in vivo.

Mucosal interplay among commensal and pathogenic bacteria: lessons from flagellin and Toll-like receptor 5

Toll-like receptors (TLR) detect pathogen-associated molecular patterns (PAMP) and play a crucial role in triggering immunity. Due to their large surfaces in direct contact with the environment, mucosal tissues are the major sites of PAMP-TLR signalling. How innate and adaptive immunity are triggered through flagellin-TLR5 interaction is the main focus of the review. In view of recent reports on genetic polymorphism, we will summarize the impact of TLR5 on the susceptibility to mucosal infections and on various immuno-pathologies. Finally, the contribution of TLRs in the induction and maintenance of mucosal homeostasis and commensal discrimination is discussed.

Human CD4+ T cells express TLR5 and its ligand flagellin enhances the suppressive capacity and expression of FOXP3 in CD4+CD25+ T regulatory cells

Germline encoded pattern recognition receptors, such as TLRs, provide a critical link between the innate and adaptive immune systems. There is also evidence to suggest that pathogen-associated molecular patterns may have the capacity to modulate immune responses via direct effects on CD4+ T cells. Given the key role of both CD4+CD25+ T regulatory (Treg) cells and the TLR5 ligand flagellin in regulating mucosal immune responses, we investigated whether TLR5 may directly influence T cell function. We found that both human CD4+CD25+ Treg and CD4+CD25- T cells express TLR5 at levels comparable to those on monocytes and dendritic cells. Costimulation of effector T cells with anti-CD3 and flagellin resulted in enhanced proliferation and production of IL-2, at levels equivalent to those achieved by costimulation with CD28. In contrast, costimulation with flagellin did not break the hyporesponsiveness of CD4+CD25+ Treg cells, but rather, potently increased their suppressive capacity and enhanced expression of FOXP3. These observations suggest that, in addition to their APC-mediated indirect effects, TLR ligands have the capacity to directly regulate T cell responses and modulate the suppressive activity of Treg cells.

Polyoma virus-like particles elicit polarized cytokine responses in APCs from tumor-susceptible and -resistant mice

PERA/Ei (PE) mice are highly susceptible to tumor induction by polyoma virus, whereas C57BR/cdj (BR) mice are highly resistant. PE mice respond to viral infection with a type 2 (IL-10) and BR mice with a type 1 (IL-12) cytokine response, underlining the importance of a sustained T cell response for effective antitumor immunity. PE and BR mice showed comparable Ab responses to the virus, indicating that a Th1 response is fully compatible with strong humoral immunity. Tumor susceptibility is dominant, and a type 2 response prevails in F1 mice derived from these strains. In this study, we show that the different cytokine responses of virus-infected hosts are recapitulated in vitro by exposure of APCs from uninfected PE, BR, and F1 animals to the virus. Importantly, virus-like particles formed from recombinant VP1, the major viral capsid protein, elicited the same host-specific cytokine responses as infectious virus. Assembly of VP1 pentamers into capsid shells is required because unassembled VP1 pentamers were ineffective. Binding of virus-like particles to sialic acid is required because pretreatment of APCs with neuraminidase prevented the response. Expression of TLR2 and TLR4 differed among different subpopulations of APCs and also between resistant and susceptible mice. Evidence is presented indicating that these TLRs play a role in mediating the host-specific cytokine responses to the virus.

Within-species flagellin polymorphism in Xanthomonas campestris pv campestris and its impact on elicitation of Arabidopsis FLAGELLIN SENSING2-dependent defenses

Bacterial flagellins have been portrayed as a relatively invariant pathogen-associated molecular pattern. We have found within-species, within-pathovar variation for defense-eliciting activity of flagellins among Xanthomonas campestris pv campestris (Xcc) strains. Arabidopsis thaliana FLAGELLIN SENSING2 (FLS2), a transmembrane leucine-rich repeat kinase, confers flagellin responsiveness. The flg22 region was the only Xcc flagellin region responsible for detectable elicitation of Arabidopsis defense responses. A Val-43/Asp polymorphism determined the eliciting/noneliciting nature of Xcc flagellins (structural gene fliC). Arabidopsis detected flagellins carrying Asp-43 or Asn-43 but not Val-43 or Ala-43, and it responded minimally for Glu-43. Wild-type Xcc strains carrying nonrecognized flagellin were more virulent than those carrying a recognized flagellin when infiltrated into Arabidopsis leaf mesophyll, but this correlation was misleading. Isogenic Xcc fliC gene replacement strains expressing eliciting or noneliciting flagellins grew similarly, both in leaf mesophyll and in hydathode/vascular colonization assays. The plant FLS2 genotype also had no detectable effect on disease outcome when previously untreated plants were infected by Xcc. However, resistance against Xcc was enhanced if FLS2-dependent responses were elicited 1 d before Xcc infection. Prior immunization was not required for FLS2-dependent restriction of Pseudomonas syringae pv tomato. We conclude that plant immune systems do not uniformly detect all flagellins of a particular pathogen species and that Xcc can evade Arabidopsis FLS2-mediated defenses unless the FLS2 system has been activated by previous infections.

AsialoGM1 and TLR5 cooperate in flagellin-induced nucleotide signaling to activate Erk1/2

Bacterial flagellin can interact with both Toll-like receptor 5 (TLR5) and the cell surface glycolipid, asialoGM1, to activate an innate immune response. The induction of mucin by flagellin in human lung epithelial cells (NCIH292) is dependent on asialoGM1 ligation, ATP receptor signaling, Ca2+ mobilization, and Erk1/2 activation. Conversely, the activation of NF-kappaB by flagellin is dependent on signaling through TLR5. These results prompted us to ask whether the flagellin-induced TLR5 signaling pathway was intersecting with or mutually independent of the nucleotide receptor pathway activated downstream of asialoGM1. Herein, we demonstrate that the release of ATP induced by flagellin is dependent on a Toll signaling cascade. Although Toll was able to activate NF-kappaB in the absence of extracellular ATP, Toll required ATP to activate Erk1/2. These results suggest interdependence between the asialoGM1 and TLR5 pathways and reveal a previously unsuspected role for autocrine extracellular ATP signaling in TLR signaling.

Bordetella bronchiseptica flagellin is a proinflammatory determinant for airway epithelial cells

Motility is an important virulence phenotype for many bacteria, and flagellin, the monomeric component of flagella, is a potent proinflammatory factor. Of the three Bordetella species, Bordetella pertussis and Bordetella parapertussis are nonmotile human pathogens, while Bordetella bronchiseptica expresses flagellin and causes disease in animals and immunocompromised human hosts. The BvgAS two-component signal transduction system regulates phenotypic-phase transition (Bvg+, Bvg-, and Bvg(i)) in bordetellae. The Bvg- phase of B. bronchiseptica is characterized by the expression of flagellin and the repression of adhesins and toxins necessary for the colonization of the respiratory tract. B. bronchiseptica naturally infects a variety of animal hosts and constitutes an excellent model to study Bordetella pathogenesis. Using in vitro coculture models of bacteria and human lung epithelial cells, we studied the effects of B. bronchiseptica flagellin on host defense responses. Our results show that B. bronchiseptica flagellin is a potent proinflammatory factor that induces chemokine, cytokine, and host defense gene expression. Furthermore, we investigated receptor specificity in the response to B. bronchiseptica flagellin. Our results show that B. bronchiseptica flagellin is able to signal effectively through both human and mouse Toll-like receptor 5.

Pathophysiological role of Toll-like receptor 5 engagement by bacterial flagellin in colonic inflammation

Commensal and enteroinvasive microbes in the human gut release bacterial flagellin, a specific microbial ligand of Toll-like receptor 5 (TLR5). However, the pathophysiological role of bacterial flagellin in gastrointestinal inflammation has not been determined. Here we evaluated the role of bacterial flagellin using native human colonic mucosa and the mouse colitis model of dextran sulfate sodium (DSS). We demonstrate that, in intact human colonic mucosa, the flagellin/TLR5 response occurs only after exposure to the basolateral, not the apical, surface, implying a basolaterally polarized TLR5 response in human colonic mucosa. In this context, flagellin exposure to injured colonic mucosa due to DSS administration in mice resulted in a TLR5-associated response evaluated by in vivo activation of mitogen-activated protein kinase/extracellular signal-related kinase 1/2 (MEK1/2) and elevated IL-6, TNF-alpha, and keratinocyte-derived chemokine production, whereas intact colonic mucosa did not respond to flagellin. Moreover, flagellin exposure to injured mouse colon in vivo, but not to intact colon, also significantly aggravated colonic inflammation, increased mouse mortality, and enhanced histopathological damage in the colonic mucosa. However, the TLR2-specific agonist, peptidoglycan or lipoteichoic acid, did not cause an inflammatory response in intact or DSS-injured mouse colon. Furthermore, intracolonic flagellin administration in mice causes severe apoptosis in colonic epithelium disrupted by DSS administration. These data suggest that intracolonic flagellin via TLR5 engagement is able to elicit inflammatory responses in disrupted colon, whereas the normal colon is not responsive to bacterial flagellin. These results demonstrate that bacterial flagellin plays an important role in the development and progress of colitis.