Flag in the crossroads: flagellin modulates innate and adaptive immunity

Effects of oral immunization with Bacillus subtilis displaying Vibrio harveyi FlgE protein on the intestinal structure and gut microbiota of grouper

The development of a novel formulation that combines vaccines and probiotics as a primary strategy to protect fish against pathogenic bacteria and reduce reliance on antibiotics is essential for addressing aquatic diseases in the future. In this study, a novel Bacillus subtilis strain Bs-CotC-FlgE was engineered through genetic modification to express Vibrio harveyi FlgE protein for use in grouper immunization. We orally administered the recombinant Bs-CotC-FlgE spores and the control Bs-CotC spores to groupers and evaluated their effects on the intestinal structure and gut microbiota. To assess the intestinal structure, histological analysis of the hindgut was performed, including measurements of villus length, villus height, and intestinal wall thickness. For gut microbiota analysis, total genomic DNA was extracted from the intestines, and the V3-V4 region of the bacterial 16S rRNA gene was amplified and sequenced using Illumina novaseq6000. The abundance of Vibrio in different groups was evaluated by analyzing the sequencing data at the genus level. The results showed a relative percent survival (RPS) of 63 % for Bs-CotC-FlgE group, significantly higher than the 29.63 % observed in Bs-CotC group (p < 0.01). Both Bs-CotC-FlgE and Bs-CotC spores significantly increased intestinal villus length, villus height, intestinal wall thickness in grouper (p < 0.01). Furthermore, gut microbiota diversity in grouper improved after spore consumption, as indicated by increased Shannon and Shannoneven indices and decreased Simpson index. At the phylum level, compared to the control group, the abundance of Actinobacteria and Firmicutes increased, while that of Ascomycetes decreased in the treatment groups. At the genus level, the abundance of Vibrio was significantly lower in the Bs-CotC-FlgE and Bs-CotC groups than in the control group (p < 0.05), and the abundance of Ralstonia, Halomonas and Bacillus increased. Notably, the abundance of Vibrio in the Bs-CotC-FlgE group was significantly lower than that in the Bs-CotC group (p < 0.05), suggesting Bs-CotC-FlgE immunization stimulates the grouper to produce specific antibodies and inhibits the attachment of Vibrio in the intestine of grouper. However, we observed that the spores can only survive gastric acid exposure and remain viable in the intestine for a limited duration. These findings enhance our understanding of the interaction between Bacillus carrier vaccines and fish gut microbiota and have potential for the development of a live vector vaccine against V. harveyi in grouper, which could contribute to reducing the use of antibiotics in aquaculture. Taken together, our results demonstrate oral immunization with B. subtilis spore expressing FlgE on the surface was a promising, safe and needle-free vaccination strategy against V. harveyi infection in grouper. The Bacillus carrier vaccine strategy can be adapted for different species and environmental conditions, offering a versatile solution to enhance disease resistance and promote sustainable aquaculture development.

Mechanistic insights into the interaction between the host gut microbiome and malaria

Malaria is a devastating infectious disease and significant global health burden caused by the bite of a Plasmodium-infected female Anopheles mosquito. Gut microbiota was recently discovered as a risk factor of severe malaria. This review entails the recent advances on the impact of gut microbiota composition on malaria severity and consequence of malaria infection on gut microbiota in mammalian hosts. Additionally, this review provides mechanistic insight into interactions that might occur between gut microbiota and host immunity which in turn can modulate malaria severity. Finally, approaches to modulate gut microbiota composition are discussed. We anticipate this review will facilitate novel hypotheses to move the malaria-gut microbiome field forward.

Molecular and Cellular Mechanisms Influenced by Postbiotics

In recent years, commensal bacteria colonizing the human body have been recognized as important determinants of health and multiple pathologic conditions. Among the most extensively studied commensal bacteria are the gut microbiota, which perform a plethora of functions, including the synthesis of bioactive products, metabolism of dietary compounds, and immunomodulation, both through attenuation and immunostimulation. An imbalance in the microbiota population, i.e., dysbiosis, has been linked to many human pathologies, including various cancer types and neurodegenerative diseases. Targeting gut microbiota and microbiome-host interactions resulting from probiotics, prebiotics, and postbiotics is a growing opportunity for the effective treatment of various diseases. As more research is being conducted, the microbiome field is shifting from simple descriptive analysis of commensal compositions to more molecular, cellular, and functional studies. Insight into these mechanisms is of paramount importance for understanding and modulating the effects that microbiota, probiotics, and their derivatives exert on host health.

Association of Emulsifier and Highly Processed Food Intake with Circulating Markers of Intestinal Permeability and Inflammation in the Cancer Prevention Study-3 Diet Assessment Sub-Study

Compelling animal studies report increased intestinal permeability, inflammation, and colorectal carcinogenesis with exposure to certain emulsifiers commonly added to processed foods, but human data are lacking. Highly processed food consumption is also associated with obesity and higher risk of chronic diseases. We cross-sectionally examined the association of emulsifier and highly processed food consumption estimated from six 24-h dietary recalls among 588 U.S. men and women over one year, with biomarkers of intestinal permeability and inflammation measured from two fasting blood samples collected six months apart. In multivariable-adjusted generalized linear models, greater emulsifier intake (g/d) was not associated with antibodies to flagellin (P-trend = 0.88), lipopolysaccharide (LPS) (P-trend = 0.56), or the combined total thereof (P-trend = 0.65) but was positively associated with an inflammatory biomarker, glycoprotein acetyls (GlycA) (P-trend = 0.02). Highly processed food intake (% kcal/d) was associated with higher anti-LPS antibodies (P-trend = 0.001) and total anti-flagellin and anti-LPS antibodies (P-trend = 0.005) but not with other biomarkers, whereas processed food intake expressed as % g/d was associated with higher GlycA (P-trend = 0.02). Our findings suggest that, broadly, highly processed food consumption may be associated with intestinal permeability biomarkers, and both emulsifier and highly processed food intakes may be associated with inflammation. Additional studies are warranted to further evaluate these relationships.Supplemental data for this article is available online at https://doi.org/10.1080/01635581.2021.1957947.

Flagellin Alleviates Airway Allergic Response by Stabilizing Eosinophils through Modulating Oxidative Stress

Eosinophil (Eo) degranulation plays a central role in the initiations of allergic attacks. Flagellin (FGN), the major component of bacterial flagella, has immune regulatory functions. This study aims to investigate the role of FGN in alleviating the allergic reaction by stabilizing Eos. A toll-like receptor 5-knockout mouse strain was employed to test the role of FGN in stabilizing Eos. An airway allergy mouse model was developed to test the administration of FGN in alleviating the airway allergy by stabilizing Eos. The results showed that FGN was required in stabilizing Eos in the airway tissues. FGN prevented specific antigen-induced Eo activation. Oxidative stress was associated with the antigen-induced Eo activation that could be counteracted by the presence of FGN. The FGN levels were lower and chymase levels were higher in the airway tissues of mice with allergic inflammation. Negative correlation was detected between the data of FGN and chymase in the lung tissues. Chymase physically contacted FGN to speed up its degradation. The administration of FGN alleviated experimental allergic inflammation in the mouse airways by stabilized Eos in the lung tissues. In conclusion, FGN contributes to Eo stabilization. The administration of FGN alleviates the experimental airway allergy. The data suggest that FGN can be a candidate to be employed in the treatment of allergic disorders.

Critical Role of Innate Immunity to Flagellin in the Absence of Adaptive Immunity

BACKGROUND

Bacterial flagellin is a major target of innate and adaptive immunity, both of which can promote and/or compensate for deficiencies in each other's function.

METHODS

To investigate the role of innate immune detection of flagellin irrespective of adaptive immunity, we examined the consequences of loss of Toll-like receptor 5 (T5) and/or Nod-like receptor 4 (N4) upon a Rag1-deficient background.

RESULTS

Mice lacking Toll-like receptor 5 and Rag1 (T5/Rag-DKO) exhibited frequent lethal Pasteurellaceae-containing abscesses that prevented breeding of these mice. Mice lacking Toll-like receptor 5, Nod-like receptor 4, and Rag1 (T5/N4/Rag-TKO) also resulted in sporadic lethal abdominal abscesses caused by similar Pasteurellaceae. In the absence of such infections, relative to Rag1-KO, T5/N4/Rag-TKO mice exhibited microbiota encroachment, low-grade inflammation, microbiota dysbiosis, and, moreover were highly prone to Citrobacter infection and developed severe colitis when adoptively transferred with colitogenic T cells. Relative proneness of T5/N4/Rag-TKO mice to T-cell colitis was ablated by antibiotics while fecal microbiota transplant from T5/N4/Rag-TKO mice to wild-type mice transferred proneness to Citrobacter infection, indicating that dysbiosis in T5/N4/Rag-TKO mice contributed to these phenotypes.

CONCLUSIONS

These results demonstrate a critical role for innate immune detection of flagellin, especially in the intestinal tract and particularly in hosts deficient in adaptive immunity.

Rapid Bladder Interleukin-10 Synthesis in Response to Uropathogenic Escherichia coli Is Part of a Defense Strategy Triggered by the Major Bacterial Flagellar Filament FliC and Contingent on TLR5

Interleukin-10 is part of the immune response to urinary tract infection (UTI) due to E. coli, and it is important in the early control of infection in the bladder. Defining the mechanism of engagement of the immune system by the bacteria that enables the protective IL-10 response is critical to exploring how we might exploit this mechanism for new infection control strategies. In this study, we reveal part of the bacterial flagellar apparatus (FliC) is an important component that is sensed by and responsible for induction of IL-10 in the response to UPEC. We show this response occurs in a TLR5-dependent manner. Using infection prevention and control trials in mice infected with E. coli, this study also provides evidence that purified FliC might be of value in novel approaches for the treatment of UTI or in preventing infection by exploiting the FliC-triggered bladder transcriptome.

Urinary tract infection (UTI) caused by uropathogenic Escherichia coli (UPEC) engages interleukin-10 (IL-10) as an early innate immune response to regulate inflammation and promote the control of bladder infection. However, the mechanism of engagement of innate immunity by UPEC that leads to elicitation of IL-10 in the bladder is unknown. Here, we identify the major UPEC flagellar filament, FliC, as a key bacterial component sensed by the bladder innate immune system responsible for the induction of IL-10 synthesis. IL-10 responses of human as well as mouse bladder epithelial cell-monocyte cocultures were triggered by flagella of three major UPEC representative strains, CFT073, UTI89, and EC958. FliC purified to homogeneity induced IL-10 in vitro and in vivo as well as other functionally related cytokines, including IL-6. The genome-wide innate immunological context of FliC-induced IL-10 in the bladder was defined using RNA sequencing that revealed a network of transcriptional and antibacterial defenses comprising 1,400 genes that were induced by FliC. Of the FliC-responsive bladder transcriptome, altered expression of il10 and 808 additional genes were dependent on Toll-like receptor 5 (TLR5), according to analysis of TLR5-deficient mice. Examination of the potential of FliC and associated innate immune signature in the bladder to boost host defense, based on prophylactic or therapeutic administration to mice, revealed significant benefits for the control of UPEC. We conclude that detection of FliC through TLR5 triggers rapid IL-10 synthesis in the bladder, and FliC represents a potential immune modulator that might offer benefit for the treatment or prevention of UPEC UTI.

IMPORTANCE Interleukin-10 is part of the immune response to urinary tract infection (UTI) due to E. coli, and it is important in the early control of infection in the bladder. Defining the mechanism of engagement of the immune system by the bacteria that enables the protective IL-10 response is critical to exploring how we might exploit this mechanism for new infection control strategies. In this study, we reveal part of the bacterial flagellar apparatus (FliC) is an important component that is sensed by and responsible for induction of IL-10 in the response to UPEC. We show this response occurs in a TLR5-dependent manner. Using infection prevention and control trials in mice infected with E. coli, this study also provides evidence that purified FliC might be of value in novel approaches for the treatment of UTI or in preventing infection by exploiting the FliC-triggered bladder transcriptome.

Physical Extraction and Fast Protein Liquid Chromatography for Purifying Flagella Filament From Uropathogenic Escherichia coli for Immune Assay

Flagella are expressed on the surface of a wide range of bacteria, conferring motility and contributing to virulence and innate immune stimulation. Host-pathogen interaction studies of the roles of flagella in infection, including due to uropathogenic Escherichia coli (UPEC), have used various methods to purify and examine the biology of the major flagella subunit protein, FliC. These studies have offered insight into the ways in which flagella proteins interact with host cells. However, previous methods used to extract and purify FliC, such as mechanical shearing, ultracentrifugation, heterologous expression in laboratory E. coli strains, and precipitation-inducing chemical treatments have various limitations; as a result, there are few observations based on highly purified, non-denatured FliC in the literature. This is especially relevant to host-pathogen interaction studies such as immune assays that are designed to parallel, as closely as possible, naturally-occurring interactions between host cells and flagella. In this study, we sought to establish a new, carefully optimized method to extract and purify non-denatured, native FliC from the reference UPEC strain CFT073 to be suitable for immune assays. To achieve purification of FliC to homogeneity, we used a mutant CFT073 strain containing deletions in four major chaperone-usher fimbriae operons (type 1, F1C and two P fimbrial gene clusters; CFT073Δ4). A sequential flagella extraction method based on mechanical shearing, ultracentrifugation, size exclusion chromatography, protein concentration and endotoxin removal was applied to CFT073Δ4. Protein purity and integrity was assessed using SDS-PAGE, Western blots with anti-flagellin antisera, and native-PAGE. We also generated a fliC-deficient strain, CFT073Δ4ΔfliC, to enable the concurrent preparation of a suitable carrier control to be applied in downstream assays. Innate immune stimulation was examined by exposing J774A.1 macrophages to 0.05-1 μg of purified FliC for 5 h; the supernatants were analyzed for cytokines known to be induced by flagella, including TNF-α, IL-6, and IL-12; the results were assessed in the context of prior literature. Macrophage responses to purified FliC encompassed significant levels of several cytokines consistent with prior literature reports. The purification method described here establishes a new approach to examine highly purified FliC in the context of host-pathogen interaction model systems.

A Review: The Fate of Bacteriocins in the Human Gastro-Intestinal Tract: Do They Cross the Gut-Blood Barrier?

The intestinal barrier, consisting of the vascular endothelium, epithelial cell lining, and mucus layer, covers a surface of about 400 m2. The integrity of the gut wall is sustained by transcellular proteins forming tight junctions between the epithelial cells. Protected by three layers of mucin, the gut wall forms a non-permeable barrier, keeping digestive enzymes and microorganisms within the luminal space, separate from the blood stream. Microorganisms colonizing the gut may produce bacteriocins in an attempt to outcompete pathogens. Production of bacteriocins in a harsh and complex environment such as the gastro-intestinal tract (GIT) may be below minimal inhibitory concentration (MIC) levels. At such low levels, the stability of bacteriocins may be compromised. Despite this, most bacteria in the gut have the ability to produce bacteriocins, distributed throughout the GIT. With most antimicrobial studies being performed in vitro, we know little about the migration of bacteriocins across epithelial barriers. The behavior of bacteriocins in the GIT is studied ex vivo, using models, flow cells, or membranes resembling the gut wall. Furthermore, little is known about the effect bacteriocins have on the immune system. It is generally believed that the peptides will be destroyed by macrophages once they cross the gut wall. Studies done on the survival of neurotherapeutic peptides and their crossing of the brain-blood barrier, along with other studies on small peptides intravenously injected, may provide some answers. In this review, the stability of bacteriocins in the GIT, their effect on gut epithelial cells, and their ability to cross epithelial cells are discussed. These are important questions to address in the light of recent papers advocating the use of bacteriocins as possible alternatives to, or used in combination with, antibiotics.

Expression of Salmonella typhimurium and Escherichia coli flagellin protein and its functional characterization as an adjuvant

BACKGROUND

Flagellin is the major structural protein monomer of bacterial flagella. Flagellin through binding to its receptor and activation of antigen presenting cells stimulates the innate and adaptive immune responses. Flagellin is used as an effective systemic or mucosal adjuvant to stimulate the immune system. Recently, the therapeutic and protective role of flagellin in some infectious diseases and cancers has been investigated. In this study, we cloned the fliC genes from Salmonella typhimurium and Escherichia coli into pET-28a vector and investigated their expression in the prokaryotic system.

METHODS

The fliC genes of S. typhimurium and E. coli were amplified by PCR with a specific oligonucleotide primer set. thse were cloned into the pET-28a vector and the recombinant pET-28a-fliC plasmids were successfully transformed into the E. coli strain BL-21(DE3). The expression of flagellin proteins in the prokaryotic cells were evaluated. Finally, Transcription of TNF-α mRNA was confirmed using Real-time PCR.

RESULTS

The expression of proteins in the prokaryotic cells were approved by SDS-PAGE and western blotting method. Further, the functional characterization of flagellin proteins were evaluated using their ability to induce increased m-RNA expression of pro-inflammatory cytokine.

CONCLUSIONS

The flagellin proteins were expressed in the prokaryotic system. These proteins can be used to link target antigens as an effective adjuvant for future DNA vaccine studies. Purified recombinant proteins in this study can also be used for therapeutic and prophylactic purposes.

Serum anti-flagellin and anti-lipopolysaccharide immunoglobulins as predictors of linear growth faltering in Pakistani infants at risk for environmental enteric dysfunction

BACKGROUND

Environmental Enteric Dysfunction (EED) in children from low-income countries has been linked to linear growth declines. There is a critical need to identify sensitive and early EED biomarkers.

OBJECTIVE

Determine whether levels of antibodies against bacterial components flagellin (flic) and lipopolysaccharide (LPS) predict poor growth.

DESIGN/METHODS

In a prospective birth cohort of 380 children in rural Pakistan blood and stool samples were obtained at ages 6 and 9 months. Linear mixed effects models were used to examine longitudinal associations between quartiles of anti-flic and anti-LPS antibodies and changes in LAZ, WAZ and WLZ scores. Spearman's correlations were measured between anti-flic and anti-LPS immunoglobulins with measures of systemic/enteric inflammation and intestinal regeneration.

RESULTS

Anti-LPS IgA correlated significantly with CRP, AGP and Reg1 serum at 6mo and with MPO at 9mo. In multivariate analysis at 6mo of age, higher anti-LPS IgA levels predicted greater declines in LAZ scores over subsequent 18mo (comparing highest to lowest quartile, β (SE) change in LAZ score/year = -0.313 (0.125), p-value = 0.013). Anti-flic Ig A in the two highest quartiles measured at 9mo of age had declines in LAZ of -0.269 (0.126), p = 0.033; and -0.306 (0.129), p = 0.018 respectively, during the subsequent 18mo of life, compared to those in the lowest quartile of anti-flic IgA.

CONCLUSIONS AND RELEVANCE

Elevated anti-flic IgA and anti-LPS IgA antibodies at 6 and 9mo, predict declines in linear growth. Systemic and enteric inflammation correlated with anti-LPS IgA provides mechanistic considerations for potential future interventions.

TLR5 signaling in murine bone marrow induces hematopoietic progenitor cell proliferation and aids survival from radiation

Administration of the bacterial protein flagellin to mice activates innate immune signaling that protects against an array of challenges, including ionizing radiation. Herein, we define the underlying mechanism for this protection. We report that flagellin treatment induces proliferation and mobilization of bone marrow cells that aid survival following irradiation. Specifically, treatment of mice or bone marrow cells ex vivo with flagellin induced Toll-like receptor 5 (TLR5)-dependent and NOD-like receptor C4-independent proliferation of Lin^-Sca-1⁺Kit⁺ (LSK) cells, which includes both hematopoietic stem cells that provide long-term repopulation (LTR) and multipotent progenitor cells (MPPs) that transiently proliferate and differentiate into a range of blood cell types. TLR5 expression on bone marrow cells was necessary and sufficient for flagellin-induced LSK proliferation. Flagellin treatment stimulated LSK proliferation by inducing a 10-fold increase in type 3 MPP (MPP3) without a concomitant increase in LTR cells. Cotransfer of 5 × 10³ fluorescence-activated cell sorted flagellin-induced MPP3 cells along with 1 × 10⁵ whole bone marrow cells to lethally irradiated mice revealed that such cells predominantly repopulated the neutrophil compartment for up to 4 week, and dramatically increased the survival rate of the bone marrow transplantation procedure. Hence, we propose the administration of MPP3 cells, elicited by flagellin, as a novel approach to prevent life-threatening neutropenia that can accompany bone marrow transplant and other myeloablative therapeutic procedures.

Functional Characteristics of the Gut Microbiome in C57BL/6 Mice Differentially Susceptible to Plasmodium yoelii

C57BL/6 mice are widely used for in vivo studies of immune function and metabolism in mammals. In a previous study, it was observed that when C57BL/6 mice purchased from different vendors were infected with Plasmodium yoelii, a causative agent of murine malaria, they exhibited both differential immune responses and significantly different parasite burdens: these patterns were reproducible when gut contents were transplanted into gnotobiotic mice. To gain insight into the mechanism of resistance, we removed whole ceca from mice purchased from two vendors, Taconic Biosciences (low parasitemia) and Charles River Laboratories (high parasitemia), to determine the combined host and microflora metabolome and metatranscriptome. With the exception of two Charles River samples, we observed ≥90% similarity in overall bacterial gene expression within vendors and ≤80% similarity between vendors. In total 33 bacterial genes were differentially expressed in Charles River mice (p-value < 0.05) relative to the mice purchased from Taconic. Included among these, fliC, ureABC, and six members of the nuo gene family were overrepresented in microbiomes susceptible to more severe malaria. Moreover, 38 mouse genes were differentially expressed in these purported genetically identical mice. Differentially expressed genes included basigin, a cell surface receptor required for P. falciparum invasion of red blood cells. Differences in metabolite pools were detected, though their relevance to malaria infection, microbial community activity, or host response is not yet understood. Our data have provided new targets that may connect gut microbial activity to malaria resistance and susceptibility phenotypes in the C57BL/6 model organism.

Rotation of Vibrio fischeri Flagella Produces Outer Membrane Vesicles That Induce Host Development

Using the squid-vibrio association, we aimed to characterize the mechanism through which Vibrio fischeri cells signal morphogenesis of the symbiotic light-emitting organ. The symbiont releases two cell envelope molecules, peptidoglycan (PG) and lipopolysaccharide (LPS) that, within 12 h of light organ colonization, act in synergy to trigger normal tissue development. Recent work has shown that outer membrane vesicles (OMVs) produced by V. fischeri are sufficient to induce PG-dependent morphogenesis; however, the mechanism(s) of OMV release by these bacteria has not been described. Like several genera of both beneficial and pathogenic bacteria, V. fischeri cells elaborate polar flagella that are enclosed by an extension of the outer membrane, whose function remains unclear. Here, we present evidence that along with the well-recognized phenomenon of blebbing from the cell's surface, rotation of this sheathed flagellum also results in the release of OMVs. In addition, we demonstrate that most of the development-inducing LPS is associated with these OMVs and that the presence of the outer membrane protein OmpU but not the LPS O antigen on these OMVs is important in triggering normal host development. These results also present insights into a possible new mechanism of LPS release by pathogens with sheathed flagella.

IMPORTANCE

Determining the function(s) of sheathed flagella in bacteria has been challenging, because no known mutation results only in the loss of this outer membrane-derived casing. Nevertheless, the presence of a sheathed flagellum in such host-associated genera as Vibrio, Helicobacter, and Brucella has led to several proposed functions, including physical protection of the flagella and masking of their immunogenic flagellins. Using the squid-vibrio light organ symbiosis, we demonstrate another role, that of V. fischeri cells require rotating flagella to induce apoptotic cell death within surface epithelium, which is a normal step in the organ's development. Further, we present evidence that this rotation releases apoptosis-triggering lipopolysaccharide in the form of outer membrane vesicles. Such release may also occur by pathogens but with different outcomes for the host.

Cloning, Expression, and Purification of Pseudomonas aeruginosa Flagellin, and Characterization of the Elicited Anti-Flagellin Antibody

BACKGROUND

Pseudomonas aeruginosa is an important opportunistic human pathogen that causes serious infections in immunocompromised hosts. The single polar flagellum is an important factor in both virulence and colonization.

OBJECTIVES

As flagellin is the major component of the flagellar filament, the main aims of the present study are to identify, clone, express, and purify the recombinant type B flagellin (r-B-flagellin) of P. aeruginosa, as well as to evaluate the functional activity of the rabbit polyclonal antibody raised against this r-B-flagellin.

MATERIALS AND METHODS

In the current experimental study, the r-B-flagellin gene was isolated from the P. aeruginosa PAO1 strain by PCR. It was cloned into the pET-28a vector and then transformed into the E. coli BL21 strain. Next, r-B-flagellin was overexpressed and affinity purified by Ni-NTA agarose-affinity chromatography, followed by on-column resolubilization. Polyclonal antisera against the recombinant flagellin were raised in rabbits, and the functional activity of the anti-r-B-flagellin antibody was determined by in vitro assays.

RESULTS

The polyclonal antibodies raised against this r-B-flagellin inhibited the motility of the homologous PAO1 strain of P. aeruginosa, which significantly decreased the invasion of the PAO1 strain into the A549 cells and also enhanced the opsonophagocytosis of this strain. However, our polyclonal antibody showed little effect on the heterologous PAK strain.

CONCLUSIONS

The r-B-flagellin carried antigenic epitopes just like the native flagellin, while the polyclonal antibody raised against it exhibited functional activity.

The role of innate immunity receptors in the pathogenesis of inflammatory bowel disease

Innate immunity constitutes the first line of defense, fundamental for the recognition and the initiation of an inflammatory response against microorganisms. The innate immune response relies on the sensing of microbial-associated molecular patterns through specialized structures such as toll-like receptors (TLRs) and the nucleotide oligomerization domain- (NOD-) like receptors (NLRs). In the gut, these tasks are performed by the epithelial barrier and the presence of adaptive and innate immune mechanisms. TLRs and NLRs are distributed throughout the gastrointestinal mucosa, being more expressed in the epithelium, and in lamina propria immune and nonimmune cells. These innate immunity receptors exhibit complementary biological functions, with evidence for pathways overlapping. However, as tolerance is the predominant physiological response in the gastrointestinal mucosa, it appears that the TLRs are relatively downregulated, while NLRs play a critical role in mucosal defense in the gut. Over the past two decades, genetic polymorphisms have been associated with several diseases including inflammatory bowel disease. Special emphasis has been given to the susceptibility to Crohn's disease, in association with abnormalities in the NOD2 and in the NLRP3/inflammasome. Nevertheless, the mechanisms underlying innate immune receptors dysfunction that result in the persistent inflammation in inflammatory bowel disease remain to be clarified.

Cloning of fliC Gene From Salmonella typhimurium in the Expression Vector pVAX1 and Evaluation of its Expression in Eukaryotic Cells

BACKGROUND

Flagellin is the main structural protein of the flagella of many pathogens including Salmonella typhimurium. It is a potent trigger of innate immune responses that enhance adaptive immune responses to a variety of protein antigens. Flagellin has intrinsic adjuvant activity mediated through toll-like receptor (TLR) 5 and is an attractive candidate for highly effective vaccine adjuvant conferring enhanced antibody and cellular immune responses to proteins or peptides. In the present study, we cloned the fliC gene from S. enterica typhimurium in eukaryote vector pVAX1 and evaluated its expression in eukaryotic cells.

OBJECTIVES

The main aim of the present study was to construct a DNA vaccine expressing fliC as an adjuvant.

MATERIALS AND METHODS

The fliC gene of S. typhimurium (ATCC 14028) was amplified by PCR with specific primers and cloned into the pPrime cloning vector and successfully subcloned into expression vector pVAX1. The recombinant plasmid pVAX-fliC was finally expressed in eukaryotic cells.

RESULTS

Cloning and subcloning of the fliC gene were confirmed by colony PCR, restriction enzymes digestion and DNA sequencing of the recombinant plasmids pPrime-fliC and pVAX-fliC. The expression of flagellin protein in eukaryotic cells was approved by immunofluorescence assay (IFA), western blotting analysis and the reverse transcriptase polymerase chain reaction (RT-PCR) method.

CONCLUSIONS

The results of this study demonstrated that the fliC gene in recombinant plasmid pVAX-fliC was successfully expressed in eukaryotic cells and produced flagellin protein, which could be used as an effective adjuvant for DNA vaccine research.

Regulation of Toll-like receptor 5 gene expression and function on mucosal dendritic cells

Toll-like receptor (TLR) 5 has been shown to maintain intestinal homeostasis and regulate host defense against enterobacterial infection. However, how TLR5 expression is regulated and its function in the intestine have not been fully elucidated. Here we demonstrate that mucosal dendritic cells (DCs), but not splenic DCs, express high levels of TLR5 protein. Alternatively spliced Tlr5 transcripts were identified but it did not explain the selective expression of TLR5 on mucosal DCs. Treatment with various bacterial ligands downregulated BMDC TLR5 expression, while retinoic acid and host stromal cell-derived signals promoted TLR5 expression in a TGF-β-independent mechanism. Signaling through TLR5 restrained regulatory T (Treg) cell generation, and accordingly, TLR5^−/− mice displayed increased frequencies of Foxp3⁺ Treg cells in the intestinal lamina propria. Our data indicate that bacterial and host factors differentially regulate DC TLR5 expression. TLR5 signaling regulates immune responses towards the microbiota via modulation of the Treg/effector T cell balance.

Interactions between microbes and the gut epithelium

Gut microbes interact with the epithelium through cell surface components, fermentation products, and extracellular secreted proteins. Host-microbial interactions primarily involve TLRs (toll-like receptors) and NLR (nucleotide-binding oligomerization domain and leucine-rich repeat containing proteins). In a strain and dose-dependent manner, several probiotic strains directly alter tight junction protein expression and/or localization in gut epithelial cells through the release of secreted compounds. Interactions between gut microbes and intestinal epithelial and immune cells are necessary for the development and maintenance of intestinal homeostasis.

The intestinal microbiota and chronic disorders of the gut

Mucosal surfaces of the gut are colonized by large numbers of heterogeneous bacteria that contribute to intestinal health and disease. In genetically susceptible individuals, a 'pathogenic community' may arise, whereby abnormal gut flora contributes to alterations in the mucosa and local immune system leading to gastrointestinal disease. These diseases include enteric infections, such as Clostridium difficile infection, small intestinal bacterial overgrowth, functional gastrointestinal disorders (including IBS), IBD and colorectal cancer. Prebiotics, probiotics and synbiotics (a combination of prebiotics and probiotics) have the capacity to reverse pathologic changes in gut flora and local immunity. Intestinal health and disease need to be thoroughly characterized to understand the interplay between the indigenous microbiota, the immune system and genetic host factors. This Review provides a broad overview of the importance of the intestinal microbiota in chronic disorders of the gut.

Role of the endothelium in inflammatory bowel diseases

Inflammatory bowel diseases (IBD) are a complex group of diseases involving alterations in mucosal immunity and gastrointestinal physiology during both initiation and progressive phases of the disease. At the core of these alterations are endothelial cells, whose continual adjustments in structure and function coordinate vascular supply, immune cell emigration, and regulation of the tissue environment. Expansion of the endothelium in IBD (angiogenesis), mediated by inflammatory growth factors, cytokines and chemokines, is a hallmark of active gut disease and is closely related to disease severity. The endothelium in newly formed or inflamed vessels differs from that in normal vessels in the production of and response to inflammatory cytokines, growth factors, and adhesion molecules, altering coagulant capacity, barrier function and blood cell recruitment in injury. This review examines the roles of the endothelium in the initiation and propagation of IBD pathology and distinctive features of the intestinal endothelium contributing to these conditions.

TLR5 functions as an endocytic receptor to enhance flagellin-specific adaptive immunity

Innate immune activation via TLR induces dendritic cell maturation and secretion of inflammatory mediators, generating favorable conditions for naïve T-cell activation. Here, we demonstrate a previously unknown function for TLR5, namely that it enhances MHC class-II presentation of flagellin epitopes to CD4(+) T cells and is required for induction of robust flagellin-specific adaptive immune responses. Flagellin-specific CD4(+) T cells expanded poorly in TLR5-deficient mice immunized with flagellin, a deficiency that persisted even when additional TLR agonists were provided. Flagellin-specific IgG responses were similarly depressed in the absence of TLR5. In marked contrast, TLR5-deficient mice developed robust flagellin-specific T-cell responses when immunized with processed flagellin peptide. Surprisingly, the adaptor molecule Myd88 was not required for robust CD4(+) T-cell responses to flagellin, indicating that TLR5 enhances flagellin-specific CD4(+) T-cell responses in the absence of conventional TLR signaling. A requirement for TLR5 in generating flagellin-specific CD4(+) T-cell activation was also observed when using an in vitro dendritic cell culture system. Together, these data uncover an Myd88-independent function for dendritic cell TLR5 in enhancing the presentation of peptides to flagellin-specific CD4(+) T cells.

Gut microbiota and related diseases: clinical features

Intestinal microbiota is essential for gut homeostasis. Specifically, the microorganisms inhabiting the gut lumen interact with the intestinal immune system, supply key nutrients for the major components of the gut wall, and modulate energy metabolism. Host-microbiome interactions can be either beneficial or deleterious, driving gastrointestinal lymphoid tissue activities and shaping gut wall structures. This overview briefly focuses on the potential role played by abnormalities in gut microbiota and relative responses of the gastrointestinal tract in the determination of important pathological conditions such as the irritable bowel syndrome, inflammatory bowel diseases and colorectal cancer.

Extracellular proteins secreted by probiotic bacteria as mediators of effects that promote mucosa-bacteria interactions

During the last few years, a substantial body of scientific evidence has accumulated suggesting that certain surface-associated and extracellular components produced by probiotic bacteria could be responsible for some of their mechanisms of action. These bacterial components would be able to directly interact with the host mucosal cells; they include exopolysaccharides, bacteriocins, lipoteichoic acids and surface-associated and extracellular proteins. Extracellular proteins include proteins that are actively transported to the bacterial surroundings through the cytoplasmic membrane, as well as those that are simply shed from the bacterial surface. Compared to the other bacterial components, the interactive ability of extracellular proteins/peptides has been less extensively studied. In this review, current findings supporting an interaction between extracellular proteins/peptides produced by probiotic bacteria (strains of the genera Bifidobacterium, Lactobacillus and Escherichia) and host mucosal cells are discussed. Research needs and future trends are also considered.

Topical flagellin protects the injured corneas from Pseudomonas aeruginosa infection

Among bacterial pathogens, Pseudomonas (P.) aeruginosa infection is the most sight threatening. The corneal innate immune responses are key mediators of the host's defense to P. aeruginosa. Using a mouse model of Pseudomonas keratitis, we evaluated the protective effects of topical application of flagellin, a ligand for Toll-Like receptor 5 (TLR5), on the development of Pseudomonas keratitis and elucidated the underlying mechanisms. Topical application of purified flagellin 6 and 24 h prior to P. aeruginosa inoculation on injured mouse corneas significantly attenuated clinical symptoms of P. aeruginosa keratitis, decreased bacterial burden, and suppressed infection induced inflammation in the B6 mouse cornea. Topical application of flagellin on wounded cornea induced PMN infiltration and markedly upregulated cathelicidin-related antimicrobial peptide (CRAMP) expression. In PMN depleted mice, flagellin promoted bacterial clearance in the cornea compared to that of the PBS treated mice, but was unable to prevent corneal perforation and systemic bacterial dissemination and sepses. Deletion of CRAMP increased corneal susceptibility to P. aeruginosa and abolished flagellin-induced protection in B6 mice. Our findings illustrate the profound protective effect of flagellin on the cornea innate defense, a response that can be exploited for prophylactic purposes to prevent contact lens associated Pseudomonas keratitis.

The rate of bloodstream infection is high in infants with short bowel syndrome: relationship with small bowel bacterial overgrowth, enteral feeding, and inflammatory and immune responses

OBJECTIVE

This pilot study in parenteral nutrition-dependent infants with short bowel syndrome (SBS) evaluated the impact of feeding route and intestinal permeability on bloodstream infection (BSI), small bowel bacterial overgrowth (SBBO), and systemic immune responses, as well as fecal calprotectin as a biomarker for SBBO.

STUDY DESIGN

Ten infants (ages 4.2-15.4 months) with SBS caused by necrotizing enterocolitis were evaluated. Nutritional assessment, breath hydrogen testing, intestinal permeability, fecal calprotectin, serum flagellin- and lipopolysaccharide-specific antibody titers, and proinflammatory cytokine concentrations (tumor necrosis factor-alpha [TNF-alpha], interleukin-1 beta, -6, and -8) were performed at baseline and at 60 and 120 days. Healthy, age-matched control subjects (n = 5) were recruited.

RESULTS

BSI incidence was high (80%), and SBBO was common (50%). SBBO increased the odds for BSI (>7-fold; P = .009). Calprotectin levels were higher in children with SBS and SBBO versus those without SBBO and healthy control subjects (P < .05). Serum TNF-alpha, was elevated at baseline versus controls. Serum TNF-alpha and interleukin-1 beta, -6, and -8 levels diminished with increased enteral nutrition. Anti-flagellin and anti-lipopolysaccharide immunoglobulin G levels in children with SBS were lower versus control subjects and rose over time.

CONCLUSION

In children with SBS, SBBO increases the risk for BSI, and systemic proinflammatory response decreases with increasing enteral feeding and weaning parenteral nutrition.

Vibrio cholerae proteome-wide screen for immunostimulatory proteins identifies phosphatidylserine decarboxylase as a novel Toll-like receptor 4 agonist

Recognition of conserved bacterial components provides immediate and efficient immune responses and plays a critical role in triggering antigen-specific adaptive immunity. To date, most microbial components that are detected by host innate immune system are non-proteinaceous structural components. In order to identify novel bacterial immunostimulatory proteins, we developed a new high-throughput approach called “EPSIA”, Expressed Protein Screen for Immune Activators. Out of 3,882 Vibrio cholerae proteins, we identified phosphatidylserine decarboxylase (PSD) as a conserved bacterial protein capable of activating host innate immunity. PSD in concentrations as low as 100 ng/ml stimulated RAW264.7 murine macrophage cells and primary peritoneal macrophage cells to secrete TNFα and IL-6, respectively. PSD-induced proinflammatory response was dependent on the presence of MyD88, a known adaptor molecule for innate immune response. An enzymatically inactive PSD mutant and heat-inactivated PSD induced ∼40% and ∼15% of IL-6 production compared to that by native PSD, respectively. This suggests that PSD induces the production of IL-6, in part, via its enzymatic activity. Subsequent receptor screening determined TLR4 as a receptor mediating the PSD-induced proinflammatory response. Moreover, no detectable IL-6 was produced in TLR4-deficient mouse macrophages by PSD. PSD also exhibited a strong adjuvant activity against a co-administered antigen, BSA. Anti-BSA response was decreased in TLR4-deficient mice immunized with BSA in combination with PSD, further proving the role of TLR4 in PSD signaling in vivo. Taken together, these results provide evidence for the identification of V. cholerae PSD as a novel TLR4 agonist and further demonstrate the potential application of PSD as a vaccine adjuvant.

Innate immune responses are the first line of defense and involve the early recognition of pathogenic microorganisms. Furthermore, these early innate responses can help shape and influence the development of more specific adaptive immune responses. One way that innate immunity is triggered is by activation of TLRs, or Toll-like Receptors. TLRs recognize a wide spectrum of microbes by binding to pathogen-associated molecular patterns (PAMPs), which are conserved microbial products. Here, we have used a high-throughput method to understand more about how a pathogen can trigger early innate immune responses and also how these early responses to infection can influence the adaptive, more specific, immune response. This technique can also be utilized for adjuvant discovery which is important in vaccine development since different adjuvants can induce or enhance different kinds of immune responses to a particular antigen. Using this method, we identified a novel bacterial protein that activates a TLR and further characterized its role as an adjuvant. Identifying the TLRs, their ligands, and the signal transduction events that they initiate has provided insight into our understanding of how the immune response to infection begins, and how these factors also collectively influence the adaptive immune response.

Successful treatment of bacterial infection hinders development of acquired immunity

Antibiotics are routinely used to control bacterial infection, but the acquisition of acquired immunity following successful treatment has rarely been examined. We developed a model that allows visualization of acquired immunity during and following antibiotic treatment of typhoid. Pathogen-specific humoral and cellular immune responses were activated rapidly in antibiotic-treated mice, but were not sustained after successful antibiotic treatment and did not confer protection to secondary infection. In marked contrast, pathogen-specific Th1 and Ab responses matured over several weeks following immunization with a live vaccine strain. The deficiency in protective immunity following antibiotic treatment could be overcome by administering flagellin during antibiotic therapy. Thus, development of protective immunity is hindered by rapid therapeutic elimination of bacteria, but can be overcome by providing additional inflammatory and/or antigenic stimuli.

Radioresistant cells expressing TLR5 control the respiratory epithelium's innate immune responses to flagellin

Bacterial products (such as endotoxins and flagellin) trigger innate immune responses through TLRs. Flagellin-induced signalling involves TLR5 and MyD88 and, according to some reports, TLR4. Whereas epithelial and dendritic cells are stimulated by flagellin in vitro, the cell contribution to the in vivo response is still unclear. Here, we studied the respective roles of radioresistant and radiosensitive cells in flagellin-induced airway inflammation in mice. We found that i.n. delivery of flagellin elicits a transient change in respiratory function and an acute, pro-inflammatory response in the lungs, characterized by TLR5- and MyD88-dependent chemokine secretion and neutrophil recruitment. In contrast, TLR4, CD14 and TRIF were not essential for flagellin-mediated responses, indicating that TLR4 does not cooperate with TLR5 in the lungs. Respiratory function, chemokine secretion and airway infiltration by neutrophils were dependent on radioresistant, TLR5-expressing cells. Furthermore, lung haematopoietic cells also responded to flagellin by activating TNF-alpha production. We suggest that the radioresistant lung epithelial cells are essential for initiating early, TLR5-dependent signalling in response to flagellin and thus triggering the lung's innate immune responses.

TLR5 stimulation is sufficient to trigger reactivation of latent HIV-1 provirus in T lymphoid cells and activate virus gene expression in central memory CD4+ T cells

When effector CD4+ T cells carrying integrated HIV-1 proviruses revert back to a resting memory state, the virus can remain silent in those cells for years. Following re-exposure to the nominal antigen or in response to other stimuli (e.g. pro-inflammatory cytokines), these cells can begin to produce virus. Here we demonstrate that TLR5 stimulation induces activation of NF-kappaB and reactivate latent HIV-1 in CD4+ T lymphoid cells. Interestingly, we report also that TLR5 engagement leads to virus gene expression in quiescent central memory CD4+ T cells, a cell population recognized as a major reservoir in infected individuals. This study supports the hypothesis that translocation of microbes that can engage pathogen recognition receptors might play a dominant role in chronic immune activation seen in HIV-1-infected individuals and promote virus replication and dissemination.

Host-microbe communication within the GI tract

The gastrointestinal tract is a biologically diverse and complicated system which carries out essential physiological functions that support human health, while at the same time maintaining itself as an isolated environment to prevent infection and systemic disease. To maintain homeostasis in the gut, communication between the host and residing microbial communities must occur to identify and eliminate potential pathogens which could colonize and cause damage through aggressive pro-inflammatory responses by the mucosal immune system. To prevent such events, a number of host and bacterial-mediated mechanisms are utilized to monitor the environment and initiate appropriate immune responses to invading pathogens. An essential component of this communication process between gastrointestinal microflora and the host involves distinguishing indigenous species from pathogens through ligand-receptor interactions which lead to various signaling events in host cells. Such events generally result in the development of mucosal immunity and immunological tolerance. While these signaling pathways provide a highly effective means of communication between the gut microflora and the host, pathogens have developed mechanisms to manipulate these pathways to evade detection by the immune system to persist and cause disease. These adaptations include cell surface modifications and the expression of various virulence factors in response to different immunological and hormonal components produced by the host.

Antiflagellin antibodies recognize the autoantigens Toll-Like Receptor 5 and Pals 1-associated tight junction protein and induce monocytes activation and increased intestinal permeability in Crohn's disease

BACKGROUND AND OBJECTIVES

Bacterial flagellin is considered an important antigen in Crohn's disease (CD) as it activates innate immunity through Toll-Like Receptor 5 (TLR5) engagement and induces an elevated adaptive immune response. Little is known about the presence of an autoimmune process in CD. We aimed to identify pathogenically relevant autoantigen targets in CD.

METHODS

We screened a random peptide library with pooled sera of patients with active CD. Transepithelial flux of [3H] mannitol in T84 human intestinal epithelial cell line was used to study the epithelial barrier function. Monocyte activation was evaluated by surface expression of activation markers and by production of pro-inflammatory cytokines. Gene modulation of T84 cells exposed to antipeptide antibodies was analysed by gene array.

RESULTS

We identified a peptide that shares homology with Salmonella typhimurium flagellin and with self-antigens such as TLR5 and cell junction protein, Pals 1-associated tight junction protein. The affinity-purified antipeptide antibodies recognized the self-antigens and induced increased intestinal epithelial cell permeability. Moreover, the antibodies induced monocyte activation upon binding TLR5. Finally, in cultured intestinal cells (T84) the purified antibodies induced the modulation of clusters of proinflammatory genes similar to the one induced by the engagement of TLR5 by its natural ligand flagellin.

CONCLUSIONS

Antibodies directed against an immunodominant peptide of flagellin recognize self-antigens and are functionally active suggesting the presence of an autoimmune process that can both facilitate loss of tolerance to intestinal microflora by increasing cell permeability and amplify the innate immunity involvement through a novel mechanism of TLR5 activation.

Stimulation of human CD4⁺ T lymphocytes via TLR3, TLR5 and TLR7/8 up-regulates expression of costimulatory and modulates proliferation

The cells of innate and adaptive immunity, although activated by different ligands, engage in cross talk to ensure a successful immune outcome. Toll-like receptors (TLRs) are key components of the innate immune system and have the ability to detect microbial infection and trigger host defence responses. Otherwise, human T lymphocytes are able to produce most TLRs. Thus, we analyze the capability of some TLR ligands to modulate the function of highly-purified CD4⁺ T cells. We found that agents acting via TLRs (poly I:C, a TLR3 ligand; flagellin, a TLR5 ligand; and R848, a TLR7/8 ligand) are able to regulate the expression of costimulatory molecules both on purified antigen presenting cells and on purified T lymphocytes. Moreover, the activation mediated by TLRs determines a kinetic expression of B7-family members such as through an inhibition of T lymphocytes delayed proliferation. These findings suggest a functional role of some invading microorganisms in regulating acquired immunity.

Microbes in gastrointestinal health and disease

Most, if not all, animals coexist with a complement of prokaryotic symbionts that confer a variety of physiologic benefits. In humans, the interaction between animal and bacterial cells is especially important in the gastrointestinal tract. Technical and conceptual advances have enabled rapid progress in characterizing the taxonomic composition, metabolic capacity, and immunomodulatory activity of the human gut microbiota, allowing us to establish its role in human health and disease. The human host coevolved with a normal microbiota over millennia and developed, deployed, and optimized complex immune mechanisms that monitor and control this microbial ecosystem. These cellular mechanisms have homeostatic roles beyond the traditional concept of defense against potential pathogens, suggesting these pathways contribute directly to the well-being of the gut. During their coevolution, the bacterial microbiota has established multiple mechanisms to influence the eukaryotic host, generally in a beneficial fashion, and maintain their stable niche. The prokaryotic genomes of the human microbiota encode a spectrum of metabolic capabilities beyond that of the host genome, making the microbiota an integral component of human physiology. Gaining a fuller understanding of both partners in the normal gut-microbiota interaction may shed light on how the relationship can go awry and contribute to a spectrum of immune, inflammatory, and metabolic disorders and may reveal mechanisms by which this relationship could be manipulated toward therapeutic ends.

Antiflagellin antibodies suggest infective participation in irritable bowel syndrome pathogenesis

Irritable bowel syndrome (IBS) is a functional disorder of multifactorial origin. Recent attention has been paid to the potential role of immune activation in intestinal sensorimotor dysfunction and symptom generation in patients with IBS. The link between immune activation and IBS is further supported by the evidence that IBS may develop after an acute episode of infectious gastroenteritis, IBS-like symptoms may precede the diagnosis or accompany a period of remission of inflammatory bowel disease (IBD), and quantitative histopathologic data demonstrate the presence of low-grade mucosal immune infiltration in a large subset of patients with IBS. These data also suggest some areas of potential overlap between IBS and IBD. The present study explored the possibility that, similarly to IBD patients, IBS patients have antibodies directed against certain components of indigenous flora, such as flagellin (the primary structural component of bacterial flagella). The authors demonstrated that, compared with healthy controls, antibodies against flagellin were recognized more frequently in patients with IBS. Furthermore, these antibodies were found more frequently in postinfectious compared with unspecific IBS. In patients with Crohn's disease, antiflagellin antibodies were detected with an increased frequency and at higher concentrations than in patients with IBS. All together, these results indicated the presence of a systemic immune activation in IBS patients, characterized by specific antibodies directed against luminal bacterial antigens. Furthermore, these results support the hypothesis that a subset of IBS presents an immune activation with pathogenic features common with IBD.

Live and ultraviolet-inactivated Lactobacillus rhamnosus GG decrease flagellin-induced interleukin-8 production in Caco-2 cells

Probiotics are widely used in the treatment and prevention of gastrointestinal problems. However, in some immune-compromised populations, the administration of live microorganisms may not be appropriate. A potential alternative to live microorganisms is to inactivate them as long as the beneficial function is retained. We hypothesized that UV-inactivated Lactobacillus rhamnosus GG (LGG) could downregulate interleukin-8 (IL-8) production in intestinal epithelial cells stimulated by the pathogenic ligand, flagellin, using similar mechanisms as live LGG. Caco-2 cells were pretreated with live or UV-inactivated LGG at 10(11) colony-forming units/L and stimulated by flagellin at a dose of 500 mug/L. IL-8 production was measured by ELISA, inhibitor of kappaB (IkappaB) and ubiquitinated-IkappaB (Ub-IkappaB) expression by immunoblotting and nuclear factor (NF) kappaB localization by immunofluorescence staining. Flagellin induced a 17-fold increase in IL-8 production compared with control (P < 0.05), whereas pretreatment with either live LGG or UV-inactivated LGG resulted in 66 and 59% decreases, respectively, compared with the flagellin group (P < 0.05). Flagellin-induced NFkappaB nuclear translocation was prevented by both live and UV-inactivated LGG. Flagellin decreased IkappaB, which was reversed by either live or UV-inactivated LGG (P < 0.05). UV-inactivated LGG decreased Ub-IkappaB expression (P < 0.05), although live LGG had no effect. This study supports the concept that UV-inactivated and live LGG are equally effective in decreasing IL-8 production in the intestinal epithelium. Although the mechanism involves different pathways, both alter cytoplasmic IkappaB, thereby inhibiting NFkappaB nuclear translocation.

Antibodies to flagellin indicate reactivity to bacterial antigens in IBS patients

One of the several possible causes of irritable bowel syndrome (IBS) is thought to be low-grade mucosal inflammation. Flagellin, the primary structural component of bacterial flagellae, was shown in inflammatory bowel disease patients to activate the innate and adaptive immunity. It has not yet been conclusively established if IBS patients show reactivity to luminal antigens. In 266 patients [112 IBS, 61 Crohn's disease (CD), 50 ulcerative colitis (UC) and 43 healthy controls (HC)], we measured antibodies to flagellin (FAB, types A4-Fla2 and Fla-X), anti-Saccharomyces cerevisiae antibodies (ASCA) (both ELISA), antipancreas antibodies (PAB) and perinuclear antineutrophil cytoplasmatic antibodies (p-ANCA) (both IF). All IBS patients had normal fecal calprotectin (mean 21 microg mL(-1), SD 6.6) and fulfilled the ROME II criteria. Frequencies of antibodies in patients with IBS, CD, UC and HC, respectively, are as follows (in per cent): antibodies against A4-Fla2: 29/48/8/7; antibodies against Fla-X: 26/52/10/7; ASCA: 6/59/0/2; p-ANCA: 0/10/52/0; and PAB: 0/28/0/0. Antibodies against A4-Fla2 and Fla-X were significantly more frequent in IBS patients than in HC (P = 0.004 and P = 0.009). Antibodies to A4-Fla2 and Fla-X were significantly more frequent in IBS patients with antecedent gastroenteritis compared to non-postinfectious IBS patients (P = 0.002 and P = 0.012). In contrast to ASCA, PAB and p-ANCA, antibodies against A4-Fla2 and Fla-X were found significantly more often in IBS patients, particularly in those with postinfectious IBS, compared to HC. This observation supports the concept that immune reactivity to luminal antigens has a putative role in the development of IBS, at least in a subset of patients.

Multiple pathogenic roles of microvasculature in inflammatory bowel disease: a Jack of all trades

The etiology of Crohn's disease and ulcerative colitis, the two major forms of inflammatory bowel disease (IBD), is still largely unknown. However, it is now clear that the abnormalities underlying pathogenesis of intestinal inflammation are not restricted to those mediated by classic immune cells but also involve nonimmune cells. In particular, advances in vascular biology have outlined a central and multifaceted pathogenic role for the microcirculation in the initiation and perpetuation of IBD. The microcirculation and its endothelial lining play a crucial role in mucosal immune homeostasis through tight regulation of the nature and magnitude of leukocyte migration from the intravascular to the interstitial space. Chronically inflamed IBD microvessels display significant alterations in microvascular physiology and function compared with vessels from healthy and uninvolved IBD intestine. The investigation into human IBD has demonstrated how endothelial activation present in chronically inflamed IBD microvessels results in a functional phenotype that also includes leakiness, chemokine and cytokine expression, procoagulant activity, and angiogenesis. This review contemplates the newly uncovered contribution of intestinal microcirculation to pathogenesis and maintenance of chronic intestinal inflammation. In particular, we assess the multiple roles of the microvascular endothelium in innate immunity, leukocyte recruitment, coagulation and perfusion, and immune-driven angiogenesis in IBD.

Multiple pathogenic roles of microvasculature in inflammatory bowel disease: a Jack of all trades

The etiology of Crohn's disease and ulcerative colitis, the two major forms of inflammatory bowel disease (IBD), is still largely unknown. However, it is now clear that the abnormalities underlying pathogenesis of intestinal inflammation are not restricted to those mediated by classic immune cells but also involve nonimmune cells. In particular, advances in vascular biology have outlined a central and multifaceted pathogenic role for the microcirculation in the initiation and perpetuation of IBD. The microcirculation and its endothelial lining play a crucial role in mucosal immune homeostasis through tight regulation of the nature and magnitude of leukocyte migration from the intravascular to the interstitial space. Chronically inflamed IBD microvessels display significant alterations in microvascular physiology and function compared with vessels from healthy and uninvolved IBD intestine. The investigation into human IBD has demonstrated how endothelial activation present in chronically inflamed IBD microvessels results in a functional phenotype that also includes leakiness, chemokine and cytokine expression, procoagulant activity, and angiogenesis. This review contemplates the newly uncovered contribution of intestinal microcirculation to pathogenesis and maintenance of chronic intestinal inflammation. In particular, we assess the multiple roles of the microvascular endothelium in innate immunity, leukocyte recruitment, coagulation and perfusion, and immune-driven angiogenesis in IBD.

Arcobacter spp. possess two very short flagellins of which FlaA is essential for motility

Like Campylobacter and Helicobacter spp., Arcobacter spp. possess two flagellin genes (flaA and flaB) located adjacent to each other. The aim of this study was to characterize the flagellin proteins of Arcobacter spp., because these proteins are known virulence factors in the Epsilonproteobacteria, to which these three species belong. With the exception of Arcobacter nitrofigilis, Arcobacter flagellins are almost half the size of those in other Epsilonproteobacteria. Arcobacter flagellin proteins lack a large part of the variable central region. The low homology observed among flagellins of different Arcobacter species indicates genetic heterology between the members of this genus. Unlike in other Epsilonproteobacteria, the transcription of flagellin genes is not regulated by sigma 28- or sigma 54-dependent promoters, which suggests that transcription must be regulated in a different way in Arcobacter spp. Mutational studies revealed that only FlaA is needed for the motility of Arcobacter spp. Quantitative PCR analysis showed that transcription of flaB is higher at 30 degrees C than at 37 degrees C. Mutation of flaB had no effect on motility or on flaA transcription while mutation of flaA abolished motility and increased the transcription of flaB. These results underline that the genus Arcobacter is an unusual taxon in the epsilon subdivision of the Proteobacteria.

Deletion of TLR5 results in spontaneous colitis in mice

Activation of TLRs by bacterial products results in rapid activation of genes encoding products designed to protect the host from perturbing microbes. In the intestine, which is colonized by a large and diverse population of commensal bacteria, TLR signaling may not function in a simple on/off mode. Here, we show that the flagellin receptor TLR5 has an essential and nonredundant role in protecting the gut from enteric microbes. Mice lacking TLR5 (TLR5KO mice) developed spontaneous colitis, as assessed by well-defined clinical, serologic, and histopathologic indicators of this disorder. Compared with WT littermates, TLR5KO mice that had not yet developed robust colitis exhibited decreased intestinal expression of TLR5-regulated host defense genes despite having an increased bacterial burden in the colon. In contrast, such TLR5KO mice displayed markedly increased colonic expression of hematopoietic-derived proinflammatory cytokines, suggesting that elevated levels of bacterial products may result in activation of other TLRs that drive colitis in TLR5KO mice. In accordance, deletion of TLR4 rescued the colitis of TLR5KO mice in that mice lacking both TLR4 and TLR5 also had elevated bacterial loads in the colon but lacked immunological, histopathological, and clinical evidence of colitis. That an engineered innate immune deficiency ultimately results in spontaneous intestinal inflammation supports the notion that an innate immune deficiency might underlie some instances of inflammatory bowel disease.

Toll-like receptor polymorphisms and susceptibility to human disease

Although several lines of evidence suggest that variation in human inflammation is genetically controlled, the genes which regulate these responses are largely unknown. TLRs (Toll-like receptors) mediate recognition of microbes, regulate activation of the innate immune response and influence the formation of adaptive immunity. Cellular and molecular studies over the past several years have identified a number of common TLR polymorphisms that modify the cellular immune response and production of cytokines in vitro. In addition, human genetic studies suggest that some of these polymorphisms are associated with susceptibility to a spectrum of diseases. In this review, we summarize studies of common TLR polymorphisms and how this work is beginning to illuminate the influence of human variation on inflammation and disease susceptibility.

Flagellin-dependent and -independent inflammatory responses following infection by enteropathogenic Escherichia coli and Citrobacter rodentium

Enteropathogenic Escherichia coli (EPEC) and the murine pathogen Citrobacter rodentium belong to the attaching and effacing (A/E) family of bacterial pathogens. These noninvasive bacteria infect intestinal enterocytes using a type 3 secretion system (T3SS), leading to diarrheal disease and intestinal inflammation. While flagellin, the secreted product of the EPEC fliC gene, causes the release of interleukin 8 (IL-8) from epithelial cells, it is unclear whether A/E bacteria also trigger epithelial inflammatory responses that are FliC independent. The aims of this study were to characterize the FliC dependence or independence of epithelial inflammatory responses to direct infection by EPEC or C. rodentium. Following infection of Caco-2 intestinal epithelial cells by wild-type and DeltafliC EPEC, a rapid activation of several proinflammatory genes, including those encoding IL-8, monocyte chemoattractant protein 1, macrophage inflammatory protein 3alpha (MIP3alpha), and beta-defensin 2, occurred in a FliC-dependent manner. These responses were accompanied by mitogen-activated protein kinase activation, as well as the Toll-like receptor 5 (TLR5)-dependent activation of NF-kappaB. At later infection time points, a subset of these proinflammatory genes (IL-8 and MIP3alpha) was also induced in cells infected with DeltafliC EPEC. The nonmotile A/E pathogen C. rodentium also triggered similar innate responses through a TLR5-independent but partially NF-kappaB-dependent mechanism. Moreover, the EPEC FliC-independent responses were increased in the absence of the locus of enterocyte effacement-encoded T3SS, suggesting that translocated bacterial effectors suppress rather than cause the FliC-independent inflammatory response. Thus, we demonstrate that infection of intestinal epithelial cells by A/E pathogens can trigger an array of proinflammatory responses from epithelial cells through both FliC-dependent and -independent pathways, expanding our understanding of the innate epithelial response to infection by these pathogens.

Decreased potency of the Vibrio cholerae sheathed flagellum to trigger host innate immunity

Vibrio cholerae is a monoflagellated gram-negative bacterium that causes the severe diarrheal disease cholera. In contrast to Salmonella enterica serovar Typhimurium infection, which is accompanied by both acute diarrhea and high-level inflammation, V. cholerae infection is largely noninflammatory in human hosts. Bacterial flagella are composed of flagellin, a highly conserved protein that is also a target of the innate immune response. Because the V. cholerae flagellum is covered by a sheath, we hypothesized that it might be less prone to activation of the innate immune response than nonsheathed flagella, such as those produced by Salmonella serovar Typhimurium. Indeed, compared with Salmonella serovar Typhimurium flagella, V. cholerae flagella demonstrated significantly reduced NF-kappaB activation in A549 human pulmonary epithelial cells. However, V. cholerae flagellin monomers, FlaD and FlaC, were almost equally potent with purified FliC, a monomer derived from Salmonella serovar Typhimurium flagella, in NF-kappaB activation. Heat- and acid-induced dissociation assays showed that Salmonella serovar Typhimurium flagella disassembled far more readily into monomeric flagellins than V. cholerae flagella, suggesting that the differential levels of NF-kappaB activation by V. cholerae and Salmonella serovar Typhimurium flagella are likely attributable to the difference in their flagellin shedding. Our results suggest that monomer dissociation of V. cholerae flagella is suppressed likely due to the presence of the sheath and that this unique structural feature of V. cholerae flagella may have evolved as a strategy to evade flagellin-triggered host innate immune responses in various host species.

Salmonella flagellin induces bystander activation of splenic dendritic cells and hinders bacterial replication in vivo

Bacterial flagellin is a target of innate and adaptive immune responses during Salmonella infection. Intravenous injection of Salmonella flagellin into C57BL/6 mice induced rapid IL-6 production and increased expression of activation markers by splenic dendritic cells. CD11b(+), CD8alpha(+), and plasmacytoid dendritic cells each increased expression of CD86 and CD40 in response to flagellin stimulation, although CD11b(+) dendritic cells were more sensitive than the other subsets. In addition, flagellin caused the rapid redistribution of dendritic cells from the red pulp and marginal zone of the spleen into the T cell area of the white pulp. Purified splenic dendritic cells did not respond directly to flagellin, indicating that flagellin-mediated activation of splenic dendritic cells occurs via bystander activation. IL-6 production, increased expression of activation markers, and dendritic cell redistribution in the spleen were dependent on MyD88 expression by bone marrow-derived cells. Avoiding this innate immune response to flagellin is important for bacterial survival, because Salmonella-overexpressing recombinant flagellin was highly attenuated in vivo. These data indicate that flagellin-mediated activation of dendritic cells is rapid, mediated by bystander activation, and highly deleterious to bacterial survival.

Dissecting the genetic components of adaptation of Escherichia coli to the mouse gut

While pleiotropic adaptive mutations are thought to be central for evolution, little is known on the downstream molecular effects allowing adaptation to complex ecologically relevant environments. Here we show that Escherichia coli MG1655 adapts rapidly to the intestine of germ-free mice by single point mutations in EnvZ/OmpR two-component signal transduction system, which controls more than 100 genes. The selective advantage conferred by the mutations that modulate EnvZ/OmpR activities was the result of their independent and additive effects on flagellin expression and permeability. These results obtained in vivo thus suggest that global regulators may have evolved to coordinate activities that need to be fine-tuned simultaneously during adaptation to complex environments and that mutations in such regulators permit adjustment of the boundaries of physiological adaptation when switching between two very distinct environments.

The mammalian intestine is a privileged physiological site to study how coevolution between hosts and the trillions of bacteria present in the microbiota has shaped the genome of each partner and promoted the development of mutualistic interactions. Herein we have used germ-free mice, a simplified albeit ecologically relevant system, to analyse intestinal adaptation of a model bacterial strain, Escherichia coli MG1655. Our results show that single point mutations in the ompB master regulator confer a striking selective adaptive advantage. OmpB comprises EnvZ, a transmembrane sensor with a dual kinase/phosphatase activity, and OmpR, a transcription factor controlling more than 100 target genes. In response to environmental changes, EnvZ modulates the phosphorylation and thereby the transcriptional activity of OmpR. We further show that the selective advantage conferred by OmpB mutations is related to their additive and independent effects on genes regulating permeability and flagellin expression, two major set of genes controlled by OmpR. These results suggest that global regulators may have evolved to coordinate physiological activities necessary for adaptation to complex environments and that mutations offer a complementary genetic mechanism to adjust the scale of the physiological regulation controlled by these regulators in distinct environments.

Detectable serum flagellin and lipopolysaccharide and upregulated anti-flagellin and lipopolysaccharide immunoglobulins in human short bowel syndrome

Gut barrier dysfunction may occur in short bowel syndrome (SBS). We hypothesized that systemic exposure to flagellin and lipopolysaccharide (LPS) in SBS might regulate specific immune responses. We analyzed serial serum samples obtained from parenteral nutrition (PN)-dependent patients with SBS versus non-SBS control serum. Serum from 23 adult SBS patients was obtained at baseline and 4, 8, 12, 16, 20, and 24 wk in a trial of modified diet with or without growth hormone. Control serum was obtained from 48 healthy adults and 37 adults requiring PN during critical illness. Serum flagellin was detected by an ELISA recognizing an array of gram-negative flagellins, and LPS was detected by limulus assay. Serum flagellin- and LPS-specific immunoglobulin levels (IgM, IgA, and IgG) were determined by ELISA. Serum flagellin and LPS were undetectable in control subjects. In contrast, serum flagellin, LPS, or both were detected in 14 SBS patients (61%) during one or more time points [flagellin alone, 5/23 (22%); LPS alone, 6/23 (26%); or flagellin + LPS, 3/23 (13%)]. Flagellin-specific serum IgM, IgA, and IgG levels were markedly increased in SBS patients compared with both control populations and remained elevated during the 6-mo study period. LPS-specific IgA was significantly higher in SBS patients compared with healthy controls; LPS-specific IgM, IgA, and IgG levels each decreased over time in association with PN weaning. We conclude that adults with PN-dependent SBS are systemically exposed to flagellin and LPS, presumably from the gut lumen. This likely regulates innate and adaptive immune responses to these specific bacterial products.

Functional characterization of chicken TLR5 reveals species-specific recognition of flagellin

Mammalian Toll-like receptor 5 (TLR5) senses flagellin of several bacterial species and activates the innate immune system. The avian TLR repertoire exhibits considerable functional diversity compared to mammalian TLRs and evidence of a functional TLR5 in the avian species is lacking. In the present study we cloned and successfully expressed chicken TLR5 (chTLR5) in HeLa cells, as indicated by laser confocal microscopy. Infection of chTLR5 transfected cells with Salmonella enterica serovar Enteritidis activated NF-kappaB in a dose- and flagellin-dependent fashion. Similar NF-kappaB activation was observed with recombinant bacterial flagellin. Targeted mutagenesis of the proline residue at position 737 in the chTLR5-TIR domain was detrimental to chTLR5 function, confirming that the observed effects were conferred via chTLR5 and the MyD88 signaling pathway. Comparison of human, mouse and chicken TLR5 activation by flagellin of S. enterica serovar Typhimurium revealed that chTLR5 consistently yielded stronger responses than human but not mouse TLR5. This species-specific reactivity was not observed with flagellin of serovar Enteritidis. The species-specific TLR5 response was nullified after targeted mutagenesis of a single amino acid (Q89A) in serovar Typhimurium flagellin, while L415A and N100A substitutions had no effect. These results show that chickens express a functional TLR5 albeit with different flagellin sensing qualities compared to human TLR5. The finding that single amino acid substitutions in bacterial flagellin can alter the species-specific TLR5 response may influence the host range and susceptibility of infection.