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

A multiepitopic theoretical fusion construct based on in-silico epitope screening of known vaccine candidates for protection against wide range of enterobacterial pathogens

Enterobacterial pathogens that have acquired antibiotic resistance genes are a leading cause of community and hospital acquired infections. In such a situation vaccination is considered as a better option to prevent such infections. In the current study reverse vaccinology approach has been used to select peptides from already known immunogenic proteins to design a chimeric construct. We selected Yersiniabactin receptor of Escherichia coli UMN026 and Flagellin of Stenotrophomonas maltophila. B-cell linear epitopes were predicted using Bepipred prediction tool. Peptide binding with reference sets of 27 alleles of MHC class I and class II was also analyzed. The predicted peptides-MHC complexes were further validated using simulation dynamics. The in-silico construction of chimera was done by restriction mapping and codon optimization. Chimera was evaluated using the immunoinformatic approach as done for the selected proteins. From the 673 amino acids of FyuA protein, a region from 1 to 492 was selected for containing more linear epitopes and the processing scores obtained were significant for MHC class I and class II binding. Similarly, from Flagellin, a region between 60 and 328 amino acids was selected and the peptides present in the selected region showed lower percentile ranks for binding with MHC molecules. The simulation studies validated the predictions of peptide-MHC complexes. The selected gene fragments accommodating maximum part of these peptides were used to design a chimaeric construct of 2454 bp. From the immunoinformatic analysis, the chimera was found to be more immunogenic in terms of increased number of B-cell and T-cell epitopes along with increased coverage of global populations with allelic variability.

Intraluminal Flagellin Differentially Contributes to Gut Dysbiosis and Systemic Inflammation following Burn Injury

Burn injury is associated with a loss of gut barrier function, resulting in systemic dissemination of gut-derived bacteria and their products. The bacterial protein and TLR5 agonist, flagellin, induces non-specific innate immune responses. Because we detected flagellin in the serum of burn patients, we investigated whether gut-derived flagellin was a primary or secondary contributor to intestinal dysfunction and systemic inflammation following burn injury. The apical surface of polarized human intestinal epithelial cells (IECs), Caco-2BBe, were exposed to 50 or 500 ng of purified flagellin and 1 x 10⁵ of an intestinal E. coli (EC) isolate as follows: 1) flagellin added 30 min prior to EC, 2) flagellin and EC added simultaneously, or 3) EC added 30 min prior to flagellin. Our results showed that luminal flagellin and EC modulated each other's biological actions, which influenced their ability to induce basolateral secretion of inflammatory cytokines and subsequent translocation of bacteria and their products. A low dose of flagellin accompanied by an enteric EC in the lumen, tempered inflammation in a dose- and time-dependent manner. However, higher doses of flagellin acted synergistically with EC to induce both intestinal and systemic inflammation that compromised barrier integrity, increasing systemic inflammation following burn injury, a process we have termed flagellemia. In a murine model of burn injury we found that oral gavage of flagellin (1 μg/mouse) significantly affected the gut microbiome after burn injury. In these mice, flagellin disseminated out of the intestine into the serum and to distal organs (mesenteric lymph nodes and lungs) where it induced secretion of monocyte chemoattractant protein (MCP-1) and CXCL1/KC (mouse equivalent of human IL-8) at 24 and 48h post-burn. Our results illustrated that gut-derived flagellin alone or accompanied by a non-pathogenic enteric EC strain can function as an initiator of luminal and systemic inflammation following burn injury.

Functional analysis of bovine TLR5 and association with IgA responses of cattle following systemic immunisation with H7 flagella

Flagellin subunits are important inducers of host immune responses through activation of TLR5 when extracellular and the inflammasome if cytosolic. Our previous work demonstrated that systemic immunization of cattle with flagella generates systemic and mucosal IgA responses. The IgA response in mice is TLR5-dependent and TLR5 can impact on the general magnitude of the adaptive response. However, due to sequence differences between bovine and human/murine TLR5 sequences, it is not clear whether bovine TLR5 (bTLR5) is able to stimulate an inflammatory response following interaction with flagellin. To address this we have examined the innate responses of both human and bovine cells containing bTLR5 to H7 flagellin from E. coli O157:H7. Both HEK293 (human origin) and embryonic bovine lung (EBL) cells transfected with bTLR5 responded to addition of H7 flagellin compared to non-transfected controls. Responses were significantly reduced when mutations were introduced into the TLR5-binding regions of H7 flagellin, including an R90T substitution. In bovine primary macrophages, flagellin-stimulated CXCL8 mRNA and secreted protein levels were significantly reduced when TLR5 transcript levels were suppressed by specific siRNAs and stimulation was reduced with the R90T-H7 variant. While these results indicate that the bTLR5 sequence produces a functional flagellin-recognition receptor, cattle immunized with R90T-H7 flagella also demonstrated systemic IgA responses to the flagellin in comparison to adjuvant only controls. This presumably either reflects our findings that R90T-H7 still activates bTLR5, albeit with reduced efficiency compared to WT H7 flagellin, or that other flagellin recognition pathways may play a role in this mucosal response.

Toll-like receptor 4 stimulation initiates an inflammatory response that decreases cardiomyocyte contractility

Toll-like receptors (TLRs) have been identified as primary innate immune receptors for the recognition of pathogen-associated molecular patterns by immune cells, initiating a primary response toward invading pathogens and recruitment of the adaptive immune response. TLRs, especially Toll-like receptor 4 (TLR4), can also be stimulated by host-derived molecules and are expressed in the cardiovascular system, thus acting as a possible key link between cardiovascular diseases and the immune system. TLR4 is involved in the acute myocardial dysfunction caused by septic shock and myocardial ischemia. We used wild-type (WT) mice, TLR4-deficient (TLR4-knockout [ko]) mice, and chimeras that underwent myeloablative bone marrow transplantation to dissociate between TLR4 expression in the heart (TLR4-ko/WT) and the immunohematopoietic system (WT/TLR4-ko). Following lipopolysaccharide (LPS) challenge (septic shock model) or coronary artery ligation, myocardial ischemia (MI) model, we found WT/TLR4-ko mice challenged with LPS or MI displayed reduced cardiac function, increased myocardial levels of interleukin-1β and tumor necrosis factor-α, and upregulation of mRNA encoding TLR4 prior to myocardial leukocyte infiltration. The cardiac function of TLR4-ko or WT/TLR4-ko mice was less affected by LPS and demonstrated reduced suppression by MI compared with WT. These results suggest that TLR4 expressed in the cardiomyocytes plays a key role in this acute phenomenon.

Stenotrophomonas maltophilia flagellin induces a compartmentalized innate immune response in mouse lung

Intranasal (i.n.) instillation of different amounts of purified Stenotrophomonas maltophilia flagellin preparation (1, 5 and 15 microg) in BALB/c mice stimulated a transient innate immune response in the lungs. This was characterized by infiltration of different kinds of leukocytes (neutrophils, monocytes and lymphocytes), production of various inflammatory mediators (tumour necrosis factor alpha, interleukin 1 beta, interleukin 10, nitric oxide, myeloperoxidase and malondialdehyde) and activated alveolar macrophages (AMs). The proinflammatory cytokine production resulted in accumulation of activated neutrophils and macrophages and their products following immunostimulation with flagellin. The activation of AMs by flagellin was non-specific as AMs obtained from flagellin-treated animals, even after 4 h of exposure, were found to engulf and kill S. maltophilia and Staphylococcus aureus efficiently compared to macrophages obtained from control animals. i.n. instillation of 5 microg flagellin resulted in the generation of an effective innate immunity compared to other flagellin doses. Our data provide strong evidence that S. maltophilia flagellin stimulates innate immunity in mouse lung.

Recent Developments in the Identification of Novel Therapeutic Targets for the Treatment of Patients with Sepsis and Septic Shock

The overall mortality in patients with sepsis is approximately 30%; this figure increases to 50% or higher in patients with septic shock, and sepsis continues to be seen as a major clinical challenge. The recent success of several important trials has fuelled interest in further therapeutic developments. Here, I review the many different strategies that are being investigated, focusing in particular on those that are in late pre-clinical or early clinical development. These can be broadly divided into three groups: strategies aimed at bacterial targets, strategies aimed at disorders of immune regulation in the host, and finally, other novel strategies based on modifying host response; which, if any, of these will prove successful in large clinical trials is unknown. Nevertheless, the fact that sepsis has finally proved tractable as a target for new drug development lends support to those who believe that at least some of the compounds identified in this paper will prove to have clinical benefit.

Immune modulators with defined molecular targets: cornerstone to optimize rational vaccine design

Vaccination remains the most valuable tool for preventing infectious diseases. However, the performance of many existing vaccines should be improved and there are diseases for which vaccines are still not available. The use of well-defined antigens for the generation of subunit vaccines has led to products with an improved safety profile. However, purified antigens are usually poorly immunogenic, making essential the use of adjuvants. Despite the fact that adjuvants have been used to increase the immunogenicity of vaccines for more than 70 years, only a handful has been licensed for human use (e.g., aluminium salts, the micro-fluidized squalene-in-water emulsion MF59 and monophosphoryl lipid A). Thus, the development of new adjuvants which are able to promote broad and sustained immune responses at systemic and mucosal levels still remains as a major challenge in vaccinology. Recent advances in our understanding of the immune system have facilitated the identification of new biological targets for screening programs aimed at the discovery of novel immune stimulators. This resulted in the identification of new candidate adjuvants, which made possible the modulation of the immune responses elicited according to specific needs. A number of promising adjuvants which are currently under preclinical or clinical development will be described in this chapter.

The interleukin-22/STAT3 pathway potentiates expression of inducible nitric-oxide synthase in human colon carcinoma cells

Inducible nitric-oxide synthase (iNOS) has been identified as a marker and mediator of disease in human colonic inflammation and carcinogenesis. Accordingly, identification of mediators that trigger iNOS in colon carcinoma/epithelial cells is an important topic of current research. Here we demonstrate that interleukin (IL)-22, a newly described member of the IL-10 cytokine family, potently synergizes with interferon (IFN)-gamma for iNOS expression in human DLD-1 colon carcinoma cells. Detection of both IL-22 receptor chains and STAT3 phosphorylation proved robust IL-22 responsiveness of these cells. Short interfering RNA technology identified STAT3 as being crucial for up-regulation of iNOS. Compared with IFNgamma, STAT1 phosphorylation by IL-22 was insufficient. IL-22 did not stabilize IL-1beta/tumor necrosis factor-alpha/IFNgamma-induced iNOS mRNA. IL-22 also failed to amplify expression of the prototypic IFNgamma-inducible parameters IL-18-binding protein and CXCL-10, indicating that IL-22 is not a general amplifier of IFNgamma functions. This assumption is furthermore supported by the observation that IL-22 was unable to enhance cellular activation of the pro-inflammatory transcription factor nuclear factor-kappaB. In contrast, IL-22 increased iNOS promoter activation as detected by using DLD-1 cells stably transfected with a corresponding 16-kb promoter construct (pNOS2(16)-Luc). IL-22 likewise enhanced iNOS in Caco-2 colon carcinoma cells. With IL-22 we introduce a novel potent determinant of iNOS expression in human colon carcinoma/epithelial cells. Considering the eminent functions of STAT3 and iNOS in inflammation and carcinogenesis, IL-22 may represent a novel target for immunotherapeutic intervention.

Interleukin-18 secretion and Th1-like cytokine responses in human peripheral blood mononuclear cells under the influence of the toll-like receptor-5 ligand flagellin

Flagellin is the major protein component of the flagella from motile bacteria and was identified as the ligand for toll-like receptor (TLR)-5. Whereas its effects on epithelial cells have been studied in detail, activation of human peripheral blood mononuclear cells (PBMC) by flagellin is characterized only partially. By using the recombinant protein of Salmonella muenchen we confirm the proinflammatory nature of flagellin as detected by nuclear factor-kappaB activation and interleukin (IL)-8 production. Aim of the current study was to elucidate in PBMC effects of flagellin on IL-18 and Th1-like cytokine responses. We report that flagellin in pathophysiologically relevant concentrations augmented release of mature IL-18 by THP-1 monocytes, PBMC, and whole blood stimulated with nigericin or by ATP-mediated P2X7 purinergic receptor activation. Further key functions of the IL-18/IL-12/interferon-gamma (IFNgamma) pathway were upregulated by flagellin. Flagellin synergized with IL-12 for production of IFN-gamma and augmented secretion of interferon-inducible protein-10, a CXC-chemokine that is key to the generation of Th1-type responses. In contrast, neither IL-18-binding protein nor IL-4 was affected. Taken together, the present data demonstrate for the first time that flagellin at concentrations that are detectable in the blood compartment during sepsis efficiently enhances the IL-18/IL-12/IFNgamma pathway and thus Th1-like cytokine responses in PBMC.

The white pulp in the setting of the septic spleen caused by different bacteria: a comparative morphometric study

In the past little attention has been paid to histomorphologic changes accompanying the phenomenon of the septic spleen, thus indirectly reinforcing the old axiom that the spleen is an organ of mystery. It is especially noteworthy that the relationship between different causative bacteria and histopathologic abnormalities of the white pulp has not been investigated. In this study morphometric analysis was performed on the white pulp of 30 spleens obtained at autopsy from individuals with premortal sepsis. A strictly defined age- and sex-matched control group was analyzed for statistical comparison. Our findings demonstrate a significant depletion of B- and T-areas in the septic spleen, accompanied by a significant tendency towards reactive germinal center hyperplasia regardless of the type of bacteria responsible. However, depletion of splenic B-areas was shown to be significantly pronounced in the setting of premortal enterococcemia in comparison with a panel of gram-negative flagellated bacteria. It is felt that certain bacterial virulence factors (e.g. flagellation and/or structural components of the cell wall) might be pathogenetically involved in the observed changes, reflecting a partially different activation of splenic lymphocytes in the setting of the septic spleen.

Flagellin enhances NK cell proliferation and activation directly and through dendritic cell-NK cell interactions

Flagellin, the principal component of bacterial flagella, is a ligand for Toll-like receptor 5 (TLR5) or TLR11 and contributes to systemic inflammation during sepsis through activation of dendritic cells (DCs) and other cells of the innate immune system. Here, we report that flagellin and the TLR4 ligand, lipopolysaccharide (LPS), induced phenotypic and functional maturation of murine bone marrow-derived DCs and enhanced DC accumulation in the draining popliteal lymph node following their footpad injection. It is interesting that flagellin injection enhanced myeloid (CD8alpha(-1)) and plasmacytoid (plasmacytoid DC antigen(+) B220(+)) DC subsets, whereas LPS only increased myeloid DCs in the draining lymph node. In addition, the footpad injection of flagellin or LPS induced significant CD4(+) T cell activation in the draining popliteal lymph node, as judged by increased CD69 or CD25 expression. We illustrate, for the first time, that flagellin also increases natural killer (NK) cell number and activation status in the draining lymph node after footpad injection. Using coculture with enriched carboxy-fluorescein diacetate succinimidyl ester-labeled NK cells, flagellin-treated DCs induce significant NK cell proliferation and activation. In fact, direct treatment of NK cells with flagellin induces a greater increase in cell proliferation than treatment with LPS. In contrast, flagellin treatment of NK cells was not a strong inducer of interferon-gamma (IFN-gamma) production, indicating that NK cell proliferation and IFN-gamma production may be regulated differentially. These data suggest that flagellin is a capable maturation agent for murine myeloid-derived DCs, and flagellin-activated DCs and flagellin itself are potent inducers of NK cell proliferation.

Management of severe sepsis and septic shock

PURPOSE OF REVIEW

Severe sepsis and septic shock are common and deadly conditions for which the epidemiology, pathogenesis, and management continue to evolve. Recent publications (2003 and early 2004) have been systematically reviewed for important new original research and scholarly reviews, with an emphasis on clinical advances in adults.

RECENT FINDINGS

Important new epidemiologic studies establish the increasing frequency (nearly 9% per year) and falling mortality rates associated with sepsis. Sepsis definitions were reviewed by a group of experts, and the principal features of the 1991 consensus conference definitions were supported, with a new framework for evaluation of sepsis proposed. New research and thoughtful reviews continue to elucidate the pathogenesis of sepsis, with emphasis on innate immunity and time-based changes in immune status, varying from hyperreactive immunity and inflammation to immune depression with enhanced risk for nosocomial infections. A comprehensive evidence-based approach to the management of severe sepsis is presented in an important document developed by representatives from many critical care and infectious disease societies. Management includes early targeted resuscitation, broad empiric antibiotic coverage and source control, effective shock evaluation and treatment, adjuvant therapy with recombinant human activated protein C and moderate-dose hydrocortisone in selected patients, and comprehensive supportive care. Recently published multicenter clinical trials for novel agents have been disappointing, particularly for a nitric oxide synthase inhibitor that effectively supported blood pressure but increased mortality.

SUMMARY

The works reviewed reflect the advances in the care of patients with sepsis.

Pseudomonas aeruginosaFlagellin and Alginate Elicit Very Distinct Gene Expression Patterns in Airway Epithelial Cells: Implications for Cystic Fibrosis Disease

Infection with the opportunistic pathogen Pseudomonas aeruginosa remains a major health concern. Two P. aeruginosa phenotypes relevant in human disease include motility and mucoidy. Motility is characterized by the presence of flagella and is essential in the establishment of acute infections, while mucoidy, defined by the production of the exopolysaccharide alginate, is critical in the development of chronic infections, such as the infections seen in cystic fibrosis patients. Indeed, chronic infection of the lung by mucoid P. aeruginosa is a major cause of morbidity and mortality in cystic fibrosis patients. We have used Calu-3 human airway epithelial cells to investigate global responses to infection with motile and mucoid P. aeruginosa. The response of airway epithelial cells to exposure to P. aeruginosa motile strains is characterized by a specific increase in gene expression in pathways controlling inflammation and host defense. By contrast, the response of airway epithelia to the stimuli presented by mucoid P. aeruginosa is not proinflammatory and, hence, may not be conducive to the effective elimination of the pathogen. The pattern of gene expression directed by flagellin, but not alginate, includes innate host defense genes, proinflammatory cytokines, and chemokines. By contrast, infection with alginate-producing P. aeruginosa results in an overall attenuation of host responses and an antiapoptotic effect.

Enteroaggregative Escherichia coli flagellin-induced interleukin-8 secretion requires Toll-like receptor 5-dependent p38 MAP kinase activation

Enteroaggregative Escherichia coli (EAEC) is an emerging enteric pathogen that causes acute and chronic diarrhoea in a number of clinical settings. EAEC diarrhoea involves bacterial aggregation, adherence to intestinal epithelial cells and elaboration of several toxigenic bacterial mediators. Flagellin (FliC-EAEC), a major bacterial surface protein of EAEC, causes interleukin (IL)-8 release from several epithelial cell lines. The host response to flagellins from E. coli and several other bacteria is mediated by Toll-like receptor 5 (TLR5), which signals through nuclear factor kappa B (NF-kappaB) to induce transcription of pro-inflammatory cytokines. p38 mitogen-activating protein (MAP) kinase (MAPK) is a member of a family of stress-related kinases that influences a diverse range of cellular functions including host inflammatory responses to microbial products. We studied the role of p38 MAPK in FliC-EAEC-induced IL-8 secretion from Caco-2 human intestinal epithelial cells and THP-1 human monocytic cells. We found that IL-8 secretion from both cell types is dependent on p38 MAPK, which is phospho-activated in response to FliC-EAEC. The role of TLR5 in p38 MAPK-dependent IL-8 secretion was verified in HEp-2 cells transiently transfected with a TLR5 expression construct. Activation of interleukin-1 receptor-associated kinase (IRAK) was also observed in Caco-2 and TLR5-transfected HEp-2 cells after exposure to FliC-EAEC. Finally, we demonstrated that pharmacological inhibition of p38 MAPK reduced IL-8 transcription and mRNA levels, but did not affect NF-kappaB activation. Collectively, our results suggest that TLR5 mediates p38 MAPK-dependent IL-8 secretion from epithelial and monocytic cells incubated with FliC-EAEC, and that this effect requires IL-8 promoter activation independent of NF-kappaB nuclear migration.