Flagellin promotes myeloid differentiation factor 88-dependent development of Th2-type response

Salmonella flagellins are potent adjuvants for intranasally administered whole inactivated influenza vaccine

Bacterial flagellins are potent inducers of innate immune responses in the mouse lung because they bind to TLR5 expressed on the apical surfaces of airway epithelial cells. TLR engagement leads to the initiation of a signaling cascade that results in a pro-inflammatory response with subsequent up-regulation of several cytokines and leads to adaptive immune responses. We examined the ability of two soluble flagellins, a monomeric flagellin expressed in Escherichia coli and a highly purified polymeric flagellin directly isolated from Salmonella, to enhance the efficacy of influenza vaccines in mice. Here we demonstrate that both flagellins co-administered intranasally with inactivated A/PR/8/34 (PR8) virus induced robust increases of systemic influenza-specific IgA and IgG titers and resulted in a more comprehensive humoral response as indicated by the increase of IgG2a and IgG2b subclass responses. Groups immunized with the adjuvanted vaccines were fully protected against high dose lethal challenge by homologous virus whereas inactivated PR8 alone conferred only partial protection. Finally we show that shortly after immunization the adjuvanted vaccines induced a dramatic increase in pro-inflammatory cytokines in the lung, resulting in extensive lung infiltration by granulocytes and monocytes/macrophages. Our results reveal a promising perspective for the use of both soluble monomeric and polymeric flagellin as mucosal vaccine adjuvants to improve protection against influenza epidemics as well as a range of other infectious diseases.

Der p2 activates airway smooth muscle cells in a TLR2/MyD88-dependent manner to induce an inflammatory response

Der p2 is a major allergen from Dermatophagoides pteronyssinus, the main species of house dust mite and a major inducer of asthma, inducing harmful respiratory inflammatory responses by activating cells in the respiratory tract, leading to an unstable status. We hypothesize that Der p2 may induce local inflammatory responses by directly affecting airway smooth muscle (ASM) cells. In this study, we demonstrated that Der p2 raised nuclear factor-kappa B (NF-kappaB) and extracellular signal-regulated kinase (ERK) 1/2 activation and induced a high level of proinflammatory cytokines expression in primary cultured ASM cells. Der p2 activated the MyD88 signaling pathway through toll-like receptor (TLR) 2, not through TLR4. Notably, Der p2 stimulated ASM cells to increase phosphorylation of ERK1/2 and expression of c-Fos, which were also important in the T helper type 2 (Th2) immune response. These results suggest that Der p2 induces asthma through the MyD88 signaling pathway in respiratory tissue.

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

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

TLR5 ligation by flagellin converts tolerogenic dendritic cells into activating antigen-presenting cells that preferentially induce T-helper 1 responses

Dendritic cells (DCs) differentiated in the presence of IL-10 preferentially induce regulatory T-cells and tolerance. Whether the tolerogenic properties displayed by these DCs (Tol-DCs) can be overcome has not been fully explored. Here we show for the first time that Tol-DCs express higher levels of TLR5 mRNA, but not TLR4 or TLR9 mRNA relative to DCs differentiated with GM-CSF and IL-4 (BM-DCs). In response to flagellin, a natural TLR-5 ligand, Tol-DCs produced IL-12 but not IL-10. Unlike Tol-DCs stimulated with LPS, which produce high levels of IL-10 and fail to generate a cognate inflammatory response in CD4(+) T-cells, flagellin-stimulated Tol-DCs promoted the differentiation of CD4(+) T cells with a T-helper 1 phenotype. The divergent T-cell outcomes induced by Tol-DCs in response to different TLR-ligands highlights not only their plasticity, but also points to TLR5 ligation as a potential strategy to overcome tolerance in environments that are otherwise conducive to immune unresponsiveness.

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.

Toll-like receptor 5- and lymphotoxin beta receptor-dependent epithelial Ccl20 expression involves the same NF-kappaB binding site but distinct NF-kappaB pathways and dynamics

Canonical and alternative NF-kappaB pathways depend on distinct NF-kappaB members and regulate expression of different gene subset in inflammatory and steady state conditions, respectively. In intestinal epithelial cells, both pathways control the transcription of the gene coding the CCL20 chemokine. Lymphotoxin beta receptor (LTbetaR) mediates long lasting CCL20 expression whereas Toll-like receptor 5 (TLR5) signals promote inducible and transient activation. Here, we investigated whether the regulation of ccl20 expression involves different promoter sites and NF-kappaB molecules in response to TLR5 and LTbetaR stimulation. In epithelial cells, both stimulation required the same promoter regions, especially the NF-kappaB binding site but involved different NF-kappaB isoforms: p65/p50 and p52/RelB, for TLR5 and LTbetaR-dependent activation, respectively. The dynamic of activation and interaction with CCL20-specific NF-kappaB site correlated with gene transcription. Similar Ccl20 expression and NF-kappaB activation was found in the small intestine of mice stimulated with TLR5 and LTbetaR agonists. In summary, different NF-kappaB pathways modulate CCL20 transcription by operating on the same NF-kappaB binding site in the same cell type.

Porcine Toll-like receptors: the front line of pathogen monitoring and possible implications for disease resistance

Toll-like receptors (TLRs) are the most famous pattern-recognition receptors (PRRs); they monitor pathogen-associated molecular patterns and play a critical role in activation of the immune system against infection. TLR gene mutations may affect the gene products in terms of their ligand-binding ability or their signal transduction ability after ligand binding; such changes have a great influence on pathogen monitoring and disease resistance. Thirteen mammalian TLRs have been identified, and genes corresponding to all 10 TLR genes identified in humans have been fully cloned in pigs. Porcine TLR gene coding sequences possess a large number of nonsynonymous single nucleotide polymorphisms (SNPs). They are concentrated in ectodomains, and may increase the variability of pathogen recognition in pig populations. We summarize the current knowledge of TLR molecules in mammals and livestock (particularly pigs) and speculate on the relationship between SNPs in porcine TLRs and their application to vaccine design and disease-resistance breeding.

Induction of adaptive immunity by flagellin does not require robust activation of innate immunity

The ability of TLR agonists to promote adaptive immune responses is attributed to their ability to robustly activate innate immunity. However, it has been observed that, for adjuvants in actual use in research and vaccination, TLR signaling is dispensable for generating humoral immunity. Here, we examined the role of TLR5 and MyD88 in promoting innate and humoral immunity to flagellin using a prime/boost immunization regimen. We observed that eliminating TLR5 greatly reduced flagellin-induced cytokine production, except for IL-18, and ablated DC maturation but did not significantly impact flagellin's ability to promote humoral immunity. Elimination of MyD88, which will ablate signaling through TLR and IL-1beta/IL-18 generated by Nod-like receptors, reduced, but did not eliminate flagellin's promotion of humoral immunity. In contrast, loss of the innate immune receptor for profilin-like protein (PLP), TLR11, greatly reduced the ability of PLP to elicit humoral immunity. Together, these results indicate that, firstly, the degree of innate immune activation induced by TLR agonists may be in great excess of that needed to promote humoral immunity and, secondly, there is considerable redundancy in mechanisms that promote the humoral immune response upon innate immune recognition of flagellin. Thus, it should be possible to design innate immune activators that are highly effective vaccine adjuvants yet avoid the adverse events associated with systemic TLR activation.

Toll-like receptors in CNS parasitic infections

Parasite infections in the central nervous system (CNS) are a major cause of morbidity and mortality worldwide, second only to HIV infection. Finding appropriate therapeutic measures to control CNS parasite infections requires an understanding of the tissue-specific host response. CNS parasitic diseases are invariably associated with persistent T-helper 1 (Th1) cytokine-dependent proinflammatory responses. Although type 1 cytokine-dependent proinflammatory responses are essential to control several types of parasite infections, their persistent production contributes to the development of neuropathology with severe consequences. A family of proteins called Toll-like receptors (TLRs) plays a pivotal role in the induction of inflammatory cytokines during infections and tissue injury. Accumulating evidence indicates that in several CNS parasitic infections such as toxoplasmosis and sleeping sickness, host responses mediated through TLRs contribute to parasite clearance and host survival. However, TLR-mediated responses can also contribute to disease severity, as exemplified in cerebral malaria, neurocysticercosis and river blindness. Thus, TLRs influence the immunopathogenesis of CNS parasitic infections by mechanisms that can either benefit the host or further contribute to CNS pathology. This chapter discusses the immunopathogenesis of parasitic infections in the CNS and the role of TLRs in this process.

Expression and functional analysis of toll-like receptors of peripheral blood cells in asthmatic patients: implication for immunopathological mechanism in asthma

BACKGROUND

We investigated the expression profile of toll-like receptors (TLRs) and TLR ligand-activated production profile of asthma-related inflammatory cytokines in asthmatic patients. The expression of TLR1-8 on monocytes, CD4+ T helper lymphocytes, CD8+ T cytotoxic lymphocytes, CD19+ B lymphocytes, and dendritic cells, and ex vivo production of cytokines from peripheral blood mononuclear cells activated by TLR ligands were measured by flow cytometry.

DISCUSSION

Ex vivo productions of TNF-alpha, IL-10, and IL-1beta by TLR4 and TLR5 ligand LPS and flagellin were significantly lower in asthmatic patients (all P < 0.05). Expression of TLR4 and TLR5 was also found to be significantly lower in asthmatic patients when compared to that of control subjects (all P < 0.05). Therefore, the decreased activation of TLR4 and TLR5 in asthmatic patients might contribute to the immunopathological mechanisms of asthma by reducing the release of Th1 and anti-inflammatory cytokines.

Effects of flagellin on the functions of follicle-associated epithelium

Bacterial flagellin activates innate immune responses by signaling through Toll-like receptor 5 and is a potential vaccine adjuvant. Mucosal lymphoid follicles, inductive sites for adaptive mucosal immune responses, are covered by a follicle-associated epithelium (FAE) specialized for the uptake of antigens. This study demonstrates that mucosal application of Salmonella dublin flagellin enhanced transepithelial transport of microparticles by the FAE of mouse Peyer's patches in vivo. Flagellin also induced rapid, matrix metalloproteinase-dependent migration of subepithelial dendritic cells (DCs) into the FAE, better positioning DCs for antigen capture. These innate responses to flagellin enhance FAE functions and may promote adaptive immune responses in the mucosa.

Incorporation of membrane-anchored flagellin into influenza virus-like particles enhances the breadth of immune responses

We have designed a membrane-anchored form of the Toll-like receptor 5 ligand flagellin, the major proinflammatory determinant of enteropathogenic Salmonella, which was found to be glycosylated and expressed on cell surfaces. A chimeric influenza virus-like particle (cVLP) vaccine candidate containing A/PR8/34 (H(1)N(1)) hemagglutinin (HA), matrix protein (M1), and the modified flagellin as a molecular adjuvant was produced. The immunogenicity, including the serum antibody levels and cellular immune responses, and the protective efficacy against homologous and heterologous live virus challenge of the resulting VLPs were tested after intramuscular administration in a mouse model. The results demonstrated that flagellin-containing VLPs elicited higher specific immunoglobulin G (IgG) responses than standard HA and M1 VLPs, indicating the adjuvant effect of flagellin. Enhanced IgG2a and IgG2b but not IgG1 responses were observed with flagellin-containing VLPs, illuminating the activation of Th1 class immunity. The adjuvant effects of flagellin were also reflected by enhanced specific cellular responses revealed by the secretion of cytokines by freshly isolated splenocyte cultures when stimulated with pools of major histocompatibility complex class I or II peptides. When immunized mice were challenged with homologous live PR8 virus, complete protection was observed for both the standard and cVLP groups. However, when a heterosubtypic A/Philippines (H(3)N(2)) virus was used for challenge, all of the standard VLP group lost at least 25% of body weight, reaching the experimental endpoint. In contrast, for the cVLP group, 67% of mice survived the challenge infection. These results reveal that cVLPs designed by incorporating flagellin as a membrane-anchored adjuvant induce enhanced cross-protective heterosubtypic immune responses. They also indicate that such cVLP vaccines are a promising new approach for protection against pandemic influenza viruses.

Toll-like receptor ligand activation of murine bone marrow-derived dendritic cells

Dendritic cells (DCs) are required for the initiation of primary immune responses. The pattern of Toll-like receptor (TLR) expression on various subsets of these cells has been shown to differ, suggestive of distinct roles in influencing immune responses. We have examined here the responses of immature DCs derived from murine bone marrow (BMDCs) to a range of TLR ligands. BMDCs cultured for 6 days in the presence of granulocyte-macrophage colony-stimulating factor were stimulated for 24 hr with ligands to TLR1-2 [Pam(3)Cys-Ser-(Lys)(4) (PAM)], TLR2-6 (macrophage-activating lipopeptide-2 (MALP-2); zymosan or peptidoglycan (PG)], TLR3 (polyinosinic-polycytidylic acid), TLR4 [lipopolysaccharide R515 (LPS)], TLR5 (flagellin), TLR7 (polyuridylic acid) and TLR9 [CpG ODN2395 (CpG)]. DC activation was monitored using membrane marker expression and analysis of culture supernatants for cytokine/chemokine release. Ligands to TLR3 and TLR7 failed to activate BMDCs. All other TLR ligands caused elevated expression of membrane markers. PAM, MALP-2 and LPS induced high-level expression of proinflammatory cytokines and chemokines. Treatment with CpG was associated with a preferential type 1 cytokine and chemokine profile. Zymosan and PG were proinflammatory but also skewed towards a type 2 pattern of cytokines and chemokines. In contrast, flagellin did not cause marked secretion by BMDCs of cytokines or chemokines. These data for BMDCs are largely consistent with the reported TLR repertoire of freshly isolated murine Langerhans cells. In addition, murine BMDCs show selective responses to TLR ligands with respect to general activation, with differentiated cytokine patterns suggestive of potential priming for divergent immune responses.

Interactions between epithelial cells and dendritic cells in airway immune responses: lessons from allergic airway disease

Micro-organisms constantly invade the human body and may form a threat to our health. Traditionally, concepts of defence mechanisms have included a protective outer layer of epithelia and a vigilant immune system searching for areas where the integrity of the outer layer may be compromised. Instead of considering these elements as two independent mechanisms, we should be treating them as a single integrated system. This review will present and discuss the role of local immune-competent cells and local epithelia in the recognition of potential pathogens and how the interaction between the two components may affect the initiation of the airway immune response. A concept emerges where airway mucosal dendritic cells act as integrators of both immunostimulatory and immunosuppressive signals that act within actively-involved mucosal tissue.

Deletion of flagellin's hypervariable region abrogates antibody-mediated neutralization and systemic activation of TLR5-dependent immunity

TLRs trigger immunity by detecting microbe-associated molecular patterns (MAMPs). Flagellin is a unique MAMP because it harbors 1) an antigenic hypervariable region and 2) a conserved domain involved in TLR5-dependent systemic and mucosal proinflammatory and adjuvant activities. In this study, the contribution of the flagellin domains in TLR5 activation was investigated. We showed that TLR5 signaling can be neutralized in vivo by flagellin-specific Abs, which target the conserved domain. However, deletions of flagellin's hypervariable region abrogated the protein's intrinsic ability to trigger the production of neutralizing Abs. The fact that MAMP-specific Abs block TLR-mediated responses shows that this type of neutralization is a novel mechanism for down-regulating innate immunity. The stimulation of mucosal innate immunity and adjuvancy to foreign Ag was not altered by the hypervariable domain deletions. In contrast, this domain is essential to trigger systemic innate immunity, suggesting that there are distinct mechanisms for TLR5 activation in systemic and mucosal compartments. In summary, specific MAMP determinants control the production of neutralizing Abs and the compartmentalization of innate responses.

Both radioresistant and hemopoietic cells promote innate and adaptive immune responses to flagellin

The TLR5 agonist flagellin induces innate and adaptive immune responses in a MyD88-dependent manner and is under development as a vaccine adjuvant. In vitro studies indicate that, compared with other bacteria-derived adjuvants, flagellin is a very potent activator of proinflammatory gene expression and cytokine production from cells of nonhemopoietic origin. However, the role of nonhemopoietic cells in promoting flagellin-induced immune responses in vivo remains unclear. To investigate the relative contributions of the nonhemopoietic (radioresistant) and the hemopoietic (radiosensitive) compartments, we measured both innate and adaptive immune responses of flagellin-treated MyD88 radiation bone marrow chimeras. We observed that radiosensitive and radioresistant cells played distinct roles in the innate response to flagellin, with the radiosensitive cells producing the majority of the TNF-alpha, IL-12, and IL-6 cytokines and the radioresistant cells most of the KC, IP-10, and MCP-1 cytokines. Direct activation of either compartment alone by flagellin initiated dendritic cell costimulatory molecule up-regulation and induced a significant humoral immune response to the protein itself as well as to coinjected OVA. However, robust humoral responses were only observed when MyD88 was present in both cell compartments. Further studies revealed that hemopoietic and nonhemopoietic expression of the cytokines TNF-alpha and IL-6, but not IL-1, played an important role in promoting flagellin-induced Ab responses. Thus, in vivo both radioresistant and hemopoietic cells play key nonredundant roles in mediating innate and adaptive immune responses to flagellin.

Toll-like receptors in the respiratory system: their roles in inflammation

Allergic airway inflammation develops in the context of innate immune cells that express Toll-like receptors (TLRs). TLRs recognize microbial components and evoke diverse responses in immune and other respiratory cells through distinct signaling pathways. Bacterial and viral infection in the airway modulates the extent of allergic inflammation. TLR stimulation controls T helper (Th) 1, Th2, and Th17 cell differentiation, cytokine production in mast cells, and activation of eosinophils via direct and indirect pathways. TLR signals in dendritic cells increase expression of major histocompatibility complex proteins and T-cell coreceptors, resulting in greater T-cell activation with Th1 bias. TLR signals in mast cells increase their release of IL-5, and TLR signals in airway epithelial cells enhance airway generation of proallergic cytokines. Although these responses play an important protective role in infection, they may exacerbate allergic inflammation. Under some conditions, TLR stimulation, especially via TLR9, reduces Th2-dependent allergic inflammation through induction of Th1 responses. Therefore, understanding the regulatory role of TLRs in the pathogenesis of allergic airway inflammation may shed light on improving inflammation control in asthmatic patients.

Do Salmonella carry spare tyres?

Salmonellae are enterobacteria that have the unique ability to change their flagellar composition by switching expression among two loci that encode the major flagellin protein. This property is not available to all Salmonella, but is species, subspecies and serotype specific. Curiously, the subsequent loss of the second locus in some lineages of Salmonella has apparently been tolerated and, indeed, has led to considerable success for some lineages. We discuss here an evolutionary model for maintenance of this unique function and the possible evolutionary advantages of loss or preservation of this mechanism. We hypothesize that the second flagellin locus is a genetic 'spare tyre' used in particular environmental circumstances.

Effects of flagellin on innate and adaptive immunity

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

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.

A comparative analysis of cytokine responses, cell surface marker expression and MAPKs in DCs matured with LPS compared with a panel of TLR ligands

Dendritic cells (DCs) are professional antigen-presenting cells that play a vital role in shaping adaptive immunity. DC maturation begins when exogenous danger signals bind to the appropriate toll-like receptor (TLR) and initiate expression of cell surface markers and the secretion of cytokines. This process occurs through defined mitogen-activated protein kinase (MAPK) signalling pathways. Of the 13 known mammalian TLRs, lipopolysaccharide (LPS), which activates TLR4, is the most commonly used ligand for the maturation of DCs in vitro. This comprehensive study measures cytokine secretion and cell surface marker expression in murine bone-marrow-derived DCs following maturation with LPS compared to DCs matured with a panel of other TLR-ligands (zymosan A (TLR2/6), PGN (TLR2), poly(I:C) (TLR3), flagellin (TLR5) and CpG-ODN1826 (TLR9)). The role of MAPK signalling pathways in the maturation process was also examined. Results demonstrate that zymosan A and CpG induce comparable cytokine and cell surface marker profiles to LPS. The remaining ligands differed significantly for cytokine and CD40 expression, but not for CD80 and CD86 expression. While there were differences for MAPK signalling pathways for all ligands, the effect of the inhibitors were broadly similar. These findings broaden our knowledge of TLR ligand-matured DCs.

Toll-like receptor (TLR)2 and TLR3 synergy and cross-inhibition in murine myeloid dendritic cells

Toll-like receptors (TLRs) play an important role in the innate recognition of pathogens by dendritic cells (DCs) and in the induction of immune responses. Few studies have been devoted to address the impact of TLR2 (a fully MyD88-dependent receptor) and TLR3 (a fully TRIF-dependent receptor) co-activation on DC functions, especially in the mouse system. Using canonical agonists, we show that TLR2 acts in concert with TLR3 to induce the synthesis of inflammatory cytokines (TNF-alpha, IL-6), of some IL-12 family members (IL-12p40, IL-12p23, IL-27p28) and of the Notch ligand Delta-4 by mouse DCs. In contrast, TLR2 interferes with the TLR3-induced expression of type I interferon stimulated genes (MIG/CXCL9, IP-10/CXCL10, GARG39) and IL-12p35. We also report that TLR2 cooperates with TLR3 to enhance the DC-mediated production of IFN-gamma by Natural Killer cells and by conventional Ag-specific T lymphocytes. To conclude, our data support the existence of TLR2 and TLR3 synergy and cross-inhibition in DCs that could be important to strengthen immune responses during infection.

Toll like receptor-5: protecting the gut from enteric microbes

The intestine is normally colonized by a large and diverse commensal microbiota and is occasionally exposed to a variety of potential pathogens. In recent years, there has been substantial progress made in identifying molecular mechanisms that normally serve to protect the intestine from such enteric bacteria and which may go awry in chronic idiopathic inflammatory diseases of the gut. One specific molecular interaction that appears to play a key role in governing bacterial-intestinal interactions is that of the bacterial protein flagellin with toll-like receptor 5. This article reviews studies performed in vitro, in mice, and in humans that indicate an important role for the flagellin-TLR5 interaction in regulating both the innate and adaptive immune responses in the intestine.

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.

Filaria/Wolbachia activation of dendritic cells and development of Th1-associated responses is dependent on Toll-like receptor 2 in a mouse model of ocular onchocerciasis (river blindness)

Toll-like receptors (TLRs) regulate dendritic cell function and activate signals that mediate the nature of the adaptive immune response. The current study examined the role of TLRs in dendritic cell activation and in regulating T cell and antibody responses to antigens from the filarial parasites Onchocerca volvulus and Brugia malayi, which cause river blindness and lymphatic filariasis, respectively. Bone-marrow-derived CD11c(+) cells from C57BL/6 and TLR4(-/-) mice produced high levels of IL-6 and RANTES, and showed elevated surface CD40 expression, whereas CD11c(+) cells from myeloid differentiation factor 88(-/-) (MyD88(-/-)), TLR2(-/-) and TLR2/4(-/-) mice were not activated. Similarly, IFN-gamma production by splenocytes from immunized TLR2(-/-) mice was significantly impaired compared with splenocytes from C57BL/6 and TLR4(-/-) mice. In contrast, there was no difference among these strains in Th2-associated responses including IL-5 production by splenocytes from immunized animals, serum IgE and IgG(1), or eosinophil infiltration into the corneal stroma. Neutrophil recruitment to the cornea and CXC chemokine production was inhibited in immunized TLR2(-/-) mice compared with C57BL/6 and TLR4(-/-) mice. Taken together, these findings demonstrate an essential role for TLR2 in filaria-induced dendritic cell activation, IFN-gamma production and neutrophil migration to the cornea, but does not affect filaria-induced Th2-associated responses.

Potent immunogenicity and efficacy of a universal influenza vaccine candidate comprising a recombinant fusion protein linking influenza M2e to the TLR5 ligand flagellin

The recognition of specific pathogen associated molecular patterns (PAMPs) by members of the Toll-like receptor (TLR) family is critical for the activation of the adaptive immune response. Thus, incorporation of PAMPs into vaccines should result in more potent, protective antigen-specific responses in the absence of adjuvants or complex formulations. Here we describe an influenza A vaccine that is refractory to the genetic instability of hemagglutinin and neuraminidase and includes a trigger of the innate immune response to enhance immunogenicity and efficacy. A recombinant protein comprising the TLR5 ligand flagellin fused to four tandem copies of the ectodomain of the conserved influenza matrix protein M2 (M2e) was expressed in Escherichia coli and purified to homogeneity. This protein, STF2.4xM2e, retained TLR5 activity and displayed the protective epitope of M2e defined by a monoclonal antibody, 14C2. Mice immunized with STF2.4xM2e in aqueous buffer, without adjuvants or other formulation additives, developed potent M2e-specific antibody responses that were quantitatively and qualitatively superior to those observed with M2e peptide delivered in alum. The antibody response was dependent on the physical linkage of the antigen to flagellin and recognized the epitope defined by monoclonal antibody 14C2, which has been shown to protect mice from challenge with influenza A virus. Moreover, immunization with STF2.4xM2e at a dose of 0.3 microg per mouse protected mice from a lethal challenge with influenza A virus, and significantly reduced weight loss and clinical symptoms. These data demonstrate that the linkage of specific TLR ligand with influenza M2e yields a vaccine candidate that offers significant promise for widespread protection against multiple influenza A virus strains.

Inhibition of airway allergic disease by co-administration of flagellin with allergen

Bacterial flagellin, which activates Toll-like receptor 5 and cytosolic pattern recognition receptor Ipaf, has a strong immunomodulatory activity. In the present study, we examined whether intranasal co-administration of flagellin with allergen could modulate established airway hyperresponsiveness and Th2 response using an ovalbumin (OVA)-sensitized mouse model. Balb/c mice sensitized with OVA were treated with OVA-flagellin (FlaB) mixture three times at 1-week intervals. Seven days after the final OVA-FlaB administration, the mice were challenged with OVA inhalation, and airway responses and OVA-specific immune responses were evaluated. The OVA-FlaB treatment significantly suppressed OVA-induced airway hyperresponsiveness, airway eosinophilic inflammation, and OVA-specific Th2 cytokine productions in splenocytes. These results indicate that flagellin co-administered with allergen can modulate airway inflammatory response through inhibition of Th2 responses, and flagellin can be considered as a component for allergen-specific immunotherapy.

Agonists of Toll-like receptors 3, 4, 7, and 9 are candidates for use as adjuvants in an outer membrane vaccine against Neisseria meningitidis serogroup B

The bacterium Neisseria meningitidis is the causative agent of meningitis and sepsis. A generally effective vaccine against N. meningitidis serogroup B is not yet available, but outer membrane vesicle vaccines are in development. These vaccines contain lipopolysaccharide (LPS). The inclusion of N. meningitidis wild-type LPS in a vaccine is controversial because of its high toxicity. Therefore, the adjuvant activity of a panel of different Toll-like receptor (TLR) agonists in combination with LPS-deficient meningococcal outer membrane complexes was compared after immunization of mice. The results demonstrate that TLR3, TLR4, TLR7, and TLR9 agonists enhance immune responses against LPS-deficient outer membrane complexes. Their adjuvant activity was characterized by higher levels of antigen-specific immunoglobulin G (IgG), IgG2a, and IgG2b; a higher IgG2a/IgG1 ratio; lower total IgE levels; and most importantly, higher serum bactericidal antibody titers compared to LPS-deficient outer membrane complexes alone.

Toll-like receptor (TLR)2 and TLR3 sensing is required for dendritic cell activation, but dispensable to control Schistosoma mansoni infection and pathology

Toll-like receptors (TLRs) play an important role in the innate recognition of pathogens by dendritic cells (DCs) and in the induction of immune responses. However, relatively little is known about their functions in innate/acquired responses to complex eukaryotic microorganisms, including helminth parasites. That Schistosoma mansoni eggs activate myeloid DCs through TLR2 and TLR3 has been shown by us and others, but the consequences of this combined activation are still unknown. We show that the engagement of both TLR2 and TLR3 by schistosome eggs is important for the production of inflammatory cytokines and interferon-stimulated genes, such as some chemokines, by DCs. Strikingly, DCs sensitized with ovalbumin in the presence of parasite eggs dramatically reduce the release of Th2-type cytokines by ovalbumin-specific T lymphocytes, an effect that fully depends on TLR3. Finally, although TLR2 and TLR3 have no role in host resistance and in egg-induced granuloma formation in S. mansoni-infected mice, they individually and additionally increase the Th1/Th2 balance of the immune response. Thus, TLR2 and TLR3 sensing is required to shape the immune response during murine schistosomiasis, but is dispensable to control infection and pathology.

Comparison of tissue-selective proinflammatory gene induction in mice infected with wild-type, DNA adenine methylase-deficient, and flagellin-deficient Salmonella enterica

Mutants of Salmonella enterica serovar Typhimurium deficient in DNA adenine methylase (Dam) are attenuated for virulence in mice and confer heightened immunity in vaccinated animals. In contrast, infection of mice with wild-type (WT) strains or flagellin-deficient mutants of Salmonella causes typhoid fever. Here we examined the bacterial load and spatiotemporal kinetics of expression of several classes of host genes in Peyer's patches, the liver, and the spleen following oral infection of mice with WT, dam mutant, or flagellin-deficient (flhC) Salmonella. The genes evaluated included inflammatory (interleukin-1beta [IL-1beta], tumor necrosis factor alpha), chemokine (macrophage inflammatory protein 2), Th1/Th2 indicator (IL-12p40, IL-4), and interferon system (beta interferon [IFN-beta], IFN-gamma, protein Mx1 GTPase, RNA-dependent protein kinase, inducible nitric oxide synthase, suppressor of cytokine signaling 1) beacons. We showed that maximal interferon system and proinflammatory gene induction occurred by 5 days after infection and that the levels were comparable for the WT and flhC strains but were significantly lower for the dam mutant. Additionally, host gene expression in systemic tissues of individual animals was dependent on the bacterial load in the Peyer's patches for mice infected with WT, dam mutant, or flhC mutant Salmonella as early as 8 h after infection. Moreover, a bacterial load threshold in the Peyer's patches was necessary to stimulate the host gene induction in the liver and spleen. Taken together, these results suggest that bacterial load and the accompanying strain-specific cytokine signature are important determinants of the host innate immune response and associated disease manifestations observed in dam mutant Salmonella-infected animals compared to the immune response and disease manifestations observed in WT and flhC mutant Salmonella-infected animals.

Phosphoinositide 3-kinase is activated by MUC1 but not responsible for MUC1-induced suppression of Toll-like receptor 5 signaling

MUC1 is a membrane-tethered mucin-like glycoprotein expressed on the surface of various mucosal epithelial cells as well as hematopoietic cells. Recently, we showed that MUC1 suppresses flagellin-induced Toll-like receptor (TLR) 5 signaling both in vivo and in vitro through cross talk with TLR5. In this study, we determined whether phosphoinositide 3-kinase (PI3K), a negative regulator of TLR5 signaling, is involved in the cross talk between MUC1 and TLR5 using various genetically modified epithelial cell lines. Our results showed 1) activation of MUC1 induced recruitment of the PI3K regulatory subunit p85 to the MUC1 cytoplasmic tail (CT) as well as Akt phosphorylation, 2) MUC1-induced Akt phosphorylation required the presence of Tyr(20) within the PI3K binding motif of the MUC1 CT, and 3) mutation of Tyr(20) or pharmacological inhibition of PI3K activation failed to block MUC1-induced suppression of TLR5 signaling. We conclude that whereas PI3K is downstream of MUC1 activation and negatively regulates TLR5 signaling, it is not responsible for MUC1-induced suppression of TLR5 signaling.

Immunobiology of toll-like receptors in allergic disease

Allergic diseases prevalence rates have increased dramatically over the last 50 years in developed countries and one explanation might be that modern practices in public health lead to a decreased exposure towards pathogens resulting in a misguided immune response. Recently, it has become evident that immune responses against pathogens are initiated by Toll-like receptors (TLRs) that recognize a variety of structures derived from viruses, bacteria, fungi or protozoa. In this review we will discuss TLR ligands, TLR signaling in regard to Th1 and Th2 immune responses, their involvement in the development and their therapeutic potential in treatment of allergic disease.

DNA vaccines: recent developments and future possibilities

The field of DNA vaccines continues to advance and several new strategies to augment the immunogenicity of DNA vaccines are under evaluation. The majority of these studies are in the early preclinical stage, but some DNA vaccines have moved into clinical trials. In this review, we describe some of the more recent efforts aimed at increasing the immunogenicity of DNA vaccines, including the use of genetic adjuvants and plasmid-based expression of viral replicons. In addition, we discuss the possibility of using DNA vaccines to address emerging infectious agents where they may provide an advantage over other vaccine strategies and we review some areas where DNA vaccines have been used to target self-antigens.

Molecular and cellular basis of microflora-host interactions

Mucosal surfaces represent the main sites in which environmental microorganisms and antigens interact with the host. In particular the intestinal mucosal surfaces are in continuous contact with a heterogeneous population of microorganisms of the endogenous flora and are exposed to food and microbes. As a result, the immune system of the host has to discriminate between pathogenic and commensal microorganisms. This article reviews the types of sentinel cells that continuously sense the environment and coordinate immune defenses as well as the mechanisms of the innate and adaptive immune systems that are activated by bacterial and viral molecular patterns leading to inflammatory, allergic, or regulatory immune response with special emphasis on probiotic bacteria.

TLRs in the Gut. III. Immune responses to flagellin in Crohn's disease: good, bad, or irrelevant?

Recent observations suggest that bacterial flagellin may be a target of the seemingly aberrant innate and adaptive immune responses that are associated with Crohn's disease. This article discusses the evidence that supports a role for flagellin in Crohn's disease and considers the potential roles of these responses in the pathophysiology of this disorder.

Intracellular signaling mechanisms regulating toll-like receptor-mediated activation of eosinophils

Activation of eosinophils by microbe-derived molecules via Toll-like receptors (TLR) potentially provides the link between microbe-induced innate immune responses and the exacerbation of allergic inflammation. We investigated the expression of TLRs and the effect of their ligands on human eosinophils. Expression of TLR1-9 was detected by Western blot and flow cytometry. Adhesion molecules, cytokines, superoxides, and eosinophlilic cationic protein (ECP) were assessed by flow cytometry, enzyme-linked immunosorbent assay, chemiluminescent method, and fluorescence immunoassay, respectively. Human eosinophils differentially expressed TLR1, -2, -4, -5, -6, -7, and -9. Peptidoglycan (PGN) (TLR2 ligand), flagellin (TLR5 ligand), and Imiquimod R837 (TLR7 ligand) could significantly upregulate cell surface expression of intercellular adhesion molecule (ICAM)-1 and CD18, and induce the release of IL-1beta, IL-6, IL-8, growth-related oncogene (GRO)-alpha, and superoxides of eosinophils. Only PGN could induce the degranulation for ECP release. However, ds poly I-C (TLR3 ligand), LPS (TLR4 ligand), ssRNA (TLR8 ligand), and CpG-DNA (TLR9 ligand) were much less effective or inactive. PGN, flagellin, and R837 could activate both nuclear factor (NF)-kappaB and extracellular signal-regulated protein kinase (ERK). PGN could activate phosphatidylinositol 3-kinase (PI3K)-Akt, and R837 both PI3K-Akt and p38 mitogen-activated protein kinase (MAPK). The induction of the release of IL-1beta, IL-6, IL-8, GRO-alpha, superoxides, and ECP by PGN, flagellin, and R837 was found to be differentially regulated by NF-kappaB, ERK, PI3K-Akt, and p38 MAPK. The above results therefore support that microbial infection may lead to the exacerbation of allergic inflammation.

The identification of two protective DNA vaccines from a panel of five plasmid constructs encoding Brucella melitensis 16M genes

Five candidate genes from the Brucella melitensis 16M genome were selected. Eukaryotic expression plasmids encoding these antigens were constructed and expression was verified in vitro from transfected Cos7 cells. Each vaccine was assessed for protective efficacy in a BALB/c mouse brucellosis infection model. From these experiments two protective DNA vaccines were identified: p-omp25 and p-ialB. The Omp25 antigen (BMEI1249) has previously been studied in terms of Brucella virulence, serodiagnosis and as a protective antigen. However, this study represents the first report of a significant protective effect achieved against B. melitensis 16M challenge using the Omp25 antigen in a DNA vaccine approach. The other protective vaccine identified in this study was p-ialB. The ialB candidate (BMEI1584) was selected based upon its' putative function as an invasion protein which was assigned due to shared identity with the invasion protein B (ialB) of Bartonella bacilliformis. This candidate has not previously been investigated with regard to Brucella virulence or pathogenesis. This study is the first report to identify the Brucella invasion protein B (BMEI1584) as a novel protective antigen for brucellosis.

Update on toll-like receptor ligands and allergy: implications for immunotherapy

Innate responses to microbes are mediated in large part via toll-like receptors (TLRs) that recognize a diverse family of ligands produced by viruses, bacteria, and fungi. Great effort has been directed toward translating this knowledge into the development of therapies for the prevention and treatment of diseases, including those fueled by allergic (Th2-biased) hypersensitivities. In this review, we consider the ways in which ligands for different TLRs influence the allergic phenotype. In addition, an update on safety and efficacy data from clinical trials of allergic patients treated with TLR9 ligand-based interventions is provided. Finally, recent experimental results that help elucidate how ambient TLR ligand exposures influence allergic risk and their relevance to the development of TLR ligand-based therapeutics are discussed. Investigations presented within this opinion paper suggest that several TLR ligands could have clinical utility in the treatment of allergic diseases, whereas other TLR ligands appear less attractive, as they facilitate development of Th2-biased hypersensitivities in murine studies.

Bacterial flagellin inhibits T cell receptor-mediated activation of T cells by inducing suppressor of cytokine signalling-1 (SOCS-1)

Flagellin, the structural component of bacterial flagella, is secreted by pathogenic and commensal bacteria, and is recognized by Toll-like receptor (TLR) 5. Flagellin is a common bacterial antigen present on most motile bacteria and is speculated to contribute to the activation of CD4+ T cells in the intestine. However, molecular mechanisms by which flagellin regulate T cell activation remains to be determined. Using Jurkat T cells or human primary T cell, we showed that flagellin stimulation induced tyrosine phosphorylation of TLR5 and activation of both mitogen-activated protein kinases and nuclear factor kappaB. In addition, stimulation by flagellin did not induce nuclear factor of activated T cells (NFAT) activation, while stimulation via the T cell receptor (TCR) leads to activation of NFAT. However, TCR-mediated NFAT activation and tyrosine phosphorylation of zeta-associated protein 70 (Zap-70) were inhibited in cells pre-stimulated by flagellin. Furthermore, flagellin stimulation induced suppressor of cytokine signalling-1 (SOCS-1), which formed a complex with Zap-70 after stimulation via TCR, and inhibition of SOCS-1 expression by SOCS-1-specific small interfering RNA reinstated TCR-mediated activation of NFAT in cells pre-stimulated with flagellin. These results collectively indicate that bacterial flagellin inhibits TCR-mediated activation of T cells by inducing SOCS-1.

The diacylated lipopeptide FSL-1 induces TLR2-mediated Th2 responses

The diacylated lipopeptide FSL-1 enhanced the generation of IgG antibodies in TLR2(+/+) mice, but not in TLR2(-/-) mice, when administered together with hen egg lysozyme as an antigen. Escherichia coli lipopolysaccharide enhanced the generation of antigen-specific antibodies in both TLR2(-/-) and TLR2(+/+) mice. In TLR2(+/+) mice, the level of enhancement due to FSL-1 was similar to that caused by lipopolysaccharide. Analysis of the IgG antibodies subclass demonstrated that the level of Th2-type IgG1 antibodies was higher than that of Th1-type IgG2a antibodies. Both FSL-1 and lipopolysaccharide induced production of IL-10 and IL-6 by splenocytes from TLR2(+/+) mice. Lipopolysaccharide also induced production of these cytokines by splenocytes from TLR2(-/-) mice, but FSL-1 did not. Neither FSL-1 nor lipopolysaccharide induced IL-12p70 production by splenocytes from either type of mice. FSL-1 upregulated B7.2 expression in B220(+) cells from TLR2(+/+) mice but not those from TLR2(-/-) mice, whereas lipopolysaccharide upregulated B7.2 expression in B220(+) cells from both types of mice. FSL-1 and, to a lesser extent, lipopolysaccharide activated mitogen-activated protein kinases in splenocytes. FSL-1 and, to a lesser extent, lipopolysaccharide induced the expression of c-Fos, which is known to be involved in Th2-type responses, in splenocytes. Thus, this study demonstrated that FSL-1 possessed TLR2-mediated Th2-type responses in vivo.

Humoral immune response to flagellin requires T cells and activation of innate immunity

Bacterial flagellin, the primary structural component of flagella, is a dominant target of humoral immunity upon infection by enteric pathogens and in Crohn's disease. To better understand how such responses may be regulated, we sought to define, in mice, basic mechanisms that regulate generation of flagellin-specific Igs. We observed that, in response to i.p. injection with flagellin, generation of flagellin-specific Ig required activation of innate immunity in that these responses were ablated in MyD88-deficient mice and that flagellin from Helicobacter pylori, which is known not to activate TLR5, also did not elicit Abs. Mice lacking alphabeta T cells (TCRbeta(null)) were completely deficient in their ability to make flagellin Abs in various contexts indicating that, in contrast to common belief, generation of flagellin-specific Ig is absolutely T cell dependent. In contrast to Ab responses to whole flagella (H serotyping), responses to flagellin monomers displayed only moderate serospecificity. Whereas neither oral nor rectal administration of flagellin elicited a strong serum Ab response, induction of colitis with dextran sodium sulfate resulted in a MyD88-dependent serum Ab response to endogenous flagellin, suggesting that, in an inflammatory milieu, TLR signaling promotes acquisition of Abs to intestinal flagellin. Thus, acquisition of a humoral immune response to flagellin requires activation of innate immunity, is T cell dependent, and can originate from flagellin in the intestinal tract in inflammatory conditions in the intestine.

Dendritic cells: translating innate to adaptive immunity

The innate immune system provides many ways to quickly resist infection. The two best-studied defenses in dendritic cells (DCs) are the production of protective cytokines-like interleukin (IL)-12 and type I interferons-and the activation and expansion of innate lymphocytes. IL-12 and type I interferons influence distinct steps in the adaptive immune response of lymphocytes, including the polarization of T-helper type 1 (Th1) CD4+ T cells, the development of cytolytic T cells and memory, and the antibody response. DCs have many other innate features that do not by themselves provide innate resistance but are critical for the induction of adaptive immunity. We have emphasized three intricate and innate properties of DCs that account for their sentinel and sensor roles in the immune system: (1) special mechanisms for antigen capture and processing, (2) the capacity to migrate to defined sites in lymphoid organs, especially the T cell areas, to initiate immunity, and (3) their rapid differentiation or maturation in response to a variety of stimuli ranging from Toll-like receptor (TLR) ligands to many other nonmicrobial factors such as cytokines, innate lymphocytes, and immune complexes. The combination of innate defenses and innate physiological properties allows DCs to serve as a major link between innate and adaptive immunity. DCs and their subsets contribute to many subjects that are ripe for study including memory, B cell responses, mucosal immunity, tolerance, and vaccine design. DC biology should continue to be helpful in understanding pathogenesis and protection in the setting of prevalent clinical problems.

Gene polymorphisms in Toll-like receptors, interleukin-10, and interleukin-10 receptor alpha and lymphoma risk

Interactions between environment and immune system play an essential role in the aetiology of immunopathologies, including lymphomas. Toll-like receptors (TLR) belong to a group of pattern recognition receptors, with importance for innate immune response and inflammatory processes. Interleukin-10 (IL-10) is a key regulatory cytokine and has been implicated in lymphomagenesis. Functional polymorphisms in these inflammation-associated genes may affect the susceptibility towards lymphoma. To test this hypothesis, we have genotyped DNA of 710 lymphoma cases and 710 controls within the context of a population-based epidemiological study for 11 functionally important single-nucleotide polymorphisms in TLR1, -2, -4, -5, -9, IL10 and IL10 receptor (IL10RA). The IL10RA Ser138Gly variant was underrepresented among lymphoma cases (odds ratio (OR)=0.81, 95 per cent confidence interval (95% CI)=0.65-1.02), mainly owing to an inverse association with Hodgkin's lymphoma (HL). The TLR2 -16933T>A variant was associated with a 2.8-fold increased risk of follicular lymphoma (95% CI=1.43-5.59) and a decreased risk of chronic lymphocytic leukaemia (OR=0.61, 95% CI=0.38-0.95). Furthermore, the TLR4 Asp299Gly variant was positively associated with the risk of mucosa-associated lymphoid tissue lymphoma (OR=2.76, 95% CI=1.12-6.81) and HL (OR=1.80, 95% CI=0.99-3.26). In conclusion, this study suggests an effect of polymorphisms in factors of the innate immune response in the aetiology of some lymphoma subtypes.

Vaccination with recombinant fusion proteins incorporating Toll-like receptor ligands induces rapid cellular and humoral immunity

Recognition of specific pathogen associated molecular patterns (PAMPs) is mediated primarily by members of the Toll-like receptor (TLR) family. Stimulation through these receptors results in quantitative and qualitative changes in antigen presentation and cellular activation, thereby linking innate and adaptive immunity. Consequently, the incorporation of TLR-ligands into vaccines could result in more potent and efficacious vaccines. To test this hypothesis, we employed a recombinant fusion protein strategy using the TLR5 ligand flagellin fused to specific antigens to promote protective immunity. These purified recombinant fusion proteins demonstrated potent TLR5-specific NF-kappaB dependent activity in vitro. Immunization of mice with the recombinant-flagellin-OVA fusion protein STF2.OVA resulted in potent antigen-specific T and B cell responses that were equal to or better than responses induced by OVA emulsified in Complete Freund's adjuvant. These included rapid and consistent antigen-specific IgG(1) and IgG(2a) antibody responses that were detectable within 7 days of immunization, and the development of protective CD8 T cell responses. Moreover, the enhanced immunogenicity to OVA is dependant on the direct fusion to flagellin, as co-delivery of OVA with flagellin unlinked failed to augment antigen-specific responses in vivo. Similar results were obtained using a recombinant fusion protein comprised of flagellin and a novel polypetide sequence containing two immuno-protective epitopes derived from the Listeria monocytogenes antigens p60 and listeriolysin O. Animals immunized with this recombinant protein demonstrated significant antigen-specific CD8 T cell responses and protection upon challenge with virulent L. monocytogenes. We conclude that immunization with PAMP:antigen fusion proteins induce rapid and potent antigen-specific responses in the absence of supplemental adjuvants. Collectively our data demonstrate that PAMP:antigen fusion proteins offer significant promise for developing recombinant protein vaccines.

Involvement of Toll-like receptor 5 in the recognition of flagellated bacteria

Toll-like receptors (TLRs) are key components of the immune system that detect microbial infection and trigger antimicrobial host defense responses. TLR5 is a sensor for monomeric flagellin, which is a component of bacterial flagella known to be a virulence factor. In this study we generated TLR5-deficient mice and investigated the role of TLR5 signaling in the detection of flagellin and antibacterial immune responses to Salmonella typhimurium and Pseudomonas aeruginosa. We found that TLR5 is essential for the recognition of bacterial flagellin both in vivo and ex vivo. TLR5 contribution to antibacterial host response to i.p. infection with S. typhimurium or intranasal administration of P. aeruginosa may be masked by TLR4 or other sensing mechanisms. By using radiation bone marrow chimera, we showed that upon i.p. injection of flagellin immune responses are mediated by lymphoid cells, whereas resident cells are required for the initiation of response upon intranasal flagellin administration. These results suggest that flagellin recognition in different organs is mediated by distinct TLR5-expressing cells and provide insights into the cooperation of the TLR5 and TLR4 signaling pathways used by the innate immune system in the recognition of bacterial pathogens.

Tlr5 is not primarily associated with susceptibility to Salmonella Typhimurium infection in MOLF/Ei mice

The extreme susceptibility to infection with Salmonella Typhimurium of wild-derived MOLF/Ei mice has been linked to one genomic region on Chromosome 1 (Ity3). A member of the Toll-like receptors family, Tlr5, located on distal Chromosome 1, was previously shown to be a candidate gene for Ity3 based on expression studies and sequencing analysis. The candidacy of Tlr5 as a Salmonella-susceptibility gene was evaluated functionally by comparing Tlr5 C57BL/6J and MOLF/Ei alleles in vitro and in vivo. In vitro studies showed that the MOLF/Ei Tlr5 allele is more transcriptionally active when the gene is removed from its natural genomic environment. This observation was supported by in vivo studies in B6.MOLF-Ity3 congenic mice that showed that mice homozygous for the MOLF/Ei allele at Ity3, including Tlr5, had an increased response to flagellin as measured by IL-6 and CXCL-1 secretion in the serum compared with parental MOLF/Ei mice. Despite the fact that both MOLF/Ei and B6.MOLF-Ity/Ity3 mice are more susceptible to Salmonella Typhimurium infection than B6.MOLF-Ity mice, they exhibit a different phenotype with respect to Tlr5 expression and Tlr5 signaling, supporting the prediction that Tlr5 is not primarily involved in the disease phenotype underlying the Ity3 locus in MOLF/Ei mice.