Airway structural cells regulate TLR5-mediated mucosal adjuvant activity

Antigen-presenting cell (APC) activation is enhanced by vaccine adjuvants. Most vaccines are based on the assumption that adjuvant activity of Toll-like receptor (TLR) agonists depends on direct, functional activation of APCs. Here, we sought to establish whether TLR stimulation in non-hematopoietic cells contributes to flagellin's mucosal adjuvant activity. Nasal administration of flagellin enhanced T-cell-mediated immunity, and systemic and secretory antibody responses to coadministered antigens in a TLR5-dependent manner. Mucosal adjuvant activity was not affected by either abrogation of TLR5 signaling in hematopoietic cells or the presence of flagellin-specific, circulating neutralizing antibodies. We found that flagellin is rapidly degraded in conducting airways, does not translocate into lung parenchyma and stimulates an early immune response, suggesting that TLR5 signaling is regionalized. The flagellin-specific early response of lung was regulated by radioresistant cells expressing TLR5 (particularly the airway epithelial cells). Flagellin stimulated the epithelial production of a small set of mediators that included the chemokine CCL20, which is known to promote APC recruitment in mucosal tissues. Our data suggest that (i) the adjuvant activity of TLR agonists in mucosal vaccination may require TLR stimulation of structural cells and (ii) harnessing the effect of adjuvants on epithelial cells can improve mucosal vaccines.     

Requirement of MyD88 signaling in keratinocytes for Langerhans cell migration and initiation of atopic dermatitis‐like symptoms in mice

Atopic dermatitis (AD) is a chronic inflammatory disease controlled by the innate and adaptive immune system. To elucidate the impact of innate immune signaling in AD, we analyzed MyD88‐deficient mice in a murine model of AD‐like dermatitis by epicutaneous sensitization with ovalbumin (OVA). Global MyD88 deficiency led to reduced epidermal thickening and diminished accumulation of macrophages within the inflamed skin. In addition, we observed impaired emigration of Langerhans cells (LCs) out of the epidermis of MyD88‐deficient mice. These findings indicate that MyD88 deficiency affects various skin‐resident cell types in the AD model. Moreover, production of IFN‐g, IL‐17, and CCL17 was reduced in skin draining lymph node cells and OVA‐specific immunoglobulin levels were lower in MyD88‐deficient mice. We further investigated the role of MyD88 in keratinocytes, as keratinocytes contribute to AD pathology. Exclusive expression of MyD88 in epidermal keratinocytes partially restored LC emigration after AD induction and expression of CCL17 in skin draining lymph nodes (LNs), but did not promote epidermal thickening nor production of IL‐17. Altogether, these data demonstrate that MyD88 signaling in keratinocytes is able to restore LC migration in an otherwise MyD88‐deficient background, and significantly contributes to the development of AD‐like dermatitis.     

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 perse, 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.     

B cell‐intrinsic MyD88 signaling controls IFN‐γ‐mediated early IgG2c class switching in mice in response to a particulate adjuvant

Adjuvants improve the potency of vaccines, but the modes of action (MOAs) of most adjuvants are largely unknown. TLR‐dependent and ‐independent innate immune signaling through the adaptor molecule MyD88 has been shown to be pivotal to the effects of most adjuvants; however, MyD88's involvement in the TLR‐independent MOAs of adjuvants is poorly understood. Here, using the T‐dependent antigen NIPOVA and a unique particulate adjuvant called synthetic hemozoin (sHZ), we show that MyD88 is required for early GC formation and enhanced antibody class‐switch recombination (CSR) in mice. Using cell‐type‐specific MyD88 KO mice, we found that IgG2c class switching, but not IgG1 class switching, was controlled by B cell‐intrinsic MyD88 signaling. Notably, IFN‐γ produced by various cells including T cells, NK cells, and dendritic cells was the primary cytokine for IgG2c CSR and B‐cell intrinsic MyD88 is required for IFN‐γ production. Moreover, IFN‐γ receptor (IFNγR) deficiency abolished sHZ‐induced IgG2c production, while recombinant IFN‐γ administration successfully rescued IgG2c CSR impairment in mice lacking B‐cell intrinsic MyD88. Together, our results show that B cell‐intrinsic MyD88 signaling is involved in the MOA of certain particulate adjuvants and this may enhance our specific understanding of how adjuvants and vaccines work.     

Toll-like receptors in innate immunity

Functional characterization of Toll-like receptors (TLRs) has established that innate immunity is a skillful system that detects invasion of microbial pathogens. Recognition of microbial components by TLRs initiates signal transduction pathways, which triggers expression of genes. These gene products control innate immune responses and further instruct development of antigen-specific acquired immunity. TLR signaling pathways are finely regulated by TIR domain-containing adaptors, such as MyD88, TIRAP/Mal, TRIF and TRAM. Differential utilization of these TIR domain-containing adaptors provides specificity of individual TLR-mediated signaling pathways. Several mechanisms have been elucidated that negatively control TLR signaling pathways, and thereby prevent overactivation of innate immunity leading to fatal immune disorders. The involvement of TLR-mediated pathways in autoimmune and inflammatory diseases has been proposed. Thus, TLR-mediated activation of innate immunity controls not only host defense against pathogens but also immune disorders.     

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‐α 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.     

MyD88 and TRIF synergistic interaction is required for TH1‐cell polarization with a synthetic TLR4 agonist adjuvant

Glucopyranosyl lipid adjuvant‐stable emulsion (GLA‐SE) is a synthetic adjuvant TLR4 agonist that promotes potent poly‐functional T_H1 responses. Different TLR4 agonists may preferentially signal via MyD88 or TIR‐domain‐containing adapter inducing IFN‐beta (TRIF) to exert adjuvant effects; however, the contribution of MyD88 and TRIF signaling to the induction of polyclonal T_H1 responses by TLR4 agonist adjuvants has not been studied in vivo. To determine whether GLA‐SE preferentially signals through MyD88 or TRIF, we evaluated the immune response against a candidate tuberculosis (TB) vaccine Ag following immunization of mice lacking either signaling adapter compared with that of wild‐type mice. We find that both MyD88 and TRIF are necessary for GLA‐SE to induce a poly‐functional T_H1 immune response characterized by CD4⁺ T cells producing IFN‐γ, TNF, and IL‐2, as well as IgG2c class switching, when paired with the TB vaccine Ag ID93. Accordingly, the protective efficacy of ID93/GLA‐SE immunization against aerosolized Mycobacterium tuberculosis was lost when either signaling molecule was ablated. We demonstrate that MyD88 and TRIF must be expressed in the same cell for the in vivo T_H1‐skewing adjuvant activity, indicating that these two signaling pathways cooperate on an intracellular level. Thus engagement of both the MyD88 and TRIF signaling pathways are essential for the effective adjuvant activity of this TLR4 agonist.     

Endotoxin can induce MyD88-deficient dendritic cells to support T(h)2 cell differentiation

Toll-like receptor (TLR) signaling activates dendritic cells (DC) to secrete proinflammatory cytokines and up-regulate co-stimulatory molecule expression, thereby linking innate and adaptive immunity. A TLR-associated adapter protein, MyD88, is essential for cytokine production induced by TLR. However, in response to a TLR4 ligand, lipopolysaccharide (LPS), MyD88-deficient (MyD88(-/-)) DC can up-regulate co-stimulatory molecule expression and enhance their T cell stimulatory activity, indicating that the MyD88-independent pathway through TLR4 can induce some features of DC maturation. In this study, we have further characterized function of LPS-stimulated, MyD88(-/-) DC. In response to LPS, wild-type DC could enhance their ability to induce IFN-gamma production in allogeneic mixed lymphocyte reaction (alloMLR). In contrast, in response to LPS, MyD88(-/-) DC augmented their ability to induce IL-4 instead of IFN-gamma in alloMLR. Impaired production of T(h)1-inducing cytokines in MyD88(-/-) DC cannot fully account for their increased T(h)2 cell-supporting ability, because absence of T(h)1-inducing cytokines in DC caused impairment of IFN-gamma, but did not lead to augmentation of IL-4 production in alloMLR. In vivo experiments with adjuvants also revealed T(h)2-skewed immune responses in MyD88(-/-) mice. These results demonstrate that the MyD88-independent pathway through TLR4 can confer on DC the ability to support T(h)2 immune responses.     

What the Myddosome structure tells us about the initiation of innate immunity

Signaling by the toll-like receptor (TLR) and interleukin-1 receptor superfamily requires the adapter protein myeloid differentiation primary response protein 88 (MyD88). The recent determination of the structure of the so-called Myddosome provides us with new insights into the structural basis for innate immune signaling. Other information on the biochemistry and genetics of MyD88 and other adapters, such as MyDD adapter-like and TRIF-related adapter molecule, allows us to describe in some detail the signaling process activated by TLRs and provides new insights into the role these important proteins play in innate immunity.     

TLR5 or NLRC4 is necessary and sufficient for promotion of humoral immunity by flagellin

The fact that some TLR-based vaccine adjuvants maintain function in TLR-deficient hosts highlights that their mechanism of function remains incompletely understood. Thus, we examined the ability of flagellin to induce cytokines and elicit/promote murine antibody responses upon deletion of the flagellin receptors TLR5 and/or NLRC4 (also referred to as IPAF) using a prime/boost regimen. In TLR5-KO mice, flagellin failed to induce NF-κB-regulated cytokines such as keratinocyte-derived chemokine (CXCL1) but induced WT levels of the inflammasome cytokine IL-18 (IL-1F4). Conversely, in NLRC4-KO mice, flagellin induced keratinocyte-derived chemokine, but not IL-18, whereas TLR5/NLRC4-DKO lacked induction of all cytokines measured. Flagellin/ovalbumin treatment resulted in high-antibody titers to both flagellin and ovalbumin in WT, TLR5-KO and DKO mice but did not elicit antibodies to either in TLR5/NLRC4-DKO mice. Thus, flagellin's ability to elicit/promote humoral immunity requires a germ-line-encoded receptor capable of recognizing this molecule. Such promotion of adaptive immunity can be effectively driven by either TLR5-mediated activation of NF-κB or NLRC4-mediated activation of the inflammasome.     

Evidence for a DC‐Specific Inhibitory Mechanism that Depends on MyD88 and SIGIRR

Dendritic cells (DC) are an essential link between the innate and adaptive immune response. To become effective antigen‐presenting cells DC need to undergo maturation, during which they up‐regulate co‐stimulatory molecules and produce cytokines. There is great interest in utilizing DC in vaccination regimes. Over recent years, Toll‐like receptor (TLR) signalling has been recognized to be one of the major inducers of DC maturation. This study describes a mutant version of the TLR adaptor molecule MyD88 (termed MyD88lpr) as a novel adjuvant for vaccination regimes. MyD88lpr specifically activates DC by disrupting a DC intrinsic inhibitory mechanism, which is dependent on single immunoglobulin IL‐1R‐related. Moreover, MyD88lpr was able to induce an IgG2a‐dominated response to a co‐expressed antigen, suggesting Th1 immunity. However, when used as a vaccine adjuvant for Influenza nucleoprotein there was no significant difference in the lung viral titres during the infection. This study describes MyD88lpr as a potential adjuvant for vaccinations, which would be able to target DC specifically.     

Rapid CD4+ T‐cell responses to bacterial flagellin require dendritic cell expression of Syk and CARD9

Toll‐like receptors (TLRs) can recognize microbial patterns and utilize adaptor molecules, such as‐MyD88 or (TRIF TIR‐domain‐containing adapter‐inducing interferon‐β), to initiate downstream signaling that ultimately affects the initiation of adaptive immunity. In addition to this inflammatory role, TLR5 expression on dendritic cells can favor antigen presentation of flagellin peptides and thus increase the sensitivity of flagellin‐specific T‐cell responses in vitro and in vivo. Here, we examined the role of alternative signaling pathways that might regulate flagellin antigen presentation in addition to MyD88. These studies suggest a requirement for spleen tyrosine kinase, a noncanonical TLR‐signaling adaptor molecule, and its downstream molecule CARD9 in regulating the sensitivity of flagellin‐specific CD4⁺ T‐cell responses in vitro and in vivo. Thus, a previously unappreciated signaling pathway plays an important role in regulating the dominance of flagellin‐specific T‐cell responses.     

MyD88 innate immune function in a zebrafish embryo infection model

Innate immunity signaling mechanisms during vertebrate embryogenesis are largely unknown. To study Toll-like receptor (TLR) signaling function in the zebrafish embryo model, we designed an experimental setup for antisense morpholino knockdown under conditions of bacterial infection. Clearance of Salmonella enterica serovar Typhimurium Ra bacteria was significantly impaired after knockdown of myeloid differentiation factor 88 (MyD88), a common adaptor protein in TLR and interleukin-1 receptor signaling. Thereby, we demonstrate for the first time that the innate immune response of the developing embryo involves MyD88-dependent signaling, which further establishes the zebrafish embryo as a model for the study of vertebrate innate immunity.     

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.     

Suppression of allergic reaction by λ-carrageenan: Toll-like receptor 4/MyD88-dependent and -independent modulation of immunity

BACKGROUND: Recognition of foreign substances by innate immunity through pattern recognition receptors (PRRs) regulates acquired immunity such as allergic reaction. Because PRRs recognize heterogeneous ligands, daily food intake can potentially regulate immune allergic reaction. OBJECTIVE: Elucidation of the effect of lambda-carrageenan on allergic reactions was aimed. METHOD: IFN-gamma and IL-4 was measured in in vitro T cell-stimulated culture. Cytokine production from macrophages in response to lambda-carrageenan was measured as indicator for innate immunity activation. Mice were immunized with OVA in alum to induce specific IgE, and then histamine release was induced by systemic injection of OVA. RESULTS: Activation of innate immunity by lambda-carrageenan is dependent on Toll-like receptor-4 (TLR4) and MyD88, in which induction of pro-inflammatory cytokines such as TNF-alpha and IL-6 was largely impaired in macrophages from TLR4- and MyD88-deficient mice. Footpad oedema, a model for in vivo inflammatory reactions, was significantly reduced in these mice. Similar to recent evidence showing a preference for the stimulation of Th1 via TLR/MyD88 signalling, lambda-carrageenan showed enhanced IFN-gamma and decreased IL-4 in stimulated T cell cultures. Interestingly, increased IFN-gamma production was still seen in TLR4- and MyD88-deficient splenocytes. Oral administration of lambda-carrageenan to immunized mice successfully decreased OVA-specific IgE, and lambda-carrageenan was also effective in previously immunized mice. Further, serum histamine release upon systemic challenge of OVA was significantly inhibited. Neither OVA-specific IgG1/IgG2a nor cytokine secretion from in vitro cultures were altered, suggesting the involvement of multiple PRRs as demonstrated by TLR4/MyD88-independent IFN-gamma up-regulation. The simultaneous feeding of OVA with lipopolysaccharide abrogated oral tolerance, but lambda-carrageenan was not only devoid of such an effect but was also found to promote oral tolerance in the absence of TLR4. CONCLUSION: lambda-Carrageenan was suggested to be a useful dietary supplement to ameliorate allergic reactions while maintaining oral tolerance-dependent intestinal homeostasis.     

MyDths and un-TOLLed truths: sensor, instructive and effector immunity to tuberculosis

Controversy exists concerning the role of Toll-like receptors and MyD88 in immunity to tuberculosis (TB). This mini-review argues that (i) Toll-like receptors are not essential for an effective immune response against TB, (ii) MyD88 is essential, but not because it transduces signals from TLRs, (iii) adaptive immunity to TB is largely TLR/MyD88-independent. Some of the discrepancies may be resolved by cogent attribution of distinct immune functions to the individual components of the TLR/MyD88 system. In mice, TLRs and MyD88 are fully dispensable in sensing Mtb infection and instructing T cell-mediated adaptive immunity, and while TLRs are also redundant during macrophage effector immunity, MyD88 is essential for efficient killing of mycobacteria. This distinction should help to molecularly pinpoint the MyD88-dependent, yet TLR-independent critical mechanisms of macrophage activation involved in intracellular growth restriction of Mtb. Disrupted IL-1R and/or IFN-gamma signaling pathways likely play a much more prominent role in explaining the exquisite susceptibility of MyD88-deficient mice to TB than the function of MyD88 as a TLR adaptor.     

Toll‐like receptor 4 agonists adsorbed to aluminium hydroxide adjuvant attenuate ovalbumin‐specific allergic airway disease: role of MyD88 adaptor molecule and interleukin‐12/interferon‐γ axis

Background Epidemiological and experimental data suggest that bacterial lipopolysaccharides (LPS) can either protect from or exacerbate allergic asthma. Lipopolysaccharides trigger immune responses through toll‐like receptor 4 (TLR4) that in turn activates two major signalling pathways via either MyD88 or TRIF adaptor proteins. The LPS is a pro‐Type 1 T helper cells (Th1) adjuvant while aluminium hydroxide (alum) is a strong Type 2 T helper cells (Th2) adjuvant, but the effect of the mixing of both adjuvants on the development of lung allergy has not been investigated. Objective We determined whether natural (LPS) or synthetic (ER‐803022) TLR4 agonists adsorbed onto alum adjuvant affect allergen sensitization and development of airway allergic disease. To dissect LPS‐induced molecular pathways, we used TLR4‐, MyD88‐, TRIF‐, or IL‐12/IFN‐γ‐deficient mice. Methods Mice were sensitized with subcutaneous injections of ovalbumin (OVA) with or without TLR4 agonists co‐adsorbed onto alum and challenged with intranasally with OVA. The development of allergic lung disease was evaluated 24 h after last OVA challenge. Results Sensitization with OVA plus LPS co‐adsorbed onto alum impaired in dose‐dependent manner OVA‐induced Th2‐mediated allergic responses such as airway eosinophilia, type‐2 cytokines secretion, airway hyper‐reactivity, mucus hyper production and serum levels of IgE or IgG1 anaphylactic antibodies. Although the levels of IgG2a, Th1‐affiliated isotype increased, investigation into the lung‐specific effects revealed that LPS did not induce a Th1 pattern of inflammation. Lipopolysaccharides impaired the development of Th2 immunity, signaling via TLR4 and MyD88 molecules and via the IL‐12/IFN‐γ axis, but not through TRIF pathway. Moreover, the synthetic TLR4 agonists that proved to have a less systemic inflammatory response than LPS also protected against allergic asthma development. Conclusion Toll‐like receptor 4 agonists co‐adsorbed with allergen onto alum down‐modulate allergic lung disease and prevent the development of polarized T cell‐mediated airway inflammation.     

Toll-like receptor 4 is a sensor for autophagy associated with innate immunity

Autophagy has recently been shown to be an important component of the innate immune response. The signaling pathways leading to activation of autophagy in innate immunity are not known. Here we showed that Toll-like receptor 4 (TLR4) served as a previously unrecognized environmental sensor for autophagy. Autophagy was induced by lipopolysaccharide (LPS) in primary human macrophages and in the murine macrophage RAW264.7 cell line. We defined a new molecular pathway in which LPS-induced autophagy was regulated through a Toll-interleukin-1 receptor domain-containing adaptor-inducing interferon-beta (TRIF)-dependent, myeloid differentiation factor 88 (MyD88)-independent TLR4 signaling pathway. Receptor-interacting protein (RIP1) and p38 mitogen-activated protein kinase were downstream components of this pathway. This signaling pathway did not affect cell viability, indicating that it is distinct from the autophagic death signaling pathway. We further showed that LPS-induced autophagy could enhance mycobacterial colocalization with the autophagosomes. This study links two ancient processes, autophagy and innate immunity, together through a shared signaling pathway.     

Both innate immunity and type 1 humoral immunity to Streptococcus pneumoniae are mediated by MyD88 but differ in their relative levels of dependence on toll-like receptor 2

Little is known regarding the role of Toll-like receptors (TLRs) in regulating protein- and polysaccharide-specific immunoglobulin (Ig) isotype production in response to an in vivo challenge with an extracellular bacterium. In this report we demonstrate that MyD88(-/-), but not TLR2(-/-), mice are markedly defective in their induction of multiple splenic proinflammatory cytokine- and chemokine-specific mRNAs after intraperitoneal (i.p.) challenge with heat-killed Streptococcus pneumoniae capsular type 14 (S. pneumoniae type 14). This is correlated with analogous responses in splenic cytokine protein release in vitro following addition of S. pneumoniae type 14. Consistent with these data, naive MyD88(-/-), but not TLR2(-/-), mice are more sensitive to killing following i.p. challenge with live S. pneumoniae type 14, relative to responses in wild-type mice. However, prior immunization of MyD88(-/-) mice with heat-killed S. pneumoniae type 14 protects against an otherwise-lethal challenge with live S. pneumoniae type 14. Surprisingly, both MyD88(-/-) and TLR2(-/-) mice exhibit striking and equivalent defects in elicitation of type 1 IgG isotypes (IgG3, IgG2b, and IgG2a), but not the type 2 IgG isotype, IgG1, specific for several protein and polysaccharide antigens, in response to i.p. challenge with heat-killed S. pneumoniae type 14. Of note, the type 1 IgG isotype titers specific for pneumococcal surface protein A are reduced in MyD88(-/-) mice but not TLR2(-/-) mice. These data suggest that distinct TLRs may differentially regulate innate versus adaptive humoral immunity to intact S. pneumoniae and are the first to implicate a role for TLR2 in shaping an in vivo type 1 IgG humoral immune response to a gram-positive extracellular bacterium.     

Toll‐like receptor 6 senses Mycobacterium avium and is required for efficient control of mycobacterial infection

Mycobacterium avium has been reported to signal through both Toll‐like receptor (TLR2) and TLR9. To investigate the role of TLR6 in innate immune responses to M. avium, TLR6, MyD88, TLR2, and TLR2/6 KO mice were infected with this pathogen. Bacterial burdens were higher in the lungs and livers of infected TLR6, TLR2, TLR2/6, and MyD88 KO mice compared with those in C57BL/6 mice, which indicates that TLR6 is required for the efficient control of M. avium infection. However, TLR6 KO spleen cells presented with normal M. avium induced IFN‐γ responses as measured by ELISA and flow cytometry. In contrast, the production of IFN‐γ in lung tissue was diminished in all studied KO mice. Furthermore, only MyD88 deficiency reduced granuloma areas in mouse livers. Moreover, we determined that TLR6 plays an important role in controlling bacterial growth within macrophages and in the production of TNF‐α, IL‐12, and IL‐6 by M. avium infected DCs. Finally, the lack of TLR6 reduced activation of MAPKs and NF‐κB in DCs. In summary, TLR6 is required for full resistance to M. avium and for the activation of DCs to produce proinflammatory cytokines.