Toll-like receptor genes are differentially expressed at the sites of infection during the progression of Johne's disease in outbred sheep

Toll-like receptors (TLR) are engaged by ligands on microbial pathogens to initiate innate and adaptive immune responses. Little is known about TLR involvement during infection with Mycobacterium avium subsp. paratuberculosis (M. ptb), the cause of Johne's disease in ruminants, although there is a profound immunopathological response in affected animals. We have analyzed the expression of 10 TLR genes relative to validated reference genes at predilection sites in ileum, jejunum and associated lymph nodes as well as in peripheral blood, to determine if TLR expression is altered in response to infection with M. ptb in outbred sheep. Previously unexposed animals from two flocks and animals from three naturally infected flocks were used with restricted maximum likelihood linear mixed modeling applied to determine significant differences. These were related to the pathologies observed at different stages of infection in exposed sheep, after allowing for other sources of variation. In most cases there were differences in TLR expression between early paucibacillary and multibacillary groups when compared to uninfected sheep, with most TLRs for the paucibacillary group having lower expression levels than the multibacillary group. Increased expression of TLR1-5, and 8 was observed in ileum or jejunum, and TLR1-4, 6, and 8 in mesenteric lymph nodes. There was a trend for increased expression of TLR1, 2, and 6-8 in PBMCs of exposed compared to non-exposed animals. Further study of TLR expression in Johne's disease in ruminants is warranted as these observed differences may help explain pathogenesis and may be useful in the future diagnosis of M. ptb infection.

Expression of cathelicidin LL-37 during Mycobacterium tuberculosis infection in human alveolar macrophages, monocytes, neutrophils, and epithelial cells

The innate immune response in human tuberculosis is not completely understood. To improve our knowledge regarding the role of cathelicidin hCAP-18/LL37 in the innate immune response to tuberculosis infection, we used immunohistochemistry, immunoelectron microscopy, and gene expression to study the induction and production of the antimicrobial peptide in A549 epithelial cells, alveolar macrophages (AM), neutrophils, and monocyte-derived macrophages (MDM) after infection with Mycobacterium tuberculosis. We demonstrated that mycobacterial infection induced the expression and production of LL-37 in all cells studied, with AM being the most efficient. We did not detect peptide expression in tuberculous granulomas, suggesting that LL-37 participates only during early infection. Through the study of Toll-like receptors (TLR) in MDM, we showed that LL-37 can be induced by stimulation through TLR-2, TLR-4, and TLR-9. This last TLR was strongly stimulated by M. tuberculosis DNA. We concluded that LL-37 may have an important role in the innate immune response against M. tuberculosis.

T cell expression of MyD88 is required for resistance to Toxoplasma gondii

Resistance to Toxoplasma gondii depends on dendritic cells to recognize this pathogen and secrete IL-12, in turn promoting IFN-gamma production from responding T cells. The adaptor protein, myeloid differentiation primary-response gene 88 (MyD88), is important for most Toll-like receptor (TLR) signaling, as well as IL-1R/IL-18R signals. There is considerable evidence that MyD88 is required for the innate sensing of T. gondii and IL-12 responses. Although Myd88(-/-) mice challenged with T. gondii have defective IL-12 and Th1 effector responses and succumb to disease, administration of IL-12 to Myd88(-/-) mice partially restores the Th1 response and yet fails to prolong survival. This finding suggested that MyD88 may mediate signals within T cells important for resistance to this pathogen. To evaluate the role of MyD88 in T cells under noncompetitive conditions, bone marrow chimeras were generated, in which the T cells lacked MyD88, but MyD88-dependent innate immune responses were intact. Upon challenge with T. gondii, these chimeric mice were more susceptible to disease, developing severe toxoplasmic encephalitis and succumbing within 30 days. Splenocytes and brain mononuclear cells isolated from infected chimeric mice produced less IFN-gamma when cultured with a T. gondii-derived antigen. The increase in susceptibility observed was independent of signals via the IL-1R and IL-18R, suggesting a role for TLRs in MyD88-mediated T cell responses to T. gondii. These observations show that, in addition to a role for MyD88 in innate responses, T cell expression of MyD88 is necessary for prolonged resistance to a pathogen.

Respiratory dendritic cells: mediators of tolerance and immunity

The respiratory tract is under constant bombardment from both innocuous and pathogenic material. The decision of how to respond to these challenges is mediated by a specialized set of antigen presenting cells within the lungs called dendritic cells (DC). Proper respiratory homeostasis requires that these respiratory DC (rDC) utilize both the local lung inflammatory environment as well as recognition of pathogen-specific patterns to determine whether to maintain homeostasis by either driving tolerance or immunity to the inhaled material. This review will focus on rDC and highlight how rDC regulate tolerance and immunity.

Tracking the elusive lymphocyte: methods of detection during adoptive immunotherapy

Adoptive immunotherapy is an attractive cancer treatment modality due to its capacity to target primary and metastatic lesions with large numbers of tumor-reactive, cytotoxic lymphocytes. The inability of fully armed lymphocytes to traffic into sites of tumor has been proposed as a causal factor for the minimal success observed clinically with this type of immunotherapy. The study of lymphocyte trafficking during adoptive immunotherapy has been limited, despite the existence of a variety of tracking methods. In murine models that simulate adoptive immunotherapy, the use of congenic mice and cell tracking dyes can be used to elucidate lymphocyte trafficking behavior. The continued development of novel technologies will further contribute to this expanding area of research.

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.

Mycobacterium tuberculosis, macrophages, and the innate immune response: does common variation matter?

Despite the discovery of the tuberculosis (TB) bacillus over 100 years ago and the availability of effective drugs for over 50 years, there remain a number of formidable challenges for controlling Mycobacterium tuberculosis (MTb). Understanding the genetic and immunologic factors that influence human susceptibility could lead to novel insights for vaccine development as well as diagnostic advances to target treatment to those who are at risk for developing active disease. Although a series of studies over the past 50 years suggests that host genetics influences resistance to TB, a comprehensive understanding of which genes and variants are associated with susceptibility is only partially understood. In this article, we review recent advances in our understanding of human variation of the immune system and its effects on macrophage function and influence on MTb susceptibility. We emphasize recent discoveries in human genetic studies and correlate these findings with efforts to understand how these variants alter the molecular and cellular functions that regulate the macrophage response to MTb.

TLR9 activation is a key event for the maintenance of a mycobacterial antigen-elicited pulmonary granulomatous response

Type 1 (Th1) granulomas can be studied in mice sensitized with mycobacterium antigens followed by challenge of agarose beads covalently coupled to purified protein derivative. TLR9 is known to play a role in the regulation of Th1 responses; thus, we investigated the role of TLR9 in granuloma formation during challenge with mycobacterium antigens and demonstrated that mice deficient in TLR9 had increased granuloma formation, but a dramatically altered cytokine phenotype. Th1 cytokine levels of IFN-gamma and IL-12 in the lungs were decreased in TLR9(-/-) mice when compared to wild-type mice. In contrast, Th2 cytokine levels of IL-4, IL-5, and IL-13 were increased in TLR9(-/-) mice. The migration of CD4(+) T cells in the granuloma was impaired, while the number of F4/80(+) macrophages was increased in TLR9(-/-) mice. Macrophages in the lungs of the TLR9-deficient animals with developing granulomas expressed significantly lower levels of the classically activated macrophage marker, nitric oxide synthase, but higher levels of the alternatively activated macrophage markers such as 'found in inflammatory zone-1' antigen and Arginase-1. These results suggest that TLR9 plays an important role in maintaining the appropriate phenotype in a Th1 granulomatous response.

NOD2 pathway activation by MDP or Mycobacterium tuberculosis infection involves the stable polyubiquitination of Rip2

The Rip2 kinase contains a caspase recruitment domain and has been implicated in the activation of the transcriptional factor NF-kappaB downstream of Toll-like receptors, Nod-like receptors, and the T cell receptor. Although Rip2 has been linked to Nod signaling, how Nod-Rip2 proteins mediate NF-kappaB activation has remained unclear. We find Rip2 required for Nod2-mediated NF-kappaB activation and to a lesser extent mitogen-activated protein kinase activation. We demonstrate that Rip2 and IkappaB kinase-gamma become stably polyubiquitinated upon treatment of cells with the NOD2 ligand, muramyl dipeptide. We also demonstrate a requirement for the E2-conjugating enzyme Ubc13, the E3 ubiquitin ligase Traf6, and the ubiquitin-activated kinase Tak1 in Nod2-mediated NF-kappaB activation. Rip2 polyubiquitination is also stimulated when macrophages are infected with live Mycobacterium tuberculosis but not when infected with heat-killed bacteria. Consistent with our data linking Rip2 to NOD and not Toll-like receptor signaling, M. tuberculosis-induced Rip2 polyubiquitination appears MyD88-independent. Collectively, these data reveal that the NOD2 pathway is ubiquitin-regulated and that Rip2 employs a ubiquitin-dependent mechanism to achieve NF-kappaB activation.

IL-1 receptor-mediated signal is an essential component of MyD88-dependent innate response to Mycobacterium tuberculosis infection

MyD88, the common adapter involved in TLR, IL-1, and IL-18 receptor signaling, is essential for the control of acute Mycobacterium tuberculosis (MTB) infection. Although TLR2, TLR4, and TLR9 have been implicated in the response to mycobacteria, gene disruption for these TLRs impairs only the long-term control of MTB infection. Here, we addressed the respective role of IL-1 and IL-18 receptor pathways in the MyD88-dependent control of acute MTB infection. Mice deficient for IL-1R1, IL-18R, or Toll-IL-1R domain-containing adaptor protein (TIRAP) were compared with MyD88-deficient mice in an acute model of aerogenic MTB infection. Although primary MyD88-deficient macrophages and dendritic cells were defective in cytokine production in response to mycobacterial stimulation, IL-1R1-deficient macrophages exhibited only a reduced IL-12p40 secretion with unaffected TNF, IL-6, and NO production and up-regulation of costimulatory molecules CD40 and CD86. Aerogenic MTB infection of IL-1R1-deficient mice was lethal within 4 wk with 2-log higher bacterial load in the lung and necrotic pneumonia but efficient pulmonary CD4 and CD8 T cell responses, as seen in MyD88-deficient mice. Mice deficient for IL-18R or TIRAP controlled acute MTB infection. These data demonstrate that absence of IL-1R signal leads to a dramatic defect of early control of MTB infection similar to that seen in the absence of MyD88, whereas IL-18R and TIRAP are dispensable, and that IL-1, together with IL-1-induced innate response, might account for most of MyD88-dependent host response to control acute MTB infection.

The pathogen recognition sensor, NOD2, is variably expressed in patients with pulmonary tuberculosis

Background

NOD2, an intracellular pathogen recognition sensor, modulates innate defences to muropeptides derived from various bacterial species, including Mycobacterium tuberculosis (MTB). Experimentally, NOD2 attenuates two key putative mycobactericidal mechanisms. TNF-α synthesis is markedly reduced in MTB-antigen stimulated-mononuclear cells expressing mutant NOD2 proteins. NOD2 agonists also induce resistance to apoptosis, and may thus facilitate the survival of MTB in infected macrophages. To further define a role for NOD2 in disease pathogenesis, we analysed NOD2 transcriptional responses in pulmonary leucocytes and mononuclear cells harvested from patients with pulmonary tuberculosis (PTB).

Methods

We analysed NOD2 mRNA expression by real-time polymerase chain-reaction in alveolar lavage cells obtained from 15 patients with pulmonary tuberculosis and their matched controls. We compared NOD2 transcriptional responses, in peripheral leucocytes, before and after anti-tuberculous treatment in 10 patients. In vitro, we measured NOD2 mRNA levels in MTB-antigen stimulated-mononuclear cells.

Results

No significant differences in NOD2 transcriptional responses were detected in patients and controls. In some patients, however, NOD2 expression was markedly increased and correlated with toll-like-receptor 2 and 4 expression. In whole blood, NOD2 mRNA levels increased significantly after completion of anti-tuberculosis treatment. NOD2 expression levels did not change significantly in mononuclear cells stimulated with mycobacterial antigens in vitro.

Conclusion

There are no characteristic NOD2 transcriptional responses in PTB. Nonetheless, the increased levels of NOD2 expression in some patients with severe tuberculosis, and the increases in expression levels within peripheral leucocytes following treatment merit further studies in selected patient and control populations.

Interleukin-12 and tuberculosis: an old story revisited

Our understanding of the role of interleukin (IL)-12 in controlling tuberculosis has expanded because of increased interest in other members of the IL-12 family of cytokines. Recent data show that IL-12, IL-23 and IL-27 have specific roles in the initiation, expansion and control of the cellular response to tuberculosis. Specifically, IL-12, and to a lesser degree IL-23, generates protective cellular responses and promotes survival, whereas IL-27 moderates the inflammatory response and is required for long-term survival. Paradoxically, IL-27 also limits bacterial control, suggesting that a balance between bacterial killing and tissue damage is required for survival. Understanding the balance between IL-12, IL-23 and IL-27 is crucial to the development of immune intervention in tuberculosis.

Immune factors and immunoregulation in tuberculosis

Pathogens causing tuberculosis and other chronic infectious diseases of major public health importance commonly have complex mechanisms involved in their persistence in the host despite specific and sometimes strong immune responses. These diseases are also associated with the lack of efficient vaccines, difficult therapeutics and a high mortality rate among susceptible individuals. Here, we will review features of the host immune response that contribute to the occurrence of disease. In addition, we propose that the immune responses observed in tuberculosis cannot be interpreted solely on the basis of a Th1-Th2 counter-regulatory paradigm since there is growing evidence that natural regulatory T cells may play an important role in the regulation of host immune responses against Mycobacterium tuberculosis. Thus, the development of more effective vaccines against this bacterial disease should take into account the role of natural regulatory T cells in the progression to severe disease and persistence of infection. Finally, new treatments based on manipulation of regulatory T cells should be investigated.

MyD88-dependent signals are essential for the host immune response in experimental brain abscess

Brain abscesses form in response to a parenchymal infection by pyogenic bacteria, with Staphylococcus aureus representing a common etiologic agent of human disease. Numerous receptors that participate in immune responses to bacteria, including the majority of TLRs, the IL-1R, and the IL-18R, use a common adaptor molecule, MyD88, for transducing activation signals leading to proinflammatory mediator expression and immune effector functions. To delineate the importance of MyD88-dependent signals in brain abscesses, we compared disease pathogenesis using MyD88 knockout (KO) and wild-type (WT) mice. Mortality rates were significantly higher in MyD88 KO mice, which correlated with a significant reduction in the expression of several proinflammatory mediators, including but not limited to IL-1beta, TNF-alpha, and MIP-2/CXCL2. These changes were associated with a significant reduction in neutrophil and macrophage recruitment into brain abscesses of MyD88 KO animals. In addition, microglia, macrophages, and neutrophils isolated from the brain abscesses of MyD88 KO mice produced significantly less TNF-alpha, IL-6, MIP-1alpha/CCL3, and IFN-gamma-induced protein 10/CXCL10 compared with WT cells. The lack of MyD88-dependent signals had a dramatic effect on the extent of tissue injury, with significantly larger brain abscesses typified by exaggerated edema and necrosis in MyD88 KO animals. Interestingly, despite these striking changes in MyD88 KO mice, bacterial burdens did not significantly differ between the two strains at the early time points examined. Collectively, these findings indicate that MyD88 plays an essential role in establishing a protective CNS host response during the early stages of brain abscess development, whereas MyD88-independent pathway(s) are responsible for pathogen containment.

Disparity in IL-12 release in dendritic cells and macrophages in response to Mycobacterium tuberculosis is due to use of distinct TLRs

The control of IL-12 production from dendritic cells (DCs) and macrophages in response to Mycobacterium tuberculosis (Mtb) is not well understood. The objective of this study was to pursue the mechanism underlying our previous report that in response to Mtb infection, DCs release abundant IL-12, whereas secretion is limited in macrophages. An initial comparison of IL-12p35 and IL-12p40 gene induction showed that p35 transcription is similar in murine bone marrow-derived DCs and macrophages, but a rapid and enhanced IL-12p40 transcription occurs only in DCs. Consistent with the p40 gene transcription profile, Mtb-induced remodeling at nucleosome 1 of the p40 promoter also occurs rapidly and extensively in DCs in comparison to macrophages. Removal of IL-10 or addition of IFNgamma enhances macrophage IL-12 release to Mtb, but without affecting the kinetics of remodeling at the macrophage p40 promoter. Furthermore, we show that Mtb-induced remodeling at the p40 promoter and IL-12 release in DCs is TLR9 dependent, and in contrast, TLR2 dependent, in macrophages. Data are also presented to demonstrate that a TLR9 agonist induces quantitatively more extensive remodeling at the IL-12p40 promoter and larger IL-12 release in comparison to a TLR2 agonist. Collectively, these findings suggest that DCs and macrophages handle Mtb differently resulting in only DCs being able to engage the more efficient TLR9 pathway for IL-12 gene induction. Our results also imply that TLR2 signaling is not a good inducer of IL-12, supporting the increasingly strong paradigm that TLR2 favors Th2 responses.

Potential functional role of plasmacytoid dendritic cells in cancer immunity

Plasmacytoid dendritic cells (pDCs), as well as myeloid dendritic cells (mDCs), have a dual role not only in initiating immune responses but also in inducing tolerance to exogenous and endogenous antigens. Tumour antigens originate from endogenous self-antigens, which are poorly immunogenic and also subject to change during tumour progression. In general, tumour antigens derived from apoptotic cells are captured by immature mDCs, antigen presentation by which is most likely to result in immune tolerance. In contrast, tumour antigens may be taken up by pDCs through Toll-like receptor 9 (TLR9) via receptor-mediated endocytosis. TLR9-dependent activation of pDCs results in the secretion of pro-inflammatory cytokines such as interleukin (IL)-12 and type I interferons (IFNs) through a MyD88-dependent pathway. Type I IFNs also activate mDCs for T-cell priming. Although pDCs recruited to the tumour site are implicated in facilitating tumour growth via immune suppression, they can be released from the tumour as a result of cell death caused by primary systemic chemotherapy, and can then be activated through TLR9. Thus, synergistically with mDCs, pDCs may also play a crucial role in mediating cancer immunity. In this review, the potential functional duality and plasticity of pDCs mediated by TLR9 ligation in cancer immunity will be discussed.

MyD88-dependent changes in the pulmonary transcriptome after infection with Chlamydia pneumoniae

Chlamydia pneumoniae, an intracellular bacterium, causes pneumonia in humans and mice. Toll-like receptors and the key adaptor molecule myeloid differentiation factor-88 (MyD88) play a critical role in inducing immunity against this microorganism and are crucial for survival. To explore the influence of MyD88 on induction of immune responses in vivo on a genome-wide level, wildtype (WT) or MyD88(-/-) mice were infected with C. pneumoniae on anesthesia, and the pulmonary transcriptome was analyzed 3 days later by microarrays. We found that the infection caused pulmonary cellular infiltration in WT but not MyD88(-/-) mice. Furthermore, it induced the transcription of 360 genes and repressed 18 genes in WT mice. Of these, 221 genes were not or weakly induced in lungs of MyD88(-/-) mice. This cluster contains primarily genes encoding for chemokines and cytokines like MIP-1alpha, MIP-2, MIP-1gamma, MCP-1, TNF, and KC and other immune effector molecules like immunoresponsive gene-1 and TLR2. Arginase was highly induced after C. pneumoniae infection and was MyD88 dependent. Genes induced by interferons were abundant in a cluster of 102 genes that were only partially MyD88 dependent. Also, lcn2 (lipocalin-2) and timp1 were represented within this cluster. Interestingly, a set of 37 genes including sprr1a was induced more strongly in MyD88(-/-) mice, and most of them are involved in the regulation of cellular replication. In summary, ex vivo analysis of the pulmonary transcriptome on infection with C. pneumoniae demonstrated a major impact of MyD88 on inflammatory responses but not on interferon-type responses and identified MyD88-independent genes involved in cellular replication.

Toll or toll-free adjuvant path toward the optimal vaccine development

Successful vaccines contain an adjuvant component that activates the innate immune system, thereby eliciting antigen-specific immune responses. Many adjuvants appear to be ligands for toll-like receptors (TLR), which are thus promising targets for the development of novel adjuvants to elicit vaccine immunogenicity. However, recent evidence suggests that some adjuvants activate the innate immune system in a TLR-independent manner possibly through other pattern recognition receptors and signaling machinery. In particular, newly identified intracellular retinoic-acid-inducible gene (RIG)-like receptors, NOD-like receptors, or even as yet unknown recognition machinery for the adjuvant may regulate TLR-independent vaccine immunogenicity. To develop optimal vaccines, it will be critical to understand how TLR-dependent and TLR-independent innate immune activation, by various adjuvants, control the consequent adaptive immune responses to vaccine.

Intracellular signalling cascades regulating innate immune responses to Mycobacteria: branching out from Toll-like receptors

Toll-like receptors (TLRs) recognize Mycobacterium tuberculosis (Mtb) or Mtb components and initiate mononuclear phagocyte responses that influence both innate and adaptive immunity. Recent studies have revealed the intracellular signalling cascades involved in the TLR-initiated immune response to mycobacterial infection. Although both TLR2 and TLR4 have been implicated in host interactions with Mtb, the relationship between specific mycobacterial molecules and various signal transduction pathways is not well understood. This review will discuss recent studies indicating critical roles for mycobacteria and mycobacterial components in regulation of mitogen-activated protein kinases and related signal transduction pathways that govern the outcome of infection and antibacterial defence. To better understand the roles of infection-induced signalling cascades in molecular pathogenesis, future studies are needed to clarify mechanisms that integrate the multiple signalling pathways that are activated by engagement of TLRs by both individual mycobacterial molecules and whole mycobacteria. These efforts will allow for the development of novel diagnostic and therapeutic modalities for tuberculosis that targets the intracellular signalling pathways permitting the replication of this nefarious pathogen.

What is the role of Toll-like receptors in bacterial infections?

Innate immunity relies on signalling by Toll-like receptors (TLRs) to alert the immune system of the presence of invading bacteria. TLR activation leads to the release of cytokines that allow for effective innate and adaptive immune responses. However, the contribution of different TLRs depends on the site of the infection and the pathogen. This review will describe the involvement of TLRs in the development of three different bacterial infections as well as our current understanding of the role of TLRs during microbial pathogenesis.

Mycobacterium tuberculosis subverts innate immunity to evade specific effectors

The macrophage is the niche of the intracellular pathogen Mycobacterium tuberculosis. Induction of macrophage apoptosis by CD4(+) or CD8(+) T cells is accompanied by reduced bacterial counts, potentially defining a host defense mechanism. We have already established that M. tuberculosis-infected primary human macrophages have a reduced susceptibility to Fas ligand (FasL)-induced apoptosis. To study the mechanisms by which M. tuberculosis prevents apoptotic signaling, we have generated a cell culture system based on PMA- and IFN-gamma-differentiated THP-1 cells recapitulating the properties of primary macrophages. In these cells, nucleotide-binding oligomerization domain 2 or TLR2 agonists and mycobacterial infection protected macrophages from apoptosis and resulted in NF-kappaB nuclear translocation associated with up-regulation of the antiapoptotic cellular FLIP. Transduction of a receptor-interacting protein-2 dominant-negative construct showed that nucleotide-binding oligomerization domain 2 is not involved in protection in the mycobacterial infection system. In contrast, both a dominant-negative construct of the MyD88 adaptor and an NF-kappaB inhibitor abrogated the protection against FasL-mediated apoptosis, showing the implication of TLR2-mediated activation of NF-kappaB in apoptosis protection in infected macrophages. The apoptosis resistance of infected macrophages might be considered as an immune escape mechanism, whereby M. tuberculosis subverts innate immunity signaling to protect its host cell against FasL(+)-specific cytotoxic lymphocytes.

Effect of phthiocerol dimycocerosate deficiency on the transcriptional response of human macrophages to Mycobacterium tuberculosis

The control of mycobacterial infections is dependent on the finely tuned synergism between the innate and adaptive immune responses. The macrophage is the major host cell for Mycobacterium tuberculosis and the degree of virulence of mycobacteria may influence the initial macrophage response to infection. The cell wall molecule, phthiocerol dimycocerosate (DIM), is an important virulence factor that influences the early growth of M. tuberculosis in the lungs. To explore the basis for this effect we have compared the early gene response of human THP-1 macrophages to infection with virulent M. tuberculosis and the DIM-deficient DeltafadD26 M. tuberculosis strain using microarrays. Detailed analysis revealed a common core of macrophage genes, which were rapidly induced following infection with both strains, and deficiency of DIM had no significant effect on this initial macrophage transcriptional responses. In addition to chemokines and pro-inflammatory cytokines, the early response genes included components of the Toll-like receptor signalling, antigen presentation and apoptotic pathways, interferon response genes, cell surface receptors and their ligands, including TNF-related apoptosis inducing ligand (TRAIL) and CD40, and other novel genes. Therefore, although fadD26 deficiency is responsible for the early attenuation of the growth of M. tuberculosis in vivo, this effect is not associated with differences in the initial macrophage transcriptional response.

Adenovirus infection triggers a rapid, MyD88-regulated transcriptome response critical to acute-phase and adaptive immune responses in vivo

Nearly 50 years ago, the discovery of interferon prompted the notion that host cells innately respond to viral invasion. Since that time, technological advances have allowed this response to be extensively characterized and dissected in vitro. However, these advances have only recently been applied to highly complex, in vivo biological systems. To this end, we exploited high-titer adenovirus (Ad) vectors to globally investigate the innate immune response to nonenveloped viral infection in vivo. Our results indicated a potent cellular transcriptome response shortly after infection, with global assessments revealing significant dysregulation in approximately 15% of the measured transcripts derived from Ad vector-transduced tissue. Bioinformatics-based transcriptome analysis revealed a complex innate response to Ad infection, with induction of proinflammatory responses (and suppression of metabolism and mitochondrial genes) akin to those observed when mice are challenged with lipopolysaccharide. Despite this commonality, there were many unique aspects of the Ad-dependent transcriptome response, including the upregulation of several RNA regulatory mechanisms and apoptosis-related pathways, accompanied by the suppression of lysosomal and endocytic genes. Our results also implicated the Toll-like receptors (TLRs) in these responses, prompting specific investigations into this pathway. By using MyD88KO mice, our results confirmed that Ad-induced dysregulation of five functionally related gene clusters are significantly dependent on this TLR adaptor gene. MyD88 deficiency also resulted in significantly diminished, although not abolished, adaptive and acute-phase immune responses to Ad, confirming the transcriptome data, as well as specifically identifying MyD88 as a significant Ad immunity amplifier and regulator in vivo.

Evidence for licensing of IFN-gamma-induced IFN regulatory factor 1 transcription factor by MyD88 in Toll-like receptor-dependent gene induction program

The recognition of microbial components by Toll-like receptors (TLRs) initiates signal transduction pathways, which trigger the expression of a series of target genes. It has been reported that TLR signaling is enhanced by cytokines such as IFN-gamma, but the mechanisms underlying this enhancement remain unclear. The MyD88 adaptor, which is essential for signaling by many TLRs, recruits members of the IFN regulatory factor (IRF) family of transcription factors, such as IRF5 and IRF7, to evoke the activation of TLR target genes. In this study we demonstrate that IRF1, which is induced by IFN-gamma, also interacts with and is activated by MyD88 upon TLR activation. We provide evidence that MyD88-associated IRF1 migrates into the nucleus more efficiently than non-MyD88-associated IRF1 and that this IRF1 selectively participates in the TLR-dependent gene induction program. The critical role of MyD88-dependent "IRF1 licensing" is underscored by the observation that the induction of a specific gene subset downstream of the TLR-MyD88 pathway, such as IFN-beta, inducible NO synthase, and IL-12p35, are impaired in Irf1-deficient cells. Thus, our present study places IRF1 as an additional member participating in MyD88 signaling and provides a mechanistic insight into the enhancement of the TLR-dependent gene induction program by IFN-gamma.

Innate Immune Activation and CD4+ T Cell Priming during Respiratory Fungal Infection

Aspergillus fumigatus is a mold that causes a spectrum of diseases, including lethal lung infections in immunocompromised humans and allergic asthma in atopic individuals. T helper 1 (Th1) CD4(+) T cells protect against invasive A. fumigatus infections whereas Th2 CD4(+) T cells exacerbate asthma upon inhalation of A. fumigatus spores. Herein, we demonstrate that A. fumigatus-specific T cells were rapidly primed in lymph nodes draining the lung and fully differentiated into interferon-gamma (IFN-gamma)-producing Th1 CD4(+) T cells upon arrival in the airways. T-bet induction in A. fumigatus-specific CD4(+) T cells was enhanced by MyD88-mediated signals in draining lymph nodes, but T cell proliferation, trafficking, and Th1 differentiation in the airways were Toll-like receptor (TLR) and MyD88 independent. Our studies demonstrate that CD4(+) T cell differentiation during respiratory fungal infection occurs incrementally, with TLR-mediated signals in the lymph node enhancing the potential for IFN-gamma production whereas MyD88-independent signals promote Th1 differentiation in the lung.

Protective and destructive immunity in the periodontium: Part 1--innate and humoral immunity and the periodontium

Based on the results of recent research in the field, the present paper will discuss the protective and destructive aspects of the innate vs. adaptive (humoral and cell-mediated) immunity associated with the bacterial virulent factors or antigenic determinants during periodontal pathogenesis. Attention will be focused on: (i) the Toll-like receptors (TLR), the innate immune repertoire for recognizing the unique molecular patterns of microbial components that trigger innate and adaptive immunity for effective host defenses, in some general non-oral vs. periodontal microbial infections; (ii) T-cell-mediated immunity, Th-cytokines, and osteoclastogenesis in periodontal disease progression; and (iii) some molecular techniques developed and used to identify critical microbial virulence factors or antigens associated with host immunity (using Actinobacillus actinomycetemcomitans and Porphyromonas gingivalis as the model species). Therefore, further understanding of the molecular interactions and mechanisms associated with the host's innate and adaptive immune responses will facilitate the development of new and innovative therapeutics for future periodontal treatments.

Toll-like receptor 9 contributes to recognition of Mycobacterium bovis Bacillus Calmette-Guérin by Flt3-ligand generated dendritic cells

Recognition of mycobacteria by the innate immune system is essential for the development of an adaptive immune response. Mycobacterial antigens stimulate antigen presenting cells (APCs) through distinct Toll-like receptors (TLRs) resulting in rapid activation of the innate immune system. The role of TLRs during infection with Mycobacterium bovis Bacillus Calmette-Guérin (BCG) has been evaluated for TLR2 and TLR4 only. Surprisingly, despite the fact that immune stimulatory CpG-motifs have been originally derived from BCG, for the vaccine strain the role of TLR9 has not been addressed before. To identify the set of TLRs involved in the recognition of BCG, we infected bone marrow-derived macrophages and bone marrow-derived dendritic cells (Flt3-ligand generated DCs) from TLR2, TLR3, TLR4, TLR7, TLR9, MyD88 knockout, TLR2/4 and TLR2/4/9 multiple knockout mice. The degree of activation and stimulation was determined by TNFalpha, IL-6 and IL-12p40 ELISA. Activation of DCs was measured by surface expression of the costimulatory molecule CD86. We observed the most dramatic reduction of the inflammatory response for TLR2-deficient antigen presenting cells. Both macrophages and DCs produce markedly decreased amounts of TNFalpha and IL-6 in the absence of TLR2 whereas no significant reduction could be observed for TLR3, 4, 7, 9 single TLR-knockouts. However, IL-12 production in DCs appears not exclusively dependent on TLR2 and only in TLR2/4/9-deficient DCs BCG-induced IL-12 is reduced to background levels. Similarly, up-regulation of CD86 is abolished only in TLR2/4/9-deficient DCs supporting a role of TLR9 in the recognition of M. bovis BCG by murine dendritic cells.

Mycobacterium tuberculosis LprA is a lipoprotein agonist of TLR2 that regulates innate immunity and APC function

TLR2 recognizes components of Mycobacterium tuberculosis (Mtb) and initiates responses by APCs that influence both innate and adaptive immunity. Mtb lipoproteins are an important class of TLR2 ligand, but only two, LpqH and LprG, have been characterized to date. In this study, we characterize a third Mtb lipoprotein, LprA, and determine its effects on host macrophages and dendritic cells. LprA is a cell wall-associated lipoprotein with no homologs outside the slow-growing mycobacteria. Using Mycobacterium smegmatis as an expression host, we purified 6x His-tagged LprA both with and without its acyl modifications. Acylated LprA had agonist activity for both human and murine TLR2 and induced expression of TNF-alpha, IL-10, and IL-12. LprA also induced dendritic cell maturation as shown by increased expression of CD40, CD80, and class II MHC (MHC-II). In macrophages, prolonged (24 h) incubation with LprA decreased IFN-gamma-induced MHC-II Ag processing and presentation, consistent with an observed decrease in MHC-II expression (macrophage viability was not affected and apoptosis was not induced by LprA). Reduced MHC-II Ag presentation may represent a negative feedback mechanism for control of inflammation that may be subverted by Mtb for immune evasion. Thus, Mtb LprA is a TLR2 agonist that induces cytokine responses and regulates APC function.

IL-32: an emerging player in the immune response network against tuberculosis?

IL-32 has emerged as an important player in innate and adaptive immune responses. Kundu and Basu discuss a new study in PLoS Medicine that explored the role of IL-32 in the context of M. tuberculosis infection.

Central role for MyD88 in the responses of microglia to pathogen-associated molecular patterns

Microglia, the innate immune effector cells of the CNS parenchyma, express TLR that recognize conserved motifs of microorganisms referred to as pathogen-associated molecular patterns (PAMP). All TLRs identified to date, with the exception of TLR3, use a common adaptor protein, MyD88, to transduce activation signals. Recently, we reported that microglial activation in response to the Gram-positive bacterium Staphylococcus aureus was not completely attenuated following TLR2 ablation, suggesting the involvement of additional receptors. To assess the functional role of alternative TLRs in microglial responses to S. aureus and its cell wall product peptidoglycan as well as the Gram-negative PAMP LPS, we evaluated primary microglia from MyD88 knockout (KO) and wild-type mice. The induction of TNF-alpha, IL-12 p40, and MIP-2 (CXCL2) expression by S. aureus- and peptidoglycan-stimulated microglia was MyD88 dependent, as revealed by the complete inhibition of cytokine production in MyD88 KO cells. In addition, the expression of additional pattern recognition receptors, including TLR9, pentraxin-3, and lectin-like oxidized LDL receptor-1, was regulated, in part, via a MyD88-dependent manner as demonstrated by the attenuated expression of these receptors in MyD88 KO microglia. Microglial activation was only partially inhibited in LPS-stimulated MyD88 KO cells, suggesting the involvement of MyD88-independent pathways. Collectively, these findings reveal the complex mechanisms for microglia to respond to diverse bacterial pathogens, which occur via both MyD88-dependent and -independent pathways.

MyD88 deficiency results in tissue-specific changes in cytokine induction and inflammation in interleukin-18-independent mice infected with Borrelia burgdorferi

Toll-like receptors (TLRs) play an important role in the control of infection with Borrelia burgdorferi. Deficiencies in TLR-2 or the shared TLR adapter molecule MyD88 have been shown to result in greatly increased bacterial burdens in mice. However, although in vitro studies have shown that the activation of TLR pathways by B. burgdorferi results in the release of inflammatory cytokines, studies in deficient mice have shown either no change or increased rather than decreased inflammation in infected animals. In this study, we looked at mechanisms to explain the increase in inflammation in the absence of MyD88. We found that MyD88-deficient mice infected with B. burgdorferi did not show increased inflammation at sites typically associated with Lyme disease (joints and heart). However, there was markedly increased inflammation in the muscles, kidneys, pancreas, and lungs of deficient animals. Muscle inflammation was typically seen perivascularly and perineuronally similar to that seen in infected humans. Chemotactic chemokines and cytokines were greatly increased in the muscle and kidneys of MyD88-deficient animals but not in the joints or heart tissue, suggesting that MyD88-independent pathways for recognizing B. burgdorferi and inducing these chemokines are present in the muscle and kidneys. Interleukin-18 signaling through MyD88 does not appear to play a role in either control of infection or inflammation.

Determinant role for Toll-like receptor signalling in acute mycobacterial infection in the respiratory tract

Toll-like receptors (TLRs) are a vital component of the innate branch of the immune system in its battle against mycobacterial infections. Extensive in vitro studies have demonstrated a role for both TLR2 and TLR4 in recognition of mycobacterial components, whereas the in vivo situation appears less clear, with results depending on the infection model. In the present work, the importance of TLR signalling in the course of mycobacterial infection was investigated in a human-like infection model using TLR-knockout mice. TLR2(-/-) and TLR4(-/-) mice infected with Mycobacterium tuberculosis by aerosol, or for the first time, intranasally with Mycobacterium bovis bacillus Calmette-Guérin (BCG), displayed increased susceptibility at an early stage of infection in the respiratory tract, while at a later stage of infection, the TLR deficiency appeared to be overcome. The higher susceptibility was correlated to impaired pro-inflammatory responses to BCG components, and reduced induction of anti-bacterial activity by infected macrophages from TLR2(-/-) mice, and to a lesser extent from TLR4(-/-) mice. These findings demonstrate a role for TLR signalling in protection against mycobacterial infection specifically in the respiratory tract at the acute phase, whereas the TLR deficiency can be compensated at a later stage of infection.

TLR9 regulates Th1 responses and cooperates with TLR2 in mediating optimal resistance to Mycobacterium tuberculosis

To investigate the role of Toll-like receptor (TLR)9 in the immune response to mycobacteria as well as its cooperation with TLR2, a receptor known to be triggered by several major mycobacterial ligands, we analyzed the resistance of TLR9^−/− as well as TLR2/9 double knockout mice to aerosol infection with Mycobacterium tuberculosis. Infected TLR9^−/− but not TLR2^−/− mice displayed defective mycobacteria-induced interleukin (IL)-12p40 and interferon (IFN)-γ responses in vivo, but in common with TLR2^−/− animals, the TLR9^−/− mice exhibited only minor reductions in acute resistance to low dose pathogen challenge. When compared with either of the single TLR-deficient animals, TLR2/9^−/− mice displayed markedly enhanced susceptibility to infection in association with combined defects in proinflammatory cytokine production in vitro, IFN-γ recall responses ex vivo, and altered pulmonary pathology. Cooperation between TLR9 and TLR2 was also evident at the level of the in vitro response to live M. tuberculosis, where dendritic cells and macrophages from TLR2/9^−/− mice exhibited a greater defect in IL-12 response than the equivalent cell populations from single TLR9-deficient animals. These findings reveal a previously unappreciated role for TLR9 in the host response to M. tuberculosis and illustrate TLR collaboration in host resistance to a major human pathogen.

Human host genetic factors in nontuberculous mycobacterial infection: lessons from single gene disorders affecting innate and adaptive immunity and lessons from molecular defects in interferon-gamma-dependent signaling

Mendelian defects in interferon-gamma (IFN-gamma) signaling most commonly lead to infection with nontuberculous mycobacteria. Mutations have been identified in the genes encoding IFN-gamma-receptor-1, IFN-gamma-receptor-2 and Stat-1. Partial and complete deficiencies in signaling are found, leading to parallel clinical, pathological, and cellular phenotypes. These rare defects have led to better molecular and mechanistic understanding of the role of IFN-gamma in the human immune system.

MyD88-mediated stabilization of interferon-gamma-induced cytokine and chemokine mRNA

The MyD88 adaptor protein is critical in Toll-like receptor and interleukin 1 receptor (IL-1R) signaling, but has not been linked to interferon-gamma receptor (IFN-gammaR) signaling. Here we demonstrate that MyD88 increased the half-life but not the synthesis of IFN-gamma-induced mRNA transcripts encoding tumor necrosis factor and IFN-gamma-inducible protein 10. IFN-gamma stimulation triggered a physical association between the IFN-gammaR1 and MyD88. Transcript stabilization required activation of mixed-lineage kinase 3 and p38 mitogen-activated protein kinase and the presence of an adenine-uridine-rich element in the transcript's 3' untranslated region. These results demonstrate a MyD88-dependent post-transcriptional mechanism through which IFN-gamma can enhance the expression of genes encoding proinflammatory molecules.

Membrane tumor necrosis factor confers partial protection to Listeria infection

Tumor necrosis factor (TNF) plays a critical role in the host response to the intracellular pathogen Listeria monocytogenes (LM). TNF exists in soluble and membrane-bound forms and exhibits both unique and overlapping activities. We examined the role of membrane TNF in the absence of secreted TNF for host resistance in knockin mice in which the endogenous TNF was replaced by a regulated, noncleavable allele (mem-TNF). Macrophages expressing mem-TNF produced nitric oxide and displayed normal bactericidal activity. Although mice completely deficient in TNF (TNF(-/-)) succumbed to LM infection within 4 days, mem-TNF mice controlled LM infection at a low dose (10(4) CFU) but succumbed at a higher dose of infection (10(5) CFU). In contrast to complete TNF deficiency, mem-TNF mice developed confined microabscesses that expressed inducible nitric oxide synthase. The transfer of lymphocytes from immunized mem-TNF, but not TNF(-/-), mice protected TNF(-/-) mice from fatal infection. Taken together the data suggest that in the absence of soluble TNF, the presence of membrane-expressed TNF on phagocytes and lymphocytes partially restores host defense to LM infection.

Toll-like receptors: sentinels of host defence against bacterial infection

Innate immunity provides a first line of host defence against infection through microbial recognition and killing while simultaneously activating a definitive adaptive immune response. Toll-like receptors (TLRs) are principal mediators of rapid microbial recognition and function mainly by detection of structural patterns that do not exist in the host. TLR2 and TLR4 were the first members of this innate immune receptor family to be strongly implicated in antibacterial host defence. Following the initial description of the mammalian TLR family, susceptibility to infection with numerous human microbial pathogens has been intensively studied using mice with engineered deletions of each of these molecules. While it has become quite clear that TLR activation is necessary for optimal host defence, a comprehensive understanding of the mechanisms by which this family of pattern recognition receptors engages protective immunity, particularly the adaptive response, is still evolving.

NOD2 and toll-like receptors are nonredundant recognition systems of Mycobacterium tuberculosis

Infection with Mycobacterium tuberculosis is one of the leading causes of death worldwide. Recognition of M. tuberculosis by pattern recognition receptors is crucial for activation of both innate and adaptive immune responses. In the present study, we demonstrate that nucleotide-binding oligomerization domain 2 (NOD2) and Toll-like receptors (TLRs) are two nonredundant recognition mechanisms of M. tuberculosis. CHO cell lines transfected with human TLR2 or TLR4 were responsive to M. tuberculosis. TLR2 knock-out mice displayed more than 50% defective cytokine production after stimulation with mycobacteria, whereas TLR4-defective mice also released 30% less cytokines compared to controls. Similarly, HEK293T cells transfected with NOD2 responded to stimulation with M. tuberculosis. The important role of NOD2 for the recognition of M. tuberculosis was demonstrated in mononuclear cells of individuals homozygous for the 3020insC NOD2 mutation, who showed an 80% defective cytokine response after stimulation with M. tuberculosis. Finally, the mycobacterial TLR2 ligand 19-kDa lipoprotein and the NOD2 ligand muramyl dipeptide synergized for the induction of cytokines, and this synergism was lost in cells defective in either TLR2 or NOD2. Together, these results demonstrate that NOD2 and TLR pathways are nonredundant recognition mechanisms of M. tuberculosis that synergize for the induction of proinflammatory cytokines.

Tuberculosis is one of the most prevalent infections worldwide, with 2 billion people believed to be infected, and 2 million deaths each year. In addition to representing a major health care problem in developing countries, concern is also growing about the increased incidence of tuberculosis in developed countries, especially in immunocompromised patients such as those with AIDS, transplantation, and immunosuppressive therapy. The present study describes the pathways that enable leukocytes to recognize M. tuberculosis, and demonstrates for the first time that NOD2, member of a new class of intracellular receptors, is an independent recognition mechanism for mycobacteria. NOD2 acts together with the earlier-described Toll-like receptors for the activation of host defenses during the encounter of leukocytes with M. tuberculosis. Understanding the mechanisms through which the cells of the immune system recognize M. tuberculosis can be an important step in designing new therapeutic approaches, as well as improving the limited success of current vaccination strategies.

Triggering TLR signaling in vaccination

Toll-like receptors (TLRs) are a family of pattern-recognition receptors that are an important link between innate and adaptive immunity. Many established, as well as experimental, vaccines incorporate ligands for TLRs, not only to protect against infectious diseases but also in therapeutic immunization against noninfectious diseases, such as cancer. We review the underlying mechanisms by which engagement of TLR signaling pathways might trigger an adaptive immune response after immunization. Although the engagement of TLR signaling pathways is a promising mechanism for boosting vaccine responses, questions of efficacy, feasibility and safety remain the subject of active investigation.

Membrane TNF confers protection to acute mycobacterial infection

Background

Tumour necrosis factor (TNF) is crucial for the control of mycobacterial infection as TNF deficient (KO) die rapidly of uncontrolled infection with necrotic pneumonia. Here we investigated the role of membrane TNF for host resistance in knock-in mice with a non-cleavable and regulated allele (mem-TNF).

Methods

C57BL/6, TNF KO and mem-TNF mice were infected with M. tuberculosis H37Rv (Mtb at 100 CFU by intranasal administration) and the survival, bacterial load, lung pathology and immunological parameters were investigated. Bone marrow and lymphocytes transfers were used to test the role of membrane TNF to confer resistance to TNF KO mice.

Results

While TNF-KO mice succumbed to infection within 4–5 weeks, mem-TNF mice recruited normally T cells and macrophages, developed mature granuloma in the lung and controlled acute Mtb infection. However, during the chronic phase of infection mem-TNF mice succumbed to disseminated infection with necrotic pneumonia at about 150 days. Reconstitution of irradiated TNF-KO mice with mem-TNF derived bone marrow cells, but not with lymphocytes, conferred host resistance to Mtb infection in TNF-KO mice.

Conclusion

Membrane expressed TNF is sufficient to allow cell-cell signalling and control of acute Mtb infection. Bone marrow cells, but not lymphocytes from mem-TNF mice confer resistance to infection in TNF-KO mice. Long-term infection control with chronic inflammation likely disrupting TNF mediated cell-cell signalling, additionally requires soluble TNF.

Host innate and Th1 responses and the bacterial factors that control Mycobacterium tuberculosis infection

It is clear that resistance against acute tuberculosis (TB) is dependent on the host's ability to generate Th1 immunity. Nevertheless, the role of host immunity in latent TB remains incompletely defined. Recent progress in elucidating host innate and adaptive immune responses to M. tuberculosis (Mtb) and their impact on latent infection includes identification of TLR2-dependent anti-inflammatory responses, a MyD88-independent, non-protective Th1 response, the formation of secondary lymphoid follicles in granulomas and the role of Th1 responses, IFN-gamma and TNF-alpha in preventing re-activation of infection; IFN-gamma also appears to be involved in activating latency genes in Mtb. When Mtb re-infects a patient, it appears to localize in established granulomas; however, different bacterial strains may behave differently. Although these advances do not provide all the answers regarding host defense mechanisms, they nevertheless bring us closer to new and better design strategies for immunotherapy and immunoprophylaxis.

Toll-like receptors and chronic lung disease

TLRs (Toll-like receptors) comprise a family of proteins whose function is principally to facilitate the detection of, and response to, pathogens. Protozoa, helminths, viruses, bacteria and fungi can all activate TLR signalling, and these signals have important roles in the activation of host defence. TLRs may also respond to products of tissue damage, providing them with roles in infective and sterile inflammation. Their role as detectors of pathogens and pathogen-associated molecules provides molecular mechanisms to underpin the observations leading to the hygiene hypothesis. Targeting of TLR signalling has implications in the control of infection, vaccine design, desensitization to allergens and down-regulation of inflammation. This review will explore TLR history, molecular signalling and the potential roles of TLRs in chronic lung disease.

Innate immunity to mycobacterial infection in mice: Critical role for toll-like receptors

Toll-like receptors (TLRs) play a critical role in the recognition of several pathogens, including Mycobacterium tuberculosis. Mycobacterial antigens recognize distinct TLRs resulting in rapid activation of cells of the innate immune system. Ablation of most of the TLR signalling as in mice deficient for the common adaptor protein MyD88 reveals that TLR is crucial for the activation of an innate immune response. MyD88-deficient mice are unable to clear virulent mycobacteria and succumb to acute necrotic pneumonia. Despite the profound defect of the innate immune response, MyD88 deficiency allows the emergence of an adaptive immunity. These data demonstrate that activation of multiple TLRs contributes to an efficient innate response to mycobacteria, while MyD88-dependent signalling is dispensable to generate adaptive immunity.

Expression of Many Immunologically Important Genes inMycobacterium tuberculosis-Infected Macrophages Is Independent of Both TLR2 and TLR4 but Dependent on IFN-αβ Receptor and STAT1

Macrophages respond to several subcellular products of Mycobacterium tuberculosis (Mtb) through TLR2 or TLR4. However, primary mouse macrophages respond to viable, virulent Mtb by pathways largely independent of MyD88, the common adaptor molecule for TLRs. Using microarrays, quantitative PCR, and ELISA with gene-disrupted macrophages and mice, we now show that viable Mtb elicits the expression of inducible NO synthase, RANTES, IFN-inducible protein 10, immune-responsive gene 1, and many other key genes in macrophages substantially independently of TLR2, TLR4, their combination, or the TLR adaptors Toll-IL-1R domain-containing adapter protein and Toll-IL-1R domain-containing adapter inducing IFN-beta. Mice deficient in both TLR2 and TLR4 handle aerosol infection with viable Mtb as well as congenic controls. Viable Mtb also up-regulates inducible NO synthase, RANTES, IFN-inducible protein 10, and IRG1 in macrophages that lack mannose receptor, complement receptors 3 and 4, type A scavenger receptor, or CD40. These MyD88, TLR2/4-independent transcriptional responses require IFN-alphabetaR and STAT1, but not IFN-gamma. Conversely, those genes whose expression is MyD88 dependent do not depend on IFN-alphabetaR or STAT1. Transcriptional induction of TNF is TLR2/4, MyD88, STAT1, and IFN-alphabetaR independent, but TNF protein release requires the TLR2/4-MyD88 pathway. Thus, macrophages respond transcriptionally to viable Mtb through at least three pathways. TLR2 mediates the responses of a numerically minor set of genes that collectively do not appear to affect the course of infection in mice; regulation of TNF requires TLR2/4 for post-transcriptional control, but not for transcriptional induction; and many responding genes are regulated through an unknown, TLR2/4-independent pathway that may involve IFN-alphabetaR and STAT1.

Regulation of Streptococcus pneumoniae distribution by Toll-like receptor 2 in vivo

The phagocyte pattern recognition receptor Toll-like receptor 2 (TLR2) and the multi-receptor adaptor MyD88 contribute to the reduction of bacterial load in infections with intra- and extra-cellular Gram-positive bacteria. Their mechanism of antibacterial action is mostly unresolved but evident in vivo by an increased pathogen burden in infected TLR2-/- and MyD88-/- compared to C57BL/6 wild type (wt) mice. We had previously observed higher bacterial numbers in brains of TLR2-/- than of wt mice with meningitis. Here we study bacteria-phagocyte interaction by comparing S. pneumoniae distribution and localization in wt and TLR2-/- brain by confocal microscopy using a green fluorescent protein-transformed encapsulated S. pneumoniae (C5017). Colony-forming units were similarly distributed in TLR2-/- and wt mice and exclusively localized in meninges and ventricles. Bacteria were more abundant in ventricles, in and around TLR2-/- than wt GLT1v+ plexus choroideus epithelial cells. S. pneumoniae were also found in and around Gr-1+ granulocytes, but never in F4/80+ macrophages, Iba1+ microglia, GFAP+ astrocytes, Meca-31+ endothelial cells or Neun+ neurons of either mouse strain. The results indicate that TLR2 does not change bacterial distribution, but may contribute to antibacterial defense by modulating S. pneumoniae adherence and uptake in plexus epithelia.

Mammalian Toll-like receptors: to immunity and beyond

Toll-like receptors (TLRs) constitute an archetypal pattern recognition system. Their sophisticated biology underpins the ability of innate immunity to discriminate between highly diverse microbial pathogens and self. However, the remarkable progress made in describing this biology has also revealed new immunological systems and processes previously hidden to investigators. In particular, TLRs appear to have a fundamental role in the generation of clonal adaptive immune responses, non-infectious disease pathogenesis and even in the maintenance of normal mammalian homeostasis. Although an understanding of TLRs has answered some fundamental questions at the host-pathogen interface, further issues, particularly regarding therapeutic modulation of these receptors, have yet to be resolved.

Chlamydia trachomatisInduces Expression of IFN-γ-Inducible Protein 10 and IFN-β Independent of TLR2 and TLR4, but Largely Dependent on MyD88

IFN-gamma-inducible protein 10 (IP-10) is a chemokine important in the attraction of T cells, which are essential for resolution of chlamydial genital tract infection. During infections with Gram-negative bacteria, the IP-10 response mediated through type I IFNs usually occurs as a result of TLR4 stimulation by bacterial LPS. However, we found that levels of IP-10 in genital tract secretions of Chlamydia trachomatis-infected female wild-type mice were similar to those of infected TLR2- and TLR4-deficient mice but significantly greater than those of infected MyD88-deficient mice. We investigated the mechanism of IP-10 and IFN-beta induction during chlamydial infection using mouse macrophages and fibroblasts infected ex vivo. The induction of IP-10 and IFN-beta was unchanged in Chlamydia-infected TLR2- and TLR4-deficient cells compared with wild-type cells. However, infection of MyD88-deficient cells resulted in significantly decreased responses. These results suggest a role for MyD88-dependent pathways in induction of IP-10 and IFN-beta during chlamydial infection. Furthermore, treatment of infected macrophages with an endosomal maturation inhibitor significantly reduced chlamydial-induced IFN-beta. Because endosomal maturation is required for MyD88-dependent intracellular pathogen recognition receptors to function, our data suggest a role for the intracellular pathogen recognition receptor(s) in induction of IFN-beta and IP-10 during chlamydial infection. Furthermore, the intracellular pathways that lead to chlamydial-induced IFN-beta function through TANK-binding kinase mediated phosphorylation and nuclear translocation of IFN regulatory factor-3.

Transforming Growth Factor-β Differentially Inhibits MyD88-dependent, but Not TRAM- and TRIF-dependent, Lipopolysaccharide-induced TLR4 Signaling

Transforming growth factor-beta1 (TGF-beta1) is a multifunctional, potent anti-inflammatory cytokine produced by many cell types that regulates cell proliferation, apoptosis, and immune responses. Toll-like receptors (TLRs) recognize various pathogen-associated molecular patterns and are therefore a pivotal component of the innate immune system. In this study we show that TGF-beta1 blocks the NF-kappaB activation and cytokine release that is stimulated by ligands for TLRs 2, 4, and 5. We further show that TGF-beta1 can specifically interfere with TLR2, -4, or -5 ligand-induced responses involving the adaptor molecule MyD88 (myeloid differentiation factor 88) but not the TRAM/TRIF signaling pathway by decreasing MyD88 protein levels in a dose- and time-dependent manner without altering its mRNA expression. The proteasome inhibitor epoxomicin abolished the MyD88 degradation induced by TGF-beta1. Furthermore, TGF-beta1 resulted in ubiquitination of MyD88 protein, suggesting that TGF-beta1 facilitates ubiquitination and proteasomal degradation of MyD88 and thereby attenuates MyD88-dependent signaling by decreasing cellular levels of MyD88 protein. These findings importantly contribute to our understanding of molecular mechanisms mediating anti-inflammatory modulation of immune responses by TGF-beta1.

TB, or not TB: that is the question -- does TLR signaling hold the answer?

Innate immunity critically depends on signaling by Toll-like receptors (TLRs) that rely heavily on an intracellular adapter protein called myeloid differentiation factor 88 (MyD88). Adaptive immune defenses are generally thought to be orchestrated by innate immune responses and so should require intact TLR-MyD88 signaling pathways. But a surprising new study in MyD88-null mice infected with Mycobacterium tuberculosis challenges this view and instead suggests that MyD88 may not be absolutely required for a normal adaptive immune response.