Evolution and integration of innate immune recognition systems: the Toll-like receptors

Toll-like receptors (TLRs) recognize specific components of micro-organisms and trigger the activation of innate immunity. TLR-mediated signaling pathways are now rapidly being elucidated. TLR signaling originates from the cytoplasmic Toll/IL-1 receptor (TIR) domain, which is conserved among all TLRs. Furthermore, TIR domain-containing adaptors, such as MyD88, TRIF, TIRAP and TRAM, play essential roles in TLR signaling. MyD88 is essential for inflammatory cytokine production via all TLRs, whereas TRIF is involved in TLR3- and TLR4-mediated MyD88-independent induction of IFN-beta. Thus, innate immunity represents a skillful system that senses microbial invasion and initiates appropriate immune responses.     

Critical coordination of innate immune defense against Toxoplasma gondii by dendritic cells responding via their Toll-like receptors

Toll-like receptors (TLRs) play an important role in host defense against a variety of microbial pathogens. We addressed the mechanism by which TLRs contribute to host defense against the lethal parasite Toxoplasma gondii by using mice with targeted inactivation of the TLR adaptor protein myeloid differentiation primary response gene 88 (MyD88) in different innate cell types. Lack of MyD88 in dendritic cells (DCs), but not in macrophages or neutrophils, resulted in high susceptibility to the T. gondii infection. In the mice deficient in MyD88 in DCs, the early IL-12 response by DCs was ablated, the IFN-γ response by natural killer cells was delayed, and the recruited inflammatory monocytes were incapable of killing the T. gondii parasites. The T-cell response, although attenuated in these mice, was sufficient to eradicate the parasite during the chronic stage, provided that defects in DC activation were compensated by IL-12 treatment early after infection. These results demonstrate a central role of DCs in orchestrating the innate immune response to an intracellular pathogen and establish that defects in pathogen recognition by DCs can predetermine sensitivity to infection.     

Toll-like receptor signaling in colorectal cancer: Carcinogenesis to cancer therapy

Toll-like receptors (TLRs) are germ line encoded innate immune sensors that recognize conserved microbial structures and host alarmins, and signal expression of major histocompatibility complex proteins, costimulatory molecules, and inflammatory mediators by macrophages, neutrophils, dendritic cells, and other cell types. These protein receptors are characterized by their ability to respond to invading pathogens promptly by recognizing particular TLR ligands, including flagellin and lipopolysaccharide of bacteria, nucleic acids derived from viruses, and zymosan of fungi. There are 2 major TLR pathways; one is mediated by myeloid differentiation factor 88 (MYD88) adaptor proteins, and the other is independent of MYD88. The MYD88-dependent pathway involves early-phase activation of nuclear factor of kappa light polypeptide gene enhancer in B-cells 1 (NF-κB1) and all the TLRs, except TLR3, have been shown to activate this pathway. TLR3 and TLR4 act via MYD88-independent pathways with delayed activation of NF-κB signaling. TLRs play a vital role in activating immune responses. TLRs have been shown to mediate inflammatory responses and maintain epithelial barrier homeostasis, and are highly likely to be involved in the activation of a number of pathways following cancer therapy. Colorectal cancer (CRC) is one of the most common cancers, and accounts for almost half a million deaths annually worldwide. Inflammation is considered a risk factor for many common malignancies including cancers of the colorectum. The key molecules involved in inflammation-driven carcinogenesis include TLRs. As sensors of cell death and tissue remodeling, TLRs may have a universal role in cancer; stimulation of TLRs to activate the innate immune system has been a legitimate therapeutic strategy for some years. TLRs 3/4/7/8/9 are all validated targets for cancer therapy, and a number of companies are developing agonists and vaccine adjuvants. On the other hand, antagonists may favor inhibition of signaling responsible for autoimmune responses. In this paper, we review TLR signaling in CRC from carcinogenesis to cancer therapy.     

TLR2 and TLR4 agonists stimulate unique repertoires of host resistance genes in murine macrophages: interferon-beta-dependent signaling in TLR4-mediated responses

That TLRs share a common MyD88-dependent signaling pathway which results in the generation of nuclear DNA-binding proteins, such as NF-kappaB, is a well-accepted paradigm. However, studies from our laboratories and others suggested that TLR4 agonists elicit a more diverse pattern of gene expression in murine macrophages than TLR2 agonists. The data presented show that activation of TLR4 by Escherichia coli LPS results in an MyD88-independent, TIRAP/Mal-dependent signaling pathway that, in turn, leads to early induction of interferon-beta (IFN-beta). IFN-beta, in turn, acts in an autocrine/paracrine fashion on the macrophage to activate STAT1-containing DNA binding complexes that participate in the induction of genes not expressed in response to natural or synthetic TLR2 agonists. These data support the hypothesis that the host response to microbes is controlled by TLRs at two levels: (i) the "sensing" of differences in microbial structures through the TLR extracellular domain; and (ii) signaling pathways that are initiated via interactions through unique intracytoplasmic regions of different TLRs with adaptor proteins.     

NF-kappaB activation by the Toll-IL-1 receptor domain protein MyD88 adapter-like is regulated by caspase-1

Toll-like receptors (TLRs)-2 and -4 are important proteins in innate immunity, recognizing microbial products and eliciting host defense responses. Both use the adapter proteins MyD88 and MyD88 adapter-like (Mal) to activate signaling pathways. Here we report that Mal but not MyD88 interacts with caspase-1, the enzyme that processes the precursors of the proinflammatory cytokines IL-1beta and IL-18. The interaction was found in a yeast two-hybrid screen and was confirmed by reciprocal GST pull-downs and coimmunoprecipitation of endogenous proteins. We were unable to implicate Mal in regulating caspase-1 activation. However, we found that Mal was cleaved by caspase-1 and that inhibition of caspase-1 activity blocked TLR2- and TLR4-mediated NF-kappaB and p38 MAP kinase activation but not IL-1 or TLR7 signaling, which are Mal independent. These responses, and the induction of TNF, were also attenuated in caspase-1-deficient cells. Finally, unlike wild-type Mal, a mutant Mal, which was not cleaved by caspase-1, was unable to signal and acted as a dominant negative inhibitor of TLR2 and TLR4 signaling. Our study therefore reveals a role for caspase-1 in the regulation of TLR2 and TLR4 signaling pathways via an effect on Mal. This functional interaction reveals an important aspect of the coordination between TLRs and caspase-1 during the innate response to pathogens.     

Regulation of innate immune responses by Toll-like receptors

Innate immune response in Drosophila is mediated by signaling through Toll receptors. In mammals, Toll-like receptors (TLRs), comprising a large family, recognize a specific pattern of microbial components. So far, the roles of TLR2, TLR4, TLR5, TLR6, and TLR9 have been revealed. The recognition of microbial components by TLRs leads to activation of innate immunity, which provokes inflammatory responses and finally the development of adaptive immunity. The inflammatory response depends on a TLR-mediated MyD88-dependent cascade. However, there seems to exist additional cascades in TLR signaling. In the case of TLR4 signaling, an MyD88-independent pathway is now being characterized. In addition to the activation of innate immune responses, TLR-mediated signaling leads to suppression of the activity of innate immune cells, represented by "lipopolysaccharide (LPS) tolerance". Progress in elucidating the molecular mechanisms for LPS tolerance has been made through the analysis of TLR-mediated signaling pathways. Thus, the activity for innate immune responses is known to be finely regulated by TLRs.     

Expression of Toll-like receptors in chronic hepatitis C virus infection

BACKGROUND: Toll-like receptors (TLRs) are involved in innate immunity. Certain viruses interact with TLRs and mediate antiviral effects as well as immune responses. The aim of this study was to investigate the effect of TLRs on pathogenesis in hepatitis C virus (HCV)-infected patients. METHODS: Peripheral blood mononuclear cells (PBMC) and CD14+ (monocytes) or CD14- cells from 25 patients with chronic liver disease and 15 healthy subjects were studied for expression of TLRs 2-9 and cytokines of extracted RNA using real-time PCR. Then TLR expression was examined in HepG2 cells transfected with entire or parts (core-NS3, NS3-NS5B) of the HCV open reading frame. TLR expression was calculated as the relative mRNA levels. RESULTS: Expression of TLRs 4, 7 and 8 in CD14+ cells of PBMC was increased in patients. Levels of tumor necrosis factor (TNF)-alpha, interleukin (IL)-6 and IL-12 p35 for PBMC were also increased in patients. When PBMC were incubated with HCV core protein, enhancement of TLR2 expression and suppression of TLR4 and TLR7 were noted in patients. Similar alteration of TLRs expression was observed in controls. Among HepG2 transfectants, only TLR3 expression was changed; it was suppressed in entire gene transfectant and enhanced in core-NS3 transfectant. Expression of some proteins related to the TLR signaling pathway was suppressed in the entire gene transfectant. CONCLUSIONS: The results suggest a correlation between expression levels of TLRs and cytokines, and chronic HCV infection. TLR3 recognizes double-stranded RNA and induces type 1 interferon synthesis. Collectively, suppressed expression of TLR3 in cells transfected with entire HCV may be responsible for continuous HCV infection, although a part of the HCV gene enhances its expression.     

Human -defensin-3 activates professional antigen-presenting cells via Toll-like receptors 1 and 2

There is increasing evidence that innate and adaptive immune responses are intimately linked. This linkage is in part mediated through the recognition of conserved microbial products by Toll-like receptors (TLRs). Detection of microbial products by TLRs can result in induction of inflammatory cytokines and activation of professional antigen-presenting cells, thereby enhancing adaptive immune responses. Here, we show that human beta-defensin-3 (hBD-3), an innate antimicrobial peptide, can induce expression of the costimulatory molecules CD80, CD86, and CD40, on monocytes and myeloid dendritic cells in a TLR-dependent manner. Activation of monocytes by hBD-3 is mediated by interaction with TLRs 1 and 2, resulting in signaling that requires myeloid differentiating factor 88 and results in IL-1 receptor-associated kinase-1 phosphorylation. In studies with HEK cells engineered to express various TLRs, we show that activation of NF-kappaB by hBD-3 depends on the expression of both TLR1 and TLR2. Thus, human TLR signaling is not restricted to recognition of microbial patterns but also can be initiated by host-derived peptides such as hBD-3.     

Inhibitory effect of miR-125b on hepatitis C virus core protein-induced TLR2/My D88 signaling in THP-1 cells

AIM: To investigate the role of mi R-125 b in regulating monocyte immune responses induced by hepatitis C virus(HCV) core protein.METHODS: Monocytic THP-1 cells were treated with various concentrations of recombinant HCV core protein, and cytokines and mi R-125 b expression in these cells were analyzed. The requirement of Tolllike receptor 2(TLR2) or My D88 gene for HCV core protein-induced immune responses was determined by the transfection of THP-1 cells with gene knockdown vectors expressing either TLR2 si RNA or My D88 si RNA. The effect of mi R-125 b overexpression on TLR2/My D88 signaling was examined by transfecting THP-1 cells with mi R-125 b mimic RNA oligos.RESULTS: In response to HCV core protein stimulation, cytokine production was up-regulated and mi R-125 b expression was down-regulated in THP-1 cells. The modulatory effect of HCV core protein on cellular events was dose-dependent and required functional TLR2 or My D88 gene. Forced mi R-125 b expression abolished the HCV core protein-induced enhancement of tumor necrosis factor-α, interleukin(IL)-6, and IL-10 expression by 66%, 54%, and 66%, respectively(P 0.001), by inhibiting My D88-mediated signaling, including phosphorylation of NF-k Bp65, ERK, and P38.CONCLUSION: The inverse correlation between mi R-125 b and cytokine expression after HCV core challenge suggests that mi R-125 b may negatively regulate HCVinduced immune responses by targeting TLR2/My D88 signaling in monocytes.     

Distinct pathways of LPS-induced NF-kappa B activation and cytokine production in human myeloid and nonmyeloid cells defined by selective utilization of MyD88 and Mal/TIRAP

How lipopolysaccharide (LPS) signals through toll-like receptors (TLRs) to induce nuclear factor (NF)-kappa B inflammatory cytokines in sepsis remains unclear. Major candidates for that process are myeloid differentiation protein 88 (MyD88) and MyD88 adaptor-like/TIR domain-containing adaptor protein (Mal/TIRAP) but their role needs to be further defined. Here, we have examined the role of MyD88 and Mal/TIRAP in primary human cells of nonmyeloid and myeloid origin as physiologically relevant systems. We found that MyD88 and Mal/TIRAP are essential for LPS-induced I kappa B alpha phosphorylation, NF-kappa B activation, and interleukin 6 (IL-6) or IL-8 production in fibroblasts and endothelial cells in a pathway that also requires IKK2. In contrast, in macrophages neither MyD88, Mal/TIRAP, nor I kappa B kinase 2 (IKK2) are required for NF-kappa B activation or tumor necrosis factor alpha (TNF alpha), IL-6, or IL-8 production, although Mal/TIRAP is still involved in the production of interferon beta (IFN beta). Differential usage of TLRs may account for that, as in macrophages but not fibroblasts or endothelial cells, TLR4 is expressed in high levels at the cell surface, and neutralization of TLR4 but not TLR2 blocks LPS signaling. These observations demonstrate for the first time the existence of 2 distinct pathways of LPS-induced NF-kappa B activation and cytokine production in human myeloid and nonmyeloid cells defined by selective utilization of TLR4, MyD88, Mal/TIRAP, and IKK2, and reveal a layer of complexity not previously expected.     

Toll-like receptor 2 controls mucosal inflammation by regulating epithelial barrier function

BACKGROUND & AIMS: Toll-like receptors (TLRs) represent a class of transmembrane pattern recognition receptors essential for microbial recognition and control of innate immune responses. Commensal bacteria play an important role in maintaining tolerance and active stability of the intestinal epithelial barrier by suppressing intestinal inflammation, yet the mechanisms of action are unknown. The aim of this study was to determine the functional relevance of TLR2 to control tight junction (TJ)-associated intestinal epithelial barrier integrity to balance mucosal homeostasis against inflammatory stress-induced damage. METHODS: TLR2 ligand (synthetic Pam(3)Cys-SK4 [PCSK])-induced activation of signaling cascades and TJ-associated distribution was assessed by using Western blotting and confocal microscopy combined with functional transfection and inhibitor studies in model intestinal epithelial cell (IEC) lines (IEC-6, Caco-2) or primary IEC cultured short-term ex vivo. DSS colitis was induced by standard protocol in wild-type, TLR2-/-, and MyD88-/- mice. Spontaneous apoptosis was assessed by terminal deoxinucleotidyl-transferase-mediated dUTP-biotin nick end-labeling. RESULTS: Data from in vitro and ex vivo models of intestinal epithelial cells revealed that TLR2 stimulation effectively preserves TJ-associated barrier assembly against stress-induced damage through promotion of PI3K/Akt-mediated cell survival via MyD88. Furthermore, in vivo studies underscored that TLR2-mediated TJ regulation critically determines susceptibility to intestinal injury and inflammation. Inflammatory stress in mice deficient of TLR2 or MyD88 induced early TJ-associated disruption interrelated with anti-apoptotic failure of the intestinal epithelial barrier. Oral treatment of colitis with the TLR2 ligand PCSK significantly suppressed mucosal inflammation and apoptosis by efficiently restoring TJ-associated integrity of the intestinal epithelium in vivo. CONCLUSION: TLR2 may provide a target to pharmacologically modulate mucosal injury and intestinal inflammation.     

Contribution of Toll-like receptor/myeloid differentiation factor 88 signaling to murine liver regeneration

Toll-like receptors (TLRs) act as innate immune signal sensors and play central roles in host defense. Myeloid differentiation factor (MyD) 88 is a common adaptor molecule required for signaling mediated by TLRs. When the receptors are activated, cells bearing TLRs produce various proinflammatory cytokines in a MyD88-dependent manner. Liver regeneration following partial hepatectomy (PH) requires innate immune responses, particularly interleukin-6 (IL-6) and tumor necrosis factor alpha (TNF-alpha) production by Kupffer cells, although the recognition and activation processes are still unknown. We investigated whether TLR/MyD88 signaling is critical for induction of innate immune responses after PH. In Myd88(-/-) mice after PH, induction of expression of immediate early genes involved in hepatocyte replication and phosphorylation of STAT3 in the liver, and production of TNF-alpha/IL-6 by and activation of NF-kappaB in the Kupffer cells were grossly subnormal and were associated with impaired liver regeneration. However, TLR2, 4 and 9, which recognize gram-negative and -positive bacterial products, are not essential for NF-kappaB activation and IL-6 production after PH, which excludes a possible contribution of TLR2/TLR4 or TLR9 to MyD88-mediated pathways. In conclusion, the TLR/MyD88 pathway is essential for incidental liver restoration, particularly its early phase.     

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.     

Interferon-beta is activated by hepatitis C virus NS5B and inhibited by NS4A, NS4B, and NS5A

Innate immunity is part of the antiviral response. Interferon (IFN)-beta plays a leading role in this system. To investigate the influence of hepatitis C virus (HCV) on innate immunity, we examined the effect of viral proteins on IFN-beta induction. HepG2 cells were co-transfected with plasmids for seven HCV proteins (core protein, NS2, NS3, NS4A, NS4B, NS5A, and NS5B) and the IFN-beta promoter luciferase. Toll-like receptor (TLR) 3 and Toll/IL-1 receptor domain-containing adapter inducing IFN-beta (TRIF) play key roles in dsRNA-mediated activation of interferon regulatory factor (IRF)-3 and IFN-beta; therefore, the participation of TLR3/TRIF in NS5B-mediated IFN induction was examined. Among seven HCV proteins, only NS5B, a viral RNA-dependent RNA polymerase (RdRp), activated the IFN-beta promoter. However, mutant NS5B without RdRp activity or template/primer association did not activate the IFN-beta promoter. Activation of the IFN-beta promoter by NS5B required the positive regulatory domain III, a binding sequence for IRF-3. Moreover, IRF-3 was phosphorylated by NS5B. Both inhibition of TLR3 expression by small interfering RNA and expression of the dominant negative form of TRIF significantly reduced NS5B-induced activation of IFN-beta. Of the six other HCV proteins, NS4A, NS4B, and NS5A efficiently inhibited this activation. HCV NS5B is a potent activator of the host innate immune system, possibly through TLR3/TRIF and synthesis of dsRNA. Meanwhile, NS4A, NS4B, and NS5A block IFN-beta induction by NS5B, which may contribute toward the persistence of this virus.     

Distinct toil-like receptor expression in monocytes and T cells in chronic HCV infection

AIM: Hepatitis C virus often establishes chronic infections. Recent studies suggest that viral and bacterial infections are more common in HCV-infected patients compared to controls. Pathogens are recognized by Toil-like receptors (TLRs) to shape adaptive and innate immune responses.METHODS: In this study, to assess the ability of HCV-infected host to recognize invading pathogens, we investigated Toil-like receptor expression in innate (monocytes) and adaptive (T cells) immune cells by real-time PCR.RESULTS: We determined that RNA levels for TLRs 2, 6. 7, 8, 9 and 10 mRNA levels were upregulated in both monocytes and T cells in HCV-infected patients compared to controls. TLR4 was only upregulated in T lymphocytes, while TLR5 was selectively increased in monocytes of HCV-infected patients. MD-2, a TLR4 coreceptor, was increased in patients' monocytes and T cells while CD14 and MyD88 were increased only in monocytes.CONCLUSION: Our data reveal novel details on TLR expression that likely relates to innate recognition of pathogens and immune defense in HCV-infected individuals.     

Distinct Toll-like receptor expression in monocytes and T cells in chronic HCV infection

AIM: Hepatitis C virus often establishes chronic infections. Recent studies suggest that viral and bacterial infections are more common in HCV-infected patients compared to controls. Pathogens are recognized by Toll-like receptors (TLRs) to shape adaptive and innate immune responses. METHODS: In this study, to assess the ability of HCV-infected host to recognize invading pathogens, we investigated Toll-like receptor expression in innate (monocytes) and adaptive (T cells) immune cells by real-time PCR. RESULTS: We determined that RNA levels for TLRs 2, 6. 7, 8, 9 and 10 mRNA levels were upregulated in both monocytes and T cells in HCV-infected patients compared to controls. TLR4 was only upregulated in T lymphocytes, while TLR5 was selectively increased in monocytes of HCV-infected patients. MD-2, a TLR4 co-receptor, was increased in patients' monocytes and T cells while CD14 and MyD88 were increased only in monocytes. CONCLUSION: Our data reveal novel details on TLR expression that likely relates to innate recognition of pathogens and immune defense in HCV-infected individuals.     

Use of murine embryonic fibroblasts to define Toll-like receptor activation and specificity

Toll-like receptors (TLRs) are critically involved in the innate immune response to bacterial, viral and fungal pathogens. We have studied human peripheral blood mononuclear cells, murine embryonic fibroblasts (MEFs) and a panel of human cell lines, including HEK, HeLa, AGS, ECV304 and U373 cells, for expression of TLR-specific mRNAs and for TLR-ligand dependent cytokine secretion. Peripheral blood cells expressed multiple TLRs; however, many studies have shown that blood contains multiple, heterogeneous cell populations with distinct patterns of TLR expression. Cell lines had variable expression of TLRs, and in most cases lacked TLR2 and TLR8 expression and only weakly expressed mRNAs for TLR5, TLR7 and TLR9. In contrast, MEFs expressed high levels of mRNA for TLR1, TLR2, TLR3, TLR4, TLR5, TLR6, TLR7, TLR8 and TLR9. MEFs were highly responsive to TLR-ligand activation and secreted high levels of both IL-6 and MCP-1 in response to TLR ligands. MEFs from mice with targeted deletions of TLR2, TLR4 and MyD88 demonstrated profound defects in their IL-6 response to their specific ligands, consistent with studies of macrophages and tissues from adult knockout animals. MEF cultures are homogenous and amenable to biochemical analysis and should allow rigorous studies of the contribution of individual TLRs to the innate immune response.     

Bone marrow-derived immune cells mediate sensitization to liver injury in a myeloid differentiation factor 88-dependent fashion

Toll-like receptors (TLRs) expressed on both immune cells and hepatocytes recognize microbial danger signals and regulate immune responses. Previous studies showed that TLR9 and TLR2 mediate Propionibacterium acnes-induced sensitization to lipopolysaccharide-triggered acute liver injury in mice. Ligand-specific activation of TLR2 and TLR9 are dependent on the common TLR adaptor, myeloid differentiation factor 88 (MyD88). Here, we dissected the role of MyD88 in parenchymal and bone marrow (BM)-derived cells in liver sensitization. Using chimeric mice with green fluorescent protein-expressing BM cells, we identified that P. acnes-induced liver inflammatory foci are of BM origin. Chimeras with MyD88-deficient BM showed no inflammatory foci after P. acnes or TLR2+TLR9 challenge, suggesting that recruitment of inflammatory cells to the liver required MyD88 expression in BM-derived cells. Further, selective MyD88 deficiency in parenchymal cells in mice with wild-type BM failed to prevent inflammatory cell infiltration. These results demonstrate that MyD88 in immune cells rather than in liver parenchymal cells plays an important role in inflammatory cell recruitment and liver injury.