Severe impairment of interleukin-1 and Toll-like receptor signalling in mice lacking IRAK-4

The kinase activity of IL-1 receptor-associated kinase 4 is required for interleukin-1 receptor/toll-like receptor-induced TAK1-dependent NFkappaB activation

Two parallel interleukin-1 (IL-1)-mediated signaling pathways have been uncovered for IL-1R-TLR-mediated NFkappaB activation: TAK1-dependent and MEKK3-dependent pathways, respectively. The TAK1-dependent pathway leads to IKKalpha/beta phosphorylation and IKKbeta activation, resulting in classic NFkappaB activation through IkappaBalpha phosphorylation and degradation. The TAK1-independent MEKK3-dependent pathway involves IKKgamma phosphorylation and IKKalpha activation, resulting in NFkappaB activation through dissociation of phosphorylated IkappaBalpha from NFkappaB without IkappaBalpha degradation. IL-1 receptor-associated kinase 4 (IRAK4) belongs to the IRAK family of proteins and plays a critical role in IL-1R/TLR-mediated signaling. IRAK4 kinase-inactive mutant failed to mediate the IL-1R-TLR-induced TAK1-dependent NFkappaB activation pathway, but mediated IL-1-induced TAK1-independent NFkappaB activation and retained the ability to activate substantial gene expression, indicating a structural role of IRAK4 in mediating this alternative NFkappaB activation pathway. Deletion analysis of IRAK4 indicates the essential structural role of the IRAK4 death domain in receptor proximal signaling for mediating IL-1R-TLR-induced NFkappaB activation.

Leishmania donovani infection down-regulates TLR2-stimulated IL-12p40 and activates IL-10 in cells of macrophage/monocytic lineage by modulating MAPK pathways through a contact-dependent mechanism

The failure of Leishmania, an intracellular pathogen, to stimulate a pro-inflammatory response following entry into macrophages has been well reported. This occurs in spite of the fact that ligands for the toll-like receptors (TLR) have been recently shown on the parasite surface and their role in disease protection well documented. The outcome of infection in leishmaniasis is determined by the Th1 versus Th2 nature of the effector response and the generation of IL-12 and IL-10 by the infected macrophages is important for this decision. We evaluated the effect of L. donovani infection of monocytes (cell line THP-1, and monocytes derived from human peripheral blood) on Pam3cys (TLR2 ligand) and lipopolysaccharide (TLR4 ligand) stimulated production of IL-12p40 and IL-10. L. donovani infection caused suppression of TLR2 and TLR4-stimulated IL-12p40, with an increase in IL-10 production. Parasites also modulated the TLR2-stimulated mitogen-activated protein kinase (MAPK) pathway by suppressing MAPK P(38) phosphorylation and activating extracellular regulated kinase (ERK)1/2 phosphorylation. These effects could be reversed either by using a MAPK P(38) activator, anisomycin, or ERK1/2 inhibitor, U0126. L. donovani caused modulation of TLR2-stimulated MAPK pathways in a contact-dependent mechanism. In addition parasite structural integrity but not viability was required for suppression of TLR2-stimulated IL-12p40 and activation of IL-10. These observations suggest that L. donovani has evolved survival strategies that subvert the pro-inflammatory response generated through TLRs.

Toll-like receptors in skin infections and inflammatory diseases

The skin is the ultimate example of the function of innate immunity, it alerts the host of danger by many systems including sensing pathogen-associated molecule patterns (PAMPs) through Toll-like receptors and other pattern recognition receptors (PRRs), yet normally provides defense without inflammation. The skin responds rapidly to invading microbes by producing antimicrobial peptides or other antimicrobial intermediates before cytokine release results in inflammation. To achieve maximal immune responses for clearing invading microbes, the activation of select PRRs in skin then initiates and shapes adaptive immune responses through the activation of dendritic cells and recruitment of T cell subsets. Importantly, cross-talk between TLRs can influence this system in several ways including augmenting or suppressing the immune response. As a consequence of their pivotal role, TLR responses need to be tightly controlled by associated negative regulators or negative feedback loops to prevent detrimental effects from TLRs overactivation. This review focuses on describing the involvement of TLRs in the development of skin infections and inflammatory diseases, and highlights the potential application of TLR agonists or antagonists in these skin diseases.

Transient membrane recruitment of IRAK-1 in response to LPS and IL-1β requires TNF R1

The transcription factor NF-κB is an essential regulator of the innate immune response that functions as the first line of defense against infections. Activation of the innate immune response by bacterial lipopolysaccharide (LPS) triggers production of tumor necrosis factor-α (TNF-α) followed by interleukin-1 (IL-1). The IL-1 receptor associated kinase-1 (IRAK-1) is an integral component of the LPS, TNF-α, and IL-1 signaling pathways that regulate NF-κB. Thus we hypothesized that IRAK-1 coordinates cellular NF-κB responses to LPS, TNF-α, and IL-1. In contrast to TNF-α where IRAK-1 subcellular localization does not change, treatment with LPS or IL-1 leads to a loss in cytoplasmic IRAK-1 with a coordinate increase in plasma membrane associated modified IRAK-1. In fibroblasts lacking the type 1 TNF-α receptor (TNF R1), IRAK-1 turnover is altered and modification of IRAK-1 in the plasma membrane is decreased in response to LPS and IL-1, respectively. When NF-κB controlled gene expression is measured, fibroblasts lacking TNF R1 are hyperresponsive to LPS, whereas a more variable response to IL-1 is seen. Further analysis of the LPS response revealed that plasma membrane-associated IRAK-1 is found in Toll 4, IL-1, and TNF R1-containing complexes. The data presented herein suggest a model whereby the TNF R1-IRAK-1 interaction integrates the cellular response to LPS, TNF-α, and IL-1, culminating in a cell poised to activate TNF-α-dependent NF-κB controlled gene expression. In the absence of TNF R1-dependent events, exposure to LPS or IL-1 leads to hyperactivation of the inflammatory response.

Pyogenic bacterial infections in humans with MyD88 deficiency

MyD88 is a key downstream adapter for most Toll-like receptors (TLRs) and interleukin-1 receptors (IL-1Rs). MyD88 deficiency in mice leads to susceptibility to a broad range of pathogens in experimental settings of infection. We describe a distinct situation in a natural setting of human infection. Nine children with autosomal recessive MyD88 deficiency suffered from life-threatening, often recurrent pyogenic bacterial infections, including invasive pneumococcal disease. However, these patients were otherwise healthy, with normal resistance to other microbes. Their clinical status improved with age, but not due to any cellular leakiness in MyD88 deficiency. The MyD88-dependent TLRs and IL-1Rs are therefore essential for protective immunity to a small number of pyogenic bacteria, but redundant for host defense to most natural infections.

Group A streptococcus activates type I interferon production and MyD88-dependent signaling without involvement of TLR2, TLR4, and TLR9

Bacterial pathogens are recognized by the innate immune system through pattern recognition receptors, such as Toll-like receptors (TLRs). Engagement of TLRs triggers signaling cascades that launch innate immune responses. Activation of MAPKs and NF-kappaB, elements of the major signaling pathways induced by TLRs, depends in most cases on the adaptor molecule MyD88. In addition, Gram-negative or intracellular bacteria elicit MyD88-independent signaling that results in production of type I interferon (IFN). Here we show that in mouse macrophages, the activation of MyD88-dependent signaling by the extracellular Gram-positive human pathogen group A streptococcus (GAS; Streptococcus pyogenes) does not require TLR2, a receptor implicated in sensing of Gram-positive bacteria, or TLR4 and TLR9. Redundant engagement of either of these TLR molecules was excluded by using TLR2/4/9 triple-deficient macrophages. We further demonstrate that infection of macrophages by GAS causes IRF3 (interferon-regulatory factor 3)-dependent, MyD88-independent production of IFN. Surprisingly, IFN is induced also by GAS lacking slo and sagA, the genes encoding cytolysins that were shown to be required for IFN production in response to other Gram-positive bacteria. Our data indicate that (i) GAS is recognized by a MyD88-dependent receptor other than any of those typically used by bacteria, and (ii) GAS as well as GAS mutants lacking cytolysin genes induce type I IFN production by similar mechanisms as bacteria requiring cytoplasmic escape and the function of cytolysins.

TLR4-mediated MyD88-dependent signaling pathway is activated by cerebral ischemia-reperfusion in cortex in mice

To study whether the signaling pathway is activated in the inflammatory reaction of cerebral ischemia-reperfusion and its mechanism. The mice were randomly divided into sham group, ischemia-reperfusion group and TLR4-blocked group with different time points of reperfusion 12h, 24h, 48h and 72h group. We observed the different expression of TLR4 mRNA and MyD88 mRNA, activation of NF-kappaB and the TNF-alpha and IL-1beta protein levels in each group at different time point after ischemia-reperfusion. Mice cerebral ischemia was induced by occlusion of common carotid arteries (CCA) bilaterally. TLR4 signaling pathway could be inhibited by specific anti-TLR4 binding protein to prevent TLR4 from interacting with its receptors. We determined the result of TLR4 antibodies-blocking and mice cerebral ischemia-reperfusion injuries by Western blot, and evaluated neuronal damage in cortex. We also determined the expression of TLR4 mRNA and MyD88 mRNA by in situ hybridization (ISH), the activation of NF-kappaB by EMSA, and the expression of TNF-alpha protein by Western blot. Anti-TLR4 binding TLR4 receptors before reperfusion was effective; There was distinct difference among each group respecting neuronal damage; The expression of TLR4 mRNA and MyD88 mRNA, the activation of NF-kappaB, and the expression of TNF-alpha protein showed clear difference as well. LR4-mediated MyD88-dependent signaling pathway activated by ischemia-reperfusion may be involved in the mechanism of ischemia-reperfusion through upregulation of NF-kappaB and TNF-alpha.

CD44 suppresses TLR-mediated inflammation

The cell adhesion molecule CD44, which is the major hyaluronan receptor, has been implicated in the binding, endocytosis, and metabolism of hyaluronan. Previous studies have revealed that CD44 plays crucial roles in a variety of inflammatory diseases. In recent years, TLRs, which are ancient microbial pattern recognition receptors, have been shown to initiate an innate immune response and have been linked to a variety of inflammatory diseases. The present study shows that CD44 negatively regulates in vivo inflammation mediated by TLRs via NF-kappaB activation, which leads to proinflammatory cytokine production. Furthermore, our results show that CD44 directly associates with TLR2 when stimulated by the TLR2 ligand zymosan and that the cytoplasmic domain of CD44 is crucial for its regulatory effect on TLR signaling. This study indicates that CD44 plays a protective role in TLR-mediated inflammation and is the first to demonstrate a direct association between CD44 and a TLR.

MyD88-induced downregulation of IRAK-4 and its structural requirements

IRAK-4 plays an essential role in Toll-like receptor (TLR)/IL-1 receptor signaling. However, its signaling and regulation mechanisms have remained elusive. We have reported previously that stimulation of TLR2, TLR4 or TLR9, but not TLR3, leads to downregulation of IRAK-4 protein. Here, we show that expression of MyD88 leads to downregulation of endogenous as well as exogenously expressed IRAK-4 protein in HEK293 cells. Expression of TRIF did not cause IRAK-4 downregulation although it induced NF-kappaB activation. Expression of either a deletion mutant of MyD88 lacking its death domain or MyD88s, neither of which induced NF-kappaB activation, did not lead to IRAK-4 downregulation. MyD88-induced downregulation was observed in an IRAK-4 mutant lacking the kinase domain, but not in another mutant lacking the death domain. These results demonstrate that downregulation of IRAK-4 requires activation of the MyD88-dependent pathway and that the death domains of both MyD88 and IRAK-4 are important for this downregulation.

IRAK-4 kinase activity-dependent and -independent regulation of lipopolysaccharide-inducible genes

IRAK-4 kinase inactive (IRAK-4 KD) knock-in mice display defects in TLR- and IL-1 receptor signaling and are resistant to LPS-induced shock. In the present study we examined the LPS-induced response in IRAK-4 KD mice in more detail. We show that IRAK-4 kinase activity is required for certain aspects of TLR-mediated signaling but not for others. We found that IRAK-4 KD cells displayed reduced JNK and p38 signaling, while NF-kappaB was activated to a normal level but with delayed kinetics compared to wild-type cells. TLR4-mediated IRF3 activation was intact in these cells. Comprehensive analysis of expression of LPS-inducible genes by microarray demonstrated that IRAK-4 KD cells were severely impaired in the expression of many pro-inflammatory genes, suggesting their dependence on IRAK-4 kinase activity. In contrast, the expression of a subset of LPS-induced genes of anti-viral response was not affected by IRAK-4 kinase deficiency. Additionally, we demonstrate that LPS-activated early expression and production of some cytokines, e.g., TNF-alpha, is partially induced in the absence of IRAK-4 kinase activity. This suggests that the partially unaffected TLR4-mediated signaling could still drive expression of these genes in early phases and that IRAK-4 kinase activity is important for a more sustained anti-bacterial response.

IRAK-4 kinase activity is required for IRAK-4-dependent innate and adaptive immune responses

Interleukin-1 receptor-associated kinase (IRAK)-4 is a serine-threonine kinase that plays an important role in innate and adaptive immune responses. While the requirement of IRAK-4 kinase activity has been studied in the context of IL-1R signaling, it is not clear whether IRAK-4 requires its kinase function for all of its roles in the immune system. IRAK-4 kinase-dead knock-in (IRAK-4KD/KD) mice were generated to further elucidate whether IRAK-4 kinase activity is required for IRAK-4 to induce cytokine production. IRAK-4KD/KD mice were impaired in their ability to produce cytokines in response to in vivo challenge with lipopolysaccharide (LPS), a potent TLR4 ligand. Cytokine production was also reduced in macrophages and dendritic cells from IRAK-4KD/KD mice in response to LPS and other TLR ligands. In addition, adaptive immune responses were impaired in IRAK-4KD/KD mice. Although in vitro T cell proliferation in response to TCR activation was unaffected in IRAK-4-deficient mice, in vivo T cell responses to lymphocytic choriomeningitits virus infection were significantly impaired in IRAK-4-knockout mice or mice expressing the kinase-dead mutant of IRAK-4. Collectively, these results indicate that IRAK-4 kinase activity is required for IRAK-4-dependent signaling in innate and adaptive immunity.

Sequential control of Toll-like receptor-dependent responses by IRAK1 and IRAK2

Members of the IRAK family of kinases mediate Toll-like receptor (TLR) signaling. Here we show that IRAK2 was essential for sustaining TLR-induced expression of genes encoding cytokines and activation of the transcription factor NF-kappaB, despite the fact that IRAK2 was dispensable for activation of the initial signaling cascades. IRAK2 was activated 'downstream' of IRAK4, like IRAK1, and TLR-induced cytokine production was abrogated in the absence of both IRAK1 and IRAK2. Whereas the kinase activity of IRAK1 decreased within 1 h of TLR2 stimulation, coincident with IRAK1 degradation, the kinase activity of IRAK2 was sustained and peaked at 8 h after stimulation. Thus, IRAK2 is critical in late-phase TLR responses, and IRAK1 and IRAK2 are essential for the initial responses to TLR stimulation.

The involvement of the interleukin-1 receptor-associated kinases (IRAKs) in cellular signaling networks controlling inflammation

Innate immunity and inflammation plays a key role in host defense and wound healing. However, Excessive or altered inflammatory processes can contribute to severe and diverse human diseases including cardiovascular disease, diabetes and cancer. The interleukin-1 receptor-associated kinases (IRAKs) are critically involved in the regulation of intracellular signaling networks controlling inflammation. Collective studies indicate that IRAKs are present in many cell types, and can mediate signals from various cell receptors including toll-like-receptors (TLRs). Consequently, diverse downstream signaling processes can be elicited following the activation of various IRAKs. Given the critical and complex roles IRAK proteins play, it is not surprising that genetic variations in human IRAK genes have been found to be linked with various human inflammatory diseases. This review intends to summarize the recent advances regarding the regulations of various IRAK proteins and their cellular functions in mediating inflammatory signaling processes.

Lys63-linked polyubiquitination of IRAK-1 is required for interleukin-1 receptor- and toll-like receptor-mediated NF-kappaB activation

Stimulation through the interleukin-1 receptor (IL-1R) and some Toll-like receptors (TLRs) induces ubiquitination of TRAF6 and IRAK-1, signaling components required for NF-kappaB and mitogen-activated protein kinase activation. Here we show that although TRAF6 and IRAK-1 acquired Lys63 (K63)-linked polyubiquitin chains upon IL-1 stimulation, only ubiquitinated IRAK-1 bound NEMO, the regulatory subunit of IkappaB kinase (IKK). The sites of IRAK-1 ubiquitination were mapped to Lys134 and Lys180, and arginine substitution of these residues impaired IL-1R/TLR-mediated IRAK-1 ubiquitination, NEMO binding, and NF-kappaB activation. K63-linked ubiquitination of IRAK-1 required enzymatically active TRAF6, indicating that it is the physiologically relevant E3. Thus, K63-linked polyubiquitination of proximal signaling proteins is a common mechanism used by diverse innate immune receptors for recruiting IKK and activating NF-kappaB.

LPS/TLR4 signal transduction pathway

The stimulation of Toll-like receptor 4 (TLR4) by lipopolysaccharide (LPS) induces the release of critical proinflammatory cytokines that are necessary to activate potent immune responses. LPS/TLR4 signaling has been intensively studied in the past few years. Here we review molecules involved in TLR4-mediated signaling, including players that are involved in the negative regulation of this important pathway.

Signalling of toll-like receptors

Since Toll-like receptor (TLR) signaling was found crucial for the activation of innate and adaptive immunity, it has been the focus of immunological research. There are at least 13 identified mammalian TLRs, to date, that share similarities in their extracellular and intracellular domains. A vast number of ligands have been identified that are specifically recognized by different TLRs. As a response the TLRs dimerize and their signaling is initiated. The molecular basis of that signaling depends on the conserved part of their intracellular domain; namely the Toll/IL-1 receptor (TIR) domain. Upon TLR dimerization a TIR-TIR structure is formed that can recruit TIR-containing intracellular proteins that mediate their signaling. For this reason these proteins are named adapters. There are five adapters identified so far named myeloid differentiation primary response protein 88 (MyD88), MyD88-adapter like (Mal) or TIR domain-containing adapter (TIRAP), TIR domain-containing adapter inducing interferon-beta (IFN-beta) (TRIF) or TIR-containing adapter molecule-1 (TICAM-1), TRIF-related adapter molecule (TRAM) or TICAM-2, and sterile alpha and HEAT-Armadillo motifs (SARM). The first four play a fundamental role in TLR-signaling, defining which pathways will be activated, depending on which of these adapters will be recruited by each TLR. Among these adapter proteins MyD88 and TRIF are now considered as the signaling ones and hence the TLR pathways can be categorized as MyD88-dependent and TRIF-dependent.

TAK-242 selectively suppresses Toll-like receptor 4-signaling mediated by the intracellular domain

TAK-242, a small-molecule antisepsis agent, has shown to suppress lipopolysaccharide (LPS)-induced inflammation. In this study, we demonstrate that TAK-242 is a selective inhibitor of Toll-like receptor (TLR)-4 signaling. TAK-242 almost completely suppressed production of nitric oxide (NO) or tumor necrosis factor (TNF)-alpha induced by a TLR4-specific ligand, ultra-pure LPS, in mouse RAW264.7, human U-937 and P31/FUJ cells, whereas this agent showed little effect on other TLR ligands, Pam(3)CSK(4) (TLR1/2), peptidoglycan (TLR2/6), double strand RNA (TLR3), R-848 (TLR7) and CpG oligonucleotide (TLR9). Furthermore, TAK-242 potently inhibited nuclear factor (NF)-kappaB activation induced by ultra-pure LPS in HEK293 cells transiently expressing TLR4 and co-receptors, myeloid differentiation protein-2 (MD2) and CD14, whereas this agent showed little effect on other TLRs, TLR1/2, TLR2/6, TLR3, TLR5, TLR7 and TLR9. TAK-242 also inhibited ligand-independent NF-kappaB activation resulting from over-expression of TLR4. Although chimera receptors, which are consist of the extracellular domain of CD4 and the intracellular domain of human or mouse TLR4, showed constitutive NF-kappaB activation, TAK-242 potently inhibited the signaling from CD4-TLR4 chimera receptors. In contrast, the NF-kappaB activation mediated by TLR4 adaptors, myeloid differentiation factor 88 (MyD88), TIR-associated protein (TIRAP), Toll/IL-1R homology (TIR)-domain-containing adaptor protein-inducing interferon-beta (TRIF) or TRIF-related adaptor molecule (TRAM) was not affected by TAK-242. TAK-242 is therefore a selective inhibitor of signaling from the intracellular domain of TLR4 and represents a novel therapeutic approach to the treatment of TLR4-mediated diseases.

Inherited human IRAK-4 deficiency: an update

Interleukin-1 receptor-associated kinase-4 (IRAK-4), a member of the IRAK family, plays an essential role in Toll-like receptor- and IL-1 receptor-mediated signaling. We briefly review inherited human IRAK-4 deficiency, a recently described primary immunodeficiency leading to recurrent, invasive, pyogenic bacteria infection, and invasive pneumococcal disease in particular.

The IRAK-catalysed activation of the E3 ligase function of Pellino isoforms induces the Lys63-linked polyubiquitination of IRAK1

The protein kinases IRAK [IL-1 (interleukin 1) receptor-associated kinase] 1 and 4 play key roles in a signalling pathway by which bacterial infection or IL-1 trigger the production of inflammatory mediators. In the present study, we demonstrate that IRAK1 and IRAK4 phosphorylate Pellino isoforms in vitro and that phosphorylation greatly enhances Pellino's E3 ubiquitin ligase activity. We show that, in vitro, Pellino 1 can combine with the E2 conjugating complex Ubc13 (ubiquitin-conjugating enzyme 13)-Uev1a (ubiquitin E2 variant 1a) to catalyse the formation of K63-pUb (Lys63-linked polyubiquitin) chains, with UbcH3 to catalyse the formation of K48-pUb chains and with UbcH4, UbcH5a or UbcH5b to catalyse the formation of pUb-chains linked mainly via Lys11 and Lys48 of ubiquitin. In IRAK1-/- cells, the co-transfection of DNA encoding wild-type IRAK1 and Pellino 2, but not inactive mutants of these proteins, induces the formation of K63-pUb-IRAK1 and its interaction with the NEMO [NF-kappaB (nuclear factor kappaB) essential modifier] regulatory subunit of the IKK (inhibitor of NF-kappaB kinase) complex, a K63-pUb-binding protein. These studies suggest that Pellino isoforms may be the E3 ubiquitin ligases that mediate the IL-1-stimulated formation of K63-pUb-IRAK1 in cells, which may contribute to the activation of IKKbeta and the transcription factor NF-kappaB, as well as other signalling pathways dependent on IRAK1/4.

Molecular Basis for the Unique Specificity of TRAF6

Tumor necrosis factor (TNF) receptor (TNFR) associated factor 6 (TRAF6) is a unique member of the TRAF family of adaptor proteins that is involved in both the TNF receptor superfamily and the interleukin-1 receptor (IL-1R)/Toll-like receptor (TLR) superfamily signal transduction pathways. The ability to mediate signals from both families of receptors implicates TRAF6 as an important regulator of a diverse range of physiological processes such as innate and adaptive immunity, bone metabolism, and the development of lymph nodes, mammary glands, skin, and the central nervous system. This chapter will highlight the structural and biochemical studies of TRAF6 in receptor interactions and discuss the potential for peptidomimetic drug application based on TRAF6 receptor binding motif.

Act1 modulates autoimmunity through its dual functions in CD40L/BAFF and IL-17 signaling

Coordinated regulation of T and B cell-mediated immune responses plays a critical role in the control and modulation of autoimmune diseases. This review is focused on the adapter molecule Act1 and its regulation of autoimmunity through its impact on both T and B cell-mediated immune responses. Whereas Act1 molecule is an important negative regulator for B cell-mediated humoral immune responses through its function in CD40L and BAFF signaling, recent studies have shown that Act1 is also a key positive signaling component for IL-17 signaling pathway, critical for T(H)17-mediated autoimmune and inflammatory responses. The dual functions of Act1 are evident in Act1-deficient mice that displayed B cell-mediated autoimmune phenotypes (including dramatic increase in peripheral B cells, lymphadenopathy and splenomegaly, hypergammaglobulinemia and Sjogren's disease in association with Lupus Nephritis), but showed resistance to T(H)17-dependent EAE and colitis. Such seemingly opposite functions of Act1 in CD40-BAFFR and IL-17R signaling are orchestrated by different domains in Act1. Whereas Act1 interacts with the IL-17R through the C-terminal SEFIR domain, Act1 is recruited to CD40 and BAFFR indirectly, which is mediated by TRAF3 through the TRAF binding site in Act1. Such delicate regulatory mechanisms may provide a common vehicle to promote balance between host defense to pathogens and tolerance to self.

Human Toll-like receptor-dependent induction of interferons in protective immunity to viruses

Five of the 10 human Toll-like receptors (TLRs) (TLR3, TLR4, TLR7, TLR8, and TLR9), and four of the 12 mouse TLRs (TLR3, TLR4, TLR7, TLR9) can trigger interferon (IFN)-alpha, IFN-beta, and IFN-lambda, which are critical for antiviral immunity. Moreover, TLR3, TLR7, TLR8, and TLR9 differ from TLR4 in two particularly important ways for antiviral immunity: they can be activated by nucleic acid agonists mimicking compounds produced during the viral cycle, and they are typically present within the cell, along the endocytic pathway, where they sense viral products in the intraluminal space. Investigations in mice have demonstrated that the TLR7/9-IFN and TLR3-IFN pathways are different and critical for protective immunity to various experimental viral infections. Investigations in humans with interleukin-1 receptor-associated kinase-4 (IRAK-4) deficiency (unresponsive to TLR7, TLR8, and TLR9), UNC-93B deficiency (unresponsive to TLR3, TLR7, TLR8, and TLR9), and TLR3 deficiency have recently shed light on the role of these two pathways in antiviral immunity in natural conditions. UNC-93B- and TLR3-deficient patients appear to be specifically prone to herpes simplex virus 1 (HSV-1) encephalitis, although clinical penetrance is incomplete, whereas IRAK-4-deficient patients appear to be normally resistant to most viruses, including HSV-1. These experiments of nature suggest that the TLR7-, TLR8-, and TLR9-dependent induction of IFN-alpha, IFN-beta, and IFN-lambda is largely redundant in human antiviral immunity, whereas the TLR3-dependent induction of IFN-alpha, IFN-beta, and IFN-lambda is critical for primary immunity to HSV-1 in the central nervous system in children but redundant for immunity to most other viral infections.

Early events in the recognition of danger signals after tissue injury

The systemic inflammatory response observed in the setting of overwhelming infection bears striking similarities to that observed in the setting of severe traumatic injury from a clinical and physiologic standpoint. Recent observations have demonstrated that these disparate clinical entities share common mediators on a molecular level. TLRs, specifically TLR4, and the endogenous molecule high-mobility group box 1 are among the mediators that are known to play a role in inflammation in the setting of sepsis. Evidence is accumulating that demonstrates that these mediators also play a role in the host response to tissue injury. Here, we highlight findings from the 7th World Conference on Trauma, Shock, Inflammation and Sepsis in Munich, Germany, in the context of this growing body of literature.

Interleukin-18: a pro-inflammatory cytokine that plays an important role in acute pancreatitis

A large body of clinical and experimental evidence suggests that cytokines play a key role in the pathogenesis of local and systemic complications of acute pancreatitis. IL-18 is a pro-inflammatory cytokine that plays a key role in many human diseases, including acute pancreatitis. This review focuses on the present understanding in IL-18 and its potential role in acute pancreatitis. IL-18 levels reflect the severity of acute pancreatitis and display a significant negative correlation with the concentrations of antioxidative damage factors, serum selenium and glutathione peroxidases (GPx). The relationship between IL-18 and other pro-inflammatory cytokines shows that IL-18 is one of the key mediators of inflammation in the pathogenesis of acute pancreatitis. Elevation of serum IL-18 levels may mediate acute pancreatitis associated liver injury. The use of IL-18 antagonists as direct routes to block IL-18 activity and P2X7 receptor antagonists and interleukin-1beta-converting enzyme (ICE) inhibitors as indirect routes to block IL-18 activity suggest that specific therapeutic inhibition of IL-18 is a promising therapeutic approach for acute pancreatitis.

Wound healing is impaired in MyD88-deficient mice: a role for MyD88 in the regulation of wound healing by adenosine A2A receptors

Synergy between Toll-like receptor (TLR) and adenosine A2A receptor (A2AR) signaling switches macrophages from production of inflammatory cytokines such as tumor necrosis factor-alpha to production of the angiogenic growth factor vascular endothelial growth factor (VEGF). We show in this study that this switch critically requires signaling through MyD88, IRAK4, and TRAF6. Macrophages from mice lacking MyD88 (MyD88(-/-)) or IRAK4 (IRAK4(-/-)) lacked responsiveness to TLR agonists and did not respond to A2AR agonists by expressing VEGF. Suppression of TRAF6 expression with siRNA in RAW264.7 macrophages also blocked their response to TLR and A2AR agonists. Excisional skin wounds in MyD88(-/-) mice healed at a markedly slower rate than wounds in wild-type MyD88(+/+) mice, showing delayed contraction, decreased and delayed granulation tissue formation, and reduced new blood vessel density. Although macrophages accumulated to higher levels in MyD88(-/-) wounds than in controls, expression of VEGF and HIF1-alpha mRNAs was elevated in MyD88(+/+) wounds. CGS21680, an A2AR agonist, promoted repair in MyD88(+/+) wounds and stimulated angiogenesis but had no significant effect on healing of MyD88(-/-) wounds. These results suggest that the synergistic interaction between TLR and A(2A)R signaling observed in vitro that switches macrophages from an inflammatory to an angiogenic phenotype also plays a role in wound healing in vivo.

IRF3-dependent type I interferon response in B cells regulates CpG-mediated antibody production

Hypomethylated CpG oligonucleotides (CpG) are not only potent adjuvants for enhancing adaptive immune responses but may also play a critical role in the development of autoimmune diseases such as Rheumatoid Arthritis (RA) and Systemic Lupus Erythematosus (SLE). Here we provide evidence that, in addition to dendritic cells, murine B lymphocytes also exhibit a type I IFN response to CpG-B. Unlike dendritic cells, B cell-mediated type I IFN induction depended on the transcription factor IRF3, but similar to dendritic cells this pathway was independent of the IRF3 kinase TBK1. Utilizing type I IFN receptor-deficient mice, we were able to demonstrate that this IFN pathway enhanced Syndecan-1 expression and IgM production and was required for IgG2a production following CpG-B stimulation. Overall, our findings identify a unique IFN pathway in B cells that may play a central role in mediating B cell biology in response to CpG, potentially implicating this pathway in autoantibody production and the pathogenesis of certain autoimmune diseases.

Calcineurin negatively regulates TLR-mediated activation pathways

In innate immunity, microbial components stimulate macrophages to produce antimicrobial substances, cytokines, other proinflammatory mediators, and IFNs via TLRs, which trigger signaling pathways activating NF-kappaB, MAPKs, and IFN response factors. We show in this study that, in contrast to its activating role in T cells, in macrophages the protein phosphatase calcineurin negatively regulates NF-kappaB, MAPKs, and IFN response factor activation by inhibiting the TLR-mediated signaling pathways. Evidence for this novel role for calcineurin was provided by the findings that these signaling pathways are activated when calcineurin is inhibited either by the inhibitors cyclosporin A or FK506 or by small interfering RNA-targeting calcineurin, and that activation of these pathways by TLR ligands is inhibited by the overexpression of a constitutively active form of calcineurin. We further found that IkappaB-alpha degradation, MAPK activation, and TNF-alpha production by FK506 were reduced in macrophages from mice deficient in MyD88, Toll/IL-1R domain-containing adaptor-inducing IFN-beta (TRIF), TLR2, or TLR4, whereas macrophages from TLR3-deficient or TLR9 mutant mice showed the same responses to FK506 as those of wild-type cells. Biochemical studies indicate that calcineurin interacts with MyD88, TRIF, TLR2, and TLR4, but not with TLR3 or TLR9. Collectively, these results suggest that calcineurin negatively regulates TLR-mediated activation pathways in macrophages by inhibiting the adaptor proteins MyD88 and TRIF, and a subset of TLRs.

Selective predisposition to bacterial infections in IRAK-4-deficient children: IRAK-4-dependent TLRs are otherwise redundant in protective immunity

Human interleukin (IL) 1 receptor-associated kinase 4 (IRAK-4) deficiency is a recently discovered primary immunodeficiency that impairs Toll/IL-1R immunity, except for the Toll-like receptor (TLR) 3- and TLR4-interferon (IFN)-alpha/beta pathways. The clinical and immunological phenotype remains largely unknown. We diagnosed up to 28 patients with IRAK-4 deficiency, tested blood TLR responses for individual leukocyte subsets, and TLR responses for multiple cytokines. The patients' peripheral blood mononuclear cells (PBMCs) did not induce the 11 non-IFN cytokines tested upon activation with TLR agonists other than the nonspecific TLR3 agonist poly(I:C). The patients' individual cell subsets from both myeloid (granulocytes, monocytes, monocyte-derived dendritic cells [MDDCs], myeloid DCs [MDCs], and plasmacytoid DCs) and lymphoid (B, T, and NK cells) lineages did not respond to the TLR agonists that stimulated control cells, with the exception of residual responses to poly(I:C) and lipopolysaccharide in MDCs and MDDCs. Most patients (22 out of 28; 79%) suffered from invasive pneumococcal disease, which was often recurrent (13 out of 22; 59%). Other infections were rare, with the exception of severe staphylococcal disease (9 out of 28; 32%). Almost half of the patients died (12 out of 28; 43%). No death and no invasive infection occurred in patients older than 8 and 14 yr, respectively. The IRAK-4-dependent TLRs and IL-1Rs are therefore vital for childhood immunity to pyogenic bacteria, particularly Streptococcus pneumoniae. Conversely, IRAK-4-dependent human TLRs appear to play a redundant role in protective immunity to most infections, at most limited to childhood immunity to some pyogenic bacteria.

Kinase-active Interleukin-1 Receptor-associated Kinases Promote Polyubiquitination and Degradation of the Pellino Family

Members of the Pellino family are interleukin-1 receptor-associated kinase (IRAK)-interacting proteins that possess RING-like domains. The presence of these domains led to the suggestion that Pellino proteins are ubiquitin-protein isopeptide ligases (E3). However, no conclusive data currently exist to prove this proposal. This study provides the first direct evidence that Pellino proteins possess E3 activity. Recombinant forms of Pellino1 and Pellino2 and both spliced variants of Pellino3 are shown in an in vitro ubiquitination assay to be E3 ligases that catalyze Lys(63)-linked polyubiquitination, with Pellino3 exhibiting the greatest ligase activity. Whereas the Pellino proteins cause polyubiquitination of IRAK-1, we also show that kinase-active members of the IRAK family (IRAK-1 and IRAK-4) promote reciprocal polyubiquitination of the Pellino proteins and that this is associated with IRAK-induced degradation of the Pellino family. In contrast, IRAK-2 (which lacks a functional kinase domain) and kinase-dead forms of IRAK-1 and IRAK-4 fail to degrade the Pellino proteins. We show that these kinase-inactive IRAK proteins can associate with Pellino proteins, thus excluding the possibility that their inability to regulate Pellino degradation is due to lack of association with the Pellino proteins. The physiological relevance of IRAK-induced degradation of Pellino proteins is confirmed by the demonstration that lipopolysaccharide causes degradation of endogenous forms of Pellino3 in peripheral blood mononuclear cells. In summary, this study not only demonstrates Pellino proteins to be E3 ligases that can catalyze Lys(63)-linked polyubiquitination but also shows bidirectional signaling between the IRAK and Pellino families and highlights a novel function for IRAK kinase activity.

Genetic variability in inflammation pathways and prostate cancer risk

Genetic susceptibility to prostate cancer has been consistently observed by a large number of studies. Recently, several pieces of evidence obtained from epidemiological and pathological studies support that chronic inflammation in prostate tissues may play a role in prostate cancer development. Multiple genes that play critical roles in inflammatory pathways have been associated with prostate cancer risk. In this article we review the key genetic findings of the associated genes. This includes 2 genes identified through family studies, ribonuclease L (RNASEL) and macrophage scavenger receptor 1 (MSR1), as well as a number of genes suggested by case-control studies, such as macrophage inhibitory cytokine-1 (MIC-1), interleukins (IL-8, IL-10), vascular endothelial growth factor (VEGF), intercellular adhesion molecule (ICAM), and Toll-like receptors (TLR-4, TLR-1-6-10 gene cluster). Overall, recent studies seem to suggest multiple genes work together to increase prostate risk, and this is consistent with the reality that inflammation is a very complex process. Thus, future studies are expected to place an emphasis on the study of gene-gene interactions. Advances in high throughput genotyping, data mining, and algorithm development are needed in order to produce interpretable results.

IRAK-2 participates in multiple toll-like receptor signaling pathways to NFkappaB via activation of TRAF6 ubiquitination

Toll-like receptor (TLR) signaling is known to involve interleukin-1 receptor-associated kinases (IRAKs), however the particular role of IRAK-2 has remained unclear. Further, although IRAK-1 was originally thought to be central for the TLR-NFkappaB signaling axis, recent data have shown that it is dispensable for NFkappaB activation for some TLRs and demonstrated an alternative role for it in interferon regulatory factor activation. Here we show that IRAK-2 is critical for the TLR-mediated NFkappaB activation pathway. The poxviral TLR antagonist A52 inhibited NFkappaB activation by TLR2, -3, -4, -5, -7, and -9 ligands, via its interaction with IRAK-2, while not affecting interferon regulatory factor activation. Knockdown of IRAK-2 expression by small interfering RNA suppressed TLR3, TLR4, and TLR8 signaling to NFkappaB in human cell lines, and importantly, TLR4-mediated chemokine production in primary human cells. IRAK-2 usage by different TLRs was distinct, because it acted downstream of the TLR adaptors MyD88 and Mal but upstream of TRIF. Expression of IRAK-2, but not IRAK-1, led to TRAF6 ubiquitination, an event critical for NFkappaB activation. Further, IRAK-2 loss-of-function mutants, which could not activate NFkappaB, were incapable of promoting TRAF6 ubiquitination. Thus we propose that IRAK-2 plays a more central role than IRAK-1 in TLR signaling to NFkappaB.

Signaling pathways downstream of pattern-recognition receptors and their cross talk

Pattern-recognition receptors (PRRs) initiate innate immunity through pathogen recognition. Serum PRRs opsonize pathogens for enhanced phagocytic clearance. Toll-like receptors (TLRs) initiate common NF-kappaB/AP-1 and distinct IRF3/7 pathways to coordinate innate immunity and to initiate adaptive immunity against diverse pathogens. Cytoplasmic caspase-recruiting domain (CARD) helicases, such as RIG-I/MDA5, mediate antiviral immunity by inducing the production of type I interferons via the adaptor IPS-1, whereas nucleotide-binding oligomerization domain (NOD)-like receptors mediate mainly antibacterial immunity by activating NF-kappaB or inflammasomes. Dectin-1 is important for antifungal immunity, promoting phagocytosis and activating NF-kappaB. Potentially harmful TLR signaling pathways can be negatively regulated by negative feedback mechanisms and also by anti-inflammatory factors such as TGFbeta, interleukin (IL)-10, and steroids. Many combinations of TLR-TLR and TLR-NOD modulate inflammatory responses. TLRs and NALP3 interplay to produce mature IL-1beta. Thus signaling pathways downstream of PRRs and their cross talk control immune responses in effective manners.

T cell activation by TLRs: a role for TLRs in the adaptive immune response

Toll-like receptor (TLR) activation is primarily thought to affect antigen-presenting cells (APCs) by inducing an innate immune response that can subsequently activate the adaptive immune system. However, there are increasing data that demonstrate expression and activation of TLRs on T cells, thus providing evidence for a direct role for TLRs in the activation of an adaptive immune response. A study recently demonstrated that Pam3CSK {N-palmitoyl-S-[2,3-bis(palmitoloxy)-(2RS)-propyl]-Cys-Ser-Lys(4)}, a TLR2 agonist lipopeptide, activates T helper 1 (T(H)1) cells and induces interferon-gamma (IFN-gamma) production, even in the absence of TLR1, which differs from its mechanism of activation of APCs. Moreover, whereas Pam3CSK-stimulated IFN-gamma production by T(H)1 cells is ablated in the absence of both myeloid differentiation marker 88 (MyD88), an adaptor protein in the TLR pathway, and interleukin-1 receptor (IL-1R)-associated kinase-4 (IRAK4), the mitogen-activated protein kinases p38 and c-Jun N-terminal kinase (JNK) are still phosphorylated. These data suggest that TLR2 activation of T(H)1 cells occurs through a mechanism different from that described for APCs and provides further evidence of direct TLR activation of the adaptive immune system.

Mass spectrometric analysis of the endogenous type I interleukin-1 (IL-1) receptor signaling complex formed after IL-1 binding identifies IL-1RAcP, MyD88, and IRAK-4 as the stable components

We investigated the composition of the endogenous ligand-bound type I interleukin-1 (IL-1) receptor (IL-1RI) signaling complex using immunoprecipitation and tandem mass spectrometry. Three proteins with approximate molecular masses of 60 (p60), 36 (p36), and 90 kDa (p90) became phosphorylated after treatment with IL-1. Phosphorylation in vitro of p60 has been reported previously, but its identity was unknown. We showed using tandem mass spectrometry that p60 is identical to interleukin-1 receptor-associated kinase (IRAK)-4. MS also enabled detection of IL-1, IL-1RI, IL-1 receptor accessory protein (IL-1RAcP), and myeloid differentiation primary response protein 88 (MyD88) in the complex. The p60 protein (IRAK-4) was the earliest component of the complex to be phosphorylated. Phosphorylated IRAK-4 from the receptor complex migrated more slowly in SDS-PAGE than its unphosphorylated form as did recombinant IRAK-4 autophosphorylated in vitro. Phosphorylation was restricted to serine and threonine residues. IRAK-4, p36, IL-1RAcP, and MyD88 bound to the liganded receptor within 15 s of activation by IL-1 and remained associated upon prolonged activation, suggesting that the signaling complex is very stable. The p90 phosphoprotein was only transiently associated with the receptor. This behavior and its size were consistent with it being IRAK-1. Our work revealed that liganding of IL-1RI causes its strong and stable association with IL-1RAcP, MyD88, and the previously unidentified protein p60 (IRAK-4). The only component of the IL-1RI signaling complex that dissociated is IRAK-1. Our study is therefore the first detailed description of the endogenous IL-1RI complex.

MyD88, IRAK1 and TRAF6 knockdown in human chondrocytes inhibits interleukin-1-induced matrix metalloproteinase-13 gene expression and promoter activity by impairing MAP kinase activation

Interleukin-1 (IL-1) is the major prototypic proinflammatory cytokine that stimulates degradation of cartilage in arthritis by inducing prominent collagen II-degrading matrix metalloproteinase-13 (MMP-13). Nothing is known about the involvement of adaptor proteins, MyD88, IRAK1 and TRAF6 in MMP-13 regulation. Here we investigated for the first time the role of these proteins in IL-1-regulated MMP-13 expression in chondrocytes. MyD88 homodimerization inhibitory peptide diminished the expression of MMP-13 gene, promoter activity, phosphorylation of mitogen-activated protein kinases (MAPKs), c-Jun and activating protein 1 (AP-1) activity. Knockdown of MyD88, IRAK1 and TRAF6 by RNA interference (RNAi) drastically down-regulated the expression of IL-1-induced MMP-13 mRNA and protein levels and MMP-13 promoter-driven luciferase activity. Non-specific control siRNA had no effect. Mechanisms of MMP-13 inhibition involved reduced phosphorylation of ERK, p38, JNK and c-Jun as well as AP-1 transcription factor binding activity. The genetic evidence presented here demonstrates that MyD88, IRAK1 and TRAF6 proteins are crucial early mediators for the IL-1-induced MMP-13 regulation through MAPK pathways and AP-1 activity. These proteins could constitute important therapeutic targets for arthritis-associated cartilage loss by MMP-13.

How important are Toll-like receptors for antimicrobial responses?

The innate immune system is the primary line of defence against invading pathogenic microbes. Toll-like receptors (TLRs) are a family of membrane receptors which play a pivotal role in sensing a wide range of invading pathogens including bacteria, fungi and viruses. TLR-deficient mice have provided us with immense knowledge on the functioning of individual TLRs. Dysregulation of TLR signalling is linked with a number of disease conditions. Disease models have helped show that targeting components of TLR signalling cascades could lead to novel therapies in the treatment of infectious diseases. In this review we focus on the evidence provided to date to explain just how important TLRs are in host defence against microbial pathogens.

Targeting toll‐like receptors for drug development: a summary of commercial approaches

Toll-like receptors (TLRs) play a fundamental role in recognizing infectious and noxious agents as well as products of tissue damage. They are capable of initiating both protective and damaging inflammatory and immune responses. Several biotechnology and pharmaceutical companies have programmes to develop new drugs that are either: agonists of TLRs to enhance immune responses against tumours and infectious agents, or to correct allergic responses; or antagonists designed to reduce inflammation due to infection or autoimmune disease. This article reviews the commercial approaches being undertaken to develop new TLR drugs.

Acute allograft rejection occurs independently of inducible heat shock protein-70

Dendritic cells (DCs) are key mediators of the innate response to transplantation. Yet, the substances that activate these cells during acute allograft rejection remain elusive. Previous work has suggested that heat shock protein (HSP)-70 is associated with acute allograft rejection. Hence, the goal of this study was to determine whether HSP-70 activates DCs and plays a critical role in acute allograft rejection in an experimental model that is dependent on innate MyD88 signaling. Our in vitro data indicate that HSP-70 does not activate DCs. In vivo transplant studies demonstrate that HSP-70 levels are not increased during acute allograft rejection and that an absence of the inducible form of HSP-70 neither delays acute allograft rejection, impairs DCs maturation, nor alters Th1 immune responses during acute allograft rejection. In conclusion, our results indicate that HSP-70 in our experimental models does not play an essential role in acute allograft rejection.

Septicemia without sepsis: inherited disorders of nuclear factor-kappa B-mediated inflammation

Septicemia is a life-threatening condition that may lead to sepsis and even septic shock. This cascade is usually accompanied by a pronounced inflammatory response, leading to high body temperature and elevated levels of laboratory markers of inflammation. However, this response can be significantly diminished in children with inherited disorders of nuclear factor (NF)-kappa B-mediated immunity. Three disease-causing genes involved in NF-kappa B activation have been identified: NEMO, IKBA, and IRAK4. Patients with anhidrotic ectodermal dysplasia and immunodeficiency, which is caused by mutations in NEMO and IKBA, have sparse hair, dry skin, and conical teeth and are at increased risk of severe infections caused by pyogenic bacteria and atypical mycobacteria. Patients with interleukin-1 receptor-associated kinase-4 deficiency are at increased risk of invasive disease due to pyogenic bacteria. An underlying defect in NF- kappa B activation should be suspected in children with bacterial septicemia accompanied by mild signs of inflammation.

Gnotobiotic IL-10-/-;NF-kappa B(EGFP) mice reveal the critical role of TLR/NF-kappa B signaling in commensal bacteria-induced colitis

Commensal bacteria and TLR signaling have been associated with the maintenance of intestinal homeostasis in dextran sodium sulfate-induced intestinal injury. The aim of this study was to determine the in vivo role of TLR/NF-kappaB activation in a model of commensal bacteria-induced T cell-mediated colitis. A NF-kappaB reporter gene mouse (NF-kappaBEGFP) (EGFP, enhanced GFP) was crossed to the colitogenic susceptible strain IL-10-/- and derived into germfree conditions using embryo-transfer technology. Germfree IL-10wt/wt;NF-kappaBEGFP and IL-10-/-;NF-kappaBEGFP mice (wt, wild type) were dual associated with the nonpathogenic commensal bacteria strains Enterococcus faecalis and Escherichia coli. EGFP was detected using macroimaging, confocal microscopy, and flow cytometry. IL-10-/-;MyD88-/- mice were used to assess E. faecalis/E. coli-induced TLR-dependent signaling and IL-23 gene expression. Dual-associated IL-10-/-;NF-kappaBEGFP mice developed severe inflammation by 7 wk. Macroscopic analysis showed elevated EGFP expression throughout the colon of bacteria-associated IL-10-/-;NF-kappaBEGFP mice. Confocal microscopy analysis revealed EGFP-positive enterocytes during the early phase of bacterial colonization (1 wk) in both IL-10wt/wt and IL-10-/- mice, while the signal shifted toward lamina propria T cells, dendritic cells, neutrophils, and macrophages in IL-10-/- mice during colitis (7 wk). The NF-kappaB inhibitor BAY 11-7085 attenuated E. faecalis/E. coli-induced EGFP expression and development of colitis. Additionally, E. faecalis/E. coli-induced NF-kappaB signaling and IL-23 gene expression were blocked in bone marrow-derived dendritic cells derived from IL-10-/-;MyD88-/- mice. We conclude that bacteria-induced experimental colitis involves the activation of TLR-induced NF-kappaB signaling derived mostly from mucosal immune cells. Blocking TLR-induced NF-kappaB activity may represent an attractive strategy to treat immune-mediated intestinal inflammation.