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

Pivotal Advance: Inhibition of MyD88 dimerization and recruitment of IRAK1 and IRAK4 by a novel peptidomimetic compound

MyD88 is an adaptor protein, which plays an essential role in the intracellular signaling elicited by IL-1R and several TLRs. Central to its function is the ability of its Toll/IL-1R translation initiation region (TIR) domain to heterodimerize with the receptor and to homodimerize with another MyD88 molecule to favor the recruitment of downstream signaling molecules such as the serine/threonine kinases IL-1R-associated kinase 1 (IRAK1) and IRAK4. Herein, we have synthesized and tested the activity of a synthetic peptido-mimetic compound (ST2825) modeled after the structure of a heptapeptide in the BB-loop of the MyD88-TIR domain, which interferes with MyD88 signaling. ST2825 inhibited MyD88 dimerization in coimmunoprecipitation experiments. This effect was specific for homodimerization of the TIR domains and did not affect homodimerization of the death domains. Moreover, ST2825 interfered with recruitment of IRAK1 and IRAK4 by MyD88, causing inhibition of IL-1beta-mediated activation of NF-kappaB transcriptional activity. After oral administration, ST2825 dose-dependently inhibited IL-1beta-induced production of IL-6 in treated mice. Finally, we observed that ST2825 suppressed B cell proliferation and differentiation into plasma cells in response to CpG-induced activation of TLR9, a receptor that requires MyD88 for intracellular signaling. Our results indicate that ST2825 blocks IL-1R/TLR signaling by interfering with MyD88 homodimerization and suggest that it may have therapeutic potential in treatment of chronic inflammatory diseases.

The interaction between the ER membrane protein UNC93B and TLR3, 7, and 9 is crucial for TLR signaling

Toll-like receptors (TLRs) sense the presence of microbial and viral pathogens by signal transduction mechanisms that remain to be fully elucidated. A single point mutation (H412R) in the polytopic endoplasmic reticulum (ER)–resident membrane protein UNC93B abolishes signaling via TLR3, 7, and 9. We show that UNC93B specifically interacts with TLR3, 7, 9, and 13, whereas introduction of the point mutation H412R in UNC93B abolishes their interactions. We establish the physical interaction of the intracellular TLRs with UNC93B in splenocytes and bone marrow–derived dendritic cells. Further, by expressing chimeric TLRs, we show that TLR3 and 9 bind to UNC93B via their transmembrane domains. We propose that a physical association between UNC93B and TLRs in the ER is essential for proper TLR signaling.

Cutting Edge: TLR2 Directly Triggers Th1 Effector Functions

Toll-like receptors recognize pathogen-associated molecular patterns, activate innate immunity, and consequently modulate adaptive immunity in response to infections. TLRs are also expressed on T cells, and it has been shown that T cell activation is modulated by TLR ligands. However, the functions of TLRs on Th1 and Th2 effector cells and the molecular mechanisms underlying TLR-mediated activation are not fully understood. We analyzed TLR functions and downstream signaling events in both effector T cells. In mouse Th1 cells the stimulation by TLR2 but not by other TLRs directly induced IFN-gamma production, cell proliferation, and cell survival without TCR stimulation, and these effects were greatly enhanced by IL-2 or IL-12 through the enhanced activation of MAPKs. In contrast, no TLR affected the function of effector Th2 cells. These results identify TLR2 as a new specific activator of Th1 cell function and imply the involvement in Th1-mediated responses.

Chronic inflammation: a failure of resolution?

Inflammation has evolved as a protective response to insult or injury, it's a primordial response that eliminates or neutralises foreign organisms or material, the resolution of inflammation encompasses the endogenous anti-inflammatory mechanisms that protect us against excessive tissue injury and promote the restoration of tissue structure and function. In fact, our well being and survival depends upon its efficiency and carefully-balanced control. In general, the innate inflammatory response initiates within minutes and, if all is well, resolves within hours. In contrast, chronic inflammation persists for weeks, months or even years. Here, we are going to discuss the key endogenous checkpoints necessary for mounting an effective yet limited inflammatory response and the crucial biochemical pathways necessary to prevent its persistence.

Direct extracellular interaction between the early secreted antigen ESAT-6 of Mycobacterium tuberculosis and TLR2 inhibits TLR signaling in macrophages

Expression of early secreted antigenic target protein 6 (ESAT-6) by Mycobacterium tuberculosis is associated with lower innate immune responses to infection. Here we show that ESAT-6 inhibited activation of transcription factor NF-kappaB and interferon-regulatory factors (IRFs) after Toll-like receptor (TLR) signaling; inhibition of TLR signaling by ESAT-6 required the kinase Akt. Direct binding of ESAT-6 to TLR2 activated Akt and prevented interaction between the adaptor MyD88 and 'downstream' kinase IRAK4, thus abrogating NF-kappaB activation. The six carboxy-terminal amino acid residues of ESAT-6 were required and sufficient for the TLR2-mediated inhibitory effect. A critical function for the carboxy-terminal peptide of ESAT-6 in restricting MyD88-dependent TLR signaling emphasizes the possibility that mimetic inhibitory peptides could be used to restrict innate immune responses in situations in which prolonged TLR signaling has deleterious effects.

Cross-talk between IRAK-4 and the NADPH oxidase

Exposure of neutrophils to LPS (lipopolysaccharide) triggers their oxidative response. However, the relationship between the signalling downstream of TLR4 (Toll-like receptor 4) after LPS stimulation and the activation of the oxidase remains elusive. Phosphorylation of the cytosolic factor p47phox is essential for activation of the NADPH oxidase. In the present study, we examined the hypothesis that IRAK-4 (interleukin-1 receptor-associated kinase-4), the main regulatory kinase downstream of TLR4 activation, regulates the NADPH oxidase through phosphorylation of p47phox. We show that p47phox is a substrate for IRAK-4. Unlike PKC (protein kinase C), IRAK-4 phosphorylates p47phox not only at serine residues, but also at threonine residues. Target residues were identified by tandem MS, revealing a novel threonine-rich regulatory domain. We also show that p47phox is phosphorylated in granulocytes in response to LPS stimulation. LPS-dependent phosphorylation of p47phox was enhanced by the inhibition of p38 MAPK (mitogen-activated protein kinase), confirming that the kinase operates upstream of p38 MAPK. IRAK-4-phosphorylated p47phox activated the NADPH oxidase in a cell-free system, and IRAK-4 overexpression increased NADPH oxidase activity in response to LPS. We have shown that endogenous IRAK-4 interacts with p47phox and they co-localize at the plasma membrane after LPS stimulation, using immunoprecipitation assays and immunofluorescence microscopy respectively. IRAK-4 was activated in neutrophils in response to LPS stimulation. We found that Thr133, Ser288 and Thr356, targets for IRAK-4 phosphorylation in vitro, are also phosphorylated in endogenous p47phox after LPS stimulation. We conclude that IRAK-4 phosphorylates p47phox and regulates NADPH oxidase activation after LPS stimulation.

Essential role of IRAK-4 protein and its kinase activity in Toll-like receptor-mediated immune responses but not in TCR signaling

Interleukin-1 receptor-associated kinase 4 (IRAK-4) was reported to be essential for the Toll-like receptor (TLR)- and T cell receptor (TCR)-mediated signaling leading to the activation of nuclear factor kappaB (NF-kappaB). However, the importance of kinase activity of IRAK family members is unclear. In this study, we investigated the functional role of IRAK-4 activity in vivo by generating mice carrying a knockin mutation (KK213AA) that abrogates its kinase activity. IRAK-4(KN/KN) mice were highly resistant to TLR-induced shock response. The cytokine production in response to TLR ligands was severely impaired in IRAK-4(KN/KN) as well as IRAK-4(-/-) macrophages. The IRAK-4 activity was essential for the activation of signaling pathways leading to mitogen-activated protein kinases. TLR-induced IRAK-4/IRAK-1-dependent and -independent pathways were involved in early induction of NF-kappaB-regulated genes in response to TLR ligands such as tumor necrosis factor alpha and IkappaBzeta. In contrast to a previous paper (Suzuki, N., S. Suzuki, D.G. Millar, M. Unno, H. Hara, T. Calzascia, S. Yamasaki, T. Yokosuka, N.J. Chen, A.R. Elford, et al. 2006. Science. 311:1927-1932), the TCR signaling was not impaired in IRAK-4(-/-) and IRAK-4(KN/KN) mice. Thus, the kinase activity of IRAK-4 is essential for the regulation of TLR-mediated innate immune responses.

Loss of DExD/H Box RNA Helicase LGP2 Manifests Disparate Antiviral Responses

The DExD/H box RNA helicase retinoic acid-inducible gene I (RIG-I) and the melanoma differentiation-associated gene 5 (MDA5) are key intracellular receptors that recognize virus infection to produce type I IFN. A third helicase gene, Lgp2, is homologous to Rig-I and Mda5 but lacks a caspase activation and recruitment domain. We generated Lgp2-deficient mice and report that the loss of this gene greatly sensitizes cells to cytosolic polyinosinic/polycytidylic acid-mediated induction of type I IFN. However, negative feedback inhibition of IFN-beta transcription was found to be normal in the absence of LGP2, indicating that LGP2 is not the primary negative regulator of type I IFN production. Our data further indicate that Lgp2-/- mice exhibited resistance to lethal vesicular stomatitis virus infection, a virus whose replicative RNA intermediates are recognized specifically by RIG-I rather than by MDA5 to trigger the production of type I IFN. However, mice lacking LGP2 were observed to exhibit a defect in type I IFN production in response to infection by the encephalomyocarditis virus, the replication of which activates MDA5-dependent innate immune responses. Collectively, our data indicate a disparate regulatory role for LGP2 in the triggering of innate immune signaling pathways following RNA virus infection.

A critical role for IRAK4 kinase activity in Toll-like receptor-mediated innate immunity

IRAK4 is a member of IL-1 receptor (IL-1R)–associated kinase (IRAK) family and has been shown to play an essential role in Toll-like receptor (TLR)–mediated signaling. We recently generated IRAK4 kinase-inactive knock-in mice to examine the role of kinase activity of IRAK4 in TLR-mediated signaling pathways. The IRAK4 kinase–inactive knock-in mice were completely resistant to lipopolysaccharide (LPS)- and CpG-induced shock, due to impaired TLR-mediated induction of proinflammatory cytokines and chemokines. Although inactivation of IRAK4 kinase activity did not affect the levels of TLR/IL-1R–mediated nuclear factor κB activation, a reduction of LPS-, R848-, and IL-1–mediated mRNA stability contributed to the reduced cytokine and chemokine production in bone marrow–derived macrophages from IRAK4 kinase–inactive knock-in mice. Both TLR7- and TLR9-mediated type I interferon production was abolished in plasmacytoid dendritic cells isolated from IRAK4 knock-in mice. In addition, influenza virus–induced production of interferons in plasmacytoid DCs was also dependent on IRAK4 kinase activity. Collectively, our results indicate that IRAK4 kinase activity plays a critical role in TLR-dependent immune responses.

RBCK1 negatively regulates tumor necrosis factor- and interleukin-1-triggered NF-kappaB activation by targeting TAB2/3 for degradation

Inflammation is a homeostatic mechanism that limits the effects of infectious agents. Tumor necrosis factor (TNF) and interleukin (IL)-1 are two cytokines that induce inflammation through activation of the transcription factor NF-kappaB. Various studies have suggested that two homologous and structurally related adapter proteins TAB2 and TAB3 play redundant roles in TNF- and IL-1-mediated NF-kappaB activation pathways. Both TAB2 and TAB3 contain CUE, coiled-coil, and nuclear protein localization 4 zinc finger (NZF) domains. The NZF domains of TAB2/3 are critical for TAB2/3 to bind to Lys(63)-linked polyubiquitin chains of other adaptor proteins, such as receptor-interacting protein and TRAF6, which are two signaling proteins essential for TNF- and IL-1-induced NF-kappaB activation, respectively. In a search for proteins containing NZF domains conserved with those of TAB2/3, we identified RBCK1, which has been shown to act as an E3 ubiquitin ligase in iron metabolism. Overexpression of RBCK1 negatively regulates TAB2/3-mediated and TNF- and IL-1-induced NF-kappaB activation, whereas knockdown of RBCK1 by RNA interference potentiates TNF- and IL-1-induced NF-kappaB activation. RBCK1 physically interacts with TAB2/3 and facilitates degradation of TAB2/3 through a proteasome-dependent process. Taken together, our findings suggest that RBCK1 is involved in negative regulation of inflammatory signaling triggered by TNF and IL-1 through targeting TAB2/3 for degradation.

Elevated levels of IL-18 in plasma and skeletal muscle in chronic obstructive pulmonary disease

The aim of this study was to test the hypothesis that systemic inflammation in patients with chronic obstructive pulmonary disease (COPD) is accompanied by enhanced interleukin 18 (IL-18) expression in skeletal muscle, which may precede muscle weight loss. Twenty patients with moderate to severe COPD [12 women, 66 +/- 9.4 years of age and forced expiratory volume in 1 second (FEV(1)) of 32% +/- 12 % of predicted value] and 20 healthy age-, gender-, and body mass index (BMI)-matched controls (10 nonsymptomatic smokers and 10 nonsmokers) were included in the study. Plasma levels of IL-18 were elevated in COPD patients (n = 20) versus healthy controls (n = 20) (221.2 pg/ml [196.0-294.2 pg/pl] vs. 164.8 pg/ml [144.4-193.3 pg/pl], p = 0.05) [corrected] and IL-18 was expressed in skeletal muscle, with IL-18 mRNA levels being elevated in biopsies from COPD patients (n = 19) versus healthy controls (n = 18) (4.3 [2.6-5.9] vs. 2.4 [1.6-3.1], p = 0.05) [corrected]. Immunohistochemical evaluation revealed a strong expression of IL-18 in Type II muscle fibers from COPD patients. Plasma levels and skeletal muscle mRNA levels of tumor necrosis factor alpha (TNF-alpha) and IL-6 did not differ between the groups. Elevated skeletal muscle expression of IL-18 was found in COPD patients with normal body weight, indicating that IL-18 potentially may be involved in the pathogenesis of COPD-associated muscle wasting.

Gene expression profiles differentiate between sterile SIRS and early sepsis

INTRODUCTION

The systemic inflammatory response syndrome (SIRS) occurs frequently in critically ill patients and presents similar clinical appearances despite diverse infectious and noninfectious etiologies. Despite similar phenotypic expression, these diverse SIRS etiologies may induce divergent genotypic expressions. We hypothesized that gene expression differences are present between sepsis and uninfected SIRS prior to the clinical appearance of sepsis.

METHODS

Critically ill uninfected SIRS patients were followed longitudinally for the development of sepsis. All patients had whole blood collected daily for gene expression analysis by Affymetrix Hg_U133 2.0 Plus microarrays. SIRS patients developing sepsis were compared with those remaining uninfected for differences in gene expression at study entry and daily for 3 days prior to conversion to sepsis. Acceptance criteria for differentially expressed genes required: >1.2 median fold change between groups and significance on univariate and multivariate analysis. Differentially expressed genes were annotated to pathways using DAVID 2.0/EASE analysis.

RESULTS

A total of 12,782 (23.4%) gene probes were differentially expressed on univariate analysis 0 to 48 hours before clinical sepsis. 626 (1.1%) probes met acceptance criteria, corresponding to 459 unique genes, 65 (14.2%) down and 395 (85.8%) up expressed. These genes annotated to 10 pathways that functionally categorized to 4 themes involving innate immunity, cytokine receptors, T cell differentiation, and protein synthesis regulation.

CONCLUSIONS

Sepsis has a unique gene expression profile that is different from uninfected inflammation and becomes apparent prior to expression of the clinical sepsis phenotype.

Development of sorbent therapy for multiple organ dysfunction syndrome (MODS)

As a syndrome, multiple organ dysfunction (MODS) is defined as an altered organ function in the setting of sepsis, septic shock or systemic inflammatory response syndrome (SIRS) and is the most common cause of death in intensive care units. Endotoxin, a constituent of cell walls of Gram-negative bacteria, plays an important role in the initiation and development of MODS. The cytokines, especially tumor necrosis factor alpha (TNF-alpha), are early regulators of the immune response and can induce the release of secondary cytokines. To remove endotoxin and TNF-alpha from patients with MODS, the adsorption method has proven to be most effective. In this review, we provide various methods of removal of endotoxins and TNF-alpha using different adsorbents.

T-independent activation-induced cytidine deaminase expression, class-switch recombination, and antibody production by immature/transitional 1 B cells

Inflammation elicits a splenic lymphopoiesis of unknown physiologic significance but one that juxtaposes developing B cells and exogenous Ag. We show that immature and transitional 1 (immature/T1) B cells constitutively express activation-induced cytidine deaminase and B lymphocyte-induced maturation protein 1 in amounts that support accelerated plasmacytic differentiation and limited class-switch recombination. In vivo, activation of immature/T1 B cells by TLR ligands or bacterial vaccine rapidly induces T1 cells to divide, proliferate, and secrete IgM, IgG, or IgA Ab; in vitro, proliferation and differentiation are substantially enhanced by B cell-activating factor. We propose that inflammation-induced extramedullary lymphopoiesis represents a specialized mechanism for innate Ab responses to microbial pathogens.

Role of IRAK4 and IRF3 in the control of intracellular infection withChlamydia pneumoniae

TLR signal transduction involves a MyD88-mediated pathway, which leads to recruitment of the IL-1 receptor (IL-1R)-associated kinase 4 (IRAK4) and Toll/IL-1R translation initiation region domain-containing adaptor-inducing IFN-beta-mediated pathway, resulting in the activation of IFN regulatory factor (IRF)3. Both pathways can lead to expression of IFN-beta. TLR-dependent and -independent signals converge in the TNF receptor-associated factor 6 (TRAF6) adaptor, which mediates the activation of NF-kappaBeta. Infection of murine bone marrow-derived macrophages (BMM) with Chlamydia pneumoniae induces IFN-alpha/beta- and NF-kappaBeta-dependent expression of IFN-gamma, which in turn, will control bacterial growth. The role of IRAK4 and IRF3 in the regulation of IFN-alpha/beta expression and NF-kappaBeta activation was studied in C. pneumoniae-infected BMM. We found that levels of IFN-alpha, IFN-beta, and IFN-gamma mRNA were reduced in infected IRAK4(-/-) BMM compared with wild-type (WT) controls. BMM also showed an IRAK4-dependent growth control of C. pneumoniae. No increased IRF3 activation was detected in C. pneumoniae-infected BMM. Similar numbers of intracellular bacteria, IFN-alpha, and IFN-gamma mRNA titers were observed in C. pneumoniae-infected IRF3(-/-) BMM. On the contrary, IFN-beta(-/-) BMM showed lower IFN-alpha and IFN-gamma mRNA levels and higher bacterial titers compared with WT controls. C. pneumoniae infection-induced activation of NF-kappaBeta and expression of proinflammatory cytokines were shown to be TRAF6-dependent but did not require IRAK4 or IRF3. Thus, our data indicate that IRAK4, but not IRF3, controls C. pneumoniae-induced IFN-alpha and IFN-gamma secretion and bacterial growth. IRAK4 and IRF3 are redundant for infection-induced NF-kappaB activation, which is regulated by TRAF6.

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.

IRAK-4 kinase activity is required for interleukin-1 (IL-1) receptor- and toll-like receptor 7-mediated signaling and gene expression

IRAK-4 is an essential component of the signal transduction complex downstream of the IL-1- and Toll-like receptors. Although regarded as the first kinase in the signaling cascade, the role of IRAK-4 kinase activity versus its scaffold function is still controversial. To investigate the role of IRAK-4 kinase function in vivo, "knock-in" mice were generated by replacing the wild type IRAK-4 gene with a mutant gene encoding kinase-deficient IRAK-4 protein (IRAK-4 KD). IRAK-4 kinase was rendered inactive by mutating the conserved lysine residues in the ATP pocket essential for coordinating ATP. Analyses of embryonic fibroblasts and macrophages obtained from IRAK-4 KD mice demonstrate lack of cellular responsiveness to stimulation with IL-1beta or a Toll-like receptor 7 (TLR7) agonist. IRAK-4 kinase deficiency prevents the recruitment of IRAK-1 to the IL-1 receptor complex and its subsequent phosphorylation and degradation. IRAK-4 KD cells are severely impaired in NFkappaB, JNK, and p38 activation in response to IL-1beta or TLR7 ligand. As a consequence, IL-1 receptor/TLR7-mediated production of cytokines and chemokines is largely absent in these cells. Additionally, microarray analysis identified IL-1beta response genes and revealed that the induction of IL-1beta-responsive mRNAs is largely ablated in IRAK-4 KD cells. In summary, our results suggest that IRAK-4 kinase activity plays a critical role in IL-1 receptor (IL-1R)/TLR7-mediated induction of inflammatory responses.

TLR2 and TLR4 agonists synergistically up-regulate SR-A in RAW264.7 through p38

It is known that macrophage scavenger receptor A (SR-A) can protect mice from endotoxemia. In addition, Escherichia coli O111:B4 LPS from Sigma (sLPS), which contains both TLR4 and TLR2 agonists, was previously reported to be able to induce SR-A expression on murine macrophage cell line RAW264.7. However, the relative role of both TLR4 and TLR2 agonists from Sigma (sLPS) in the up-regulation of SR-A on RAW264.7 is still undefined. Here, we found that sLPS could only slightly up-regulate SR-A on RAW264.7 following removing its TLR4 and TLR2 agonists, respectively. In contrast, the combination of TLR4 agonist uLPS (re-extracted sLPS) and TLR2 agonist Pam3CSK4 dramatically induced SR-A expression, and synergistically promoted RAW264.7 to bind and internalize FITC-LPS specifically through SR-A. The combination had no such effect either on TLR2 or TLR4 expression, and incubation with IL-6, IL-10, IL-12 or TNF-alpha alone could not induce SR-A expression on RAW264.7. In addition, treatment with a NF-kappaB inhibitor pyrrolidine dithiocarbamate (PDTC) could only weakly suppress the up-regulation of SR-A by the combination. However, the combination synergistically promoted MAPK p38 phosphorylation, and p38 specific inhibitor SB203580 completely suppressed its inducible effect on SR-A expression. Hence, we demonstrated that up-regulation of SR-A by sLPS was resulted from the cooperation of its TLR4 and TLR2 agonists through p38, and we also presented a novel synergy effect of TLR2 and TLR4 agonists.

Cutting Edge: IL-1 Receptor-Associated Kinase 4 Structures Reveal Novel Features and Multiple Conformations

IL-1R-associated kinase (IRAK)4 plays a central role in innate and adaptive immunity, and is a crucial component in IL-1/TLR signaling. We have determined the crystal structures of the apo and ligand-bound forms of human IRAK4 kinase domain. These structures reveal several features that provide opportunities for the design of selective IRAK4 inhibitors. The N-terminal lobe of the IRAK4 kinase domain is structurally distinctive due to a loop insertion after an extended N-terminal helix. The gatekeeper residue is a tyrosine, a unique feature of the IRAK family. The IRAK4 structures also provide insights into the regulation of its activity. In the apo structure, two conformations coexist, differing in the relative orientation of the two kinase lobes and the position of helix C. In the presence of an ATP analog only one conformation is observed, indicating that this is the active conformation.

THE IN VIVO ANTITUMORAL EFFECTS OF LIPOPOLYSACCHARIDE AGAINST GLIOBLASTOMA MULTIFORME ARE MEDIATED IN PART BY TOLL-LIKE RECEPTOR 4

OBJECTIVE

Toll-like receptor 4 (Tlr-4) mediates many biological effects of lipopolysaccharide (LPS), which has antitumoral effects on glioblastoma both in vivo and in vitro. However, the precise role of Tlr-4 in these antitumoral effects remains unknown.

METHODS

The role of Tlr-4 in the antitumoral effect of LPS on glioblastomas was assessed in wild-type BALB/c mice and in Tlr-4 knockout (KO) BALB/c mice. Mice were implanted with DBT glioblastoma cells intracranially or subcutaneously, were treated with intratumoral LPS, and were assessed by histopathological examination for degrees of tumor progression and inflammation. Flow cytometry and Western blotting with antibodies to the Tlr-4 receptor and flow cytometry to the related CD14 moiety were performed to quantitate the expression levels of these two receptors by glioblastoma cells.

RESULTS

For subcutaneous tumors, LPS caused near complete tumor elimination in wild-type mice, but only a 50% reduction in Tlr-4 KO mice. For mice implanted with intracranial glioblastomas, LPS increased survival times modestly in wild-type mice, but showed no benefit in the Tlr-4 KO mice. There were no histological differences among wild-type and Tlr-4 KO mice, except for tumor size. In both models, an early neutrophilic and later macrophage-rich inflammatory infiltrate were seen after LPS administration. Quantitative flow cytometry and Western blotting showed no Tlr-4 receptor or CD14 expression in murine and human glioblastoma cells in vitro, and Western blotting suggested that Tlr-4 effects are mediated by nontumoral elements such as microglia and inflammatory cells.

CONCLUSION

LPS-induced antitumoral effects on glioblastoma multiforme are mediated, in part, by the Tlr-4 receptor. Further understanding of this process may lead to novel treatment strategies for this uniformly fatal disease.

Crystal structures of IRAK-4 kinase in complex with inhibitors: a serine/threonine kinase with tyrosine as a gatekeeper

Interleukin-1 (IL-1) receptor-associated kinase-4 (IRAK-4) is a serine/threonine kinase that plays an essential role in signal transduction by Toll/IL-1 receptors (TIRs). Here, we report the crystal structures of the phosphorylated human IRAK-4 kinase domain in complex with a potent inhibitor and with staurosporine to 2.0 and 2.2 A, respectively. The structures reveal that IRAK-4 has a unique tyrosine gatekeeper residue that interacts with the conserved glutamate from helix alphaC. Consequently, helix alphaC is "pulled in" to maintain the active orientation, and the usual pre-existing hydrophobic back pocket of the ATP-binding site is abolished. The peptide substrate-binding site is more open when compared with other protein kinases due to a marked movement of helix alphaG. The pattern of phosphate ligand interactions in the activation loop bears a close resemblance to that of a tyrosine kinase. Our results provide insights into IRAK-4 function and the design of selective inhibitors.

Toll like receptors and viruses

Toll like receptors (TLRs) are 'pattern recognition proteins' that discriminate between self and non-self. They interact with products of infectious agents to activate cells of the innate immune system and also stimulate the adaptive immune system. Intracellular and extracellular TLRs recognise a wide range of viruses leading to the production of different cytokines. In this paper, we summarise the types of TLRs that recognise viruses, the cytokines produced, the specific cell types involved and the activation pathways that have been characterised. We also provide an overview of the viruses demonstrated to interact with TLRs. A greater understanding of relationships between viruses or viral products and TLRs should improve understanding of pathogenesis and lead to the development of new anti-viral therapies.

Identifying and hurdling obstacles to translational research

Although there is overwhelming pressure from funding agencies and the general public for scientists to bridge basic and translational studies, the fact remains that there are significant hurdles to overcome in order to achieve this goal. The purpose of this Opinion article is to examine the nature of these hurdles and to provide food for thought on the main obstacles that impede this process.

Genomics of host-pathogen interactions

The complete sequences of hundreds of microbial genomes have provided drug discovery pipelines with thousands of new potential drug targets. Their availability has also stimulated the development of a variety of innovative approaches that allow functional studies to be performed on the entire genome of an organism. This chapter describes how these approaches have been applied to the analysis of host-pathogen interactions and discusses how such studies might facilitate the development of new antibiotics.

Interleukin-1 (IL-1)-induced TAK1-dependent Versus MEKK3-dependent NFkappaB activation pathways bifurcate at IL-1 receptor-associated kinase modification

Interleukin-1 (IL-1) receptor-associated kinase (IRAK) is phosphorylated after it is recruited to the receptor, subsequently ubiquitinated, and eventually degraded upon IL-1 stimulation. Although a point mutation changing lysine 134 to arginine (K134R) in IRAK abolished IL-1-induced IRAK ubiquitination and degradation, mutations of serines and threonines adjacent to lysine 134 to alanines ((S/T)A (131-144)) reduced IL-1-induced IRAK phosphorylation and abolished IRAK ubiquitination. Through the study of these IRAK modification mutants, we uncovered two parallel IL-1-mediated signaling pathways for NFkappaB activation, TAK1-dependent and MEKK3-dependent, respectively. These two pathways bifurcate at the level of IRAK modification. The TAK1-dependent pathway leads to IKKalpha/beta phosphorylation and IKKbeta activation, resulting in classical NFkappaB activation through IkappaBalpha phosphorylation and degradation. The TAK1-independent MEKK3-dependent pathway involves IKKgamma phosphorylation and IKKalpha activation, resulting in NFkappaB activation through IkappaBalpha phosphorylation and subsequent dissociation from NFkappaB but without IkappaBalpha degradation. These results provide significant insight to our further understanding of NFkappaB activation pathways.

Recognition and signaling by toll-like receptors

Toll-like receptors (TLRs) are transmembrane proteins that detect invading pathogens by binding conserved, microbially derived molecules and that induce signaling cascades for proinflammatory gene expression. A critical component of the innate immune system, TLRs utilize leucine-rich-repeat motifs for ligand binding and a shared cytoplasmic domain to recruit the adaptors MyD88, TRIF, TIRAP, and/or TRAM for downstream signaling. Despite significant domain conservation, TLRs induce gene programs that lead not only to the robust production of general proinflammatory mediators but also to the production of unique effectors, which provide pathogen-tailored immune responses. Here we review the mechanisms by which TLRs recognize pathogens and induce distinct signaling cascades.

IL-21-induced Bepsilon cell apoptosis mediated by natural killer T cells suppresses IgE responses

Epidemiological studies have suggested that the recent increase in the incidence and severity of immunoglobulin (Ig)E-mediated allergic disorders is inversely correlated with Mycobacterium bovis bacillus Calmette Guerin (BCG) vaccination; however, the underlying mechanisms remain uncertain. Here, we demonstrate that natural killer T (NKT) cells in mice and humans play a crucial role in the BCG-induced suppression of IgE responses. BCG-activated murine Vα14 NKT cells, but not conventional CD4 T cells, selectively express high levels of interleukin (IL)-21, which preferentially induces apoptosis in Bɛ cells. Signaling from the IL-21 receptor increases the formation of a complex between Bcl-2 and the proapoptotic molecule Bcl-2–modifying factor, resulting in Bɛ cell apoptosis. Similarly, BCG vaccination induces IL-21 expression by human peripheral blood mononuclear cells (PBMCs) in a partially NKT cell–dependent fashion. BCG-activated PBMCs significantly reduce IgE production by human B cells. These findings provide new insight into the therapeutic effect of BCG in allergic diseases.

IRAK-4 mutation (Q293X): rapid detection and characterization of defective post-transcriptional TLR/IL-1R responses in human myeloid and non-myeloid cells

Innate immunodeficiency has recently been reported as resulting from the Q293X IRAK-4 mutation with consequent defective TLR/IL-1R signaling. In this study we report a method for the rapid allele-specific detection of this mutation and demonstrate both cell type specificity and ligand specificity in defective IL-1R-associated kinase (IRAK)-4-deficient cellular responses, indicating differential roles for this protein in human PBMCs and primary dermal fibroblasts and in LPS, IL-1beta, and TNF-alpha signaling. We demonstrate transcriptional and post-transcriptional defects despite NF-kappaB signaling and intact MyD88-independent signaling and propose that dysfunctional complex 1 (IRAK1/TRAF6/TAK1) signaling, as a consequence of IRAK-4 deficiency, generates specific defects in MAPK activation that could underpin this patient's innate immunodeficiency. These studies demonstrate the importance of studying primary human cells bearing a clinically relevant mutation; they underscore the complexity of innate immune signaling and illuminate novel roles for IRAK-4 and the fundamental importance of accessory proinflammatory signaling to normal human innate immune responses and immunodeficiencies.

IRAK-4 – a shared NF-κB activator in innate and acquired immunity

The human body is protected against external pathogens by two immune systems: innate and acquired immunities. Whereas innate immunity exhibits immediate responses to external pathogens by recognizing pathogen-associated molecular patterns (PAMPs), adaptive immunity uses T cells to recognize and defend against pathogens by developing effector cells, antibodies and memory cells. Although each system seems to possess distinct activation mechanisms, interleukin-1 receptor-associated kinase (IRAK)-4 is essential for NF-kappaB activation in Toll-like receptor (TLR) and T-cell receptor (TCR) signaling pathways. This implies possible crosstalk between innate and acquired immunities, and evolutionary development that resulted in the use of innate signaling molecules by the acquired immune system. Here, we discuss the impact of these evolutionarily conserved molecules on innate and acquired immunity, and their potential as drug targets for the simultaneous modulation of both immunities.

HIV-1 infection of mononuclear phagocytic cells: the case for bacterial innate immune deficiency in AIDS

HIV-1 infection of mononuclear phagocytic cells, comprising monocytes, macrophages, and dendritic cells, has been the subject of extensive research over the past 20 years. The roles of mononuclear phagocytic cells in transmission of HIV-1 infection and as reservoirs of actively replicating virus have received particular attention. Experimental data have also accumulated about the effects of HIV-1 on the physiological function of mononuclear phagocytic cells, particularly their role in innate immunity to bacteria. The effects of HIV-1 on bacterial innate immune responses by mononuclear phagocytic cells are discussed here together with reports of direct interactions between HIV-encoded products and bacterial innate immune signalling pathways. These reports demonstrate mechanisms for HIV-mediated disruption of innate immune responses by mononuclear phagocytic cells that could provide novel therapeutic targets in HIV-infected patients. The clinical urgency is highlighted by greatly increased risk of invasive bacterial disease in this population, even in the era of highly active antiretroviral therapy. HIV-mediated injury to bacterial innate immunity provides an experimental paradigm that could broaden our overall understanding of innate immunity and be used to study responses to pathogens other than bacteria.

Oligonucleotides suppress IL-8 in skin keratinocytes in vitro and offer anti-inflammatory properties in vivo

DNA codes for genetic information. Furthermore, recent findings suggest that DNA offers additional function, particularly in the recognition of microorganisms. In this study, we investigated two classes of oligodeoxynucleotides (ODN) in skin keratinocytes; namely, an ODN comprising two cytidine-phosphate-guanosine (CpG) motifs (CpG-1-phosphorothioate (PTO)) and a poly-cytidine (Non-CpG-5-PTO) as control. Both fluorescence-tagged ODN were rapidly taken up by cells and accumulated already after 5 minutes in perinuclear compartments. In order to test whether ODN convey immunological effects in keratinocytes, secretion of IL-8 was measured. Interestingly, both CpG-1-PTO and Non-CpG-5-PTO suppressed basal and tumor necrosis factor alpha-induced IL-8 levels measured in cell culture supernatants. Experiments using deletion mutant revealed a critical length of approximately 16 nucleotides conveying IL-8 suppression. Studies regarding the ODN backbone offered that PTO bondings are critical for significant IL-8 suppression. In order to substantiate the anti-inflammatory response, a contact hypersensitivity mouse model was utilized. Topical application of Non-CpG-5-PTO-containing ointments reduced ear thickness in sensitized mice. Taken together, these findings suggest an anti-inflammatory effect of ODN in epithelial cells in vitro and in vivo, indicating that DNA molecules offer distinct biological activities restricted to the physiological compartment applied. This effect seems to be independent from Toll-like receptor 9.

Glycine blunts transplantative liver ischemia-reperfusion injury by downregulating interleukin 1 receptor associated kinase-4

AIM

To determine whether glycine could downregulate interleukin 1 receptor associated kinase-4 (IRAK-4) expression to interfere with lipopolysaccharides (LPS) signal transduction and blunt transplantative liver ischemia-reperfusion injury (I/RI).

METHODS

SD rats were randomly divided into two groups: donor animals of the glycine group (n=40) were given glycine (1.5 mL; 300 mmol/L, iv) 1 h before harvest, and the control group were treated with 1.5 mL physiological saline (n= 40). Orthotopic liver transplantation was then performed according to the Kamada technique. Ten animals in each group were followed up for 7 d after surgery to assess survival. The remaining animals in each group were divided into 3 subgroups (n=10) at 1h, 2 h and 6 h after portal vein reperfusion. Levels of LPS, serum aspartate aminotransferase (AST), alanine aminotransferase (ALT) and total bilirubin in portal circulation, as well as IRAK-4 and TNF-alpha expression, NF-kappaB transcriptional activity and morphological study of liver tissues were analyzed.

RESULTS

Reperfusion resulted in a significant elevation of LPS concentrations in each group persisting to the end of our study. However, glycine, which led to improved survival rate and liver function, significantly alleviated liver parenchyma cell damage by downregulating IRAK-4, TNF-alpha expression and NF-kappaB transcriptional activity compared with the control group.

CONCLUSION

Glycine can attenuate hepatic I/RI by downregulating IRAK-4 to interfere with LPS signal transduction.

Regulation and function of IKK and IKK-related kinases

Members of the nuclear factor kappa B (NF-kappaB) family of dimeric transcription factors (TFs) regulate expression of a large number of genes involved in immune responses, inflammation, cell survival, and cancer. NF-kappaB TFs are rapidly activated in response to various stimuli, including cytokines, infectious agents, and radiation-induced DNA double-strand breaks. In nonstimulated cells, some NF-kappaB TFs are bound to inhibitory IkappaB proteins and are thereby sequestered in the cytoplasm. Activation leads to phosphorylation of IkappaB proteins and their subsequent recognition by ubiquitinating enzymes. The resulting proteasomal degradation of IkappaB proteins liberates IkappaB-bound NF-kappaB TFs, which translocate to the nucleus to drive expression of target genes. Two protein kinases with a high degree of sequence similarity, IKKalpha and IKKbeta, mediate phosphorylation of IkappaB proteins and represent a convergence point for most signal transduction pathways leading to NF-kappaB activation. Most of the IKKalpha and IKKbeta molecules in the cell are part of IKK complexes that also contain a regulatory subunit called IKKgamma or NEMO. Despite extensive sequence similarity, IKKalpha and IKKbeta have largely distinct functions, due to their different substrate specificities and modes of regulation. IKKbeta (and IKKgamma) are essential for rapid NF-kappaB activation by proinflammatory signaling cascades, such as those triggered by tumor necrosis factor alpha (TNFalpha) or lipopolysaccharide (LPS). In contrast, IKKalpha functions in the activation of a specific form of NF-kappaB in response to a subset of TNF family members and may also serve to attenuate IKKbeta-driven NF-kappaB activation. Moreover, IKKalpha is involved in keratinocyte differentiation, but this function is independent of its kinase activity. Several years ago, two protein kinases, one called IKKepsilon or IKK-i and one variously named TBK1 (TANK-binding kinase), NAK (NF-kappaB-activated kinase), or T2K (TRAF2-associated kinase), were identified that exhibit structural similarity to IKKalpha and IKKbeta. These protein kinases are important for the activation of interferon response factor 3 (IRF3) and IRF7, TFs that play key roles in the induction of type I interferon (IFN-I). Together, the IKKs and IKK-related kinases are instrumental for activation of the host defense system. This Review focuses on the functions of IKK and IKK-related kinases and the molecular mechanisms that regulate their activities.

The role of caspases in photoreceptor cell death of the retinoschisin-deficient mouse

Early schisis cavities in the retinal bipolar cell layer accompanied by progressive loss of cone and rod photoreceptor cells are the hallmark of the retinoschisin-deficient (Rs1h(-/Y)) murine retina. With this study we aimed at elucidating the molecular events underlying the photoreceptor cell death in this established murine model of X-linked juvenile retinoschisis. We show that photoreceptor degeneration in the Rs1h(-/Y) mouse is due to apoptotic events peaking around postnatal day 18. Cell death is accompanied by increased expression of initiator and inflammatory caspases but not by downstream effector caspases. The strong induction of caspase-1 (Casp1) prompted us to explore its involvement in the apoptotic process. We therefore generated double knock-out mice deficient for both retinoschisin and Casp1. No direct influence of the Casp1 genotype on apoptosis could be identified although striking differences in the overall number of resident microglia were observed independent of the Rs1h genotype.

Toll-like receptors: from the discovery of NFkappaB to new insights into transcriptional regulations in innate immunity

Toll-like receptors (TLRs) are key components of the innate immune system, functioning as pattern recognition receptors that recognise a wide range of microbial pathogens. TLRs represent a primary line of defence against invading pathogens in mammals, plants and insects. Recognition of microbial components by TLRs triggers a cascade of cellular signals that culminates in the activation of NFkappaB which leads to inflammatory gene expression and clearance of the infectious agent. The history of NFkappaB began with the TLR4 ligand lipopolysaccharide (LPS), a component of the cell wall of Gram-negative bacteria, since this was the stimulus first used to activate NFkappaB in pre-B-cells. However, since those early days it has been a circuitous route, made possible by drawing on information provided by many different fields, that has led us not only to the discovery of TLRs but also to an understanding of the complex pathways that lead from TLR ligation to NFkappaB activation. In this review we will summarize the current knowledge of TLR-mediated NFkappaB activation, and also the recent discoveries that subtle differences in kappaB binding sequences and NFkappaB dimer formation result in specific gene expression profiles.

TLR signaling

The Toll-like receptor (TLR) family plays an instructive role in innate immune responses against microbial pathogens, as well as the subsequent induction of adaptive immune responses. TLRs recognize specific molecular patterns found in a broad range of microbial pathogens such as bacteria and viruses, triggering inflammatory and antiviral responses and dendritic cell maturation, which result in the eradication of invading pathogens. Individual TLRs interact with different combinations of adapter proteins and activate various transcription factors such as nuclear factor (NF)-kappaB, activating protein-1 and interferon regulatory factors, driving a specific immune response. This review outlines the recent advances in our understanding of TLR-signaling pathways and their roles in immune responses. Further, we also discuss a new concept of TLR-independent mechanisms for recognition of microbial pathogens.

Intracellular TLR signaling: a structural perspective on human disease

TLRs are crucial sensors of microbial infection. Maintaining structural integrity of TLR signaling components is essential for subsequent immunological protection. Alterations to the structure of these signaling molecules are often associated with profound clinical outcomes and susceptibility to various infectious diseases. These changes in structure are sometimes the result of a single nucleotide polymorphism (SNP). Numerous SNPs have been found in components of the TLR signaling pathway. Recently, the medical consequences and effects on TLR signaling of several of these SNPs have been elucidated. In addition, there have been numerous structures solved that are important to our understanding of the TLR signaling pathway at the molecular level. The scope of this review is to tie together current structural, biochemical, and genetic information of TLR signaling.

Cellular reprogramming by gram-positive bacterial components: a review

LPS tolerance has been the focus of extensive scientific and clinical research over the last several decades in an attempt to elucidate the sequence of changes that occur at a molecular level in tolerized cells. Tolerance to components of gram-positive bacterial cell walls such as bacterial lipoprotein and lipoteichoic acid is a much lesser studied, although equally important, phenomenon. This review will focus on cellular reprogramming by gram-positive bacterial components and examines the alterations in cell surface receptor expression, changes in intracellular signaling, gene expression and cytokine production, and the phenomenon of cross-tolerance.

Interactions of sequence variants in interleukin-1 receptor-associated kinase4 and the toll-like receptor 6-1-10 gene cluster increase prostate cancer risk

Chronic or recurrent inflammation has been suggested as a causal factor in several human malignancies, including prostate cancer. Genetic predisposition is also a strong risk factor in the development of prostate cancer. In particular, Toll-like receptors (TLR), especially the TLR6-1-10 gene cluster, are involved in prostate cancer development. Interleukin-1 receptor-associated kinases (IRAK) 1 and 4 are critical components in the TLR signaling pathway. In this large case-control study, we tested two hypotheses: (a) sequence variants in IRAK1 and IRAK4 are associated with prostate cancer risk and (b) sequence variants in IRAK1/4 and TLR1-6-10 interacts and confers a stronger risk to prostate cancer. We analyzed 11 single nucleotide polymorphisms (four in IRAK1 and seven in IRAK4) among 1,383 newly diagnosed prostate cancer patients and 780 population controls in Sweden. Although the single-nucleotide polymorphisms in IRAK1 and IRAK4 alone were not significantly associated with prostate cancer risk, one single-nucleotide polymorphism in IRAK4, when combined with the high-risk genotype at TLR6-1-10, conferred a significant excess risk of prostate cancer. In particular, men with the risk genotype at TLR6-1-10 and IRAK4-7987 CG/CC had an odds ratio of 9.68 (P = 0.03) when compared with men who had wild-type genotypes. Our findings suggest synergistic effects between sequence variants in IRAK4 and the TLR 6-1-10 gene cluster. Although this study was based on a priori hypothesis and was designed to address many common issues facing this type of study, our results need confirmation in even larger studies.

Oxidative stress generated by hemorrhagic shock recruits Toll-like receptor 4 to the plasma membrane in macrophages

Oxidative stress generated by ischemia/reperfusion is known to prime inflammatory cells for increased responsiveness to subsequent stimuli, such as lipopolysaccharide (LPS). The mechanism(s) underlying this effect remains poorly elucidated. These studies show that alveolar macrophages recovered from rodents subjected to hemorrhagic shock/resuscitation expressed increased surface levels of Toll-like receptor 4 (TLR4), an effect inhibited by adding the antioxidant N-acetylcysteine to the resuscitation fluid. Consistent with a role for oxidative stress in this effect, in vitro H₂O₂ treatment of RAW 264.7 macrophages similarly caused an increase in surface TLR4. The H₂O₂-induced increase in surface TLR4 was prevented by depleting intracellular calcium or disrupting the cytoskeleton, suggesting the involvement of receptor exocytosis. Further, fluorescent resonance energy transfer between TLR4 and the raft marker GM1 as well as biochemical analysis of the raft components demonstrated that oxidative stress redistributes TLR4 to lipid rafts in the plasma membrane. Preventing the oxidant-induced movement of TLR4 to lipid rafts using methyl-β-cyclodextrin precluded the increased responsiveness of cells to LPS after H₂O₂ treatment. Collectively, these studies suggest a novel mechanism whereby oxidative stress might prime the responsiveness of cells of the innate immune system.

Systemic inflammatory response syndrome (SIRS): molecular pathophysiology and gene therapy

In recent years, extensive basic science research has led to a clear understanding of the molecular mechanisms contributing to the pathophysiology of sepsis. Sepsis is now defined as a systemic inflammatory response syndrome (SIRS) in which there is an identifiable focus of infection. SIRS can be also precipitated by non-infective events such as trauma, pancreatitis, and surgery. As a consequence of an overactive SIRS response, the function of various organ systems may be compromised, resulting in multiple organ dysfunction syndrome (MODS) and death. Production and activation of multiple proinflammatory genes are likely to play a key role in the pathogenesis of MODS development. This review article focuses on the molecular mechanisms and components involved in the pathogenesis of severe sepsis. This includes cellular targets of sepsis-inducing bacterial products and their signaling pathways with a major emphasis on transcription factors and new therapeutic approaches to severe sepsis.

Toll-like receptors: applications to dermatologic disease

Toll-like receptors are a recently identified group of receptors that are an important component of the immune system. Thus far, ten different receptors have been identified and have unique tissue distribution, ligand binding properties, cellular signaling pathways, and cytokine production profiles. Importantly, ligand binding has been shown to regulate both the adaptive and host immune response; thus, defects in this pathway have the potential to lead to increased susceptibility to infection and inflammatory dysregulation. In this article, the burgeoning literature pertinent to the discovery and signaling mechanisms are reviewed in addition to the discussion of the important role Toll-like receptors may play in the pathogenesis of numerous skin diseases.

LEARNING OBJECTIVE

At the completion of this learning activity, participants should be familiar with the role of Toll-like receptors in host defenses and their relevance to dermatologic diseases.

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.

A genome-wide Drosophila RNAi screen identifies DYRK-family kinases as regulators of NFAT

Precise regulation of the NFAT (nuclear factor of activated T cells) family of transcription factors (NFAT1-4) is essential for vertebrate development and function. In resting cells, NFAT proteins are heavily phosphorylated and reside in the cytoplasm; in cells exposed to stimuli that raise intracellular free Ca2+ levels, they are dephosphorylated by the calmodulin-dependent phosphatase calcineurin and translocate to the nucleus. NFAT dephosphorylation by calcineurin is countered by distinct NFAT kinases, among them casein kinase 1 (CK1) and glycogen synthase kinase 3 (GSK3). Here we have used a genome-wide RNA interference (RNAi) screen in Drosophila to identify additional regulators of the signalling pathway leading from Ca2+-calcineurin to NFAT. This screen was successful because the pathways regulating NFAT subcellular localization (Ca2+ influx, Ca2+-calmodulin-calcineurin signalling and NFAT kinases) are conserved across species, even though Ca2+-regulated NFAT proteins are not themselves represented in invertebrates. Using the screen, we have identified DYRKs (dual-specificity tyrosine-phosphorylation regulated kinases) as novel regulators of NFAT. DYRK1A and DYRK2 counter calcineurin-mediated dephosphorylation of NFAT1 by directly phosphorylating the conserved serine-proline repeat 3 (SP-3) motif of the NFAT regulatory domain, thus priming further phosphorylation of the SP-2 and serine-rich region 1 (SRR-1) motifs by GSK3 and CK1, respectively. Thus, genetic screening in Drosophila can be successfully applied to cross evolutionary boundaries and identify new regulators of a transcription factor that is expressed only in vertebrates.

Pathophysiology of interleukin-1 receptor-associated kinase-M: implications in refractory state

PURPOSE OF REVIEW

The pseudo-kinase interleukin-1 receptor-associated kinase-M has emerged as a critical molecule in the down-regulation of inflammatory responses. Dysregulation of the toll-like receptor-interleukin-1 receptor-associated kinase system, and in particular interleukin-1 receptor-associated kinase-M up-regulation, are associated with a number of pathologies. This review highlights recent findings on interleukin-1 receptor-associated kinase-M reported in biomedical literature.

RECENT FINDINGS

Interleukin-1 receptor-associated kinase-M plays a critical role in generating a refractory state of the immune system following monocytes/macrophages encounter with bacteria or tumor cells. This state has been demonstrated so far in patients who suffer from sepsis, leukemia, and acute coronary syndrome, and seems to be associated with interleukin-1 receptor-associated kinase-M overexpression in their circulating monocytes. In addition, the pseudo-kinase represents a central regulator of osteoclast differentiation and activation, and might thus be related to the onset of osteoporosis.

SUMMARY

Interleukin-1 receptor-associated kinase-M is involved in the control of endotoxin tolerance in monocytes, in osteoporosis, as well as in the deactivation of tumor-infiltrating macrophages. Additionally, patients who suffer from several pathologies related to inflammatory responses express high levels of this molecule in their circulating monocytes. Human monocytes treated with a nitric oxide donor also express large amounts of interleukin-1 receptor-associated kinase-M, apparently under the control of tumor necrosis factor-alpha. This mechanism could explain the induction of interleukin-1 receptor-associated kinase-M in monocytes from patients who suffer from an inflammatory pathology.

TLR8-mediated NF-kappaB and JNK activation are TAK1-independent and MEKK3-dependent

TLR8-mediated NF-kappaB and IRF7 activation are abolished in human IRAK-deficient 293 cells and IRAK4-deficient fibroblast cells. Both wild-type and kinase-inactive mutants of IRAK and IRAK4, respectively, restored TLR8-mediated NF-kappaB and IRF7 activation in the IRAK- and IRAK4-deficient cells, indicating that the kinase activity of IRAK and IRAK4 is probably redundant for TLR8-mediated signaling. We recently found that TLR8 mediates a unique NF-kappaB activation pathway in human 293 cells and mouse embryonic fibroblasts, accompanied only by IkappaBalpha phosphorylation and not IkappaBalpha degradation, whereas interleukin (IL)-1 stimulation causes both IkappaBalpha phosphorylation and degradation. The intermediate signaling events mediated by IL-1 (including IRAK modifications and degradation and TAK1 activation) were not detected in cells stimulated by TLR8 ligands. TLR8 ligands trigger similar levels of IkappaBalpha phosphorylation and NF-kappaB and JNK activation in TAK1(-/-) mouse embryo fibroblasts (MEFs) as compared with wild-type MEFs, whereas lack of TAK1 results in reduced IL-1-mediated NF-kappaB activation and abolished IL-1-induced JNK activation. The above results indicate that although TLR8-mediated NF-kappaB and JNK activation are IRAK-dependent, they do not require IRAK modification and are TAK1-independent. On the other hand, TLR8-mediated IkappaBalpha phosphorylation, NF-kappaB, and JNK activation are completely abolished in MEKK3(-/-) MEFs, whereas IL-1-mediated signaling was only moderately reduced in these deficient MEFs as compared with wild-type cells. The differences between IL-1R- and TLR8-mediated NF-kappaB activation are also reflected at the level of IkappaB kinase (IKK) complex. TLR8 ligands induced IKKgamma phosphorylation, whereas IKKalpha/beta phosphorylation and IKKgamma ubiquitination that can be induced by IL-1 were not detected in cells treated with TLR8 ligands. We postulate that TLR8-mediated MEKK3-dependent IKKgamma phosphorylation might play an important role in the activation of IKK complex, leading to IkappaBalpha phosphorylation.

CpG DNA-mediated Induction of Acute Liver Injury in d-Galactosamine-sensitized Mice

Unmethylated CpG motifs present in bacterial DNA (CpG DNA) induce innate inflammatory responses, including rapid induction of proinflammatory cytokines. Although innate inflammatory responses induced by CpG DNA and other pathogen-associated molecular patterns are essential for the eradication of infectious microorganisms, excessive activation of innate immunity is detrimental to the host. In this study, we demonstrate that CpG DNA, but not control non-CpG DNA, induces a fulminant liver failure with subsequent shock-mediated death by promoting massive apoptotic death of hepatocytes in D-galactosamine (D-GalN)-sensitized mice. Inhibition of mitochondrial membrane permeability transition pore opening or caspase 9 activity in vivo protects D-GalN-sensitized mice from the CpG DNA-mediated liver injury and death. CpG DNA enhanced production of proinflammatory cytokines in D-GalN-sensitized mice via a TLR9/MyD88-dependent pathway. In addition, CpG DNA failed to induce massive hepatocyte apoptosis and subsequent fulminant liver failure and death in D-GalN-sensitized mice that lack TLR9, MyD88, tumor necrosis factor (TNF)-alpha, or TNF receptor I but not interleukin-6 or -12p40. Taken together, our results provide direct evidence that CpG DNA induces a severe acute liver injury and shock-mediated death through the mitochondrial apoptotic pathway-dependent death of hepatocytes caused by an enhanced production of TNF-alpha through a TLR9/MyD88 signaling pathway in D-GalN-sensitized mice.

MyD88 and TNF receptor-associated factor 6 are critical signal transducers in Helicobacter pylori-infected human epithelial cells

Helicobacter pylori induces NF-kappaB activation, leading to mucosal inflammation via cag pathogenicity island. Although recent studies have implicated several candidate proteins of both H. pylori and host, the molecular mechanism by which H. pylori activates NF-kappaB remains unclear. The aim of this study was to analyze the mechanism of cag pathogenicity island-mediated NF-kappaB activation in epithelial cells. The responses of human cell lines and mouse embryonic fibroblasts to infection with wild-type H. pylori or cagE mutant were investigated. The effect of small interfering RNAs (siRNAs) for several NF-kappaB signaling intermediate molecules was evaluated in H. pylori-induced IkappaBalpha phosphorylation and IL-8 production. Protein interactions of exogenously expressed TNFR-associated factor 6 (TRAF6) and MyD88 or receptor-interacting protein 2 and nucleotide-binding oligomerization domain 1 or those of endogenous IkappaB kinase, TGF-beta-activated kinase 1 (TAK1), and TRAF6 were assessed by immunoprecipitation. Cag pathogenicity island-dependent NF-kappaB activation was observed in human cell lines, but not in mouse fibroblasts. In human epithelial cells, H. pylori-induced IkappaBalpha phosphorylation and IL-8 production were severely inhibited by siRNAs directed against TAK1, TRAF6, and MyD88. In contrast, siRNAs for TRAF2, IL-1R-associated kinases 1 and 4, and cell surface receptor proteins did not affect these responses. H. pylori infection greatly enhanced MyD88 and TRAF6 complex formation in a cag-dependent manner, but did not enhance Nod1 and receptor-interacting protein 2 complex formation. H. pylori also induced TAK1 and TRAF6 complexes. These results suggest that the cag pathogenicity island of H. pylori is a cell type-specific NF-kappaB activator. TAK1, TRAF6, and MyD88 are important signal transducers in H. pylori-infected human epithelial cells.

A critical role for the innate immune signaling molecule IRAK-4 in T cell activation

IRAK-4 is a protein kinase that is pivotal in mediating signals for innate immune responses. Here, we report that IRAK-4 signaling is also essential for eliciting adaptive immune responses. Thus, in the absence of IRAK-4, in vivo T cell responses were significantly impaired. Upon T cell receptor stimulation, IRAK-4 is recruited to T cell lipid rafts, where it induces downstream signals, including protein kinase C activation through the association with Zap70. This signaling pathway was found to be required for optimal activation of nuclear factor kappaB. Our findings suggest that T cells use this critical regulator of innate immunity for the development of acquired immunity, suggesting that IRAK-4 may be involved in direct signal cross talk between the two systems.