Complete dependence on IRAK4 kinase activity in TLR2, but not TLR4, signaling pathways underlies decreased cytokine production and increased susceptibility to Streptococcus pneumoniae infection in IRAK4 kinase-inactive mice

The IRAK1/IRF5 axis initiates IL-12 response by dendritic cells and control of Toxoplasma gondii infection

Activation of endosomal Toll-like receptor (TLR) 7, TLR9, and TLR11/12 is a key event in the resistance against the parasite Toxoplasma gondii. Endosomal TLR engagement leads to expression of interleukin (IL)-12 via the myddosome, a protein complex containing MyD88 and IL-1 receptor-associated kinase (IRAK) 4 in addition to IRAK1 or IRAK2. In murine macrophages, IRAK2 is essential for IL-12 production via endosomal TLRs but, surprisingly, Irak2-/- mice are only slightly susceptible to T. gondii infection, similar to Irak1-/- mice. Here, we report that upon T. gondii infection IL-12 production by different cell populations requires either IRAK1 or IRAK2, with conventional dendritic cells (DCs) requiring IRAK1 and monocyte-derived DCs (MO-DCs) requiring IRAK2. In both populations, we identify interferon regulatory factor 5 as the main transcription factor driving the myddosome-dependent IL-12 production during T. gondii infection. Consistent with a redundant role of DCs and MO-DCs, mutations that affect IL-12 production in both cell populations show high susceptibility to infection in vivo.

Understanding the immune profile of sudden infant death syndrome - proteomic perspectives

AIM

The aim of this study was to investigate a panel of immune proteins in cases of sudden infant death syndrome (SIDS). It was hypothesised that, in at least a subset of SIDS, a dysregulated immune response may be a contributing factor leading to death.

METHODS

The subjects included 46 SIDS cases and 41 controls autopsied at the Department of Forensic Sciences, Norway. The causes of death in the controls were accidents/trauma. Samples of cerebrospinal fluid (CSF) were analysed quantitatively by Proximity Extension Assay (PEA).

RESULTS

Initial results revealed that normalised protein expression differed in 35 proteins. For the purposes of this report five proteins that are involved in immune system were selected for analysis: IFNLR1 (p = 0.003), IL10 (p = 0.007), IRAK4 (p < 0.001) and IL6 (p = 0.035); all had lower protein concentrations in SIDS cases compared to controls except for CD28 (p = 0.024) which had higher protein concentrations in SIDS cases.

CONCLUSION

The results confirm previous studies indicating that a dysregulation of the immune system may be a predisposing factor for SIDS. The results may indicate that these aberrant protein concentrations could lead to an inadequate response to immune triggers and uncontrolled defence mechanisms towards the common cold or other non-fatal infections.

The IRAK-M death domain: a tale of three surfaces

The anti-inflammatory interleukin-1 receptor associated kinase-M (IRAK-M) is a negative regulator of MyD88/IRAK-4/IRAK-1 signaling. However, IRAK-M has also been reported to activate NF-κB through the MyD88/IRAK-4/IRAK-M myddosome in a MEKK-3 dependent manner. Here we provide support that IRAK-M uses three surfaces of its Death Domain (DD) to activate NF-κB downstream of MyD88/IRAK-4/IRAK-M. Surface 1, with central residue Trp74, binds to MyD88/IRAK-4. Surface 2, with central Lys60, associates with other IRAK-M DDs to form an IRAK-M homotetramer under the MyD88/IRAK-4 scaffold. Surface 3; with central residue Arg97 is located on the opposite side of Trp74 in the IRAK-M DD tetramer, lacks any interaction points with the MyD88/IRAK-4 complex. Although the IRAK-M DD residue Arg97 is not directly involved in the association with MyD88/IRAK-4, Arg97 was responsible for 50% of the NF-κB activation though the MyD88/IRAK-4/IRAK-M myddosome. Arg97 was also found to be pivotal for IRAK-M's interaction with IRAK-1, and important for IRAK-M's interaction with TRAF6. Residue Arg97 was responsible for 50% of the NF-κB generated by MyD88/IRAK-4/IRAK-M myddosome in IRAK-1/MEKK3 double knockout cells. By structural modeling we found that the IRAK-M tetramer surface around Arg97 has excellent properties that allow formation of an IRAK-M homo-octamer. This model explains why mutation of Arg97 results in an IRAK-M molecule with increased inhibitory properties: it still binds to myddosome, competing with myddosome IRAK-1 binding, while resulting in less NF-κB formation. The findings further identify the structure-function properties of IRAK-M, which is a potential therapeutic target in inflammatory disease.

Porphyromonas gingivalis induction of TLR2 association with Vinculin enables PI3K activation and immune evasion

Porphyromonas gingivalis is a Gram-negative anaerobic bacterium that thrives in the inflamed environment of the gingival crevice, and is strongly associated with periodontal disease. The host response to P. gingivalis requires TLR2, however P. gingivalis benefits from TLR2-driven signaling via activation of PI3K. We studied TLR2 protein-protein interactions induced in response to P. gingivalis, and identified an interaction between TLR2 and the cytoskeletal protein vinculin (VCL), confirmed using a split-ubiquitin system. Computational modeling predicted critical TLR2 residues governing the physical association with VCL, and mutagenesis of interface residues W684 and F719, abrogated the TLR2-VCL interaction. In macrophages, VCL knock-down led to increased cytokine production, and enhanced PI3K signaling in response to P. gingivalis infection, effects that correlated with increased intracellular bacterial survival. Mechanistically, VCL suppressed TLR2 activation of PI3K by associating with its substrate PIP2. P. gingivalis induction of TLR2-VCL led to PIP2 release from VCL, enabling PI3K activation via TLR2. These results highlight the complexity of TLR signaling, and the importance of discovering protein-protein interactions that contribute to the outcome of infection.

Regulation of innate immune signaling by IRAK proteins

The Toll-like receptors (TLRs) and interleukin-1 receptors (IL-1R) families are of paramount importance in coordinating the early immune response to pathogens. Signaling via most TLRs and IL-1Rs is mediated by the protein myeloid differentiation primary-response protein 88 (MyD88). This signaling adaptor forms the scaffold of the myddosome, a molecular platform that employs IL-1R-associated kinase (IRAK) proteins as main players for transducing signals. These kinases are essential in controlling gene transcription by regulating myddosome assembly, stability, activity and disassembly. Additionally, IRAKs play key roles in other biologically relevant responses such as inflammasome formation and immunometabolism. Here, we summarize some of the key aspects of IRAK biology in innate immunity.

Post-Transcriptional Control of mRNA Metabolism and Protein Secretion: The Third Level of Regulation within the NF-κB System

The NF-κB system is a key transcriptional pathway that regulates innate and adaptive immunity because it triggers the activation and differentiation processes of lymphocytes and myeloid cells during immune responses. In most instances, binding to cytoplasmic inhibitory IκB proteins sequesters NF-κB into an inactive state, while a plethora of external triggers activate three complex signaling cascades that mediate the release and nuclear translocation of the NF-κB DNA-binding subunits. In addition to these cytosolic steps (level 1 of NF-κB regulation), NF-κB activity is also controlled in the nucleus by signaling events, cofactors and the chromatin environment to precisely determine chromatin recruitment and the specificity and timing of target gene transcription (level 2 of NF-κB regulation). Here, we discuss an additional layer of the NF-κB system that manifests in various steps of post-transcriptional gene expression and protein secretion. This less-studied regulatory level allows reduction of (transcriptional) noise and signal integration and endows time-shifted control of the secretion of inflammatory mediators. Detailed knowledge of these steps is important, as dysregulated post-transcriptional NF-κB signaling circuits are likely to foster chronic inflammation and contribute to the formation and maintenance of a tumor-promoting microenvironment.

The IRAK4 scaffold integrates TLR4-driven TRIF and MYD88 signaling pathways

Interleukin-1 receptor-associated kinases (IRAKs) -4, -2, and -1 are involved in transducing signals from Toll-like receptors (TLRs) via the adaptor myeloid differentiation primary-response protein 88 (MYD88). How MYD88/IRAK4/2/1 complexes are formed, their redundancies, and potential non-enzymatic roles are subjects of debate. Here, we examine the hierarchical requirements for IRAK proteins in the context of TLR4 activation and confirmed that the kinase activity of IRAK4 is essential for MYD88 signaling. Surprisingly, the IRAK4 scaffold is required for activation of the E3 ubiquitin ligase TNF receptor-associated factor 6 (TRAF6) by both MYD88 and TIR domain-containing adaptor protein inducing IFN-β (TRIF), a unique adaptation in the TLR4 response. IRAK4 scaffold is, therefore, essential in integrating MYD88 and TRIF in TLR4 signaling.

Efficacy of Sishen Wan on dinitrobenzene sulfonic acid-induced ulcerative colitis and its effect on toll-like receptor 2/interleukin-1 receptor-associated kinase-4/nuclear factor-κB signal pathway

OBJECTIVE

To investigate the therapeutic effect of Sishen Wan (, SSW) on ulcerative colitis (UC) induced by dinitrobenzene sulfonic acid and its effect on toll-like receptor 2/interleukin-1 receptor-associated kinase-4/nuclear factor-κB (TLR2/IRAK4/NF-κB) sig-naling pathway in colonic tissue.

METHODS

In this study, 120 Sprague-Dawley rats were randomly divided into blank and model groups. The experimental UC model in rats was established by subcutaneous injection of hydrocortisone + senna gavage for 21 d + dinitrobenzene sulfonic acid (DNBS)/ ethanol solution enema. The successful model rats were randomly divided into the model group; mesalazine (0.36 g/kg) group; and high-, medium-, and low- dose SSW (24, 12, and 6 g/kg) groups. The model and blank groups were gavaged with equal volumes of distilled water once a day for 21 d. The general condition of the rats was observed, and the body mass, fecal properties, and occult blood were recorded for calculating the disease activity index (DAI) score. The colonic tissue of the rats was collected, and its general morphology and pathological form were noted for obtaining the colonic mucosal injury index (CMDI) score. Hematoxylin-eosin staining was used to view the pathological changes of the colon tissue in each group, apoptosis of the cells was detected using terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling staining, and quantitative real-time polymerase chain reaction was used to measure the expressions of TLR2, myeloid differentiation primary response gene 88 (MyD88), IRAK4, and NF-κB p65 mRNA in the colon tissue. The expressions of TLR2, MyD88, IRAK4, and NF-κB p65 protein were detected using western blotting and immunohistochemistry assay, and the levels of interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α) in the colon tissue were determined using enzyme linked immunosorbent assay.

RESULTS

Compared with the blank group, the general condition of the model group was relatively poor. The DAI and CMDI scores of the model group increased significantly (< 0.01), the glands and intestinal mucosa disappeared partially, and several inflammatory cells infiltrated and gathered in the mucosal layer and base layer of the rats in the model group. Furthermore, the cell apoptosis and expression levels of TLR2, MyD88, IRAK4, and NF-κB p65 mRNA and protein in the colon tissue of rats in the model group increased significantly (< 0.01). The levels of IL-1β and TNF-α increased significantly in the colon tissue of rats in the model group (< 0.01). After treatment with SSW, compared with the model group, the general condition of the UC rats improved. Moreover, the DAI and CMDI scores of the UC rats decreased significantly (< 0.05), and the pathological changes in the colon tissue of the UC rats tended to be normal. The cell apoptosis and expression levels of TLR2, MyD88, IRAK4, and NF-κB p65 mRNA and protein in the colon tissue of the UC rats decreased gradually ( < 0.01), and the levels of IL-1β and TNF-α decreased significantly (< 0.01).

CONCLUSION

SSW can improve the general condition and alleviate the intestinal mucosal injury of UC model rats. Additionally, SSW can inhibit the TLR2/IRAK4/ NF-κB signaling pathway, but further studies are required to confirm it.

Reviewing the importance of TLR-NLRP3-pyroptosis pathway and mechanism of experimental NLRP3 inflammasome inhibitors

Cells encounter continuous challenges due to tissue insult caused by endogenous and/or exogenous stimuli. Among the mechanisms set in place to counterbalance the tissue insult, innate immunity is always at the forefront. Cells of innate immunity efficiently recognize the 'danger signals' via a specialized set of membrane-bound receptors known as Toll-like receptors. Once this interaction is established, toll-like receptor passes on the responsibility to cytosolic NOD-like receptors through a cascade of signalling pathways. Subsequently, NOD-like receptors assemble to a specialized multiprotein intracellular complex, that is inflammasome. Inflammasome activates Caspase-1 and Gasdermin-D which initiate pyroptotic cell death in the affected tissue by two simultaneous mechanisms. Being a protease, caspase-1 cleaves and activates pro-inflammatory cytokines IL-1β and IL-18. On the other hand, Gasdermin-D causes proteolytic cleavage which forms a pore in the cell membrane. This review highlights the molecular events ranging from recognition of stimuli to pyroptosis. The review is also an attempt to discuss the mechanisms of the most specific experimental NLRP3 inhibitors.

Mechanism of the fungal-like particles in the inhibition of adipogenesis in 3T3-L1 adipocytes

The dynamic ability of adipocytes in adipose tissue to store lipid in response to changes in the nutritional input and inflammatory elicitors has a major impact on human health. Previously, we established laminarin-coated beads or LCB as an inflammatory elicitor for adipocytes. However, it was not clear whether LCB inhibits lipid accumulation in adipocytes. Here, we show that LCB acts in the early stage of adipogenesis through both interleukin-1 receptor-associated kinases (IRAK) and spleen tyrosine kinase (SYK) pathways, resulting in the activation of the AMP-activated protein kinase (AMPK) and nuclear factor-κB (NF-κB) complexes, which subsequently cause cell cycle arrest, downregulation of the key transcription factors and enzymes responsible for adipogenesis, inhibition of adipogenesis, and stimulation of an inflammatory response. While LCB could effectively block lipid accumulation during the early stage of adipogenesis, it could stimulate an inflammatory response at any stage of differentiation. Additionally, our results raise a possibility that toll-like receptor 2 (TLR2) and C-type lectin domain family 7 member A (CLEC7A/Dectin-1) might be potential β-glucan receptors on the fat cells. Together, we present the mechanism of LCB, as fungal-like particles, that elicits an inflammatory response and inhibits adipogenesis at the early stage of differentiation.

Classically activated mouse macrophages produce methylglyoxal that induces a TLR4- and RAGE-independent proinflammatory response

The highly reactive compound methylglyoxal (MG) can cause direct damage to cells and tissues by reacting with cellular macromolecules. MG has been identified as a biomarker associated with increased sepsis-induced mortality. Patients undergoing septic shock have significantly elevated circulating MG levels compared to postoperative patients and healthy controls. Furthermore, MG has been implicated in the development of type II diabetes mellitus and Alzheimer's disease. Because MG is generated during glycolysis, we hypothesized that MG may be produced by classically activated (M1) macrophages, possibly contributing to the inflammatory response. LPS and IFN-γ-treated macrophages acquired an M1 phenotype (as evidenced by M1 markers and enhanced glycolysis) and formed MG adducts, MG-H1, MG-H2, and MG-H3, which were detected using antibodies specific for MG-modified proteins (methylglyoxal 5-hydro-5-methylimidazolones). MG adducts were also increased in the lungs of LPS-treated mice. Macrophages treated with LPS and IFN-γ also exhibited decreased expression of glyoxalase 1 (Glo1), an enzyme that metabolizes MG. Concentrations of exogenous, purified MG > 0.5 mM were toxic to macrophages; however, a nontoxic dose of 0.3 mM induced TNF-α and IL-1β, albeit to a lesser extent than LPS stimulation. Despite prior evidence that MG adducts may signal through "receptor for advanced glycation endproducts" (RAGE), MG-mediated cell death and cytokine induction by exogenous MG was RAGE-independent in primary macrophages. Finally, RAGE-deficient mice did not exhibit a significant survival advantage following lethal LPS injection. Overall, our evidence suggests that MG may be produced by M1 macrophages during sepsis, following IFN-γ-dependent down-regulation of Glo1, contributing to over-exuberant inflammation.

Phosphorylation of Microglial IRF5 and IRF4 by IRAK4 Regulates Inflammatory Responses to Ischemia

Background: Interferon Regulatory Factor (IRF) 5 and 4 play a determinant role in regulating microglial pro- and anti-inflammatory responses to cerebral ischemia. How microglial IRF5 and IRF4 signaling are activated has been elusive. We hypothesized that interleukin-1 receptor associated kinase 4 (IRAK4) phosphorylates and activates IRF5 and IRF4 in ischemic microglia. We aimed to explore the upstream signals of the two IRFs, and to determine how the IRAK4-IRF signaling regulates the expression of inflammatory mediators, and impacts neuropathology. Methods: Spontaneously Immortalized Murine (SIM)-A9 microglial cell line, primary microglia and neurons from C57BL/6 WT mice were cultured and exposed to oxygen-glucose deprivation (OGD), followed by stimulation with LPS or IL-4. An IRAK4 inhibitor (ND2158) was used to examine IRAK4′s effects on the phosphorylation of IRF5/IRF4 and the impacts on neuronal morphology by co-immunoprecipitation (Co-IP)/Western blot, ELISA, and immunofluorescence assays. Results: We confirmed that IRAK4 formed a Myddosome with MyD88/IRF5/IRF4, and phosphorylated both IRFs, which subsequently translocated into the nucleus. Inhibition of IRAK4 phosphorylation quenched microglial pro-inflammatory response primarily, and increased neuronal viability and neurite lengths after ischemia. Conclusions: IRAK4 signaling is critical for microglial inflammatory responses and a potential therapeutic target for neuroinflammatory diseases including cerebral ischemia.

Oleanolic Acid Acetate Alleviates Symptoms of Experimental Autoimmune Encephalomyelitis in Mice by Regulating Toll-Like Receptor 2 Signaling

Toll-like receptor 2 (TLR2) is expressed by several immune cells in the central nervous system and plays an important role in neuroinflammation. TLR2 upregulation has been reported in multiple sclerosis patients and in experimental autoimmune encephalomyelitis (EAE), a mouse model for multiple sclerosis. Therefore, modulating TLR2 signaling can be an effective treatment strategy against MS. Oleanolic acid acetate (OAA) has antiinflammatory and immunomodulatory effects. Hence, this study aimed to examine the effects of OAA on TLR2 signaling and neuroinflammation in EAE. EAE was induced in C57/BL6 mice using synthesized myelin oligodendrocyte glycoprotein (MOG)_35-55 peptide, and OAA was administered daily. Hind limb paralysis and inflammatory cell infiltration were observed in the spinal cords of EAE mice. Moreover, T-cell proliferation was significantly stimulated in splenic cells from EAE mice. The expression of proinflammatory cytokines in the spinal cord was upregulated, and their serum protein levels were increased in EAE mice. Furthermore, upregulation of TLR2 and downstream signaling molecules was observed in the spinal cord. These pathological changes were reversed by OAA treatment. Our results suggest that OAA might have promising therapeutic properties and that the TLR signaling pathway is an effective therapeutic target against multiple sclerosis.

Nucleic Acid-Sensing Toll-Like Receptors Play a Dominant Role in Innate Immune Recognition of Pneumococci

Streptococcus pneumoniae (or pneumococcus) is a highly prevalent human pathogen. Toll-like receptors (TLRs) function as immune sensors that can trigger host defenses against this bacterium. Defects in TLR-activated signaling pathways, including deficiency in the adaptor protein myeloid differentiation factor 88 (MyD88), are associated with markedly increased susceptibility to infection. However, the individual MyD88-dependent TLRs predominantly involved in antipneumococcal defenses have not been identified yet. Here we find that triple knockout mice simultaneously lacking TLR7, TLR9, and TLR13, which sense the presence of bacterial DNA (TLR9) and RNA (TLR7 and TLR13) in the phagolysosomes of phagocytic cells, display a phenotype that largely resembles that of MyD88-deficient mice and rapidly succumb to pneumococcal pneumonitis due to defective neutrophil influx into the lung. Accordingly, TLR7/9/13 triple knockout resident alveolar macrophages were largely unable to respond to pneumococci with the production of neutrophil-attracting chemokines and cytokines. Mice with single deficiencies of TLR7, TLR9, or TLR13 showed unaltered ability to control lung infection but were moderately more susceptible to encephalitis, in association with a decreased ability of microglia to mount cytokine responses in vitro Our data point to a dominant, tissue-specific role of nucleic acid-sensing pathways in innate immune recognition of S. pneumoniae and also show that endosomal TLRs are largely capable of compensating for the absence of each other, which seems crucial to prevent pneumococci from escaping immune recognition. These results may be useful to develop novel strategies to treat infections by antibiotic-resistant pneumococci based on stimulation of the innate immune system.IMPORTANCE The pneumococcus is a bacterium that frequently causes infections in the lungs, ears, sinus cavities, and meninges. During these infections, body defenses are triggered by tissue-resident cells that use specialized receptors, such as Toll-like receptors (TLRs), to sense the presence of bacteria. We show here that pneumococci are predominantly detected by TLRs that are located inside intracellular vacuoles, including endosomes, where these receptors can sense the presence of nucleic acids released from ingested bacteria. Mice that simultaneously lacked three of these receptors (specifically, TLR7, TLR9, and TLR13) were extremely susceptible to lung infection and rapidly died after inhalation of pneumococci. Moreover, tissue-resident macrophages from these mice were impaired in their ability to respond to the presence of pneumococci by producing inflammatory mediators capable of recruiting polymorphonuclear leucocytes to infection sites. This information may be useful to develop drugs to treat pneumococcal infections, particularly those caused by antibiotic-resistant strains.

Relative Contributions of Extracellular and Internalized Bacteria to Early Macrophage Proinflammatory Responses to Streptococcus pneumoniae

Both intracellular immune sensing and extracellular innate immune sensing have been implicated in initiating macrophage proinflammatory cytokine responses to Streptococcus pneumoniae The S. pneumoniae capsule, a major virulence determinant, prevents phagocytosis, and we hypothesized that this would reduce activation of host innate inflammatory responses by preventing activation of intracellular proinflammatory signaling pathways. We investigated this hypothesis in human monocyte-derived macrophages stimulated with encapsulated or isogenic unencapsulated mutant S. pneumoniae Unexpectedly, despite strongly inhibiting bacterial internalization, the capsule resulted in enhanced inflammatory cytokine production by macrophages infected with S. pneumoniae Experiments using purified capsule material and a Streptococcus mitis mutant expressing an S. pneumoniae serotype 4 capsule indicated these differences required whole bacteria and were not due to proinflammatory effects of the capsule itself. Transcriptional profiling demonstrated relatively few differences in macrophage gene expression profiles between infections with encapsulated S. pneumoniae and those with unencapsulated S. pneumoniae, largely limited to reduced expression of proinflammatory genes in response to unencapsulated bacteria, predicted to be due to reduced activation of the NF-κB family of transcription factors. Blocking S. pneumoniae internalization using cytochalasin D had minimal effects on the inflammatory response to S. pneumoniae Experiments using murine macrophages indicated that the affected genes were dependent on Toll-like receptor 2 (TLR2) activation, although not through direct stimulation of TLR2 by capsule polysaccharide. Our data demonstrate that the early macrophage proinflammatory response to S. pneumoniae is mainly dependent on extracellular bacteria and reveal an unexpected proinflammatory effect of encapsulated S. pneumoniae that could contribute to disease pathogenesis.IMPORTANCE Multiple extra- and intracellular innate immune receptors have been identified that recognize Streptococcus pneumoniae, but the relative contributions of intra- versus extracellular bacteria to the inflammatory response were unknown. We have shown that intracellular S. pneumoniae contributes surprisingly little to the inflammatory responses, with production of important proinflammatory cytokines largely dependent on extracellular bacteria. Furthermore, although we expected the S. pneumoniae polysaccharide capsule to block activation of the host immune system by reducing bacterial internalization and therefore activation of intracellular innate immune receptors, there was an increased inflammatory response to encapsulated compared to unencapsulated bacteria, which is likely to contribute to disease pathogenesis.

Influenza "Trains" the Host for Enhanced Susceptibility to Secondary Bacterial Infection

Enhanced susceptibility to 2° bacterial infections following infection with influenza virus is a global health concern that accounts for many hospitalizations and deaths, particularly during pandemics. The complexity of the impaired host immune response during 2° bacterial infection has been widely studied. Both type I IFN and neutrophil dysfunction through decreased chemokine production have been implicated as mechanisms underlying enhanced susceptibility to 2° bacterial infections. Our findings support the conclusion that selective suppression of CXCL1/CXCL2 represents an IFN-β-mediated “training” of the macrophage transcriptional response to TLR2 agonists and that blocking of TLR4 therapeutically with Eritoran after influenza virus infection reverses this suppression by blunting influenza-induced IFN-β.

We previously reported that the Toll-like receptor 4 (TLR4) antagonist Eritoran blocks acute lung injury (ALI) therapeutically in mouse and cotton rat models of influenza. However, secondary (2°) bacterial infection following influenza virus infection is associated with excess morbidity and mortality. Wild-type (WT) mice infected with mouse-adapted influenza A/Puerto Rico/8/34 virus (PR8) and, 7 days later, with Streptococcus pneumoniae serotype 3 (Sp3) exhibited significantly enhanced lung pathology and lethality that was reversed by Eritoran therapy after PR8 infection but before Sp3 infection. Cotton rats infected with nonadapted pH1N1 influenza virus and then superinfected with methicillin-resistant Staphylococcus aureus also exhibited increased lung pathology and serum high-mobility-group box 1 (HMGB1) levels, both of which were blunted by Eritoran therapy. In mice, PR8 infection suppressed Sp3-induced CXCL1 and CXCL2 mRNA, reducing neutrophil infiltration and increasing the bacterial burden, all of which were reversed by Eritoran treatment. While beta interferon (IFN-β)-deficient (IFN-β^−/−) mice are highly susceptible to PR8, they exhibited delayed death upon Sp3 superinfection, indicating that while IFN-β was protective against influenza, it negatively impacted the host response to Sp3. IFN-β-treated WT macrophages selectively suppressed Sp3-induced CXCL1/CXCL2 transcriptionally, as evidenced by reduced recruitment of RNA polymerase II to the CXCL1 promoter. Thus, influenza establishes a “trained” state of immunosuppression toward 2° bacterial infection, in part through the potent induction of IFN-β and its downstream transcriptional regulation of chemokines, an effect reversed by Eritoran.

IRAK4 activity controls immune responses to intracellular bacteria Listeria monocytogenes and Mycobacterium smegmatis

IL-1 receptor-associated kinase (IRAK) 4 is a central enzyme of the TLR pathways. This study tested the hypothesis that IRAK4 kinase activity is prerequisite for regulating innate immunity during infections with intracellular bacteria. To this end, we analyzed responses of macrophages obtained from mice expressing wild-type (WT) IRAK4 or its kinase-inactive K213M mutant (IRAK4^KI ) upon infection with intracellular bacteria Listeria monocytogenes or Mycobacterium smegmatis. In contrast to robust induction of cytokines by macrophages expressing kinase-sufficient IRAK4, IRAK4^KI macrophages expressed decreased TNF-α, IL-6, IL-1β, and C-C motif chemokine ligand 5 upon infection with L. monocytogenes or M. smegmatis. Bacterial infection of IRAK4^KI macrophages led to attenuated activation of IRAK1, MAPKs and NF-κB, impaired induction of inducible NO synthase mRNA and secretion of NO, but resulted in elevated microbial burdens. Compared with WT animals, systemic infection of IRAK4^KI mice with M. smegmatis or L. monocytogenes resulted in decreased levels of serum IL-6 and CXCL-1 but increased bacterial burdens in the spleen and liver. Thus, a loss of IRAK4 kinase activity underlies deficient cytokine and microbicidal responses during infection with intracellular bacteria L. monocytogenes or M. smegmatis via impaired activation of IRAK1, MAPKs, and NF-κB but increases bacterial burdens, correlating with decreased induction of NO.

Impact of Fungi on Immune Responses

Spores and fungal fragments found in indoor and outdoor environments originate from opportunistic fungi and they can contribute to inflammatory responses, causing a broad range of symptoms. Papers were selected and reviewed with an emphasis on the molecular mechanisms involved in the effect of fungi on immune cells, especially mast cells (MCs). Fungi can bind to antibodies and complement them, allowing them to be recognized by cells of the innate immune system, including macrophages, dendritic cells, and MCs, which are then stimulated via Toll-like receptor signaling. Fungi can cause diseases mediated by MCs and aggravate allergic inflammation. Immunosuppressed subjects can be particularly susceptible to developing diseases caused by opportunistic fungi. Mold also liberates mycotoxins that could be on volatile spores and stimulate MCs to secrete pro-inflammatory cytokines/chemokines, but this mechanism is not known. Fungi can activate the immune system directly or through mycotoxins, leading to stimulation of immune cells and chronic neuroinflammatory symptoms. Some of these processes may be inhibited by the new anti-inflammatory cytokine interleukin 37.

TLR2, TLR3 and TLR5 regulation of pro-inflammatory and pro-labour mediators in human primary myometrial cells

Preterm birth continues to be a significant global health care issue, due to our lack of understanding of the mechanisms that drive human labour and delivery. Toll-like receptors (TLRs) are essential in triggering an inflammatory response in human gestational tissues, leading to the production of pro-inflammatory and pro-labour mediators, and thus preterm birth. The aims of this study were to determine whether the adaptor molecules associated with TLR2, TLR3 and TLR5 signalling are involved in human myometrium. Primary human myometrial cells were transfected with siRNA against TIRAP, IRAK1, IRAK4, TAK1and stimulated with bacterial product fsl-1 (TLR2); TRIF, TRADD, TRAF6, RIP1, TAK1 and stimulated with dsRNA viral analogue poly(I:C) (TLR3); IRAK1, IRAK4, TAK1 and stimulated with bacterial product flagellin (TLR5), and assayed for production of pro-inflammatory and pro-labour mediators. Cells transfected with TIRAP, IRAK1, IRAK4 or TAK1 all showed a decrease in fsl-1-induced expression of cytokines (IL-1α, IL-1β, IL-6), chemokines (GRO-α, IL-8, MCP-1), adhesion molecule ICAM-1, cyclooxygenase (COX)-2 mRNA and release of PGF_2α and MMP-9 expression. Cells transfected with TRIF, TRAF6, RIP1 or TAK1 all decreased production of poly(I:C)-induced IL-1α, IL-1β, IL-6, GRO-α, IL-8, MCP-1, ICAM-1 and MMP-9 expression. Cells transfected with IRAK1, IRAK4 or TAK1 all showed decreased expression of flagellin-induced cytokine and chemokine expression, ICAM-1 and MMP-9 expression. Lastly, transfection with these siRNAs decreased fsl-1, poly(I:C) and flagellin-induced NF-κB transcriptional activity. Our study signifies that these adaptor molecules are necessary for the proper production of cytokines, chemokines and pro-labour mediators after TLR ligation.

The θ-defensin retrocyclin 101 inhibits TLR4- and TLR2-dependent signaling and protects mice against influenza infection

A member of the θ‐defensin family protects mice during infection with influenza, suggesting a new strategy for viral therapy in humans.

Despite widespread use of annual influenza vaccines, seasonal influenza‐associated deaths number in the thousands each year, in part because of exacerbating bacterial superinfections. Therefore, discovering additional therapeutic options would be a valuable aid to public health. Recently, TLR4 inhibition has emerged as a possible mechanism for protection against influenza‐associated lethality and acute lung injury. Based on recent data showing that rhesus macaque θ‐defensins could inhibit TLR4‐dependent gene expression, we tested the hypothesis that a novel θ‐defensin, retrocyclin (RC)‐101, could disrupt TLR4‐dependent signaling and protect against viral infection. In this study, RC‐101, a variant of the humanized θ‐defensin RC‐1, blocked TLR4‐mediated gene expression in mouse and human macrophages in response to LPS, targeting both MyD88‐ and TRIF‐dependent pathways. In a cell‐free assay, RC‐101 neutralized the biologic activity of LPS at doses ranging from 0.5 to 50 EU/ml, consistent with data showing that RC‐101 binds biotinylated LPS. The action of RC‐101 was not limited to the TLR4 pathway because RC‐101 treatment of macrophages also inhibited gene expression in response to a TLR2 agonist, Pam3CSK4, but failed to bind that biotinylated agonist. Mouse macrophages infected in vitro with mouse‐adapted A/PR/8/34 influenza A virus (PR8) also produced lower levels of proinflammatory cytokine gene products in a TLR4‐independent fashion when treated with RC‐101. Finally, RC‐101 decreased both the lethality and clinical severity associated with PR8 infection in mice. Cumulatively, our data demonstrate that RC‐101 exhibits therapeutic potential for the mitigation of influenza‐related morbidity and mortality, potentially acting through TLR‐dependent and TLR‐independent mechanisms.

Deficiency in IRAK4 activity attenuates manifestations of murine Lupus

Interleukin-1 receptor-associated kinase (IRAK) 4 mediates host defense against infections. As an active kinase, IRAK4 elicits full spectra of myeloid differentiation primary response protein (MyD) 88-dependent responses, while kinase-inactive IRAK4 induces a subset of cytokines and negative regulators whose expression is not regulated by mRNA stability. IRAK4 kinase activity is critical for resistance against Streptococcus pneumoniae, but its involvement in autoimmunity is incompletely understood. In this study, we determined the role of IRAK4 kinase activity in murine lupus. Lupus development in BXSB mice expressing the Y chromosome autoimmunity accelerator (Yaa) increased basal and Toll-like receptor (TLR) 4/7-induced phosphorylation of mitogen-activated protein kinases, p65 nuclear factor-κB (NF-κB), enhanced tumor necrosis factor (TNF)-α and C-C motif chemokine ligand (CCL) 5 gene expression in splenic macrophages, but decreased levels of Toll-interacting protein and IRAK-M, without affecting IRAK4 or IRAK1 expression. Mice harboring kinase-inactive IRAK4 on the lupus-prone Yaa background manifested blunted TLR signaling in macrophages and reduced glomerulonephritis, splenomegaly, serum anti-nuclear antibodies, numbers of splenic macrophages, total and TNF-α⁺ dendritic cells, activated T- and B-lymphocytes, and lower TNF-α expression in macrophages compared with lupus-prone mice with functional IRAK4. Thus, IRAK4 kinase activity contributes to murine lupus and could represent a new therapeutic target.

Type I interferon licenses enhanced innate recognition and transcriptional responses to Franciscella tularensis live vaccine strain

The innate inflammatory response to Francisella tularensis (Ft) in macrophages is significantly governed by the expression of type I interferon (IFN). Previously, the proteolytic processing and maturation of pro-IL-1β protein was shown to depend upon type I IFN expression. We show in this report that paracrine type I IFN can profoundly enhance innate recognition and TLR-dependent transcriptional responses to Ft infection upstream of its role in inflammasome regulation in both primary human monocyte-derived macrophages and primary murine peritoneal macrophages but not murine bone marrow-derived macrophages. This type I IFN-enhanced response is synergistic with TLR2 transcriptional responses, partially TLR2-independent, but strictly MyD88-dependent.

Transcriptional and Linkage Analyses Identify Loci that Mediate the Differential Macrophage Response to Inflammatory Stimuli and Infection

Macrophages display flexible activation states that range between pro-inflammatory (classical activation) and anti-inflammatory (alternative activation). These macrophage polarization states contribute to a variety of organismal phenotypes such as tissue remodeling and susceptibility to infectious and inflammatory diseases. Several macrophage- or immune-related genes have been shown to modulate infectious and inflammatory disease pathogenesis. However, the potential role that differences in macrophage activation phenotypes play in modulating differences in susceptibility to infectious and inflammatory disease is just emerging. We integrated transcriptional profiling and linkage analyses to determine the genetic basis for the differential murine macrophage response to inflammatory stimuli and to infection with the obligate intracellular parasite Toxoplasma gondii. We show that specific transcriptional programs, defined by distinct genomic loci, modulate macrophage activation phenotypes. In addition, we show that the difference between AJ and C57BL/6J macrophages in controlling Toxoplasma growth after stimulation with interferon gamma and tumor necrosis factor alpha mapped to chromosome 3, proximal to the Guanylate binding protein (Gbp) locus that is known to modulate the murine macrophage response to Toxoplasma. Using an shRNA-knockdown strategy, we show that the transcript levels of an RNA helicase, Ddx1, regulates strain differences in the amount of nitric oxide produced by macrophage after stimulation with interferon gamma and tumor necrosis factor. Our results provide a template for discovering candidate genes that modulate macrophage-mediated complex traits.

Macrophages provide a first line of defense against invading pathogens and play an important role in the initiation and resolution of immune responses. When in contact with pathogens or immune factors, such as cytokines, macrophages assume activation states that range between pro-inflammatory (classical activation) and anti-inflammatory (alternative activation). Even though it is known that macrophages from different individuals are biased towards one of the various activation states, the genetic factors that define individual differences in macrophage activation are not fully understood. Additionally, although macrophages are important in infectious disease pathogenesis, how individual differences in macrophage activation contribute to individual differences in susceptibility to infectious disease is just emerging. We used macrophages from genetically segregating mice to show that discrete transcriptional programs, which are modulated by specific genomic regions, modulate differences in macrophage activation. Murine macrophages differences in controlling Toxoplasma growth mapped to chromosome 3, proximal to the Guanylate binding protein (Gbp) locus that is known to modulate the murine macrophage response to Toxoplasma. Using a shRNA-mediated knockdown approach, we show that the DEAD box polypeptide 1 (Ddx1) modulates nitric oxide production in macrophages stimulated with interferon gamma and tumor necrosis factor. These findings are a step towards the identification of genes that regulate macrophage phenotypes and disease outcome.

Skin fibroblasts from individuals with Chediak-Higashi Syndrome (CHS) exhibit hyposensitive immunogenic response

Background

Chediak-Higashi Syndrome (CHS) is a rare autosomal recessive disease characterized by immunodeficiency, oculocutaneous albinism, neurological dysfunction, and early death. Individuals with CHS present with increased susceptibility to infections of the skin, upper-respiratory tract, gastrointestinal tract, and oral tissues. Classical CHS is caused by mutations in the gene encoding lysosomal trafficking regulator (LYST). Although defects in cytotoxic T cell lytic secretory granule secretion and neutrophil phagocytosis are suggested to contribute to the immunodeficiency in CHS, the underlying molecular mechanisms are unknown. We hypothesized that skin fibroblasts from CHS subjects exhibit impaired immune response due to defective trafficking of inflammatory factors.

Methods and results

Primary skin fibroblasts from CHS subjects or healthy controls were assessed for genes encoding inflammatory response factors using PCR array. At baseline, we found CD14, IL1R1 and TLR-1 were down-regulated significantly (≥2 fold change) and the genes encoding TLR-3, IL-1β and IL-6 were up-regulated in CHS cells compared to control cells. When challenged with E. coli lipopolysaccharide (LPS), CHS cells were less responsive than control cells, with only 8 genes significantly up-regulated (3–68 fold change) compared to baseline values, whereas 28 genes in control cells were significantly up-regulated at a much higher magnitude (3–4,629 fold change). In addition, 50% of the genes significantly up-regulated in LPS-treated control cells were significantly lower in LPS-treated CHS cells. IL-6, a fibroblast-derived proinflammatory cytokine essential for fighting infections was significantly lower in culture media of CHS cells with or without LPS. Furthermore, Western blot and immunofluorescent staining revealed that TLR-2 and TLR-4 were diminished on cell membranes of CHS cells and dissociated from Rab11a.

Conclusions

For the first time, results from our study indicate defective trafficking of TLR-2 and TLR-4 contributes to the hyposensitive response of CHS skin fibroblasts to immunogenic challenge, providing a potential therapeutic target for clinical intervention in CHS.

Receptor proximal kinases in NF-κB signaling as potential therapeutic targets in cancer and inflammation

Many signaling pathways leading to activation of transcription factors and gene expression are characterized by phosphorylation events mediated by specific kinases. The transcription factor NF-κB plays a key role in multiple cellular processes, including immune signaling, inflammation, development, proliferation and survival. Dysregulated NF-κB activation is associated with autoimmunity, chronic inflammation and cancer. Activation of NF-κB requires IκB kinase (IKK)α or β, the activity of which is regulated via phosphorylation by specific IKK kinases and by autophosphorylation. Receptor specificity is further obtained by the use of multiple upstream receptor proximal kinases. We review the identities of several IKK regulatory kinases as well as the proposed molecular mechanisms. In addition, we discuss the potential for therapeutic targeting of some of these kinases in the context of inflammatory diseases and cancer.

The role of macrophages in the innate immune response to Streptococcus pneumoniae and Staphylococcus aureus: mechanisms and contrasts

Macrophages are critical mediators of innate immune responses against bacteria. The Gram-positive bacteria Streptococcus pneumoniae and Staphylococcus aureus express a range of virulence factors, which challenge macrophages' immune competence. We review how macrophages respond to this challenge. Macrophages employ a range of strategies to phagocytose and kill each pathogen. When the macrophages capacity to clear bacteria is overwhelmed macrophages play important roles in orchestrating the inflammatory response through pattern recognition receptor-mediated responses. Macrophages also ensure the inflammatory response is tightly constrained, to avoid tissue damage, and play an important role in downregulating the inflammatory response once initial bacterial replication is controlled.

A novel benzenediamine derivate rescued mice from experimental sepsis by attenuating proinflammatory mediators via IRAK4

We designed and synthesized a novel benzenediamine derivate, FC-99, that was tested for its ability to protect mice from experimental sepsis. Moreover, we sought to determine whether FC-99 could control a bacterial infection and to clarify the mechanism by which FC-99 inhibited LPS-activated macrophages. The effects of FC-99 on inflammation were evaluated in two experimental sepsis models and in cultured macrophages. Microarrays and docking and molecular dynamics simulations were used to determine the target of FC-99. Surface plasmon resonance and molecular detection were performed to confirm the direct interaction of FC-99 with its target. FC-99 protected mice from experimental sepsis. The mice that received FC-99 exhibited a diminished inflammatory response, had a lower local bacterial burden, and experienced a significantly improved survival rate. Genome-wide transcriptional profiling of FC-99-treated macrophages identified IRAK4 as a drug-regulated gene involved in LPS/TLR4 signaling. A computer search and calculations indicated that IRAK4 directly interacted with FC-99. Surface plasmon resonance, IRAK4-regulated signaling pathway analysis, and gene expression profiling of proinflammatory mediators confirmed the direct interaction between FC-99 and IRAK4. FC-99 is a potential therapeutic molecule for sepsis that alleviated experimental sepsis by directly inhibiting IRAK4 activation, which represents a novel target for sepsis therapy.

The TLR and IL-1 signalling network at a glance

Toll-like receptors (TLRs) and the receptors for interleukin (IL)-1, IL-18 and IL-33 are required for defence against microbial pathogens but, if hyper-activated or not switched off efficiently, can cause tissue damage and inflammatory and autoimmune diseases. Understanding how the checks and balances in the system are integrated to fight infection without the network operating out of control will be crucial for the development of improved drugs to treat these diseases in the future. In this Cell Science at a Glance article and the accompanying poster, I provide a brief overview of how one of these intricate networks is controlled by the interplay of protein phosphorylation and protein ubiquitylation events, and the mechanisms in myeloid cells that restrict and terminate its activation to prevent inflammatory and autoimmune diseases. Finally, I suggest a few protein kinases that have been neglected as drug targets, but whose therapeutic potential should be explored in the light of recent advances in our understanding of their roles in the innate immune system.

MicroRNA-93 inhibits inflammatory cytokine production in LPS-stimulated murine macrophages by targeting IRAK4

Endotoxin-induced uveitis (EIU) is an animal model of acute ocular inflammation for the study of human endogenous anterior uveitis. The mechanisms accounting for the development of ocular inflammation remain hazy. MicroRNAs (mi-RNAs) have been proposed as novel regulators of inflammation. It remains unclear whether a microRNA-mediated regulatory mechanism is involved in LPS-induced EIU. In this study, we report that miR-93 expression in the eyes of EIU rats and LPS-stimulated macrophages is significantly decreased. We also show that miR-93 inhibits NF-κB activation and pro-inflammatory cytokines by targeting IRAK4 expression. We further demonstrate that miR-93 inhibits IRAK4 expression by binding directly to the 3'-UTR of IRAK4. Our findings suggest that miR-93 is a negative regulator of the immune response in EIU.

Bacterial interference with canonical NFκB signalling

The human body is constantly challenged by a variety of commensal and pathogenic micro-organisms that trigger the immune system. Central in the first line of defence is the pattern-recognition receptor (PRR)-induced stimulation of the NFκB pathway, leading to NFκB activation. The subsequent production of pro-inflammatory cytokines and/or antimicrobial peptides results in recruitment of professional phagocytes and bacterial clearance. To overcome this, bacteria have developed mechanisms for targeted interference in every single step in the PRR–NFκB pathway to dampen host inflammatory responses. This review aims to briefly overview the PRR–NFκB pathway in relation to the immune response and give examples of the diverse bacterial evasion mechanisms including changes in the bacterial surface, decoy production and injection of effector molecules. Targeted regulation of inflammatory responses is needed and bacterial molecules developed for immune evasion could provide future anti-inflammatory agents.

IRAK4 kinase activity is not required for induction of endotoxin tolerance but contributes to TLR2-mediated tolerance

Prior exposure to LPS induces "endotoxin tolerance" that reprograms TLR4 responses to subsequent LPS challenge by altering expression of inflammatory mediators. Endotoxin tolerance is thought to limit the excessive cytokine storm and prevent tissue damage during sepsis but renders the host immunocompromised and susceptible to secondary infections. Tolerance initiated via one TLR can affect cellular responses to challenge via the same TLR ("homotolerance") or through different TLRs ("heterotolerance"). IRAK4, an essential component of the MyD88-dependent pathway, functions as a kinase and an adapter, activating subsets of divergent signaling pathways. In this study, we addressed mechanistically the role of IRAK4 kinase activity in TLR4- and TLR2-induced tolerance using macrophages from WT versus IRAK4(KDKI) mice. Whereas IRAK4 kinase deficiency decreased LPS signaling, it did not prevent endotoxin tolerance, as endotoxin pretreatment of WT and IRAK4(KDKI) macrophages inhibited LPS-induced MAPK phosphorylation, degradation of IκB-α and recruitment of p65 to the TNF-α promoter, expression of proinflammatory cytokines, and increased levels of A20 and IRAK-M. Pretreatment of WT macrophages with Pam3Cys, a TLR2-TLR1 agonist, ablated p-p38 and p-JNK in response to challenge with Pam3Cys and LPS, whereas IRAK4(KDKI) macrophages exhibited attenuated TLR2-elicited homo- and heterotolerance at the level of MAPK activation. Thus, IRAK4 kinase activity is not required for the induction of endotoxin tolerance but contributes significantly to TLR2-elicited homo- and heterotolerance.

MCPIP1 negatively regulates toll-like receptor 4 signaling and protects mice from LPS-induced septic shock

Septic shock is one of leading causes of morbidity and mortality in hospital patients. However, genetic factors predisposing to septic shock are not fully understood. Our previous work showed that MCP-induced protein 1 (MCPIP1) was induced by lipopolysaccharides (LPSs), which then negatively regulates LPS-induced inflammatory signaling in vitro. Here we report that although MCPIP1 was induced by various toll-like receptor (TLR) ligands in macrophages, MCPIP1-deficient mice are extremely susceptible to TLR4 ligand (LPS)-induced septic shock and death, but not to the TLR2, 3, 5 and 9 ligands-induced septic shock. Consistently, LPS induced tumor necrosis factor α (TNFα) production in MCPIP1-deficient mice was 20-fold greater than that in their wild-type littermates. Further analysis revealed that MCPIP1-deficient mice developed severe acute lung injury after LPS injection and JNK signaling was highly activated in MCPIP1-deficient lungs after LPS stimulation. Finally, macrophage-specific MCPIP1 transgenic mice were partially protected from LPS-induced septic shock, suggesting that inflammatory cytokines from sources other than macrophages may significantly contribute to the pathogenesis of LPS-induced septic shock. Taken together, these results suggest that MCPIP1 selectively suppresses TLR4 signaling pathway and protects mice from LPS-induced septic shock.