A TH17-intrinsic IL-1β-STAT5 axis drives steroid resistance in autoimmune neuroinflammation

Steroid resistance poses a major challenge for the management of autoimmune neuroinflammation. T helper 17 (TH17) cells are widely implicated in the pathology of steroid resistance; however, the underlying mechanisms are unknown. In this study, we identified that interleukin-1 receptor (IL-1R) blockade rendered experimental autoimmune encephalomyelitis (EAE) mice sensitive to dexamethasone (Dex) treatment. Interleukin-1β (IL-1β) induced a signal transducer and activator of transcription 5 (STAT5)-mediated steroid-resistant transcriptional program in TH17 cells, which promoted inflammatory cytokine production and suppressed Dex-induced anti-inflammatory genes. TH17-specific deletion of STAT5 ablated the IL-1β-induced steroid-resistant transcriptional program and rendered EAE mice sensitive to Dex treatment. IL-1β synergized with Dex to promote the STAT5-dependent expression of CD69 and the development of central nervous system (CNS)-resident CD69+ TH17 cells. Combined IL-1R blockade and Dex treatment ablated CNS-resident TH17 cells, reduced EAE severity, and prevented relapse. CD69+ tissue-resident TH17 cells were also detected in brain lesions of patients with multiple sclerosis. These findings (i) demonstrate that IL-1β-STAT5 signaling in TH17 cells mediates steroid resistance and (ii) identify a therapeutic strategy for reversing steroid resistance in TH17-mediated CNS autoimmunity.

TRAF4 is crucial for ST2+ memory Th2 cell expansion in IL-33-driven airway inflammation

Tumor necrosis factor receptor-associated factor 4 (TRAF4) is an important regulator of type 2 responses in the airway; however, the underlying cellular and molecular mechanisms remain elusive. Herein, we generated T cell-specific TRAF4-deficient (CD4-cre Traf4fl/fl) mice and investigated the role of TRAF4 in memory Th2 cells expressing IL-33 receptor (ST2, suppression of tumorigenicity 2) (ST2+ mTh2 cells) in IL-33-mediated type 2 airway inflammation. We found that in vitro-polarized TRAF4-deficient (CD4-cre Traf4fl/fl) ST2+ mTh2 cells exhibited decreased IL-33-induced proliferation as compared with TRAF4-sufficient (Traf4fl/fl) cells. Moreover, CD4-cre Traf4fl/fl mice showed less ST2+ mTh2 cell proliferation and eosinophilic infiltration in the lungs than Traf4fl/fl mice in the preclinical models of IL-33-mediated type 2 airway inflammation. Mechanistically, we discovered that TRAF4 was required for the activation of AKT/mTOR and ERK1/2 signaling pathways as well as the expression of transcription factor Myc and nutrient transporters (Slc2a1, Slc7a1, and Slc7a5), signature genes involved in T cell growth and proliferation, in ST2+ mTh2 cells stimulated by IL-33. Taken together, the current study reveals a role of TRAF4 in ST2+ mTh2 cells in IL-33-mediated type 2 pulmonary inflammation, opening up avenues for the development of new therapeutic strategies.

Mincle-GSDMD-mediated release of IL-1β small extracellular vesicles from hepatic macrophages in ethanol-induced liver injury

BACKGROUND

Macrophage-inducible C-type lectin (Mincle) is expressed on hepatic macrophages and senses ethanol (EtOH)-induced danger signals released from dying hepatocytes and promotes IL-1β production. However, it remains unclear what and how EtOH-induced Mincle ligands activate downstream signaling events to mediate IL-1β release and contribute to alcohol-associated liver disease (ALD). In this study, we investigated the association of circulating β-glucosylceramide (β-GluCer), an endogenous Mincle ligand, with severity of ALD and examined the mechanism by which β-GluCer engages Mincle on hepatic macrophages to release IL-1β in the absence of cell death and exacerbates ALD.

METHOD AND RESULTS

Concentrations of β-GluCer were increased in serum of patients with severe AH and correlated with disease severity. Challenge of hepatic macrophages with lipopolysaccharide and β-GluCer induced formation of a Mincle and Gsdmd-dependent secretory complex containing chaperoned full-length gasdermin D (Hsp90-CDC37-NEDD4) with polyubiquitinated pro-IL-1β and components of the Caspase 8-NLRP3 inflammasome loaded as cargo in small extracellular vesicles (sEVs). Gao-binge EtOH exposure to wild-type, but not Mincle-/- and Gsdmd-/-, mice increased release of IL-1β-containing sEVs from liver explant cultures. Myeloid-specific deletion of Gsdmd similarly decreased the formation of sEVs by liver explant cultures and protected mice from EtOH-induced liver injury. sEVs collected from EtOH-fed wild-type, but not Gsdmd-/-, mice promoted injury of cultured hepatocytes and, when injected into wild-type mice, aggravated Gao-binge EtOH-induced liver injury.

CONCLUSION

β-GluCer functions as a danger-associated molecular pattern activating Mincle-dependent gasdermin D-mediated formation and release of IL-1β-containing sEVs, which in turn exacerbate hepatocyte cell death and contribute to the pathogenesis of ALD.

TRAF4 is crucial for the propagation of ST2+ memory Th2 cells in IL-33-mediated type 2 airway inflammation

Tumor necrosis factor receptor (TNF)-associated factor 4 (TRAF4) is an important regulator of type 2 responses in the airway; however, the underlying cellular and molecular mechanisms remain elusive. Herein, we generated T cell-specific TRAF4-deficient (CD4cre-Traf4fl/fl) mice and investigated the role of TRAF4 in interleukin (IL)-33 receptor (ST2, suppression of tumorigenicity 2)-expressing memory Th2 cells (ST2+ mTh2) in IL-33-mediated type 2 airway inflammation. We found that in vitro polarized TRAF4-deficient (CD4cre- Traf4fl/fl) ST2+ mTh2 cells exhibited decreased IL-33-induced proliferation as compared with TRAF4-sufficient (Traf4fl/fl) cells. Moreover, CD4cre-Traf4fl/fl mice showed less ST2+ mTh2 cell proliferation and eosinophilic infiltration in the lungs than Traf4fl/fl mice in the preclinical models of IL-33-mediated type 2 airway inflammation. Mechanistically, we discovered that TRAF4 was required for the activation of AKT/mTOR and ERK1/2 signaling pathways as well as the expression of transcription factor Myc and nutrient transporters (Slc2a1, Slc7a1, and Slc7a5), signature genes involved in T cell growth and proliferation, in ST2+ mTh2 cells stimulated by IL-33. Taken together, the current study reveals a previously unappreciated role of TRAF4 in ST2+ mTh2 cells in IL-33-mediated type 2 pulmonary inflammation, opening up avenues for the development of new therapeutic strategies.

Mechanisms of Corticosteroid Resistance in Type 17 Asthma

IL-17A plays an important role in the pathogenesis of asthma, particularly the neutrophilic corticosteroid (CS)-resistant subtype of asthma. Clinical studies suggest that a subset of asthma patients, i.e., Th17/IL-17A-mediated (type 17) CS-resistant neutrophilic asthma, may improve with Th17/IL-17A pathway blockade. However, little is known about the mechanisms underlying type 17 asthma and CS response. In this article, we show that blood levels of lipocalin-2 (LCN2) and serum amyloid A (SAA) levels are positively correlated with IL-17A levels and are not inhibited by high-dose CS usage in asthma patients. In airway cell culture systems, IL-17A induces these two secreted proteins, and their induction is enhanced by CS. Furthermore, plasma LCN2 and SAA levels are increased in mice on a preclinical type 17 asthma model, correlated to IL-17A levels, and are not reduced by glucocorticoid (GC). In the mechanistic studies, we identify CEBPB as the critical transcription factor responsible for the synergistic induction of LCN2 and SAA by IL-17A and GC. IL-17A and GC collaboratively regulate CEBPB at both transcriptional and posttranscriptional levels. The posttranscriptional regulation of CEBPB is mediated in part by Act1, the adaptor and RNA binding protein in IL-17A signaling, which directly binds CEBPB mRNA and inhibits its degradation. Overall, our findings suggest that blood LCN2 and SAA levels may be associated with a type 17 asthma subtype and provide insight into the molecular mechanism of the IL-17A-Act1/CEBPB axis on these CS-resistant genes.

The Ras GTPase-activating-like protein IQGAP1 bridges Gasdermin D to the ESCRT system to promote IL-1β release via exosomes

IL-1β can exit the cytosol as an exosomal cargo following inflammasome activation in intestinal epithelial cells (IECs) in a Gasdermin D (GSDMD)-dependent manner. The mechanistic connection linking inflammasome activation and the biogenesis of exosomes has so far remained largely elusive. Here, we report the Ras GTPase-activating-like protein IQGAP1 functions as an adaptor, bridging GSDMD to the endosomal sorting complexes required for transport (ESCRT) machinery to promote the biogenesis of pro-IL-1β-containing exosomes in response to NLPR3 inflammasome activation. We identified IQGAP1 as a GSDMD-interacting protein through a non-biased proteomic analysis. Functional investigation indicated the IQGAP1-GSDMD interaction is required for LPS and ATP-induced exosome release. Further analysis revealed that IQGAP1 serves as an adaptor which bridges GSDMD and associated IL-1β complex to Tsg101, a component of the ESCRT complex, and enables the packaging of GSDMD and IL-1β into exosomes. Importantly, this process is dependent on an LPS-induced increase in GTP-bound CDC42, a small GTPase known to activate IQGAP1. Taken together, this study reveals IQGAP1 as a link between inflammasome activation and GSDMD-dependent, ESCRT-mediated exosomal release of IL-1β.

Dominant-Negative Form of SIGIRR: SIGIRRΔE8 Promotes Tumor Growth Through Regulation of Metabolic Pathways

Colorectal carcinoma is the leading cause of cancer-related death. Previously we have shown that tumor suppressor single immunoglobulin interleukin-1-related receptor (SIGIRR) is frequently inactivated in human colorectal cancer by the increased expression of a novel SIGIRR isoform (SIGIRR^ΔE8). SIGIRR^ΔE8 showed increased retention in the cytoplasm and loss of complex glycan modification compared to the full-length SIGIRR. Now we found that the arginine residues located in the C-terminus of SIGIRR^ΔE8 serve as an endoplasmic reticulum retention signal and are required for resident protein ribophorin 1 (RPN1) interaction. In addition, we found that SIGIRR^ΔE8 exerts a direct impact on cell metabolism through interaction with the adenosine triphosphate synthase in the colorectal cancer cells. SIGIRR^ΔE8 expression promoted the metabolic shift through upregulation of mammalian target of rapamycin signaling pathway and dysregulation of mitochondrial function to promote survival and proliferation of colon cancer cells in xenograft model.

TH17 cells promote CNS inflammation by sensing danger signals via Mincle

The C-type lectin receptor Mincle is known for its important role in innate immune cells in recognizing pathogen and damage associated molecular patterns. Here we report a T cell-intrinsic role for Mincle in the pathogenesis of experimental autoimmune encephalomyelitis (EAE). Genomic deletion of Mincle in T cells impairs TH17, but not TH1 cell-mediated EAE, in alignment with significantly higher expression of Mincle in TH17 cells than in TH1 cells. Mechanistically, dying cells release β-glucosylceramide during inflammation, which serves as natural ligand for Mincle. Ligand engagement induces activation of the ASC-NLRP3 inflammasome, which leads to Caspase8-dependent IL-1β production and consequentially TH17 cell proliferation via an autocrine regulatory loop. Chemical inhibition of β-glucosylceramide synthesis greatly reduces inflammatory CD4+ T cells in the central nervous system and inhibits EAE progression in mice. Taken together, this study indicates that sensing of danger signals by Mincle on TH17 cells plays a critical role in promoting CNS inflammation.

GSDMB is increased in IBD and regulates epithelial restitution/repair independent of pyroptosis

Gasdermins are a family of structurally related proteins originally described for their role in pyroptosis. Gasdermin B (GSDMB) is currently the least studied, and while its association with genetic susceptibility to chronic mucosal inflammatory disorders is well established, little is known about its functional relevance during active disease states. Herein, we report increased GSDMB in inflammatory bowel disease, with single-cell analysis identifying epithelial specificity to inflamed colonocytes/crypt top colonocytes. Surprisingly, mechanistic experiments and transcriptome profiling reveal lack of inherent GSDMB-dependent pyroptosis in activated epithelial cells and organoids but instead point to increased proliferation and migration during in vitro wound closure, which arrests in GSDMB-deficient cells that display hyper-adhesiveness and enhanced formation of vinculin-based focal adhesions dependent on PDGF-A-mediated FAK phosphorylation. Importantly, carriage of disease-associated GSDMB SNPs confers functional defects, disrupting epithelial restitution/repair, which, altogether, establishes GSDMB as a critical factor for restoration of epithelial barrier function and the resolution of inflammation.

Epithelial-derived gasdermin D mediates nonlytic IL-1β release during experimental colitis

Gasdermin D (GSDMD) induces pyroptosis via the pore-forming activity of its N-terminal domain, cleaved by activated caspases associated with the release of IL-1β. Here, we report a nonpyroptotic role of full-length GSDMD in guiding the release of IL-1β-containing small extracellular vesicles (sEVs) from intestinal epithelial cells (IECs). In response to caspase-8 inflammasome activation, GSDMD, chaperoned by Cdc37/Hsp90, recruits the E3 ligase, NEDD4, to catalyze polyubiquitination of pro-IL-1β, serving as a signal for cargo loading into secretory vesicles. GSDMD and IL-1β colocalize with the exosome markers CD63 and ALIX intracellularly, and GSDMD and NEDD4 are required for release of CD63+ sEVs containing IL-1β, GSDMD, NEDD4, and caspase-8. Importantly, increased expression of epithelial-derived GSDMD is observed both in patients with inflammatory bowel disease (IBD) and those with experimental colitis. While GSDMD-dependent release of IL-1β-containing sEVs is detected in cultured colonic explants from colitic mice, GSDMD deficiency substantially attenuates disease severity, implicating GSDMD-mediated release of IL-1β sEVs in the pathogenesis of intestinal inflammation, such as that observed in IBD.

Inflammation mobilizes copper metabolism to promote colon tumorigenesis via an IL-17-STEAP4-XIAP axis

Copper levels are known to be elevated in inflamed and malignant tissues. But the mechanism underlying this selective enrichment has been elusive. In this study, we report a axis by which inflammatory cytokines, such as IL-17, drive cellular copper uptake via the induction of a metalloreductase, STEAP4. IL-17-induced elevated intracellular copper level leads to the activation of an E3-ligase, XIAP, which potentiates IL-17-induced NFκB activation and suppresses the caspase 3 activity. Importantly, this IL-17-induced STEAP4-dependent cellular copper uptake is critical for colon tumor formation in a murine model of colitis-associated tumorigenesis and STEAP4 expression correlates with IL-17 level and XIAP activation in human colon cancer. In summary, this study reveals a IL-17-STEAP4-XIAP axis through which the inflammatory response induces copper uptake, promoting colon tumorigenesis.

TLR5 participates in the TLR4 receptor complex and promotes MyD88-dependent signaling in environmental lung injury

Lung disease causes significant morbidity and mortality, and is exacerbated by environmental injury, for example through lipopolysaccharide (LPS) or ozone (O3). Toll-like receptors (TLRs) orchestrate immune responses to injury by recognizing pathogen- or danger-associated molecular patterns. TLR4, the prototypic receptor for LPS, also mediates inflammation after O3, triggered by endogenous hyaluronan. Regulation of TLR4 signaling is incompletely understood. TLR5, the flagellin receptor, is expressed in alveolar macrophages, and regulates immune responses to environmental injury. Using in vivo animal models of TLR4-mediated inflammations (LPS, O3, hyaluronan), we show that TLR5 impacts the in vivo response to LPS, hyaluronan and O3. We demonstrate that immune cells of human carriers of a dominant negative TLR5 allele have decreased inflammatory response to O3 exposure ex vivo and LPS exposure in vitro. Using primary murine macrophages, we find that TLR5 physically associates with TLR4 and biases TLR4 signaling towards the MyD88 pathway. Our results suggest an updated paradigm for TLR4/TLR5 signaling.

TRAF Regulation of IL-17 Cytokine Signaling

Tumor necrosis factor receptor (TNFR)-associated factors or (TRAFs) are important mediators of Interleukin-17 (IL-17) cytokine signaling and contribute to driving tissue responses that are crucial for protective immunity but are often implicated in immunopathology. By amplifying tissue immune activity, IL-17 cytokine pathways contribute to maintaining barrier function as well as activation of innate and adaptive immunity necessary for host defense. IL-17 receptors signaling is orchestrated in part, by the engagement of TRAFs and the subsequent unlocking of downstream cellular machinery that can promote pathogen clearance or contribute to immune dysregulation, chronic inflammation, and disease. Originally identified as signaling adaptors for TNFR superfamily, TRAF proteins can mediate the signaling of a variety of intercellular and extracellular stimuli and have been shown to regulate the downstream activity of many cytokine receptors including receptors for IL-1β, IL-2, IL-6, IL-17, IL-18, IL-33, type I IFNs, type III IFNs, GM-CSF, M-CSF, and TGF-β Toll-like receptors (TLRs), NOD-like receptors (NLRs), RIG-I- like receptors, and C-type lectin receptors. This review will focus on discussing studies that reveal our current understanding of how TRAFs mediate and regulate biochemical activities downstream of the IL-17 cytokines signaling.

IL-17A Recruits Rab35 to IL-17R to Mediate PKCα-Dependent Stress Fiber Formation and Airway Smooth Muscle Contractility

IL-17A is a critical proinflammatory cytokine for the pathogenesis of asthma including neutrophilic pulmonary inflammation and airway hyperresponsiveness. In this study, by cell type-specific deletion of IL-17R and adaptor Act1, we demonstrated that IL-17R/Act1 exerts a direct impact on the contraction of airway smooth muscle cells (ASMCs). Mechanistically, IL-17A induced the recruitment of Rab35 (a small monomeric GTPase) and DennD1C (guanine nucleotide exchange factor [GEF]) to the IL-17R/Act1 complex in ASMCs, resulting in activation of Rab35. Rab35 knockdown showed that IL-17A-induced Rab35 activation was essential for protein kinase Cα (PKCα) activation and phosphorylation of fascin at Ser39 in ASMCs, allowing F-actin to interact with myosin to form stress fibers and enhance the contraction induced by methacholine. PKCα inhibitor or Rab35 knockdown indeed substantially reduced IL-17A-induced stress fiber formation in ASMCs and attenuated IL-17A-enhanced, methacholine-induced contraction of airway smooth muscle. Taken together, these data indicate that IL-17A promotes airway smooth muscle contraction via direct recruitment of Rab35 to IL-17R, followed by PKCα activation and stress fiber formation.

IRAK2 directs stimulus-dependent nuclear export of inflammatory mRNAs

Expression of inflammatory genes is determined in part by post-transcriptional regulation of mRNA metabolism but how stimulus- and transcript-dependent nuclear export influence is poorly understood. Here, we report a novel pathway in which LPS/TLR4 engagement promotes nuclear localization of IRAK2 to facilitate nuclear export of a specific subset of inflammation-related mRNAs for translation in murine macrophages. IRAK2 kinase activity is required for LPS-induced RanBP2-mediated IRAK2 sumoylation and subsequent nuclear translocation. Array analysis showed that an SRSF1-binding motif is enriched in mRNAs dependent on IRAK2 for nuclear export. Nuclear IRAK2 phosphorylates SRSF1 to reduce its binding to target mRNAs, which promotes the RNA binding of the nuclear export adaptor ALYREF and nuclear export receptor Nxf1 loading for the export of the mRNAs. In summary, LPS activates a nuclear function of IRAK2 that facilitates the assembly of nuclear export machinery to export selected inflammatory mRNAs to the cytoplasm for translation.

The innate immune system is the body’s first line of defense against invading microbes. Some immune cells carry specific receptor proteins called Toll-like receptors that can identify microbes and the signals they emit. As soon as the receptors have detected a threat – for example through sensing oily molecules that make up the cell membranes of microbes – they produce signaling proteins called cytokines and chemokines to alert other immune cells.

The DNA in the cell’s nucleus carries the instructions needed to make proteins. To produce proteins, including cytokines and chemokines, the information first has to be transferred into mRNA templates, which carry the instructions to the sites in the cell where the proteins are made. Cytokine and chemokine mRNAs are generally short-lived, but previous studies in 2009 and 2011 have shown that an enzyme called IRAK2 can stabilize them to make them last longer. IRAK enzymes are activated by the Toll-like receptors after a threat has been detected. However, until now it was not known whether IRAK2 also helps to transport the mRNAs of cytokines and chemokines out of the cell nucleus.

Using immune cells of mice, Zhou et al. – including some of the researchers involved in the previous studies – discovered that IRAK2 helped to export the mRNAs of cytokines and chemokines from the immune cell nucleus into the surrounding cell fluid. The Toll-like receptors recognized the oily molecules of the microbes and consequently activated IRAK2, which lead to IRAK2 being moved into the cell nucleus.

Once activated, IRAK2 helped to assemble the export machinery that moved selected mRNAs out of the nucleus to build the proteins. To do so, IRAK2 stopped a destabilizing protein from binding to the mRNA, so that instead the export machinery could transport the mRNA of the cytokines and chemokines out of the cell nucleus.

A next step will be to test whether IRAK2 is required to guide exported mRNA tothe sites in the cell where the proteins are made. This new insight could help to develop new treatments for various diseases. For example, diseases in which the immune system attacks the cells of the body, rather than invaders, can be caused by too many cytokines and chemokines. Since IRAK2 helps to control the availability of cytokines and chemokines it may in future be used as a new drug target.

IL-17 induced NOTCH1 activation in oligodendrocyte progenitor cells enhances proliferation and inflammatory gene expression

NOTCH1 signalling contributes to defective remyelination by impairing differentiation of oligodendrocyte progenitor cells (OPCs). Here we report that IL-17 stimulation induces NOTCH1 activation in OPCs, contributing to Th17-mediated demyelinating disease. Mechanistically, IL-17R interacts with NOTCH1 via the extracellular domain, which facilitates the cleavage of NOTHC1 intracellular domain (NICD1). IL-17-induced NOTCH1 activation results in the interaction of IL-17R adaptor Act1 with NICD1, followed by the translocation of the Act1–NICD1 complex into the nucleus. Act1–NICD1 are recruited to the promoters of several NOTCH1 target genes (including STEAP4, a metalloreductase important for inflammation and cell proliferation) that are specifically induced in the spinal cord by Th17 cells. A decoy peptide disrupting the IL-17RA–NOTCH1 interaction inhibits IL-17-induced NOTCH1 activation and attenuates Th17-mediated experimental autoimmune encephalitis (EAE). Taken together, these findings demonstrate critical crosstalk between the IL-17 and NOTCH1 pathway, regulating Th17-induced inflammatory and proliferative genes to promote demyelinating disease.

NOTCH signalling stimulates oligodendrocyte progenitor cell proliferation but how this regulates demyelinating disease is unclear. Here, the authors show that an IL-17 adaptor protein, Act1, interacts with the C-terminal fragment of NOTCH1 (NICD) to activate cell proliferation and an inflammatory response.

Human Colon Tumors Express a Dominant-Negative Form of SIGIRR That Promotes Inflammation and Colitis-Associated Colon Cancer in Mice

BACKGROUND & AIMS

Single immunoglobulin and toll-interleukin 1 receptor (SIGIRR), a negative regulator of the Toll-like and interleukin-1 receptor (IL-1R) signaling pathways, controls intestinal inflammation and suppresses colon tumorigenesis in mice. However, the importance of SIGIRR in human colorectal cancer development has not been determined. We investigated the role of SIGIRR in development of human colorectal cancer.

METHODS

We performed RNA sequence analyses of pairs of colon tumor and nontumor tissues, each collected from 68 patients. Immunoblot and immunofluorescence analyses were used to determine levels of SIGIRR protein in primary human colonic epithelial cells, tumor tissues, and colon cancer cell lines. We expressed SIGIRR and mutant forms of the protein in Vaco cell lines. We created and analyzed mice that expressed full-length (control) or a mutant form of Sigirr (encoding SIGIRR(N86/102S), which is not glycosylated) specifically in the intestinal epithelium. Some mice were given azoxymethane (AOM) and dextran sulfate sodium to induce colitis-associated cancer. Intestinal tissues were collected and analyzed by immunohistochemical and gene expression profile analyses.

RESULTS

RNA sequence analyses revealed increased expression of a SIGIRR mRNA isoform, SIGIRR(ΔE8), in colorectal cancer tissues compared to paired nontumor tissues. SIGIRR(ΔE8) is not modified by complex glycans and is therefore retained in the cytoplasm-it cannot localize to the cell membrane or reduce IL1R signaling. SIGIRR(ΔE8) interacts with and has a dominant-negative effect on SIGIRR, reducing its glycosylation, localization to the cell surface, and function. Most SIGIRR detected in human colon cancer tissues was cytoplasmic, whereas in nontumor tissues it was found at the cell membrane. Mice that expressed SIGIRR(N86/102S) developed more inflammation and formed larger tumors after administration of azoxymethane and dextran sulfate sodium than control mice; colon tissues from these mutant mice expressed higher levels of the inflammatory cytokines IL-17A and IL-6 had activation of the transcription factors STAT3 and NFκB. SIGIRR(N86/102S) expressed in colons of mice did not localize to the epithelial cell surface.

CONCLUSION

Levels of SIGIRR are lower in human colorectal tumors, compared with nontumor tissues; tumors contain the dominant-negative isoform SIGIRR(ΔE8). This mutant protein blocks localization of full-length SIGIRR to the surface of colon epithelial cells and its ability to downregulate IL1R signaling. Expression of SIGIRR(N86/102S) in the colonic epithelium of mice increases expression of inflammatory cytokines and formation and size of colitis-associated tumors.

IL-27, targeting antigen-presenting cells, promotes Th17 differentiation and colitis in mice

T helper type 17 (Th17) cells have been implicated in autoimmunity and inflammatory bowel disease (IBD). Antigen-presenting cell (APC) -derived cytokines such as interleukin (IL)-1β and IL-6 are key mediators supporting Th17 differentiation, yet how these factors are induced in vivo remains unclear. Here, we show that IL-27 acting on APCs enhances IL-6 and IL-1β production and Th17 differentiation. IL-27Rα-/- T-cell receptor (TCR)β-/- recipients fail to develop gut inflammation following naive CD4 T-cell transfer, whereas IL-27Rα+/+ TCRβ-/- recipients develop severe colitis. Investigation of T-cell responses exhibits that IL-27Rα-/- TCRβ-/- mice do not support Th17 differentiation with significantly decreased levels of IL-6 and IL-1β by APCs. Our study has identified a novel proinflammatory role for IL-27 in vivo that promotes Th17 differentiation by inducing Th17-supporting cytokines in APCs.

HuR is required for IL-17-induced Act1-mediated CXCL1 and CXCL5 mRNA stabilization

IL-17, a major inflammatory cytokine plays a critical role in the pathogenesis of many autoimmune inflammatory diseases. In this study, we report a new function of RNA-binding protein HuR in IL-17-induced Act1-mediated chemokine mRNA stabilization. HuR deficiency markedly reduced IL-17-induced chemokine expression due to increased mRNA decay. Act1-mediated HuR polyubiquitination was required for the binding of HuR to CXCL1 mRNA, leading to mRNA stabilization. Although IL-17 induced the coshift of Act1 and HuR to the polysomal fractions in a sucrose gradient, HuR deficiency reduced the ratio of translation-active/translation-inactive IL-17-induced chemokine mRNAs. Furthermore, HuR deletion in distal lung epithelium attenuated IL-17-induced neutrophilia. In summary, HuR functions to couple receptor-proximal signaling to posttranscriptional machinery, contributing to IL-17-induced inflammation.

Epithelial cell-specific Act1 adaptor mediates interleukin-25-dependent helminth expulsion through expansion of Lin(-)c-Kit(+) innate cell population

Interleukin-25 (IL-25 or IL-17E), a member of the structurally related IL-17 family, functions as an important mediator of T helper 2 cell-type (type 2) responses. We examined the cell type-specific role of IL-25-induced Act1-mediated signaling in protective immunity against helminth infection. Targeted Act1 deficiency in epithelial cells resulted in a marked delay in worm expulsion and abolished the expansion of the Lin(-)c-Kit(+) innate cell population in the mesenteric lymph node, lung, and liver. Th2 cell-inducing cytokine (IL-25 and IL-33) expression were reduced in the intestinal epithelial cells from the infected and IL-25-injected epithelial-specific Act1-deficient mice. Adoptive transfer of Lin(-)c-Kit(+) cells or combined injection of IL-25 and IL-33 restored the type 2 responses in these mice. Taken together, these results suggest that epithelial-specific Act1 mediates the expansion of the Lin(-)c-Kit(+) innate cell population through the positive-feedback loop of IL-25, initiating the type 2 immunity against helminth infection.

SIGIRR, a negative regulator of colon tumorigenesis

Inappropriate activation of the Toll-IL-1R (TL-IL-1) signaling by commensal bacteria contributes to the pathogenesis of inflammatory bowel diseases and colitis-associated cancer. Recent studies have identified SIGIRR as a negative regulator of TL-IL-1 signaling. It dampens intestinal inflammation and tumorigenesis in the colon. In this review, we will discuss the role of SIGIRR in different cell types and the mechanisms underlying its tumor suppressor function.

T cell-derived Act1 is necessary for IL-25-mediated Th2 responses and allergic airway inflammation

The cellular and molecular mechanisms driven by IL-25 and its cognate receptor IL-17RB necessary for the promotion of Th2-mediating pathogenic pulmonary inflammation remains to be defined. We have previously reported the critical role of the U-box-type E3 ubiquitin ligase Act1 (1) for the downstream signaling of the IL-17 cytokine family including the Th2-promoting cytokine IL-25 (IL-17E) (2). In this study, we report that IL-25-driven but not conventional IL-4-driven Th2 polarization and cytokine production is impaired in Act1-deficient T cells. Also, Act1 deficiency in the T cell compartment results in the abrogation of eosinophilic airway infiltration as well as airway hyperresponsiveness in mouse models of Ag-induced airway inflammation. The in vivo generation of Ag-specific Th2 cytokine-producing cells is defective in the absence of Act1 expression in T cells after OVA/aluminum hydroxide immunization. Notably, the production of OVA-specific IgG(1) but not IgG(2a) or IgE is also impaired. At the molecular level, we report that IL-25-mediated induction of Th2 master regulator GATA-3 and the transcription factor GFI-1 is attenuated in Act1-deficient T cells. Taken together, our findings indicate that Act1 expression in T cells is required for cellular and humoral Th2-mediated allergic responses and the development of airway hyperresponsiveness, in part, through Act1's function in IL-25-induced development of Th2 T cells.

Treatment with IL-17 prolongs the half-life of chemokine CXCL1 mRNA via the adaptor TRAF5 and the splicing-regulatory factor SF2 (ASF)

Interleukin 17 (IL-17) promotes the expression of chemokines and cytokines via the induction of gene transcription and post-transcriptional stabilization of mRNA. We show here that IL-17 enhanced the stability of chemokine CXCL1 mRNA and other mRNAs through a pathway that involved the adaptor Act1, the adaptors TRAF2 or TRAF5 and the splicing factor SF2 (also known as alternative splicing factor (ASF)). TRAF2 and TRAF5 were necessary for IL-17 to signal the stabilization of CXCL1 mRNA. Furthermore, IL-17 promoted the formation of complexes of TRAF5-TRAF2, Act1 and SF2 (ASF). Overexpression of SF2 (ASF) shortened the half-life of CXCL1 mRNA, whereas depletion of SF2 (ASF) prolonged it. SF2 (ASF) bound chemokine mRNA in unstimulated cells, whereas the SF2 (ASF)-mRNA interaction was much lower after stimulation with IL-17. Our findings define an IL-17-induced signaling pathway that links to the stabilization of selected mRNA species through Act1, TRAF2-TRAF5 and the RNA-binding protein SF2 (ASF).

The inducible kinase IKKi is required for IL-17-dependent signaling associated with neutrophilia and pulmonary inflammation

Interleukin 17 (IL-17) is critical in the pathogenesis of inflammatory and autoimmune diseases. Here we report that Act1, the key adaptor for the IL-17 receptor (IL-7R), formed a complex with the inducible kinase IKKi after stimulation with IL-17. Through the use of IKKi-deficient mice, we found that IKKi was required for IL-17-induced expression of genes encoding inflammatory molecules in primary airway epithelial cells, neutrophilia and pulmonary inflammation. IKKi deficiency abolished IL-17-induced formation of the complex of Act1 and the adaptors TRAF2 and TRAF5, activation of mitogen-activated protein kinases (MAPKs) and mRNA stability, whereas the Act1-TRAF6-transcription factor NF-κB axis was retained. IKKi was required for IL-17-induced phosphorylation of Act1 on Ser311, adjacent to a putative TRAF-binding motif. Substitution of the serine at position 311 with alanine impaired the IL-17-mediated Act1-TRAF2-TRAF5 interaction and gene expression. Thus, IKKi is a kinase newly identified as modulating IL-17 signaling through its effect on Act1 phosphorylation and consequent function.

The receptor SIGIRR suppresses Th17 cell proliferation via inhibition of the interleukin-1 receptor pathway and mTOR kinase activation

Interleukin-1 (IL-1)-mediated signaling in T cells is essential for T helper 17 (Th17) cell differentiation. We showed here that SIGIRR, a negative regulator of IL-1 receptor and Toll-like receptor signaling, was induced during Th17 cell lineage commitment and governed Th17 cell differentiation and expansion through its inhibitory effects on IL-1 signaling. The absence of SIGIRR in T cells resulted in increased Th17 cell polarization in vivo upon myelin oligodendrocyte glycoprotein (MOG(35-55)) peptide immunization. Recombinant IL-1 promoted a marked increase in the proliferation of SIGIRR-deficient T cells under an in vitro Th17 cell-polarization condition. Importantly, we detected increased IL-1-induced phosphorylation of JNK and mTOR kinase in SIGIRR-deficient Th17 cells compared to wild-type Th17 cells. IL-1-induced proliferation was abolished in mTOR-deficient Th17 cells, indicating the essential role of mTOR activation. Our results demonstrate an important mechanism by which SIGIRR controls Th17 cell expansion and effector function through the IL-1-induced mTOR signaling pathway.

The essential role of single Ig IL-1 receptor-related molecule/Toll IL-1R8 in regulation of Th2 immune response

A novel cytokine IL-33, an IL-1 family member, signals via ST2 receptor and promotes Th2 responses, through the activation of NF-kappaB and MAP kinases. Previous studies reported that single Ig IL-1R-related molecule (SIGIRR)/Toll IL-1R8 acts as negative regulator for TLR-IL-1R-mediated signaling. We now found that SIGIRR formed a complex with ST2 upon IL-33 stimulation and specifically inhibited IL-33/ST2-mediated signaling in cell culture model. Furthermore, IL-33-induced Th2 response was enhanced in SIGIRR-deficient mice compared with that in wild-type control mice, suggesting a negative regulatory role of SIGIRR in IL-33/ST2 signaling in vivo. Similar to ST2, SIGIRR was highly expressed in in vitro polarized Th2 cells, but not Th1 cells. SIGIRR-deficient Th2 cells produce higher levels of Th2 cytokines, including IL-5, IL-4, and IL-13, than that in wild-type cells. Moreover, SIGIRR-deficient mice developed stronger Th2 immune response in OVA-challenged asthma model. Taken together, our results suggest that SIGIRR plays an important role in the regulation of Th2 response in vivo, possibly through its impact on IL-33-ST2-mediated signaling.

The critical role of epithelial-derived Act1 in IL-17- and IL-25-mediated pulmonary inflammation

IL-25 initiates, promotes, and augments Th2 immune responses. In this study, we report that Act1, a key component in IL-17-mediated signaling, is an essential signaling molecule for IL-25 signaling. Although Act1-deficient mice showed reduced expression of KC (CXCL1) and neutrophil recruitment to the airway compared with wild-type mice in response to IL-17 stimulation, Act1 deficiency abolished IL-25-induced expression of IL-4, IL-5, IL-13, eotaxin-1 (CCL11), and pulmonary eosinophilia. Using a mouse model of allergic pulmonary inflammation, we observed diminished Th2 responses and lung inflammation in Act1-deficient mice compared with wild-type mice. Importantly, Act1 deficiency in epithelial cells reduced the phenotype of allergic pulmonary inflammation due to loss of IL-17-induced neutrophilia and IL-25-induced eosinophilia, respectively. These results demonstrate the essential role of epithelial-derived Act1 in allergic pulmonary inflammation through the distinct impact of the IL-17R-Act1 and IL-25R-Act1 axes. Such findings are crucial for the understanding of pathobiology of atopic diseases, including allergic asthma, which identifies Act1 as a potential therapeutic target.

Interleukin-1 receptor-associated kinase 2 is critical for lipopolysaccharide-mediated post-transcriptional control

IRAK2, a member of the interleukin-1 receptor-associated kinase (IRAK) family, has been implicated in Toll-like receptor (TLR)-mediated signaling. We generated IRAK2-deficient mice to examine its function in detail. These mice are resistant to lipopolysaccharide-induced septic shock, because of impaired TLR4-mediated induction of pro-inflammatory cytokines and chemokines. Although IRAK2 deficiency did not affect TLR4-mediated NFkappaB activation, a reduction of lipopolysaccharide (LPS)-mediated mRNA stabilization contributed to the reduced cytokine and chemokine production observed in bone marrow-derived macrophages from IRAK2-deficient mice. Furthermore, the ratios of LPS-induced cytokine and chemokine mRNAs in translation-active (polysomal) versus translation-inactive (free ribosomes) pools were reduced in IRAK2-deficient macrophages compared with wild type macrophages. Importantly, LPS-induced phosphorylation of MKK3/6, MNK1, and eIF4E was significantly reduced in IRAK2-deficient macrophages compared with wild type macrophages. Moreover, LPS stimulation induced an interaction of IRAK2 with TRAF6, MKK3/6, and MK2, implicating a critical role for mitogen-activated protein kinase signaling in LPS-induced IRAK2-mediated post-transcriptional control. These results reveal that IRAK2 is required for LPS-mediated post-transcriptional control of cytokine and chemokine expression, which plays an essential role in TLR4-induced septic shock.

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.

The Toll-interleukin-1 receptor member SIGIRR regulates colonic epithelial homeostasis, inflammation, and tumorigenesis

Despite constant contact with the large population of commensal bacteria, the colonic mucosa is normally hyporesponsive to these potentially proinflammatory signals. Here we report that the single immunoglobulin IL-1 receptor-related molecule (SIGIRR), a negative regulator for Toll-IL-1R signaling, plays a critical role in gut homeostasis, intestinal inflammation, and colitis-associated tumorigenesis by maintaining the microbial tolerance of the colonic epithelium. SIGIRR-deficient (Sigirr(-/-)) colonic epithelial cells displayed commensal bacteria-dependent homeostatic defects, as shown by constitutive upregulation of inflammatory genes, increased inflammatory responses to dextran sulfate sodium (DSS) challenge, and increased Azoxymethane (AOM)+DSS-induced colitis-associated tumorigenesis. Gut epithelium-specific expression of the SIGIRR transgene in the SIGIRR-deficient background reduced the cell survival of the SIGIRR-deficient colon epithelium, abrogated the hypersensitivity of the Sigirr(-/-) mice to DSS-induced colitis, and reduced AOM+DSS-induced tumorigenesis. Taken together, our results indicate that epithelium-derived SIGIRR is critical in controlling the homeostasis and innate immune responses of the colon to enteric microflora.

In situ detection of DNA and mRNA of human cytomegalovirus to distinguish different forms of viral infection in leukocytes

In situ PCR and in situ reverse transcription PCR (RT-PCR) were applied to discriminate between latent and productive infection of human cytomegalovirus (HCMV) in leukocytes. We investigated 28 samples, in which viral pp65 antigen was detected only in the cytoplasm of leukocytes. Additionally we assayed 12 specimens lacking pp65 antigen. Using nested PCR (nPCR), viral DNA was detected in 27 samples. In six samples the results of nPCR were unreadable due to the presence of polymerase inhibitors. By application of in situ PCR, we were able to confirm the presence of viral DNA in the nucleus and/or cytoplasm. Productive infection was recognized in 20 samples in which transcripts for late viral genes were detected. Among the 20 samples negative by in situ RT-PCR, we recognized phagocytosis of viral particles in eight and the latent form of HCMV infection in five.

In situ detection of DNA and mRNA of human cytomegalovirus to distinguish different forms of viral infection in leukocytes

In situ PCR and in situ reverse transcription PCR (RT-PCR) were applied to discriminate between latent and productive infection of human cytomegalovirus (HCMV) in leukocytes. We investigated 28 samples, in which viral pp65 antigen was detected only in the cytoplasm of leukocytes. Additionally we assayed 12 specimens lacking pp65 antigen. Using nested PCR (nPCR), viral DNA was detected in 27 samples. In six samples the results of nPCR were unreadable due to the presence of polymerase inhibitors. By application of in situ PCR, we were able to confirm the presence of viral DNA in the nucleus and/or cytoplasm. Productive infection was recognized in 20 samples in which transcripts for late viral genes were detected. Among the 20 samples negative by in situ RT-PCR, we recognized phagocytosis of viral particles in eight and the latent form of HCMV infection in five.

In situ RT-PCR can distinguish between productive and latent cytomegalovirus infection in the blood cells of bone marrow transplant recipients

Thirty-four peripheral blood leukocyte samples from bone marrow transplant (BMT) recipients were examined for Human cytomegalovirus (HCMV) phosphoprotein 65 (pp65), DNA and late transcripts. Twenty seven samples were positive for pp65 in the cytoplasm by immunofluorescent assay (IFA). Viral DNA was confirmed in 26 samples by nested PCR (nPCR). Using in situ RT-PCR, viral late transcripts were found in 19 samples, positive also by IFA and nPCR; these samples were considered indicative of productive viral infection. Five samples, positive by nPCR but negative by IFA and in situ RT-PCR, were considered to represent latent viral infection. In 8 samples, positive by IFA and nPCR but negative by in situ RT-PCR, apparently phagocytosis of viral particles took place.