The circular RNA circBCL2L1 regulates innate immune responses via microRNA-mediated downregulation of TRAF6 in teleost fish

Growing numbers of studies have shown that circular RNAs (circRNAs) can function as regulatory factors to regulate the innate immune response, cell proliferation, cell migration, and other important processes in mammals. However, the function and regulatory mechanism of circRNAs in lower vertebrates are still unclear. Here, we discovered a novel circRNA derived from the gene encoding Bcl-2-like protein 1 (BCL2L1) gene, named circBCL2L1, which was related to the innate immune responses in teleost fish. Results indicated that circBCL2L1 played essential roles in host antiviral immunity and antibacterial immunity. Our study also identified a microRNA, miR-30c-3-3p, which could inhibit the innate immune response by targeting inflammatory mediator TRAF6. And TRAF6 is a key signal transduction factor in innate immune response mediated by TLRs. Moreover, we also found that the antiviral and antibacterial effects inhibited by miR-30c-3-3p could be reversed with the expression of circBCL2L1. Our data revealed that circBCL2L1 functioned as a competing endogenous RNA (ceRNA) of TRAF6 by competing for binding with miR-30c-3-3p, leading to activation of the NF-κB/IRF3 inflammatory pathway and then enhancing the innate immune responses. Our results suggest that circRNAs can play an important role in the innate immune response of teleost fish.

Rutin inhibited the advanced glycation end products-stimulated inflammatory response and extra-cellular matrix degeneration via targeting TRAF-6 and BCL-2 proteins in mouse model of osteoarthritis

BACKGROUND

Osteoarthritis (OA) is degenerative joint disorder mainly characterized by long-term pain with limited activity of joints, the disease has no effective preventative therapy. Rutin (RUT) is a flavonoid compound, present naturally. The flavonoid shows range of biological activities such as anti-inflammatory and anti-cancer effect. We screened RUT for its activity against osteoarthritis with in vivo and in vitro models of osteoarthritis.

METHODS

Animal model of OA was developed using C57BL/6 mice by surgical destabilization of medial meniscus. For in vitro studies the human articular cartilage tissues were used which were collected from osteoarthritis patients and were processed to isolate chondrocytes. The chondrocytes were submitted to advanced glycation end products (AGEs) for inducing osteoarthritis in vitro. Cell viability was done by CCK-8 assay, ELISA analysis for MMP13, collage II, PGE2, IL-6, TNF-α, ADAMTS-5 and MMP-13. Western blot analysis was done for expression of proteins and in silico analysis was done by docking studies.

RESULTS

Pretreatment of RT showed no cytotoxic effect and also ameliorated the AGE mediated inflammatory reaction on human chondrocytes in vitro. Treatment of RT inhibited the levels of COX-2 and iNOS in AGE exposed chondrocytes. RT decreased the AGE mediated up-regulation of IL-6, NO, TNF-α and PGE-2 in a dose dependent manner. Pretreatment of RT decreased the extracellular matrix degradation, inhibited expression of TRAF-6 and BCL-2 the NF-κB/MAPK pathway proteins. The treatment of RT in mice prevented the calcification of cartilage tissues, loss of proteoglycans and also halted the narrowing of joint space is mice subjected to osteoarthritis. The in-silico analysis suggested potential binding affinity of RT with TRAF-6 and BCL-2.

CONCLUSION

In brief RT inhibited AGE-induced inflammatory reaction and also degradation of ECM via targeting the NF-κB/MAPK pathway proteins BCL-2 and TRAF-6. RT can be a potential molecule in treating OA.

microRNA-489 negatively modulates RIG-I signaling pathway via targeting TRAF6 in miiuy croaker after poly(I:C) stimulation

The innate immune response is first line of host defense against pathogen invasion. However, excessive activation of immune responses may cause autoimmune diseases and excessive inflammation. Retinoic acid-inducible gene I (RIG-I) is an important cytoplasmic pathogen recognition receptor that is activated on virus infection. TNF-receptor-associated factor 6 (TRAF6) plays an essential role in the RIG-I-mediated signaling pathway. MicroRNAs (miRNAs) are noncoding RNAs that are emerging as important regulators of immune responses. In this study, we found that the overexpression of miR-489 mimics and pre-miR-489 significantly suppressed the luciferase activity of the wild-type TRAF6 3'UTR, whereas mutant-type led to a complete abrogation of the negative effect. In addition, we also observed that miR-489 can negatively regulate TRAF6 at the level of translation. More importantly, we demonstrated that miR-489 is a negative regulator of TRAF6 involved in the immune response to poly(I:C) stimulation. These common findings indicated that miR-489 plays a regulatory role in host-virus interactions by targeting TRAF6. Overall, all of the present results provide direct evidence that miR-489 is involved in the regulation of TRAF6 expression in miiuy croaker, which will help to better understand the complex regulatory networks of teleost fish.

TRAF6 Regulates the Immunosuppressive Effects of Myeloid-Derived Suppressor Cells in Tumor-Bearing Host

Myeloid-derived suppressor cells (MDSCs) are immature heterogeneous cells derived from the bone marrow and they are the major component of the tumor-induced immunosuppressive environment. Tumor necrosis factor receptor-associated factor 6 (TRAF6), an E3 ubiquitin ligase, catalyzes the polyubiquitination of target proteins. TRAF6 plays a critical role in modulating the immune system. However, whether TRAF6 is involved in the regulation of MDSCs has not been thoroughly elucidated to date. In this study, we found that the expression of TRAF6 in MDSCs derived from tumor tissue was significantly upregulated compared with that of MDSCs from spleen of tumor-bearing mice. Knockdown of TRAF6 remarkably attenuated the immunosuppressive effects of MDSCs. Mechanistically, TRAF6 might improve the immunosuppression of MDSCs by mediating K63-linked polyubiquitination and phosphorylation of signal transducer and activator of transcription 3 (STAT3). Additionally, it was discovered that the accumulation of MDSCs was abnormal in peripheral blood of lung cancer patients. TRAF6 and arginase 1 were highly expressed in MDSCs of patients with lung cancer. Taken together, our study demonstrated that TRAF6 participates in promoting the immunosuppressive function of MDSCs and provided a potential target for antitumor immunotherapy.

Molecular characterization and functional analysis of TRAF6 in the spotted sea bass (Lateolabrax maculatus)

Tumor necrosis factor receptor-associated factor 6 (TRAF6) is a crucial adapter protein in the toll-like receptor signaling pathway that triggers downstream molecules involved in innate immunity. Although TRAF6 has been well studied in mammals, the molecular information and function of TRAF6 in fish is still limited. Here, we identified and analyzed a TRAF6 homolog (LmTRAF6) from the spotted sea bass (Lateolabrax maculatus). Similar to its counterparts in mammals and other fish species, LmTRAF6 shares the domain topology containing one N-terminal RING, two TRAF-type zinc fingers, a coiled-coil region and a C-terminal MATH domain. Despite a sequence similarity of 60% with mammalian TRAF6s, LmTRAF6 shares higher similarities with teleost homologs (~68%-93%). The coding region of LmTRAF6 gene contains seven exons and six introns, which is consistent to the genetic organization in grouper and rock bream, but not in zebrafish, common carp and tetrapods (the sixth intron was lost resulting in a combined exon). Quantitative real-time polymerase chain reaction analysis revealed that LmTRAF6 transcripts were ubiquitously expressed in all tested tissues and upregulated after Vibrio. harveyi and S. agalactiae infection. LmTRAF6 could assist HEK293T cells to survive by inhibiting apoptosis under both V. harveyi and S. agalactiae stimulation. Intracellular localization showed that LmTRAF6 was localized mainly in the cytoplasm. Overexpression of wild-type (WT) LmTRAF6 and the truncated form of △MATH increased the ability of NF-κB in HEK293T cells, whereas truncations, including the △RING and △coiled-coil domain, did not significantly activate NF-κB, indicating that the RING finger and coiled-coil domain play crucial roles in downstream signal transduction. In addition, overexpression of LmTRAF6-WT significantly increased the activation of NF-κB in HEK293T cells under V. harveyi and S. agalactiae stimulation. These results suggest that LmTRAF6 activates NF-κB and plays a potential role in the immune defense system against bacterial infection.

Molecular characterization and expression analysis of tumor necrosis factor receptor-associated factor 6 (traf6) like gene involved in antibacterial innate immune of fresh water crayfish, Procambarus clarkii

In invertebrates, innate immunity was the crucial defending pattern against pathogenic microorganisms. For the past few years, Toll or Toll like receptors (TLRs) signaling pathway was studied extensively in crustaceans. Among the components of Toll or Toll like receptors (TLRs) signaling pathway, tumor necrosis factor receptor-associated factor 6 (TRAF6) acted as an important cytoplasmic adaptor, which was conserved from Drosophila to human. In this study, a new traf6 like gene was cloned from hepatopancreas of P. clarkii. After challenged respectively by S. aureus or E. ictaluri, the expression profiles were studied. And the results showed that the mRNA transcript of Pc-traf6 like gene was up-regulated significantly in the hemocytes, hepatopancreas, gills, and intestine of crayfish. After Pc-traf6 like gene was knocked down, the expression levels of transcription factor (Dorsal) and some crucial immunity effectors (ALF 3, Lysozyme 1, Lectin 1, and Crustin 2) in TLRs signaling pathway were dramatically suppressed. Simultaneously, the survival rate of crayfish challenged respectively by S. aureus or E. ictaluri was significantly decreased in RNAi assay. All these results indicated that Pc-traf6 like gene played an important role in regulating the expression of downstream effectors in the TLRs signaling pathway of crayfish.

B Cell Endosomal RAB7 Promotes TRAF6 K63 Polyubiquitination and NF-κB Activation for Antibody Class-Switching

Upon activation by CD40 or TLR signaling, B lymphocytes activate NF-κB to induce activation-induced cytidine deaminase and, therefore, Ig class switch DNA recombination, as central to the maturation of the Ab and autoantibody responses. In this study, we show that NF-κB activation is boosted by colocalization of engaged immune receptors, such as CD40, with RAB7 small GTPase on mature endosomes, in addition to signals emanating from the receptors localized on the plasma membrane, in mouse B cells. In mature endosomes, RAB7 directly interacts with TRAF6 E3 ubiquitin ligase, which catalyzes K63 polyubiquitination for NF-κB activation. RAB7 overexpression in Cd19+/creRosa26fl-STOP-fl-Rab7 mouse B cells upregulates K63 polyubiquitination activity of TRAF6, enhances NF-κB activation and activation-induced cytidine deaminase induction, and boosts IgG Ab and autoantibody levels. This, together with the extensive intracellular localization of CD40 and the strong correlation of RAB7 expression with NF-κB activation in mouse lupus B cells, shows that RAB7 is an integral component of the B cell NF-κB activation machinery, likely through interaction with TRAF6 for the assembly of "intracellular membrane signalosomes."

TRAF6 suppresses the apoptosis of hemocytes by activating pellino in Crassostrea hongkongensis

Tumor necrosis factor receptor-associated factor 6 (TRAF6), an E3 ubiquitin ligase, participates in both innate and adaptive immunity and regulates the apoptotic process. In this study, we observed that an ortholog of TRAF6 could inhibit the activity of p53 and suppress the apoptotic process in the Hong Kong oyster, Crassostrea hongkongensis. To investigate the possible molecular mechanism of the ChTRAF6-induced antiapoptotic effect, a GST pull-down screening assay was conducted, and ChPellino was found to physically interact with ChTRAF6. In addition, the interaction between them was confirmed by Co-immunoprecipitation. Furthermore, western blotting revealed that the phosphorylation level of ChPellino was decreased after the RNAi of ChTRAF6, demonstrating that ChTRAF6 may be an upstream regulator of Pellino activation. Furthermore, the apoptosis level of hemocytes increased after ChPellino knockdown, and ChPellino overexpression suppressed ChTRAF6-dependent p53 activation. Taken together, these results indicate that ChPellino plays a critical role in suppressing ChTRAF6-dependent anti-apoptosis in the hemocytes of Crassostrea hongkongensis.

Molecular cloning and functional analysis of TRAF6 from Yangzhou great white goose Anser anser

TNF receptor-associated factor 6 (TRAF6) is an adaptor protein and an E3 ubiquitin ligase mediating multiple cell signaling pathway activation in a context-dependent manner. TRAF6 plays critical roles in innate immune response and regulates function of antigen-presenting cells. Here, we cloned the goose TRAF6 (goTRAF6) gene from a healthy Yangzhou great white goose (Anser anser), which had a typical TRAF structure and shared a high-sequence identity with TRAF6 of other birds. Quantitative real-time PCR revealed that goTRAF6 mRNA was broadly expressed in all the studied tissues, with highest expression in the heart and pectoral muscle. Overexpression of goTRAF6 caused NF-κB activation in a dose-dependent manner and substantially upregulated IFN-β expression in HEK293T cells. Following Toll-like receptor (TLR) ligand stimulation of goose peripheral blood mononuclear cells, goTRAF6 and downstream inflammatory cytokine mRNA levels considerably up-regulated, especially at early stages. Salmonella Enteritidis challenge caused overexpression of goTRAF6 and cytokine mRNA in all the examined organs. These findings demonstrated that goTRAF6 played a substantial role in TLR-TRAF6 signaling cascade, and further contributed to the antibacterial-responses in host.

Total glucosides of paeony improve complete freund's adjuvant-induced rheumatoid arthritis in rats by inhibiting toll-like receptor 2-mediated tumor necrosis factor receptor-associated factor 6/ nuclear factor-kappa B pathway activation

OBJECTIVE

To investigate the mechanism underlying anti-inflammatory and immunoregulatory effect of total glucosides of paeony (TGP) based on toll-like receptor 2 (TLR2) mediated tumor necrosis factor (TNF) receptor-associated factor 6 (TRAF6)/nuclear factor-kappa B (NF-κB) pathway activation in rats with rheumatoid arthritis.

METHODS

Adjuvant arthritis (AA) model was developed by complete freund's adjuvant (CFA) immunization. TGP (100, 50, 25 mg/kg) and celecoxib (2.8 mg/kg) were administered by intragastric administration for 21 d. Right hind paw swelling was assessed every 2 d. After 21 d, synovial changes of the ankle were detected by histopathology. CD4+ and CD8+ T cell amounts in peripheral blood were measured by flow-cytometrically. Gene and protein levels of toll-like receptor (TLR)2, TRAF6, tumor necrosis factor ligand superfamily member 6 (FASLG) in the spleen were assessed by RT-qPCR and Western Bolt, respectively. Nuclear expression of NF-κB p65 was detected by NF-κB p65 Assay Kit.

RESULTS

Paw swelling and synovium lesions were obviously aggravated in AA rats. These symptoms were significantly relieved by TGP. The ratio of CD4+/CD8+ T cell was increased in AA rats, while TGP reduced this increased ratio. Gene and protein levels of splenic TLR2, TFAR6 and FASLG, and nuclear NF-κB p65 in AA rats were significantly increased, but overtly inhibited by TGP.

CONCLUSION

These findings suggest that TGP's anti-inflammatory effect onRA in rats with CFA may be related to the downregulation of TLR2/TRAF6/NF-κB pathway and the regulation of T cell subsets.

Molecular cloning and expression analysis of MyD88 and TRAF6 in Qihe crucian carp Carassius auratus

Myeloid differentiation factor 88 (MyD88) and tumor necrosis factor receptor-associated factor 6 (TRAF6) are two critical signal transducers in toll-like receptor (TLR) pathway. In the present study, we identified and characterized the homologues of MyD88 and TRAF6 in Qihe crucian carp Carassius auratus, termed as CaMyD88 and CaTRAF6, respectively, and examined their roles during pathogenic infection. Full-length cDNA of CaMyD88 was 2463 bp, including a 191 bp 5'-untranslated region (UTR), a 1417 bp 3'-UTR, and an 855 bp open reading frame (ORF) encoding for a putative protein with 284 amino acids. Full-length cDNA of CaTRAF6 was identified to be 2555 bp, consisting of a 52 bp 5'-UTR, an 871 bp 3'-UTR, and a 1632 bp ORF encoding a protein of 543 amino acids. Deduced amino acid sequences of CaMyD88 and CaTRAF6 contained the typical domains (CaMyD88: death domain and TIR domain; CaTRAF6: one RING-type zinc finger domain, two TRAF-type zinc finger domains, one coiled-coil region, and one conserved C-terminal meprin and TRAF homology domain) as in other fish. Quantitative Real-Time PCR (qRT-PCR) analysis revealed that both CaMyD88 and CaTRAF6 were ubiquitously expressed throughout the development stages and appeared to be developmentally regulated. In addition, CaMyD88 and CaTRAF6 had a broadly distribution of expression in all examined eleven tissues of healthy fish, although the transcript levels varied among the different tissues. Moreover, it was found that mRNA expressions of CaMyD88 and CaTRAF6 were generally up-regulated after stimulation by polyI:C, flagellin, and Aeromonas hydrophila in spite of the down-regulation appeared at some time points or tissues. These results indicated that CaMyD88 and CaTRAF6 play the critical roles in the immune defense of Qihe crucian carp against pathogenic invasion. The present findings will provide the valuable information for understanding the innate immune responses of Qihe crucian carp and contribute to develop the preventive way against pathogens.

Effect of high fat diet on NF-кB microRNA146a negative feedback loop in ovalbumin-sensitized rats

The present study aimed to investigate the role of microRNA-146a and its adapter proteins [interleukin-1 receptor-associated kinase 1 (IRAK1) and tumor necrosis factor (TNF) receptor-associated factor 6 (TRAF6)] in the pathogenesis of ovalbumin (OVA)-sensitized rats in association with the diet-induced obesity condition. Twenty male Wistar rats were divided into four groups: control with normal diet (ND), OVA-sensitized with normal diet (S + ND), high-fat diet (HFD), and OVA-sensitized with high-fat diet (S + HFD). All the animals were fed for 8 weeks with standard pelts or high-fat diet, and were then sensitized and challenged with OVA or saline for another 4 weeks. The tracheal responsiveness to methacholine, serum protein levels, and lipid profile levels was measured by the ELISA method. Moreover, the gene expression level of microRNA-146a (miR-146a) was measured in the lung tissue of the rats using the real-time PCR method. Maximum response to methacholin increased in the S + HFD group in compared with ND, S + ND, and HFD groups (P < 0.05 to P < 0.001). Moreover, in the S + HFD group the mRNA expression levels of miRNA-146a increased in the lung tissue (P < 0.001). In addition, the protein analysis results showed that IRAK1, TRAF6, NF-kB, and IL-1β protein levels were high in the S + HFD group compared to the ND and HFD groups; however, in compared with the S + ND group, only the IL-1β protein level was higher in the S + HFD group (P < 0.05). These results suggest that a defect in the NF-kB-miR-146a negative feedback loop may be involved in the pathogenesis of obesity associated with OVA-sensitized condition. © 2018 BioFactors, 45(1):75-84, 2019.

Molecular cloning and functional characterization of TRAF6 and TAK1 in rainbow trout, Oncorhynchus mykiss

TRAF6 and TAK1 are known to play important roles in vertebrate innate immunity as molecular bridge, linking upstream toll-like receptors (TLRs) with the downstream MAPK and NF-κB signalling pathways. However, their roles in TLR signalling pathway have yet to be fully described in fish. Here we identified genes encoding TRAF6 (OmTRAF6) and TAK1 (OmTAK1) from rainbow trout, Oncorhynchus mykiss, and examined their roles during pathogenic infections. The deduced amino acid sequences of OmTRAF6 and OmTAK1 contained the characteristic domains conserved in the TRAF and TAK1 families, respectively (OmTRAF6: RING, two TRAF-type zinc fingers, CCR and MATH domains; OmTAK1: STKc and CCR domains). In RTH-149 cells, the expression of OmTRAF6 and OmTAK1 was increased by stimulation with Edwardsiella tarda and LPS. Silencing of OmTRAF6 and OmTAK1 in RTH-149 cells negatively regulated the LPS-induced phosphorylation of p38 MAPK and JNK. TAK1 inhibitor (5z)-7-Oxozeaenol significantly decreased the LPS-induced activation of NF-κB in RTH-149 cells. In addition, silencing of OmTRAF6 and OmTAK1 significantly decreased the expression of MAPKs and NF-κB downstream target genes induced by LPS in RTH-149 cells. These findings suggest that OmTRAF6 and OmTAK1 might function like those of mammals to regulate bacteria-triggered signalling pathway in fish.

Molecular characterization and expression analysis of tumor necrosis factor receptor-associated factors 3 and 6 in large yellow croaker (Larimichthys crocea)

The large yellow croaker (Larimichthys crocea) has a well-developed innate immune system. To gain a better understanding of the defense mechanisms involved in this system, we studied tumor necrosis factor receptor-associated factors (TRAFs), which play important roles in the Toll-like receptor (TLR) pathway. We characterized the full-length open reading frames and protein structures of TRAF3 and TRAF6 to determine their identities, and conducted phylogenetic analysis to determine their evolutionary relationships. To assess the roles of TRAFs in innate immune responses in the large yellow croaker, we performed quantitative reverse-transcription PCR (qRT-PCR) to characterize expression profiles in a range of tissues at different stages after challenge with polyinosinic polycytidylic acid (poly I:C) and Vibrio anguillarum. Following poly I:C challenge, the expression levels of TRAF3 and TRAF6 were highest in the kidneys and lowest in the spleen, whereas after infection with V. anguillarum, TRAF6 expression was the highest in the kidneys and lowest in the liver.

Activation of RIG-I-Mediated Antiviral Signaling Triggers Autophagy Through the MAVS-TRAF6-Beclin-1 Signaling Axis

Autophagy has been implicated in innate immune responses against various intracellular pathogens. Recent studies have reported that autophagy can be triggered by pathogen recognizing sensors, including Toll-like receptors and cyclic guanosine monophosphate-adenosine monophosphate synthase, to participate in innate immunity. In the present study, we examined whether the RIG-I signaling pathway, which detects viral infections by recognizing viral RNA, triggers the autophagic process. The introduction of polyI:C into the cytoplasm, or Sendai virus infection, significantly induced autophagy in normal cells but not in RIG-I-deficient cells. PolyI:C transfection or Sendai virus infection induced autophagy in the cells lacking type-I interferon signaling. This demonstrated that the effect was not due to interferon signaling. RIG-I-mediated autophagy diminished by the deficiency of mitochondrial antiviral signaling protein (MAVS) or tumor necrosis factor receptor-associated factor (TRAF)6, showing that the RIG-I-MAVS-TRAF6 signaling axis was critical for RIG-I-mediated autophagy. We also found that Beclin-1 was translocated to the mitochondria, and it interacted with TRAF6 upon RIG-I activation. Furthermore, Beclin-1 underwent K63-polyubiquitination upon RIG-I activation, and the ubiquitination decreased in TRAF6-deficient cells. This suggests that the RIG-I-MAVS-TRAF6 axis induced K63-linked polyubiquitination of Beclin-1, which has been implicated in triggering autophagy. As deficient autophagy increases the type-I interferon response, the induction of autophagy by the RIG-I pathway might also contribute to preventing an excessive interferon response as a negative-feedback mechanism.

Identification of tumor necrosis factor receptor-associated factor 6 in the pearl mussel Hyriopsis cumingii and its involvement in innate immunity and pearl sac formation

Tumor necrosis factor receptor-associated factor 6 (TRAF6) acts as a central intracellular signal adapter molecule that mediates the tumor necrosis factor receptor superfamily and the interleukin-1 receptor/Toll-like receptor family in vertebrates and invertebrates. In the present study, HcTRAF6, a molluscan homologue of TRAF6 from Hyriopsis cumingii, has been cloned and identified. The entire open reading frame of HcTRAF6 was found to comprise a 1965-bp region that encodes a predicted protein of 654 amino acids, which contains conserved characteristic domains including a RING domain, two TRAF-type zinc finger domains, a typical coiled coil and the MATH domain. Phylogenetic analysis revealed that HcTRAF6 was aggregated closely with CsTRAF6 from Cyclina sinensis in the invertebrate cluster of mollusks. Further, qRT-PCR analysis showed that HcTRAF6 mRNA was extensively distributed in mussel tissues with a high expression in gills. After immune stimulation with Aeromonas hydrophila and lipopolysaccharides, the transcription of HcTRAF6 was obviously induced in the gills and hemocytes. In addition, significant fluctuation in HcTRAF6 expression was observed in the pearl sac, gills and hemocytes after mantle implantation. These findings confirmed its role in the alloimmune response. Dual-luciferase reporter assay showed that over-expression of HcTRAF6 could enhance the activity of the NF-κB reporter in a dose-dependent manner. Further, the RNA interference showed that the up-regulation of antimicrobial peptides in anti-bacterial infection was strongly suppressed in HcTRAF6-silenced mussels and that depletion of HcTRAF inhibited the elimination of A. hydrophila. All these findings together prove that HcTRAF6 functions as an efficient regulator in innate immune mechanisms against invading pathogens and the alloimmune mechanism after mantle implantation in H. cumingii.

Biological characterization, expression, and functional analysis of tumor necrosis factor receptor-associated factor 6 in Nile tilapia (Oreochromis niloticus)

Nile tilapia (Oreochromis niloticus) is a pivotal economic fish that has been plagued by Streptococcus agalactiae infections for many years. Tumor necrosis factor receptor-associated factor 6 (TRAF6) is a crucial adaptor molecule of the interleukin-1 receptor/Toll-like receptor (IL-1/TLR) superfamily, which can trigger downstream signaling cascades involved in innate immunity. In this study, the full-length cDNA of TRAF6 was cloned from O. niloticus (named On-TRAF6), which has an open reading frame of 1716 bp, and encode a polypeptide of 571 amino acids. The predicted amino acid sequence of On-TRAF6 contained the characteristic motifs of TRAF proteins, including a Zinc finger of RING-type, two Zinc fingers of TRAF-type, and a MATH (meprin and TRAF homology) domain. Multiple sequence alignment revealed that On-TRAF6 shares a relatively high level of identity with those of other fishes (64-98%). In healthy tilapia, mRNA expression of On-TRAF6 could be detected in all the examined tissues and the highest expression occurred in the spleen. Moreover, we found that On-TRAF6 was involved immune response of Nile tilapia following the stimulation with Streptococcus agalactiae and polyinosinic: polycytidylic acid (Poly I:C) when determined by using qPCR. The result of subcellular localization showed that On-TRAF6 distributed in the cytoplasm, and over-expression of On-TRAF6 could strongly activated NF-кB pathway in HEK293T cells. These findings indicated that On-TRAF6 may play an important role in the immune response to intracellular bacteria in Nile tilapia.

Molecular cloning and functional analysis of tumor necrosis factor receptor-associated factor 6 (TRAF6) in thick shell mussel, Mytilus coruscus

Tumor necrosis factor receptor-associated factor 6 (TRAF6) is one of the key adapter molecules in Toll-like receptor signal transduction that triggers downstream cascades involved in innate immunity. Despite of the well study in vertebrates, there is few data ascribe to this TRAF member in invertebrates, especially in bivalves. In the present study, a novel TRAF6 homologue termed McTRAF6 was firstly characterized in Mytilus coruscus. Like its counterparts in mammals, McTRAF6 shared the domain topology containing one RING domain, two zinc finger domains, one coiled-coil region and a MATH domain. McTRAF6 transcripts predominantly expressed in gills, digestive glands and hemocytes in M. coruscus, and were significantly up-regulated in hemocytes after challenge with lipopolysaccharide (LPS) and polyinosine-polycytidylic acid (poly I:C). Further, the subcellular localization in cytoplasm and the activation of Nk-κB or ISRE luciferase reporter by overexpressed McTRAF6 were identified in HEK293T cells. These results collectively indicate that McTRAF6 is a member of TRAF6 subfamily and plays a potential role in immune defense system against pathogenic agents invasions in thick shell mussel. To our knowledge, this is the first report on component of TLR signaling pathway in thick shell mussel, providing further evidence for the existence of TLR pathway in M. coruscus and contribute to clarify the innate immune system of thick shell mussel.

Th9 Cells Represent a Unique Subset of CD4+ T Cells Endowed with the Ability to Eradicate Advanced Tumors

The antitumor effector T helper 1 (Th1) and Th17 cells represent two T cell paradigms: short-lived cytolytic Th1 cells and "stem cell-like" memory Th17 cells. We report that Th9 cells represent a third paradigm-they are less-exhausted, fully cytolytic, and hyperproliferative. Only tumor-specific Th9 cells completely eradicated advanced tumors, maintained a mature effector cell signature with cytolytic activity as strong as Th1 cells, and persisted as long as Th17 cells in vivo. Th9 cells displayed a unique Pu.1-Traf6-NF-κB activation-driven hyperproliferative feature, suggesting a persistence mechanism rather than an antiapoptotic strategy. Th9 antitumor efficacy depended on interleukin-9 and upregulated expression of Eomes and Traf6. Thus, tumor-specific Th9 cells are a more effective CD4+ T cell subset for adoptive cancer therapy.

sNASP inhibits TLR signaling to regulate immune response in sepsis

Many Toll-like receptors (TLRs) signal through TNF receptor-associated factor 6 (TRAF6) to activate innate immune responses. Here, we show that somatic nuclear autoantigenic sperm protein (sNASP) binds to TRAF6 to prevent TRAF6 autoubiquitination in unstimulated macrophages. Following LPS stimulation, a complex consisting of sNASP, TRAF6, IRAK4, and casein kinase 2 (CK2) is formed. CK2 phosphorylates sNASP at serine 158, allowing sNASP to dissociate from TRAF6. Free TRAF6 is then autoubiquitinated, followed by activation of downstream signaling pathways. In sNasp S158A knockin (S158A-KI) mice, LPS-treated macrophages could not phosphorylate sNASP, which remained bound to TRAF6. S158A-KI mice were more susceptible to sepsis due to a marked reduction in IL-1β, TNF-α, and IFN-γ production accompanied by an inability to clear bacteria and recruit leukocytes. Furthermore, phosphorylation-regulated release of sNASP from TRAF6 is observed following activation of TLR-1, -2, -4, -5, and -6. Thus, sNASP is a negative regulator of TLR signaling to modulate the innate immune response.

Molecular cloning, characterization and function of a germinal center kinase MST4 gene from Litopenaeus vannamei in response to Vibrio alginolyticus challenge in TLR-TRAF6 signaling pathway

The serine/threonine protein kinase MST4 plays multiple roles in the regulation of signaling pathways that govern cellular processes including mitosis, migration, homeostasis, polarity, proliferation, differentiation and apoptosis. Here we report the identification and characterization of the full-length sequence of LvMST4 from the shrimp L. vannamei, and investigations into its role in the shrimp's immune response to infection by the pathogenic bacterium Vibrio alginolyticus. Subcellular localization assays demonstrated the enzyme's presence in the shrimp's cytoplasm, and tissue-specific expression analysis revealed that it is expressed ubiquitously but at different levels in different tissues. Infection with V. alginolyticus increased LvMST4 expression and induced a rapid response via the TLR-TRAF6 signaling pathway, causing a decline in the total hemocyte count (THC) and an increase in respiratory burst (RB) activity. In non-infected shrimp, RNAi silencing of LvMST4 with dsRNA had no significant effect on THC but seemed to activate the TRAF6-MKK6-p38 pathway and reduced RB activity. In shrimp challenged with V. alginolyticus, LvMST4 silencing reduced bacterial clearance and increased the initial upregulation of LvTRAF6 while reducing the expression of LvMKK6 and Lvp38. LvMST4 silencing also slightly reduced the THC but caused pronounced increases in RB activity and cumulative mortality. These findings suggest that LvMST4 contributes to antimicrobial responses via the TLR-TRAF6 signal pathway, and helps maintain immunological homeostasis in L. vannamei.

Characterization of the black carp TRAF6 signaling molecule in innate immune defense

Tumor necrosis factor receptor-associated factor 6 (TRAF6) plays a vital role in the innate immune response of higher vertebrates. To elucidate its function in teleost fish, TRAF6 homologue of black carp (Mylopharyngodon piceus) has been cloned and characterized in this study. Black carp TRAF6 (bcTRAF6) transcription in Mylopharyngodon piceus fin (MPF) cells was up-regulated in response to both poly (I:C) treatment and viral infection, but was suppressed by LPS stimulation. bcTRAF6 migrated around 72 KDa in immunoblot analysis and was identified as a cytosolic protein suggested to be associated with vesicles scattering in the cytoplasm. Reporter assay demonstrated that NF-κB instead of IFN was activated by bcTRAF6; and EPC cells expressing bcTRAF6 presented the same cytopathic effect (CPE) ratio to that of control cells. When co-expressed with bcMAVS, bcTRAF6 was redistributed and overlapped with the subcellular location of bcMAVS. It was interesting that bcMAVS mediated the IFN induction was up-regulated by low input of bcTRAF6 but down-regulated by high input of bcTRAF6. Taken together, the data generated in this paper supported the conclusion that bcTRAF6 associated with bcMAVS and was recruited into bcMAVS mediated signaling during host innate immune response.

ALPK1 controls TIFA/TRAF6-dependent innate immunity against heptose-1,7-bisphosphate of gram-negative bacteria

During infection by invasive bacteria, epithelial cells contribute to innate immunity via the local secretion of inflammatory cytokines. These are directly produced by infected cells or by uninfected bystanders via connexin-dependent cell-cell communication. However, the cellular pathways underlying this process remain largely unknown. Here we perform a genome-wide RNA interference screen and identify TIFA and TRAF6 as central players of Shigella flexneri and Salmonella typhimurium-induced interleukin-8 expression. We show that threonine 9 and the forkhead-associated domain of TIFA are necessary for the oligomerization of TIFA in both infected and bystander cells. Subsequently, this process triggers TRAF6 oligomerization and NF-κB activation. We demonstrate that TIFA/TRAF6-dependent cytokine expression is induced by the bacterial metabolite heptose-1,7-bisphosphate (HBP). In addition, we identify alpha-kinase 1 (ALPK1) as the critical kinase responsible for TIFA oligomerization and IL-8 expression in response to infection with S. flexneri and S. typhimurium but also to Neisseria meningitidis. Altogether, these results clearly show that ALPK1 is a master regulator of innate immunity against both invasive and extracellular gram-negative bacteria.

Epithelial cells line internal body cavities of multicellular organisms. They represent the first line of defense against various pathogens including bacteria and viruses. They can sense the presence of invasive pathogens and initiate the recruitment of immune cells to infected tissues via the local secretion of soluble factors, called chemokines. Although this phenomenon is essential for the development of an efficient immune response, the molecular mechanism underlying this process remains largely unknown. Here we demonstrate that the host proteins ALPK1, TIFA and TRAF6 act sequentially to activate the transcription factor NF-κB and regulate the production of chemokines in response to infection by the pathogens Shigella flexneri, Salmonella typhimurium and Neisseria meningitidis. In addition, we show that the production of chemokines is triggered after detection of the bacterial monosaccharide heptose-1,7-bisphosphate, found in gram-negative bacteria. In conclusion, our study uncovers a new molecular mechanism controlling inflammation during infection by gram-negative bacteria and identifies potential targets for treatments aiming at modulating inflammation during infection.

Lung Altered Expression of IL-1β mRNA and Its Signaling Pathway Molecules in Obese-asthmatic Male Wistar Rats

Epidemiological and clinical studies indicate a close relationship between obesity and asthma. Here, we determined the impact of diet-induced obesity on the expression levels of IL-1β, IRAK-1 and TRAF-6 mRNA as well as IL-1β protein level and pathological changes in male Wistar rat's lung after sensitization with ovalbumin (OVA). Twenty male Wistar rats divided into four groups, control with normal diet (C+ND), OVA-sensitized with normal diet (S+ND), control with high-fat diet (C+HFD), and OVA-sensitized with high-fat diet (S+HFD). All rats fed for 12 weeks with standard pellets or high-fat diet while sensitization and challenging with OVA or saline were done for groups in the last month. In the end of intervention, lung was isolated and tested for the expression levels of IL-1β, IRAK-1 and TRAF-6 mRNA with real time-PCR method, and pathological changes were determined. Diet-induced obesity groups showed increased weight, obesity indexes and lipid profiles The expression levels of IL-1β mRNA in OVA-sensitization groups (S+ND and S+HFD) showed a significant increase compared with other groups. Also in S+HFD group, expression level of IRAK-1 and TRAF-6 mRNA were markedly higher than other groups (p<0.001). The pathological changes were marked in sensitized groups compared to non-sensitized groups; with marked increase in obese sensitized rat. The results showed that high fat diet caused overexpression of IL-1β, IRAK-1 and TRAF-6 mRNA as well as IL-1β protein in an experimental model of asthma. Our results suggest that obese-asthmatic conditions may lead to the local production and activation of pro-inflammatory agents.

miR-146a ameliorates liver ischemia/reperfusion injury by suppressing IRAK1 and TRAF6

A critical role of the Toll-like receptor(TLR) and its downstream molecules, including IL-1 receptor-associated kinase 1(IRAK1) and tumor necrosis factor receptor- associated factor 6(TRAF6), in the pathogenesis of liver ischemia/reperfusion (I/R) injury has been documented. Recently a microRNA, miR-146a, was identified as a potent negative regulator of the TLR signaling pathway. In this study, we investigated the role of miR-146a to attenuate TLR signaling and liver I/R injury in vivo and in vitro. miR-146a was decreased in mice Kupffer cells following hepatic I/R, whereas IRAK1 and TRAF6 increased. Overexpression of miR-146a directly decreased IRAK1 and TRAF6 expression and attenuated the release of proinflammatory cytokines through the inactivation of NF-κB P65 in hypoxia/reoxygenation (H/R)-induced macrophages, RAW264.7 cells. Knockdown experiments demonstrated that IRAK1 and TRAF6 are two potential targets for reducing the release of proinflammatory cytokines. Moreover, co-culture assays indicated that miR-146a decreases the apoptosis of hepatocytes after H/R. In vivo administration of Ago-miR-146a, a stable version of miR-146a in vivo, protected against liver injury in mice after I/R via inactivation of the TLR signaling pathway. We conclude that miR-146a ameliorates liver ischemia/reperfusion injury in vivo and hypoxia/reoxygenation injury in vitro by directly suppressing IRAK1 and TRAF6.

MicroRNA-146a provides feedback regulation of lyme arthritis but not carditis during infection with Borrelia burgdorferi

MicroRNAs have been shown to be important regulators of inflammatory and immune responses and are implicated in several immune disorders including systemic lupus erythematosus and rheumatoid arthritis, but their role in Lyme borreliosis remains unknown. We performed a microarray screen for expression of miRNAs in joint tissue from three mouse strains infected with Borrelia burgdorferi. This screen identified upregulation of miR-146a, a key negative regulator of NF-κB signaling, in all three strains, suggesting it plays an important role in the in vivo response to B. burgdorferi. Infection of B6 miR-146a-/- mice with B. burgdorferi revealed a critical nonredundant role of miR-146a in modulating Lyme arthritis without compromising host immune response or heart inflammation. The impact of miR-146a was specifically localized to the joint, and did not impact lesion development or inflammation in the heart. Furthermore, B6 miR-146a-/- mice had elevated levels of NF-κB-regulated products in joint tissue and serum late in infection. Flow cytometry analysis of various lineages isolated from infected joint tissue of mice showed that myeloid cell infiltration was significantly greater in B6 miR-146a-/- mice, compared to B6, during B. burgdorferi infection. Using bone marrow-derived macrophages, we found that TRAF6, a known target of miR-146a involved in NF-κB activation, was dysregulated in resting and B. burgdorferi-stimulated B6 miR-146a-/- macrophages, and corresponded to elevated IL-1β, IL-6 and CXCL1 production. This dysregulated protein production was also observed in macrophages treated with IL-10 prior to B. burgdorferi stimulation. Peritoneal macrophages from B6 miR-146a-/- mice also showed enhanced phagocytosis of B. burgdorferi. Together, these data show that miR-146a-mediated regulation of TRAF6 and NF-κB, and downstream targets such as IL-1β, IL-6 and CXCL1, are critical for modulation of Lyme arthritis during chronic infection with B. burgdorferi.

Grass carp (Ctenopharyngodon idella) TRAF6 and TAK1: molecular cloning and expression analysis after Ichthyophthirius multifiliis infection

Ichthyophthirius multifiliis, a pathogenic ciliate parasite, infects almost all freshwater fish species and causes significant economic losses. Tumor necrosis factor receptor-associated factor 6 (TRAF6) and transforming growth factor-β-activated kinase 1 (TAK1) are two important signaling molecules involved in toll-like receptor (TLR) signal transduction. To date, the roles of TRAF6 and TAK1 in host defense against fish parasites are still poorly understood. In the present study, TRAF6 (CiTRAF6) and TAK1 (CiTAK1) were identified from grass carp (Ctenopharyngodon idella). The full-length cDNA sequence of CiTRAF6 (2250 bp) includes an open reading frame (ORF) of 1629 bp, which shows a high similarity to that of Cyprinus carpio TRAF6 and encodes a putative protein of 542 amino acids containing one RING domain, two zinc fingers, one coiled-coil region, and one MATH domain. The full-length CiTAK1 cDNA sequence is 2768 bp and includes an ORF of 1626 bp that encodes a putative protein of 541 amino acids containing a conserved serine/threonine protein kinase catalytic domain and a coiled-coil region. Phylogenetic analysis showed that CiTRAF6 and CiTAK1 were clustered with TRAF6 and TAK1 of other teleosts, respectively. CiTRAF6 and CiTAK1 were both constitutively expressed in all examined tissues but with varied expression levels. The highest expressions of CiTRAF6 and CiTAK1 were in the head kidney and spleen, respectively. The expression profiles of CiTRAF6 and CiTAK1 were detected in grass carp after I. multifiliis infection. Expressions of both genes were significantly up-regulated in the skin, gill, head kidney, and spleen at most time points after infection, indicating that CiTRAF6 and CiTAK1 may play essential roles in grass carp defense against I. multifiliis.

The innate immune sensor NLRC3 attenuates Toll-like receptor signaling via modification of the signaling adaptor TRAF6 and transcription factor NF-κB

Several members of the NLR family of sensors activate innate immunity. In contrast, we found here that NLRC3 inhibited Toll-like receptor (TLR)-dependent activation of the transcription factor NF-κB by interacting with the TLR signaling adaptor TRAF6 to attenuate Lys63 (K63)-linked ubiquitination of TRAF6 and activation of NF-κB. We used bioinformatics to predict interactions between NLR and TRAF proteins, including interactions of TRAF with NLRC3. In vivo, macrophage expression of Nlrc3 mRNA was diminished by the administration of lipopolysaccharide (LPS) but was restored when cellular activation subsided. To assess biologic relevance, we generated Nlrc3(-/-) mice. LPS-treated Nlrc3(-/-) macrophages had more K63-ubiquitinated TRAF6, nuclear NF-κB and proinflammatory cytokines. Finally, LPS-treated Nlrc3(-/-) mice had more signs of inflammation. Thus, signaling via NLRC3 and TLR constitutes a negative feedback loop. Furthermore, prevalent NLR-TRAF interactions suggest the formation of a 'TRAFasome' complex.

NLRX1 negatively regulates TLR-induced NF-κB signaling by targeting TRAF6 and IKK

Tight regulation of NF-κB signaling is essential for innate and adaptive immune responses, yet the molecular mechanisms responsible for its negative regulation are not completely understood. Here, we report that NLRX1, a NOD-like receptor family member, negatively regulates Toll-like receptor-mediated NF-κB activation. NLRX1 interacts with TRAF6 or IκB kinase (IKK) in an activation signal-dependent fashion. Upon LPS stimulation, NLRX1 is rapidly ubiquitinated, disassociates from TRAF6, and then binds to the IKK complex, resulting in inhibition of IKKα and IKKβ phosphorylation and NF-κB activation. Knockdown of NLRX1 in various cell types markedly enhances IKK phosphorylation and the production of NF-κB-responsive cytokines after LPS stimulation. We further provide in vivo evidence that NLRX1 knockdown in mice markedly enhances susceptibility to LPS-induced septic shock and plasma IL-6 level. Our study identifies a previously unrecognized role for NLRX1 in the negative regulation of TLR-induced NF-κB activation by dynamically interacting with TRAF6 and the IKK complex.

NLRX1 negatively regulates TLR-induced NF-κB signaling by targeting TRAF6 and IKK

Tight regulation of NF-κB signaling is essential for innate and adaptive immune responses, yet the molecular mechanisms responsible for its negative regulation are not completely understood. Here, we report that NLRX1, a NOD-like receptor family member, negatively regulates Toll-like receptor-mediated NF-κB activation. NLRX1 interacts with TRAF6 or IκB kinase (IKK) in an activation signal-dependent fashion. Upon LPS stimulation, NLRX1 is rapidly ubiquitinated, disassociates from TRAF6, and then binds to the IKK complex, resulting in inhibition of IKKα and IKKβ phosphorylation and NF-κB activation. Knockdown of NLRX1 in various cell types markedly enhances IKK phosphorylation and the production of NF-κB-responsive cytokines after LPS stimulation. We further provide in vivo evidence that NLRX1 knockdown in mice markedly enhances susceptibility to LPS-induced septic shock and plasma IL-6 level. Our study identifies a previously unrecognized role for NLRX1 in the negative regulation of TLR-induced NF-κB activation by dynamically interacting with TRAF6 and the IKK complex.

Molecular mechanisms of TNFR-associated factor 6 (TRAF6) utilization by the oncogenic viral mimic of CD40, latent membrane protein 1 (LMP1)

Latent membrane protein 1 (LMP1), encoded by Epstein-Barr virus, is required for EBV-mediated B cell transformation and plays a significant role in the development of posttransplant B cell lymphomas. LMP1 has also been implicated in exacerbation of autoimmune diseases such as systemic lupus erythematosus. LMP1 is a constitutively active functional mimic of the tumor necrosis factor receptor superfamily member CD40, utilizing tumor necrosis factor receptor-associated factor (TRAF) adaptor proteins to induce signaling. However, LMP1-mediated B cell activation is amplified and sustained compared with CD40. We have previously shown that LMP1 and CD40 use TRAFs 1, 2, 3, and 5 differently. TRAF6 is important for CD40 signaling, but the role of TRAF6 in LMP1 signaling in B cells is not clear. Although TRAF6 binds directly to CD40, TRAF6 interaction with LMP1 in B cells has not been characterized. Here we tested the hypothesis that TRAF6 is a critical regulator of LMP1 signaling in B cells, either as part of a receptor-associated complex and/or as a cytoplasmic adaptor protein. Using TRAF6-deficient B cells, we determined that TRAF6 was critical for LMP1-mediated B cell activation. Although CD40-mediated TRAF6-dependent signaling does not require the TRAF6 receptor-binding domain, we found that LMP1 signaling required the presence of this domain. Furthermore, TRAF6 was recruited to the LMP1 signaling complex via the TRAF1/2/3/5 binding site within the cytoplasmic domain of LMP1.

Litopenaeus vannamei tumor necrosis factor receptor-associated factor 6 (TRAF6) responds to Vibrio alginolyticus and white spot syndrome virus (WSSV) infection and activates antimicrobial peptide genes

Tumor necrosis factor receptor (TNFR)-associated factor 6 (TRAF6) is a key signaling adaptor protein not only for the TNFR superfamily but also for the Interleukin-1 receptor/Toll-like receptor (IL-1/TLR) superfamily. To investigate TRAF6 function in invertebrate innate immune responses, Litopenaeus vannamei TRAF6 (LvTRAF6) was identified and characterized. The full-length cDNA of LvTRAF6 is 2823bp long, with an open reading frame (ORF) encoding a putative protein of 594 amino acids, including a RING-type Zinc finger, two TRAF-type Zinc fingers, a coiled-coil region, and a meprin and TRAF homology (MATH) domain. The overall amino acid sequence identity between LvTRAF6 and other known TRAF6s is 22.2-33.3%. Dual luciferase reporter assays in Drosophila S2 cells revealed that LvTRAF6 could activate the promoters of antimicrobial peptide genes (AMPs), including Drosophila Attacin A and Drosomycin, and shrimp Penaeidins. Real-time quantitative PCR (qPCR) indicated that LvTRAF6 was constitutively expressed in various tissues of L. vannamei. After Vibrio alginolyticus and white spot syndrome virus (WSSV) challenge, LvTRAF6 was down-regulated, though with different expression patterns in the intestine compared to other tissues. After WSSV challenge, LvTRAF6 was up-regulated 2.7- and 2.3-fold over the control at 3h in gills and hepatopancreas, respectively. These results indicated that LvTRAF6 may play a crucial role in antibacterial and antiviral responses via regulation of AMP gene expression.

Roles of TRAF6 in CD40 signaling

CD40 provides signals crucial to the activation of antigen-presenting cells during humoral and cell-mediated immune responses. A complex cohort of proteins interacts with the cytoplasmic domain of CD40 and mediates signaling. One member of this cohort is TNF receptor associated factor six (TRAF6). TRAF6 contributes to the CD40-mediated activation of NF-kappaB, stress-activated protein kinases, and perhaps other signaling molecules. TRAF6 may have roles as an adapter molecule, an activator of mitogen-activated protein kinases, and as a repressor of certain signaling circuits. Establishing the significance and interplay of these roles will lead to a more complete understanding of mechanisms important to the CD40-mediated activation of the immune system and will reveal novel targets for the development of therapeutic agents.

A Novel Mechanism for TNFR-Associated Factor 6-Dependent CD40 Signaling

Members of the TNFR family play critical roles in the regulation of the immune system. One member of the family critical for efficient activation of T-dependent humoral immune responses is CD40, a cell surface protein expressed by B cells and other APC. The cytoplasmic domain of CD40 interacts with several members of the TNFR-associated factor (TRAF) family, which link CD40 to intracellular signaling pathways. TRAF2 and 6 appear to play particularly important roles in CD40 signaling. Previous studies suggest that the two molecules have certain overlapping roles in signaling, but that unique roles for each molecule also exist. To better define the roles of TRAF2 and TRAF6 in CD40 signaling, we used somatic cell gene targeting to generate TRAF-deficient mouse B cell lines. A20.2J cells deficient in TRAF6 exhibit marked defects in CD40-mediated JNK activation and the up-regulation of CD80. Our previous experiments with TRAF2-deficient B cell lines suggest that TRAF6 and TRAF2 may have redundant roles in CD40-mediated NF-kappaB activation. Consistent with this hypothesis, we found CD40-mediated activation of NF-kappaB intact in TRAF6-deficient cells and defective in cells lacking both TRAF2 and TRAF6. Interestingly, we found that TRAF6 mutants defective in CD40 binding were able to restore CD40-mediated JNK activation and CD80 up-regulation in TRAF6-deficient cells, indicating that TRAF6 may be able to contribute to certain CD40 signals without directly binding CD40.

Rapid CD40-mediated rescue from CD95-induced apoptosis requires TNFR-associated factor-6 and PI3K

The activation molecule CD40 and the death receptor CD95/Fas play important roles in regulating B cells so that effective antimicrobial immunity occurs without autoimmunity. CD40 signaling increases CD95 expression, sensitizing cells to apoptosis, but sustained CD40 signals rescue B cells from CD95 killing. Here we describe a mechanism of early CD40-mediated rescue from CD95-induced apoptosis in B cells. Maximal rescue was achieved when CD40 signals were given within 1-2 h of initiating CD95 apoptosis. CD40 signaling did not block association of Fas-associated death domain-containing protein with CD95, but decreased CD95-induced activation of caspases 3 and 8. Rapid CD40 rescue did not require NF-kappaB activation and was independent of de novo protein synthesis, but was dependent upon active PI3 K. Signaling via a CD40 mutant that does not bind TNFR-associated factor (TRAF)1, TRAF2, and TRAF3 rescued B cells from CD95-induced apoptosis. TRAF1/2/3-independent rescue was confirmed in B cell lines made deficient in these TRAF molecules by gene targeting. In contrast, CD40 rescue was completely abrogated in TRAF6-deficient B cells, which showed reduced activation of Akt in response to CD40 engagement. These results reveal a new rapid mechanism to balance B cell activation and apoptosis.

Connecting mitochondria and innate immunity

Viral infection results in the activation of multiple signaling pathways, but how these pathways are coordinated remains a mystery. Two studies, one published in this issue of Cell (Seth et al., 2005) and the other in Molecular Cell (Xu et al., 2005), identify a new intracellular signaling protein that is required for activating type I interferon expression in response to viral infection. In addition,Seth et al. (2005) show that the function of this protein, which they call MAVS, requires that it be localized to the mitochondria. This observation establishes an unexpected link between innate immunity and an organelle with evolutionary origins in aerobic bacteria.

The TRAF6, but not the TRAF2/3, binding domain of CD40 is required for cytokine production in human lung fibroblasts

Fibroblasts are key effector cells in inciting inflammation, wound healing, and scarring. CD40, a member of the TNF receptor superfamily, mediates intercellular communication between fibroblasts and cells that express CD154 (CD40L), including T lymphocytes and platelets. To better understand the mechanisms by which CD40 regulates fibroblast function in inflammation and scarring, we examined the ability of CD40 cytoplasmic tail regions (CD40ct) containing the TRAF6 or the TRAF2/3 binding domains to regulate cytokine and chemokine expression by primary human lung fibroblasts. The full-length human CD40ct, the first 35 amino acids of the CD40ct encompassing the TRAF6 binding site (1-35), and amino acids 35-53 containing the TRAF2/TRAF3 binding domain were expressed in human lung fibroblasts as fusion proteins with the extracellular domain of human CD8alpha by retroviral transduction. The TRAF6, but not the TRAF2/3, binding domain was found to regulate IL-8 and IL-6 production, and induce activation of NF-kappaB and Jun kinase in lung fibroblasts, demonstrating for the first time that CD40ct domains can function independently to regulate pro-inflammatory responses of primary human fibroblasts. Thus, targeting TRAF6 function through pharmacological intervention may represent a viable strategy for modulating localized inflammation.

Linking signalling pathways, thymic stroma integrity and autoimmunity

Medullary thymic epithelial cells (mTECs) are indispensable for self-tolerance to peripheral organs by virtue of their expression of a host of tissue-restricted self-antigens. The full extent of this promiscuous gene expression is confined to functionally mature mTECs. Consequently, any interference with signalling pathways directing the differentiation and/or proliferation of this mature subset will affect the scope of central tolerance and potentially predispose to autoimmunity. In a recent study, tumour necrosis factor receptor-associated factor 6 (TRAF6) has been identified as an essential component of a new signalling pathway directing mTEC development.

Common and distinct signalling cascades in the production of tumour necrosis factor-alpha and interleukin-13 induced by lipopolysaccharide in RBL-2H3 cells

BACKGROUND

Activation of mast cells by lipopolysaccharide (LPS) results in the production of TNF-alpha and IL-13. TNF-alpha and IL-13 are key mediators in the development of neutrophilic and allergic inflammation, respectively. LPS-induced TNF-alpha and IL-13 production in mast cells has been reported to be mediated by Toll-like receptor 4 (TLR4) signalling, but differences in signal transduction mechanisms leading to the production of these cytokines are not clearly defined.

OBJECTIVE

We investigated the molecular mechanisms responsible for LPS-induced TNF-alpha and IL-13 production in mast cells.

METHODS

TNF-alpha and IL-13 production by LPS was assessed by transfecting RBL-2H3 cells with dominant-negative (DN) expression vectors.

RESULTS

Transfection of RBL-2H3 cells with plasmids encoding DN mutants of myeloid differentiation protein (MyD88) and TNFR-associated factor (TRAF6) inhibited both LPS-induced TNF-alpha and IL-13 production. IkappaBalpha-DN inhibited LPS-induced production of TNF-alpha, but not IL-13. We also found that inhibition of p38 kinase suppressed both TNF-alpha and IL-13 induction by LPS, and inhibition of JNK reduced IL-13 production, but not TNF-alpha. Furthermore, we found that protein kinase R (PKR) was activated by LPS in these cells. Treatment with 2-aminopurine, a PKR inhibitor, attenuated LPS-induced nuclear factor-kappaB activation and TNF-alpha production, whereas inhibition of PKR had little effect on IL-13 production.

CONCLUSION

These findings indicate that the production of TNF-alpha and IL-13 by LPS required TLR4/MyD88/TRAF6 signalling as a common pathway of mast cell-mediated inflammation. We furthermore found that TNF-alpha and IL-13 production were differentially regulated by signalling cascades through PKR and mitogen-activated protein kinases downstream of TRAF6 in mast cells.

Dependence of self-tolerance on TRAF6-directed development of thymic stroma

The microenvironments of the thymus are generated by thymic epithelial cells (TECs) and are essential for inducing immune self-tolerance or developing T cells. However, the molecular mechanisms that underlie the differentiation of TECs and thymic compartmentalization are not fully understood. Here we show that deficiency in the tumor necrosis factor receptor-associated factor (TRAF) 6 results in disorganized distribution of medullary TECs (mTECs) and the absence of mature mTECs. Engraftment of thymic stroma of TRAF6(-/-) embryos into athymic nude mice induced autoimmunity. Thus, TRAF6 directs the development of thymic stroma and represents a critical point of regulation for self-tolerance and autoimmunity.