Cutting Edge: Role of TANK-Binding Kinase 1 and Inducible IκB Kinase in IFN Responses against Viruses in Innate Immune Cells

IKKε and TBK1 prevent RIPK1 dependent and independent inflammation

TBK1 and IKKε regulate multiple cellular processes including anti-viral type-I interferon responses, metabolism and TNF receptor signaling. However, the relative contributions and potentially redundant functions of IKKε and TBK1 in cell death, inflammation and tissue homeostasis remain poorly understood. Here we show that IKKε compensates for the loss of TBK1 kinase activity to prevent RIPK1-dependent and -independent inflammation in mice. Combined inhibition of IKKε and TBK1 kinase activities caused embryonic lethality that was rescued by heterozygous expression of kinase-inactive RIPK1. Adult mice expressing kinase-inactive versions of IKKε and TBK1 developed systemic inflammation that was induced by both RIPK1-dependent and -independent mechanisms. Combined inhibition of IKKε and TBK1 kinase activities in myeloid cells induced RIPK1-dependent cell death and systemic inflammation mediated by IL-1 family cytokines. Tissue-specific studies showed that IKKε and TBK1 were required to prevent cell death and inflammation in the intestine but were dispensable for liver and skin homeostasis. Together, these findings revealed that IKKε and TBK1 exhibit tissue-specific functions that are important to prevent cell death and inflammation and maintain tissue homeostasis.

Molecular and functional characterization of golden pompano (Trachinotus blochii) TBK1 on IFN regulation

The golden pompano (Trachinotus blochii), a pivotal commercial marine species in China, has gained significant popularity worldwide. However, accompanied with rapid growth and high density aquaculture, golden pompano has been seriously threatened by Nervous necrosis virus (NNV), while its molecular biology research regarding the innate immune system remains unexplored, which is crucial for understanding the activation of interferon (IFN) production and antiviral responses. In this study, we aimed to identify the characterization and function of golden pompano TANK-binding kinase 1 (gpTBK1), thereby providing evidence of the conservation of this classical factor in the RLR pathway among marine fish. Initially, we found the expression of gpTBK1 upregulation in diseased golden pompano with NNV infection and we successfully cloned the full-length open reading frame (ORF) of gpTBK1, consisting of 2172 nucleotides encoding 723 amino acids, from the head kidney. Subsequent analysis of the amino acid sequence revealed homology between gpTBK1 and other fish TBK1 proteins, with conserved N-terminal Serine/Threonine protein kinases catalytic domain (S_TKc) and C-terminal coiled coil domain (CCD). Moreover, the expression pattern showed that gpTBK1 exhibited ubiquitous expression across all evaluated tissues. Furthermore, functional identification experiments indicated that gpTBK1 activated interferon promoters' activity in golden pompano and induced the expression of downstream IFN-stimulated genes (ISGs). Notably, gpTBK1 was found to co-localize and interact with gpIRF3 in the cytoplasm. Collectively, these data provide a comprehensive analysis of the characterization and functional role of gpTBK1 in promoting interferon production. This research may facilitate the further study of the innate antiviral response, particularly the anti-NNV mechanisms, in golden pompano.

Dietary supplementation of exopolysaccharides from Lactobacillus rhamnosus GCC-3 improved the resistance of zebrafish against spring viremia of carp virus infection

Spring viremia of carp virus (SVCV) can cause high mortality of fish. The aim of this study was to investigate the effects of Lactobacillus rhamnosus GCC-3 exopolysaccharides (GCC-3 EPS) on zebrafish (Danio rerio) infected with SVCV and elucidate the underlying mechanisms. Zebrafish were fed with a control diet or diet supplemented with 0.5% and 1% of GCC-3 EPS for 2 weeks. The results showed that supplementation of GCC-3 EPS significantly improved the survival rate of zebrafish compared with the control group. In addition, dietary 0.5% and 1% GCC-3 EPS significantly up-regulated the expression of genes related to type I interferon (IFN) antiviral immunity. Consistent with in vivo results, GCC-3 EPS significantly inhibited SVCV replication in zebrafish embryonic fibroblast (ZF4) cells while significantly increased the expression of type I IFN signaling pathway related genes. Furthermore, knocking down TANK-binding kinase 1 significantly blocked the antiviral effect of GCC-3 EPS. Dietary GCC-3 EPS improved gut microbiota, and the culture supernatant of GCC-3 EPS-associated microbiota significantly inhibited SVCV replication in ZF4 cells compared with the control-microbiota counterpart. In conclusion, our results indicate that dietary GCC-3 EPS can improve the resistance of zebrafish against SVCV infection, and the mechanism may involve enhanced type I interferon signaling.

IKKε isoform switching governs the immune response against EV71 infection

The reciprocal interactions between pathogens and hosts are complicated and profound. A comprehensive understanding of these interactions is essential for developing effective therapies against infectious diseases. Interferon responses induced upon virus infection are critical for establishing host antiviral innate immunity. Here, we provide a molecular mechanism wherein isoform switching of the host IKKε gene, an interferon-associated molecule, leads to alterations in IFN production during EV71 infection. We found that IKKε isoform 2 (IKKε v2) is upregulated while IKKε v1 is downregulated in EV71 infection. IKKε v2 interacts with IRF7 and promotes IRF7 activation through phosphorylation and translocation of IRF7 in the presence of ubiquitin, by which the expression of IFNβ and ISGs is elicited and virus propagation is attenuated. We also identified that IKKε v2 is activated via K63-linked ubiquitination. Our results suggest that host cells induce IKKε isoform switching and result in IFN production against EV71 infection. This finding highlights a gene regulatory mechanism in pathogen-host interactions and provides a potential strategy for establishing host first-line defense against pathogens.

DDX3X Links NLRP11 to the Regulation of Type I Interferon Responses and NLRP3 Inflammasome Activation

Tight regulation of inflammatory cytokine and interferon (IFN) production in innate immunity is pivotal for optimal control of pathogens and avoidance of immunopathology. The human Nod-like receptor (NLR) NLRP11 has been shown to regulate type I IFN and pro-inflammatory cytokine responses. Here, we identified the ATP-dependent RNA helicase DDX3X as a novel binding partner of NLRP11, using co-immunoprecipitation and LC-MS/MS. DDX3X is known to enhance type I IFN responses and NLRP3 inflammasome activation. We demonstrate that NLRP11 can abolish IKKϵ-mediated phosphorylation of DDX3X, resulting in lower type I IFN induction upon viral infection. These effects were dependent on the LRR domain of NLRP11 that we mapped as the interaction domain for DDX3X. In addition, NLRP11 also suppressed NLRP3-mediated caspase-1 activation in an LRR domain-dependent manner, suggesting that NLRP11 might sequester DDX3X and prevent it from promoting NLRP3-induced inflammasome activation. Taken together, our data revealed DDX3X as a central target of NLRP11, which can mediate the effects of NLRP11 on type I IFN induction as well as NLRP3 inflammasome activation. This expands our knowledge of the molecular mechanisms underlying NLRP11 function in innate immunity and suggests that both NLRP11 and DDX3X might be promising targets for modulation of innate immune responses.

A novel role of Zebrafish TMEM33 in negative regulation of interferon production by two distinct mechanisms

The transmembrane protein 33 (TMEM33) was originally identified as an endoplasmic reticulum (ER) protein that influences the tubular structure of the ER and modulates intracellular calcium homeostasis. However, the role of TMEM33 in antiviral immunity in vertebrates has not been elucidated. In this article, we demonstrate that zebrafish TMEM33 is a negative regulator of virus-triggered interferon (IFN) induction via two mechanisms: mitochondrial antiviral signaling protein (MAVS) ubiquitination and a decrease in the kinase activity of TANK binding kinase 1 (TBK1). Upon stimulation with viral components, tmem33 was remarkably upregulated in the zebrafish liver cell line. The IFNφ1 promoter (IFNφ1pro) activity and mRNA level induced by retinoic acid-inducible gene (RIG)-I-like receptors (RLRs) were significantly inhibited by TMEM33. Knockdown of TMEM33 increased host ifn transcription. Subsequently, we found that TMEM33 was colocalized in the ER and interacted with the RLR cascades, whereas MAVS was degraded by TMEM33 during the K48-linked ubiquitination. On the other hand, TMEM33 reduced the phosphorylation of mediator of IFN regulatory factor 3 (IRF3) activation (MITA)/IRF3 by acting as a decoy substrate of TBK1, which was also phosphorylated. A functional domain assay revealed that the N-terminal transmembrane domain 1 (TM1) and TM2 regions of TMEM33 were necessary for IFN suppression. Finally, TMEM33 significantly attenuated the host cellular antiviral capacity by blocking the IFN response. Taken together, our findings provide insight into the different mechanisms employed by TMEM33 in cellular IFN-mediated antiviral process.

Targeting TANK-binding kinase 1 (TBK1) in cancer

INTRODUCTION

TANK-binding kinase 1 (TBK1) is a Ser/Thr kinase with a central role in coordinating the cellular response to invading pathogens and regulating key inflammatory signaling cascades. While intact TBK1 signaling is required for successful anti-viral signaling, dysregulated TBK1 signaling has been linked to a variety of pathophysiologic conditions, including cancer. Several lines of evidence support a role for TBK1 in cancer pathogenesis, but the specific roles and regulation of TBK1 remain incompletely understood. A key challenge is the diversity of cellular processes that are regulated by TBK1, including inflammation, cell cycle, autophagy, energy homeostasis, and cell death. Nevertheless, evidence from pre-clinical cancer models suggests that targeting TBK1 may be an effective strategy for anti-cancer therapy in specific settings.

AREAS COVERED

This review provides an overview of the roles and regulation of TBK1 with a focus on cancer pathogenesis and drug targeting of TBK1 as an anti-cancer strategy. Relevant literature was derived from a PubMed search encompassing studies from 1999 to 2020.

EXPERT OPINION

TBK1 is emerging as a potential target for anti-cancer therapy. Inhibition of TBK1 alone may be insufficient to restrain the growth of most cancers; hence, combination strategies will likely be necessary. Improved understanding of tumor-intrinsic and tumor-extrinsic TBK1 signaling will inform novel therapeutic strategies.

Systematic analysis of the IL-17 receptor signalosome reveals a robust regulatory feedback loop

IL‐17 mediates immune protection from fungi and bacteria, as well as it promotes autoimmune pathologies. However, the regulation of the signal transduction from the IL‐17 receptor (IL‐17R) remained elusive. We developed a novel mass spectrometry‐based approach to identify components of the IL‐17R complex followed by analysis of their roles using reverse genetics. Besides the identification of linear ubiquitin chain assembly complex (LUBAC) as an important signal transducing component of IL‐17R, we established that IL‐17 signaling is regulated by a robust negative feedback loop mediated by TBK1 and IKKε. These kinases terminate IL‐17 signaling by phosphorylating the adaptor ACT1 leading to the release of the essential ubiquitin ligase TRAF6 from the complex. NEMO recruits both kinases to the IL‐17R complex, documenting that NEMO has an unprecedented negative function in IL‐17 signaling, distinct from its role in NF‐κB activation. Our study provides a comprehensive view of the molecular events of the IL‐17 signal transduction and its regulation.

Multitasking Kinase RIPK1 Regulates Cell Death and Inflammation

Receptor-interacting serine threonine kinase 1 (RIPK1) is a widely expressed kinase that is essential for limiting inflammation in both mice and humans. Mice lacking RIPK1 die at birth from multiorgan inflammation and aberrant cell death, whereas humans lacking RIPK1 are immunodeficient and develop very early-onset inflammatory bowel disease. In contrast to complete loss of RIPK1, inhibiting the kinase activity of RIPK1 genetically or pharmacologically prevents cell death and inflammation in several mouse disease models. Indeed, small molecule inhibitors of RIPK1 are in phase I clinical trials for amyotrophic lateral sclerosis, and phase II clinical trials for psoriasis, rheumatoid arthritis, and ulcerative colitis. This review focuses on which signaling pathways use RIPK1, how activation of RIPK1 is regulated, and when activation of RIPK1 appears to be an important driver of inflammation.

Implications of Selective Autophagy Dysfunction for ALS Pathology

Amyotrophic lateral sclerosis (ALS) is a lethal neurodegenerative disorder that progressively affects motor neurons in the brain and spinal cord. Due to the biological complexity of the disease, its etiology remains unknown. Several cellular mechanisms involved in the neurodegenerative process in ALS have been found, including the loss of RNA and protein homeostasis, as well as mitochondrial dysfunction. Insoluble protein aggregates, damaged mitochondria, and stress granules, which contain RNA and protein components, are recognized and degraded by the autophagy machinery in a process known as selective autophagy. Autophagy is a highly dynamic process whose dysregulation has now been associated with neurodegenerative diseases, including ALS, by numerous studies. In ALS, the autophagy process has been found deregulated in both familial and sporadic cases of the disease. Likewise, mutations in genes coding for proteins involved in the autophagy machinery have been reported in ALS patients, including selective autophagy receptors. In this review, we focus on the role of selective autophagy in ALS pathology.

Functional Characterization of Dark Sleeper (Odontobutis obscura) TBK1 on IFN Regulation

In East Asia, the dark sleeper, Odontobutis obscura (O. obscura) is a crucial commercial species of freshwater fish; however, its molecular biology research is still undeveloped, including its innate immune system, which is pivotal to antiviral responses. In this study, we cloned and identified the characterization and kinase function of dark sleeper TANK-binding kinase 1 (TBK1), supplementing the evidence of the conservation of this classical factor in fish. First, the ORF of Odontobutis obscurus (O. obscura) TBK1 (OdTBK1) was cloned from liver tissue by RACE-PCR. Subsequent nucleic acid and amino acid sequence analysis suggested that OdTBK1 is homologous with other fish TBK1, and the N-terminal Serine/Threonine protein kinases catalytic domain (S_TKc) and C-terminal coiled coil domain (CCD) are conserved. Subsequently, the cellular distribution demonstrated that OdTBK1 was located in the cytoplasm region. With regard to the identification of functions, OdTBK1 activated several interferon (IFN) promoters' activity and induced downstream IFN-stimulated genes (ISGs) expression. In a canonical manner, wild-type OdTBK1 significantly phosphorylated interferon regulatory factor 3 (IRF3) but failed when the N-terminal region was truncated. Furthermore, overexpression of OdTBK1 decreased viral proliferation remarkably. Collectively, these data systematically analyzed the characterization and function of OdTBK1, initiating the study of the innate antiviral response of dark sleeper.

Zebrafish NDRG1a Negatively Regulates IFN Induction by Promoting the Degradation of IRF7

Viral infection activates the transcription factor IFN regulatory factor 7 (IRF7), which plays a critical role in the induction of IFNs and innate antiviral immune response. How virus-induced IFN signaling is controlled in fish is not fully understood. In this study, we demonstrate that N-myc downstream-regulated gene 1a (NDRG1a) in zebrafish plays a role as a negative regulator for virus-triggered IFN induction. First, the activation of the IFN promoter stimulated by the polyinosinic-polycytidylic acid or spring viremia of carp virus was decreased by the overexpression of NDRG1a. Second, NDRG1a interacted with IRF7 and blocked the IFN transcription activated by IRF7. Furthermore, NDRG1a was phosphorylated by TANK-binding kinase 1 (TBK1) and promoted the K48-linked ubiquitination and degradation of IRF7. Finally, the overexpression of NDRG1a blunted the transcription of several IFN-stimulated genes, resulting in the host cells becoming susceptible to spring viremia of carp virus infection. Our findings suggest that fish NDRG1a negatively regulates the cellular antiviral response by targeting IRF7 for ubiquitination and degradation, providing insights into the novel role of NDRG1a on the innate antiviral immune response in fish.

TBK1 Suppresses RIPK1-Driven Apoptosis and Inflammation during Development and in Aging

Aging is a major risk factor for both genetic and sporadic neurodegenerative disorders. However, it is unclear how aging interacts with genetic predispositions to promote neurodegeneration. Here, we investigate how partial loss of function of TBK1, a major genetic cause for amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) comorbidity, leads to age-dependent neurodegeneration. We show that TBK1 is an endogenous inhibitor of RIPK1 and the embryonic lethality of Tbk1-/- mice is dependent on RIPK1 kinase activity. In aging human brains, another endogenous RIPK1 inhibitor, TAK1, exhibits a marked decrease in expression. We show that in Tbk1+/- mice, the reduced myeloid TAK1 expression promotes all the key hallmarks of ALS/FTD, including neuroinflammation, TDP-43 aggregation, axonal degeneration, neuronal loss, and behavior deficits, which are blocked upon inhibition of RIPK1. Thus, aging facilitates RIPK1 activation by reducing TAK1 expression, which cooperates with genetic risk factors to promote the onset of ALS/FTD.

Toll-like receptor (TLR)4 signalling induces myeloid differentiation primary response gene (MYD) 88 independent pathway in avian species leading to type I interferon production and antiviral response

Engagement of toll-like receptor (TLR)4 ligand, lipopolysaccharide (LPS) with TLR4 in mammals activates two downstream intracellular signaling routes; the myeloid differentiation primary response gene (MyD)88 dependent and independent pathways. However, existence of the later pathway leading to production of type I interferons (IFNs) in avian species has been debated due to conflicting observations. The objective of our study was to investigate whether LPS induces type I IFN production in chicken macrophages leading to antiviral response attributable to type I IFN. We found that LPS elicits type I IFN response dominated by IFN-β production. We also found that reduction in infectious laryngotracheitis virus (ILTV) replication by LPS-mediated antiviral response is attributable to type I IFNs in addition to nitric oxide (NO). Our findings imply that LPS elicits both MyD88 dependent and independent pathways in chicken macrophages consequently eliciting anti-ILTV response attributable to production of both type I IFNs and NO.

Molecular characterization, expression of chicken TBK1 gene and its effect on IRF3 signaling pathway

TRAF family member-associated NF-κB activator (TANK)-binding kinase1 (TBK1) is a serine-threonine kinase at the crossroads of multiple interferon (IFN)-inducing signaling pathways in innate immunity. The importance of TBK1 in antiviral immunity is well established in mammal models, but in chicken, the molecular characterization and potential function of TBK1 remain unclear. In the present study, the open-reading frame (ORF) of chicken TBK1 (chTBK1) was cloned and characterized. The sequencing results revealed that the chTBK1 ORF consists of 2190 base pairs (bp) encoding a deduced protein of 729 amino acid residues. Multiple sequence alignment analysis demonstrated chTBK1 similarity to other birds and mammals, which indicates that it is evolutionarily conserved. Quantitative real-time PCR (qRT-PCR) results showed that chTBK1 was ubiquitously expressed in chicken tissues and expression was especially high in immune tissues. In addition, the expression of chTBK1 was significantly up-regulated by infection with avian leukosis virus subgroup J (ALV-J) both in vivo and in chicken embryo fibroblasts (CEFs) challenged with ALV-J or stimulated with poly I:C in vitro. Consistent with the activation of chTBK1, the interferon regulatory factor 3 (IRF3) and IFNβ gene in CEFs were also up-regulated after challenge with ALV-J or polyI:C. In contrast, the expression of IRF3 and IFNβ in CEFs was significantly reduced by siRNA targeting the chTBK1 gene compared with a negative control (NC) during ALV-J infection or polyI:C transfection. In conclusion, our results demonstrated that chTBK1 may be an important immunoregulator for IRF3 and IFNβ induction in response to viral stimulation in chicken.

A distinct subset of plasmacytoid dendritic cells induces activation and differentiation of B and T lymphocytes

Plasmacytoid dendritic cells (pDCs) are known mainly for their secretion of type I IFN upon viral encounter. We describe a CD2^hiCD5⁺CD81⁺ pDC subset, distinguished by prominent dendrites and a mature phenotype, in human blood, bone marrow, and tonsil, which can be generated from CD34⁺ progenitors. These CD2^hiCD5⁺CD81⁺ cells express classical pDC markers, as well as the toll-like receptors that enable conventional pDCs to respond to viral infection. However, their gene expression profile is distinct, and they produce little or no type I IFN upon stimulation with CpG oligonucleotides, likely due to their diminished expression of IFN regulatory factor 7. A similar population of CD5⁺CD81⁺ pDCs is present in mice and also does not produce type I IFN after CpG stimulation. In contrast to conventional CD5^-CD81^- pDCs, human CD5⁺CD81⁺ pDCs are potent stimulators of B-cell activation and antibody production and strong inducers of T-cell proliferation and Treg formation. These findings reveal the presence of a discrete pDC population that does not produce type I IFN and yet mediates important immune functions previously attributed to all pDCs.

Oncolytic measles virus induces tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-mediated cytotoxicity by human myeloid and plasmacytoid dendritic cells

Attenuated measles virus (MV) is currently being evaluated in clinical trials as an oncolytic therapeutic agent. Originally used for its lytic activity against tumor cells, it is now admitted that the effectiveness of MV also lies in its ability to initiate antitumor immune responses through the activation of dendritic cells (DCs). In this study, we investigated the capacity of oncolytic MV to convert human blood myeloid CD1c⁺ DCs and plasmacytoid DCs (pDCs) into cytotoxic effectors. We found that MV induces the expression of the cytotoxic protein TNF-related apoptosis-inducing ligand (TRAIL) on the surface of DCs. We demonstrate that the secretion of interferon-α (IFN-α) by DCs in response to MV is responsible for this TRAIL expression. Several types of PRRs (pattern recognition receptors) have been implicated in MV genome recognition, including RLRs (RIG-I-like receptors) and TLRs (Toll-like receptors). We showed that CD1c⁺ DCs secrete modest amounts of IFN-α and express TRAIL in an RLR-dependent manner upon exposure to MV. In pDCs, MV is recognized by RLRs and also by TLR7, leading to the secretion of high amounts of IFN-α and TRAIL expression. Finally, we showed that MV-stimulated DCs induce TRAIL-mediated cell death of Jurkat cells, confirming their acquisition of cytotoxic functions. Our results demonstrate that MV can activate cytotoxic myeloid CD1c⁺ DCs and pDCs, which may participate to the antitumor immune response.

Viral entry route determines how human plasmacytoid dendritic cells produce type I interferons

Although plasmacytoid dendritic cells (pDCs) represent a rare immune cell type, they are the most important source of type I interferons (IFNs) upon viral infection. Phagocytosed RNA viruses and RNA virus-infected cells are detected by pDCs with the endosomal pattern recognition receptor (PRR) toll-like receptor 7 (TLR7). We showed that replication of the yellow fever live vaccine YF-17D in human pDCs and pDC-like cell lines stimulated type I IFN production through RIG-I (retinoic acid-inducible gene I), a member of the RIG-I-like receptor (RLR) family of cytosolic PRRs. Thus, human pDCs sense replicative viral RNA. In contrast, direct contact between pDCs and YF-17D-infected cells stimulated a TLR7-dependent, viral replication-independent production of type I IFN. We also showed that the RLR pathway was dampened by the activities of interleukin-1 receptor-associated kinases 1 and 4 (IRAK1 and IRAK4), which are downstream effectors of the TLR7 pathway, suggesting that both kinases play opposing roles downstream of specific PRRs. Together, these data suggest that a virus can stimulate either TLR or RLR signaling in the same cell, depending on how its nucleic acid content is delivered.

Interferon regulatory factors: critical mediators of human lupus

The pathogenesis of systemic lupus erythematosus (SLE) is multifactorial, and the interferon regulatory factors (IRFs) play an important role. Autoantibodies formed in SLE target nuclear antigens, and immune complexes formed by these antibodies contain nucleic acid. These immune complexes can activate antiviral pattern recognition receptors (PRRs), resulting in the downstream activation of IRFs, which can induce type I interferon (IFN-I) and other inflammatory mediators. Genetic variations in IRFs have been associated with susceptibility to SLE, and current evidence supports the idea that these polymorphisms are gain of function in humans. Recent studies suggest that these genetic variations contribute to the break in humoral tolerance that allows for nucleic acid binding autoantibodies, and that the same polymorphisms also augment IFN-I production in the presence of these autoantibody immune complexes, forming a feed-forward loop. In this review, we will outline major features of the PRR/IRF systems and describe the role of the IRFs in human SLE pathogenesis.

Pivotal role of RNA-binding E3 ubiquitin ligase MEX3C in RIG-I-mediated antiviral innate immunity

The RIG-I-like receptors, retinoic acid inducible gene-1 (RIG-I), melanoma differentiation-associated protein 5, and laboratory of genetics and physiology-2, are cytoplasmic sensors for RNA viruses that mediate the antiviral innate immune responses. We demonstrate that really interesting new gene-finger domain- and K homology domain-containing MEX3C regulates RIG-I function. MEX3C colocalizes with RIG-I in the stress granules of virally infected cells, and its overexpression causes the lysine-63-linked ubiquitination of RIG-I and activates IFN-β promoter. Embryonic fibroblast cells, macrophages, and conventional dendritic cells derived from Mex3c-deficient mice showed defective production of type I IFN after infection with RNA viruses that are recognized by RIG-I. These results demonstrate that MEX3C is an E3 ubiquitin ligase that modifies RIG-I in stress granules and plays a critical role in eliciting antiviral immune responses.

Cytosolic-DNA-mediated, STING-dependent proinflammatory gene induction necessitates canonical NF-κB activation through TBK1

STING (stimulator of interferon genes) is known to control the induction of innate immune genes in response to the recognition of cytosolic DNA species, including the genomes of viruses such as herpes simplex virus 1 (HSV-1). However, while STING is essential for protection of the host against numerous DNA pathogens, sustained STING activity can lead to lethal inflammatory disease. It is known that STING utilizes interferon regulatory factor 3 (IRF3) and nuclear factor κB (NF-κB) pathways to exert its effects, although the signal transduction mechanisms remain to be clarified fully. Here we demonstrate that in addition to the activation of these pathways, potent induction of the Jun N-terminal protein kinase/stress-activated protein kinase (JNK/SAPK) pathway was similarly observed in response to STING activation by double-stranded DNA (dsDNA). Furthermore, TANK-binding kinase 1 (TBK1) associated with STING was found to facilitate dsDNA-mediated canonical activation of NF-κB as well as IRF3 to promote proinflammatory gene transcription. The triggering of NF-κB function was noted to require TRAF6 activation. Our findings detail a novel dsDNA-mediated NF-κB activation pathway facilitated through a STING-TRAF6-TBK1 axis and suggest a target for therapeutic intervention to plausibly stimulate antiviral activity or, alternatively, avert dsDNA-mediated inflammatory disease.

IMPORTANCE

The IKK complex, which is composed of two catalytic subunits, IKKα and IKKβ, has been suggested to be essential for the activation of canonical NF-κB signaling in response to various stimuli, including cytokines (e.g., interleukin-1α [IL-1α] and tumor necrosis factor alpha [TNF-α]), Toll-like receptor (TLR) ligands (e.g., lipopolysaccharide [LPS]), and dsRNAs derived from viruses, or a synthetic analog. STING has been identified as a critical signaling molecule required for the detection of cytosolic dsDNAs derived from pathogens and viruses. However, little is known about how cytosolic dsDNA triggers NF-κB signaling. In the present study, we demonstrate that TBK1, identified as an IKK-related kinase, may predominantly control the activation of NF-κB in response to dsDNA signaling via STING through the IKKαβ activation loop. Thus, our results establish TBK1 as a downstream kinase controlling dsDNA-mediated IRF3 and NF-κB signaling dependent on STING.

Hantavirus GnT elements mediate TRAF3 binding and inhibit RIG-I/TBK1-directed beta interferon transcription by blocking IRF3 phosphorylation

Hantaviruses successfully replicate in primary human endothelial cells by restricting the early induction of beta interferon (IFN-β) and interferon-stimulated genes (ISGs). Gn proteins from NY-1V, ANDV, and TULV, but not PHV, harbor elements in their 142-residue cytoplasmic tails (GnTs) that inhibit RIG-I/MAVS/TBK1-TRAF3-directed IFN-β induction. Here, we define GnT interactions and residues required to inhibit TRAF3-TBK1-directed IFN-β induction and IRF3 phosphorylation. We observed that GnTs bind TRAF3 via residues within the TRAF-N domain (residues 392 to 415) and that binding is independent of the MAVS-interactive TRAF-C domain (residues 415 to 568). We determined that GnT binding to TRAF3 is mediated by C-terminal degrons within NY-1V or ANDV GnTs and that mutations that add degrons to TULV or PHV GnTs confer TRAF3 binding. Further analysis of GnT domains revealed that TRAF3 binding is a discrete GnT function, independent of IFN regulation, and that residues 15 to 42 from the NY-1V GnT C terminus are required for inhibiting TBK1-directed IFN-β transcription. Mutagenesis of the NY-1V GnT revealed that altering tyrosine 627 (Y627A/S/F) abolished GnT regulation of RIG-I/TBK1-directed IRF3 phosphorylation and transcriptional responses of ISRE, κB, and IFN-β promoters. Moreover, GnTs from NY-1V, ANDV, and TULV, but not PHV, inhibited RIG-I-directed IRF3 phosphorylation. Collectively, these findings suggest a novel role for GnTs in regulating RIG-I/TBK1 pathway-directed IRF3 phosphorylation and IFN-β induction and define virulence determinants within GnTs that may permit the attenuation of pathogenic hantaviruses. IMPORTANCE These findings provide a mechanism for selected hantavirus GnT interactions to regulate RIG-I/TBK1 signaling responses required for IFN-β induction by inhibiting TBK1 phosphorylation of IRF3. These studies culminate in showing that a single GnT residue, Y627, is required for the NY-1V GnT to inhibit RIG-I/TBK1-directed IRF3 phosphorylation and IFN-β induction. These findings define a potential virulence determinant within the NY-1V GnT that may permit hantavirus attenuation.

Innate immune response induced by baculovirus attenuates transgene expression in mammalian cells

The baculovirus Autographa californica nucleopolyhedrovirus (AcNPV) has been widely used to achieve a high level of foreign gene expression in insect cells, as well as for efficient gene transduction into mammalian cells without any replication. In addition to permitting efficient gene delivery, baculovirus has been shown to induce host innate immune responses in various mammalian cells and in mice. In this study, we examined the effects of the innate immune responses on gene expression by recombinant baculoviruses in cultured cells. The reporter gene expression in IRF3-deficient mouse embryonic fibroblasts (MEFs) infected with the recombinant baculovirus was shown to be enhanced in accordance with the suppression of beta interferon (IFN-β) production. Furthermore, efficient gene transduction by the recombinant baculovirus was achieved in MEFs deficient for stimulator of interferon genes (STING), TANK binding kinase 1 (TBK1), IFN regulatory factor 3 (IRF3), or IFN-β promoter stimulator 1 (IPS-1), but not in those deficient for IRF7, MyD88, or Z-DNA binding protein 1 (ZBP1)/DAI. Enhancement of gene expression by the recombinant baculovirus was also observed in human hepatoma cell lines replicating hepatitis C virus (HCV), in which innate immunity was impaired by the cleavage of IPS-1 by the viral protease. In addition, infection with the recombinant baculovirus expressing the BH3-only protein, BIMS, a potent inducer of apoptosis, resulted in a selective cell death in the HCV replicon cells. These results indicate that innate immune responses induced by infection with baculovirus attenuate transgene expression, and this characteristic might be useful for a selective gene transduction into cells with impaired innate immunity arising from infection with various viruses.

IL-17 receptor adaptor protein Act1/CIKS plays an evolutionarily conserved role in antiviral signaling

Double-stranded RNA-induced antiviral gene expression in mammalian cells requires activation of IFN regulatory factor 3 (IRF3). In this study, we show that the IL-17R adaptor protein Act1/CIKS is involved in this process. Small interfering RNA-mediated knockdown of Act1 in primary human skin fibroblasts specifically attenuates expression of IFN-β and IFN-stimulated antiviral genes induced by a synthetic viral mimic, polyinosinic-polycytidylic acid. Ectopic expression of Act1 potentiates the IRF3-driven expression of a synthetic reporter construct as well as the induction of antiviral genes. We demonstrate that this effect is dependent on the ability of Act1 to functionally and physically interact with IκB kinase ε (IKKε), a known IRF3 kinase, and IRF3: 1) Act1 binds IKKε and IRF3; 2) Act1-induced IRF3 activation can be blocked specifically by coexpression of a catalytically inactive mutant of IKKε; and 3) mutants of IRF3, either lacking the C terminus or mutated at the key phosphorylation sites, important for its activation by IKKε, do not support Act1-dependent IRF3 activation. We also show that a zebrafish Act1 protein is able to trigger antiviral gene expression in human cells, which suggests an evolutionarily conserved function of vertebrate Act1 in the host defense against viruses. On the whole, our study demonstrates that Act1 is a component of antiviral signaling.

Heterozygous TBK1 mutations impair TLR3 immunity and underlie herpes simplex encephalitis of childhood

Two unrelated children with HSE carry distinct heterozygous mutations in the gene encoding TANK-binding kinase 1.

Childhood herpes simplex virus-1 (HSV-1) encephalitis (HSE) may result from single-gene inborn errors of TLR3 immunity. TLR3-dependent induction of IFN-α/β or IFN-λ is crucial for protective immunity against primary HSV-1 infection in the central nervous system (CNS). We describe here two unrelated children with HSE carrying different heterozygous mutations (D50A and G159A) in TBK1, the gene encoding TANK-binding kinase 1, a kinase at the crossroads of multiple IFN-inducing signaling pathways. Both mutant TBK1 alleles are loss-of-function but through different mechanisms: protein instability (D50A) or a loss of kinase activity (G159A). Both are also associated with an autosomal-dominant (AD) trait but by different mechanisms: haplotype insufficiency (D50A) or negative dominance (G159A). A defect in polyinosinic-polycytidylic acid–induced TLR3 responses can be detected in fibroblasts heterozygous for G159A but not for D50A TBK1. Nevertheless, viral replication and cell death rates caused by two TLR3-dependent viruses (HSV-1 and vesicular stomatitis virus) were high in fibroblasts from both patients, and particularly so in G159A TBK1 fibroblasts. These phenotypes were rescued equally well by IFN-α2b. Moreover, the IFN responses to the TLR3-independent agonists and viruses tested were maintained in both patients’ peripheral blood mononuclear cells and fibroblasts. The narrow, partial cellular phenotype thus accounts for the clinical phenotype of these patients being limited to HSE. These data identify AD partial TBK1 deficiency as a new genetic etiology of childhood HSE, indicating that TBK1 is essential for the TLR3- and IFN-dependent control of HSV-1 in the CNS.

ORF45 of Kaposi's sarcoma-associated herpesvirus inhibits phosphorylation of interferon regulatory factor 7 by IKKε and TBK1 as an alternative substrate

Open reading frame 45 (ORF45) of Kaposi's sarcoma-associated herpesvirus (KSHV) is an immediate-early and tegument protein that plays critical roles in antagonizing host antiviral responses. We have previously shown (Zhu et al, Proc. Natl. Acad. Sci. U. S. A., 99:5573-5578, 2002) that ORF45 suppresses activation of interferon regulatory factor 7 (IRF7), a crucial regulator of type I interferon gene expression, by blocking its virus-induced phosphorylation and nuclear accumulation. We report here further characterization of the mechanisms by which ORF45 inhibits IRF7 phosphorylation. In most cell types, IRF7 is phosphorylated and activated by IKKε and TBK1 after viral infection. We found that phosphorylation of IRF7 on Ser477 and Ser479 by IKKε or TBK1 is inhibited by ORF45. The inhibition is specific to IRF7 because phosphorylation of its close relative IRF3 is not affected by ORF45, implying that ORF45 does not inactivate the kinases directly. In fact, we found that ORF45 is phosphorylated efficiently on Ser41 and Ser162 by IKKε and TBK1. We demonstrated that ORF45 competes with the associated IRF7 and inhibits its phosphorylation by IKKε or TBK1 by acting as an alternative substrate.

Essential role for IKKβ in production of type 1 interferons by plasmacytoid dendritic cells

Plasmacytoid dendritic cells (pDCs) are characterized by their ability to produce high levels of type 1 interferons in response to ligands that activate TLR7 and TLR9, but the signaling pathways required for IFN production are incompletely understood. Here we exploit the human pDC cell line Gen2.2 and improved pharmacological inhibitors of protein kinases to address this issue. We demonstrate that ligands that activate TLR7 and TLR9 require the TAK1-IKKβ signaling pathway to induce the production of IFNβ via a pathway that is independent of the degradation of IκBα. We also show that IKKβ activity, as well as the subsequent IFNβ-stimulated activation of the JAK-STAT1/2 signaling pathway, are essential for the production of IFNα by TLR9 ligands. We further show that TLR7 ligands CL097 and R848 fail to produce significant amounts of IFNα because the activation of IKKβ is not sustained for a sufficient length of time. The TLR7/9-stimulated production of type 1 IFNs is inhibited by much lower concentrations of IKKβ inhibitors than those needed to suppress the production of NFκB-dependent proinflammatory cytokines, such as IL-6, suggesting that drugs that inhibit IKKβ may have a potential for the treatment of forms of lupus that are driven by self-RNA and self-DNA-induced activation of TLR7 and TLR9, respectively.

Characterization of chicken melanoma differentiation-associated gene 5 (MDA5) from alternative translation initiation

The present study was undertaken to identify and characterize chicken melanoma differentiation-associated gene 5 (MDA5) from alternative translation initiation. The alternatively translated chicken MDA5 had an open reading frame of 3309 base pairs, shorter than the predicted chicken MDA5 sequence by 549 base pairs and longer than the recently published chicken MDA5 (GU570144) by 303 base pairs. The domain architecture was conserved among MDA5 molecules from various vertebrates. The alternatively translated chicken MDA5 shared high genetic identity to zebrafinch MDA5, but not to fish or mammalian MDA5. Various concentrations of chicken MDA5 transcripts were detected in different chicken tissues, with the highest concentration found in the intestine (1.4×10(10)copy/mg tissue). Chicken interferon (IFN-β) expression was dependent on the dose of pCDNA3.1-MDA5-long transfection in DF1 cells without double-stranded RNA stimulation. DF1 cells transfected with polyinosinic-polycytidylic acid (poly(I:C)) had significantly (p<0.05) increased fold changes in chicken MDA5 and IFN-β mRNA expression as compared to those in the controls. DF1 cells incubated with poly(I:C) did not have significant (p>0.05) changes in mRNA expression of chicken MDA5 and IFN-β. Chicken MDA5 mRNA level did not change significantly (p>0.05) in DF1 cells treated with imiquimod or bacterial DNA. The results indicate that although chicken MDA5 is phylogenetically different from mammalian MDA5, the domain architecture remains conserved. Chicken MDA5 can be detected in various tissues and functions to signal the activation of chicken IFN-β when overexpressing or being stimulated by poly(I:C).

Emerging role of ubiquitination in antiviral RIG-I signaling

Detection of viruses by the innate immune system involves the action of specialized pattern recognition receptors. Intracellular RIG-I receptors sense the presence of viral nucleic acids in infected cells and trigger signaling pathways that lead to the production of proinflammatory and antiviral proteins. Over the past few years, posttranslational modification of RIG-I and downstream signaling proteins by different types of ubiquitination has been found to be a key event in the regulation of RIG-I-induced NF-κB and interferon regulatory factor 3 (IRF3) activation. Multiple ubiquitin ligases, deubiquitinases, and ubiquitin binding scaffold proteins contribute to both positive and negative regulation of the RIG-I-induced antiviral immune response. A better understanding of the function and activity of these proteins might eventually lead to the development of novel therapeutic approaches for management of viral diseases.

Systematic discovery of TLR signaling components delineates viral-sensing circuits

Deciphering the signaling networks that underlie normal and disease processes remains a major challenge. Here, we report the discovery of signaling components involved in the Toll-like receptor (TLR) response of immune dendritic cells (DCs), including a previously unkown pathway shared across mammalian antiviral responses. By combining transcriptional profiling, genetic and small-molecule perturbations, and phosphoproteomics, we uncover 35 signaling regulators, including 16 known regulators, involved in TLR signaling. In particular, we find that Polo-like kinases (Plk) 2 and 4 are essential components of antiviral pathways in vitro and in vivo and activate a signaling branch involving a dozen proteins, among which is Tnfaip2, a gene associated with autoimmune diseases but whose role was unknown. Our study illustrates the power of combining systematic measurements and perturbations to elucidate complex signaling circuits and discover potential therapeutic targets.

Cellular and molecular biology of optineurin

Optineurin is a gene linked to glaucoma, amyotrophic lateral sclerosis, other neurodegenerative diseases, and Paget's disease of bone. This review describes the characteristics of optineurin and summarizes the cellular and molecular biology investigations conducted so far on optineurin. Data from a number of laboratories indicate that optineurin is a cytosolic protein containing 577 amino acid residues. Interacting with proteins such as myosin VI, Rab8, huntingtin, transferrin receptor, and TANK-binding kinase 1, optineurin is involved in basic cellular functions including protein trafficking, maintenance of the Golgi apparatus, as well as NF-κB pathway, antiviral, and antibacteria signaling. Mutation or alteration of homeostasis of optineurin (such as overexpression or knockdown) results in adverse consequences in the cells, leading to the development of neurodegenerative diseases including glaucoma.

Functional characterization of protein domains common to animal viruses and mouse

Background

Many viruses contain genes that originate from their hosts. Some of these acquired genes give viruses the ability to interfere with host immune responses by various mechanisms. Genes of host origin that appear commonly in viruses code for proteins that span a wide range of functions, from kinases and phosphotases, to cytokines and their receptors, to ubiquitin ligases and proteases. While many important cases of such lateral gene transfer in viruses have been documented, there has yet to be a genome-wide survey of viral-encoded genes acquired from animal hosts.

Results

Here we carry out such a survey in order to gain insight into the host immune system. We made the results available in the form of a web-based tool that allows viral-centered or host-centered queries to be performed (http://imm.ifrec.osaka-u.ac.jp/musvirus/). We examine the relationship between acquired genes and immune function, and compare host-virus homology with gene expression data in stimulated dendritic cells and T-cells. We found that genes whose expression changes significantly during the innate antiviral immune response had more homologs in animal virus than genes whose expression did not change or genes involved in the adaptive immune response.

Conclusions

Statistics gathered from the MusVirus database support earlier reports of gene transfer from host to virus and indicate that viruses are more likely to acquire genes involved in innate antiviral immune responses than those involved in acquired immune responses.

Ebolavirus VP35 suppresses IFN production from conventional but not plasmacytoid dendritic cells

Ebolaviruses naturally infect a wide variety of cells including macrophages and dendritic cells (DCs), and the resulting cytokine and interferon-α/β (IFN) responses of infected cells are thought to influence viral pathogenesis. The VP35 protein impairs RIG-I-like receptor-dependent signaling to inhibit IFN production, and this function has been suggested to promote the ineffective host immune response characteristic of ebolavirus infection. To assess the impact of VP35 on innate immunity in biologically relevant primary cells, we used a recombinant Newcastle disease virus encoding VP35 (NDV/VP35) to infect macrophages and conventional DCs, which primarily respond to RNA virus infection via RIG-I-like pathways. VP35 suppressed not only IFN but also tumor necrosis factor (TNF)-α secretion, which are normally produced from these cells upon NDV infection. Additionally, in cells susceptible to the activity of VP35, IRF7 activation is impaired. In contrast, NDV/VP35 infection of plasmacytoid DCs, which activate IRF7 and produce IFN through TLR-dependent signaling, leads to robust IFN production. When plasmacytoid DCs deficient for TLR signaling were infected, NDV/VP35 was able to inhibit IFN production. Consistent with this, VP35 was less able to inhibit TLR-dependent versus RIG-I-dependent signaling in vitro. These data demonstrate that ebolavirus VP35 suppresses both IFN and cytokine production in multiple primary human cell types. However, cells that utilize the TLR pathway can circumvent this inhibition, suggesting that the presence of multiple viral sensors enables the host to overcome viral immune evasion mechanisms.

Polyubiquitin binding to optineurin is required for optimal activation of TANK-binding kinase 1 and production of interferon β

TANK-binding kinase (TBK1) is essential for transcription of the interferon (IFN) β gene in response to lipopolysaccharide (LPS) and double-stranded RNA, but the molecular mechanisms that underlie the activation of TBK1 are incompletely understood. Previously, we identified the NF-κB essential modulator (NEMO)-related polyubiquitin-binding protein, optineurin (OPTN), as a novel binding partner of TBK1. To determine whether the ubiquitin-binding function of OPTN is involved in regulating TBK1 and IFNβ production, we generated a mouse in which wild-type optineurin was replaced by the polyubiquitin binding-defective mutant, OPTN(D477N/D477N). In this study, we found that LPS or poly(I:C)-induced TBK1 activity was significantly reduced in bone marrow-derived macrophage (BMDM) from OPTN(D477N/D477N) mice. Consistent with this, the phosphorylation of IFN regulatory factor 3 (IRF3) and the production of IFNβ mRNA and secretion were reduced. Stimulation of BMDMs with LPS triggered the phosphorylation of OPTN, which was reversed by phosphatase treatment and prevented by pharmacological inhibition of both the canonical IκB kinases (IKKα/β) and the IKK-related kinases (TBK1/IKKε). In contrast, LPS-stimulated phosphorylation of OPTN(D477N) was markedly reduced in BMDMs from OPTN(D477N/D477N) mice, and inhibition of the canonical IKKs alone prevented phosphorylation, providing further evidence that ubiquitin binding to OPTN contributes to LPS-induced TBK1 activation. TBK1 and IKKβ phosphorylated OPTN preferentially at Ser-177 and Ser-513, respectively, in vitro. In conclusion, our results suggest that OPTN binds to polyubiquitylated species formed in response to LPS and poly(I:C), enhancing the activation of TBK1 that is required for optimal phosphorylation of IRF3 and production of IFNβ.

DDX1, DDX21, and DHX36 helicases form a complex with the adaptor molecule TRIF to sense dsRNA in dendritic cells

The innate immune system detects viral infection predominantly by sensing viral nucleic acids. We report the identification of a viral sensor, consisting of RNA helicases DDX1, DDX21, and DHX36, and the adaptor molecule TRIF, by isolation and sequencing of poly I:C-binding proteins in myeloid dendritic cells (mDCs). Knockdown of each helicase or TRIF by shRNA blocked the ability of mDCs to mount type I interferon (IFN) and cytokine responses to poly I:C, influenza A virus, and reovirus. Although DDX1 bound poly I:C via its Helicase A domain, DHX36 and DDX21 bound the TIR domain of TRIF via their HA2-DUF and PRK domains, respectively. This sensor was localized within the cytosol, independent of the endosomes. Thus, the DDX1-DDX21-DHX36 complex represents a dsRNA sensor that uses the TRIF pathway to activate type I IFN responses in the cytosol of mDCs.

NF-kappa B RelA subunit is crucial for early IFN-beta expression and resistance to RNA virus replication

RNA virus infection results in expression of type 1 IFNs, especially IFN-alpha/beta, which play a crucial role in host antivirus responses. Type 1 IFNs are induced in a cell type-specific manner through TLR and RIG-I-like receptor pathways, both of which activate IFN regulatory factors (IRFs) and NF-kappaB transcription factors. Although NF-kappaB activation and association with the IFN-beta promoter after RNA virus infection is well documented, our previous work showed that, surprisingly, NF-kappaB is not essential for IFN-beta gene expression. Thus, the actual function of NF-kappaB in IFN-beta expression and virus replication is not clear. In this study, we found Newcastle disease virus and vesicular stomatitis virus replication is enhanced in mouse embryonic fibroblasts (MEFs) lacking the NF-kappaB RelA subunit. Increased virus replication was traced to a specific requirement for RelA in early virus-induced IFN-beta expression. At these time points, when IFN-beta expression is ~100-fold less than peak levels, impaired IFN-beta production delayed IFN-induced gene expression, resulting in increased virus replication in RelA(-/-) MEFs. Importantly, our results show that RelA requirement is crucial only when IRF3 activation is low. Thus, high levels of activated IRF3 expression are sufficient for induction of IFN-beta in RelA(-/-) MEFs, transcriptional synergism with the coactivator CREB-binding protein, and rescue of susceptibility to virus. Together, these findings indicate that NF-kappaB RelA is not crucial for regulating overall IFN-beta production, as previously believed; instead, RelA is specifically required only during a key early phase after virus infection, which substantially impacts the host response to virus infection.

Molecular cloning and immune responsive expression of MDA5 gene, a pivotal member of the RLR gene family from grass carp Ctenopharyngodon idella

The cytoplasmic helicase protein MDA5 (melanoma-differentiation-associated gene-5) recognizes long molecules of viral double-stranded RNA (dsRNA) and single-stranded RNA with 5' triphosphate and mediates type I interferon secretion. In the present study, the first MDA5 gene in fish was cloned and characterized from grass carp Ctenopharyngodon idella. The full length of the C. idella MDA5 (CiMDA5) cDNA is 3233 nucleotides in length and encodes a polypeptide of 961 amino acids. The deduced amino acid sequence contained six main structural domains: a CARD (caspase activation and recruitment domain), a DEXDc (DEAD/DEAH box helicase domain), a ResIII (conserved restriction domain of bacterial type III restriction enzyme), two HELICc (helicase superfamily c-terminal domain) and an RD (regulatory domain). The CiMDA5 mRNA was widespread expression in the tested tissues, was high level in spleen, skin and gill tissues, and was up-regulated by GCRV injection by semi-quantitative RT-PCR assay. The CiMDA5 transcripts in spleen were significantly up-regulated at 12 h (1.80 folds, P < 0.05), reached the crest at 24 h (7.48 folds, P < 0.05), and then recovered to normal level at 48 h post-injection (P > 0.05) of grass carp reovirus (GCRV). In liver, the temporal expression of CiMDA5 mRNA was significantly increased at 48 h (5.00 folds, P < 0.05) and returned to control level at 72 h (P > 0.05) after GCRV challenge. These data implied that CiMDA5 involved in the early stage of antiviral innate immune defense to GCRV in grass carp, and provided new insight into the evolution research of RLR (RIG-I like receptor) gene family.

SUMOylation-dependent localization of IKKepsilon in PML nuclear bodies is essential for protection against DNA-damage-triggered cell death

The IKK-related kinase IKKepsilon contributes to the antiviral response and can function as an oncogene that is frequently amplified in breast cancer. Here we report on an additional role of IKKepsilon as a mediator protecting from DNA-damage-induced cell death. Genotoxic stress allows for kinase-dependent entry of IKKepsilon into the nucleus, where IKKepsilon-dependent PML phosphorylation is a prerequisite for retention of this kinase in PML nuclear bodies. Within these subnuclear structures IKKepsilon inducibly colocalizes with TOPORS, which functions as a SUMO E3 ligase mediating SUMOylation of IKKepsilon at lysine 231. SUMO modification of IKKepsilon is required to trigger phosphorylation of nuclear substrates including NF-kappaB p65, thereby contributing to the antiapoptotic function of NF-kappaB in response to DNA damage.

Polo-like kinase 1 (PLK1) regulates interferon (IFN) induction by MAVS

The mitochondria-bound adapter MAVS participates in IFN induction by recruitment of downstream partners such as members of the TRAF family, leading to activation of NF-kappaB, and the IRF3 pathways. A yeast two-hybrid search for MAVS-interacting proteins yielded the Polo-box domain (PBD) of the mitotic Polo-like kinase PLK1. We showed that PBD associates with two different domains of MAVS in both dependent and independent phosphorylation events. The phosphodependent association requires the phosphopeptide binding ability of PBD. It takes place downstream of the proline-rich domain of MAVS, within an STP motif, characteristic of the binding of PLK1 to its targets, where the central Thr234 residue is phosphorylated. Its phosphoindependent association takes place at the C terminus of MAVS. PLK1 strongly inhibits the ability of MAVS to activate the IRF3 and NF-kappaB pathways and to induce IFN. Reciprocally, depletion of PLK1 can increase IFN induction in response to RIG-I/SeV or RIG-I/poly(I)-poly(C) treatments. This inhibition is dependent on the phosphoindependent association of PBD at the C terminus of MAVS where it disrupts the association of MAVS with its downstream partner TRAF3. IFN induction was strongly inhibited in cells arrested in G2/M by nocodazole, which provokes increased expression of endogenous PLK1. Interestingly, depletion of PLK1 from these nocodazole-treated cells could restore, at least partially, IFN induction. Altogether, these data demonstrate a new function for PLK1 as a regulator of IFN induction and provide the basis for the development of inhibitors preventing the PLK1/MAVS association to sustain innate immunity.

Crosstalk between components of the innate immune system: promoting anti-microbial defenses and avoiding immunopathologies

Because it reaches full functional efficacy rapidly upon encounter with a pathogen, the innate immune system is considered as the first line of defense against infections. The sensing of microbes or of transformed or infected cells, through innate immune recognition receptors (referred to as activating I2R2), initiates pro-inflammatory responses and innate immune effector functions. Other I2R2 with inhibitory properties bind self-ligands constitutively expressed in host. However, this dichotomy in the recognition of foreign or induced self versus constitutive self by I2R2 is not always respected in certain non-infectious conditions reminiscent of immunopathologies. In this review, we discuss that immune mechanisms have evolved to avoid inappropriate inflammatory disorders in individuals. Molecular crossregulation exists between components of I2R2 signaling pathways, and intricate interactions between cells from both innate and adaptive immune systems set the bases of controlled immune responses. We also pinpoint that, like T or B cells, some cells of the innate immune system must go through education processes to prevent autoreactivity. In addition, we illustrate how gene expression profiling of immune cell types is a useful tool to find functional homologies between cell subsets of different species and to speculate about unidentified functions of these cells in the responses to pathogen infections.

Cellular models for the screening and development of anti-hepatitis C virus agents

Investigations on the biology of hepatitis C virus (HCV) have been hampered by the lack of small animal models. Efforts have therefore been directed to designing practical and robust cellular models of human origin able to support HCV replication and production in a reproducible, reliable and consistent manner. Many different models based on different forms of virions and hepatoma or other cell types have been described including virus-like particles, pseudotyped particles, subgenomic and full length replicons, virion productive replicons, immortalised hepatocytes, fetal and adult primary human hepatocytes. This review focuses on these different cellular models, their advantages and disadvantages at the biological and experimental levels, and their respective use for evaluating the effect of antiviral molecules on different steps of HCV biology including virus entry, replication, particles generation and excretion, as well as on the modulation by the virus of the host cell response to infection.

Ubiquitin-regulated recruitment of IkappaB kinase epsilon to the MAVS interferon signaling adapter

Induction of the antiviral interferon response is initiated upon recognition of viral RNA structures by the RIG-I or Mda-5 DEX(D/H) helicases. A complex signaling cascade then converges at the mitochondrial adapter MAVS, culminating in the activation of the IRF and NF-kappaB transcription factors and the induction of interferon gene expression. We have previously shown that MAVS recruits IkappaB kinase epsilon (IKKepsilon) but not TBK-1 to the mitochondria following viral infection. Here we map the interaction of MAVS and IKKepsilon to the C-terminal region of MAVS and demonstrate that this interaction is ubiquitin dependent. MAVS is ubiquitinated following Sendai virus infection, and K63-linked ubiquitination of lysine 500 (K500) of MAVS mediates recruitment of IKKepsilon to the mitochondria. Real-time PCR analysis reveals that a K500R mutant of MAVS increases the mRNA level of several interferon-stimulated genes and correlates with increased NF-kappaB activation. Thus, recruitment of IKKepsilon to the mitochondria upon MAVS K500 ubiquitination plays a modulatory role in the cascade leading to NF-kappaB activation and expression of inflammatory and antiviral genes. These results provide further support for the differential role of IKKepsilon and TBK-1 in the RIG-I/Mda5 pathway.

Cutting Edge: TLR-Dependent viral recognition along with type I IFN positive feedback signaling masks the requirement of viral replication for IFN-{alpha} production in plasmacytoid dendritic cells

Plasmacytoid dendritic cells (pDCs) recognize RNA virus infection via TLRs and consequently produce vast amounts of type I IFN. Because nucleic acid-sensing TLRs reside in the intracellular membrane compartment, it is presumable that pDCs do not require cytoplasmic viral replication to recognize the infection. By checking Newcastle disease virus (NDV) RNA abundance in GFP(+) and GFP(-) pDCs from Ifna6gfp mice, we found that NDV replication was not detected in IFN-producing pDCs. GFP(+) pDC was induced in response to replication-incompetent NDV. In contrast, the replication-incompetent NDV failed to induce IFN-producing pDCs in type I IFNR-deficient mice. The lack of IFNR signaling led to the replication of NDV and the subsequent RIG-I-like helicase-dependent IFN-alpha production in pDCs. These results showed that detection of viruses via TLRs together with a type I IFN feedback system circumvents the requirement for viral replication-dependent recognition in pDCs.

Characterization of the interferon-producing cell in mice infected with Listeria monocytogenes

Production of type I interferons (IFN-I, mainly IFNα and IFNβ) is a hallmark of innate immune responses to all classes of pathogens. When viral infection spreads to lymphoid organs, the majority of systemic IFN-I is produced by a specialized “interferon-producing cell” (IPC) that has been shown to belong to the lineage of plasmacytoid dendritic cells (pDC). It is unclear whether production of systemic IFN-I is generally attributable to pDC irrespective of the nature of the infecting pathogen. We have addressed this question by studying infections of mice with the intracellular bacterium Listeria monocytogenes. Protective innate immunity against this pathogen is weakened by IFN-I activity. In mice infected with L. monocytogenes, systemic IFN-I was amplified via IFN-β, the IFN-I receptor (IFNAR), and transcription factor interferon regulatory factor 7 (IRF7), a molecular circuitry usually characteristic of non-pDC producers. Synthesis of serum IFN-I did not require TLR9. In contrast, in vitro–differentiated pDC infected with L. monocytogenes needed TLR9 to transcribe IFN-I mRNA. Consistent with the assumption that pDC are not the producers of systemic IFN-I, conditional ablation of the IFN-I receptor in mice showed that most systemic IFN-I is produced by myeloid cells. Furthermore, results obtained with FACS-purified splenic cell populations from infected mice confirmed the assumption that a cell type with surface antigens characteristic of macrophages and not of pDC is responsible for bulk IFN-I synthesis. The amount of IFN-I produced in the investigated mouse lines was inversely correlated to the resistance to lethal infection. Based on these data, we propose that the engagement of pDC, the mode of IFN-I mobilization, as well as the shaping of the antimicrobial innate immune response by IFN-I differ between intracellular pathogens.

Type I Interferons (IFN-I) are cytokines produced by the innate immune system immediately after intrusion of a pathogen. To produce large quantities of IFN-I once an infection is starting to spread throughout the body, the innate immune system employs a specialized “interferon-producing cell” (IPC). In the case of viral infections, IFN-I protect the host organism from rapid viral replication and spread. Conversely, organisms that cannot produce IFN-I are exquisitely sensitive to viral infections. Intriguingly, the opposite has been reported for the pathogen Listeria monocytogenes. Like virus, this bacterium replicates within cells of the host organism and stimulates IFN-I synthesis. Unlike virus, however, IFN-I sensitize the infected host to lethal pathology resulting from L. monocytogenes infection. In this article, we show that all tested molecules contributing to IFN-I production in Listeria-infected mice are responsible for a corresponding increase in mortality. We address the question of which cell type is responsible for producing vast quantities of IFN-I that can be measured in the serum of mice infected with Listeria. We show that these are not IPC, but rather macrophages, cells specialized to ingest and kill bacteria. We conclude that the engagement of cells for IFN-I production and also the effect of IFN-I on innate immunity is determined by the tropism and lifestyle of a particular pathogen.

TANK-binding kinase-1 plays an important role during in vitro and in vivo type I IFN responses to DNA virus infections

TANK-binding kinase-1 (TBK1) and the inducible IkappaB kinase (IKK-i) have recently been shown to activate type I IFN responses elicited by intracellular detection of RNA or DNA from infecting viruses. Detection of viral RNA is mediated by retinoic acid inducible gene-I or melanoma differentiation-associated gene-5 pathways in which TBK1 and IKK-i have been demonstrated to play redundant roles in IFN activation. In this study, we have examined whether such redundancy occurs in the type I IFN response to DNA viral challenges by examining induction of IFNs and IFN-mediated signaling and gene programs in TBK1(-/-) macrophages. In contrast to the normal IFN responses in TBK1(-/-) macrophages infected with an RNA virus, IFN responses were severely abrogated during DNA virus infections in TBK1(-/-) macrophages. Because both TBK1 and IKK-i are expressed in macrophages, our studies suggest that TBK1 and IKK-i differ functionally in DNA virus-mediated IFN responses; however, they are redundant in RNA virus-mediated IFN responses. Confirmatively, reconstitution of TBK1(-/-)IKK-i(-/-) fibroblasts revealed that TBK1 rescued IFN responses to transfected B-DNA to a much stronger degree than IKK-i. Finally, we demonstrate the requirement for the TBK1-IFN regulatory factor-3 pathway in host defense against a DNA virus infection in vivo.

TLR7-dependent and FcgammaR-independent production of type I interferon in experimental mouse lupus

Increased type I interferon (IFN-I) production and IFN-stimulated gene (ISG) expression are linked to the pathogenesis of systemic lupus erythematosus (SLE). Although the mechanisms responsible for dysregulated IFN-I production in SLE remain unclear, autoantibody-mediated uptake of endogenous nucleic acids is thought to play a role. 2,6,10,14-tetramethylpentadecane (TMPD; also known as pristane) induces a lupus-like disease in mice characterized by immune complex nephritis with autoantibodies to DNA and ribonucleoproteins. We recently reported that TMPD also causes increased ISG expression and that the development of the lupus is completely dependent on IFN-I signaling (Nacionales, D.C., K.M. Kelly-Scumpia, P.Y. Lee, J.S. Weinstein, R. Lyons, E. Sobel, M. Satoh, and W.H. Reeves. 2007. Arthritis Rheum. 56:3770–3783). We show that TMPD elicits IFN-I production, monocyte recruitment, and autoantibody production exclusively through a Toll-like receptor (TLR) 7– and myeloid differentiation factor 88 (MyD88)–dependent pathway. In vitro studies revealed that TMPD augments the effect of TLR7 ligands but does not directly activate TLR7 itself. The effects of TMPD were amplified by the Y-linked autoimmune acceleration cluster, which carries a duplication of the TLR7 gene. In contrast, deficiency of Fcγ receptors (FcγRs) did not affect the production of IFN-I. Collectively, the data demonstrate that TMPD-stimulated IFN-I production requires TLR7/MyD88 signaling and is independent of autoantibody-mediated uptake of ribonucleoproteins by FcγRs.