Inhibition of Antiviral Innate Immunity by Avibirnavirus VP3 via Blocking TBK1-TRAF3 Complex Formation and IRF3 Activation

Chicken hnRNPK suppresses interferon production, thereby enhancing IBDV replication

Heterogeneous ribonucleoprotein K (hnRNPK) is a well-known RNA-binding protein initially identified for its role in inhibiting the growth of various human tumors. Members of the hnRNP family have also been implicated in both interferon production and RNA virus replication. However, the role of chicken hnRNPK (chhnRNPK) in the replication of Infectious Bursal Disease Virus (IBDV) remains unclear. In this study, we identified chhnRNPK as a protein that interacts with genomic double-stranded RNA (dsRNA). Following IBDV infection, chhnRNPK was recruited to the virus replication complex in the cytoplasm. Furthermore, chhnRNPK expression inhibited dsRNA-induced interferon production, specifically at the mitochondrial antiviral signaling protein (MAVS) step. Overexpression of chhnRNPK significantly enhanced virus replication, while knockdown of chhnRNPK increased dsRNA-induced interferon production and subsequently disrupted IBDV replication. Collectively, these findings suggest that chhnRNPK promotes IBDV replication by interacting with genomic dsRNA, highlighting a novel host factor that regulates viral replication.

Infectious bursal disease virus affecting interferon regulatory factor 7 signaling through VP3 protein to facilitate viral replication

Interferon regulatory factor 7 (IRF7)-mediated type I interferon antiviral response is crucial for regulating the host following viral infection in chickens. Infectious bursal disease virus (IBDV) is a double-stranded RNA virus that induces immune suppression and high mortality rates in chickens aged 3-6 weeks. Previous studies have shown that IBDV infection antagonizes the type I interferon production to facilitate viral replication in the cell, and IRF7 signaling might play an important role. However, the underlying mechanisms that enable IBDV to block the IRF7 pathway remain unclear. In this study, we found that IRF7 and IFN-β expression were suppressed in DF-1 cells during infection with very virulent IBDV (vvIBDV), but not with attenuated IBDV, while the virus continued to replicate. Overexpression of IRF7 inhibits IBDV replication while knocking down IRF7 promotes IBDV replication. Overexpression of IRF7 couldn't compensate the IRF7 protein level in vvIBDV-infected cells, which suggested that IRF7 protein was degraded by IBDV infection. By using inhibitors, the degradation of IRF7 was found to be related to the proteasome pathway. Further study revealed that IRF7 was observed to interact and colocalize with the IBDV VP3 protein. Consistent with IBDV infection results, IBDV VP3 protein was observed to inhibit the IRF7-IFN-β expression, affect the degradation of IRF7 protein via proteasome pathway. All these results suggest that the IBDV exploits IRF7 by affecting its expression and proteasome degradation via the viral VP3 protein to facilitate viral replication in the cells. These findings revealed a novel mechanism that IBDV uses to evade host antiviral defense.

A novel role of ETV6 as a pro-viral factor in host response by inhibiting TBK1 phosphorylation

E26-transforming specific (ETS) variant 6 (ETV6) is a transcription factor regulating the expression of interferon stimulating genes (ISGs) and involved in the embryonic development and hematopoietic regulation, but the role of ETV6 in host response to virus infection is not clear. In this study, we show that ETV6 was upregulated in DF-1 cells with poly(I:C) stimulation or IBDV, AIV and ARV infection via engagement of dsRNA by MDA5. Overexpression of ETV6 in DF-1 cells markedly inhibited IBDV-induced type I interferon (IFN-I) and ISGs expressions. In contrast, knockdown, or knockout of ETV6 remarkably inhibited IBDV replication via promoting IFN-I response. Furthermore, our data show that ETV6 negatively regulated host antiviral response to IBDV infection by interaction with TANK binding kinase 1 (TBK1) and subsequently inhibited its phosphorylation. These results uncovered a novel role of ETV6 as a pro-viral factor in host response by inhibiting TBK1 phosphorylation, furthering our understandings of RNA virus immunosuppression and providing a valuable clue to the development of antiviral reagents for the control of avian RNA virus infection.

Indobufen alleviates ischemic stroke injury by regulating transcription factor NRF2 and inhibiting ATG5 expression

BACKGROUND

Ischemic stroke (IS) is a detrimental neurological disease and IS lacks valuable methods to recover body function. Indobufen (IND) could alleviate IS. However, the possible mechanism remains undefined.

METHODS

SH-SY5Y cells were cultured under the oxygen-glucose deprivation/reoxygenation (OGD/R) environment and then were treated with small interfering RNA (siRNA) of NRF2 and ATG5. The influence of various concentrations of IND (50 μM, 100 μM, 200 μM, and 400 μM) was determined by 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide. Levels of superoxide dismutase (SOD) and malonaldehyde (MDA) were examined by ELISA. Reactive oxygen species (ROS) production was determined by DCFH-DA staining. The protein levels of LC3II/LC3I, Beclin1, p62, NRF2, and ATG5 were detected by western blot.

RESULTS

IND increased cell viability, while depressed the rate of apoptosis in SH-SY5Y cells of OGD/R environment. IND inhibited autophagy by suppressing the levels of LC3II/LC3I, Beclin1 protein, and increasing p62 protein expression in SH-SY5Y cells of OGD/R environment. IND limited the contents of ROS and MDA, while amplifying the activity of SOD in SH-SY5Y cells with OGD/R exposure. IND also promoted NRF2 expression in OGD/R environment.

CONCLUSION

IND could inhibit autophagy, oxidative stress, and apoptosis in SH-SY5Y cells with OGD/R exposure, further alleviating IS injury by regulating transcription factor NRF2 and inhibiting ATG5 expression.

Exploring type I interferon pathway: virulent vs. attenuated strain of African swine fever virus revealing a novel function carried by MGF505-4R

African swine fever virus represents a significant reemerging threat to livestock populations, as its incidence and geographic distribution have surged over the past decade in Europe, Asia, and Caribbean, resulting in substantial socio-economic burdens and adverse effects on animal health and welfare. In a previous report, we described the protective properties of our newly thermo-attenuated strain (ASFV-989) in pigs against an experimental infection of its parental Georgia 2007/1 virulent strain. In this new study, our objective was to characterize the molecular mechanisms underlying the attenuation of ASFV-989. We first compared the activation of type I interferon pathway in response to ASFV-989 and Georgia 2007/1 infections, employing both in vivo and in vitro models. Expression of IFN-α was significantly increased in porcine alveolar macrophages infected with ASFV-989 while pigs infected with Georgia 2007/1 showed higher IFN-α than those infected by ASFV-989. We also used a medium-throughput transcriptomic approach to study the expression of viral genes by both strains, and identified several patterns of gene expression. Subsequently, we investigated whether proteins encoded by the eight genes deleted in ASFV-989 contribute to the modulation of the type I interferon signaling pathway. Using different strategies, we showed that MGF505-4R interfered with the induction of IFN-α/β pathway, likely through interaction with TRAF3. Altogether, our data reveal key differences between ASFV-989 and Georgia 2007/1 in their ability to control IFN-α/β signaling and provide molecular mechanisms underlying the role of MGF505-4R as a virulence factor.

RNA-seq transcriptome analysis provides new insights into the negative effects of tannic acid on the intestinal function of Brandt's voles (Lasiopodomys brandtii)

Tannic acid (TA), a significant plant secondary metabolite, is contained in the daily food of Brandt's voles. Its adverse effect on gut function has been shown in earlier research, but the underlying molecular mechanisms remain uncertain. In this study, male Brandt's vole (13 weeks old) were divided into two groups and given 0 (control) or 1,200 (TA-treated) mg•kg-1 TA for 18 days. Then RNA sequencing was used to conduct a thorough transcriptome analysis on the duodenum, jejunum, and ileum of Brandt's voles. Results showed that TA significantly increased serum total cholesterol concentration (P < 0.05) and decreased the nutrient digestibility (P < 0.05) of Brandt's voles. Furthermore, there were 174 differentially expressed genes (DEGs) in the duodenum, 96 DEGs in the jejunum, and 88 DEGs in the ileum between the control and TA-treated groups. Enrichment analysis revealed that many genes associated with bile secretion, fat digestion and absorption, innate immune response, and tight junction such as ABCG2, ABCG8, PEAK1, and IFR2, etc. were altered after TA treatment, which were verified by quantitative real-time PCR. These findings suggested that TA can change the expression of intestinal genes, thereby, altering nutrition metabolism and immunological function, eventually hindering the growth of Brandt's voles. The results of this study provide a theoretical basis for explaining how TA affects the gut function of Brandt's voles at the molecular level.

Protein kinase Cdc7 supports viral replication by phosphorylating Avibirnavirus VP3 protein

IMPORTANCE

The Avibirnavirus infectious bursal disease virus is still an important agent which largely threatens global poultry farming industry economics. VP3 is a multifunctional scaffold structural protein that is involved in virus morphogenesis and the regulation of diverse cellular signaling pathways. However, little is known about the roles of VP3 phosphorylation during the IBDV life cycle. In this study, we determined that IBDV infection induced the upregulation of Cdc7 expression and phosphorylated the VP3 Ser13 site to promote viral replication. Moreover, we confirmed that the negative charge addition of phosphoserine on VP3 at the S13 site was essential for IBDV proliferation. This study provides novel insight into the molecular mechanisms of VP3 phosphorylation-mediated regulation of IBDV replication.

Human OTUD6B positively regulates type I IFN antiviral innate immune responses by deubiquitinating and stabilizing IRF3

IMPORTANCE

Interferon (IFN) regulatory factor (IRF3) is one of the key factors for type I IFN transcription. To sophisticatedly regulate type I IFN antiviral immune response, IRF3 activity is closely controlled by a variety of post-translational modifications. However, the regulatory mechanisms are still not fully elucidated. In the present study, we found that human deubiquitinase OTUD6B positively regulates IRF3-mediated antiviral immune response. OTUD6B can stabilize the IRF3 protein level via hydrolyzing (Lys33)-linked polyubiquitin at Lys315. More importantly, mice with OTUD6B overexpression exhibited more resistance to RNA virus infection. Thus, unlike the previous report that zebrafish OTUD6B negatively regulates the antiviral response by suppressing K63-linked ubiquitination of IRF3 and IRF7, we demonstrate that human OTUD6B actually enhances type I IFN response and has the potential for antiviral therapy.

Reduced NR2F2 Expression in the Host Response to Infectious Bursal Disease Virus Infection Suppressed Viral Replication by Enhancing Type I Interferon Expression by Targeting SOCS5

Nuclear receptors are ligand-activated transcription factors that play an important role in regulating innate antiviral immunity and other biological processes. However, the role of nuclear receptors in the host response to infectious bursal disease virus (IBDV) infection remains elusive. In this study, we show that IBDV infection or poly(I·C) treatment of DF-1 or HD11 cells markedly decreased nuclear receptor subfamily 2 group F member 2 (NR2F2) expression. Surprisingly, knockdown, knockout, or inhibition of NR2F2 expression in host cells remarkably inhibited IBDV replication and promoted IBDV/poly(I·C)-induced type I interferon and interferon-stimulated genes expression. Furthermore, our data show that NR2F2 negatively regulates the antiviral innate immune response by promoting the suppressor of cytokine signaling 5 (SOCS5) expression. Thus, reduced NR2F2 expression in the host response to IBDV infection inhibited viral replication by enhancing the expression of type I interferon by targeting SOCS5. These findings reveal that NR2F2 plays a crucial role in antiviral innate immunity, furthering our understanding of the mechanism underlying the host response to viral infection. IMPORTANCE Infectious bursal disease (IBD) is an immunosuppressive disease causing considerable economic losses to the poultry industry worldwide. Nuclear receptors play an important role in regulating innate antiviral immunity. However, the role of nuclear receptors in the host response to IBD virus (IBDV) infection remains elusive. Here, we report that NR2F2 expression decreased in IBDV-infected cells, which consequently reduced SOCS5 expression, promoted type I interferon expression, and suppressed IBDV infection. Thus, NR2F2 serves as a negative factor in the host response to IBDV infection by regulating SOCS5 expression, and intervention in the NR2F2-mediated host response by specific inhibitors might be employed as a strategy for prevention and treatment of IBD.

DDX5 inhibits type I IFN production by promoting degradation of TBK1 and disrupting formation of TBK1 - TRAF3 complex

DExD/H-box helicase (DDX) 5 belongs to the DExD/H-box helicase family. DDX family members play differential roles in the regulation of innate antiviral immune response. However, whether DDX5 is involved in antiviral immunity remains unclear. In this study, we found that DDX5 serves as a negative regulator of type I interferon (IFN) response. Overexpression of DDX5 inhibited IFN production induced by Spring viremia of carp virus (SVCV) and poly(I:C) and enhanced virus replication by targeting key elements of the RLR signaling pathway (MAVS, MITA, TBK1, IRF3 and IRF7). Mechanistically, DDX5 directly interacted with TBK1 to promote its autophagy-mediated degradation. Moreover, DDX5 was shown to block the interaction between TRAF3 and TBK1, hence preventing nuclear translocation of IRF3. Together, these data shed light on the roles of DDX5 in regulating IFN response.

The Formation and Function of Birnaviridae Virus Factories

The use of infectious bursal disease virus (IBDV) reverse genetics to engineer tagged reporter viruses has revealed that the virus factories (VFs) of the Birnaviridae family are biomolecular condensates that show properties consistent with liquid-liquid phase separation (LLPS). Although the VFs are not bound by membranes, it is currently thought that viral protein 3 (VP3) initially nucleates the formation of the VF on the cytoplasmic leaflet of early endosomal membranes, and likely drives LLPS. In addition to VP3, IBDV VFs contain VP1 (the viral polymerase) and the dsRNA genome, and they are the sites of de novo viral RNA synthesis. Cellular proteins are also recruited to the VFs, which are likely to provide an optimal environment for viral replication; the VFs grow due to the synthesis of the viral components, the recruitment of other proteins, and the coalescence of multiple VFs in the cytoplasm. Here, we review what is currently known about the formation, properties, composition, and processes of these structures. Many open questions remain regarding the biophysical nature of the VFs, as well as the roles they play in replication, translation, virion assembly, viral genome partitioning, and in modulating cellular processes.

FBXO11 amplifies type I interferon signaling to exert antiviral effects by facilitating the assemble of TRAF3-TBK1-IRF3 complex

As the key component of host innate antiviral immunity, type I interferons (IFN-Is) exert multiple antiviral effects by inducing hundreds of IFN-stimulated genes. However, the precise mechanism involved in host sensing of IFN-I signaling priming is particularly complex and remains incompletely resolved. This research identified F-box protein 11 (FBXO11), a component of the E3-ubiquitin ligase SKP/Cullin/F-box complex, acted as an important regulator of IFN-I signaling priming and antiviral process against several RNA/DNA viruses. FBXO11 functioned as an essential enhancer of IFN-I signaling by promoting the phosphorylation of TBK1 and IRF3. Mechanistically, FBXO11 facilitated the assembly of TRAF3-TBK1-IRF3 complex by mediating the K63 ubiquitination of TRAF3 in a NEDD8-dependent manner to amplify the activation of IFN-I signaling. Consistently, the NEDD8-activating enzyme inhibitor MLN4921 could act as a blocker for FBXO11-TRAF3-IFN-I axis of signaling. More significantly, examination of clinical samples of chronic hepatitis B virus (HBV) infection and public transcriptome database of severe acute respiratory syndrome coronavirus-2-, HBV-, and hepatitis C virus-infected human samples revealed that FBXO11 expression was positively correlated with the stage of disease course. Taken together, these findings suggest that FBXO11 is an amplifier of antiviral immune responses and might serve as a potential therapeutic target for a number of different viral diseases.

The landscape of differential splicing and transcript alternations in severe COVID-19 infection

Viral infections can modulate the widespread alternations of cellular splicing, favouring viral replication within the host cells by overcoming host immune responses. However, how SARS-CoV-2 induces host cell differential splicing and affects the landscape of transcript alternation in severe COVID-19 infection remains elusive. Understanding the differential splicing and transcript alternations in severe COVID-19 infection may improve our molecular insights into the SARS-CoV-2 pathogenesis. In this study, we analysed the publicly available blood and lung transcriptome data of severe COVID-19 patients, blood transcriptome data of recovered COVID-19 patients at 12-, 16- and 24-week postinfection and healthy controls. We identified a significant transcript isoform switching in the individual blood and lung RNA-seq data of severe COVID-19-infected patients and 25 common genes that alter their transcript isoform in both blood and lung samples. Altered transcripts show significant loss of the open reading frame, functional domains and switch from coding to noncoding transcript, impacting normal cellular functions. Furthermore, we identified the expression of several novel recurrent chimeric transcripts in the blood samples from severe COVID-19 patients. Moreover, the analysis of the isoform switching into blood samples from recovered COVID-19 patients highlights that there is no significant isoform switching in 16- and 24-week postinfection, and the levels of expressed chimeric transcripts are reduced. This finding emphasizes that SARS-CoV-2 severe infection induces widespread splicing in the host cells, which could help the virus alter the host immune responses and facilitate the viral replication within the host and the efficient translation of viral proteins.

TRAF6 autophagic degradation by avibirnavirus VP3 inhibits antiviral innate immunity via blocking NFKB/NF-κB activation

Ubiquitination is an important reversible post-translational modification. Many viruses hijack the host ubiquitin system to enhance self-replication. In the present study, we found that Avibirnavirus VP3 protein was ubiquitinated during infection and supported virus replication by ubiquitination. Mass spectrometry and mutation analysis showed that VP3 was ubiquitinated at residues K73, K135, K158, K193, and K219. Virus rescue showed that ubiquitination at sites K73, K193, and K219 on VP3 could enhance the replication abilities of infectious bursal disease virus (IBDV), and that K135 was essential for virus survival. Binding of the zinc finger domain of TRAF6 (TNF receptor associated factor 6) to VP3 mediated K11- and K33-linked ubiquitination of VP3, which promoted its nuclear accumulation to facilitate virus replication. Additionally, VP3 could inhibit TRAF6-mediated NFKB/NF-κB (nuclear factor kappa B) activation and IFNB/IFN-β (interferon beta) production to evade host innate immunity by inducing TRAF6 autophagic degradation in an SQSTM1/p62 (sequestosome 1)-dependent manner. Our findings demonstrated a macroautophagic/autophagic mechanism by which Avibirnavirus protein VP3 blocked NFKB-mediated IFNB production by targeting TRAF6 during virus infection, and provided a potential drug target for virus infection control.Abbreviations: ATG: autophagy related; BafA1: bafilomycin A₁; CALCOCO2/NDP52: calcium binding and coiled-coil domain 2; Cas9: CRISPR-associated protein 9; CHX: cycloheximide; Co-IP: co-immunoprecipitation; CRISPR: clustered regularly interspaced short palindromic repeats; GAPDH: glyceraldehyde-3-phosphate dehydrogenase; GST: glutathione S-transferase; IBDV: infectious bursal disease virus; IF: indirect immunofluorescence; IFNB/IFN-β: interferon beta; mAb: monoclonal antibody; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MOI: multiplicity of infection; MS: mass spectrometry; NFKB/NF-κB: nuclear factor kappa B; NBR1: NBR1 autophagy cargo receptor; OPTN: optineurin; pAb: polyclonal antibody; PRRs: pattern recognition receptors; RNF125: ring finger protein 125; RNF135/Riplet: ring finger protein 135; SQSTM1/p62: sequestosome 1; TAX1BP1: tax1 binding protein1; TCID50: 50% tissue culture infective dose; TRAF3: TNF receptor associated factor 3; TRAF6: TNF receptor associated factor 6; TRIM25: tripartite motif containing 25; Ub: ubiquitin; Wort: wortmannin; WT: wild type.

Interaction between chicken TRIM25 and MDA5 and their role in mediated antiviral activity against IBDV infection

Infectious Bursal Disease Virus (IBDV) is the causative agent of an immunosuppressive disease that affects domestic chickens (Gallus gallus) severely affecting poultry industry worldwide. IBDV infection is characterized by a rapid depletion of the bursal B cell population by apoptosis and the atrophy of this chief lymphoid organ. Previous results from our laboratory have shown that exposure of infected cells to type I IFN leads to an exacerbated apoptosis, indicating an important role of IFN in IBDV pathogenesis. It has been described that recognition of the dsRNA IBDV genome by MDA5, the only known cytoplasmic pattern recognition receptor for viral RNA in chickens, leads to type I IFN production. Here, we confirm that TRIM25, an E3 ubiquitin ligase that leads to RIG-I activation in mammalian cells, significantly contributes to positively regulate MDA5-mediated activation of the IFN-inducing pathway in chicken DF-1 cells. Ectopic expression of chTRIM25 together with chMDA5 or a deletion mutant version exclusively harboring the CARD domains (chMDA5 2CARD) enhances IFN-β and NF-ĸB promoter activation. Using co-immunoprecipitation assays, we show that chMDA5 interacts with chTRIM25 through the CARD domains. Moreover, chTRIM25 co-localizes with both chMDA5 and chMDA5 2CARD, but not with chMDA5 mutant proteins partially or totally lacking these domains. On the other hand, ablation of endogenous chTRIM25 expression reduces chMDA5-induced IFN-β and NF-ĸB promoter activation. Interestingly, ectopic expression of either wild-type chTRIM25, or a mutant version (chTRIM25 C59S/C62S) lacking the E3 ubiquitin ligase activity, restores the co-stimulatory effect of chMDA5 in chTRIM25 knockout cells, suggesting that the E3-ubiquitin ligase activity of chTRIM25 is not required for its downstream IFN-β and NF-ĸB activating function. Also, IBDV-induced expression of IFN-β, Mx and OAS genes was reduced in chTRIM25 knockout as compared to wild-type cells, hence contributing to the enhancement of IBDV replication. Enhanced permissiveness to replication of other viruses, such as avian reovirus, Newcastle disease virus and vesicular stomatitis virus was also observed in chTRIM25 knockout cells. Additionally, chTRIM25 knockout also results in reduced MAVS-induced IFN-β promoter stimulation. Nonetheless, similarly to its mammalian counterpart, chTRIM25 overexpression in wild-type DF-1 cells causes the degradation of ectopically expressed chMAVS.

Chicken TAX1BP1 suppresses type I interferon production via degrading chicken MAVS and facilitates infectious bursal diseases virus replication

Mammalian TAX1BP1 (TAX1 binding protein 1), originally identified as a partner of the HTLV-1 viral oncoprotein, functions in regulation of cellular cytokine production. TAX1BP1 plays an important signal transduction regulator, specifically modulating innate immune signaling pathways including NF-B and IRF3. The function of TAX1BP1, which regulates the innate immune response in mammals, has been well studied in previous reports, but the role of chicken TAX1BP1 (chTAX1) in IFN regulation and infectious bursal disease virus (IBDV) replication is still unclear. In this report, chTAX1 was successfully cloned and sub-inserted into a eukaryotic expression vector. The critical regions of chTAX1, such as LC3 binding motif, ubiquitin binding motif, are highly conserved compared to other organisms. We also found that chTAX1 inhibits IFN expression by promoting degradation of chicken MAVS (chMAVS). In addition, the distribution of chTAX1 altered and translocated to co-localize with both VP1 and VP3 after IBDV infection. Overexpression of chTAX1 promotes IBDV replication and knockdown of chTAX1 by RNA interference suppresses IBDV replication. In summary, our data initially indicate that chTAX1 is a suppressor of IFN expression as well as a promoter of IBDV replication.

Nectin4 antagonises type I interferon production by targeting TRAF3 for autophagic degradation and disrupting TRAF3-TBK1 complex formation

Autophagy, a conserved cellular degradative process, plays a crucial role in innate immunity during viral infections. Nervous necrosis virus (NNV), a leading cause of fish diseases with morbidity and mortality, triggers cell autophagy to promote viral replication; however, the details of how NNV utilises autophagy to facilitate its own replication remain largely unexplored. Here, we investigated the mechanism by which the sea perch Nectin4 (LjNectin4), a receptor of NNV, regulates autophagy and the innate immune system by targeting TNFR-associated factor 3 (TRAF3). Our data demonstrated that LjNectin4 directly binds to the NNV capsid protein and facilitates NNV entry, indicating that LjNectin4 functions as an NNV receptor. Moreover, LjNectin4 promoted NNV replication by inhibiting key elements of the RLR signalling pathway (MDA5, MAVS, TRAF3, TBK1, and IRF3)-induced IFN response. Mechanistically, LjNectin4 directly interacted with TRAF3 and promoted its autophagy-mediated lysosomal degradation. Domain mapping of the interaction between TRAF3 and LjNectin4 or TBK1 showed that both LjNectin4 and TBK1 interacted with the ZF2 and TRAF-C domains of TRAF3, suggesting that LjNectin4 blocked TRAF3-TBK1 complex formation. Collectively, our study revealed that NNV utilises LjNectin4 to suppress IFN production by mediating TRAF3 autophagic degradation and disrupting the TRAF3-TBK1 complex, thereby promoting NNV replication.

Advances on Innate Immune Evasion by Avian Immunosuppressive Viruses

Innate immunity is not only the first line of host defense against pathogenic infection, but also the cornerstone of adaptive immune response. Upon pathogenic infection, pattern recognition receptors (PRRs) of host engage pathogen-associated molecular patterns (PAMPs) of pathogens, which initiates IFN production by activating interferon regulatory transcription factors (IRFs), nuclear factor-kappa B (NF-κB), and/or activating protein-1 (AP-1) signal transduction pathways in host cells. In order to replicate and survive, pathogens have evolved multiple strategies to evade host innate immune responses, including IFN-I signal transduction, autophagy, apoptosis, necrosis, inflammasome and/or metabolic pathways. Some avian viruses may not be highly pathogenic but they have evolved varied strategies to evade or suppress host immune response for survival, causing huge impacts on the poultry industry worldwide. In this review, we focus on the advances on innate immune evasion by several important avian immunosuppressive viruses (infectious bursal disease virus (IBDV), Marek’s disease virus (MDV), avian leukosis virus (ALV), etc.), especially their evasion of PRRs-mediated signal transduction pathways (IFN-I signal transduction pathway) and IFNAR-JAK-STAT signal pathways. A comprehensive understanding of the mechanism by which avian viruses evade or suppress host immune responses will be of help to the development of novel vaccines and therapeutic reagents for the prevention and control of infectious diseases in chickens.

Regulation of IRF3 activation in Human Antiviral Signalling Pathways

The interferon regulatory factor (IRF) family of transcription factors play a vital role in the human innate antiviral immune responses with production of interferons (IFNs) as a hallmark outcome of activation. In recent years, IRF3 has been considered a principal early regulator of type I IFNs (TI-IFNs) directly downstream of intracellular virus sensing. Despite decades of research on IRF-activating pathways, many questions remain on the regulation of IRF3 activation. The kinases IκB kinase epsilon (IKKε) and TANK-binding kinase-1 (TBK1) and the scaffold proteins TRAF family member-associated NF-kappa-B activator (TANK), NF-kappa-B-activating kinase-associated protein 1 (NAP1) and TANK-binding kinase 1-binding protein 1 (TBKBP1)/similar to NAP1 TBK1 adaptor (SINTBAD) are believed to be core components of an IRF3-activation complex yet their contextual involvement and complex composition are still unclear. This review will give an overview of antiviral signaling pathways leading to the activation of IRF3 and discuss recent developments in our understanding of its proximal regulation.

DeSUMOylation of Apoptosis Inhibitor 5 by Avibirnavirus VP3 Supports Virus Replication

SUMOylation is a reversible posttranslational modification involved in the regulation of diverse biological processes. Growing evidence suggests that virus infection can interfere with the SUMOylation system. In the present study, we discovered that apoptosis inhibitor 5 (API5) is a SUMOylated protein. Amino acid substitution further identified that Lys404 of API5 was the critical residue for SUMO3 conjugation. Moreover, we found that Avibirnavirus infectious bursal disease virus (IBDV) infection significantly decreased SUMOylation of API5. In addition, our results further revealed that viral protein VP3 inhibited the SUMOylation of API5 by targeting API5 and promoting UBC9 proteasome-dependent degradation through binding to the ubiquitin E3 ligase TRAF3. Furthermore, we revealed that wild-type but not K404R mutant API5 inhibited IBDV replication by enhancing MDA5-dependent IFN-β production. Taken together, our data demonstrate that API5 is a UBC9-dependent SUMOylated protein and deSUMOylation of API5 by viral protein VP3 aids in viral replication.