IFN-λ uniquely promotes CD8 T cell immunity against SARS-CoV-2 relative to type I IFN.

Optimization of protective immune responses against SARS-CoV-2 remains an urgent worldwide priority. In this regard, type III IFN (IFN-λ) restricts SARS-CoV-2 infection in vitro, and treatment with IFN-λ limits infection, inflammation, and pathogenesis in murine models. Furthermore, IFN-λ has been developed for clinical use to limit COVID-19 severity. However, whether endogenous IFN-λ signaling has an effect on SARS-CoV-2 antiviral immunity and long-term immune protection in vivo is unknown. In this study, we identified a requirement for IFN-λ signaling in promoting viral clearance and protective immune programming in SARS-CoV-2 infection of mice. Expression of both IFN and IFN-stimulated gene (ISG) in the lungs were minimally affected by the absence of IFN-λ signaling and correlated with transient increases in viral titers. We found that IFN-λ supported the generation of protective CD8 T cell responses against SARS-CoV-2 by facilitating accumulation of CD103+ DC in lung draining lymph nodes (dLN). IFN-λ signaling specifically in DCs promoted the upregulation of costimulatory molecules and the proliferation of CD8 T cells. Intriguingly, antigen-specific CD8 T cell immunity to SARS-CoV-2 was independent of type I IFN signaling, revealing a nonredundant function of IFN-λ. Overall, these studies demonstrate a critical role for IFN-λ in protective innate and adaptive immunity upon infection with SARS-CoV-2 and suggest that IFN-λ serves as an immune adjuvant to support CD8 T cell immunity.

Histone deacetylase 8 promotes innate antiviral immunity through deacetylation of RIG-I.

Histone deacetylates family proteins have been studied for their function in regulating viral replication by deacetylating non-histone proteins. RIG-I (Retinoic acid-inducible gene I) is a critical protein in RNA virus-induced innate antiviral signaling pathways. Our previous research showed that HDAC8 (histone deacetylase 8) involved in innate antiviral immune response, but the underlying mechanism during virus infection is still unclear. In this study, we showed that HDAC8 was involved in the regulation of vesicular stomatitis virus (VSV) replication. Over-expression of HDAC8 inhibited while knockdown promoted VSV replication. Further exploration demonstrated that HDAC8 interacted with and deacetylated RIG-I, which eventually lead to enhance innate antiviral immune response. Collectively, our data clearly demonstrated that HDAC8 inhibited VSV replication by promoting RIG-I mediated interferon production and downstream signaling pathway.

Interferon subverts an AHR-JUN axis to promote CXCL13+ T cells in lupus.

Systemic lupus erythematosus (SLE) is prototypical autoimmune disease driven by pathological T cell-B cell interactions1,2. Expansion of T follicular helper (TFH) and T peripheral helper (TPH) cells, two T cell populations that provide help to B cells, is a prominent feature of SLE3,4. Human TFH and TPH cells characteristically produce high levels of the B cell chemoattractant CXCL13 (refs. 5,6), yet regulation of T cell CXCL13 production and the relationship between CXCL13+ T cells and other T cell states remains unclear. Here, we identify an imbalance in CD4+ T cell phenotypes in patients with SLE, with expansion of PD-1+/ICOS+ CXCL13+ T cells and reduction of CD96hi IL-22+ T cells. Using CRISPR screens, we identify the aryl hydrocarbon receptor (AHR) as a potent negative regulator of CXCL13 production by human CD4+ T cells. Transcriptomic, epigenetic and functional studies demonstrate that AHR coordinates with AP-1 family member JUN to prevent CXCL13+ TPH/TFH cell differentiation and promote an IL-22+ phenotype. Type I interferon, a pathogenic driver of SLE7, opposes AHR and JUN to promote T cell production of CXCL13. These results place CXCL13+ TPH/TFH cells on a polarization axis opposite from T helper 22 (TH22) cells and reveal AHR, JUN and interferon as key regulators of these divergent T cell states.

The nuclear localization signal of monkeypox virus protein P2 orthologue is critical for inhibition of IRF3-mediated innate immunity.

The Orthopoxvirus (OPXV) genus of the Poxviridae includes human pathogens variola virus (VARV), monkeypox virus (MPXV), vaccinia virus (VACV), and a number of zoonotic viruses. A number of Bcl-2-like proteins of VACV are involved in escaping the host innate immunity. However, little work has been devoted to the evolution and function of their orthologues in other OPXVs. Here, we found that MPXV protein P2, encoded by the P2L gene, and P2 orthologues from other OPXVs, such as VACV protein N2, localize to the nucleus and antagonize interferon (IFN) production. Exceptions to this were the truncated P2 orthologues in camelpox virus (CMLV) and taterapox virus (TATV) that lacked the nuclear localization signal (NLS). Mechanistically, the NLS of MPXV P2 interacted with karyopherin α-2 (KPNA2) to facilitate P2 nuclear translocation, and competitively inhibited KPNA2-mediated IRF3 nuclear translocation and downstream IFN production. Deletion of the NLS in P2 or orthologues significantly enhanced IRF3 nuclear translocation and innate immune responses, thereby reducing viral replication. Moreover, deletion of NLS from N2 in VACV attenuated viral replication and virulence in mice. These data demonstrate that the NLS-mediated translocation of P2 is critical for P2-induced inhibition of innate immunity. Our findings contribute to an in-depth understanding of the mechanisms of OPXV P2 orthologue in innate immune evasion.

Human SARS-CoV-2 challenge uncovers local and systemic response dynamics.

The COVID-19 pandemic is an ongoing global health threat, yet our understanding of the dynamics of early cellular responses to this disease remains limited1. Here in our SARS-CoV-2 human challenge study, we used single-cell multi-omics profiling of nasopharyngeal swabs and blood to temporally resolve abortive, transient and sustained infections in seronegative individuals challenged with pre-Alpha SARS-CoV-2. Our analyses revealed rapid changes in cell-type proportions and dozens of highly dynamic cellular response states in epithelial and immune cells associated with specific time points and infection status. We observed that the interferon response in blood preceded the nasopharyngeal response. Moreover, nasopharyngeal immune infiltration occurred early in samples from individuals with only transient infection and later in samples from individuals with sustained infection. High expression of HLA-DQA2 before inoculation was associated with preventing sustained infection. Ciliated cells showed multiple immune responses and were most permissive for viral replication, whereas nasopharyngeal T cells and macrophages were infected non-productively. We resolved 54 T cell states, including acutely activated T cells that clonally expanded while carrying convergent SARS-CoV-2 motifs. Our new computational pipeline Cell2TCR identifies activated antigen-responding T cells based on a gene expression signature and clusters these into clonotype groups and motifs. Overall, our detailed time series data can serve as a Rosetta stone for epithelial and immune cell responses and reveals early dynamic responses associated with protection against infection.

Interleukin 27, Similar to Interferons, Modulates Gene Expression of Tripartite Motif (TRIM) Family Members and Interferes with Mayaro Virus Replication in Human Macrophages.

BACKGROUND: The Tripartite motif (TRIM) family includes more than 80 distinct human genes. Their function has been implicated in regulating important cellular processes, including intracellular signaling, transcription, autophagy, and innate immunity. During viral infections, macrophages are key components of innate immunity that produce interferons (IFNs) and IL27. We recently published that IL27 and IFNs induce transcriptional changes in various genes, including those involved in JAK-STAT signaling. Furthermore, IL27 and IFNs share proinflammatory and antiviral pathways in monocyte-derived macrophages (MDMs), resulting in both common and unique expression of inflammatory factors and IFN-stimulated genes (ISGs) encoding antiviral proteins. Interestingly, many TRIM proteins have been recognized as ISGs in recent years. Although it is already very well described that TRIM expression is induced by IFNs, it is not fully understood whether TRIM genes are induced in macrophages by IL27. Therefore, in this study, we examined the effect of stimulation with IL27 and type I, II, and III IFNs on the mRNA expression profiles of TRIM genes in MDMs. METHODS: We used bulk RNA-seq to examine the TRIM expression profile of MDMs treated with IFNs or IL27. Initially, we characterized the expression patterns of different TRIM subfamilies using a heatmap. Subsequently, a volcano plot was employed to identify commonly differentially expressed TRIM genes. Additionally, we conducted gene ontology analysis with ClueGO to explore the biological processes of the regulated TRIMs, created a gene-gene interaction network using GeneMANIA, and examined protein-protein interactions with the STRING database. Finally, RNA-seq data was validated using RT-qPCR. Furthermore, the effect of IL27 on Mayaro virus replication was also evaluated. RESULTS: We found that IL27, similar to IFNs, upregulates several TRIM genes' expression in human macrophages. Specifically, we identified three common TRIM genes (TRIM19, 21, and 22) induced by IL27 and all types of human IFNs. Additionally, we performed the first report of transcriptional regulation of TRIM19, 21, 22, and 69 genes in response to IL27. The TRIMs involved a broad range of biological processes, including defense response to viruses, viral life cycle regulation, and negative regulation of viral processes. In addition, we observed a decrease in Mayaro virus replication in MDMs previously treated with IL27. CONCLUSIONS: Our results show that IL27, like IFNs, modulates the transcriptional expression of different TRIM-family members involved in the induction of innate immunity and an antiviral response. In addition, the functional analysis demonstrated that, like IFN, IL27 reduced Mayaro virus replication in MDMs. This implies that IL27 and IFNs share many similarities at a functional level. Moreover, identifying distinct TRIM groups and their differential expressions in response to IL27 provides new insights into the regulatory mechanisms underlying the antiviral response in human macrophages.

PELI2 is a negative regulator of STING signaling that is dynamically repressed during viral infection.

The innate immune cGAS-STING pathway is activated by cytosolic double-stranded DNA (dsDNA), a ubiquitous danger signal, to produce interferon, a potent anti-viral and anti-cancer cytokine. However, STING activation must be tightly controlled because aberrant interferon production leads to debilitating interferonopathies. Here, we discover PELI2 as a crucial negative regulator of STING. Mechanistically, PELI2 inhibits the transcription factor IRF3 by binding to phosphorylated Thr354 and Thr356 on the C-terminal tail of STING, leading to ubiquitination and inhibition of the kinase TBK1. PELI2 sets a threshold for STING activation that tolerates low levels of cytosolic dsDNA, such as that caused by silenced TREX1, RNASEH2B, BRCA1, or SETX. When this threshold is reached, such as during viral infection, STING-induced interferon production temporarily downregulates PELI2, creating a positive feedback loop allowing a robust immune response. Lupus patients have insufficient PELI2 levels and high basal interferon production, suggesting that PELI2 dysregulation may drive the onset of lupus and other interferonopathies.

Type III interferons in innate and adaptive immunity in the respiratory tract.

Lambda interferons (IFNλs), also termed type III interferons (IFNs) or interleukins-28/29, have been in the shadow of type I IFNs for a long time. Their common induction mechanisms and signalling cascades with type I IFNs have made difficult the unwinding of their unique nonredundant functions. However, this is now changing with mounting evidence supporting a major role of IFNλs as a specialized antiviral defense system in the body, mediating protection at mucosal barrier surfaces while limiting immunopathology. Here, we review the latest progress on the complex activities of IFNλs in the respiratory tract, focusing on their multiple effects in IFNλ receptor-expressing cells, the modulation of innate and adaptive immune responses in the context of infections and respiratory diseases, and their similarities and differences with type I IFNs. We also discuss their potential in therapeutic applications and the most recent developments in that direction.

Singapore grouper iridovirus VP146 modulates the cGAS-STING signaling pathway to escape the interferon immune response.

Singapore grouper iridovirus (SGIV) is a large double-stranded DNA virus that has caused significant economic losses to the grouper aquaculture industry. So far, the structure and function of SGIV proteins have been successively reported. In the present paper, the protein of SGIV VP146 was cloned and identified. VP146 was whole-cell distributed in GS cells. VP146 promoted SGIV replication and inhibited the transcription of interferon-related genes as well as pro-inflammatory cytokines in GS cells. In addition, VP146 was involved in the regulation of the cGAS-STING signaling pathway, and decreased cGAS-STING induced the promoter of ISRE and NF-κB. VP146 interacted with the proteins of cGAS, STING, TBK1, and IRF3 from grouper, but did not affect the binding of grouper STING to grouper TBK1 and grouper IRF3. Interestingly, grouper STING was able to affect the intracellular localization of VP146. Four segment structural domains of grouper STING were constructed, and grouper STING-CTT could affect the intracellular localization of VP146. VP146 had no effect on the self-binding of EcSITNG, nor on the binding of EcSTING to EcTBK1 and EcIRF3. Together, the results demonstrated that SGIV VP146 modulated the cGAS-STING signaling pathway to escape the interferon immune response.

Two cytokine receptor family B (CRFB) members in orange-spotted grouper Epinephelus coioides, EcCRFB3 and EcCRFB4, negatively regulate interferon immune responses to assist nervous necrosis virus replication.

Receptors of type I interferon (IFNR) play a vital role in the antiviral immune response. However, little is known about the negative regulatory role of the IFNR. Nervous necrosis virus (NNV) is one of the most significant viruses in cultured fish, resulting in great economic losses for the aquaculture industry. In this study, two orange-spotted grouper (Epinephelus coioides) cytokine receptor family B (CRFB) members, EcCRFB3 and EcCRFB4 were cloned and characterized from NNV infected grouper brain (GB) cells. The open reading frame (ORF) of EcCRFB3 consists of 852 bp encoding 283 amino acids, while EcCRFB4 has an ORF of 990 bp encoding 329 amino acids. The mRNA levels of EcCRFB3 or EcCRFB4 were significantly upregulated after NNV infection and the stimulation of poly (I:C) or NNV-encoded Protein A. In addition, EcCRFB3 or EcCRFB4 overexpression facilitated NNV replication, whereas EcCRFB3 or EcCRFB4 silencing resisted NNV replication. Overexpressed EcCRFB3 or EcCRFB4 inhibited the expression of IFN-I-induced ISGs. Taken together, our research provides the first evidence in fish demonstrating the role of IFNRs to regulate the IFN signaling pathway negatively. Our findings enrich the understanding of the functions of IFNRs and reveal a novel escape mechanism of NNV.

Interferon-Stimulated Genes that Target Retrovirus Translation.

The innate immune system, particularly the interferon (IFN) system, constitutes the initial line of defense against viral infections. IFN signaling induces the expression of interferon-stimulated genes (ISGs), and their products frequently restrict viral infection. Retroviruses like the human immunodeficiency viruses and the human T-lymphotropic viruses cause severe human diseases and are targeted by ISG-encoded proteins. Here, we discuss ISGs that inhibit the translation of retroviral mRNAs and thereby retrovirus propagation. The Schlafen proteins degrade cellular tRNAs and rRNAs needed for translation. Zinc Finger Antiviral Protein and RNA-activated protein kinase inhibit translation initiation factors, and Shiftless suppresses translation recoding essential for the expression of retroviral enzymes. We outline common mechanisms that underlie the antiviral activity of multifunctional ISGs and discuss potential antiretroviral therapeutic approaches based on the mode of action of these ISGs.

Interferon-Regulated Expression of Cellular Splicing Factors Modulates Multiple Levels of HIV-1 Gene Expression and Replication.

Type I interferons (IFN-Is) are pivotal in innate immunity against human immunodeficiency virus I (HIV-1) by eliciting the expression of IFN-stimulated genes (ISGs), which encompass potent host restriction factors. While ISGs restrict the viral replication within the host cell by targeting various stages of the viral life cycle, the lesser-known IFN-repressed genes (IRepGs), including RNA-binding proteins (RBPs), affect the viral replication by altering the expression of the host dependency factors that are essential for efficient HIV-1 gene expression. Both the host restriction and dependency factors determine the viral replication efficiency; however, the understanding of the IRepGs implicated in HIV-1 infection remains greatly limited at present. This review provides a comprehensive overview of the current understanding regarding the impact of the RNA-binding protein families, specifically the two families of splicing-associated proteins SRSF and hnRNP, on HIV-1 gene expression and viral replication. Since the recent findings show specifically that SRSF1 and hnRNP A0 are regulated by IFN-I in various cell lines and primary cells, including intestinal lamina propria mononuclear cells (LPMCs) and peripheral blood mononuclear cells (PBMCs), we particularly discuss their role in the context of the innate immunity affecting HIV-1 replication.

Transcriptome Analysis in Air-Liquid Interface Porcine Respiratory Epithelial Cell Cultures Reveals That the Betacoronavirus Porcine Encephalomyelitis Hemagglutinating Virus Induces a Robust Interferon Response to Infection.

Porcine hemagglutinating encephalomyelitis virus (PHEV) replicates in the upper respiratory tract and tonsils of pigs. Using an air-liquid interface porcine respiratory epithelial cells (ALI-PRECs) culture system, we demonstrated that PHEV disrupts respiratory epithelia homeostasis by impairing ciliary function and inducing antiviral, pro-inflammatory cytokine, and chemokine responses. This study explores the mechanisms driving early innate immune responses during PHEV infection through host transcriptome analysis. Total RNA was collected from ALI-PRECs at 24, 36, and 48 h post inoculation (hpi). RNA-seq analysis was performed using an Illumina Hiseq 600 to generate 100 bp paired-end reads. Differential gene expression was analyzed using DeSeq2. PHEV replicated actively in ALI-PRECs, causing cytopathic changes and progressive mucociliary disruption. Transcriptome analysis revealed downregulation of cilia-associated genes such as CILK1, DNAH11, LRRC-23, -49, and -51, and acidic sialomucin CD164L2. PHEV also activated antiviral signaling pathways, significantly increasing the expression of interferon-stimulated genes (RSAD2, MX1, IFIT, and ISG15) and chemokine genes (CCL5 and CXCL10), highlighting inflammatory regulation. This study contributes to elucidating the molecular mechanisms of the innate immune response to PHEV infection of the airway epithelium, emphasizing the critical roles of the mucociliary, interferon, and chemokine responses.

Research progress on the nonstructural protein 1 (NS1) of influenza a virus.

Influenza A virus (IAV) is the leading cause of highly contagious respiratory infections, which poses a serious threat to public health. The non-structural protein 1 (NS1) is encoded by segment 8 of IAV genome and is expressed in high levels in host cells upon IAV infection. It is the determinant of virulence and has multiple functions by targeting type Ι interferon (IFN-I) and type III interferon (IFN-III) production, disrupting cell apoptosis and autophagy in IAV-infected cells, and regulating the host fitness of influenza viruses. This review will summarize the current research on the NS1 including the structure and related biological functions of the NS1 as well as the interaction between the NS1 and host cells. It is hoped that this will provide some scientific basis for the prevention and control of the influenza virus.

Type III interferon drives pathogenicity to Staphylococcus aureus via the airway epithelium.

Type III interferon signaling contributes to the pathogenesis of the important human pathogen Staphylococcus aureus in the airway. Little is known of the cellular factors important in this response. Using Ifnl2-green fluorescent protein reporter mice combined with flow cytometry and cellular depletion strategies, we demonstrate that the alveolar macrophage is the primary producer of interferon lambda (IFN-λ) in response to S. aureus in the airway. Bone marrow chimeras showed reduced bacterial burden in IFN-λ receptor (IFNLR1)-deficient recipient mice, indicative that non-hematopoietic cells were important for pathogenesis, in addition to significant reductions in pulmonary inflammation. These observations were confirmed through the use of an airway epithelial-specific IFNLR knockout mouse. Our data suggest that upon entry to the airway, S. aureus activates alveolar macrophages to produce type III IFN that is subsequently sensed by the airway epithelium. Future steps will determine how signaling from the epithelium then exerts its influence on bacterial clearance. These results highlight the important, yet sometimes detrimental, role of type III IFN signaling during infection and the impact the airway epithelium plays during host-pathogen interactions.IMPORTANCEThe contribution of type III interferon signaling to the control of bacterial infections is largely unknown. We have previously demonstrated that it contributes to the pathogenesis of acute Staphylococcus aureus respiratory infection. In this report, we document the importance of two cell types that underpin this pathogenesis. We demonstrate that the alveolar macrophage is the cell that is responsible for the production of type III interferon and that this molecule is sensed by airway epithelial cells, which impacts both bacterial clearance and induction of inflammation. This work sheds light on the first two aspects of this important pathogenic cascade.

Nervous necrosis virus capsid protein and Protein A dynamically modulate the fish cGAS-mediated IFN signal pathway to facilitate viral evasion.

Nervous necrosis virus (NNV), an aquatic RNA virus belonging to Betanodavirus, infects a variety of marine and freshwater fishes, leading to massive mortality of cultured larvae and juveniles and substantial economic losses. The enzyme cyclic guanosine monophosphate-adenosine monophosphate synthase (cGAS) is widely recognized as a central component in the innate immune response to cytosolic DNA derived from different pathogens. However, little is known about the response of cGAS to aquatic RNA viruses. This study found that Epinephelus coioides cGAS (EccGAS) overexpression inhibited NNV replication, whereas EccGAS silencing promoted NNV replication. The anti-NNV activity of EccGAS was involved in interferon (IFN) signaling activation including tumor necrosis factor receptor-associated factor family member-associated NF-kappa-B activator-binding kinase 1 (TBK1) phosphorylation, interferon regulatory factor 3 (IRF3) nuclear translocation, and the subsequent induction of IFNc and ISGs. Interestingly, NNV employed its capsid protein (CP) or Protein A (ProA) to negatively or positively modulate EccGAS-mediated IFN signaling by simultaneously targeting EccGAS. CP interacted with EccGAS via the arm-P, S-P, and SD structural domains and promoted its polyubiquitination with K48 and K63 linkages in an EcUBE3C (the ubiquitin ligase)-dependent manner, ultimately leading to EccGAS degradation. Conversely, ProA bound to EccGAS and inhibited its ubiquitination and degradation. In regulating EccGAS protein content, CP's inhibitory action was more pronounced than ProA's protective effect, allowing successful NNV replication. These novel findings suggest that NNV CP and ProA dynamically modulate the EccGAS-mediated IFN signaling pathway to facilitate the immune escape of NNV. Our findings shed light on a novel mechanism of virus-host interaction and provide a theoretical basis for the prevention and control of NNV.IMPORTANCEAs a well-known DNA sensor, cGAS is a pivotal component in innate anti-viral immunity to anti-DNA viruses. Although there is growing evidence regarding the function of cGAS in the resistance to RNA viruses, the mechanisms by which cGAS participates in RNA virus-induced immune responses in fish and how aquatic viruses evade cGAS-mediated immune surveillance remain elusive. Here, we investigated the detailed mechanism by which EccGAS positively regulates the anti-NNV response. Furthermore, NNV CP and ProA interacted with EccGAS, regulating its protein levels through ubiquitin-proteasome pathways, to dynamically modulate the EccGAS-mediated IFN signaling pathway and facilitate viral evasion. Notably, NNV CP was identified to promote the ubiquitination of EccGAS via ubiquitin ligase EcUBE3C. These findings unveil a novel strategy for aquatic RNA viruses to evade cGAS-mediated innate immunity, enhancing our understanding of virus-host interactions.

In search of a function for human type III interferons: insights from inherited and acquired deficits.

The essential and redundant functions of human type I and II interferons (IFNs) have been delineated over the last three decades by studies of patients with inborn errors of immunity or their autoimmune phenocopies, but much less is known about type III IFNs. Patients with cells that do not respond to type III IFNs due to inherited IL10RB deficiency display no overt viral disease, and their inflammatory disease phenotypes can be explained by defective signaling via other interleukine10RB-dependent pathways. Moreover, patients with inherited deficiencies of interferon-stimulated gene factor 3 (ISGF-3) (STAT1, STAT2, IRF9) present viral diseases also seen in patients with inherited deficiencies of the type I IFN receptor (IFNAR1/2). Finally, patients with autoantibodies neutralizing type III IFNs have no obvious predisposition to viral disease. Current findings thus suggest that type III IFNs are largely redundant in humans. The essential functions of human type III IFNs, particularly in antiviral defenses, remain to be discovered.

SGIV VP82 inhibits the interferon response by degradation of IRF3 and IRF7.

During virus-host co-evolution, viruses have developed multiple strategies to dampen IFN response and prevent its antiviral activity in host cells. To date, the interactions between host IFN response and the immune evasion strategies exploited by fish iridoviruses still remain largely uncertain. Here, a potential immune evasion protein candidate of Singapore grouper iridovirus (SGIV), VP82 (encoded by SGIV ORF82) was screened and its roles during viral replication were investigated in detail. Firstly, VP82 overexpression dramatically decreased IFN or ISRE promoter activity and the transcription levels of IFN stimulated genes (ISGs) stimulated by grouper cyclic GMP-AMP synthase (EccGAS)/stimulator of interferon genes (EcSTING), TANK-binding kinase 1 (EcTBK1), IFN regulatory factor 3 (EcIRF3)and EcIRF7. Secondly, Co-IP assays indicated that VP82 interacted with EcIRF3 and EcIRF7, but not EcSTING and EcTBK1, which was consistent with the co-localization between VP82 and EcIRF3 or EcIRF7. Furthermore, VP82 promoted the degradation of EcIRF3 and EcIRF7 in a dose-dependent manner via the autophagy pathway. Finally, VP82 overexpression accelerated SGIV replication, evidenced by the increased transcriptions of viral core genes and viral production. Moreover, the antiviral action of EcIRF3 or EcIRF7 was significantly depressed in VP82 overexpressed cells. Together, VP82 was speculated to exert crucial roles for SGIV replication by inhibiting the IFN response via the degradation of IRF3 and IRF7. Our findings provided new insights into understanding the immune evasion strategies utilized by fish iridovirus through IFN regulation.

SFTSV nucleoprotein mediates DNA sensor cGAS degradation to suppress cGAS-dependent antiviral responses.

Cyclic GMP-AMP synthase (cGAS) is an important DNA pattern recognition receptor that senses double-stranded DNA derived from invading pathogens or self DNA in cytoplasm, leading to an antiviral interferon response. A tick-borne Bunyavirus, severe fever with thrombocytopenia syndrome virus (SFTSV), is an RNA virus that causes a severe emerging viral hemorrhagic fever in Asia with a high case fatality rate of up to 30%. However, it is unclear whether cGAS interacts with SFTSV infection. In this study, we found that SFTSV infection upregulated cGAS RNA transcription and protein expression, indicating that cGAS is an important innate immune response against SFTSV infection. The mechanism of cGAS recognizing SFTSV is by cGAS interacting with misplaced mitochondrial DNA in the cytoplasm. Depletion of mitochondrial DNA significantly inhibited cGAS activation under SFTSV infection. Strikingly, we found that SFTSV nucleoprotein (N) induced cGAS degradation in a dose-dependent manner. Mechanically, N interacted with the 161-382 domain of cGAS and linked the cGAS to LC3. The cGAS-N-LC3 trimer was targeted to N-induced autophagy, and the cGAS was degraded in autolysosome. Taken together, our study discovered a novel antagonistic mechanism of RNA viruses, SFTSV is able to suppress the cGAS-dependent antiviral innate immune responses through N-hijacking cGAS into N-induced autophagy. Our results indicated that SFTSV N is an important virulence factor of SFTSV in mediating host antiviral immune responses. IMPORTANCE: Severe fever with thrombocytopenia syndrome virus (SFTSV) is a tick-borne RNA virus that is widespread in East and Southeast Asian countries with a high fatality rate of up to 30%. Up to now, many cytoplasmic pattern recognition receptors, such as RIG-I, MDA5, and SAFA, have been reported to recognize SFTSV genomic RNA and trigger interferon-dependent antiviral responses. However, current knowledge is not clear whether SFTSV can be recognized by DNA sensor cyclic GMP-AMP synthase (cGAS). Our study demonstrated that cGAS could recognize SFTSV infection via ectopic mitochondrial DNA, and the activated cGAS-stimulator of interferon genes signaling pathway could significantly inhibit SFTSV replication. Importantly, we further uncovered a novel mechanism of SFTSV to inhibit innate immune responses by the degradation of cGAS. cGAS was degraded in N-induced autophagy. Collectively, this study illustrated a novel virulence factor of SFTSV to suppress innate immune responses through autophagy-dependent cGAS degradation.

Regulation of type I and type III interferon induction in response to pathogen sensing.

Type I and III interferons (IFN-I and IFN-III) have a central role in the early antimicrobial response against invading pathogens. Induction of IFN-Is and IFN-IIIs arises due to the sensing by pattern recognition receptors of pathogen-associated molecular patterns (from micro-organisms) or of damage-associated molecular patterns (DAMPs; produced by host cells). Here, we review recent developments on how IFN-I and IFN-III expression is stimulated by different pathogens and how the signalling pathways leading to IFN induction are tightly regulated. We also summarise the growing knowledge of the sensing pathways that lead to IFN-I and IFN-III induction in response to severe acute respiratory syndrome coronavirus 2.