Herpes Simplex Virus 1 Abrogates the cGAS/STING-Mediated Cytosolic DNA-Sensing Pathway via Its Virion Host Shutoff Protein, UL41

HSV-1 employs UL56 to antagonize expression and function of cGAMP channels

DNA sensing is important for antiviral immunity. The DNA sensor cGAS synthesizes 2'3'-cyclic GMP-AMP (cGAMP), a second messenger that activates STING, which induces innate immunity. cGAMP not only activates STING in the cell where it is produced but cGAMP also transfers to other cells. Transporters, channels, and pores (including SLC19A1, SLC46A2, P2X7, ABCC1, and volume-regulated anion channels (VRACs)) release cGAMP into the extracellular space and/or import cGAMP. We report that infection with multiple human viruses depletes some of these cGAMP conduits. This includes herpes simplex virus 1 (HSV-1) that targets SLC46A2, P2X7, and the VRAC subunits LRRC8A and LRRC8C for degradation. The HSV-1 protein UL56 is necessary and sufficient for these effects that are mediated at least partially by proteasomal turnover. UL56 thereby inhibits cGAMP uptake via VRAC, SLC46A2, and P2X7. Taken together, HSV-1 antagonizes intercellular cGAMP transfer. We propose that this limits innate immunity by reducing cell-to-cell communication via the immunotransmitter cGAMP.

Tegument protein UL3 of bovine herpesvirus 1 suppresses antiviral IFN-I signaling by targeting STING for autophagic degradation

Bovine herpesvirus 1 (BoHV-1) is a highly contagious pathogen which causes infectious bovine rhinotracheitis in cattle worldwide. Although it has the ability to evade the host's antiviral innate immune response and establish persistent latent infections, the mechanisms are not fully understood, especially the function of the tegument protein to escape innate immunity and participate in viral replication. In this study, we showed that overexpression of tegument protein UL3 facilitates BoHV-1 replication and suppresses the expression of type-I interferon (IFN-I) and IFN-stimulated genes. Then, STING was identified as the target by which UL3 inhibits the IFN-I signaling pathway, and STING was degraded through the UL3-induced autophagy pathway. Furthermore, overexpression of UL3 promotes the expression of the autophagy-related protein ATG101, thereby inducing autophagy. Further study showed that UL3 enhances the interaction between ATG101 and STING, and then the degradation of STING was reversed following ATG101 silencing in UL3-overexpressing cells during BoHV-1 infection. Our research results demonstrate a novel function of UL3 in regulating host's antiviral response and provide a potential mechanism for BoHV-1 immune evasion.

Multi-targeted loss of the antigen presentation molecule MR1 during HSV-1 and HSV-2 infection

The major histocompatibility complex (MHC), Class-I-related (MR1) molecule presents microbiome-synthesized metabolites to Mucosal-associated invariant T (MAIT) cells, present at sites of herpes simplex virus (HSV) infection. During HSV type 1 (HSV-1) infection there is a profound and rapid loss of MR1, in part due to expression of unique short 3 protein. Here we show that virion host shutoff RNase protein downregulates MR1 protein, through loss of MR1 transcripts. Furthermore, a third viral protein, infected cell protein 22, also downregulates MR1, but not classical MHC-I molecules. This occurs early in the MR1 trafficking pathway through proteasomal degradation. Finally, HSV-2 infection results in the loss of MR1 transcripts, and intracellular and surface MR1 protein, comparable to that seen during HSV-1 infection. Thus HSV coordinates a multifaceted attack on the MR1 antigen presentation pathway, potentially protecting infected cells from MAIT cell T cell receptor-mediated detection at sites of primary infection and reactivation.

The cGAS-STING pathway in viral infections: a promising link between inflammation, oxidative stress and autophagy

The host defence responses play vital roles in viral infection and are regulated by complex interactive networks. The host immune system recognizes viral pathogens through the interaction of pattern-recognition receptors (PRRs) with pathogen-associated molecular patterns (PAMPs). As a PRR mainly in the cytoplasm, cyclic GMP-AMP synthase (cGAS) senses and binds virus DNA and subsequently activates stimulator of interferon genes (STING) to trigger a series of intracellular signalling cascades to defend against invading pathogenic microorganisms. Integrated omic and functional analyses identify the cGAS-STING pathway regulating various host cellular responses and controlling viral infections. Aside from its most common function in regulating inflammation and type I interferon, a growing body of evidence suggests that the cGAS-STING signalling axis is closely associated with a series of cellular responses, such as oxidative stress, autophagy, and endoplasmic reticulum stress, which have major impacts on physiological homeostasis. Interestingly, these host cellular responses play dual roles in the regulation of the cGAS-STING signalling axis and the clearance of viruses. Here, we outline recent insights into cGAS-STING in regulating type I interferon, inflammation, oxidative stress, autophagy and endoplasmic reticulum stress and discuss their interactions with viral infections. A detailed understanding of the cGAS-STING-mediated potential antiviral effects contributes to revealing the pathogenesis of certain viruses and sheds light on effective solutions for antiviral therapy.

Nonstructural protein VP2 of chicken anemia virus triggers IFN-β expression via host cGAS

Chicken anemia virus (CAV) constitutes an important economic threat for the poultry industry. Advancing the understanding of the pathogenic process of CAV infection, we had previously demonstrated that CAV VP1 has the ability to inhibit expression of IFN-β via cGAS-STING signalling pathway. Here to go further to reveal this regulatory role of viral phosphatase VP2, we have performed protein-protein interaction assays with cGAS adaptors, as well as IFN-β induction screenings. Contrary to VP1, VP2 of CAV stimulates the expression of IFN-β, a regulatory effect more closely associated with cGAS (in the context of the cGAS-STING axis) than with STING, TBK1 or IRF7. The results reported here offer new insights about the molecular mechanisms that varied viral proteins act in a timely manner on the host during CAV infection.

The arms race between bacteria CBASS and bacteriophages

The Bacterial Cyclic oligonucleotide-Based Anti-phage Signaling System (CBASS) is an innate immune system that induces cell suicide to defend against phage infections. This system relies on cGAS/DncV-like nucleotidyltransferases (CD-NTase) to synthesize cyclic oligonucleotides (cOs) and CD-NTase-associated proteins (Caps) to execute cell death through DNA cleavage, membrane damage, and NAD depletion, thereby inhibiting phage replication. Ancillary proteins expressed in CBASS, in combination with CD-NTase, ensure the normal synthesis of cOs and prepare CD-NTase for full activation by binding to phage genomes, proteins, or other unknown products. To counteract cell death induced by CBASS, phage genes encode immune evasion proteins that curb Cap recognition of cOs, allowing for phage replication, assembly, and propagation in bacterial cells. This review provides a comprehensive understanding of CBASS immunity, comparing it with different bacterial immune systems and highlighting the interplay between CBASS and phage. Additionally, it explores similar immune escape methods based on shared proteins and action mechanisms between prokaryotic and eukaryotic viruses.

Inhibition of AIM2 inflammasome activation by SOX/ORF37 promotes lytic replication of Kaposi's sarcoma-associated herpesvirus

Inflammasomes are one kind of important innate immune defense against viral and bacterial infections. Several inflammasome-forming sensors detect molecular patterns of invading pathogens and then trigger inflammasome activation and/or pyroptosis in infected cells, and viruses employ unique strategies to hijack or subvert inflammasome activation. Infection with herpesviruses induces the activation of diverse inflammasomes, including AIM2 and IFI16 inflammasomes; however, how Kaposi's sarcoma-associated herpesvirus (KSHV) counteracts inflammasome activation largely remains unclear. Here, we reveal that the KSHV ORF37-encoded SOX protein suppresses AIM2 inflammasome activation independent of its viral DNA exonuclease activity and host mRNA turnover. SOX interacts with the AIM2 HIN domain through the C-terminal Motif VII region and disrupts AIM2:dsDNA polymerization and ASC recruitment and oligomerization. The Y443A or F444A mutation of SOX abolishes the inhibition of AIM2 inflammasome without disrupting SOX nuclease activity, and a short SOX peptide is capable of inhibiting AIM2 inflammasome activation; consequently, infection with SOX-null, Y443A, or F444A Bac16 recombinant viruses results in robust inflammasome activation, suppressed lytic replication, and increased pyroptosis in human lymphatic endothelial cells in an AIM2-dependent manner. These results reveal that KSHV SOX suppresses AIM2 inflammasome activation to promote KSHV lytic replication and inhibit pyroptosis, representing a unique mechanism for evasion of inflammasome activation during KSHV lytic cycle.

African swine fever virus QP383R dampens type I interferon production by promoting cGAS palmitoylation

Cyclic GMP-AMP synthase (cGAS) recognizes viral DNA and synthesizes cyclic GMP-AMP (cGAMP), which activates stimulator of interferon genes (STING/MITA) and downstream mediators to elicit an innate immune response. African swine fever virus (ASFV) proteins can antagonize host immune responses to promote its infection. Here, we identified ASFV protein QP383R as an inhibitor of cGAS. Specifically, we found that overexpression of QP383R suppressed type I interferons (IFNs) activation stimulated by dsDNA and cGAS/STING, resulting in decreased transcription of IFNβ and downstream proinflammatory cytokines. In addition, we showed that QP383R interacted directly with cGAS and promoted cGAS palmitoylation. Moreover, we demonstrated that QP383R suppressed DNA binding and cGAS dimerization, thus inhibiting cGAS enzymatic functions and reducing cGAMP production. Finally, the truncation mutation analysis indicated that the 284-383aa of QP383R inhibited IFNβ production. Considering these results collectively, we conclude that QP383R can antagonize host innate immune response to ASFV by targeting the core component cGAS in cGAS-STING signaling pathways, an important viral strategy to evade this innate immune sensor.

The role of STING signaling in central nervous system infection and neuroinflammatory disease

The cyclic guanosine monophosphate-adenosine monophosphate (GMP-AMP) synthase-Stimulator of Interferon Genes (cGAS-STING) pathway is a critical innate immune mechanism for detecting the presence of double-stranded DNA (dsDNA) and prompting a robust immune response. Canonical cGAS-STING activation occurs when cGAS, a predominantly cytosolic pattern recognition receptor, binds microbial DNA to promote STING activation. Upon STING activation, transcription factors enter the nucleus to cause the production of Type I interferons, inflammatory cytokines whose primary function is to prime the host for viral infection by producing a number of antiviral interferon-stimulated genes. While the pathway was originally described in viral infection, more recent studies have implicated cGAS-STING signaling in a number of different contexts, including autoimmune disease, cancer, injury, and neuroinflammatory disease. This review focuses on how our understanding of the cGAS-STING pathway has evolved over time with an emphasis on the role of STING-mediated neuroinflammation and infection in the nervous system. We discuss recent findings on how STING signaling contributes to the pathology of pain, traumatic brain injury, and stroke, as well as how mitochondrial DNA may promote STING activation in common neurodegenerative diseases. We conclude by commenting on the current knowledge gaps that should be filled before STING can be an effective therapeutic target in neuroinflammatory disease. This article is categorized under: Neurological Diseases > Molecular and Cellular Physiology Infectious Diseases > Molecular and Cellular Physiology Immune System Diseases > Molecular and Cellular Physiology.

Tegument protein UL21 of alpha-herpesvirus inhibits the innate immunity by triggering CGAS degradation through TOLLIP-mediated selective autophagy

Alpha-herpesvirus causes lifelong infections and serious diseases in a wide range of hosts and has developed multiple strategies to counteract the host defense. Here, we demonstrate that the tegument protein UL21 (unique long region 21) in pseudorabies virus (PRV) dampens type I interferon signaling by triggering the degradation of CGAS (cyclic GMP-AMP synthase) through the macroautophagy/autophagy-lysosome pathway. Mechanistically, the UL21 protein scaffolds the E3 ligase UBE3C (ubiquitin protein ligase E3C) to catalyze the K27-linked ubiquitination of CGAS at Lys384, which is recognized by the cargo receptor TOLLIP (toll interacting protein) and degraded in the lysosome. Additionally, we show that the N terminus of UL21 in PRV is dominant in destabilizing CGAS-mediated innate immunity. Moreover, viral tegument protein UL21 in herpes simplex virus type 1 (HSV-1) also displays the conserved inhibitory mechanisms. Furthermore, by using PRV, we demonstrate the roles of UL21 in degrading CGAS to promote viral infection in vivo. Altogether, these findings describe a distinct pathway where alpha-herpesvirus exploits TOLLIP-mediated selective autophagy to evade host antiviral immunity, highlighting a new interface of interplay between the host and DNA virus.Abbreviations: 3-MA: 3-methyladenine; ACTB: actin beta; AHV-1: anatid herpesvirus 1; ATG7: autophagy related 7; ATG13: autophagy related 13; ATG101: autophagy related 101; BHV-1: bovine alphaherpesvirus 1; BNIP3L/Nix: BCL2 interacting protein 3 like; CALCOCO2/NDP52: calcium binding and coiled-coil domain 2; CCDC50: coiled-coil domain containing 50; CCT2: chaperonin containing TCP1 subunit 2; CGAS: cyclic GMP-AMP synthase; CHV-2: cercopithecine herpesvirus 2; co-IP: co-immunoprecipitation; CQ: chloroquine; CRISPR: clustered regulatory interspaced short palindromic repeat; Cas9: CRISPR-associated system 9; CTD: C-terminal domain; Ctrl: control; DAPI: 4',6-diamidino-2-phenylindole; DBD: N-terminal DNA binding domain; DMSO: dimethyl sulfoxide; DYNLRB1: dynein light chain roadblock-type 1; EHV-1: equine herpesvirus 1; gB: glycoprotein B; GFP: green fluorescent protein; H&E: hematoxylin and eosin; HSV-1: herpes simplex virus 1; HSV-2: herpes simplex virus 2; IB: immunoblotting; IRF3: interferon regulatory factor 3; lenti: lentivirus; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MARCHF9: membrane associated ring-CH-type finger 9; MG132: cbz-leu-leu-leucinal; NBR1: NBR1 autophagy cargo receptor; NC: negative control; NEDD4L: NEDD4 like E3 ubiquitin protein ligase; NH4Cl: ammonium chloride; OPTN: optineurin; p-: phosphorylated; PFU: plaque-forming unit; Poly(dA:dT): Poly(deoxyadenylic-deoxythymidylic) acid; PPP1: protein phosphatase 1; PRV: pseudorabies virus; RB1CC1/FIP200: RB1 inducible coiled-coil 1; RNF126: ring finger protein 126; RT-PCR: real-time polymerase chain reaction; sgRNA: single guide RNA; siRNA: small interfering RNA; SQSTM1/p62: sequestosome 1; STING1: stimulator of interferon response cGAMP interactor 1; TBK1: TANK binding kinase 1; TOLLIP: toll interacting protein; TRIM33: tripartite motif containing 33; UL16: unique long region 16; UL21: unique long region 21; UL54: unique long region 54; Ub: ubiquitin; UBE3C: ubiquitin protein ligase E3C; ULK1: unc-51 like autophagy activating kinase 1; Vec: vector; VSV: vesicular stomatitis virus; VZV: varicella-zoster virus; WCL: whole-cell lysate; WT: wild-type; Z-VAD: carbobenzoxy-valyl-alanyl-aspartyl-[O-methyl]-fluoromethylketone.

Chicken infectious anemia virus (CIAV) VP1 antagonizes type I interferon (IFN-I) production by inhibiting TBK1 phosphorylation

Chicken infectious anemia virus (CIAV) infection induces immunosuppression or subclinical immunosuppression in chickens. CIAV infection has been reported to repress type I interferon (IFN-I) expression, but the underlying mechanisms are not yet understood. Here we reported that VP1, the capsid protein of CIAV, the major immunogenic protein that triggers the production of neutralizing antibodies in chickens, inhibited type I interferon (IFN-I) expression induced by cGAS-STING signaling. We showed that VP1 inhibited TBK1 phosphorylation and down stream signal transduction, leading to the inhibition of IFN-I expression. Subsequently, we demonstrated that VP1 interacted with TBK1. Finally, we clarified that aa 120-150 in VP1 was essential for VP1 to interact with TBK1 and inhibit cGAS-STING signaling. These findings will help us further understand the pathogenesis of CIAV in chickens.

Role of Innate Interferon Responses at the Ocular Surface in Herpes Simplex Virus-1-Induced Herpetic Stromal Keratitis

Herpes simplex virus type 1 (HSV-1) is a highly successful pathogen that primarily infects epithelial cells of the orofacial mucosa. After initial lytic replication, HSV-1 enters sensory neurons and undergoes lifelong latency in the trigeminal ganglion (TG). Reactivation from latency occurs throughout the host’s life and is more common in people with a compromised immune system. HSV-1 causes various diseases depending on the site of lytic HSV-1 replication. These include herpes labialis, herpetic stromal keratitis (HSK), meningitis, and herpes simplex encephalitis (HSE). HSK is an immunopathological condition and is usually the consequence of HSV-1 reactivation, anterograde transport to the corneal surface, lytic replication in the epithelial cells, and activation of the host’s innate and adaptive immune responses in the cornea. HSV-1 is recognized by cell surface, endosomal, and cytoplasmic pattern recognition receptors (PRRs) and activates innate immune responses that include interferons (IFNs), chemokine and cytokine production, as well as the recruitment of inflammatory cells to the site of replication. In the cornea, HSV-1 replication promotes type I (IFN-α/β) and type III (IFN-λ) IFN production. This review summarizes our current understanding of HSV-1 recognition by PRRs and innate IFN-mediated antiviral immunity during HSV-1 infection of the cornea. We also discuss the immunopathogenesis of HSK, current HSK therapeutics and challenges, proposed experimental approaches, and benefits of promoting local IFN-λ responses.

African Swine Fever Virus L83L Negatively Regulates the cGAS-STING-Mediated IFN-I Pathway by Recruiting Tollip To Promote STING Autophagic Degradation

African swine fever (ASF) is a devastating infectious disease of pigs caused by the African swine fever virus (ASFV), which poses a great danger to the global pig industry. Many viral proteins can suppress with interferon signaling to evade the host's innate immune responses. Therefore, the development of an effective vaccine against ASFV has been dampened. Recent studies have suggested that the L83L gene may be integrated into the host genome, weakening the host immune system, but the underlying mechanism is unknown. Our study found that L83L negatively regulates the cGAS-STING-mediated type I interferon (IFN-I) signaling pathway. Overexpression of L83L inhibited IFN-β promoter and ISRE activity, and knockdown of L83L induced higher transcriptional levels of interferon-stimulated genes (ISGs) and phosphorylation levels of IRF3 in primary porcine alveolar macrophages. Mechanistically, L83L interacted with cGAS and STING to promote autophagy-lysosomal degradation of STING by recruiting Tollip, thereby blocking the phosphorylation of the downstream signaling molecules TBK1, IRF3, and IκBα and reducing IFN-I production. Altogether, our study reveals a negative regulatory mechanism involving the L83L-cGAS-STING-IFN-I axis and provides insights into an evasion strategy involving autophagy and innate signaling pathways employed by ASFV. IMPORTANCE African swine fever virus (ASFV) is a large double-stranded DNA virus that primarily infects porcine macrophages. The ASFV genome encodes a large number of immunosuppressive proteins. Current options for the prevention and control of this pathogen remain pretty limited. Our study showed that overexpression of L83L inhibited the cGAS-STING-mediated type I interferon (IFN-I) signaling pathway. In contrast, the knockdown of L83L during ASFV infection enhanced IFN-I production in porcine alveolar macrophages. Additional analysis revealed that L83L protein downregulated IFN-I signaling by recruiting Tollip to promote STING autophagic degradation. Although L83L deletion has been reported to have little effect on viral replication, its immune evade mechanism has not been elucidated. The present study extends our understanding of the functions of ASFV-encoded pL83L and its immune evasion strategy, which may provide a new basis for developing a live attenuated vaccine for ASF.

Spermine enhances antiviral and anticancer responses by stabilizing DNA binding with the DNA sensor cGAS

Self-nonself discrimination is vital for the immune system to mount responses against pathogens while maintaining tolerance toward the host and innocuous commensals during homeostasis. Here, we investigated how indiscriminate DNA sensors, such as cyclic GMP-AMP synthase (cGAS), make this self-nonself distinction. Screening of a small-molecule library revealed that spermine, a well-known DNA condenser associated with viral DNA, markedly elevates cGAS activation. Mechanistically, spermine condenses DNA to enhance and stabilize cGAS-DNA binding, optimizing cGAS and downstream antiviral signaling. Spermine promotes condensation of viral, but not host nucleosome, DNA. Deletion of viral DNA-associated spermine, by propagating virus in spermine-deficient cells, reduced cGAS activation. Spermine depletion subsequently attenuated cGAS-mediated antiviral and anticancer immunity. Collectively, our results reveal a pathogenic DNA-associated molecular pattern that facilitates nonself recognition, linking metabolism and pathogen recognition.

Function and regulation of cGAS-STING signaling in infectious diseases

The efficacious detection of pathogens and prompt induction of innate immune signaling serve as a crucial component of immune defense against infectious pathogens. Over the past decade, DNA-sensing receptor cyclic GMP-AMP synthase (cGAS) and its downstream signaling adaptor stimulator of interferon genes (STING) have emerged as key mediators of type I interferon (IFN) and nuclear factor-κB (NF-κB) responses in health and infection diseases. Moreover, both cGAS-STING pathway and pathogens have developed delicate strategies to resist each other for their survival. The mechanistic and functional comprehension of the interplay between cGAS-STING pathway and pathogens is opening the way for the development and application of pharmacological agonists and antagonists in the treatment of infectious diseases. Here, we briefly review the current knowledge of DNA sensing through the cGAS-STING pathway, and emphatically highlight the potent undertaking of cGAS-STING signaling pathway in the host against infectious pathogenic organisms.

When herpes simplex virus encephalitis meets antiviral innate immunity

Herpes simplex virus (HSV) is the most common pathogen of infectious encephalitis, accounting for nearly half of the confirmed cases of encephalitis. Its clinical symptoms are often atypical. HSV PCR in cerebrospinal fluid is helpful for diagnosis, and the prognosis is usually satisfactory after regular antiviral treatment. Interestingly, some patients with recurrent encephalitis have little antiviral effect. HSV PCR in cerebrospinal fluid is negative, but glucocorticoid has a significant effect after treatment. Specific antibodies, such as the NMDA receptor antibody, the GABA receptor antibody, and even some unknown antibodies, can be isolated from cerebrospinal fluid, proving that the immune system contributes to recurrent encephalitis, but the specific mechanism is still unclear. Based on recent studies, we attempt to summarize the relationship between herpes simplex encephalitis and innate immunity, providing more clues for researchers to explore this field further.

HSV Replication: Triggering and Repressing STING Functionality

Herpes simplex virus (HSV) has persisted within human populations due to its ability to establish both lytic and latent infection. Given this, human hosts have evolved numerous immune responses to protect against HSV infection. Critical in this defense against HSV, the host protein stimulator of interferon genes (STING) functions as a mediator of the antiviral response by inducing interferon (IFN) as well as IFN-stimulated genes. Emerging evidence suggests that during HSV infection, dsDNA derived from either the virus or the host itself ultimately activates STING signaling. While a complex regulatory circuit is in operation, HSV has evolved several mechanisms to neutralize the STING-mediated antiviral response. Within this review, we highlight recent progress involving HSV interactions with the STING pathway, with a focus on how STING influences HSV replication and pathogenesis.

Research advances in cGAS-stimulator of interferon genes pathway and central nervous system diseases: Focus on new therapeutic approaches

Cyclic GMP-AMP synthase (cGAS), a crucial innate immune sensor, recognizes cytosolic DNA and induces stimulator of interferon genes (STING) to produce type I interferon and other proinflammatory cytokines, thereby mediating innate immune signaling. The cGAS-STING pathway is involved in the regulation of infectious diseases, anti-tumor immunity, and autoimmune diseases; in addition, it plays a key role in the development of central nervous system (CNS) diseases. Therapeutics targeting the modulation of cGAS-STING have promising clinical applications. Here, we summarize the cGAS-STING signaling mechanism and the recent research on its role in CNS diseases.

Duck Enteritis Virus Protein Kinase US3 Inhibits DNA Sensing Signaling by Phosphorylating Interferon Regulatory Factor 7

The cytosolic DNA sensing pathway mediates innate immune defense against infection by many DNA viruses; however, viruses have evolved multiple strategies to evade the host immune response. Duck enteritis virus (DEV) causes an acute and contagious disease with high mortality in waterfowl. The mechanisms employed by DEV to block the DNA sensing pathway are not well understood. Here, we sought to investigate the role of DEV US3, a serine/threonine protein kinase, in the inhibition of DNA sensing. We found that ectopic expression of DEV US3 significantly inhibited the production of IFN-β and expression of interferon-stimulated genes induced by interferon-stimulatory DNA and poly(dA-dT). US3 also inhibited viral DNA-triggered IFN-β activation and promoted DEV replication in duck embryo fibroblasts, while knockdown of US3 during DEV infection enhances the IFN-β response and suppresses viral replication. US3 inhibited the DNA-sensing signaling pathway by targeting interferon regulatory factor 7 (IRF7), and the kinase activity of US3 was indispensable for its inhibitory function. Furthermore, we found that US3 interacts with the activation domain of IRF7, phosphorylating IRF7, blocking its dimerization and nuclear translocation, and finally leading to the inhibition of IFN-β production. These findings expand our knowledge on DNA sensing in ducks and reveal a novel mechanism whereby DEV evades host antiviral immunity. IMPORTANCE Duck enteritis virus (DEV) is a duck alphaherpesvirus that causes an acute and contagious disease with high mortality, resulting in substantial economic losses in the commercial waterfowl industry. The evasion of DNA-sensing pathway-mediated antiviral innate immunity is essential for the persistent infection and replication for many DNA viruses. However, the strategies used by DEV to block the DNA-sensing pathway are not well understood. In this study, DEV US3 protein kinase was demonstrated to inhibit the DNA-sensing signaling via binding to the activation domain of interferon regulatory factor 7 (IRF7), which induced the hyperphosphorylation of IRF7 and abolished IRF7 dimerization and nuclear translocation. Our findings provide insights into how duck herpesviral kinase counteracts host antiviral innate immunity to ensure viral replication and spread.

Duck Enteritis Virus Inhibits the cGAS-STING DNA-Sensing Pathway To Evade the Innate Immune Response

Cyclic GMP-AMP synthase (cGAS), a key DNA sensor, detects cytosolic viral DNA and activates the adaptor protein stimulator of interferon genes (STING) to initiate interferon (IFN) production and host innate antiviral responses. Duck enteritis virus (DEV) is a duck alphaherpesvirus that causes an acute and contagious disease with high mortality in waterfowl. In the present study, we found that DEV inhibits host innate immune responses during the late phase of viral infection. Furthermore, we screened DEV proteins for their ability to inhibit the cGAS-STING DNA-sensing pathway and identified multiple viral proteins, including UL41, US3, UL28, UL53, and UL24, which block IFN-β activation through this pathway. The DEV tegument protein UL41, which exhibited the strongest inhibitory effect, selectively downregulated the expression of interferon regulatory factor 7 (IRF7) by reducing its mRNA accumulation, thereby inhibiting the DNA-sensing pathway. Ectopic expression of UL41 markedly reduced viral DNA-triggered IFN-β production and promoted viral replication, whereas deficiency of UL41 in the context of DEV infection increased the IFN-β response to DEV and suppressed viral replication. In addition, ectopic expression of IRF7 inhibited the replication of the UL41-deficient virus, whereas IRF7 knockdown facilitated its replication. This study is the first report identifying multiple viral proteins encoded by a duck DNA virus, which inhibit the cGAS-STING DNA-sensing pathway. These findings expand our knowledge of DNA sensing in ducks and reveal a mechanism through which DEV antagonizes the host innate immune response. IMPORTANCE Duck enteritis virus (DEV) is a duck alphaherpesvirus that causes an acute and contagious disease with high mortality, resulting in substantial economic losses in the commercial waterfowl industry. The evasion of DNA-sensing pathway-mediated antiviral innate immunity is essential for the persistent infection and replication of many DNA viruses. However, the mechanisms used by DEV to modulate the DNA-sensing pathway remain poorly understood. In the present study, we found that DEV encodes multiple viral proteins to inhibit the cGAS-STING DNA-sensing pathway. The DEV tegument protein UL41 selectively diminished the accumulation of interferon regulatory factor 7 (IRF7) mRNA, thereby inhibiting the DNA-sensing pathway. Loss of UL41 potently enhanced the IFN-β response to DEV and impaired viral replication in ducks. These findings provide insights into the host-virus interaction during DEV infection and help develop new live attenuated vaccines against DEV.

Engineering antiviral immune-like systems for autonomous virus detection and inhibition in mice

The ongoing COVID-19 pandemic has demonstrated that viral diseases represent an enormous public health and economic threat to mankind and that individuals with compromised immune systems are at greater risk of complications and death from viral diseases. The development of broad-spectrum antivirals is an important part of pandemic preparedness. Here, we have engineer a series of designer cells which we term autonomous, intelligent, virus-inducible immune-like (ALICE) cells as sense-and-destroy antiviral system. After developing a destabilized STING-based sensor to detect viruses from seven different genera, we have used a synthetic signal transduction system to link viral detection to the expression of multiple antiviral effector molecules, including antiviral cytokines, a CRISPR-Cas9 module for viral degradation and the secretion of a neutralizing antibody. We perform a proof-of-concept study using multiple iterations of our ALICE system in vitro, followed by in vivo functionality testing in mice. We show that dual output ALICE_SaCas9+Ab system delivered by an AAV-vector inhibited viral infection in herpetic simplex keratitis (HSK) mouse model. Our work demonstrates that viral detection and antiviral countermeasures can be paired for intelligent sense-and-destroy applications as a flexible and innovative method against virus infection.

Singapore Grouper Iridovirus VP131 Drives Degradation of STING-TBK1 Pathway Proteins and Negatively Regulates Antiviral Innate Immunity

Iridoviruses are large DNA viruses which cause great economic losses to the aquaculture industry and serious threats to ecological diversity worldwide. Singapore grouper iridovirus (SGIV), a novel member of the genus Ranavirus, causes high mortality in grouper aquaculture. Previous work on genome annotation demonstrated that SGIV contained numerous uncharacterized or hypothetical open reading frames (ORFs), whose functions remained largely unknown. Here, we reported that the protein encoded by SGIV ORF131R (VP131) was localized predominantly within the endoplasmic reticulum (ER). Ectopic expression of GFP-VP131 significantly enhanced SGIV replication, while VP131 knockdown decreased viral infection in vitro, suggesting that VP131 functioned as a proviral factor during SGIV infection. Overexpression of GFP-VP131 inhibited the interferon (IFN)-1 promoter activity and mRNA level of IFN-related genes induced by poly(I:C), Epinephelus coioides cyclic GMP/AMP synthase (EccGAS)/stimulator of IFN genes (EcSTING), TANK-binding kinase 1 (EcTBK1), or melanoma differentiation-associated gene 5 (EcMDA5), whereas such activation induced by mitochondrial antiviral signaling protein (EcMAVS) was not affected. Moreover, VP131 interacted with EcSTING and degraded EcSTING through both the autophagy-lysosome pathway and ubiquitin-proteasome pathway, and targeted for the K63-linked ubiquitination. Of note, we also found that EcSTING significantly accelerated the formation of GFP-VP131 aggregates in co-transfected cells. Finally, GFP-VP131 inhibited EcSTING- or EcTBK1-induced antiviral activity upon red-spotted grouper nervous necrosis virus (RGNNV) infection. Together, our results demonstrated that the SGIV VP131 negatively regulated the IFN response by inhibiting EcSTING-EcTBK1 signaling for viral evasion. IMPORTANCE STING has been identified as a critical factor participating in the innate immune response which recruits and phosphorylates TBK1 and IFN regulatory factor 3 (IRF3) to induce IFN production and defend against viral infection. However, viruses also distort the STING-TBK1 pathway to negatively regulate the IFN response and facilitate viral replication. Here, we reported that SGIV VP131 interacted with EcSTING within the ER and degraded EcSTING, leading to the suppression of IFN production and the promotion of SGIV infection. These results for the first time demonstrated that fish iridovirus evaded the host antiviral response via abrogating the STING-TBK1 signaling pathway.

Bovine delta papillomavirus E5 oncoprotein negatively regulates the cGAS-STING signaling pathway in cattle in a spontaneous model of viral disease

Persistent infection and tumorigenesis by papillomaviruses (PVs) require viral manipulation of various cellular processes, including those involved in innate immune responses. The cyclic guanosine monophosphate-adenosine monophosphate synthase-stimulator of interferon genes (cGAS-STING) pathway has emerged as an essential innate immune sensing system, that recognizes DNA and trigger potent antiviral effector responses. In this study, we found that bovine PV (BPV) E5 protein, the major oncoprotein of bovine delta PVs, interacts with STING but not with cGAS in a spontaneous BPV infection of neoplastic urothelial cells of cattle. Real-time RT-PCR revealed a significant reduction in both cGAS and STING transcripts in E5-expressing cells. Furthermore, western blot (WB) analysis failed to detect any variation in the expression of interferon-inducible protein 16 (IFI16), an upstream effector of the STING pathway. A ternary complex composed of E5/STING/IFI16 was also observed. Co-immunoprecipitation studies showed that STING interacts with a protein network composed of total and phosphorylated TANK-binding kinase 1 (TBK1), total and phosphorylated interferon regulatory factor 3 (IRF3), IRF7, IKKα, IKKβ, IKKϵ, ELKS, MEKK3, and TAK1. RT-qPCR revealed a significant reduction in TBK1 mRNA levels in BPV-infected cells. WB analysis revealed significantly reduced expression levels of pTBK1, which is essential for the activation and phosphorylation of IRF3, a prerequisite for the latter to enter the nucleus to activate type 1 IFN genes. WB also revealed significantly down-expression of IKKα, IKKβ, IKKϵ, and overexpression of IRF7, ELKS, MEKK3, and TAK1in BPV-positive urothelial cells compared with that in uninfected healthy cells. Phosphorylated p65 (p-p65) was significantly reduced in both the nuclear and cytosolic compartments of BPV-infected cells compared with that in uninfected urothelial cells. Our results suggest that the innate immune signaling pathway mediated by cGAS-STING is impaired in cells infected with BPV. Therefore, effective immune responses are not elicited against these viruses, which facilitates persistent viral infection and subsequent tumorigenesis.

African swine fever virus M1249L protein antagonizes type I interferon production via suppressing phosphorylation of TBK1 and degrading IRF3

Cyclic GMP-AMP synthase (cGAS) is a major DNA sensor. The recognition of cytosolic DNA by cGAS triggers a robust innate immune response that restricts the replication of diverse viral pathogens through the type I interferon (IFN) and nuclear factor-κB (NF-κB) pathways. African swine fever virus (ASFV) is a large and complex DNA virus reported to strongly inhibit the cGAS-STING signaling pathway. Herein, 12 ASFV structural proteins were screened to determine their effects on the cGAS-STING pathway. Ectopic expression of the ASFV caspid protein M1249L significantly inhibited the IFN-β promoter activity induced by the cGAS-STING pathway in a dose-dependent manner. And it could also downregulate the levels of IFN-β and several interferon-stimulating genes (ISGs) induced by cGAS-STING and 2'3'-cGAMP. Moreover, ASFV M1249L also suppressed phosphorylation of TBK1 by cGAS and STING overexpression. Further study showed that M1249L co-localized and interacted with interferon regulatory factor 3 (IRF3), which led to induce IRF3 degradation by lysosomal pathway. Taken together, our study revealed a novel strategy utilized by ASFV for cGAS-STING-related immune evasion.

The Role of Viral RNA Degrading Factors in Shutoff of Host Gene Expression

Many viruses induce shutoff of host gene expression (host shutoff) as a strategy to take over cellular machinery and evade host immunity. Without host shutoff activity, these viruses generally replicate poorly in vivo, attesting to the importance of this antiviral strategy. In this review, we discuss one particularly advantageous way for viruses to induce host shutoff: triggering widespread host messenger RNA (mRNA) decay. Viruses can trigger increased mRNA destruction either directly, by encoding RNA cleaving or decapping enzymes, or indirectly, by activating cellular RNA degradation pathways. We review what is known about the mechanism of action of several viral RNA degradation factors. We then discuss the consequences of widespread RNA degradation on host gene expression and on the mechanisms of immune evasion, highlighting open questions. Answering these questions is critical to understanding how viral RNA degradation factors regulate host gene expression and how this process helps viruses evade host responses and replicate.

Post-Translational Modifications of cGAS-STING: A Critical Switch for Immune Regulation

Innate immune mechanisms initiate immune responses via pattern-recognition receptors (PRRs). Cyclic GMP-AMP synthase (cGAS), a member of the PRRs, senses diverse pathogenic or endogenous DNA and activates innate immune signaling pathways, including the expression of stimulator of interferon genes (STING), type I interferon, and other inflammatory cytokines, which, in turn, instructs the adaptive immune response development. This groundbreaking discovery has rapidly advanced research on host defense, cancer biology, and autoimmune disorders. Since cGAS/STING has enormous potential in eliciting an innate immune response, understanding its functional regulation is critical. As the most widespread and efficient regulatory mode of the cGAS-STING pathway, post-translational modifications (PTMs), such as the covalent linkage of functional groups to amino acid chains, are generally considered a regulatory mechanism for protein destruction or renewal. In this review, we discuss cGAS-STING signaling transduction and its mechanism in related diseases and focus on the current different regulatory modalities of PTMs in the control of the cGAS-STING-triggered innate immune and inflammatory responses.

Regulation of cGAS/STING signaling and corresponding immune escape strategies of viruses

Innate immunity is the first line of defense against invading external pathogens, and pattern recognition receptors (PRRs) are the key receptors that mediate the innate immune response. Nowadays, there are various PRRs in cells that can activate the innate immune response by recognizing pathogen-related molecular patterns (PAMPs). The DNA sensor cGAS, which belongs to the PRRs, plays a crucial role in innate immunity. cGAS detects both foreign and host DNA and generates a second-messenger cGAMP to mediate stimulator of interferon gene (STING)-dependent antiviral responses, thereby exerting an antiviral immune response. However, the process of cGAS/STING signaling is regulated by a wide range of factors. Multiple studies have shown that viruses directly target signal transduction proteins in the cGAS/STING signaling through viral surface proteins to impede innate immunity. It is noteworthy that the virus utilizes these cGAS/STING signaling regulators to evade immune surveillance. Thus, this paper mainly summarized the regulatory mechanism of the cGAS/STING signaling pathway and the immune escape mechanism of the corresponding virus, intending to provide targeted immunotherapy ideas for dealing with specific viral infections in the future.

Nanocarriers for effective delivery: modulation of innate immunity for the management of infections and the associated complications

Innate immunity is the first line of defense against invading pathogens. Innate immune cells can recognize invading pathogens through recognizing pathogen-associated molecular patterns (PAMPs) via pattern recognition receptors (PRRs). The recognition of PAMPs by PRRs triggers immune defense mechanisms and the secretion of pro-inflammatory cytokines such as TNF-α, IL-1β, and IL-6. However, sustained and overwhelming activation of immune system may disrupt immune homeostasis and contribute to inflammatory disorders. Immunomodulators targeting PRRs may be beneficial to treat infectious diseases and their associated complications. However, therapeutic performances of immunomodulators can be negatively affected by (1) high immune-mediated toxicity, (2) poor solubility and (3) bioactivity loss after long circulation. Recently, nanocarriers have emerged as a very promising tool to overcome these obstacles owning to their unique properties such as sustained circulation, desired bio-distribution, and preferred pharmacokinetic and pharmacodynamic profiles. In this review, we aim to provide an up-to-date overview on the strategies and applications of nanocarrier-assisted innate immune modulation for the management of infections and their associated complications. We first summarize examples of important innate immune modulators. The types of nanomaterials available for drug delivery, as well as their applications for the delivery of immunomodulatory drugs and vaccine adjuvants are also discussed.

A Baseline Cellular Antiviral State Is Maintained by cGAS and Its Most Frequent Naturally Occurring Variant rs610913

Upon recognition of aberrantly located DNA, the innate immune sensor cyclic GMP-AMP synthase (cGAS) activates stimulator of IFN genes (STING)/IFN regulatory factor (IRF)3-driven antiviral responses. In this study, we characterized the ability of a specific variant of the human cGAS-encoding gene MB21D1, rs610913, to alter cGAS-mediated DNA sensing and viral infection. rs610913 is a frequent G>T polymorphism resulting in a P261H exchange in the cGAS protein. Data from the International Collaboration for the Genomics of HIV suggested that rs610913 nominally associates with HIV-1 acquisition in vivo. Molecular modeling of cGAS(P261H) hinted toward the possibility for an additional binding site for a potential cellular cofactor in cGAS dimers. However, cGAS(wild-type [WT]) or cGAS(P261H)-reconstituted THP-1 cGAS knockout cells shared steady-state expression of IFN-stimulated genes, as opposed to cells expressing the enzymatically inactive cGAS(G212A/S213A). Accordingly, cGAS(WT) and cGAS(P261H) cells were less susceptible to lentiviral transduction and infection with HIV-1, HSV-1, and Chikungunya virus as compared with cGAS knockout or cGAS(G212A/S213A) cells. Upon DNA challenge, innate immune activation appeared to be mildly reduced upon expression of cGAS(P261H) compared with cGAS(WT). Finally, DNA challenge of PBMCs from donors homozygously expressing rs610913 provoked a trend toward a slightly reduced type I IFN response as compared with PBMCs from GG donors. Taken together, the steady-state activity of cGAS maintains a baseline antiviral state rendering cells more refractory to IFN-stimulated gene-sensitive viral infections. rs610913 failed to grossly differ phenotypically from the WT gene, suggesting that cGAS(P261H) and WT cGAS share a similar ability to sense viral infections in vivo.

Insights on the cGAS-STING Signaling Pathway During Herpesvirus Infections

Herpesviruses belong to large double-stranded DNA viruses. They are under a wide range of hosts and establish lifelong infection, which creates a burden on human health and animal health. Innate immunity is the host’s innate defense ability. Activating the innate immune signaling pathway and producing type I interferon is the host’s first line of defense against infectious pathogens. Emerging evidence indicates that the cGAS-STING signaling pathway plays an important role in the innate immunity in response to herpesvirus infections. In parallel, because of the constant selective pressure imposed by host immunity, herpesvirus also evolves to target the cGAS-STING signaling pathway to inhibit or escape the innate immune responses. In the current review, we insight on the classical cGAS-STING signaling pathway. We describe the activation of cGAS-STING signaling pathway during herpesvirus infections and strategies of herpesvirus targeting this pathway to evade host antiviral response. Furthermore, we outline the immunotherapy boosting cGAS-STING signaling pathway.

African Swine Fever Virus Structural Protein p17 Inhibits cGAS-STING Signaling Pathway Through Interacting With STING

African swine fever virus (ASFV) encodes more than 150 proteins, which establish complex interactions with the host for the benefit of the virus in order to evade the host’s defenses. However, currently, there is still a lack of information regarding the roles of the viral proteins in host cells. Here, our data demonstrated that ASFV structural protein p17 exerts a negative regulatory effect on cGAS-STING signaling pathway and the STING signaling dependent anti-HSV1 and anti-VSV functions. Further, the results indicated that ASFV p17 was located in ER and Golgi apparatus, and interacted with STING. ASFV p17 could interfere the STING to recruit TBK1 and IKKϵ through its interaction with STING. It was also suggested that the transmembrane domain (amino acids 39–59) of p17 is required for interacting with STING and inhibiting cGAS-STING pathway. Additionally, with the p17 specific siRNA, the ASFV induced IFN-β, ISG15, ISG56, IL-6 and IL-8 gene transcriptions were upregulated in ASFV infected primary porcine alveolar macrophages (PAMs). Taken together, ASFV p17 can inhibit the cGAS-STING pathway through its interaction with STING and interference of the recruitment of TBK1 and IKKϵ. Our work establishes the role of p17 in the immune evasion and thus provides insights on ASFV pathogenesis.

Nuclear soluble cGAS senses double-stranded DNA virus infection

The DNA sensor cGAS detects cytosolic DNA and instigates type I interferon (IFN) expression. Recent studies find that cGAS also localizes in the nucleus and binds the chromatin. Despite the mechanism controlling nuclear cGAS activation is well elucidated, whether nuclear cGAS participates in DNA sensing is unclear. Here, we report that herpes simplex virus 1 (HSV-1) infection caused the release of cGAS from the chromatin into the nuclear soluble fraction. Like its cytosolic counterpart, the leaked nuclear soluble cGAS also sensed viral DNA, produced cGAMP, and induced mRNA expression of type I IFN and interferon-stimulated genes. Consistently, the nuclear soluble cGAS limited HSV-1 infection. Furthermore, enzyme-deficient mutation (D307A) or cGAS inhibitor RU.251 abolished nuclear cGAS-mediated innate immune responses, suggesting that enzymatic activity is also required for nuclear soluble cGAS. Taken all together, our study demonstrates that nuclear soluble cGAS acts as a nuclear DNA sensor detecting nuclear-replicating DNA viruses.

Pseudorabies virus tegument protein UL13 recruits RNF5 to inhibit STING-mediated antiviral immunity

Pseudorabies virus (PRV) has evolved various immune evasion mechanisms that target host antiviral immune responses. However, it is unclear whether and how PRV encoded proteins modulate the cGAS-STING axis for immune evasion. Here, we show that PRV tegument protein UL13 inhibits STING-mediated antiviral signaling via regulation of STING stability. Mechanistically, UL13 interacts with the CDN domain of STING and recruits the E3 ligase RING-finger protein 5 (RNF5) to promote K27-/K29-linked ubiquitination and degradation of STING. Consequently, deficiency of RNF5 enhances host antiviral immune responses triggered by PRV infection. In addition, mutant PRV lacking UL13 impaired in antagonism of STING-mediated production of type I IFNs and shows attenuated pathogenicity in mice. Our findings suggest that PRV UL13 functions as an antagonist of IFN signaling via a novel mechanism by targeting STING to persistently evade host antiviral responses.

Induction of type I interferons mediated by cGAS-STING axis is critical for host against DNA virus infection whereas herpesviruses employ multiple strategies to antagonize this signaling pathway for immune evasion. Herein, our findings provide strong evidence that PRV tegument protein UL13 functions as a suppressor of STING-mediated antiviral response via recruitment of E3 ligase RNF5 to induce K27-/K29-linked ubiquitination and degradation of STING. Therefore, our study reveals a novel evasion strategy of PRV against host defense and suggests UL13 could be a promising target for development of gene-deleted vaccine for pseudorabies.

HSV-2-Specific Human Female Reproductive Tract Tissue Resident Memory T Cells Recognize Diverse HSV Antigens

Antigen-specific T_RM persist and protect against skin or female reproductive tract (FRT) HSV infection. As the pathogenesis of HSV differs between humans and model organisms, we focus on humans with well-characterized recurrent genital HSV-2 infection. Human CD8+ T_RM persisting at sites of healed human HSV-2 lesions have an activated phenotype but it is unclear if T_RM can be cultivated in vitro. We recovered HSV-specific T_RM from genital skin and ectocervix biopsies, obtained after recovery from recurrent genital HSV-2, using ex vivo activation by viral antigen. Up to several percent of local T cells were HSV-reactive ex vivo. CD4 and CD8 T cell lines were up to 50% HSV-2-specific after sorting-based enrichment. CD8 T_RM displayed HLA-restricted reactivity to specific HSV-2 peptides with high functional avidities. Reactivity to defined peptides persisted locally over several month and was quite subject-specific. CD4 T_RM derived from biopsies, and from an extended set of cervical cytobrush specimens, also recognized diverse HSV-2 antigens and peptides. Overall we found that HSV-2-specific T_RM are abundant in the FRT between episodes of recurrent genital herpes and maintain competency for expansion. Mucosal sites are accessible for clinical monitoring during immune interventions such as therapeutic vaccination.

HBx inhibits DNA sensing signaling pathway via ubiquitination and autophagy of cGAS

BACKGROUND

Cyclic GMP-AMP synthase (cGAS) is a crucial DNA sensor and plays an important role in host antiviral innate immune responses. During hepatitis B virus (HBV) infection, the cGAS signaling pathway can suppress HBV replication. As an important regulatory protein of HBV, hepatitis B virus X protein (HBx) may serve as an antagonistic character to the cGAS/STING signaling pathway. In this study, we aim to investigate the functional role of HBx in the cGAS/STING signaling pathway.

METHODS

The effects of HBx on IFN-β promoter activity were measured by Dual-luciferase reporter assays. Ubiquitination and autophagy were analyzed by Western-blot and Co-immunoprecipitation assays.

RESULTS

Our results show that HBx down-regulates IFN-I production by directly promoting ubiquitination and autophagy degradation of cGAS.

CONCLUSIONS

HBV can antagonize host cGAS DNA sensing to promote HBV replication and provide novel insights to develop novel approaches against HBV infection.

The Interplay between Viruses and Host DNA Sensors

DNA virus infections are often lifelong and can cause serious diseases in their hosts. Their recognition by the sensors of the innate immune system represents the front line of host defence. Understanding the molecular mechanisms of innate immunity responses is an important prerequisite for the design of effective antivirotics. This review focuses on the present state of knowledge surrounding the mechanisms of viral DNA genome sensing and the main induced pathways of innate immunity responses. The studies that have been performed to date indicate that herpesviruses, adenoviruses, and polyomaviruses are sensed by various DNA sensors. In non-immune cells, STING pathways have been shown to be activated by cGAS, IFI16, DDX41, or DNA-PK. The activation of TLR9 has mainly been described in pDCs and in other immune cells. Importantly, studies on herpesviruses have unveiled novel participants (BRCA1, H2B, or DNA-PK) in the IFI16 sensing pathway. Polyomavirus studies have revealed that, in addition to viral DNA, micronuclei are released into the cytosol due to genotoxic stress. Papillomaviruses, HBV, and HIV have been shown to evade DNA sensing by sophisticated intracellular trafficking, unique cell tropism, and viral or cellular protein actions that prevent or block DNA sensing. Further research is required to fully understand the interplay between viruses and DNA sensors.

Duck plague virus UL41 protein inhibits RIG-I/MDA5-mediated duck IFN-β production via mRNA degradation activity

Retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs) are cytosolic pattern recognition receptors that initiate innate antiviral immunity. Recent reports found that duck RLRs significantly restrict duck plague virus (DPV) infection. However, the molecular mechanism by which DPV evades immune responses is unknown. In this study, we first found that the DPV UL41 protein inhibited duck interferon-β (IFN-β) production mediated by RIG-I and melanoma differentiation-associated gene 5 (MDA5) by broadly downregulating the mRNA levels of important adaptor molecules, such as RIG-I, MDA5, mitochondrial antiviral signalling protein (MAVS), stimulator of interferon gene (STING), TANK-binding kinase 1 (TBK1), and interferon regulatory factor (IRF) 7. The conserved sites of the UL41 protein, E229, D231, and D232, were responsible for this activity. Furthermore, the DPV CHv-BAC-ΔUL41 mutant virus induced more duck IFN-β and IFN-stimulated genes (Mx, OASL) production in duck embryo fibroblasts (DEFs) than DPV CHv-BAC parent virus. Our findings provide insights into the molecular mechanism underlying DPV immune evasion.

DDIT3 antagonizes innate immune response to promote bovine alphaherpesvirus 1 replication via the DDIT3-SQSTM1-STING pathway

DNA damage-inducible transcript 3 (DDIT3), a transcription factor, is typically involved in virus replication control. We are the first to report that DDIT3 promotes the replication of bovine viral diarrhea virus, an RNA virus, by inhibiting innate immunity. However, whether the DDIT3 gene participates in DNA virus replication by regulating innate immunity remains unclear. This study reported that DDIT3 suppressed the innate immune response caused by DNA viruses to promote bovine herpesvirus 1 (BoHV-1) replication. After BoHV-1 infection of Madin-Darby bovine kidney (MDBK) cells, upregulated expression of DDIT3 induced SQSTM1-mediated autophagy and promoted STING degradation. Overexpression of the SQSTM1 protein effectively reduced STING protein levels, whereas SQSTM1 knockdown increased STING protein levels. Coimmunoprecipitation experiments and confocal laser scanning microscopy revealed that the SQSTM1 protein interacts with and colocalizes with STING. Knockdown of SQSTM1 expression in DDIT3-overexpressing cell lines restored STING protein levels. Moreover, a dual-luciferase reporter assay revealed that DDIT3 directly binds to the bovine SQSTM1 promoter and induces SQSTM1 transcription. Overexpression of SQSTM1 promoted BoHV-1 replication by inhibiting IFN-β and IFN-stimulated genes (ISGs) production; silencing of SQSTM1 promoted the expression of IFN-β and ISGs to inhibit BoHV-1 replication. In conclusion, DDIT3 targets STING via SQSTM1-mediated autophagy to promote BoHV-1 replication. These results suggest a novel mechanism by which DDIT3 regulates DNA virus replication by targeting innate immunity. DDIT3 antagonizes the innate immune response to promote bovine alphaherpesvirus 1 replication via the DDIT3-SQSTM1-STING pathway.

The Regulatory Network of Cyclic GMP-AMP Synthase-Stimulator of Interferon Genes Pathway in Viral Evasion

Virus infection has been consistently threatening public health. The cyclic GMP-AMP synthase (cGAS)-Stimulator of Interferon Genes (STING) pathway is a critical defender to sense various pathogens and trigger innate immunity of mammalian cells. cGAS recognizes the pathogenic DNA in the cytosol and then synthesizes 2'3'-cyclic GMP-AMP (2'3'cGAMP). As the second messenger, cGAMP activates STING and induces the following cascade to produce type I interferon (IFN-I) to protect against infections. However, viruses have evolved numerous strategies to hinder the cGAS-STING signal transduction, promoting their immune evasion. Here we outline the current status of the viral evasion mechanism underlying the regulation of the cGAS-STING pathway, focusing on how post-transcriptional modifications, viral proteins, and non-coding RNAs involve innate immunity during viral infection, attempting to inspire new targets discovery and uncover potential clinical antiviral treatments.

Oncolytic Virotherapy: From Bench to Bedside

Oncolytic viruses are naturally occurring or genetically engineered viruses that can replicate preferentially in tumor cells and inhibit tumor growth. These viruses have been considered an effective anticancer strategy in recent years. They mainly function by direct oncolysis, inducing an anticancer immune response and expressing exogenous effector genes. Their multifunctional characteristics indicate good application prospects as cancer therapeutics, especially in combination with other therapies, such as radiotherapy, chemotherapy and immunotherapy. Therefore, it is necessary to comprehensively understand the utility of oncolytic viruses in cancer therapeutics. Here, we review the characteristics, antitumor mechanisms, clinical applications, deficiencies and associated solutions, and future prospects of oncolytic viruses.

African Swine Fever Virus pI215L Negatively Regulates cGAS-STING Signaling Pathway through Recruiting RNF138 to Inhibit K63-Linked Ubiquitination of TBK1

African swine fever is a severe animal infectious disease caused by African swine fever virus (ASFV), and the morbidity and mortality associated with virulent ASFV isolates are as high as 100%. Previous studies showed that the ability of ASFV to antagonize IFN production is closely related to its pathogenicity. Here, we report that ASFV HLJ/18 infection induced low levels of type I IFN and inhibited cGMP-AMP-induced type I IFN production in porcine alveolar macrophages that were isolated from specific pathogen-free Landrace piglets. Subsequently, an unbiased screen was performed to screen the ASFV genes with inhibitory effects on the type I IFN production. ASFV pI215L, a viral E2 ubiquitin-conjugating enzyme, was identified as one of the strongest inhibitory effectors on the production of type I IFN. Knockdown of pI215L expression inhibited ASFV replication and enhanced IFN-β production. However, inhibition of type I IFN production by pI215L was independent of its E2 enzyme activity. Furthermore, we found that pI215L inhibited type I IFN production and K63-linked polyubiquitination of TANK-binding kinase 1 through pI215L-binding RING finger protein 138 (RNF138). ASFV pI215L enhanced the interaction between RNF138 and RNF128 and promoted RNF138 to degrade RNF128, which resulted in reduced K63-linked polyubiquitination of TANK-binding kinase 1 and type І IFN production. Taken together, our findings reveal a novel immune escape mechanism of ASFV, which provides a clue to the design and development of an immune-sensitive attenuated live vaccine.

Pathogenesis and virulence of herpes simplex virus

Two of the most prevalent human viruses worldwide, herpes simplex virus type 1 and type 2 (HSV-1 and HSV-2, respectively), cause a variety of diseases, including cold sores, genital herpes, herpes stromal keratitis, meningitis and encephalitis. The intrinsic, innate and adaptive immune responses are key to control HSV, and the virus has developed mechanisms to evade them. The immune response can also contribute to pathogenesis, as observed in stromal keratitis and encephalitis. The fact that certain individuals are more prone than others to suffer severe disease upon HSV infection can be partially explained by the existence of genetic polymorphisms in humans. Like all herpesviruses, HSV has two replication cycles: lytic and latent. During lytic replication HSV produces infectious viral particles to infect other cells and organisms, while during latency there is limited gene expression and lack of infectious virus particles. HSV establishes latency in neurons and can cause disease both during primary infection and upon reactivation. The mechanisms leading to latency and reactivation and which are the viral and host factors controlling these processes are not completely understood. Here we review the HSV life cycle, the interaction of HSV with the immune system and three of the best-studied pathologies: Herpes stromal keratitis, herpes simplex encephalitis and genital herpes. We also discuss the potential association between HSV-1 infection and Alzheimer's disease.

Leaked Mitochondrial C1QBP Inhibits Activation of the DNA Sensor cGAS

Cytosolic DNA from pathogens activates the DNA sensor cyclic GMP-AMP (cGAMP) synthase (cGAS) that produces the second messenger, cGAMP. cGAMP triggers a signal cascade leading to type I IFN expression. Host DNA is normally restricted in the cellular compartments of the nucleus and mitochondria. Recent studies have shown that DNA virus infection triggers mitochondrial stress, leading to the release of mitochondrial DNA to the cytosol and activation of cGAS; however, the regulatory mechanism of mitochondrial DNA-mediated cGAS activation is not well elucidated. In this study, we analyzed cGAS protein interactome in mouse RAW264.7 macrophages and found that cGAS interacted with C1QBP. C1QBP predominantly localized in the mitochondria and leaked into the cytosol during DNA virus infection. The leaked C1QBP bound the NTase domain of cGAS and inhibited cGAS enzymatic activity in cells and in vitro. Overexpression of the cytosolic form of C1QBP inhibited cytosolic DNA-elicited innate immune responses and promoted HSV-1 infection. By contrast, deficiency of C1QBP led to the elevated innate immune responses and impaired HSV-1 infection. Taken together, our study suggests that C1QBP is a novel cGAS inhibitor hidden in the mitochondria.

How dendritic cells sense and respond to viral infections

The ability of dendritic cells (DCs) to sense viral pathogens and orchestrate a proper immune response makes them one of the key players in antiviral immunity. Different DC subsets have complementing functions during viral infections, some specialize in antigen presentation and cross-presentation and others in the production of cytokines with antiviral activity, such as type I interferons. In this review, we summarize the latest updates concerning the role of DCs in viral infections, with particular focus on the complex interplay between DC subsets and severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2). Despite being initiated by a vast array of immune receptors, DC-mediated antiviral responses often converge towards the same endpoint, that is the production of proinflammatory cytokines and the activation of an adaptive immune response. Nonetheless, the inherent migratory properties of DCs make them a double-edged sword and often viral recognition by DCs results in further viral dissemination. Here we illustrate these various aspects of the antiviral functions of DCs and also provide a brief overview of novel antiviral vaccination strategies based on DCs targeting.

cGAS-STING-mediated sensing pathways in DNA and RNA virus infections: crosstalk with other sensing pathways

Viruses cause a variety of diseases in humans and other organisms. The most important defense mechanism against viral infections is initiated when the viral genome is sensed by host proteins, and this results in interferon production and pro-inflammatory cytokine responses. The sensing of the viral genome or its replication intermediates within host cells is mediated by cytosolic proteins. For example, cGAS and IFI16 recognize non-self DNA, and RIG-I and MDA5 recognize non-self RNA. Once these sensors are activated, they trigger a cascade of reactions activating downstream molecules, which eventually results in the transcriptional activation of type I and III interferons, which play a critical role in suppressing viral propagation, either by directly limiting their replication or by inducing host cells to inhibit viral protein synthesis. The immune response against viruses relies solely upon sensing of viral genomes and their downstream signaling molecules. Although DNA and RNA viruses are sensed by distinct classes of receptor proteins, there is a possibility of overlap between the viral DNA and viral RNA sensing mechanisms. In this review, we focus on various host sensing molecules and discuss the associated signaling pathways that are activated in response to different viral infections. We further highlight the possibility of crosstalk between the cGAS-STING and the RIG-I-MAVS pathways to limit viral infections. This comprehensive review delineates the mechanisms by which different viruses evade host cellular responses to sustain within the host cells.

Herpes simplex virus 1 evades cellular antiviral response by inducing microRNA-24, which attenuates STING synthesis

STING is a nodal point for cellular innate immune response to microbial infections, autoimmunity and cancer; it triggers the synthesis of the antiviral proteins, type I interferons. Many DNA viruses, including Herpes Simplex Virus 1 (HSV1), trigger STING signaling causing inhibition of virus replication. Here, we report that HSV1 evades this antiviral immune response by inducing a cellular microRNA, miR-24, which binds to the 3’ untranslated region of STING mRNA and inhibits its translation. Expression of the gene encoding miR-24 is induced by the transcription factor AP1 and activated by MAP kinases in HSV1-infected cells. Introduction of exogenous miR-24 or prior activation of MAPKs, causes further enhancement of HSV1 replication in STING-expressing cells. Conversely, transfection of antimiR-24 inhibits virus replication in those cells. HSV1 infection of mice causes neuropathy and death; using two routes of infection, we demonstrated that intracranial injection of antimiR-24 alleviates both morbidity and mortality of the infected mice. Our studies reveal a new immune evasion strategy adopted by HSV1 through the regulation of STING and demonstrates that it can be exploited to enhance STING’s antiviral action.

The type I interferon system is the first line of cellular antiviral innate immune response. Virus infection is recognized by various pattern recognition receptors in the infected cell and it activates the interferon system to inhibit virus replication. However, viruses have evolved various mechanisms to evade the cellular immune response and enhance viral replication. Our study uncovers an immune evasion strategy used by the Herpes Simplex virus to circumvent the cGAS/STING signaling pathway which is the pivotal innate immune response to combat DNA virus replication. miR-24 induction by HSV1 targets STING and hence, dampens Type I Immune response against the virus. The induction of miR-24 is regulated by virus induced MAPK activation, which are also required during early lytic cycles of HSV1 replication and is indispensable for HSV1 reactivation from latency in neurons; depicting a new direct co-relation between MAPK activation and HSV1 replication orchestrated through cellular miR-24. Silencing of miR-24 in mice brain curtails viral replication and disease severity. Overall, these results indicate possible therapeutic use of stable antimiR-24 against HSV1 and other diseases that are alleviated by STING.

PCV2 targets cGAS to inhibit type I interferon induction to promote other DNA virus infection

Viruses use diverse strategies to impair the antiviral immunity of host in order to promote infection and pathogenesis. Herein, we found that PCV2 infection promotes the infection of DNA viruses through inhibiting IFN-β induction in vivo and in vitro. In the early phase of infection, PCV2 promotes the phosphorylation of cGAS at S278 via activation of PI3K/Akt signaling, which directly silences the catalytic activity of cGAS. Subsequently, phosphorylation of cGAS at S278 can facilitate the K48-linked poly-ubiquitination of cGAS at K389, which can been served as a signal for recognizing by the ubiquitin-binding domain of histone deacetylase 6 (HDAC6), to promote the translocation of K48-ubiquitinated-cGAS from cytosol to autolysosome depending on the deacetylase activity of HDAC6, thereby eventually resulting in a markedly increased cGAS degradation in PCV2 infection-induced autophagic cells relative to Earle’s Balanced Salt Solution (EBSS)-induced autophagic cells (a typical starving autophagy). Importantly, we found that PCV2 Cap and its binding protein gC1qR act as predominant regulators to promote porcine cGAS phosphorylation and HDAC6 activation through mediating PI3K/AKT signaling and PKCδ signaling activation. Based on this finding, gC1qR-binding activity deficient PCV2 mutant (PCV2RmA) indeed shows a weakened inhibitory effect on IFN-β induction and a weaker boost effect for other DNA viruses infection compared to wild-type PCV2. Collectively, our findings illuminate a systematic regulation mechanism by which porcine circovirus counteracts the cGAS-STING signaling pathway to inhibit the type I interferon induction and promote DNA virus infection, and identify gC1qR as an important regulator for the immunosuppression induced by PCV2.

PCV2 is well known for its ability to induce immunosuppression in pigs. However, how PCV2 infection interferes cGAS-STING signaling is still poorly understood. Herein, we demonstrate that PCV2 infection can phosphorylate porcine cGAS via gC1qR-mediated PI3K/AKT signaling to silence the catalytic activity of cGAS, while activates PKCδ signaling to promote histone deacetylase 6 (HDAC6) activation depending on the assistance of gC1qR. Subsequently, phosphorylation of cGAS facilitates the poly-ubiquitination of cGAS, then ubiquitinated-cGAS proteins are recruited and transported to autolysosome by activated HDAC6 depending on its deacetylase activity and ubiquitin-binding function, thereby eventually resulting in the autophagic degradation of cGAS in PCV2-infected cells. This study reveals that PCV2 can inhibit the activation of cGAS signaling pathway through two different mechanisms at different stages of infection and clarifies the internal relationship and cooperation model between these two mechanisms.

Local Immune Control of Latent Herpes Simplex Virus Type 1 in Ganglia of Mice and Man

Herpes simplex virus type 1 (HSV-1) is a prevalent human pathogen. HSV-1 genomes persist in trigeminal ganglia neuronal nuclei as chromatinized episomes, while epithelial cells are typically killed by lytic infection. Fluctuations in anti-viral responses, broadly defined, may underlay periodic reactivations. The ganglionic immune response to HSV-1 infection includes cell-intrinsic responses in neurons, innate sensing by several cell types, and the infiltration and persistence of antigen-specific T-cells. The mechanisms specifying the contrasting fates of HSV-1 in neurons and epithelial cells may include differential genome silencing and chromatinization, dictated by variation in access of immune modulating viral tegument proteins to the cell body, and protection of neurons by autophagy. Innate responses have the capacity of recruiting additional immune cells and paracrine activity on parenchymal cells, for example via chemokines and type I interferons. In both mice and humans, HSV-1-specific CD8 and CD4 T-cells are recruited to ganglia, with mechanistic studies suggesting active roles in immune surveillance and control of reactivation. In this review we focus mainly on HSV-1 and the TG, comparing and contrasting where possible observational, interventional, and in vitro studies between humans and animal hosts.

Innate Immune Responses to Herpesvirus Infection

Infection of a host cell by an invading viral pathogen triggers a multifaceted antiviral response. One of the most potent defense mechanisms host cells possess is the interferon (IFN) system, which initiates a targeted, coordinated attack against various stages of viral infection. This immediate innate immune response provides the most proximal defense and includes the accumulation of antiviral proteins, such as IFN-stimulated genes (ISGs), as well as a variety of protective cytokines. However, viruses have co-evolved with their hosts, and as such, have devised distinct mechanisms to undermine host innate responses. As large, double-stranded DNA viruses, herpesviruses rely on a multitude of means by which to counter the antiviral attack. Herein, we review the various approaches the human herpesviruses employ as countermeasures to the host innate immune response.