Human Cytomegalovirus Tegument Protein UL82 Inhibits STING-Mediated Signaling to Evade Antiviral Immunity

Marek's disease virus protein kinase US3 inhibits DNA-sensing antiviral innate immunity via abrogating activation of NF-κB

Marek's disease virus (MDV) is an avian alphaherpesvirus associated with Marek's disease, an immunosuppressive and lymphoproliferative disease in chickens. The DNA sensing pathway mediates innate immune defense against infection by many DNA-containing pathogens, while viruses have evolved multiple strategies to evade the host immune response to survive in host cells. This study found that ectopic expression of MDV protein kinase US3 inhibited beta interferon (IFN-β) and interleukin-6 (IL-6) production induced by interferon-stimulatory and viral DNA. US3 was further shown to abolish the nuclear factor κB (NF-κB) activation. The US3 kinase activity was indispensable for its inhibitory function, as the kinase-dead US3 mutant (US3K220A) did not inhibit NF-κB activation. Further studies showed that US3 interacted with the Rel homology domains of the NF-κB subunits p65 and p50, which phosphorylated these transcription factors and blocked their nuclear translocation. Finally, US3 deficiency promoted IFN-β and IL-6 production, resulting in reduced viral replication and lower MDV-specific lesion incidence during MDV infection in chickens. Altogether, these findings reveal a novel mechanism for MDV to evade host antiviral immunity.IMPORTANCEMarek's disease virus (MDV) is an oncogenic avian alphaherpesvirus that causes an economically important disease affecting the health and welfare of poultry worldwide. Whereas human herpesviruses have been shown to evolve various strategies to inhibit the DNA sensing signaling for the evasion of the host's innate immunity, little is known regarding the mechanism for MDV to regulate this pathway. In this study, MDV US3 protein kinase was demonstrated to inhibit the activation of NF-κB in the DNA sensing pathway via binding to the Rel homology domains of the NF-κB subunits p65 and p50, which hyperphosphorylated these transcription factors and abolished their nuclear translocation. This is an important finding toward a better understanding of the functions of avian alphaherpesviruses encoded US3 protein kinase.

Universal STING Pathway-Activating Complexes Counteract Viral Immune Evasion and Boost Antiviral Responses

Viruses have evolved multiple mechanisms to counteract the stimulator of the interferon genes (STING) pathway, resulting in the suppression of antiviral responses. Accordingly, in addition to developing STING agonist analogs with enhanced stability and deliverability, overcoming the defective STING function in virus-infected cells is essential for defense against viral infections. In this study, we developed STING pathway-activating complexes (SPAC) based on bioinspired vesicles that display and multimerize STING molecules with a specific affinity for agonist binding. As a broad-spectrum antiviral agent, this universal STING mimic triggers IFN-I signaling independently of endogenous STING. In infectious models, including CMV and SARS-CoV-2 infection, both prophylactic and therapeutic regimens of SPAC can reduce viral load and disease severity. These results indicate that SPAC, functioning as a host-targeted immune modulator, provides the distinct advantage of broad-spectrum therapy against infectious diseases caused by both DNA and RNA viruses, particularly those with strong STING antagonistic functions. The endogenous STING-independent activation mechanisms of SPAC may provide a universal therapy for infectious diseases, potentially serving as a candidate option to defend against future pandemics of "Disease X".

Grass carp reovirus VP4 manipulates TOLLIP to degrade STING for inhibition of IFN production

Although fish possess an effective interferon (IFN) system to defend against viral infection, grass carp reovirus (GCRV) still causes epidemic hemorrhagic disease and tremendous economic loss in grass carp. Therefore, it is necessary to investigate the immune escape strategies employed by GCRV. In this study, we show that the structural protein VP4 of GCRV (encoded by the S6 segment) significantly restricts IFN expression by degrading stimulator of IFN genes (STING) through the autophagy-lysosome-dependent pathway. First, overexpression of VP4 inhibited the expression of IFN induced by GCRV and polyinosinic-polycytidylic acid (poly I:C) at both the promoter and mRNA levels. Second, VP4 was found to associate with STING, and the N-terminal transmembrane domain is essential for this interaction. Additionally, VP4 dramatically blocked STING-induced IFN expression and weakened its antiviral capacity. Further mechanistic studies revealed that VP4 degrades STING via the autophagy-lysosome pathway in a dose-dependent manner. Interestingly, toll-interacting protein (TOLLIP), a selective autophagy receptor, was found to interact with VP4 and reduce VP4-mediated STING degradation after tollip knockdown. Finally, overexpression of VP4 facilitated GCRV proliferation, while its depletion had the opposite effect. These findings indicate that GCRV VP4 recruits TOLLIP to degrade STING and achieve immune escape. This enhances our comprehension of aquatic virus pathogenesis.

IMPORTANCE

Upon virus invasion, fish cells employ a multitude of strategies to defend against infection. Consequently, viruses have evolved a plethora of tactics to evade host antiviral mechanisms. To date, fewer studies have been conducted on the immune evasion mechanism of grass carp reovirus (GCRV). In this study, we demonstrate that VP4 of GCRV-873 inhibits interferon expression by interacting with stimulator of IFN gene and degrading it in an autophagy-lysosome-dependent manner through the manipulation of the selective autophagy receptor toll-interacting protein. The findings of this study contribute to our understanding of the novel evasion mechanisms of GCRV and widen our knowledge of the virus-host interactions in lower vertebrates.

Human cytomegalovirus UL82 promotes cell cycle progression of colorectal cancer by upregulating AGR2

The correlation between persistent human cytomegalovirus (HCMV) infection and poor prognosis in colorectal cancer (CRC) patients has garnered increasing attention. UL82 is a tegument protein of HCMV, and our previous research indicated that the presence of UL82 is significantly associated with reduced overall survival in CRC patients. However, the mechanism by which UL82 affects the prognosis of CRC patients remains unclear. In this study, we investigated the role of UL82 in CRC progression through both in vitro and in vivo experiments, and revealed its downstream regulatory pathways by integrating transcriptomics, metabolomics, and proteomics. Our findings first revealed that UL82 significantly promoted CRC cell proliferation by increasing the proportion of cells in the S phase of the cell cycle. Additionally, UL82 enhanced the expression of the oncogene AGR2, while knockdown of AGR2 abolished the proliferative effect of UL82. Interestingly, UL82 interacted with the transcription factor DDX5, which transcriptionally inhibited AGR2 expression. Furthermore, this UL82-AGR2 axis promoted nucleotide metabolism in CRC cells by enhancing the levels of nucleotide synthesis enzymes DTYMK, RRM2, and TYMS. In conclusion, our study suggests that the UL82/DDX5 complex may promote nucleotide metabolism and cell cycle progression of CRC by upregulating AGR2 and UL82 may serve as a potential prognostic biomarker for CRC patients.

Innate Sensing of Viral Nucleic Acids and Their Use in Antiviral Vaccine Development

Viruses pose a significant threat to humans by causing numerous infectious and potentially fatal diseases. Understanding how the host's innate immune system recognizes viruses is essential to understanding pathogenesis and ways to control viral infection. Innate immunity also plays a critical role in shaping adaptive immune responses induced by vaccines. Recently developed adjuvants often include nucleic acids that stimulate pattern recognition receptors which are essential components of innate immunity necessary for activating antigen-presentation cells and thereby bridging innate and adaptive immunity. Therefore, understanding viral nucleic acid sensing by cytosolic sensors is essential, as it provides the potential means for developing new vaccine strategies, including effective adjuvants.

Infection-induced lysine lactylation enables herpesvirus immune evasion

Aerobic glycolysis is a hallmark of many viral infections, leading to substantial accumulation of lactate. However, the regulatory roles of lactate during viral infections remain poorly understood. Here, we report that human cytomegalovirus (HCMV) infection leverages lactate to induce widespread protein lactylation and promote viral spread. We establish that lactyllysine is enriched in intrinsically disordered regions, regulating viral protein condensates and immune signaling transduction. Dynamic lactylation of immune factors suppresses immunity, a feature we show to be shared for HCMV and herpes simplex virus 1 infections, through regulation of RNA binding protein 14 and interferon-γ-inducible protein 16 (IFI16). K90 lactylation of the viral DNA sensor IFI16 inhibits recruitment of the DNA damage response kinase DNA-PK, preventing IFI16-driven virus gene repression and cytokine induction. Together, we characterize global protein lactylation dynamics during virus infection, finding that virus-induced lactate contributes to its immune evasion through direct inhibition of immune signaling pathways.

African swine fever viral proteins that inhibit cGAS-STING pathway and type-I interferon production

African swine fever virus (ASFV) is the causative agent of a lethal disease in pigs. Highly virulent strains of ASFV are known to suppress the induction of type I interferons (IFNs), while naturally attenuated strains do not exhibit this capability. Thus, the ability to suppress IFN is assumed to be associated with viral virulence. ASFV genome encodes many proteins capable of disrupting crucial components of host immune response pathways. Notably, these viral proteins interfere with the induction of type I IFNs by targeting various steps of the cGAS-STING signaling pathway. Additionally, certain viral proteins impede the expression of interferon-stimulated genes by interfering with the JAK-STAT pathway. Consequently, ASFV proteins hamper both IFN production and the induction of antiviral responses by IFNs. This review article summarizes the viral proteins responsible for suppressing various steps of the cGAS-STING and JAK-STAT signaling pathways and discusses the potential application of this knowledge to the rational design of a live-attenuated ASFV vaccine.

Corilagin inhibits human cytomegalovirus infection and replication via activating the cGAS-STING signaling pathway in vitro and in vivo

AIM

The existence of human cytomegalovirus (HCMV) is extremely widespread, causing serious diseases in patients with low immune function. The purpose of this study is to explore the efficacy and mechanism of Corilagin in the control of CMV infection, in order to provide scientific basis for the control of CMV infection.

METHODS

Our study employed an animal model in Balb/c mice, infected with MCMV, alongside cellular models in HFF cells and THP-1 cells, stimulated with HCMV. The expression of cGAS-STING signaling pathway molecules was detected in liver tissue, lung tissue, serum, cells and cell supernatant. The liver function and histopathological changes of mice were evaluated.

RESULTS

In vivo and in vitro experiments showed that Corilagin significantly inhibits CMV levels and attenuates pathological damage in liver and lung tissues in vivo, and similarly inhibits viral load in cells in vitro. Corilagin promotes the expression levels of STING and its downstream molecules in vivo and in vitro. Inhibition/down-regulation of STING significantly promotes CMV replication, on the contrary, activation/up-regulation of STING inhibits CMV replication, and Corilagin also promotes the expression levels of molecules related to the cGAS-STING signaling pathway in the above cases.

CONCLUSION

Corilagin could effectively inhibit the infection and replication of CMV in vitro and in vivo, which may be through the activation of cGAS-STING signaling pathway.

Progress of cGAS-STING signaling pathway-based modulation of immune response by traditional Chinese medicine in clinical diseases

The cGAS-STING signaling pathway is a critical component of the innate immune response, playing a significant role in various diseases. As a central element of this pathway, STING responds to both endogenous and exogenous DNA stimuli, triggering the production of interferons and pro-inflammatory cytokines to enhance immune defenses against tumors and pathogens. However, dysregulated activation of the STING pathway is implicated in the pathogenesis of multiple diseases, including autoinflammation, viral infections, and cancer. Traditional Chinese Medicines (TCMs), which have a long history of use, have been associated with positive effects in disease prevention and treatment. TCM formulations (e.g., Lingguizhugan Decoction, Yi-Shen-Xie-Zhuo formula) and active compounds (e.g., Glabridin, Ginsenoside Rd) can modulate the cGAS-STING signaling pathway, thereby influencing the progression of inflammatory, infectious, or oncological diseases. This review explores the mechanisms by which TCMs interact with the cGAS-STING pathway to regulate immunity, focusing on their roles in infectious diseases, malignancies, and autoimmune disorders.

Human cytomegalovirus: pathogenesis, prevention, and treatment

Human cytomegalovirus (HCMV) infection remains a significant global health challenge, particularly for immunocompromised individuals and newborns. This comprehensive review synthesizes current knowledge on HCMV pathogenesis, prevention, and treatment strategies. We examine the molecular mechanisms of HCMV entry, focusing on the structure and function of key envelope glycoproteins (gB, gH/gL/gO, gH/gL/pUL128-131) and their interactions with cellular receptors such as PDGFRα, NRP2, and THBD. The review explores HCMV's sophisticated immune evasion strategies, including interference with pattern recognition receptor signaling, modulation of antigen presentation, and regulation of NK and T cell responses. We highlight recent advancements in developing neutralizing antibodies, various vaccine strategies (live-attenuated, subunit, vector-based, DNA, and mRNA), antiviral compounds (both virus-targeted and host-targeted), and emerging cellular therapies such as TCR-T cell approaches. By integrating insights from structural biology, immunology, and clinical research, we identify critical knowledge gaps and propose future research directions. This analysis aims to stimulate cross-disciplinary collaborations and accelerate the development of more effective prevention and treatment strategies for HCMV infections, addressing a significant unmet medical need.

ER: a critical hub for STING signaling regulation

The Stimulator of Interferon Genes (STING) has a crucial role in mediating the immune response against cytosolic double-stranded DNA (dsDNA) and its activation is critically involved in various diseases. STING is synthesized, modified, and resides in the endoplasmic reticulum (ER), and its ER exit is intimately connected with its signaling. The ER, primarily known for its roles in protein folding, lipid synthesis, and calcium storage, has been identified as a pivotal platform for the regulation of a wide range of STING functions. In this review, we discuss the emerging factors that regulate STING in the ER and examine the interplay between STING signaling and ER pathways, highlighting the impacts of such regulations on immune responses and their potential implications in STING-related disorders.

Functions of the UL51 protein during the herpesvirus life cycle

The herpesvirus UL51 protein is a multifunctional tegument protein involved in the regulation of multiple aspects of the viral life cycle. This article reviews the biological characteristics of the UL51 protein and its functions in herpesviruses, including participating in the maintenance of the viral assembly complex (cVAC) during viral assembly, affecting the production of mature viral particles and promoting primary and secondary envelopment, as well as its positive impact on viral cell-to-cell spread (CCS) through interactions with multiple viral proteins and its key role in the proliferation and pathogenicity of the virus in the later stage of infection. This paper discusses how the UL51 protein participates in the life cycle of herpesviruses and provides new ideas for further research on UL51 protein function.

STRAP upregulates antiviral innate immunity against PRV by targeting TBK1

Serine/threonine kinase receptor-associated protein (STRAP) serves as a scaffold protein and is engaged in a variety of cellular activities, although its importance in antiviral innate immunity is unknown. We discovered that STRAP works as an interferon (IFN)-inducible positive regulator, facilitating type I IFN signaling during pseudorabies virus infection. Mechanistically, STRAP interacts with TBK1 to activate type I IFN signaling. Both the CT and WD40 7 - 6 domains contribute to the function of STRAP. Furthermore, TBK1 competes with PRV-UL50 for binding to STRAP, and STRAP impedes the degradation of TBK1 mediated by PRV-UL50, thereby increasing the interaction between STRAP and TBK1. Overall, these findings reveal a previously unrecognized role for STRAP in innate antiviral immune responses during PRV infection. STRAP could be a potential therapeutic target for viral infectious diseases.

The battle between host antiviral innate immunity and immune evasion by cytomegalovirus

Cytomegalovirus (CMV) has successfully established a long-lasting latent infection in humans due to its ability to counteract the host antiviral innate immune response. During coevolution with the host, the virus has evolved various evasion techniques to evade the host's innate immune surveillance. At present, there is still no vaccine available for the prevention and treatment of CMV infection, and the interaction between CMV infection and host antiviral innate immunity is still not well understood. However, ongoing studies will offer new insights into how to treat and prevent CMV infection and its related diseases. Here, we update recent studies on how CMV evades antiviral innate immunity, with a focus on how CMV proteins target and disrupt critical adaptors of antiviral innate immune signaling pathways. This review also discusses some classic intrinsic cellular defences that are crucial to the fight against viral invasion. A comprehensive review of the evasion mechanisms of antiviral innate immunity by CMV will help investigators identify new therapeutic targets and develop vaccines against CMV infection.

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.

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.

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.

Crystal structure of the tegument protein UL82 (pp71) from human cytomegalovirus

Human cytomegalovirus (HCMV) is an opportunistic pathogen that infects a majority of the world population. It may cause severe disease in immunocompromised people and lead to pregnancy loss or grave disabilities of the fetus upon congenital infection. For effective replication and lifelong persistence in its host, HCMV relies on diverse functions of its tegument protein UL82, also known as pp71. Up to now, little is known about the molecular mechanisms underlying the multiple functions of this crucial viral protein. Here, we describe the X-ray structure of full-length UL82 to a resolution of 2.7 Å. A single polypeptide chain of 559 amino acids mainly folds into three ß-barrels. We show that UL82 forms a dimer in the crystal as well as in solution. We identify point mutations that disturb the dimerization interface and show that the mutant protein is monomeric in solution and upon expression in human cells. On the basis of the three-dimensional structure, we identify structural homologs of UL82 from other herpesviruses and analyze whether their functions are preserved in UL82. We demonstrate that UL82, despite its structural homology to viral deoxyuridinetriphosphatases (dUTPases), does not possess dUTPase activity. Prompted by the structural homology of UL82 to the ORF10 protein of murine herpesvirus 68 (MHV68), which is known to interact with the RNA export factor ribonucleic acid export 1 (Rae1), we performed coimmunoprecipitations and demonstrated that UL82 indeed interacts with Rae1. This suggests that HCMV UL82 may play a role in mRNA export from the nucleus similar to ORF10 encoded by the gammaherpesviruses MHV68.

IUPHAR ECR review: The cGAS-STING pathway: Novel functions beyond innate immune and emerging therapeutic opportunities

Stimulator of interferon genes (STING) is a crucial innate immune sensor responsible for distinguishing pathogens and cytosolic DNA, mediating innate immune signaling pathways to defend the host. Recent studies have revealed additional regulatory functions of STING beyond its innate immune-related activities, including the regulation of cellular metabolism, DNA repair, cellular senescence, autophagy and various cell deaths. These findings highlight the broader implications of STING in cellular physiology beyond its role in innate immunity. Currently, approximately 10 STING agonists have entered the clinical stage. Unlike inhibitors, which have a maximum inhibition limit, agonists have the potential for infinite amplification. STING signaling is a complex process that requires precise regulation of STING to ensure balanced immune responses and prevent detrimental autoinflammation. Recent research on the structural mechanism of STING autoinhibition and its negative regulation by adaptor protein complex 1 (AP-1) provides valuable insights into its different effects under physiological and pathological conditions, offering a new perspective for developing immune regulatory drugs. Herein, we present a comprehensive overview of the regulatory functions and molecular mechanisms of STING beyond innate immune regulation, along with updated details of its structural mechanisms. We discuss the implications of these complex regulations in various diseases, emphasizing the importance and feasibility of targeting the immunity-dependent or immunity-independent functions of STING. Moreover, we highlight the current trend in drug development and key points for clinical research, basic research, and translational research related to STING.

Discovery of a Novel Antiviral Effect of the Restriction Factor SPOC1 against Human Cytomegalovirus

The chromatin-remodeler SPOC1 (PHF13) is a transcriptional co-regulator and has been identified as a restriction factor against various viruses, including human cytomegalovirus (HCMV). For HCMV, SPOC1 was shown to block the onset of immediate-early (IE) gene expression under low multiplicities of infection (MOI). Here, we demonstrate that SPOC1-mediated restriction of IE expression is neutralized by increasing viral titers. Interestingly, our study reveals that SPOC1 exerts an additional antiviral function beyond the IE phase of HCMV replication. Expression of SPOC1 under conditions of high MOI resulted in severely impaired viral DNA replication and viral particle release, which may be attributed to inefficient viral transcription. With the use of click chemistry, the localization of viral DNA was investigated at late time points after infection. Intriguingly, we detected a co-localization of SPOC1, RNA polymerase II S5P and polycomb repressor complex 2 (PRC2) components in close proximity to viral DNA in areas that are hypothesized to harbor viral transcription sites. We further identified the N-terminal domain of SPOC1 to be responsible for interaction with EZH2, a subunit of the PRC2 complex. With this study, we report a novel and potent antiviral function of SPOC1 against HCMV that is efficient even with unrestricted IE gene expression.

ASFV pA151R negatively regulates type I IFN production via degrading E3 ligase TRAF6

African swine fever (ASF) caused by African swine fever virus (ASFV) is a highly mortal and hemorrhagic infectious disease in pigs. Previous studies have indicated that ASFV modulates interferon (IFN) production. In this study, we demonstrated that ASFV pA151R negatively regulated type I IFN production. Ectopic expression of pA151R dramatically inhibited K63-linked polyubiquitination and Ser172 phosphorylation of TANK-binding kinase 1 (TBK1). Mechanically, we demonstrated that E3 ligase TNF receptor-associated factor 6 (TRAF6) participated in the ubiquitination of TBK1 in cGAS-STING signaling pathway. We showed that pA151R interacted with TRAF6 and degraded it through apoptosis pathway, leading to the disruption of TBK1 and TRAF6 interaction. Moreover, we clarified that the amino acids H102, C109, C132, and C135 in pA151R were crucial for pA151R to inhibit type I interferon production. In addition, we verified that overexpression of pA151R facilitated DNA virus Herpes simplex virus 1 (HSV-1) replication by inhibiting IFN-β production. Importantly, knockdown of pA151R inhibited ASFV replication and enhanced IFN-β production in porcine alveolar macrophages (PAMs). Our findings will help understand how ASFV escapes host antiviral immune responses and develop effective ASFV vaccines.

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.

Herpes Simplex Virus-1 targets the 2'-3'cGAMP importer SLC19A1 as an antiviral countermeasure

To establish a successful infection, herpes simplex virus-1 (HSV-1), a virus with high seropositivity in the human population, must undermine host innate and intrinsic immune defense mechanisms, including the stimulator of interferon genes (STING) pathway. Recently it was discovered that not only de novo produced intracellular 2'-3'cGAMP, but also extracellular 2'-3'cGAMP activates the STING pathway by being transported across the cell membrane via the folate transporter, SLC19A1, the first identified extracellular antiporter of this signaling molecule. We hypothesized that the import of exogenous 2'-3'cGAMP functions to establish an antiviral state like that seen with the paracrine antiviral activities of interferon. Further, to establish a successful infection, HSV-1 must undermine this induction of the STING pathway by inhibiting the biological functions of SLC19A1. Herein, we report that treatment of the monocytic cell line, THP-1 cells, epithelial cells (ARPE-19) and SH-SY5Y neuronal cell line with exogenous 2'-3'cGAMP induces interferon production and establishes an antiviral state. Using either pharmaceutical inhibition or genetic knockout of SLC19A1 blocks the 2'-3'cGAMP-induced antiviral state. Additionally, HSV-1 infection results in the reduction of SLC19A1 transcription, translation, and importantly, the rapid removal of SLC19A1 from the cell surface of infected cells. Our data indicate SLC19A1 functions as a newly identified antiviral mediator for extracellular 2'-3'cGAMP which is undermined by HSV-1. This work presents novel and important findings about how HSV-1 manipulates the host's immune environment for viral replication and discovers details about an antiviral mechanism which information could aid in the development of better antiviral drugs in the future.

Importance

HSV-1 has evolved multiple mechanisms to neutralize of the host's innate and intrinsic defense pathways, such as the STING pathway. Here, we identified an antiviral response in which extracellular 2'-3'cGAMP triggers IFN production via its transporter SLC19A1. Moreover, we report that HSV-1 blocks the functions of this transporter thereby impeding the antiviral response, suggesting exogenous 2'-3'cGAMP can act as an immunomodulatory molecule in uninfected cells to activate the STING pathway, and priming an antiviral state, similar to that seen in interferon responses. The details of this mechanism highlight important details about HSV-1 infections. This work presents novel findings about how HSV-1 manipulates the host's immune environment for viral replication and reveals details about a novel antiviral mechanism. These findings expand our understanding of how viral infections undermine host responses and may help in the development of better broad based antiviral drugs in the future.

Mitochondrial DNA-triggered innate immune response: mechanisms and diseases

Various cellular stress conditions trigger mitochondrial DNA (mtDNA) release from mitochondria into the cytosol. The released mtDNA is sensed by the cGAS-MITA/STING pathway, resulting in the induced expression of type I interferon and other effector genes. These processes contribute to the innate immune response to viral infection and other stress factors. The deregulation of these processes causes autoimmune diseases, inflammatory metabolic disorders and cancer. Therefore, the cGAS-MITA/STING pathway is a potential target for intervention in infectious, inflammatory and autoimmune diseases as well as cancer. In this review, we focus on the mechanisms underlying the mtDNA-triggered activation of the cGAS-MITA/STING pathway, the effects of the pathway under various physiological and pathological conditions, and advances in the development of drugs that target cGAS and MITA/STING.

DNA damage response(DDR): a link between cellular senescence and human cytomegalovirus

The DNA damage response (DDR) is a signaling cascade that is triggered by DNA damage, involving the halting of cell cycle progression and repair. It is a key event leading to senescence, which is characterized by irreversible cell cycle arrest and the senescence-associated secretory phenotype (SASP) that includes the expression of inflammatory cytokines. Human cytomegalovirus (HCMV) is a ubiquitous pathogen that plays an important role in the senescence process. It has been established that DDR is necessary for HCMV to replicate effectively. This paper reviews the relationship between DDR, cellular senescence, and HCMV, providing new sights for virus-induced senescence (VIS).

Human cytomegalovirus UL36 inhibits IRF3-dependent immune signaling to counterbalance its immunoenhancement as apoptotic inhibitor

Apoptotic inhibition and immune evasion have particular importance to efficient viral infection, while a dilemma often faced by viruses is that inhibiting apoptosis can up-regulate antiviral immune signaling. Herein, we uncovered that in addition to inhibiting caspase-8/extrinsic apoptosis, human cytomegalovirus (HCMV)-encoded UL36 suppresses interferon regulatory factor 3 (IRF3)-dependent immune signaling by directly targeting IRF3 to abrogate IRF3 interaction with stimulator of interferon genes or TANK-binding kinase 1 and inhibit IRF3 phosphorylation/activation. Although UL36-mediated caspase-8/extrinsic apoptosis inhibition enhances immune signaling, the immunosuppressing activity of UL36 counterbalances this immunoenhancing "side effect" undesirable for virus. Furthermore, we used mutational analyses to show that only the wild-type, but not the UL36 mutant losing either inhibitory activity, is sufficient to support effective HCMV replication in cells, showing the functional importance of the dual inhibition by UL36 for the HCMV life cycle. Together, our findings demonstrate a sophisticated mechanism by which HCMV tightly controls innate immune signaling and extrinsic apoptosis for efficient infection.

P26 enhances baculovirus gene delivery by modulating the mammalian antiviral response

Poxins are poxviral proteins that act by degrading 2´3´-cGAMP, a key molecule of cGAS-STING axis that drives and amplifies the antiviral response. Previous works have described some poxin homologous among lepidopteran and baculoviral genes. In particular, P26, a poxin homologous from AcMNPV retains the 2´3´-cGAMP degradation activity in vitro. In this work, we demonstrated that the antiviral activity triggered by baculovirus was disrupted by the transient expression of P26 in murine and human cell lines, and the effect of this action is not only on IFN-β production but also on the induction of IFN-λ. Besides, we proved P26 functionality in a stable-transformed cell line where the protein was constitutively expressed, preventing the production of IFN-β induced by baculovirus and resulting in an improvement in the transduction efficiency by the attenuation of the antiviral activity. Finally, we incorporated P26 into budded virions by capsid display or passive incorporation, and the results showed that both strategies resulted in an improvement of 3-17 times in the efficiency of transgene expression in murine fibroblasts. Our results suggest that the incorporation of P26 to budded baculoviral vectors is a very promising tool to modulate negatively the innate antiviral cellular response and to improve the efficiency of gene delivery in mammalian cells. KEY POINTS: • P26 affects baculovirus-induced IFN-β and IFN-λ production in mammalian cells. • Murine fibroblasts expressing P26 are more susceptible to transduction by baculovirus. • Incorporation of P26 into the virion improves gene delivery efficiency of baculovirus.

cGAS-STING signaling pathway in intestinal homeostasis and diseases

The intestinal mucosa is constantly exposed to commensal microbes, opportunistic pathogens, toxins, luminal components and other environmental stimuli. The intestinal mucosa consists of multiple differentiated cellular and extracellular components that form a critical barrier, but is also equipped for efficient absorption of nutrients. Combination of genetic susceptibility and environmental factors are known as critical components involved in the pathogenesis of intestinal diseases. The innate immune system plays a critical role in the recognition and elimination of potential threats by detecting pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs). This host defense is facilitated by pattern recognition receptors (PRRs), in which the cyclic GMP-AMP synthase-stimulator of interferon genes (cGAS-STING) pathway has gained attention due to its role in sensing host and foreign double-stranded DNA (dsDNA) as well as cyclic dinucleotides (CDNs) produced by bacteria. Upon binding with dsDNA, cGAS converts ATP and GTP to cyclic GMP-AMP (cGAMP), which binds to STING and activates TANK binding kinase 1 (TBK1) and interferon regulatory factor 3 (IRF3), inducing type I interferon (IFN) and nuclear factor kappa B (NF-κB)-mediated pro-inflammatory cytokines, which have diverse effects on innate and adaptive immune cells and intestinal epithelial cells (IECs). However, opposite perspectives exist regarding the role of the cGAS-STING pathway in different intestinal diseases. Activation of cGAS-STING signaling is associated with worse clinical outcomes in inflammation-associated diseases, while it also plays a critical role in protection against tumorigenesis and certain infections. Therefore, understanding the context-dependent mechanisms of the cGAS-STING pathway in the physiopathology of the intestinal mucosa is crucial for developing therapeutic strategies targeting the cGAS-STING pathway. This review aims to provide insight into recent findings of the protective and detrimental roles of the cGAS-STING pathway in intestinal diseases.

TRIM26 positively affects hepatitis B virus replication by inhibiting proteasome-dependent degradation of viral core protein

Chronic hepatitis B virus (HBV) infection is a major medical concern worldwide. Current treatments for HBV infection effectively inhibit virus replication; however, these treatments cannot cure HBV and novel treatment-strategies should be necessary. In this study, we identified tripartite motif-containing protein 26 (TRIM26) could be a supportive factor for HBV replication. Small interfering RNA-mediated TRIM26 knockdown (KD) modestly attenuated HBV replication in human hepatocytes. Endogenous TRIM26 physically interacted with HBV core protein (HBc), but not polymerase and HBx, through the TRIM26 SPRY domain. Unexpectedly, TRIM26 inhibited HBc ubiquitination even though TRIM26 is an E3 ligase. HBc was degraded by TRIM26 KD in Huh-7 cells, whereas the reduction was restored by a proteasome inhibitor. RING domain-deleted TRIM26 mutant (TRIM26ΔR), a dominant negative form of TRIM26, sequestered TRIM26 from HBc, resulting in promoting HBc degradation. Taking together, this study demonstrated that HBV utilizes TRIM26 to avoid the proteasome-dependent HBc degradation. The interaction between TRIM26 and HBc might be a novel therapeutic target against HBV infection.

Insights into the Transcriptome of Human Cytomegalovirus: A Comprehensive Review

Human cytomegalovirus (HCMV) is a widespread pathogen that poses significant risks to immunocompromised individuals. Its genome spans over 230 kbp and potentially encodes over 200 open-reading frames. The HCMV transcriptome consists of various types of RNAs, including messenger RNAs (mRNAs), long non-coding RNAs (lncRNAs), circular RNAs (circRNAs), and microRNAs (miRNAs), with emerging insights into their biological functions. HCMV mRNAs are involved in crucial viral processes, such as viral replication, transcription, and translation regulation, as well as immune modulation and other effects on host cells. Additionally, four lncRNAs (RNA1.2, RNA2.7, RNA4.9, and RNA5.0) have been identified in HCMV, which play important roles in lytic replication like bypassing acute antiviral responses, promoting cell movement and viral spread, and maintaining HCMV latency. CircRNAs have gained attention for their important and diverse biological functions, including association with different diseases, acting as microRNA sponges, regulating parental gene expression, and serving as translation templates. Remarkably, HCMV encodes miRNAs which play critical roles in silencing human genes and other functions. This review gives an overview of human cytomegalovirus and current research on the HCMV transcriptome during lytic and latent infection.

The battle between the innate immune cGAS-STING signaling pathway and human herpesvirus infection

The incidence of human herpesvirus (HHVs) is gradually increasing and has affected a wide range of population. HHVs can result in serious consequences such as tumors, neonatal malformations, sexually transmitted diseases, as well as pose an immense threat to the human health. The cGAS-STING pathway is one of the innate immune pattern-recognition receptors discovered recently. This article discusses the role of the cGAS-STING pathway in human diseases, especially in human herpesvirus infections, as well as highlights how these viruses act on this pathway to evade the host immunity. Moreover, the author provides a comprehensive overview of modulators of the cGAS-STING pathway. By focusing on the small molecule compounds based on the cGAS-STING pathway, novel targets and concepts have been proposed for the development of antiviral drugs and vaccines, while also providing a reference for the investigation of disease models related to the cGAS-STING pathway. HHV is a double-stranded DNA virus that can trigger the activation of intracellular DNA sensor cGAS, after which the host cells initiate a cascade of reactions that culminate in the secretion of type I interferon to restrict the viral replication. Meanwhile, the viral protein can interact with various molecules in the cGAS-STING pathway. Viruses can evade immune surveillance and maintain their replication by inhibiting the enzyme activity of cGAS and reducing the phosphorylation levels of STING, TBK1 and IRF3 and suppressing the interferon gene activation. Activators and inhibitors of the cGAS-STING pathway have yielded numerous promising research findings in vitro and in vivo pertaining to cGAS/STING-related disease models. However, there remains a dearth of small molecule modulators that have been successfully translated into clinical applications, which serves as a hurdle to be overcome in the future.

Viral evasion of the interferon response at a glance

Re-emerging and new viral pathogens have caused significant morbidity and mortality around the world, as evidenced by the recent monkeypox, Ebola and Zika virus outbreaks and the ongoing COVID-19 pandemic. Successful viral infection relies on tactical viral strategies to derail or antagonize host innate immune defenses, in particular the production of type I interferons (IFNs) by infected cells. Viruses can thwart intracellular sensing systems that elicit IFN gene expression (that is, RIG-I-like receptors and the cGAS-STING axis) or obstruct signaling elicited by IFNs. In this Cell Science at a Glance article and the accompanying poster, we review the current knowledge about the major mechanisms employed by viruses to inhibit the activity of intracellular pattern-recognition receptors and their downstream signaling cascades leading to IFN-based antiviral host defenses. Advancing our understanding of viral immune evasion might spur unprecedented opportunities to develop new antiviral compounds or vaccines to prevent viral infectious diseases.

The Many Ways to Deal with STING

The stimulator of interferon genes (STING) is an adaptor protein involved in the activation of IFN-β and many other genes associated with the immune response activation in vertebrates. STING induction has gained attention from different angles such as the potential to trigger an early immune response against different signs of infection and cell damage, or to be used as an adjuvant in cancer immune treatments. Pharmacological control of aberrant STING activation can be used to mitigate the pathology of some autoimmune diseases. The STING structure has a well-defined ligand binding site that can harbor natural ligands such as specific purine cyclic di-nucleotides (CDN). In addition to a canonical stimulation by CDNs, other non-canonical stimuli have also been described, whose exact mechanism has not been well defined. Understanding the molecular insights underlying the activation of STING is important to realize the different angles that need to be considered when designing new STING-binding molecules as therapeutic drugs since STING acts as a versatile platform for immune modulators. This review analyzes the different determinants of STING regulation from the structural, molecular, and cell biology points of view.

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.

Genome-Wide CRISPR-Cas9 Screen Identifies SMCHD1 as a Restriction Factor for Herpesviruses

Intrinsic immunity is the frontline of host defense against invading pathogens. To combat viral infection, mammalian hosts deploy cell-intrinsic effectors to block viral replication prior to the onset of innate and adaptive immunity. In this study, SMCHD1 is identified as a pivotal cellular factor that restricts Kaposi's sarcoma-associated herpesvirus (KSHV) lytic reactivation through a genome-wide CRISPR-Cas9 knockout screen. Genome-wide chromatin profiling revealed that SMCHD1 associates with the KSHV genome, most prominently the origin of lytic DNA replication (ORI-Lyt). SMCHD1 mutants defective in DNA binding could not bind ORI-Lyt and failed to restrict KSHV lytic replication. Moreover, SMCHD1 functioned as a pan-herpesvirus restriction factor that potently suppressed a wide range of herpesviruses, including alpha, beta, and gamma subfamilies. SMCHD1 deficiency facilitated the replication of a murine herpesvirus in vivo. These findings uncovered SMCHD1 as a restriction factor against herpesviruses, and this could be harnessed for the development of antiviral therapies to limit viral infection. IMPORTANCE Intrinsic immunity represents the frontline of host defense against invading pathogens. However, our understanding of cell-intrinsic antiviral effectors remains limited. In this study, we identified SMCHD1 as a cell-intrinsic restriction factor that controlled KSHV lytic reactivation. Moreover, SMCHD1 restricted the replication of a wide range of herpesviruses by targeting the origins of viral DNA replication (ORIs), and SMCHD1 deficiency facilitated the replication of a murine herpesvirus in vivo. This study helps us to better understand intrinsic antiviral immunity, which may be harnessed to develop new therapeutics for the treatment of herpesvirus infection and the related diseases.

Unlocking STING as a Therapeutic Antiviral Strategy

Invading pathogens have developed weapons that subvert physiological conditions to weaken the host and permit the spread of infection. Cells, on their side, have thus developed countermeasures to maintain cellular physiology and counteract pathogenesis. The cyclic GMP-AMP (cGAMP) synthase (cGAS) is a pattern recognition receptor that recognizes viral DNA present in the cytosol, activating the stimulator of interferon genes (STING) protein and leading to the production of type I interferons (IFN-I). Given its role in innate immunity activation, STING is considered an interesting and innovative target for the development of broad-spectrum antivirals. In this review, we discuss the function of STING; its modulation by the cellular stimuli; the molecular mechanisms developed by viruses, through which they escape this defense system; and the therapeutical strategies that have been developed to date to inhibit viral replication restoring STING functionality.

African Swine Fever Virus E184L Protein Interacts with Innate Immune Adaptor STING to Block IFN Production for Viral Replication and Pathogenesis

African swine fever is one of the most serious viral diseases that affects domestic and wild pigs. The causative agent, African swine fever virus (ASFV), has evolved sophisticated immune evasion mechanisms that target both innate and adaptive immune responses. However, the underlying molecular mechanisms have not been fully understood. Here, we report that ASFV E184L protein inhibits host innate immune response via targeting the stimulator of IFN genes (STING)-mediated signaling pathway in both human embryonic kidney HEK-293T cells and porcine pulmonary alveolar macrophages. E184L interacts with STING, impairing dimerization and oligomerization of STING but not affecting its puncta formation at the perinuclear region. Furthermore, E184L disrupts STING-TBK1-IRF3 complex formation, leading to inhibition of STING phosphorylation, and IRF3 dimerization and nuclear translocation. The 1-20 aa region in E184L is essential for E184L-STING interaction and blocking IL-1β and type I IFN production. Deletion of E184L in ASFV considerably impairs antagonistic function of the virus in suppression of the STING-mediated antiviral response, an effect that is reversible by introduction of E184L. Importantly, the virulence of mutant ASFV lacking E184L is reduced in pigs compared with its parental virus due to induction of higher IFN production in vivo. Our findings indicate that ASFV E184L is an important antagonist of IFN signaling to evade host innate immune antiviral responses, which improves our understanding of immune evasion mechanisms of ASFV.

Varicella-Zoster Virus ORF39 Transmembrane Protein Suppresses Interferon-Beta Promoter Activation by Interacting with STING

Varicella-Zoster virus (VZV) causes varicella in primary infection of children and zoster during reactivation in adults. Type I interferon (IFN) signaling suppresses VZV growth, and stimulator of interferon genes (STING) plays an important role in anti-VZV responses by regulating type I IFN signaling. VZV-encoded proteins are shown to inhibit STING-mediated activation of the IFN-β promoter. However, the mechanisms by which VZV regulates STING-mediated signaling pathways are largely unknown. In this study, we demonstrate that the transmembrane protein encoded by VZV open reading frame (ORF) 39 suppresses STING-mediated IFN-β production by interacting with STING. In IFN-β promoter reporter assays, ORF39 protein (ORF39p) inhibited STING-mediated activation of the IFN-β promoter. ORF39p interacted with STING in co-transfection assays, and this interaction was comparable to that of STING dimerization. The cytoplasmic N-terminal 73 amino acids region of ORF39P was not necessary for ORF39 binding and suppression of STING-mediated IFN-β activation. ORF39p also formed a complex containing both STING and TBK1. A recombinant VZV expressing HA-tagged ORF39 was produced using bacmid mutagenesis and showed similar growth to its parent virus. During HA-ORF39 virus infection, the expression level of STING was markedly reduced, and HA-ORF39 interacted with STING. Moreover, HA-ORF39 also colocalized with glycoprotein K (encoded by ORF5) and STING at the Golgi during virus infection. Our results demonstrate that the transmembrane protein ORF39p of VZV plays a role in evading the type I IFN responses by suppressing STING-mediated activation of the IFN-β promoter.

β-adrenoreceptor-triggered PKA activation negatively regulates the innate antiviral response

Upon viral infection, cytoplasmic pattern recognition receptors detect viral nucleic acids and activate the adaptor protein VISA/MAVS- or MITA/STING-mediated innate antiviral response. Whether and how the innate antiviral response is regulated by neuronal endocrine functions is unclear. Here, we show that viral infection reduced the serum levels of the β-adrenergic hormones epinephrine and norepinephrine as well as the cellular levels of their receptors ADRB1 and ADRB2. We further show that an increase in epinephrine/norepinephrine level inhibited the innate antiviral response in an ADRB1-/2-dependent manner. Mechanistically, epinephrine/norepinephrine stimulation activated the downstream kinase PKA, which catalyzed the phosphorylation of MITA at S241, S243 and T263, inhibiting MITA activation and suppressing the innate immune response to DNA virus. In addition, phosphorylation of VISA at T54 by PKA antagonized the innate immune response to RNA virus. These findings reveal the regulatory mechanisms of innate antiviral responses by epinephrine/norepinephrine and provide a possible explanation for increased host susceptibility to viral infection in stressful and anxiety-promoting situations.

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.

Subviral Dense Bodies of Human Cytomegalovirus Induce an Antiviral Type I Interferon Response

(1) Background: Cells infected with the human cytomegalovirus (HCMV) produce subviral particles, termed dense bodies (DBs), both in-vitro and in-vivo. They are released from cells, comparable to infectious virions, and are enclosed by a membrane that resembles the viral envelope and mediates the entry into cells. To date, little is known about how the DB uptake influences the gene expression in target cells. The purpose of this study was to investigate the impact of DBs on cells, in the absence of a viral infection. (2) Methods: Mass spectrometry, immunoblot analyses, siRNA knockdown, and a CRISPR-CAS9 knockout, were used to investigate the changes in cellular gene expression following a DB exposure; (3) Results: A number of interferon-regulated genes (IRGs) were upregulated after the fibroblasts and endothelial cells were exposed to DBs. This upregulation was dependent on the DB entry and mediated by the type I interferon signaling through the JAK-STAT pathway. The induction of IRGs was mediated by the sensing of the DB-introduced DNA by the pattern recognition receptor cGAS. (4) Conclusions: The induction of a strong type I IFN response by DBs is a unique feature of the HCMV infection. The release of DBs may serve as a danger signal and concomitantly contribute to the induction of a strong, antiviral immune response.

SARS-CoV-2 ORF10 antagonizes STING-dependent interferon activation and autophagy

A characteristic feature of COVID-19, the disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, is the dysregulated immune response with impaired type I and III interferon (IFN) expression and an overwhelming inflammatory cytokine storm. RIG-I-like receptors (RLRs) and cGAS-STING signaling pathways are responsible for sensing viral infection and inducing IFN production to combat invading viruses. Multiple proteins of SARS-CoV-2 have been reported to modulate the RLR signaling pathways to achieve immune evasion. Although SARS-CoV-2 infection also activates the cGAS-STING signaling by stimulating micronuclei formation during the process of syncytia, whether SARS-CoV-2 modulates the cGAS-STING pathway requires further investigation. Here, we screened 29 SARS-CoV-2-encoded viral proteins to explore the viral proteins that affect the cGAS-STING signaling pathway and found that SARS-CoV-2 open reading frame 10 (ORF10) targets STING to antagonize IFN activation. Overexpression of ORF10 inhibits cGAS-STING-induced interferon regulatory factor 3 phosphorylation, translocation, and subsequent IFN induction. Mechanistically, ORF10 interacts with STING, attenuates the STING-TBK1 association, and impairs STING oligomerization and aggregation and STING-mediated autophagy; ORF10 also prevents the endoplasmic reticulum (ER)-to-Golgi trafficking of STING by anchoring STING in the ER. Taken together, these findings suggest that SARS-CoV-2 ORF10 impairs the cGAS-STING signaling by blocking the translocation of STING and the interaction between STING and TBK1 to antagonize innate antiviral immunity.

Nuclear antiviral innate responses at the intersection of DNA sensing and DNA repair

The coevolution of vertebrate and mammalian hosts with DNA viruses has driven the ability of host cells to distinguish viral from cellular DNA in the nucleus to induce intrinsic immune responses. Concomitant viral mechanisms have arisen to inhibit DNA sensing. At this virus-host interface, emerging evidence links cytokine responses and cellular homeostasis pathways, particularly the DNA damage response (DDR). Nuclear DNA sensors, such as the interferon (IFN)-γ inducible protein 16 (IFI16), functionally intersect with the DDR regulators ataxia telangiectasia mutated (ATM) and DNA-dependent protein kinase (DNA-PK). Here, we discuss accumulating knowledge for the DDR-innate immunity signaling axis. Through the lens of this infection-driven signaling axis, we present host and viral molecular strategies acquired to regulate autoinflammation and antiviral responses.

L-Citrulline Supplementation Restrains Ferritinophagy-Mediated Ferroptosis to Alleviate Iron Overload-Induced Thymus Oxidative Damage and Immune Dysfunction

L-citrulline (L-cit) is a key intermediate in the urea cycle and is known to possess antioxidant and anti-inflammation characteristics. However, the role of L-cit in ameliorating oxidative damage and immune dysfunction against iron overload in the thymus remains unclear. This study explored the underlying mechanism of the antioxidant and anti-inflammation qualities of L-cit on iron overload induced in the thymus. We reported that L-cit administration could robustly alleviate thymus histological damage and reduce iron deposition, as evidenced by the elevation of the CD8+ T lymphocyte number and antioxidative capacity. Moreover, the NF-κB pathway, NCOA4-mediated ferritinophagy, and ferroptosis were attenuated. We further demonstrated that L-cit supplementation significantly elevated the mTEC1 cells' viability and reversed LDH activity, iron levels, and lipid peroxidation caused by FAC. Importantly, NCOA4 knockdown could reduce the intracellular cytoplasmic ROS, which probably relied on the Nfr2 activation. The results subsequently indicated that NCOA4-mediated ferritinophagy was required for ferroptosis by showing that NCOA4 knockdown reduced ferroptosis and lipid ROS, accompanied with mitochondrial membrane potential elevation. Intriguingly, L-cit treatment significantly inhibited the NF-κB pathway, which might depend on restraining ferritinophagy-mediated ferroptosis. Overall, this study indicated that L-cit might target ferritinophagy-mediated ferroptosis to exert antioxidant and anti-inflammation capacities, which could be a therapeutic strategy against iron overload-induced thymus oxidative damage and immune dysfunction.

Chicken anemia virus VP1 negatively regulates type I interferon via targeting interferon regulatory factor 7 of the DNA-sensing pathway

The cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) signaling pathway plays a vital role in sensing viral DNA in the cytosol, stimulating type I interferon (IFN) production and triggering the innate immune response against DNA virus infection. However, viruses have evolved effective inhibitors to impede this sensing pathway. Chicken anemia virus (CAV), a nonenveloped ssDNA virus, is a ubiquitous pathogen causing great economic losses to the poultry industry globally. CAV infection is reported to downregulate type I IFN induction. However, whether the cGAS-STING signal axis is used by CAV to regulate type I IFN remains unclear. Our results demonstrate that CAV infection significantly elevates the expression of cGAS and STING at the mRNA level, whereas IFN-β levels are reduced. Furthermore, IFN-β activation was completely blocked by the structural protein VP1 of CAV in interferon stimulatory DNA (ISD) or STING-stimulated cells. VP1 was further confirmed as an inhibitor by interacting with interferon regulatory factor 7 (IRF7) by binding its C-terminal 143-492 aa region. IRF7 dimerization induced by TANK binding kinase 1 (TBK1) could be inhibited by VP1 in a dose-dependent manner. Together, our study demonstrates that CAV VP1 is an effective inhibitor that interacts with IRF7 and antagonizes cGAS-STING pathway-mediated IFN-β activation. These findings reveal a new mechanism of immune evasion by CAV.

Human Cytomegalovirus UL24 and UL43 Cooperate to Modulate the Expression of Immunoregulatory UL16 Binding Protein 1

The human cytomegalovirus (HCMV) UL24 and UL43 are tegument proteins that have recently been shown to interact with each other in a yeast two-hybrid system. By their overexpression in MRC5 cells, we demonstrate that these viral proteins interact with several important host proteins, especially Dicer and trans-activation response RNA binding protein. As these hots proteins are involved in regulating the production of cellular micro-RNAs, the cytomegalovirus (CMV) proteins could interfere with their actions to favor viral replication directly or through an immune escape mechanism. Double knockout of UL24 and UL43 does not show a remarkable effect on CMV entry or replication, but it significantly downregulates the expression of CMV-encoded miR-UL59, which is thought to regulate the expression of a downstream target UL16 binding protein 1 (ULBP1). Interestingly, the double knockout increases the expression of the ULBP1 recognized by the NKG2D activating receptor of natural killer cells. This study investigates the potential role of several proteins encoded by HCMV in regulating the host cellular environment to favor escape from immunity, and it also provides some basis for the future development of RNA-targeted small molecules to control HCMV infection.

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.

Human Cytomegalovirus Modifies Placental Small Extracellular Vesicle Composition to Enhance Infection of Fetal Neural Cells In Vitro

Although placental small extracellular vesicles (sEVs) are extensively studied in the context of pregnancy, little is known about their role during viral congenital infection, especially at the beginning of pregnancy. In this study, we examined the consequences of human cytomegalovirus (hCMV) infection on sEVs production, composition, and function using an immortalized human cytotrophoblast cell line derived from first trimester placenta. By combining complementary approaches of biochemistry, electron microscopy, and quantitative proteomic analysis, we showed that hCMV infection increases the yield of sEVs produced by cytotrophoblasts and modifies their protein content towards a potential proviral phenotype. We further demonstrate that sEVs secreted by hCMV-infected cytotrophoblasts potentiate infection in naive recipient cells of fetal origin, including human neural stem cells. Importantly, these functional consequences are also observed with sEVs prepared from an ex vivo model of infected histocultures from early placenta. Based on these findings, we propose that placental sEVs could be important actors favoring viral dissemination to the fetal brain during hCMV congenital infection.