Viruses transfer the antiviral second messenger cGAMP between cells

The cGAS-STING pathway in HIV-1 and Mycobacterium tuberculosis coinfection

Mycobacterium tuberculosis (M. tuberculosis) infection is the most common opportunistic infection in human immunodeficiency virus-1 (HIV-1)-infected individuals, and the mutual reinforcement of these two pathogens may accelerate disease progression and lead to rapid mortality. Therefore, HIV-1/M. tuberculosis coinfection is one of the major global public health concerns. HIV-1 infection is the greatest risk factor for M. tuberculosis infection and increases the likelihood of endogenous relapse and exogenous reinfection with M. tuberculosis. Moreover, M. tuberculosis further increases HIV-1 replication and the occurrence of chronic immune activation, accelerating the progression of HIV-1 disease. Exploring the pathogenesis of HIV-1/M. tuberculosis coinfections is essential for the development of novel treatments to reduce the global burden of tuberculosis. Innate immunity, which is the first line of host immune defense, plays a critical role in resisting HIV-1 and M. tuberculosis infections. The role of the cyclic guanosine monophosphate-adenosine monophosphate synthase (cGAS)-stimulator of interferon genes (STING) signaling pathway, which is a major DNA-sensing innate immune signaling pathway, in HIV-1 infection and M. tuberculosis infection has been intensively studied. This paper reviews the role of the cGAS-STING signaling pathway in HIV-1 infection and M. tuberculosis infection and discusses the possible role of this pathway in HIV-1/M. tuberculosis coinfection to provide new insight into the pathogenesis of HIV-1/M. tuberculosis coinfection and the development of novel therapeutic strategies.

Regulation of the cGAS-STING Pathway

The cGAS-cGAMP-STING pathway is essential for immune defense against pathogens. Upon binding DNA, cGAS synthesizes cGAMP, which activates STING, leading to potent innate immune effector responses. However, lacking specific features to distinguish between self and nonself DNA, cGAS-STING immunity requires precise regulation to prevent aberrant activation. Several safeguard mechanisms acting on different levels have evolved to maintain tolerance to self DNA and ensure immune homeostasis under normal conditions. Disruption of these safeguards can lead to erroneous activation by self DNA, resulting in inflammatory conditions but also favorable antitumor immunity. Insights into structural and cellular checkpoints that control and terminate cGAS-STING signaling are essential for comprehending and manipulating DNA-triggered innate immunity in health and disease.

HSV-1 virions and related particles: biogenesis and implications in the infection

Virion formation and egress are sophisticated processes that rely on the spatial and temporal organization of host cell membranes and the manipulation of host machineries involved in protein sorting, membrane bending, fusion, and fission. These processes result in the formation of infectious virions, defective particles, and various vesicle-like structures. In herpes simplex virus 1 (HSV-1) infections, virions and capsid-less particles, known as light (L)-particles, are formed. HSV-1 infection also stimulates the release of particles that resemble extracellular vesicles (EVs). In productively infected cells, most EVs are generated through the CD63 tetraspanin biogenesis pathway and lack viral components. A smaller subset of EVs, generated through the endosomal sorting complexes required for transport (ESCRT) pathway, contains both viral and host factors. Viral mechanisms tightly regulate EV biogenesis, including the inhibition of autophagy-a process critical for increased production of CD63+ EVs during HSV-1 infection. Mutant viruses that fail to suppress autophagy instead promote microvesicle production from the plasma membrane. Additionally, the viral protein ICP0 (Infected Cell Protein 0) enhances EV biogenesis during HSV-1 infection. The different types of particles can be separated by density gradients due to their distinct biophysical properties. L-particles and ESCRT+ EVs display a pro-viral role, supporting viral replication, whereas CD63+ EVs exhibit antiviral effects. Overall, these studies highlight that HSV-1 infection yields numerous and diverse particles, with their type and composition shaped by the ability of the virus to evade host responses. These particles likely shape the infectious microenvironment and determine disease outcomes.

cGAS-STING DNA-sensing in inflammatory bowel diseases

Inflammatory bowel diseases (IBD) are chronic, incurable pathologies with unknown causes, affecting millions of people. Pediatric-onset IBD, starting before the age of 18 years, are increasing, with more aggressive and extensive features than adult-onset IBD. These differences remain largely unexplained. Intestinal mucosal damage, cell death, DNA release from nuclear, mitochondrial, or microbiota sources, and DNA-sensing activating the cGAS-STING pathway may contribute to disease evolution. Increased colonic cGAS and STING are increasingly reported in experimental and human IBD. However, limited knowledge of the mechanisms involved hinders the development of new therapeutic options. Here, we discuss recent advances and unresolved questions regarding DNA release, DNA sensor activation, and the role and therapeutic potential of the cGAS-STING pathway in inflammatory colitis.

The antiviral activity of myricetin against pseudorabies virus through regulation of the type I interferon signaling pathway

The type I interferon signaling pathway constitutes a pivotal component of the innate immune response, encompassing the cGAS/STING and JAK/STAT pathways. Drugs that affect the body's innate immune response could potentially be used as broad-spectrum antivirals. In this study, the antiviral activities of 25 flavonoids against pseudorabies virus (PRV) were tested in PK-15 cells. Eight active flavonoids were identified, with IC50 values ranging from 23.24 to 323.09 µM. Subsequently, the regulatory effects of these flavonoids on the cGAS/STING pathway in PRV-infected cells were investigated. It was found that Myricetin significantly increased the transcriptional levels of cGAS, STING, IRF3, and IFN-β, which had been reduced by PRV infection. The regulation of the type I interferon signaling pathways by myricetin following PRV infection was further investigated through the production of cGAMP and the assessment of transcriptional and protein levels of pivotal genes and proteins. To confirm the activation of the innate immune response, a dual luciferase gene reporter study found that the expression of the IFN-β promoter in the myricetin-treated group was significantly elevated in a cellular model of type I interferon signaling pathway, and the contents of IFN-β were also significantly higher than those observed in the infected-untreated group in a PRV-infected mice model. Moreover, the transcriptional and protein levels of key genes and proteins in cell and mouse models exhibited analogous outcomes to those observed in PRV-infected cells. These findings suggest that myricetin can effectively activate the type I interferon signaling pathway, thereby enhancing the innate immune response during PRV infection.

IMPORTANCE

PRV, belonging to the Herpesviridae family, is an easily overlooked zoonotic pathogen that can threaten human health. The immunoprotective efficacy of conventional vaccines is significantly reduced due to the continuous mutation of the PRV genome, which constantly generates new viral strains. Therefore, there is a need to develop potent therapeutic drugs. PRV is capable of evading the host's natural immunity by suppressing the host's type I interferon signaling pathway, and the search for drugs that activate natural immunity can induce the body to produce type I IFN interferon and exert antiviral effects. Accordingly, the present study sought to identify active compounds from flavonoids that modulate the type I IFN interferon signaling pathway and thus inhibit the proliferation of PRV, which provides a new idea for the development of anti-PRV drugs from flavonoids that modulate the type I IFN interferon signaling pathway to enhance the body's antiviral immunity.

Anatomical, subset, and HIV-dependent expression of viral sensors and restriction factors

We developed viral sensor and restriction factor-cytometry by time of flight (VISOR-CyTOF), which profiles 19 viral sensors and restriction factors (VISORs) simultaneously in single cells, and applied it to 41 postmortem tissues from people with HIV. Mucosal myeloid cells are well equipped with SAMHD1 and sensors of viral capsid and DNA while CD4+ T cells are not. In lymph node CD4+ Tfh, VISOR expression patterns reflect those favoring integration but blocking HIV gene expression, thus favoring viral latency. We also identify small subsets of bone marrow-, lung-, and gut-associated CD4+ T and myeloid cells expressing high levels of restriction factors targeting most stages of the HIV replication cycle. In vitro, HIV preferentially fuses to CD4+ T cells with a permissive VISOR profile, but early induction of select VISORs by T1IFN prevents productive HIV infection. Our findings document the diverse patterns of VISOR profiles across tissues and cellular subsets and define their association with susceptibility to HIV.

HIV-1 Vpu and SARS-CoV-2 ORF3a proteins disrupt STING-mediated activation of antiviral NF-κB signaling

Activation of the stimulator of interferon genes (STING) pathway by cytosolic DNA leads to the activation of the transcription factors interferon regulatory factor 3 (IRF3) and nuclear factor κB (NF-κB). Although many viruses produce proteins that inhibit IRF3-dependent antiviral responses, some viruses produce proteins that inhibit STING-induced NF-κB activation without blocking IRF3 activation. Here, we found that STING-activated, NF-κB-dependent, and IRF3-independent innate immunity inhibited the replication of the DNA virus herpes simplex virus type 1 (HSV-1), the RNA virus coxsackievirus A16 (CV-A16), and the retrovirus HIV-1. The HIV-1 nonstructural protein Vpu bound to STING and prevented it from interacting with the upstream NF-κB pathway kinase inhibitor of NF-κB subunit β (IKKβ), thus blocking NF-κB signaling. This function of Vpu was conserved among Vpu proteins from diverse HIV-1 and simian immunodeficiency virus strains and was distinct from its action in disrupting other host antiviral pathways. Furthermore, the ORF3a protein from the coronavirus SARS-CoV-2 also promoted viral replication by interacting with STING and blocking STING-induced activity of NF-κB but not of IRF3. These findings demonstrate that diverse viral proteins have convergently evolved to selectively inhibit NF-κB-mediated innate immunity downstream of STING activation, suggesting that targeting this pathway may represent a promising antiviral strategy.

Regulation of cGAS-STING signalling and its diversity of cellular outcomes

The cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) signalling pathway, which recognizes both pathogen DNA and host-derived DNA, has emerged as a crucial component of the innate immune system, having important roles in antimicrobial defence, inflammatory disease, ageing, autoimmunity and cancer. Recent work suggests that the regulation of cGAS-STING signalling is complex and sophisticated. In this Review, we describe recent insights from structural studies that have helped to elucidate the molecular mechanisms of the cGAS-STING signalling cascade and we discuss how the cGAS-STING pathway is regulated by both activating and inhibitory factors. Furthermore, we summarize the newly emerging understanding of crosstalk between cGAS-STING signalling and other signalling pathways and provide examples to highlight the wide variety of cellular processes in which cGAS-STING signalling is involved, including autophagy, metabolism, ageing, inflammation and tumorigenesis.

Cytosolic delivery of innate immune agonists

Solute carrier proteins (SLCs) are pivotal for maintaining cellular homeostasis by transporting small molecules across cellular membranes. Recent discoveries have uncovered their involvement in modulating innate immunity, particularly within the cytosol. We review emerging evidence that links SLC transporters to cytosolic innate immune recognition and highlight their role in regulating inflammation. We explore how SLC transporters influence the activation of endosomal Toll-like receptors, cytosolic NODs, and STING sensors. Understanding the contribution of SLCs to innate immune recognition provides insight into their fundamental biological functions and opens new avenues to develop possible therapeutic interventions for autoimmune and inflammatory diseases. This review aims to discuss current knowledge and identify key gaps in this rapidly evolving field.

Import of extracellular 2'-3'cGAMP by the folate transporter, SLC19A1, establishes an antiviral response that limits herpes simplex virus-1

Recently it was discovered that extracellular 2'-3'cGAMP can activate the STING pathway in a cGAS-independent fashion by being transported across the cell membrane via the folate transporter, SLC19A1, the first identified extracellular antiporter of this critical signaling molecule in cancer cells. We hypothesized that this non-canonical activation of STING pathway would function to establish an antiviral state similar to that seen with the paracrine antiviral activities of interferon. Herein, we report that treatment of the monocytic cell line, THP-1 cells and SH-SY5Y neuronal cell line with exogenous 2'-3'cGAMP induces interferon production and establishes an antiviral state that limits herpes simplex virus-1 (HSV-1), a ubiquitous virus with high seropositivity in the human population. Using either pharmaceutical inhibition or genetic knockout of SLC19A1 blocks the 2'-3'cGAMP-induced inhibition of viral replication. Our data indicate SLC19A1 functions as a newly identified antiviral mediator for extracellular 2'-3'cGAMP. This work presents novel and important findings about an antiviral mechanism which information could aid in the development of better antiviral drugs in the future.

Silybin attenuates avermectin-induced oxidative damage in carp respiration by modulating the cGAS-STING pathway and endoplasmic reticulum stress

Avermectin is a commonly used insect repellent for aquaculture and crops, but it is easy to remain in the aquatic environment, causing organism disorders, inflammation, and even death. This resulted in significant economic losses to the carp aquaculture industry. Silybin has antioxidant, anti-inflammatory, and anti-apoptotic properties. However, it is unclear whether Silybin counteracts gill damage caused by avermectin exposure. Therefore, we modeled avermectin exposure and Silybin intervention by adding 2.404 μg/L avermectin to water and 400 mg/kg of Silybin to feed. Gill tissue was collected and analyzed in depth during a 30-day experimental period. The results showed that avermectin exposure induced structural disorganization of gill filaments and led to increased reactive oxygen species, inhibition of antioxidant functions, induction of inflammatory responses, and endoplasmic reticulum stress in addition to the endogenous apoptotic pathway. In contrast, Silybin effectively alleviated pathological changes and reduced reactive oxygen species levels, thereby attenuating oxidative stress and endogenous apoptosis and inhibiting endoplasmic reticulum stress pathways. In addition, Silybin reduced avermectin-induced gill tissue inflammation in carp, and it is considered that it might modulate the cGAS-STING pathway. In summary, Silybin alleviates avermectin-induced oxidative damage within the carp's respiratory system by modulating the cGAS-STING pathway and endoplasmic reticulum stress. The main goal is to understand how Silybin reduces oxidative damage caused by avermectin in carp gills, offering management strategies. Concurrently, the current study proposes that Silybin can serve as a dietary supplement to reduce the risks brought on by repellent buildup in freshwater aquaculture.

Compound Danshen Dripping Pill effectively alleviates cGAS-STING-triggered diseases by disrupting STING-TBK1 interaction

BACKGROUND

The cyclic GMP-AMP synthase (cGAS)-stimulator of interferon (IFN) genes (STING) pathway is critical in the innate immune system and can be mobilized by cytosolic DNA. The various inflammatory and autoimmune diseases progression is highly correlated with aberrant cGAS-STING pathway activation. While some cGAS-STING pathway inhibitor were identified, there are no drugs that can be applied to the clinic. Compound Danshen Dripping Pill (CDDP) has been successfully used in clinic around the world, but the most common application is limited to cardiovascular disease. Therefore, the purpose of the present investigation was to examine whether CDDP inhibits the cGAS-STING pathway and could be used as a therapeutic agent for multiple cGAS-STING-triggered diseases.

METHODS

BMDMs, THP1 cells or Trex1-/- BMDMs were stimulated with various cGAS-STING-agonists after pretreatment with CDDP to detect the function of CDDP on IFN-β and ISGs productionn. Next, we detect the influence on IRF3 and P65 nuclear translocation, STING oligomerization and STING-TBK1-IRF3 complex formation of CDDP. Additionally, the DMXAA-mediated activation mice model of cGAS-STING pathway was used to study the effects of CDDP. Trex1-/- mice model and HFD-mediated obesity model were established to clarify the efficacy of CDDP on inflammatory and autoimmune diseases.

RESULTS

CDDP efficacy suppressed the IRF3 phosphorylation or the generation of IFN-β, ISGs, IL-6 and TNF-α. Mechanistically, CDDP did not influence the STING oligomerization and IRF3-TBK1 and STING-IRF3 interaction, but remarkably eliminated the STING-TBK1 interaction, ultimately blocking the downstream responses. In addition, we also clarified that CDDP could suppress cGAS-STING pathway activation triggered by DMXAA, in vivo. Consistently, CDDP could alleviate multi-organ inflammatory responses in Trex1-/- mice model and attenuate the inflammatory disorders, incleding obesity-induced insulin resistance.

CONCLUSION

CDDP is a specifically cGAS-STING pathway inhibitor. Furthermore, we provide novel mechanism for CDDP and discovered a clinical agent for the therapy of cGAS-STING-triggered inflammatory and autoimmune diseases.

Early signaling pathways in virus-infected cells

Virus infection activates specific pattern recognition receptors and immune signal transduction, resulting in pro-inflammatory cytokine production and activation of innate immunity. We describe here the molecular organization of early signaling pathways downstream of viral recognition, including conformational changes, post-translational modifications, formation of oligomers, and generation of small-molecule second messengers. Such molecular organization allows tight regulation of immune signal transduction, characterized by swift but transient responses, nonlinearity, and signal amplification. Pathologies of early immune signaling caused by genomic mutations illustrate the fine regulation of the immune transduction cascade.

The Role of Extracellular Vesicles in Pandemic Viral Infections

Extracellular vesicles (EVs), of diverse origin and content, are membranous structures secreted by a broad range of cell types. Recent advances in molecular biology have highlighted the pivotal role of EVs in mediating intercellular communication, facilitated by their ability to transport a diverse range of biomolecules, including proteins, lipids, DNA, RNA and metabolites. A striking feature of EVs is their ability to exert dual effects during viral infections, involving both proviral and antiviral effects. This review explores the dual roles of EVs, particularly in the context of pandemic viruses such as HIV-1 and SARS-CoV-2. On the one hand, EVs can enhance viral replication and exacerbate pathogenesis by transferring viral components to susceptible cells. On the other hand, they have intrinsic antiviral properties, including activation of immune responses and direct inhibition of viral infection. By exploring these contrasting functions, our review emphasizes the complexity of EV-mediated interactions in viral pathogenesis and highlights their potential as targets for therapeutic intervention. The insights obtained from investigating EVs in the context of HIV-1 and SARS-CoV-2 provide a deeper understanding of viral mechanisms and pathologies, and offer a new perspective on managing and mitigating the impact of these global health challenges.

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.

Stimulating STING for cancer therapy: Taking the extracellular route

Ten years ago, the second messenger cGAMP was discovered as the activator of the anti-cancer STING pathway. The characterization of cGAMP's paracrine action and dominant extracellular hydrolase ENPP1 cemented cGAMP as an intercellular immunotransmitter that coordinates the innate and adaptive immune systems to fight cancer. In this Perspective, I look back at a decade of discovery of extracellular cGAMP biology and drug development aiming to supply or preserve extracellular cGAMP for cancer treatment. Reviewing our understanding of the cell type-specific regulatory mechanisms of STING agonists, including their transporters and degradation enzymes, I explain on a molecular and cellular level the successes and challenges of direct STING agonists for cancer therapy. Based on what we know now, I propose new ways to stimulate the STING pathway in a manner that is not only cancer specific, but also cell type specific to fully harness the anti-cancer effect of cGAMP while avoiding collateral damage.

Tumor Exosomal ENPP1 Hydrolyzes cGAMP to Inhibit cGAS-STING Signaling

To evade immune surveillance, tumor cells express ectonucleotide pyrophosphatase phosphodiesterase 1 (ENPP1) on the surface of their membrane, which degrades extracellular cyclic GMP-AMP (cGAMP), thereby inhibiting the cyclic GMP-AMP synthase (cGAS) stimulator of interferon gene (STING) DNA-sensing pathway. To fully understand this tumor stealth mechanism, it is essential to determine whether other forms of ENPP1 with hydrolytic cGAMP activity also are present in the tumor microenvironment to regulate this innate immune pathway. Herein, it is reported that various tumor-derived exosomes carry ENPP1, and can hydrolyze synthetic 2'3'-cGAMP and endogenous 2'3'-cGAMP produced by cells to inhibit cGAS-STING pathway in immune cells. Moreover, tumor exosomal ENPP1 also can hydrolyze 2'3'-cGAMP bound to LL-37 (an effective transporter of 2'3'-cGAMP) to inhibit STING signaling. Furthermore, high expression of ENPP1 in exosomes is observed isolated from human breast and lung cancer tissue, and tumor exosomal ENPP1 inhibited the immune infiltration of CD8+ T cells and CD4+ T cells. The results elucidate the essential function of tumor exosomal ENPP1 in the cGAS-STING pathway, furthering understanding of the crosstalk between the tumor cells and immune system.

Cutting-edge approaches for targeted drug delivery in breast cancer: beyond conventional therapies

Breast cancer is a global health challenge with staggering statistics underscoring its pervasive impact. The burden of this disease is measured in terms of its prevalence and the challenges it poses to healthcare systems, necessitating a closer look at its epidemiology and impact. Current breast cancer treatments, including surgery, chemotherapy, radiation therapy, and targeted therapies, have made significant strides in improving patient outcomes. However, they are not without limitations, often leading to adverse effects and the development of drug resistance. This comprehensive review delves into the complex landscape of breast cancer, including its incidence, current treatment modalities, and the inherent limitations of existing therapeutic approaches. It also sheds light on the promising role of nanotechnology, encompassing both inorganic and organic nanoparticles equipped with the ability to selectively deliver therapeutic agents to tumor sites, in the battle against breast cancer. The review also addresses the emerging therapies, their associated challenges, and the future prospects of targeted drug delivery in breast cancer management.

How enzyme-centered approaches are advancing research on cyclic oligo-nucleotides

Cyclic nucleotides are the most diversified category of second messengers and are found in all organisms modulating diverse pathways. While cAMP and cGMP have been studied over 50 years, cyclic di-nucleotide signaling in eukaryotes emerged only recently with the anti-viral molecule 2´3´cGAMP. Recent breakthrough discoveries have revealed not only the astonishing chemical diversity of cyclic nucleotides but also surprisingly deep-rooted evolutionary origins of cyclic oligo-nucleotide signaling pathways and structural conservation of the proteins involved in their synthesis and signaling. Here we discuss how enzyme-centered approaches have paved the way for the identification of several cyclic nucleotide signals, focusing on the advantages and challenges associated with deciphering the activation mechanisms of such enzymes.

cGAS-STING, an important signaling pathway in diseases and their therapy

Since cyclic guanosine monophosphate-adenosine monophosphate synthase (cGAS)-stimulator of interferon genes (STING) signaling pathway was discovered in 2013, great progress has been made to elucidate the origin, function, and regulating mechanism of cGAS-STING signaling pathway in the past decade. Meanwhile, the triggering and transduction mechanisms have been continuously illuminated. cGAS-STING plays a key role in human diseases, particularly DNA-triggered inflammatory diseases, making it a potentially effective therapeutic target for inflammation-related diseases. Here, we aim to summarize the ancient origin of the cGAS-STING defense mechanism, as well as the triggers, transduction, and regulating mechanisms of the cGAS-STING. We will also focus on the important roles of cGAS-STING signal under pathological conditions, such as infections, cancers, autoimmune diseases, neurological diseases, and visceral inflammations, and review the progress in drug development targeting cGAS-STING signaling pathway. The main directions and potential obstacles in the regulating mechanism research and therapeutic drug development of the cGAS-STING signaling pathway for inflammatory diseases and cancers will be discussed. These research advancements expand our understanding of cGAS-STING, provide a theoretical basis for further exploration of the roles of cGAS-STING in diseases, and open up new strategies for targeting cGAS-STING as a promising therapeutic intervention in multiple diseases.

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.

DNA-PKcs is required for cGAS/STING-dependent viral DNA sensing in human cells

To mount an efficient interferon response to virus infection, intracellular pattern recognition receptors (PRRs) sense viral nucleic acids and activate anti-viral gene transcription. The mechanisms by which intracellular DNA and DNA viruses are sensed are relevant not only to anti-viral innate immunity, but also to autoinflammation and anti-tumour immunity through the initiation of sterile inflammation by self-DNA recognition. The PRRs that directly sense and respond to viral or damaged self-DNA function by signaling to activate interferon regulatory factor (IRF)-dependent type one interferon (IFN-I) transcription. We and others have previously defined DNA-dependent protein kinase (DNA-PK) as an essential component of the DNA-dependent anti-viral innate immune system. Here, we show that DNA-PK is essential for cyclic GMP-AMP synthase (cGAS)- and stimulator of interferon genes (STING)-dependent IFN-I responses in human cells during stimulation with exogenous DNA and infection with DNA viruses.

Diverse drug delivery systems for the enhancement of cancer immunotherapy: an overview

Despite the clear benefits demonstrated by immunotherapy, there is still an inevitable off-target effect resulting in serious adverse immune reactions. In recent years, the research and development of Drug Delivery System (DDS) has received increased prominence. In decades of development, DDS has demonstrated the ability to deliver drugs in a precisely targeted manner to mitigate side effects and has the advantages of flexible control of drug release, improved pharmacokinetics, and drug distribution. Therefore, we consider that combining cancer immunotherapy with DDS can enhance the anti-tumor ability. In this paper, we provide an overview of the latest drug delivery strategies in cancer immunotherapy and briefly introduce the characteristics of DDS based on nano-carriers (liposomes, polymer nano-micelles, mesoporous silica, extracellular vesicles, etc.) and coupling technology (ADCs, PDCs and targeted protein degradation). Our aim is to show readers a variety of drug delivery platforms under different immune mechanisms, and analyze their advantages and limitations, to provide more superior and accurate targeting strategies for cancer immunotherapy.

Cell-to-cell communications of cGAS-STING pathway in tumor immune microenvironment

Targeting cyclic guanosine monophosphate-adenosine monophosphate synthase (cGAS)-stimulator of interferon genes (STING) pathway is a promising strategy for tumor treatment. The pattern recognition receptor cGAS identifies dsDNA and catalyzes the formation of a second messenger 2'3'-cyclic guanosine monophosphate-adenosine monophosphate (cGAMP), activating the downstream interferons and pro-inflammatory cytokines through the adaptor protein STING. Notably, in tumor immune microenvironment, key components of cGAS-STING pathway are transferred among neighboring cells. The intercellular transmission under these contexts serves to sustain and amplify innate immune responses while facilitating the emergence of adaptive immunity. The membrane-based system, including extracellular vesicles transport, phagocytosis and membrane fusion transmit dsDNA, cGAMP and activated STING, enhances the immune surveillance and inflammatory responses. The membrane proteins, including a specific protein channel and intercellular gap junctions, transfer cGAMP and dsDNA, which are crucial to regulate immune responses. The ligand-receptor interactions for interferon transmission amplifies the anti-tumor response. This review elaborates on the regulatory mechanisms of cell-to-cell communications of cGAS-STING pathway in tumor immune microenvironment, explores how these mechanisms modulate immunological processes and discusses potential interventions and immunotherapeutic strategies targeting these signaling cascades.

Exosomes multifunctional roles in HIV-1: insight into the immune regulation, vaccine development and current progress in delivery system

Human Immunodeficiency Virus (HIV-1) is known to establish a persistent latent infection. The use of combination antiretroviral therapy (cART) can effectively reduce the viral load, but the treatment can be costly and may lead to the development of drug resistance and life-shortening side effects. It is important to develop an ideal and safer in vivo target therapy that will effectively block viral replication and expression in the body. Exosomes have recently emerged as a promising drug delivery vehicle due to their low immunogenicity, nanoscale size (30-150nm), high biocompatibility, and stability in the targeted area. Exosomes, which are genetically produced by different types of cells such as dendritic cells, neurons, T and B cells, epithelial cells, tumor cells, and mast cells, are designed for efficient delivery to targeted cells. In this article, we review and highlight recent developments in the strategy and application of exosome-based HIV-1 vaccines. We also discuss the use of exosome-based antigen delivery systems in vaccine development. HIV-1 antigen can be loaded into exosomes, and this modified cargo can be delivered to target cells or tissues through different loading approaches. This review also discusses the immunological prospects of exosomes and their role as biomarkers in disease progression. However, there are significant administrative and technological obstacles that need to be overcome to fully harness the potential of exosome drug delivery systems.

CBASS to cGAS-STING: The Origins and Mechanisms of Nucleotide Second Messenger Immune Signaling

Host defense against viral pathogens is an essential function for all living organisms. In cell-intrinsic innate immunity, dedicated sensor proteins recognize molecular signatures of infection and communicate to downstream adaptor or effector proteins to activate immune defense. Remarkably, recent evidence demonstrates that much of the core machinery of innate immunity is shared across eukaryotic and prokaryotic domains of life. Here, we review a pioneering example of evolutionary conservation in innate immunity: the animal cGAS-STING (cyclic GMP-AMP synthase-stimulator of interferon genes) signaling pathway and its ancestor in bacteria, CBASS (cyclic nucleotide-based antiphage signaling system) antiphage defense. We discuss the unique mechanism by which animal cGLRs (cGAS-like receptors) and bacterial CD-NTases (cGAS/dinucleotide-cyclase in Vibrio (DncV)-like nucleotidyltransferases) in these pathways link pathogen detection with immune activation using nucleotide second messenger signals. Comparing the biochemical, structural, and mechanistic details of cGAS-STING, cGLR signaling, and CBASS, we highlight emerging questions in the field and examine evolutionary pressures that may have shaped the origins of nucleotide second messenger signaling in antiviral defense.

cGAS‐STING signaling in cancer: Regulation and therapeutic targeting

Immunotherapy has revolutionized antitumor therapy. Since the discovery of stimulators of interferon genes (STING), efforts have been made to elucidate their mechanisms and physiological functions and explore the potential of STING as a therapeutic target in immune‐related diseases and malignant tumors. In recent years, STING agonists have become a popular research topic. Activation of the cyclic GMP–AMP synthase (cGAS)‐STING pathway produces large amounts of type I interferons, which play key roles in activating innate and acquired immune responses. The cGAS‐STING pathway influences almost all aspects of tumorigenesis and has great antitumor potential. In addition, the activation of the cGAS‐STING pathway is associated with tumor regression, prolonged survival of patients with cancer, and enhanced immunotherapy. Given the positive role of STING in antitumor immunity, the development of STING‐targeted drugs is important. In this review, we summarize the activation and potential mechanisms of the cGAS‐STING pathway, discuss the association of the cGAS‐STING pathway with tumors and autoimmune diseases, and highlight research progress, clinical applications, and combination drug strategies for STING agonists.

Structure-based mechanisms of 2'3'-cGAMP intercellular transport in the cGAS-STING immune pathway

Upon activation by double-stranded DNA (dsDNA), the cytosolic dsDNA sensor cyclic GMP-AMP synthase (cGAS) synthesizes the diffusible cyclic dinucleotide 2'3'-cGAMP (cyclic GMP-AMP), which subsequently binds to the adaptor STING, triggering a cascade of events leading to an inflammatory response. Recent studies have highlighted the role of 2'3'-cGAMP as an 'immunotransmitter' between cells, a process facilitated by gap junctions as well as by specialized membrane-spanning importer and exporter channels. This review highlights recent advances from a structural perspective of intercellular trafficking of 2'3'-cGAMP, with particular emphasis on the binding of importer SLC19A1 to 2'3'-cGAMP, as well as the significance of associated folate nutrients and antifolate therapeutics. This provides a path forward for structure-guided understanding of the transport cycle in immunology, as well as for candidate targeting approaches towards therapeutic intervention in inflammation.

cGAMP the travelling messenger

2'3'-cGAMP is a key molecule in the cGAS-STING pathway. This cyclic dinucleotide is produced by the cytosolic DNA sensor cGAS in response to the presence of aberrant dsDNA in the cytoplasm which is associated with microbial invasion or cellular damage. 2'3'-cGAMP acts as a second messenger and activates STING, the central hub of DNA sensing, to induce type-I interferons and pro-inflammatory cytokines necessary for responses against infection, cancer or cellular stress. Classically, detection of pathogens or danger by pattern recognition receptors (PRR) was thought to signal and induce the production of interferon and pro-inflammatory cytokines in the cell where sensing occurred. These interferon and cytokines then signal in both an autocrine and paracrine manner to induce responses in neighboring cells. Deviating from this dogma, recent studies have identified multiple mechanisms by which 2'3'-cGAMP can travel to neighboring cells where it activates STING independent of DNA sensing by cGAS. This observation is of great importance, as the cGAS-STING pathway is involved in immune responses against microbial invaders and cancer while its dysregulation drives the pathology of a wide range of inflammatory diseases to which antagonists have been elusive. In this review, we describe the fast-paced discoveries of the mechanisms by which 2'3'-cGAMP can be transported. We further highlight the diseases where they are important and detail how this change in perspective can be applied to vaccine design, cancer immunotherapies and treatment of cGAS-STING associated disease.

RELA tunes innate-like interferon I/III responses in human T cells

In innate immune cells, intracellular sensors such as cGAS-STING stimulate type I/III interferon (IFN) expression, which promotes antiviral defense and immune activation. However, how IFN-I/III expression is controlled in adaptive cells is poorly understood. Here, we identify a transcriptional rheostat orchestrated by RELA that confers human T cells with innate-like abilities to produce IFN-I/III. Despite intact cGAS-STING signaling, IFN-I/III responses are stunted in CD4+ T cells compared with dendritic cells or macrophages. We find that lysine residues in RELA tune the IFN-I/III response at baseline and in response to STING stimulation in CD4+ T cells. This response requires positive feedback driven by cGAS and IRF7 expression. By combining RELA with IRF3 and DNA demethylation, IFN-I/III production in CD4+ T cells reaches levels observed in dendritic cells. IFN-I/III production provides self-protection of CD4+ T cells against HIV infection and enhances the elimination of tumor cells by CAR T cells. Therefore, innate-like functions can be tuned and leveraged in human T cells.

Primary role of type I interferons for the induction of functionally optimal antigen-specific CD8+ T cells in HIV infection

BACKGROUND

CD8⁺ T cells equipped with a full arsenal of antiviral effector functions are critical for effective immune control of HIV-1. It has nonetheless remained unclear how best to elicit such potent cellular immune responses in the context of immunotherapy or vaccination. HIV-2 has been associated with milder disease manifestations and more commonly elicits functionally replete virus-specific CD8⁺ T cell responses compared with HIV-1. We aimed to learn from this immunological dichotomy and to develop informed strategies that could enhance the induction of robust CD8⁺ T cell responses against HIV-1.

METHODS

We developed an unbiased in vitro system to compare the de novo induction of antigen-specific CD8⁺ T cell responses after exposure to HIV-1 or HIV-2. The functional properties of primed CD8⁺ T cells were assessed using flow cytometry and molecular analyses of gene transcription.

FINDINGS

HIV-2 primed functionally optimal antigen-specific CD8⁺ T cells with enhanced survival properties more effectively than HIV-1. This superior induction process was dependent on type I interferons (IFNs) and could be mimicked via the adjuvant delivery of cyclic GMP-AMP (cGAMP), a known agonist of the stimulator of interferon genes (STING). CD8⁺ T cells elicited in the presence of cGAMP were polyfunctional and highly sensitive to antigen stimulation, even after priming from people living with HIV-1.

INTERPRETATION

HIV-2 primes CD8⁺ T cells with potent antiviral functionality by activating the cyclic GMP-AMP synthase (cGAS)/STING pathway, which results in the production of type I IFNs. This process may be amenable to therapeutic development via the use of cGAMP or other STING agonists to bolster CD8⁺ T cell-mediated immunity against HIV-1.

FUNDING

This work was funded by INSERM, the Institut Curie, and the University of Bordeaux (Senior IdEx Chair) and by grants from Sidaction (17-1-AAE-11097, 17-1-FJC-11199, VIH2016126002, 20-2-AEQ-12822-2, and 22-2-AEQ-13411), the Agence Nationale de la Recherche sur le SIDA (ECTZ36691, ECTZ25472, ECTZ71745, and ECTZ118797), and the Fondation pour la Recherche Médicale (EQ U202103012774). D.A.P. was supported by a Wellcome Trust Senior Investigator Award (100326/Z/12/Z).

Interplay between RNA viruses and cGAS/STING axis in innate immunity

While the function of cGAS/STING signalling axis in the innate immune response to DNA viruses is well deciphered, increasing evidence demonstrates its significant contribution in the control of RNA virus infections. After the first evidence of cGAS/STING antagonism by flaviviruses, STING activation has been detected following infection by various enveloped RNA viruses. It has been discovered that numerous viral families have implemented advanced strategies to antagonize STING pathway through their evolutionary path. This review summarizes the characterized cGAS/STING escape strategies to date, together with the proposed mechanisms of STING signalling activation perpetrated by RNA viruses and discusses possible therapeutic approaches. Further studies regarding the interaction between RNA viruses and cGAS/STING-mediated immunity could lead to major discoveries important for the understanding of immunopathogenesis and for the treatment of RNA viral infections.

Molecular mechanisms of mitochondrial DNA release and activation of the cGAS-STING pathway

In addition to constituting the genetic material of an organism, DNA is a tracer for the recognition of foreign pathogens and a trigger of the innate immune system. cGAS functions as a sensor of double-stranded DNA fragments and initiates an immune response via the adaptor protein STING. The cGAS-STING pathway not only defends cells against various DNA-containing pathogens but also modulates many pathological processes caused by the immune response to the ectopic localization of self-DNA, such as cytosolic mitochondrial DNA (mtDNA) and extranuclear chromatin. In addition, macrophages can cause inflammation by forming a class of protein complexes called inflammasomes, and the activation of the NLRP3 inflammasome requires the release of oxidized mtDNA. In innate immunity related to inflammasomes, mtDNA release is mediated by macropores that are formed on the outer membrane of mitochondria via VDAC oligomerization. These macropores are specifically formed in response to mitochondrial stress and tissue damage, and the inhibition of VDAC oligomerization mitigates this inflammatory response. The rapidly expanding area of research on the mechanisms by which mtDNA is released and triggers inflammation has revealed new treatment strategies not only for inflammation but also, surprisingly, for neurodegenerative diseases such as amyotrophic lateral sclerosis.

Medicinal plant-derived mtDNA via nanovesicles induces the cGAS-STING pathway to remold tumor-associated macrophages for tumor regression

Plant-derived nanovesicles (PDNVs) have been proposed as a major mechanism for the inter-kingdom interaction and communication, but the effector components enclosed in the vesicles and the mechanisms involved are largely unknown. The plant Artemisia annua is known as an anti-malaria agent that also exhibits a wide range of biological activities including the immunoregulatory and anti-tumor properties with the mechanisms to be further addressed. Here, we isolated and purified the exosome-like particles from A. annua, which were characterized by nano-scaled and membrane-bound shape and hence termed artemisia-derived nanovesicles (ADNVs). Remarkably, the vesicles demonstrated to inhibit tumor growth and boost anti-tumor immunity in a mouse model of lung cancer, primarily through remolding the tumor microenvironment and reprogramming tumor-associated macrophages (TAMs). We identified plant-derived mitochondrial DNA (mtDNA), upon internalized into TAMs via the vesicles, as a major effector molecule to induce the cGAS-STING pathway driving the shift of pro-tumor macrophages to anti-tumor phenotype. Furthermore, our data showed that administration of ADNVs greatly improved the efficacy of PD-L1 inhibitor, a prototypic immune checkpoint inhibitor, in tumor-bearing mice. Together, the present study, for the first time, to our knowledge, unravels an inter-kingdom interaction wherein the medical plant-derived mtDNA, via the nanovesicles, induces the immunostimulatory signaling in mammalian immune cells for resetting anti-tumor immunity and promoting tumor eradication.

STAM transports STING oligomers into extracellular vesicles, down-regulating the innate immune response

Stimulator of interferon genes (STING) mediates the innate immune response against damaged endogenous double-strand DNA and exogenous virus infection. The location of STING is critical to the accurate control of defence signalling pathways. Recently, the effects of extracellular vesicles (EVs) in the regulation of innate immune signalling have been reported. Nevertheless, the particular roles played by STING in EVs and the related mechanisms have remained largely unknown. Herein, we report that when STING was activated in cells, EVs derived from these cells carried STING oligomers. Signal transducing adapter molecule 1 (STAM) was found to be a STING transporter that directly interacted with STING and facilitated STING transport into EVs. Importantly, the translocation of STING into EVs was a mechanism by which STING was degraded, suppressing the innate immune response. In summary, we elucidated the mechanism and function of the translocation of STING into EVs, adding to the understanding of STING activity regulation.

Exosomes: from biology to immunotherapy in infectious diseases

Exosomes are extracellular vesicles derived from the endosomal compartment, which are released by all kinds of eukaryotic and prokaryotic organisms. These vesicles contain a variety of biomolecules that differ both in quantity and type depending on the origin and cellular state. Exosomes are internalized by recipient cells, delivering their content and thus contributing to cell-cell communication in health and disease. During infections exosomes may exert a dual role, on one hand, they can transmit pathogen-related molecules mediating further infection and damage, and on the other hand, they can protect the host by activating the immune response and reducing pathogen spread. Selective packaging of pathogenic components may mediate these effects. Recently, quantitative analysis of samples by omics technologies has allowed a deep characterization of the proteins, lipids, RNA, and metabolite cargoes of exosomes. Knowledge about the content of these vesicles may facilitate their therapeutic application. Furthermore, as exosomes have been detected in almost all biological fluids, pathogenic or host-derived components can be identified in liquid biopsies, making them suitable for diagnosis and prognosis. This review attempts to organize the recent findings on exosome composition and function during viral, bacterial, fungal, and protozoan infections, and their contribution to host defense or to pathogen spread. Moreover, we summarize the current perspectives and future directions regarding the potential application of exosomes for prophylactic and therapeutic purposes.

The emerging role of exosomes in innate immunity, diagnosis and therapy

Exosomes, which are nano-sized transport bio-vehicles, play a pivotal role in maintaining homeostasis by exchanging genetic or metabolic information between different cells. Exosomes can also play a vital role in transferring virulent factors between the host and parasite, thereby regulating host gene expression and the immune interphase. The association of inflammation with disease development and the potential of exosomes to enhance or mitigate inflammatory pathways support the notion that exosomes have the potential to alter the course of a disease. Clinical trials exploring the role of exosomes in cancer, osteoporosis, and renal, neurological, and pulmonary disorders are currently underway. Notably, the information available on the signatory efficacy of exosomes in immune-related disorders remains elusive and sporadic. In this review, we discuss immune cell-derived exosomes and their application in immunotherapy, including those against autoimmune connective tissue diseases. Further, we have elucidated our views on the major issues in immune-related pathophysiological processes. Therefore, the information presented in this review highlights the role of exosomes as promising strategies and clinical tools for immune regulation.

Selective STING stimulation in dendritic cells primes antitumor T cell responses

T cells that recognize tumor antigens are crucial for mounting antitumor immune responses. Induction of antitumor T cells in immunogenic tumors depends on STING, the intracellular innate immune receptor for cyclic guanosine monophosphate-adenosine monophosphate (cGAMP) and related cyclic dinucleotides (CDNs). However, the optimal way to leverage STING activation in nonimmunogenic tumors is still unclear. Here, we show that cGAMP delivery by intratumoral injection of virus-like particles (cGAMP-VLP) led to differentiation of circulating tumor-specific T cells, decreased tumor regulatory T cells (T_regs), and antitumoral responses that synergized with PD1 blockade. By contrast, intratumoral injection of the synthetic CDN ADU-S100 led to tumor necrosis and systemic T cell activation but simultaneously depleted immune cells from injected tumors and induced minimal priming of circulating tumor-specific T cells. The antitumor effects of cGAMP-VLP required type 1 conventional dendritic cells (cDC1), whereas ADU-S100 eliminated cDC1 from injected tumors. cGAMP-VLP preferentially targeted STING in dendritic cells at a 1000-fold smaller dose than ADU-S100. Subcutaneous administration of cGAMP-VLP showed synergy when combined with PD1 blockade or a tumor T_reg-depleting antibody to elicit systemic tumor-specific T cells and antitumor activity, leading to complete and durable tumor eradication in the case of tumor T_reg depletion. These findings show that cell targeting of STING stimulation shapes the antitumor T cell response and identify a therapeutic strategy to enhance T cell-targeted immunotherapy.

SARS-CoV-2, HIV, and HPV: Convergent evolution of selective regulation of cGAS-STING signaling

Recognizing aberrant cytoplasmic double-stranded DNA and stimulating innate immunity is essential for the host's defense against viruses and tumors. Cyclic GMP-AMP (cGAMP) synthase (cGAS) is a cytosolic DNA sensor that synthesizes the second messenger 2'3'-cGAMP and subsequently activates stimulator of interferon genes (STING)-mediated activation of TANK-binding kinase 1 (TBK1)/interferon regulatory factor 3 (IRF3) and the production of type I interferon (IFN-I). Both the cGAS-STING-mediated IFN-I antiviral defense and the countermeasures developed by diverse viruses have been extensively studied. However, recent studies have revealed a convergent evolutionary feature of severe acute respiratory syndrome coronavirus 2 and human immunodeficiency virus (HIV) viral proteins in terms of the selective regulation of cGAS-STING-mediated nuclear factor-κB (NF-κB) signaling without any effect on cGAS-STING-mediated TBK1/IRF3 activation and IFN production. The potential beneficial effect of this cGAS-STING-mediated, NF-κB-dependent antiviral effect, and the possible detrimental effect of IFN-I in the pathogenesis of coronavirus disease 2019 and HIV infection deserve more attention and future investigation.

Density dependent regulation of inflammatory responses in macrophages

Macrophage distribution density is tightly regulated within the body, yet the importance of macrophage crowding during in vitro culture is largely unstudied. Using a human induced pluripotent stem cell (iPSC)-derived macrophage model of tissue resident macrophages, we characterize how increasing macrophage culture density changes their morphology and phenotype before and after inflammatory stimulation. In particular, density drives changes in macrophage inflammatory cytokine and chemokine secretion in both resting and activated states. This density regulated inflammatory state is also evident in blood monocyte derived-macrophages, the human monocytic THP-1 immortalized cell line, and iPSC-derived microglia. Density-dependent changes appear to be driven by a transferable soluble factor, yet the precise mechanism remains unknown. Our findings highlight cell plating density as an important but frequently overlooked consideration of in vitro macrophage research relevant to a variety of fields ranging from basic macrophage cell biology to disease studies.

The cGAS-STING pathway and cancer

The cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) pathway has emerged as a critical innate immune pathway that, following engagement by DNA, promotes distinct immune effector responses that can impact virtually all aspects of tumorigenesis, from malignant cell transformation to metastasis. Here we address how natural tumor-associated processes and traditional cancer therapies are shaped by cGAS-STING signaling, and how this contributes to beneficial or detrimental outcomes of cancer. We consider current efforts to target the cGAS-STING axis in tumors and highlight new frontiers in cGAS-STING biology to inspire thinking about their connection to cancer.

ABCC1 transporter exports the immunostimulatory cyclic dinucleotide cGAMP

The DNA sensor cyclic GMP-AMP synthase (cGAS) is important for antiviral and anti-tumor immunity. cGAS generates cyclic GMP-AMP (cGAMP), a diffusible cyclic dinucleotide that activates the antiviral response through the adaptor protein stimulator of interferon genes (STING). cGAMP cannot passively cross cell membranes, but recent advances have established a role for extracellular cGAMP as an "immunotransmitter" that can be imported into cells. However, the mechanism by which cGAMP exits cells remains unknown. Here, we identifed ABCC1 as a direct, ATP-dependent cGAMP exporter in mouse and human cells. We show that ABCC1 overexpression enhanced cGAMP export and limited STING signaling and that loss of ABCC1 reduced cGAMP export and potentiated STING signaling. We demonstrate that ABCC1 deficiency exacerbated cGAS-dependent autoimmunity in the Trex1-/- mouse model of Aicardi-Goutières syndrome. Thus, ABCC1-mediated cGAMP export is a key regulatory mechanism that limits cell-intrinsic activation of STING and ameliorates STING-dependent autoimmune disease.

Cyclic Nucleotide Signaling in Phage Defense and Counter-Defense

Advances in our understanding of prokaryotic antiphage defense mechanisms in the past few years have revealed a multitude of new cyclic nucleotide signaling molecules that play a crucial role in switching infected cells into an antiviral state. Defense pathways including type III CRISPR (clustered regularly interspaced palindromic repeats), CBASS (cyclic nucleotide-based antiphage signaling system), PYCSAR (pyrimidine cyclase system for antiphage resistance), and Thoeris all use cyclic nucleotides as second messengers to activate a diverse range of effector proteins. These effectors typically degrade or disrupt key cellular components such as nucleic acids, membranes, or metabolites, slowing down viral replication kinetics at great cost to the infected cell. Mechanisms to manipulate the levels of cyclic nucleotides are employed by cells to regulate defense pathways and by viruses to subvert them. Here we review the discovery and mechanism of the key pathways, signaling molecules and effectors, parallels and differences between the systems, open questions, and prospects for future research in this area.

Genistein Targets STING-Driven Antiviral Responses

Cytoplasmic detection of DNA by cyclic GMP-AMP (cGAMP) synthase (cGAS) is an essential component of antiviral responses. Upon synthesis, cGAMP binds to the stimulator of interferon (IFN) genes (STING) in infected and adjacent cells through intercellular transfer by connexins forming gap-junctions, eliciting a strong IFN-β-driven antiviral response. We demonstrate here that Genistein, a flavonoid compound naturally occurring in soy-based foods, inhibits cGAS-STING antiviral signaling at two levels. First, Genistein pretreatment of cGAMP-producing cells inhibited gap-junction intercellular communication, resulting in reduced STING responses in adjacent cells. In addition, Genistein directly blocked STING activation by the murine agonist DMXAA, by decreasing the interaction of STING with TBK1 and IKKε. As a result, Genistein attenuated STING signaling in human and mouse cells, dampening antiviral activity against Semliki Forest Virus infection. Collectively, our findings identify a previously unrecognized proviral activity of Genistein mediated via its inhibitory effects at two levels of cGAS-STING signaling.

New Insights in the Interplay Between African Swine Fever Virus and Innate Immunity and Its Impact on Viral Pathogenicity

The continuous spread of African swine fever virus (ASFV) in Europe and Asia represents a major threat to livestock health, with billions of dollars of income losses and major perturbations of the global pig industry. One striking feature of African swine fever (ASF) is the existence of different forms of the disease, ranging from acute with mortality rates approaching 100% to chronic, with mild clinical manifestations. These differences in pathogenicity have been linked to genomic alterations present in attenuated ASFV strains (and absent in virulent ones) and differences in the immune response of infected animals. In this mini-review, we summarized current knowledge on the connection between ASFV pathogenicity and the innate immune response induced in infected hosts, with a particular focus on the pathways involved in ASFV detection. Indeed, recent studies have highlighted the key role of the DNA sensor cGAS in ASFV sensing. We discussed what other pathways may be involved in ASFV sensing and inflammasome activation and summarized recent findings on the viral ASFV genes involved in the modulation of the interferon (IFN) and nuclear factor kappa B (NF-κB) pathways.

African Swine Fever Virus EP364R and C129R Target Cyclic GMP-AMP To Inhibit the cGAS-STING Signaling Pathway

African swine fever virus (ASFV) is a highly pathogenic swine DNA virus with high mortality that causes African swine fever (ASF) in domestic pigs and wild boars. For efficient viral infection, ASFV has developed complex strategies to evade key components of antiviral innate immune responses. However, the immune escape mechanism of ASFV remains unclear. Upon ASFV infection, cyclic GMP-AMP (2′,3′-cGAMP) synthase (cGAS), a cytosolic DNA sensor, recognizes ASFV DNA and synthesizes the second messenger 2′,3′-cGAMP, which triggers interferon (IFN) production to interfere with viral replication. In this study, we demonstrated a novel immune evasion mechanism of ASFV EP364R and C129R, which blocks cellular cyclic 2′,3′-cGAMP-mediated antiviral responses. ASFV EP364R and C129R with nuclease homology inhibit IFN-mediated responses by specifically interacting with 2′,3′-cGAMP and exerting their phosphodiesterase (PDE) activity to cleave 2′,3′-cGAMP. Particularly notable is that ASFV EP364R had a region of homology with the stimulator of interferon genes (STING) protein containing a 2′,3′-cGAMP-binding motif and point mutations in the Y76S and N78A amino acids of EP364R that impaired interaction with 2′,3′-cGAMP and restored subsequent antiviral responses. These results highlight a critical role for ASFV EP364R and C129R in the inhibition of IFN responses and could be used to develop ASFV live attenuated vaccines.

IMPORTANCE African swine fever (ASF) is a highly contagious hemorrhagic disease in domestic pigs and wild boars caused by African swine fever virus (ASFV). ASF is a deadly epidemic disease in the global pig industry, but no drugs or vaccines are available. Understanding the pathogenesis of ASFV is essential to developing an effective live attenuated ASFV vaccine, and investigating the immune evasion mechanisms of ASFV is crucial to improve the understanding of its pathogenesis. In this study, for the first time, we identified the EP364R and C129R, uncharacterized proteins that inhibit type I interferon signaling. ASFV EP364R and C129R specifically interacted with 2′,3′-cGAMP, the mammalian second messenger, and exerted phosphodiesterase activity to cleave 2′,3′-cGAMP. In this study, we discovered a novel mechanism by which ASFV inhibits IFN-mediated antiviral responses, and our findings can guide the understanding of ASFV pathogenesis and the development of live attenuated ASFV vaccines.

cGAS/cGAMP/STING signal propagation in the tumor microenvironment: key role for myeloid cells in antitumor immunity

Cyclic GMP-AMP synthase (cGAS), second messenger 2'3'-cyclic GMP-AMP (cGAMP) and stimulator of interferon genes (STING) are fundamental for sensing cytoplasmic double stranded DNA. Radiotherapy treatment induces large amounts of nuclear and mitochondrial DNA damage and results in the presence of DNA fragments in the cytoplasm, activating the cGAS/STING pathway. Triggering of the cGAS/STING pathway in the tumor microenvironment (TME) results in the production of type I interferons (IFNs). Type I IFNs are crucial for an effective antitumor defense, with myeloid cells as key players. Many questions remain on how these myeloid cells are activated and in which cells (tumor versus myeloid) in the TME the signaling pathway is initiated. The significance of cGAS/STING signaling in the onco-immunology field is being recognized, emphasized by the frequent occurrence of mutations in or silencing of genes in this pathway. We here review several mechanisms of cGAS/STING signal propagation in the TME, focusing on tumor cells and myeloid cells. Cell-cell contact-dependent interactions facilitate the transfer of tumor-derived DNA and cGAMP. Alternatively, transport routes via the extracellular space such as extracellular vesicles, and channel-mediated cGAMP transfer to and from the extracellular space contribute to propagation of cGAS/STING signal mediators DNA and cGAMP. Finally, we discuss regulation of extracellular cGAMP. Altogether, we provide a comprehensive overview of cGAS/cGAMP/STING signal propagation from tumor to myeloid cells in the TME, revealing novel targets for combinatorial treatment approaches with conventional anticancer therapies like radiotherapy.

Varicella-Zoster virus ORF9 is an antagonist of the DNA sensor cGAS

Varicella-Zoster virus (VZV) causes chickenpox and shingles. Although the infection is associated with severe morbidity in some individuals, molecular mechanisms that determine innate immune responses remain poorly defined. We found that the cGAS/STING DNA sensing pathway was required for type I interferon (IFN) induction during VZV infection and that recognition of VZV by cGAS restricted its replication. Screening of a VZV ORF expression library identified the essential VZV tegument protein ORF9 as a cGAS antagonist. Ectopically or virally expressed ORF9 bound to endogenous cGAS leading to reduced type I IFN responses to transfected DNA. Confocal microscopy revealed co-localisation of cGAS and ORF9. ORF9 and cGAS also interacted directly in a cell-free system and phase-separated together with DNA. Furthermore, ORF9 inhibited cGAMP production by cGAS. Taken together, these results reveal the importance of the cGAS/STING DNA sensing pathway for VZV recognition and identify a VZV immune antagonist that partially but directly interferes with DNA sensing via cGAS.