SIRT2 negatively regulates the cGAS-STING pathway by deacetylating G3BP1

SIRT2, a cytoplasmic member of the Sirtuin family, has important roles in immunity and inflammation. However, its function in regulating the response to DNA virus infection remains elusive. Here, we find that SIRT2 is a unique regulator among the Sirtuin family that negatively modulates the cGAS-STING-signaling pathway. SIRT2 is down-regulated after Herpes simplex virus-1 (HSV-1) infection, and SIRT2 deficiency markedly elevates the expression levels of type I interferon (IFN). SIRT2 inhibits the DNA binding ability and droplet formation of cGAS by interacting with and deacetylating G3BP1 at K257, K276, and K376, leading to the disassembly of the cGAS-G3BP1 complex, which is critical for cGAS activation. Administration of AGK2, a selective SIRT2 inhibitor, protects mice from HSV-1 infection and increases the expression of IFN and IFN-stimulated genes. Our study shows that SIRT2 negatively regulates cGAS activation through G3BP1 deacetylation, suggesting a potential antiviral strategy by modulating SIRT2 activity.

The MC160 protein of the molluscum contagiosum virus dampens cGAS/STING-induced interferon-β activation

Molluscum contagiosum virus (MCV) is a poxvirus that causes benign, persistent skin lesions. MCV encodes a variety of immune evasion molecules to dampen host immune responses. Two of these proteins are the MC159 and MC160 proteins. Both MC159 and MC160 contain two tandem death effector domains and share homology to the cellular FLIPs, FADD, and procaspase-8. MC159 and MC160 dampen several innate immune responses such as NF-κB activation and mitochondrial antiviral signaling (MAVS)-mediated induction of type 1 interferon (IFN). The type 1 IFN response is also activated by the cytosolic DNA sensors cyclic GMP-AMP synthase (cGAS) and stimulator of interferon genes (STING). Both cGAS and STING play a vital role in sensing a poxvirus infection. In this study, we demonstrate that there are nuanced differences between both MC160 and MC159 in terms of how the viral proteins modulate the cGAS/STING and MAVS pathways. Specifically, MC160 expression, but not MC159 expression, dampens cGAS/STING-mediated induction of IFN in HEK 293 T cells. Further, MC160 expression prevented the K63-ubiquitination of both STING and TBK1, a kinase downstream of cGAS/STING. Ectopic expression of the MC160 protein, but not the MC159 protein, resulted in a measurable decrease in the TBK1 protein levels as detected via immunoblotting. Finally, using a panel of MC160 truncation mutants, we report that the MC160 protein requires both DEDs to inhibit cGAS/STING-induced activation of IFN-β. Our model indicates MC160 likely alters the TBK1 signaling complex to decrease IFN-β activation at the molecular intersection of the cGAS/STING and MAVS signaling pathways.

Terbuthylazine exposure induces innate immune response and inflammation through activating cGAS-STING/NF-κB pathway in myocardium of broiler chicken (Gallus gallus)

Terbuthylazine (TBA), a triazine herbicide, is extensively employed in agriculture for its wide range of effectiveness. However, prolonged utilization of TBA can pose a potential hazard to animals and human health. Here, a total of 180 broiler chickens (Gallus gallus) were stochastically assigned to three groups (control group, 0.4 mg/kg TBA group, and 4 mg/kg TBA group) for investigating the impact of TBA on cardiotoxicity. The results revealed that TBA exposure resulted in pathological alterations in the myocardium. Moreover, TBA exposure activated cGAS-STING pathway and markedly elevated the mRNA and protein expression levels of innate immune response (cGAS, STING, TBK1, and IRF3) in myocardium. Additionally, NF-κB signal was also activated under TBA exposure, which was characterized by the increasing mRNA expression levels of NF-κB, IKKα and the protein expression levels of p-NF-κB/NF-κB, IKKα, p-IκBα/IκBα in the TBA treatment groups. Meanwhile, the expression of pro-inflammatory cytokines (TNF-α and IL-1β) were also significantly increased. In summary, our findings suggested that cGAS-STING/NF-κB pathway functionated in the innate immune response and inflammation in myocardium brought on by TBA exposure, which provided new insights into the TBA toxicology.

Eukaryotic CD-NTase, STING, and viperin proteins evolved via domain shuffling, horizontal transfer, and ancient inheritance from prokaryotes

Animals use a variety of cell-autonomous innate immune proteins to detect viral infections and prevent replication. Recent studies have discovered that a subset of mammalian antiviral proteins have homology to antiphage defense proteins in bacteria, implying that there are aspects of innate immunity that are shared across the Tree of Life. While the majority of these studies have focused on characterizing the diversity and biochemical functions of the bacterial proteins, the evolutionary relationships between animal and bacterial proteins are less clear. This ambiguity is partly due to the long evolutionary distances separating animal and bacterial proteins, which obscures their relationships. Here, we tackle this problem for 3 innate immune families (CD-NTases [including cGAS], STINGs, and viperins) by deeply sampling protein diversity across eukaryotes. We find that viperins and OAS family CD-NTases are ancient immune proteins, likely inherited since the earliest eukaryotes first arose. In contrast, we find other immune proteins that were acquired via at least 4 independent events of horizontal gene transfer (HGT) from bacteria. Two of these events allowed algae to acquire new bacterial viperins, while 2 more HGT events gave rise to distinct superfamilies of eukaryotic CD-NTases: the cGLR superfamily (containing cGAS) that has since diversified via a series of animal-specific duplications and a previously undefined eSMODS superfamily, which more closely resembles bacterial CD-NTases. Finally, we found that cGAS and STING proteins have substantially different histories, with STING protein domains undergoing convergent domain shuffling in bacteria and eukaryotes. Overall, our findings paint a picture of eukaryotic innate immunity as highly dynamic, where eukaryotes build upon their ancient antiviral repertoires through the reuse of protein domains and by repeatedly sampling a rich reservoir of bacterial antiphage genes.

RVX-208, an inducer of Apolipoprotein A-I, inhibits the particle production of hepatitis B virus through activation of cGAS-STING pathway

BACKGROUND

Previously, we have demonstrated that Apolipoprotein A-I (ApoA-I) could inhibit the secretion of Hepatitis B virus (HBV), suggesting that stimulation of ApoA-I may block particle production. In the present study, we evaluated the anti-HBV effect of RVX-208, a small-molecule stimulator of ApoA-I gene expression.

METHODS

RVX-208 was used to treat HepG2.2.15 cell, a HepG2 derived cell line stably producing HBV virus. Real-time PCR was performed to examine the HBV DNA levels. Magnetic particles, which were coated with anti-HBS or anti-HBE antibody, were used to examine the HBsAg and HBeAg levels in the supernatant of cultured HepG2.2.15 cells in combination with the enzyme conjugates that were prepared with horseradish peroxidase labelled anti-HBS or anti-HBE antibody in a double antibody sandwich manner. RNA-seq, immunoblots and real-time PCR were used to analyze the functional mechanism of RVX-208.

RESULTS

RVX-208 could elevate the ApoA-I protein levels in HepG2.2.15 cells. In the meantime, RVX-208 significantly repressed HBV DNA, HBsAg and HBeAg levels in the supernatants of HepG2.2.15 cells. RNA-seq data revealed that RVX-208 treatment not only affected the cholesterol metabolism, which is closely related to ApoA-I, but also regulated signalling pathways that are associated with antiviral immune response. Moreover, mechanistic studies demonstrated that RVX-208 could activate cGAS-STING pathway and upregulate the transcription of a series of interferons, pro-inflammatory cytokines and chemokines with antiviral potential that are at the downstream of cGAS-STING pathway.

CONCLUSION

Our study demonstrated that RVX-208, an inducer of ApoA-I, could suppress HBV particle production through activation of cGAS-STING pathway.

Targeting mitotic regulators in cancer as a strategy to enhance immune recognition

Eukaryotic DNA has evolved to be enclosed within the nucleus to protect the cellular genome from autoinflammatory responses driven by the immunogenic nature of cytoplasmic DNA. Cyclic GMP-AMP Synthase (cGAS) is the cytoplasmic dsDNA sensor, which upon activation of Stimulator of Interferon Genes (STING), mediates production of pro-inflammatory interferons (IFNs) and interferon stimulated genes (ISGs). However, although this pathway is crucial in detection of viral and microbial genetic material, cytoplasmic DNA is not always of foreign origin. It is now recognised that specifically in genomic instability, a hallmark of cancer, extranuclear material in the form of micronuclei (MN) can be generated as a result of unresolved DNA lesions during mitosis. Activation of cGAS-STING in cancer has been shown to regulate numerous tumour-immune interactions such as acquisition of 'immunologically hot' phenotype which stimulates immune-mediated elimination of transformed cells. Nonetheless, a significant percentage of poorly prognostic cancers is 'immunologically cold'. As this state has been linked with low proportion of tumour-infiltrating lymphocytes (TILs), improving immunogenicity of cold tumours could be clinically relevant by exhibiting synergy with immunotherapy. This review aims to present how inhibition of vital mitotic regulators could provoke cGAS-STING response in cancer and improve the efficacy of current immunotherapy regimens.

Interferon induction by STING requires its translocation to the late endosomes

To combat microbial infections, mammalian cells use a variety of innate immune response pathways to induce synthesis of anti-microbial proteins. The cGAS/STING pathway recognizes cytoplasmic viral or cellular DNA to elicit signals that lead to type I interferon and other cytokine synthesis. cGAMP, synthesized by DNA-activated cGAS, activates the ER-associated protein, STING, which oligomerizes and translocates to other intracellular membrane compartments to trigger different branches of signaling. We have reported that, in the ER, EGFR-mediated phosphorylation of Tyr245 of STING is required for its transit to the late endosomes, where it recruits and activates the transcription factor IRF3 required for IFN induction. In the current study, we inquired whether STING Tyr245 phosphorylation per se or STING's location in the late endosomes was critical for its ability to recruit IRF3 and induce IFN. Using pharmacological inhibitors or genetic ablation of proteins that are essential for specific steps of STING trafficking, we demonstrated that the presence of STING in the late endosomal membranes, even without Tyr245 phosphorylation, was sufficient for IRF3-mediated IFN induction.

Ginsenoside Rg3 enriches SCFA-producing commensal bacteria to confer protection against enteric viral infection via the cGAS-STING-type I IFN axis

The microbiota-associated factors that influence host susceptibility and immunity to enteric viral infections remain poorly defined. We identified that the herbal monomer ginsenoside Rg3 (Rg3) can shape the gut microbiota composition, enriching robust short-chain fatty acid (SCFA)-producing Blautia spp. Colonization by representative Blautia coccoides and Blautia obeum could protect germ-free or vancomycin (Van)-treated mice from enteric virus infection, inducing type I interferon (IFN-I) responses in macrophages via the MAVS-IRF3-IFNAR signaling pathway. Application of exogenous SCFAs (acetate/propionate) reproduced the protective effect of Rg3 and Blautia spp. in Van-treated mice, enhancing intracellular Ca2+- and MAVS-dependent mtDNA release and activating the cGAS-STING-IFN-I axis by stimulating GPR43 signaling in macrophages. Our findings demonstrate that macrophage sensing of metabolites from specific commensal bacteria can prime the IFN-I signaling that is required for antiviral functions.

STING activation in TET2-mutated hematopoietic stem/progenitor cells contributes to the increased self-renewal and neoplastic transformation

Somatic loss-of-function mutations of the dioxygenase Ten-eleven translocation-2 (TET2) occur frequently in individuals with clonal hematopoiesis (CH) and acute myeloid leukemia (AML). These common hematopoietic disorders can be recapitulated in mouse models. However, the underlying mechanisms by which the deficiency in TET2 promotes these disorders remain unclear. Here we show that the cyclic guanosine monophosphate-adenosine monophosphate synthase (cGAS)-stimulator of interferon genes (STING) pathway is activated to mediate the effect of TET2 deficiency in dysregulated hematopoiesis in mouse models. DNA damage arising in Tet2-deficient hematopoietic stem/progenitor cells (HSPCs) leads to activation of the cGAS-STING pathway which in turn promotes the enhanced self-renewal and development of CH. Notably, both pharmacological inhibition and genetic deletion of STING suppresses Tet2 mutation-induced aberrant hematopoiesis. In patient-derived xenograft (PDX) models, STING inhibition specifically attenuates the proliferation of leukemia cells from TET2-mutated individuals. These observations suggest that the development of CH associated with TET2 mutations is powered through chronic inflammation dependent on the activated cGAS-STING pathway and that STING may represent a potential target for intervention of relevant hematopoietic diseases.

SMURF1 activates the cGAS/STING/IFN-1 signal axis by mediating YY1 ubiquitination to accelerate the progression of lupus nephritis

Aggravated endoplasmic reticulum stress (ERS) and apoptosis in podocytes play an important role in lupus nephritis (LN) progression, but its mechanism is still unclear. Herein, the role of SMURF1 in regulating podocytes apoptosis and ERS during LN progression were investigated. MRL/lpr mice was used as LN model in vivo. HE staining was performed to analyze histopathological changes. Mouse podocytes (MPC5 cells) were treated with serum IgG from LN patients (LN-IgG) to construct LN model in vitro. CCK8 assay was adopted to determine the viability. Cell apoptosis was measured using flow cytometry and TUNEL staining. The interactions between SMURF1, YY1 and cGAS were analyzed using ChIP and/or dual-luciferase reporter gene and/or Co-IP assays. YY1 ubiquitination was analyzed by ubiquitination analysis. Our results found that SMURF1, cGAS and STING mRNA levels were markedly increased in serum samples of LN patients, while YY1 was downregulated. YY1 upregulation reduced LN-IgG-induced ERS and apoptosis in podocytes. Moreover, SMURF1 upregulation reduced YY1 protein stability and expression by ubiquitinating YY1 in podocytes. Rescue studies revealed that YY1 knockdown abrogated the inhibition of SMURF1 downregulation on LN-IgG-induced ERS and apoptosis in podocytes. It was also turned out that YY1 alleviated podocytes injury in LN by transcriptional inhibition cGAS/STING/IFN-1 signal axis. Finally, SMURF1 knockdown inhibited LN progression in vivo. In short, SMURF1 upregulation activated the cGAS/STING/IFN-1 signal axis by regulating YY1 ubiquitination to facilitate apoptosis in podocytes during LN progression.

The poxvirus F17 protein counteracts mitochondrially orchestrated antiviral responses

Poxviruses are unusual DNA viruses that replicate in the cytoplasm. To do so, they encode approximately 100 immunomodulatory proteins that counteract cytosolic nucleic acid sensors such as cGAMP synthase (cGAS) along with several other antiviral response pathways. Yet most of these immunomodulators are expressed very early in infection while many are variable host range determinants, and significant gaps remain in our understanding of poxvirus sensing and evasion strategies. Here, we show that after infection is established, subsequent progression of the viral lifecycle is sensed through specific changes to mitochondria that coordinate distinct aspects of the antiviral response. Unlike other viruses that cause extensive mitochondrial damage, poxviruses sustain key mitochondrial functions including membrane potential and respiration while reducing reactive oxygen species that drive inflammation. However, poxvirus replication induces mitochondrial hyperfusion that independently controls the release of mitochondrial DNA (mtDNA) to prime nucleic acid sensors and enables an increase in glycolysis that is necessary to support interferon stimulated gene (ISG) production. To counter this, the poxvirus F17 protein localizes to mitochondria and dysregulates mTOR to simultaneously destabilize cGAS and block increases in glycolysis. Our findings reveal how the poxvirus F17 protein disarms specific mitochondrially orchestrated responses to later stages of poxvirus replication.

Membrane traffic governs the STING inflammatory signalling

The cGAS-STING innate immune pathway has recently emerged as a critical driver of inflammation in a variety of settings, such as virus infection, cellular stress and tissue damage. The pathway detects microbial and host-derived double-stranded DNA (dsDNA) in the cytosol, and triggers the production of the type I interferons through the activation of IRF3. The detailed mechanistic and biochemical understanding of the pathway has enabled the development of pharmacological agents for the treatment of chronic inflammation and cancer. STING is an endoplasmic reticulum (ER)-localized transmembrane protein. Upon emergence of cytosolic dsDNA, STING exits the ER and migrates sequentially to the Golgi, recycling endosomes and lysosomes. Importantly, the intracellular translocation of STING is essential for the activation and inactivation of the STING signalling. In this review, I summarize the recent insights into the regulators of the membrane traffic of STING and STING-associated autoinflammatory diseases.

Regulation of STING activity in DNA sensing by ISG15 modification

Sensing of human immunodeficiency virus type 1 (HIV-1) DNA is mediated by the cyclic GMP-AMP synthase-stimulator of interferon genes (cGAS-STING) signaling axis. Signal transduction and regulation of this cascade is achieved by post-translational modifications. Here we show that cGAS-STING-dependent HIV-1 sensing requires interferon-stimulated gene 15 (ISG15). ISG15 deficiency inhibits STING-dependent sensing of HIV-1 and STING agonist-induced antiviral response. Upon external stimuli, STING undergoes ISGylation at residues K224, K236, K289, K347, K338, and K370. Inhibition of STING ISGylation at K289 suppresses STING-mediated type Ⅰ interferon induction by inhibiting its oligomerization. Of note, removal of STING ISGylation alleviates gain-of-function phenotype in STING-associated vasculopathy with onset in infancy (SAVI). Molecular modeling suggests that ISGylation of K289 is an important regulator of oligomerization. Taken together, our data demonstrate that ISGylation at K289 is crucial for STING activation and represents an important regulatory step in DNA sensing of viruses and autoimmune responses.

cGAS-STING signaling in the tumor microenvironment

The cGAS-STING signaling is an important pathway involved in the regulation of tumor microenvironment, which affects many cellular functions including immune activation. Its role in combating tumor progression is widely recognized, especially with its function in inducing innate and adaptive immune responses, on which many immunotherapies have been developed. However, a growing number of findings also suggest a diversity of its roles in shaping tumor microenvironment, including functions that promote tumor progression. Here, we summarize the functions of the cGAS-STING signaling in tumor microenvironment to maintain tumor survival and proliferation through facilitating the forming of an immunosuppressive tumor microenvironment and discuss the current advances of STING-related immunotherapies.

SMPDL3A links cholesterol metabolism to the cGAS-STING pathway

Liver X receptor (LXR), well known for its role in cholesterol metabolism, also has anti-inflammatory properties. In this issue of Immunity, Hou et al. demonstrate that LXR signaling induces SMPDL3A, a cGAMP-degrading enzyme that restricts cGAS-cGAMP-STING innate immune signaling, providing a mechanistic link between lipid metabolism and inflammation.

DTX3L mediated ubiquitination of cGAS suppresses antitumor immunity in pancreatic cancer

The global incidence of pancreatic cancer is associated with a high mortality rate and one of the lowest survival rates among all types of cancer. The clinical management modalities for pancreatic cancer encompass surgical intervention, chemotherapy, radiation therapy, targeted therapy, immunotherapy, or a combination thereof. Nevertheless, the diagnosis of pancreatic cancer often occurs at an advanced stage, thereby restricting treatment options and diminishing the prospects of achieving a cure. The cGAS-STING pathway has emerged as a potential target for antitumor therapy due to its role in promoting immune responses against cancer cells. Activation of the cGAS-STING pathway in tumor cells can lead to the production of pro-inflammatory cytokines and type I interferons, which can enhance the recruitment and activation of immune cells to the tumor microenvironment. The cGAS protein was detected in only a half of tumor tissues in pancreatic cancer patients and the underlying mechanism is still elusive. In this study, we have identified the E3 ligase DTX3L as a key regulator of cGAS-STING signaling in pancreatic cancer cells by mediating the ubiquitination and degradation of cGAS. The expression levels of DTX3L were found to be upregulated in pancreatic tumor tissues and correlated with a poor prognosis for patients with pancreatic cancer. Silencing of DTX3L resulted in enhanced activation of the cGAS-STING signaling pathway and improved antitumor immunity for pancreatic cancer, suggesting that targeting the DTX3L-cGAS axis could hold promise for the treatment of this disease.

Mutant RB1 enhances therapeutic efficacy of PARPis in lung adenocarcinoma by triggering the cGAS/STING pathway

Poly (ADP-ribose) polymerase inhibitors (PARPis) are approved for cancer therapy according to their synthetic lethal interactions, and clinical trials have been applied in non-small cell lung cancer. However, the therapeutic efficacy of PARPis in lung adenocarcinoma (LUAD) is still unknown. We explored the effect of a mutated retinoblastoma gene (RB1) on PARPi sensitivity in LUAD. Bioinformatic screening was performed to identify PARPi-sensitive biomarkers. Here, we showed that viability of LUAD cell lines with mutated RB1 was significantly decreased by PARPis (niraparib, rucaparib, and olaparib). RB1 deficiency induced genomic instability, prompted cytosolic double-stranded DNA (dsDNA) formation, activated the cGAS/STING pathway, and upregulated downstream chemokines CCL5 and CXCL10, triggering immune cell infiltration. Xenograft experiments indicated that PARPi treatment reduced tumorigenesis in RB1-KO mice. Additionally, single-cell RNA sequencing analysis showed that malignant cells with downregulated expression of RB1 had more communications with other cell types, exhibiting activation of specific signaling such as GAS, IFN response, and antigen-presenting and cytokine activities. Our findings suggest that RB1 mutation mediates the sensitivity to PARPis through a synthetic lethal effect by triggering the cGAS/STING pathway and upregulation of immune infiltration in LUAD, which may be a potential therapeutic strategy.

An IKBKE variant conferring functional cGAS/STING pathway deficiency and susceptibility to recurrent HSV-2 meningitis

The mechanisms underlying susceptibility to recurrent herpes simplex virus type 2 (HSV-2) meningitis remain incompletely understood. In a patient experiencing multiple episodes of HSV-2 meningitis, we identified a monoallelic variant in the IKBKE gene, which encodes the IKKε kinase involved in induction of antiviral IFN genes. Patient cells displayed impaired induction of IFN-β1 (IFNB1) expression upon infection with HSV-2 or stimulation with double-stranded DNA (dsDNA) and failed to induce phosphorylation of STING, an activation marker of the DNA-sensing cyclic GMP-AMP synthase/stimulator of IFN genes (cGAS/STING) pathway. The patient allele encoded a truncated IKKε protein with loss of kinase activity and also capable of exerting dominant-negative activity. In stem cell-derived microglia, HSV-2-induced expression of IFNB1 was dependent on cGAS, TANK binding kinase 1 (TBK1), and IKBKE, but not TLR3, and supernatants from HSV-2-treated microglia exerted IKBKE-dependent type I IFN-mediated antiviral activity upon neurons. Reintroducing wild-type IKBKE into patient cells rescued IFNB1 induction following treatment with HSV-2 or dsDNA and restored antiviral activity. Collectively, we identify IKKε to be important for protection against HSV-2 meningitis and suggest a nonredundant role for the cGAS/STING pathway in human antiviral immunity.

Mechanism and therapeutic potential of targeting cGAS-STING signaling in neurological disorders

DNA sensing is a pivotal component of the innate immune system that is responsible for detecting mislocalized DNA and triggering downstream inflammatory pathways. Among the DNA sensors, cyclic GMP-AMP synthase (cGAS) is a primary player in detecting cytosolic DNA, including foreign DNA from pathogens and self-DNA released during cellular damage, culminating in a type I interferon (IFN-I) response through stimulator of interferon genes (STING) activation. IFN-I cytokines are essential in mediating neuroinflammation, which is widely observed in CNS injury, neurodegeneration, and aging, suggesting an upstream role for the cGAS DNA sensing pathway. In this review, we summarize the latest developments on the cGAS-STING DNA-driven immune response in various neurological diseases and conditions. Our review covers the current understanding of the molecular mechanisms of cGAS activation and highlights cGAS-STING signaling in various cell types of central and peripheral nervous systems, such as resident brain immune cells, neurons, and glial cells. We then discuss the role of cGAS-STING signaling in different neurodegenerative conditions, including tauopathies, Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis, as well as aging and senescence. Finally, we lay out the current advancements in research and development of cGAS inhibitors and assess the prospects of targeting cGAS and STING as therapeutic strategies for a wide spectrum of neurological diseases.

Mycobacterium tuberculosis suppresses host DNA repair to boost its intracellular survival

Mycobacterium tuberculosis (Mtb) triggers distinct changes in macrophages, resulting in the formation of lipid droplets that serve as a nutrient source. We discover that Mtb promotes lipid droplets by inhibiting DNA repair responses, resulting in the activation of the type-I IFN pathway and scavenger receptor-A1 (SR-A1)-mediated lipid droplet formation. Bacterial urease C (UreC, Rv1850) inhibits host DNA repair by interacting with RuvB-like protein 2 (RUVBL2) and impeding the formation of the RUVBL1-RUVBL2-RAD51 DNA repair complex. The suppression of this repair pathway increases the abundance of micronuclei that trigger the cyclic GMP-AMP synthase (cGAS)/stimulator of interferon genes (STING) pathway and subsequent interferon-β (IFN-β) production. UreC-mediated activation of the IFN-β pathway upregulates the expression of SR-A1 to form lipid droplets that facilitate Mtb replication. UreC inhibition via a urease inhibitor impaired Mtb growth within macrophages and in vivo. Thus, our findings identify mechanisms by which Mtb triggers a cascade of cellular events that establish a nutrient-rich replicative niche.

Engineering and Delivery of cGAS-STING Immunomodulators for the Immunotherapy of Cancer and Autoimmune Diseases

The cyclic GMP-AMP synthase-stimulator interferon gene (cGAS-STING) pathway is an emerging therapeutic target for the prophylaxis and therapy of a variety of diseases, ranging from cancer, infectious diseases, to autoimmune disorders. As a cytosolic double stranded DNA (dsDNA) sensor, cGAS can bind with relatively long dsDNA, resulting in conformational change and activation of cGAS. Activated cGAS catalyzes the conversion of adenosine triphosphate (ATP) and guanosine triphosphate (GTP) into cGAMP, a cyclic dinucleotide (CDN). CDNs, including 2'3'-cGAMP, stimulate adapter protein STING on the endoplasmic membrane, triggering interferon regulatory factor 3 (IRF3) phosphorylation and nuclear factor kappa B (NF-κB) activation. This results in antitumor and antiviral type I interferon (IFN-I) responses. Moreover, cGAS-STING overactivation and the resulting IFN-I responses have been associated with a number of inflammatory and autoimmune diseases. This makes cGAS-STING appealing immunomodulatory targets for the prophylaxis and therapy of various related diseases. However, drug development of CDNs and CDN derivatives is challenged by their limited biostability, difficult formulation, poor pharmacokinetics, and inefficient tissue accumulation and cytosolic delivery. Though recent synthetic small molecular CDN- or non-CDN-based STING agonists have been reported with promising preclinical therapeutic efficacy, their therapeutic efficacy and safety remain to be fully evaluated preclinically and clinically. Therefore, it is highly desirable and clinically significant to advance drug development for cGAS-STING activation by innovative approaches, such as drug delivery systems and drug development for pharmacological immunomodulation of cGAS. In this Account, we summarize our recent research in the engineering and delivery of immunostimulatory or immunoregulatory modulators for cGAS and STING for the immunotherapy of cancer and autoimmune diseases. To improve the delivery efficiency of CDNs, we developed ionizable and pH-responsive polymeric nanocarriers to load STING agonists, aiming to improve the cellular uptake and facilitate the endosomal escape to induce efficient STING activation. We also codelivered STING agonists with complementary immunostimulatants in nanoparticle-in-hydrogel composites to synergetically elicit potent innate and adaptive antitumor responses that eradicate local and distant large tumors. Further, taking advantage of the simplicity of manufacturing and the established nucleic acid delivery system, we developed oligonucleotide-based cGAS agonists as immunostimulant immunotherapeutics as well as adjuvants for peptide antigens for cancer immunotherapy. To suppress the overly strong proinflammatory responses associated with cGAS-STING overactivation in some of the autoimmune disorders, we devised nanomedicine-in-hydrogel (NiH) that codelivers a cGAS inhibitor and cell-free DNA (cfDNA)-scavenging cationic nanoparticles (cNPs) for systemic immunosuppression in rheumatoid arthritis (RA) therapy. Lastly, we discussed current drug development by targeting cGAS-STING for cancer, infectious diseases, and autoimmune diseases, as well as the potential opportunities for utilizing cGAS-STING pathway for versatile applications in disease treatment.

FEN1 inhibitor SC13 promotes CAR-T cells infiltration into solid tumours through cGAS-STING signalling pathway

It is well known that chimeric antigen receptor T-cell immunotherapy (CAR-T-cell immunotherapy) has excellent therapeutic effect in haematological tumours, but it still faces great challenges in solid tumours, including inefficient T-cell tumour infiltration and poor functional persistence. Flap structure-specific endonuclease 1 (FEN1), highly expressed in a variety of cancer cells, plays an important role in both DNA replication and repair. Previous studies have reported that FEN1 inhibition is an effective strategy for cancer treatment. Therefore, we hypothesized whether FEN1 inhibitors combined with CAR-T-cell immunotherapy would have a stronger killing effect on solid tumours. The results showed that low dose of FEN1 inhibitors SC13 could induce an increase of double-stranded broken DNA (dsDNA) in the cytoplasm. Cytosolic dsDNA can activate the cyclic GMP-AMP synthase-stimulator of interferon gene signalling pathway and increase the secretion of chemokines. In vivo, under the action of FEN1 inhibitor SC13, more chemokines were produced at solid tumour sites, which promoted the infiltration of CAR-T cells and improved anti-tumour immunity. These findings suggest that FEN1 inhibitors could enable CAR-T cells to overcome poor T-cell infiltration and improve the treatment of solid tumours.

Biochemical characterization of mRNA capping enzyme from Faustovirus

The mammalian mRNA 5' cap structures play important roles in cellular processes such as nuclear export, efficient translation, and evading cellular innate immune surveillance and regulating 5'-mediated mRNA turnover. Hence, installation of the proper 5' cap is crucial in therapeutic applications of synthetic mRNA. The core 5' cap structure, Cap-0, is generated by three sequential enzymatic activities: RNA 5' triphosphatase, RNA guanylyltransferase, and cap N7-guanine methyltransferase. Vaccinia virus RNA capping enzyme (VCE) is a heterodimeric enzyme that has been widely used in synthetic mRNA research and manufacturing. The large subunit of VCE D1R exhibits a modular structure where each of the three structural domains possesses one of the three enzyme activities, whereas the small subunit D12L is required to activate the N7-guanine methyltransferase activity. Here, we report the characterization of a single-subunit RNA capping enzyme from an amoeba giant virus. Faustovirus RNA capping enzyme (FCE) exhibits a modular array of catalytic domains in common with VCE and is highly efficient in generating the Cap-0 structure without an activation subunit. Phylogenetic analysis suggests that FCE and VCE are descended from a common ancestral capping enzyme. We found that compared to VCE, FCE exhibits higher specific activity, higher activity toward RNA containing secondary structures and a free 5' end, and a broader temperature range, properties favorable for synthetic mRNA manufacturing workflows.

'From R-lupus to cancer': Reviewing the role of R-loops in innate immune responses

Cells possess an inherent and evolutionarily conserved ability to detect and respond to the presence of foreign and pathological 'self' nucleic acids. The result is the stimulation of innate immune responses, signalling to the host immune system that defence mechanisms are necessary to protect the organism. To date, there is a vast body of literature describing innate immune responses to various nucleic acid species, including dsDNA, ssDNA and ssRNA etc., however, there is limited information available on responses to R-loops. R-loops are 3-stranded nucleic acid structures that form during transcription, upon DNA damage and in various other settings. Emerging evidence suggests that innate immune responses may also exist for the detection of R-loop related nucleic acid structures, implicating R-loops as drivers of inflammatory states. In this review, we aim to summarise the evidence indicating that R-loops are immunogenic species that can trigger innate immune responses in physiological and pathological settings and discuss the implications of this in the study of various diseases and therapeutic development.

ARF1 prevents aberrant type I interferon induction by regulating STING activation and recycling

Type I interferon (IFN) signalling is tightly controlled. Upon recognition of DNA by cyclic GMP-AMP synthase (cGAS), stimulator of interferon genes (STING) translocates along the endoplasmic reticulum (ER)-Golgi axis to induce IFN signalling. Termination is achieved through autophagic degradation or recycling of STING by retrograde Golgi-to-ER transport. Here, we identify the GTPase ADP-ribosylation factor 1 (ARF1) as a crucial negative regulator of cGAS-STING signalling. Heterozygous ARF1 missense mutations cause a previously unrecognized type I interferonopathy associated with enhanced IFN-stimulated gene expression. Disease-associated, GTPase-defective ARF1 increases cGAS-STING dependent type I IFN signalling in cell lines and primary patient cells. Mechanistically, mutated ARF1 perturbs mitochondrial morphology, causing cGAS activation by aberrant mitochondrial DNA release, and leads to accumulation of active STING at the Golgi/ERGIC due to defective retrograde transport. Our data show an unexpected dual role of ARF1 in maintaining cGAS-STING homeostasis, through promotion of mitochondrial integrity and STING recycling.