Search

The detection of intracellular DNA has emerged to be a key event in the innate immune response to viruses and intracellular bacteria, and during conditions of sterile inflammation and autoimmunity. One of the consequences of the detection of DNA as a 'stranger' and a 'danger' signal is the production of type I interferons and pro-inflammatory cytokines. Much work has been dedicated to the elucidation of the signalling cascades that activate this DNA-induced gene expression programme. However, while many proteins have been proposed to act as sensors for intracellular DNA in recent years, none has been met with universal acceptance, and a theory linking all the recent observations is, as yet, lacking. This review presents the evidence for the various interferon-inducing DNA receptors proposed to date, and examines the hypotheses that might explain why so many different receptors appear to be involved in the innate immune recognition of intracellular DNA.

The innate immune defence of multicellular organisms against microbial pathogens requires cellular collaboration. Information exchange allowing immune cells to collaborate is generally attributed to soluble protein factors secreted by pathogen-sensing cells. Cytokines, such as type I interferons (IFNs), serve to alert non-infected cells to the possibility of pathogen challenge1. Moreover, in conjunction with chemokines they can instruct specialized immune cells to contain and eradicate microbial infection. Several receptors and signalling pathways exist that couple pathogen sensing to the induction of cytokines, whereas cytosolic recognition of nucleic acids seems to be exquisitely important for the activation of type I IFNs, master regulators of antiviral immunity2. Cytosolic DNA is sensed by the receptor cyclic GMP-AMP (cGAMP) synthase (cGAS), which catalyses the synthesis of the second messenger cGAMP(2′-5′)3, 4, 5, 6, 7, 8. This molecule in turn activates the endoplasmic reticulum (ER)-resident receptor STING9, 10, 11, thereby inducing an antiviral state and the secretion of type I IFNs. Here we find in murine and human cells that cGAS-synthesized cGAMP(2′-5′) is transferred from producing cells to neighbouring cells through gap junctions, where it promotes STING activation and thus antiviral immunity independently of type I IFN signalling. In line with the limited cargo specificity of connexins, the proteins that assemble gap junction channels, most connexins tested were able to confer this bystander immunity, thus indicating a broad physiological relevance of this local immune collaboration. Collectively, these observations identify cGAS-triggered cGAMP(2′-5′) transfer as a novel host strategy that serves to rapidly convey antiviral immunity in a transcription-independent, horizontal manner.

Summary The presence of microbial or self DNA in the cytoplasm of mammalian cells is a danger signal detected by the DNA sensor cyclic-GMP-AMP (cGAMP) synthase (cGAS), which catalyzes the production of cGAMP that in turn serves as a second messenger to activate innate immune responses. Here we show that endogenous cGAMP in mammalian cells contains two distinct phosphodiester linkages, one between 2′-OH of GMP and 5′-phosphate of AMP, and the other between 3′-OH of AMP and 5′-phosphate of GMP. This molecule, termed 2′3′-cGAMP, is unique in that it binds to the adaptor protein STING with a much greater affinity than cGAMP molecules containing other combinations of phosphodiester linkages. The crystal structure of STING bound to 2′3′-cGAMP revealed the structural basis of this high-affinity binding and a ligand-induced conformational change in STING that may underlie its activation.

Detection of cytoplasmic DNA represents one of the most fundamental mechanisms of the innate immune system to sense the presence of microbial pathogens1. Moreover, erroneous detection of endogenous DNA by the same sensing mechanisms has an important pathophysiological role in certain sterile inflammatory conditions2, 3. The endoplasmic-reticulum-resident protein STING is critically required for the initiation of type I interferon signalling upon detection of cytosolic DNA of both exogenous and endogenous origin4, 5, 6, 7, 8. Next to its pivotal role in DNA sensing, STING also serves as a direct receptor for the detection of cyclic dinucleotides, which function as second messenger molecules in bacteria9, 10, 11, 12, 13. DNA recognition, however, is triggered in an indirect fashion that depends on a recently characterized cytoplasmic nucleotidyl transferase, termed cGAMP synthase (cGAS), which upon interaction with DNA synthesizes a dinucleotide molecule that in turn binds to and activates STING14, 15. We here show in vivo and in vitro that the cGAS-catalysed reaction product is distinct from previously characterized cyclic dinucleotides. Using a combinatorial approach based on mass spectrometry, enzymatic digestion, NMR analysis and chemical synthesis we demonstrate that cGAS produces a cyclic GMP-AMP dinucleotide, which comprises a 2′-5′ and a 3′-5′ phosphodiester linkage >Gp(2′-5′)Ap(3′-5′)>. We found that the presence of this 2′-5′ linkage was required to exert potent activation of human STING. Moreover, we show that cGAS first catalyses the synthesis of a linear 2′-5′-linked dinucleotide, which is then subject to cGAS-dependent cyclization in a second step through a 3′-5′ phosphodiester linkage. This 13-membered ring structure defines a novel class of second messenger molecules, extending the family of 2′-5′-linked antiviral biomolecules.

The presence of foreign DNA in the cytosol of mammalian cells elicits a potent antiviral interferon response. Recently, cytosolic DNA was proposed to induce the synthesis of cyclic GMP-AMP (cGAMP) upon binding to an enzyme called cGAMP synthase (cGAS). cGAMP activates an interferon response by binding to a downstream receptor called STING. Here, we identify natural variants of human STING (hSTING) that are poorly responsive to cGAMP yet,unexpectedly, are normally responsive to DNA andcGAS signaling. We explain this paradox by demonstrating that the cGAS product is actually a noncanonical cyclic dinucleotide, cyclic [G(2'-5')pA(3'-5')p], which contains a single 2'-5' phosphodiester bond. Cyclic [G(2'-5')pA(3'-5')p] potently activates diverse hSTING receptors and, therefore, may be a useful adjuvant or immunotherapeutic. Our results indicate that hSTING variants have evolved to distinguish conventional (3'-5') cyclic dinucleotides, known to be produced mainly by bacteria, from the noncanonical cyclic dinucleotide produced by mammalian cGAS. © 2013 The Authors.

SummaryInnate immune recognition of foreign nucleic acids induces protective interferon responses. Detection of cytosolic DNA triggers downstream immune signaling through activation of cyclic GMP-AMP synthase (cGAS). We report here the crystal structure of human cGAS, revealing an unanticipated zinc-ribbon DNA-binding domain appended to a core enzymatic nucleotidyltransferase scaffold. The catalytic core of cGAS is structurally homologous to the RNA-sensing enzyme, 2′-5′ oligo-adenylate synthase (OAS), and divergent C-terminal domains account for specific ligand-activation requirements of each enzyme. We show that the cGAS zinc ribbon is essential for STING-dependent induction of the interferon response and that conserved amino acids displayed within the intervening loops are required for efficient cytosolic DNA recognition. These results demonstrate that cGAS and OAS define a family of innate immunity sensors and that structural divergence from a core nucleotidyltransferase enables second-messenger responses to distinct foreign nucleic acids.

The presence of DNA in the cytoplasm of mammalian cells is a danger signal that triggers host immune responses such as the production of type I interferons. Cytosolic DNA induces interferons through the production of cyclic guanosine monophosphate–adenosine monophosphate (cyclic GMP-AMP, or cGAMP), which binds to and activates the adaptor protein STING. Through biochemical fractionation and quantitative mass spectrometry, we identified a cGAMP synthase (cGAS), which belongs to the nucleotidyltransferase family. Overexpression of cGAS activated the transcription factor IRF3 and induced interferon-β in a STING-dependent manner. Knockdown of cGAS inhibited IRF3 activation and interferon-β induction by DNA transfection or DNA virus infection. cGAS bound to DNA in the cytoplasm and catalyzed cGAMP synthesis. These results indicate that cGAS is a cytosolic DNA sensor that induces interferons by producing the second messenger cGAMP.

Cytosolic DNA induces type I interferons and other cytokines that are important for antimicrobial defense but can also result in autoimmunity. This DNA signaling pathway requires the adaptor protein STING and the transcription factor IRF3, but the mechanism of DNA sensing is unclear. We found that mammalian cytosolic extracts synthesized cyclic guanosine monophosphate-adenosine monophosphate (cyclic GMP-AMP, or cGAMP) in vitro from adenosine triphosphate and guanosine triphosphate in the presence of DNA but not RNA. DNA transfection or DNA virus infection of mammalian cells also triggered cGAMP production. cGAMP bound to STING, leading to the activation of IRF3 and induction of interferon-β. Thus, cGAMP functions as an endogenous second messenger in metazoans and triggers interferon production in response to cytosolic DNA.

The recognition of microbial nucleic acids is a major mechanism by which the immune system detects pathogens. Cyclic GMP-AMP (cGAMP) synthase (cGAS) is a cytosolic DNA sensor that activates innate immune responses through production of the second messenger cGAMP, which activates the adaptor STING. The cGAS-STING pathway not only mediates protective immune defense against infection by a large variety of DNA-containing pathogens but also detects tumor-derived DNA and generates intrinsic antitumor immunity. However, aberrant activation of the cGAS pathway by self DNA can also lead to autoimmune and inflammatory disease. Thus, the cGAS pathway must be properly regulated. Here we review the recent advances in understanding of the cGAS-STING pathway, focusing on the regulatory mechanisms and roles of this pathway in heath and disease.

Objective Type I interferon (IFN) is implicated in the pathogenesis of systemic lupus erythematosus (SLE) and interferonopathies such as Aicardi-Goutieres syndrome. A recently discovered DNA-activated type I IFN pathway, cyclic GMP-AMP synthase (cGAS), has been linked to Aicardi-Goutieres syndrome and mouse models of lupus. The aim of this study was to determine whether the cGAS pathway contributes to type I IFN production in patients with SLE. Methods SLE disease activity was measured by the Safety of Estrogens in Lupus Erythematosus National Assessment version of the Systemic Lupus Erythematosus Disease Activity Index. Expression of messenger RNA for cGAS and IFN-stimulated genes (ISGs) was determined by quantitative polymerase chain reaction analysis. Cyclic GMP-AMP (cGAMP) levels were examined by multiple reaction monitoring with ultra-performance liquid chromatography tandem mass spectrometry. Results Expression of cGAS in peripheral blood mononuclear cells (PBMCs) was significantly higher in SLE patients than in normal controls (n = 51 and n = 20 respectively; P

The latency-associated nuclear antigen (LANA) of Kaposi sarcoma herpesvirus (KSHV) is mainly localized and functions in the nucleus of latently infected cells, playing a pivotal role in the replication and maintenance of latent viral episomal DNA. In addition, N-terminally truncated cytoplasmic isoforms of LANA, resulting from internal translation initiation, have been reported, but their function is unknown. Using coimmunoprecipitation and MS, we found the cGMP-AMP synthase (cGAS), an innate immune DNA sensor, to be a cellular interaction partner of cytoplasmic LANA isoforms. By directly binding to cGAS, LANA, and particularly, a cytoplasmic isoform, inhibit the cGAS-STING-dependent phosphorylation of TBK1 and IRF3 and thereby antagonize the cGAS-mediated restriction of KSHV lytic replication. We hypothesize that cytoplasmic forms of LANA, whose expression increases during lytic replication, inhibit cGAS to promote the reactivation of the KSHV from latency. This observation points to a novel function of the cytoplasmic isoforms of LANA during lytic replication and extends the function of LANA from its role during latency to the lytic replication cycle.

Interferons and the IL-1 family of cytokines have important roles in host defense against invading viruses and bacteria. Inflammasomes, multimeric cytosolic sensors of infection, are required for IL-1β and IL-18 processing and release. Interferons, IL-1β, and IL-18 are also implicated in autoimmune disease and chronic inflammation. Although independent but complementary pathways induce these cytokine subsets during infection, in some circumstances the cross-talk between these key inflammatory mediators is a particular requirement for effective host defense. In this review we will summarize recent discoveries concerning the potentiation of inflammasome responses by type I interferons, particularly in patients with gram-negative bacterial infections, and reflect on the molecular mechanisms of IFN-β's immunosuppressive effects through modulation of inflammasome and IL-1β signaling in patients with tuberculosis and multiple sclerosis.

Summary STING is an essential signaling molecule for DNA and cyclic di-GMP (c-di-GMP)-mediated type I interferon (IFN) production via TANK-binding kinase 1 (TBK1) and interferon regulatory factor 3 (IRF3) pathway. It contains an N-terminal transmembrane region and a cytosolic C-terminal domain (CTD). Here, we describe crystal structures of STING CTD alone and complexed with c-di-GMP in a unique binding mode. The strictly conserved aa 153–173 region was shown to be cytosolic and participated in dimerization via hydrophobic interactions. The STING CTD functions as a dimer and the dimerization was independent of posttranslational modifications. Binding of c-di-GMP enhanced interaction of a shorter construct of STING CTD (residues 139–344) with TBK1. This suggests an extra TBK1 binding site, other than serine 358. This study provides a glimpse into the unique architecture of STING and sheds light on the mechanism of c-di-GMP-mediated TBK1 signaling.

Detection of foreign materials is the first step of successful immune responses. Stimulator of interferon genes (STING) was shown to directly bind cyclic diguanylate monophosphate (c-di-GMP), a bacterial second messenger, and to elicit strong interferon responses. Here we elucidate the structural features of the cytosolic c-di-GMP binding domain (CBD) of STING and its complex with c-di-GMP. The CBD exhibits an α + β fold and is a dimer in the crystal and in solution. Surprisingly, one c-di-GMP molecule binds to the central crevice of a STING dimer, using a series of stacking and hydrogen bonding interactions. We show that STING is autoinhibited by an intramolecular interaction between the CBD and the C-terminal tail (CTT) and that c-di-GMP releases STING from this autoinhibition by displacing the CTT. The structures provide a remarkable example of pathogen-host interactions in which a unique microbial molecule directly engages the innate immune system.

SummaryThe type I interferon (IFN) response initiated by detection of nucleic acids is important for antiviral defense but is also associated with specific autoimmune diseases. Mutations in the human 3′ repair exonuclease 1 (Trex1) gene cause Aicardi-Goutières syndrome (AGS), an IFN-associated autoimmune disease. However, the source of the type I IFN response and the precise mechanisms of disease in AGS remain unknown. Here, we demonstrate that Trex1 is an essential negative regulator of the STING-dependent antiviral response. We used an in vivo reporter of IFN activity in Trex1-deficient mice to localize the initiation of disease to nonhematopoietic cells. These IFNs drove T cell-mediated inflammation and an autoantibody response that targeted abundant, tissue-restricted autoantigens. However, B cells contributed to mortality independently of T cell-mediated tissue damage. These findings reveal a stepwise progression of autoimmune disease in Trex1-deficient mice, with implications for the treatment of AGS and related disorders.

The type I interferon response protects cells against invading viral pathogens. The cellular factors that mediate this defence are the products of interferon-stimulated genes (ISGs). Although hundreds of ISGs have been identified since their discovery more than 25 years ago, only a few have been characterized with respect to antiviral activity. For most ISG products, little is known about their antiviral potential, their target specificity and their mechanisms of action. Using an overexpression screening approach, here we show that different viruses are targeted by unique sets of ISGs. We find that each viral species is susceptible to multiple antiviral genes, which together encompass a range of inhibitory activities. To conduct the screen, more than 380 human ISGs were tested for their ability to inhibit the replication of several important human and animal viruses, including hepatitis C virus, yellow fever virus, West Nile virus, chikungunya virus, Venezuelan equine encephalitis virus and human immunodeficiency virus type-1. Broadly acting effectors included IRF1, C6orf150 (also known as MB21D1), HPSE, RIG-I (also known as DDX58), MDA5 (also known as IFIH1) and IFITM3, whereas more targeted antiviral specificity was observed with DDX60, IFI44L, IFI6, IFITM2, MAP3K14, MOV10, NAMPT (also known as PBEF1), OASL, RTP4, TREX1 and UNC84B (also known as SUN2). Combined expression of pairs of ISGs showed additive antiviral effects similar to those of moderate type I interferon doses. Mechanistic studies uncovered a common theme of translational inhibition for numerous effectors. Several ISGs, including ADAR, FAM46C, LY6E and MCOLN2, enhanced the replication of certain viruses, highlighting another layer of complexity in the highly pleiotropic type I interferon system.

Type I interferons (IFNs) are central regulators of the innate and adaptive immune responses to viral and bacterial infections. Type I IFNs are induced upon cytosolic detection of microbial nucleic acids, including DNA, RNA, and the bacterial second messenger cyclic-di-GMP (c-di-GMP). In addition, a recent study demonstrated that the intracellular bacterial pathogen Listeria monocytogenes stimulates a type I IFN response due to cytosolic detection of bacterially secreted c-di-AMP. The transmembrane signaling adaptor Sting (Tmem173, Mita, Mpys, Eris) has recently been implicated in the induction of type I IFNs in response to cytosolic DNA and/or RNA. However, the role of Sting in response to purified cyclic dinucleotides or during in vivo L. monocytogenes infection has not been addressed. In order to identify genes important in the innate immune response, we have been conducting a forward genetic mutagenesis screen in C57BL/6 mice using the mutagen N -ethyl- N -nitrosourea (ENU). Here we describe a novel mutant mouse strain, Goldenticket ( Gt ), that fails to produce type I IFNs upon L. monocytogenes infection. By genetic mapping and complementation experiments, we found that Gt mice harbor a single nucleotide variant (T596A) of Sting that functions as a null allele and fails to produce detectable protein. Analysis of macrophages isolated from Gt mice revealed that Sting is absolutely required for the type I interferon response to both c-di-GMP and c-di-AMP. Additionally, Sting is required for the response to c-di-GMP and L. monocytogenes in vivo . Our results provide new functions for Sting in the innate interferon response to pathogens.

The innate immune system is critical for the early detection of invading pathogens and for initiating cellular host defence countermeasures, which include the production of type I interferon (IFN)1–3. However, little is known about how the innate immune system is galvanized to respond to DNA-based microbes. Here we show that STING (stimulator of interferon genes) is critical for the induction of IFN by non-CpG intracellular DNA species produced by various DNA pathogens after infection4. Murine embryonic fibroblasts, as well as antigen presenting cells such as macrophages and dendritic cells (exposed to intracellular B-form DNA, the DNA virus herpes simplex virus 1 (HSV-1) or bacteria Listeria monocytogenes), were found to require STING to initiate effective IFN production. Accordingly, Sting-knockout mice were susceptible to lethal infection after exposure to HSV-1. The importance of STING in facilitating DNA-mediated innate immune responses was further evident because cytotoxic T-cell responses induced by plasmid DNA vaccination were reduced in Sting-deficient animals. In the presence of intracellular DNA, STING relocalized with TANK-binding kinase 1 (TBK1) from the endoplasmic reticulum to perinuclear vesicles containing the exocyst component Sec5 (also known as EXOC2). Collectively, our studies indicate that STING is essential for host defence against DNA pathogens such as HSV-1 and facilitates the adjuvant activity of DNA-based vaccines.

The cellular innate immune system is essential for recognizing pathogen infection and for establishing effective host defence. But critical molecular determinants responsible for facilitating an appropriate immune response-following infection with DNA and RNA viruses, for example-remain to be identified. Here we report the identification, following expression cloning, of a molecule (STING; stimulator of interferon genes) that appears essential for effective innate immune signalling processes. It comprises five putative transmembrane regions, predominantly resides in the endoplasmic reticulum and is able to activate both NF-kappaB and IRF3 transcription pathways to induce expression of type I interferon (IFN-alpha and IFN-beta ) and exert a potent anti-viral state following expression. In contrast, loss of STING rendered murine embryonic fibroblasts extremely susceptible to negative-stranded virus infection, including vesicular stomatitis virus. Further, STING ablation abrogated the ability of intracellular B-form DNA, as well as members of the herpesvirus family, to induce IFN-beta, but did not significantly affect the Toll-like receptor (TLR) pathway. Yeast two-hybrid and co-immunoprecipitation studies indicated that STING interacts with RIG-I and with SSR2 (also known as TRAPbeta), which is a member of the translocon-associated protein (TRAP) complex required for protein translocation across the endoplasmic reticulum membrane following translation. Ablation by RNA interference of both TRAPbeta and translocon adaptor SEC61beta was subsequently found to inhibit STING's ability to stimulate expression of IFN-beta. Thus, as well as identifying a regulator of innate immune signalling, our results imply a potential role for the translocon in innate signalling pathways activated by select viruses as well as intracellular DNA.

Central to innate immunity is the sensing of pathogen-associated molecular patterns by cytosolic and membrane-associated receptors. In particular, DNA is a potent activator of immune responses during infection or tissue damage, and evidence indicates that, in addition to the membrane-associated Toll-like receptor 9, an unidentified cytosolic DNA sensor(s) can activate type I interferon (IFN) and other immune responses. Here we report on a candidate DNA sensor, previously named DLM-1 (also called Z-DNA binding protein 1 (ZBP1)), for which biological function had remained unknown; we now propose the alternative name DAI (DNA-dependent activator of IFN-regulatory factors). The artificial expression of otherwise IFN-inducible DAI (DLM-1/ZBP1) in mouse fibroblasts selectively enhances the DNA-mediated induction of type I IFN and other genes involved in innate immunity. On the other hand, RNA interference of messenger RNA for DAI (DLM-1/ZBP1) in cells inhibits this gene induction programme upon stimulation by DNA from various sources. Moreover, DAI (DLM-1/ZBP1) binds to double-stranded DNA and, by doing so, enhances its association with the IRF3 transcription factor and the TBK1 serine/threonine kinase. These observations underscore an integral role of DAI (DLM-1/ZBP1) in the DNA-mediated activation of innate immune responses, and may offer new insight into the signalling mechanisms underlying DNA-associated antimicrobial immunity and autoimmune disorders.

TREX1 is an exonuclease that digests DNA in the cytoplasm. Loss-of-function mutations of TREX1 are linked to Aicardi-Goutieres Syndrome (AGS) and systemic lupus erythematosus (SLE) in humans. Trex1(-/-) mice exhibit autoimmune and inflammatory phenotypes that are associated with elevated expression of interferon (IFN)-induced genes (ISGs). Cyclic GMP-AMP (cGAMP) synthase (cGAS) is a cytosolic DNA sensor that activates the IFN pathway. Upon binding to DNA, cGAS is activated to catalyze the synthesis of cGAMP, which functions as a second messenger that binds and activates the adaptor protein STING to induce IFNs and other cytokines. Here we show that genetic ablation of cGas in Trex1(-/-) mice eliminated all detectable pathological and molecular phenotypes, including ISG induction, autoantibody production, aberrant T-cell activation, and lethality. Even deletion of just one allele of cGas largely rescued the phenotypes of Trex1(-/-) mice. Similarly, deletion of cGas in mice lacking DNaseII, a lysosomal enzyme that digests DNA, rescued the lethal autoimmune phenotypes of the DNaseII(-/-) mice. Through quantitative mass spectrometry, we found that cGAMP accumulated in mouse tissues deficient in Trex1 or DNaseII and that this accumulation was dependent on cGAS. These results demonstrate that cGAS activation causes the autoimmune diseases in Trex1(-/-) and DNaseII(-/-) mice and suggest that inhibition of cGAS may lead to prevention and treatment of some human autoimmune diseases caused by self-DNA.

Rheumatoid arthritis (RA) is characterized by inflammatory cell infiltration, fibroblast-like synoviocytes (FLS) invasive proliferation, and joint destruction. Cyclic GMP-AMP synthase (cGAS) is a cytosolic DNA sensor that induces immune activation. In this study, we examined whether cGAS plays a role in RA FLS. In this study, cGAS was overexpressed in RA-FLS compared with OA FLS. TNFαstimulation induced cGAS expression in RA FLS. Overexpression of cGAS promoted the proliferation and knockdown of cGAS inhibited the proliferation of RA FLS. cGAS overexpression enhanced the production of proinflammatory cytokines and matrix metalloproteinases (MMPs) as well as AKT and ERK phosphorylation in TNFα-stimulated FLS. In contrast, cGAS silencing inhibited production of proinflammatory cytokines and matrix metalloproteinases (MMPs) as well as AKT and ERK phosphorylation in TNFα-stimulated FLS. These results suggest that cGAS activates the AKT and ERK pathways to promote the inflammatory response of RA FLS, and the development of strategies targeting cGAS may have therapeutic potential for human RA.

SummaryNucleic acid recognition upon viral infection triggers type I interferon production. Viral RNA is detected by both endosomal, TLR-dependent and cytosolic, RIG-I/MDA5-dependent pathways. TLR9 is the only known sensor of foreign DNA; it is unknown whether innate immune recognition of DNA exists in the cytosol. Here we present evidence that cytosolic DNA activates a potent type I interferon response to the invasive bacterium Listeria monocytogenes. The noninvasive Legionella pneumophila triggers an identical response through its type IV secretion system. Activation of type I interferons by cytosolic DNA is TLR independent and requires IRF3 but occurs without detectable activation of NF-κB and MAP kinases. Microarray analyses reveal a unique but overlapping gene-expression program activated by cytosolic DNA compared to TLR9- and RIG-I/MDA5-dependent responses. These findings define an innate immune response to DNA linked to type I interferon production.

Type I IFN is strongly implicated in the pathogenesis of systemic autoimmune diseases, such as lupus, and rare monogenic IFNopathies, including Aicardi–Goutières syndrome. Recently, a new DNA-activated pathway involving the enzyme cyclic GMP-AMP synthase (cGAS) was described and potentially linked to Aicardi–Goutières syndrome. To identify drugs that could potentially inhibit cGAS activity, we performed in silico screening of drug libraries. By computational analysis, we identified several antimalarial drugs (AMDs) that were predicted to interact with the cGAS/dsDNA complex. Our studies validated that several AMDs were effective inhibitors of IFN-β production and that they functioned by inhibiting dsDNA stimulation of cGAS. Because AMDs have been widely used in human diseases and have an excellent safety profile, our findings suggest new therapeutic strategies for the treatment of severe debilitating diseases associated with type I IFNs due to cGAS activation.

Cyclic GMP–AMP synthase (cGAS), which belongs to the nucleotidyltransferase family, recognizes cytosolic DNA and induces the type I interferon (IFN) pathway through the synthesis of the second messenger cGAMP. In this study, porcine cGAS (p-cGAS) was identified and its tissue distribution, subcellular localization, and functions in innate immunity were characterized. The coding sequence of p-cGAS is 1494 bp long, encodes 497 amino acids, and is most similar (74%) to Bos taurus cGAS. p-cGAS mRNA is abundant in the spleen, duodenum, jejunum, and ileum. The subcellular distribution of p-cGAS is not only in the cytosol, but also on the endoplasmic reticulum (ER) membrane. The overexpression of wild-type p-cGAS in porcine kidney epithelial cells, but not its catalytically inactive mutants, induced IFN-β expression, which was dependent on STING and IRF3. However, the downregulation of p-cGAS by RNA interference markedly reduced IFN-β expression after pseudorabies virus (PRV) infection or poly(dA:dT) transfection. These results demonstrate that p-cGAS is an important DNA sensor, required for IFN-β activation.

Summary Cyclic dinucleotides (CDNs) play key roles as second messengers and signaling molecules in bacteria and metazoans. The newly identified dinucleotide cyclase in Vibrio cholerae (DncV) produces three different CDNs containing two 3′–5′ phosphodiester bonds, and its predominant product is cyclic GMP-AMP, whereas mammalian cyclic GMP-AMP synthase (cGAS) produces only cyclic GMP-AMP containing mixed 2′–5′ phosphodiester bonds. We report the crystal structures of V. cholerae and Escherichia coli DncV in complex with various nucleotides in the pre-reaction states. The high-resolution structures revealed that DncV preferably recognizes ATP and GTP as acceptor and donor nucleotides, respectively, in the first nucleotidyl transfer reaction. Considering the recently reported intermediate structures, our pre-reaction state structures provide the precise mechanism of 3′–5′ linked cyclic AMP-GMP production in bacteria. A comparison with cGAS in the pre-reaction states suggests that the orientation of the acceptor nucleotide primarily determines the distinct linkage specificities between DncV and cGAS.

The crystal structures of human STING in the apo and c-di-GMP–bound states, supported by mutagenesis and biochemical data, reveal that c-di-GMP binds to preformed dimeric STING. c-di-GMP prolongs STING phosphorylation in vitro, which may contribute to downstream IFN signaling. These findings aid in understanding the innate immune response to bacterial infection. STING (stimulator of interferon genes) is an innate immune sensor of cyclic dinucleotides that regulates the induction of type I interferons. STING's C-terminal domain forms a V-shaped dimer and binds a cyclic diguanylate monophosphate (c-di-GMP) at the dimer interface by both direct and solvent-mediated hydrogen bonds. Guanines of c-di-GMP stack against the phenolic rings of a conserved tyrosine, and mutations at the c-di-GMP binding surface reduce nucleotide binding and affect signaling.

Retroviruses can selectively trigger an array of innate immune responses through various PRR. The identification and the characterization of the molecular basis of retroviral DNA sensing by the DNA sensors IFI16 and cGAS has been one of the most exciting developments in viral immunology in recent years. DNA sensing by these cytosolic sensors not only leads to the initiation of the type I interferon (IFN) antiviral response and the induction of the inflammatory response, but also triggers cell death mechanisms including pyroptosis and apoptosis in retrovirus-infected cells, thereby providing important insights into the pathophysiology of chronic retroviral infection. Host restriction factors such as SAMHD1 and Trex1 play important roles in regulating innate immune sensing, and have led to the idea that innate immune defense and host restriction actually converge at different levels to determine the outcome of retroviral infection. In this review, we discuss the sensing of retroviruses by cytosolic DNA sensors, the relevance of host factors during retroviral infection, and the interplay between host factors and the innate antiviral response in different cell types, within the context of two human pathogenic retroviruses – human immunodeficiency virus (HIV-1) and human T cell-leukemia virus type I (HTLV-1).

IFN-β has been implicated as an effector of oviduct pathology resulting from genital chlamydial infection in the mouse model. In this study, we investigated the role of cytosolic DNA and engagement of DNA sensors in IFN-β expression during chlamydial infection. We determined that three-prime repair exonuclease-1, a host 3′ to 5′ exonuclease, reduced IFN-β expression significantly during chlamydial infection using small interfering RNA and gene knockout fibroblasts, implicating cytosolic DNA as a ligand for this response. The DNA sensor cyclic GMP–AMP synthase (cGAS) has been shown to bind cytosolic DNA to generate cyclic GMP–AMP, which binds to the signaling adaptor stimulator of IFN genes (STING) to induce IFN-β expression. We determined that cGAS is required for IFN-β expression during chlamydial infection in multiple cell types. Interestingly, although infected cells deficient for STING or cGAS alone failed to induce IFN-β, coculture of cells depleted for either STING or cGAS rescued IFN-β expression. These data demonstrate that cyclic GMP–AMP produced in infected cGAS+STING− cells can migrate into adjacent cells via gap junctions to function in trans in cGAS−STING+ cells. Furthermore, we observed cGAS localized in punctate regions on the cytosolic side of the chlamydial inclusion membrane in association with STING, indicating that chlamydial DNA is most likely recognized outside the inclusion as infection progresses. These novel findings provide evidence that cGAS-mediated DNA sensing directs IFN-β expression during Chlamydia trachomatis infection and suggest that effectors from infected cells can directly upregulate IFN-β expression in adjacent uninfected cells during in vivo infection, contributing to pathogenesis.

The innate immune system utilizes pattern-recognition receptors (PRRs) to detect the invasion of pathogens and initiate host antimicrobial responses such as the production of type I interferons and proinflammatory cytokines. Nucleic acids, which are essential genetic information carriers for all living organisms including viral, bacterial, and eukaryotic pathogens, are major structures detected by the innate immune system. However, inappropriate detection of self nucleic acids can result in autoimmune diseases. PRRs that recognize nucleic acids in cells include several endosomal members of the Toll-like receptor family and several cytosolic sensors for DNA and RNA. Here, we review the recent advances in understanding the mechanism of nucleic acid sensing and signaling in the cytosol of mammalian cells as well as the emerging role of cytosolic nucleic acids in autoimmunity.

The type I interferon (IFN) response protects cells from viral infection by inducing hundreds of interferon-stimulated genes (ISGs), some of which encode direct antiviral effectors. Recent screening studies have begun to catalogue ISGs with antiviral activity against several RNA and DNA viruses. However, antiviral ISG specificity across multiple distinct classes of viruses remains largely unexplored. Here we used an ectopic expression assay to screen a library of more than 350 human ISGs for effects on 14 viruses representing 7 families and 11 genera. We show that 47 genes inhibit one or more viruses, and 25 genes enhance virus infectivity. Comparative analysis reveals that the screened ISGs target positive-sense single-stranded RNA viruses more effectively than negative-sense single-stranded RNA viruses. Gene clustering highlights the cytosolic DNA sensor cyclic GMP-AMP synthase (cGAS, also known as MB21D1) as a gene whose expression also broadly inhibits several RNA viruses. In vitro, lentiviral delivery of enzymatically active cGAS triggers a STING-dependent, IRF3-mediated antiviral program that functions independently of canonical IFN/STAT1 signalling. In vivo, genetic ablation of murine cGAS reveals its requirement in the antiviral response to two DNA viruses, and an unappreciated contribution to the innate control of an RNA virus. These studies uncover new paradigms for the preferential specificity of IFN-mediated antiviral pathways spanning several virus families.

Cyclic GMP-AMP synthase (cGAS) plays a central role in sensing aberrantly localized cytoplasmic dsDNA. Upon binding dsDNA, cGAS synthesizes the cyclic dinucleotide cGAMP, which acts as a second messenger to initiate a signaling pathway for the induction of interferon genes. Given that cytoplasmic DNA can originate from bacteria or virus, cGAS is essential for innate immunity against pathogenic infection. Conversely, cGAS detection of mislocalized nuclear or mitochondrial DNA can lead to hyper-inflammation. Recent studies indicate that cGAS can potentiate autoimmunity in cases of lupus, Aicardi-Goutières syndrome, and other related diseases—suggesting that targeting cGAS activity may be therapeutically beneficial. We report the discovery of a first-in-class generation of cGAS small molecule inhibitor, RU.365, identified in a high-throughput screen. Its structure-activity relationship, and selectivity and potency in cellular assays, was characterized. Moreover, we solved the structure of cGAS in complex with dsDNA and RU.365 or its close structural analog within the active site. Finally, we demonstrate that the inhibitor can reduce constitutive expression of interferonin macrophages derived from a mouse model of autoimmunity. RU.365 will be useful towards understanding the biological roles of cGAS and can serve as a molecular scaffold for development of future autoimmune therapies. Note: This abstract was not presented at the conference. Citation Format: Pu Gao. Small molecule inhibition of cyclic GMP-AMP synthase reduces interferon expression in macrophages from an autoimmune mouse model [abstract]. In: Proceedings of the Second CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; 2016 Sept 25-28; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(11 Suppl):Abstract nr B145.

Alarm Bells The presence of DNA in the cytosol of mammalian cells is a danger signal, indicating, for example, that a DNA-containing virus has infected the cell. This signal triggers an innate immune response, which involves the expression of type I interferons, and is critical for antiviral immunity and responses to DNA vaccines. Cyclic GMP-AMP synthase (cGAS) was recently identified as a sensor of cytosolic DNA. Li et al. (p. 1390 , published online 29 August) now use knockout mice to provide genetic evidence that, in multiple cell types, cGAS is the primary DNA sensor required for the type I interferon response in vivo.

HIV Detection Is a (c)GAS Despite it being one of the most highly studied viruses, there are still many unknowns when it comes to HIV—including how it triggers the innate immune response. Gao et al. (p. 903 , published 8 August) now demonstrate that the DNA sensor cyclic GMP-AMP synthase (cGAS) detects HIV infection. Reverse-transcribed HIV DNA triggers cGAS and downstream activation of antiviral immunity. Detection of HIV, as well as the retroviruses simian immunodeficiency virus and murine leukemia virus, was abrogated in mouse and human cells deficient in cGAS—suggesting that cGAS may be a critical activator of innate immunity in response to retroviral infection.

Cytosolic DNA arising from intracellular bacterial or viral infections is a powerful pathogen-associated molecular pattern (PAMP) that leads to innate immune host defence by the production of type I interferon and inflammatory cytokines. Recognition of cytosolic DNA by the recently discovered cyclic-GMP-AMP (cGAMP) synthase (cGAS) induces the production of cGAMP to activate the stimulator of interferon genes (STING). Here we report the crystal structure of cGAS alone and in complex with DNA, ATP and GTP along with functional studies. Our results explain the broad DNA sensing specificity of cGAS, show how cGAS catalyses dinucleotide formation and indicate activation by a DNA-induced structural switch. cGAS possesses a remarkable structural similarity to the antiviral cytosolic double-stranded RNA sensor 2'-5'oligoadenylate synthase (OAS1), but contains a unique zinc thumb that recognizes B-form double-stranded DNA. Our results mechanistically unify dsRNA and dsDNA innate immune sensing by OAS1 and cGAS nucleotidyl transferases.

Cytosolic double-stranded DNA (dsDNA) stimulates the production of type I interferon (IFN) through the endoplasmic reticulum (ER)-resident adaptor protein STING (stimulator of IFN genes), which activates the transcription factor interferon regulatory factor 3 (IRF3); however, how STING activates IRF3 is unclear. Here, we showed that STING stimulates phosphorylation of IRF3 by the kinase TBK1 (TANK-binding kinase 1) in an in vitro reconstitution system. With this system, we identified a carboxyl-terminal region of STING that was both necessary and sufficient to activate TBK1 and stimulate the phosphorylation of IRF3. We also found that STING interacted with both TBK1 and IRF3 and that mutations in STING that selectively disrupted its binding to IRF3 abrogated phosphorylation of IRF3 without impairing the activation of TBK1. These results suggest that STING functions as a scaffold protein to specify and promote the phosphorylation of IRF3 by TBK1. This scaffolding function of STING (and possibly of other adaptor proteins) may explain why IRF3 is activated in only a subset of signaling pathways that activate TBK1.

The innate immune system detects infection by using germline-encoded receptors that are specific for conserved microbial molecules. The recognition of microbial ligands leads to the production of cytokines, such as type I interferons (IFNs), that are essential for successful pathogen elimination. Cytosolic detection of pathogen-derived DNA is one major mechanism of inducing IFN production, and this process requires signalling through TANK binding kinase 1 (TBK1) and its downstream transcription factor, IFN-regulatory factor 3 (IRF3). In addition, a transmembrane protein called STING (stimulator of IFN genes; also known as MITA, ERIS, MPYS and TMEM173) functions as an essential signalling adaptor, linking the cytosolic detection of DNA to the TBK1-IRF3 signalling axis. Recently, unique nucleic acids called cyclic dinucleotides, which function as conserved signalling molecules in bacteria, have also been shown to induce a STING-dependent type I IFN response. However, a mammalian sensor of cyclic dinucleotides has not been identified. Here we report evidence that STING itself is an innate immune sensor of cyclic dinucleotides. We demonstrate that STING binds directly to radiolabelled cyclic diguanylate monophosphate (c-di-GMP), and we show that unlabelled cyclic dinucleotides, but not other nucleotides or nucleic acids, compete with c-di-GMP for binding to STING. Furthermore, we identify mutations in STING that selectively affect the response to cyclic dinucleotides without affecting the response to DNA. Thus, STING seems to function as a direct sensor of cyclic dinucleotides, in addition to its established role as a signalling adaptor in the IFN response to cytosolic DNA. Cyclic dinucleotides have shown promise as novel vaccine adjuvants and immunotherapeutics, and our results provide insight into the mechanism by which cyclic dinucleotides are sensed by the innate immune system.

The detection of intracellular microbial DNA is critical to appropriate innate immune responses; however, knowledge of how such DNA is sensed is limited. Here we identify IFI16, a PYHIN protein, as an intracellular DNA sensor that mediates the induction of interferon-β (IFN-β). IFI16 directly associated with IFN-β-inducing viral DNA motifs. STING, a critical mediator of IFN-β responses to DNA, was recruited to IFI16 after DNA stimulation. Lowering the expression of IFI16 or its mouse ortholog p204 by RNA-mediated interference inhibited gene induction and activation of the transcription factors IRF3 and NF-κB induced by DNA and herpes simplex virus type 1 (HSV-1). IFI16 (p204) is the first PYHIN protein to our knowledge shown to be involved in IFN-β induction. Thus, the PYHIN proteins IFI16 and AIM2 form a new family of innate DNA sensors we call 'AIM2-like receptors' (ALRs).

Bacterial (Interferon)ce Intracellular bacterial pathogens, such as Listeria monocytogenes , are detected in the cytosol of host immune cells, where they induce a host response that is often dependent on microbial secretion systems. Woodward et al. (p. 1703 , published online 27 May) now show that L. monocytogenes produce and release cyclic diadenosine monophosphate into the host cytosol, which induces the production of host type I interferon. Because a number of intracellular pathogens contain the protein machinery to generate this nucleotide and also activate this same innate immune pathway, a common molecular mechanism may exist for host detection of cytosolic bacterial pathogens.

Microbial nucleic acids are critical for the induction of innate immune responses, a host defense mechanism against infection by microbes. Recent studies have indicated that double-stranded DNA (dsDNA) induces potent innate immune responses via the induction of type I IFN (IFN) and IFN-inducible genes. However, the regulatory mechanisms underlying dsDNA-triggered signaling are not fully understood. Here we show that the translocation and assembly of the essential signal transducers, stimulator of IFN genes (STING) and TANK-binding kinase 1 (TBK1), are required for dsDNA-triggered innate immune responses. After sensing dsDNA, STING moves from the endoplasmic reticulum (ER) to the Golgi apparatus and finally reaches the cytoplasmic punctate structures to assemble with TBK1. The addition of an ER-retention signal to the C terminus of STING dampens its ability to induce antiviral responses. We also show that STING co-localizes with the autophagy proteins, microtubule-associated protein 1 light chain 3 (LC3) and autophagy-related gene 9a (Atg9a), after dsDNA stimulation. The loss of Atg9a, but not that of another autophagy-related gene (Atg7), greatly enhances the assembly of STING and TBK1 by dsDNA, leading to aberrant activation of the innate immune response. Hence Atg9a functions as a regulator of innate immunity following dsDNA stimulation as well as an essential autophagy protein. These results demonstrate that dynamic membrane traffic mediates the sequential translocation and assembly of STING, both of which are essential processes required for maximal activation of the innate immune response triggered by dsDNA.

We report here the identification and characterization of a protein, ERIS, an endoplasmic reticulum (ER) IFN stimulator, which is a strong type I IFN stimulator and plays a pivotal role in response to both non–self-cytosolic RNA and dsDNA. ERIS (also known as STING or MITA) resided exclusively on ER membrane. The ER retention/retrieval sequence RIR was found to be critical to retain the protein on ER membrane and to maintain its integrity. ERIS was dimerized on innate immune challenges. Coumermycin-induced ERIS dimerization led to strong and fast IFN induction, suggesting that dimerization of ERIS was critical for self-activation and subsequent downstream signaling.

Cytoplasmic DNA triggers activation of the innate immune system. Although 'downstream' signaling components have been characterized, the DNA-sensing components remain elusive. Here we present a systematic proteomics screen for proteins that associate with DNA, 'crossed' to a screen for transcripts induced by interferon-beta, which identified AIM2 as a candidate cytoplasmic DNA sensor. AIM2 showed specificity for double-stranded DNA. It also recruited the inflammasome adaptor ASC and localized to ASC 'speckles'. A decrease in AIM2 expression produced by RNA-mediated interference impaired DNA-induced maturation of interleukin 1beta in THP-1 human monocytic cells, which indicated that endogenous AIM2 is required for DNA recognition. Reconstitution of unresponsive HEK293 cells with AIM2, ASC, caspase-1 and interleukin 1beta showed that AIM2 was sufficient for inflammasome activation. Our data suggest that AIM2 is a cytoplasmic DNA sensor for the inflammasome.