STING cyclic dinucleotide sensing originated in bacteria

cGLRs Join Their Cousins of Pattern Recognition Receptor Family to Regulate Immune Homeostasis

Pattern recognition receptors (PRRs) recognize danger signals such as PAMPs/MAMPs and DAMPs to initiate a protective immune response. TLRs, NLRs, CLRs, and RLRs are well-characterized PRRs of the host immune system. cGLRs have been recently identified as PRRs. In humans, the cGAS/STING signaling pathway is a part of cGLRs. cGAS recognizes cytosolic dsDNA as a PAMP or DAMP to initiate the STING-dependent immune response comprising type 1 IFN release, NF-κB activation, autophagy, and cellular senescence. The present article discusses the emergence of cGLRs as critical PRRs and how they regulate immune responses. We examined the role of cGAS/STING signaling, a well-studied cGLR system, in the activation of the immune system. The following sections discuss the role of cGAS/STING dysregulation in disease and how immune cross-talk with other PRRs maintains immune homeostasis. This understanding will lead to the design of better vaccines and immunotherapeutics for various diseases, including infections, autoimmunity, and cancers.

Tracing the evolutionary origins of antiviral immunity

Animal and bacterial cells use shared mechanisms to defend against viruses. Analyzing 3 families of immune genes, a new study in PLOS Biology illuminates this evolutionary connection and traces the emergence of antiviral signaling across domains of life.

cGAS-STING-mediated novel nonclassic antiviral activities

Stimulatorof interferon genes (STING) is an intracellular sensor of cyclic dinucleotides involved in the innate immune response against pathogen- or self-derived DNA. For years, interferon (IFN) induction of cyclic GMP-AMP synthase (cGAS)-STING has been considered as a canonical pattern defending the host from viral invasion. The mechanism of the cGAS-STING-IFN pathway has been well-illustrated. However, other signalling cascades driven by cGAS-STING have emerged in recent years and some of them have been found to possess antiviral ability independent of IFN. Here, we summarize the current progress on cGAS-STING-mediated nonclassic antiviral activities with an emphasis on the nuclear factor-κB and autophagy pathways, which are the most-studied pathways. In addition, we briefly present the primordial function of the cGAS-STING pathway in primitive species to show the importance of IFN-unrelated antiviral activity from an evolutionary angle. Finally, we discuss open questions that need to be solved for further exploitation of this field.

Evolution of the Major Components of Innate Immunity in Animals

Innate immunity is present in all animals. In this review, we explore the main conserved mechanisms of recognition and innate immune responses among animals. In this sense, we discuss the receptors, critical for binding to pathogen-associated molecular patterns (PAMPs) or danger-associated molecular patterns (DAMPs); the downstream signaling proteins; and transcription factors that govern immune responses. We also highlight conserved inflammatory mediators that are induced after the recognition of DAMPs and PAMPs. At last, we discuss the mechanisms that are involved in the regulation and/or generation of reactive oxygen species (ROS), influencing immune responses, like heme-oxygenases (HOs).

Nanomaterial-encapsulated STING agonists for immune modulation in cancer therapy

The cGAS-STING signaling pathway has emerged as a critical mediator of innate immune responses, playing a crucial role in improving antitumor immunity through immune effector responses. Targeting the cGAS-STING pathway holds promise for overcoming immunosuppressive tumor microenvironments (TME) and promoting effective tumor elimination. However, systemic administration of current STING agonists faces challenges related to low bioavailability and potential adverse effects, thus limiting their clinical applicability. Recently, nanotechnology-based strategies have been developed to modulate TMEs for robust immunotherapeutic responses. The encapsulation and delivery of STING agonists within nanoparticles (STING-NPs) present an attractive avenue for antitumor immunotherapy. This review explores a range of nanoparticles designed to encapsulate STING agonists, highlighting their benefits, including favorable biocompatibility, improved tumor penetration, and efficient intracellular delivery of STING agonists. The review also summarizes the immunomodulatory impacts of STING-NPs on the TME, including enhanced secretion of pro-inflammatory cytokines and chemokines, dendritic cell activation, cytotoxic T cell priming, macrophage re-education, and vasculature normalization. Furthermore, the review offers insights into co-delivered nanoplatforms involving STING agonists alongside antitumor agents such as chemotherapeutic compounds, immune checkpoint inhibitors, antigen peptides, and other immune adjuvants. These platforms demonstrate remarkable versatility in inducing immunogenic responses within the TME, ultimately amplifying the potential for antitumor immunotherapy.

cGAS goes viral: A conserved immune defense system from bacteria to humans

To survive, all organisms need the ability to accurately recognize and neutralize pathogens. As a result, many of the fundamental strategies that our innate immune system uses to fight infection have deep evolutionary roots. The innate immune sensor cyclic-GMP-AMP synthase (cGAS), an enzyme that plays a critical role in our bodies by sensing and signaling in response to microbial infection, is broadly conserved and has functional homologs in many vertebrates, invertebrates, and even bacteria. In this review, we will provide an overview of cGAS and cGAS-like signaling in eukaryotes before discussing cGAS-like homologs in bacteria.

From bacteria to biomedicine: Developing therapies exploiting NAD+ metabolism

Nicotinamide adenine dinucleotide (NAD+) serves as a critical cofactor in cellular metabolism and redox reactions. Bacterial pathways rely on NAD+ participation, where its stability and concentration govern essential homeostasis and functions. This review delves into the role and metabolic regulation of NAD+ in bacteria, highlighting its influence on physiology and virulence. Notably, we explore enzymes linked to NAD+ metabolism as antibacterial drug targets and vaccine candidates. Moreover, we scrutinize NAD+'s medical potential, offering insights for its application in biomedicine. This comprehensive assessment informs future research directions in the dynamic realm of NAD+ and its biomedical significance.

cGAS-STING at the crossroads in cancer therapy

DNA is highly immunogenic, both exogenous and endogenous DNA can activate the pathogen-associated molecular pattern (PAMP) and danger-associated molecular pattern (DAMP), respectively, and hence activate the evolutionarily conserved cGAS-STING pathway for inflammatory responses. The cGAS-STING signaling pathway plays a very important role in the pathogenesis and progression of neoplastic diseases. For cancer therapy, there are some discrepancies on whether cGAS-STING should be inhibited or activated. Deregulated cGAS-STING signaling pathway might be the origin and pathogenesis of tumor, understanding and modulating cGAS-STING signaling holds great promise for cancer therapy. In this review article, we discuss the molecular mechanisms underlying cGAS-STING deregulation, highlighting the tumor inhibiting and promoting roles and challenges with cGAS-STING agonists in the context of cancer therapies.

Nucleic-acid-triggered NADase activation of a short prokaryotic Argonaute

Argonaute (Ago) proteins mediate RNA- or DNA-guided inhibition of nucleic acids1,2. Although the mechanisms used by eukaryotic Ago proteins and long prokaryotic Ago proteins (pAgos) are known, that used by short pAgos remains elusive. Here we determined the cryo-electron microscopy structures of a short pAgo and the associated TIR-APAZ proteins (SPARTA) from Crenotalea thermophila (Crt): a free-state Crt-SPARTA; a guide RNA-target DNA-loaded Crt-SPARTA; two Crt-SPARTA dimers with distinct TIR organization; and a Crt-SPARTA tetramer. These structures reveal that Crt-SPARTA is composed of a bilobal-fold Ago lobe that connects with a TIR lobe. Whereas the Crt-Ago contains a MID and a PIWI domain, Crt-TIR-APAZ has a TIR domain, an N-like domain, a linker domain and a trigger domain. The bound RNA-DNA duplex adopts a B-form conformation that is recognized by base-specific contacts. Nucleic acid binding causes conformational changes because the trigger domain acts as a 'roadblock' that prevents the guide RNA 5' ends and the target DNA 3' ends from reaching their canonical pockets; this disorders the MID domain and promotes Crt-SPARTA dimerization. Two RNA-DNA-loaded Crt-SPARTA dimers form a tetramer through their TIR domains. Four Crt-TIR domains assemble into two parallel head-to-tail-organized TIR dimers, indicating an NADase-active conformation, which is supported by our mutagenesis study. Our results reveal the structural basis of short-pAgo-mediated defence against invading nucleic acids, and provide insights for optimizing the detection of SPARTA-based programmable DNA sequences.

Structural insights into the regulation, ligand recognition, and oligomerization of bacterial STING

The cyclic GMP-AMP synthase (cGAS)/stimulator of interferon gene (STING) signaling pathway plays a critical protective role against viral infections. Metazoan STING undergoes multilayers of regulation to ensure specific signal transduction. However, the mechanisms underlying the regulation of bacterial STING remain unclear. In this study, we determined the crystal structure of anti-parallel dimeric form of bacterial STING, which keeps itself in an inactive state by preventing cyclic dinucleotides access. Conformational transition between inactive and active states of bacterial STINGs provides an on-off switch for downstream signaling. Some bacterial STINGs living in extreme environment contain an insertion sequence, which we show codes for an additional long lid that covers the ligand-binding pocket. This lid helps regulate anti-phage activities. Furthermore, bacterial STING can bind cyclic di-AMP in a triangle-shaped conformation via a more compact ligand-binding pocket, forming spiral-shaped protofibrils and higher-order fibril filaments. Based on the differences between cyclic-dinucleotide recognition, oligomerization, and downstream activation of different bacterial STINGs, we proposed a model to explain structure-function evolution of bacterial STINGs.

Acquired stress resilience through bacteria-to-nematode interdomain horizontal gene transfer

Natural selection drives the acquisition of organismal resilience traits to protect against adverse environments. Horizontal gene transfer (HGT) is an important evolutionary mechanism for the acquisition of novel traits, including metazoan acquisitions in immunity, metabolic, and reproduction function via interdomain HGT (iHGT) from bacteria. Here, we report that the nematode gene rml-3 has been acquired by iHGT from bacteria and that it enables exoskeleton resilience and protection against environmental toxins in Caenorhabditis elegans. Phylogenetic analysis reveals that diverse nematode RML-3 proteins form a single monophyletic clade most similar to bacterial enzymes that biosynthesize L-rhamnose, a cell-wall polysaccharide component. C. elegans rml-3 is highly expressed during larval development and upregulated in developing seam cells upon heat stress and during the stress-resistant dauer stage. rml-3 deficiency impairs cuticle integrity, barrier functions, and nematode stress resilience, phenotypes that can be rescued by exogenous L-rhamnose. We propose that interdomain HGT of an ancient bacterial rml-3 homolog has enabled L-rhamnose biosynthesis in nematodes, facilitating cuticle integrity and organismal resilience to environmental stressors during evolution. These findings highlight a remarkable contribution of iHGT on metazoan evolution conferred by the domestication of a bacterial gene.

Metabolite itaconate in host immunoregulation and defense

Metabolic states greatly influence functioning and differentiation of immune cells. Regulating the metabolism of immune cells can effectively modulate the host immune response. Itaconate, an intermediate metabolite derived from the tricarboxylic acid (TCA) cycle of immune cells, is produced through the decarboxylation of cis-aconitate by cis-aconitate decarboxylase in the mitochondria. The gene encoding cis-aconitate decarboxylase is known as immune response gene 1 (IRG1). In response to external proinflammatory stimulation, macrophages exhibit high IRG1 expression. IRG1/itaconate inhibits succinate dehydrogenase activity, thus influencing the metabolic status of macrophages. Therefore, itaconate serves as a link between macrophage metabolism, oxidative stress, and immune response, ultimately regulating macrophage function. Studies have demonstrated that itaconate acts on various signaling pathways, including Keap1-nuclear factor E2-related factor 2-ARE pathways, ATF3-IκBζ axis, and the stimulator of interferon genes (STING) pathway to exert antiinflammatory and antioxidant effects. Furthermore, several studies have reported that itaconate affects cancer occurrence and development through diverse signaling pathways. In this paper, we provide a comprehensive review of the role IRG1/itaconate and its derivatives in the regulation of macrophage metabolism and functions. By furthering our understanding of itaconate, we intend to shed light on its potential for treating inflammatory diseases and offer new insights in this field.

Innate immunity: the bacterial connection

Pathogens have fueled the diversification of intracellular defense strategies that collectively define cell-autonomous innate immunity. In bacteria, innate immunity is manifested by a broad arsenal of defense systems that provide protection against bacterial viruses, called phages. The complexity of the bacterial immune repertoire has only been realized recently and is now suggesting that innate immunity has commonalities across the tree of life: many components of eukaryotic innate immunity are found in bacteria where they protect against phages, including the cGAS-STING pathway, gasdermins, and viperins. Here, I summarize recent findings on the conservation of innate immune pathways between prokaryotes and eukaryotes and hypothesize that bacterial defense mechanisms can catalyze the discovery of novel molecular players of eukaryotic innate immunity.

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.

At the Crossroads of the cGAS-cGAMP-STING Pathway and the DNA Damage Response: Implications for Cancer Progression and Treatment

The evolutionary conserved DNA-sensing cGAS-STING innate immunity pathway represents one of the most important cytosolic DNA-sensing systems that is activated in response to viral invasion and/or damage to the integrity of the nuclear envelope. The key outcome of this pathway is the production of interferon, which subsequently stimulates the transcription of hundreds of genes. In oncology, the situation is complex because this pathway may serve either anti- or pro-oncogenic roles, depending on context. The prevailing understanding is that when the innate immune response is activated by sensing cytosolic DNA, such as DNA released from ruptured micronuclei, it results in the production of interferon, which attracts cytotoxic cells to destroy tumors. However, in tumor cells that have adjusted to significant chromosomal instability, particularly in relapsed, treatment-resistant cancers, the cGAS-STING pathway often supports cancer progression, fostering the epithelial-to-mesenchymal transition (EMT). Here, we review this intricate pathway in terms of its association with cancer progression, giving special attention to pancreatic ductal adenocarcinoma and gliomas. As the development of new cGAS-STING-modulating small molecules and immunotherapies such as oncolytic viruses involves serious challenges, we highlight several recent fundamental discoveries, such as the proton-channeling function of STING. These discoveries may serve as guiding lights for potential pharmacological advancements.

Single phage proteins sequester TIR- and cGAS-generated signaling molecules

Prokaryotic anti-phage immune systems use TIR (toll/interleukin-1 receptor) and cGAS (cyclic GMP-AMP synthase) enzymes to produce 1"-3'/1"-2' glycocyclic ADPR (gcADPR) and cyclid di-/trinucleotides (CDNs and CTNs) signaling molecules that limit phage replication, respectively 1-3. However, how phages neutralize these common systems is largely unknown. Here, we show that Thoeris anti-defense proteins Tad1 4 and Tad2 5 both have anti-CBASS activity by simultaneously sequestering CBASS cyclic oligonucleotides. Strikingly, apart from binding Thoeris signals 1"-3' and 1"-2' gcADPR, Tad1 also binds numerous CBASS CDNs/CTNs with high affinity, inhibiting CBASS systems using these molecules in vivo and in vitro. The hexameric Tad1 has six binding sites for CDNs or gcADPR, which are independent from two high affinity binding sites for CTNs. Tad2 also sequesters various CDNs in addition to gcADPR molecules, inhibiting CBASS systems using these CDNs. However, the binding pockets for CDNs and gcADPR are different in Tad2, whereby a tetramer can bind two CDNs and two gcADPR molecules simultaneously. Taken together, Tad1 and Tad2 are both two-pronged inhibitors that, alongside anti-CBASS protein 2, establish a paradigm of phage proteins that flexibly sequester a remarkable breadth of cyclic nucleotides involved in TIR- and cGAS-based anti-phage immunity.

MicroRNA-4691-3p inhibits the inflammatory response by targeting STING in human dental pulp cells: A laboratory investigation

AIM

The regulation of human dental pulp inflammation is not fully understood. This study aims to investigate the effect of miR-4691-3p on the cGAS-STING signalling cascade and its downstream cytokines production in human dental pulp cells (HDPCs).

METHODOLOGY

Normal dental pulp tissue and pulp tissue with irreversible pulpitis from third molars were collected. HDPCs were isolated from pulp tissue. The expression of STING mRNA and miR-4691-3p was measured by quantitative real-time PCR. Bioinformatic computation via TargetScanHuman 8.0 and a luciferase reporter assay was used to identify the targets of miR-4691-3p. A miR-4691-3p mimic and inhibitor were used to upregulate or downregulate miR-4691-3p expression in HDPCs. HDPCs were transfected with c-di-AMP, c-di-GMP, cGAMP, interferon stimulatory DNA (ISD) and bacterial genomic DNA. Immunoblot was performed to detect the phosphorylation of TBK1, p65 and IRF3. Enzyme-linked immunoassay was performed to detect the cytokines including IFN-β, TNF or IL-6 downstream of cGAS-STING.

RESULTS

MiR-4691-3p expression was increased in human dental pulp tissue with irreversible pulpitis. Treatment of HDPCs using recombinant human IFN-β, TNF or IL-6 also upregulated miR-4691-3p. The bioinformatic prediction and luciferase reporter assay confirmed that STING was a direct target of miR-4691-3p. The miR-4691-3p mimic suppressed STING expression, the phosphorylation of TBK1, p65 and IRF3, and the IFN-β, TNF or IL-6 production. In contrast, the miR-4691-3p inhibitor enhanced the STING expression, the phosphorylation of TBK1, p65 and IRF3 and the IFN-β, TNF or IL-6 production.

CONCLUSIONS

MiR-4691-3p negatively regulates the cGAS-STING pathway by directly targeting STING. This provides insight to utilize miRNA-dependent regulatory effect to treat endodontic disease as well as STING-dependent systemic inflammatory disease.

The role of cGAS-STING signaling in pulmonary fibrosis and its therapeutic potential

Pulmonary fibrosis is a progressive and ultimately fatal lung disease, exhibiting the excessive production of extracellular matrix and aberrant activation of fibroblast. While Pirfenidone and Nintedanib are FDA-approved drugs that can slow down the progression of pulmonary fibrosis, they are unable to reverse the disease. Therefore, there is an urgent demand to develop more efficient therapeutic approaches for pulmonary fibrosis. The intracellular DNA sensor called cyclic guanosine monophosphate-adenosine monophosphate (cGAMP) synthase (cGAS) plays a crucial role in detecting DNA and generating cGAMP, a second messenger. Subsequently, cGAMP triggers the activation of stimulator of interferon genes (STING), initiating a signaling cascade that leads to the stimulation of type I interferons and other signaling molecules involved in immune responses. Recent studies have highlighted the involvement of aberrant activation of cGAS-STING contributes to fibrotic lung diseases. This review aims to provide a comprehensive summary of the current knowledge regarding the role of cGAS-STING pathway in pulmonary fibrosis. Moreover, we discuss the potential therapeutic implications of targeting the cGAS-STING pathway, including the utilization of inhibitors of cGAS and STING.

GAS-STING: a classical DNA recognition pathways to tumor therapy

Cyclic GMP-AMP synthetase (cGAS), recognized as the primary DNA sensor within cells, possesses the capability to identify foreign DNA molecules along with free DNA fragments. This identification process facilitates the production of type I IFNs through the activator of the interferon gene (STING) which induces the phosphorylation of downstream transcription factors. This action characterizes the most archetypal biological functionality of the cGAS-STING pathway. When treated with anti-tumor agents, cells experience DNA damage that triggers activation of the cGAS-STING pathway, culminating in the expression of type I IFNs and associated downstream interferon-stimulated genes. cGAS-STING is one of the important innate immune pathways,the role of type I IFNs in the articulation between innate immunity and T-cell antitumour immunity.type I IFNs promote the recruitment and activation of inflammatory cells (including NK cells) at the tumor site.Type I IFNs also can promote the activation and maturation of dendritic cel(DC), improve the antigen presentation of CD4+T lymphocytes, and enhance the cross-presentation of CD8+T lymphocytes to upregulating anti-tumor responses. This review discussed the cGAS-STING signaling and its mechanism and biological function in traditional tumor therapy and immunotherapy.

GSDMD promotes neutrophil extracellular traps via mtDNA-cGAS-STING pathway during lung ischemia/reperfusion

Lung ischemia/reperfusion injury (LIRI) is a complex pathophysiological process, with the histopathological hallmark of neutrophils migrating into the lungs. Neutrophil extracellular traps (NETs) have been suggested to exert a critical role in the pathogenesis of inflammation and infection in humans and animals, while the exact functions and underlying mechanisms of NETs in LIRI remain insufficiently elucidated. In this study, we investigated the role of pore-forming protein gasdermin D (GSDMD) on NETs release in LIRI induced by lung ischemia/reperfusion (I/R). We found that disulfiram, a GSDMD inhibitor, dramatically reduced NETs release and pathological injury in lung I/R in vivo and in vitro. Additionally, GSDMD caused mitochondrial DNA (mtDNA) leaking into the neutrophil cytosol, and then the cytoplasmic mtDNA activated the cGAS-STING signaling pathway and stimulated NETs formation in lung I/R. Furthermore, inhibition of cGAS/STING pathway could inhibit cytosol mtDNA mediated NETs formation.

The highly diverse antiphage defence systems of bacteria

Bacteria and their viruses have coevolved for billions of years. This ancient and still ongoing arms race has led bacteria to develop a vast antiphage arsenal. The development of high-throughput screening methods expanded our knowledge of defence systems from a handful to more than a hundred systems, unveiling many different molecular mechanisms. These findings reveal that bacterial immunity is much more complex than previously thought. In this Review, we explore recently discovered bacterial antiphage defence systems, with a particular focus on their molecular diversity, and discuss the ecological and evolutionary drivers and implications of the existing diversity of antiphage defence mechanisms.

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.

The virus-induced cyclic dinucleotide 2'3'-c-di-GMP mediates STING-dependent antiviral immunity in Drosophila

In mammals, the enzyme cGAS senses the presence of cytosolic DNA and synthesizes the cyclic dinucleotide (CDN) 2'3'-cGAMP, which triggers STING-dependent immunity. In Drosophila melanogaster, two cGAS-like receptors (cGLRs) produce 3'2'-cGAMP and 2'3'-cGAMP to activate STING. We explored CDN-mediated immunity in 14 Drosophila species covering 50 million years of evolution and found that 2'3'-cGAMP and 3'2'-cGAMP failed to control infection by Drosophila C virus in D. serrata and two other species. We discovered diverse CDNs produced in a cGLR-dependent manner in response to viral infection in D. melanogaster, including 2'3'-c-di-GMP. This CDN was a more potent STING agonist than cGAMP in D. melanogaster and it also activated a strong antiviral transcriptional response in D. serrata. Our results shed light on the evolution of cGLRs in flies and provide a basis for understanding the function and regulation of this emerging family of pattern recognition receptors in animal innate immunity.

Advances in structure-guided mechanisms impacting on the cGAS-STING innate immune pathway

The metazoan cGAS-STING innate immunity pathway is triggered in response to cytoplasmic double-stranded DNA (dsDNA), thereby providing host defense against microbial pathogens. This pathway also impacts on autoimmune diseases, cellular senescence and anti-tumor immunity. The cGAS-STING pathway was also observed in the bacterial antiviral immune response, known as the cyclic oligonucleotide (CDN)-based anti-phage signaling system (CBASS). This review highlights a structure-based mechanistic perspective of recent advances in metazoan and bacterial cGAS-STING innate immune signaling by focusing on the cGAS sensor, cGAMP second messenger and STING adaptor components, thereby elucidating the specificity, activation, regulation and signal transduction features of the pathway.

TET2 inhibits the proliferation and metastasis of lung adenocarcinoma cells via activation of the cGAS-STING signalling pathway

BACKGROUND

Effective identification and development of new molecular methods for the diagnosis, treatment and prognosis of lung adenocarcinoma (LUAD) remains an urgent clinical need. DNA methylation patterns at cytosine bases in the genome are closely related to gene expression, and abnormal DNA methylation is frequently observed in various cancers. The ten-eleven translocation (TET) enzymes oxidize 5-methylcytosine (5mC) and promote locus-specific DNA methylation reversal. This study aimed to explore the role of the TET2 protein and its downstream effector, 5-hmC/5-mC DNA modification, in LUAD progression.

METHODS

The expression of TET2 was analysed by real-time PCR, Western blotting and immunohistochemistry. The 5-hmC DNA content was determined by a colorimetric kit. Activation of the cGAS-STING signalling pathway was evaluated by Western blotting. CCK-8, wound healing and Transwell assays were performed to evaluate the effect of TET2 on cell proliferation, migration and invasion abilities. A xenograft model was used to analyse the effect of TET2 on the tumorigenic ability of A549 cells.

RESULTS

TET2 overexpression decreased proliferation and metastasis of A549 and H1975 cells in vitro and in vivo. However, TET2 knockdown dramatically enhanced the proliferation, migration and invasion of A549 and H1975 cells. Mechanistically, activation of the cGAS-STING signalling pathway is critical for the TET2-mediated suppression of LUAD cell tumorigenesis and metastasis.

CONCLUSION

In this study, we demonstrate a tumour suppressor role of TET2 in LUAD, providing new potential molecular therapeutic targets and clinical therapies for patients with non-small cell lung cancer.

Recent trends in STING modulators: Structures, mechanisms, and therapeutic potential

The cyclic GMP-AMP synthase stimulator (cGAS)-stimulator of interferon gene (STING) signaling pathway has an integral role in the host immune response through DNA sensing followed by inducing a robust innate immune defense program. STING has become a promising therapeutic target associated with multiple diseases, including various inflammatory diseases, cancer, and infectious diseases, among others. Thus, modulators of STING are regarded as emerging therapeutic agents. Recent progress has been made in STING research, including recently identified STING-mediated regulatory pathways, the development of a new STING modulator, and the new association of STING with disease. In this review, we focus on recent trends in the development of STING modulators, including structures, mechanisms, and clinical application.

Structural insights into mechanisms of Argonaute protein-associated NADase activation in bacterial immunity

Nicotinamide adenine dinucleotide (NAD+) is a central metabolite in cellular processes. Depletion of NAD+ has been demonstrated to be a prevalent theme in both prokaryotic and eukaryotic immune responses. Short prokaryotic Argonaute proteins (Agos) are associated with NADase domain-containing proteins (TIR-APAZ or SIR2-APAZ) encoded in the same operon. They confer immunity against mobile genetic elements, such as bacteriophages and plasmids, by inducing NAD+ depletion upon recognition of target nucleic acids. However, the molecular mechanisms underlying the activation of such prokaryotic NADase/Ago immune systems remain unknown. Here, we report multiple cryo-EM structures of NADase/Ago complexes from two distinct systems (TIR-APAZ/Ago and SIR2-APAZ/Ago). Target DNA binding triggers tetramerization of the TIR-APAZ/Ago complex by a cooperative self-assembly mechanism, while the heterodimeric SIR2-APAZ/Ago complex does not assemble into higher-order oligomers upon target DNA binding. However, the NADase activities of these two systems are unleashed via a similar closed-to-open transition of the catalytic pocket, albeit by different mechanisms. Furthermore, a functionally conserved sensor loop is employed to inspect the guide RNA-target DNA base pairing and facilitate the conformational rearrangement of Ago proteins required for the activation of these two systems. Overall, our study reveals the mechanistic diversity and similarity of Ago protein-associated NADase systems in prokaryotic immune response.

PCSK9 causes inflammation and cGAS/STING pathway activation in diabetic nephropathy

Our previous research revealed that an increase in PCSK9 is linked to aggravated inflammation in the kidneys of mice affected by a high-fat diet and streptozotocin (HFD/STZ) or in HGPA-induced HK-2 cells. Furthermore, the cGAS/STING pathway has been reported to be involved in diabetic nephropathy (DN). Therefore, in this study, we aimed to examine the correlation between the proinflammatory effect of PCSK9 and the cGAS/STING pathway in DN. We used PCSK9 mAbs to inhibit PCSK9 in vivo and PCSK9 siRNA in vitro and measured the inflammatory phenotype in HFD/STZ-treated mice or HGPA-induced HK-2 cells, and observed decreased blood urea nitrogen, creatinine, UACR, and kidney injury in response to the PCSK9 mAb in HFD/STZ-treated mice. Moreover, IL-1 β, MCP-1, and TNF-α levels were reduced by the PCSK9 mAb in vivo and PCSK9 siRNA in vitro. We observed increased mtDNA damage and activation of the cGAS-STING signaling pathway during DN, as well as the downstream targets p-TBK1, p-NF-κB p65, and IL-1β. In a further experiment with an HGPA-induced DN model in HK-2 cells, we revealed that mtDNA damage was increased, which led to the activation of the cGAS/STING system and its downstream targets. Notably, the cGAS-STING signaling pathway was inhibited by the PCSK9 mAb in vivo and PCSK9 siRNA in vitro. In addition, inhibition of STING with C-176 in HGPA-induced HK-2 cells markedly blocked inflammation. In conclusion, we report for the first time that PCSK9 triggers mitochondrial DNA damage and activates the cGAS-STING pathway in DN, which leads to a series of inflammation cascades. PCSK9-targeted intervention can effectively reduce DN inflammation and delay its progression. Moreover, the inhibition of STING significantly abrogated the inflammation triggered by HGPA in HK-2 cells.

Eukaryotic antiviral immune proteins arose via convergence, horizontal transfer, and ancient inheritance

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 anti-phage 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 three 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 truly ancient immune proteins, likely inherited since the last eukaryotic common ancestor and possibly longer. In contrast, we find other immune proteins that arose via at least four independent events of horizontal gene transfer (HGT) from bacteria. Two of these events allowed algae to acquire new bacterial viperins, while two more HGT events gave rise to distinct superfamilies of eukaryotic CD-NTases: the Mab21 superfamily (containing cGAS) which has 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 STINGs arising via 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 anti-phage genes.

Acquired stress resilience through bacteria-to-nematode horizontal gene transfer

Natural selection drives acquisition of organismal resilience traits to protect against adverse environments. Horizontal gene transfer (HGT) is an important evolutionary mechanism for the acquisition of novel traits, including metazoan acquisition of functions in immunity, metabolism, and reproduction via interdomain HGT (iHGT) from bacteria. We report that the nematode gene rml-3, which was acquired by iHGT from bacteria, enables exoskeleton resilience and protection against environmental toxins in C. elegans. Phylogenetic analysis reveals that diverse nematode RML-3 proteins form a single monophyletic clade most highly similar to bacterial enzymes that biosynthesize L-rhamnose to build cell wall polysaccharides. C. elegans rml-3 is regulated in developing seam cells by heat stress and stress-resistant dauer stage. Importantly, rml-3 deficiency impairs cuticle integrity, barrier functions and organismal stress resilience, phenotypes that are rescued by exogenous L-rhamnose. We propose that iHGT of an ancient bacterial rml-3 homolog enables L-rhamnose biosynthesis in nematodes that facilitates cuticle integrity and organismal resilience in adaptation to environmental stresses during evolution. These findings highlight the remarkable contribution of iHGT on metazoan evolution that is conferred by the domestication of bacterial genes.

Crystal structure and functional implications of cyclic di-pyrimidine-synthesizing cGAS/DncV-like nucleotidyltransferases

Purine-containing nucleotide second messengers regulate diverse cellular activities. Cyclic di-pyrimidines mediate anti-phage functions in bacteria; however, the synthesis mechanism remains elusive. Here, we determine the high-resolution structures of cyclic di-pyrimidine-synthesizing cGAS/DncV-like nucleotidyltransferases (CD-NTases) in clade E (CdnE) in its apo, substrate-, and intermediate-bound states. A conserved (R/Q)xW motif controlling the pyrimidine specificity of donor nucleotide is identified. Mutation of Trp or Arg from the (R/Q)xW motif to Ala rewires its specificity to purine nucleotides, producing mixed purine-pyrimidine cyclic dinucleotides (CDNs). Preferential binding of uracil over cytosine bases explains the product specificity of cyclic di-pyrimidine-synthesizing CdnE to cyclic di-UMP (cUU). Based on the intermediate-bound structures, a synthetic pathway for cUU containing a unique 2'3'-phosphodiester linkage through intermediate pppU[3'-5']pU is deduced. Our results provide a framework for pyrimidine selection and establish the importance of conserved residues at the C-terminal loop for the specificity determination of CD-NTases.

A conserved family of immune effectors cleaves cellular ATP upon viral infection

During viral infection, cells can deploy immune strategies that deprive viruses of molecules essential for their replication. Here, we report a family of immune effectors in bacteria that, upon phage infection, degrade cellular adenosine triphosphate (ATP) and deoxyadenosine triphosphate (dATP) by cleaving the N-glycosidic bond between the adenine and sugar moieties. These ATP nucleosidase effectors are widely distributed within multiple bacterial defense systems, including cyclic oligonucleotide-based antiviral signaling systems (CBASS), prokaryotic argonautes, and nucleotide-binding leucine-rich repeat (NLR)-like proteins, and we show that ATP and dATP degradation during infection halts phage propagation. By analyzing homologs of the immune ATP nucleosidase domain, we discover and characterize Detocs, a family of bacterial defense systems with a two-component phosphotransfer-signaling architecture. The immune ATP nucleosidase domain is also encoded within diverse eukaryotic proteins with immune-like architectures, and we show biochemically that eukaryotic homologs preserve the ATP nucleosidase activity. Our findings suggest that ATP and dATP degradation is a cell-autonomous innate immune strategy conserved across the tree of life.

Bacterial Shedu immune nucleases share a common enzymatic core regulated by diverse sensor domains

Prokaryotes encode diverse anti-bacteriophage immune systems, including the single-protein Shedu nuclease. Here we reveal the structural basis for activation of Bacillus cereus Shedu. In the inactive homotetramer, a key catalytic residue in Shedu's nuclease domain is sequestered away from the catalytic site. Activation involves a conformational change that completes the active site and promotes assembly of a homo-octamer for coordinated double-strand DNA cleavage. Removal of Shedu's N-terminal domain ectopically activates the enzyme, suggesting that this domain allosterically inhibits Shedu in the absence of infection. Bioinformatic analysis of nearly 8,000 Shedu homologs reveals remarkable diversity in their N-terminal regulatory domains: we identify 79 domain families falling into eight functional classes, including diverse nucleic acid binding, enzymatic, and other domains. Together, these data reveal Shedu as a broad family of immune nucleases with a common nuclease core regulated by diverse N-terminal domains that likely respond to a range of infection-related signals.

Run, Ribosome, Run: From Compromised Translation to Human Health

Significance: Translation is an essential cellular process, and diverse signaling pathways have evolved to deal with problems arising during translation. Erroneous stalls and unresolved ribosome collisions are implicated in many pathologies, including neurodegeneration and metabolic dysregulation. Recent Advances: Many proteins involved in detection and clearance of stalled and collided ribosomes have been identified and studied in detail. Ribosome profiling techniques have revealed extensive and nonprogrammed ribosome stalling and leaky translation into the 3' untranslated regions of mRNAs. Impairment of protein synthesis has been linked to aging in yeast and mice. Critical Issues: Ribosomes act as sensors of cellular states, but the molecular mechanisms, as well as physiological relevance, remain understudied. Most of our current knowledge stems from work in yeast and simple multicellular organisms such as Caenorhabditis elegans, while we are only beginning to comprehend the role of ribosome surveillance in higher organisms. As an example, the ribotoxic stress response, a pathway responding to global translational stress, has been studied mostly in response to small translation inhibitors and ribotoxins, and has only recently been explored in physiological settings. This review focuses on ribosome-surveillance pathways and their importance for cell and tissue homeostasis upon naturally occurring insults such as oxidative stress, nutrient deprivation, and viral infections. Future Directions: A better insight into the physiological roles of ribosome-surveillance pathways and their crosstalk could lead to an improved understanding of human pathologies and aging. Antioxid. Redox Signal. 39, 336-350.

Origin of the OAS-RNase L innate immune pathway before the rise of jawed vertebrates via molecular tinkering

Discriminating self from nonself is fundamental to immunity. Yet, it remains largely elusive how the mechanisms of self and nonself discrimination originated. Sensing double-stranded RNA as nonself, the 2',5'-oligoadenylate synthetase (OAS)-ribonuclease L (RNase L) pathway represents a crucial component of innate immunity. Here, we combine phylogenomic and functional analyses to show that the functional OAS-RNase L pathway likely originated through tinkering with preexisting proteins before the rise of jawed vertebrates during or before the Silurian period (444 to 419 Mya). Multiple concerted losses of OAS and RNase L occurred during the evolution of jawed vertebrates, further supporting the ancient coupling between OAS and RNase L. Moreover, both OAS and RNase L genes evolved under episodic positive selection across jawed vertebrates, suggesting a long-running evolutionary arms race between the OAS-RNase L pathway and microbes. Our findings illuminate how an innate immune pathway originated via molecular tinkering.

Functional tumor cell-intrinsic STING, not host STING, drives local and systemic antitumor immunity and therapy efficacy following cryoablation

BACKGROUND

Despite its potential utility in delivering direct tumor killing and in situ whole-cell tumor vaccination, tumor cryoablation produces highly variable and unpredictable clinical response, limiting its clinical utility. The mechanism(s) driving cryoablation-induced local antitumor immunity and the associated abscopal effect is not well understood.

METHODS

The aim of this study was to identify and explore a mechanism of action by which cryoablation enhances the therapeutic efficacy in metastatic tumor models. We used the subcutaneous mouse model of the rhabdomyosarcoma (RMS) cell lines RMS 76-9STINGwt or RMS 76-9STING-/-, along with other murine tumor models, in C57BL/6 or STING-/- (TMEM173-/- ) mice to evaluate local tumor changes, lung metastasis, abscopal effect on distant tumors, and immune cell dynamics in the tumor microenvironment (TME).

RESULTS

The results show that cryoablation efficacy is dependent on both adaptive immunity and the STING signaling pathway. Contrary to current literature dictating an essential role of host-derived STING activation as a driver of antitumor immunity in vivo, we show that local tumor control, lung metastasis, and the abscopal effect on distant tumor are all critically dependent on a functioning tumor cell-intrinsic STING signaling pathway, which induces inflammatory chemokine and cytokine responses in the cryoablated TME. This reliance extends beyond cryoablation to include intratumoral STING agonist therapy. Additionally, surveys of gene expression databases and tissue microarrays of clinical tumor samples revealed a wide spectrum of expressions among STING-related signaling components.

CONCLUSIONS

Tumor cell-intrinsic STING pathway is a critical component underlying the effectiveness of cryoablation and suggests that expression of STING-related signaling components may serve as a potential therapy response biomarker. Our data also highlight an urgent need to further characterize tumor cell-intrinsic STING pathways and the associated downstream inflammatory response evoked by cryoablation and other STING-dependent therapy approaches.

Expanding evolutionary theories of ageing to better account for symbioses and interactions throughout the Web of Life

How, when, and why organisms age are fascinating issues that can only be fully addressed by adopting an evolutionary perspective. Consistently, the main evolutionary theories of ageing, namely the Mutation Accumulation theory, the Antagonistic Pleiotropy theory, and the Disposable Soma theory, have formulated stimulating hypotheses that structure current debates on both the proximal and ultimate causes of organismal ageing. However, all these theories leave a common area of biology relatively under-explored. The Mutation Accumulation theory and the Antagonistic Pleiotropy theory were developed under the traditional framework of population genetics, and therefore are logically centred on the ageing of individuals within a population. The Disposable Soma theory, based on principles of optimising physiology, mainly explains ageing within a species. Consequently, current leading evolutionary theories of ageing do not explicitly model the countless interspecific and ecological interactions, such as symbioses and host-microbiomes associations, increasingly recognized to shape organismal evolution across the Web of Life. Moreover, the development of network modelling supporting a deeper understanding on the molecular interactions associated with ageing within and between organisms is also bringing forward new questions regarding how and why molecular pathways associated with ageing evolved. Here, we take an evolutionary perspective to examine the effects of organismal interactions on ageing across different levels of biological organisation, and consider the impact of surrounding and nested systems on organismal ageing. We also apply this perspective to suggest open issues with potential to expand the standard evolutionary theories of ageing.

The evolutionary success of regulated cell death in bacterial immunity

Bacteria employ a complex arsenal of immune mechanisms to defend themselves against phages. Recent studies demonstrate that these immune mechanisms frequently involve regulated cell death in response to phage infection. By sacrificing infected cells, this strategy prevents the spread of phages within the surrounding population. In this review, we discuss the principles of regulated cell death in bacterial defense, and show that over 70% of sequenced prokaryotes employ this strategy as part of their defensive arsenals. We highlight the modularity of defense systems involving regulated cell death, explaining how shuffling between phage-sensing and cell-killing protein domains dominates their evolution. Some of these defense systems are the evolutionary ancestors of key components of eukaryotic immunity, highlighting their importance in shaping the evolutionary trajectory of immune systems across the tree of life.

The arms race between bacteria CBASS and bacteriophages

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

Bacterial cGAS-like enzymes produce 2',3'-cGAMP to activate an ion channel that restricts phage replication

The mammalian innate immune system uses cyclic GMP-AMP synthase (cGAS) to synthesize the cyclic dinucleotide 2',3'-cGAMP during antiviral and antitumor immune responses. 2',3'-cGAMP is a nucleotide second messenger that initiates inflammatory signaling by binding to and activating the stimulator of interferon genes (STING) receptor. Bacteria also encode cGAS/DncV-like nucleotidyltransferases (CD-NTases) that produce nucleotide second messengers to initiate antiviral (antiphage) signaling. Bacterial CD-NTases produce a wide range of cyclic oligonucleotides but have not been documented to produce 2',3'-cGAMP. Here we discovered bacterial CD-NTases that produce 2',3'-cGAMP to restrict phage replication. Bacterial 2',3'-cGAMP binds to CD-NTase associated protein 14 (Cap14), a transmembrane protein of unknown function. Using electrophysiology, we show that Cap14 is a chloride-selective ion channel that is activated by 2',3'-cGAMP binding. Cap14 adopts a modular architecture, with an N-terminal transmembrane domain and a C-terminal nucleotide-binding SAVED domain. Domain-swapping experiments demonstrated the Cap14 transmembrane region could be substituted with a nuclease, thereby generating a biosensor that is selective for 2',3'-cGAMP. This study reveals that 2',3'-cGAMP signaling extends beyond metazoa to bacteria. Further, our findings suggest that transmembrane proteins of unknown function in bacterial immune pathways may broadly function as nucleotide-gated ion channels.

cGLRs are a diverse family of pattern recognition receptors in innate immunity

Cyclic GMP-AMP synthase (cGAS) is an enzyme in human cells that controls an immune response to cytosolic DNA. Upon binding DNA, cGAS synthesizes a nucleotide signal 2'3'-cGAMP that activates STING-dependent downstream immunity. Here, we discover that cGAS-like receptors (cGLRs) constitute a major family of pattern recognition receptors in innate immunity. Building on recent analysis in Drosophila, we identify >3,000 cGLRs present in nearly all metazoan phyla. A forward biochemical screening of 150 animal cGLRs reveals a conserved mechanism of signaling including response to dsDNA and dsRNA ligands and synthesis of isomers of the nucleotide signals cGAMP, c-UMP-AMP, and c-di-AMP. Combining structural biology and in vivo analysis in coral and oyster animals, we explain how synthesis of distinct nucleotide signals enables cells to control discrete cGLR-STING signaling pathways. Our results reveal cGLRs as a widespread family of pattern recognition receptors and establish molecular rules that govern nucleotide signaling in animal immunity.

Interplay between human STING genotype and bacterial NADase activity regulates inter-individual disease variability

Variability in disease severity caused by a microbial pathogen is impacted by each infection representing a unique combination of host and pathogen genomes. Here, we show that the outcome of invasive Streptococcus pyogenes infection is regulated by an interplay between human STING genotype and bacterial NADase activity. S. pyogenes-derived c-di-AMP diffuses via streptolysin O pores into macrophages where it activates STING and the ensuing type I IFN response. However, the enzymatic activity of the NADase variants expressed by invasive strains suppresses STING-mediated type I IFN production. Analysis of patients with necrotizing S. pyogenes soft tissue infection indicates that a STING genotype associated with reduced c-di-AMP-binding capacity combined with high bacterial NADase activity promotes a 'perfect storm' manifested in poor outcome, whereas proficient and uninhibited STING-mediated type I IFN production correlates with protection against host-detrimental inflammation. These results reveal an immune-regulating function for bacterial NADase and provide insight regarding the host-pathogen genotype interplay underlying invasive infection and interindividual disease variability.

Regulation of cGAS and STING signaling during inflammation and infection

Stimulator of interferon genes (STING) is a sensor of cyclic dinucleotides including cyclic GMP-AMP, which is produced by cyclic GMP-AMP synthase (cGAS) in response to cytosolic DNA. The cGAS-STING signaling pathway regulates both innate and adaptive immune responses, as well as fundamental cellular functions such as autophagy, senescence, and apoptosis. Mutations leading to constitutive activation of STING cause devastating human diseases. Thus, the cGAS-STING pathway is of great interest because of its role in diverse cellular processes and because of the potential therapeutic implications of targeting cGAS and STING. Here, we review molecular and cellular mechanisms of STING signaling, and we propose a framework for understanding the immunological and other cellular functions of STING in the context of disease.

Conserved domains can be found across distinct phage defence systems

Bacteria are continuously exposed to predation from bacteriophages (phages) and, in response, have evolved a broad range of defence systems. These systems can prevent the replication of phages and other mobile genetic elements (MGE). Defence systems are often encoded together in genomic loci defined as "defence islands", a tendency that has been extensively exploited to identify novel antiphage systems. In the last few years, >100 new antiphage systems have been discovered, and some display homology to components of the immune systems of plants and animals. In many instances, prediction tools have found domains with similar predicted functions present as different combinations within distinct antiphage systems. In this Perspective Article, we review recent reports describing the discovery and the predicted domain composition of several novel antiphage systems. We discuss several examples of similar protein domains adopted by different antiphage systems, including domains of unknown function (DUFs), domains involved in nucleic acid recognition and degradation, and domains involved in NAD+ depletion. We further discuss the potential evolutionary advantages that could have driven the independent acquisition of these domains by different antiphage systems.

Viral recognition and the antiviral interferon response

Interferons (IFNs) are antiviral cytokines that play a key role in the innate immune response to viral infections. In response to viral stimuli, cells produce and release interferons, which then act on neighboring cells to induce the transcription of hundreds of genes. Many of these gene products either combat the viral infection directly, e.g., by interfering with viral replication, or help shape the following immune response. Here, we review how viral recognition leads to the production of different types of IFNs and how this production differs in spatial and temporal manners. We then continue to describe how these IFNs play different roles in the ensuing immune response depending on when and where they are produced or act during an infection.

A host of armor: Prokaryotic immune strategies against mobile genetic elements

Prokaryotic adaptation is strongly influenced by the horizontal acquisition of beneficial traits via mobile genetic elements (MGEs), such as viruses/bacteriophages and plasmids. However, MGEs can also impose a fitness cost due to their often parasitic nature and differing evolutionary trajectories. In response, prokaryotes have evolved diverse immune mechanisms against MGEs. Recently, our understanding of the abundance and diversity of prokaryotic immune systems has greatly expanded. These defense systems can degrade the invading genetic material, inhibit genome replication, or trigger abortive infection, leading to population protection. In this review, we highlight these strategies, focusing on the most recent discoveries. The study of prokaryotic defenses not only sheds light on microbial evolution but also uncovers novel enzymatic activities with promising biotechnological applications.

TIR enzymatic functions: signaling molecules and receptor mechanisms

The evolutionarily conserved Toll/Interleukin-1 Receptor (TIR) domains across kingdoms of prokaryotes, plants, and animals play critical roles in innate immunity. Recent studies have revealed the enzymatic functions of TIRs, the structural bases of TIRs as holoenzymes, and the identity of TIR-generated small signaling molecules and their receptors, which significantly advanced our understanding on TIR-mediated immune signaling pathways. We reviewed the most up-to-date findings in TIR enzymatic functions from the perspectives of signaling molecules and receptor mechanisms.

TIR domain-associated nucleotides with functions in plant immunity and beyond

TIR (Toll/interlukin-1 receptor) domains are found in archaea, bacteria and eukaryotes, featured in proteins generally associated with immune functions. In plants, they are found in a large group of NLRs (nucleotide-binding leucine-rich repeat receptors), NLR-like proteins and TIR-only proteins. They are also present in effector proteins from phytopathogenic bacteria that are associated with suppression of host immunity. TIR domains from plants and bacteria are enzymes that cleave NAD+ (nicotinamide adenine dinucleotide, oxidized form) and other nucleotides. In dicot plants, TIR-derived signalling molecules activate downstream immune signalling proteins, the EDS1 (enhanced disease susceptibility 1) family proteins, and in turn helper NLRs. Recent work has brought major advances in understanding how TIR domains work, how they produce signalling molecules and how these products signal.