Cyclic Dinucleotides Research Articles

Discovery and Characterization of the ddx41 Gene in Atlantic Salmon: Evolutionary Implications, Structural Functions, and Innate Immune Responses to Piscirickettsia salmonis and Renibacterium salmoninarum Infections.

The innate immune response in Salmo salar, mediated by pattern recognition receptors (PRRs), is crucial for defending against pathogens. This study examined DDX41 protein functions as a cytosolic/nuclear sensor for cyclic dinucleotides, RNA, and DNA from invasive intracellular bacteria. The investigation determined the existence, conservation, and functional expression of the ddx41 gene in S. salar. In silico predictions and experimental validations identified a single ddx41 gene on chromosome 5 in S. salar, showing 83.92% homology with its human counterpart. Transcriptomic analysis in salmon head kidney confirmed gene transcriptional integrity. Proteomic identification through mass spectrometry characterized three unique peptides with 99.99% statistical confidence. Phylogenetic analysis demonstrated significant evolutionary conservation across species. Functional gene expression analysis in SHK-1 cells infected by Piscirickettsia salmonis and Renibacterium salmoninarum indicated significant upregulation of DDX41, correlated with increased proinflammatory cytokine levels and activation of irf3 and interferon signaling pathways. In vivo studies corroborated DDX41 activation in immune responses, particularly when S. salar was challenged with P. salmonis, underscoring its potential in enhancing disease resistance. This is the first study to identify the DDX41 pathway as a key component in S. salar innate immune response to invading pathogens, establishing a basis for future research in salmonid disease resistance.

Lipo/TK-CDN/TPP/Y6 nanoparticles inhibit cutaneous melanoma formation

Stimulation of the innate immune stimulator of interferon genes (STING) pathway has been shown to boost anti-tumour immunity. Nevertheless, the systemic delivery of STING agonists to the tumour presents challenges. Therefore, we designed a cyclic dinucleotide (CDN)-based drug delivery system (DDS) combined photothermal therapy (PTT)/photodynamic therapy (PDT)/immunotherapy for cutaneous melanoma. We coencapsulated a reactive oxygen species (ROS)-responsive prodrug thioketone-linked CDN (TK-CDN), and photoresponsive agents chlorin E6 (Y6) within mitochondria-targeting reagent triphenylphosphonium (TPP)-modified liposomes (Lipo/TK-CDN/TPP/Y6). Lipo/TK-CDN/TPP/Y6 exhibited a photothermal effect similar to Y6, along with a superior cellular uptake rate. Upon endocytosis by B16F10 cells, Lipo/TK-CDN/TPP/Y6 generated large amounts of ROS under laser irradiation for PDT. Mice bearing B16F10 tumours were intravenously injected with Lipo/TK-CDN/TPP/Y6 and exposed to irradiation, resulting in a substantial inhibition of tumour growth. Exploration of the mechanism of anti-tumour action showed that Lipo/TK-CDN/TPP/Y6 had a stronger stimulation of STING activation and anti-tumour immune cell infiltration compared to other groups. Hence, the Lipo/TK-CDN/TPP/Y6 nanoparticles offer great potential as a DDS for targeted and on-demand drug release at tumour sites. These nanoparticles exhibit promise as a candidate for precise and controllable combination therapy in the treatment of tumours.

Unraveling the Binding Mode of Cyclic Adenosine-Inosine Monophosphate (cAIMP) to STING through Molecular Dynamics Simulations.

The stimulator of interferon genes (STING) plays a significant role in immune defense and protection against tumor proliferation. Many cyclic dinucleotide (CDN) analogues have been reported to regulate its activity, but the dynamic process involved when the ligands activate STING remains unclear. In this work, all-atom molecular dynamics simulations were performed to explore the binding mode between human STING (hSTING) and four cyclic adenosine-inosine monophosphate analogs (cAIMPs), as well as 2',3'-cGMP-AMP (2',3'-cGAMP). The results indicate that these cAIMPs adopt a U-shaped configuration within the binding pocket, forming extensive non-covalent interaction networks with hSTING. These interactions play a significant role in augmenting the binding, particularly in interactions with Tyr167, Arg238, Thr263, and Thr267. Additionally, the presence of hydrophobic interactions between the ligand and the receptor further contributes to the overall stability of the binding. In this work, the conformational changes in hSTING upon binding these cAIMPs were also studied and a significant tendency for hSTING to shift from open to closed state was observed after binding some of the cAIMP ligands.

Intravenous delivery of STING agonists using acid-sensitive polycationic polymer-modified lipid nanoparticles for enhanced tumor immunotherapy

Although cancer immunotherapy has made great strides in the clinic, it is still hindered by the tumor immunosuppressive microenvironment (TIME). The stimulator of interferon genes (STING) pathway which can modulate TIME effectively has emerged as a promising therapeutic recently. However, the delivery of most STING agonists, specifically cyclic dinucleotides (CDNs), is performed intratumorally due to their insufficient pharmacological properties, such as weak permeability across cell membranes and vulnerability to nuclease degradation. To expand the clinical applicability of CDNs, a novel pH-sensitive polycationic polymer-modified lipid nanoparticle (LNP-B) system was developed for intravenous delivery of CDNs. LNP-B significantly extended the circulation of CDNs and enhanced the accumulation of CDNs within the tumor, spleen, and tumor-draining lymph nodes compared with free CDNs thereby triggering the STING pathway of dendritic cells and repolarizing pro-tumor macrophages. These events subsequently gave rise to potent anti-tumor immune reactions and substantial inhibition of tumors in CT26 colon cancer-bearing mouse models. In addition, due to the acid-sensitive property of the polycationic polymer, the delivery system of LNP-B was more biocompatible and safer compared with lipid nanoparticles formulated with an indissociable cationic DOTAP (LNP-D). These findings suggest that LNP-B has great potential in the intravenous delivery of CDNs for tumor immunotherapy.

A phase 1 dose-escalation and expansion study of an intratumorally administered dual STING agonist (ONM-501) alone and in combination with cemiplimab in patients with advanced solid tumors and lymphomas.

TPS2693 Background: The Stimulator of Interferon Genes (STING) pathway has garnered significant interest as a target for anticancer interventions due to its role in driving the production of pro-inflammatory cytokines and activating tumor-specific cell killing. Cyclic dinucleotides (CDNs) such as cGAMP, the endogenous STING agonist, are not ideal clinical candidates for exogenous routes of delivery. Early trials showed limited efficacy with CDNs due to rapid clearance, degradation and/or off-target toxicity. ONM-501 is comprised of a STING-activating pH-sensitive PC7A polymer conjugated with cGAMP. At physiologic pH, ONM-501 forms a micelle, preventing degradation of cGAMP. In the acidic tumor microenvironment, ONM-501 effects STING agonism through a multi-faceted mode of action: 1) endocytosis of nanoparticles and pH-activated micelle dissociation and payload release in endolysosomes enhances intracellular delivery of cGAMP 2) the combination of cGAMP canonical binding, and PC7A polymer noncanonical binding on STING yields synergistic STING activation and 3) stabilization of STING by PC7A polymers delays its degradation and prolongs STING activation. In pre-clinical models, these factors combine to help generate a stronger and more prolonged innate immune response that more effectively translates to a robust adaptive immune response. ONM-501 is being investigated in this first-in-human Phase 1 trial as monotherapy and combination therapy with the anti-PD1 antibody, cemiplimab, for patients with solid tumors and lymphomas. Methods: ONM5001 (NCT06022029) is a first-in-human, open-label, multi-center clinical trial. Adult patients with solid tumors or lymphomas for which no standard therapy exists or for which patients decline palliative standard therapy who have at least one injectable lesion are eligible for this study. This Phase 1 study consists of three parts: monotherapy dose escalation; combination therapy dose finding; and combination therapy dose expansion in specific tumor indication(s) that will be selected based on data from mono- and combination therapy dose finding. Each dosing cycle of ONM-501 is 21 days: patients receive an intratumoral injection of ONM-501 weekly for three weeks, followed by three weeks without injection. Cemiplimab is administered intravenously during combination therapy arms according to standard administration protocol, once every three weeks. The primary endpoint is to evaluate safety of ONM-501 monotherapy as evidenced by the number of dose limiting toxicities and treatment-emergent adverse events, and to determine the recommended dose for expansion. Secondary endpoints include characterization of pharmacokinetics and pharmacodynamics, as well as initial efficacy based on RECIST evaluation criteria. Enrollment began in November 2023. Clinical trial information: NCT06022029 .

Structural and functional characterization of the bacterial Cap3 enzyme in deconjugation and regulation of the cyclic dinucleotide transferase CD-NTase

The Cyclic GMP-AMP synthase (cGAS) and cGAS/DncV-like nucleotidyltransferase (CD-NTase) enzymes belong to the key components of the innate immune sensor system that generates cyclic dinucleotide molecules in response to danger signals. Recently, it was discovered that CD-NTase in bacteria can undergo conjugation to protein substrates via an E1/E2 enzyme-mediated process, resembling ubiquitin modification system. Subsequently, these CD-NTase conjugated molecules will be hydrolyzed by the Cap3 enzyme in the same gene cluster. However, the experimental structure of bacterial CD-NTase recognized by Cap3 is unknown. Here, we first determined the crystal structure of the Cap3 enzyme in complex with the C-terminal tail of CD-NTase. Our structural and enzymatic analysis revealed that the C-terminal tail of CD-NTase is both necessary and sufficient for the Cap3-mediated hydrolysis of CD-NTase from its substrates. Interestingly, we further observed that after the hydrolysis reaction, the terminal glycine residue of the CD-NTase C-terminal tail was sequentially removed by Cap3, indicating that Cap3 might play a role in quenching the CD-NTase conjugation reaction. Our work provides experimental evidence elucidating the interaction between Cap3 and CD-NTase, and suggests a potential role for Cap3 in the bacterial Cyclic-oligonucleotide-based anti-phage signaling system (CBASS).

A metabolic switch orchestrated by IL-18 and the cyclic dinucleotide cGAMP programs intestinal tolerance

Tissues are exposed to diverse inflammatory challenges that shape future inflammatory responses. While cellular metabolism regulates immune function, how metabolism programs and stabilizes immune states within tissues and tunes susceptibility to inflammation is poorly understood. Here, we describe an innate immune metabolic switch that programs long-term intestinal tolerance. Intestinal interleukin-18 (IL-18) stimulation elicited tolerogenic macrophages by preventing their proinflammatory glycolytic polarization via metabolic reprogramming to fatty acid oxidation (FAO). FAO reprogramming was triggered by IL-18 activation of SLC12A3 (NCC), leading to sodium influx, release of mitochondrial DNA, and activation of stimulator of interferon genes (STING). FAO was maintained in macrophages by a bistable switch that encoded memory of IL-18 stimulation and by intercellular positive feedback that sustained the production of macrophage-derived 2'3'-cyclic GMP-AMP (cGAMP) and epithelial-derived IL-18. Thus, a tissue-reinforced metabolic switch encodes durable immune tolerance in the gut and may enable reconstructing compromised immune tolerance in chronic inflammation.

Nucleic Acid Sensing by STING Induces an IFN-like Antiviral Response in a Marine Invertebrate.

The cytosolic detection of pathogen-derived nucleic acids has evolved as an essential strategy for host innate immune defense in mammals. One crucial component in this process is the stimulator of IFN genes (STING), which acts as a vital signaling adaptor, connecting the cytosolic detection of DNA by cyclic GMP-AMP (cGAMP) synthase (cGAS) to the downstream type I IFN signaling pathway. However, this process remains elusive in invertebrates. In this study, we present evidence demonstrating that STING, an ortholog found in a marine invertebrate (shrimp) called Litopenaeus vannamei, can directly detect DNA and initiate an IFN-like antiviral response. Unlike its homologs in other eukaryotic organisms, which exclusively function as sensors for cyclic dinucleotides, shrimp STING has the ability to bind to both double-stranded DNA and cyclic dinucleotides, including 2'3'-cGAMP. In vivo, shrimp STING can directly sense DNA nucleic acids from an infected virus, accelerate IFN regulatory factor dimerization and nuclear translocation, induce the expression of an IFN functional analog protein (Vago4), and finally establish an antiviral state. Taken together, our findings unveil a novel double-stranded DNA-STING-IKKε-IRF-Vago antiviral axis in an arthropod, providing valuable insights into the functional origins of DNA-sensing pathways in evolution.

Bis-2′-F-cGSASMP isomers encapsulated in cytidinyl/cationic lipids act as potent in situ autologous tumor vaccines

Cancer immunotherapy has greatly improved the prognosis of tumor-bearing patients. Nevertheless, cancer patients exhibit low response rates to current immunotherapy drugs, such as PD1 and PDL1 antibodies. Cyclic dinucleotide analogs are a promising class of immunotherapeutic agents. In this study, in situ autologous tumor vaccines, composed of Bis-2′-F-cGSASMP phosphonothioate isomers (FGA-di-pS-2 or FGA-di-pS-4) and cytidinyl/cationic lipids (Mix), were constructed. Intravenous (i.v.) and intratumoral (i.t.) injection of FGA-di-pS-2/Mix or FGA-di-pS-4/Mix enhanced the immunogenic cell death of tumor cells in vivo, leading to the exposure and presentation of whole tumor antigens, inhibiting tumor growth in both LLC and EO771 tumor in situ murine models and increasing their survival rates to 50% and 23%, respectively. Furthermore, the tumor-bearing mice after treatment showed potent immune memory efficacy and exhibited 100% protection against tumor rchallenge. i.v. administration of FGA-di-pS-2/Mix potently promoted DC maturation, M1 macrophage polarization and CD8+ T-cell activation and decreased the proportion of Treg cells in the tumor microenvironment. Notably, two doses of ICD-debris (generated by FGA-di-pS-2 or 4/Mix-treated LLC cells) protected 100% of mice from tumor growth. These tumor vaccines showed promising results and may serve as personalized cancer vaccinations in the future.

STING pathway as a cancer immunotherapy: Progress and challenges in activating anti-tumor immunity.

The stimulator of the interferon genes (STING) signaling pathway plays a crucial role in innate immunity by detecting cytoplasmic DNA and initiating antiviral host defense mechanisms. The STING cascade is triggered when the enzyme cyclic GMP-AMP synthase (cGAS) binds cytosolic DNA and synthesizes the secondary messenger cGAMP. cGAMP activates the endoplasmic reticulum adaptor STING, leading to the activation of kinases TBK1 and IRF3 that induce interferon production. Secreted interferons establish an antiviral state in infected and adjacent cells. Beyond infections, aberrant DNA in cancer cells can also activate the STING pathway. Preclinical studies have shown that pharmacological STING agonists like cyclic dinucleotides elicit antitumor immunity when administered intratumorally by provoking innate and adaptive immunity. Combining STING agonists with immune checkpoint inhibitors may improve outcomes by overcoming tumor immunosuppression. First-generation STING agonists encountered challenges like poor pharmacokinetics, limited tumor specificity, and systemic toxicity. The development of the next-generation STING-targeted drugs to realize the full potential of engaging this pathway for cancer treatment can be a solution to overcome the current challenges, but further studies are required to determine optimal applications and combination regimens for the clinic. Notably, the controlled activation of STING is needed to preclude adverse effects. This review explores the mechanisms and effects of STING activation, its role in cancer immunotherapy, and current challenges.

Abstract 5883: Role of major facilitative folate transporters SLC19A1 and SLC46A1 in cyclic dinucleotide transport and STING signaling

Abstract Activation of stimulator of interferon genes (STING) activates the transcription factor pIRF3, promoting gene transcription encoding type I interferons including IFNβ that stimulate broader immune responses. Targeting the STING pathway for cancer is especially compelling given its importance to the malignant phenotype. Cyclic dinucleotides (CDNs) function as STING agonists. Inspired by preclinical studies, two phase I clinical trials with CDNs including ADU-S100 were initiated but concluded early due to lack of anti-tumor efficacy. The reduced folate carrier (RFC; SLC19A1) has been identified as one of the primary means to transport CDNs through the cell membrane, although a study suggested that CDN and folate binding to RFC was as least partly distinct. While depletion of the proton-coupled folate transporter (PCFT; SLC46A1) in THP-1 cells had nominal effects on CDN stimulation, overexpressing PCFT increased STING signaling. Limited clinical responses may reflect the impact of physiologic variables governing RFC and PCFT not previously considered. Understanding the mechanisms of CDN uptake by RFC and PCFT under physiologic conditions may elucidate the role of CDNs on STING signaling. To study the impact of RFC levels on CDN transport and STING signaling, we used an engineered R1-11/Tet-on-RFC HeLa cell model induced with doxycycline. In R1-11/Tet-on-RFC cells, ADU-S100 showed a low affinity for binding to RFC, as measured by direct competition for transport with [3H]methotrexate. RFC dose-dependence of ADU-S100 uptake and STING activation was confirmed by monitoring IFN-β (qPCR) and pIRF3 (Westerns). Maximum induction of pIRF3/IFNβ was seen in THP-1 and R1-11/Tet-on-RFC cells by ADU-S100 treatment at pH 7.2 (RFC pH optimum) in the presence of 25 nM leucovorin. Induction of pIRF3/IFNβ by ADU-S100 in THP-1 and R1-11/Tet-on-RFC cells were partially abolished by treatment with 200 μM of leucovorin or 10 μM of PT523 (RFC-specific inhibitor), suggesting the transport of folates and CDNs may share common albeit distinct mechanisms. ADU-S100 treatment showed no pIRF3 or IFN-β production in an engineered R1-11/Tet-on-PCFT HeLa model expressing high levels of PCFT, suggesting no direct transport by PCFT, even under optimal pH conditions. Interestingly, pIRF3/IFN-β induction was enhanced in R1-11/Tet-on-RFC/PCFT cells expressing constitutive PCFT in addition to RFC, compared to R1-11/Tet-on-RFC cells that express comparable RFC without PCFT. This may reflect previously unrecognized functional interactions between RFC and PCFT. In conclusion, RFC mediates ADU-S100 uptake and STING activation under physiologic conditions and this process is enhanced by concomitant expression of PCFT. Additional characterization of key molecular and biochemical determinants of CDN-based cancer therapeutics may lead to improved approaches for CDN therapy based on enhanced cellular uptake. Citation Format: Holly McQuithey, Madelyn Brzezinski, Carrie O’Connor, Larry Matherly, Zhanjun Hou. Role of major facilitative folate transporters SLC19A1 and SLC46A1 in cyclic dinucleotide transport and STING signaling [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 5883.

Abstract 3187: Enhancing STING activation by cyclic dinucleotide-manganese particles for systemic cancer immunotherapy

Abstract Background: Activating the innate immune pathway of stimulator of interferon genes (STING) can elicit potent anti-tumor immunity via the production of type-I interferons (IFN-I). However, cyclic-dinucleotide (CDN) STING agonists have undesirable pharmacological properties. Most CDNs in clinical development are administered intratumorally, thus precluding their utility against advanced cancer. Here, we report the development of a new nanoparticle system for the systemic delivery of CDN STING agonists with potent efficacy, favorable pharmaceutical properties, and acceptable safety profiles. Methods: Various metal ions were screened for synergy with CDNs for IFN-I response from bone marrow-derived dendritic cells. Lipid nanoparticles carrying CDN and manganese ions (termed SNP) were synthesized. Mice bearing syngenetic tumors (including CT26 and B16F10) as well as genetically engineered mouse models were treated by intravenous administration of SNP, followed by tumor monitoring and immune profiling of the tumor microenvironment. White New Zealand rabbits bearing VX2 tumors as well as healthy mongrel dogs were treated with SNP and evaluated. Lastly, human tumor biopsies derived from head and neck squamous cell carcinoma (HNSCC) patients were incubated with SNP or soluble STING agonists and analyzed for immune activation. Results: We identified that Mn2+ achieved strong synergy with CDN STING agonists for inducing IFN-I production. SNP carrying CDN and Mn2+ was dosed intravenously in mice, leading to robust IFN-I response, remodeling of the immunosuppressive tumor microenvironment, and expansion of anti-tumor CD8+ T cells. Intravenous SNP therapy resulted in robust anti-tumor efficacy in multiple murine tumor models, including CT26, B16F10, and a genetically engineered mouse model of MMTV-PyMT triple-negative breast cancer. We have also observed robust anti-tumor efficacy of SNP against VX2 squamous carcinomas in rabbits and have demonstrated the safety of intravenous SNP therapy in healthy dogs. Mechanistically, the efficacy of SNP therapy was dependent on the host STING expression but independent of the tumor STING expression. In addition, the anti-tumor efficacy of SNP was decreased in mice lacking Ifnar1 or Ifngr1 expression in the B16F10 melanoma model, showing the crucial effects of IFN-I and IFN-II responses. Moreover, SNP treatment led to robust IFN-I responses from fresh human tumor biopsies derived from HNSCC patients. Conclusions: SNP empowers highly effective systemic cancer immunotherapy via nanotechnology. It also underscores the potential of utilizing metal ions for immune-mediated disease treatment. Citation Format: Xingwu Zhou, Xiaoqi Sun, Wang Gong, Ziye Wan, Teresa Krieger-Burke, Vilma Yuzbasiyan-Gurkan, Yu Leo Lei, James Moon. Enhancing STING activation by cyclic dinucleotide-manganese particles for systemic cancer immunotherapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 3187.

Abstract 488: Modulation of ENPP1 activity and 2'3'-cGAMP degradation in ovarian cancer cell lines via loadable pre-formed lipid nanoparticles

Abstract Ovarian cancer is a complex heterogeneous disease with various subtypes and diverse molecular mechanisms of pathogenesis. Among different forms, epithelial ovarian cancer is the most common, representing up to 90% of total reported cases. Due to the lack of early diagnostic tools, the prognosis is mainly poor, as most cases are typically identified in advanced stages. Thus, a more thorough understanding of the inherent complexities of the disease, in terms of its biochemical etiology, will allow for identification of specific molecular and genetic profiles that could lead to more effective treatment plans. Recent studies have shown that ENPP1 may play a role in cancer cell proliferation, migration, and invasion in ovarian and other types of cancer. ENPP1 is a glycosylated type II transmembrane protein acting as a pyrophosphatase and phosphodiesterase with broad specificity. ENPP1 substrates include mononucleotides and cyclic dinucleotides such as the second messenger, 2'3'-cGAMP, which binds to and activates STING. 2'3'-cGAMP is exported extracellularly by cancerous cells into the tumor microenvironment where uptake into neighboring cells can activate their STING-dependent type 1 interferon pathway and promote antitumoral immune responses. ENPP1 can hydrolyze extracellular 2'3'-cGAMP however, allowing cancer cells to evade such an immune response. In this study, we assessed the activity of ENPP1 in CaOV-3, SK-OV-3, and PA-1 ovarian cancer cell lines using a fluorometric cell-based assay. The effects of ENPP1 inhibition via a chemical inhibitor on extracellular 2'3'-cGAMP levels were characterized and compared with siRNA-mediated knockdown of ENPP1 via loadable pre-formed lipid nanoparticles, which enable small-scale encapsulation and transfection of nucleic acid cargo. In addition, we explored whether uptake of exogenous 2'3'-cGAMP into THP-1 and RAW immune cell lines can be enhanced through delivery via preformed loadable lipid nanoparticles. Corresponding immune cell activation was also assessed using immunoblotting and RT-PCR. Our results demonstrate how modulation of ENPP1 activity and 2'3'-cGAMP levels in ovarian cancer cell lines using unconventional, pre-formed loadable lipid nanoparticle transfection reagent can aid in elucidating mechanisms of immune evasion in cancer cells. Citation Format: David Taylor, Jiajia Ji, Phillip Rzeczycki, Margaret L. Collins, Lauren Clements, Aileen R. Ariosa. Modulation of ENPP1 activity and 2'3'-cGAMP degradation in ovarian cancer cell lines via loadable pre-formed lipid nanoparticles [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 488.

Abstract 2057: Monitoring cGAMP Biochemically and in cell using homogenous HTS formatted bioluminescent assay

Abstract The STimulator of INterferon Genes (STING) is a transmembrane protein located at the endoplasmic reticulum, which serves as a molecular hub in response to cytoplasmic cyclic dinucleotide (CDN) second messengers such as cyclic GMP-AMP (cGAMP). Upon its generation via cyclic GMP-AMP synthase (cGAS) by a wide array of pathogens that provide extracellular microbial DNA and by intracellular (nuclear and mitochondrial) DNA, it binds directly to the STING protein resulting in the activation of the STING pathway. This activation by cytosolic DNA initiates a cascade of events, cGAS-STING-TBK1-IRF3 signaling, that induces type 1 IFN response, and eventually the signaling is terminated by degradation of pathway components and clearance of stimulatory DNA. Given its significant role as an adaptor for DNA sensors to pathogens and its involvement in immune sensing, tumor growth control, and control of autoinflammatory and autoimmune disorders, the STING pathway has become a very important drug target. Credible evidence suggests that the cGAS–cGAMP–STING pathway makes fundamental contributions to at least three major cancer therapies: radiation therapy, chemotherapy, and immunotherapy, and it thus represents a promising drug target. Therefore, there is a need to identify lead compounds that effectively modulate human STING for further drug development. Towards this goal, we have developed a bioluminescent assay to monitor the concentration of cGAMP in intracellular as well as extracellular compartments. The assay relies on the new concept of Protein Complementation Assay (PCA) using nanoluciferase (nanoluc) complementation technology where a small peptide (SmBiT) derived from nanoluc linked to cGAMP through a linker (cGAMP tracer) can be recognized by an anti cGAMP antibody that is linked to a nanoluc complementary fragment (LgBiT) resulting in bioluminescence upon the addition of nanoluc substrate. The presence of free cGAMP that is generated in the reaction competes with cGAMP tracer and decreases bioluminescence. We have optimized the assay for higher sensitivity down to less than 10 nM cGAMP and less than one ng of cGAS and thus, and it is suitable for testing compounds that modulate cGAS activity. We also improved cell lysis condition to increase recovery of cGAMP that is generated intracellularly using several cGAS activators and correlating its concentration with cytokine release. We envision using this assay to monitor cGAMP presence in sera and spinal fluids of patients who have ALS and Alzheimer diseases. The assay is homogenous, with minimal or no false hits using LOPAC screening and HTS formatted and can be completed in less than one hour. Thus, this assay provides valuable contribution for investigating modulators of the STING pathway and facilitates the search for novel therapeutics. Citation Format: Said A. Goueli, kevin Hsiao, Nate Murray, dareen Mikheil, Matt Larsen, Hui Wang. Monitoring cGAMP Biochemically and in cell using homogenous HTS formatted bioluminescent assay [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 2057.

Evolutionary immunology to explore original antiviral strategies.

Over the past 25 years, the field of evolutionary developmental biology (evo-devo) has used genomics and genetics to gain insight on the developmental mechanisms underlying the evolution of morphological diversity of animals. Evo-devo exploits the key insight that conserved toolkits of development (e.g. Hox genes) are used in animals to produce genetic novelties that provide adaptation to a new environment. Like development, immunity is forged by interactions with the environment, namely the microbial world. Yet, when it comes to the study of immune defence mechanisms in invertebrates, interest primarily focuses on evolutionarily conserved molecules also present in humans. Here, focusing on antiviral immunity, we argue that immune genes not conserved in humans represent an unexplored resource for the discovery of new antiviral strategies. We review recent findings on the cGAS-STING pathway and explain how cyclic dinucleotides produced by cGAS-like receptors may be used to investigate the portfolio of antiviral genes in a broad range of species. This will set the stage for evo-immuno approaches, exploiting the investment in antiviral defences made by metazoans over hundreds of millions of years of evolution. This article is part of the theme issue 'Sculpting the microbiome: how host factors determine and respond to microbial colonization'.

Exploration of the Binding Mechanism of Cyclic Dinucleotide Analogs to Stimulating Factor Proteins and the Implications for Subsequent Analog Drug Design.

Cyclic dinucleotides (CDNs) are cyclic molecules consisting of two nucleoside monophosphates linked by two phosphodiester bonds, which act as a second messenger and bind to the interferon gene stimulating factor (STING) to activate the downstream signaling pathway and ultimately induce interferon secretion, initiating an anti-infective immune response. Cyclic dinucleotides and their analogs are lead compounds in the immunotherapy of infectious diseases and tumors, as well as immune adjuvants with promising applications. Many agonists of pathogen recognition receptors have been developed as effective adjuvants to optimize vaccine immunogenicity and efficacy. In this work, the binding mechanism of human-derived interferon gene-stimulating protein and its isoforms with cyclic dinucleotides and their analogs was theoretically investigated using computer simulations and combined with experimental results in the hope of providing guidance for the subsequent synthesis of cyclic dinucleotide analogs.

STING dependent BAX-IRF3 signaling results in apoptosis during late-stage Coxiella burnetii infection

STING (STimulator of Interferon Genes) is a cytosolic sensor for cyclic dinucleotides (CDNs) and initiates an innate immune response upon binding to CDNs. Coxiella burnetii is a Gram-negative obligate intracellular bacterium and the causative agent of the zoonotic disease Q fever. The ability of C. burnetii to inhibit host cell death is a critical factor in disease development. Previous studies have shown that C. burnetii inhibits host cell apoptosis at early stages of infection. However, during the late-stages of infection, there is host cell lysis resulting in the release of bacteria to infect bystander cells. Thus, we investigated the role of STING during late-stages of C. burnetii infection and examined STING’s impact on host cell death. We show that the loss of STING results in higher bacterial loads and abrogates IFNβ and IL6 induction at 12 days post-infection. The absence of STING during C. burnetii infection significantly reduces apoptosis through decreased caspase-8 and -3 activation. During infection, STING activates IRF3 which interacts with BAX. BAX then translocates to the mitochondria, which is followed by mitochondrial membrane depolarization. This results in increased cytosolic mtDNA in a STING-dependent manner. The presence of increased cytosolic mtDNA results in greater cytosolic 2′-3′ cGAMP, creating a positive feedback loop and leading to further increases in STING activation and its downstream signaling. Taken together, we show that STING signaling is critical for BAX-IRF3-mediated mitochondria-induced apoptosis during late-stage C. burnetii infection.

Abstract 2306 Signaling by cyclic and linear dinucleotides

Abstract 2306 Signaling by cyclic and linear dinucleotides

Abstract 2281 Discovery of divergent activity in cyclic dinucleotide signaling

Abstract 2281 Discovery of divergent activity in cyclic dinucleotide signaling

Profiling of cyclic di‐adenyl and ‐guanyl nucleotides and their precursors and degradation products in bacteria using LC–MS/MS

AbstractThe 3,3′‐linked cyclic dinucleotides (CDNs) of adenosine monophosphate (AMP) and guanosine monophosphate (GMP), cyclic di‐AMP (c‐di‐AMP), cyclic di‐GMP (c‐di‐GMP), and c‐GMP–AMP (cGAMP) are second messenger molecules in bacteria that regulate processes, such as biofilm formation, motility, virulence, stress response, and cell wall homeostasis. To analyze the profiles of the three CDNs together with their breakdown and precursor molecules, 5′‐phosphoadenylyl‐(3′ → 5′)adenine (pApA), 5′‐guanylyl‐(3′ → 5′)guanine (pGpG), 5′‐AMP, 3′‐ and 5′‐GMP, adenosine triphosphate (ATP), and GTP, we established an LC–MS/MS‐based approach for semi‐quantification and profiling. Weak anion exchange solid‐phase extraction was employed to improve selectivity and instrumental signal/noise of CDNs as well as pApA and pGpG. CDNs were analyzed using reverse‐phase UHPLC–MS/MS, whereas all other nucleotides were analyzed using hydrophilic interaction chromatography (HILIC)–MS/MS. The instrument limit of quantification ranged from 0.72 (c‐di‐AMP) to 60 nM (ATP and GTP). We applied this method to the analysis of the nine nucleotides in eight bacterial strains and found that the profiles varied widely in terms of both absolute and relative concentrations. Thus, CDN concentrations were generally <1 pmol/mg biomass, and the hydrolysis products, pApA and pGpG, were detected at lower pmol/mg concentrations. The presented method is a relatively simple and straightforward approach to profiling nucleotides with the rationale of comparing their relative levels between populations of bacterial strains.