Innate immunity in host cells must be rapidly activated to combat invading microbes. Upon RIG-I activation, the transcription of type I interferons is induced within one hour in virus-infected cells. Previous studies have shown that endogenous MAVS spreads signals via aggregation on the mitochondrial membrane, whereas truncated recombinant MAVS forms prion-like filaments in vitro. How MAVS transmits signals so quickly, and the molecular architecture of its membrane aggregates, remains elusive. Here, we report that activated MAVS forms fibrils encircling its resident mitochondrion or connecting neighboring mitochondria with a “ladder-like” structure, allowing the activation of dormant MAVS on encountered mitochondria. This “intermitochondrial activation” process promotes a rapid antiviral response in cells to overcome the immediate danger caused by viruses. Moreover, stuck MAVS fibrils between mitochondria have limited cytosolic protein access and thus relay signals poorly. This study demonstrated that prion-like MAVS fibrils cluster in mitochondria to ensure a rapid antiviral response.

Nod-like receptor family pyrin domain-containing 3 (NLRP3) is activated by many stimuli, and its dysfunction is involved in various inflammatory diseases. Activation of NLRP3 is thought to happen via a multistep process involving phase separation, conformational opening and oligomerization. However, how NLRP3 is released from its autorepressed conformation remains elusive. Here we report that activating molecule in Beclin1-regulated autophagy protein 1 (AMBRA1), previously known for its role in autophagy, bound NLRP3 to scaffold and allosterically activate NLRP3. AMBRA1 engaged the leucine-rich repeat and helical domain 2 subdomains of NLRP3 through its β-propeller domain and destabilized the closed, inactive conformation of NLRP3, facilitating adenosine triphosphate binding and transition of NLRP3 to the active state. AMBRA1 deficiency in monocytes or macrophages impaired NLRP3 activation and reduced inflammatory responses in mouse models of endotoxic shock, colitis and sepsis. Nanobodies blocking the interaction between AMBRA1 and NLRP3 inhibited NLRP3 activation, underscoring the therapeutic potential of targeting this interaction. Our study revealed the role of AMBRA1 in NLRP3 inflammasome assembly and activation, offering potential pharmacological targets for related diseases. Jiang and colleagues show that the adaptor protein AMBRA1 binds NLRP3 to destabilize the closed, inactive conformation of NLRP3 and facilitate binding of ATP and transition to the active state of NLRP3.

The cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) signaling pathway is a crucial component of the innate immune system, responsible for detecting cytosolic double-stranded DNA (dsDNA) from both pathogen invasion and host damage, thereby initiating a robust type I-interferons (IFNs) response. In this review, we summarize the complex and stringent mechanisms governing the activation and regulation of the cGAS-STING pathway. We describe the structural basis of cGAS activation by dsDNA and its catalytic synthesis of 2'3'-cGAMP, and highlight the DNA-independent activation of cGAS by manganese (Mn2 +), which exhibits a distinct catalytic mechanism. We also discuss recent advances in the regulatory mechanisms of cGAS. The binding of 2'3'-cGAMP triggers STING translocation from the ER to the Golgi apparatus, where sulfated glycosaminoglycans (sGAGs) act as an essential second ligand to promote STING polymerization. Following this, a second translocation from the trans-Golgi network (TGN) to endosomes is required for its full activation. Conversely, supranormal concentrations of 2'3'-cGAMP induce the formation of ER-localized STING biocondensates, which restrict activation and thus prevent an excessive immune response. Dysregulation of the cGAS-STING pathway has been implicated in diverse human health conditions, including infection, autoimmune disorders, neurodegeneration, ageing, and cancer. Understanding these activation and regulatory mechanisms will inform the development of novel therapeutic strategies.

Objective: Clinical use of stimulator of interferon genes (STING) agonists has challenges due to poor responsiveness and variable efficacy. Therefore, identifying tumor types that are sensitive to these agents and clarifying the underlying mechanisms are essential. Methods: In vitro screening was performed to identify tumor types that are sensitive to STING agonists. The non-nucleotide agonist, SR-717, and the macrocyclic agonist, E7766, were compared for efficacy. Complementary in vivo and in vitro studies, including gene-knockout models, HMGN2-knockout Neuro-2A and CT-2A cells apoptosis assays, and murine tumor models, were then performed. These experiments focused on the mechanism by which SR-717 mediates antitumor effects and emphasized the role of STING signaling-induced high-mobility group nucleosome-binding protein 2 (HMGN2). In addition, the potential of HMGN2 as a prognostic biomarker was assessed. Results: Neuroblastomas and glioblastomas, two nervous system tumors, were shown to be sensitive to STING agonists. SR-717 exhibited greater antitumor efficacy compared to E7766. Mechanistic studies indicated that STING agonists promote apoptosis through activation of the intrinsic STING-signal transducer and activator of transcription 1 (STAT1)-HMGN2 axis within tumor cells. Ectopic expression of HMGN2 in melanoma cells, which naturally lack HMGN2, led to significant apoptosis. Furthermore, analysis of The Cancer Genome Atlas and Gene Expression Omnibus databases revealed positive correlation between elevated HMGN2 expression and patient survival, supporting the utility of HMGN2 as a prognostic biomarker. Conclusions: This study clarified the mechanism underlying the potent antitumor activity of SR-717 in nervous system tumors through activation of the STING-STAT1-HMGN2 signaling pathway and demonstrated that SR-717 has superior efficacy compared to E7766. In addition, HMGN2 was shown to exhibit translational potential as a prognostic biomarker for patient survival.

Immune responses need to be tightly controlled to avoid excessive inflammation and prevent unwanted host damage. Here we report that germinal center kinase MST4 responded dynamically to bacterial infection and acted as a negative regulator of inflammation. We found that MST4 directly interacted with and phosphorylated the adaptor TRAF6 to prevent its oligomerization and autoubiquitination. Accordingly, MST4 did not inhibit lipopolysaccharide-induced cytokine production in Traf6(-/-) embryonic fibroblasts transfected to express a mutant form of TRAF6 that cannot be phosphorylated at positions 463 and 486 (with substitution of alanine for threonine at those positions). Upon developing septic shock, mice in which MST4 was knocked down showed exacerbated inflammation and reduced survival, whereas heterozygous deletion of Traf6 (Traf6(+/-)) alleviated such deleterious effects. Our findings reveal a mechanism by which TRAF6 is regulated and highlight a role for MST4 in limiting inflammatory responses.

The Type VI secretion system (T6SS) is a key virulence mechanism utilized by many Gram-negative bacteria to mediate the microbial competition and host pathogenesis. Despite the identification of diverse T6SS effectors targeting eukaryotic or prokaryotic cells, the trans-kingdom T6SS effectors that simultaneously target both eukaryotic and prokaryotic cells remain rarely reported. In this study, it is demonstrated that Yersinia pseudotuberculosis (Yptb) T6SS secretes a DNase effector, TkeA, which induces apoptosis in host cells. The translocation of TkeA into host cells causes nuclear DNA damage. This, in turn, activates the DNA-sensing cyclic GMP-AMP synthase (cGAS)/stimulator of interferon genes (STING) pathway. The activation of the cGAS-STING pathway by TkeA subsequently triggers apoptosis in host cells via extrinsic pathways, with tumor necrosis factor (TNF) signaling playing a critical role. Additionally, TkeA enhances bacterial competition by targeting rival bacteria, thereby promoting host colonization. These findings reveal that the transkingdom T6SS effector TkeA executes a "one weapon, two battlefields" strategy, acting as a trans-kingdom effector that enhances interbacterial competition while inducing apoptosis in host cells through the activation of the cGAS-STING-TNF axis. This highlights a previously unrecognized dimension of bacterial virulence strategies and expands the understanding of host-pathogen interactions involving T6SS effectors.

Abscopal effect remains rare and clinically unpredictable because of immunosuppressive mechanisms and inadequate immune activation. This phenomenon may stem from cyclic GMP-AMP synthase (cGAS) sequestration within the nucleus, where high-affinity nucleosome-DNA interactions potently suppress cGAS activation. Our study elucidates the capacity of manganese (Mn2+) to overcome this limitation through cytoplasmic relocalization and potentiation of cGAS activity, thereby enabling abscopal responses. We demonstrated that Mn2+ disrupts nucleosomal constraints on cGAS, permitting robust double-stranded DNA (dsDNA) sensing and activation. This molecular reprogramming amplifies cytosolic cGAS-STING signaling cascades and IFN-I production in irradiated tumors, potentiating systemic antitumor immunity. Notably, Mn2+ exerts direct immunostimulatory effects on adaptive immunity by activating T cells, creating synergistic therapeutic benefits. Preliminary clinical observations in advanced metastatic malignancies have demonstrated that Mn2+ augments abscopal responses during radiotherapy. These findings mechanistically delineate the dual role of Mn2+ in modulating tumor-intrinsic cGAS-STING activation and immune effector functions, supporting its therapeutic application in combination with radioimmunotherapy regimens to overcome tumor microenvironment immunosuppression and induce abscopal effect.

NLRP3 inflammasome is activated by diverse stimuli including infections, intracellular and environmental irritants. How NLRP3 senses these unrelated stimuli and what activates NLRP3 remain unknown. Here we report that signal-dependent NLRP3 phase separation initiated its activation, in which the palmitoyltransferase ZDHHC7-mediated tonic NLRP3 palmitoylation and an IDR region in the FISNA domain of NLRP3 play important roles. Moreover, three conserved hydrophobic residues in the IDR critically mediate multivalent weak interactions. NLRP3-activating stimuli including K+ efflux and NLRP3-interacting molecules imiquimod, palmitate, and cardiolipin all cause NLRP3 conformational change and induce its phase separation and activation in cells and/or in vitro. Surprisingly, amphiphilic molecules like di-alcohols used to inhibit biomolecular phase separation and chemotherapeutic drugs doxorubicin and paclitaxel activate NLRP3 independently of ZDHHC7 by directly inducing NLRP3 phase separation. Mechanistically, amphiphilic molecules decrease the solubility of both palmitoylated and non-palmitoylated NLRP3 to directly induce its phase separation and activation while NLRP3 palmitoylation reduces its solubility to some extent without activation. Therefore, ZDHHC7-mediated NLRP3 palmitoylation in resting cells licenses its activation by lowering the threshold for NLRP3 phase separation in response to any of the diverse stimuli whereas NLRP3 solubility-reducing molecules like di-alcohols and chemotherapeutic drugs activate NLRP3 directly. The signal-induced NLRP3 phase separation likely provides the simplest and most direct mechanistic basis for NLRP3 activation.

Inflammasome activation drives pyroptotic cell death and the release of inflammatory cytokines, and many diseases involve its overactivation. Zinc is essential for all organisms as a trace element, but its functions in innate immunity remain undefined. Here, we reported that Zn2+ inhibits caspase-1 to hinder inflammasome activation. We first identified the zinc exporter solute carrier family 30 member 1 (SLC30A1) as an inflammasome regulator, using a genome-wide CRISPR-Cas9-mediated screen. SLC30A1 deficiency suppressed multiple inflammasomes by increasing intracellular levels of Zn2+, which bound and inhibited caspase-1 at its active site residues H237, C244 and C285. Mutation of these residues almost completely blocked zinc binding. Similarly, Zn2+ also inhibited caspase-4/5/11-mediated noncanonical inflammasome activation. Importantly, zinc supplementation significantly relieved cecal ligation and puncture (CLP)-induced sepsis, Imiquimod (IMQ)-induced psoriasis and Alzheimer’s disease. Thus, zinc might be used to treat inflammasome-related diseases as a broad-spectrum inflammasome inhibitor.

Autoimmune diseases are immune disorders in which the immune system mistakenly targets and attacks the normal cells, tissues, and organs of the patients. The etiology of autoimmune diseases is complex and multi-factorial. This review aims to provide a general introduction to the immunological functions of inflammasomes and describe the role of inflammasomes in disorders characterized by self-directed inflammation, with a particular focus on several common autoimmune diseases. We reviewed current research on the mechanisms of inflammasome activation and inflammasomes' contribution to autoimmune diseases. Inhibitors targeting inflammasome components were also explored for their potential in therapeutic applications. With increasing research on mechanisms of inflammation of the pathologic conditions, accumulated evidence suggests that the aberrant or uncontrolled activation of inflammasomes contributes to the pathogenesis and development of autoimmune diseases. Additionally, inflammasome-targeting drugs have shown promise in treating autoimmune diseases. Insights into the mechanisms governing inflammasome activation and their roles in autoimmune diseases could contribute to the development of novel anti-inflammatory drugs for the prevention and treatment of autoimmune diseases with enhanced targeting precision and reduced adverse reactions.

The STING pathway plays a critical role in tumor immunosurveillance. However, the precise mechanisms by which STING regulates gamma delta (γδ) T cell function during tumor progression remain unclear. Herein, we find that tumor-derived cyclic GMP-AMP (cGAMP) activates a distinct STING pathway by inducing TBK1-mediated phosphorylation of Eomes in γδ T cells during the early stage of tumor development is demonstrated. This activation leads to interferon-gamma (IFN-γ) production and consequent tumor surveillance. However, at advanced stages of tumor progression, the accumulation of immune-suppressive cytokine transforming growth factor-beta (TGF-β) downregulates STING levels, compromising the function of γδ T cells. Notably, the synergism between TGF-β inhibition and STING agonists effectively counteracts the immunosuppressive tumor microenvironment, thereby augmenting the antitumoral effects of γδ T cells. These findings present a novel mechanism involving STING-mediated IFN-γ production in γδ T cells and hold significant implications for the development of potent immunotherapeutic approaches against cancer.

Platinum-resistant or refractory ovarian cancer (PROC) remains without immunotherapy approval and dismal prognosis, emphasizing the urgent need for novel therapies. This phase 2, single-blind, placebo-controlled, randomized trial evaluated the safety and efficacy of manganese chloride or placebo plus sintilimab, nab-paclitaxel and cisplatin in these patients. 84 patients were randomized to the Mn2+ (n=55) or placebo (n=29) arm. The primary endpoint of objective response rate (ORR) was met at 61.8% in the Mn2+ and 13.8% in placebo group. The secondary endpoints of median PFS (9.8 vs. 3.9 months), OS (21.4 vs. 7.9 months) and DOR (14.9 vs. 1.8 months) were significantly prolonged in the Mn2+ group. No significant differences in AEs and quality of life were document during the treatment period. The serum cytokines and scRNA-seq evidenced the cGAS-STING agonist function of Mn2+. Our study supported Mn2+-priming immunochemotherapy as a promising treatment regimen for PROC patients. ClinicalTrials.gov identifier: NCT03989336.

Divalent metal ions such as magnesium (Mg2+), manganese (Mn2+), and zinc (Zn2+) play important roles in regulating innate immune responses. Lipopolysaccharide stimulation led to increased intracellular Mn and Zn in macrophages. However, the effect of those metal ions in regulating lipopolysaccharide-induced innate immune responses remains unclear. Here, we uncovered that both Mn2+ and Zn2+ have immunostimulatory effects, which could potentiate the lipopolysaccharide-induced expression of interferon-stimulated genes (ISGs), cytokines and pro-inflammatory genes in a dose-dependent manner. Enhancement of lipopolysaccharide-induced innate immune gene expression by Mn2+ varies between 10 % and 900 %. Conversely, the chelating of Mn2+ almost totally diminished Mn2+-enhanced lipopolysaccharide-induced gene expression. In addition, Mn2+ exerted its ability to potentiate LPS-induced innate immune gene expression regardless of slight pH changes. Importantly, we found that Mn2+ potentiates lipopolysaccharide-induced immune responses independent of TLR4 but partially relies on cGAS-STING pathway. Further in vivo study showed that colloidal Mn2+ salt (Mn jelly [MnJ]) pretreatment exacerbated lipopolysaccharide-induced septic shock and mice death. In conclusion, we demonstrated that Mn2+ plays an essential role in boosting lipopolysaccharide-induced innate immune responses. These findings greatly expand the current understanding of the immunomodulatory potential of divalent metal Mn2+ and may provide a potential therapeutic target to prevent excessive immune responses.

The cGAS-STING pathway mediates cytoplasmic DNA-triggered innate immunity. STING activation is initiated by cyclic-GMP-AMP (cGAMP)-induced translocation from the endoplasmic reticulum and sulfated glycosaminoglycans-induced polymerization at the Golgi. Here, we examine the mechanisms underlying STING transport and activation beyond the Golgi. A genome-wide CRISPR-Cas9 screen identified Armadillo-like helical domain-containing protein 3 (ARMH3) as critical for STING activation. Upon cGAMP-triggered translocation, ARMH3 interacted with STING at the Golgi and recruited phosphatidylinositol 4-kinase beta (PI4KB) to synthesize PI4P, which directed STING Golgi-to-endosome trafficking via PI4P-binding proteins AP-1 and GGA2. Disrupting PI4P-dependent lipid transport through RNAi of other PI4P-binding proteins impaired STING activation. Consistently, disturbed lipid composition inhibited STING activation, whereas aberrantly elevated cellular PI4P led to cGAS-independent STING activation. Armh3(fl/fll)Lyz(Cre/Cre) mice were susceptible to DNA virus challenge in vivo. Thus, ARMH3 bridges STING and PIK4B to generate PI4P for STING transportation and activation, an interaction conserved in all eukaryotes.

Dual-specificity phosphatase 6 (DUSP6) serves a specific and conserved function on the dephosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2). We previously identified Dusp6 as a regenerative repressor during zebrafish heart regeneration, therefore we propose to investigate the role of this repressor in mammalian cardiac repair. Utilizing a rat strain harboring Dusp6 nonsense mutation, rat neutrophil-cardiomyocyte co-culture, bone marrow transplanted rats and neutrophil-specific Dusp6 knockout mice, we find that Dusp6 deficiency improves cardiac outcomes by predominantly attenuating neutrophil-mediated myocardial damage in acute inflammatory phase after myocardial infarction. Mechanistically, Dusp6 is transcriptionally activated by p38-C/EBPβ signaling and acts as an effector for maintaining p-p38 activity by down-regulating pERK and p38-targeting phosphatases DUSP1/DUSP16. Our findings provide robust animal models and novel insights for neutrophil-mediated cardiac damage and demonstrate the potential of DUSP6 as a therapeutic target for post-MI cardiac remodeling and other relevant inflammatory diseases.

STING, an endoplasmic reticulum (ER) transmembrane protein, mediates innate immune activation upon cGAMP stimulation and is degraded through autophagy. Here, we report that activated STING could be transferred between cells to promote antitumor immunity, a process triggered by RAB22A-mediated non-canonical autophagy. Mechanistically, RAB22A engages PI4K2A to generate PI4P that recruits the Atg12–Atg5–Atg16L1 complex, inducing the formation of ER-derived RAB22A-mediated non-canonical autophagosome, in which STING activated by agonists or chemoradiotherapy is packaged. This RAB22A-induced autophagosome fuses with RAB22A-positive early endosome, generating a new organelle that we name Rafeesome ( R AB22A-mediated non-canonical a utophagosome f used with e arly e ndo some ). Meanwhile, RAB22A inactivates RAB7 to suppress the fusion of Rafeesome with lysosome, thereby enabling the secretion of the inner vesicle of the autophagosome bearing activated STING as a new type of extracellular vesicle that we define as R-EV ( R AB22A-induced e xtracellular v esicle). Activated STING-containing R-EVs induce IFNβ release from recipient cells to the tumor microenvironment, promoting antitumor immunity. Consistently, RAB22A enhances the antitumor effect of the STING agonist diABZI in mice, and a high RAB22A level predicts good survival in nasopharyngeal cancer patients treated with chemoradiotherapy. Our findings reveal that Rafeesome regulates the intercellular transfer of activated STING to trigger and spread antitumor immunity, and that the inner vesicle of non-canonical autophagosome originated from ER is secreted as R-EV, providing a new perspective for understanding the intercellular communication of organelle membrane proteins.

The cGAS-STING pathway is responsible for cytoplasmic double-stranded DNA (dsDNA) -triggered innate immunity and involved in the pathology of various diseases including infection, autoimmune diseases, neurodegeneration and cancer. Understanding the activation and regulatory mechanisms of this pathway is critical to develop therapeutic strategies toward these diseases. Here, we review the signal transduction, cellular functions and regulations of cGAS and STING, particularly highlighting the latest understandings on the activation of cGAS by dsDNA and/or Manganese (Mn2+), STING trafficking, sulfated glycosaminoglycans (sGAGs)-induced STING polymerization and activation, and also regulation of the cGAS-STING pathway by different biocondensates formed via phase separation of proteins from host cells and viruses.

ABSTRACT The cyclic guanosine monophosphate (GMP)–adenosine monophosphate (AMP) synthetase (cGAS)–stimulator of interferon genes (STING) pathway, comprising the DNA sensor cGAS, the second messenger cyclic GMP–AMP (cGAMP), and the endoplasmic reticulum (ER) adaptor protein STING, detects cytoplasmic double-stranded DNA (dsDNA) to trigger type I-interferon responses for host defense against pathogens. Previous studies defined a model for the allosteric activation of cGAS by DNA-binding, but recent work reveals other layers of mechanisms to regulate cGAS activation such as the phase condensation and metal ions, especially the discovery of Mn2+ as a cGAS activator. Activation of the 2′3′-cGAMP sensor STING requires translocating from the ER to the Golgi apparatus. The sulfated glycosaminoglycans at the Golgi are found to be the second STING ligand promoting STING oligomerization and activation in addition to 2′3′-cGAMP, while surpassed levels of 2′3′-cGAMP induce ER-located STING to form a highly organized ER membranous condensate named STING phase-separator to restrain STING activation. Here, we summarize recent advances in the regulation of cGAS–STING activation and their implications in physiological or pathological conditions, particularly focusing on the emerging complexity of the regulation.

Significance Although emerging evidence suggests that the STING-mediated immune response pathway plays a crucial role in microbial pathogen infection, few bacterial effectors have been reported to target this pathway. Here, we identified a T6SS-secreted micropeptide, TssS, which is crucial for the pathogenesis of Yptb . Distinct from traditional bacterial effectors that target host proteins or other macromolecules, TssS inhibits STING oligomerization and downstream signaling pathways by chelating Mn 2+ . Thus, TssS mediates a previously unrecognized immune evasion mechanism by modulating the availability of immunostimulatory Mn 2+ in host cells. This finding reveals a strategy to modulate the STING pathway by microbial pathogens, provides a new perspective on the role of T6SS in pathogenesis, and highlights the importance of micropeptides in pathogen–host interactions.