Regulation and function of the cGAS-MITA/STING axis in health and disease

Activation of the cGAS-STING-IRF3 Axis by Type I and II Interferons Contributes to Host Defense

Interferons (IFNs) activate JAK-STAT pathways to induce downstream effector genes for host defense against invaded pathogens and tumors. Here both type I (β) and II (γ) IFNs are shown that can activate the transcription factor IRF3 in parallel with STAT1. IRF3-deficiency impairs transcription of a subset of downstream effector genes induced by IFN-β and IFN-γ. Mechanistically, IFN-induced activation of IRF3 is dependent on the cGAS-STING-TBK1 axis. Both IFN-β and IFN-γ cause mitochondrial DNA release into the cytosol. In addition, IFNs induce JAK1-mediated tyrosine phosphorylation of cGAS at Y214/Y215, which is essential for its DNA binding activity and signaling. Furthermore, deficiency of cGAS, STING, or IRF3 impairs IFN-β- or IFN-γ-mediated antiviral and antitumor activities. The findings reveal a novel IRF3 activation pathway parallel with the canonical STAT1/2 activation pathways triggered by IFNs and provide an explanation for the pleiotropic roles of the cGAS-STING-IRF3 axis in host defense.

Palmitoylation of KSHV pORF55 is required for Golgi localization and efficient progeny virion production

Kaposi's sarcoma-associated herpesvirus (KSHV) is a double-stranded DNA virus etiologically associated with multiple malignancies. Both latency and sporadic lytic reactivation contribute to KSHV-associated malignancies, however, the specific roles of many KSHV lytic gene products in KSHV replication remain elusive. In this study, we report that ablation of ORF55, a late gene encoding a tegument protein, does not impact KSHV lytic reactivation but significantly reduces the production of progeny virions. We found that cysteine 10 and 11 (C10 and C11) of pORF55 are palmitoylated, and the palmytoilation is essential for its Golgi localization and secondary envelope formation. Palmitoylation-defective pORF55 mutants are unstable and undergo proteasomal degradation. Notably, introduction of a putative Golgi localization sequence to these palmitoylation-defective pORF55 mutants restores Golgi localization and fully reinstates KSHV progeny virion production. Together, our study provides new insight into the critical role of pORF55 palmitoylation in KSHV progeny virion production and offers potential therapeutic targets for the treatment of related malignancies.

Disulfiram ameliorates STING/MITA-dependent inflammation and autoimmunity by targeting RNF115

STING (also known as MITA) is an adaptor protein that mediates cytoplasmic DNA-triggered signaling, and aberrant activation of STING/MITA by cytosolic self-DNA or gain-of-function mutations causes severe inflammation. Here, we show that STING-mediated inflammation and autoimmunity are promoted by RNF115 and alleviated by the RNF115 inhibitor disulfiram (DSF). Knockout of RNF115 or treatment with DSF significantly inhibit systemic inflammation and autoimmune lethality and restore immune cell development in Trex1-/- mice and STINGN153S/WT bone marrow chimeric mice. In addition, knockdown or pharmacological inhibition of RNF115 substantially downregulate the expression of IFN-α, IFN-γ and proinflammatory cytokines in PBMCs from patients with systemic lupus erythematosus (SLE) who exhibit high concentrations of dsDNA in peripheral blood. Mechanistically, knockout or inhibition of RNF115 impair the oligomerization and Golgi localization of STING in various types of cells transfected with cGAMP and in organs and cells from Trex1-/- mice. Interestingly, knockout of RNF115 inhibits the activation and Golgi localization of STINGN153S as well as the expression of proinflammatory cytokines in myeloid cells but not in endothelial cells or fibroblasts. Taken together, these findings highlight the RNF115-mediated cell type-specific regulation of STING and STINGN153S and provide potential targeted intervention strategies for STING-related autoimmune diseases.

Evasion of host defense by Brucella

Brucella , an adept intracellular pathogen, causes brucellosis, a zoonotic disease leading to significant global impacts on animal welfare and the economy. Regrettably, there is currently no approved and effective vaccine for human use. The ability of Brucella to evade host defenses is essential for establishing chronic infection and ensuring stable intracellular growth. Brucella employs various mechanisms to evade and undermine the innate and adaptive immune responses of the host through modulating the activation of pattern recognition receptors (PRRs), inflammatory responses, or the activation of immune cells like dendritic cells (DCs) to inhibit antigen presentation. Moreover, it regulates multiple cellular processes such as apoptosis, pyroptosis, and autophagy to establish persistent infection within host cells. This review summarizes the recently discovered mechanisms employed by Brucella to subvert host immune responses and research progress on vaccines, with the aim of advancing our understanding of brucellosis and facilitating the development of more effective vaccines and therapeutic approaches against Brucella .

Enhancement of the viability of T cells electroporated with DNA via osmotic dampening of the DNA-sensing cGAS-STING pathway

Viral delivery of DNA for the targeted reprogramming of human T cells can lead to random genomic integration, and electroporation is inefficient and can be toxic. Here we show that electroporation-induced toxicity in primary human T cells is mediated by the cytosolic pathway cGAS-STING (cyclic guanosine monophosphate-adenosine monophosphate (cGAMP) synthase-stimulator of interferon genes). We also show that an isotonic buffer, identified by screening electroporation conditions, that reduces cGAS-STING surveillance allowed for the production of chimaeric antigen receptor (CAR) T cells with up to 20-fold higher CAR T cell numbers than standard electroporation and with higher antitumour activity in vivo than lentivirally generated CAR T cells. The osmotic pressure of the electroporation buffer dampened cGAS-DNA interactions, affecting the production of the STING activator 2'3'-cGAMP. The buffer also led to superior efficiencies in the transfection of therapeutically relevant primary T cells and human haematopoietic stem cells. Our findings may facilitate the optimization of electroporation-mediated DNA delivery for the production of genome-engineered T cells.

Mitochondrial DNA-triggered innate immune response: mechanisms and diseases

Various cellular stress conditions trigger mitochondrial DNA (mtDNA) release from mitochondria into the cytosol. The released mtDNA is sensed by the cGAS-MITA/STING pathway, resulting in the induced expression of type I interferon and other effector genes. These processes contribute to the innate immune response to viral infection and other stress factors. The deregulation of these processes causes autoimmune diseases, inflammatory metabolic disorders and cancer. Therefore, the cGAS-MITA/STING pathway is a potential target for intervention in infectious, inflammatory and autoimmune diseases as well as cancer. In this review, we focus on the mechanisms underlying the mtDNA-triggered activation of the cGAS-MITA/STING pathway, the effects of the pathway under various physiological and pathological conditions, and advances in the development of drugs that target cGAS and MITA/STING.

PD-1 signaling negatively regulates the common cytokine receptor γ chain via MARCH5-mediated ubiquitination and degradation to suppress anti-tumor immunity

Combination therapy with PD-1 blockade and IL-2 substantially improves anti-tumor efficacy comparing to monotherapy. The underlying mechanisms responsible for the synergistic effects of the combination therapy remain enigmatic. Here we show that PD-1 ligation results in BATF-dependent transcriptional induction of the membrane-associated E3 ubiquitin ligase MARCH5, which mediates K27-linked polyubiquitination and lysosomal degradation of the common cytokine receptor γ chain (γc). PD-1 ligation also activates SHP2, which dephosphorylates γcY357, leading to impairment of γc family cytokine-triggered signaling. Conversely, PD-1 blockade restores γc level and activity, thereby sensitizing CD8+ T cells to IL-2. We also identified Pitavastatin Calcium as an inhibitor of MARCH5, which combined with PD-1 blockade and IL-2 significantly improves the efficacy of anti-tumor immunotherapy in mice. Our findings uncover the mechanisms by which PD-1 signaling antagonizes γc family cytokine-triggered immune activation and demonstrate that the underlying mechanisms can be exploited for increased efficacy of combination immunotherapy of cancer.

STING is a cell-intrinsic metabolic checkpoint restricting aerobic glycolysis by targeting HK2

Evasion of antitumour immunity is a hallmark of cancer. STING, a putative innate immune signalling adaptor, has a pivotal role in mounting antitumour immunity by coordinating innate sensing and adaptive immune surveillance in myeloid cells. STING is markedly silenced in various human malignancies and acts as a cell-intrinsic tumour suppressor. How STING exerts intrinsic antitumour activity remains unclear. Here, we report that STING restricts aerobic glycolysis independent of its innate immune function. Mechanistically, STING targets hexokinase II (HK2) to block its hexokinase activity. As such, STING inhibits HK2 to restrict tumour aerobic glycolysis and promote antitumour immunity in vivo. In human colorectal carcinoma samples, lactate, which can be used as a surrogate for aerobic glycolysis, is negatively correlated with STING expression level and antitumour immunity. Taken together, this study reveals that STING functions as a cell-intrinsic metabolic checkpoint that restricts aerobic glycolysis to promote antitumour immunity. These findings have important implications for the development of STING-based therapeutic modalities to improve antitumour immunotherapy.

STING Promotes Intestinal IgA Production by Regulating Acetate-producing Bacteria to Maintain Host-microbiota Mutualism

BACKGROUND

Intestinal Immunoglobulin A (IgA) is crucial in maintaining host-microbiota mutualism and gut homeostasis. It has been shown that many species of gut bacteria produce cyclic dinucleotides, along with an abundance of microbiota-derived DNA present within the intestinal lumen, which triggers the tonic activation of the cytosolic cGAS-STING pathway. However, the role of STING in intestinal IgA remains poorly understood. We further investigated whether and how STING affects intestinal IgA response.

METHODS

Intestinal IgA was determined between wild-type (WT) mice and Sting-/- mice in steady conditions and upon enteric Citrobacter rodentium infection. STING agonists were used to stimulating B cells or dendritic cells in vitro. Gut microbiota composition was examined by 16S ribosomal RNA gene sequencing. Bacteria metabolomics functional analyses was performed by PICRUSt2. Fecal short-chain fatty acid (SCFA) was determined by Mass spectrometry and Cedex Bio Analyzer. Gut bacteria from WT mice and Sting-/- mice were transferred into germ-free mice and antibiotic-pretreated mice.

RESULTS

Intestinal IgA response was impaired in Sting-/- mice. However, STING agonists did not directly stimulate B cells or dendritic cells to induce IgA. Interestingly, Sting-/- mice displayed altered gut microbiota composition with decreased SCFA-producing bacteria and downregulated SCFA fermentation pathways. Transfer of fecal bacteria from Sting-/- mice induced less IgA than that from WT mice in germ-free mice and antibiotic-pretreated mice, which is mediated by GPR43. Acetate, the dominant SCFA, was decreased in Sting-/- mice, and supplementation of acetate restored intestinal IgA production in Sting-/- mice.

CONCLUSIONS

STING promotes intestinal IgA by regulating acetate-producing gut bacteria.

MITA/STING-mediated antiviral immunity and autoimmunity: the evolution, mechanism, and intervention

Mediator of IRF3 activation (MITA, also known as stimulator of interferon genes (STING) and endoplasmic reticulum interferon stimulator (ERIS)) is an ER-associated protein that senses cellular and bacterium-derived cyclic dinucleotide (CDN), leading to induction of type-I interferons (IFNs) and innate immune responses against viruses and bacteria. Recently, it has become clear that sensing of CDN and induction of autophagy are two evolutionarily conserved functions of MITA, predating its role in mediating type-I IFN induction. Studies have shown that MITA-mediated signaling promotes a number of autoimmune disorders caused by gene mutations in human. Here, we summarize the most recent progress on MITA-mediated signaling in a view of evolution and highlight the roles of MITA in human inflammatory disorders caused by gene mutations and in genetically modified mouse models. We also briefly introduce the chemicals targeting MITA and discuss their potential in treatment of MITA-mediated inflammatory diseases. Finally, we propose several key questions that should be addressed for targeting MITA for treatment of related autoimmune diseases.

Multifaceted Roles of the E3 Ubiquitin Ligase RING Finger Protein 115 in Immunity and Diseases

Ubiquitination is a post-translational modification that plays essential roles in various physiological and pathological processes. Protein ubiquitination depends on E3 ubiquitin ligases that catalyze the conjugation of ubiquitin molecules on lysine residues of targeted substrates. RING finger protein 115 (RNF115), also known as breast cancer associated gene 2 (BCA2) and Rab7-interacting RING finger protein (Rabring7), has been identified as a highly expressed protein in breast cancer cells and tissues. Later, it has been demonstrated that RNF115 catalyzes ubiquitination of a series of proteins to modulate a number of signaling pathways, and thereby regulates viral infections, autoimmunity, cell proliferation and death and tumorigenesis. In this review, we introduce the identification, expression and activity regulation of RNF115, summarize the substrates and functions of RNF115 in different pathways, and discuss the roles of RNF115 as a biomarker or therapeutic target in diseases.

RNF115 Inhibits the Post-ER Trafficking of TLRs and TLRs-Mediated Immune Responses by Catalyzing K11-Linked Ubiquitination of RAB1A and RAB13

The subcellular localization and intracellular trafficking of Toll-like receptors (TLRs) critically regulate TLRs-mediated antimicrobial immunity and autoimmunity. Here, it is demonstrated that the E3 ubiquitin ligase RNF115 inhibits the post-endoplasmic reticulum (ER) trafficking of TLRs and TLRs-mediated immune responses by catalyzing ubiquitination of the small GTPases RAB1A and RAB13. It is shown that the 14-3-3 chaperones bind to AKT1-phosphorylated RNF115 and facilitate RNF115 localizing on the ER and the Golgi apparatus. RNF115 interacts with RAB1A and RAB13 and catalyzes K11-linked ubiquitination on the Lys49 and Lys61 residues of RAB1A and on the Lys46 and Lys58 residues of RAB13, respectively. Such a modification impairs the recruitment of guanosine diphosphate (GDP) dissociation inhibitor 1 (GDI1) to RAB1A and RAB13, a prerequisite for the reactivation of RAB proteins. Consistently, knockdown of RAB1A and RAB13 in Rnf115^+/+ and Rnf115^-/- cells markedly inhibits the post-ER and the post-Golgi trafficking of TLRs, respectively. In addition, reconstitution of RAB1A^K49/61R or RAB13^K46/58R into Rnf115^+/+ cells but not Rnf115^-/- cells promotes the trafficking of TLRs from the ER to the Golgi apparatus and from the Golgi apparatus to the cell surface, respectively. These findings uncover a common and step-wise regulatory mechanism for the post-ER trafficking of TLRs.

Herpes simplex virus protein UL56 inhibits cGAS-Mediated DNA sensing to evade antiviral immunity

After herpes simplex virus type 1 (HSV-1) infection, the cytosolic sensor cyclic GMP-AMP synthase (cGAS) recognizes DNA and catalyzes synthesis of the second messenger 2'3'-cGAMP. cGAMP binds to the ER-localized adaptor protein MITA (also known as STING) to activate downstream antiviral responses. Conversely, HSV-1-encoded proteins evade antiviral immune responses via a wide variety of delicate mechanisms, promoting viral replication and pathogenesis. Here, we identified HSV-1 envelop protein UL56 as a negative regulator of cGAS-mediated innate immune responses. Overexpression of UL56 inhibited double-stranded DNA-triggered antiviral responses, whereas UL56-deficiency increased HSV-1-triggered induction of downstream antiviral genes. UL56-deficiency inhibited HSV-1 replication in wild-type but not MITA-deficient cells. UL56-deficient HSV-1 showed reduced replication in the brain of infected mice and was less lethal to infected mice. Mechanistically, UL56 interacted with cGAS and inhibited its DNA binding and enzymatic activity. Furthermore, we found that UL56 homologous proteins from different herpesviruses had similar roles in antagonizing cGAS-mediated innate immune responses. Our findings suggest that UL56 is a component of HSV-1 evasion of host innate immune responses by antagonizing the DNA sensor cGAS, which contributes to our understanding of the comprehensive mechanisms of immune evasion by herpesviruses.