MPYS is required for IFN response factor 3 activation and type I IFN production in the response of cultured phagocytes to bacterial second messengers cyclic-di-AMP and cyclic-di-GMP

STING Polymer Structure Reveals Mechanisms for Activation, Hyperactivation, and Inhibition

How the central innate immune protein, STING, is activated by its ligands remains unknown. Here, using structural biology and biochemistry, we report that the metazoan second messenger 2'3'-cGAMP induces closing of the human STING homodimer and release of the STING C-terminal tail, which exposes a polymerization interface on the STING dimer and leads to the formation of disulfide-linked polymers via cysteine residue 148. Disease-causing hyperactive STING mutations either flank C148 and depend on disulfide formation or reside in the C-terminal tail binding site and cause constitutive C-terminal tail release and polymerization. Finally, bacterial cyclic-di-GMP induces an alternative active STING conformation, activates STING in a cooperative manner, and acts as a partial antagonist of 2'3'-cGAMP signaling. Our insights explain the tight control of STING signaling given varying background activation signals and provide a therapeutic hypothesis for autoimmune syndrome treatment.

Distinct Dynamic and Conformational Features of Human STING in Response to 2'3'-cGAMP and c-di-GMP

The human stimulator of interferon genes protein (hSTING) can bind cyclic dinucleotides (CDNs) to activate the production of type I interferons and inflammatory cytokines. These CDNs can be either bacterial secondary messengers, 3'3'-CDNs, or endogenous 2'3'-cGAMP. cGAMP, with a unique 2'-5' bond, is the most potent activator of hSTING among all CDNs. However, current understanding of the molecular principles underlying the unique ability of 2'3'-cGAMP to potently activate hSTINGs other than 3'3'-CDNs remains incomplete. In this work, molecular dynamics simulations were used to provide an atomistic picture of the binding of 2'3'-cGAMP and one 3'3'-CDN (c-di-GMP) to hSTING. The results suggest that hSTING binds more strongly to 2'3'-cGAMP than to c-di-GMP, which prefers to bind with a more open and flexible state of hSTING. Finally, a potential "dock-lock-anchor" mechanism is proposed for the activation of hSTING upon the binding of a potent ligand. It is believed that deep insights into understanding the binding of hSTING with 3'3'-CDNs and the endogenous 2'3'-cGAMP would help to establish the principles underlying powerful 2'3'-cGAMP signaling and the nature of hSTING activation, as well as related drug design.

cGAS facilitates sensing of extracellular cyclic dinucleotides to activate innate immunity

Cyclic dinucleotides (CDNs) are important second messenger molecules in prokaryotes and eukaryotes. Within host cells, cytosolic CDNs are detected by STING and alert the host by activating innate immunity characterized by type I interferon (IFN) responses. Extracellular bacteria and dying cells can release CDNs, but sensing of extracellular CDNs (eCDNs) by mammalian cells remains elusive. Here, we report that endocytosis facilitates internalization of eCDNs. The DNA sensor cGAS facilitates sensing of endocytosed CDNs, their perinuclear accumulation, and subsequent STING-dependent release of type I IFN Internalized CDNs bind cGAS directly, leading to its dimerization, and the formation of a cGAS/STING complex, which may activate downstream signaling. Thus, eCDNs comprise microbe- and danger-associated molecular patterns that contribute to host-microbe crosstalk during health and disease.

USP20 Promotes Cellular Antiviral Responses via Deconjugating K48-Linked Ubiquitination of MITA

Mediator of IRF3 activation ([MITA] also known as STING) is a direct sensor of cyclic dinucleotide and critically mediates cytoplasmic DNA--triggered innate immune signaling. The activity of MITA is extensively regulated by ubiquitination and deubiquitination. In this study, we report that USP20 interacts with and removes K48-linked ubiquitin chains from MITA after HSV-1 infection, thereby stabilizing MITA and promoting cellular antiviral responses. Deletion of USP20 accelerates HSV-1-induced degradation of MITA and impairs phosphorylation of IRF3 and IκBα as well as subsequent induction of type I IFNs and proinflammatory cytokines after HSV-1 infection or cytoplasmic DNA challenge. Consistently, Usp20 ^-/- mice produce decreased type I IFNs and proinflammatory cytokines, exhibit increased susceptibility to lethal HSV-1 infection, and aggravated HSV-1 replication compared with Usp20 ^+/+ mice. In addition, complement of MITA into Usp20 ^-/- cells fully restores HSV-1-triggered signaling and inhibits HSV-1 infection. These findings suggest a crucial role of USP20 in maintaining the stability of MITA and promoting innate antiviral signaling.

The herpesviral antagonist m152 reveals differential activation of STING-dependent IRF and NF-κB signaling and STING's dual role during MCMV infection

Cytomegaloviruses (CMVs) are master manipulators of the host immune response. Here, we reveal that the murine CMV (MCMV) protein m152 specifically targets the type I interferon (IFN) response by binding to stimulator of interferon genes (STING), thereby delaying its trafficking to the Golgi compartment from where STING initiates type I IFN signaling. Infection with an MCMV lacking m152 induced elevated type I IFN responses and this leads to reduced viral transcript levels both in vitro and in vivo. This effect is ameliorated in the absence of STING. Interestingly, while m152 inhibits STING‐mediated IRF signaling, it did not affect STING‐mediated NF‐κB signaling. Analysis of how m152 targets STING translocation reveals that STING activates NF‐κB signaling already from the ER prior to its trafficking to the Golgi. Strikingly, this response is important to promote early MCMV replication. Our results show that MCMV has evolved a mechanism to specifically antagonize the STING‐mediated antiviral IFN response, while preserving its pro‐viral NF‐κB response, providing an advantage in the establishment of an infection.

Chronic neurodegeneration induces type I interferon synthesis via STING, shaping microglial phenotype and accelerating disease progression

Type I interferons (IFN‐I) are the principal antiviral molecules of the innate immune system and can be made by most cell types, including central nervous system cells. IFN‐I has been implicated in neuroinflammation during neurodegeneration, but its mechanism of induction and its consequences remain unclear. In the current study, we assessed expression of IFN‐I in murine prion disease (ME7) and examined the contribution of the IFN‐I receptor IFNAR1 to disease progression. The data indicate a robust IFNβ response, specifically in microglia, with evidence of IFN‐dependent genes in both microglia and astrocytes. This IFN‐I response was absent in stimulator of interferon genes (STING^−/−) mice. Microglia showed increased numbers and activated morphology independent of genotype, but transcriptional signatures indicated an IFNAR1‐dependent neuroinflammatory phenotype. Isolation of microglia and astrocytes demonstrated disease‐associated microglial induction of Tnfα, Tgfb1, and of phagolysosomal system transcripts including those for cathepsins, Cd68, C1qa, C3, and Trem2, which were diminished in IFNAR1 and STING deficient mice. Microglial increases in activated cathepsin D, and CD68 were significantly reduced in IFNAR1^−/− mice, particularly in white matter, and increases in COX‐1 expression, and prostaglandin synthesis were significantly mitigated. Disease progressed more slowly in IFNAR1^−/− mice, with diminished synaptic and neuronal loss and delayed onset of neurological signs and death but without effect on proteinase K‐resistant PrP levels. Therefore, STING‐dependent IFN‐I influences microglial phenotype and influences neurodegenerative progression despite occurring secondary to initial degenerative changes. These data expand our mechanistic understanding of IFN‐I induction and its impact on microglial function during chronic neurodegeneration.

Immature lung TNFR2- conventional DC 2 subpopulation activates moDCs to promote cyclic di-GMP mucosal adjuvant responses in vivo

Cyclic dinucleotides (CDNs), including cyclic di-GMP (CDG), are promising vaccine adjuvants in preclinical/clinical trials. The in vivo mechanisms of CDNs are not clear. Here we investigated the roles of lung DC subsets in promoting CDG mucosal adjuvant responses in vivo. Using genetically modified mice and adoptive cell transfer, we identified lung conventional DC 2 (cDC2) as the central player in CDG mucosal responses. We further identified two functionally distinct lung cDC2 subpopulations: TNFR2+pRelB+ and TNFR2-pRelB- cDC2. The TNFR2+ cDC2 were mature and migratory upon intranasal CDG administration while the TNFR2- cDC2 were activated but not mature. Adoptive cell transfer showed that TNFR2- cDC2 mediate the antibody responses of CDG, while the TNFR2+ cDC2 generate Th1/17 responses. Mechanistically, immature TNFR2- cDC2 activate monocyte-derived DCs (moDCs), which do not take up intranasally administered CDG. moDCs promote CDG-induced generation of T follicular helper- and germinal center B cells in the lungs. Our data revealed a previously undescribed in vivo mode of DCs action, whereby an immature lung TNFR2- cDC2 subpopulation directs the non-migratory moDCs to generate CDG mucosal responses in the lung.

Discriminating cyclic from linear nucleotides - CRISPR/Cas-related cyclic hexaadenosine monophosphate as a case study

Nucleic acids exist in biological systems as linear and cyclic forms and in most cases the biology of the cyclic form is different from the linear form of exactly the same sequence. Case examples are cyclic nucleotides, second messengers in both prokaryotes and eukaryotes whereby the cyclic forms account for their interesting biological profiles and the actions of the cyclic nucleotides are terminated upon phosphodiesterase hydrolysis into linear forms. For mono and dinucleotides, it has been shown that vast conformational changes that accompany the hydrolysis of the cyclized form allow for discrimination between the cyclized and linear forms. As the ring size increases, it becomes difficult to use conformational or structural differences alone to discriminate between cyclic and linear nucleotides. Here we reveal that for the recently discovered CRISPR/Cas-related cyclic hexaadenosine monophosphate, it is possible to discriminate between the cyclized and linear forms. The structures of c-HexaAMP and linear form are different in acidic media and this structural difference facilitated a simple and practical detection platform for this interesting and new bacterial immunity-related molecule, and we also demonstrate that it is possible to distinguish between linear and cyclized polynucleotides using simple spectroscopic techniques, such as CD and fluorescence-based methods.

Molecular Characterization, Expression and Functional Analysis of Chicken STING

Innate immunity is an essential line of defense against pathogen invasion which is gained at birth, and the mechanism involved is mainly to identify pathogen-associated molecular patterns through pattern recognition receptors. STING (stimulator of interferon genes) is a signal junction molecule that hosts the perception of viral nucleic acids and produces type I interferon response, which plays a crucial role in innate immunity. However, relatively few studies have investigated the molecular characterization, tissue distribution, and potential function of STING in chickens. In this study, we cloned the full-length cDNA of chicken STING that is composed of 1341 bp. Sequence analyses revealed that STING contains a 1140-bp open-reading frame that probably encodes a 379-amino acid protein. Multiple sequence alignments showed that the similarity of the chicken STING gene to other birds is higher than that of mammals. Real-time polymerase chain reaction (PCR) assays revealed that STING is highly expressed in the spleen, thymus and bursa of fabricious in chickens. Furthermore, we observed that STING expression was significantly upregulated both in vitro and in vivo following infection with Newcastle disease virus (NDV). STING expression was also significantly upregulated in chicken embryo fibroblasts upon stimulation with poly(I:C) or poly(dA:dT). Taken together, these findings suggest that STING plays an important role in antiviral signaling pathways in chickens.

Role of the cGAS-STING pathway in cancer development and oncotherapeutic approaches

The cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) pathway mediates anti-microbial innate immunity by inducing the production of type I interferons (IFNs) and inflammatory cytokines upon recognition of microbial DNA. Recent studies reveal that self-DNA from tumors and by-products of genomic instability also activates the cGAS-STING pathway and either promotes or inhibits tumor development. This has led to the development of cancer therapeutics using STING agonists alone and in combination with conventional cancer treatment or immune checkpoint targeting. On the other hand, for cancers lacking the cGAS-STING pathway and thus a regular innate immunity response, oncolytic virus therapy has been shown to have therapeutic potential. We here review and discuss the dichotomous roles of the cGAS-STING pathway in cancer development and therapeutic approaches.

B Cell-Intrinsic STING Signaling Triggers Cell Activation, Synergizes with B Cell Receptor Signals, and Promotes Antibody Responses

Generation of protective immune responses requires coordinated stimulation of innate and adaptive immune responses. An important mediator of innate immunity is stimulator of IFN genes (STING, MPYS, MITA), a ubiquitously but differentially expressed adaptor molecule that functions in the relay of signals initiated by sensing of cytosolic DNA and bacterial cyclic dinucleotides (CDNs). Whereas systemic expression of STING is required for CDN-aided mucosal Ab responses, its function in B cells in particular is unclear. In this study, we show that B cells can be directly activated by CDNs in a STING-dependent manner in vitro and in vivo. Direct activation of B cells by CDNs results in upregulation of costimulatory molecules and cytokine production and this can be accompanied by caspase-dependent cell death. CDN-induced cytokine production by B cells and other cell types also contributes to activation and immune responses. Type I IFN is primarily responsible for this indirect stimulation although other cytokines may contribute. BCR and STING signaling pathways act synergistically to promote Ab responses independent of type I IFN. B cell expression of STING is required for optimal in vivo IgG and mucosal IgA Ab responses induced by T cell-dependent Ags and cyclic-di-GMP but plays no discernable role in Ab responses in which alum is used as an adjuvant. Thus, STING functions autonomously in B cells responding to CDNs, and its activation synergizes with Ag receptor signals to promote B cell activation.

Cyclic Dimeric Guanosine Monophosphate: Activation and Inhibition of Innate Immune Response

Cyclic dimeric guanosine monophosphate (c-di-GMP) is a universally conserved second messenger that contributes to the pathogenicity of numerous bacterial species. In recent years, growing evidence has shown that bacterial extracellular c-di-GMP can interact with the innate immune system and regulate host immune responses. This review summarizes our current understanding on the dual roles of bacterial c-di-GMP in pathogen-host interaction: activation of the antibacterial innate immune response through the cytosolic surveillance pathway and inhibition of innate immune defense for iron restriction.

Cyclic di-AMP-mediated interaction between Mycobacterium tuberculosis ΔcnpB and macrophages implicates a novel strategy for improving BCG vaccination

Cyclic di-AMP (c-di-AMP) has been shown to play an important role in bacterial physiology and pathogen-host interactions. We previously reported that deletion of the sole c-di-AMP phosphodiesterase-encoding gene (cnpB) in Mycobacterium tuberculosis (Mtb) led to significant virulence attenuation. In this study, we found that ΔcnpB of M. bovisbacillus Calmette-Guerin (BCG) was unable to secrete c-di-AMP, which differs from Mtb ΔcnpB. We infected bone marrow-derived macrophages (BMDMs) with c-di-AMP-associated mutants generated from both Mtb and BCG. Our results showed that upon infection with Mtb ΔcnpB, BMDMs of wildtype mice secreted a large amount of interferon-β (IFN-β) post-infection similarly as we reported previously. In contrast, the response was less pronounced with BMDMs isolated from cGAS-/- mice and was nearly abolished with BMDMs prepared from STING-/- mice. Deletion of the region of difference 1 (RD1) locus in Mtb ΔcnpB did not alter the c-di-AMP secretion of ΔcnpB but eliminated the IFN-β production in the infected cells. In contrast, neither BCG ΔcnpB nor a recombinant BCG ΔcnpB with a pRD1 cosmid induced a type I interferon response. Interestingly, multiple studies have demonstrated that type I IFN enhances BCG's immunity. Thus, amending BCG based on our findings might improve BCG vaccination.

Cytosolic DNA Sensing in Organismal Tumor Control

Besides constituting a first layer of defense against microbial challenges, the detection of cytosolic DNA is fundamental for mammalian organisms to control malignant transformation and tumor progression. The accumulation of DNA in the cytoplasm can initiate the proliferative inactivation (via cellular senescence) or elimination (via regulated cell death) of neoplastic cell precursors. Moreover, cytosolic DNA sensing is intimately connected to the secretion of cytokines that support innate and adaptive antitumor immunity. Here, we discuss the molecular mechanisms whereby cytosolic DNA enables cell-intrinsic and -extrinsic oncosuppression, and their relevance for the development of novel therapeutic approaches that reinstate anticancer immunosurveillance.

Detection of Microbial Infections Through Innate Immune Sensing of Nucleic Acids

Microbial infections are recognized by the innate immune system through germline-encoded pattern recognition receptors (PRRs). As most microbial pathogens contain DNA and/or RNA during their life cycle, nucleic acid sensing has evolved as an essential strategy for host innate immune defense. Pathogen-derived nucleic acids with distinct features are recognized by specific host PRRs localized in endolysosomes and the cytosol. Activation of these PRRs triggers signaling cascades that culminate in the production of type I interferons and proinflammatory cytokines, leading to induction of an antimicrobial state, activation of adaptive immunity, and eventual clearance of the infection. Here, we review recent progress in innate immune recognition of nucleic acids upon microbial infection, including pathways involving endosomal Toll-like receptors, cytosolic RNA sensors, and cytosolic DNA sensors. We also discuss the mechanisms by which infectious microbes counteract host nucleic acid sensing to evade immune surveillance.

ALK is a therapeutic target for lethal sepsis

Sepsis, a life-threatening organ dysfunction caused by infection, is a major public health concern with limited therapeutic options. We provide evidence to support a role for anaplastic lymphoma kinase (ALK), a tumor-associated receptor tyrosine kinase, in the regulation of innate immunity during lethal sepsis. The genetic disruption of ALK expression diminishes the stimulator of interferon genes (STING)-mediated host immune response to cyclic dinucleotides in monocytes and macrophages. Mechanistically, ALK directly interacts with epidermal growth factor receptor (EGFR) to trigger serine-threonine protein kinase AKT phosphorylation and activate interferon regulatory factor 3 (IRF3) and nuclear factor κB (NF-κB) signaling pathways, enabling STING-dependent rigorous inflammatory responses. Moreover, pharmacological or genetic inhibition of the ALK-STING pathway confers protection against lethal endotoxemia and sepsis in mice. The ALK pathway is up-regulated in patients with sepsis. These findings uncover a key role for ALK in modulating the inflammatory signaling pathway and shed light on the development of ALK-targeting therapeutics for lethal systemic inflammatory disorders.

Transcriptomic and Quantitative Proteomic Analyses Provide Insights Into the Phagocytic Killing of Hemocytes in the Oyster Crassostrea gigas

As invertebrates lack an adaptive immune system, they depend to a large extent on their innate immune system to recognize and clear invading pathogens. Although phagocytes play pivotal roles in invertebrate innate immunity, the molecular mechanisms underlying this killing remain unclear. Cells of this type from the Pacific oyster Crassostrea gigas were classified efficiently in this study via fluorescence-activated cell sorting (FACS) based on their phagocytosis of FITC-labeled latex beads. Transcriptomic and quantitative proteomic analyses revealed a series of differentially expressed genes (DEGs) and proteins present in phagocytes; of the 352 significantly high expressed proteins identified here within the phagocyte proteome, 262 corresponding genes were similarly high expressed in the transcriptome, while 140 of 205 significantly low expressed proteins within the proteome were transcriptionally low expressed. A pathway crosstalk network analysis of these significantly high expressed proteins revealed that phagocytes were highly activated in a number of antimicrobial-related biological processes, including oxidation–reduction and lysosomal proteolysis processes. A number of DEGs, including oxidase, lysosomal protease, and immune receptors, were also validated in this study using quantitative PCR, while seven lysosomal cysteine proteases, referred to as cathepsin Ls, were significantly high expressed in phagocytes. Results show that the expression level of cathepsin L protein in phagocytes [mean fluorescence intensity (MFI): 327 ± 51] was significantly higher (p < 0.01) than that in non-phagocytic hemocytes (MFI: 83 ± 26), while the cathepsin L protein was colocalized with the phagocytosed Vibrio splendidus in oyster hemocytes during this process. The results of this study collectively suggest that oyster phagocytes possess both potent oxidative killing and microbial disintegration capacities; these findings provide important insights into hemocyte phagocytic killing as a component of C. gigas innate immunity.

The cGAS/STING Pathway Detects Streptococcus pneumoniae but Appears Dispensable for Antipneumococcal Defense in Mice and Humans

Streptococcus pneumoniae is a frequent colonizer of the upper respiratory tract and a leading cause of bacterial pneumonia. The innate immune system senses pneumococcal cell wall components, toxin, and nucleic acids, which leads to production of inflammatory mediators to initiate and control antibacterial defense. Here, we show that the cGAS (cyclic GMP-AMP [cGAMP] synthase)-STING pathway mediates detection of pneumococcal DNA in mouse macrophages to primarily stimulate type I interferon (IFN) responses. Cells of human individuals carrying HAQ TMEM173, which encodes a common hypomorphic variant of STING, were largely or partly defective in inducing type I IFNs and proinflammatory cytokines upon infection. Subsequent analyses, however, revealed that STING was dispensable for restricting S. pneumoniae during acute pneumonia in mice. Moreover, explorative analyses did not find differences in the allele frequency of HAQ TMEM173 in nonvaccinated pneumococcal pneumonia patients and healthy controls or an association of HAQ TMEM173 carriage with disease severity. Together, our results indicate that the cGAS/STING pathway senses S. pneumoniae but plays no major role in antipneumococcal immunity in mice and humans.

TNFα and Radioresistant Stromal Cells Are Essential for Therapeutic Efficacy of Cyclic Dinucleotide STING Agonists in Nonimmunogenic Tumors

The cGAS-STING cytosolic DNA sensing pathway may play an integral role in the initiation of antitumor immune responses. Studies evaluating the immunogenicity of various cyclic dinucleotide (CDN) STING agonists administered by intratumoral (i.t.) injection showed potent induction of inflammation, tumor necrosis, and, in some cases, durable tumor-specific adaptive immunity. However, the specific immune mechanisms underlying these responses remain incompletely defined. The majority of these studies have focused on the effect of CDNs on immune cells but have not conclusively interrogated the role of stromal cells in the acute rejection of the CDN-injected tumor. Here, we revealed a mechanism of STING agonist-mediated tumor response that relied on both stromal and immune cells to achieve tumor regression and clearance. Using knockout and bone marrow chimeric mice, we showed that although bone marrow-derived TNFα was necessary for CDN-induced necrosis, STING signaling in radioresistant stromal cells was also essential for CDN-mediated tumor rejection. These results provide evidence for crosstalk between stromal and hematopoietic cells during CDN-mediated tumor collapse after i.t. administration. These mechanistic insights may prove critical in the clinical development of STING agonists. Cancer Immunol Res; 6(4); 422-33. ©2018 AACR.

WITHDRAWN: Research progress of cGAS-STING pathway in infectious diseases

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A robust microparticle platform for a STING-targeted adjuvant that enhances both humoral and cellular immunity during vaccination

Most FDA-approved adjuvants for infectious agents boost humoral but not cellular immunity, and have poorly-understood mechanisms. Stimulator of interferon genes (STING, also known as MITA, MPYS, or ERIS) is an exciting adjuvant target due to its role in cyclic dinucleotide (CDN)-driven anti-viral immunity; however, a major hindrance is STING's cytosolic localization which requires intracellular delivery of its agonists. As a result, STING agonists administered in a soluble form have elicited suboptimal immune responses. Delivery of STING agonists via particle platforms has proven a more successful strategy, but the opportunity for improved formulations and bioactivity remains. In this study we evaluated the adjuvant activity of the potent STING agonist, CDN 3'3'-cGAMP (cGAMP), encapsulated in acid-sensitive acetalated dextran (Ace-DEX) polymeric microparticles (MPs) which passively target antigen-presenting cells for intracellular release. This formulation was superior to all particle delivery systems evaluated and maintained its bioactivity following a sterilizing dose of gamma irradiation. Compared to soluble cGAMP, the Ace-DEX cGAMP MPs enhanced type-I interferon responses nearly 1000-fold in vitro and 50-fold in vivo, caused up to a 104-fold boost in antibody titers, increased Th1-associated responses, and expanded germinal center B cells and memory T cells. Furthermore, the encapsulated cGAMP elicited no observable toxicity in animals and achieved protective immunity against a lethal influenza challenge seven months post-immunization when using CDN adjuvant doses up to 100-fold lower than previous reports. For these reasons, Ace-DEX MP-encapsulated cGAMP represents a potent vaccine adjuvant of humoral and cellular immunity.

A Comparison of Oral and Intravenous Mouse Models of Listeriosis

Listeria monocytogenes is one of several enteric microbes that is acquired orally, invades the gastric mucosa, and then disseminates to peripheral tissues to cause systemic disease in humans. Intravenous (i.v.) inoculation of mice with L. monocytogenes has been the most widely-used small animal model of listeriosis over the past few decades. The infection is highly reproducible and has been invaluable in deciphering mechanisms of adaptive immunity in vivo, particularly CD8⁺ T cell responses to intracellular pathogens. However, the i.v. model completely bypasses the gut phase of the infection. Recent advances in generating both humanized mice and murinized bacteria, as well as the development of a foodborne route of transmission has reignited interest in studying oral models of listeriosis. In this review, we analyze previously published reports to highlight both the similarities and differences in tissue colonization and host response to infection using either oral or i.v. inoculation.

IFN Regulatory Factor 3 Balances Th1 and T Follicular Helper Immunity during Nonlethal Blood-Stage Plasmodium Infection

Differentiation of CD4⁺ Th cells is critical for immunity to malaria. Several innate immune signaling pathways have been implicated in the detection of blood-stage Plasmodium parasites, yet their influence over Th cell immunity remains unclear. In this study, we used Plasmodium-reactive TCR transgenic CD4⁺ T cells, termed PbTII cells, during nonlethal P. chabaudi chabaudi AS and P. yoelii 17XNL infection in mice, to examine Th cell development in vivo. We found no role for caspase1/11, stimulator of IFN genes, or mitochondrial antiviral-signaling protein, and only modest roles for MyD88 and TRIF-dependent signaling in controlling PbTII cell expansion. In contrast, IFN regulatory factor 3 (IRF3) was important for supporting PbTII expansion, promoting Th1 over T follicular helper (Tfh) differentiation, and controlling parasites during the first week of infection. IRF3 was not required for early priming by conventional dendritic cells, but was essential for promoting CXCL9 and MHC class II expression by inflammatory monocytes that supported PbTII responses in the spleen. Thereafter, IRF3-deficiency boosted Tfh responses, germinal center B cell and memory B cell development, parasite-specific Ab production, and resolution of infection. We also noted a B cell-intrinsic role for IRF3 in regulating humoral immune responses. Thus, we revealed roles for IRF3 in balancing Th1- and Tfh-dependent immunity during nonlethal infection with blood-stage Plasmodium parasites.

The common HAQ STING variant impairs cGAS-dependent antibacterial responses and is associated with susceptibility to Legionnaires' disease in humans

The cyclic GMP-AMP synthase (cGAS)-STING pathway is central for innate immune sensing of various bacterial, viral and protozoal infections. Recent studies identified the common HAQ and R232H alleles of TMEM173/STING, but the functional consequences of these variants for primary infections are unknown. Here we demonstrate that cGAS- and STING-deficient murine macrophages as well as human cells of individuals carrying HAQ TMEM173/STING were severely impaired in producing type I IFNs and pro-inflammatory cytokines in response to Legionella pneumophila, bacterial DNA or cyclic dinucleotides (CDNs). In contrast, R232H attenuated cytokine production only following stimulation with bacterial CDN, but not in response to L. pneumophila or DNA. In a mouse model of Legionnaires’ disease, cGAS- and STING-deficient animals exhibited higher bacterial loads as compared to wild-type mice. Moreover, the haplotype frequency of HAQ TMEM173/STING, but not of R232H TMEM173/STING, was increased in two independent cohorts of human Legionnaires’ disease patients as compared to healthy controls. Our study reveals that the cGAS-STING cascade contributes to antibacterial defense against L. pneumophila in mice and men, and provides important insight into how the common HAQ TMEM173/STING variant affects antimicrobial immune responses and susceptibility to infection.

Interferons (IFNs) and pro-inflammatory cytokines are key regulators of gene expression and antibacterial defense during Legionella pneumophila infection. Here we demonstrate that production of these mediators was largely or partly dependent on the cyclic GMP-AMP synthase (cGAS)-STING pathway in human and murine cells. Cells of individuals carrying the common HAQ allele of TMEM173/STING were strongly impaired in their ability to respond to L. pneumophila, bacterial DNA or cyclic dinucleotides (CDNs), whereas the R232H allele was only attenuated in sensing of exogenous CDNs. Importantly, cGAS and STING contributed to antibacterial defense in mice during L. pneumophila lung infection, and the allele frequency of HAQ TMEM173/STING, but not of R232H TMEM173/STING, was increased in two independent cohorts of human Legionnaires’ disease patients as compared to healthy controls. Hence, sensing of bacterial DNA by the cGAS/STING pathway contributes to antibacterial defense against L. pneumophila infection, and the hypomorphic variant HAQ TMEM173/STING is associated with increased susceptibility to Legionnaires’ disease in humans.

The ER membrane adaptor ERAdP senses the bacterial second messenger c-di-AMP and initiates anti-bacterial immunity

Cyclic diadenylate monophosphate (c-di-AMP) is secreted by bacteria as a secondary messenger. How immune cells detect c-di-AMP and initiate anti-bacterial immunity remains unknown. We found that the endoplasmic reticulum (ER) membrane adaptor ERAdP acts as a direct sensor for c-di-AMP. ERAdP-deficient mice were highly susceptible to Listeria monocytogenes infection and exhibited reduced pro-inflammatory cytokines. Mechanistically, c-di-AMP bound to the C-terminal domain of ERAdP, which in turn led to dimerization of ERAdP, resulting in association with and activation of the kinase TAK1. TAK1 activation consequently initiated activation of the transcription factor NF-κB to induce the production of pro-inflammatory cytokines in innate immune cells. Moreover, double-knockout of ERAdP and TAK1 resulted in heightened susceptibility to L. monocytogenes infection. Thus, ERAdP-mediated production of pro-inflammatory cytokines is critical for controlling bacterial infection.

cGAS drives noncanonical-inflammasome activation in age-related macular degeneration

Geographic atrophy is a blinding form of age-related macular degeneration characterized by death of the retinal pigmented epithelium (RPE). In this disease, the RPE displays evidence of DICER1 deficiency, resultant accumulation of endogenous Alu retroelement RNA, and NLRP3 inflammasome activation. How the inflammasome is activated in this untreatable disease is largely unknown. Here we demonstrate that RPE degeneration in human cell culture and in mouse models is driven by a non-canonical inflammasome pathway that results in activation of caspase-4 (caspase-11 in mice) and caspase-1, and requires cyclic GMP-AMP synthase (cGAS)-dependent interferon-β (IFN-β) production and gasdermin D-dependent interleukin-18 (IL-18) secretion. Reduction of DICER1 levelsor accumulation of Alu RNA triggers cytosolic escape of mitochondrial DNA, which engages cGAS. Moreover, caspase-4, gasdermin D, IFN-β, and cGAS levels are elevated in the RPE of human eyes with geographic atrophy. Collectively, these data highlight an unexpected role for cGAS in responding to mobile element transcripts, reveal cGAS-driven interferon signaling as a conduit for mitochondrial damage-induced inflammasome activation, expand the immune sensing repertoire of cGAS and caspase-4 to non-infectious human disease, and identify new potential targets for treatment of a major cause of blindness.

Cyclic Dinucleotides in the Scope of the Mammalian Immune System

First discovered in prokaryotes and more recently in eukaryotes, cyclic dinucleotides (CDNs) constitute a unique branch of second messenger signaling systems. Within prokaryotes CDNs regulate a wide array of different biological processes, whereas in the vertebrate system CDN signaling is largely dedicated to activation of the innate immune system. In this book chapter we summarize the occurrence and signaling pathways of these small-molecule second messengers, most importantly in the scope of the mammalian immune system. In this regard, our main focus is the role of the cGAS-STING axis in the context of microbial infection and sterile inflammation and its implications for therapeutic applications.

Innate Immune Recognition: Implications for the Interaction of Francisella tularensis with the Host Immune System

The intracellular bacterial pathogen Francisella tularensis causes serious infectious disease in humans and animals. Moreover, F. tularensis, a highly infectious pathogen, poses a major concern for the public as a bacterium classified under Category A of bioterrorism agents. Unfortunately, research has so far failed to develop effective vaccines, due in part to the fact that the pathogenesis of intracellular bacteria is not fully understood and in part to gaps in our understanding of innate immune recognition processes leading to the induction of adaptive immune response. Recent evidence supports the concept that immune response to external stimuli in the form of bacteria is guided by the primary interaction of the bacterium with the host cell. Based on data from different Francisella models, we present here the basic paradigms of the emerging innate immune recognition concept. According to this concept, the type of cell and its receptor(s) that initially interact with the target constitute the first signaling window; the signals produced in the course of primary interaction of the target with a reacting cell act in a paracrine manner; and the innate immune recognition process as a whole consists in a series of signaling windows modulating adaptive immune response. Finally, the host, in the strict sense, is the interacting cell.

Innate immunity related pathogen recognition receptors and chronic hepatitis B infection

Innate immunity consists of several kinds of pathogen recognition receptors (PRRs), which participate in the recognition of pathogens and consequently activation of innate immune system against pathogens. Recently, several investigations reported that PRRs may also play key roles in the induction/stimulation of immune system related complications in microbial infections. Hepatitis B virus (HBV), as the main cause of viral hepatitis in human, can induce several clinical forms of hepatitis B and also might be associated with hepatic complications such as cirrhosis and hepatocellular carcinoma (HCC). Based on the important roles of PRRs in the eradication of microbial infections including viral infections and their related complications, it appears that the molecules may be a main part of immune responses against viral infections including HBV and participate in the HBV related complications. Thus, this review article has brought together information regarding the roles of PRRs in immunity against HBV and its complications.

Innate immune sensing of cytosolic chromatin fragments through cGAS promotes senescence

Cellular senescence is triggered by various distinct stresses and characterized by a permanent cell cycle arrest. Senescent cells secrete a variety of inflammatory factors, collectively referred to as the senescence-associated secretory phenotype (SASP). The mechanism(s) underlying the regulation of the SASP remains incompletely understood. Here we define a role for innate DNA sensing in the regulation of senescence and the SASP. We find that cyclic GMP-AMP synthase (cGAS) recognizes cytosolic chromatin fragments in senescent cells. The activation of cGAS, in turn, triggers the production of SASP factors via stimulator of interferon genes (STING), thereby promoting paracrine senescence. We demonstrate that diverse stimuli of cellular senescence engage the cGAS-STING pathway in vitro and we show cGAS-dependent regulation of senescence following irradiation and oncogene activation in vivo. Our findings provide insights into the mechanisms underlying cellular senescence by establishing the cGAS-STING pathway as a crucial regulator of senescence and the SASP.

Uterotubal junction prevents chlamydial ascension via innate immunity

Ascension to the oviduct is necessary for Chlamydia to induce tubal infertility. Using the Chlamydia muridarum induction of hydrosalpinx mouse model, we have demonstrated a significant role of the uterotubal junction in preventing chlamydial ascending infection. First, delivery of C. muridarum to either side of the uterotubal junction resulted in significant reduction in live organisms from the tissues on the opposite sides. However, the recovery yields remained similar among different sections of the uterine horn. These observations suggest that the uterotubal junction may function as a barrier between the uterine horn and oviduct. Second, deficiency in innate immunity signaling pathways mediated by either MyD88 or STING significantly compromised the uterotubal junction barrier function, permitting C. muridarum to spread freely between uterine horn and oviduct. Finally, transcervical inoculation of C. muridarum led to significantly higher incidence of bilateral hydrosalpinges in the STING-deficient mice while the same inoculation mainly induced unilateral hydrosalpinx in the wild type mice, suggesting that the STING pathway-dependent uterotubal junction plays a significant role in preventing tubal pathology. Thus, we have demonstrated for the first time that the uterotubal junction is a functional barrier for preventing tubal infection by a sexually transmitted agent, providing the first in vivo evidence for detecting chlamydial infection by the STING pathway.

Sensing of viral and endogenous RNA by ZBP1/DAI induces necroptosis

Nucleic acids are potent triggers for innate immunity. Double‐stranded DNA and RNA adopt different helical conformations, including the unusual Z‐conformation. Z‐DNA/RNA is recognised by Z‐binding domains (ZBDs), which are present in proteins implicated in antiviral immunity. These include ZBP1 (also known as DAI or DLM‐1), which induces necroptosis, an inflammatory form of cell death. Using reconstitution and knock‐in models, we report that mutation of key amino acids involved in Z‐DNA/RNA binding in ZBP1's ZBDs prevented necroptosis upon infection with mouse cytomegalovirus. Induction of cell death was cell autonomous and required RNA synthesis but not viral DNA replication. Accordingly, ZBP1 directly bound to RNA via its ZBDs. Intact ZBP1‐ZBDs were also required for necroptosis triggered by ectopic expression of ZBP1 and caspase blockade, and ZBP1 cross‐linked to endogenous RNA. These observations show that Z‐RNA may constitute a molecular pattern that induces inflammatory cell death upon sensing by ZBP1.

Grass Carp Reovirus VP41 Targets Fish MITA To Abrogate the Interferon Response

Although fish possess an efficient interferon (IFN) system to defend against aquatic virus infection, grass carp reovirus (GCRV) still causes hemorrhagic disease in grass carp. To date, GCRV's strategy for evading the fish IFN response is still unknown. Here, we report that GCRV VP41 inhibits fish IFN production by suppressing the phosphorylation of mediator of IFN regulatory factor 3 (IRF3) activation (MITA). First, the activation of the IFN promoter (IFNpro) stimulated by mitochondrial antiviral signaling protein (MAVS) and MITA was decreased by the overexpression of VP41, whereas such activation induced by TANK-binding kinase 1 (TBK1) was not affected. Second, VP41 was colocalized in the cellular endoplasmic reticulum (ER) and associated with MITA. Furthermore, as a phosphorylation substrate of TBK1, VP41 significantly decreased the phosphorylation of MITA. Truncation assays indicated that the transmembrane (TM) region of VP41 was indispensable for the suppression of IFNpro activity. Finally, after infection with GCRV, VP41 blunted the transcription of host IFN and facilitated viral RNA synthesis. Taken together, our findings suggest that GCRV VP41 prevents the fish IFN response by attenuating the phosphorylation of MITA for viral evasion.IMPORTANCE MITA is thought to act as an adaptor protein to facilitate the phosphorylation of IRF3 by TBK1 upon viral infection, and it plays a critical role in innate antiviral responses. Here, we report that GCRV VP41 colocalizes with MITA at the ER and reduces MITA phosphorylation by acting as a decoy substrate of TBK1, thus inhibiting IFN production. These findings reveal GCRV's strategy for evading the host IFN response for the first time.

Regulating STING in health and disease

The presence of cytosolic double-stranded DNA molecules can trigger multiple innate immune signalling pathways which converge on the activation of an ER-resident innate immune adaptor named "STimulator of INterferon Genes (STING)". STING has been found to mediate type I interferon response downstream of cyclic dinucleotides and a number of DNA and RNA inducing signalling pathway. In addition to its physiological function, a rapidly increasing body of literature highlights the role for STING in human disease where variants of the STING proteins, as well as dysregulated STING signalling, have been implicated in a number of inflammatory diseases. This review will summarise the recent structural and functional findings of STING, and discuss how STING research has promoted the development of novel therapeutic approaches and experimental tools to improve treatment of tumour and autoimmune diseases.

USP13 negatively regulates antiviral responses by deubiquitinating STING

STING (also known as MITA) is critical for host defence against viruses and the activity of STING is regulated by ubiquitination. However, the deubiquitination of STING is not fully understood. Here, we show that ubiquitin-specific protease 13 (USP13) is a STING-interacting protein that catalyses deubiquitination of STING. Knockdown or knockout of USP13 potentiates activation of IRF3 and NF-κB and expression of downstream genes after HSV-1 infection or transfection of DNA ligands. USP13 deficiency results in impaired replication of HSV-1. Consistently, USP13 deficient mice are more resistant than wild-type littermates to lethal HSV-1 infection. Mechanistically, USP13 deconjugates polyubiquitin chains from STING and prevents the recruitment of TBK1 to the signalling complex, thereby negatively regulating cellular antiviral responses. Our study thus uncovers a function of USP13 in innate antiviral immunity and provides insight into the regulation of innate immunity.

Involvement of GATA1 and Sp3 in the activation of the murine STING gene promoter in NIH3T3 cells

Stimulator of Interferon Gene (STING) is a key mediator of innate immune signaling. STING plays a pivotal role in the pathogenesis of many diseases including infectious diseases, auto-immune diseases and cancer. Many studies have been carried out recently in the field of STING-regulated pathway, however, rarely of transcriptional mechanisms. To characterize the murine STING (mSTING) promoter, we cloned a series of different nucleotide sequences of the 5'-flanking region of the mSTING gene. Transient transfection of promoter-reporter recombinant plasmids and luciferase assay illustrated the region (-77/+177) relative to the transcription start site (TSS) of the mSTING gene was sufficient for full promoter activity. This region contains GATA1, IK2, Sp1/Sp3 and STAT putative transcription factor binding sites. Mutation of GATA1 or Sp1/Sp3 sites led to obvious decrease of the mSTING promoter activity. Overexpression of GATA1 and Sp3 enhanced the mSTING promoter activity, whereas knockdown of GATA1 and Sp3 by a siRNA strategy significantly reduced the transcription activity. Chromatin immunoprecipitation assays demonstrated that GATA1 and Sp3 interact with the mSTING promoter in vivo. These results provided the first analysis of mSTING promoter and demonstrated that transcription factor GATA1 and Sp3 positively regulate the basal transcription of the mSTING gene.

STING-IRF3 Triggers Endothelial Inflammation in Response to Free Fatty Acid-Induced Mitochondrial Damage in Diet-Induced Obesity

OBJECTIVE

Metabolic stress in obesity induces endothelial inflammation and activation, which initiates adipose tissue inflammation, insulin resistance, and cardiovascular diseases. However, the mechanisms underlying endothelial inflammation induction are not completely understood. Stimulator of interferon genes (STING) is an important molecule in immunity and inflammation. In the present study, we sought to determine the role of STING in palmitic acid-induced endothelial activation/inflammation.

APPROACH AND RESULTS

In cultured endothelial cells, palmitic acid treatment activated STING, as indicated by its perinuclear translocation and binding to interferon regulatory factor 3 (IRF3), leading to IRF3 phosphorylation and nuclear translocation. The activated IRF3 bound to the promoter of ICAM-1 (intercellular adhesion molecule 1) and induced ICAM-1 expression and monocyte-endothelial cell adhesion. When analyzing the upstream signaling, we found that palmitic acid activated STING by inducing mitochondrial damage. Palmitic acid treatment caused mitochondrial damage and leakage of mitochondrial DNA into the cytosol. Through the cytosolic DNA sensor cGAS (cyclic GMP-AMP synthase), the mitochondrial damage and leaked cytosolic mitochondrial DNA activated the STING-IRF3 pathway and increased ICAM-1 expression. In mice with diet-induced obesity, the STING-IRF3 pathway was activated in adipose tissue. However, STING deficiency (Stinggt/gt ) partially prevented diet-induced adipose tissue inflammation, obesity, insulin resistance, and glucose intolerance.

CONCLUSIONS

The mitochondrial damage-cGAS-STING-IRF3 pathway is critically involved in metabolic stress-induced endothelial inflammation. STING may be a potential therapeutic target for preventing cardiovascular diseases and insulin resistance in obese individuals.

Cyclic dinucleotides modulate induced type I IFN responses in innate immune cells by degradation of STING

The cyclic dinucleotides, GMP-AMP (cGAMP) and c-di-AMP [bis-(3',5')-cyclic dimeric AMP], are potent type I IFN inducers via STING-TBK1-IRF3 cascade. They are promising adjuvants that promote antigen-specific humoral and cellular immune responses in different preclinical models; however, an optimal outcome of vaccination depends on a balanced immune activation. Here, we characterize the process of IFN-β induction by c-di-AMP and cGAMP in an in vitro model on the basis of primary mouse dendritic cells. Results obtained show decreased IFN-β production upon prolonged cell stimulation. We demonstrate that this effect depends on c-di-AMP/cGAMP-mediated down-regulation of stimulator of IFN gene (STING) protein levels. These results were confirmed by using human peripheral blood mononuclear cell-derived dendritic cells. Studies performed to explore the potential mechanism of STING modulation suggested proteolytic degradation to be a contributing factor to the observed decrease in cellular STING levels. Our work contributes to the elucidation of the molecular mode of action of vaccine constituents, which, in turn, is a prerequisite for the rational design of vaccines with predictable efficacy and safety profiles-Rueckert, C., Rand, U., Roy, U., Kasmapour, B., Strowig, T., Guzmán, C. A. Cyclic dinucleotides modulate induced type I IFN responses in innate immune cells by degradation of STING.

Cyclic GMP-AMP as an Endogenous Second Messenger in Innate Immune Signaling by Cytosolic DNA

The innate immune system functions as the first line of defense against invading bacteria and viruses. In this context, the cGAS/STING [cyclic guanosine monophosphate (GMP)-adenosine monophosphate (AMP) synthase/STING] signaling axis perceives the nonself DNA associated with bacterial and viral infections, as well as the leakage of self DNA by cellular dysfunction and stresses, to elicit the host's immune responses. In this pathway, the noncanonical cyclic dinucleotide 2',3'-cyclic GMP-AMP (2',3'-cGAMP) functions as a second messenger for signal transduction: 2',3'-cGAMP is produced by the enzyme cGAS upon its recognition of double-stranded DNA, and then the 2',3'-cGAMP is recognized by the receptor STING to induce the phosphorylation of downstream factors, including TBK1 (TANK binding kinase 1) and IRF3 (interferon regulatory factor 3). Numerous crystal structures of the components of this cGAS/STING signaling axis have been reported and these clarify the structural basis for their signal transduction mechanisms. In this review, we summarize recent progress made in the structural dissection of this signaling pathway and indicate possible directions of forthcoming research.

Monophosphoryl Lipid A Enhances Efficacy of a Francisella tularensis LVS-Catanionic Nanoparticle Subunit Vaccine against F. tularensis Schu S4 Challenge by Augmenting both Humoral and Cellular Immunity

Francisella tularensis, a bacterial biothreat agent, has no approved vaccine in the United States. Previously, we showed that incorporating lysates from partially attenuated F. tularensis LVS or fully virulent F. tularensis Schu S4 strains into catanionic surfactant vesicle (V) nanoparticles (LVS-V and Schu S4-V, respectively) protected fully against F. tularensis LVS intraperitoneal (i.p.) challenge in mice. However, we achieved only partial protection against F. tularensis Schu S4 intranasal (i.n.) challenge, even when employing heterologous prime-boost immunization strategies. We now extend these findings to show that both LVS-V and Schu S4-V immunization (i.p./i.p.) elicited similarly high titers of anti-F. tularensis IgG and that the titers could be further increased by adding monophosphoryl lipid A (MPL), a nontoxic Toll-like receptor 4 (TLR4) adjuvant that is included in several U.S. FDA-approved vaccines. LVS-V+MPL immune sera also detected more F. tularensis antigens than LVS-V immune sera and, after passive transfer to naive mice, significantly delayed the time to death against F. tularensis Schu S4 subcutaneous (s.c.) but not i.n. challenge. Active immunization with LVS-V+MPL (i.p./i.p.) also increased the frequency of gamma interferon (IFN-γ)-secreting activated helper T cells, IFN-γ production, and the ability of splenocytes to control intramacrophage F. tularensis LVS replication ex vivo Active LVS-V+MPL immunization via heterologous routes (i.p./i.n.) significantly elevated IgA and IgG levels in bronchoalveolar lavage fluid and significantly enhanced protection against i.n. F. tularensis Schu S4 challenge (to ∼60%). These data represent a significant step in the development of a subunit vaccine against the highly virulent type A strains.

Crosstalk Between Apoptosis and Autophagy: Environmental Genotoxins, Infection, and Innate Immunity

Autoimmune disorders constitute a major and growing health concern. However, the genetic and environmental factors that contribute to or exacerbate disease symptoms remain unclear. Type I interferons (IFNs) are known to break immune tolerance and be elevated in the serum of patients with autoimmune diseases such as lupus. Extensive work over the past decade has characterized the role of a protein termed stimulator of interferon genes, or STING, in mediating IFN expression and activation in response to cytosolic DNA and cyclic dinucleotides. Interestingly, this STING-dependent innate immune pathway both utilizes and is targeted by the cell's autophagic machinery. Given that aberrant interplay between the apoptotic and autophagic machineries contributes to deregulation of the STING-dependent pathway, IFN-regulated autoimmune phenotypes may be influenced by the combined exposure to environmental carcinogens and pathogenic microorganisms and viruses. This review therefore summarizes recent data regarding these important issues in the field of autoimmunity.

A prosurvival DNA damage-induced cytoplasmic interferon response is mediated by end resection factors and is limited by Trex1

Radiotherapy and chemotherapy are effective treatment methods for many types of cancer, but resistance is common. Recent findings indicate that antiviral type I interferon (IFN) signaling is induced by these treatments. However, the underlying mechanisms still need to be elucidated. Expression of a set of IFN-stimulated genes comprises an IFN-related DNA damage resistance signature (IRDS), which correlates strongly with resistance to radiotherapy and chemotherapy across different tumors. Classically, during viral infection, the presence of foreign DNA in the cytoplasm of host cells can initiate type I IFN signaling. Here, we demonstrate that DNA-damaging modalities used during cancer therapy lead to the release of ssDNA fragments from the cell nucleus into the cytosol, engaging this innate immune response. We found that the factors that control DNA end resection during double-strand break repair, including the Bloom syndrome (BLM) helicase and exonuclease 1 (EXO1), play a major role in generating these DNA fragments and that the cytoplasmic 3'-5' exonuclease Trex1 is required for their degradation. Analysis of mRNA expression profiles in breast tumors demonstrates that those with lower Trex1 and higher BLM and EXO1 expression levels are associated with poor prognosis. Targeting BLM and EXO1 could therefore represent a novel approach for circumventing the IRDS produced in response to cancer therapeutics.

Promising Targets for Cancer Immunotherapy: TLRs, RLRs, and STING-Mediated Innate Immune Pathways

Malignant cancers employ diverse and intricate immune evasion strategies, which lead to inadequately effective responses of many clinical cancer therapies. However, emerging data suggest that activation of the tolerant innate immune system in cancer patients is able, at least partially, to counteract tumor-induced immunosuppression, which indicates triggering of the innate immune response as a novel immunotherapeutic strategy may result in improved therapeutic outcomes for cancer patients. The promising innate immune targets include Toll-like Receptors (TLRs), RIG-I-like Receptors (RLRs), and Stimulator of Interferon Genes (STING). This review discusses the antitumor properties of TLRs, RLRs, and STING-mediated innate immune pathways, as well as the promising innate immune targets for potential application in cancer immunotherapy.

The Common R71H-G230A-R293Q Human TMEM173 Is a Null Allele

TMEM173 encodes MPYS/STING and is an innate immune sensor for cyclic dinucleotides (CDNs) playing a critical role in infection, inflammation, and cancer. The R71H-G230A-R293Q (HAQ) of TMEM173 is the second most common human TMEM173 allele. In this study, using data from the 1000 Genomes Project we found that homozygous HAQ individuals account for ∼16.1% of East Asians and ∼2.8% of Europeans whereas Africans have no homozygous HAQ individuals. Using B cells from homozygous HAQ carriers, we found, surprisingly, that HAQ/HAQ carriers express extremely low MPYS protein and have a decreased TMEM173 transcript. Consequently, the HAQ/HAQ B cells do not respond to CDNs. We subsequently generated an HAQ knock-in mouse expressing a mouse equivalent of the HAQ allele (mHAQ). The mHAQ mouse has decreased MPYS protein in B cells, T cells, Ly6Chi monocytes, bone marrow-derived dendritic cells, and lung tissue. The mHAQ mouse also does not respond to CDNs in vitro and in vivo. Lastly, Pneumovax 23, with an efficacy that depends on TMEM173, is less effective in mHAQ mice than in wild type mice. We conclude that HAQ is a null TMEM173 allele. Our findings have a significant impact on research related to MPYS-mediated human diseases and medicine.

Cell-Based Screen Identifies Human Interferon-Stimulated Regulators of Listeria monocytogenes Infection

The type I interferon (IFN) activated transcriptional response is a critical antiviral defense mechanism, yet its role in bacterial pathogenesis remains less well characterized. Using an intracellular pathogen Listeria monocytogenes (Lm) as a model bacterial pathogen, we sought to identify the roles of individual interferon-stimulated genes (ISGs) in context of bacterial infection. Previously, IFN has been implicated in both restricting and promoting Lm growth and immune stimulatory functions in vivo. Here we adapted a gain-of-function flow cytometry based approach to screen a library of more than 350 human ISGs for inhibitors and enhancers of Lm infection. We identify 6 genes, including UNC93B1, MYD88, AQP9, and TRIM14 that potently inhibit Lm infection. These inhibitors act through both transcription-mediated (MYD88) and non-transcriptional mechanisms (TRIM14). Further, we identify and characterize the human high affinity immunoglobulin receptor FcγRIa as an enhancer of Lm internalization. Our results reveal that FcγRIa promotes Lm uptake in the absence of known host Lm internalization receptors (E-cadherin and c-Met) as well as bacterial surface internalins (InlA and InlB). Additionally, FcγRIa-mediated uptake occurs independently of Lm opsonization or canonical FcγRIa signaling. Finally, we established the contribution of FcγRIa to Lm infection in phagocytic cells, thus potentially linking the IFN response to a novel bacterial uptake pathway. Together, these studies provide an experimental and conceptual basis for deciphering the role of IFN in bacterial defense and virulence at single-gene resolution.

While the type I interferon response is known to be activated by both viruses and bacteria, it has mostly been characterized in terms of its antiviral properties. Listeria monocytogenes, an opportunistic Gram-positive bacterial pathogen with up to 50% mortality rate and a variety of clinical manifestations, is a potent activator of interferon secretion. In mouse models, interferon has been previously implicated in both restricting and promoting L. monocytogenes infection. Here, we utilized a high-throughput flow-cytometry based approach to screen a library of human interferon I stimulated genes (ISGs) and identified regulators of L. monocytogenes infection. These include inhibitors that act through both transcriptional (MYD88) and transcription-independent (TRIM14) mechanisms. Strikingly, expression of the human high affinity immunoglobulin receptor FcγRIa (CD64) was found to potently enhance L. monocytogenes infection. Both biochemical and cellular studies indicate that FcγRIa increases primary invasion of L. monocytogenes through a previously uncharacterized IgG-independent internalization mechanism. Together, these studies provide an important insight into the complex role of interferon response in bacterial virulence and host defense.

Mechanisms of transcriptional activation of the stimulator of interferon genes by transcription factors CREB and c-Myc

Stimulator of interferon genes (STING) plays an important role in host defense, autoimmune disease, osteoclast differentiation and anti-tumor response. Although many downstream targets have been studied in depth, the regulation of STING gene expression remains largely unknown. Here we demonstrate that transcription factors CREB and c-Myc maintain the transcriptional activity of STING. By 5'-rapid amplification of cDNA ends analysis, we identified the transcriptional start site (TSS) of STING. We illustrated that the region -124/+1 relative to TSS was sufficient for full promoter activity by a series of 5' deletion promoter constructs. Transcriptional activity of the STING minimal promoter was dependent on CREB and c-Myc binding motifs and was abolished after mutation of these two DNA elements. Chromatin immunoprecipitation assays demonstrated that transcription factors CREB and c-Myc bind to STING promoter in vivo. Overexpression of CREB and c-Myc increased the STING promoter activity. Meanwhile, knocking-down of CREB and c-Myc by a small interfering RNA (siRNA) strategy markedly reduced endogenous STING expression. In summary, these results demonstrated that transcription factors CREB and c-Myc are involved in the regulation of STING transcription.

AMP-activated Kinase (AMPK) Promotes Innate Immunity and Antiviral Defense through Modulation of Stimulator of Interferon Genes (STING) Signaling

The host protein Stimulator of Interferon Genes (STING) has been shown to be essential for recognition of both viral and intracellular bacterial pathogens, but its regulation remains unclear. Previously, we reported that mitochondrial membrane potential regulates STING-dependent IFN-β induction independently of ATP synthesis. Because mitochondrial membrane potential controls calcium homeostasis, and AMP-activated protein kinase (AMPK) is regulated, in part, by intracellular calcium, we postulated that AMPK participates in STING activation; however, its role has yet to be been defined. Addition of an intracellular calcium chelator or an AMPK inhibitor to either mouse macrophages or mouse embryonic fibroblasts (MEFs) suppressed IFN-β and TNF-α induction following stimulation with the STING-dependent ligand 5,6-dimethyl xanthnone-4-acetic acid (DMXAA). These pharmacological findings were corroborated by showing that MEFs lacking AMPK activity also failed to up-regulate IFN-β and TNF-α after treatment with DMXAA or the natural STING ligand cyclic GMP-AMP (cGAMP). As a result, AMPK-deficient MEFs exhibit impaired control of vesicular stomatitis virus (VSV), a virus sensed by STING that can cause an influenza-like illness in humans. This impairment could be overcome by pretreatment of AMPK-deficient MEFs with type I IFN, illustrating that de novo production of IFN-β in response to VSV plays a key role in antiviral defense during infection. Loss of AMPK also led to dephosphorylation at Ser-555 of the known STING regulator, UNC-51-like kinase 1 (ULK1). However, ULK1-deficient cells responded normally to DMXAA, indicating that AMPK promotes STING-dependent signaling independent of ULK1 in mouse cells.

Cyclic di-AMP Released from Staphylococcus aureus Biofilm Induces a Macrophage Type I Interferon Response

Staphylococcus aureus is a leading cause of community- and nosocomial-acquired infections, with a propensity for biofilm formation. S. aureus biofilms actively skew the host immune response toward an anti-inflammatory state; however, the biofilm effector molecules and the mechanism(s) of action responsible for this phenomenon remain to be fully defined. The essential bacterial second messenger cyclic diadenylate monophosphate (c-di-AMP) is an emerging pathogen-associated molecular pattern during intracellular bacterial infections, as c-di-AMP secretion into the infected host cytosol induces a robust type I interferon (IFN) response. Type I IFNs have the potential to exacerbate infectious outcomes by promoting anti-inflammatory effects; however, the type I IFN response to S. aureus biofilms is unknown. Additionally, while several intracellular proteins function as c-di-AMP receptors in S. aureus, it has yet to be determined if any extracellular role for c-di-AMP exists and its release during biofilm formation has not yet been demonstrated. This study examined the possibility that c-di-AMP released during S. aureus biofilm growth polarizes macrophages toward an anti-inflammatory phenotype via type I interferon signaling. DacA, the enzyme responsible for c-di-AMP synthesis in S. aureus, was highly expressed during biofilm growth, and 30 to 50% of total c-di-AMP produced from S. aureus biofilm was released extracellularly due to autolytic activity. S. aureus biofilm c-di-AMP release induced macrophage type I IFN expression via a STING-dependent pathway and promoted S. aureus intracellular survival in macrophages. These findings identify c-di-AMP as another mechanism for how S. aureus biofilms promote macrophage anti-inflammatory activity, which likely contributes to biofilm persistence.