Vaccinia E5 is a major inhibitor of the DNA sensor cGAS

The DNA sensor cyclic GMP-AMP synthase (cGAS) is critical in host antiviral immunity. Vaccinia virus (VACV) is a large cytoplasmic DNA virus that belongs to the poxvirus family. How vaccinia virus antagonizes the cGAS-mediated cytosolic DNA-sensing pathway is not well understood. In this study, we screened 80 vaccinia genes to identify potential viral inhibitors of the cGAS/Stimulator of interferon gene (STING) pathway. We discovered that vaccinia E5 is a virulence factor and a major inhibitor of cGAS. E5 is responsible for abolishing cGAMP production during vaccinia virus (Western Reserve strain) infection of dendritic cells. E5 localizes to the cytoplasm and nucleus of infected cells. Cytosolic E5 triggers ubiquitination of cGAS and proteasome-dependent degradation via interacting with cGAS. Deleting the E5R gene from the Modified vaccinia virus Ankara (MVA) genome strongly induces type I IFN production by dendritic cells (DCs) and promotes DC maturation, and thereby improves antigen-specific T cell responses.

A Baseline Cellular Antiviral State Is Maintained by cGAS and Its Most Frequent Naturally Occurring Variant rs610913

Upon recognition of aberrantly located DNA, the innate immune sensor cyclic GMP-AMP synthase (cGAS) activates stimulator of IFN genes (STING)/IFN regulatory factor (IRF)3-driven antiviral responses. In this study, we characterized the ability of a specific variant of the human cGAS-encoding gene MB21D1, rs610913, to alter cGAS-mediated DNA sensing and viral infection. rs610913 is a frequent G>T polymorphism resulting in a P261H exchange in the cGAS protein. Data from the International Collaboration for the Genomics of HIV suggested that rs610913 nominally associates with HIV-1 acquisition in vivo. Molecular modeling of cGAS(P261H) hinted toward the possibility for an additional binding site for a potential cellular cofactor in cGAS dimers. However, cGAS(wild-type [WT]) or cGAS(P261H)-reconstituted THP-1 cGAS knockout cells shared steady-state expression of IFN-stimulated genes, as opposed to cells expressing the enzymatically inactive cGAS(G212A/S213A). Accordingly, cGAS(WT) and cGAS(P261H) cells were less susceptible to lentiviral transduction and infection with HIV-1, HSV-1, and Chikungunya virus as compared with cGAS knockout or cGAS(G212A/S213A) cells. Upon DNA challenge, innate immune activation appeared to be mildly reduced upon expression of cGAS(P261H) compared with cGAS(WT). Finally, DNA challenge of PBMCs from donors homozygously expressing rs610913 provoked a trend toward a slightly reduced type I IFN response as compared with PBMCs from GG donors. Taken together, the steady-state activity of cGAS maintains a baseline antiviral state rendering cells more refractory to IFN-stimulated gene-sensitive viral infections. rs610913 failed to grossly differ phenotypically from the WT gene, suggesting that cGAS(P261H) and WT cGAS share a similar ability to sense viral infections in vivo.

cGAS-like receptor-mediated immunity: the insect perspective

The cGAS-STING pathway plays a central role in the detection of DNA in the cytosol of mammalian cells and activation of immunity. Although the early evolutionary origin of this pathway in animals has been noted, its ancestral functions have remained elusive so far. We review here new findings in invertebrates establishing a role in sensing and signaling infection, triggering potent transcriptional responses, in addition to autophagy. Results from flies and moths/butterflies point to the importance of STING signaling in antiviral immunity in insects. The recent characterization of cGAS-like receptors in Drosophila reveals the plasticity of this family of pattern-recognition receptors, able to accommodate ligands different from DNA and to produce cyclic dinucleotides beyond 2'3'-cGAMP.

cGAS exacerbates Schistosoma japonicum infection in a STING-type I IFN-dependent and independent manner

Schistosomiasis, which is caused by infection with Schistosoma spp., is characterized by granuloma and fibrosis in response to egg deposition. Pattern recognition receptors are important to sense invading Schistosoma, triggering an innate immune response, and subsequently shaping adaptive immunity. Cyclic GMP-AMP synthase (cGAS) was identified as a major cytosolic DNA sensor, which catalyzes the formation of cyclic GMP-AMP (cGAMP), a critical second messenger for the activation of the adaptor protein stimulator of interferon genes (STING). The engagement of STING by cGAMP leads to the activation of TANK-binding kinase 1 (TBK1), interferon regulatory factor 3 (IRF3), and the subsequent type I interferon (IFN) response. cGAS is suggested to regulate infectious diseases, autoimmune diseases, and cancer. However, the function of cGAS in helminth infection is unclear. In this study, we found that Cgas deficiency enhanced the survival of mice infected with S. japonicum markedly, without affecting the egg load in the liver. Consistently, Cgas deletion alleviated liver pathological impairment, reduced egg granuloma formation, and decreased fibrosis severity. In contrast, Sting deletion reduced the formation of egg granulomas markedly, but not liver fibrosis. Notably, Cgas or Sting deficiency reduced the production of IFNβ drastically in mice infected with S. japonicum. Intriguingly, intravenous administration of recombinant IFNβ exacerbated liver damage and promoted egg granuloma formation, without affecting liver fibrosis. Clodronate liposome-mediated depletion of macrophages indicated that macrophages are the major type of cells contributing to the induction of the type I IFN response during schistosome infection. Moreover, cGAS is important for type I IFN production and phosphorylation of TBK1 and IRF3 in response to stimulation with S. japonicum egg- or adult worm-derived DNA in macrophages. Our results clarified the immunomodulatory effect of cGAS in the regulation of liver granuloma formation during S. japonicum infection, involving sensing schistosome-derived DNA and producing type I IFN. Additionally, we showed that cGAS regulates liver fibrosis in a STING-type I–IFN-independent manner.

The sensing of invading pathogens by pattern recognition receptors (PRRs) is important for the host to mount an immune response. Cytosolic DNA receptor cGAS has been documented as critical for the induction of innate immunity, manifesting as a type I IFN response. However, little is known about the role of cGAS or type I IFN in the process of helminth infection. In this study, we identified an important role of the cGAS-STING-type I IFN signaling axis in driving schistosome infection-induced liver inflammation. Moreover, we revealed a hitherto unknown role of cGAS in the regulation of liver fibrosis during Schistosoma infection, a process that is independent of STING. Our study revealed cGAS as a novel functional receptor for the recognition of invading Schistosoma, paving the way for the development of novel strategies to treat schistosomiasis.

The Alternatively Spliced Isoforms of Key Molecules in the cGAS-STING Signaling Pathway

Alternative splicing of pre-mRNA increases transcriptome and proteome diversity by generating distinct isoforms that encode functionally diverse proteins, thus affecting many biological processes, including innate immunity. cGAS-STING signaling pathway, whose key molecules also undergo alternative splicing, plays a crucial role in regulating innate immunity. Protein isoforms of key components in the cGAS-STING-TBK1-IRF3 axis have been detected in a variety of species. A chain of evidence showed that these protein isoforms exhibit distinct functions compared to their normal counterparts. The mentioned isoforms act as positive or negative modulators in interferon response via distinct mechanisms. Particularly, we highlight that alternative splicing serves a vital function for the host to avoid the overactivation of the cGAS-STING signaling pathway and that viruses can utilize alternative splicing to resist antiviral response by the host. These findings could provide insights for potential alternative splicing-targeting therapeutic applications.

PCV2 targets cGAS to inhibit type I interferon induction to promote other DNA virus infection

Viruses use diverse strategies to impair the antiviral immunity of host in order to promote infection and pathogenesis. Herein, we found that PCV2 infection promotes the infection of DNA viruses through inhibiting IFN-β induction in vivo and in vitro. In the early phase of infection, PCV2 promotes the phosphorylation of cGAS at S278 via activation of PI3K/Akt signaling, which directly silences the catalytic activity of cGAS. Subsequently, phosphorylation of cGAS at S278 can facilitate the K48-linked poly-ubiquitination of cGAS at K389, which can been served as a signal for recognizing by the ubiquitin-binding domain of histone deacetylase 6 (HDAC6), to promote the translocation of K48-ubiquitinated-cGAS from cytosol to autolysosome depending on the deacetylase activity of HDAC6, thereby eventually resulting in a markedly increased cGAS degradation in PCV2 infection-induced autophagic cells relative to Earle’s Balanced Salt Solution (EBSS)-induced autophagic cells (a typical starving autophagy). Importantly, we found that PCV2 Cap and its binding protein gC1qR act as predominant regulators to promote porcine cGAS phosphorylation and HDAC6 activation through mediating PI3K/AKT signaling and PKCδ signaling activation. Based on this finding, gC1qR-binding activity deficient PCV2 mutant (PCV2RmA) indeed shows a weakened inhibitory effect on IFN-β induction and a weaker boost effect for other DNA viruses infection compared to wild-type PCV2. Collectively, our findings illuminate a systematic regulation mechanism by which porcine circovirus counteracts the cGAS-STING signaling pathway to inhibit the type I interferon induction and promote DNA virus infection, and identify gC1qR as an important regulator for the immunosuppression induced by PCV2.

PCV2 is well known for its ability to induce immunosuppression in pigs. However, how PCV2 infection interferes cGAS-STING signaling is still poorly understood. Herein, we demonstrate that PCV2 infection can phosphorylate porcine cGAS via gC1qR-mediated PI3K/AKT signaling to silence the catalytic activity of cGAS, while activates PKCδ signaling to promote histone deacetylase 6 (HDAC6) activation depending on the assistance of gC1qR. Subsequently, phosphorylation of cGAS facilitates the poly-ubiquitination of cGAS, then ubiquitinated-cGAS proteins are recruited and transported to autolysosome by activated HDAC6 depending on its deacetylase activity and ubiquitin-binding function, thereby eventually resulting in the autophagic degradation of cGAS in PCV2-infected cells. This study reveals that PCV2 can inhibit the activation of cGAS signaling pathway through two different mechanisms at different stages of infection and clarifies the internal relationship and cooperation model between these two mechanisms.

SARS-CoV-2 ORF9b antagonizes type I and III interferons by targeting multiple components of the RIG-I/MDA-5-MAVS, TLR3-TRIF, and cGAS-STING signaling pathways

The suppression of types I and III interferon (IFN) responses by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) contributes to the pathogenesis of coronavirus disease 2019 (COVID-19). The strategy used by SARS-CoV-2 to evade antiviral immunity needs further investigation. Here, we reported that SARS-CoV-2 ORF9b inhibited types I and III IFN production by targeting multiple molecules of innate antiviral signaling pathways. SARS-CoV-2 ORF9b impaired the induction of types I and III IFNs by Sendai virus and poly (I:C). SARS-CoV-2 ORF9b inhibited the activation of types I and III IFNs induced by the components of cytosolic dsRNA-sensing pathways of RIG-I/MDA5-MAVS signaling, including RIG-I, MDA-5, MAVS, TBK1, and IKKε, rather than IRF3-5D, which is the active form of IRF3. SARS-CoV-2 ORF9b also suppressed the induction of types I and III IFNs by TRIF and STING, which are the adaptor protein of the endosome RNA-sensing pathway of TLR3-TRIF signaling and the adaptor protein of the cytosolic DNA-sensing pathway of cGAS-STING signaling, respectively. A mechanistic analysis revealed that the SARS-CoV-2 ORF9b protein interacted with RIG-I, MDA-5, MAVS, TRIF, STING, and TBK1 and impeded the phosphorylation and nuclear translocation of IRF3. In addition, SARS-CoV-2 ORF9b facilitated the replication of the vesicular stomatitis virus. Therefore, the results showed that SARS-CoV-2 ORF9b negatively regulates antiviral immunity and thus facilitates viral replication. This study contributes to our understanding of the molecular mechanism through which SARS-CoV-2 impairs antiviral immunity and provides an essential clue to the pathogenesis of COVID-19.

Nuclear cGAS: guard or prisoner?

cGAS, an innate immune sensor of cellular stress, recognizes double-stranded DNA mislocalized in the cytosol upon infection, mitochondrial stress, DNA damage, or malignancy. Early models suggested that cytosolic localization of cGAS prevents autoreactivity to nuclear and mitochondrial self-DNA, but this paradigm has shifted in light of recent findings of cGAS as a predominantly nuclear protein tightly bound to chromatin. This has raised the question how nuclear cGAS is kept inactive while being surrounded by chromatin, and what function nuclear localization of cGAS may serve in the first place? Cryo-EM structures have revealed that cGAS interacts with nucleosomes, the minimal units of chromatin, mainly via histones H2A/H2B, and that these protein-protein interactions block cGAS from DNA binding and thus prevent autoreactivity. Here, we discuss the biological implications of nuclear cGAS and its interaction with chromatin, including various mechanisms for nuclear cGAS inhibition, release of chromatin-bound cGAS, regulation of different cGAS pools in the cell, and chromatin structure/chromatin protein effects on cGAS activation leading to cGAS-induced autoimmunity.

TREX1 as a Novel Immunotherapeutic Target

Mutations in the TREX1 3' → 5' exonuclease are associated with a spectrum of autoimmune disease phenotypes in humans and mice. Failure to degrade DNA activates the cGAS-STING DNA-sensing pathway signaling a type-I interferon (IFN) response that ultimately drives immune system activation. TREX1 and the cGAS-STING DNA-sensing pathway have also been implicated in the tumor microenvironment, where TREX1 is proposed to degrade tumor-derived DNA that would otherwise activate cGAS-STING. If tumor-derived DNA were not degraded, the cGAS-STING pathway would be activated to promote IFN-dependent antitumor immunity. Thus, we hypothesize TREX1 exonuclease inhibition as a novel immunotherapeutic strategy. We present data demonstrating antitumor immunity in the TREX1 D18N mouse model and discuss theory surrounding the best strategy for TREX1 inhibition. Potential complications of TREX1 inhibition as a therapeutic strategy are also discussed.

Unique and complementary suppression of cGAS-STING and RNA sensing- triggered innate immune responses by SARS-CoV-2 proteins

The emergence of SARS-CoV-2 has resulted in the COVID-19 pandemic, leading to millions of infections and hundreds of thousands of human deaths. The efficient replication and population spread of SARS-CoV-2 indicates an effective evasion of human innate immune responses, although the viral proteins responsible for this immune evasion are not clear. In this study, we identified SARS-CoV-2 structural proteins, accessory proteins, and the main viral protease as potent inhibitors of host innate immune responses of distinct pathways. In particular, the main viral protease was a potent inhibitor of both the RLR and cGAS-STING pathways. Viral accessory protein ORF3a had the unique ability to inhibit STING, but not the RLR response. On the other hand, structural protein N was a unique RLR inhibitor. ORF3a bound STING in a unique fashion and blocked the nuclear accumulation of p65 to inhibit nuclear factor-κB signaling. 3CL of SARS-CoV-2 inhibited K63-ubiquitin modification of STING to disrupt the assembly of the STING functional complex and downstream signaling. Diverse vertebrate STINGs, including those from humans, mice, and chickens, could be inhibited by ORF3a and 3CL of SARS-CoV-2. The existence of more effective innate immune suppressors in pathogenic coronaviruses may allow them to replicate more efficiently in vivo. Since evasion of host innate immune responses is essential for the survival of all viruses, our study provides insights into the design of therapeutic agents against SARS-CoV-2.

ATM inhibition enhances cancer immunotherapy by promoting mtDNA leakage and cGAS/STING activation

Novel approaches are needed to boost the efficacy of immune checkpoint blockade (ICB) therapy. Ataxia telangiectasia mutated (ATM) protein plays a central role in sensing DNA double-stranded breaks (DSBs) and coordinating their repair. Recent data indicated that ATM might be a promising target to enhance ICB therapy. However, the molecular mechanism involved has not been clearly elucidated. Here, we show that ATM inhibition could potentiate ICB therapy by promoting cytoplasmic leakage of mitochondrial DNA (mtDNA) and activation of the cGAS/STING pathway. We show that genetic depletion of ATM in murine cancer cells delayed tumor growth in syngeneic mouse hosts in a T cell-dependent manner. Furthermore, chemical inhibition of ATM potentiated anti-PD-1 therapy of mouse tumors. ATM inhibition potently activated the cGAS/STING pathway and enhanced lymphocyte infiltration into the tumor microenvironment by downregulating mitochondrial transcription factor A (TFAM), which led to mtDNA leakage into the cytoplasm. Moreover, our analysis of data from a large patient cohort indicated that ATM mutations, especially nonsense mutations, predicted for clinical benefits of ICB therapy. Our study therefore provides strong evidence that ATM may serve as both a therapeutic target and a biomarker to enable ICB therapy.

A Nuclear Export Signal Is Required for cGAS to Sense Cytosolic DNA

The cyclic GMP-AMP (cGAMP) synthase (cGAS) is a key DNA sensor that initiates STING-dependent signaling to produce type I interferons through synthesizing the secondary messenger 2'3'-cGAMP. In this study, we confirm previous studies showing that cGAS is located both in the cytoplasm and in the nucleus. Nuclear accumulation is observed when leptomycin B is used to block the exportin, CRM1 protein. As a result, leptomycin B impairs the production of interferons in response to DNA stimulation. We further identify a functional nuclear export signal (NES) in cGAS, 169LEKLKL174. Mutating this NES leads to the sequestration of cGAS within the nucleus and the loss of interferon response to cytosolic DNA treatment, and it further determines the key amino acid to L172. Collectively, our data demonstrate that the cytosolic DNA-sensing function of cGAS depends on its presence within the cytoplasm, which is warranted by a functional NES.

Lymphocyte Changes in Severe COVID-19: Delayed Over-Activation of STING?

Upon recognition of microbial DNA or self-DNA, the cyclic-GMP-AMP synthase (cGAS) of the host catalyzes the production of the cyclic dinucleotide cGAMP. cGAMP is the main activator of STING, stimulator of interferon genes, leading to interferon synthesis through the STING-TBK1-IRF3 pathway. STING is also a hub for activation of NF-κB and autophagy. The present review details the striking similarities between T and B cell responses in severe coronavirus disease 2019 (COVID-19) and both animal or human models of STING gain of function (SAVI syndromes: STING-associated vasculopathy with onset in infancy). Those similarities may be further clues for a delayed activation of STING in severe COVID-19 patients, due to DNA damages following severe acute respiratory syndrome coronaviruses (SARS-CoV-2) infection and unusual role of STING in SARS-CoV-2 control. In early stages, Th2 differentiation are noticed in both severe COVID-19 and SAVI syndromes; then, CD4+ and CD8+ T cells functional exhaustion/senescent patterns due to TCR hyper-responsiveness are observed. T cell delayed over-responses can contribute to pneumonitis and delayed cytokine secretion with over-production of IL-6. Last, STING over-activation induces progressive CD4+ and CD8+ T lymphopenia in SAVI syndromes, which parallels what is observed in severe COVID-19. ACE2, the main receptor of SARS-CoV-2, is rarely expressed in immune cells, and it has not been yet proven that some human lymphocytes could be infected by SARS-CoV-2 through CD147 or CD26. However, STING, expressed in humans T cells, might be triggered following excessive transfer of cGAMP from infected antigen presenting cells into activated CD4+ and CD8+ T cells lymphocytes. Indeed, those lymphocytes highly express the cGAMP importer SLC19A1. Whereas STING is not expressed in human B cells, B cells counts are much less affected, either in COVID-19 or SAVI syndromes. The recognition of delayed STING over-activation in severe COVID-19 patients could prompt to target STING with specific small molecules inhibitors already designed and/or aspirin, which inhibits cGAS.

The Impact of Radiation-Induced DNA Damage on cGAS-STING-Mediated Immune Responses to Cancer

Radiotherapy is a major modality used to combat a wide range of cancers. Classical radiobiology principles categorize ionizing radiation (IR) as a direct cytocidal therapeutic agent against cancer; however, there is an emerging appreciation for additional antitumor immune responses generated by this modality. A more nuanced understanding of the immunological pathways induced by radiation could inform optimal therapeutic combinations to harness radiation-induced antitumor immunity and improve treatment outcomes of cancers refractory to current radiotherapy regimens. Here, we summarize how radiation-induced DNA damage leads to the activation of a cytosolic DNA sensing pathway mediated by cyclic GMP-AMP (cGAMP) synthase (cGAS) and stimulator of interferon genes (STING). The activation of cGAS-STING initiates innate immune signaling that facilitates adaptive immune responses to destroy cancer. In this way, cGAS-STING signaling bridges the DNA damaging capacity of IR with the activation of CD8+ cytotoxic T cell-mediated destruction of cancer-highlighting a molecular pathway radiotherapy can exploit to induce antitumor immune responses. In the context of radiotherapy, we further report on factors that enhance or inhibit cGAS-STING signaling, deleterious effects associated with cGAS-STING activation, and promising therapeutic candidates being investigated in combination with IR to bolster immune activation through engaging STING-signaling. A clearer understanding of how IR activates cGAS-STING signaling will inform immune-based treatment strategies to maximize the antitumor efficacy of radiotherapy, improving therapeutic outcomes.

Baculovirus Transduction in Mammalian Cells Is Affected by the Production of Type I and III Interferons, Which Is Mediated Mainly by the cGAS-STING Pathway

The baculovirus Autographa californica multiple nucleopolyhedrovirus is an insect virus with a circular double-stranded DNA genome, which, among other multiple biotechnological applications, is used as an expression vector for gene delivery in mammalian cells. Nevertheless, the nonspecific immune response triggered by viral vectors often suppresses transgene expression. To understand the mechanisms involved in that response, in the present study, we studied the cyclic GMP-AMP synthase-stimulator of interferon genes (cGAS-STING) pathway by using two approaches: the genetic edition through CRISPR/Cas9 technology of genes encoding STING or cGAS in NIH/3T3 murine fibroblasts and the infection of HEK293 and HEK293 T human epithelial cells, deficient in cGAS and in cGAS and STING expression, respectively. Overall, our results suggest the existence of two different pathways involved in the establishment of the antiviral response, both dependent on STING expression. Particularly, the cGAS-STING pathway resulted in the more relevant production of beta interferon (IFN-β) and IFN-λ1 in response to baculovirus infection. In human epithelial cells, IFN-λ1 production was also induced in a cGAS-independent and DNA-protein kinase (DNA-PK)-dependent manner. Finally, we demonstrated that these cellular responses toward baculovirus infection affect the efficiency of transduction of baculovirus vectors.IMPORTANCE Baculoviruses are nonpathogenic viruses that infect mammals, which, among other applications, are used as vehicles for gene delivery. Here, we demonstrated that the cytosolic DNA sensor cGAS recognizes baculoviral DNA and that the cGAS-STING axis is primarily responsible for the attenuation of transduction in human and mouse cell lines through type I and type III IFNs. Furthermore, we identified DNA-dependent protein kinase (DNA-PK) as a cGAS-independent and alternative DNA cytosolic sensor that contributes less to the antiviral state in baculovirus infection in human epithelial cells than cGAS. Knowledge of the pathways involved in the response of mammalian cells to baculovirus infection will improve the use of this vector as a tool for gene therapy.

Chikungunya virus antagonizes cGAS-STING mediated type-I interferon responses by degrading cGAS

Chikungunya virus (CHIKV) is a mosquito-borne alphavirus known to cause epidemics resulting in predominantly symptomatic infections, which in rare cases cause long term debilitating arthritis and arthralgia. Significant progress has been made in understanding the roles of canonical RNA sensing pathways in the host recognition of CHIKV; however, less is known regarding antagonism of CHIKV by cytosolic DNA sensing pathways like that of cyclic GMP-AMP synthase (cGAS) and Stimulator of Interferon Genes (STING). With the use of cGAS or STING null cells we demonstrate that the pathway restricts CHIKV replication in fibroblasts and immune cells. We show that DNA accumulates in the cytoplasm of infected cells and that CHIKV blocks DNA dependent IFN-β transcription. This antagonism of DNA sensing is via an early autophagy-mediated degradation of cGAS and expression of the CHIKV capsid protein is sufficient to induce cGAS degradation. Furthermore, we identify an interaction of CHIKV nsP1 with STING and map the interaction to 23 residues in the cytosolic loop of the adaptor protein. This interaction stabilizes the viral protein and increases the level of palmitoylated nsP1 in cells. Together, this work supports previous publications highlighting the relevance of the cGAS-STING pathway in the early detection of (+)ssRNA viruses and provides direct evidence that CHIKV interacts with and antagonizes cGAS-STING signaling.

The intracellular DNA sensors cGAS and IFI16 do not mediate effective antiviral immune responses to HSV-1 in human microglial cells

Glia play a key role in immunosurveillance within the central nervous system (CNS) and can recognize a wide range of pathogen-associated molecular patterns (PAMPS) via members of multiple pattern recognition receptor (PRR) families. Of these, the expression of cytosolic/nuclear RNA and DNA sensors by glial cells is of particular interest as their ability to interact with intracellular nucleic acids suggests a critical role in the detection of viral pathogens. The recently discovered DNA sensors cyclic GMP-AMP synthase (cGAS) and interferon gamma-inducible protein 16 (IFI16) have been reported to be important for the recognition of DNA pathogens such as herpes simplex virus-1 (HSV-1) in peripheral human cell types, and we have recently demonstrated that human glia express cGAS and its downstream adaptor molecule stimulator of interferon genes (STING). Here, we have demonstrated that human microglial cells functionally express cGAS and exhibit robust constitutive IFI16 expression. While cGAS serves as a significant component in IRF3 activation and IFN-β production by human microglial cells in response to foreign intracellular DNA, IFI16 is not required for such responses. Surprisingly, neither of these sensors mediate effective antiviral responses to HSV-1 in microglia, and this may be due, at least in part, to viral suppression of cGAS and/or IFI16 expression. As such, this ability may represent an important HSV immune evasion strategy in glial cells, and approaches that mitigate such suppression might represent a novel strategy to limit HSV-1-associated neuropathology.

PARP1-cGAS-NF-κB pathway of proinflammatory macrophage activation by extracellular vesicles released during Trypanosoma cruzi infection and Chagas disease

Trypanosoma cruzi (T. cruzi) is the etiological agent of Chagas cardiomyopathy. In the present study, we investigated the role of extracellular vesicles (Ev) in shaping the macrophage (Mφ) response in progressive Chagas disease (CD). We purified T. cruzi Ev (TcEv) from axenic parasite cultures, and T. cruzi-induced Ev (TEv) from the supernatants of infected cells and plasma of acutely and chronically infected wild-type and Parp1-/- mice. Cultured (Raw 264.7) and bone-marrow Mφ responded to TcEV and TEv with a profound increase in the expression and release of TNF-α, IL-6, and IL-1β cytokines. TEv produced by both immune (Mφ) and non-immune (muscle) cells were proinflammatory. Chemical inhibition or genetic deletion of PARP1 (a DNA repair enzyme) significantly depressed the TEv-induced transcriptional and translational activation of proinflammatory Mφ response. Oxidized DNA encapsulated by TEv was necessary for PARP1-dependent proinflammatory Mφ response. Inhibition studies suggested that DNA-sensing innate immune receptors (cGAS>>TLR9) synergized with PARP1 in signaling the NFκB activation, and inhibition of PARP1 and cGAS resulted in >80% inhibition of TEv-induced NFκB activity. Histochemical studies showed intense inflammatory infiltrate associated with profound increase in CD11b+CD68+TNF-α+ Mφ in the myocardium of CD wild-type mice. In comparison, chronically infected Parp1-/- mice exhibited low-to-moderate tissue inflammation, >80% decline in myocardial infiltration of TNF-α+ Mφ, and no change in immunoregulatory IL-10+ Mφ. We conclude that oxidized DNA released with TEv signal the PARP1-cGAS-NF-κB pathway of proinflammatory Mφ activation and worsens the chronic inflammatory pathology in CD. Small molecule antagonists of PARP1-cGAS signaling pathway would potentially be useful in reprogramming the Mφ activation and controlling the chronic inflammation in CD.

Barrier-to-Autointegration Factor 1 Protects against a Basal cGAS-STING Response

Although the pathogen recognition receptor pathways that activate cell-intrinsic antiviral responses are well delineated, less is known about how the host regulates this response to prevent sustained signaling and possible immune-mediated damage. Using a genome-wide CRISPR-Cas9 screening approach to identify host factors that modulate interferon-stimulated gene (ISG) expression, we identified the DNA binding protein Barrier-to-autointegration factor 1 (Banf1), a previously described inhibitor of retrovirus integration, as a modulator of basal cell-intrinsic immunity. Ablation of Banf1 by gene editing resulted in chromatin activation near host defense genes with associated increased expression of ISGs, including Oas2, Rsad2 (viperin), Ifit1, and ISG15 The phenotype in Banf1-deficient cells occurred through a cGAS-, STING-, and IRF3-dependent signaling axis, was associated with reduced infection of RNA and DNA viruses, and was reversed in Banf1 complemented cells. Confocal microscopy and biochemical studies revealed that a loss of Banf1 expression resulted in higher level of cytosolic double-stranded DNA at baseline. Our study identifies an undescribed role for Banf1 in regulating the levels of cytoplasmic DNA and cGAS-dependent ISG homeostasis and suggests possible therapeutic directions for promoting or inhibiting cell-intrinsic innate immune responses.IMPORTANCE Although the interferon (IFN) signaling pathway is a key host mechanism to restrict infection of a diverse range of viral pathogens, its unrestrained activity either at baseline or in the context of an immune response can result in host cell damage and injury. Here, we used a genome-wide CRISPR-Cas9 screen and identified the DNA binding protein Barrier-to-autointegration factor 1 (Banf1) as a modulator of basal cell-intrinsic immunity. A loss of Banf1 expression resulted in higher level of cytosolic double-stranded DNA at baseline, which triggered IFN-stimulated gene expression via a cGAS-STING-IRF3 axis that did not require type I IFN or STAT1 signaling. Our experiments define a regulatory network in which Banf1 limits basal inflammation by preventing self DNA accumulation in the cytosol.

Grass carp cGASL negatively regulates fish IFN response by targeting MITA

Mammalian cyclic GMP-AMP synthase (cGAS) senses double-stranded (ds) DNA in the cytosol to activate the innate antiviral response. In the present study, a cGAS-like gene, namely cGASL, was cloned from grass carp Ctenopharyngodon idellus, and its role as a negative regulator of the IFN response was revealed. Phylogenetic analysis indicated that cGASL was evolutionarily closest to cGAS, but was not a true ortholog of cGAS. Overexpression of cGASL inhibited poly I:C-stimulated grass carp (gc)IFN1pro and ISRE activities. In addition, MITA-, but not TBK1-mediated activation of gcIFN1pro was impaired by cGASL. Co-immunoprecipitation and Western blot experiments indicated that cGASL interacted with MITA and TBK1, resulting in a reduction in the phosphorylation of MITA. Lastly, overexpression of cGASL reduced the transcriptional levels of several IFN-stimulated genes activated by MITA. Collectively, these data suggest that cGASL is a negative regulator of IFN response by targeting MITA in fish.

Filament-like Assemblies of Intracellular Nucleic Acid Sensors: Commonalities and Differences

Self versus non-self discrimination by innate immune sensors is critical for mounting effective immune responses against pathogens while avoiding harmful auto-inflammatory reactions against the host. Foreign DNA and RNA sensors must discriminate between self versus non-self nucleic acids, despite their shared building blocks and similar physicochemical properties. Recent structural and biochemical studies suggest that multiple steps of filament-like assembly are required for the functions of several nucleic acid sensors. Here, we discuss ligand discrimination and oligomerization of RIG-I-like receptors, AIM2-like receptors, and cGAS. We discuss how filament-like assembly allows for robust and accurate discrimination of self versus non-self nucleic acids and how these assemblies enable sensing of multiple distinct features in foreign nucleic acids, including structure, length, and modifications. We also discuss how individual receptors differ in their assembly and disassembly mechanisms and how these differences contribute to the diversity in nucleic acid specificity and pathogen detection strategies.

The cGAS-cGAMP-STING Pathway: A Molecular Link Between Immunity and Metabolism

It has been appreciated for many years that there is a strong association between metabolism and immunity in advanced metazoan organisms. Distinct immune signatures and signaling pathways have been found not only in immune but also in metabolic cells. The newly discovered DNA-sensing cGAS-cGAMP-STING pathway mediates type I interferon inflammatory responses in immune cells to defend against viral and bacterial infections. Recent studies show that this pathway is also activated by host DNA aberrantly localized in the cytosol, contributing to increased sterile inflammation, insulin resistance, and the development of nonalcoholic fatty liver disease (NAFLD). Potential interactions of the cGAS-cGAMP-STING pathway with mTORC1 signaling, autophagy, and apoptosis have been reported, suggesting an important role of the cGAS-cGAMP-STING pathway in the networking and coordination of these important biological processes. However, the regulation, mechanism of action, and tissue-specific role of the cGAS-cGAMP-STING signaling pathway in metabolic disorders remain largely elusive. It is also unclear whether targeting this signaling pathway is effective for the prevention and treatment of obesity-induced metabolic diseases. Answers to these questions would provide new insights for developing effective therapeutic interventions for metabolic diseases such as insulin resistance, NAFLD, and type 2 diabetes.

ATG16L1 orchestrates interleukin-22 signaling in the intestinal epithelium via cGAS-STING

A coding variant of the inflammatory bowel disease (IBD) risk gene ATG16L1 has been associated with defective autophagy and deregulation of endoplasmic reticulum (ER) function. IL-22 is a barrier protective cytokine by inducing regeneration and antimicrobial responses in the intestinal mucosa. We show that ATG16L1 critically orchestrates IL-22 signaling in the intestinal epithelium. IL-22 stimulation physiologically leads to transient ER stress and subsequent activation of STING-dependent type I interferon (IFN-I) signaling, which is augmented in Atg16l1 ΔIEC intestinal organoids. IFN-I signals amplify epithelial TNF production downstream of IL-22 and contribute to necroptotic cell death. In vivo, IL-22 treatment in Atg16l1 ΔIEC and Atg16l1 ΔIEC/Xbp1 ΔIEC mice potentiates endogenous ileal inflammation and causes widespread necroptotic epithelial cell death. Therapeutic blockade of IFN-I signaling ameliorates IL-22-induced ileal inflammation in Atg16l1 ΔIEC mice. Our data demonstrate an unexpected role of ATG16L1 in coordinating the outcome of IL-22 signaling in the intestinal epithelium.

Herpes Simplex Virus 1 Tegument Protein VP22 Abrogates cGAS/STING-Mediated Antiviral Innate Immunity

Cytosolic DNA arising from intracellular pathogens is sensed by cyclic GMP-AMP synthase (cGAS) and triggers a powerful innate immune response. However, herpes simplex virus 1 (HSV-1), a double-stranded DNA virus, has developed multiple mechanisms to attenuate host antiviral machinery and facilitate viral infection and replication. In the present study, we found that HSV-1 tegument protein VP22 acts as an inhibitor of cGAS/stimulator of interferon genes (cGAS/STING)-mediated production of interferon (IFN) and its downstream antiviral genes. Our results showed that ectopic expression of VP22 decreased cGAS/STING-mediated IFN-β promoter activation and IFN-β production. Infection with wild-type (WT) HSV-1, but not VP22-deficient virus (ΔVP22), inhibited immunostimulatory DNA (ISD)-induced activation of the IFN signaling pathway. Further study showed that VP22 interacted with cGAS and inhibited the enzymatic activity of cGAS. In addition, stable knockdown of cGAS facilitated the replication of ΔVP22 virus but not the WT. In summary, our findings indicate that HSV-1 VP22 acts as an antagonist of IFN signaling to persistently evade host innate antiviral responses.IMPORTANCE cGAS is very important for host defense against viral infection, and many viruses have evolved ways to target cGAS and successfully evade the attack by the immune system of their susceptible host. This study demonstrated that HSV-1 tegument protein VP22 counteracts the cGAS/STING-mediated DNA-sensing antiviral innate immunity signaling pathway by inhibiting the enzymatic activity of cGAS. The findings in this study will expand our understanding of the interaction between HSV-1 replication and the host DNA-sensing signaling pathway.

Delicate regulation of the cGAS-MITA-mediated innate immune response

Although it has long been demonstrated that cytosolic DNA is a potent immune stimulant, it is only in recent years that the molecular mechanisms of DNA-stimulated innate immune responses have emerged. Studies have established critical roles for the DNA sensor cyclic GMP-AMP synthase (cGAS) and the adapter protein MITA/STING in the innate immune response to cytosolic DNA or DNA viruses. Although the regulation of cGAS-MITA/STING-mediated signaling remains to be fully investigated, understanding the processes involved may help to explain the mechanisms of innate immune signaling events and perhaps autoinflammatory diseases and to provide potential therapeutic targets for drug intervention. In this review, we summarize recent progress on the regulation of the cGAS-MITA/STING-mediated innate immune response to DNA viruses at the organelle-trafficking, post-translational and transcriptional levels.

STAG2 deficiency induces interferon responses via cGAS-STING pathway and restricts virus infection

Cohesin is a multi-subunit nuclear protein complex that coordinates sister chromatid separation during cell division. Highly frequent somatic mutations in genes encoding core cohesin subunits have been reported in multiple cancer types. Here, using a genome-wide CRISPR-Cas9 screening approach to identify host dependency factors and novel innate immune regulators of rotavirus (RV) infection, we demonstrate that the loss of STAG2, an important component of the cohesin complex, confers resistance to RV replication in cell culture and human intestinal enteroids. Mechanistically, STAG2 deficiency results in spontaneous genomic DNA damage and robust interferon (IFN) expression via the cGAS-STING cytosolic DNA-sensing pathway. The resultant activation of JAK-STAT signaling and IFN-stimulated gene (ISG) expression broadly protects against virus infections, including RVs. Our work highlights a previously undocumented role of the cohesin complex in regulating IFN homeostasis and identifies new therapeutic avenues for manipulating the 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.

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.

The E3 ubiquitin ligase RNF185 facilitates the cGAS-mediated innate immune response

The cyclic GMP-AMP synthase (cGAS), upon cytosolic DNA stimulation, catalyzes the formation of the second messenger 2'3'-cGAMP, which then binds to stimulator of interferon genes (STING) and activates downstream signaling. It remains to be elucidated how the cGAS enzymatic activity is modulated dynamically. Here, we reported that the ER ubiquitin ligase RNF185 interacted with cGAS during HSV-1 infection. Ectopic-expression or knockdown of RNF185 respectively enhanced or impaired the IRF3-responsive gene expression. Mechanistically, RNF185 specifically catalyzed the K27-linked poly-ubiquitination of cGAS, which promoted its enzymatic activity. Additionally, Systemic Lupus Erythematosus (SLE) patients displayed elevated expression of RNF185 mRNA. Collectively, this study uncovers RNF185 as the first E3 ubiquitin ligase of cGAS, shedding light on the regulation of cGAS activity in innate immune responses.

SENP7 Potentiates cGAS Activation by Relieving SUMO-Mediated Inhibition of Cytosolic DNA Sensing

Cyclic GMP-AMP (cGAMP) synthase (cGAS, a.k.a. MB21D1), a cytosolic DNA sensor, catalyzes formation of the second messenger 2’3’-cGAMP that activates the stimulator of interferon genes (STING) signaling. How the cGAS activity is modulated remains largely unknown. Here, we demonstrate that sentrin/SUMO-specific protease 7 (SENP7) interacted with and potentiated cGAS activation. The small ubiquitin-like modifier (SUMO) was conjugated onto the lysine residues 335, 372 and 382 of cGAS, which suppressed its DNA-binding, oligomerization and nucleotidyl-transferase activities. SENP7 reversed this inhibition via catalyzing the cGAS de-SUMOylation. Consistently, silencing of SENP7 markedly impaired the IRF3-responsive gene expression induced by cGAS-STING axis. SENP7-knockdown mice were more susceptible to herpes simplex virus 1 (HSV-1) infection. SENP7 was significantly up-regulated in patients with SLE. Our study highlights the temporal modulation of the cGAS activity via dynamic SUMOylation, uncovering a novel mechanism for fine-tuning the STING signaling in innate immunity.

The Cyclic GMP-AMP (cGAMP) synthase (cGAS, a.k.a. MB21D1) is critical for monitoring the pathogen-derived DNA upon microbial infection. Its activity should be dynamically modulated in case the inadvertent recognition of the aberrant self nucleic acids in cytosol leads to severe autoimmune diseases. Protein posttranslational modifications dynamically shape the strength and duration of the immune signaling pathways. It is intriguing to explore whether SUMOylation could modulate the cGAS-initiated signaling. In this study, we characterized sentrin/SUMO-specific protease 7 (SENP7) to specifically potentiate the cGAS activation. Upon microbial DNA challenge, the small ubiquitin-like modifier (SUMO) was conjugated onto cGAS, which suppressed its DNA-binding, oligomerization and nucleotidyl-transferase activities. SENP7 reversed this inhibition via catalyzing the de-SUMOylation of cGAS. Our study sheds new light on the dynamic function of the SUMOylation in cytosolic DNAs-triggered innate immunity response.

S6K-STING interaction regulates cytosolic DNA-mediated activation of the transcription factor IRF3

Cytosolic DNA-mediated activation of the transcription factor IRF3 is a key event in host antiviral responses. Here we found that infection with DNA viruses induced interaction of the metabolic checkpoint kinase mTOR downstream effector and kinase S6K1 and the signaling adaptor STING in a manner dependent on the DNA sensor cGAS. We further demonstrated that the kinase domain, but not the kinase function, of S6K1 was required for the S6K1-STING interaction and that the TBK1 critically promoted this process. The formation of a tripartite S6K1-STING-TBK1 complex was necessary for the activation of IRF3, and disruption of this signaling axis impaired the early-phase expression of IRF3 target genes and the induction of T cell responses and mucosal antiviral immunity. Thus, our results have uncovered a fundamental regulatory mechanism for the activation of IRF3 in the cytosolic DNA pathway.

cGAS Senses Human Cytomegalovirus and Induces Type I Interferon Responses in Human Monocyte-Derived Cells

Human cytomegalovirus (HCMV) infections of healthy individuals are mostly unnoticed and result in viral latency. However, HCMV can also cause devastating disease, e.g., upon reactivation in immunocompromised patients. Yet, little is known about human immune cell sensing of DNA-encoded HCMV. Recent studies indicated that during viral infection the cyclic GMP/AMP synthase (cGAS) senses cytosolic DNA and catalyzes formation of the cyclic di-nucleotide cGAMP, which triggers stimulator of interferon genes (STING) and thus induces antiviral type I interferon (IFN-I) responses. We found that plasmacytoid dendritic cells (pDC) as well as monocyte-derived DC and macrophages constitutively expressed cGAS and STING. HCMV infection further induced cGAS, whereas STING expression was only moderately affected. Although pDC expressed particularly high levels of cGAS, and the cGAS/STING axis was functional down-stream of STING, as indicated by IFN-I induction upon synthetic cGAMP treatment, pDC were not susceptible to HCMV infection and mounted IFN-I responses in a TLR9-dependent manner. Conversely, HCMV infected monocyte-derived cells synthesized abundant cGAMP levels that preceded IFN-I production and that correlated with the extent of infection. CRISPR/Cas9- or siRNA-mediated cGAS ablation in monocytic THP-1 cells and primary monocyte-derived cells, respectively, impeded induction of IFN-I responses following HCMV infection. Thus, cGAS is a key sensor of HCMV for IFN-I induction in primary human monocyte-derived DC and macrophages.

STING-Dependent Cytosolic DNA Sensing Promotes Radiation-Induced Type I Interferon-Dependent Antitumor Immunity in Immunogenic Tumors

Ionizing radiation-mediated tumor regression depends on type I interferon (IFN) and the adaptive immune response, but several pathways control I IFN induction. Here, we demonstrate that adaptor protein STING, but not MyD88, is required for type I IFN-dependent antitumor effects of radiation. In dendritic cells (DCs), STING was required for IFN-? induction in response to irradiated-tumor cells. The cytosolic DNA sensor cyclic GMP-AMP (cGAMP) synthase (cGAS) mediated sensing of irradiated-tumor cells in DCs. Moreover, STING was essential for radiation-induced adaptive immune responses, which relied on type I IFN signaling on DCs. Exogenous IFN-? treatment rescued the cross-priming by cGAS or STING-deficient DCs. Accordingly, activation of STING by a second messenger cGAMP administration enhanced antitumor immunity induced by radiation. Thus radiation-mediated antitumor immunity in immunogenic tumors requires a functional cytosolic DNA-sensing pathway and suggests that cGAMP treatment might provide a new strategy to improve radiotherapy.

Modified vaccinia virus Ankara triggers type I IFN production in murine conventional dendritic cells via a cGAS/STING-mediated cytosolic DNA-sensing pathway

Modified vaccinia virus Ankara (MVA) is an attenuated poxvirus that has been engineered as a vaccine against infectious agents and cancers. Our goal is to understand how MVA modulates innate immunity in dendritic cells (DCs), which can provide insights to vaccine design. In this study, using murine bone marrow-derived dendritic cells, we assessed type I interferon (IFN) gene induction and protein secretion in response to MVA infection. We report that MVA infection elicits the production of type I IFN in murine conventional dendritic cells (cDCs), but not in plasmacytoid dendritic cells (pDCs). Transcription factors IRF3 (IFN regulatory factor 3) and IRF7, and the positive feedback loop mediated by IFNAR1 (IFN alpha/beta receptor 1), are required for the induction. MVA induction of type I IFN is fully dependent on STING (stimulator of IFN genes) and the newly discovered cytosolic DNA sensor cGAS (cyclic guanosine monophosphate-adenosine monophosphate synthase). MVA infection of cDCs triggers phosphorylation of TBK1 (Tank-binding kinase 1) and IRF3, which is abolished in the absence of cGAS and STING. Furthermore, intravenous delivery of MVA induces type I IFN in wild-type mice, but not in mice lacking STING or IRF3. Treatment of cDCs with inhibitors of endosomal and lysosomal acidification or the lysosomal enzyme Cathepsin B attenuated MVA-induced type I IFN production, indicating that lysosomal enzymatic processing of virions is important for MVA sensing. Taken together, our results demonstrate a critical role of the cGAS/STING-mediated cytosolic DNA-sensing pathway for type I IFN induction in cDCs by MVA. We present evidence that vaccinia virulence factors E3 and N1 inhibit the activation of IRF3 and the induction of IFNB gene in MVA-infected cDCs.

Modified vaccinia virus Ankara (MVA) is an attenuated vaccinia strain with large deletions of the parental genome that render it non-replicative in mammalian cells. MVA is a safe and effective vaccine against both smallpox and monkeypox. MVA has been investigated as a vaccine vector for infectious diseases and cancers. Dendritic cells (DCs) play important roles in innate and adaptive immunity. A better understanding of how MVA is detected by innate immune sensors in DCs would guide the development of more effective MVA-based vaccines. We report our findings that MVA infection induces the production of type I interferon (IFN) in conventional dendritic cells via a cytosolic DNA-sensing pathway mediated by the newly discovered DNA sensor cGAS, its adaptor STING, and transcription factors IRF3 and IRF7. By contrast, wild-type vaccinia virus fails to activate this pathway. Furthermore, we show that vaccinia virulence factors E3 and N1 play inhibitory roles in the cytosolic DNA-sensing pathway.

Uterine epithelial cells specifically induce interferon-stimulated genes in response to polyinosinic-polycytidylic acid independently of estradiol

Interferon β (IFNβ) is an antiviral cytokine secreted in response to pathogenic exposure that creates a restrictive intracellular environment through the action of downstream interferon-stimulated genes (ISG). The objective of this study was to examine the expression of IFNβ and ISG in both human uterine epithelial cells (UEC) and the ECC-1 uterine epithelial cell line and determine if expression changes with TLR stimulation and hormone exposure. Stimulation of primary uterine epithelial cells and ECC-1 cells with the TLR3 agonist poly (I:C) induced the mRNA expression of IFNβ, MxA, OAS2 and PKR. Other TLR agonists including imiquimod and CpG had no effect on either IFNβ or ISG expression. In contrast to ECC-1 cell responses which were slower, maximal IFNβ upregulation in UEC occurred 3 hours post-stimulation and preceded the ISG response which peaked approximately 12 hours after poly (I:C) exposure. Unexpectedly, estradiol, either alone or prior to treatment with poly (I:C), had no effect on IFNβ or ISG expression. Blockade of the IFN receptor abrogated the upregulation of MxA, OAS2 and PKR. Furthermore, neutralizing antibodies against IFNβ partially inhibited the upregulation of all three ISG. Estradiol, directly and in the presence of poly (I:C) had no effect on IFNβ and ISG expression. These results indicate that uterine epithelial cells are important sentinels of the innate immune system and demonstrate that uterine epithelial cells are capable of mounting a rapid IFN-mediated antiviral response that is independent of estradiol and is therefore potentially sustained throughout the menstrual cycle to aid in the defense of the uterus against potential pathogens.

Generation and evaluation of an anti-REV1 monoclonal antibody

Continuous exposure of cells to exogenous and endogenous agents produces many types of DNA damage during normal cell cycles. Post-replication repair, consisting of error-free and error-prone sub-pathways, is required for tolerance of such DNA damage. REV1 plays a crucial role in regulation of the error-prone pathway. To facilitate analysis of its cellular functions, we here generated a mouse Rev1 monoclonal antibody, called D6, which also recognizes human REV1. The epitope for the antibody could be mapped between 860-877 amino acid residues of human REV1, which was located outside of the conserved catalytic domain. Although the antibody unfortunately could not specifically detect endogenous mouse and human REV1 by western blotting and immunohistochemistry, we established a method to identify endogenous human REV1 by immunoprecipitation-western blotting analysis combining D6 and separately generated polyclonal antibodies.

Identification of a putative membrane-interacting domain of CTP:phosphocholine cytidylyltransferase from rat liver

A putative membrane-interacting domain of CTP:phosphocholine cytidylyltransferase (CT) was identified using two peptide-specific antibodies. One antibody (SA2) was raised against the N-terminus of CT (amino acid residues 1-17) and the other antibody (SA209) against an alpha-helical domain of the enzyme (amino acid residues 247-257). Both antibodies quantitatively immunoprecipitated CT from rat liver cytosol and showed specificity towards CT when octylglucoside extracts of rat liver cytosol were assessed by Western blot analysis. However, further experiments revealed that the antibodies had different characteristics. Whereas the antibody directed against the N-terminus of CT (SA2) did not influence CT/membrane interaction, the new antibody (SA209) against the alpha-helical domain of the enzyme interfered with this interaction. Our results provide experimental evidence that the alpha-helical domain (amino acid residues 228-287) of CT may serve as a membrane-interacting domain.

Immunological characterization, lipid dependence, and subcellular localization of CTP:phosphoethanolamine cytidylyltransferase purified from rat liver. Comparison with CTP:phosphocholine cytidylyltransferase

CTP:phosphoethanolamine cytidylyltransferase (ET) (ethanolamine-phosphate cytidylyltransferase, EC 2.7.7.14), which is generally considered as the rate-regulatory enzyme of phosphatidylethanolamine synthesis via the CDP-ethanolamine pathway, was purified to homogeneity from a rat liver postmicrosomal supernatant. A polyclonal antibody was raised against the enzyme in rabbits and subsequently purified by affinity chromatography. The affinity-purified antibody recognized one single immunoreactive 49.6-kDa protein band on SDS-polyacrylamide gel. The enzyme showed an isoelectric point at a pH of 6.5 and was sensitive to various sulfhydryl reagents. Cross-reactivity experiments of ET and CTP:phosphocholine cytidylyltransferase (CT) (choline-phosphate cytidylyltransferase, EC 2.7.7.15) with their corresponding antibodies showed that these enzymes were immunologically distinct. In contrast with the well known lipid dependence of CT, the activities of both purified and cytosolic ETs were not affected by the presence of various phospholipid preparations. Differential centrifugation studies as well as release experiments with digitonin-permeabilized hepatocytes demonstrated that ET, unlike CT, is not associated with cellular organelles. However, amino acid analysis of ET revealed a high content of hydrophobic amino acids, suggesting a possible association of this enzyme with some kind of cellular structure in the hepatocyte.

Expression of rat CTP:phosphocholine cytidylyltransferase in insect cells using a baculovirus vector

CTP:phosphocholine cytidylyltransferase (CT) is a key regulatory enzyme in phosphatidylcholine biosynthesis. We constructed a recombinant baculovirus (bCT) containing rat CT cDNA under the control of the polyhedrin promoter. Crude cell extracts of Spodoptera frugiperda (Sf9) cells infected with bCT possessed 250-fold higher specific activities for CT compared to rat liver cytosol, and CT protein constituted 3-6% of the total cellular protein. The 42-kDa form of CT predicted from the cDNA sequence was the first immunoreactive CT protein detected at Day 2 after infection and this form continued to accumulate until Day 5. On Day 3 following infection, a 37-kDa protein immunologically related to CT began to accumulate, indicating that CT was being degraded. The active, 42-kDa form of CT was purified to homogeneity in a single step using hydroxyapatite chromatography. Antibodies raised against recombinant CT were employed to quantitatively extract and assay CT activity in mammalian cell lines. The baculovirus expression system is suitable for the preparation of large amounts of protein for investigating the structure, function, and regulation of CT.

Identification of a protein complex between choline-phosphate cytidylyltransferase and a 112-kDa protein in rat liver

Antisera raised against purified cytidylyltransferase (CT) immunoprecipitated CT activity from liver cytosol and detected the M(r) 45,000 subunit of CT on Western blots. Antisera detected a M(r) 112,000 protein on Western blots of liver cytosol. This protein was not detected in purified CT and was not detected by preimmune serum. The 112-kDa antibodies, isolated by affinity chromatography, did not immunoprecipitate CT activity. Antiserum raised against an N-terminal sequence of CT and antibodies raised against an internal sequence of CT immunoprecipitated CT activity but did not detect the 112-kDa protein. These results showed that the 112-kDa protein was not a form of CT. We concluded that the antiserum probably contained anti-idiotypic antibodies that recognized CT binding sites on the 112-kDa protein. Purified CT that was conjugated to horseradish peroxidase bound to crude 112-kDa protein immobilized on nitrocellulose. The binding was competitively reduced by purified CT and by affinity-purified antibodies to the 112-kDa protein. CT and 112-kDa protein coeluted from DEAE-Sepharose. When the putative 112-kDa protein-CT complex was chromatographed on a second DEAE-Sepharose column or on a Bio-Gel A-1.5m column, CT activity and 112-kDa protein were eluted together. Chromatography of the complex on hydroxylapatite dissociated most of the complex, producing CT free of the 112-kDa protein. We conclude that the 112-kDa protein is a CT-binding protein. The formation and/or dissociation of this complex may be important in the regulation of CT.

Insensitivity of barley endosperm ADP-glucose pyrophosphorylase to 3-phosphoglycerate and orthophosphate regulation

Crude extracts of starchy endosperm from barley (Hordeum vulgare cv Bomi) contained high pyrophosphorolytic activity (up to 0.5 mumol of glucose-1-P formed min-1 mg-1 of protein) of ADP-glucose pyrophosphorylase (AGP) when assayed in the absence of 3-phosphoglycerate (3-PGA). This high activity was observed regardless of whether AGP had been extracted in the presence or absence of various protease inhibitors or other protectants. Western blot analysis using antibodies specific for either the small or large subunit of the enzyme demonstrated that the large, 60-kD subunit was prone to proteolysis in crude extracts, with a half-time of degradation at 4 degrees C (from 60 to 53 to 51 kD) on the order of minutes. The presence of high concentrations of protease inhibitors decreased, but did not prevent this proteolysis. The small, 51-kD subunit of barley endosperm AGP was relatively resistant to proteolysis, both in the presence or absence of protease inhibitors. For the crude, nonproteolyzed enzyme, 3-PGA acted as a weak activator of the ADP-glucose synthetic reaction (about 25% activation), whereas in the reverse reaction (pyrophosphorolysis) it served as an inhibitor rather than an activator. For both the synthetic and pyrophosphorolytic reactions, inorganic phosphate (Pi) acted as a weak competitive or mixed inhibitor of AGP. The relative insensitivity to 3-PGA/Pi regulation has been observed with both the nonproteolyzed crude enzyme and partially purified (over 60-fold) AGP, the latter characterized by two bands for the large subunit (molecular masses of 53 and 51 kD) and one band for the small subunit (51 kD). Addition of 3-PGA to assays of the partially purified, proteolyzed enzyme had little or no effect on the Km values of all substrates of AGP, but it reduced the Hill coefficient for ATP (from 2.1 to 1.0). These findings are discussed with respect to previous reports on the structure and regulation of higher plant AGP.

Immunolocalization of membrane-associated CTP:phosphocholine cytidylyltransferase in phosphatidylcholine-deficient Chinese hamster ovary cells

The location of CTP:phosphocholine cytidylyltransferase in Chinese hamster ovary (CHO) cells made deficient in phosphatidylcholine was determined by immunofluorescence techniques. A rabbit polyclonal antibody was raised against a synthetic peptide corresponding to the amino-terminal 17 amino acid residues of rat liver cytidylyltransferase. The antibody recognized both native and denatured cytidylyltransferase from both rat liver and CHO cells. CHO cells were treated with phospholipase C to alter the lipid composition of the plasma membrane and to elicit translocation of cytidylyltransferase from the less active soluble pool to an activated membrane fraction. Visualization of cytidylyltransferase by indirect immunofluorescence revealed staining of the nuclear envelope in phospholipase C-treated cells but not in untreated cells. CHO cells were also starved for choline and supplemented with a choline analogue to provide an alternative technique of rendering the cells phosphatidylcholine-deficient. Although this treatment should affect different cellular membranes than those affected by phospholipase C treatment, cytidylyltransferase still translocated to the nuclear envelope, as shown by indirect immunofluorescence. These results indicate that activated, membrane-bound cytidylyltransferase is associated with the nuclear membrane and suggest that the nuclear membrane may be a site of de novo phosphatidylcholine synthesis.

Evidence that cyclic AMP-induced inhibition of phosphatidylcholine biosynthesis is caused by a decrease in cellular diacylglycerol levels in cultured rat hepatocytes

The mechanism by which glucagon and cAMP analogues inhibit phosphatidylcholine biosynthesis was investigated in rat hepatocytes. The studies were facilitated by preparation of an antibody to a synthetic peptide (D-F-V-A-H-D-D-I-P-Y-S-S-A) corresponding to residues 164-176 of CTP:phosphocholine cytidylyl-transferase. The antibody, which was purified by affinity chromatography, quantitatively immunoprecipitated cytidylyltransferase from rat liver cytosol. Various analogues of cAMP had no effect on the labeling of cytidylyltransferase with 32Pi in rat hepatocytes. Nor did the cAMP analogues have any effect on the distribution of cytidylyltransferase between cytosol and membranes. These results indicate that the supply of CDP-choline does not limit phosphatidylcholine biosynthesis in hepatocytes treated with cAMP analogues. A decreased supply of diacylglycerol was considered as an alternative mechanism for inhibition of phosphatidylcholine biosynthesis. An approximately 30% decrease in diacylglycerol concentration was observed in hepatocytes treated with the cAMP analogues or glucagon, compared with controls. A similar decrease of phosphatidylcholine biosynthesis was observed. The cAMP-mediated decrease in diacylglycerol levels and inhibition of phosphatidylcholine biosynthesis were reversed by addition of 0.5-1.5 mM oleic acid to the treated hepatocytes. A correlation coefficient of 0.93 was calculated between the levels of diacylglycerol and the rate of phosphatidylcholine biosynthesis. In another approach, the diacylglycerol levels were increased by an inhibitor of diacylglycerol lipase (U-57908) which also reversed the cAMP effects on diacylglycerol levels and phosphatidylcholine biosynthesis. We conclude that the cAMP-mediated inhibition of phosphatidylcholine biosynthesis was not due to an effect on the phosphorylation of cytidylyltransferase. Instead, phosphatidylcholine biosynthesis appears to be inhibited due to a decreased level of diacylglycerol, a substrate for CDP-choline: 1,2-diacylglycerol cholinephosphotransferase.

Regulation of CTP:phosphocholine cytidylyltransferase activity and subcellular location by phosphorylation in Chinese hamster ovary cells. The effect of phospholipase C treatment

The phosphorylation state of cytidylyltransferase in Chinese hamster ovary (CHO) cells was correlated with its subcellular distribution and activity in vivo. Western blot analysis of soluble and particulate fractions from control and phospholipase C-treated cells revealed slower migrating forms of cytidylyltransferase present only in the soluble fraction of control cells. These were abolished by incubating the soluble fraction at 37 degrees C in the presence of 5 mM Mg2+ but persisted if 135 mM NaF was present in the incubation. CHO cells were labeled with 32Pi, and cytidylyltransferase was immunoprecipitated from soluble and particulate fractions from control and phospholipase C-treated cells. The slower migrating forms of cytidylyltransferase, present in the soluble fraction of control cells, were phosphorylated at multiple sites. Although an equivalent amount of cytidylyltransferase was immunoprecipitated from the particulate fraction of phospholipase C-treated cells, it contained little 32P. Pretreatment of the CHO cells with okadaic acid, an inhibitor of type 1 and 2A phosphatases, prevented the stimulation of cytidylyltransferase in vivo by phospholipase C. These results demonstrate that dephosphorylation of soluble cytidylyltransferase is required for the phospholipase C-mediated translocation of cytidylyltransferase in CHO cells.

Preparation of synthetic polypeptide domains of carcinoembryonic antigen and their use in epitope mapping

Genes encoding the four principal polypeptide domains (N, A1-B1, A2-B2, and A3-B3) of carcinoembryonic antigen (CEA) were synthesized and expressed in Escherichia coli as fusion products with bacterial CMP-KDO synthetase (CKS). The four synthetic fusion proteins were purified in high yield and used as targets in Western blots for 11 anti-CEA MAbs and to compete with immobilized CEA for binding to four of these MAbs. Each of the MAbs showed strong binding to one or more of the fusion proteins. In Western blots, MAbs H19C91 and 4230 bound only to CKS-N. MAbs H8C2 and H11C35 bound only CKS-A1-B1, and MAbs T84.66, H46C136, and H21C83 appeared to be specific for CKS-A3-B3. None of the MAbs tested bound only to CKS-A2-B2. However, two MAbs bound both CKS-A1-B1 and CKS-A3-B3 and one MAb (3519) bound to all three of the repeated domains. Since these three domains exhibit over 90% amino acid sequence homology, the latter results were not surprising. The competition studies largely confirmed the results of Western blots but did show some MAb-fusion protein interactions not observed in Western blots. These competition studies also allowed estimation of the relative affinities of the MAbs for the synthetic domains and for native CEA. These studies demonstrated that epitopes in CEA recognized by the MAbs in this study are peptide in nature and that the fusion proteins are of utility in the localization of the epitopes on the polypeptide chain of CEA.

Antibodies to ADP-ATP carrier--an autoantigen in myocarditis and dilated cardiomyopathy--impair cardiac function

The adenosine diphosphate (ADP)-adenosine triphosphate (ATP) carrier of the inner mitochondrial membrane is identified as an autoantigen in myocarditis and dilated cardiomyopathy. Sera of patients with these diseases contain autoantibodies to the ADP-ATP carrier capable of inhibiting nucleotide transport in vitro. Recently, an antibody-related infringement of energy metabolism was shown in intact perfused hearts isolated from guinea pigs immunized with the ADP-ATP carrier. A decreased cytosolic-mitochondrial difference of the phosphorylation potential of ATP was measured that originated from a reduction in mitochondrial-cytosolic nucleotide transport. Nonimmunized animals did not show these changes in energy metabolism, despite being in a comparable metabolic state and performing equal external heart work. To establish whether antibodies to the ADP-ATP carrier can also alter cardiac function, hemodynamic parameters of isolated hearts of guinea pigs that were preimmunized with the carrier protein were measured. Cardiac metabolism was stimulated by exposing the hearts to a high calcium concentration in conjunction with a maximum elevation of the afterload. Mean aortic pressure, stroke volume, stroke work, and external heart work were found to be lowered significantly (p less than 0.005). The external heart work of the immunized hearts reached only about 20% of the level performed by control hearts. Myocardial oxygen consumption was lowered 2.5-fold, whereas the extent of lactate production was found to be more than doubled. These results show a diminished cardiac performance of hearts from animals immunized with the ADP-ATP carrier. Our findings demonstrate that autoimmunity to the ADP-ATP carrier may contribute to the pathophysiology of dilated cardiomyopathy as a subsequent stage of myocarditis by causing an autoantibody-mediated reduction in cardiac function on the basis of an imbalance between energy delivery and demand.

Antibodies to the ADP/ATP carrier, an autoantigen in myocarditis and dilated cardiomyopathy, penetrate into myocardial cells and disturb energy metabolism in vivo

We identified the ADP/ATP carrier, located within the inner mitochondrial membrane, to be an organ- and conformation-specific autoantigen in myocarditis and dilated cardiomyopathy. We also showed that autoantibodies to the ADP/ATP carrier inhibit the nucleotide transport in vitro. Specific binding of the autoantibodies to the carrier was demonstrated by radioimmunoassay and the immunoblot technique; the inhibition of the nucleotide transport was determined by the inhibitor stop method. To establish if these autoantibodies might also affect cardiac energy metabolism in vivo, we measured whether they are capable of penetrating into myocytes and whether subcellular ATP/ADP ratios and phosphorylation potentials of ATP change in hearts of guinea pigs that have been immunized with the isolated ADP/ATP carrier. An intracellular deposition of autoantibodies was observed by direct immunofluorescence and by immunoperoxidase staining on cryosections of the myocardial tissue of animals immunized with the ADP/ATP carrier. Furthermore, binding of autoantibodies to mitochondrial membrane structures was shown by immunoelectron-microscopic methods. The cytosolic and intramitochondrial distribution of adenine nucleotides in stimulated, isolated perfused hearts of guinea pigs immunized with the ADP/ATP carrier was measured by nonaqueous fractionation. Compared with controls performing equal external heart work, the cytosolic ATP decreased in the immunized animals, whereas the mitochondrial ATP increased strongly; ADP concentrations showed an opposite change. Thus, a resultant cytosolic decrease and a marked mitochondrial increase of the ATP/ADP ratio was established. As a consequence, the cytosolic-mitochondrial phosphorylation potential of ATP was diminished. These findings demonstrate that antibodies against intracellular antigens are able to penetrate into living cells, and that autoimmunity to the ADP/ATP carrier may contribute to the pathophysiology of myocarditis and dilated cardiomyopathy by causing an autoantibody-mediated imbalance between intracellular energy delivery and demand.