Pan-viral specificity of IFN-induced genes reveals new roles for cGAS in innate immunity

Redox Activation of Mitochondrial DAMPs and the Metabolic Consequences for Development of Autoimmunity

Significance: Reactive oxygen species (ROS) are well known to promote innate immune responses during and in the absence of microbial infections. However, excessive or prolonged exposure to ROS provokes innate immune signaling dysfunction and contributes to the pathogenesis of many autoimmune diseases. The relatively high basal expression of pattern recognition receptors (PRRs) in innate immune cells renders them prone to activation in response to minor intrinsic or extrinsic ROS misbalances in the absence of pathogens. Critical Issues: A prominent source of ROS are mitochondria, which are also major inter-organelle hubs for innate immunity activation, since most PRRs and downstream receptor molecules are directly located either at mitochondria or at mitochondria-associated membranes. Due to their ancestral bacterial origin, mitochondria can also act as quasi-intrinsic self-microbes that mimic a pathogen invasion and become a source of danger-associated molecular patterns (DAMPs) that triggers innate immunity from within. Recent Advances: The release of mitochondrial DAMPs correlates with mitochondrial metabolism changes and increased generation of ROS, which can lead to the oxidative modification of DAMPs. Recent studies suggest that ROS-modified mitochondrial DAMPs possess increased, persistent immunogenicity. Future Directions: Herein, we discuss how mitochondrial DAMP release and oxidation activates PRRs, changes cellular metabolism, and causes innate immune response dysfunction by promoting systemic inflammation, thereby contributing to the onset or progression of autoimmune diseases. The future goal is to understand what the tipping point for DAMPs is to become oxidized, and whether this is a road without return. Antioxid. Redox Signal. 36, 441-461.

Iron Activates cGAS-STING Signaling and Promotes Hepatic Inflammation

Iron deposition and chronic inflammation are associated with chronic liver diseases, such as alcoholic liver disease, nonalcoholic fatty liver disease, and chronic hepatitis B and C. However, the relationship between iron deposition and chronic inflammation in these diseases is still unclear. In the current study, we aimed to investigate the effect of iron on chronic inflammation in HepG2 cells and mice liver. We demonstrated that iron treatment enhanced the expression of cGAS, STING, and their downstream targets, including TBK1, IRF-3, and NF-κB in HepG2 cells and mice liver. We also found that treatment of HepG2 cells and mice with ferric ammonium citrate increased the expression of inflammatory cytokines, such as IFN-β. Finally, we found that genes involved in iron metabolism and the STING signaling pathway were up-regulated in liver cancer tissues, and the survival time of patients with high expression of these genes in tumor tissues was significantly shortened. These results suggest that iron overload may promote the progress of the chronic liver disease by activating cGAS-STING-mediated chronic inflammation, which provides a new idea for the development of drugs for the treatment of the chronic liver disease.

Mitochondria and Viral Infection: Advances and Emerging Battlefronts

Mitochondria are dynamic organelles vital for energy production with now appreciated roles in immune defense. During microbial infection, mitochondria serve as signaling hubs to induce immune responses to counteract invading pathogens like viruses. Mitochondrial functions are central to a variety of antiviral responses including apoptosis and type I interferon signaling (IFN-I). While apoptosis and IFN-I mediated by mitochondrial antiviral signaling (MAVS) are well-established defenses, new dimensions of mitochondrial biology are emerging as battlefronts during viral infection. Increasingly, it has become apparent that mitochondria serve as reservoirs for distinct cues that trigger immune responses and that alterations in mitochondrial morphology may also tip infection outcomes. Furthermore, new data are foreshadowing pivotal roles for classic, homeostatic facets of this organelle as host-virus interfaces, namely, the tricarboxylic acid (TCA) cycle and electron transport chain (ETC) complexes like respiratory supercomplexes. Underscoring the importance of "housekeeping" mitochondrial activities in viral infection is the growing list of viral-encoded inhibitors including mimics derived from cellular genes that antagonize these functions. For example, virologs for ETC factors and several enzymes from the TCA cycle have been recently identified in DNA virus genomes and serve to pinpoint new vulnerabilities during infection. Here, we highlight recent advances for known antiviral functions associated with mitochondria as well as where the next battlegrounds may be based on viral effectors. Collectively, new methodology and mechanistic insights over the coming years will strengthen our understanding of how an ancient molecular truce continues to defend cells against viruses.

The Emerging Role of RNA Modifications in the Regulation of Antiviral Innate Immunity

Posttranscriptional modifications have been implicated in regulation of nearly all biological aspects of cellular RNAs, from stability, translation, splicing, nuclear export to localization. Chemical modifications also have been revealed for virus derived RNAs several decades before, along with the potential of their regulatory roles in virus infection. Due to the dynamic changes of RNA modifications during virus infection, illustrating the mechanisms of RNA epigenetic regulations remains a challenge. Nevertheless, many studies have indicated that these RNA epigenetic marks may directly regulate virus infection through antiviral innate immune responses. The present review summarizes the impacts of important epigenetic marks on viral RNAs, including N6-methyladenosine (m⁶A), 5-methylcytidine (m⁵C), 2ʹ-O-methylation (2ʹ-O-Methyl), and a few uncanonical nucleotides (A-to-I editing, pseudouridine), on antiviral innate immunity and relevant signaling pathways, while highlighting the significance of antiviral innate immune responses during virus infection.

Small molecule STING inhibition improves myocardial infarction remodeling

AIMS

Myocardial infarction (MI) is a major global cause of death. Massive cell death leads to inflammation, which is necessary for ensuing wound healing. Extensive inflammation, however, promotes infarct expansion and adverse remodeling. The DNA sensing receptor cyclic GMP-AMP synthase and its downstream signaling effector stimulator of interferon genes (cGAS-STING) is central in innate immune reactions in infections or autoimmunity. Cytosolic double-strand DNA activates the pathway and down-stream inflammatory responses. Recent papers demonstrated that this pathway is also active following MI and that its genetic targeting improves outcome. Thus, we investigated if pharmacologic pathway inhibition is protective after MI in order to test its translational potential.

MAIN METHODS

We investigated novel and selective small-molecule STING inhibitors that inhibit STING palmitoylation and multimerization and thereby downstream pathway activation in a preclinical murine MI model. We assessed structural and functional cardiac remodeling, infarct expansion and fibrosis, as well as cardiomyocyte hypertrophy and the expression of inflammatory genes.

KEY FINDINGS

Pharmacologic STING inhibition did not reduce mortality due to myocardial rupture in non-reperfused MI. Infarct size at day one was comparable. However, three weeks of pharmacologic STING inhibition after reperfused MI decreased infarct expansion and scarring, increased left ventricular systolic function to levels approaching normal values, and reduced myocardial hypertrophy.

SIGNIFICANCE

Selective small-molecule STING inhibition after myocardial infarction has the potential to improve wound healing responses and pathological remodeling and thereby attenuate the development of ischemic heart failure.

RNA viruses and the cGAS-STING pathway: reframing our understanding of innate immune sensing

The past decade has provided critical information about the cytoplasmic innate immune sensing pathway of cyclic GMP-AMP synthase (cGAS) and stimulator of interferon genes (STING). These discoveries have broadened our understanding of the interconnectedness of the cGAS-STING pathway with autophagy, programmed cell death, Rig-I-like receptor (RLR) signaling, DNA independent interferon induction, and how this pathway responds to RNA virus infection. These advances highlight how multiple families of RNA viruses are restricted by and in turn have mechanisms to inhibit cGAS-STING dependent type-I interferon (IFN-I) induction. Here we review recent discoveries of how and why the cGAS-STING pathway responds to infection with RNA viruses, novel findings of RNA viral antagonism of the cGAS-STING innate immune sensing pathway, and attempt to provide context for a shift in thinking as to how critical this DNA sensing pathway is for the restriction of a wide range of RNA viruses.

Biochemistry, Cell Biology, and Pathophysiology of the Innate Immune cGAS-cGAMP-STING Pathway

In the decade since the discovery of the innate immune cyclic GMP-AMP synthase (cGAS)- 2'3'-cyclic GMP-AMP (cGAMP)- stimulator of interferon genes (STING) pathway, its proper activation and dysregulation have been rapidly implicated in many aspects of human disease. Understanding the biochemical, cellular, and regulatory mechanisms of this pathway is critical to developing therapeutic strategies that either harness it to boost defense or inhibit it to prevent unwanted inflammation. In this review, we first discuss how the second messenger cGAMP is synthesized by cGAS in response to double-stranded DNA and cGAMP's subsequent activation of cell-type-dependent STING signaling cascades with differential physiological consequences. We then review how cGAMP as an immunotransmitter mediates tightly controlled cell-cell communication by being exported from producing cells and imported into responding cells via cell-type-specific transporters. Finally, we review mechanisms by which the cGAS-cGAMP-STING pathway responds to different sources of mislocalized double-stranded DNA in pathogen defense, cancer, and autoimmune diseases. Expected final online publication date for the Annual Review of Biochemistry, Volume 91 is June 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Role of Mitochondrial Nucleic Acid Sensing Pathways in Health and Patho-Physiology

Mitochondria, in symbiosis with the host cell, carry out a wide variety of functions from generating energy, regulating the metabolic processes, cell death to inflammation. The most prominent function of mitochondria relies on the oxidative phosphorylation (OXPHOS) system. OXPHOS heavily influences the mitochondrial-nuclear communication through a plethora of interconnected signaling pathways. Additionally, owing to the bacterial ancestry, mitochondria also harbor a large number of Damage Associated Molecular Patterns (DAMPs). These molecules relay the information about the state of the mitochondrial health and dysfunction to the innate immune system. Consequently, depending on the intracellular or extracellular nature of detection, different inflammatory pathways are elicited. One group of DAMPs, the mitochondrial nucleic acids, hijack the antiviral DNA or RNA sensing mechanisms such as the cGAS/STING and RIG-1/MAVS pathways. A pro-inflammatory response is invoked by these signals predominantly through type I interferon (T1-IFN) cytokines. This affects a wide range of organ systems which exhibit clinical presentations of auto-immune disorders. Interestingly, tumor cells too, have devised ingenious ways to use the mitochondrial DNA mediated cGAS-STING-IRF3 response to promote neoplastic transformations and develop tumor micro-environments. Thus, mitochondrial nucleic acid-sensing pathways are fundamental in understanding the source and nature of disease initiation and development. Apart from the pathological interest, recent studies also attempt to delineate the structural considerations for the release of nucleic acids across the mitochondrial membranes. Hence, this review presents a comprehensive overview of the different aspects of mitochondrial nucleic acid-sensing. It attempts to summarize the nature of the molecular patterns involved, their release and recognition in the cytoplasm and signaling. Finally, a major emphasis is given to elaborate the resulting patho-physiologies.

Distinct Cellular Tropism and Immune Responses to Alphavirus Infection

Alphaviruses are emerging and reemerging viruses that cause disease syndromes ranging from incapacitating arthritis to potentially fatal encephalitis. While infection by arthritogenic and encephalitic alphaviruses results in distinct clinical manifestations, both virus groups induce robust innate and adaptive immune responses. However, differences in cellular tropism, type I interferon induction, immune cell recruitment, and B and T cell responses result in differential disease progression and outcome. In this review, we discuss aspects of immune responses that contribute to protective or pathogenic outcomes after alphavirus infection. Expected final online publication date for the Annual Review of Immunology, Volume 40 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

A Variety of Nucleic Acid Species Are Sensed by cGAS, Implications for Its Diverse Functions

Cyclic GMP-AMP synthase (cGAS) recognizes double-stranded DNA (dsDNA) derived from invading pathogens and induces an interferon response via activation of the key downstream adaptor protein stimulator of interferon genes (STING). This is the most classic biological function of the cGAS-STING signaling pathway and is critical for preventing pathogenic microorganism invasion. In addition, cGAS can interact with various types of nucleic acids, including cDNA, DNA : RNA hybrids, and circular RNA, to contribute to a diverse set of biological functions. An increasing number of studies have revealed an important relationship between the cGAS-STING signaling pathway and autophagy, cellular senescence, antitumor immunity, inflammation, and autoimmune diseases. This review details the mechanism of action of cGAS as it interacts with different types of nucleic acids, its rich biological functions, and the potential for targeting this pathway to treat various diseases.

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

The innate immune systems detect pathogens via pattern-recognition receptors including nucleic acid sensors and non-nucleic acid sensors. Cyclic guanosine monophosphate-adenosine monophosphate (cGAMP) synthase (cGAS, also known as MB21D1) is a cytosolic DNA sensor that recognizes double-stranded DNA (dsDNA) and catalyzes the synthesis of 2',3'-cGAMP. Subsequently, 2',3'-cGAMP binds to the adaptor protein mediator of IRF3 activation (MITA, also known as STING, MPYS, ERIS, and TMEM173) to activate downstream signaling cascades. The cGAS-MITA/STING signaling critically mediates immune responses against DNA viruses, retroviruses, bacteria, and protozoan parasites. In addition, recent discoveries have extended our understanding of the roles of the cGAS-MITA/STING pathway in autoimmune diseases and cancers. Here, we summarize the identification and activation of cGAS and MITA/STING, present the updated functions and regulatory mechanisms of cGAS-MITA/STING signaling and provide a comprehensive understanding of the cGAS-MITA/STING axis in autoimmune diseases and cancers.

Host cell-intrinsic innate immune recognition of SARS-CoV-2

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) emerged at the end of 2019 and caused the pandemic of coronavirus disease 2019 (COVID-19). Basic and clinical investigations indicate that severe forms of COVID-19 are due in part to dysregulated immune responses to virus infection. The innate immune system is the first line of host defense against most virus infections, with pathogen recognition receptors detecting SARS-CoV-2 RNA and protein components and initiating pro-inflammatory and antiviral responses. Notwithstanding this response, SARS-CoV-2 proteins evade, inhibit, and skew innate immune signaling early in infection. In this review, we highlight the components of cell-based recognition of SARS-CoV-2 infection and the mechanisms employed by the virus to modulate these innate immune host defense pathways.

Innate immunity: the first line of defense against SARS-CoV-2

The coronavirus disease 2019 (COVID-19) pandemic, caused by severe acute respiratory syndrome coronavirus (SARS-CoV)-2, continues to cause substantial morbidity and mortality. While most infections are mild, some patients experience severe and potentially fatal systemic inflammation, tissue damage, cytokine storm and acute respiratory distress syndrome. The innate immune system acts as the first line of defense, sensing the virus through pattern recognition receptors and activating inflammatory pathways that promote viral clearance. Here, we discuss innate immune processes involved in SARS-CoV-2 recognition and the resultant inflammation. Improved understanding of how the innate immune system detects and responds to SARS-CoV-2 will help identify targeted therapeutic modalities that mitigate severe disease and improve patient outcomes.

Assessing Mitochondrial DNA Release into the Cytosol and Subsequent Activation of Innate Immune-related Pathways in Mammalian Cells

Mitochondria have emerged as key drivers of mammalian innate immune responses, functioning as signaling hubs to trigger inflammation and orchestrating metabolic switches required for phagocyte activation. Mitochondria also contain damage-associated molecular patterns (DAMPs), molecules that share similarity with pathogen-associated molecular patterns (PAMPs) and can engage innate immune sensors to drive inflammation. The aberrant release of mitochondrial DAMPs during cellular stress and injury is an increasingly recognized trigger of inflammatory responses in human diseases. Mitochondrial DNA (mtDNA) is a particularly potent DAMP that engages multiple innate immune sensors, although mounting evidence suggests that cytosolic mtDNA is primarily detected via the cyclic GMP-AMP synthase-stimulator of interferon genes (cGAS-STING) pathway. cGAS and STING are widely expressed in mammalian cells and serve as key regulators of type I interferon and cytokine expression in both infectious and inflammatory diseases. Despite growing roles for the mtDNA-cGAS-STING axis in human disease, assays to quantify mtDNA release into the cytosol and approaches to link mtDNA to cGAS-STING signaling are not standardized, which increases the possibility for experimental artifacts and misinterpretation of data. Here, we present a series of protocols for assaying the release of mtDNA into the cytosol and subsequent activation of innate immune signaling in mammalian cells. We highlight genetic and pharmacological approaches to induce and inhibit mtDNA release from mitochondria. We also describe immunofluorescence microscopy and cellular fractionation assays to visualize morphological changes in mtDNA and quantify mtDNA accumulation in the cytosol. Finally, we include protocols to examine mtDNA-dependent cGAS-STING activation by RT-qPCR and western blotting. These methods can be performed with standard laboratory equipment and are highly adaptable to a wide range of mammalian cell types. They will permit researchers working across the spectrum of biological and biomedical sciences to accurately and reproducibly measure cytosolic mtDNA release and resulting innate immune responses. © 2022 Wiley Periodicals LLC. Basic Protocol 1: siRNA-mediated knockdown of TFAM to induce mtDNA instability, cytosolic release, and activation of the cGAS-STING pathway Alternate Protocol: Pharmacological induction of mtDNA release and cGAS-STING activation using ABT-737 and Q-VD-OPH Basic Protocol 2: Isolation and quantitation of DNA from cytosolic, mitochondrial, and nuclear fractions Basic Protocol 3: Pharmacological inhibition of mtDNA replication and release.

Second Messenger 2'3'-cyclic GMP-AMP (2'3'-cGAMP):Synthesis, transmission, and degradation

Cyclic GMP-AMP synthase (cGAS) senses foreign DNA to produce 2'3'-cyclic GMP-AMP (2'3'-cGAMP). 2'3'-cGAMP is a second messenger that binds and activates the adaptor protein STING, which triggers the innate immune response. As a STING agonist, the small molecule 2'3'-cGAMP plays pivotal roles in antiviral defense and has adjuvant applications, and anti-tumor effects. 2'3'-cGAMP and its analogs are thus putative targets for immunotherapy and are currently being testedin clinical trials to treat solid tumors. However, several barriers to further development have emerged from these studies, such as evidence of immune and inflammatory side-effects, poor pharmacokinetics, and undesirable biodistribution. Here, we review the status of 2'3'-cGAMP research and outline the role of 2'3'-cGAMP in immune signaling, adjuvant applications, and cancer immunotherapy, as well as various 2'3'-cGAMP detection methods.

Flaviviruses: Innate Immunity, Inflammasome Activation, Inflammatory Cell Death, and Cytokines

The innate immune system is the host’s first line of defense against the invasion of pathogens including flavivirus. The programmed cell death controlled by genes plays an irreplaceable role in resisting pathogen invasion and preventing pathogen infection. However, the inflammatory cell death, which can trigger the overflow of a large number of pro-inflammatory cytokines and cell contents, will initiate a severe inflammatory response. In this review, we summarized the current understanding of the innate immune response, inflammatory cell death pathway and cytokine secretion regulation during Dengue virus, West Nile virus, Zika virus, Japanese encephalitis virus and other flavivirus infections. We also discussed the impact of these flavivirus and viral proteins on these biological processes. This not only provides a scientific basis for elucidating the pathogenesis of flavivirus, but also lays the foundation for the development of effective antiviral therapies.

cGAS-STING effectively restricts murine norovirus infection but antagonizes the antiviral action of N-terminus of RIG-I in mouse macrophages

Although cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) signaling has been well recognized in defending DNA viruses, the role of cGAS-STING signaling in regulating infection of RNA viruses remains largely elusive. Noroviruses, as single-stranded RNA viruses, are the main causative agents of acute viral gastroenteritis worldwide. This study comprehensively investigated the role of cGAS-STING in response to murine norovirus (MNV) infection. We found that STING agonists potently inhibited MNV replication in mouse macrophages partially requiring the JAK/STAT pathway that induced transcription of interferon (IFN)-stimulated genes (ISGs). Loss- and gain-function assays revealed that both cGAS and STING were necessary for host defense against MNV propagation. Knocking out cGAS or STING in mouse macrophages led to defects in induction of antiviral ISGs upon MNV infection. Overexpression of cGAS and STING moderately increased ISG transcription but potently inhibited MNV replication in human HEK293T cells ectopically expressing the viral receptor CD300lf. This inhibitory effect was not affected by JAK inhibitor treatment or expression of different MNV viral proteins. Interestingly, STING but not cGAS interacted with mouse RIG-I, and attenuated its N-terminus-mediated anti-MNV effects. Our results implicate an essential role for mouse cGAS and STING in regulating innate immune response and defending MNV infection. This further strengthens the evidence of cGAS-STING signaling in response to RNA virus infection.

SARS-CoV-2 infection induces a pro-inflammatory cytokine response through cGAS-STING and NF-κB

SARS-CoV-2 is a novel virus that has rapidly spread, causing a global pandemic. In the majority of infected patients, SARS-CoV-2 leads to mild disease; however, in a significant proportion of infections, individuals develop severe symptoms that can lead to long-lasting lung damage or death. These severe cases are often associated with high levels of pro-inflammatory cytokines and low antiviral responses, which can cause systemic complications. Here, we have evaluated transcriptional and cytokine secretion profiles and detected a distinct upregulation of inflammatory cytokines in infected cell cultures and samples taken from infected patients. Building on these observations, we found a specific activation of NF-κB and a block of IRF3 nuclear translocation in SARS-CoV-2 infected cells. This NF-κB response was mediated by cGAS-STING activation and could be attenuated through several STING-targeting drugs. Our results show that SARS-CoV-2 directs a cGAS-STING mediated, NF-κB-driven inflammatory immune response in human epithelial cells that likely contributes to inflammatory responses seen in patients and could be therapeutically targeted to suppress severe disease symptoms.

From high-throughput to therapeutic: host-directed interventions against influenza viruses

Influenza viruses are simultaneously supported and antagonized by factors within the host cell. This close relationship is the theoretical basis for future antivirals that target the host rather than the virus itself, a concept termed host-directed therapeutics. Genetic screening has led to the identification of host factors capable of modulating influenza virus infections, and these factors represent candidate targets for host-directed antiviral strategies. Despite advances in understanding host targets, however, there are currently no host-directed interventions for influenza viruses in clinical use. In this brief review, we discuss some host factors identified in knockout/knockdown and overexpression screens that could potentially be targeted as host-directed influenza intervention strategies. We further comment on the feasibility of changing gene expression in the respiratory tract with RNA delivery vectors and transient CRISPR-mediated gene targeting.

Evolution of RNA sensing receptors in birds

Birds are important hosts for many RNA viruses, including influenza A virus, Newcastle disease virus, West Nile virus and coronaviruses. Innate defense against RNA viruses in birds involves detection of viral RNA by pattern recognition receptors. Several receptors of different classes are involved, such as endosomal toll-like receptors and cytoplasmic retinoic acid-inducible gene I-like receptors, and their downstream adaptor proteins. The function of these receptors and their antagonism by viruses is well established in mammals; however, this has received less attention in birds. These receptors have been characterized in a few bird species, and the completion of avian genomes will permit study of their evolution. For each receptor, functional work has established ligand specificity and activation by viral infection. Engagement of adaptors, regulation by modulators and the supramolecular organization of proteins required for activation are incompletely understood in both mammals and birds. These receptors bind conserved nucleic acid agonists such as single- or double-stranded RNA and generally show purifying selection, particularly the ligand binding regions. However, in birds, these receptors and adaptors differ between species, and between individuals, suggesting that they are under selection for diversification over time. Avian receptors and signalling pathways, like their mammalian counterparts, are targets for antagonism by a variety of viruses, intent on escape from innate immune responses.

Primary Tumor Suppression and Systemic Immune Activation of Macrophages through the Sting Pathway in Metastatic Skin Tumor

PURPOSE

Agonists of the stimulator of interferon genes (STING) play a key role in activating the STING pathway by promoting the production of cytokines. In this study, we investigated the antitumor effects and activation of the systemic immune response of treatment with DMXAA (5,6-dimethylxanthenone-4-acetic acid), a STING agonist, in EML4-ALK lung cancer and CT26 colon cancer.

MATERIALS AND METHODS

The abscopal effects of DMXAA in the treatment of metastatic skin nodules were assessed. EML4-ALK lung cancer and CT26 colon cancer models were used to evaluate these effects after DMXAA treatment. To evaluate the expression of macrophages and T cells, we sacrificed the tumor-bearing mice after DMXAA treatment and obtained the formalin-fixed paraffin-embedded (FFPE) tissue and tumor cells. Immunohistochemistry and flow cytometry were performed to analyze the expression of each FFPE and tumor cell.

RESULTS

We observed that highly infiltrating immune cells downstream of the STING pathway had increased levels of chemokines after DMXAA treatment. In addition, the levels of CD80 and CD86 in antigen-presenting cells were significantly increased after STING activation. Furthermore, innate immune activation altered the systemic T cell-mediated immune responses, induced proliferation of macrophages, inhibited tumor growth, and increased numbers of cytotoxic memory T cells. Tumor-specific lymphocytes also increased in number after treatment with DMXAA.

CONCLUSION

The abscopal effect of DMXAA treatment on the skin strongly reduced the spread of EML4-ALK lung cancer and CT26 colon cancer through the STING pathway and induced the presentation of antigens.

Recent Advances in Bunyavirus Reverse Genetics Research: Systems Development, Applications, and Future Perspectives

Bunyaviruses are members of the Bunyavirales order, which is the largest group of RNA viruses, comprising 12 families, including a large group of emerging and re-emerging viruses. These viruses can infect a wide variety of species worldwide, such as arthropods, protozoans, plants, animals, and humans, and pose substantial threats to the public. In view of the fact that a better understanding of the life cycle of a highly pathogenic virus is often a precondition for developing vaccines and antivirals, it is urgent to develop powerful tools to unravel the molecular basis of the pathogenesis. However, biosafety level −3 or even −4 containment laboratory is considered as a necessary condition for working with a number of bunyaviruses, which has hampered various studies. Reverse genetics systems, including minigenome (MG), infectious virus-like particle (iVLP), and infectious full-length clone (IFLC) systems, are capable of recapitulating some or all steps of the viral replication cycle; among these, the MG and iVLP systems have been very convenient and effective tools, allowing researchers to manipulate the genome segments of pathogenic viruses at lower biocontainment to investigate the viral genome transcription, replication, virus entry, and budding. The IFLC system is generally developed based on the MG or iVLP systems, which have facilitated the generation of recombinant infectious viruses. The MG, iVLP, and IFLC systems have been successfully developed for some important bunyaviruses and have been widely employed as powerful tools to investigate the viral replication cycle, virus–host interactions, virus pathogenesis, and virus evolutionary process. The majority of bunyaviruses is generally enveloped negative-strand RNA viruses with two to six genome segments, of which the viruses with bipartite and tripartite genome segments have mostly been characterized. This review aimed to summarize current knowledge on reverse genetic studies of representative bunyaviruses causing severe diseases in humans and animals, which will contribute to the better understanding of the bunyavirus replication cycle and provide some hints for developing designed antivirals.

B Cell Intrinsic STING Signaling Is Not Required for Autoreactive Germinal Center Participation

Germinal centers (GCs) are induced microanatomical structures wherein B cells undergo affinity maturation to improve the quality of the antibody response. Although GCs are crucial to appropriate humoral responses to infectious challenges and vaccines, many questions remain about the molecular signals driving B cell participation in GC responses. The cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) pathway is an important mediator of type I interferon and proinflammatory cytokine responses during infection and cellular stress. Recent studies have reported important roles for STING in B cell responses, including an impact on GC B cells and downstream antibody responses, which could have great consequences for vaccine design and understanding STING-associated interferonopathies. GCs are also involved in untoward reactions to autoantigens in a plethora of autoimmune disorders, and it is generally thought that these responses coopt the mechanisms used in foreign antigen-directed GCs. Here, we set out to investigate the importance of the cGAS-STING pathway in autoreactive B cell responses. In a direct competition scenario in a murine mixed bone marrow chimera model of autoreactive GCs, we find that B cell intrinsic deficiency of cGAS, STING, or the type I interferon receptor IFNAR, does not impair GC participation, whereas Toll-like receptor (TLR)-7 deficiency mediates a near-complete block. Our findings suggest that physiological B cell responses are strictly sustained by signals linked to BCR-mediated endocytosis. This wiring of B cell signals may enable appropriate antibody responses, while at the same time restricting aberrant antibody responses during infections and in autoimmune or autoinflammatory settings.

Crosstalk Between Autophagy and the cGAS-STING Signaling Pathway in Type I Interferon Production

Innate immunity is the front-line defense against infectious microorganisms, including viruses and bacteria. Type I interferons are pleiotropic cytokines that perform antiviral, antiproliferative, and immunomodulatory functions in cells. The cGAS–STING pathway, comprising the main DNA sensor cyclic guanosine monophosphate/adenosine monophosphate synthase (cGAS) and stimulator of IFN genes (STING), is a major pathway that mediates immune reactions and is involved in the strong induction of type I IFN production, which can fight against microbial infections. Autophagy is an evolutionarily conserved degradation process that is required to maintain host health and facilitate capture and elimination of invading pathogens by the immune system. Mounting evidence indicates that autophagy plays an important role in cGAS–STING signaling pathway-mediated type I IFN production. This review briefly summarizes the research progress on how autophagy regulates the cGAS–STING pathway, regulating type I IFN production, with a particular focus on the crosstalk between autophagy and cGAS–STING signaling during infection by pathogenic microorganisms.

Roles of Emerging RNA-Binding Activity of cGAS in Innate Antiviral Response

cGAS, a DNA sensor in mammalian cells, catalyzes the generation of 2'-3'-cyclic AMP-GMP (cGAMP) once activated by the binding of free DNA. cGAMP can bind to STING, activating downstream TBK1-IRF-3 signaling to initiate the expression of type I interferons. Although cGAS has been considered a traditional DNA-binding protein, several lines of evidence suggest that cGAS is a potential RNA-binding protein (RBP), which is mainly supported by its interactions with RNAs, RBP partners, RNA/cGAS-phase-separations as well as its structural similarity with the dsRNA recognition receptor 2'-5' oligoadenylate synthase. Moreover, two influential studies reported that the cGAS-like receptors (cGLRs) of fly Drosophila melanogaster sense RNA and control 3'-2'-cGAMP signaling. In this review, we summarize and discuss in depth recent studies that identified or implied cGAS as an RBP. We also comprehensively summarized current experimental methods and computational tools that can identify or predict RNAs that bind to cGAS. Based on these discussions, we appeal that the RNA-binding activity of cGAS cannot be ignored in the cGAS-mediated innate antiviral response. It will be important to identify RNAs that can bind and regulate the activity of cGAS in cells with or without virus infection. Our review provides novel insight into the regulation of cGAS by its RNA-binding activity and extends beyond its DNA-binding activity. Our review would be significant for understanding the precise modulation of cGAS activity, providing the foundation for the future development of drugs against cGAS-triggering autoimmune diseases such as Aicardi-Gourtières syndrome.

A defective viral genome strategy elicits broad protective immunity against respiratory viruses

RNA viruses generate defective viral genomes (DVGs) that can interfere with replication of the parental wild-type virus. To examine their therapeutic potential, we created a DVG by deleting the capsid-coding region of poliovirus. Strikingly, intraperitoneal or intranasal administration of this genome, which we termed eTIP1, elicits an antiviral response, inhibits replication, and protects mice from several RNA viruses, including enteroviruses, influenza, and SARS-CoV-2. While eTIP1 replication following intranasal administration is limited to the nasal cavity, its antiviral action extends non-cell-autonomously to the lungs. eTIP1 broad-spectrum antiviral effects are mediated by both local and distal type I interferon responses. Importantly, while a single eTIP1 dose protects animals from SARS-CoV-2 infection, it also stimulates production of SARS-CoV-2 neutralizing antibodies that afford long-lasting protection from SARS-CoV-2 reinfection. Thus, eTIP1 is a safe and effective broad-spectrum antiviral generating short- and long-term protection against SARS-CoV-2 and other respiratory infections in animal models.

Shiftless inhibits flavivirus replication in vitro and is neuroprotective in a mouse model of Zika virus pathogenesis

Flaviviruses such as Zika virus and West Nile virus have the potential to cause severe neuropathology if they invade the central nervous system. The type I interferon response is well characterized as contributing to control of flavivirus-induced neuropathogenesis. However, the interferon-stimulated gene (ISG) effectors that confer these neuroprotective effects are less well studied. Here, we used an ISG expression screen to identify Shiftless (SHFL, C19orf66) as a potent inhibitor of diverse positive-stranded RNA viruses, including multiple members of the Flaviviridae (Zika, West Nile, dengue, yellow fever, and hepatitis C viruses). In cultured cells, SHFL functions as a viral RNA-binding protein that inhibits viral replication at a step after primary translation of the incoming genome. The murine ortholog, Shfl, is expressed constitutively in multiple tissues, including the central nervous system. In a mouse model of Zika virus infection, Shfl ^-/- knockout mice exhibit reduced survival, exacerbated neuropathological outcomes, and increased viral replication in the brain and spinal cord. These studies demonstrate that Shfl is an important antiviral effector that contributes to host protection from Zika virus infection and virus-induced neuropathological disease.

Over-expression screen of interferon-stimulated genes identifies RARRES3 as a restrictor of Toxoplasma gondii infection

Toxoplasma gondii is an important human pathogen infecting an estimated one in three people worldwide. The cytokine interferon gamma (IFNγ) is induced during infection and is critical for restricting T. gondii growth in human cells. Growth restriction is presumed to be due to the induction of interferon-stimulated genes (ISGs) that are upregulated to protect the host from infection. Although there are hundreds of ISGs induced by IFNγ, their individual roles in restricting parasite growth in human cells remain somewhat elusive. To address this deficiency, we screened a library of 414 IFNγ induced ISGs to identify factors that impact T. gondii infection in human cells. In addition to IRF1, which likely acts through the induction of numerous downstream genes, we identified RARRES3 as a single factor that restricts T. gondii infection by inducing premature egress of the parasite in multiple human cell lines. Overall, while we successfully identified a novel IFNγ induced factor restricting T. gondii infection, the limited number of ISGs capable of restricting T. gondii infection when individually expressed suggests that IFNγ-mediated immunity to T. gondii infection is a complex, multifactorial process.

The MAVS Immune Recognition Pathway in Viral Infection and Sepsis

Significance: It is estimated that close to 50 million cases of sepsis result in over 11 million annual fatalities worldwide. The pathognomonic feature of sepsis is a dysregulated inflammatory response arising from viral, bacterial, or fungal infections. Immune recognition of pathogen-associated molecular patterns is a hallmark of the host immune defense to combat microbes and to prevent the progression to sepsis. Mitochondrial antiviral signaling protein (MAVS) is a ubiquitous adaptor protein located at the outer mitochondrial membrane, which is activated by the cytosolic pattern recognition receptors, retinoic acid-inducible gene I (RIG-I) and melanoma differentiation associated gene 5 (MDA5), following binding of viral RNA agonists. Recent Advances: Substantial progress has been made in deciphering the activation of the MAVS pathway with its interacting proteins, downstream signaling events (interferon [IFN] regulatory factors, nuclear factor kappa B), and context-dependent type I/III IFN response. Critical Issues: In the evolutionary race between pathogens and the host, viruses have developed immune evasion strategies for cleavage, degradation, or blockade of proteins in the MAVS pathway. For example, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) M protein and ORF9b protein antagonize MAVS signaling and a protective type I IFN response. Future Directions: The role of MAVS as a sensor for nonviral pathogens, host cell injury, and metabolic perturbations awaits better characterization in the future. New technical advances in multidimensional single-cell analysis and single-molecule methods will accelerate the rate of new discoveries. The ultimate goal is to manipulate MAVS activities in the form of immune-modulatory therapies to combat infections and sepsis. Antioxid. Redox Signal. 35, 1376-1392.

Single-cell analysis of arthritogenic alphavirus-infected human synovial fibroblasts links low abundance of viral RNA to induction of innate immunity and arthralgia-associated gene expression

Infection by (re-)emerging RNA arboviruses including Chikungunya virus (CHIKV) and Mayaro virus primarily cause acute febrile disease and transient polyarthralgia. However, in a significant subset of infected individuals, debilitating arthralgia persists for weeks over months up to years. The underlying immunopathogenesis of chronification of arthralgia upon primary RNA-viral infection remains unclear. Here, we analysed cell-intrinsic responses to ex vivo arthritogenic alphaviral infection of primary human synovial fibroblasts isolated from knee joints, one the most affected joint types during acute and chronic CHIKV disease. Synovial fibroblasts were susceptible and permissive to alphaviral infection. Base-line and exogenously added type I interferon (IFN) partially and potently restricted infection, respectively. RNA-seq revealed a CHIKV infection-induced transcriptional profile that comprised upregulation of expression of several hundred IFN-stimulated and arthralgia-mediating genes. Single-cell virus-inclusive RNA-seq uncovered a fine-tuned switch from induction to repression of cell-intrinsic immune responses depending on the abundance of viral RNA in an individual cell. Specifically, responses were most pronounced in cells displaying low-to-intermediate amounts of viral RNA and absence of virus-encoded, fluorescent reporter protein expression, arguing for efficient counteraction of innate immunity in cells expressing viral antagonists at sufficient quantities. In summary, cell-intrinsic sensing of viral RNA that potentially persists or replicates at low levels in synovial fibroblasts and other target cell types in vivo may contribute to the chronic arthralgia induced by alphaviral infections. Our findings might advance our understanding of the immunopathophysiology of long-term pathogenesis of RNA-viral infections.

JAK-STAT Pathway: A Novel Target to Tackle Viral Infections

Modulation of the antiviral innate immune response has been proposed as a putative cellular target for the development of novel pan-viral therapeutic strategies. The Janus kinase–signal transducer and activator of transcription (JAK-STAT) pathway is especially relevant due to its essential role in the regulation of local and systemic inflammation in response to viral infections, being, therefore, a putative therapeutic target. Here, we review the extraordinary diversity of strategies that viruses have evolved to interfere with JAK-STAT signaling, stressing the relevance of this pathway as a putative antiviral target. Moreover, due to the recent remarkable progress on the development of novel JAK inhibitors (JAKi), the current knowledge on its efficacy against distinct viral infections is also discussed. JAKi have a proven efficacy against a broad spectrum of disorders and exhibit safety profiles comparable to biologics, therefore representing good candidates for drug repurposing strategies, including viral infections.

The Evasion of Antiviral Innate Immunity by Chicken DNA Viruses

The innate immune system constitutes the first line of host defense. Viruses have evolved multiple mechanisms to escape host immune surveillance, which has been explored extensively for human DNA viruses. There is growing evidence showing the interaction between avian DNA viruses and the host innate immune system. In this review, we will survey the present knowledge of chicken DNA viruses, then describe the functions of DNA sensors in avian innate immunity, and finally discuss recent progresses in chicken DNA virus evasion from host innate immune responses.

DNA hypomethylation leads to cGAS-induced autoinflammation in the epidermis

DNA methylation is a fundamental epigenetic modification, important across biological processes. The maintenance methyltransferase DNMT1 is essential for lineage differentiation during development, but its functions in tissue homeostasis are incompletely understood. We show that epidermis-specific DNMT1 deletion severely disrupts epidermal structure and homeostasis, initiating a massive innate immune response and infiltration of immune cells. Mechanistically, DNA hypomethylation in keratinocytes triggered transposon derepression, mitotic defects, and formation of micronuclei. DNA release into the cytosol of DNMT1-deficient keratinocytes activated signaling through cGAS and STING, thus triggering inflammation. Our findings show that disruption of a key epigenetic mark directly impacts immune and tissue homeostasis, and potentially impacts our understanding of autoinflammatory diseases and cancer immunotherapy.

Neuroimmune Evasion of Zika Virus to Facilitate Viral Pathogenesis

Zika virus (ZIKV), which preferentially targets neural stem and progenitor cells (NSCs) especially in developing brain, is causally associated with fetal microcephaly, intrauterine retardation, and other congenital malformations in humans. However, there are, so far, no effective drugs and vaccines against ZIKV epidemics, warranting an enhanced understanding of ZIKV biology. Immune response is essential for neuronal cells to combat viral invasion. In turn, neurotropic ZIKV has developed a complex strategy of neuroimmune evasion to facilitate viral pathogenesis, especially developmental impairment in embryonic brain. Here, we review not only overall knowledge of ZIKV-related immune responses, but also current advances in our understanding of immune evasion in ZIKV infection. We also review several specific mechanisms underlying ZIKV protein-mediated immune evasion for viral pathogenesis.

DNA Repair Inhibition Leads to Active Export of Repetitive Sequences to the Cytoplasm Triggering an Inflammatory Response

Adult-onset neurodegenerative diseases are often accompanied by evidence of a chronic inflammation that includes activation of microglial cells and altered levels of brain cytokines. Aspects of this response are likely secondary reactions to neurodegeneration, but for many illnesses the inflammation may itself be an early and even causative disease event. In such cases, the inflammation is referred to as "sterile" as it occurs in the absence of an actual bacterial or viral pathogen. A potent trigger of sterile inflammation in CNS microglia has been shown to be the presence of DNA in the cytoplasm (cytoDNA) induced either by direct DNA damage or by inhibited DNA repair. We have shown that cytoDNA comes from the cell nucleus as a result of insufficient DNA damage repair. Using wild-type and Atm ^-/- mouse microglia, we extend these observations here by showing that its genomic origins are not random, but rather are heavily biased toward transcriptionally inactive, intergenic regions, in particular repetitive elements and AT-rich sequences. Once released from the genome, in both males and females, we show that cytoDNA is actively exported to the cytoplasm by a CRM1-dependent mechanism. In the cytoplasm, it is degraded either by a cytosolic exonuclease, Trex1, or an autophagy pathway that ends with degradation in the lysosome. Blocking the accumulation of cytoDNA prevents the emergence of the sterile inflammation reaction. These findings offer new insights into the emergence of sterile inflammation and offer novel approaches that may be of use in combatting a wide range of neurodegenerative conditions.SIGNIFICANCE STATEMENT Sterile inflammation describes a state where the defenses of the immune system are activated in the absence of a true pathogen. A potent trigger of this unorthodox response is the presence of DNA in the cytoplasm, which immune cells interpret as an invading virus or pathogen. We show that when DNA damage increases, fragments of the cell's own genome are actively exported to the cytoplasm where they are normally degraded. If this degradation is incomplete an immune reaction is triggered. Both age and stress increase DNA damage, and as age-related neurodegenerative diseases are frequently accompanied by a chronic low-level inflammation, strategies that reduce the induction of cytoplasmic DNA or speed its clearance become attractive therapeutic targets.

Human IFIT3 Protein Induces Interferon Signaling and Inhibits Adenovirus Immediate Early Gene Expression

Interferons (IFNs) are one of the hallmarks of host antiviral immunity. IFNs exert their antiviral activities through the induction of IFN-stimulated genes (ISGs) and antiviral proteins; however, the mechanism by which ISGs inhibit adenovirus (Ad) replication is not clearly understood. IFNs repress Ad immediate early gene expression and, consequently, all subsequent aspects of the viral life cycle. In this study, we found that IFN-induced protein with tetratricopeptide repeats 3, IFIT3 (ISG60), restricts Ad replication. IFIT3 repressed Ad E1A immediate early gene expression but did not alter Ad genome entry into the nucleus. Expression of IFIT3 led to phosphorylation of TBK1, IRF3, and STAT1; increased expression of IFNβ and ISGs; and required IFIT1 and IFIT2 partner proteins. During RNA virus infections, it is known that IFIT3 stimulates IFN production through mitochondrial antiviral signaling (MAVS)-mediated activation of TBK1 which synergizes activation of IRF3 and NF-κB. MAVS or TBK1 depletion in cells expressing IFIT3 blocked IFN signaling and reversed the Ad replication restriction. In addition, STING depletion phenocopied the effect suggesting that IFIT3 activates the STING pathway with cross talk to the MAVS pathway. This occurs independently of viral pathogen-associated molecular patterns (PAMPs). These results demonstrate that the expression of a single ISG, IFIT3, activates IFN signaling and establishes a cellular antiviral state independent of viral PAMPs. IMPORTANCE IFITs belong to a family of IFN-induced proteins that have broad antiviral functions, primarily studied with RNA viruses leaving a gap of knowledge on the effects of these proteins on DNA viruses. In this study we show that IFIT3, with its partner proteins IFIT1 and IFIT2, specifically restricts replication of human Ad, a DNA virus, by stimulating IFNβ production via the STING and MAVS pathways. This effect enhanced the IFN response and is independent of viral PAMPs. These results reveal a novel mechanism of activation of IFN signaling to enhance cellular antiviral responses.

Differential roles of interferons in innate responses to mucosal viral infections

Interferons (IFNs) are among the first vertebrate immune pathways activated upon viral infection and are crucial for control of viral replication and dissemination, especially at mucosal surfaces as key locations for host exposure to pathogens. Inhibition of viral establishment and spread at and from these mucosal sites is paramount for preventing severe disease, while concomitantly limiting putative detrimental effects of inflammation. Here, we compare the roles of type I, II, and III IFNs in regulating three archetypal viruses - norovirus, herpes simplex virus, and severe acute respiratory virus coronavirus 2 (SARS-CoV-2) - which infect distinct mammalian mucosal tissues. Emerging paradigms include highly specific roles for IFNs in limiting local versus systemic infection, synergistic activities, and a spectrum of protective versus detrimental effects of IFNs during the infection response.

A prenylated dsRNA sensor protects against severe COVID-19

Inherited genetic factors can influence the severity of COVID-19, but the molecular explanation underpinning a genetic association is often unclear. Intracellular antiviral defenses can inhibit the replication of viruses and reduce disease severity. To better understand the antiviral defenses relevant to COVID-19, we used interferon-stimulated gene (ISG) expression screening to reveal that 2′-5′-oligoadenylate synthetase 1 (OAS1), through ribonuclease L, potently inhibits severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). We show that a common splice-acceptor single-nucleotide polymorphism (Rs10774671) governs whether patients express prenylated OAS1 isoforms that are membrane-associated and sense-specific regions of SARS-CoV-2 RNAs or if they only express cytosolic, nonprenylated OAS1 that does not efficiently detect SARS-CoV-2. In hospitalized patients, expression of prenylated OAS1 was associated with protection from severe COVID-19, suggesting that this antiviral defense is a major component of a protective antiviral response.

Cytoplasmic DNA: sources, sensing, and role in aging and disease

Endogenous cytoplasmic DNA (cytoDNA) species are emerging as key mediators of inflammation in diverse physiological and pathological contexts. Although the role of endogenous cytoDNA in innate immune activation is well established, the cytoDNA species themselves are often poorly characterized and difficult to distinguish, and their mechanisms of formation, scope of function and contribution to disease are incompletely understood. Here, we summarize current knowledge in this rapidly progressing field with emphases on similarities and differences between distinct cytoDNAs, their underlying molecular mechanisms of formation and function, interactions between cytoDNA pathways, and therapeutic opportunities in the treatment of age-associated diseases.

ACE2 and Innate Immunity in the Regulation of SARS-CoV-2-Induced Acute Lung Injury: A Review

Despite the protracted battle against coronavirus acute respiratory infection (COVID-19) and the rapid evolution of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), no specific and effective drugs have to date been reported. Angiotensin-converting enzyme 2 (ACE2) is a zinc metalloproteinase and a critical modulator of the renin-angiotensin system (RAS). In addition, ACE2 has anti-inflammatory and antifibrosis functions. ACE has become widely known in the past decade as it has been identified as the primary receptor for SARS-CoV and SARS-CoV-2, being closely associated with their infection. SARS-CoV-2 primarily targets the lung, which induces a cytokine storm by infecting alveolar cells, resulting in tissue damage and eventually severe acute respiratory syndrome. In the lung, innate immunity acts as a critical line of defense against pathogens, including SARS-CoV-2. This review aims to summarize the regulation of ACE2, and lung host cells resist SARS-CoV-2 invasion by activating innate immunity response. Finally, we discuss ACE2 as a therapeutic target, providing reference and enlightenment for the clinical treatment of COVID-19.

Downregulation of mitochondrial biogenesis by virus infection triggers antiviral responses by cyclic GMP-AMP synthase

In general, in mammalian cells, cytosolic DNA viruses are sensed by cyclic GMP-AMP synthase (cGAS), and RNA viruses are recognized by retinoic acid-inducible gene I (RIG-I)-like receptors, triggering a series of downstream innate antiviral signaling steps in the host. We previously reported that measles virus (MeV), which possesses an RNA genome, induces rapid antiviral responses, followed by comprehensive downregulation of host gene expression in epithelial cells. Interestingly, gene ontology analysis indicated that genes encoding mitochondrial proteins are enriched among the list of downregulated genes. To evaluate mitochondrial stress after MeV infection, we first observed the mitochondrial morphology of infected cells and found that significantly elongated mitochondrial networks with a hyperfused phenotype were formed. In addition, an increased amount of mitochondrial DNA (mtDNA) in the cytosol was detected during progression of infection. Based on these results, we show that cytosolic mtDNA released from hyperfused mitochondria during MeV infection is captured by cGAS and causes consequent priming of the DNA sensing pathway in addition to canonical RNA sensing. We also ascertained the contribution of cGAS to the in vivo pathogenicity of MeV. In addition, we found that other viruses that induce downregulation of mitochondrial biogenesis as seen for MeV cause similar mitochondrial hyperfusion and cytosolic mtDNA-priming antiviral responses. These findings indicate that the mtDNA-activated cGAS pathway is critical for full innate control of certain viruses, including RNA viruses that cause mitochondrial stress.

Viruses exert their pathogenicity by targeting various cellular components in infected cells. In response, host cells have evolved strategies to sense intracellular pathogen-associated molecules, such as nucleic acids derived from infected virus, and trigger subsequent antiviral responses to counteract infection. Measles virus (MeV), the causative agent of human measles, is the most highly contagious virus, killing 300 children per day worldwide; thus MeV has been targeted for eradication by the World Health Organization. In the present study, we found that MeV causes downregulation of mitochondrial biogenesis accompanied with aberrant hyperfusion of mitochondria in the infected cells. Furthermore, we show that cytoplasmic release of mitochondrial DNA activates DNA sensor molecule, cGAS, in addition to the innate immune response induced by the viral component. Importantly, this phenomenon was also observed for viruses, both RNA and DNA, which target mitochondrial biogenesis. Our study provides new insights into the mitochondrial stress by virus infection and an important host defense system to suppress viral propagation.

Zika Virus Infection of Pregnant Ifnar1-/- Mice Triggers Strain-Specific Differences in Fetal Outcomes

Zika virus (ZIKV) is a flavivirus that causes a constellation of adverse fetal outcomes collectively termed congenital Zika syndrome (CZS). However, not all pregnancies exposed to ZIKV result in an infant with apparent defects. During the 2015 to 2016 American outbreak of ZIKV, CZS rates varied by geographic location. The underlying mechanisms responsible for this heterogeneity in outcomes have not been well defined. Therefore, we sought to characterize and compare the pathogenic potential of multiple Asian-/American-lineage ZIKV strains in an established Ifnar1-/- pregnant mouse model. Here, we show significant differences in the rate of fetal demise following maternal inoculation with ZIKV strains from Puerto Rico, Panama, Mexico, Brazil, and Cambodia. Rates of fetal demise broadly correlated with maternal viremia but were independent of fetus and placenta virus titer, indicating that additional underlying factors contribute to fetal outcome. Our results, in concert with those from other studies, suggest that subtle differences in ZIKV strains may have important phenotypic impacts. With ZIKV now endemic in the Americas, greater emphasis needs to be placed on elucidating and understanding the underlying mechanisms that contribute to fetal outcome. IMPORTANCE Zika virus (ZIKV) transmission has been reported in 87 countries and territories around the globe. ZIKV infection during pregnancy is associated with adverse fetal outcomes, including birth defects, microcephaly, neurological complications, and even spontaneous abortion. Rates of adverse fetal outcomes vary between regions, and not every pregnancy exposed to ZIKV results in birth defects. Not much is known about how or if the infecting ZIKV strain is linked to fetal outcomes. Our research provides evidence of phenotypic heterogeneity between Asian-/American-lineage ZIKV strains and provides insight into the underlying causes of adverse fetal outcomes. Understanding ZIKV strain-dependent pathogenic potential during pregnancy and elucidating underlying causes of diverse clinical sequelae observed during human infections is critical to understanding ZIKV on a global scale.

Constitutive TRIM22 Expression in the Respiratory Tract Confers a Pre-Existing Defence Against Influenza A Virus Infection

The induction of antiviral effector proteins as part of a homeostatically controlled innate immune response to infection plays a critical role in limiting the propagation and transmission of respiratory pathogens. However, the prolonged induction of this immune response can lead to lung hyperinflammation, tissue damage, and respiratory failure. We hypothesized that tissues exposed to the constant threat of infection may constitutively express higher levels of antiviral effector proteins to reduce the need to activate potentially harmful innate immune defences. By analysing transcriptomic data derived from a range of human tissues, we identify lung tissue to express constitutively higher levels of antiviral effector genes relative to that of other mucosal and non-mucosal tissues. By using primary cell lines and the airways of rhesus macaques, we show the interferon-stimulated antiviral effector protein TRIM22 (TRIpartite Motif 22) to be constitutively expressed in the lung independently of viral infection or innate immune stimulation. These findings contrast with previous reports that have shown TRIM22 expression in laboratory-adapted cell lines to require interferon stimulation. We demonstrate that constitutive levels of TRIM22 are sufficient to inhibit the onset of human and avian influenza A virus (IAV) infection by restricting the onset of viral transcription independently of interferon-mediated innate immune defences. Thus, we identify TRIM22 to confer a pre-existing (intrinsic) intracellular defence against IAV infection in cells derived from the respiratory tract. Our data highlight the importance of tissue-specific and cell-type dependent patterns of pre-existing immune gene expression in the intracellular restriction of IAV from the outset of infection.

cGAS-STING-mediated sensing pathways in DNA and RNA virus infections: crosstalk with other sensing pathways

Viruses cause a variety of diseases in humans and other organisms. The most important defense mechanism against viral infections is initiated when the viral genome is sensed by host proteins, and this results in interferon production and pro-inflammatory cytokine responses. The sensing of the viral genome or its replication intermediates within host cells is mediated by cytosolic proteins. For example, cGAS and IFI16 recognize non-self DNA, and RIG-I and MDA5 recognize non-self RNA. Once these sensors are activated, they trigger a cascade of reactions activating downstream molecules, which eventually results in the transcriptional activation of type I and III interferons, which play a critical role in suppressing viral propagation, either by directly limiting their replication or by inducing host cells to inhibit viral protein synthesis. The immune response against viruses relies solely upon sensing of viral genomes and their downstream signaling molecules. Although DNA and RNA viruses are sensed by distinct classes of receptor proteins, there is a possibility of overlap between the viral DNA and viral RNA sensing mechanisms. In this review, we focus on various host sensing molecules and discuss the associated signaling pathways that are activated in response to different viral infections. We further highlight the possibility of crosstalk between the cGAS-STING and the RIG-I-MAVS pathways to limit viral infections. This comprehensive review delineates the mechanisms by which different viruses evade host cellular responses to sustain within the host cells.

Interferon b drives intestinal regeneration after radiation

The cGAS-STING cytosolic DNA sensing pathway is critical for host defense. Here, we report that cGAS-STING–dependent type 1 interferon (IFN) response drives intestinal regeneration and animal recovery from radiation injury. STING deficiency has no effect on radiation-induced DNA damage or crypt apoptosis but abrogates epithelial IFN-β production, local inflammation, innate transcriptional response, and subsequent crypt regeneration. cGAS KO, IFNAR1 KO, or CCR2 KO also abrogates radiation-induced acute crypt inflammation and regeneration. Impaired intestinal regeneration and survival in STING-deficient mice are fully rescued by a single IFN-β treatment given 48 hours after irradiation but not by wild-type (WT) bone marrow. IFN-β treatment remarkably improves the survival of WT mice and Lgr5+ stem cell regeneration through elevated compensatory proliferation and more rapid DNA damage removal. Our findings support that inducible IFN-β production in the niche couples ISC injury and regeneration and its potential use to treat acute radiation injury.

Overview on Chikungunya Virus Infection: From Epidemiology to State-of-the-Art Experimental Models

Chikungunya virus (CHIKV) is currently one of the most relevant arboviruses to public health. It is a member of the Togaviridae family and alphavirus genus and causes an arthritogenic disease known as chikungunya fever (CHIKF). It is characterized by a multifaceted disease, which is distinguished from other arbovirus infections by the intense and debilitating arthralgia that can last for months or years in some individuals. Despite the great social and economic burden caused by CHIKV infection, there is no vaccine or specific antiviral drugs currently available. Recent outbreaks have shown a change in the severity profile of the disease in which atypical and severe manifestation lead to hundreds of deaths, reinforcing the necessity to understand the replication and pathogenesis processes. CHIKF is a complex disease resultant from the infection of a plethora of cell types. Although there are several in vivo models for studying CHIKV infection, none of them reproduces integrally the disease signature observed in humans, which is a challenge for vaccine and drug development. Therefore, understanding the potentials and limitations of the state-of-the-art experimental models is imperative to advance in the field. In this context, the present review outlines the present knowledge on CHIKV epidemiology, replication, pathogenesis, and immunity and also brings a critical perspective on the current in vitro and in vivo state-of-the-art experimental models of CHIKF.

Effects of Overexpression of the Egyptian Fruit Bat Innate Immune Genes on Filovirus Infections in the Host Cells

Bats constitute a large and diverse group of mammals with unique characteristics. One of these is the ability of bats to maintain various pathogens, particularly viruses, without evidence of disease. The innate immune system has been implicated as one of the important components involved in this process. However, in contrast to the human innate immune system, little data is available for bats. In the present study we generated 23 fusion constructs of innate immune genes of Egyptian fruit bat (Rousettus aegyptiacus) with mCherry as a fluorescent reporter. We evaluated the effects of overexpressing these genes on the replication of Marburg and Ebola viruses in the Egyptian fruit bat cell line R06EJ. Both viruses were substantially inhibited by overexpression of type I, II and III interferons, as well as by DDX58 (RIG-I), IFIH1, and IRF1. Our observations suggest that the broad antiviral activity of these genes reported previously in human cells is conserved in Egyptian fruit bats and these possess anti-filovirus activities that may contribute to the efficient virus clearance.

Proteomic landscape of Japanese encephalitis virus-infected fibroblasts

Advances in proteomics have enabled a comprehensive understanding of host-pathogen interactions. Here we have characterized Japanese encephalitis virus (JEV) infection-driven changes in the mouse embryonic fibroblast (MEF) proteome. Through tandem mass tagging (TMT)-based mass spectrometry, we describe changes in 7.85 % of the identified proteome due to JEV infection. Pathway enrichment analysis showed that proteins involved in innate immune sensing, interferon responses and inflammation were the major upregulated group, along with the immunoproteasome and poly ADP-ribosylation proteins. Functional validation of several upregulated anti-viral innate immune proteins, including an active cGAS-STING axis, was performed. Through siRNA depletion, we describe a crucial role of the DNA sensor cGAS in restricting JEV replication. Further, many interferon-stimulated genes (ISGs) were observed to be induced in infected cells. We also observed activation of TLR2 and inhibition of TLR2 signalling using TLR1/2 inhibitor CU-CPT22-blocked production of inflammatory cytokines IL6 and TNF-α from virus-infected N9 microglial cells. The major proteins that were downregulated by infection were involved in cell adhesion (collagens), transport (solute carrier and ATP-binding cassette transporters), sterol and lipid biosynthesis. Several collagens were found to be transcriptionally downregulated in infected MEFs and mouse brain. Collectively, our data provide a bird's-eye view into how fibroblast protein composition is rewired following JEV infection.

Cooperative DNA binding mediated by KicGAS/ORF52 oligomerization allows inhibition of DNA-induced phase separation and activation of cGAS

Cyclic GMP-AMP synthase (cGAS) is a key DNA sensor that detects aberrant cytosolic DNA arising from pathogen invasions or genotoxic stresses. Upon binding to DNA, cGAS is activated and catalyzes the synthesis of cyclic GMP-AMP (cGAMP), which induces potent antimicrobial and antitumor responses. Kaposi sarcoma-associated herpesvirus (KSHV) is a human DNA tumor virus that causes Kaposi sarcoma and several other malignancies. We previously reported that KSHV inhibitor of cGAS (KicGAS) encoded by ORF52, inhibits cGAS enzymatic activity, but the underlying mechanisms remained unclear. To define the inhibitory mechanisms, here we performed in-depth biochemical and functional characterizations of KicGAS, and mapped its functional domains. We found KicGAS self-oligomerizes and binds to double stranded DNA cooperatively. This self-oligomerization is essential for its DNA binding and cGAS inhibition. Interestingly, KicGAS forms liquid droplets upon binding to DNA, which requires collective multivalent interactions with DNA mediated by both structured and disordered domains coordinated through the self-oligomerization of KicGAS. We also observed that KicGAS inhibits the DNA-induced phase separation and activation of cGAS. Our findings reveal a novel mechanism by which DNA viruses target the host protein phase separation for suppression of the host sensing of viral nucleic acids.