STING is required for host defense against neuropathological West Nile virus infection

The STING Signaling: A Novel Target for Central Nervous System Diseases

The canonical cyclic GMP-AMP (cGAMP) synthase (cGAS)-Stimulator of Interferon Genes (STING) pathway has been widely recognized as a crucial mediator of inflammation in many diseases, including tumors, infections, and tissue damage. STING signaling can also be activated in a cGAS- or cGAMP-independent manner, although the specific mechanisms remain unclear. In-depth studies on the structural and molecular biology of the STING pathway have led to the development of therapeutic strategies involving STING modulators and their targeted delivery. These strategies may effectively penetrate the blood-brain barrier (BBB) and target STING signaling in multiple central nervous system (CNS) diseases in humans. In this review, we outline both canonical and non-canonical pathways of STING activation and describe the general mechanisms and associations between STING activity and CNS diseases. Finally, we discuss the prospects for the targeted delivery and clinical application of STING agonists and inhibitors, highlighting the STING signaling pathway as a novel therapeutic target in CNS diseases.

Progress Update on STING Agonists as Vaccine Adjuvants

Low antigen immunogenicity poses a significant challenge in vaccine development, often leading to inadequate immune responses and reduced vaccine efficacy. Therefore, the discovery of potent immune-enhancing adjuvants is crucial. STING (stimulator of interferon genes) agonists are a promising class of adjuvants which have been identified in various immune cells and are activated in response to DNA fragments, triggering a broad range of type-I interferon-dependent immune responses. Integrating STING agonists with vaccine components is an ideal strategy to bolster vaccine-induced immunity to infections and cancer cells. Several STING agonists are currently under investigation in preclinical studies and clinical trials; however, some have shown limited efficacy, while others exhibit off-target effects. To ensure safety, they are typically delivered with carriers that exhibit high biocompatibility and insolubility. In this review, we present the latest research on natural and synthetic STING agonists that have been effectively used in vaccine development, and summarize their application in adjuvant preventive and therapeutic vaccines. Additionally, we discuss the safety of STING agonists as vaccine adjuvants by reviewing potential delivery strategies. Overall, incorporating STING agonists into vaccine formulations represents a significant advancement in vaccine research with the potential to significantly enhance immune responses and improve vaccine efficacy. However, ongoing research is still required to identify the most effective and safe delivery strategies for STING agonists, as well as to evaluate their long-term safety and efficacy in clinical trials.

Subcellular determinants of orthoflavivirus protease activity

Orthoflaviviruses are small, enveloped, positive-sense RNA viruses that cause over 500 million infections globally each year for which there are no antiviral treatments. The viral protease is an attractive target for therapeutics due to its critical functions throughout infection. Many studies have reported on the structure, function, and importance of orthoflavivirus proteases; However, the molecular determinants for cleavage of intracellular substrates by orthoflavivirus proteases and how these factors affect viral fitness are unknown. In this study, we used our fluorescent, protease-activity reporter system to investigate the subcellular determinants involved in orthoflavivirus protease cleavage. By modifying our reporter platform, we identified endoplasmic reticulum (ER) subdomain localization and membrane proximity of the substrate cut site as two previously uncharacterized molecular determinants for cleavage. We also altered the amino acid composition of the reporter cut site to introduce sequences present at the cytoplasmic junctions within orthoflavivirus polyproteins and found that each protease processed the sequence located at the junction between NS4A and the 2K peptide least efficiently. Live-cell imaging revealed that cleavage of the NS4A|2K sequence is significantly delayed compared to the capsid cleavage sequence. We further determined that introducing a more efficient cleavage sequence into the NS4A|2K junctions of orthoflavivirus infectious clones abolished virus recovery. Overall, this study identifies ER subdomain localization and membrane proximity of the cut site as molecular determinants for cleavage by orthoflavivirus proteases and provides insight into the role that sequence specificity plays in the coordinated processing of the viral polyprotein and establishing productive infections.

Importance

Orthoflaviviruses are the most prevalent and dangerous arthropod-borne viruses (arboviruses) leading to over 500 million global infections annually. Orthoflavivirus infection can cause severe pathologies, including hemorrhagic conditions and neurological disease, that lead to hundreds of thousands of deaths each year. The viral protease complex, responsible for processing the viral polyprotein into its functional subunits, is an attractive target for antiviral therapeutic development. Despite extensive research efforts on these viral protein complexes, all protease inhibitor candidates have fallen short of clinical efficacy, highlighting a considerable gap in knowledge of the viral protease's complex intracellular activity. The significance of our research is in characterizing the subcellular determinants associated with orthoflavivirus protease cleavage efficiency and how these factors can influence viral fitness. These findings contribute to closing this gap in knowledge of the mechanisms of orthoflavivirus proteases which can ultimately lead to the successful development of targeted antivirals.

A central role of stimulator of interferon genes' adaptor protein in defensive immune response

Cytotoxic DNAs, methylation, histones and histones binding proteins are speculated to induce DNA sensors. Under stressed condition, the antigenic patterns, PAMPs and DAMPs, trigger the hyperactive innate response through DNA, DNA-RNA hybrids, oligonucleotides, histones and mtDNA to initiate cGAMP-STING-IFN I cascade. HSV -1&2, HIV, Varicella- Zoster virus, Polyomavirus, Cytomegalovirus, and KSHV negatively regulate the STING-MAVS-TBK-1/1KKE pathway. Implications in STING-PKR-ER regulation often run into causing senescence and organ fibrosis. Post-translational modifications such as, phosphorylation, ubiquitination, SUMOylation, hydrolysis etc. downstream the processing of cGAS-STING that determine the fate of disease prognosis. Self-DNA under normal circumstances is removed through DNase III action; however, its deficiency is the great cause of RA diseases. Regular STING activation in chronic diseases could lead to exacerbate the neurodegenerative disorders due to constant mtDNA leakage. 2' 3' cGAMP or CDN or its associates are being explored as STING agonist therapeutics to treat solid/metastatic tumors to help infiltrate the immune cells, cytokines and chemokines to regulate the protective response. Liposomes, polymer nanoparticles, and cell-derived nanoparticles are also meant to increase the drug efficiency and stability for desired immune response to enhance the IFN I production. This review highlights the implications of cGAMP-STING- IFN I cascade and related pathways involved in the disease prognosis, therapeutics and considering the gaps on different aspects to utilize its greater potential in disease control.

Recent insights on pattern recognition receptors and the interplay of innate immune responses against West Nile Virus infection

The recent outbreaks of neurotropic West Nile Virus (WNV) in humans are of grave public health concern, requiring a thorough understanding of the host immune response to develop effective therapeutic interventions. Innate immunity contributes to the primary immune response against WNV infection aimed at controlling and eliminating the virus from the body. As soon as WNV infects the body, pattern recognition receptors (PRRs) recognize viral pathogen-associated molecular patterns, particularly viral RNA, and initiate innate immune responses. This review explores the diverse PRRs in sensing WNV infection and orchestrating immune defenses. Specifically, this paper reviews the role of PRRs in WNV infection, encompassing both findings from mouse models and current clinical studies. Activation of PRRs triggers signaling pathways that induce the expression of antiviral proteins to inhibit viral replication. Understanding the intricacies of the immune response is crucial for developing effective vaccines and therapeutic interventions against WNV infection.

Unraveling the cGAS/STING signaling mechanism: impact on glycerolipid metabolism and diseases

The cyclic GMP-AMP synthase (cGAS) and its downstream effector, the stimulator of interferon genes (STING), are crucial components of the innate immune response, traditionally recognized for their role in detecting cytosolic DNA from pathogens and damaged host cells. However, recent research indicates that the cGAS-STING pathway also significantly impacts metabolic processes, particularly glycerolipid metabolism. Glycerolipids are essential for energy storage and cellular membrane integrity, and their dysregulation is linked to metabolic disorders such as obesity, insulin resistance, and non-alcoholic fatty liver disease (NAFLD). Both cGAS and STING are expressed in various metabolic tissues, suggesting a potential role in lipid homeostasis. Chronic activation of the cGAS-STING pathway may promote inflammatory states that exacerbate insulin resistance and lipid accumulation, forming a feedback loop of metabolic dysfunction. This review explores the emerging relationship between cGAS/STING signaling and glycerolipid metabolism, discussing the mechanisms through which this pathway influences lipid regulation and the potential for therapeutic interventions. By integrating insights from immunology and metabolism, we aim to provide a comprehensive understanding of how the cGAS-STING axis may serve as a novel target for addressing metabolic disorders and enhancing metabolic health outcomes.

The Role of cGAS-STING Signalling in Metabolic Diseases: from Signalling Networks to Targeted Intervention

The cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) is a crucial innate defence mechanism against viral infection in the innate immune system, as it principally induces the production of type I interferons. Immune responses and metabolic control are inextricably linked, and chronic low-grade inflammation promotes the development of metabolic diseases. The cGAS-STING pathway activated by double-stranded DNA (dsDNA), cyclic dinucleotides (CDNs), endoplasmic reticulum stress (ER stress), mitochondrial stress, and energy imbalance in metabolic cells and immune cells triggers proinflammatory responses and metabolic disorders. Abnormal overactivation of the pathway is closely associated with metabolic diseases such as obesity, nonalcoholic fatty liver disease (NAFLD), insulin resistance and cardiovascular diseases (CVDs). The interaction of cGAS-STING with other pathways, such as the nuclear factor-kappa B (NF-κB), Jun N-terminal kinase (JNK), AMP-activated protein kinase (AMPK), mammalian target of rapamycin (mTOR), autophagy, pyroptosis and insulin signalling pathways, is considered an important mechanism by which cGAS-STING regulates inflammation and metabolism. This review focuses on the link between immune responses related to the cGAS-STING pathway and metabolic diseases and cGAS-STING interaction with other pathways for mediating signal input and affecting output. Moreover, potential inhibitors of the cGAS-STING pathway and therapeutic prospects against metabolic diseases are discussed. This review provides a comprehensive perspective on the involvement of STING in immune-related metabolic diseases.

Recent trends in STING modulators: Structures, mechanisms, and therapeutic potential

The cyclic GMP-AMP synthase stimulator (cGAS)-stimulator of interferon gene (STING) signaling pathway has an integral role in the host immune response through DNA sensing followed by inducing a robust innate immune defense program. STING has become a promising therapeutic target associated with multiple diseases, including various inflammatory diseases, cancer, and infectious diseases, among others. Thus, modulators of STING are regarded as emerging therapeutic agents. Recent progress has been made in STING research, including recently identified STING-mediated regulatory pathways, the development of a new STING modulator, and the new association of STING with disease. In this review, we focus on recent trends in the development of STING modulators, including structures, mechanisms, and clinical application.

The Many Ways to Deal with STING

The stimulator of interferon genes (STING) is an adaptor protein involved in the activation of IFN-β and many other genes associated with the immune response activation in vertebrates. STING induction has gained attention from different angles such as the potential to trigger an early immune response against different signs of infection and cell damage, or to be used as an adjuvant in cancer immune treatments. Pharmacological control of aberrant STING activation can be used to mitigate the pathology of some autoimmune diseases. The STING structure has a well-defined ligand binding site that can harbor natural ligands such as specific purine cyclic di-nucleotides (CDN). In addition to a canonical stimulation by CDNs, other non-canonical stimuli have also been described, whose exact mechanism has not been well defined. Understanding the molecular insights underlying the activation of STING is important to realize the different angles that need to be considered when designing new STING-binding molecules as therapeutic drugs since STING acts as a versatile platform for immune modulators. This review analyzes the different determinants of STING regulation from the structural, molecular, and cell biology points of view.

The role of STING signaling in central nervous system infection and neuroinflammatory disease

The cyclic guanosine monophosphate-adenosine monophosphate (GMP-AMP) synthase-Stimulator of Interferon Genes (cGAS-STING) pathway is a critical innate immune mechanism for detecting the presence of double-stranded DNA (dsDNA) and prompting a robust immune response. Canonical cGAS-STING activation occurs when cGAS, a predominantly cytosolic pattern recognition receptor, binds microbial DNA to promote STING activation. Upon STING activation, transcription factors enter the nucleus to cause the production of Type I interferons, inflammatory cytokines whose primary function is to prime the host for viral infection by producing a number of antiviral interferon-stimulated genes. While the pathway was originally described in viral infection, more recent studies have implicated cGAS-STING signaling in a number of different contexts, including autoimmune disease, cancer, injury, and neuroinflammatory disease. This review focuses on how our understanding of the cGAS-STING pathway has evolved over time with an emphasis on the role of STING-mediated neuroinflammation and infection in the nervous system. We discuss recent findings on how STING signaling contributes to the pathology of pain, traumatic brain injury, and stroke, as well as how mitochondrial DNA may promote STING activation in common neurodegenerative diseases. We conclude by commenting on the current knowledge gaps that should be filled before STING can be an effective therapeutic target in neuroinflammatory disease. This article is categorized under: Neurological Diseases > Molecular and Cellular Physiology Infectious Diseases > Molecular and Cellular Physiology Immune System Diseases > Molecular and Cellular Physiology.

Interplay between RNA viruses and cGAS/STING axis in innate immunity

While the function of cGAS/STING signalling axis in the innate immune response to DNA viruses is well deciphered, increasing evidence demonstrates its significant contribution in the control of RNA virus infections. After the first evidence of cGAS/STING antagonism by flaviviruses, STING activation has been detected following infection by various enveloped RNA viruses. It has been discovered that numerous viral families have implemented advanced strategies to antagonize STING pathway through their evolutionary path. This review summarizes the characterized cGAS/STING escape strategies to date, together with the proposed mechanisms of STING signalling activation perpetrated by RNA viruses and discusses possible therapeutic approaches. Further studies regarding the interaction between RNA viruses and cGAS/STING-mediated immunity could lead to major discoveries important for the understanding of immunopathogenesis and for the treatment of RNA viral infections.

Innate sensing of picornavirus infection involves cGAS-STING-mediated antiviral responses triggered by mitochondrial DNA release

Cyclic GMP-AMP synthase (cGAS) plays a key role in the innate immune responses to both DNA and RNA virus infection. Here, we found that enterovirus 71 (EV-A71), Seneca Valley virus (SVV), and foot-and-mouth disease virus (FMDV) infection triggered mitochondria damage and mitochondrial DNA (mtDNA) release in vitro and vivo. These responses were mediated by picornavirus 2B proteins which induced mtDNA release during viral replication. SVV infection caused the opening of mitochondrial permeability transition pore (mPTP) and led to voltage-dependent anion channel 1 (VDAC1)- and BCL2 antagonist/killer 1 (Bak) and Bak/BCL2-associated X (Bax)-dependent mtDNA leakage into the cytoplasm, while EV-A71 and FMDV infection induced mPTP opening and resulted in VDAC1-dependent mtDNA release. The released mtDNA bound to cGAS and activated cGAS-mediated antiviral immune response. cGAS was essential for inhibiting EV-A71, SVV, and FMDV replication by regulation of IFN-β production. cGAS deficiency contributed to higher mortality of EV-A71- or FMDV-infected mice. In addition, we found that SVV 2C protein was responsible for decreasing cGAS expression through the autophagy pathway. The 9th and 153rd amino acid sites in 2C were critical for induction of cGAS degradation. Furthermore, we also show that EV-A71, CA16, and EMCV 2C antagonize the cGAS-stimulator of interferon genes (STING) pathway through interaction with STING, and highly conserved amino acids Y155 and S156 were critical for this inhibitory effect. In conclusion, these data reveal novel mechanisms of picornaviruses to block the antiviral effect mediated by the cGAS-STING signaling pathway, which will provide insights for developing antiviral strategies against picornaviruses.

UBXN3B Controls Immunopathogenesis of Arthritogenic Alphaviruses by Maintaining Hematopoietic Homeostasis

Ubiquitin regulatory X domain-containing proteins (UBXN) might be involved in diverse cellular processes. However, their in vivo physiological functions remain largely elusive. We recently showed that UBXN3B positively regulated stimulator-of-interferon-genes (STING)-mediated innate immune responses to DNA viruses. Herein, we reported the essential role of UBXN3B in the control of infection and immunopathogenesis of two arthritogenic RNA viruses, Chikungunya (CHIKV) and O'nyong'nyong (ONNV) viruses. Ubxn3b deficient (Ubxn3b-/-) mice presented higher viral loads, more severe foot swelling and immune infiltrates, and slower clearance of viruses and resolution of inflammation than the Ubxn3b+/+ littermates. While the serum cytokine levels were intact, the virus-specific immunoglobulin G and neutralizing antibody levels were lower in the Ubxn3b-/- mice. The Ubxn3b-/- mice had more neutrophils and macrophages, but much fewer B cells in the ipsilateral feet. Of note, this immune dysregulation was also observed in the spleens and blood of uninfected Ubxn3b-/- mice. UBXN3B restricted CHIKV replication in a cell-intrinsic manner but independent of type I IFN signaling. These results demonstrated a dual role of UBXN3B in the maintenance of immune homeostasis and control of RNA virus replication. IMPORTANCE The human genome encodes 13 ubiquitin regulatory X (UBX) domain-containing proteins (UBXN) that might participate in diverse cellular processes. However, their in vivo physiological functions remain largely elusive. Herein, we reported an essential role of UBXN3B in the control of infection and immunopathogenesis of arthritogenic alphaviruses, including Chikungunya virus (CHIKV), which causes acute and chronic crippling arthralgia, long-term neurological disorders, and poses a significant public health problem in the tropical and subtropical regions worldwide. However, there are no approved vaccines or specific antiviral drugs. This was partly due to a poor understanding of the protective and detrimental immune responses elicited by CHIKV. We showed that UBXN3B was critical for the control of CHIKV replication in a cell-intrinsic manner in the acute phase and persistent immunopathogenesis in the post-viremic stage. Mechanistically, UBXN3B was essential for the maintenance of hematopoietic homeostasis during viral infection and in steady-state.

Post-Translational Modifications of cGAS-STING: A Critical Switch for Immune Regulation

Innate immune mechanisms initiate immune responses via pattern-recognition receptors (PRRs). Cyclic GMP-AMP synthase (cGAS), a member of the PRRs, senses diverse pathogenic or endogenous DNA and activates innate immune signaling pathways, including the expression of stimulator of interferon genes (STING), type I interferon, and other inflammatory cytokines, which, in turn, instructs the adaptive immune response development. This groundbreaking discovery has rapidly advanced research on host defense, cancer biology, and autoimmune disorders. Since cGAS/STING has enormous potential in eliciting an innate immune response, understanding its functional regulation is critical. As the most widespread and efficient regulatory mode of the cGAS-STING pathway, post-translational modifications (PTMs), such as the covalent linkage of functional groups to amino acid chains, are generally considered a regulatory mechanism for protein destruction or renewal. In this review, we discuss cGAS-STING signaling transduction and its mechanism in related diseases and focus on the current different regulatory modalities of PTMs in the control of the cGAS-STING-triggered innate immune and inflammatory responses.

cGAS- Stimulator of Interferon Genes Signaling in Central Nervous System Disorders

Cytosolic nucleic acid sensors contribute to the initiation of innate immune responses by playing a critical role in the detection of pathogens and endogenous nucleic acids. The cytosolic DNA sensor cyclic-GMP-AMP synthase (cGAS) and its downstream effector, stimulator of interferon genes (STING), mediate innate immune signaling by promoting the release of type I interferons (IFNs) and other inflammatory cytokines. These biomolecules are suggested to play critical roles in host defense, senescence, and tumor immunity. Recent studies have demonstrated that cGAS-STING signaling is strongly implicated in the pathogenesis of central nervous system (CNS) diseases which are underscored by neuroinflammatory-driven disease progression. Understanding and regulating the interactions between cGAS-STING signaling and the nervous system may thus provide an effective approach to prevent or delay late-onset CNS disorders. Here, we present a review of recent advances in the literature on cGAS-STING signaling and provide a comprehensive overview of the modulatory patterns of the cGAS-STING pathway in CNS disorders.

cGAS-STING-mediated IFN-I response in host defense and neuro-inflammatory diseases

The presence of foreign or misplaced nucleic acids is a danger signal that triggers innate immune responses through activating cytosolic DNA sensor cyclic GMP-AMP synthase (cGAS) and binding to its downstream signaling effector stimulator of interferon genes (STING). Then the cGAS-STING pathway activation links nucleic acid sensing to immune responses and pathogenic entities clearance. However, overactivation of this signaling pathway leads to fatal immune disorders and contributes to the progression of many human inflammatory diseases. Therefore, optimal activation of this pathway is crucial for the elimination of invading pathogens and the maintenance of immune homeostasis. In this review, we will summarize its fundamental roles in initiating host defense against invading pathogens and discuss its pathogenic roles in multiple neuro-inflammatory diseases, such as Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), amyotrophic lateral sclerosis (ALS), multiple sclerosis (MS) and other neurodegenerative diseases.

Traumatic Brain Injury Induces cGAS Activation and Type I Interferon Signaling in Aged Mice

Aging adversely affects inflammatory processes in the brain, which has important implications in the progression of neurodegenerative disease. Following traumatic brain injury (TBI), aged animals exhibit worsened neurological function and exacerbated microglial-associated neuroinflammation. Type I Interferons (IFN-I) contribute to the development of TBI neuropathology. Further, the Cyclic GMP-AMP Synthase (cGAS) and Stimulator of Interferon Genes (STING) pathway, a key inducer of IFN-I responses, has been implicated in neuroinflammatory activity in several age-related neurodegenerative diseases. Here, we set out to investigate the effects of TBI on cGAS/STING activation, IFN-I signaling and neuroinflammation in young and aged C57Bl/6 male mice. Using a controlled cortical impact model, we evaluated transcriptomic changes in the injured cortex at 24 hours post-injury, and confirmed activation of key neuroinflammatory pathways in biochemical studies. TBI induced changes were highly enriched for transcripts that were involved in inflammatory responses to stress and host defense. Deeper analysis revealed that TBI increased expression of IFN-I related genes (e.g. Ifnb1, Irf7, Ifi204, Isg15) and IFN-I signaling in the injured cortex of aged compared to young mice. There was also a significant age-related increase in the activation of the DNA-recognition pathway, cGAS, which is a key mechanism to propagate IFN-I responses. Finally, enhanced IFN-I signaling in the aged TBI brain was confirmed by increased phosphorylation of STAT1, an important IFN-I effector molecule. This age-related activation of cGAS and IFN-I signaling may prove to be a mechanistic link between microglial-associated neuroinflammation and neurodegeneration in the aged TBI brain.

STING1 is essential for an RNA-virus triggered autophagy

While the functions of STING1 (stimulator of interferon response cGAMP interactor 1) during DNA virus infection had been well documented, the roles STING1 plays during RNA viruses infection is obscure. Infection with foot-and-mouth disease virus (FMDV), a well-known picornavirus, induces endoplasmic reticulum (ER) stress response and autophagy. Here, we found that the FMDV-induced integrated stress response originates from the cellular pattern recognition receptor DDX58/RIG-I (DExD/H-box helicase 58). DDX58 transmits signals to the ER-anchored adaptor protein STING1, which specifically activates the EIF2AK3/PERK (eukaryotic translation initiation factor 2A)-dependent integrated stress response and finally leads to reticulophagy and degradation of STING1 itself. Knockdown/knockout of STING1 or EIF2AK3 suppresses FMDV genome replication and viral protein expression. Reticulophagy induction by STING1 does not require its translocation to the Golgi or IFN response activation. However, STING1 polymerization is necessary for the FMDV-induced integrated stress response and reticulophagy. Our work illustrated the signaling cascades that mediate the cellular stress response to FMDV infection and indicated that induction of autophagy in response to both DNA and RNA virus infection may be an evolutionarily conserved function of STING1. Abbreviations: ATF6: activating transcription factor 6; CGAS: cyclic GMP-AMP synthase; DDX58/RIG-I: DExD/H-box helicase 58; EIF2A/eIF2α: eukaryotic translation initiation factor 2A; EIF2AK3/PERK: eukaryotic translation initiation factor 2 alpha kinase 2; ER: endoplasmic reticulum; ERN1/IRE1: endoplasmic reticulum to nucleus signaling 1; FMD: foot-and-mouth disease; FMDV: foot-and-mouth disease virus; IFIH1/MDA5: interferon induced with helicase C domain 1; MAP1LC3B/LC3B: microtubule associated protein 1 light chain 3 beta; MAVS: mitochondrial antiviral signaling protein; MOI: multiplicity of infection; RETREG1/FAM134B: reticulophagy regulator 1; STING1: stimulator of interferon response cGAMP interactor 1; TCID50: 50% tissue culture infectious dose; XBP1: X-box binding protein 1.

Decreased antiviral immune response within the central nervous system of aged mice is associated with increased lethality of West Nile virus encephalitis

West Nile virus (WNV) is an emerging pathogen that causes disease syndromes ranging from a mild flu‐like illness to encephalitis. While the incidence of WNV infection is fairly uniform across age groups, the risk of lethal encephalitis increases with advanced age. Prior studies have demonstrated age‐related, functional immune deficits that limit systemic antiviral immunity and increase mortality; however, the effect of age on antiviral immune responses specifically within the central nervous system (CNS) is unknown. Here, we show that aged mice exhibit increased peripheral organ and CNS tissue viral burden, the latter of which is associated with alterations in activation of both myeloid and lymphoid cells compared with similarly infected younger animals. Aged mice exhibit lower MHCII expression by microglia, and higher levels of PD1 and lower levels of IFNγ expression by WNV‐specific CD8⁺ T cells in the CNS and CD8⁺CD45⁺ cells. These data indicate that the aged CNS exhibits limited local reactivation of T cells during viral encephalitis, which may lead to reduced virologic control at this site.

Unraveling the Link Between Mitochondrial Dynamics and Neuroinflammation

Neuroinflammatory and neurodegenerative diseases are a major public health problem worldwide, especially with the increase of life-expectancy observed during the last decades. For many of these diseases, we still lack a full understanding of their etiology and pathophysiology. Nonetheless their association with mitochondrial dysfunction highlights this organelle as an important player during CNS homeostasis and disease. Markers of Parkinson (PD) and Alzheimer (AD) diseases are able to induce innate immune pathways induced by alterations in mitochondrial Ca2+ homeostasis leading to neuroinflammation. Additionally, exacerbated type I IFN responses triggered by mitochondrial DNA (mtDNA), failures in mitophagy, ER-mitochondria communication and mtROS production promote neurodegeneration. On the other hand, regulation of mitochondrial dynamics is essential for CNS health maintenance and leading to the induction of IL-10 and reduction of TNF-α secretion, increased cell viability and diminished cell injury in addition to reduced oxidative stress. Thus, although previously solely seen as power suppliers to organelles and molecular processes, it is now well established that mitochondria have many other important roles, including during immune responses. Here, we discuss the importance of these mitochondrial dynamics during neuroinflammation, and how they correlate either with the amelioration or worsening of CNS disease.

The Complexity of the cGAS-STING Pathway in CNS Pathologies

Neuroinflammation driven by type-I interferons in the CNS is well established to exacerbate the progression of many CNS pathologies both acute and chronic. The role of adaptor protein Stimulator of Interferon Genes (STING) is increasingly appreciated to instigate type-I IFN-mediated neuroinflammation. As an upstream regulator of type-I IFNs, STING modulation presents a novel therapeutic opportunity to mediate inflammation in the CNS. This review will detail the current knowledge of protective and detrimental STING activity in acute and chronic CNS pathologies and the current therapeutic avenues being explored.

SARS-CoV-2 Immuno-Pathogenesis and Potential for Diverse Vaccines and Therapies: Opportunities and Challenges

Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) is a novel coronavirus that emerged from Wuhan, China in late 2019 causing coronavirus disease-19 (COVID-19). SARS-CoV-2 infection begins by attaching to angiotensin-converting enzyme 2 receptor (ACE2) via the spike glycoprotein, followed by cleavage by TMPRSS2, revealing the viral fusion domain. Other presumptive receptors for SARS-CoV-2 attachment include CD147, neuropilin-1 (NRP1), and Myeloid C-lectin like receptor (CLR), each of which might play a role in the systemic viral spread. The pathology of SARS-CoV-2 infection ranges from asymptomatic to severe acute respiratory distress syndrome, often displaying a cytokine storm syndrome, which can be life-threatening. Despite progress made, the detailed mechanisms underlying SARS-CoV-2 interaction with the host immune system remain unclear and are an area of very active research. The process's key players include viral non-structural proteins and open reading frame products, which have been implicated in immune antagonism. The dysregulation of the innate immune system results in reduced adaptive immune responses characterized by rapidly diminishing antibody titers. Several treatment options for COVID-19 are emerging, with immunotherapies, peptide therapies, and nucleic acid vaccines showing promise. This review discusses the advances in the immunopathology of SARS-CoV-2, vaccines and therapies under investigation to counter the effects of this virus, as well as viral variants.

Innate Immune Sensing of Viruses and Its Consequences for the Central Nervous System

Viral infections remain a global public health concern and cause a severe societal and economic burden. At the organismal level, the innate immune system is essential for the detection of viruses and constitutes the first line of defense. Viral components are sensed by host pattern recognition receptors (PRRs). PRRs can be further classified based on their localization into Toll-like receptors (TLRs), C-type lectin receptors (CLR), retinoic acid-inducible gene-I (RIG-I)-like receptors (RLRs), NOD-like receptors (NLRs) and cytosolic DNA sensors (CDS). TLR and RLR signaling results in production of type I interferons (IFNα and -β) and pro-inflammatory cytokines in a cell-specific manner, whereas NLR signaling leads to the production of interleukin-1 family proteins. On the other hand, CLRs are capable of sensing glycans present in viral pathogens, which can induce phagocytic, endocytic, antimicrobial, and pro- inflammatory responses. Peripheral immune sensing of viruses and the ensuing cytokine response can significantly affect the central nervous system (CNS). But viruses can also directly enter the CNS via a multitude of routes, such as the nasal epithelium, along nerve fibers connecting to the periphery and as cargo of infiltrating infected cells passing through the blood brain barrier, triggering innate immune sensing and cytokine responses directly in the CNS. Here, we review mechanisms of viral immune sensing and currently recognized consequences for the CNS of innate immune responses to viruses.

In-Depth Analysis of Genetic Variation Associated with Severe West Nile Viral Disease

West Nile virus (WNV) is a mosquito-borne virus which causes symptomatic disease in a minority of infected humans. To identify novel genetic variants associated with severe disease, we utilized data from an existing case-control study of WNV and included population controls for an expanded analysis. We conducted imputation and gene-gene interaction analysis in the largest and most comprehensive genetic study conducted to date for West Nile neuroinvasive disease (WNND). Within the imputed West Nile virus dataset (severe cases n = 381 and asymptomatic/mild controls = 441), we found novel loci within the MCF.2 Cell Line Derived Transforming Sequence Like (MCF2L) gene (rs9549655 and rs2297192) through the individual loci analyses, although none reached statistical significance. Incorporating population controls from the Wisconsin Longitudinal Study on Aging (n = 9012) did not identify additional novel variants, a possible reflection of the cohort's inclusion of individuals who could develop mild or severe WNV disease upon infection. Many of the top gene-gene interaction results were intergenic, with currently undefined biological roles, highlighting the need for further investigation into these regions and other identified gene targets in severe WNND. Further studies including larger sample sizes and more diverse populations reflective of those at risk are needed to fully understand the genetic architecture of severe WNDD and provide guidance on viable targets for therapeutic and vaccine development.

Nanomedicinal delivery of stimulator of interferon genes agonists: recent advances in virus vaccination

The discovery of stimulator of interferon genes (STING) and their agonists as primary components that link antiviral innate and adaptive immunity has motivated growing research on STING agonist-mediated immunotherapy and vaccine development. To overcome the delivery challenge in shuttling highly polar STING agonists, typically in the form of cyclic dinucleotides, to target cells and to STING proteins in cellular cytosol, numerous nanoformulation strategies have been implemented for effective STING activation. While many STING-activating nanoparticles are developed to enhance anticancer immunotherapy, their adoption as vaccine adjuvant has vastly propelled antiviral vaccination efforts against challenging public health threats, including HIV, influenza and coronaviruses. In light of the COVID-19 pandemic that has thrusted vaccine development into the public spotlight, this review highlights advances in nanomedicinal STING agonist delivery with an emphasis on their applications in antiviral vaccination.

A Critical Role for STING Signaling in Limiting Pathogenesis of Chikungunya Virus

The stimulator of interferon gene (STING) pathway controls both DNA and RNA virus infection. STING is essential for induction of innate immune responses during DNA virus infection, while its mechanism against RNA virus remains largely elusive. We show that STING signaling is crucial for restricting chikungunya virus infection and arthritis pathogenesis. Sting-deficient mice (Stinggt/gt) had elevated viremia throughout the viremic stage and viral burden in feet transiently, with a normal type I IFN response. Stinggt/gt mice presented much greater foot swelling, joint damage, and immune cell infiltration than wild-type mice. Intriguingly, expression of interferon-γ and Cxcl10 was continuously upregulated by approximately 7 to 10-fold and further elevated in Stinggt/gt mice synchronously with arthritis progression. However, expression of chemoattractants for and activators of neutrophils, Cxcl5, Cxcl7, and Cxcr2 was suppressed in Stinggt/gt joints. These results demonstrate that STING deficiency leads to an aberrant chemokine response that promotes pathogenesis of CHIKV arthritis.

Skin Viral Infections: Host Antiviral Innate Immunity and Viral Immune Evasion

The skin is an active immune organ that functions as the first and largest site of defense to the outside environment. Serving as the primary interface between host and pathogen, the skin’s early immune responses to viral invaders often determine the course and severity of infection. We review the current literature pertaining to the mechanisms of cutaneous viral invasion for classical skin-tropic, oncogenic, and vector-borne skin viruses. We discuss the skin’s evolved mechanisms for innate immune viral defense against these invading pathogens, as well as unique strategies utilized by the viruses to escape immune detection. We additionally explore the roles that demographic and environmental factors, such as age, biological sex, and the cutaneous microbiome, play in altering the host immune response to viral threats.

Detection of Viral Infections by Innate Immunity

Pattern recognition receptors (PRRs) and inflammasomes are a key part of the anti-viral innate immune system as they detect conserved viral pathogen-associated molecular patterns (PAMPs). A successful host response to viral infections critically depend on the initial activation of PRRs by viruses, mainly by viral DNA and RNA. The signalling pathways activated by PRRs leads to the expression of pro-inflammatory cytokines, to recruit immune cells, and type I and type III interferons which leads to the induction of interferon stimulated genes (ISG), powerful virus restriction factors that establish the "antiviral state". Inflammasomes contribute to anti-viral responses through the maturation of interleukin (IL)-1 and IL-18 and through triggering pyroptotic cell death. The activity of the innate immune system along with the adaptive immune response normally leads to successful virus elimination, although disproportionate innate responses contribute to viral pathology. In this review we will discuss recent insights into the influence of PRR activation and inflammasomes on viral infections and what this means for the mammalian host. We will also comment on how specific PRRs and inflammasomes may be relevant to how SARS-CoV-2, the virus responsible for the current COVID-19 pandemic, interacts with host innate immunity.

Innate Immune DNA Sensing of Flaviviruses

Flaviviruses are arthropod-borne RNA viruses that have been used extensively to study host antiviral responses. Often selected just to represent standard single-stranded positive-sense RNA viruses in early studies, the Flavivirus genus over time has taught us how truly unique it is in its remarkable ability to target not just the RNA sensory pathways but also the cytosolic DNA sensing system for its successful replication inside the host cell. This review summarizes the main developments on the unexpected antagonistic strategies utilized by different flaviviruses, with RNA genomes, against the host cyclic GAMP synthase (cGAS)/stimulator of interferon genes (STING) cytosolic DNA sensing pathway in mammalian systems. On the basis of the recent advancements on this topic, we hypothesize that the mechanisms of viral sensing and innate immunity are much more fluid than what we had anticipated, and both viral and host factors will continue to be found as important factors contributing to the host innate immune system in the future.

West Nile Virus Restriction in Mosquito and Human Cells: A Virus under Confinement

West Nile virus (WNV) is an emerging neurotropic flavivirus that naturally circulates between mosquitoes and birds. However, WNV has a broad host range and can be transmitted from mosquitoes to several mammalian species, including humans, through infected saliva during a blood meal. Although WNV infections are mostly asymptomatic, 20% to 30% of cases are symptomatic and can occasionally lead to severe symptoms, including fatal meningitis or encephalitis. Over the past decades, WNV-carrying mosquitoes have become increasingly widespread across new regions, including North America and Europe, which constitutes a public health concern. Nevertheless, mosquito and human innate immune defenses can detect WNV infection and induce the expression of antiviral effectors, so-called viral restriction factors, to control viral propagation. Conversely, WNV has developed countermeasures to escape these host defenses, thus establishing a constant arms race between the virus and its hosts. Our review intends to cover most of the current knowledge on viral restriction factors as well as WNV evasion strategies in mosquito and human cells in order to bring an updated overview on WNV-host interactions.

The continued threat of emerging flaviviruses

Flaviviruses are vector-borne RNA viruses that can emerge unexpectedly in human populations and cause a spectrum of potentially severe diseases including hepatitis, vascular shock syndrome, encephalitis, acute flaccid paralysis, congenital abnormalities and fetal death. This epidemiological pattern has occurred numerous times during the last 70 years, including epidemics of dengue virus and West Nile virus, and the most recent explosive epidemic of Zika virus in the Americas. Flaviviruses are now globally distributed and infect up to 400 million people annually. Of significant concern, outbreaks of other less well-characterized flaviviruses have been reported in humans and animals in different regions of the world. The potential for these viruses to sustain epidemic transmission among humans is poorly understood. In this Review, we discuss the basic biology of flaviviruses, their infectious cycles, the diseases they cause and underlying host immune responses to infection. We describe flaviviruses that represent an established ongoing threat to global health and those that have recently emerged in new populations to cause significant disease. We also provide examples of lesser-known flaviviruses that circulate in restricted areas of the world but have the potential to emerge more broadly in human populations. Finally, we discuss how an understanding of the epidemiology, biology, structure and immunity of flaviviruses can inform the rapid development of countermeasures to treat or prevent human infections as they emerge.

Amino acid 159 of the envelope protein affects viral replication and T-cell infiltration by West Nile virus in intracranial infection

West Nile virus (WNV) is an important cause of viral encephalitis in birds and animals, including humans. Amino acid 159 of the envelope (E) protein is reportedly implicated in the different levels of neurovirulence in mice infected with WNV NY99 or Eg101. We investigated the role of amino acid 159 of the E protein in the pathogenesis of WNV infection. We produced recombinant WNV with the structural proteins of the NY99 or Eg101 strain (NY-WT or EgCME-WT) and mutant viruses with substitutions of amino acid 159 of the E protein (NY-E-V159I or EgCME-E-I159V). The NY-WT and NY-E-V159I or EgCME-WT and EgCME-E-I159V titers in culture supernatant were similar. The mortality rate and viral titer in the brains of mice inoculated intraperitoneally with NY-WT or NY-E-V159I were also similar. In contrast, the mortality rate and viral titer in the brains of mice inoculated intracranially with EgCME-E-I159V were significantly higher than those of mice inoculated with EgCME-WT. The numbers of CD3-positive and CD8-positive T cells were greater in brains inoculated with EgCME-E-I159V than in those inoculated with EgCME-WT. Therefore, amino acid 159 of the E protein modulates the pathogenicity of WNV by affecting viral replication and T-cell infiltration in the brain.

An Acute Stress Model in New Zealand White Rabbits Exhibits Altered Immune Response to Infection with West Nile Virus

The immune competence of an individual is a major determinant of morbidity in West Nile virus (WNV)-infection. Previously, we showed that immunocompetent New Zealand White rabbits (NZWRs; Oryctolagus cuniculus) are phenotypically resistant to WNV-induced disease, thus presenting a suitable model for study of virus-control mechanisms. The current study used corticosteroid-treated NZWRs to model acute “stress”-related immunosuppression. Maximal effects on immune parameters were observed on day 3 post dexamethasone-treatment (pdt). However, contrary to our hypothesis, intradermal WNV challenge at this time pdt produced significantly lower viremia 1 day post-infection (dpi) compared to untreated controls, suggestive of changes to antiviral control mechanisms. To examine this further, RNAseq was performed on RNA extracted from draining lymph node—the first site of virus replication and immune detection. Unaffected by dexamethasone-treatment, an early antiviral response, primarily via interferon (IFN)-I, and induction of a range of known and novel IFN-stimulated genes, was observed. However, treatment was associated with expression of a different repertoire of IFN-α-21-like and IFN-ω-1-like subtypes on 1 dpi, which may have driven the different chemokine response on 3 dpi. Ongoing expression of Toll-like receptor-3 and transmembrane protein-173/STING likely contributed to signaling of the treatment-independent IFN-I response. Two novel genes (putative HERC6 and IFIT1B genes), and the SLC16A5 gene were also highlighted as important component of the transcriptomic response. Therefore, the current study shows that rabbits are capable of restricting WNV replication and dissemination by known and novel robust antiviral mechanisms despite environmental challenges such as stress.