RIG-I Signaling via MAVS Is Dispensable for Survival in Lethal Influenza Infection In Vivo

Cytosolic nucleic acid sensing as driver of critical illness: mechanisms and advances in therapy

Nucleic acids from both self- and non-self-sources act as vital danger signals that trigger immune responses. Critical illnesses such as acute respiratory distress syndrome, sepsis, trauma and ischemia lead to the aberrant cytosolic accumulation and massive release of nucleic acids that are detected by antiviral innate immune receptors in the endosome or cytosol. Activation of receptors for deoxyribonucleic acids and ribonucleic acids triggers inflammation, a major contributor to morbidity and mortality in critically ill patients. In the past decade, there has been growing recognition of the therapeutic potential of targeting nucleic acid sensing in critical care. This review summarizes current knowledge of nucleic acid sensing in acute respiratory distress syndrome, sepsis, trauma and ischemia. Given the extensive research on nucleic acid sensing in common pathological conditions like cancer, autoimmune disorders, metabolic disorders and aging, we provide a comprehensive summary of nucleic acid sensing beyond critical illness to offer insights that may inform its role in critical conditions. Additionally, we discuss potential therapeutic strategies that specifically target nucleic acid sensing. By examining nucleic acid sources, sensor activation and function, as well as the impact of regulating these pathways across various acute diseases, we highlight the driving role of nucleic acid sensing in critical illness.

MAVS disruption impairs downstream signaling and results in higher virus replication levels of salmonid alphavirus subtype 3 but not infectious pancreatic necrosis virus in vitro

The mitochondrial anti-viral signaling (MAVS) protein is an intermediary adaptor protein of retinoic acid-inducible gene-1 (RIG-I) like receptor (RLR) signaling, which activates the transcription factor interferon (IFN) regulatory factor 3 (IRF3) and NF-kB to produce type I IFNs. MAVS expression has been reported in different fish species, but few studies have shown its functional role in anti-viral responses to fish viruses. In this study, we used the transcription activator-like effector nuclease (TALEN) as a gene editing tool to disrupt the function of MAVS in Chinook salmon (Oncorhynchus tshawytscha) embryonic cells (CHSE) to understand its role in induction of interferon I responses to infections with the (+) RNA virus salmonid alphavirus subtype 3 (SAV-3), and the dsRNA virus infectious pancreatic necrosis virus (IPNV) infection. A MAVS-disrupted CHSE clone with a 7-aa polypeptide (GVFVSRV) deletion mutation at the N-terminal of the CARD domain infected with SAV-3 resulted in significantly lower expression of IRF3, IFNa, and ISGs and increased viral titer (1.5 log10) compared to wild-type. In contrast, the IPNV titer in MAVS-disrupted cells was not different from the wild-type. Furthermore, overexpression of salmon MAVS in MAVS-disrupted CHSE cells rescued the impaired type I IFN-mediated anti-viral effect against SAV-3.

Pyrogallol protects against influenza A virus-triggered lethal lung injury by activating the Nrf2-PPAR-γ-HO-1 signaling axis

Pyrogallol, a natural polyphenol compound (1,2,3-trihydroxybenzene), has shown efficacy in the therapeutic treatment of disorders associated with inflammation. Nevertheless, the mechanisms underlying the protective properties of pyrogallol against influenza A virus infection are not yet established. We established in this study that pyrogallol effectively alleviated H1N1 influenza A virus-induced lung injury and reduced mortality. Treatment with pyrogallol was found to promote the expression and nuclear translocation of nuclear factor erythroid-2-related factor 2 (Nrf2) and peroxisome proliferator-activated receptor gamma (PPAR-γ). Notably, the activation of Nrf2 by pyrogallol was involved in elevating the expression of PPAR-γ, both of which act synergistically to enhance heme oxygenase-1 (HO-1) synthesis. Blocking HO-1 by zinc protoporphyrin (ZnPP) reduced the suppressive impact of pyrogallol on H1N1 virus-mediated aberrant retinoic acid-inducible gene-I-nuclear factor kappa B (RIG-I-NF-κB) signaling, which thus abolished the dampening effects of pyrogallol on excessive proinflammatory mediators and cell death (including apoptosis, necrosis, and ferroptosis). Furthermore, the HO-1-independent inactivation of janus kinase 1/signal transducers and activators of transcription (JAK1/STATs) and the HO-1-dependent RIG-I-augmented STAT1/2 activation were both abrogated by pyrogallol, resulting in suppression of the enhanced transcriptional activity of interferon-stimulated gene factor 3 (ISGF3) complexes, thus prominently inhibiting the amplification of the H1N1 virus-induced proinflammatory reaction and apoptosis in interferon-beta (IFN-β)-sensitized cells. The study provides evidence that pyrogallol alleviates excessive proinflammatory responses and abnormal cell death via HO-1 induction, suggesting it could be a potential agent for treating influenza.

Early IFN-β administration protects cigarette smoke exposed mice against lethal influenza virus infection without increasing lung inflammation

During influenza A virus (IAV) infection, it is unclear whether type I interferons (IFNs) have defensive antiviral effects or contribute to immunopathology in smokers. We treated nonsmoking (NS) and cigarette smoke (CS)-exposed mice intranasally with early (prophylactic) or late (therapeutic) IFN-β. We compared the mortality and innate immune responses of the treated mice following challenge with IAV. In NS mice, both early and late IFN-β administration decreased the survival rate in mice infected with IAV, with late IFN-β administration having the greatest effect on survival. In contrast, in CS-exposed mice, early IFN-β administration significantly increased survival during IAV infection while late IFN-β administration did not alter mortality. With regards to inflammation, in NS mice, IFN-β administration, especially late administration, significantly increased IAV-induced inflammation and lung injury. Early IFN-β administration to CS-exposed mice did not increase IAV-induced inflammation and lung injury as occurred in NS mice. Our results demonstrate, although IFN-β administration worsens the susceptibility of NS mice to influenza infection with increased immunopathology, early IFN-β administration to CS-exposed mice, which have suppression of the intrinsic IFN response, improved outcomes during influenza infection.

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.

Circular RNA circSamd4a Regulates Antiviral Immunity in Teleost Fish by Upregulating STING through Sponging miR-29a-3p

Circular RNAs (circRNAs) are a subgroup of endogenous noncoding RNA that is covalently closed rings and widely expressed. In recent years, there is accumulating evidence indicating that circRNAs are a class of important regulators, which play an important role in various biological processes. However, the biological functions and regulation mechanism of circRNAs in lower vertebrates are little known. In this study, we discovered a circRNA Samd4a (circSamd4a) that is related to the antiviral immune response of teleost fish. It can act as a key regulator of the host's antiviral response and play a key role in inhibiting Sininiperca chuatsi rhabdovirus replication. Further studies have shown that circSamd4a may act as a competing endogenous RNA, which can enhance the STING-mediated NF-κB/IRF3 signaling pathway by adsorbing miR-29a-3p, thereby enhancing the antiviral immune response. Therefore, circSamd4a plays an active regulatory role in the antiviral immune response of bony fish. Our research results provide a strong foundation for circular RNA to play a regulatory role in the antiviral immune response of teleost fish.

Review of Influenza Virus Vaccines: The Qualitative Nature of Immune Responses to Infection and Vaccination Is a Critical Consideration

Influenza viruses have affected the world for over a century, causing multiple pandemics. Throughout the years, many prophylactic vaccines have been developed for influenza; however, these viruses are still a global issue and take many lives. In this paper, we review influenza viruses, associated immunological mechanisms, current influenza vaccine platforms, and influenza infection, in the context of immunocompromised populations. This review focuses on the qualitative nature of immune responses against influenza viruses, with an emphasis on trained immunity and an assessment of the characteristics of the host–pathogen that compromise the effectiveness of immunization. We also highlight innovative immunological concepts that are important considerations for the development of the next generation of vaccines against influenza viruses.

Human Herpesvirus 6B U26 Inhibits the Activation of the RLR/MAVS Signaling Pathway

U26 is one of the roseolovirus unique genes with unknown function. Human herpesvirus 6B (HHV-6B) pU26 is predicted to be an 8-transmembrane protein containing a mitochondrion location signal. Here, we analyzed U26 function during HHV-6B infection and find that (i) HHV-6B U26 is expressed at a very early stage during HHV-6B infection, and knockdown of it results in a significant decrease of HHV-6B progeny virus production; (ii) U26 inhibits the activation of the retinoic acid-inducible gene I (RIG-I)-like receptor (RLR)/mitochondrial antiviral signaling protein (MAVS) signaling pathway, an important anti-HHV-6B infection innate immune response, by targeting MAVS protein for degradation; and (iii) a portion of U26 locates to the mitochondria, which could affect the mitochondrial membrane potential and finally leads to MAVS degradation. These findings indicate that HHV-6B U26 is a novel antagonistic viral factor against host innate antiviral immunity.IMPORTANCE HHV-6B (human herpesvirus 6B) is well known to evade host antiviral responses and establish a lifelong latent infection. How HHV-6B evades RNA recognition is still poorly understood. Our results indicate that HHV-6 U26 plays a vital role in RLR/MAVS signaling pathway activity. Knockout of endogenous MAVS could facilitate HHV-6B replication. The findings in this study could provide new insights into host-virus interactions and help develop a new therapy against HHV-6B infection.

Host genetic susceptibility to viral infections: the role of type I interferon induction

The innate immune response is the major front line of defense against viral infections. It involves hundreds of genes with antiviral properties which expression is induced by type I interferons (IFNs) and are therefore called interferon stimulated genes (ISGs). Type I IFNs are produced after viral recognition by pathogen recognition receptors, which trigger a cascade of activation events. Human and mouse studies have shown that defective type I IFNs induction may hamper the ability to control viral infections. In humans, moderate to high-effect variants have been identified in individuals with particularly severe complications following viral infection. In mice, functional studies using knock-out alleles have revealed the specific role of most genes of the IFN pathway. Here, we review the role of the molecular partners of the type I IFNs induction pathway and their implication in the control of viral infections and of their complications.

Regulation of Host Immune Responses against Influenza A Virus Infection by Mitogen-Activated Protein Kinases (MAPKs)

Influenza is a major respiratory viral disease caused by infections from the influenza A virus (IAV) that persists across various seasonal outbreaks globally each year. Host immune response is a key factor determining disease severity of influenza infection, presenting an attractive target for the development of novel therapies for treatments. Among the multiple signal transduction pathways regulating the host immune activation and function in response to IAV infections, the mitogen-activated protein kinase (MAPK) pathways are important signalling axes, downstream of various pattern recognition receptors (PRRs), activated by IAVs that regulate various cellular processes in immune cells of both innate and adaptive immunity. Moreover, aberrant MAPK activation underpins overexuberant production of inflammatory mediators, promoting the development of the “cytokine storm”, a characteristic of severe respiratory viral diseases. Therefore, elucidation of the regulatory roles of MAPK in immune responses against IAVs is not only essential for understanding the pathogenesis of severe influenza, but also critical for developing MAPK-dependent therapies for treatment of respiratory viral diseases. In this review, we will summarise the current understanding of MAPK functions in both innate and adaptive immune response against IAVs and discuss their contributions towards the cytokine storm caused by highly pathogenic influenza viruses.

Pattern Recognition Receptor Signaling and Innate Responses to Influenza A Viruses in the Mallard Duck, Compared to Humans and Chickens

Mallard ducks are a natural host and reservoir of avian Influenza A viruses. While most influenza strains can replicate in mallards, the virus typically does not cause substantial disease in this host. Mallards are often resistant to disease caused by highly pathogenic avian influenza viruses, while the same strains can cause severe infection in humans, chickens, and even other species of ducks, resulting in systemic spread of the virus and even death. The differences in influenza detection and antiviral effectors responsible for limiting damage in the mallards are largely unknown. Domestic mallards have an early and robust innate response to infection that seems to limit replication and clear highly pathogenic strains. The regulation and timing of the response to influenza also seems to circumvent damage done by a prolonged or dysregulated immune response. Rapid initiation of innate immune responses depends on viral recognition by pattern recognition receptors (PRRs) expressed in tissues where the virus replicates. RIG-like receptors (RLRs), Toll-like receptors (TLRs), and Nod-like receptors (NLRs) are all important influenza sensors in mammals during infection. Ducks utilize many of the same PRRs to detect influenza, namely RIG-I, TLR7, and TLR3 and their downstream adaptors. Ducks also express many of the same signal transduction proteins including TBK1, TRIF, and TRAF3. Some antiviral effectors expressed downstream of these signaling pathways inhibit influenza replication in ducks. In this review, we summarize the recent advances in our understanding of influenza recognition and response through duck PRRs and their adaptors. We compare basal tissue expression and regulation of these signaling components in birds, to better understand what contributes to influenza resistance in the duck.

Die Another Way: Interplay between Influenza A Virus, Inflammation and Cell Death

Influenza A virus (IAV) is a major concern to human health due to the ongoing global threat of a pandemic. Inflammatory and cell death signalling pathways play important roles in host defence against IAV infection. However, severe IAV infections in humans are characterised by excessive inflammation and tissue damage, often leading to fatal disease. While the molecular mechanisms involved in the induction of inflammation during IAV infection have been well studied, the pathways involved in IAV-induced cell death and their impact on immunopathology have not been fully elucidated. There is increasing evidence of significant crosstalk between cell death and inflammatory pathways and a greater understanding of their role in host defence and disease may facilitate the design of new treatments for IAV infection.