The viral RNA recognition sensor RIG-I is degraded during encephalomyocarditis virus (EMCV) infection

Multiple functions of the nonstructural protein 3D in picornavirus infection

3D polymerase, also known as RNA-dependent RNA polymerase, is encoded by all known picornaviruses, and their structures are highly conserved. In the process of picornavirus replication, 3D polymerase facilitates the assembly of replication complexes and directly catalyzes the synthesis of viral RNA. The nuclear localization signal carried by picornavirus 3D polymerase, combined with its ability to interact with other viral proteins, viral RNA and cellular proteins, indicate that its noncatalytic role is equally important in viral infections. Recent studies have shown that 3D polymerase has multiple effects on host cell biological functions, including inducing cell cycle arrest, regulating host cell translation, inducing autophagy, evading immune responses, and triggering inflammasome formation. Thus, 3D polymerase would be a very valuable target for the development of antiviral therapies. This review summarizes current studies on the structure of 3D polymerase and its regulation of host cell responses, thereby improving the understanding of picornavirus-mediated pathogenesis caused by 3D polymerase.

The Main Protease of Middle East Respiratory Syndrome Coronavirus Induces Cleavage of Mitochondrial Antiviral Signaling Protein to Antagonize the Innate Immune Response

Mitochondrial antiviral signaling protein (MAVS) is a crucial signaling adaptor in the sensing of positive-sense RNA viruses and the subsequent induction of the innate immune response. Coronaviruses have evolved multiple mechanisms to evade this response, amongst others, through their main protease (Mpro), which is responsible for the proteolytic cleavage of the largest part of the viral replicase polyproteins pp1a and pp1ab. Additionally, it can cleave cellular substrates, such as innate immune signaling factors, to dampen the immune response. Here, we show that MAVS is cleaved in cells infected with Middle East respiratory syndrome coronavirus (MERS-CoV), but not in cells infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). This cleavage was independent of cellular negative feedback mechanisms that regulate MAVS activation. Furthermore, MERS-CoV Mpro expression induced MAVS cleavage upon overexpression and suppressed the activation of the interferon-β (IFN-β) and nuclear factor-κB (NF-κB) response. We conclude that we have uncovered a novel mechanism by which MERS-CoV downregulates the innate immune response, which is not observed among other highly pathogenic coronaviruses.

A Bacterial Quorum Sensing Regulated Protease Inhibits Host Immune Responses by Cleaving Death Domains of Innate Immune Adaptors

Innate immune adaptor proteins are critical components of the innate immune system that propagate pro-inflammatory responses from their upstream receptors, and lead to pathogen clearance from the host. Bacterial pathogens have developed strategies to survive inside host cells without triggering the innate immune surveillance in ways that are still not fully understood. Here, it is reported that Pseudomonas aeruginosa induces its quorum sensing mechanism after macrophage engulfment. Further investigation of its secretome identified a quorum sensing regulated product, LasB, is responsible for innate immune suppression depending on the MyD88-mediated signaling. Moreover, it is showed that this specific type of pathogen-mediated innate immune suppression is due to the enzymatic digestion of the death domains of the innate immune adaptors, mainly MyD88, and attributed to LasB's large substrate binding groove. Lastly, it is demonstrated that the secretion of LasB from P. aeruginosa directly contributed to MyD88 degradation within macrophages. Hence, it is discovered an example of bacterial quorum sensing-regulated cellular innate immune suppression by direct cleavage of immune adaptors.

Orbivirus NS4 Proteins Play Multiple Roles to Dampen Cellular Responses

Non-structural protein 4 (NS4) of insect-borne and tick-borne orbiviruses is encoded by genome segment 9, from a secondary open reading frame. Though a protein dispensable for bluetongue virus (BTV) replication, it has been shown to counter the interferon response in cells infected with BTV or African horse sickness virus. We further explored the functional role(s) of NS4 proteins of BTV and the tick-borne Great Island virus (GIV). We show that NS4 of BTV or GIV helps an E3L deletion mutant of vaccinia virus to replicate efficiently in interferon-treated cells, further confirming the role of NS4 as an interferon antagonist. Our results indicate that ectopically expressed NS4 of BTV localised with caspase 3 within the nucleus and was found in a protein complex with active caspase 3 in a pull-down assay. Previous studies have shown that pro-apoptotic caspases (including caspase 3) suppress type I interferon response by cleaving mediators involved in interferon signalling. Our data suggest that orbivirus NS4 plays a role in modulating the apoptotic process and/or regulating the interferon response in mammalian cells, thus acting as a virulence factor in pathogenesis.

Theilovirus 3C Protease Cleaves the C-Terminal Domain of the Innate Immune RNA Sensor, Melanoma Differentiation-Associated Gene 5, and Impairs Double-Stranded RNA-Mediated IFN Response

Melanoma differentiation-associated gene 5 (MDA5), a member of the retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs), has pivotal roles in innate immune responses against many positive-stranded RNA viruses, including picornavirus and coronavirus. Upon engagement with dsRNA derived from viral infection, MDA5 initiates coordinated signal transduction leading to type I IFN induction to restrict viral replication. In this study, we describe a targeted cleavage events of MDA5 by the 3C protease from Theilovirus. Upon ectopic expression of theilovirus 3C protease from Saffold virus or Theiler's murine encephalomyelitis virus but not encephalomyocarditis virus, fragments of cleaved MDA5 were observed in a dose-dependent manner. When enzymatically inactive Theilovirus 3C protease was expressed, MDA5 cleavage was completely abrogated. Mass spectrometric analysis identified two cleavage sites at the C terminus of MDA5, cleaving off one of the RNA-binding domains. The same cleavage pattern was observed during Theilovirus infection. The cleavage of MDA5 by Theilovirus protease impaired ATP hydrolysis, RNA binding, and filament assembly on RNA, resulting in dysfunction of MDA5 as an innate immune RNA sensor for IFN induction. Furthermore, the cleavage-resistant MDA5 mutant against the 3C protease showed an enhanced IFN response during Saffold virus infection, indicating that Theilovirus has a strategy to circumvent the antiviral immune response by cleaving MDA5 using 3C protease. In summary, these data suggest MDA5 cleavage by 3C protease as a novel immune evasive strategy of Theilovirus.

DHAV 3CD targets IRF7 and RIG-I proteins to block the type I interferon upstream signaling pathway

Duck hepatitis A virus type 1 (DHAV-1) is an acute, highly lethal infectious agent that infects ducklings and causes up to 95% mortality in ducklings up to 1 week of age, posing a significant economic threat to the duck farming industry. Previous studies have found that the proteolytic enzyme 3 C encoded by DHAV-1 can inhibit the IRF7 protein from blocking the upstream signaling pathway of the type I interferon to promote viral replication. However, there are still few studies on the mechanism of DHAV-1 in immune evasion. Here, we demonstrate that the DHAV-1 3CD protein can interact with IRF7 protein and reduce IRF7 protein expression without directly affecting IRF7 protein nuclear translocation. Further studies showed that the 3CD protein could reduce the expression of RIG-I protein without affecting its transcription level. Furthermore, we found that the 3CD protein interacted with the N-terminal structural domain of RIG-I protein, interfered with the interaction between RIG-I and MAVS, and degraded RIG-I protein through the proteasomal degradation pathway, thereby inhibiting its mediated antiviral innate immunity to promote DHAV-1 replication. These data suggest a novel immune evasion mechanism of DHAV-1 mediated by the 3CD protein, and the results of this experiment are expected to improve the understanding of the biological functions of the viral precursor protein and provide scientific data to elucidate the mechanism of DHAV-1 infection and pathogenesis.

Porcine Sapelovirus 3Cpro Inhibits the Production of Type I Interferon

Porcine sapelovirus (PSV) is the causative pathogen of reproductive obstacles, acute diarrhea, respiratory distress, or severe polioencephalomyelitis in swine. Nevertheless, the pathogenicity and pathogenic mechanism of PSV infection are not fully understood, which hinders disease prevention and control. In this study, we found that PSV was sensitive to type I interferon (IFN-β). However, PSV could not activate the IFN-β promoter and induce IFN-β mRNA expression, indicating that PSV has evolved an effective mechanism to block IFN-β production. Further study showed that PSV inhibited the production of IFN-β by cleaving mitochondrial antiviral signaling (MAVS) and degrading melanoma differentiation-associated gene 5 (MDA5) and TANK-binding kinase 1 (TBK1) through viral 3Cpro. In addition, our study demonstrated that PSV 3Cpro degrades MDA5 and TBK1 through its protease activity and cleaves MAVS through the caspase pathway. Collectively, our results revealed that PSV inhibits the production of type I interferon to escape host antiviral immunity through cleaving and degrading the adaptor molecules.

Surgical Strikes on Host Defenses: Role of the Viral Protease Activity in Innate Immune Antagonism

As a frontline defense mechanism against viral infections, the innate immune system is the primary target of viral antagonism. A number of virulence factors encoded by viruses play roles in circumventing host defenses and augmenting viral replication. Among these factors are viral proteases, which are primarily responsible for maturation of viral proteins, but in addition cause proteolytic cleavage of cellular proteins involved in innate immune signaling. The study of these viral protease-mediated host cleavages has illuminated the intricacies of innate immune networks and yielded valuable insights into viral pathogenesis. In this review, we will provide a brief summary of how proteases of positive-strand RNA viruses, mainly from the Picornaviridae, Flaviviridae and Coronaviridae families, proteolytically process innate immune components and blunt their functions.

DHAV-1 Blocks the Signaling Pathway Upstream of Type I Interferon by Inhibiting the Interferon Regulatory Factor 7 Protein

Duck hepatitis A virus (DHAV), which mainly infects 1- to 4-week-old ducklings, has a fatality rate of 95% and poses a huge economic threat to the duck industry. However, the mechanism by which DHAV-1 regulates the immune response of host cells is rarely reported. This study examined whether DHAV-1 contains a viral protein that can regulate the innate immunity of host cells and its specific regulatory mechanism, further exploring the mechanism by which DHAV-1 resists the host immune response. In the study, the dual-luciferase reporter gene system was used to screen the viral protein that regulates the host innate immunity and the target of this viral protein. The results indicate that the DHAV-1 3C protein inhibits the pathway upstream of interferon (IFN)-β by targeting the interferon regulatory factor 7 (IRF7) protein. In addition, we found that the 3C protein inhibits the nuclear translocation of the IRF7 protein. Further experiments showed that the 3C protein interacts with the IRF7 protein through its N-terminus and that the 3C protein degrades the IRF7 protein in a caspase 3-dependent manner, thereby inhibiting the IFN-β-mediated antiviral response to promote the replication of DHAV-1. The results of this study are expected to serve as a reference for elucidating the mechanisms of DHAV-1 infection and pathogenicity.

Running With Scissors: Evolutionary Conflicts Between Viral Proteases and the Host Immune System

Many pathogens encode proteases that serve to antagonize the host immune system. In particular, viruses with a positive-sense single-stranded RNA genome [(+)ssRNA], including picornaviruses, flaviviruses, and coronaviruses, encode proteases that are not only required for processing viral polyproteins into functional units but also manipulate crucial host cellular processes through their proteolytic activity. Because these proteases must cleave numerous polyprotein sites as well as diverse host targets, evolution of these viral proteases is expected to be highly constrained. However, despite this strong evolutionary constraint, mounting evidence suggests that viral proteases such as picornavirus 3C, flavivirus NS3, and coronavirus 3CL, are engaged in molecular 'arms races' with their targeted host factors, resulting in host- and virus-specific determinants of protease cleavage. In cases where protease-mediated cleavage results in host immune inactivation, recurrent host gene evolution can result in avoidance of cleavage by viral proteases. In other cases, such as recently described examples in NLRP1 and CARD8, hosts have evolved 'tripwire' sequences that mimic protease cleavage sites and activate an immune response upon cleavage. In both cases, host evolution may be responsible for driving viral protease evolution, helping explain why viral proteases and polyprotein sites are divergent among related viruses despite such strong evolutionary constraint. Importantly, these evolutionary conflicts result in diverse protease-host interactions even within closely related host and viral species, thereby contributing to host range, zoonotic potential, and pathogenicity of viral infection. Such examples highlight the importance of examining viral protease-host interactions through an evolutionary lens.

Host protein, HSP90β, antagonizes IFN-β signaling pathway and facilitates the proliferation of encephalomyocarditis virus in vitro

Encephalomyocarditis virus (EMCV) is a small, non-enveloped, single stranded RNA virus which infects a wide variety of mammalian species, and has zoonotic importance. Many host proteins are known to regulate EMCV proliferation by interacting with its structural or nonstructural proteins, but the regulatory role and mechanism of heat shock protein 90β (HSP90β), in EMCV infection has not been reported yet. Here, we report that overexpression of HSP90β significantly promotes the growth and proliferation of EMCV in vitro. On the contrary, down-regulation of HSP90β by RNAi or geldanamycin inhibits EMCV replication. HSP90β suppresses IFN-β responses in the RLRs pathway by targeting the expression of the key adaptor molecules MAVS, TBK1, and IRF3, but not MDA5. This study demonstrates the firsthand information that HSP90β plays a positive role in viral proliferation by inhibiting EMCV induced IFN-β production. Collectively, the results reveal new insights into HSP90β-assisted progression of EMCV infection.

Picornavirus 3C - a protease ensuring virus replication and subverting host responses

The protease 3C is encoded by all known picornaviruses, and the structural features related to its protease and RNA-binding activities are conserved; these contribute to the cleavage of viral polyproteins and the assembly of the viral RNA replication complex during virus replication. Furthermore, 3C performs functions in the host cell through its interaction with host proteins. For instance, 3C has been shown to selectively 'hijack' host factors involved in gene expression, promoting picornavirus replication, and to inactivate key factors in innate immunity signaling pathways, inhibiting the production of interferon and inflammatory cytokines. Importantly, 3C maintains virus infection by subtly subverting host cell death and modifying critical molecules in host organelles. This Review focuses on the molecular mechanisms through which 3C mediates physiological processes involved in virus-host interaction, thus highlighting the picornavirus-mediated pathogenesis caused by 3C.

MDA5 cleavage by the Leader protease of foot-and-mouth disease virus reveals its pleiotropic effect against the host antiviral response

The RIG-I-like receptor (RLR) melanoma differentiation-associated gene 5 (MDA5) plays a key role in triggering innate antiviral response during infection by RNA viruses. MDA5 activation leads to transcription induction of type-I interferon (IFN) and proinflammatory cytokines. MDA5 has also been associated with autoimmune and autoinflammatory diseases by dysfunctional activation of innate immune response in the absence of infection. Here, we show how foot-and-mouth disease virus (FMDV) counteracts the specific antiviral effect exerted by MDA5 targeting the protein for cleavage by the viral Leader protease (Lpro). MDA5 overexpression had an inhibitory effect on FMDV infection in IFN-competent cells. Remarkably, immunostimulatory viral RNA co-immunoprecipitated with MDA5 in infected cells. Moreover, specific cleavage of MDA5 by Lpro was detected in co-transfected cells, as well as during the course of FMDV infection. A significant reduction in IFN induction associated with MDA5 cleavage was detected by comparison with a non-cleavable MDA5 mutant protein with preserved antiviral activity. The Lpro cleavage site in MDA5 was identified as the RGRAR sequence in the conserved helicase motif VI, coinciding with that recently reported for Lpro in LGP2, another member of the RLRs family involved in antiviral defenses. Interestingly, specific mutations within the MDA5 Lpro target sequence have been associated with immune disease in mice and humans. Our results reveal a pleiotropic strategy for immune evasion based on a viral protease targeting phylogenetically conserved domains of immune sensors. Identification of viral strategies aimed to disrupt MDA5 functionality may also contribute to develop new treatment tools for MDA5-related disorders.

Innate immune evasion by picornaviruses

The family Picornaviridae comprises a large number of viruses that cause disease in broad spectrum of hosts, which have posed serious public health concerns worldwide and led to significant economic burden. A comprehensive understanding of the virus-host interactions during picornavirus infections will help to prevent and cure these diseases. Upon picornavirus infection, host pathogen recognition receptors (PRRs) sense viral RNA to activate host innate immune responses. The activated PRRs initiate signal transduction through a series of adaptor proteins, which leads to activation of several kinases and transcription factors, and contributes to the consequent expression of interferons (IFNs), IFN-inducible antiviral genes, as well as various inflammatory cytokines and chemokines. In contrast, to maintain viral replication and spread, picornaviruses have evolved several elegant strategies to block innate immune signaling and hinder host antiviral response. In this review, we will summarize the recent progress of how the members of family Picornaviridae counteract host immune response through evasion of PRRs detection, blocking activation of adaptor molecules and kinases, disrupting transcription factors, as well as counteraction of antiviral restriction factors. Such knowledge of immune evasion will help us better understand the pathogenesis of picornaviruses, and provide insights into developing antiviral strategies and improvement of vaccines.

Regulation of Apoptosis by Enteroviruses

Enterovirus infection can cause a variety of diseases and severely impair the health of humans, animals, poultry, and other organisms. To resist viral infection, host organisms clear infected cells and viruses via apoptosis. However, throughout their long-term competition with host cells, enteroviruses have evolved a series of mechanisms to regulate the balance of apoptosis in order to replicate and proliferate. In the early stage of infection, enteroviruses mainly inhibit apoptosis by regulating the PI3K/Akt pathway and the autophagy pathway and by impairing cell sensors, thereby delaying viral replication. In the late stage of infection, enteroviruses mainly regulate apoptotic pathways and the host translation process via various viral proteins, ultimately inducing apoptosis. This paper discusses the means by which these two phenomena are balanced in enteroviruses to produce virus-favoring conditions – in a temporal sequence or through competition with each other. This information is important for further elucidation of the relevant mechanisms of acute infection by enteroviruses and other members of the picornavirus family.

Role of pattern recognition receptors and interferon-beta in protecting bat cell lines from encephalomyocarditis virus and Japanese encephalitis virus infection

Bats are potential natural hosts of Encephalomyocarditis virus (EMCV) and Japanese encephalitis virus (JEV). Bats appear to have some unique features in their innate immune system that inhibit viral replication causing limited clinical symptoms, and thus, contributing to the virus spill over to humans. Here, kidney epithelial cell lines derived from four bat species (Pteropus dasymallus, Rousettus leschenaultii, Rhinolophus ferrumequinum, and Miniopterus fuliginosus) and two non-bat species (Homo sapiens and Mesocricetus auratus) were infected with EMCV and JEV. The replication of EMCV and JEV was lower in the bat cell lines derived from R. leschenaultii, R. ferrumequinum, and M. fuliginosus with a higher expression level of pattern recognition receptors (PRRs) (TLR3, RIG-I, and MDA5) and interferon-beta (IFN-β) than that in the non-bat cell lines and a bat cell line derived from P. dasymallus. The knockdown of TLR3, RIG-I, and MDA5 in Rhinolophus bat cell line using antisense RNA oligonucleotide led to decrease IFN-β expression and increased viral replication. These results suggest that TLR3, RIG-I, and MDA5 are important for antiviral response against EMCV and JEV in Rhinolophus bats.

Apoptosis and Autophagy in Picornavirus Infection

Cell death is a fundamental process in maintaining cellular homeostasis, which can be either accidental or programed. Programed cell death depends on the specific signaling pathways, resulting in either lytic or non-lytic morphology. It exists in two primary forms: apoptosis and autophagic cell death. Apoptosis is a non-lytic and selective cell death program, which is executed by caspases in response to non-self or external stimuli. In contrast, autophagy is crucial for maintaining cellular homeostasis via the degradation and recycling of cellular components. These two mechanisms also function in the defense against pathogen attack. However, picornaviruses have evolved to utilize diverse strategies and target critical components to regulate the apoptotic and autophagic processes for optimal replication and the release from the host cell. Although an increasing number of investigations have shown that the apoptosis and autophagy are altered in picornavirus infection, the mechanism by which viruses take advantage of these two processes remains unknown. In this review, we discuss the mechanisms of picornavirus executes cellular apoptosis and autophagy at the molecular level and the relationship between these interactions and viral pathogenesis.

Porcine deltacoronavirus nucleocapsid protein antagonizes IFN-β production by impairing dsRNA and PACT binding to RIG-I

Porcine deltacoronavirus (PDCoV) is an emerging swine enteropathogenic coronavirus that causes watery diarrhea, vomiting and mortality in newborn piglets. Previous studies have suggested that PDCoV infection antagonizes RIG-I-like receptor (RLR)-mediated IFN-β production to evade host innate immune defense, and PDCoV-encoded nonstructural protein nsp5 and accessory protein NS6 are associated with this process. However, whether the structural protein(s) of PDCoV also antagonize IFN-β production remains unclear. In this study, we found that PDCoV nucleocapsid (N) protein, the most abundant viral structural protein, suppressed Sendai virus (SEV)-induced IFN-β production and transcription factor IRF3 activation, but did not block IFN-β production induced by overexpressing RIG-I/MDA5. Furthermore, study revealed that PDCoV N protein interacted with RIG-I and MDA5 in an in vitro overexpression system and evident interactions between N protein and RIG-I could be detected in the context of PDCoV infection, which interfered with the binding of dsRNA and protein activator of protein kinase R (PACT) to RIG-I. Together, our results demonstrate that PDCoV N protein is an IFN antagonist and utilizes diverse strategies to attenuate RIG-I recognition and activation.

An update on enterovirus 71 infection and interferon type I response

Enteroviruses are members of Pichornaviridae family consisting of human enterovirus group A, B, C, and D as well as nonhuman enteroviruses. Hand, foot, and mouth disease (HFMD) is a serious disease which is usually seen in the Asia-Pacific region in children. Enterovirus 71 and coxsackievirus A16 are two important viruses responsible for HFMD which are members of group A enterovirus. IFN α and β are two cytokines, which have a major activity in the innate immune system against viral infections. Most of the viruses have some weapons against these cytokines. EV71 has two main proteases called 2A and 3C, which are important for polyprotein processing and virus maturation. Several studies have indicated that they have a significant effect on different cellular pathways such as interferon production and signaling pathway. The aim of this study was to investigate the latest findings about the interaction of 2A and 3C protease of EV71 and IFN production/signaling pathway and their inhibitory effects on this pathway.

Camouflage and interception: how pathogens evade detection by intracellular nucleic acid sensors

Intracellular DNA and RNA sensors play a vital part in the innate immune response to viruses and other intracellular pathogens, causing the secretion of type I interferons, cytokines and chemokines from infected cells. Pathogen RNA can be detected by retinoic-acid inducible gene I-like receptors in the cytosol, whereas cytosolic DNA is recognized by DNA sensors such as cyclic GMP-AMP synthase (cGAS). The resulting local immune response, which is initiated within hours of infection, is able to eliminate many pathogens before they are able to establish an infection in the host. For this reason, all viruses, and some intracellular bacteria and protozoa, need to evade detection by nucleic acid sensors. Immune evasion strategies include the sequestration and modification of nucleic acids, and the inhibition or degradation of host factors involved in innate immune signalling. Large DNA viruses, such as herpesviruses, often use multiple viral proteins to inhibit signalling cascades at several different points; for instance herpes simplex virus 1 targets both DNA sensors cGAS and interferon-γ-inducible protein 16, as well as the adaptor protein STING (stimulator of interferon genes) and other signalling factors in the pathway. Viruses with a small genome encode only a few immunomodulatory proteins, but these are often multifunctional, such as the NS1 protein from influenza A virus, which inhibits RNA sensing in multiple ways. Intracellular bacteria and protozoa can also be detected by nucleic acid sensors. However, as the type I interferon response is not always beneficial for the host under these circumstances, some bacteria subvert, rather than evade, these signalling cascades for their own gain.

Cell-type specificity and functional redundancy of RIG-I-like receptors in innate immune sensing of Coxsackievirus B3 and encephalomyocarditis virus

The contributions of RIG-I and MDA5 receptors to sensing viruses of the Picornaviridae family were investigated. The picornaviruses encephalomyocarditis virus (EMCV) and Coxsackievirus B3 (CVB3) are detected by both MDA5 and RIG-I in bone marrow derived macrophages. In macrophages from wild type mice, type I IFN is produced early after infection; IFNβ synthesis is reduced in the absence of each sensor, while IFNα production is reduced in the absence of MDA5. EMCV and CVB3 do not replicate in murine macrophages, and their detection is different in murine embryonic fibroblasts (MEFs), in which the viruses replicate to high titers. In MEFs RIG-I was essential for the expression of type I IFNs but contributes to increased yields of CVB3, while MDA5 inhibited CVB3 replication but in an IFN independent manner. These observations demonstrate functional redundancy within the innate immune response to picornaviruses.

The Strategy of Picornavirus Evading Host Antiviral Responses: Non-structural Proteins Suppress the Production of IFNs

Viral infections trigger the innate immune system to produce interferons (IFNs), which play important role in host antiviral responses. Co-evolution of viruses with their hosts has favored development of various strategies to evade the effects of IFNs, enabling viruses to survive inside host cells. One such strategy involves inhibition of IFN signaling pathways by non-structural proteins. In this review, we provide a brief overview of host signaling pathways inducing IFN production and their suppression by picornavirus non-structural proteins. Using this strategy, picornaviruses can evade the host immune response and replicate inside host cells.

Type I Interferon Induced and Antagonized by Foot-and-Mouth Disease Virus

Viral infections trigger the innate immune system, serving as the first line of defense, and are characterized by the production of type I interferon (IFN). Type I IFN is expressed in a broad spectrum of cells and tissues in the host and includes various subtypes (IFN-α, IFN-β, IFN-δ, IFN-ε, IFN-κ, IFN-τ, IFN-ω, IFN-ν, and IFN-ζ). Since the discovery of type I IFN, our knowledge of the biology of type I IFN has accumulated immensely, and we now have a substantial amount of information on the molecular mechanisms of the response and induction of type I IFN, as well as the strategies utilized by viruses to evade the type I IFN response. Foot-and-mouth disease virus (FMDV) can selectively alter cellular pathways to promote viral replication and evade antiviral immune activation of type I IFN. RNA molecules generated by FMDV are sensed by the cellular receptor for pathogen-associated molecular patterns (PAMPs). FMDV preferentially activates different sensor molecules and various signal transduction pathways. Based on knowledge of the virus or RNA pathogen specificity as well as the function-structure relationship of RNA sensing, it is necessary to summarize numerous signaling adaptors that are reported to participate in the regulation of IFN gene activation.

The ubiquitin-protein ligase E6AP/UBE3A supports early encephalomyocarditis virus replication

Many viruses make use of, and even direct, the ubiquitin-proteasome system to facilitate the generation of a cellular environment favorable for virus replication, while host cells use selected protein ubiquitylation pathways for antiviral defense. Relatively little information has been acquired, however, regarding the extent to which protein ubiquitylation determines the replication success of picornaviruses. Here we report that the ubiquitin-protein ligase E6AP/UBE3A, recently shown to be a participant in encephalomyocarditis virus (EMCV) 3C protease concentration regulation, also facilitates the early stages of EMCV replication, probably by a mechanism that does not involve 3C protease ubiquitylation. Using stably transfected E6AP knockdown cells, we found that reduced E6AP concentration extends the time required for infected cells to undergo the morphological changes caused by virally induced pathogenesis and to begin the production of infectious virions. This lag in virion production is accompanied by a corresponding delay in the appearance of detectable levels of viral proteins and RNA. We also found, by using both immunofluorescence microscopy and cell fractionation, that E6AP is partially redistributed from the nucleus to the cytoplasm in EMCV-infected cells, thereby increasing its availability to participate in cytoplasmic virus replication processes.

Comparison of Porcine Airway and Intestinal Epithelial Cell Lines for the Susceptibility and Expression of Pattern Recognition Receptors upon Influenza Virus Infection

Influenza viruses infect the epithelial cells of the swine respiratory tract. Cell lines derived from the respiratory tract of pigs could serve as an excellent in vitro model for studying the pathogenesis of influenza viruses. In this study, we examined the replication of influenza viruses in the MK1-OSU cell line, which was clonally derived from pig airway epithelium. MK1-OSU cells expressed both cytokeratin and vimentin proteins and displayed several sugar moieties on the cell membrane. These cells also expressed both Sial2-3Gal and Sial2-6Gal receptors and were susceptible to swine influenza A, but not to human B and C viruses. Interestingly, these cells were also permissive to infection by influenza D virus that utilized 9-O-acetylated glycans. To study the differences in the expression of pattern recognition receptors (PRRs) upon influenza virus infection in the respiratory and digestive tract, we compared the protein expression of various PRRs in MK1-OSU cells with that in the SD-PJEC cell line, a clonally derived cell line from the porcine jejunal epithelium. Toll-like receptor 7 (TLR-7) and melanoma differentiation-associated protein 5 (MDA5) receptors showed decreased expression in influenza A infected MK1-OSU cells, while only TLR-7 expression decreased in SD-PJEC cells. Further research is warranted to study the mechanism behind the virus-mediated suppression of these proteins. Overall, this study shows that the porcine respiratory epithelial cell line, MK1-OSU, could serve as an in-vitro model for studying the pathogenesis and innate immune responses to porcine influenza viruses.

The Roles of Picornavirus Untranslated Regions in Infection and Innate Immunity

Viral genomes have evolved to maximize their potential of overcoming host defense mechanisms and to induce a variety of disease syndromes. Structurally, a genome of a virus consists of coding and noncoding regions, and both have been shown to contribute to initiation and progression of disease. Accumulated work in picornaviruses has stressed out the importance of the noncoding RNAs, or untranslated 5′- and 3′-regions (UTRs), in both replication and translation of viral genomes. Unsurprisingly, defects in these processes have been reported to cause viral attenuation and affect viral pathogenicity. However, substantial evidence suggests that these untranslated RNAs may influence the outcome of the host innate immune response. This review discusses the involvement of 5′- and 3′-terminus UTRs in induction and regulation of host immunity and its consequences for viral life cycle and virulence.

Intracellular Antiviral Immunity

Innate immunity is traditionally thought of as the first line of defense against pathogens that enter the body. It is typically characterized as a rather weak defense mechanism, designed to restrict pathogen replication until the adaptive immune response generates a tailored response and eliminates the infectious agent. However, intensive research in recent years has resulted in better understanding of innate immunity as well as the discovery of many effector proteins, revealing its numerous powerful mechanisms to defend the host. Furthermore, this research has demonstrated that it is simplistic to strictly separate adaptive and innate immune functions since these two systems often work synergistically rather than sequentially. Here, we provide a broad overview of innate pattern recognition receptors in antiviral defense, with a focus on the TRIM family, and discuss their signaling pathways and mechanisms of action with special emphasis on the intracellular antibody receptor TRIM21.

VPg unlinkase/TDP2 in cardiovirus infected cells: Re-localization and proteolytic cleavage

Cardioviruses cause diseases in many animals including, in rare cases, humans. Although they share common features with all picornaviruses, cardioviruses have unique properties that distinguish them from other family members, including enteroviruses. One feature shared by all picornaviruses is the covalent attachment of VPg to the 5' end of genomic RNA via a phosphotyrosyl linkage. For enteroviruses, this linkage is cleaved by a host cell protein, TDP2. Since TDP2 is divergently required during enterovirus infections, we determined if TDP2 is necessary during infection by the prototype cardiovirus, EMCV. We found that EMCV yields are reduced in the absence of TDP2. We observed a decrease in viral protein accumulation and viral RNA replication in the absence of TDP2. In contrast to enterovirus infections, we found that TDP2 is modified at peak times of EMCV infection. This finding suggests a unique mechanism for cardioviruses to regulate TDP2 activity during infection.

Differential effects of SUMO1 and SUMO3 on PKR activation and stability

Double-stranded RNA (dsRNA)-dependent protein kinase (PKR) is a serine/threonine kinase that exerts its own phosphorylation and the phosphorylation of the α subunit of the protein synthesis initiation factor eIF-2α. PKR was identified as a target of SUMOylation and the triple PKR-SUMO deficient mutant on Lysine residues K60-K150-K440 has reduced PKR activity. We report that SUMO1 and SUMO3 expression exert differential effects on PKR localization, activation and stability. SUMO1 or SUMO3 did not alter the repartition of PKR in the cytoplasm and the nucleus. However, in SUMO3-expressing cells PKR was found more concentrated around the perinuclear membrane and was recruited from small speckles to nuclear dots. Interestingly, SUMO1 expression alone resulted in PKR and eIF-2α activation, whereas SUMO3 reduced PKR and eIF-2α activation upon viral infection or dsRNA transfection. In addition, encephalomyocarditis virus (EMCV) enhanced PKR conjugation to SUMO1 and SUMO3 but only SUMO3 expression promoted caspase-dependent EMCV-induced PKR degradation. Furthermore, the higher EMCV-induced PKR activation by SUMO1 was correlated with an inhibition of EMCV. Importantly SUMO1, by inducing PKR activation in the absence of viral infection, and SUMO3, by counteracting both PKR activation and stability upon viral infection, shed a new light on the differential effects of SUMO-modified PKR.

Picornaviruses and Apoptosis: Subversion of Cell Death

Infected cells can undergo apoptosis as a protective response to viral infection, thereby limiting viral infection. As viruses require a viable cell for replication, the death of the cell limits cellular functions that are required for virus replication and propagation. Picornaviruses are single-stranded RNA viruses that modify the host cell apoptotic response, probably in order to promote viral replication, largely as a function of the viral proteases 2A, 3C, and 3CD. These proteases are essential for viral polyprotein processing and also cleave cellular proteins. Picornavirus proteases cleave proapoptotic adaptor proteins, resulting in downregulation of apoptosis. Picornavirus proteases also cleave nucleoporins, disrupting the orchestrated manner in which signaling pathways use active nucleocytoplasmic trafficking, including those involved in apoptosis. In addition to viral proteases, the transmembrane 2B protein alters intracellular ion signaling, which may also modulate apoptosis. Overall, picornaviruses, via the action of virally encoded proteins, exercise intricate control over and subvert cell death pathways, specifically apoptosis, thereby allowing viral replication to continue.

RNA-virus proteases counteracting host innate immunity

Virus invasion triggers host immune responses, in particular, innate immune responses. Pathogen‐associated molecular patterns of viruses (such as dsRNA, ssRNA, or viral proteins) released during virus replication are detected by the corresponding pattern‐recognition receptors of the host, and innate immune responses are induced. Through production of type‐I and type‐III interferons as well as various other cytokines, the host innate immune system forms the frontline to protect host cells and inhibit virus infection. Not surprisingly, viruses have evolved diverse strategies to counter this antiviral system. In this review, we discuss the multiple strategies used by proteases of positive‐sense single‐stranded RNA viruses of the families Picornaviridae, Coronaviridae, and Flaviviridae, when counteracting host innate immune responses.

Disruption of MDA5-Mediated Innate Immune Responses by the 3C Proteins of Coxsackievirus A16, Coxsackievirus A6, and Enterovirus D68

Coxsackievirus A16 (CV-A16), CV-A6, and enterovirus D68 (EV-D68) belong to the Picornaviridae family and are major causes of hand, foot, and mouth disease (HFMD) and pediatric respiratory disease worldwide. The biological characteristics of these viruses, especially their interplay with the host innate immune system, have not been well investigated. In this study, we discovered that the 3Cpro proteins from CV-A16, CV-A6, and EV-D68 bind melanoma differentiation-associated gene 5 (MDA5) and inhibit its interaction with MAVS. Consequently, MDA5-triggered type I interferon (IFN) signaling in the retinoic acid-inducible gene I-like receptor (RLR) pathway was blocked by the CV-A16, CV-A6, and EV-D68 3Cpro proteins. Furthermore, the CV-A16, CV-A6, and EV-D68 3Cpro proteins all cleave transforming growth factor β-activated kinase 1 (TAK1), resulting in the inhibition of NF-κB activation, a host response also critical for Toll-like receptor (TLR)-mediated signaling. Thus, our data demonstrate that circulating HFMD-associated CV-A16 and CV-A6, as well as severe respiratory disease-associated EV-D68, have developed novel mechanisms to subvert host innate immune responses by targeting key factors in the RLR and TLR pathways. Blocking the ability of 3Cpro proteins from diverse enteroviruses and coxsackieviruses to interfere with type I IFN induction should restore IFN antiviral function, offering a potential novel antiviral strategy.IMPORTANCE CV-A16, CV-A6, and EV-D68 are emerging pathogens associated with hand, foot, and mouth disease and pediatric respiratory disease worldwide. The pathogenic mechanisms of these viruses are largely unknown. Here we demonstrate that the CV-A16, CV-A6, and EV-D68 3Cpro proteins block MDA5-triggered type I IFN induction. The 3Cpro proteins of these viruses bind MDA5 and inhibit its interaction with MAVS. In addition, the CV-A16, CV-A6, and EV-D68 3Cpro proteins cleave TAK1 to inhibit the NF-κB response. Thus, our data demonstrate that circulating HFMD-associated CV-A16 and CV-A6, as well as severe respiratory disease-associated EV-D68, have developed a mechanism to subvert host innate immune responses by simultaneously targeting key factors in the RLR and TLR pathways. These findings indicate the potential merit of targeting the CV-A16, CV-A6, and EV-D68 3Cpro proteins as an antiviral strategy.

Molecular Mechanisms of Foot-and-Mouth Disease Virus Targeting the Host Antiviral Response

Foot-and-mouth disease virus (FMDV) is the causative agent of an acute vesicular disease affecting pigs, cattle and other domestic, and wild animals worldwide. The aim of the host interferon (IFN) response is to limit viral replication and spread. Detection of the viral genome and products by specialized cellular sensors initiates a signaling cascade that leads to a rapid antiviral response involving the secretion of type I- and type III-IFNs and other antiviral cytokines with antiproliferative and immunomodulatory functions. During co-evolution with their hosts, viruses have acquired strategies to actively counteract host antiviral responses and the balance between innate response and viral antagonism may determine the outcome of disease and pathogenesis. FMDV proteases Lpro and 3C have been found to antagonize the host IFN response by a repertoire of mechanisms. Moreover, the putative role of other viral proteins in IFN antagonism is being recently unveiled, uncovering sophisticated immune evasion strategies different to those reported to date for other members of the Picornaviridae family. Here, we review the interplay between antiviral responses induced by FMDV infection and viral countermeasures to block them. Research on strategies used by viruses to modulate immunity will provide insights into the function of host pathways involved in defense against pathogens and will also lead to development of new therapeutic strategies to fight virus infections.

Host and Viral Modulation of RIG-I-Mediated Antiviral Immunity

Innate immunity is the first line of defense against invading pathogens. Rapid and efficient detection of pathogen-associated molecular patterns via pattern-recognition receptors is essential for the host to mount defensive and protective responses. Retinoic acid-inducible gene-I (RIG-I) is critical in triggering antiviral and inflammatory responses for the control of viral replication in response to cytoplasmic virus-specific RNA structures. Upon viral RNA recognition, RIG-I recruits the mitochondrial adaptor protein mitochondrial antiviral signaling protein, which leads to a signaling cascade that coordinates the induction of type I interferons (IFNs), as well as a large variety of antiviral interferon-stimulated genes. The RIG-I activation is tightly regulated via various posttranslational modifications for the prevention of aberrant innate immune signaling. By contrast, viruses have evolved mechanisms of evasion, such as sequestrating viral structures from RIG-I detections and targeting receptor or signaling molecules for degradation. These virus–host interactions have broadened our understanding of viral pathogenesis and provided insights into the function of the RIG-I pathway. In this review, we summarize the recent advances regarding RIG-I pathogen recognition and signaling transduction, cell-intrinsic control of RIG-I activation, and the viral antagonism of RIG-I signaling.

Roles of the Picornaviral 3C Proteinase in the Viral Life Cycle and Host Cells

The Picornaviridae family comprises a large group of non-enveloped viruses that have a major impact on human and veterinary health. The viral genome contains one open reading frame encoding a single polyprotein that can be processed by viral proteinases. The crucial 3C proteinases (3C(pro)s) of picornaviruses share similar spatial structures and it is becoming apparent that 3C(pro) plays a significant role in the viral life cycle and virus host interaction. Importantly, the proteinase and RNA-binding activity of 3C(pro) are involved in viral polyprotein processing and the initiation of viral RNA synthesis. In addition, 3C(pro) can induce the cleavage of certain cellular factors required for transcription, translation and nucleocytoplasmic trafficking to modulate cell physiology for viral replication. Due to interactions between 3C(pro) and these essential factors, 3C(pro) is also involved in viral pathogenesis to support efficient infection. Furthermore, based on the structural conservation, the development of irreversible inhibitors and discovery of non-covalent inhibitors for 3C(pro) are ongoing and a better understanding of the roles played by 3C(pro) may provide insights into the development of potential antiviral treatments. In this review, the current knowledge regarding the structural features, multiple functions in the viral life cycle, pathogen host interaction, and development of antiviral compounds for 3C(pro) is summarized.

HDAC6 regulates cellular viral RNA sensing by deacetylation of RIG-I

RIG-I is a key cytosolic sensor that detects RNA viruses through its C-terminal region and activates the production of antiviral interferons (IFNs) and proinflammatory cytokines. While posttranslational modification has been demonstrated to regulate RIG-I signaling activity, its significance for the sensing of viral RNAs remains unclear. Here, we first show that the RIG-I C-terminal region undergoes deacetylation to regulate its viral RNA-sensing activity and that the HDAC6-mediated deacetylation of RIG-I is critical for viral RNA detection. HDAC6 transiently bound to RIG-I and removed the lysine 909 acetylation in the presence of viral RNAs, promoting RIG-I sensing of viral RNAs. Depletion of HDAC6 expression led to impaired antiviral responses against RNA viruses, but not against DNA viruses. Consequently, HDAC6 knockout mice were highly susceptible to RNA virus infections compared to wild-type mice. These findings underscore the critical role of HDAC6 in the modulation of the RIG-I-mediated antiviral sensing pathway.

3C Protease of Enterovirus D68 Inhibits Cellular Defense Mediated by Interferon Regulatory Factor 7

Human enterovirus 68 (EV-D68) is a member of the EV-D species, which belongs to the EV genus of the Picornaviridae family. Over the past several years, clusters of EV-D68 infections have occurred worldwide. A recent outbreak in the United States is the largest one associated with severe respiratory illness and neurological complication. Although clinical symptoms are recognized, the virus remains poorly understood. Here we report that EV-D68 inhibits innate antiviral immunity by downregulation of interferon regulatory factor 7 (IRF7), an immune factor with a pivotal role in viral pathogenesis. This process depends on 3C(pro), an EV-D68-encoded protease, to mediate IRF7 cleavage. When expressed in host cells, 3C(pro) targets Q167 and Q189 within the constitutive activation domain, resulting in cleavage of IRF7. Accordingly, wild-type IRF7 is fully active. However, IRF7 cleavage abrogated its capacity to activate type I interferon expression and limit replication of EV-D68. Notably, IRF7 cleavage strictly requires the protease activity of 3C(pro). Together, these results suggest that a dynamic interplay between 3C(pro) and IRF7 may determine the outcome of EV-D68 infection.

IMPORTANCE

EV-D68 is a globally emerging pathogen, but the molecular basis of EV-D68 pathogenesis is unclear. Here we report that EV-D68 inhibits innate immune responses by targeting an immune factor, IRF7. This involves the 3C protease encoded by EV-D68, which mediates the cleavage of IRF7. These observations suggest that the 3C(pro)-IRF7 interaction may represent an interface that dictates EV-D68 infection.

Induction of functional Brm protein from Brm knockout mice

Once the knockout of the Brm gene was found to be nontumorigenic in mice, the study of BRM's involvement in cancer seemed less important compared with that of its homolog, Brg1. This has likely contributed to the disparity that has been observed in the publication ratio between BRG1 and BRM. We show that a previously published Brm knockout mouse is an incomplete knockout whereby a truncated isoform of Brm is detected in normal tissue and in tumors. We show that this truncated Brm isoform has functionality comparable to wild type Brm. By immunohistochemistry (IHC), this truncated Brm is undetectable in normal lung tissue and is minimal to very low in Brmnull tumors. However, it is significant in a subset (~40%) of Brg1/Brm double knockout (DKO) tumors that robustly express this truncated BRM, which in part stems from an increase in Brm mRNA levels. Thus, it is likely that this mutant mouse model does not accurately reflect the role that Brm plays in cancer development. We suggest that the construction of a completely new mouse Brm knockout, where Brm is functionally absent, is needed to determine whether or not Brm is actually tumorigenic and if Brm might be a tumor suppressor.

Interferon induction by RNA viruses and antagonism by viral pathogens

Interferons are a group of small proteins that play key roles in host antiviral innate immunity. Their induction mainly relies on host pattern recognition receptors (PRR). Host PRR for RNA viruses include Toll-like receptors (TLR) and retinoic acid-inducible gene I (RIG-I) like receptors (RLR). Activation of both TLR and RLR pathways can eventually lead to the secretion of type I IFNs, which can modulate both innate and adaptive immune responses against viral pathogens. Because of the important roles of interferons, viruses have evolved multiple strategies to evade host TLR and RLR mediated signaling. This review focuses on the mechanisms of interferon induction and antagonism of the antiviral strategy by RNA viruses.

Hepatitis A virus 3C protease cleaves NEMO to impair induction of beta interferon

NEMO (NF-κB essential modulator) is a bridging adaptor indispensable for viral activation of interferon (IFN) antiviral response. Herein, we show that hepatitis A virus (HAV) 3C protease (3Cpro) cleaves NEMO at the Q304 residue, negating its signaling adaptor function and abrogating viral induction of IFN-β synthesis via the retinoic acid-inducible gene I/melanoma differentiation-associated protein 5 (RIG-I/MDA5) and Toll-like receptor 3 (TLR3) pathways. NEMO cleavage and IFN antagonism, however, were lost upon ablation of the catalytic activity of 3Cpro. These data describe a novel immune evasion mechanism of HAV.

Induction and suppression of innate antiviral responses by picornaviruses

The family Picornaviridae comprises of small, non-enveloped, positive-strand RNA viruses and contains many human and animal pathogens including enteroviruses (e.g. poliovirus, coxsackievirus, enterovirus 71 and rhinovirus), cardioviruses (e.g. encephalomyocarditis virus), hepatitis A virus and foot-and-mouth disease virus. Picornavirus infections activate a cytosolic RNA sensor, MDA5, which in turn, induces a type I interferon response, a crucial component of antiviral immunity. Moreover, picornaviruses activate the formation of stress granules (SGs), large aggregates of preassembled mRNPs (messenger ribonucleoprotein particles) to temporarily store these molecules upon cellular stress. Meanwhile, picornaviruses actively suppress these antiviral responses to ensure efficient replication. In this review we provide an overview of the induction and suppression of the MDA5-mediated IFN-α/β response and the cellular stress pathway by picornaviruses.

Differential restriction patterns of mRNA decay factor AUF1 during picornavirus infections

During infection by picornaviruses, the cellular environment is modified to favour virus replication. This includes the modification of specific host proteins, including the recently discovered viral proteinase cleavage of mRNA decay factor AU-rich binding factor 1 (AUF1). This cellular RNA-binding protein was shown previously to act as a restriction factor during poliovirus, rhinovirus and coxsackievirus infection. During infection by these viruses, AUF1 relocalizes to the cytoplasm and is cleaved by the viral 3C/3CD proteinase. In this study, we demonstrated that replication of encephalomyocarditis virus (EMCV), a picornavirus belonging to the genus Cardiovirus, is AUF1 independent. During EMCV infection, AUF1 relocalized to the cytoplasm; however, unlike what is seen during enterovirus infections, AUF1 was not cleaved to detectable levels, even at late times after infection. This suggests that AUF1 does not act broadly as an inhibitor of picornavirus infections but may instead act as a selective restriction factor targeting members of the genus Enterovirus.

Enterovirus 2Apro targets MDA5 and MAVS in infected cells

RIG-I-like receptors (RLRs) MDA5 and RIG-I are key players in the innate antiviral response. Upon recognition of viral RNA, they interact with MAVS, eventually inducing type I interferon production. The interferon induction pathway is commonly targeted by viruses. How enteroviruses suppress interferon production is incompletely understood. MDA5 has been suggested to undergo caspase- and proteasome-mediated degradation during poliovirus infection. Additionally, MAVS is reported to be cleaved during infection with coxsackievirus B3 (CVB3) by the CVB3 proteinase 3C(pro), whereas MAVS cleavage by enterovirus 71 has been attributed to 2A(pro). As yet, a detailed examination of the RLR pathway as a whole during any enterovirus infection is lacking. We performed a comprehensive analysis of crucial factors of the RLR pathway, including MDA5, RIG-I, LGP2, MAVS, TBK1, and IRF3, during infection of CVB3, a human enterovirus B (HEV-B) species member. We show that CVB3 inhibits the RLR pathway upstream of TBK1 activation, as demonstrated by limited phosphorylation of TBK1 and a lack of IRF3 phosphorylation. Furthermore, we show that MDA5, MAVS, and RIG-I all undergo proteolytic degradation in CVB3-infected cells through a caspase- and proteasome-independent manner. We convincingly show that MDA5 and MAVS cleavages are both mediated by CVB3 2A(pro), while RIG-I is cleaved by 3C(pro). Moreover, we show that proteinases 2A(pro) and 3C(pro) of poliovirus (HEV-C) and enterovirus 71 (HEV-A) exert the same functions. This study identifies a critical role of 2A(pro) by cleaving MDA5 and MAVS and shows that enteroviruses use a common strategy to counteract the interferon response in infected cells.

IMPORTANCE

Human enteroviruses (HEVs) are important pathogens that cause a variety of diseases in humans, including poliomyelitis, hand, foot, and mouth disease, viral meningitis, cardiomyopathy, and more. Like many other viruses, enteroviruses target the host immune pathways to gain replication advantage. The MDA5/MAVS pathway is responsible for recognizing enterovirus infections in the host cell and leads to expression of type I interferons (IFN-I), crucial antiviral signaling molecules. Here we show that three species of HEVs all employ the viral proteinase 2A (2A(pro)) to proteolytically target MDA5 and MAVS, leading to an efficient blockade upstream of IFN-I transcription. These observations suggest that MDA5/MAVS antagonization is an evolutionarily conserved and beneficial mechanism of enteroviruses. Understanding the molecular mechanisms of enterovirus immune evasion strategies will help to develop countermeasures to control infections with these viruses in the future.

Master sensors of pathogenic RNA - RIG-I like receptors

Initiating the immune response to invading pathogens, the innate immune system is constituted of immune receptors (pattern recognition receptors, PRR) that sense microbe-associated molecular patterns (MAMPs). Detection of pathogens triggers intracellular defense mechanisms, such as the secretion of cytokines or chemokines to alarm neighboring cells and attract or activate immune cells. The innate immune response to viruses is mostly based on PRRs that detect the unusual structure, modification or location of viral nucleic acids. Most of the highly pathogenic and emerging viruses are RNA genome-based viruses, which can give rise to zoonotic and epidemic diseases or cause viral hemorrhagic fever. As viral RNA is located in the same compartment as host RNA, PRRs in the cytosol have to discriminate between viral and endogenous RNA by virtue of their structure or modification. This challenging task is taken on by the homologous cytosolic DExD/H-box family helicases RIG-I and MDA5, which control the innate immune response to most RNA viruses. This review focuses on the molecular basis for RIG-I like receptor (RLR) activation by synthetic and natural ligands and will discuss controversial ligand definitions.

Dengue virus subverts the interferon induction pathway via NS2B/3 protease-IκB kinase epsilon interaction

Dengue is the world's most common mosquito-borne viral infection and a leading cause of morbidity throughout the tropics and subtropics. Viruses are known to evade the establishment of an antiviral state by regulating the activation of interferon regulatory factor 3 (IRF3), a critical transcription factor in the alpha/beta interferon induction pathway. Here, we show that dengue virus (DENV) circumvents the induction of the retinoic acid-inducible gene I-like receptor (RLR) pathway during infection by blocking serine 386 phosphorylation and nuclear translocation of IRF3. This effect is associated with the expression of nonstructural 2B/3 protein (NS2B/3) protease in human cells. Using interaction assays, we found that NS2B/3 interacts with the cellular IκB kinase ε (IKKε). Docking computational analysis revealed that in this interaction, NS2B/3 masks the kinase domain of IKKε and potentially affects its functionality. This observation is supported by the DENV-associated inhibition of the kinase activity of IKKε. Our data identify IKKε as a novel target of DENV NS2B/3 protease.

Enter at your own risk: how enteroviruses navigate the dangerous world of pattern recognition receptor signaling

Enteroviruses are the most common human viral pathogens worldwide. This genus of small, non-enveloped, single stranded RNA viruses includes coxsackievirus, rhinovirus, echovirus, and poliovirus species. Infection with these viruses can induce mild symptoms that resemble the common cold, but can also be associated with more severe syndromes such as poliomyelitis, neurological diseases including aseptic meningitis and encephalitis, myocarditis, and the onset of type I diabetes. In humans, polarized epithelial cells lining the respiratory and/or digestive tracts represent the initial sites of infection by enteroviruses. Control of infection in the host is initiated through the engagement of a variety of pattern recognition receptors (PRRs). PRRs act as the sentinels of the innate immune system and serve to alert the host to the presence of a viral invader. This review assembles the available data annotating the role of PRRs in the response to enteroviral infection as well as the myriad ways by which enteroviruses both interrupt and manipulate PRR signaling to enhance their own replication, thereby inducing human disease.

Implications of Age-Dependent Immune Responses to Enterovirus 71 Infection for Disease Pathogenesis and Vaccine Design

Epidemics of enterovirus serotype 71 (EV71) infection in Asia appear to be increasing in size and severity, and there is increasing concern for pandemic spread. Efforts are underway to develop an effective EV71 vaccine. However, the immunologic correlates of protection against EV71 infection are not fully understood, and studies suggest that severe complications may result from a combination of pathological immune responses and direct viral effects. Severe disease and death typically occur only in young children, which is likely due in part to a lack of EV71-specific adaptive immunity but possibly also due to age-dependent hyperactive innate immune responses. Infants are the primary targets of EV71 vaccination strategies. Therefore, studies are needed to understand the interplay between age, immunopathology, and severity of EV71 infection to distinguish protective from harmful immune responses and to guide the development of effective EV71 vaccines. This review summarizes our current understanding and outlines the next steps forward.

Toscana virus NSs protein inhibits the induction of type I interferon by interacting with RIG-I

Toscana virus (TOSV) is a phlebovirus, of the Bunyaviridae family, that is responsible for central nervous system (CNS) injury in humans. Previous data have shown that the TOSV NSs protein is a gamma interferon (IFN-β) antagonist when transiently overexpressed in mammalian cells, inhibiting IRF-3 induction (G. Gori Savellini, F. Weber, C. Terrosi, M. Habjan, B. Martorelli, and M. G. Cusi, J. Gen. Virol. 92:71-79, 2011). In this study, we investigated whether an upstream sensor, which has a role in the signaling cascade leading to the production of type I IFN, was involved. We found a significant decrease in RIG-I protein levels in cells overexpressing TOSV NSs, suggesting that the nonstructural protein interacts with RIG-I and targets it for proteasomal degradation. In fact, the MG-132 proteasome inhibitor was able to restore IFN-β promoter activation in cells expressing NSs, demonstrating the existence of an evasion mechanism based on inhibition of the RIG-I sensor. Furthermore, a C-terminal truncated NSs protein (ΔNSs), although able to interact with RIG-I, did not affect the RIG-I-mediated IFN-β promoter activation, suggesting that the NSs domains responsible for RIG-I-mediated signaling and interaction with RIG-I are mapped on different regions. These results contribute to identify a novel mechanism for bunyaviruses by which TOSV NSs counteracts the early IFN response.

Antiviral type I and type III interferon responses in the central nervous system

The central nervous system (CNS) harbors highly differentiated cells, such as neurons that are essential to coordinate the functions of complex organisms. This organ is partly protected by the blood-brain barrier (BBB) from toxic substances and pathogens carried in the bloodstream. Yet, neurotropic viruses can reach the CNS either by crossing the BBB after viremia, or by exploiting motile infected cells as Trojan horses, or by using axonal transport. Type I and type III interferons (IFNs) are cytokines that are critical to control early steps of viral infections. Deficiencies in the IFN pathway have been associated with fatal viral encephalitis both in humans and mice. Therefore, the IFN system provides an essential protection of the CNS against viral infections. Yet, basal activity of the IFN system appears to be low within the CNS, likely owing to the toxicity of IFN to this organ. Moreover, after viral infection, neurons and oligodendrocytes were reported to be relatively poor IFN producers and appear to keep some susceptibility to neurotropic viruses, even in the presence of IFN. This review addresses some trends and recent developments concerning the role of type I and type III IFNs in: i) preventing neuroinvasion and infection of CNS cells; ii) the identity of IFN-producing cells in the CNS; iii) the antiviral activity of ISGs; and iv) the activity of viral proteins of neurotropic viruses that target the IFN pathway.

Enterovirus 71 protease 2Apro targets MAVS to inhibit anti-viral type I interferon responses

Enterovirus 71 (EV71) is the major causative pathogen of hand, foot, and mouth disease (HFMD). Its pathogenicity is not fully understood, but innate immune evasion is likely a key factor. Strategies to circumvent the initiation and effector phases of anti-viral innate immunity are well known; less well known is whether EV71 evades the signal transduction phase regulated by a sophisticated interplay of cellular and viral proteins. Here, we show that EV71 inhibits anti-viral type I interferon (IFN) responses by targeting the mitochondrial anti-viral signaling (MAVS) protein--a unique adaptor molecule activated upon retinoic acid induced gene-I (RIG-I) and melanoma differentiation associated gene (MDA-5) viral recognition receptor signaling--upstream of type I interferon production. MAVS was cleaved and released from mitochondria during EV71 infection. An in vitro cleavage assay demonstrated that the viral 2A protease (2A(pro)), but not the mutant 2A(pro) (2A(pro)-110) containing an inactivated catalytic site, cleaved MAVS. The Protease-Glo assay revealed that MAVS was cleaved at 3 residues between the proline-rich and transmembrane domains, and the resulting fragmentation effectively inactivated downstream signaling. In addition to MAVS cleavage, we found that EV71 infection also induced morphologic and functional changes to the mitochondria. The EV71 structural protein VP1 was detected on purified mitochondria, suggesting not only a novel role for mitochondria in the EV71 replication cycle but also an explanation of how EV71-derived 2A(pro) could approach MAVS. Taken together, our findings reveal a novel strategy employed by EV71 to escape host anti-viral innate immunity that complements the known EV71-mediated immune-evasion mechanisms.