MDA5 detects the double-stranded RNA replicative form in picornavirus-infected cells

Enterovirus Control of Translation and RNA Granule Stress Responses

Enteroviruses such as poliovirus (PV) and coxsackievirus B3 (CVB3) have evolved several parallel strategies to regulate cellular gene expression and stress responses to ensure efficient expression of the viral genome. Enteroviruses utilize their encoded proteinases to take over the cellular translation apparatus and direct ribosomes to viral mRNAs. In addition, viral proteinases are used to control and repress the two main types of cytoplasmic RNA granules, stress granules (SGs) and processing bodies (P-bodies, PBs), which are stress-responsive dynamic structures involved in repression of gene expression. This review discusses these processes and the current understanding of the underlying mechanisms with respect to enterovirus infections. In addition, the review discusses accumulating data suggesting linkage exists between RNA granule formation and innate immune sensing and activation.

Enterovirus 71 induces dsRNA/PKR-dependent cytoplasmic redistribution of GRP78/BiP to promote viral replication

GRP78/BiP is an endoplasmic reticulum (ER) chaperone protein with the important function of maintaining ER homeostasis, and the overexpression of GRP78/BiP alleviates ER stress. Our previous studies showed that infection with enterovirus 71 (EV71), a (+)RNA picornavirus, induced GRP78/BiP upregulation; however, ectopic GRP78/BiP overexpression in ER downregulates virus replication and viral particle formation. The fact that a virus infection increases GRP78/BiP expression, which is unfavorable for virus replication, is counterintuitive. In this study, we found that the GRP78/BiP protein level was elevated in the cytoplasm instead of in the ER in EV71-infected cells. Cells transfected with polyinosinic-polycytidylic acid, a synthetic analog of replicative double-stranded RNA (dsRNA), but not with viral proteins, also exhibited upregulation and elevation of GRP78/BiP in the cytosol. Our results further demonstrate that EV71 infections induce the dsRNA/protein kinase R-dependent cytosolic accumulation of GRP78/BiP. The overexpression of a GRP78/BiP mutant lacking a KDEL retention signal failed to inhibit both dithiothreitol-induced eIF2α phosphorylation and viral replication in the context of viral protein synthesis and viral titers. These data revealed that EV71 infection might cause upregulation and aberrant redistribution of GRP78/BiP to the cytosol, thereby facilitating virus replication.

Filament assemblies in foreign nucleic acid sensors

Helical filamentous assembly is ubiquitous in biology, but was only recently realized to be broadly employed in the innate immune system of vertebrates. Accumulating evidence suggests that the filamentous assemblies and helical oligomerization play important roles in detection of foreign nucleic acids and activation of the signaling pathways to produce antiviral and inflammatory mediators. In this review, we focus on the helical assemblies observed in the signaling pathways of RIG-I-like receptors (RLRs) and AIM2-like receptors (ALRs). We describe ligand-dependent oligomerization of receptor, receptor-dependent oligomerization of signaling adaptor molecules, and their functional implications and regulations.

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.

Innate Immunity Evasion by Enteroviruses: Insights into Virus-Host Interaction

Enterovirus genus includes multiple important human pathogens, such as poliovirus, coxsackievirus, enterovirus (EV) A71, EV-D68 and rhinovirus. Infection with EVs can cause numerous clinical conditions including poliomyelitis, meningitis and encephalitis, hand-foot-and-mouth disease, acute flaccid paralysis, diarrhea, myocarditis and respiratory illness. EVs, which are positive-sense single-stranded RNA viruses, trigger activation of the host antiviral innate immune responses through pathogen recognition receptors such as retinoic acid-inducible gene (RIG-I)-likeand Toll-like receptors. In turn, EVs have developed sophisticated strategies to evade host antiviral responses. In this review, we discuss the interplay between the host innate immune responses and EV infection, with a primary focus on host immune detection and protection against EV infection and viral strategies to evade these antiviral immune responses.

Rhinoviruses and Respiratory Enteroviruses: Not as Simple as ABC

Rhinoviruses (RVs) and respiratory enteroviruses (EVs) are leading causes of upper respiratory tract infections and among the most frequent infectious agents in humans worldwide. Both are classified in the Enterovirus genus within the Picornaviridae family and they have been assigned to seven distinct species, RV-A, B, C and EV-A, B, C, D. As viral infections of public health significance, they represent an important financial burden on health systems worldwide. However, the lack of efficient antiviral treatment or vaccines against these highly prevalent pathogens prevents an effective management of RV-related diseases. Current advances in molecular diagnostic techniques have revealed the presence of RV in the lower respiratory tract and its role in lower airway diseases is increasingly reported. In addition to an established etiological role in the common cold, these viruses demonstrate an unexpected capacity to spread to other body sites under certain conditions. Some of these viruses have received particular attention recently, such as EV-D68 that caused a large outbreak of respiratory illness in 2014, respiratory EVs from species C, or viruses within the newly-discovered RV-C species. This review provides an update of the latest findings on clinical and fundamental aspects of RV and respiratory EV, including a summary of basic knowledge of their biology.

Innate Immunity and Immune Evasion by Enterovirus 71

Enterovirus 71 (EV71) is a major infectious disease affecting millions of people worldwide and it is the main etiological agent for outbreaks of hand foot and mouth disease (HFMD). Infection is often associated with severe gastroenterological, pulmonary, and neurological diseases that are most prevalent in children. Currently, no effective vaccine or antiviral drugs exist against EV71 infection. A lack of knowledge on the molecular mechanisms of EV71 infection in the host and the virus-host interactions is a major constraint to developing specific antiviral strategies against this infection. Previous studies have identified and characterized the function of several viral proteins produced by EV71 that interact with the host innate immune proteins, including type I interferon signaling and microRNAs. These interactions eventually promote efficient viral replication and increased susceptibility to the disease. In this review we discuss the functions of EV71 viral proteins in the modulation of host innate immune responses to facilitate viral replication.

Type I Interferon Response Limits Astrovirus Replication and Protects against Increased Barrier Permeability In Vitro and In Vivo

Little is known about intrinsic epithelial cell responses against astrovirus infection. Here we show that human astrovirus type 1 (HAstV-1) infection induces type I interferon (beta interferon [IFN-β]) production in differentiated Caco2 cells, which not only inhibits viral replication by blocking positive-strand viral RNA and capsid protein synthesis but also protects against HAstV-1-increased barrier permeability. Excitingly, we found similar results in vivo using a murine astrovirus (MuAstV) model, providing new evidence that virus-induced type I IFNs may protect against astrovirus replication and pathogenesis in vivo.

IMPORTANCE

Human astroviruses are a major cause of pediatric diarrhea, yet little is known about the immune response. Here we show that type I interferon limits astrovirus infection and preserves barrier permeability both in vitro and in vivo. Importantly, we characterized a new mouse model for studying astrovirus replication and pathogenesis.

Both RIG-I and MDA5 detect alphavirus replication in concentration-dependent mode

Alphaviruses are a family of positive-strand RNA viruses that circulate on all continents between mosquito vectors and vertebrate hosts. Despite a significant public health threat, their biology is not sufficiently investigated, and the mechanisms of alphavirus replication and virus-host interaction are insufficiently understood. In this study, we have applied a variety of experimental systems to further understand the mechanism by which infected cells detect replicating alphaviruses. Our new data strongly suggest that activation of the antiviral response by alphavirus-infected cells is determined by the integrity of viral genes encoding proteins with nuclear functions, and by the presence of two cellular pattern recognition receptors (PRRs), RIG-I and MDA5. No type I IFN response is induced in their absence. The presence of either of these PRRs is sufficient for detecting virus replication. However, type I IFN activation in response to pathogenic alphaviruses depends on the basal levels of RIG-I or MDA5.

Proteolysis of MDA5 and IPS-1 is not required for inhibition of the type I IFN response by poliovirus

BACKGROUND

The type I interferon (IFN) response is a critical component of the innate immune response to infection by RNA viruses and is initiated via recognition of viral nucleic acids by RIG-like receptors (RLR). Engagement of these receptors in the cytoplasm initiates a signal transduction pathway leading to activation of the transcription factors NF-κB, ATF-2 and IRF-3 that coordinately upregulate transcription of type I IFN genes, such as that encoding IFN-β. In this study the impact of poliovirus infection on the type I interferon response has been examined.

METHODS

The type I IFN response was assessed by measuring IFN-β mRNA levels using qRT-PCR and normalizing to levels of β-actin mRNA. The status of host factors involved in activation of the type I IFN response was examined by immunoblot, immunofluorescence microcopy and qRT-PCR.

RESULTS

The results show that poliovirus infection results in induction of very low levels of IFN-β mRNA despite clear activation of NF-κB and ATF-2. In contrast, analysis of IRF-3 revealed no transcriptional induction of an IRF-3-responsive promoter or homodimerization of IRF-3 indicating it is not activated in poliovirus-infected cells. Exposure of poliovirus-infected cells to poly(I:C) results in lower levels of IFN-β mRNA synthesis and IRF-3 activation compared to mock-infected cells. Analysis of MDA-5 and IPS-1 revealed that these components of the RLR pathway were largely intact at times when the type I IFN response was suppressed.

CONCLUSIONS

Collectively, these results demonstrate that poliovirus infection actively suppresses the host type I interferon response by blocking activation of IRF-3 and suggests that this is not mediated by cleavage of MDA-5 or IPS-1.

Integrative Genomics-Based Discovery of Novel Regulators of the Innate Antiviral Response

The RIG-I-like receptor (RLR) pathway is essential for detecting cytosolic viral RNA to trigger the production of type I interferons (IFNα/β) that initiate an innate antiviral response. Through systematic assessment of a wide variety of genomics data, we discovered 10 molecular signatures of known RLR pathway components that collectively predict novel members. We demonstrate that RLR pathway genes, among others, tend to evolve rapidly, interact with viral proteins, contain a limited set of protein domains, are regulated by specific transcription factors, and form a tightly connected interaction network. Using a Bayesian approach to integrate these signatures, we propose likely novel RLR regulators. RNAi knockdown experiments revealed a high prediction accuracy, identifying 94 genes among 187 candidates tested (~50%) that affected viral RNA-induced production of IFNβ. The discovered antiviral regulators may participate in a wide range of processes that highlight the complexity of antiviral defense (e.g. MAP3K11, CDK11B, PSMA3, TRIM14, HSPA9B, CDC37, NUP98, G3BP1), and include uncharacterized factors (DDX17, C6orf58, C16orf57, PKN2, SNW1). Our validated RLR pathway list (http://rlr.cmbi.umcn.nl/), obtained using a combination of integrative genomics and experiments, is a new resource for innate antiviral immunity research.

LGP2 synergy with MDA5 in RLR-mediated RNA recognition and antiviral signaling

Mammalian cells have the ability to recognize virus infection and mount a powerful antiviral response. Pattern recognition receptor proteins detect molecular signatures of virus infection and activate antiviral signaling. The RIG-I-like receptor (RLR) proteins are expressed in the cytoplasm of nearly all cells and specifically recognize virus-derived RNA species as a molecular feature discriminating the pathogen from the host. The RLR family is composed of three homologous proteins, RIG-I, MDA5, and LGP2. All RLRs have the ability to detect virus-derived dsRNA and hydrolyze ATP, but display individual differences in enzymatic activity, intrinsic ability to recognize RNA, and mechanisms of activation. Emerging evidence suggests that MDA5 and RIG-I utilize distinct mechanisms to form oligomeric complexes along dsRNA. Aligning of their signaling domains creates a platform capable of propagating and amplifying antiviral signaling responses. LGP2 with intact ATP hydrolysis is critical for the MDA5-mediated antiviral response, but LGP2 lacks the domains essential for activation of antiviral signaling, leaving the role of LGP2 in antiviral signaling unclear. Recent studies revealed a mechanistic basis of synergy between LGP2 and MDA5 leading to enhanced antiviral signaling. This review briefly summarizes the RLR system, and focuses on the relationship between LGP2 and MDA5, describing in detail how these two proteins work together to detect foreign RNA and generate a fully functional antiviral response.

Intracellular detection of viral nucleic acids

Successful clearance of a microbial infection depends on the concerted action of both the innate and adaptive arms of the immune system. Accurate recognition of an invading pathogen is the first and most crucial step in eliciting effective antimicrobial defense mechanisms. In recent years, remarkable progress has been made towards understanding the molecular details of how the innate immune system recognizes microbial signatures, commonly called pathogen-associated molecular patterns (PAMPs). For viral pathogens, nucleic acids-both viral genomes and viral replication products-represent a major class of PAMPs that trigger antiviral host responses via activation of germline-encoded innate immune receptors. Here we summarize recent advances in intracellular innate sensing mechanisms of viral RNA and DNA.

Kinetic discrimination of self/non-self RNA by the ATPase activity of RIG-I and MDA5

BACKGROUND

The cytoplasmic RIG-like receptors are responsible for the early detection of viruses and other intracellular microbes by activating the innate immune response mediated by type I interferons (IFNs). RIG-I and MDA5 detect virus-specific RNA motifs with short 5'-tri/diphosphorylated, blunt-end double-stranded RNA (dsRNA) and >0.5-2 kb long dsRNA as canonical agonists, respectively. However, in vitro, they can bind to many RNA species, while in cells there is an activation threshold. As SF2 helicase/ATPase family members, ATP hydrolysis is dependent on co-operative RNA and ATP binding. Whereas simultaneous ATP and cognate RNA binding is sufficient to activate RIG-I by releasing autoinhibition of the signaling domains, the physiological role of the ATPase activity of RIG-I and MDA5 remains controversial.

RESULTS

A cross-analysis of a rationally designed panel of RNA binding and ATPase mutants and truncated receptors, using type I IFN promoter activation as readout, allows us to refine our understanding of the structure-function relationships of RIG-I and MDA5. RNA activation of RIG-I depends on multiple critical RNA binding sites in its helicase domain as confirmed by functional evidence using novel mutations. We found that RIG-I or MDA5 mutants with low ATP hydrolysis activity exhibit constitutive activity but this was fully reverted when associated with mutations preventing RNA binding to the helicase domain. We propose that the turnover kinetics of the ATPase domain enables the discrimination of self/non-self RNA by both RIG-I and MDA5. Non-cognate, possibly self, RNA binding would lead to fast ATP turnover and RNA disassociation and thus insufficient time for the caspase activation and recruitment domains (CARDs) to promote downstream signaling, whereas tighter cognate RNA binding provides a longer time window for downstream events to be engaged.

CONCLUSIONS

The exquisite fine-tuning of RIG-I and MDA5 RNA-dependent ATPase activity coupled to CARD release allows a robust IFN response from a minor subset of non-self RNAs within a sea of cellular self RNAs. This avoids the eventuality of deleterious autoimmunity effects as have been recently described to arise from natural gain-of-function alleles of RIG-I and MDA5.

Assembly-driven activation of the AIM2 foreign-dsDNA sensor provides a polymerization template for downstream ASC

AIM2 recognizes foreign dsDNA and assembles into the inflammasome, a filamentous supramolecular signalling platform required to launch innate immune responses. We show here that the pyrin domain of AIM2 (AIM2^PYD) drives both filament formation and dsDNA binding. In addition, the dsDNA-binding domain of AIM2 also oligomerizes and assists in filament formation. The ability to oligomerize is critical for binding dsDNA, and in turn permits the size of dsDNA to regulate the assembly of the AIM2 polymers. The AIM2^PYD oligomers define the filamentous structure, and the helical symmetry of the AIM2^PYD filament is consistent with the filament assembled by the PYD of the downstream adaptor ASC. Our results suggest that the role of AIM2^PYD is not autoinhibitory, but generating a structural template by coupling ligand binding and oligomerization is a key signal transduction mechanism in the AIM2 inflammasome.

The AIM2 inflammasome complex is essential for defence against a number of human pathogens but how it assembles upon recognition of foreign DNA remains incompletely understood. Here Morrone et al. suggest the AIM2 pyrin domain acts in both DNA binding and filament assembly to generate a structural template for complex assembly.

Modulation of the immune response to respiratory viruses by vitamin D

Background: Vitamin D deficiency has been shown to be independently associated with increased risk of viral acute respiratory infection (ARI) in a number of observational studies, and meta-analysis of clinical trials of vitamin D supplementation for prevention of ARI has demonstrated protective effects. Several cellular studies have investigated the effects of vitamin D metabolites on immune responses to respiratory viruses, but syntheses of these reports are lacking. Scope: In this article, we review the literature reporting results of in vitro experiments investigating immunomodulatory actions of vitamin D metabolites in human respiratory epithelial cells infected with respiratory viruses. Key findings: Vitamin D metabolites do not consistently influence replication or clearance of rhinovirus, respiratory syncytial virus (RSV) or influenza A virus in human respiratory epithelial cell culture, although they do modulate expression and secretion of type 1 interferon, chemokines including CXCL8 and CXCL10 and pro-inflammatory cytokines, such as TNF and IL-6. Future research: More studies are needed to clarify the effects of vitamin D metabolites on respiratory virus-induced expression of cell surface markers mediating viral entry and bacterial adhesion to respiratory epithelial cells.

dsRNA-activation of TLR3 and RLR signaling: gene induction-dependent and independent effects

Double-stranded (ds) RNA has diverse roles in host defense and disease prevention. dsRNA, produced by viral replication, elicits strong antiviral responses in host; similar protective responses can also be triggered by cellular dsRNA produced by necrotic, apoptotic, or otherwise stressed, uninfected cells. dsRNA is recognized in the cell by a large family of dsRNA-binding proteins, among which are the pattern recognition receptors (PRRs), toll-like receptor 3 (TLR3), and retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs). TLR3 signals from the endosomal membrane where it senses extracellular dsRNA that has been endocytosed, whereas RLRs signal from the cytoplasm using a mitochondrial adaptor protein. In this review, we will summarize the signaling pathways used by these 2 PRRs, which lead to the activation of specific transcription factors and the induction of many proinflammatory and antiviral genes. However, it is becoming increasingly clear that all host responses are not mediated by the products of these induced genes; signal-dependent post-translational modifications of existing proteins can also profoundly change cellular properties. We will discuss how Src activation by TLR3 changes cell migration, adhesion, and proliferation rates and how IRF-3 activation by RLR triggers a gene induction-independent pro-apoptotic pathway that provides strong antiviral protection.

Unc93b1 -Dependent Endosomal Toll-Like Receptor Signaling Regulates Inflammation and Mortality during Coxsackievirus B3 Infection

Coxsackievirus strain B serotype 3 (CVB3)-induced myocarditis is an important human disease that causes permanent tissue damage and can lead to death from acute infection or long-term morbidity caused by chronic inflammation. The timing and magnitude of immune activation following CVB3 infection can mediate a positive host outcome or increase tissue pathology. To better elucidate the role of endosomal Toll-like receptor (TLR) signaling in acute CVB3 infection, we studied mice with a loss-of-function mutation, known as Letr for 'loss of endosomal TLR response', in Unc93b1, which is a chaperone protein for TLR3, TLR7 and TLR9. Using Unc93b1(Letr/)(Letr) mice, we determined that Unc93b1-dependent TLR activation was essential for the survival of acute CVB3-induced myocarditis. We also determined that a lack of endosomal TLR signaling was associated with a higher viral load in target organs and that it increased inflammation, necrosis and fibrosis in cardiac tissue. Loss of Unc93b1 function was also associated with increased cardiac expression of Ifn-b and markers of tissue injury and fibrosis including Lcn2 and Serpina3n early after CVB3 infection. These observations establish a significant role for Unc93b1 in the regulation of the host inflammatory response to CVB3 infection and also reveal potential mediators of host tissue damage that merit further investigation in acute viral myocarditis.

MDA5-filament, dynamics and disease

Melanoma Differentiation-Associated gene 5 (MDA5), encoded by the gene IFIH1, is a cytoplasmic sensor for viral double-stranded RNAs (dsRNAs). MDA5 activates the type I interferon signaling pathway upon detection of long viral dsRNA generated during replication of picornaviruses. Studies have shown that MDA5 forms a filament along the length of dsRNA and utilizes ATP-dependent filament dynamics to discriminate between self versus non-self on the basis of dsRNA length. This review summarizes our current understanding of how the MDA5 filament assembles and disassembles, how this filament dynamics are utilized in dsRNA length-dependent signaling, and how dysregulated filament dynamics lead to pathogenesis of immune disorders.

Cytoplasmic sensing of viral nucleic acids

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Viral nucleic acids (NAs) are targeted by cellular proteins with diverse functions.

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NA sensing proteins are forming a three-layered defence system.

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NA localisation and modifications synergistically activate defence systems.

Viruses are the most abundant pathogens on earth. A fine-tuned framework of intervening pathways is in place in mammalian cells to orchestrate the cellular defence against these pathogens. Key for this system is sensor proteins that recognise specific features associated with nucleic acids of incoming viruses. Here we review the current knowledge on cytoplasmic sensors for viral nucleic acids. These sensors induce expression of cytokines, affect cellular functions required for virus replication and directly target viral nucleic acids through degradation or sequestration. Their ability to respond to a given nucleic acid is based on both the differential specificity of the individual proteins and the downstream signalling or adaptor proteins. The cooperation of these multiple proteins and pathways plays a key role in inducing successful immunity against virus infections.

Sensing viral RNAs by Dicer/RIG-I like ATPases across species

Induction of antiviral immunity in vertebrates and invertebrates relies on members of the RIG-I-like receptor and Dicer families, respectively. Although these proteins have different size and domain composition, members of both families share a conserved DECH-box helicase domain. This helicase, also known as a duplex RNA activated ATPase, or DRA domain, plays an important role in viral RNA sensing. Crystallographic and electron microscopy studies of the RIG-I and Dicer DRA domains indicate a common structure and that similar conformational changes are induced by dsRNA binding. Genetic and biochemical studies on the function and regulation of DRAs reveal similarities, but also some differences, between viral RNA sensing mechanisms in nematodes, flies and mammals.

International Union of Basic and Clinical Pharmacology. XCVI. Pattern recognition receptors in health and disease

Since the discovery of Toll, in the fruit fly Drosophila melanogaster, as the first described pattern recognition receptor (PRR) in 1996, many families of these receptors have been discovered and characterized. PRRs play critically important roles in pathogen recognition to initiate innate immune responses that ultimately link to the generation of adaptive immunity. Activation of PRRs leads to the induction of immune and inflammatory genes, including proinflammatory cytokines and chemokines. It is increasingly clear that many PRRs are linked to a range of inflammatory, infectious, immune, and chronic degenerative diseases. Several drugs to modulate PRR activity are already in clinical trials and many more are likely to appear in the near future. Here, we review the different families of mammalian PRRs, the ligands they recognize, the mechanisms of activation, their role in disease, and the potential of targeting these proteins to develop the anti-inflammatory therapeutics of the future.

Inflammatory responses to infection: the Dutch contribution

At any given moment, our body is under attack by a large variety of pathogens, which aim to enter and use our body to propagate and disseminate. The extensive cellular and molecular complexity of our immune system enables us to efficiently eliminate invading pathogens or at least develop a condition in which propagation of the microorganism is reduced to a minimum. Yet, the evolutionary pressure on pathogens to circumvent our immune defense mechanisms is immense, which continuously leads to the development of novel pathogenic strains that challenge the health of mankind. Understanding this battle between pathogen and the immune system has been a fruitful area of immunological research over the last century and will continue to do so for many years. In this review, which has been written on the occasion of the 50th anniversary of the Dutch Society for Immunology, we provide an overview of the major contributions that Dutch immunologists and infection biologists have made in the last decades on the inflammatory response to viral, bacterial, fungal or parasitic infections. We focus on those studies that have addressed both the host and the pathogen, as these are most interesting from an immunological point of view. Although it is not possible to completely cover this comprehensive research field, this review does provide an interesting overview of Dutch research on inflammatory responses to infection.

Regulation of RIG-I-like receptor signaling by host and viral proteins

Vertebrate innate immunity is characterized by an effective immune surveillance apparatus, evolved to sense foreign structures, such as proteins or nucleic acids of invading microbes. RIG-I-like receptors (RLRs) are key sensors of viral RNA species in the host cell cytoplasm. Activation of RLRs in response to viral RNA triggers an antiviral defense program through the production of hundreds of antiviral effector proteins including cytokines, chemokines, and host restriction factors that directly interfere with distinct steps in the virus life cycle. To avoid premature or abnormal antiviral and proinflammatory responses, which could have harmful consequences for the host, the signaling activities of RLRs and their common adaptor molecule, MAVS, are delicately controlled by cell-intrinsic regulatory mechanisms. Furthermore, viruses have evolved multiple strategies to modulate RLR-MAVS signal transduction to escape from immune surveillance. Here, we summarize recent progress in our understanding of the regulation of RLR signaling through host factors and viral antagonistic proteins.

Activation and regulation of pathogen sensor RIG-I

Cells are equipped with a large set of pattern recognition receptors or sensors that detect foreign molecules such as pathogenic nucleic acids and initiate proinflammatory and antimicrobial innate immune responses. RIG-I is a cytosolic sensor that detects 5'-triphosphate double-stranded RNAs produced during infection. RIG-I is responsible for mounting an antimicrobial response against a variety of viruses and intracellular bacteria. RIG-I contains an intricate structural architecture that allows for efficient signaling downstream in the pathway and autoregulation. The RIG-I-mediated antimicrobial pathway is highly regulated in cells requiring various cofactors, negative regulators, and posttranslational modifications. Modulation of RIG-I and RIG-I-mediated signaling in cells by pathogens to evade recognition and activation of the antimicrobial pathway highlights the essential nature of RIG-I in the innate immune response.

The innate immune sensor LGP2 activates antiviral signaling by regulating MDA5-RNA interaction and filament assembly

Cytoplasmic pattern recognition receptors detect non-self RNAs during virus infections and initiate antiviral signaling. One receptor, MDA5, possesses essential signaling domains, but weak RNA binding. A second receptor, LGP2, rapidly detects diverse dsRNA species, but lacks signaling domains. Accumulating evidence suggests LGP2 and MDA5 work together to detect viral RNA and generate a complete antiviral response, but the basis for their cooperation has been elusive. Experiments presented here address this gap in antiviral signaling, revealing that LGP2 assists MDA5-RNA interactions leading to enhanced MDA5-mediated antiviral signaling. LGP2 increases the initial rate of MDA5-RNA interaction and regulates MDA5 filament assembly, resulting in the formation of more numerous, shorter MDA5 filaments that are shown to generate equivalent or greater signaling activity in vivo than the longer filaments containing only MDA5. These findings provide a mechanism for LGP2 coactivation of MDA5 and a biological context for MDA5-RNA filaments in antiviral responses.

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.

Antiviral RNA recognition and assembly by RLR family innate immune sensors

Virus-encoded molecular signatures, such as cytosolic double-stranded or otherwise biochemically distinct RNA species, trigger cellular antiviral signaling. Cytoplasmic proteins recognize these non-self RNAs and activate signal transduction pathways that drive the expression of virus-induced genes, including the primary antiviral cytokine, IFNβ, and diverse direct and indirect antiviral effectors. One important group of cytosolic RNA sensors known as the RIG-I-like receptors (RLRs) is comprised of three proteins that are similar in structure and function. The RLR proteins, RIG-I, MDA5, and LGP2, share the ability to recognize nucleic acid signatures produced by virus infections and activate antiviral signaling. Emerging evidence indicates that RNA detection by RLRs culminates in the assembly of dynamic multimeric ribonucleoprotein (RNP) complexes. These RNPs can act as signaling platforms that are capable of propagating and amplifying antiviral signaling responses. Despite their common domain structures and similar abilities to induce antiviral responses, the RLRs differ in their enzymatic properties, their intrinsic abilities to recognize RNA, and their ability to assemble into filamentous complexes. This molecular specialization has enabled the RLRs to recognize and respond to diverse virus infections, and to mediate both unique and overlapping functions in immune regulation.

Sensing viral invasion by RIG-I like receptors

Cellular responses to pathogen invasion are crucial for maintaining cell homeostasis and survival. The interferon (IFN) system is one of the most effective cellular responses to viral intrusion in mammals. Viral recognition by innate immune sensors activates the antiviral IFN system. Retinoic acid-inducible gene I (RIG-I) like receptors (RLRs) are DExD/H box RNA helicases that sense viral invasion. RLRs recognize cytoplasmic viral RNAs and trigger antiviral responses, resulting in production of type I IFN and inflammatory cytokines. Unique and common sensing mechanisms among RLRs have been reported. In this review, recent progress in the understanding of antiviral responses by RLRs is summarized and discussed.

MDA5 and LGP2: accomplices and antagonists of antiviral signal transduction

Mammalian cells have the ability to recognize virus infection and mount a powerful antiviral transcriptional response that provides an initial barrier to replication and impacts both innate and adaptive immune responses. Retinoic acid-inducible gene I (RIG-I)-like receptor (RLR) proteins mediate intracellular virus recognition and are activated by viral RNA ligands to induce antiviral signal transduction. While the mechanisms of RIG-I regulation are already well understood, less is known about the more enigmatic melanoma differentiation-associated 5 (MDA5) and laboratory of genetics and physiology 2 (LGP2). Emerging evidence suggests that these two RLRs are intimately associated as both accomplices and antagonists of antiviral signal transduction.

c-FLIP-Short reduces type I interferon production and increases viremia with coxsackievirus B3

Cellular FLIP (c-FLIP) is an enzymatically inactive paralogue of caspase-8 and as such can block death receptor-induced apoptosis. However, independent of death receptors, c-FLIP-Long (c-FLIP_L) can heterodimerize with and activate caspase-8. This is critical for promoting the growth and survival of T lymphocytes as well as the regulation of the RIG-I helicase pathway for type I interferon production in response to viral infections. Truncated forms of FLIP also exist in mammalian cells (c-FLIP_S) and certain viruses (v-FLIP), which lack the C-terminal domain that activates caspase-8. Thus, the ratio of c-FLIP_L to these short forms of FLIP may greatly influence the outcome of an immune response. We examined this model in mice transgenically expressing c-FLIP_S in T cells during infection with Coxsackievirus B3 (CVB3). In contrast to our earlier findings of reduced myocarditis and mortality with CVB3 infection of c-FLIP_L-transgenic mice, c-FLIP_S-transgenic mice were highly sensitive to CVB3 infection as manifested by increased cardiac virus titers, myocarditis score, and mortality compared to wild-type C57BL/6 mice. This observation was paralleled by a reduction in serum levels of IL-10 and IFN-α in CVB3-infected c-FLIP_S mice. In vitro infection of c-FLIP_S T cells with CVB3 confirmed these results. Furthermore, molecular studies revealed that following infection of cells with CVB3, c-FLIP_L associates with mitochondrial antiviral signaling protein (MAVS), increases caspase-8 activity and type I IFN production, and reduces viral replication, whereas c-FLIP_S promotes the opposite phenotype.

Coxsackievirus cloverleaf RNA containing a 5' triphosphate triggers an antiviral response via RIG-I activation

Upon viral infections, pattern recognition receptors (PRRs) recognize pathogen-associated molecular patterns (PAMPs) and stimulate an antiviral state associated with the production of type I interferons (IFNs) and inflammatory markers. Type I IFNs play crucial roles in innate antiviral responses by inducing expression of interferon-stimulated genes and by activating components of the adaptive immune system. Although pegylated IFNs have been used to treat hepatitis B and C virus infections for decades, they exert substantial side effects that limit their use. Current efforts are directed toward the use of PRR agonists as an alternative approach to elicit host antiviral responses in a manner similar to that achieved in a natural infection. RIG-I is a cytosolic PRR that recognizes 5' triphosphate (5'ppp)-containing RNA ligands. Due to its ubiquitous expression profile, induction of the RIG-I pathway provides a promising platform for the development of novel antiviral agents and vaccine adjuvants. In this study, we investigated whether structured RNA elements in the genome of coxsackievirus B3 (CVB3), a picornavirus that is recognized by MDA5 during infection, could activate RIG-I when supplied with 5'ppp. We show here that a 5'ppp-containing cloverleaf (CL) RNA structure is a potent RIG-I inducer that elicits an extensive antiviral response that includes induction of classical interferon-stimulated genes, as well as type III IFNs and proinflammatory cytokines and chemokines. In addition, we show that prophylactic treatment with CVB3 CL provides protection against various viral infections including dengue virus, vesicular stomatitis virus and enterovirus 71, demonstrating the antiviral efficacy of this RNA ligand.

Hijacking of RIG-I signaling proteins into virus-induced cytoplasmic structures correlates with the inhibition of type I interferon responses

Recognition of viral pathogens by the retinoic acid-inducible gene I (RIG-I)-like receptor (RLR) family results in the activation of type I interferon (IFN) responses. To avoid this response, most viruses have evolved strategies that target different essential steps in the activation of host innate immunity. In this study, we report that the nonstructural protein NSs of the newly described severe fever with thrombocytopenia syndrome virus (SFTSV) is a potent inhibitor of IFN responses. The SFTSV NSs protein was found to inhibit the activation of the beta interferon (IFN-β) promoter induced by viral infection and by a RIG-I ligand. Astonishingly, we found that SFTSV NSs interacts with and relocalizes RIG-I, the E3 ubiquitin ligase TRIM25, and TANK-binding kinase 1 (TBK1) into SFTSV NSs-induced cytoplasmic structures. Interestingly, formation of these SFTSV NSs-induced structures occurred in the absence of the Atg7 gene, a gene essential for autophagy. Furthermore, confocal microscopy studies revealed that these SFTSV NSs-induced structures colocalize with Rab5 but not with Golgi apparatus or endoplasmic reticulum markers. Altogether, the data suggest that sequestration of RIG-I signaling molecules into endosome-like structures may be the mechanism used by SFTSV to inhibit IFN responses and point toward a novel mechanism for the suppression of IFN responses.

IMPORTANCE

The mechanism by which the newly described SFTSV inhibits host antiviral responses has not yet been fully characterized. In this study, we describe the redistribution of RIG-I signaling components into virus-induced cytoplasmic structures in cells infected with SFTSV. This redistribution correlates with the inhibition of host antiviral responses. Further characterization of the interplay between the viral protein and components of the IFN responses could potentially provide targets for the rational development of therapeutic interventions.

In vivo ligands of MDA5 and RIG-I in measles virus-infected cells

RIG-I-like receptors (RLRs: RIG-I, MDA5 and LGP2) play a major role in the innate immune response against viral infections and detect patterns on viral RNA molecules that are typically absent from host RNA. Upon RNA binding, RLRs trigger a complex downstream signaling cascade resulting in the expression of type I interferons and proinflammatory cytokines. In the past decade extensive efforts were made to elucidate the nature of putative RLR ligands. In vitro and transfection studies identified 5'-triphosphate containing blunt-ended double-strand RNAs as potent RIG-I inducers and these findings were confirmed by next-generation sequencing of RIG-I associated RNAs from virus-infected cells. The nature of RNA ligands of MDA5 is less clear. Several studies suggest that double-stranded RNAs are the preferred agonists for the protein. However, the exact nature of physiological MDA5 ligands from virus-infected cells needs to be elucidated. In this work, we combine a crosslinking technique with next-generation sequencing in order to shed light on MDA5-associated RNAs from human cells infected with measles virus. Our findings suggest that RIG-I and MDA5 associate with AU-rich RNA species originating from the mRNA of the measles virus L gene. Corresponding sequences are poorer activators of ATP-hydrolysis by MDA5 in vitro, suggesting that they result in more stable MDA5 filaments. These data provide a possible model of how AU-rich sequences could activate type I interferon signaling.

Identification of an LGP2-associated MDA5 agonist in picornavirus-infected cells

The RIG-I-like receptors RIG-I, LGP2, and MDA5 initiate an antiviral response that includes production of type I interferons (IFNs). The nature of the RNAs that trigger MDA5 activation in infected cells remains unclear. Here, we purify and characterise LGP2/RNA complexes from cells infected with encephalomyocarditis virus (EMCV), a picornavirus detected by MDA5 and LGP2 but not RIG-I. We show that those complexes contain RNA that is highly enriched for MDA5-stimulatory activity and for a specific sequence corresponding to the L region of the EMCV antisense RNA. Synthesis of this sequence by in vitro transcription is sufficient to generate an MDA5 stimulatory RNA. Conversely, genomic deletion of the L region in EMCV generates viruses that are less potent at stimulating MDA5-dependent IFN production. Thus, the L region antisense RNA of EMCV is a key determinant of innate immunity to the virus and represents an RNA that activates MDA5 in virally-infected cells. DOI: http://dx.doi.org/10.7554/eLife.01535.001.

The influence of CpG and UpA dinucleotide frequencies on RNA virus replication and characterization of the innate cellular pathways underlying virus attenuation and enhanced replication

Most RNA viruses infecting mammals and other vertebrates show profound suppression of CpG and UpA dinucleotide frequencies. To investigate this functionally, mutants of the picornavirus, echovirus 7 (E7), were constructed with altered CpG and UpA compositions in two 1.1–1.3 Kbase regions. Those with increased frequencies of CpG and UpA showed impaired replication kinetics and higher RNA/infectivity ratios compared with wild-type virus. Remarkably, mutants with CpGs and UpAs removed showed enhanced replication, larger plaques and rapidly outcompeted wild-type virus on co-infections. Luciferase-expressing E7 sub-genomic replicons with CpGs and UpAs removed from the reporter gene showed 100-fold greater luminescence. E7 and mutants were equivalently sensitive to exogenously added interferon-β, showed no evidence for differential recognition by ADAR1 or pattern recognition receptors RIG-I, MDA5 or PKR. However, kinase inhibitors roscovitine and C16 partially or entirely reversed the attenuated phenotype of high CpG and UpA mutants, potentially through inhibition of currently uncharacterized pattern recognition receptors that respond to RNA composition. Generating viruses with enhanced replication kinetics has applications in vaccine production and reporter gene construction. More fundamentally, the findings introduce a new evolutionary paradigm where dinucleotide composition of viral genomes is subjected to selection pressures independently of coding capacity and profoundly influences host–pathogen interactions.

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.

Sensing microbial RNA in the cytosol

The innate immune system faces the difficult task of keeping a fine balance between sensitive detection of microbial presence and avoidance of autoimmunity. To this aim, key mechanisms of innate responses rely on isolation of pathogens in specialized subcellular compartments, or sensing of specific microbial patterns absent from the host. Efficient detection of foreign RNA in the cytosol requires an additional layer of complexity from the immune system. In this particular case, innate sensors should be able to distinguish self and non-self molecules that share several similar properties. In this review, we discuss this interplay between cytosolic pattern recognition receptors and the microbial RNA they detect. We describe how microbial RNAs gain access to the cytosol, which receptors they activate and counter-strategies developed by microorganisms to avoid this response.

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.

Activation and evasion of antiviral innate immunity by hepatitis C virus

Hepatitis C virus (HCV) chronically infects 130-170 million people worldwide and is a major public health burden. HCV is an RNA virus that infects hepatocytes within liver, and this infection is sensed as non-self by the intracellular innate immune response to program antiviral immunity to HCV. HCV encodes several strategies to evade this antiviral response, and this evasion of innate immunity plays a key role in determining viral persistence. This review discusses the molecular mechanisms of how the intracellular innate immune system detects HCV infection, including how HCV pathogen-associated molecular patterns are generated during infection and where they are recognized as foreign by the innate immune system. Further, this review highlights the key innate immune evasion strategies used by HCV to establish persistent infection within the liver, as well as how host genotype influences the outcome of HCV infection. Understanding these HCV-host interactions is key in understanding how to target HCV during infection and for the design of more effective HCV therapies at the immunological level.

RIG-I forms signaling-competent filaments in an ATP-dependent, ubiquitin-independent manner

Retinoic acid-inducible gene 1 (RIG-I) and melanoma differentiation-associated protein 5 (MDA5) are paralogous receptors for viral double-stranded RNA (dsRNA) with divergent specificity. We have previously shown that MDA5 forms filaments upon viral dsRNA recognition and that this filament formation is essential for interferon signal activation. Here, we show that while RIG-I binds to a dsRNA end as a monomer in the absence of ATP, it assembles in the presence of ATP into a filament that propagates from the dsRNA end to the interior. Furthermore, RIG-I filaments directly stimulate mitochondrial antiviral signaling (MAVS) filament formation without any cofactor, such as polyubiquitin chains, and forced juxtaposition of the isolated signaling domain of RIG-I, as it would be in the filament, is sufficient to activate interferon signaling. Our findings thus define filamentous architecture as a common yet versatile molecular platform for divergent viral RNA detection and proximity-induced signal activation by RIG-I and MDA5.

Small RNAs tackle large viruses: RNA interference-based antiviral defense against DNA viruses in insects

The antiviral RNA interference (RNAi) pathway processes viral double-stranded RNA (dsRNA) into viral small interfering RNAs (vsiRNA) that guide the recognition and cleavage of complementary viral target RNAs. In RNA virus infections, viral replication intermediates, dsRNA genomes or viral structured RNAs have been implicated as Dicer-2 substrates. In a recent publication, we demonstrated that a double-stranded DNA virus, Invertebrate iridescent virus 6, is a target of the Drosophila RNAi machinery, and we proposed that overlapping converging transcripts base pair to form the dsRNA substrates for vsiRNA biogenesis. Here, we discuss the role of RNAi in antiviral defense to DNA viruses in Drosophila and other invertebrate model systems.

Encephalomyocarditis virus disrupts stress granules, the critical platform for triggering antiviral innate immune responses

In response to stress, cells induce ribonucleoprotein aggregates, termed stress granules (SGs). SGs are transient loci containing translation-stalled mRNA, which is eventually degraded or recycled for translation. Infection of some viruses, including influenza A virus with a deletion of nonstructural protein 1 (IAVΔNS1), induces SG-like protein aggregates. Previously, we showed that IAVΔNS1-induced SGs are required for efficient induction of type I interferon (IFN). Here, we investigated SG formation by different viruses using green fluorescent protein (GFP)-tagged Ras-Gap SH3 domain binding protein 1 (GFP-G3BP1) as an SG probe. HeLa cells stably expressing GFP-G3BP1 were infected with different viruses, and GFP fluorescence was monitored live with time-lapse microscopy. SG formations by different viruses was classified into 4 different patterns: no SG formation, stable SG formation, transient SG formation, and alternate SG formation. We focused on encephalomyocarditis virus (EMCV) infection, which exhibited transient SG formation. We found that EMCV disrupts SGs by cleavage of G3BP1 at late stages of infection (>8 h) through a mechanism similar to that used by poliovirus. Expression of a G3BP1 mutant that is resistant to the cleavage conferred persistent formation of SGs as well as an enhanced induction of IFN and other cytokines at late stages of infection. Additionally, knockdown of endogenous G3BP1 blocked SG formation with an attenuated induction of IFN and potentiated viral replication. Taken together, our findings suggest a critical role of SGs as an antiviral platform and shed light on one of the mechanisms by which a virus interferes with host stress and subsequent antiviral responses.

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.

Cytosolic sensing of viruses

Cells are equipped with mechanisms that allow them to rapidly detect and respond to viruses. These defense mechanisms rely partly on receptors that monitor the cytosol for the presence of atypical nucleic acids associated with virus infection. RIG-I-like receptors detect RNA molecules that are absent from the uninfected host. DNA receptors alert the cell to the abnormal presence of that nucleic acid in the cytosol. Signaling by RNA and DNA receptors results in the induction of restriction factors that prevent virus replication and establish cell-intrinsic antiviral immunity. In light of these formidable obstacles, viruses have evolved mechanisms of evasion, masking nucleic acid structures recognized by the host, sequestering themselves away from the cytosol or targeting host sensors, and signaling adaptors for deactivation or degradation. Here, we detail recent advances in the molecular understanding of cytosolic nucleic acid detection and its evasion by viruses.

MDA5 localizes to stress granules, but this localization is not required for the induction of type I interferon

Virus infection can initiate a type I interferon (IFN-α/β) response via activation of the cytosolic RNA sensors retinoic acid-inducible gene-I (RIG-I) and melanoma differentiation-associated gene 5 (MDA5). Furthermore, it can activate kinases that phosphorylate eukaryotic translation initiation factor 2α (eIF2α), which leads to inhibition of (viral) protein translation and formation of stress granules (SG). Most viruses have evolved mechanisms to suppress these cellular responses. Here, we show that a mutant mengovirus expressing an inactive leader (L) protein, which we have previously shown to be unable to suppress IFN-α/β, triggered SG formation in a protein kinase R (PKR)-dependent manner. Furthermore, we show that infection of cells that are defective in SG formation yielded higher viral RNA levels, suggesting that SG formation acts as an antiviral defense mechanism. Since the induction of both IFN-α/β and SG is suppressed by mengovirus L, we set out to investigate a potential link between these pathways. We observed that MDA5, the intracellular RNA sensor that recognizes picornaviruses, localized to SG. However, activation of the MDA5 signaling pathway did not trigger and was not required for SG formation. Moreover, cells that were unable to form SG-by protein kinase R (PKR) depletion, using cells expressing a nonphosphorylatable eIF2α protein, or by drug treatment that inhibits SG formation-displayed a normal IFN-α/β response. Thus, although MDA5 localizes to SG, this localization seems to be dispensable for induction of the IFN-α/β pathway.

[New aspects on the pathogenesis of myositis]

Idiopathic inflammatory myopathies (IIM) are chronic inflammatory diseases of muscle characterized by proximal muscle weakness. There are three main groups of diseases, dermatomyositis, polymyositis and inclusion body myositis. The muscle tissue is invaded by the humoral autoantibody producing immune system (B-cells) and by the cellular immune system with autoaggressive and inflammation modulating cells (e.g. dendritic cells, monocytes/macrophages, CD4 + and CD8 + T-cells and natural killer cells). The presence of specific or associated autoantibodies and inflammatory cellular infiltrates with cytotoxic and immune autoreactive properties are characteristic for IIM diseases. The pathogenesis is still unknown; nevertheless, there are several hints that exogenic factors might be involved in initiation and disease progression and bacterial, fungal and viral infections are thought to be possible initiators. Up to now information on prognostic markers to help with decision-making for individual treatment are limited. In addition, there has been only limited therapeutic success including conventional or novel drugs and biologicals and comparative validation studies are needed using similar outcome measurements. Moreover, to facilitate the use and development of novel therapies, elaboration of intracellular and cell-specific regulation could be useful to understand the etiopathogenesis and allow a better diagnosis, prognosis and possibly also a prediction for individualized subgroup treatment.