Murine coronavirus induces type I interferon in oligodendrocytes through recognition by RIG-I and MDA5

Pangolins Lack IFIH1/MDA5, a Cytoplasmic RNA Sensor That Initiates Innate Immune Defense Upon Coronavirus Infection

Zoonotic infections are an imminent threat to human health. Pangolins were recently identified as carriers and intermediate hosts of coronaviruses. Previous research has shown that infection with coronaviruses activates an innate immune response upon sensing of viral RNA by interferon-induced with helicase C domain 1 (IFIH1), also known as MDA5. Here, we performed a comparative genomics study of RNA sensor genes in three species of pangolins. DDX58/RIG-I, a sensor of cytoplasmic viral RNA and toll-like receptors (TLR) 3, 7, and 8, which bind RNA in endosomes, are conserved in pangolins. By contrast, IFIH1 a sensor of intracellular double-stranded RNA, has been inactivated by mutations in pangolins. Likewise, Z-DNA-binding protein (ZBP1), which senses both Z-DNA and Z-RNA, has been lost during the evolution of pangolins. These results suggest that the innate immune response to viruses differs significantly between pangolins and other mammals, including humans. We put forward the hypothesis that loss of IFIH1 and ZBP1 provided an evolutionary advantage by reducing inflammation-induced damage to host tissues and thereby contributed to a switch from resistance to tolerance of viral infections in pangolins.

RIG-I-like receptors: their regulation and roles in RNA sensing

Retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs) are key sensors of virus infection, mediating the transcriptional induction of type I interferons and other genes that collectively establish an antiviral host response. Recent studies have revealed that both viral and host-derived RNAs can trigger RLR activation; this can lead to an effective antiviral response but also immunopathology if RLR activities are uncontrolled. In this Review, we discuss recent advances in our understanding of the types of RNA sensed by RLRs in the contexts of viral infection, malignancies and autoimmune diseases. We further describe how the activity of RLRs is controlled by host regulatory mechanisms, including RLR-interacting proteins, post-translational modifications and non-coding RNAs. Finally, we discuss key outstanding questions in the RLR field, including how our knowledge of RLR biology could be translated into new therapeutics.

The RNA-sensing retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs) are important inducers of type I interferons and other antiviral immune mediators. Here, Jan Rehwinkel and Michaela Gack explain how members of the RLR family are regulated and reflect on the importance of the RLRs in viral infection, autoimmunity and cancer.

Expression Pattern Analysis of Antiviral Genes and Inflammatory Cytokines in PEDV-Infected Porcine Intestinal Epithelial Cells

Porcine diarrhea disease in newborn and suckling piglets due to infection with porcine epidemic diarrhea virus (PEDV) is a leading cause of economic loss in the pig industry globally. In this study, we investigated the molecular mechanism of the host innate immune response to PEDV infection. The expression dynamics of antiviral genes (e.g., RIG-1, PKR, OAS1, Mx1, and Mx2) and inflammatory cytokines (e.g., IFN-α, IFN-β, TNF-α, IL-6, IL-8, and IL-12) in porcine small intestinal epithelial (IPEC-J2) cells were analyzed following PEDV stimulation. The results showed that the expression of antiviral genes (e.g., PKR, OAS1, and Mx2) and inflammatory cytokines (e.g., IFN-α and TNF-α) were significantly reduced within 0–4 h post-infection (P < 0.05). However, all antiviral genes and inflammatory cytokines were up-regulated from 12 to 24 h (P < 0.05), and cytopathic changes were observed during this time. The expression of RIG-1, PKR, OAS1, Mx1, and Mx2 were significantly and positively correlated to each other during the entire infection (P < 0.01). The results suggested that the RIG-1, PKR, OAS1, Mx1, and Mx2 genes may play an important role in PEDV infection in piglets. Initially, PEDV displayed cellular invasion by inhibiting IFN-α transcription and interfering with the antiviral function of PKR, OAS1, and Mx2, ultimately induced an intense inflammatory response. The relationship between antiviral genes and inflammatory cytokines with PEDV infection at the cellular level provides a reference for studying the mechanism of resistance to PEDV infection in piglets.

Activation of C-Type Lectin Receptor and (RIG)-I-Like Receptors Contributes to Proinflammatory Response in Middle East Respiratory Syndrome Coronavirus-Infected Macrophages

BACKGROUND

Human infection with Middle East respiratory syndrome coronavirus (MERS-CoV) poses an ongoing threat to public health worldwide. The studies of MERS patients with severe disease and experimentally infected animals showed that robust viral replication and intensive proinflammatory response in lung tissues contribute to high pathogenicity of MERS-CoV. We sought to identify pattern recognition receptor (PRR) signaling pathway(s) that mediates the inflammatory cascade in human macrophages upon MERS-CoV infection.

METHODS

The potential signaling pathways were manipulated individually by pharmacological inhibition, small interfering ribonucleic acid (siRNA) depletion, and antibody blocking. The MERS-CoV-induced proinflammatory response was evaluated by measuring the expression levels of key cytokines and/or chemokines. Reverse transcription-quantitative polymerase chain reaction assay, flow cytometry analysis, and Western blotting were applied to evaluate the activation of related PRRs and engagement of adaptors.

RESULTS

MERS-CoV replication significantly upregulated C-type lectin receptor (CLR) macrophage-inducible Ca2+-dependent lectin receptor (Mincle). The role of Mincle for MERS-CoV-triggered cytokine/chemokine induction was established based on the results of antibody blockage, siRNA depletion of Mincle and its adaptor spleen tyrosine kinase (Syk), and Syk pharmacological inhibition. The cytokine and/or chemokine induction was significantly attenuated by siRNA depletion of retinoic acid-inducible-I-like receptors (RLR) or adaptor, indicating that RLR signaling also contributed to MERS-CoV-induced proinflammatory response.

CONCLUSIONS

The CLR and RLR pathways are activated and contribute to the proinflammatory response in MERS-CoV-infected macrophages.

Transmissible Gastroenteritis Virus Infection Up-Regulates FcRn Expression via Nucleocapsid Protein and Secretion of TGF-β in Porcine Intestinal Epithelial Cells

Transmissible gastroenteritis virus (TGEV) is a porcine intestinal coronavirus that causes fatal severe watery diarrhea in piglets. The neonatal Fc receptor (FcRn) is the only IgG transport receptor, its expression on mucosal surfaces is triggered upon viral stimulation, which significantly enhances mucosal immunity. We utilized TGEV as a model pathogen to explore the role of FcRn in resisting viral invasion in overall intestinal mucosal immunity. TGEV induced FcRn expression by activating NF-κB signaling in porcine small intestinal epithelial (IPEC-J2) cells, however, the underlying mechanisms are unclear. First, using small interfering RNAs, we found that TGEV up-regulated FcRn expression via TLR3, TLR9 and RIG-I. Moreover, TGEV induced IL-1β, IL-6, IL-8, TGF-β, and TNF-α production. TGF-β-stimulated IPEC-J2 cells highly up-regulated FcRn expression, while treatment with a JNK-specific inhibitor down-regulated the expression. TGEV nucleocapsid (N) protein also enhanced FcRn promoter activity via the NF-κB signaling pathway and its central region (aa 128–252) was essential for FcRn activation. Additionally, N protein-mediated FcRn up-regulation promotes IgG transcytosis. Thus, TGEV N protein and TGF-β up-regulated FcRn expression, further clarifying the molecular mechanism of up-regulation of FcRn expression by TGEV.

Swine acute diarrhea syndrome coronavirus (SADS-CoV) antagonizes interferon-β production via blocking IPS-1 and RIG-I

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SADS-CoV failed to induce IFN-β expression and inhibited poly (I:C)-or SEV-mediated IFN-β production in IPEC-J2 cells.

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SADS-CoV interrupted poly (I:C)-induced phosphorylation and nuclear translocation of IRF3 and NF-κB.

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SADS-CoV failed to block IRF3, TBK1 and IKKε activity.

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SADS-CoV impeded IFN-β induction mediated by IPS-1 and RIG-I.

Swine acute diarrhea syndrome coronavirus (SADS-CoV), a newly emerging enteric coronavirus, is considered to be associated with swine acute diarrhea syndrome (SADS) which has caused significantly economic losses to the porcine industry. Interactions between SADS-CoV and the host innate immune response is unclear yet. In this study, we used IPEC-J2 cells as a model to explore potential evasion strategies employed by SADS-CoV. Our results showed that SADS-CoV infection failed to induce IFN-β production, and inhibited poly (I:C) and Sendai virus (SeV)-triggered IFN-β expression. SADS-CoV also blocked poly (I:C)-induced phosphorylation and nuclear translocation of IRF-3 and NF-κB. Furthermore, SADS-CoV did not interfere with the activity of IFN-β promoter stimulated by IRF3, TBK1 and IKKε, but counteracted its activation induced by IPS-1 and RIG-I. Collectively, this study is the first investigation that shows interactions between SADS-CoV and the host innate immunity, which provides information of the molecular mechanisms underlying SASD-CoV infection.

Novel Functions of IFI44L as a Feedback Regulator of Host Antiviral Responses

We describe a novel function for the interferon (IFN)-induced protein 44-like (IFI44L) gene in negatively modulating innate immune responses induced after virus infections. Furthermore, we show that decreasing IFI44L expression impairs virus production and that IFI44L expression negatively modulates the antiviral state induced by an analog of double-stranded RNA (dsRNA) or by IFN treatment. The mechanism likely involves the interaction of IFI44L with cellular FK506-binding protein 5 (FKBP5), which in turn interacts with kinases essential for type I and III IFN responses, such as inhibitor of nuclear factor kappa B (IκB) kinase alpha (IKKα), IKKβ, and IKKε. Consequently, binding of IFI44L to FKBP5 decreased interferon regulatory factor 3 (IRF-3)-mediated and nuclear factor kappa-B (NF-κB) inhibitor (IκBα)-mediated phosphorylation by IKKε and IKKβ, respectively. According to these results, IFI44L is a good target for treatment of diseases associated with excessive IFN levels and/or proinflammatory responses and for reduction of viral replication.IMPORTANCE Excessive innate immune responses can be deleterious for the host, and therefore, negative feedback is needed. Here, we describe a completely novel function for IFI44L in negatively modulating innate immune responses induced after virus infections. In addition, we show that decreasing IFI44L expression impairs virus production and that IFI44L expression negatively modulates the antiviral state induced by an analog of dsRNA or by IFN treatment. IFI44L binds to the cellular protein FKBP5, which in turn interacts with kinases essential for type I and III IFN induction and signaling, such as the kinases IKKα, IKKβ, and IKKε. IFI44L binding to FKBP5 decreased the phosphorylation of IRF-3 and IκBα mediated by IKKε and IKKβ, respectively, providing an explanation for the function of IFI44L in negatively modulating IFN responses. Therefore, IFI44L is a candidate target for reducing virus replication.

Porcine transmissible gastroenteritis virus inhibits NF-κB activity via nonstructural protein 3 to evade host immune system

Background

Transmissible gastroenteritis virus (TGEV), a member of the family Coronaviridae, causes lethal watery diarrhea in piglets. Previous studies have revealed that the coronaviruses develop various strategies to evade the host innate immunity through the inhibition of nuclear factor kappa B (NF-κB) signaling pathway. However, the ability of TGEV to inhibit the host innate immune response by modulating the NF-κB signaling pathway is not clear.

Methods

In this study, a dual luciferase reporter assay was used to confirm the inhibition of NF-κB by TGEV infection and to identify the major viral proteins involved in the inhibition of NF-κB signaling. Real-time quantitative PCR was used to quantify the mRNA expression of inflammatory factors. The deubiquitination of Nsp3 domains and its effect on IκBα and p65 were analyzed by western blotting. The ubiquitination level of IκBα was analyzed by immunoprecipitation.

Results

In ST and IPEC-J2 cells, TGEV exhibited a dose-dependent inhibition of NF-κB activity. Individual TGEV protein screening revealed the high potential of non-structural protein 3 (Nsp3) to inhibit NF-κB signaling, and leading to the downregulation of the NF-κB-induced cytokine production. We demonstrated that the inhibitory effect of Nsp3 was mainly mediated through the suppression of IκBα degradation as well as the inhibition of p65 phosphorylation and nuclear translocation. Furthermore, the amino acid residues at positions 590–1,215 in Nsp3 were demonstrated to inhibit the degradation of IκBα by inhibiting the IκBα ubiquitination.

Conclusion

TGEV infection can inhibit the activation of the NF-κB signaling pathway, which is mainly mediated by Nsp3 through the canonical pathway. The amino acid residues at positions 590–1,215 in Nsp3 compose the critical domain that mediates NF-κB inhibition. We speculate that this inhibitory effect is likely to be related to the structure of PLP2 with deubiquitinating enzyme activity of the amino acid residues at positions 590–1,215 in Nsp3. Our study provides a better understanding of the TGEV-mediated innate immune modulation and lays the basis for studies on the pathogenesis of coronavirus.

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.

Host Factors in Coronavirus Replication

Coronaviruses are pathogens with a serious impact on human and animal health. They mostly cause enteric or respiratory disease, which can be severe and life threatening, e.g., in the case of the zoonotic coronaviruses causing severe acute respiratory syndrome (SARS) and Middle East Respiratory Syndrome (MERS) in humans. Despite the economic and societal impact of such coronavirus infections, and the likelihood of future outbreaks of additional pathogenic coronaviruses, our options to prevent or treat coronavirus infections remain very limited. This highlights the importance of advancing our knowledge on the replication of these viruses and their interactions with the host. Compared to other +RNA viruses, coronaviruses have an exceptionally large genome and employ a complex genome expression strategy. Next to a role in basic virus replication or virus assembly, many of the coronavirus proteins expressed in the infected cell contribute to the coronavirus-host interplay. For example, by interacting with the host cell to create an optimal environment for coronavirus replication, by altering host gene expression or by counteracting the host’s antiviral defenses. These coronavirus–host interactions are key to viral pathogenesis and will ultimately determine the outcome of infection. Due to the complexity of the coronavirus proteome and replication cycle, our knowledge of host factors involved in coronavirus replication is still in an early stage compared to what is known for some other +RNA viruses. This review summarizes our current understanding of coronavirus–host interactions at the level of the infected cell, with special attention for the assembly and function of the viral RNA-synthesising machinery and the evasion of cellular innate immune responses.

PEDV nsp16 negatively regulates innate immunity to promote viral proliferation

Type-I IFNs (IFN-I) provide a key mediator of innate antiviral response during virus proliferation. Porcine epidemic diarrhea virus (PEDV), which causes diarrhea in swine of all ages, is a worldwide-distributed alphacoronavirus with economic importance. Here, we screened PEDV RNA modification enzymes involved in regulating antiviral response. Whereas the PEDV nsp13 barely regulates type I IFN, inflammatory cytokines (IL-6, TNF-a) and MHCII, nsp16 and nsp14 (to a lesser extent) down-regulate these antiviral effectors. Importantly, we found nsp16 KDKE tetrad appears to play a key role in interferon inhibition by mutating the D129 catalytic residue. Mechanistically, nsp16 down-regulates the activities of RIG-I and MDA5 mediated IFN-β and ISRE. In turn, the mRNA levels of IFIT family members (IFIT1, IFIT2, IFIT3) was inhibited in cells overexpressing nsp16. In addition, nsp10 enhanced the inhibitory effect of nsp16 on IFN-β. Altogether these results indicate PEDV nsp16 negatively regulates innate immunity to promote viral proliferation. Findings from this study provides novel perspective to advance the understanding in the pathogenesis of PEDV.

Porcine transmissible gastroenteritis virus nonstructural protein 2 contributes to inflammation via NF-κB activation

Transmissible gastroenteritis virus (TGEV) infection causes acute enteritis in swine of all ages, and especially in suckling piglets. Small intestinal inflammation is considered a central event in the pathogenesis of TGEV infections, and nuclear factor-kappa B (NF-κB) is a key transcription factor in the inflammatory response. However, it is unclear whether NF-κB is crucial for inducing inflammation during a TGEV infection. Our results show that NF-κB was activated in swine testicular (ST) cells and intestinal epithelial cell lines J2 (IPEC-J2) cells infected with TGEV, which is consistent with the up-regulation of pro-inflammatory cytokines. Treatment of TGEV-infected ST cells and IPEC-J2 cells with the NF-κB-specific inhibitor caused the down-regulation of pro-inflammatory cytokine expression, but did not significantly affect TGEV replication. Individual TGEV protein screening results demonstrated that Nsp2 exhibited a high potential for activating NF-κB and enhancing the expression of pro-inflammatory cytokines. Functional domain analyzes indicated that the first 120 amino acid residues of Nsp2 were essential for NF-κB activation. Taken together, these data suggested that NF-κB activation was a major contributor to TGEV infection-induced inflammation, and that Nsp2 was the key viral protein involved in the regulation of inflammation, with amino acids 1–120 playing a critical role in activating NF-κB.

Abbreviations: TCID50: 50% tissue culture infectious dose; DMEM: Dulbecco’s Modified Eagle Medium; eNOS: Endothelial nitric oxide synthase; FBS: fetal bovine serum; IFA: Indirect immunofluorescence; IκB: inhibitor of nuclear factor kappa-B; IL: interleukin; IPEC-J2: intestinal epithelial cell lines J2; IKK: IκB kinase; Luc: luciferase reporter gene; mAbs: monoclonal antibodies; MOI: multiple of infection; Nsp: nonstructural protein; NF-κB: nuclear factor-kappa ; ORFs: open reading frames; PBS: phosphate-buffered saline; p65 p-p65: phosphorylated; RT-PCR: reverse transcription PC; SeV: Sendai virus; ST: swine testicular; TGEV: Transmissible gastroenteritis virus; TNF-α: tumor necrosis factor α; UV-TGEV: Ultraviolet light-inactivated TGEV; ZnF: zinc finger

Nucleocapsid protein from porcine epidemic diarrhea virus isolates can antagonize interferon-λ production by blocking the nuclear factor-κB nuclear translocation

Porcine epidemic diarrhea virus (PEDV) is a highly infectious pathogen that can cause severe diseases in pigs and result in enormous economic losses in the worldwide swine industry. Previous studies revealed that PEDV exhibits an obvious capacity for modulating interferon (IFN) signaling or expression. The newly discovered type III IFN, which plays a crucial role in antiviral immunity, has strong antiviral activity against PEDV proliferation in IPEC-J2 cells. In this study, we aimed to investigate the effect of PEDV nucleocapsid (N) protein on type III IFN-λ. We found that the N proteins of ten PEDV strains isolated between 2013 and 2017 from different local farms shared high nucleotide identities, while the N protein of the CV777 vaccine strain formed a monophyletic branch in the phylogenetic tree. The N protein of the epidemic strain could antagonize type III IFN, but not type I or type II IFN expression induced by polyinosinic-polycytidylic acid (poly(I:C)) in IPEC-J2 cells. Subsequently, we demonstrated that the inhibition of poly(I:C)-induced IFN-λ3 production by PEDV N protein was dependent on the blocking of nuclear factor-κB (NF-κB) nuclear translocation. These findings might help increase understanding of the pathogenesis of PEDV and its mechanisms for evading the host immune response.

A Balancing Act: MDA5 in Antiviral Immunity and Autoinflammation

Induction of interferons during viral infection is mediated by cellular proteins that recognise viral nucleic acids. MDA5 is one such sensor of virus presence and is activated by RNA. MDA5 is required for immunity against several classes of viruses, including picornaviruses. Recent work showed that mutations in the IFIH1 gene, encoding MDA5, lead to interferon-driven autoinflammatory diseases. Together with observations made in cancer cells, this suggests that MDA5 detects cellular RNAs in addition to viral RNAs. It is therefore important to understand the properties of the RNAs which activate MDA5. New data indicate that RNA length and secondary structure are features sensed by MDA5. We review these developments and discuss how MDA5 strikes a balance between antiviral immunity and autoinflammation.

MDA5 is a pattern-recognition receptor for RNA and induces a type I interferon response.

MDA5 is activated in a variety of clinically relevant settings. This includes infection with ssRNA, dsRNA, and dsDNA viruses; several autoimmune and autoinflammatory diseases, such as type 1 diabetes and Aicardi–Goutières syndrome; and some forms of cancer treatment.

Synthetic, viral, and cellular RNAs can all activate MDA5. The latter may include transcripts from endogenous retroelements such as Alu repeats.

Length and secondary structure are important features that determine whether an RNA molecule is detected by MDA5. Indeed, long, base-paired RNA molecules potently activate MDA5 in the test tube.

Alpha/Beta Interferon (IFN-α/β) Signaling in Astrocytes Mediates Protection against Viral Encephalomyelitis and Regulates IFN-γ-Dependent Responses

The contribution of distinct central nervous system (CNS) resident cells to protective alpha/beta interferon (IFN-α/β) function following viral infections is poorly understood. Based on numerous immune regulatory functions of astrocytes, we evaluated the contribution of astrocyte IFN-α/β signaling toward protection against the nonlethal glia- and neuronotropic mouse hepatitis virus (MHV) strain A59. Analysis of gene expression associated with IFN-α/β function, e.g., pattern recognition receptors (PRRs) and interferon-stimulated genes (ISGs), revealed lower basal mRNA levels in brain-derived astrocytes than in microglia. Although astrocytes poorly induced Ifnβ mRNA following infection, they upregulated various mRNAs in the IFN-α/β pathway to a higher extent than microglia, supporting effective IFN-α/β responsiveness. Ablation of the IFN-α/β receptor (IFNAR) in astrocytes using mGFAPcre IFNAR^fl/fl mice resulted in severe encephalomyelitis and mortality, coincident with uncontrolled virus replication. Further, virus spread was not restricted to astrocytes but also affected microglia and neurons, despite increased and sustained Ifnα/β and ISG mRNA levels within the CNS. IFN-γ, a crucial mediator for MHV control, was not impaired in infected mGFAPcre IFNAR^fl/fl mice despite reduced T cell CNS infiltration. Unexpectedly however, poor induction of IFN-γ-dependent major histocompatibility complex (MHC) class II expression on microglia supported that defective IFN-γ signaling contributes to uncontrolled virus replication. A link between sustained elevated IFN-α/β and impaired responsiveness to IFN-γ supports the novel concept that temporally limited early IFN-α/β responses are critical for effective antiviral IFN-γ function. Overall, our results imply that IFN-α/β signaling in astrocytes is not only critical in limiting early CNS viral spread but also promotes protective antiviral IFN-γ function.IMPORTANCE An antiviral state established by IFN-α/β contains initial viral spread as adaptive immunity develops. While it is apparent that the CNS lacks professional IFN-α/β producers and that resident cells have distinct abilities to elicit innate IFN-α/β responses, protective interactions between inducer and responder cells require further investigation. Infection with a glia- and neuronotropic coronavirus demonstrates that astrocytes mount a delayed but more robust response to infection than microglia, despite their lower basal mRNA levels of IFN-α/β-inducing components. Lethal, uncontrolled viral dissemination following ablation of astrocyte IFN-α/β signaling revealed the importance of IFN-α/β responses in a single cell type for protection. Sustained global IFN-α/β expression associated with uncontrolled virus did not suffice to protect neurons and further impaired responsiveness to protective IFN-γ. The results support astrocytes as critical contributors to innate immunity and the concept that limited IFN-α/β responses are critical for effective subsequent antiviral IFN-γ function.

The nucleocapsid proteins of mouse hepatitis virus and severe acute respiratory syndrome coronavirus share the same IFN-β antagonizing mechanism: attenuation of PACT-mediated RIG-I/ MDA5 activation

Coronaviruses (CoVs) are a huge threat to both humans and animals and have evolved elaborate mechanisms to antagonize interferons (IFNs). Nucleocapsid (N) protein is the most abundant viral protein in CoV-infected cells, and has been identified as an innate immunity antagonist in several CoVs, including mouse hepatitis virus (MHV) and severe acute respiratory syndrome (SARS)-CoV. However, the underlying molecular mechanism(s) remain unclear. In this study, we found that MHV N protein inhibited Sendai virus and poly(I:C)-induced IFN-β production by targeting a molecule upstream of retinoic acid-induced gene I (RIG-I) and melanoma differentiation gene 5 (MDA5). Further studies showed that both MHV and SARS-CoV N proteins directly interacted with protein activator of protein kinase R (PACT), a cellular dsRNA-binding protein that can bind to RIG-I and MDA5 to activate IFN production. The N–PACT interaction sequestered the association of PACT and RIG-I/MDA5, which in turn inhibited IFN-β production. However, the N proteins from porcine epidemic diarrhea virus (PEDV) and porcine reproductive and respiratory syndrome virus (PRRSV), which are also classified in the order Nidovirales, did not interact and counteract with PACT. Taken together, our present study confirms that both MHV and SARS-CoV N proteins can perturb the function of cellular PACT to circumvent the innate antiviral response. However, this strategy does not appear to be used by all CoVs N proteins.

Structural insights into the Middle East respiratory syndrome coronavirus 4a protein and its dsRNA binding mechanism

Middle East respiratory syndrome coronavirus (MERS-CoV) has evolved to navigate through the sophisticated network of a host's immune system. The immune evasion mechanism including type 1 interferon and protein kinase R-mediated antiviral stress responses has been recently attributed to the involvement of MERS-CoV protein 4a (p4a) that masks the viral dsRNA. However, the structural mechanism of how p4a recognizes and establishes contacts with dsRNA is not well explained. In this study, we report a dynamic mechanism deployed by p4a to engage the viral dsRNA and make it unavailable to the host immune system. Multiple variants of p4a-dsRNA were created and investigated through extensive molecular dynamics procedures to highlight crucial interfacial residues that may be used as potential pharmacophores for future drug development. The structural analysis revealed that p4a exhibits a typical αβββα fold structure, as found in other dsRNA-binding proteins. The α1 helix and the β1-β2 loop play a crucial role in recognizing and establishing contacts with the minor grooves of dsRNA. Further, mutational and binding free energy analyses suggested that in addition to K63 and K67, two other residues, K27 and W45, might also be crucial for p4a-dsRNA stability.

Cellular RNA Helicase DDX1 Is Involved in Transmissible Gastroenteritis Virus nsp14-Induced Interferon-Beta Production

Transmissible gastroenteritis virus (TGEV), an enteropathogenic coronavirus (CoV) of porcine, causes lethal watery diarrhea and severe dehydration in piglets and leads to severe economic losses in the swine industry. Unlike most CoVs that antagonize type I interferon (IFN) production, previous studies showed that TGEV infection induces IFN-I production both in vivo and in vitro. However, the underlying mechanism(s) remain largely unknown. In this study, we found that TGEV infection significantly facilitated IFN-β production as well as activation of the transcription factors IFN regulatory factor 3 (IRF3) and nuclear factor-kappaB (NF-κB) in porcine kidney (PK-15) cells. Screening of TGEV-encoded proteins demonstrated that non-structural protein 14 (nsp14) was the most potent IFN-β inducer and induced IFN-β production mainly by activating NF-κB but not IRF3. Further analysis showed that nsp14 interacted with DDX1, a member of the DExD/H helicase family. Knockdown of DDX1 by specific small interfering RNA (siRNA) significantly decreased nsp14-induced IFN-β production and NF-κB activation. Furthermore, TGEV-induced IFN-β production and IFN-stimulated gene (ISG) expression were decreased in cells transfected with DDX1-specific siRNA, indicating the vital role of DDX1 to TGEV-induced IFN-β responses. In summary, our data revealed a potential coactivator role of host RNA helicase DDX1 to the induction of IFN-β response initiated by TGEV and demonstrated that nsp14 is an important IFN inducer among the TGEV-encoded proteins.

Inhibition of NF-κB activity by the porcine epidemic diarrhea virus nonstructural protein 1 for innate immune evasion

Porcine epidemic diarrhea virus emerged in the US is known to suppress the type I interferons response during infection. In the present study using porcine epithelial cells, we showed that PEDV inhibited both NF-κB and proinflammatory cytokines. PEDV blocked the p65 activation in infected cells and suppressed the PRD II-mediated NF-κB activity. Of the total of 22 viral proteins, nine proteins were identified as NF-κB antagonists, and nsp1 was the most potent suppressor of proinflammatory cytokines. Nsp1 interfered the phosphorylation and degradation of IκBα, and thus blocked the p65 activation. Mutational studies demonstrated the essential requirements of the conserved residues of nsp1 for NF-κB suppression. Our study showed that PEDV inhibited NF-κB activity and nsp1 was a potent NF-κB antagonist for suppression of both IFN and early production of pro-inflammatory cytokines.

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PEDV inhibits type I IFNs and NF-κB-mediated pro-inflammatory cytokines.

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PEDV blocks p65 nuclear translocation in virus-infected cells.

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Among 22 viral proteins, nsp1, nsp3, nsp5, nsp7, nsp14, nsp15, nsp16, ORF3, and E are NF-κB antagonists.

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Nsp1 suppresses pro-inflammatory cytokines and p65 activation by blocking IκBα phosphorylation.

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The conserved residues of nsp1 are crucial for NF-κB suppression.

Recurrent rhinovirus infections in a child with inherited MDA5 deficiency

MDA5 is a cytosolic sensor of double-stranded RNA (ds)RNA including viral byproducts and intermediates. We studied a child with life-threatening, recurrent respiratory tract infections, caused by viruses including human rhinovirus (HRV), influenza virus, and respiratory syncytial virus (RSV). We identified in her a homozygous missense mutation in IFIH1 that encodes MDA5. Mutant MDA5 was expressed but did not recognize the synthetic MDA5 agonist/(ds)RNA mimic polyinosinic-polycytidylic acid. When overexpressed, mutant MDA5 failed to drive luciferase activity from the IFNB1 promoter or promoters containing ISRE or NF-κB sequence motifs. In respiratory epithelial cells or fibroblasts, wild-type but not knockdown of MDA5 restricted HRV infection while increasing IFN-stimulated gene expression and IFN-β/λ. However, wild-type MDA5 did not restrict influenza virus or RSV replication. Moreover, nasal epithelial cells from the patient, or fibroblasts gene-edited to express mutant MDA5, showed increased replication of HRV but not influenza or RSV. Thus, human MDA5 deficiency is a novel inborn error of innate and/or intrinsic immunity that causes impaired (ds)RNA sensing, reduced IFN induction, and susceptibility to the common cold virus.

Targeting Coronaviral Replication and Cellular JAK2 Mediated Dominant NF-κB Activation for Comprehensive and Ultimate Inhibition of Coronaviral Activity

Tylophorine-based compounds exert broad spectral, potent inhibition of coronaviruses. NF-κB activation is a common pro-inflammatory response of host cells to viral infection. The aims of this study were to (i) find an effective combination treatment for coronaviral infections through targeting of the virus per se and cellular NF-κB activity; and (ii) to study the underling mechanisms. We found that tylophorine-based compounds target the TGEV viral RNA and effectively inhibit TGEV replication. NF-κB inhibition also leads to anti-TGEV replication. NF-κB activation induced by TGEV infection was found to be associated with two convergent pathways, IKK-2_IκBα/p65 and JAK2 mediated p65 phosphorylation, in swine testicular cells. JAK2 inhibition either by CYT387 (a JAK family inhibitor) or by silencing JAK2-expression revealed a dominant JAK2 mediated p65 phosphorylation pathway for NF-κB activation and resulted in NF-κB inhibition, which overrode the IκBα regulation via the IKK-2. Finally, tylophorine-based compounds work cooperatively with CYT387 to impart comprehensive anti-TGEV activities. The combination treatment, wherein a tylophorine compound targets TGEV and a JAK2 inhibitor blocks the alternative dominant NF-κB activation mediated by JAK2, is more effective and comprehensive than either one alone and constitutes a feasible approach for the treatment of SARS-CoV or MERS-CoV.

Transmissible gastroenteritis virus infection induces NF-κB activation through RLR-mediated signaling

Transmissible gastroenteritis virus (TGEV) is a porcine enteric coronavirus which causes lethal severe watery diarrhea in piglets. The pathogenesis of TGEV is strongly associated with inflammation. In this study, we found that TGEV infection activates transcription factors NF-κB, IRF3 and AP-1 in a time- and dose-dependent manner in porcine kidney cells. Treatment with the NF-κB-specific inhibitor BAY11-7082 significantly decreased TGEV-induced proinflammatory cytokine production, but did not affect virus replication. Phosphorylation of NF-κB subunit p65 and proinflammatory cytokine production were greatly decreased after knockdown of retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs) or its adaptors MAVS and STING, while only slight reduction was observed in cells following silencing of Toll-like receptor adaptors, MyD88 and TRIF. Furthermore, TGEV infection significantly upregulated mRNA expression of RIG-I and MDA5. Taken together, our results indicate that the RLR signaling pathway is involved in TGEV-induced inflammatory responses.

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Transmissible gastroenteritis virus (TGEV) infection activates NF-κB.

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Inhibition of NF-κB activation does not affect TGEV replication.

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RLR signaling pathway is involved in TGEV-induced inflammatory responses.

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TGEV infection significantly upregulates mRNA expression of RIG-I and MDA5.

Interaction of SARS and MERS Coronaviruses with the Antiviral Interferon Response

Severe Acute Respiratory Syndrome (SARS) and Middle East Respiratory Syndrome (MERS) are the most severe coronavirus (CoV)-associated diseases in humans. The causative agents, SARS-CoV and MERS-CoV, are of zoonotic origin but may be transmitted to humans, causing severe and often fatal respiratory disease in their new host. The two coronaviruses are thought to encode an unusually large number of factors that allow them to thrive and replicate in the presence of efficient host defense mechanisms, especially the antiviral interferon system. Here, we review the recent progress in our understanding of the strategies that highly pathogenic coronaviruses employ to escape, dampen, or block the antiviral interferon response in human cells.

Brain heterogeneity leads to differential innate immune responses and modulates pathogenesis of viral infections

The central nervous system (CNS) is a highly complex organ with highly specialized cell subtypes. Viral infections often target specific structures of the brain and replicate in certain regions. Studies in mice deficient in type I Interferon (IFN) receptor or IFN-β have highlighted the importance of the type I IFN system against viral infections and non-viral autoimmune disorders in the CNS. Direct antiviral effects of type I IFNs appear to be crucial in limiting early spread of a number of viruses in CNS tissues. Increased efforts have been made to characterize IFN expression and responses in the brain. In this context, it is important to identify cells that produce IFN, decipher pathways leading to type I IFN expression and to characterize responding cells. In this review we give an overview about region specific aspects that influence local innate immune responses. The route of entry is critical, but also the susceptibility of different cell types, heterogeneity in subpopulations and micro-environmental cues play an important role in antiviral responses. Recent work has outlined the tremendous importance of type I IFNs, particularly in the limitation of viral spread within the CNS. This review will address recent advances in understanding the mechanisms of local type I IFN production and response, in the particular context of the CNS.

Responses of the Toll-like receptor and melanoma differentiation-associated protein 5 signaling pathways to avian infectious bronchitis virus infection in chicks

Avian infectious bronchitis virus (IBV) is a Gammacoronavirus in the family Coronaviridae and causes highly contagious respiratory disease in chickens. Innate immunity plays significant roles in host defense against IBV. Here, we explored the interaction between IBV and the host innate immune system. Severe histopathological lesions were observed in the tracheal mucosa at 3-5 days post inoculation (dpi) and in the kidney at 8 dpi, with heavy viral loads at 1-11 and 1-28 dpi, respectively. The expression of mRNAs encoding Toll-like receptor (TLR) 3 and TLR7 were upregulated at 3-8 dpi, and that of TIR-domain-containing adapter-inducing interferon (IFN) β (TRIF) was upregulated at 21 dpi in the trachea and kidney. Myeloid differentiation primary response protein 88 (MyD88) was upregulated in the trachea during early infection. Tumor necrosis factor receptor-associated factor (TRAF) 3 and TRAF6 were upregulated expression in both tissues. Moreover, melanoma differentiation-associated protein 5 (MDA5), laboratory of genetics and physiology 2 (LGP2), stimulator of IFN genes (STING), and mitochondrial antiviral signaling protein (MAVS), as well as TANK binding kinase 1 (TBK1), inhibitor of kappaB kinase (IKK) ε, IKKα, IKKβ, IFN regulatory factor (IRF) 7, nuclear factor of kappaB (NF-ĸB), IFN-α, IFN-β, various interleukins(ILs), and macrophage inflammatory protein-1β (MIP-1β) were significantly upregulated in the trachea and downregulated in the kidney. These results suggested that the TLR and MDA5 signaling pathways and innate immune cytokine were induced after IBV infection. Additionally, consistent responses to IBV infection were observed during early infection, with differential and complicated responses in the kidney.

Porcine Epidemic Diarrhea Virus 3C-Like Protease Regulates Its Interferon Antagonism by Cleaving NEMO

Porcine epidemic diarrhea virus (PEDV) is an enteropathogenic coronavirus causing lethal watery diarrhea in piglets. Since 2010, a PEDV variant has spread rapidly in China, and it emerged in the United States in 2013, posing significant economic and public health concerns. The ability to circumvent the interferon (IFN) antiviral response, as suggested for PEDV, promotes viral survival and regulates pathogenesis of PEDV infections, but the underlying mechanisms remain obscure. Here, we show that PEDV-encoded 3C-like protease, nsp5, is an IFN antagonist that proteolytically cleaves the nuclear transcription factor kappa B (NF-κB) essential modulator (NEMO), an essential adaptor bridging interferon-regulatory factor and NF-κB activation. NEMO is cleaved at glutamine 231 (Q231) by PEDV, and this cleavage impaired the ability of NEMO to activate downstream IFN production and to act as a signaling adaptor of the RIG-I/MDA5 pathway. Mutations specifically disrupting the cysteine protease activity of PEDV nsp5 abrogated NEMO cleavage and the inhibition of IFN induction. Structural analysis suggests that several key residues outside the catalytic sites of PEDV nsp5 probably impact NEMO cleavage by modulating potential interactions of nsp5 with their substrates. These data show that PEDV nsp5 disrupts type I IFN signaling by cleaving NEMO. Previously, we and others demonstrated that NEMO is also cleaved by 3C or 3C-like proteinases of picornavirus and artertivirus. Thus, NEMO probably represents a prime target for 3C or 3C-like proteinases of different viruses.

IMPORTANCE

The continued emergence and reemergence of porcine epidemic diarrhea virus (PEDV) underscore the importance of studying how this virus manipulates the immune responses of its hosts. During coevolution with its hosts, PEDV has acquired mechanisms to subvert host innate immune responses for its survival advantage. At least two proteins encoded by PEDV have been identified as interferon (IFN) antagonists, papain-like protease (PLP) and N protein. Here, we report that the PEDV nsp5 gene, which encodes the 3C-like protease of PEDV, is another IFN antagonist. Mechanistically, the cysteine protease activity of PEDV nsp5 mediates proteolysis of NEMO, the key adaptor for IFN synthesis, and NEMO is cleaved at glutamine 231 (Q231). The new molecular details and determinants impacting NEMO scission by PEDV nsp5 delineated in this study are fundamental to our understanding of critical virus-host interactions that determine PEDV pathogenesis.

A molecular arms race between host innate antiviral response and emerging human coronaviruses

Coronaviruses have been closely related with mankind for thousands of years. Communityacquired human coronaviruses have long been recognized to cause common cold. However, zoonotic coronaviruses are now becoming more a global concern with the discovery of highly pathogenic severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS) coronaviruses causing severe respiratory diseases. Infections by these emerging human coronaviruses are characterized by less robust interferon production. Treatment of patients with recombinant interferon regimen promises beneficial outcomes, suggesting that compromised interferon expression might contribute at least partially to the severity of disease. The mechanisms by which coronaviruses evade host innate antiviral response are under intense investigations. This review focuses on the fierce arms race between host innate antiviral immunity and emerging human coronaviruses. Particularly, the host pathogen recognition receptors and the signal transduction pathways to mount an effective antiviral response against SARS and MERS coronavirus infection are discussed. On the other hand, the counter-measures evolved by SARS and MERS coronaviruses to circumvent host defense are also dissected. With a better understanding of the dynamic interaction between host and coronaviruses, it is hoped that insights on the pathogenesis of newly-identified highly pathogenic human coronaviruses and new strategies in antiviral development can be derived.

Suppression of type I interferon production by porcine epidemic diarrhea virus and degradation of CREB-binding protein by nsp1

Type I interferons (IFN-α/β) are the major components of the innate immune response of hosts, and in turn many viruses have evolved to modulate the host response during infection. We found that the IFN-β production was significantly suppressed during PEDV infection in cells. To identify viral IFN antagonists and to study their suppressive function, viral coding sequences for the entire structural and nonstructural proteins were cloned and expressed. Of 16 PEDV nonstructural proteins (nsps), nsp1, nsp3, nsp7, nsp14, nsp15 and nsp16 were found to inhibit the IFN-β and IRF3 promoter activities. The sole accessory protein ORF3, structure protein envelope (E), membrane (M), and nucleocapsid (N) protein were also shown to inhibit such activities. PEDV nsp1 did not interfere the IRF3 phosphorylation and nuclear translocation but interrupted the enhanceosome assembly of IRF3 and CREB-binding protein (CBP) by degrading CBP. A further study showed that the CBP degradation by nsp1 was proteasome-dependent. Our data demonstrate that PEDV modulates the host innate immune responses by degrading CBP and suppressing ISGs expression.

Gliopathy of Demyelinating and Non-Demyelinating Strains of Mouse Hepatitis Virus

Demyelination in the central nervous system induced by neurovirulent strains of Mouse Hepatitis Virus (MHV) is mediated by the viral spike glycoprotein, but it is not clear whether the mechanism of this disease pathology involves direct viral infection of oligodendrocytes. Detailed studies of glial cell tropism of MHV are presented, demonstrating that direct MHV infection of oligodendrocytes differs between demyelinating (RSA59) and non-demyelinating (RSMHV2) viral strains both in vitro and in vivo. Our results indicate that direct injury of mature oligodendrocytes is an important mechanism of virus-induced demyelination. In vivo, RSA59 infection was identified in spinal cord gray and white matter, but infected oligodendrocytes were restricted to white matter. In contrast, RSMHV2 infection was restricted to gray matter neurons and was not localized to oligodendrocytes. In vitro, RSA59 can infect both oligodendrocyte precursors and differentiated oligodendrocytes, whereas RSMHV2 can infect oligodendrocyte precursors but not differentiated oligodendrocytes. Viral spreading through axonal means to white matter and release of the demyelinating strain MHV at the nerve end is critical for oligodendrocytes infection and subsequent demyelination. Understanding the mechanisms by which known viruses effect demyelination in this animal model has important therapeutic implications in the treatment of human demyelinating disease.

Pathogen-Associated Molecular Pattern Recognition of Hepatitis C Virus Transmitted/Founder Variants by RIG-I Is Dependent on U-Core Length

Despite the introduction of direct-acting antiviral (DAA) drugs against hepatitis C virus (HCV), infection remains a major public health concern because DAA therapeutics do not prevent reinfection and patients can still progress to chronic liver disease. Chronic HCV infection is supported by a variety of viral immune evasion strategies, but, remarkably, 20% to 30% of acute infections spontaneously clear prior to development of adaptive immune responses, thus implicating innate immunity in resolving acute HCV infection. However, the virus-host interactions regulating acute infection are unknown. Transmission of HCV involves one or a few transmitted/founder (T/F) variants. In infected hepatocytes, the retinoic acid-inducible gene I (RIG-I) protein recognizes 5' triphosphate (5'ppp) of the HCV RNA and a pathogen-associated molecular pattern (PAMP) motif located within the 3' untranslated region consisting of poly-U/UC. PAMP binding activates RIG-I to induce innate immune signaling and type 1 interferon antiviral defenses. HCV poly-U/UC sequences can differ in length and complexity, suggesting that PAMP diversity in T/F genomes could regulate innate immune control of acute HCV infection. Using 14 unique poly-U/UC sequences from HCV T/F genomes recovered from acute-infection patients, we tested whether RIG-I recognition and innate immune activation correlate with PAMP sequence characteristics. We show that T/F variants are recognized by RIG-I in a manner dependent on length of the U-core motif of the poly-U/UC PAMP and are recognized by RIG-I to induce innate immune responses that restrict acute infection. PAMP recognition of T/F HCV variants by RIG-I may therefore impart innate immune signaling and HCV restriction to impact acute-phase-to-chronic-phase transition.

IMPORTANCE

Recognition of nonself molecular patterns such as those seen with viral nucleic acids is an essential step in triggering the immune response to virus infection. Innate immunity is induced by hepatitis C virus infection through the recognition of viral RNA by the cellular RIG-I protein, where RIG-I recognizes a poly-uridine/cytosine motif in the viral genome. Variation within this motif may provide an immune evasion strategy for transmitted/founder viruses during acute infection. Using 14 unique poly-U/UC sequences from HCV T/F genomes recovered from acutely infected HCV patients, we demonstrate that RIG-I binding and activation of innate immunity depend primarily on the length of the uridine core within this motif. T/F variants found in acute infection contained longer U cores within the motif and could activate RIG-I and induce innate immune signaling sufficient to restrict viral infection. Thus, recognition of T/F variants by RIG-I could significantly impact the transition from acute to chronic infection.

RIG-I Mediates an Antiviral Response to Crimean-Congo Hemorrhagic Fever Virus

In the cytoplasm, the retinoic acid-inducible gene I (RIG-I) senses the RNA genomes of several RNA viruses. RIG-I binds to viral RNA, eliciting an antiviral response via the cellular adaptor MAVS. Crimean-Congo hemorrhagic fever virus (CCHFV), a negative-sense RNA virus with a 5'-monophosphorylated genome, is a highly pathogenic zoonotic agent with significant public health implications. We found that, during CCHFV infection, RIG-I mediated a type I interferon (IFN) response via MAVS. Interfering with RIG-I signaling reduced IFN production and IFN-stimulated gene expression and increased viral replication. Immunostimulatory RNA was isolated from CCHFV-infected cells and from virion preparations, and RIG-I coimmunoprecipitation of infected cell lysates isolated immunostimulatory CCHFV RNA. This report serves as the first description of a pattern recognition receptor for CCHFV and highlights a critical signaling pathway in the antiviral response to CCHFV.

IMPORTANCE

CCHFV is a tick-borne virus with a significant public health impact. In order for cells to respond to virus infection, they must recognize the virus as foreign and initiate antiviral signaling. To date, the receptors involved in immune recognition of CCHFV are not known. Here, we investigate and identify RIG-I as a receptor involved in initiating an antiviral response to CCHFV. This receptor initially was not expected to play a role in CCHFV recognition because of characteristics of the viral genome. These findings are important in understanding the antiviral response to CCHFV and support continued investigation into the spectrum of potential viruses recognized by RIG-I.

Porcine epidemic diarrhea virus inhibits dsRNA-induced interferon-β production in porcine intestinal epithelial cells by blockade of the RIG-I-mediated pathway

Background

The lack of optimal porcine cell lines has severely impeded the study and progress in elucidation of porcine epidemic diarrhea virus (PEDV) pathogenesis. Vero cell, an African green monkey kidney cell line, was often used to isolate and propagate PEDV. Nonetheless, the target cells of PEDV in vivo are intestinal epithelial cells, during infection, intestinal epithelia would be damaged and resulted in digestive disorders. The immune functions of porcine epithelial cells and interactions with other immune cell populations display a number of differences compared to other species. Type I interferon (IFN) plays an important role in antiviral immune response. Limited reports showed that PEDV could inhibit type I interferon production. In this study, porcine small intestinal epithelial cells (IECs), the target cells of PEDV, were used as the infection model in vitro to identify the possible molecular mechanisms of PEDV-inhibition IFN-β production.

Results

PEDV not only failed to induce IFN-β expression, but also inhibited dsRNA-mediated IFN-β production in IECs. As the key IFN-β transcription factors, we found that dsRNA-induced activation of IFN regulatory factor 3 (IRF-3) was inhibited after PEDV infection, but not nuclear factor-kappaB (NF-κB). To identify the mechanism of PEDV intervention with dsRNA-mediated IFN-β expression more accurately, the role of individual molecules of RIG-I signaling pathway were investigated. In the upstream of IRF-3, TANK-binding kinase 1 (TBK1)-or inhibitor of κB kinase-ε (IKKε)-mediated IFN-β production was not blocked by PEDV, while RIG-I-and its adapter molecule IFN-β promoter stimulator 1 (IPS-1)-mediated IFN-β production were completely inhibited after PEDV infection.

Conclusion

Taken together, our data demonstrated for the first time that PEDV infection of its target cell line, IECs, inhibited dsRNA-mediated IFN-β production by blocking the activation of IPS-1 in RIG-I-mediated pathway. Our studies offered new visions in understanding of the interaction between PEDV and host innate immune system.

Myd88 Initiates Early Innate Immune Responses and Promotes CD4 T Cells during Coronavirus Encephalomyelitis

Myd88 signaling is critical to the control of numerous central nervous system (CNS) infections by promoting both innate and adaptive immune responses. Nevertheless, the extent to which Myd88 regulates type I interferon (IFN) versus proinflammatory factors and T cell function, as well as the anatomical site of action, varies extensively with the pathogen. CNS infection by neurotropic coronavirus with replication confined to the brain and spinal cord induces protective IFN-α/β via Myd88-independent activation of melanoma differentiation-associated gene 5 (MDA5). However, a contribution of Myd88-dependent signals to CNS pathogenesis has not been assessed. Infected Myd88(-/-) mice failed to control virus, exhibited enhanced clinical disease coincident with increased demyelination, and succumbed to infection within 3 weeks. The induction of IFN-α/β, as well as of proinflammatory cytokines and chemokines, was impaired early during infection. However, defects in both IFN-α/β and select proinflammatory factors were rapidly overcome prior to T cell recruitment. Myd88 deficiency also specifically blunted myeloid and CD4 T cell recruitment into the CNS without affecting CD8 T cells. Moreover, CD4 T cells but not CD8 T cells were impaired in IFN-γ production. Ineffective virus control indeed correlated most prominently with reduced antiviral IFN-γ in the CNS of Myd88(-/-) mice. The results demonstrate a crucial role for Myd88 both in early induction of innate immune responses during coronavirus-induced encephalomyelitis and in specifically promoting protective CD4 T cell activation. In the absence of these responses, functional CD8 T cells are insufficient to control viral spread within the CNS, resulting in severe demyelination.

IMPORTANCE

During central nervous system (CNS) infections, signaling through the adaptor protein Myd88 promotes both innate and adaptive immune responses. The extent to which Myd88 regulates antiviral type I IFN, proinflammatory factors, adaptive immunity, and pathology is pathogen dependent. These results reveal that Myd88 protects from lethal neurotropic coronavirus-induced encephalomyelitis by accelerating but not enhancing the induction of IFN-α/β, as well as by promoting peripheral activation and CNS accumulation of virus-specific CD4 T cells secreting IFN-γ. By controlling both early innate immune responses and CD4 T cell-mediated antiviral IFN-γ, Myd88 signaling limits the initial viral dissemination and is vital for T cell-mediated control of viral loads. Uncontrolled viral replication in the absence of Myd88 leads to severe demyelination and pathology despite overall reduced inflammatory responses. These data support a vital role of Myd88 signaling in protective antimicrobial functions in the CNS by promoting proinflammatory mediators and T cell-mediated IFN-γ production.

RIG-I in RNA virus recognition

Antiviral immunity is initiated upon host recognition of viral products via non-self molecular patterns known as pathogen-associated molecular patterns (PAMPs). Such recognition initiates signaling cascades that induce intracellular innate immune defenses and an inflammatory response that facilitates development of the acquired immune response. The retinoic acid-inducible gene I (RIG-I) and the RIG-I-like receptor (RLR) protein family are key cytoplasmic pathogen recognition receptors that are implicated in the recognition of viruses across genera and virus families, including functioning as major sensors of RNA viruses, and promoting recognition of some DNA viruses. RIG-I, the charter member of the RLR family, is activated upon binding to PAMP RNA. Activated RIG-I signals by interacting with the adapter protein MAVS leading to a signaling cascade that activates the transcription factors IRF3 and NF-κB. These actions induce the expression of antiviral gene products and the production of type I and III interferons that lead to an antiviral state in the infected cell and surrounding tissue. RIG-I signaling is essential for the control of infection by many RNA viruses. Recently, RIG-I crosstalk with other pathogen recognition receptors and components of the inflammasome has been described. In this review, we discuss the current knowledge regarding the role of RIG-I in recognition of a variety of virus families and its role in programming the adaptive immune response through cross-talk with parallel arms of the innate immune system, including how RIG-I can be leveraged for antiviral therapy.

Identification of a gamma interferon-activated inhibitor of translation-like RNA motif at the 3' end of the transmissible gastroenteritis coronavirus genome modulating innate immune response

A 32-nucleotide (nt) RNA motif located at the 3′ end of the transmissible gastroenteritis coronavirus (TGEV) genome was found to specifically interact with the host proteins glutamyl-prolyl-tRNA synthetase (EPRS) and arginyl-tRNA synthetase (RRS). This RNA motif has high homology in sequence and secondary structure with the gamma interferon-activated inhibitor of translation (GAIT) element, which is located at the 3′ end of several mRNAs encoding proinflammatory proteins. The GAIT element is involved in the translation silencing of these mRNAs through its interaction with the GAIT complex (EPRS, heterogeneous nuclear ribonucleoprotein Q, ribosomal protein L13a, and glyceraldehyde 3-phosphate dehydrogenase) to favor the resolution of inflammation. Interestingly, we showed that the viral RNA motif bound the GAIT complex and inhibited the in vitro translation of a chimeric mRNA containing this RNA motif. To our knowledge, this is the first GAIT-like motif described in a positive RNA virus. To test the functional role of the GAIT-like RNA motif during TGEV infection, a recombinant coronavirus harboring mutations in this motif was engineered and characterized. Mutations of the GAIT-like RNA motif did not affect virus growth in cell cultures. However, an exacerbated innate immune response, mediated by the melanoma differentiation-associated gene 5 (MDA5) pathway, was observed in cells infected with the mutant virus compared with the response observed in cells infected with the parental virus. Furthermore, the mutant virus was more sensitive to beta interferon than the parental virus. All together, these data strongly suggested that the viral GAIT-like RNA motif modulates the host innate immune response.

The innate immune response is the first line of antiviral defense that culminates with the synthesis of interferon and proinflammatory cytokines to limit virus replication. Coronaviruses encode several proteins that interfere with the innate immune response at different levels, but to date, no viral RNA counteracting antiviral response has been described. In this work, we have characterized a 32-nt RNA motif located at the 3′ end of the TGEV genome that specifically interacted with EPRS and RRS. This RNA motif presented high homology with the GAIT element, involved in the modulation of the inflammatory response. Moreover, the disruption of the viral GAIT-like RNA motif led to an exacerbated innate immune response triggered by MDA5, indicating that the GAIT-like RNA motif counteracts the host innate immune response. These novel findings may be of relevance for other coronaviruses and could serve as the basis for the development of novel antiviral strategies.

High secretion of interferons by human plasmacytoid dendritic cells upon recognition of Middle East respiratory syndrome coronavirus

The Middle East respiratory syndrome coronavirus (MERS-CoV) emerged in 2012 as the causative agent of a severe respiratory disease with a fatality rate of approximately 30%. The high virulence and mortality rate prompted us to analyze aspects of MERS-CoV pathogenesis, especially its interaction with innate immune cells such as antigen-presenting cells (APCs). Particularly, we analyzed secretion of type I and type III interferons (IFNs) by APCs, i.e., B cells, macrophages, monocyte-derived/myeloid dendritic cells (MDDCs/mDCs), and by plasmacytoid dendritic cells (pDCs) of human and murine origin after inoculation with MERS-CoV. Production of large amounts of type I and III IFNs was induced exclusively in human pDCs, which were significantly higher than IFN induction by severe acute respiratory syndrome (SARS)-CoV. Of note, IFNs were secreted in the absence of productive replication. However, receptor binding, endosomal uptake, and probably signaling via Toll-like receptor 7 (TLR7) were critical for sensing of MERS-CoV by pDCs. Furthermore, active transcription of MERS-CoV N RNA and subsequent N protein expression were evident in infected pDCs, indicating abortive infection. Taken together, our results point toward dipeptidyl peptidase 4 (DPP4)-dependent endosomal uptake and subsequent infection of human pDCs by MERS-CoV. However, the replication cycle is stopped after early gene expression. In parallel, human pDCs are potent IFN-producing cells upon MERS-CoV infection. Knowledge of such IFN responses supports our understanding of MERS-CoV pathogenesis and is critical for the choice of treatment options.

IMPORTANCE

MERS-CoV causes a severe respiratory disease with high fatality rates in human patients. Recently, confirmed human cases have increased dramatically in both number and geographic distribution. Understanding the pathogenesis of this highly pathogenic CoV is crucial for developing successful treatment strategies. This study elucidates the interaction of MERS-CoV with APCs and pDCs, particularly the induction of type I and III IFN secretion. Human pDCs are the immune cell population sensing MERS-CoV but secrete significantly larger amounts of IFNs, especially IFN-α, than in response to SARS-CoV. A model for molecular virus-host interactions is presented outlining IFN induction in pDCs. The massive IFN secretion upon contact suggests a critical role of this mechanism for the high degree of immune activation observed during MERS-CoV infection.

Activation of the chicken type I interferon response by infectious bronchitis coronavirus

Coronaviruses from both the Alphacoronavirus and Betacoronavirus genera interfere with the type I interferon (IFN) response in various ways, ensuring the limited activation of the IFN response in most cell types. Of the gammacoronaviruses that mainly infect birds, little is known about the activation of the host immune response. We show that the prototypical Gammacoronavirus, infectious bronchitis virus (IBV), induces a delayed activation of the IFN response in primary renal cells, tracheal epithelial cells, and a chicken cell line. In fact, Ifnβ expression is delayed with respect to the peak of viral replication and the accompanying accumulation of double-stranded RNA (dsRNA). In addition, we demonstrate that MDA5 is the primary sensor for Gammacoronavirus infections in chicken cells. Furthermore, we provide evidence that accessory proteins 3a and 3b of IBV modulate the response at the transcriptional and translational levels. Finally, we show that, despite the lack of activation of the IFN response during the early phase of IBV infection, the signaling of nonself dsRNA through both MDA5 and TLR3 remains intact in IBV-infected cells. Taken together, this study provides the first comprehensive analysis of host-virus interactions of a Gammacoronavirus with avian innate immune responses.

IMPORTANCE

Our results demonstrate that IBV has evolved multiple strategies to avoid the activation of the type I interferon response. Taken together, the present study closes a gap in the understanding of host-IBV interaction and paves the way for further characterization of the mechanisms underlying immune evasion strategies as well as the pathogenesis of gammacoronaviruses.

To sense or not to sense viral RNA--essentials of coronavirus innate immune evasion

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Coronaviruses employ versatile mechanisms to evade sensing of viral RNA.

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Cap-methylation of viral RNA facilitates innate immune evasion.

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There is increasing evidence that virus-encoded ribonucleases impact innate immune responses.

An essential function of innate immunity is to distinguish self from non-self and receptors have evolved to specifically recognize viral components and initiate the expression of antiviral proteins to restrict viral replication. Coronaviruses are RNA viruses that replicate in the host cytoplasm and evade innate immune sensing in most cell types, either passively by hiding their viral signatures and limiting exposure to sensors or actively, by encoding viral antagonists to counteract the effects of interferons. Since many cytoplasmic viruses exploit similar mechanisms of innate immune evasion, mechanistic insight into the direct interplay between viral RNA, viral RNA-processing enzymes, cellular sensors and antiviral proteins will be highly relevant to develop novel antiviral targets and to restrict important animal and human infections.

Middle east respiratory syndrome coronavirus 4a protein is a double-stranded RNA-binding protein that suppresses PACT-induced activation of RIG-I and MDA5 in the innate antiviral response

Middle East respiratory syndrome coronavirus (MERS-CoV) is an emerging pathogen that causes severe disease in human. MERS-CoV is closely related to bat coronaviruses HKU4 and HKU5. Evasion of the innate antiviral response might contribute significantly to MERS-CoV pathogenesis, but the mechanism is poorly understood. In this study, we characterized MERS-CoV 4a protein as a novel immunosuppressive factor that antagonizes type I interferon production. MERS-CoV 4a protein contains a double-stranded RNA-binding domain capable of interacting with poly(I · C). Expression of MERS-CoV 4a protein suppressed the interferon production induced by poly(I · C) or Sendai virus. RNA binding of MERS-CoV 4a protein was required for IFN antagonism, a property shared by 4a protein of bat coronavirus HKU5 but not by the counterpart in bat coronavirus HKU4. MERS-CoV 4a protein interacted with PACT in an RNA-dependent manner but not with RIG-I or MDA5. It inhibited PACT-induced activation of RIG-I and MDA5 but did not affect the activity of downstream effectors such as RIG-I, MDA5, MAVS, TBK1, and IRF3. Taken together, our findings suggest a new mechanism through which MERS-CoV employs a viral double-stranded RNA-binding protein to circumvent the innate antiviral response by perturbing the function of cellular double-stranded RNA-binding protein PACT. PACT targeting might be a common strategy used by different viruses, including Ebola virus and herpes simplex virus 1, to counteract innate immunity.

IMPORTANCE

Middle East respiratory syndrome coronavirus (MERS-CoV) is an emerging and highly lethal human pathogen. Why MERS-CoV causes severe disease in human is unclear, and one possibility is that MERS-CoV is particularly efficient in counteracting host immunity, including the sensing of virus invasion. It will therefore be critical to clarify how MERS-CoV cripples the host proteins that sense viruses and to compare MERS-CoV with its ancestral viruses in bats in the counteraction of virus sensing. This work not only provides a new understanding of the abilities of MERS-CoV and closely related bat viruses to subvert virus sensing but also might prove useful in revealing new strategies for the development of vaccines and antivirals.

PKR mediated regulation of inflammation and IL-10 during viral encephalomyelitis

Double-stranded RNA-dependent protein kinase (PKR) regulates antiviral activity, immune responses, apoptosis and neurotoxicity. Gliatropic coronavirus infection induced PKR activation in infected as well uninfected cells within the central nervous system (CNS). However, PKR deficiency only modestly increased viral replication and did not affect IFN-α/β or IL-1β expression. Despite reduced Il-6, Ccl5, and Cxcl10 mRNA, protein levels remained unaltered. Furthermore, PKR deficiency selectively reduced IL-10 production in CD4, but not CD8 T cells, without affecting CNS pathology. The results demonstrate the ability of PKR to balance neuroinflammation by selectively modulating key cytokines and chemokines in CNS resident and CD4 T cells.

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.

Transcriptome analysis of chicken kidney tissues following coronavirus avian infectious bronchitis virus infection

Background

Infectious bronchitis virus (IBV), a prototype of the Coronaviridae family, is an economically important causative agent of infectious bronchitis in chickens and causes an acute and highly contagious upper respiratory tract infections that may lead to nephritis. However, the molecular antiviral mechanisms of chickens to IBV infection remain poorly understood. In this study, we conducted global gene expression profiling of chicken kidney tissue after nephropathogenic IBV infection to better understand the interactions between host and virus.

Results

IBV infection contributed to differential expression of 1777 genes, of which 876 were up-regulated and 901 down-regulated in the kidney compared to those of control chickens and 103 associated with immune and inflammatory responses may play important roles in the host defense response during IBV infection. Twelve of the altered immune-related genes were confirmed by real-time RT-PCR. Gene ontology category, KEGG pathway, and gene interaction networks (STRING analysis) were analyzed to identify relationships among differentially expressed genes involved in signal transduction, cell adhesion, immune responses, apoptosis regulation, positive regulation of the I-kappaB kinase/NF-kappaB cascade and response to cytokine stimulus. Most of these genes were related and formed a large network, in which IL6, STAT1, MYD88, IRF1 and NFKB2 were key genes.

Conclusions

Our results provided comprehensive knowledge regarding the host transcriptional response to IBV infection in chicken kidney tissues, thereby providing insight into IBV pathogenesis, particularly the involvement of innate immune pathway genes associated with IBV infection.

Middle East respiratory syndrome coronavirus accessory protein 4a is a type I interferon antagonist

Middle East respiratory syndrome coronavirus (MERS-CoV) causes severe acute respiratory infection with as yet unclear epidemiology. We previously showed that MERS-CoV counteracts parts of the innate immune response in human bronchiolar cells. Here we analyzed accessory proteins 3, 4a, 4b, and 5 for their abilities to inhibit the type I interferon response. Accessory protein 4a was found to block interferon induction at the level of melanoma differentiation-associated protein 5 (MDA5) activation presumably by direct interaction with double-stranded RNA.

Oligodendroglia are limited in type I interferon induction and responsiveness in vivo

Type I interferons (IFNα/β) provide a primary defense against infection. Nevertheless, the dynamics of IFNα/β induction and responsiveness by central nervous system (CNS) resident cells in vivo in response to viral infections are poorly understood. Mice were infected with a neurotropic coronavirus with tropism for oligodendroglia and microglia to probe innate antiviral responses during acute encephalomyelitis. Expression of genes associated with the IFNα/β pathways was monitored in microglia and oligodendroglia purified from naïve and infected mice by fluorescent activated cell sorting. Compared with microglia, oligodendroglia were characterized by low basal expression of mRNA encoding viral RNA sensing pattern recognition receptors (PRRs), IFNα/β receptor chains, interferon sensitive genes (ISG), as well as kinases and transcription factors critical in IFNα/β signaling. Although PRRs and ISGs were upregulated by infection in both cell types, the repertoire and absolute mRNA levels were more limited in oligodendroglia. Furthermore, although oligodendroglia harbored higher levels of viral RNA compared with microglia, Ifnα/β was only induced in microglia. Stimulation with the double stranded RNA analogue poly I:C also failed to induce Ifnα/β in oligodendroglia, and resulted in reduced and delayed induction of ISGs compared with microglia. The limited antiviral response by oligodendroglia was associated with a high threshold for upregulation of Ikkε and Irf7 transcripts, both central to amplifying IFNα/β responses. Overall, these data reveal that oligodendroglia from the adult CNS are poor sensors of viral infection and suggest they require exogenous IFNα/β to establish an antiviral state.

Recent progress in studies of arterivirus- and coronavirus-host interactions

Animal coronaviruses, such as infectious bronchitis virus (IBV), and arteriviruses, such as porcine reproductive and respiratory syndrome virus (PRRSV), are able to manifest highly contagious infections in their specific native hosts, thereby arising in critical economic damage to animal industries. This review discusses recent progress in studies of virus-host interactions during animal and human coronavirus and arterivirus infections, with emphasis on IBV-host cell interactions. These interactions may be directly involved in viral replication or lead to the alteration of certain signaling pathways, such as cell stress response and innate immunity, to facilitate viral replication and pathogenesis.

SARS coronavirus pathogenesis: host innate immune responses and viral antagonism of interferon

SARS-CoV is a pathogenic coronavirus that emerged from a zoonotic reservoir, leading to global dissemination of the virus. The association SARS-CoV with aberrant cytokine, chemokine, and Interferon Stimulated Gene (ISG) responses in patients provided evidence that SARS-CoV pathogenesis is at least partially controlled by innate immune signaling. Utilizing models for SARS-CoV infection, key components of innate immune signaling pathways have been identified as protective factors against SARS-CoV disease, including STAT1 and MyD88. Gene transcription signatures unique to SARS-CoV disease states have been identified, but host factors that regulate exacerbated disease phenotypes still remain largely undetermined. SARS-CoV encodes several proteins that modulate innate immune signaling through the antagonism of the induction of Interferon and by avoidance of ISG effector functions.

Coronavirus papain-like proteases negatively regulate antiviral innate immune response through disruption of STING-mediated signaling

Viruses have evolved elaborate mechanisms to evade or inactivate the complex system of sensors and signaling molecules that make up the host innate immune response. Here we show that human coronavirus (HCoV) NL63 and severe acute respiratory syndrome (SARS) CoV papain-like proteases (PLP) antagonize innate immune signaling mediated by STING (stimulator of interferon genes, also known as MITA/ERIS/MYPS). STING resides in the endoplasmic reticulum and upon activation, forms dimers which assemble with MAVS, TBK-1 and IKKε, leading to IRF-3 activation and subsequent induction of interferon (IFN). We found that expression of the membrane anchored PLP domain from human HCoV-NL63 (PLP2-TM) or SARS-CoV (PLpro-TM) inhibits STING-mediated activation of IRF-3 nuclear translocation and induction of IRF-3 dependent promoters. Both catalytically active and inactive forms of CoV PLPs co-immunoprecipitated with STING, and viral replicase proteins co-localize with STING in HCoV-NL63-infected cells. Ectopic expression of catalytically active PLP2-TM blocks STING dimer formation and negatively regulates assembly of STING-MAVS-TBK1/IKKε complexes required for activation of IRF-3. STING dimerization was also substantially reduced in cells infected with SARS-CoV. Furthermore, the level of ubiquitinated forms of STING, RIG-I, TBK1 and IRF-3 are reduced in cells expressing wild type or catalytic mutants of PLP2-TM, likely contributing to disruption of signaling required for IFN induction. These results describe a new mechanism used by CoVs in which CoV PLPs negatively regulate antiviral defenses by disrupting the STING-mediated IFN induction.

Arterivirus and nairovirus ovarian tumor domain-containing Deubiquitinases target activated RIG-I to control innate immune signaling

The innate immune response constitutes the first line of defense against viral infection and is extensively regulated through ubiquitination. The removal of ubiquitin from innate immunity signaling factors by deubiquitinating enzymes (DUBs) therefore provides a potential opportunity for viruses to evade this host defense system. It was previously found that specific proteases encoded by the unrelated arteri- and nairoviruses resemble the ovarian tumor domain-containing (OTU) family of DUBs. In arteriviruses, this domain has been characterized before as a papain-like protease (PLP2) that is also involved in replicase polyprotein processing. In nairoviruses, the DUB resides in the polymerase protein but is not essential for RNA replication. Using both in vitro and cell-based assays, we now show that PLP2 DUB activity is conserved in all members of the arterivirus family and that both arteri- and nairovirus DUBs inhibit RIG-I-mediated innate immune signaling when overexpressed. The potential relevance of RIG-I-like receptor (RLR) signaling for the innate immune response against arterivirus infection is supported by our finding that in mouse embryonic fibroblasts, the production of beta interferon primarily depends on the recognition of arterivirus RNA by the pattern-recognition receptor MDA5. Interestingly, we also found that both arteri- and nairovirus DUBs inhibit RIG-I ubiquitination upon overexpression, suggesting that both MDA5 and RIG-I have a role in countering infection by arteriviruses. Taken together, our results support the hypothesis that arteri- and nairoviruses employ their deubiquitinating potential to inactivate cellular proteins involved in RLR-mediated innate immune signaling, as exemplified by the deubiquitination of RIG-I.

Toll-like receptors 2 and 3 agonists differentially affect oligodendrocyte survival, differentiation, and myelin membrane formation

Toll-like receptors (TLRs) play a key role in controlling innate immune responses to a wide variety of pathogen-associated molecules as well as endogenous signals. In addition, TLR expression within nonimmune cells has been recognized as as modulator of cell behavior. In this study we have addressed the question of whether functional TLRs are expressed on oligodendrocytes, the myelinating cells of the central nervous system. Primary cultures of rat oligodendrocytes at different maturation stages were found to express TLR2 and, to lesser extent, TLR3. Immunocytochemical analysis revealed that both TLRs were localized at the cell body and primary processes and were excluded from myelin-like membranes. Interestingly, innate immune receptor ligands were able to modulate oligodendrocyte survival, differentiation, and myelin-like membrane formation, indicating that TLRs on oligodendrocytes are functional. In highly purified oligodendrocytes cultures, the TLR2 agonist zymosan promoted survival, differentiation, and myelin-like membrane formation, whereas poly-I:C, a TLR3 ligand, was a potent inducer of apoptosis. Together, these data indicate that, in addition to other neural cell types, also oligodendrocytes express functional TLRs, which play a role in regulating various aspects of oligodendrocyte behavior.