Cardiovirus leader proteins are functionally interchangeable and have evolved to adapt to virus replication fitness

Picornavirus security proteins promote the release of extracellular vesicle enclosed viruses via the modulation of host kinases

The discovery that extracellular vesicles (EVs) serve as carriers of virus particles calls for a reevaluation of the release strategies of non-enveloped viruses. Little is currently known about the molecular mechanisms that determine the release and composition of EVs produced by virus-infected cells, as well as conservation of these mechanisms among viruses. We previously described an important role for the Leader protein of the picornavirus encephalomyocarditis virus (EMCV) in the induction of virus-carrying EV subsets with distinct molecular and physical properties. EMCV L acts as a 'viral security protein' by suppressing host antiviral stress and type-I interferon (IFN) responses. Here, we tested the ability of functionally related picornavirus proteins of Theilers murine encephalitis virus (TMEV L), Saffold virus (SAFV L), and coxsackievirus B3 (CVB3 2Apro), to rescue EV and EV-enclosed virus release when introduced in Leader-deficient EMCV. We show that all viral security proteins tested were able to promote virus packaging in EVs, but that only the expression of EMCV L and CVB3 2Apro increased overall EV production. We provide evidence that one of the main antiviral pathways counteracted by this class of picornaviral proteins, i.e. the inhibition of PKR-mediated stress responses, affected EV and EV-enclosed virus release during infection. Moreover, we show that the enhanced capacity of the viral proteins EMCV L and CVB3 2Apro to promote EV-enclosed virus release is linked to their ability to simultaneously promote the activation of the stress kinase P38 MAPK. Taken together, we demonstrate that cellular stress pathways involving the kinases PKR and P38 are modulated by the activity of non-structural viral proteins to increase the release EV-enclosed viruses during picornavirus infections. These data shed new light on the molecular regulation of EV production in response to virus infection.

MicroRNA-detargeting proves more effective than leader gene deletion for improving safety of oncolytic Mengovirus in a nude mouse model

A dual microRNA-detargeted oncolytic Mengovirus, vMC24NC, proved highly effective against a murine plasmacytoma in an immunocompetent syngeneic mouse model; however, there remains the concern of escape mutant development and the potential for toxicity in severely immunocompromised cancer patients when it is used as an oncolytic virus. Therefore, we sought to compare the safety and efficacy profiles of an attenuated Mengovirus containing a virulence gene deletion versus vMC24NC in an immunodeficient xenograft mouse model of human glioblastoma. A Mengovirus construct, vMC24ΔL, wherein the gene coding for the leader protein, a virulence factor, was deleted, was used for comparison. The vMC24ΔL induced significant levels of toxicity following treatment of subcutaneous human glioblastoma (U87-MG) xenografts as well as when injected intracranially in athymic nude mice, reducing the overall survival. The in vivo toxicity of vMC24ΔL was associated with viral replication in nervous and cardiac tissue. In contrast, microRNA-detargeted vMC24NC demonstrated excellent efficacy against U87-MG subcutaneous xenografts and improved overall survival significantly compared to that of control mice without toxicity. These results reinforce microRNA-detargeting as an effective strategy for ameliorating unwanted toxicities of oncolytic picornaviruses and substantiate vMC24NC as an ideal candidate for clinical development against certain cancers in both immunocompetent and immunodeficient hosts.

The location of sensing determines the pancreatic β-cell response to the viral mimetic dsRNA

Viral infection is an environmental trigger that has been suggested to initiate pancreatic β-cell damage, leading to the development of autoimmune diabetes. Viruses potently activate the immune system and can damage β cells by either directly infecting them or stimulating the production of secondary effector molecules (such as proinflammatory cytokines) during bystander activation. However, how and where β cells recognize viruses is unclear, and the antiviral responses that are initiated following virus recognition are incompletely understood. In this study, we show that the β-cell response to dsRNA, a viral replication intermediate known to activate antiviral responses, is determined by the cellular location of sensing (intracellular versus extracellular) and differs from the cellular response to cytokine treatment. Using biochemical and immunological methods, we show that β cells selectively respond to intracellular dsRNA by expressing type I interferons (IFNs) and inducing apoptosis, but that they do not respond to extracellular dsRNA. These responses differ from the activities of cytokines on β cells, which are mediated by inducible nitric oxide synthase expression and β-cell production of nitric oxide. These findings provide evidence that the antiviral activities of type I IFN production and apoptosis are elicited in β cells via the recognition of intracellular viral replication intermediates and that β cells lack the capacity to respond to extracellular viral intermediates known to activate innate immune responses.

IFN-λ Decreases Murid Herpesvirus-4 Infection of the Olfactory Epithelium but Fails to Prevent Virus Reactivation in the Vaginal Mucosa

Murid herpesvirus-4 (MuHV-4), a natural gammaherpesvirus of rodents, can infect the mouse through the nasal mucosa, where it targets sustentacular cells and olfactory neurons in the olfactory epithelium before it propagates to myeloid cells and then to B cells in lymphoid tissues. After establishment of latency in B cells, viral reactivation occurs in the genital tract in 80% of female mice, which can lead to spontaneous sexual transmission to co-housed males. Interferon-lambda (IFN-λ) is a key player of the innate immune response at mucosal surfaces and is believed to limit the transmission of numerous viruses by acting on epithelial cells. We used in vivo plasmid-mediated IFN-λ expression to assess whether IFN-λ could prophylactically limit MuHV-4 infection in the olfactory and vaginal mucosae. In vitro, IFN-λ decreased MuHV-4 infection in cells that overexpressed IFN-λ receptor 1 (IFNLR1). In vivo, prophylactic IFN-λ expression decreased infection of the olfactory epithelium but did not prevent virus propagation to downstream organs, such as the spleen where the virus establishes latency. In the olfactory epithelium, sustentacular cells readily responded to IFN-λ. In contrast, olfactory neurons did not respond to IFN-λ, thus, likely allowing viral entry. In the female genital tract, columnar epithelial cells strongly responded to IFN-λ, as did most vaginal epithelial cells, although with some variation from mouse to mouse. IFN-λ expression, however, failed to prevent virus reactivation in the vaginal mucosa. In conclusion, IFN-λ decreased MuHV-4 replication in the upper respiratory epithelium, likely by protecting the sustentacular epithelial cells, but it did not protect olfactory neurons and failed to block virus reactivation in the genital mucosa.

Species Specificity of Type III Interferon Activity and Development of a Sensitive Luciferase-Based Bioassay for Quantitation of Mouse Interferon-λ

The type III interferon (IFN-λ) family includes 4 IFN-λ subtypes in man. In the mouse, only the genes coding for IFN-λ2 and -λ3 are present. Unlike mouse and human type I IFNs (IFN-α/β), which exhibit strong species specificity, type III IFNs were reported to act in a cross-specific manner. We reexamined the cross-specificity and observed that mouse and human IFN-λ exhibit some species specificity, although much less than type I IFNs. Mouse IFN-λ3 displayed clear species specificity, being 25-fold less active in human cells than the closely related mouse IFN-λ2. This specificity likely depends on amino acids in α helices A and F that diverged from other IFN-λ sequences. Human IFN-λ4, in contrast, retained high activity in mouse cells. We next developed a firefly luciferase-based reporter cell line, named Fawa-λ-luc, to detect IFN-λ in biological fluids with high specificity and sensitivity. Fawa-λ-luc cells, derived from mouse epithelial cells that are responsive to IFN-λ, were made nonresponsive to type I IFNs by inactivation of the Ifnar2 gene and strongly responsive to IFN-λ by overexpression of the mouse IFNLR1. This bioassay was as sensitive as a commercially available enzyme-linked immunosorbent assay in detecting mouse IFN-λ in cell culture supernatant, as well as in serum and bronchoalveolar lavage samples of virus-infected mice. The assay also enabled the sensitive detection of human IFN-λ activity, including that of the divergent IFN-λ4 with a bias, however, due to variable activity of IFN-λ subtypes.

Encephalomyocarditis virus 3C protease attenuates type I interferon production through disrupting the TANK-TBK1-IKKε-IRF3 complex

TRAF family member-associated NF-κB activator (TANK) is a scaffold protein that assembles into the interferon (IFN) regulator factor 3 (IRF3)-phosphorylating TANK-binding kinase 1 (TBK1)-(IκB) kinase ε (IKKε) complex, where it is involved in regulating phosphorylation of the IRF3 and IFN production. However, the functions of TANK in encephalomyocarditis virus (EMCV) infection-induced type I IFN production are not fully understood. Here, we demonstrated that, instead of stimulating type I IFN production, the EMCV-HB10 strain infection potently inhibited Sendai virus- and polyI:C-induced IRF3 phosphorylation and type I IFN production in HEK293T cells. Mechanistically, EMCV 3C protease (EMCV 3C) cleaved TANK and disrupted the TANK-TBK1-IKKε-IRF3 complex, which resulted in the reduction in IRF3 phosphorylation and type I IFN production. Taken together, our findings demonstrate that EMCV adopts a novel strategy to evade host innate immune responses through cleavage of TANK.

Saffold virus, an emerging human cardiovirus

Saffold virus (SAFV) is an emerging human cardiovirus that has been shown to be ubiquitous. Initial studies of SAFV focused on respiratory and gastrointestinal infection; however, it has also recently been associated with diverse clinical symptoms including the endocrine, cardiovascular, and neurological systems. Given the systemic nature of SAFV, and its high prevalence, understanding its pathogenicity and clinical impact is of utmost importance. This comprehensive review highlights and discusses recent developments in epidemiology, human pathogenicity, animal, and molecular studies related to SAFV. It also provides detailed insights into the neuropathogenicity of SAFV. We argue that human studies have been confounded by coinfections and therefore require support from robust molecular and animal research. Thereby, we aim to provide foresight into further research to better understand this emerging virus.

Cellular microRNAs Repress Vesicular Stomatitis Virus but Not Theiler's Virus Replication

Picornavirus’ genomic RNA is a positive-stranded RNA sequence that also serves as a template for translation and replication. Cellular microRNAs were reported to interfere to different extents with the replication of specific picornaviruses, mostly acting as inhibitors. However, owing to the high error rate of their RNA-dependent RNA-polymerases, picornavirus quasi-species are expected to evolve rapidly in order to lose any detrimental microRNA target sequence. We examined the genome of Theiler’s murine encephalomyelitis virus (TMEV) for the presence of under-represented microRNA target sequences that could have been selected against during virus evolution. However, little evidence for such sequences was found in the genome of TMEV and introduction of the most under-represented microRNA target (miR-770-3p) in TMEV did not significantly affect viral replication in cells expressing this microRNA. To test the global impact of cellular microRNAs on viral replication, we designed a strategy based on short-term Dicer inactivation in mouse embryonic fibroblasts. Short-term Dicer inactivation led to a >10-fold decrease in microRNA abundance and strongly increased replication of Vesicular stomatitis virus (VSV), which was used as a microRNA-sensitive control virus. In contrast, Dicer inactivation did not increase TMEV replication. In conclusion, cellular microRNAs appear to exert little influence on Theiler’s virus fitness.

The Pathogenesis of Saffold Virus in AG129 Mice and the Effects of Its Truncated L Protein in the Central Nervous System

Saffold Virus (SAFV) is a human cardiovirus that has been suggested to cause severe infection of the central nervous system (CNS). Compared to a similar virus, Theiler’s murine encephalomyelitis virus (TMEV), SAFV has a truncated Leader (L) protein, a protein essential in the establishment of persistent CNS infections. In this study, we generated a chimeric SAFV by replacing the L protein of SAFV with that of TMEV. We then compared the replication in cell cultures and pathogenesis in a mouse model. We showed that both SAFV and chimeric SAFV are able to infect Vero and Neuro2a cells well, but only chimeric SAFV was able to infect RAW264.7. We then showed that mice lacking IFN-α/β and IFN-γ receptors provide a good animal model for SAFV infection, and further identified the locality of the infection to the ventral horn of the spine and several locations in the brain. Lastly, we showed that neither SAFV nor chimeric SAFV causes persistence in this model. Overall, our results provide a strong basis on which the mechanisms underlying Saffold virus induced neuropathogenesis can be further studied and, hence, facilitating new information about its pathogenesis.

The Interferon-Inducible Mouse Apolipoprotein L9 and Prohibitins Cooperate to Restrict Theiler's Virus Replication

Apolipoprotein L9b (Apol9b) is an interferon-stimulated gene (ISG) that has antiviral activity and is weakly expressed in primary mouse neurons as compared to other cell types. Here, we show that both Apol9 isoforms (Apol9b and Apol9a) inhibit replication of Theiler’s murine encephalomyelitis virus (TMEV) but not replication of vesicular stomatitis virus (VSV), Murid herpesvirus-4 (MuHV-4), or infection by a lentiviral vector. Apol9 genes are strongly expressed in mouse liver and, to a lesser extent, in pancreas, adipose tissue and intestine. Their expression is increased by type I interferon and viral infection. In contrast to genuine apolipoproteins that are involved in lipid transport, ApoL9 has an intracytoplasmic localization and does not seem to be secreted. The cytoplasmic localization of ApoL9 is in line with the observation that ApoL9 inhibits the replication step of TMEV infection. In contrast to human ApoL6, ApoL9 did not sensitize cells to apoptosis, in spite of the presence of a conserved putative BH3 domain, required for antiviral activity. ApoL9a and b isoforms interact with cellular prohibitin 1 (Phb1) and prohibitin 2 (Phb2) and this interaction might contribute to ApoL9 antiviral activity. Knocking down Phb2 slightly increased TMEV replication, irrespective of ApoL9 overexpression. The antiviral activity of prohibitins against TMEV contrasts with the pro-viral activity of prohibitins observed for VSV and reported previously for Dengue 2 (DENV-2), Chikungunya (CHIKV) and influenza H5N1 viruses. ApoL9 is thus an example of ISG displaying a narrow antiviral range, which likely acts in complex with prohibitins to restrict TMEV replication.

Genomic Changes in an Attenuated ZB Strain of Foot-and-Mouth Disease Virus Serotype Asia1 and Comparison with Its Virulent Parental Strain

The molecular basis of attenuation of foot-and-mouth disease virus (FMDV) serotype Asia1 ZB strain remains unknown. To understand the genetic changes of attenuation, we compared the entire genomes of three different rabbit-passaged attenuated ZB strains (ZB/CHA/58(att), ZBRF168, and ZBRF188) and their virulent parental strains (ZBCF22 and YNBS/58). The results showed that attenuation may be brought about by 28 common amino acid substitutions in the coding region, with one nucleotide point mutation in the 5′-untranslated region (5′-UTR) and another one in the 3′-UTR. In addition, a total of 21 nucleotides silent mutations had been found after attenuation. These substitutions, alone or in combination, may be responsible for the attenuated phenotype of the ZB strain in cattle. This will contribute to elucidation of attenuating molecular basis of the FMDV ZB strain.

Binding interactions between the encephalomyocarditis virus leader and protein 2A

The leader (L) and 2A proteins of cardioviruses are the primary antihost agents produced during infection. For encephalomyocarditis virus (EMCV), the prototype of the genus Cardiovirus, these proteins interact independently with key cellular partners to bring about inhibition of active nucleocytoplasmic trafficking and cap-dependent translation, respectively. L and 2A also bind each other and require this cooperation to achieve their effects during infection. Recombinant L and 2A interact with 1:1 stoichiometry at a KD (equilibrium dissociation constant) of 1.5 μM. The mapped contact domains include the amino-proximal third of 2A (first 50 amino acids) and the central hinge region of L. This contact partially overlaps the L segment that makes subsequent contact with Ran GTPase in the nucleus, and Ran can displace 2A from L. The equivalent proteins from Theiler's murine encephalomyelitis virus (TMEV; BeAn) and Saffold virus interact similarly in any subtype combination, with various affinities. The data suggest a mechanism whereby L takes advantage of the nuclear localization signal in the COOH region of 2A to enhance its trafficking to the nucleus. Once there, it exchanges partners in favor of Ran. This required cooperation during infection explains many observed codependent phenotypes of L and 2A mutations.

IMPORTANCE

Cardiovirus pathogenesis phenotypes vary dramatically, from asymptomatic, to mild gastrointestinal (GI) distress, to persistent demyelination and even encephalitic death. Leader and 2A are the primary viral determinants of pathogenesis, so understanding how these proteins cooperate to induce such a wide variety of outcomes for the host is of great important and interest to the field of virology, especially to those who use TMEV as a murine model for multiple sclerosis.

Picornavirus infection leading to immunosuppression

Viruses, such as HIV, hepatitis A, poliovirus, coxsackievirus B3 and foot-and-mouth disease virus, use a variety of mechanisms to suppress the human immune system in order to evade clearance by the host. Therefore, investigating how a few changes in the viral genome of a nonlethal virus can lead to an alteration in disease, from survivable to immunosuppression and death, would provide valuable information into viral pathogenesis. In addition, we propose that gaining a better insight into how these viruses suppress an antiviral immune response could lead to viral-based therapeutics to combat specifc autoimmune diseases such as multiple sclerosis and Type 1 diabetes.

Human but not mouse hepatocytes respond to interferon-lambda in vivo

The type III interferon (IFN) receptor is preferentially expressed by epithelial cells. It is made of two subunits: IFNLR1, which is specific to IFN-lambda (IFN-λ) and IL10RB, which is shared by other cytokine receptors. Human hepatocytes express IFNLR1 and respond to IFN-λ. In contrast, the IFN-λ response of the mouse liver is very weak and IFNLR1 expression is hardly detectable in this organ. Here we investigated the IFN-λ response at the cellular level in the mouse liver and we tested whether human and mouse hepatocytes truly differ in responsiveness to IFN-λ. When monitoring expression of the IFN-responsive Mx genes by immunohistofluorescence, we observed that the IFN-λ response in mouse livers was restricted to cholangiocytes, which form the bile ducts, and that mouse hepatocytes were indeed not responsive to IFN-λ. The lack of mouse hepatocyte response to IFN-λ was observed in different experimental settings, including the infection with a hepatotropic strain of influenza A virus which triggered a strong local production of IFN-λ. With the help of chimeric mice containing transplanted human hepatocytes, we show that hepatocytes of human origin readily responded to IFN-λ in a murine environment. Thus, our data suggest that human but not mouse hepatocytes are responsive to IFN-λ in vivo. The non-responsiveness is an intrinsic property of mouse hepatocytes and is not due to the mouse liver micro-environment.

Inefficient type I interferon-mediated antiviral protection of primary mouse neurons is associated with the lack of apolipoprotein l9 expression

We examined the antiviral response promoted by type I interferons (IFN) in primary mouse neurons. IFN treatment of neuron cultures strongly upregulated the transcription of IFN-stimulated genes but conferred a surprisingly low resistance to infection by neurotropic viruses such as Theiler's murine encephalomyelitis virus (TMEV) or vesicular stomatitis virus (VSV). Response of primary mouse neurons to IFN treatment was heterogeneous, as many neurons failed to express the typical IFN response marker Mx1 after IFN treatment. This heterogeneous response of primary neurons correlated with a low level of basal expression of IFN-stimulated genes, such as Stat1, that are involved in signal transduction of the IFN response. In addition, transcriptomic analysis identified 15 IFN-responsive genes whose expression was low in IFN-treated primary neurons compared to that of primary fibroblasts derived from the same mice (Dhx58, Gvin1, Sp100, Ifi203 isoforms 1 and 2, Irgm2, Lgals3bp, Ifi205, Apol9b, Ifi204, Ifi202b, Tor3a, Slfn2, Ifi35, Lgals9). Among these genes, the gene coding for apolipoprotein L9b (Apol9b) displayed antiviral activity against Theiler's virus when overexpressed in L929 cells or in primary neurons. Accordingly, knocking down Apol9b expression in L929 cells increased viral replication. Therefore, we identified a new antiviral protein induced by interferon, ApoL9b, whose lack of expression in primary neurons likely contributes to the high sensitivity of these cells to viral infection.

IMPORTANCE

The type I interferon (IFN) response is an innate immune defense mechanism that is critical to contain viral infection in the host until an adaptive immune response can be mounted. Neurons are a paradigm for postmitotic, highly differentiated cells. Our data show that primary mouse neurons that are exposed to type I interferon remain surprisingly susceptible to viral infection. On one hand, the low level of basal expression of some factors in neurons might prevent a rapid response of these cells. On the other hand, some genes that are typically activated by type I interferon in other cell types are expressed at much lower levels in neurons. Among these genes is the gene encoding apolipoprotein L9, a protein that proved to have antiviral activity against the neurotropic Theiler's murine encephalomyelitis virus. Our data suggest important functional differences in the IFN response mounted by specific cell populations.

Encephalomyocarditis virus leader is phosphorylated by CK2 and syk as a requirement for subsequent phosphorylation of cellular nucleoporins

Encephalomyocarditis virus and Theilovirus are species in the Cardiovirus genus of the Picornaviridae family. For all cardioviruses, the viral polyprotein is initiated with a short Leader (L) protein unique to this genus. The nuclear magnetic resonance (NMR) structure of LE from encephalomyocarditis virus (EMCV) has been determined. The protein has an NH2-proximal CHCC zinc finger, a central linker, and a contiguous, highly acidic motif. The theiloviruses encode the same domains, with one or two additional, COOH-proximal domains, characteristic of the human Saffold viruses (SafV) and Theiler's murine encephalomyelitis viruses (TMEV), respectively. The expression of a cardiovirus L, in recombinant form, or during infection/transfection, triggers an extensive, cell-dependent, antihost phosphorylation cascade, targeting nucleoporins (Nups) that form the hydrophobic core of nuclear pore complexes (NPC). The consequent inhibition of active nucleocytoplasmic trafficking is potent and prevents the host from mounting an effective antiviral response. For this inhibition, the L proteins themselves must be phosphorylated. In cells (extracts or recombinant form), LE was shown to be phosphorylated at Thr47 and Tyr41. The first reaction (Thr47), catalyzed by casein kinase 2 (CK2), is an obligatory precedent to the second event (Tyr41), catalyzed by spleen tyrosine kinase (Syk). Site mutations in LE, or kinase-specific inhibitors, prevented LE phosphorylation and subsequent Nup phosphorylation. Parallel experiments with LS (SafV-2) and LT (TMEV BeAn) proteins confirmed the general cardiovirus requirement for L phosphorylation, but CK2 was not the culpable kinase. It is likely that LS and LT are both activated by alternative kinases in different cell types, probably reactive within the Theilo-specific domains. IMPORTANCE An understanding of the diverse methods used by viruses to interfere with cellular processes is important because they can teach us how to control virus infections. This report shows how viruses in the same genus use different cellular enzymes to phosphorylate their proteins. If these processes are interfered with, the viruses are severely disabled.

Mutation of the Theiler's virus leader protein zinc-finger domain impairs apoptotic activity in murine macrophages

The Theiler's murine encephalomyelitis virus (TMEV) leader (L) protein zinc-finger domain was mutated to study its role in cell death in infection of the murine macrophage cell line M1-D, revealing that an intact zinc-finger domain is required for full apoptotic activity. A functional L zinc-finger domain was also required for activation of p38 MAPK that results in phosphorylation and activation of p53, and in turn, alteration of the conformation of the anti-apoptotic proteins Puma and Mcl-1, leading to the release of pro-apoptotic Bax and apoptosis through the intrinsic pathway. TMEV infection also inhibits host protein synthesis, a stress shown by others to induce apoptosis. Since inhibition of host protein synthesis follows rather than precedes activation of MKK3/6 and p38, it seems less likely that it triggers apoptosis in infected cells. Finally, we showed that the levels of reactive oxygen species following infection were consistent with apoptotic rather than necrotic cell death. Thus, these experiments support an important role for the TMEV L protein zinc-finger domain in apoptosis in an infected murine macrophage line.

IFN-ε is constitutively expressed by cells of the reproductive tract and is inefficiently secreted by fibroblasts and cell lines

Type-I interferons (IFNs) form a large family of cytokines that primarily act to control the early development of viral infections. Typical type-I IFN genes, such as those encoding IFN-α or IFN-β are upregulated by viral infection in many cell types. In contrast, the gene encoding IFN-ε was reported to be constitutively expressed by cells of the female reproductive tract and to contribute to the protection against vaginal infections with herpes simplex virus 2 and Chlamydia muridarum. Our data confirm the lack of induction of IFN-ε expression after viral infection and the constitutive expression of IFN-ε by cells of the female but also of the male reproductive organs. Interestingly, when expressed from transfected expression plasmids in 293T, HeLa or Neuro2A cells, the mouse and human IFN-ε precursors were inefficiently processed and secretion of IFN-ε was minimal. Analysis of chimeric constructs produced between IFN-ε and limitin (IFN-ζ) showed that both the signal peptide and the mature moiety of IFN-ε contribute to poor processing of the precursor. Immunofluorescent detection of FLAG-tagged IFN-ε in transfected cells suggested that IFN-ε and chimeric proteins were defective for progression through the secretory pathway. IFN-ε did not, however, act intracellularly and impart an antiviral state to producing cells. Given the constitutive expression of IFN-ε in specialized cells and the poor processing of IFN-ε precursor in fibroblasts and cell lines, we hypothesize that IFN-ε secretion may require a co-factor specifically expressed in cells of the reproductive organs, that might secure the system against aberrant release of this IFN.

Evasion of antiviral innate immunity by Theiler's virus L* protein through direct inhibition of RNase L

Theiler's virus is a neurotropic picornavirus responsible for chronic infections of the central nervous system. The establishment of a persistent infection and the subsequent demyelinating disease triggered by the virus depend on the expression of L*, a viral accessory protein encoded by an alternative open reading frame of the virus. We discovered that L* potently inhibits the interferon-inducible OAS/RNase L pathway. The antagonism of RNase L by L* was particularly prominent in macrophages where baseline oligoadenylate synthetase (OAS) and RNase L expression levels are elevated, but was detectable in fibroblasts after IFN pretreatment. L* mutations significantly affected Theiler's virus replication in primary macrophages derived from wild-type but not from RNase L-deficient mice. L* counteracted the OAS/RNase L pathway through direct interaction with the ankyrin domain of RNase L, resulting in the inhibition of this enzyme. Interestingly, RNase L inhibition was species-specific as Theiler's virus L* protein blocked murine RNase L but not human RNase L or RNase L of other mammals or birds. Direct RNase L inhibition by L* and species specificity were confirmed in an in vitro assay performed with purified proteins. These results demonstrate a novel viral mechanism to elude the antiviral OAS/RNase L pathway. By targeting the effector enzyme of this antiviral pathway, L* potently inhibits RNase L, underscoring the importance of this enzyme in innate immunity against Theiler's virus.

Theiler's virus is a murine picornavirus (same family as poliovirus) which has a striking ability to establish persistent infections of the central nervous system. To do so, the virus has to counteract the immune response of the host and particularly the potent response mediated by interferon. We observed that a protein encoded by Theiler's virus, the L* protein, inhibited the RNase L pathway, one of the best-characterized pathways mediating the antiviral IFN response. In contrast to previously identified viral antagonists of this pathway, L* was found to act directly on RNase L, the effector enzyme of the pathway. L* activity was found to be species-specific as it inhibited murine but not human RNase L. We confirmed the species-specificity and the direct interaction between L* and RNase L in vitro, using purified proteins. Acting at the effector step in the pathway allows L* to block RNase L activity efficiently. This suggests that RNase L is particularly important to control Theiler's virus replication in vivo. Another virus, mouse hepatitis virus (MHV), was recently shown to interfere with RNase L activation. Theiler's virus and MHV share a marked tropism for macrophages which may suggest that the RNase L pathway is particularly important in this cell type.

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.

Encephalomyocarditis virus Leader protein hinge domain is responsible for interactions with Ran GTPase

Encephalomyocarditis virus (EMCV), a Cardiovirus, initiates its polyprotein with a short 67 amino acid Leader (L) sequence. The protein acts as a unique pathogenicity factor, with anti-host activities which include the triggering of nuclear pore complex hyperphosphorylation and direct binding inhibition of the active cellular transport protein, Ran GTPase. Chemical modifications and protein mutagenesis now map the Ran binding domain to the L hinge-linker region, and in particular, to amino acids 35-40. Large deletions affecting this region were shown previously to diminish Ran binding. New point mutations, especially K35Q, D37A and W40A, preserve the intact L structure, abolish Ran binding and are deficient for nucleoporin (Nup) hyperphosphorylation. Ran itself morphs through multiple configurations, but reacts most effectively with L when in the GDP format, preferably with an empty nucleotide binding pocket. Therefore, L:Ran binding, mediated by the linker-hinge, is a required step in L-induced nuclear transport inhibition.

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

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

The encephalomyocarditis virus

The encephalomyocarditis virus (EMCV) is a small non-enveloped single-strand RNA virus, the causative agent of not only myocarditis and encephalitis, but also neurological diseases, reproductive disorders and diabetes in many mammalian species. EMCV pathogenesis appears to be viral strain- and host-specific, and a better understanding of EMCV virulence factors is increasingly required. Indeed, EMCV is often used as a model for diabetes and viral myocarditis, and is also widely used in immunology as a double-stranded RNA stimulus in the study of Toll-like as well as cytosolic receptors. However, EMCV virulence and properties have often been neglected. Moreover, EMCV is able to infect humans albeit with a low morbidity. Progress on xenografts, such as pig heart transplantation in humans, has raised safety concerns that need to be explored. In this review we will highlight the biology of EMCV and all known and potential virulence factors.

The fecal viral flora of wild rodents

The frequent interactions of rodents with humans make them a common source of zoonotic infections. To obtain an initial unbiased measure of the viral diversity in the enteric tract of wild rodents we sequenced partially purified, randomly amplified viral RNA and DNA in the feces of 105 wild rodents (mouse, vole, and rat) collected in California and Virginia. We identified in decreasing frequency sequences related to the mammalian viruses families Circoviridae, Picobirnaviridae, Picornaviridae, Astroviridae, Parvoviridae, Papillomaviridae, Adenoviridae, and Coronaviridae. Seventeen small circular DNA genomes containing one or two replicase genes distantly related to the Circoviridae representing several potentially new viral families were characterized. In the Picornaviridae family two new candidate genera as well as a close genetic relative of the human pathogen Aichi virus were characterized. Fragments of the first mouse sapelovirus and picobirnaviruses were identified and the first murine astrovirus genome was characterized. A mouse papillomavirus genome and fragments of a novel adenovirus and adenovirus-associated virus were also sequenced. The next largest fraction of the rodent fecal virome was related to insect viruses of the Densoviridae, Iridoviridae, Polydnaviridae, Dicistroviriade, Bromoviridae, and Virgaviridae families followed by plant virus-related sequences in the Nanoviridae, Geminiviridae, Phycodnaviridae, Secoviridae, Partitiviridae, Tymoviridae, Alphaflexiviridae, and Tombusviridae families reflecting the largely insect and plant rodent diet. Phylogenetic analyses of full and partial viral genomes therefore revealed many previously unreported viral species, genera, and families. The close genetic similarities noted between some rodent and human viruses might reflect past zoonoses. This study increases our understanding of the viral diversity in wild rodents and highlights the large number of still uncharacterized viruses in mammals.

Rodents are the natural reservoir of numerous zoonotic viruses causing serious diseases in humans. We used an unbiased metagenomic approach to characterize the viral diversity in rodent feces. In addition to diet-derived insect and plant viruses mammalian viral sequences were abundant and diverse. Most notably, multiple new circular viral DNA families, two new picornaviridae genera, and the first murine astrovirus and picobirnaviruses were characterized. A mouse kobuvirus was a close relative to the Aichi virus human pathogen. This study significantly increases the known genetic diversity of eukaryotic viruses in rodents and provides an initial description of their enteric viromes.

The leader protein of cardioviruses inhibits stress granule assembly

Stress granules (SG) are cytoplasmic aggregates of stalled translation preinitiation complexes that form in cells exposed to various environmental stresses. Here, we show that stress granules assemble in cells infected with Theiler's murine encephalomyelitis virus (TMEV) mutants carrying alterations in the leader (L) protein, but not in cells infected with wild-type TMEV. Stress granules also formed in STAT1-deficient cells, suggesting that SG formation was not a consequence of increased type I interferon (IFN) production when cells were infected with the mutant virus. Ectopic expression of the wild-type L protein was sufficient to inhibit stress granule formation induced by sodium arsenite or thapsigargin treatment. In conclusion, TMEV infection induces stress granule assembly, but this process is inhibited by the L protein. Unlike poliovirus-induced stress granules, TMEV-induced stress granules did not contain the nuclear protein Sam68 but contained polypyrimidine tract binding protein (PTB), an internal ribosome entry site (IRES)-interacting protein. Moreover, G3BP was not degraded and was found in SG after TMEV infection, suggesting that SG content could be virus specific. Despite the colocalization of PTB with SG and the known interaction of PTB with viral RNA, in situ hybridization and immunofluorescence assays failed to detect viral RNA trapped in infection-induced SG. Recombinant Theiler's viruses expressing the L protein of Saffold virus 2 (SAFV-2), a closely related human theilovirus, or the L protein of mengovirus, an encephalomyocarditis virus (EMCV) strain, also inhibited infection-induced stress granule assembly, suggesting that stress granule antagonism is a common feature of cardiovirus L proteins.

Theiler's virus L* protein is targeted to the mitochondrial outer membrane

The L* protein encoded by Theiler's murine encephalomyelitis virus (TMEV) is a unique example of a picornaviral protein encoded by an alternative open reading frame. This protein is an important determinant of TMEV persistence in the mouse central nervous system. We showed that in infected cells, L* is partitioned between the cytosol and the mitochondria. In mitochondria, L* is anchored in the outer membrane and exposed to the cytosol. The targeting of L* to mitochondria is independent of other viral components and likely depends on a conformational signal. L* targeting to mitochondria might involve chaperones of the Hsp70 family, as these proteins are shown to interact.

Viral security proteins: counteracting host defences

Viral reproduction involves not only replication but also interactions with host defences. Although various viral proteins can take part in counteracting innate and adaptive immunity, many viruses possess a subset of proteins that are specifically dedicated to counter-defensive activities. These proteins are sometimes referred to as 'virulence factors', but here we argue that the term 'security proteins' is preferable, for several reasons.

The concept of security proteins of RNA-containing viruses can be considered using the leader (L and L^*) and 2A proteins of picornaviruses as examples. The picornaviruses are a large group of human and animal viruses that include important pathogens such as poliovirus, hepatitis A virus and foot-and-mouth disease virus. The genomes of different picornaviruses have a similar organization, in which the genes for L and 2A occupy fixed positions upstream and downstream of the capsid genes, respectively.

Both L and 2A are extremely heterogeneous with respect to size, sequence and biochemical properties. The similarly named proteins can be completely unrelated to each other in different viral genera, and the variation can be striking even among members of the same genus. A subset of picornaviruses lacks L altogether.

The properties and functions of L and 2A of many picornaviruses are unknown, but in those viruses that have been investigated sufficiently it has been found that these proteins can switch off various aspects of host macromolecular synthesis and specifically suppress mechanisms involved in innate immunity. Thus, notwithstanding their unrelatedness, the security proteins carry out similar biological functions. It is proposed that other picornavirus L and 2A proteins that have not yet been investigated should also be primarily involved in security activities.

The L, L^* and 2A proteins are dispensable for viral reproduction, but their elimination or inactivation usually renders the viruses less pathogenic. The phenotypic changes associated with inactivation of security proteins are much less pronounced in cells or organisms that have innate immunity deficiencies. In several examples, the decreased fitness of a virus in which a security protein has been inactivated could be rescued by the experimental introduction of an unrelated security protein.

It can be argued that L and 2A were acquired by different picornaviruses independently, and possibly by exploiting different mechanisms, late in the evolution of this viral family.

It is proposed that the concept of security proteins is of general relevance and can be applied to viruses other than picornaviruses. The hallmarks of security proteins are: structural and biochemical unrelatedness in related viruses or even absence in some of them; dispensability of the entire protein or its functional domains for viral viability; and, for mutated versions of the proteins, fewer detrimental effects on viral reproduction in immune-compromised hosts than in immune-competent hosts.

Viral security proteins are structurally and biochemically unrelated proteins that function to counteract host defences. Here, Agol and Gmyl consider the impact of the picornavirus security proteins on viral reproduction, pathogenicity and evolution.

Interactions with host defences are key aspects of viral infection. Various viral proteins perform counter-defensive functions, but a distinct class, called security proteins, is dedicated specifically to counteracting host defences. Here, the properties of the picornavirus security proteins L and 2A are discussed. These proteins have well-defined positions in the viral polyprotein, flanking the capsid precursor, but they are structurally and biochemically unrelated. Here, we consider the impact of these two proteins, as well as that of a third security protein, L^*, on viral reproduction, pathogenicity and evolution. The concept of security proteins could serve as a paradigm for the dedicated counter-defensive proteins of other viruses.

Different strains of Theiler's murine encephalomyelitis virus antagonize different sites in the type I interferon pathway

The DA strain of Theiler's murine encephalomyelitis virus (TMEV), a member of the Cardiovirus genus of the family Picornaviridae, causes persistent infection in susceptible mice, associated with restricted expression of viral proteins, and induces a demyelinating disease of the central nervous system. DA-induced demyelinating disease serves as a model of human multiple sclerosis because of similarities in pathology and because host immune responses contribute to pathogenesis in both disorders. In contrast, the GDVII strain of TMEV causes acute lethal encephalitis with no virus persistence. Cardiovirus L is a multifunctional protein that blocks beta interferon (IFN-beta) gene transcription. We show that both DA L and GDVII L disrupt IFN-beta gene transcription induction by IFN regulatory factor 3 (IRF-3) but do so at different points in the signaling pathway. DA L blocks IFN-beta gene transcription downstream of mitochondrial antiviral signaling protein (MAVS) but upstream of IRF-3 activation, while GDVII L acts downstream of IRF-3 activation. Both DA L and GDVII L block IFN-beta gene transcription in infected mice; however, IFN-beta mRNA is expressed at low levels in the central nervous systems of mice persistently infected with DA. The particular level of IFN-beta mRNA expression set by DA L as well as other factors in the IRF-3 pathway may play a role in virus persistence, inflammation, and the restricted expression of viral proteins during the late stage of demyelinating disease.

Differential IFN-alpha/beta production suppressing capacities of the leader proteins of mengovirus and foot-and-mouth disease virus

Picornaviruses encompass a large family of RNA viruses. Some picornaviruses possess a leader (L) protein at the N-terminus of their polyprotein. The L proteins of encephalomyocarditis virus, a cardiovirus, and foot-and-mouth disease virus (FMDV), an aphthovirus, are both dispensable for replication and their major function seems to be the suppression of antiviral host cell responses. Previously, we showed that the L protein of mengovirus, a strain of encephalomyocarditis virus, inhibits antiviral responses by inhibiting type I interferon (IFN-alpha/beta) gene transcription. The L protein of the FMDV is a protease (L(pro)) that cleaves cellular factors to reduce cytokine and chemokine mRNA production and to inhibit cap-dependent cellular host mRNA translation, thereby limiting the production of proteins with antiviral activity. In this study, we constructed a viable chimeric mengovirus that expresses FMDV L(pro) in place of the authentic L protein in order to compare the efficiency of the immune evasion mechanisms mediated by L and L(pro) respectively. We show that in this mengovirus background the L protein is more potent than FMDV L(pro) in suppressing IFN-alpha/beta responses. Yet, FMDV L(pro) is important to antagonize infection-limiting responses both in vitro and in vivo.

Random mutagenesis defines a domain of Theiler's virus leader protein that is essential for antagonism of nucleocytoplasmic trafficking and cytokine gene expression

The leader protein of cardioviruses, Theiler's murine encephalomyelitis virus (TMEV) and encephalomyocarditis virus (EMCV), is a multifunctional protein known to antagonize type I interferon expression and to interfere with nucleocytoplasmic trafficking of host proteins and mRNA. This protein plays an important role in the capacity of TMEV to establish persistent infection of the central nervous system. Mutant forms of the TMEV leader protein were generated by random mutagenesis and selected after retroviral transduction on the basis of the loss of the highly toxic nature of this protein. Selected mutations define a short C-terminal domain of the leader conserved in TMEV and Saffold virus but lacking in the EMCV leader and thus called the Theilo domain. Mutations in this domain had a dramatic impact on TMEV L protein activity. Like the zinc finger mutation, Theilo domain mutations affected all of the activities of the L protein tested: interferon gene transcription and IRF-3 dimerization antagonism, alteration of nucleocytoplasmic trafficking, nucleoporin 98 hyperphosphorylation, and viral persistence in vivo. This suggests that the Zn finger and the Theilo domain of the protein cooperate for function. Moreover, the fact that all of the activities tested were affected by these mutations suggests that the various leader protein functions are somehow coupled.

Different subcellular localization of Theiler's murine encephalomyelitis virus leader proteins of GDVII and DA strains in BHK-21 cells

The highly virulent GDVII strain of Theiler's murine encephalomyelitis virus causes acute and fatal encephalomyelitis, whereas the DA strain causes mild encephalomyelitis followed by a chronic inflammatory demyelinating disease with virus persistence. The differences in the amino acid sequences of the leader protein (L) of the DA and GDVII strains are greater than those for any other viral protein. We examined the subcellular distribution of DA L and GDVII L tagged with the FLAG epitope in BHK-21 cells. Wild-type GDVII L was localized predominantly in the cytoplasm, whereas wild-type DA L showed a nucleocytoplasmic distribution. A series of the L mutant experiments demonstrated that the zinc finger domain, acidic domain, and C-terminal region of L were necessary for the nuclear accumulation of DA L. A GDVII L mutant with a deletion of the serine/threonine (S/T)-rich domain showed a nucleocytoplasmic distribution, in contrast to the predominant cytoplasmic distribution of wild-type GDVII L. A chimeric DA/GDVII L, D/G, which encodes the N region of DA L including the zinc finger domain and acidic domain, followed by the GDVII L sequence including the S/T-rich domain, was distributed exclusively throughout the cytoplasm but not in the nucleus, as observed with wild-type GDVII L. Another chimeric L, G/D (which is the converse of the D/G construct), accumulated in the nucleus as well as the cytoplasm, as was observed for wild-type DA L. The findings suggest that the differential distribution of DA L and GDVII L is determined primarily by the S/T-rich domain. The S/T-rich domain may be important for the viral activity through the regulation of the subcellular distribution of L.

Genome analysis and development of infectious cDNA clone of a virulence-attenuated strain of foot-and-mouth disease virus type Asia 1 from China

The RNA genome sequence of the rabbit passage-attenuated strain of foot-and-mouth disease virus (FMDV) Asia 1, ZB/CHA/58(att), was determined to be 8165 nt in length excluding the poly(C) tract in the 5' UTR and the poly(A) tail at the 3' end. ZB/CHA/58(att) was most similar to the vaccine strain Asia 1/YNBS/58 in genome sequence and there were no deletions or insertions within the deduced polyprotein between ZB/CHA/58(att) and YNBS/58, but there were a total of 25 substitutions at the amino acid level and an extra 19-nt stretch in the 5' UTR was found in ZB/CHA/58(att). An infectious full-length cDNA clone of ZB/CHA/58(att) was developed. Infectious virus could be recovered in BHK-21 cells transfected with the synthetic viral RNA transcribed in vitro. The plaque morphology, growth kinetics and antigenic profile of the infectious clone-derived virus (termed tZB) were indistinguishable from those induced by the parental virus. Furthermore, the virulence properties of ZB/CHA/58(att) and tZB were found to be highly similar in the mouse model. The availability of genome sequence information and infectious cDNA clone of the FMDV ZB/CHA/58(att) lays a new ground for further investigation of FMDV virulence determinants and development of new potent vaccine to FMD.

Inhibition of mRNA export and dimerization of interferon regulatory factor 3 by Theiler's virus leader protein

Theiler's murine encephalomyelitis virus (TMEV or Theiler's virus) is a neurotropic picornavirus that can persist lifelong in the central nervous system of infected mice, causing a chronic inflammatory demyelinating disease. The leader (L) protein of the virus is an important determinant of viral persistence and has been shown to inhibit transcription of type I interferon (IFN) genes and to cause nucleocytoplasmic redistribution of host proteins. In this study, it was shown that expression of the L protein shuts off synthesis of the reporter proteins green fluorescent protein and firefly luciferase, suggesting that it induces a global shut-off of host protein expression. The L protein did not inhibit transcription or translation of the reporter genes, but blocked cellular mRNA export from the nucleus. This activity correlated with the phosphorylation of nucleoporin 98 (Nup98), an essential component of the nuclear pore complex. In contrast, the data confirmed that the L protein inhibited IFN expression at the transcriptional level, and showed that transcription of other chemokine or cytokine genes was affected by the L protein. This transcriptional inhibition correlated with inhibition of interferon regulatory factor 3 (IRF-3) dimerization. Whether inhibition of IRF-3 dimerization and dysfunction of the nuclear pore complex are related phenomena remains an open question. In vivo, IFN antagonism appears to be an important role of the L protein early in infection, as a virus bearing a mutation in the zinc finger of the L protein replicated as efficiently as the wild-type virus in type I IFN receptor-deficient mice, but had impaired fitness in IFN-competent mice.

Modulation of viral replication in macrophages persistently infected with the DA strain of Theiler's murine encephalomyelitis virus

Background

Demyelinating strains of Theiler's murine encephalomyelitis virus (TMEV) such as the DA strain are the causative agents of a persistent infection that induce a multiple sclerosis-like disease in the central nervous system of susceptible mice. Viral persistence, mainly associated with macrophages, is considered to be an important disease determinant that leads to chronic inflammation, demyelination and autoimmunity. In a previous study, we described the establishment of a persistent DA infection in RAW macrophages, which were therefore named DRAW.

Results

In the present study we explored the potential of diverse compounds to modulate viral persistence in these DRAW cells. Hemin was found to increase viral yields and to induce cell lysis. Enviroxime and neutralizing anti-TMEV monoclonal antibody were shown to decrease viral yields, whereas interferon-α and interferon-γ completely cleared the persistent infection. We also compared the cytokine pattern secreted by uninfected RAW, DRAW and interferon-cured DRAW macrophages using a cytokine protein array. The chemokine RANTES was markedly upregulated in DRAW cells and restored to a normal expression level after abrogation of the persistent infection with interferon-α or interferon-γ. On the other hand, the chemokine MCP-1 was upregulated in the interferon-cured DRAW cells.

Conclusion

We have identified several compounds that modulate viral replication in an in vitro model system for TMEV persistence. These compounds now await further testing in an in vivo setting to address fundamental questions regarding persistent viral infection and immunopathogenesis.

IFN-lambda (IFN-lambda) is expressed in a tissue-dependent fashion and primarily acts on epithelial cells in vivo

Interferons (IFN) exert antiviral, immunomodulatory and cytostatic activities. IFN-alpha/beta (type I IFN) and IFN-lambda (type III IFN) bind distinct receptors, but regulate similar sets of genes and exhibit strikingly similar biological activities. We analyzed to what extent the IFN-alpha/beta and IFN-lambda systems overlap in vivo in terms of expression and response. We observed a certain degree of tissue specificity in the production of IFN-lambda. In the brain, IFN-alpha/beta was readily produced after infection with various RNA viruses, whereas expression of IFN-lambda was low in this organ. In the liver, virus infection induced the expression of both IFN-alpha/beta and IFN-lambda genes. Plasmid electrotransfer-mediated in vivo expression of individual IFN genes allowed the tissue and cell specificities of the responses to systemic IFN-alpha/beta and IFN-lambda to be compared. The response to IFN-lambda correlated with expression of the alpha subunit of the IFN-lambda receptor (IL-28R alpha). The IFN-lambda response was prominent in the stomach, intestine and lungs, but very low in the central nervous system and spleen. At the cellular level, the response to IFN-lambda in kidney and brain was restricted to epithelial cells. In contrast, the response to IFN-alpha/beta was observed in various cell types in these organs, and was most prominent in endothelial cells. Thus, the IFN-lambda system probably evolved to specifically protect epithelia. IFN-lambda might contribute to the prevention of viral invasion through skin and mucosal surfaces.

The mengovirus leader protein blocks interferon-alpha/beta gene transcription and inhibits activation of interferon regulatory factor 3

Viral infection of mammalian cells triggers the synthesis and secretion of type I interferons (i.e. IFN-alpha/beta), which induce the transcription of genes that cause cells to adopt an antiviral state. Many viruses have adapted mechanisms to evade IFN-alpha/beta-mediated responses. The leader protein of mengovirus, a picornavirus, has been implicated as an IFN-alpha/beta antagonist. Here, we show that the leader inhibits the transcription of IFN-alpha/beta and that both the presence of a zinc finger motif in its N-terminus and phosphorylation of threonine-47 are required for this function. Transcription of IFN-alpha/beta genes relies on the activity of a number of transcription factors, including interferon regulatory factor 3 (IRF-3). We show that the leader interferes with the transactivation activity of IRF-3 by interfering with its dimerization. Accordingly, mutant viruses with a disturbed leader function were impaired in their ability to suppress IFN-alpha/beta transcription in vivo. By consequence, the leader mutant viruses had an impaired ability to replicate and spread in normal mice but not in IFNAR-KO mice, which are incapable of mounting an IFN-alpha/beta-dependent antiviral response. These results suggest that the leader, by suppressing IRF3-mediated IFN-alpha/beta production, plays an important role in replication and dissemination of mengovirus in its host.

Theiler's virus strain-dependent induction of innate immune responses in RAW264.7 macrophages and its influence on viral clearance versus viral persistence

Infection of susceptible mice with the DA strain of Theiler's murine encephalomyelitis virus (TMEV) induces a persistent central nervous system infection accompanied by demyelination that resembles multiple sclerosis. In contrast, Theiler's GDVII strain does not persist, because infected animals either clear the virus or die. Previously, the authors have shown that in vitro infection of RAW264.7 macrophages displays a similar strain-dependent outcome, resulting in the establishment of a persistent infection with the DA strain and clearance of the GDVII strain. Here, the authors show that when RAW264.7 cells were infected with both strains, the antiviral response triggered by the GDVII virus interfered with the DA virus' ability to induce a persistent infection. Treatment of cells with 2-aminopurine, a protein kinase R inhibitor, increased GDVII virus yields in contrast to DA virus yields. By comparing the antiviral activity of RAW264.7 macrophages against TMEV, it was found that GDVII-infected macrophages mounted a five times more potent antiviral response than the DA-infected ones, indicating that there are strain-dependent differences in the induction of host innate immune responses. Measurements of interferon (IFN) production confirmed this finding. In addition, it was found that the macrophages' antiviral response is dependent on the multiplicity of infection. The antiviral activity resulting from GDVII-infected macrophages could be partially neutralized with antibodies against IFN-alpha or IFN-gamma, but not with an anti-IFN-beta antibody. Because only a partial neutralization was reached, the authors speculate that apart from the investigated IFNs, other cellular factors contribute to the observed antiviral activity. Taken together, these results demonstrate the importance of host innate immune responses in determining the balance between viral clearance and viral persistence.

Theiler's murine encephalomyelitis virus leader protein amino acid residue 57 regulates subgroup-specific virus growth on BHK-21 cells

Strains of Theiler's murine encephalomyelitis virus (TMEV) are divided into two subgroups, TO and GDVII. TMEV strains show subgroup-specific virus growth and cell tropism and induce subgroup-specific diseases. Using site-directed mutagenesis, we demonstrated that the amino acid at position 57 of the leader protein (L(57)), which is located at the most N-terminal part of the polyprotein, regulates subgroup-specific virus growth on BHK-21 cells. Further study suggested that L(57) may regulate viral RNA encapsidation, although it does not affect the synthesis of viral proteins or the assembly of viral intermediates.