Synthesis of (2'-5')oligoadenylate and activation of an endoribonuclease in interferon-treated HeLa cells infected with reovirus

The Unusual Role of Ribonuclease L in Innate Immunity

Ribonuclease L is an endonuclease that is activated as part of the dsRNA-driven innate immune response. Active RNase L cleaves pathogenic RNAs as a way to eliminate infections. However, there are additional and unexpected ways that RNase L causes changes in the host that promote an immune response and contribute to its role in host defense. Central to these unconventional mechanisms is the observation that RNase L also degrades the mRNA of the host. In turn, mRNA fragments that RNase L generates can be translated. This causes activation of a ribosome collision sensor that leads to downstream signaling and cell death. Additionally, the liberation of RNA binding proteins after RNA decay appears to affect gene expression. In this review, we discuss these and other recent advances that focus on novel and unusual ways RNase L contributes to innate immunity.

How Many Mammalian Reovirus Proteins are involved in the Control of the Interferon Response?

As with most viruses, mammalian reovirus can be recognized and attacked by the host-cell interferon response network. Similarly, many viruses have developed resistance mechanisms to counteract the host-cell response at different points of this response. Reflecting the complexity of the interferon signaling pathways as well as the resulting antiviral response, viruses can-and often have-evolved many determinants to interfere with this innate immune response and allow viral replication. In the last few years, it has been evidenced that mammalian reovirus encodes many different determinants that are involved in regulating the induction of the interferon response or in interfering with the action of interferon-stimulated gene products. In this brief review, we present our current understanding of the different reovirus proteins known to be involved, introduce their postulated modes of action, and raise current questions that may lead to further investigations.

IRF3 deficiency impacts granzyme B expression and maintenance of memory T cell function in response to viral infection

The role of interferon regulatory factor 3 (IRF3) in the innate immune response to infection has been well studied. However, less is known about IRF3 signaling in shaping the adaptive T cell response. To determine the role of IRF3 in the generation and maintenance of effective anti-viral T cell responses, mice deficient in IRF3 were infected with a potentially persistent virus, Theiler's murine encephalomyelitis virus (TMEV) or with a model acute infection, influenza A virus (IAV). IRF3 was required to prevent TMEV persistence and induce robust TMEV specific effector T cell responses at the site of infection. This defect was more pronounced in the memory phase with an apparent lack of TMEV-specific memory T cells expressing granzyme B (GrB) in IRF3 deficient mice. In contrast, IRF3 had no effect on antigen specific T cell responses at the effector stage during IAV infection. However, memory T cell responses to IAV were also impaired in IRF3 deficient mice. Furthermore, addition of cytokines during peptide restimulation could not restore GrB expression in IRF3 deficient memory T cells. Taken together, IRF3 plays an important role in the maintenance of effective anti-viral T cell memory responses.

RNase-L control of cellular mRNAs: roles in biologic functions and mechanisms of substrate targeting

RNase-L is a mediator of type 1 interferon-induced antiviral activity that has diverse and critical cellular roles, including the regulation of cell proliferation, differentiation, senescence and apoptosis, tumorigenesis, and the control of the innate immune response. Although RNase-L was originally shown to mediate the endonucleolytic cleavage of both viral and ribosomal RNAs in response to infection, more recent evidence indicates that RNase-L also functions in the regulation of cellular mRNAs as an important mechanism by which it exerts its diverse biological functions. Despite this growing body of work, many questions remain regarding the roles of mRNAs as RNase-L substrates. This review will survey known and putative mRNA substrates of RNase-L, propose mechanisms by which it may selectively cleave these transcripts, and postulate future clinical applications.

Role of ribonuclease L in viral pathogen-associated molecular pattern/influenza virus and cigarette smoke-induced inflammation and remodeling

Interactions between cigarette smoke (CS) exposure and viral infection play an important role(s) in the pathogenesis of chronic obstructive pulmonary disease and a variety of other disorders. A variety of lines of evidence suggest that this interaction induces exaggerated inflammatory, cytokine, and tissue remodeling responses. We hypothesized that the 2'-5' oligoadenylate synthetase (OAS)/RNase L system, an innate immune antiviral pathway, plays an important role in the pathogenesis of these exaggerated responses. To test this hypothesis, we characterize the activation of 2'-5' OAS in lungs from mice exposed to CS and viral pathogen-associated molecular patterns (PAMPs)/live virus, alone and in combination. We also evaluated the inflammatory and remodeling responses induced by CS and virus/viral PAMPs in lungs from RNase L null and wild-type mice. These studies demonstrate that CS and viral PAMPs/live virus interact in a synergistic manner to stimulate the production of select OAS moieties. They also demonstrate that RNase L plays a critical role in the pathogenesis of the exaggerated inflammatory, fibrotic, emphysematous, apoptotic, TGF-β1, and type I IFN responses induced by CS plus virus/viral PAMP in combination. These studies demonstrate that CS is an important regulator of antiviral innate immunity, highlight novel roles of RNase L in CS plus virus induced inflammation, tissue remodeling, apoptosis, and cytokine elaboration and highlight pathways that may be operative in chronic obstructive pulmonary disease and mechanistically related disorders.

Temperature and density dependent induction of a cytopathic effect following infection with non-cytopathic HAV strains

Hepatitis A virus infection and growth in cultured cells is protracted, cell-type restricted, and generally not accompanied by the appearance of a cytopathic effect, with the exception of some culture-adapted strains. We demonstrate that the non-cytopathic HAV strain HM175/clone 1 can be induced to exhibit a cytopathic phenotype in both persistently or acutely infected cells under co-dependent conditions of lower incubation temperature (<34°C) and reduced cell density in both monkey (FRhK-4) and human (A549) cells. This phenotype is not virus-strain restricted, as it was also observed in cells infected with HAV strains, HAS-15 and LSH/S. Cytopathic effect was accompanied by rRNA cleavage, indicating activation of the RNase L pathway, viral negative strand synthesis, caspase-3 activation, and apoptosis. The results indicate that a cytopathic phenotype may be present in some HAV strains that can be induced under appropriate conditions, suggesting the potential for development of a plaque assay for this virus.

Histology, immunocytochemistry and qRT-PCR analysis of Atlantic salmon, Salmo salar L., post-smolts following infection with infectious pancreatic necrosis virus (IPNV)

Infectious pancreatic necrosis (IPN) is a very serious viral disease in terms of its impact on production of Atlantic salmon, Salmo salar L., fry and post-smolts. Post-smolts of Atlantic salmon were injected with infectious pancreatic necrosis virus (IPNV) and cohabited with naive fish to produce natural infection. Cohabitant fish were sampled every 2 days, up to day 36 post-infection (p.i.). From 90 cohabitant fish, 11 (12.2%) were positive by immunohistochemistry (IHC). The first detection of IPNV by IHC occurred on day 16 p.i. which coincided with the onset of mortality in this group. Besides the pancreas, the liver was found to be a key target organ for IPNV. For the first time, the virus was observed in the islets of Langerhans and in the kidney corpuscles of Stannius which suggests that the virus could affect the fish's metabolism. The liver of two fish, which showed the most widespread presence of IPNV by IHC, had a pathology including focal necrosis and widespread presence of apoptotic hepatocytes, many of which did not stain for virus by IHC. Up-regulation of cytokine gene expression was found only in the IHC-positive (IHC+ve) fish and reflected the level of infection as determined by IHC positivity of the liver. In most fish, interferon (IFN), Mx, γIFN and γIP were up-regulated in liver and kidney, while only IFN and Mx were up-regulated in gill. IL1β and TNFα were not induced in any tissue. The gill showed variable levels of constitutive expression of IL1β and γIFN. The two fish with liver pathology had the highest level of IFN expression, especially relative to the level of Mx expression, in the liver compared with the other IHC+ve fish which did not have a liver pathology. The results suggest that following widespread infection of hepatocytes, the cells may over-produce IFN, resulting in apoptosis of neighbouring cells with subsequent death from liver failure.

Accessory protein 5a is a major antagonist of the antiviral action of interferon against murine coronavirus

The type I interferon (IFN) response plays an essential role in the control of in vivo infection by the coronavirus mouse hepatitis virus (MHV). However, in vitro, most strains of MHV are largely resistant to the action of this cytokine, suggesting that MHV encodes one or more functions that antagonize or evade the IFN system. A particular strain of MHV, MHV-S, exhibited orders-of-magnitude higher sensitivity to IFN than prototype strain MHV-A59. Through construction of interstrain chimeric recombinants, the basis for the enhanced IFN sensitivity of MHV-S was found to map entirely to the region downstream of the spike gene, at the 3' end of the genome. Sequence analysis revealed that the major difference between the two strains in this region is the absence of gene 5a from MHV-S. Creation of a gene 5a knockout mutant of MHV-A59 demonstrated that a major component of IFN resistance maps to gene 5a. Conversely, insertion of gene 5a, or its homologs from related group 2 coronaviruses, at an upstream genomic position in an MHV-A59/S chimera restored IFN resistance. This is the first demonstration of a coronavirus gene product that can protect that same virus from the antiviral state induced by IFN. Neither protein kinase R, which phosphorylates eukaryotic initiation factor 2, nor oligoadenylate synthetase, which activates RNase L, was differentially activated in IFN-treated cells infected with MHV-A59 or MHV-S. Thus, the major IFN-induced antiviral activities that are specifically inhibited by MHV, and possibly by other coronaviruses, remain to be identified.

Viruses and the cellular RNA decay machinery

The ability to control cellular and viral gene expression, either globally or selectively, is central to a successful viral infection, and it is also crucial for the host to respond and eradicate pathogens. In eukaryotes, regulation of message stability contributes significantly to the control of gene expression and plays a prominent role in the normal physiology of a cell as well as in its response to environmental and pathogenic stresses. Not surprisingly, emerging evidence indicates that there are significant interactions between the eukaryotic RNA turnover machinery and a wide variety of viruses. Interestingly, in many cases viruses have evolved mechanisms not only to evade eradication by these pathways, but also to manipulate them for enhanced viral replication and gene expression. Given our incomplete understanding of how many of these pathways are normally regulated, viruses should be powerful tools to help deconstruct the complex networks and events governing eukaryotic RNA stability. Copyright © 2010 John Wiley & Sons, Ltd.

This article is categorized under: 1

RNA Turnover and Surveillance > Turnover/Surveillance Mechanisms

2

RNA in Disease and Development > RNA in Disease

Activation of the 2-5OAS/RNase L pathway in CVB1 or HAV/18f infected FRhK-4 cells does not require induction of OAS1 or OAS2 expression

The latent, constitutively expressed protein RNase L is activated in coxsackievirus and HAV strain 18f infected FRhK-4 cells. Endogenous oligoadenylate synthetase (OAS) from uninfected and virus infected cell extracts synthesizes active forms of the triphosphorylated 2-5A oligomer (the only known activator of RNase L) in vitro and endogenous 2-5A is detected in infected cell extracts. However, only the largest OAS isoform, OAS3, is readily detected throughout the time course of infection. While IFNbeta treatment results in an increase in the level of all three OAS isoforms in FRhK-4 cells, IFNbeta pretreatment does not affect the temporal onset or enhancement of RNase L activity nor inhibit virus replication. Our results indicate that CVB1 and HAV/18f activate the 2-5OAS/RNase L pathway in FRhK-4 cells during permissive infection through endogenous levels of OAS, but contrary to that reported for some picornaviruses, CVB1 and HAV/18f replication is insensitive to this activated antiviral pathway.

Diverse functions of RNase L and implications in pathology

The endoribonuclease L (RNase L) is the effector of the 2-5A system, a major enzymatic pathway involved in the molecular mechanism of interferons (IFNs). RNase L is a very unusual nuclease with a complex mechanism of regulation. It is a latent enzyme, expressed in nearly every mammalian cell type. Its activation requires its binding to a small oligonucleotide, 2-5A. 2-5A is a series of unique 5'-triphosphorylated oligoadenylates with 2'-5' phosphodiester bonds. By regulating viral and cellular RNA expression, RNase L plays an important role in the antiviral and antiproliferative activities of IFN and contributes to innate immunity and cell metabolism. The 2-5A/RNase L pathway is implicated in mediating apoptosis in response to viral infections and to several types of external stimuli. Several recent studies have suggested that RNase L could have a role in cancer biology and evidence of a tumor suppressor function of RNase L has emerged from studies on the genetics of hereditary prostate cancer.

Systemic innate immune responses following intrapulmonary delivery of CpG oligodeoxynucleotides in sheep

Mucosal delivery of CpG oligodeoxynucleotide (ODN) in mice has been shown to induce potent innate immunostimulatory responses and protection against infection. We evaluated the efficacy of CpG ODN in stimulating systemic innate immune responses in sheep following delivery to the pulmonary mucosa. Intrapulmonary (IPM) administration of B-Class CpG ODN in saline induced transient systemic responses which included increased rectal temperatures, elevated serum 2'5'-A synthetase and haptoglobin concentrations. The ODN dose required to induce detectable systemic responses following IPM delivery could be reduced by approximately 80% if the CpG ODN was administered in 30% emulsigen instead of saline. Intrapulmonary B-Class CpG ODN formulated in 30% emulsigen produced similar effects when compared to those seen following SC injection. These responses were CpG ODN-specific since control GpC ODN did not induce any detectable response. Intrapulmonary administration of both B-Class and the newly described C-Class CpG ODN produced similar effects indicating that both classes of CpG ODN were comparably effective in stimulating innate immune system following mucosal delivery. Administration of CpG ODN directly into the lungs or delivery of CpG ODN via an intratracheal (IT) infusion also produced similar systemic responses. These observations support the conclusion that mucosal delivery of CpG ODN is an effective route for induction of systemic acute phase responses and antiviral effector molecules in large animals, and may be helpful in controlling systemic infections.

Sendai virus infection induces efficient adaptive immunity independently of type I interferons

Adaptive immunity in response to virus infection involves the generation of Th1 cells, cytotoxic T cells, and antibodies. This type of immune response is crucial for the clearance of virus infection and for long-term protection against reinfection. Type I interferons (IFNs), the primary innate cytokines that control virus growth and spreading, can influence various aspects of adaptive immunity. The development of antiviral immunity depends on many viral and cellular factors, and the extent to which type I IFNs contribute to the generation of adaptive immunity in response to a viral infection is controversial. Using two strains (Cantell and 52) of the murine respiratory Sendai virus (SeV) with differential abilities to induce type I IFN production from infected cells, together with type I IFN receptor-deficient mice, we examined the role of type I IFNs in the generation of adaptive immunity. Our results show that type I IFNs facilitate virus clearance and enhance the migration and maturation of dendritic cells after SeV infection in vivo; however, soon after infection, mice clear the virus from their lungs and efficiently generate cytotoxic T cells independently of type I IFN signaling. Furthermore, animals that are unresponsive to type I IFN develop long-term anti-SeV immunity, including CD8+ T cells and antibodies. Significantly, this memory response is able to protect mice against challenge with a lethal dose of virus. In conclusion, our results show that primary and secondary anti-SeV adaptive immunities are developed normally in the absence of type I IFN responsiveness.

RNase L plays a role in the antiviral response to West Nile virus

Alleles at the Flv locus determine disease outcome after a flavivirus infection in mice. Although comparable numbers of congenic resistant and susceptible mouse embryo fibroblasts (MEFs) are infected by the flavivirus West Nile virus (WNV), resistant MEFs produce approximately 100- to 150-fold lower titers than susceptible ones and flavivirus titers in the brains of resistant and susceptible animals can differ by >10,000-fold. The Flv locus was previously identified as the 2'-5' oligoadenylate synthetase 1b (Oas1b) gene. Oas gene expression is up-regulated by interferon (IFN), and after activation by double-stranded RNA, some mouse synthetases produce 2-5A, which activates latent RNase L to degrade viral and cellular RNAs. To determine whether the lower levels of intracellular flavivirus genomic RNA from resistant mice detected in cells at all times after infection were mediated by RNase L, RNase L activity levels in congenic resistant and susceptible cells were compared. Similar moderate levels of RNase L activation by transfected 2-5A were observed in both types of uninfected cells. After WNV infection, the mRNAs of IFN-beta and three Oas genes were up-regulated to similar levels in both types of cells. However, significant levels of RNase L activity were not detected until 72 h after WNV infection and the patterns of viral RNA cleavage products generated were similar in both types of cells. When RNase L activity was down-regulated in resistant cells via stable expression of a dominant negative RNase L mutant, approximately 5- to 10-times-higher yields of WNV were produced. Similarly, about approximately 5- to 10-times-higher virus yields were produced by susceptible C57BL/6 RNase L-/- cells compared to RNase L+/+ cells that were either left untreated or pretreated with IFN and/or poly(I) . poly(C). The data indicate that WNV genomic RNA is susceptible to RNase L cleavage and that RNase L plays a role in the cellular antiviral response to flaviviruses. The results suggest that RNase L activation is not a major component of the Oas1b-mediated flavivirus resistance phenotype.

Involvement of the interferon-regulated antiviral proteins PKR and RNase L in reovirus-induced shutoff of cellular translation

Cellular translation is inhibited following infection with most strains of reovirus, but the mechanisms responsible for this phenomenon remain to be elucidated. The extent of host shutoff varies in a strain-dependent manner; infection with the majority of strains leads to strong host shutoff, while infection with strain Dearing results in minimal inhibition of cellular translation. A genetic study with reassortant viruses and subsequent biochemical analyses led to the hypothesis that the interferon-induced, double-stranded RNA-activated protein kinase, PKR, is responsible for reovirus-induced host shutoff. To directly determine whether PKR is responsible for reovirus-induced host shutoff, we used a panel of reovirus strains and mouse embryo fibroblasts derived from knockout mice. This approach revealed that PKR contributes to but is not wholly responsible for reovirus-induced host shutoff. Studies with cells lacking RNase L, the endoribonuclease component of the interferon-regulated 2',5'-oligoadenylate synthetase-RNase L system, demonstrated that RNase L also down-regulates cellular protein synthesis in reovirus-infected cells. In many viral systems, PKR and RNase L have well-characterized antiviral functions. An analysis of reovirus replication in cells lacking these molecules indicated that, while they contributed to host shutoff, neither PKR nor RNase L exerted an antiviral effect on reovirus growth. In fact, some strains of reovirus replicated more efficiently in the presence of PKR and RNase L than in their absence. Data presented in this report illustrate that the inhibition of cellular translation following reovirus infection is complex and involves multiple interferon-regulated gene products. In addition, our results suggest that reovirus has evolved effective mechanisms to avoid the actions of the interferon-stimulated antiviral pathways that include PKR and RNase L and may even benefit from their expression.

CpG oligodeoxynucleotide induction of antiviral effector molecules in sheep

Immunostimulatory CpG oligodeoxynucleotide (ODN) can protect mice against infection by many pathogens but the mechanisms mediating disease protection are not well defined. Furthermore, the mechanisms of CpG ODN induced disease protection in vivo have not been investigated in other species. We investigated the induction of antiviral effector molecules in sheep treated with a class B CpG ODN (2007). Subcutaneous injection of ODN 2007 induced a dose-dependent increase in serum levels of the antiviral effector molecule, 2'5'-A synthetase. Peak levels of enzyme were observed 4 days following ODN injection and enzyme levels remained elevated for the following 3-5 days. Repeated ODN injections induced a more sustained elevation of serum 2'5'-A synthetase activity. Finally, formulation of ODN 2007 in emulsigen increased the level of serum 2'5'-A synthetase activity and this response was CpG-specific. Elevated serum 2'5'-A synthetase activity suggested that CpG ODN acted through the induction of either interferon (IFN)-alpha or IFN-gamma. ODN 2007 did not induce detectable levels of IFN-alpha or IFN-gamma when incubated with peripheral blood mononuclear cells, but both IFN-alpha and IFN-gamma were detected following stimulation of lymph node cells with ODN 2007. CpG ODN induction of 2'5'-A synthetase in vitro correlated with the secretion of both IFN-alpha and IFN-gamma. Furthermore, immunohistochemical staining of skin revealed a marked cellular infiltration at the site of ODN 2007 injection. This cellular infiltration was CpG-specific and consisted of primarily CD172(+) myeloid cells. Many of the cells recruited to the site of ODN 2007 injection expressed IFN-alpha and some IFN-gamma. These observations support the conclusion that localized cell recruitment and activation contribute to CpG ODN induction of antiviral effector molecules, such as interferon and 2'5'-A synthetase.

Innate immune responses induced by CpG oligodeoxyribonucleotide stimulation of ovine blood mononuclear cells

Examples exist in the literature that demonstrate that treatment with immunostimulatory cytosine-phosphate-guanosine (CpG)-DNA can protect mice against infection by intracellular pathogens. There are, however, few studies reporting that CpG-DNA offers similar disease protection in other species. In this study, we assessed the potential of a class A and class B CpG oligodeoxynucleotide (ODN) to induce innate immune responses in sheep, an outbred species. Using peripheral blood mononuclear cells, we have for the first time demonstrated CpG-ODN-induced innate immune responses, including natural-killer-like activity [non-major histocompatibility complex (MHC)-restricted cytotoxicity], interferon-alpha secretion and 2'-5'A oligoadenylate synthetase activity, that could contribute to immune protection in sheep. The type and magnitude of these responses were dependent on ODN class and non-MHC-restricted killing was not associated with interferon-gamma production. The latter observation is in contrast with observations reported for mice and humans. These observations support the conclusion that differences in CpG-ODN-induced responses exist among species and that specific ODN sequences can significantly influence innate immune responses.

Interferon regulatory factor-1, interferon-beta, and reovirus-induced myocarditis

Viral myocarditis is an important human disease, and reovirus-induced myocarditis in mice provides an excellent model to study direct viral damage to the heart. Previously, we showed that reovirus induction of and sensitivity to interferon-beta (IFN-beta) is an important determinant of viral pathogenicity in the heart and that the transcription factor interferon regulatory factor-3 (IRF-3) is required for reovirus induction of IFN-beta in primary cardiac myocyte cultures. Given several lines of evidence suggesting a possible distinctive environment for IRFs in the heart, we have now focused on IRF-1. Previous studies demonstrated that viruses, double-stranded-RNA (dsRNA), and IFN-alpha/beta can each induce IRF-1 and that IRF-1 plays a role in dsRNA, but perhaps not viral, induction of IFN-alpha/beta. Importantly, none of these studies used a virus with a dsRNA genome (such as reovirus), none of them used a highly differentiated nonlymphoid cell type, and none of them addressed whether viral induction of IRF-1 is direct or is mediated through viral induction of IFN-beta. Indeed, as recently as this year it has been assumed that viral induction of IRF-1 is direct. Here, we found that reovirus induced IRF-1 in primary cardiac myocyte cultures, but that IRF-1 was not required for reovirus induction of IFN-beta. Surprisingly, we found that reovirus failed to induce IRF-1 in the absence of the IFN-alpha/beta response. This provides the first evidence that viruses may not induce IRF-1 directly. Finally, nonmyocarditic reovirus strains induced more cardiac lesions in mice deficient for IRF-1 than they did in wildtype mice, directly demonstrating a protective role for IRF-1. Together, the results indicate that while IRF-1 is downstream of the IFN-beta response, it plays an important protective role against viral myocarditis.

Interferon inhibits dengue virus infection by preventing translation of viral RNA through a PKR-independent mechanism

Previously, we demonstrated that pretreatment of cells with interferon (IFN) alpha + gamma or beta + gamma inhibited dengue virus (DV) replication. In this study, experiments were performed to better define the mechanism by which IFN blocks the accumulation of dengue virus (DV) RNA. Pretreatment of human hepatoma cells with IFN beta + gamma did not significantly alter virus attachment, viral entry, or nucleocapsid penetration into the cytoplasm. The inhibitory effect of IFN was retained even when naked DV RNA was transfected directly into cells, confirming that steps associated with viral entry were not the primary target of IFN action. Biosynthetic labeling experiments revealed that IFN abolished the translation of infectious viral RNA that occurred prior to RNA replication. Subcellular fractionation experiments demonstrated that IFN did not significantly alter the ability of viral RNA to attach to ribosomes. The antiviral effect of IFN appeared independent of the IFN-induced, double-stranded RNA-activated protein kinase (PKR) and RNase L, as genetically deficient PKR- RNase L- cells that were infected by DV retained sensitivity to inhibition by IFN. We conclude that IFN prevents DV infection by inhibiting translation of the infectious viral RNA through a novel, PKR-independent mechanism.

RNase L inhibitor is induced during human immunodeficiency virus type 1 infection and down regulates the 2-5A/RNase L pathway in human T cells

The interferon-regulated 2-5A/RNase L pathway plays a major role in the antiviral and antiproliferative activities of these cytokines. Several viruses, however, have evolved strategies to escape the antiviral activity of the 2-5A/RNase L pathway. In this context, we have cloned a cDNA coding for the RNase L inhibitor (RLI), a protein that specifically inhibits RNase L and whose regulated expression in picornavirus-infected cells down regulates the activity of the 2-5A/RNase L pathway. We show here that RLI increases during the course of human immunodeficiency virus type 1 (HIV-1) infection, which may be related to the downregulation of RNase L activity that has been described to occur in HIV-infected cells. In order to establish a possible causal relationship between these observations, we have stably transfected H9 cells with RLI sense or antisense cDNA-expressing vectors. The overexpression of RLI causes a decrease in RNase L activity and a twofold enhancement of HIV production. This increase in HIV replication correlates with an increase in HIV RNA and proteins. In contrast, reduction of RLI levels in RLI antisense cDNA-expressing clones reverses the inhibition of RNase L activity associated with HIV multiplication and leads to a threefold decrease in the viral load. This anti-HIV activity correlated with a decrease in HIV RNA and proteins. These findings demonstrate that the level of RLI, via its modulation of RNase L activity, can severely impair HIV replication and suggest the involvement of RLI in the inhibition of the 2-5A/RNase L system observed during HIV infection.

Antisense RNA: function and fate of duplex RNA in cells of higher eukaryotes

There is ample evidence that cells of higher eukaryotes express double-stranded RNA molecules (dsRNAs) either naturally or as the result of viral infection or aberrant, bidirectional transcriptional readthrough. These duplex molecules can exist in either the cytoplasmic or nuclear compartments. Cells have evolved distinct ways of responding to dsRNAs, depending on the nature and location of the duplexes. Since dsRNA molecules are not thought to exist naturally within the cytoplasm, dsRNA in this compartment is most often associated with viral infections. Cells have evolved defensive strategies against such molecules, primarily involving the interferon response pathway. Nuclear dsRNA, however, does not induce interferons and may play an important posttranscriptional regulatory role. Nuclear dsRNA appears to be the substrate for enzymes which deaminate adenosine residues to inosine residues within the polynucleotide structure, resulting in partial or full unwinding. Extensively modified RNAs are either rapidly degraded or retained within the nucleus, whereas transcripts with few modifications may be transported to the cytoplasm, where they serve to produce altered proteins. This review summarizes our current knowledge about the function and fate of dsRNA in cells of higher eukaryotes and its potential manipulation as a research and therapeutic tool.

Reoviruses and the interferon system

Reovirus induces IFN, and reovirus is sensitive to the antiviral actions of IFN. The characteristics of the IFN-inducing capacity of reovirus, and the antiviral actions of IFN exerted against reovirus, are dependent upon the specific combination of reovirus strain, host cell line, and IFN type. Responses, both IFN induction and IFN action, differ quantitatively if not qualitatively and are dependent upon the virus, cell, and IFN combination. Stable natural dsRNA, identified as the form of nucleic acid that constitutes the reovirus genome, is centrally involved in the function of at least three IFN-induced enzymes. Protein phosphorylation by PKR, RNA editing by the ADAR adenosine deaminase, and RNA degradation by the 2',5'-oligoA pathway all involve dsRNA either as an effector or as a substrate. Considerable evidence implicates PKR as a particularly important contributor to the IFN-induced antiviral state displayed at the level of the single virus-infected cell, where the translation of viral mRNA is often observed to be inhibited following treatment with IFN-alpha/beta. In the whole animal infected with reovirus, elevated cellular immune responses mediated by enhanced expression of MHC class I and class II antigens induced by IFN-alpha/beta or IFN-gamma may contribute significantly to the overall antiviral response.

The 2-5A system: modulation of viral and cellular processes through acceleration of RNA degradation

The 2-5A system is an RNA degradation pathway that can be induced by the interferons (IFNs). Treatment of cells with IFN activates genes encoding several double-stranded RNA (dsRNA)-dependent synthetases. These enzymes generate 5'-triphosphorylated, 2',5'-phosphodiester-linked oligoadenylates (2-5A) from ATP. The effects of 2-5A in cells are transient since 2-5A is unstable in cells due to the activities of phosphodiesterase and phosphatase. 2-5A activates the endoribonuclease 2-5A-dependent RNase L, causing degradation of single-stranded RNA with moderate specificity. The human 2-5A-dependent RNase is an 83.5 kDa polypeptide that has little, if any, RNase activity, unless 2-5A is present. 2-5A binding to RNase L switches the enzyme from its off-state to its on-state. At least three 2',5'-linked oligoadenylates and a single 5'-phosphoryl group are required for maximal activation of the RNase. Even though the constitutive presence of 2-5A-dependent RNase is observed in nearly all mammalian cell types, cellular amounts of 2-5A-dependent mRNA and activity can increase after IFN treatment. One well-established role of the 2-5A system is as a host defense against some types of viruses. Since virus infection of cells results in the production and secretion of IFNs, and since dsRNA is both a frequent product of virus infection and an activator of 2-5A synthesis, the replication of encephalomyocarditis virus, which produces dsRNA during its life cycle, is greatly suppressed in IFN-treated cells as a direct result of RNA decay by the activated 2-5A-dependent RNase. This review covers the organic chemistry, enzymology, and molecular biology of 2-5A and its associated enzymes. Additional possible biological roles of the 2-5A system, such as in cell growth and differentiation, human immunodeficiency virus replication, heat shock, atherosclerotic plaque, pathogenesis of Type I diabetes, and apoptosis, are presented.

RNase L mediates the antiviral effect of interferon through a selective reduction in viral RNA during encephalomyocarditis virus infection

The 2',5'-oligoadenylate (2-5A) system is an RNA degradation pathway which plays an important role in the antipicornavirus effects of interferon (IFN). RNase L, the terminal component of the 2-5A system, is thought to mediate this antiviral activity through the degradation of viral RNA; however, the capacity of RNase L to selectively target viral RNA has not been carefully examined in intact cells. Therefore, the mechanism of RNase L-mediated antiviral activity was investigated following encephalomyocarditis virus (EMCV) infection of cell lines in which expression of transfected RNase L was induced or endogenous RNase L activity was inhibited. RNase L induction markedly enhanced the anti-EMCV activity of IFN via a reduction in EMCV RNA. Inhibition of endogenous RNase L activity inhibited this reduction in viral RNA. RNase L had no effect on IFN-mediated protection from vesicular stomatitis virus. RNase L induction reduced the rate of EMCV RNA synthesis, suggesting that RNase L may target viral RNAs involved in replication early in the virus life cycle. The RNase L-mediated reduction in viral RNA occurred in the absence of detectable effects on specific cellular mRNAs and without any global alteration in the cellular RNA profile. Extensive rRNA cleavage, indicative of high levels of 2-5A, was not observed in RNase L-induced, EMCV-infected cells; however, transfection of 2-5A into cells resulted in widespread degradation of cellular RNAs. These findings provide the first demonstration of the selective capacity of RNase L in intact cells and link this selective activity to cellular levels of 2-5A.

Reovirus induction of and sensitivity to beta interferon in cardiac myocyte cultures correlate with induction of myocarditis and are determined by viral core proteins

Reovirus-induced acute myocarditis in mice serves as a model to investigate non-immune-mediated mechanisms of viral myocarditis. We have used primary cardiac myocyte cultures infected with a large panel of myocarditic and nonmyocarditic reassortant reoviruses to identify determinants of viral myocarditic potential. Here, we report that while both myocarditic and nonmyocarditic reoviruses kill cardiac myocytes, viral myocarditic potential correlates with viral spread through cardiac myocyte cultures and with cumulative cell death. To address the role of secreted interferon (IFN), we added anti-IFN-alpha/beta antibody to infected cardiac myocyte cultures. Antibody benefited nonmyocarditic more than myocarditic virus spread (P < 0.001), and this benefit was associated with the reovirus M1 and L2 genes. There was no benefit for a differentiated skeletal muscle cell line culture (C2C12 cells), suggesting cell type specificity. IFN-beta induction in reovirus-infected cardiac myocyte cultures correlated with viral myocarditic potential (P = 0.006) and was associated with the reovirus M1, S2, and L2 genes. Sensitivity to the antiviral effects of IFN-alpha/beta added to cardiac myocyte cultures also correlated with viral myocarditic potential (P = 0.004) and was associated with the same reovirus genes. Several reoviruses induced IFN-beta levels discordant with their myocarditic phenotypes, and for those tested, sensitivity to IFN-alpha/beta compensated for the anomalous induction levels. Thus, the combination of induction of and sensitivity to IFN-alpha/beta is a determinant of reovirus myocarditic potential. Finally, a nonmyocarditic reovirus induced cardiac lesions in mice depleted of IFN-alpha/beta, demonstrating that IFN-alpha/beta is a determinant of reovirus-induced myocarditis. This provides the first identification of reovirus genes associated with IFN induction and sensitivity and provides the first evidence that IFN-beta can be a determinant of viral myocarditis and reovirus disease.

Repression of retrovirus-mediated transgene expression by interferons: implications for gene therapy

Retrovirus-mediated gene transfer is commonly used in gene therapy protocols and has the potential to provide long-term expression of the transgene. Although expression of a retrovirus-delivered transgene is satisfactory in cultured cells, it has been difficult to achieve consistent and high-level expression in vivo. In this investigation, we explored the possibility of modulating transgene expression by host-derived cytokines. Normal human keratinocytes and dermal fibroblasts were transduced with recombinant retroviruses expressing a reporter gene (lacZ). Treatment of transduced cells with a proinflammatory cytokine, gamma interferon (IFN-gamma), significantly reduced lacZ expression to less than 25% of that of nontreated cells. The inhibition was concentration dependent (peak at 5 ng/ml) and time dependent (maximal at 16 h for transcript and 24 h for protein); expression remained repressed in the continued presence of IFN-gamma but returned to normal levels 24 h after IFN-gamma withdrawal. The decrease in beta-galactosidase activity appeared to result from decrease in steady-state lacZ mRNA levels. Inhibitors of transcription and translation blocked IFN-gamma-induced repression, suggesting involvement of newly synthesized protein intermediates. Similar results were obtained by treatment of transduced cells with IFN-alpha but not with other proinflammatory cytokines, including tumor necrosis factor alpha, interleukin-2 (IL-1), IL-4, and granulocyte colony-stimulating factor. Although the level of lacZ mRNA was reduced by >70% following IFN treatment, the rate of lacZ transcription was not significantly different from that for nontreated cells. These results suggest that IFN-mediated regulation of transgene expression is at a posttranscriptional level. Interestingly, IFN-gamma also suppressed transgene expression driven by a cellular promoter (involucrin) inserted in an internal position in the retroviral vector. The presence of the overlapping 3' untranslated regions in transcripts initiated from the internal promoter and the long terminal repeat is suggestive of a posttranscriptional regulation, likely at the level of RNA stabilization. These results provide direct evidence for modulatory effects of IFNs on retrovirus-mediated transgene expression and suggest that gene therapy results may be altered by host inflammatory responses.

Double-stranded RNA is a trigger for apoptosis in vaccinia virus-infected cells

The vaccinia virus E3L gene codes for double-stranded RNA (dsRNA) binding proteins which can prevent activation of the dsRNA-dependent, interferon-induced protein kinase PKR. Activated PKR has been shown to induce apoptosis in HeLa cells. HeLa cells infected with vaccinia virus with the E3L gene deleted have also been shown to undergo apoptosis, whereas HeLa cells infected with wild-type vaccinia virus do not. In this report, using virus recombinants expressing mutant E3L products or alternative dsRNA binding proteins, we show that suppression of induction of apoptosis correlates with functional binding of proteins to dsRNA. Infection of HeLa cells with ts23, which leads to synthesis of increased dsRNA at restrictive temperature, induced apoptosis at restrictive but not permissive temperatures. Treatment of cells with cytosine arabinoside, which blocks the late buildup of dsRNA in vaccinia virus-infected cells, prevented induction of apoptosis by vaccinia virus with E3L deleted. Cells transfected with dsRNA in the absence of virus infection also underwent apoptosis. These results suggest that dsRNA is a trigger that can initiate a suicide response in virus-infected and perhaps uninfected cells.

Effect of interferon and 2',5'-oligoadenylates on rotavirus RNA synthesis

Based on the antiviral effect of interferon on rotavirus replication the inhibitory effect of 2',5'-oligoadenylates on MRNA and double-stranded RNA synthesis was studied using an in vitro assay. The chemically synthesized oligonucleotides were used to determine several characteristics of the inhibitory effect, such as chain length, presence of phosphate residues at the 5'-end, and the 2',5'-phosphodiester bond itself. In vitro transcription was inhibited by oligos with 5 or more adenine residues at a final concentration of 100 microM or greater. This result makes rotavirus transcriptase different from other viruses in which the inhibitory effects are associated with dinucleotides and trinucleotides. The inhibitory effect was increased when the oligo contained a phosphate residue at the 5'-end; in this case, inhibition was also seen at lower oligo concentrations as well as at shorter oligo chain length. The study of the kinetics of inhibition showed that the inhibition by p(A2'p5')(3)3A was competitive with a Ki value of 256 microM. The effect of the oligonucleotides on the in vitro viral RNA replication showed that the 2',5'-oligoadenylates were not able to significantly inhibit the in vitro rotavirus RNA synthesis. The lack of inhibition in the in vitro assay was very peculiar since RNA transcription and replication involves the viral RNA polymerase, VP1.

Mechanisms of viral inhibition by interferons

Interferons (IFNs) are a family of related proteins grouped in four species (alpha, beta, gamma and omega) according to their cellular origin, inducing agents and antigenic and functional properties. Their binding to specific receptors leads to the activation of signal transduction pathways that stimulate a defined set of genes, whose products are eventually responsible for the IFN antiviral effects. Their action against viruses is a complex phenomenon. It has been reported that IFNs restrict virus growth at the levels of penetration, uncoating, synthesis of mRNA, protein synthesis and assembly. This review will attempt to evaluate evidence of the involvement of the IFN-inducible proteins in the expression of the antiviral state against RNA or DNA viruses.

The 2-5A system and HIV infection

2',5'-Oligoadenylates (2-5A) have an essential role in the establishment of the antiviral state of a cell exposed to virus infection. The key enzymes of the 2-5A system are the 2-5A forming 2',5'-oligoadenylate synthetase (2-5OAS), the activity of which depends on the presence of viral or cellular double-stranded RNA (dsRNA), and the 2-5A-activated ribonuclease (RNase L). Basic research in recent years has shown that the 2-5A system is a promising target for anti-HIV chemotherapy, particularly due to its interaction with double-stranded segments within HIV RNA. Two new strategies have been developed which yield a selective antiviral effect of 2-5A against HIV-1 infection: (1) development of 2-5A analogues displaying a dual mode of action (activation of RNase L and inhibition of HIV-1 RT) and (2) intracellular immunization of cells against HIV-1 infection by application of the HIV-1-LTR--2-5OAS hybrid gene. A further strategy is the inhibition of DNA topoisomerase I by longer 2-5A oligomers.

(2'-5')Oligoadenylate and intracellular immunity against retrovirus infection

1. The double-stranded RNA-dependent 2',5'-oligoadenylate (2-5A) synthetase/ribonuclease L (RNase L) system plays an essential role in the establishment of the antiviral state of a cell exposed to virus infection. 2. Until recently, the application of 2-5A derivatives to reinforce this system seemed to be limited mainly due to the low specificity of RNase L for viral RNA. 3. Two new strategies have been developed which yield a selective antiviral effect of 2-5As at least against human immunodeficiency virus-1 (HIV-1) infection: (i) an "intracellular immunization" approach using 2-5A synthetase cDNA linked to HIV trans-acting response element (TAR) and (ii) inhibition of retroviral reverse transcriptase activity by 2-5A analogues.

Determination of the level of the core of 2',5'-oligoadenylates by high performance liquid chromatography

A simple and rapid method for analysis of the core of 2',5'-oligoadenylates, mainly based on the use of high performance liquid chromatography (HPLC), is described. Perchloric acid extracts of tissues or cells were first treated with nuclease P1. Portions of the extracts were then digested with alkaline phosphatase. HPLC analysis of the extracts was performed on a column system composed of an Ultrasphere ODS precolumn (4.6 x 45 mm) and an Ultrasphere Octyl column (4.6 x 250 mm) by stepwise elution using a 50 mM ammonium phosphate buffer, pH 7, containing 3.5 and 7% methanol. Three species of the core of 2',5'-oligoadenylates (dimer, trimer and tetramer) from a number of samples were eluted separately with 7% methanol, and the concentration of each core was directly estimated using constant values calculated with the standard core. The level of the core of 2',5'-oligoadenylates in tissues and cells determined by our method is similar to that reported by other authors who used biological, radiobinding or radioimmunological assays.

The Lang strain of reovirus serotype 1 and the Dearing strain of reovirus serotype 3 differ in their sensitivities to beta interferon

Replication of the Dearing strain of reovirus serotype 3 in mouse L cells was decreased 17- to 100-fold when a saturating dose of beta interferon (1,000 IU/ml) was used. Replication of the Lang strain of reovirus serotype 1 was inhibited only two- to threefold under similar conditions. It therefore appears that closely related strains of reovirus differ in their sensitivities to beta interferon treatment of mouse L cells.

Expression of cellular genes in CD4 positive lymphoid cells infected by the human immunodeficiency virus, HIV-1: evidence for a host protein synthesis shut-off induced by cellular mRNA degradation

We have investigated the effects of HIV-1 infection on cellular gene expression in two different human CD4 positive lymphoid cell lines: CEM and C8166 cells. As a prerequisite for this study it was necessary to develop virus-cell culture systems in which greater than 90% of the cells could be near synchronously infected by HIV-1. Further, since HIV-1 is a cytopathic virus, it was essential that cellular gene expression be examined in virus-infected cells which remained viable. After meeting these requirements, we measured cellular RNA and protein levels in virus-infected lymphocytes. In the cell lines examined the levels of cellular protein synthesis markedly decreased at times when viral-specific protein synthesis was increasing. Both Northern and slot blot analysis revealed that the declines in host protein synthesis were due, at least in part, to declines in steady state levels of cellular mRNAs. Runoff assays with nuclei isolated from infected cells demonstrated that the decreases in cellular mRNA levels were not due to declines in cellular RNA polymerase II transcription rates. To determine if the decreases in cellular protein synthesis also might be due to specific translational controls exerted by HIV-1, we compared the polysome association of cellular RNAs in infected and uninfected C8166 cells. The polysome distribution of cellular mRNAs was virtually identical in mock- and HIV-1-infected cells although, as expected, the total amount of cellular mRNAs were significantly lower in virus-infected cells. Taken together, these results suggest that HIV-1 may encode mechanisms to inhibit cellular protein synthesis, likely as a result of cellular mRNA degradation, rather than specific blocks in cellular mRNA translation.

Binding of Tat protein to TAR region of human immunodeficiency virus type 1 blocks TAR-mediated activation of (2'-5')oligoadenylate synthetase

The TAR sequence of the 5' leader of HIV-1 long terminal repeat-directed mRNA was found to be able to bind to and to activate double-stranded RNA-dependent (2'-5')A synthetase. Binding of TAR to the purified synthetase in vitro was abolished by addition of HIV-1 Tat protein, which binds to this sequence with a high affinity. Inhibition of TAR-mediated activation of (2'-5')A synthetase by Tat was prevented in the presence of the Zn2+ and Cd2+ chelators o-phenanthroline and penicillamine, which did not impair TAR-synthetase interaction. Transient expression assays of bacterial chloramphenicol acetyltransferase (CAT) gene in HeLa cells revealed that the levels of both CAT mRNA and CAT protein decreased after treatment of the cells with interferon, if CAT gene was linked to HIV-1 TAR segment. Cotransfection of the cells with a tat sequence containing plasmid rendered CAT gene expression insensible to the action of interferon.

In vitro translational activity of messenger-RNA isolated from mice treated with the interferon inducer, polyriboinosinic acid.polyribocytidylic acid

Treatment of mice with interferon and interferon inducers causes down regulation of a number of hepatic proteins. In a previous publication it was demonstrated that these treatments depress hepatic protein synthesis and increase protein degradation, particularly of the endoplasmic reticulum Gooderham NJ and Mannering GJ, Arch Biochem Biophys 250: 418-425, 1986. In the present study the effects of polyriboinosinic acid.polyribocytidylic acid (poly IC) treatment on mouse hepatic RNA levels and the translation of this RNA in a cell-free system were examined. Poly IC treatment of mice increased hepatic poly (A+) RNA levels. The translation of isolated poly(A+)RNA was evaluated at various intervals after the administration of poly IC. Translation was marginally increased at 3-6 hr after treatment and depressed after 12-18 hr. Antibodies were employed to examine the effects of poly IC treatment on specific polypeptides in order to evaluate the in vitro translation of mRNAs for tyrosine aminotransferase and albumin; translation of these proteins was biphasic with pronounced depression. These studies indicate that in vivo interferon may regulate gene expression by altering levels of hepatic proteins via increased transcription and decreased translation.

Accurate processing of human pre-rRNA in vitro

We report here that the mature 5' terminus of human 18S rRNA is generated in vitro by a two-step processing reaction. In the first step, SP6 transcripts were specifically cleaved in HeLa cell nucleolar extract at three positions near the external transcribed spacer (ETS)-18S boundary. Of these cleavage sites, two were major and the other was minor. RNase T1 fingerprint and secondary nuclease analyses placed the two major cleavage sites 3 and 8 bases upstream from the mature 5' end of 18S rRNA and the minor cleavage site 1 base into the 18S sequence. All three cleavages yielded 5'-hydroxyl, 2'-3'-cyclic phosphate termini and were 5' of adenosine residues in the sequence UACCU, which was repeated three times near the ETS-18S boundary. In the second step, the initial cleavage product containing 3 bases of ETS was converted to an RNA with a 5' terminus identical to that of mature 18S RNA by an activity found in HeLa cell cytoplasmic extracts.

Effect of 2',5'-oligoadenylate on herpes simplex virus-infected cells and preventive action of 2',5'-oligoadenylate on the lethal effect of HSV-2

Antiviral effects of ppp(A2'p)nA, (2-5A) on herpes simplex virus type 1 or type 2 (HSV-1 or HSV-2)-infected baby hamster kidney fibroblasts (BHK cells) and HSV-2-infected female guinea pigs were examined. 2-5A was introduced into BHK cells in the form of a calcium phosphate precipitate and as an ointment of polyethylene glycol (PEG) into guinea pig vagina. Under optimum conditions, 2-5A trimer and other 2-5A derivatives inhibited over 90% of HSV-1 syncytia formation and over 50% of HSV-2 plaque formation. The growth of uninfected cells was only slightly and transiently inhibited under these conditions. Northern analysis of viral immediate early mRNAs and cellular mRNAs showed that all transcripts determined were reduced in amount by the 2-5A trimer treatment. The reduction in level of viral mRNAs (ICP4, ICP22, and ICP47) by 2-5A trimer was significantly more than that of cellular mRNAs (represented by beta-actin). HSV-2 (strain 186) inoculation into the vagina of female guinea pigs causes severe symptoms in the genital area and high mortality. Topically applied 2-5A trimer almost completely prevented the lethal effect of HSV-2. These data are discussed from the viewpoint of mechanism of interferon action.

Accurate processing of human pre-rRNA in vitro

We report here that the mature 5' terminus of human 18S rRNA is generated in vitro by a two-step processing reaction. In the first step, SP6 transcripts were specifically cleaved in HeLa cell nucleolar extract at three positions near the external transcribed spacer (ETS)-18S boundary. Of these cleavage sites, two were major and the other was minor. RNase T1 fingerprint and secondary nuclease analyses placed the two major cleavage sites 3 and 8 bases upstream from the mature 5' end of 18S rRNA and the minor cleavage site 1 base into the 18S sequence. All three cleavages yielded 5'-hydroxyl, 2'-3'-cyclic phosphate termini and were 5' of adenosine residues in the sequence UACCU, which was repeated three times near the ETS-18S boundary. In the second step, the initial cleavage product containing 3 bases of ETS was converted to an RNA with a 5' terminus identical to that of mature 18S RNA by an activity found in HeLa cell cytoplasmic extracts.

Modulation of ppp(A2'p)nA-dependent RNase by a temperature-sensitive mutant of vaccinia virus

Activation of the ppp(A2'p)nA (2-5A)-dependent RNase was investigated during the abortive infection of BSC40 cells by a temperature-sensitive mutant of vaccinia virus, ts22. At the nonpermissive temperature, ts22 has an abortive late phenotype. At the onset of late-viral-gene expression, viral mRNA is degraded and rRNA is cleaved into discrete fragments in the absence of prior interferon treatment (R. F. Pacha and R. C. Condit, J. Virol. 56:395-403, 1985). Concomitant with rRNA cleavage, an increase in 2-5A occurred late during infection. Discrete 18S- and 28S-rRNA degradation products from BSC40 cells infected with ts22 at the nonpermissive temperature comigrated in denaturing agarose gels with rRNA cleaved fragments produced by the activation of 2-5A-dependent RNase in uninfected cells transfected with exogenous 2-5A. An increase in 2-5A levels and a similar discrete and characteristic degradation of rRNA were observed in BSC40 cells infected with wild-type vaccinia virus in the presence of isatin-beta-thiosemicarbazone. The results show that the ts22 lesion and the action of isatin-beta-thiosemicarbazone may affect the same pathway, leading to the activation of latent 2-5A-dependent RNase and resulting in indiscriminate RNA degradation and inhibition of viral replication.

Mechanism of interferon action: studies on the activation of protein phosphorylation and the inhibition of translation in cell-free systems

We describe the ability of reovirus messenger RNA (mRNA) to serve as a template for translation and as an activator of protein phosphorylation in cell-free extracts prepared from untreated and from interferon (IFN)-treated mouse fibroblast L cells. In vitro transcribed reovirus mRNA was purified by column chromatography on CF-11 cellulose. This procedure removed trace amounts of double-stranded RNA (dsRNA) [0.01%-0.1%] present in mRNA preparations purified solely by extensive LiCl precipitation. In the absence of added dsRNA, CF-11 cellulose-purified reovirus mRNA did not detectably activate phosphorylation of either ribosome-associated protein P1 or the alpha subunit of protein synthesis initiation factor eIF-2 in S-10 extracts prepared from L cells; the CF-11 cellulose-purified reovirus mRNA was translated more efficiently than was LiCl-purified reovirus mRNA in these extracts. Highly purified CF-11 reovirus mRNA was, however, translated less efficiently by S-10 extracts prepared from IFN-treated L cells than by extracts prepared from untreated L cells, suggesting that the inefficient translation by IFN-treated extracts was an integral property of reovirus mRNA. Increasing the secondary structure of reovirus mRNA by substituting bromouridine (Br-uridine) for uridine in the mRNA caused an increased inhibition of mRNA binding to ribosomes in extracts prepared from IFN-treated as compared to untreated cells. The mechanism of inhibition of translation of CF-11 cellulose-purified reovirus mRNA in IFN-treated systems remains to be established.

Reovirus type 3 synthesizes proteins in interferon-treated HeLa cells without reversing the antiviral state

Treatment of HeLa cells with human lymphoblastoid interferon (IFN-alpha) does not inhibit reovirus type 3 protein synthesis during virus infection. In contrast, reovirus translation is blocked by treatment of L cells with mouse IFN-alpha. The (2'-5')A synthetase activity is induced in HeLa cells by IFN-alpha treatment and is activated after reovirus infection, since cell lysates from these cells synthesize in vitro (2'-5')A oligonucleotides. The IFN-induced protein kinase activity is also triggered in those lysates upon dsRNA addition. Thus, contrary to DNA-containing viruses, such as vaccinia virus or adenovirus, reovirus infection does not destroy or reverse the IFN-induced antiviral state. In support of this conclusion, superinfection with poliovirus or vesicular stomatitis virus of reovirus-infected HeLa cells treated with IFN leads only to a blockade of translation of the former viruses. These results provide a remarkable example where in the same cells doubly infected with two different viruses, the antiviral state induced by IFN-alpha is manifested by selectively inhibiting translation of one kind of virus (poliovirus or vesicular stomatitis virus) without affecting the translation of reovirus type 3. In addition, these results indicate that the resistance of reovirus translation to inhibition by IFN is different from the mechanism of resistance induced by DNA-containing viruses.

Induction of an antiviral state by interferon in the absence of elevated levels of 2,5-oligo(A) synthetase and eIF-2 kinase

A series of clones has been derived from an interferon-resistant murine cell line, Ltk- aprt-, and their antiviral properties have been characterized. In the parental Ltk- aprt- line interferon is unable to establish antiviral properties or to increase the levels of 2,5-oligo(A) synthetase, the 2,5-oligo(A)-activated endonuclease F, 2',5'-phosphodiesterase, or eIF-2 kinase. However, interferon did prevent replication of vesicular stomatitis, Mengo virus, and reovirus in some of the derivative cell lines. The effect of interferon on the levels of the enzymes of the 2,5-oligo(A) and eIF-2 kinase pathways did not correlate directly with the antiviral properties of these cell clones. Greatly increased levels of 2,5-oligo(A) synthetase occurred in one clone without activation of an antiviral state. Another clone exhibited antiviral activity without detectably increased 2,5-oligo(A) synthetase activity. Changes in the levels of endonuclease F and 2',5'-phosphodiesterase were slight in all the clones examined. Neither 2,5-oligo(A) synthetase nor eIF-2 kinase levels were altered by interferon in another clone and yet an antiviral state was established and prevented replication of vesicular stomatitis, Mengo virus, and reovirus. The results show that mechanisms other than the 2,5-oligo(A) and eIF-2 kinase pathways are likely to contribute to the antiviral effects of interferon.