Interferon production and inhibition of host synthesis in cells infected with vesicular stomatitis virus

Interferon production by Viral, Bacterial & Yeast system: A comparative overview in 2023

Interferons play a critical role in the innate immune response against several infections and play a key role in the control of a variety of viral and bacterial infectious diseases such as hepatitis, covid-19, cancer, and multiple sclerosis. Therefore, natural or synthetic IFN production is important and had three common methods, including bacterial fermentation, animal cell culture, and recombinant nucleic acid technology. However, the safety, purity, and accuracy of the most preferred INF production systems have not been extensively studied. This study provides a comprehensive comparative overview of interferon production in various systems that include viral, bacterial, yeast, and mammalian. We aim to determine the most efficient, safe, and accurate interferon production system available in the year 2023. The mechanisms of artificial interferon production were reviewed in various organisms, and the types and subtypes of interferons produced by each system were compared. Our analysis provides a comprehensive overview of the similarities and differences in interferon production and highlights the potential for developing new therapeutic strategies to combat infectious diseases. This review article offers the diverse strategies used by different organisms in producing and utilizing interferons, providing a framework for future research into the evolution and function of this critical immune response pathway.

Evidence of Transcriptional Shutoff by Pathogenic Viral Haemorrhagic Septicaemia Virus in Rainbow Trout

The basis of pathogenicity of viral haemorrhagic septicaemia virus (VHSV) was analysed in the transcriptome of a rainbow trout cell line inoculated with pathogenic and non-pathogenic VHSV isolates. Although both VHSV isolates showed similar viral replication patterns, the number of differentially expressed genes was 42-fold higher in cells inoculated with the non-pathogenic VHSV at 3 h post inoculation (hpi). Infection with the non-pathogenic isolate resulted in Gene Ontologies (GO) enrichment of terms such as immune response, cytokine-mediated signalling pathway, regulation of translational initiation, unfolded protein binding, and protein folding, and induced an over-representation of the p53, PPAR, and TGF-β signalling pathways. Inoculation with the pathogenic isolate resulted in the GO enrichment of terms related to lipid metabolism and the salmonella infection KEGG pathway involved in the rearrangement of the cytoskeleton. Antiviral response was evident at 12hpi in cells infected with the pathogenic isolate. Overall, the data showed a delay in the response of genes involved in immune responses and viral sensing in cells inoculated with the pathogenic isolate and suggest transcriptional shutoff and immune avoidance as a critical mechanism of pathogenicity in VHSV. These pathways offer opportunities to further understand and manage VHSV pathogenicity in rainbow trout.

Selective regulation in ribosome biogenesis and protein production for efficient viral translation

As intracellular parasites, viruses depend heavily on host cell structures and their functions to complete their life cycle and produce new viral particles. Viruses utilize or modulate cellular translational machinery to achieve efficient replication; the role of ribosome biogenesis and protein synthesis in viral replication particularly highlights the importance of the ribosome quantity and/or quality in controlling viral protein synthesis. Recently reported studies have demonstrated that ribosome biogenesis factors (RBFs) and ribosomal proteins (RPs) act as multifaceted regulators in selective translation of viral transcripts. Here we summarize the recent literature on RBFs and RPs and their association with subcellular redistribution, post-translational modification, enzyme catalysis, and direct interaction with viral proteins. The advances described in this literature establish a rationale for targeting ribosome production and function in the design of the next generation of antiviral agents.

Porcine Reproductive and Respiratory Syndrome Virus Infection Induces both eIF2α Phosphorylation-Dependent and -Independent Host Translation Shutoff

Porcine reproductive and respiratory syndrome virus (PRRSV) is an Arterivirus that has caused tremendous economic losses in the global swine industry since it was discovered in the late 1980s. Inducing host translation shutoff is a strategy used by many viruses to optimize their replication and spread. Here, we demonstrate that PRRSV infection causes host translation suppression, which is strongly dependent on viral replication. By screening PRRSV-encoded nonstructural proteins (nsps), we found that nsp2 participates in the induction of host translation shutoff and that its transmembrane (TM) domain is required for this process. nsp2-induced translation suppression is independent of protein degradation pathways and the phosphorylation of eukaryotic initiation factor 2α (eIF2α). However, the overexpression of nsp2 or its TM domain significantly attenuated the mammalian target of rapamycin (mTOR) signaling pathway, an alternative pathway for modulating host gene expression. PRRSV infection also attenuated the mTOR signaling pathway, and PRRSV-induced host translation shutoff could be partly reversed when the attenuated mTOR phosphorylation was reactivated by an activator of the mTOR pathway. PRRSV infection still negatively regulated the host translation when the effects of eIF2α phosphorylation were completely reversed. Taken together, our results demonstrate that PRRSV infection induces host translation shutoff and that nsp2 is associated with this process. Both eIF2α phosphorylation and the attenuation of the mTOR signaling pathway contribute to PRRSV-induced host translation arrest.IMPORTANCE Viruses are obligate parasites, and the production of progeny viruses relies strictly on the host translation machinery. Therefore, the efficient modulation of host mRNA translation benefits viral replication, spread, and evolution. In this study, we provide evidence that porcine reproductive and respiratory syndrome virus (PRRSV) infection induces host translation shutoff and that the viral nonstructural protein nsp2 is associated with this process. Many viruses induce host translation shutoff by phosphorylating eukaryotic initiation factor 2α (eIF2α). However, PRRSV nsp2 does not induce eIF2α phosphorylation but attenuates the mTOR signaling pathway, another pathway regulating the host cell translational machinery. We also found that PRRSV-induced host translation shutoff was partly reversed by eliminating the effects of eIF2α phosphorylation or reactivating the mTOR pathway, indicating that PRRSV infection induces both eIF2α phosphorylation-dependent and -independent host translation shutoff.

Role of Viral Hemorrhagic Septicemia Virus Matrix (M) Protein in Suppressing Host Transcription

Viral hemorrhagic septicemia virus (VHSV) is a pathogenic fish rhabdovirus found in discrete locales throughout the Northern Hemisphere. VHSV infection of fish cells leads to upregulation of the host's virus detection response, but the virus quickly suppresses interferon (IFN) production and antiviral gene expression. By systematically screening each of the six VHSV structural and nonstructural genes, we identified matrix protein (M) as the virus' most potent antihost protein. Only M of VHSV genotype IV sublineage b (VHSV-IVb) suppressed mitochondrial antiviral signaling protein (MAVS) and type I IFN-induced gene expression in a dose-dependent manner. M also suppressed the constitutively active simian virus 40 (SV40) promoter and globally decreased cellular RNA levels. Chromatin immunoprecipitation (ChIP) studies illustrated that M inhibited RNA polymerase II (RNAP II) recruitment to gene promoters and decreased RNAP II C-terminal domain (CTD) Ser2 phosphorylation during VHSV infection. However, transcription directed by RNAP I to III was suppressed by M. To identify regions of functional importance, M proteins from a variety of VHSV strains were tested in cell-based transcriptional inhibition assays. M of a particular VHSV-Ia strain, F1, was significantly less potent than IVb M at inhibiting SV40/luciferase (Luc) expression yet differed by just 4 amino acids. Mutation of D62 to alanine alone, or in combination with an E181-to-alanine mutation (D62A E181A), dramatically reduced the ability of IVb M to suppress host transcription. Introducing either M D62A or D62A E181A mutations into VHSV-IVb via reverse genetics resulted in viruses that replicated efficiently but exhibited less cytotoxicity and reduced antitranscriptional activities, implicating M as a primary regulator of cytopathicity and host transcriptional suppression.IMPORTANCE Viruses must suppress host antiviral responses to replicate and spread between hosts. In these studies, we identified the matrix protein of the deadly fish novirhabdovirus VHSV as a critical mediator of host suppression during infection. Our studies indicated that M alone could block cellular gene expression at very low expression levels. We identified several subtle mutations in M that were less potent at suppressing host transcription. When these mutations were engineered back into recombinant viruses, the resulting viruses replicated well but elicited less toxicity in infected cells and activated host innate immune responses more robustly. These data demonstrated that VHSV M plays an important role in mediating both virus-induced cell toxicity and viral replication. Our data suggest that its roles in these two processes can be separated to design effective attenuated viruses for vaccine candidates.

Resistance to Rhabdoviridae Infection and Subversion of Antiviral Responses

Interferon (IFN) treatment induces the expression of hundreds of IFN-stimulated genes (ISGs). However, only a selection of their products have been demonstrated to be responsible for the inhibition of rhabdovirus replication in cultured cells; and only a few have been shown to play a role in mediating the antiviral response in vivo using gene knockout mouse models. IFNs inhibit rhabdovirus replication at different stages via the induction of a variety of ISGs. This review will discuss how individual ISG products confer resistance to rhabdoviruses by blocking viral entry, degrading single stranded viral RNA, inhibiting viral translation or preventing release of virions from the cell. Furthermore, this review will highlight how these viruses counteract the host IFN system.

A piscine birnavirus induces inhibition of protein synthesis in CHSE-214 cells primarily through the induction of eIF2α phosphorylation

Inhibition of protein synthesis represents one of the antiviral mechanisms employed by cells and it is also used by viruses for their own propagation. To what extent members of the Birnaviridae family employ such strategies is not well understood. Here we use a type-strain of the Aquabirnavirus, infectious pancreatic necrosis virus (IPNV), to investigate this phenomenon in vitro. CHSE-214 cells were infected with IPNV and at 3, 12, 24, and 48 hours post infection (hpi) before the cells were harvested and labeled with S³⁵ methionine to assess protein synthesis. eIF2α phosphorylation was examined by Western blot while RT-qPCR was used to assess virus replication and the expression levels of IFN-α, Mx1 and PKR. Cellular responses to IPNV infection were assessed by DNA laddering, Caspase-3 assays and flow cytometry. The results show that the onset and kinetics of eIF2α phosphorylation was similar to that of protein synthesis inhibition as shown by metabolic labeling. Increased virus replication and virus protein formation was observed by 12 hpi, peaking at 24 hpi. Apoptosis was induced in a small fraction (1−2%) of IPNV-infected CHSE cells from 24 hpi while necrotic/late apoptotic cells increased from 10% by 24 hpi to 59% at 48 hpi, as shown by flow cytometry. These results were in accordance with a small decline in cell viability by 24hpi, dropping below 50% by 48 hpi. IPNV induced IFN-α mRNA upregulation by 24 hpi while no change was observed in the expression of Mx1 and PKR mRNA. Collectively, these findings show that IPNV induces inhibition of protein synthesis in CHSE cells through phosphorylation of eIF2α with minimal involvement of apoptosis. The anticipation is that protein inhibition is used by the virus to evade the host innate antiviral responses.

Faster replication and higher expression levels of viral glycoproteins give the vesicular stomatitis virus/measles virus hybrid VSV-FH a growth advantage over measles virus

VSV-FH is a hybrid vesicular stomatitis virus (VSV) with a deletion of its G glycoprotein and encoding the measles virus (MV) fusion (F) and hemagglutinin (H) envelope glycoproteins. VSV-FH infects cells expressing MV receptors and is fusogenic and effective against myeloma xenografts in mice. We evaluated the fusogenic activities of MV and VSV-FH in relationship to the density of receptor on the target cell surface and the kinetics of F and H expression in infected cells. Using a panel of cells expressing increasing numbers of the MV receptor CD46, we evaluated syncytium size in MV- or VSV-FH-infected cells. VSV-FH is not fusogenic at low CD46 density but requires less CD46 for syncytium formation than MV. The size of each syncytium is larger in VSV-FH-infected cells at a specific CD46 density. While syncytium size reached a plateau and did not increase further in MV-infected CHO cells expressing ≥4,620 CD46 copies/cell, there was a corresponding increase in syncytium size with increases in CD46 levels in VSV-FH-infected CD46-expressing CHO (CHO-CD46) cells. Further analysis in VSV-FH-infected cell lines shows earlier and higher expression of F and H mRNAs and protein. However, VSV-FH cytotoxic activity was reduced by pretreatment of the cells with type I interferon. In contrast, the cytopathic effects are not affected in MV-infected cells. In summary, VSV-FH has significant advantages over MV as an oncolytic virus due to its higher viral yield, faster replication kinetics, and larger fusogenic capabilities but should be used in cancer types with defective interferon signaling pathways.

IMPORTANCE

We studied the cytotoxic activity of a vesicular stomatitis/measles hybrid virus (VSV-FH), which is superior to that of measles virus (MV), in different cancer cell lines. We determined that viral RNA and protein were produced faster and in higher quantities in VSV-FH-infected cells. This resulted in the formation of larger syncytia, higher production of infectious particles, and a more potent cytopathic effect in permissive cells. Importantly, VSV-FH, similar to MV, can discriminate between low- and high-expressing CD46 cells, a phenotype important for cancer therapy as the virus will be able to preferentially infect cancer cells that overexpress CD46 over low-CD46-expressing normal cells.

Rhabdovirus evasion of the interferon system

The family Rhabdoviridae contains important pathogens of humans, livestock, and crops, including the insect-transmitted vesicular stomatitis virus (VSV) and the neurotropic rabies virus (RV), which is directly transmitted between mammals. In spite of a highly similar organization of RNA genomes, proteins, and virus particles, cell biology of VSV and RV is divergent in several aspects, particularly with respect to their interplay with the cellular host defense. While infection with both rhabdoviruses is recognized via viral triphosphate RNAs by the cytoplasmic RNA helicase/translocase RIG-I, the viral counteractions to limit the response are contrasting. VSV infection is characterized by a rapid general shutdown of host gene expression and severe cytopathic effects, due to multiple activities of the matrix (M) protein affecting host polymerase functions and mRNA nuclear export, and by rapid and high-level virus replication. In contrast, RV spread and transmission relies on preserving the integrity of host cells, particularly of neurons. While a general cell shutdown by RV M is not observed, RV phosphoprotein (P) has developed independent functions to interfere with activation of IRFs and with STAT signaling. The molecular mechanisms employed are different from those of the paramyxovirus P gene products serving similar functions, and illustrate evolution of IFN antagonists to specifically support virus survival in the natural niches.

A temperature sensitive VSV identifies L protein residues that affect transcription but not replication

To investigate the polymerase components selectively involved in transcription versus replication of vesicular stomatitis virus (VSV), we sequenced the polymerase gene of a conditionally RNA defective, temperature sensitive VSV: ts(G)114, which has a phenotype upon shift from permissive to non-permissive temperature of shut-down of mRNA transcription and unaffected genome replication. Sequence analysis of the ts(G)114 L gene identified three altered amino acid residues in the L protein. These three changes were specifically engineered individually and in combinations into a functional cDNA clone encoding the VSV genome and tested for association with the temperature sensitive and RNA defective phenotypes in the background of recovered engineered viruses. The data presented in this study show a specific amino acid substitution in domain II of the VSV L protein that significantly affects total RNA synthesis, but when in combination with two additional amino acid substitutions identified in the ts(G)114 L protein, leads to a specific reduction in mRNA transcription, but not replication.

Vesicular stomatitis virus infection alters the eIF4F translation initiation complex and causes dephosphorylation of the eIF4E binding protein 4E-BP1

Vesicular stomatitis virus (VSV) modulates protein synthesis in infected cells in a way that allows the translation of its own 5'-capped mRNA but inhibits the translation of host mRNA. Previous data have shown that inactivation of eIF2alpha is important for VSV-induced inhibition of host protein synthesis. We tested whether there is a role for eIF4F in this inhibition. The multisubunit eIF4F complex is involved in the regulation of protein synthesis via phosphorylation of cap-binding protein eIF4E, a subunit of eIF4F. Translation of host mRNA is significantly reduced under conditions in which eIF4E is dephosphorylated. To determine whether VSV infection alters the eIF4F complex, we analyzed eIF4E phosphorylation and the association of eIF4E with other translation initiation factors, such as eIF4G and the translation inhibitor 4E-BP1. VSV infection of HeLa cells resulted in the dephosphorylation of eIF4E at serine 209 between 3 and 6 h postinfection. This time course corresponded well to that of the inhibition of host protein synthesis induced by VSV infection. Cells infected with a VSV mutant that is delayed in the ability to inhibit host protein synthesis were also delayed in dephosphorylation of eIF4E. In addition to decreasing eIF4E phosphorylation, VSV infection also resulted in the dephosphorylation and activation of eIF4E-binding protein 4E-BP1 between 3 and 6 h postinfection. Analysis of cap-binding complexes showed that VSV infection reduced the association of eIF4E with the eIF4G scaffolding subunit at the same time as its association with 4E-BP1 increased and that these time courses correlated with the dephosphorylation of eIF4E. These changes in the eIF4F complex occurred over the same time period as the onset of viral protein synthesis, suggesting that activation of 4E-BP1 does not inhibit translation of viral mRNAs. In support of this idea, VSV protein synthesis was not affected by the presence of rapamycin, a drug that blocks 4E-BP1 phosphorylation. These data show that VSV infection results in modifications of the eIF4F complex that are correlated with the inhibition of host protein synthesis and that translation of VSV mRNAs occurs despite lowered concentrations of the active cap-binding eIF4F complex. This is the first noted modification of both eIF4E and 4E-BP1 phosphorylation levels among viruses that produce capped mRNA for protein translation.

Cytopathogenesis and inhibition of host gene expression by RNA viruses

Many viruses interfere with host cell function in ways that are harmful or pathological. This often results in changes in cell morphology referred to as cytopathic effects. However, pathogenesis of virus infections also involves inhibition of host cell gene expression. Thus the term "cytopathogenesis," or pathogenesis at the cellular level, is meant to be broader than the term "cytopathic effects" and includes other cellular changes that contribute to viral pathogenesis in addition to those changes that are visible at the microscopic level. The goal of this review is to place recent work on the inhibition of host gene expression by RNA viruses in the context of the pathogenesis of virus infections. Three different RNA virus families, picornaviruses, influenza viruses, and rhabdoviruses, are used to illustrate common principles involved in cytopathogenesis. These examples were chosen because viral gene products responsible for inhibiting host gene expression have been identified, as have some of the molecular targets of the host. The argument is made that the role of the virus-induced inhibition of host gene expression is to inhibit the host antiviral response, such as the response to double-stranded RNA. Viral cytopathogenesis is presented as a balance between the host antiviral response and the ability of viruses to inhibit that response through the overall inhibition of host gene expression. This balance is a major determinant of viral tissue tropism in infections of intact animals.

Selective induction of the monocyte-attracting chemokines MCP-1 and IP-10 in vesicular stomatitis virus-infected human monocytes

It is characteristic of viral infections that monocytes/macrophages and lymphocytes infiltrate infected tissue, and neutrophils are absent. CC and non-ELR CXC chemokines predominantly attract mononuclear leukocytes, whereas the ELR motif-expressing CXC chemokines primarily act on neutrophils. To investigate the general role of chemokines in viral diseases, we determined their release and expression patterns after infection of human monocytes with vesicular stomatitis virus (VSV). Human monocytes were productively infected by VSV. Surprisingly, VSV did not induce the release of the proinflammatory cytokines tumor necrosis factor-alpha (TNF-alpha), interleukin-1beta (IL-1beta), and IL-6. In contrast, we found a strong induction of the CC chemokine monocyte chemotactic protein-1 (MCP-1) and the non-ELR CXC chemokine interferon-gamma (IFN-gamma) inducible protein-10 (IP-10) by VSV on the gene and protein level. The expression and release of the neutrophil chemoattractants IL-8 and growth-related oncogene-alpha (GRO-alpha) remained unaffected after VSV infection. Our results indicate that the typical monocyte and lymphocyte-dominated leukocyte infiltration of virus-infected tissue is based on a selective induction of mononuclear leukocyte-attracting chemokines.

Phenotypic consequences of rearranging the P, M, and G genes of vesicular stomatitis virus

The nonsegmented negative-strand RNA viruses (order Mononegavirales) include many important human pathogens. The order of their genes, which is highly conserved, is the major determinant of the relative levels of gene expression, since genes that are close to the single promoter site at the 3' end of the viral genome are transcribed at higher levels than those that occupy more distal positions. We manipulated an infectious cDNA clone of the prototypic vesicular stomatitis virus (VSV) to rearrange three of the five viral genes, using an approach which left the viral nucleotide sequence otherwise unaltered. The central three genes in the gene order, which encode the phosphoprotein P, the matrix protein M, and the glycoprotein G, were rearranged into all six possible orders. Viable viruses were recovered from each of the rearranged cDNAs. The recovered viruses were examined for their levels of gene expression, growth potential in cell culture, and virulence in mice. Gene rearrangement changed the expression levels of the encoded proteins in concordance with their distance from the 3' promoter. Some of the viruses with rearranged genomes replicated as well or slightly better than wild-type virus in cultured cells, while others showed decreased replication. All of the viruses were lethal for mice, although the time to symptoms and death following inoculation varied. These data show that despite the highly conserved gene order of the Mononegavirales, gene rearrangement is not lethal or necessarily even detrimental to the virus. These findings suggest that the conservation of the gene order observed among the Mononegavirales may result from immobilization of the ancestral gene order due to the lack of a mechanism for homologous recombination in this group of viruses. As a consequence, gene rearrangement should be irreversible and provide an approach for constructing viruses with novel phenotypes.

Activity of vesicular stomatitis virus M protein mutants in cell rounding is correlated with the ability to inhibit host gene expression and is not correlated with virus assembly function

In addition to its role in virus assembly, the matrix (M) protein of vesicular stomatitis virus (VSV) is involved in virus-induced cell rounding and inhibition of host-directed gene expression. Previous experiments have shown that two M protein mutants genetically dissociate the ability of M protein to inhibit host-directed gene expression from its function in virus assembly. M protein from tsO82 virus is fully functional in virus assembly but defective in the inhibition of host-directed gene expression, while the MN1 deletion mutant, which lacks amino acids 4-21, inhibits host-directed gene expression but cannot function in virus assembly. Experiments presented here compared cell rounding induced by these two mutant M proteins to that of wt M protein. BHK cells were transfected with M protein mRNA transcribed in vitro, and the extent of cell rounding was evaluated at 24 hr posttransfection. The MN1 protein was nearly as effective as wt M protein in the induction of cell rounding, while tsO82 M protein expressed from transfected RNA was not able to induce cell rounding above that observed in negative controls without M protein, although it did cause BHK cells to have a less elongated shape. These results indicate that the ability of MN1 and tsO82 M proteins to induce cell rounding is not correlated with their virus assembly function. Instead the cell rounding activity of these mutants is correlated with their ability to inhibit host-directed gene expression. Previous data suggesting that these two cytopathic activities could be dissociated can be readily accounted for by quantitative differences in M protein expression required. Infection of either BHK cells or L cells with tsO82 virus induced cell rounding, although cell rounding was delayed relative to that following infection with wt VSV, suggesting that tsO82 M protein retains some cytopathic activity. The distribution of actin, vimentin, and tubulin in transfected cells was determined by fluorescence microscopy. In cells transfected with tsO82 M mRNA, these cytoskeletal elements were indistinguishable from those of negative control transfected cells. In cells rounded as a result of transfection with wt M or MN1 mRNA, actin-containing filaments were reorganized into a thick perinuclear ring but were not depolymerized. In contrast, tubulin and vimentin appeared to be diffusely distributed throughout the cytoplasm of rounded cells. These results support the idea that cell rounding induced by M protein results from the depolymerization of microtubules and/or intermediate filaments.

The vesicular stomatitis virus matrix protein inhibits transcription from the human beta interferon promoter

In cells infected by wild-type (wt) vesicular stomatitis virus (VSV) Indiana, host transcription is severely inhibited. DNA cotransfection studies have implicated the VSV matrix (M) protein in this process (B. L. Black and D. S. Lyles, J. Virol. 66:4058-4064, 1992). The M protein inhibited transcription not only from viral promoters in plasmids but also from the chromosomally integrated human immunodeficiency virus type 1 (HIV-1) provirus promoter (S.-Y. Paik, A. C. Banerjea, G. G. Harmison, C.-J. Chen, and M. Schubert, J. Virol. 69:3529-3537, 1995). In this study, we investigated the effect of wt VSV M protein on expression of a reporter gene under control of a cellular promoter (beta-interferon [IFN-beta] promoter), using double transient transfections in BHK and COS-1 cells. The cellular IFN-beta promoter was as susceptible to the inhibitory effect of the M protein as the viral promoters used previously. Viral proteins N, P, and G had no significant effect on reporter gene expression. The M protein gene from VSV mutant T1026R1, which is defective in host transcription inhibition, was cloned and sequenced, and its effect on reporter gene expression was tested. The mutant M protein had a methionine-to-arginine change at position 51 in the protein sequence and did not inhibit transcription from either the IFN-beta promoter or viral promoters. This VSV mutant is a good inducer of IFN, as opposed to the wt virus, which suppresses IFN induction. These results show that the M protein inhibits transcription from cellular as well as viral promoters and that the M protein does not regulate the IFN promoter any differently from viral promoters. While the M protein may play a role in IFN gene regulation, other viral or cellular factors that provide specificity to the induction process must also be involved.

Effect of vesicular stomatitis virus matrix protein on host-directed translation in vivo

Vesicular stomatitis virus infection causes a rapid and potent inhibition of both host transcription and translation. Recently, the viral matrix (M) protein was shown to inhibit host-directed transcription in vivo in the absence of any other viral component (B. L. Black and D. S. Lyles, J. Virol. 66:4058-4064, 1992). The goal of this study was to determine the effect of M protein on host-directed translation. In vitro-transcribed mRNAs encoding M protein and chloramphenicol acetyltransferase (CAT) were cotransfected into BHK cells to determine the effect of M protein expression on translation of CAT mRNA. The results presented here show that M protein did not inhibit host-directed translation of CAT mRNA. On the contrary, this study gave the unexpected result that M protein actually stimulated host-directed translation under the same conditions in which it potently inhibited host-directed transcription. Under these conditions, the combined effect on host gene expression was a greater-than-20-fold inhibition. Furthermore, the enhancement of host translation mediated by M protein was genetically correlated with M protein's ability to inhibit host transcription. Thus, the results of this study establish that M protein does not inhibit host protein synthesis under the same conditions in which it potently inhibits host transcription and suggest that the inhibition of transcription and that of translation by vesicular stomatitis virus require separate viral gene products.

Interferon induction by viruses. XV. Biological characteristics of interferon induction-suppressing particles of vesicular stomatitis virus

A single interferon (IFN) induction-suppressing particle (ISP) of vesicular stomatitis virus (VSV) blocked completely the yield of IFN in a cell otherwise programmed to produce IFN. With mouse L cells as hosts, one lethal hit of UV radiation (D37 = 52.5 ergs/mm2) to the VSV genome sufficed to inactivate ISP activity; however, with "aged" primary chick embryo cells as hosts, it took 198 lethal hits (D37 = 10,395 ergs/mm2). ISP expression in chick cells did not require virus replication or amplified RNA synthesis, but did involve functional virion-associated L protein. ISP in chick cells also were capable of inhibiting, in a multiplicity-dependent manner, the plaquing efficiency of two viruses that require cellular polymerase II (pol II) for replication, e.g., pseudorabies and influenza. The refractory state to IFN inducibility that resulted from infection of chick cells with ISP (VSV tsO5 [UV = 100 hits]) was still extant after 6 days. In contrast, the plaquing efficiency of pseudorabies virus returned to control levels by 5 h after ISP infection. Chick cells infected with UV ISP remained viable, served as hosts for the replication of other viruses, and could be subcultured. Models are presented to account for these contrasting effects. The involvement of viral plus-strand leader RNA as an inhibitor of cellular pol II-dependent RNA synthesis, and the multifunctional activities of the virion-associated L protein, are discussed as possible molecules involved in the action of ISP in chick cells.

Interferon induction by viruses. XIII. Detection and assay of interferon induction-suppressing particles

An assay is described for detecting and measuring the capacity of a single virus particle to suppress completely the yield of interferon from a cell otherwise competent to product interferon. The assay defines virus populations in terms of a newly quantifiable attribute--interferon induction-suppressing particles (ISP). The antagonistic actions of interferon-inducing particles and interferon induction-suppressing particles are thought to determine the interferon-inducing capacity intrinsic to a virus population.

Further studies of the RNA synthesis phenotype selected during persistent infection with vesicular stomatitis virus

Vesicular stomatitis virus (VSV) isolated from two independently established lines of persistently infected mouse L cells expressed an altered phenotype of RNA synthesis at 37 degrees, the temperature at which the persistently infected cultures were maintained (T.K. Frey and J.S. Youngner, 1982, J. Virol. 44, 167-174). In comparison to the viruses used to initiate the two lines, wild-type (wt) VSV and ts-0-23 (ts-, RNA+ complementation group III), the VSV expressing this RNA phenotype synthesized much less mRNA but equal or greater amounts of 40 S genomic RNA (rt- phenotype). In the line initiated with wt-VSV, at 17 days after initiation, when 85% of the clones were ts-, 36% of the ts- clones were rt-. By 63 days the VSV-PI population was uniformly ts- and rt- and this phenotype prevailed for at least 2 years of persistence. In the line initiated with ts-0-23, the rt- phenotype was stable for at least 3 years of persistence. To study the relationship of the ts- and rt- phenotypes which were coselected during persistence, ts+ revertants of a ts- rt- VSV-PI clone were isolated. All of the ts+ revertants expressed a wt-VSV phenotype of RNA synthesis at 37 degrees (rt+), indicating that the two phenotypic markers may be pleiotropic manifestations of the same mutation. rt-VSV inhibited host cell RNA and protein synthesis more slowly than did wt-VSV. However, rt-VSV synthesized equivalent or greater amounts of all the virus proteins, compared to wt-VSV, despite the reduced amount of mRNA transcription. The attenuated shutoff of host cell macromolecular synthesis by rt- VSV and the concomitant efficient 40 S genome replication and virus protein synthesis may in part explain the selective advantage of the rt- mutation during persistence. The rt- phenotype was not unique to persistent infection; ts- rt- mutants also evolved during serial undiluted passages of wt-VSV in L cells and one ts- rt- mutant was identified in a group of spontaneous mutants isolated from a wt-VSV stock.

Interferon induction by viruses. IX. Antagonistic activities of virus particles modulated interferon production

Cells generally responded to viral inducers of interferon (IFN) by generating a dose (multiplicity of infection)-response (interferon-yield) curve which showed a good fit to a model in which the fraction of interferon-yielders corresponded to the fraction of cells infected with one or more particles. These were defined as interferon-inducing particles (IFPs). The IFN-producing capacity of cells infected with these particles was modulated upon coinfection with a second type of virus particle, those with the capacity to suppress IFN production. These were termed interferon induction-suppressing particles (ISPs). In cells coinfected with IFP and ISP the latter were dominant phenotypically. In some systems ISP activity was expressed by IFP when the latter were present at two or more particles per cell. The existence of virus particles with antagonistic activities for interferon induction provides a mechanism for regulating the production of IFN and hence the potential outcome of a virus infection.

Inhibition of synthesis of ribosomal proteins and of ribosome assembly after infection of L cells with vesicular stomatitis virus

The effect of infection of mouse L cells by vesicular stomatitis virus on the synthesis of ribosomal proteins was investigated using two-dimensional polyacrylamide gel electrophoresis to analyze the ribosomal proteins. It was found that the synthesis of nearly all of the cytoplasmic ribosomal proteins examined was inhibited by infection and mostly to the same extent. Analysis of the ribosomal proteins extracted from intact ribosomes indicated that infection also reduces the incorporation of all the ribosomal proteins tested into assembled ribosomes. The inhibition of ribosome assembly was greater than the inhibition of synthesis of ribosomal proteins, suggesting that some other factor was also limiting the assembly of ribosomes. As shown in this report, infection also inhibits ribosomal RNA production. Thus, the decreased assembly of ribosomes in infected cells probably results from the inhibition of synthesis of both ribosomal proteins and ribosomal RNA.

Inhibition of RNA synthesis in mouse myeloma cells infected with vesicular stomatitis virus

Infection of mouse myeloma cells (MPC-11) with vesicular stomatitis (VS) virus resulted in rapid and marked reduction in cellular RNA synthesis considerably before cell viability was compromised. Mouse myeloma cells responded maximally to viral infection at a multiplicity of 1 and were considerably more se;sitive to shut-off of RNA synthesis than were mouse L cells or BHK-21 cells. This inhibition of cellular RNA synthesis was shown not to be caused by differential membrane permeability of infected and uninfected MPC-11 cells to [3H]uridine, nor was it due to greater degradation of previously synthesized RNA. VS viral infection appeared not to impede transport of newly synthesized nuclear RNA to the cytoplasm; moreover, infected cells accumulated polyadenylated mRNA at the same rate as did uninfected cells. Polyacrylamide gel electrophoresis of newly synthesized nuclear RNA demonstrated that the polydisperse nature and size distribution were not affected by VS viral infection. Isolated nuclei of infected MPC-11 cells also inhibited greatly impaired capacity to synthesize RNA despite the absence of cytoplasmic factors. Infected-cell cytosol did not inhibit transcription by uninfected-cell nuclei, nor did uninfected-cell cytosol reverse viral inhibition of nuclear transcription. Studies with alpha-amanitin revealed that VS viral infection inhibited the activity of polymerases I, II, and III, but only polymerase II was affected progressively throughout infection and to a much greater extent. These data suggest that, even at low multiplicities of infection, VS virus rapidly shuts off cellular RNA synthesis at the level of nuclear transcription.

Interferon production and response to exogenous interferon in two cell lines of mouse brain origin persistently infected with Sendai virus

Two persistently infected cell lines established from C3H mouse brain cells infected in vivo with Sendai virus were shown to differ with respect to interferon (IF) production and response to exogenous IF. MB/Sen carrier cells contained 1-5 per cent antigen positive cells when examined by immunofluorescence, and virus was occasionally recovered from the culture medium. MB/SenAS carrier cells were maintained with 0.16 per cent Sendai antiserum in the supernatant medium. All MB/SenAS cells contained viral antigen and infectious virus was present in the culture medium. MB/Sen released IF spontaneously into the culture medium. Further IF production could be stimulated in MB/Sen by superinfection with Newcastle disease virus (NDV) or vesicular stomatitis virus (VSV). Exogenous IF provided good protection against VSV challenge. In contrast, MB/SenAS produced no IF spontaneously but could be stimulated by NDV and VSV to produce IF. Exogenous IF failed to reduce the amount of VSV released into the supernatant fluid. Replication of VSV was restricted in MB/SenAS as shown by a 2.3 log10 lower virus yield compared to MB/Sen.

Translation of individual host mRNA's in MPC-11 cells is differentially suppressed after infection by vesicular stomatitis virus

Infection of MPC-11 mouse plasmacytoma cells by vesicular stomatitis virus results in 30 to 35% reduction in [35S]methionine incorporation into total proteins within 30 min postinfection. By 6 h postinfection, total protein synthesis is reduced by 80 to 90%. However, even by 30 min postinfection, a differential suppression of the synthesis of individual host protein is observed. The synthesis of the immunoglobin G (IgG) heavy chain (H), and, in particular, the synthesis of IgG light chain (L), is considerably more resistant to vesicular stomatitis virus-induced inhibition than is the synthesis of the non-IgG proteins as a whole; e.g., when the synthesis of non-IgG proteins was reduced by 41%, the synthesis of the H and L chains was reduced by 28 and 7%, respectively. Furthermore, these alterations in the relative synthesis of the L chain, H chain, and non-IgG are comparable to the alterations previously observed in uninfected MPC-11 cells when the overall rate of polypeptide chain initiation was selectively reduced (D.L. Nuss and G. Koch, 1976). These results are discussed in terms of the strategy of virus-directed suppression of host mRNA translation.

Inhibition of pseudorabies virus replication by vesicular stomicles virus I. Activity of infectious and inactivated B particles

Infectious B particles of vesicular stomatitis virus (VSV) are capable of inhibiting the replication of pseudorabies virus (PSR) in a variety of cell lines. Even under conditions of an abortive infection in a continuous line of rabbit cornea cells (RC-6O), B particles interfere with the replication of PSR with high efficiency. Particle per cell dose-response analysis of B particle populations revealed that the number of VSV particles capable of inhibiting PSR replication exceeds the number of PFU by a factor of 32 to 64. When B particles are treated with UV irradiation, a drastic increase in the multiplicity of infection is required to inhibit PSR replication. Whereas one infective B particles per cell is sufficient to prevent replication of PSR, 800 to 1,000 VSV particles rendered noninfective by UV irradiation are required to compensate for the loss of VSV synthetic activity that results from irradiation. Temperature-sensitive mutants representing five complementation groups of VSV were tested at low multiplicities of infection for their effect on PSR replication at the nonpermissive temperature. Generally, the ability of the different complementation groups to amplify virion products at the nonpermissive temperature is associated with their ability to inhibit PSR replication. These results imply that at low multiplicities of infection, amplification of infecting VSV components is necessary for inhibition of PSR replication., but at high multiplicities of infection with VSV, a virion component can prevent PSR replication in the absence of de novo VSV RNA or protein synthesis.

Inhibition of pseudorabies virus replication by vesicular stomatitis virus. II Activity of defective interfering particles

Purified defective interfering (DI) particles of vesicular stomatitis virus (VSV) inhibit the replication of a heterologous virus, pseudorabies virus (PSR), in hamster (BHK-21) and rabbit (RC-60) cell lines. In contrast to infectious B particles of VSV, UV irradiation of DI particles does not reduce their ability to inhibit PSR replication. However, UV irradiation progressively reduces the ability of DI particles to cause homologous interference with B particle replication. Pretreatment with interferon does not affect the ability of DI particles to inhibit PSR replication in a rabbit cell line (RC-60) in which RNA, but not DNA, viruses are sensitive to the action of interferon. Under similar conditions of interferon pretreatment, the inhibition of PSR by B particles is blocked. These data suggest that de novo VSV RNA or protein synthesis is not required for the inhibition of PSR replication by DI particles. DI particles that inhibit PSR replication also inhibit host RNA and protein synthesis in BHK-21 and RC-60 cells. Based on the results described and data in the literature, it is proposed that the same component of VSV B and DI particles is responsible for most, if not all, of the inhibitory activities of VSV, except homologous interference.

Differential inhibition of host protein synthesis in L cells infected with RNA - temperature-sensitive mutants of vesicular stomatitis virus

The response of mouse L cells to infection with wild-type (wt) and temperature-sensitive (ts) mutants of vesicular stomatitis virus was monitored by sodium dodecyl sulfate-polyacrylamide gel electrophoresis to delineate the synthesis of host cell and viral proteins. Experiments utilized transcriptase mutants of complementation group I (ts114 and ts13), a group IV mutant (ts44) that is restricted in total RNA synthesis (RNA-1) but not in primary transcription, and a group II mutant (ts52) variably restricted in RNA synthesis (RNA +/-). L cells infected with ts mutants at permissive temperature exhibited the wt response of progressive inhibition of host cell protein synthesis accompanied by accumulation of all five viral proteins. Mutant ts44 (IV) also switched off cell protein synthesis at restrictive temperature and accumulated all five viral proteins, but with disproportionate ratios of N and G proteins. At restrictive temperature, cells infected with group I ts mutants failed to accumulate any viral protein and did not exhibit significant reduction in host cell protein synthesis. These data suggest that vesicular stomatitis virus inhibits cell protein synthesis at a stage of viral infection after transcription and possibly translation but preceding replication of progeny viral RNA.

Abortive infection of a rabbit cornea cell line by vesicular stomatitis virus: conversion to productive infection by superinfection with vaccinia virus

An abortive infection of a rabbit cornea cell line (RC-60) by vesicular stomatitis virus (VSV), yielding less than 1 PFU/cell, was converted to a productive infection, yielding 1,900 PFU/cell, when cells were superinfected with vaccinia. Studies on the synthesis of VSV-directed RNA in RC-60 cells suggest that the abortive infection by VSV alone may be due in part to (i) a limited production of 40S virion RNA and (ii) a markedly reduced activity of virion-bound transcriptase activity in RC-60 cells compared to the activity in mouse L cells, a permissive host for VSV. No recognizable VSV structures, except a small amount of viral core structures, were produced by the abortive infection. In contrast, double infection of RC-60 cells with VSV and vaccinia in the presence of hydroxyurea resulted in the production of infective B particles of VSV. Although the function supplied by vaccinia responsible for the productive replication of VSV in double infected RC-60 cells has not been identified, metabolic inhibitor studies indicate that continuous vaccinia-dependent RNA synthesis is required for maximal production of infective VSV. The possibility is considered that vaccinia may supply a product or function required for VSV replication which is ordinarily supplied by the host but which is lacking in RC-60 cells.

Scanning electron microscopic studies of virus-infected cells. I. Cytopathic effects and maturation of vesicular stomatitis virus in L2 cells

L2 cells infected with vesicular stomatitis virus under single-cycle conditions have been studied by scanning electron microscopy after preparation by the critical point drying technique. Three dimensional images of intact cells show bullet-shaped vesicular stomatitis virus virions budding singly and in radiating clusters both from the plasma membrane between cellular microvilli and from the sides of microvilli. Virus-induced cytopathic effects observed by scanning electron microscopy include intermeshing of microvilli, loss of filipodia which attach cells to the substrate, and rounding up and detachment of infected cells from the substrate.

Analysis of uridine incorporation in chicken embryo cells infected by vesicular stomatitis virus and its temperature-sensitive mutants: uridine transport

The shut-off of RNA synthesis in chicken embryo cells, after infection with vesicular stomatitis virus, is partially due to a reduced capacity of the infected cells to transport uridine. Permeability to uridine decreases exponentially after infection. This loss of ability to transport uridine may be caused either by structural components of the input virions or may result from the expression of the viral gene products. In the latter case, only minor levels of viral transcription is sufficient to modify cellular permeability, since, even at low multiplicities, RNA minus temperature-sensitive (ts) mutants of vesicular stomatitis virus bring about a significant diminution of uridine incorporation in cells infected under nonpermissive conditions. Experiments with mutants of group III suggest that the M protein of the viral envelope may play a role in the sequence of events that modifies uridine transport. In addition to this cause of the diminution of incorporation of uridine by infected cells, another mechanism is noted which requires protein synthesis.

In vitro synthesis of proteins by membrane-bound polyribosomes from vesicular stomatitis virus-infected HeLa cells

Membrane-bound polysomes from vesicular stomatitis virus (VSV)-infected HeLa cells synthesize predominantly three proteins in an in vitro protein synthesizing system. These three proteins have different molecular weights than the viral structural proteins, i.e., 115,000, 88,000, and 72,000. Addition of preincubated L or HeLa cell S10 or HeLa cell crude initiation factors stimulates amino acid incorporation and, furthermore, alters the pattern of proteins synthesized. Stimulated membrane-bound polysomes synthesize predominantly viral protein G and lesser amounts of N, NS, and M. In vitro synthesized proteins G and N are very similar to virion proteins G and N based on analysis of tryptic methionine-labeled peptides. Most methionine-labeled tryptic peptides of virion G protein contain no carbohydrate moieties, since about 90% of sugar-labeled peptides co-chromatograph with only about 10% of methionine-labeled peptides. Sucrose gradient analysis of the labeled RNA present in VSV-infected membrane-bound polysomes reveals a relative enrichment in a class of viral RNA sedimenting slightly faster than the total population of the 13 to 15S mRNA, as compared to a VSV-infected crude cytoplasmic extract. A number of proteins, other than the viral structural proteins, are synthesized in the cytoplasm of five lines of VSV-infected cells. One of these proteins has the same molecular weight as the major in vitro synthesized protein, P(88). In vitro synthesized protein P(88) does not appear to be a precursor of viral structural proteins G, N, or M based on pulse-chase experiments and tryptic peptide mapping. Nonstimulated membrane-bound polysomes from uninfected HeLa cells synthesize the same size distribution of proteins as nonstimulated VSV-infected membrane-bound polysomes.

RNA synthesis by vesicular stomatitis virus and a small plaque mutant: effects of cycloheximide

The synthesis of viral RNA by wild-type vesicular stomatitis virus (L(1)VSV) and a small, plaque-size mutant (S(2)VSV) was studied in vitro and in chicken embryo (CE) and mouse L-cell cultures. Virus-specific RNA synthesized in CE or L cells infected with either L(1) or S(2)VSV at low multiplicity was of the same size classes, 12 to 15S, 28S, and 38S. The major differences were in the proportion of RNA produced of each size class. L(1)VSV always synthesized larger proportions of 38S RNA, and S(2)VSV produced larger proportions of 12 to 15S RNA. Both S(2) and L(1)VSV exhibited RNA transcriptase activity in vitro and in cell culture. The products of the in vitro reaction were the same, 12 to 15S for both. The products of the virion-associated transcriptase in CE or L-cell cultures in the presence of cycloheximide were also the same for both viruses but differed from the in vitro products in that 28S and 12 to 15S RNA were made. The effects of addition of cycloheximide at various times after infection demonstrated that new protein synthesis is required early (0-2 h) for both S(2) and L(1)VSV to initiate and maintain the normal rate of viral RNA synthesis. However, the overall rate of RNA synthesis in L(1)VSV infections became independent of protein synthesis after 2 h whereas the rate in S(2)VSV infections did not. With either virus, synthesis of 38S RNA did not occur in the absence of protein synthesis. Moreover, continuous 38S RNA production required continuous protein synthesis. Production of 38S RNA ceased within 30 min after addition of cycloheximide to S(2) (-) or L(1)VSV-infected CE or L cells that had already begun to synthesize the 38S form. The cycloheximide-induced cessation of 38S RNA synthesis was accompanied by a marked increase in production of 12 to 15S and 28S RNA in L(1)VSV-infected cells, but no increase in synthesis of small RNA species occurred in S(2)VSV-infected cells.

Inhibition of protein synthesis in L cells infected with vesicular stomatitis virus

The inhibition of protein synthesis in L cells by vesicular stomatitis virus (VSV) requires the synthesis of new protein subsequent to virus infection. However, two mechanisms may be involved in the inhibition of cell protein synthesis by VSV: an initial, multiplicity-dependent, ultraviolet-insensitive inhibition and a progressive, ultraviolet-sensitive inhibition.