Antiviral activity of a synthetic analog of prostaglandin A in mice infected with influenza A virus

We have previously shown that prostaglandins of the A series potently inhibit virus replication in several virus-host systems in vitro. In the present report we have studied the effect of a long-acting synthetic analog of PGA, 16,16-dimethyl-PGA2(Di-M-PGA2), on virus infection in vivo, using as a model Balb/c mice infected with influenza A (PR8) virus. Depending upon the dose of viral inoculum, PR8 virus caused the death of 50 to 100% of the animals in a period of 8-20 days. Di-M-PGA2-treatment significantly increased mouse survival by an average of 40%, independently of the dose of inoculum and the age of the animals. The fact that Di-M-PGA2-treatment decreased virus titers in the lungs and did not alter the host immune response, suggested that PGA's therapeutic action was due to suppression of virus replication. Finally, two anti-inflammatory compounds, which inhibit prostaglandin synthesis, aspirin and indomethacin, were shown not to significantly alter mouse survival in this system.

Stability of vaccinia virus DNA during persistent infections: accumulation of left-end deletions and of tandem repeats at both ends of the viral genome and prevention by interferon

The stability of the large vaccinia virus genome (122 MDa) has been studied in long-term cultures of persistently infected Friend erythroleukemia (FEL) cells. Deletions of about 8 MDa at the left terminus of vaccinia DNA are generated at early passages, and are maintained even after 2 years of continuous cell passages. The generation of deletions is followed by a stable accumulation of tandem repeats up to 6 MDa at the left end and up to 2 MDa at the right end of vaccinia DNA. Neither translocations nor rearrangements of DNA are observed during persistent infection. A recombinational mechanism within the tandem repeats or a mechanism similar to that described for adenovirus might explain the reiteration of tandem repeats at both ends. Significantly, in persistently infected cultures continuously treated with interferon (IFN) both the deletion and the large accumulation of tandem repeats were completely blocked. We suggest that reiteration of tandem repeats at the termini of vaccinia DNA might provide signals for more efficient virus multiplication in FEL cells.

Structural and functional studies of a 39,000-Mr immunodominant protein of vaccinia virus

Little is known about the nature of poxvirus proteins involved in the host immune response. Screening a lambda gt11 expression library of genomic rabbit poxvirus DNA with hyperimmune rabbit anti-vaccinia virus serum and selection of monospecific antibodies identified a highly antigenic viral protein of about 39,000 molecular weight (39K protein). The same-size protein of vaccinia virus was also identified with a monoclonal antibody (MAb B6) obtained from hybridomas generated after fusion of hyperimmunized mouse spleen cells with mouse myeloma cells. Structural analysis revealed that the 39K protein is an acidic polypeptide, that it can exist in two molecular forms because of intramolecular disulfide linkages, and that it is part of the virus core. This protein shares antigenic determinants with a cytoplasmic component(s) from uninfected cells. Functional studies revealed that the 39K protein is synthesized at late times postinfection and appears to be required for virus assembly. This protein is highly conserved in members of the Orthopoxvirus group, but in cowpox virus, a 41K virion protein was specifically recognized by antibodies that reacted against the vaccinia virus 39K protein. Significantly, during long-term passages of Friend erythroleukemia cells persistently infected with vaccinia virus, some virus mutants were found to increase or decrease by about 2 kilodaltons the size of the 39K protein. Mapping analysis localized sequences encoding the 39K protein in a rifampin-sensitive gene cluster between the two major core-associated viral polypeptides, 4a and 4b. The fact that the 39K core protein of vaccinia virus elicits strong humoral immune response, induces antibodies that react against a host component(s), and is subjected to genetic variability suggests that this protein has important biological functions.

Selective inhibition of protein synthesis by synthetic and vaccinia virus-core synthesized poly(riboadenylic acids)

Studies were undertaken to compare the effect of poly(A)s from various sources on selective inhibition of protein synthesis in the reticulocyte lysate system programmed with viral and cellular mRNAs. RNA synthesized in vitro by vaccinia virus cores in the presence of only ATP inhibited overall HeLa cell polypeptide synthesis by over 80% with a minimal effect on translation of vaccinia virus mRNAs. Hybridization of the [alpha-32P]AMP-labeled RNA made in vitro by vaccinia virus cores in the presence of only ATP, showed no complementary to HindIII restriction fragments of vaccinia virus DNA indicating that the in vitro product was poly(A). Fractionation of synthetic and core-synthesized poly(A) into three size classes showed that the larger the size of poly(A), the greater its inhibitory activity of protein synthesis in the cell-free system. Inhibition of translation of mRNAs from vaccinia virus-infected HeLa cells was also observed in the presence of poly(A). However, virus-induced polypeptide synthesis was more resistant to the effect of poly(A) than were cellular polypeptides. Oligo(dT) when added to the reticulocyte lysate system was capable of reversing the inhibition of protein synthesis caused by both core-synthesized poly(A) and core-transcribed RNAs. These results indicate that poly(A) synthesized by the virion-associated enzyme has inhibitory properties similar to those of synthetic poly(A).

Mapping and nucleotide sequence of the vaccinia virus gene that encodes a 14-kilodalton fusion protein

A library of rabbit poxvirus DNA fragments contained in the expression cloning vector lambda gt11 was screened with monoclonal antibodies that react specifically against a 14-kilodalton envelope protein of vaccinia virus and rabbit poxvirus. The 14-kilodalton protein appears to play an important role in virus penetration at the level of cell fusion; it also elicits neutralizing antibodies, and it forms covalently linked trimers on the surface of virions and in infected cells (Rodriguez et al., J. Virol. 56:482-488, 1985; Rodriguez et al., J. Virol. 61:395-404, 1987). Two recombinant bacteriophages expressing beta-galactosidase fusion proteins were isolated. Restriction enzyme analysis and hybridization studies mapped the 14-kilodalton encoding sequences in the middle of vaccinia virus HindIII A DNA fragment. Nucleotide sequence analysis revealed an open reading frame (ATG) preceded by a characteristic TAA sequence of late genes. The sequence spans 330 nucleotides and codes for a protein with a molecular weight of 12,500 and an isoelectric point of 6.3. There are two small hydrophobic regions, one at the C terminus (11 amino acids) and the other at the N terminus (5 amino acids). The protein contains two cysteines for oligomer formation and one glycosylation site. Inspection of the deduced amino acid sequence of the 14-kilodalton protein revealed consensus sites with the hemagglutinin precursor of influenza A virus and with adenylate kinase and cytochrome c of various species.

Resistance of vaccinia virus to interferons: modulation of the 2-5A system in interferon-treated, vaccinia virus infected cells

In this study we have examined if resistance of vaccinia virus to interferon (IFN) correlates with virus-induced alterations of the 2-5A system. We have shown that in various IFN-treated vaccinia virus infected cells of mouse, monkey and human origins, the intracellular levels of 2-5A are low early in infection but exhibit a sharp rise late in infection. In spite of the presence of 2-5A, activation of the 2-5A dependent RNase, as measured by the rRNA cleavage assay, does not occur or is delayed in the course of virus infection. However, when cycloheximide, an inhibitor of protein synthesis is added at the time of virus infection, extensive cleavage or rRNA is observed in IFN-treated, infected cells. If cycloheximide is added at various times after virus infection, rRNA cleavage is gradually prevented and a virus-induced inhibitor of the 2-5A system can be detected between 1-2 hr post infection. A function encoded by a ts 22 mutant of vaccinia virus blocked rRNA cleavage. Restriction of rRNA cleavage during virus infection correlated with dephosphorylation of 2-5A. Our findings suggest that modulation of the 2-5A system by vaccinia virus involves the production of an activator and simultaneous synthesis of an inhibitor(s). Viral ds-RNA is likely to be the activator while a function encoded by ts 22 mutant is involved in inhibition of the 2-5A system. Other viral functions (ATPase and phosphatase) may also be involved in modifications of the 2-5A system by regulating 2-5A levels and altering the integrity of 2-5A. Modifications of the 2-5A system, during vaccinia virus infection might contribute to the resistance of this cytoplasmic DNA virus to IFN.

Virus attenuation and identification of structural proteins of vaccinia virus that are selectively modified during virus persistence

To investigate the genetic stability of vaccinia virus DNA, we have tested whether alterations occurred in the polypeptide composition of this complex virus during persistent infections. We found that variants isolated at various passages in Friend erythroleukemia cells persistently infected with vaccinia virus contained, in addition to an 8-megadalton (MDa) deletion on the left terminus of the viral genome, major alterations in the sizes of three structural proteins with molecular masses of about 39, 21, and 14 kDa. Alterations in isoelectric points were also observed in proteins of 48, 27, and 14 kDa. The 14-kDa protein is part of the virus envelope, and the variants increased the size of this protein from 0.5 to 3 kDa with increasing passage number. Alteration in size of the 14-kDa protein is a dominant trait since it appeared in the whole virus population by passage 48. With more passages, some variants were found to increase or decrease the size of a 39-kDa core protein by about 2 kDa and to decrease the size of an envelope protein of 21 kDa by about 2 kDa. These three proteins were immunogenic in mice and elicited a strong host immune response. Major alterations in the sizes of these proteins were prevented by continuous treatment of the persistently infected cultures with interferon. However, after interferon was removed, protein modifications appeared with increasing passage number. Generation of the 8-MDa deletion and alterations in the size of the 14-kDa protein correlated with a marked decrease in virulence of these variants. Our findings suggest that during virus persistence, specific mutations are introduced in the vaccinia virus genome that lead to protein alterations and to highly attenuated viruses.

A 14K envelope protein of vaccinia virus with an important role in virus-host cell interactions is altered during virus persistence and determines the plaque size phenotype of the virus

The phenomenon of genetic variability and attenuation or virulence of poxviruses is poorly understood. We have identified mutants of vaccinia virus from untreated and interferon (IFN)-treated persistently infected Friend erythroleukemia (FEL) cells that have major alterations in the size of a virus structural protein. This protein is part of the virus envelope as documented with specific monoclonal antibody (mAbC3). This protein, under reducing conditions, has a molecular weight of about 14,000 (14K) Da in wild-type virus but gained 1.5 K in mutants from untreated, persistently infected cells and about 0.5 K in mutants from IFN-treated persistently infected cells. Under nonreducing conditions, this protein forms covalently linked oligomers which also differ in size between wild-type and mutant viruses. The 14K protein elicits humoral immune response as assessed by immunoblots of two-dimensional SDS-PAGE analysis using rabbit anti-vaccinia serum. Two molecular forms of the 14K protein with different isoelectric points were found only in mutants from untreated, persistently infected cells. Protein modifications were the result of DNA sequence alterations in the virus population since the 15.5 K protein could be reverted to 14K after marker-rescue with the cloned 14K encoding gene. We provide direct evidence that changes in size of the 14K envelope protein are responsible for the small plaque size phenotype of these variants. From our previous studies (J. F. Rodriguez, R. Janeczko, and M. Esteban, 1985, J. Virol. 56, 352-356; J. F. Rodriguez, E. Paez, and M. Esteban, 1987, J. Virol. 61, 395-404) and those described here, we established that the 14K protein plays an important role in virus-host cell interactions. This is because this protein is part of the virus envelope, is highly antigenic, elicits neutralizing antibodies, has a role in virus penetration acting in cell fusion, is involved in the virus plaque size phenotype and is highly conserved among Orthopoxviruses. In addition, because mutants with altered 14K protein that have gained 1.5K in size are attenuated (S. Dallo and M. Esteban, 1987, Virology 159, 408-422), it suggests that a modified form of this protein contributes to the decrease of the virulence of vaccinia virus.

Isolation and characterization of attenuated mutants of vaccinia virus

Attenuated variants of vaccinia virus with two specific and stable genetic markers were obtained after long-term passage of persistently infected Friend erythroleukemia (FEL) cells. The genetic markers were an 8-MDa deletion on the left HindIII-C terminus of the viral genome and sequence alterations localized in the middle of the HindIII-A DNA fragment. This latter genetic marker led to small plaque size phenotype of these variants. The mode of replication of these variants in tissue culture cells and their virulence in mice were analyzed. In cultured cells, these variants have greatly reduced virus yields in cell lines of different origins. These variants penetrate into cells, synthesize early and late viral proteins, and replicate their DNA with about the same efficiency as wild-type virus. The defect of these variants appears at some step during virus maturation. When groups of BALB/c mice were inoculated intraperitoneally (ip) with these variants, 50% of the mice survived with greater than or equal to 1 X 10(9) plaque-forming units (PFU) as opposed to about 50% survival for mice inoculated with 1 X 10(6) PFU of wild-type virus. Mice inoculated with these variants were fully protected when challenged ip with lethal doses of wild-type virus. The reduced virulence of these variants correlated with the 8-MDa deletion; in addition, the plaque size phenotype marker contributes to a further decrease of the virulence of vaccinia virus. Due to their limited virus production and protective immune response, these variants may be potentially useful as vaccines.

Studies on the mechanism of entry of vaccinia virus in animal cells

In order to study the mechanism of entry of vaccinia virus into cells the fate of virion associated polypeptides was investigated during infection of african green monkey kidney (BSC-40) cells with 35 S-methionine labelled virus. Approximately 12-15 percent of the virion polypeptides were degraded to acid-soluble products by 3 hours post-infection. Proteolysis was inhibited (50 percent) by methylamine, suggesting a lysosomal site of degradation. Neither methylamine or chloroquine inhibited virus infectivity or uncoating indicating a non-acid endocytic mechanism of entry. Subcellular fractionation studies on density gradients indicated that the bulk of the input virion polypeptides were associated with the plasma membrane fraction. In addition, input virion DNA was partially resolved from the membrane fraction. The results are most consistent with a mechanism of entry involving fusion of the virus with the plasma membrane.

Role of prostaglandins and non-steroid anti-inflammatory drugs in the pathogenicity of vaccinia virus

The effect of prostaglandins (PGs) of the A series (A1 and dimethyl PGA2), E1, D2, F2 alpha and PGI2 (prostacyclin) and of inhibitors of PG synthesis (aspirin and indomethacin) on the pathogenicity of vaccinia virus was studied in BALB/c mice. PGs of the A series, D2 and F2 alpha conferred little or no protection to mice against the lethal effects of vaccinia virus. Mice treated with PGE1 showed a dramatic increase in mortality after viral infection. However, when mice were treated with PGI2, their survival was greatly enhanced. Mice treated with aspirin and indomethacin showed a marked increase in mortality. Increased mortality correlated with higher virus yields in target tissues (spleen) and with inhibition of antibody response, whereas the increase in survival correlated with lower virus yields and with normal antibody responses. The significance of our findings is that PGI2 can block the outcome of the disease caused by vaccinia virus whereas other PGs and their inhibitors not only worsen the disease, but may activate and enhance viral infections through immune suppression.

A 14,000-Mr envelope protein of vaccinia virus is involved in cell fusion and forms covalently linked trimers

A monoclonal antibody, MAbC3, that reacts with a 14,000-molecular-weight envelope protein (14K protein) of vaccinia virus completely inhibited virus-induced cell fusion during infection. Immunoblot and immunofluorescence studies revealed that the 14K protein was synthesized at about 6 to 7 h postinfection and transported from the cytoplasm to the cell surface. Synthesis and transport of the 14K protein during infection occurred in the presence of rifampin, an inhibitor of virus maturation. One- and two-dimensional gel electrophoretic analyses demonstrated that the 14K protein forms largely trimers (42K) that are covalently linked by disulfide bonds. The facts that MAbC3 prevents virus uncoating and blocks virus-induced cell fusion but does not prevent virus attachment to cells and the 14K envelope protein forms trimers all suggest that this protein plays major role in virus penetration.

Molecular cloning, encoding sequence, and expression of vaccinia virus nucleic acid-dependent nucleoside triphosphatase gene

A rabbit poxvirus genomic library contained within the expression vector lambda gt11 was screened with polyclonal antiserum prepared against vaccinia virus nucleic acid-dependent nucleoside triphosphatase (NTPase)-I enzyme. Five positive phage clones containing from 0.72- to 2.5-kilobase-pair (kbp) inserts expressed a beta-galactosidase fusion protein that was reactive by immunoblotting with the NTPase-I antibody. Hybridization analysis allowed the location of this gene within the vaccinia HindIIID restriction fragment. From the known nucleotide sequence of the 16-kbp vaccinia HindIIID fragment, we identified a region that contains a 1896-base open reading frame coding for a 631-amino acid protein. Analysis of the complete sequence revealed a highly basic protein, with hydrophilic COOH and NH2 termini, various hydrophobic domains, and no significant homology to other known proteins. Translational studies demonstrate that NTPase-I belongs to a late class of viral genes. This protein is highly conserved among Orthopoxviruses.

Mode of sensitivity and resistance of vaccinia virus replication to interferon

In this study we show that vaccinia virus replication can be sensitive or resistant to interferon (IFN) in the same strain of mouse L cells. When IFN-treated L cells were maintained in suspension culture, infection led to a rapid inhibition of both viral and cellular protein synthesis together with breakdown of viral RNA and of rRNA. When IFN-treated L cells were maintained in monolayer culture, infection did not lead to significant inhibition of viral protein or RNA synthesis and breakdown of viral or of rRNA was not observed. The resistance of vaccinia virus replication to IFN was not dependent on the input multiplicity or state of growth of the cells (actively dividing or resting). Qualitative and quantitative differences in viral transcription and translation were observed between the two virus-cell systems. Our findings are consistent with the hypothesis that the sensitivity or resistance of vaccinia virus to IFN is mediated by specific viral products that act as activators or selective inhibitors of, at least, the dsRNA-dependent ppp(A2'p)nA synthetase/RNase system.

Isolation and characterization of neutralizing monoclonal antibodies to vaccinia virus

Cells producing neutralizing monoclonal antibodies (mAbs) to UV-inactivated vaccinia virus strain WR were derived by fusion of hyperimmunized mouse spleen cells with mouse myeloma cells. Three mAbs that reacted strongly with purified virus envelopes as determined by enzyme-linked immunosorbent assay were studied. The three mAbs recognized a 14,000-molecular-weight (14K) envelope protein of vaccinia virus and were shown to be immunoglobulin G2b (mAbC3 and mAbB11) and immunoglobulin M (mAbF11). By using ascites, one of the antibodies, mAbC3, neutralized (50%) virus infectivity with a titer of about 10(-4), whereas the others exhibited lower neutralization titers of 10(-2) to 10(-3). The binding of the mAbs to vaccinia virus did not alter virus attachment to cells. However, virus uncoating was extensively blocked by mAbC3, whereas mAbB11 and mAbF11 had little or no effect. The three mAbs recognized a similar 14K protein in cowpox, rabbitpox, and vaccinia Elstree strains, indicating a high degree of protein conservation among orthopoxviruses. Based on the binding of mAbs to V-8 protease cleavage products of the 14K protein, the extent of protein recognition for other poxviruses, and differences in the degree of virus neutralization and of virus uncoating into cells, we suggest that the three mAbs recognize different domains of vaccinia 14K viral envelope protein. Furthermore, our findings indicate that the 14K protein may play a role in virus penetration.

Interferon prevents the generation of spontaneous deletions at the left terminus of vaccinia virus DNA

In this report we have shown that Friend erythroleukemia cells persistently infected with vaccinia virus maintain the persistent infection even after 1 year of continuous interferon (IFN) treatment. The persistently infected cultures were responsive to IFN as determined by their ability to induce 2-5A synthetase, to increase the intracellular levels of 2-5A, and to cause rRNA cleavage. While large deletions at the left terminus of vaccinia DNA occurred readily in the virus population from untreated cells, IFN completely suppressed the generation of these spontaneous deletions. Removal of IFN from these cultures led to the appearance of similar deletions at the left terminus of the viral genome. The regions deleted contain more than half of the left-end inverted terminal repetition of the vaccinia genome. These findings show that IFN alters specific events associated with the generation of vaccinia DNA deletions.

Biochemical and electron microscopic studies of the transcription of vaccinia DNA by RNA polymerase from Escherichia coli: localization and characterization of transcriptional complexes

We used the prokaryotic Escherichia coli RNA polymerase to determine if vaccinia DNA might provide recognition sites for the bacterial binding and initiation. Electron microscopic studies of the interaction of E. coli RNA polymerase with vaccinia DNA and molecular hybridization analysis of the transcription products formed after 3 or 5 min of in vitro incubation showed that: there were 30-40 sites on the template where the polymerase could bind and initiate cRNA synthesis; the entire coding capacity of the genome was utilized for cRNA synthesis; transcription was asymmetric; cRNA molecules were similar in size to the transcripts synthesized by the vaccinia virus RNA polymerase in vitro and in vivo; cRNA contains sequences in common with 'pre-early', 'early', and 'late' in vivo RNA; 'self-annealing' of cRNA in the presence or absence of RNA synthesized in vitro by the virion associated RNA polymerase showed that less than 1% dsRNA product could be detected suggesting that initially the same strand(s) was copied by the viral and bacterial enzymes; no differences in the frequency with which sequences represented in the Hind III fragments of vaccinia DNA were transcripted with time of in vitro incubation could be detected. These findings strongly suggest that the bacterial enzyme might recognize truly viral promotors. With extended in vitro incubations of the E. coli RNA polymerase with vaccinia DNA the control of transcription was found to diminish. This was correlated with an increase in the size of the transcripts and the synthesis of significant amounts of self-complementary RNA, indicating that symmetrical transcription was occurring. The dsRNA species recovered after self-annealing the cRNA from a 30 min in vitro reaction mixture were found to contain sequences which hybridized to some portion of all the Hind III restriction fragments of vaccinia DNA. The methods described here might be useful for the localization and characterization of promotor sequences in the genome of vaccinia virus, as well as for studies on sequence conservation between members of the Poxvirus genus.

Inhibitory effect of interferon on the genetic and oncogenic transformation by viral and cellular genes

The rodent established cell lines LTk- and NIH 3T3 have been used as recipients in gene transfer experiments to study the effect of interferon treatment on the genetic and oncogenic transformation by several genes of viral and cellular origin. Our results show that interferon severely inhibits, to a similar extent, the stable transformation of Ltk- and NIH 3T3 cells by the chicken thymidine kinase (tk) gene, Ecogpt gene, simian virus 40, v-Ha-ras, and human c-Ha-ras and c-Ki-ras oncogenes. These results are consistent with an inhibition by interferon at the level of stabilization or integration, or both, of exogenous DNA sequences in the recipient cells, with an apparent effect on gene expression.

Interferon inhibits marker rescue of vaccinia virus

In this investigation we have examined the effect of human interferon (IFN) type alpha on the ability of vaccinia virus to recombine within infected African green monkey kidney cells (BSC-40). We measured by marker rescue, the extent of insertion of cloned 5-kb Hind III-J restriction fragment of wild-type vaccinia DNA into the genome of temperature-sensitive mutants. We showed that IFN at doses of 100-1000 U/ml inhibited the replication of vesicular stomatitis virus (VSV) and polio viruses but not of vaccinia virus. Vaccinia virus adsorption, penetration, uncoating, protein synthesis, and yields were not inhibited. However, marker rescue of vaccinia virus was inhibited by IFN. This inhibition was not related to IFN-mediated changes in uptake of exogenous DNA or enhanced degradation of the transfected DNA. These results suggest that IFN affects homologous vaccinia DNA recombination.

Indiscriminate degradation of RNAs in interferon-treated, vaccinia virus-infected mouse L cells

In this report we used Northern blot hybridization analysis to characterize the fate of several species of viral RNA transcribed from internal and terminal regions of vaccinia DNA in interferon-treated, infected mouse L cells grown in suspension. All species of viral RNAs were expressed but were reduced in amount. Larger-sized RNAs were reduced more than smaller-sized RNAs. This reduction appears to be related to the activation of the interferon-mediated double-stranded RNA-dependent 2-5A synthetase-endoribonuclease system, as the rRNA cleavage pattern characteristic of this system was observed early in infection and in cell extracts in response to exogenous 2-5A. Thus, in interferon-treated, vaccinia-infected mouse L cells in suspension, there is indiscriminate degradation of viral and cellular RNAs, and this RNA breakdown might play a role in the interferon-mediated inhibition of protein synthesis.

Expression of cloned vaccinia virus DNA sequences introduced into animal cells

Individual cloned HindIII fragments of vaccinia virus DNA were introduced into cells by DNA-mediated gene transfer. The presence of individual fragments in the different transformants was confirmed by Southern blot hybridization analysis. Several of the transformants were found to express viral sequences at various levels. The sizes of the transcripts containing vaccinia virus sequences were highly heterogeneous, with no discrete species of RNA. Positive clones contained vaccinia virus sequences in both the poly(A)+ and poly(A)- RNA fractions, although the prevalence of these sequences was variable in the two fractions. The S1 nuclease map of the 5' end of the transcripts from transformants containing the HindIII-J fragment revealed a unique 5' end, similar to RNA from virus-infected cells. In contrast, analysis of the 3' end of RNAs from these transformants showed a high degree of heterogeneity, which might explain the heterogeneity found in Northern blot patterns. In this report, it is shown for the first time that in cells transformed with vaccinia virus DNA there is proper initiation for, at least, the viral thymidine kinase gene.

Nature and mode of action of vaccinia virus products that block activation of the interferon-mediated ppp(A2'p)nA-synthetase

In this report it has been shown that inhibition of the 2-5A synthetase in IFN-treated, vaccinia virus-infected mouse L and human HeLa S3 cells is related to specific viral functions. This inhibition occurs concomitantly with degradation of ATP and with dephosphorylation of ppp(A2'p)nA. At least two viral-mediated enzyme activities are thought to be involved in this process, an ATPase and a phosphatase. The ATPase activity was established after determining the extent of hydrolysis of ATP, the nature of 2-5A, and the relative abundance of the different oligomers. Cytoplasmic cell extracts and purified vaccinia virions were bound to poly (I):(C) agarose, incubated with [3H]ATP, [alpha-32P]ATP, or [gamma-32P]ATP, and the extent of hydrolysis of ATP was determined by TLC. Authentic 2-5A and the relative abundance of the various oligomers were characterized by enzymatic and alkali treatments and identification by TLC and HPLC analysis. The phosphatase activity was measured by TLC after determining the degree of dephosphorylation of 2-5A from the extent of labeling at the 5'-OH termini with [gamma-32P]ATP and polynucleotide kinase. While free 5'-OH termini were not observed in oligomers synthesized with bound poly (I):(C) agarose enzyme fractions from IFN-treated, uninfected cells, a strong phosphorylation was found in oligomers from IFN-treated, infected cells. These findings suggest that it is the contribution of these viral enzyme activities that renders vaccinia virus resistant to interferons.

Effect of interferon on integrity of vaccinia virus and ribosomal RNA in infected cells

The state of vaccinia and ribosomal RNAs in IFN-treated, vaccinia virus-infected mouse L cells grown in suspension is examined. In these cells a drastic inhibition (approximately 90%) of both viral and cellular protein synthesis occurs after virus infection of IFN-treated cells. The findings show that (1) primary transcription of vaccinia virus is not impaired by IFN, but is rather enhanced; (2) viral RNAs produced in IFN-treated, infected cells are predominantly of the early class; (3) these viral RNAs can be translated in a cell-free protein-synthesizing system; (4) in IFN-treated, infected cells there is extensive cleavage of 28 and 18 S rRNA at early times post infection, resulting in a characteristic cleavage pattern; (5) cleavage of rRNA is viral RNA dependent. The results indicate that in this virus-cell system the IFN-mediated inhibition of vaccinia and cellular protein synthesis is correlated with an alteration in ribosomal integrity.

Resistance of vaccinia virus to interferon is related to an interference phenomenon between the virus and the interferon system

In this investigation the sensitivity of vaccinia virus to interferon (IFN) has been examined in cultured cells. In a variety of mouse and human cells of different origins vaccinia virus functions (RNA, protein, and virus yields) were found to be relatively resistant to IFN. In these systems, the levels of the IFN-mediated enzyme activities (2-5A synthetase and protein kinase) were severely impaired by the virus. This virus-mediated inhibitory effect developed with time after infection and was dependent on viral protein synthesis. Mixed infections between vaccinia virus and viruses (VSV or polio) which are sensitive to IFN showed that both protein synthesis and virus yields were not inhibited. These findings show that vaccinia virus can overcome the antiviral action of IFN and that viral gene functions appear to be involved in this interference phenomenon.

Induction of an anti-viral response and 2',5'-oligo A synthetase by interferon in several thymidine kinase-deficient cell lines

It has previously been shown that a strain of thymidine kinase (tk)-deficient mouse L-929 cells was unable to respond to murine beta-interferon by induction of an anti-viral state and synthesis of double-stranded, RNA-dependent enzymes. Sensitivity to interferon can be restored by introducing into the cells a segment of Herpes simplex virus DNA containing the viral tk gene. It is shown here that not all Ltk(-) cell strains are resistant to interferon, suggesting that expression of a tk gene is not a prerequisite for response to interferon. Introduction of various genes into the resistant Ltk(-) strain, either alone or together with DNA containing the Herpes virus tk gene, leads to restoration of interferon sensitivity only when tk-containing DNA is inserted, showing that the activation of interferon responsiveness is not an artifact of the gene transfer, selection, and cloning procedures. The results imply that a component of the Herpes virus DNA introduced into the cells is able to activate interferon sensitivity.

Selective inhibition of viral gene expression as the mechanism of the antiviral action of PGA1 in vaccinia virus-infected cells

We have previously shown that prostaglandins of the A series are potent inhibitors of the replication of several animal viruses in cultured cells. In this report we have studied the mechanism of the antiviral action of PGA1 in vaccinia virus-infected mouse L cells, where there is an alteration in both the rate and extent of the synthesis of some virus proteins. When cytoplasmic RNAs from PGA-treated, vaccinia virus-infected cells were translated in cell-free systems, similar selective inhibition of the synthesis of some viral polypeptides was observed. The lack of translation of some viral RNAs was not due to an impairment of the methylation process nor to a difference in ionic requirements. PGA1, even at doses as high as 10 micrograms/ml, did not exert any direct inhibitory action on transcription in vitro as measured in two cell-free systems, and had no effect on primary transcription-translation of vaccinia virus RNAs when assayed in coupled cell-free systems. Southern blot hybridization analysis of cytoplasmic RNAs to EcoRI restriction fragments of vaccinia DNA showed that PGA1 was able to induce major changes in the pattern of RNA transcripts during the course of viral infection. We propose that changes in the transcription programme of vaccinia virus RNAs could be due either to an alteration of specific viral proteins that regulate transcription by direct binding of PGA1, or to the synthesis and/or activation of a host product that mediates the antiviral action.

Defective vaccinia virus particles in interferon-treated infected cells

Vaccinia virus particles formed in interferon-treated, infected cells have been isolated. These particles have been characterized with regard to polypeptide composition, and ability to adsorb, penetrate, uncoat, and synthesize proteins in infected cells. As determined by one-dimensional SDS-PAGE analysis, with interferon concentrations of 100-500 u/ml, the pattern of [35S]methionine-labeled virion proteins was not altered; higher doses of interferon resulted in decreased labeling of some proteins. However, interferon doses of 100-500 u/ml decreased phosphorylation of vaccinia virus basic core polypeptide (P-11) by 30-70%; the same doses of interferon decreased the labeling of virus glycoproteins by 40-80%. Virus purified from interferon-treated cells adsorb, penetrate, and uncoat to a lesser extent than virus control. During infection to cells, these virus particles caused shutoff and synthesized the same spectrum of viral proteins as normal virus. These findings show that there are protein alterations in vaccinia virus particles isolated from interferon-treated, infected cells. These alterations may contribute to limit the spread of virus infection.

Analysis of replicating vaccinia DNA in interferon-treated, virus-infected cells

The effect of interferon (IFN) on the replication of vaccinia DNA has been examined. Studies were carried out in infected mouse L cells grown in monolayer cultures. We examined discontinuous synthesis of small DNA fragments, ligation of the small fragments to form intermediate-sized DNA molecules, completion of unit-length DNA molecules, and introduction of crosslinks at each end of the newly replicated DNA molecules late in infection. We measured by pulse-label and pulse-chase experiments the extent of incorporation of 3H-thymidine into DNA and the conversion of small size DNA into mature and cross-linked viral DNA molecules. Interferon inhibited the initiation of viral DNA and this correlated with an overall inhibition of protein synthesis. Interferon inhibited elongation of viral DNA but this did not correlate with an inhibition of protein synthesis. The effect on elongation was not the result of increased degradation of newly synthesized DNA. It is suggested that the effects of IFN on initiation and elongation of vaccinia DNA may be the result of a decrease in the availability of enzymes or factors involved in the regulation of viral DNA synthesis.

Prostaglandin A inhibits the replication of vesicular stomatitis virus: effect on virus glycoprotein

Prostaglandins of the A series were found to strongly suppress the replication of vesicular stomatitis virus (VSV) in mouse L fibroblasts. The highest non-toxic dose of PGA1, 4 micrograms/ml, decreased VSV production by 93.6%. At this dose, PGA1 did not alter DNA, RNA or protein synthesis in uninfected L cells for periods up to 24 h, whereas it further suppressed protein synthesis and slightly increased RNA synthesis in VSV-infected cells. The presence of PGA1 during virus adsorption, with no treatment after infection, reduced VSV yields by 63.6%. However, the presence of PGA1 during an early step of VSV replication was not essential for the antiviral action to occur (PGA1 treatment could be started 1 to 2 h post-infection). Apart from a slight overall inhibition of virus protein synthesis, PGA1 strongly suppressed the synthesis of the VSV glycoprotein G; moreover, it produced an alteration in the mobility of this protein in SDS-polyacrylamide gels. We propose that this slight decrease in molecular weight (about 4000) of the G protein in the presence of PGA1 could be due to an alteration in the glycosylation process.

The relationship between the antiviral action of interferon and prostaglandins in virus-infected murine cells

The relationship between prostaglandins (PG) and interferon (IFN) was investigated. IFN induced the synthesis of immunoreactive PGE and PGA at early and late stages, respectively, of vaccinia virus infection in mouse L fibroblasts. Only species-specific IFN possessed this activity and PG synthesis was stimulated in virus-infected cells, while normal L cells were not affected. The vaccinia virus infection did not significantly alter PG synthesis in the absence of IFN. Indomethacin increased the rate of vaccinia virus replication and partially inhibited the IFN-induced protection of L cells. The addition of exogenous PGA1 only partially reversed this effect. Finally, short-term PGA treatment induced the synthesis of two enzymes (protein kinase and 2,5A synthetase) thought to be partially responsible for the antiviral action of interferon. These findings suggest that a prostaglandin or PG-related compound seems to mediate at least one aspect of IFN action.

Electron microscopic studies of transcriptional complexes released from vaccinia cores during RNA-synthesis in vitro: methods for fractionation of transcriptional complexes

Electron microscopic (EM) and biochemical methods were employed to study the transcriptional complexes present in detergent lysates of vaccinia virus cores actively synthesizing RNA in vitro. When processed and examined in the EM, 14 'transcriptional sites' could be observed on full-length DNA templates. Fractionation of lysates by equilibrium density centrifugation in CsSO4, chromatography on hydroxyapatite columns or by sedimentation in sucrose gradients, allowed isolation of DNA templates associated with transcripts but these manipulations often resulted in fragmentation of the DNA template or promoted the release of transcripts from the template. It is suggested that RNA transcripts remain associated with the template in regions of supercoiling. These regions, in turn, may be maintained by DNA-protein interactions which are compromised as the transcriptional complexes are fractionated and purified.

Thymidine kinase genes and the induction of anti-viral responses by interferon

A mouse fibroblast cell-line deficient in thymidine kinase (Ltk(-) aprt(-)) fails to show an anti-viral response when treated with interferon. After introduction of a viral tk gene into these cells the resultant clones showed normal responses to interferon. However, one such tk-containing clone (C6) spontaneously lost its ability to respond to interferon by inducing an antiviral state although it retained its ability to induce the enzyme oligo(2'-5' A)-synthetase. This sub-clone (6A) still expressed thymidine kinase activity but restriction endonuclease analysis indicated an alteration in the sequences flanking the exogenous viral tk gene. Our results suggest that a modification in the exogenous viral DNA sequences led to a loss of interferon sensitivity.

Induction of an antiviral response by interferon requires thymidine kinase

Mouse fibroblastoid cells (Ltk-) that lack thymidine kinase (tk) activity are unable to respond to murine beta-interferon by establishing antiviral activity or inducing the double-stranded RNA-dependent enzymes, oligo[(2'-5')A] polymerase and Mr 68,000 protein kinase. In contrast, the parental L-929 cell line or clonal derivatives of Ltk- cells into which the herpes virus tk gene was introduced by DNA-mediated gene transfer respond normally to interferon in developing resistance to viral infection and in inducing double-stranded RNA-dependent enzymes. Further evidence for a role of tk in the response to interferon was obtained by isolating revertants of tk+ clones that lost the herpes virus tk gene during growth in BrdUrd-containing medium. In such revertant sublines both tk enzyme activity and viral tk genes were undetectable and treatment with interferon failed to produce an antiviral effect or induce synthesis of the double-stranded RNA-dependent enzymes. Our results indicate that the ability of mouse L cells to respond to beta-interferon is dependent upon the presence of a functional tk gene. We propose that the induction of antiviral responses by interferon stringently requires a metabolite, the level of which is determined by tk activity. The system described may provide a means for elucidating the mechanisms by which responses to interferon are induced.

Antiviral effect of prostaglandins of the A series: inhibition of vaccinia virus replication in cultured cells

Prostaglandins of the A series potently inhibited the production of vaccinia virus in mouse L fibroblasts. With the highest non-toxic dose of PGA1, 4 micrograms/ml, the replication of the virus was inhibited by 95 . 3%. The antiviral activity was dose-dependent and specific for the A series. At the dose used, PGA1 was not toxic to uninfected cells and did not alter cell metabolism as measured by DNA, RNA and protein synthesis. PGA1 did not influence the adsorption of the virus by the host cells and the antiviral activity was not dependent on the presence of PGA1 during the early stages of infection. PGA treatment delayed and partially inhibited virus DNA synthesis and, while it did not produce any change in the pattern of protein synthesis in uninfected cells, it altered both the rate and the pattern of virus protein synthesis. We conclude that PGA1 selectively inhibits one or more steps involved in the replication of vaccinia virus in mouse L fibroblasts.

Studies on the mechanisms of vaccina virus cytopathic effects. I. Inhibition of protein synthesis in infected cells is associated with virus-induced RNA synthesis

The mechanism of vaccinia virus-induced inhibition of protein synthesis was studied in LLC-MK2, HeLa and L cells. Removal of cycloheximide (300 microgram/ml) from cells infected at a multiplicity of infection (m.o.i.) of 300 particles/cell at 4 h after infection resulted in the resumption of both host and virus protein synthesis in LLC-MK2 cells, but not in HeLa and L cells. In order to determine whether virus-induced RNA synthesis, which occurs in infected cells in the presence of cycloheximide, is related to the inhibition of protein synthesis, (cut-off), the rate of virus-induced RNA synthesis in the presence of cycloheximide was measured in all three cell types. In L cells and HeLa cells the rate of virus-induced RNA synthesis increased with time, whereas in LLC-MK2 cells it remained constant for at least 4 h. However, when higher multiplicities (900 and 2700 particles/cell) were used to infect LLC-MK2 cells, the rate of RNA synthesis in the presence of cycloheximide did increase with time and was greater at the higher multiplicity. Under these conditions there was a direct relationship between the extent of virus RNA synthesis and the degree of cut-off after the removal of cycloheximide. In HeLa and L cells infected at 300 particles/cell, the longer the exposure to cycloheximide, the greater was the cut-off observed upon removal of the drug. As was the case the LLC-MK2 cells, there was a direct relationship between the rate of RNA synthesis and the degree of inhibition of protein synthesis. Since virus-induced RNA synthesis occurs in the presence of cycloheximide, the effects of actinomycin D and cordycepin on host polypeptide synthesis were tested. Inhibition of host cell protein synthesis was virtually abolished when HeLA cells were infected in the presence of cordycepin (50 and 25 microgram/ml) or actinomycin D (20 microgram/ml). These results indicate that, as the rate of virus-induced RNA synthesis increased, regardless of the type of cell used, protein synthesis was inhibited at earlier times and to a greater extent. These observations are consistent with the hypothesis that the cut-off phenomenon is related to the synthesis of an early virus-induced RNA(s).