Inhibition of protein synthesis by vaccinia virus. III. The effect of ultraviolet-irradiated virus on the inhibition of protein synthesis

Involvement of the cellular phosphatase DUSP1 in vaccinia virus infection

Poxviruses encode a large variety of proteins that mimic, block or enhance host cell signaling pathways on their own benefit. It has been reported that mitogen-activated protein kinases (MAPKs) are specifically upregulated during vaccinia virus (VACV) infection. Here, we have evaluated the role of the MAPK negative regulator dual specificity phosphatase 1 (DUSP1) in the infection of VACV. We demonstrated that DUSP1 expression is enhanced upon infection with the replicative WR virus and with the attenuated VACV viruses MVA and NYVAC. This upregulation is dependent on early viral gene expression. In the absence of DUSP1 in cultured cells, there is an increased activation of its molecular targets JNK and ERK and an enhanced WR replication. Moreover, DUSP1 knock-out (KO) mice are more susceptible to WR infection as a result of enhanced virus replication in the lungs. Significantly, MVA, which is known to produce non-permissive infections in most mammalian cell lines, is able to grow in DUSP1 KO immortalized murine embryo fibroblasts (MEFs). By confocal and electron microscopy assays, we showed that in the absence of DUSP1 MVA morphogenesis is similar as in permissive cell lines and demonstrated that DUSP1 is involved at the stage of transition between IVN and MV in VACV morphogenesis. In addition, we have observed that the secretion of pro-inflammatory cytokines at early times post-infection in KO mice infected with MVA and NYVAC is increased and that the adaptive immune response is enhanced in comparison with WT-infected mice. Altogether, these findings reveal that DUSP1 is involved in the replication and host range of VACV and in the regulation of host immune responses through the modulation of MAPKs. Thus, in this study we demonstrate that DUSP1 is actively involved in the antiviral host defense mechanism against a poxvirus infection.

Phosphorylation is a post-translational modification that is highly conserved throughout the animal kingdom. Viruses have evolved to acquire their own kinases and phosphatases and to be able to modulate host phosphorylation mechanisms on their benefit. DUSP1 is an early induced gene that belongs to the superfamily of Dual-specificity phosphatases and provides an essential negative feedback regulation of MAPKs. DUSP1 is involved in innate and adaptive immune responses against different bacteria and parasites infections. The use of Knock-out technology has allowed us to understand the role of DUSP1 in the context of VACV infection both in cultured cells and in the in vivo mouse model. Here, we have showed that DUSP1 expression is upregulated during VACV infection and that DUSP1 plays an important role in VACV replication. Interestingly, we have demonstrated that the VACV attenuated virus MVA is able to grow in immortalized murine embryo fibroblasts in the absence of DUSP1. In vivo results showed that VACV replication-competent WR pathogenesis is enhanced in the absence of DUSP1. Furthermore, we have demonstrated that DUSP1 is involved in the host innate and adaptive responses against VACV. Altogether, we have presented a novel role for DUSP1 in VACV replication and anti-VACV host immune response.

Activation of the PI3K/Akt pathway early during vaccinia and cowpox virus infections is required for both host survival and viral replication

Viral manipulation of the transduction pathways associated with key cellular functions such as actin remodeling, microtubule stabilization, and survival may favor a productive viral infection. Here we show that consistent with the vaccinia virus (VACV) and cowpox virus (CPXV) requirement for cytoskeleton alterations early during the infection cycle, PBK/Akt was phosphorylated at S473 [Akt(S473-P)], a modification associated with the mammalian target of rapamycin complex 2 (mTORC2), which was paralleled by phosphorylation at T308 [Akt(T308-P)] by PI3K/PDK1, which is required for host survival. Notably, while VACV stimulated Akt(S473-P/T308-P) at early (1 h postinfection [p.i.]) and late (24 h p.i.) times during the infective cycle, CPXV stimulated Akt at early times only. Pharmacological and genetic inhibition of PI3K (LY294002) or Akt (Akt-X and a dominant-negative form of Akt-K179M) resulted in a significant decline in virus yield (from 80% to >/=90%). This decline was secondary to the inhibition of late viral gene expression, which in turn led to an arrest of virion morphogenesis at the immature-virion stage of the viral growth cycle. Furthermore, the cleavage of both caspase-3 and poly(ADP-ribose) polymerase and terminal deoxynucleotidyl transferase-mediated deoxyuridine nick end labeling assays confirmed that permissive, spontaneously immortalized cells such as A31 cells and mouse embryonic fibroblasts (MEFs) underwent apoptosis upon orthopoxvirus infection plus LY294002 treatment. Thus, in A31 cells and MEFs, early viral receptor-mediated signals transmitted via the PI3K/Akt pathway are required and precede the expression of viral antiapoptotic genes. Additionally, the inhibition of these signals resulted in the apoptosis of the infected cells and a significant decline in viral titers.

The orthopoxvirus 68-kilodalton ankyrin-like protein is essential for DNA replication and complete gene expression of modified vaccinia virus Ankara in nonpermissive human and murine cells

Modified vaccinia virus Ankara (MVA) is a highly attenuated and replication-deficient vaccinia virus (VACV) that is being evaluated as replacement smallpox vaccine and candidate viral vector. MVA lacks many genes associated with virulence and/or regulation of virus tropism. The 68-kDa ankyrin-like protein (68k-ank) is the only ankyrin repeat-containing protein that is encoded by the MVA genome and is highly conserved throughout the Orthopoxvirus genus. We showed previously that 68k-ank is composed of ankyrin repeats and an F-box-like domain and forms an SCF ubiquitin ligase complex together with the cellular proteins Skp1a and Cullin-1. We now report that 68k-ank (MVA open reading frame 186R) is an essential factor for completion of the MVA intracellular life cycle in nonpermissive human and murine cells. Infection of mouse NIH 3T3 and human HaCaT cells with MVA with a deletion of the 68k-ank gene (MVA-Delta68k-ank) was characterized by an extensive reduction of viral intermediate RNA and protein, as well as late transcripts and drastically impaired late protein synthesis. Furthermore, infections with MVA-Delta68k-ank failed to induce the host protein shutoff that is characteristic of VACV infections. Although we demonstrated that proteasome function in general is essential for the completion of the MVA molecular life cycle, we found that a mutant 68k-ank protein with a deletion of the F-box-like domain was able to fully complement the deficiency of MVA-Delta68k-ank to express all classes of viral genes. Thus, our data demonstrate that the 68k-ank protein contains another critical domain that may function independently of SCF ubiquitin ligase complex formation, suggesting multiple activities of this interesting regulatory protein.

Cellular and biochemical differences between two attenuated poxvirus vaccine candidates (MVA and NYVAC) and role of the C7L gene

The poxvirus strains NYVAC and MVA are two candidate vectors for the development of vaccines against a broad spectrum of diseases. Although these attenuated virus strains have proven to be safe in animals and humans, little is known about their comparative behavior in vitro. In contrast with MVA, NYVAC infection triggers greater cytopathic effect in a range of permissive and nonpermissive cell lines. The yields of NYVAC cell-associated virus in permissive cells (BHK-21) were slightly reduced compared with those of MVA infection. During the course of infection in HeLa cells, there is a translational block induced by NYVAC late in infection, which correlated with a marked increase in phosphorylation levels of the initiation factor eIF-2alpha. In contrast to MVA, the synthesis of certain late viral proteins was only blocked in NYVAC-infected HeLa cells. Electron microscopy (EM) analysis revealed that morphogenesis of NYVAC in HeLa cells was blocked at the stage of formation of immature viral forms. Phase-contrast microscopy, EM, flow cytometry, and rRNA analyses demonstrated that contrary to MVA, NYVAC infection induces potent apoptosis, a phenomenon dependent on activation of caspases and RNase L. Apoptosis induced by NYVAC was prevented when the virus gene C7L was placed back into the NYVAC genome, recovering the ability of NYVAC to replicate in HeLa cells and maintaining the attenuated phenotype in mice. Overall, our findings demonstrate distinct behavior between NYVAC and MVA strains in cultured cells, as well as a new role for the C7L viral gene as an inhibitor of apoptosis in NYVAC infection.

Vaccinia virus blocks gamma interferon signal transduction: viral VH1 phosphatase reverses Stat1 activation

We have analyzed the effects of vaccinia virus (VV) on gamma interferon (IFN-gamma) signal transduction. Infection of cells with VV 1 to 2 h prior to treatment with IFN-gamma inhibits phosphorylation and nuclear translocation of Stat1 and consequently blocks accumulation of mRNAs normally induced by IFN-gamma. While phosphorylation of other proteins in the IFN-gamma pathway was not affected, activation of Stat1 by other ligand-receptor systems was also blocked by VV. This block of Stat1 activation was dose dependent, and although viral protein synthesis was not required, entry and uncoating of viral cores appear to be needed to block the accumulation of phosphorylated Stat1. These results suggest that a virion component is responsible for the effect. VV virions contain a phosphatase (VH1) that is sensitive to the phosphatase inhibitor Na(3)VO(4) but not to okadaic acid. Addition of Na(3)VO(4) but not okadaic acid restored normal Stat1 phosphorylation levels in VV-infected cells. Moreover, virions containing reduced levels of VH1 were unable to block the IFN-gamma signaling pathway. In vitro studies show that the phosphatase can bind and dephosphorylate Stat1, indicating that this transcription factor can be a substrate for VH1. Our results reveal a novel mechanism by which VV interferes with the onset of host immune responses by blocking the IFN-gamma signal cascade through the dephosphorylating activity of the viral phosphatase VH1.

The antiviral activity exerted by vaccinia virus on the growth of herpes simplex virus in BS-C-1 cells

The growth of herpes simplex virus type 2 (HSV-2) in BS-C-1 cells, was inhibited following super-infection with vaccinia virus. This inhibition was efficiently induced by both the intracellular mature virus (IMV) form of vaccinia virus and the extracellular enveloped virus (EEV), containing an additional external viral membrane. Treatment of vaccinia IMV with the detergents NP-40, Brij-58 or n-octyl-alpha-D-glucopyranoside, abolished its ability to inhibit the growth of HSV-2. Ultraviolet irradiation of vaccinia virus, that completely inactivated the infectivity of the virus, resulted in partial loss of the capability to inhibit the growth of HSV-2: 16-fold more irradiated virus was needed for the inhibition. Electron microscopy showed that the irradiated vaccinia virus adsorbed and penetrated into the HSV-infected cells but remained morphologically intact within the cells for at least 22 h. When the steps in the growth of HSV affected by the irradiated vaccinia virus were followed, it was found that while the synthesis of HSV DNA was partially decreased, the synthesis of HSV proteins was very strongly inhibited and virus particles were not formed.

Characterization of small nontranslated polyadenylylated RNAs in vaccinia virus-infected cells

Host protein synthesis is selectively inhibited in vaccinia virus-infected cells. This inhibition has been associated with the production of a group of small, nontranslated, polyadenylylated RNAs (POLADS) produced during the early part of virus infection. The inhibitory function of POLADS is associated with the poly(A) tail of these small RNAs. To determine the origin of the 5'-ends of POLADS, reverse transcription was performed with POLADS isolated from VV-infected cells at 1 hr and 3.5 hr post infection. The cDNAs of these POLADS were cloned into plasmids (pBS or pBluescript II KS +/-), and their nucleotide composition was determined by DNA sequencing. The results of this investigation show the following: There is no specific gene encoding for POLADS. The 5' ends of POLADS may be derived from either viral or cellular RNAs. Any RNA sequence including tRNAs, small nuclear RNAs and 5'ends of mRNAs can become POLADS if they acquire a poly(A) tail at their 3' ends during infection. This nonspecific polyadenylylation found in vaccinia virus-infected cells is probably conducted by vaccinia virus poly(A)+ polymerase. No consensus sequence is found on the 5' ends of POLADS for polyadenylylation. The 5' ends of POLADS have no direct role in their inhibitory activity of protein synthesis.

Killing of Burkitt-lymphoma-derived Daudi cells by ultraviolet-inactivated vaccinia virus

Interaction of active and UV-inactivated vaccinia virus at high multiplicity caused cytological changes and inhibition in cellular protein and DNA synthesis, thus arresting the multiplication of Burkitt-lymphoma-derived Daudi cells and eventually killing the cells. Adsorption to the cells but the lack of penetration was evident by immunofluorescence, electron microscopy and [3H]thymidine-labeled virus incorporation. Viral DNA synthesis or virus replication was not demonstrated. Thus, it appears that the massive adsorption of viral particles, active or UV-inactivated, or possibly a "toxic" component that resides in the virion, damages the plasma membrane and may be responsible for killing the cells by a mechanism of lysis from without.

Polyadenylated RNA sequences produced in vaccinia virus-infected cells under aberrant conditions inhibit protein synthesis in vitro

We have previously demonstrated that small nontranslated polyadenylated RNAs (POLADS) produced in vaccinia virus (VV)-infected cells inhibit the translation of cellular mRNAs, but minimally affect the translation of VV mRNAs in a cell-free protein synthesizing system. Infection of HeLa cells with ultraviolet-irradiated vaccinia virus or infection in the presence of actinomycin D (ACD) amplifies the synthesis of POLADS compared to the amount produced in cells infected under normal conditions. The effect of these POLADS on translation was studied in the reticulocyte lysate system. Polyadenylated RNAs isolated from cells infected with wild-type virus (V-POLADS) had a greater inhibitory effect on HeLa cell protein synthesis than on VV protein synthesis. Polyadenylated sequences obtained from cells infected with ultraviolet-irradiated virus (UV-POLADS) or from cells infected in the presence of ACD (ACD-POLADS), however, inhibited translation of both HeLa and viral mRNAs. Ultraviolet-POLADS and ACD-POLADS were found to possess, on average, longer poly(A) tails than V-POLADS. The inhibition of translation of both host and viral mRNAs effected by V-POLADS, UV-POLADS, and ACD-POLADS was reversed by poly(A) binding protein.

Mechanism of selective translation of vaccinia virus mRNAs: differential role of poly(A) and initiation factors in the translation of viral and cellular mRNAs

We have recently demonstrated that the poly(A) moieties of short RNAs obtained from both in vitro transcription and from vaccinia virus (VV)-infected cells exhibit dissimilar effects on the in vitro translation of cellular and VV mRNAs (R. Bablanian, G. Coppola, P. Masters, and A. K. Banerjee, Virology 148:375-380, 1986; M. J. Su and R. Bablanian, Virology 179:679-693, 1990). In the present study, we have investigated the roles of poly(A), m7GTP, and initiation factors in the mechanism of selective translation of VV mRNAs. The effects of unfractionated poly(A) [termed poly(A)un, with various chain lengths up to 3,000 nucleotides] and a 150- to 300-nucleotide fraction of synthetic poly(A) [termed poly(A)150-300] on the translation of HeLa cell mRNAs and early and late VV mRNAs were studied. Both the poly(A)un and the poly(A)150-300 completely inhibited the translation of HeLa cell mRNAs obtained from total cytoplasmic RNA in the nuclease-treated reticulocyte lysates. Viral mRNAs from total cytoplasmic RNA also were slightly inhibited (15 to 38%) by the poly(A)un, whereas the poly(A)150-300 had no significant effect on their translation. The translation of oligo(dT)-cellulose-selected HeLa mRNAs was as sensitive to inhibition by poly(A)150-300 as the mRNAs found in total cytoplasmic RNA. However, the translations of oligo(dT)-cellulose-selected viral mRNAs become more sensitive to the inhibitory effect of poly(A)150-300 than the translations of viral mRNAs found in the total cytoplasmic RNA. Both HeLa and VV mRNAs became more resistant to the poly(A)-mediated inhibition when these mRNAs were deadenylated, but the relative resistance to inhibition by poly(A)150-300 of deadenylated VV mRNAs was much greater than that of HeLa cell mRNAs. The translation of VV mRNAs was significantly less inhibited than the translation of HeLa mRNAs when the cap analog, m7GTP, was added to the cell-free system. The inhibition of HeLa cell mRNA translation by both poly(A)un and poly(A)150-300 was completely restored when poly(A)-binding protein (PAB) was added to the cell-free translational system. The addition of eukaryotic initiation factor 4A (eIF-4A) did not restore translation when poly(A)un was used to inhibit translation; however, inhibition by poly(A)150-300 was significantly reversed by this initiation factor. The reversal of poly (A)-mediated inhibition of HeLa cell mRNA translation was additive when PAB was used together with eIF-4A. Early VV mRNA translation was only slightly inhibited by poly(A)un (15%), and this inhibition was completely reversed by either PAB or eIF-4A.(ABSTRACT TRUNCATED AT 400 WORDS)

Polyadenylated RNA sequences from vaccinia virus-infected cells selectively inhibit translation in a cell-free system: structural properties and mechanism of inhibition

The mechanism of vaccinia virus-induced selective inhibition of host cell protein synthesis was studied in a nonpermissive (Chinese hamster ovary, CHO) and in a permissive mouse cell line ( L cells). Small polyadenylated RNAs obtained from uninfected and infected cells were fractionated into six size classes by polyacrylamide gel electrophoresis. The RNAs from the first two largest fractions (greater than 500 nucleotide, nt) were translated into some low-molecular-weight polypeptides, whereas, the RNAs from the remaining fractions (400-500, 300-400, 200-300, and 100-200 nt) had no translational activity in reticulocyte lysates. When these nontranslating polyadenylated short sequences (POLADS) were added to the cell-free system together with HeLa cell mRNAs, translation was inhibited from 70%, by the 400- to 500-nt fraction, to about 20%, by the 100- to 200-nt fraction. The degree of inhibition of protein synthesis was clearly dependent on the size of POLADS. The translation of vaccinia virus mRNAs in the cell-free system was inhibited by about 25% with the 400- to 500-nt fraction, by 5% with the 300- to 400-nt fraction, while the smaller size POLADS had no inhibitory effect. The inhibition of HeLa cell and vaccinia virus mRNA translation by POLADS was reversed by the simultaneous addition of oligo(dT) to the cell-free system. POLADS were also obtained from uninfected cells, but they inhibited the translation of HeLa cell and vaccinia virus mRNAs to a much lesser extent. The removal of the poly(A) moiety from POLADS by treatment with ribonuclease H and oligo(dT) abolished their inhibitory effect on HeLa cell mRNA translation. The average length of the poly(A) tails of POLADS obtained from infected cells was longer than that of POLADS from normal cells. Inhibition of HeLa cell mRNA translation mediated by POLADS in the cell-free system was reversed (approximately 70%) by addition of crude initiation factors (ribosomal salt wash, RSW). Significantly, inhibition of translation of POLADS was reversed (greater than 90%) by addition of purified poly(A) binding protein (PAB). Purified initiation factor 4A (eIF-4A) also reversed this inhibition, but to a lesser extent than RSW and PAB. Our results show that the translation of vaccinia virus mRNAs is resistant to POLADS, suggesting that POLADS, by virtue of their long poly(A) tails, may sequester PAB and thus, play a role in selective inhibition.

Superinfection exclusion of vaccinia virus in virus-infected cell cultures

Vaccinia virus-infected BSC 40 cells do not permit the replication of superinfecting vaccinia virus. The extent of superinfecting virus propagation depends on the time of superinfection; there is 90% exclusion by 4 hr after the initial infection, and more than 99% by 6 hr. When superinfection is attempted at 6 hr after infection, the superinfecting virus is incapable of carrying out DNA replication or early gene transcription, demonstrating that an early event in the virus life cycle is inhibited. The rate of adsorption of the superinfecting virus is unaltered which shows that exclusion is affected at a point between adsorption and early gene transcription. In order to exclude superinfection, the primary infecting virus does not require replication of its DNA or expression of its late genes but it must express one or more early genes.

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).

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.

Vaccinia virus proteins on the plasma membranes of infected cells. IV. Studies employing L cells infected with ultraviolet-irradiated vaccinia virions

As measured by in vitro, 51Cr-release assays, the expression on plasma membranes of two, immediate-early, vaccinia virus-specified cell-surface antigens, with mol wt of 25K-27K and 16K-17K, could be directly correlated with the susceptibility of target cells to lysis by vaccinia virus-specific cytotoxic T cells.

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 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.

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.

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.

Specific inhibition of vaccinia virus growth by 2'-O-methyladenosine: isolation of a drug-resistant virus mutant

2'-O-methyladenosine (Am) specifically inhibits growth of vaccinia virus on cultured monkey kidney (BSC40) cells. Specificity has been demonstrated by the isolation of an Am-resistant mutant of vaccinia which forms plaques on Am-treated monolayers of BSC40 cells under conditions where wild type (wt) plaque formation is inhibited. Am inhibits virus growth at an early stage of infection; host shut off and early virus protein synthesis are inhibited by the drug.