Inhibition of Sindbis virus replication in Aedes albopictus cells by virazole (ribavirin) and its reversal by actinomycin: a correction

In vitro Antiviral Activity of Ribavirin against Picornaviruses

The in vitro antiviral activity of ribavirin (Rby; 1-β-D-ribof uranosyl-1,2,4-triazole-3-carboxamide) against selected members of the Picornaviridae is described. When antiviral activity was determined by reduction of infectious virus progeny, Rbv was active against human rhinovirus type 2 (HRV-2) and poliovirus type 1 (Polio-1) in both a drug concentration and multiplicity of infection-(MOI) dependent response. However, an antiviral activity rating assay based on the reductions of cellular cytopathic effects (CPE) indicated that Rbv was active against human rhinoviruses, but was less active against polioviruses. Prophylactic administration of Rbv significantly improved virus yield reduction, especially in Polio-1. SDS Polyacrylamide gel electrophoresis (SDS-PAGE) of HRV-2- and Polio-1-infected cell lysates demonstrated that Rbv inhibited the synthesis of viral-specific proteins. Although actinomycin D (Act D) did not significantly influence Picornavirus yields, when added concomitantly with Rbv, Act D reversed Rbv's antiviral effect.

Cell type mediated resistance of vesicular stomatitis virus and Sendai virus to ribavirin

Ribavirin (RBV) is a synthetic nucleoside analog with broad spectrum antiviral activity. Although RBV is approved for the treatment of hepatitis C virus, respiratory syncytial virus, and Lassa fever virus infections, its mechanism of action and therapeutic efficacy remains highly controversial. Recent reports show that the development of cell-based resistance after continuous RBV treatment via decreased RBV uptake can greatly limit its efficacy. Here, we examined whether certain cell types are naturally resistant to RBV even without prior drug exposure. Seven different cell lines from various host species were compared for RBV antiviral activity against two nonsegmented negative-strand RNA viruses, vesicular stomatitis virus (VSV, a rhabdovirus) and Sendai virus (SeV, a paramyxovirus). Our results show striking differences between cell types in their response to RBV, ranging from virtually no antiviral effect to very effective inhibition of viral replication. Despite differences in viral replication kinetics for VSV and SeV in the seven cell lines, the observed pattern of RBV resistance was very similar for both viruses, suggesting that cellular rather than viral determinants play a major role in this resistance. While none of the tested cell lines was defective in RBV uptake, dramatic variations were observed in the long-term accumulation of RBV in different cell types, and it correlated with the antiviral efficacy of RBV. While addition of guanosine neutralized RBV only in cells already highly resistant to RBV, actinomycin D almost completely reversed the RBV effect (but not uptake) in all cell lines. Together, our data suggest that RBV may inhibit the same virus via different mechanisms in different cell types depending on the intracellular RBV metabolism. Our results strongly point out the importance of using multiple cell lines of different origin when antiviral efficacy and potency are examined for new as well as established drugs in vitro.

The 126- and 183-kilodalton proteins of tobacco mosaic virus, and not their common nucleotide sequence, control mosaic symptom formation in tobacco

Nucleotide substitutions at two positions within the open reading frame encoding the 126-kDa protein in the attenuated masked (M) strain of tobacco mosaic tobamovirus (TMV) to those found in the virulent U1-TMV genome led to the induction of near U1-TMV-like symptoms on leaves of Nicotiana tabacum L. cv. Xanthi nn by progeny virus (M. H. Shintaku, S. A. Carter, Y. Bao, and R. S. Nelson, Virology 221:218-225, 1996). In this study, further site-directed mutations were made at these positions within the M strain cDNA to determine whether the protein or nucleotide sequence directly controlled the symptom phenotype. The protein and not the nucleotide sequence directly controlled the symptom phenotype when amino acid 360 within the 126-kDa protein sequence was altered and likely controlled the symptom phenotype when amino acid 601 was altered. The effects of the substitutions at amino acid position 360 on viral protein accumulation were studied by pulse-labeling proteins in infected protoplasts. Accumulation of the 126- and 183-kDa proteins was less for an attenuated mutant than for two virulent mutants, but the viral movement protein and coat protein accumulated to levels reported to be sufficient for normal systemic symptom development. The size of necrotic local lesions on N. tabacum L. cv. Xanthi NN was negatively correlated with symptom development and accumulation of the 126-kDa protein for these mutants. With reference to this last finding, an explanation of the cause of the differing symptoms induced by these viruses is presented.

The alphaviruses: gene expression, replication, and evolution

The alphaviruses are a genus of 26 enveloped viruses that cause disease in humans and domestic animals. Mosquitoes or other hematophagous arthropods serve as vectors for these viruses. The complete sequences of the +/- 11.7-kb plus-strand RNA genomes of eight alphaviruses have been determined, and partial sequences are known for several others; this has made possible evolutionary comparisons between different alphaviruses as well as comparisons of this group of viruses with other animal and plant viruses. Full-length cDNA clones from which infectious RNA can be recovered have been constructed for four alphaviruses; these clones have facilitated many molecular genetic studies as well as the development of these viruses as expression vectors. From these and studies involving biochemical approaches, many details of the replication cycle of the alphaviruses are known. The interactions of the viruses with host cells and host organisms have been exclusively studied, and the molecular basis of virulence and recovery from viral infection have been addressed in a large number of recent papers. The structure of the viruses has been determined to about 2.5 nm, making them the best-characterized enveloped virus to date. Because of the wealth of data that has appeared, these viruses represent a well-characterized system that tell us much about the evolution of RNA viruses, their replication, and their interactions with their hosts. This review summarizes our current knowledge of this group of viruses.

Reversal of the antiviral activity of ribavirin against Sindbis virus in Ae. albopictus mosquito cells

Earlier work in our laboratory has shown that the replication of Sindbis virus in Aedes albopictus mosquito cells is inhibited by ribavirin (Rbv) and mycophenolic acid (MPA) (Sarver and Stollar (1978) Virology 91, 267-282; Malinoski and Stollar (1980) Virology 102, 473-476). We report here that the antiviral effect of Rbv and MPA can be reversed by depriving infected cells of methionine or isoleucine, or by treating them with fluorodeoxyuridine (FUdR) or cycloleucine. We suggest that, as was the case when the antiviral activity of Rbv was reversed by actinomycin D (Malinoski and Stollar (1981a) Virology 110, 281-291), these effects may be mediated by changes in the GTP pools of treated cells.

The complete nucleotide sequence of cell fusing agent (CFA): homology between the nonstructural proteins encoded by CFA and the nonstructural proteins encoded by arthropod-borne flaviviruses

Cell fusing agent (CFA) is an RNA virus originally isolated from a line of Aedes aegypti mosquito cells. Although our characterization of the virus many years ago showed that it resembled the flaviviruses, there was no detectable serological cross-reaction with members of the genus flavivirus. Furthermore, unlike the well-studied members of the genus flavivirus, CFA did not replicate in any of several vertebrate cell lines tested. We have now determined the nucleotide sequence of the CFA genome. Comparison of the predicted amino acid sequence of the CFA polyprotein with viral protein sequences in Genbank, has made it apparent that CFA should now be assigned to the family Flaviviridae, genus flavivirus. The homology between CFA proteins and those of other flaviviruses was highest for NS5 (45%) and NS3 (34%). Little homology was found for the structural proteins. Thus, CFA is only distantly related to the other flaviviruses for which there is sequence information; nevertheless, with respect to their hydrophobicity plots, the CFA polyprotein and the polyproteins of other flaviviruses are remarkably similar. We suggest that CFA is an insect virus, which was present in the embryos from which the Ae. aegypti cell line was established. Thus, CFA seems to be the first member of the family Flaviviridae, genus flavivirus, to be identified as an insect virus.

Selective inhibition of arthropod-borne and arenaviruses in vitro by 3′-fluoro-3′-deoxyadenosine

A novel nucleoside analog, 3'-fluoro-3'-deoxyadenosine (3'F3'dAdo), was evaluated for antiviral activity against several arthropod-borne and arenaviruses in Vero cell culture. The following 50% inhibitory concentrations (EC50) of virus plaque formation were obtained against the test viruses: Semliki Forest (10.3 microM) and Venezuelan equine encephalitis (5.3 microM) alphaviruses, lymphocytic choriomeningitis (7.7 microM) and Pichinde (greater than 32 microM) arenaviruses, Punta Toro (greater than 32 microM) and San Angelo (1.6 microM) bunyaviruses, banzi flavivirus (4.0 microM), and Colorado tick fever orbivirus (0.6 microM). By comparison, the broad-spectrum antiviral agent ribavirin was active against lymphocytic choriomeningitis (18 microM), Pichinde (24 microM), Punta Toro (114 microM), and San Angelo (99 microM) viruses, but was less active against the other 4 viruses (greater than 200 microM). Vero cell proliferation and thymidine and uridine incorporation into replicating Vero cells were inhibited by 50% with 3'F3'dAdo concentrations of 36, 45, and 32 microM, respectively. In virus yield reduction assays, increasing the multiplicity of infections of Semliki Forest and Venezuelan equine encephalitis viruses reduced the inhibitory activity of 3'F3'dAdo. Using the same assay, 3'F3'dAdo was found to enhance Punta Toro virus replication up to 5-fold relative to the untreated control. By adding the nucleoside transport inhibitor nitrobenzylthioinosine (100 microM) to the culture medium, antiviral activity against the two alphaviruses was eliminated, indicating that 3'F3'dAdo uses the nucleoside transport system for cell entry. When actinomycin D (5 microM) was used to greatly suppress cellular RNA synthesis in Semliki Forest virus-infected and uninfected cells, 3'F3'dAdo preferentially inhibited viral RNA synthesis. The results of these studies indicate 3'F3'dAdo is a selective inhibitor of most of the viruses tested and should be a promising candidate for in vivo evaluations.

Antiviral activity and mode of action of ribavirin 5'-sulfamate against Semliki Forest virus

Ribavirin 5'-sulfamate, a nucleotide analog, inhibited Semliki Forest virus cytopathology by 50% at 10 microM, whereas ribavirin was inactive at less than or equal to 1 mM. Actinomycin D did not reverse (antagonize) the effect of ribavirin 5'-sulfamate against the virus. The compound inhibited amino acid incorporation into macromolecules of uninfected cells but had no appreciable effect on uridine incorporation. Infected cells treated with actinomycin D and nucleotide analog were inhibited in amino acid and uridine incorporation. The compound blocked the formation of the viral RNA polymerase protein in cells, which could account for the inhibited synthesis of new viral RNA. By electrophoresis, inhibition of the synthesis of viral proteins was more pronounced than the inhibition of cellular polypeptides. The analog inhibited the translation of mRNA to protein. Most animals treated intraperitoneally for 7 days with ribavirin 5'-sulfamate at 20 and 40 mg/kg/day starting 2 h before intraperitoneal Semliki Forest virus inoculation survived the otherwise lethal infection.

Metabolism of ribavirin in respiratory syncytial virus-infected and uninfected cells

The metabolism of ribavirin to its mono-, di-, and triphosphate derivatives was examined in uninfected and respiratory syncytial virus-infected cells. The degree of phosphorylation was dose dependent upon extracellular ribavirin concentration. The major species formed was the triphosphate, with mono- and diphosphates being approximately 12 and 4% of the triphosphate, respectively. Amounts of triphosphate formed in infected cells were up to 2.6-fold greater than those in uninfected cells. Upon drug removal, ribavirin triphosphate degradation was very rapid, with decay half-lives of 70 to 100 min. Actinomycin D inhibited triphosphate production and also neutralized the antiviral effect of ribavirin.

Effect of ribavirin on macromolecular synthesis in vesicular stomatitis virus-infected cells

Ribavirin at 200 micrograms/ml inhibited vesicular stomatitis virus (VSV) growth in Chinese hamster ovary (CHO) cells by 2.5 logs. To determine the mechanism of this inhibition, viral macromolecular synthesis was examined. VSV primary transcription remained unaffected, but overall VSV RNA synthesis decreased by 40 to 60%. When ribavirin was added 1.5 h after infection, inhibition of progeny production was partially lost, indicating that the antiviral effect was on an early stage after primary transcription. Inhibition of RNA polymerization by premature chain termination was not evident. Viral translation, on the other hand, was reduced by 95% with an inhibition of every protein species. Furthermore, viral RNA synthesized in the presence of ribavirin did not translate well in an in vitro translation system. In contrast, uninfected CHO cells treated with ribavirin showed a greater sensitivity in RNA synthesis than in protein synthesis. This suggests that the cellular translational machinery was not directly affected. Short-term treatment of cells resulted in negligible toxicity, but after 24 h there was marked alteration of cellular integrity. These results, taken together with data on other viruses, suggest that in the presence of ribavirin, dysfunctional VSV mRNA was synthesized, resulting in its failure to be translated. The selective antiviral effects of ribavirin and its relative lack of toxicity for host cells may be predicted on the basis of mRNA turnover and the requirements for de novo functional mRNA.

Exclusion of superinfecting homologous virus by Sindbis virus-infected Aedes albopictus (mosquito) cells

The infection of tissue-cultured Aedes albopictus (mosquito) cells by an alphavirus ultimately results in a persistently infected cell population which can be maintained in the laboratory for years. One characteristic of this culture is that it will not support the replication of superinfecting homologous virus. We have previously shown that mosquito cells persistently infected with Sindbis virus produce an antiviral agent which when applied to uninfected mosquito cells suppresses Sindbis virus replication. The exclusion of virus replication in the antiviral-agent-treated cells is similar to the phenomenon of homologous interference described in alphavirus-infected vertebrate cells. In this study we examined the expression of homologous interference in three lines of mosquito cells and compared the expression of homologous interference to the effects of the antiviral activity. The cell lines were found to differ in their ability to express homologous interference, and evidence suggests that the mosquito cells may suppress replication by homologous interference or by the action of the antiviral agent.

Requirement of cell nucleus for Sindbis virus replication in cultured Aedes albopictus cells

The ability of Sindbis virus to grow in enucleated BHK-21 (vertebrate) and Aedes albopictus (invertebrate) cells was tested to determine the dependence of this virus upon nuclear function in these two phylogenetically unrelated hosts. Although both cell types could be demonstrated to produce viable cytoplasts (enucleated cells) which produced virus-specific antigen subsequent to infection. BHK cytoplasts produced a significant number of progeny virions, whereas mosquito cytoplasts did not. The production of vesicular stomatitis virus in mosquito cells was not significantly reduced by enucleation. That such a host function was not essential for vesicular stomatitis virus growth in insect cells is supported by the observation that the production of this virus by mosquito cells is not actinomycin D sensitive. This result agrees with a previously published report in which it was shown that Sindbis virus maturation in invertebrate cells is inhibited by actinomycin D, indicating a possible requirement for host cell nuclear function (Scheefers-Borchel et al., Virology, 110:292-301, 1981).