Poliovirus type II produced from cloned cDNA is infectious in mice

Proteinase 2Apro is essential for enterovirus replication in type I interferon-treated cells

The Picornaviridae family comprises a diverse group of small RNA viruses that cause a variety of human and animal diseases. Some of these viruses are known to induce cleavage of components of the innate immune system and to inhibit steps in the interferon pathway that lead to the production of type I interferon. There has been no study of the effect of picornaviral infection on the events that occur after interferons have been produced. To determine whether members of the Enterovirus genus can antagonize the antiviral activity of interferon-stimulated genes (ISGs), we pretreated cells with alpha interferon (IFN-alpha) and then infected the cells with poliovirus type 1, 2, or 3; enterovirus type 70; or human rhinovirus type 16. We found that these viruses were able to replicate in IFN-alpha-pretreated cells but that replication of vesicular stomatitis virus, a Rhabdovirus, and encephalomyocarditis virus (EMCV), a picornavirus of the Cardiovirus genus, was completely inhibited. Although EMCV is sensitive to IFN-alpha, coinfection of cells with poliovirus and EMCV leads to EMCV replication in IFN-alpha-pretreated cells. The enteroviral 2A proteinase (2A(pro)) is essential for replication in cells pretreated with interferon, because amino acid changes in this protein render poliovirus sensitive to IFN-alpha. The addition of the poliovirus 2A(pro) gene to the EMCV genome allowed EMCV to replicate in IFN-alpha-pretreated cells. These results support an inhibitory role for 2A(pro) in the most downstream event in interferon signaling, the antiviral activities of ISGs.

Enterovirus 70 receptor utilization is controlled by capsid residues that also regulate host range and cytopathogenicity

Enterovirus type 70, an etiologic agent of acute hemorrhagic conjunctivitis, may bind different cellular receptors depending on cell type. To understand how EV70-receptor interaction is controlled, we studied two variants of the virus with distinct receptor utilization. EV70-Rmk, derived by passage in rhesus monkey kidney cells, replicates poorly in HeLa cells and does not cause cytopathic effects. Decay accelerating factor (DAF) is not a cell receptor for EV70-Rmk. Passage of EV70-Rmk in HeLa cells lead to isolation of EV70-Dne, which does not replicate in rhesus monkey kidney cells but grows to high titers in HeLa cells and causes cytopathic effects. DAF is sufficient for cell entry of EV70-Dne. EV70-Rmk replicates in human eye and brain-derived cell lines, whereas the Dne strain replicates only in HeLa cells and in conjunctiva-derived 15C4 cells. The two EV70 strains differ by five amino acid changes in the viral capsid. Single substitution of four of the five EV70-Rmk amino acids with the residue from EV70-Dne leads to lytic replication in HeLa cells. Conversely, substitution of any of the five EV70-Dne amino acids with the EV70-Rmk amino acid does not alter replication in HeLa cells. Three of these capsid amino acids are predicted to be located in the canyon encircling the fivefold axis of symmetry, one amino acid is found at the fivefold axis of symmetry, and one is located the interior of the capsid. The five EV70 residues define a region of the capsid that controls viral host range, DAF utilization, and cytopathogenicity.

An intragenic revertant of a poliovirus 2C mutant has an uncoating defect

A revertant was isolated from a temperature-sensitive poliovirus 2C mutant, 2C-31, which is defective in viral RNA synthesis. This revertant, called 2C-31R1, grew well at 39 degrees C and was not defective in RNA synthesis. However, in contrast to its parental mutant, 2C-31R1 was cold sensitive and could hardly grow at all at 32 degrees C. Analysis of a single-cycle growth revealed that 2C-31R1 was defective in virion uncoating at 32 degrees C, and a substantial amount (more than 30%) of input viruses could be recovered as infectious particles from an infected cell lysate up to 6 h postinfection. The uncoating defect and the inability to grow at cold temperatures could be overcome by a brief incubation at the permissive temperature (39 degrees C) before the infection was continued at 32 degrees C. cDNA cloning and mix-and-match recombination experiments indicated that the defect in uncoating was the result of two secondary point mutations, seven nucleotides apart, in the 2C-coding sequence downstream of the inserted linker which is the original mutation in the parental 2C-31 genome. Another revertant, 2C-31R3, isolated from the same 2C-31 stock, was not defective in uncoating and appeared to be a secondary revertant that contained an intragenic suppressor for the uncoating defect. The uncoating defect of 2C-31R1 could be complemented by type 2 poliovirus. These results suggested that protein 2C, in addition to its role in viral RNA synthesis, has a function in determining virion structure.

Construction and characterization of poliovirus subgenomic replicons

Poliovirus RNAs containing in-frame deletions within the capsid-coding region were produced by in vitro transcription of altered poliovirus type 1 cDNA by using bacteriophage T7 RNA polymerase. Three RNAs were transcribed that contained deletions of 2,317 nucleotides (bases 747 to 3064), 1,781 nucleotides (bases 1,175 to 2,956), and 1,295 nucleotides (bases 1,175 to 2,470). All three subgenomic RNAs replicated after transfection into HeLa cells, demonstrating that sequences encoding the capsid polypeptides are not essential for viral RNA replication in vivo. Viral RNA containing the largest deletion (R1) replicated approximately three times better than full-length RNA produced in vitro. Northern blot (RNA blot) hybridization analysis of total cellular RNA from HeLa cells at different times after transfection with R1 demonstrated the presence of increasing amounts of the expected 5.1-kilobase subgenomic RNA. Analysis by immunoprecipitation of viral proteins induced after transfection of R1 RNA into HeLa cells revealed the presence of proteins 2Apro, 2C, and 3Dpol and its precursors, suggesting that the polyprotein cleavages are similar to those occurring in virus-infected cells. Replication of P2/Lansing virion RNA was inhibited by cotransfection with the R1 replicon, as demonstrated by hybridization analysis with a serotype-specific oligonucleotide probe. A higher level of inhibition of RNA replication was observed when P2/Lansing RNA was cotransfected into HeLa cells with truncated R1 transcripts (R1-PvuII) that were missing 395 3' nucleotides and a poly(A) tail. These internally and terminally deleted RNAs inhibited the replication of subgenomic replicons R1, R2, and R3 and caused a reduction in plaque size when cotransfected with P1/Mahoney or P2/Lansing viral RNA, suggesting that individual cells had received both RNAs. No inhibition of plaque size was observed when replicon RNAs were used that were missing 1,384 or 1,839 3' nucleotides or contained plasmid-derived sequences downstream of the 3' poly(A). The trans-acting inhibitory effect of R1-PvuII on the replication of poliovirus P2/Lansing RNA did not involve entry of RNA into cells and appeared to reduce viral translation and RNA synthesis late in the infection cycle.

Defined recombinants of poliovirus and coxsackievirus: sequence-specific deletions and functional substitutions in the 5'-noncoding regions of viral RNAs

We describe the isolation of a variant of a polio--coxsackie recombinant virus (PCV110) containing a genomic RNA with a chimeric 5'-noncoding region. The variant virus [designated PCV110(1)] has growth and biosynthetic properties that are quite different from the original, temperature-sensitive isolate of the recombinant virus [designated PCV110(4)]. Nucleotide sequencing of the 5'-noncoding region of RNA from PCV110(1) revealed a 4-base deletion within the substituted coxsackievirus region of the chimeric genome that may contribute to the loss of temperature sensitivity of this variant recombinant virus. In addition, we have generated new recombinant viruses that contain (1) coxsackievirus sequences within the N66-N627 region of the poliovirus genome and (2) coxsackievirus sequences substituted from N1-N627 in the poliovirus genome. These recombinant viruses are not temperature sensitive for growth at 37 degrees and have biosynthetic properties similar to those of wild-type poliovirus. Our results provide evidence that replicase recognition signals encoded in the 5' noncoding regions of enterovirus genomic RNAs are not strictly sequence specific.

Reduced mouse neurovirulence of poliovirus type 2 Lansing antigenic variants selected with monoclonal antibodies

The Lansing strain of poliovirus type 2 is a mouse-adapted virus that induces a fatal paralytic disease in mice after intracerebral inoculation. Our previous results indicated that the mouse-adapted phenotype maps to the Lansing viral capsid. To further define regions of the capsid that are specifically involved in the infection of mice, antigenic variants resistant to neutralization with monoclonal antibodies were selected, and their mouse neurovirulence was studied. The monoclonal antibodies used were directed against antigenic site 1, an immunodominant loop of capsid polypeptide VP1 located on the virion surface. Ten of twenty-two variants selected had lower intracerebral neurovirulence in mice when compared to the parental virus. Four of the ten antigenic variants with reduced neurovirulence were temperature sensitive (ts) for replication in HeLa cells, while the remaining six variants replicated in HeLa cells as well as the parent virus. Two ts+ variants that were studied had a reduced ability to replicate in the mouse brain. There was no difference in the histopathology and pattern of involvement in the central nervous system of one variant compared to the parent virus. In three variants, reduction of neurovirulence correlated with specific amino acid substitutions at positions 100 and 101 of VP1, located within antigenic site 1. The ts phenotype in three variants was associated with a single amino acid deletion at position 105. Virus recovered from the brain of paralyzed mice that had been inoculated with the antigenic variants was characterized to identify the virus causing disease. In most cases, brain isolates resembled the inoculated virus in neurovirulence and amino acid sequence at the antigenic site. Virus recovered from brains of paralyzed mice that had been inoculated with the ts variants was either ts+ or cold sensitive, and had become more neurovirulent. These results suggest that specific amino acid changes within an antigenic site on the virion surface may result in reduction of mouse neurovirulence without affecting viral replication in cultured cells.

Hepatitis A virus cDNA and its RNA transcripts are infectious in cell culture

A full-length cDNA copy of an attenuated, cell culture-adapted hepatitis A virus (HAV HM-175/7 MK-5) genome was constructed in the PstI site of plasmid vector pBR322. Transfection of monkey kidney cells with this plasmid failed to induce the production of hepatitis A virus (HAV). The HAV cDNA was excised from pBR322 and inserted, without the oligo(dG) X oligo(dC) tails, into an RNA transcription vector to yield plasmid pHAV/7. Transfection of monkey kidney cells with pHAV/7 DNA induced HAV infection. Transfection with RNA transcripts produced in vitro from pHAV/7 yielded about 10-fold more HAV than did transfection with pHAV/7 DNA. Marmosets inoculated with transfection-derived virus developed anti-HAV antibodies and had liver enzyme patterns that closely resembled the liver enzyme patterns seen in animals inoculated with virus from a comparable level of cell culture passage. Infectious RNA transcripts from HAV cDNA should be useful for studying the molecular basis of cell culture adaptation and attenuation as well as for studying specific viral functions.

Complete nucleotide sequence of the genome of coxsackievirus B1

The complete nucleotide sequence of the genome of the coxsackievirus B1, a human enterovirus that belongs to the Picornaviridae, was determined by using molecular cloning and rapid sequence analysis techniques. Sequence analysis of the cloned cDNAs revealed that the virion RNA was 7389 nucleotides long and polyadenylylated at the 3' terminus. Similar to other picornavirus genomes, a single large open reading frame was identified. The translated sequence starts at nucleotide position 742 and ends at 7287 of the genome. Thus, the viral polyprotein should consist of 2182 amino acids. When the predicted amino acid sequence of the viral polyprotein was compared with those of other human enteroviruses such as polioviruses, a striking sequence homology was observed, especially in viral proteins 1B, 2C, and 3D. This allowed us to predict precise map locations of the viral structural and nonstructural proteins on the genome, although two proteolytic processing sites, between 1D and 2A and between 2B and 2C, were obscure. The result presented here implied important information with respect to the genetical variation of human enteroviruses.

The mechanism of RNA recombination in poliovirus

We have investigated RNA recombination among poliovirus genomes by analyzing both intratypic and intertypic recombinant crosses involving the same defined genetic markers. Sequence analysis of the recombinant junctions of 13 nonsibling intertypic recombinants showed that intertypic RNA recombination is not site-specific, nor does it require extensive homology between the recombining parents at the crossover site. To discriminate between breaking-rejoining and copy choice mechanisms of RNA recombination, we have inhibited the replication of the recombining parents independently and found opposite effects on the frequency of genetic recombination in intratypic crosses. The results strongly support a copy choice mechanism for RNA recombination, in which the viral RNA polymerase switches templates during negative strand synthesis.