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The inability of certain poliovirus strains to infect mice can be overcome by the expression of human poliovirus receptors in mice or by the presence of a particular amino acid sequence of the B‐C loop of the viral capsid protein VP1. We have identified changes in an additional capsid structure that permit host‐restricted poliovirus strains to infect mice. Variants of the mouse‐virulent P2/Lansing strain were constructed containing amino acid changes, deletions and insertions in the B‐C loop of VP1. These variants were attenuated in mice, demonstrating the importance of the B‐C loop sequence in host range. Passage of two of the B‐C loop variants in mice led to the selection of viruses that were substantially more virulent. The increased neurovirulence of these strains was mapped to two different suppressor mutations in the N‐terminus of VP1. Whereas the B‐C loop of VP1 is highly exposed on the surface of the capsid, near the five‐fold axis of symmetry, the suppressor mutations are in the interior of the virion, near the three‐fold axis. Introduction of the suppressor mutations into the genome of the mouse‐avirulent P1/Mahoney strain resulted in neurovirulent viruses, demonstrating that the P2/Lansing B‐C loop sequence is not required to infect mice. Because the internal host range determinants are in a structure known to be important in conformational transitions of the virion, the host range of poliovirus may be determined by the ability of virions to undergo transitions catalyzed by cell receptors.

The inability of certain poliovirus strains to infect mice can be overcome by the expression of human poliovirus receptors in mice or by the presence of a particular amino acid sequence of the B‐C loop of the viral capsid protein VP1. We have identified changes in an additional capsid structure that permit host‐restricted poliovirus strains to infect mice. Variants of the mouse‐virulent P2/Lansing strain were constructed containing amino acid changes, deletions and insertions in the B‐C loop of VP1. These variants were attenuated in mice, demonstrating the importance of the B‐C loop sequence in host range. Passage of two of the B‐C loop variants in mice led to the selection of viruses that were substantially more virulent. The increased neurovirulence of these strains was mapped to two different suppressor mutations in the N‐terminus of VP1. Whereas the B‐C loop of VP1 is highly exposed on the surface of the capsid, near the five‐fold axis of symmetry, the suppressor mutations are in the interior of the virion, near the three‐fold axis. Introduction of the suppressor mutations into the genome of the mouse‐avirulent P1/Mahoney strain resulted in neurovirulent viruses, demonstrating that the P2/Lansing B‐C loop sequence is not required to infect mice. Because the internal host range determinants are in a structure known to be important in conformational transitions of the virion, the host range of poliovirus may be determined by the ability of virions to undergo transitions catalyzed by cell receptors.

Poliovirus polysomal RNA is naturally uncapped, and as such, its translation must bypass any 5' cap-dependent ribosome recognition event. To elucidate the manner by which poliovirus mRNA is translated, we have determined the translational efficiencies of a series of deletion mutants within the 5' noncoding region of the mRNA. We found striking differences in translatability among the altered mRNAs when assayed in mock-infected and poliovirus-infected HeLa cell extracts. The results identify a functional cis-acting element within the 5' noncoding region of the poliovirus mRNA which enables it to translate in a cap-independent fashion. The major determinant of this element maps between nucleotides 320 and 631 of the 5' end of the poliovirus mRNA. We also show that this region (320 to 631), when fused to a heterologous mRNA, can function in cis to render the mRNA cap independent in translation.

The attenuated phenotype of Sabin 3 poliovirus compared with its neurovirulent progenitor strain has been largely accounted for by mutations in the genome at positions 472 and 2034 (G. D. Westrop, K. A. Wareham, D. M. A. Evans, G. Dunn, P. D. Minor, D. I. Magrath, F. Taffs, S. Marsden, M. A. Skinner, G. C. Schild, and J. W. Almond, J. Virol. 63:1338-1344, 1989). By sequencing vaccine virus RNA, we recently identified another Sabin 3-specific mutation at position 2493 (U----C), which predicts an Ile----Thr change at the sixth residue of VP1 (C. Weeks-Levy, J. M. Tatem, S. J. DiMichele, W. Waterfield, A. F. Georgiu, and S. J. Mento, Virology 185:934-937, 1991). Viruses generated by using cDNAs which represent the vaccine sequence (LED3) and a derivative (VR318) possessing a single base change to the wild-type nucleotide (U) at 2493 were used to determine the impact of the 2493 mutation on virus phenotype. The VP1 proteins of LED3 and VR318 viruses were distinguishable by denaturing electrophoretic analysis. LED3 produced smaller plaques in Vero cells than VR318 virus did. Neurovirulence testing of these cDNA-derived viruses in monkeys demonstrated that the 2493 mutation in LED3 virus is attenuating.

The poliovirus receptor (Pvr) is a member of the immunoglobulin superfamily of proteins, but its function in the cell is not known. Southern blot hybridization analysis indicated that the murine genome contains a sequence homolog of pvr. As a first step toward using the murine pvr homolog (mph) to study the function of Pvr, murine genomic and cDNA clones encoding mph were isolated. mph encodes a polypeptide with extensive sequence similarity to the extracellular domains of the human PVR. mph mRNAs of 2.0 and 3.0 kb are transcribed in the adult mouse brain, the spinal cord, the spleen, the kidney, the heart, and the liver. The Mph protein does not function as a receptor for poliovirus. However, substitution of domain 1 of the Mph protein with the corresponding sequence from pvr produced a chimeric receptor that could bind poliovirus and lead to productive infection. By constructing pvr-mph chimeras, it will be possible to identify the contact points of poliovirus within domain 1 of Pvr. Identification of the ligand and the cellular function of the Mph protein may help us understand the role of Pvr in the cell.

The identification of a monoclonal antibody, AF3, which recognizes a single isoform of the cell surface protein CD44 and preferentially blocks binding of serotype 2 poliovirus to HeLa cells, suggested that CD44 might be an accessory molecule to Pvr, the cell receptor for poliovirus, and that it could play a role in the function of the poliovirus receptor site. We show here that only AF3 blocks binding of serotype 2 poliovirus to HeLa cells and, in contrast to a previously published report, that the anti-CD44 monoclonal antibodies A3D8 and IM7 are unable to block binding of poliovirus. To determine whether CD44 is involved in poliovirus infection, we analyzed the replication of all three serotypes of poliovirus in human neuroblastoma cells which lack or express CD44 and in mouse neuroblastoma cells which lack Pgp-1, the mouse homolog of human CD44, and which express Pvr. All three poliovirus serotypes replicate with normal kinetics and to normal levels in the absence or presence of CD44 or in the absence of Pgp-1. Furthermore, the binding affinity constants of all three poliovirus serotypes for Pvr are unaffected by the presence or absence of CD44 in the human neuroblastoma cell line. We conclude that CD44 and Pgp-1 are not required for poliovirus replication and are unlikely to be involved in poliovirus pathogenesis.

Poliovirus initiates infection of primate cells by binding to the poliovirus receptor, Pvr. Mouse cells do not bind poliovirus but express a Pvr homolog, Mph, that does not function as a poliovirus receptor. Previous work has shown that the first immunoglobulin-like domain of the Pvr protein contains the virus binding site. To further identify sequences of Pvr important for its interaction with poliovirus, stable cell lines expressing mutated Pvr molecules were examined for their abilities to bind virus and support virus replication. Substitution of the amino-terminal domain of Mph with that of Pvr yields a molecule that can function as a poliovirus receptor. Cells expressing this chimeric receptor have normal binding affinity for poliovirus, yet the kinetics of virus replication are delayed. Results of virus alteration assays indicate that this chimeric receptor is defective in converting native virus to 135S altered particles. This defect is not observed with cells expressing receptor recombinants that include Pvr domains 1 and 2. Because altered particles are believed to be an intermediate in poliovirus entry, these findings suggest that Pvr domains 2 and 3 participate in early stages of infection. Additional mutants were made by substituting variant Mph residues for the corresponding residues in Pvr. The results were interpreted by using a model of Pvr predicted from the known structures of other immunoglobulin-like V-type domains. Analysis of stable cell lines expressing the mutant proteins revealed that virus binding is influenced by mutations in the predicted C'-C" loop, the C" beta-strand, the C"-D loop, and the D-E loop. Mutations in homologous regions of the immunoglobulin-like CD4 molecule alter its interaction with gp120 of human immunodeficiency virus type 1. Cells expressing Pvr mutations on the predicted C" edge do not develop cytopathic effect during poliovirus infection, suggesting that poliovirus-induced cytopathic effect may be induced by the virus-receptor interaction.

The first step in poliovirus replication is binding of virus to a cellular receptor. Mouse L cells, which are resistant to poliovirus infection because they do not bear a poliovirus receptor, were transformed with HeLa cell (human) DNA to poliovirus sensitivity at a frequency of approximately 1 in 50,000 transformants. Monoclonal antibody directed against the HeLa cell poliovirus receptor site was used in rosette assays to identify poliovirus-sensitive L-cell transformants in a background of L-cell tk+ transformants. A cloned cell line, CM-1, was isolated that displayed a surface component recognized by the anti-poliovirus receptor antibody. CM-1 cells were susceptible to infection with all three poliovirus serotypes, and infection could be blocked by the antireceptor antibody. Poliovirus formed plaques in CM-1 and HeLa cells with equal efficiency. CM-1 and HeLa cells produced infectious poliovirus at a similar rate, although yield of virus in CM-1 cells was about 33% less than the yield in HeLa cells. These results suggest that DNA encoding the HeLa cell poliovirus receptor has been introduced into mouse cells, resulting in the expression of the receptor and susceptibility to poliovirus infection.

Portions of the cellular receptor for poliovirus were expressed in Escherichia coli as fusion proteins with the product of the trpE gene. One of two antireceptor antisera obtained by immunizing rabbits with the fusion proteins blocked poliovirus infection. Western immunoblot analyses demonstrated that poliovirus receptor-related proteins were expressed in HeLa cells and a variety of human tissues, including those that are not sites of poliovirus replication. Tissue-specific variation in electrophoretic mobility, immunoreactivity, and subunit arrangement of poliovirus receptor-related proteins was observed. These results demonstrate that poliovirus tissue tropism cannot be explained by a limited distribution of receptor polypeptide, but may be the result of alternative splicing, posttranslational modifications, or both. In addition, the widespread but heterogeneous expression of the receptor suggests that the protein may have an important endogenous function.

The complete 7410 nucleotide sequence of poliovirus type I genome was obtained from cloned cDNA. Double-stranded poliovirus cDNA was synthesized and inserted into the Pst I site of plasmid pBR322, and three clones were derived that together provided DNA copies of the entire poliovirus genome. Two of the clones contained inserts of 2.5 and 6.5 kilobases and represented all but the 5' 115 bases of poliovirus RNA. A third clone was generated from primer-extended DNA and contained sequences from the 5' end of the viral RNA. An open reading frame that was identified in the nucleotide sequence starting 743 bases from the 5' end of the RNA and extending to a termination codon 71 bases from the 3' end contained known poliovirus polypeptide sequence.

To examine the interaction of the poliovirus receptor (PVR) with virus and the role of the PVR in virus entry, the PVR was expressed in insect cells. Poliovirus bound to insect cells infected with a recombinant baculovirus (AcPVR) carrying cDNA encoding the PVR. Antibodies raised against PVR expressed in bacteria immunoprecipitated a 67-kilodalton polypeptide from cytoplasmic extracts of AcPVR-infected cells. Treatment of AcPVR-infected cells with tunicamycin revealed that the PVR is a glycoprotein containing N-glycosidic linkages and that carbohydrate accounts for nearly 50% of its molecular weight as estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. When PVR was solubilized from AcPVR-infected insect cells and incubated with poliovirus, viral infectivity was neutralized. Sedimentation analysis revealed that irreversibly altered 135S particles were formed after incubation of poliovirus at 37 degrees C with solubilized extracts of AcPVR-infected insect cells. These results demonstrate that poliovirus eclipse may result from interaction with the cell receptor at neutral pH in the absence of membranes and suggest that soluble receptors may be effective antiviral agents against picornaviruses.

It was shown that all eight RNA segments of influenza B viruses are most likely monocistronic and code for eight virus-specific polypeptides. A genetic map of the influenza B virus genome was established, and six polypeptides (P1 protein, nucleoprotein, hemagglutinin, neuraminidase, M protein, and nonstructural protein) were unambiguously assigned to specific RNA segments. Molecular weight estimates of the eight individual genes are obtained by using the glyoxal method. These results suggest that each influenza B virus RNA segment has a greater molecular weight than the influenza A virus RNA segment which codes for the analogous gene product.

The 5' noncoding region of poliovirus RNA contains sequences necessary for translation and replication. These functions are probably carried out by recognition of poliovirus RNA by cellular and/or viral proteins. Using a mobility-shift electrophoresis assay and 1,10-phenanthroline/Cu+ footprinting, we demonstrate specific binding of cytoplasmic factors with a sequence from nucleotides 510-629 within the 5' untranslated region (UTR). Complex formation was also observed with a second sequence (nucleotides 97-182) within the 5' UTR. These two regions of the 5' UTR appear to be recognized by distinct cell factors as determined by competition analysis and the effects of ionic strength on complex formation. However, both complexes contain eukaryotic initiation factor 2 alpha, as revealed by their reaction with specific antibody.

Mutations in the 5' untranslated regions (5'-UTRs) of all three serotypes of the Sabin vaccine strains are known to be major determinants of the attenuation phenotype. To further understand the functional basis of the attenuation phenotype caused by mutations in the 5'-UTR, we studied their effects on viral replication, translation, and the interaction of the viral RNA with cell proteins. A mutation at base 472 (C472U), which attenuates neurovirulence in primates and mice, was previously found to reduce viral replication and translation in neuroblastoma cells but not in HeLa cells. This mutation reduced cross-linking of the poliovirus 5'-UTR to polypyrimidine tract-binding protein (pPTB) in neuroblastoma cells but not in HeLa cells. These defects were absent in a neurovirulent virus with C at nucleotide 472. When C472U and an additional mutation, G482A, were introduced into the 5'-UTR, the resulting virus was more attenuated, had a replication and translation defect in both HeLa cells and neuroblastoma cells, and cross-linked poorly to pPTB from both cell types. A neurovirulent revertant of this virus (carrying U472C, G482A, and C529U) no longer had a replication defect in HeLa and SH-SY5Y cell lines and cross-linked with pPTB to wild-type levels. The results suggest that the attenuating effects of the mutation C472U may result from an impaired interaction of the 5'-UTR with pPTB in neural cells, which reduces viral translation and replication. Introduction of a second mutation, G482A, into the 5'-UTR extends this defect to HeLa cells.

To identify sequences of the cellular poliovirus receptor (PVR) involved in viral infection, mutant PVR cDNAs were constructed and assayed for biological activity in mouse L cells. To confirm that mutant PVRs reached the cell surface, an immunological tag, consisting of part of CH3 from human immunoglobulin G1, was engineered into the PVR. Deletion of PVR amino acids 256 to 320 or 385 to the carboxy terminus yielded receptors that were able to support poliovirus infection. PVRs lacking amino acids 40 to 136 or 137 to 256 were expressed at the cell surface but were not active as receptors for poliovirus. The results show that immunoglobulin-type domain 3 and the extreme carboxy terminus of the PVR are not required for viral receptor function, but sequences within the two amino-terminal domains contribute to the initiation of poliovirus infection.

Cotransfection of poliovirus RNA and R1, a poliovirus subgenomic RNA containing a deletion of nearly all of the capsid region, resulted in surviving cells, in contrast to the complete cell death observed after transfection with viral RNA. Cells that survived the cotransfection grew into colonies, produced infectious poliovirus, and underwent cycles of cell lysis (crisis periods) where less than 1% of the cells survived, followed by periods of growth. Poliovirus evolved during the persistent infection as judged by changes in plaque size. After passage for 6 months, a stable line called SOFIA emerged that no longer produced infectious virus and did not contain viral proteins or viral RNA. Cells frozen in liquid N2 while still in crisis and recultured 4 months later (named SOFIA N2) were also stabilized. After infection with poliovirus, SOFIA N2 cells showed a delay in the development of cytopathic effect, viral production, and cellular death when compared with HeLa cells. In contrast, SOFIA cells did not develop cytopathic effect and produced 10,000 times less virus than SOFIA N2 or HeLa cells. Viral production was delayed in SOFIA and SOFIA N2 cells transfected with poliovirus RNA when compared with HeLa cells, suggesting the presence of an intracellular block to poliovirus replication. Analysis of the cellular receptor for poliovirus by virus binding, an enzyme-linked immunosorbent assay, and in situ rosette assays with an antireceptor monoclonal antibody showed that receptors were expressed in SOFIA N2 cells but not in SOFIA cells. Echovirus 6, an enterovirus which uses a different cellular receptor, formed small plaques on SOFIA cells. Vesicular stomatitis virus formed plaques of similar size on SOFIA and HeLa cells, suggesting that the intracellular block was specific for enteroviruses. Cotransfection of the subgenomic replicon R1 with poliovirion RNA therefore resulted in the selection of HeLa cell variants containing blocks to poliovirus replication at the level of receptor and within the cell.

Replication of the infectious RNA genome of poliovirus is accomplished in cells by the viral RNA polymerase through negative-strand RNA intermediates. Full-length negative-strand poliovirus RNA was synthesized in vitro by transcription of infectious poliovirus cDNA with bacteriophage SP6 DNA-dependent RNA polymerase. When provided with this negative-strand RNA as template, the poliovirus RNA-dependent RNA polymerase synthesized full-length positive-strand molecules. The positive-strand RNAs synthesized in vitro were infectious when transfected into HeLa cells. In contrast, positive-strand copies of poliovirus RNA synthesized in vitro by SP6 polymerase, using a poliovirus cDNA template, were not infectious. Production of infectious positive-strand RNA by the poliovirus polymerase was not observed when magnesium or negative-strand RNA template was omitted from the reaction mixture. Infectivity of the product RNA was not destroyed by DNase treatment. The specific infectivity in HeLa cells of in vitro-synthesized positive-strand RNA was 4 X 10(4) plaque-forming units/micrograms of RNA.

Although the initial site of poliovirus replication in humans is the intestine, previously isolated transgenic mice which carry the human poliovirus receptor (PVR) gene (TgPVR mice), which develop poliomyelitis after intracerebral inoculation, are not susceptible to infection by the oral route. The low levels of PVR expressed in the TgPVR mouse intestine might explain the absence of poliovirus replication at that site. To ascertain whether PVR is the sole determinant of poliovirus susceptibility of the mouse intestine, we have generated transgenic mice by using the promoter for rat intestine fatty acid binding protein to direct PVR expression in mouse gut. Pvr was detected by immunohistochemistry in the enterocytes and M cells of transgenic mouse (TgFABP-PVR) small intestine. Upon oral inoculation with poliovirus, no increase in virus titer was detected in the feces of TgFABP-PVR mice, and no virus replication was observed in the small intestine, although poliovirus replicated in the brain after intracerebral inoculation. The failure of poliovirus to replicate in the TgFABP-PVR mouse small intestine was not due to lack of virus binding sites, because poliovirus could attach to fragments of small intestine from these mice. These results indicate that the inability of poliovirus to replicate in the mouse alimentary tract is not solely due to the absence of virus receptor, and other factors are involved in determining the ability of poliovirus to replicate in the mouse gut.

In the current model of poliovirus entry, the initial interaction of the native virion with its cellular receptor is followed by a transition to an altered form, which then acts as an intermediate in viral entry. While the native virion sediments at 160S in a sucrose gradient, the altered particle sediments at 135S, has lost the coat protein VP4, and has become more hydrophobic. Altered particles can be found both associated with cells and in the culture medium. It has been hypothesized that the cell-associated 135S particle releases the viral genome into the cell cytoplasm and that nonproductive transitions to the 135S form are responsible for the high particle-to-PFU ratio observed for polioviruses. At 25 degrees C, a temperature at which the transition to 135S particles does not occur, the P1/Mahoney strain of poliovirus was unable to replicate, and cold-adapted (ca) mutants were selected from the population. These mutants have not gained the ability to convert to 135S particles at 25 degrees C, and the block to wild-type (wt) infection at low temperatures is not at the level of cellular entry. The particle-to-PFU ratio of poliovirus does not change at 25 degrees C in the absence of alteration. Three independent amino acid changes in the 2C coding region were identified in ca mutants, at positions 218 (Val to Ile), 241 (Arg to Ala), and 309 (Met to Val). Introduction of any of these mutations individually into wt poliovirus by site-directed mutagenesis confers the ca phenotype. All three serotypes of the Sabin vaccine strains and the P3/Leon strain of poliovirus also exhibit the ca phenotype. These results do not support a model of poliovirus entry into cells that includes an obligatory transition to the 135S particle.

To identify determinants of attenuation in the poliovirus type 1 Sabin vaccine strain, a series of recombinant viruses were constructed by using infectious cDNA clones of the virulent type 1 poliovirus P1/Mahoney and the attenuated type 1 vaccine strain P1/Sabin. Intracerebral inoculation of these viruses into transgenic mice which express the human receptor for poliovirus identified regions of the genome that conferred reduced neurovirulence. Exchange of smaller restriction fragments and site-directed mutagenesis were used to identify the nucleotide changes responsible for attenuation. P1/Sabin mutations at nucleotides 935 of VP4, 2438 of VP3, and 2795 and 2879 of VP1 were all shown to be determinants of attenuation. The recombinant viruses and site-directed mutants were also used to identify the nucleotide changes which are involved in the temperature sensitivity of P1/Sabin. Determinants of this phenotype in HeLa cells were mapped to changes at nucleotides 935 of VP4, 2438 of VP3, and 2741 of VP1. The 3Dpol gene of P1/Sabin, which contains three amino acid differences from its parent P1/Mahoney, also contributes to the temperature sensitivity of P1/Sabin; however, mutants containing individual amino acid changes grew as well as P1/Mahoney at elevated temperatures, suggesting that either some combination or all three changes are required for temperature sensitivity. In addition, the 3'-noncoding region of P1/Sabin augments the temperature-sensitive phenotype conferred by 3Dpol. Although nucleotide 2741, 3Dpol, and the 3'-noncoding region of P1/Sabin contribute to the temperature sensitivity of P1/Sabin, they do not contribute to attenuation in transgenic mice expressing the poliovirus receptor, demonstrating that determinants of attenuation and temperature sensitivity can be genetically separated.

Mutations in the predicted C'-C"-D edge of the first immunoglobulin-like domain of the poliovirus receptor were previously shown to eliminate poliovirus binding. To identify capsid residues that expand receptor recognition, 16 poliovirus suppressor mutants were selected that replicate in three different mutant receptor-expressing cell lines as well as in cells expressing the wild-type receptor. Sequence analysis of the mutant viruses revealed three capsid residues that enable poliovirus to utilize defective receptors. Two residues are in regions of the capsid that are known to regulate receptor binding and receptor-mediated conformational transitions. A third residue is located in a highly exposed loop on the virion surface that controls poliovirus host range in mice by influencing receptor recognition. One of the suppressor mutations enables the primate-restricted P1/Mahoney strain to paralyze mice by enabling the virus to recognize a receptor in the mouse central nervous system. Capsid mutations that suppress receptor defects may exert their effect at the binding site or may improve receptor binding by regulating structural transitions of the capsid.

A monoclonal antibody, AF3, was previously shown to specifically inhibit poliovirus binding to HeLa cells and to detect a 100-kDa glycoprotein only in cell lines and tissues permissive for poliovirus infection. These results suggested that the 100-kDa protein may be involved in the pathogenesis of poliomyelitis and the cellular function of the poliovirus receptor site. To study further the role of the 100-kDa protein in poliovirus attachment, immunoaffinity purification, amino acid sequencing, and cDNA cloning were undertaken. The results demonstrate that antibody AF3 reacts with the lymphocyte homing receptor CD44, a multifunctional cell surface glycoprotein involved in the homing of circulating lymphocytes to lymph nodes and the modulation of lymphocyte adhesion and activation. Antibody AF3 reacts with a subset of CD44 molecules (AF3CD44H), which appears to be a small fraction of the heterogeneously glycosylated CD44 molecules expressed on hematopoietic and nonhematopoietic cells. Anti-CD44 monoclonal antibodies, previously reported to induce CD44-mediated modulation of lymphocyte activation and adhesion, compete with 125I-AF3 in binding assays, demonstrating functional overlap among the epitopes. The anti-CD44 monoclonal antibody A3D8, which binds to a greater molecular weight range of CD44 than does AF3, inhibits poliovirus binding to a similar degree. CD44 does not act as a poliovirus receptor, since CD44-expressing mouse L-cell transformants did not bind poliovirus. The poliovirus receptor and AF3CD44H may be noncovalently associated, or they may interact through the cytoskeleton or signal transduction pathways.

A base change from C to U at position 472 of the 5' noncoding region of the poliovirus genome is known to be a major determinant of attenuation in the P3/Sabin vaccine strain. To determine the biochemical basis for the attenuated phenotype imparted by this mutation, a cell line in which replication of neurovirulent and attenuated viruses could be distinguished was identified. A pair of P3/Sabin-P2/Lansing viral recombinants that differ only at position 472 was used; the viruses replicated equally well in HeLa cells, but the virus with a U at base 472 was attenuated in mice. In the human neuroblastoma cell line SH-SY5Y, recombinants with a U at base 472 replicated to approximately 10-fold-lower titers than did neurovirulent viruses with a C at this position. Analysis of viral RNA and protein synthesis indicated that translation of the attenuated viral RNA was specifically reduced in SH-SY5Y cells.

A mouse model for poliomyelitis was used to identify genomic sequences that attenuate neurovirulence of poliovirus strain P2/P712. This type 2 strain is avirulent in primates and mice yet grows as well as virulent strains in cell culture. The approach used was to exchange portions of the genome of the mouse-virulent P2/Lansing strain with the corresponding region from P2/P712 to identify sequences that could attenuate Lansing neurovirulence in mice. A full-length infectious cDNA of P2/P712 was assembled and used to construct recombinants between P2/P712 and P2/Lansing. The results of neurovirulence testing of 11 recombinants indicated that strong attenuating determinants are located in the 5' noncoding region of P2/P712 and a region encoding capsid protein VP1 and 2Apro, 2B, and part of 2C. An attenuating determinant was further localized to between nucleotides 456 and 628 of P2/P712. A third sequence from P2/P712, nucleotides 752 to 2268, encoding VP4, VP2, and part of VP3, was weakly attenuating. The sequence from nucleotide 4454, approximately halfway through the 2C-coding region, to the end of the P2/P712 genome did not contain attenuating determinants. Nucleotide sequence analysis revealed that P2/P712 differs from the type 2 Sabin vaccine strain by only 22 nucleotides. Six differences lead to amino acid changes in the coding region, and four differences are in the 5' noncoding region. These studies show that, like the type 1 and type 3 Sabin vaccine strains, the attenuated type 2 strain P712 contains multiple attenuating sequences, including strongly attenuating sequences in the 5' noncoding region of the genome.

A mutation in the genome of poliovirus type 3 that is known to reduce neurovirulence in humans similarly reduces neurovirulence in mice when incorporated into a mouse-adapted-human poliovirus recombinant. Viral recombinants with a uracil at nucleotide position 472 in the 5'-noncoding regions of their genomes are unable to replicate in the mouse brain. Viral recombinants with a cytosine at this position are neurovirulent in mice. Neurovirulence of poliovirus in mice may therefore prove to be a useful indicator of the genetic stability of new attenuating mutations created by site-directed mutagenesis.

Four RNA fragments of approximately 1,000 to 1,200 nucleotides, representing both the 5' and 3' termini of poliovirus plus- and minus-strand RNAs, were generated by transcription of poliovirus cDNA by using bacteriophage SP6 RNA polymerase. The copying of these templates by the poliovirus replicase invariably produced RNA products approximately twice the size of the templates. In experiments with templates uniformly labeled with 32P it was shown that some of the apparently double-length products were generated by extension from an internal site of the template. Filter hybridization of the labeled in vitro-synthesized products with various unlabeled templates suggested a second mechanism by which double-length molecules could be synthesized; the results can be best explained by de novo synthesis of the first strand by copying of the template RNA, followed by snap-back of the newly synthesized RNA, generating a template-primer structure for the synthesis of the second strand. Highly purified poliovirus replicase was able to support the synthesis of double-length RNA products in response to these templates. These reactions did not require host factor. In contrast, synthesis of genome-length copies of poliovirion RNA by the same replicase was absolutely dependent on added host factor. The synthesis of double-length RNA products did not require either the 3'-terminal poly(A) of plus RNA or sequences within the 3' termini of both plus- and minus-strand RNAs.

Intracerebral inoculation of mice with poliovirus type 2 Lansing induces a fatal paralysis, while most other poliovirus strains are unable to cause disease in the mouse. To determine the molecular basis for Lansing virus neurovirulence, we determined the complete nucleotide sequence of the Lansing viral genome from cloned cDNA. The deduced amino acid sequence was compared with that of two mouse-avirulent strains. There are 83 amino acid differences between the Lansing and Sabin type 2 strain and 179 differences between the Lansing and Mahoney type 1 strain scattered throughout the genome. To further localize Lansing sequences important for mouse neurovirulence, four intertypic recombinants were isolated by exchanging DNA restriction fragments between the Lansing 2 and Mahoney 1 infectious poliovirus cDNA clones. Plasmids were transfected into HeLa cells, and infectious recombinant viruses were recovered. All four recombinant viruses, which contained the Lansing capsid region and different amounts of the Mahoney genome, were neurovirulent for 18- to 21-day-old Swiss-Webster mice by the intracerebral route. The genome of neurovirulent recombinant PRV5.1 contained only nucleotides 631 to 3413 from Lansing, encoding primarily the viral capsid proteins. Therefore, the ability of Lansing virus to cause paralysis in mice is due to the viral capsid. The Lansing capsid sequence differs from that of the mouse avirulent Sabin 2 strain at 32 of 879 amino acid positions: 1 in VP4, 5 in VP2, 4 in VP3, and 22 in VP1.

Expression of the human poliovirus receptor (PVR) in transgenic mice results in susceptibility to poliovirus infection. In the primate host, poliovirus infection is characterized by restricted tissue tropism. To determine the pattern of poliovirus tissue tropism in PVR transgenic mice, PVR gene expression and susceptibility to poliovirus infection were examined by in situ hybridization. PVR RNA is expressed in transgenic mice at high levels in neurons of the central and peripheral nervous system, developing T lymphocytes in the thymus, epithelial cells of Bowman's capsule and tubules in the kidney, alveolar cells in the lung, and endocrine cells in the adrenal cortex, and it is expressed at low levels in intestine, spleen, and skeletal muscle. After infection, poliovirus replication was detected only in neurons of the brain and spinal cord and in skeletal muscle. These results demonstrated that poliovirus tissue tropism is not governed solely by expression of the PVR gene nor by accessibility of cells to virus. Although transgenic mouse kidney tissue expressed poliovirus binding sites and was not a site of poliovirus replication, when cultivated in vitro, kidney cells developed susceptibility to infection. Identification of the changes in cultured kidney cells that permit poliovirus infection may provide information on the mechanism of poliovirus tissue tropism.

A line of HeLa cells (SOFIA) was previously isolated that is resistant to poliovirus infection and does not express functional virus binding sites at the cell surface. The expression of the poliovirus receptor (PVR) gene in SOFIA cells was examined to determine the molecular basis for the failure of these cells to express PVRs. Southern blot analysis of genomic DNA revealed that the PVR gene in SOFIA cells did not contain gross alterations. However, PVR transcripts were not detected in Northern (RNA) blot analysis of SOFIA cell RNA. In vitro nuclear run-on analysis showed that transcription of PVR-specific RNA was reduced in SOFIA cells. Treatment of SOFIA cells with 5-azacytidine restored susceptibility to poliovirus infection, which correlated with the appearance of PVRs at the cell surface, as detected with anti-PVR monoclonal antibody D171. PVR RNA was detected in clones derived from 5-azacytidine-treated SOFIA cells. SOFIA cells were converted to poliovirus sensitivity at a rate of 5 to 7%, suggesting that down regulation of PVR expression involved few cellular targets. Resistance of SOFIA cells to poliovirus infection therefore appears to result from down regulation of PVR RNA, leading to lack of PVR expression at the cell surface. Methylation may play a role in regulating the expression of the PVR gene, which is not essential for survival of HeLa cells.

The poliovirus P2/P712 strain is an attenuated virus that is closely related to the type 2 Sabin vaccine strain. By using a mouse model for poliomyelitis, sequences responsible for attenuation of the P2/P712 strain were previously mapped to the 5' noncoding region of the genome and a central region encoding VP1, 2Apro, 2B, and part of 2C. To identify specific determinants that attenuate the P2/P712 strain, recombinants between this virus and the mouse-adapted P2/Lansing were constructed and their neurovirulence in mice was determined. By using this approach, the attenuation determinant in the central region was mapped to capsid protein VP1. Candidate attenuating sequences in VP1 and the 5' noncoding region were identified by comparing the P2/P712 sequence with that of vaccine-associated isolate P2/P117, and the P2/117 sequences were introduced into the P2/Lansing-P2/P712 recombinants by site-directed mutagenesis. Results of neurovirulence assays in mice indicate that an A at nucleotide 481 in the 5' noncoding region and isoleucine (Ile) at position 143 of capsid protein VP1 are the major determinants of attenuation of P2/P712. These determinants also attenuated neurovirulence in transgenic mice expressing human poliovirus receptors, a new model for poliomyelitis in which virulent viruses are not host restricted. These results demonstrate that A-481 and Ile-143 are general determinants of attenuation.

A poliovirus type 2 Lansing mutant was constructed by inserting 6 base pairs into the 2Apro region of an infectious cDNA clone, resulting in the addition of a leucine and threonine into the polypeptide sequence. The resulting small-plaque mutant, 2A-2, had a reduced viral yield in HeLa cells and synthesized viral proteins inefficiently. Infection with the mutant did not lead to specific inhibition of host cell protein synthesis early in infection, and this defect was attributed to a failure to induce cleavage of the cap-binding complex protein p220. At late times after infection with the mutant virus, both cellular and viral protein syntheses were severely inhibited. To explain this global inhibition of protein synthesis, the phosphorylation state of the alpha subunit of eucaryotic initiation factor 2 (eIF-2 alpha) was examined. eIF-2 alpha was phosphorylated in both R2-2A-2- and wild-type-virus-infected cells, indicating that poliovirus does not encode a function that blocks phosphorylation of eIF-2 alpha. The kinetics and extent of eIF-2 alpha phosphorylation correlated with the production of double-stranded RNA in infected cells, suggesting that eIF-2 alpha is phosphorylated by P1/eIF-2 alpha kinase. When HeLa cells were infected with R2-2A-2 in the presence of 2-aminopurine, a protein kinase inhibitor, much higher virus titers were produced, cleavage of p220 occurred, and host cell protein synthesis was specifically inhibited. Since phosphorylation of eIF-2 alpha was not inhibited by 2-aminopurine, we propose that 2-aminopurine rescues the ability of R2-2A-2 to induce cleavage of p220 by inhibition of a second as yet unidentified kinase.

Poliovirus mRNA contains a long 5' noncoding region of about 750 nucleotides (the exact number varies among the three virus serotypes), which contains several AUG codons upstream of the major initiator AUG. Unlike most eucaryotic mRNAs, poliovirus does not contain a m7GpppX (where X is any nucleotide) cap structure at its 5' end and is translated by a cap-independent mechanism. To study the manner by which poliovirus mRNA is expressed, we examined the translational efficiencies of a series of deletion mutants within the 5' noncoding region of the mRNA. In this paper we report striking translation system-specific differences in the ability of the altered mRNAs to be translated. The results suggest the existence of an inhibitory cis-acting element(s) within the 5' noncoding region of poliovirus (between nucleotides 70 and 381) which restricts mRNA translation in reticulocyte lysate, wheat germ extract, and Xenopus oocytes, but not in HeLa cell extracts. In addition, we show that HeLa cell extracts contain a trans-acting factor(s) that overcomes this restriction.

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.

Recombinants between two different influenza C viruses were isolated. In MDCK (canine kidney) cells, one strain, C/JJ/50, caused lytic plaques, whereas C/JHG/66 virus did not produce clear plaques. From a mixed infection of MDCK cells with C/JHG/66 virus and UV-inactivated C/JJ/50 virus, clones were isolated which possessed the clear-plaque phenotype. Fingerprint analyses indicated that the RNAs of parent viruses had different oligonucleotide patterns and that one of the clones derived from the mixed infection was formed by reassortment of parental genes. This recombinant clone most likely inherited RNAs 1, 2, 3, 6, and 7 from C/JGH/66 virus and RNAs 4 and 5 from C/JJ/50 virus.

Double-stranded cDNA was synthesized from hepatitis A virus (HAV) RNA and inserted into the Pst I site of pBR322. Restriction endonuclease digestion and cross-hybridization of fragments yielded a map of overlapping cloned cDNAs that included at least 99% of the viral genome. Molecular clones containing HAV cDNA were identified by hybridizing cloned cDNA to electrophoretically resolved RNA from uninfected and HAV-infected tissue culture cells. Cloned cDNA probes specifically hybridized to RNA from infected cells, and the predominant species identified had the characteristic genomic length of picornaviral RNA (approximately equal to 7,500 nucleotides). A partial sequence from the 3' end of the genome revealed 414 bases in an open reading frame followed by two closely spaced stop codons, a 60-base noncoding region, and a tract of poly(A).