Use of type 1/type 2 chimeric polioviruses to study determinants of poliovirus type 1 neurovirulence in a mouse model

The Picornaviridae Family: Knowledge Gaps, Animal Models, Countermeasures, and Prototype Pathogens

Picornaviruses are nonenveloped particles with a single-stranded RNA genome of positive polarity. This virus family includes poliovirus, hepatitis A virus, rhinoviruses, and Coxsackieviruses. Picornaviruses are common human pathogens, and infection can result in a spectrum of serious illnesses, including acute flaccid myelitis, severe respiratory complications, and hand-foot-mouth disease. Despite research on poliovirus establishing many fundamental principles of RNA virus biology and the first transgenic animal model of disease for infection by a human virus, picornaviruses are understudied. Existing knowledge gaps include, identification of molecules required for virus entry, understanding cellular and humoral immune responses elicited during virus infection, and establishment of immune-competent animal models of virus pathogenesis. Such knowledge is necessary for development of pan-picornavirus countermeasures. Defining enterovirus A71 and D68, human rhinovirus C, and echoviruses 29 as prototype pathogens of this virus family may provide insight into picornavirus biology needed to establish public health strategies necessary for pandemic preparedness.

Oral poliovirus vaccine evolution and insights relevant to modeling the risks of circulating vaccine-derived polioviruses (cVDPVs)

The live, attenuated oral poliovirus vaccine (OPV) provides a powerful tool for controlling and stopping the transmission of wild polioviruses (WPVs), although the risks of vaccine-associated paralytic polio (VAPP) and circulating vaccine-derived poliovirus (cVDPV) outbreaks exist as long as OPV remains in use. Understanding the dynamics of cVDPV emergence and outbreaks as a function of population immunity and other risk factors may help to improve risk management and the development of strategies to respond to possible outbreaks. We performed a comprehensive review of the literature related to the process of OPV evolution and information available from actual experiences with cVDPV outbreaks. Only a relatively small fraction of poliovirus infections cause symptoms, which makes direct observation of the trajectory of OPV evolution within a population impractical and leads to significant uncertainty. Despite a large global surveillance system, the existing genetic sequence data largely provide information about transmitted virulent polioviruses that caused acute flaccid paralysis, and essentially no data track the changes that occur in OPV sequences as the viruses transmit largely asymptomatically through real populations with suboptimal immunity. We updated estimates of cVDPV risks based on actual experiences and identified the many limitations in the existing data on poliovirus transmission and immunity and OPV virus evolution that complicate modeling. Modelers should explore the space of potential model formulations and inputs consistent with the available evidence and future studies should seek to improve our understanding of the OPV virus evolution process to provide better information for policymakers working to manage cVDPV risks.

Mechanisms controlling virulence thresholds of mixed viral populations

The propensity of RNA viruses to revert attenuating mutations contributes to disease and complicates vaccine development. Despite the presence of virulent revertant viruses in some live-attenuated vaccines, disease from vaccination is rare. This suggests that in mixed viral populations, attenuated viruses may limit the pathogenesis of virulent viruses, thus establishing a virulence threshold. Here we examined virulence thresholds using mixtures of virulent and attenuated viruses in a transgenic mouse model of poliovirus infection. We determined that a 1,000-fold excess of the attenuated Sabin strain of poliovirus was protective against disease induced by the virulent Mahoney strain. Protection was induced locally, and inactivated virus conferred protection. Treatment with a poliovirus receptor-blocking antibody phenocopied the protective effect of inactivated viruses in vitro and in vivo, suggesting that one mechanism controlling virulence thresholds may be competition for a viral receptor. Additionally, the type I interferon response reduces poliovirus pathogenesis; therefore, we examined virulence thresholds in mice lacking the alpha/beta interferon receptor. We found that the attenuated virus was virulent in immunodeficient mice due to the enhanced replication and reversion of attenuating mutations. Therefore, while the type I interferon response limits the virulence of the attenuated strain by reducing replication, protection from disease conferred by the attenuated strain in immunocompetent mice can occur independently of replication. Our results identified mechanisms controlling the virulence of mixed viral populations and indicate that live-attenuated vaccines containing virulent virus may be safe, as long as virulent viruses are present at levels below a critical threshold.

Growth kinetic analysis of bi-recombinant poliovirus vaccine strains

Attenuated strains of Sabin poliovirus vaccine replicate in the human gut and in rare cases may cause vaccine-associated paralytic poliomyelitis (VAPP). Mutations at specific sites of the genome and recombination between Sabin strains may result in the loss of the attenuated phenotype of OPV (Oral Poliovirus Vaccine) strains and the acquisition of traits characteristic of wild polioviruses, such as increased neurovirulence and loss of temperature sensitivity. In this study, we determined the phenotypic traits such as temperature sensitivity and growth kinetics of eight OPV isolates (six bi-recombinant and two non-recombinant). The growth phenotype of each isolate as well as of Sabin vaccine strains in Hep2 cell line at two different temperatures (37 and 40 degrees C) was evaluated using two different assays, RCT test (Reproductive Capacity at different Temperatures) and one-step growth curve analysis. Moreover, the nucleotide and amino acid positions in the genomes of the isolates that have been identified as being involved in the attenuated and thermo sensitive phenotype of Sabin vaccine strains were investigated. Mutations that result in loss of the attenuated and thermo sensitive phenotype of Sabin vaccine strains were identified in the genomes of all isolates. Both mutations and recombination events correlated well with the reverted phenotypic traits of OPV-derivatives. In the post-eradication era of wild polioviruses, the identification and the characterization (genomic and phenotypic) of vaccine-derived polioviruses become increasingly important in order to prevent cases or even outbreaks of paralytic poliomyelitis caused by neurovirulent strains.

The molecular basis of mouse adaptation by human enterovirus 71

A mouse-adapted strain of human enterovirus 71 (HEV71) was selected by serial passage of a HEV71 clinical isolate (HEV71-26M) in Chinese hamster ovary (CHO) cells (CHO-26M) and in newborn BALB/c mice (MP-26M). Despite improved growth in CHO cells, CHO-26M did not show increased virulence in newborn BALB/c mice compared with HEV71-26M. By contrast, infection of newborn mice with MP-26M resulted in severe disease of high mortality. Skeletal muscle was the primary site of replication in mice for both viruses. However, MP-26M infection induced severe necrotizing myositis, whereas CHO-26M infection caused only mild inflammation. MP-26M was also isolated from whole blood, heart, liver, spleen and brain of infected mice. CHO-26M harboured a single mutation within the open reading frame (ORF), resulting in an amino acid substitution of K149→I in the VP2 capsid protein; two further ORF mutations that resulted in amino acid substitutions were identified in MP-26M, located within the VP1 capsid protein (G145→E) and the 2C protein (K216→R). Infectious cDNA clone-derived mutant virus populations containing the mutations identified in CHO-26M and MP-26M were generated in order to study the molecular basis of CHO cell and mouse adaptation. The VP2 (K149→I) change was responsible only for improved growth in CHO cells and did not lead to increased virulence in mice. Of the two amino acid substitutions identified in MP-26M, the VP1 (G145→E) mutation alone was sufficient to increase virulence in mice to the level observed in MP-26M-infected mice.

Cooperative effect of the attenuation determinants derived from poliovirus sabin 1 strain is essential for attenuation of enterovirus 71 in the NOD/SCID mouse infection model

Enterovirus 71 (EV71) is a causative agent of hand, foot, and mouth disease and is also associated with serious neurological disorders. An attenuated EV71 strain [EV71(S1-3')] has been established in the cynomolgus monkey infection model; this strain contains the attenuation determinants derived from the type 1 poliovirus vaccine strain, Sabin 1 [PV1(Sabin)], in the 5' nontranslated region (NTR), 3D polymerase, and 3' NTR. In this study, we analyzed the effect of the attenuation determinants of PV1(Sabin) on EV71 infection in a NOD/SCID mouse infection model. We isolated a mouse-adapted EV71 strain [EV71(NOD/SCID)] that causes paralysis of the hind limbs in 3- to 4-week-old NOD/SCID mice by adaptation of the virulent EV71(Nagoya) strain in the brains of NOD/SCID mice. A single mutation at nucleotide 2876 that caused an amino acid change in capsid protein VP1 (change of the glycine at position 145 to glutamic acid) was essential for the mouse-adapted phenotype in NOD/SCID mice. Next, we introduced attenuation determinants derived from PV1(Sabin) along with the mouse adaptation mutation into the EV71(Nagoya) genome. In 4-week-old mice, the determinants in the 3D polymerase and 3' NTR, which are the major temperature-sensitive determinants, had a strong effect on attenuation. In contrast, the effect of individual determinants was weak in 3-week-old NOD/SCID mice, and all the determinants were required for substantial attenuation. These results suggest that a cooperative effect of the attenuation determinants of PV1(Sabin) is essential for attenuated neurovirulence of EV71.

Effects of Vaccine Strain Mutations in Domain V of the Internal Ribosome Entry Segment Compared in the Wild Type Poliovirus Type 1 Context

Initiation of poliovirus (PV) protein synthesis is governed by an internal ribosome entry segment structured into several domains including domain V, which is accepted to be important in PV neurovirulence because it harbors an attenuating mutation in each of the vaccine strains developed by A. Sabin. To better understand how these single point mutations exert their effects, we placed each of them into the same genomic context, that of PV type 1. Only the mutation equivalent to the Sabin type 3 strain mutation resulted in significantly reduced viral growth both in HeLa and neuroblastoma cells. This correlated with poor translation efficiency in vitro and could be explained by a structural perturbation of the domain V of the internal ribosome entry segment, as evidenced by RNA melting experiments. We demonstrated that reduced cell death observed during infection by this mutant is due to the absence of inhibition of host cell translation. We confirmed that this shut-off is correlated principally with cleavage of eIF4GII and not eIF4GI and that this cleavage is significantly impaired in the case of the defective mutant. These data support the previously reported conclusion that the 2A protease has markedly different affinities for the two eIF4G isoforms.

Poliovirus cell entry: common structural themes in viral cell entry pathways

Structural studies of polio- and closely related viruses have provided a series of snapshots along their cell entry pathways. Based on the structures and related kinetic, biochemical, and genetic studies, we have proposed a model for the cell entry pathway for polio- and closely related viruses. In this model a maturation cleavage of a capsid protein precursor locks the virus in a metastable state, and the receptor acts like a transition-state catalyst to overcome an energy barrier and release the mature virion from the metastable state. This initiates a series of conformational changes that allow the virus to attach to membranes, form a pore, and finally release its RNA genome into the cytoplasm. This model has striking parallels with emerging models for the maturation and cell entry of more complex enveloped viruses such as influenza virus and HIV.

Molecular genetic analysis of revertants from a poliovirus mutant that is specifically adapted to the mouse spinal cord

SA virus, a mutant of the Mahoney strain of type 1 poliovirus (PV1/Mahoney), replicates specifically in the spinal cords of mice and causes paralysis, although the PV1/Mahoney strain does not show any mouse neurovirulence (Q. Jia, S. Ohka, K. Iwasaki, K. Tohyama, and A. Nomoto, J. Virol. 73:6041-6047, 1999). The key mutation site for the mouse neurovirulence of SA was mapped to nucleotide (nt) 928 of the genome (A to G), resulting in the amino acid substitution of Met for Ile at residue 62 within the capsid protein VP4 (VP4062). A small-plaque phenotype of SA appears to be indicative of its mouse-neurovirulent phenotype. To identify additional amino acid residues involved in the host range determination of PV, a total of 14 large-plaque (LP) variants were isolated from a single point mutant, Mah/I4062M, that showed the SA phenotype. All the LP variants no longer showed any mouse neurovirulence when delivered via an intraspinal inoculation route. Of these, 11 isolates had a back mutation at nt 928 (G to A) that restored the nucleotide of the PV1/Mahoney type. The reversions of the remaining three isolates (LP8, LP9, and LP14) were mediated by a second site mutation. Molecular genetic analysis involving recombinants between Mah/I4062M and the LP variants revealed that the mere substitution of an amino acid residue at position 107 in VP1 (Val to Leu) (LP9), position 33 in VP2 (Val to Ile) (LP14), or position 231 in VP3 (Ile to Thr) (LP8) was sufficient to restore the PV1/Mahoney phenotype. These amino acid residues are located either on the surface or inside of the virus particle. Our results indicate that the mouse neurovirulence of PV is determined by the virion surface structure, which is formed by all four capsid proteins.

Isolation and molecular characterization of a poliovirus type 1 mutant that replicates in the spinal cords of mice

The Mahoney strain of poliovirus type 1 (OM) is generally unable to cause paralysis in mice. We isolated a mouse-adapted mutant, PV1/OM-SA (SA), from the spinal cord of a mouse that had been intracerebrally inoculated with OM. SA showed mouse neurovirulence only with intraspinal inoculation, and the infected mice developed a flaccid paralysis, which was indistinguishable from that observed in poliovirus-sensitive transgenic mice inoculated with OM. SA antigens were detected in neurons of the spinal cords of the infected mice. Nucleotide (nt) sequence analysis revealed 9 nt changes on the SA genome, resulting in three amino acid (a.a.) substitutions, i.e., one each in the capsid proteins VP4 and VP1 and in the noncapsid protein 2C. To identify the key mutation site(s) for the mouse neurovirulence, virus recombinants between OM and SA were constructed by using infectious cDNA clones of these two viruses and tested for their mouse neurovirulence after inoculation via an intraspinal route. The results indicated that a mutation at nt 928 (replacement of A with G), resulting in a substitution of Met for Ile at a.a. 62 within VP4, was responsible for conferring the mouse neurovirulence phenotype of the mutant SA. The mutation in VP4 may render the virus accessible to a molecule that acts as a virus receptor and is located on the surfaces of neurons of the mouse spinal cord. This molecule appears not to be expressed in the mouse brain.

Mapping the genetic determinants of pathogenicity and plaque phenotype in swine vesicular disease virus

A series of recombinant viruses were constructed using infectious cDNA clones of the virulent J1'73 (large plaque phenotype) and the avirulent H/3'76 (small plaque phenotype) strains of swine vesicular disease virus to identify the genetic determinants of pathogenicity and plaque phenotype. Both traits could be mapped to the region between nucleotides (nt) 2233 and 3368 corresponding to the C terminus of VP3, the whole of VP1, and the N terminus of 2A. In this region, there are eight nucleotide differences leading to amino acid changes between the J1'73 and the H/3'76 strains. Site-directed mutagenesis of individual nucleotides from the virulent to the avirulent genotype and vice versa indicated that A at nt 2832, encoding glycine at VP1-132, and G at nt 3355, encoding arginine at 2APRO-20, correlated with a large-plaque phenotype and virulence in pigs, irrespective of the origin of the remainder of the genome. Of these two sites, 2APRO-20 appeared to be the dominant determinant for the large-plaque phenotype but further studies are required to elucidate their relative importance for virulence in pigs.

Evolution of the Sabin type 1 poliovirus in humans: characterization of strains isolated from patients with vaccine-associated paralytic poliomyelitis

Attenuated strains of the Sabin oral poliovirus vaccine replicate in the human gut and in rare cases cause vaccine-associated paralytic poliomyelitis (VAPP). Reversion of vaccine strains toward a pathogenic phenotype is probably one of the main causes of VAPP, a disease most frequently associated with type 3 and type 2 strains and more rarely with the type 1 (Sabin 1) strain. To identify the determinants and mechanisms of safety versus pathogenicity of the Sabin 1 strain, we characterized the genetic and phenotypic changes in six Sabin 1-derived viruses isolated from immunocompetent patients with VAPP. The genomes of these strains carried either few or numerous mutations from the original Sabin 1 genome. As assessed in transgenic mice carrying the human poliovirus receptor (PVR-Tg mice), all but one strain had lost the attenuated phenotype. Four strains presented only a moderate neurovirulent phenotype, probably due at least in part to reversions to the wild-type genotype, which were detected in the 5' noncoding region of the genome. The reversions found in most strains at nucleotide position 480, are known to be associated with an increase in neurovirulence. The construction and characterization of Sabin 1 mutants implicated a reversion at position 189, found in one strain, in the phenotypic change. The presence of 71 mutations in one neurovirulent strain suggests that a vaccine-derived strain can survive for a long time in humans. Surprisingly, none of the strains analyzed were as neurovirulent to PVR-Tg mice as was the wild-type parent of Sabin 1 (Mahoney) or a previously identified neurovirulent Sabin 1 mutant selected at a high temperature in cultured cells. Thus, in the human gut, the Sabin 1 strain does not necessarily evolve toward the genetic characteristics and high neuropathogenicity of its wild-type parent.

Structural studies of poliovirus mutants that overcome receptor defects

In order to better understand the process of cell entry for non-enveloped viruses, we have solved the crystal structures of five poliovirus mutants which can infect cells expressing mutant poliovirus receptors. Four of these structures have been solved from frozen crystals using cryocrystallographic data collection methods. The mutations have a range of structural consequences, from small local perturbations to significant loop rearrangements. All of the mutant viruses are more labile to conversion to an apparent cell entry intermediate, suggesting that these mutant viruses could compensate for the suboptimal receptors by lowering the thermal energy required to undergo the receptor-mediated conformational change.

Structure of poliovirus type 2 Lansing complexed with antiviral agent SCH48973: comparison of the structural and biological properties of three poliovirus serotypes

BACKGROUND

Polioviruses are human pathogens and the causative agents of poliomyelitis. Polioviruses are icosahedral single-stranded RNA viruses, which belong to the picornavirus family, and occur as three distinct serotypes. All three serotypes of poliovirus can infect primates, but only type 2 can infect mice. The crystal structures of a type 1 and a type 3 poliovirus are already known. Structural studies of poliovirus type 2 Lansing (PV2L) were initiated to try to enhance our understanding of the differences in host range specificity, antigenicity and receptor binding among the three serotypes of poliovirus.

RESULTS

The crystal structure of the mouse neurovirulent PV2L complexed with a potent antiviral agent, SCH48973, was determined at 2.9 A resolution. Structural differences among the three poliovirus serotypes occur primarily in the loop regions of the viral coat proteins (VPs), most notably in the loops of VP1 that cluster near the fivefold axes of the capsid, where the BC loop of PV2L is disordered. Unlike other known structures of enteroviruses, the entire polypeptide chain of PV2L VP4 is visible in the electron density and RNA bases are observed stacking with conserved aromatic residues (Tyr4020 and Phe4046) of VP4. The broad-spectrum antiviral agent SCH48973 is observed binding in a pocket within the beta-barrel of VP1, in approximately the same location that natural 'pocket factors' bind to polioviruses. SCH48973 forms predominantly hydrophobic interactions with the pocket residues.

CONCLUSIONS

Some of the conformational changes required for infectivity and involved in the control of capsid stability and neurovirulence in mice may occur in the vicinity of the fivefold axis of the poliovirus, where there are significant structural differences among the three poliovirus serotypes in the surface exposed loops of VP1 (BC, DE, and HI). A surface depression is located at the fivefold axis of PV2L that is not present in the other two poliovirus serotypes. The observed interaction of RNA with VP4 supports the observation that loss of VP4 ultimately leads to the loss of viral RNA. A model is proposed that suggests dual involvement of the virion fivefold and pseudo-threefold axes in receptor-mediated initiation of infection by picornaviruses.

Mapping of mutations contributing to the temperature sensitivity of the Sabin 1 vaccine strain of poliovirus

The temperature-sensitive and attenuated phenotypes of the Sabin type 1 vaccine strain of poliovirus result from numerous point mutations which occurred in the virulent Mahoney virus parent. One of these mutations is located in a 3D polymerase (3Dpol) codon (U-6203-->C, Tyr-73-->His) and is involved in attenuation in common mice (M. Tardy-Panit, B. Blondel, A. Martin, F. Tekaia, F. Horaud, and F. Delpeyroux, J. Virol. 67:4630-4638, 1993). This mutation also appears to contribute to temperature sensitivity, in association with at least 1 other of the 10 mutations of the 3'-terminal part of the genome including the 3Dpol coding and 3' noncoding regions. To map the other mutation(s), we constructed poliovirus mutants by mutagenesis and recombination of Mahoney and Sabin 1 cDNAs. Characterization of these poliovirus mutants showed that a second mutation in a 3Dpol codon (C-7071-->U, Thr-362-->Ile) contributes to temperature sensitivity. A mutation in the 3' noncoding region of the genome (A-7441-->G), alone or linked to another mutation (U-7410-->C), also appeared to be involved in this phenotype. The temperature-sensitive effect associated with the 3'-terminal part of the Sabin 1 genome results from the cumulative and/or synergistic effects of at least three genetic determinants, i.e., the His-73 and Ile-362 codons of 3Dpol and nucleotide G-7441. Sequence analysis of strains isolated from patients with vaccine-associated paralytic poliomyelitis showed that these genetic determinants are selected against in vivo, although the Ile-362 codon appeared to be more stable than either the His-73 codon or G-7441. These genetic determinants may contribute to the safety of Sabin 1 in vaccines.

Determinants of attenuation and temperature sensitivity in the type 1 poliovirus Sabin vaccine

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.

Poliovirus variants selected on mutant receptor-expressing cells identify capsid residues that expand receptor recognition

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.

Structure of the complex between the Fab fragment of a neutralizing antibody for type 1 poliovirus and its viral epitope

The crystal structure of the complex between the Fab fragment of C3, a neutralizing antibody for poliovirus, and a peptide corresponding to the viral epitope has been determined at 3.0 A resolution. Although this antibody was originally raised to heat inactivated (noninfectious) virus particles, it strongly neutralizes the Mahoney strain of type 1 poliovirus. Eleven peptide residues are well-defined in the electron-density map and form two type I beta-turns in series. At the carboxyl end, the peptide is bound snugly in the antibody-combining site and adopts a conformation that differs significantly from the structure of the corresponding residues in the virus. Structural comparisons between the peptide in the complex and the viral epitope suggests that on binding to infectious virions, this antibody may induce structural changes important for neutralization.

High diversity of poliovirus strains isolated from the central nervous system from patients with vaccine-associated paralytic poliomyelitis

To establish the etiology of vaccine-associated paralytic poliomyelitis (VAPP), isolates from the central nervous system (CNS) from eight patients with VAPP were compared with stool isolates from the same patients. The vaccine (Sabin) origin was checked for all of the available isolates. Unique and similar strains were recovered from paired stool and CNS samples for five of the eight VAPP cases and the three wild-type cases included in the study. In the remaining three VAPP cases, the stool samples and, in one case, the CNS samples contained mixtures of strains. In two of these cases an equivalent of the CNS isolate was found among the strains separated by plaque purification from stool mixtures, and in one case different strains were isolated from CNS and stool. This shows that the stool isolate in VAPP might not be always representative of the etiologic agent of the neurological disease. A wide variety of poliovirus vaccine genomic structures appeared to be implicated in the etiology of VAPP. Of nine CNS vaccine-derived strains, four were nonrecombinant and five were recombinant (vaccine/vaccine or even vaccine/nonvaccine). The neuropathogenic potential of the isolates was evaluated in transgenic mice sensitive to poliovirus. All of the CNS-isolated strains lost the attenuated phenotype of the Sabin strains. However, for half of them, the neurovirulence was lower than expected, suggesting that the degree of neurovirulence for transgenic mice is not necessarily correlated with the neuropathogenicity in humans.

Substitutions in the capsids of poliovirus mutants selected in human neuroblastoma cells confer on the Mahoney type 1 strain a phenotype neurovirulent in mice

Poliovirus (PV) type 1 mutants selected in human neuroblastoma cells persistently infected (PVpi) with the wild-type Mahoney strain exhibited a mouse-neurovirulent phenotype. Four of the five substitutions present in the capsid proteins of a PVpi were demonstrated to extend the host range of the Mahoney strain to mice. These new mouse-neurovirulent determinants were located in the three-dimensional structure of the viral capsid; two of them (residues 142 of VP2 and 60 of VP3) were located in loops exposed at the surface of the protein shell, whereas the other two (residues 43 of VP1 and 62 of VP4) were located on the inside of the capsid. VP1 residue 43 and VP2 residue 142 substitutions were also selected in a PVpi derived from the attenuated Sabin strain. We suggest that the selective pressure of human neuroblastoma cell factor(s) involved in early steps of PV multiplication could be responsible for the arising of amino acid substitutions which confer adaptation to the mouse central nervous system to PV.

Characterization of the N-terminal part of the neutralizing antigenic site I of coxsackievirus B4 by mutation analysis of antigen chimeras

Coxsackievirus B3 (CVB3) as a potential RNA virus vector for the presentation of foreign antigenic epitopes was further characterized. Insertion mutagenesis of infectious CVB3 cDNA yielded viable antigen chimeras containing variant BC loops of VP1 of coxsackievirus B4 (CVB4). Analysis of three antigen chimeras allowed the mapping of the N-terminal part of the neutralizing antigenic site 1 (N-Ag1) of CVB4 which is located in the BC loop of the structural protein VP1. A significant neutralization of a viable chimera with the deletion of CVB4-specific amino acid Ser-83 at the amino terminus of the VP1 BC loop was obtained with CVB4 serotype-specific polyclonal antisera. This neutralization was reduced after further deletion of the adjacent Ala-84, suggesting that this amino acid either constitutes the beginning of N-Ag1 of CVB4 or is essential for the conformation of the adjacent epitope. In contrast, exchange of amino acid Ser-86 to alanine, in the middle of the BC loop, led to complete loss of reactivity with CVB4-specific antibodies, demonstrating the importance of this residue for binding of CVB4 neutralizing antisera. Furthermore, we observed that manipulations of the VP1 BC loop resulted in increased thermolability of the viable chimeras in comparison to CVB3, although replication efficiencies were similar.

Transgenic mice carrying the human poliovirus receptor: new animal models for study of poliovirus neurovirulence

Recombinant viruses between the virulent Mahoney and attenuated Sabin 1 strains of poliovirus type 1 were subjected to neurovirulence tests using a transgenic (Tg) mouse line, ICR-PVRTg1, that carried the human poliovirus receptor gene. The Tg mice were inoculated intracerebrally with these recombinant viruses and observed for clinical signs, histopathological lesions, and viral antigens as parameters of neurovirulence of the viruses. These parameters observed in the Tg mice were different for different inoculated viruses. Dose-dependent incidences of paralysis and of death were observed in the Tg mice inoculated with any viruses used. This indicates that values of 50% lethal dose are useful to score a wide range of neurovirulence of poliovirus. The neurovirulence of individual viruses estimated by the Tg mouse model had a strong correlation with those estimated by monkey model. Consequently, the mouse tests identified the neurovirulence determinants on the genome of poliovirus that had been identified by monkey tests. In addition, the mouse tests revealed new neurovirulence determinants, that is, different nucleotides between the two strains at positions 189 and 21 and/or 935 in the 5'-proximal 1,122 nucleotides. The Tg mice used in this study may be suitable for replacing monkeys for investigating poliovirus neurovirulence.

A mutation in the RNA polymerase of poliovirus type 1 contributes to attenuation in mice

The attenuated Sabin strain of poliovirus type 1 (PV-1) differs from the neurovirulent PV-1 Mahoney strain by 55 nucleotide mutations. Only one of these mutations (A-480-->G, in the 5' noncoding (5' NC) region of the genome, is well characterized, and it confers a strong attenuating effect. We attempted to identify genetic attenuation determinants in the 3'-terminal part of the Sabin 1 genome including the 3D polymerase (3Dpol) gene and the 3' NC region. Previous studies suggested that some of the 11 mutations in this region of the Sabin 1 genome, and in particular a mutation in the polymerase gene (U-6203-->C, Tyr-73-->His), are involved to some extent in the attenuation of PV-1. We analyzed the attenuating effect in the mouse model by using the mouse-adapted PV-1/PV-2 chimeric strain v510 (a Mahoney strain carrying nine amino acids of the VP1 capsid protein from the Lansing strain of PV-2). Mutagenesis of locus 6203 was performed on the original v510 (U-6203-->C) and also on a hybrid v510/Sabin 1 (C-6203-->U) carrying the downstream 1,840 nucleotides of the Sabin 1 genome including the 3Dpol and 3' NC regions. Statistical analysis of disease incidence and time to disease onset in numerous mice inoculated with these strains strongly suggested that nucleotide C-6203 is involved in the attenuation of the Sabin 1 strain. Results also suggested that, among the mutations located in the 3Dpol and 3' NC regions, nucleotide C-6203 may be the principal or the only one to be involved in attenuation in this mouse model. We also found that the effect of C-6203 was weaker than that of nucleotide G-480; the two nucleotides acted independently and may have a cumulative effect on attenuation. The U-6203-->C substitution also appeared to contribute to the thermosensitivity of the Sabin 1 strain.

Molecular characterization of mouse-virulent poliovirus type 1 Mahoney mutants: involvement of residues of polypeptides VP1 and VP2 located on the inner surface of the capsid protein shell

Most poliovirus (PV) strains, including PV PV-1/Mahoney, are unable to cause paralysis in mice. Determinants for restriction of PV-1/Mahoney in mice have been identified by manipulating PV-1 cDNA and located on the viral capsid protein VP1. These determinants consist of a highly exposed amino acid sequence on the capsid surface corresponding to the B-C loop (M. Murray, J. Bradley, X. Yang, E. Wimmer, E. Moss, and V. Racaniello, Science 241:213-215, 1988; A. Martin, C. Wychowski, T. Couderc, R. Crainic, J. Hogle, and M. Girard, EMBO J. 7:2839-2847, 1988) and of residues belonging to the N-terminal sequence located on the inner surface of the protein shell (E. Moss and V. Racaniello, EMBO J. 10:1067-1074, 1991). Using an in vivo approach, we isolated two mouse-neurovirulent PV-1 mutants in the mouse central nervous system after a single passage of PV-1/Mahoney inoculated by the intracerebral route. Both mutants were subjected to two additional passages in mice, plaque purified, and subsequently characterized. The two cloned mutants, Mah-NK13 and Mah-NL32, retained phenotypic characteristics of the parental PV-1/Mahoney, including epitope map, heat lability, and temperature sensitivity. Mah-NK13 exhibited slightly smaller plaques than did the parental virus. The nucleotide sequences of the mutant genomes were determined, and mutations were identified. Mutations were independently introduced into the parental PV-1/Mahoney genome by single-site mutagenesis. Mutated PV-1/Mahoney viruses were then tested for their neurovirulence in mice. A single amino acid substitution in the capsid proteins VP1 (Thr-22-->Ile) and VP2 (Ser-31-->Thr) identified in the Mah-NK13 and Mah-NL32 genomes, respectively, conferred the mouse-virulent phenotype to the mouse-avirulent PV-1/Mahoney. Ile-22 in VP1 was responsible for the small-plaque phenotype of Mah-NK13. Both mutations arose during the first passage in the mouse central nervous system. We thus identified a new mouse adaptation determinant on capsid protein VP1, and we showed that at least one other capsid protein, VP2, could also express a mouse adaptation determinant. Both determinants are located in the inside of the three-dimensional structure of the viral capsid. They may be involved in the early steps of mouse nerve cell infection subsequent to receptor attachment.

Multiple amino acids in the capsid structure of canine parvovirus coordinately determine the canine host range and specific antigenic and hemagglutination properties

Canine parvovirus (CPV) and feline panleukopenia virus (FPV) are over 98% similar in DNA sequence but have specific host range, antigenic, and hemagglutination (HA) properties which were located within the capsid protein gene. In vitro mutagenesis and recombination were used to prepare 16 different recombinant genomic clones, and viruses derived from those clones were analyzed for their in vitro host range, antigenic, and HA properties. The region of CPV from 59 to 91 map units determined the ability to replicate in canine cells. A complex series of interactions was observed among the individual sequence differences between 59 and 73 map units. The canine host range required that VP2 amino acids (aa) 93 and 323 both be the CPV sequence, and those two CPV sequences introduced alone into FPV greatly increased viral replication in canine cells. Changing any one of aa 93, 103, or 323 of CPV to the FPV sequence either greatly decreased replication in canine cells or resulted in an inviable plasmid. The Asn-Lys difference of aa 93 alone was responsible for the CPV-specific epitope recognized by monoclonal antibodies. An FPV-specific epitope was affected by aa 323. Amino acids 323 and 375 together determined the pH dependence of HA. Amino acids involved in the various specific properties were all around the threefold spikes of the viral particle.

Poliovirus chimeras expressing sequences from the principal neutralization domain of human immunodeficiency virus type 1

Sequences from the principal neutralization domain of human immunodeficiency virus type 1 (HIV-1) strain LAI or RF have been expressed in antigenic site 1 of the capsid of the Sabin strain of poliovirus type 1. A number of the resulting chimeras were viable. Viable variants bearing mutations within the insertion site spontaneously arose from several nonviable chimeras. In general, these mutations result in a decrease in positive charge in the substituted antigenic site 1. Two of the chimeras were genetically stable and have been further characterized. Both chimeras were neutralized by various HIV-1 neutralizing antibodies. In rabbits, both chimeras produced high levels of antibodies which react with HIV-1 gp120/160 in immunoprecipitation and enzyme-linked immunosorbent assays. One of the chimeras (HIV-1LAI) produced a significant but weak HIV-1 neutralizing response.