Poliovirus type 1/type 3 antigenic hybrid virus constructed in vitro elicits type 1 and type 3 neutralizing antibodies in rabbits and monkeys

EV-D68 virus-like particle vaccines elicit cross-clade neutralizing antibodies that inhibit infection and block dissemination

Enterovirus D68 (EV-D68) causes severe respiratory illness in children and can result in a debilitating paralytic disease known as acute flaccid myelitis. No treatment or vaccine for EV-D68 infection is available. Here, we demonstrate that virus-like particle (VLP) vaccines elicit a protective neutralizing antibody against homologous and heterologous EV-D68 subclades. VLP based on a B1 subclade 2014 outbreak strain elicited comparable B1 EV-D68 neutralizing activity as an inactivated viral particle vaccine in mice. Both immunogens elicited weaker cross-neutralization against heterologous viruses. A B3 VLP vaccine elicited more robust neutralization of B3 subclade viruses with improved cross-neutralization. A balanced CD4⁺ T helper response was achieved using a carbomer-based adjuvant, Adjuplex. Nonhuman primates immunized with this B3 VLP Adjuplex formulation generated robust neutralizing antibodies against homologous and heterologous subclade viruses. Our results suggest that both vaccine strain and adjuvant selection are critical elements for improving the breadth of protective immunity against EV-D68.

Increasing Type 1 Poliovirus Capsid Stability by Thermal Selection

Poliomyelitis is a highly infectious disease caused by poliovirus (PV). It can result in paralysis and may be fatal. Integrated global immunization programs using live-attenuated oral (OPV) and/or inactivated (IPV) PV vaccines have systematically reduced its spread and paved the way for eradication. Immunization will continue posteradication to ensure against reintroduction of the disease, but there are biosafety concerns for both OPV and IPV. They could be addressed by the production and use of virus-free virus-like particle (VLP) vaccines that mimic the "empty" capsids (ECs) normally produced in viral infection. Although ECs are antigenically indistinguishable from mature virus particles, they are less stable and readily convert into an alternative conformation unsuitable for vaccine purposes. Stabilized ECs, expressed recombinantly as VLPs, could be ideal candidate vaccines for a polio-free world. However, although genome-free PV ECs have been expressed as VLPs in a variety of systems, their inherent antigenic instability has proved a barrier to further development. In this study, we selected thermally stable ECs of type 1 PV (PV-1). The ECs are antigenically stable at temperatures above the conversion temperature of wild-type (wt) virions. We have identified mutations on the capsid surface and in internal networks that are responsible for EC stability. With reference to the capsid structure, we speculate on the roles of these residues in capsid stability and postulate that such stabilized VLPs could be used as novel vaccines.

IMPORTANCE

Poliomyelitis is a highly infectious disease caused by PV and is on the verge of eradication. There are biosafety concerns about reintroduction of the disease from current vaccines that require live virus for production. Recombinantly expressed virus-like particles (VLPs) could address these inherent problems. However, the genome-free capsids (ECs) of wt PV are unstable and readily change antigenicity to a form not suitable as a vaccine. Here, we demonstrate that the ECs of type 1 PV can be stabilized by selecting heat-resistant viruses. Our data show that some capsid mutations stabilize the ECs and could be applied as candidates to synthesize stable VLPs as future genome-free poliovirus vaccines.

A novel inactivated enterovirus 71 vaccine can elicit cross-protective immunity against coxsackievirus A16 in mice

Hand, foot, and mouth disease (HFMD) is a highly contagious disease that mainly affects infants and children. Enterovirus 71 (EV71) and coxsackievirus A16 (CA16) are the major pathogens of HFMD. Two EV71 vaccines were recently licensed in China and the administration of the EV71 vaccines is believed to significantly reduce the number of HFMD-related severe or fatal cases. However, a monovalent EV71 vaccine cannot cross-protect against CA16 infection, this may result in that it cannot effectively control the overall HFMD epidemic. In this study, a chimeric EV71, whose VP1/210-225 epitope was replaced by that of CA16, was constructed using a reverse genetics technique to produce a candidate EV71/CA16 bivalent vaccine strain. The chimeric EV71 was infectious and showed similar growth characteristics as its parental strain. The replacement of the VP1/210-225 epitope did not significantly affect the antigenicity and immunogenicity of EV71. More importantly, the chimeric EV71 could induce protective immunity against both EV71 and CA16, and protect neonatal mice against either EV71 or CA16 lethal infections, the chimeric EV71 constructed in this study was shown to be a feasible and promising candidate bivalent vaccine against both EV71 and CA16. The construction of a chimeric enterovirus also provides an alternative platform for broad-spectrum HFMD vaccines development.

Expression of recombinant human α-lactalbumin in milk of transgenic cloned pigs is sufficient to enhance intestinal growth and weight gain of suckling piglets

Human α-lactalbumin (HLA) has very high nutritional value and important physiological functions during the neonatal period. The peptides derived from HLA provide diverse health benefits including antimicrobial, antiviral, immune-modulating, and antihypertensive effects. Thus, it is worth investigating the effects on offspring development of increasing HLA in milk. In this study, we found that recombinant human α-lactalbumin (rHLA) exhibits efficient inhibition of dipeptidyl peptidase-IV (DPP-IV) activity in an in vitro simulated gastrointestinal digestion system. Using a BAC clone containing the complete HLA gene as a candidate vector, we generated two lines of transgenic cloned sows via somatic cell nuclear transfer that over-expressed rHLA. The average concentrations of rHLA in milk from the two lines of transgenic cloned sows were 2.24 ± 0.71 mg/ml and 2.67 ± 1.29 mg/ml. The feeding experiments revealed that rHLA represses dipeptidyl peptidase-IV (DPP-IV) activity in vivo. Furthermore, the piglets reared by rHLA transgenic cloned sows exhibit better performance in gain of body weight and intestine growth than the control piglets reared by non-transgenic sows. Therefore, these findings indicate that rHLA could serve as a natural precursor for a DPP-IV inhibitor, and the transgenic technology that produced the over-expression of rHLA could be a useful method for pig breeders to improve lactation performance.

Recombinant Encephalomyocarditis Viruses Elicit Neutralizing Antibodies against PRRSV and CSFV in Mice

Encephalomyocarditis virus (EMCV) is capable of infecting a wide range of species and the infection can cause myocarditis and reproductive failure in pigs as well as febrile illness in human beings. In this study, we introduced the entire ORF5 of the porcine reproductive and respiratory syndrome virus (PRRSV) or the neutralization epitope regions in the E2 gene of the classical swine fever virus (CSFV), into the genome of a stably attenuated EMCV strain, T1100I. The resultant viable recombinant viruses, CvBJC3m/I-ΔGP5 and CvBJC3m/I-E2, respectively expressed partial PRRSV envelope protein GP5 or CSFV neutralization epitope A1A2 along with EMCV proteins. These heterologous proteins fused to the N-terminal of the nonstructural leader protein could be recognized by anti-GP5 or anti-E2 antibody. We also tested the immunogenicity of these fusion proteins by immunizing BALB/c mice with the recombinant viruses. The immunized animals elicited neutralizing antibodies against PRRSV and CSFV. Our results suggest that EMCV can be engineered as an expression vector and serve as a tool in the development of novel live vaccines in various animal species.

Crystal Structures of Yeast-Produced Enterovirus 71 and Enterovirus 71/Coxsackievirus A16 Chimeric Virus-Like Particles Provide the Structural Basis for Novel Vaccine Design against Hand-Foot-and-Mouth Disease

Human enterovirus 71 (EV71) and coxsackievirus A16 (CVA16) are the two major causative agents for hand-foot-and-mouth disease (HFMD). Previously, we demonstrated that a virus-like particle (VLP) for EV71 produced from Saccharomyces cerevisiae is a potential vaccine candidate against EV71 infection, and an EV71/CVA16 chimeric VLP can elicit protective immune responses against both virus infections. Here, we presented the crystal structures of both VLPs, showing that both the linear and conformational neutralization epitopes identified in EV71 are mostly preserved on both VLPs. The replacement of only 4 residues in the VP1 GH loop converted strongly negatively charged surface patches formed by portions of the SP70 epitope in EV71 VLP into a relatively neutral surface in the chimeric VLP, which likely accounted for the additional neutralization capability of the chimeric VLP against CVA16 infection. Such local variations in the amino acid sequences and the surface charge potential are also present in different types of polioviruses. In comparison to EV71 VLP, the chimeric VLP exhibits structural changes at the local site of amino acid replacement and the surface loops of all capsid proteins. This is consistent with the observation that the VP1 GH loop located near the pseudo-3-fold junction is involved in extensive interactions with other capsid regions. Furthermore, portions of VP0 and VP1 in EV71 VLP are at least transiently exposed, revealing the structural flexibility of the VLP. Together, our structural analysis provided insights into the structural basis of enterovirus neutralization and novel vaccine design against HFMD and other enterovirus-associated diseases.

IMPORTANCE

Our previous studies demonstrated that the enterovirus 71 (EV71) virus-like particle (VLP) produced from yeast is a vaccine candidate against EV71 infection and that a chimeric EV71/coxsackievirus A16 (CVA16) VLP with the replacement of 4 amino acids in the VP1 GH loop can confer protection against both EV71 and CVA16 infections. This study reported the crystal structures of both the EV71 VLP and the chimeric EV71/CVA16 VLP and revealed that the major neutralization epitopes of EV71 are mostly preserved in both VLPs. In addition, the mutated VP1 GH loop in the chimeric VLP is well exposed on the particle surface and exhibits a surface charge potential different from that contributed by the original VP1 GH loop in EV71 VLP. Together, this study provided insights into the structural basis of enterovirus neutralization and evidence that the yeast-produced VLPs can be developed into novel vaccines against hand-foot-and-mouth disease (HFMD) and other enterovirus-associated diseases.

Crystal structures of enterovirus 71 (EV71) recombinant virus particles provide insights into vaccine design

Hand-foot-and-mouth disease (HFMD) remains a major health concern in the Asia-Pacific regions, and its major causative agents include human enterovirus 71 (EV71) and coxsackievirus A16. A desirable vaccine against HFMD would be multivalent and able to elicit protective responses against multiple HFMD causative agents. Previously, we have demonstrated that a thermostable recombinant EV71 vaccine candidate can be produced by the insertion of a foreign peptide into the BC loop of VP1 without affecting viral replication. Here we present crystal structures of two different naturally occurring empty particles, one from a clinical C4 strain EV71 and the other from its recombinant virus containing an insertion in the VP1 BC loop. Crystal structure analysis demonstrated that the inserted foreign peptide is well exposed on the particle surface without significant structural changes in the capsid. Importantly, such insertions do not seem to affect the virus uncoating process as illustrated by the conformational similarity between an uncoating intermediate of another recombinant virus and that of EV71. Especially, at least 18 residues from the N terminus of VP1 are transiently externalized. Altogether, our study provides insights into vaccine development against HFMD.

A novel universal neutralizing monoclonal antibody against enterovirus 71 that targets the highly conserved "knob" region of VP3 protein

Hand, foot and mouth disease caused by enterovirus 71(EV71) leads to the majority of neurological complications and death in young children. While putative inactivated vaccines are only now undergoing clinical trials, no specific treatment options exist yet. Ideally, EV71 specific intravenous immunoglobulins could be developed for targeted treatment of severe cases. To date, only a single universally neutralizing monoclonal antibody against a conserved linear epitope of VP1 has been identified. Other enteroviruses have been shown to possess major conformational neutralizing epitopes on both the VP2 and VP3 capsid proteins. Hence, we attempted to isolate such neutralizing antibodies against conformational epitopes for their potential in the treatment of infection as well as differential diagnosis and vaccine optimization. Here we describe a universal neutralizing monoclonal antibody that recognizes a conserved conformational epitope of EV71 which was mapped using escape mutants. Eight escape mutants from different subgenogroups (A, B2, B4, C2, C4) were rescued; they harbored three essential mutations either at amino acid positions 59, 62 or 67 of the VP3 protein which are all situated in the “knob” region. The escape mutant phenotype could be mimicked by incorporating these mutations into reverse genetically engineered viruses showing that P59L, A62D, A62P and E67D abolish both monoclonal antibody binding and neutralization activity. This is the first conformational neutralization epitope mapped on VP3 for EV71.

Over the last decade, EV71 has emerged as a major cause of severe hand, foot and mouth disease in the Asia-Pacific region, occasionally leading to fatal brain stem encephalitis in young children. The rapid progression and high mortality of severe EV71 infection makes it vital to identify neutralization epitopes and putative therapeutic monoclonal antibodies. In this study we mapped the first conformational neutralization epitope on the VP3 protein of EV71. This epitope was confirmed by introducing the mutations into reverse genetically engineered viruses which abolished neutralization with monoclonal antibody (mAb)10D3. The importance of this novel neutralization epitope lies in the optimization of putative EV71 vaccines because the VP3 knob could be incorporated together with VP1 into a bivalent subunit vaccine. Further, the universal recognition of a conserved site on EV71 VP3 and not CVA16 makes mAb 10D3 a valuable tool for differential diagnosis of hand, foot and mouth disease. An additional hope is that mAb 10D3 could be used as a therapeutic intravenous immunoglobulin (IVIG).

Thermal stability of RNA phage virus-like particles displaying foreign peptides

Background

To be useful for genetic display of foreign peptides a viral coat protein must tolerate peptide insertions without major disruption of subunit folding and capsid assembly. The folding of the coat protein of RNA phage MS2 does not normally tolerate insertions in its AB-loop, but an engineered single-chain dimer readily accepts them as long as they are restricted to one of its two halves.

Results

Here we characterize the effects of peptide insertions on the thermal stabilities of MS2 virus-like particles (VLPs) displaying a variety of different peptides in one AB-loop of the coat protein single-chain dimer. These particles typically denature at temperatures around 5-10°C lower than unmodified VLPs. Even so, they are generally stable up to about 50°C. VLPs of the related RNA phage PP7 are cross-linked with intersubunit disulfide bonds and are therefore significantly more stable. An AB-loop insertion also reduces the stability of PP7 VLPs, but they only begin to denature above about 70°C.

Conclusions

VLPs assembled from MS2 single-chain dimer coat proteins with peptide insertions in one of their AB-loops are somewhat less stable than the wild-type particle, but still resist heating up to about 50°C. Because they possess disulfide cross-links, PP7-derived VLPs provide an alternate platform with even higher stability.

Viable polioviruses that encode 2A proteins with fluorescent protein tags

The 2A proteins of the Picornaviridae enterovirus genus are small cysteine proteinases that catalyze essential cleavages in the viral polyprotein in cis and in several cellular proteins in trans. In addition, 2A has been implicated in the process of viral RNA replication, independent of its protease functions. We have generated viable polioviruses that encode 2A proteins containing fluorescent protein tag insertions at either of two sites in the 2A protein structure. Viruses containing an insertion of Discosoma sp. red fluorescent protein (DsRed) after residue 144 of 2A, near the C terminus, produced plaques only slightly smaller than wild-type (wt) virus. The polyprotein harboring the 2A-DsRed fusion protein was efficiently and accurately cleaved; fluorescent 2A proteinase retained protease activity in trans and supported translation and replication of viral RNA, both in vitro and in infected cells. Intracellular membrane reorganization to support viral RNA synthesis was indistinguishable from that induced by wt virus. Infected cells exhibited strong red fluorescence from expression of the 2A-DsRed fusion protein, and the progeny virus was stable for three to four passages, after which deletions within the DsRed coding sequence began to accumulate. Confocal microscopic imaging and analysis revealed a portion of 2A-DsRed in punctate foci concentrated in the perinuclear region that colocalized with replication protein 2C. The majority of 2A, however, was associated with an extensive structural matrix throughout the cytoplasm and was not released from infected cells permeabilized with digitonin.

In Planta production of immunogenic poliovirus peptide using tobacco mosaic virus-based vector system

The tobacco mosaic virus (TMV) provides an attractive means of producing foreign peptides in plants. In this study, a TMV-based vector was designed such that a fragment encoding 15 amino acids of the poliovirus peptide (PVP) derived from the viral capsid proteins VP3 and VP1 of poliovirus type 1 Sabin was inserted downstream of the six-base 3' context nucleotide sequence of the TMV coat protein (CP) gene. This design allowed readthrough at the amber stop codon, thereby producing the chimeric TMV particle with both intact CP and CP-fusion protein (CP-PVP) in Nicotiana tabacum cv. Samsun infected with the TMV vector. The TMVCP-PVP virus particle induced antibodies against PVP as well as TMVCP in mice after intraperitoneal immunization. These data illustrate the potential of the readthrough translation system with TMVCP for antigen presentation and vaccine production.

In vitro disassembly of a parvovirus capsid and effect on capsid stability of heterologous peptide insertions in surface loops

We have analyzed the in vitro disassembly of the capsid of the minute virus of mice, and the stability of capsid chimeras carrying heterologous epitope insertions. Upon heating in a physiological buffer, empty capsids formed by 60 copies of protein VP2 underwent first a reversible conformational change with a small enthalpy change detected by fluorescence. This change was associated with, but not limited to, externalization of the VP2 N terminus. Irreversible capsid dissociation as detected by changes in fluorescence, hemagglutination activity, and electrophoretic mobility occurred at much higher temperatures. Differential scanning calorimetry in the same conditions indicated that the dissociation/denaturation transition involved a high enthalpy change and proceeded through one or more intermediates. In contrast, in the presence of 1.5 M guanidinium chloride, heat-induced disassembly fitted a two-state irreversible process. Both thermally and chemically induced dissociation/denaturation yielded a form that had lost a part of the tertiary structure, but still retained the native secondary structure. Data from chemical dissociation indicates this form may correspond to a molten globule-like monomeric state of the capsid protein. All five antigenic peptide insertions attempted in exposed loops, despite being perhaps among the least disruptive, led to defects in folding/assembly of the capsid and, in most cases, to reduced capsid stability against thermal dissociation. The results with one of the simplest viral capsids reveal a complex pathway for disassembly, and a reduction in capsid assembly and stability upon insertion of peptides, even within the most exposed capsid loops.

Isolation of an intertypic poliovirus capsid recombinant from a child with vaccine-associated paralytic poliomyelitis

The isolation of a capsid intertypic poliovirus recombinant from a child with vaccine-associated paralytic poliomyelitis is described. Virus 31043 had a Sabin-derived type 3-type 2-type 1 recombinant genome with a 5'-end crossover point within the capsid coding region. The result was a poliovirus chimera containing the entire coding sequence for antigenic site 3a derived from the Sabin type 2 strain. The recombinant virus showed altered antigenic properties but did not acquire type 2 antigenic characteristics. The significance of the presence in nature of such poliovirus chimeras and the consequences for the current efforts to detect potentially dangerous vaccine-derived poliovirus strains are discussed in the context of the global polio eradication initiative.

Surface expression of an immunodominant malaria protein B cell epitope by yellow fever virus

The yellow fever 17D virus (YF17D) has several characteristics that are desirable for the development of new, live attenuated vaccines. We approached its development as a vector for heterologous antigens by studying the expression of a humoral epitope at the surface of the E protein based on the results of modelling its three-dimensional structure. This model indicated that the most promising insertion site is between beta-strands f and g, a site that is exposed at the external surface of the virus. The large deletion of six residues from the fg loop of the E protein from yellow fever virus, compared to tick-born encephalitis virus, leaves space at the dimer interface for a large insertion without creating steric hindrance. We have tested this hypothesis by inserting a model humoral epitope from the circumsporozoite protein of Plasmodium falciparum consisting of triple NANP repeats. Recombinant virus (17D/8) expressing this insertion flanked by two glycine residues at each end, is specifically neutralized by a monoclonal antibody to the model epitope. Furthermore, mouse antibodies raised to the recombinant virus recognize the parasite protein in an ELISA assay. Serial passage analysis confirmed the genetic stability of the insertion made in the viral genome and the resulting 17D/8 virus is significantly more attenuated in mouse neurovirulence tests than the 17DD vaccine. The fg loop belongs to the dimerization domain of the E protein and lies at the interface between monomers. This domain undergoes a low pH transition, which is related to the fusion of the viral envelope to the endosome membrane. It is conceivable that a slower rate of fusion, resulting from the insertion close to the dimer interface, may delay the onset of virus production and thereby lead to a milder infection of the host. This would account for the more attenuated phenotype of the recombinant virus in the mouse model and lower extent of replication in cultured cells. The vectorial capacity of the yellow fever virus is being further explored for the expression and presentation of other epitopes, including those mediating T-cell responses.

Genetic recombination of poliovirus in vitro and in vivo: temperature-dependent alteration of crossover sites

Genetic recombination that occurs with high frequency during poliovirus genome replication is a process whose molecular mechanism is poorly understood. Studies of genetic recombination in a cell-free system in vitro and in infected tissue culture cells in vivo have led to the unexpected observation that temperature strongly influences the loci at which cross-over between the two recombining RNA strands occurs. Specifically, cross-over between two genetically marked RNA strands in vitro and in vivo at 34 degrees C occurred over a wide range of the genome. In contrast, recombination in vivo at 37 and 40 degrees C yielded cross-over patterns that had shifted dramatically to a region encoding nonstructural proteins. Preferential selection of recombinants at 37 and 40 degrees C was ruled out by analyses of the growth kinetics of the recombinants. During the studies of recombination in the cell-free system we found that there is a direct correlation between the ability of a poliovirus RNA molecule to replicate in the cell-free system and its capacity to complement de novo virus synthesis programmed by another viral RNA.

Expression of foreign proteins by poliovirus polyprotein fusion: analysis of genetic stability reveals rapid deletions and formation of cardioviruslike open reading frames

Using a strategy developed by R. Andino, D. Silvera, S. D. Suggett, P. I. Achacoso, C. J. Miller, D. Baltimore, and M. B. Feinberg (Science 265:1448-1451, 1994), we constructed recombinant polioviruses by fusing the open reading frame (ORF) of the green fluorescent protein gene (gfp) of Aequorea victoria or the gag gene (encoding p17-p24) of human immunodeficiency virus type 1 (HIV-1) to the N terminus of the poliovirus polyprotein. All poliovirus expression vectors constructed by us and those obtained from Andino et al. were found to be severely impaired in viral replication and genetically unstable. Upon replication, inserted sequences were rapidly deleted as early as the first growth cycle in HeLa cells. However, the vector viruses did not readily revert to the wild-type sequence but rather retained some of the insert plus the artificial 3Cpro/3CDpro cleavage site, engineered between the heterologous sequence and the poliovirus polyprotein, to give rise to genotypes reminiscent of cardioviruses. These virus variants that carry a small leader polypeptide were now relatively stable, and they grew better than their progenitor strains. Reverse transcription followed by PCR and sequence analysis of the genomic RNAs reproducibly revealed a few preferred genotypes among the isolated deletion variants. The remaining truncated inserts were retained through subsequent passages. In the immediate vicinity of the deletion borders, we observed short direct sequence repeats that we propose are involved in aligning RNA strands for illegitimate (nonhomologous) RNA recombination during minus-strand synthesis. On the basis of our results, which are at variance with published data, the utility of poliovirus vectors to express proteins > 10 kDa in size through fusion with the polyprotein needs to be reevaluated.

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.

The yellow fever 17D vaccine virus: molecular basis of viral attenuation and its use as an expression vector

The yellow fever (YF) virus is the prototype flavivirus. The use of molecular techniques has unraveled the basic mechanisms of viral genome structure and expression. Recent trends in flavivirus research include the use of infectious clone technology with which it is possible to recover virus from cloned cDNA. Using this technique, mutations can be introduced at any point of the viral genome and their resulting effect on virus phenotype can be assessed. This approach has opened new possibilities to study several biological viral features with special emphasis on the issue of virulence/attenuation of the YF virus. The feasibility of using YF virus 17D vaccine strain, for which infectious cDNA is available, as a vector for the expression of heterologous antigens is reviewed.

Human rhinovirus 3 at 3.0 A resolution

BACKGROUND

The over 100 serotypes of human rhinoviruses (HRV) are major causative agents of the common cold in humans. These HRVs can be roughly divided into a major and minor group according to their cellular receptors. They can also be divided into two antiviral groups, A and B, based on their sensitivity to different capsid-binding antiviral compounds. The crystal structures of HRV14 and HRV16, major-receptor group rhinoviruses, as well as HRV1A, a minor-receptor group rhinovirus, were determined previously. Sequence comparisons had shown that HRV14 seemed to be an outlier among rhinoviruses. Furthermore, HRV14 was the only virus with no cellular 'pocket factor' in a hydrophobic pocket which is targeted by many capsid-binding antiviral compounds and is thought to regulate viral stability. HRV3, another major-receptor group virus, was chosen for study because it is one of a subset of serotypes that best represents the drug sensitivity of most rhinovirus serotypes. Both HRV3 and HRV14 belong to antiviral group A, while HRV16 and HRV1A belong to antiviral group B.

RESULTS

HRV3 was found to be very similar to HRV14 in sequence and structure. Like HRV14, crystallized HRV3 also has no bound pocket factor. The structure of HRV3 complexed with an antiviral compound, WIN56291, was also determined and found to be similar to the same antiviral compound complexed with HRV14.

CONCLUSIONS

The amino-acid sequence and structural similarity between HRV3 and HRV14 suggests that rhinoviruses in the same antiviral group have similar amino-acid sequences and structures. The similar amino-acid composition in the pocket region and the viral protein VP1 N termini in all known group B HRV sequences suggests that these viruses may all contain pocket factors and ordered N-terminal amphipathic helices in VP1. Both of these factors contribute to viral stability, which is consistent with the observations that group B rhinoviruses have a higher chance of successful transmission from one host to another and is a possible explanation for the observed higher pathogenicity of these rhinoviruses.

Theiler's virus as a vector for foreign gene delivery

DA strain and other strains of the TO subgroup of Theiler's murine encephalomyelitis viruses are members of the Cardiovirus genus of picornaviruses and produce a persistent demyelinating disease in mice. A recent study from our laboratory (W.-P. Kong, G. D. Ghadge, and R. P. Roos, Proc. Natl. Acad. Sci. USA 91:1796-1800, 1994) demonstrated that the leader, which is encoded at the N terminus of the Theiler's murine encephalomyelitis virus polyprotein, can be partially replaced by foreign sequences as well as completely deleted, with no loss of infectivity in BHK-21 cells. In this study, we have inserted up to 724 nucleotides into the leader coding region of an infectious DA clone. Recombinant viruses were produced, and the inserts were shown to be stable for at least three passages in BHK-21 cells.

A single amino acid substitution in the capsid protein VP1 of coxsackievirus B3 (CVB3) alters plaque phenotype in Vero cells but not cardiovirulence in a mouse model

We previously described a large plaque attenuant (p14V-1) derived from a cardiovirulent Coxsackievirus B3 (CVB3) and showed that there were no major determinants of either attenuation or plaque phenotype in the 5' nontranslated region (5'NTR). Part of the region encoding the last 124 amino acids of VP3 and the first 106 amino acids of VP1 of the attenuant was then sequenced and compared to the wild-type. Three nucleotide changes were found in the VP1 coding region: a silent single base change at nucleotide position 2467 (C to U) and a double-base change at position 2690-1 (AA to GT), which leads to a change from lysine to serine at amino acid position 80. This mutation maps to the begining of B-C loop of the three-dimensional structure of VP1 of CVB3, where a distinct surface projection is formed. Two infectious chimeric cDNA clones were constructed, based on a cardiovirulent cDNA construct. In one construct, the 5'NTR and the VP3-VP1 region were from p14V-1 and in the other, only the VP3-VP1 region was from this attenuant. Both chimeric viruses produced large plaques on Vero cell monolayers, similar to p14V-1 but larger than the prototypic cardiovirulent virus. In vivo experiments showed that both chimeric viruses induced myocarditis in a murine model, similar to wild-type virus. We conclude that mutation serine-80 in capsid protein VP1 of p14V-1 is a determinant of the large plaque phenotype but is not responsible for attenuation.

Poliovirus hybrids expressing neutralization epitopes from variable domains I and IV of the major outer membrane protein of Chlamydia trachomatis elicit broadly cross-reactive C. trachomatis-neutralizing antibodies

Trachoma and sexually transmitted diseases caused by Chlamydia trachomatis are major health problems worldwide. Epitopes from the variable domains of the major outer membrane protein are candidates for vaccine development. We have constructed hybrid polioviruses expressing sequences from major outer membrane protein variable domains I and IV. Antisera to the hybrids could, in combination, strongly neutralize 8 of the 12 C. trachomatis serovars most commonly associated with oculogenital infections and weakly neutralize the others.

Vaccines prepared from chimeras of foot-and-mouth disease virus (FMDV) induce neutralizing antibodies and protective immunity to multiple serotypes of FMDV

The G-H loop of VP1 (residues 132 to 159) of foot-and-mouth disease virus (FMDV) is a prominent feature on the virion surface and has an important role in vaccine efficacy, generation of antigenic variants, and cell binding. Using an infectious cDNA of FMDV, we have constructed serotype A viruses in which the G-H loop has been substituted with the homologous sequences from serotype O or C. These chimeric viruses replicated to high titer and displayed plaque morphologies similar to those of wild-type viruses, demonstrating that the functions provided by the loop can be readily exchanged between serotypes. Monoclonal antibody analyses showed that epitopes contained within the loop were transferred to the chimeras and that epitopes encoded by the type A backbone were maintained. Chemically inactivated vaccines prepared from chimeric viruses induced antibodies in guinea pigs that neutralized both type A and either type O or type C viruses. Swine inoculated with the A/C chimera vaccine also produced cross-reactive antibodies, were protected from challenge with the type A virus, and partially protected against challenge with type C. These studies emphasize the importance of epitopes outside of the G-H loop in protective immunity in swine, which is a natural host of FMDV.

Protection against lethal lymphocytic choriomeningitis virus (LCMV) infection by immunization of mice with an influenza virus containing an LCMV epitope recognized by cytotoxic T lymphocytes

The reverse genetics system has made it possible to modify the influenza virus genome. By this method, we were able to assess influenza virus as a vaccine vector for protecting BALB/c mice against otherwise lethal lymphocytic choriomeningitis virus (LCMV) infection. A single dose of influenza virus [A/WSN/33 (H1N1)] bearing a cytotoxic T-lymphocyte-specific epitope of the LCMV nucleoprotein (residues 116 to 127) in the neuraminidase stalk protected mice against LCMV challenge for at least 4 months. The immunity was mediated by cytotoxic T lymphocytes and was haplotype specific, indicating that the observed protective response was solely a consequence of prior priming with the H-2d LCMV nucleoprotein epitope expressed in the recombinant influenza virus. We also found that as many as 58 amino acids could be inserted into the neuraminidase stalk without loss of viral function. These findings demonstrate the potential of influenza virus as a vaccine vector, with the neuraminidase stalk as a repository for foreign epitopes.

Attenuated poliovirus strain as a live vector: expression of regions of rotavirus outer capsid protein VP7 by using recombinant Sabin 3 viruses

The ability to express heterologous antigens from attenuated poliovirus strains suggests the potential for use as live vectored vaccines. Full- or partial-length sequences of the gene encoding rotavirus major outer capsid protein VP7 were cloned into the open reading frame of a full-length cDNA copy of poliovirus Sabin type 3. They were inserted either at the 5' end or immediately after the capsid protein coding region, at the junction between precursors P1 and P2. A protease cleavage site for 3C protease was introduced 3' to the foreign sequences to enable proteolytic processing of the antigen from the poliovirus polyprotein. Infectious viruses were generated from several of the DNA constructs, and the presence of the foreign gene sequences was confirmed by reverse transcription of the viral RNA and PCR amplification. Viruses with inserts of about 300 bases maintained the foreign sequences during passage in Vero cells. Viruses carrying larger sequences were unstable, and deletions were generated within the foreign sequences. Expression of the VP7 polypeptides was demonstrated by immunoprecipitation with specific antiserum of labeled proteins from cells infected with Sabin 3 recombinant viruses. Comparative studies of RNA synthesis showed similar kinetics for Sabin 3 and the Sabin 3/VP7 recombinants. One-step growth curves showed that production of recombinant viruses was slower than that of Sabin 3 and that the final titers were 1 to 1.5 logs lower. Accumulation of VP7-containing precursors in infected cells suggests that slow cleavage at the engineered 3C protease site may be a limiting step in the growth of these recombinant Sabin polioviruses and may influence the permissible size of foreign sequence to be inserted.

Polioviruses containing picornavirus type 1 and/or type 2 internal ribosomal entry site elements: genetic hybrids and the expression of a foreign gene

A picornavirus hybrid genome was constructed in which the internal ribosomal entry site (IRES) of encephalomyocarditis virus was inserted between the 5' non-translated region and the open reading frame of poliovirus (PV), type 1 (Mahoney). Upon transfection into HeLa cells, the hybrid RNA replicated and yielded a derivative of PV (W1-PNENPO). The PV IRES could be removed from pPNENPO, which resulted in a hybrid picornavirus (W1-P108ENPO) in which the translation of the PV open reading frame normally promoted by the type 1 IRES of PV was promoted by the type 2 IRES of encephalomyocarditis virus. This result indicates that these elements are not likely to contain cis-acting elements necessary for PV replication or encapsidation. A foreign gene (bacterial chloramphenicol acetyltransferase, CAT) was inserted into pPNENPO cDNA between the PV and encephalomyocarditis virus IRES elements. The dicistronic RNA replicated in HeLa cells and yielded a derivative of PV (W1-DICAT) with a genome 17% longer than that of wild-type PV. CAT assays and immunoblot analyses showed that the viral RNA efficiently expressed the foreign gene in cell culture. The CAT activity diminished somewhat with each passage of the dicistronic virus, an observation which suggested that the inserted gene had a deleterious effect on viral replication. However, even after five virus passages, a significant quantity of the foreign gene was still expressed. Insertion of the open reading frame of luciferase (67 kDa) resulted in an RNA species that replicated and expressed luciferase for up to 20 hr after transfection. However, this elongated RNA was not encapsidated.

Use of random systematic mutagenesis to generate viable human rhinovirus 14 chimeras displaying human immunodeficiency virus type 1 V3 loop sequences

Random systematic mutagenesis was used to generate a library of human rhinovirus 14 chimeras that each display a segment from the V3 loop of human immunodeficiency virus type 1. The sequence XXIGPGRAXX, where X could be any of the 20 amino acids, was inserted at the neutralizing immunogenic site II of human rhinovirus 14 between VP2 residues 159 and 160. Twenty-five unique chimeric viruses were isolated, and the identity of their randomized residues was determined. A nonrandom amino acid distribution that may reflect structural requirements for viability was observed at the randomized positions. Fifteen of 25 chimeras were neutralized by one or more of a panel of four anti-human immunodeficiency virus type 1 V3 loop antibody preparations, indicating that antigenicity had been successfully transplanted. Libraries of chimeric viruses produced by using the techniques described may be a source of vaccines and other immunotherapeutic reagents. The random systematic mutagenesis methodology described should be generally useful for the rapid transplantation of foreign sequences into viral coat and other proteins to produce libraries containing members with the desired properties.

A poliovirus hybrid expressing a neutralization epitope from the major outer membrane protein of Chlamydia trachomatis is highly immunogenic

Trachoma and sexually transmitted diseases caused by Chlamydia trachomatis are major health problems worldwide. Epitopes on the major outer membrane protein (MOMP) of C. trachomatis have been identified as important targets for the development of vaccines. In order to examine the immunogenicity of a recombinant vector expressing a chlamydial epitope, a poliovirus hybrid was constructed in which part of neutralization antigenic site I of poliovirus type 1 Mahoney (PV1-M) was replaced by a sequence from variable domain I of the MOMP of C. trachomatis serovar A. The chlamydial sequence included the neutralization epitope VAGLEK. This hybrid was viable, grew very well compared with PV1-M, and expressed both poliovirus and chlamydial antigenic determinants. When inoculated into rabbits, this hybrid was highly immunogenic, inducing a strong response against both PV1-M and C. trachomatis serovar A. Antichlamydia titers were 10- to 100-fold higher than the titers induced by equimolar amounts of either purified MOMP or a synthetic peptide expressing the VAGLEK epitope. Furthermore, rabbit antisera raised against this hybrid neutralized chlamydial infectivity both in vitro, for hamster kidney cells, and passively in vivo, for conjunctival epithelia of cynomolgus monkeys. Because poliovirus infection induces a strong mucosal immune response in primates and humans, these results indicate that poliovirus-chlamydia hybrids could become powerful tools for the study of mucosal immunity to chlamydial infection and for the development of recombinant chlamydial vaccines.

Replication of poliovirus RNA containing two VPg coding sequences leads to a specific deletion event

Studies of the poliovirus genome-linked protein VPg have shown that this small viral protein is required for replication of virus-specific RNA (Q. Reuer, R. J. Kuhn, and E. Wimmer, J. Virol. 64:2967-2975, 1990). To understand the mechanism of RNA replication, we constructed a recombinant poliovirus genome encoding two tandemly arranged VPg coding sequences that were nearly identical in both nucleotide and amino acid sequence. Following transfection of this two-VPg-containing RNA into HeLa cells, we found a specific and selective deletion in the progeny virus genome. Sequence analysis of the recovered viral RNA indicated that the complete nucleotide sequence encoding the second (3C-proximal) VPg coding sequences was removed, restoring the authentic genome sequences in the poliovirus genome. Analysis of viral RNAs following transfection suggested that the deletion event occurred during genome replication. Deletion could have occurred via homologous recombination between two VPg sequences or via intramolecular deletion with loop-out of the template. In vitro translation of the two-VPg-containing transcript RNA indicated aberrant processing of the viral polyprotein. This result suggested that selection of the wild-type genotype in the transfected cells may occur at the level of viral protein synthesis.

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.

Infectious enveloped RNA virus antigenic chimeras

Random insertion mutagenesis has been used to construct infectious Sindbis virus structural protein chimeras containing a neutralization epitope from a heterologous virus, Rift Valley fever virus. Insertion sites, permissive for recovery of chimeric viruses with growth properties similar to the parental virus, were found in the virion E2 glycoprotein and the secreted E3 glycoprotein. For the E2 chimeras, the epitope was expressed on the virion surface and stimulated a partially protective immune response to Rift Valley fever virus infection in vivo. Besides providing a possible approach for developing live attenuated vaccine viruses, insertion of peptide ligands into virion surface proteins may ultimately allow targeting of virus infection to specific cell types.

Mapping of a neutralizing antigenic site of Coxsackievirus B4 by construction of an antigen chimera

A neutralizing antigenic site of coxsackievirus B4 (CVB4) was identified by construction of an antigen chimera between coxsackievirus B3 (CVB3) and CVB4. This chimera, designated CVB3/4, was constructed by inserting five amino acids of the putative BC loop of the structural protein VP1 of CVB4 into the corresponding loop of CVB3 by site-directed mutagenesis of infectious recombinant CVB3 cDNA. The chimeric cDNA was capable of inducing an infectious cycle upon transfection of permissive host cells. The resulting chimeric virus CVB3/4 was neutralized and precipitated by CVB4 and CVB3 serotype-specific polyclonal antisera, demonstrating that it unifies antigenic properties of both coxsackievirus serotypes. In addition, the chimera elicited antibodies in rabbits which were capable of neutralizing the two coxsackievirus serotypes CVB3 and CVB4. The insertion of the CVB4-specific antigenic site into the BC loop of CVB3 reduces the efficiency of viral replication, resulting in a small-plaque morphology of the virus chimera. In summary, these data give evidence for the presence of a serotype-specific neutralizing antigenic site in the BC loop of VP1 of CVB4 (amino acids 81 to 89). Our findings suggest that the construction of intertypic chimeras can be used as a tool for the identification of antigenic sites of coxsackieviruses. The retained immunogenicity of the mapped CVB4-specific antigenic epitope, when expressed in CVB3, indicates that CVB3 can be used as a RNA virus vector for heterologous antigenic sites.

Chimeric polioviruses that include sequences derived from two independent antigenic sites of foot-and-mouth disease virus (FMDV) induce neutralizing antibodies against FMDV in guinea pigs

Five poliovirus recombinants containing sequences corresponding to foot-and-mouth disease virus (FMDV) antigenic sites were constructed. Viable virus was recovered from four of these plasmids, in which the VP1 beta B-beta C loop (antigenic site 1) of poliovirus type 1 Sabin had been replaced with sequences derived from the VP1 beta G-beta H loop (antigenic site 1) of FMDV O1 Kaufbeuren (O1K), chimera O1.1 (residues 141 to 154), chimera O1.2 (residues 147 to 156), and chimera O1.3 (residues 140 to 160) or from the beta B-beta C loop of VP1 (antigenic site 3) in chimera O3.1 (residues 40 to 49). One chimera (O1.3) was neutralized by FMDV-specific polyclonal serum and monoclonal antibodies directed against antigenic site 1 of FMDV. Chimeras O1.3 and O3.1 induced site-specific FMDV-neutralizing antibodies in guinea pigs. Chimera O1.3 was capable of inducing a protective response against FMDV challenge in some guinea pigs.

Poliovirus vaccines. A continuing challenge

The control of poliomyelitis remains a provocative challenge. Alternative vaccination schedules, continuing research toward better vaccines, and ongoing international scientific, epidemiologic, and economic collaboration may make it possible to provide effective immunization for all children of the world and eventually may eradicate poliomyelitis worldwide, a goal set forward by the Expanded Programme on Immunization of the World Health Organization.

Evidence suggesting that virulence maps to the P1 region of the coxsackievirus B4 genome

A chimeric virus containing the P1 region of a virulent variant of coxsackievirus B4 and the P2 and P3 regions of a nonvirulent strain was constructed from cDNA clones. The chimeric virus induced pancreatitis with concurrent hypoglycemia similar to that observed in mice infected with the virulent variant.

An antigen chimera of poliovirus induces antibodies against human papillomavirus type 16

It has been established that the surface of poliovirus type 1 can be extensively modified to incorporate antigenic domains from other poliovirus serotypes and from unrelated viruses. The fact that the modified (chimeric) viruses exhibit dual antigenicity and immunogenicity led us to explore the possibility of using the Sabin vaccine strain of poliovirus type 1 as a vector for the presentation of antigenic domains from human papillomavirus type 16 (HPV-16), a virus associated with the development of cervical carcinoma. We report here the construction and characterization of a chimeric poliovirus containing a 16-residue sequence derived from the major capsid protein (L1) of HPV-16. This virus chimera stimulated the production in rabbits of antibodies which recognized the HPV-16-derived peptide and an L1 fusion protein synthesized in Escherichia coli and detected HPV-16 in human biopsy material by immunoperoxidase staining. The possibility that poliovirus-HPV chimeras could be used as vaccines against HPV-16 is discussed.

Improved distribution of antigenic site specificity of poliovirus-neutralizing antibodies induced by a protease-cleaved immunogen in mice

Previous studies showed that the distribution of antigenic site specificity of neutralizing antibodies to type 3 poliovirus obtained with the inactivated poliovirus vaccine can be deficient as compared with that obtained following poliovirus infection. This observation was shown by the relatively low capacity of sera from inactivated-poliovirus-vaccine-immunized persons to neutralize poliovirus cleaved at antigenic site 1. We investigated possibilities for improving the situation in a mouse model. Balb/c mice were immunized with intact or trypsin-cleaved type 3 poliovirus (Saukett strain). Sera from mice immunized with the intact virus readily neutralized the intact virus but neutralized the cleaved virus only rarely. In contrast, cleaved-virus-immunized mice produced antibodies that were able to neutralize the cleaved virus as well as the intact one. Mice immunized with a 100-fold-higher dose of the intact virus produced significant levels of antibodies to the cleaved virus, too. Somewhat surprisingly, mice immunized with high doses of the cleaved virus produced antibodies specific for the intact loop between beta sheets B and C of VP1 (virion protein 1), which should be cleaved in the immunogen. This was shown by a higher titer of antibodies to intact Saukett virus than to the corresponding cleaved virus, as well as to a type 1/type 3 hybrid poliovirus in which only the BC loop amino acids were derived from type 3 poliovirus. The cleavage-induced enhanced availability of antigenic determinants residing outside the BC loop was also shown by increased neutralization titers of monoclonal antibodies specific for some of these other determinants. These results indicate that by using a trypsin-cleaved type 3 poliovirus as a parenteral immunogen, it is possible to change the distribution of antigenic site specificities of neutralizing antibodies to resemble that following poliovirus infection.

Construction of a poliovirus type 1/type 2 antigenic hybrid by manipulation of neutralization antigenic site II

There are three serotypes of poliovirus, poliovirus type 1 (PV-1), PV-2, and PV-3. These viruses each display four distinct neutralization antigenic sites, designated N-AgI, N-AgII, N-AgIIIA, and N-AgIIIB. It has been demonstrated previously that part of N-AgI can be replaced with heterogeneous amino acid sequences, resulting in hybrid viruses expressing heterogeneous antigenic determinants. To study whether hybrid viruses could be constructed by modifying another antigenic site, a part of N-AgII (amino acids 158 to 173 of VP2) of PV-1(Mahoney) was replaced with the equivalent sequence from PV-2(Lansing). The resulting hybrid was viable and expressed both PV-1 and PV-2 antigenic determinants. When inoculated into rabbits, the hybrid induced neutralizing antibodies against both PV-1 and PV-2, showing that amino acids 158 to 173 of VP2 are able to function as an antigenic site independent of the rest of N-AgII. Manipulation of N-AgII represents a useful alternative method for the production of hybrid polioviruses.

Insertion of short hepatitis virus A amino acid sequences into poliovirus antigenic determinants results in viable progeny

In an infectious poliovirus cDNA construct, the determinant encoding antigenic epitope N-Ag1 (in a loop located between two beta-strands in poly-peptide VP1) was altered by site-directed mutagenesis, to be partially similar with the determinants for presumptive epitopes in polypeptides VP1 or VP3 of hepatitis A virus (HAV). The modified constructs proved to be infectious. However, another construct, in which the same locus encoded a 'nonsense' and a relatively hydrophobic amino acid sequence, exhibited no infectivity. These data showed the feasibility of the insertion of foreign sequences in a specific antigenically active locus of the poliovirus icosahedron, and suggest some limitations with respect to the sequences to be 'transplanted'.

Chimeric picornavirus polyproteins demonstrate a common 3C proteinase substrate specificity

Cross-species proteolytic processing was demonstrated by the 3C proteinases of human rhinovirus 14 and coxsackievirus B3 on poliovirus-specific polypeptide precursors. Chimeric picornavirus cDNA genomes were constructed in a T7 transcription vector in which the poliovirus 3C coding region was substituted with the corresponding allele from human rhinovirus 14 or coxsackievirus B3. In vitro translation and processing of the polypeptides encoded by the chimeric genomes demonstrated that the proteolytic processing of poliovirus P2 region (nonstructural) proteins could be functionally substituted by the heterologous proteinases. In contrast, the 3C proteinase activities expressed from the chimeric genomes were incapable of recognizing the poliovirus-specific processing sites within the capsid precursor. Since the amino acid sequences flanking and inclusive of the P2 region cleavage sites of the three viruses are not stringently conserved, these results provide evidence for the existence of common conformational determinants necessary for 3C-mediated processing.

The antigenic structure of poliovirus

We have solved the structure of the Mahoney strain of type 1 and the Sabin (attenuated vaccine) strain of type 3 poliovirus by X-ray crystallographic methods. By providing a three-dimensional framework for the interpretation of a wealth of experimental data, the structures have yielded insight into the architecture and assembly of the virus particle, have provided information regarding the entry of virus into susceptible cells, and defined the sites on the virus particle that are recognized by neutralizing monoclonal antibodies. Thus locating mutations in variants selected for resistance to neutralizing monoclonal antibodies has defined three antigenic sites of the surface of the virion, and provided clues as to the mechanisms by which viruses escape neutralization. Finally, comparison of the structures of the two strains, together with analysis of sequences of many poliovirus strains, have begun to define the structural changes associated with serotypic differences between polioviruses.

An engineered poliovirus chimaera elicits broadly reactive HIV-1 neutralizing antibodies

The Sabin type 1 vaccine strain of poliovirus is probably the safest and most successful live-attenuated vaccine virus used in humans. Its widespread use since the early 1960s has contributed significantly to the virtual eradication of poliomyelitis in developed countries. We have reported previously the construction of an intertypic antigen chimaera of poliovirus, based on the Sabin 1 strain, and proposed that this virus could be modified to express on its surface antigenic determinants from other pathogens. We describe here the construction and characterization of a poliovirus antigen chimaera containing an epitope from the transmembrane glycoprotein (gp41) of human immunodeficiency virus type 1 (HIV-1). In antibody absorption experiments, the virus chimaera inhibited neutralization of HIV-1 by antipeptide monoclonal antibodies specific for the gp41 epitope and significantly reduced the group specific neutralizing activity of HIV-1-positive human sera. Rabbit antisera raised by subcutaneous injection of the polio/HIV chimaera in adjuvant was shown to be specific for HIV-1 gp41 in peptide-binding assays and by western blotting. Moreover, the antisera neutralized a wide range of American and African HIV-1 isolates and also inhibited virus-induced cell fusion. Monoclonal antibodies against the HIV-1 derived regions of the chimaera also neutralized HIV-1. These results establish the potential of using poliovirus for the presentation of foreign antigens and suggest that Sabin 1 poliovirus/HIV chimaeras could offer an approach to the development of an HIV vaccine.

Genetic variation occurring on the genome of an in vitro insertion mutant of poliovirus type 1

An insertion sequence of 72 nucleotides prepared from a polylinker sequence of plasmid pUC18 was introduced at nucleotide position 702 of the 5' noncoding sequence (742 nucleotides long) of the genome of the Sabin strain of poliovirus type 1 by using an infectious cDNA clone of the virus strain. The insertion mutant thus obtained showed a small-plaque phenotype compared with that of the parent virus. Apparent revertants (large-plaque variants) were easily generated from the insertion mutant. Nucleotide sequence analysis was performed on the revertant genomes to determine the mutation(s) by which the plaque size of the parent virus was regained. Some large-plaque variants lacked genomic sequences including all or a part of the insertion sequence. A computer-aided search for secondary structures with respect to the deletion sites detected possible supporting sequences which provided fairly stable secondary structures at the deletion sites. This result was consistent with our supporting sequence-loop model which had been proposed as a new copy-choice model for the generation of genetic rearrangements occurring on single-stranded RNA genomes (S. Kuge, I. Saito, and A. Nomoto, J. Mol. Biol. 192:473-487, 1986). The other large-plaque variants had point mutations at any one of three positions of an AUG existing in the insertion sequence. A small-plaque phenotype was observed when an AUG codon was inserted in frame or out of frame with regard to the initiation site of viral polyprotein synthesis. Our data strongly suggest that an AUG sequence in this genome region is deleterious for efficient poliovirus replication.