Localization of a poliovirus type 1 neutralization epitope in viral capsid polypeptide VP1

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.

Effects of the thermal denaturation of Sabin-derived inactivated polio vaccines on the D-antigenicity and the immunogenicity in rats

The efficacy of a Sabin-derived inactivated polio vaccine (sIPV) can be evaluated by measuring the immunogenicity and the contents of D-antigens, which induce the neutralizing antibodies. The immunogenic potency test in rats was done as a national assay in Japan. The two manufacturers of sIPV in Japan have performed both assays since development, and there is no clear discrepancy between the results obtained in the two assays. To further know the relationship between the two assays, we analyzed the effects of the heat treatment of sIPV on the D-antigenicity and the immunogenicity. We observed that the marginal D-antigen that remained after the thermal treatment was capable of inducing relatively high neutralizing antibodies in rats. This indicates that the measurement of the D-antigen contents as part of the quality control of sIPV is more sensitive and appropriate to detect denatured vaccines.

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.

Conformational shift of a major poliovirus antigen confirmed by immuno-cryogenic electron microscopy

Small, interfacial conformational changes occur in some Ag-Ab interactions. Using cryogenic electron microscopy (cryo-EM), we have demonstrated such changes in a major antigenic site of a poliovirus capsid protein. During cell entry, native human poliovirus (160S particle) converts to a cell entry intermediate (135S particle) and later to an RNA-released (80S) particle. By mixing particles with Fabs of the neutralizing C3 mAb, we labeled the external loop connecting the B and C β-strands (BC loop) of the capsid protein VP1 (residues 95-105) in the 160S and 135S states. We then determined three-dimensional structures by cryo-EM and enhanced their interpretability by fitting high-resolution coordinates of C3 Fab and the capsid proteins into the density maps. Binding of C3 to either 160S or 135S particles caused residues of the BC loop, located on the tip of a prominent peak known as the "mesa," to move by an estimated 5 Å. C3 Abs are neutralizing and can bind bivalently. The orientation of the bound Fabs in our reconstructions suggests that C3 neutralizes poliovirus by binding two adjacent BC loops on the same mesa and inhibiting conformational changes in the viral capsid.

High-titred neutralizing antibodies to human enterovirus 71 preferentially bind to the N-terminal portion of the capsid protein VP1

Human enterovirus 71 has emerged as an important pathogen of children in the Asia Pacific region, and it may be important to consider the development of a vaccine against this virus. Human cord serum was used as a source of neutralizing antibodies to determine whether the N- or C-terminal half of the VP1 capsid protein was more likely to harbour neutralizing determinants. Cord sera from 205 individuals were tested for neutralizing antibodies against human enterovirus 71 in an indirect ELISA against recombinant VP1 antigen as well as the N- and C-terminal portions of VP1 antigen. High-titred human neutralizing antibodies were significantly more reactive with the N-terminal half of VP1 than weak or negative sera. The N-terminal half of human enterovirus 71 is likely to have important neutralizing antibody determinants and should be investigated further in vaccine development efforts.

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.

Protection against lethal enterovirus 71 infection in newborn mice by passive immunization with subunit VP1 vaccines and inactivated virus

Enterovirus 71 (EV71), the newest member of Enteroviridae, is notable for its etiological role in epidemics of severe neurological diseases in children. Developing effective vaccines is considered a top choice among all control measures. We compared the inactivated virus vaccine (10 microg protein/mouse) with subunit vaccines--VP1 DNA vaccine (100 microg/mouse) or recombinant VP1 protein (10 microg/mouse)--in its ability to elicit maternal antibody and to provide protection against lethal infection of EV71 in suckling mice. Prior to gestation, all three groups of vaccinated dams possessed similar levels of neutralizing antibody. With a challenge dose of 2300 LD(50) virus/mouse, suckling mice born to dams immunized with inactivated virus showed 80% survival. The subunit vaccines provided protection only at a lower challenge dosage of 230 LD(50) per mouse, with 40% survival for DNA vaccine and 80% survival for VP1 protein. The cytokine profile produced by splenocytes showed a high level of IL-4 in the inactivated virus group, high levels of IFN-gamma and IL-12 in the DNA vaccine group, and high levels of IL-10 and IFN-gamma in the VP1 protein group. Overall, the inactivated virus elicited a much greater magnitude of immune response than the subunit vaccines, including total IgG, all four IgG subtypes, and T-helper-cell responses; these antibodies were shown to be protective against lethal infection when passively transferred to susceptible newborn mice. Our data indicated that inactivated virus is the choice of vaccine preparation capable of fulfilling the demand for effective control, and that VP1 subunit vaccines remain promising vaccine strategies that require further refinement.

Minor displacements in the insertion site provoke major differences in the induction of antibody responses by chimeric parvovirus-like particles

An antigen-delivery system based on hybrid virus-like particles (VLPs) formed by the self-assembly of the capsid VP2 protein of canine parvovirus (CPV) and expressing foreign peptides was investigated. In this report, we have studied the effects of inserting the poliovirus C3:B epitope in the four loops and the C terminus of the CPV VP2 on the particle structure and immunogenicity. Epitope insertions in the four loops allowed the recovery of capsids in all of the mutants. However, only insertions of the C3:B epitope in VP2 residue 225 of the loop 2 were able to elicit a significant anti-peptide antibody response, but not poliovirus-neutralizing antibodies, probably because residue 225 is located in an small depression of the surface. To fine modulate the insertion site in loop 2, a cassette-mutagenesis was carried out to insert the epitope in adjacent positions 226, 227, and 228. The epitope C3:B inserted into these positions was well recognized by the specific monoclonal antibody C3 by immunoelectron microscopy. BALB/c mice immunized with these chimeric C3:B CPV:VLPs were able to elicit an strong neutralizing antibody response (>3 log(10) units) against poliovirus type 1 (Mahoney strain). Therefore, minor displacements in the insertion place cause dramatic changes in the accessibility of the epitope and the induction of antibody responses.

Engineering parvovirus-like particles for the induction of B-cell, CD4(+) and CTL responses

An antigen delivery system based on hybrid recombinant parvovirus-like particles (VLPs) formed by the self-assembly of the capsid VP2 protein of porcine (PPV) or canine parvovirus (CPV) expressed in insect cells with the baculovirus system has been developed. PPV:VLPs containing a CD8(+) epitope from the LCMV nucleoprotein evoked a potent CTL response and were able to protect mice against a lethal infection with the virus. Also, PPV:VLPs containing the C3:T epitope from poliovirus elicited a CD4(+)3 log(10) units) against poliovirus. The possibility of combining different types of epitopes in different positions of a single particle to stimulate different branches of the immune system paves the way to the production of more potent vaccines in a simple and cheap way.

RGD inclusion in the hexon monomer provides adenovirus type 5-based vectors with a fiber knob-independent pathway for infection

Hypervariable region 5 (HVR5) is a hydrophilic, serotypically nonconserved loop of the hexon monomer which extrudes from the adenovirus (Ad) capsid. We have replaced the HVR5 sequence of Ad5 with that of heterologous peptides and studied their effects on virus viability and peptide accessibility. A poliovirus model epitope was first inserted in a series of nine "isogenic" viruses that differed in their flanking spacers. Whereas virus productivity was not profoundly altered by any of these modifications, immunoprecipitation experiments under nondenaturing conditions demonstrated that epitope recognition by its cognate monoclonal antibody (C3 MAb) was strongly linker dependent and correlated perfectly with the ability of C3 MAb to inhibit transgene delivery and expression. An alphav-specific ligand (DCRGDCF) was then inserted in a suitable linker context to investigate whether hexon-modified capsids would enhance the transduction of cells displaying limiting amounts of the virus attachment receptors. Interestingly, although hexon has never been implicated in Ad entry, the modified virus significantly increased the transduction of human vascular smooth muscle cells in vitro. Competition experiments with 293 cells saturated with recombinant knob further indicated that the hexon-modified virus could use an additional, knob-independent pathway for entry. We concluded that genetic engineering of the Ad5 hexon monomer constitutes a novel and feasible approach to equip the virus with additional targeting ligands.

Persistent poliovirus infection of human fetal brain cells

It has been suggested that poliovirus (PV), the causative agent of poliomyelitis, could persist in surviving patients. We have previously shown that PV can persistently infect some human cell lines in vitro, particularly neuroblastoma cell lines. We report here an ex vivo model in which PV can persistently infect primary cultures of human fetal brain cells. Two mutations involving capsid residues 142 of VP2 and 95 of VP1 were repeatedly selected during the persistent infections. These residues are located in capsid regions known to be involved in interactions between PV and its receptor. During the first week after infection, viral antigens were found in cells of both the neuronal and glial lineages. In contrast, 2 weeks after infection, viral antigens were detected almost exclusively in cells of the neuronal lineage. They were detected predominantly in cells expressing a marker of early commitment to the neuronal lineage, MAP-5, particularly in neuroblasts. Viral antigens were also found in immature progenitors expressing a neuroepithelium marker, nestin, and in cells expressing a marker of postmitotic neurons, MAP-2. The presence of viral antigens in postmitotic neurons suggests that PV can persist in neurons of patients who have survived poliomyelitis.

Phage display of intact domains at high copy number: a system based on SOC, the small outer capsid protein of bacteriophage T4

Peptides fused to the coat proteins of filamentous phages have found widespread applications in antigen display, the construction of antibody libraries, and biopanning. However, such systems are limited in terms of the size and number of the peptides that may be incorporated without compromising the fusion proteins' capacity to self-assemble. We describe here a system in which the molecules to be displayed are bound to pre-assembled polymers. The polymers are T4 capsids and polyheads (tubular capsid variants) and the display molecules are derivatives of the dispensable capsid protein SOC. In one implementation, SOC and its fusion derivatives are expressed at high levels in Escherichia coli, purified in high yield, and then bound in vitro to separately isolated polyheads. In the other, a positive selection vector forces integration of the modified soc gene into a soc-deleted T4 genome, leading to in vivo binding of the display protein to progeny virions. The system is demonstrated as applied to C-terminal fusions to SOC of (1) a tetrapeptide; (2) the 43-residue V3 loop domain of gp120, the human immunodeficiency virus type-1 (HIV-1) envelope glycoprotein; and (3) poliovirus VP1 capsid protein (312 residues). SOC-V3 displaying phage were highly antigenic in mice and produced antibodies reactive with native gp120. That the fusion protein binds correctly to the surface lattice was attested in averaged electron micrographs of polyheads. The SOC display system is capable of presenting up to approximately 10(3) copies per capsid and > 10(4) copies per polyhead of V3-sized domains. Phage displaying SOC-VP1 were isolated from a 1:10(6) mixture by two cycles of a simple biopanning procedure, indicating that proteins of at least 35 kDa may be accommodated.

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.

Genetic insertion and exposure of a reporter epitope in the ferrichrome-iron receptor of Escherichia coli K-12

The ferrichrome-iron receptor of Escherichia coli K-12 is FhuA (M(r), 78,992), the first component of an energy-dependent, high-affinity iron uptake pathway. FhuA is also the cognate receptor for bacteriophages T5, T1, phi 80, and UC-1, for colicin M and microcin 25, and for albomycin. To probe the topological organization of FhuA which enables recognition of these different ligands, we generated a library of 16 insertion mutations within the fhuA gene. Each insertion spliced a 13-amino-acid antigenic determinant (the C3 epitope of poliovirus) at a different position within FhuA. Immunoblotting of outer membranes with anti-FhuA and anti-C3 antibodies indicated that 15 of 16 FhuA.C3 proteins were present in the outer membrane in amounts similar to that observed for plasmid-encoded wild-type FhuA. One chimeric protein with the C3 epitope inserted after amino acid 440 of FhuA was present in the outer membrane in greatly reduced amounts. Strains overexpressing FhuA.C3 proteins were subjected to flow cytometric analysis using anti-FhuA monoclonal antibodies. Such analysis showed that (i) the chimeric proteins were properly localized and (ii) the wild-type FhuA protein structure had not been grossly altered by insertion of the C3 epitope. Twelve of sixteen strains expressing FhuA.C3 proteins were proficient in ferrichrome transport and remained sensitive to FhuA-specific phages. Three FhuA.C3 proteins, with insertions after amino acid 321, 405, or 417 of FhuA, were detected at the cell surface by flow cytometry using anti-C3 antibodies. These three chimeric proteins were all biologically active. We conclude that amino acids 321, 405, and 417 are surface accessible in wild-type FhuA.

CS31A capsule-like antigen as an exposure vector for heterologous antigenic determinants

CS31A is a multimeric surface protein surrounding certain enterotoxigenic and septicemic bovine Escherichia coli strains. The possibility of using CS31A as a carrier of foreign antigenic determinants was investigated. Introduction of heterologous viral epitopes into the CS31A major subunit, ClpG, was successfully performed in the V3 region of the molecule which encodes for an immunodominant linear epitope. E. coli K-12 strains producing the hybrid CS31A molecules or the purified chimeric proteins were used for mice immunization. By using the C epitope derived from the S protein of the porcine transmissible gastroenteritis virus, significant antiviral antibody titers were elicited and seroneutralization of the virus was demonstrated, confirming that the molecular environment in V3 is favorable for antigen presentation. These results indicate that synthesis of CS31A hybrid proteins by the wild-type strain 31A could become a powerful tool for the development of oral vaccines directed against mucosal pathogens.

Identification of T-helper epitopes in the VP1 capsid protein of poliovirus

Poliovirus-specific T lymphocytes were isolated from virus-immunized mice of different H-2 haplotypes. Immunological characterization of this population indicates that the effector population involved in the observed poliovirus-specific proliferative response was that of CD4-positive T-helper cells. Proliferative responses also were induced within these T-lymphocyte populations upon stimulation with either purified VP1 capsid protein or VP1 synthetic peptides. By using these synthetic peptides, several T-helper epitopes were identified. Generally, proliferative responses were observed in three regions of VP1. Two regions spanning VP1 residues 86 to 120 and 201 to 241 were recognized by T lymphocytes from BALB/c (H-2d), C57BL/6 (H-2b), and C3H/HeJ (H-2k) backgrounds. Analyses using synthetic peptides of nonoverlapping sequences indicated that the region spanning residues 201 to 241 may contain several T epitopes and may account for the strong proliferative response observed. In addition, for two of the three haplotypes examined, T epitopes were observed within residues 7 to 24 of VP1. Additional epitopes which appeared to be restricted to specific H-2 backgrounds were identified. T epitopes within VP1 that are common between different strains of mice appeared to lie within previously identified neutralizing antigenic sites in poliovirus.

Identification of a T-cell epitope adjacent to neutralization antigenic site 1 of poliovirus type 1

Proliferative T-cell responses to poliovirus in various strains of mice have been analyzed by using either killed purified virus or capsid protein VP1 synthetic peptides. Following immunization of mice with inactivated poliovirus type 1 (PV1), a specific proliferative response of their lymph node CD4+ T cells was obtained after in vitro stimulation with purified virus. In mice immunized with PV1, PV2, or PV3, a strong cross-reactivity of the T-cell responses was observed after in vitro stimulation with heterologous viruses. By using various strategies, a dominant T-cell epitope was identified in the amino acid 103 to 115 region of capsid polypeptide VP1, close by the C3 neutralization epitope. The T-cell response to VP1 amino acids 103 to 115 is H-2 restricted: H-2d mice are responders, whereas H-2k and H-2b mice do not respond to this T-cell epitope. Immunization of BALB/c (H-2d) mice with the uncoupled p86-115 peptide, which represents VP1 amino acids 86 to 115 and contains both the T-cell epitope and the C3 neutralization epitope, induced poliovirus-specific B- and T-cell responses. Moreover, these mice developed poliovirus neutralizing antibodies.

Permissive sites and topology of an outer membrane protein with a reporter epitope

We are developing a genetic approach to study with a single antibody the folding and topology of LamB, an integral outer membrane protein from Escherichia coli K-12. This approach consists of inserting the same reporter foreign antigenic determinant (the C3 epitope from poliovirus) at different sites of LamB so that the resulting hybrid proteins have essentially kept the in vivo biological properties of LamB and therefore its cellular location and structure; the corresponding sites are called permissive sites. A specific monoclonal antibody can then be used to examine the position of the reporter epitope with respect to the protein and the membrane. We present an improved and efficient procedure that led us to identify eight new permissive sites in LamB. These sites appear to be distributed on both sides of the membrane. At one of them (after residue 253), the C3 epitope was detected on intact bacteria, providing the first direct argument for exposure of the corresponding LamB region at the cell surface. At this site as well as at four others (after residues 183, 219, 236, and 352), the C3 epitope could be detected with the C3 monoclonal antibody at the surface of the extracted trimeric LamB-C3 hybrid proteins. We provide a number of convergent arguments showing that the hybrid proteins are not strongly distorted with respect to the wild-type protein so that the conclusions drawn are also valid for this protein. These conclusions are essentially in agreement with the proposed folding model for the LamB protein. They agree, in particular, with the idea that regions 183 and 352 are exposed to the periplasm. In addition, they suggest that region 236 is buried at the external face of the outer membrane and that region 219 is exposed to the periplasm. Including the 3 sites previously determined, 11 permissive sites are now available in LamB, including 3 at the cell surface and most probably at least 3 in the periplasm. We discuss the nature of such sites, the generalization of this approach to other proteins, and possible applications.

Structural factors modulate the activity of antigenic poliovirus sequences expressed on hybrid hepatitis B surface antigen particles

We have studied the functional expression of antigenic poliovirus fragments carried by various hybrid hepatitis B surface antigen (HBsAg) particles. Several constructions were made by using two different insertion sites in the HBsAg molecule (amino acid positions 50 and 113) and two different sequences, one derived from poliovirus type 1 (PV-1) and the other from PV-2. The inserted fragments each encompassed residues 93 to 103 of the capsid protein VP1, a segment which includes the linear part of the neutralization antigenic site 1 of the poliovirus. The antigenicity and immunogenicity of the hybrid particles were evaluated and compared in terms of poliovirus neutralization. A high level of antigenic and immunogenic activity of the PV-1 fragment was obtained by insertion at position 113 but not at position 50 of HBsAg. However, a cooperative effect was observed when two PV-1 fragments were inserted at both positions of the same HBsAg molecule. Antibodies elicited by the PV-2 fragment inserted at amino acid position 113 did not bind or neutralize the corresponding poliovirus strain. They did, however, bind a chimeric poliovirus in which the homologous antigenic fragment of PV-1 had been replaced by that of PV-2. The only virions that were neutralized by these antibodies were certain mutants carrying amino acid substitutions within the PV-2 fragment. These results show that position, constraints from the carrier protein, and nature of the inserted sequences are critically important in favoring or limiting the expression of antigenic fragments as viral neutralization immunogens.

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.

Cell-induced conformational change in poliovirus: externalization of the amino terminus of VP1 is responsible for liposome binding

Upon attachment to susceptible cells, poliovirus and a number of other picornaviruses undergo conformational transitions which result in changes in antigenicity, increased protease sensitivity, the loss of the internal capsid protein VP4, and a loss of the ability to attach to cells. These conformationally altered particles have been characterized by using a number of sequence-specific probes, including two proteases, a panel of antiviral monoclonal antibodies, and a panel of antisera against synthetic peptides which correspond to sequences from the capsid protein VP1. With these probes, cell-altered virus is clearly distinguishable from native and heat-inactivated virions. The probes also demonstrate that the cell-induced conformational change alters the accessibility of several regions of the virus. In particular, the amino terminus of VP1, which is entirely internal in the native virion, becomes externalized. Unlike native and heat-inactivated virus, cell-altered virions are able to attach to liposomes. The exposed amino terminus of VP1 is shown to be responsible for liposome attachment. We propose that during infection the amino terminus of VP1 inserts into endosomal membranes and thus plays a role in the mechanism of cell entry.

Molecular basis for linkage of a continuous and discontinuous neutralization epitope on the structural polypeptide VP2 of poliovirus type 1

We obtained neutralizing monoclonal antibodies against a continuous neutralization epitope on VP2 of poliovirus type 1 strain Mahoney by using a combined in vivo-in vitro immunization procedure. The antibody-binding site was mapped to amino acid residues within the peptide segment (residues 164 through 170) of VP2 by competition with synthetic peptide and sequencing of resistant mutants. Cross-neutralization of these mutants with another neutralizing monoclonal antibody revealed a linkage of the continuous epitope and a discontinuous neutralization epitope involving both loops of the double-loop structure of VP2 at the twofold axis on the surface of the virion.

Mapping of viral epitopes with prokaryotic expression products

Several systems are available for the expression of foreign gene sequences in Escherichia coli. We describe the use of prokaryotic expression products of viral gene fragments in order to identify the regions that specify the binding sites of antibodies. This approach is particularly successful if the antigenicity does not depend on the native protein, but only on the amino acid sequence, i.e., if the epitope is sequential. Combining prokaryotic expression with the use of synthetic peptides often permits a fast and accurate mapping of an epitope. The occurrence of immunodominant sequential epitopes on the surfaces of viruses seems to be a widespread phenomenon.

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.

Evidence for a complex structure of neutralization antigenic site I of poliovirus type 1 Mahoney

We have selected neutralization escape mutants by using a monoclonal antibody (nt-MAb) against a sequential epitope between amino acids 93 through 104 (neutralization antigenic site I) of poliovirus type 1 Mahoney. The majority of mutants were also resistant against five strain-specific nt-MAbs which recognized conformation-dependent epitopes, suggesting that the neutralization antigenic site I must be involved in the formation of such epitopes. An analysis of all mutants by the binding of nt-MAbs and by isoelectric focusing of VP1 allowed discrimination of five classes of mutants. Sequence analysis of mutant RNAs revealed point mutations and deletions in the antibody-binding site.

Three-dimensional structure of poliovirus serotype 1 neutralizing determinants

Antigenic mutants of poliovirus (Sabin strain, serotype 1) were isolated by the resistance of the virus to anti-Sabin neutralizing monoclonal antibodies. The amino acid replacements within the capsid protein sequence causing the altered antigenicity were identified for each of 63 isolates. The mutations cluster into distinct nonoverlapping peptide segments that group into three general immunological phenotypes on the basis of cross-neutralization analyses with 15 neutralizing anti-Sabin monoclonal antibodies. Location of the mutated amino acid residues within the three-dimensional structure of the virion indicates that the majority of these amino acid residues are highly exposed and located within prominent structural features of the viral surface. Those mutated amino acid residues that are less accessible to antibody interaction are often involved in hydrogen bonds or salt bridges that would stabilize the local tertiary structure of the antigenic site. The interactions of the peptide segments that form these neutralizing sites suggest specific models for the generation of neutralization-resistant variants and for the interaction between the viral surface and antibody.

Construction of a chimaeric type 1/type 2 poliovirus by genetic recombination

A chimaeric poliovirus carrying a type-2-specific neutralization epitope on a type 1 capsid was created by site-directed mutagenesis of the Mahoney strain of poliovirus type 1. An EcoRV and a HindIII restriction sites were first constructed in the cDNA of poliovirus type 1 at nucleotide positions 2756 and 2786, respectively, i.e. on either side of the sequence encoding neutralization epitope C3 (VP1 amino acids 93-103), which is part of neutralization site NImI. The cDNA sequence framed by the two sites was next taken out and replaced by custom-made oligonucleotides encoding the equivalent region of VP1 from the Lansing strain of poliovirus type 2. The DNA from the plasmid carrying such a hybrid construct was transfected onto CV1 cells generating a chimaeric virus, v510. Neutralization of v510 with a panel of monoclonal antibodies showed that v510 has lost the poliovirus type 1 C3 epitope but acquired a new, poliovirus type-2-specific neutralization epitope. Preliminary results indicate that v510 also shows neurovirulence for mice, which is a specific trait of the Lansing strain of poliovirus type 2.

Regions of poliovirus protein VP1 produced in Escherichia coli induce neutralizing antibodies

Poliovirus type 1 cDNA was prepared from viral RNA encoding the VP1 capsid region of the virus by using a specific DNA primer and was cloned in Escherichia coli. DNA fragments corresponding to VP1 amino acid positions 129 to 302 (pPM5k3), 52 to 302 (pPMhae3), and 24 to 129 (pPMDxba) were incorporated into plasmid vectors designed to express Trp LE-poliovirus VP1 fusion proteins under the control of the inducible tryptophan promoter-operator system. Induction of bacterial cultures containing the plasmids resulted in the production of fusion proteins which accounted for 21% (pPMhae3), 68% (pPM5k3), and 27% (pPMDxba) of the total cell protein. The proteins were purified, and each reacted with polyclonal antibodies raised against intact virions as measured by an enzyme-linked immunosorbent assay. The sera from rabbits immunized with the bacterially produced fusion proteins pPMDxba and pPMhae3 contained poliovirus-neutralizing antibodies.

Modulation of humoral response to a 12-amino-acid site on the poliovirus virion

Most monoclonal antibodies to poliovirus 3 but not poliovirus 1 require a single 12-amino-acid sequence in virion protein VP1 for neutralization (site 1). None of the available monoclonal antibodies requiring this site bound virions after tryptic cleavage of site 1. This result allowed the amount of site 1-specific antibodies to be determined in an antiserum by comparing its reactivity with virus and trypsin-cleaved virus. Antisera to poliovirus 3 Sabin strain (PS3) but not poliovirus 1 Sabin showed site 1 immunodominance, consistent with the frequency of isolation of site 1-specific monoclonal antibodies to these viruses. Cleavage of site 1 prior to immunization dramatically reduced the immunogenicity of this site in PS3. However, the antiserum against trypsin-cleaved PS3 still had a high neutralization titer, demonstrating that sites other than site 1 can elicit a neutralizing response to PS3. Other antisera to PS3 showed significant variability in the response to site 1, indicating that other factors, such as the genetic background of inbred mouse strains, the species immunized, and the immunization protocol, also affect immunodominance. In particular, a serum from a human infant recently immunized with oral trivalent vaccine had little response to site 1.

Genetic analysis of the attenuation phenotype of poliovirus type 1

Seven different recombinant viruses from the virulent Mahoney and the attenuated Sabin parental strains of type 1 poliovirus were constructed in vitro by using infectious cDNA clones. Monkey neurovirulence tests (lesion score, spread value, and incidence of paralysis) using these recombinant viruses revealed that the loci influencing attenuation were spread over several areas of the viral genome, including the 5' noncoding region. In vitro phenotypic marker tests corresponding to temperature sensitivity of growth (rct marker), plaque size, and dependency of growth on bicarbonate concentration (d marker) were performed to identify the genomic loci of these determinants and to investigate their correlation with attenuation. Determinants of temperature sensitivity mapped to many areas of the viral genome and expressed strong but not perfect correlation with attenuation. Recombinant viruses with Sabin-derived capsid proteins showed a small-plaque phenotype, and their growth was strongly dependent on bicarbonate concentration, suggesting that these determinants map to the genomic region encoding the viral capsid proteins. Plaque size and the d marker, however, were found to be poor indicators of attenuation. Moreover, virion surface characteristics such as immunogenicity and antigenicity had little or no correlation with neurovirulence. Nevertheless, viruses carrying Sabin-derived capsid proteins had an apparent tendency to exhibit less neurovirulence in tests on monkeys compared with recombinants carrying Mahoney-derived capsid proteins. Our results suggest that the extent of viral multiplication in the central nervous system of the test animals might be one of the most important factors determining neurovirulence. Moreover, we conclude that the expression of the attenuated phenotype of the Sabin 1 strain of poliovirus is the result of several different biological characteristics. Finally, none of the in vitro phenotypic markers alone can serve as a good indicator of neurovirulence or attenuation.

Mutations conferring resistance to neutralization with monoclonal antibodies in type 1 poliovirus can be located outside or inside the antibody-binding site

Antigenic variants resistant to eight neutralizing monoclonal antibodies were selected from wild (Mahoney) and attenuated (Sabin) type 1 infectious poliovirions. Cross-immunoprecipitation revealed interrelationships between epitopes which were not detected by cross-neutralization. Operational analysis of antigenic variants showed that seven of eight neutralization epitopes studied were interrelated. Only one neutralization epitope, named Kc, varied independently from all the others. This latter, recognized by C3 neutralizing monoclonal antibody, was present not only on infectious virions but also on heat-denatured (C-antigenic) particles and on isolated capsid protein VP1. Loss of the neutralization function of an epitope did not necessary result from the loss of its antibody-binding capacity. Such potential, but not functional, neutralization epitopes exist naturally on Mahoney and Sabin 1 viruses. Their antibody-binding property could be disrupted by isolating antigenic variants in the presence of the nonneutralizing monoclonal antibody and anti-mouse immunoglobulin antibodies. Single-point mutations responsible for the acquisition of resistance to neutralization in the antigenic variants were located by sequence analyses of their genomes. Mutants selected in the presence of C3 neutralizing monoclonal antibody always had the mutation located inside the antibody-binding site (residues 93 through 103 of VP1) at the amino acid position 100 of VP1. On the contrary, antigenic variants selected in the presence of neutralizing monoclonal antibodies reacting only with D-antigenic particles had mutations situated in VP3, outside the antibody-binding site (residues 93 through 103 of VP1). The complete conversion of the Mahoney to the Sabin 1 epitope map resulted from a threonine-to-lysine substitution at position 60 of VP3.

Structure of a human common cold virus and functional relationship to other picornaviruses

We report the first atomic resolution structure of an animal virus, human rhinovirus 14. It is strikingly similar to known icosahedral plant RNA viruses. Four neutralizing immunogenic regions have been identified. These, and corresponding antigenic sequences of polio and foot-and-mouth disease viruses, reside on external protrusions. A large cleft on each icosahedral face is probably the host cell receptor binding site.

In vitro phenotypic markers of a poliovirus recombinant constructed from infectious cDNA clones of the neurovirulent Mahoney strain and the attenuated Sabin 1 strain

Infectious cDNA corresponding to the entire genome of the attenuated Sabin strain of type 1 poliovirus has been inserted into EcoRI site of bacterial plasmid pBR325. Two consecutive PstI fragments (nucleotide positions 1814 to 3421) of the infectious cDNA of the Sabin 1 strain were replaced by the corresponding DNA fragments prepared from an infectious DNA clone of the genome of the virulent Mahoney strain of poliovirus type 1. The exchanged segment encodes capsid protein VP1 and part of capsid protein VP3, a region in which a large number of amino acid differences between the attenuated Sabin and the parental, neurovirulent Mahoney strain cluster. The recombinant virus was obtained by DNA transfection of HeLa S3 cells, and several in vitro phenotypes of the virus were compared with those of the parental viruses. The recombinant virus was recognized by a neutralizing monoclonal antibody specific to the Mahoney strain. Growth of the Sabin strain of poliovirus has been shown to be quite dependent upon the bicarbonate concentration (d marker). The growth of the recombinant virus, however, was not highly dependent upon the concentration of bicarbonate in cell culture media, and thus resembled that of the Mahoney strain. On the other hand, the temperature-sensitive multiplication (rct marker) and the small-plaque morphology of the recombinant virus corresponded to the phenotype of the Sabin 1 strain. The in vitro recombination of infectious cDNA clones of genomic RNA and subsequent analysis of the growth properties of the recombinant virus have allowed us to correlate specific mutations in the genome of an RNA virus with certain biological characteristics of that virus.

Identification of a new neutralization antigenic site on poliovirus coat protein VP2

Major neutralization antigenic sites have been previously mapped by us on VP1, the largest capsid protein of poliovirus type 1. Here we report the first identification of the primary sequence of a neutralization antigenic site on capsid protein VP2. Inspection of the amino acid sequence of VP2 led to the selection and synthesis of a peptide (n = 12) that, after linking to a carrier protein, induced an antiviral neutralizing antibody response in rabbits. The response was augmented by a single subsequent inoculation of intact virus; thus, the peptide was also capable of priming the production of neutralizing antibodies. These antibodies were directed only against the site specified by the synthetic peptide. Although the VP2-specific neutralization antigenic site appears not to be strongly immunogenic in the intact virion, it can nevertheless contribute to neutralization of poliovirus. This observation may be important for the development of peptide vaccines.

The impact of new technologies on vaccine development

The ability to move genetic determinants between species using in vitro gene-manipulation techniques has opened up new approaches to vaccine development. This has rapidly grown into an exciting area of research in both academic and industrial laboratories. There are numerous scientific challenges which require multidisciplinary teams to solve problems in creating new immunogens. This has challenged our existing knowledge about protein structure and conformation, microbial pathogenicity and the immune system. Recombinant-DNA techniques are invaluable as tools of analysis and antigen production. The surface of micro-organisms can also be minutely explored with the use of synthetic peptides and monoclonal antibodies. Nevertheless, these new technologies do not allow us to circumvent the need for detailed understanding of pathogens and the disease process. What is apparent from the work carried out so far is that there are few easy answers to vaccine development and it is not realistic to expect rapid solutions to these problems. As there are many potential targets for constructing novel vaccines for both human and animal diseases, it is helpful to establish some priorities. There is a tendency to look at the existing effective vaccines and simply direct research at producing them more economically or with enhanced safety and stability. The advantage of this approach is that considerable background work will have already been carried out establishing the basis for the application of recombinant DNA techniques. However, this can also lead to conflicts (often within the same institute or company) between the new and old technologies. This could be to the detriment of the new technologies which are still only partly developed and may not be good enough yet to compete with existing vaccines in cost or efficacy. The more ambitious, and eventually more rewarding, approach is to attempt to develop new vaccines where none had existed before. There is a vast untapped market, especially in the parasitic diseases, but the scientific problems may be considerable and much more background work is likely to be necessary. Indeed, most of the work in this area is more accurately referred to as basic research rather than vaccine development as totally new, effective vaccines are still some way off. Having directed research towards a specific organism or disease there are still many options available as to the scientific strategy to adopt. As discussed in this review it may be possible to consider subunits, synthetic antigens and live (attenuated or heterologous) organisms as possible vaccines.(ABSTRACT TRUNCATED AT 400 WORDS)

Epitopes on foot-and-mouth disease virus outer capsid protein VP1 involved in neutralization and cell attachment

Foot-and-mouth disease virus structural protein VP1 elicits neutralizing and protective antibody and is probably the viral attachment protein which interacts with cellular receptor sites on cultured cells. To study the relationships between epitopes on the molecule related to neutralization and cell attachment, we tested monoclonal antibodies prepared against type A12 virus, isolated A12 VP1, and a CNBr-generated A12 VP1 fragment for neutralization and effect on viral absorption. The antibodies selected for analysis neutralized viral infectivity with varying efficiencies. One group of antibodies caused a high degree of viral aggregation and inhibited the adsorption of virus to cells by 50 to 70%. A second group of antibodies caused little or no viral aggregation but inhibited the adsorption of virus to cells by 80 to 90%. One antibody, which is specific for the intact virion, caused little viral aggregation and had no effect on the binding of virus to specific cellular receptor sites. Thus, at least three antigenic areas on the surface of foot-and-mouth disease virus which were involved in neutralization were demonstrated. One of the antigenic sites appears to have been responsible for interaction with the cellular receptor sites on the surface of susceptible cells.

Method to map antigenic determinants recognized by monoclonal antibodies: localization of a determinant of virus neutralization on the feline leukemia virus envelope protein gp70

A method is presented whereby antigenic determinants recognized by specific monoclonal antibodies can be mapped to specific sites on a protein sequence with high resolution. Short DNase I-generated DNA fragments encoding portions of the protein of interest are molecularly cloned into the EcoRI site of the beta-galactosidase gene of phage lambda Charon 16 so as to obtain expression of random protein fragments as fusion proteins. The monoclonal antibody is used to screen the phage library to isolate phage expressing the specific antigenic determinant. DNA of immunoreactive phage can be analyzed rapidly and subcloned to allow DNA sequence determination. The method is generally applicable and permits antigenic determinants of functionally interesting monoclonal antibodies to be mapped and related to specific protein sequences. We have used this procedure to determine the region of the feline leukemia virus envelope protein gp70 recognized by a virus-neutralizing monoclonal antibody, cl.25. Antibody binding was mapped to a 14-amino acid region in the amino-terminal half of gp70. This region may be directly involved in an essential function of the gp70 protein, perhaps in gp70-mediated host recognition functions. Synthetic peptides derived from this region may provide useful vaccine antigens for the prevention of feline leukemia virus-associated disease in cats.