C terminus of infectious bursal disease virus major capsid protein VP2 is involved in definition of the T number for capsid assembly

Infectious bursal disease virus (IBDV), a member of the Birnaviridae family, is a double-stranded RNA virus. The IBDV capsid is formed by two major structural proteins, VP2 and VP3, which assemble to form a T=13 markedly nonspherical capsid. During viral infection, VP2 is initially synthesized as a precursor, called VPX, whose C end is proteolytically processed to the mature form during capsid assembly. We have computed three-dimensional maps of IBDV capsid and virus-like particles built up by VP2 alone by using electron cryomicroscopy and image-processing techniques. The IBDV single-shelled capsid is characterized by the presence of 260 protruding trimers on the outer surface. Five classes of trimers can be distinguished according to their different local environments. When VP2 is expressed alone in insect cells, dodecahedral particles form spontaneously; these may be assembled into larger, fragile icosahedral capsids built up by 12 dodecahedral capsids. Each dodecahedral capsid is an empty T=1 shell composed of 20 trimeric clusters of VP2. Structural comparison between IBDV capsids and capsids consisting of VP2 alone allowed the determination of the major capsid protein locations and the interactions between them. Whereas VP2 forms the outer protruding trimers, VP3 is found as trimers on the inner surface and may be responsible for stabilizing functions. Since elimination of the C-terminal region of VPX is correlated with the assembly of T=1 capsids, this domain might be involved (either alone or in cooperation with VP3) in the induction of different conformations of VP2 during capsid morphogenesis.

Inactivated recombinant plant virus protects dogs from a lethal challenge with canine parvovirus

A vaccine based upon a recombinant plant virus (CPMV-PARVO1), displaying a peptide derived from the VP2 capsid protein of canine parvovirus (CPV), has previously been described. To date, studies with the vaccine have utilized viable plant chimaeric particles (CVPs). In this study, CPMV-PARVO1 was inactivated by UV treatment to remove the possibility of replication of the recombinant plant virus in a plant host after manufacture of the vaccine. We show that the inactivated CVP is able to protect dogs from a lethal challenge with CPV following parenteral immunization with the vaccine. Dogs immunized with the inactivated CPMV-PARVO1 in adjuvant displayed no clinical signs of disease and shedding of CPV in faeces was limited following CPV challenge. All immunized dogs elicited high titres of peptide-specific antibody, which neutralized CPV in vitro. Levels of protection, virus shedding and VP2-specific antibody were comparable to those seen in dogs immunized with the same VP2- peptide coupled to keyhole limpet hemocyanin (KLH). Since plant virus-derived vaccines have the potential for cost-effective manufacture and are not known to replicate in mammalian cells, they represent a viable alternative to current replicating vaccine vectors for development of both human and veterinary vaccines.

High-yield expression of a viral peptide vaccine in transgenic plants

A high-yield production of a peptide vaccine in transgenic plants is described here. A 21-mer peptide, which confers protection to dogs against challenge with virulent canine parvovirus, has been expressed in transgenic plants as an amino-terminal translational fusion with the GUS gene. Transformants were selected on the basis of their GUS activities, showing expression levels of the recombinant protein up to 3% of the total leaf soluble protein, a production yield comparable to that obtained with the same epitope expressed by chimeric plant viruses. The immunogenicity of the plant-derived peptide was demonstrated in mice immunized either intraperitoneally or orally with transgenic plant extracts, providing the suitability of the GUS fusions approach for low-cost production of peptide vaccines.

Antigenic properties and diagnostic potential of baculovirus-expressed infectious bursal disease virus proteins VPX and VP3

The routine technique for detecting antibodies specific to infectious bursal disease virus (IBDV) is a serological evaluation by enzyme-linked immunosorbent assay (ELISA) with preparations of whole virions as the antigens. To avoid using complete virus in the standard technique, we have developed two new antigens through the expression of the VPX and VP3 genes in insect cells. VPX and especially VP3 were expressed at high levels in insect cells and simple to purify. The immunogenicity of both proteins was similar to that of the native virus. VPX was able to elicit neutralizing antibodies but VP3 was not. Purified VPX and VP3 were tested in an indirect ELISA with more than 300 chicken sera. There was an excellent correlation between the results of the ELISA using VPX and those of the two commercial kits. VP3 did not perform as well as VPX, and the linear correlation was significantly lower. A comparison with the standard reference technique, seroneutralization, showed that the indirect ELISA was more sensitive. Therefore, VPX-based ELISA is a good alternative to conventional ELISAs that use whole virions.

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.

Antigenic profile of African horse sickness virus serotype 4 VP5 and identification of a neutralizing epitope shared with bluetongue virus and epizootic hemorrhagic disease virus

African horse sickness virus (AHSV) causes a fatal disease in horses. The virus capsid is composed of a double protein layer, the outermost of which is formed by two proteins: VP2 and VP5. VP2 is known to determine the serotype of the virus and to contain the neutralizing epitopes. The biological function of VP5, the other component of the capsid, is unknown. In this report, AHSV VP5, expressed in insect cells alone or together with VP2, was able to induce AHSV-specific neutralizing antibodies. Moreover, two VP5-specific monoclonal antibodies (MAbs) that were able to neutralize the virus in a plaque reduction assay were generated. To dissect the antigenic structure of AHSV VP5, the protein was cloned in Escherichia coli using the pET3 system. The immunoreactivity of both MAbs, and horse and rabbit polyclonal antisera, with 17 overlapping fragments from VP5 was analyzed. The most immunodominant region was found in the N-terminal 330 residues of VP5, defining two antigenic regions, I (residues 151-200) and II (residues 83-120). The epitopes were further defined by PEPSCAN analysis with 12mer peptides, which determined eight antigenic sites in the N-terminal half of the molecule. Neutralizing epitopes were defined at positions 85-92 (PDPLSPGE) for MAb 10AE12 and at 179-185 (EEDLRTR) for MAb 10AC6. Epitope 10AE12 is highly conserved between the different orbiviruses. MAb 10AE12 was able to recognize bluetongue virus VP5 and epizootic hemorrhagic disease virus VP5 by several techniques. These data will be especially useful for vaccine development and diagnostic purposes.

Antigenic structure of the capsid protein of rabbit haemorrhagic disease virus

Rabbit haemorrhagic disease virus (RHDV) causes an important disease in rabbits. The virus capsid is composed of a single 60 kDa protein. The capsid protein gene was cloned in Escherichia coli using the pET3 system, and the antigenic structure of RHDV VP60 was dissected using 11 monoclonal antibodies (MAbs) and 12 overlapping fragments of the protein expressed in E. coli. Two antigenic regions were found. Ten out of the 11 MAbs recognized different discontinuous epitopes in the most immunodominant region of the viral capsid. This domain was located between residues 31 and 250 of the VP60 N terminus. The other MAb revealed the presence of an antigenic site within 102 aa of the C terminus. This MAb did not recognize the major cleavage product of the full-length 60 kDa protein. These results indicate that, in contrast to other caliciviruses such as Norwalk virus (NV), the 36 kDa cleavage product probably forms the N-terminal region of VP60. However, as in NV, the cleavage region appears to be the most immunodominant region.

Development of an antigen presentation system based on plum pox potyvirus

The development of an antigen presentation system based on the plum pox potyvirus (PPV) is here described. The amino-terminal part of PPV capsid protein was chosen as the site for expression of foreign antigenic peptides. Modifications in this site were engineered to avoid the capability of natural transmission by aphids of this PPV vector. As a first practical attempt, different forms of an antigenic peptide (single and tandem repetition) from the VP2 capsid protein of canine parvovirus (CPV) were expressed. Both chimeras are able to infect Nicotiana clevelandii plants with similar characteristics to wild-type virus and remain genetically stable after several plant passages. The antigenicity of purified chimeric virions was demonstrated, proving the suitability of this system for diagnostic purposes. Moreover, mice and rabbits immunized with chimeric virions developed CPV-specific antibodies, which showed neutralizing activity.

Serologic markers in early stages of African horse sickness virus infection

Fifteen horses were experimentally infected with African horse sickness virus (AHSV) serotype 4. To learn more about the time course of production and specificity of AHSV-specific antibodies, sera were analyzed by immunoblot analysis. Only animals that survived for more than 9 days were able to develop a humoral immune response detectable by immunoblotting. The earliest serological markers corresponded mainly to VP5, VP6, and NS2 and to a lesser extent to VP3, NS1, and NS3. Neutralizing antibodies to VP2 were not detected by immunoblotting, suggesting that they are mostly conformation dependent. VP7-specific antibodies were detected later in infection. These results make NS2 and VP6 the most attractive candidates for the rapid diagnosis of the infection.

Full protection against African horsesickness (AHS) in horses induced by baculovirus-derived AHS virus serotype 4 VP2, VP5 and VP7

African horsesickness virus serotype 4 (AHSV-4) outer capsid protein VP2, or VP2 and VP5 plus inner capsid protein VP7, derived from single or dual recombinant baculovirus expression vectors were used in different combinations to immunize horses. When the proteins were purified by affinity chromatography, the combination of all three proteins induced low levels of neutralizing antibodies and conferred protection against virulent virus challenge. However, purified VP2 or VP2 and VP5 in the absence of VP7 failed to induce neutralizing antibodies and protection. Immunization with non-purified proteins enhanced the titres of neutralizing antibodies. Again, the combination of the three proteins was able to confer total protection to immunized horses, which showed absence of viraemia. The antigenicity of recombinant VP2 was analysed with a collection of 30 MAbs. Both purified and unpurified recombinant VP2 proteins showed different antigenic patterns in comparison to that of VP2 on virions. An immunization experiment with four more horses confirmed these results. The vaccine described here would not only prevent the disease, but would drastically reduce the propagation of the virus by vectors.

Identification of a linear neutralization domain in the protein VP2 of African horse sickness virus

Overlapping fragments of the outermost capsid protein VP2 of African horse sickness virus serotype 4 (AHSV-4) have been expressed in Escherichia coli. Horse sera from infected and vaccinated animals, rabbit sera, and mice monoclonal antibodies specific for AHSV were used to screen these fragments for antigenic regions. The screening revealed that the major antigenic domain of the AHSV-4 VP2 is localized in a central region (amino acids 200 to 413) and that both the N-terminal region (aa 1-159) and the half C-terminal region (aa 414-1060) are not immunogenic. All the fragments containing a region between amino acids 253 and 413 (fragment H) were able to elicit consistently high titers of neutralizing antibodies. The ability of several subfragments of this region to evoke neutralizing antibodies indicates the presence of several sites inside this domain. However, neutralizing antibodies in sera of horse infected or vaccinated with attenuated viruses were not absorbed by fragment H, indicating that this domain is not immunodominant in AHSV. This information might be useful in designing a subunit vaccine against AHSV infection.