A synthetic peptide derived from the amino acid sequence of canine parvovirus structural proteins which defines a B cell epitope and elicits antiviral antibody in BALB c mice

Cryo EM structures map a post vaccination polyclonal antibody response to canine parvovirus

Canine parvovirus (CPV) is an important pathogen that emerged by cross-species transmission to cause severe disease in dogs. To understand the host immune response to vaccination, sera from dogs immunized with parvovirus are obtained, the polyclonal antibodies are purified and used to solve the high resolution cryo EM structures of the polyclonal Fab-virus complexes. We use a custom software, Icosahedral Subparticle Extraction and Correlated Classification (ISECC) to perform subparticle analysis and reconstruct polyclonal Fab-virus complexes from two different dogs eight and twelve weeks post vaccination. In the resulting polyclonal Fab-virus complexes there are a total of five distinct Fabs identified. In both cases, any of the five antibodies identified would interfere with receptor binding. This polyclonal mapping approach identifies a specific, limited immune response to the live vaccine virus and allows us to investigate the binding of multiple different antibodies or ligands to virus capsids.

Structural comparison of different antibodies interacting with parvovirus capsids

The structures of canine parvovirus (CPV) and feline parvovirus (FPV) complexed with antibody fragments from eight different neutralizing monoclonal antibodies were determined by cryo-electron microscopy (cryoEM) reconstruction to resolutions varying from 8.5 to 18 A. The crystal structure of one of the Fab molecules and the sequence of the variable domain for each of the Fab molecules have been determined. The structures of Fab fragments not determined crystallographically were predicted by homology modeling according to the amino acid sequence. Fitting of the Fab and virus structures into the cryoEM densities identified the footprints of each antibody on the viral surface. As anticipated from earlier analyses, the Fab binding sites are directed to two epitopes, A and B. The A site is on an exposed part of the surface near an icosahedral threefold axis, whereas the B site is about equidistant from the surrounding five-, three-, and twofold axes. One antibody directed to the A site binds CPV but not FPV. Two of the antibodies directed to the B site neutralize the virus as Fab fragments. The differences in antibody properties have been linked to the amino acids within the antibody footprints, the position of the binding site relative to the icosahedral symmetry elements, and the orientation of the Fab structure relative to the surface of the virus. Most of the exposed surface area was antigenic, although each of the antibodies had a common area of overlap that coincided with the positions of the previously mapped escape mutations.

Different mechanisms of antibody-mediated neutralization of parvoviruses revealed using the Fab fragments of monoclonal antibodies

Antibody binding and neutralization are major host defenses against viruses, yet the mechanisms are often not well understood. Eight monoclonal antibodies and their Fab fragments were tested for neutralization of canine parvovirus and feline panleukopenia virus. All IgGs neutralized >85% of virus infectivity. Two Fabs neutralized when present at 5 nM, while the others gave little or no neutralization even at 20-100 nM. The antibodies bind two antigenic sites on the capsids which overlap the binding site of the host transferrin receptor (TfR). There was no specific correlation between Fab binding affinity and neutralization. All Fabs reduced capsid binding of virus to purified feline TfR in vitro, but the highly neutralizing Fabs were more efficient competitors. All partially prevented binding and uptake of capsids by feline TfR on cells. The virus appears adapted to allow some infectivity in the presence of at least low levels of antibodies.

Large-scale production, purification and crystallization of wild-type adeno-associated virus-2

Adeno-associated virus-2 (AAV-2) has long been recognized as a potential vector for human gene therapy. Although much progress has been made in the molecular virology of AAV-2, structural studies of AAV-2 have been hampered by the low efficiency of virus production in culture, the low purity of preparations, and the low solubility of pure virus particles in solution. Methods of larger scale AAV-2 production have been developed through adaptation to suspension culture and re-optimization of the times of infection and transfection with respect to particle production. The methods allow the purification of 10mg ( approximately 10(15) particles) of AAV-2 per preparation at approximately 99% purity as judged by SDS-PAGE. This was sufficient for the screening of conditions for the formation of diffraction-grade crystals, ultimately leading to an atomic structure for AAV-2.

Expression of Aleutian mink disease parvovirus capsid proteins in defined segments: localization of immunoreactive sites and neutralizing epitopes to specific regions

The capsid proteins of the ADV-G isolate of Aleutian mink disease parvovirus (ADV) were expressed in 10 nonoverlapping segments as fusions with maltose-binding protein in pMAL-C2 (pVP1, pVP2a through pVP2i). The constructs were designed to capture the VP1 unique sequence and the portions analogous to the four variable surface loops of canine parvovirus (CPV) in individual fragments (pVP2b, pVP2d, pVP2e, and pVP2g, respectively). The panel of fusion proteins was immunoblotted with sera from mink infected with ADV. Seropositive mink infected with either ADV-TR, ADV-Utah, or ADV-Pullman reacted preferentially against certain segments, regardless of mink genotype or virus inoculum. The most consistently immunoreactive regions were pVP2g, pVP2e, and pVP2f, the segments that encompassed the analogs of CPV surface loops 3 and 4. The VP1 unique region was also consistently immunoreactive. These findings indicated that infected mink recognize linear epitopes that localized to certain regions of the capsid protein sequence. The segment containing the hypervariable region (pVP2d), corresponding to CPV loop 2, was also expressed from ADV-Utah. An anti-ADV-G monoclonal antibody and a rabbit anti-ADV-G capsid antibody reacted exclusively with the ADV-G pVP2d segment but not with the corresponding segment from ADV-Utah. Mink infected with ADV-TR or ADV-Utah also preferentially reacted with the pVP2d sequence characteristic of that virus. These results suggested that the loop 2 region may contain a type-specific linear epitope and that the epitope may also be specifically recognized by infected mink. Heterologous antisera were prepared against the VP1 unique region and the four segments capturing the variable surface loops of CPV. The antisera against the proteins containing loop 3 or loop 4, as well as the anticapsid antibody, neutralized ADV-G infectivity in vitro and bound to capsids in immune electron microscopy. These results suggested that regions of the ADV capsid proteins corresponding to surface loops 3 and 4 of CPV contain linear epitopes that are located on the external surface of the ADV capsid. Furthermore, these linear epitopes contain neutralizing determinants. Computer comparisons with the CPV crystal structure suggest that these sequences may be adjacent to the threefold axis of symmetry of the viral particle.

Detection of canine parvovirus antigens with antibodies to synthetic peptides

Antibodies produced in rabbits against an 18-amino acid peptide (peptide 1, NSLPQSEGATNFGDIGVP) of capsid protein VP2/residues 292-309 of canine parvovirus (CPV) or against an 18-amino acid peptide (peptide 2, GKRNTVLFHGPASTKGKS) of nonstructural protein NS1/residues 391-409 of CPV identified, in immunofluorescence analysis, viral antigens in canine A 72 cells infected with CPV. Antibodies to peptide 2 also identified viral antigens in bovine cells infected with bovine parvovirus. In western blot analysis, antibodies to peptide 1 and peptide 2 also detected viral antigens derived from blue fox parvovirus, feline parvovirus, mink enteritis virus and raccoon dog parvovirus. The peptide antibodies could be used as convenient tools in diagnosis of infections caused by CPV or closely related viruses affecting cats, minks, blue foxes and raccoon dogs.

Peptide vaccine against canine parvovirus: identification of two neutralization subsites in the N terminus of VP2 and optimization of the amino acid sequence

The N-terminal domain of the major capsid protein VP2 of canine parvovirus was shown to be an excellent target for development of a synthetic peptide vaccine, but detailed information about number of epitopes, optimal length, sequence choice, and site of coupling to the carrier protein was lacking. Therefore, several overlapping peptides based on this N terminus were synthesized to establish conditions for optimal and reproducible induction of neutralizing antibodies in rabbits. The specificity and neutralizing ability of the antibody response for these peptides were determined. Within the N-terminal 23 residues of VP2, two subsites able to induce neutralizing antibodies and which overlapped by only two glycine residues at positions 10 and 11 could be discriminated. The shortest sequence sufficient for neutralization induction was nine residues. Peptides longer than 13 residues consistently induced neutralization, provided that their N termini were located between positions 1 and 11 of VP2. The orientation of the peptides at the carrier protein was also of importance, being more effective when coupled through the N terminus than through the C terminus to keyhole limpet hemocyanin. The results suggest that the presence of amino acid residues 2 to 21 (and probably 3 to 17) of VP2 in a single peptide is preferable for a synthetic peptide vaccine.

B19 parvovirus

B19 parvovirus is pathogenic in man and causes a variety of clinical illnesses, among them several haematological diseases. Acute infection of a host with underlying haemolysis produces transient aplastic crisis; of the midtrimester fetus, hydrops fetalis; and of an immunocompromised patient, pure red cell aplasia. The target of B19 parvovirus infection is the human erythroid progenitor cell. Infection is cytotoxic due to expression of the viral nonstructural protein. The virus can be propagated in cultures of human bone marrow, blood, and fetal liver. Humoral immunity normally terminates infection, and commercially available immunoglobulin can be used to treat persistent infection. Recombinant capsids, produced in a baculovirus system, are suitable as a vaccine reagent.

Effective induction of neutralizing antibodies with the amino terminus of VP2 of canine parvovirus as a synthetic peptide

Fourteen synthetic peptides corresponding to previously mapped antigenic sites in VP2 of canine parvovirus (CPV) were used for immunization of rabbits to identify antiviral properties favourable for inclusion into a vaccine. Most antipeptide antisera obtained were reactive with viral protein, and with one of them it was possible to locate the hypothetical amino terminus of VP3 within positions 15-31 of VP2. Virus-neutralizing antibodies were only obtained with two overlapping 15-mer peptides corresponding in sequence to the amino terminus of VP2 (MSDGAVQPDGGQPAVRNERAT). Antibodies in the neutralizing sera bound most strongly to amino acids of the sequence DGGQPAV within the N-terminus of VP2, indicating that efforts to develop a synthetic vaccine against CVP should be focused on this stretch of amino acids. The two peptides induced long-lasting immunity (at least 8 months) using either Freund's adjuvant or aluminium hydroxide plus Quil A. Thus, this approach delineated the exact peptide sequence useful for vaccines applied to the amino-terminal region of VP2. These findings in experimental animals form a solid basis for exploration of a synthetic peptide vaccine against parvovirus infection in dogs, minks or cats.

The structure of a neutralized virus: canine parvovirus complexed with neutralizing antibody fragment

BACKGROUND

Members of the Parvovirus genus cause a variety of diseases in mammals, including humans. One of the major defences against viral infection is the presence of neutralizing antibodies that prevent virus particles from infecting target cells. The mechanism of neutralization is not well understood. We therefore studied the structure of canine parvovirus (CPV) complexed with the Fab fragment of a neutralizing antibody, A3B10, using image reconstruction of electron micrographs of vitrified samples, together with the already known structure of CPV from X-ray crystallographic data.

RESULTS

The structure of the complex of CPV with Fab A3B10 has been determined to 23 A resolution. The known CPV atomic structure was subtracted from the electron density of the complex, and the difference map was used to fit the atomic coordinates of a known Fab fragment, HyHEL-5. The long axis of each Fab molecule is oriented in a near radial direction, inclined away from the two-fold axes. The viral epitope consists of 14 amino acid residues found in loops 1, 2 and 3 on the capsid surface, which include previously identified escape mutations.

CONCLUSIONS

The mode of Fab binding suggests that the A3B10 neutralizing antibody cannot bind bivalently to the capsid across the two-fold axes, consistent with the observation that whole A3B10 antibody readily precipitates CPV. Since Fab A3B10 can also neutralize the virus, mechanisms of neutralization such as interference with cell attachment, cell entry, or uncoating, must be operative.

First peptide vaccine providing protection against viral infection in the target animal: studies of canine parvovirus in dogs

A synthetic peptide vaccine which protects dogs against challenge with virulent canine parvovirus is described. The amino acid sequence used was discovered in previous studies on the immunogenic properties of previously mapped antigenic sites and represents the amino-terminal region of viral protein VP2. As with marker vaccines, it is possible to discriminate between vaccinated dogs that have not been exposed to the virus and dogs that have been infected with the virus. The protective mechanism can be explained by a humoral response against the peptide aided by T-cell epitopes contained in the carrier protein used for peptide coupling. This is the first example of a synthetic peptide vaccine that induces protection in target animals.

Neutralizing linear epitopes of B19 parvovirus cluster in the VP1 unique and VP1-VP2 junction regions

Presentation of linear epitopes of the B19 parvovirus capsid proteins as peptides might be a useful vaccine strategy. We produced overlapping fusion proteins to span the viral capsid sequence, inoculated rabbits, and determined whether the resulting antisera contained antibodies that neutralized the ability of the virus to infect human erythroid progenitor cells. Antibodies that bound to virus in an enzyme-linked immunosorbent assay were present in antisera raised against 10 of 11 peptides; strongest activity was found for antisera against the carboxyl-terminal half of the major capsid protein. However, strong neutralizing activity was elicited in animals immunized with peptides from the amino-terminal portion of the unique region of the minor capsid protein and peptides containing the sequence of the junction region between the minor and major capsid proteins. The development of neutralizing activity in animals was elicited most rapidly with the fusion peptide from the first quarter of the unique region. A 20-amino-acid region of the unique region of the minor capsid protein was shown to contain a neutralizing epitope. Multiple antigenic peptides, based on the sequence of the unique region and produced by covalent linkage through a polylysine backbone, elicited strong neutralizing antibody responses. Synthetic peptides and fusion proteins containing small regions of the unique portion of the minor capsid protein might be useful as immunogens in a human vaccine against B19 parvovirus.

B-cell epitopes of canine parvovirus: distribution on the primary structure and exposure on the viral surface

Ten antigenic sites on canine parvovirus (CPV) were mapped with a complete set of overlapping nonapeptides of the capsid proteins VP1 and VP2: five of these sites were recognized by sera from CPV-infected dogs, three were recognized by a rabbit anti-CPV antiserum, and two were recognized by murine monoclonal anti-CPV antibodies. A region covering the first 21 amino-terminal amino acid residues of VP2 was recognized by three sera from infected dogs, one neutralizing rabbit antiserum, and one neutralizing murine monoclonal antibody. Immunoabsorption experiments with full virions indicated that at least 6 of the 10 antigenic sites are located on the surface. Of these six, three sites occur in the amino terminus of VP2. When superimposed on the three-dimensional structure of canine parvovirus (J. Tsao, M. S. Chapman, M. Agbandje, W. Keller, K. Smith, H. Wu, M. Luo, T. J. Smith, M. G. Rossmann, R. W. Compans, and C. R. Parrish, Science 251:1456-1464, 1991), the other three epitopes are located on two loops of VP2 which form the highly exposed "spike" around the threefold-symmetry axis of the virus. Thus, these regions (amino terminus and loops 1 and 3) are of interest as major target sites for induction of neutralizing antibodies.

Mapping specific functions in the capsid structure of canine parvovirus and feline panleukopenia virus using infectious plasmid clones

DNA sequences between 0 and 98.8 genome map units (m.u.) from canine parvovirus (CPV) and feline panleukopenia virus (FPV) were cloned into plasmid vectors to form infectious molecular clones. Those plasmids were transfected into permissive cells and viruses recovered were shown to contain intact genomes, having regenerated the complete viral 5' ends up to 100 m.u. The viruses derived from the plasmids were compared to the original viruses, and shown to be indistinguishable in antigenic type, hemagglutination (HA) type and host range. The plasmid origin of the viruses was shown by preparing recombinant clones between CPV and FPV, and demonstrating the recombinant nature of the resulting viruses by restriction mapping and by sequencing viral DNA across the recombination sites. The sequences of our wild-type isolates CPV-d and FPV-b were completed, revealing 50 nucleotide sequence differences, of which 16 determined coding changes--5 in NS-1,2 in NS-2, and 9 in VP-2 protein. The sequences of the 5' ends (95.3-100 m.u.) of both viruses were also determined. Analysis of recombinant viruses mapped both CPV- and FPV-specific antigenic epitopes, the pH dependence of HA, and sequences affecting canine host range of the viruses within the VP-1 and VP-2 structural protein genes. Most of the specific changes were shown to be either on, or within one amino acid of, the surface of the virus capsid, indicating that the exposed surface of the parvovirus capsid plays an important role in determining a number of virus functions. The specific epitopes were affected by differences in a raised area on the capsid ("threefold spike"), while the pH dependence of HA difference was adjacent to a depression in the surface of the capsid at the twofold axis of symmetry.