Development of a Novel Canine Parvovirus Vaccine Capable of Stimulating Protective Immunity in Four-Week-Old Puppies in the Face of High Levels of Maternal Antibodies

Many highly effective vaccines have been developed to protect dogs against disease caused by canine parvovirus, but despite this vaccine interference by maternally derived antibodies continues to cause immunisation failure. To help overcome this limitation we have developed a novel, recombinant canine parvovirus type 2c vaccine strain, based on the structural and non-structural elements of an established type 2 vaccine. This novel CPV-2c vaccine strain has unique efficacy in the field, it is able to induce sterilising immunity in naïve animals 3 days after vaccination and is able to overcome very high levels of maternally derived antibodies from 4 weeks of age-thus closing the immunity gap to canine parvovirus infection in young puppies. The vaccine strain, named 630a, has been combined with an established canine distemper virus Onderstepoort vaccine strain to produce a new bivalent vaccine (Nobivac DP PLUS), intended to immunise very young puppies in the face of high levels of maternally derived antibody. Here, we describe the onset of immunity and maternal antibody interference studies that support the unique efficacy of the strain, and present overdose studies in both dogs and cats that demonstrate the vaccine to be safe.

Generation and immunogenicity of virus-like particles based on mink enteritis virus capsid protein VP2 expressed in Sf9 cells

Mink enteritis virus (MEV) is a parvovirus that causes acute enteritis in mink. The capsid protein VP2 of MEV is a major immunogenicity that is important for disease prevention. In this study, this protein was expressed in Spodoptera frugiperda 9 cells using a recombinant baculovirus system and was observed to self-assemble into virus-like particles (VLPs) with a high hemagglutination (HA) titer (1:2¹⁶). A single-dose injection of VLPs (HA titer, 1:256) resulted in complete protection of mink against virulent MEV challenge for at least 180 days. These data suggest that these MEV VLPs could be used as a vaccine for the prevention of viral enteritis in mink.

Isolation and sequence analysis of the complete H gene of canine distemper virus from domestic dogs in Henan Province, China

Eighteen canine distemper virus (CDV) isolates were obtained from clinical samples in Henan province, China, between 2012 and 2016. These viruses could not be recognized by 1A4, a monoclonal antibody specific for the H protein of CDV vaccine strains. The complete haemagglutinin (H) genes of all 18 isolates were sequenced, and phylogenetic analysis showed that they segregated into two clusters within the Asia-1 genotype. Moreover, the H genes of four viruses were found to lack a potential N-glycosylation site at position 309, which is the most conserved site within the Asia-1 genotype of CDV, and a novel potential N-glycosylation site (amino acids 517–519) was found in strain HL013, which has not been reported previously. These results will help in achieving a better understanding of the evolution of CDV in China.

Genetic characterization of parvoviruses in domestic cats in Henan province, China

Feline panleukopenia virus (FPV) and canine parvovirus (CPV) infections are highly contagious and cause serious enteric diseases, with high fatality rates of cats and dogs. Given the importance of cats as a potential source of genetic diversity for parvoviruses, parvovirus strains detected in cats with gastroenteritis signs were analysed, and molecular characterisation, sequence analysis and phylogeny were evaluated on the VP2 gene. The results showed that FPV, new CPV-2a, and CPV-2 are co-circulating in the cat population in Henan province of China. Moreover, CPV-2 strains (F2016020, and F2016021) with Ser297Ala substitution in VP2 protein was for the first time detected in cats with clinical gastroenteritis. This study provided new important findings about the evolutionary of parvovirus infection in domestic cats.

Novel bivalent vectored vaccine for control of myxomatosis and rabbit haemorrhagic disease

A novel, recombinant myxoma virus-rabbit haemorrhagic disease virus (RHDV) vaccine has been developed for the prevention of myxomatosis and rabbit haemorrhagic disease (RHD). A number of laboratory studies are described illustrating the safety and efficacy of the vaccine following subcutaneous administration in laboratory rabbits from four weeks of age onwards. In these studies, both vaccinated and unvaccinated control rabbits were challenged using pathogenic strains of RHD and myxoma viruses, and 100 per cent of the vaccinated rabbits were protected against both myxomatosis and RHD.

Canine parvovirus type 2 vaccine protects against virulent challenge with type 2c virus

The ability of dogs vaccinated with a live attenuated CPV type 2 (Nobivac Intervet) vaccine to resist challenge with a current CPV2c isolate was investigated. Six SPF beagle dogs were given the minimum recommended course of vaccination, comprising a single inoculation of vaccine (Nobivac Lepto+Nobivac Pi) at 8-10 weeks of age followed 3 weeks later with a parvovirus vaccine in combination with distemper, adenovirus and parainfluenza virus (Nobivac DHPPi) and a repeat leptospirosis vaccine. Six control dogs were kept unvaccinated. All animals were challenged orally with a type 2c isolate of CPV and monitored for clinical signs, virus shedding, white blood cell fluctuations and serological responses. All vaccinated dogs were fully protected; showing no clinical signs nor shedding challenge virus in the faeces, in contrast to control animals, which displayed all the typical signs of infection with pathogenic CPV and shed challenge virus in the faeces.

Potential for broad-spectrum protection against feline calicivirus using an attenuated myxoma virus expressing a chimeric FCV capsid protein

It has previously been demonstrated that recombinant myxoma viruses expressing FCV capsid protein are capable of eliciting protective responses against virulent FCV challenge, following vaccination, in cats. An attempt was made to produce a bivalent myxoma recombinant expressing the capsid protein genes of both FCV strains F9 and LS015. The FCV capsid protein genes were inserted into the myxoma growth factor gene (MGF) locus, and the serine protease inhibitor (SERP 2) gene locus. Subsequent recombination between myxoma-FCV viruses resulted in a recombinant expressing a chimeric form of the capsid protein. Nonetheless, cats immunised with this myxoma-FCV recombinant demonstrate high levels of serum neutralising antibodies against both F9 and LS015 strains. Such a chimeric vaccine may provide effective protection against a wide range of FCV strains.

Vaccination of cats with an attenuated recombinant myxoma virus expressing feline calicivirus capsid protein

Myxoma virus, a member of the Poxviridae family (genus Leporipoxvirus) is the agent responsible for myxomatosis in the European rabbit. Recombinant myxoma viruses expressing the capsid gene of an F9 strain of feline calicivirus (FCV) were constructed from an apathogenic, laboratory attenuated, isolate of myxoma virus. The FCV capsid genes were recombined into the myxoma growth factor (MGF) locus of the myxoma genome and expressed from synthetic poxvirus promoters. Myxoma virus is unable to replicate productively in feline cells in vitro, however, cells infected with recombinant viruses do express the heterologous antigens from both late and early/late synthetic promoters. Cats immunised with myxoma-FCV recombinant virus generated high levels of serum neutralising antibody and were protected from disease on subsequent challenge with virulent FCV. Furthermore, there was no evidence of transmission of myxoma-FCV recombinant virus from vaccinated to non-vaccinated cats. These results demonstrate the potential of myxoma virus as a safe vaccine vector for use in non-lepori species and in particular the cat.

Protection against feline immunodeficiency virus using replication defective proviral DNA vaccines with feline interleukin-12 and -18

A molecular clone of the Glasgow-8 isolate of FIV (FIVGL8) was rendered replication defective by an in-frame deletion in either reverse transcriptase (deltaRT) or integrase (deltaIN) genes for use as DNA vaccines. To test the ability of these multi-gene vaccines to protect against two feline immunodeficiency virus (FIV) isolates of differing virulence, cats were immunized using either DNA vaccine alone or co-administered with interleukin-12 (IL-12) and/or interleukin-18 (IL-18) cytokine DNA. Animals were challenged sequentially with FIV-Petaluma (FIVPET) an FIV isolate of relatively low virulence and subsequently with the more virulent FIVGL8. A proportion of vaccinates (5/18 deltaIN and 2/12 deltaRT) were protected against primary challenge with FIV(PET). Five of the vaccinated-protected cats were re-challenged with FIV(PET); four (all deltaIN) remained free of viraemia whilst all naive controls became viraemic. Following subsequent challenge with the more virulent FIVGL8 these four vaccinated-protected animals all became viraemic but showed lower proviral loads than naive cats. This study suggests that while our current DNA vaccines may not produce sterilizing immunity against more virulent isolates of FIV, they may nevertheless significantly reduce the impact of infection.

Generation of E3-deleted canine adenoviruses expressing canine parvovirus capsid by homologous recombination in bacteria

E3-deleted canine adenovirus type 1 (CAV-1) was generated by homologous recombination in bacterial cells, using an antibiotic resistance marker to facilitate the recovery of recombinants. This marker was flanked by unique restriction endonuclease sites, which allowed its subsequent removal and the insertion of cassettes expressing the canine parvovirus capsid at the E3 locus. Infectious virus was recovered following transfection of canine cells and capsid expression was observed by RT-PCR from one of the virus constructs. A second construct, containing a different promoter, showed delayed growth and genome instability which, based on the size difference between these inserts, suggests a maximum packaging size of 106 to 109% wild-type genome size for CAV-1.

Vaccination with inactivated virus but not viral DNA reduces virus load following challenge with a heterologous and virulent isolate of feline immunodeficiency virus

It has been shown that cats can be protected against infection with the prototypic Petaluma strain of feline immunodeficiency virus (FIV(PET)) using vaccines based on either inactivated virus particles or replication-defective proviral DNA. However, the utility of such vaccines in the field is uncertain, given the absence of consistent protection against antigenically distinct strains and the concern that the Petaluma strain may be an unrepresentative, attenuated isolate. Since reduction of viral pathogenicity and dissemination may be useful outcomes of vaccination, even in the absence of complete protection, we tested whether either of these vaccine strategies ameliorates the early course of infection following challenge with heterologous and more virulent isolates. We now report that an inactivated virus vaccine, which generates high levels of virus neutralizing antibodies, confers reduced virus loads following challenge with two heterologous isolates, FIV(AM6) and FIV(GL8). This vaccine also prevented the marked early decline in CD4/CD8 ratio seen in FIV(GL8)-infected cats. In contrast, DNA vaccines based on either FIV(PET) or FIV(GL8), which induce cell-mediated responses but no detectable antiviral antibodies, protected a fraction of cats against infection with FIV(PET) but had no measurable effect on virus load when the infecting virus was FIV(GL8). These results indicate that the more virulent FIV(GL8) is intrinsically more resistant to vaccinal immunity than the FIV(PET) strain and that a broad spectrum of responses which includes virus neutralizing antibodies is a desirable goal for lentivirus vaccine development.

The receptor binding site of feline leukemia virus surface glycoprotein is distinct from the site involved in virus neutralization

The external surface glycoprotein (SU) of feline leukemia virus (FeLV) contains sites which define the viral subgroup and induce virus-neutralizing antibodies. The subgroup phenotypic determinants have been located to a small variable region, VR1, towards the amino terminus of SU. The sites which function as neutralizing epitopes in vivo are unknown. Recombinant SU proteins were produced by using baculoviruses that contained sequences encoding the SUs of FeLV subgroup A (FeLV-A), FeLV-C, and two chimeric FeLVs (FeLV-215 and FeLV-VC) in which the VR1 domain of FeLV-A had been replaced by the corresponding regions of FeLV-C isolates. The recombinant glycoproteins, designated Bgp70-A, -C, -215, and -VC, respectively, were similar to their wild-type counterparts in several immunoblots and inhibited infection of susceptible cell lines in a subgroup-specific manner. Thus, Bgp70-A interfered with infection by FeLV-A, whereas Bgp70-C, -VC, and -215 did not. Conversely, Bgp70-C, -VC, and -215 blocked infection with FeLV-C, while Bgp70-A had no effect. These results indicate that the site on SU which binds to the FeLV cell surface receptor was preserved in the recombinant glycoproteins. It was also found that the recombinant proteins were able to bind naturally occurring neutralizing antibodies. Bgp70-A, -VC, and -215 interfered with the action of anti-FeLV-A neutralizing antibodies, whereas Bgp70-C did not. Furthermore, Bgp70-C interfered with the action of anti-FeLV-C neutralizing antibodies, while the other proteins did not. These results indicate that the neutralizing epitope(s) of FeLV SU lies outside the subgroup-determining VR1 domain.

A mutant of canine adenovirus type 2 with a duplication of the E1a region exhibits altered expression of early region 4

The genomic DNA of a vaccine strain of canine adenovirus type 2 (Vaxitas; ICI Tasman) has been shown to contain two copies of the E1a region, the second being at the far right end of the genome. DNA sequence analysis of the right terminal 2.8 kbp of this vaccine strain showed that numerous point mutations have occurred in the second copy, which would preclude the synthesis of any functional products. However, expression vectors in which the E1a promoter from the right terminus were linked to the chloramphenicol acetyltransferase gene showed that the promoter was fully functional. Furthermore, the activity of the reiterated E1a promoter was considerably greater than that of the normal E4 promoter. This dramatic change in the regulation of E4 expression may be an important factor in determining the altered host cell specificity displayed by this vaccine strain virus.

Identification and nucleotide sequence of the early region 1 from canine adenovirus types 1 and 2

The genome of canine adenovirus type 1 (CAV-1) has been cloned and restriction maps compiled. These maps are compared with those of canine adenovirus type 2 (CAV-2). The left ends of both genomes were further characterised by DNA sequence analysis. Several features of the DNA sequence and predicted polypeptide sequence are similar to those of the human adenoviruses. The level of homology observed across the E1 regions appears to be of the same order as the overall DNA similarity between CAV-1 and CAV-2 (75%). Transfection experiments using the presumptive E1a containing region of CAV-2 suggests that it encodes a transactivating function typical of the human adenovirus E1a genes.

Molecular cloning and restriction endonuclease mapping of two strains of canine adenovirus type 2

The DNA of a field isolate and of a vaccine strain of canine adenovirus type 2 (CAV-2) were analysed by digestion with several restriction endonucleases. The PstI restriction fragments of the field isolate (CAV-2 Glasgow) and the vaccine strain were cloned into the plasmid pBR322. Physical maps of the two viral genomes were constructed by molecular hybridization of PstI, EcoRI, SmaI, BamHI and KpnI digests of the viral DNA with the cloned PstI fragments. The restriction profile of CAV-2 Glasgow was shown to be virtually identical to those of the two prototype CAV-2 strains, Toronto A26/61 and Manhattan. However, the restriction fragment pattern of the vaccine strain of CAV-2 showed characteristic alterations, in particular additional sequences at or near the genome termini.

Isolation of canine adenovirus-2 from the faeces of dogs with enteric disease and its unambiguous typing by restriction endonuclease mapping

Fifty-four faecal specimens obtained from kennelled dogs with diarrhoeal disease were used to inoculate a range of cell types in tissue culture. Particles resembling adenovirus virions were seen in three specimens and 22 stools yielded an adenovirus upon culture. Viral DNA from each isolate was digested with the restriction endonucleases Bam H1 and Pst 1. Agarose gel electrophoresis revealed identical restriction patterns for all isolates. One isolate, 9228, was selected as a prototype and was compared with reference strains of canine adenovirus-2 (CAV-2) (Manhattan and Toronto A26/61) and CAV-1. Isolate 9228 was clearly distinct from CAV-1 but identical to both Manhattan and Toronto (A26/61) strains of CAV-2. However, restriction site polymorphism was observed in 9228 following digestion with Hpa II. Isolate 9228 was similarly compared with all commercially available vaccinal isolates of CAV-2 and was shown to be clearly distinct from these.

Construction and characterization of guaB-lacZ fusions in Escherichia coli K12

GuaB-lacZ fusion strains of Escherichia coli K12 have been constructed using the Casadaban (1976) gene-fusion technique. The major modification to the procedure was the removal of the guanine requirement of Mu-induced guaA auxotrophs by introduction of a ColEl-gua+ plasmid prior to selection of the fusion. Conversion to prototrophy was necessary to overcome problems associated with repression of the gua operon by guanine added to selection media. Induction of the lysogenic fusion strains yielded plaque-forming lambda transducing phages that carried the lac genes and either the complete guaB gene and the gua promoter or only a distal portion of guaB.