Empty Canine Parvovirus Capsids Having CPV Recombinant VP2 and Vaccines Having Such Capsids

The present invention relates in general to viral proteins and to assays and vaccines using the same and, in particular, to a protein related to the major antigen (VP2) of the Canine Parvovirus (CPV) capsid. Such protein was produced in an expression vector of baculoviruses multiplied in a cell culture of a permissive host. The protein obtained with the instant invention is singularly characterized in forming empty VP2 capsids that are useful in vaccine formulation.

The Canine Parvovirus (CPV) belongs to the autonomous parvoviruses, causing severe enteritis in dogs of all ages and myocarditis in puppies less than 12 weeks old. CPV was first isolated in 1978 (Burtonboy, G. et al., Arch. Virol. 61:1-11, 1979; Appel et al., Vet. Rec. 105. 156-179, 1979). It is believed to have arisen as a natural variant of the feline panleukopenia virus (FPLV) or the mink enteritis virus (MEV). CPV infection is controlled using conventional vaccines based on live or inactivated viruses. However, since dogs are vaccinated before pregnancy, maternal antibodies can block attenuated live vaccine replication. Autonomous parvoviruses provide a good vector for recombinant subunit vaccine production for several reasons, inter alia:

1. The capsid proteins are structurally simple (no glycosylation, phosphorylation or acetylation is required).

2. Humoral response appears to adequately control viral dissemination given the relative efficiency of inactivated vaccines.

Protein and DNA sequence studies and serologic studies show an enhanced antigenic and genetic homology between CPV, FPLV, MEV and the Raccoon Parvovirus (Tratschin et al., J. Gen Virol. 61:33-41. 1982. Carlsson et al., J. Virol., 55, 574-582, 1985. Parrish et al., Arch. Virol. 72, 267-278, 1982. Reed et al., J. Virol. 62:266-276, 1988). Despite this homology they are exquisitely specific in the "in vivo" host, although "in vitro" all viruses replicate in cat kidney cells (Appel et al., Vet. Rec. 105, 156-179, 1979. Trastschin et al., J. Gen. Virol., 61:33-41, 1982). The CPV capsid contains two proteins with broadly overlapping amino acid sequences, VP1 (82-84 KDa) and VP2 (67.70 KDa) (Paradiso et al. J. Gen. Virol. 39, 800-807, 1982. Surleraux et al. Arch Virol., 82, 233-240, 1984. J. Gen. Virol. 62, 113-125, 1982. Surleraux et al., Arch. Virol. 82, 233-240, 1984). The parvovirus capsid has a diameter of 22 nm and holds some 10 VP1 copies and some 60 VP2 copies (Wobble et al., Biochemistry 23, 6565-6569, 1984), arranged as either homo- or heterodimers (Paradiso, J. Virol., 46, 94-102, 1983) though the precise capsid structure is unknown. VP2 in full capsids (holding DNA) is preferentially broken down by proteolytic digestion into 63-67 KDa VP3 (Paradiso et al., Gen. Virol. 39, 800-807, 1982. Surleaux et al. Arch. Virol., 82, 233.240, 1984) after capsid assembly (Paradiso, J. Virol., 39, 800-807, 1981).

Our laboratory has of late been researching into the immunogenicity of various fragments of the proteins making up the CPV viral capsid which has resulted in new VP2 protein and VP2 and VP1 fragment-based recombinant vaccines being described. These findings are summarised in Spanish patent application number 9002074 for: "PRODUCTION OF CANINE PARVOVIRUS VACCINES BY VIRAL POLYPEPTIDE EXPRESSION IN E. coli OR BY CHEMICAL SYNTHESIS" with J. I. Casal et al. as inventors.

The said patent application relates to the expression of such products in E. coli bacterial vectors.

New large-scale protein production vectors have however been recently discovered based upon the replication of recombinant baculoviruses derived from the Autographa californica nuclear polyhedrosis virus (ACMNPV) in culture insect cells. The state of the art for these vectors is summed up in two scientific papers as follows:

1. LucKow, V. A. & Summers, M. D. (1988). Trends in the development of baculovirus expression vectors. Bio/Technology 6, 47-55.

2. J. Vialard et al. (1990). Synthesis of the membrane fusion and Hemagglutinin Proteins of Measles virus, using a novel baculovirus vector containing the .beta.-galactosidase gene. J. Virol. 64. 37-50.

Furthermore, European Patent Application number 0 341 611 by Cornell Research Foundation, Inc. and Boyce Thompson Institute for Plant Research, Inc., describes the obtention of CPV subunit vaccines using a baculovirus expression vector other than that used in the present invention, and the said European Patent moreover fails to mention that the proteins obtained are capable of forming empty VP2 capsids thereby drawing a fundamental distinction with the instant invention inasmuch as the VP2 protein obtained under our invention is able to form empty VP2 capsids. In consequence of the above their immunogenic and hemagglutinant capacity is clearly superior as the following description will show. This added capacity of the VP2 protein obtained with our invention is further convenient in that other viral protein epitopes can be introduced in the said capsids by genetic manipulation of the recombinant baculoviruses, or by chemical manipulation of the actual capsids.

The advantages of VP2 protein synthesis in a baculovirus vector over E. coli synthesis are remarkable, namely a greater fidelity as to the identity of the protein obtained, a greater solubility of the product obtained, fusion proteins need not be used, etc. These are clearly advancements of and improvement to processes for obtaining recombinant subunit vaccines as regards previous processes.

The present invention puts forth a new process for producing a recombinant subunit vaccine to protect dogs from CPV, cats from FPLV and minks from MEV. The new vaccine produced thus can contain:

i) the CPV VP2 protein produced in an expression vector of baculoviruses multiplied in a cell culture of a permissive host (this protein hereinafter optionally referred to as "VP2 hereof"); or

ii) empty VP2 capsids formed by assembly of the VP2 hereof.

The VP2 protein hereof is singularly characterized in forming empty VP2 capsids, optionally incorporating other viral protein epitopes by genetic manipulation of the recombinant baculoviruses or chemical manipulation of the actual capsids.

The object of the invention is therefore a new process for obtaining new improved subunit vaccines capable of protecting dogs, cats and minks respectively from CPV, FPLV and MEV infections. As aforesaid the said vaccines can either contain the VP2 protein hereof or empty VP2 capsids formed by the said VP2 protein hereof, inasmuch as the said empty capsids have an enhanced hemagglutination activity and are highly immunogenic, excelling other recombinant proteins of these viruses produced heretofore in any other vector. The new vaccines the invention provides and being one of their objects can contain either the said empty capsids with an immunologically acceptable diluent, with or without an adjuvant, or the VP2 protein hereof together with a diluent and an adjuvant.

Since the said empty VP2 capsids can be chemically or genetically manipulated to introduce other unrelated viral protein or peptide epitopes therein, the use of the said capsids both for CPV, FPLV and MEV vaccinal purposes and modified to incorporate other epitopes, thereby to provide a polyvalent vaccine, are further additional objects of this invention.

The VP2 protein obtained with the invention and the empty VP2 capsids it can form can be useful in diagnosis to detect the presence of specific CPV antibodies or to induce polyclonal or monoclonal antibodies capable of CPV detection. The use of the VP2 protein hereof and of the chimeric capsids it can form for the above purposes is a further object of the present invention.

An additional object of this invention is a recombinant baculovirus and the process for obtaining the same, capable of producing a CPV VP2 recombinant protein identical to the viral protein, as shown in antigenic reactivity assays and other biological functionality assays. The recombinant baculovirus was called AcMNPV.pCPVEx17 and filed on 2.3.91 with the European Collection of Animal Cell Cultures, (ECACC), at Porton Down, Salisbury, Wiltshire SP4 OJG (Great Britain), accession number V91030212.

A further object of the invention is the new baculovirus transfer vector (pCPVEx17) containing the nucleic acid sequence coding for the VP2 hereof. With a process known as AcMNPV wild strain homologous recombination this new vector leads to the said AcMNPV.pCPVEx17 recombinant baculovirus.

This invention also provides the nucleic acid sequence coding for the VP2 protein of the invention (FIG. 1).

The empty CPV chimeric capsids formed by autoassembly of the CPV recombinant VP2 proteins are yet another object of this invention.

SEQ. ID. NO. 1 and FIGS. 1A-E shows the nucleotide sequence coding for the VP2 hereof and the amino acid sequence thereof. The nucleotide sequence is shown in direction 5'3' from left to right. The amino acids have been designated using the generally accepted three-letter code.

FIG. 2 shows the construction of the pCPVEx17 transfer vector, pointing out the appropriate manipulations for inserting the CPV VP2 gene in the pJVP10Z plasmid.

FIG. 3 shows the presence of empty VP2 capsids formed by aggregation of the VP2 protein hereof, as observed under an electron microscope.

The invention provides a new process for obtaining a recombinant subunit vaccine serving to protect against infections due to Canine Parvovirus or related viruses such as FPLV and MEV. The new vaccine can contain the CPV VP2 recombinant protein produced in an expression vector of baculoviruses multiplied in a cell culture of lepidoptera or other permissive host, or empty VP2 capsids formed by aggregating the said recombinant VP2.

The invention also provides a recombinant baculovirus capable of expressing the CPV VP2 when inoculated in a permissive host, and the process for obtaining the said recombinant baculovirus.

The obtention of the recombinant baculovirus basically comprises the following steps:

a) Preparing the gene coding for the CPV VP2;

b) inserting the VP2 gene in a baculovirus transfer vector;

c) transfecting permissive host cells with the said baculovirus transfer vector holding the VP2 gene; and

d) selecting the recombinant baculovirus expressing the CPV VP2 protein.

The recombinant baculovirus obtained thus and the proteins and capsids produced are additionally characterised.

These steps will be described in detail hereinafter. The attached Sequence Listing is incorporated by reference hereto.

In a preferred embodiment the gene coding for the CPV VP2 protein is prepared in accordance with the protocol set forth in above-mentioned Spanish patent application number 9002074 and inserted in the restriction site NheI of the ACMNPV derived pJVP10Z plasmid, thereby to obtain a baculovirus transfer vector. In our invention the pCPVEx17 vector proved to have the CPV DNA adequately oriented to be expressed by the AcMNPV virus polyhedrin promoter.

The pCPVEx17 vector was used to co-transfect permissive host cells, with the AcMNPV virus wild-type strain DNA. Reference could, inter alia, be made to cells of lepidotera or their larvae. In a preferred embodiment of this invention Spodoptera frugiperda (S. frugiperda) cells, generally from the Sf9 strain, were transfected with pCPVEx17, though it can naturally be assumed that similar results would be achieved transfecting other permissive cells for recombinant baculovirus replication.

After transfection, the recombinant baculoviruses were selected after removing and titrating the floats produced in confluent monolayers of S. frugiperda cells. The blue plates with no trace of the viral polyhedrin under a light microscope were collected and back-titrated on S. frugiperda cells to obtain the recombinant baculoviruses. The AcMNPV.pCPVEx17 recombinant baculovirus is capable of expressing the CPV VP2 recombinant protein (VP2 hereof) and was filed with the ECACC, accession number V91030212. The deposit of the recombinant baculovirus has been made on Mar. 2, 1991 with the European Collection of Animal Cell Cultures (ECACC) at the PHLS Centre of Applied Microbiology and Research, Porton Down, Salisbury, Wilts. SP4 OJG, United Kingdom.

A Dot Blot assay was used to verify that the VP2 gene had been adequately integrated into the said recombinant baculovirus genome.

The proteins expressed by the S. frugiperda cells infected with the recombinant baculovirus were analysed by electrophoresis in SDS-polyacrylamide 8% to 15% gradient gels and were stained with Coomassie blue to observe a major presence of a protein with an estimated molecular weight of 67 KDa, equivalent to that of the viral VP2 in the recombinant virus plate. Immunodetection assays showed that the anti-CPV polyclonal antisera reacted with the VP2 expressed by the recombinant baculovirus. It was also found that neutralizing monoclonal antibodies also recognized the recombinant VP2. It can in light of these results be said that the VP2 hereof expressed by the recombinant baculovirus in S. frugiperda cells is antigenically undistinguishable from the viral VP2.

The VP2 protein obtained with the above-described process can be used for diagnosis purposes to detect the presence of specific CPV antibodies or to induce polyclonal or monoclonal antibodies capable of CPV detection. They can further be used to immunize animals to CPV and other related viruses. ELISA assays have shown that immunized animal sera recognized the viral antigens while hemagglutination inhibition assays (HI) showed that sera from animals immunized with the purified VP2 protein obtained with this invention offered HI titres equal to or greater than those obtained with other commercially available vaccines, though in some cases the response was faster and/or considerably greater when the animals were immunized with the VP2 obtained with our process (see table I).

It has also been ascertained that the VP2 protein hereof expressed by the AcMNPC.pCPVEx17 recombinant baculovirus induces antisera capable of neutralizing CPV and protecting cell monolayers up to a dilution of 1:2000 equivalent to sera from animals that are hyperimmunized or have recovered from natural infections.

Based on the results obtained, the VP2 protein expressed by the recombinant baculovirus vector hereof can be used to be formulated in vaccines in order to protect animals from infections caused by CPV and/or related viruses. These vaccines can be both passive and active. A passive vaccine could be obtained immunizing animals with the recombinant and purified VP2 hereof and then isolating polyclonal antibodies from the said VP2 which could, when purified, be used in therapeutic or prophylactic applications. An active vaccine can be prepared resuspending the VP2 hereof in an immunologically acceptable diluent with an adjuvant.

It was submitted above that the VP2 protein obtained with the process of this invention is peculiar in that it can be aggregated, working pursuant to our conditions, and form pseudo-viral empty VP2 capsids of regular and uniform structure and with a size of 22 nm, as shown by electron microscopy. No-one had to date described the "in vitro" formation of pseudo-viral capsids in Canine Parvoviruses using only the VP2 protein thereof. This much allows the recombinant VP2 proteins obtained to be easily purified. Furthermore, the empty capsids formed by VP2 assembly have an enhanced hemagglutination activity and are highly immunogenic, more so than other CPV recombinant proteins produced heretofore in other vectors. The said capsids can hence be formulated to be used in vaccines capable of protecting animals from infections caused by CPV and/or related viruses (FPLV, MEV). Broadly speaking, an active vaccine can be prepared resuspending the said capsids in an immunologically acceptable diluent, with or without an adjuvant. An important feature of these empty VP2 capsids, that could be obvious to someone skilled in the art, is that they can be chemically or genetically manipulated to introduce the protein epitopes of other viruses, infection of which is to be protected, thereby to work as a polyvalent vaccine.

Phosphate buffer saline (PBS) solutions or other like saline solutions could be used as an immunologically acceptable diluent. The adjuvant used could be alumina gel suspensions or other adjuvants regularly used in formulating vaccines.

1. Obtaining Recombinant Baculoviruses Expressing the CPV VP2 Gene

1.1. Preparing the VP2 gene

Isolation and cloning of the gene coding for the VP2 from the canine Parvovirus genome was described in detail in aforesaid Spanish patent application number 9002074. The said patent application describes the construction of a genomic clone containing a copy of the CPV genome from the 780916 CPV attenuated strain (Cornell University) by means of a process comprising donation of the 3' end, the central region and the 5' end into three plasmids that were then digested with the appropriate restriction enzymes to isolate the CPV genome fragments, that were isolated and ligated to each other. The structural protein VP2 gene was cloned to obtain the pCPV12 plasmid to facilitate structural gene expression.

1.2. VP2 gene insertion in a baculovirus transfer vector

A plasmidic vector AcMNPV derived with an unique cloning site NheI locus (pJVP10Z plasmid, Vialard, J. et al., J. Virol. 64, 37-50, 1990) was donated by Dr Chris Richardson (NRC. Quebec. Canada) and used to clone the XbaI fragment obtained from the pCPV12 as described in FIG. 2. The said figure shows how the pCPV12 HaII fragment containing the gene coding for the CPV VP2 was cloned in pMTL24 vector AccI site flanked by two XbaI sites, leading to the pCPV13 plasmid. The XbaI fragment of such plasmid was then inserted into the pJPV10Z NheI site. The plasmids obtained with the VP2 gene inserted were purified in accordance with the alkalyne lysis method (Brimboim & Doly. Nucleic Acids Res. 7, 1513-1523. 1979) and characterized by restriction endonuclease mapping. The pCPVEx17 recombinant proved to have adequately oriented CPV DNA for expression thereof by the AcMNPV virus polyhedrin promoter.

1.3. Recombinant virus transfection and selection

S. frugiperda cells were transfected with purified DNA mixtures from wild-type AcMNPV and pCPVEx17 plasmidic DNA in accordance with the process described by Burand et al. Virology 101. 286-290 (1980). AcMNPV DNA (1 .mu.g) purified by the method of Smith and Summers (Virology 123, 393-406. 1983) was mixed with two different quantities of plasmidic DNA (1 and 5 .mu.g) and taken to 750 .mu.l with Hepes buffered saline solution (25 mM Hepes, pH 7.1, 140 mM NaCl and 125 mM CaCl.sub.2). The DNA solution was inoculated onto monolayers of 2.times.10.sup.6 S. frugiperda cells and incubated for 4 h at room temperature. The medium was then removed and 5 ml of medium containing 10% calf foetal serum added. After incubating for 4 days the supernatants were collected and titrated in confluent monolayers of S. frugiperda cells. To improve detection of the recombinant plaques the X-gal blue indicator was added to the agarose. Blue plates showing no traces of occlusion bodies (viral polyhedrin) under a light microscope were collected and back-titrated on S. frugiperda cells to obtain the recombinant viruses. After a third plating, high titre stocks of the recombinant viruses (10.sup.7-8 pfu/ml) were obtained.

The recombinant baculovirus was called AcMNPV.pCPVEx17 and filed with the European Collection of Animal Cell Cultures (ECACC), accession number V91030212.

2. Dot Blot Assay

A Dot Blot assay was made to determine whether the VP2 gene had been integrated in the recombinant baculovirus genome, as follows.

To obtain DNA from the recombinant baculovirus the S. frugiperda cells were infected with the said virus at multiplicity of infection of 5 PFU/cell and incubated at 27.degree. C. for 48 hours. The infected cells were collected, sonicated and centrifuged at 10000 rpm for 10 min to eliminate cell debris. The supernatants were used as starting material for the assays.

A volume of 100 .mu.l was denatured with 10 .mu.l of NaOH 1M, boiled for 5 min and placed immediately on ice. The mixture was neutralized with 10 .mu.l of PO.sub.4 H.sub.2 Na 1M.H.sub.2 Na 1M. A 20.times.SSC solution was added immediately to obtain a final 6.times.SSC concentration (SSC, citrate saline solution).

The solution was transferred to a nitrocellulose filter that had previously been moistened with 6.times.SSC. It was washed with more 6.times.SSC and dried at 37.degree. C. for 30 min. The DNA was fixed to the nitrocellulose filter under an U.V. light for 2-3 min. The membranes were then hybridized with a specific probe of the VP2 region marked with Phosphorous-32 labeled probe at 37.degree. C. overnight. It was then washed with decreasing SSC solutions and autoradiographed.

A strong sign of hybridization was observed only in the case of the cups containing floats from the cultures infected with recombinant viruses, which indicated that the VP2 gene had been integrated in the viral genome.

3. Protein and Immunodetection Analysis

S. frugiperda cells were infected with recombinant baculovirus at a multiplicity of 5 PFU/cell and incubated at 27.degree. C. for 48 h. The cells were collected by centrifuging at 1000 rpm for 10 min, washed twice with phosphate buffered saline (PBS) solution pH 7.4 and resuspended at 1.times.10.sup.6 cells/ml with lysis buffer (5% sodium dodecyl sulphate (SDS), 1% .beta.-mercaptoethanol and 17.4% glycerol). The samples were charged in SDS-polyacrylamide 8 to 15% gradient gels for electrophoresis and stained with Coomasie blue or transferred to nitrocellulose membranes for immunodetection. The Coomasie blue tint showed the major presence of a protein with a virtual molecular weight of 67 KDa, equivalent to that of the VP2 viral protein in the recombinant virus plate.

For immunodetection the proteins were transferred to nitrocellulose membranes in accordance with previously described methods (Burnette, Anal. Biochem. 112. 195-203, 1981. Towbin et al., Proc. Natl. Acad. Sci. USA 76. 4350-4354. 1979). Protein transfer was made in a PhastSystem (Pharmacy) apparatus. In general 25 mA/gel were used for 10-15 minutes. The nitrocellulose strips were blocked with 3% powdered skim milk in Tris HCl 20 mM pH 7.5, NaCl 500 mM (TBS) for 30 min at room temperature. The strips were then incubated for an hour at room temperature with the first anti-CPV antiserum (specific antisera or monoclonal antibodies), washed with TBS-0.05% TWEEN-20, a sorbitan derivative, namely, sorbitan mono-9-octadecenoate poly(oxy-1,2-ethanediyl)derivative, commercialized by Sigma Chemicals Company for 30 min at room temperature and incubated with anti-mouse rabbit serum where anti-CPV murine monoclonal antibodies was used as the first antibody at a dilution of 1:1000 for an hour at room temperature, or with anti-rabbit goat immunoglobulins marked with biotin (1:1000) where rabbit antisera was used as a first antibody. The strips were washed again and allowed to react with either protein A, marked with peroxidase, at a dilution of 1:1000 for an hour at room temperature, or streptavidin-peroxidase (1:2000) for 30 min at room temperature. After a thorough wash, the filters were developed with a TBS solution containing 0.5 mg/ml of 4-chloro-1-naphthol (Sigma), 17% (v/v) of methanol and 0.015% of hydrogen peroxide in TBS until visible bands appeared. The reaction was stopped treating the strips with distilled water.

Wherever specificities could be determined by immunoblot analysis all anti-CPV polyclonal antisera reacted with the VP2 protein expressed in baculovirus. The most significant feature is that all the neutralising monoclonal antibodies working in this assay also recognize recombinant VP2. This assay proves that the recombinant VP2 protein is antigenically undistinguishable from the viral VP2.

3.1. Recombinant protein and capsid purification

S. frugiperda cells were infected with AcMNPV.pCPVEx17 recombinant virus at a multiplicity of infection of 5-10 PFU/cell and incubated at 27.degree. C. for 48-72 h. The cells were collected by centrifuging at 1000 rpm for 10 min, washed twice in a phosphate buffered saline solution pH 7.4 and resuspended at 2.times.10.sup.7 cells/ml in a 25 mM pH 9.5 bicarbonate buffer. The resuspended cells were broken down by sonication and centrifuged at 10,000 rpm for 10 min to eliminate cell remains. The supernatant containing the recombinant VP2 protein can be purified using its autoaggregating capacity to form empty capsids. They are to this end either purified by 20% ammonium sulphate precipitation or the empty capsids are centrifuged on CsCl gradients at 45,000 rpm for 14 h. Capsids offer a buoyant density (.rho.) of 1.30 g/cm.sup.3 when banded in CsCl gradient. Preparation purity was determined by electrophoresis in SDS-polyacrylamide gels as per the method described above and VP2 protein purity turned out to be in excess of 99%.

4. Hemagglutination Activity

Recombinant protein functionality was analysed by a hemagglutination assay (HA). This activity is only associated to the particulate character of the product, thereby clearly distinguishing the same from previous ones. To this end 50 .mu.l are mixed in a solution containing several factor two dilutions of a viral preparation with 50 .mu.l of a solution containing 1% pig erythrocytes in phosphate buffer pH 6.5 and left at 4.degree. C. for at least 4 h in round bottom plates.

The results show that the preparation of VP2 capsids has an HA titre of 10.sup.5 units/ml.

5. Confirming the Presence of Capsids by Electron Microscopy

A purified VP2 preparation was stained by negative contrast with 2% uranyl acetate and observed under an electron microscope at a magnifying power of 40,000.times.2.5, the presence of a large number of pseudo-viral chimeric particles of regular and uniform structure and with a size of roughly 22 nm being observed. (FIG. 3).

6. Immunizing Animals

Two New Zealand rabbits weighing 2 kg were intramuscularly immunized three times (days 0, 15 and 30) with 100 .mu.g of a purified VP2 preparation. The first time in complete Freund adjuvant, the second and third times with Freund's incomplete adjuvant. A week after the last immunization the rabbit was bled and the serum obtained tested with an ELISA assay, an hemagglutination inhibition assay (IHA) and an in vitro CPV neutralization assay, as described hereinbelow.

On the other hand, 22 Beagle dogs, aged 45 days, grouped into four batches, were subcutaneously immunized with two doses of semi-purified VP2 capsid preparations (with .beta.-galactosidase). Protein concentration was calculated by the Bradford assay. The second dose was provided 28 days after the first immunization. The antigen was adjuvanted with ALHYDROGEL, an aluminum hydroxide gel, commercialized by superflos (Denmark) QUIL-A, otherwise known as "Quillaic acid", a saponin adjuvant, commercialized by Superflos (Denmark), both products of Superfos of Denmark, (50 .mu.g/dog) or a combination of both. The dogs were bled twice a week until two months after the last injection. Two dogs were immunized with an inactivated commercial vaccine. A sentinel dog was used as a negative control to check for accidental CPV exposure.

The results are set out in table I. Dogs in group I were given 100 .mu.g of VP2 capsids per inoculation. Groups II, III and IV were respectively given 50, 25 and 10 .mu.g per immunization.

The presence of antibodies in the dog sera was determined by an ELISA assay and by their capacity to inhibit viral HA, according to a process described hereinafter. All dogs immunized with our antigen displayed anti-CPV antibodies at various levels at the different adjuvant combinations considered (Table I). The highest antibody titres were nevertheless obtained when Quil A.TM. was used, alone or combined with ALHYDROGEL, an aluminum hydroxide gel. This combination was most effective at low doses (10 .mu.g) of VP2 capsids.

HI and neutralization assays were used to assess the capacity of these preparations to induce CPV protection in dogs. Pollock and Carmichael (Cornell Vet. 72, 16-35, 1982) had previously shown that there is a good correlation between HI titre and protection; dogs showing HI>80 titres are considered refractory to CPV infection. All dogs vaccinated with our preparation showed high HI titres, between 150 and 5,120 (Table I). Sufficient HI titres lasted for over 2 months. A good correlation was found between HI titres and monolayer protection neutralisation. The titres obtained are in both cases far higher, even at lower doses (10 .mu.g/dog) than what is needed to obtain a protective response in dogs. The titres were similar to those achieved with sera of dogs who had recovered from parvovirosis infection and much higher than those obtained with the vaccinated control. No anti-CPV antibodies were detected in the serum of dogs that were not inoculated.

A) ELISA

The presence of specific CPV antibodies in the serum of immunized animals was determined by means of an indirect ELISA assay. The antigen used was both purified virus and purified VP2 protein. Briefly, polystyrene plates were coated with 0.5 .mu.g of virus or 0.25 .mu.g/well of VP2 in 100 .mu.l of carbonate buffer (0.05M, pH 9.6) at 4.degree. C. overnight. The plates were washed with PBS (0.15M NaCl in 0.1M sodium phosphate, pH 7.4) containing 0.05% TWEEN-20.TM. and incubated with the antiserum for 2 h at 37.degree. C., washed again and incubated with either anti-rabbit goat IgG marked with biotin, where rabbit sera was used, or with protein A labeled with peroxidase if dog sera were used, for 1 h at room temperature. The antibody marked with biotin was later incubated with streptavidin marked with peroxidase for 30 min at room temperature. The plates were washed again and the reaction developed with o-phenylenediamine as a substrate for the peroxidase, for 10 min in darkness, and read at 450 nm in a multichannel spectrophotometer. The results obtained were as follows:

For the rabbit sera, the ELISA titre obtained was 1/6400 for the complete virus.

For the dog sera, the antisera recognised the viral antigen in an ELISA assay perfectly, with complete virus titres of up to 1/5120 for the 100 .mu.g dose.

B) Hemagglutination Inhibition (HI)

The sera were inactivated at 56.degree. C. for 30 min and adsorbed with 0.25% Kaolin for 1 h at room temperature whereupon the immunized dog sera was diluted (factor two) in a phosphate buffer pH 6.5, starting with a 1:20 dilution. 25 .mu.l of each of the previous dilutions were mixed with 25 .mu.l of a virus dilution containing 4 units of HA, and incubated for 2 h at 37.degree. C. 50 .mu.l of 1% pig erythrocytes were then added in a phosphate buffer pH 6.5 and left at 4.degree. C. for at least 4 h in round bottom plates.

C) "In vitro" CPV Neutralization

Neutralization was determined by means of a monolayer protection test. Briefly, a known quantity of CPV (100 units of HA) was incubated with rabbit antiserum at different dilutions for 2 hours at 37.degree. C. The samples were then inoculated on CRFK susceptible cell monolayers for 90 min at 37.degree. C. The monolayers were covered with 1 ml of agarose at 1% in a fresh medium. Five days after infection the cells were fixed with 10% formaldehyde in PBS for 20 min, the agarose was removed and the remaining cells stained with 1% violet crystal in 50% ethanol for 20 min. The level of protection was assessed by screening the infected monolayers.

The rabbit antiserum induced by the pCPVEx17 plasmid protein is able to protect the cellular monolayer from the infection up to a dilution of 1:2000, equivalent to sera from animals that are hyperimmunized or have recovered from natural infections.

7. Protecting Dogs Immunized with VP2 Capsids from Virulent CPV Virus Infection

In order to determine the capacity of the VP2 recombinant particles to induce protection in dogs, six dogs were infected 6 weeks after the second injection with 1 ml of virulent CPV infected dog faeces, diluted twice in PBS buffer, by oro-nasal inoculation. The clinical signs of the disease were monitored for 17 days post-infection. Rectal temperatures were recorded from day 3 post-infection. Blood samples were taken at intervals and checked for the presence of viral antibodies and for HA viremia and infection of susceptible cell cultures (Table II). The virus recovered from the dog faeces was identified as CPV by hemagglutination. All dogs immunized with the viral antigen were immune to viral infection. None of them developed any visible clinical symptom of disease or viremia. The sentinel and the vaccinated dog however offered an enhanced humoral response which indicated viral replication.

Given the significant genetic and immunologic relationship between CPV, the feline panleukopenia virus (cats) and the mink enteritis virus (minks) it could reasonably be said that the same VP2 particles may be used to render cats and minks immune to parvovirus, as is the case with conventional vaccines.

The dogs used in the challenge test were immunized as follows: 1. Sentinel dog. 2. Dog vaccinated with an inactivated commercial vaccine. 3. Dog vaccinated with 50 .mu.g of VP2 adjuvanted with Quil-A (quillaic acid). 4. Dog vaccinated with 50 .mu.g of VP2 adjuvanted with alumina and Quil-A(quillaic acid). 5. Dog vaccinated with 25 .mu.g of VP2 adjuvanted with alumina and Quil-A(quillaic acid). 6. Dog vaccinated with 10 .mu.g of VP2 adjuvanted with alumina and Quil-A(quillaic acid).

8. Vaccine Formulation

A passive vaccine can be obtained immunizing animals with the purified recombinant VP2 vaccines as described herein. Polyclonal antibodies directed against this VP2 can be isolated from serum, milk or other animal bodily fluids. These antibodies can then be purified and used for therapeutic or prophylactic applications.

An active vaccine can be prepared resuspending the recombinant VP2 capsids described herein in an immunologically acceptable diluent such as PBS and an adjuvant such as ALHYDROGEL.TM. or QuillA.TM.. Initial and second injections or oral administration of the vaccinal solution can be used to confer immunity.

An active vaccine can also be prepared resuspending the empty capsids in an immunologically acceptable diluent with or without an adjuvant. Anyone skilled in the art will clearly see that these VP2 capsids formed only by VP2 can be chemically or genetically manipulated to introduce other viral protein epitopes and hence function as a polyvalent vaccine.

9. Conclusions

The AcMNPV.pCPVEx17 baculovirus is capable of producing a recombinant VP2 absolutely identical to the viral VP2 protein, as shown with the DNA sequence, molecular weight estimate and antigenic characterization. The VP2 obtained herein with our process is also remarkably capable of forming empty capsids, thereby providing the same with a hemagglutination and immunogenic activity that is clearly greater than in other previously described recombinant proteins, as shown with the animal immunisation tests herein described.

This enhanced immunogenic capacity can be used by those skilled in the art to present other viral protein epitopes, that can be introduced therein by either chemical or genetic manipulation of the recombinant baculoviruses.

Translation of the keys to the figures

FIG. 2

(a) Isolate agarose gel fragments.

(b) Ligate.

(c) Phosphatase.

Having described the object of the present invention what is deemed as the essence thereof is now contained in the following