NUCLEOTIDE SEQUENCES OF PESTIVIRUS STRAINS, POLYPEPTIDES ENCODED BY THESE SEQUENCES AND USE THEREOF FOR DIAGNOSIS AND PREVENTION OF PESTIVIRUS INFECTIONS

Abstract: The invention provides a nucleotide sequence corresponding to a classical swine fever virus (CSFV) genome or a part or a mutant thereof, which comprises at least a part of the nucleotide sequence of the CSFV C-strain depicted in SEQ ID No. 1, or a complement or RNA equivalent of such nucleotide sequence, or which comprises a nucleotide sequence encoding at least the amino acid sequence 268-494 of the amino acid sequence depicted in SEQ ID No. 1, or a complement or RNA equivalent of such nucleotide sequence. Also provided is a pestivirus polypeptide corresponding to the amino acid sequence 690-1063 of SEQ ID No. 1 or part thereof, which contains a mutation in one of the epitopes within amino acid sequences 691-750 or 785-870, said mutation altering said epitope. Further provided is a method of determining the presence of a test substance capable of specifically binding with a binding site of a binding partner, in a sample, by means of competition of said test substance with a measurable amount of a reference substance capable of specifically binding with the same binding site of said binding partner, comprising: (1) contacting said sample with (a) said reference substance bound to a solid carrier, (b) the binding partner of said reference substance, said binding partner molecule containing at least two identical binding sites for said reference substance, and (c) said reference substance provided with a label; (2) measuring the degree of binding of said label to said carrier.

Description: Nucleotide sequences of pestivirus strains, polypeptides encoded by these sequences and use thereof for diagnosis and prevention of pestivirus infections. Field of the invention The invention discloses a method for the construction of a full-length DNAcopy of the genome of the C-strain, a classical swine fever vaccine strain, and transcription of RNA thereof which after transfection in cells gives rise to synthesis of infectious C- strain virus. The invention also comprises C-strain derived (pestivirus) vaccines, as well as subunit vaccines against pestivirus and diagnostic means and methods in relation to pestivirus infections. The invention furthermore provides a method of detecting an immunoactive substance in a sample by means of a competitive assay. Background of the invention Classical swine fever (CSF) or hog cholera is a highly contagious and often fatal disease of pigs which is characterised by fever and haemorrhages and can run an acute or chronic course (Van Oirschot. 1986. Hog cholera, p.289-300. In Diseases of Swine. Iowa State University Press, Ames). Outbreaks of the disease occur intermittently in several European and other countries and can cause large economic losses. Vaccination of pigs with a live attenuated Classical swine fever virus (CSFV) vaccine strain, the "Chinese" strain (C-strain), protects pigs against CSF (Terpstra and Wensvoort. 1988. Vet. Microbiol. 16: 123-128). A major drawback of vaccinating pigs with the conventional vaccines, of which the C-strain is one, is that these vaccinated pigs cannot be distinguished serologically from pigs infected with a CSFV field strain. The C-strain, however, is considered one of the most effective and safe live vaccines. Addition of a (serological) marker to the C-strain would be highly advantageous and would improve the vaccine. CSFV is a member of the Pestivirus genus of the Fiaviviridae (Francki, R.I.B. et al. 1991. Ftaviviridae, p. 223-233. In Fifth report of the International Committee on Taxonomy of Viruses. Archiv. Virol. Suppl. 2, Springer Verlag, Vienna). The other two members of the Pestivirus genus, which are structurally, antigenically and genetically closely related to CSFV, are Bovine viral diarrhoea virus (BVDV) mainly affecting cattle, and Border disease virus (BDV) mainly affecting sheep (Moennig and Plagemann, 1992. Adv. Virus Res. 41: 53-98; Moormann et al., 1990. Virology 177: 184-198; Becher et al. 1994. Virology 198: 542-551). The genomes o pestiviruses consist of a positive strand RNA molecule of about 12.5 kb (Renard et al. 1985. DNA 4: 429-438; Moormann and Hulst 1988. Virus Res. 11: 281-291 ; Becher et al. 1994. Virology 198: 542-551). The positive strand RNA genomes of several non-cytopathogenic BVDV strains, however, may be considerably larger (Meyers et al. 1991. Virology 180: 602-616; Meyers et al. 1992. Virology 191: 368-386; Qi et al. 1992. Virology 189: 285-292). An inherent property of viruses with a positive strand RNA genome is that their genomic RNA is infectious, i.e. after transfection of this RNA in cells that support viral replication infectious virus is produced. As expected, the genomic (viral) RNA of pesti- viruses is also infectious (Moennig and Plagemann, 1992. Adv. Virus Res. 41: 53-98). For several years recombinant DNA technology has allowed in vitro transcription of cloned DNA. This possibility has opened the way to synthesize infectious RNA in vitro from a DNA copy of the genome of a positive strand RNA virus. It is well known in the field of molecular engineering that DNA, in contrast to RNA, is easily manipulated by site directed mutagenesis. Hence, the availability of the technique to produce synthetic infectious RNA has greatly enhanced the study of e.g. replication, virulence, patho- genesis, RNA recombination, vector development, and antiviral strategies of the positive strand RNA viruses. However, application of the technology may cause severe problems. The nature of these problems has been described in a recent review by Boyer and Haenni. 1994. (Virology 198: 415-426). In fact, the success or failure to construct a full- length DNA copy of the genome of a positive strand RNA virus and to produce synthetic infectious RNA from such a full-length DNA copy cannot be reliably predicted. Summary of the invention The invention provides nucleotide sequences corresponding to a CSFV genome which comprise at least a part of the nucleotide sequence of the CSFV C-strain depicted in SEQ ID No. 1 , or a complement or RNA equivalent of such nucleotide sequence, or mutants thereof. Also provided are degenerate nucleotide sequences having different nucleotides but encoding the same amino acids. The invention also covers polypeptides encoded by these nucleotide sequences, and vaccine strains, the genome of which contains such a nucleotide sequence, in particular a recombinant virus strain based on transcripts of a full-length DNA copy of the genome of the CSFV C-strain. Partial nucleotide sequences as indicated above are also useful, in particular those which contain a mutation in the structural region of the virus genome, i.e. in the nucleotide sequence encoding amino acids 1-1063 of the sequence depicted in SEQ ID No. 1. The mutation may be a substitution by a corresponding part of the genome of another pestivirus strain, a substitution of one or amino acids, or a deletion. The mutation may also be an inserted or substituted heterologous nucleotide sequence altering the translation strategy of the CSFV nucleotide sequence or altering the processing of a polypeptide encoded by the CSFV nucleotide sequence. Furthermore, the mutation may be an inserted or substituted heterologous nucleotide sequence encoding a polypeptide inducing immunity against another pathogen; in this case the CSFV sequence is used as a vector for heterologous immunogens. The invention is also concerned with nucleotide sequences of a pestivirus genome in general or a part or a mutant thereof, which sequences contain a mutation in a subregion of the E1 protein, i.e. in the nucleotide sequence encoding amino acids corresponding to amino acids 691 -750 or 785-870 of the sequence depicted in SEQ ID No. 1 , as well as the polypeptides encoded by these nucleotide sequences. These polypeptides are particularly useful for protecting animals against a pestivirus infection in such a way so as to allow a diagnosis which distinguishes between animals infected with field strains of the pestivirus and vaccinated animals. The invention is furthermore concerned with vaccines containing a nucleotide sequence, a polypeptide, or a vaccine strain as indicated above, as well as to diagnostic compositions containing a nucleotide sequence or a polypeptide as mentioned above, or an antibody raised against such polypeptide. The invention also relates to methods and means for diagnosis of pestivirus infections, especially with such means and methods which distinguish between infected animals infected and vaccinated animals. The invention also provides a method for determining test substances, such as an antibody or an antigen in an immunoassay, by means of a specific binding test, wherein a specifically binding reference substance in immobilised form and the same specifically binding reference substance in labeled form are used. Detailed description of the invention The invention provides the complete cDNA sequence of the RNA genome of the "Chinese" strain (C-strain; EP-A-351901) of CSFV. This allows the construction of a full-length DNA copy of this sequence, of which synthetic RNA can be transcribed that after transfection in suitable cells, such as SK6-M cells (Kasza, L et al. 1972. Res. Vet. Sci., 13: 46-51; EP-A-351901) gives rise to synthesis of infectious C-strain virus. The use of this finding for the development of modified C-strain vaccines, e.g. vaccines which contain a (serological) marker, is described. Although the invention is illustrated for one CSFV strain, it is also applicable and useful for other pestivirus strains by exchanging specific genomic segments, described below, between the other pestivirus and the CSFV C-strain, or by constructing an "infectious" DNA copy of the other pestivirus. The nucleotide sequence of a DNA copy of the genomic RNA of the C-strain is depicted in SEQ ID No. 1. The numerals mentioned in the text are all related to this sequence and may differ slightly in the sequences of other pestiviruses. The nucleotide sequence is 12,311 nucleotides in length and contains one large open reading frame (ORF) of 11 ,694 nucleotides encoding a polyprotein of 3,898 amino acids. The size of this ORF is the same as that of the genomes of CSFV strains Brescia (Moormann et al. 1990. Virology 177: 184-198) and Aifort (Meyers et al. 1989. Virology 171: 555-567). The ORF starts with the ATG at nucleotide positions 374 to 376 and stops at the TGA codon at nucleotide positions 12,068 to 12,070. The 5' non-coding region which precedes the ORF is 373 nucleotides in length. Its sequence is highly conserved between strains Brescia, Aifort and C (Fig. 2), and the predicted secondary structure of this region resembles that of the 5' non-coding region of hepatitis C virus (Brown et al. 1992. Nucleic Acids Res. 20: 5041-5045), another member of the Flaviviridae. The 5' non-coding region of hepatitis C virus has been shown to contain an internal ribosome entry site (Tsukiyama-Kohara et al. 1992. J. Virol. 66: 1476-1483). Such sites have important regulatory functions (Agol. 1991. Adv. Virus. Res. 40: 103-180). The analogy with hepatitis C virus indicates that the 5' non-coding region of CSFV also contains an internal ribosome entry site, which is located approximately between nucleotides 124 and 374 of the sequence of SEQ ID No. 1 , as important regulatory element. The internal ribosome entry site may be used as a site for mutation in order to attenuate the virus, as well as for altering the translation strategy of the ORF. A second important region regulating replication of pestiviruses is the 3' non- coding region. Upon alignment of the C-strain sequence with the sequences of strains Brescia and Aifort, a sequence of 13 nucleotides unique to the C-strain was observed in this region (Figure 2B). This unique sequence TTTTCTTTTTTTT is located from nucleotide positions 12,128 to 12,140 in the sequence of SEQ ID No. 1. It is the only insertion of more than two nucleotides in a row observed in the sequence of the C strain compared to the sequences of strains Brescia and Aifort. For the rest, the sequences in the 3' non-coding regions of three CSFV strains are highly homologous. The overall homology between sequences in this region is lower when CSFV strains and BVDV strains are compared. Nevertheless, it is clear that the TTTTCTTTTTTTT sequence of the C-strain is also absent in the sequences of the 3' non-coding regions of the BVDV strains. The TTTTCTTTTTTTT sequence therefore appears to be unique to the genome of the C-strain, and will provide an excellent marker for a C-strain specific sequence. This sequence can be used as a basis for nucleotide probes, and for sequence determ¬ ination, to identify C-strain specific pestiviruses. Therefore, all pestivirus strains having this sequence in their 3' non-coding region (not necessarily at an identical position as in the C-strain) are considered related to the C-strain, and are also part of the invention. A crucial parameter for infectivity of transcripts of a DNA copy of the genome of a pestivirus is the amino acid sequence. In this respect, two aspects regarding the cloning and sequencing of RNA viruses in general, and pestiviruses in particular, had to be considered. First, the mutation frequency of the genome of positive strand RNA viruses is high (about 1/10 4 nucleotides during replication), and therefore no stock of virus or viral RNA preparation is ever clonal with regard to the viral RNA it contains. Among these RNA molecules there may also be molecules which are noninfectious. If this were caused by premature stop codons in the large open reading frame, this would be easily recognised. Also mutations affecting active sites of viral enzymes, or known structures of proteins would be recognizable. However, where tine relation between the amino acid sequence and the function and structure of a protein is unknown, which is the case with most of the pestivirus proteins, it is impossible to predict which amino acid is valid and which one is not. Second, mutations may have been introduced during cDNA synthesis. Therefore, the genome of the C-strain was cloned and sequenced indepen¬ dently twice. Regions with discrepancies between the sequences were cloned and sequenced at least thrice (compare Figure 1). The sequence which was encountered twice at a particular position was regarded as the correct one at that position. The necessity of this approach for the generation of infectious transcripts of a DNA copy of the genome of the C-strain is demonstrated by the following finding. Full-length DNA copy pPRKflc-113, composed after the second round of cloning and sequencing (Fig. 3), appeared to be noninfectious. After cloning and sequencing of regions with dis- crepancies between the sequences of cDNA clones of the first and second round, there appeared to be five amino acids which were different in the full-length copy of the second round cDNA clones and the sequence of the C-strain considered correct. After correction of these five amino acids in pPRKflc-113, clone pPRKflc-133 was obtained which generated infectious transcripts (Figure 4). The 5 differences are located at amino acid positions 1414 (Val→Ala); 2718 (Gly— Asp); 2877 (Val— Met); 3228 (Leu— et); 3278 (Tyr→Glu). The amino acids encoded at these positions by the cDNA sequence which is noninfectious are indicated before the arrow, amino acids at these positions in the copy that is infectious are indicated after the arrow (SEQ ID No. 1). Whether each of the amino acid changes individually will abolish infectivity of the C-strain DNA copy will have to be determined by analysing infectivity of transcripts with individual mutations of each of the five amino acids. However, this finding shows that small differences in the amino acid sequence may be crucial for infectivity of transcripts of a DNA copy of the genome of the C-strain. It also indicates that preparing infectious transcripts of a copy of the sequence of a pestivirus may in practice appear to be impossible because of small differences in sequences (even at the one amino acid level) which may go unnoticed. C-strain derived mutants that are suitable for (marker) vaccine development are part of the invention. T ey may contain mutations like deletions, insertions, (multiple) nucleotide mutations, and inserted and/or exchanged genomic fragments originating from other pestivirus strains, in the nucleotide sequence described in SEQ ID No. 1. The sequence of the C-strain can be divided in four regions suitable for mutation and/or exchange. Region one is the 5' non-coding sequence running from nucleotides 1 to 373. Region two encodes the structural proteins N ro ~C-E2-E3-E1 and runs from nucleotides 374 to 3563. Region three encodes the nonstructural proteins and runs from nucleotides 3564 to 12068. Region four is the 3' non-coding sequence which runs from nucleotides 12069 to 12311. One region that is particularly suitable for making C-strain marker vaccines comprises the genomic region encoding the structural proteins N r0 -C-E2-E3-E1. This region is located between amino acids 1 and 1063 in the sequence of SEQ ID No. 1. Preferred subregions of this part of the genome are specified by the following amino acid sequences 1-168 (N pr0 ), 169 to 267 (C), 268 to 494 (E2), 495 to 689 (E3), and 690 to 1063 (E1), or parts thereof. As an example the N-terminal antigenic part of the region encoding E1 of the C-strain, running from amino acid 690 to 877, was exchanged with the corresponding region of E1 of strain Brescia (Figure 4, pPRKflc-h6). The newly generated C-strain derivative is infectious and can be discriminated from the wild-type strain and from strain Brescia through reaction with C-strain and Brescia specific mono¬ clonal antibodies, directed against E1 and E2; as an example, the resulting C-strain reacts with monoclonal antibodies specific for E1 of strain Brescia (Table 1). Thus, the antigenic properties of this new mutant have changed with respect to the parent virus, demonstrating that exchanging the N-terminal half of E1 of the C-strain with that of another CSFV strain is one approach to the development of a C-strain marker vaccine. However, the invention is not restricted to exchange of N-terminal halves of E1 between the C-strain and other CSFV strains. The N-terminal halves of E1 from any other pesti¬ virus strain may be exchanged with corresponding parts of E1 of the C-strain. In this respect, E1 sequences of pestivirus strains which are isolated from pigs, but belong to an antigenic group other than the C-strain, are particularly suitable. Examples of such strains, which were selected on the basis of cross-neutralisation, include strains "Van EE", "Stam", "SF UK 87", "Wisman", and "5250" (Wensvoort et al. 1989. Vet. Microbiol. 20: 291-306; Wensvoort. 1992. In: Report on meeting of national swine laboratories within the European Community. 16-17 June 1992. VI/405992-EN(PVET/EN/1479) 1992, p59-62). The N-terminal half of E1 has been shown to contain three distinct antigenic domains, A, B and C, located on distinct parts of the E1 protein and each reacting with strongly neutralizing monoclonal antibodies (Wensvoort. 1989. J. Gen. Virol. 70: 2865- 2876; Van Rijn et al. 1992. Vet. Microbiol. 33: 221-230; Van Rijn et al. 1993. J. Gen. Virol. 74: 2053-2060). Epitopes conserved among 94 CSFV strains tested, map to domain A, whereas the epitopes of domains B and C are non-conserved (Wensvoort. 1989. J. Gen. Virol. 70: 2865-2876). Mapping of epitopes with hybrids of the E1 genes of strains Brescia and C (Van Rijn et al. 1992. Vet. Microbiol. 33: 221-230), and with deletion mutants of E1 of strain Brescia, suggest that domains A and B + C form two distinct antigenic units in the N-terminal half of E1 (Van Rijn et al. 1993. J. Gen. Virol. 74: 2053-2060). This suggestion was further supported by the finding that the six cys- teines located at positions 693, 737, 792, 818, 828, and 856, in the N-terminal half of E1 are critical for the correct folding of E1. However, at least Cys 792 is not crucial for infectivity of strain Brescia, because a monoclonal antibody resistant mutant of this virus was isolated with a Cys→Arg mutation at this position (Van Rijn et al. 1993. Presentation and abstract at the 9th International Congress of Virology, 8-13 August, Glasgow, Scotland). Whereas small changes in the amino acid sequence may abolish infectivity of the RNA of the C-strain (see Example 2), the cysteine change at position 792 shows that an amino acid change at a position which is less predicted to be suitable for modification without loss of function, may still result in a viable virus mutant. Thus, the effects of a particular amino acid change on the properties of the virus will have to be determined empirically for each amino acid in the sequence of strain C. This again shows that no obvious target sequences for modification of the C-strain, e.g. for marker vaccine development, can be identified on the basis of previously published information. Essential to the development of C-strain marker vaccines is the possibility to differentiate serologically between vaccinated pigs and pigs infected with a CSFV field strain. It was shown previously that a live attenuated pseudorabies virus vector express¬ ing E1 , or immunoaffinity purified E1. expressed in insect cells with a baculovirus vector, induces a protective immune response in pigs against hog cholera (WO 91/00352; Van ZijI et al. 1991. J. Virol. 65: 2761-2765; Hulst et al. 1993. J. Virol. 67: 5435-5442). It was surprisingly found that mutants of E1 with a deleted A domain or with deleted B + C domains (Figure 5), also induce a protective immune response in pigs against hog cholera (Table 2). This indicates that protective immunity induced by the vaccine strain does not depend on neutralizing antibodies against both domains A and B + C. There- fore, pestivirus strain mutants having exchanged or mutated only the A domain, or only the B + C domains, or parts thereof, with the corresponding region of another pestivirus, preferably but not exclusively a pestivirus isolated from pigs belonging to a different antigenic group than the C-strain (for examples see above), are also part of the invention. The region of E1 covering domain A and suitable for exchange or mutation, is located between amino acids 785 and 870. Parts of this region may also be suitably exchanged or mutated, e.g. the subregions located between amino acids 785 and 830 and between amino acids 829 and 870. The region of E1 covering domains B + C and suitable for exchange or mutation is located between amino acids 691 and 750. Parts of this region may also be suitably exchanged or mutated, e.g. the subregions located between amino acids 691 and 718 and between amino acids 717 and 750. Animals infected with pestiviruses develop antibodies against EΞ2 (Kwang et al., 1992. Vet. Microbiol. 32: 281-292; Wensvoort. unpublished observation). Therefore, a second region suitable for (marker) vaccine development via mutation (deletions, insertions, point mutations), or exchange of corresponding genetic material with an antigenically different pestivirus, or with a pestivirus belonging to a different antigenic group, is the region encoding E2. The C-strain may also be used as a vector for the insertion and expression of heterologous genetic material (sequences). For vector development, heterologous genetic material inserted into the C-strain serves to alter translation strategy of the large ORF and processing of the polyprotein encoded by this ORF. An example of a sequence suitable for altering the translation strategy of the large ORF is a sequence specifying an internal Ribosome Entry Site (IRES) (Duke et al. 1992. J. Virol. 66: 1602-1609, and references therein). An example of a sequence suitable for altering processing of the polyprotein is a signal sequence responsible for translocation of proteins exported from the cell or inserted into membranes, across the membrane of the endoplasmatic reticuium (Blobel. 1980. Proc. Natl. Acad. Sci. U.SΛ. 77: 1496-1500; Kreil. 1981. Annu. Rev. Biochem. 50: 317-348). Signal sequences are cleaved by cellular signal peptidases. However, sequences encoding cleavage sites of viral proteases may as well be used to alter processing of the polyprotein. Sequences inserted and expressed by a C-strain vector may be used as a marker to identify vaccinated pigs, or may be used to protect pigs against the pathogen from which the heterologous inserted sequence originates. Marker sequences are preferably highly antigenic and belonging to microorganisms not replicating in pigs. They may encode known complete gene products (e.g. capsid or envelope proteins) or antigenic parts of these gene products (e.g. epitopes). Preferably marker sequences originate from viruses belonging to the families: Adenoviridae, Arenaviridae, Arteriviridae, Bunyaviridae, Caiiciviridae, Circoviridae, Coronaviridae, Flaviviridae, Hepadnaviridae, Herpesviridae, Orthomyxoviridae, Paramyxoviridae, Papovaviridae, Rhabdoviridae, Parvoviridae, Poxviridae, Picornaviridae, Reoviridae, Retroviridae, and Togaviridae. However, marker sequences may also encode artificial antigens not normally encountered in nature, or histochemical markers like Escherichia colt /3-galactosidase, Drosophila alcohol dehydrogenase, human placental alkaline phosphatase, firefly lucif erase, and chloramphenicol acetyltransferase. Heterologous genetic material encoding one or more proteins inducing protective immunity against disease caused by the pathogen corresponding with the heterologous genetic material may be derived from other pestivirus strains, including sequences of strains specified above, porcine parvovirus, porcine respiratory coronavirus, transmissible gastro-enteritis virus, porcine reproductive and respiratory syndrome virus (Lelystad virus, EP.92200781.0), Aujeszky's disease virus (pseudorabies virus), porcine endemic diarrhoea virus, and porcine influenza virus, and bacteria, such as Pasteurella multocida, Bordetella bronchiseptica, Actinobacillus pleuropneumoniae, Streptococcus suis, Treponema hyodysenteria, Escherichia coli, Leptospira, and mycoplasmata, such as M. hyopneumoniae and M. lyorhinis. Suitable sites for insertion of heterologous sequences in the C-strain, but not the only ones, are located between amino acid residues 170 and 171, between residues 690 and 69 , and between residues 691 and 692 and are indicated in SEQ ID No. 1. The invention also includes diagnostic tests which can be used to discriminate between pigs vaccinated with a marker vaccine, or a subunit vaccine containing (mutated) E1 and/or (mutated) E2, and pigs infected with a pestivirus field strain. Suitable forms of such differential diagnostic tests are described in Examples 4 and 5. In the conventional non-discrimiminatory CSFV ELISA test, E1 is used as antigen in the complex trapping blocking (CTB) ELISA assay described by Wensvoort et al., 1988. (Vet. Microbiol. 17: 129-140). This prior art CTB-ELISA, also called Liquid Phase Blocking ELISA, or double antibody sandwich ELISA, uses two monoclonal antibodies (Mabs) which were raised against E1 of CSFV strain Brescia. The epitope for Mab b3, which is located within domain A, is conserved among CSFV strains, whereas the epitope of Mab b8, which is located within domain C, is nonconserved (Wensvoort. 1989. J. Gen. Virol. 70: 2865-2876). The above CTB-ELISA is sensitive, reliable and specifically detects CSFV specific antibodies in pigs infected with a pestivirus. Thus, the test differentiates between pigs infected with a CSFV strain and pigs infected with e.g. a BVDV strain. However, the test does not differentiate between pigs infected with a CSFV field strain and pigs vaccinated with the C-strain vaccine. Also this test is not suitable in conjunction with an E1 subunit vaccine whether live or dead. One test according to the invention is a modified CTB-ELISA, based on only one MAb, e.g. MAb b3. Such a CTB-ELISA, based on only one Mab which Mab is used for binding of the antigen to the surface of an ELISA plate as well as competition with a field serum has not yet been described and is an essential part of this invention. Now that the principle of this test has been described, it can be usefully applied to the development of diagnostic kits for the detection of other antibodies including antibodies against other viruses or other diseases, or antibodies which are indicative for other conditions of the human or animal body. The finding is therefore useful for all CTB- ELISA's, or ELISA's based on the same principle as a CTB-ELISA, which are developed on the basis of a single Mab and a dimerised or muttimerised antigen. The claimed test method is also applicable to the determination of other members of pairs of specifically binding partner molecules, such as activators/receptors, enzymes inhibitors and the like, wherein one of the partners has at least two identical binding sites. Thus the invention also comprises a method of determining the presence of a test substance (e.g. antibody) capable of specifically binding with a binding site of a binding partner (e.g. antigen), in a sample, by means of competition of said test substance with a measurable amount of a reference substance (antibody) capable of specifically binding with the same binding site of said binding partner, comprising (1) contacting said sample with (a) said reference substance (antibody) bound to a solid carrier, (b) the binding partner (antigen) of said reference substance, said binding partner molecule containing at least two identical binding sites for said reference substance, and (c) said reference substance (antibody) provided with a label; (2) measuring the degree of separation of said label from said carrier. As an example, said binding partner (antigen) to said reference substance (antibody), containing at least two identical binding sites is a dimer of a binding partner (antigen) to said reference substance. Using the same principle, the invention also comprises a method of determining the presence of a test substance (antigen) having at least two identical binding sites per molecule for specifically binding with a binding partner (antibody), in a sample, comprising (1) contacting said sample with (a) said binding partner (antibody) bound to a solid carrier, and (b) said binding partner (antibody) provided with a label; (2) measuring the degree of binding of said label to said carrier. In these methods, the antibodies and antigens are only referred to by way of example; they may be substituted by other specifically binding partner molecules. Further provided is a diagnostic kit containing: (a) a reference monoclonal antibody bound to a solid carrier, (b) said reference monoclonal antibody provided with a label; and optionally (c) an antigen to said reference antibody containing at least two identical binding sites for said reference antibody; or a complex between said components (a) and/or (b) and (c); as well as further components for carrying out a competitive immunological assay. The method is suitable as a differential diagnostic test in conjunction with an E1 subunit vaccine, which has a deletion in one or more epitopes of E1 , e.g. domain A. The test is also suitable in conjunction with subunit E1 of which the A domain has been mutated such that antibodies induced against such mutated A domain do not compete with Mab b3 for the epitope of Mab b3. Furthermore, the test is suitable in conjunction with a modified C-strain or other CSFV strain vaccines with a deletion in domain A, with a domain A which has been exchanged with that of a pestivirus belonging to a different antigenic group as CSFV (see above), or with a domain A which has been mutated such that antibodies directed against that domain do not compete with Mab b3 for the epitope of Mab b3. Although the test is described and exemplified for domain A of E1 , a similar test based on only Mab b8 can be used in conjunction with a vaccine with a deletion in domains B + C or domain C, with domain B + C or domain C which has been exchanged with that of a pestivirus belonging to a different antigenic group as CSFV (see above), or with domain B + C or domain C which has been mutated such that antibodies directed against those domains do not compete with Mab b8 for the epitope of Mab b8. The test 12 is illustrated in conjunction with Mab b3 or Mab b8 of strain Brescia. However, the test may be usefully set up with other Mabs directed against domain A or domains B + C of E1 of strain Brescia or against domain A or domains B + C of any other CSFV strain, but also with Mabs against analogous domains in E1 of any other pestivirus. The test can also be based on epitopes of E2 (see Example 5). Antigens suitable in the (modi¬ fied) CTB-ELISAs according to the invention are preferably dimers or multimers of E1 (plus or minus a 3-TMR) or E2 (see Example 5) of CSFV strains reacting with Mab b3 or Mab b8 or similar MAbs directed against E2 epitopes. In the case of a vaccine with a mutated A domain, dimers or multimers of the antigen used for the diagnostic test may be synthesised by the deletion B + C construct (see Example 5), or in the case of a vaccine with mutated B + C domains, dimers or multimers of the antigen used for the diagnostic test may be synthesised by the deletion A construct (compare Figure 5 for constructs; compare Examples 4 and 5). The dimerised (or multimerised) form of the E1 antigen is believed to be based on disulphide bridges formed by cysteine residues in the C-terminal part of E1. It allows a very sensitive immunoassay, as tho dimerised antigen molecule contains two copies of the epitope of one Mab. Thus, this one Mab can be used for immobilising the dimerised antigen via one epitope, and for labeling the dimerised antigen via the other epitope. Competition by sample serum antibodies raised as a result of field strain infection inhibits binding of the labeled antibody to the antigen, and thus results in a sensitive test for the presence of such antibodies. The invention also relates to diagnostic kits based on this method, which kit comprises E1- or E2- based antigens, and (enzyme-) labeled and immobilised monoclonal antibodies of the same type directed at an E1 or E2 epitope, as well as further conventional components (plates, diluents, enzyme substrate, colouring agents, etc.) for carrying out an immunoassay of the competition type. The vaccine according to the invention contains a nucleotide sequence as described above, either as such or as a vaccine strain or in a vector or host organism, or a polypeptide as described above, in an amount effective for producing protection against a pestivirus infection. The vaccine can also be a multipurpose vaccine com- prising other immunogens or nucleotides encoding these. The vaccines can furthermore contain conventional carriers, adjuvants, solubilizers, emulsifiers, preservatives etc. The vaccines according to the invention can be prepared by conventional methods. The method of the invention for the production of infectious transcripts of a full- length DNA copy of the genome of a CSFV strain, the C-strain, is useful for any other C-strain derived, or pestivirus strain. The method, described here for a live attenuated O 95/35380 -.«-,-,„ PCT NL95/00214 13 CSFV vaccine strain, may also be very usefully applied to in vitro attenuate (modify) the C-strain or any other CSFV or pestivirus strain, for vaccine purposes. The C-strain vaccine according to the invention allows serological discrimination between vaccinated pigs and pigs infected with a CSFV field strain. Marker vaccines of any other CSFV-strain or pestivirus strain may equally well be obtained using the methods of the invention. Such marker vaccines may be developed for instance by mutating (deletions, point mutations, insertions) the region encoding E1 , or the N- terminal half of E1 , or domains A or B + C of E1 , or the region encoding E2 of the C- strain, or analogous regions in the genomes of C-strain derived, or other pestivirus strains, or by exchanging these regions with the corresponding regions of antigenically different pestiviruses or of pestiviruses belonging to a different antigenic group. An alternative approach to the development of a C-strain marker vaccine is to add to its genome heterologous genetic material expressing a highly antigenic protein or epitope(s) of a microorganism not replicating in pigs, or of artificial nature and not normally occurring in pigs. Furthermore such heterologous genetic material may encode antigens inducing protective immunity against a disease caused by a microorganism pathogenic for pigs. Therefore, application of the C-strain, or strains derived from the C-strain, or whatever other pestivirus strain, as a vector for the expression of heterologous antigens inducing protection against a particular disease in a host organism, the host organism being a mammal, is also part of the invention. The construction of recombinant C-strain viruses expressing heterologous sequences and suitable sites for insertion of these heterologous sequences are described above. Analogous recombinant viruses can be made for C- strain derived viruses, or for any other pestivirus. These viruses are therefore also part of the invention. An essential part of the invention relates to the immunogenic potential of subunit E1 with deletions in domain A, or domains B + C. As summarised in Table 2, both of these mutant E1s are capable of inducing protective immunity in pigs against challenge with a lethal dose of the highly virulent Brescia strain. The use of mutants of E1 containing deletions or other mutations in domains A and B + C as dead subunit vaccine, or as live subunit vaccine expressed by a vector system in the vaccinated animal, against CSF, is also part of the invention. Also mutated E1 together with other antigenic CSFV proteins, e.g. E2 or a mutated form of E2, is suitable as dead or live subunit vaccine (see above). 14 The invention also includes diagnostic tests which can be used to discriminate between pigs vaccinated with a CSFV marker vaccine, or a subunit vaccine containing (mutated) E1 and/or (mutated) E2, and pigs infected with a pestivirus field strain. Such a diagnostic test may be based on serology, antigen detection, or nucleic acid detection. The choice which test is appropriate in a given case is amongst others dependent on the specificity of the marker used. One suitable form of a serological diagnostic test is the modified CTB-ELISA, described in example 4. According to the invention, this method, based on a CTB-ELISA using a single antibody, is not restricted to the use in the context of CSFV or other pestiviruses, but is also applicable to the determination of other antibodies for other diagnostic purposes in the human or animal field, as well as to the determination of other specifically binding substances. An example of a suitable antigen detection test in conjunction with a C-strain marker vaccine is a test detecting CSFV field strain E1 and not vaccine strain E1 in the blood of pigs. If the A domain of the C-strain has been modified by e.g. exchange of this domain with that of a pestivirus strain belonging to a different antigenic group than CSFV, such a test may be based on monoclonal antibodies recognizing conserved epi¬ topes of the A domain of CSFV. However, if the E2 region of the C-strain is modified for marker vaccine deve¬ lopment, a serological or antigenic diagnostic test accompanying such a vaccine detects differences between vaccinated and infected animals, in relation to the modified E2 region. Such a diagnostic test thus uses E2 specific sequences as an antigen. These E2 specific sequences may originate from the parent C-strain (see example 5), from CSFV strains which are antigenically different from the C-strain, or from pestiviruses belonging to a different antigenic group than CSFV. However, these E2 specific sequences may also be obtained via mutation (deletion(s), insertion(s), or point mutation(s)) of native E2 of any pestivirus, or may consist of (mutated) parts of E2 of any pestivirus. Dimeric E2 and multimeric EE2 may be used as antigen in a diagnostic test (see example 5). Also EΞ2 in conjunction with one monoclonal antibody (compare Examples 4 and 5) may be used in a CTB-ELISA test, the principle of which has been described above. A diagnostic test based on E2 is described in Example 5. Where an antigen detecting kit is to detect pestivirus E2 and is based on one Mab, such test kist preferably contains an antibody recognising a conserved epitope on E2. Such tests are also part of the invention. Finally, a diagnostic test may be based on the specific detection of a region of CSFV field strains which is modified in the C-strain. Suitable techniques for this test include nucleic acid hybridisation, e.g. with specific probes, and/or amplification, e.g. with the polymerase chain reaction. Alternatively, C-strain sequences may be distinguished from CSFV field strain sequences by PCR amplification of (a part of) the 3' non-coding region containing the TTTTCTTTTTTTT sequence unique to the C-strain genome. If the C-strain is modified by insertion of a heterologous marker sequence, any form of a diagnostic test based on this sequence, e.g. based on the antigen, epitope(s), or histochemical product encoded by this sequence, or based on detection of the heterologous genetic information via nucleic acid hybridisation techniques, e.g. specific probes, and/or amplification techniques, like the polymerase chain reaction, is also part of the invention. Example 1 Molecular cloning and sequencing of the genome of the C-strain. Cells and virus Swine kidney cells (SK6-M, EP-A-351901) were grown in Eagle's basal medium containing 5% fetal bovine serum (FBS) and antibiotics. FBS was tested for the presence of BVDV and BVDV antibodies as described (Moormann etal. 1990. Virology 177: 184-198). Only sera free from BVDV and BVDV antibodies were used. The "Chinese" vaccine strain (C-strain) of Classical swine fever virus (CSFV) was adapted to SK6-M cells as described in EP-A-351901. The strain designated "Cedipest" is noncytopathic and was biologically cloned by threefold endpoint dilution. After three amplification steps a cloned virus stock with a liter of 3,5.10 6 TClD 50 /ml was produced. Isolation of cytoplasmic RNA nf SK-fi cells infected with the C-strain. Intracellular RNA from cells infected with the C-strain was isolated essentially as described (Moormann et al. 1990. Virology 177: 184-198). Briefly, monolayers of SK6-M cells in 162 cm 2 bottles (Costar) were infected with Cedipest at a multiplicity of infection (m.o.i.) of 5 TCID^ per cell. Subsequently, cells were incubated for 1.5 hr at 37°C, and fresh medium was added to a final volume of 40 ml. After 7 hrs Actinomycin D was added to a final concentration of 1 μg/ml. After 24 hrs cells were washed twice with ice cold phosphate buffered saline (PBS), and lysed in ice- cold lysisbuffer (50 mM Tris-HCl pH 8.2, 0.14 M NaCI, 2 mM MgCI-, 5 mM DTT, 0.5% [v/v] NP-40, 0.5%[w/v] Na-deoxycholate, and 10 M vanadyl ribonucleoside complexes (New England Biolabs)). The lysates were centrifuged (4 β C, 5 min., 4000 g) and the supernatant was treated with proteinase K (250 μg ml, final concentration) for 30 min. at 37°C, extracted twice with phenol, chloroform, and isoamyl alcohol (49:49:2), and extracted once with chloroform and isoamyl alcohol (24:1). RNA was stored in ethanol. 16 Synthesis and amplification of cDNA. One to two μg of cytoplasmic RNA of cells infected with the C-strain, and 20 pmol (-)sense primer were incubated with 1 μl 10 mM methylmercury hydroxide for 10 min. at room temperature. The denaturated RNA was then incubated with 1 μl 286 mM β-mercaptoethanol for 5 min. at room temperature. The RNA was reverse transcri¬ bed with 200-400 units M-MLV reverse transcriptase deficient of RNase H (Promega) for 45 min. at 42 °C in 1 M-MLV reverse transcriptase buffer (50 mM Tris-HCI pH 8.3, 75 mM KCI, 3 mM MgCI- and 10 mM DTT), 40 U rRNasin (Promega), and 80 μM of dATP, dGTP, dCTP and dTTP. The final reaction volume was 25 μl. The samples were overlaid with 30 μl of mineral oil (Sigma). After reverse transcription (RT) the samples were denaturated for 10 min. at 94°C. Portions of 2.5 μl of each RT-reaction were amplified in a polymerase chain reaction (PCR) of 39 cycles (cycle: 94°C, 60 sec; 55°C, 60 sec. and 72°C, 1-2 min.) in 100 μl Taq polymerase buffer (supplied by the manufacturer of Taq polymerase) contai- ning 1 μM of the (+) as well as the (-) sense primer, 200 μM of each of the four dNTPs, and 2.5 U Taq DNA polymerase (Boehringer Mannheim). The samples were overlaid with 75 μl of mineral oil (Sigma). Cloning of cDNA covering the complete genome of the C-strain. The genome of the C-strain was cloned indepently twice. During the first round of cloning (Figure 1A), primers for first strand cDNA synthesis and PCR were selected on the basis of homology between the sequences of the CSFV strains Brescia (Moormann et al. 1990. Virology 177: 184-198) and Aifort (Meyers et al. 1989. Virology 171:555-567), and the BVDV strains Osloss (Renard et al. EP 0208672) and NADL (Collett et al. 1988. Virology. 165: 191 -199). The sizes of the cDNA fragments were chosen between 0.5-2.5 kb in order to obtain optimal amplification. Gel purified amplification products were treated with T4 DNA polymerase and Klenow DNA poly¬ merase I, and phosphorylated with T4 polynucleotide kinase. Thereafter, cDNA frag¬ ments were ligated with T4 ligase into the Smal site of pGEM4z-blue. In the second round of cloning (Figure 1B), primers were selected from the sequence of the cDNA clones obtained after the first round of cloning. Where possible, primers contained restriction sites suitable for cloning of the amplified cDNA fragments. After RT and PCR amplification (see above), cDNA fragments were either cut with two different restriction enzymes, or blunted and phosphorylated (as described above) atone end, and digested with a suitable restriction enzyme at the other end. If it was not O 95/35380 PCΪ7NL95/00214 17 possible to use PCR introduced restriction sites located in the primers, a site within the amplified cDNA fragment was chosen for cloning. After gel purification, PCR products were ligated into gel purified pGEM4z-blue (Promega) or pGEM5zf(+) (Promega), digested with restriction enzymes creating ends compatible with those of the PCR products. To obtain cDNA clones containing the ultimate 5' and 3' ends of the genome of the C-strain, we used the 3'-5' ligation method (Mandl et al. 1991. Journal of Virology 65:4070-4077). Cytoplasmic RNA was isolated from cells infected with the C-strain as described above, and was further purified through a 5.7 CsCI cushion (Moormann and Hulst. 1988. Virus Res. 11: 281-291). Based on results suggesting that there is no Cap structure at the 5' end of the BVDV genome (Brock et al. 1992. J. Virol. Meth. 38: 39- 46), genomic RNA of the C-strain was ligated without previous treatment with pyro- phosphatase. Eight μg of RNA was ligated in a reaction mix of 50 mM Tris-HCI pH 8.0, 10 mM MgCI2, 10 mM DTT, 20 U rRNasin (Promega), 10 μg/ml BSA (RNase free) and 1 mM ATP, using 10 U of T4 RNA ligase (New England Biolabs). The mixture was incubated for 4 hrs at 37 β C. RNA was extracted with phenol/chloroform, precipitated with ethanol, pelleted, and resuspended in RNase-free water. Portions of 2 μg RNA were reverse transcribed and amplified as described above. Portions of 2μl of each PCR were reamplified using a nested set of primers. For reverse transcription, a (-)sense primer was used hybridizing to the 5' noncoding region. For the two PCR amplification steps we used (+)sense primers hybridizing the 3' noncoding region and (-)sense primers hybridizing to the 5' noncoding region. After extraction with phenol/chloroform and ethanol precipitation, PCR products were digested with A/col (incorporated in the (+)sense primer used in the nested PCR) and Eag\ (nucleotide 81 in the sequence of SEQ ID No. 1), and ligated into the Nco\-Eag sites of pUC21 (Vieira and Messing. 1991. Gene 100: 189-194). All modification and cloning procedures used in Example 1 were carried out essentially as described (Sambrook etal. 1989. Molecular cloning: A Laboratory Manual. Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y.). Restriction enzymes and DNA modifying enzymes were commercially purchased and used as described by the suppliers. Plasmids were transformed and maintained in Escherichia coli strain DH5σ (Hanahan. 1985. in DNA cloning 1: 109-135). Sequencing nf H")NA rlnnes Plasmid DNA used for sequencing was extracted and purified either by alkaline 18 lysis and LiCI precipitation, or by CsCl centrifugation (Sambrook et al. 1989. Molecular Cloning: A Laboratory Manual. Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y.). The T7 polymerase based sequencing kit (Pharmacia) was used for direct double stranded sequencing of plasmid DNA. In addition to the SP6. T7, and universal pUC/M13 forward and reverse primers, oligonucleotide primers were used based on the sequence of CSFV strain Brescia (Moormann ef al. 1990. Virology 177: 184-198). Primers were synthesised with a Cyclone DNA synthesizer (New Brunswick Scientific) or with a 392 DNA/RNA synthesizer (Applied Biosystems). Sequence reactions were analysed on a 6% acrylamide gel containing 8 M urea. Sequence data were analysed with a Compaq 386 computer using Speedreader hardware, and PCgene software (Intelligenetics Inc, Applied Imaging Corp., Geneva, Switzerland) and with an Apple Macintosh computer using the program MacMollytefra. Considering the possibility of sequence errors or differences caused by Taq polymerase or heterogeneity of C-strain RNA, the entire genomic sequence of cDNA clones of the C-strain was determined by sequencing a minimum of two cDNA clones, obtained after independent PCR reactions. If differences were observed between the nucleotide sequences of two clones of a particular region, the consensus nucleotide sequence of that region was determined by sequencing a third cDNA clone obtained after another independent PCR reaction (Figure 1A). Example 2 Generation of infectious transcripts of a full-length DNA copy of the genome of the C-strain. Construction of cDNA clone pPRKflc-113. A full-length DNA copy of the genomic RNA of the C-strain was composed according to the scheme depicted in Figure 3. First, two subclones, one (pPRc64) containing the cDNA sequence of the 5' half of the genome (nucleotides 1-5,560), and the other (pPRc111) containing the cDNA sequence of the 3' half of the genome (nucleotides 5,463-12,311) were constructed. Initially construction of the full-length cDNA clone was tried in pGEM4z-blue. However, this approach failed because of instability of the full-length insert in this vector. To increase stability of the clones, inserts of the 5' and 3' half clones were recloned in a derivative of the low copy number vector pOK12 (Vieira and Messing. 1991. Gene 100: 189-194), resulting in pPRc108 and pPRc123, respectively. To this end pOK12 was modified by deleting most of the restriction sites of the multiple cloning site (MCS), and the T7 promoter sequence. The resulting vector, pPRK, which was used for all further full-length cloning, still contains unique Spel, Notl, Eag\, SamHI, EcoRV, EcoRI, and Xba\ sites in the MCS. In detail, the construction of full-length clone pPRKflc-113 proceeded as follows (Figure 3). Inserts of plasmids pPRc45 and pPRc46 were joined at the Hpa\ site, located at nucleotide position 1249 in the sequence of the C-strain (SEQ ID No. 1), resulting in plasmid pPRc49. The insert of pPRc49 was subsequently joined with the insert of pPRc44 at the Nsh site located at nucleotide position 3241 (SEQ ID No. 1), resulting in pPRc63. The 5' half clone pPRc64 (nucleotide 1 to 5560, SEQ ID No. 1) was constructed by joining the insert of pPRc63 with an amplified (PCR) cDNA fragment of the ultimate 5' region of the genomic RNA of the C-strain as follows. A 5' end (+)sense primer was synthesised containing an EcoRI and a Sa/I site followed by the T7 RNA polymerase promoter sequence and the first 23 nucleotides of the genomic RNA of the C-strain. This primer and a (-) sense primer of the second round of cloning were used to amplify a cDNA fragment that was digested with EcoRI and Xho\ cloned into EcoRI-XΛol (nucleo¬ tide 216 in SEQ ID No. 1) digested pPRc63. Finally, the insert of pPRc64 was recloned into EcoRI-X_>al digested pPRK resulting in pPRc108. The 3' half clone pPRc111 (nucleotide 5,463 to 12,311, SEQ ID No. 1) was constructed by joining 4 second round clones (pPRc67, 53, 58, and 55) and one first round clone (pPRc14). The inserts of pPRc67 and pPRc53 were joined at the Nhe site located at nucleotide position 7,778, resulting in pPRc71. The inserts of pPRc55 and pPRc58 were joined at the Apal site located at nucleotide position 10,387, resulting in pPRc65. The inserts of pPRc65 and pPRc14 were subsequently joined at the Aflll site at nucleotide position 11 ,717, resulting in pPRc73. The insert of pPRc73 was joined with the insert of pPRc71 at the Psti site located at nucleotide position 8,675, resulting in pPRc79. Then, the insert of pPRc79, which contains the complete 3' terminal sequence of the cDNA of the C-strain, was modified such that an Srtl site was introduced which after digestion generated the exact 3' end of the C-strain cDNA sequence (for exact run-off transcription at the 3'end). To achieve this, a 3' end (-) sense primer was syn¬ thesised containing an Srtl and an Xbal site and 18 nucleotides complementary to the 3' terminal sequence of the genomic RNA of the C-strain. This primer and a (+)sense primer of the first round of cloning were used to amplify a cDNA fragment. This fragment was digested with Spel (nucleotide position 11,866, SEQ ID No. 1) and Xba\ and cloned into Spel-Xoal digested pPRc79, resulting in pPRd 11. Full-length cDNA clone pPRKflc-113, finally, was constructed by inserting the C-strain specific Λteol 5532 -Xbal mcs fragment of pPRd 11 into Ncol 5532 -Xύal cs digested pPRc108. Constπirtinn of fiill-lenqth clone pPRKflc-133. Full-length cDNA clone ρPRKflc-113 still had, besides silent nucleotide mutations, 5 point mutations leading to amino acid changes compared to the amino acid sequence determined from the sequence of at least two first round cDNA clones. These 5 point mutations in pPRKflc-113 were changed to the predominant sequence (2 out of 3) through exchange of affected DNA fragments with corresponding DNA fragments containing the predominant sequence. The 5' half cDNA clone pPRc108, with a point mutation at nucleotide position 4,516, was changed by replacing the Scal 3 13 -JVcoi 5532 fragment of pPRc108 with that of pPRd 24. Clone pPRd 24 was made by exchanging the Pvull 4 85 -A nel 5065 fragment of pPRc44 by the corresponding fragment of pPRc32 (compare Figure 1). The new 5' half cDNA clone was designated pPRc129. For cloning purposes a 3' half clone was constructed by deleting the 5' part of the C-strain sequence of pPRKflc-113 from the Sail site in the vector (compare Figure 3) up to the Hpa\ site at nucleotide position 5,509 (SEQ ID No. 1), resulting in pPRc123. In pPRc123 mutations at nucleotide positions 8,526, 9,002, 10,055, and 10,205 had to be changed. The mutation at position 8,526 was restored in two steps. First, the Apal 8,506 -Psfl 8,675 fragment of pPRc53 was exchanged with that of pPRc90, resulting in pPRc125. Second, the Λ/nel 7,378 -Psfl 8,675 fragment of pPRc123 was exchanged with that of pPRc125, resulting in pPRc127. To be able to restore the 3 mutations at positions 9,002, 10,055, and 10,205, we first modified pPRc58 such that the Fspl site in the vector was deleted. To this end the EcoRl mcs -Mfel fragment of pPRc58 was deleted [Nde\ cuts in pGEM4z-blue), resulting in pPRc126. Plasmid pPRc126 was used for restoring the mutations at positions 10,055 and 10,205 by replacing its Sad 9,975 -Apal 10,38? fragment with the corresponding fragment of pPRc96, resulting in pPRc128. The mutation at position 9002 was restored by replacing the Aatll-Fspl 9016 (Aafll cuts in pGEM4z-blue) of pPRc128 with the Aafll- Fspl 9,016 fragment of pPRc90, resulting in ρPRc130. Finally, the Psπ 8 - 675 -Apal 10,387 fragment of pPRc127 was replaced with the corresponding fragment of pPRc130, resul- ting in plasmid pPRc132. All subcloning steps in which single mutations were changed were verified by sequencing. Full-length clone pPRKflc-133 was constructed by inserting the Nco 5 ' 532 - Xi.al mcs fragment of pPRc132 into Λ/col 5,532 -Xbal mcs digested pPRc129. Construction of a hybrid full-length clone pPRKflc-h6. Antigenically different but viable C-strain mutants can be made from pPRKflc- 133, by exchanging part of the E1 gene of this construct with that of CSFV strain Brescia. To this end, the Λ/πel 2 ' 443 -Afllll 2,999 fragment of pPRc129 was replaced with the corresponding fragment of pPEh6 (van Rijn ef al., 1992), resulting in the 5' half hybrid clone pPRc139. Hybrid full-length clone pPRKflc-h6 was constructed by inserting the A col 5,532 -X_»al cs fragment of pPRc132 into pPRc139. This clone now contained the antigenic region of E1 of CSFV-strain Brescia including a unique Bglll site. All modification and cloning procedures used in Example 2 were carried out essentially as described (Sambrook et al. 1989. Molecular cloning: a laboratory manual. Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y.). Restriction enzymes and DNA modifying enzymes were commercially purchased and used as described by the suppliers. Plasmids were transformed and maintained in Escherichia coli strain DH5σ (Hanahan. 1985. in DNA cloning 1 : 109-135). In vitro RNA transcription Plasmid DNA used for in vitro RNA transcription was purified using Qiagen columns (Westburg), according to manufacturers conditions. After linearisation with Xbal or Sril, plasmid DNA was extracted with phenol and chloroform, precipitated with ethanol, vacuum dried and dissolved in an appropriate volume of RNase-free water. One μg of linearised plasmid DNA was used as template for in vitro transcrip¬ tion. RNA was synthesised at 37°C for 1 hr in 100 μl reaction mixtures containing 40 mM Tris-HCI pH 7.5, 6 mM MgCl 2 , 2 mM Spermidine, 10 mM DTT, 100 U rRNasin (Pro¬ mega), 0.5 mM each of ATP, GTP, CTP, UTP and 170 Units of T7 RNA polymerase (Pharmacia). Template DNA was removed by digestion with RNase-free DNasel (Pharmacia) for 15 min. at 37°C, followed by extraction with phenol and chloroform, and ethanol precipitation. The RNA was dissolved in 20 μl RNase-free H 2 O, and quantitated by UV measurement at 260 nm. RNA transfection The RNA transfection mix was composed by gently mixing 50 μl of a lipofectine (Gibco BRL) dilution (10 μg lipofectine in RNase-free H-O) and 50 μl of an RNA solution (1 μg RNA in RNase-free H-O), and incubation of this mix at room temperature for 15 minutes. Subconfluent monolayers of SK6-cells in φ 35 mm, 6-well tissue culture plates (Greiner) were used for RNA transfection. The cells were washed twice with Dulbecco's modified Eagles medium (DMEM). Then 1 ml of DMEM was added to the cells, followed by the RNA transfection mix. After incubation for 16 hrs at 37 β C, the medium was replaced by 2 ml DMEM supplemented with 5% FBS. Incubation was continued for another 3 days at 37 β C. Then cells were immunostained with CSFV specific monoclonal antibodies (Mabs) by the immunoperoxidase monolayer assay (IPMA) as described by Wensvoort et al. (Vet. Microbiol. 1986. 12: 101-108). Characterisation of recombinant C-strain viruses. The supernatants of transfected cells were brought on confluent monolayers of SK6-cells in wells of φ 35 mm, and incubated for 5 days at 37°C. Cells of the trans- fected monolayers were trypsinised and diluted 7.5 times with DMEM and grown for another 7 days at 37°C in 75 cm 2 flasks (Costar). Hereafter, virus stocks were prepared by freeze-thawing the cells twice, clarifying cell suspensions by centrifugation at 5,000 x g for 10 min. at 4°C, and harvesting of the supernatants. Virus was characterised by IPMA, and by restriction analysis of RT-PCR ampii- fied viral fragments. After infection of SK6-cells with viruses FLc-h6 and Fic-133, monolayers were incubated for 4 days at 37°C. Subsequently, monolayers were immunostained using Mabs directed against conserved (Mab b3, domain A) and non- conserved (Mabs b5 and b6, domains B+C) epitopes on E1 of Brescia, and with Mabs specific for the C-strain and directed against E1 (Mab c2) or E2 (Mab c5) (Wensvoort, G. 1989. In Thesis, pp 99-113, Utrecht, The Netherlands). Monolayers of SK6-cells in¬ fected with native Brescia virus or native C-strain virus were controls in this assay. The results are presented in Table 1 , and are as expected. Briefly, Mab b3 recognizes an epitope on E1 conserved among CSFV strains, and therefore recognizes all strains in Table 1. Mabs b5 and b6 do not recognize E1 of the C-strain and thus only react with strains Brescia and Flc-h6. In contrast, Mabc2 does not recognize E1 of strain Brescia, and thus only reacts with strains C and FLc-133. Finally, Mab c5 does not recognize E2 of strain Brescia, and therefore reacts with all viruses in Table 1 except strain Brescia. The genomic RNA of virus FLc-h6 should contain a unique Spill site, which is located in the E1 gene (see above). To check for the presence of this site, cytoplasmic RNA was isolated from SK6-cells infected with recombinant virus FLc-h6, or infected with FLc-133, PCR-amplified as described above, using primers described by Van Rijn er /., 1993. J. Gen. Virol. 74:2053-2060), and digested with Bgh . Indeed, the amplified fragment of 1,091 basepairs of FLc-h6 was cut by Bgl\\, resulting in fragments of 590 and 501 basepairs, whereas the amplified fragment of FLc-133 remained intact. Example 3 Immunisation of pigs with deletion mutants of E1. Construction and expression of deletion mutants of E1 of CSFV strain Brescia. It was previously shown that TMR-less E1 of CSFV strain Brescia, expressed by insect cells, induces a protective immune response in pigs against CSF (Hulst etal., 1993. J. Virol. 67: 5435-5442). Two distinct antigenic units, A and B+C, in the N- terminal half of E1 , which induce neutralizing antibodies against CSFV, were also defined (Wensvoort. 1989. J. Gen. Virol.70: 2865-2876; Van Rijn ef al. 1992. Vet. Microbiol. 33: 221-230; Van Rijn et al. 1993. J. Gen. Virol. 74: 2053-2060). Moreover, neutralizing antibodies directed against domain A and domains B+C act synergistically in neutralizing CSFV (Wensvoort. 1989. J. Gen. Virol.70: 2865-2876). To evaluate the immunogenicity of mutant E1s with a deletion of domains B+C or with a deletion of domain A, relevant constructs in a bacuiovirus vector were made, and expressed mutant proteins were tested in pigs. Baculoviruses expressing mutant E1 s were constructed via overlap recombina¬ tion of wild type AcNPV (Autographa californica nuclear poiyhedrosis virus) DNA and DNA of transfer vector pAcMoδ containing the sequence encoding a particular mutant E1. Transfer vector pAcMoδ was derived from pAcAs3 (Vlak et al., 1990. Virology 179: 312-320) by inserting a T directly 5' of the first base (G) of the unique Bam λ site of the latter vector. In this way an ATG start codon was generated overlapping the first G of the BamHI site. Messenger RNA is transcribed from heterologous sequences inserted into the BamHI site by the AcNPV p10 promoter. The sequences encoding mutant E1s were derived from the E1 insert of pPRb2 (Van Rijn et al., 1992. Vet. Microbiol. 33: 221-230) via PCR amplification. To this end two primers were constructed which contained a BamHI site in their sequence. The 5' end (+ sense) primer has the sequence 5-AGA TTG GAT CCT AAA GTA TTA AGA GGA CAG QT-3' (SEQ ID No. 2). The underlined sequence in this primer is identical to nucleotides 2362-2381 in the sequence of strain Brescia (Moormann ef al., 1990. Virology 177: 184-198), bold letters indicate the BamHI site. The 3* end (-sense) primer contains a stop-codon adjacent to the BamHI site. It has the sequence 5'-TA GTC GGATCC TTA GAA TTC TGC GAA GTA ATC TGA-3 1 (SEQ ID No. 3). The underlined sequence in this primer is complementary to nucleotides 3433-3453 in the sequence of strain Brescia (Moormann etal., 1990. Virology 177: 184-198); bold letters indicate the BamHI site, and letters in italics indicate the stop-codon. Amplified sequences were cloned into the BamHI site of pAcMcθ and checked for a correct orientation in the vector by restriction enzyme analysis. The correct transfer vector was designated pPAb11. Overlap recombination between AcNPV DNA and DNA of pPAbl 1 , and selection and purification of a baculovirus vector expressing E1 was performed as described (Hulst etal., 1993. J. Virol. 67: 5435-5442). Further character- isation of E1 in radioimmunoprecipitation assays, and with E1 specific Mabs was also described by Hulst et al. (J. Virol., 1993. 67: 5435-5442). The resulting recombinant baculovirus expresses wild-type Brescia E1 without a TMR (compare 2 nd bar from top in Figure 3). This TMR-less E1 is secreted from the cells (Hulst etal., 1993. J. Virol. 67: 5435-5442). Deletion of the region encoding domains B+C from the E1 gene of pPAbl 1 was achieved by exchanging the Nhe\-Bgl\\ fragment of this construct with the corresponding fragment of pPEh14 (Van Rijn etal., 1993. J. Gen. Virol. 74: 2053-2060). The resulting transfer vector was designated pPAb16. It contains a deletion in the E1 gene running from codon 693 to 746. Similarly, the region encoding domain A was deleted from pPAb11 by exchanging the Nhe\-Bgl\\ fragment of pPAb11 with the corresponding fragment of pPEh18 (Van Rijn ef al., 1993. J. Gen. Virol. 74: 2053-2060), resulting in transfer vector pPAb12. pPAb12 contains a deletion in the E1 gene running from codon 800 to 864. Recombinant baculoviruses expressing the deleted E1s were constructed, selected, and characterised with regard to their E1 expression products, as described above. Immunisation and challenge exposure of pigs. Groups of four (or two) specific-pathogen-f ree (SPF) , 6 to 8 week old pigs were vaccinated intramuscularly on day 0 with 1 ml of a double water-oil emulsion containing 4 μg (mutant) E1 , and revaccinated on day 28 with 1 ml of a double water-oil emulsion containing 15 μg (mutant) E1 (Table 2). The construction of mutant E1 containing a deletion in domain A, or a deletion in domain B/C, and of wild type E1 is described above and specified by the constructs depicted in Figure 5. For the first vaccination on day 0, supernatant of insect cells infected with the appropriate recombinant baculoviru- ses was used. The amount of E1 in the supernatant was calibrated as described before (Hulst ef al., 1993. J. Virol. 67: 5435-5442). For revaccination on day 28, E1 was immunoaffinity-purified from the supernatant of the infected insect cells (Hulst ef al., 1993. ibid). Pigs of all vaccinated groups, and an unvaccinated control group of two SPF animals, were challenged intranasally with 100 LD 50 of CSFV strain Brescia 456610 (Terpstra and Wensvoort. 1988. Vet. Microbiol. 16: 123-128). This challenge dose leads to acute disease in unprotected pigs characterised by high fever and thrombocytopenia starting at days 3 to 5 and to death at days 7 to 11. Heparinised (EDTA) blood samples were taken on days 40, 42, 45, 47, 49 , 51 , 53, and 56 after vaccination, and analysed for thrombocytes and CSFV virus as described (Hulst et al., 1993. ibid). Serum blood samples were taken on days 0, 21 , 28, 42, and 56 and tested in the CTB-ELISA (Wens¬ voort et al., 1988. Vet. Microbiol. 17: 129-140) and in the neutralizing peroxidase-linked assay (NPLA, Terpstra et al. 1984. Vet. Microbiol. 9: 113-120), to detect (neutralizing) antibodies against CSFV. Test results in the CTB-ELISA are expressed as the percent- age inhibition of a standard signal; <30% inhibition is negative, 30-50% inhibition is doubtful, >50% inhibition is positive. NPLA titers are expressed as the reciprocal of the serum dilution that neutralised 100 TCID 50 of strain Brescia in 50% of the replicate cultures. All animals were observed daily for signs of disease, and body temperatures were measured. Clinical signs of disease were: Fever, anorexia.leukopenia, thrombo¬ cytopenia, and paralysis. Example 4 Development of a CTB-ELISA (CTB-DIF) for CSFV, based on one monoclonal antibody. Description of the diagnostic test. This example describes a CTB-ELISA (CTB-DIF), which is a modification of the existing CTB-ELISA (Wensvoort et al., 1988. Vet. Microbiol. 17: 129-140) for the detection of CSFV specific antibodies. The CTB-DIF is based on the finding that SF21 cells infected with a recombinant baculovirus expressing E1 -TMR, efficiently secrete dimerised E1 into the medium. This dimerised secreted E1 was detected when media of cells infected with the above baculovirus was analysed on western blot after electrophoresis in SDS-PAGE under non-reduced conditions. For E1 specific Mabs, two copies of an epitope are present on dimers of E1 (one on each monomer). Thus, in conjunction with the dimerised antigen a particular E1 specific Mab can be used as capture antibody, coated to the wall of a microtiter plate well, as well as detecting, horseradish peroxidase (HRPO) conjugated antibody. The CTB-DIF is shown to be useful in conjunction with an E1 subunit vaccine which has a deletion in domain A (see Fig. 5 for construct) and is shown to distinguish 26 between CSFV specific antibodies induced in pigs vaccinated with E1 with a deleted domain A, and CSFV specific antibodies induced in pigs infected with low-virulent CSFV strains Henken, Zoelen, Bergen, 331 , and Cedipest (EP-A-351901). Four SPF pigs, numbered 766, 786, 789, and 770, were vaccinated with mutant E1 containing a deletion in domain A, as described in example 3 (see also Table 2), and challenged with virulent CSFV strain Brescia on day 44 after vaccination. Sera taken on days 28, 42, and 56 after vaccination, were tested. Sera against the low-virulent CSFV strains were also prepared in groups of four SPF pigs. Sera from pigs infected with strains Henken, Zoelen, Bergen, and 331 were tested at days 0, 21 , 28 and 42 after infection. Sera from pigs vaccinated with the Cedipest vaccine were tested at days 0, 44, 72, and 170 after vaccination. Three different serological tests were performed with the above sera. Test 1 is the neutralizing peroxidase-linked assay (NPLA) described by Terpstra et al. 1984. (Vet. Microbiol. 9: 113-120), to detect neutralizing antibodies against CSFV. Test 2 is the CTB-ELISA (Wensvoort et al., 1988. Vet. Microbiol. 17: 129-140), to routinely detect antibodies against CSFV. The CTB-DIF uses Mab b3 (also known as CVI-HCV-39.5) (Wensvoort. 1989. J. Gen. Virol. 70:2865-2876.), which recognizes an epitope located in domain A1 of E of CSFV. The wells of an ELISA plate are coated with Mab b3 (dilution 1 :2.000) (capture antibody). After the wells are washed, Mab b3, conjugated to (HRPO), (dilution 1 :4.000) (detecting antibody), is added to the wells. Media of Sf21 cells infected with a baculovirus producing E1 -TMR and containing dimerised E1 to a concentration of 20 μg/ml is diluted 1 :500, and pre-incubated with the test serum (diluted 1 :2.5). The serum- antigen mixture is then added to the conjugate in the wells of the coated ELISA plate. After incubation, the wells are washed again and the chromogen-substrate solution is added. If both the capture and conjugated Mab have bound to the antigen, the HRPO induces a chromogenic reaction, indicating that the test serum is negative for CSFV antibodies. If the epitope on the antigen is blocked by antibodies from the test serum, the HRPO-conjugate will be washed away and the wells will remain clear, indicating the test serum contains antibodies against CSFV, domain A1. The results with the three different serological tests are indicated in Table 3. Sera of pigs vaccinated with E1 with a deletion in domain A, do react in the NPLA and CTB-ELISA, and not in the CTB-DIF, on day 42 after vaccination. After challenge with virulent CSFV strain Brescia sera of the same pigs react positively in all the 3 tests on day 56 after vaccination (day 12 after challenge), indicating that a booster response has taken place after challenge. Starting on day 21 after infection, sera from pigs vaccinated with strains Henken, Zoelen, Bergen, and 331 react positively in the NPLA, the CTB-ELISA, and the CTB-DIF. Starting on day 44 after vaccination, the same holds true for pigs vaccinated with the Cedipest vaccine strain. Thus, the CTB-DIF exactly performs as desired, and is suited to accompany a CSFV marker vaccine with a mutated domain A of E1 , such that antibodies directed against this mutated domain A do not compete with Mab b3 for the epitope of Mab b3. The antigen used in the CTB-DIF is the dimerised TMR-less wild type Brescia E1 depicted in Figure 5. However, dimerised E1 synthesised by the "deletion domains B+C" construct of Figure 5 is also suitable as an antigen in the test. Example 5 Comparison of CTB-ELISA's for CSFV based on E1 and E2. Description of the diagnostic tests This example describes a modification of the CTB-DIF of example 4, and a CTB-ELISA based on E2 of CSFV, and compares the sensitivity of these ELISAs with 3 other CTB-ELlSAs detecting antibodies directed against E1 and the NPLA (Terpstra et al. 1984. Vet. Microbiol. 9: 113-120). The CTB-DIF of example 4, called E1-Bac-DIF in Tables 4 to 8, uses intact TMR-less E1 synthesized in insect cells (SF21 cells) as an antigen. The modification of E1-Bac-D1F, called E1-Bac-dBC-DIF, uses TMR-less E1 synthesized in insect cells (SF 21 cells) with a deleted domain B + C (compare figure 5) as an antigen. As established on western blot, TMR-less E1 with deleted domains B + C is secreted from the cell as a dimer (results not shown). Test E1-bac-dBC-DIF is performed as follows. The wells of an ELISA plate are coated with Mab b3 (dilution 1 :4,000) (capture antibody), 16 h at 37 β C, and washed. Medium containing dimerised antigen E1 -dBC to a concen¬ tration of 20 μg/ml is diluted 1 :50, and pre-incubated with the test serum (dilution 1 -2.5) (0.5 h at 37 β C). The serum-antigen mixture is then added to the coated ELISA plate. After incubation, 1 h at 37 β C, the wells are washed and Mab b3, conjugated to HRPO (dilution 1:1,000) (detection antibody), is added. After incubation, 1 h at 37 β C, the wells are washed again and the chromogen-substrate solution is added. The chromogenic reaction is performed for 10 minutes at room temperature. The interpretation of the chromogenic reaction is the same as explained in example 4. Other CTB-ELlSAs detecting antibodies directed against E1 of CSFV described in Tables 4 to 8 are the E1-CSFV ELISA, using native E1 from CSFV infected cells as antigen (Wensvoort et al., 1988. Vet. Microbiol. 17:129-140); the E1-Bac and E1-Bac- DIF ELISAs, use TMR-less E1 synthesized in insect cells as antigen. The E1 -CSFV and E1-Bac ELISAs use CSFV Mabs b3 and b8 (Wensvoort 1989. J. Gen. Virol. 70: 2,865- 2,876) as capture and detection antibody, respectively, whereas the E1-Bac-DIF ELISA uses only Mab b3 as both capture and detection antibody. The E1-CSFV ELISA is per¬ formed exactly as described by Wensvoort et al., 1988. (Vet. Microbiol. 17:129-140). The E1-Bac, and E1-Bac-DIF ELISAs are performed as described above for the E1- Bac-dBC-DIF with the following modifications. In the E1 -Bac ELISA the antigen used is a 1 :400 dilution of dimerized E1 present in the medium of SF21 cells, infected with the relevant E1 baculovirus construct (compare figure 5) , at a concentration of 20 μg/ml. Mab b8 which is conjugated to HRPO is the detection antibody in tinis ELISA, and is used at a dilution of 1 :1000. The E1-Bac-DIF ELISA uses the same antigen as the E1-Bac ELISA but at a dilution of 1 :200. HRPO conjugated Mab b3 is used as detection antibody in this ELISA at a dilution of 1 :1 ,000. The E2-Bac ELISA uses CSFV E2 antigen synthesized in SF21 cells infected with the Bac CE2 construct (Hulst et al., 1994. Virology 200: 558-565). Because E2 is not secreted from the infected insect cells, the lysate of these cells is used. Like E1 , most of E2 is found as dimerized molecules when lysates of infected cells are analyzed under non-denaturing conditions in SDS-PAGE gels (results not shown). The CTB- ELISA developed on the basis of this E2 antigen performs optimally in conjunction with Mabs C5 and C12 (Wensvoort, G. 1989. In Thesis, pp99-113, Utrecht). However, also E2 in conjunction with only Mab C5 or Mab C12 may be used. In a competition assay Mabs C5 and C12 inhibit each other with regard to binding to E2. This indicates that these Mabs recognize the same, or overlapping epitopes on E2 (results not shown). The E2-Bac ELISA is performed as follows. Mab C12 is diluted 1:1 ,000, and coated to the wells of an ELISA plate (16 h at 37 °C). Hereafter, wells are washed. Lysates of SF21 cells infected with Bac CE2, diluted 1 :1,250, are preincubated with the test serum (1:1) for 0.5 h at 37 β C. The serum-antigen mixture is then added to the wells of the coated plates and incubated for 1 h at 37 β C. Subsequently plates are washed and incubated with Mab C5 conjugated to HRPO (dilution 12,000). After 1h at 37 β C plates are washed again and the chromogen-substrate solution is added. The chromogenic reaction is performed for 10 minutes at room temperature. The interpretation of the chromogenic reaction is the same as explained in example 4. All above described dilutions are performed in NPLA buffer + 4% PS (Terpstra et al., Vet. Microbiol. 9: 113-120). 29 Table 4 shows the results of the analysis of sera of 3 SPF pigs vaccinated with the Cedipest vaccine with the above described CTB-ELISAs and the NPLA. Sera were analyzed at days 0, 16, 23, 30, 37, 44, 50, 72, 113, 141 , and 170 after vaccination. Tables 5 to 8 show the results of the analysis with the above described CTB-ELISAs and the NPLA of sera of groups of 5 SPF pigs infected with the low-virulent CSFV strains 331 , Bergen, Henken, and Zoelen, respectively. Sera were analyzed at days 0, 10, 14, 17, 24, 28, 35, and 42 after infection. Starting at day 16 after vaccination, sera from pigs vaccinated with the Cedipest strain react in each of the 5 CTB-ELISAs as well as in the NPLA. At tinis time point the sensitivity of the E2-Bac ELISA and the E1-Bac- dBC-DIF is as good, if not better, tinan tinat of the otiner 3 CTB-ELISAs. From day 37 after vaccination up till day 170, all sera react consistently (positive) in the 5 CTB- ELISAs as well as the NPLA. Sera of pigs infected with the low-virulent CSFV strains also react in all 5 CTB-ELISAs as well as in the NPLA. With an occasional exception consistency in the reaction of the sera in the 5 CTB-ELISAs and the NPLA is observed from day 21 after infection up till day 42. More sera of animals infected with low virulent strains need to be analyzed to be able to conclude whether there are significant differences between the sensitivity of the 5 CTB-ELISAs early after infection (up till day 17). It can be concluded that the E2-Bac ELISA and the E1 -Bac-CTB-DIF ELISA both perform as desired. Therefore the E2-Bac ELISA is suitable to accompany a CSFV marker vaccine (eg. subunit E1, whether mutated or not, a C-strain marker vaccine modified in the E2 region) which does not induce antibodies tinat compete with the Mabs in tinis ELISA. The E1-Bac-dBC-DIF ELISA is as suitable as the E1-Bac-DIF ELISA (CTB-DIF ELISA of example 4) to accompany a CSFV marker vaccine with a mutated domain A of E1 , such tinat antibodies directed against this mutated domain A do not compete with Mab b3 for the epitope of Mab b3. Description of the figures Figure 1. Schematic representation of the cDNA clones used to determine the nucleotide sequence of the C-strain. Figure 1A indicates the first round cDNA clones (see text). cDNA clones with numbers 32, 90, and 96 were used to change pPRKflc-113 into pPRKflc-133 (see example 2). Clone 14 was the only first round cDNA clone used for construction of pPRKflc-113 (see Figure 3). Figure 1B indicates second round cDNA clones (see text). The numbered second round cDNA clones were used to construct 30 pPRKflc-113 (see SEQ ID No. 1). Positions of the cDNA with respeci to the nucleotide sequence of the genome of the C-strain are indicated by the scale bar (in kilobases) at the bottom of the figure. A schematic representation of the currently identified genes of CSFV, and their organisation in the CSFV genome is indicated at the top of the figure. There is no consensus yet among workers in the field about the nomenclature of pestivirus proteins. The E2 protein as described here is also called gp42 (Tamura et al. 1993. Virology 193: 1-10), gp44/48 (Thiel et al. 1991. J. Virol. 65: 4705-4712) or EO (Rϋmenapf et al. 1993. J. Virol. 67: 3288-3294). The E3 protein is also called gp25 (Tamura et al. 1993. Virology 193: 1-10), gp33 (Thiel et al. 1991. J. Virol. 65: 4705- 4712) or E1 (Rϋmenapf et al. 1993. J. Virol. 67: 3288-3294). The E1 protein of tinis invention is also called gp53 (Tamura et al. 1993. Virology 193: 1-10), gp55 (Thiel et al. 1991. J. Virol. 65: 4705-4712), gp51-54 (Moormann etal. 1990. Virology 177: 184-198) and E2 (Rϋmenapf et al. 1993. J. Virol. 67: 3288-3294). The N-terminal autoprotease N pr0 of CSFV (p20 of BVDV, Wiskerchen et al. 1991. J. Virol. 64: 4508-4514), also called p23, was identified by Thiel et al. 1991. (J. Virol. 65: 4705-4712). Cleavage of the recognition sequence, which is conserved among pestiviruses, of this protease results in the N-terminus of C (Stark et al. 1993. J. Virol. 67: 7088-7095). Figure 2. Alignment of the nucleotide sequences of the 5' (A) and 3' (B) non-coding regions of CSFV strains Brescia, Aifort, and C. Except for the first 12 nucleotides, the 5' non-coding sequence of strain Brescia has been described by Moormann et al., 1990. Virology 177: 184-198. The first 12 nucleotides of the 5' non-coding region of strain Brescia have not been published before. Like the ultimate 5' and 3' sequences of the genome of the C-strain, they were determined with the 3-5' RNA ligation method described in Example 1 of this patent application. Except for the first 9 nucleotides, the 5' non-coding sequence of strain Aifort has been described by Meyers et al., 1989. Virology 171: 555-567. The first 9 nucleotides of the genome of strain Aifort were published by Meyers in a Thesis entitled: "Virus der Klassischen Schweinepest: Genomanalyse und Vergleich mit dem Virus der Bovinen Viralen Diarrhδe". 1990. Tubingen, Germany. The sequences of the 3' non-coding regions of strains Brescia and Aifort have been described by Moormann et al., 1990. Virology 177: 184-198 and Meyers et al., 1989. Virology 171: 555-567, respectively. The ATG start codon and the TGA stopcodon of the large ORF (compare SEQ ID No. 1), are underlined. Figure 3. Construction scheme of full-length cDNA clone pPRKflc-113. Clone numbers have been explained in the legend of Figure 1. Fusion sites of inserts of clones are indicated by vertical lines. The sites corresponding with these lines are indicated at the bottom of tine figure. Underlined clone numbers indicate cDNA clones having pOK12 (Vieira and Messing. 1991. Gene 100: 189-194) derived vector sequences (see Figure 4). The 5' and 3' ends of pPRKflc-113 were tailor made via PCR amplification of cDNA fragments (see text Example 2). The amplified fragments are indicated with PCR. The scale bar at the bottom of the figure, and the schematic representation of the genome organisation of CSFV, have been described in the legend of Figure 1. Figure 4. Schematic representation of the vector sequences and full-length cDNA inserts in clones pPRKflc-113, pPRKflc-133, and pPRKflc-h6. The construction of vector pPRK, a derivative of pOK12 (Vieira and Messing. 1991. Gene 100: 189-194), has been described in Example 2. Kan R , kanamycin resistance gene; ORl, origin of replication; 'i, gene encoding repressor of B-galactosidase gene; PO, promoter/operator region of β- galactosidase gene; lacZ, part of the β-galactosidase gene encoding the « subunit of β-galactosidase. Several restriction sites of the vector, and the sequences directly flanking the full-length inserts in the vector, are indicated. Relevant sites have been described in the text of Example 2. The lollypops and numbers in pPRKflc-113 corres¬ pond to the nucleotides of the five codons which were changed in this construct, re¬ sulting in pPRKflc-133. The latter construct has the sequence as indicated in SEQ ID No. 1. The black box in pPRKflc-h6 indicates the region of E1 of pPRKflc-133 that was exchanged with the corresponding region of strain Brescia. Whether transcripts derived from a particular full-length construct are infectious (+) or not (-) is indicated to the right of the construct. T7, T7 promoter sequence. Inserts of full-length constructs are indicated in relation to a scale bar (in kilobases) representing the nucleotide sequence of the C-strain as indicated in SEQ ID No. 1. Figure 5. Schematic representation of mutant E1 proteins expressed in insect cells with a baculovirus vector. All E1 proteins are encoded by tine nucleotide sequence of strain Brescia (Moormann et al.. 1990. Virology 177: 184-198), and start at their N-terminus with the Lys at codon position 668 in the large ORF of this sequence. The C-terminus of native E1 is the Leu at codon position 1 ,063 in the large ORF, whereas tine C-termini of the three other E1 proteins are located at amino acid position 1 ,031. The dotted boxes in the bars represent the N-terminal signal sequence, running from amino acid residues 668 to 689, the internal hydrophobic sequence, running from amino acid residues 806 to 826, and the C-terminal transmembrane region (TMR), located in the region running from amino acid residues 1 ,032 to 1 ,063, of E1. The deleted amino acid sequences in mutant E1s with a deleted B+C or A domain are indicated by interruptions in the bars representing these proteins. The location of these deletions in relation to the amino acid sequence of E1 can be determined from the scale bar at the bottom of the figure. The scale bar indicates the location of E1 in the amino acid sequence encoded by the large ORF of strain Brescia. TABLE 1. Characterisation of recombinant C-strain viruses Mabs specific for CSFV directed against E1 directed against E2 conserved Brescia "C" "C" virus epitopes specific specific specific epitopes epitopes epitopes "C + + + Brescia + + - - FLc-133 + + + FLc-h6 + + - + TABLE 2. Vaccination of | 3igs with deletion mutants of E1 % inhibition in the CTB-ELISA on days: Nβutralizinq antibody titer on days: Results of CSFV challenqβ: Pig no. Construct 0 21 28 42 56 0 21 28 42 56 Disease Viremia Death used 766 Deletion 0 12 0 78 99 <12.5 <25 <25 3.200 > 3,200 „. _ _- _ _ — — 768 Domain 0 0 0 74 99 <12.5 <25 <25 2,400 > 3,200 _ _ 769 A 0 16 0 99 99 < 12.5 <25 <25 2,400 > 3,200 770 0 14 0 83 99 < 12.5 <25 <25 400 > 3,200 — — 771 Deletion 0 36 25 100 99 <12.5 <25 <25 300 > 3,200 _ _ 772 Domain 0 25 29 100 99 < 12.5 <25 <25 1,200 > 3,200 773 B + C 0 7 0 100 99 <12.5 <25 <25 200 150 en 774 0 49 52 99 99 < 12.5 <25 <25 300 > 3,200 — - 792 Wild type 0 35 31 98 100 <12.5 2,000 1,200 > 3,200 > 3.200 + 794 BresciaEI 0 48 40 98 100 < 12.5 <25 <25 1,600 > 3,200 + 775 None 0 2 0 0 < 12.5 <25 <25 <25 + + + + + + + 795 0 0 0 0 <12.5 <25 <25 <25 + + + + + + + Groups of four (or two) SPF pigs were inoculated intramuscularly on day 0 with 4 μg and on day 28 with 15//g modified E1 protein. On day 42, the pigs were challenged intranasally with 100 LD50 of CSF strain Brescia 456610. All animals were observed daily for signs of disease. Blood samples were taken at days 0, 21, 28, 42, and 56 and tested in the CTB-ELISA and the neutralisation test for the detection of antibodies against CSFV (Terpstra et al., 1984. Vet. Microbiol. 9: 113-120). Clinical signs of disease were : Fever, anorexia, leukopeπia, thrombocytopenia, and paralysis and the presence or © absence of these signs is indicated in that order by a + or - in the table listed under disease. oo en O TABLE 3. Differential diagnostic ELISA test for CSFV Serum DPV or DPI' NPLA" CTB-EUSA C CTB-DIF' 766 2 8 < 25 9 27 768 28 < 25 0 27 769 2 8 < 25 17 0 770 28 <25 1 1 0 766 4 2 3200 61 0 768 4 2 2400 52 0 769 42 2400 99 2Θ 770 42 400 65 0 766 56 > 3200 99 65 768 56 > 3200 100 104 769 56 > 3200 101 105 770 56 >3200 101 104 Henken 0 < 12.5 5 18 Henken 21 50 76 47 Henken 28 75 92 88 Henken 42 300 100 102 Zoelen 0 < 12.5 0 0 Zoelen 17 37.5 85 71 Zoelen 2 1 150 90 80 Zoelen 42 400 100 108 Bergen 0 < 12.5 1 49 Bergen 21 25 96 100 Bergen 28 100 99 95 Bergen 42 300 100 103 331 0 18.75 4 15 331 21 100 92 90 331 28 300 99 99 331 42 300 100 105 Cedipest 0 < 12.5 0 19 Cedipest 44 75 85 93 Cedipest 72 50 89 102 Cedipest 170 150 98 106 * ) DPV: days post vaccination; DPI: days post infection b ) NPLA titers are expressed as the reciprocal of the serum dilution neutralizing 100 TCID 50 of HCV strain Brescia in 50% of the replicate cultures (Terpstra et al. 1984. Vet Microbiol. 9:1 13- 120). e ) Complex trapping blocking-ELISA, CTB-ELISA, or differential CTB-ELISA, CTB-DIF. Test results are expressed as the percentage inhibition of a standard signal; <30% inhibition is negative, 30- 50% inhibition is doubtful, > 50% inhibition is positive. TABLE 4. Comparison of CTB-ELISA's with CSFV strain Cedipest sera strain pig DPV NPLA" CTB-ELISA C or E1 E1-Bac E1-Bac El-Bac E2-Bac DPI CSFV DIF dBC-DIF Cedi- 1 0 <12.5 21 ND ND ND 0 pest 2 0 <12.5 28 8 0 0 0 3 0 <12.5 25 0 0 0 0 1 16 25 60 0 56 62 79 2 16 25 66 51 26 76 79 3 16 19 11 15 0 11 62 1 23 25 54 66 60 68 79 2 23 50 81 57 54 75 74 3 23 25 25 37 32 54 74 1 30 50 76 87 80 81 75 2 30 75 87 ND ND ND ND 3 30 19 60 40 28 57 82 1 37 50 82 90 80 87 85 2 37 50 87 84 61 85 85 3 37 19 49 62 63 79 77 1 44 75 84 94 99 92 88 2 44 75 90 89 74 93 92 3 44 25 66 68 79 93 90 1 50 75 86 93 92 98 89 2 50 150 91 95 96 97 91 3 50 19 74 67 58 95 88 1 72 50 86 92 94 99 81 2 72 200 94 96 93 100 89 3 72 25 53 76 66 99 73 1 113 75 94 99 100 100 92 2 113 200 94 98 100 99 89 3 113 75 93 99 100 96 84 1 141 75 91 100 91 100 89 2 141 150 76 100 95 100 85 3 141 50 87 94 95 100 85 1 190 150 92 97 100 100 94 2 170 150 85 97 100 100 86 3 170 150 74 88 84 98 88 a, b: For explanation see Table 3 footnotes a and b, respectively . c: CTB-ELISA's E1 CSFV, E1-Bac, E1-Bac-D1F, E1-Bac-dBC-DlF and E2-Bac are explained on Example 5. Test results are expressed as the percentage inhibition of a standard signal; <30% inhibition is negative, 30-50% inhibition is doubtful, >50% inhibition is positive. TABLE 5. Comparison of CTB-ELISA's with CSFV strain 331 sera strain pig DPI' NPLA" CTB-ELISA C E1 E1-Bac E1-Bac E1-Bac E2-Bac CSFV DIF dBC-DIF 331 1 0 <12.5 0 0 0 8 0 2 0 <12.5 4 5 0 0 0 3 0 <19 4 13 0 0 0 4 0 <12.5 , 0 14 0 0 0 5 0 <12.5 5 11 11 0 0 1 10 <12.5 0 13 0 14 13 2 10 <12.5 0 11 0 0 13 3 10 <12.5 0 20 16 24 31 4 10 <12.5 0 29 0 9 26 5 10 <12.5 0 24 7 38 18 1 14 <12.5 0 34 9 34 8 2 14 <12.5 2 18 0 0 36 3 14 19 28 67 22 60 60 4 14 19 35 77 37 60 10 5 14 25 32 99 74 90 23 1 17 19 7 84 53 69 0 2 17 <12.5 23 0 0 0 4 3 17 25 63 93 62 87 54 4 17 37 55 82 36 80 0 5 17 37 69 100 84 94 3 1 21 37 57 84 100 50 39 2 21 37 29 52 0 26 3 3 21 100 76 93 100 96 96 4 21 50 76 90 100 91 63 5 21 75 65 ND ND ND 72 1 28 75 73 95 100 96 96 2 28 25 59 89 100 79 58 3 28 300 78 100 100 100 100 4 26 150 74 93 100 80 82 5 28 100 72 98 100 100 91 1 35 75 83 95 100 97 98 2 35 150 80 98 100 100 96 3 35 300 80 99 100 100 100 4 35 200 81 ND ND ND ND 5 35 150 82 98 100 100 90 1 42 150 81 98 100 96 99 2 42 200 80 100 94 100 98 3 42 300 79 94 100 100 100 4 42 200 79 97 100 99 100 5 42 150 80 99 100 100 90 a, b, c: See footnotes of Table 4. TABLE 7. Comparison of CTB-ELISA's with CSFV strain Henken sera strain pig DPI' NPLA" CTB-ELISA e E1 E1-Bac E1-Bac E1-Bac E2-Bac CSFV DIF dBC-DIF Henken 1 0 <12.5 0 0 0 0 0 2 0 <12.5 0 0 0 0 0 3 0 <12.5 1 0 0 0 0 4 0 <12.5 0 2 0 0 0 5 0 <12.5 ' 0 0 0 0 0 1 10 <12.5 0 0 0 0 0 2 10 <12.5 3 6 1 1 25 3 10 <12.5 0 5 12 0 52 4 10 <12.5 0 6 0 4 27 5 10 <12.5 0 12 0 0 8 1 14 <12.5 0 0 6 1 0 2 14 <12.5 0 54 22 29 10 3 14 50 5 57 67 100 20 4 14 <12.5 0 7 0 34 0 5 14 <12.5 0 12 10 9 0 1 17 <12.5 0 0 0 7 0 2 17 12.5 48 73 26 63 53 3 17 75 75 100 94 100 35 4 17 19 7 56 0 60 23 5 17 12.5 0 29 0 16 15 1 21 <12.5 29 0 0 0 0 2 21 50 75 ND ND ND ND 3 21 300 84 ND ND ND ND 4 21 19 36 68 76 78 34 5 21 50 63 83 61 82 32 1 28 <12.5 0 0 0 0 0 2 28 75 80 92 100 100 92 3 28 600 80 99 100 100 82 4 28 50 58 93 100 100 100 5 28 50 79 99 100 100 100 1 35 <12.5 22 13 13 0 1 2 35 200 78 95 100 100 82 3 35 300 78 100 100 100 75 4 35 75 75 98 100 100 100 5 35 150 80 98 100 100 97 1 42 <12.5 17 12 9 0 0 2 42 400 79 ND ND ND ND 3 42 400 79 ND ND ND ND 4 42 400 79 98 100 100 100 5 42 300 82 98 100 100 90 a, b. c. See footnotes of Table 4. TABLE 8. Comparison of CTB-ELISA's with CSFV strain Zoelen sera strain pig DPI* NPLA" CTB-ELISA e E1 E1-Bac E1-Bac E1-Bac E2-Bac CSFV D1F dBC-DIF Zoelen 1 0 <12.5 0 0 0 0 0 2 0 <12.5 0 5 0 0 0 3 0 <12.5 14 4 0 0 0 4 0 19 6 0 0 0 0 5 0 <12.5 16 35 0 19 0 1 10 <12.5 0 16 8 3 31 2 10 <12.5 0 14 0 0 15 3 10 <12.5 0 10 2 8 24 4 10 19 12 8 0 0 22 5 10 <12.5 0 27 27 12 24 1 14 19 19 60 18 75 41 2 14 <12.5 4 36 4 34 10 3 14 <12.5 19 26 14 23 12 4 14 25 26 91 40 92 39 5 14 12.5 0 50 16 41 0 1 17 37 61 96 76 91 64 2 17 19 29 94 62 73 0 3 17 12.5 23 41 16 45 4 4 17 37 65 97 82 95 0 5 17 37 48 90 60 75 0 1 21 150 78 95 100 99 84 2 21 19 68 89 100 94 58 3 21 37 60 73 99 77 0 4 21 75 75 92 100 95 46 c 21 37 54 ND ND ND 57 1 28 200 75 100 100 100 100 2 28 150 76 100 100 100 89 3 28 75 77 97 100 100 56 4 28 300 79 97 100 100 84 5 28 100 67 100 100 100 80 1 35 400 82 100 100 100 100 2 35 150 72 100 100 100 91 3 35 200 81 98 100 100 60 4 35 150 80 94 100 100 77 5 35 100 79 99 100 100 89 1 42 400 82 93 100 100 100 2 42 200 67 99 100 100 78 3 42 400 83 94 100 100 84 4 42 300 82 99 100 100 86 5 42 150 82 94 100 100 32 a, b, c. See footnotes of Table 4.

Claims: Claims 1. Nucleotide sequence corresponding to a classical swine fever virus (CSFV) genome or a part or a mutant thereof, characterised in that it comprises at least a part of the nucleotide sequence of the CSFV C-strain depicted in SEQ ID No. 1, or a complement or RNA equivalent of such nucleotide sequence. 2. Nucleotide sequence corresponding to a classical swine fever virus (CSFV) genome or a part or a mutant thereof, characterised in that it comprises a nucleotide sequence encoding at least the amino acid sequence 268-494 of the amino acid sequence depicted in SEQ ID No. 1 , or a complement or RNA equivalent of such nucleotide sequence. 3. Nucleotide sequence according to claim 1 or 2, containing a mutation in the nucleotide sequence encoding amino acids 1-1063 of the sequence depicted in SEQ ID No. 1. 4. Nucleotide sequence according to claim 3, containing a mutation in the nucleotide sequence encoding amino acids 690-870. 5. Nucleotide sequence according to any one of claims 1 -4, wherein said mutation is a substitution by a corresponding part of the genome of another pestivirus strain. 6. Nucleotide sequence according to any one of claims 1 -4, wherein said mutation is a deletion, an insertion or a mutation resulting in a substitution of one or more amino acids encoded by the nucleotide sequence. 7 Nucleotide sequence according to any one of claims 1 -4, wherein said mutation is an insertion of a heterologous nucleotide sequence altering the translation strategy of the CSFV nucleotide sequence or altering the processing of a polypeptide encoded by the CSFV nucleotide sequence. 8. Nucleotide sequence according to any one of claims 1 -4, wherein said mutation is an insertion of a heterologous nucleotide sequence encoding a polypeptide inducing immunity against another pathogen. 41 9. Nucleotide sequence according to any one of claims 1 -4, wherein said mutation is an insertion of a heterologous nucleotide sequence encoding a marker polypeptide. 10. Polypeptide encoded by the nucleotide sequence according to any one of claims 1-9. 11. Pestivirus polypeptide corresponding to the amino acid sequence 690-1063 of SEQ ID No. 1 or part thereof, characterised in that it contains a mutation in one of the epitopes within amino acid sequences 691-750 or 785-870, said mutation altering said epitope. 12. Nucleotide sequence encoding the polypeptide according to claim 11. 13. Vaccine strain, the genome of which is derived from a full-length DNA copy, and/or an infectious transcript thereof, of the nucleotide sequence according to any one of claims 1 -8. 14. Vaccine containing a polynucleotide having the sequence according to any one of claims 1-9 or 12, a polypeptide according to claim 10 or 11, or a vaccine strain according to claim 13. 15. Diagnostic composition containing at least a nucleotide sequence according to any one of claims 1-9, a polypeptide according to claim 10 or 11 , and/or an antibody raised against a polypeptide according to claim 10 or 11. 16. Use of the nucleotide sequence TTTTCTTTTTTTT as a marker for CSFV C-strain derived sequences. 17. Method of diagnosis distinguishing pestivirus infected animals from vaccinated animals, said vaccinated animals being vaccinated with a pestivirus polypeptide or pesti¬ virus strain containing a mutation in aminoacid sequence 268-494 or 690-1063 of SEQ ID No. 1 , wherein a test sample is contacted with a pestivirus antigen corresponding to aminoacid sequence 268-494 or aminoacid sequence 690-1063 or part thereof and with an antibody directed against an epitope of said pestivirus antigen, which epitope is not functionally present in the mutated polypeptide or pestivirus strain used for vaccination. 18. Method according to claim 17, wherein said pestivirus antigen is a dimerised or multimerised polypeptide and part of said antibody is immobilised and another part of said antibody is labeled. 19. Method according to claim 17 or 18, wherein said pestivirus polypeptide and said antigen correspond to aminoacid sequence 690-1063, and said epitope is located between amino acids 785 and 870. 20. Method of determining the presence of a test substance capable of specifically binding with a binding site of a binding partner, in a sample, by means of competition of said test substance with a measurable amount of a reference substance capable of specifically binding with the same binding site of said binding partner, which comprises: (1) contacting said sample with (a) the binding partner of said reference substance, said binding partner molecule containing at least two identical binding sites for said reference substance, (b) said reference substance bound to a solid carrier, and (c) said reference substance provided with a label; (2) measuring the degree of binding of said label to said carrier. 21. Method according to claim 20, wherein said binding partner is a dimer or multimer of a binding partner to said reference substance. 22. Method of determining the presence of a test substance having at least two identical binding sites per molecule for spec ' rfally binding with a binding partner, in a sample, which comprises: (1) contacting said sample with (a) said binding partner bound to a solid carrier, and (b) said binding partner substance provided with a label; (2) measuring the degree of binding of said label to said carrier. 23. Diagnostic kit containing: (a) a reference antibody bound to a solid carrier, (b) said reference antibody provided with a label; and optionally (c) an antigen to said reference antibody containing at least two identical binding sites for said reference antibody; or a complex between said components (a), (b) and (c); as well as further components for carrying out a competitive immunological assay.

Inventors: MOORMANN ROBERTUS JACOBUS MARI, RIJN PETRUS ANTONIUS VAN, MOORMANN, ROBERTUS, JACOBUS, MARIA, VAN RIJN, PETRUS, ANTONIUS
Applicants: INST VEEHOUDERIJ DIERGEZONDH, MOORMANN ROBERTUS J M, RIJN PETRUS A VAN, INSTITUUT VOOR VEEHOUDERIJ EN DIERGEZONDHEID (ID-D, MOORMANN, ROBERTUS, JACOBUS, MARIA, VAN RIJN, PETRUS, ANTONIUS
Application year: 1995
Main classification: C12N15
IPC codes: A61K39, A61P31, C07H21, C07K14, C12N7, C12N15, C12P21, C12Q1, C12R1, G01N33