Abstract:
The invention relates to the field for Arteriviruses and vaccines directed against infections caused by these viruses. The invention provides an Arteriviruses-like particle comprising at least a first structural protein derived from a first Arterivirus and a second structural protein wherein the second structural protein is at least partly not derived from said first Arterivirus.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
         [0001]    This application is a continuation of PCT International Patent Appln. No. PCT/NL01/00382, filed on May 21, 2001, designating the United States of America, and published, in English, as International Publication No. WO 01/90363 A1 (Nov. 29, 2001), the contents of the entirety of which is incorporated by this reference.  
         TECHNICAL FIELD  
         [0002]    The invention generally relates to veterinary medicine, and particularly to Arteriviruses and vaccines directed against infections caused by these viruses.  
         BACKGROUND  
         [0003]    Porcine reproductive and respiratory syndrome virus (PRRSV) is a positive-strand RNA virus that belongs to the family of arteriviruses together with equine arteritis virus (EAV), lactate dehydrogenase-elevating virus (LDV) and simian hemorrhagic fever virus (SHFV, 14). PRRSV causes reproductive failure in pregnant sows and respiratory problems in piglets (20). It causes huge economic losses in -pig populations world wide. EAV causes reproductive failure and abortions in mares, and leads to persistently infected stallions. Infections with LDV or SHFV are mainly of importance as infections of experimental animals in the laboratory.  
           [0004]    Vaccination against these Arterivirus infections is often cumbersome. Killed vaccines, in general, are not effective enough for most purposes, and although live-attenuated Arterivirus vaccines are available, it has been shown that some of these are not safe and still spread. Furthermore, these vaccines can not be distinguished from wild type field virus.  
           [0005]    The genome of PRRSV, as an example of an Arterivirus genome, is 15.1 kb in length and contains genes encoding the RNA dependent RNA polymerase (ORFIa and ORF1b) and genes encoding structural proteins (ORFs 2 to 7; (14), (11)). Other Arterivirus genomes are somewhat smaller, but share the same genomic build-up, in that all synthesize subgenomic messenger RNA encoding the structural proteins.  
           [0006]    The ORFs 2, 3, and 4 encode glycoproteins designated GP2, GP3, and GP4,respectively. ORF5 encodes the major envelope glycoprotein, designated GP5, ORF6encodes the membrane protein M, and ORF7 encodes the nueleocapsid protein N. An additional structural protein (GP2b) is encoded by a small OFR, ORF2b. The analysis of the genome sequence of PRRSV isolates from Europe and North America, and their reactivity with monoclonal antibodies has indicated that; isolates from these continents are genetically distinct and must have diverged from a common ancestor relatively long ago (15).  
         DISCLOSURE OF THE INVENTION  
         [0007]    The invention provides an Arterivirus-like particle comprising at least a first structural protein derived from a first Arterivirus and a second structural protein wherein said second structural protein is at least partly not derived from said first Arterivirus. In a preferred embodiment, the invention provides a chimeric Arterivirus that is composed of parts originating from at least two different arteriviruses. Said parts are encoded by genes (or parts thereof) originating from said different arteriviruses, and that are preferably at least partly exchanged or substituted for each other. (Note that substitution does nor comprise a mere addition of a second structural protein (such as is disclosed in de Vries et al Virol. 270:84-97) where a stretch of nucleic acids encoding a non-Arteriviris protein fragment is inserted in the full genome of an Arterivirus, thereby extending said genome without an exchange of parts as provided herein. In a preferred embodiment of the invention said chimeric arterivirus as provided exhibits distinct characteristics of the composing arteriviruses.  
           [0008]    Said second part that is not derived from the first Arterivirus can for example comprise a fully but preferably only partially artificial or synthetic sequence, encoding in frame a stretch of amino acids of distinct length allowing for functional dimerisation with said first structural protein as shown herein, thereby allowing heterodimerisation. A heterodimer is a composition of two different interacting peptide chains. The interaction may for example consist of both Van derWaals forces or covalent disulfide bonds, but are not limited to this. It was found that said heterodimerisation, preferably of two glycoproteins, or of a glycoprotein and the matrix or membrane protein, enhances the structural integrity of the resulting chimeric virus particle, thereby allowing a better presentation of immunologically important domains on the particle and making it a better vaccine constituent.  
           [0009]    Besides that said part being involved in heterodimerisation should be a structural protein (non-structural proteins are no part of the particle) it is thus preferred that said part that is not derived from a first Arterivirus at least has a certain measure of homology with said second Arterivirus, e.g. to allow for functional dimerisation. A further condition relevant for heterodimerisation is that in general the nucleoprotein (N) should not be involved, the nucleoprotein of particles as provided in EP 0 839 912 does not contribute to the phenomenon. However, such a particle as provided herein can for example be based on an infectious cDNA clone of an Arterivirus (13; EP 0 839 912), as also described in WO 98/55626 where a recombinant virus is described comprising a combination of non-structural proteins (from genes encoding open reading frames  1   a  and  1   b,  such as the viral poymerase) of a first Arterivirus with the structural proteins (from genes encoding open reading frames 2 to 7) of a second. An infectious clone is an excellent tool for site-directed mutagenesis and is important for projects whose aim is to construct new live vaccines against Arteriviruses. Herein we for example provide a so-called marker vaccine by mutagenesis of the genome, so that, in the case of for example PRRSV, vaccinated pigs (i.e. vaccinated with a vaccine as provided herein) can be distinguished or discriminated from field virus-infected pigs on the basis of differences in serum antibodies, and vice-versa, on the basis of differences in serum antibodies. Such discrimination can in particular well be done when said second structural protein is at least partly not derived from said first Arterivirus, and antibodies directed against said artificial, synthetic or heterologous part can thus be detected, or, alternatively, vaccinated animals are detectable in diagnostic tests by the absence of antibodies directed against the homologous, now absent, structural protein or part thereof. It is preferred that said second structural protein is the nucleocapsid (N) protein since antibodies directed against N are often overabundant, especially in natural infections, and allow for discrimination of vaccinated from non-vaccinated but infected animals. In particular the invention provides a particle wherein said second structural protein is at last partly derived from a second Arterivirus, or at least has a certain measure (e.g. &gt;50%) of homology with said second Arterivirus. A particle as provided herein is also called an inter-Arterivirus or -virus-like chimeric particle, and can of course also comprise stretches on nucleic acid that are not Arterivirus derived, for example encoding non-Arterivirus pathogens or antigens thereof. Particularly useful is such a particle wherein said first and second structural protein comprise a heterodimer, e.g. linked by a disulfide bridge between two cysteines. Most preferred is a particle according to the invention wherein said first or second structural protein comprises a integral membrane protein (M) or part thereof.  
           [0010]    The M protein (18 kDa) is non-glycosylated and is the most conserved structural protein of arteriviruses. For PRRSV, its topology and membrane-associated function is first suggested by Meulenberg et al (14). The N-terminal half of the protein is suggested to have three potential membrane-spanning regions, the N-terminus comprises an ectodomain part, the C-terminus comprises an endodomain part. A stretch of 16 amino acids is exposed at the virion surface. For LDV, the M protein has been identified as class III membrane protein (5). The M protein is assumed to play an important role in virus assembly and budding. In the ER, it forms disulfide-linked heterodimers (3, 4, 10) with the major glycoprotein GP5 (25-42 kDa), encoded by ORF5. In addition, disulfide-linked M protein homodimers can also be formed, however, they are in general thought not to be incorporated into virions (3).  
           [0011]    In another embodiment, the invention provides a particle wherein said first or second structural protein comprises a glycoprotein (GP) or part thereof, such as GP2, GP2b, GP3, GP4 or, preferably, GP5. GP5 is the major glycoprotein of arteriviruses and is suggested to be a class I glycoprotein (5). It contains a signal peptide and after processing the protein consists of a short N-terminal ectodomain, a segment that crosses the membrane three times, and a C-terminal endodomain. In addition, the ectodomain contains N-glycosylation sites (12). Recently, the major neutralisation epitope of LDV was mapped to the putative ectodomain (30 aa) of the ORF5 glycoprotein (8). For EAV, the ectodomain of GP5, which is somewhat larger than with LDV, also contains a neutralization epitope.  
           [0012]    Since the cysteine residue in the short N-terminal ectodomain of the M protein is naturally involved in the formation of an intermolecular disulfide bridge with a cysteine residue in the ectodomain of the glycoprotein encoded by ORF5, thereby providing a heterodimer, the invention provides for a close to native chimeric particle wherein said first structural protein comprises GP5 or part thereof and said second structural protein comprises a membrane protein (M) or part thereof. Preferably, the invention provides a PRRSV-like particle for the generation of vaccines against PRRS, thus the invention provides a particle wherein said first Arterivirus comprises porcine reproductive and respiratory syndrome virus (PRRSV). In the detailed description a particle according to the invention is provided wherein said second Arterivirus comprises lactate dehydrogenase-elevating virus (LDV), however, it can also be turned around, in that the GP5, or part thereof, preferred is the above identified ectodomain, is LDV derived and the M, or part thereof, preferred is the above identified ectodomain, is PRRSV derived, as long as the heterodimer ca be established by for example disulfide bridge formation. Of course, other Arteriviruses can be used as first and/or second Arterivirus as explained herein, whereby said second Arterivirus may be of the same genus but of another strain or serotype of said first Arterivirus. For PRRSV, it has also been shown that a disulfide bond between the M protein and the GP5 protein is formed (10). This cysteine residue of the M protein is highly conserved between all arteriviruses. For LDV, it has been shown that virions, after treatment with 5-10 mM DTT to disrupt disulfide bonds, lost their infectivity (4). For EAV, the same results were observed (3).  
           [0013]    The invention also provides nucleic acid encoding at least a first structural protein derived from a first Arterivirus and a second structural protein wherein said second structural protein is at least partly not derived from said first Arterivirus wherein said first and second structural protein allow for incorporation in an Arterivirus-like particle. Such nucleic acid or transcripts thereof as provided herein allow the production in a host cell, such as a BHK-21 cell, or a macrophage, of a particle according to the invention. Particles according to the invention provided with a nucleic acid according to the invention are herewith also provided, see for example tables 2 and 3 wherein infection of macrophages with chimeric particles as provided herein is shown.  
           [0014]    The invention also provides a vaccine comprising such a particle, nucleic acid, or host cell according to the invention. For the purpose of vaccine development, the invention provides a method for attenuation of the virus and one of the accomplishments is reduced viral infectivity. In particular a method is provided obtaining an attenuated Arterivirus (a vaccine) comprising a first Arterivirus with a structural protein that is at least partly not derived from said first Arterivirus, preferably, although not necessarily, as shown herein above, a method wherein said structural protein is at least partly derived from a second Arterivirus, such as wherein said structural protein comprises a heterodimer with another structural protein. When one of said structural proteins comprises a membrane protein (M) or part thereof such dimerisation is particularly useful, at least in those case wherein another one of said structural proteins comprises a glycoprotein, such as GP5, or part thereof.  
           [0015]    This is done by reducing the stability of the interaction between the M protein and the GP5 protein, thereby reducing infectivity. In particular, we have determined that the first cysteine residue (in PRRSV at position 8, see FIG. 1) of the ectodomain of the M protein of Arterivirus is essential for the viral life cycle, since no infectious virus was produced from mutants lacking this cysteine. This residue is essential for the disulfide bond between the M protein and GP5 and heterodimerisation between these two structural proteins is essential either for proper virus assembly or for virus entry for example by the interaction of the virus with a receptor. Therefore, we show that the cysteine residue at position 8 (or a similar position relative to the position shown herein for PRRSV) of the ectodomain of the M protein is essential to maintain full infectivity. For this purpose, we substituted this cysteine residue by a serine residue and secondly, we deleted this residue, both by using the infectious cDNA clone of PRRSV (13). RNA transcripts of these so-called mutant full-length cDNA constructs were tested on their ability to express the viral proteins after transfection into BHK-21 cells, and on their ability to generate infectious virus. In addition, several other mutations of the ectodomain of the M protein were introduced in the infectious cDNA clone of LV, including the exchange of the ectodomain of LV by that of LDV, a related arterivirus (FIG. 1) As can be seen from for example tables 2 and 3, wild-type or parent particles can be differentiated from chimeric particles by comparing distinct patterns of reactivity with antibodies; likewise animals infected with field virus can be differentiated from animals vaccinated with such chimeric particles can be differentiated with diagnostic tests utilising such distinct patterns of reactivity. Suitable antigen for such a diagnostic test would be an antigenic part of the wild-type virus that is not or only partly present in the vaccine. For example, for the vaccines described in the detailed description, an 16-18 amino acid stretch, or antigenic parts thereof of the ectodomain of M can be used, in combination with antibodies having similar specificity as Mabs 126.3 or 126.4. The invention thus also provides a method for controlling or eradicating an Arterivirus infection in a population of animals comprising testing samples (e.g. bloodsamples) of animals vaccinated with a vaccine according to the invention for the presence or absence of antibodies differentiating such animals from animals infected with a wild-type Arterivirus, e.g. by applying routine cull and control measures.  
           [0016]    The invention is further explained in the detailed description herein without limiting the invention. 
       
    
    
     LEGENDS  
       [0017]    [0017]FIG. 1. Comparison of the amino acid sequences of the M proteins of the arteriviruses EAV, LDV-P, PRRSV-Ter Huurne, PRRSV-VR2332, and SHFV.  
         [0018]    [0018]FIG. 2 GP5-Mprotein costructs  
         [0019]    [0019]FIG. 3 Growth curves of deletion mutants  
     
    
     DETAILED DESCRIPTION  
       [0020]    Materials &amp; Methods  
         [0021]    Cells and Viruses.  
         [0022]    BHK-21 cells were grown in BHK-21 medium (Gibco BRL), completed with 5% FBS, 10% tryptose phosphate broth (Gibco BRL), 20 mM Hepes pH 7.4 (Gibco BRL) and 200 mM glutamine, 10 U/ml penicillin and 10 μg/ml streptomycin. Porcine alveolar lung macrophages (PAMs) were maintained in MCA-RPMI-1640 medium, containing 10% FBS, 100 μg/ml kanamycin, 50 U/ml penicillin and 50 μg/ml streptomycin. Virus stocks were produced by serial passage of recombinant LV viruses secreted in the culture supernatant of tranfected BHK-21 cells on PAMs. Virus was harvested when PAMs displayed cytopathic effect (cpe) usually 48 hours after infection. Virus titers (expressed as 50% tissue culture infective doses [TCID50] per ml) were determined on PAMs using end point dilution (19).  
         [0023]    Construction of Mutations in the Ectodomain of the M Protein of PRRSV.  
         [0024]    PCR-mutagenesis was used to mutate amino acids of the ectodomain of the M protein in the PacI-mutant of the genome-length cDNA clone of LV (pABV437) (13). The primers used are listed in Table 1. The PCR fragments were digested with StuI and HpaI and ligated into these sites of pABV651, a subclone of pABV437 containing the region encoding the structural proteins of PRRSV. Standard cloning procedures were performed essentially as described by (17). Transformation conditions were used as described by Sambrook et al. (17). Sequence analysis was performed to confirm the inserted mutations. Clones containing the correct inserts were digested with AatII and HpaI and ligated into the appropriate sites of pABV437.  
         [0025]    First, the cysteine residue at position 8 in the ectodomain of the M protein was substituted by a serine residue by PCR-mutagenesis with primers LV217 and LV93, resulting in subclone pABV702 and full-length clone pABV705. In addition, this cysteine residue was deleted from the ectodomain of M by PCR-directed mutagenesis with primers LV227 and LV93. This resulted in subclone pABV703 and full-length cDNA clone pABV706. Second, the complete ectodomain of the M protein (amino acids 1 to 16) was replaced by the ectodomain of LDV using primers LV218 and LV93. The designed clones were named pABV704 (subclone) and pABV707 (full-length cDNA clone). Third, several other amino acid substitutions and deletions in the ectodomain of ORF6 were created, using LV 219 to LV226 as forward primers and LV93 as reversed primer, resulting in subclones pABV732 till pABV736 and full-length cDNA clones pABV737 till pAB743.  
         [0026]    Sequence Analysis.  
         [0027]    The regions of the subclones originating from the PCR products were analyzed by nucleotide sequencing. Sequences were determined with the PRISM Ready Dye Deoxy Terminator cycle sequencing kit and the ABI PRISM 310 Genetic Analyzer (Perkin Elmer).  
         [0028]    In vitro Transcription and Transfection of BHK-21 Cells.  
         [0029]    The constructed full-length genomic cDNA clones and derivatives thereof were linearized with PvuI and in vitro transcribed using T7 RNA polymerase (9). BHK-21 cells were transfected with the resulting RNA by electroporation as described before (13). The medium was harvested 24 h after transfection, and BHK-21 cells were washed with PBS, dried and stored at −20° C. until the IPMA was performed.  
         [0030]    Infection of PAMs  
         [0031]    To rescue infectious virus, the culture supernatant of BHK-21 cells was harvested 24 hours after transfection and used to inoculate PAMs. After 1 hour the inoculum was removed and fresh culture medium was added. Approximately 24 hours after infection the culture supernatant was harvested and PAMs were washed with PBS, dried and stored at −20° C. until the immuno peroxidase monolayer assay was performed.  
         [0032]    Immuno Peroxidase Monolayer Assay (IPMA).  
         [0033]    Immunostaining of BHK-21 cells and PAMs was performed by the methods described by Wensvoort et al. (19), in order to determine transient expression and infectious virus, respectively. A panel of monoclonal antibodies (MAbs) (126.3, 126.4, 122.9, 126.12, 126.6 (18)) directed to unknown antigenic sites of the M protein were used to study the expression of the M protein and the presence of antigenic sites thereon. MAbs 122.14, 122.1, and 122.17 (18) (directed against GP3, GP4, and the N protein respectively), were used to detect the expression of other PRRSV proteins.  
         [0034]    Analysis of the Production of Non-Infectious Virus of the Recombinant RNA Transcripts.  
         [0035]    From the culture supernatant of transfected BHK-21 cells, viral RNA was isolated to determine whether the full-length cDNA recombinants were packaged into viruses or virus-like particles, which were non-infectious. A volume of 500 μl proteinase K buffer (100 mM Tris-HCl [pH 7.2], 25 mM EDTA, 300 mM NaCl, 2% [wt/vol] sodium dodecyl sulfate) and 0.2 mg Proteinase K was added to 500 μl supernatant. After incubation for 30 minutes at 37° C., the RNA was extracted with phenol-chloroform and precipitated with ethanol. The RNA was reverse transcribed with primer LV76. Then, PCR was performed with primers LV35 and LV7 to amplify fragments comprising the region in which the mutations were introduced. Sequence analysis was performed to determine whether the mutations introduced in the cDNA clone were also present in the isolated viral RNA  
         [0036]    Radio Immuno Precipitation (RIP).  
         [0037]    The expression of GP5 and the M protein were analyzed by metabolic labeling of transfected BHK-21 cells, followed by immunoprecipitation using peptide sera or MAbs directed against GP5 or the M protein, respectively, essentially as described by Meulenberg et al [Meulenberg, 1996 #10]. In addition, the co-precipitation of both proteins was investigated by lyzing the cells under non-reducing conditions. The samples were analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) using a 14% denaturing acrylamide gel.  
         [0038]    Results  
         [0039]    In order to test whether the disulfide bond between the ectodomains of GP5 and the M protein of PRRSV is essential for viral infection, we substituted amino acid residue 8 of the M protein (10), by a serine residue. In addition, this cysteine residue was deleted from the ectodomain of the M protein. The cysteine substitution and deletion mutations were subsequently introduced in the infectious clone pABV437 of the Lelystad virus isolate of PRRSV, resulting in plasmids pABV705 (C→S) and pABV706 (C→deletion). The RNA transcripts of these full-length cDNA clones were transfected into BHK-21 cells and the expression of the viral proteins was examined. In both cases, the cells stained positive in IPMA with the GP3, GP4, and N specific MAbs (table 2). In addition, MAb 126.12 directed against the M protein resulted in positive staining. Two other MAbs directed against the M protein, 126.3 and 126.4, stained BHK-21 cells transfected with transcripts from pABV705, but not those transfected with transcripts from pABV706 (table 2). This indicated that these MAbs were directed against the ectodomain of the M protein, or at least directed against (a) peptide fragment(s) comprising some of the 18 amino acids comprising said domain. The supernatants of the transfected cells were used to infect PAMs to rescue infectious virus. However, no staining of any of the MAbs could be detected on PAMs 24 hours after transfection (table 3). In addition, no cytopathogenic effect (cpe) could be induced. In conclusion, full-length cDNA transcripts of PRRSV lacking the cysteine residue at position 8 of the M protein, either by substitution or deletion, were able to replicate and express the viral proteins in BHK-21 cells, but unable to produce infectious virus.  
         [0040]    Second, the ectodomain of the M protein was exchanged by the ectodomain of LDV, resulting in the full-length cDNA clone pABV707. BHK-21 cells transfected with transcripts from this PRRS recombinant could be stained with MAbs against GP3, GP4, and the N protein, MAb 126.12 directed against the M protein, but not with the MAbs 126.3 and 126.4 (table 2). This confirmed the above described results, that these MAbs reacted with the ectodomain of the M protein. To test the production of infectious chimeric virus, PAMs were infected with the supernatant of the transfected BHK-21 cells. In IPMA, PAMs could be stained with all but MAbs 126.3 and 126.4 (table 3). In conclusion, the ectodomain of the M protein can be replaced by the ectodomain of LDV, resulting in the production of a chimeric virus, which still infects porcine alveolar macrophages. Studies on coronaviruses suggest that all domains of the M protein are important for coronavirus assembly (1). The amino-terminal domain of the M protein, which is exposed on the outside of the virus, plays a role in virus assembly. In addition, the carboxy-terminal domain, located inside the virus envelope, is also important for virus assembly by interacting with the nucleocapsid. This domain is also crucial for the assembly of the viral envelope. However, they showed that the amino-terminal domain of the M protein was not involved in the interaction between the M protein and the S protein (2). This indicates that the association between the proteins takes place at the level of the membrane, possibly also involving part of the M proteins carboxy-terminal domain. For another coronavirus, TGEV, MAbs against the carboxyterminus of the M protein have been described to neutralise virus infectivity (16), indicating that the C-terminal domain of the M protein is exposed on the outside of the virus particle. This topology of the M protein probably coexists with the structure currently described for the M protein of coronaviruses, which consists of an exposed amino terminus and an intravirion carboxy-terminal domain. In our recent study, we are mutating other amino acids in the ectodomain of the M protein. We show that distinct deletions or mutations result in a weakening of the disulfide bond between the M protein and GP5. These constructs show in general normal replication and expression of the structural proteins, resulting in an immune response comparable to wild type. However, fewer virus particles will be produced. Also it results in the production of virus particles, which are impaired in the infection of the macrophage. In both cases, it results in a virus, which is considered to be a safe vaccine for protection of pigs against for example PRRSV. Our results also showed that mutations in the ectodomain of the M protein can result in the generation of a marker vaccine, since replacement with the LDV ectodomain, as well as deletion of some of its amino acids, such as the deletion of the cysteine residue resulted in the loss of the binding of two MAbs. So mutation of the virus at this epitope results in the generation of a marker vaccine. In this study we also showed that PRRSV transcripts containing the ectodomain of the M protein of LDV, generated an infectious, chimeric virus, also useful as a (marker) vaccine.  
         [0041]    Materials and Methods  
         [0042]    Further Construction of Mutations in the Ectodomain of the M Protein of PRRSV.  
         [0043]    First, the cysteine residues at position 50, 111, and 117 in GP5 were substituted by serine residues. For subsitution of amino acid 50, PCR-mutagenesis was performed with primers LV32 and LV303 for the first fragment and with primers LV302 and LV182 for the second fragment. For subsitution of amino acid 111, PCR-mutagenesis was performed with primers LV32 and LV311 for the first fragment and with primers LV310 and LV182 for the second fragment. For subsitution of amino acid 117, PCR-mutagenesis was performed with primers LV32 and LV313 for the first fragment and with primers LV312 and LV182 for the second fragment. The fragments were fused and-amplified using the most 5′ and 3′ primers. The resulting fragments were cloned using BstXI and NheI in pABV651, and from the resulting clones, the AatII-HpaI fragment was cloned into the appropriate sites of pABV437. This resulted in pABV858, 861, and 859 for the cysteine residues 50, 111, and 117, respectively.  
         [0044]    Second, the region from amino acid 9 till 16 was deleted from the ectodomain of the M protein. PCR was performed using primers LV32 and LV306. The fragment was digested with BstXI-NheI and cloned into these sites of pABV651. From this clone, the AatII-HpaI fragment was cloned into the corresponding sites of pABV437, resulting in pABV855.  
         [0045]    Third, the region encoding the ectodomain of the M protein of LV was substituted by that of other arteriviruses. For introduction of the VR2332 ectodomain, two sequential PCRs were performed with primers LV32 and PRRSV57 and with primers LV32 and PRRSV58. Cloning of the PCR fragment with BstXI and NheI into pABV651 and from this resulting clone with AatII and HpaI into pABV437 resulted in the full-length clone pABV857. For introduction of the ectodomain of M of EAV, we performed sequential PCRs with primers LV32 and PRRSV59 and with primers LV32 and PRRSV60. The resulting fragment was cloned with BstXI and NheI into pABV651, and from the resulting clone with AatII and HpaI into pABV437, resulting in pABV856.  
         [0046]    Forth, the overlap between LV ORF5 and 6 was removed by performing PCR with primers LV32 and LV358. The resulting PCR fragment was cloned into the BstXI and StuI sites of pABV651. From the resulting clone, the AatII-HpaI fragment was introduced into pABV437, resulting in pABV871. In this clone, the ectodomains of other arteriviruses were introduced. For introduction of the ectodomain of the M protein of VR2332, two PCR fragments were generated, one using LV32 and LV357 and one using LV356 and 118U250. For introduction of the ectodomain of the M protein of EAV, PCR fragments were generated with primers LV32 and LV361 and with primers LV360 and 118U250. The PCR fragments were fused and amplified with primers LV32 and 118U250. Both PCR fragments were digested with BstXI and HpaI, and ligated into these sites of pABV651. The resulting clones were digested with AatII and HpaI, and the fragments were ligated into these sites of pABV437. This resulted in clone pABV872 for the ectodomain of the M protein of VR2332 and in pABV873 for the ectodomain of the M protein of EAV.  
         [0047]    The primers used are listed in Table 4.  
         [0048]    Results  
         [0049]    Full-Length cDNA Clones Containing Deletions in the Ectodomain of the M Protein.  
         [0050]    RNA transcripts of pABV738 (aa 15&amp;16 deletion), pABV739 (aa 15 deletion), pABV740 (aa 15 Q to E), pABV741 (aa 9 deletion), and pABV742 (aa 5 deletion) were transfected into BHK-21 cells and tested for the expression of the structural proteins 24 hours after transfection in IPMA. For all mutants, expression of GP3, GP4, and N was detected. Two MAbs against the M protein (126.3 and 126.4) did not stain the transfected cells, in contrast to another Mab against the M protein (126.12), which stained the cells positive. The culture supernatant of the transfected cells was used to infect PAMs. Staining 24 hours after infection showed expression of the N protein for all mutants. This indicates that all mutants produced viable virus.  
         [0051]    In addition, a mutant in which the coding region for amino acid 9 till 16 from the M protein was deleted was constructed, resulting in pABV855. Transfection of its RNA transcripts into BHK-21 cells showed expression of all the structural proteins of LV. MAbs 126.3 and 126.4, however, did not stain the transfected cells. After inoculation of PAMs with the culture supernatant of the transfected cells, no expression of the structural proteins was detected. In conclusion, no viable virus was produced.  
         [0052]    Mutations of Cysteine Residues in the GP5 Protein.  
         [0053]    Cysteine residues 50, 111, and 117 of GP5 were changed into serine residues, resulting in the full-length cDNA clones pABV858, pABV 861, and in pABV 859, respectively. Transfection of RNA transcripts in BHK-21 cells showed for all mutants expression of the structural proteins, as detected in IPMA 24 hours after transfection. PAMs were inoculated with the culture supernatant of the transfected cells and stained in IPMA 24 hours after infection. Cells stained positive when PAMs were inoculated with culture supernatant of BHK-21 cells transfected with RNA transcripts of pABV861 and 859, in contrast to PAMs inoculated with culture supernatant of BHK-21 cells transfected with RNA transcripts of pABV858, for which no positive staining was observed. In conclusion, the cysteine residue at position 50 of GP5 is essential for the production of viable virus, and residues 111 and 117 are not.  
         [0054]    Introduction of the Ectodomain of the M Protein of Other Arteriviruses.  
         [0055]    Since introduction of the ectodomain of the M protein of LDV resulted in the production of viable virus, we now inserted the ectodomain of the M protein of VR2332 and that of EAV into the infectious cDNA clone of LV, resulting in pABV857 and pABV856, respectively (FIG. 2A). However, both introductions of these sequences introduced mutations in the C-terminus of the GP5 protein, since the coding sequences for GP5 and M, ORF5 and 6, respectively, overlap. Transfection of their RNA transcripts showed for both mutants expression of the structural proteins. However, staining of PAMs infected with the culture supernatant of transfected BHK-21 cells was negative. In conclusion, no viable virus is produced from these chimeric arteriviruses.  
         [0056]    Removal of the Overlap Between ORF5 and 6 and Introduction of Chimeric Sequences.  
         [0057]    Since introduction of the ectodomain of M of VR2332 and EAV also introduced mutations in the region encoding the C-terminus of GP5, we removed the overlap between ORFs5 and 6 from the infectious cDNA clone of LV. In this way, we wanted to create a region in ORF6 at which arterivirus sequences could be introduced without disturbing the coding sequence of ORF5. First, the overlap between ORF5 and 6 was removed in the infectious cDNA clone, resulting in pABV871 (FIG. 2B). Transfection of its RNA transcripts into BHK-21 cells revealed that the structural proteins were expressed, indicating that both replication and transcription were not disturbed. Infection of PAMs with the culture supernatant of transfected BHK-21 cells showed that infectious virus was produced since structural protein expression was detected by IPMA and cpe was observed. Second, the ectodomain of the M protein of VR2332 and that of EAV were introducted in this construct, resulting in pABV872 and pABV873 (FIG. 2B). Their RNA transcripts were transfected into BHK-21 cells. All, but 126.3 and 126.4, MAbs stained the transfected cells positive. PAMs infected with the culture supernatant of transfected BHK-21 cells showed expression of all structural proteins in IPMA. These results indicate that the ectodomain of the M protein of other arteriviruses, providing that the C-terminus of the GP5 was left intact, could be functionally exchanged by that of the ectodomain of the LV M protein.  
         [0058]    Genetic Stability of Chimeric Arteriviruses.  
         [0059]    In order to investigate whether the viruses generated from pABV707, 738, 741, and 742, 871, 872 and pABV873 were stably maintained in vitro, they were serially passaged on PAMs. The viral RNA was isolated from the culture supernatant after 5 passages, and studied by genetic analysis. The viral RNA was reversely transcribed and the region flanking the introduced deletions was amplified by PCR. Sequence analysis of the fragment showed that for each mutant the introduced mutations were still present and that no additional mutations had been introduced in the flanking regions during in vitro passages. These results indicate that the deletions were maintained stably during in vitro passaging on PAMs.  
         [0060]    Growth characteristics were determined for vABV707, vABV741, and vABV742 in a growth curve and compared with those of wild type vABV437. PAMs were infected with passage 5 at a multiplicity of infection of 0.05, and the culture medium was harvested at various time intervals. Virus titers were determined by end point dilution on macrophages. In all cases, we observed that the growth rates were similar, however, the amount of viable virus inclined faster after reaching its highest titer. This result might indicate that the generated viruses are thermolabile which may be a further useful property for vaccine purposes.  
                                                       TABLE 1                           Primers used in PCR-mutagenesis and sequencing                    Primer           Primer       Orientation/           (nt.)   Sequence of primer a     location   Purpose                    39U247   5′ GCCAAGGCAACACAATCTGC 3′   −   14368   Sequencing       LV7   5′ AATGTAAAGGAAGAGCTCAGAA 3′   −   14222   PCR on RT-PCR viral       LV8   5′ ACTTTATCATTGGATCGAGCA 3′   −   14673   RNA       LV17   5′ CCCTTGACGAGCTCTTCGGC 3′   +   14045   Sequencing       LV35   5′ GATTACGCGTGCTGCTAAAAATTGC 3′   +   13867   Sequencing       LV76   5′ TCTAGGAATTCTAGACGATCG(T) 40  3′   −   15088   PCR on       LV93   5′ ACTTTATCATTGGATCCAGCA-3′   −   14581   RT-CR on RT-PCR       LV198   5′ TTTTCCGGGCATACTTGAC 3′   +   14086   Viral RNA       LV217   5′ AATGGG AGGCCT AGACGATTTTTCCAACGA 3′   +   14086   Reverse primer cloning       LV218   5′   +   14086   Sequencing       LV219   AATGGG AGGCCT AGAA TTTTGTGATCAAACTTCCTGGTATCA     +   14086   M protein a.a. 8 C to S       LV220   GCTCGTGCTAGCG 3′   +   14086   M protein a.a. 1-16 LV       LV221   5′   +   14086   to LDV       LV222   AATGGG AGGCCT AGACGATTTTTTGCAACGATCCTATCGCCGC   +   14086   M protein a.a. 16 K       LV225   ACAACTCGTGCTA 3′   +   14086   deletion       LV226   5′   +   14086   M protein a.a. 15/16       LV227   AATGGG AGGCCT AGACGATTTTTGCAACGATCCTATCGCCGC   +   14086   QK deletion           ACTCGTGCTA 3′           M protein a.a. 9 N           5′ AATGGG AGGCCT AGACGATTTTTGCGATCCTATCGCC 3′           deletion           5′ AATGGG AGGCCT AGATTTTTTGCAAC 3′           M protein a.a. 9 N           5′           deletion           AATGGG AGGCCT AGACGATTTTTGCAACGATCCTATCGCCGC           M protein a.a. 15 Q           AAAGCTCGTG 3′           deletion           5′           M protein a.a. 15 Q to           AATGGG AGGCCT AGACGATTTTTGCAACGATCCTATCGCCGC           E           AGAAAAGCTC 3′           M protein aa. 8 C→           5′ AATGGG AGGCCT AGACGATTTTAACGATCCT           deletion                          
 
         [0061]    [0061]                                             TABLE 2                           Staining of BHK-21 transfected with transcripts from pABV437, 705, 706, and 707.            pABV   GP3 (122.14)   GP4 (122.1)   M (126.3)   M (126.4)   M (126.12)   N (122.17)               437   +   +   +   +   +   +       705   +   +   +   +   +   +       706   +   +   −   −   +   +       707   +   +   −   −   +   +                                    
         [0062]    [0062]                                 TABLE 3                           Staining of PAMs infected with supernatant of transfected       BHK-21 cells with pABV437, 705, 706, and 707.                pABV   M (126.3)   N (122.17)                       437   +   +           705   −   −           706   −   −           707   +   +                                                
         [0063]    [0063]                                                           TABLE 4                           Sequences of the primers used to introduce deletions by PCR, and primers used to sequence the introduced mutations.                    Orien-   Purpose           Primer   Sequence of the primer a     tation   (pABV)   Location                    39U247   5′ GCCAAGGCAACACAATCTGC 3′   −   sequencing   14368       118U250   5′ CAGCCAGGGGAAAATGTGGC 3′   −   sequencing/PCR   14745       LV17   5′ CCCTTGACGAGCTCTTCGGC 3′   +   sequencing/PCR   14045       LV32   5′ GATTGGATCCATTCTCTTGGCAATATG 3′   +   sequencing/PCR   13466       LV75   5′ TCTAGGAATTCTAGACGATCG 3′   −   PCR   15088       LV76   5′ TCTAGGAATTCTAGACGATCG(T) 40  3′   −   RT-PCR   15088       LV93   5′ ACTTTATCATTGGATCCAGCA 3′   −   PCR   14581       LV182   5′ GGATTGAAAATGCAATTAATTAATCATGTAT 3′   −   PCR   14257       LV198   5′ TTTTCCCGGGCATACTTGAC 3′   +   Sequencing   14086       PRRSV57   5′ TGCTATCATGACAGAAGTCATCTAAGGACGACCCCATTGCTCAG 3′   −   857   14132       PRRSV58   5′ GCTAAAG GCTAGC ACGAGCTTTTGTGGAGCCGTGCTATCATGAC 3′   −   857   14132       PRRSV59   5′ ATCCCGTCACCACAAAATGAATCTATGGCTCCCATTGGTCAG 3′   −   856   14132       PRRSV60   5′ GCTAAAG GCTAGC ACGAGCTCACCTAAAATCCCGTCACCA 3′   −   856   14132       LV302   5′ CTTGACGATATCAGAGCTGAATGGG 3′   +   858   13630       LV303   5′ CCCATTCAGCTCTGATATCGTCAAG 3′   −   858   13630       LV306   5′ GCTAAG GCTAGC ACGAGGCAAAAATCGTC 3′   −   855   14132       LV310   5′ GTACGTACTCTCAAGCGTC 3′   +   861   13814       LV311   5′ GACGCTTGAGAGTACGTAC 3′   −   861   13814       LV312   5′ CTACGGCGCTTCAGCTTTCG 3′   +   859   13832       LV313   5′ CGAAAGCTGAAGCGCGGTAG 3′   −   859   13832       LV356   5′ GCAGTGGGAGGCCTGATGGGGTCGTCCTTAG 3′   +   872   14083       LV357   5′ CTAAGGACGACCCCATCAGGCCTCCCACTGC 3′   −   872   14083       LV358   5′ CGTCTAGGCCTCCCATCAAGCTTCCCACTGC 3′   −   871   14083       LV360   5′ GCAGTGGGAGGCCTGATGGGAGCCATAGATTC 3′   +   873   14083       LV361   5′ GAATCTATGGCTCCCATCAGGCCTCCCACTGC 3′   −   873   14083                            
       Vaccination Examples  
       [0064]    Intranasal Inoculation of Wild-Type PRRSV (EU en US-Type) After Vaccination of 8-Week Old Pigs with Specified PRRSV-Mutants; Virus Kinetics and Antibody Response  
         [0065]    Introduction  
         [0066]    The Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) causes abortion and poor litter quality in third trimester pregnant sows. Moreover, it may cause respiratory disease in young pigs. Infection of late term pregnant sows (80-95 days) with PRRSV can cause profound reproductive failure, especially due to a high level of mortality among the off-spring of these sows at birth and during the first week after birth. PRRSV is a ubiquitous pathogen. Two distinct antigenic types can be distinguished, i.e. the European and the American type. Clinical effects after a PRRSV infection depend on the type of strain involved. Vaccination of pigs with a PRRS vaccine influences the way a PRRSV-challenge works out on an animal and a farm level. The level and duration of viraemia, and shedding of the field-virus is reduced by this vaccination.  
         [0067]    For the development of a second generation PRRS vaccine, new candidates are to be tested. Therefore, 8-week old pigs were vaccinated with a number of specified PRRSV-mutants (recombinant viruses), after which a PRRSV-challenge was given. Kinetics of this virus exposure is scored in terms of level and duration of viremia and booster responses, both in a homologous and heterologous set-up.  
         [0068]    Aims of the Study  
         [0069]    The determination of the immunological efficacy and safety of defined PRRSV-mutants used as a vaccine in a vaccination-(homologous and heterologous) challenge model. Along with this, mutant immunogenicity was tested.  
         [0070]    Study Design  
         [0071]    Four PRRSV mutants were tested which all full-filled the following criteria:  
         [0072]    genetic stability after 5 passages in-vitro (cell cultures)  
         [0073]    genetic stability after 3 weeks of exposure to animals  
         [0074]    immunogenicity (as determined by IDEXX elisa)  
         [0075]    The following mutants were tested:  
         [0076]    vABV707: LDV-PRRS chimeric virus (ectodomain of M exchange)  
         [0077]    vABV741: aa9 deletion of the M-protein of PRRSV  
         [0078]    vABV746: 18 nucleotide deletions at the C-terminal part of ORF7  
         [0079]    vABV688: mutations at position 88-95 of ORF2  
         [0080]    As a positive control, the following virus was used:  
         [0081]    vABV437: wild-type recombinant of Lelystad virus  
         [0082]    Each Mutant was Tested in Two Groups Each Consisting of 5 SPF-Pigs of 8 Weeks Old.  
         [0083]    All groups were completely segregated without any contact with each other. Two naive sentinel pigs (so, one per each mutant-group) were united with these vaccinated pigs 24 hours after vaccination and removed and killed 28 days thereafter.  
         [0084]    In the 2 groups (per mutant) each consisting of 5 vaccinates, two animals were challenged with wild-type virus (i.e. Lelystad virus (LV-tH) as a representative of an European strain of PRRSV or SDSU#73 as a representative of an American (US) strain of PRRSV), at day 28 post-vaccination.  
         [0085]    The other three vaccinates were separated from these challenged animals for 24 hours and re-united thereafter. 28 days after challenge, all pigs were removed and destroyed.  
         [0086]    vABV437 served as a positive control. A challenge control was included for 14 days starting at the moment of challenge in order to control challenge efficacy with LV-tH and SDSU#73, Animals were treated as described for the other animals during the challenge phase.  
         [0087]    The allocation of the pigs is outlined in Table 1.  
                                     TABLE 1                           Allocation of pigs to designated groups. Each mutant group       consisted of 5 vaccinated pigs and 1 sentinel (*so each       PRRSV-mutant had two groups). Groups 11 and 12 served       as challenge control groups (**) consisting of 5 animals       per group; only two of these pigs were intranasally       exposed to LV-tH or SDSU#73. All mutant groups       were housed in isolation recombinant facilities,       whereas the wild-type groups were housed in standard       isolation facilities.                        N           Group   Challenge   Vaccination   animals   Stables               1 + 2   LV-tH/   707   12*    2 (geb. 46)           SDSU#73       3 + 4   LV-tH/   741   12*    2 (HRW-           SDSU#73           223.030/40)       5 + 6   LV-tH/   746   12*    2 (HRW-           SDSU#73           223.050/60)       7 + 8   LV-tH/   688   12*    2 (HRW-           SDSU#73           223.070/80)        9 + 10   LV-tH/   437   12*    2 (EHW)           SDSU#73       11 + 12   LV-tH/   —   10**   2 (EHW)           SDSU#73                  
 
         [0088]    The vaccines were administered intramuscularly according to a SOP (2 ml deep intramuscularly in the neck halfway between the shoulder and the right ear; min titer 10 5  TCID 50 /ml). All inoculae were titrated before and after usage and were stored on melting ice at all times.  
         [0089]    Experimental Animals  
         [0090]    70 SPF pigs of 8-weeks old, tested free of PRRSV.  
         [0091]    Execution of the Study (Table 2)  
                         TABLE 2                           Course of the study valid for each of the mutant groups.            Day   Action               −5 till 0   Acclimatisation of animals       −2   Serum sampling for IDEXX-ELISA       Daily   General clinical status       0   Vaccination of 5 animals per group (2 ml intramuscular)       1   Sentinels       3 × per week   Serum sampling for virus isolation (3 × per week)       sampling   and INDEXX-ELISA           (1 × week)       Dag 28   Removal of sentinels and challenge of 2 vaccinates           with LV-tH or US virus (in stable 1 and           2 per mutant group, respectively)       3 × per week   Serum sampling for virus isolation (3 × per week)       sampling   and INDEXX-ELISA           (1 × week)       56   Finalization; destruction of pigs                  
 
         [0092]    Results  
         [0093]    No adverse reactions were noted after exposure of the mutant virus or wild-type viruses to the pigs in each of the groups.  
         [0094]    Tables 3 and 4 show the results of the PRRS virus isolation from serum and calculated viraemia scores. Incidences of viraemia at defined sampling points were determined by virus isolation on porcine alveolar macrophages using routine and published techniques;  
         [0095]    Virus positivity at a serum sample dilution of 1:10 was designated (+), and (++) means virus positivity at a serum sample dilution of 1:100. These results were used to calculate a group total “viraemia score” as (type 1) the percentage of the virus-exposed animals in each group (each virus positive animal at each time-point=1 point, so a max score of 100% (=12/12) can be obtained, and (type 2) as the percentage of maximal viraemia of the exposed animals. In the latter case, a max score of 100% (=24/24) can be obtained based upon the fact that max viraemia is scored as 2 points (1:100 dilution of the samples) for each individual animal. All mutant virus groups showed a reduced type 1 and type 2 viremia score as compared to vABV437. vABV707 vaccinated pigs showed a reduced type 1 and type 2 viraemia score prior to challenge as compared to the score of the pigs in all other groups. At the moment of challenge no animals were shown to be viraemic any more. All sentinels became viraemic and sero-converted, meaning that the viruses shedded from the exposed pigs to the sentinels. It is shown that primary exposure of the mutant viruses to the pigs renders an effective immunological response as determined by a near complete prevention of viraemia after homologous wild-type challenge and a firm reduction of viraemia after heteroogous challenge as compared to challenge controls. Vaccinated sentinels were effectively protected.  
         [0096]    No differences could be documented in serological responses after vaccination and challenge between each of the groups studied.  
         [0097]    Challenge controls all show viraemia during the course of the 14-day study, where the viraemia is most predominant in the intranasally exposed pigs.  
                                                                           TABLE 3                           Type 1 viraemia score. A group total “viraemia score” was       calculated as the percentage of the virus-exposed animals in       each group. Each virus positive animal at each       time-point = 1 point, so a max score of       100% (={fraction (12/12)}) can be obtained.                                    Wild-       dpi   vABV707   vABV741   vABV746   vABV688   vABV437   type                    0    0, 0    0, 0    0, 0    0, 0    0, 0           2    0, 0    8, 3   25, 0    16, 7    75, 0       4   16, 7    83, 3   91, 7    75, 0   100, 0       7   91, 7    83, 3   91, 7   100, 0   100, 0       9   91, 7    91, 7   91, 7    83, 3   100, 0       11   50, 0   100, 0   66, 7   100, 0   100, 0       14   66, 7    83, 3   83, 3    83, 3   100, 0       16   33, 3    58, 3   58, 3    66, 7    75, 0       18   41, 7    16, 7   25, 0    33, 3    50, 0       21   25, 0    8, 3   33, 3    16, 7    91, 7       23   25, 0    16, 7   25, 0    0, 0    41, 7       25    8, 3.    0, 0    0, 0    16, 7    16, 7       28    0, 0    0,0    0, 0    0, 0    0, 0   0       30   10, 0    0,0   30, 0    30, 0    10, 0   0       32   20, 0    0, 0   10, 0    20, 0    40, 0   40       35   20, 0    10, 0   10, 0    20, 0    20, 0   60       37    0, 0    30, 0    0, 0    20, 0    20, 0   90       39   10, 0    0, 0    0, 0    0, 0    30, 0   90       42    0, 0    0, 0    0, 0    0, 0    10, 0   100       44    0, 0    0, 0    0, 0    0, 0    0, 0       46    0, 0    0, 0    0, 0    0, 0    0, 0       49    0, 0    0, 0    0, 0    0, 0    0, 0       51    0, 0    0, 0    0, 0    0, 0    0, 0       53    0, 0    0, 0    0, 0    0, 0    0, 0       56    0, 0    0, 0    0, 0    0, 0    0, 0                  
 
         [0098]    [0098]                                                                           TABLE 4                           Type 2 viraemia score, calculated as the percentage of maximal       viraemia of the exposed animals. A max score of 100% (={fraction (24/24)})       can be obtained based upon the fact that max viraemia is scored       as 2 points (1:100 dilution of the samples) for each       individual animal at each time point.                                    Wild-       dpi   vABV707   vABV741   vABV746   vABV688   vABV437   type                    0    0, 0    0, 0    0, 0    0, 0    0, 0           2    0, 0    4, 2   12, 5    8, 3   37, 5       4    8, 3   50, 0   54, 2   50, 0   70, 8       7   45, 8   58, 3   62, 5   66, 7   83, 3       9   54, 2   50, 0   45, 8   50, 0   58, 3       11   25, 0   70, 8   37, 5   54, 2   95, 8       14   33, 3   62, 5   41, 7   45, 8   70, 8       16   16, 7   45, 8   33, 3   33, 3   41, 7       18   20, 8    8, 3   12, 5   16, 7   37, 5       21   12, 5    8, 3   16, 7    8, 3   50, 0       23   12, 5    8, 3    8, 3    0, 0   41, 7       25    4, 2    0, 0    0, 0    8, 3    8, 3       28    0, 0    0, 0    0, 0    0, 0    0, 0   0       30    5, 0    0, 0   15, 0   15, 0    5, 0   0       32   10, 0    0, 0    5, 0   10, 0   20, 0   40       35   10, 0    5, 0    5, 0   10, 0   10, 0   60       37    0, 0   15, 0    0, 0   10, 0   10, 0   90       39    5, 0    0, 0    0, 0    0, 0   15, 0   90       42    0, 0    0, 0    0, 0    0, 0    5, 0   100       44    0, 0    0, 0    0, 0    0, 0    0, 0       46    0, 0    0, 0    0, 0    0, 0    0, 0       49    0, 0    0, 0    0, 0    0, 0    0, 0       51    0, 0    0, 0    0, 0    0, 0    0, 0       53    0, 0    0, 0    0, 0    0, 0    0, 0       56    0, 0    0, 0    0, 0    0, 0    0, 0                    
         [0099]    Conclusion  
         [0100]    The studied recombinant mutant PRRS viruses show a reduced virulence as determined by a reduction of viraemia (length and height) as compared to wild-type (vABv437). All mutants instigate an effective immune response for the protection of pigs against a wild-type field PRRSV. The homologous protection seems to be somewhat more effective than the heterologous one. vABV707 seems to be the most suitable vaccine from among tested viruses.  
         [0101]    The humoral response is measurable by a commercial ELISA (IDEXX) in all cases. No adverse reactions are elicited.  
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