| PATENT NUMBER | This data is not available for free |
| PATENT GRANT DATE | 31.12.02 |
| PATENT TITLE |
Infectious cDNA clone of North American porcine reproductive and respiratory syndrome (PRRS) virus and uses thereof |
| PATENT ABSTRACT |
The invention provides isolated polynucleotide molecules, including plasmids; viral vectors; and transfected host cells that comprise a DNA sequence encoding an infectious RNA sequence encoding a North American PRRS virus; and also North American PRRS viruses encoded thereby. The invention further provides isolated infectious RNA molecules encoding a North American PRRS virus. The invention also provides isolated polynucleotide molecules, infectious RNA molecules, viral vectors, and transfected host cells encoding genetically-modified North American PRRS viruses; and genetically-modified North American PRRS viruses encoded thereby. The invention also provides vaccines comprising such plasmids, RNA molecules, viral vectors, and North American PRRS viruses, and methods of using these vaccines in swine and in other animals. Also provided are isolated polynucleotide molecules, viral vectors, and transfected host cells that comprise a nucleotide sequence encoding a peptide of a North American PRRS virus. These viral vectors and transfected host cell lines are useful in providing peptides to compensate for mutated peptide coding sequences of DNA sequences encoding genetically-modified North American PRRS viruses so that functional virions can be generated. |
| PATENT INVENTORS | This data is not available for free |
| PATENT ASSIGNEE | This data is not available for free |
| PATENT FILE DATE | December 22, 1999 |
| PATENT REFERENCES CITED |
Meulenberg, J.J.M., et al. "An infectious cDNA clone of Porcine Reproductive and Respiratory Syndrome Virus" Chapter 24, pp. 199-206 Coronaviruses and Arteriviruses (Advances in Experimental Medicine and Biology, vol. 440), 1998. Meulenberg J. J. M., et al., Infectious transcripts from cloned genome-length cDNA of porcine reproductive and respiratory syndrome virus. J. Virol. 72(1):pp. 380-387, 1998. Meulenberg J. J. M., et al., Posttranslational processing and identification of a neutralization domain of the gp(4) protein encoded by ORF4 of Lelystad virus. J. Virol. 71(8): pp. 6061-6067, 1997. Nelsen, C. J., et al., Porcine reproductive and respiratory syndrome virus comparison: Divergent evolution on two continents. J. Virol. 73 (1): pp. 270-280, 1999. Murtaugh, M. P., et al. Comparison of the Structural Protein Coding Sequences of the VR-2332 and Lelystad virus strain of the PRRS virus, Journal Arch. Virol. 140(8), pp. 1451-1460 (1995). Kapur, V., et al., Genetic Variation in Porcine reproductive and Respiratory Syndrome Virus Isolates in the Midwestern United States, Journal of Gen. Virol. 77, pp. 1271-1276 (1996). Allende, R. et al., North American and European Porcine Reproductive and Respiratory Syndrome Viruses Differ in Non-Structural Protein Coding Regions, Journal Gen. Virol. 80 (Pt 2), 307-315 (1999). Kwang, Jimmy, et al., Cloning, Expression, and Sequence Analysis of the OrF4 Gene of the Porcine Reproductive and Respiratory Syndrome Virus MN-1b, J Vet Diagn. Invest 6, pp. 293-296 (1994). Mardassi, H., et al., Molecular Analysis of the ORF's 3 to 7 of Porcine Reproductive and Respiratory Syndrome Virus, Quebec Reference Strain, Arch Virol 140, pp. 1405-1418 (1995). Meng, Xiang-Jin, Molecular Cloning and Nucleotide Sequencing of the 3'-Terminal Genomic RNA of the Porcine Reproductive and Respiratory Syndrome Virus, Journal of General Virology 75, pp. 1795-1801 (1994). Allende, R., et al.., P1-19, Fifth International Symposium on Positive Strand RNA Viruses (St. Petersburg, Florida, May 1998). Faaberg, K. S., et al., P2-59, Fifth International Symposium on Positive Strand RNA Viruses (St. Petersburg, Florida, May 1998). Nelson, C., et al. P5-1, American Society of Virology, 17.sup.th Annual Meeting (Vancouver, Canada, Jul. 1998). Spatz, S. J., et al., Conference of Researcher Working on Animal Diseases (Chicago, Nov. 1998). J.M., Janneke, et al., Lelystad Virus, the Causative Agent of Porcine Epidemic Abortion and Respiratory Syndrome (PEARS), Is Related to LDV and EAV, Virology, (1993), 192, pp. 62-72. Andreyev VG., Wesley RD., Mengeling WL., Vorwald AC., Lager KM., Genetic variation and phylogenetic relationships of 22 porcine reproductive and respiratory syndrome virus (PRRSV) fields starins based on sequence analysis of open reading frame 5, Arch Virol 142:993-1001, 1997. Ausubel et al., 1989, Current Protocols In Molecular Biology, Greene Publishing Associates & Wiley Interscience, NY. Collins JE., Benfield DA., Christianson WT., Harris L., Hennings JC., Shaw DP., Goyal SM., McCullough S., Morrison RB., Joo HS., Gorcyca D., Chladek D. Isolation of swine infertility and respiratory syndrome virus (isolate ATCC VR-2332) in North America and experimental reproduction of the disease in gnotobiotic pigs, J Vet Diagn Invest 4:117-126, 1992. A. Domb et al., 1992, Polymers for Advanced Technologies, 3:279-292. Kim HS., Kwang J., Yoon IJ., Joo HS., Frey, ML. Enhanced replication of porcine reproductive and respiratory syndrome (PRRS) virus in a homogeneous subpopulation of MA-104 cell line, Arch Virol 133:477-483, 1993. Meng XJ., Paul PS., Halbur PG., Lum MA., Phylogenetic analyses of the putative M (ORF 6) and N (ORF 7) genes of porcine reproductive and respiratory syndrome virus (PRRSV): implication for the existence of two genotypes of PRRSV in the U.S.A. and Europe, Arch Virol 140:745-755, 1995. Morozov I., Meng XJ., Paul PS., Sequence analysis of open reading frames (ORFs) 2 to 4 of a U.S. isolate of porcine reproductive and respiratory syndrome virus, Arch Virol 140:1313-1319, 1995. Rossow KD., Porcine Reproductive and Respiratory Syndrome, Vet Pathol 35:1-20 (1998). Sambrook et al., 1989, Molecular Cloning: A Laboratory Manual, 2.sup.nd ed., Cold Springs Harbor Laboratory Press, Cold Springs Harbor, NY. Snider EJ., Meulenberg, JJM. The molecular biology of arteriviruses, Journal of General Virology, 79:961-979, 1998. Suarez P., Zardoya R., Martin MJ., Prieto C., Dopazo J., Solana A., Castro JM. Phylogenetic relationships of European strains of porcine reproducitve and respiratory syndrome virus (PRRSV) inferred from DNA sequences of putative ORF-5 and ORF-7 genes, Virus Research 42:159-165, 1996. Terpstra C., Wensvoort G., Pol JMA. Experimental reproduction of porcine epidemic abortion and respiratory syndrome (mystery swine disease) by infection with Lelystad virus: Koch's postulates fulfilled, The Veterinary Quarterly, vol. 13, No. 3, pp. 131-136, Jul. 1991. Wensvoort G., Terpstra C., Pol JMA., ter Laak EA., Bloemraad M., de Kluyver EP., Kragten C., van Buiten L., den Besten A., Wagenaar F., Broekhuijsen JM., Moonen PLJM., Zetstra T., de Boer EA., Tibben HJ., de Jong MF., van't Veld P., Groenland GJR., van Gennep JA., Voets MTh., Verheijden JHM., Braamskamp J. Mystery swine disease in the Netherlands: the isolation of Lelystad virus, The Veterinary Quarterly, vol. 13, No. 3, pp. 121-130, Jul. 1991. Zimmerman JJ., Yoon, KJ., Willis RW., Swenson SL., General overview of PRRSV: A perspective from the United States, Veterinary Microbiology 55:187-196, 1997. Inis et al. (eds.), 1995, PCR Strategies, Academic Press, Inc., San Diego (Table of Contents Only). Maniatis et al., 1989, Molecular Cloning, A Laboratory Manual, Cold Springs Harbor Laboratory Press, Cold Springs Harbor, NY (Table of Contents Only). Chasin M., Domb A. "Polyanhydrides as Drug Delivery Systems" in Biodegradable Polymers as Drug Delivery Systems, vol. 45, pp. 43-70, 1990. Den Boon, J.A. et al., 1991, J. Virol. 65(6):2910-2920. Erlich (ed), 1992, PCR Technology, Oxford University Press, New York (Table of Contents only). Kreutz, L.C. "Cellular membrance factors are the major determinants of porcine reproductive and respiratory syndrome virus tropism," Virus Research 53:121-128, 1998. Van Dinten, L.D. et al., 1997, Proc. Natl. Acad. Sci. USA, 94(3):991-996. |
| PATENT PARENT CASE TEXT | This data is not available for free |
| PATENT CLAIMS |
What is claimed is: 1. An isolated polynucleotide molecule comprising a DNA sequence encoding an infectious RNA molecule encoding a North American PRRS virus, wherein said DNA sequence is SEQ ID NO:1 or a sequence that hybridizes to the complement of SEQ ID NO:1 under highly stringent condition, wherein said highly stringent conditions comprise hybridization to filter-bound DNA in 0.5 M NaHPO.sub.4, 7% SDS, 1 mM EDTA at 65.degree. C., and washing in 0.1.times.SSC/0.1% SDS at 68.degree. C. 2. A transfected host cell comprising a DNA sequence encoding an infectious RNA molecule encoding a North American PRRS virus, wherein said DNA sequence is SEQ ID NO:1 or a sequence that hybridizes to the complement of SEQ ID NO:1 under highly stringent condition, wherein said highly stringent conditions comprise hybridization to filter-bound DNA in 0.5 M NaHPO.sub.4, 7% SDS, 1 mM EDTA at 65.degree. C., and washing in 0.1.times.SSC/0.1% SDS at 68.degree. C., which transfected host cell is capable of expressing the encoded North American PRRS virus. 3. An isolated polynucleotide molecule comprising a DNA sequence encoding an infectious RNA molecule encoding a North American PRRS virus, wherein said DNA sequence is SEQ ID NO: 1. 4. An isolated polynucleotide molecule in the form of a plasmid, wherein said isolated polynucleotide molecule comprises a DNA sequence encoding an infectious RNA molecule encoding a North American PRRS virus, wherein said DNA sequence is SEQ ID NO: 1. 5. An isolated infectious RNA molecule encoded by an isolated polynucleotide molecule comprising SEQ ID NO: 1, which infectious RNA molecule encodes a North American PRRS virus. 6. A recombinant North American PRRS virus encoded by an isolated polynucleotide molecule comprising a DNA sequence encoding an infectious RNA molecule encoding a North American PRRS virus, wherein said DNA sequence is SEQ ID NO: 1. -------------------------------------------------------------------------------- |
| PATENT DESCRIPTION |
FIELD OF THE INVENTION The present invention is in the field of animal health and is directed to infectious cDNA clones of positive polarity RNA viruses and the construction of vaccines, in particular, swine vaccines, using such cDNA clones. BACKGROUND OF THE INVENTION Porcine reproductive and respiratory syndrome (PRRS) is a new disease of swine, first described in 1987 in North America and in 1990 in Europe. The disease has since spread to Asia and affects most of the major swine producing countries of the world. Primary symptoms are reproductive problems in sows and gilts, including late term abortions, stillbirths and mummies, and litters of small weak pigs which are born viremic and often fail to survive. In addition, the syndrome manifests itself as a respiratory disease in young pigs which spreads horizontally and causes fever, lethargy, labored breathing, loss of appetite, slow growth, and occasionally death, often in association with other respiratory pathogens. The disease furthermore can be transmitted to sows and gilts via the semen of infected boars, either naturally or by artificial insemination. For these, and other reasons, PRRS has proven to be a difficult disease to control and therefore one of the most economically damaging diseases to the swine industry. The causative agent of PRRS is the PRRS virus, which exists as two genetically and serologically distinct types (Murtaugh, M. P. et al., 1995, Arch-Virol. 140, 1451-1460; Suarez, P. et al., 1996, Virus Research 42:159-165). The two types are believed to have first entered swine populations independently, one in North America and the other in Europe, in the1980's, from unknown biological reservoirs, possibly of rodent or avian origin. The European type, represented by the prototype "Lelystad Virus", was isolated and sequenced in the Netherlands in 1991 (Terpstra, C. et al., 1991, Vet. Quart. 13:131-136; Wensvoort, G. et al., 1991, Vet. Quart. 13:121-130; Wensvoort, G. et al., WO 92/213751992 (PCT/NL92/00096), 1992; Meulenberg, J. J. M. et al., 1993, Virol. 192:62-72). Both the North American PRRS virus and the European PRRS virus are classified within the family Arteriviridae, which also includes equine arteritis virus, lactate dehydrogenase-elevating virus, and simian haemorrhagic fever virus. The arteriviruses are in turn placed within the order Nidovirales, which also includes the coronaviruses and toroviruses. The nidoviruses are enveloped viruses having genomes consisting of a single strand of positive polarity RNA. The genomic RNA of a positive-stranded RNA virus fulfills the dual role in both storage and expression of genetic information. No DNA is involved in replication or transcription in nidoviruses. The reproduction of nidoviral genomic RNA is thus a combined process of genome replication and mRNA transcription. Moreover, some proteins are translated directly from the genomic RNA of nidoviruses. The molecular biology of the family Arteriviridae has recently been reviewed by Snijder and Meulenberg (Snijder, E. J. and Meulenberg, J. J. M., 1998, Journal of General Virology 79:961-979). Currently available commercial vaccines against PRRS are either conventional modified live virus (cell culture, attenuated) or conventional killed (inactivated cell culture preparations of virulent virus). Several of these vaccines have been criticized based on safety and/or efficacy concerns. The development of a second generation of PRRS vaccines, based upon specific additions, deletions, and other modifications to the PRRS genome, is therefore highly desirable. However, since the PRRS viruses do not include any DNA intermediates during their replication, such vaccines have thus far awaited the construction of full-length cDNA clones of PRRS viruses for manipulation by molecular biology techniques at the DNA level. Very recently, a full-length infectious cDNA clone of the European PRRS virus has been reported (Meulenberg, J. J. M. et al., 1998, supra; Meulenberg, J. J. M. et al., 1988, J. Virol. 72, 380-387.). The preceding publications, as well as all other references discussed below in this application, are hereby incorporated by reference in their entireties. SUMMARY OF THE INVENTION The subject invention provides an isolated polynucleotide molecule comprising a DNA sequence encoding an infectious RNA molecule encoding a North American PRRS virus, wherein said DNA sequence is SEQ ID NO:1 or is a sequence homologous thereto. The subject invention further provides an isolated infectious RNA molecule encoded by the isolated polynucleotide molecule recited above, and isolated infectious RNA molecules homologous thereto, which isolated infectious RNA molecules each encode a North American PRRS virus. The subject invention further provides the above-recited isolated polynucleotide molecule encoding the infectious RNA molecule in the form of a vector such as a plasmid. The subject invention further provides a viral vector comprising a DNA encoding an infectious RNA molecule encoding a North American PRRS virus, wherein said DNA sequence is SEQ ID NO:1 or is a DNA sequence homologous thereto. The subject invention further provides a transfected host cell comprising a DNA sequence encoding an infectious RNA molecule encoding a North American PRRS virus, wherein said DNA sequence is SEQ ID NO:1 or is a DNA sequence homologous thereto, which transfected host cell is capable of expressing the encoded North American PRRS virus. The subject invention further provides a method for making a genetically modified North American PRRS virus, which method comprises mutating a DNA sequence of the present invention encoding an infectious RNA molecule encoding the North American PRRS virus, and expressing the genetically modified North American PRRS virus therefrom subsequent to said mutation. The subject invention further provides an isolated polynucleotide molecule comprising a DNA sequence encoding an infectious RNA molecule encoding a genetically modified North American PRRS virus. In a preferred embodiment, the PRRS virus is genetically modified such that when it infects a porcine animal it is: a) unable to produce PRRS in the animal, and b) able to elicit an effective immunoprotective response against infection by a PRRS virus in the animal. In a particular embodiment, the DNA sequence is SEQ ID NO:1 or a sequence homologous thereto, except for that it contains one or more mutations that genetically disable the encoded PRRS virus in its ability to produce PRRS. The subject invention further provides an isolated infectious RNA molecule encoded by the isolated polynucleotide molecule recited above, and isolated infectious RNA molecules homologous thereto, which isolated infectious RNA molecules each encode a genetically modified North American PRRS virus, disabled in its ability to produce PRRS. The subject invention further provides a genetically modified North American PRRS virus encoded by an infectious RNA molecule as recited above, which genetically modified North American PRRS virus is disabled such that when it infects a porcine animal it is unable to produce PRRS in the animal, yet is able to elicit an effective immunoprotective response against infection by a PRRS virus in the animal. The subject invention further provides a viral vector comprising a DNA sequence encoding an infectious RNA molecule encoding a genetically modified North American PRRS virus as recited above. The subject invention further provides a transfected host cell comprising a DNA sequence encoding an infectious RNA molecule encoding a genetically modified North American PRRS virus as recited above. The subject invention further provides a vaccine for protecting a porcine animal from infection by a PRRS virus, which vaccine comprises a genetically modified North American PRRS virus as recited above; an infectious RNA molecule as recited above encoding the genetically modified North American PRRS virus; an isolated polynucleotide molecule recited above, in the form of a plasmid, encoding the genetically modified North American PRRS virus; or the above-recited viral vector encoding the genetically modified North American PRRS virus; in an amount effective to produce immunoprotection against infection by a PRRS virus; and a carrier acceptable for veterinary use. The subject invention further provides a method for protecting a porcine animal from infection by a PRRS virus, which comprises vaccinating the animal with an amount of the above-recited vaccine that is effective to produce immunoprotection against infection by a PRRS virus. The present invention further provides any of the aforementioned polynucleotide molecules further comprising a nucleotide sequence encoding a heterologous antigenic epitope, as well as corresponding infectious RNA molecules, vectors, transfected host cells, genetically modified North American PRRS viruses, vaccines, and methods of administering such vaccines to mammals and/or birds. Such heterologous antigenic epitopes can be from any antigenic epitope, presently known in the art, or to be determined in the future. In a non-limiting embodiment, such antigenic epitope is from a pathogen capable of pathogenically infecting a bird or mammal other than porcine animal, for example a human, and is capable of inducing an effective immunoprotective response against said pathogen. In another non-limiting embodiment, such antigenic epitope is from a swine pathogen other than a North American PRRS virus and is capable of inducing an effective immunoprotective response against said swine pathogen. In another non-limiting embodiment, such antigenic epitope is a detectable antigenic epitope. The present invention further provides any of the aforementioned polynucleotide molecules, but which lack one or more detectable antigenic epitopes, as well as corresponding infectious RNA molecules, vectors, transfected host cells, genetically modified North American PRRS viruses, vaccines, and methods of administering such vaccines to mammals and/or birds. The subject invention further provides an isolated polynucleotide molecule comprising one or more nucleotide sequences that encode a peptide encoded by a North American PRRS virus, wherein the genome sequence of said North American PRRS virus is SEQ ID NO:1 or a sequence homologous thereto. The subject invention further provides a transfected host cell comprising one or more nucleotide sequences that encode a peptide encoded by a North American PRRS virus, wherein the genome sequence of said North American PRRS virus is SEQ ID NO:1 or a sequence homologous thereto. The subject invention further provides a genetically modified Nidovirales virus that is able to elicit an effective immunoprotective response in a mammal or a bird vaccinated therewith, which genetically modified Nidovirales virus is prepared by obtaining an isolated polynucleotide molecule comprising a DNA sequence encoding an infectious RNA molecule encoding a wild-type Nidovirales virus, genetically mutating the DNA sequence so as to obtain an isolated polynucleotide molecule comprising a DNA sequence encoding an infectious RNA molecule encoding a genetically modified Nidovirales virus which virus is unable to produce a pathogenic infection yet is able to elicit an effective immunoprotective response against infection by the wild-type Nidovirales virus in a mammal or a bird, and expressing the genetically modified Nidovirales virus from the isolated polynucleotide molecule so obtained. The subject invention further provides a method for preparing a genetically modified Nidovirales virus that is capable of eliciting an immunoprotective response in a mammal or a bird vaccinated therewith, which method comprises obtaining an isolated polynucleotide molecule comprising a DNA sequence encoding an infectious RNA molecule encoding a wild-type Nidovirales virus, genetically mutating the DNA so as to obtain an isolated polynucleotide molecule comprising a DNA sequence encoding an infectious RNA molecule encoding a genetically modified Nidovirales virus which virus is unable to produce a pathogenic infection yet able to elicit an effective immunoprotective response against infection by the wild-type Nidovirales virus in a mammal or a bird, and expressing the genetically modified Nidovirales virus from the isolated polynucleotide molecule so obtained. The subject invention further provides a vaccine for protecting a mammal or a bird from infection by a Nidovirales virus, which vaccine comprises a genetically modified Nidovirales virus as described above in an amount effective to elicit an effective immunoprotective response against the wild-type Nidovirales virus in a mammal or a bird vaccinated therewith, and a carrier acceptable for pharmaceutical or veterinary use. BRIEF DESCRIPTION OF THE FIGURES FIG. 1: Cloning strategy for construction of full-length infectious cDNA clone of North American PRRS virus, pT7P129A. Arrowheads represent T7 promoter sequences. FIG. 2: Serum viremia following infection with P129A or recombinant PRRS virus rP129A-1. Determined by plaque assay on MARC-145 cells. The lower limit of detection is 5 pfu/ml (or 0.7 on the log scale). FIG. 3: Anti-PRRS virus serum antibody following infection with P129A or recombinant PRRS virus rP129A-1. Determined by HerdChek PRRS ELISA assay (IDEXX (Westbrook, Me. USA)). DETAILED DESCRIPTION OF THE INVENTION Production and manipulation of the isolated polynucleotide molecules described herein are within the skill in the art and can be carried out according to recombinant techniques described, among other places, in Maniatis, et al., 1989, Molecular Cloning, A Laboratory Manual, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.; Ausubel, et al., 1989, Current Protocols In Molecular Biology, Greene Publishing Associates & Wiley Interscience, N.Y.; Sambrook, et al., f1989, Molecular Cloning: A Laboratory Manual, 2d ed., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.; Innis et al. (eds), 1995, PCR Strategies, Academic Press, Inc., San Diego; and Erlich (ed), 1992, PCR Technology, Oxford University Press, N.Y., all of which are incorporated herein by reference. A. Isolated Polynucleotide Molecules and RNA Molecules Encoding a North American PRRS Virus, and Isolated Polynucleotide Molecules and RNA Molecules Encoding Genetically Modified North American PRRS Viruses: The subject invention provides isolated polynucleotide molecules comprising DNA sequences that encode infectious RNA molecules that encode a North American PRRS virus. The present invention further provides isolated polynucleotide molecules comprising DNA sequences that encode infectious RNA molecules that encode genetically modified North American PRRS viruses. In particular, the subject invention provides an isolated polynucleotide molecule comprising a DNA sequence encoding an infectious RNA molecule that encodes a North American PRRS virus, wherein said DNA sequence is SEQ ID NO:1 or a sequence homologous thereto. In a preferred embodiment, the present invention provides an isolated polynucleotide molecule comprising a DNA sequence encoding an infectious RNA molecule that encodes a North American PRRS virus, wherein said DNA sequence is the sequence beginning with and including nucleotide 1 through and including nucleotide 15,416 of SEQ ID NO:1, except that the nucleotide corresponding to nucleotide 12,622 of SEQ ID NO:1 is a guanine instead of an adenine and the nucleotide corresponding to nucleotide 1,559 of SEQ ID NO:1 is a thymine instead of a cytosine. Said DNA sequence encodes an infectious RNA molecule that is the RNA genome of the North American PRRS isolate P129. It is understood that terms herein referring to nucleic acid molecules such as "isolated polynucleotide molecule", "nucleotide sequence", "open reading frame (ORF)", and the like, unless otherwise specified, include both DNA and RNA molecules and include both single-stranded and double-stranded molecules. Also, when reference to a particular sequence from the "Sequence Listing" section of the subject application is made, it is intended, unless otherwise specified, to refer to both the DNA of the "Sequence Listing", as well as RNA corresponding to the DNA sequence, and includes sequences complementary to the DNA and RNA sequences. In such contexts in this application, "corresponding to" refers to sequences of DNA and RNA that are identical to one another but for the fact that the RNA sequence contains uracil in place of thymine and the backbone of the RNA molecule contains ribose instead of deoxyribose. For example, SEQ ID NO:1 is a DNA sequence corresponding to the RNA genome of a North American PRRS virus. Thus, a DNA sequence complementary to the DNA sequence set forth in SEQ ID NO:1 is a template for, i.e. is complementary to or "encodes", the RNA genome of the North American PRRS virus (i.e., RNA that encodes the North American PRRS virus). Nonetheless, a reference herein to SEQ ID NO:1 includes both the RNA sequence corresponding to SEQ ID NO:1 and a DNA sequence complementary to SEQ ID NO:1. Furthermore, when reference is made herein to sequences homologous to a sequence in the Sequence Listing, it is to be understood that sequences homologous to a sequence corresponding to the sequence in the Sequence Listing and sequences homologous to a sequence complementary to the sequence in the Sequence Listing are also included. An "infectious RNA molecule", for purposes of the present invention, is an RNA molecule that encodes the necessary elements for viral replication, transcription, and translation into a functional virion in a suitable host cell, provided, if necessary, with a peptide or peptides that compensate for any genetic modifications, e.g. sequence deletions, in the RNA molecule. An "isolated infectious RNA molecule" refers to a composition of matter comprising the aforementioned infectious RNA molecule purified to any detectable degree from its naturally occurring state, if such RNA molecule does indeed occur in nature. Likewise, an "isolated polynucleotide molecule" refers to a composition of matter comprising a polynucleotide molecule of the present invention purified to any detectable degree from its naturally occurring state, if any. For purposes of the present invention, the nucleotide sequence of a second polynucleotide molecule (either RNA or DNA) is "homologous" to the nucleotide sequence of a first polynucleotide molecule where the nucleotide sequence of the second polynucleotide molecule encodes the same polyaminoacid as the nucleotide sequence of the first polynucleotide molecule as based on the degeneracy of the genetic code, or when it encodes a polyaminoacid that is sufficiently similar to the polyaminoacid encoded by the nucleotide sequence of the first polynucleotide molecule so as to be useful in practicing the present invention. For purposes of the present invention, a polynucleotide molecule is useful in practicing the present invention where it can be used as a diagnostic probe to detect the presence of the North American PRRS virus in a fluid or tissue sample of an infected pig, e.g. by standard hybridization or amplification techniques. It is to be understood that the polyaminoacid encoded by the nucleotide sequence of the polynucleotide molecule can comprise a group of two or more polyaminoacids. Generally, the nucleotide sequence of a second polynucleotide molecule is homologous to the nucleotide sequence of a first polynucleotide molecule if it has at least about 70% nucleotide sequence identity to the nucleotide sequence of the first polynucleotide molecule as based on the BLASTN algorithm (National Center for Biotechnology Information, otherwise known as NCBI, (Bethesda, Md., USA) of the United States National Institute of Health). Preferably, a homologous nucleotide sequence has at least about 75% nucleotide sequence identity, even more preferably at least about 85% nucleotide sequence identity. Since the genetic code is degenerate, a homologous nucleotide sequence can include any number of "silent" base changes, i.e. nucleotide substitutions that nonetheless encode the same amino acid. A homologous nucleotide sequence can further contain non-silent mutations, i.e. base substitutions, deletions, or additions resulting in amino acid differences in the encoded polyaminoacid, so long as the sequence remains at least about 70% identical to the polyaminoacid encoded by the first nucleotide sequence or otherwise is useful for practicing the present invention. Homologous nucleotide sequences can be determined by comparison of nucleotide sequences, for example by using BLASTN, above. Alternatively, homologous nucleotide sequences can be determined by hybridization under selected conditions. For example, the nucleotide sequence of a second polynucleotide molecule is homologous to SEQ ID NO:1 if it hybridizes to the complement of SEQ ID NO:1 under moderately stringent conditions, e.g., hybridization to filter-bound DNA in 0.5 M NaHPO.sub.4, 7% sodium dodecyl sulfate (SDS), 1 mM EDTA at 65.degree. C., and washing in 0.2.times.SSC/0.1% SDS at 42.degree. C. (see Ausubel et al. above), or conditions which will otherwise result in hybridization of sequences that encode a North American PRRS virus as defined below. In another embodiment, a second nucleotide sequence is homologous to SEQ ID NO:1 if it hybridizes to the complement of SEQ ID NO:1 under highly stringent conditions, e.g. hybridization to filter-bound DNA in 0.5 M NaHPO.sub.4, 7% SDS, 1 mM EDTA at 65.degree. C., and washing in 0.1.times.SSC/0.1% SDS at 68.degree. C. (Ausubel et al., above). It is furthermore to be understood that the isolated polynucleotide molecules and the isolated RNA molecules of the present invention include both synthetic molecules and molecules obtained through recombinant techniques, such as by in vitro cloning and transcription. As used herein, the term "PRRS" encompasses disease symptoms in swine caused by a PRRS virus infection. Examples of such symptoms include, but are not limited to, abortion in pregnant females, and slow growth, respiratory difficulties, loss of appetite, and mortality in young pigs. As used herein, a PRRS virus that is "unable to produce PRRS" refers to a virus that can infect a pig, but which does not produce any disease symptoms normally associated with a PRRS infection in the pig, or produces such symptoms, but to a lesser degree, or produces a fewer number of such symptoms, or both. The terms "porcine" and "swine" are used interchangeably herein and refer to any animal that is a member of the family Suidae such as, for example, a pig. "Mammals" include any warm-blooded vertebrates of the Mammalia class, including humans. The term "PRRS virus", as used herein, unless otherwise indicated, means any strain of either the North American or European PRRS viruses. The term "North American PRRS virus" means any PRRS virus having genetic characteristics associated with a North American PRRS virus isolate, such as, but not limited to the PRRS virus that was first isolated in the United States around the early 1990's (see, e.g., Collins, J. E., et al., 1992, J. Vet. Diagn. Invest. 4:117-126); North American PRRS virus isolate MN-1b (Kwang, J. et al., 1994, J.Vet.Diagn.Invest. 6:293-296); the Quebec IAF-exp91 strain of PRRS (Mardassi, H. et al., 1995, Arch.Virol. 140:1405-1418); and North American PRRS virus isolate VR 2385 (Meng, X.-J et al., 1994, J.Gen.Virol. 75:1795-1801). Genetic characteristics refers to genomic nucleotide sequence similarity and aminoacid sequence similarity shared by North American PRRS virus strains. For purposes of the present invention, a North American PRRS virus is a virus that is encoded by an RNA sequence the same as or homologous to SEQ ID NO:1, wherein the term "homologous" is as defined previously. Thus, strains of North American PRRS viruses have, preferably, at least about 70% genomic nucleotide sequence identity with SEQ ID NO:1, and more preferably at least about 75% genomic nucleotide sequence identity with SEQ ID NO:1, at least about 85% genomic nucleotide sequence identity with SEQ ID NO:1 being even more preferred. The term "European PRRS virus" refers to any strain of PRRS virus having the genetic characteristics associated with the PRRS virus that was first isolated in Europe around 1991 (see, e.g., Wensvoort, G., et al., 1991, Vet. Q. 13:121-130). "European PRRS virus" is also sometimes referred to in the art as "Lelystad virus". Unless otherwise indicated, a North American PRRS virus is "useful in practicing the present invention" if its characteristics are within the definition of a North American PRRS virus set forth herein. For example, a virus encoded by one of the isolated polynucleotide molecules of the present invention is a "North American PRRS virus useful in practicing the present invention" if it, e.g., has genetic characteristics associated with a North American PRRS virus. Other polyaminoacids are "useful in practicing the present invention", e.g., peptides encoded by polynucleotide sequences homologous to North American PRRS virus ORFs, if they can compensate for an RNA molecule encoding a genetically modified PRRS virus, deficient in a gene essential for expressing functional PRRS virions, in a transfected host cell so that functional PRRS virions can be generated by the cell. The term "open reading frame", or "ORF", as used herein, means the minimal nucleotide sequence required to encode a particular PRRS virus protein without an intervening stop codon. Terms such as "suitable host cell" and "appropriate host cell", unless otherwise indicated, refer to cells into which RNA molecules (or isolated polynucleotide molecules or viral vectors comprising DNA sequences encoding such RNA molecules) of the present invention can be transformed or transfected. "Suitable host cells" for transfection with such RNA molecules, isolated polynucleotide molecules, or viral vectors, include mammalian, particularly porcine, and avian cells, and are described in further detail below. A "functional virion" is a virus particle that is able to enter a cell capable of hosting a PRRS virus, and express genes of its particular RNA genome (either an unmodified genome or a genetically modified genome as described herein) within the cell. Cells capable of hosting a PRRS virus include porcine alveolar macrophage cells and MARC 145 monkey kidney cells. Other mammalian or avian cells, especially other porcine cells, may also serve as suitable host cells for PRRS virions. The isolated polynucleotide molecules of the present invention encode North American PRRS viruses that can be used to prepare live, killed, or attenuated vaccines using art-recognized methods for protecting swine from infection by a PRRS virus, as described in further detail below. These isolated polynucleotide molecules are also useful as vectors for delivering heterologous genes into mammals, including swine, or birds, as is also described in detail below. Furthermore, these isolated polynucleotide molecules are useful because they can be mutated using molecular biology techniques to encode genetically-modified North American PRRS viruses useful, inter alia, as vaccines for protecting swine from PRRS infection. Such genetically-modified North American PRRS viruses, as well as vaccines comprising them, are also described in further detail below. Accordingly, the subject invention further provides a method for making a genetically modified North American PRRS virus, which method comprises mutating the DNA sequence encoding an infectious RNA molecule which encodes the North American PRRS virus as described above, and expressing the genetically modified North American PRRS virus using a suitable expression system. A North American PRRS virus, either wild-type or genetically modified, can be expressed from an isolated polynucleotide molecule using suitable expression systems generally known in the art, examples of which are described in this application. For example, the isolated polynucleotide molecule can be in the form of a plasmid capable of expressing the encoded virus in a suitable host cell in vitro, as is described in further detail below. The term "genetically modified", as used herein and unless otherwise indicated, means genetically mutated, i.e. having one or more nucleotides replaced, deleted and/or added. Polynucleotide molecules can be genetically mutated using recombinant techniques known to those of ordinary skill in the art, including by site-directed mutagenesis, or by random mutagenesis such as by exposure to chemical mutagens or to radiation, as known in the art. In one embodiment, genetic modification of the North American PRRS virus of the present invention renders the virus unable to replicate effectively, or reduces its ability to replicate effectively, in a bird or mammal in which the wild-type virus otherwise can effectively replicate. In another embodiment, the genetically modified North American PRRS virus of the present invention remains able to replicate effectively in birds or mammals infected therewith. "Effective replication" means the ability to multiply and produce progeny viruses (virions) in an infected animal, i.e. the ability to "productively infect" an animal. The subject invention further provides an isolated polynucleotide molecule comprising a DNA sequence encoding an infectious RNA molecule which encodes a genetically modified North American PRRS virus that is unable to produce PRRS in a porcine animal, wherein the DNA sequence encoding the infectious RNA molecule encoding said North American PRRS virus is SEQ ID NO:1 or a sequence homologous thereto, except that it contains one or more mutations that genetically disable the encoded PRRS virus in its ability to produce PRRS. "Genetically disabled" means that the PRRS virus is unable to produce PRRS in a swine animal infected therewith. In one embodiment, the genetically modified North American PRRS virus disabled in its ability to cause PRRS is able to elicit an effective immunoprotective response against infection by a PRRS virus in a swine animal. Accordingly, the subject invention also provides an isolated polynucleotide molecule comprising a DNA sequence encoding an infectious RNA molecule which encodes a North American PRRS virus that is genetically modified such that when it infects a porcine animal it: a) is unable to produce PRRS in the animal, and b) is able to elicit an effective immunoprotective response against infection by a PRRS virus in the animal, wherein the DNA sequence encoding said North American PRRS virus is SEQ ID NO:1 or a sequence homologous thereto, except that it contains one or more mutations that genetically disable the encoded PRRS virus in its ability to produce PRRS. The term "immune response" for purposes of this invention means the production of antibodies and/or cells (such as T lymphocytes) that are directed against, or assist in the decomposition or inhibition of, a particular antigenic epitope or particular antigenic epitopes. The phrases "an effective immunoprotective response", "immunoprotection", and like terms, for purposes of the present invention, mean an immune response that is directed against one or more antigenic epitopes of a pathogen so as to protect against infection by the pathogen in a vaccinated animal. For purposes of the present invention, protection against infection by a pathogen includes not only the absolute prevention of infection, but also any detectable reduction in the degree or rate of infection by a pathogen, or any detectable reduction in the severity of the disease or any symptom or condition resulting from infection by the pathogen in the vaccinated animal as compared to an unvaccinated infected animal. An effective immunoprotective response can be induced in animals that have not previously been infected with the pathogen and/or are not infected with the pathogen at the time of vaccination. An effective immunoprotective response can also be induced in an animal already infected with the pathogen at the time of vaccination. An "antigenic epitope" is, unless otherwise indicated, a molecule that is able to elicit an immune response in a particular animal or species. Antigenic epitopes are proteinaceous molecules, i.e. polyaminoacid sequences, optionally comprising non-protein groups such as carbohydrate moieties and/or lipid moieties. The term "pathogenically infecting" used herein refers to the ability of a pathogen to infect an animal and cause a disease in the animal. As an example, a PRRS virus is capable of pathogenically infecting a porcine animal since it can cause PRRS in swine. However, although a PRRS virus may be able to infect, either productively or non-productively, a bird or another mammal, such as a human, it does not pathogenically infect any animal other than a porcine animal since it does not cause any disease in animals other than porcine animals. The genetically modified North American PRRS viruses encoded by the above-described isolated polynucleotide molecules are, in one embodiment, able to elicit an effective immunoprotective response against infection by a PRRS virus. Such genetically modified North American PRRS viruses are preferably able to elicit an effective immunoprotective response against any strain of PRRS viruses, including both European and North American strains. In one embodiment, the mutation or mutations in the isolated polynucleotide molecule encoding the genetically disabled North American PRRS virus are non-silent and occur in one or more open reading frames of the nucleotide sequence encoding the North American PRRS virus; i.e., the mutation or mutations occur in one or more of the sequences within the nucleotide sequence encoding the North American PRRS virus that are the same as or homologous to ORFs 1a, 1b, 2, 3, 4, 5, 6, or 7 of SEQ ID NO:1. In another embodiment, the mutation or mutations occur in one or more noncoding regions of the North American PRRS virus genome, such as, for example, in the leader sequence of the North American PRRS virus genome; i.e., the mutation or mutations occur within the sequence that is the same as or homologous to the sequence of nucleotides 1-191 of SEQ ID NO:1. In the same isolated polynucleotide molecule, mutations can occur in both coding and noncoding regions. As used herein, unless otherwise indicated, "noncoding regions" of the nucleotide sequence encoding the North American PRRS virus refer to those sequences of RNA that are not translated into a protein and those sequences of cDNA that encode such RNA sequences. Coding regions refer to those sequences of RNA from which North American PRRS virus proteins are expressed, and also refer to cDNA that encodes such RNA sequences. Likewise, "ORFs" refer both to RNA sequences that encode North American PRRS virus proteins and to cDNA sequence encoding such RNA sequences. Determining suitable locations for a mutation or mutations that will encode a North American PRRS virus that is genetically disabled so that it is unable to produce PRRS yet remains able to elicit an effective immunoprotective response against infection by a PRRS virus can be made based on the SEQ ID NO:1 provided herein. One of ordinary skill can refer to the sequence of the infectious cDNA clone of North American PRRS virus provided by this invention, make sequence changes which will result in a mutation, and test the viruses encoded thereby both for their ability to produce PRRS in swine, and to elicit an effective immunoprotective response against infection by a PRRS virus. In so doing, one of ordinary skill can refer to techniques known in the art and also those described and/or exemplified herein. For example, an ORF of the sequence encoding the infectious RNA molecule encoding the North American PRRS virus can be mutated and the resulting genetically modified North American PRRS virus tested for its ability to cause PRRS. The ORF of a North American PRRS virus encodes proteins as follows: ORF la encodes a polyprotein comprising protease function; ORF 1b encodes a polyprotein comprising replicase (RNA polymerase) and helicase functions; ORFs 2, 3, and 4 encode small membrane glycoproteins; ORF 5 encodes a major envelope glycoprotein; ORF 6 encodes a nonglycosylated integral membrane protein; and ORF 7 encodes a nucleocapsid protein. Genetic mutations of one or more of these ORFs can be used in preparing the genetically modified North American PRRS viruses described infra. The subject invention also provides an isolated polynucleotide molecule comprising a DNA sequence encoding an infectious RNA molecule encoding a North American PRRS virus that is genetically modified such that it comprises one or more heterologous antigenic epitopes, wherein the DNA sequence encoding the RNA molecule encoding the North American PRRS virus is SEQ ID NO:1 or a sequence homologous thereto, and further comprising one or more additional nucleotide sequences that each encode a heterologous antigenic epitope, and wherein each heterologous antigenic epitope is capable of inducing an effective immunoprotective response against a particular pathogen in a mammal or a bird. A pathogen against which an effective immunoprotective response can be induced by means of the above recited aspect of the present invention is any pathogen, such as a virus, bacteria, fungus, or protozoan, capable of causing a disease in a mammal or bird, which pathogen comprises or has associated therewith one or more antigenic epitopes which can be used to induce an effective immunoprotective response against the pathogen in the mammal or bird. The term "heterologous antigenic epitope" for purposes of the present invention means an antigenic epitope, as defined above, not normally found in a wild-type North American PRRS virus. A nucleotide sequence encoding a heterologous antigenic epitope can be inserted into a North American PRRS viral genome using known recombinant techniques. Antigenic epitopes useful as heterologous antigenic epitopes for the present invention include additional North American PRRS virus antigenic epitopes, antigenic epitopes from European PRRS viruses, antigenic epitopes from swine pathogens other than PRRS viruses, or antigenic epitopes from pathogens that pathogenically infect birds or mammals other than swine, including humans. Sequences encoding such antigenic epitopes are known in the art or are provided herein. For example, a second North American PRRS virus envelope protein, encoded by North American PRRS ORF 5 described herein, can be inserted into a DNA sequence encoding an RNA molecule encoding a North American PRRS virus of the present invention to generate a genetically modified North American PRRS virus comprising an additional envelope protein as a heterologous antigenic epitope. Such a genetically modified North American PRRS virus can be used to induce a more effective immunoprotective response against PRRS viruses in a porcine animal vaccinated therewith. Examples of an antigenic epitope from a swine pathogen other than a North American PRRS virus include, but are not limited to, an antigenic epitope from a swine pathogen selected from the group consisting of European PRRS, porcine parvovirus, porcine circovirus, a porcine rotavirus, swine influenza, pseudorabies virus, transmissible gastroenteritis virus, porcine respiratory coronavirus, classical swine fever virus, African swine fever virus, encephalomyocarditis virus, porcine paramyxovirus, Actinobacillus pleuropneumoni, Bacillus anthraci, Bordetella bronchiseptica, Clostridium haemolyticum, Clostridium perfringens, Clostridium tetani, Escherichia coli, Erysipelothdix rhusiopathiae, Haemophilus parasuis, Leptospira spp., Mycoplasma hyopneumoniae, Mycoplasma hyorhinis, Pasteurella haemolytica, Pasteurella multocida, Salmonella choleraesuis, Salmonella typhimurium, Streptococcus equismilis, and Streptococcus suis. Nucleotide sequences encoding antigenic epitopes from the aforementioned swine pathogens are known in the art and can be obtained from public gene databases such as GenBank (http://www.ncbi.nlm.nih.gov/Web/Genbank/index.html) provided by NCBI. If the heterologous antigenic epitopes are antigenic epitopes from one or more other swine pathogens, then the isolated polynucleotide molecule can further contain one or more mutations that genetically disable the encoded PRRS virus in its ability to produce PRRS. Such isolated polynucleotide molecules and the viruses they encode are useful for preparing vaccines for protecting swine against the swine pathogen or pathogens from which the heterologous antigenic epitopes are derived. In a preferred embodiment, the genetically modified North American PRRS is able to elicit an effective immunoprotective response against infection by a PRRS virus in a porcine animal. Such isolated polynucleotide molecules and the viruses they encode are useful for preparing dual-function vaccines for protecting swine against infection by both a North American PRRS virus and the swine pathogen or pathogens from which the heterologous antigenic epitopes are derived. In another preferred embodiment, the genetically modified North American PRRS virus useful in a dual-function vaccine is genetically disabled. The isolated polynucleotide molecules of the present invention comprising nucleotide sequences encoding heterologous antigenic epitopes can be prepared as described above based on the sequence encoding a North American PRRS virus described herein using known techniques in molecular biology. In a further preferred embodiment, a heterologous antigenic epitope of the genetically modified North American PRRS virus of the present invention is a detectable antigenic epitope. Such isolated polynucleotide molecules and the North American PRRS viruses they encode are useful, inter alia, for studying PRRS infections in swine, determining successfully vaccinated swine, and/or for distinguishing vaccinated swine from swine infected by a wild-type PRRS virus. Preferably, such isolated polynucleotide molecules further contain one or more mutations that genetically disable the encoded PRRS virus in its ability to produce PRRS, and more preferably are able to elicit an effective immunoprotective response in a porcine animal against infection by a PRRS virus. Heterologous antigenic epitopes that are detectable, and the sequences that encode them, are known in the art. Techniques for detecting such antigenic epitopes are also known in the art and include serological detection of antibody specific to the heterologous antigenic epitope by means of, for example, Western blot, ELISA, or fluorescently labeled antibodies capable of binding to the antibodies specific to the heterologous antigenic epitope. Techniques for serological detection useful in practicing the present invention can be found in texts recognized in the art, such as Coligan, J. E., et al. (eds), 1998, Current Protocols in Immunology, John Willey & Sons, Inc., which is hereby incorporated by reference in its entirety. Alternatively, the heterologous antigenic epitope itself can be detected by, for example, contacting samples that potentially comprise the antigenic epitope with fluorescently-labeled antibodies or radioactively-labeled antibodies that specifically bind to the antigenic epitopes. The present invention further provides an isolated polynucleotide molecule comprising a DNA sequence encoding an infectious RNA molecule which encodes a genetically modified North American PRRS virus that detectably lacks a North American PRRS virus antigenic epitope, wherein the DNA sequence encoding the RNA molecule encoding the North American PRRS virus is SEQ ID NO:1 or a sequence homologous thereto, except that it lacks one or more nucleotide sequences encoding a detectable North American PRRS virus antigenic epitope. Such isolated polynucleotide molecules are useful for distinguishing between swine infected with a recombinant North American PRRS virus of the present invention and swine infected with a wild-type PRRS virus. For example, animals vaccinated with killed, live or attenuated North American PRRS virus encoded by such an isolated polynucleotide molecule can be distinguished from animals infected with wild-type PRRS based on the absence of antibodies specific to the missing antigenic epitope, or based on the absence of the antigenic epitope itself: If antibodies specific to the missing antigenic epitope, or if the antigenic epitope itself, are detected in the animal, then the animal was exposed to and infected by a wild-type PRRS virus. Means for detecting antigenic epitopes and antibodies specific thereto are known in the art, as discussed above. Preferably, such an isolated polynucleotide molecule further contains one or more mutations that genetically disable the encoded PRRS virus in its ability to produce PRRS. More preferably, the encoded virus remains able to elicit an effective immunoprotective response against infection by a PRRS virus. B. Plasmids Encoding a North American PRRS Virus or a Genetically Modified North American PRRS Virus: The present invention also provides any of the above-described isolated polynucleotide molecules in the form of a plasmid capable of expressing the North American PRRS virus encoded thereby. Plasmids of the present invention can express the encoded North American PRRS virus outside of a living organism, to produce North American PRRS viruses of the invention useful, inter alia, for preparing vaccines. In one embodiment, a plasmid of the present invention capable of expressing a North American PRRS virus outside of a living organism is a plasmid wherein transcription of viral RNA therefrom occurs in vitro (i.e. extracellularly); the resulting viral RNA molecule is transfected into a suitable host cell using known mechanisms of transfection, such as electroporation, lipofection (in some cases using a commercially available reagent, such as Lipofectin.TM. (Life Technologies Inc., Rockville, Md., USA)), or DEAE dextran mediated transfection. Other methods of transfection are known in the art and can be employed in the present invention. An example of such a plasmid for in vitro transcription of North American PRRS viral RNA is the plasmid pT7P129A (ATCC Accession No. 203488). Any promoter useful for in vitro transcription can be used in such plasmids of this invention. T7 is one such promoter, but other promoters can be used, such as an SP6 promoter or a T3 promoter. The sequences of such promoters can be artificially synthesized or cloned from commercially available plasmids. Suitable plasmids for preparing such plasmids capable of expressing North American PRRS virus include, but are not limited to, general purpose cloning vector plasmids such as pCR2.1 (Invitrogen, Carlsbad, Calif., USA), pBR322, and pUC18. A nucleotide sequence of the present invention encoding the North American PRRS virus can be inserted into any of these plasmids using known recombinant techniques. Other plasmids into which the polynucleotide molecules of the present invention can be inserted will be recognized by those of ordinary skill in the art. Suitable conditions for in vitro transcription of viral RNA from any of the above-described recombinant plasmids comprising a DNA sequence encoding an infectious RNA molecule encoding a North American PRRS virus depends on the type of plasmid, for example, its particular promoter, and can be ascertained by one of ordinary skill in the art. For example, if a plasmid of the present invention is based on a pCR2.1 plasmid comprising a T7 promoter, then an example of suitable conditions for in vitro transcription includes reacting the plasmid with T7 RNA polymerase and ribonucleotides in a standard buffer and incubating the reaction at 37.degree. C. for about 30 minutes. In some cases, commercial kits are available for transcribing RNA from a particular plasmid, and such kits can be used in the present invention. The reaction mixture following transcription can be directly transfected into a suitable host cell without purification, or the transcribed North American PRRS virus RNA can be purified by known RNA purification techniques, for example by organic (e.g. phenol) extraction and alcohol (e.g. ethanol or isopropanol) precipitation, prior to transfection. Practically any mammalian or avian cell culture can be transfected with the North American PRRS virus RNA obtained as described above in order to generate a first round of North American PRRS virions. An example of cells which one might find particularly useful because of their ready availability and ease of use are BHK (baby hamster kidney) cells. However, if one wishes to generate a cell culture capable of sustained production of North American PRRS virions, then porcine alveolar macrophage cells or MARC-145 cells (Kim, H. S., et al., supra) are preferred since these cells excrete high levels of new generation PRRS virions subsequent to PRRS virus infection. Other cell lines derived from the MA-104 cell line may also be used for sustained generation of North American PRRS virions of the present invention. Primary porcine alveolar macrophage cells can be obtained by lung lavages from pigs, and the MARC-145 monkey kidney cell line can be obtained from the National Veterinary Services Laboratories otherwise known as NVSL (Ames, Iowa, USA). In another embodiment, a plasmid capable of expressing a North American PRRS virus of the present invention outside of a living organism is a plasmid which is transfected into a suitable host cell, for example by electroporation or lipofection, transcription of the infectious RNA molecule and expression of the North American PRRS virus therefrom occurring within the transfected host cell. The transfected host cell therefore generates North American PRRS virions. Such a completely cellular method has heretofore never been disclosed or suggested for any virus within the order of Nidovirales. Because of possible cryptic splicing and termination sequences present in the RNA genome of viruses of the Nidovirales order, a completely cellular method of expressing a Nidovirales virus was believed unlikely. Cryptic sequences include RNA splice donor and splice acceptor sequences, which could cause inappropriate splicing of the RNA transcript, as well as polyadenylation sequences, which could cause premature termination by the cellular RNA polymerase II. The present invention demonstrates, however, that the presence of such sequences in a plasmid comprising a cDNA clone of a Nidovirus does not prevent the plasmid's ability to express the Nidovirus when the plasmid is directly transfected into a suitable host cell. Accordingly, the subject invention also provides plasmids and a completely cellular method for expressing a Nidovirales virus, wherein the plasmid comprises: a) a DNA sequence encoding an infectious RNA molecule encoding the Nidovirales virus; and b) a promoter capable of transcribing said encoding sequence in a cell, wherein said promoter is in operative association with the DNA sequence encoding the infectious RNA molecule. The method comprises transfecting a suitable host cell with such a plasmid, subjecting the transfected host cell to conditions suitable for expression of gene sequences transfected therein, and collecting the expressed Nidovirales virus therefrom. An example of a plasmid suitable for completely cellular expression of North American PRRS virus outside of a living organism is the plasmid pCMV-S-P129 (ATCC Accession No. 203489). In a preferred embodiment, the promoter of such a plasmid is a CMV promoter. In a preferred embodiment, a plasmid of the invention suitable for a completely cellular method of expressing a Nidovirales virus comprises a eukaryotic promoter, such as a CMV promoter, immediately upstream and adjacent to the nucleotide sequence encoding the Nidovirales virus. In a preferred embodiment, the nucleotide sequence encoding the Nidovirales virus encodes a PRRS virus, either European or North American. Other examples of Nidovirales viruses that can be expressed by means of the above-described completely cellular method include other Arteriviruses such as, equine arteritis virus, lactate dehydrogenase-elevating virus, and simian haemorrhagic fever virus; viruses that are members of the genus Coronaviridae, such as, but not limited to, feline infectious peritinitis virus, feline enteric coronavirus, canine coronavirus, bovine coronavirus, porcine respiratory coronavirus, turkey coronavirus, porcine transmissible gastroenteritis virus, human coronavirus, murine hepatitis virus, and avian infectious bronchitis virus; and members of the genus Toroviridae, such as, but not limited to, Berne virus, Breda virus, and human torovirus. Thus, plasmids suitable for completely cellular expression comprising a nucleotide sequence encoding one of these viruses are also encompassed by the present invention. Suitable plasmids that can be used to prepare recombinant plasmids of the present invention for completely cellular expression outside of a living organism of a Nidovirales virus, such as a PRRS virus, include virtually any plasmid useful for transfection and expression in eukaryotic cells. An examples of a plasmid suitable for preparing recombinant plasmids of the present invention for completely cellular expression of a Nidovirales virus is the plasmid pCMVbeta (Clontech, Palo Alto, Calif., USA). Other plasmids which are able to transfect and express genes in eukaryotic cells which can be used to prepare plasmids of the present invention include, but are not limited to, pcDNA3.1, pRc/RSV, and pZeoSV2 (all from Invitrogen); and pCMV-Sport3 and pSV-Sport1 (both from Life Technologies Inc.). However, almost any eukaryotic expression vector will work for the present invention. Constructs based on cosmids can also be used for completely cellular ex vivo expression of a Nidovirales virus. Suitable host cells for the completely cellular method of the present invention for expressing PRRS virus include porcine alveolar macrophage cells and the MARC-145 cells, described above. Methods of transfecting these cells with a plasmid are basically the same as those methods for transfecting cells with viral RNA described above. Such methods include, but are not limited to, electroporation, lipofection, DEAE dextran mediated transfection, and calcium phosphate coprecipitation. Once host cells, such as porcine alveolar macrophage cells or a MARC-145 cells, have been transfected according to the subject invention, either with viral RNA or with a plasmid comprising a nucleotide sequence encoding a virus, then the cells can be frozen at about -80.degree. C. or below for storage for up to several years. For longer periods of time, i.e. decades, storage in liquid nitrogen is preferred. If relatively frequent use of the encoded virus is envisioned, then cells hosting the virus can also be maintained (unfrozen) in culture using known techniques, for shorter periods of time. Moreover, viral particles excreted by such cells can be stored frozen at about -80.degree. C. or below as a source of virus. Transfection of such cell lines with the polynucleotide molecule encoding the virus can be confirmed if desired, for example, by testing exhausted medium excreted by the cell line for a PRRS virus antigen using an immunofluorescent antibody test. Antibodies which are specific for PRRS virus antigens are known in the art (see, e.g., Collins, E. J., et al., WO 93/03760 Mar. 4, 1993). In another embodiment, a plasmid of the present invention comprising a nucleotide sequence encoding a North American PRRS virus is suitable for in vivo expression of the North American PRRS virus, i.e. expression in a living organism. Plasmids which can be used for preparing recombinant plasmids for in vivo expression of a North American PRRS virus include, but are not limited to the plasmids capable of transfecting eukaryotic cells described above, such as pCMVbeta. Animals that can be transfected with plasmids of the present invention include mammals and birds. If the animal is other than a porcine animal, for example, a mallard duck, then the plasmid can comprise a nucleotide sequence encoding a North American PRRS virus comprising further antigenic epitopes from pathogens which are capable of pathogenically infecting the animal; in such a case, the plasmid will encode a North American PRRS virus serving as a vector for transporting epitopes into the animal. If the animal is a porcine animal, then the plasmid can usefully encode any of the North American PRRS viruses described herein, including the genetically-modified North American PRRS viruses described herein. C. Viral Vectors Encoding a North American PRRS Virus, Including Viral Vectors Encoding Genetically Modified North American PRRS Viruses: The present invention also provides viral vectors comprising a DNA sequence encoding an infectious RNA molecule encoding any of the North American PRRS viruses described herein, including the genetically-modified North American PRRS viruses described herein. Such viral vectors are useful for transfecting eukaryotic cells for production of PRRS viruses of the present invention outside of a living organism, or for transfecting swine, or other mammals, or avians, with the sequence encoding the North American PRRS virus, for in vivo expression of the North American PRRS virus therein. Some examples of viruses that can be used as vectors for preparing the viral vectors of the present invention include, but are not limited to, swine viruses such as, but not limited to, swine pox virus, pseudorabies virus, or African swine fever virus. Such swine viruses can be obtained from The National Veterinary Services Laboratories (Ames, Iowa, USA) of the United States Department of Agriculture; the American Type Culture Collection, otherwise known as the ATCC (Manassas, Va., USA); and other known sources. Recombinant viral vectors based on suitable swine viruses such as the aforementioned swine viruses are useful for transfecting swine animals with a nucleotide sequence encoding a North American PRRS virus of the present invention. Viral vectors comprising a DNA sequence encoding an infectious RNA molecule encoding a North American PRRS virus of the present invention based on these and other viruses can be prepared using known recombinant techniques described in texts such as those cited previously in this application. D. Transfected Host cells Encoding or a Genetically Modified North American PRRS Viruses: The present invention also provides transfected host cells that comprise a DNA sequence encoding an infectious RNA molecule encoding any of the North American PRRS viruses described herein, including the genetically-modified North American PRRS viruses described herein, which transfected host cells are capable of expressing the North American PRRS virus. Such transfected host cells are useful for producing North American PRRS viruses of the present invention. Examples of transfected host cells of the present invention include the transfected porcine alveolar macrophage cells and the transfected MARC-145 cells described above. Other transfected host cells of the invention include, but are not limited to, transfected MA-104 cells and other derivatives of MA-104 cells that are transfected; transfected Baby Hamster Kidney (BHK) cells; transfected Chinese Hamster Ovary (CHO) cells; and African Green Monkey kidney cells other than MA-104 cells or MARC-1 45 cells, such as VERO cells; that are transfected. E. North American PRRS Viruses, Including Genetically Modified North American PRRS Viruses: The present invention also provides North American PRRS viruses as described herein, including genetically-modified North American PRRS viruses as described herein, expressed and/or encoded by any of the above-described isolated polynucleotide molecules, RNA molecules, plasmids, viral vectors, or transfected host cells. In certain situations, for example where the North American PRRS virus is to be used in a vaccine for swine and the North American PRRS virus has not been genetically modified as described above so as to be unable to cause PRRS, it is desirable to treat the North American PRRS virus, for example by inactivating or attenuating it, so that it is unable to cause PRRS in swine to which it is administered. Known methods can be used to inactivate a North American PRRS virus of the present invention so that it is unable to cause PRRS in an animal. Examples of such methods include, but are not limited to, treatment with formaldehyde, BEI (binary ethyleneimine), or BPL (beta-propiolactone). Methods of attenuation are also known in the art, and such methods can be used to attenuate a North American PRRS virus of the present invention. A North American PRRS virus of the present invention can, for example, be attenuated by serial passage in cell culture. If a North American PRRS virus of the present invention is for use in an animal other than a porcine animal, or if it has been genetically modified as described herein so that it is unable to produce PRRS in a porcine animal, then it is not necessary to treat the virus as described in the preceding paragraph prior to using it in a vaccine. F. Vaccines and Uses Thereof: The present invention also provides vaccines comprising North American PRRS viruses, including genetically modified North American PRRS viruses disabled in their ability to produce PRRS in a swine animal as described herein; infectious RNA molecules and plasmids encoding such North American PRRS viruses as described herein; and viral vectors encoding such North American PRRS viruses and isolated RNA molecules as described herein. The invention also provides methods for protecting animals from infection comprising vaccination with such vaccines. In a preferred embodiment, the subject invention provides a vaccine comprising a genetically modified North American PRRS virus comprising one or more heterologous antigenic epitopes as described herein, an infectious RNA molecule encoding such a genetically modified North American PRRS virus, or a plasmid as described herein encoding such a genetically modified North American PRRS virus, in an amount effective to elicit an immunoprotective response against infection by the pathogen or pathogens from which the heterologous antigenic epitope(s) are derived, and a carrier acceptable for pharmaceutical or veterinary use. Such vaccines can be used to protect from infection a mammal or a bird capable of being pathogenically infected by the pathogen or pathogens from which the heterologous antigenic epitope(s) are derived. Accordingly, the subject invention also provides a method for protecting a mammal or a bird from infection by a pathogen, which comprises vaccinating the mammal or bird with an amount of the vaccine described in the preceding paragraph effective to elicit an immunoprotective response in the mammal or bird from infection by the pathogen. In a further preferred embodiment, the vaccine comprises a genetically modified North American PRRS virus, or an infectious RNA molecule or plasmid encoding such a genetically modified North American PRRS virus, comprising or encoding one or more heterologous antigenic epitopes from a swine pathogen other than a North American PRRS virus These vaccines are useful for protecting swine from infection by the swine pathogen or pathogens from which the heterologous antigenic epitope(s) are derived. If such a vaccine comprises the genetically modified North American PRRS virus, the genetic modification of the North American PRRS virus preferably preferably renders the virus unable to cause PRRS in swine. In another preferred embodiment, the genetically modified North American PRRS virus in the vaccine is able to elicit an immunoprotective response against infection by a PRRS virus, thus providing a dual-vaccine for swine, protecting swine from infection by the swine pathogen or pathogens from which the heterologous antigenic epitope(s) are derived as well as from infection by a PRRS virus. If the vaccine comprises an infectious RNA molecule or a plasmid encoding a genetically-modified North American PRRS virus comprising one or more heterologous antigenic epitopes from another swine pathogen, then the sequence encoding the infectious RNA molecule encoding the genetically modified PRRS virus preferably comprises one or more further mutations that genetically disable the encoded North American PRRS virus so that it is unable to cause PRRS. In another preferred embodiment, the encoded genetically modified, disabled North American PRRS virus is able to elicit an immunoprotective response against a PRRS infection in a swine animal, thus providing a dual-vaccine for swine, able to protect swine from infection by the swine pathogen or pathogens from which the heterologous antigenic epitope(s) are derived as well as from infection by a PRRS virus. All of these vaccines also further comprise a carrier acceptable for veterinary use. The present invention further provides a method for protecting a porcine animal from infection by a swine pathogen or pathogens other than a North American PRRS virus and, optionally, for simultaneously protecting the swine animal from infection by a PRRS virus, which comprises vaccinating the animal with an amount of a vaccine described in the preceding paragraph effective to elicit an immunoprotective response in the swine animal from an infection by the swine pathogen or pathogens and optionally, by a PRRS virus. Vaccines of the present invention can be formulated following accepted convention to include acceptable carriers for animals, including humans (if applicable), such as standard buffers, stabilizers, diluents, preservatives, and/or solubilizers, and can also be formulated to facilitate sustained release. Diluents include water, saline, dextrose, ethanol, glycerol, and the like. Additives for isotonicity include sodium chloride, dextrose, mannitol, sorbitol, and lactose, among others. Stabilizers include albumin, among others. Other suitable vaccine vehides and additives, including those that are particularly useful in formulating modified live vaccines, are known or will be apparent to those skilled in the art. See, e.g., Remington's Pharmaceutical Science, 18th ed., 1990, Mack Publishing, which is incorporated herein by reference. Vaccines of the present invention can further comprise one or more additional immunomodulatory components such as, e.g., an adjuvant or cytokine, among others. Non-limiting examples of adjuvants that can be used in the vaccine of the present invention include the RIBI adjuvant system (Ribi Inc., Hamilton, Mont.), alum, mineral gels such as aluminum hydroxide gel, oil-in-water emulsions, water-in-oil emulsions such as, e.g., Freund's complete and incomplete adjuvants, Block copolymer (CytRx, Atlanta Ga.), QS-21 (Cambridge Biotech Inc., Cambridge Mass.), SAF-M (Chiron, Emeryville Calif.), AMPHIGEN.RTM. adjuvant, saponin, Quil A or other saponin fraction, monophosphoryl lipid A, and Avridine lipid-amine adjuvant. Non-limiting examples of oil-in-water emulsions useful in the vaccine of the invention include modified SEAM62 and SEAM 1/2 formulations. Modified SEAM62 is an oil-in-water emulsion containing 5% (v/v) squalene (Sigma), 1% (v/v) SPAN.RTM. 85 detergent (ICI Surfactants), 0.7% (v/v) TWEEN.RTM. 80 detergent (ICI Surfactants), 2.5% (v/v) ethanol, 200 pg/ml Quil A, 100 .mu.g/ml cholesterol, and 0.5% (v/v) lecithin. Modified SEAM 1/2 is an oil-in-water emulsion comprising 5% (v/v) squalene, 1% (v/v) SPAN.RTM. 85 detergent, 0.7% (v/v) Tween 80 detergent, 2.5% (v/v) ethanol, 100 .mu.g/ml Quil A, and 50 .mu.g/ml cholesterol. Other immunomodulatory agents that can be included in the vaccine include, e.g., one or more interleukins, interferons, or other known cytokines. Vaccines of the present invention can optionally be formulated for sustained release of the virus, infectious RNA molecule, plasmid, or viral vector of the present invention. Examples of such sustained release formulations include virus, infectious RNA molecule, plasmid, or viral vector in combination with composites of biocompatible polymers, such as, e.g., poly(lactic acid), poly(lactic-co-glycolic acid), methylcellulose, hyaluronic acid, collagen and the like. The structure, selection and use of degradable polymers in drug delivery vehicles have been reviewed in several publications, including A. Domb et al., 1992, Polymers for Advanced Technologies 3: 279-292, which is incorporated herein by reference. Additional guidance in selecting and using polymers in pharmaceutical formulations can be found in texts known in the art, for example M. Chasin and R. Langer (eds), 1990, "Biodegradable Polymers as Drug Delivery Systems" in: Drugs and the Pharmaceutical Sciences, Vol. 45, M. Dekker, N.Y., which is also incorporated herein by reference. Alternatively, or additionally, the virus, plasmid, or viral vector can be microencapsulated to improve administration and efficacy. Methods for microencapsulating antigens are well-known in the art, and include techniques described, e.g., in U.S. Pat. No. 3,137,631; U.S. Pat. No. 3,959,457; U.S. Pat. No. 4,205,060; U.S. Pat. No. 4,606,940; U.S. Pat. No. 4,744,933; U.S. Pat. No. 5,132,117; and International Patent Publication WO 95/28227, all of which are incorporated herein by reference. Liposomes can also be used to provide for the sustained release of virus, plasmid, or viral vector. Details concerning how to make and use liposomal formulations can be found in, among other places, U.S. Pat. No. 4,016,100; U.S. Pat. No. 4,452,747; U.S. Pat. No. 4,921,706; U.S. Pat. No. 4,927,637; U.S. Pat. No. 4,944,948; U.S. Pat. No. 5,008,050; and U.S. Pat. No. 5,009,956, all of which are incorporated herein by reference. An effective amount of any of the above-described vaccines can be determined by conventional means, starting with a low dose of virus, plasmid or viral vector, and then increasing the dosage while monitoring the effects. An effective amount may be obtained after a single administration of a vaccine or after multiple administrations of a vaccine. Known factors can be taken into consideration when determining an optimal dose per animal. These include the species, size, age and general condition of the animal, the presence of other drugs in the animal, and the like. The actual dosage is preferably chosen after consideration of the results from other animal studies. One method of detecting whether an adequate immune response has been achieved is to determine seroconversion and antibody titer in the animal after vaccination. The timing of vaccination and the number of boosters, if any, will preferably be determined by a doctor or veterinarian based on analysis of all relevant factors, some of which are described above. The effective dose amount of virus, infectious RNA molecule, plasmid, or viral vector, of the present invention can be determined using known techniques, taking into account factors that can be determined by one of ordinary skill in the art such as the weight of the animal to be vaccinated. The dose amount of virus of the present invention in a vaccine of the present invention preferably ranges from about 10.sup.1 to about 10.sup.9 pfu (plaque forming units), more preferably from about 10.sup.2 to about 10.sup.8 pfu, and most preferably from about 10.sup.3 to about 10.sup.7 pfu. The dose amount of a plasmid of the present invention in a vaccine of the present invention preferably ranges from about 0.1 .mu.g to about 100 mg, more preferably from about 1.mu.g to about 10 mg, even more preferably from about 10.mu.g to about 1 mg. The dose amount of an infectious RNA molecule of the present invention in a vaccine of the present invention preferably ranges from about 0.1 to about 100 mg, more preferably from about 1 .mu.g to about 10 mg, even more preferably from about 10 .mu.g to about 1 mg. The dose amount of a viral vector of the present invention in a vaccine of the present invention preferably ranges from about 10.sup.1 pfu to about 10.sup.9 pfu, more preferably from about 10.sup.2 pfu to about 10.sup.8 pfu, and even more preferably from about 10.sup.3 to about 10.sup.7 pfu. A suitable dosage size ranges from about 0.5 ml to about 10 ml, and more preferably from about 1 ml to about 5 ml. The present invention further provides a method of preparing a vaccine comprising a North American PRRS virus, infectious RNA molecule, plasmid, or viral vector described herein, which method comprises combining an effective amount of one of the North American PRRS virus, infectious RNA molecule, plasmid, or viral vector of the present invention, with a carrier acceptable for pharmaceutical or veterinary use. It is to be understood that the term "North American PRRS viruses of the present invention" and like terms, unless otherwise indicated, include any of the genetically modified North American PRRS viruses described herein as well as the unmodified North American PRRS virus described herein encoded by SEQ ID NO:1 or a sequence homologous thereto. G. Isolated Polynucleotide Molecules and Transfected Host Cells Encoding North American PRRS Virus Peptides, and Methods for Making Functional North American PRRS Virions: The present invention also provides an isolated polynucleotide molecule comprising one or more nucleotide sequences that encode a peptide encoded by a North American PRRS virus, wherein the genome sequence of said North American PRRS virus is the same as or homologous to an RNA molecule corresponding to SEQ ID NO:1. As used herein, terms such as "North American PRRS virus peptide" mean a peptide that is expressed by a North American PRRS virus. Such a peptide can be, but is not necessarily, specific to North American PRRS viruses. In a preferred embodiment, an isolated polynucleotide molecule of the present invention encoding a North American PRRS virus peptide comprises a sequence or sequences independently selected from SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:8, SEQ ID NO:9, and a sequence homologous to any of said sequences. Such isolated polynucleotide molecules are useful in plasmids or for preparing viral vectors for transfecting a suitable host cell to create a "helper" cell comprising one or more nucleotide sequences that encode a peptide encoded by a North American PRRS virus, wherein the genome sequence of said North American PRRS virus is the same as or homologous to an RNA sequence corresponding to SEQ ID NO:1, which helper cell is useful in the preparation of functional virions of genetically modified North American PRRS viruses of the present invention, which viruses have been genetically modified as described above so that they are missing from their RNA genome the sequence(s) encoding the peptide or peptides encoded by the helper cell. Accordingly, the subject invention also includes plasmids and viral vectors comprising one or more nucleotide sequences encoding a peptide encoded by a North American PRRS virus, wherein the genome sequence of said North American PRRS virus is the same as or homologous to an RNA sequence corresponding to SEQ ID NO:1. Such plasmids of the invention can be based on those plasmids described above useful for preparing plasmids comprising a nucleotide sequence encoding a North American PRRS virus. Such viral vectors of the invention can be based on, for example, retrovirus vectors or adeno-associated viral vectors. These plasmids and viral vectors are useful for preparing the helper cells described in the preceding paragraph. The present invention also thus provides a helper cell, i.e., a transfected host cell comprising one or more nucleotide sequences that encode a peptide encoded by a North American PRRS virus, wherein the genome sequence of said North American PRRS virus is the same as or homologous to an RNA sequence corresponding to SEQ ID NO:1. In preferred embodiments, the transfected host cell comprises a nucleotide sequence or sequences independently selected from SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:8, SEQ ID NO:9 and a sequence homologous to any of said sequences. These helper cells are useful as described above for providing peptides for completing infectious RNA molecules deficient in sequences encoding the peptide(s) encoded by the helper cell so that functional virions of a genetically modified North American PRRS virus can be generated by the cell. Suitable cells for this aspect of the invention include the cells described above which are suitable for expressing North American PRRS viruses, such as the porcine alveolar macrophage cells and the MARC-145 cells. However, practically any mammalian or avian cell can be used. As discussed above, if one wishes to obtain a transfected host cell capable of reinfection by PRRS virions, and thus able to generate multiple generations of functional genetically modified North American PRRS virions, porcine alveolar macrophage cells and MARC-145 cells are preferred. The subject invention thus further provides a method for generating a functional virion of a genetically modified North American PRRS virus encoded by an RNA sequence corresponding to SEQ ID NO:1 or a sequence homologous thereto comprising one or more mutations that disable one or more peptide coding sequences, which method comprises transfecting a helper cell as described in the preceding paragraph with an infectious RNA molecule, plasmid, or viral vector encoding the genetically modified North American PRRS virus, which helper cell comprises a nucleotide sequence or sequences encoding the North American PRRS virus peptide or peptides of the disabled peptide coding sequence(s) of the genetically modified North American PRRS virus. The subject invention further provides a method for generating a functional virion of a genetically modified North American PRRS virus encoded by an RNA sequence corresponding to SEQ ID NO:1 or a sequence homologous thereto comprising one or more mutations in one or more peptide coding sequences, which method comprises transfecting a suitable cell with both an infectious RNA molecule, plasmid, or viral vector encoding the genetically modified North American PRRS virus and a helper virus that expresses in the cell the North American PRRS virus peptide or peptides of the mutated peptide coding sequence or sequences of the modified North American PRRS virus, and separating genetically modified North American PRRS virions from the helper virus. Methods of separation are known in the art and include use of conditional lethal helper viruses that distinguish (kill) the helper virus under certain conditions wherein the expressed North American PRRS virus is not killed. Other known methods of separation include physical methods of separation subsequent to expression of both helper virions and North American PRRS virions, for example by density gradient centrifugation. Suitable cells for this method of the invention include the cells capable of infection by a PRRS virus, such as porcine alveolar macrophage cells and MARC-145 cells. Other cells which are able to be infected by a PRRS virus are described previously in this application, for example the MA-104 cell line or other cell lines derived from the MA-104 cell line. An example of a helper virus is a PRRS virus, preferably a North American PRRS virus isolate, such as P129. Any virus, either wild-type, isolated, or recombinant, that expresses PRRS peptides can be used as a helper virus. Infectious RNA molecules, plasmids, and viral vectors that encode a genetically modified North American PRRS virus useful in either of the above methods for generating a functional virion of a genetically modified North American PRRS virus are those infectious RNA molecules, plasmids, and viral vectors described in this application. H. Immunoprotective Genetically Modified Nidovirales Viruses and Vaccines Comprising Them: The subject invention further provides a genetically modified Nidovirales virus that is capable of eliciting an immunoprotective response in a mammal or a bird vaccinated therewith, which genetically modified Nidovirales virus is prepared by obtaining an isolated polynucleotide molecule comprising a DNA sequence encoding an infectious RNA molecule encoding a wild-type Nidovirales virus, genetically mutating the DNA sequence encoding the infectious RNA molecule encoding the wild-type Nidovirales virus so as to obtain an isolated polynucleotide molecule comprising a DNA sequence encoding an infectious RNA molecule encoding a genetically modified Nidovirales virus which virus is able to elicit an effective immunoprotective response against infection by the wild-type Nidovirales virus in a mammal or a bird, and expressing the genetically modified Nidovirales virus from the isolated polynucleotide molecule so obtained. The subject invention further provides a method for preparing a genetically modified Nidovirales virus that is capable of eliciting an immunoprotective response in a mammal or a bird vaccinated therewith, which method comprises obtaining an isolated polynucleotide molecule comprising a DNA sequence encoding an infectious RNA molecule encoding a wild-type Nidovirales virus, genetically mutating the DNA sequence encoding the infectious RNA molecule encoding the wild-type Nidovirales virus so as to obtain an isolated polynucleotide molecule comprising a DNA sequence encoding an infectious RNA molecule encoding a genetically modified Nidovirales virus which virus is able to elicit an effective immunoprotective response against infection by the wild-type Nidovirales virus in a mammal or a bird, and expressing the genetically modified Nidovirales virus from the isolated polynucleotide molecule so obtained. DNA sequences encoding infectious RNA molecule encoding a Nidovirales virus include the DNA sequences which are the same as or homologous to SEQ ID NO:1 described herein, which encode a North American PRRS virus. DNA sequences encoding infectious RNA molecules encoding Nidovirales viruses other than a North American PRRS virus are known in the art and can be obtained from known sources, for example the genetic databases cited above. Other examples of some DNA sequences encoding infectious RNA molecules encoding Nidovirales viruses which can be used in the subject invention include the DNA sequence encoding European PRRS virus described in Meulenberg, J. J. M. et al., 1993, supra; the DNA sequence encoding equine arteritis virus described in van Dinten, L. D., et al., 1997, Proc. Natl. Acad. Sci. USA, 94(3):991-6; and the DNA sequence encoding equine arteritis virus described in den Boon, J. A., et al., 1991, J. Virol. 65(6):2910-20, all of which references are hereby incorporated in their entireties into the present application. Nidovirales viruses which can be genetically modified by means of the present invention can be any Nidovirales virus for which a coding DNA sequence is obtained. Some examples of Nidovirales viruses are described in previous sections of the present application. Once a DNA sequence encoding an infectious RNA molecule encoding a Nidovirales virus is obtained, isolated polynucleotide molecules, for example plasmids, comprising the DNA sequence encoding an infectious RNA molecule encoding a Nidovirales virus can be synthesized using recombinant techniques described above. The sequences encoding the infectious RNA molecule encoding a Nidovirales virus can be genetically mutated as described above for the North American PRRS virus so as to obtain Nidovirales viruses comprising genetic modifications such as an inability to cause disease in an animal infected therewith, inclusion of heterologous antigenic epitopes that are able to elicit an immunoprotective response in an animal, inclusion of heterologous antigenic epitopes that are detectable, and deletion of detectable antigenic epitopes. Such genetic modifications are described above. Genetic mutations so as to encode genetically modified Nidovirales viruses can occur in coding and/or noncoding regions of the DNA sequence encoding the infectious RNA molecule encoding the Nidovirales virus. In one embodiment, one or more genetic mutations occur in an ORF encoding a viral peptide of the Nidovirales virus. "Wild-type Nidovirales virus", as used herein, means a Nidovirales virus whose genome has not intentionally been genetically mutated, such as a Nidovirales virus occurring in nature and isolates thereof. The subject invention further provides a vaccine for protecting a mammal or a bird from infection by a Nidovirales virus, which comprises a genetically modified Nidovirales virus as described above in an amount effective to elicit an effective immunoprotective response against the wild-type Nidovirales virus in a mammal or a bird vaccinated therewith, and a carrier acceptable for pharmaceutical or veterinary use. These vaccines of the present invention can be formulated using the techniques for formulating vaccines described above. The following examples are provided to merely illustrate aspects of the subject invention. They are not intended, and should not be construed, to limit the invention set forth in the claims and more fully described herein |
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