Mutant porcine reproductive and respiratory syndrome virus

1. An isolated polynucleotide that comprises:

(a) a DNA molecule or its complement, that encodes a genetically modified PRRS virus, wherein the encoding sequence of the E protein thereof has been mutated, such that the conserved cysteine residue that corresponds to residue 54 in North American genotype viruses and residue 51 in European genotype viruses, is either deleted, or replaced with a non-cysteine residue, wherein the virus is attenuated as a result of said modification, and with the proviso that the non-cysteine residue is not tyrosine; or;

(b) an infectious RNA molecule encoding the genetically modified PRRS virus of (a).

2. The isolated polynucleotide of claim 1 , wherein the original codon corresponding to said conserved cysteine residue of said E protein has been deleted, or mutated to provide a resultant codon for serine or alanine, said polynucleotide optionally further comprising an additional codon mutation within two codons of the codon for the conserved cysteine to minimize (1) the probability of reversion of said resultant codon back to a cysteine codon, or (2) the mutation of a codon adjacent to said original cysteine codon to a further cysteine codon.

3. The polynucleotide according to claim 2 wherein:

(a) said serine codon differs by at least 2 nucleotides from any cysteine codon; or

(b) said alanine codon differs by 3 nucleotides from any cysteine codon; or

(c) an adjacent codon, that is one or two codons upstream or downstream from said conserved cysteine codon, is mutated such that said adjacent codon still encodes the same amino acid, but at least two nucleotides thereof would need to be changed to before said adjacent codon would encode cysteine; or

(d) an adjacent codon, that is one or two codons upstream or downstream from said conserved cysteine codon, is mutated such that said adjacent codon encodes a different amino acid, and at least two nucleotides thereof would need to be changed before said adjacent codon would encode cysteine.

4. The polynucleotide of claim 1 wherein the codon for the conserved cysteine has been replaced by a codon for serine, threonine, or methionine.

5. A genetically modified PRRS virus encoded by any of the polynucleotides of claim 1 .

6. A genetically modified PRRS virus encoded by any of the polynucleotides of claims 2 .

7. A genetically modified PRRS virus encoded by any of the polynucleotides of claim 3 .

8. A genetically modified PRRS virus encoded by any of the polynucleotides of claim 4 .

9. The virus of claim 5 wherein the modification results in a small plaque phenotype virus.

10. A method of inducing an immune response against PRRS virus in a porcine animal, which comprises administering an effective immunizing amount of the virus of claim 5 .

11. An immunogenic composition that comprises the polynucleotide of claim 1 , and a carrier acceptable for veterinary use.

12. An immunogenic composition that comprises the genetically modified PRRS virus of claim 5 , and a carrier acceptable for veterinary use.

13. A method for making a genetically modified North American PRRS virus, which method comprises: a) mutating a DNA sequence encoding an infectious RNA molecule which encodes a PRRS virus, to produce a genetically modified PRRS virus wherein the conserved cysteine in the E protein that corresponds to residue 54 in North American genotype viruses and residue 51 in European genotype viruses has been changed to a non-cysteine residue; and b) introducing the genetically modified North American PRRS virus into a host cell capable of supporting PRRS virus replication.

14. The method of claim 13 wherein the host cell capable of supporting PRRS replication is a MARC-145 cell.

15. The method of claim 13 wherein the host cell capable of supporting PRRS replication is comprised within a live porcine animal.

16. An isolated host cell transfected with the polynucleotide of claim 1 .

17. An isolated host cell infected with the virus of claim 5 .