Construct for producing empty virus capsids

1. A construct which, when expressed in a eukaryotic host cell, produces empty picornavirus capsids, the construct comprising: (i) a nucleic acid encoding a picornavirus capsid precursor protein; (ii) a nucleic acid encoding protease 3C, capable of cleaving the picornavirus capsid precursor protein, wherein the protease includes a mutation at Cys142 which reduces its capsid precursor protein cleaving activity; and (iii) a control element between the nucleic acid encoding the picornavirus capsid precursor protein and the nucleic acid encoding protease 3C which reduces expression of the protease compared to expression if the control element were absent such that, when the construct is present in the eukaryotic host cell, the control element causes the protease to be expressed at a level sufficient to cleave the picornavirus capsid precursor protein, but not sufficient to induce significant toxicity in the host cell, wherein the control element is a retrovirus frameshift site.

2. A construct according to claim 1 , wherein the control element is the HIV-1 frameshift site.

3. A construct according to claim 1 , wherein the control element causes a frameshift in about 5% of the mRNAs translated.

4. A construct according to claim 1 wherein the protease with the mutation has approximately 3-fold lower capsid precursor protein cleaving activity than wild-type protease 3C.

5. A construct according to claim 1 which, when expressed in a host cell, produces empty Foot and Mouth Disease Virus (FMDV) capsids.

6. A construct according to claim 5 , wherein the picornavirus capsid precursor protein is P1.

7. A vector comprising the construct of claim 1 .

8. A vector according to claim 7 , which is a baculovirus transfer vector, a DNA vector, a plasmid vector or a viral vector.

9. An isolated host cell comprising the construct of claim 1 .

10. An isolated host cell which produces a vector that comprises the construct of claim 1 , said host cell comprising nucleic acid encoding virion proteins and a plasmid comprising the construct of claim 1 .

11. An isolated host cell according to claim 9 , which is capable of producing empty picornavirus capsids.

12. An isolated host cell according to claim 10 which is an insect cell or a mammalian cell.

13. A method for producing empty picornavirus capsids which comprises the following steps:

(i) expressing in a eukaryotic host cell a construct comprising:

(a) a nucleic acid encoding a picornavirus capsid precursor protein;

(b) a nucleic acid encoding a protease capable of cleaving the picornavirus capsid precursor protein, wherein the protease is a mutant 3C protease that includes a mutation at Cys142 which reduces its capsid precursor protein cleaving activity; and

(c) a control element between the nucleic acid encoding the picornavirus capsid precursor protein and the nucleic acid encoding protease 3C which reduces expression of the protease compared to expression if the control element were absent such that, when the construct is present in the eukaryotic host cell, the control element causes the protease to be expressed at a level sufficient to cleave the picornavirus capsid precursor protein, but not sufficient to induce significant toxicity in the host cell, wherein the control element is a retrovirus frameshift site; and

(ii) harvesting empty picornavirus capsids produced by the eukaryotic host cell.

14. The method according to claim 13 further comprising incorporating the empty picornavirus capsids in a vaccine.

15. A method of inducing an immune response in a subject, which comprises the step of expressing the construct of claim 1 in the subject such that empty picornavirus capsids are produced in vivo.

16. A method of inducing an immune response in a subject according to claim 15 , wherein the construct is administered to the subject by DNA vaccination or using a viral vector.

17. A eukaryotic expression construct which comprises:

(i) a nucleic acid encoding a picornavirus capsid precursor protein;

(ii) a nucleic acid encoding protease 3C, capable of cleaving the picornavirus capsid precursor protein, wherein the encoded protease includes a mutation at Cys142 which reduces its capsid precursor protein cleaving activity, and

(iii) a retrovirus frameshift site between the nucleic acid encoding the picornavirus capsid precursor protein and the nucleic acid encoding protease 3C such that, when the construct is translated: if the ribosome does not undergo a frameshift it produces a truncated product which comprises the picornavirus capsid precursor protein but not the protease; but if the ribosome does undergo a frameshift, both the picornavirus capsid precursor protein and the protease are translated.