Retrovirus packaging cell

1. A retrovirus packaging cell which expresses a temperature sensitive RNA-dependent-RNA polymerase (RdRp), wherein the temperature sensitive RdRp comprises the sequence shown as SEQ ID NO: 2 or a variant thereof.

2. A retrovirus packaging cell according to claim 1 wherein the RdRp is encoded by a nucleic acid sequence which is stably integrated into the genome of the packaging cell.

3. A retrovirus packaging cell according to claim 1 wherein the RdRp is an alphavirus RdRp.

4. A retrovirus packaging cell according to claim 1 wherein the RdRp is a Sindbis virus RdRp.

5. A retrovirus producer cell which is a packaging cell according to claim 1 further comprising a retroviral nucleic acid transfer vector which comprises at least one RdRp promoter element which enables the retroviral nucleic acid transfer vector to be replicated by the RdRp.

6. A retrovirus producer cell according to claim 5 wherein the retroviral nucleic acid transfer vector comprises the following structure:

5′ P1-rPSG- 5 dLTR-P2-NOI- 3 dLTR-PSG 3′

in which

P1 is a eukaryotic promoter which drives transcription

rPSG is a sequence which acts as a negative sense sub-genomic promoter for the RdRp when at the 3′ end of the negative sense strand;

5 dLTR is a 5′ retrovirus long terminal repeat where the U3 region is non-functional;

P2 is a eukaryotic promoter which drives expression of the NOI;

NOI is a nucleotide sequence of interest;

3 dLTR is a self-inactivating 3′ retrovirus long terminal repeat with a non-functional U3;

PSG is a sequence which acts as a sub-genomic promoter for RdRp as a positive sense.

7. A retrovirus packaging cell according to claim 1 which comprises at least one retroviral helper element nucleotide sequence comprising the following structure:

5 ′P-rPSG-RetroP-PSG 3′   i)

in which

P is a eukaryotic promoter;

rPSG is a sequence which acts as a negative sense sub-genomic promoter for the RdRp when at the 3′ end of the negative sense strand;

RetroP is nucleic acid sequence which encodes a retrovirus protein; and

PSG is a sequence which acts as a sub-genomic promoter for RdRp as positive sense; or

5′ P-rPSG-rRetroP-PSG 3′   ii)

in which

P is a eukaryotic promoter;

rPSG is a sequence which acts as a negative sense sub-genomic promoter for the RdRp when at the 3′ end of the negative sense strand;

rRetroP is nucleic acid sequence which encodes a retrovirus protein in reverse orientation;

PSG is a sequence which acts as a sub-genomic promoter for RdRp as a positive sense; or

5′ P-MG-STOP-iPSG-RetroP-PSG 3′   iii)

in which

P is a eukaryotic promoter;

MG is an open reading frame of a non-toxic marker gene;

STOP is a stop signal;

iPSG is a reverse-orientation RdRp sub-genomic promoter which works internally;

RetroP is nucleic acid sequence which encodes a retrovirus protein; and

PSG is a sequence which acts as a sub-genomic promoter for RdRp as a positive sense;

or

5 ′P-rPSG-MG-STOP-iPSG-RetroP-PSG 3′

in which

P is a eukaryotic promoter;

rPSG is a sequence which acts as a negative sense sub-genomic promoter for the RdRp when at the 3′ end of the negative sense strand

MG is an open reading frame of a non-toxic marker gene

STOP is a stop signal or a series of stop signals

iPSG is a reverse-orientation RdRp sub-genomic promoter which works internally

RetroP is nucleic acid sequence which encodes a retrovirus protein; and

PSG is a sequence which acts as a sub-genomic promoter for RdRp as a positive sense.

8. A retrovirus packaging cell or producer cell according to claim 1 which is a lentiviral packaging cell or a producer cell.

9. A retroviral nucleic acid transfer vector which comprises at least one RdRp promoter element which enables the retroviral nucleic acid transfer vector to be replicated by the RdRp, wherein the RdRp comprises the sequence shown as SEQ ID NO: 2 or a variant thereof.

10. A retroviral nucleic acid transfer vector of claim 9 comprising the following structure:

5 P1-rPSG- 5 dLTR-P2-NOI- 3 dLTR-PSG 3′

in which

P1 is a eukaryotic promoter which drives transcription

rPSG is a sequence which acts as a negative sense sub-genomic promoter for the RdRp when at the 3′ end of the negative sense strand;

5 dLTR is a 5′ retrovirus long terminal repeat where the U3 region is non-functional;

P2 is a eukaryotic promoter which drives expression of the NOI;

NOI is a nucleotide sequence of interest;

3 dLTR is a self-inactivating 3′ retrovirus long terminal repeat with a non-functional U3;

PSG is a sequence which acts as a sub-genomic promoter for RdRp as a positive sense.

11. A retroviral helper element nucleotide sequence comprising at least one RdRp promoter element which enables a nucleic acid sequence which encodes a retroviral protein to be replicated by the RdRp, wherein the RdRp comprises the sequence shown as SEQ ID NO: 2 or a variant thereof.

12. A plasmid comprising a retroviral nucleic acid transfer vector as defined in claim 9 .

13. A method for making a packaging cell which comprises the step of introducing a nucleic acid sequence encoding a RdRp as defined in claim 1 into a cell, such that the cell expresses the RdRp.

14. A method for making a producer cell which comprises the step of introducing retroviral nucleic acid transfer vector as defined in claim 5 into a retrovirus packaging cell.

15. A method for making a retrovirus vector which comprises the step of culturing a producer cell as defined in claim 14 , and isolating the retrovirus vector.

16. A method according to claim 15 which comprises culturing the producer cell at a relatively low temperature and isolating the retrovirus vector.

17. A method according to claim 15 which comprises expanding the population of producer cells by culturing the cells at a relatively high temperature and subsequently culturing the producer cells at a relatively low temperature and isolating the retrovirus vector.

18. A method for expanding a population of packaging cells or producer cells according to claim 1 by culturing the cells at a relatively high temperature.

19. A method for increasing the production of retroviral proteins in a packaging cell or producer cell according to claim 1 by decreasing the temperature of the culture medium.