Method and means for purifying retroviral vectors

1. A producer cell which expresses a tagging protein at the cell surface, such that retroviral vectors that bud from the producer cell are tagged with the tagging protein, wherein the tagging protein comprises:

i) a binding domain that comprises amino acids 2-15 of SEQ ID NO: 2 and binds to streptavidin,

ii) a membrane targeting domain that comprises a transmembrane domain and an endodomain; and

iii) a spacer connecting the binding domain to the membrane targeting domain, wherein the spacer comprises a CD8 stalk or an equivalent thereof, wherein the tagging protein has binding affinity for streptavidin that facilitates purification of the tagged retroviral vectors from cellular supernatant with a capture moiety comprising streptavidin; and wherein the binding domain binds streptavidin with a lower affinity than biotin, so that biotin may be used to elute retroviral vectors produced by the cell and captured with the capture moiety.

2. The producer cell according to claim 1 , wherein the spacer comprises a CD8 stalk or an equivalent thereof.

3. The producer cell according to claim 1 , wherein the tagging protein also comprises a cleavage site, cleavage at which releases retroviral vectors bound to a capture moiety that comprises streptavidin.

4. The producer cell according to claim 3 , wherein the tagging protein comprises a linker between the binding domain and the spacer, which linker is or comprises the cleavage site.

5. The producer cell according to claim 3 , wherein the cleavage site is a thrombin cleavage site.

6. The producer cell according to claim 1 , wherein the transmembrane domain and endodomain are the same as the transmembrane and endodomain of a retroviral vector envelope protein.

7. The producer cell according to claim 6 , which comprises the transmembrane domain and endodomain from RD-PRO envelope protein as set out in amino acids 509-565 of SEQ ID NO: 39.

8. A retroviral vector which comprises a producer cell-derived tagging protein, said tagging protein comprising:

i) a binding domain that comprises amino acids 2-15 of SEQ ID NO: 2 and binds to streptavidin,

ii) a membrane targeting domain that comprises a transmembrane domain and an endodomain; and

iii) a spacer connecting the binding domain to the membrane targeting domain, wherein the spacer comprises a CD8 stalk or an equivalent thereof, and wherein the binding domain binds streptavidin with a lower affinity than biotin, so that biotin may be used to elute the retroviral vector following capture with a capture moiety comprising streptavidin.

9. The producer cell according to claim 1 which comprises genes encoding retroviral Gag, Pol and Env proteins stably integrated within the cell genome.

10. A producer cell which expresses a tagging protein at the cell surface, such that retroviral vectors produced by the cell are tagged with the tagging protein, wherein the tagging protein comprises:

i) a binding domain that comprises two or more streptavidin-binding epitopes, comprising at least one linker between the streptavidin-binding epitopes, each of said streptavidin-binding epitopes comprising amino acids 2-15 of SEQ ID NO: 2, and that binds streptavidin; and

ii) a membrane targeting domain comprising a glycosylphosphatidylinositol (GPI) anchor;

wherein the tagging protein has binding affinity for streptavidin that facilitates purification of the tagged retroviral vectors from cellular supernatant with a capture moiety comprising streptavidin; and

wherein the binding domain binds streptavidin with a lower affinity than biotin, so that biotin may be used to elute retroviral vectors produced by the cell and captured with the capture moiety.

11. The producer cell according to claim 10 , wherein the tagging protein further comprises a linker between the binding domain and the GPI anchor, which linker is or comprises a cleavage site.

12. The producer cell according to claim 10 , wherein the tagging protein further comprises a linker between the binding domain and the GPI anchor, which linker is or comprises a cleavage site.

13. A producer cell according to claim 10 , wherein the binding domain comprises the amino acid sequence shown as SEQ ID NO: 5.

14. The producer cell according to claim 10 , which comprises genes encoding retroviral Gag, Pol and Env proteins stably integrated within the cell genome.

15. A retroviral vector which comprises a producer cell-derived tagging protein, said tagging protein comprising:

i) a binding domain that comprises two or more streptavidin-binding epitopes, each of said streptavidin-binding epitopes comprising amino acids 2-15 of SEQ ID NO: 2, and further comprises at least one linker between the streptavidin-binding epitopes, wherein the binding domain binds streptavidin; and

ii) a membrane targeting domain comprising a glycosylphosphatidylinositol (GPI) anchor;

wherein the tagging protein has binding affinity for streptavidin that facilitates purification of the tagged retroviral vectors from cellular supernatant with a capture moiety comprising streptavidin; and

wherein the binding domain binds streptavidin with a lower affinity than biotin, so that biotin may be used to elute retroviral vectors captured with a capture moiety comprising streptavidin.

16. The producer cell according to claim 8 , wherein the spacer comprises a CD 8 stalk.

17. The retroviral vector according to claim 10 , wherein the tagging protein further comprises a linker between the binding domain and the GPI anchor, which linker is or comprises a cleavage site.

18. A method for making a producer cell according to claim 1 , which comprises the step of introducing a nucleic acid which encodes the tagging protein into a cell, such that the tagging protein is expressed at the cell surface.

19. A method for purifying a retroviral vector according to claim 8 which comprises a step of capturing the retroviral vector using a capture moiety that comprises streptavidin.

20. The method according to claim 19 , which also comprises a step of eluting the retroviral vector from the capture moiety with biotin.