RNA expression cassette and cells for making alphavirus particles
Strategies for increasing the productivity of alphavirus packaging cell lines and of reducing the possibility that replication competent virus may be generated during large scale production of recombinant alphavirus particles.
1. An RNA expression cassette comprising a first and second transcription unit, wherein:
(a) the first transcription unit comprises a first alphavirus subgenomic promoter operably linked to a first coding sequence which encodes a mutant capsid protein of a first alphavirus, but not a glycoprotein of the first alphavirus, wherein the mutant capsid protein has reduced autoproteolytic activity;
(b) the second transcription unit comprises a second alphavirus subgenomic promoter operably linked to a second coding sequence which encodes non-structural proteins 1-4 of a second alphavirus,
wherein the expression cassette enhances production of replicon particles while minimizing generation of replication competent viral particles (RCVs) when used in a suitable packaging cell line.
2. The expression cassette of claim 1 wherein the first transcription unit is 5′ to the second transcription unit.
3. The expression cassette of claim 1 wherein the first transcription unit is 3′ to the second transcription unit.
4. The expression cassette of claim 1 wherein said first coding sequence encodes a hybrid capsid protein.
5. The expression cassette of claim 1 wherein the mutation is selected from the group consisting of His141Ala, Asp147Ala, Asp163Ala, Ser215Ala, ΔHis141, ΔAsp147, ΔAsp163, ΔSer215, and ΔTrp264 and combinations thereof, numbered according to SEQ ID NO:1.
6. The expression cassette of claim 1 wherein at least one of the first and second alphaviruses is a Sindbis virus.
7. The expression cassette of claim 1 wherein at least one of the first and second alphaviruses is a Venezuelan equine encephalitis (VEE) virus.
8. The expression cassette of claim 1 wherein at least one of the first and second subgenomic promoters is a VEE subgenomic promoter, a Sindbis virus subgenomic promoter, an Eastern equine encephalitic (EEE) subgenomic promoter, or a Semliki Forest virus subgenomic promoter.
9. The expression cassette of claim 1 further comprising a selectable marker.
10. The expression cassette of claim 1 further comprising an internal ribosome entry site (IRES).
11. The expression cassette of claim 1 wherein the first coding sequence comprises a sequence encoding SEQ ID NO:2.
12. The expression cassette of claim 1 wherein the second transcription unit comprises a mutation in at least one of R331 or R332 in Nsp4 numbered according to SEQ ID NO:9.
13. An isolated host cell comprising a first RNA expression cassette, wherein the first expression cassette comprises:
(a) a first transcription unit comprising a first alphavirus subgenomic promoter operably linked to a first coding sequence which encodes glycoprotein or a capsid protein, but not both, of a first alphavirus; and
(b) a second transcription unit comprising a second alphavirus subgenomic promoter operably linked to a second coding sequence which encodes non-structural proteins 1-4 of a second alphavirus,
wherein the host cell is adapted for large-scale production of replicon particles and has enhanced production of replicon particles while minimizing generation of replication competent viral particles (RCVS).