Successful production of pseudotyped rAAV vectors using a modified baculovirus expression system

Abstract

Scalable production of rAAV vectors remains a major obstacle to the clinical application of this prototypical gene therapy vector. A recently developed baculovirus-based production protocol (M. Urabe et al., 2002, Hum. Gene Ther. 13, 1935-1943) found limited applications due to the system's design. Here we report a detailed analysis of the stability of the original baculovirus system components BacRep, BacVP, and transgene cassette-containing BacGFP. All of the baculovirus helpers analyzed were prone to passage-dependent loss-of-function deletions resulting in considerable decreases in rAAV titers. To alleviate the instability and to extend the baculovirus platform to other rAAV serotypes, we have modified both Rep- and Cap-encoding components of the original system. The modifications include a parvoviral phospholipase A2 domain swap allowing production of infectious rAAV8 vectors in vivo. Alternatively, an infectious rAAV8 (or rAAV5) vector incorporating the AAV2 VP1 capsid protein in a mosaic vector particle with AAV8 capsid proteins was produced using a novel baculovirus vector. In this vector, the level of AAV2 VP1 expression is controlled with a "riboswitch," a self-cleaving ribozyme controlled by toyocamycin in the "ON" mode. The redesigned baculovirus system improves our capacity for rAAV manufacturing by making this production platform more applicable to other existing serotypes.

FIG. 1

Western blot analysis of Rep proteins expressed in Sf9 cells by individual BacRep baculovirus helper plaque isolates (lanes 1 through 10). Isolate 5 (circled) was selected and propagated for the passage stability test (shown in Fig. 2).

FIG. 2

Western blot analysis of Rep proteins expressed in Sf9 cells by BacRep, BacRep52, or BacRep78 baculovirus helpers. Cells were infected with serially passaged baculovirus stocks (P1 through P5) at an m.o.i. of 5.

FIG. 3

Schematic representation of the baculovirus helper vectors. (A) BacRep78 and BacRep52 as two separate helpers; (B) VP1up domain swapping between AAV2 and AAV8 BacVP helpers; (C) toyocamycin-regulated VP1 AAV2 baculovirus helper cassette. The scissors illustrate the position of a self-cleaving HH Rz control element.

FIG. 4

Western blot analysis of Rep proteins expressed in Sf9 cells by BacRep, BacRep52, or BacRep78 baculovirus helpers individually or upon co-infection with other baculovirus helpers (m.o.i. of 5 each). Lane 1, positive control (a lysate from 293 cells transfected with pIM45 [21]; lanes 2 through 6 contain lysates from Sf9 cells infected with, lane 2, BacRep; lane 3, BacRep52; lane 4, BacRep78; lane 5, BacRep78 + BacRep52; lane 6, BacRep78 + BacRep52 + BacVP + BacGFP (the latter vector also contains a strong baculovirus p10 promoter driving the GFP gene inside the transgene cassette [15]).

FIG. 5

Passaging stability analysis of ITR-containing transgene cassette (BacGFP). (A) Analysis of rescued rAAV cassette. Sf9 cells were infected with BacGFP from consecutive passage stocks (m.o.i. of 5 each) in addition to BacRep (P2, m.o.i. of 5). Forty-eight hours postinfection, DNA was prepared by Hirt DNA extraction, resolved using a 1.2% agarose gel, transferred to a nylon filter, and hybridized with a ³²P-labeled GFP probe. (B) Analysis of rAAV2-GFP titers of vector stocks prepared using BacGFP P2 through P5 helpers. Sf9 cells were co-infected with BacVP and BacRep (P2, m.o.i. of 5 each). In addition, cells were co-infected with BacGFP at the indicated passages (m.o.i. 5 of each). Seventy-two hours postinfection, cells were harvested and rAAV infectious titers in crude cell lysates were calculated using GFP fluorescence assay using C12 cells co-infected with Ad5 (m.o.i. of 10) [18].

FIG. 6

Western blot analysis of AAV2 capsid proteins expressed in Sf9 cells by BacVP helper. Sf9 cells were infected with BacVP (m.o.i. of 5) of consecutive passages, as indicated. Seventy-two hours postinfection, cells were harvested and cell lysates were analyzed by Western blotting as described under Materials and Methods.

FIG. 7

Analysis of the capsid protein VP1 content and the respective VP1up phospholipase A2 activity in rAAV vector stocks produced in 293 cells vs Sf9 cells. (A) Silver stain of polyacrylamide gel analysis (10% PAAG) of purified rAAV stocks prepared in HEK 293 and Sf9 cells. Lane 1, rAAV2-GFP/HEK 293; lane 2, rAAV2-GFP/Sf9; lane 3, rAAV5-GFP/HEK 293; lane 4, rAAV5-GFP/Sf9; lane 5, rAAV2/5-GFP/Sf9; lane 6, rAAV8-GFP/HEK 293; lane 7, rAAV8-GFP/Sf9; lane 8, rAAV2/8-GFP/Sf9; lane 9, mAAV8/2-GFP/Sf9. (B) Western blotting analysis (4–14% gradient PAAG) of rAAV8-GFP vector preparations (samples in lanes 1–4 are the same as in (A), lanes 6–9). For comparison purpose, baculovirus-produced rAAV2-GFP was analyzed next to mosaic mAAV8/2-GFP (compare the mobility of the respective VP1s marked by asterisks in lanes 4 and 5. (C)– Thin-layer chromatography of phospholipase A2 activity of virus produced in 293 cells vs Sf9 cells. The same amounts of rAAV particles (approximately 10¹⁰ drp) as in (A) were analyzed by the assay as described under Materials and Methods. Samples in lanes 1–9 correspond to the samples in lanes 1–9 in (A). Lane marked Bee venom contains positive control sample—1 ng of a bee venom phospholipase (Sigma). (D) Average values from two independent phospholipase assays quantified using phosphoimaging analysis. Samples are the same as in (A) and (C). The lower phospholipase activity of rAAV2/293 vs rAAV2/Sf9 reflects the inadvertently smaller amount of particles added to the electrophoresis well and the assay reaction (see A).

FIG. 8

Transduction of murine livers in vivo with rAAV8 or rAAV2/8. Mice were injected with 10¹² drp rAAV8-GFP. (A) HEK 293 cell-derived rAAV8-GFP; (B) Sf9 cell-derived rAAV8-GFP; (C) Sf9 cell-produced chimeric rAAV2/8; (D) Sf9 cell-derived mosaic mAAV8/2. (E and F) Specificity of the GFP fluorescence was confirmed by the absence of fluorescence in the same field with a rhodamine filter.