Efficient FLPe recombinase enables scalable production of helper-dependent adenoviral vectors with negligible helper-virus contamination

Abstract
Helper-dependent (HD), high-capacity adenoviruses are one of the most efficient and safe gene therapy vectors, capable of mediating long-term expression1,2,3,4,5,6,7,8,9,10,11,12. Currently, the most widely used system for HD vector production avoids significant contamination with helper virus by using producer cells stably expressing a nuclear-targeted Cre recombinase and an engineered first-generation helper virus with parallel loxP sites flanking its packaging signal1,3,4,5,6,7,8,9,10,11,12. The system requires a final, density-based separation of HD and residual helper viruses by ultracentrifugation to reduce contaminating helper virus to low levels. This separation step hinders large-scale production of clinical-grade HD virus13. By using a very efficient recombinase, in vitro–evolved FLPe (ref. 14), to excise the helper virus packaging signal in the producer cells, we have developed a scalable HD vector production method. FLP has previously been shown to mediate maximum levels of excision close to 100% compared to 80% for Cre (ref. 15). Utilizing a common HD plasmid backbone1,7,8,10,11,12, the FLPe-based system reproducibly yielded HD virus with the same low levels of helper virus contamination before any density-based separation by ultracentrifugation. This should allow large-scale production of HD vectors using column chromatography–based virus purification13.

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