This invention responds to a long felt need, by providing in one embodiment, a helper virus based on the Ad2 serotype for use in the Cre/loxP system for the generation of Ad vectors deleted of all Ad protein coding sequences. Using this and helper virus based on Ad5, genetically identical hdAd that differ only in the virion protein components, which are derived from the helper virus, were produced. The vectors have identical expression characteristics in vitro, regardless of the serotype, and the sequential use of hdAd of different serotypes allows for successful repeat vector administration in vivo.
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