Use of helper-dependent adenoviral vectors of alternative serotypes permits repeat vector administration

We have developed a new helper adenovirus (Ad) based on serotype 2, Ad2LC8cCARP, for use in the Cre/loxP system (Parks et al. Proc Natl Acad Sci USA, 1996; 93: 13565–13570) to generate Ad vectors deleted of all protein coding sequences (helper-dependent Ad vectors (hdAd)). A comparison of Ad2LC8cCARP and our original helper virus (based on serotype 5, Ad5LC8cluc) showed that the two helper viruses amplified hdAd with a similar efficiency, and resulted in a similar yield and purity after large-scale preparation of vector. In vitro, the resulting hdAd2 had a similar transduction efficiency and expression kinetics of transgene (β-gal) as the hdAd5. An important feature of the helper-dependent system is that all virion components, except the virion DNA, derive from the helper virus. Consequently, vectors produced with help from Ad2LC8cCARP were not neutralized by antibodies against Ad5, and vectors produced with Ad5 helper were resistant to neutralizing antibodies against Ad2. Analysis of transgene expression in mouse liver after intravenous injection of the Ad2-based hdAd showed that the vector could efficiently transduce the liver, and produce high levels of a foreign transgene, similar to those expressed by the hdAd generated with the Ad5 helper virus. Mice immunized with hdAd2 produced Ad2-neutralizing antibodies, which did not cross-react with hdAd5. To determine if successful repeat Ad vector administration could be achieved by sequential use of alternative Ad serotypes, we injected mice with hdAd2 (hSEAP) followed 3 months later by a lacZ-expressing hdAd of either the same or different serotype. Repeated administration of hdAd2 resulted in a 30- to 100-fold reduction in transgene expression compared with naive animals. In contrast, no decrease in transgene expression was observed when the second vector was of a different serotype. These results demonstrate that effective vector readministration can be achieved by the sequential use of hdAds based on alternative serotypes.