Adenoviral Type 35 and 26 Vectors with a Bidirectional Expression Cassette in the E1 Region Show an Improved Genetic Stability Profile and Potent Transgene-Specific Immune Response.

Genetic vaccines based on replication-incompetent adenoviral (AdV) vectors are currently in clinical development. Monovalent AdV vectors express one antigen from an expression cassette placed in most cases in the E1 region. For many vaccines, inclusion of several antigens is necessary in order to raise protective immunity and/or target more than one pathogen or pathogen strain. On the basis of the current technology, a mix of several monovalent vectors can be employed. However, a mix of the standard monovalent AdV vectors may not be optimal with respect to manufacturing costs and the final dose per vector in humans. Alternatively, a variety of bivalent recombinant AdV vector approaches is described in the literature. It remains unclear whether all strategies are equally suitable for clinical development while preserving all the beneficial properties of the monovalent AdV (e.g., immunogenic potency). Therefore, a thorough assessment of different bivalent AdV strategies was performed in a head-to-head fashion compared with the monovalent benchmark. The vectors were tested for rescue efficiency, genetic stability, transgene expression, and potency to induce transgene-specific immune responses. We report that the vector expressing multiple antigens from a bidirectional expression cassette in E1 shows a better genetic stability profile and a potent transgene-specific immune response compared with the other tested bivalent vectors.