Adenovirus (AdV)-based vectors are popular experimental vaccine vectors, but despite their ability to induce strong immune responses, their application is impeded by wide-spread pre-existing immunity against many AdV types that can impair or even abrogate induction of transgene-specific immune responses. Therefore, the development of vectors based on AdV types with low seroprevalence is important for effective AdV-based immunization in humans.
We investigated the immunization efficacy of vectors based on AdV types 48 and 50 in the ovalbumin (ova) model as well as the Friend retrovirus (FV) model, which allows testing the protective effect of vaccine-induced immunity. Using ova-encoding vectors, we found a significantly lower induction of ova-specific CD8+ T cells and antibody responses by Ad48- and Ad50-based vectors compared to Ad5. Similarly, we found a reduced induction of FV-specific CD8+ T cell responses in Ad48- and Ad50.Leader-Gag-immunized mice compared to Ad5; however, some of those mice were able to control the FV infection, and protection correlated with the level of neutralizing antibodies 10 days after FV challenge. Analyses of AdV-specific antibodies and CD8+ T cells induced by the individual AdV types revealed a high level of cross-reactivity, and the efficacy of Ad48-based immunization was impaired in Ad5 pre-immune mice.
Our results show that immunity induced by Ad48- and Ad50-based vectors is reduced compared to Ad5, and is sufficient only in some of the immunized mice to control FV infection. A high level of cross-reactivity suggests that AdV pre-immunity must be considered even when applying rare AdV based vectors.
AdV-based vectors are important tools for the development of vaccines against a wide range of pathogens. While AdV vectors are generally considered safe and highly effective, their application can be severely impaired by pre-existing immunity due to wide-spread seroprevalence of some AdV types. The characterization of different AdV types with regard to immunogenicity and efficacy in challenge models is of great importance for the development of improved AdV-based vectors that allow for efficient immunization despite anti-AdV immunity. We show that immunity induced by an Ad48-based vector is inferior to Ad5, but can still mediate control of an FV infection in highly FV-susceptible mice. However, the efficacy of Ad48-based immunization was impaired in Ad5 pre-immune mice. Importantly, we found cross-reactivity of both humoral and cellular immune responses raised by the individual AdV types, suggesting that switching to a different AdV type may not be sufficient to circumvent pre-existing anti-AdV immunity.