We present a new type of adenoviral vector that both encodes and displays a vaccine antigen on the capsid, thus combining in itself gene-based and protein vaccination; this vector resulted in an improved vaccination outcome in the Friend virus (FV) model. For presentation of the envelope protein gp70 of Friend murine leukemia virus on the adenoviral capsid, gp70 was fused to the adenovirus capsid protein IX. When compared to vaccination with conventional FV Env-and Gag-encoding adenoviral vectors, vaccination with the adenoviral vector that encodes and displays pIX-gp70 combined with an FV Gag-encoding vector resulted in significantly improved protection against systemic FV challenge infection, with highly controlled viral loads in plasma and spleen. This improved protection correlated with improved neutralizing antibody titers and stronger CD4 ؉ T-cell responses. Using a vector that displays gp70 without encoding it, we found that while the antigen display on the capsid alone was sufficient to induce high levels of binding antibodies, in vivo expression was necessary for the induction of neutralizing antibodies. This new type of adenovirus-based vaccine could be a valuable tool for vaccination.Adenoviruses have been a focus of interest as vaccine vectors for more than a decade and have been tested in various preclinical and clinical studies for vaccination against viral and bacterial infections (reviewed in reference 38). This interest is based on the ability of adenoviral vectors to induce high antibody titers and robust cytotoxic T-lymphocyte (CTL) responses and on the high immunogenicity of the vector, which might have an adjuvant effect on vaccination (17). Adenoviral vectors have also been extensively evaluated for immunization against HIV (reviewed in reference 1), where they were used either alone or in combination with plasmid DNA or protein in prime-boost immunizations. However, vaccination with adenoviral vectors against HIV showed no effectiveness in a large phase IIb study (4), but it is conceivable that the observed lack of effectiveness was due to the choice of vaccine antigen rather than the vector itself, as the vaccine relied exclusively on the induction of CTL responses, and the outcome was unexpected given previous results from studies in nonhuman primates (33,42). The findings of the phase IIb study brought about a shift of focus from the CTL response to a more balanced immune response, including neutralizing antibodies, that is now expected to be necessary for protection from HIV infection.Apart from adenoviral vectors that encode vaccine antigens, there have also been approaches to modify adenoviral capsid proteins to include antigenic epitopes. These were mostly inserted into external loops of the hexon protein (5,22,25,26,43), which is the main component of the adenovirus capsid, but also other components of the capsid, such as fiber, protein IX, and penton base, have been evaluated (22). These studies showed that incorporation of single epitopes into capsid proteins of adenovirus leads ...