Heterologous prime-boost vaccine using antigen-loaded microparticles and adenovirus (encoding antigen) enhances cellular immune responses and antitumor activity.

Heterologous prime-boost vaccines have the potential to promote higher immune responses than homologous prime-boost vaccines and were used in this murine study to investigate the effect on the magnitude of the cellular (and humoral) antigen-specific immune responses and antitumor efficacy when a microparticle formulation (prime) is combined with an adenoviral vaccine (boost). Specifically, the prime comprised chick egg ovalbumin (OVA; 25 µg/dose), used here as a model tumor antigen (TA), encapsulated in microparticles (∼700 nm diameter) made from the biodegradable polymer, 50:50 poly(lactic-co-glycolic acid) (PLGA); while attenuated adenovirus (type 5) encoding OVA (Ad5OVA; 108 PFU/dose) was employed as the boost. The ability of OVA-loaded microparticles to enhance OVA-specific antibody responses, OVA-specific CD3 + CD8 + T cell responses and antitumor activity (i.e., protection against OVA-expressing tumor-challenge) to the heterologous prime-boost vaccine was investigated; and it was found that this prime-boost combination could significantly enhance OVA-specific cellular responses compared to all other vaccination groups and was the only group to confer a significant survival advantage over the unvaccinated group (naïve) in a prophylactic animal tumor model. This finding illustrates the potential for combining TA-loaded PLGA-based microparticles with other vaccine formats to improve tumor-specific cellular immune responses.