Recently, several injectable scaffold-based cancer vaccines have been developed that can recruit and activate host dendritic cells (DCs) and generate potent antitumor responses. However, the optimal timing of adjuvant delivery, particularly of the commonly used cytosine-phosphodiester-guanine-oligonucleotide (CpG-ODN), for scaffold-based cancer vaccines remains unknown. We hypothesized that optimally timed CpG-ODN delivery will lead to enhanced immune responses, and designed a cryogel vaccine system where CpG-ODN release can be triggered on-demand by ultrasound. CpG-ODN was first condensed with polyethylenimine and then adsorbed to cryogels. Little adsorbed CpG-ODN was released in vitro. Ultrasound stimulation triggered continuous CpG-ODN release, at an enhanced rate even after ultrasound was turned off, with minimal burst release. In vivo, ultrasound stimulation four days post-vaccination induced a significantly higher antigen-specific cytotoxic T-lymphocyte (CTL) response compared to control mice. Furthermore, ultrasound stimulation at this time point generated a significantly higher IgG2a/c antibody titer than all the groups except ultrasound stimulation eight days post-vaccination. This optimal timing of ultrasound-triggered release coincided with peak DC accumulation in the cryogels. By enabling temporal control of vaccine components through release on-demand, this system is a promising platform to study the optimal timing of delivery of immunomodulatory agents for cancer vaccination.
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