217. Development of an Effective Cancer Vaccine Platform Using Attenuated Salmonella to Deliver Recombinant Tumor-Associated Antigens

Abstract

Inadequate antigen delivery is one of the major limitations of modern cancer vaccine vectors. To overcome this challenge, we exploited Salmonella Pathogenicity Island 2 (SPI2) and its type III secretion system (T3SS) to deliver a tumor-associated antigen (TAA) of choice into the cytosol of antigen-presenting cells (APC) in situ. In a recent report, we demonstrated that an attenuated strain of S. typhimurium, MvP728, which was engineered to express SPI2-regulated oncoprotein survivin, induced potent CD8 T-cell-mediated antitumor response that was curative in a murine model of highly aggressive lymphoma. In further development of this technology for clinical use, the vaccine has been transferred from an experimental strain of S. typhimurium to a clinically validated strain of S. typhi, CVD908. To adapt CVD908 to stably express recombinant antigens without antibiotic-dependent selection, we used a recently reported plasmid stabilization system that encodes the single-stranded binding protein (SSB), an essential protein in DNA metabolism, which was deleted from the bacterial chromosome. The SPI2-regulated expression cassette was then cloned into the SSB plasmid, so that the resultant construct maintained bacterial vector stability while expressing and translocating TAAs. We found that CVD908Dssb vector effectively infects human dendritic cells in vitro and translocates recombinant human survivin and MYCN oncoproteins into their cytosol. Furthermore, CVD908Dssb remains stable and immunogenic in mice, providing further support for choosing this strain as a basis for an effective cancer vaccine platform.