Development of an Effective Cancer Vaccine Platform Using Attenuated Salmonella typhi

Abstract

Abstract Poor antigen delivery is one of the major limitations of modern cancer vaccine vectors. To overcome this challenge, we exploited Salmonella Pathogenicity Island 2 and type III secretion system to deliver tumor-associated antigen (TAAs) into the cytosol of antigen-presenting cells in situ. In a recent report, we demonstrated that an attenuated strain of S. typhimurium, which was engineered to express SPI2-regulated oncoprotein survivin (SVN), 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 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 CVD908Δssb vector effectively infects human dendritic cells in vitro and translocates recombinant SVN and MYCN oncoproteins into their cytosol. Furthermore, CVD908Δssb remains stable in mice and induces generation of antigen-specific CD8 T cells. Therapeutic vaccination of tumor-bearing mice with SVN or MYCN vaccine produced potent antitumor activity in murine models of lymphoma or neuroblastoma. The results justify clinical testing of CVD908Δssb-based SVN and MYCN vaccines in cancer patients.