Oral immunization with Listeria monocytogenes vaccine enhances immunotherapy for protective immunity in murine models of colorectal cancer

Background Colorectal cancer (CRC) is a leading cause of cancer-related death and remains a significant global health challenge. Cancer vaccines have emerged as a promising immunotherapy for long-term tumor control. While Listeria monocytogenes (Lm)-based intravenous vaccines can generate tumor-reactive CD8 T cells, clinical trial success has been limited. Here, we sought to determine whether in vivo targeting of gastrointestinal tissues with foodborne delivery of Lm-based cancer vaccines controlled tumor growth in murine models of CRC. Methods The ActA and InlB virulence genes were deleted from a mouse-adapted Lm strain expressing ovalbumin and containing an internalin A mutation (InlA^M Lm-ova) that allows epithelial cell invasion of mice to generate an oral vaccine administered via consumption of inoculated bread. Immunogenicity and safety were tested in C57Bl/6 mice. Vaccine efficacy was evaluated with CRC tumors delivered by colonoscopy-guided orthotopic transplantation into the colon submucosa. Microsatellite instability high MC38 cell line expressing ovalbumin or genetically engineered microsatellite stable AKPS (Apc^KO Kras ^G12D Trp53^KO Smad4^KO) organoids expressing low levels of ovalbumin (lo^SIIN) were used. Vaccines were tested in prophylactic and therapeutic settings and in the context of immune checkpoint inhibitors (ICI). Results Oral immunization induced a robust CD8 T cell response that was similar in magnitude and phenotype to the fully virulent Lm. Immunized mice did not lose weight, and Lm was contained to intestinal tissues. Mice prophylactically immunized with the vaccine were protected from CRC tumors. Therapeutic immunization of mice bearing lo^SIIN AKPS tumors revealed curtailed growth of the local tumor but did not improve survival. Immunization with anti-programmed cell death protein-1 and anti-cytotoxic T-lymphocyte-associated protein 4 controlled tumors when coupled with therapeutic immunization. Protection correlated with accumulation of ova-specific CD8 T cells within the tumor. Conclusions Oral Lm-based cancer vaccines targeting CRC elicit robust, widely disseminated, and persistent tumor-specific immune responses in mice. These vaccines limit CRC development when administered prophylactically and provide tumor control when administered therapeutically with ICI. Thus, oral delivery of Lm-based cancer vaccines coupled with ICI may provide improved control of CRC progression in clinical application.