H(2)O(2)-Inactivated Salmonella typhimurium RE88 Strain as a New Cancer Vaccine Carrier: Evaluation in a Mouse Model of Cancer

PURPOSE: This study aimed to describe a novel cancer vaccine developed using H(2)O(2)-inactivated Salmonella typhimurium RE88 [with deletions of AroA (the first enzyme in the aromatic amino acid biosynthesis pathway) and DNA adenine methylase] as the carrier. METHODS: The pVLT33 plasmid was used to engineer an RE88 strain induced to express ovalbumin (OVA) by isopropylthiogalactoside (RE88-pVLT33-OVA). The immune responses and anticancer effects of H(2)O(2)-inactivated RE88-pVLT33-OVA were compared with those of non-inactivated RE88-pVLT33-OVA and OVA (positive control) in mice carrying OVA-expressing tumors (EG7-OVA) cells. RESULTS: Anti-ovalbumin IgG (immunoglobulin G) titer following vaccination with H(2)O(2)-inactivated RE88-pVLT33-OVA was higher for subcutaneous than for intragastric vaccination. When subcutaneous administration was used, H(2)O(2)-inactivated RE88-pVLT33-OVA (2 × 10(9) CFU (colony forming units)/mouse) achieved an anti-ovalbumin IgG titer higher than that for the same dose of RE88-pVLT33-OVA and comparable to that for 10 µg ovalbumin (positive control). The binding of mouse serum antibodies to EG7-OVA cells was stronger for H(2)O(2)-inactivated RE88-pVLT33-OVA (2 × 10(9) CFU/mouse) than for 10 µg ovalbumin. Furthermore, subcutaneous vaccination with H(2)O(2)-inactivated RE88-pVLT33-OVA (2 × 10(9) CFU/mouse) induced greater activation of splenic T cells and more extensive tumor infiltration with CD4(+)/CD8(+) T cells compared with 10 µg ovalbumin (positive control). The mice vaccinated subcutaneously with H(2)O(2)-inactivated RE88-pVLT33-OVA at a dose of 2 × 10(8) or 6 × 10(8) CFU/mouse had smaller tumors compared with mice in the negative control groups. Tumor weight in mice vaccinated with H(2)O(2)-inactivated RE88-pVLT33-OVA at a dose of 2 × 10(9) CFU/mouse was significantly lower than that in both negative control groups (P < 0.05) and decreased with the increasing dose of H(2)O(2)-inactivated RE88-pVLT33-OVA. H(2)O(2)-inactivated RE88-pVLT33-OVA was potentially safer than the non-inactivated strain, could carry exogenous antigens, and had specific epitopes that could be exploited as natural adjuvants to facilitate the induction of cellular and humoral immune responses. CONCLUSION: It was anticipated that H(2)O(2)-inactivated RE88-pVLT33-OVA could be used as a novel delivery system for new cancer vaccines.