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

Abstract

Purpose

This study aimed to describe a novel cancer vaccine developed using H ₂O 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 ₂O 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 ₂O ₂-inactivated RE88-pVLT33-OVA was higher for subcutaneous than for intragastric vaccination. When subcutaneous administration was used, H ₂O 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 ₂O ₂-inactivated RE88-pVLT33-OVA (2 × 10 ⁹ CFU/mouse) than for 10 µg ovalbumin. Furthermore, subcutaneous vaccination with H ₂O 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 ₂O 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 ₂O 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 ₂O ₂-inactivated RE88-pVLT33-OVA. H ₂O 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 ₂O ₂-inactivated RE88-pVLT33-OVA could be used as a novel delivery system for new cancer vaccines.