B subunit of Shiga toxin-based vaccines synergize with α- galactosylceramide to break tolerance against self antigen and elicit antiviral immunity

The nontoxic B subunit of Shiga toxin (STxB) targets in vivo Ag to dendritic cells that preferentially express the glycolipid Gb3 receptor. After administration of STxB chemically coupled to OVA (STxB-OVA) or E7, a polypeptide derived from HPV, in mice, we showed that the addition of α- galactosylceramide (α-GalCer) resulted in a dramatic improvement of the STxB Ag delivery system, as reflected by the more powerful and longer lasting CD8⁺ T cell response observed even at very low dose of immunogen (50 ng). This synergy was not found with other adjuvants (CpG, poly(I:C), IFN-α) also known to promote dendritic cell maturation. With respect to the possible mechanism explaining this synergy, mice immunized with α-GalCer presented in vivo the OVA^257-264/K^b complex more significantly and for longer period than mice vaccinated with STxB alone or mixed with other adjuvants. To test whether this vaccine could break tolerance against self Ag, OVA transgenic mice were immunized with STxB-OVA alone or mixed with α-GalCer. Although no CTL induction was observed after immunization of OVA transgenic mice with STxB-OVA, tetramer assay clearly detected specific anti-OVA CD8⁺ T cells in 8 of 11 mice immunized with STxB-OVA combined with α-GalCer. In addition, vaccination with STxB-OVA and α-GalCer conferred strong protection against a challenge with vaccinia virus encoding OVA with virus titers in the ovaries reduced by 5 log compared with nonimmunized mice. STxB combined with α-GalCer therefore appears as a promising vaccine strategy to more successfully establish protective CD8⁺ T cell memory against intracellular pathogens and tumors.