Chemotherapy enhances CD8⁺ T cell-mediated antitumor immunity induced by vaccination with vaccinia virus

The use of immunotherapy or chemotherapy alone is generally ineffective against well-established tumors. To overcome this intrinsic resistance against therapy for tumors, we have attempted to combine immunotherapy with chemotherapy. In this study, we tried to induce a rapid antitumor effect via chemoimmunotherapy using a vaccinia viral vaccine as an immunotherapeutic agent with anticancer agents including epigallocatechin-3-gallate (EGCG) and conventional anticancer drugs. Although a combination of vaccinia-mediated vaccination and chemotherapy led to a strong inhibition of tumor growth, monotherapy alone failed to completely cure tumors. In contrast, intravenous injection of cisplatin (CDDP) or cyclophosphamide (CTX) after vaccinia virus vaccination led to complete regression of the established tumors. Interestingly, anticancer drugs appear to augment the antitumor effect of the vaccinia virus-mediated immunotherapy. This effect is mainly associated with the enhanced tumor-specific CD8⁺ T cell immune response induced by vaccinia virus, which was demonstrated by antibody depletion. However, anticancer drugs alone failed to induce a significant enhancement of the tumor-specific CD8⁺ T cell immune response. Taken together, these results suggest that combining vaccinia virus-based immunotherapy with anticancer drugs is particularly effective against established tumors by increasing the tumor antigen-specific CD8⁺ T cell immune response, which is primed by vaccinia virus-mediated vaccination.