Abstract
One of the major obstacles to achieving complete eradication of tumors, even in the presence of circulating tumor-specific immunity, is the tumor-induced immunosuppressive environment, which includes myeloid-derived suppressor cells and regulatory T cells. Attenuated microorganisms have emerged as candidates for a novel anti-cancer approach in which they enhance anti-cancer immunity by boosting the innate immune system. Herein, we will discuss current innate-immunity activating strategies for anti-cancer therapy, with a focus on our recently reported approach involving the use of intratumoral injection of recombinant attenuated Salmonella enterica serovar Typhimurium vaccine; this approach elicits transformation of immunosuppressive myeloid-derived suppressor cells into TNF-α-secreting cells with characteristics of neutrophils, and reduces the generation of regulatory T cells, particularly in the presence of tumor-specific cytotoxic T lymphocytes.