Proceedings Of The National Academy Of Sciences Of The United States Of America
Bacillus Calmette–Guérin (BCG) immunotherapy for bladder cancer is the only bacterial cancer therapy approved for clinical use. Although presumed to induce T cell-mediated immunity, whether tumor elimination depends on bacteria-specific or tumor-specific immunity is unknown. Herein we show that BCG-induced bladder tumor elimination requires CD4 and CD8 T cells, although augmentation or inhibition of bacterial antigen-specific T cell responses does not alter the efficacy of BCG-induced tumor elimination. In contrast, BCG stimulates long-term tumor-specific immunity that primarily depends on CD4 T cells. We demonstrate that BCG therapy results in enhanced effector function of tumor-specific CD4 T cells, mainly through enhanced production of IFN-γ. Accordingly, BCG-induced tumor elimination and tumor-specific immune memory require tumor cell expression of the IFN-γ receptor, but not MHC class II. Our findings establish that a bacterial immunotherapy for cancer is capable of inducing tumor immunity, an antitumor effect that results from enhanced function of tumor-specific CD4 T cells, and ultimately requires tumor-intrinsic IFN-γ signaling, via a mechanism that is distinct from other tumor immunotherapies.