Programmable bacteria synergize with PD-1 blockade to overcome cancer cell-intrinsic immune resistance mechanisms

Interferon-gamma (IFN-gamma) is a potent cytokine critical for response to immunotherapy, yet conventional methods to systemically deliver this cytokine have been hindered by severe dose-limiting toxicities. Here, we engineered a strain of probiotic bacteria that home to tumors and locally release IFN-gamma. A single intratumoral injection of these IFN-gamma-producing bacteria was sufficient to drive systemic tumor antigen-specific antitumor immunity, without observable toxicity. Although cancer cells use various resistance mechanisms to evade immune responses, bacteria-derived IFN-gamma overcame primary resistance to programmed cell death 1 (PD-1) blockade via activation of cytotoxic Foxp3-CD4(+) and CD8(+) T cells. Moreover, by activating natural killer (NK) cells, bacteria-derived IFN-gamma also overcame acquired resistance mechanisms to PD-1 blockade, specifically loss-of-function mutations in IFN-gamma signaling and antigen presentation pathways. Collectively, these results demonstrate the promise of combining IFN-gamma-producing bacteria with PD-1 blockade as a therapeutic strategy for overcoming immunotherapy-resistant, locally advanced, and metastatic disease.