Improving SBRT by re-wiring immunosuppressive neutrophils in murine pancreatic ductal adenocarcinoma

Radiation is used to treat pancreatic ductal adenocarcinoma (PDAC) in the locally-advanced setting. Stereotactic body radiation therapy (SBRT), in particular, has shown improved outcomes against conventional RT in a number of clinical trials. One key cell type involved in the response to RT are neutrophils. These innate immune cells are the first responders to tissue damage and infiltrate irradiated tumors in high numbers to rectify injury. Neutrophils typically possess an immunosuppressive, wound healing phenotype in this scenario, which allows the tumor to recover and actively suppress immunological efforts to eradicate the disease. Here, we aimed to elucidate the role of neutrophils in a murine model of PDAC treated with SBRT. Mice harboring PDAC tumors were treated with targeted SBRT and neutrophils were determined to be significantly increased in the tissue when compared to unirradiated controls. Additionally, phenotypic analysis determined that these cells were largely immunosuppressive and depletion studies confirmed they played a key role in acquired radioresistance in our model. In order to establish whether these infiltrating cells could be re-wired to contribute to anti-tumor immunity, we utilized a novel combination therapy consisting of SBRT and microspheres containing recombinant IL-12 to attempt to repolarize these cells. Transcriptomic analysis confirmed intratumoral neutrophils underwent considerable changes indicative of an immunostimulatory, anti-tumor phenotype following treatment. Moreover, depleting these cells resulted in a loss of treatment efficacy, suggesting that neutrophil re-wiring was vital for the therapeutic outcome. This study highlights neutrophils as key players in SBRT and IL-12 treatment and confirms they can act as a double-edged sword depending on the treatment employed.