Brain metastasis of osteosarcoma are rare but carry a dismal prognosis. Despite the advances in both systemic immunotherapy and localized radiation, it is still difficult to treat brain metastasis, with less than 12 months of survival from the time of diagnosis for most patients. Currently, there is interest in combining strategies to take advantage of the potential synergy. In this study, the mouse model of metastatic osteosarcoma to brain was used to explore the ability of local radiation and anti-PD-1 blockade to induce beneficial anti-tumor immune responses against distant, unirradiated brain metastatic tumors. Immune markers from the peripheral blood and tumor tissue were analyzed by flow cytometry, real-time PCR and western blot. The combination treatment produced a stronger systemic anti-tumor response than either treatment alone, shown by the reduced tumor burden and larger numbers of cytotoxic CD8+ T cells in the unirradiated tumors, indicating an abscopal effect. These data suggested that combination treatment of irradiation with anti-PD-1 immunotherapy can induce abscopal anti-tumor responses and improve both local and distant control.
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