Low-dose metronomic gemcitabine pretreatments overcome the resistance of breast cancer to immune checkpoint therapy

Aims: Immunotherapy has revolutionized cancer management. However, response to immunotherapy is heterogeneous. Thus, strategies to improve antitumor immune responses in resistant tumors, such as breast cancer, are urgently needed. Methods: Established murine tumors were treated with anti-CTLA4 or anti-PD-1 alone or combined with metronomic gemcitabine (met-GEM). Tumor vascular function, immune cell tumor infiltration and gene transcription were determined. Results: Low-dose met-GEM (2 mg/kg) treatments improved tumor vessel perfusion and increased tumor-infiltrating T cells. Notably, low-dose met-GEM pretreatments converted resistant tumors to respond to immunotherapy. Moreover, combined therapy reduced tumor vessel density, improved tumor vessel perfusion, increased T-cell tumor infiltration and upregulated the expression of some anticancer genes. Conclusion: Low-dose met-GEM pretreatment reconditioned the tumor immune microenvironment and improved immunotherapy efficacy in murine breast cancer.Breast cancer is the most commonly diagnosed cancer in women globally. However, only a small subset of breast cancer patients benefits from treatment with immunotherapy. Thus, strategies aiming to enhance the antitumor effects of immunotherapies in breast cancer are an urgent requirement. We found that low-dose metronomic gemcitabine treatments improved tumor vessel function and increased tumor-infiltrating T cells, and did not deplete myeloid-derived suppressor cells, but did not affect tumor growth in the murine breast cancer model. Notably, however, low-dose metronomic gemcitabine pretreatments sensitized breast cancers to immunotherapy. Our findings provide important insights into the optimal strategies for combining immunotherapy with improved tumor vessel perfusion.