Combination of CTLA-4 blockade with MUC1 mRNA nanovaccine induces enhanced anti-tumor CTL activity by modulating tumor microenvironment of triple negative breast cancer

The immunosuppressive tumor microenvironment (TME) is the main reason for the failure of many immunotherapies that directly stimulate anti-tumor immune response. Anti-CTLA-4 antibody may reduce effector regulatory T (Treg) cell numbers and their suppressive activity in the TME. We have previously reported that combination of anti-CTLA-4 antibody with MUC1 mRNA nanovaccine may mutually enhance each single treatment. But the enhancement mechanism of therapeutic efficacy of MUC1 mRNA nanovaccine plus anti-CTLA-4 monoclonal antibody (mAb) is unknown. In this study, anti-tumor CTL activity induced by combination of CTLA-4 Blockade with MUC1 mRNA nanovaccine and immunosuppressive factors in the TME of triple negative breast cancer were investigated. The results demonstrated that combined therapy with nanovaccine and anti-CTLA-4 mAb could induce stronger anti-tumor CTL response than each monotherapy, result in significantly decreased numbers of myeloid-derived suppressor cells (MDSC), Treg cells, tumor-associated fibroblasts (TAFs) and tumor vasculature in the TME, downregulated levels of interleukin-6, tumor necrosis factor-α and transforming growth factor-β, and significantly upregulated levels of IFN-γ and interleukin-12 as well as increased number of CD8(+) T cell, and appear more effective than either nanovaccine or anti-CTLA-4 mAb alone at increasing level of apoptosis in tumor cells. In addition, combination immunotherapy could significantly downregulated the signal transducer and activator of transcription 3 (STAT3) signal pathway. Therefore, it can be concluded that combination of CTLA-4 blockade with MUC1 mRNA nanovaccine enhances anti-tumor cytotoxic T-lymphocyte activity by reducing immunosuppressive TME and inhibiting tumor-promoting STAT3 signaling pathway.