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Cancer cells escape immune destruction. From this perspective, myeloid-derived suppressor cells (MDSCs), which are immunosuppressive in various cancers including breast cancer (BC), are significant. However, the precise mechanisms are unknown. We isolated HLA-DR - CD33 +  MDSCs and CD3 +  T cells from BC patients’ peripheral blood and healthy donors through MACS and immunophenotyped by flow cytometry. Transfection of short-interfering RNAs and treatment with a TLR7/8 agonist altered pathway activities  in vitro . Gene expression was analyzed using qRT-PCR, western blotting, and immunohistochemistry. Our findings showed an association between the progression of BC and increased levels of circulating HLA-DR - CD33 +  MDSCs. These cells strongly suppress both autologous and analogous CD3 +  T cell proliferation and enter the tumor microenvironment. We also identified increased STAT3 signaling and increased IDO and IL-10 expression in BC-derived MDSCs as immunosuppression mechanisms. Further, STAT3 inhibition and TLR7/8 pathway stimulation reduce the immunosuppressive activity of patient-derived MDSCs on T cells by inducing MDSC repolarization and differentiation into mature myeloid cells. This also alters the expression of critical cytokines and transcription factors in CD3 +  T cells and, importantly, reduces breast cancer cells’ proliferation. Finally, while chemotherapy is able to significantly reduce circulating MDSCs’ level in patients with breast cancer, these MDSCs remained highly T cell-suppressive. We identified a novel molecular mechanism of MDSC-mediated immunosuppression. STAT3 inhibition and TLR7/8 pathway stimulation in MDSCs repolarize and suppress MDSCs from breast cancer patients. This offers new opportunities for BC immunotherapy.

STAT3 Silencing and TLR7/8 Pathway Activation Repolarize and Suppress Myeloid-Derived Suppressor Cells From Breast Cancer Patients