216. TLR9-Targeted STAT3 Silencing Abrogates Immunosuppressive Activity of Myeloid-Derived Suppressor Cells from Prostate Cancer Patients

Abstract

Recent advances in immunotherapy of advanced human cancers underscored the need to eliminate tumor-induced immunosuppression. The myeloid-derived suppressor cells (MDSCs) are important negative regulators of T cell responses in advanced solid tumors, such as prostate cancers. Targeting MDSCs proved challenging due to heterogeneity and phenotypic similarities between myeloid cell lineages. Here, we identify a population of Lin–CD15HICD33LO granulocytic MDSCs that accumulate in patients’ blood during prostate cancer progression from localized to metastatic disease. The prostate cancer-associated MDSCs potently inhibit autologous CD8+ T cell proliferation, IFNg and Granzyme-B production. The circulating MDSCs have high levels of activated STAT3, a central immune checkpoint regulator. As shown by immunofluorescent microscopy, the CD15+/pSTAT3+ cells are also important component of immune infiltrate in primary tumor and tumor-draining lymph node tissues in these patients. We previously generated an original strategy to silence genes specifically in TLR9+ cells using naked CpG-siRNA conjugates. Here, we demonstrate that the CpG-STAT3siRNA is quickly internalized by human MDSCs effectively reducing STAT3 expression. The TLR9-dependent STAT3 blocking abrogates immunosuppressive effects of patients-derived MDSCs on CD8+ T cells. Our further studies demonstrated that these effects resulted from reduced expression and enzymatic activity of Arginase-1, a downstream STAT3 target gene and T cell inhibitor. Overall, we demonstrate the feasibility of using targeted STAT3 blockade to alleviate MDSC-mediated immunosuppression without depletion of myeloid cells in prostate cancer patients. We anticipate that further development of this oligonucleotide strategy will generate safer and more effective immunotherapy for advanced prostate cancers and potentially other solid tumors.