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

Abstract

<div>Abstract<p><b>Purpose:</b> Recent advances in immunotherapy of advanced human cancers underscored the need to address and eliminate tumor immune evasion. The myeloid-derived suppressor cells (MDSC) are important inhibitors of T-cell responses in solid tumors, such as prostate cancers. However, targeting MDSCs proved challenging due to their phenotypic heterogeneity.</p><p><b>Experimental Design:</b> Myeloid cell populations were evaluated using flow cytometry on blood samples, functional assays, and immunohistochemical/immunofluorescent stainings on specimens from healthy subjects, localized and metastatic castration-resistant prostate cancer patients.</p><p><b>Results:</b> Here, we identify a population of Lin<sup>−</sup>CD15<sup>HI</sup>CD33<sup>LO</sup> granulocytic MDSCs that accumulate in patients' circulation during prostate cancer progression from localized to metastatic disease. The prostate cancer–associated MDSCs potently inhibit autologous CD8<sup>+</sup> T cells' proliferation and production of IFNγ and granzyme-B. The circulating MDSCs have high levels of activated STAT3, which is a central immune checkpoint regulator. The granulocytic pSTAT3<sup>+</sup> cells are also detectable in patients' prostate tissues. We previously generated an original strategy to silence genes specifically in Toll-like Receptor-9 (TLR9) positive myeloid cells using CpG-siRNA conjugates. We demonstrate that human granulocytic MDSCs express TLR9 and rapidly internalize naked CpG-<i>STAT3</i>siRNA, thereby silencing <i>STAT3</i> expression. STAT3 blocking abrogates immunosuppressive effects of patients-derived MDSCs on effector CD8<sup>+</sup> T cells. These effects depended on reduced expression and enzymatic activity of Arginase-1, a downstream STAT3 target gene and a potent T-cell inhibitor.</p><p><b>Conclusions:</b> Overall, we demonstrate the accumulation of granulocytic MDSCs with prostate cancer progression and the feasibility of using TLR9-targeted <i>STAT3</i>siRNA delivery strategy to alleviate MDSC-mediated immunosuppression. <i>Clin Cancer Res; 21(16); 3771–82. ©2015 AACR</i>.</p></div>