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TLR9-Targeted STAT3 Silencing Abrogates Immunosuppressive Activity of Myeloid-Derived Suppressor Cells from Prostate Cancer Patients #MMPMID25967142
Hossain DM; Pal SK; Moreira D; Duttagupta P; Zhang Q; Won H; Jones J; D'Apuzzo M; Forman S; Kortylewski M
Clin Cancer Res 2015[Aug]; 21 (16): 3771-82 PMID25967142show ga
PURPOSE: 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. EXPERIMENTAL DESIGN: 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. RESULTS: Here, we identify a population of Lin(-)CD15(HI)CD33(LO) 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(+) T cells' proliferation and production of IFNgamma and granzyme-B. The circulating MDSCs have high levels of activated STAT3, which is a central immune checkpoint regulator. The granulocytic pSTAT3(+) 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-STAT3siRNA, thereby silencing STAT3 expression. STAT3 blocking abrogates immunosuppressive effects of patients-derived MDSCs on effector CD8(+) 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. CONCLUSIONS: Overall, we demonstrate the accumulation of granulocytic MDSCs with prostate cancer progression and the feasibility of using TLR9-targeted STAT3siRNA delivery strategy to alleviate MDSC-mediated immunosuppression.
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Clin+Cancer+Res 2015 ; 21 (16): 3771-82
