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

• Published in: Clinical cancer research Vol. 21; no. 16; pp. 3771 - 3782
• Main Authors: Hossain, Dewan M. S., Pal, Sumanta K., Moreira, Dayson, Duttagupta, Priyanka, Zhang, Qifang, Won, Haejung, Jones, Jeremy, D'Apuzzo, Massimo, Forman, Stephen, Kortylewski, Marcin
• Format: Journal Article
• Language: English
• Published: United States 15.08.2015
• Subjects: CD8-Positive T-Lymphocytes - immunology; CD8-Positive T-Lymphocytes - metabolism; CD8-Positive T-Lymphocytes - pathology; Cell Lineage - immunology; Cell Proliferation - genetics; Gene Silencing; Humans; Immunosuppression; Lymphocyte Activation - immunology; Male; Myeloid Cells - immunology; Myeloid Cells - metabolism; Neoplastic Cells, Circulating - immunology; Neoplastic Cells, Circulating - pathology; Prostatic Neoplasms - blood; Prostatic Neoplasms - genetics; Prostatic Neoplasms - immunology; Prostatic Neoplasms - pathology; Prostatic Neoplasms - therapy; STAT3 Transcription Factor - biosynthesis; STAT3 Transcription Factor - genetics; Toll-Like Receptor 9 - biosynthesis; Toll-Like Receptor 9 - genetics; Tumor Escape - genetics
• ISSN: 1078-0432; 1557-3265
• DOI: 10.1158/1078-0432.CCR-14-3145

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−CD15HICD33LO 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 IFNγ 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. Clin Cancer Res; 21(16); 3771–82. ©2015 AACR.