title: TLR9-Targeted STAT3 Silencing Abrogates Immunosuppressive Activity of Myeloid-Derived Suppressor Cells from Prostate Cancer Patients.
abstract: OBJECTIVE
  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.
  
  
  METHODS
  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 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.
authors.affiliation: Department of Cancer Immunotherapeutics & Tumor Immunology, Beckman Research Institute at City of Hope, Duarte, California.
authors.forename: Dewan M S
authors.initials: DM
authors.lastname: Hossain
authors.affiliation: Department of Medical Oncology and Experimental Therapeutics, Beckman Research Institute at City of Hope, Duarte, California. mkortylewski@coh.org spal@coh.org.
authors.forename: Sumanta K
authors.initials: SK
authors.lastname: Pal
authors.affiliation: Department of Cancer Immunotherapeutics & Tumor Immunology, Beckman Research Institute at City of Hope, Duarte, California.
authors.forename: Dayson
authors.initials: D
authors.lastname: Moreira
authors.affiliation: Department of Cancer Immunotherapeutics & Tumor Immunology, Beckman Research Institute at City of Hope, Duarte, California.
authors.forename: Priyanka
authors.initials: P
authors.lastname: Duttagupta
authors.affiliation: Department of Cancer Immunotherapeutics & Tumor Immunology, Beckman Research Institute at City of Hope, Duarte, California.
authors.forename: Qifang
authors.initials: Q
authors.lastname: Zhang
authors.affiliation: Department of Cancer Immunotherapeutics & Tumor Immunology, Beckman Research Institute at City of Hope, Duarte, California.
authors.forename: Haejung
authors.initials: H
authors.lastname: Won
authors.affiliation: Department of Cancer Biology, Beckman Research Institute at City of Hope, Duarte, California.
authors.forename: Jeremy
authors.initials: J
authors.lastname: Jones
authors.affiliation: Department of Pathology, Beckman Research Institute at City of Hope, Duarte, California.
authors.forename: Massimo
authors.initials: M
authors.lastname: D'Apuzzo
authors.affiliation: Department of Hematology and Hematopoietic Cell Transplantation, Beckman Research Institute at City of Hope, Duarte, California.
authors.forename: Stephen
authors.initials: S
authors.lastname: Forman
authors.affiliation: Department of Cancer Immunotherapeutics & Tumor Immunology, Beckman Research Institute at City of Hope, Duarte, California. mkortylewski@coh.org spal@coh.org.
authors.forename: Marcin
authors.initials: M
authors.lastname: Kortylewski
mesh_terms: D018414:CD8-Positive T-Lymphocytes / Q000276:immunology / Q000378:metabolism / Q000473:pathology; D019070:Cell Lineage / Q000276:immunology; D049109:Cell Proliferation / Q000235:genetics; D020868:Gene Silencing; D006801:Humans; D007165:Immunosuppression Therapy; D008213:Lymphocyte Activation / Q000276:immunology; D008297:Male; D022423:Myeloid Cells / Q000276:immunology / Q000378:metabolism*; D009360:Neoplastic Cells, Circulating / Q000276:immunology / Q000473:pathology; D011471:Prostatic Neoplasms / Q000097:blood / Q000235:genetics* / Q000276:immunology / Q000473:pathology / Q000628:therapy; D050796:STAT3 Transcription Factor / Q000096:biosynthesis / Q000235:genetics*; D051217:Toll-Like Receptor 9 / Q000096:biosynthesis / Q000235:genetics*; D019139:Tumor Escape / Q000235:genetics
chemical_list: D050796:STAT3 Transcription Factor; C494086:STAT3 protein, human; C495371:TLR9 protein, human; D051217:Toll-Like Receptor 9