Highlights d CD8 + T cell infiltration in tumors is associated with CCL5 and CXCL9 coexpression d CCL5 is expressed in tumor cells and CXCL9 is induced in APCs in response to IFN-g d CCL5 hi CXCL9 hi tumors are immunoreactive and respond to checkpoint blockade d Cancer cells negatively regulate CCL5 expression by epigenetic silencing mechanisms

Purpose Recent advances in immunotherapy of advanced human cancers underscored the need to address and eliminate tumor immune evasion. The myeloid-derived suppressor cells (MDSCs) 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 tumor/tumor-draining lymph node 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.

Summary Type I interferon (IFN), essential for spontaneous T cell priming against solid tumors, is generated through recognition of tumor DNA by STING. Interestingly, we observe that type I IFN is not elicited in animals with disseminated acute myeloid leukemia (AML). Further, survival of leukemia-bearing animals is not diminished in the absence of type I IFN signaling, suggesting that STING may not be triggered by AML. However, the STING agonist, DMXAA, induces expression of IFN-β and other inflammatory cytokines, promotes dendritic cell (DC) maturation, and results in the striking expansion of leukemia-specific T cells. Systemic DMXAA administration significantly extends survival in two AML models. The therapeutic effect of DMXAA is only partially dependent on host type I IFN signaling, suggesting that other cytokines are important. A synthetic cyclic dinucleotide that also activates human STING provided a similar anti-leukemic effect. These data demonstrate that STING is a promising immunotherapeutic target in AML.