A significant fraction of advanced prostate cancer (PCa) patients treated with androgen deprivation therapy (ADT) experience relapse with relentless progression to lethal metastatic castration-resistant prostate cancer (mCRPC)1. Immune checkpoint blockade (ICB) using antibodies against cytotoxic-T-lymphocyte-associated protein 4 (CTLA4) or programmed cell death 1/programmed cell death 1 ligand 1 (PD1/PD-L1) generates durable therapeutic responses in a significant subset of patients across a variety of cancer types2. However, mCRPC showed overwhelming de novo resistance to ICB3–5, motivating a search for targeted therapies that overcome this resistance. Myeloid-derived suppressor cells (MDSCs) are known to play important roles in tumor immune evasion6. Circulating MDSC abundance correlates with PSA levels and metastasis in PCa patients7–9. Mouse models of PCa show that MDSCs (CD11b+ Gr1+) promote tumor initiation10 and progression11. These observations prompted us to hypothesize that robust immunotherapy responses in mCRPC may be elicited by the combined actions of ICB agents together with targeted agents that neutralize MDSCs yet preserve T cell function. Here we developed a novel chimeric mouse model of mCRPC to efficiently test combination therapies in an autochthonous setting. Combination of anti-CTLA4 and anti-PD1 engendered only modest efficacy. Targeted therapy against mCRPC-infiltrating MDSCs, using multikinase inhibitors such as cabozantinib and BEZ235, also showed minimal anti-tumor activities. Strikingly, primary and metastatic CRPC showed robust synergistic responses when ICB was combined with MDSC-targeted therapy. Mechanistically, combination therapy efficacy stemmed from the upregulation of IL-1ra and suppression of MDSC-promoting cytokines secreted by PCa cells. These observations illuminate a clinical path hypothesis for combining ICB with MDSC-targeted therapies in the treatment of mCRPC.

Highlights d Oncogenic KRAS promotes an immune-suppressive profile in CRC d IRF2 is a key downstream target of oncogenic KRASmediating immune suppression d IRF2 suppresses MDSC migration and infiltration by targeting the CXCL3-CXCR2 axis d Enforced IRF2 expression or CXCR2 inhibition overcomes anti-PD1 resistance in CRC

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Purpose: Natural killer (NK)-cell-based immunotherapy may overcome obstacles to effective T-cell-based immunotherapy such as the presence of genomic alterations in IFN response genes and antigen presentation machinery. All immunotherapy approaches may be abrogated by the presence of an immunosuppressive tumor microenvironment present in many solid tumor types, including head and neck squamous cell carcinoma (HNSCC). Here, we studied the role of myeloid-derived suppressor cells (MDSC) in suppressing NK-cell function in HNSCC.Experimental Design: The ability of peripheral and tumorinfiltrating MDSC from mice bearing murine oral cancer 2 (MOC2) non-T-cell-inflamed tumors and from patients with HNSCC to suppress NK-cell function was studied with real-time impedance and ELISpot assays. The therapeutic efficacy of SX-682, a smallmolecule inhibitor of CXCR1 and CXCR2, was assessed in combination with adoptively transferred NK cells.Results: Mice bearing MOC2 tumors pathologically accumulate peripheral CXCR2 þ neutrophilic-MDSC (PMN-MDSC) that traffic into tumors and suppress NK-cell function through TGFb and production of H 2 O 2 . Inhibition of MDSC trafficking with orally bioavailable SX-682 significantly abrogated tumor MDSC accumulation and enhanced the tumor infiltration, activation, and therapeutic efficacy of adoptively transferred murine NK cells. Patients with HNSCC harbor significant levels of circulating and tumor-infiltrating CXCR1/2 þ CD15 þ PMN-MDSC and CD14 þ monocytic-MDSC. Tumor MDSC exhibited greater immunosuppression than those in circulation. HNSCC tumor MDSC immunosuppression was mediated by multiple, independent, cell-specific mechanisms including TGFb and nitric oxide.Conclusions: The clinical study of CXCR1/2 inhibitors in combination with adoptively transferred NK cells is warranted.

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