Metastatic castration-resistant prostate cancer (mCRPC) tumors are characterized by an abundant, tumor- promoting immune infiltrate that is composed of immune suppressive tumor-associated macrophages (TAMs). These cells promote angiogenesis, suppress T cell recruitment and/or activation and promote metastasis in mCRPC. T cells exclusion by TAMs in mCRPC leads to resistance of immune checkpoint therapy. Annually, nearly 270,000 men will be diagnosed with prostate cancer (PCa) in the U.S. in 2021 and more than 33,000 men will die from this disease. Therefore, mCRPC is an unmet medical need and there is an urgent need to develop innovative therapeutics that would target immune suppressive TAMs that will enhance responses to therapies and benefit mCRPC patients. We sought to re-establish an anti-tumor immune response by targeting surface receptors on TAMs that help drive PCa progression. Using human PCa tumor microarrays, we published that the macrophage mannose receptor (CD206) increased during PCa progression to mCRPC. Our team then selectively targeted of CD206 using RP-182, a 10-mer amphipathic analog of host defense peptide that selectively induces a conformational switch of CD206 expressed on TAMs. RP-182-mediated induction of CD206 in human and murine TAMs elicits a program of endocytosis, phagosomelysosome formation, cancer cell phagocytosis, and reprograms immune-suppressive TAMs to an anti-tumor inflammatory phenotype. In syngeneic and autochthonous murine cancer models, RP-182 suppressed tumor growth, extended survival, and was an effective combination partner with chemo- or immune checkpoint therapy. In the proposed studies, we will evaluate the premise that CD206+ TAMs play an essential role in PCa tumor progression and therapeutic resistance. We hypothesize that the immune-suppressive CD206+ TAMs drive PCa resistance to immunotherapies and therapeutic inhibition of CD206+ TAMs will re-educate TAMs to the pro-inflammatory phenotype, enhance anti-tumor immune responses and will synergize with chemo- or immune checkpoint therapy. The specific aims of this proposal are: 1) Establish the contribution of CD206+ TAM function in PCa tumorigenesis and immune evasion; 2) Elucidate how inhibition of CD206+ TAMs may synergize wirh anti-PD- L1 to enhance anti-tumor immunity 3) Correlate CD206+ TAM infiltrate with Nivolumab response, PCa bone metastasis and patient outcomes. Utilizing a humanized mice and a novel syngeneic mCRPC mouse model in which tumors are heavily infiltrated with CD206+ TAMs and refractory to immune checkpoint therapy, we will characterize the changes in adaptive and immune responses after CD206 is inhibited both in vitro and in vivo. In the third aim, we will measure CD206+ TAM expression in human PCa from mCRPC bone and single-cell RNA sequenced PCa tumors from a Nivolumab clinical trial. Targeting CD206+ TAMs in syngeneic murine tumor models and patient-derived xenotransplantation models in humanized mice has great potential to alter the tumor microenvironment and enhance anti-tumor immune responses that may lead to novel therapeutics for mCRPC.

PROJECT NARRATIVE Mannose Receptor (CD206) positive tumor associated macrophages within the metastatic castration resistant prostate cancer (mCRPC) tumor microenvironment correlate with disease progression and play a critical role in suppressing adaptive immune responses and CD8+ T cell exclusion leading to tumor cell growth, invasion, and survival. Our team has demonstrated that CD206+ tumor associated macrophages control immune suppression in tumors, correlate with disease progression and inhibitory targeting of this receptor stimulates anti-tumor immune responses that synergize with chemo- and immune checkpoint therapy. The overall goal of the proposed research is to delineate the mechanistic role of CD206+ tumor associated macrophages in its regulation of suppressing anti-tumor immune responses that lead to prostate tumor growth, metastasis, therapeutic resistance and use these discoveries to develop novel therapeutic strategies.