Unraveling myeloid-mediated mechanisms of immunotherapy resistance in prostate cancer bone metastases at single-cell resolution

Fred Hutchinson Cancer Center, Seattle WA

Investigators

Abstract

PROJECT SUMMARY Most patients with metastatic castration-resistant prostate cancer (mCRPC) are resistant to immune checkpoint inhibitors (ICIs), posing a significant clinical challenge that necessitates understanding the mechanisms behind this resistance. Immunosuppressive myeloid cells have been recognized in tumors, but their heterogeneity has made CSF1R inhibitors and other broad targeting approaches largely ineffective. My recent prostate cancer studies identified a distinct subset of tumor-associated macrophages, expressing high levels of SPP1 transcripts (SPP1hi-TAMs) that encode the osteogenic protein osteopontin (OPN). These cells become significantly more prevalent during disease progression and contribute to resistance to ICIs through adenosine signaling, particularly within the prostate. Despite bone being the most common site (>70%) of metastases in mCRPC patients and the poorer immunotherapy responses observed in patients with bone metastases, the role of bone- associated myeloid cells in resistance is unclear. My preliminary data identify granulocytes with high expression of CD62L and CXCR2 (CD62LhiCXCR2hi-gran) as key myeloid cells promoting immunosuppression through IL- 1R signaling in culture. I hypothesize that targeting CD62LhiCXCR2hi-gran and their corresponding molecular signals could enhance the efficacy of ICIs in mCRPC patients with bone metastases. I now propose to utilize intraosseous CRPC mouse models to determine whether CD62LhiCXCR2hi-gran are critical for immunotherapy resistance in vivo. By employing adaptive transfer experiments and pharmacologic interventions in mice and in situ hybridization imaging of patient bone biopsies, I expect to determine if these granulocytes mediate resistance to ICIs via the IL-1R pathway, consistent with our initial findings. Moreover, I aim to investigate whether this suppression mechanism is associated with spatial changes during disease progression by performing 10x Genomics Xenium in situ spatial transcriptomics analysis on formalin-fixed paraffin-embedded of patient and mouse tissues. I also plan to determine whether prostate-prevalent SPP1hi-TAMs can modulate the bone- associated myeloid landscape through long-range communication, particularly with OPN, during the progression of prostate cancer, and whether inhibiting these interactions can enhance the efficacy of ICIs for treating prostate cancer in bone. Preliminary results indicate that CD62LhiCXCR2hi-gran not only increase in abundance but also undergo transcriptional reprogramming during disease progression, with various soluble factors, including CCL5, being the most abundant drivers of myelopoiesis derived from CRPC cells. I now plan to determine whether signaling through these factors, particularly the CCR5-CCL5 axis, is necessary and/or sufficient for pathogenic myelopoiesis in mCRPC bone metastases, and whether it could serve as an immunotherapeutic target. Through these studies, my goal is to identify novel cellular and molecular therapeutic targets for mCRPC bone metastases, with the potential to translate into more effective immunotherapies.