Stereotactic ablative radiotherapy (SABR) has shown durable response rates in a subset of prostate cancer patients presenting with low metastatic burden (oligometastasis). By delivering a highly focused dose of radiation, SABR optimizes local control and induces a systemic antitumor immune response which causes tumor regression in non-irradiated metastases at distant sites (called abscopal response). However, cases of abscopal response remain scarce and the disease in majority of patients progresses by developing wide-spread metastasis. Therefore, it is crucial to elucidate the underlying mechanisms of resistance to SABR in non-responder patients and develop more effective combination therapies. By analyzing plasma samples from metastatic castration-resistant prostate cancer (mCRPC) patients, we have observed that radiotherapy induces release of tumor fragments, called extracellular vesicles (EVs). Patients with high levels of tumor-derived EVs were more at risk of developing metastases following radiotherapy. We hypothesize that radiotherapy can inhibit a systemic antitumor immune response by inducing the release of immunosuppressive tumor-derived EVs in prostate cancer. Prostate cancer cell lines (PC3 and DU145) were treated with single dose or fractionated radiotherapy. Levels of EVs were measured with Apogee Micro60-Plus nanoscale flow cytometer and the molecular composition of EVs was analyzed by proteomics and western-blot. Co-culture of prostate cancer cells with CD8+-T cells isolated from healthy donors was used to evaluate the impact of EVs on T-cell activity.For the first time, we identified irradiated PC3 and DU145 prostate cancer cells with both single dose and fractionated radiotherapy exhibiting significant increases in B7H3 protein expression. In accordance with total B7H3 protein level changes, the levels of surface B7H3 protein expression in irradiated PC3 and DU145 cells also significantly increased. Lastly, not only did radiation cause increases in the levels of PC3 and DU145 cell EV release, but also increased B7H3 protein enrichment in those EVs. Given the increases in the B7H3 protein levels in irradiated PC3 and DU145 cell surface and EVs, B7H3 enriched- PC3 and DU145 cell EV treatment to CD8+ T-cells caused significant decreases in the proliferation rates of CD8+ T-cells compared to the proliferation rates of CD8+ T-cells without EV treatment or with non-irradiated PC3 and DU145 cell-derived EV treatment.We anticipate that this proposed study can unveil novel cellular mechanisms underscoring the role of B7H3-enriched prostate cancer cell EVs in halting the radiation-induced anti-tumor immune response. This study warrants investigation into potential therapeutic strategies paired with SABR, making more efficacious anti-metastatic prostate cancer treatments.

Citation Format: Yohan Kim, Roxane R. Lavoie, Haidong dong, Sean Park, Fabrice Lucien-Matteoni. Radiotherapy inhibits the antitumor immune response through release of immunosuppressive tumor-derived extracellular vesicles in prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 675.