Abstract A069: Radiotherapy remodels the tumor microenvironment via a TREM2-associated macrophage program that can be therapeutically targeted with TREM2 blockade

Abstract The prostate cancer (PCa) tumor microenvironment (TME) is enriched in myeloid cells that drive progression and therapy resistance. Tumor-associated macrophages (TAM) can promote tumor growth by suppressing antitumor immunity. We have previously shown that stereotactic body radiotherapy (SBRT) induces inflammatory remodeling of the PCa TME with enrichment of a population of radiation-induced TREM2+ TAM. Herein, we characterize the immunological impact of TREM2-expressing TAM on intratumoral T-cell function and explore the therapeutic potential of TREM2 blockade in the context of radiotherapy. We performed 10x scRNA-seq on prostatectomy specimens from high-risk PCa patients treated with preoperative SBRT or non-irradiated controls. THP-1 and bone marrow–derived macrophages (BMDMs) were used to assess radiation-induced immune response. In vivo studies in C57BL/6 mice bearing PTEN-/-SPOPmutCHD1del organoid-derived or B6CaP flank tumors assessed immune changes after SBRT. Anti-TREM2 mAb treatment +/- SBRT was used to evaluate the immune remodeling of the TME. scRNA-seq analysis of human samples revealed enrichment of a myeloid cluster expressing a damage-associated macrophage signature (Trem2, Apoe, Spp1, Lgals3) in SBRT-treated versus controls (35.6% vs 5.7%). Irradiation increased TREM2 expression in BMDMs (p<0.01) and THP-1 cells under M0 and TAM-like conditions. When co-cultured with irradiated (vs. non-irradiated) BMDM-M0 cells, CD8+ T-cells demonstrate a decrease in CD8+ T-cell proliferation (p<0.012) and IFN-γ production (p<0.0005). Further, CD8+ T-cells co-cultured with TREM2hi TAMs (vs. TREM2lo) from irradiated organoid PCa tumors demonstrated a higher proportion of T-cells expressing Tim-3+ (p<0.001) and PD-1+ (p<0.01), suggesting that TREM2hi TAMs may induce a terminally exhausted phenotype in CD8+ T-cells. In vivo studies demonstrated that SBRT reduces tumor volume and was associated with a corresponding increase in intra-tumoral density of TREM2+ TAMs and FoxP3+ regulatory T-cells (Tregs) (p<0.008) in both organoid-derived PCa and B6CaP flank models. Tumor lysate from irradiated tumors demonstrated elevated soluble TREM2 (p < 0.028) and TGF-β1 (p < 0.034) levels. scRNA-seq of irradiated and non-irradiated organoid-derived flank tumors confirmed a radiation-induced anti-inflammatory TAM program (Arg1, Mrc1, Msr1, Tgfbi), corroborating our human PCa data. The combination of TREM2 blockade and SBRT remodeled the TME with a reduction in the density of intratumoral FoxP3+ Tregs and reduced Treg:Teff ratio as well as fewer TAMs expressing Arg1 and TGF-β1, collectively suggesting abrogation of SBRT-induced immunosuppression. Our findings in irradiated human PCa and subsequent validation studies in preclinical PCa models suggest that SBRT promotes a TREM2-driven anti-inflammatory macrophage program that remodels the immune TME. TREM2 blockade represents a potential strategy to reverse immune suppression and promote anti-tumor immunity when combined with SBRT. Citation Format: Janny A. Villa-Pulgarin, Un In Chan, Jiansheng Wu, Fabio Socciarelli, Jeffrey Kraynak, Sidney Wolfe, Hubert Pakula, Luigi Marchionni, Francesca Khani, Massimo Loda, Himanshu Nagar, Christopher Barbieri, Ariel E. Marciscano. Radiotherapy remodels the tumor microenvironment via a TREM2-associated macrophage program that can be therapeutically targeted with TREM2 blockade [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Innovations in Prostate Cancer Research and Treatment; 2026 Jan 20-22; Philadelphia PA. Philadelphia (PA): AACR; Cancer Res 2026;86(2_Suppl):Abstract nr A069.