3243 - Modulation of Anti-Tumor Immune Responses by PD-1-laIL2 Sequentially Combined with Radiotherapy in a Metastatic Prostate Cancer Model
Presenter(s)
H. Chen1, C. Han2, X. S. Gao1, S. Li1, and M. W. Ma1; 1Department of Radiation Oncology, Peking University First Hospital, Beijing, China, 2International Cancer Institute, Peking University, Beijing, China
Purpose/Objective(s): Immune checkpoint blockade has limited efficacy in "cold" tumors like metastatic castration-resistant prostate cancer (mCRPC). While combining radiotherapy (RT) with immunotherapy may yield superior efficacy. This study aims to investigate the therapeutic efficacy and optimal sequencing of a novel radio-immunotherapy combination (PD-1-laIL2, a low-affinity IL-2 paired with anti-PD-1, with RT) in a preclinical mCRPC model and to elucidate the underlying immunological mechanisms.
Materials/Methods: Subcutaneous RM-1 (androgen-independent prostate cancer) tumors were established in male C57BL/6 mice. A dual-tumor model assessed abscopal effects: primary (RT-treated) and secondary (micrometastases) tumors. Sequencing optimization: Mice were randomized into six groups: Control, RT alone (10 Gy×3), PD-1-laIL2 alone, PD-1-laIL2?RT (pre-RT), PD-1-laIL2+RT (concurrent), and RT?PD-1-laIL2 (post-RT). Tumor growth and survival were monitored. Mechanistic study: Based on sequencing results, the lead schedule (PD-1-laIL2?RT) was compared to the control and anti-PD-1 (same schedule). Tumors were harvested pre-RT for flow cytometry (lymphoid/myeloid subsets).
Results: Sequencing optimization: PD-1-laIL2?RT significantly outperformed all other schedules in primary tumor control, survival, and secondary tumor inhibition (p<0.05). Immune priming (pre-RT): Before RT, PD-1-laIL2?RT tumors exhibited distinct immunological signatures vs. control and anti-PD-1: (1) Enhanced lymphocyte recruitment: increased CD45.2? infiltration, with higher CD8?, CD4? T cells, and NK cells (p<0.05). (2) T cell differentiation: elevated PD-1/TIM-3 mean fluorescence intensity (MFI) on CD8? T cells (p<0.05), indicating a pushed activation/exhaustion phenotype. (3) Myeloid remodeling: downregulated MHC-II on tumor-associated macrophages (TAMs) and dendritic cells (DCs), with unchanged myeloid subset proportions. This may reflect DC migration to lymph nodes or TAM functional repolarization following robust T cell priming. Notably, PD-1-laIL2 induced stronger immune activation signatures than anti-PD-1 under the same schedule.
Conclusion: Sequential administration of PD-1-laIL2 before radiotherapy in a preclinical model recapitulates mCRPC, enhancing both local control and systemic anti-tumor immunity. Pre-RT immune priming creates a tumor microenvironment characterized by lymphocyte activation (increased infiltration, pushed T cell differentiation) coupled with myeloid remodeling (downregulated MHC-II). The paradoxical coexistence of tumor regression with elevated T cell exhaustion markers suggests that PD-1hiTIM-3hi CD8? T cells in this context represent recently activated, tumor-specific effectors rather than terminally dysfunctional cells. These findings provide a mechanistic rationale for optimizing sequencing in radio-immunotherapy combinations for prostate cancer, with direct implications for clinical trial design.