2479 - Early HFRT With Pulsed LDRT and PD-1 Blockade Drives Systemic Antitumor Control via Coordinated Effector-Memory CD8? T-Cell Redistribution

Purpose/Objective(s): Recent randomized phase II trials combining low-dose radiotherapy (LDRT) with immune checkpoint blockade in metastatic colorectal and lung cancers failed to produce consistent abscopal tumor responses, likely reflecting limited spatial irradiation and suboptimal treatment integration. We sought to define an optimized spatial and temporal radiotherapy–immunotherapy configuration and to delineate the immune programs enabling coordinated intertumoral antitumor responses.

Materials/Methods: Syngeneic bilateral tumor models were used to systematically compare distinct HFRT–LDRT–anti–PD-1 integration strategies, including dual- and trimodal regimens with alternative sequencing. Hypofractionated radiotherapy (HFRT) was delivered to the primary tumor and pulsed LDRT to the secondary tumor. Tumor growth was monitored longitudinally, with bilateral tumor control as the primary endpoint. Immune profiling was performed by multiparameter flow cytometry and single-cell RNA sequencing with paired T-cell receptor (TCR) analysis. Ligand–receptor interactions were inferred computationally, and functional blockade of lymphocyte trafficking and candidate chemokine signaling pathways was employed to determine the requirement for Tem redistribution.

Results: Among five integration strategies, a trimodal regimen incorporating early HFRT to the primary tumor with pulsed LDRT to the secondary tumor and anti–PD-1 therapy achieved superior bilateral tumor control compared with alternative strategies and was accompanied by marked enrichment of intratumoral CD8? effector-memory T cells (Tem) in both primary and distant tumors. Clonal overlap between primary and distant Tem increased approximately twofold in the LDRT-containing regimen (Morisita index 0.49 vs 0.25 in controls), demonstrating enhanced interlesional sharing. Pharmacologic inhibition of lymphocyte egress abrogated abscopal tumor control without overt systemic immunosuppression, establishing a functional requirement for immune trafficking. Shared Tem exhibited a migration-associated transcriptional program, and cell–cell interaction analyses identified a chemokine signaling axis mediating Tem redistribution. These findings were independently supported in a multicenter randomized cohort of stage IVB esophageal squamous cell carcinoma treated with first-line PD-1–based therapy, where LDRT exposure was associated with significantly increased Tem enrichment.

Conclusion: Early HFRT integrated with pulsed LDRT and anti–PD-1 therapy programs systemic antitumor immunity by driving coordinated intertumoral redistribution of effector-memory CD8? T cells. These findings establish immune trafficking as a therapeutically targetable determinant of bilateral tumor control and provide a translational framework for spatially optimized radiotherapy–immunotherapy strategies in patients with multi-lesional disease.