S2523
Radiobiology – Immuno-radiobiology
ESTRO 2026
Radiation Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Dresden, Germany. 4 German Cancer Consortium (DKTK), Partner site Dresden, German Cancer Research Center (DKFZ), Heidelberg, Germany. 5 Institute of Radiopharmaceutical Cancer Research, Helmholtz- Zentrum Dresden-Rossendorf (HZDR), Dresden, Germany. 6 Mildred Scheel Early Career Center, Faculty of Medicine Carl Gustav Carus, Technische Universität Dresden, Dresden, Germany Purpose/Objective: Personalized strategies for patients with HPV-negative locally advanced (LA) head and neck squamous cell carcinoma (HNSCC) are urgently needed to overcome radioresistance and recurrence. Our study focuses on investigating the molecular mechanisms connecting HNSCC stemness and radioresistance driven by the CD98hc-associated amino acid transporter network (CD98hc-LAT1/LAT2/xCT, CD98hc-AAT). We target it with modular, universal chimeric antigen-receptor (UniCAR) T cell-immunotherapy in combination with radiotherapy (RT) to develop innovative and efficient treatment strategies. Material/Methods: CD98hc-AAT protein levels were evaluated by immunohistochemistry in tumor tissues from patients with LA-HNSCC treated with post-operative radiochemotherapy (n=91) and analyzed regarding their association with loco-regional control (LRC). Radiobiological analyses were conducted after CRISPR/Cas9 and siRNA-mediated knockdown of CD98hc-AATs with 2D radiobiological assays in several HPV-negative HNSCC cell lines. The mechanisms driven by CD98hc-AATs, regulation of the CSC phenotype, and potential interplays were analyzed using gene expression, aldehyde dehydrogenase (ALDH) activity, and chromatin immunoprecipitation assay (ChIP). We targeted CD98hc in multicellular radioresistant tumor spheroid cultures and patient- derived tumor organoids (PDTOs) using UniCAR with a short-lived target module (TM) in combination with fractionated RT. The therapeutic potential of CD98- targeting via the UniCAR system with or without its combination with standard radiochemotherapy was evaluated in a comprehensive in vivo study. Results: Expression levels of all four transporters are significantly associated with LRC in patients with LA- HNSCC. CD98hc downregulation increased radiosensitivity, reduced clonogenic survival, and impaired sphere formation across all analyzed cell lines. Disruption of the CD98hc-AAT network modulated CSC features through ALDH1A1/ALDH1A3 axes. Through a feedback loop, ALDH-RAR signaling regulated CD98hc and LAT1 gene transcription. Treatment with ALDH products, retinoic acid isomers
differ from controls, suggesting limited induction of long-term immune memory. Survival curves analysis revealed median survivals of 25 days for controls, 39 days for SF-SBRT, and not reached for MF-SBRT. Hazard ratios confirmed improved survival with MF- SBRT (HR = 0.14, p = 0.0002) and SF-SBRT (HR = 0.23, p = 0.0004) compared to controls, with superiority of MF-SBRT over SF-SBRT (HR = 0.16, p = 0.0006). These results were further investigated through flow cytometry, where significant increases in activated CD44+ and exhausted PD-1+ CD4+ and CD8+ tumor- infiltrating lymphocytes were observed for MF-SBRT compared to SF-SBRT and controls. This was also amplified between day 3 and 7 following MF-SBRT. Similar immune remodeling patterns were found in the spleen. Conclusion: MF-SBRT reduces tumor growth and increases T cell responses more effectively than SF-SBRT in a subcutaneous NSCLC mouse model, promoting a more immunogenic tumor microenvironment that may later exhibit signs of T cell exhaustion. These findings reinforce the hypothesis that SBRT, particularly when administered in a fractionated regimen, could benefit from combination with immune checkpoint inhibitors such as anti-PD-1 to sustain durable anti-tumor immunity. References: 1. Simone CB, Daly ME, Redman MW, Hsieh MH, Gray JE, Hesketh PJ, et al. SWOG/NRG S1914: Randomized phase III trial of induction/consolidation atezolizumab + SBRT versus SBRT alone in high risk, early-stage NSCLC. J Clin Oncol. 2025;43(16_suppl):8003. https://doi.org/10.1200/JCO.2 025.43.16_suppl.8002. Demaria S, Guha C, Schoenfeld J, Morris Z, Monjazeb A, Sikora A, et al. Radiation dose and fraction in immunotherapy: one-size regimen does not fit all settings, so how does one choose? J Immunother Cancer. 2021;9(4):e002038. https://doi.org/10.1136/jitc-2020-002038 Keywords: NSCLC, SBRT, immunomodulation CD98hc-Associated Amino Acid Transporters: Essential Regulators of Radioresistance and Immune Response in HNSCC Ay ş e Sedef Köseer 1,2 , Melike Demir 1,2 , Mareike Gruhn 1,3 , Jacqueline Nathansen 1 , Steffen Löck 1,2 , Mechthild Krause 1,2 , Annett Linge 1,4 , Claudia Arndt 5,6 , Anna Dubrovska 1,2 1 Faculty of Medicine and University Hospital Carl Gustav Carus, OncoRay – National Center for Radiation Research in Oncology, Dresden, Germany. 2 Institute of Radiooncology, Helmholtz-Zentrum Dresden - Rossendorf, Dresden, Germany. 3 Radiotherapy and Proffered Paper 819
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