FLASH Irradiation Modulates Immune Responses and Accelerates Lung Recovery: A Single‐Cell Perspective

Radiation therapy (RT) is essential for treating thoracic malignancies but often causes significant lung damage. FLASH-RT, an ultra-high dose rate irradiation technique, shows potential in reducing radiation-induced lung injury (RILI) while maintaining tumor control. However, the underlying immune mechanisms remain poorly understood. This study investigates the immune and cellular responses to FLASH-RT versus conventional dose rate (CONV) RT during the early phase of RILI. Using single-cell RNA sequencing (scRNA-seq), a dynamic landscape of the lung microenvironment is pictured during RILI within one-week post-irradiation. The analysis reveals that FLASH-RT induces a more immediate but transient cellular response, while CONV-RT causes sustained inflammation. FLASH irradiation significantly reduces neutrophil infiltration compared to CONV irradiation, particularly within the pro-inflammatory Ccrl2⁺ subset. FLASH irradiation also triggers stronger activation of CD4⁺ CD40L⁺ Th cells, which are critical for regulating immune responses and balancing inflammation. Moreover, FLASH irradiation attenuates pro-inflammatory activation and intercellular signaling of Mefv⁺ monocytes, thereby restraining excessive macrophage-driven inflammation. Additionally, FLASH irradiation enhances TGF-β signaling and epithelial-mesenchymal transition (EMT) in alveolar type 1 (AT1) cells, promoting tissue repair. These findings highlight FLASH-RT's superior immune modulation and reparative potential, providing valuable insights into its clinical application for minimizing radiation damage and enhancing lung recovery.