2938 - FLASH-RT has Advantages over Conventional-RT in Promoting DC Homing to Lymph Nodes and T Cell Activation

Z. Zhou¹, K. Hu², and F. Zhang³; ¹Department of radiation oncology, Peking Union Medical College Hospital. Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing, People’s Republic of China, Beijing, China, ²Department of Radiation Oncology, Peking Union Medical College Hospital, Chinese Academy of Medical Sciences & Peking Union Medical College, Beijing, China, ³Department of radiation oncology, Peking Union Medical College Hospital. Chinese Academy of Medical Sciences & Peking Union Medical College, Beijing, China

Purpose/Objective(s): FLASH-RT is a major technological innovation in the field of radiotherapy. More studies are needed to explore the biological effects of FLASH-RT, especially in radioimmunology. Dendritic cells (DCs) are one of the most important antigen presenting cells, playing an immune surveillance function. When stimulated by antigens, DCs leave the parenchyma and migrate to lymph tissues to perform antigen presentation. Therefore, whether DCs can migrate to T cell enriched areas efficiently is a key step in determining the immune function in the tumor microenvironment. However, it is still unclear whether FLASH irradiation affects the homing ability of DCs to lymph nodes and its specific molecular mechanism. The purpose of this study was to explore the effects of FLASH irradiation on the homing ability of DCs to lymph tissues and the activation of T cells, and to reveal its molecular mechanism. Materials/Methods: Using electron beam as the irradiation source, DCs were irradiated with 20 Gy conventional dose rate(0.2Gy/s or FLASH irradiation(600Gy/s). Then, the effects of irradiation on the maturation phenotype of DCs were analyzed by flow cytometry. The effects of irradiation on the cytoskeleton of DCs were analyzed by immunofluorescence method. The Ca2+ flow in DCs was detected by Ca2+ion probe. The signaling pathways related to irradiation-induced DC cytoskeleton rearrangement were detected by Western Blot; The in vivo homing ability of DC to lymph nodes was detected by in vivo imaging technique. The ability of DC to activate T cells was detected by adoptive DC immunoassay.

Results: During the 72-hour observation period, neither conventional dose rate irradiation at 20 Gy gradient dose nor flash irradiation affected the survival of DC. The expressions of CD40, CD80, CD86, CXCR4 and CCR7 on the surface of DC were up-regulated after irradiation, and the secretion of pro-inflammatory factor TNF-a was increased. Compared with the conventional dose rate irradiation group, CCR7 was further increased in the FLASH group. Animal experiments confirmed that FLASH irradiation could significantly promote the homing of peripheral tissue DC and circulating DC to lymph tissue. Further studies found that the enhanced cytoskeleton rearrangement ability of DC and the up-regulated expression of surface chemokine receptors were the main factors promoting the migration ability of DC after FLASH irradiation. The activation of Ca2+ pathway was the main mechanism of cytoskeleton rearrangement of DC. Compared with the conventional dose rate irradiation group, DC had stronger in vivo T cell activation ability after FLASH irradiation, as shown by the increase in the number and proportion of OVA257-264 specific CD8+ T cells and the up-regulated expression of reactive CD8+ T cell surface activation markers CD44/CD69 and intracellular TNF-a.

Conclusion: Single high dose of FLASH irradiation has obvious advantages over conventional dose rate irradiation in promoting DC homing to lymph nodes and T cell activation.