超高剂量率FLASH放疗（FLASH）产生FLASH效应的能力为提高放疗治疗指数提供了可能。免疫反应的贡献经常被假设为 FLASH 抗肿瘤功效和肿瘤杀伤的一定比例，但尚未得到严格评估。 研究免疫反应作为抗肿瘤的潜在重要机制为了利用 FLASH 的作用，将各种小鼠肿瘤模型皮下或原位移植到免疫功能正常的小鼠或中度和重度免疫功能低下的小鼠中。使用FLASH（≥2000 Gy/s）和常规剂量率（0.1 Gy/s，CONV）对小鼠进行单剂量（20 Gy）或大分割方案（3×8 Gy；2×6 Gy）局部照射，含/不含抗 CTLA-4。随着时间的推移，监测肿瘤生长，并进行免疫分析。20 Gy FLASH 和 CONV 在延缓免疫功能正常和中度免疫缺陷宿主的肿瘤生长方面具有同等效果，并且将肿瘤倍增时间延长至 >14 天，而对照动物则为 >7 天。无论微环境如何（皮下侧腹与正位肺），通过大分割方案获得了类似的观察结果。有趣的是，在免疫功能严重受损的小鼠中，20 Gy FLASH 保留了抗肿瘤活性，并显着将肿瘤倍增时间延长至 >14 天，而对照动物则为 >8 天，这表明可能存在独立于免疫反应的抗肿瘤机制。对肿瘤微环境的分析显示，两种照射方式后的免疫特征相似，淋巴细胞显着减少约 40%，而骨髓细胞相应增加。此外，与未受辐射的对照动物相比，FLASH 和 CONV 并未增加肿瘤中 TGF-β1 的水平。此外，当获得完整且持久的抗肿瘤反应（> 140天）时，两种照射方式都能够产生长期的免疫记忆反应。目前的结果清楚地证明了多种免疫活性和免疫活性的肿瘤反应免疫缺陷小鼠模型在很大程度上与剂量率无关，同时与免疫反应在 FLASH 抗肿瘤功效中的主要作用相矛盾。因此，我们的研究表明，FLASH 在调节抗肿瘤免疫反应方面与 CONV 一样有效，可用作免疫调节剂。版权所有 © 2023。由 Elsevier Inc. 出版。

The capability of ultra-high dose rate FLASH radiotherapy (FLASH) to generate the FLASH effect has opened the possibility to enhance the therapeutic index of radiotherapy. The contribution of the immune response has frequently been hypothesized to account for a certain fraction of the anti-tumor efficacy and tumor kill of FLASH but has yet to be rigorously evaluated.To investigate the immune response as a potentially important mechanism of the anti-tumor effect of FLASH, various murine tumor models were grafted either subcutaneously or orthotopically into immunocompetent mice, or in moderately and severely immunocompromised mice. Mice were locally irradiated with single dose (20 Gy) or hypo-fractionated regimens (3 × 8 Gy; 2 × 6 Gy), using FLASH (≥ 2000 Gy/s) and Conventional dose rates (0.1 Gy/s, CONV), with/without anti-CTLA-4. Tumor growth was monitored over time, and immune profiling performed.20 Gy FLASH and CONV were isoeffective in delaying tumor growth in immunocompetent and moderately immunodeficient hosts, and increased tumor doubling time to >14 days vs. >7 days in control animals. Similar observations were obtained with hypo-fractionated scheme, regardless the microenvironment (subcutaneous flank vs. ortho lungs). Interestingly, in profoundly immunocompromised mice, 20 Gy FLASH retained anti-tumor activity and significantly increased tumor doubling time to >14 days vs. >8 days in control animals, suggesting a possible anti-tumor mechanism independent of the immune response. Analysis of the tumor microenvironment showed similar immune profiles after both irradiation modalities with significant decrease of lymphoid cells by ∼40 % and a corresponding increase of myeloid cells. In addition, FLASH and CONV did not increase TGF-β1 levels in tumors as compared to unirradiated control animals. Furthermore, when a complete and long-lasting anti-tumor response was obtained (> 140 days), both modalities of irradiation were able to generate a long-term immunological memory response.The present results clearly document that the tumor responses across multiple immunocompetent and immunodeficient mouse models are largely dose rate independent, and simultaneously contradict a major role of the immune response in the anti-tumor efficacy of FLASH. Therefore, our study indicates that FLASH is as potent as CONV in modulating anti-tumor immune response and can be used as an immunomodulatory agent.Copyright © 2023. Published by Elsevier Inc.