局部复发是肿瘤放射治疗失败的主要原因，了解其机制迫在眉睫。免疫逃逸是肿瘤放疗后复发的关键。髓系抑制细胞（MDSCs）可通过NO抑制T淋巴细胞功能使肿瘤免疫逃逸，进而促进肿瘤进展。我们前期实验发现：MDSCs高表达iNOS/NO与肺癌放疗后局部复发密切相关。由此我们设想：肿瘤放疗后募集MDSCs，并高表达iNOS产生大量NO，进而抑制局部CD8+T淋巴细胞浸润及其功能，最终抑制抗肿瘤免疫应答而进入逃逸期，促进局部复发。阻断MDSCs对CD8+T淋巴细胞作用可抑制肿瘤进一步佐证了该设想。本项目拟从动物实验了解MDSCs与肿瘤放疗后局部复发间关系，接着探讨MDSCs对肿瘤放疗后CD8+T淋巴细胞的抑制作用，最后研究PDE5/iNOS/NO轴对MDSCs抑制CD8+T淋巴细胞的调控作用，明确MDSCs可重构免疫微环境促进肿瘤放疗后复发，为解释肿瘤放疗后局部复发的机制和治疗提供理论依据。

Recurrence after irradiation is the leading cause of failure for radiotherapy, so uncover the underlying mechanism has gained its importance. The loss and dysfunction of T lymphocytes to tumor can broke the local immune balance, the immunologic escape of tumor cells after irradiation is the key step for recurrence. Some recent studies revealed that myeloid-derived suppressor cells(MDSCs) can inhibit CD8+T lymphocytes function through the PDE5/ iNOS/NO axis, thereby promoting tumor progression.Our previous research work also found that MDSCs especially monocyte subsets could express iNOS/NO and were closely related with tumor recurrence after radiotherapy of lung cancer.In this regard, we propose a hypothesis: Lung cancer after radiotherapy may alter the tumor microenvironment,The changes of tumor can recruit MDSCs through the SDF-1a/CXCR4 axis, MDSCs can improve the expression of NO through iNOS, NO then could inhibit the tumor infiltrating and function of CD8+T lymphocytes，making the immune tolerance of tumor cells, finally promote tumor local recurrence after radiotherapy. This project is going to study the relationship between MDSCs and recurrence of lung cancer after radiotherapy ,then investigate the role of PDE5/iNOS/NO axis in the inhibiting processes of MDSCs to CD8+ T lymphocyte in the tumor after radiotherapy, finally illuminate whether the inhibitory effect of blocking MDSCs on CD8+ T lymphocytes can delay tumor recurrence after radiotherapy or not. This research will open a new horizon for mechanism of recurrence after irradiation in lung cancer and provide a new way to reduce it.