肿瘤免疫监视新机制探究：IFN-γ诱导肿瘤干细胞休眠

More and more evidences from clinical and preclinical studies showed that tumor dormancy exists. However, the underlying mechanism involved in tumor dormancy is extremely limited. The immunosurveillance of immune system on tumor is mainly due to the elimination of tumor. It is very important to know whether immune system could regulate tumor dormancy in the field of tumor immunity. We previously found an effective method to isolate and amplify tumor stem cells in vitro by soft 3D fibrin gel. By using soft 3D fibrin gel system, we found that conditioned medium from activated tumor specific CD8+T cells induced B16 melanoma stem-like cells into dormancy, suggesting that tumor specific immune response can regulate tumor dormancy by releasing some kinds of cytokines. Further study showed that IFN-γinduced B16 stem cells into dormancy which was inhibited by the antagonist of indoleamine 2,3- dioxygenase (IDO1). Moreover, kynurenine (the intermediated product of tryptophan metabolism catalyzed by IDO1) also induced tumor stem cells into dormancy. Based on the above results, we hypothesize that IFN-γregulated tumor stem cell dormancy through the IDO signalling pathway. This project will help us understand how immune system monitors tumor dormancy, which will open new avenues for therapeutic intervention against tumor.

越来越多的临床及临床前期研究证明肿瘤休眠状态的存在，然而迄今为止，肿瘤休眠发生机制依然不清。免疫系统对肿瘤的监视作用往往归因于其对肿瘤细胞的杀伤，是否免疫系统能够调节肿瘤休眠是肿瘤免疫领域一个非常值得探究的课题。申请者所在课题组前期建立了纤维蛋白三维软胶筛选、扩增肿瘤干细胞技术，利用该技术申请人发现，肿瘤特异性CD8+ T细胞激活后的上清液能够导致B16黑色素瘤干细胞进入休眠，表明肿瘤特异性免疫反应可以通过释放某类因子调节肿瘤干细胞的休眠。进一步研究发现，IFN-γ可诱导B16干细胞进入休眠，且能够被吲哚胺2,3-双加氧酶（IDO1）抑制剂所打破。IDO1催化色氨酸代谢的中间产物犬尿氨酸也能诱导肿瘤干细胞进入休眠。基于此，本课题推测IFN-γ通过吲哚胺2,3-双加氧酶信号通路调节肿瘤干细胞休眠。本课题的开展将有助于回答机体免疫系统是如何监视肿瘤休眠这一基本问题，为免疫防控肿瘤提供新思路。

肿瘤细胞休眠是肿瘤研究领域的谜团，被认为是临床肿瘤复发和转移的关键环节，目前对于其形成机理完全不清楚。课题组利用前期所建立的三维纤维蛋白软胶（soft 3D fibrin gels）技术，成功地在体外建立起免疫诱导的肿瘤再生细胞休眠模型。利用该休眠模型该课题组发现，对于分化的肿瘤细胞IFN-γ通过STAT1信号通路激活caspase 3和caspase 7导致细胞凋亡；但是对于肿瘤再生细胞（TRCs）而言，IFN-γ却上调IDO1和AhR表达，AhR作为转录因子直接上调p27的表达，p27作为细胞周期抑制因子，一方面阻断细胞增殖，另一方面又可以与STAT1相结合，抑制STAT1的活性以及STAT1介导的细胞凋亡，从而使细胞进入休眠状态。然而，通过阻断IDO1/AhR代谢通路以阻断p27的上调表达，能够恢复IFN-γ-STAT1信号通路传导的凋亡信号，导致杀伤休眠的TRCs。这一全新机理的阐明，为目前克服临床上不能被清除的休眠的肿瘤种子细胞提供了新的方向，并为肿瘤免疫治疗开辟了新途径。