腺苷、调节性T细胞在胃癌的免疫调控中的进展

时林森， 沙索友， 邵永， 朱孝成

作者单位： 221006 江苏 徐州，徐州医科大学附属医院 胃肠外科

综述与讲座

腺苷、调节性T细胞在胃癌的免疫调控中的进展

时林森， 沙索友， 邵永， 朱孝成

肿瘤免疫治疗是一种有效的手段。调节性T细胞(Treg)可通过细胞表面外核苷酸酶(CD39，CD73)降解ATP生成腺苷(ADO)，ADO和细胞表面A2aR结合后可激活细胞内腺苷酸环化酶合成cAMP，从而进一步抑制效应T细胞(Teff)的免疫活性。胃癌作为常见的消化道恶性肿瘤之一，其免疫治疗尚处于临床前期或起步阶段。近年来，关于腺苷在调节性T细胞对胃癌免疫调控中的作用日益受到重视，作者对腺苷、T细胞与胃癌的研究进展进行了综述。

胃肿瘤； 腺苷； T淋巴细胞,调节性； 免疫

肿瘤免疫治疗是继手术、放疗、化疗后又一有效治疗手段，更是被2013年Science杂志列为当年十大科学突破之首[1]。胃癌作为我国最常见的消化道恶性肿瘤之一，其免疫治疗尚处于临床前期或起步阶段[2]。深入研究肿瘤微环境中复杂的作用机制，是肿瘤免疫治疗的基础[3]。近年来，关于腺苷(adenosine，ADO)在调节性T细胞(Treg)对胃癌免疫调控中的作用日益受到重视，现对ADO、Treg与胃癌的研究进展做一概述。

1 腺苷受体家族

ADO是机体ATP代谢产物，细胞内ADO参与细胞的能量代谢、核算合成等生理过程，而在细胞外则是重要的信号传导因子。正常生理状态下，ADO的生成和细胞摄取及降解保持动态平衡。在炎症及肿瘤组织缺氧条件下，ATP不充分裂解，低氧诱导因子-α(HIF-α)诱导组织外核苷酸酶(CD39、CD73)的过表达，使组织ADO水平明显升高。在肿瘤微环境中，其不仅可以抑制免疫杀伤细胞的抗肿瘤免疫应答，而且可以促进免疫抑制细胞的极化和增殖，有利于肿瘤新生血管形成，从而促进肿瘤生长(图1)[4]。

ADO受体的生理作用主要是通过与其受体结合实现的[5]。其属于G蛋白偶联受体超家族，共有7个跨膜结构域，与胞内GTP结合蛋白相偶联，共有A1R、A2aR、A2bR及A3R四种腺苷受体(adenosine receptors，ARs)[6]。ARs几乎在机体所有细胞均有表达，但不同组织及器官其功能各异[7]。其中A2aR与ADO的亲和力最强，ADO可通过直接结合Teff表面A2aR激活细胞内腺苷酸环化酶生成cAMP或结合Treg表面A2aR生成cAMP，然后通过细胞缝隙连接进入Teff。阻断或基因敲除A2aR可明显降低T细胞内cAMP浓度，小鼠体内淋巴瘤和黑素瘤的生长明显受到抑制[8]。由于缺血、缺氧、炎症等因素影响，ARs的表达会明显上调[9]。

2 Treg与肿瘤免疫调控

Treg作为CD4+T细胞的亚群之一，约占人体外周血CD4+T细胞总数的5%，根据其起源及作用机制又分为中心性Treg(nTreg)和外周性Treg(iTreg)[10]。在健康个体中，nTreg可维持机体的免疫耐受和免疫平衡，疾病状态下iTreg则替代nTreg对外来应激作出功能改变并调控Teff产生免疫反应[11]。Treg对机体免疫应答的调控主要通过以下途径来实现：①通过产生细胞因子IL-10，IL-35、转移生长因子-β(TGF-β)等来调控；②裂解抗原提呈细胞(APCs)和Teff；③通过细胞毒T淋巴抗原-4(CTLA-4)和凋亡受体-1(PD-1)介导的细胞接触反应抑制机体炎症反应[12-13]。其在机体的移植免疫耐受、炎症反应、自身免疫性疾病中起着重要的作用[14-15]。Treg与其他T细胞亚群相比，外核苷酸酶(CD39、CD73)的表达明显升高，CD39可降解ATP/ADP生成AMP，CD73可裂解AMP进一步生成ADO激活Treg细胞的抑制活性[16]。CD39、CD73在多种免疫细胞Treg、CD8+T细胞、B细胞均高表达[17-19]，而肿瘤细胞表面CD73的表达和预后存在相关性[20-23]。基因敲除CD73后，小鼠体内结肠癌、淋巴瘤、乳腺癌、黑素瘤的生长和转移明显受到抑制[24-25]。

图1 ADO、Treg与肿瘤免疫调控肿瘤缺氧微环境中HIF-α诱导外核苷酸酶(CD39，CD73)表达，使组织ADO水平明显升高。ADO可通过直接结合Teff表面A2aR激活细胞内腺苷酸环化酶生成cAMP或结合Treg表面A2aR生成cAMP，然后通过细胞缝隙连接进入Teff。通过减少促炎性细胞因子IFN-γ、TNF-α、IL-6表达，增加炎症抑制因子TGF-β、IL-10等合成；上调免疫关键节点受体PD-1，LAG-3的表达；诱导Treg细胞分化从而抑制Teff活性

3 腺苷与免疫效应细胞

在缺氧和ADO富集的肿瘤微环境中，ADO主要通过免疫细胞表面广泛表达的腺苷受体尤其是A2aR来抑制免疫效应细胞，如CD8+T细胞、NK细胞、NKT细胞、树突细胞、巨噬细胞等的功能[17，26-27]。cAMP作为细胞内第二信使，可通过cAMP蛋白激酶-A(PKA)淋巴细胞特异性络氨酸激酶(Lck)/肉瘤基因蛋白激酶(Src激酶)途径来抑制机体免疫应答[28]。ADO和细胞表面A2aR结合后可激活细胞内腺苷酸环化酶合成cAMP[29]，cAMP通过活化PKA介导的磷酸化又可激活Lck和Src激酶[30]。而Lck和Src激酶的免疫抑制主要通过以下途径实现：①减少促炎性细胞因子干扰素-γ、肿瘤坏死因子-α、IL-6表达，炎症抑制因子转化生长因子-β、IL-10等合成增加；②上调免疫关键节点受体PD-1，LAG-3的表达；③诱导Treg细胞分化从而抑制Teff活性[31]。使用A2aR激动剂可明显抑制T细胞和NK细胞免疫功能，这种抑制功能在去除A2aR激动剂仍然持续存在[32]。Cekic等[33]研究发现，A2aR缺失的T细胞虽可明显增强其抗肿瘤活性，但却导致T细胞的半衰期明显缩短，而过早凋亡的T细胞又可导致肿瘤的复发和进展。

4 腺苷与胃癌调控

胃癌的发生是一个多因素、多步骤、复杂的生理病理过程，具体发生机制尚不清楚。但大量致癌物，如蒽环类、亚硝酸盐、H.pylori感染导致的胃黏膜慢性炎症及癌前病变是其主要原因，而肿瘤细胞的免疫逃避则是其中重要的一环[34]。H.pylori感染和75%的胃癌发生存在相关性，其感染胃黏膜细胞后，Treg可抑制胃黏膜CD4+细胞的免疫应答，导致胃黏膜炎症和溃疡形成，同时胃癌细胞可通过转化生长因子-β诱导Treg分化增殖[35-37]。但是在肿瘤发生后，Treg抑制宿主的免疫应答则会促进肿瘤的进展和转移[38-40]。胃癌患者外周血和肿瘤组织中，Treg的比例较非肿瘤个体明显升高[41-43]，而在接受根治性手术后2个月，其比值降至正常。关于Treg和胃癌预后的关系目前尚有争议，Kono等[44]认为，胃癌患者外周血Treg比例随着胃癌进展而逐渐降低，因此，Treg比例和胃癌患者预后存在负相关[44]。而Haas等[45]的研究则表明，胃癌组织中较高的Treg浸润程度和胃癌患者术后生存之间呈正比[45]。国内已有学者证实Treg可通过CD8+T细胞调控胃癌细胞生长[46]。

由于胃癌生理病理的多样性及机体免疫调控网络的复杂性，目前关于Treg对胃癌免疫调控的具体机制尚不十分清楚，腺苷及其受体是否参与了Treg导致肿瘤免疫效应细胞(CD8+T细胞、NK细胞等)失活的具体过程目前知之甚少。Öztürk等[47]的研究证实，水飞蓟宾提取物可激活胃癌组织中腺苷脱氨酶，从而起到治疗胃癌的效果。

5 胃癌免疫治疗现状及展望

肿瘤的免疫治疗近年来取得了飞速进展，针对关键节点受体抑制剂、嵌合抗原的T细胞受体疗法、肿瘤浸润淋巴细胞过继疗法等免疫治疗方法在肿瘤的治疗中已经有了一定疗效[48-51]，但大都存在反应率低、疗效不确切等缺点。ADO及其受体在肿瘤免疫逃避中发挥重要作用，针对CD73、A2aR的免疫治疗在乳腺癌等肿瘤的临床前期实验中已取得了积极结果[25，52-54]。胃癌作为一种免疫原性较弱的恶性肿瘤，如何选择合适的治疗人群和恰当的治疗手段是免疫治疗成功的关键。胃癌的基因组图谱分型为其免疫治疗提供了新的视角。根据分型，胃癌可分为EB病毒阳性型、微卫星不稳定型、基因稳定型和染色体不稳定型[55]。其中EB病毒阳性型已证实和PD-L1/2高表达相关，亦有临床试验证实，微卫星不稳定型中的超突变亚型对免疫治疗疗效较好[56]。因此，胃癌的免疫治疗下一步将着眼于：①胃癌的免疫调控机制和基因分型之间有何关系？②如何根据胃癌的基因分型选择特异性的免疫治疗手段？③免疫治疗和胃癌的现有治疗，如化疗、分子靶向药物、甚至是免疫治疗药物自身之间的联合能否提高胃癌治疗的有效率？

鉴于ADO及其受体、Treg在肿瘤免疫调控中处于重要位置，而在胃癌发生、转移、复发的免疫逃避方面仍有大量未知领域和探索空间。因此进一步阐明ADO、Treg在胃癌免疫耐受中的作用机制，有望为胃癌的免疫治疗提供新的靶点和更有效途径，也为晚期、常规治疗疗效欠佳的胃癌患者带来新的希望。

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江苏省自然科学基金项目(BK201508)

作者单位： 221006 江苏 徐州，徐州医科大学附属医院 胃肠外科

朱孝成，Email: zxchl@yahoo.com

10.3969/j.issn.1674-4136.2017.02.017

1674-4136(2017)02-0125-04

2016-12-25][本文编辑：李庆]