生命科学   2018, Vol. 30 Issue (9): 939-949.  DOI: 10.13376/j.cbls/2018113.
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专刊:基因编辑技术2.0

引用本文 [复制中英文]

穆伟, 李娜, 王皓毅. 基因编辑在T细胞治疗中的应用. 生命科学, 2018, 30(9): 939-949. DOI: 10.13376/j.cbls/2018113.
[复制中文]
MU Wei, LI Na, WANG Hao-Yi. Gene editing in T cell therapy. Chinese Bulletin of Life Sciences, 2018, 30(9): 939-949. DOI: 10.13376/j.cbls/2018113.
[复制英文]

基金项目

中科院战略先导计划(XDA16010205);国家自然科学基金项目(81773269)

作者简介

王皓毅,中国科学院动物研究所研究员、博士生导师,“青年千人计划”入选者。2009年于美国华盛顿大学获得分子细胞生物学博士学位,2009年至2014年在麻省理工学院Whitehead研究所Rudolf Jaenisch实验室进行博士后研究,2014年5月加入中国科学院动物研究所,担任基因工程技术研究组组长。致力于新型基因工程技术的开发和现有基因编辑技术的优化和应用,相关研究成果发表在CellNature BiotechnologyGenome ResearchCell ResearchGenetics等学术期刊 。

通信作者

王皓毅, E-mail: wanghaoyi@ioz.ac.cn

文章历史

收稿日期:2018-08-31
基因编辑在T细胞治疗中的应用
穆伟 1,2, 李娜 1, 王皓毅 1,2     
(1 中国科学院动物研究所干细胞与生殖生物学国家重点实验室,北京 100101)
(2 中国科学院大学存济医学院,北京 100049)
摘要:过继性免疫细胞治疗是一个迅速发展的领域,在癌症、自身免疫病和感染性疾病的治疗方面显示出了巨大的潜力。近年来基因编辑技术的发展使得在人类原代细胞中进行各种遗传修饰成为可能,将基因编辑技术与过继性免疫细胞疗法相结合将有助于开发更安全有效的免疫细胞疗法。现系统介绍不同类型的基因编辑技术以及它们在T细胞中的应用,总结主要的研究进展,讨论现阶段存在的问题和未来的发展方向。
关键词基因编辑    T细胞    过继性细胞治疗    
Gene editing in T cell therapy
MU Wei 1,2, LI Na 1, WANG Hao-Yi 1,2     
(1 State Key Laboratory of Stem Cell and Reproductive Biology, Institute of Zoology, Chinese Academy of Sciences, Beijing 100101, China)
(2 Savaid Medical School, University of Chinese Academy of Sciences, Beijing 100049, China)
Abstract: Adoptive immunotherapy is a rapidly developing field that has shown great promise in the treatment of cancer, autoimmune diseases and infectious diseases. In recent years, the development of gene editing technology has made it possible to carry out various genetic modifications in human primary cells, and combining gene editing technology with adoptive immunotherapy will help to develop safer and more effective immunotherapy. Here we review the different types of gene editing technologies and their applications in T cell therapies, summarize the main research progress, and discuss the challenges and future prospects.
Key words: gene editing    T cells    adoptive cell therapy (ACT)    

使用自体或者异体淋巴细胞的过继性细胞(ACT)治疗在各种疾病中显示出良好的效果。随着基础研究和临床数据的积累,这一疗法的潜力和局限性正变得更加清晰。早期基于ACT的免疫疗法是从肿瘤中获取肿瘤浸润淋巴细胞(TIL)进行体外扩增,然后回输至患者以治疗转移性黑色素瘤[1]。结合临床上对宿主淋巴细胞的清除,TIL治疗达到了50%的客观反应(OR)率[2-4]。然而,分离技术要求高和体外制备周期长等因素都制约了TIL疗法的广泛应用。为了产生靶向特定肿瘤抗原的特异性T细胞,研究人员从TIL中鉴定出了能够识别主要组织相容性复合物(MHC)递呈的抗原肽的T细胞受体(TCR)的基因序列[5]。通过转染编码TCR基因的病毒产生TCR工程化的淋巴细胞(TCR-T),并且在2006年的临床研究中首次成功用于治疗转移性黑色素瘤[6]。因为TCR-T识别的是MHC递呈的抗原多肽,该治疗只能应用于MHC多态性一致的患者,而且许多肿瘤会通过下调MHC表达和加工缺陷抗原来降低抗原的递呈和识别,从而影响治疗效果。嵌合抗原受体T细胞(CAR-T)疗法是一种不依赖于MHC的过继性细胞疗法,但其只能识别细胞表面的抗原。抗原嵌合受体(CAR)的概念最初是由Zelig Eshhar提出的[7],它由三部分组成:胞外区,通常含有来源于抗体的一段单链可变片段(ScFv); 跨膜区; 含有CD3ζ信号转导结构域和一个或多个共刺激分子,如CD28、4-1BB、CD27和OX40的胞内区[8-13]。ScFv与肿瘤细胞表面靶抗原结合后,通过CD3ζ的信号转导结构域激活CAR-T细胞,特异性杀伤肿瘤细胞。CAR-T细胞疗法在治疗B细胞急性淋巴细胞白血病(B-ALL)的早期阶段临床研究中已取得了显著的成功,其完全应答率为70%~ 90%[14-18]。然而,目前CAR-T细胞疗法对于实体瘤的治疗仍面临着巨大的挑战[8, 19-25]。随着基因编辑技术的发展,使得在人类原代T细胞中进行各种遗传修饰从而开发更安全有效的T细胞疗法成为可能。本文就目前的基因编辑技术及其在T细胞免疫治疗中的应用展开论述。

1 基因编辑技术

在20世纪80年代,基于细胞同源重组途径的基因打靶技术被开发用来修改基因组中的特定位点; 然而,同源重组介导的基因打靶发生的概率很低,平均每106个细胞中有一个重组事件发生[26]。Maria Jasin实验室的开创性工作表明,在哺乳动物细胞基因组靶位点引入DNA双链断裂(DSB)可以导致小片段碱基插入或缺失突变,也可以显著提高同源重组的效率[27-28]。因此,为了进行高效精确的基因修饰,需要开发具有以下三个特征的工具酶:(1)具有足够的特异性以识别基因组中特定位点; (2)有效地造成DNA双链断裂; (3)可被编程靶向任意基因组位点。工具酶诱导的DNA断裂可通过以下两种途径修复:非同源末端连接(NHEJ)和同源定向修复(HDR)。NHEJ是一种较容易发生错误的修复途径,它将断裂末端直接连接,会导致碱基的插入或(和)缺失(Indel)[29],该机制可用于通过插入或(和)缺失来改变靶位点的阅读框从而实现破坏靶基因的目的。HDR修复途径是依赖于同源重组的机制,在同源序列存在时,修复系统以同源序列为模板修复断裂位点[30]。目前最为常用的基因编辑工具酶有锌指核酸酶(ZFN)、转录激活样效应核酸酶(TALEN)和CRISPR-Cas核酸酶。

1.1 ZFN

锌指(zinc finger)是一类常见于DNA结合蛋白中的结构域,每个锌指由大约30个氨基酸组成,可以折叠成ββα结构,通过锌离子与保守的Cys2His2残基鳌合来稳定自身结构[31]。锌指核酸酶由两个结构域组成:DNA结合结构域和DNA切割结构域。每个锌指结构域识别DNA上三联碱基,DNA结合结构域通常由3~6个独立的锌指组成,可识别9~18个碱基对。DNA切割结构域为FokI的核酸酶结构域,其以二聚体的形式介导DNA双链的断裂[32-34]。因此,只有当一对ZFN以适当的距离和方向结合特定的DNA序列才能造成目标位点的DNA双链断裂。

锌指核酸酶已被广泛用于修饰各种类型动植物的基因组[35]。由于其蛋白和编码基因较小,因此在体内递送中具有一定的优势,已被用于临床试验,如靶向人类CD4+ T细胞中C-C基序趋化因子受体5 (CCR5)以治疗艾滋病[36],本文后面会有更为详细的介绍。锌指核酸酶也具有较为明显的局限性:(1)脱靶效应较高,ZFN的锌指重复序列在识别靶DNA序列时,会被靶DNA的核苷酸组分以及每个锌指内部或外部的氨基酸序列所影响,从而使得识别的特异性发生改变,影响基因编辑效率,同时增加脱靶切割的可能性[37];(2)设计难度大,虽然已有多种策略,如“模块化组装”[38]、工程化的单聚体库(OPEN)系统及其衍生物[39-40]来简化ZFN的设计,但是实际上针对特定DNA序列设计一对具有高特异性的ZFN仍然具有很大的难度。

1.2 TALEN

转录激活剂样效应子(TALE)是在植物致病性黄单胞细菌中发现的,它们通过细菌Ⅲ型分泌系统注入植物细胞并与靶启动子结合以操纵宿主基因的转录。TALE的DNA结合结构域具有多个高度重复性的单元,每个单元由高度保守的33或34个氨基酸组成,而其第12和13个氨基酸的可变性较大,主要参与特定核苷酸的识别[41-42]。每个TALE重复单元都可以识别目标DNA序列的一个特定碱基,因此通过这些重复序列的重排可以产生不同的DNA结合特异性[41-44]。将TALE的DNA结合结构域与FokI的核酸酶结构域融合就形成了TALEN (TALE nuclease)。

与ZFN类似,TALEN也是以二聚体的形式对靶序列DNA进行切割,而其简单的DNA识别代码使得TALEN比ZFN更易于设计,从而迅速得到广泛应用[32, 43, 45]。TALEN已经在人类体细胞和多能干细胞中显示出强大的的基因修饰能力[45-46]。尽管设计比ZFN简单,TALEN也有其局限性:主要表现在蛋白较大,递送比较困难,TALEN需要34个氨基酸来特异性识别单个碱基对。如果要识别基因组中独特的靶序列,TALEN的大小将使载体的递送具有挑战性。除此之外,TALEN重复序列的长度和高度重复性也使得分子克隆有较高的难度,并且序列容易发生重组,而将其编码序列包装至病毒系统中也具有较高难度[47]

1.3 CRISPR-Cas

CRISPR-Cas系统是存在于细菌和古细菌中的适应性免疫系统,是细菌在抵御外源噬菌体和病毒入侵过程中进化出来的自然防御机制[48-50]。细菌或古细菌的免疫防御过程主要包括三个阶段:第一阶段,从外源核酸中取得序列片段并插入到CRISPR阵列中; 第二阶段是CRISPR阵列的转录以及导向RNA (gRNA)的成熟; 第三阶段即是降解外源核酸的阶段,成熟的导向RNA将核酸内切酶或者核糖核酸酶带至与其互补的靶位点处,随后进行靶序列的切割或者降解[51-52]。虽然不同细菌中的CRISPR-Cas系统都遵循这三个阶段,但是成熟的导向RNA及效应分子具有多样性[53]。目前所有已知的CRISPR- Cas系统分为两大类,依据其基因座和特征蛋白的不同又可进一步分成不同的类型和亚型[54-55]。第一类CRISPR-Cas系统包括Ⅰ、Ⅲ和Ⅳ型,是细菌和古细菌中最丰富的类型,它们采用多亚基效应复合物发挥作用[54]。第二类CRISPR-Cas系统较少见,包括Ⅱ、Ⅴ和Ⅵ型[56]。这类系统利用导向RNA指导单个Cas蛋白对靶序列进行识别和切割。由于该系统的简单性,其作为基因编辑工具得到了广泛的研究和应用[57-58]

1.3.1 CRISPR-Cas9

CRISPR-Cas9系统属于二类Ⅱ型,是在化脓链球菌(Streptococcus pyogenes)和嗜热链球菌(Streptococcus thermophilus)的研究中建立起来的基因编辑平台[58-59]。该系统主要包括三种组分:(1)核酸内切酶Cas9蛋白,它有两个核酸酶结构域HNH和RuvC,用于切割和降解外源核酸; (2)两个小RNA,CRISPR RNA (crRNA)和反式激活crRNA (tracrRNA),引导Cas9蛋白至特定的靶位点介导双链断裂。将crRNA-tracrRNA融合形成sgRNA进一步简化了该系统[58]。这种Cas9-sgRNA系统是目前应用最广泛的基因编辑工具。与大多数已知的DNA结合蛋白不同,Cas9是一种RNA导向的核酸酶,其序列特异性主要来源于它的gRNA和DNA位点之间的碱基配对以及SpCas9和原型间隔区相邻基序(PAM)之间的直接相互作用[58, 60-62]。因此,只需通过改变sgRNA的核苷酸序列,即可将Cas9蛋白导向至不同靶点。sgRNA设计的简易性超越了之前的两种基因编辑工具所依赖的蛋白序列的靶向特异性[63-64]。CRISPR-Cas9系统的发展极大简化了原核和真核生物的基因编辑,其作为高效的基因编辑工具在各个物种包括细菌、真菌、植物和动物中得到了广泛应用[59, 65-70]

1.3.2 CRISPR-Cpf1

Zetsche等[71]发现在二类Ⅴ型系统中,Francisella novicida U112的CRISPR-Cas基因座能够编码FnCpf1蛋白,其在gRNA的引导下产生靶向切割,可用于人类细胞系中的基因编辑。CRISPR-Cpf1和CRISPR- SpCas9系统工作的差异主要体现在三个方面:第一,gRNA不同,Cpf1仅需要单个的crRNA发挥导向功能,而不需要tracrRNA; 第二,PAM序列不同,Cpf1的PAM序列富含T,而SpCas9蛋白的PAM序列富含G; 第三,产生的DNA双链断裂末端不同,SpCas9在靶位点处切割产生平末端,而Cpf1产生4或者5个碱基的黏性末端[71]。序列分析显示Cpf1蛋白仅包含一个Ruvc核酸酶结构域,该结构域的缺失会导致Cpf1的完全失活[71]。2016年,关于Acidaminococcus sp. BV3L6 (AsCpf1)的晶体结构的研究揭示了负责切割靶链的核酸酶结构域,该结构域保守残基的突变会消除其核酸酶的双链断裂能力,而仅保留切口酶(nickase)的活性[72]。在多样化的二类CRISPR系统中AsCpf1及其他效应蛋白的发现拓展了RNA导向的核酸内切酶工具的应用[54-55, 71]

2 基因编辑技术在T细胞治疗中的应用

接下来将就基因编辑在疾病方面的应用做总结和讨论,主要集中在对于病毒传染病和肿瘤的治疗方面。

2.1 基因编辑的T细胞在治疗人类免疫缺陷病毒(HIV)感染中的应用 2.1.1 HIV受体的敲除

CD4+ T细胞是人类免疫缺陷病毒感染的主要靶点[73]。病毒可介导细胞凋亡、诱导自身免疫反应等[74-75],导致CD4 + T细胞数量减少、细胞免疫丧失、体液免疫应答和后天免疫机能丧失综合症[76]。HIV通过病毒包膜蛋白与CD4+ T细胞的CCR5或C-X-C基序趋化因子受体4 (CXCR4)结合进入宿主细胞,在大多数情况下,HIV通过CCR5受体进入从而引发原发感染。在感染的后期,包膜蛋白的突变使其对CXCR4受体有趋向性。天然存在的CCR5突变等位基因(CCR5Δ32)以高频率(大约10%)存在于高加索人群中,这种突变阻止了CCR5在细胞表面的表达,使得CCR5Δ32基因型纯合子个体对HIV感染具有高度抗性[77-78]

由于很难找到纯合的具有CCR5Δ32突变的人类白细胞抗原(HLA)配对的供体,因此使用基因编辑来治疗HIV感染更具有可行性。在自体未感染的CD4+ T细胞或CD34+造血干细胞中敲除CCR5基因的替代策略是非常有吸引力的。CCR5敲除的T细胞在体外和移植的免疫缺陷小鼠中显示出良好的增殖和抗HIV效应。与输注的未修饰的T细胞相比,基因修饰的CD4+ T细胞治疗显著降低了HIV感染的外周血中的病毒载量,并增加了CD4+ T细胞的数量。研究表明,CCR5敲除的CD4+ T细胞的输注可以重建患者的免疫系统,从而产生持久有效的抗HIV效应,为使用基因编辑技术治疗HIV感染提供了重要依据[79](图 1)。

HIV通过识别CD4受体和CCR5/CXCR4辅助受体来感染宿主细胞。进入细胞质后,将其RNA基因组逆转录成DNA,然后整合到宿主基因组中,经过转录翻译形成新的病毒。基因编辑已被用于敲除CCR5和(或)CXCR4基因,以消除病毒进入细胞所必需的共同受体。另外,靶向病毒的LTR区可以将HIV原病毒从宿主基因组中去除。 图 1 基因编辑的T细胞在治疗HIV感染中的应用

C-X-C基序趋化因子受体4 (CXCR4)是HIV感染后期的另一重要辅助受体[80]。敲除T细胞中的CXCR4基因是获得对HIV感染抗性的有利策略[81]。敲除CXCR4的CD4+ T细胞具有正常的增殖和功能,并显示出对利用CXCR4受体的病毒的抗性[82-83],而CCR5CXCR4双敲除的T细胞展现出了对所有HIV毒株的抗性[84](图 1)。

2.1.2 清除感染细胞中的HIV原病毒

上述方法的主要限制是它们只能阻止感染扩散,而不能消除整合到基因组中的HIV原病毒。由于潜伏在感染细胞中的储库是治疗的主要障碍,靶向并破坏感染细胞中整合的前病毒DNA是潜在的治疗策略。ZFN、TALEN和CRISPR-Cas9系统都曾被用于靶向HIV基因组的不同区域以减少不同细胞系中的HIV含量[36, 85-88]。HIV中的长末端重复序列(LTR)是基因编辑治疗非常有吸引力的靶标,这些序列存在于原病毒基因组的5'和3'末端。因此,靶向HIV基因组的LTR处,可导致大部分HIV基因组包括编码所有病毒蛋白的序列的完全缺失[76, 89-92] (图 1)。

2.2 基因编辑的T细胞在治疗肿瘤中的应用 2.2.1 T细胞中免疫检查点的敲除

ACT治疗实体瘤主要面临三大障碍。首先,肿瘤抗原的异质性和不特异性。由于靶抗原也在正常组织中表达,因此细胞疗法会产生脱靶毒性。例如在临床靶向碳酸酐酶Ⅸ (CAIX)的CAR-T治疗研究中观察到了它的脱靶效应,这主要是由于CAIX在肾细胞癌中高表达,但在正常组织中也以低水平表达[22, 93]。其次,T细胞不能有效归巢并渗入肿瘤组织。第三,外源输注的T细胞被肿瘤免疫抑制性微环境抑制,导致其功能衰竭和丧失[94]。由免疫检查点介导的免疫抑制信号,如PD-1CTLA-4LAG-3TIM-3组成的肿瘤微环境,在促进肿瘤免疫逃逸中发挥重要作用[94]。早期研究已证实,使用阻断免疫检查点的单克隆抗体可挽救T细胞耗竭并恢复T细胞功能[95-100]。基于这些发现,免疫检查点阻断疗法已被用于临床试验,并取得令人兴奋的结果,对于标准治疗无效的晚期肿瘤患者产生了部分或完全消退[101-102]。不幸的是,持续的肿瘤消退并不常见,并且在许多患者中没有效果。为了打破免疫抑制微环境,ACT和单克隆抗体联合治疗是一种有吸引力的策略[94]。然而免疫检查点抑制剂的长期使用可能破坏免疫耐受,导致严重的副作用。随着基因编辑技术的发展,在ACT之前敲除免疫检查点基因,从而产生对实体瘤的免疫抑制微环境无反应的更强大的T细胞是一种更值得尝试的策略。

PD-1是最有希望的基因编辑靶点,它在T细胞活化后表达,并与相应的配体PD-L1/PD-L2结合从而抑制T细胞活化信号[103]。在慢性感染或肿瘤微环境中,PD-1会在T细胞上高表达并损害其功能,这是T细胞耗竭的重要特征[104]。由于各种肿瘤细胞表面均表达PD-1配体[105-106],这为使用基因编辑技术消除T细胞中的PD-1抵抗免疫抑制提供了依据。Beane等[107]使用ZFN敲除了从黑色素瘤患者分离的TIL中的PD-1PD-1的敲除不影响T细胞亚群和增殖,但它导致了更强的抗原特异性肿瘤细胞杀伤和体外细胞因子释放。Menger等[108]使用TALEN敲除从黑色素瘤和纤维肉瘤小鼠分离的肿瘤抗原特异性T细胞上的PD-1,结果发现PD-1敲除T细胞在肿瘤部位持续的时间更久,并且与未敲除的T细胞相比,能更好地控制肿瘤进展。CRISPR-Cas9系统也用于敲除人原代T细胞中的PD-1基因,并可增强基因修饰的T细胞功能[109-111]。本研究组以及另一研究组也使用CRISPR-Cas9敲除了CAR-T细胞中的PD-1,结果发现CAR-T细胞表面上的PD-1蛋白表达显著降低,而细胞增殖和免疫表型不受影响。基因编辑的CAR-T细胞在体内和体外均表现出更强的抗肿瘤作用。除PD-1之外,许多临床前研究发现TIM-3LAG-3PD-1在调节免疫反应和介导肿瘤逃逸中发挥协同作用,同时阻断PD-1LAG-3TIM-3具有协同增强的抗肿瘤作用[112-113]

通过基因编辑方法去除免疫检查点,可增强CAR-T细胞功能。但除了免疫检查点,FOXP3+调节T细胞(regulatory T cells, Treg)[114-115]、髓系抑制细胞(myeloid derived suppressor cells, MDSC)[116]等免疫抑制细胞,TGF-β[117]、IL-10[118]等发挥免疫抑制作用的细胞因子,IDO (indoleamine-2, 3-dioxygenase)[119]、腺苷[120]等可溶性成分,葡萄糖的缺乏,低氧等,都构成了“敌对”的肿瘤微环境,如何应用基因编辑工具去除免疫微环境对CAR-T细胞的抑制作用也是目前研究的热点及重点(图 2)。

T细胞在CAR识别位于癌细胞表面的抗原后被激活,并释放功能性细胞因子[IFN-γ、白细胞介素2(IL-2)]、穿孔素、颗粒酶等来消除癌细胞。然而在肿瘤微环境(TME)中,抑制性受体、Treg细胞、MDSC等免疫抑制细胞,TGF-β、IL-10等细胞因子,IDO等可溶性成分等,都构成了“敌对”的肿瘤微环境,抑制T细胞的功能。通过基因编辑敲除免疫抑制受体如PD-1、CTLA-4、TIM-3和LAG-3,TME对T细胞的免疫抑制效应部分解除,也使T细胞功能更强。使用基因编辑工具去除其他免疫微环境对CAR-T细胞的抑制作用也是目前研究的热点及重点。 图 2 基因编辑的T细胞在治疗肿瘤中的应用
2.2.2 通用型CAR-T的制备

目前CAR-T细胞主要使用患者自身的T细胞制备。这涉及为每个患者分离、修饰和扩增T细胞,整个过程耗时且费用昂贵。此外,对于新生儿、老年人或恶病质患者,通常难以获得具有良好质量的T细胞以产生患者所需的特异性CAR-T细胞。总而言之,自体T细胞的质量会影响这种疗法的广泛应用。

一种可能的解决方案是使用源自健康供体的T细胞产生通用型CAR-T细胞。为了实现这一点,需要消除同种异体CAR-T细胞上的TCR以避免移植物抗宿主病(GVHD)的发生,并且需要去除CAR-T细胞上的人类白细胞抗原Ⅰ类(HLA I)以尽量减少其免疫原性。为此,需要在T细胞中进行有效的多重基因编辑来敲除TCR、MHC或其他分子的表达。

TCR蛋白由α和β两条链组成,α链由单个基因TRAC编码,β链由两个TCRB基因编码,α链和β链形成二聚体以维持T细胞表面TCR的表达和功能。因此,敲除TRAC基因是消除T细胞表面TCR的最直接的方法。目前主要的基因编辑技术ZFN[121]、TALEN[122-124]和CRISPR-Cas9系统[125-127]已全部成功用于敲除CAR-T细胞上的TCR。TCR敲除的CAR-T细胞在体外和体内维持抗原特异性肿瘤细胞杀伤功能,对同种异体抗原的免疫应答显著降低,并且没有在小鼠中引起GVHD[122]。而在首次人类临床应用中,经过TALEN编辑的缺失TCR和CD52的抗CD19的CAR-T细胞被成功用于治疗复发难治性CD19+ B细胞急性淋巴细胞白血病的两名婴儿,证明了通用CAR-T细胞的临床效果。但是两位患者都出现了皮肤GVHD症状,这突出表明了进一步改善CAR-T细胞治疗的需要[128]

MHC I类分子也可以在T细胞表面被消除,通过敲除HLA I异二聚体的细胞表面表达必需的β2-微球蛋白来避免或延迟转移细胞的排斥[125-127, 129]。本研究组和其他研究组也使用CRISPR-Cas9技术在CAR-T细胞中进行了TCR和HLA的同时敲除。此外,本研究组同时敲除免疫检查点PD-1以开发更强大的通用CAR-T细胞[125-127]

其他需要考虑的是,HLA I类分子的缺失可能引发针对同种异体T细胞的NK细胞的应答。一种可能的解决方案是过表达其他HLA I类分子以避免NK细胞活化。有研究组通过敲除CD52[122]或脱氧胞苷激酶(dCK)基因[123],而不是敲除HLA I类分子,使CAR-T细胞获得对阿仑单抗嘌呤核苷酸类似物的抗性。这些药物通常用于化疗去除淋巴细胞,这可以确保移除患者的淋巴细胞,同时维持注入的同种异体CAR-T细胞。

2.2.3 定点整合CAR-T细胞的制备

基因编辑的另一个重要应用是开发定点整合的转基因CAR-T细胞。CAR基因大部分通过逆转录病毒或慢病毒转导导入T细胞基因组中,基因组中随机插入和高度可变的CAR表达水平[130]具有潜在的诱发肿瘤的风险。研究人员已将外源基因整合到人类淋巴细胞和造血干细胞的CCR5基因座和腺相关病毒位点1 (AAVS1)中,这是人类基因组中安全的位点,可以在不影响周围基因表达的情况下稳定表达转基因[131-132]。2017年的两项研究使用基因编辑工具酶在TRAC基因座产生缺口,随后转染含CAR基因的AAV定点插入到TRAC基因座,这不仅使T细胞中更均匀地表达CAR,而且还增强了T细胞效力,抑制了抑制信号转导、终末分化和衰竭[133-134]。值得注意的是,Roth等[135]通过电转的方式将Cas9蛋白和sgRNA以及双链DNA模板的混合物导入T细胞,真正实现了在无病毒载体的情况下,将长DNA片段快速且高效地定点整合到CRISPR-Cas9靶向的基因组位置,并且保持了T细胞的活力和功能。

基因编辑的T细胞无论在科学实验还是临床研究中对肿瘤和HIV的治疗都展示出一定优势。但是基因编辑的T细胞在临床治疗上还存在一定的问题。最主要的是基因编辑的安全性问题。目前有大量的研究工作集中在提高编辑效率和降低脱靶性上[136-139],但是针对不同的个体基因编辑的效率及脱靶效应也不近相同[140],所以要针对不同个体定制基因编辑策略。另一方面,机体的免疫系统如何应答基因编辑的细胞仍是待解决的问题。虽然仍存在问题,但是基因编辑技术和过继细胞免疫治疗的结合仍是肿瘤治疗和免疫相关疾病治疗的发展前景和趋势。

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