综述丨靶向肿瘤免疫微环境的肿瘤免疫疗法

肿瘤严重影响人类的健康，是人类的第二大杀手。肿瘤的形成可分为两个层次，第一个层次是一个正常的细胞变成一个肿瘤细胞，第二个层次是肿瘤细胞发展为肿瘤组织，过去40多年以抑癌基因与促癌基因为主的研究已经把细胞癌变的原理解释的比较清楚了，但是肿瘤细胞怎样形成肿瘤组织的还不是很清楚，而肿瘤的微环境是其中的关键。肿瘤微环境中的细胞可以分为免疫细胞、肿瘤相关的成纤维细胞、血管形成细胞三个类群[1],近年来肿瘤免疫发展很快，以PD-1、CAR-T为代表的肿瘤免疫疗法革新了肿瘤治疗，本文对靶向肿瘤微环境的肿瘤免疫疗法进行综述。肿瘤微环境中免疫细胞可分为三大类，第一类是只有抑癌作用的CTL (cytotoxic lymphocyte）和NK（natural killer cell），第二类是既有促癌又有抑癌作用的巨噬细胞、中性粒细胞，第三类是只有促癌作用的MDSC (Myeloid-derived suppressor cells) 、Treg（Regulatory T cell）、Mast cell等。

靶向CTL的免疫疗法
靶向CTLA-4（cytotoxic T-lymphocyte-associated protein 4）的免疫疗法

CTLA-4是在1987年首次被克隆出来[2]，其配体CD80也在4年后被确定[3],随后发现了CTLA-4的免疫抑制能力[4]。CTLA通过两种机制抑制T细胞的活化，第一T细胞表面的CTLA-4比CD28结合CD80/86的能力要强，抑制T细胞第二活化信号的传导，第二CD80/86与CTLA-4结合后，CTLA-4向T细胞提供抑制信号[5]。2010年CTLA-4抗体lpilimumab的首个III临床结果表明，lpilimumab能够延长晚期黑色素瘤患者的生命[6]，lpilimumab与2011年获FDA批准上市，成为第一个免疫检查点抑制剂。

靶向PD-1（programmed cell death protein 1）/PD-L1的免疫疗法

PD-1是在1992年首次被克隆出来[7]，其配体PD-L1与1999年被发现[8]，并在同一年发现PD-1有免疫抑制功能[9]。相比于CTLA-4及其配体，PD-1及其受体都是在肿瘤微环境中才表达，理论上具有更小的副作用，随后的临床试验结果也证明了这一点。2014年PD-1抗体Opdivo、keytruda相继通过FDA批准，并随后在多种类型的肿瘤治疗中大放异彩，另外靶向PD-L1的Tecentriq、Bavencio、Imfinzi单抗也在2017年先后获FDA得上市申请，PD-1/L1已经成了最热的肿瘤治疗靶点。

靶向IDO（indoleamine-(2,3)-dioxygenase）的免疫疗法

IDO通过多种方式抑制T细胞的活性，第一IDO的高表达导致细胞局部色氨酸耗竭，诱导T细胞停滞于G1期，抑制T细胞的增殖活化[10]；第二色氨酸的耗竭可以加强调节性T细胞介导的免疫抑制[11]；第三IDO作用下色氨酸的代谢产物N-甲基犬尿酸存在细胞毒性，可以直接溶解T细胞[12]。尽管IDO抑制剂可以通过多种方式起到激活T细胞的作用，但是单独使用IDO抑制剂并不能带来临床上的益处，与其他抗癌药物连用成为了其唯一出路，但是多个临床联合用药实验都表现出了令人失望的数据，IDO的前景似乎一片灰暗。

靶向NK细胞的免疫疗法
靶向NK抑制分子的免疫疗法

KIRs（killer cell immunoglobulin-like receptors）与其配体MHC-1（major histocompatibility complex-1）结合能够抑制NK细胞的细胞毒活性[13]，单克隆抗体IPH2101是KIRs的抑制剂，临床前研究表明IPH2101能够促进NK细胞消灭肿瘤[14][15]，I/II的临床结果表明IPH2101并不会引起严重的副作用[16][17]，但是其作为单一药物的治疗效果不够明显，联合疗法也许能给其带去一些希望。CD94/NKG2A异源二聚体与其配体HLA-E结合后亦能抑制NK细胞的细胞毒活性，且HLA-E在多种肿瘤中表达量上升[18][19]，针对NKG2A的单克隆抗体IPH2201正在进行多个临床研究。TIGIT（T cell immunoglobulin and immunoreceptor tyrosine-based inhibitory motif domain）与其受体CD155结合后能抑制NK细胞的细胞毒活性[20][21]，田志刚、孙汭组研究表明TIGIT单抗可以显著延长小鼠的生存期[22]。

靶向NK细胞激活信号的免疫疗法

尽管NK细胞表面有很多激活型受体，例如NCRs（the natural cytotoxicity receptors）系列的NKp30、NKp44和NKp46，CD226等，但是想要激活这些受体并不容易。NK细胞表面会表达CD16，它能与单克隆抗体的Fc结合，引发ADCC作用，基于此原理的α-CD20,α-GD2, α-Her2, and α-EGFR单克隆抗体已经成功上市，并且各种双特异性单抗也在紧密研发中。NKG2D（natural killergroup 2D）与其配体MICA 、MICB作用能够提升NK细胞的细胞毒活性[23]，但是MICA 和MICB被蛋白酶裂解能够封闭此激活作用，使用单抗抑制MICA 和MICB的裂解能够提升小鼠的生存率[24]。

靶向巨噬细胞的免疫疗法
激活巨噬细胞的吞噬活性

巨噬细胞表面的SIRPα（signal regulatory protein alpha）与肿瘤细胞表面的CD47结合可以抑制巨噬细胞的吞噬能力，鉴于巨噬细胞可占肿瘤组织中50%[25]，解放巨噬细胞可能带来巨大的临床效益。CD47的单克隆抗体Hu5F9-G4在小鼠模型中延长了小鼠的生存时间，并且没有严重的副作用[26]，目前已有Forty Seven等至少5家公司在进行针对CD47的临床研究。

消灭M2巨噬细胞

巨噬细胞可以分为抗肿瘤的M1型巨噬细胞和促进肿瘤的M2型巨噬细胞，M2巨噬细胞可以促进肿瘤血管增长[27]、侵袭与转移[28]、增强癌细胞的化疗抵抗性[29]，,而肿瘤微环境中主要以M2型巨噬细胞为主，巨噬细胞具有很大的可塑性[30]，将M2型巨噬细胞转变为M1型巨噬细胞可显著改善肿瘤微环境。肿瘤细胞借助IIa类HDAC来调节巨噬细胞的增殖与分化，通常情况下这种酶会使巨噬细胞向M2型转变[31]，HDAC的抑制剂TMP195可以减少小鼠体内M2型巨噬细胞的数量,并且提高化疗与PD-1抑制剂的疗效与耐受性[32]。CSF-1R[33]、TIE2对于巨噬细胞的存活很关键，抑制它们的活性可以减少M2巨噬细胞的免疫抑制功能，相关临床试验正在进行中。

靶向MDSC的免疫疗法
MDSC可以通过多种方式来抑制免疫反应，第一其释放的活性氧分子与活性氮分子能够破坏CD3与TCR的相互作用[34]、IL2[35]、MHC-I与TCR的识别[36]来抑制T细胞的激活，第二其能够耗竭T细胞活化增殖所需的必须氨基酸[37][38]，第三它抑制T细胞的招募与在肿瘤内的移动[39][40]，第四它通过分泌多种细胞因子促进Treg细胞的产生[41][42][43]。APOE可以与MDSC细胞表面的受体LRP8结合，促进MDSC的死亡[44]，此外APOE还可以促进血管增生与肿瘤转移[45]，是很好的靶点，目前正在临床实验中。

靶向Treg的免疫疗法
Treg能够与肿瘤发展有很大关系[46][47]，在免疫抑制中起到非常重要的作用。CCR4在Treg细胞表面高表达而在T细胞表面极少表达[48]，它在受肿瘤组织中分泌的CCL22作用下将细胞招募到肿瘤组织，抑制CCR4可以增强抗肿瘤活性，相关临床试验正在进行中。GITR（glucocorticoid-induced TNF-receptor family related protein）和OX40是共刺激活化信号受体，其在Treg中组成型高表达而在静息的CD4+/CD8+细胞中短暂表达，阻断GITR、OX40与其配体的相互作用能够抑制Treg的免疫抑制活性[49][50]，相关临床试验正在进行中。

联合治疗
由于这些免疫细胞之间具有相似性，一个靶标可以出现在不同的细胞表面，靶向不同细胞上的同一靶标可能引起协同、拮抗两种截然不同的结果。CTL、NK、Treg细胞表面都可以表达CTLA-4，CTLA-4单抗不仅可以直接解除CTLA-4对CTL、NK细胞的抑制作用[5][51]，还可以通过减少Treg的细胞数来间接地提升CTL、NL细胞的抗肿瘤活性[52]，此时靶向CTLA-4的单抗在不同细胞中发挥着协同效应。TIM3是一个免疫检查点，它在CTL细胞与NK上都有表达，TIM3在CTL上表达意味着CTL功能紊乱，而在NK细胞上表达与NK细胞的成熟有关，虽然靶向CTL上的TIM3能逆转T细胞耗竭[53]，但是靶向NK细胞上TIM3的实验结果却有些矛盾[54][55]，靶向TIM3的免疫疗法需进一步明确对NK细胞的影响。CD25对CTL于Treg的增殖活化都很重要[56][57]，引起靶向CD25会引发拮抗反应，不是很好的免疫靶点。

肿瘤免疫微环境由多种免疫细胞共同塑造，并且这些细胞之间可以相互影响[58]，同时靶向不同的细胞具有协同效应。虽然目前只有针对T细胞的免疫药物上市，免疫联合用药也因此在很大程度被局限在T细胞上，但是随着靶向其他免疫细胞的药物上市，联合用药的空间将快速增加。

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