第二节 抗CTLA-4抗体不良反应机制

彭 玲 董晓荣

华中科技大学同济医学院附属协和医院

目前,临床广泛应用的免疫治疗药物是PD-1/PD-L1抑制剂和CTLA-4抑制剂(抗CTLA-4抗体),免疫治疗在带来众所期望疗效的同时,也带来了有别于以往治疗的不良反应。本节重点阐述抗CTLA-4抗体不良反应的发生机制。目前抗CTLA-4抗体有两种:伊匹木单抗(ipilimumab)和曲美木单抗(tremelimumab)。

T细胞表面的共刺激受体CD28与抗原提呈细胞(APC)表面共刺激分子CD80/86结合,提供T细胞活化增殖信号,同时活化的T细胞表面诱导性表达CTLA-4,竞争性结合APC表面CD80/86,抑制T细胞过度活化及炎症因子过度释放,维持机体免疫平衡。除竞争性结合CD80/CD86分子外,CTLA-4与其配体结合后激活下游信号通路,通过降低白细胞介素(interleukin,IL)-2等表达抑制抗原活化的T细胞增殖。天然存在的Foxp3+CD4+Tregs对机体维持自身免疫耐受和免疫稳态至关重要。CTLA-4在Foxp3+CD4+Tregs组成性高表达,作为免疫抑制因子,它可以阻止免疫系统攻击正常组织,发挥免疫抑制功能。抗CTLA-4抗体可打破自身免疫的外周耐受,导致免疫系统攻击正常组织和器官,产生免疫相关不良反应(immune-related adverse effects,irAEs)。对CTLA-4的基础研究发现,小鼠敲除CTLA-4基因后会因淋巴增殖和多器官组织结构破坏而死亡,而清除小鼠体内CD4+T细胞而非CD8+T细胞小鼠可以存活,因此推测CD4+T细胞参与敲除CTLA-4后导致相关疾病。抗CTLA-4抗体会导致自身反应性记忆T细胞活化,促进循环T细胞增殖,增加CD4+和CD8+T细胞库的多样性。对治疗早期T细胞库变化进行探索发现,治疗初期新检测出的T细胞克隆及增殖与irAEs的发生相关。T细胞克隆性的变化先于irAEs发生,这意味着,通过早期检测新的T细胞克隆,可以预测irAEs的发生。在抗CTLA-4治疗黑色素瘤模型中,增强的抗体依赖细胞介导细胞毒性(antibody-dependent cell-mediated cytotoxicity,ADCC)效应及肿瘤微环境中调节性T细胞(regulatory T cells,Tregs)的选择性消耗可以增加抗肿瘤效应,但同时也能增加irAEs发生风险。抗CTLA-4抗体导致irAEs发生的潜在机制还包括:针对肿瘤和健康组织中存在的交叉抗原特异性T细胞活性增加,预先存在的自身抗体水平增加,炎性细胞因子水平升高以及由于抗CTLA-4抗体与在正常组织(如垂体)表达的CTLA-4直接结合,通过增强补体介导的炎症导致组织损伤,但具体发生机制仍需深入研究。抗CTLA-4抗体导致的irAEs多见于皮肤、胃肠道、内分泌系统,也可见于神经系统、肺、心脏、肝脏、肾脏等。目前发现,抗CTLA-4抗体治疗导致的irAEs常发生于初次治疗3个周期内,即接受治疗10周左右,但不同的器官有其特殊性,irAEs发生时间有所不同。抗CTLA-4的irAEs总发生率为72%,高级别irAEs总发生率为24%,出现irAEs的风险与剂量相关,如3mg/kg的伊匹木单抗(ipilimumab)和10mg/kg的伊匹木单抗对应的所有级别irAEs的发生率分别为61%和79%,0.86%的伊匹木单抗使用者因irAEs导致死亡。

本节将对irAEs在各系统中的表现和潜在可能机制进行简要总结。

一、皮肤

皮肤毒性是抗CTLA-4抗体治疗过程中最常见的免疫相关不良反应,出现时间早于其他irAEs,表现为瘙痒、皮疹,甚至皮肤坏死、白癜风等。有多达半数患者诉抗CTLA-4抗体使用过程中有皮肤不良反应,皮疹发生率为43%~45%,且当抗CTLA-4抗体与抗PD-1抗体联用时,皮疹发生率显著提高,皮肤毒性发生时间更早,更加严重,因而在多种抗体联合治疗肿瘤时要及早防范皮肤损害。大部分皮肤毒性表现为1/2级毒性,通过对症治疗可以恢复,因此不需要中断免疫检查点抑制剂(immune checkpoint inhibitors,ICIs)治疗,但这需要临床医生早发现并及时干预。3/4级毒性反应多发生在免疫联合治疗中,但发生率 < 5%。白癜风一般属于不可逆皮肤损害,中断或终止免疫治疗和对症处理并不能缓解白癜风表现。然而有文献报道,白癜风表现可能预示肿瘤对免疫治疗有较好的反应。糖皮质激素对皮肤损害治疗效果良好,提示免疫亢进是皮肤毒性反应的重要机制。病理研究发现,皮肤浅表血管周围有T淋巴细胞浸润,以CD3+/CD4+T淋巴细胞为主,并可延展至表皮组织,散在于坏死的角质细胞和簇状嗜酸性粒细胞中。使用抗CTLA-4抗体的患者外周血嗜酸性粒细胞增加,促成皮疹形成。因此,T淋巴细胞过度活化和嗜酸性粒细胞增多是抗CTLA-4抗体导致皮肤毒性的原因所在。

二、胃肠道

胃肠道毒性是抗CTLA-4抗体发生频率最高、最严重的irAEs之一,包括口腔溃疡、食管炎、胃炎、结肠炎、腹泻等。接受抗CTLA-4治疗的患者腹泻和结肠炎发生率分别为27%~54%、8%~22%,多发生在首次治疗后35~49d。大多数患者病变累及乙状结肠和直肠。内镜下多表现为黏膜水肿、红斑、溃疡、渗出、糜烂或出血。临床主要表现为腹泻、腹痛、血便、恶心,发热和呕吐较少见,少数患者可出现致死性肠穿孔。病理活检显示,肠道固有层大量淋巴细胞浸润,多为CD8+T淋巴细胞,伴随浆细胞和嗜酸性粒细胞增多,肠隐窝结构被破坏,隐窝炎、隐窝脓肿。血清IL-17和外周血嗜酸性粒细胞增加与胃肠道irAEs相关。有研究表明,肠道Tregs诱导分化需要CTLA-4与CD80/CD86分子介导,这些Tregs参与维持肠道共生菌群。有文献报道,肠道菌群与免疫性结肠炎相关,拟杆菌降低结肠炎发生率,而粪杆菌增加结肠炎发生率。因此,调节肠道菌群对改善免疫相关性肠炎也有一定作用。抗CTLA-4抗体使用过程中会耗竭肠黏膜Tregs,同时效应T细胞过度激活,循环记忆T细胞增加,肠黏膜淋巴细胞浸润增加,出现肠道免疫稳态失衡,从而导致肠炎发生。有研究发现,抗CTLA-4治疗的患者CD4+T细胞明显增加,肠炎严重程度也与患者自身的基因多态性相关。

三、内分泌系统

抗CTLA-4抗体治疗时,甲状腺功能紊乱的发生率为1%~5%。欧洲肿瘤内科学会年会(European Society for Medical Oncology,ESMO)irAEs管理指南指出,使用ICIs后,可能会出现甲状腺功能减退(简称甲减)及甲状腺功能亢进(简称甲亢),联合抗PD-1抗体治疗时发生率增高,前者更为多见,后者往往随着病情发展会转成甲状腺功能减退。具体的病理学机制还不明确,多认为是T细胞介导,而非B细胞介导。

垂体炎是抗CTLA-4抗体导致的常见内分泌毒性表现,发生率约为3.2%,需要尽早干预,否则可能导致严重后果。抗CTLA-4抗体介导的内分泌毒性表现,如垂体炎,最早出现在接受免疫治疗后的7~8周,通常发生在接受免疫治疗12周,也有文献报道在接受伊匹木单抗(ipilimumab)治疗19个月后出现垂体炎。免疫性垂体炎在抗CTLA-4抗体治疗过程中比较常见。抗CTLA-4抗体与垂体内分泌细胞上的CTLA-4结合后被单核巨噬细胞系统吞噬,进而激活补体介导的免疫炎症反应,影响垂体功能。病理检查提示大量免疫细胞包括淋巴细胞、浆细胞浸润垂体,垂体体积增大,随后出现萎缩,因此磁共振成像(magnetic resonance imaging,MRI)/计算机断层扫描(computer tomography,CT)可表现为垂体增大和强化增强。免疫性垂体炎症状与腺垂体功能减退导致相关激素缺乏和垂体占位效应有关,最常累及下丘脑-垂体-肾上腺轴、下丘脑-垂体-甲状腺轴及下丘脑-垂体-性腺轴,常表现为头疼、乏力、勃起功能障碍和性欲减退,但视觉障碍少见。50%~60%垂体-甲状腺轴和垂体-性腺轴可以恢复,而垂体-肾上腺轴较少得到改善,因此有些患者需要永久性激素替代治疗。

1型糖尿病以及原发性肾上腺皮质功能减退并不常见,早发现、早干预常预后良好。抗CTLA-4抗体导致的1型糖尿病可能和糖尿病自身抗体有关,也可能与患者自身基因多态性和遗传易感性有关。原发性肾上腺皮质功能不全(primary adrenal insufficiency,PAI)极其罕见,表现为皮质醇水平较低而促肾上腺皮质激素(adrenocorticotropic hormone,ACTH)水平升高,其机制尚不清楚,可能与肾上腺皮质抗体升高有关,继发性肾上腺皮质功能不全可由垂体功能不全引起。

除了甲状腺功能减退及糖尿病外,大部分内分泌相关irAEs达到一定级别,需要糖皮质激素治疗。

四、神经系统

抗CTLA-4抗体引发的神经系统毒性发生率为3.8%,大多数为1~2级,3~4级irAEs发生率低于1%,中位发生时间为6周。抗CTLA-4抗体治疗神经毒性表现为周围神经病变、神经炎、脑炎或吉兰-巴雷综合征及罕见的严重性神经毒性,即使中断抗CTLA-4抗体治疗,神经系统毒性仍会存在。因此,必须尽早发现并紧急干预,对危重甚至威胁生命的患者应该永久性停止抗CTLA-4抗体治疗。病理学研究显示,抗CTLA-4抗体可导致神经血管周围炎症,炎症可累及神经系统本身。免疫组化结果显示,病变组织内可见CD4+T细胞和CD8+T细胞浸润,由此推测抗CTLA-4抗体导致的神经毒性由T细胞免疫介导参与。

五、肺脏

免疫相关性肺炎在抗CTLA-4抗体单药治疗中的发生率约为1%,主要症状包括呼吸困难、咳嗽、发热、胸痛、低氧血症、虚弱,35%患者可无临床症状。影像学表现可为隐源性机化性肺炎(cryptogenic organizing pneumonia,COP)、磨玻璃样改变(ground glass opacity,GGO)、非特异性间质性肺炎(nonspecific interstitial pneumonia,NSIP)、过敏性肺炎(hypersensitivity pneumonia,HP)和其他非特异性肺炎。病理诊断方面目前证据不足,不能直接由病理诊断来判定是否为治疗导致的免疫相关性肺炎,需结合临床综合分析。

一项抗CTLA-4抗体联合激素去势治疗前列腺癌的研究发现,发生irAEs的患者外周血中的自身反应性T细胞克隆增殖,T细胞多样性增加,CD8+T细胞克隆扩增先于2~3级irAEs,说明肺毒性可能由抗原特异性细胞毒性T细胞功能亢进造成。病理检查发现,患者的肺部和支气管肺泡灌洗液(bronchoalveolar lavage fluid,BALF)含有大量T淋巴细胞,以CD8+T细胞为主,且BALF中中枢记忆性T细胞增加,Tregs细胞上的CTLA-4和PD-L1表达降低,表明抗CTLA-4抗体治疗会使肺内免疫表型由免疫抑制向促炎方向转化。因此,抗CTLA-4抗体可能会促进肺内炎症发展,最终造成肺纤维化,甚至呼吸衰竭。

六、心脏

ICIs引起的心肌炎、心包炎和心血管异常并不常见,发生率估计不超过1%。最常见的免疫相关心脏毒性表现是免疫性心肌炎,可与肌炎、重症肌无力伴随发生。其发生中位时间在免疫初始治疗后34d,一般在3个月内出现,可表现为轻度疲劳,伴或不伴肌无力。患者发生心肌炎后,可导致心力衰竭和致死性心律失常,死亡率极高,需引起重视。临床试验发现,抗PD-1/PD-L1基础上加用抗CTLA-4患者死亡风险增加近2倍。心肌活检是诊断免疫性心肌炎的金标准,可见大量CD4+T细胞、CD8+T细胞、CD68+巨噬细胞浸润。CTLA-4基因敲除小鼠心肌中有CD4+和CD8+T淋巴细胞浸润,提示抗CTLA-4抗体可能会导致细胞介导的自身免疫性心肌炎发生。

七、肝脏

抗CTLA-4抗体治疗导致的肝毒性一般表现为无症状的谷草转氨酶(glutamic-oxaloacetic transaminase,GOT)/谷丙转氨酶(glutamic-pyruvic transaminase,GPT)升高,伴或不伴胆红素升高,发生率 < 10%,严重者可出现肝衰竭甚至死亡,较为罕见。免疫性肝炎通常出现在首次用药后8~12周。肝活检显示肝炎主要以小叶炎症为主,门静脉炎症较轻,一般没有胆管损伤,组织中有弥漫性T淋巴细胞浸润,主要是CD3+/CD8+T淋巴细胞,提示肝毒性可能主要由CD8+T细胞介导的免疫损伤促成。

八、肾脏

抗CTLA-4抗体导致肾损害的比例不高,发生率不足1%,通常表现为急性间质性肾炎,一般发生在免疫治疗后的2~6个月,平均3个月。研究表明,抗CTLA-4抗体(伊匹木单抗)会抑制Tregs功能,打破局部免疫稳态,引起肾组织炎症细胞浸润,导致急性间质性肾炎(acute interstitial nephritis,AIN)、肉芽肿病变,还可使得肾单位损伤、足细胞病变,造成与膜性肾病相似的肾病综合征甚至狼疮性肾炎,抗CTLA-4抗体(伊匹木单抗)引起的垂体炎可导致泛垂体功能减退,继发肾上腺功能不全,导致电解质紊乱,出现低钠血症。

九、血液系统

血液系统irAEs包括免疫性血小板减少性紫癜(immunologic thrombocytopenic purpura,ITP)、溶血性贫血(hemolytic anemia,HA)、再生障碍性贫血、纯红细胞发育不良、中性粒细胞减少症、全血细胞减少、炎症释放综合征伴嗜血综合征,这可能与自身免疫反应有关。血液系统irAEs很罕见,抗CTLA-4抗体的发生率约为0.5%,低于抗PD-1/PD-L1抗体,但通常表现为严重不良反应,死亡率约为14%。常见的严重致死性不良反应包括炎症释放综合征伴嗜血综合征、全血细胞减少和再生障碍性贫血。有文献综述报道,血液系统毒性出现在免疫治疗开始后1~84周,平均出现时间在免疫治疗开始后10周左右。

十、风湿系统

自身抗CTLA-4抗体被发现存在于各种自身免疫性疾病中,包括系统性红斑狼疮、类风湿关节炎和干燥综合征(Sjögren syndrome),说明抗CTLA-4抗体可能会引起风湿系统性疾病。临床试验也发现,使用抗CTLA-4抗体会促成炎症性关节炎,关节痛和肌肉痛也并不少见,这些可能与体内抗CTLA-4抗体浓度过高有关。

十一、眼

抗CTLA-4抗体(伊匹木单抗)治疗引起免疫功能失调时,患者会出现眼部症状。抗CTLA-4抗体(伊匹木单抗)眼毒性的发生率为1.3%,包括前葡萄膜炎、视神经病变、Grave综合征样眼病和Vogt-Koyanagi-Harada(VKH)样综合征,以及其他一些自身免疫性反应引起的罕见性病变。

总之,抗CTLA-4抗体导致的irAEs可累及各个系统及器官,大部分为低级别irAEs,给予对症处理而不需要中断免疫治疗,然而部分高级别irAEs需要中断甚至终止免疫治疗,并需要大剂量激素和免疫抑制剂处理,如不及时处理可危及患者生命。早期发现并及时干预不但可以减少因irAEs导致的免疫治疗中断,而且可以减少免疫抑制剂及激素的使用。然而irAEs表现复杂多样,目前尚无有效早期预测irAEs的标志物。了解irAEs的机制及在其各个系统的临床表现,有助于临床医生早期发现irAEs并及时处理,从而提高免疫治疗抗肿瘤效应。

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