2018, Vol. 13
人体免疫系统中最早被用于临床治疗的就是抗体。早在20世纪初，白喉抗毒素就曾以马血清制备的形式用于治疗白喉患者。虽然白喉杆菌是革兰阳性菌，可用青霉素类药物治疗，但其产生的外毒素引起的心肌中毒等症状必须用白喉抗毒素治疗。1901年，Bering也因为发现了白喉抗毒素和应用血清疗法而获得首届诺贝尔生理学或医学奖[1]。在化学药物(包括抗生素)被发现前，抗体是有效治疗手段，一些抗菌抗体(如针对不同血清型肺炎链球菌的抗体)曾用于治疗。用动物制备的抗体进行治疗时最棘手的问题是发生过敏反应，如过敏性休克、血清病等。因此，化学药物(包括抗生素)治疗兴起后，大部分感染治疗中抗体治疗被摒弃。但作为抗毒素抗体，在肉毒杆菌感染及少数厌氧梭状芽胞杆菌感染如破伤风的治疗中，抗体仍为首选。在病毒感染特别是突发的感染如严重急性呼吸综合征(severe acute respiratory syndrome，SARS)、埃博拉病毒(Ebola virus)感染中，对个别病例曾用过抗体治疗，并取得了较好效果，但未被正式纳入治疗方案。此外，针对一些可能是自身免疫性疾病或免疫紊乱疾病如川崎病，早期大剂量静脉注射免疫球蛋白是有效治疗方法。随着科学技术的发展，基因重组单克隆抗体(简称单抗)及人源化抗体、全人源抗体正逐步取代动物血清来源抗体[2]。治疗性抗体在抗肿瘤领域已取得显著效果和广泛应用，而针对微生物感染的抗体则较多用于预防，如偶然人类免疫缺陷病毒(human immunodeficiency virus，HIV)感染、乙型肝炎、甲型肝炎、麻疹等[3]。但随着大量耐药微生物的出现及一些持续性病毒感染病例的增多，抗微生物感染领域中治疗性单抗正被积极研发并推进临床研究。
2 单抗在感染治疗中的应用目前，单抗治疗病毒感染的效果还相当有限。第1个由美国食品药品管理局(Food and Drug Administration，FDA)批准用于临床的抗体是一种抗呼吸道合胞病毒(respiratory syncytial virus，RSV)的免疫球蛋白多克隆抗体(简称多抗)RSV-IGIV(RSV immune globulin intravenous)，仅用于低体重的RSV感染高危新生儿，作为被动预防[4]。以后改用RSV人源化单抗帕利珠单抗(Palivizumab)，其比多抗有更好的保护作用，且可显著减少不良反应。
广谱中和抗体的发现与建立，在推进单抗治疗感染性疾病中发挥了重要作用。例如，抗反转录酶药物用于治疗HIV-1感染已达20年以上，辅以抗体的预防与治疗作用将进一步控制HIV感染，并协助持续抑制病毒复制，可能对细胞中的贮存病毒库有抑制作用[5]。在HIV感染中，病毒包膜糖蛋白(envelope，Env)gp120、gp41分子可诱生特异抗体，但因HIV突变可逃逸相应抗体的中和作用。随着病毒抗原的不断变异，感染者体内的抗体也会不断随变异的毒株而表现出多元化，即感染者体内抗体可出现更为广谱的中和效果。虽然患者体内的病毒并不被其体内广谱中和抗体所中和而得以继续复制，但这些广谱中和抗体对猴免疫缺陷病毒(simian immunodeficiency virus，SIV)有预防和治疗效果[6]，且对其他HIV毒株有广谱中和作用。如VRC01广谱中和抗体就有阻断病毒结合CD4受体的作用，可中和90%的不同HIV毒株，半数抑制浓度(half maximal inhibitory concentration，IC50)达0.33 μg/mL[7-8]。
针对广泛存在的狂犬病毒，来自疫苗注射者的多抗是多年暴露后的抗体，产量极其有限。现已获得几株有广谱中和作用的人源单抗(RVC20和RVC58)，可中和35株狂犬病毒及25株狂犬病毒属(Lyssavirus)的其他非狂犬病毒[9]。
在呼吸道感染方面，对于新出现的流感病毒，在新型疫苗尚未成批提供应用之前，生物技术制备的广谱中和抗体可为流感防治提供新的思路。例如，MEDI8852单抗可与甲型流感病毒血凝素(hemagglutinin，HA)茎部(stalk)高度保守的表位结合，通过阻断HA的构象变化来阻止HA介导的膜融合，从而中和所有甲型流感病毒株，目前已进入Ⅱ期临床研究[10]。此外，还有CR6261单抗(针对H5N1、H1N1等死亡率高的流感病毒的保护性抗体)、CR8020单抗(可中和H7N9的广谱中和单抗)等已在研究或进入临床研究中[11]。
除上述单抗外，针对一些突发性传染病如埃博拉、寨卡的单抗也可用于急性预防或治疗感染。西非埃博拉疫情暴发中，2014—2015年有28 652人被感染，约11 325人死亡。当时应急特批了鸡尾酒式单抗ZMapp(3种单抗：13C6、2G4和4G7)作为治疗[12]。标准临床研究结果显示，尽管ZMapp治疗组比对照组死亡率降低40%，但受限于较小的临床研究样本(71人)，未能对ZMapp是否有效作出确切结论。此后，从1例埃博拉病毒感染恢复者的B细胞中获得了有更强中和效应的单抗，但需进一步考核其预防及治疗效果。寨卡病毒与小头畸形症的紧密关联大大推进了寨卡疫苗研发，已发现单抗Z23、Z3L1等可结合于病毒包膜的Ⅰ、Ⅱ或Ⅲ结构域表位，有潜在治疗作用[13]。
针对乙型肝炎病毒(hepatitis B virus，HBV)的单抗主要在HBV流行地区开展。夏宁邵等报道了针对HBV外膜蛋白一个特定表位的鼠源单抗E6F6，并改造为人源化，发现其具有中和作用，在转基因鼠中表现出很强的治疗效果[14]。北京李文辉团队也研发了针对HBV PreS1的单抗，不仅能阻断HBV感染细胞，还可在动物实验中抑制病毒复制[15]。复旦大学基础医学院医学分子病毒学教育部/卫生部重点实验室协同中国疾病预防控制中心病毒病预防控制所从HBV疫苗接种者的B细胞中筛选获得了可中和HBV表面抗原(HBV surface antigen，HBsAg)的高活性中和抗体G12[16]。目前，这些单抗正在申报临床研究，以考核其临床疗效与价值。
3 单抗治疗的重要机制——Fab与Fc以往对单抗治疗机制的研究仅限于其Fab段可中和相应微生物的抗原部分，具有明确的保护作用。结合抗体后的病原体可被吞噬细胞消化，还可通过激活自然杀伤(natural killer，NK)细胞或补体产生抗体依赖细胞介导的细胞毒作用(antibody-dependent cell-mediated cytotoxicity，ADCC)或补体介导的细胞毒作用(complement-dependent cytotoxicity，CDC)[17-19]。近年来，人们更重视抗体Fc段在调控机体免疫系统中发挥的更多、更复杂的作用。已知数种免疫细胞如树突细胞(dendritic cell，DC)、巨噬细胞、B细胞等具有Fc受体，抗体可通过Fc受体激活多种免疫细胞，特别是经抗原呈递细胞加工与呈递后作用于不同类型T细胞，通过Th1细胞激活有效的细胞免疫，并促进γ干扰素(interferon γ，IFN-γ)、肿瘤坏死因子α(tumor necrosis factor α，TNF-α)等细胞因子的调控作用[20]。Fc段可通过一定机制降低调节性T细胞(regulatory T cell，Treg)功能，上调细胞毒性T细胞(cytotoxic T cell，CTL)的杀伤作用，还可通过上调B细胞功能以促进抗体的形成与分泌[21-23]。此外，Fc段中糖链的不同组成对抗体发挥作用也有不同的影响[24-25]。
4 结语随着对抗体分子结构与功能研究的进一步开展及对抗体介导免疫机制的揭示，治疗性抗体有更大的发展空间与应用前景，但必须考虑以下几个问题。①由于研发与生产人源单抗的经费较高，一些能简便生产抗体的技术，如用原核细胞表达单链抗体或新结构抗体，或用植物作为表达载体的技术可能在降低成本方面有所作为[26-27]。②由于Fc段糖链在抗体发挥免疫调控作用中有新的贡献，人工改造糖链可获得更有效的单抗，在抗微生物、抗肿瘤、抗自身免疫疾病方面有更好的临床效果[28-29]。③鉴于人源单抗针对的抗原表位一般较为单一或局限，研究几株抗体联合或鸡尾酒式应用，将是临床研究考核的重要内容。④抗体治疗属被动免疫治疗范畴，一般必须多次注射，出现疗效减退及不良反应难以避免。如何有效地将抗体治疗与药物或其他治疗方法联用，应及早进行临床考核。⑤可考虑应用抗原-抗体复合物型免疫治疗，一方面充分利用抗体Fc段的免疫调节功能，另一方面利用抗体介导抗原进入抗原呈递细胞的免疫调节作用。这一技术已在机制及应用方面得以阐明[30-34]，并在我国免疫复合物型HBV治疗性疫苗中证实其有一定的临床疗效[35-36]。
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