人类免疫缺陷病毒潜伏库定量检测技术的研究进展
2016-03-05金姗何涌泉杨瑜张晓燕
金姗,何涌泉,杨瑜, 张晓燕
复旦大学附属公共卫生临床中心,上海 201508
人类免疫缺陷病毒潜伏库定量检测技术的研究进展
金姗,何涌泉,杨瑜, 张晓燕
复旦大学附属公共卫生临床中心,上海 201508
摘要:以静息CD4+T细胞为主的人类免疫缺陷病毒(human immunodeficiency virus,HIV)潜伏库的清除已成为治愈HIV-1感染的主要障碍,人们迫切需要建立一种高通量、可靠的、高灵敏度的方法来定量检测病毒潜伏库的真实大小。本文就目前关于HIV潜伏库的多种定量检测方法进行综述。
关键词:人类免疫缺陷病毒;潜伏库;定量检测
尽管高效抗反转录病毒治疗(highly active antiretroviral therapy,HAART)能有效阻断人类免疫缺陷病毒(human immunodeficiency virus,HIV)感染患者体内的病毒复制,使血浆中病毒载量控制在极低水平(<50拷贝/ml),然而在治疗中断后的2~8周内,多数患者会出现血浆病毒载量反弹[1]。原因是HIV在感染急性期就形成了以静息记忆CD4+T细胞为主的病毒潜伏库(viral reservoir)。病毒潜伏库的细胞包括效应记忆T细胞(effective memory T cell,TEM)[2]、中心记忆T细胞(central memory T cell,TCM)、过渡型记忆T细胞(transitional memory T cell)[3]和记忆性干细胞样T细胞(memory stem cell-like T cell)[4]。此外,对血浆中残余HIV序列的研究发现[2-3],巨噬细胞、滤泡树突细胞、小神经胶质细胞、星型胶质细胞等也可能成为潜伏库的组成部分。这些病毒潜伏库细胞的半衰期较长(44个月),在治疗中断后成为感染复发的主要源头[5]。目前有多种HIV潜伏库的清除策略,如引蛇出洞疗法(shock and kill)[6]、基因修饰技术[7]、基因打靶技术[8]等,但主要问题是如何评价这些干预措施的有效性。因此,人们迫切需要建立一种高通量、可靠的、高灵敏度的方法来定量检测病毒潜伏库的真实大小,从而用于临床药物动力学效果的评价和完全治愈的判定[9]。本文主要综述目前HIV潜伏库的多种定量检测方法。
1基于体外培养实验的HIV潜伏库定量检测技术
1.1病毒扩增实验
VOA法(viral outgrowth assay)目前被认为是测量潜伏状态下携带复制型前病毒的静息CD4+T细胞拷贝数的金标准[10]。从接受HAART的患者外周血单个核细胞(peripheral blood mononuclear cell,PBMC)中分选的静息CD4+T细胞在无外界刺激时不产生病毒颗粒。将稀释后的静息CD4+T细胞与经γ线照射的健康人PBMC按比例混合,再加入一种很强的促细胞分裂素——植物凝集素(phytohemagglutinin,PHA)共培养,经γ线照射后的PBMC可增强高浓度PHA下CD4+T细胞的激活效率[11],在共培养过程中PBMC逐渐死亡,仅CD4+T细胞存活。在上述存活的CD4+T细胞中加入淋巴母细胞或持续增殖的细胞系MOLT-4/ CCR5共培养2~3周,CD4+T细胞内病毒不断复制并释放,使培养上清液中病毒含量达到可检测水平。取培养上清液,用酶联免疫吸附试验(enzyme-linked immunosorbent assay,ELISA)最快7 d便可检测到病毒颗粒,或通过高灵敏度的反转录-聚合酶链反应(reverse transcriptase-polymerase chain reaction,RT-PCR)检测具有复制能力的子代病毒。VOA法也有局限性,例如不能检测出有缺陷的前病毒、需大量血液(120~180 mL)、需在生物安全三级(biosafety level 3,BSL-3)实验室中进行、只能检测一轮刺激后的病毒载量而低估了潜伏库的实际大小[12]。
1.2以病毒RNA为指标的定量检测
在HIV-1复制早期,HIV前病毒基因的主要存在形式为多剪接mRNA转录子,在转录过程中多剪接mRNA转录子转化为未剪接或单剪接的mRNA 片段。一些mRNA转录子翻译成为病毒蛋白,还有一些全长转录子被组装入新的病毒颗粒[13]。基于体外培养实验检测HIV潜伏库,将患者CD4+T细胞在板上进行有限稀释,使每孔至多有一个产毒细胞,T细胞经最大程度活化后,用定量PCR(quantitative PCR,qPCR)检测细胞产生病毒RNA的频率,可检测到的mRNA包括病毒复制过程中的未剪接mRNA、多剪接mRNA及培养上清液中的HIV-1 mRNA。培养上清液中的mRNA反映细胞释放可复制型病毒的频率,使定量检测HIV潜伏库的效果更精确[14]。最新研究表明,平均1.5%的HIV-1前病毒可重新活化产生病毒颗粒,但来源不同的两例患者分别有6.8%和8.2%的前病毒可重新活化产生未剪接的HIV-1 RNA[10,13]。这类定量检测方法的优点在于缩短了细胞培养所需时间,与VOA法类似的是只检测出一轮T细胞活化下的病毒RNA,且能检测出一些有缺陷但产生非感染性病毒颗粒的前病毒。这些方法可能混淆假阴性和假阳性结果,不能准确定量HIV潜伏库的大小[13]。
2基于PCR的HIV潜伏库定量检测技术
2.1qPCR检测前病毒DNA
PCR已普遍应用于定量检测静息状态下的HIV,为VOA法提供了一种补充途径。其中qPCR广泛用于定量检测PBMC[14-15]中的前病毒DNA。以外周血目的细胞群的总DNA为模板,以HIV-1基因上的一段保守序列为探针,进行qPCR检测,与已知拷贝数的前病毒DNA标准曲线进行比对,估算感染细胞数量。通过qPCR检测每个细胞中基因组的两个拷贝来反映样本中的总细胞数目,采用前病毒DNA拷贝数目结合总细胞数目可估算藏匿HIV-1 DNA的细胞频率。该方法同样适用于检测肠相关淋巴组织(gut-associated lymphoid tissue,GALT)中的前病毒DNA[16-17]。
2.2微滴数字PCR检测前病毒DNA
新一代PCR——微滴数字PCR(ddPCRTM)[18]可将反应体系无限稀释至1 000万份,其中每个微滴或不含待检核酸靶病毒分子,或只含一个待检靶病毒分子[19]。经PCR扩增后,对每个微滴逐个进行检测,有荧光信号的微滴判为1,没有荧光信号的微滴判为0,根据泊松分布原理及阳性微滴的比例即可计算出靶病毒分子的起始拷贝数或浓度。与传统qPCR相比,ddPCRTM测量HIV前病毒DNA的精确率更高[20-21],可检测外周血中含量极低的病毒序列[19,22],且无需标准品即可检测出靶病毒分子的启始拷贝数或浓度[19]。
2.3Alu-PCR检测整合形式的前病毒DNA
Alu-PCR用于区分PBMC和CD4+T细胞中整合的病毒基因与线性未整合的病毒基因[22]。以外周血目的细胞群的总DNA为模板,以基因组中普遍存在的Alu重复序列和HIV-1gag基因为双引物,采用Alu-PCR放大整合状态的HIV-1基因,随后以HIV的长末端重复序列(long terminal repeat,LTR)为引物用套式PCR检测。对于不同的感染细胞,整合位点与Alu序列的距离不相同,导致对前病毒DNA的一轮PCR放大效果也不相同。为解决这个问题,以细胞不同的HIV-1整合位点与Alu序列的不同距离作整合位点标准曲线,除去距离 Alu序列太远的前病毒[22]。此外,还可用linker ligation PCR[23]和反向PCR[24]检测整合形式的前病毒DNA。
2.4qPCR和ddPCRTM检测2-LTR HIV环
潜伏形成后,HIV将RNA反转录成cDNA,形成整合前复合体(preintegration complex,PIC)。部分PIC进入细胞核,没有整合入人基因组,而是在核内呈游离状态,形成两种形式:1-LTR环和2-LTR环[25-26]。这两种形式不能整合入人类基因组,也不能产生感染性病毒颗粒,不是潜伏库的一部分。但2-LTR环仍被用于近期感染情况和病毒复制情况的研究。以2-LTR交叉处侧翼序列为引物,可用ddPCRTM、qPCR对PBMC中2-LTR环进行测量[10,27],但目前对2-LTR环的稳定性还存在极大争议[28]。
3单拷贝实验检测血中HIV载量
尽管HAART能将血浆中的HIV载量控制在低至检测线以下,但仍能在低病毒载量血浆中检测到HIV-1 RNA,表明血浆中存在低水平持续复制的HIV。Palmer及其同事研究出了高灵敏度的单拷贝实验(single-copy assay,SCA),用以定量检测血浆中的残留HIV,监测患者血浆中的HIV-1 反弹[29]。取患者血液(至少7 mL血浆),进行连续稀释,直至最终理论上HIV RNA<1 拷贝/mL。样本中加入RCAS(一种禽流感反转录病毒)作为标准。从病毒颗粒中提取RNA,以反转录cDNA为模板,HIV-1gag区为探针,行RT-PCR检测。样本中HIV-1水平由已知RNA拷贝数的HIV-1标准曲线测定。
研究表明,血浆中的残留HIV对目前HAART药物极其敏感[30],且主要来源于稳定的病毒潜伏库而非正在复制的病毒。因此,血浆中的残留病毒反映HAART前潜伏库细胞的病毒产量。然而,能产生血浆病毒的细胞类型还不清楚,血浆病毒与潜伏库之间的关系也尚未明了。SCA是一个非常好的能检测血浆中持续存在病毒的工具,但在临床清除实验中不能精确定量患者潜伏库的动态改变[29-30]。
4基于反弹时间的HIV潜伏库定量检测
最初认为经过几年HAART即可完全治愈HIV-1感染,然而稳定存在的病毒潜伏库的发现表明患者必须一直接受HAART,以防止血浆病毒反弹[1]。例如,两例“柏林患者”因淋巴癌接受骨髓移植而停药,停药后12周和32周分别出现了病毒反弹[31]。又如“密西西比婴儿”在出生后30 h使用了3种抗反转录药物治疗,1个月后血浆病毒降低至检测线以下[31];停止治疗后,超过2年未在体内检测到病毒;然后在停止治疗后27个月时出现了病毒反弹。这3例患者都不存在HIV-1特异性免疫应答,证明小部分HIV潜伏库由于移植或早期治疗而显著推迟了反弹时间。
尽管中断治疗是判断患者是否痊愈的唯一方法,但以HIV反弹时间来定量检测潜伏库大小仍受到伦理问题和停药后不可预测因素的限制。相比于持续接受HAART的患者,中断治疗的患者有更高的发病率,恶性肿瘤疾病的发生率增加,且无法预测停药后数月或数年可能发生的病变[32-33]。因此,以中断HAART来评估HIV潜伏库的缩减仍困难重重。
5HIV潜伏库定量检测技术面临的挑战
关于定量检测HIV潜伏库的挑战主要来源于两个方面。第一是生物性,定量HIV潜伏库均基于临床样本,而临床样本无法代表个体内HIV潜伏库的情况;第二是技术性,临床测量HIV潜伏库都是寻找一种可靠的高精确性、可靠性、特异性、重复性技术来判断经过干预策略后潜伏库的变化[34]。目前为止,还没有一种方法能准确定量HIV潜伏库实际大小。作为定量检测HIV潜伏库的金标准,VOA法能估计出HIV潜伏库的最小范围,但不能检测出所有可复制型病毒的潜伏感染细胞[35],低估了HIV潜伏库的实际大小,可能误导临床停止用药,而一旦停止用药,最终将引起病毒反弹。除VOA法外,还有很多基于PCR的技术可测量HIV-1前病毒DNA。例如,qPCR被广泛用来测量未分离的PBMC[14-15]中整合或未整合形式的HIV前病毒DNA,但不能区分复制型病毒与有缺陷的前病毒,高估了HIV潜伏库大小,不能判断HIV潜伏库何时已清除干净,何时可停止用药。而新的ddPCRTM技术[18]被应用于定量总HIV DNA和2-LTR环,但其费用高,效率较低。对于整合形式的HIV DNA,最好的测量方法就是Alu-PCR;但由于gag探针缺乏特异性,一个样本需42个PCR,过程过于繁琐[22]。对于细胞内的HIV RNA,常采用如组蛋白去乙酰酶抑制剂等药物活化潜伏感染细胞,产生病毒转录子;但目前尚无法区分一些转录子是持续保持潜伏状态还是进行低水平复制,也不能准确量化HIV潜伏库。对于血浆中的残留病毒,一般采用高灵敏度的SCA进行检测;但血浆病毒与潜伏库之间的关系仍未明了。而基于反弹时间测量HIV潜伏库大小,则需考虑到停药风险与疾病进程相关问题。虽然以上实验均提供了一些新的视角来定量HIV潜伏库,但缺乏标准来比较各实验数据,尚没有一项实验可真正准确量化HIV潜伏库的实际大小。
6结语
目前关于HIV潜伏库的定量检测研究主要集中在对外周血的探索,然而外周血中真正被感染的CD4+T细胞并不多,不能代表整个HIV潜伏库的情况,且残留病毒血中的病毒序列也不来源于稳定的CD4+T细胞潜伏库[36],这使得量化HIV潜伏库充满不确定性。虽然已有研究表明在中枢神经系统(central nervous system,CNS)[37]、GALT[38]和淋巴组织(lymphoid organ)[39]中HIV保持低水平持续复制,但关于组织区域化与外周血之间的病毒进化关系尚未明了,因此要精确量化HIV潜伏库的大小,还需在潜伏库建立和调控机制方面进一步探索。
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为了促进及帮助本刊读者提高专业英语写作水平,本期刊出1句英文科技词句,下一期将刊出其对应的中文词句,供读者对比学习。
Malaria is the most important parasitic disease in people, and a major cause of morbidity and mortality in tropical regions. WHO has declared malaria control a global development priority and has changed its focus from containment and control to elimination. Drug resistance inPlasmodiumspecies poses a major obstacle. Resistance inPlasmodiumfalciparum, the main cause of malarial death, has rendered several first-line antimalarial drugs (first chloroquine, then sulfadoxine-pyrimethamine, and in some areas amodiaquine) largely ineffective.
Corresponding author. ZHANG Xiaoyan, E-mail:zhang_xycn2002@yahoo.com.cn
·综述·
Research progress on measuring the latent reservoir of human immunodeficiency virus
JIN Shan, HE Yongquan, YANG Yu, ZHANG Xiaoyan
Shanghai Public Health Clinical Center Affiliated to Fudan University, Shanghai 201508, China
Abstract:The latent viral reservoir in resting CD4+T cells is widely recognized as a major barrier to cure human immunodeficiency virus type 1 (HIV-1) infection. A high-throughput, reliable and sensitive assay that can accurately measure the true size of the viral reservoir is urgently needed. In this review, multiple measurement methods of HIV latent reservoir are summarized.
Key words:Human immunodeficiency virus; Reservoir; Quantitative detection
收稿日期:(2015-11-02)
通信作者:张晓燕
基金项目:“十二五”国家科技重大专项(2013ZX10001-002)