流行性乙型脑炎病毒与寨卡病毒交叉免疫的特点及意义

Zika virus (ZIKV) has caused explosive outbreaks in more than 70 countries including many previously unaffected regions. Of special concern is the newly identified link between congenital malformations (especially microcephaly) and Guillain-Barrés syndrome and ZIKV infections. So far, there is no specific vaccine for prevention. ZIKV is a member of the genus Flavivirus of the family Flaviviridae, which includes some of the most important human-pathogenic mosquito-borne viruses, such as Japanese encephalitis (JE) virus (JEV). All flaviviruses are antigenically related to various degrees, and immunological cross-reactions have been implicated not only in cross-protection but also in infection enhancement phenomena (antibody-dependent enhancement, ADE) that may exacerbate disease in humans. This is of special relevance in regions where different flaviviruses cocirculate, allowing sequential infections to occur on the background of immunity to an antigenically related flavivirus. Cocirculation is found in many parts of the world and specifically also in regions where ZIKV recently emerged. China has integrated JE vaccine into national immunization program from 2007. It is obviously urgent to elucidate the contribution of the background of JEV-elicited immunity to ZIKV infection. Therefore, this study aims to investigate the effect of JEV infection and vaccines in the context of JEV-specific cross-protection or cross-antibody mediated ADE to ZIKV infection. First, the potential cross-protection to ZIKV will be evaluated in relevant mouse models after JE vaccination. Second, the possible ADE to ZIKV will be investigated by in vitro assay and mouse models after JE vaccination. Furthermore, the serologic survey on JE convalescent patients and JE vaccinated individuals will be used to analyze the characteristics of cross-reactive immune responses to ZIKV. The findings will raise concerns in the context of cross-immunity among flaviviruses and the possible spread of ZIKV to JE vaccination region. The results of the potential cross-protection or the possible ADE induced by JE vaccines will provide important information in terms of ZIKV prevention or precaution, respectively. New insights into the relationship between JEV and ZIKV should provide the foundation for progress made in developing ZIKV and other flaviviruses vaccines.

寨卡病毒（ZIKV）已在70余国流行，因与新生儿小头畸形及成人格林巴利综合征等关系密切而倍受关注，但尚无疫苗预防。ZIKV与流行性乙型脑炎（乙脑）病毒（JEV）同属蚊媒黄病毒，亲缘关系较近，病毒间的交叉免疫反应既可能提供交叉保护，也可能介导感染增强（ADE）。我国自2007年已将乙脑疫苗接种纳入计划免疫，因而人群普遍存在的针对JEV的免疫本底对寨卡疫情的影响亟待阐明。本项目拟：1. 利用小鼠模型，从体液免疫、细胞免疫及保护性实验等方面评价乙脑疫苗对ZIKV是否具有交叉保护作用；2. 利用体外实验和小鼠模型，评价有无ADE作用导致ZIKV感染增强；3. 利用乙脑患者恢复期及乙脑疫苗接种者血清，研究人源抗JEV抗体对ZIKV的交叉反应性质。研究结果将阐明JEV与ZIKV之间交叉免疫反应的特点及可能机制，为“老苗新用”、ZIKV疫情评估提供依据，同时为评价ZIKV及其它黄病毒的疫苗提供重要参考。

寨卡病毒（ZIKV）已在70余国流行，因与新生儿小头畸形及成人格林巴利综合征关系密切而倍受关注，但尚无疫苗预防。中国目前仅有ZIKV感染的输入性病例，但已从蚊虫中发现ZIKV，因此存在一定流行风险。ZIKV与流行性乙型脑炎（乙脑）病毒（JEV）同属蚊媒黄病毒，亲缘关系较近，JEV与ZIKV具有诱生交叉免疫反应的结构基础，二者氨基酸同源性可达56.1%，其中包膜E蛋白和非结构蛋白同源性可分别达到55.2%和68.0%，而该两种蛋白分别为诱导黄病毒间交叉反应性体液免疫和细胞免疫的主要抗原蛋白。黄病毒间的交叉免疫反应既可能提供交叉保护，也可能介导感染增强（ADE）。我国已将乙脑疫苗接种纳入国家计划免疫，因而需要了解针对JEV的免疫本底对寨卡疫情的影响、阐明JEV与ZIKV之间交叉免疫反应的特点及可能机制，为ZIKV疫情风险评估提供依据。本项目利用小鼠模型，从体液免疫、细胞免疫、ADE水平及保护性实验等方面评价乙脑疫苗对ZIKV的交叉反应特点；利用JEV抗体阳性血清，研究人源抗JEV抗体对ZIKV的交叉反应性质。结果显示，乙脑疫苗可诱导具有完全免疫活性的BALB/c小鼠及C57BL/6小鼠产生抗JEVIgG抗体并具有交叉结合ZIKV的能力，但未显示出中和活性。减毒活疫苗（LAV）免疫后的小鼠血清经被动转移给乳鼠不提供显著的抗ZIKV的交叉保护作用，并可在体外介导对小鼠生存没有不利影响的抗体依赖性增强效应。LAV可诱导小鼠产生针对ZIKV的Th1和Th2交叉反应性免疫应答；LAV接种Ifnar1-/-（A6）小鼠后的攻毒实验证实疫苗对小鼠抵抗ZIKV感染具有交叉保护作用；LAV免疫后的脾细胞过继转移A6小鼠后，可提供抗ZIKV的被动保护作用。因此，主要介导乙脑疫苗抗ZIKV感染的交叉保护作用的是疫苗诱导的细胞免疫应答。此外，在人体血清学研究中发现，在JE患者恢复期血清中可检出低效价ZIKV交叉反应性中和抗体，而JE疫苗接种者血清中无明显抗ZIKV交叉中和活性，研究结果与动物实验结果相符合。研究结果对于可能受ZIKV影响的JEV流行区域或JE疫苗广泛接种地区进行ZIKV预防及预警提供重要信息，并为开发针对ZIKV和JEV的新型二价疫苗开辟新视野。