Distinct virulence of Rift Valley fever phlebovirus strains from different genetic lineages in a mouse model

Rift Valley fever phlebovirus (RVFV) causes high rates of abortions and fetal malformations in ruminants, and hemorrhagic fever, encephalitis, or blindness in humans. Viral transmission occurs via mosquito vectors in endemic areas, which necessitates regular vaccination of susceptible livestock animals to prevent the RVF outbreaks. Although ZH501 strain has been used as a challenge strain for past vaccine efficacy studies, further characterization of other RVFV strains is important to optimize ruminant and nonhuman primate RVFV challenge models. This study aimed to characterize the virulence of wild-type RVFV strains belonging to different genetic lineages in outbred CD1 mice. Mice were intraperitoneally infected with 1x103 PFU of wild-type ZH501, Kenya 9800523, Kenya 90058, Saudi Arabia 200010911, OS1, OS7, SA75, Entebbe, or SA51 strains. Among them, mice infected with SA51, Entebbe, or OS7 strain showed rapid dissemination of virus in livers and peracute necrotic hepatitis at 2–3 dpi. Recombinant SA51 (rSA51) and Zinga (rZinga) strains were recovered by reverse genetics, and their virulence was also tested in CD1 mice. The rSA51 strain reproduced peracute RVF disease in mice, whereas the rZinga strain showed a similar virulence with that of rZH501 strain. This study showed that RVFV strains in different genetic lineages display distinct virulence in outbred mice. Importantly, since wild-type RVFV strains contain defective-interfering RNA or various genetic subpopulations during passage from original viral isolations, recombinant RVFV strains generated by reverse genetics will be better suitable for reproducible challenge studies for vaccine development as well as pathological studies.裂谷热病毒（Rift Valley fever phlebovirus, RVFV）可导致反刍动物出现高比例流产与胎儿畸形，亦可引发人类出血热、脑炎或失明。该病毒通过流行地区的蚊媒传播，因此需定期为易感家畜接种疫苗以防控裂谷热（RVF）暴发。尽管既往疫苗效力研究常以ZH501毒株作为攻毒株，但对其他RVFV毒株进行进一步表征，对于优化反刍动物与非人灵长类的RVFV攻毒模型至关重要。本研究旨在表征不同遗传谱系的野生型RVFV毒株在远交系CD1小鼠（outbred CD1 mice）中的毒力。研究人员以1×10³噬斑形成单位（plaque forming unit, PFU）的野生型ZH501、肯尼亚9800523、肯尼亚90058、沙特阿拉伯200010911、OS1、OS7、SA75、Entebbe及SA51毒株对小鼠进行腹腔感染。其中，感染SA51、Entebbe或OS7毒株的小鼠，在感染后2-3天（days post infection, dpi）即可出现病毒在肝脏内快速播散，以及超急性坏死性肝炎。研究人员通过反向遗传学技术拯救得到重组SA51（rSA51）与Zinga（rZinga）毒株，并在CD1小鼠中测试其毒力。重组SA51毒株可在小鼠中复刻超急性RVF病症，而重组Zinga毒株的毒力与重组ZH501毒株相当。本研究证实，不同遗传谱系的RVFV毒株在远交系CD1小鼠中表现出显著的毒力差异。值得注意的是，野生型RVFV毒株在原代病毒分离传代过程中，会携带缺陷干扰RNA（defective-interfering RNA）或多种遗传亚群；因此通过反向遗传学技术获得的重组RVFV毒株，更适用于可重复的疫苗研发攻毒研究与病理学研究。