Division of Virology, Department of Pathology, University of Cambridge, Addenbrooke's Hospital Level 5, Hills Road, Cambridge, CB2 0QQ, UK.
Division of Infection and Immunity, University College London, London, WC1E 6BT, UK.
J Gen Virol. 2021 Sep;102(9). doi: 10.1099/jgv.0.001651.
Murine norovirus (MNV) is widely used as a model for studying norovirus biology. While MNV isolates vary in their pathogenesis, infection of immunocompetent mice mostly results in persistent infection. The ability of a virus to establish a persistent infection is dependent on its ability to subvert or avoid the host immune response. Previously, we described the identification and characterization of virulence factor 1 (VF1) in MNV, and demonstrated its role as an innate immune antagonist. Here, we explore the role of VF1 during persistent MNV infection in an immunocompetent host. Using reverse genetics, we generated MNV-3 viruses carrying a single or a triple termination codon inserted in the VF1 ORF. VF1-deleted MNV-3 replicated to comparable levels to the wildtype virus in tissue culture. Comparative studies between MNV-3 and an acute MNV-1 strain show that MNV-3 VF1 exerts the same functions as MNV-1 VF1, but with reduced potency. C57BL/6 mice infected with VF1-deleted MNV-3 showed significantly reduced replication kinetics during the acute phase of the infection, but viral loads rapidly reached the levels seen in mice infected with wildtype virus after phenotypic restoration of VF1 expression. Infection with an MNV-3 mutant that had three termination codons inserted into VF1, in which reversion was suppressed, resulted in consistently lower replication throughout a 3 month persistent infection in mice, suggesting a role for VF1 in viral fitness . Our results indicate that VF1 expressed by a persistent strain of MNV also functions to antagonize the innate response to infection. We found that VF1 is not essential for viral persistence, but instead contributes to viral fitness in mice. These data fit with the hypothesis that noroviruses utilize multiple mechanisms to avoid and/or control the host response to infection and that VF1 is just one component of this.
鼠诺如病毒（MNV）被广泛用作研究诺如病毒生物学的模型。虽然 MNV 分离株在发病机制上有所不同，但免疫功能正常的小鼠感染后大多会导致持续性感染。病毒建立持续性感染的能力取决于其逃避或规避宿主免疫反应的能力。此前，我们描述了 MNV 中毒力因子 1（VF1）的鉴定和特征，并证明了其作为先天免疫拮抗剂的作用。在这里，我们在免疫功能正常的宿主中探索了 VF1 在持续性 MNV 感染中的作用。我们使用反向遗传学，生成了携带单个或三个终止密码子插入 VF1 ORF 的 MNV-3 病毒。VF1 缺失的 MNV-3 在组织培养中复制到与野生型病毒相当的水平。MNV-3 与急性 MNV-1 株的比较研究表明，MNV-3 VF1 发挥与 MNV-1 VF1 相同的功能，但效力降低。感染 VF1 缺失的 MNV-3 的 C57BL/6 小鼠在感染的急性期显示出明显降低的复制动力学，但在 VF1 表达表型恢复后，病毒载量迅速达到感染野生型病毒的水平。感染 VF1 中有三个终止密码子插入的 MNV-3 突变体，其中回复被抑制，导致在 3 个月的持续性感染中病毒复制始终较低，表明 VF1 在病毒适应性中起作用。我们的结果表明，持续性 MNV 株表达的 VF1 也能拮抗感染的先天反应。我们发现 VF1 对于病毒的持续性不是必需的，但对于小鼠中的病毒适应性有贡献。这些数据符合这样一种假设，即诺如病毒利用多种机制来避免和/或控制宿主对感染的反应，而 VF1 只是其中的一个组成部分。
