Department of Paraclinical Science, Veterinary Faculty, University Eduardo Mondlane, Maputo, Mozambique; Department of Veterinary Tropical Diseases, Faculty of Veterinary Science, University of Pretoria, South Africa.
Center for Emerging Zoonotic and Parasitic Diseases, National Institute for Communicable Diseases of the National Health Laboratory Service, Sandringham, South Africa; Centre for Viral Zoonoses, Department of Medical Virology, School of Medicine, Faculty of Health Sciences, University of Pretoria, South Africa.
Virus Res. 2018 Apr 2;249:31-44. doi: 10.1016/j.virusres.2018.03.005. Epub 2018 Mar 9.
The NSs protein encoded by the S segment of Rift Valley fever virus (RVFV) is the major virulence factor, counteracting the host innate antiviral defence. It contains five highly conserved cysteine residues at positions 39, 40, 149, 178 and 194, which are thought to stabilize the tertiary and quaternary structure of the protein. Here, we report significant differences between clinical, virological, histopathological and host gene responses in BALB/c mice infected with wild-type RVFV (wtRVFV) or a genetic mutant having a double cysteine-to-serine substitution at residues 39 and 40 of the NSs protein (RVFV-C39S/C40S). Mice infected with the wtRVFV developed a fatal acute disease; characterized by high levels of viral replication, severe hepatocellular necrosis, and massive up-regulation of transcription of genes encoding type I and -II interferons (IFN) as well as pro-apoptotic and pro-inflammatory cytokines. The RVFV-C39S/C40S mutant did not cause clinical disease and its attenuated virulence was consistent with virological, histopathological and host gene expression findings in BALB/c mice. Clinical signs in mice infected with viruses containing cysteine-to-serine substitutions at positions 178 or 194 were similar to those occurring in mice infected with the wtRVFV, while a mutant containing a substitution at position 149 caused mild, non-fatal disease in mice. As mutant RVFV-C39S/C40S showed an attenuated phenotype in mice, the molecular mechanisms behind this attenuation were further investigated. The results show that two mechanisms are responsible for the attenuation; (1) loss of the IFN antagonistic propriety characteristic of the wtRVFV NSs and (2) the inability of the attenuated mutant to degrade Proteine Kinase R (PKR).
裂谷热病毒（RVFV）S 片段编码的 NSs 蛋白是主要的毒力因子，它能拮抗宿主的先天抗病毒防御。该蛋白含有 5 个高度保守的半胱氨酸残基，分别位于位置 39、40、149、178 和 194，这些残基被认为可以稳定蛋白的三级和四级结构。在这里，我们报道了 BALB/c 小鼠感染野生型 RVFV（wtRVFV）或 NSs 蛋白第 39 和 40 位的两个半胱氨酸突变为丝氨酸的遗传突变体（RVFV-C39S/C40S）后临床、病毒学、组织病理学和宿主基因反应的显著差异。感染 wtRVFV 的小鼠发生致命性急性疾病；其特征是高水平的病毒复制、严重的肝细胞坏死以及编码 I 型和 II 型干扰素（IFN）以及促凋亡和促炎细胞因子的基因转录的大量上调。RVFV-C39S/C40S 突变体不会引起临床疾病，其减毒特性与 BALB/c 小鼠的病毒学、组织病理学和宿主基因表达研究一致。在感染位置 178 或 194 半胱氨酸突变为丝氨酸的病毒的小鼠中出现的临床症状与感染 wtRVFV 的小鼠相似，而感染位置 149 突变的病毒则导致小鼠发生轻度、非致命性疾病。由于突变 RVFV-C39S/C40S 在小鼠中表现出减毒表型，因此进一步研究了这种减毒的分子机制。结果表明，有两种机制导致了减毒：（1）丧失 wtRVFV NSs 的 IFN 拮抗特性，（2）减毒突变体无法降解蛋白激酶 R（PKR）。
