Mutation of adjacent cysteine residues in the NSs protein of Rift Valley fever virus results in loss of virulence in mice.

Author(s): Dr. Gabby Monteiro

Author information

1
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. Electronic address: gaby.monteiro@sacids.org.
2
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.
3
Department of Virology, Wageningen Bioveterinary Research, Lelystad, the Netherlands.
4
Department of Paraclinical Science, Faculty of Veterinary Science, University of Pretoria, South Africa.
5
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; School of Pathology, Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, South Africa.

Abstract

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).

 

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