Elucidating Mechanisms of Immunity to Nipah Virus Infection through Generation of Attenuated Viruses and a Single-Cycle Vectored Vaccine

Nipah virus (NiV) is an emerging paramyxovirus that has caused outbreaks with high case-fatality rates in South and Southeast Asia. Mechanisms of NiV virulence are poorly understood, and there is no licensed vaccine nor treatment. Accessory proteins produced from the NiV P gene through co-transcriptional gene editing (V and W) inhibit multiple molecules in the type-I interferon (IFN-I) induction and response pathways to modulate the host innate immune response to NiV infection. Previously, ferrets infected with a recombinant NiV (rNiV) lacking V survived an otherwise lethal NiV challenge via an unknown mechanism. Mutation of the V gene of the related canine distemper virus prevented binding of V protein to melanoma differentiation-associated protein 5 (MDA5) and attenuated virulence in an otherwise lethal ferret model. The NiV V-MDA5 binding site and the effects of blocking this interaction on virulence were previously unknown. The work described here identified amino acid I414 in NiV V as a critical residue for binding to MDA5 through co-immunoprecipitation/western blot and IFN-β dual luciferase reporter assays in a plasmid overexpression system. Subsequently, rNiV lacking the ability to bind to MDA5 and signal transducer and activator of transcription 1 (STAT1) were recovered, characterized in cell culture with and without IFN-I pretreatment, and used in an experimental infection model in ferrets. Interestingly, 25% of ferrets infected with the rNiV lacking V survived challenge with a higher virus dose than in previous studies, while 75% of ferrets infected with a rNiV lacking the ability to bind to MDA5 and STAT1 survived. These experiments identified MDA5 and STAT1 together as important targets for NiV virulence. Additionally, previous NiV vaccine candidates have shown efficacy against NiV challenge in a variety of animal models, but no virus-vectored vaccines have been tested for efficacy shortly prior to challenge, as in an outbreak scenario. Therefore, a vesicular stomatitis virus-vectored NiV vaccine was rescued and tested in African green monkeys. Animals were protected from lethal challenge with NiV when the vaccine was given seven or three days prior. The vaccine is non-replicative and yet works rapidly in a single dose with no adjuvants. Combined, the experiments described here will advance understanding of NiV virulence and development of effective vaccines against this deadly infection.