Search

Orthobunyaviruses such as Cache Valley virus (CVV) and Kairi virus (KRIV) are important animal pathogens. Periodic outbreaks of CVV have resulted in the significant loss of lambs on North American farms, whilst KRIV has mainly been detected in South and Central America with little overlap in geographical range. Vaccines or treatments for these viruses are unavailable. One approach to develop novel vaccine candidates is based on the use of reverse genetics to produce attenuated viruses that elicit immune responses but cannot revert to full virulence. The full genomes of both viruses were sequenced to obtain up to date genome sequence information. Following sequencing, minigenome systems and reverse genetics systems for both CVV and KRIV were developed. Both CVV and KRIV showed a wide in vitro cell host range, with BHK-21 cells a suitable host cell line for virus propagation and titration. To develop attenuated viruses, the open reading frames of the NSs proteins were disrupted. The recombinant viruses with no NSs protein expression induced the production of type I interferon (IFN), indicating that for both viruses NSs functions as an IFN antagonist and that such attenuated viruses could form the basis for attenuated viral vaccines. To assess the potential for reassortment between CVV and KRIV, which could be relevant during vaccination campaigns in areas of overlap, we attempted to produce M segment reassortants by reverse genetics. We were unable to obtain such viruses, suggesting that it is an unlikely event., Author summary Cache Valley and Kairi viruses (CVV and KRIV; Peribunyaviridae, Orthobunyavirus) are important animal pathogens of the Americas. In this study we developed reverse genetics systems to study and manipulate viral genomes of both viruses. Viral genomes were mutated to prevent the expression of the NSs protein, a key virulence factor and antagonist of the type 1 interferon (IFN) system. Replication studies in IFN producing cell lines showed slower growth of the NSs-deletion carrying viruses compared to wild type virus. In contrast, in IFN-deficient cell lines growth of both viral types was comparable, highlighting the role of NSs as an IFN antagonist in both CVV and KRIV. We also demonstrated using genetic studies that CVV and KRIV are unlikely to combine by reassortment to form novel viruses at least for one combination tested here, and propose that such recombinant viruses would be suitable live attenuated vaccine candidates.