Deletion mutants of Schmallenberg virus are avirulent and protect from virus challenge

Since its emergence, Schmallenberg virus (SBV), a novel insect-transmitted orthobunyavirus which predominantly infects ruminants, caused a large epidemic in European livestock. Newly developed inactivated vaccines are available, but highly efficacious and safe live vaccines are still missing. Here, the properties of novel recombinant SBV-mutants lacking the non-structural proteins NSs (rSBVΔNSs), NSm (rSBVΔNSm) or both of them (rSBVΔNSs/ΔNSm) were tested in vitro and in vivo in type I interferon receptor knock-out mice (IFNAR-/-) and in a vaccination/challenge trial in cattle. As for other bunyaviruses, both non-structural proteins of SBV are not essential for viral growth in vitro. In interferon-defective BHK-21 cells rSBVΔNSs and rSBVΔNSm replicated to comparable level as the parental rSBV, the double mutant virus, however, showed a mild growth defect resulting in lower final virus titers. Additionally, both mutants with a NSs-deletion induced high levels of interferon and showed a marked growth defect in interferon-competent sheep SFT-R cells. Nevertheless, in IFNAR-/- mice, all mutants were virulent, with the highest mortality rate for rSBVΔNSs and a reduced virulence for the NSm-deleted virus. In cattle, SBV lacking NSm caused viraemia and seroconversion comparable to wild-type virus, while the NSs and the combined NSs/NSm deletion mutant induced no detectable virus replication or clinical disease after immunization. Furthermore, three out of four cattle immunized once with the NSs deletion mutant and all animals vaccinated with the virus lacking both non-structural proteins were fully protected against a challenge infection. Therefore, the double deletion mutant will provide the basis for further developments of safe and efficacious modified live SBV-vaccines which could be also a model for other viruses of the Simbu-serogroup and related orthobunyaviruses.