Recombinant Semliki Forest virus particles expressing louping ill virus antigens induce a better protective response than plasmid-based DNA vaccines or an inactivated whole particle vaccine.

Louping ill virus (LIV) infection of mice was used as a model to evaluate the protective efficacy of Semliki Forest virus (SFV)-based vaccines in comparison to a standard DNA vaccine and a commercial chemically inactivated vaccine. The recombinant SFV-based vaccines consisted of suicidal particles and a naked layered DNA/RNA construct. The nucleic acid vaccines expressed the spike precursor prME and the nonstructural protein 1 (NS1) antigens of LIV. Three LIV strains of graded virulence for mice were used for challenge. One of these was a naturally occurring antibody escape variant. All vaccines tested induced humoral immunity but gave varying levels of protection against lethal challenge. Only recombinant SFV particles administered twice gave full protection against neuronal degeneration and encephalitis induced by two of the three challenge strains, and partial protection against the highly virulent strain, whereas the other vaccines tested gave lower levels of partial protection.