Recombinant Semliki Forest virus particles encoding the prME or NS1 proteins of louping ill virus protect mice from lethal challenge

Recombinant Semliki Forest virus (rSFV) vaccines encoding louping ill virus (LIV) genes prME and NS1 were examined. Cells transfected with rSFV-prME R NA showed correct processing of the precursor prME and the release into the medium of M and E proteins in particulate form, whilst rSFV-NS1-transfecte d cells secreted glycosylated, heat-labile NS1 dimers. Mice immunized with rSFV particles produced antibodies against prME and NS1 that were mainly of the IgG2a subtype, indicating that a T-helper 1 immune response was induce d. Immunization with prME- or NS1-encoding particles induced T-cell prolife ration. Mice vaccinated intraperitoneally (i.p.) with rSFV-prME and/or rSFV -NS1 were significantly protected from lethal i.p. challenge by two strains of LIV, the virulent L1/31 strain, from which the commercial LIV vaccine i s derived, and the less-virulent LI/I antibody-escape variant. Intranasal ( i.n.) vaccination was protective for rSFV-prME only against L1/31 challenge and not against challenge with LI/I, Immunization with rSFV-NS1 was protec tive against i.p. and i.n. challenge with both virus strains when given i.p ., but was not protective when given i.n. For unvaccinated mice infected wi th LIV, all animals showing clinical signs had severe degenerative and infl ammatory lesions in the central nervous system. None of the rSFV-vaccinated mice that survived challenge showed central nervous system pathology, with the exception of mild leptomeningitis in a minority of LI/31-infected mice . This suggests that protection following immunization with rSFV must occur at early stages of LIV infection.