The use of attenuated classical swine fever virus (CSFV) strains as live
vaccines is no longer allowed for the control of
classical swine fever in Europe, due to the inability to differentiate between infected and vaccinated animals (Differentiating Infected from Vaccinated Animals; DIVA), except as emergency
vaccines or as bait
vaccines for wild boars. Thus, the establishment of a DIVA
vaccine(s) is of pivotal importance for the control of this
infectious disease. In this study, recombinant versions of the live-
attenuated vaccine strain CSFV Riems were generated by replacing parts of the E2 gene with the corresponding sequence of border disease virus strain Gifhorn. Three
cDNA clones were constructed: pRiems-ABC-Gif, pRiems-A-Gif and pRiems-BC-Gif. Infectious particles were obtained from clones pRiems-ABC-Gif and pRiems-BC-Gif only, whereas transfected
RNA from clone pRiems-A-Gif behaved like a replicon. Based on its ability to be differentiated in vitro from wild-type CSFV by mAbs, vRiems-ABC-Gif was assessed for immunogenicity and protection against challenge
infection in pigs. Before challenge, no CSFV-specific anti-E2
antibodies could be detected with commercial E2-blocking ELISAs in vRiems-ABC-Gif-vaccinated animals, whereas vRiems-vaccinated pigs developed high titres of anti-E2
antibodies, confirming the marker properties of this
vaccine candidate. After oral vaccination, only partial protection against challenge
infection was observed in the vRiems-ABC-Gif vaccinees, whereas all intramuscularly vaccinated animals and all vRiems-vaccinated animals were fully protected. These experiments suggest that the strategy of exchanging specific antigenic
epitopes among pestiviruses is a promising tool for the development of new CSFV
marker vaccines.