An avirulent chimeric Pestivirus with altered cell tropism protects pigs against lethal infection with classical swine fever virus

A chimeric Pestivirus was constructed using an infectious cDNA clone of bovine viral diarrhea virus (BVDV) [J. Virol. 70 (1996) 8606]. After deletion of the envelope protein E2-encoding region, the respective sequence of classical swine fever virus (CSFV) strain Alfort 187 was inserted in-frame resulting in plasmid pA/CP7E2alf. After transfection of in vitro-transcribed CP7E2alf RNA, autonomous replication of chimeric RNA in bovine and porcine cell cultures was observed. Efficient growth of chimeric CP7E2alf virus, however, could only be demonstrated on porcine cells, and in contrast to the parental BVDV strain CP7, CP7E2alf only inefficiently infected and propagated in bovine cells. The virulence, immunogenicity, and 'marker vaccine' properties of the generated chimeric CP7E2alf virus were determined in an animal experiment using 27 pigs. After intramuscular inoculation of 1 x 107 TCID50, CP7E2alf proved to be completely avirulent, and neither viremia nor virus transmission to contact animals was observed; however, CSFV-specific neutralizing antibodies were detected from day 11 after inoculation. In addition, sera from all animals reacted positive in an E2-specific CSFV-antibody ELISA, but were negative for CSFV-ERNS-specific antibodies as determined with a CSFV marker ELISA. After challenge infection with highly virulent CSFV strain Eystrup, pigs immunized with CP7E2alf were fully protected against clinical signs of CSFV infection, viremia, and shedding of challenge virus, and almost all animals scored positive in a CSFV marker ELISA. From our results, we conclude that chimeric CP7E2alf may not only serve as a tool for a better understanding of Pestivirus attachment, entry, and assembly, but also represents an innocuous and efficacious modified live CSFV 'marker vaccine'