Expression of a Foreign Epitope by Porcine Reproductive and Respiratory Syndrome Virus

The potential of porcine reproductive and respiratory syndrome virus (PRRSV) as a viral vector was explored by the insertion of a sequence encoding a foreign antigen into the infectious cDNA clone of the Lelystad virus isolate. An epitope of the hemagglutinin (HA) protein of human influenza A virus was introduced at the 5′ end and at the 3′ end of ORF7, in each case resulting in a fusion protein between the HA epitope and the nucleocapsid (N) protein. Furthermore, in the construct carrying the HA sequences at the 5′ end of ORF7, additional in-frame insertions encoding the autoprotease 2A of foot-and-mouth disease virus were made between the HA and ORF7 sequences to ensure the generation of a functional N protein from its hybrid precursor. When RNA transcripts from these full-length cDNA clones were transfected into BHK-21 cells, they were each found to replicate, to express the HA epitope, and to produce progeny virus. However, fusion of the HA epitope directly to the nucleocapsid protein either at the N terminus or at the C terminus adversely affected both the viability and the genetic stability of the recombinant PRRS viruses. Serial passage of the recombinant viruses on porcine alveolar macrophages demonstrated that these viruses had lost (part of) the HA epitope at passage four. In contrast, in the PRRS viruses expressing the HA epitope from a precursor cleavable by the autoprotease 2A peptide, the HA epitope was still intact after four passages, and no effect on the viability of these viruses was observed. Immunoprecipitation and pulse chase experiments revealed the efficient and presumably cotranslational cleavage of the HA epitope from the N protein by the 2A protease. Our results demonstrate the feasibility of using PRRSV as a viral vector that might be suitable for the delivery of antigens from other pathogens to the immune system of the pig.