Porcine B-cells recognize epitopes that are conserved between the structural proteins of American- and European-type porcine reproductive and respiratory syndrome virus

By selecting phage display libraries with immune sera from experimentally infected pigs, porcine B-cell epitopes in the open reading frame (ORF) 2, 3, 5 and 6 proteins of European-type porcine reproductive and respiratory syndrome virus (PRRSV) were identified. The sequences of all the epitopes were well conserved in European-type PRRSV and even between European- and American-type PRRSV. Accordingly, sera from pigs infected with American-type PRRSV cross-reacted with the European-type epitopes. Thus, this study showed, for the first time, the presence of highly conserved epitopes in the matrix protein and envelope glycoproteins of PRRSV. ORF5 and 6 epitopes localized to protein parts that are predicted to be hidden in PRRSV virions. In contrast, ORF2 and 3 epitopes localized to putative protein ectodomains. Due to the interesting localization, the sequence surrounding the ORF2 and 3 epitopes was subjected to closer scrutiny. A heptad motif, VSRRIYQ, which is present in a single copy in ORF2 and 3 proteins, was identified; this arrangement is completely conserved in all European-type PRRSV sequences available. The VSRRIYQ repeat motif colocalized closely with one of the ORF2 epitopes and secondary structure modelling showed that this segment of the ORF2 protein could form an amphipathic helix. Intriguingly, a mutation associated with virulence/attenuation of an American vaccine strain of PRRSV also localized to this ORF2 protein segment and affected the hydrophobic face of the predicted amphipathic helix. Further work is needed to determine whether these findings delineate a functional domain in the PRRSV ORF2 protein.