swIFN-β promotes genetic mutation of porcine reproductive and respiratory syndrome virus in Marc-145

This result, for the first time, demonstrates that under swine interferon-β (swIFN-β) immune pressure molecular variation of porcine reproductive and respiratory syndrome virus (PRRSV) accelerates. PRRSV-A1 strain was continuously propagated in Marc-145 cells primed with swIFN-β for 20 passages to develop RRRSV-A1βf20 strains while PRRSV-A1f20 was control without swIFN-β respectively. NSP2, ORF3, ORF5 and ORF6 genes of these strains were amplified, cloned and sequenced. Sequencing analysis indicated that PRRSV-A1βf20 had 43 nucleotide mutations while PRRSV-A1f20 had only 14 in NSP2, ORF3, ORF5 and ORF6. The ratio of mutative nucleotides between them was 307%. Amino acid variations were 24 in RRRSV-A1βf20 and 5 in PRRSV-A1, so that mutation in RRRSV-A1βf20 appeared to be 4.8 times faster than that in PRRSV-A1. The ratio of non-synonymous (NS) mutations to synonymous (S) mutations, named NS/S, was 3 in RRRSV-A1βf20 and 0.625 in PRRSV-A1f20. The nucleotide mutation rate of NSP2 ORF3 ORF5 and ORF6 of PRRSV-A1βf20 were 3.8%, 2.3%, 4.5%, 0.57% and their NS/S were 5, 2, 6 and 1 respectively, which is much higher than those in PRRSV-A1f20. Thus the mutative frequency of PRRSV passaged under swIFN-β immune pressure was significantly faster than that without swIFN-β. The immuno-pressure of swIFN-β accelerated genetic variation on PRRSV ORF5, NSP2 and ORF3, but not on the ORF6 gene. PRRSV NSP2 and ORF5 genes may play a key role in escaping from inhibition of swIFN-β.