Highly pathogenic porcine reproductive and respiratory syndrome virus GP5 B antigenic region is not a neutralizing antigenic region

In 2006, highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) caused great economic losses emerged in China and continues to be a threat for the pig industry. B antigenic region (AR) (37SHL/FQLIYNL45) of GP5 was considered to be a major linear neutralizing AR in PRRSV classical strains. However, peptide-purified antibodies against this AR did not neutralize PRRSV in a recent report. Compared with classical PRRSV, one amino acid mutation (L/F39→I39) was found in B AR of HP-PRRSV. To study the ability of B AR of HP-PRRSV to induce neutralizing antibody (NA) in vitro and in vivo, rabbit antisera against B AR with and without the mutation and pig hyperimmune sera with high titer of NAs against HP-PRRSV were prepared. Immunofluorescence assays (IFA) showed that the two rabbit antisera both had reactivity to classical PRRSV CH-1a and HP-PRRSV HuN4 with no observable difference in IFA titer. However, antisera did not have neutralizing activity against classical PRRSV CH-1a and HP-PRRSV HuN4. No correlation was observed between the levels of anti-B AR peptide antibodies and NAs in pig hyperimmune sera that were detected by indirect ELISA and virus neutralization, respectively. B AR peptide-specific serum antibodies had no neutralizing activity and, GST-B fusion protein could not inhibit neutralization of NAs in pig hyperimmune sera. Based on these findings, we conclude that B AR of HP-PRRSV is not a neutralizing AR of HP-PRRSV GP5.