Seroneutralization of porcine reproductive and respiratory syndrome virus correlates with antibody response to the GP(5) major envelope glycoprotein

To determine the structural protein of the porcine reproductive and respira tory syndrome virus (PRRSV) involved in the production of neutralizing anti bodies following clinical infection, correlation was studied between virus neutralization capability of convalescent pig sera and antibody response to the open reading frames (ORFs) 3-, 4-, 5-, and 7-encoded proteins GP(3), G P(4), GP(5), and N, respectively. Individual virus genes were cloned into t he pGEX-4T-1 vector, and the recombinant viral proteins were expressed in E scherichia coli fused to the glutathione S-transferase (GST) protein. The r esulting GST-ORF3, GST-ORF4, GST-ORF5, and GST-ORF7 recombinant fusion prot eins were purified by electroelution and used as antigens for serologic tes ting by indirect enzyme-linked immunosorbent assay and western immunoblotti ng. The overall antibody (IgG and IgM) titers to PRRSV of pooled convalesce nt pig sera were first determined by indirect immunofluorescence, and then sera with specific IgG titers >1:1,024 were tested for their specific virus neutralization activity and reactivity to individual recombinant fusion pr oteins. Except for the early immune response las revealed by the presence o f specific IgM), neutralizing titers were correlated with anti-GP(5) titers but not with anti-GP(3) and anti-GP(4) titers. The correlation between vir us neutralization and anti-GP(5) titers was significant (r = 0.811, P less than or equal to 0.001).