To determine the structural protein of the porcine reproductive and respiratory syndrome virus (PRRSV) involved in the production of neutralizing antibodies 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 GP3, GP4, GP5, and N, respectively. Individual virus genes were cloned into the pGEX-4T-1 vector, and the recombinant viral proteins were expressed in Escherichia coli fused to the glutathione S-transferase (GST) protein. The resulting GST-ORF3, GST-ORF4, GST-ORF5, and GST-ORF7 recombinant fusion proteins were purified by electroelution and used as antigens for serologic testing by indirect enzyme-linked immunosorbent assay and western immunoblotting. The overall antibody (IgG and IgM) titers to PRRSV of pooled convalescent 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 proteins. Except for the early immune response (as revealed by the presence of specific IgM), neutralizing titers were correlated with anti-GP5 titers but not with anti-GP3 and anti-GP4 titers. The correlation between virus neutralization and anti-GP5 titers was significant (r = 0.811, P ≤ 0.001).
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