Characterization of antigenic regions in the porcine reproductive and respiratory syndrome virus by the use of peptide-specific serum antibodies.

The porcine reproductive and respiratory syndrome virus (PRRSV) is an RNA virus that causes reproductive failure in sows and boars, and respiratory disease in pigs of all ages. Antibodies against several viral envelope proteins are produced upon infection, and the glycoproteins GP4 and GP5 are known targets for virus neutralization. Still, substantial evidence points to the presence of more, yet unidentified neutralizing antibody targets in the PRRSV envelope proteins. The current study aimed to identify and characterize linear antigenic regions (ARs) within the entire set of envelope proteins of the European prototype PRRSV strain Lelystad virus (LV). Seventeen LV-specific antisera were tested in pepscan analysis on GP2, E, GP3, GP4, GP5 and M, resulting in the identification of twenty-one ARs that are capable of inducing antibodies upon infection in pigs. A considerable number of these ARs correspond to previously described epitopes in different European- and North-American-type PRRSV strains. Remarkably, the largest number of ARs was found in GP3, and two ARs in the GP3 ectodomain consistently induced antibodies in a majority of infected pigs. In contrast, all remaining ARs, except for a highly immunogenic epitope in GP4, were only recognized by one or a few infected animals. Sensitivity to antibody-mediated neutralization was tested for a selected number of ARs by in vitro virus-neutralization tests on alveolar macrophages with peptide-purified antibodies. In addition to the known neutralizing epitope in GP4, two ARs in GP2 and one in GP3 turned out to be targets for virus-neutralizing antibodies. No virus-neutralizing antibody targets were found in E, GP5 or M. Since the neutralizing AR in GP3 induced antibodies in a majority of infected pigs, the immunogenicity of this AR was studied more extensively, and it was demonstrated that the corresponding region in GP3 of virus strains other than LV also induces virus-neutralizing antibodies. This study provides new insights into PRRSV antigenicity, and contributes to the knowledge on protective immunity and immune evasion strategies of the virus.