The Ability Of B Epitope In GP5 Protein Of Highly Pathogenic Porcine Reproduction And Respiratory Syndrome Virus To Induce Neutralizing Antibody

Porcine reproductive and respiratory syndrome (PRRS) is a serious swine disease which causes economically great losses to the swine industry worldwide. The etiological agent, PRRS virus (PRRSV), possesses some mystery characteristics, such as antibody dependent enhancement (ADE), persistent infection, delay in the production of neutralizing antibodies and so on. In 2006, the highly pathogenic PRRSV (HP-PRRSV) that caused great losses was emerged in China.B epitope (³⁷SHL/FQLIYNL⁴⁵) was considered to be a major linear neutralizing epitope in the GP5 protein of PRRSV classical strains. Compared with classical PRRSV strains, one amino acid mutation (L/F³⁹I) was found in the B epitope of HP-PRRSV strains. To study the antigenic variation of the B epitope after the mutation and the ability of the B epitope to induce neutralizing antibody (NA) against HP-PRRSV, the B epitope peptides with and without the mutation (L/F³⁹I) were synthesized and immunized in rabbits to prepare the anti-peptide serums, respectively. The nucleotide sequence encoding the B epitope peptide was cloned into the pGEX-6P-1 vector to construct recombinant plasmid named as pGEX-B. B epitope and GST fusion protein (GST-B) was obtained after induced. IFA result showed that the two rabbit anti-peptide serums both had reactivity to PRRSV CH-1a strain and HP-PRRSV HuN4 strain with IFA titer no difference, but didn't have neutralizing activity. GST-B fusion protein was expressed correctly after analyzed by SDS-PAGE and Western blot. To confirm the ability of B epitope to induce NAs, pig hyperimmune serums with high titer of NAs against PRRSV were prepared by hyperimmunization of swine with HP-PRRSV HuN4 vaccine strain (HuN4-F112) firstly and virulent strain (HuN4-F5). The level of anti-B epitope peptide antibodies and NAs in pig hyperimmune serums was determined by indirect ELISA and virus neutralization test, respectively. In addition, the result of indirect ELISA was confirmed by Western blot. The level of anti-B epitope peptide antibodies had little change among pigs in different periods, however, the level of NAs experienced a process of increasing first and then decline. There was no correlation between the two of them. In addition, pig hyperimmune serums were added with GST-B fusion and then detected by neutralization test. The result indicated that GST-B fusion protein did not have the ability of inhibiting NAs. Base on these findings,we conclude that there is no difference in antigenicity of B epitope of PRRSV classical strains and mutant strains. Furthermore, we conclude that the B epitope of HP-PRRSV strains is not the primary neutralization epitope. Our results provide an important clue for further study.