Pigs exposed to GP5 protein of PRRSV by means of DNA immunization develop specific neutralizing and protecting antibodies. Herein, we
report on the consequences of codon bias, and on the favorable outcome of the systematic replacement of native codons of PRRSV ORF5 gene
with codons chosen to reflect more closely the codon preference of highly expressed mammalian genes. Therefore, a synthetic PRRSV ORF5
gene (synORF5) was constructed in which 134 nucleotide substitutions were made in comparison to wild-type gene (wtORF5), such that
59% (119) of wild-type codons were replaced with known preferable codons in mammalian cells. In vitro expression in mammalian cells of
synORF5 was considerably increased comparatively to wtORF5, following infection with tetracycline inducible replication-defective human
adenoviral vectors (hAdVs). After challenge inoculation, SPF pigs vaccinated twice with recombinant hAdV/synORF5 developed earlier and
higher antibody titers, including virus neutralizing antibodies to GP5 than pigs vaccinated with hAdV/wtORF5. Data obtained from animal
inoculation studies suggest direct correlation between expression levels of immunogenic structural viral proteins and immune response
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