A continuous epitope from transmissible gastroenteritis virus S protein fused to E. coli heat-labile toxin B subunit expressed by attenuated Salmonella induces serum and secretory immunity

Antigenic site D from the spike protein of transmissible gastroenteritis virus (TGEV), which is a continuous epitope critical in neutralization, has been expressed as a fusion protein with E. coli heat-labile toxin B subunit (LT-B) in attenuated S. typhimurium. Synthetic peptides containing the sequence of site D induced TGEB neutralizing antibodies when inoculated subcutaneously in both rabbits and swine. A synthetic oligonucleotide encoding residues 373–398 of TGEV S protein, including antigenic site D, was cloned in frame with the 3′ end of LT-B gene, into a plasmid used to transform S. typhimurium Δasd (χ)3730. A collection of 6 recombinant plasmids designated pYALTB-D I–VI encoding LTB-site D fusions with a variable number of site D sequences were selected. Four of the 6 LTB-site D fusion products expressed in S. typhimurium(χ)3730 formed oligomers (pentamers) that dissociated at > 70°. S. typhimurium(χ)3730 (pYALTB-D) V and VI expressed the oligomer forming products with higher antigenicity. Partially purified LTB-site D fusion product expressed from S. typhimurium(χ)3730 (pYALTB-D) V induced anti-TGEV neutralizing antibodies in rabbits. Recombinant vaccine strain S. typhimuriumΔcyaΔcrpΔasd(χ)3987 transformed with plasmid pYALTB-D V expressed constitutively products that formed oligomers presumably containing 20 copies of site D, and showed a high stability in vitro. This recombinant strain was orally inoculated in rabbits and induced TGEV specific antibodies in both serum and intestinal secretion.