Synthetic peptides induce antibody against a host-protective antigen of Echinococcus granulosus

The immunogenicity of four synthetic peptides was investigated in sheep. The sequences of the peptides (6, 12/13, 21/22 and 24) were derived from linear, antibody-binding epitopes of the EG95 recombinant protein, a host-protective antigen of the parasite Echinococcus granulosus. Sheep were immunised with either free peptide or peptide conjugated to diphtheria toxoid. All sheep responded to both conjugated and unconjugated forms of the peptides. For two of the four peptides (6 and 21/22), the amount of antibody elicited was significantly greater for the conjugated form of the peptides than for the corresponding unconjugated forms. For the other two peptides (12/13 and 24), peak antibody levels to both forms of the peptide were equivalent. Maximal antibody titres against peptides 6, 12/13 and 21/22 were established after only one immunisation and were not boosted by a second dose. Antisera to all four peptides reacted with the recombinant antigen, and three of the four peptides generated antibodies, which bound to the native parasite oncosphere antigen. Antisera raised against the peptides were unable to kill the parasite in in vitro culture, although each of the peptides could be used to affinity purify lethal antibody from antisera raised against the recombinant protein. These results indicate that peptides 6, 12/13, 21/22 and 24 of the EG95 recombinant vaccine are immunogenic and suggest that they are associated with host-protective epitopes.