Expression of disulphide-bridge-dependent conformational epitopes and immunogenicity of the carboxy-terminal 19 kDa domain of Plasmodium yoelii merozoite surface protein-1 in live attenuated Salmonella vaccine strains

The 19 kDa carboxy-terminal domain of Plasmodium yoelii merozoite surface p rotein-1 (MSP1(19)) was expressed in Salmonella vaccine strains as a carbox yterminal fusion to fragment C of tetanus toxin (TetC), This study demonstr ates that antibodies that recognize disulphide-dependent conformational epi topes in native MSP1 react with the TetC-MSP1(19) fusion protein expressed in Salmonella, The proper folding of MSP1(19) polypeptide is dependent on b oth the Salmonella host strain and the protein to which the MSP1(19) polype ptide is fused. Serum from mice immunized with Salmonella typhimurium C5aro D expressing TetC-MSP1(19) recognized native MSP1 as shown by immunofluores cence with P. yoelii-infected erythrocytes, Antibody levels to MSP1(19) wer e highest in out-bred mice immunized with S. typhimurium C5aroD carrying pT ECH2-MSP1(19) and antibody was mostly directed against reduction-sensitive conformational epitopes, However, antibody levels were lower than in BALB/c mice immunized with a glutathione S-transferase (GST)-MSP1(19) fusion prot ein in Freund's adjuvant, and which were protected against P, yoelii challe nge infection. In challenge experiments with P. yoelii the Salmonella-immun ized mice were not protected, probably reflecting the magnitude of the anti body response. The results of this study have important implications in the design of live multivalent bacterial vaccines against eukaryotic pathogens .