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 protein-1 (MSP1(19)) was expressed in Salmonella vaccine strains as a carboxy-terminal fusion to fragment C of tetanus toxin (TetC). This study demonstrates that antibodies that recognize disulphide-dependent conformational epitopes in native MSP1 react with the TetC-MSP1(19) fusion protein expressed in Salmonella. The proper folding of MSP1(19) polypeptide is dependent on both the Salmonella host strain and the protein to which the MSP1(19) polypeptide is fused. Serum from mice immunized with Salmonella typhimurium C5aroD expressing TetC-MSP1(19) recognized native MSP1 as shown by immunofluorescence with P. yoelii-infected erythrocytes. Antibody levels to MSP1(19) were highest in out-bred mice immunized with S. typhimurium C5aroD carrying pTECH2-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 protein in Freund's adjuvant, and which were protected against P. yoelii challenge infection. In challenge experiments with P. yoelii the Salmonella-immunized mice were not protected, probably reflecting the magnitude of the antibody response. The results of this study have important implications in the design of live multivalent bacterial vaccines against eukaryotic pathogens.