Comparison of the protective efficacy of yeast-derived and Escherichia coli-derived recombinant merozoite surface protein 4/5 against lethal challenge by Plasmodium yoelii

The gene encoding the Plasmodium yoelii homologue of P. falciparum merozoite surface proteins 4 (MSP4) and 5 (MSP5) has been expressed in Escherichia coli and Saccharomyces cerevisiae. The protein contains a single epidermal growth factor (EGF)-like domain and is expressed in a form lacking the predicted N-terminal signal and glycosyl phosphatidylinositol (GPI) attachment sequences. The recombinant protein derived from E. coli (EcMSP4/5) was highly effective at protecting mice against lethal challenge with 10⁵ parasites of the P. yoelii YM strain. In contrast, the protective efficacy of yeast-derived MSP4/5 (yMSP4/5) was considerably less. The antibody titres in both groups were significantly different with mice immunised with yeast-derived protein showing significantly lower pre-challenge antibody responses. There was a significant inverse correlation between antibody levels as measured by ELISA and peak parasitaemia. Mice immunised with EcMSP4/5 produced anti-PyMSP4/5 antibodies predominantly of the IgG2a and IgG2b isotypes, whereas, mice immunised with yMSP4/5 mainly produced antibodies of the IgG1 isotype. The differences in antibody titres and subtype distribution may account for the observed differences in protective efficacy of these protein preparations. Levels of protective efficacy of MSP4/5 were compared with that obtained using P. yoelii MSP1 produced in S. cerevisiae. Levels of protection induced by E. coli derived MSP4/5 were superior to those induced by MSP1 which in turn were better than those induced by yeast-derived MSP4/5.