Antibody responses stimulated in rabbits, guinea-pigs and mice by recombinant and synthetic portions of a 75 kDa malarial merozoite protein

The 75 kDa heat-shock-related protein (p75) of Plasmodium falciparum is an abundant, highly conserved, merozoite surface protein. A bacterial clone, C7, produces a polypeptide (C7Ag) of ∼ 30 kDa representing the C-terminal 40% of p75. In several species of animals, the C7Ag stimulated high titre IgG antibodies which cross-react with p75. Two major portions of the C7Ag, theoretically predicted to have strong secondary structural preferences, were modelled with four synthetic peptides. An alpha-helical, hydrophilic region, modelled with a 28-mer, proved a poor immunogen in guinea-pigs and several strains of inbred mice, even though it had been a strong immunogen in rabbits. A disulphide-bonded region of the C7Ag was modelled with three peptides of increasing length, namely 49-, 64- and 76-amino acid residues. In general, the order of immunogenicity was 49 < 64 < 76- mer. Antibodies to the 76-mer and the 64-mer reacted strongly with the native parasite protein. The data also suggested that the 76-mer was a good model for the region of the molecule it was made to represent.