Epitopic Specificity of the Human Immune Response to the Invariant Region of a Polymorphic Plasmodium faciparum Merozoite Surface Antigen

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Merozoite surface antigen 2 (MSA2) is a Plasmodium falciparum vaccine candidate. Antibody responses to MSA2 in Melanesians naturally exposed to P. falciparum were investigated by ELISA using the recombinant MSA2 protein known as Ag 1609 and 65 overlapping synthetic peptides spanning predominantly the conserved N and C terminal regions of MSA2. Significant differences were obtained between control and malaria exposed subjects in the ability of sera to bind to Ag 1609. The median absorbance obtained with control sera was 0.36 whereas that obtained with sera from malaria exposed subjects was 1.8. The sera of ninety-five percent of malaria exposed subjects reacted significantly with Ag 1609. The percentage reactivity of the control sera with Ag 1609 was 39%. Overall the serological responses to the synthetic peptides were low and well defined B epitope regions were not delineated. The median absorbances were in general higher for the malaria exposed group than the controls with these differences being significant for five N terminal peptides and 14 C terminal peptides. One peptide (KECTDGNK) at the conserved C terminal end was identified for further investigation as a possible immunodiagnostic epitope. Immunisation of mice with an allelic form of Ag 1609 (namely Ag 1615) produced antibodies to the conserved C terminal of MSA2 but not to the N terminal.