Extensive antigenic polymorphism within the repeat sequence of the Plasmodium falciparum merozoite surface protein 1 block 2 is incorporated in a minimal polyvalent immunogen.

Polymorphism in pathogen antigens presents a complex challenge for vaccine design. A prime example is the N-terminal block 2 region of the Plasmodium falciparum merozoite surface protein 1 (MSP1), to which allele-specific antibodies have been associated with protection from malaria. In a Zambian population studied here, 49 of 91 alleles sampled were of the K1-like type (the most common of three block 2 types in all African populations), and most of these had unique sequences due to variation in tri- and hexapeptide repetitive motifs. There were significant negative correlations between allelic sequence lengths of different regions of the repeats, so the complete repeat sequence had less length variation than its component parts, suggesting a constraint on overall length. Diverse epitopes recognized by three murine monoclonal antibodies and 24 individual human sera were then mapped by using a comprehensive panel of synthetic peptides, revealing epitopes in all regions of the repeats. To incorporate these different epitopes in a single molecule, a composite sequence of minimal overall length (78 amino acids) was then designed and expressed as a recombinant antigen. More human immune sera reacted with this "K1-like Super Repeat" antigen than with proteins consisting of single natural allelic sequences, and immunization of mice elicited antibodies that recognized a range of five cultured parasite lines with diverse K1-like MSP1 block 2 repeat sequences. Thus, complex allelic polymorphism was deconstructed and a minimal composite polyvalent antigen was engineered, delivering a designed candidate sequence for inclusion in a malaria vaccine.