Merozoite surface protein-1 epitopes recognized by antibodies that inhibit Plasmodium falciparum merozoite dispersal

Serum antibodies from malaria immune donors can inhibit merozoite dispersal by forming immune complexes through surface-accessible regions of membrane associated antigens. Such merozoite forms are referred to as immune clusters of merozoites (ICM). Antibodies dissociated from ICM of Plasmodium falciparum identify a restricted subset of antigens, including merozoite surface protein-1 (MSP-1). We performed epitope mapping by comparing the reactivity of whole immune sera and ICM-derived antibodies in immunoblotting assays, using fourteen overlapping recombinant MSP-1 fragments, and by ELISA, using each of the 1720 octapeptides encoded within MSP-1. Antibodies in immune sera reacted with thirteen recombinant fragments and hundreds of octapeptides, but antibodies derived from ICM reacted with only six recombinant fragments and twenty octapeptides. Recombinant fragment recognition by ICM-derived antibodies was delimited to three regions 150-200 residues long, with seven of the octapeptide epitopes also mapping to these regions. The octapeptides recognized most strongly by antibodies in whole serum corresponded to the degenerate repeats near the N-terminus of MSP-1, however, neither recombinant fragments, nor octapeptides containing these degenerate repeats, were recognized by ICM-derived antibodies. Compared to reactions with recombinant fragments, the reactions observed with octapeptides were weak and may represent low-affinity mimetopes or cross-reactions. Alternatively, they may represent reactions with a portion of an epitope assembled from more than one non-contiguous peptide. These results suggest that ICM-derived antibodies can be used to map surface-accessible epitopes on MSP-1 and that the recombinant fragments with which they react are appropriate candidates for further evaluation as components of a malaria vaccine.