Plasmodium falciparum MSP3 Exists in a Complex on the Merozoite Surface and Generates Antibody Response during Natural Infection.

Plasmodium falciparum merozoite surface protein 3 (MSP3) is an abundantly expressed secreted merozoite surface protein and a leading malaria vaccine candidate antigen. However, it is unclear how MSP3 is retained on the surface of merozoites without a glycosylphosphatidylinositol (GPI) anchor or a transmembrane domain. In the present study, we identified an MSP3-associated network on the Plasmodium merozoite surface by immunoprecipitation of Plasmodium merozoite lysate using antibody to the N terminus of MSP3 (anti-MSP3N) followed by mass spectrometry analysis. The results suggested the association of MSP3 with other merozoite surface proteins: MSP1, MSP6, MSP7, RAP2, and SERA5. Protein-protein interaction studies by enzyme-linked immunosorbent assay (ELISA) and surface plasmon resonance (SPR) analysis showed that MSP3 complex consists of MSP1, MSP6, and MSP7 proteins. Immunological characterization of MSP3 revealed that MSP3N is strongly recognized by hyperimmune serum from African and Asian populations. Furthermore, we demonstrate that human antibodies, affinity purified against recombinant MSP3N (rMSP3N), promote opsonic phagocytosis of merozoites in cooperation with monocytes. At nonphysiological concentrations, anti-MSP3N antibodies inhibited the growth of P. falciparum in vitro Together, the data suggest that MSP3 and especially its N-terminal region containing known B/T cell epitopes are targets of naturally acquired immunity against malaria and also comprise an important candidate for a multisubunit malaria vaccine.