Rice‐produced MSP142 of Plasmodium falciparum elicits antibodies that inhibit parasite growth in vitro

Many malaria antigens contain multiple disulphide bonds involved in the formation of inhibitory B‐cell epitopes. Producing properly folded malaria antigens in sufficient quantities for vaccination is often a challenge. The 42‐kDa fragment of Plasmodium falciparum merozoite surface protein 1 (MSP1₄₂) is such a kind of malaria antigen. In this study, we investigated the expression of MSP1₄₂ in a rice system (9522, a cultivar of Oryza sativa ssp. japonica), which was used as a bioreactor for protein production. The MSP1₄₂ gene was synthesized according to rice‐preferred codons and transformed into rice plants via an Agrobacterium‐mediated method. The recombinant antigen was efficiently expressed in rice seeds with a level up to 1.56% of total soluble protein and was recognized by both the conformational monoclonal antibody 5.2 (mAb5.2) and the pooled sera of P. falciparum malaria patients. Rabbits were immunized intramuscularly with the purified MSP1₄₂ formulated with Freund's adjuvant. High antibody titres against MSP1₄₂ were elicited. The rabbit immune sera reacted well with the native protein of P. falciparum parasite and strongly inhibited the in vitro growth of blood‐stage P. falciparum parasites, demonstrating that transgenic rice can become an efficient bioreactor for the production of malaria vaccine antigens.