Antigenicity of recombinant proteins derived from Plasmodium falciparum merozoite surface protein 1.

We have expressed seven recombinant antigens representing two N-terminal regions of the polymorphic merozoite surface protein 1 (MSP-1) of Plasmodium falciparum. The antigens include the MAD20 and Palo Alto forms of the relatively conserved Block 1 region, and variants of the Block 2 region from isolates 3D7, Palo Alto FUP, MAD20, Wellcome and RO33, that are representative of a range or amino acid sequence diversity in this most polymorphic section of MSP-1. All recombinant antigens have been able to immunise mice to produce polyclonal antibodies which specifically recognise parasite MSP-1 in indirect immunofluorescence assays and in Western blots. The recombinant antigens also react appropriately in ELISA with murine monoclonal antibodies specific for variant epitopes in Block 2 of MSP-1. These results show that the antigenic structure of the recombinant proteins is similar to that of the native MSP-1 product from parasites. Importantly, human sera from malaria-exposed individuals contain IgG antibodies that recognise very specifically one or another of the Block 2 types, showing that different Block 2 types are immunogenic, antigenically distinct and distinguishable when presented during natural infections. In contrast, the conserved Block 1 is rarely recognised by human antibodies.