Antibodies to a merozoite surface protein promote multiple invasion of red blood cells by malaria parasites

The 40-50 kDa merozoite surface antigen (MSA2) is a candidate molecule for use in a malaria vaccine. The gene for MSA2 from the 3D7 isolate of Plasmodium falciparum was amplified by polymerase chain reaction and cloned into the bacterial expression vector pGEX-3X to obtain a fusion protein of MSA2 with Schistosoma japonicum glutathione S-transferase. The recombinant fusion protein was used to immunize rabbits. After four injections, the sera had Western blotting and immunofluorescence titres of 10(-6). Immune sera, and immunoglobulin (Ig)G, F(ab)'2, F(ab) prepared from the immune sera, were assessed for their effects on the growth of 3D7 parasites in vitro by microscopy and a 3H]-hypoxanthine incorporation assay. The antibodies did not significantly inhibit red blood cell invasion and parasite growth when added to cultures as 10% v/v serum or as immunoglobulin preparations at concentrations up to 200 microg ml(-1). However, in the presence of IgG or F(ab)'2, but not F(ab), antibodies to MSA2, the proportions of red blood cells invaded by more than one merozoite increased significantly. Multiple invasion is attributed to merozoites cross-linked by bivalent antibodies, attaching to and subsequently invading the same red cell. These observations have a bearing on the evasion of host immune responses by the parasite and the use of full-length recombinant MSA2 protein in a malaria vaccine.