Human antibodies to a Mr 155,000 Plasmodium falciparum antigen efficiently inhibit merozoite invasion.

IgG from a donor clinically immune to Plasmodium falciparum malaria strongly inhibited reinvasion in vitro of human erythrocytes by the parasite. When added to monolayers of glutaraldehyde-fixed and air-dried erythrocytes infected with the parasite, this IgG also displayed a characteristic immunofluorescence restricted to the surface of infected erythrocytes. Elution of the IgG adsorbed to such monolayers gave an antibody fraction that was 40 times more efficient in the reinvasion inhibition assay (50% inhibition titer, less than 1 microgram/ml) than the original IgG preparation. The major antibody in this eluate was directed against a parasite-derived antigen of Mr 155,000 (Pf 155) deposited by the parasite in the erythrocyte membrane in the course of invasion. A detailed study of IgG fractions from 11 donors with acute P. falciparum malaria or clinical immunity revealed the existence of an excellent correlation between their capacities to stain the surface of infected erythrocytes, their titers in reinvasion inhibition, and the presence of antibodies to Pf 155 as detected by immunoblotting. No such correlations were seen when the IgG fractions were analyzed for immunofluorescence of intracellular parasites or for the presence of antibodies to other parasite antigens as detected by immunoprecipitation of [35S]methionine-labeled and NaDodSO4/PAGE-separated parasite extracts. The results suggest that Pf 155 has an important role in the process of erythrocyte infection and that host antibodies to this antigen may efficiently interfere with this process.