Anti-Plasmodium falciparum Antibodies Acquired by Residents in a Holoendemic Area of Liberia During Development of Clinical Immunity

Sera from 48 children and adolescents (2-15 years of age), residing in a malaria holoendemic area of Liberia were investigated for specificities and isotypes of anti-P. falciparum antibodies. No clear-cut relationship to the development of clinical immunity was found when the overall antibody activities to total parasite antigens were determined by enzyme-linked immunosorbent assay (ELISA). Although there was a certain rise of IgM, total IgG- and IgG2 antibody activities, this was most pronounced at ages when a clinical but nonsterile immunity is already present. When the sera were investigated by immunoprecipitation of 35S-methionine labeled parasite polypeptides, the total number of parasite antigens precipitated was similar at all ages. Analysis by indirect immunofluorescence (IFA), registering antibodies to intracellular parasite antigens, revealed no age-dependent changes in antibody titers. In contrast, when the sera were assayed by a novel IFA, specific for a restricted number of parasite antigens in the membrane of infected erythrocytes, the frequency of positive sera as well as the anti-P. falciparum titers rose in parallel with the development of clinical immunity. Thus, these antigens appeared to be important inducers of protective immune responses and may be suitable candidates for a vaccine against the asexual blood stages of P. falciparum.