Antibodies to a Plasmodium falciparum blood-stage antigen as a tool for predicting the protection levels of two malaria-exposed populations.

To evaluate the ability of antibodies to Plasmodium falciparum ring-infected erythrocyte surface antigen (Pf155/RESA) epitopes to discriminate between individuals well protected or poorly protected against malaria, a receiver operating characteristic analysis was performed in two populations living in Madagascar and Malawi. The definition of protection was based on longitudinal measurements of clinical malarial attacks during the season of high malaria transmission in the Madagascar study, and on a cross-sectional measurement of parasitemia in the Malawi study. Antibodies to peptides reproducing the 4-mer, 8-mer, and 11-mer of the Pf155/RESA were tested for their reactivities using the Falcon assay screening test-enzyme-linked immunosorbent assay. Maximal detection of poorly protected individuals (specificity = 100%) corresponded to high cutoff antibody titers (range = 1.65-3.0 optical density [OD] units in the Madagascar study and 0.67-1.42 OD units in the Malawi study) and a sensitivity less than 50%. For a given sensitivity of 50%, specificity ranged from 55% to 62% in the Madagascar study, and from 67% to 94% in the Malawi study. The antibody cutoff titers corresponding to minimal misclassification rates ranged from 0.24 to 1.73 OD units in the Madagascar study and from 0.15 to 0.55 OD units in the Malawi study. For each antibody, the highest detectability value as measured by the area under the curve was obtained for anti-R11 in the Malawi study (0.838). In demonstrating such qualities, antibodies to Pf155/RESA epitopes could be used for screening poorly protected populations in which malaria control programs have to be implemented.