Background: The targets and mechanisms of human immunity to malaria are poorly understood, which poses a major barrier to malaria vaccine development. Antibodies play a key role in human immunity and may act by inhibiting receptor-binding functions of key merozoite invasion ligands. Antibodies to the major invasion ligand and vaccine candidate, erythrocyte-binding antigen 175 (EBA-175), have been linked with protection, but how these antibodies function has not been established.
Methods: We developed 2 new assays that quantify the ability of antibodies to inhibit binding of EBA-175 to its erythrocyte receptor, glycophorin A, using either native or recombinant EBA-175. Binding-inhibitory antibodies were evaluated in a longitudinal cohort study of Papua New Guinean children and related to risk of malaria, age, infection status, and markers of parasite exposure.
Results: Binding-inhibition assays (BIAs) were reproducible, and the 2 assays had a high level of agreement. Inhibitory antibodies were common among children, acquired in association with markers of increasing parasite exposure, and high in those children with active infection. Inhibitory antibodies correlated with total immunoglobulin G levels to the EBA-175 binding domain (region II). Importantly, binding-inhibitory antibodies were significantly associated with protection from symptomatic malaria when measured using either BIA.
Conclusions: Findings suggest that naturally acquired binding-inhibitory antibodies are an important functional mechanism that contributes to protection against malaria and further supports the potential of EBA-175 as a vaccine candidate. Identifying vaccines and approaches that induce potent binding-inhibitory antibodies may be a valuable strategy in the development of highly efficacious malaria vaccines.
Keywords: EBA-175; Plasmodium falciparum; antibodies; binding inhibition; malaria.
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