Identification and Characterization of Functional Human Monoclonal Antibodies to Duffy-Binding Protein.

invasion of reticulocytes relies on distinct receptor-ligand interactions between the parasite and host erythrocytes. Engagement of the highly polymorphic domain II of the Duffy-binding protein (DBPII) with the erythrocyte's Duffy Ag receptor for chemokines (DARC) is essential. Some -exposed individuals acquired Abs to DBPII that block DBPII-DARC interaction and inhibit reticulocyte invasion, and Ab levels correlate with protection against malaria. To better understand the functional characteristics and fine specificity of protective human Abs to DBPII, we sorted single DBPII-specific IgG memory B cells from three individuals with high blocking activity to DBPII. We identified 12 DBPII-specific human mAbs from distinct lineages that blocked DBPII-DARC binding. All mAbs were strain transcending and targeted known binding motifs of DBPII with DARC. Eleven mAbs competed with each other for binding, indicating recognition of the same or overlapping epitopes. Naturally acquired blocking Abs to DBPII from individuals with high levels residing in different endemic areas worldwide competed with mAbs, suggesting broadly shared recognition sites. We also found that mAbs inhibited entry into reticulocytes in vitro. These findings suggest that IgG memory B cell activity in individuals with strain-transcending Abs to DBPII display a limited clonal response with inhibitory blocking directed against a distinct region of the molecule.