Characterizing Inhibitory Antibodies:
Towards vaccine design, we are interested in understanding the molecular mechanisms of antibody-mediated neutralization. Several antibodies target the parasite EBL-proteins, but only a subset of the antibodies are neutralizing and can block invasion. Our work will determine the epitopes recognized by neutralizing antibodies, and characterize the mechanism of inhibition.
Antibodies that target PfEBA-175
A strongly-inhibitory antibody R217 (red below) binds to the glycan-binding sites and proposed dimer interface of PfEBA-175. In contrast, a weakly inhibitory antibody R218 (green below) binds to residues far removed from receptor-binding regions. This work suggests inhibitory antibodies target functional regions of invasion proteins.
Mapping epitopes from R217 (red) and R218 (green) on the crystal structure of PfEBA-175 Region II with the glycan binding residues in yellow/orange and the proposed dimer interface residues in blue/purple.
Understanding the Erythrocyte Binding Like family of invasion proteins:
Attachment of the parasite to a red blood cell requires several interactions mediated by the Erythrocyte-binding like family (EBL). We are in the process of characterizing these interactions and events during erythrocyte invasion. A detailed understanding of the interactions is necessary to develop methods to inhibit invasion leading to antibody therapeutics and vaccine design.
PfEBA-140 and Glycophorin C
Structure of PfEBA-140 alone: Erythrocyte binding antigen 140 (PfEBA-140, BAEBL, EBL-2) is an EBL ligand that recognizes glycophorin C (GpC) on the red blood cell. The crystal and solution structure of PfEBA-140 shows distinct differences to PfEBA-175 that account for receptor specificity.
Crystal structure of PfEBA-140 (yellow/blue) overlayed with PfEBA-175 (green/purple)
PfEBA-140 in complex with glycans:The structure of PfEBA-140 in complex with glycans from the receptor Glycophorin C reveal novel glycan binding pockets different from other EBL ligands and sialic acid binding proteins. Polymorphisms in one of the glycan binding pockets may explain receptor switchin by PfEBA-140.
Crystal structure of PfEBA-140 in complex with glycans (mesh).
PvDBP and DARC
Unlike Plasmodium falciparum which has several redudant EBL ligands, Plasmodium vivax is reliant on the Duffy Binding Protein (PvDBP) engagement of the Duffy Antigen/Receptor for Chemokines (DARC) on red blood cells for invasion. We have solved the crystal strucutre of of PvDBP and show that receptor binding drives dimerization of PvDBP. This analysis also demonstrates that naturally acquired immunity targets the dimer interface and putative DARC binding site in PvDBP.
PvDBP dimer structure. Epitopes that block receptor binding, from individuals with naturally acquired immunity, are shown in red and brown.
PfEBA-175 and Glycophorin A
The EBL family member erythrocyte binding antigen 175 (PfEBA-175) is a parasite surface protein and leading vaccine candidate for Plasmodium falciparum malaria. PfEBA-175 binds to the sugars of glycophorin A (GpA) on the red blood cell. The crystal structure of PfEBA-175 in complex with glycans from GpA suggest this ligand dimerizes upon binding and opens new avenues for therapeutic intervention.
Crystal structure of the binding domain of EBA-175 in complex with sialyllactose (red mesh)