Date Published: May 23, 2013
Publisher: Public Library of Science
Author(s): Edwin Chen, May M. Paing, Nichole Salinas, B. Kim Lee Sim, Niraj H. Tolia, Joe Smith.
Disrupting erythrocyte invasion by Plasmodium falciparum is an attractive approach to combat malaria. P. falciparum EBA-175 (PfEBA-175) engages the host receptor Glycophorin A (GpA) during invasion and is a leading vaccine candidate. Antibodies that recognize PfEBA-175 can prevent parasite growth, although not all antibodies are inhibitory. Here, using x-ray crystallography, small-angle x-ray scattering and functional studies, we report the structural basis and mechanism for inhibition by two PfEBA-175 antibodies. Structures of each antibody in complex with the PfEBA-175 receptor binding domain reveal that the most potent inhibitory antibody, R217, engages critical GpA binding residues and the proposed dimer interface of PfEBA-175. A second weakly inhibitory antibody, R218, binds to an asparagine-rich surface loop. We show that the epitopes identified by structural studies are critical for antibody binding. Together, the structural and mapping studies reveal distinct mechanisms of action, with R217 directly preventing receptor binding while R218 allows for receptor binding. Using a direct receptor binding assay we show R217 directly blocks GpA engagement while R218 does not. Our studies elaborate on the complex interaction between PfEBA-175 and GpA and highlight new approaches to targeting the molecular mechanism of P. falciparum invasion of erythrocytes. The results suggest studies aiming to improve the efficacy of blood-stage vaccines, either by selecting single or combining multiple parasite antigens, should assess the antibody response to defined inhibitory epitopes as well as the response to the whole protein antigen. Finally, this work demonstrates the importance of identifying inhibitory-epitopes and avoiding decoy-epitopes in antibody-based therapies, vaccines and diagnostics.
PfEBA-175 is a P. falciparum parasite ligand that binds to its receptor GpA on erythrocytes in a sialic acid-dependent manner –. This binding event is necessary for erythrocyte invasion and consequently PfEBA-175 is a leading vaccine candidate –. PfEBA-175 has also paved the way for the concept and development of a P. falciparum receptor blockade vaccine , , . Within PfEBA-175, region II (RII) is sufficient for GpA binding and is comprised of two Duffy Binding Like (DBL) domains , F1 and F2 .
The first ever structures of EBL ligands in complex with inhibitory antibodies presented here elucidates the dynamic process of receptor-ligand interactions during invasion. In addition, these findings provide an explanation of the difference in potency of R217 and R218. We demonstrate that R217 engages critical receptor-binding residues in PfEBA-175 with a demonstrated role in erythrocyte binding . The functional regions include additional glycan binding residues and the proposed dimer interface of RII. In addition, R217 directly prevents GpA engagement by RII. Thus, we conclude the R217 directly blocks GpA binding as it engages functional residues in RII. On the other hand, the R218 epitope is far removed from proposed functional regions and mutation of residues in the epitope have no effect on erythrocyte binding. R218 is also unable to directly prevent GpA binding at any concentration tested. Thus, we conclude R218 does not directly inhibit GpA binding by RII as it engages non-functional regions. While this study was under review, a phage display approach to map epitopes targeted by anti-PfEBA175 monoclonal antibodies identified residues contacted by R217 . The structural and mapping studies presented here are consistent with the phage display results.