Erythrocyte invasion-neutralising antibodies prevent Plasmodium falciparum RH5 from binding to basigin-containing membrane protein complexes

Abstract

Basigin is an essential host receptor for invasion of Plasmodium falciparum into human erythrocytes, interacting with parasite surface protein PfRH5. PfRH5 is a leading blood-stage malaria vaccine candidate and a target of growth-inhibitory antibodies. However, basigin is not alone on the erythrocyte surface. Instead, we show that it is exclusively found in one of two macromolecular complexes, bound predominantly to either plasma membrane Ca2+-ATPase 1/4, PMCA1/4, or monocarboxylate transporter 1, MCT1. PfRH5 binds to either of these complexes with a higher affinity than to isolated basigin ectodomain, making it likely that these are the physiological targets of PfRH5. PMCA-mediated Ca2+ export is not affected by PfRH5, ruling this out as the mechanism underlying changes in calcium flux at the interface between an erythrocyte and the invading parasite. However, our studies rationalise the function of the most effective growth inhibitory antibodies targeting PfRH5. While these antibodies do not reduce the binding of PfRH5 to monomeric basigin, they do reduce its binding to basigin-PMCA and basigin-MCT complexes. This indicates that the most effective PfRH5-targeting antibodies inhibit growth by sterically blocking the essential interaction of PfRH5 with basigin in its physiological context.

Structure of the PfRCR complex which bridges the malaria parasite and erythrocyte during invasion

“…To understand how these antibodies block the invasion process, we mapped their binding sites onto a model of PfRCR. As erythrocyte basigin is all found in a stable complex with either plasma membrane calcium transporters (PMCAs) or with the monocarboxylate transporter (MCT1) 4 , we combined our structure of PfRCR with that of the PfRH5-basigin complex 5 and with structures of basigin-MCT1 32 and basigin-PMCA 33 to generate composite models of the PfRCR complex on the erythrocyte surface (Fig. 4b and Extended Data Fig.…”

“…PfRH5- and PfCyRPA-targeting neutralising antibodies are proposed to act by sterically blocking the approach of PfRCR to the erythrocyte membrane, preventing its binding to basigin and any potentially unknown surface receptors 14, 18 . Indeed, the most effective PfRH5- binding neutralising antibodies block PfRH5 from binding to basigin-PMCA and basigin-MCT1 complexes 4 . There is no known erythrocyte binding partner for PfCyRPA and the mechanism by which invasion blocking anti-PfCyRPA antibodies act is uncertain.…”

“…These stages of erythrocyte invasion require multiple host-parasite interactions, many of which are mediated by protein families with redundant function 1 . However, no strain of Plasmodium falciparum has yet been found which can invade erythrocytes when the interaction between PfRH5 3 and membrane complexes containing the erythrocyte receptor basigin [3][4][5] is prevented. Indeed, both PfRH5 and basigin are targets of invasion-neutralising antibodies 3,[6][7][8] , and immunisation with PfRH5 is protective in an Aotus model of malaria 9 and delays the onset of symptoms in a human challenge model 10 .…”

The Dual Action of Human Antibodies Specific toPlasmodium falciparumPfRH5 and PfCyRPA: Blocking Invasion and Inactivating Extracellular Merozoites

“…Anti-PfRH5 mAbs were isolated from volunteers immunized in the first human Phase Ia PfRH5 vaccine trial [24]. Individual mAbs were characterized: clone R5.004 was found to be strongly neutralizing and to bind directly to the basigin-binding site of PfRH5 [4]; whilst clone R5.008 was found to be moderately neutralizing possibly by hindering PfRH5’s access to basigin through steric clashes with the RBC membrane or basigin’s RBC binding partners PCMA or MCT1 (S1 Fig A and B) [4, 25]. It has recently been discovered that PfRH5 must have its pro-sequence cleaved by plasmepsin X before it can engage basigin [26].…”

“…The anti-PfCyRPA mAbs were also very potent at inhibiting invasion using the invasions per egress and invasions per contact measurements with the Cy.007 Fabs being especially effective. This indicates that the Cy.007 Fab may be able to access its epitope far more effectively in the confined space between the merozoite apex and the RBC bound basigin complex [25] than the whole 150 kDa IgG even though the Fab would have less avidity than parental mAb. Of the invasions which did occur, Cy.009 behaved (in a dose-dependent manner) similarly to the anti- PfRH5 mAbs by delaying the start of invasion after the end of deformation, suggesting Cy.009 may inhibit PCRCR complex formation or the binding of PCRCR to basigin [4, 6].…”