Inhibitory Humoral Responses to the Plasmodium falciparum Vaccine Candidate EBA-175 Are Independent of the Erythrocyte Invasion Pathway

Abstract

dPlasmodium falciparum utilizes multiple ligand-receptor interactions for invasion. The invasion ligand EBA-175 is being developed as a major blood-stage vaccine candidate. EBA-175 mediates parasite invasion of host erythrocytes in a sialic acid-dependent manner through its binding to the erythrocyte receptor glycophorin A. In this study, we addressed the ability of naturally acquired human antibodies against the EBA-175 RII erythrocyte-binding domain to inhibit parasite invasion of ex vivo isolates, in relationship to the sialic acid dependence of these parasites. We have determined the presence of antibodies to the EBA-175 RII domain by enzyme-linked immunosorbent assay (ELISA) in individuals from areas of Senegal where malaria is endemic with high and low transmission. Using affinity-purified human antibodies to the EBA-175 RII domain from pooled patient plasma, we have measured the invasion pathway as well as the invasion inhibition of clinical isolates from Senegalese patients in ex vivo assays. Our results suggest that naturally acquired anti-EBA-175 RII antibodies significantly inhibit invasion of Senegalese parasites and that these responses can be significantly enhanced through limiting other ligand-receptor interactions. However, the extent of this functional inhibition by EBA-175 antibodies is not associated with the sialic acid dependence of the parasite strain, suggesting that erythrocyte invasion pathway usage by parasite strains is not driven by antibodies targeting the EBA-175/glycophorin A interaction. This work has implications for vaccine design based on the RII domain of EBA-175 in the context of alternative invasion pathways.

A quantitative assay for binding and inhibition of Plasmodium falciparum Erythrocyte Binding Antigen 175 reveals high affinity binding depends on both DBL domains

“…RII binding to erythrocytes is inhibited by an anti-GpA antibody, α-2,3-sialyllactose and the neutralizing antibody R217 demonstrating a specific interaction with the host receptor. In addition, recombinant RII is recognized by sera from individuals living in endemic areas, and antibodies affinity purified by RII prevent parasite growth [18]. These studies further validate the potential for the use of recombinant RII described here in vaccine development and diagnostics to measure the immune response.…”

“…Plasmodium falciparum Erythrocyte Binding Antigen of 175 kDa (PfEBA-1751) is a parasite protein ligand that binds to the erythrocyte receptor Glycophorin A during the blood-stage of the parasite life cycle [1–5]. PfEBA-175 is therefore an important antibody target and vaccine candidate [6–18]. …”

“…No binding to erythrocytes for F1 alone was observed [3]. In addition to a direct role in red blood cell engagement, RII is a target for neutralizing antibodies [6–18]. …”

Critical Glycosylated Residues in Exon Three of Erythrocyte Glycophorin A Engage Plasmodium falciparum EBA-175 and Define Receptor Specificity

“…Although EBL ligands are functionally redundant, PfEBA-175 is an excellent vaccine candidate (44, 45) because it is a major invasion pathway (46) and antibodies that target PfEBA-175 inhibit parasite growth of multiple strains of P. falciparum , including those that use PfEBA-175-independent invasion pathways (47). Toward development of more efficacious vaccines, greater understanding of interactions between surface receptors and ligands is needed to better focus vaccine responses.…”

Far from the Madding Crowd: the Molecular Basis for Immunological Escape ofPlasmodium falciparum

“…On the other hand, EBA-175 RII (where cHABPs 1779 and 1783 are located) is recognised by antibodies in individuals having naturally acquired immunity in an age-dependent manner and inhibits invasion of P. falciparum clinical isolates to some degree (Okenu et al, 2000;El Sahly et al, 2010;Badiane et al, 2013;Irani et al, 2015). High-titre antibodies against EBA-175 RII are associated with protection from clinical malaria in children (Okenu et al, 2000;McCarra et al, 2011), although such association is not clear in some studies or has not been observed in groups having low incidence of disease (Osier et al, 2008;Richards et al, 2010) It has been suggested that EBA-175 RII polymorphism has shown immune selection aimed at maintaining genetic diversity in parasite populations.…”