The Plasmodium falciparum Erythrocyte Invasion Ligand Pfrh4 as a Target of Functional and Protective Human Antibodies against Malaria

Linda Reiling,Brendan S Crabb,Ivo Mueller,Christine Langer,John Donelson,Pascal Michon,Watcharee Chokejindachai,Freya J I Fowkes,Alan F Cowman,Peter M Siba,Wai-Hong Tham,Danny W Wilson,Alyssa E Barry,Jack S Richards,Livingstone Tavul,James G Beeson,Janine Stubbs

doi:10.1371/journal.pone.0045253

Abstract

BackgroundAcquired antibodies are important in human immunity to malaria, but key targets remain largely unknown. Plasmodium falciparum reticulocyte-binding-homologue-4 (PfRh4) is important for invasion of human erythrocytes and may therefore be a target of protective immunity.MethodsIgG and IgG subclass-specific responses against different regions of PfRh4 were determined in a longitudinal cohort of 206 children in Papua New Guinea (PNG). Human PfRh4 antibodies were tested for functional invasion-inhibitory activity, and expression of PfRh4 by P. falciparum isolates and sequence polymorphisms were determined.ResultsAntibodies to PfRh4 were acquired by children exposed to P. falciparum malaria, were predominantly comprised of IgG1 and IgG3 subclasses, and were associated with increasing age and active parasitemia. High levels of antibodies, particularly IgG3, were strongly predictive of protection against clinical malaria and high-density parasitemia. Human affinity-purified antibodies to the binding region of PfRh4 effectively inhibited erythrocyte invasion by P. falciparum merozoites and antibody levels in protected children were at functionally-active concentrations. Although expression of PfRh4 can vary, PfRh4 protein was expressed by most isolates derived from the cohort and showed limited sequence polymorphism.ConclusionsEvidence suggests that PfRh4 is a target of antibodies that contribute to protective immunity to malaria by inhibiting erythrocyte invasion and preventing high density parasitemia. These findings advance our understanding of the targets and mechanisms of human immunity and evaluating the potential of PfRh4 as a component of candidate malaria vaccines.

Highlights

Malaria due to Plasmodium falciparum remains a major global health burden and a leading cause of death worldwide among children under five [1,2]
Antibodies to Plasmodium falciparum reticulocyte-binding-homologue-4 (PfRh4).2 and PfRh4.9 are Associated with Age and Concurrent Parasitemia
The prevalence antibodies to PfRh4.2 and PfRh4.9 was significantly higher in older children (p,0.01 and p,0.001, respectively), and among children who were parasitemic at time of sample collection (p = 0.042 and p,0.001 for PfRh4.2 and PfRh4.9, respectively), consistent with the specific acquisition of antibodies from P. falciparum exposure

Summary

Introduction

Malaria due to Plasmodium falciparum remains a major global health burden and a leading cause of death worldwide among children under five [1,2]. The repertoire of invasion ligands includes two major families, the P. falciparum reticulocyte-binding homologues (PfRh), and erythrocyte binding antigens (EBAs) [3,4]. The ability of P. falciparum to vary the expression and/or use of EBA and PfRh proteins enables the use of alternate invasion pathways [5,6], facilitating immune evasion that enables P. falciparum to cause repeated and chronic infections [7]. Acquired antibodies are important in human immunity to malaria, but key targets remain largely unknown. Plasmodium falciparum reticulocyte-binding-homologue-4 (PfRh4) is important for invasion of human erythrocytes and may be a target of protective immunity

Methods

Results

Discussion

Conclusion