The study of antigenic targets of naturally-acquired immunity is essential to identify and prioritize antigens for further functional characterization. We measured total IgG antibodies to 38 P. vivax antigens, investigating their relationship with prospective risk of malaria in a cohort of 1–3 years old Papua New Guinean children. Using simulated annealing algorithms, the potential protective efficacy of antibodies to multiple antigen-combinations, and the antibody thresholds associated with protection were investigated for the first time. High antibody levels to multiple known and newly identified proteins were strongly associated with protection (IRR 0.44–0.74, p<0.001–0.041). Among five-antigen combinations with the strongest protective effect (>90%), EBP, DBPII, RBP1a, CyRPA, and PVX_081550 were most frequently identified; several of them requiring very low antibody levels to show a protective association. These data identify individual antigens that should be prioritized for further functional testing and establish a clear path to testing a multicomponent P. vivax vaccine.

Understanding how malaria parasites gain entry into human red blood cells is essential for developing strategies to stop blood stage infection. Plasmodium vivax preferentially invades reticulocytes, which are immature red blood cells. The organism has two erythrocyte-binding protein families: namely, the Duffy-binding protein (PvDBP) and the reticulocyte-binding protein (PvRBP) families. Several members of the PvRBP family bind reticulocytes, specifically suggesting a role in mediating host cell selectivity of P. vivax. Here, we present, to our knowledge, the first high-resolution crystal structure of an erythrocyte-binding domain from PvRBP2a, solved at 2.12 Å resolution. The monomeric molecule consists of 10 α-helices and one short β-hairpin, and, although the structural fold is similar to that of PfRh5-the essential invasion ligand in Plasmodium falciparum-its surface properties are distinct and provide a possible mechanism for recognition of alternate receptors. Sequence alignments of the crystallized fragment of PvRBP2a with other PvRBPs highlight the conserved placement of disulfide bonds. PvRBP2a binds mature red blood cells through recognition of an erythrocyte receptor that is neuraminidase-and chymotrypsinresistant but trypsin-sensitive. By examining the patterns of sequence diversity within field isolates, we have identified and mapped polymorphic residues to the PvRBP2a structure. Using mutagenesis, we have also defined the critical residues required for erythrocyte binding. Characterization of the structural features that govern functional erythrocyte binding for the PvRBP family provides a framework for generating new tools that block P. vivax blood stage infection.parasite invasion | X-ray crystallography | SAXS | reticulocyte binding protein | malaria T he most widely distributed recurring malaria infections globally are caused by Plasmodium vivax, which accounts for 80-100 million malaria infections per year (1). The majority of clinical symptoms associated with malaria are due to blood stage infection (2). The merozoite forms of malaria parasites invade human erythrocytes through a multistep process that involves initial contact with the red blood cell, apical reorientation of the merozoite, and the formation of a tight junction that moves progressively toward the posterior end of the parasite until host cell membrane fusion is completed. These steps in invasion are dependent on specific interactions between parasite adhesins and their cognate erythrocyte receptors (reviewed in ref.3).P. vivax preferentially invades reticulocytes: i.e., immature red blood cells (4). The basis of host cell selectivity by merozoites from Plasmodium spp. seems to be mediated primarily by families of adhesin proteins. The two erythrocyte-binding protein families of P. vivax are called the Duffy-binding protein (PvDBP) and reticulocyte-binding protein (PvRBP) families (5). In laboratory-adapted P. vivax strains, there is only one PvDBP protein in P. vivax that binds to Duffy antigen receptor for chemokines (DARC) (6...