Evidence That the Erythrocyte Invasion Ligand PfRh2 is a Target of Protective Immunity against Plasmodium falciparum Malaria

Abstract

Abs targeting blood-stage Ags of Plasmodium falciparum are important in acquired immunity to malaria, but major targets remain unclear. The P. falciparum reticulocyte-binding homologs (PfRh) are key ligands used by merozoites during invasion of erythrocytes. PfRh2a and PfRh2b are functionally important members of this family and may be targets of protective immunity, but their potential role in human immunity has not been examined. We expressed eight recombinant proteins covering the entire PfRh2 common region, as well as PfRh2a- and PfRh2b-specific regions. Abs were measured among a cohort of 206 Papua New Guinean children who were followed prospectively for 6 mo for reinfection and malaria. At baseline, Abs were associated with increasing age and active infection. High levels of IgG to all PfRh2 protein constructs were strongly associated with protection from symptomatic malaria and high-density parasitemia. The predominant IgG subclasses were IgG1 and IgG3, with little IgG2 and IgG4 detected. To further understand the significance of PfRh2 as an immune target, we analyzed PfRh2 sequences and found that polymorphisms are concentrated in an N-terminal region of the protein and seem to be under diversifying selection, suggesting immune pressure. Cluster analysis arranged the sequences into two main groups, suggesting that many of the haplotypes identified may be antigenically similar. These findings provide evidence suggesting that PfRh2 is an important target of protective immunity in humans and that Abs act by controlling blood-stage parasitemia and support its potential for vaccine development.

Acquisition of Antibodies against Plasmodium falciparum Merozoites and Malaria Immunity in Young Children and the Influence of Age, Force of Infection, and Magnitude of Response

“…Similarly, the prevalences of IgG subclass responses (33.9% to 63.9%) differed among the different antigens. In previous studies in PNG (23,24,49), we found that the dominant IgG subclass responses among PNG children are comprised of IgG1 and IgG3, with very little IgG2 or IgG4, consistent with reports from African studies (18,(50)(51)(52). Therefore, only IgG1 and IgG3 responses were examined in detail in the current study.…”

Identification and Prioritization of Merozoite Antigens as Targets of Protective Human Immunity to Plasmodium falciparum Malaria for Vaccine and Biomarker Development

“…Although some children had multiple episodes of parasitemia or symptomatic malaria, the analysis presented examined the time to first symptomatic episode only. This is consistent with previous analytic approaches used in this cohort (48)(49)(50)(51). A range of demographic, clinical, and biological variables was assessed as potential confounders of associations between Abs and malaria outcomes.…”

“…All statistical analyses were performed using STATA 9.2 (StataCorp) or Prism 5.01 (GraphPad) software. Ab responses to MSP1-19, MSP2, AMA1, EBAs, PfRh2, and PfRh4 in this population were reported previously (48)(49)(50)(51), and the data were reanalyzed for comparison with other Ags and in combination analyses.…”

Using Population Genetics to Guide Malaria Vaccine Design

“…Another example is the PfRh invasion ligand family. One member that appears to be an important target of acquired immunity, PfRh2, has significant polymorphism in the N-terminal region [171], but studies suggest it is possible to target a C-terminal conserved stretch of the protein in vaccine design [172]. The challenge of addressing diversity in vaccine development is greatest for PfEMP1, which is expressed on the surface of infected RBCs.…”

“…More recently discovered antigens demonstrate significant potential as vaccine candidates including members of the Rh family of proteins (Rh1, 2a, 2b, 4 and 5) [171,172] and the Rh-interacting protein, RIPr [173], Serine repeat antigen 5 (SERA5, reviewed in [174]); MSP6 [175], MSP7 [176] and the pregnancy-associated malaria vaccine candidate, var2csa [177][178][179]. For some of these antigens, diversity is a significant issue that needs to be evaluated.…”