Plasmodium falciparum merozoites invade erythrocytes through a multistep process. Two protein families of invasion ligands have been identified in P. falciparum: PfEBAs and PfRhs. Previous data suggest that isolate from Papua have varied invasion profile based on receptors membrane sensitivity based on particular enzymatic treatment. We investigated antibody responses to P. falciparum antigens, determined whether PfEBAs and PfRhs are targets of acquired immunity to falciparum malaria, and perform next generation sequencing (NGS)-amplicon sequencing to identify polymorphism in those invasion proteins.
Antibody responses (IgG) against P. falciparum antigens (3D7 lysate) were tested from 95 symptomatic individuals by using ELISA. Individuals with uncomplicated malaria showed significantly higher antibody responses compared to individuals with severe malaria (p=0.0147). Antibody responses were also measured to 7 P. falciparum recombinant antigens: PfEBA-175, PfEBA-140, PfRh2, PfRh4, PfRh5, PfRipr C-terminal, and PfRipr N-terminal. Findings from this study revealed that increasing level of acquired antibodies to all P. falciparum recombinant antigens tested were strongly associated with reduced risk of having high parasite densities after adjustment to age.
We have also explored the sequencing polymorphisms of PfEBA175 and PfEBA140 by amplicon sequencing to examine the extent of polymorphisms of these invasion ligands. Results from targeted sequencing showed that out of 30 PfEBA-175 sequences, 21 (70%) had haplotypes with two variants XP_001349207.2:p.(Lys932Glu) and XP_001349207.2:p.(Gly1100Asp). Out of 24 PfEBA140 sequences, 8 (33%) of them had a single polymorphism XP_001349859.1:p.(Asn779Ser). Although those variants may alter the biochemical properties of the amino acids, the overall sequences are relatively conserved.
Our results demonstrated that 7 protein are targets of naturally acquired immunity to malaria and having a high level of antibodies to those antigens were associated with reduced parasitemia levels after adjusted by age. The relatively conserved sequence of PfEBA175 and PfEBA140 supports the inclusions of these antigens into malaria vaccine components.