Is the single variant form of Plasmodium vivax Duffy binding protein-II (PvDBP-II) adequate for inclusion in a PvDBP-II-based vaccine?

The binding domain of Duffy protein (DBP-II) is a leading vaccine candidate of Plasmodium vivax. In order to develop a successful vivax malaria vaccine based on DBP-II, the antigenic diversity and also naturally occurring functional antibodies to different PvDBP-II variant types in the various populations must be known. To define whether the polymorphisms in PvDBP-II influenced the nature of functional antibody responses, this investigation was designed to evaluate naturally acquired antibodies to five circulating variant forms of DBP-II antigens in infected individuals with P. vivax living in hypoendemic areas in Iran. Sequence diversity of pvdbp-II gene was performed in 63 Iranian P. vivax isolates collected during 2008-2012. The sequencing analysis showed twenty two single nucleotide polymorphisms in PvDBP-II, resulting in 16 different haplotypes among the Iranian P. vivax isolates. Five of 16 genetically distinct variants were expressed in E. coli, and anti-DBP-II responses were measured in P. vivax-infected individuals (n = 202). Also, by performing immune-depletion ELISA experiments, antibody responses to the conserved sites of all five allelic forms were evaluated using the corresponding and non-corresponding patients’ sera (n = 20). ELISA results revealed that naturally acquired anti-PvDBP-II IgG were recognized all five expressed variant forms with no statistically difference (P > 0.05, Cochran’s Q test). The antibody depletion experiments also showed presence of the cross-reactive antibody responses to heterologous variants of PvDBP-II in Iranian individuals who were infected with distinct allelic forms of the PvDBP-II. Finally, all five examined variant forms of DBP-II were expressed transiently on the surface of COS-7 cells to determine whether people exposed to vivax malaria acquire antibodies that have the ability to block erythrocyte cytoadherence to PvDBP-II. The anti-DBP-II IgG block heterologous and homologous expressed DBP-II function, indicating that the protective immunity against PvDBP-II binding is not strain specific. In conclusion, the present results indicate that the single variant of PvDBP-II is adequate to be included in a PvDBP-II-based vaccine.