Identification and characterization of Pv50, a novel Plasmodium vivax merozoite surface protein

Plasmodium vivax contains approximately 5400 coding genes, more than 40% of which code for hypothetical proteins that have not been functionally characterized. In a previous preliminary screening using pooled serum samples, numerous hypothetical proteins were selected from among those that were highly transcribed in the schizont-stage of parasites, and highly antigenic P. vivax candidates including hypothetical proteins were identified. However, their immunological and functional activities in P. vivax remain unclear. From these candidates, we investigated a P. vivax 50-kDa protein (Pv50, PVX_087140) containing a highly conserved signal peptide that shows high transcription levels in blood-stage parasites.

Recombinant Pv50 was expressed in a cell-free expression system and used for IgG prevalence analysis of patients with vivax malaria and healthy individuals. Immune responses were analyzed in immunized mice and mouse antibodies were used to detect the subcellular localization of the protein in blood-stage parasites by immunofluorescence assay. A protein array method was used to evaluate protein-protein interactions to predict protein functional activities during the invasion of parasites into erythrocytes. Recombinant Pv50 showed IgG prevalence in patient samples with a sensitivity of 42.9% and specificity of 93.8% compared to that in healthy individuals. The non-cytophilic antibodies IgG1 and IgG3 were the major components involved in the antibody response in Pv50-immunized mice. Pv50 localized on the surface of merozoites and a specific interaction between Pv50 and PvMSP1 was detected, suggesting that Pv50-PvMSP1 forms a heterodimeric complex in P. vivax.

Increased immune responses caused by native P. vivax parasites were detected, confirming its immunogenic effects. This study provides a method for detecting new malaria antigens, and Pv50 may be a vivax malaria vaccine candidate with PvMSP1.