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Background  The Duffy-binding protein II of  Plasmodium vivax  ( PvDBPII ) has been considered as an attractive target for vaccine-mediated immunity despite a possible highly polymorphic nature. Among seven  PvDBP  domains, domain II has been shown to exhibit a high rate of nonsynonymous polymorphism, which has been suggested to be a potential immune (antibody binding) evasion mechanism. This study aimed to determine the extent of genetic polymorphisms and positive natural selection at domain II of the  PvDBP  gene among a sampling of Thai  P. vivax  isolates.    Methods  The  PvDBPII  gene was PCR amplified and the patterns of polymorphisms were characterized from 30 Thai  P. vivax  isolates using DNA cloning and sequencing. Phylogenetic analysis of the sequences and positive selection were done using DnaSP ver 4.0 and MEGA ver 4.0 packages.    Results  This study demonstrated a high rate of nonsynonymous polymorphism. Using Sal I as the reference strain, a total of 30 point-mutations were observed in the  PvDBPII  gene among the set of Thai  P. vivax  isolates, of which 25 nonsynonymous and five synonymous were found. The highest frequency of polymorphism was found in five variant amino acids (residues D384G, R390H, L424I, W437R, I503K) with the variant L424I having the highest frequency. The difference between the rates of nonsynonymous and synonymous mutations estimated by the Nei and Gojobori's method suggested that  PvDBPII  antigen appears to be under selective pressure. Phylogenetic analysis of  PvDBPII  Thai  P. vivax  isolates to others found internationally demonstrated six distinct allele groups. Allele groups 4 and 6 were unique to Thailand.    Conclusion  Polymorphisms within  PvDBPII  indicated that Thai vivax malaria parasites are genetically diverse. Phylogenetic analysis of DNA sequences using the Neighbour-Joining method demonstrated that Thai isolates shared distinct alleles with  P. vivax  isolates from different geographical areas. The study reported here will be valuable for the development of  PvDBPII -based malaria vaccine.

Polymorphism patterns in Duffy-binding protein among Thai Plasmodium vivax isolates