An engineered vaccine of the Plasmodium vivax Duffy binding protein enhances induction of broadly neutralizing antibodies.

Background: The Duffy binding protein (DBP) plays a vital role in P. vivax merozoite invasion of reticulocytes. DBP is essential during the process of junction formation and DBP dimerization with its cognate receptor, the Duffy antigen receptor for chemokines (DARC), is key to its function. Due to its functional importance, DBP is considered a prime vaccine candidate for blood-stage vivax malaria. However, variations in dominant B-cell epitopes at the dimer interface of DBP represent a major challenge that may compromise efficacy of a DBP vaccine. Specifically, there is the tendency for immunity to be strain specific. Our hypothesis is that polymorphic dominant B-cell epitopes of DBP represent an evasion mechanism to misdirect immune responses away from functional, conserved epitopes. Consistent with this interpretation, the dominant B cell epitopes in DBP are polymorphic surface-exposed motifs but these variable residues are generally not important for binding the erythrocyte receptor DARC. Despite these shortcomings, DBPII appears to contain conserved epitopes that are targets of broadly neutralizing anti-DBPII antibodies. Our objective is to design a DBP mutant that will focus immune response to these conserved neutralizing epitopes to overcome strain-specific immunity. Methods: We previously engineered a DBP mutant, termed DEKnull that lacked highly charged residues of an immunodominant variant B-cell epitope. The DEKnull antigen partially overcame strain-specific immunity. In this current study, we have evaluated three additional novel engineered DBP immunogens that altered: (i) polymorphic residues, (DEKnull-2); (ii) functional DARC-binding residues, (DEKnull-3); and (iii) residues important for dimerization, (DEKnull-4). Results: We showed that each antigen design altered the nature of functional antibodies elicited and importantly DEKnull-2 induced more broadly neutralizing antibodies against a range of diverse allelic variants than the native DBPII-Sal1 reference allele. Anti-DEKnul-2 antibodies blocked merozoite invasion of human reticulocytes. Conclusion: We engineered a next generation DBP mutant immunogen and validated that the prime targets of protective immunity are conformational epitopes at the dimer interface. Our data indicate a potential approach that can be used generally to improve efficacy of other malaria vaccine candidates.