Date Published: June 13, 2013
Publisher: Public Library of Science
Author(s): Sowmya Sampath, Chris Carrico, Joel Janes, Sairam Gurumoorthy, Claire Gibson, Martin Melcher, Chetan E. Chitnis, Ruobing Wang, William R. Schief, Joseph D. Smith, James W. Kazura.
Glycan masking is an emerging vaccine design strategy to focus antibody responses to specific epitopes, but it has mostly been evaluated on the already heavily glycosylated HIV gp120 envelope glycoprotein. Here this approach was used to investigate the binding interaction of Plasmodium vivax Duffy Binding Protein (PvDBP) and the Duffy Antigen Receptor for Chemokines (DARC) and to evaluate if glycan-masked PvDBPII immunogens would focus the antibody response on key interaction surfaces. Four variants of PVDBPII were generated and probed for function and immunogenicity. Whereas two PvDBPII glycosylation variants with increased glycan surface coverage distant from predicted interaction sites had equivalent binding activity to wild-type protein, one of them elicited slightly better DARC-binding-inhibitory activity than wild-type immunogen. Conversely, the addition of an N-glycosylation site adjacent to a predicted PvDBP interaction site both abolished its interaction with DARC and resulted in weaker inhibitory antibody responses. PvDBP is composed of three subdomains and is thought to function as a dimer; a meta-analysis of published PvDBP mutants and the new DBPII glycosylation variants indicates that critical DARC binding residues are concentrated at the dimer interface and along a relatively flat surface spanning portions of two subdomains. Our findings suggest that DARC-binding-inhibitory antibody epitope(s) lie close to the predicted DARC interaction site, and that addition of N-glycan sites distant from this site may augment inhibitory antibodies. Thus, glycan resurfacing is an attractive and feasible tool to investigate protein structure-function, and glycan-masked PvDBPII immunogens might contribute to P. vivax vaccine development.
Plasmodium vivax invasion of human reticulocytes is strongly dependent on an interaction between the P. vivax Duffy Binding Protein (PvDBP) and the Duffy Antigen Receptor for Chemokines (DARC) on the reticulocyte surface . DARC-negative individuals are highly resistant to P. vivax infection  and the DARC-null phenotype has independently arisen in different human populations , . Although an alternative pathway of P. vivax invasion has recently been described , , DARC-null carriers have reduced susceptibility to P. vivax infection ,  and the FyA DARC allele shows reduced binding to PvDBP and is more susceptible to antibody blocking . Thus, the PvDBP-DARC interaction has a critical role in P. vivax infection making it an attractive vaccine target.
Efforts to make a blood stage malaria vaccine have proven extremely challenging because of antigen polymorphism, redundant pathways of RBC invasion, and the rapid kinetics of parasite invasion . Although P. vivax is highly restricted to reticulocytes and theoretically poses fewer challenges for blood stage vaccine development than P. falciparum, PvDBP is polymorphic ,  and vaccine-induced antibodies are only partially effective at inhibiting the PvDBP-DARC binding interaction , , –. Therefore, new immunogen design strategies and adjuvants  are required to enhance neutralizing antibody responses and confer cross-strain protection.