Evidence for Globally Shared, Cross-reacting Polymorphic Epitopes in the Pregnancy Malaria Vaccine Candidate VAR2CSA

Seattle Biomedical Research Institute, 307 Westlake Ave N, Suite 500, Seattle, WA 98109-5219; Malaria Vaccine Development Branch, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Twinbrook I, 5640 Fishers Lane, Rockville, Maryland 20852; Center for Medical Parasitology, University of Copenhagen, Copenhagen, Denmark; Department of Pathobiology, University of Washington, Seattle, WA 98195, USA

^* To whom correspondence should be addressed. Email: joseph.smith{at}sbri.org.

	Abstract

Pregnancy-associated malaria (PAM) is characterized by the placental sequestration of Plasmodium falciparum-infected erythrocytes (IEs) with the ability to bind to chondroitin sulfate A (CSA). VAR2CSA is a leading candidate for a pregnancy malaria vaccine, but its large size (350 kDa) and extensive polymorphism may pose a challenge to vaccine development. In this study, rabbits were immunized with individual VAR2CSA Duffy binding-like (DBL) domains expressed in Pichia pastoris or var2csa plasmid DNA and sera were screened on different CSA-binding parasite lines. Rabbit antibodies to three recombinant proteins (DBL1, DBL3, DBL6) and four plasmid DNAs (DBL1, DBL3, DBL5 and DBL6) reacted with homologous FCR3-CSA IEs. By comparison, antibodies to the DBL4 domain were unable to react with native VAR2CSA protein unless it was first partially proteolyzed with trypsin or chymotrypsin. To investigate the antigenic relationship of geographically diverse CSA-binding isolates, rabbit immune sera were screened on four heterologous CSA-binding lines from different continental origins. Antibodies did not target conserved epitopes exposed in all VAR2CSA alleles, however, anti-sera to several DBL domains cross-reacted on parasite isolates that had polymorphic loops in common with the homologous immunogen. This study demonstrates that VAR2CSA contains common polymorphic epitopes that are shared between geographically diverse CSA-binding lines.