Date Published: December 18, 2007
Publisher: Public Library of Science
Author(s): Brian T Grimberg, Rachanee Udomsangpetch, Jia Xainli, Amy McHenry, Tasanee Panichakul, Jetsumon Sattabongkot, Liwang Cui, Moses Bockarie, Chetan Chitnis, John Adams, Peter A Zimmerman, Christopher L King, James Beeson
Abstract: BackgroundPlasmodium vivax invasion requires interaction between the human Duffy antigen on the surface of erythrocytes and the P. vivax Duffy binding protein (PvDBP) expressed by the parasite. Given that Duffy-negative individuals are resistant and that Duffy-negative heterozygotes show reduced susceptibility to blood-stage infection, we hypothesized that antibodies directed against region two of P. vivax Duffy binding protein (PvDBPII) would inhibit P. vivax invasion of human erythrocytes.Methods and FindingsUsing a recombinant region two of the P. vivax Duffy binding protein (rPvDBPII), polyclonal antibodies were generated from immunized rabbits and affinity purified from the pooled sera of 14 P. vivax–exposed Papua New Guineans. It was determined by ELISA and by flow cytometry, respectively, that both rabbit and human antibodies inhibited binding of rPvDBPII to the Duffy antigen N-terminal region and to Duffy-positive human erythrocytes. Additionally, using immunofluorescent microscopy, the antibodies were shown to attach to native PvDBP on the apical end of the P. vivax merozoite. In vitro invasion assays, using blood isolates from individuals in the Mae Sot district of Thailand, showed that addition of rabbit anti-PvDBPII Ab or serum (antibodies against, or serum containing antibodies against, region two of the Plasmodium vivax Duffy binding protein) (1:100) reduced the number of parasite invasions by up to 64%, while pooled PvDBPII antisera from P. vivax–exposed people reduced P. vivax invasion by up to 54%.ConclusionsThese results show, for what we believe to be the first time, that both rabbit and human antibodies directed against PvDBPII reduce invasion efficiency of wild P. vivax isolated from infected patients, and suggest that a PvDBP-based vaccine may reduce human blood-stage P. vivax infection.
Partial Text: Plasmodium vivax accounts for at least half of all malaria cases in Latin America, Oceania, and Asia ; 70 to 80 million clinical P. vivax cases occur worldwide annually. While Plasmodium falciparum uses a complex array of receptors to invade human erythrocytes [2–6], erythrocyte invasion by P. vivax, and the closely related simian parasite Plasmodium knowlesi, are understood to depend upon interaction with the Duffy blood group antigen [7,8]. In the homologous P. knowlesi system, merozoites interact with Duffy-negative human red blood cells, but are unable to invade [8,9]. In Africa, where Duffy-negativity has reached fixation in many different ethnicities, transmission of P. vivax malaria is uncommon [1,10]. Of further interest, in Papua New Guinea, heterozygous carriers of a Duffy-negative allele are shown to express half the amount of the Duffy antigen on erythrocytes compared to wild-type homozygotes , and exhibit reduced susceptibility to P. vivax blood-stage infection . These observations suggest that completely or partially disrupting access to the Duffy antigen reduces the ability of the parasite to invade new erythrocytes and may constrain P. vivax parasitemia.
This study demonstrates that anti-PvDBPII Ab obtained from humans exposed to P. vivax, or artificially induced in rabbits, can partially inhibit P. vivax merozoite invasion in short-term cultures. Our results establish region II of PvDBP as a prominent ligand engaging the Duffy antigen on human red blood cells, making it a potential vaccine candidate against P. vivax. These studies also demonstrate, for what we believe to be the first time, the utility of short-term P. vivax cultures derived from human isolates for measuring the invasion-inhibitory potential of antibodies directed against a specific merozoite antigen. This approach can be used to test additional antibodies targeting other P. vivax merozoite invasion ligands to evaluate potential alternative antigens as vaccine candidates similar to studies performed more routinely for P. falciparum [42,43].