Antibodies raised against receptor-binding domain of Plasmodium knowlesi Duffy binding protein inhibit erythrocyte invasion.

Erythrocyte invasion by malaria parasites requires specific receptor-ligand interactions. Plasmodium vivax and Plasmodium knowlesi are completely dependent on binding the Duffy blood group antigen to invade human erythrocytes. P. knowlesi invades rhesus erythrocytes by multiple pathways using the Duffy antigen as well as alternative receptors. Plasmodium falciparum binds sialic acid residues on glycophorin A as well as other sialic acid-independent receptors to invade human erythrocytes. Parasite proteins that mediate these interactions belong to a family of erythrocyte binding proteins, which includes the P. vivax Duffy binding protein, 175 kDa P. falciparum erythrocyte binding antigen (EBA-175), P. knowlesi alpha protein, which binds human and rhesus Duffy antigens, and P. knowlesi beta and gamma proteins, which bind Duffy-independent receptors on rhesus erythrocytes. The receptor-binding domains of these proteins lie in conserved, N-terminal, cysteine-rich regions that are referred to as region II. Here, we have examined the feasibility of inhibiting erythrocyte invasion with antibodies directed against receptor-binding domains of erythrocyte binding proteins. Region II of P. knowelsi alpha protein (Pk(alpha)RII), which binds the Duffy antigen, was expressed as a secreted protein in insect cells and purified from culture supernatants. Rabbit antibodies raised against recombinant Pk(alpha)RII were tested for inhibition of erythrocyte binding and invasion. Antibodies raised against Pk(alpha)RII inhibit P. knowlesi invasion of both human and rhesus erythrocytes. These data provide support for the development of recombinant vaccines based on the homologous binding domains of P. vivax Duffy binding protein and P. falciparum EBA-175.