Abstract

The Plasmodium knowlesi circumsporozoite (CS) protein contains a repetitive immunodominant epitope. Here we show that the serum of rabbits repeatedly immunized with P. knowlesi sporozoites contains antibodies which bind to immobilized synthetic peptides ('C2', 'N2', and 'charged') representing two different polar regions of the CS polypeptide. These reactions are specific since the binding is inhibited only by the homologous peptides. Antisporozoite antibodies were isolated from the rabbit serum by affinity chromatography on Sepharose beads coupled to two synthetic peptides, 'C2' and 'charged'. Both purified antibodies recognized the CS protein and the intracellular precursors as shown by Western blotting analysis using sporozoite extracts. These results demonstrate that the corresponding areas of the native CS molecule are immunogenic, accessible to interaction with antibody, and therefore constitute potential targets for vaccine development. In addition, the present findings confirm the published amino acid sequence of a large portion of the CS protein which has been deduced from the nucleotide sequence of the corresponding gene.