Synthesis and immunological characterization of 104-mer and 102-mer peptides corresponding to the N- and C-terminal regions of the Plasmodium falciparum CS protein.

We investigated the immunogenicity and the conformational properties of the non-repetitive sequences of the Plasmodium falciparum circumsporozoite (CS) protein. Two polypeptides of 104 and 102 amino acids long, covering, respectively, the N- and C-terminal regions of the CS protein, were synthesized using solid phase Fmoc chemistry. The crude polypeptides were purified by a combination of size exclusion chromatography and RP-HPLC. Sera of mice immunized with the free polypeptides emulsified in incomplete Freund's adjuvant strongly reacted with the synthetic polypeptides as well as with native CS protein as judged by ELISA and IFAT assays. Most importantly, these antisera inhibited the sporozoite invasion of hepatoma cells. In addition, sera derived from donors living in a malaria endemic area recognized the CS 104- and 102-mers. Conformational studies of the CS polypeptides were also performed by circular dichroism spectroscopy showing the presence of a weakly ordered structure that can be increased by addition of trifluoroethanol. The obtained results indicate that the synthetic CS polypeptides and the natural CS protein share some common antigenic determinants and probably have similar conformation. The approach used in this study might be useful for the development of a synthetic malaria vaccine.