IMMUNOGENICITY AND ANTIGENIC REACTIVITY OF A CARRIER-FREE SYNTHETIC PEPTIDE COMPLEX DERIVED FROM A 70-KDA PLASMODIUM-FALCIPARUM EXOANTIGEN

A 70-kDa Plasmodium falciparum exoantigen was purified from supernatan t fluids of continuous in vitro P. falciparum cultures using sequentia l cation-exchange and high performance liquid chromatographic procedur es. The purified protein was then digested with chymotrypsin and amino -acid sequences were determined for the resulting fragments. Four pept ides (termed C2, C3, C5, and C10) were subsequently selected for synth esis based on their predictability for antigenic sites. The peptides w ere effectively used as a synthetic immunogen (SPf70) when they had be en copolymerized with glutaraldehyde in the absence of a carrier. When given with Freund's adjuvant, the synthetic peptide complex was found to be highly immunogenic in rabbits. Serologic reactivity to the pept ide complex and peptides C2 and C5 was uniformly high, followed by the responses to peptides C3 and C10. Peptide antigenicity was also asses sed with human anti-P.falciparum sera from malaria-endemic regions of Uganda and Venezuela. Enzyme-linked immunosorbent assay (ELISA) data s howed that anti-P.falciparum antibodies were specific for and reactive to the peptides. The specificity of the rabbit anti-SPf70 antibodies for P. falciparum antigen was shown by immunoprecipitation of metaboli cally labeled proteins and by immunoblotting. Herein we describe the p eptide sequences of a 70-kDa P.falciparum exoantigen (Pf70) that, when synthesized and constructed as a copolymer (SPf70), are capable of in ducing the formation of antibodies that are reactive with the native m alarial protein. The high immunogenicity and antigenic reactivity of S Pf70 indicate the potential use of this synthetic peptide polymer as a n immunogen and a diagnostic reagent.