A protective merozoite protein ofPlasmodium falciparumshares an epitope with surface antigens ofParamecium.

APlasmodium falciparumcDNA expression clone,λPf9, had been identified earlier as a protective epitope, using anti-λPf9 antibodies and combinatorial phagotopes. A segment of the Pf9 gene showed homology withParameciumimmobilization surface antigens such as 51B, 51A and 156G. A synthetic Pf9-peptide was designed from this region, and specific antibodies were raised. Each of these anti-Pf9 antibodies and combinatorial reagents, as well as anti-Paramecium51B antibodies, recognized the Pf9-peptide on ELISA, and the same protein band in parasite immunoblots. TheP. falciparumprotein was released from the merozoite membrane fraction on treatment with PI-PLC, indicating the presence of a GPI anchor. Anti-Pf9-peptide antibodies specifically inhibited the growth ofP. falciparumin culture. Immunofluorescence assays showed the reactivity of anti-Pf9-peptide sera withP. falciparummerozoites and gametocytes, as well as on the surface ofParamecium tetraurelia. The Pf9-peptide was able to induce proliferation of splenic lymphocytes obtained from mice infected with the rodent malarial parasitesPlasmodium bergheiandPlasmodium yoelii.These results point towardsPlasmodiumPf9 as a conserved novel protective protein, sharing an epitope withParameciumsurface antigens. [ABSTRACT FROM AUTHOR]