Human immune response in Plasmodium falciparum malaria. Synthetic peptides corresponding to known epitopes of the Pf155/RESA antigen induce production of parasite-specific antibodies in vitro.

Autologous cell mixtures containing T cells, B cells, and adherent accessory cells from individuals primed to the malaria parasite Plasmodium falciparum by repeated natural infections were investigated for induction of Ig and antibody secretion in vitro. In vitro activation of cell cultures with two synthetic peptides corresponding to immunodominant T cell epitopes of the merozoite Ag ring-infected erythrocyte surface Ag (Mr 155,000) (Pf155/RESA), one from its carboxyl-terminal repeat and one from its nonrepeated amino-terminal region, gave rise to significant IgG secretion. Supernatants from lymphocyte cultures activated with either one of these peptides contained antibodies reacting with P. falciparum Ag in immunofluorescence assays and with Pf155/RESA peptides in a slot blot assay. No anti-P. falciparum antibodies were induced in the medium controls by lymphocyte stimulation with either tetanus toxoid or PWM. Induction in vitro of anti-Pf155/RESA antibodies was correlated with the presence of such antibodies in the sera of the lymphocyte donors, suggesting that the induction of antibody secretion reflected a secondary response in vitro of in vivo primed cells. Inspection of antibody profiles in individual donors revealed that the peptide corresponding to a sequence in the 3' repeat region induced anti-Pf155/RESA peptide antibodies reacting with identical or related and cross-reacting sequences in the 3' or 5' repeat region of the molecule. In contrast, the peptide corresponding to a nonrepeated T cell epitope in the amino terminus of the molecule only induced antibodies to an immunodominant amino-terminal B cell epitope partly overlapping with the T cell reactive sequence. Similar findings were made in the lymphocyte donors' plasma, frequently displaying significant correlations between antibody reactivities to the repeat peptides but not between these reactivities and those to the amino-terminal peptide. The marked specificity of this antibody formation in vitro suggests an underlying process of cognate recognition involving Ag-specific T and B cells reacting with different segments of the inducer peptide. The present experimental system should be well suited for identification of Th epitopes capable of inducing the production of antibodies of defined specificity in the human system.