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The definition of human T-cell antigenic sites is important for subunit vaccine development of a peptide immunogen if the goal is to allow antibody boosting during infection or to stimulate antibody-independent T-ceil immunity. To identify such sites on the circumsporozoite (CS) protein of Plasmodium falciparum, 29 overlapping synthetic peptides spanning the entire CS protein were made and tested for their ability to stimulate peripheral blood lymphocytes from 35 adults living in a P. falciparum malaria-endemic region of West Africa. Three immunodominant domains were located outside the repetitive region. These domains, however, occurred in the polymorphic regions of the molecule, suggesting that parasite mutation and selection has occurred in response to immune pressure from T cells. Such polymorphism may impose an obstacle for vaccine development.Malaria is initiated with the inoculation of sporozoites from infected mosquitoes. These travel to the liver where they commence the hepatic or exoerythrocytic stage of the life cycle. The basis for an anti-sporozoite malaria vaccine (1) is to prevent sporozoites from entering hepatocytes or to destroy infected hepatocytes prior to release of merozoites, which would then commence the asexual stage of the life cycle within erythrocytes.Sporozoites are coated with the circumsporozoite (CS) protein, of which the molecular sequence for four Plasmodiumfalciparum strains has been determined (2-4). Antibodies to the central repetitive region of the molecule can prevent sporozoite-induced infection, but high titers of antibody are required (5)(6)(7)(8). In addition to blocking antibody, effector T cells can mediate anti-sporozoite immunity in mice immunized with irradiated sporozoites through an antibody-independent mechanism (6, 9, 10).It was originally thought that the development of an anti-sporozoite vaccine would not be plagued by the problem of parasite polymorphism, since the central repetitive region of the molecule has been shown to consist of invariant repeats (Asn-Ala-Asn-Pro) for a large number of isolates (11). The nine amino acid variations observed between the 7G8 clone first published (2) and the sequences from other clones (3, 4) were also outside the central repetitive epitope. One cluster of variation, however, overlapped a region, Th2R, which was identified in mice as a T-cell helper epitope for production of antibody to the P. falciparum CS protein (12).While regions of the CS protein of P. falciparum that stimulate murine T cells have been described (12-15), it was not known which regions could stimulate human T cells and if these T-cell epitopes would fall in regions of parasite polymorphism. It is important to know and include the human T-cell sites in a vaccine based on the CS protein, both because of the need to stimulate antibody-independent T-cell immunity and also to allow antibody boosting by sporozoite infections to maintain high antibody titers. To search for such antigenic regions, we constructed overlapping synthetic peptides spanning...

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