Recognition of different domains of the Plasmodium falciparum CS protein by the sera of naturally infected individuals compared with those of sporozoite-immunized volunteers.

The fine specificities of antibodies to the circumsporozoite (CS) protein of Plasmodium falciparum, present in the sera of volunteers immunized with irradiated P. falciparum sporozoites, were defined and compared to those of sera from persons living in a malaria-endemic area in West Africa. The specificity of these anti-CS antibodies was determined by ELISA, using recombinant proteins and synthetic peptides containing repeat and nonrepeat sequences of this CS protein. All 10 serum samples of the five sporozoite-immunized volunteers displayed very high antibody titers to the immunodominant repeat (NANP)n of the CS protein. However, only three of the serum samples of these vaccinees reacted with a single nonrepeat region and only at low titers. In contrast, a high percentage of sera from adults living in the malaria-endemic area who had been exposed to sporozoites, as well as liver and blood stages of P. falciparum, had high antibody levels, not only to the repeats but also to several nonrepeat regions of the CS protein. Furthermore, a number of sera from children living in this endemic area displayed appreciable levels of antibodies to the nonrepeat regions, in the absence of any antirepeat reactivity. Sera of Saimiri monkeys, which had undergone multiple blood-induced P. falciparum infections, consistently contained high titers of antibodies to several nonrepeat sequences of the CS protein, whereas only a few of these sera had low titers of antirepeat antibodies. Antibody binding sites, in nonrepeat regions, were mapped using synthetic polymers containing multiple copies of selected C-terminal sequences of the P. falciparum CS protein. The binding to sporozoites of antibodies to nonrepeat regions of the CS protein was determined. The basis for the differences in antibody binding sites of sera from persons immunized with irradiated sporozoites, compared to those from an endemic area, is discussed.