We have used panels of monoclonal antibodies to circumsporozoite (CS) proteins of Plasmodium falciparium, P. vivax, and P. knowlesi to determine the number of topographically independent epitopes of these antigens. The results of competition binding assays indicated that single regions of the CS molecules were recognized by the homologous monoclonal antibodies. Competition binding assays were also used to study the specificity of antibodies contained in the sera of humans and monkeys that had developed sterile immunity after immunization with irradiated, intact sporozoites. We found that single monoclonal antibodies inhibited 70-95% of the specific binding of the polyclonal antibodies to crude extracts of sporozoites. It appears, therefore, that CS proteins are among the most immunogenic constituents of sporozoites, and that a single region of these molecules contains most of the immunogenic activity. An additional finding was that the immunodominant region of CS molecules is multivalent with regard to the expression of a single epitope. This was demonstrated by the ability of monomers of CS proteins to bind simultaneously two or more molecules of the same monoclonal antibody.

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