Dissection of the human antibody response to the malaria antigen Pf155/RESA into epitope specific components

Abstract

The development of vaccines is presently receiving major attention in malaria research. As it is not possible to base malaria vaccines on the use of killed or attenuated organisms, the vaccines which are being developed are subunit vaccines in which the immunogens consist of defined parasite antigens or antigenic fragments. Since protective immunity to malaria involves both antibody-dependent and antibody-independent mechanisms, the immunogens in a subunit vaccine must have the capacity to induce relevant B- and T-cell responses in the majority of vaccinees. In turn, this requires good knowledge of these responses in humans who have acquired immunity through natural infection. In this paper we have summarized our recent work on the dissection into epitope-specific components of the human antibody response to the Plasmodium falciparum antigen Pf155/RESA, a recognized candidate for a vaccine against the asexual blood stages of this parasite. Epitope mapping of the antigen by means of short synthetic peptides led to the identification in several molecular regions of short amino acid sequences constituting linear and probably immunodominant B-cell epitopes. The antigenically most active region was located in the C-terminus of the molecule. This region, which consists of approximately 40 related, 4- or 8-amino acid long repeats, induced higher antibody concentrations in a larger number of malaria-immune donors than any of the other regions. A large fraction of these antibodies bound to short synthetic peptides representing the major repeat motifs of Pf155/RESA. Although these repeats are made up of closely related amino acid sequences, the antibody response to them was highly polyclonal, indicating the presence of several linear and probably also conformational epitopes which gave rise to a variety of cross-reacting as well as monospecific antibodies. Further analysis revealed that the levels of antibodies differing in specificity and/or avidity for different peptides varied independently of each other in individual donors. In an area (Liberia) where malaria transmission is holoendemic and perennial, these antibody profiles remained constant when individual donors were followed over several years. Since the C-terminal repeat region of Pf155/RESA is conserved in different P. falciparum strains, the results reflect differences in the genetic regulation of epitope-specific host responses rather than antigenic differences between infecting parasites. In donors living in an area with high but seasonal malaria transmission, antibody levels usually drop to lower levels when there is no transmission.(ABSTRACT TRUNCATED AT 400 WORDS)