Development of a vaccine that contains antigens of sexual stages of Plasmodium falciparum may contribute to the control of malaria infection by reducing the number of infectious mosquitoes. Previously it has been shown that particular surface proteins of P. falciparum sexual and sporogonic stages can function as targets for transmission-blocking (TB) immunity (Rener et al. 1983, Vermeulen et al. 1985a. One of these proteins has a molecular mass of 25 kD (pfs 25) and is expressed on gametes, zygotes, and ookinetes which develop in the mosquito midgut. Anti-pfs 25 monoclonal antibodies (MoAbs 32F61 and 32F81) have been produced which prevent maturation of ookinetes into oocysts, thus blocking transmission of the parasite.Identification of peptide sequences that are recognized by TB anti-pfs 25 antibodies may form an appropriate starting point for the development of a TB vaccine. Such an option might be attractive for a number of reasons. Naturally occurring anti-pfs 25 antibodies have so far not been detected in sera from endemic malaria regions (Carter et al. 1989, Beckers et al. unpublished), which can be explained by the fact that pfs 25 is only expressed in the mosquito. As a consequence pfs 25 may have evaded immune selective pressure in human populations. This possibility is further supported by the notion that comparison of pfs 25 DNA sequences in eight different P. falciparum isolates showed no substantial polymorphism in any of these strains (Kaslow, Quakyi & Keister 1989). Incorporation of pfs 25 epitopes in a vaccine may thus enhance prospects for an effective TB immune response. Since the antibody response to this protein is not influenced by natural infections, evaluation of a pfs 25-based vaccine programme might be possible in malaria endemic regions.Correspondence: Dr A.van Amerongen.
426A.van Amerongen et al.The cloning of a cDNA encoding pfs 25 has recently been achieved in our research programme (Schoenmakers et al. unpublished data) and the nucleotide sequence was found to be identical to the one that has been published by Kaslow and co-workers ( I 988). From the primary amino acid structure it is difficult to predict sequences that are recognized by specific monoclonal antibodies, since most B-cell epitopes have a configuration determined by non-contiguous amino acids. This is probably also true for the epitopes recognized by MoAbs 32F81 and 32F61. The binding sites for these MoAbs are similar or at least closely related. Based on the assumption that at least a few residues of the 32F81 and/or 32F61 binding epitopes might be linear, it was decided to screen overlapping nonapeptides of pfs 25 for binding with these TB MoAbs. For this purpose a novel method (PEPSCAN) was used and the reactivities of the peptides were determined in an adapted ELISA.Antibodies. TB MoAbs 32F61 and 32F8 1 were prepared as described (Vermeulen et al. 1985a).PEPSCAN. All overlapping nonapeptides based on the amino acid sequence of pfs 25 (Kaslow et al. 1988, Schoenmakers et al. unpublished results) were synthesized ...
