Immunology Letters, 23 (1989/1990) 305-310 Elsevier IMLET 01345 Characterisation of an inhibitory monoclonal antibody-defined epitope on a malaria vaccine candidate antigen R. Ramasamy, G. Jones and R. Lord Queensland Institute of Medical Research, Bramston Terrace, Brisbane, Austrialia (Received 24 May 1989; revision received 14 November 1989; accepted 16 November 1989) 1. Summary A monoclonal antibody that recognises a recently characterised 45-kDa merozoite surface antigen of the human malaria parasite Plasrnodiurnfalciparurn inhibits the growth of the asexual blood stages of the parasite in vitro. The corresponding epitope has been determined by testing the reactivity of the anti- body with sequentially overlapping octapeptides. A synthetic peptide containing the epitope elicits anti- bodies that react with the native antigen. Epitope mapping in this manner is useful in the design of syn- thetic vaccines against malaria. 2. Introduction Molecules present on the surface of merozoites are potentially important for generating protective im- munity against malaria, because of their accessibili- ty to the antibodies and effector cells of the host im- mune system. Indeed, a 185-200-kDa glycoprotein precursor to the major merozoite surface antigens (PMMSA) of the human malaria parasite Plasmodi- urn falciparum [1] has been shown to protect im- munised monkeys against falciparum malaria [2-4]. A different molecule, a 45-kDa glycosylated and myristilated smaller surface antigen (GYMSSA) has been identified on P.falciparum merozoites with a monoclonal antibody 8F6-49 [5]. This antigen, Key words." Epitope; Malaria; Merozoite surface; Monoclonal antibody; Plasmodium falciparum Correspondence to: Professor Ranjan Ramasamy, Institute of Fundamental Studies, Hantana Road, Kandy, Sri Lanka. designated QF122, has been shown to be recognised by two other monoclonal antibodies (mAbs), 8G10- 48 and 9E3-48, that inhibit the in vitro growth of the asexual blood stages of the parasite [6]. The epitopes recognised by the two inhibitory mAbs was deter- mined by scanning a complete set of overlapping oc- tapeptides of an amino acid sequence derived from a cDNA clone cL2122 that was detected by the two mAbs in a colony immunoassay. The amino acid se- quence Ser-Thr-Asn-Ser (STNS) was shown to con- tain the most critical part of the epitope recognised by the two mAbs [6]. The complete sequence of GYMSSA has since been determined from an in- dependently isolated cDNA clone that is unrelated to cL2122 except for the shared STNS sequence [7]. We report here that mAb 8F6-49 recognises a different epitope in GYMSSA and also inhibits the in vitro growth of asexual blood stages. In addition, a synthetic peptide containing the epitope recog- nised by mAb 8F6-49 is shown to be capable of elicit- ing murine antibodies that react with the parent mol- ecule on merozoites. The delineation of epitopes in this manner is of value in the design of a synthetic vaccine against malaria. 3. Materials and Methods 3.1. Parasite culture and immunofluorescence The culturing of the FCQ-27 isolate of P. falcipa- rum and immunofluorescence was performed essen- tially as previously described [5]. Murine sera were used at 1:50 and 1:1000 dilutions for the im- munofluorescence on fixed late-stage parasites. 0165-2478 / 90 / $ 3.50 © 1990 Elsevier Science Publishers B.V. (Biomedical Division) 305