Inhibitory and blocking monoclonal antibody epitopes on merozoite surface protein 1 of the malaria parasite Plasmodium falciparum

Merozoite surface protein 1 (MSP-1) is a precursor to major antigens on the surface of Plasmodium spp. merozoites, which are involved in erythrocyte b inding and invasion. MSP-1 is initially processed into smaller fragments; a nd at the time of erythrocyte invasion one of these of 42 kDa (MSP-1(42)) i s subjected to a second processing, producing 33 kDa and 19 kDa fragments ( MSP-1(33) and MSP-1(19)). Certain MSP-1-specific monoclonal antibodies (mAb s) react with conformational epitopes contained within the two epidermal gr owth factor domains that comprise MSP-1(19), and are classified as either i nhibitory (inhibit processing of MSP-1(42) and erythrocyte invasion), block ing (block the binding and function of the inhibitory mAb), or neutral (nei ther inhibitory nor blocking). We have mapped the epitopes for inhibitory m Abs 12.8 and 12.10, and blocking mAbs such as 1E1 and 7.5 by using site-dir ected mutagenesis to change specific amino acid residues in MSP-1(19) and a bolish antibody binding, and by using PEPSCAN to measure the reaction of th e antibodies with every octapeptide within MSP-1(42). Twenty-six individual amino acid residue changes were made and the effect of each on the binding of mAbs was assessed by Western blotting and BIAcore analysis. Individual changes had either no effect, or reduced, or completely abolished the bindi ng of individual mAbs. No two antibodies had an identical pattern of reacti vity with the modified proteins. Using PEPSCAN each mAb reacted with a numb er of octapeptides, most of which were derived from within the first epider mal growth factor domain, although 1E1 also reacted with peptides spanning the processing site. When the single amino acid changes and the reactive pe ptides were mapped onto the three-dimensional structure of MSP-1(19), it wa s apparent that the epitopes for the mAbs could be defined more fully by us ing a combination of both mutagenesis and PEPSCAN than by either method alo ne, and differences in the fine specificity of binding for all the differen t antibodies could be distinguished. The incorporation of several specific amino acid changes enabled the design of proteins that bound inhibitory but not blocking antibodies. These may be suitable for the development of MSP- 1-based vaccines against malaria. (C) 2001 Academic Press.