Malaria parasite-inhibitory antibody epitopes on Plasmodium falciparum merozoite surface protein-1(19) mapped by TROSY NMR.

Plasmodium falciparum merozoite surface protein 1 (MSP1)(19), the C-terminal fragment of merozoite surface protein 1, is a leading candidate antigen for development of a vaccine against the blood stages of the malaria parasite. Many human and animal studies have indicated the importance of MSP1(19)-specific immune responses. Anti-MSP1(19) antibodies can prevent invasion of red blood cells by P. falciparum parasites in vitro. However, the fine specificity of anti-MSP1(19) antibodies is also important, as only a fraction of monoclonal antibodies (mAbs) have parasite-inhibitory activity in vitro. Human sera from malaria-endemic locations show strong MSP1(19) reactivity, but individual serum samples vary greatly in inhibitory activity. NMR is an excellent method for studying protein-protein interactions, and has been used widely to study binding of peptides representing known epitopes (as well as non-protein antigens) to antibodies and antibody fragments. The recent development of transverse relaxation optimized spectroscopy (TROSY) and related methods has significantly extended the maximum size limit of molecules that can be studied by NMR. TROSY NMR experiments produce high quality spectra of Fab complexes that allow the mapping of epitopes by the chemical shift perturbation technique on a complete, folded protein antigen such as MSP1(19). We studied the complexes of P. falciparum MSP1(19) with Fab fragments from three monoclonal antibodies. Two of these antibodies have parasite-inhibitory activity in vitro, while the third is non-inhibitory. NMR epitope mapping showed a close relationship between binding sites for the two inhibitory antibodies, distinct from the location of the non-inhibitory antibody. Together with a previously published crystal structure of the P. falciparum MSP1(19) complex with the Fab fragment of another non-inhibitory antibody, these results revealed a surface on MSP1(19) where inhibitory antibodies bind. This information will be useful in evaluating the anti-MSP1(19) immune response in natural populations from endemic areas, as well as in vaccine trials. It will also be valuable for optimizing the MSP1(19) antigen by rational vaccine design. This work also shows that TROSY NMR techniques are very effective for mapping conformational epitopes at the level of individual residues on small- to medium-sized proteins, provided that the antigen can be expressed in a system amenable to stable isotope labelling, such as bacteria or yeast.