Rhoptry-associated Protein 1-binding Monoclonal Antibody Raised Against a Heterologous Peptide Sequence Inhibits Plasmodium Falciparum Growth in Vitro

Monoclonal antibodies (MAbs) specific for Plasmodium falciparum rhoptry-associated protein 1 (RAP-1) were generated and tested for inhibition of parasite growth in vitro. The majority of indirect immunofluorescence assay (IFA)-positive MAbs raised against recombinant RAP-1 positions 23 to 711 (rRAP-1(23-711)) recognized epitopes located in the immunodominant N-terminal third of RAP-1. MAbs specific for the building block 35.1 of the synthetic peptide malaria vaccine SPf66 also yielded an IFA staining pattern characteristic for rhoptry-associated proteins and reacted specifically with rRAP-1 and parasite-derived RAP-1 molecules p67 and p82. Cross-reactivity with RAP-1 was blocked by the 35.1 peptide. Epitope mapping with truncated rRAP-1 molecules and overlapping peptides identified the linear RAP-1 sequence Y218KYSL222 as a target of the anti-35.1 MAbs. This sequence lacks primary sequence similarity with the 35.1 peptide (YGGPANKKNAG). Cross-reactivity of the anti-35.1 MAbs thus appears to be associated with conformational rather than sequence homology. While the anti-35.1 MAb SP8.18 exhibited parasite growth-inhibitory activity, none of the tested anti-rRAP-1(23-711) MAbs inhibited parasite growth, independently of their fine specificity for the RAP-1 sequences at positions 33 to 42, 213 to 222, 243 to 247, 280 to 287, or 405 to 446. The growth-inhibitory activity of MAb SP8.18 was, however, accelerated by noninhibitory anti-RAP-1 MAbs. Results demonstrate that in addition to fine specificity, other binding parameters are also crucial for the inhibitory potential of an antibody.