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-ass ociated protein 1 (RAP-1) were generated and tested for inhibition of paras ite 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 thi rd of RAP-1. MAbs specific for the building block 35.1 of the synthetic pep tide malaria vaccine SPf66 also yielded an IFA staining pattern characteris tic for rhoptry-associated proteins and reacted specifically with rRAP-1 an d 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 mol ecules and overlapping peptides identified the linear RAP-1 sequence Y218KY SL222 as a target of the anti-35.1 MAbs. This sequence lacks primary sequen ce similarity with the 35.1 peptide (YGGPANKKNAG). Cross-reactivity of the anti-35.1 MAbs thus appears to be associated with conformational rather tha n sequence homology. While the anti-35.1 MAb SP8.18 exhibited parasite grow th-inhibitory activity, none of the tested anti-rRAP-1(23-711) MAbs inhibit ed parasite growth, independently of their fine specificity for the RAP-1 s equences 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, accelera ted by noninhibitory anti-RAP-1 MAbs. Results demonstrate that in addition to fine specificity, other binding parameters are also crucial for the inhi bitory potential of an antibody.