ANALYSIS OF A HUMAN MONOCLONAL-ANTIBODY REACTIVE WITH MULTIPLE PLASMODIUM-FALCIPARUM ANTIGEN REPEAT SEQUENCES USING A SOLID-PHASE AFFINITY ASSAY

A solid-phase affinity assay was set up for the determination of the a ffinity of the interaction between the human monoclonal antibody (mAb) 33G2 and peptides corresponding to repeated sequences in three blood stage antigens of the malaria parasite Plasmodium falciparum. The epit ope of this mAb is of interest due to the parasite blocking capacity o f the mAb. Previous studies with PEPSCAN have defined the minimal epit ope for the mAb as the pentapeptide VTEEI, a sequence frequently found in antigen Pf332. In the previous study, epitopes responsible for the cross-reactivity of the mAb with antigens Pf155/RESA and Pf11.1 were also identified. In the affinity assay described herein, the mAb was c oated on a solid phase and binding of a labelled peptide was displaced by homologous or heterologous peptides. The affinity of peptides corr esponding to Pf332 increased with increasing length, and the highest a ffinity was displayed by a dimer (23 amino acids) of a Pf332 repeat (K = 1.9 x 10(8) m-1). Peptide length did not influence the binding of p eptides corresponding to the Pf155/RESA and Pf11.1 repeats, which had lower affinities comparable to that of the shortest Pf332 octapeptide (K = 2.2 x 10(4) M-1). Only peptides containing binding sites as defin ed by PEPSCAN analysis showed a measurable binding. When using peptide s as inhibitors in peptide ELISA, binding correlated with the affinity of the peptides, but only the high affinity peptides were inhibitory. In contrast, a poor correlation was found when peptides were used dir ectly for coating in ELISA. Although the results of the affinity deter minations were in concordance with the PEPSCAN analysis regarding the specificity of the mAb, they indicate that the optimal binding site ma y be more complex than revealed by PEPSCAN. This may have implications for selecting peptide sequences comprising the mAb 33G2 epitope for t he construction of vaccine immunogens.