Modification of a human monoclonal antibody Fab fragment specific for Plasmodium falciparum 19-kDa C-terminal merozoite surface protein 1 by site-directed mutagenesis.

We recently produced human monoclonal antibody Fab fragments specific for the 19-kDa C-terminal merozoite surface protein 1 of Plasmodium falciparum in a bacterial expression system. The effect of single amino acid modifications in the third complementarity-determining regions of the heavy and light chains on affinity was examined in one of the Fab fragments, Pf25. Recombination polymerase chain reaction was used to modify Tyr(92) or Ile(97) in the light chain and Val(101) or Trp(107) in the heavy chain. No effective replacements for Tyr(92) and Val(101) were found, but possible substitutions of Ile(97) with Gly, Leu, Glu, Ala and Ser, and of Trp(107) with Arg and Ser were demonstrated. Of these modified Fab fragments, the affinities of Fabs with Ile(97)-Leu and Trp(107)-Ser mutations were slightly higher than that of the original Fab. The effects of these modifications on the antigen-antibody interaction are discussed.