Improved Affinity of a Human Anti-Entamoeba histolyticaGal/GalNAc Lectin Fab Fragment by a Single Amino Acid Modification of the Light Chain

Abstract

We previously produced, inEscherichia coli, a human monoclonal antibody Fab fragment, CP33, specific for the galactose- andN-acetyl-d-galactosamine-inhibitable lectin ofEntamoeba histolytica. To prepare antibodies with a higher affinity to the lectin, recombination PCR was used to exchange Ser⁹¹and Arg⁹⁶in the third complementarity-determining region of the light chain with other amino acids. The screening of 200 clones of each exchange by an indirect fluorescent antibody test showed that 14 clones for Ser⁹¹and nine clones for Arg⁹⁶reacted strongly withE. histolyticatrophozoites. Sequence analyses revealed that the substituted amino acids at Ser⁹¹were Ala in five clones, Gly in three clones, Pro in two clones, and Val in two clones, while the amino acid at position 96 was substituted with Leu in three clones. The remaining eight clones exhibited no amino acid change at position 91 or 96. These mutant Fab fragments were purified and subjected to a surface plasmon resonance assay to measure the affinity of these proteins to the cysteine-rich domain of lectin. Pro or Gly substitution for Ser⁹¹caused an increased affinity of the Fab, but substitution with Ala or Val did not. The replacement of Arg⁹⁶with Leu did not affect affinity. These results demonstrate that modification of antibody genes by recombination PCR is a useful method for affinity maturation and that amino acid substitution at position 91 yields Fabs with increased affinity for the lectin.