Recognition of Lewis X by Anti-Lex Monoclonal Antibody 1G5F6

mAbs directed toward the Lewis X (Le x ) determinant have been shown to display different specificities, depending on the presentation of Le x o the immune system. Of interest is the murine anti-Le x mAb IG5F6, generated against the O chain polysaccharide of Helicobacter pylori hat contains polymeric Le x structures. The mAb was found to have a higher affinity for polymeric Le x over monomeric Le x In this study, we explore the recognition of monomeric Le x by IG5F6 using a panel of Le x analogues in which N -acetyl-d-glucosamine, l-fucose, or d-galactose (D-Gal) are replaced with d-glucose and/or l-rhamnose. Our studies show that all analogues were weaker inhibitors than the Le x Ag, indicating that all three residues are essential in the recognition of Le x by mAb IG5F6. We explored the involvement of 4″-OH of d-Gal in the binding with IG5F6 using a panel of 4″-modified Le x analogues. Although the 4″-OH is only involved in a weak polar interaction, we conclude that the D-Gal residue in Le x is primarily involved in aromatic stacking interactions with the Ab binding site. We compared these results to our work with mAb SH1. Although stacking interactions between D-Gal and an aromatic residue was also suggested for SH1, an H-bond involving the 4″-OH was identified that is not found in the binding of IG5F6 to Le x Thus, anti-Le x mAbs SH1 and IG5F6 bind to Le x in different manners, even though the hydrophobic patch displayed by the β-galactoside in Le x is essential in both cases for their binding to Le x .