Recognition of Lewis X by Anti-Le x Monoclonal Antibody IG5F6.

mAbs directed toward the Lewis X (Le ^x ) determinant have been shown to display different specificities, depending on the presentation of Le ^x to the immune system. Of interest is the murine anti-Le ^x mAb IG5F6, generated against the O chain polysaccharide of Helicobacter pylori that 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 .
(Copyright © 2019 by The American Association of Immunologists, Inc.)