Understanding the Recognition of Lewis X by Anti-Lex Monoclonal Antibodies

Recognition of Dimeric Lewis X by Anti-Dimeric Lex Antibody SH2

The carbohydrate antigen dimeric Lewis X (DimLe^x), which accumulates in colonic and liver adenocarcinomas, is a valuable target to develop anti-cancer therapeutics. Using the native DimLe^x antigen as a vaccine would elicit an autoimmune response against the Le^x antigen found on normal, healthy cells. Thus, we aim to study the immunogenic potential of DimLe^x and search internal epitopes displayed by DimLe^x that remain to be recognized by anti-DimLe^x monoclonal antibodies (mAbs) but no longer possess epitopes recognized by anti-Le^x mAbs. In this context, we attempted to map the epitope recognized by anti-DimLe^x mAb SH2 by titrations and competitive inhibition experiments using oligosaccharide fragments of DimLe^x as well as Le^x analogues. We compare our results with that reported for anti-Le^x mAb SH1 and anti-polymeric Le^x mAbs 1G5F6 and 291-2G3-A. While SH1 recognizes an epitope localized to the non-reducing end Le^x trisaccharide, SH2, 1G5F6, and 291-2G3-A have greater affinity for DimLe^x conjugates than for Le^x conjugates. We show, however, that the Le^x trisaccharide is still an important recognition element for SH2, which (like 1G5F6 and 291-2G3-A) makes contacts with all three sugar units of Le^x. In contrast to mAb SH1, anti-polymeric Le^x mAbs make contact with the GlcNAc acetamido group, suggesting that epitopes extend further from the non-reducing end Le^x. Results with SH2 show that this epitope is only recognized when DimLe^x is presented by glycoconjugates. We have reported that DimLe^x adopts two conformations around the β-d-GlcNAc-(1→3)-d-Gal bond connecting the Le^x trisaccharides. We propose that only one of these conformations is recognized by SH2 and that this conformation is favored when the hexasaccharide is presented as part of a glycoconjugate such as DimLe^x-bovine serum albumin (DimLe^x-BSA). Proper presentation of the oligosaccharide candidate via conjugation to a protein or lipid is essential for the design of an anti-cancer vaccine or immunotherapeutic based on DimLe^x.

Recognition of Lewis X by Anti-Lex 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.

Structural biology of antibody recognition of carbohydrate epitopes and potential uses for targeted cancer immunotherapies

Monoclonal antibodies represent the most successful class of biopharmaceuticals for the treatment of cancer. Mechanisms of action of therapeutic antibodies are very diverse and reflect their ability to engage in antibody-dependent effector mechanisms, internalize to deliver cytotoxic payloads, and display direct effects on cells by lysis or by modulating the biological pathways of their target antigens. Importantly, one of the universal changes in cancer is glycosylation and carbohydrate-binding antibodies can be produced to selectively recognize tumor cells over normal tissues. A promising group of cell surface antibody targets consists of carbohydrates presented as glycolipids or glycoproteins. In this review, we outline the basic principles of antibody-based targeting of carbohydrate antigens in cancer. We also present a detailed structural view of antibody recognition and the conformational properties of a series of related tissue-blood group (Lewis) carbohydrates that are being pursued as potential targets of cancer immunotherapy.

Carbohydrate-mimetic peptides for pan anti-tumor responses

Molecular mimicry is fundamental to biology and transcends to many disciplines ranging from immune pathology to drug design. Structural characterization of molecular partners has provided insight into the origins and relative importance of complementarity in mimicry. Chemical complementarity is easy to understand; amino acid sequence similarity between peptides, for example, can lead to cross-reactivity triggering similar reactivity from their cognate receptors. However, conformational complementarity is difficult to decipher. Molecular mimicry of carbohydrates by peptides is often considered one of those. Extensive studies of innate and adaptive immune responses suggests the existence of carbohydrate mimicry, but the structural basis for this mimicry yields confounding details; peptides mimicking carbohydrates in some cases fail to exhibit both chemical and conformational mimicry. Deconvolution of these two types of complementarity in mimicry and its relationship to biological function can nevertheless lead to new therapeutics. Here, we discuss our experience examining the immunological aspects and implications of carbohydrate-peptide mimicry. Emphasis is placed on the rationale, the lessons learned from the methodologies to identify mimics, a perspective on the limitations of structural analysis, the biological consequences of mimicking tumor-associated carbohydrate antigens, and the notion of reverse engineering to develop carbohydrate-mimetic peptides in vaccine design strategies to induce responses to glycan antigens expressed on cancer cells.