The hydration features of carbohydrate determinants of Lewis antigens

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.

Antibody recognition of aberrant glycosylation on the surface of cancer cells

Carbohydrate-binding antibodies and carbohydrate-based vaccines are being actively pursued as targeted immunotherapies for a broad range of cancers. Recognition of tumor-associated carbohydrates (glycans) by antibodies is predominantly towards terminal epitopes on glycoproteins and glycolipids on the surface of cancer cells. Crystallography along with complementary experimental and computational methods have been extensively used to dissect antibody recognition of glycan epitopes commonly found in cancer. We provide an overview of the structural biology of antibody recognition of tumor-associated glycans and propose potential rearrangements of these targets in the membrane that could dictate the complex biological activities of these antibodies against cancer cells.

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.

Synthesis of 4" manipulated Lewis X trisaccharide analogues

Three analogues of the Le^x trisaccharide antigen (β-D-Galp(1→4)[α-L-Fucp(1→3)]-D-GlcNAcp) in which the galactosyl residue is modified at O-4 as a methyloxy, deoxychloro or deoxyfluoro, were synthesized. We first report the preparation of the modified 4-OMe, 4-Cl and 4-F trichloroacetimidate galactosyl donors and then report their use in the glycosylation of an N-acetylglucosamine glycosyl acceptor. Thus, we observed that the reactivity of these donors towards the BF₃·OEt₂-promoted glycosylation at O-4 of the N-acetylglucosamine glycosyl acceptors followed the ranking 4-F > 4-OAc ≈ 4-OMe > 4-Cl. The resulting disaccharides were deprotected at O-3 of the glucosamine residue and fucosylated, giving access to the desired protected Le^x analogues. One-step global deprotection (Na/NH₃) of the protected 4”-methoxy analogue, and two-step deprotections (removal of a p-methoxybenzyl with DDQ, then Zemplén deacylation) of the 4”-deoxychloro and 4”-deoxyfluoro protected Le^x analogues gave the desired compounds in good yields.

A possible role for metallic ions in the carbohydrate cluster recognition displayed by a Lewis Y specific antibody

Background

Lewis Y (Le^y) is a blood group-related carbohydrate that is expressed at high surface densities on the majority of epithelial carcinomas and is a promising target for antibody-based immunotherapy. A humanized Le^y-specific antibody (hu3S193) has shown encouraging safety, pharmacokinetic and tumor-targeting properties in recently completed Phase I clinical trials.

Methodology/Principal Findings

We report the three-dimensional structures for both the free (unliganded) and bound (Le^y tetrasaccharide) hu3S193 Fab from the same crystal grown in the presence of divalent zinc ions. There is no evidence of significant conformational changes occurring in either the Le^y carbohydrate antigen or the hu3S193 binding site, which suggests a rigid fit binding mechanism. In the crystal, the hu3S193 Fab molecules are coordinated at their protein-protein interface by two zinc ions and in solution aggregation of Fab can be initiated by zinc, but not magnesium ions. Dynamic light scattering revealed that zinc ions could initiate a sharp transition from hu3S193 Fab monomers to large multimeric aggregates in solution.

Conclusions/Significance

Zinc ions can mediate interactions between hu3S193 Fab in crystals and in solution. Whether metallic ion mediated aggregation of antibody occurs in vivo is not known, but the present results suggest that similar clustering mechanisms could occur when hu3S193 binds to Le^y on cells, particularly given the high surface densities of antigen on the target tumor cells.