GalNAc-Tyrosine Is a Ligand of Plant Lectins, Antibodies, and Human and Murine Macrophage Galactose-Type Lectins

Controlling Antigen Fate in Therapeutic Cancer Vaccines by Targeting Dendritic Cell Receptors

Antigen-presenting cells (APCs) orchestrate immune responses and are therefore of interest for the targeted delivery of therapeutic vaccines. Dendritic cells (DCs) are professional APCs that excel in presentation of exogenous antigens toward CD4+ T helper cells, as well as cytotoxic CD8+ T cells. DCs are highly heterogeneous and can be divided into subpopulations that differ in abundance, function, and phenotype, such as differential expression of endocytic receptor molecules. It is firmly established that targeting antigens to DC receptors enhances the efficacy of therapeutic vaccines. While most studies emphasize the importance of targeting a specific DC subset, we argue that the differential intracellular routing downstream of the targeted receptors within the DC subset should also be considered. Here, we review the mouse and human receptors studied as target for therapeutic vaccines, focusing on antibody and ligand conjugates and how their targeting affects antigen presentation. We aim to delineate how targeting distinct receptors affects antigen presentation and vaccine efficacy, which will guide target selection for future therapeutic vaccine development.

Molecular Recognition of GalNAc in Mucin-Type O-Glycosylation

ConspectusN-Acetylgalactosamine (GalNAc)-type O-glycosylation is an essential posttranslational modification (PTM) that plays fundamental roles in biology. Malfunction of this PTM is exemplified by the presence of truncated O-glycans in cancer. For instance, the glycoprotein MUC1 is overexpressed in many tumor tissues and tends to carry simple oligosaccharides that allow for the presentation of different tumor-associated antigens, such as the Tn or sTn antigens (GalNAc-α-1-O-Thr/Ser and Neu5Acα2-6GalNAcα1-O-Ser/Thr, respectively). In other cases, such as tumoral calcinosis associated with O-glycosylation of the fibroblast growth factor 23, O-glycans are absent or less abundant. Significant progress has been made in determining the three-dimensional structures of biomolecules that recognize GalNAc, such as antibodies, lectins, mucinases, GalNAc-transferases, and other glycosyltransferases. Analysis of the complexes between these entities and GalNAc-containing glycopeptides, in most cases derived from crystallographic or NMR analysis, provides an understanding of the key structural elements that control molecular recognition of these glycopeptides. Here, we describe and compare the binding sites of these proteins in detail, focusing on how the GalNAc moieties interact selectively with them. We also summarize the differences and similarities in GalNAc recognition. In general, the recognition of GalNAc-containing glycopeptides is determined by hydrogen bonds between hydroxyl groups and the N-acetyl group of GalNAc with proteins, as well as CH-π contacts in which the hydrophobic α-face of the sugar and the methyl group of NHAc can be involved. The latter interaction usually provides the basis for selectivity. It is worth noting that binding of these glycopeptides depends primarily on recognition of the sugar moiety, with some exceptions such as a few anti-MUC1 antibodies that primarily recognize the peptide backbone and use the sugar to facilitate shape complementarity or to establish a limited number of interactions with the protein. Focusing specifically on the GalNAc moiety, we can observe that there is some degeneracy of interactions within the same protein families, likely due to substrate flexibility. However, when all studied proteins are considered together, despite the commonalities within each protein family, no pattern can be discerned between the different families, apart from the presence of common residues such as Tyr, His, or Asp, which are responsible for hydrogen bonds. The lack of a pattern can be anticipated, given the diverse functions of mucinases, glycosyltransferases, antibodies, and lectins. Finally, it is important to point out that the conformational differences observed in solution in glycopeptides bearing GalNAc-α-1-O-Ser or GalNAc-α-1-O-Thr also can be found in the bound state. This unique characteristic is exploited, for instance, by the enzyme C1GalT1 to broadly glycosylate both acceptor substrates. The findings summarized in this review may contribute to the rational structure-guided development of therapeutic vaccines, novel diagnostic tools for early cancer detection, and new cancer treatments for cancer with tailored anti-Tn or anti-STn antibodies or new drugs to inhibit GalNAc-T isoenzymes.

Glycan microarrays from construction to applications

Through their specific interactions with proteins, cellular glycans play key roles in a wide range of physiological and pathological processes. One of the main goals of research in the areas of glycobiology and glycomedicine is to understand glycan-protein interactions at the molecular level. Over the past two decades, glycan microarrays have become powerful tools for the rapid evaluation of interactions between glycans and proteins. In this review, we briefly describe methods used for the preparation of glycan probes and the construction of glycan microarrays. Next, we highlight applications of glycan microarrays to rapid profiling of glycan-binding patterns of plant, animal and pathogenic lectins, as well as other proteins. Finally, we discuss other important uses of glycan microarrays, including the rapid analysis of substrate specificities of carbohydrate-active enzymes, the quantitative determination of glycan-protein interactions, discovering high-affinity or selective ligands for lectins, and identifying functional glycans within cells. We anticipate that this review will encourage researchers to employ glycan microarrays in diverse glycan-related studies.

Development of a GalNAc-Tyrosine-Specific Monoclonal Antibody and Detection of Tyrosine O-GalNAcylation in Numerous Human Tissues and Cell Lines

Glycosylation is a vital post-translational modification involved in a range of biological processes including protein folding, signaling, and cell-cell interactions. In 2011, a new type of O-linked glycosylation was discovered, wherein the side-chain oxygen of tyrosine is modified with a GalNAc residue (GalNAc-Tyr). At present, very little is known about GalNAc-Tyr prevalence, function, or biosynthesis. Herein, we describe the design and synthesis of a GalNAc-Tyr-derived hapten and its use in generating a GalNAc-Tyr selective monoclonal antibody. The antibody, G10C, has an unusually high affinity (app KD = 100 pM) and excellent selectivity for GalNAc-Tyr. We also obtained a crystal structure of the G10C Fab region in complex with 4-nitrophenyl-N-acetyl-α-d-galactosaminide (a small molecule mimic of GalNAc-Tyr) providing insights into the structural basis for high affinity and selectivity. Using this antibody, we discovered that GalNAc-Tyr is widely expressed in most human tissues, indicating that it is a ubiquitous and underappreciated post-translational modification. Localization to specific cell types and organ substructures within those tissues indicates that GalNAc-Tyr is likely regulated in a cell-specific manner. GalNAc-Tyr was also observed in a variety of cell lines and primary cells but was only present on the external cell surface in certain cancer cell lines, suggesting that GalNAc-Tyr localization may be altered in cancer cells. Collectively, the results shed new light on this under-studied form of glycosylation and provide access to new tools that will enable expanded biochemical and clinical investigations.

The Shape of Nanostructures Encodes Immunomodulation of Carbohydrate Antigen and Vaccine Development

Gold nanoparticles (AuNPs) have shown remarkable potential for vaccine development, but the influence of the size and shape of nanoparticles modulating the T-cell-dependent carbohydrate antigen processing and immunomodulation is poorly investigated. Here, we described how different shapes and sizes of gold nanostructures carrying adjuvant modulate carbohydrate-based antigen processing in murine dendritic cells (mDCs) and subsequent T-cell activation produce a robust antibody response. As a prototype, CpG-adjuvant-coated spherical and rod- and star-shaped AuNPs were conjugated to the tripodal Tn-glycopeptide antigen to study their DC uptake and activation of T-cells in a DCs/T-cell co-culture assay. Our results showed that the spherical and star-shaped AuNPs displayed relatively weak receptor-mediated uptake and endosomal sequestration; however, they induced a high level of T helper-1 (Th1) biasing immune responses compared with rod-shaped AuNPs. Furthermore, the in vivo administration of AuNPs showed that the small spherical and star-shaped AuNPs induced an effective anti-Tn-glycopeptide immunoglobulin (IgG) antibody response compared with rod-shaped AuNPs. These results indicated that one could obtain superior carbohydrate vaccines by varying the shape and size parameters of nanostructures.

A Stable Gold Nanoparticle-Based Vaccine for the Targeted Delivery of Tumor-Associated Glycopeptide Antigens

We have developed a novel antigen delivery system based on polysaccharide-coated gold nanoparticles (AuNPs) targeted to antigen presenting cells (APCs) expressing Dectin-1. AuNPs were synthesized de-novo using yeast-derived β-1,3-glucans (B13G) as the reductant and passivating agent in a microwave-catalyzed procedure yielding highly uniform and serum-stable particles. These were further functionalized with both a peptide and a specific glycosylated form from the tandem repeat sequence of mucin 4 (MUC4), a glycoprotein overexpressed in pancreatic tumors. The glycosylated sequence contained the Thomsen-Friedenreich disaccharide, a pan-carcinoma, Tumor-Associated Carbohydrate Antigen (TACA), which has been a traditional target for antitumor vaccine design. These motifs were prepared with a cathepsin B protease cleavage site (Gly-Phe-Leu-Gly), loaded on the B13G-coated particles and these constructs were examined for Dectin-1 binding, APC processing and presentation in a model in vitro system and for immune responses in mice. We showed that these particles elicit strong in vivo immune responses through the production of both high-titer antibodies and priming of antigen-recognizing T-cells. Further examination showed that a favorable antitumor balance of expressed cytokines was generated, with limited expression of immunosuppressive Il-10. This system is modular in that any range of antigens can be conjugated to our particles and efficiently delivered to APCs expressing Dectin-1.

Sweet systems: technologies for glycomic analysis and their integration into systems biology

Found in virtually every organism, glycans are essential molecules that play important roles in almost every aspect of biology. The composition of glycome, the repertoire of glycans in an organism or a biological sample, is often found altered in many diseases, including cancer, infectious diseases, metabolic and developmental disorders. Understanding how glycosylation and glycomic changes enriches our knowledge of the mechanisms of disease progression and sheds light on the development of novel therapeutics. However, the inherent diversity of glycan structures imposes challenges on the experimental characterization of glycomes. Advances in high-throughput glycomic technologies enable glycomic analysis in a rapid and comprehensive manner. In this review, we discuss the analytical methods currently used in high-throughput glycomics, including mass spectrometry, liquid chromatography and lectin microarray. Concomitant with the technical advances is the integration of glycomics into systems biology in the recent years. Herein we elaborate on some representative works from this recent trend to underline the important role of glycomics in such integrated approaches to disease.

Enhanced Binding and Reduced Immunogenicity of Glycoconjugates Prepared via Solid-State Photoactivation of Aliphatic Diazirine Carbohydrates

Biological conjugation is an important tool employed for many basic research and clinical applications. While useful, common methods of biological conjugation suffer from a variety of limitations, such as (a) requiring the presence of specific surface-exposed residues, such as lysines or cysteines, (b) reducing protein activity, and/or (c) reducing protein stability and solubility. Use of photoreactive moieties including diazirines, azides, and benzophenones provide an alternative, mild approach to conjugation. Upon irradiation with UV and visible light, these functionalities generate highly reactive carbenes, nitrenes, and radical intermediates. Many of these will couple to proteins in a non-amino-acid-specific manner. The main hurdle for photoactivated biological conjugation is very low yield. In this study, we developed a solid-state method to increase conjugation efficiency of diazirine-containing carbohydrates to proteins. Using this methodology, we produced multivalent carbohydrate-protein conjugates with unaltered protein charge and secondary structure. Compared to carbohydrate conjugates prepared with amide linkages to lysine residues using standard NHS conjugation, the photoreactive prepared conjugates displayed up to 100-fold improved binding to lectins and diminished immunogenicity in mice. These results indicate that photoreactive bioconjugation could be especially useful for in vivo applications, such as lectin targeting, where high binding affinity and low immunogenicity are desired.

Molecular Recognition in C-Type Lectins: The Cases of DC-SIGN, Langerin, MGL, and L-Sectin

Carbohydrates play a pivotal role in intercellular communication processes. In particular, glycan antigens are key for sustaining homeostasis, helping leukocytes to distinguish damaged tissues and invading pathogens from healthy tissues. From a structural perspective, this cross-talk is fairly complex, and multiple membrane proteins guide these recognition processes, including lectins and Toll-like receptors. Since the beginning of this century, lectins have become potential targets for therapeutics for controlling and/or avoiding the progression of pathologies derived from an incorrect immune outcome, including infectious processes, cancer, or autoimmune diseases. Therefore, a detailed knowledge of these receptors is mandatory for the development of specific treatments. In this review, we summarize the current knowledge about four key C-type lectins whose importance has been steadily growing in recent years, focusing in particular on how glycan recognition takes place at the molecular level, but also looking at recent progresses in the quest for therapeutics.

ASGR1 and Its Enigmatic Relative, CLEC10A

The large family of C-type lectin (CLEC) receptors comprises carbohydrate-binding proteins that require Ca²⁺ to bind a ligand. The prototypic receptor is the asialoglycoprotein receptor-1 (ASGR1, CLEC4H1) that is expressed primarily by hepatocytes. The early work on ASGR1, which is highly specific for N-acetylgalactosamine (GalNAc), established the foundation for understanding the overall function of CLEC receptors. Cells of the immune system generally express more than one CLEC receptor that serve diverse functions such as pathogen-recognition, initiation of cellular signaling, cellular adhesion, glycoprotein turnover, inflammation and immune responses. The receptor CLEC10A (C-type lectin domain family 10 member A, CD301; also called the macrophage galactose-type lectin, MGL) contains a carbohydrate-recognition domain (CRD) that is homologous to the CRD of ASGR1, and thus, is also specific for GalNAc. CLEC10A is most highly expressed on immature DCs, monocyte-derived DCs, and alternatively activated macrophages (subtype M2a) as well as oocytes and progenitor cells at several stages of embryonic development. This receptor is involved in initiation of T_H1, T_H2, and T_H17 immune responses and induction of tolerance in naïve T cells. Ligand-mediated endocytosis of CLEC receptors initiates a Ca²⁺ signal that interestingly has different outcomes depending on ligand properties, concentration, and frequency of administration. This review summarizes studies that have been carried out on these receptors.

A Structurally Simple Vaccine Candidate Reduces Progression and Dissemination of Triple-Negative Breast Cancer

The Tn antigen is a well-known tumor-associated carbohydrate determinant, often incorporated in glycopeptides to develop cancer vaccines. Herein, four copies of a conformationally constrained mimetic of the antigen TnThr (GalNAc-Thr) were conjugated to the adjuvant CRM197, a protein licensed for human use. The resulting vaccine candidate, mime[4]CRM elicited a robust immune response in a triple-negative breast cancer mouse model, correlated with high frequency of CD4+ T cells and low frequency of M2-type macrophages, which reduces tumor progression and lung metastasis growth. Mime[4]CRM-mediated activation of human dendritic cells is reported, and the proliferation of mime[4]CRM-specific T cells, in cancer tissue and peripheral blood of patients with breast cancer, is demonstrated. The locked conformation of the TnThr mimetic and a proper presentation on the surface of CRM197 may explain the binding of the conjugate to the anti-Tn antibody Tn218 and its efficacy to fight cancer cells in mice.

H-type lectins - Structural characteristics and their applications in diagnostics, analytics and drug delivery

Lectins are ubiquitous carbohydrate-binding proteins that interact with sugar moieties in a highly specific manner. H-type lectins represent a new group of lectins that were identified in invertebrates. These lectins share structural homology and bind mainly to N-acetylgalactosamine (GalNAc). Recent structural studies on the H-type lectins provided a detailed description of the GalNAc-lectin interaction that is already exploited in a number of biomedical applications. Two members of the H-type lectin family, Helix pomatia agglutinin (HPA) and Helix aspersa agglutinin (HAA), have already been extensively used in many diagnostic tests due their ability to specifically recognize GalNAc. This ability is especially important because aberrant glycosylation patterns of proteins expressed by cancer cells contain GalNAc. In addition, H-type lectins were utilized in diagnostics of other non-cancer diseases and represent great potential as components of drug delivery systems. Here, we present an overview of the H-type lectins and their applications in diagnostics, analytics and drug delivery.

Enhanced Immune Response Against the Thomsen-Friedenreich Tumor Antigen Using a Bivalent Entirely Carbohydrate Conjugate

The Thomsen-Friedenreich (TF) antigen is a key target for the development of anticancer vaccines, and this ongoing challenge remains relevant due to the poor immunogenicity of the TF antigen. To overcome this challenge, we adopted a bivalent conjugate design which introduced both the TF antigen and the Thomsen-nouveau (Tn) antigen onto the immunologically relevant polysaccharide A1 (PS A1). The immunological results in C57BL/6 mice revealed that the bivalent, Tn-TF-PS A1 conjugate increased the immune response towards the TF antigen as compared to the monovalent TF-PS A1. This phenomenon was first observed with enzyme-linked immunosorbent assay (ELISA) where the bivalent conjugate generated high titers of IgG antibodies where the monovalent conjugate generated an exclusive IgM response. Fluorescence-activated cell sorting (FACS) analysis also revealed increased binding events to the tumor cell lines MCF-7 and OVCAR-5, which are consistent with the enhanced tumor cell lysis observed in a complement dependent cytotoxicity (CDC) assay. The cytokine profile generated by the bivalent construct revealed increased pro-inflammatory cytokines IL-17 and IFN-γ. This increase in cytokine concentration was matched with an increase in cytokine producing cells as observed by ELISpot. We hypothesized the mechanisms for this phenomenon to involve the macrophage galactose N-acetylgalactosamine specific lectin 2 (MGL2). This hypothesis was supported by using biotinylated probes and recombinant MGL2 to measure carbohydrate-protein interactions.

Characterization of Macrophage Galactose-type Lectin (MGL) ligands in colorectal cancer cell lines

BACKGROUND

The Ca²⁺-dependent C-type lectin receptor Macrophage Galactose-type Lectin (MGL) is highly expressed by tolerogenic dendritic cells (DC) and macrophages. MGL exhibits a high binding specificity for terminal alpha- and beta-linked GalNAc residues found in Tn, sTn and LacdiNAc antigens. These glycan epitopes are often overexpressed in colorectal cancer (CRC), and, as such, MGL can be used to discriminate tumor from the corresponding healthy tissues. Moreover, the high expression of MGL ligands is associated with poor disease-free survival in stage III of CRC tumors. Nonetheless, the glycoproteins expressed by tumor cells that are recognized by MGL have hitherto remained elusive.

METHODS

Using a panel of three CRC cell lines (HCT116, HT29 and LS174T), recapitulating CRC diversity, we performed FACS staining and pull-down assays using a recombinant soluble form of MGL (and a mutant MGL as control) combined with mass spectrometry-based (glyco)proteomics.

RESULTS

HCT116 and HT29, but not LS174T, are high MGL-binding CRC cell lines. On these cells, the major cell surface binding proteins are receptors (e.g. MET, PTK7, SORL1, PTPRF) and integrins (ITGB1, ITGA3). From these proteins, several N- and/or O-glycopeptides were identified, of which some carried either a LacdiNAc or Tn epitope.

CONCLUSIONS

We have identified cell surface MGL-ligands on CRC cell lines.

GENERAL SIGNIFICANCE

Advances in (glyco)proteomics have led to identification of candidate key mediators of immune-evasion and tumor growth in CRC.

"Stuck on sugars - how carbohydrates regulate cell adhesion, recognition, and signaling"

We have explored the fundamental biological processes by which complex carbohydrates expressed on cellular glycoproteins and glycolipids and in secretions of cells promote cell adhesion and signaling. We have also explored processes by which animal pathogens, such as viruses, bacteria, and parasites adhere to glycans of animal cells and initiate disease. Glycans important in cell signaling and adhesion, such as key O-glycans, are essential for proper animal development and cellular differentiation, but they are also involved in many pathogenic processes, including inflammation, tumorigenesis and metastasis, and microbial and parasitic pathogenesis. The overall hypothesis guiding these studies is that glycoconjugates are recognized and bound by a growing class of proteins called glycan-binding proteins (GBPs or lectins) expressed by all types of cells. There is an incredible variety and diversity of GBPs in animal cells involved in binding N- and O-glycans, glycosphingolipids, and proteoglycan/glycosaminoglycans. We have specifically studied such molecular determinants recognized by selectins, galectins, and many other C-type lectins, involved in leukocyte recruitment to sites of inflammation in human tissues, lymphocyte trafficking, adhesion of human viruses to human cells, structure and immunogenicity of glycoproteins on the surfaces of human parasites. We have also explored the molecular basis of glycoconjugate biosynthesis by exploring the enzymes and molecular chaperones required for correct protein glycosylation. From these studies opportunities for translational biology have arisen, involving production of function-blocking antibodies, anti-glycan specific antibodies, and synthetic glycoconjugates, e.g. glycosulfopeptides, that specifically are recognized by GBPs. This invited short review is based in part on my presentation for the IGO Award 2019 given by the International Glycoconjugate Organization in Milan.

Novel Technologies for Quantitative O-Glycomics and Amplification/Preparation of Cellular O-Glycans

Mucin-type O-glycosylation (O-glycans, O-glycome) characterized by GalNAc linked to Serine/Threonine or even tyrosine residues in proteins is one of the major types of glycosylations. In animals, O-glycans on glycoproteins participate in many critical biological processes such as cell adhesion, development, and immunity. Importantly, the O-glycome is different in a tissue/cell-specific manner, and often altered in cells at their pathological states; and this alteration, in turn, affects cellular properties and functions. Clearly, the Functional O-glycomics, which concerns biological roles of O-glycans, requires a comprehensive understanding of O-glycome. Structural and/or quantitative analysis of O-glycans, however, is an unmet demand because no enzyme can universally release O-glycans from glycoproteins. Furthermore, the preparation of complex O-glycans for biological studies is even more challenging. To meet these demands, we have developed a novel technology termed Cellular O-glycome Reporter/Amplification (CORA) for profiling cellular O-glycan structures and amplifying/preparing complex O-glycans from cultured cells. In this chapter, we describe the recent advances of CORA: quantitative-CORA (qCORA) and preparative-CORA (pCORA). qCORA takes the strategy of “metabolic stable isotopic labeling O-glycome of culture cells (SILOC),” and pCORA adapts cells to “O-glycan factories” when supplied with R-α-GalNAc(Ac)3 derivatives. qCORA and pCORA technologies can facilitate the cellular O-glycomics and functional O-glycomics studies.

Development of a Multiplex Glycan Microarray Assay and Comparative Analysis of Human Serum Anti-Glycan IgA, IgG, and IgM Repertoires

Serum antibodies that recognize carbohydrate antigens play a fundamental role in immune defense, homeostasis, and autoimmunity. In addition, they serve as potential biomarkers for a variety of medical applications. For most anti-glycan antibodies found in human serum, however, the origins, regulation, and biological significance are not well understood. Antibody subpopulations that are relevant to a particular biological process or disease are often difficult to identify from the myriad of anti-glycan antibodies present in human serum. While prior studies have examined anti-glycan IgG and/or IgM repertoires, little is known about IgA repertoires or how IgA, IgG, and IgM are related. In this study, we describe the development of a multiplex assay to simultaneously detect IgA, IgG, and IgM on a glycan microarray and its application to studying anti-glycan repertoires in healthy subjects. The multiplex glycan microarray assay revealed unique insights and systems-level relationships that would be difficult to uncover using traditional approaches. In particular, we found that anti-glycan IgA, IgG, and IgM expression levels appear to be tightly regulated, coordinated within individuals, and stable over time. Additionally, our results help define natural fluctuations over time, which is critical for identifying changes that are beyond normal biological variation.

Synthesis of aryl-thioglycopeptides through chemoselective Pd-mediated conjugation

We describe herein a Pd-catalyzed methodology for the thioglycoconjugation of iodoaryl peptides and aminoacids. This operationally simple process occurs under semi-aqueous conditions and displays wide substrate scope. The strategy has been successfully applied to both the thioglycosylation of unprotected peptides and the generation of thioglyco-aminoacid building blocks, including those suitable for solid phase peptide synthesis. To demonstrate the broad potential of this technique for late stage functionalization, we successfully incorporated challenging unprotected β-S-GlcNAc- and α-S-GalNAc-derivatives into very long unprotected peptides. This study opens the way to new applications in chemical biology, considering the well-recognized advantages of S-glycosides over O-glycosides in terms of resistance towards both enzymatic and chemical degradation.

Identification of a secondary binding site in human macrophage galactose-type lectin by microarray studies: Implications for the molecular recognition of its ligands

The human macrophage galactose-type lectin (MGL) is a C-type lectin characterized by a unique specificity for terminal GalNAc residues present in the tumor-associated Tn antigen (αGalNAc-Ser/Thr) and its sialylated form, the sialyl-Tn antigen. However, human MGL has multiple splice variants, and whether these variants have distinct ligand-binding properties is unknown. Here, using glycan microarrays, we compared the binding properties of the short MGL 6C (MGL^short) and the long MGL 6B (MGL^long) splice variants, as well as of a histidine-to-threonine mutant (MGL^short H259T). Although the MGL^short and MGL^long variants displayed similar binding properties on the glycan array, the MGL^short H259T mutant failed to interact with the sialyl-Tn epitope. As the MGL^short H259T variant could still bind a single GalNAc monosaccharide on this array, we next investigated its binding characteristics to Tn-containing glycopeptides derived from the MGL ligands mucin 1 (MUC1), MUC2, and CD45. Strikingly, in the glycopeptide microarray, the MGL^short H259T variant lost high-affinity binding toward Tn-containing glycopeptides, especially at low probing concentrations. Moreover, MGL^short H259T was unable to recognize cancer-associated Tn epitopes on tumor cell lines. Molecular dynamics simulations indicated that in WT MGL^short, His²⁵⁹ mediates H bonds directly or engages the Tn-glycopeptide backbone through water molecules. These bonds were lost in MGL^short H259T, thus explaining its lower binding affinity. Together, our results suggest that MGL not only connects to the Tn carbohydrate epitope, but also engages the underlying peptide via a secondary binding pocket within the MGL carbohydrate recognition domain containing the His²⁵⁹ residue.

Proteomic analysis of Tn-bearing glycoproteins from different stages of melanoma cells reveals new biomarkers

Cutaneous melanoma, the most aggressive form of skin cancer, responds poorly to conventional therapy. The appearance of Tn antigen-modified proteins in cancer is correlated with metastasis and poor prognoses. The Tn determinant has been recognized as a powerful diagnostic and therapeutic target, and as an object for the development of anti-tumor vaccine strategies. This study was designed to identify Tn-carrying proteins and reveal their influence on cutaneous melanoma progression. We used a lectin-based strategy to purify Tn antigen-enriched cellular glycoproteome, the LC-MS/MS method to identify isolated glycoproteins, and the DAVID bioinformatics tool to classify the identified proteins. We identified 146 different Tn-bearing glycoproteins, 88% of which are new. The Tn-glycoproteome was generally enriched in proteins involved in the control of ribosome biogenesis, CDR-mediated mRNA stabilization, cell-cell adhesion and extracellular vesicle formation. The differential expression patterns of Tn-modified proteins for cutaneous primary and metastatic melanoma cells supported nonmetastatic and metastatic cell phenotypes, respectively. To our knowledge, this study is the first large-scale proteomic analysis of Tn-bearing proteins in human melanoma cells. The identified Tn-modified proteins are related to the biological and molecular nature of cutaneous melanoma and may be valuable biomarkers and therapeutic targets.

Glycopeptides as Targets for Dendritic Cells: Exploring MUC1 Glycopeptides Binding Profile toward Macrophage Galactose-Type Lectin (MGL) Orthologs

The macrophage galactose-type lectin (MGL) recognizes glycan moieties exposed by pathogens and malignant cells. Particularly, mucin-1 (MUC1) glycoprotein presents an altered glycosylation in several cancers. To estimate the ability of distinct MGL orthologs to recognize aberrant glycan cores in mucins, we applied evanescent-field detection to a versatile MUC1-like glycopeptide microarray platform. Here, as binding was sequence-dependent, we demonstrated that not only sugars but also peptide region impact the recognition of murine MGL1 (mMGL1). In addition, we observed for all three MGL orthologs that divalent glycan presentation increased the binding. To assess the utility of the glycopeptide binders of the MGL orthologs for MGL targeting, we performed uptake assays with fluorescein-MUC1 using murine dendritic cells. A diglycosylated MUC1 peptide was preferentially internalized in an MGL-dependent fashion, thus showing the utility for divalent MGL targeting. These findings may be relevant to a rational design of antitumor vaccines targeting dendritic cells via MGL.