Tumour-associated glycan modifications of antigen enhance MGL2 dependent uptake and MHC class I restricted CD8 T cell responses

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.

Aberrant Glycosylation as Immune Therapeutic Targets for Solid Tumors

Glycosylation occurs at all major types of biomolecules, including proteins, lipids, and RNAs to form glycoproteins, glycolipids, and glycoRNAs in mammalian cells, respectively. The carbohydrate moiety, known as glycans on glycoproteins and glycolipids, is diverse in their compositions and structures. Normal cells have their unique array of glycans or glycome which play pivotal roles in many biological processes. The glycan structures in cancer cells, however, are often altered, some having unique structures which are termed as tumor-associated carbohydrate antigens (TACAs). TACAs as tumor biomarkers are glycan epitopes themselves, or glycoconjugates. Some of those TACAs serve as tumor glyco-biomarkers in clinical practice, while others are the immune therapeutic targets for treatment of cancers. A monoclonal antibody (mAb) to GD2, an intermediate of sialic-acid containing glycosphingolipids, is an example of FDA-approved immune therapy for neuroblastoma indication in young adults and many others. Strategies for targeting the aberrant glycans are currently under development, and some have proceeded to clinical trials. In this review, we summarize the currently established and most promising aberrant glycosylation as therapeutic targets for solid tumors.

Research progress on O-GlcNAcylation in the occurrence, development, and treatment of colorectal cancer

For a long time, colorectal cancer (CRC) has been ranked among the top cancer-related mortality rates, threatening human health. As a significant post-translational modification, O-GlcNAcylation plays an essential role in complex life activities. Related studies have found that the occurrence, development, and metastasis of CRC are all related to abnormal O-GlcNAcylation and participate in many critical biological processes, such as gene transcription, signal transduction, cell growth, and differentiation. Recently, nucleotide sugar analogs, tumor-specific carbohydrate vaccine, SIRT1 longevity gene, dendritic cells as targets, and NOTCH gene have become effective methods to induce antitumor therapy. Not long ago, checkpoint kinase 1 and checkpoint kinase 2 were used as therapeutic targets for CRC, but there are still many problems to be solved. With an in-depth study of protein chip, mass spectrometry, chromatography, and other technologies, O-GlcNAcylation research will accelerate rapidly, which may provide new ideas for the research and development of antitumor drugs and the discovery of new CRC diagnostic markers.

Identification of Tn antigen O-GalNAc-expressing glycoproteins in human carcinomas using novel anti-Tn recombinant antibodies

The Tn antigen is a neoantigen abnormally expressed in many human carcinomas and expression correlates with metastasis and poor survival. To explore its biomarker potential, new antibodies are needed that specifically recognize this antigen in tumors. Here we generated two recombinant antibodies to the Tn antigen, Remab6 as a chimeric human IgG1 antibody and ReBaGs6 as a murine IgM antibody and characterized their specificities using multiple biochemical and biological approaches. Both Remab6 and ReBaGs6 recognize clustered Tn structures, but most importantly do not recognize glycoforms of human IgA1 that contain potential cross-reactive Tn antigen structures. In flow cytometry and immunofluorescence analyses, Remab6 recognizes human cancer cell lines expressing the Tn antigen, but not their Tn-negative counterparts. In immunohistochemistry (IHC), Remab6 stains many human cancers in tissue array format but rarely stains normal tissues and then mostly intracellularly. We used these antibodies to identify several unique Tn-containing glycoproteins in Tn-positive Colo205 cells, indicating their utility for glycoproteomics in future biomarker studies. Thus, recombinant Remab6 and ReBaGs6 are useful for biochemical characterization of cancer cells and IHC of tumors and represent promising tools for Tn biomarker discovery independently of recognition of IgA1.

Pustulan Activates Chicken Bone Marrow-Derived Dendritic Cells In Vitro and Promotes Ex Vivo CD4+ T Cell Recall Response to Infectious Bronchitis Virus

Infectious bronchitis virus (IBV) is a highly contagious avian coronavirus. IBV causes substantial worldwide economic losses in the poultry industry. Vaccination with live-attenuated viral vaccines, therefore, are of critical importance. Live-attenuated viral vaccines, however, exhibit the potential for reversion to virulence and recombination with virulent field strains. Therefore, alternatives such as subunit vaccines are needed together with the identification of suitable adjuvants, as subunit vaccines are less immunogenic than live-attenuated vaccines. Several glycan-based adjuvants directly targeting mammalian C-type lectin receptors were assessed in vitro using chicken bone marrow-derived dendritic cells (BM-DCs). The β-1-6-glucan, pustulan, induced an up-regulation of MHC class II (MHCII) cell surface expression, potentiated a strong proinflammatory cytokine response, and increased endocytosis in a cation-dependent manner. Ex vivo co-culture of peripheral blood monocytes from IBV-immunised chickens, and BM-DCs pulsed with pustulan-adjuvanted recombinant IBV N protein (rN), induced a strong recall response. Pustulan-adjuvanted rN induced a significantly higher CD4⁺ blast percentage compared to either rN, pustulan or media. However, the CD8⁺ and TCRγδ⁺ blast percentage were significantly lower with pustulan-adjuvanted rN compared to pustulan or media. Thus, pustulan enhanced the efficacy of MHCII antigen presentation, but apparently not the cross-presentation on MHCI. In conclusion, we found an immunopotentiating effect of pustulan in vitro using chicken BM-DCs. Thus, future in vivo studies might show pustulan as a promising glycan-based adjuvant for use in the poultry industry to contain the spread of coronaviridiae as well as of other avian viral pathogens.

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.

MGL1 Receptor Plays a Key Role in the Control of T. cruzi Infection by Increasing Macrophage Activation through Modulation of ERK1/2, c-Jun, NF-κB and NLRP3 Pathways

Macrophage galactose-C type lectin (MGL)1 receptor is involved in the recognition of Trypanosoma cruzi (T. cruzi) parasites and is important for the modulation of the innate and adaptive immune responses. However, the mechanism by which MGL1 promotes resistance to T. cruzi remains unclear. Here, we show that MGL1 knockout macrophages (MGL1^-/- Mφ) infected in vitro with T. cruzi were heavily parasitized and showed decreased levels of reactive oxygen species (ROS), nitric oxide (NO), IL-12 and TNF-α compared to wild-type macrophages (WT Mφ). MGL1^-/- Mφ stimulated in vitro with T. cruzi antigen (TcAg) showed low expression of TLR-2, TLR-4 and MHC-II, which resulted in deficient splenic cell activation compared with similar co-cultured WT Mφ. Importantly, the activation of p-ERK1/2, p-c-Jun and p-NF-κB p65 were significantly reduced in MGL1^-/- Mφ exposed to TcAg. Similarly, procaspase 1, caspase 1 and NLRP3 inflammasome also displayed a reduced expression that was associated with low IL-β production. Our data reveal a previously unappreciated role for MGL1 in Mφ activation through the modulation of ERK1/2, c-Jun, NF-κB and NLRP3 signaling pathways, and to the development of protective innate immunity against experimental T. cruzi infection.

Immature O-glycans recognized by the macrophage glycoreceptor CLEC10A (MGL) are induced by 4-hydroxy-tamoxifen, oxidative stress and DNA-damage in breast cancer cells

BACKGROUND

Ligands of the C-type lectin CLEC10A such as Tn and sialyl-Tn representing early intermediates of O-glycosylation are hallmarks of many human malignancies. A variety of regulatory mechanisms underlying their expression are being discussed.

METHODS

CLEC10A ligands were detected in various tissues and cells using the recombinant glycan-binding domain of CLEC10A. In normal breast and endometrium, presence of ligands was correlated to the female cycle. Estrogen- and stress dependent induction of CLEC10A ligands was analyzed in MCF7 and T47D cells exposed to 4-hydroxy-tamoxifen (Tam), zeocin and hydrogen peroxide. The expression and localization of CLEC10A ligands was analyzed by Western blot and immunofluorescence. In breast cancer patients CLEC10A ligand expression and survival was correlated by Kaplan-Meyer analysis.

RESULT

We observed binding of CLEC10A in normal endometrial and breast tissues during the late phase of the female hormonal cycle suggesting a suppressive effect of female sex hormones on CLEC10A ligand expression. Accordingly, CLEC10A ligands were induced in MCF7- and T47D breast cancer cells after Tam treatment and accumulated on the cell surface and in the endosomal/lysosomal compartment. Phagocytosis experiments indicate that macrophages preferentially internalize CLEC10A ligands coated beads and Tam treated MCF7 cells. CLEC10A ligands were also expressed after the addition of zeocin and hydrogen-peroxide. Each substance induced the production of ROS indicating reactive oxygen species as a unifying mechanism of CLEC10A ligand induction. Mechanistically, increased expression of GalNAc-transferase 6 (GalNT6) and translocation of GalNT2 and GalNT6 from cis- towards trans-Golgi compartment was observed, while protein levels of COSMC and T-synthase remained unaffected. In breast cancer patients, positivity for CLEC10A staining in tumor tissues was associated with improved outcome and survival.

CONCLUSION

CLEC10A ligands are inducible by hormone depletion, 4-hydroxy-tamoxifen and agents inducing DNA damage and oxidative stress. Our results indicate that CLEC10A acts as a receptor for damaged and dead cells and may play an important role in the uptake of cell debris by macrophages and dendritic cells.

CLEC10A Is a Specific Marker for Human CD1c+ Dendritic Cells and Enhances Their Toll-Like Receptor 7/8-Induced Cytokine Secretion

Dendritic cells (DCs) are major players for the induction of immune responses. Apart from plasmacytoid DCs (pDCs), human DCs can be categorized into two types of conventional DCs: CD141⁺ DCs (cDC1) and CD1c⁺ DCs (cDC2). Defining uniquely expressed surface markers on human immune cells is not only important for the identification of DC subpopulations but also a prerequisite for harnessing the DC subset-specific potential in immunomodulatory approaches, such as antibody-mediated antigen targeting. Although others identified CLEC9A as a specific endocytic receptor for CD141⁺ DCs, such a receptor for CD1c⁺ DCs has not been discovered, yet. By performing transcriptomic and flow cytometric analyses on human DC subpopulations from different lymphohematopoietic tissues, we identified CLEC10A (CD301, macrophage galactose-type C-type lectin) as a specific marker for human CD1c⁺ DCs. We further demonstrate that CLEC10A rapidly internalizes into human CD1c⁺ DCs upon binding of a monoclonal antibody directed against CLEC10A. The binding of a CLEC10A-specific bivalent ligand (the MUC-1 peptide glycosylated with N-acetylgalactosamine) is limited to CD1c⁺ DCs and enhances the cytokine secretion (namely TNFα, IL-8, and IL-10) induced by TLR 7/8 stimulation. Thus, CLEC10A represents not only a candidate to better define CD1c⁺ DCs—due to its high endocytic potential—CLEC10A also exhibits an interesting candidate receptor for future antigen-targeting approaches.

Targeting C-type lectin receptors: a high-carbohydrate diet for dendritic cells to improve cancer vaccines

There is a growing understanding of why certain patients do or do not respond to checkpoint inhibition therapy. This opens new opportunities to reconsider and redevelop vaccine strategies to prime an anticancer immune response. Combination of such vaccines with checkpoint inhibitors will both provide the fuel and release the brake for an efficient anticancer response. Here, we discuss vaccine strategies that use C-type lectin receptor (CLR) targeting of APCs, such as dendritic cells and macrophages. APCs are a necessity for the priming of antigen-specific cytotoxic and helper T cells. Because CLRs are natural carbohydrate-recognition receptors highly expressed by multiple subsets of APCs and involved in uptake and processing of Ags for presentation, these receptors seem particularly interesting for targeting purposes.

The Biology and Underlying Mechanisms of Cross-Presentation of Exogenous Antigens on MHC-I Molecules

To monitor the health of cells, the immune system tasks antigen-presenting cells with gathering antigens from other cells and bringing them to CD8 T cells in the form of peptides bound to MHC-I molecules. Most cells would be unable to perform this function because they use their MHC-I molecules to exclusively present peptides derived from the cell's own proteins. However, the immune system evolved mechanisms for dendritic cells and some other phagocytes to sample and present antigens from the extracellular milieu on MHC-I through a process called cross-presentation. How this important task is accomplished, its role in health and disease, and its potential for exploitation are the subject of this review.

A Bitter Sweet Symphony: Immune Responses to Altered O-glycan Epitopes in Cancer

The appearance of aberrant glycans on the tumor cell surface is one of the emerging hallmarks of cancer. Glycosylation is an important post-translation modification of proteins and lipids and is strongly affected by oncogenesis. Tumor-associated glycans have been extensively characterized regarding their composition and tumor-type specific expression patterns. Nevertheless whether and how tumor-associated glycans contribute to the observed immunomodulatory actions by tumors has not been extensively studied. Here, we provide a detailed overview of the current knowledge on how tumor-associated O-glycans affect the anti-tumor immune response, thereby focusing on truncated O-glycans present on epithelial tumors and mucins. These tumor-associated O-glycans and mucins bind a variety of lectin receptors on immune cells to facilitate the subsequently induction of tolerogenic immune responses. We, therefore, postulate that tumor-associated glycans not only support tumor growth, but also actively contribute to immune evasion.

The Ligands of C-Type Lectins

In this chapter, a comprehensive overview of the known ligands for the C-type lectins (CTLs) is provided. Emphasis has been placed on the chemical structure of the glycans that bind to the different CTLs and the amount of structural variation (or overlap) that each CTL can tolerate. In this way, both the synthetic carbohydrate chemist and the immunologist can more readily gain insight into the existing structure-activity space for the CTL ligands and, ideally, see areas of synergy that will help identify and refine the ligands for these receptors.

Rational Design of Targeted Next-Generation Carriers for Drug and Vaccine Delivery

Pattern recognition receptors on innate immune cells play an important role in guiding how cells interact with the rest of the organism and in determining the direction of the downstream immune response. Recent advances have elucidated the structure and function of these receptors, providing new opportunities for developing targeted drugs and vaccines to treat infections, cancers, and neurological disorders. C-type lectin receptors, Toll-like receptors, and folate receptors have attracted interest for their ability to endocytose their ligands or initiate signaling pathways that influence the immune response. Several novel technologies are being developed to engage these receptors, including recombinant antibodies, adoptive immunotherapy, and chemically modified antigens and drug delivery vehicles. These active targeting technologies will help address current challenges facing drug and vaccine delivery and lead to new tools to treat human diseases.

Targeting C-Type Lectin Receptors for Cancer Immunity

C-type lectin receptors (CLRs) are a large family of soluble and trans-membrane pattern recognition receptors that are widely and primarily expressed on myeloid cells. CLRs are important for cell-cell communication and host defense against pathogens through the recognition of specific carbohydrate structures. Similar to a family of Toll-like receptors, CLRs signaling are involved in the various steps for initiation of innate immune responses and promote secretion of soluble factors such as cytokines and interferons. Moreover, CLRs contribute to endocytosis and antigen presentation, thereby fine-tune adaptive immune responses. In addition, there may also be a direct activation of acquired immunity. On the other hand, glycans, such as mannose structures, Lewis-type antigens, or GalNAc are components of tumor antigens and ligate CLRs, leading to immunoregulation. Therefore, agonists or antagonists of CLRs signaling are potential therapeutic reagents for cancer immunotherapy. We aim to overview the current knowledge of CLRs signaling and the application of their ligands on tumor-associating immune response.

Cancer intelligence acquired (CIA): tumor glycosylation and sialylation codes dismantling antitumor defense

Aberrant glycosylation is a key feature of malignant transformation and reflects epigenetic and genetic anomalies among the multitude of molecules involved in glycan biosynthesis. Although glycan biosynthesis is not template bound, altered tumor glycosylation is not random, but associated with common glycosylation patterns. Evidence suggests that acquisition of distinct glycosylation patterns evolves from a 'microevolutionary' process conferring advantages in terms of tumor growth, tumor dissemination, and immune escape. Such glycosylation modifications also involve xeno- and hypersialylation. Xeno-autoantigens such as Neu5Gc-gangliosides provide potential targets for immunotherapy. Hypersialylation may display 'enhanced self' to escape immunosurveillance and involves several not mutually exclusive inhibitory pathways that all rely on protein-glycan interactions. A better understanding of tumor 'glycan codes' as deciphered by lectins, such as siglecs, selectins, C-type lectins and galectins, may lead to novel treatment strategies, not only in cancer, but also in autoimmune disease or transplantation.

Glycosylation characteristics of colorectal cancer

Glycans on proteins and lipids are known to alter with malignant transformation. The study of these may contribute to the discovery of biomarkers and treatment targets as well as understanding of cancer biology. We here describe the change of glycosylation specifically defining colorectal cancer with view on N-glycans, O-glycans, and glycosphingolipid glycans in colorectal cancer cells and tissues as well as patient sera. Glycan alterations observed in colon cancer include increased β1,6-branching and correlating higher abundance of (poly-)N-acetyllactosamine extensions of N-glycans as well as an increase in (truncated) high-mannose type glycans, while bisected structures decrease. Colorectal cancer-associated O-glycan changes are predominated by reduced expression of core 3 and 4 glycans, whereas higher levels of core 1 glycans, (sialyl) T-antigen, (sialyl) Tn-antigen, and a generally higher density of O-glycans are observed. Specific changes for glycosphingolipid glycans are lower abundances of disialylated structures as well as globo-type glycosphingolipid glycans with exception of Gb3. In general, alterations affecting all discussed glycan types are increased sialylation, fucosylation as well as (sialyl) Lewis-type antigens and type-2 chain glycans. As a consequence, interactions with glycan-binding proteins can be affected and the biological function and cellular consequences of the altered glycosylation with regard to tumorigenesis, metastasis, modulation of immunity, and resistance to antitumor therapy will be discussed. Finally, analytical approaches aiding in the field of glycomics will be reviewed with focus on binding assays and mass spectrometry.

Glycan-mediated modification of the immune response

Aberrantly glycosylated tumor antigens represent promising targets for the development of anti-cancer vaccines, yet how glycans influence immune responses is poorly understood. Recent studies have demonstrated that GalNAc-glycosylation enhances antigen uptake by dendritic cells as well as CD4⁺ T-cell and humoral responses, but prevents CD8⁺ T-cell activation. Here, we briefly discuss the relevance of glycans as candidate targets for anti-cancer vaccines.

Novel insights into the immunomodulatory role of the dendritic cell and macrophage-expressed C-type lectin MGL

Based on their ability to balance tolerance and inflammation, antigen presenting cells, such as dendritic cells and macrophages contribute to the maintenance of immune homeostasis as well as the instigation of immune activation. Acting as key sensors of tissue integrity and pathogen invasion, they are well equipped with a wide variety of pattern recognition receptors, to which the C-type lectin family also belongs. C-type lectins are glycan-binding receptors that mediate cell-cell communication and pathogen recognition, besides participating in the endocytosis of antigens for presentation to T cells and the fine-tuning of immune responses. Here we review the current state-of-the-art on the dendritic cell and macrophage-expressed C-type lectin macrophage galactose-type lectin (MGL), highlighting the binding specificities, signaling properties and modulation of innate and adaptive immunity by its human and murine orthologues.

Biological evaluation of multivalent lewis X-MGL-1 interactions

Myeloid C-type lectin receptors (CLRs) expressed by antigen-presenting cells are pattern-recognition receptors involved in the recognition of pathogens as well as of self-antigens. The interaction of carbohydrate ligands with a CLR can trigger immune responses. Although several CLR ligands are known, there is limited insight into CLR targeting by carbohydrate ligands. The weak affinity of lectin-carbohydrate interactions often renders multivalent carbohydrate presentation necessary. Here, we have analyzed the impact of multivalent presentation of the trisaccharide Lewis X (Le(X) ) epitope on its interaction with the CLR macrophage galactose-type lectin-1 (MGL-1). Glycan arrays, including N-glycan structures with terminal Le(X) , were prepared by enzymatic extension of immobilized synthetic core structures with two recombinant glycosyltransferases. Incubation of arrays with an MGL-1-hFc fusion protein showed up to tenfold increased binding to multiantennary N-glycans displaying Le(X) structures, compared to monovalent Le(X) trisaccharide. Multivalent presentation of Le(X) on the model antigen ovalbumin (OVA) led to increased cytokine production in a dendritic cell /T cell coculture system. Furthermore, immunization of mice with Le(X) -OVA conjugates modulated cytokine production and the humoral response, compared to OVA alone. This study provides insights into how multivalent carbohydrate-lectin interactions can be exploited to modulate immune responses.

Targeting C-type lectin receptors with multivalent carbohydrate ligands

C-type lectin receptors (CLRs) represent a large receptor family including collectins, selectins, lymphocyte lectins, and proteoglycans. CLRs share a structurally homologous carbohydrate-recognition domain (CRD) and often bind carbohydrates in a Ca²⁺-dependent manner. In innate immunity, CLRs serve as pattern recognition receptors (PRRs) and bind to the glycan structures of pathogens and also to self-antigens. In nature, the low affinity of CLR/carbohydrate interactions is overcome by multivalent ligand presentation at the surface of cells or pathogens. Thus, multivalency is a promising strategy for targeting CLR-expressing cells and, indeed, carbohydrate-based targeting approaches have been employed for a number of CLRs, including asialoglycoprotein receptor (ASGPR) in the liver, or DC-SIGN expressed by dendritic cells. Since CLR engagement not only mediates endocytosis but also influences intracellular signaling pathways, CLR targeting may allow for cell-specific drug delivery and also the modulation of cellular functions. Glyconanoparticles, glycodendrimers, and glycoliposomes were successfully used as tools for CLR-specific targeting. This review will discuss different approaches for multivalent CLR ligand presentation and aims to highlight how CLR targeting has been employed for cell specific drug delivery. Major emphasis is directed towards targeting of CLRs expressed by antigen-presenting cells to modulate immune responses.

Tumor-Associated Glycans and Immune Surveillance

Changes in cell surface glycosylation are a hallmark of the transition from normal to inflamed and neoplastic tissue. Tumor-associated carbohydrate antigens (TACAs) challenge our understanding of immune tolerance, while functioning as immune targets that bridge innate immune surveillance and adaptive antitumor immunity in clinical applications. T-cells, being a part of the adaptive immune response, are the most popular component of the immune system considered for targeting tumor cells. However, for TACAs, T-cells take a back seat to antibodies and natural killer cells as first-line innate defense mechanisms. Here, we briefly highlight the rationale associated with the relative importance of the immune surveillance machinery that might be applicable for developing therapeutics.

MGL signaling augments TLR2-mediated responses for enhanced IL-10 and TNF-α secretion

DCs orchestrate immune responses to infectious pathogens and disturbances in tissue integrity. Equipped with C-type lectins, DCs can respond to environmental changes in glycosylation. Many C-type lectins are capable of modulating TLR activation, thereby facilitating tailor-made immune reactions. Here, we investigated the signaling properties of the C-type lectin MGL and show that MGL engagement by agonistic antibodies or carbohydrate ligands couples to TLR signal transduction for increased IL-10 and TNF-α secretion by human monocyte-derived DCs. MGL triggering especially synergized with TLR2-induced pathways, leading to elevated IL-10 mRNA levels and enhanced TNF-α mRNA stability. In addition, MGL signaling promoted phosphorylation of the MAPK ERK and the transcription factor CREB. Whereas specific inhibitors of p90RSK blocked the MGL-induced cytokine secretion, AP-1 was not involved. Strikingly, NF-κB was only crucial for the IL-10 response and dispensable for TNF-α production. Together, our results demonstrate that MGL activation of the ERK-p90RSK-CREB axis converges with TLR2-induced pathways, thereby fine-tuning the DC maturation phenotype.

Organization of the extracellular portion of the macrophage galactose receptor: a trimeric cluster of simple binding sites for N-acetylgalactosamine

The properties of the human macrophage galactose receptor have been investigated. Specificity for N-acetylgalactosamine (GalNAc) residues with exposed 3- and 4-hydroxyl groups explains virtually all of the results obtained from a recently expanded array of synthetic glycans and is consistent with a model for the structure of the binding site. This simple interaction is sufficient to explain the ability of the receptor to bind to tumor-cell glycans bearing Tn and sialyl-Tn antigens, but not to more elaborate O-linked glycans that predominate on normal cells. This specificity also allows for binding of parasite glycans and screening of an array of bacterial outer membrane oligosaccharides confirms that the receptor binds to a subset of these structures with appropriately exposed GalNAc residues. A key feature of the receptor is the clustering of binding sites in the extracellular portion of the protein, which retains the trimeric structure observed in the cell membrane. Chemical crosslinking, gel filtration, circular dichroism analysis and differential scanning calorimetry demonstrate that this trimeric structure of the receptor is stabilized by an α-helical coiled coil that extends from the surface of the membrane to the globular carbohydrate-recognition domains. The helical neck domains form independent trimerization domains. Taken together, these results indicate that the macrophage galactose receptor shares many of the features of serum mannose-binding protein, in which clusters of monosaccharide-binding sites serve as detectors for a simple epitope that is not common on endogenous cell surface glycans but that is abundant on the surfaces of tumor cells and certain pathogens.

Cancer associated aberrant protein O-glycosylation can modify antigen processing and immune response

Aberrant glycosylation of mucins and other extracellular proteins is an important event in carcinogenesis and the resulting cancer associated glycans have been suggested as targets in cancer immunotherapy. We assessed the role of O-linked GalNAc glycosylation on antigen uptake, processing, and presentation on MHC class I and II molecules. The effect of GalNAc O-glycosylation was monitored with a model system based on ovalbumin (OVA)-MUC1 fusion peptides (+/- glycosylation) loaded onto dendritic cells co-cultured with IL-2 secreting OVA peptide-specific T cell hybridomas. To evaluate the in vivo response to a cancer related tumor antigen, Balb/c or B6.Cg(CB)-Tg(HLA-A/H2-D)2Enge/J (HLA-A2 transgenic) mice were immunized with a non-glycosylated or GalNAc-glycosylated MUC1 derived peptide followed by comparison of T cell proliferation, IFN-γ release, and antibody induction. GalNAc-glycosylation promoted presentation of OVA-MUC1 fusion peptides by MHC class II molecules and the MUC1 antigen elicited specific Ab production and T cell proliferation in both Balb/c and HLA-A2 transgenic mice. In contrast, GalNAc-glycosylation inhibited the presentation of OVA-MUC1 fusion peptides by MHC class I and abolished MUC1 specific CD8+ T cell responses in HLA-A2 transgenic mice. GalNAc glycosylation of MUC1 antigen therefore facilitates uptake, MHC class II presentation, and antibody response but might block the antigen presentation to CD8+ T cells.

Targeting of macrophage galactose-type C-type lectin (MGL) induces DC signaling and activation

Dendritic cells (DCs) sense the microenvironment through several types of receptors recognizing pathogen-associated molecular patterns. In particular, C-type lectins, expressed by distinct subsets of DCs, recognize and internalize specific carbohydrate antigen in a Ca(2+) -dependent manner. Targeting of these receptors is becoming an efficient strategy of delivering antigens in DC-based anticancer immunotherapy. Here we investigated the role of the macrophage galactose type C-lectin receptor (MGL), expressed by immature DCs (iDCs), as a molecular target for α-N-acetylgalactosamine (GalNAc or Tn)-carrying tumor-associated antigens to improve DC performance. MGL expressed by ex vivo-generated iDCs from healthy donors was engaged by a 60-mer MUC1(9Tn) -glycopeptide as a Tn-carrying tumor-associated antigen, and an anti-MGL antibody, as a specific MGL binder. We demonstrated that MGL engagement induced homotrimers and homodimers, triggering the phosphorylation of extracellular signal-regulated kinase 1,2 (ERK1,2) and nuclear factor-κB activation. Analysis of DC phenotype and function demonstrated that MGL engagement improved DC performance as antigen-presenting cells, promoting the upregulation of maturation markers, a decrease in phagocytosis, an enhancement of motility, and most importantly an increase in antigen-specific CD8(+) T-cell activation. These results demonstrate that the targeting of MGL receptor on human DCs has an adjuvant effect and that this strategy can be used to design novel anticancer vaccines.

Regulatory circuits mediated by lectin-glycan interactions in autoimmunity and cancer

Numerous regulatory programs have been identified that contribute to the restoration of homeostasis at the conclusion of immune responses and to safeguarding against the detrimental effects of chronic inflammation and autoimmune pathology. Malignant cells may usurp these pathways to create immunosuppressive networks that thwart antitumor responses. Herein we review the role of endogenous lectins (C-type lectins, siglecs, and galectins) and specific N- and O-glycans generated by the coordinated action of glycosyltransferases and glycosidases that together promote regulatory signals that control immune cell homeostasis. We also discuss the mechanisms by which glycan-dependent regulatory programs integrate into canonical circuits that amplify or silence immune responses related to autoimmunity and neoplastic disease.

Signaling by myeloid C-type lectin receptors in immunity and homeostasis

Myeloid cells are key drivers of physiological responses to pathogen invasion or tissue damage. Members of the C-type lectin receptor (CLR) family stand out among the specialized receptors utilized by myeloid cells to orchestrate these responses. CLR ligands include carbohydrate, protein, and lipid components of both pathogens and self, which variably trigger endocytic, phagocytic, proinflammatory, or anti-inflammatory reactions. These varied outcomes rely on a versatile system for CLR signaling that includes tyrosine-based motifs that recruit kinases, phosphatases, or endocytic adaptors as well as nontyrosine-based signals that modulate the activation of other pathways or couple to the uptake machinery. Here, we review the signaling properties of myeloid CLRs and how they impact the role of myeloid cells in innate and adaptive immunity.

Differential glycosylation of MUC1 and CEACAM5 between normal mucosa and tumour tissue of colon cancer patients

Altered glycosylation in epithelial cancers may play an important role in tumour progression, as it may affect tumour cell migration and antigen presentation by antigen presenting cells. We specifically characterise the glycosylation patterns of two tumour antigens that are highly expressed in cancer tissue and often detected in their secreted form in serum: the epithelial mucin MUC1 and carcinoembryonic antigen (CEA, also called CEACAM5). We analysed 48 colorectal cancer patients, comparing normal colon and tumour epithelium within each patient. Lectin binding was studied by a standardised CEA/MUC1 capture ELISA, using several plant lectins, and the human C-type lectins MGL and DC-SIGN, and Galectin-3. Peanut agglutinin (PNA) bound to MUC1 from tumour tissue in particular, suggests increased expression of the Thomsen-Friedenreich antigen (TF-antigen) (Core 1, Galβ1-3GalNAc-Ser/Thr). Only small amounts of Tn-antigen (GalNAcα-Ser/Thr) expression was observed, but the human C-type lectin MGL showed increased binding to tumour-associated MUC1. Furthermore, sialylation was greatly enhanced. In sharp contrast, tumour-associated CEA (CEACAM5) contained high levels of the blood-group related carbohydrates, Lewis X and Lewis Y. This correlated strongly with the interaction of the human C-type lectin DC-SIGN to tumour-associated CEA, suggesting that CEA can be recognized and taken up by antigen presenting cells. In addition, increased mannose expression was observed and branched N-glycans were prominent, and this correlated well with human Galectin-3 binding. These data demonstrate that individual tumour antigens contain distinct glycan structures associated with cancer and, since glycans affect cellular interactions with its microenvironment, this may have consequences for progression of the disease.

C-type Lectin Receptors for Tumor Eradication: Future Directions

Dendritic cells are key regulators in directing immune responses and therefore are under extensive research for the induction of anti-tumor responses. DCs express a large array of receptors by which they scan their surroundings for recognition and uptake of pathogens. One of the receptor-families is the C-type lectins (CLR), which bind carbohydrate structures and internalize antigens upon recognition. Intracellular routing of antigen through CLR enhances loading and presentation of antigen through MHC class I and II, inducing antigen-specific CD4⁺ and CD8⁺ T-cell proliferation and skewing T-helper cells. These characteristics make CLRs very interesting targets for DC-based immunotherapy. Profound research has been done on targeting specific tumor antigens to CLR using either antibodies or the natural ligands such as glycan structures. In this review we will focus on the current data showing the potency of CLR-targeting and discuss improvements that can be achieved to enhance anti-tumor activity in the near future.