Human B-lymphocytes express alpha2-6-sialylated 6-sulfo-N-acetyllactosamine serving as a preferred ligand for CD22/Siglec-2

The ligand interactions of B cell Siglecs are involved in the prevention of autoimmunity to sialylated self-antigens and in the quality control of signaling-competent B cells

Sialic acid-binding immunoglobulin-like lectins (Siglecs) are a family of membrane molecules that recognize sialic acid. Most of them are inhibitory receptors that inhibit immune-cell activation by recognizing sialic acid as a self-motif. Human B cells express CD22 (also known as Siglec-2), Siglec-5, Siglec-6 and Siglec-10 whereas mouse B cells express CD22 and Siglec-G (ortholog of human Siglec-10). Siglecs recognize both sialylated molecules expressed on the same cell (cis-ligands) and those expressed by other cells (trans-ligands). In Guillain-Barré syndrome (GBS), antibody production to gangliosides (which are sialic acid-containing glycolipids) expressed by neurons plays a pathogenic role. A Siglec-10 variant deficient in recognition of gangliosides is genetically associated with GBS, suggesting that Siglec-10 induces self-tolerance to gangliosides by recognizing gangliosides as trans-ligands. Recognition of the BCR as a cis-ligand by Siglec-G and CD22 suppresses BCR signaling in B-1 cells and conventional B cells, respectively. This signal suppression prevents excess expansion of B-1 cells and is involved in the quality control of signaling-competent B cells by setting a threshold for tonic signaling during B cell development. CD22 recognizes other cis-ligands including CD22 and β7 integrin. Interaction of CD22 with other CD22 molecules induces CD22 clustering that suppresses CD22-mediated signal inhibition upon BCR ligation, and interaction with β7 integrin maintains its function in the gut-homing of B cells. Taken together, interactions of B cell Siglecs with multiple trans- and cis-ligands play important roles in B cell homeostasis and immune responses.

Sequence variety in the CC' loop of Siglec-8/9/3 determines the recognitions to sulfated oligosaccharides

Siglecs are important lectins found in different types of immune cells and function as regulatory molecules by recognizing self-associated glycans and converting extracellular interactions into signals for inhibiting immune cell functions. Although many Siglecs have been found to show broad specificities and recognize different types of sulfated oligosaccharides, Siglec-8 and Siglec-9 displayed a high degree of specificity for sialyl N-acetyllactosamine (sLacNAc) with sulfations at O6-positions of the galactose (6'-sulfation) and N-acetylglucosamine (6-sulfation), respectively. Siglec-3 was recently discovered to bind sLacNAc both sulfations. In addition to a conserved arginine residue for binding to sialic acid residue, the sequence variety in the CC' loop may provide binding specificities to sulfated oligosaccharides in Siglecs. Thus, the present study employed molecular models to study the impact of different residues in the CC' loops of Siglec-8/9/3 to the recognitions of 6-sulfations in Gal and/or GlcNAc of sLacNAc. The negatively charged residues in the CC' loop of Siglec-9 formed unfavorable electrostatic repulsions with the 6-sulfate in Gal and resulted no recognitions, in contrast to the favorable interactions formed between the positively charged residues in the CC' loop of Siglec-8 and the 6-sulfate in Gal resulting strong specificity. A two-state binding model was proposed for Siglec-3 recognizing 6-sulfations in Gal and GlcNAc of sLacNAc, as the neutral residues in the CC' loop of Siglec-3 could not form strong favorable interactions to lock the 6-sulfate in Gal within a single binding pose or strong unfavorable interactions to repel the 6-sulfate in Gal. The oligosaccharide adopted two distinctive binding poses and oriented the sulfate groups to form interactions with residues in the CC' loop and G-strand. The present study provided a structural mechanism for the sequence variety in the CC' loop of Siglec-8/9/3 determining the recognitions to the sulfated oligosaccharides and offered insights into the binding specificities for Siglecs.

Siglec cis-ligands and their roles in the immune system

Sialic acid-binding immunoglobulin-like lectins are a family of membrane molecules primarily expressed in immune cells. Most of them are inhibitory receptors containing immunoreceptor tyrosine-based inhibition motifs in the cytoplasmic tail. On the cell surface, sialic acid-binding immunoglobulin-like lectins are mostly bound by sialylated glycans on membrane molecules expressed in the same cell (cis-ligands). Although ligands of sialic acid-binding immunoglobulin-like lectins are not efficiently identified by conventional methods such as immunoprecipitation, in situ labeling including proximity labeling is useful in identifying both cis-ligands and the sialylated ligands expressed by other cells (trans-ligands) of sialic acid-binding immunoglobulin-like lectins. Interaction of the inhibitory sialic acid-binding immunoglobulin-like lectins with cis-ligands including both those with and without signaling function modulates the inhibitory activity of sialic acid-binding immunoglobulin-like lectins by multiple different ways. This interaction also modulates signaling function of the cis-ligands. So far, little is known about the role of the interaction between sialic acid-binding immunoglobulin-like lectins and the cis-ligands. Nonetheless, recent studies showed that the inhibitory activity of CD22 (also known as Siglec-2) is regulated by endogenous ligands, most likely cis-ligands, differentially in resting B cells and those in which B-cell antigen receptor is ligated. This differential regulation plays a role in quality control of signaling-competent B cells and also partial restoration of B-cell antigen receptor signaling in immunodeficient B cells.

Glycome dynamics in T and B cell development: basic immunological mechanisms and clinical applications

Glycans cover the surfaces of all mammalian cells through a process called glycosylation. Nearly all proteins and receptors that integrate the intricate series of co-stimulatory/inhibitory pathways of the immune system are glycosylated. Growing evidence indicates that the development of the immune system at the origins of T and B cell development is tightly regulated by glycosylation. In this opinion, we hypothesize that the glycome composition of developing T and B cells is developmentally regulated. We discuss how glycans play fundamental roles in lymphocyte development and how glycans early define T and B cell functionality in multiple aspects of adaptive immunity. These advances can provide opportunities for the discovery of novel disease factors and more effective candidate treatments for various conditions.

Expression of Homing Receptors in IgM+IgD+CD27+ B Cells and Their Frequencies in Appendectomized and/or Tonsillectomized Individuals

BACKGROUND

In humans, blood circulating IgM⁺IgD⁺CD27⁺ B cells are considered analogous to those described in the marginal zone of the spleen and are involved in important immunological processes. The homing receptors they express, and the organs involved in their development (for example, intestinal organs in rabbits) are only partially known. We recently reported that this population is heterogeneous and composed of at least two subsets: one expressing high levels of IgM - IgM^hi B cells - and another low levels - IgM^lo B cells.

OBJECTIVES

To evaluate the expression of homing receptors on IgD⁺CD27⁺ IgM^hi and IgM^lo B cells and quantify their frequencies in blood of control and appendectomized and/or tonsillectomized volunteers.

MATERIALS AND METHODS

Using spectral flow cytometry, the simultaneous expression of 12 previously reported markers that differentiate IgM^hi B cells and IgM^lo B cells and of α4β7, CCR9, CD22 and CCR10 were evaluated in blood circulating B cells of control and appendectomized and/or tonsillectomized volunteers.

RESULTS

The existence of phenotypically defined IgM^lo and IgM^hi B cell subsets was confirmed. They differentially expressed intestinal homing receptors, and the expression of α4β7 and CCR9 seems to determine new IgM subpopulations. IgM^lo and IgM^hi B cells were detected at lower frequencies in the appendectomized and/or tonsillectomized volunteers relative to controls.

CONCLUSIONS

Human blood circulating IgD⁺CD27⁺ IgM^lo and IgM^hi B cell subsets differentially express homing receptors, and it is necessary to investigate if mucosal organs are important in their development.

Coordinated changes in glycosylation regulate the germinal center through CD22

Germinal centers (GCs) are essential for antibody affinity maturation. GC B cells have a unique repertoire of cell surface glycans compared with naive B cells, yet functional roles for changes in glycosylation in the GC have yet to be ascribed. Detection of GCs by the antibody GL7 reflects a downregulation in ligands for CD22, an inhibitory co-receptor of the B cell receptor. To test a functional role for downregulation of CD22 ligands in the GC, we generate a mouse model that maintains CD22 ligands on GC B cells. With this model, we demonstrate that glycan remodeling plays a critical role in the maintenance of B cells in the GC. Sustained expression of CD22 ligands induces higher levels of apoptosis in GC B cells, reduces memory B cell and plasma cell output, and delays affinity maturation of antibodies. These defects are CD22 dependent, demonstrating that downregulation of CD22 ligands on B cells plays a critical function in the GC.

Siglec Signaling in the Tumor Microenvironment

Sialic acid-binding immunoglobulin-like lectins (Siglecs) are a family of receptors that recognize sialoglycans - sialic acid containing glycans that are abundantly present on cell membranes. Siglecs are expressed on most immune cells and can modulate their activity and function. The majority of Siglecs contains immune inhibitory motifs comparable to the immune checkpoint receptor PD-1. In the tumor microenvironment (TME), signaling through the Siglec-sialoglycan axis appears to be enhanced through multiple mechanisms favoring tumor immune evasion similar to the PD-1/PD-L1 signaling pathway. Siglec expression on tumor-infiltrating immune cells appears increased in the immune suppressive microenvironment. At the same time, enhanced Siglec ligand expression has been reported for several tumor types as a result of aberrant glycosylation, glycan modifications, and the increased expression of sialoglycans on proteins and lipids. Siglec signaling has been identified as important regulator of anti-tumor immunity in the TME, but the key factors contributing to Siglec activation by tumor-associated sialoglycans are diverse and poorly defined. Among others, Siglec activation and signaling are co-determined by their expression levels, cell surface distribution, and their binding preferences for cis- and trans-ligands in the TME. Siglec binding preference are co-determined by the nature of the proteins/lipids to which the sialoglycans are attached and the multivalency of the interaction. Here, we review the current understanding and emerging conditions and factors involved in Siglec signaling in the TME and identify current knowledge gaps that exist in the field.

CD22 Controls Germinal Center B Cell Receptor Signaling, Which Influences Plasma Cell and Memory B Cell Output

Germinal center reactions are established during a thymus-dependent immune response. Germinal center (GC) B cells are rapidly proliferating and undergo somatic hypermutation in Ab genes. This results in the production of high-affinity Abs and establishment of long-lived memory cells. GC B cells show lower BCR-induced signaling when compared with naive B cells, but the functional relevance is not clear. CD22 is a member of the Siglec family and functions as an inhibitory coreceptor on B cells. Interestingly, GC B cells downregulate sialic acid forms that serve as high-affinity ligands for CD22, indicating a role for CD22 ligand binding during GC responses. We studied the role of CD22 in the GC with mixed bone marrow chimeric mice and found a disadvantage of CD22^-/- GC B cells during the GC reaction. Mechanistic investigations ruled out defects in dark zone/light zone distribution and affinity maturation. Rather, an increased rate of apoptosis in CD22^-/- GC B cells was responsible for the disadvantage, also leading to a lower GC output in plasma cells and memory B cells. CD22^-/- GC B cells showed a clearly increased calcium response upon BCR stimulation, which was almost absent in wild-type GC B cells. We conclude that the differential expression of the low-affinity cis CD22 ligands in the GC normally results in a strong attenuation of BCR signaling in GC B cells, probably due to higher CD22-BCR interactions. Therefore, attenuation of BCR signaling by CD22 is involved in GC output and B cell fate.

Hyposialylation Must Be Considered to Develop Future Therapies in Autoimmune Diseases

Autoimmune disease development depends on multiple factors, including genetic and environmental. Abnormalities such as sialylation levels and/or quality have been recently highlighted. The adjunction of sialic acid at the terminal end of glycoproteins and glycolipids is essential for distinguishing between self and non-self-antigens and the control of pro- or anti-inflammatory immune reactions. In autoimmunity, hyposialylation is responsible for chronic inflammation, the anarchic activation of the immune system and organ lesions. A detailed characterization of this mechanism is a key element for improving the understanding of these diseases and the development of innovative therapies. This review focuses on the impact of sialylation in autoimmunity in order to determine future treatments based on the regulation of hyposialylation.

Siglecs as Immune Cell Checkpoints in Disease

Sialic acid-binding immunoglobulin-type lectins (Siglecs) are expressed on the majority of white blood cells of the immune system and play critical roles in immune cell signaling. Through recognition of sialic acid-containing glycans as ligands, they help the immune system distinguish between self and nonself. Because of their restricted cell type expression and roles as checkpoints in immune cell responses in human diseases such as cancer, asthma, allergy, neurodegeneration, and autoimmune diseases they have gained attention as targets for therapeutic interventions. In this review we describe the Siglec family, its roles in regulation of immune cell signaling, current efforts to define its roles in disease processes, and approaches to target Siglecs for treatment of human disease.

The β-galactoside α2,6-sialyltranferase 1 (ST6GAL1) inhibits the colorectal cancer metastasis by stabilizing intercellular adhesion molecule-1 via sialylation

Background: Colorectal cancer (CRC) is one of the most frequent malignancies of the digestive system. Elevated expression of β-galactoside α2,6-sialyltranferase 1 (ST6GAL1) has been observed in multiple cancers. But the mechanism of how ST6GAL1 might affect cancer cells remains to be clarified. Our previous study recognized intercellular adhesion molecule-1(ICAM-1) as a probable substrate of ST6GAL1 through mass spectrometry (MS) analysis. ICAM-1 is related to tumor metastasis in various cancers.

Methods: First, ST6GAL1 was overexpressed and knocked down to perform transwell and wound healing assays, and the results were further confirmed in vivo. Based on the results of MS, GO and KEGG analysis were applied to reveal the connection between ST6GAL1 and ICAM-1. Immunoblot and tissue microarrays were administered to investigate the expression of ICAM-1 in different stages of CRC. Next, PCR, lectin precipitation and cycloheximide (CHX) were used to demonstrate the mechanism of ST6GAL1 on ICAM-1. Moreover, we investigated the sialylation on soluble ICAM in serum and its connection to tumor staging.

Results: Overexpression of ST6GAL1 inhibited the migratory ability, while knockdown of ST6GAL1 cells had the reverse effect. Moreover, nude mice injected with ST6GAL1-knockdown cells harvested more liver metastases. Based on the GO and KEGG analysis, data from TCGA database showed a positive correlation between ST6GAL1 and ICAM-1. ICAM-1 also demonstrated a significant decrease in stage III/IV compared with stage I/II tumors. Our results revealed that ST6GAL1 could increase the stability of ICAM-1 through sialylation but had little influence on transcriptional level. Additionally, results of serum lectin precipitation revealed a correlation between the level of sialylation on soluble ICAM and CRC staging.

Conclusion: This study illustrated that ST6GAL1 inhibited the metastatic ability of CRC by stabilizing ICAM-1 via sialylation and demonstrated a correlation between CRC staging and the sialylation on soluble ICAM-1 in serum.

Distinct substrate specificities of human GlcNAc-6-sulfotransferases revealed by mass spectrometry-based sulfoglycomic analysis

Sulfated glycans are known to be involved in several glycan-mediated cell adhesion and recognition pathways. Our mRNA transcript analyses on the genes involved in synthesizing GlcNAc-6-O-sulfated glycans in human colon cancer tissues indicated that GlcNAc6ST-2 (CHST4) is preferentially expressed in cancer cells compared with nonmalignant epithelial cells among the three known major GlcNAc-6-O-sulfotransferases. On the contrary, GlcNAc6ST-3 (CHST5) was only expressed in nonmalignant epithelial cells, whereas GlcNAc6ST-1 (CHST2) was expressed equally in both cancerous and nonmalignant epithelial cells. These results suggest that 6-O-sulfated glycans that are synthesized only by GlcNAc6ST-2 may be highly colon cancer-specific, as supported by immunohistochemical staining of cancer cells using the MECA-79 antibody known to be relatively specific to the enzymatic reaction products of GlcNAc6ST-2. By more precise MS-based sulfoglycomic analyses, we sought to further infer the substrate specificities of GlcNAc6STs via a definitive mapping of various sulfo-glycotopes and O-glycan structures expressed in response to overexpression of transfected GlcNAc6STs in the SW480 colon cancer cell line. By detailed MS/MS sequencing, GlcNAc6ST-3 was shown to preferentially add sulfate onto core 2-based O-glycan structures, but it does not act on extended core 1 structures, whereas GlcNAc6ST-1 prefers core 2-based O-glycans to extended core 1 structures. In contrast, GlcNAc6ST-2 could efficiently add sulfate onto both extended core 1- and core 2-based O-glycans, leading to the production of unique sulfated extended core 1 structures such as R-GlcNAc(6-SO3-)β1-3Galβ1-4GlcNAc(6-SO3-)β1-3Galβ1-3GalNAcα, which are good candidates to be targeted as cancer-specific glycans.

CD22: A Regulator of Innate and Adaptive B Cell Responses and Autoimmunity

CD22 (Siglec 2) is a receptor predominantly restricted to B cells. It was initially characterized over 30 years ago and named “CD22” in 1984 at the 2nd International workshop in Boston (1). Several excellent reviews have detailed CD22 functions, CD22-regulated signaling pathways and B cell subsets regulated by CD22 or Siglec G (2–4). This review is an attempt to highlight recent and possibly forgotten findings. We also describe the role of CD22 in autoimmunity and the great potential for CD22-based immunotherapeutics for the treatment of autoimmune diseases such as systemic lupus erythematosus (SLE).

Distinctive and Complementary MS2 Fragmentation Characteristics for Identification of Sulfated Sialylated N-Glycopeptides by nanoLC-MS/MS Workflow

High sensitivity identification of sulfated glycans carried on specific sites of glycoproteins is an important requisite for investigation of molecular recognition events involved in diverse biological processes. However, aiming for resolving site-specific glycosylation of sulfated glycopeptides by direct LC-MS² sequencing is technically most challenging. Other than the usual limiting factors such as lower abundance and ionization efficiency compared to analysis of non-glycosylated peptides, confident identification of sulfated glycopeptides among the more abundant non-sulfated glycopeptides requires additional considerations in the selective enrichment and detection strategies. Metal oxide has been applied to enrich phosphopeptides and sialylated glycopeptides, but its use to capture sulfated glycopeptides has not been investigated. Likewise, various complementary MS² fragmentation modes have yet to be tested against sialylated and non-sialylated sulfoglycopeptides due to limited appropriate sample availability. In this study, we have investigated the feasibility of sequencing tryptic sulfated N-glycopeptide and its MS² fragmentation characteristics by first optimizing the enrichment methods to allow efficient LC-MS detection and MS² analysis by a combination of CID, HCD, ETD, and EThcD on hybrid and tribrid Orbitrap instruments. Characteristic sulfated glyco-oxonium ions and direct loss of sulfite from precursors were detected as evidences of sulfate modification. It is anticipated that the technical advances demonstrated in this study would allow a feasible extension of our sulfoglycomic analysis to sulfoglycoproteomics. Graphical Abstract ᅟ.

Biochemical, Cellular, Physiological, and Pathological Consequences of Human Loss of N-Glycolylneuraminic Acid

About 2-3 million years ago, Alu-mediated deletion of a critical exon in the CMAH gene became fixed in the hominin lineage ancestral to humans, possibly through a stepwise process of selection by pathogen targeting of the CMAH product (the sialic acid Neu5Gc), followed by reproductive isolation through female anti-Neu5Gc antibodies. Loss of CMAH has occurred independently in some other lineages, but is functionally intact in Old World primates, including our closest relatives, the chimpanzee. Although the biophysical and biochemical ramifications of losing tens of millions of Neu5Gc hydroxy groups at most cell surfaces remains poorly understood, we do know that there are multiscale effects functionally relevant to both sides of the host-pathogen interface. Hominin CMAH loss might also contribute to understanding human evolution, at the time when our ancestors were starting to use stone tools, increasing their consumption of meat, and possibly hunting. Comparisons with chimpanzees within ethical and practical limitations have revealed some consequences of human CMAH loss, but more has been learned by using a mouse model with a human-like Cmah inactivation. For example, such mice can develop antibodies against Neu5Gc that could affect inflammatory processes like cancer progression in the face of Neu5Gc metabolic incorporation from red meats, display a hyper-reactive immune system, a human-like tendency for delayed wound healing, late-onset hearing loss, insulin resistance, susceptibility to muscular dystrophy pathologies, and increased sensitivity to multiple human-adapted pathogens involving sialic acids. Further studies in such mice could provide a model for other human-specific processes and pathologies involving sialic acid biology that have yet to be explored.

Structural characterisation of high affinity Siglec-2 (CD22) ligands in complex with whole Burkitt's lymphoma (BL) Daudi cells by NMR spectroscopy

Siglec-2 undergoes constitutive endocytosis and is a drug target for autoimmune diseases and B cell-derived malignancies, including hairy cell leukaemia, marginal zone lymphoma, chronic lymphocytic leukaemia and non-Hodgkin's lymphoma (NHL). An alternative to current antibody-based therapies is the use of liposomal nanoparticles loaded with cytotoxic drugs and decorated with Siglec-2 ligands. We have recently designed the first Siglec-2 ligands (9-biphenylcarboxamido-4-meta-nitrophenyl-carboxamido-Neu5Acα2Me, 9-BPC-4-mNPC-Neu5Acα2Me) with simultaneous modifications at C-4 and C-9 position. In the current study we have used Saturation Transfer Difference (STD) NMR spectroscopy to monitor the binding of 9-BPC-4-mNPC-Neu5Acα2Me to Siglec-2 present on intact Burkitt's lymphoma Daudi cells. Pre-treatment of cells with periodate resulted in significantly higher STD NMR signal intensities for 9-BPC-4-mNPC-Neu5Acα2Me as the cells were more susceptible to ligand binding because cis-binding on the cell surface was removed. Quantification of STD NMR effects led to a cell-derived binding epitope of 9-BPC-4-mNPC-Neu5Acα2Me that facilitated the design and synthesis of C-2, C-3, C-4 and C-9 tetra-substituted Siglec-2 ligands showing an 88-fold higher affinity compared to 9-BPC-Neu5Acα2Me. This is the first time a NMR-based binding study of high affinity Siglec-2 (CD22) ligands in complex with whole Burkitt's lymphoma Daudi cells has been described that might open new avenues in developing tailored therapeutics and personalised medicine.

Sialic acids and autoimmune disease

An important underlying mechanism that contributes to autoimmunity is the loss of inhibitory signaling in the immune system. Sialic acid-recognizing Ig superfamily lectins or Siglecs are a family of cell surface proteins largely expressed in hematopoietic cells. The majority of Siglecs are inhibitory receptors expressed in immune cells that bind to sialic acid-containing ligands and recruit SH2-domain-containing tyrosine phosphatases to their cytoplasmic tails. They deliver inhibitory signals that can contribute to the constraining of immune cells, and thus protect the host from autoimmunity. The inhibitory functions of CD22/Siglec-2 and Siglec-G and their contributions to tolerance and autoimmunity, primarily in the B lymphocyte context, are considered in some detail in this review. The relevance to autoimmunity and unregulated inflammation of modified sialic acids, enzymes that modify sialic acid, and other sialic acid-binding proteins are also reviewed.

Leukocyte Trafficking to the Small Intestine and Colon

Leukocyte trafficking to the small and large intestines is tightly controlled to maintain intestinal immune homeostasis, mediate immune responses, and regulate inflammation. A wide array of chemoattractants, chemoattractant receptors, and adhesion molecules expressed by leukocytes, mucosal endothelium, epithelium, and stromal cells controls leukocyte recruitment and microenvironmental localization in intestine and in the gut-associated lymphoid tissues (GALTs). Naive lymphocytes traffic to the gut-draining mesenteric lymph nodes where they undergo antigen-induced activation and priming; these processes determine their memory/effector phenotypes and imprint them with the capacity to migrate via the lymph and blood to the intestines. Mechanisms of T-cell recruitment to GALT and of T cells and plasmablasts to the small intestine are well described. Recent advances include the discovery of an unexpected role for lectin CD22 as a B-cell homing receptor GALT, and identification of the orphan G-protein-coupled receptor 15 (GPR15) as a T-cell chemoattractant/trafficking receptor for the colon. GPR15 decorates distinct subsets of T cells in mice and humans, a difference in species that could affect translation of the results of mouse colitis models to humans. Clinical studies with antibodies to integrin α4β7 and its vascular ligand mucosal vascular addressin cell adhesion molecule 1 are proving the value of lymphocyte trafficking mechanisms as therapeutic targets for inflammatory bowel diseases. In contrast to lymphocytes, cells of the innate immune system express adhesion and chemoattractant receptors that allow them to migrate directly to effector tissue sites during inflammation. We review the mechanisms for innate and adaptive leukocyte localization to the intestinal tract and GALT, and discuss their relevance to human intestinal homeostasis and inflammation.

Unmasking of CD22 Co-receptor on Germinal Center B-cells Occurs by Alternative Mechanisms in Mouse and Man

CD22 is an inhibitory B-cell co-receptor whose function is modulated by sialic acid (Sia)-bearing glycan ligands. Glycan remodeling in the germinal center (GC) alters CD22 ligands, with as yet no ascribed biological consequence. Here, we show in both mice and humans that loss of high affinity ligands on GC B-cells unmasks the binding site of CD22 relative to naive and memory B-cells, promoting recognition of trans ligands. The conserved modulation of CD22 ligands on GC B-cells is striking because high affinity glycan ligands of CD22 are species-specific. In both species, the high affinity ligand is based on the sequence Siaα2-6Galβ1-4GlcNAc, which terminates N-glycans. The human ligand has N-acetylneuraminic acid (Neu5Ac) as the sialic acid, and the high affinity ligand on naive B-cells contains 6-O-sulfate on the GlcNAc. On human GC B-cells, this sulfate modification is lost, giving rise to lower affinity CD22 ligands. Ligands of CD22 on naive murine B-cells do not contain the 6-O-sulfate modification. Instead, the high affinity ligand for mouse CD22 has N-glycolylneuraminic acid (Neu5Gc) as the sialic acid, which is replaced on GC B-cells with Neu5Ac. Human naive and memory B-cells express sulfated glycans as high affinity CD22 ligands, which are lost on GC B-cells. In mice, Neu5Gc-containing glycans serve as high affinity CD22 ligands that are replaced by Neu5Ac-containing glycans on GC B-cells. Our results demonstrate that loss of high affinity CD22 ligands on GC B-cells occurs in both mice and humans through alternative mechanisms, unmasking CD22 relative to naive and memory B-cells.

Efficient Mapping of Sulfated Glycotopes by Negative Ion Mode nanoLC-MS/MS-Based Sulfoglycomic Analysis of Permethylated Glycans

We have previously developed the enabling techniques for sulfoglycomics based on mass spectrometry (MS) analysis of permethylated glycans, which preserves the attractive features of more reliable MS/MS sequencing compared with that performed on native glycans, while providing an easy way to separate and hence enrich the sulfated glycans. Unlike LC-MS/MS analysis of native glycans in negative ion mode that has been more widely in use, the characteristics and potential benefits of similar applications based on permethylated sulfated glycans have not been fully investigated. We report here the important features of reverse phase-based nanoLC-MS/MS analysis of permethylated sulfated glycans in negative ion mode and demonstrate that complementary sets of diagnostic fragment ions afforded can allow rapid identification of various fucosylated, sialylated, sulfated glycotopes and definitive determination of the location of sulfate in a way difficult to achieve by other means. A parallel acquisition of both higher collision energy and trap-based MS(2) coupled with a product dependent MS(3) is conceivably the most productive sulfoglycomic workflow currently possible and the manually curated fragmentation characteristics presented here will allow future developments in automating data analysis.

Significance of β-Galactoside α2,6 Sialyltranferase 1 in Cancers

Altered glycosylation is a common feature of cancer cells. It takes a variety of forms, which includes loss of expression or excessive expression of some structures, the accumulation of precursors, the appearance of novel structures, etc. Notably, these changes in glycan structure do not occur as a random consequence of disorder biology. Only a limited subset of oligosaccharides is found frequently enriched on the tumor cell surface and implicated in different tumor phenotypes. Among these, altered sialylation has long been associated with metastatic cell behaviors such as invasion and enhanced cell survival and accumulating evidence points to the alteration occurring in the sialic acid linkage to other sugars, which normally exists in three main configurations: α2,3, α2,6, and α2,8, catalyzed by a group of sialyltransferases. The aberrant expression of all three configurations has been described in cancer progression. However, the increased α2,6 sialylation catalyzed by β-galactoside α2,6 sialyltranferase 1 (ST6Gal I) is frequently observed in many types of the cancers. In this review, we describe the findings on the role of ST6Gal I in cancer progression, and highlight in particular the knowledge of how ST6Gal I-mediated α2,6 sialylated glycans or sialylated carrier proteins regulate cell signaling to promote the malignant phenotype of human carcinoma.

Siglec-mediated regulation of immune cell function in disease

All mammalian cells display a diverse array of glycan structures that differ from those that are found on microbial pathogens. Siglecs are a family of sialic acid-binding immunoglobulin-like receptors that participate in the discrimination between self and non-self, and that regulate the function of cells in the innate and adaptive immune systems through the recognition of their glycan ligands. In this Review, we describe the recent advances in our understanding of the roles of Siglecs in the regulation of immune cell function in infectious diseases, inflammation, neurodegeneration, autoimmune diseases and cancer.

CD22 and Siglec-G regulate inhibition of B-cell signaling by sialic acid ligand binding and control B-cell tolerance

CD22 and Siglec-G are two B-cell expressed members of the Siglec (sialic acid-binding immunoglobulin (Ig)-like lectin) family and are potent inhibitors of B-cell signaling. Genetic approaches have provided evidence that this inhibition of B-cell antigen receptor (BCR) signaling by Siglecs is dependent on ligand binding to sialic acids in specific linkages. The cis-ligand-binding activity of CD22 leads to homo-oligomer formation, which are to a large extent found in membrane domains that are distinct from those containing the BCR. In contrast, Siglec-G is recruited via sialic acid binding to the BCR. This interaction of Siglec-G with mIgM leads to an inhibitory function that seems to be specific for B-1 cells. Both CD22 and Siglec-G control B-cell tolerance and loss of these proteins, its ligands or its inhibitory pathways can increase the susceptibility for autoimmune diseases. CD22 is a target protein both in B-cell leukemias and lymphomas, as well as in B-cell mediated autoimmune diseases. Both antibodies and synthetic chemically modified sialic acids are currently tested to target Siglecs on B cells.

KSGal6ST generates galactose-6-O-sulfate in high endothelial venules but does not contribute to L-selectin-dependent lymphocyte homing

The addition of sulfate to glycan structures can regulate their ability to serve as ligands for glycan-binding proteins. Although sulfate groups present on the monosaccharides glucosamine, uronate, N-acetylglucosamine and N-acetylgalactosamine are recognized by defined receptors that mediate important functions, the functional significance of galactose-6-O-sulfate (Gal6S) is not known. However, in vitro studies using synthetic glycans and sulfotransferase overexpression implicate Gal6S as a binding determinant for the lymphocyte homing receptor, L-selectin. Only two sulfotransferases have been shown to generate Gal6S, namely keratan sulfate galactose 6-O-sulfotransferase (KSGal6ST) and chondroitin 6-O-sulfotransferase-1 (C6ST-1). In the present study, we use mice deficient in KSGal6ST and C6ST-1 to test whether Gal6S contributes to ligand recognition by L-selectin in vivo. First, we establish that KSGal6ST is selectively expressed in high endothelial venules (HEVs) in lymph nodes and Peyer's patches. We also determine by mass spectrometry that KSGal6ST generates Gal6S on several classes of O-glycans in peripheral lymph nodes. Furthermore, KSGal6ST, but not C6ST-1, is required for the generation of the Gal6S-containing glycan, 6,6'-disulfo-3'sLN (Siaα2→3[6S]Galβ1→4[6S]GlcNAc) or a closely related structure in lymph node HEVs. Nevertheless, L-selectin-dependent short-term homing of lymphocytes is normal in KSGal6ST-deficient mice, indicating that the Gal6S-containing structures we detected do not contribute to L-selectin ligand recognition in this setting. These results refine our understanding of the biological ligands for L-selectin and introduce a mouse model for investigating the functions of Gal6S in other contexts.

Siglec-8 as a drugable target to treat eosinophil and mast cell-associated conditions

Siglecs (sialic acid immunoglobulin-like lectins) are members of the immunoglobulin gene family that contain sialoside binding N-terminal domains. They are cell surface proteins found predominantly on cells of the immune system. Among them, Siglec-8 is uniquely expressed by human eosinophils and mast cells, as well as basophils. Engaging this structure with antibodies or glycan ligands results in apoptosis in human eosinophils and inhibition of release of preformed and newly generated mediators from human mast cells without affecting their survival. Pro-apoptotic effects are also seen when its closest functional paralog, Siglec-F, on mouse eosinophils is similarly engaged in vitro, and beneficial effects are observed after administration of Siglec-F antibody using models of eosinophilic pulmonary and gastrointestinal inflammation in vivo. Siglec-8 targeting may thus provide a means to specifically inhibit or deplete these cell types. Cell-directed therapies are increasingly sought after by the pharmaceutical industry for their potential to reduce side effects and increase safety. The challenge is to identify suitable targets on the cell type of interest, and selectively deliver a therapeutic agent. By targeting Siglec-8, monoclonal antibodies and glycan ligand-conjugated nanoparticles may be ideally suited for treatment of eosinophil and mast cell-related diseases, such as asthma, chronic rhinosinusitis, chronic urticaria, hypereosinophilic syndromes, mast cell and eosinophil malignancies and eosinophilic gastrointestinal disorders.

Induction of 6-sulfated glycans with cell adhesion activity via T-bet and GATA-3 in human helper T cells

BACKGROUND

Cell surface 6-sulfated glycans play important roles in various immunological events through cell-to-cell interactions. The 6-sulfation process is mediated by 6-sulfotransferase family isoenzymes. We previously demonstrated that GlcNAc6ST-1, one of the isoenzyme genes, is induced by GATA-3 and NF-κB in human helper T (Th) cells. However, transcriptional regulation of HEC-GlcNAc6ST, another isoenzyme important in Th cells, remains unclear.

METHODS

5'-RACE analysis, chromatin immunoprecipitation, and reporter assays were performed to reveal transcriptional regulation of HEC-GlcNAc6ST. RNA-knockdown and forced expression experiments were performed to demonstrate the contribution of HEC-GlcNAc6ST to the 6-sulfated glycan expression.

RESULTS

We identified potential binding sites of Sp1, T-bet, and GATA-3 in the HEC-GlcNAc6ST promoter. Reporter assays indicated that transfection of Sp1 enhanced the activity, whereas mithramycin A, an Sp1-specific inhibitor, repressed it. Transfection of T-bet increased the activity, which was inhibited by introducing a mutation into the potential T-bet binding site. GATA-3 alone could not elevate the activity, although co-transfection of protein kinase A, which is known to enhance IL-5 transcription in Th2 cells through phosphorylation of GATA-3, caused elevation. RNA-knockdown and forced expression of HEC-GlcNAc6ST in Jurkat cells down- and up-regulated α2,6-sialylated 6-sulfo N-acetyllactosamine, a preferential ligand for B-cell-specific CD22 antigen, respectively. From these results, we concluded that T-bet and GATA-3 as well as Sp1 control the expression of glycan with cell-adhesion activity by regulating HEC-GlcNAc6ST transcription in Th cells.

GENERAL SIGNIFICANCE

These results may provide a clue to biological regulation of Th-cell interaction with selectins and other carbohydrate-recognition molecules by T-bet and GATA-3.

Colonic epithelial cells express specific ligands for mucosal macrophage immunosuppressive receptors siglec-7 and -9

Immune cells are known to express specific recognition molecules for cell surface glycans. However, mechanisms involved in glycan-mediated cell-cell interactions in mucosal immunity have largely been left unaccounted for. We found that several glycans preferentially expressed in nonmalignant colonic epithelial cells serve as ligands for sialic acid-binding Ig-like lectins (siglecs), the immunosuppressive carbohydrate-recognition receptors carried by immune cells. The siglec ligand glycans in normal colonic epithelial cells included disialyl Lewis(a), which was found to have binding activity to both siglec-7 and -9, and sialyl 6-sulfo Lewis(x), which exhibited significant binding to siglec-7. Expression of these siglec-7/-9 ligands was impaired upon carcinogenesis, and they were replaced by cancer-associated glycans sialyl Lewis(a) and sialyl Lewis(x), which have no siglec ligand activity. When we characterized immune cells expressing siglecs in colonic lamina propriae by flow cytometry and confocal microscopy, the majority of colonic stromal immune cells expressing siglec-7/-9 turned out to be resident macrophages characterized by low expression of CD14/CD89 and high expression of CD68/CD163. A minor subpopulation of CD8(+) T lymphocytes also expressed siglec-7/-9. Siglec-7/-9 ligation suppressed LPS-induced cyclooxygenase-2 expression and PGE(2) production by macrophages. These results suggest that normal glycans of epithelial cells exert a suppressive effect on cyclooxygenase-2 expression by resident macrophages, thus maintaining immunological homeostasis in colonic mucosal membranes. Our results also imply that loss of immunosuppressive glycans by impaired glycosylation during colonic carcinogenesis enhances inflammatory mediator production.

Siglecs as sensors of self in innate and adaptive immune responses

Siglecs are expressed on most white blood cells of the immune system and are known to modulate the activity of cell signaling receptors via regulatory motifs in their cytoplasmic domains. This immunoglobulin subfamily of coreceptors recognize sialic acid containing glycans as ligands, which are found on glycoproteins and glycolipids of all mammalian cells. By virtue of their ability to recognize this common structural element, siglecs are increasingly recognized for their ability to help immune cells distinguish between self and nonself, and dampen autoimmune responses.

Detecting breakdown points in metabolic networks

BACKGROUND

A complex network of biochemical reactions present in an organism generates various biological moieties necessary for its survival. It is seen that biological systems are robust to genetic and environmental changes at all levels of organization. Functions of various organisms are sustained against mutational changes by using alternative pathways. It is also seen that if any one of the paths for production of the same metabolite is hampered, an alternate path tries to overcome this defect and helps in combating the damage.

METHODOLOGY

Certain physical, chemical or genetic change in any of the precursor substrate of a biochemical reaction may damage the production of the ultimate product. We employ a quantitative approach for simulating this phenomena of causing a physical change in the biochemical reactions by performing external perturbations to 12 metabolic pathways under carbohydrate metabolism in Saccharomyces cerevisae as well as 14 metabolic pathways under carbohydrate metabolism in Homo sapiens. Here, we investigate the relationship between structure and degree of compatibility of metabolites against external perturbations, i.e., robustness. Robustness can also be further used to identify the extent to which a metabolic pathway can resist a mutation event. Biological networks with a certain connectivity distribution may be very resilient to a particular attack but not to another. The goal of this work is to determine the exact boundary of network breakdown due to both random and targeted attack, thereby analyzing its robustness. We also find that compared to various non-standard models, metabolic networks are exceptionally robust. Here, we report the use of a 'Resilience-based' score for enumerating the concept of 'network-breakdown'. We also use this approach for analyzing metabolite essentiality providing insight into cellular robustness that can be further used for future drug development.

RESULTS

We have investigated the behavior of metabolic pathways under carbohydrate metabolism in S. cerevisae and H. sapiens against random and targeted attack. Both random as well as targeted resilience were calculated by formulating a measure, that we termed as 'Resilience score'. Datasets of metabolites were collected for 12 metabolic pathways belonging to carbohydrate metabolism in S. cerevisae and 14 metabolic pathways belonging to carbohydrate metabolism in H. sapiens from Kyoto Encyclopedia for Genes and Genomes (KEGG).

Glycan microarrays for decoding the glycome

In the last decade, glycan microarrays have revolutionized the analysis of the specificity of glycan-binding proteins (GBPs), providing information that simultaneously illuminates the biology mediated by them and decodes the informational content of the glycome. Numerous methods have emerged for arraying glycans in a "chip" format, and glycan libraries have been assembled that address the diversity of the human glycome. Such arrays have been successfully used for analysis of GBPs, which mediate mammalian biology, host-pathogen interactions, and immune recognition of glycans relevant to vaccine production and cancer antigens. This review covers the development of glycan microarrays and applications that have provided insights into the roles of mammalian and microbial GBPs.

Novel O-linked glycans containing 6'-sulfo-Gal/GalNAc of MUC1 secreted from human breast cancer YMB-S cells: possible carbohydrate epitopes of KL-6(MUC1) monoclonal antibody

Human serum Krebs von den Lugen-6 (KL-6) antigen is a MUC1 glycoprotein (KL-6/MUC1) recognized by anti-KL-6 monoclonal antibody (KL-6/mAb) and has been utilized as a diagnostic marker for interstitial pneumonia. KL-6/mAb is thought to recognize the specific glycopeptides sequence of MUC1, but the precise glycan structure of the epitope is unclear. In this study, we determined the carbohydrate structures of KL-6/MUC1 to search the carbohydrate epitopes for KL-6/mAb. KL-6/MUC1 was purified from the culture medium of human breast cancer YMB-S cells by KL-6/mAb-affinity chromatography; the O-linked glycan structures were determined in combination with paper electrophoresis, several lectin column chromatographies, sialidase digestion and methanolysis. KL-6/MUC1 contained core 1 and extended core 1 glycans modified with one or two sialic acid/sulfate residues. Based on these structures, several synthetic glycans binding to anti-KL-6/mAb were compared with one another by surface plasmon resonance. Sequentially, related radiolabeled oligosaccharides were enzymatically synthesized and analyzed for binding to a KL-6/mAb-conjugated affinity column. 3'-sialylated, 6'-sulfated LNnT [Neu5Acα2-3(SO(3)(-)-6)Galβ1-4GlcNAcβ1-3Galβ1-4Glc], 3'-sialylated, 6-sulfated core 1 [Neu5Acα2-3Galβ1-3(SO(3)(-)-6)GalNAc] and disulfated core 1 SO(3)(-)-3Galβ1-3(SO(3)(-)-6)GalNAc exhibited substantial affinity for KL-6/mAb, and 3'-sulfated core 1 derivatives [SO(3)(-)-3Galβ1-3(±Neu5Acα2-6)GalNAc] and 3'-sialylated core 1 weakly interacted with KL-6/mAb. These results indicated that the possible carbohydrate epitopes of KL-6/mAb involve not only 3'-sialylated core 1 but also novel core 1 and extended core 1 with sulfate and sialic acid residues. Epitope expressing changes with suppression or over-expression of the Gal6ST (Gal 6-O-sulfotransferase) gene, suggesting that Gal6ST is involved in the biosynthesis of the unique epitopes of KL-6/mAb.

The sialome--far more than the sum of its parts

The glycome is defined as the glycan repertoire of cells, tissues, and organisms, as found under specified conditions. The vastly diverse glycome is generated by a nontemplate driven biosynthesis, which is indirectly encoded in the genome, and very dynamic. Due to this overwhelming diversity, glycomic analysis must be approached at different hierarchical levels of complexity. In this review five such levels of complexity and the experimental approaches used for analysis at each level are discussed for a subclass of the glycome: the sialome. The sialome, in analogy to the canopy of a forest, covers the cell membrane with diverse array of complex sialylated structures. Sialome complexity includes modification of sialic acid core structure (the leaves and flowers), the linkage to the underlying sugar (the stems), the identity, and arrangement of the underlying glycans (the branches), the structural attributes of the underlying glycans (the trees), and finally, the spatial organization of the sialoglycans in relation to components of the intact cell surface (the forest). Understanding the full complexity of the sialome thus requires combined analyses at multiple levels, that is, the sialome is far more than the sum of its parts.

Epigenetic silencing of the sulfate transporter gene DTDST induces sialyl Lewisx expression and accelerates proliferation of colon cancer cells

Colon cancer cells express the carbohydrate determinant sialyl Lewis(x), while they exhibit markedly decreased the expression of its sulfated derivative, sialyl 6-sulfo Lewis(x). In contrast, normal colonic epithelial cells strongly express sialyl 6-sulfo Lewis(x), but they virtually do not express sialyl Lewis(x). Impaired sulfation was therefore suggested to occur during the course of malignant transformation of colonic epithelial cells and was assumed to be responsible for the increased sialyl Lewis(x) expression in cancers. To elucidate the molecular biological background of the impaired sulfation in cancers, we studied the expression levels of mRNA for 6-O-sulfotransferase isoenzymes, PAPS synthases and transporters, and a cell membrane sulfate transporter, DTDST, in cancer tissues. The most striking decrease in cancer cells compared with nonmalignant epithelial cells was noted in the transcription of the DTDST gene (P = 0.0000014; n = 20). Most cultured colon cancer cells had a diminished DTDST transcription, which was restored when cultured with histone deacetylase inhibitors. Suppression of DTDST transcription under the control of a tet-off inducible promoter resulted in increased sialyl Lewis(x) expression and reduced sialyl 6-sulfo Lewis(x) expression. Unexpectedly, the growth rate of the cancer cells was markedly enhanced when transcription of DTDST was suppressed. These results show that the decrease in the transcription of the sulfate transporter gene is the major cause of decreased expression of sialyl 6-sulfo Lewis(x) and increased expression of sialyl Lewis(x) in colon cancers. The results also suggest that the diminished DTDST expression is closely related to enhanced proliferation of cancer cells.

Anti-oligosaccharide antibodies as tools for studying sulfated sialoglycoconjugate ligands for siglecs and selectins

Several sulfated sialoglycoconjugates have recently been shown to serve as ligands for selectins and siglecs. For instance, alpha2-->3 sialylated 6-sulfo-Lewis x was found to serve as a ligand for selectins on skin-homing helper memory T cells, and alpha2-->6 sialylated 6-sulfo-LacNAc to be a preferred ligand for CD22/siglec-2 on human naïve B cells. Monoclonal antibodies specific to sulfated sialoglycoconjugates are effective tools to dissect these ligands on minor subsets of human leukocytes.

Advances in the biology and chemistry of sialic acids

Sialic acids are a subset of nonulosonic acids, which are nine-carbon alpha-keto aldonic acids. Natural existing sialic acid-containing structures are presented in different sialic acid forms, various sialyl linkages, and on diverse underlying glycans. They play important roles in biological, pathological, and immunological processes. Sialobiology has been a challenging and yet attractive research area. Recent advances in chemical and chemoenzymatic synthesis, as well as large-scale E. coli cell-based production, have provided a large library of sialoside standards and derivatives in amounts sufficient for structure-activity relationship studies. Sialoglycan microarrays provide an efficient platform for quick identification of preferred ligands for sialic acid-binding proteins. Future research on sialic acid will continue to be at the interface of chemistry and biology. Research efforts not only will lead to a better understanding of the biological and pathological importance of sialic acids and their diversity but also could lead to the development of therapeutics.

In situ trans ligands of CD22 identified by glycan-protein photocross-linking-enabled proteomics

CD22, a regulator of B-cell signaling, is a siglec that recognizes the sequence NeuAcalpha2-6Gal on glycoprotein glycans as ligands. CD22 interactions with glycoproteins on the same cell (in cis) and apposing cells (in trans) modulate its activity in B-cell receptor signaling. Although CD22 predominantly recognizes neighboring CD22 molecules as cis ligands on B-cells, little is known about the trans ligands on apposing cells. We conducted a proteomics scale study to identify candidate trans ligands of CD22 on B-cells by UV photocross-linking CD22-Fc chimera bound to B-cell glycoproteins engineered to carry sialic acids with a 9-aryl azide moiety. Using mass spectrometry-based quantitative proteomics to analyze the cross-linked products, 27 glycoproteins were identified as candidate trans ligands. Next, CD22 expressed on the surface of one cell was photocross-linked to glycoproteins on apposing B-cells followed by immunochemical analysis of the products with antibodies to the candidate ligands. Of the many candidate ligands, only the B-cell receptor IgM was found to be a major in situ trans ligand of CD22 that is selectively redistributed to the site of cell contact upon interaction with CD22 on the apposing cell.

A closer look into the GL7 antigen: its spatio-temporally selective differential expression and localization in lymphoid cells and organs in human

The GL7 epitope was originally described as part of a late lymphocyte activation antigen expressed in mouse and widely used since then as a marker of germinal center. Here we report on its differential expression by rat and human immune cells and lymphoid organs. Expression pattern of the GL7 epitope in rats is similar to that described earlier in mice, namely that GL7 antigen appears only on lymphocytes after 48h activation. In humans lymphocytes, but not the differentiated cells of myeloid origin, express this epitope. The GL7 epitope is up-regulated upon in vitro activation of primary T cells, while a slightly decreased expression is found on B lymphocytes. Fluorescent immunohistochemistry shows discrete location of GL7(hi) cells in human tonsil. GL7 antibody intensely stains CD19(+), IgD(+), IgM(low) B lymphocytes found at the margin of B cell follicles. The GL7 epitope is constitutively and highly raft-associated in human lymphoid cells. Strong neuraminidase- and partial papain-sensitivity of the GL7 epitope on human lymphocytes indicates a sialic acid-containing epitope linked either to one (or more) membrane protein(s) or to lipids. The lymphocyte-restricted GL7 epitope expression and the activation-dependent bi-directional change in the amount of the epitope suggest a functional role for GL7 epitope linked to carbohydrate-based immunoregulation.

Mass spectrometric analysis of sulfated N- and O-glycans

Sulfated N- and O-glycans carried on a myriad of cell-surface adhesion molecules and receptors are often not detected by current approaches in mass spectrometry (MS)-based glycomic mapping of cells and tissues. This is in part due to a natural lower abundance, compounded further by their negatively charged nature, which adversely disfavors their ionization and detection amid a sea of often much more abundant, nonsulfated, sialylated glycans. However, this particular limitation can actually be taken advantage of to effect highly selective enrichment and sensitive MS screening in negative ion mode, provided the ubiquitous sialic acids can first be neutralized. It has been demonstrated that permethylation not only fulfills this role adequately but further confers better MS/MS fragmentation characteristics for more efficient structural mapping and sequencing. Protocols and general practical considerations are described here which would enable one to readily prepare permethylated sulfated glycans, fractionate them away from the more abundant nonsulfated ones in simple steps for high-sensitivity MS analysis, and sensibly interpret the initial sulfoglycomic screening data thus obtained.

Multivalent ligands for siglecs

Siglecs have emerged as an important family of immunomodulatory glycan-binding proteins that can bind sialoside ligands both on the same cell surface, in cis, and on other cells, in trans. Expression of siglecs varies among a variety of immune cells, and tools to probe siglecs on these cells are crucial to understanding their function. In designing synthetic ligands, competition by cis ligands requires the use of multivalency to achieve sufficient avidity to stably bind siglecs on native cells. This chapter describes the use of multivalent ligands to probe cell surfaces, as well as to investigate ligand binding to recombinant siglecs.

Dual specificity of Langerin to sulfated and mannosylated glycans via a single C-type carbohydrate recognition domain

Langerin is categorized as a C-type lectin selectively expressed in Langerhans cells, playing roles in the first line of defense against pathogens and in Birbeck granule formation. Although these functions are thought to be exerted through glycan-binding activity of the C-type carbohydrate recognition domain, sugar-binding properties of Langerin have not been fully elucidated in relation to its biological functions. Here, we investigated the glycan-binding specificity of Langerin using comprehensive glycoconjugate microarray, quantitative frontal affinity chromatography, and conventional cell biological analyses. Langerin showed outstanding affinity to galactose-6-sulfated oligosaccharides, including keratan sulfate, while it preserved binding activity to mannose, as a common feature of the C-type lectins with an EPN motif. By a mutagenesis study, Lys-299 and Lys-313 were found to form extended binding sites for sulfated glycans. Consistent with the former observation, the sulfated Langerin ligands were found to be expressed in brain and spleen, where the transcript of keratan sulfate 6-O-sulfotransferase is expressed. Moreover, such sulfated ligands were up-regulated in glioblastoma relative to normal brain tissues, and Langerin-expressing cells were localized in malignant brain tissues. Langerin also recognized pathogenic fungi, such as Candida and Malassezia, expressing heavily mannosylated glycans. These observations provide strong evidence that Langerin mediates diverse functions on Langerhans cells through dual recognition of sulfated as well as mannosylated glycans by its uniquely evolved C-type carbohydrate-recognition domain.

The repertoire of glycan determinants in the human glycome

The number of glycan determinants that comprise the human glycome is not known. This uncertainty arises from limited knowledge of the total number of distinct glycans and glycan structures in the human glycome, as well as limited information about the glycan determinants recognized by glycan-binding proteins (GBPs), which include lectins, receptors, toxins, microbial adhesins, antibodies, and enzymes. Available evidence indicates that GBP binding sites may accommodate glycan determinants made up of 2 to 6 linear monosaccharides, together with their potential side chains containing other sugars and modifications, such as sulfation, phosphorylation, and acetylation. Glycosaminoglycans, including heparin and heparan sulfate, comprise repeating disaccharide motifs, where a linear sequence of 5 to 6 monosaccharides may be required for recognition. Based on our current knowledge of the composition of the glycome and the size of GBP binding sites, glycoproteins and glycolipids may contain approximately 3000 glycan determinants with an additional approximately 4000 theoretical pentasaccharide sequences in glycosaminoglycans. These numbers provide an achievable target for new chemical and/or enzymatic syntheses, and raise new challenges for defining the total glycome and the determinants recognized by GBPs.

Siglecs as targets for therapy in immune-cell-mediated disease

The sialic-acid-binding immunoglobulin-like lectins (siglecs) comprise a family of receptors that are differentially expressed on leukocytes and other immune cells. The restricted expression of several siglecs to one or a few cell types makes them attractive targets for cell-directed therapies. The anti-CD33 (also known as Siglec-3) antibody gemtuzumab (Mylotarg) is approved for the treatment of acute myeloid leukemia, and antibodies targeting CD22 (Siglec-2) are currently in clinical trials for treatment of B cell non-Hodgkins lymphomas and autoimmune diseases. Because siglecs are endocytic receptors, they are well suited for a 'Trojan horse' strategy, whereby therapeutic agents conjugated to an antibody, or multimeric glycan ligand, bind to the siglec and are efficiently carried into the cell. Although the rapid internalization of unmodified siglec antibodies reduces their utility for induction of antibody-dependent cellular cytotoxicity or complement-mediated cytotoxicity, antibody binding of Siglec-8, Siglec-9 and CD22 has been demonstrated to induce apoptosis of eosinophils, neutrophils and depletion of B cells, respectively. Here, we review the properties of siglecs that make them attractive for cell-targeted therapies.

Ligation of tumour-produced mucins to CD22 dramatically impairs splenic marginal zone B-cells

CD22 [Siglec-2 (sialic acid-binding, immunoglobulin-like lectin-2)], a negative regulator of B-cell signalling, binds to alpha2,6- sialic acid-linked glycoconjugates, including a sialyl-Tn antigen that is one of the typical tumour-associated carbohydrate antigens expressed on various mucins. Many epithelial tumours secrete mucins into tissues and/or the bloodstream. Mouse mammary adenocarcinoma cells, TA3-Ha, produce a mucin named epiglycanin, but a subline of them, TA3-St, does not. Epiglycanin binds to CD22 and inhibits B-cell signalling in vitro. The in vivo effect of mucins in the tumour-bearing state was investigated using these cell lines. It should be noted that splenic MZ (marginal zone) B-cells were dramatically reduced in the mice bearing TA3-Ha cells but not in those bearing TA3-St cells, this being consistent with the finding that the thymus-independent response was reduced in these mice. When the mucins were administered to normal mice, a portion of them was detected in the splenic MZ associated with the MZ B-cells. Furthermore, administration of mucins to normal mice clearly reduced the splenic MZ B-cells, similar to tumour-bearing mice. These results indicate that mucins in the bloodstream interacted with CD22, which led to impairment of the splenic MZ B-cells in the tumour-bearing state.