Ablation of CD22 in ligand-deficient mice restores B cell receptor signaling

Sialic acid in the regulation of blood cell production, differentiation and turnover

Sialic acid is a unique sugar moiety that resides in the distal and most accessible position of the glycans on mammalian cell surface and extracellular glycoproteins and glycolipids. The potential for sialic acid to obscure underlying structures has long been postulated, but the means by which such structural changes directly affect biological processes continues to be elucidated. Here, we appraise the growing body of literature detailing the importance of sialic acid for the generation, differentiation, function and death of haematopoietic cells. We conclude that sialylation is a critical post-translational modification utilized in haematopoiesis to meet the dynamic needs of the organism by enforcing rapid changes in availability of lineage-specific cell types. Though long thought to be generated only cell-autonomously within the intracellular ER-Golgi secretory apparatus, emerging data also demonstrate previously unexpected diversity in the mechanisms of sialylation. Emphasis is afforded to the mechanism of extrinsic sialylation, whereby extracellular enzymes remodel cell surface and extracellular glycans, supported by charged sugar donor molecules from activated platelets.

Siglec-15/sialic acid axis as a central glyco-immune checkpoint in breast cancer bone metastasis

Immunotherapy is a promising approach for treating metastatic breast cancer (MBC), offering new possibilities for therapy. While checkpoint inhibitors have shown great progress in the treatment of metastatic breast cancer, their effectiveness in patients with bone metastases has been disappointing. This lack of efficacy seems to be specific to the bone environment, which exhibits immunosuppressive features. In this study, we elucidate the multiple roles of the sialic acid-binding Ig-like lectin (Siglec)-15/sialic acid glyco-immune checkpoint axis in the bone metastatic niche and explore potential therapeutic strategies targeting this glyco-immune checkpoint. Our research reveals that elevated levels of Siglec-15 in the bone metastatic niche can promote tumor-induced osteoclastogenesis as well as suppress antigen-specific T cell responses. Next, we demonstrate that antibody blockade of the Siglec-15/sialic acid glyco-immune checkpoint axis can act as a potential treatment for breast cancer bone metastasis. By targeting this pathway, we not only aim to treat bone metastasis but also inhibit the spread of metastatic cancer cells from bone lesions to other organs.

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.

Hyper α2,6-Sialylation Promotes CD4+ T-Cell Activation and Induces the Occurrence of Ulcerative Colitis

α2,6-sialylation, catalyzed by α2,6-sialyltransferase (ST6GAL1), plays a pivotal role in immune responses. However, the role of ST6GAL1 in the pathogenesis of ulcerative colitis (UC) remains unknown. ST6GAL1 mRNA is highly expressed in UC tissues compared with the corresponding adjacent normal tissues, and α2,6-sialylation is significantly increased in the colon tissues of patients with UC. The expression of ST6GAL1 and proinflammatory cytokines, such as interleukin (IL)-2, IL-6, IL-17, and interferon-gamma, is also increased. The number of CD4+ T cells increases in UC patients. St6gal1 gene knockout (St6gal1-/- ) rats are established by clustered regularly interspaced short palindromic repeats (CRISPR)-associated gene knockout system. St6gal1 deficiency reduces the levels of pro-inflammatory cytokines and alleviates colitis symptoms in UC model rats. Ablation of α2,6-sialylation inhibits the transport of the TCR to lipid rafts and suppresses CD4+ T-cell activation. The attenuation of TCR signaling downregulates the expression of NF-κB in ST6GAL1-/- CD4+ T-cells. Moreover, NF-κB could bind to the ST6GAL1 promoter to increase its transcription. Ablation of ST6GAL1 downregulates the expression of NF-κB and reduces the production of proinflammatory cytokines to relieve UC pathogenesis, which is a potential novel target for the clinical treatment of UC.

Siglecs as potential targets of therapy in human mast cell- and/or eosinophil-associated diseases

Siglecs (sialic acid-binding immunoglobulin-like lectins) are a family of vertebrate glycan-binding cell-surface proteins. The majority mediate cellular inhibitory activity once engaged by specific ligands or ligand-mimicking molecules. As a result, Siglec engagement is now of interest as a strategy to therapeutically dampen unwanted cellular responses. When considering allergic inflammation, human eosinophils and mast cells express overlapping but distinct patterns of Siglecs. For example, Siglec-6 is selectively and prominently expressed on mast cells while Siglec-8 is highly specific for both eosinophils and mast cells. This review will focus on a subset of Siglecs and their various endogenous or synthetic sialoside ligands that regulate eosinophil and mast cell function and survival. It will also summarize how certain Siglecs have become the focus of novel therapies for allergic and other eosinophil- and mast cell-related diseases.

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.

Targeting N-linked Glycosylation for the Therapy of Aggressive Lymphomas

Diffuse large B-cell lymphoma (DLBCL) can be subdivided into the activated B-cell (ABC) and germinal center B cell-like (GCB) subtypes. Self-antigen engagement of B-cell receptors (BCR) in ABC tumors induces their clustering, thereby initiating chronic active signaling and activation of NF-κB and PI3 kinase. Constitutive BCR signaling is essential in some GCB tumors but primarily activates PI3 kinase. We devised genome-wide CRISPR-Cas9 screens to identify regulators of IRF4, a direct transcriptional target of NF-κB and an indicator of proximal BCR signaling in ABC DLBCL. Unexpectedly, inactivation of N-linked protein glycosylation by the oligosaccharyltransferase-B (OST-B) complex reduced IRF4 expression. OST-B inhibition of BCR glycosylation reduced BCR clustering and internalization while promoting its association with CD22, which attenuated PI3 kinase and NF-κB activation. By directly interfering with proximal BCR signaling, OST-B inactivation killed models of ABC and GCB DLBCL, supporting the development of selective OST-B inhibitors for the treatment of these aggressive cancers.

SIGNIFICANCE

DLBCL depends on constitutive BCR activation and signaling. There are currently no therapeutics that target the BCR directly and attenuate its pathologic signaling. Here, we unraveled a therapeutically exploitable, OST-B-dependent glycosylation pathway that drives BCR organization and proximal BCR signaling. This article is highlighted in the In This Issue feature, p. 1749.

The role of N-glycosylation in B-cell biology and IgG activity. The aspects of autoimmunity and anti-inflammatory therapy

The immune system is strictly regulated by glycosylation through the addition of highly diverse and dynamically changing sugar structures (glycans) to the majority of immune cell receptors. Although knowledge in the field of glycoimmunology is still limited, numerous studies point to the key role of glycosylation in maintaining homeostasis, but also in reflecting its disruption. Changes in oligosaccharide patterns can lead to impairment of both innate and acquired immune responses, with important implications in the pathogenesis of diseases, including autoimmunity. B cells appear to be unique within the immune system, since they exhibit both innate and adaptive immune activity. B cell surface is rich in glycosylated proteins and lectins which recognise glycosylated ligands on other cells. Glycans are important in the development, selection, and maturation of B cells. Changes in sialylation and fucosylation of cell surface proteins affect B cell signal transduction through BCRs, CD22 inhibitory coreceptor and Siglec-G. Plasmocytes, as the final stage of B cell differentiation, produce and secrete immunoglobulins (Igs), of which IgGs are the most abundant N-glycosylated proteins in human serum with the conserved N-glycosylation site at Asn297. N-oligosaccharide composition of the IgG Fc region affects its secretion, structure, half-life and effector functions (ADCC, CDC). IgG N-glycosylation undergoes little change during homeostasis, and may gradually be modified with age and during ongoing inflammatory processes. Hyperactivated B lymphocytes secrete autoreactive antibodies responsible for the development of autoimmunity. The altered profile of IgG N-glycans contributes to disease progression and remission and is sensitive to the application of therapeutic substances and immunosuppressive agents. In this review, we focus on the role of N-glycans in B-cell biology and IgG activity, the rearrangement of IgG oligosaccharides in aging, autoimmunity and immunosuppressive therapy.

Siglec receptors as new immune checkpoints in cancer

Cancer immunotherapy in the form of immune checkpoint inhibitors and cellular therapies has improved the treatment and prognosis of many patients. Nevertheless, most cancers are still resistant to currently approved cancer immunotherapies. New approaches and rational combinations are needed to overcome these resistances. There is emerging evidence that Siglec receptors could be regarded as new immune checkpoints and targets for cancer immunotherapy. In this review, we summarize the experimental evidence supporting Siglec receptors as new immune checkpoints in cancer and discuss their mechanisms of action, as well as current efforts to target Siglec receptors and their interactions with sialoglycan Siglec-ligands.

High-Throughput Analysis Reveals miRNA Upregulating α-2,6-Sialic Acid through Direct miRNA-mRNA Interactions

Chemical biology has revealed the importance of sialic acids as a major signal in physiology and disease. The terminal modification α-2,6-sialic acid is controlled by the enzymes ST6GAL1 and ST6GAL2. Dysregulation of this glycan impacts immunological recognition and cancer development. microRNAs (miRNA, miR), noncoding RNAs that downregulate protein expression, are important regulators of glycosylation. Using our recently developed high-throughput fluorescence assay (miRFluR), we comprehensively mapped the miRNA regulatory landscape of α-2,6-sialyltransferases ST6GAL1 and ST6GAL2. We found, contrary to expectations, the majority of miRNAs upregulate ST6GAL1 and α-2,6-sialylation in a variety of cancer cells. In contrast, miRNAs that regulate ST6GAL2 were predominantly downregulatory. Mutational analysis identified direct binding sites in the 3'-untranslated region (UTR) responsible for upregulation, confirming it is a direct effect. The miRNA binding proteins AGO2 and FXR1 were required for upregulation. Our results upend common assumptions surrounding miRNA, arguing that upregulation by these noncoding RNA is common. Indeed, for some proteins, upregulation may be the dominant function of miRNA. Our work also suggests that upregulatory miRNAs enhance overexpression of ST6GAL1 and α-2,6-sialylation, providing another potential pathway to explain the dysregulation observed in cancer and other disease states.

Sialic acid and food allergies: The link between nutrition and immunology

Food allergies (FA), a major public health problem recognized by the World Health Organization, affect an estimated 3%-10% of adults and 8% of children worldwide. However, effective treatments for FA are still lacking. Recent advances in glycoimmunology have demonstrated the great potential of sialic acids (SAs) in the treatment of FA. SAs are a group of nine-carbon α-ketoacids usually linked to glycoproteins and glycolipids as terminal glycans. They play an essential role in modulating immune responses and may be an effective target for FA intervention. As exogenous food components, sialylated polysaccharides have anti-FA effects. In contrast, as endogenous components, SAs on immunoglobulin E and immune cell surfaces contribute to the pathogenesis of FA. Given the lack of comprehensive information on the effects of SAs on FA, we reviewed the roles of endogenous and exogenous SAs in the pathogenesis and treatment of FA. In addition, we considered the structure-function relationship of SAs to provide a theoretical basis for the development of SA-based FA treatments.

Characterization of sialylation-related long noncoding RNAs to develop a novel signature for predicting prognosis, immune landscape, and chemotherapy response in colorectal cancer

Aberrant sialylation plays a key biological role in tumorigenesis and metastasis, including tumor cell survival and invasion, immune evasion, angiogenesis, and resistance to therapy. It has been proposed as a possible cancer biomarker and a potential therapeutic target of tumors. Nevertheless, the prognostic significance and biological features of sialylation-related long noncoding RNAs (lncRNAs) in colorectal cancer (CRC) remain unclear. This study aimed to develop a novel sialylation-related lncRNA signature to accurately evaluate the prognosis of patients with CRC and explore the potential molecular mechanisms of the sialylation-related lncRNAs. Here, we identified sialylation-related lncRNAs using the Pearson correlation analysis on The Cancer Genome Atlas (TCGA) dataset. Univariate and stepwise multivariable Cox analysis were used to establish a signature based on seven sialylation-related lncRNAs in the TCGA dataset, and the risk model was validated in the Gene Expression Omnibus dataset. Kaplan-Meier curve analysis revealed that CRC patients in the low-risk subgroup had a better survival outcome than those in the high-risk subgroup in the training set, testing set, and overall set. Multivariate analysis demonstrated that the sialylation-related lncRNA signature was an independent prognostic factor for overall survival, progression-free survival, and disease-specific survival prediction. The sialylation lncRNA signature-based nomogram exhibited a robust prognostic performance. Furthermore, enrichment analysis showed that cancer hallmarks and oncogenic signaling were enriched in the high-risk group, while inflammatory responses and immune-related pathways were enriched in the low-risk group. The comprehensive analysis suggested that low-risk patients had higher activity of immune response pathways, greater immune cell infiltration, and higher expression of immune stimulators. In addition, we determined the sialylation level in normal colonic cells and CRC cell lines by flow cytometry combined with immunofluorescence, and verified the expression levels of seven lncRNAs using real-time quantitative polymerase chain reaction. Finally, combined drug sensitivity analysis using the Genomics of Drug Sensitivity in Cancer, Cancer Therapeutics Response Portal, and Profiling Relative Inhibition Simultaneously in Mixtures indicated that the sialylation-related lncRNA signature could serve as a potential predictor for chemosensitivity. Collectively, this is the first sialylation lncRNA-based signature for predicting the prognosis, immune landscape, and chemotherapeutic response in CRC, and may provide vital guidance to facilitate risk stratification and optimize individualized therapy for CRC patients.

NEU1 and NEU3 enzymes alter CD22 organization on B cells

The B cell membrane expresses sialic-acid-binding immunoglobulin-like lectins, also called Siglecs, that are important for modulating immune response. Siglecs have interactions with sialoglycoproteins found on the same membrane (cis-ligands) that result in homotypic and heterotypic receptor clusters. The regulation and organization of these clusters, and their effect on cell activation, is not clearly understood. We investigated the role of human neuraminidase enzymes NEU1 and NEU3 on the clustering of CD22 on B cells using confocal microscopy. We observed that native NEU1 and NEU3 activity influence the cluster size of CD22. Using single-particle tracking, we observed that NEU3 activity increased the lateral mobility of CD22, which was in contrast to the effect of exogenous bacterial NEU enzymes. Moreover, we show that native NEU1 and NEU3 activity influenced cellular Ca²⁺ levels, supporting a role for these enzymes in regulating B cell activation. Our results establish a role for native NEU activity in modulating CD22 organization and function on B cells.

Targeting Human CD22/Siglec-2 with Dimeric Sialosides as Novel Oligosaccharide Mimetics

Significant interest in the development of high-affinity ligands for Siglecs exists due to the various therapeutically relevant functions of these proteins. Here, we report a new strategy to develop and design Siglec ligands as disialyl-oligosaccharide mimetics exemplified on Siglec-2 (CD22). We report insights into development of dimeric ligands with high affinity and avidity to cell surface-expressed CD22, assay development, tool compounds, structure activity relationships, and biological data on calcium flux regulation in B-cells. The binding modes of selected ligands have been modeled based on state-of-the-art molecular dynamics simulations on the microsecond timescale, providing detailed views on ligand binding and opening a new perspective on drug design efforts for Siglecs. High-avidity dimeric ligands containing a linker opening the way towards bispecifics are presented as well.

N-Glycosylation and Inflammation; the Not-So-Sweet Relation

Chronic inflammation is the main feature of many long-term inflammatory diseases such as autoimmune diseases, metabolic disorders, and cancer. There is a growing number of studies in which alterations of N-glycosylation have been observed in many pathophysiological conditions, yet studies of the underlying mechanisms that precede N-glycome changes are still sparse. Proinflammatory cytokines have been shown to alter the substrate synthesis pathways as well as the expression of glycosyltransferases required for the biosynthesis of N-glycans. The resulting N-glycosylation changes can further contribute to disease pathogenesis through modulation of various aspects of immune cell processes, including those relevant to pathogen recognition and fine-tuning the inflammatory response. This review summarizes our current knowledge of inflammation-induced N-glycosylation changes, with a particular focus on specific subsets of immune cells of innate and adaptive immunity and how these changes affect their effector functions, cell interactions, and signal transduction.

Sialic acids on B cells are crucial for their survival and provide protection against apoptosis

Sialic acids (Sias) on the B cell membrane are involved in cell migration, in the control of the complement system and, as sialic acid-binding immunoglobulin-like lectin (Siglec) ligands, in the regulation of cellular signaling. We studied the role of sialoglycans on B cells in a mouse model with B cell-specific deletion of cytidine monophosphate sialic acid synthase (CMAS), the enzyme essential for the synthesis of sialoglycans. Surprisingly, these mice showed a severe B cell deficiency in secondary lymphoid organs. Additional depletion of the complement factor C3 rescued the phenotype only marginally, demonstrating a complement-independent mechanism. The B cell survival receptor BAFF receptor was not up-regulated, and levels of activated caspase 3 and processed caspase 8 were high in B cells of Cmas-deficient mice, indicating ongoing apoptosis. Overexpressed Bcl-2 could not rescue this phenotype, pointing to extrinsic apoptosis. These results show that sialoglycans on the B cell surface are crucial for B cell survival by counteracting several death-inducing pathways.

Suppressing Immune Responses Using Siglec Ligand-Decorated Anti-receptor Antibodies

The sialic acid-binding immunoglobulin-type lectins (Siglecs) are expressed predominantly on white blood cells and participate in immune cell recognition of self. Most Siglecs contain cytoplasmic inhibitory immunoreceptor tyrosine-based inhibitory motifs characteristic of inhibitory checkpoint co-receptors that suppress cell signaling when they are recruited to the immunological synapse of an activating receptor. Antibodies to activatory receptors typically activate immune cells by ligating the receptors on the cell surface. Here, we report that the conjugation of high affinity ligands of Siglecs to antibodies targeting activatory immune receptors can suppress receptor-mediated activation of immune cells. Indeed, B-cell activation by antibodies to the B-cell receptor IgD is dramatically suppressed by conjugation of anti-IgD with high affinity ligands of a B-cell Siglec CD22/Siglec-2. Similarly, degranulation of mast cells induced by antibodies to IgE, which ligate the IgE/FcεR1 receptor complex, is suppressed by conjugation of anti-IgE to high affinity ligands of a mast cell Siglec, CD33/Siglec-3 (CD33L). Moreover, the anti-IgE-CD33L suppresses anti-IgE-mediated systemic anaphylaxis of sensitized humanized mice and prevents anaphylaxis upon subsequent challenge with anti-IgE. The results demonstrate that attachment of ligands of inhibitory Siglecs to anti-receptor antibodies can suppress the activation of immune cells and modulate unwanted immune responses.

Multifunctional, Multivalent PIC Polymer Scaffolds for Targeting Antigen-Specific, Autoreactive B Cells

Multivalent scaffolds that carry multiple molecules with immunophenotyping or immunomodulatory properties are invaluable tools for studying and modulating specific functions of human immune responses. So far, streptavidin–biotin-based tetramers have been widely used for B-cell immunophenotyping purposes. However, the utility of these tetramers is limited by their tetravalency, the inherent immunogenicity of streptavidin (a bacterial protein that can potentially be recognized by B cells), and the limited feasibility to functionalize these reagents. This has rendered tetramers suboptimal for studying rare, in particular, antigen-specific B-cell populations in the context of clinical applications. Here, we used polyisocyanopeptides (PICs), multivalent polymeric scaffolds functionalized with around 50 peptide antigens, to detect autoreactive B cells in the peripheral blood of patients with rheumatoid arthritis. To explore the potential immunomodulatory functionalities, we functionalized PICs with autoantigenic peptides and a trisaccharide CD22 ligand to inhibit autoreactive B-cell activation through interference with the B-cell receptor activation pathway, as evidenced by reduced phospho-Syk expression upon PIC binding. Given the possibilities to functionalize PICs, our data demonstrate that the modular and versatile character of PIC scaffolds makes them promising candidates for future clinical applications in B-cell-mediated diseases.

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.

The inhibitory coreceptor CD22 restores B cell signaling by developmentally regulating Cd45-/- immunodeficient B cells

The protein tyrosine phosphatase CD45 plays a crucial role in B cell antigen receptor (BCR) signaling by activating Src family kinases. Cd45^-/- mice show altered B cell development and a phenotype likely due to reduced steady-state signaling; however, Cd45^-/- B cells show relatively normal BCR ligation-induced signaling. In our investigation of how BCR signaling was restored in Cd45^-/- cells, we found that the coreceptor CD22 switched from an inhibitory to a stimulatory function in these cells. We disrupted the ability of CD22 to interact with its ligands in Cd45^-/- B cells by generating Cd45^-/-St6galI^-/- mice, which cannot synthesize the glycan ligand of CD22, or by treating Cd45^-/- B cells in vitro with the sialoside GSC718, which inhibits ligand binding to CD22. BCR ligation-induced signaling was reduced by ST6GalI deficiency, but not by GSC718 treatment, suggesting that CD22 restored BCR ligation-induced signaling in Cd45^-/- mature B cells by altering cellular phenotypes during development. CD22 was required for the increase in the surface amount of IgM-BCR on Cd45^-/- B cells, which augmented signaling. Because B cell survival depends on steady-state BCR signaling, IgM-BCR abundance was likely increased by the selective survival of IgM-BCR^hi Cd45^-/- B cells because of CD22-mediated signaling under conditions of substantially reduced steady-state signaling. Because the amount of surface IgM-BCR is increased on B cells from patients with other BCR signaling deficiencies, including X-linked agammaglobulinemia, our findings suggest that CD22 may contribute to the partial restoration of B cell function in these patients.

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.

Modulation of Siglec-7 Signaling Via In Situ-Created High-Affinity cis-Ligands

Sialic acid-binding immunoglobulin-like lectins, also known as Siglecs, have recently been designated as glyco-immune checkpoints. Through their interactions with sialylated glycan ligands overexpressed on tumor cells, inhibitory Siglecs on innate and adaptive immune cells modulate signaling cascades to restrain anti-tumor immune responses. However, the elucidation of the mechanisms underlying these processes is just beginning. We find that when human natural killer (NK) cells attack tumor cells, glycan remodeling occurs on the target cells at the immunological synapse. This remodeling occurs through both the transfer of sialylated glycans from NK cells to target tumor cells and the accumulation of de novo synthesized sialosides on the tumor cells. The functionalization of NK cells with a high-affinity ligand of Siglec-7 leads to multifaceted consequences in modulating a Siglec-7-regulated NK-activation. At high levels of ligand, an enzymatically added Siglec-7 ligand suppresses NK cytotoxicity through the recruitment of Siglec-7 to an immune synapse, whereas at low levels of ligand an enzymatically added Siglec-7 ligand triggers the release of Siglec-7 from the cell surface into the culture medium, preventing a Siglec-7-mediated inhibition of NK cytotoxicity. These results suggest that a glycan engineering of NK cells may provide a means to boost NK effector functions for related applications.

Siglec Ligands

A dense and diverse array of glycans on glycoproteins and glycolipids decorate all cell surfaces. In vertebrates, many of these carry sialic acid, in a variety of linkages and glycan contexts, as their outermost sugar moiety. Among their functions, glycans engage complementary glycan binding proteins (lectins) to regulate cell physiology. Among the glycan binding proteins are the Siglecs, sialic acid binding immunoglobulin-like lectins. In humans, there are 14 Siglecs, most of which are expressed on overlapping subsets of immune system cells. Each Siglec engages distinct, endogenous sialylated glycans that initiate signaling programs and regulate cellular responses. Here, we explore the emerging science of Siglec ligands, including endogenous sialoglycoproteins and glycolipids and synthetic sialomimetics. Knowledge in this field promises to reveal new molecular pathways controlling cell physiology and new opportunities for therapeutic intervention.

The Protein Tyrosine Phosphatase SHP-1 (PTPN6) but Not CD45 (PTPRC) Is Essential for the Ligand-Mediated Regulation of CD22 in BCR-Ligated B Cells

CD22 is an inhibitory B cell coreceptor that regulates B cell development and activation by downregulating BCR signaling through activation of SH2-containing protein tyrosine phosphatase-1 (SHP-1). CD22 recognizes α2,6 sialic acid as a specific ligand and interacts with α2,6 sialic acid-containing membrane molecules, such as CD45, IgM, and CD22, expressed on the same cell. Functional regulation of CD22 by these endogenous ligands enhances BCR ligation-induced signaling and is essential for normal B cell responses to Ags. In this study, we demonstrate that CD45 plays a crucial role in CD22-mediated inhibition of BCR ligation-induced signaling. However, disruption of ligand binding of CD22 enhances CD22 phosphorylation, a process required for CD22-mediated signal inhibition, upon BCR ligation in CD45^-/- as well as wild-type mouse B cells but not in mouse B cells expressing a loss-of-function mutant of SHP-1. This result indicates that SHP-1 but not CD45 is required for ligand-mediated regulation of CD22. We further demonstrate that CD22 is a substrate of SHP-1, suggesting that SHP-1 recruited to CD22 dephosphorylates nearby CD22 as well as other substrates. CD22 dephosphorylation by SHP-1 appears to be augmented by homotypic CD22 clustering mediated by recognition of CD22 as a ligand of CD22 because CD22 clustering increases the number of nearby CD22. Our results suggest that CD22 but not CD45 is an endogenous ligand of CD22 that enhances BCR ligation-induced signaling through SHP-1-mediated dephosphorylation of CD22 in CD22 clusters.

Probing the binding specificities of human Siglecs by cell-based glycan arrays

Siglecs are a family of sialic acid-binding receptors expressed by cells of the immune system and a few other cell types capable of modulating immune cell functions upon recognition of sialoglycan ligands. While human Siglecs primarily bind to sialic acid residues on diverse types of glycoproteins and glycolipids that constitute the sialome, their fine binding specificities for elaborated complex glycan structures and the contribution of the glycoconjugate and protein context for recognition of sialoglycans at the cell surface are not fully elucidated. Here, we generated a library of isogenic human HEK293 cells with combinatorial loss/gain of individual sialyltransferase genes and the introduction of sulfotransferases for display of the human sialome and to dissect Siglec interactions in the natural context of glycoconjugates at the cell surface. We found that Siglec-4/7/15 all have distinct binding preferences for sialylated GalNAc-type O-glycans but exhibit selectivity for patterns of O-glycans as presented on distinct protein sequences. We discovered that the sulfotransferase CHST1 drives sialoglycan binding of Siglec-3/8/7/15 and that sulfation can impact the preferences for binding to O-glycan patterns. In particular, the branched Neu5Acα2-3(6-O-sulfo)Galβ1-4GlcNAc (6'-Su-SLacNAc) epitope was discovered as the binding epitope for Siglec-3 (CD33) implicated in late-onset Alzheimer's disease. The cell-based display of the human sialome provides a versatile discovery platform that enables dissection of the genetic and biosynthetic basis for the Siglec glycan interactome and other sialic acid-binding proteins.

The clinical impact of glycobiology: targeting selectins, Siglecs and mammalian glycans

Carbohydrates - namely glycans - decorate every cell in the human body and most secreted proteins. Advances in genomics, glycoproteomics and tools from chemical biology have made glycobiology more tractable and understandable. Dysregulated glycosylation plays a major role in disease processes from immune evasion to cognition, sparking research that aims to target glycans for therapeutic benefit. The field is now poised for a boom in drug development. As a harbinger of this activity, glycobiology has already produced several drugs that have improved human health or are currently being translated to the clinic. Focusing on three areas - selectins, Siglecs and glycan-targeted antibodies - this Review aims to tell the stories behind therapies inspired by glycans and to outline how the lessons learned from these approaches are paving the way for future glycobiology-focused therapeutics.

Targeting glyco-immune checkpoints for cancer therapy

Introduction: Targeting immune checkpoints with antibodies has significantly improved the outcome of cancer patients, but only few patients have long-term benefits from currently used PD-1/PD-L1 and CTLA-4 inhibitors. New approaches are needed to increase the number of patients going into long-term remission after cancer immunotherapy. Glyco-immune checkpoints are new targets for cancer immunotherapy. They are defined as immune-modulatory pathways including interactions of glycans with glycan-binding proteins or lectins. The most prominent pathway is the sialoglycan-Siglec axis and inhibitors of this axis are already successfully tested in early clinical trials.Area covered: Here, we summarize the current knowledge on glyco-immune checkpoints with a focus on the sialoglycan-Siglec axis. We also provide an overview on current approaches to clinically target glyco-immune checkpoints and give an outlook for the further clinical development of glyco-immune checkpoint targeting agents.Expert opinion: Glyco-immune checkpoints are interesting new targets to improve cancer immunotherapy. Antibodies targeting the sialoglycan-Siglec axis are already in clinical development. Other approaches with higher risk of toxicity including tumor-targeted sialidases are in late stage pre-clinical development. Despite the challenges, targeting of glyco-immune checkpoints could lead to the development of a new class of drugs providing improved anti-cancer immunity and eventually benefit cancer patients.

N-Glycan Branching Decouples B Cell Innate and Adaptive Immunity to Control Inflammatory Demyelination

B cell depletion potently reduces episodes of inflammatory demyelination in multiple sclerosis (MS), predominantly through loss of innate rather than adaptive immunity. However, molecular mechanisms controlling innate versus adaptive B cell function are poorly understood. N-glycan branching, via interactions with galectins, controls endocytosis and signaling of cell surface receptors to control cell function. Here we report that N-glycan branching in B cells dose dependently reduces pro-inflammatory innate responses by titrating decreases in Toll-like receptor-4 (TLR4) and TLR2 surface expression via endocytosis. In contrast, a minimal level of N-glycan branching maximizes surface retention of the B cell receptor (BCR) and the CD19 co-receptor to promote adaptive immunity. Branched N-glycans inhibit antigen presentation by B cells to reduce T helper cell-17 (TH17)/TH1 differentiation and inflammatory demyelination in mice. Thus, N-glycan branching negatively regulates B cell innate function while promoting/maintaining adaptive immunity via BCR, providing an attractive therapeutic target for MS.

ST6GAL1 polymorphisms influence susceptibility and progression of IgA nephropathy in a Chinese Han population

BACKGROUND

ST6GAL1 has been identified as a novel susceptibility gene for IgA nephropathy (IgAN) in a previous genome-wide association study. The present study is aimed at exploring whether the genetic polymorphisms of ST6GAL1 gene correlate with IgAN susceptibility, clinical phenotypes and progression in a Chinese Han population.

METHODS

Twenty-six single nucleotide polymorphisms (SNPs) of ST6GAL1 were genotyped in 1000 biopsy-proven IgAN patients and 1000 control subjects of Chinese Han population using Sequenom MassARRAY iPLEX. A logistic regression analysis with age and sex as covariates was performed to evaluate the effects of ST6GAL1 gene polymorphisms on IgAN susceptibility. Kaplan-Meier method and Cox proportional hazard models were applied to analyze the associations between genetic variants and renal survival.

RESULTS

We found that rs7634389 (OR = 1.24, 95 % CI = 1.02-1.50, p_dominant = 0.034) and rs6784233 (OR = 1.23, 95 % CI = 1.05-1.45, p_additive = 0.013; OR = 1.28, 95 % CI = 1.05-1.55, p_dominant = 0.014) were associated with susceptibility of IgAN. In addition, rs7634389 was correlated with hyperuricemia (OR = 1.27, p =  0.012) and segmental glomerulosclerosis (OR = 1.21, p = 0.047) in IgAN patients. Furthermore, rs7634389 was independently associated with renal survival after adjustments for multiple confounders (hazard ratio [HR] = 0.51, 95 % CI = 0.33-0.78, p =  0.002). Haplotype analysis for ST6GAL1 polymorphisms confirmed their associations with the susceptibility to IgAN.

CONCLUSIONS

Our research suggested that ST6GAL1 gene polymorphisms were implicated in IgAN susceptibility and clinical outcome in a Chinese Han population.

Sialylation of immunoglobulin E is a determinant of allergic pathogenicity

Approximately one-third of the world's population suffers from allergies¹. Exposure to allergens crosslinks immunoglobulin E (IgE) antibodies that are bound to mast cells and basophils, triggering the release of inflammatory mediators, including histamine². Although IgE is absolutely required for allergies, it is not understood why total and allergen-specific IgE concentrations do not reproducibly correlate with allergic disease^3-5. It is well-established that glycosylation of IgG dictates its effector function and has disease-specific patterns. However, whether IgE glycans differ in disease states or affect biological activity is completely unknown⁶. Here we perform an unbiased examination of glycosylation patterns of total IgE from individuals with a peanut allergy and from non-atopic individuals without allergies. Our analysis reveals an increase in sialic acid content on total IgE from individuals with a peanut allergy compared with non-atopic individuals. Removal of sialic acid from IgE attenuates effector-cell degranulation and anaphylaxis in several functional models of allergic disease. Therapeutic interventions-including removing sialic acid from cell-bound IgE with a neuraminidase enzyme targeted towards the IgE receptor FcεRI, and administering asialylated IgE-markedly reduce anaphylaxis. Together, these results establish IgE glycosylation, and specifically sialylation, as an important regulator of allergic disease.

Blood-Borne ST6GAL1 Regulates Immunoglobulin Production in B Cells

Humoral immunity is an effective but metabolically expensive defense mechanism. It is unclear whether systemic cues exist to communicate the dynamic need for antigen presentation and immunoglobulin production. Here, we report a novel role for the liver-produced, acute phase reactant ST6GAL1 in IgG production. B cell expression of ST6GAL1, a sialyltransferase mediating the attachment of α2,6-linked sialic acids on N-glycans, is classically implicated in the dysregulated B cell development and immunoglobulin levels of St6gal1-deficient mice. However, the blood-borne pool of ST6GAL1, upregulated during systemic inflammation, can also extrinsically modify leukocyte cell surfaces. We show that B cell independent, extracellular ST6GAL1 enhances B cell IgG production and increases blood IgG titers. B cells of mice lacking the hepatocyte specific St6gal1 promoter have reduced sialylation of cell surface CD22 and CD45 and produce less IgG upon stimulation. Sialylation of B cells by extracellular ST6GAL1 boosts expression of IgM, IgD, and CD86, proliferation, and IgG production in vitro. In vivo, elevation of blood ST6GAL1 enhances B cell development and systemic IgG in a CD22-dependent manner. Our data point to a function of an extracellular glycosyltransferase in promoting humoral immunity. Manipulation of systemic ST6GAL1 may represent an effective therapeutic approach for humoral insufficiency.

CD22 Expression in B-Cell Acute Lymphoblastic Leukemia: Biological Significance and Implications for Inotuzumab Therapy in Adults

CD22 is a surface molecule expressed early during the ontogeny of B cells in the bone marrow and spleen, and can be found on B cells isolated from the different lymphoid compartments in humans. CD22 is expressed by most blasts from the majority (60-90%) of B-cell acute lymphoblastic leukemia (B-ALL). Current therapies in adults with newly diagnosed B-ALL are associated with complete remission (CR) rates of 50-90%. However, 30-60% of these patients relapse, and only 25-40% achieve disease-free survival of three years or more. Chemotherapy regimens for patients with refractory/relapsed B-ALL are associated with CR rates ranging from 31% to 44%. Novel immune-targeted therapies, such as blinatumomab and inotuzumab (a humanized anti-CD22 monoclonal antibody conjugated to the cytotoxic antibiotic agent calicheamicin), provide potential means of circumventing chemo-refractory B-ALL cells through novel mechanisms of action. Eighty percent of inotuzumab-treated B-ALL patients may achieve a CR state. This review is focused on the biological and clinical activities of CD22 antibodies in B-ALL, and provides evidence about the potential role played by qualitative and quantitative analysis of the CD22 molecule on individual B-ALL blasts in predicting the depletion of leukemic cells, and, ultimately, leading to better clinical response rates.

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 sialoglycan-Siglec glyco-immune checkpoint - a target for improving innate and adaptive anti-cancer immunity

Introduction: During cancer progression, tumor cells develop several mechanisms to prevent killing and to shape the immune system into a tumor-promoting environment. One of such regulatory mechanism is the overexpression of sialic acid (Sia) on carbohydrates of proteins and lipids on tumor cells. Sia-containing glycans or sialoglycans were shown to inhibit immune effector functions of NK cells and T cells by engaging inhibitory Siglec receptors on the surface of these cells. They can also modulate the differentiation of myeloid cells into tumor-promoting M2 macrophages. Areas covered: We review the role of sialoglycans in cancer and introduce the Siglecs, their expression on different immune cells and their interaction with cancer-associated sialoglycans. The targeting of this sialoglycan-Siglec glyco-immune checkpoint is discussed along with potential therapeutic approaches. Pubmed was searched for publications on Siglecs, sialic acid, and cancer. Expert opinion: The targeting of sialoglycan-Siglec interactions has become a major focus in cancer research. New approaches have been developed that directly target sialic acids in tumor lesions. Targeted sialidases that cleave sialic acid specifically in the tumor, have already shown efficacy; efforts targeting the sialoglycan-Siglec pathway for improvement of CAR T cell therapy are ongoing. The sialoglycan-Siglec immune checkpoint is a promising new target for cancer immunotherapy.

Coordinated roles for glycans in regulating the inhibitory function of CD22 on B cells

CD22 is an inhibitory B cell co-receptor that recognizes sialic acid-containing glycoconjugates as ligands. Interactions with its glycan ligands are key to regulating the ability of CD22 to modulate B cell function, the most widely explored of which is antagonizing B cell receptor (BCR) signaling. Most importantly, interactions of CD22 with ligands on the same cell (cis) control the organization of CD22 on the cell surface, which minimizes co-localization with the BCR. In contrast with the modest ability of CD22 to intrinsically dampen BCR signaling, glycan ligands presented on another cell (trans) along with an antigen drawn CD22 and the BCR together within an immunological synapse, strongly inhibiting BCR signaling. New concepts are emerging for how CD22 controls B cell function, such as changes in glycosylation at different stages of B cell differentiation, specifically on GC B cells. Related to these changes, new players, such galectin-9, have been discovered that regulate cell surface nanoclusters of CD22. Roles of glycan ligands in controlling CD22 are the primary focus of this review as we highlight the ability of CD22 to modulate B cell function.

N-Linked Glycosylation Regulates CD22 Organization and Function

The organization and clustering of cell surface proteins plays a critical role in controlling receptor signaling; however, the biophysical mechanisms regulating these parameters are not well understood. Elucidating these mechanisms is highly significant to our understanding of immune function in health and disease, given the importance of B cell receptor (BCR) signaling in directing B cells to produce antibodies for the clearance of pathogens, and the potential deleterious effects of dysregulated BCR signaling, such as in B cell malignancies or autoimmune disease. One of main inhibitory co-receptors on B cells is CD22, a sialic-acid binding protein, which interacts homotypically with other sialylated CD22 molecules, as well as heterotypically with IgM and CD45. Although the importance of CD22 in attenuating BCR signaling is well established, we still do not fully understand what mediates CD22 organization and association to BCRs. CD22 is highly glycosylated, containing 12 N-linked glycosylation sites on its extracellular domain, the function of which remain to be resolved. We were interested in how these glycosylation sites mediate homotypic vs. heterotypic interactions. To this end, we mutated five out of the six N-linked glycosylation residues on CD22 localized closest to the sialic acid binding site. Glycan site N101 was not mutated as this resulted in lack of CD22 expression. We used dual-color super-resolution imaging to investigate the impact of altered glycosylation of CD22 on the nanoscale organization of CD22 and its association with BCR. We show that mutation of these five glycosylation sites increased the clustering tendency of CD22 and resulted in higher density CD22 nanoclusters. Consistent with these findings of altered CD22 organization, we found that mutation of N-glycan sites attenuated CD22 phosphorylation upon BCR stimulation, and consequently, increased BCR signaling. Importantly, we identified that these sites may be ligands for the soluble secreted lectin, galectin-9, and are necessary for galectin-9 mediated inhibition of BCR signaling. Taken together, these findings implicate N-linked glycosylation in the organization and function of CD22, likely through regulating heterotypic interactions between CD22 and its binding partners.

B Cell Siglecs-News on Signaling and Its Interplay With Ligand Binding

CD22 and Siglec-G are members of the Siglec family. Both are inhibitory co-receptors on the surface of B cells and inhibit B-cell receptor induced signaling, characterized by inhibition of the calcium mobilization and cellular activation. CD22 functions predominantly as an inhibitor on conventional B cells, while Siglec-G is an important inhibitor on the B1a-cell subset. These two B-cell Siglecs do not only inhibit initial signaling, but also have an important function in preventing autoimmunity, as double deficient mice develop a lupus-like phenotype with age. Siglecs are characterized by their conserved ability to bind terminal sialic acid of glycans on the cell surface, which is important to regulate the inhibitory role of Siglecs. While CD22 binds α2,6-linked sialic acids, Siglec-G can bind both α2,6-linked and α2,3-linked sialic acids. Interestingly, ligand binding is differentially regulating the ability of CD22 and Siglec-G to control B-cell activation. Within the last years, quite a few studies focused on the different functions of B-cell Siglecs and the interplay of ligand binding and signal inhibition. This review summarizes the role of CD22 and Siglec-G in regulating B-cell receptor signaling, membrane distribution with the importance of ligand binding, preventing autoimmunity and the role of CD22 beyond the naïve B-cell stage. Additionally, this review article features the long time discussed interaction between CD45 and CD22 with highlighting recent data, as well as the interplay between CD22 and Galectin-9 and its influence on B-cell receptor signaling. Moreover, therapeutical approaches targeting human CD22 will be elucidated.

Ligand Recognition Determines the Role of Inhibitory B Cell Co-receptors in the Regulation of B Cell Homeostasis and Autoimmunity

B cells express various inhibitory co-receptors including CD22, CD72, and Siglec-G. These receptors contain immunoreceptor tyrosine-based inhibition motifs (ITIMs) in the cytoplasmic region. Although many of the inhibitory co-receptors negatively regulate BCR signaling by activating SH2-containing protein tyrosine phosphatase 1 (SHP-1), different inhibitory co-receptors have distinct functional properties. CD22, Siglec-G, and CD72 preferentially regulate tonic signaling in conventional B cells, B-1 cell homeostasis, and development of lupus-like disease, respectively. CD72 recognizes RNA-related lupus self-antigen Sm/RNP as a ligand. This ligand recognition recruits CD72 to BCR in Sm/RNP-reactive B cells thereby suppressing production of anti-Sm/RNP autoantibody involved in the pathogenesis of lupus. In contrast, Siglec-G recognizes α2,3 as well as α2,6 sialic acids whereas CD22 recognizes α2,6 sialic acid alone. Because glycoproteins including BCR are dominantly glycosylated with α2,3 sialic acids in B-1 cells, Siglec-G but not CD22 recruits BCR as a ligand specifically in B-1 cells, and regulates B-1 cell homeostasis by suppressing BCR signaling in B-1 cells. Thus, recognition of distinct ligands determines functional properties of different inhibitory B cell co-receptors.

CD22-Binding Synthetic Sialosides Regulate B Lymphocyte Proliferation Through CD22 Ligand-Dependent and Independent Pathways, and Enhance Antibody Production in Mice

Sialic acid-binding immunoglobulin-like lectins (Siglecs) are expressed in various immune cells and most of them carry signaling functions. High-affinity synthetic sialoside ligands have been developed for various Siglecs. Therapeutic potentials of the nanoparticles and compounds that contain multiple numbers of these sialosides and other reagents such as toxins and antigens have been demonstrated. However, whether immune responses can be regulated by monomeric sialoside ligands has not yet been known. CD22 (also known as Siglec-2) is an inhibitory molecule preferentially expressed in B lymphocytes (B cells) and is constitutively bound and functionally regulated by α2,6 sialic acids expressed on the same cell (cis-ligands). Here, we developed synthetic sialosides GSC718 and GSC839 that bind to CD22 with high affinity (IC₅₀ ~100 nM), and inhibit ligand binding of CD22. When B cells are activated by B cell antigen receptor (BCR) ligation, both GSC718 and GSC839 downregulate proliferation of B cells, and this regulation requires both CD22 and α2,6 sialic acids. This result suggests that these sialosides regulate BCR ligation-induced B cell activation by reversing endogenous ligand-mediated regulation of CD22. By contrast, GSC718 and GSC839 augment B cell proliferation induced by TLR ligands or CD40 ligation, and this augmentation requires CD22 but not α2,6 sialic acids. Thus, these sialosides appear to enhance B cell activation by directly suppressing the inhibitory function of CD22 independently of endogenous ligand-mediated regulation. Moreover, GSC839 augments B cell proliferation that depends on both BCR ligation and CD40 ligation as is the case for in vivo B cell responses to antigens, and enhanced antibody production to the extent comparable to CpG oligonuleotides or a small amount of alum. Although these known adjuvants induce production of the inflammatory cytokines or accumulation of inflammatory cells, CD22-binding sialosides do not. Thus, synthetic sialosides that bind to CD22 with high-affinity modulate B cell activation through endogenous ligand-dependent and independent pathways, and carry an adjuvant activity without inducing inflammation.

Proximity labeling of cis-ligands of CD22/Siglec-2 reveals stepwise α2,6 sialic acid-dependent and -independent interactions

Lectins expressed on the cell surface are often bound and regulated by the membrane molecules containing the glycan ligands on the same cell (cis-ligands). However, molecular nature and function of cis-ligands are generally poorly understood partly because of weak interaction between lectins and glycan ligands. Cis-ligands are most extensively studied in CD22 (also known as Siglec-2), an inhibitory B lymphocyte receptor specifically recognizing α2,6 sialic acids. CD22, CD45 and IgM are suggested to be ligands of CD22. Here we labeled molecules in the proximity of CD22 in situ on B cell surface using biotin-tyramide. Molecules including CD22, CD45 and IgM were labeled in wild-type but not ST6GalI^-/- B cells that lack α2,6 sialic acids, indicating that these molecules associate with CD22 by lectin-glycan interaction, and are therefore cis-ligands. In ST6GalI^-/- B cells, these cis-ligands are located in a slightly more distance from CD22. Thus, the lectin-glycan interaction recruits cis-ligands already located in the relative proximity of CD22 through non-lectin-glycan interaction to the close proximity. Moreover, cis-ligands are labeled in Cmah^-/- B cells that lack Neu5Gc preferred by mouse CD22 as efficiently as in wild-type B cells, indicating that very low affinity lectin-glycan interaction is sufficient for recruiting cis-ligands, and can be detected by proximity labeling. Thus, proximity labeling with tyramide appears to be a useful method to identify cis-ligands and to analyze their interaction with the lectins.

Human CD22 Inhibits Murine B Cell Receptor Activation in a Human CD22 Transgenic Mouse Model

CD22, a sialic acid-binding Ig-type lectin (Siglec) family member, is an inhibitory coreceptor of the BCR with established roles in health and disease. The restricted expression pattern of CD22 on B cells and most B cell lymphomas has made CD22 a therapeutic target for B cell-mediated diseases. Models to better understand how in vivo targeting of CD22 translates to human disease are needed. In this article, we report the development of a transgenic mouse expressing human CD22 (hCD22) in B cells and assess its ability to functionally substitute for murine CD22 (mCD22) for regulation of BCR signaling, Ab responses, homing, and tolerance. Expression of hCD22 on transgenic murine B cells is comparable to expression on human primary B cells, and it colocalizes with mCD22 on the cell surface. Murine B cells expressing only hCD22 have identical calcium (Ca²⁺) flux responses to anti-IgM as mCD22-expressing wild-type B cells. Furthermore, hCD22 transgenic mice on an mCD22^-/- background have restored levels of marginal zone B cells and Ab responses compared with deficiencies observed in CD22^-/- mice. Consistent with these observations, hCD22 transgenic mice develop normal humoral responses in a peanut allergy oral sensitization model. Homing of B cells to Peyer's patches was partially rescued by expression of hCD22 compared with CD22^-/- B cells, although not to wild-type levels. Notably, Siglec-engaging antigenic liposomes formulated with an hCD22 ligand were shown to prevent B cell activation, increase cell death, and induce tolerance in vivo. This hCD22 transgenic mouse will be a valuable model for investigating the function of hCD22 and preclinical studies targeting hCD22.

Glycobiology of Eosinophilic Inflammation: Contributions of Siglecs, Glycans, and Other Glycan-Binding Proteins

The historical focus on protein-protein interactions in biological systems, at the expense of attention given to interactions between other classes of molecules, has overlooked important and clinically relevant processes and points of potential clinical intervention. For example, the significance of protein-carbohydrate interactions, especially in the regulation of immune responses, has recently received greater recognition and appreciation. This review discusses several ways by which cell-surface lectin-glycan interactions can modulate eosinophil function, particularly at the levels of eosinophil recruitment and survival, and how such interactions can be exploited therapeutically. A primary focus is on discoveries concerning Siglec-8, a glycan-binding protein selectively expressed on human eosinophils, and its closest functional paralog in the mouse, Siglec-F. Recent advances in the synthesis of polymeric ligands, the identification of physiological ligands for Siglec-8 and Siglec-F in the airway, and the determination of the basis of glycan ligand discrimination of Siglec-8 are discussed. Important similarities and differences between these siglecs are outlined. Eosinophil expression of additional glycan-binding proteins or their glycan ligands, including interactions involving members of the selectin, galectin, and siglec families, is summarized. The roles of these molecules in eosinophil recruitment, survival, and inflammation are described. Finally, the modulation of these interactions and potential therapeutic exploitation of glycan-binding proteins and their ligands to ameliorate eosinophil-associated diseases are considered.

Sialylation of N-glycans: mechanism, cellular compartmentalization and function

Sialylated N-glycans play essential roles in the immune system, pathogen recognition and cancer. This review approaches the sialylation of N-glycans from three perspectives. The first section focuses on the sialyltransferases that add sialic acid to N-glycans. Included in the discussion is a description of these enzymes' glycan acceptors, conserved domain organization and sequences, molecular structure and catalytic mechanism. In addition, we discuss the protein interactions underlying the polysialylation of a select group of adhesion and signaling molecules. In the second section, the biosynthesis of sialic acid, CMP-sialic acid and sialylated N-glycans is discussed, with a special emphasis on the compartmentalization of these processes in the mammalian cell. The sequences and mechanisms maintaining the sialyltransferases and other glycosylation enzymes in the Golgi are also reviewed. In the final section, we have chosen to discuss processes in which sialylated glycans, both N- and O-linked, play a role. The first part of this section focuses on sialic acid-binding proteins including viral hemagglutinins, Siglecs and selectins. In the second half of this section, we comment on the role of sialylated N-glycans in cancer, including the roles of β1-integrin and Fas receptor N-glycan sialylation in cancer cell survival and drug resistance, and the role of these sialylated proteins and polysialic acid in cancer metastasis.

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.

B-cell-independent sialylation of IgG

IgG carrying terminal α2,6-linked sialic acids added to conserved N-glycans within the Fc domain by the sialyltransferase ST6Gal1 accounts for the anti-inflammatory effects of large-dose i.v. Ig (IVIg) in autoimmunity. Here, B-cell-specific ablation of ST6Gal1 in mice revealed that IgG sialylation can occur in the extracellular environment of the bloodstream independently of the B-cell secretory pathway. We also discovered that secreted ST6Gal1 is produced by cells lining central veins in the liver and that IgG sialylation is powered by serum-localized nucleotide sugar donor CMP-sialic acid that is at least partially derived from degranulating platelets. Thus, antibody-secreting cells do not exclusively control the sialylation-dependent anti-inflammatory function of IgG. Rather, IgG sialylation can be regulated by the liver and platelets through the corresponding release of enzyme and sugar donor into the cardiovascular circulation.

Cell type-specific glycosylation of Orai1 modulates store-operated Ca2+ entry

N-glycosylation of cell surface proteins affects protein function, stability, and interaction with other proteins. Orai channels, which mediate store-operated Ca(2+) entry (SOCE), are composed of N-glycosylated subunits. Upon activation by Ca(2+) sensor proteins (stromal interaction molecules STIM1 or STIM2) in the endoplasmic reticulum, Orai Ca(2+) channels in the plasma membrane mediate Ca(2+) influx. Lectins are carbohydrate-binding proteins, and Siglecs are a family of sialic acid-binding lectins with immunoglobulin-like repeats. Using Western blot analysis and lectin-binding assays from various primary human cells and cancer cell lines, we found that glycosylation of Orai1 is cell type-specific. Ca(2+) imaging experiments and patch-clamp experiments revealed that mutation of the only glycosylation site of Orai1 (Orai1N223A) enhanced SOCE in Jurkat T cells. Knockdown of the sialyltransferase ST6GAL1 reduced α-2,6-linked sialic acids in the glycan structure of Orai1 and was associated with increased Ca(2+) entry in Jurkat T cells. In human mast cells, inhibition of sialyl sulfation altered the N-glycan of Orai1 (and other proteins) and increased SOCE. These data suggest that cell type-specific glycosylation influences the interaction of Orai1 with specific lectins, such as Siglecs, which then attenuates SOCE. In summary, the glycosylation state of Orai1 influences SOCE-mediated Ca(2+) signaling and, thus, may contribute to pathophysiological Ca(2+) signaling observed in immune disease and cancer.

Nanoscale organization and dynamics of the siglec CD22 cooperate with the cytoskeleton in restraining BCR signalling

Receptor organization and dynamics at the cell membrane are important factors of signal transduction regulation. Using super-resolution microscopy and single-particle tracking, we show how the negative coreceptor CD22 works with the cortical cytoskeleton in restraining BCR signalling. In naïve B cells, we found endogenous CD22 to be highly mobile and organized into nanodomains. The landscape of CD22 and its lateral diffusion were perturbed either in the absence of CD45 or when the CD22 lectin domain was mutated. To understand how a relatively low number of CD22 molecules can keep BCR signalling in check, we generated Brownian dynamic simulations and supported them with ex vivo experiments. This combined approach suggests that the inhibitory function of CD22 is influenced by its nanoscale organization and is ensured by its fast diffusion enabling a "global BCR surveillance" at the plasma membrane.