Metabolic sialic acid blockade lowers the activation threshold of moDCs for TLR stimulation

Overcoming the novel glycan-lectin checkpoints in tumor microenvironments for the success of the cross-presentation-based immunotherapy

In pursuit of meeting the ever-rising demand for cancer therapies, cross-presentation-based glyconanovaccines (GNVs) targeting C-type lectin receptors (CLRs) on DCs have shown significant potential as cutting-edge cancer immunotherapy. GNVs are an attractive approach to induce anti-cancer cytotoxic T lymphocyte responses. Despite immune checkpoints (ICs) being well established and an obstacle to the success of GNVs, glycan-lectin circuits are emerging as unique checkpoints due to their immunomodulatory functions. Given the role of aberrant tumor glycosylation in promoting immune evasion, mitigating these effects is crucial for the efficacy of GNVs. Lectins, such as siglecs and galectins, are detrimental to the tumor immune landscape as they promote an immunosuppressive TME. From this perspective, this review aims to explore glycan-lectin ICs and their influence on the efficacy of GNVs. We aim to discuss various ICs in the TME followed by drawbacks of immune checkpoint inhibitors (ICIs). We will also emphasize the altered glycosylation profile of tumors, addressing their immunosuppressive nature along with ways in which CLRs, siglecs, and galectins contribute to immune evasion and cancer progression. Considering the resistance towards ICIs, current and prospective approaches for targeting glycan-lectin circuits and future prospects of these endeavors in harnessing the full potential of GNVs will also be highlighted.

Insights into stability, dimerisation, and ligand binding properties of Siglec-7: Isotope labelling in HEK293 cells for protein characterisation by NMR spectroscopy

Siglec-7, an immune checkpoint receptor, has emerged as a promising target for cancer immunotherapy due to its involvement in the regulation of immune and inflammatory responses. However, while its participation in immunoediting and immune evasion is well established, understanding its biological context, relevant ligands, and associated signalling pathways remains limited. Understanding these aspects is crucial for the development of effective immunotherapies targeting Siglec-7. In this study, three expression constructs of Siglec-7 were designed, expressed, and characterised, including an analysis of the oligomeric state of its extracellular domain. The N-terminal V-set Ig carbohydrate recognition domain was also produced in an isotopically double-labelled (13C,15N) mammalian cell growth medium. Two stable constructs suitable for biophysical and structural studies were identified. These findings reveal the noncovalent dimerisation of Siglec-7, offering new insights into its possible ligand interactions, signal transduction mechanisms, or receptor/ligand clustering. The dimerisation of Siglec-7 may be essential to achieve multivalent, high-avidity interactions with glycoconjugates, which may result in enhanced or alternative signalling processes within the NK cell immune synapse. In addition, a detailed protocol for generating double-labelled Siglec-7 in HEK293 cells, which may apply to other proteins under similar conditions, was described. These findings contribute to a better understanding of the biophysical and structural properties of Siglec-7 and are key to the design of more precise and effective cancer immunotherapies targeting Siglec-7.

New insights into the immunomodulatory potential of sialic acid on monocyte-derived dendritic cells

Sialic acids at the cell surface of dendritic cells (DCs) play an important immunomodulatory role, and their manipulation enhances DC maturation, leading to heightened T cell activation. Particularly, at the molecular level, the increased stability of surface MHC-I molecules in monocyte-derived DCs (MoDCs) underpins an improved DC: T cell interaction. In this study, we focused on the impact of sialic acid remodelling by treatment with Clostridium perfringens sialidase on MoDCs' phenotypic and functional characteristics. Our investigation juxtaposes this novel approach with the conventional cytokine-based maturation regimen commonly employed in clinical settings.Notably, C. perfringens sialidase remarkably increased MHC-I levels compared to other sialidases having different specificities, supporting the idea that higher MHC-I is due to the cleavage of specific sialoglycans on cell surface proteins. Sialidase treatment induced rapid elevated surface expression of MHC-I, MHC-II and CD40 within an hour, a response not fully replicated by 48 h cytokine cocktail treatment. These increases were also observable 48 h post sialidase treatment. While CD86 and PD-L1 showed significant increases after 48 h of cytokine maturation, 48 h post sialidase treatment showed a higher increase in CD86 and shorter increase in PD-L1. CCR-7 expression was significantly increased 48 h after sialidase treatment but not significantly affected by cytokine maturation. Both treatments promoted higher secretion of the IL-12 cytokine. However, the cytokine cocktail induced a more pronounced IL-12 production. SNA lectin staining analysis demonstrated that the sialic acid profile is significantly altered by sialidase treatment, but not by the cytokine cocktail, which causes only slight sialic acid upregulation. Notably, the lipid-presenting molecules CD1a, CD1b and CD1c remained unaffected by sialidase treatment in MoDCs, a finding also further supported by experiments performed on C1R cells. Inhibition of endogenous sialidases Neu1 and Neu3 during MoDC differentiation did not affect surface MHC-I expression and cytokine secretion. Yet, sialidase activity in MoDCs was minimal, suggesting that sialidase inhibition does not significantly alter MHC-I-related functions. Our study highlights the unique maturation profile induced by sialic acid manipulation in MoDCs. These findings provide insights into the potential of sialic acid manipulation as a rapid immunomodulatory strategy, offering promising avenues for targeted interventions in inflammatory contexts.

Novel genetically glycoengineered human dendritic cell model reveals regulatory roles of α2,6-linked sialic acids in DC activation of CD4+ T cells and response to TNFα

Dendritic cells (DCs) are central for the initiation and regulation of appropriate immune responses. While several studies suggest important regulatory roles of sialoglycans in DC biology, our understanding is still inadequate primarily due to a lack of appropriate models. Previous approaches based on enzymatic- or metabolic-glycoengineering and primary cell isolation from genetically modified mice have limitations related to specificity, stability, and species differences. This study addresses these challenges by introducing a workflow to genetically glycoengineer the human DC precursor cell line MUTZ-3, described to differentiate and maturate into fully functional dendritic cells, using CRISPR-Cas9, thereby providing and validating the first isogenic cell model for investigating glycan alteration on human DC differentiation, maturation, and activity. By knocking out (KO) the ST6GAL1 gene, we generated isogenic cells devoid of ST6GAL1-mediated α(2,6)-linked sialylation, allowing for a comprehensive investigation into its impact on DC function. Glycan profiling using lectin binding assay and functional studies revealed that ST6GAL1 KO increased the expression of important antigen presenting and co-stimulatory surface receptors and a specifically increased activation of allogenic human CD4 + T cells. Additionally, ST6GAL1 KO induces significant changes in surface marker expression and cytokine response to TNFα-induced maturation, and it affects migration and the endocytic capacity. These results indicate that genetic glycoengineering of the isogenic MUTZ-3 cellular model offers a valuable tool to study how specific glycan structures influence human DC biology, contributing to our understanding of glycoimmunology.

Targeting Siglec-Sialylated MUC1 Immune Axis in Cancer

Siglecs play a key role in mediating cell-cell interactions via the recognition of different sialylated glycoconjugates, including tumor-associated MUC1, which can lead to the activation or inhibition of the immune response. The activation occurs through the signaling of Siglecs with the cytoplasmic immunoreceptor tyrosine-based activation motif (ITAM)-containing proteins, while the inhibition signal is a result of the interaction of intracellular immunoreceptor tyrosine-based inhibition motif (ITIM)-bearing receptors. The interaction of tumor-associated MUC1 sialylated glycans with Siglecs via ITIM motifs decreases antitumor immunity. Consequently, these interactions are expected to play a key role in tumor evasion. Efforts to modulate the response of immune cells by blocking the immune-suppressive effects of inhibitory Siglecs, driving immune-activating Siglecs, and/or altering the synthesis and expression of the sialic acid glycocalyx are new therapeutic strategies deserving further investigation. We will highlight the role of Siglec's family receptors in immune evasion through interactions with glycan ligands in their natural context, presented on the protein such as MUC1, factors affecting their fine binding specificities, such as the role of multivalency either at the ligand or receptor side, their spatial organization, and finally the current and future therapeutic interventions targeting the Siglec-sialylated MUC1 immune axis in cancer.

Targeting myeloid cells for cancer immunotherapy: Siglec-7/9/10/15 and their ligands

Advances in immunotherapy have revolutionized cancer treatment, yet many patients do not show clinical responses. While most immunotherapies target T cells, myeloid cells are the most abundant cell type in solid tumors and are key orchestrators of the immunosuppressive tumor microenvironment (TME), hampering effective T cell responses. Therefore, unraveling the immune suppressive pathways within myeloid cells could unveil new avenues for cancer immunotherapy. Over the past decade, Siglec receptors and their ligand, sialic acids, have emerged as a novel immune checkpoint on myeloid cells. In this review, we highlight key findings on how sialic acids modify immunity in the TME through engagement of Siglec-7/9/10/15 expressed on myeloid cells, and how the sialic acid-Siglec axis can be targeted for future cancer immunotherapies.

Interactions between Siglec-8 and endogenous sialylated cis ligands restrain cell death induction in human eosinophils and mast cells

Sialic acid-binding immunoglobulin-like lectin (Siglec)-8 is a sialoside-binding receptor expressed by eosinophils and mast cells that exhibits priming status- and cell type-dependent inhibitory activity. On eosinophils that have been primed with IL-5, GM-CSF, or IL-33, antibody ligation of Siglec-8 induces cell death through a pathway involving the β2 integrin-dependent generation of reactive oxygen species (ROS) via NADPH oxidase. In contrast, Siglec-8 engagement on mast cells inhibits cellular activation and mediator release but reportedly does not impact cell viability. The differences in responses between cytokine-primed and unprimed eosinophils, and between eosinophils and mast cells, to Siglec-8 ligation are not understood. We previously found that Siglec-8 binds to sialylated ligands present on the surface of the same cell (so-called cis ligands), preventing Siglec-8 ligand binding in trans. However, the functional relevance of these cis ligands has not been elucidated. We therefore explored the potential influence of cis ligands of Siglec-8 on both eosinophils and mast cells. De-sialylation using exogenous sialidase profoundly altered the consequences of Siglec-8 antibody engagement on both cell types, eliminating the need for cytokine priming of eosinophils to facilitate cell death and enabling Siglec-8-dependent mast cell death without impacting anti-Siglec-8 antibody binding. The cell death process licensed by de-sialylation resembled that characterized in IL-5-primed eosinophils, including CD11b upregulation, ROS production, and the activities of Syk, PI3K, and PLC. These results implicate cis ligands in restraining Siglec-8 function on eosinophils and mast cells and reveal a promising approach to the selective depletion of mast cells in patients with mast cell-mediated diseases.

Siglec-9 is an inhibitory receptor on human mast cells in vitro

BACKGROUND

Mast cell activation is critical for the development of allergic diseases. Ligation of sialic acid-binding immunoglobin-like lectins (Siglecs), such as Siglec-6, -7, and -8 as well as CD33, have been shown to inhibit mast cell activation. Recent studies showed that human mast cells express Siglec-9, an inhibitory receptor also expressed by neutrophils, monocytes, macrophages, and dendritic cells.

OBJECTIVE

We aimed to characterize Siglec-9 expression and function in human mast cells in vitro.

METHODS

We assessed the expression of Siglec-9 and Siglec-9 ligands on human mast cell lines and human primary mast cells by real-time quantitative PCR, flow cytometry, and confocal microscopy. We used a clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) gene editing approach to disrupt the SIGLEC9 gene. We evaluated Siglec-9 inhibitory activity on mast cell function by using native Siglec-9 ligands, glycophorin A (GlycA), and high-molecular-weight hyaluronic acid, a monoclonal antibody against Siglec-9, and coengagement of Siglec-9 with the high-affinity receptor for IgE (FcεRI).

RESULTS

Human mast cells express Siglec-9 and Siglec-9 ligands. SIGLEC9 gene disruption resulted in increased expression of activation markers at baseline and increased responsiveness to IgE-dependent and IgE-independent stimulation. Pretreatment with GlycA or high-molecular-weight hyaluronic acid followed by IgE-dependent or -independent stimulation had an inhibitory effect on mast cell degranulation. Coengagement of Siglec-9 with FcεRI in human mast cells resulted in reduced degranulation, arachidonic acid production, and chemokine release.

CONCLUSIONS

Siglec-9 and its ligands play an important role in limiting human mast cell activation in vitro.

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.

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.

Pomalidomide enhances the maturation of dendritic cells derived from healthy donors and multiple myeloma patients

Objective: To explore the effect of pomalidomide on the maturation of monocyte-derived dendritic cells (moDCs) from healthy donors (HDs) and multiple myeloma (MM) patients. Methods: MoDCs were generated by the incubation of monocytes from peripheral blood mononuclear cells (PBMCs) for 7 days in a medium consisting of 800 U/ml granulocyte-macrophage colony stimulating factor (GM-CSF), 500 U/ml interleukin-4 (IL-4), RPMI 1,640 medium, 5% human serum, 100 U/ml penicillin and 0.1 mg/ml streptomycin. Meanwhile, the incubation system was administrated with 10 µM pomalidomide or 1 × PBS as the control group. On the eighth day, cells were harvested and analyzed by flow cytometry. The CD80⁺CD86⁺ cell population in total cells was gated as moDCs in the FACS analyzing system. After that, the expression of CD40 and HLA-DR on moDCs was analyzed. Meanwhile, the supernatant from the incubation system was evaluated for the secretion of cytokines interleukin-12 (IL-12), tumor necrosis factor-α (TNF-α), and macrophage inflammatory protein 1α (MIP-1α) by enzyme-linked immunosorbent assay (ELISA). Results: When analyzing all the HD-moDCs together (n = 15), pomalidomide significantly increased the mean fluorescence intensity (MFI) of CD40 expression and HLA-DR expression on moDCs compared with the control group (p = 0.003, p = 0.040). Meanwhile, the proportion of CD40⁺ moDCs and HLA-DR⁺ moDCs in total moDCs was significantly higher in the pomalidomide group than in the control group (p = 0.008, p = 0.032). When analyzing all MM patient-moDCs together (n = 11), pomalidomide significantly increased the MFI of CD40 expression and HLA-DR expression on moDCs compared with the control group (p = 0.047, p = 0.006). Meanwhile, the proportion of HLA-DR⁺ moDCs in total DCs was significantly higher in the pomalidomide group than in the control group (p < 0.001). Moreover, HD-moDCs (n = 8) treated with pomalidomide secreted 192% IL-12, 110% TNF-α, and 112% MIP-1α of the untreated moDCs (p = 0.020, p = 0.006, p = 0.055). However, when analyzing MM patient-moDCs (n = 10) together, the secretion of IL-12, TNF-α and MIP-1α from moDCs showed no significant difference between the pomalidomide group and the control group (p = 0.458, p = 0.377, p = 0.248). Conclusion: In vitro, 10 µM pomalidomide enhances the maturation of moDCs derived from both HDs and MM patients. Pomalidomide shows potential to be applied as a DC adjuvant for DC-based immunotherapy, such as the DC vaccine and DC cell therapy in MM.

Chemical tools to track and perturb the expression of sialic acid and fucose monosaccharides

The biosynthesis of glycans is a highly conserved biological process and found in all domains of life. The expression of cell surface glycans is increasingly recognized as a target for therapeutic intervention given the role of glycans in major pathologies such as cancer and microbial infection. Herein, we summarize our contributions to the development of unnatural monosaccharide derivatives to infiltrate and alter the expression of both mammalian and bacterial glycans and their therapeutic application.

Whole-cell tumor vaccines desialylated to uncover tumor antigenic Gal/GalNAc epitopes elicit anti-tumor immunity

BACKGROUND

Aberrant sialoglycans on the surface of tumor cells shield potential tumor antigen epitopes, escape recognition, and suppress activation of immunocytes. α2,3/α2,6Gal- and α2,6GalNAc (Gal/GalNAc)-linked sialic acid residues of sialoglycans could affect macrophage galactose-type lectins (MGL) mediated-antigen uptake and presentation and promote sialic acid-binding immunoglobulin-like lectins (Siglecs) mediated-immunosuppression. Desialylating sialoglycans on tumor cells could present tumor antigens with Gal/GalNAc residues and overcome glyco-immune checkpoints. Thus, we explored whether vaccination with desialylated whole-cell tumor vaccines (DWCTVs) triggers anti-tumor immunity in ovarian cancer (OC).

METHODS

Sialic acid (Sia) and Gal/GalNAc residues on OC A2780, OVCAR3, and ID8 cells treated with α2-3 neuraminidase (α2-3NA) and α2-6NA, and Sigec-9 or Siglec-E and MGL on DCs pulsed with desialylated OC cells were identified using flow cytometry (FCM); RT-qPCR determined IFNG expression of T cells, TRBV was sequenced using Sanger sequencing and cytotoxicity of αβ T cells was measured with LDH assay; Anti-tumor immunity in vivo was validated via vaccination with desialylated whole-cell ID8 vaccine (ID8 DWCTVs).

RESULTS

Gal/GalNAc but not Sia residues were significantly increased in the desialylated OC cells. α2-3NA-modified DWCTV increased MGL but decreased Siglec-9 or Siglec E expression on DCs. MGL^bright/Siglec-9^dim DCs significantly up-regulated IFNG expression and CD4/CD8 ratio of T cells and diversified the TCR repertoire of αβ T-cells that showed enhanced cytotoxic activity. Vaccination with α2-3NA-modified ID8 DWCTVs increased MGL^bright/Siglec-E^dim DCs in draining lymph nodes, limited tumor growth, and extended survival in tumor-challenged mice.

CONCLUSION

Desialylated tumor cell vaccine could promote anti-tumor immunity and provide a strategy for OC immunotherapy in a clinical setting.

Polysialic Acid Sustains the Hypoxia-Induced Migration and Undifferentiated State of Human Glioblastoma Cells

Gliomas are the most common primary malignant brain tumors. Glioblastoma, IDH-wildtype (GBM, CNS WHO grade 4) is the most aggressive form of glioma and is characterized by extensive hypoxic areas that strongly correlate with tumor malignancy. Hypoxia promotes several processes, including stemness, migration, invasion, angiogenesis, and radio- and chemoresistance, that have direct impacts on treatment failure. Thus, there is still an increasing need to identify novel targets to limit GBM relapse. Polysialic acid (PSA) is a carbohydrate composed of a linear polymer of α2,8-linked sialic acids, primarily attached to the Neural Cell Adhesion Molecule (NCAM). It is considered an oncodevelopmental antigen that is re-expressed in various tumors. High levels of PSA-NCAM are associated with high-grade and poorly differentiated tumors. Here, we investigated the effect of PSA inhibition in GBM cells under low oxygen concentrations. Our main results highlight the way in which hypoxia stimulates polysialylation in U87-MG cells and in a GBM primary culture. By lowering PSA levels with the sialic acid analog, F-NANA, we also inhibited GBM cell migration and interfered with their differentiation influenced by the hypoxic microenvironment. Our findings suggest that PSA may represent a possible molecular target for the development of alternative pharmacological strategies to manage a devastating tumor like GBM.

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.

Structure-Activity Relationship of Metabolic Sialic Acid Inhibitors and Labeling Reagents

Sialic acids cap the glycans of cell surface glycoproteins and glycolipids. They are involved in a multitude of biological processes, and aberrant sialic acid expression is associated with several pathologies, such as cancer. Strategies to interfere with the sialic acid biosynthesis can potentially be used for anticancer therapy. One well-known class of sialylation inhibitors is peracetylated 3-fluorosialic acids. We synthesized 3-fluorosialic acid derivatives modified at the C-4, C-5, C-8, and C-9 position and tested their inhibitory potency in vitro. Modifications at C-5 lead to increased inhibition, compared to the natural acetamide at this position. These structure–activity relationships could also be applied to improve the efficiency of sialic acid metabolic labeling reagents by modification of the C-5 position. Hence, these results improve our understanding of the structure–activity relationships of sialic acid glycomimetics and their metabolic processing.

Sialic acid blockade in dendritic cells enhances CD8+ T cell responses by facilitating high-avidity interactions

Sialic acids are negatively charged carbohydrates that cap the glycans of glycoproteins and glycolipids. Sialic acids are involved in various biological processes including cell-cell adhesion and immune recognition. In dendritic cells (DCs), the major antigen-presenting cells of the immune system, sialic acids emerge as important regulators of maturation and interaction with other lymphocytes including T cells. Many aspects of how sialic acids regulate DC functions are not well understood and tools and model systems to address these are limited. Here, we have established cultures of murine bone marrow-derived DCs (BMDCs) that lack sialic acid expression using a sialic acid-blocking mimetic Ac53FaxNeu5Ac. Ac53FaxNeu5Ac treatment potentiated BMDC activation via toll-like receptor (TLR) stimulation without affecting differentiation and viability. Sialic acid blockade further increased the capacity of BMDCs to induce antigen-specific CD8+ T cell proliferation. Transcriptome-wide gene expression analysis revealed that sialic acid mimetic treatment of BMDCs induces differential expression of genes involved in T cell activation, cell-adhesion, and cell-cell interactions. Subsequent cell clustering assays and single cell avidity measurements demonstrated that BMDCs with reduced sialylation form higher avidity interactions with CD8+ T cells. This increased avidity was detectable in the absence of antigens, but was especially pronounced in antigen-dependent interactions. Together, our data show that sialic acid blockade in BMDCs ameliorates maturation and enhances both cognate T cell receptor-MHC-dependent and independent T cell interactions that allow for more robust CD8+ T cell responses.

Re-Expression of Poly/Oligo-Sialylated Adhesion Molecules on the Surface of Tumor Cells Disrupts Their Interaction with Immune-Effector Cells and Contributes to Pathophysiological Immune Escape

Glycans linked to surface proteins are the most complex biological macromolecules that play an active role in various cellular mechanisms. This diversity is the basis of cell-cell interaction and communication, cell growth, cell migration, as well as co-stimulatory or inhibitory signaling. Our review describes the importance of neuraminic acid and its derivatives as recognition elements, which are located at the outermost positions of carbohydrate chains linked to specific glycoproteins or glycolipids. Tumor cells, especially from solid tumors, mask themselves by re-expression of hypersialylated neural cell adhesion molecule (NCAM), neuropilin-2 (NRP-2), or synaptic cell adhesion molecule 1 (SynCAM 1) in order to protect themselves against the cytotoxic attack of the also highly sialylated immune effector cells. More particularly, we focus on α-2,8-linked polysialic acid chains, which characterize carrier glycoproteins such as NCAM, NRP-2, or SynCam-1. This characteristic property correlates with an aggressive clinical phenotype and endows them with multiple roles in biological processes that underlie all steps of cancer progression, including regulation of cell-cell and/or cell-extracellular matrix interactions, as well as increased proliferation, migration, reduced apoptosis rate of tumor cells, angiogenesis, and metastasis. Specifically, re-expression of poly/oligo-sialylated adhesion molecules on the surface of tumor cells disrupts their interaction with immune-effector cells and contributes to pathophysiological immune escape. Further, sialylated glycoproteins induce immunoregulatory cytokines and growth factors through interactions with sialic acid-binding immunoglobulin-like lectins. We describe the processes, which modulate the interaction between sialylated carrier glycoproteins and their ligands, and illustrate that sialic acids could be targets of novel therapeutic strategies for treatment of cancer and immune diseases.

Novel Virulence Role of Pneumococcal NanA in Host Inflammation and Cell Death Through the Activation of Inflammasome and the Caspase Pathway

Streptococcus pneumoniae is one of most deadly Gram-positive bacterium that causes significant mortality and morbidity worldwide. Intense inflammation and cytotoxicity is a hallmark of invasive pneumococcal disease. Pneumococcal NanA has been shown to exaggerate the production of inflammatory cytokines via unmasking of inhibitory Siglec-5 from its sialyl cis-ligands. To further investigate the mechanistic role of NanA and Siglec-5 in pneumococccal diseases, we systemically analyzed genes and signaling pathways differentially regulated in macrophages infected with wild type and NanA-deficient pneumococcus. We found that NanA-mediated desialylation impairs the Siglec-5-TLR-2 interaction and reduces the recruitment of phosphatase SHP-1 to Siglec-5. This dysregulated crosstalk between TLR-2 and inhibitory Siglec-5 exaggerated multiple inflammatory and death signaling pathways and consequently caused excessive inflammation and cytotoxicity in the infected macrophage. Collectively, our results reveal a novel virulence role of NanA in pneumococcal pathogenesis and suggest that targeting NanA activity may ameliorate the pneumococcus-mediated inflammation and cytotoxicity in severe invasive pneumococcal diseases.

Modulation of immune cell reactivity with cis-binding Siglec agonists

Inflammatory pathologies caused by phagocytes lead to numerous debilitating conditions, including chronic pain and blindness due to age-related macular degeneration. Many members of the sialic acid-binding immunoglobulin-like lectin (Siglec) family are immunoinhibitory receptors whose agonism is an attractive approach for antiinflammatory therapy. Here, we show that synthetic lipid-conjugated glycopolypeptides can insert into cell membranes and engage Siglec receptors in cis, leading to inhibitory signaling. Specifically, we construct a cis-binding agonist of Siglec-9 and show that it modulates mitogen-activated protein kinase (MAPK) signaling in reporter cell lines, immortalized macrophage and microglial cell lines, and primary human macrophages. Thus, these cis-binding agonists of Siglecs present a method for therapeutic suppression of immune cell reactivity.

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.

Current and emerging immunotherapeutic approaches to the treatment of multiple myeloma

Multiple myeloma (MM) has a worldwide incidence of 1-5/100,000/year. Outcomes have improved significantly in recent years following incorporation of immunomodulatory drugs and proteasome inhibitors into standard-of-care regimes. MM is profoundly immunosuppressive, enabling immune evasion, proliferation and disease progression. The role of the immune system in MM is becoming increasingly characterized and understood, and numerous therapies are under development or in routine clinical use targeting these elements of MM pathogenesis. In this review we discuss the immunosuppressive effects of MM, then the therapies targeting these defects. Specifically, we review the monoclonal and bispecific antibodies, alongside adoptive cellular therapies currently under investigation.

Expression profiling of immune inhibitory Siglecs and their ligands in patients with glioma

Gliomas appear to be highly immunosuppressive tumors, with a strong myeloid component. This includes MDSCs, which are a heterogeneous, immature myeloid cell population expressing myeloid markers Siglec-3 (CD33) and CD11b and lacking markers of mature myeloid cells including MHC II. Siglec-3 is a member of the sialic acid-binding immunoglobulin-like lectin (Siglec) family and has been suggested to promote MDSC expansion and suppression. Siglecs form a recently defined family of receptors with potential immunoregulatory functions but only limited insight in their expression on immune regulatory cell subsets, prompting us to investigate Siglec expression on MDSCs. We determined the expression of different Siglec family members on monocytic-MDSCs (M-MDSCs) and polymorphnuclear-MDSCs (PMN-MDSCs) from blood of glioma patients and healthy donors, as well as from patient-derived tumor material. Furthermore, we investigated the presence of sialic acid ligands for these Siglecs on MDSCs and in the glioma tumor microenvironment. Both MDSC subsets express Siglec-3, -5, -7 and -9, with higher levels of Siglec-3, -7 and -9 on M-MDSCs and higher Siglec-5 levels on PMN-MDSCs. Similar Siglec expression profiles were found on MDSCs from healthy donors. Furthermore, the presence of Siglec-5 and -9 was also confirmed on PMN-MDSCs from glioma tissue. Interestingly, freshly isolated glioma cells predominantly expressed sialic acid ligands for Siglec-7 and -9, which was confirmed in situ. In conclusion, our data show a distinct Siglec expression profile for M- and PMN-MDSCs and propose possible sialic acid-Siglec interactions between glioma cells and MDSCs in the tumor microenvironment.

Combined sialic acid and histone deacetylase (HDAC) inhibitor treatment up-regulates the neuroblastoma antigen GD2

Neuroblastoma cells highly express the disialoganglioside GD2, a tumor-associated carbohydrate antigen, which is only sparsely expressed on healthy tissue. GD2 is a primary target for the development of immunotherapy for neuroblastoma. Immunotherapy with monoclonal anti-GD2 antibodies has proven safety and efficacy in clinical trials and is included in the standard treatment for children with high-risk neuroblastoma. Strategies to modulate GD2 expression in neuroblastoma could further improve anti-GD2-targeted immunotherapy. Here, we report that the cellular sialylation pathway, as well as epigenetic reprogramming, strongly modulates GD2 expression in human and mouse neuroblastoma cell lines. Recognition of GD2 by the 14G2a antibody is sialic acid-dependent and was blocked with the fluorinated sialic acid mimetic Ac₅3F_axNeu5Ac. Interestingly, sialic acid supplementation using a cell-permeable sialic acid analogue (Ac₅Neu5Ac) boosted GD2 expression without or with minor alterations in overall cell surface sialylation. Furthermore, sialic acid supplementation with Ac₅Neu5Ac combined with various histone deacetylase (HDAC) inhibitors, including vorinostat, enhanced GD2 expression in neuroblastoma cells beyond their individual effects. Mechanistic studies revealed that Ac₅Neu5Ac supplementation increased intracellular CMP-Neu5Ac concentrations, thereby providing higher substrate levels for sialyltransferases. Furthermore, HDAC inhibitor treatment increased mRNA expression of the sialyltransferases GM3 synthase (ST3GAL5) and GD3 synthase (ST8SIA1), both of which are involved in GD2 biosynthesis. Our findings reveal that sialic acid analogues and HDAC inhibitors enhance GD2 expression and could potentially be employed to boost anti-GD2 targeted immunotherapy in neuroblastoma patients.

Human-like NSG mouse glycoproteins sialylation pattern changes the phenotype of human lymphocytes and sensitivity to HIV-1 infection

Background

The use of immunodeficient mice transplanted with human hematopoietic stem cells is an accepted approach to study human-specific infectious diseases such as HIV-1 and to investigate multiple aspects of human immune system development. However, mouse and human are different in sialylation patterns of proteins due to evolutionary mutations of the CMP-N-acetylneuraminic acid hydroxylase (CMAH) gene that prevent formation of N-glycolylneuraminic acid from N-acetylneuraminic acid. How changes in the mouse glycoproteins’ chemistry affect phenotype and function of transplanted human hematopoietic stem cells and mature human immune cells in the course of HIV-1 infection are not known.

Results

We mutated mouse CMAH in the NOD/scid-IL2Rγ_c^−/− (NSG) mouse strain, which is widely used for the transplantation of human cells, using the CRISPR/Cas9 system. The new strain provides a better environment for human immune cells. Transplantation of human hematopoietic stem cells leads to broad B cells repertoire, higher sensitivity to HIV-1 infection, and enhanced proliferation of transplanted peripheral blood lymphocytes. The mice showed no effect on the clearance of human immunoglobulins and enhanced transduction efficiency of recombinant adeno-associated viral vector rAAV2/DJ8.

Conclusion

NSG-cmah^−/− mice expand the mouse models suitable for human cells transplantation, and this new model has advantages in generating a human B cell repertoire. This strain is suitable to study different aspects of the human immune system development, provide advantages in patient-derived tissue and cell transplantation, and could allow studies of viral vectors and infectious agents that are sensitive to human-like sialylation of mouse glycoproteins.

Electronic supplementary material

The online version of this article (10.1186/s12865-018-0279-3) contains supplementary material, which is available to authorized users.

Modulation of Immune Tolerance via Siglec-Sialic Acid Interactions

One of the key features of the immune system is its extraordinary capacity to discriminate between self and non-self and to respond accordingly. Several molecular interactions allow the induction of acquired immune responses when a foreign antigen is recognized, while others regulate the resolution of inflammation, or the induction of tolerance to self-antigens. Post-translational signatures, such as glycans that are part of proteins (glycoproteins) and lipids (glycolipids) of host cells or pathogens, are increasingly appreciated as key molecules in regulating immunity vs. tolerance. Glycans are sensed by glycan binding receptors expressed on immune cells, such as C-type lectin receptors (CLRs) and Sialic acid binding immunoglobulin type lectins (Siglecs), that respond to specific glycan signatures by triggering tolerogenic or immunogenic signaling pathways. Glycan signatures present on healthy tissue, inflamed and malignant tissue or pathogens provide signals for “self” or “non-self” recognition. In this review we will focus on sialic acids that serve as “self” molecular pattern ligands for Siglecs. We will emphasize on the function of Siglec-expressing mononuclear phagocytes as sensors for sialic acids in tissue homeostasis and describe how the sialic acid-Siglec axis is exploited by tumors and pathogens for the induction of immune tolerance. Furthermore, we highlight how the sialic acid-Siglec axis can be utilized for clinical applications to induce or inhibit immune tolerance.

Immunoregulatory Siglec ligands are abundant in human and mouse aorta and are up-regulated by high glucose

AIM

Inflammation is a driving force in development of atherosclerosis, and hyperglycemia is a significant risk factor for angiopathy. Siglec-9, expressed on human neutrophils and macrophages, engages specific glycan ligands on tissues to diminish ongoing inflammation.

MATERIALS AND METHOD

Siglec-9 ligands on human aorta were characterized and the effects of high glucose exposure on the expression of ligands for Siglec-9 on human umbilical vein endothelial cells (HUV-EC-C) in vitro and ligands for the comparable siglec (Siglec-E) on mouse aorta in vivo were studied.

KEY FINDINGS

Siglec-9 ligands were expressed broadly on human aorta, as well as on HUV-EC-C. Siglec-9 ligands on HUV-EC-C were sharply up-regulated under high glucose exposure in vitro, as were Siglec-E ligands on the aortas of hyperglycemic mice. Exposure of HUV-EC-C to high-glucose resulted in consistent inhibitory changes in co-cultured macrophages including increased apoptosis and decreased phagocytosis. Control of Siglec-9 ligand expression on HUV-EC-C was downstream of changes in an enzyme involved in their biosynthesis, UDP-galactose-4-epimerase (GALE) and increased cellular N-acetylgalactosamine. The alteration of GALE was associated with the regulatory microRNA hsa-let-7f.

SIGNIFICANCE

We conclude that exposure to high-glucose results in up-regulation of immune inhibitory Siglec-9 sialoglycan ligands on aorta and HUV-EC-C cells downstream of altered GALE and GalNAc expression, resulting in up-regulation of apoptosis and decrease of phagocytic activity of macrophages. Changes in Siglec-9 sialoglycan ligand expression on vascular endothelial cells may be a natural response to the initial steps of atherosclerosis and might be a potential target to regulate inflammation in diabetic angiopathy.

Sialic Acid Blockade Suppresses Tumor Growth by Enhancing T-cell-Mediated Tumor Immunity

Sialic acid sugars on the surface of cancer cells have emerged as potent immune modulators that contribute to the immunosuppressive microenvironment and tumor immune evasion. However, the mechanisms by which these sugars modulate antitumor immunity as well as therapeutic strategies directed against them are limited. Here we report that intratumoral injections with a sialic acid mimetic Ac₅3F_axNeu5Ac block tumor sialic acid expression in vivo and suppress tumor growth in multiple tumor models. Sialic acid blockade had a major impact on the immune cell composition of the tumor, enhancing tumor-infiltrating natural killer cell and CD8⁺ T-cell numbers while reducing regulatory T-cell and myeloid regulatory cell numbers. Sialic acid blockade enhanced cytotoxic CD8⁺ T-cell-mediated killing of tumor cells in part by facilitating antigen-specific T-cell-tumor cell clustering. Sialic acid blockade also synergized with adoptive transfer of tumor-specific CD8⁺ T cells in vivo and enhanced CpG immune adjuvant therapy by increasing dendritic cell activation and subsequent CD8⁺ T-cell responses. Collectively, these data emphasize the crucial role of sialic acids in tumor immune evasion and provide proof of concept that sialic acid blockade creates an immune-permissive tumor microenvironment for CD8⁺ T-cell-mediated tumor immunity, either as single treatment or in combination with other immune-based intervention strategies.Significance: Sialic acid sugars function as important modulators of the immunosuppressive tumor microenvironment that limit potent antitumor immunity.Graphical Abstract: http://cancerres.aacrjournals.org/content/canres/78/13/3574/F1.large.jpg Cancer Res; 78(13); 3574-88. ©2018 AACR.

Using the glycan toolbox for pathogenic interventions and glycan immunotherapy

Glycans play a crucial role to discern between self and foreign entities by providing key recognition elements for C-type lectin receptors (CLRs) and Siglec receptors expressed on immune cells. The glycan recognition of CLRs has illustrated a potent immune modulatory role affecting not only innate pathogen binding and immune signalling, but also Thelper differentiation, cytokine production and antigen presentation. This broad range of influence has implicated glycans in the pathogenesis of infectious diseases but also revealed their extraordinary properties in cancer. Glycan binding by CLRs and Siglecs can be exploited for immunotherapy and the design of glycan-based therapeutics and their multivalent requirements will aspire new biotechnological approaches to effectively interfere in immunological processes in cancer and infectious diseases.

Therapeutic potential of carbohydrates as regulators of macrophage activation

It is well established for a broad range of disease states, including cancer and Mycobacterium tuberculosis infection, that pathogenesis is bolstered by polarisation of macrophages towards an anti-inflammatory phenotype, known as M2. As these innate immune cells are relatively long-lived, their re-polarisation to pro-inflammatory, phagocytic and bactericidal "classically activated" M1 macrophages is an attractive therapeutic approach. On the other hand, there are scenarios where the resolving inflammation, wound healing and tissue remodelling properties of M2 macrophages are beneficial - for example the successful introduction of biomedical implants. Although there are numerous endogenous and exogenous factors that have an impact on the macrophage polarisation spectrum, this review will focus specifically on prominent macrophage-modulating carbohydrate motifs with a view towards highlighting structure-function relationships and therapeutic potential.