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

Integrated chemoenzymatic synthesis of a comprehensive sulfated ganglioside glycan library to decipher functional sulfoglycomics and sialoglycomics

Ganglioside glycans are ubiquitous and complex biomolecules that are involved in a wide range of biological functions and disease processes. Variations in sialylation and sulfation render the structural complexity and diversity of ganglioside glycans, and influence protein-carbohydrate interactions. Structural and functional insights into the biological roles of these glycans are impeded due to the limited accessibility of well-defined structures. Here we report an integrated chemoenzymatic strategy for expeditious and systematic synthesis of a comprehensive 65-membered ganglioside glycan library covering all possible patterns of sulfation and sialylation. This strategy relies on the streamlined modular assembly of three common sialylated precursors by highly stereoselective iterative sialylation, modular site-specific sulfation through flexible orthogonal protecting-group manipulations and enzymatic-catalysed diversification using three sialyltransferase modules and a galactosidase module. These diverse ganglioside glycans enable exploration into their structure-function relationships using high-throughput glycan microarray technology, which reveals that different patterns of sulfation and sialylation on these glycans mediate their unique binding specificities.

Limited impact of cancer-derived gangliosides on anti-tumor immunity in colorectal cancer

Aberrant glycosylation is a key mechanism employed by cancer cells to evade immune surveillance, induce angiogenesis and metastasis, among other hallmarks of cancer. Sialic acids, distinctive terminal glycan structures located on glycoproteins or glycolipids, are prominently upregulated across various tumor types, including colorectal cancer (CRC). Sialylated glycans modulate anti-tumor immune responses through their interactions with Siglecs, a family of glycan-binding receptors with specificity for sialic acid-containing glycoconjugates, often resulting in immunosuppression. In this paper, we investigated the immunomodulatory function of ST3Gal5, a sialyltransferase that catalyzes the addition of α2-3 sialic acids to glycosphingolipids, since lower expression of ST3Gal5 is associated with better survival of CRC patients. We employed CRISPR/Cas9 to knock out the ST3Gal5 gene in two murine CRC cell lines MC38 and CT26. Glycomics analysis confirmed the removal of sialic acids on glycolipids, with no discernible impact on glycoprotein sialylation. Although knocking out ST3Gal5 in both cell lines did not affect in vivo tumor growth, we observed enhanced levels of regulatory T cells in CT26 tumors lacking ST3Gal5. Moreover, we demonstrate that the absence of ST3Gal5 affected size and blood vessel density only in MC38 tumors. In summary, we ascertain that sialylation of glycosphingolipids has a limited influence on the anti-tumor immune response in CRC, despite detecting alterations in the tumor microenvironment, possibly due to a shift in ganglioside abundance.

Site-specific immobilization of the endosialidase reveals QSOX2 is a novel polysialylated protein

Polysialic acid (polySia) is a linear polymer of α2,8-linked sialic acid residues that is of fundamental biological interest due to its pivotal roles in the regulation of the nervous, immune, and reproductive systems in healthy human adults. PolySia is also dysregulated in several chronic diseases, including cancers and mental health disorders. However, the mechanisms underpinning polySia biology in health and disease remain largely unknown. The polySia-specific hydrolase, endoneuraminidase NF (EndoN), and the catalytically inactive polySia lectin EndoNDM, have been extensively used for studying polySia. However, EndoN is heat stable and remains associated with cells after washing. When studying polySia in systems with multiple polysialylated species, the residual EndoN that cannot be removed confounds data interpretation. We developed a strategy for site-specific immobilization of EndoN on streptavidin-coated magnetic beads. We showed that immobilizing EndoN allows for effective removal of the enzyme from samples, while retaining hydrolase activity. We used the same strategy to immobilize the polySia lectin EndoNDM, which enabled the enrichment of polysialylated proteins from complex mixtures such as serum for their identification via mass spectrometry. We used this methodology to identify a novel polysialylated protein, QSOX2, which is secreted from the breast cancer cell line MCF-7. This method of site-specific immobilization can be utilized for other enzymes and lectins to yield insight into glycobiology.

The sialic acid-Siglec immune checkpoint: an opportunity to enhance immune responses and therapy effectiveness in melanoma

Modulation of immune responses through immune checkpoint blockade has revolutionized cutaneous melanoma treatment. However, it is still the case that not all patients respond successfully to these therapies, indicating the presence of as yet unknown resistance mechanisms. Hence, it is crucial to find novel targets to improve therapy efficacy. One of the described resistance mechanisms is regulated by immune inhibitory Siglec receptors, which are engaged by the carbohydrates sialic acids expressed on tumour cells, contributing to programmed cell death protein-1 (PD1)-like immune suppression mechanisms. In this review, we provide an overview on the regulation of sialic acid synthesis, its expression in melanoma, and the contribution of the sialic acid-Siglec axis to tumour development and immune suppressive mechanisms in the tumour microenvironment. Finally, we highlight potential sialic acid-Siglec axis-related therapeutics to improve the treatment of melanoma.

Comparison of Siglec-1 protein networks and expression patterns in sperm and male reproductive tracts of mice, rats, and humans

Background

Sialic acid-binding immunoglobulin-like lectin 1 (Siglec-1) is a transmembrane glycoprotein involved in the sialic acid (Sia)-dependent regulation of the immune system. Siglec-1 expression has recently been identified in the male reproductive tract (MRT) of several species, including humans, cattle, horses, and sheep, and may play a role in modulating fertility in a Sia-dependent manner.

Materials and Methods

In this study, protein-protein interaction (PPI) analysis of Siglec-1 was conducted to identify associated network protein conservation, and the expression of Siglec-1 in the MRT of mice and rats, including their accessory sex glands and spermatozoa was determined by immunostaining.

Results

Network analysis of proteins with Siglec-1 in mice and rats demonstrated significant similarity to human Siglec-1 networks, suggesting a similar conservation of network proteins between these species and, hence, a potential conservation role in immune modulation and function. Specific immunostaining patterns of mouse and rat testes, epididymis, ductus deferens, accessory sex gland tissues, and sperm were detected using human Siglec-1. These results confirmed that the human Siglec-1 antibody could cross-react with mouse and rat Siglec-1, suggesting that the specific expression patterns of Siglec-1 in the MRT and sperm of both mice and rats are similar to those observed in other species.

Conclusions

The conservation of Siglec-1 expression patterns in sperm and within the MRT and the similarity of protein networks for Siglec-1 across species suggest that Siglec-1 may function in a similar manner across species. These results also suggest that rodents may serve as a valuable model system for exploring the function of Siglecs in the reproductive system across species and their potential role in modulating fertility in a Sia-dependent manner.

Bioinformatic analysis reveals prognostic value and immunotherapy potential of Siglec-15 in laryngeal squamous cell carcinoma

Background

Laryngeal squamous cell carcinoma (LSCC) is the ultimate common malignant head and neck cancer with dismal prognosis. The expression pattern and clinical significance of Siglec-15 (Sialic acid-binding immunoglobulin-like lectin 15) in LSCC are poorly understood. In order to lay the groundwork for future immune-related research on Siglec-15 in LSCC, we set out to study its expression and prognostic importance in the disease, as well as to use bioinformatics to investigate the immune features modulated by Siglec-15 in LSCC.

Methods

① In order to get the gene expression profile and clinical data for TCGA head and neck cancer (TCGA-HNSC), you may access the relevant data from UCSC xena and use 110 cases of laryngeal cancer as a training set. Two datasets, GSE27020 and GSE25727, were obtained from the GEO databank and utilized as validation sets. These datasets include expression profiles and clinical information. The Siglec-15 gene and immune characteristics were analyzed by bioinformatics methods. ② Retrospectively collected routine paraffin specimens from patients with pathological diagnosis of squamous cell carcinoma from December 2012 to November 2015 in Sun Yat-sen Memorial Hospital and fresh frozen tissue of patients from June 2021 to March 2022. Immunohistochemistry method, immunofluorescence technique and real-time quantitative PCR was used to examine the difference of Siglec-15 appearance in LSCC tissue and adjacent tissue, and its correlation of prognosis, clinic pathological characteristics and CD8+T lymphocyte infiltration. Using human laryngeal cancer cell line (LCC), we studied the influence of Siglec-15 in cell proliferation and invasion.

Results

We identified Siglec-15 was upregulated in LSCC. The patients in Siglec-15 high expression group had a poor overall survival (OS) based on the clinical information from TGCA and 111 LSCC patients that hospitalized in Sun Yat-sen Memorial Hospital. The COX regression analysis indicated Siglec-15 as an independent predictor for poor prognosis of LSCC. Bioinformatic analysis suggested that the high expression of Siglec-15 shape an immune suppressive tumor microenvironment (TEM), leading to poor response to immunotherapy in LSCC. Siglec-15 enhanced cell invasion and proliferation, as we showed in vitro.

Conclusion

Our study support Siglec-15 as a potential predictor for LSCC prognosis and an attractive target for LSCC immunotherapy.

The Potential of Siglecs and Sialic Acids as Biomarkers and Therapeutic Targets in Tumor Immunotherapy

The dysregulation of sialic acid is closely associated with oncogenesis and tumor progression. Most tumor cells exhibit sialic acid upregulation. Sialic acid-binding immunoglobulin-like lectins (Siglecs) are receptors that recognize sialic acid and are expressed in various immune cells. The activity of Siglecs in the tumor microenvironment promotes immune escape, mirroring the mechanisms of the well-characterized PD-1/PD-L1 pathway in cancer. Cancer cells utilize sialic acid-linked glycans to evade immune surveillance. As Siglecs exhibit similar mechanisms as the established immune checkpoint inhibitors (ICIs), they are potential therapeutic targets for different forms of cancer, especially ICI-resistant malignancies. Additionally, the upregulation of sialic acid serves as a potential tumor biomarker. This review examines the feasibility of using sialic acid and Siglecs for early malignant tumor detection and discusses the potential of targeting Siglec-sialic acid interaction as a novel cancer therapeutic strategy.

Targeting Breast Cancer with N-Acetyl-D-Glucosamine: Integrating Machine Learning and Cellular Assays for Promising Results

BACKGROUND

Breast cancer is a common cancer with high mortality rates. Early diagnosis is crucial for reducing the prognosis and mortality rates. Therefore, the development of alternative treatment options is necessary.

OBJECTIVE

This study aimed to investigate the inhibitory effect of N-acetyl-D-glucosamine (D-GlcNAc) on breast cancer using a machine learning method. The findings were further confirmed through assays on breast cancer cell lines.

METHODS

MCF-7 and 4T1 cell lines (ATCC) were cultured in the presence and absence of varying concentrations of D-GlcNAc (0.5 mM, 1 mM, 2 mM, and 4 mM) for 72 hours. A xenograft mouse model for breast cancer was established by injecting 4T1 cells into mammary glands. D-GlcNAc (2 mM) was administered intraperitoneally to mice daily for 28 days, and histopathological effects were evaluated at pre-tumoral and post-tumoral stages.

RESULTS

Treatment with 2 mM and 4 mM D-GlcNAc significantly decreased cell proliferation rates in MCF-7 and 4T1 cell lines and increased Fas expression. The number of apoptotic cells was significantly higher than untreated cell cultures (p < 0.01 - p < 0.0001). D-GlcNAc administration also considerably reduced tumour size, mitosis, and angiogenesis in the post-treatment group compared to the control breast cancer group (p < 0.01 - p < 0.0001). Additionally, molecular docking/dynamic analysis revealed a high binding affinity of D-GlcNAc to the marker protein HER2, which is involved in tumour progression and cell signalling.

CONCLUSION

Our study demonstrated the positive effect of D-GlcNAc administration on breast cancer cells, leading to increased apoptosis and Fas expression in the malignant phenotype. The binding affinity of D-GlcNAc to HER2 suggests a potential mechanism of action. These findings contribute to understanding D-GlcNAc as a potential anti-tumour agent for breast cancer treatment.

SIGLEC-5/14 Inhibits CD11b/CD18 Integrin Activation and Neutrophil-Mediated Tumor Cell Cytotoxicity

Since the successful introduction of checkpoint inhibitors targeting the adaptive immune system, monoclonal antibodies inhibiting CD47-SIRPα interaction have shown promise in enhancing anti-tumor treatment efficacy. Apart from SIRPα, neutrophils express a broad repertoire of inhibitory receptors, including several members of the sialic acid-binding receptor (SIGLEC) family. Here, we demonstrate that interaction between tumor cell-expressed sialic acids and SIGLEC-5/14 on neutrophils inhibits antibody-dependent cellular cytotoxicity (ADCC). We observed that conjugate formation and trogocytosis, both essential processes for neutrophil ADCC, were limited by the sialic acid-SIGLEC-5/14 interaction. During neutrophil-tumor cell conjugate formation, we found that inhibition of the interaction between tumor-expressed sialic acids and SIGLEC-5/14 on neutrophils increased the CD11b/CD18 high affinity conformation. By dynamic acoustic force measurement, the binding between tumor cells and neutrophils was assessed. The interaction between SIGLEC-5/14 and the sialic acids was shown to inhibit the CD11b/CD18-regulated binding between neutrophils and antibody-opsonized tumor cells. Moreover, the interaction between sialic acids and SIGLEC-5/14-consequently hindered trogocytosis and tumor cell killing. In summary, our results provide evidence that the sialic acid-SIGLEC-5/14 interaction is an additional target for innate checkpoint blockade in the tumor microenvironment.

Exploiting innate immunity for cancer immunotherapy

Immunotherapies have revolutionized the treatment paradigms of various types of cancers. However, most of these immunomodulatory strategies focus on harnessing adaptive immunity, mainly by inhibiting immunosuppressive signaling with immune checkpoint blockade, or enhancing immunostimulatory signaling with bispecific T cell engager and chimeric antigen receptor (CAR)-T cell. Although these agents have already achieved great success, only a tiny percentage of patients could benefit from immunotherapies. Actually, immunotherapy efficacy is determined by multiple components in the tumor microenvironment beyond adaptive immunity. Cells from the innate arm of the immune system, such as macrophages, dendritic cells, myeloid-derived suppressor cells, neutrophils, natural killer cells, and unconventional T cells, also participate in cancer immune evasion and surveillance. Considering that the innate arm is the cornerstone of the antitumor immune response, utilizing innate immunity provides potential therapeutic options for cancer control. Up to now, strategies exploiting innate immunity, such as agonists of stimulator of interferon genes, CAR-macrophage or -natural killer cell therapies, metabolic regulators, and novel immune checkpoint blockade, have exhibited potent antitumor activities in preclinical and clinical studies. Here, we summarize the latest insights into the potential roles of innate cells in antitumor immunity and discuss the advances in innate arm-targeted therapeutic strategies.

Targeting myeloid checkpoint Siglec-10 reactivates antitumor immunity and improves anti-programmed cell death 1 efficacy in gastric cancer

OBJECTIVE

Immunotherapy has not yielded satisfactory therapeutic responses in gastric cancer (GC). However, targeting myeloid checkpoints holds promise for expanding the potential of immunotherapy. This study aims to evaluate the critical role of Siglec-10+ tumor-associated macrophages (TAMs) in regulating antitumor immunity and to explore the potential of the myeloid checkpoint Siglec-10 as an interventional target.

DESIGN

Siglec-10+ TAMs were assessed based on immunohistochemistry on tumor microarrays and RNA-sequencing data. Flow cytometry, RNA sequencing, and single-cell RNA-sequencing analysis were employed to characterize the phenotypic and transcriptional features of Siglec-10+ TAMs and their impact on CD8+ T cell-mediated antitumor immunity. The effectiveness of Siglec-10 blockade, either alone or in combination with anti-programmed cell death 1 (PD-1), was evaluated using an ex vivo GC tumor fragment platform based on fresh tumor tissues.

RESULTS

Siglec-10 was predominantly expressed on TAMs in GC, and associated with tumor progression. In Zhongshan Hospital cohort, Siglec-10+ TAMs predicted unfavorable prognosis (n=446, p<0.001) and resistance to adjuvant chemotherapy (n=331, p<0.001), which were further validated in exogenous cohorts. In the Samsung Medical Center cohort, Siglec-10+ TAMs demonstrated inferior response to pembrolizumab in GC (n=45, p=0.008). Furthermore, Siglec-10+ TAMs exhibited an immunosuppressive phenotype and hindered T cell-mediated antitumor immune response. Finally, blocking Siglec-10 reinvigorated the antitumor immune response and synergistically enhances anti-PD-1 immunotherapy in an ex vivo GC tumor fragment platform.

CONCLUSIONS

In GC, the myeloid checkpoint Siglec-10 contributes to the regulation of immunosuppressive property of TAMs and promotes the depletion of CD8+ T cells, ultimately facilitating immune evasion. Targeting Siglec-10 represents a potential strategy for immunotherapy in GC.

The cancer-associated glycan polysialic acid is dysregulated in systemic sclerosis and is associated with fibrosis

OBJECTIVE

Systemic sclerosis (SSc) is a rare but deadly disease characterized by autoimmunity, vasculopathy, and fibrosis. Fibrotic complications associated with SSc correlate with severe morbidity and mortality. Previous studies in SSc have identified fibroblasts as the primary drivers of fibrosis; however, the mechanism(s) promoting this are not well understood. Aberrant glycosylation, particularly polysialylation (polySia), has been described as a prominent feature of aggressive cancers. Inspired by this observation, we aimed to determine if polySia is dysregulated in various forms of SSc.

METHODS

All patients with SSc met the 2013 ACR/EULAR. Patients were sub-classified into limited cutaneous (lSSc, N = 5 or 46 patients for polySia quantification in the dermis or serum; respectively), diffuse cutaneous (dSSc, N = 11 or 18 patients for polySia quantification in the dermis or serum; respectively), or patients with dSSc treated with an autologous stem cell transplantation (post-ASCT, N = 4 patients for quantification in the dermis). Dermal polySia levels were measured via immunofluorescence microscopy in 10 μm dermal sections, quantified in each group (healthy volunteers (HC), lSSc, dSSc, and post-ASCT) and correlated with skin fibrosis (via the modified Rodnan skin score (mRSS)). Similarly, serum polySia was quantified in each group, and correlated with the mRSS.

RESULTS

Dermal polySia levels were highest in patients with dSSc (compared to HC < 0.001), and correlated with the degree of fibrosis in all of the groups (P = 0.008). Serum polySia was higher in all SSc groups (p < 0.001) and correlated with the severity of mRSS (p < 0.0001).

CONCLUSION

Polysia is more abundant in the skin and sera from patients with SSc and correlates with the degree of skin fibrosis. The aberrant expression of polySia highlights its potential use as a biomarker in patients with progressive forms of SSc. Dysregulated polySia levels in SSc further emphasizes the cancer-like phenotype present in SSc, which may promote fibrosis and immune dysregulation.

Roles of the Siglec family in bone and bone homeostasis

Tremendous progress has been seen in the study of the role of sialic acid binding im-munoglobulin type lectins (Siglecs) in osteoimmunology in the past two decades. Interest in Siglecs as immune checkpoints has grown from the recognition that Siglecs have relevance to human disease. Siglecs play important roles in inflammation and cancer, and play key roles in immune cell signaling. By recognizing common sialic acid containing glycans on glycoproteins and glycolipids as regulatory receptors for immune cell signals, Siglecs are expressed on most immune cells and play important roles in normal homeostasis and self-tolerance. In this review, we describe the role that the siglec family plays in bone and bone homeostasis, including the regulation of osteoclast differentiation as well as recent advances in inflammation, cancer and osteoporosis. Particular emphasis is placed on the relevant functions of Siglecs in self-tolerance and as pattern recognition receptors in immune responses, thereby potentially providing emerging strategies for the treatment of bone related diseases.

Recent advances in the development of sialyltransferase inhibitors to control cancer metastasis: A comprehensive review

Metastasis accounts for the majority of cancer-associated mortalities, representing a huge health and economic burden. One of the mechanisms that enables metastasis is hypersialylation, characterized by an overabundance of sialylated glycans on the tumor surface, which leads to repulsion and detachment of cells from the original tumor. Once the tumor cells are mobilized, sialylated glycans hijack the natural killer T-cells through self-molecular mimicry and activatea downstream cascade of molecular events that result in inhibition of cytotoxicity and inflammatory responses against cancer cells, ultimately leading to immune evasion. Sialylation is mediated by a family of enzymes known as sialyltransferases (STs), which catalyse the transfer of sialic acid residue from the donor, CMP-sialic acid, onto the terminal end of an acceptor such as N-acetylgalactosamine on the cell-surface. Upregulation of STs increases tumor hypersialylation by up to 60% which is considered a distinctive hallmark of several types of cancers such as pancreatic, breast, and ovarian cancer. Therefore, inhibiting STs has emerged as a potential strategy to prevent metastasis. In this comprehensive review, we discuss the recent advances in designing novel sialyltransferase inhibitors using ligand-based drug design and high-throughput screening of natural and synthetic entities, emphasizing the most successful approaches. We analyse the limitations and challenges of designing selective, potent, and cell-permeable ST inhibitors that hindered further development of ST inhibitors into clinical trials. We conclude by analysing emerging opportunities, including advanced delivery methods which further increase the potential of these inhibitors to enrich the clinics with novel therapeutics to combat metastasis.

Sialic Acids on Tumor Cells Modulate IgA Therapy by Neutrophils via Inhibitory Receptors Siglec-7 and Siglec-9

Immunotherapy with targeted therapeutic antibodies is often ineffective in long-term responses in cancer patients due to resistance mechanisms such as overexpression of checkpoint molecules. Similar to T lymphocytes, myeloid immune cells express inhibitory checkpoint receptors that interact with ligands overexpressed on cancer cells, contributing to treatment resistance. While CD47/SIRPα-axis inhibitors in combination with IgA therapy have shown promise, complete tumor eradication remains a challenge, indicating the presence of other checkpoints. We investigated hypersialylation on the tumor cell surface as a potential myeloid checkpoint and found that hypersialylated cancer cells inhibit neutrophil-mediated tumor killing through interactions with sialic acid-binding immunoglobulin-like lectins (Siglecs). To enhance antibody-dependent cellular cytotoxicity (ADCC) using IgA as therapeutic, we explored strategies to disrupt the interaction between tumor cell sialoglycans and Siglecs expressed on neutrophils. We identified Siglec-9 as the primary inhibitory receptor, with Siglec-7 also playing a role to a lesser extent. Blocking Siglec-9 enhanced IgA-mediated ADCC by neutrophils. Concurrent expression of multiple checkpoint ligands necessitated a multi-checkpoint-blocking approach. In certain cancer cell lines, combining CD47 blockade with desialylation improved IgA-mediated ADCC, effectively overcoming resistance that remained when blocking only one checkpoint interaction. Our findings suggest that a combination of CD47 blockade and desialylation may be necessary to optimize cancer immunotherapy, considering the upregulation of checkpoint molecules by tumor cells to evade immune surveillance.

Disruption of the sialic acid/Siglec-9 axis improves antibody-mediated neutrophil cytotoxicity towards tumor cells

Upregulation of surface expressed sialoglycans on tumor cells is one of the mechanisms which promote tumor growth and progression. Specifically, the interactions of sialic acids with sialic acid-binding immunoglobulin-like lectins (Siglecs) on lymphoid or myeloid cells transmit inhibitory signals and lead to suppression of anti-tumor responses. Here, we show that neutrophils express among others Siglec-9, and that EGFR and HER2 positive breast tumor cells express ligands for Siglec-9. Treatment of tumor cells with neuraminidases or a sialyl transferase inhibitor significantly reduced binding of a soluble recombinant Siglec-9-Fc fusion protein, while EGFR and HER2 expression remained unchanged. Importantly, the cytotoxic activity of neutrophils driven by therapeutic EGFR or HER2 antibodies in vitro was increased by blocking the sialic acid/Siglec interaction, either by reducing tumor cell sialylation or by a Siglec-9 blocking antibody containing an effector silenced Fc domain. In vivo a short-term xenograft mouse model confirmed the improved therapeutic efficacy of EGFR antibodies against sialic acid depleted, by a sialyltransferase inhibitor, tumor cells compared to untreated cells. Our studies demonstrate that sialic acid/Siglec interactions between tumor cells and myeloid cells can impair antibody dependent tumor cell killing, and that Siglec-9 on polymorphonuclear cells (PMN) is critically involved. Considering that PMN are often a highly abundant cell population in the tumor microenvironment, Siglec-9 constitutes a promising target for myeloid checkpoint blockade to improve antibody-based tumor immunotherapy.

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.

MYC-driven synthesis of Siglec ligands is a glycoimmune checkpoint

The Siglecs (sialic acid-binding immunoglobulin-like lectins) are glycoimmune checkpoint receptors that suppress immune cell activation upon engagement of cognate sialoglycan ligands. The cellular drivers underlying Siglec ligand production on cancer cells are poorly understood. We find the MYC oncogene causally regulates Siglec ligand production to enable tumor immune evasion. A combination of glycomics and RNA-sequencing of mouse tumors revealed the MYC oncogene controls expression of the sialyltransferase St6galnac4 and induces a glycan known as disialyl-T. Using in vivo models and primary human leukemias, we find that disialyl-T functions as a "don't eat me" signal by engaging macrophage Siglec-E in mice or the human ortholog Siglec-7, thereby preventing cancer cell clearance. Combined high expression of MYC and ST6GALNAC4 identifies patients with high-risk cancers and reduced tumor myeloid infiltration. MYC therefore regulates glycosylation to enable tumor immune evasion. We conclude that disialyl-T is a glycoimmune checkpoint ligand. Thus, disialyl-T is a candidate for antibody-based checkpoint blockade, and the disialyl-T synthase ST6GALNAC4 is a potential enzyme target for small molecule-mediated immune therapy.

The Blessed Union of Glycobiology and Immunology: A Marriage That Worked

In this article, we discuss the main aspects regarding the recognition of cell surface glycoconjugates and the immunomodulation of responses against the progression of certain pathologies, such as cancer and infectious diseases. In the first part, we talk about different aspects of glycoconjugates and delve deeper into the importance of N-glycans in cancer immunotherapy. Then, we describe two important lectin families that have been very well studied in the last 20 years. Examples include the sialic acid-binding immunoglobulin (Ig)-like lectins (siglecs), and galectins. Finally, we discuss a topic that needs to be better addressed in the field of glycoimmunology: the impact of oncofetal antigens on the cells of the immune system. New findings in this area are of great importance for advancement, especially in the field of oncology, since it is already known that cellular interactions mediated by carbohydrate-carbohydrate and/or carbohydrate proteins are able to modulate the progression of different types of cancer in events that compromise the functionality of the immune responses.

Targeting the Siglec-Sialic Acid Immune Axis in Cancer: Current and Future Approaches

The sialic acid-binding immunoglobulin-like lectin (Siglec)-sialic acid immune axis is an evolutionarily conserved immunoregulatory pathway that provides a mechanism for establishing self-recognition and combatting invasive pathogens. Perturbations in the pathway lead to many immune dysregulated diseases, including autoimmunity, neurodegeneration, allergic conditions, and cancer. The purpose of this review is to provide a brief overview of the relationship between Siglecs and sialic acid as they relate to human health and disease, to consider current Siglec-based therapeutics, and to discuss new therapeutic approaches targeting the Siglec-sialic acid immune axis, with a focus on cancer.

Targeted nanomedicines remodeling immunosuppressive tumor microenvironment for enhanced cancer immunotherapy

Cancer immunotherapy has significantly flourished and revolutionized the limited conventional tumor therapies, on account of its good safety and long-term memory ability. Discouragingly, low patient response rates and potential immune-related side effects make it rather challenging to literally bring immunotherapy from bench to bedside. However, it has become evident that, although the immunosuppressive tumor microenvironment (TME) plays a pivotal role in facilitating tumor progression and metastasis, it also provides various potential targets for remodeling the immunosuppressive TME, which can consequently bolster the effectiveness of antitumor response and tumor suppression. Additionally, the particular characteristics of TME, in turn, can be exploited as avenues for designing diverse precise targeting nanomedicines. In general, it is of urgent necessity to deliver nanomedicines for remodeling the immunosuppressive TME, thus improving the therapeutic outcomes and clinical translation prospects of immunotherapy. Herein, we will illustrate several formation mechanisms of immunosuppressive TME. More importantly, a variety of strategies concerning remodeling immunosuppressive TME and strengthening patients' immune systems, will be reviewed. Ultimately, we will discuss the existing obstacles and future perspectives in the development of antitumor immunotherapy. Hopefully, the thriving bloom of immunotherapy will bring vibrancy to further exploration of comprehensive cancer treatment.

Mass spectrometry-based shotgun glycomics for discovery of natural ligands of glycan-binding proteins

The non-covalent associations of complex carbohydrates (glycans) with glycan-binding proteins mediate many important physiological and pathophysiological processes. Identifying these interactions is essential to understanding their diverse biological functions and enables the development of new disease treatments and diagnostics. Knowledge of the repertoire of glycans recognized by most glycan-binding proteins and their affinities is incomplete. Mass spectrometry-based screening of natural glycan libraries has emerged as a promising approach to defining the glycan interactome of glycan-binding proteins. Here, we review recent advances in mass spectrometry-based natural library screening that have led to the discovery of glycan ligands of endogenous and exogenous proteins and illuminated their binding specificities.

Insights into the Role of Sialylation in Cancer Metastasis, Immunity, and Therapeutic Opportunity

Sialylation is an enzymatic process that covalently attaches sialic acids to glycoproteins and glycolipids and terminates them by creating sialic acid-containing glycans (sialoglycans). Sialoglycans, usually located in the outmost layers of cells, play crucial biological roles, notably in tumor transformation, growth, metastasis, and immune evasion. Thus, a deeper comprehension of sialylation in cancer will help to facilitate the development of innovative cancer therapies. Cancer sialylation-related articles have consistently increased over the last four years. The primary subjects of these studies are sialylation, cancer, immunotherapy, and metastasis. Tumor cells activate endothelial cells and metastasize to distant organs in part by the interactions of abnormally sialylated integrins with selectins. Furthermore, cancer sialylation masks tumor antigenic epitopes and induces an immunosuppressive environment, allowing cancer cells to escape immune monitoring. Cytotoxic T lymphocytes develop different recognition epitopes for glycosylated and nonglycosylated peptides. Therefore, targeting tumor-derived sialoglycans is a promising approach to cancer treatments for limiting the dissemination of tumor cells, revealing immunogenic tumor antigens, and boosting anti-cancer immunity. Exploring the exact tumor sialoglycans may facilitate the identification of new glycan targets, paving the way for the development of customized cancer treatments.

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.

Endothelial Glycocalyx

The glycocalyx is a polysaccharide structure that protrudes from the body of a cell. It is primarily conformed of glycoproteins and proteoglycans, which provide communication, electrostatic charge, ionic buffering, permeability, and mechanosensation-mechanotransduction capabilities to cells. In blood vessels, the endothelial glycocalyx that projects into the vascular lumen separates the vascular wall from the circulating blood. Such a physical location allows a number of its components, including sialic acid, glypican-1, heparan sulfate, and hyaluronan, to participate in the mechanosensation-mechanotransduction of blood flow-dependent shear stress, which results in the synthesis of nitric oxide and flow-mediated vasodilation. The endothelial glycocalyx also participates in the regulation of vascular permeability and the modulation of inflammatory responses, including the processes of leukocyte rolling and extravasation. Its structural architecture and negative charge work to prevent macromolecules greater than approximately 70 kDa and cationic molecules from binding and flowing out of the vasculature. This also prevents the extravasation of pathogens such as bacteria and virus, as well as that of tumor cells. Due to its constant exposure to shear and circulating enzymes such as neuraminidase, heparanase, hyaluronidase, and matrix metalloproteinases, the endothelial glycocalyx is in a continuous process of degradation and renovation. A balance favoring degradation is associated with a variety of pathologies including atherosclerosis, hypertension, vascular aging, metastatic cancer, and diabetic vasculopathies. Consequently, ongoing research efforts are focused on deciphering the mechanisms that promote glycocalyx degradation or limit its syntheses, as well as on therapeutic approaches to improve glycocalyx integrity with the goal of reducing vascular disease. © 2022 American Physiological Society. Compr Physiol 12: 1-31, 2022.

The genetics of autoimmune Addison disease: past, present and future

Autoimmune Addison disease is an endocrinopathy that is fatal if not diagnosed and treated in a timely manner. Its rarity has hampered unbiased studies of the predisposing genetic factors. A 2021 genome-wide association study, explaining up to 40% of the genetic susceptibility, has revealed new disease loci and reproduced some of the previously reported associations, while failing to reproduce others. Credible risk loci from both candidate gene and genome-wide studies indicate that, like one of its most common comorbidities, type 1 diabetes mellitus, Addison disease is primarily caused by aberrant T cell behaviour. Here, we review the current understanding of the genetics of autoimmune Addison disease and its position in the wider field of autoimmune disorders. The mechanisms that could underlie the effects on the adrenal cortex are also discussed.

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.

Glycobiology of Cancer: Sugar Drives the Show

Cancer development and progression is associated with aberrant changes in cellular glycosylation. Cells expressing altered glycan-structures are recognized by cells of the immune system, favoring the induction of inhibitory immune processes which subsequently promote tumor growth and spreading. Here, we discuss about the importance of glycobiology in modern medicine, taking into account the impact of altered glycan structures expressed in cancer cells as potential glycobiomarkers of disease, as well as on cancer development and progression.

Bioinformatics Analyses Identify the Therapeutic Potential of ST8SIA6 for Colon Cancer

Sialylation of glycoproteins is modified by distinct sialyltransferases such as ST3Gal, ST6Gal, ST6GalNAc, or ST8SIA with α2,3-, α2,6-, or α2,8-linkages. Alteration of these sialyltransferases causing aberrant sialylation is associated with the progression of colon cancer. However, among the ST8- sialyltransferases, the role of ST8SIA6 in colon cancer remains poorly understood. In this study, we explored the involvement of ST8SIA6 in colon cancer using multiple gene databases. The relationship between ST8SIA6 expression and tumor stages/grades was investigated by UALCAN analysis, and Kaplan–Meier Plotter analysis was used to analyze the expression of ST8SIA6 on the survival outcome of colon cancer patients. Moreover, the biological functions of ST8SIA6 in colon cancer were explored using LinkedOmics and cancer cell metabolism gene DB. Finally, TIMER and TISMO analyses were used to delineate ST8SIA6 levels in tumor immunity and immunotherapy responses, respectively. ST8SIA6 downregulation was associated with an advanced stage and poorly differentiated grade; however, ST8SIA6 expression did not affect the survival outcomes in patients with colon cancer. Gene ontology analysis suggested that ST8SIA6 participates in cell surface adhesion, angiogenesis, and membrane vesicle trafficking. In addition, ST8SIA6 levels affected immunocyte infiltration and immunotherapy responses in colon cancer. Collectively, these results suggest that ST8SIA6 may serve as a novel therapeutic target towards personalized medicine for colon cancer.

Siglec Receptors Modulate Dendritic Cell Activation and Antigen Presentation to T Cells in Cancer

Interactions between sialylated glycans and sialic acid-binding immunoglobulin-like lectin (Siglec) receptors have been recently described as potential new immune checkpoint that can be targeted to improve anticancer immunity. Myeloid cells have been reported to express a wide range of different Siglecs; however, their expression and functions on cancer-associated dendritic cells (DCs) were not fully characterized. We found that classical conventional DCs (cDCs) from cancer patient samples have a high expression of several inhibitory Siglecs including Siglec-7, Siglec-9, and Siglec-10. In subcutaneous murine tumor models, we also found an upregulation of the inhibitory Siglec-E receptor on cancer-associated cDCs. DC lines and bone marrow-derived DCs (BMDCs) with expression of these inhibitory Siglecs showed impaired maturation states on transcriptome and protein level. Furthermore, ablation of these inhibitory Siglecs from DCs enhanced their capability to prime antigen-specific T cells and induce proliferation. Our work provides a deeper understanding of the influence of inhibitory Siglecs on DCs and reveals a potential new target to improve cancer immunotherapy.

Surface glycan targeting for cancer nano-immunotherapy

Cancer immunotherapy is an emerging therapeutic strategy for cancer treatment. Most of the immunotherapeutics approved by the FDA regulate the innate immune system and associated immune cell activity, with immune check inhibitors in particular having transformed the field of cancer immunotherapy due to their significant clinical potential. However, previously reported immunotherapeutics have exhibited undesirable side effects, including autoimmune toxicity and inflammation. Controlling these deleterious responses and designing therapeutics that can precisely target specific regions are thus crucial to improving the efficacy of cancer immunotherapies. Recent studies have reported that cancer cells employ glycan-immune checkpoint interactions to modulate immune cell activity. Thus, the recognition of cancer glycan moieties such as sialoglycans may improve the anticancer activity of immune cells. In this review, we discuss recent advances in cancer immunotherapies involving glycans and glycan-targeting technologies based on nanomaterial-assisted local delivery systems.

Peripheral immune cells in NAFLD patients: A spyhole to disease progression

Nonalcoholic fatty liver disease (NAFLD) is a worldwide leading cause of chronic liver disease, but we still lack ideal non-invasive tools for diagnosis and evaluation of nonalcoholic steatohepatitis (NASH) and related liver fibrosis in NAFLD population. Systemic immune dysregulations such as metabolic inflammation are believed to play central role in the development of NAFLD, signifying the hope of utilizing quantitative and phenotypic changes in peripheral immune cells among NAFLD patients as a diagnostic tool of NASH and fibrosis. In this review, we summarize the known changes in peripheral immune cells from NAFLD/NASH patients and their potential relationship with NAFLD and NASH progression. Potential challenges and possible solutions for further clinical translation are also discussed.

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.

Survival Advantage Following TAG-72 Antigen-Directed Cancer Surgery in Patients With Colorectal Carcinoma: Proposed Mechanisms of Action

Patients with colorectal carcinoma (CRC) continue to have variable clinical outcomes despite undergoing the same surgical procedure with curative intent and having the same pathologic and clinical stage. This problem suggests the need for better techniques to assess the extent of disease during surgery. We began to address this problem 35 years ago by injecting patients with either primary or recurrent CRC with 125I-labeled murine monoclonal antibodies against the tumor-associated glycoprotein-72 (TAG-72) and using a handheld gamma-detecting probe (HGDP) for intraoperative detection and removal of radioactive, i.e., TAG-72-positive, tissue. Data from these studies demonstrated a significant difference in overall survival data (p < 0.005 or better) when no TAG-72-positive tissue remained compared to when TAG-72-positive tissue remained at the completion of surgery. Recent publications indicate that aberrant glycosylation of mucins and their critical role in suppressing tumor-associated immune response help to explain the cellular mechanisms underlying our results. We propose that monoclonal antibodies to TAG-72 recognize and bind to antigenic epitopes on mucins that suppress the tumor-associated immune response in both the tumor and tumor-draining lymph nodes. Complete surgical removal of all TAG-72-positive tissue serves to reverse the escape phase of immunoediting, allowing a resetting of this response that leads to improved overall survival of the patients with either primary or recurrent CRC. Thus, the status of TAG-72 positivity after resection has a significant impact on patient survival.

Siglecs as Therapeutic Targets in Cancer

Hypersialylation is a common post-translational modification of protein and lipids found on cancer cell surfaces, which participate in cell-cell interactions and in the regulation of immune responses. Sialic acids are a family of nine-carbon α-keto acids found at the outermost ends of glycans attached to cell surfaces. Given their locations on cell surfaces, tumor cells aberrantly overexpress sialic acids, which are recognized by Siglec receptors found on immune cells to mediate broad immunomodulatory signaling. Enhanced sialylation exposed on cancer cell surfaces is exemplified as "self-associated molecular pattern" (SAMP), which tricks Siglec receptors found on leukocytes to greatly down-regulate immune responsiveness, leading to tumor growth. In this review, we focused on all 15 human Siglecs (including Siglec XII), many of which still remain understudied. We also highlighted strategies that disrupt the course of Siglec-sialic acid interactions, such as antibody-based therapies and sialic acid mimetics leading to tumor cell depletion. Herein, we introduced the central roles of Siglecs in mediating pro-tumor immunity and discussed strategies that target these receptors, which could benefit improved cancer immunotherapy.

Targeting sialylation to treat central nervous system diseases

Sialic acid-binding immunoglobulin-type lectins (SIGLECs) are membrane receptors that are preferentially expressed on immune cells and recognize sialylated proteins, lipids, and RNA. Sialic acids and signaling through SIGLECs are increasingly recognized for their essential roles in immune system homeostasis as well as nervous system development, plasticity, and repair. Dysregulated sialylation and SIGLEC dysfunctions contribute to several chronic diseases of the central nervous system (CNS) in which current therapeutic options are very limited. While only a few therapies targeting SIGLECs are currently being tested in clinical trials, the area emerged as one of the most dynamic and active fields in glycobiology and drug development. This review highlights recent insights into sialic acid and SIGLEC function in CNS pathologies and illustrates opportunities and challenges for the development of sialic acid-based and SIGLEC-targeted therapies for neurological diseases.

Glycoproteins Presenting Galactose and N-Acetylgalactosamine in Human Seminal Plasma as Potential Players Involved in Immune Modulation in the Fertilization Process

In light of recent research, there is increasing evidence showing that extracellular semen components have a significant impact on the immune reaction of the female partner, leading to the tolerogenic response enabling the embryo development and implantation as well as further progress of healthy pregnancy. Seminal plasma glycoproteins are rich in the unique immunomodulatory glycoepitopes that may serve as ligands for endogenous lectins that decorate the surface of immune cells. Such interaction may be involved in modulation of the maternal immune response. Among immunomodulatory glycans, Lewis type antigens have been of interest for at least two decades, while the importance of T/Tn antigens and related structures is still far from understanding. In the current work, we applied two plant lectins capable of distinguishing glycoepitopes with terminal GalNAc and Gal to identify glycoproteins that are their efficient carriers. By means of lectin blotting and lectin affinity chromatography followed by LC-MS, we identified lactotransferrin, prolactin inducible protein as well as fibronectin and semenogelins 1 and 2 as lectin-reactive. Net-O-glycosylation analysis results indicated that the latter three may actually carry T and/or Tn antigens, while in the case of prolactin inducible protein and lactotransferrin LacdiNAc and lactosamine glycoepitopes were more probable. STRING bioinformatics analysis linked the identified glycoproteins in the close network, indicating their involvement in immune (partially innate) processes. Overall, our research revealed potential seminal plasma ligands for endogenous Gal/GalNAc specific lectins with a possible role in modulation of maternal immune response during fertilization.

Siglec-6 as a New Potential Immune Checkpoint for Bladder Cancer Patients

Among the growing family of inhibitory receptors regulating immunity, sialic acid-binding immunoglobulin domain-containing lectins (Siglecs) have recently emerged as immunoregulatory receptors recognizing sialylated ligands on tumor cell surface. However, their role in the immunoregulation of bladder cancer (BCa) remains unknown. Here, we determined the presence of eight Siglec ligands (SLs) on bladder nontumor and tumor cell lines. S2L, S3L, and S6L were not expressed, and few bladder tumor cell lines expressed S5L and S14L. In contrast, S7L and S10L were upregulated on all bladder tumor cell lines. We found a discrepency in S9L expression by nontumor cell lines, which is however highly expressed by bladder tumor cell lines. Notably, expression of S5L, S6L, and S14L was increased upon bacillus Calmette-Guérin (BCG) infection. Furthermore, we analyzed the expression of Siglecs on T cells from healthy donors and BCa patients. Circulating T cells only expressed Siglec-6, which is upregulated in non-muscle-invasive BCa patients. In addition, BCG therapy induced the overexpression of Siglec-6 by urinary CD8⁺ T cells. In vitro functional assays suggested that Siglecs may decrease cytotoxic functions of effector CD8⁺ T cells. Finally, analyses from two BCa datasets (The Cancer Genome Atlas and UROMOL cohorts) showed that Siglec-6 is associated with tumor progression and poor survival. Our findings indicate that Siglec-6 might be a new target for BCa treatments. PATIENT SUMMARY: We investigated the expression of Siglecs, a family of immunoregulatory receptors, in bladder cancer patients. We observed that the expression of Siglec-6 is increased on circulating and urinary T cells of non-muscle-invasive bladder cancer patients. We also showed that Siglec-6 is associated with lower survival in bladder cancer patients and might contribute to bladder cancer recurrence.

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.

Discovery, Function, and Therapeutic Targeting of Siglec-8

Siglecs (sialic acid-binding immunoglobulin-like lectins) are single-pass cell surface receptors that have inhibitory activities on immune cells. Among these, Siglec-8 is a CD33-related family member selectively expressed on human mast cells and eosinophils, and at low levels on basophils. These cells can participate in inflammatory responses by releasing mediators that attract or activate other cells, contributing to the pathogenesis of allergic and non-allergic diseases. Since its discovery in 2000, initial in vitro studies have found that the engagement of Siglec-8 with a monoclonal antibody or with selective polyvalent sialoglycan ligands induced the cell death of eosinophils and inhibited mast cell degranulation. Anti-Siglec-8 antibody administration in vivo to humanized and transgenic mice selectively expressing Siglec-8 on mouse eosinophils and mast cells confirmed the in vitro findings, and identified additional anti-inflammatory effects. AK002 (lirentelimab) is a humanized non-fucosylated IgG1 antibody against Siglec-8 in clinical development for mast cell- and eosinophil-mediated diseases. AK002 administration has safely demonstrated the inhibition of mast cell activity and the depletion of eosinophils in several phase 1 and phase 2 trials. This article reviews the discovery and functions of Siglec-8, and strategies for its therapeutic targeting for the treatment of eosinophil- and mast cell-associated diseases.

Current Status on Therapeutic Molecules Targeting Siglec Receptors

The sialic acid-binding immunoglobulin-type of lectins (Siglecs) are receptors that recognize sialic acid-containing glycans. In the majority of the cases, Siglecs are expressed on immune cells and play a critical role in regulating immune cell signaling. Over the years, it has been shown that the sialic acid-Siglec axis participates in immunological homeostasis, and that any imbalance can trigger different pathologies, such as autoimmune diseases or cancer. For all this, different therapeutics have been developed that bind to Siglecs, either based on antibodies or being smaller molecules. In this review, we briefly introduce the Siglec family and we compile a description of glycan-based molecules and antibody-based therapies (including CAR-T and bispecific antibodies) that have been designed to therapeutically targeting Siglecs.

Regulation of ST6GAL1 sialyltransferase expression in cancer cells

The ST6GAL1 sialyltransferase, which adds α2-6 linked sialic acids to N-glycosylated proteins, is overexpressed in a wide range of human malignancies. Recent studies have established the importance of ST6GAL1 in promoting tumor cell behaviors such as invasion, resistance to cell stress and chemoresistance. Furthermore, ST6GAL1 activity has been implicated in imparting cancer stem cell characteristics. However, despite the burgeoning interest in the role of ST6GAL1 in the phenotypic features of tumor cells, insufficient attention has been paid to the molecular mechanisms responsible for ST6GAL1 upregulation during neoplastic transformation. Evidence suggests that these mechanisms are multifactorial, encompassing genetic, epigenetic, transcriptional and posttranslational regulation. The purpose of this review is to summarize current knowledge regarding the molecular events that drive enriched ST6GAL1 expression in cancer cells.

Tools to study and target the Siglec-sialic acid axis in cancer

Siglecs are widely expressed on leucocytes and bind to ubiquitously presented glycans containing sialic acids (sialoglycans). Most Siglecs carry an immunoreceptor tyrosine-based inhibition motif (ITIM) and elicit an inhibitory intracellular signal upon ligand binding. A few Siglec receptors can, however, recruit immunoreceptor tyrosine-based activation motif (ITAM)-containing factors, which activate cells. The role of hypersialylation (the enhanced expression of sialoglycans) has recently been explored in cancer progression. Mechanistic studies have shown that hypersialylation on cancer cells can engage inhibitory Siglecs on the surface of immune cells and induce immunosuppression. These recent studies strongly suggest that the Siglec-sialic acid axis can act as a potential target for cancer immunotherapy. Moreover, the use of new tools and techniques is facilitating these studies. In this review, we summarise techniques used to study Siglecs, including different mouse models, monoclonal antibodies, Siglec fusion proteins, and sialoglycan arrays. Furthermore, we discuss the recent major developments in the study of Siglecs in cancer immunosuppression, tools, and techniques used in targeting the Siglec-sialic acid axis and the possibility of clinical intervention.

Tumor Derived SIGLEC Family Genes May Play Roles in Tumor Genesis, Progression, and Immune Microenvironment Regulation

Background

SIGLEC family genes can also be expressed on tumor cells in different cancer types, and though it has been found that SIGLEC genes expressed by immune cells can be exploited by tumors to escape immune surveillance, functions of tumor derived SIGLEC expression in tumor microenvironment (TME) were barely investigated, which could play roles in cancer patients' survival.

Methods

Using bioinformatic analysis, mutation status of SIGLEC family genes was explored through the cBioPortal database, and expression of them in different tumors was explored through the UALCAN database. The GEPIA database was used to compare SIGLEC family genes' mRNA between cancers and to generate a highly correlated gene list in tumors. A KM-plotter database was used to find the association between SIGLEC genes and survival of patients. The associations between SIGLEC family genes' expression, immune infiltration, and immune regulators' expression in TME were generated and examined by the TIMER 2.0 database; the differential fold changes of SIGLEC family genes in specific oncogenic mutation groups of different cancer types were also yielded by TIMER 2.0. The networks of SIGLEC family genes and highly correlated genes were constructed by the STRING database, and gene ontology and pathway annotation of SIGLEC family highly correlated genes were performed through the DAVID database.

Results

SIGLEC family genes were highly mutated and amplified in melanoma, endometrial carcinoma, non-small cell lung cancer, bladder urothelial carcinoma, and esophagogastric adenocarcinoma, while deep deletion of SIGLEC family genes was common in diffuse glioma. Alteration of SIGLEC family genes demonstrated different levels in specific tumors, and oncogenic mutation in different cancer types could influence SIGLEC family genes' expression. Most SIGLEC family genes were related to patients' overall survival and progression free survival. Also, tumor derived SIGLEC family genes were related to tumor immune cell infiltration and may regulate TME by influencing chemokine axis.

Conclusion

Our computational analysis showed SIGLEC family genes expressed by tumor cells were associated with tumor behaviors, and they may also influence TME through chemokine axis, playing vital roles in patients' survival. Further experiments targeting tumor derived SIGLEC family genes are needed to confirm their influences on tumor growth, metastasis, and immune environment regulation.

Insights into the role of sialylation in cancer progression and metastasis

Upregulation of sialyltransferases—the enzymes responsible for the addition of sialic acid to growing glycoconjugate chains—and the resultant hypersialylation of up to 40–60% of tumour cell surfaces are established hallmarks of several cancers, including lung, breast, ovarian, pancreatic and prostate cancer. Hypersialylation promotes tumour metastasis by several routes, including enhancing immune evasion and tumour cell survival, and stimulating tumour invasion and migration. The critical role of enzymes that regulate sialic acid in tumour cell growth and metastasis points towards targeting sialylation as a potential new anti-metastatic cancer treatment strategy. Herein, we explore insights into the mechanisms by which hypersialylation plays a role in promoting metastasis, and explore the current state of sialyltransferase inhibitor development.

Tumor resistance mechanisms and their consequences on γδ T cell activation

Human γδ T lymphocytes are predominated by two major subsets, defined by the variable domain of the δ chain. Both, Vδ1 and Vδ2 T cells infiltrate in tumors and have been implicated in cancer immunosurveillance. Since the localization and distribution of tumor-infiltrating γδ T cell subsets and their impact on survival of cancer patients are not completely defined, this review summarizes the current knowledge about this issue. Different intrinsic tumor resistance mechanisms and immunosuppressive molecules of immune cells in the tumor microenvironment have been reported to negatively influence functional properties of γδ T cell subsets. Here, we focus on selected tumor resistance mechanisms including overexpression of cyclooxygenase (COX)-2 and indolamine-2,3-dioxygenase (IDO)-1/2, regulation by tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)/TRAIL-R4 pathway and the release of galectins. These inhibitory mechanisms play important roles in the cross-talk of γδ T cell subsets and tumor cells, thereby influencing cytotoxicity or proliferation of γδ T cells and limiting a successful γδ T cell-based immunotherapy. Possible future directions of a combined therapy of adoptively transferred γδ T cells together with γδ-targeting bispecific T cell engagers and COX-2 or IDO-1/2 inhibitors or targeting sialoglycan-Siglec pathways will be discussed and considered as attractive therapeutic options to overcome the immunosuppressive tumor microenvironment.

Tumor-associated carbohydrates and immunomodulatory lectins as targets for cancer immunotherapy

During oncogenesis, tumor cells present specific carbohydrate chains that are new targets for cancer immunotherapy. Whereas these tumor-associated carbohydrates (TACA) can be targeted with antibodies and vaccination approaches, TACA including sialic acid-containing glycans are able to inhibit anticancer immune responses by engagement of immune receptors on leukocytes. A family of immune-modulating receptors are sialic acid-binding Siglec receptors that have been recently described to inhibit antitumor activity mediated by myeloid cells, natural killer cells and T cells. Other TACA-binding receptors including selectins have been linked to cancer progression. Recent studies have shown that glycan-lectin interactions can be targeted to improve cancer immunotherapy. For example, interactions between the immune checkpoint T cell immunoglobulin and mucin-domain containing-3 and the lectin galectin-9 are targeted in clinical trials. In addition, an antibody against the lectin Siglec-15 is being tested in an early clinical trial. In this review, we summarize the previous and current efforts to target TACA and to inhibit inhibitory immune receptors binding to TACA including the Siglec-sialoglycan axis.

Coronaviruses: Is Sialic Acid a Gate to the Eye of Cytokine Storm? From the Entry to the Effects

Coronaviruses (CoVs) are a diverse family of the enveloped human and animal viruses reported as causative agents for respiratory and intestinal infections. The high pathogenic potential of human CoVs, including SARS-CoV, MERS-CoV and SARS-CoV-2, is closely related to the invasion mechanisms underlying the attachment and entry of viral particles to the host cells. There is increasing evidence that sialylated compounds of cellular glycocalyx can serve as an important factor in the mechanism of CoVs infection. Additionally, the sialic acid-mediated cross-reactivity with the host immune lectins is known to exert the immune response of different intensity in selected pathological stages. Here, we focus on the last findings in the field of glycobiology in the context of the role of sialic acid in tissue tropism, viral entry kinetics and immune regulation in the CoVs infections.