Targeting hypersialylation in multiple myeloma represents a novel approach to enhance NK cell-mediated tumor responses

MicroRNA:Siglec crosstalk in cancer progression

Aberrant Siglec expression in the tumour microenvironment has been implicated in tumour malignancies and can impact tumour behaviour and patient survival. Further to this, engagement with sialoglycans induces masked antigen recognition and promotes immune evasion, highlighting deregulated immune function. This necessitates the elucidation of their expression profiles in tumour progression. MicroRNAs (miRNAs) mediated targeting represents a novel approach to further elucidate Siglec potential and clinical relevance. Although miRNA activity in Siglec expression remains limited, we highlight current literature detailing miRNA:Siglec interactions within the tumour landscape and provide insights for possible diagnostic and therapeutic strategies in targeting the Siglec/sialic acid axis.

All-trans retinoic acid in hematologic disorders: not just acute promyelocytic leukemia

All-trans retinoic acid (ATRA) plays a role in tissue development, neural function, reproduction, vision, cell growth and differentiation, tumor immunity, and apoptosis. ATRA can act by inducing autophagic signaling, angiogenesis, cell differentiation, apoptosis, and immune function. In the blood system ATRA was first used with great success in acute promyelocytic leukemia (APL), where ATRA differentiated leukemia cells into mature granulocytes. ATRA can play a role not only in APL, but may also play a role in other hematologic diseases such as immune thrombocytopenia (ITP), myelodysplastic syndromes (MDS), non-APL acute myeloid leukemia (AML), aplastic anemia (AA), multiple myeloma (MM), etc., especially by regulating mesenchymal stem cells and regulatory T cells for the treatment of ITP. ATRA can also increase the expression of CD38 expressed by tumor cells, thus improving the efficacy of daratumumab and CD38-CART. In this review, we focus on the mechanism of action of ATRA, its role in various hematologic diseases, drug combinations, and ongoing clinical trials.

Glycosylation in the tumor immune response: the bitter side of sweetness

Glycosylation is the most structurally diverse form of post-translational modification (PTM) of proteins that affects a myriad of cellular processes. As a pivotal regulator of protein homeostasis, glycosylation notably impacts the function of proteins, spanning from protein localization and stability to protein-protein interactions. Aberrant glycosylation is a hallmark of cancer, and extensive studies have revealed the multifaceted roles of glycosylation in tumor growth, migration, invasion and immune escape Over the past decade, glycosylation has emerged as an immune regulator in the tumor microenvironment (TME). Here, we summarize the intricate interplay between glycosylation and the immune system documented in recent literature, which orchestrates the regulation of the tumor immune response through endogenous lectins, immune checkpoints and the extracellular matrix (ECM) in the TME. In addition, we discuss the latest progress in glycan-based cancer immunotherapy. This review provides a basic understanding of glycosylation in the tumor immune response and a theoretical framework for tumor immunotherapy.

Anticancer approach by targeted activation of a global inhibitor of sialyltransferases with acrolein

Cells are covered with a thick layer of sugar molecules known as glycans. Abnormal glycosylation is a hallmark of cancer, and hypersialylation increases tumor metastasis by promoting immune evasion and inducing tumor cell invasion and migration. Inhibiting sialylation is thus a potential anticancer treatment strategy. However, targeting sialic acids is difficult because of the lack of selective delivery tools. Here, we present a prodrug strategy for selectively releasing the global inhibitor of sialylation peracetylated 3Fax-Neu5Ac (PFN) in cancer cells using the reaction between phenyl azide and endogenous acrolein, which is overproduced in most cancer cells. The prodrug significantly suppressed tumor growth in mice as effectively as PFN without causing kidney dysfunction, which is associated with PFN. The use of sialylated glycans as immune checkpoints is gaining increasing attention, and the proposed method for precisely targeting aberrant sialylation provides a novel avenue for expanding current cancer treatments.

Synthesis of α-Hydroxy-1,2,3-Triazole-linked Sialyltransferase Inhibitors and Evaluation of Selectivity Towards ST3GAL1, ST6GAL1 and ST8SIA2

Tumour-derived sialoglycans, bearing the charged nonulosonic sugar sialic acid at their termini, play a critical role in tumour cell adhesion and invasion, as well as evading cell death and immune surveillance. Sialyltransferases (ST), the enzymes responsible for the biosynthesis of sialylated glycans, are highly upregulated in cancer, with tumour hypersialylation strongly correlated with tumour growth, metastasis and drug resistance. As a result, desialylation of the tumour cell surface using either targeted delivery of a pan-ST inhibitor (or sialidase) or systemic delivery of a non-toxic selective ST inhibitors are being pursued as potential new anti-metastatic strategies against multiple cancers including pancreatic, ovarian, breast, melanoma and lung cancer. Herein, we have employed molecular modelling to give insights into the selectivity observed in a series of selective ST inhibitors that incorporate a uridyl ring in place of the cytidine of the natural donor (CMP-Neu5Ac) and replace the charged phosphodiester linker of classical ST inhibitors with a neutral α-hydroxy-1,2,3-triazole linker. The inhibitory activities of the nascent compounds were determined against recombinant human ST enzymes (ST3GAL1, ST6GAL1, ST8SIA2) showing promising activity and selectivity towards specific ST sub-types. Our ST inhibitors are non-toxic and show improved synthetic accessibility and drug-likeness compared to earlier nucleoside-based ST inhibitors.

Regulation of sialic acid metabolism in cancer

Sialic acid, the terminal structure of cell surface glycans, has essential functions in regulating immune response, cell-to-cell communication, and cell adhesion. More importantly, an increased level of sialic acid, termed hypersialylation, has emerged as a commonly observed phenotype in cancer. Therefore, targeting sialic acid ligands (sialoglycans) and their receptors (Siglecs) may provide a new therapeutic approach for cancer immunotherapy. We highlight the complexity of the sialic acid metabolism and its involvement in malignant transformation within individual cancer subtypes. In this review, we focus on the dysregulation of sialylation, the intricate nature of sialic acid synthesis, and clinical perspective. We aim to provide a brief insight into the mechanism of hypersialylation and how our understanding of these processes can be leveraged for the development of novel therapeutics.

Increased Siglec-9/Siglec-9L interactions on NK cells predict poor HCC prognosis and present a targetable checkpoint for immunotherapy

BACKGROUND & AIMS

Natural killer (NK) cell-based anti-hepatocellular carcinoma (HCC) therapy is an increasingly attractive approach that warrants further study. Siglec-9 interacts with its ligand (Siglec-9L) and restrains NK cell functions, suggesting it is a potential therapeutic target. However, in situ Siglec-9/Siglec-9L interactions in HCC have not been reported, and a relevant interventional strategy is lacking. Herein, we aim to illustrate Siglec-9/Siglec-9L-mediated cell sociology and identify small-molecule inhibitors targeting Siglec-9 that could improve the efficacy of NK cell-based immunotherapy for HCC.

METHODS

Multiplexed immunofluorescence staining was performed to analyze the expression pattern of Siglec-7, -9 and their ligands in HCC tissues. Then we conducted docking-based virtual screening combined with bio-layer interferometry assays to identify a potent small-molecule Siglec-9 inhibitor. The therapeutic potential was further evaluated in vitro and in hepatoma-bearing NCG mice.

RESULTS

Siglec-9 expression, rather than Siglec-7, was markedly upregulated on tumor-infiltrating NK cells, which correlated significantly with reduced survival of patients with HCC. Moreover, the number of Siglec-9L+ cells neighboring Siglec-9+ NK cells was increased in HCC tissues and was also associated with tumor recurrence and reduced survival, further suggesting that Siglec-9/Siglec-9L interactions are a potential therapeutic target in HCC. In addition, we identified a small-molecule Siglec-9 inhibitor MTX-3937 which inhibited phosphorylation of Siglec-9 and downstream SHP1 and SHP2. Accordingly, MTX-3937 led to considerable improvement in NK cell function. Notably, MTX-3937 enhanced cytotoxicity of both human peripheral and tumor-infiltrating NK cells. Furthermore, transfer of MTX-3937-treated NK92 cells greatly suppressed the growth of hepatoma xenografts in NCG mice.

CONCLUSIONS

Our study provides the rationale for HCC treatment by targeting Siglec-9 on NK cells and identifies a promising small-molecule inhibitor against Siglec-9 that enhances NK cell-mediated HCC surveillance.

IMPACT AND IMPLICATIONS

Herein, we found that Siglec-9 expression is markedly upregulated on tumor-infiltrating natural killer (TINK) cells and correlates with reduced survival in patients with hepatocellular carcinoma (HCC). Moreover, the number of Siglec-9L+ cells neighboring Siglec-9+ NK cells was increased in HCC tissues and was also associated with tumor recurrence and reduced survival. More importantly, we identified a small-molecule inhibitor targeting Siglec-9 that augments NK cell functions, revealing a novel immunotherapy strategy for liver cancer that warrants further clinical investigation.

Redistribution of the glycocalyx exposes phagocytic determinants on apoptotic cells

Phagocytes remove dead and dying cells by engaging "eat-me" ligands such as phosphatidylserine (PtdSer) on the surface of apoptotic targets. However, PtdSer is obscured by the bulky exofacial glycocalyx, which also exposes ligands that activate "don't-eat-me" receptors such as Siglecs. Clearly, unshielding the juxtamembrane "eat-me" ligands is required for the successful engulfment of apoptotic cells, but the mechanisms underlying this process have not been described. Using human and murine cells, we find that apoptosis-induced retraction and weakening of the cytoskeleton that anchors transmembrane proteins cause an inhomogeneous redistribution of the glycocalyx: actin-depleted blebs emerge, lacking the glycocalyx, while the rest of the apoptotic cell body retains sufficient actin to tether the glycocalyx in place. Thus, apoptotic blebs can be engaged by phagocytes and are targeted for engulfment. Therefore, in cells with an elaborate glycocalyx, such as mucinous cancer cells, this "don't-come-close-to-me" barrier must be removed to enable clearance by phagocytosis.

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.

Natural killer cells affect the natural course, drug resistance, and prognosis of multiple myeloma

Multiple myeloma (MM), a stage-developed plasma cell malignancy, evolves from monoclonal gammopathy of undetermined significance (MGUS) or smoldering MM (SMM). Emerging therapies including immunomodulatory drugs, proteasome inhibitors, monoclonal antibodies, chimeric antigen-T/natural killer (NK) cells, bispecific T-cell engagers, selective inhibitors of nuclear export, and small-molecule targeted therapy have considerably improved patient survival. However, MM remains incurable owing to inevitable drug resistance and post-relapse rapid progression. NK cells with germline-encoded receptors are involved in the natural evolution of MGUS/SMM to active MM. NK cells actively recognize aberrant plasma cells undergoing malignant transformation but are yet to proliferate during the elimination phase, a process that has not been revealed in the immune editing theory. They are potential effector cells that have been neglected in the therapeutic process. Herein, we characterized changes in NK cells regarding disease evolution and elucidated its role in the early clinical monitoring of MM. Additionally, we systematically explored dynamic changes in NK cells from treated patients who are in remission or relapse to explore future combination therapy strategies to overcome drug resistance.

The glycoimmune checkpoint receptor Siglec-7 interacts with T-cell ligands and regulates T-cell activation

Siglec-7 (sialic acid-binding immunoglobulin-like lectin 7) is a glycan-binding immune receptor that is emerging as a significant target of interest for cancer immunotherapy. The physiological ligands that bind Siglec-7, however, remain incompletely defined. In this study, we characterized the expression of Siglec-7 ligands on peripheral immune cell subsets and assessed whether Siglec-7 functionally regulates interactions between immune cells. We found that disialyl core 1 O-glycans are the major immune ligands for Siglec-7 and that these ligands are particularly highly expressed on naïve T-cells. Densely glycosylated sialomucins are the primary carriers of these glycans, in particular a glycoform of the cell-surface marker CD43. Biosynthesis of Siglec-7-binding glycans is dynamically controlled on different immune cell subsets through a genetic circuit involving the glycosyltransferase GCNT1. Siglec-7 blockade was found to increase activation of both primary T-cells and antigen-presenting dendritic cells in vitro, indicating that Siglec-7 binds T-cell glycans to regulate intraimmune signaling. Finally, we present evidence that Siglec-7 directly activates signaling pathways in T-cells, suggesting a new biological function for this receptor. These studies conclusively demonstrate the existence of a novel Siglec-7-mediated signaling axis that physiologically regulates T-cell activity. Going forward, our findings have significant implications for the design and implementation of therapies targeting immunoregulatory Siglec receptors.

PSGL-1 decorated with sialyl Lewisa/x promotes high affinity binding of myeloma cells to P-selectin but is dispensable for E-selectin engagement

Dissemination of multiple myeloma into the bone marrow proceeds through sequential steps mediated by a variety of adhesion molecules and chemokines that eventually results in the extravasation of malignant plasma cells into this protective niche. Selectins are a class of C-type lectins that recognize carbohydrate structures exposed on blood borne cells and participate in the first step of the extravasation cascade, serving as brakes to slow down circulating cells enabling them to establish firm adhesion onto the endothelium. Myeloma cells enriched for the expression of selectin ligands present an aggressive disease in vivo that is refractory to bortezomib treatment and can be reverted by small molecules targeting E-selectin. In this study, we have defined the molecular determinants of the selectin ligands expressed on myeloma cells. We show that PSGL-1 is the main protein carrier of sialyl Lewisa/x-related structures in myeloma. PSGL-1 decorated with sialyl Lewisa/x is essential for P-selectin binding but dispensable for E-selectin binding. Moreover, sialylation is required for E-selectin engagement whereas high affinity binding to P-selectin occurs even in the absence of sialic acid. This study provides further knowledge on the biology of selectin ligands in myeloma, opening the way to their clinical application as diagnostic tools and therapeutic targets.

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.

Glycosyltransferases as versatile tools to study the biology of glycans

All cells are decorated with complex carbohydrate structures called glycans that serve as ligands for glycan-binding proteins (GBPs) to mediate a wide range of biological processes. Understanding the specific functions of glycans is key to advancing an understanding of human health and disease. However, the lack of convenient and accessible tools to study glycan-based interactions has been a defining challenge in glycobiology. Thus, the development of chemical and biochemical strategies to address these limitations has been a rapidly growing area of research. In this review, we describe the use of glycosyltransferases (GTs) as versatile tools to facilitate a greater understanding of the biological roles of glycans. We highlight key examples of how GTs have streamlined the preparation of well-defined complex glycan structures through chemoenzymatic synthesis, with an emphasis on synthetic strategies allowing for site- and branch-specific display of glyco-epitopes. We also describe how GTs have facilitated expansion of glyco-engineering strategies, on both glycoproteins and cell surfaces. Coupled with advancements in bioorthogonal chemistry, GTs have enabled selective glyco-epitope editing of glycoproteins and cells, selective glycan subclass labeling, and the introduction of novel biomolecule functionalities onto cells, including defined oligosaccharides, antibodies, and other proteins. Collectively, these approaches have contributed great insight into the fundamental biological roles of glycans and are enabling their application in drug development and cellular therapies, leaving the field poised for rapid expansion.

Single-cell functional genomics reveals determinants of sensitivity and resistance to natural killer cells in blood cancers

Cancer cells can evade natural killer (NK) cell activity, thereby limiting anti-tumor immunity. To reveal genetic determinants of susceptibility to NK cell activity, we examined interacting NK cells and blood cancer cells using single-cell and genome-scale functional genomics screens. Interaction of NK and cancer cells induced distinct activation and type I interferon (IFN) states in both cell types depending on the cancer cell lineage and molecular phenotype, ranging from more sensitive myeloid to less sensitive B-lymphoid cancers. CRISPR screens in cancer cells uncovered genes regulating sensitivity and resistance to NK cell-mediated killing, including adhesion-related glycoproteins, protein fucosylation genes, and transcriptional regulators, in addition to confirming the importance of antigen presentation and death receptor signaling pathways. CRISPR screens with a single-cell transcriptomic readout provided insight into underlying mechanisms, including regulation of IFN-γ signaling in cancer cells and NK cell activation states. Our findings highlight the diversity of mechanisms influencing NK cell susceptibility across different cancers and provide a resource for NK cell-based therapies.

Unraveling the impact of sialic acids on the immune landscape and immunotherapy efficacy in pancreatic cancer

BACKGROUND

Pancreatic ductal adenocarcinoma (PDAC) is one of the deadliest cancers. Despite the successful application of immune checkpoint blockade in a range of human cancers, immunotherapy in PDAC remains unsuccessful. PDAC is characterized by a desmoplastic, hypoxic and highly immunosuppressive tumor microenvironment (TME), where T-cell infiltration is often lacking (immune desert), or where T cells are located distant from the tumor islands (immune excluded). Converting the TME to an immune-inflamed state, allowing T-cell infiltration, could increase the success of immunotherapy in PDAC.

METHOD

In this study, we use the KPC3 subcutaneous PDAC mouse model to investigate the role of tumor-derived sialic acids in shaping the tumor immune landscape. A sialic acid deficient KPC3 line was generated by genetic knock-out of the CMAS (cytidine monophosphate N-acetylneuraminic acid synthetase) enzyme, a critical enzyme in the synthesis of sialic acid-containing glycans. The effect of sialic acid-deficiency on immunotherapy efficacy was assessed by treatment with anti-programmed cell death protein 1 (PD-1) and agonistic CD40.

RESULT

The absence of sialic acids in KPC3 tumors resulted in increased numbers of CD4+ and CD8+ T cells in the TME, and reduced frequencies of CD4+ regulatory T cells (Tregs) within the T-cell population. Importantly, CD8+ T cells were able to infiltrate the tumor islands in sialic acid-deficient tumors. These favorable alterations in the immune landscape sensitized sialic acid-deficient tumors to immunotherapy, which was ineffective in sialic acid-expressing KPC3 tumors. In addition, high expression of sialylation-related genes in human pancreatic cancer correlated with decreased CD8+ T-cell infiltration, increased presence of Tregs, and poorer survival probability.

CONCLUSION

Our results demonstrate that tumor-derived sialic acids mediate T-cell exclusion within the PDAC TME, thereby impairing immunotherapy efficacy. Targeting sialic acids represents a potential strategy to enhance T-cell infiltration and improve immunotherapy outcomes in PDAC.

Cancer cell-intrinsic PD-1: Its role in malignant progression and immunotherapy

Programmed cell death protein-1 (PD-1), also called CD279, is coded by the PDCD1 gene and is constitutively expressed on the surface of immune cells. As a receptor and immune checkpoint, PD-1 can bind to programmed death ligand-1/programmed death ligand-2 (PD-L1/PD-L2) in tumor cells, leading to tumor immune evasion. Anti-PD-1 and anti-PD-L1 are important components in tumor immune therapy. PD-1 is also expressed as an intrinsic variant (iPD-1) in cancer cells where it plays important roles in malignant progression as proposed by recent studies. However, iPD-1 has received much less attention compared to PD-1 expressed on immune cells although there is an unmet medical need for fully elucidating the mechanisms of actions to achieve the best response in tumor immunotherapy. iPD-1 suppresses tumorigenesis in non-small cell lung cancer (NSCLC) and colon cancer, whereas it promotes tumorigenesis in melanoma, hepatocellular carcinoma (HCC), pancreatic ductal adenocarcinoma (PDAC), thyroid cancer (TC), glioblastoma (GBM), and triple-negative breast cancer (TNBC). In this review, we focus on the role of iPD-1 in tumorigenesis and development and its molecular mechanisms. We also deeply discuss nivolumab-based combined therapy in common tumor therapy. iPD-1 may explain the different therapeutic effects of anti-PD-1 treatment and provide critical information for use in combined anti-tumor approaches.

Enhancing personalized immune checkpoint therapy by immune archetyping and pharmacological targeting

Immune checkpoint inhibitors (ICIs) are an expanding class of immunotherapeutic agents with the potential to cure cancer. Despite the outstanding clinical response in patient subsets, most individuals become refractory or develop resistance. Patient stratification and personalized immunotherapies are limited by the absence of predictive response markers. Recent findings show that dominant patterns of immune cell composition, T-cell status and heterogeneity, and spatiotemporal distribution of immune cells within the tumor microenvironment (TME) are becoming essential determinants of prognosis and therapeutic response. In this context, ICIs also function as investigational tools and proof of concept, allowing the validation of the identified mechanisms. After reviewing the current state of ICIs, this article will explore new comprehensive predictive markers for ICIs based on recent discoveries. We will discuss the recent establishment of a classification of TMEs into immune archetypes as a tool for personalized immune profiling, allowing patient stratification before ICI treatment. We will discuss the developing comprehension of T-cell diversity and its role in shaping the immune profile of patients. We describe the potential of strategies that score the mutual spatiotemporal modulation between T-cells and other cellular components of the TME. Additionally, we will provide an overview of a range of synthetic and naturally occurring or derived small molecules. We will compare compounds that were recently identified by in silico prediction to wet lab-validated drug candidates with the potential to function as ICIs and/or modulators of the cellular components of the TME.

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.

Sialylation: A Cloak for Tumors to Trick the Immune System in the Microenvironment

The tumor microenvironment (TME), where the tumor cells incite the surrounding normal cells to create an immune suppressive environment, reduces the effectiveness of immune responses during cancer development. Sialylation, a type of glycosylation that occurs on cell surface proteins, lipids, and glycoRNAs, is known to accumulate in tumors and acts as a "cloak" to help tumor cells evade immunological surveillance. In the last few years, the role of sialylation in tumor proliferation and metastasis has become increasingly evident. With the advent of single-cell and spatial sequencing technologies, more research is being conducted to understand the effects of sialylation on immunity regulation. This review provides updated insights into recent research on the function of sialylation in tumor biology and summarizes the latest developments in sialylation-targeted tumor therapeutics, including antibody-mediated and metabolic-based sialylation inhibition, as well as interference with sialic acid-Siglec interaction.

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.

Sialofucosylation Enables Platelet Binding to Myeloma Cells via P-Selectin and Suppresses NK Cell-Mediated Cytotoxicity

Multiple myeloma (MM) is a plasma cell disorder that develops in the bone marrow (BM) and is characterized by uncontrolled proliferation and the ability to disseminate to different sites of the skeleton. Sialofucosylated structures, particularly Sialyl Lewis a/x (SLe^a/x), facilitate the homing of MM cells into the BM, leading to resistance to bortezomib in vivo. Platelets have been shown to play an important role in tumor metastasis. Platelets can bind to the surface of cancer cells, forming a "cloak" that protects them from the shear stress of the bloodstream and natural killer (NK) cell-mediated cytotoxicity. In this study, we showed that the presence of SLe^a/x induced a strong binding of MM cells to P-selectin, leading to specific and direct interactions with platelets, which could be inhibited by a P-selectin-blocking antibody. Importantly, platelets surrounded SLe^a/x-enriched MM cells, protecting them from NK cell-mediated cytotoxicity. The interactions between the platelets and MM cells were also detected in BM samples obtained from MM patients. Platelet binding to SLe^a/x-enriched MM cells was increased in patients with symptomatic disease and at relapse. These data suggest an important role of SLe^a/x and platelets in MM disease progression and resistance to therapy.

Human sialoglycan ligands for immune inhibitory Siglecs

Most human Siglecs (sialic acid binding immunoglobulin-like lectins) are expressed on the surfaces of overlapping subsets of immune cells, and most carry immunoreceptor tyrosine-based inhibitory domains on their intracellular motifs. When immune inhibitory Siglecs bind to complementary sialoglycans in their local milieu, engagement results in down-regulation of the immune response. Siglecs have come under scrutiny as potential targets of drugs to modify the course of inflammation (and other immune system responses) and as immune checkpoints in cancer. Human Siglecs bind to endogenous human sialoglycans. The identities of these endogenous human sialoglycan immune regulators are beginning to emerge, along with some general principles that may inform future investigations in this area. Among these principles is the finding that a cell type or tissue may express a ligand for a particular Siglec on a single or a very few of its sialoglycoproteins. The selected protein carrier for a particular Siglec may be unique in a certain tissue, but vary tissue-to-tissue. The binding affinity of endogenous Siglec ligands may surpass that of its binding to synthetic sialoglycan determinants by several orders of magnitude. Since most human Siglecs have evolved rapidly and are distinct from those in most other mammals, this review describes endogenous human Siglec ligands for several human immune inhibitory Siglecs. As the identities of these immune regulatory sialoglycan ligands are defined, additional opportunities to target Siglecs therapeutically may emerge.

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.

Roles of Siglecs in neurodegenerative diseases

Microglia are resident myeloid cells in the central nervous system (CNS) with a unique developmental origin, playing essential roles in developing and maintaining the CNS environment. Recent studies have revealed the involvement of microglia in neurodegenerative diseases, such as Alzheimer's disease, through the modulation of neuroinflammation. Several members of the Siglec family of sialic acid recognition proteins are expressed on microglia. Since the discovery of the genetic association between a polymorphism in the CD33 gene and late-onset Alzheimer's disease, significant efforts have been made to elucidate the molecular mechanism underlying the association between the polymorphism and Alzheimer's disease. Furthermore, recent studies have revealed additional potential associations between Siglecs and Alzheimer's disease, implying that the reduced signal from inhibitory Siglec may have an overall protective effect in lowering the disease risk. Evidences suggesting the involvement of Siglecs in other neurodegenerative diseases are also emerging. These findings could help us predict the roles of Siglecs in other neurodegenerative diseases. However, little is known about the functionally relevant Siglec ligands in the brain, which represents a new frontier. Understanding how microglial Siglecs and their ligands in CNS contribute to the regulation of CNS homeostasis and pathogenesis of neurodegenerative diseases may provide us with a new avenue for disease prevention and intervention.

Live-Cell Glycocalyx Engineering

A heavy layer of glycans forms a brush matrix bound to the outside of all the cells in our bodies; it is referred to as the "sugar forest" or glycocalyx. Beyond the increased appreciation of the glycocalyx over the past two decades, recent advances in engineering the glycocalyx on live cells have spurred the creation of cellular drugs and novel medical treatments. The development of new tools and techniques has empowered scientists to manipulate the structures and functions of cell-surface glycans on target cells and endow target cells with desired properties. Herein, we provide an overview of live-cell glycocalyx engineering strategies for controlling the cell-surface molecular repertory to suit therapeutic applications, even though the realm of this field remains young and largely unexplored.

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.

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.

Inhibitory Siglec-sialic acid interactions in balancing immunological activation and tolerance during viral infections

Siglecs are a family of emerging glyco-immune checkpoints. Inhibiting them can enhance the functions of several types of immune cells, whereas engaging them can reduce hyper-inflammation and hyper-activation of immune functions. Siglec-sialoglycan interactions play an important role in modulating immunological functions during cancer, however, their roles in regulating immunological equilibrium during viral infections is less clear. In this review, we discuss the documented and potential roles of inhibitory Siglecs in balancing immune activation and tolerance during viral infections and consider how this balance could affect both the desired anti-viral immunological functions and the unwanted hyper- or chronic inflammation. Finally, we discuss the opportunities to target the Siglec immunological switches to reach an immunological balance during viral infections: inhibiting specific Siglec-sialoglycan interactions when maximum anti-viral immune responses are needed, or inducing other interactions when preventing excessive inflammation or reducing chronic immune activation are the goals.

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.

Measuring the multifaceted roles of mucin-domain glycoproteins in cancer

Mucin-domain glycoproteins are highly O-glycosylated cell surface and secreted proteins that serve as both biochemical and biophysical modulators. Aberrant expression and glycosylation of mucins are known hallmarks in numerous malignancies, yet mucin-domain glycoproteins remain enigmatic in the broad landscape of cancer glycobiology. Here we review the multifaceted roles of mucins in cancer through the lens of the analytical and biochemical methods used to study them. We also describe a collection of emerging tools that are specifically equipped to characterize mucin-domain glycoproteins in complex biological backgrounds. These approaches are poised to further elucidate how mucin biology can be understood and subsequently targeted for the next generation of cancer therapeutics.

Overcoming tumor resistance mechanisms in CAR-NK cell therapy

Despite the impressive results of autologous CAR-T cell therapy in refractory B lymphoproliferative diseases, CAR-NK immunotherapy emerges as a safer, faster, and cost-effective approach with no signs of severe toxicities as described for CAR-T cells. Permanently scrutinized for its efficacy, recent promising data in CAR-NK clinical trials point out the achievement of deep, high-quality responses, thus confirming its potential clinical use. Although CAR-NK cell therapy is not significantly affected by the loss or downregulation of its CAR tumor target, as in the case of CAR-T cell, a plethora of common additional tumor intrinsic or extrinsic mechanisms that could also disable NK cell function have been described. Therefore, considering lessons learned from CAR-T cell therapy, the emergence of CAR-NK cell therapy resistance can also be envisioned. In this review we highlight the processes that could be involved in its development, focusing on cytokine addiction and potential fratricide during manufacturing, poor tumor trafficking, exhaustion within the tumor microenvironment (TME), and NK cell short in vivo persistence on account of the limited expansion, replicative senescence, and rejection by patient’s immune system after lymphodepletion recovery. Finally, we outline new actively explored alternatives to overcome these resistance mechanisms, with a special emphasis on CRISPR/Cas9 mediated genetic engineering approaches, a promising platform to optimize CAR-NK cell function to eradicate refractory cancers.

Molecular Basis and Role of Siglec-7 Ligand Expression on Chronic Lymphocytic Leukemia B Cells

Siglec-7 (sialic acid-binding immunoglobulin-like lectin 7) is an immune checkpoint-like glycan recognition protein on natural killer (NK) cells. Cancer cells often upregulate Siglec ligands to subvert immunosurveillance, but the molecular basis of Siglec ligands has been elusive. In this study, we investigated Siglec-7 ligands on chronic lymphocytic leukemia (CLL) B cells. CLL B cells express higher levels of Siglec-7 ligands compared with healthy donor B cells, and enzymatic removal of sialic acids or sialomucins makes them more sensitive to NK cell cytotoxicity. Gene knockout experiments have revealed that the sialyltransferase ST6GalNAc-IV is responsible for the biosynthesis of disialyl-T (Neu5Acα2-3Galβ1-3[Neu5Acα2-6]GalNAcα1-), which is the glycotope recognized by Siglec-7, and that CD162 and CD45 are the major carriers of this glycotope on CLL B cells. Analysis of public transcriptomic datasets indicated that the low expression of GCNT1 (encoding core 2 GlcNAc transferase, an enzyme that competes against ST6GalNAc-IV) and high expression of ST6GALNAC4 (encoding ST6GalNAc-IV) in CLL B cells, together enhancing the expression of the disialyl-T glycotope, are associated with poor patient prognosis. Taken together, our results determined the molecular basis of Siglec-7 ligand overexpression that protects CLL B cells from NK cell cytotoxicity and identified disialyl-T as a potential prognostic marker of CLL.