Negative regulation of T cell receptor signaling by Siglec-7 (p70/AIRM) and Siglec-9

Recent advances in sialic acid-based active targeting chemoimmunotherapy promoting tumor shedding: a systematic review

Tumors have always been a major public health concern worldwide, and attempts to look for effective treatments have never ceased. Sialic acid is known to be a crucial element for tumor development and its receptors are highly expressed on tumor-associated immune cells, which perform significant roles in establishing the immunosuppressive tumor microenvironment and further boosting tumorigenesis, progression, and metastasis. Obviously, it is essential to consider sophisticated crosstalk between tumors, the immune system, and preparations, and understand the links between pharmaceutics and immunology. Sialic acid-based chemoimmunotherapy enables active targeting drug delivery via mediating the recognition between the sialic acid-modified nano-drug delivery system represented by liposomes and sialic acid-binding receptors on tumor-associated immune cells, which inhibit their activity and utilize their homing ability to deliver drugs. Such a "Trojan horse" strategy has remarkably improved the shortcomings of traditional passive targeting treatments, unexpectedly promoted tumor shedding, and persistently induced robust immunological memory, thus highlighting its prospective application potential for targeting various tumors. Herein, we review recent advances in sialic acid-based active targeting chemoimmunotherapy to promote tumor shedding, summarize the current viewpoints on the tumor shedding mechanism, especially the formation of durable immunological memory, and analyze the challenges and opportunities of this attractive approach.

Engagement of sialylated glycans with Siglec receptors on suppressive myeloid cells inhibits anticancer immunity via CCL2

The overexpression of sialic acids on glycans, called hypersialylation, is a common alteration found in cancer cells. Sialylated glycans can enhance immune evasion by interacting with sialic acid-binding immunoglobulin-like lectin (Siglec) receptors on tumor-infiltrating immune cells. Here, we investigated the effect of sialylated glycans and their interaction with Siglec receptors on myeloid-derived suppressor cells (MDSCs). We found that MDSCs derived from the blood of lung cancer patients and tumor-bearing mice strongly express inhibitory Siglec receptors and are highly sialylated. In murine cancer models of emergency myelopoiesis, Siglec-E knockout in myeloid cells resulted in prolonged survival and increased tumor infiltration of activated T cells. Targeting suppressive myeloid cells by blocking Siglec receptors or desialylation strongly reduced their suppressive potential. We further identified CCL2 as a mediator involved in T-cell suppression upon interaction between sialoglycans and Siglec receptors on MDSCs. Our results demonstrated that sialylated glycans inhibit anticancer immunity by modulating CCL2 expression.

Gene Expression Profiling in Pediatric Appendicitis

Importance

Appendicitis is the most common indication for urgent surgery in the pediatric population, presenting across a range of severity and with variable complications. Differentiating simple appendicitis (SA) and perforated appendicitis (PA) on presentation may help direct further diagnostic workup and appropriate therapy selection, including antibiotic choice and timing of surgery.

Objective

To provide a mechanistic understanding of the differences in disease severity of appendicitis with the objective of developing improved diagnostics and treatments, specifically for the pediatric population.

Design, Setting, and Participants

The Gene Expression Profiling of Pediatric Appendicitis (GEPPA) study was a single-center prospective exploratory diagnostic study with transcriptomic profiling of peripheral blood collected from a cohort of children aged 5 to 17 years with abdominal pain and suspected appendicitis between November 2016 and April 2017 at the Alberta Children's Hospital in Calgary, Alberta, Canada, with data analysis reported in August 2023. There was no patient follow-up in this study.

Exposure

SA, PA, or nonappendicitis abdominal pain.

Main Outcomes and Measures

Blood transcriptomics was used to develop a hypothesis of underlying mechanistic differences between SA and PA to build mechanistic hypotheses and blood-based diagnostics.

Results

Seventy-one children (mean [SD] age, 11.8 [3.0] years; 48 [67.6%] male) presenting to the emergency department with abdominal pain and suspected appendicitis were investigated using whole-blood transcriptomics. A central role for immune system pathways was revealed in PA, including a dampening of major innate interferon responses. Gene expression changes in patients with PA were consistent with downregulation of immune response and inflammation pathways and shared similarities with gene expression signatures derived from patients with sepsis, including the most severe sepsis endotypes. Despite the challenges in identifying early biomarkers of severe appendicitis, a 4-gene signature that was predictive of PA compared to SA, with an accuracy of 85.7% (95% CI, 72.8-94.1) was identified.

Conclusions

This study found that PA was complicated by a dysregulated immune response. This finding should inform improved diagnostics of severity, early management strategies, and prevention of further postsurgical complications.

Dissecting the Ability of Siglecs To Antagonize Fcγ Receptors

Fcγ receptors (FcγRs) play key roles in the effector function of IgG, but their inappropriate activation plays a role in several disease etiologies. Therefore, it is critical to better understand how FcγRs are regulated. Numerous studies suggest that sialic acid-binding immunoglobulin-type lectins (Siglecs), a family of immunomodulatory receptors, modulate FcγR activity; however, it is unclear of the circumstances in which Siglecs can antagonize FcγRs and which Siglecs have this ability. Using liposomes displaying selective ligands to coengage FcγRs with a specific Siglec, we explore the ability of Siglec-3, Siglec-5, Siglec-7, and Siglec-9 to antagonize signaling downstream of FcγRs. We demonstrate that Siglec-3 and Siglec-9 can fully inhibit FcγR activation in U937 cells when coengaged with FcγRs. Cells expressing Siglec mutants reveal differential roles for the immunomodulatory tyrosine-based inhibitory motif (ITIM) and immunomodulatory tyrosine-based switch motif (ITSM) in this inhibition. Imaging flow cytometry enabled visualization of SHP-1 recruitment to Siglec-3 in an ITIM-dependent manner, while SHP-2 recruitment is more ITSM-dependent. Conversely, both cytosolic motifs of Siglec-9 contribute to SHP-1/2 recruitment. Siglec-7 poorly antagonizes FcγR activation for two reasons: masking by cis ligands and differences in its ITIM and ITSM. A chimera of the Siglec-3 extracellular domains and Siglec-5 cytosolic tail strongly inhibits FcγR when coengaged, providing evidence that Siglec-5 is more like Siglec-3 and Siglec-9 in its ability to antagonize FcγRs. Additionally, Siglec-3 and Siglec-9 inhibited FcγRs when coengaged by cells displaying ligands for both the Siglec and FcγRs. These results suggest a role for Siglecs in mediating FcγR inhibition in the context of an immunological synapse, which has important relevance to the effectiveness of immunotherapies.

Non B Cell-Derived Immunoglobulins in Intestinal Tract

Intestinal epithelium constitutes a barrier to the unrestricted movement of pathogens, and other detrimental substances from the external world (gut lumen) into the interstitial environment. Intestinal epithelial cells obstruct harmful substances passing through the epithelium as a physical and chemical barrier; Moreover, the epithelial cells can express Toll-like receptors (TLRs) and cytokines to exert innate immune function. In addition, high levels of immunoglobulin A (IgA) and other antibodies exist in the intestinal mucosa, maintaining intestinal immune homeostasis in conjunction with intestinal probiotics. Traditionally, these antibodies have been deemed to be secreted by submucosal plasma cells. Nonetheless, in recent years, it has been demonstrated that intestinal epithelial cells produce a substantial amount of Igs, especially IgA or free Ig light chains, which are involved in intestinal immune homeostasis and the survival of normal epithelial cells. Furthermore, mounting evidence affirms that many human carcinoma cells, including colorectal cancer (CRC), can overexpress Igs, particularly IgG. Cancer-derived Igs exhibit a unique V(D)J rearrangement pattern distinct from B cell-derived Ig; moreover, this cancer cell-derived IgG also has a unique sialic acid modification on the 162 site of CH1 domain (SIA-IgG). The SIA-IgG plays a crucial role in promoting cancer initiation, progression, metastasis, and tumour immune escape. Simultaneously, CRC cells can also express free Ig light chains, which promote colitis, colitis-associated colon carcinogenesis, and CRC progression. Therefore, Igs expressed by CRC cells could be a potential target for diagnosing and preventing the transformation of inflammation into cancer, as well as treating CRC.

Tetra- and Hexavalent Siglec-8 Ligands Modulate Immune Cell Activation

Carbohydrate-binding proteins are generally characterized by poor affinities for their natural glycan ligands, predominantly due to the shallow and solvent-exposed binding sites. To overcome this drawback, nature has exploited multivalency to strengthen the binding by establishing multiple interactions simultaneously. The development of oligovalent structures frequently proved to be successful, not only for proteins with multiple binding sites, but also for proteins that possess a single recognition domain. Herein we present the syntheses of a number of oligovalent ligands for Siglec-8, a monomeric I-type lectin found on eosinophils and mast cells, alongside the thermodynamic characterization of their binding. While the enthalpic contribution of each binding epitope was within a narrow range to that of the monomeric ligand, the entropy penalty increased steadily with growing valency. Additionally, we observed a successful agonistic binding of the tetra- and hexavalent and, to an even larger extent, multivalent ligands to Siglec-8 on immune cells and modulation of immune cell activation. Thus, triggering a biological effect is not restricted to multivalent ligands but could be induced by low oligovalent ligands as well, whereas a monovalent ligand, despite binding with similar affinity, showed an antagonistic effect.

Combinatorial multimer staining and spectral flow cytometry facilitate quantification and characterization of polysaccharide-specific B cell immunity

Bacterial capsular polysaccharides are important vaccine immunogens. However, the study of polysaccharide-specific immune responses has been hindered by technical restrictions. Here, we developed and validated a high-throughput method to analyse antigen-specific B cells using combinatorial staining with fluorescently-labelled capsular polysaccharide multimers. Concurrent staining of 25 cellular markers further enables the in-depth characterization of polysaccharide-specific cells. We used this assay to simultaneously analyse 14 Streptococcus pneumoniae or 5 Streptococcus agalactiae serotype-specific B cell populations. The phenotype of polysaccharide-specific B cells was associated with serotype specificity, vaccination history and donor population. For example, we observed a link between non-class switched (IgM+) memory B cells and vaccine-inefficient S. pneumoniae serotypes 1 and 3. Moreover, B cells had increased activation in donors from South Africa, which has high-incidence of S. agalactiae invasive disease, compared to Dutch donors. This assay allows for the characterization of heterogeneity in B cell immunity that may underlie immunization efficacy.

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

BACKGROUND

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

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

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

Siglecs family used by pathogens for immune escape may engaged in immune tolerance in pregnancy

The Siglecs family is a group of type I sialic acid-binding immunoglobulin-like receptors that regulate cellular signaling by recognizing sialic acid epitopes. Siglecs are predominantly expressed on the surface of leukocytes, where they play a crucial role in regulating immune activity. Pathogens can exploit inhibitory Siglecs by utilizing their sialic acid components to promote invasion or suppress immune functions, facilitating immune evasion. The establishing of an immune-balanced maternal-fetal interface microenvironment is essential for a successful pregnancy. Dysfunctional immune cells may lead to adverse pregnancy outcomes. Siglecs are important for inducing a phenotypic switch in leukocytes at the maternal-fetal interface toward a less toxic and more tolerant phenotype. Recent discoveries regarding Siglecs in the reproductive system have drawn further attention to their potential roles in reproduction. In this review, we primarily discuss the latest advances in understanding the impact of Siglecs as immune regulators on infections and pregnancy.

Targeting the Siglec-sialic acid axis promotes antitumor immune responses in preclinical models of glioblastoma

Glioblastoma (GBM) is the most aggressive form of primary brain tumor, for which effective therapies are urgently needed. Cancer cells are capable of evading clearance by phagocytes such as microglia- and monocyte-derived cells through engaging tolerogenic programs. Here, we found that high expression of sialic acid-binding immunoglobulin-like lectin 9 (Siglec-9) correlates with reduced survival in patients with GBM. Using microglia- and monocyte-derived cell-specific knockouts of Siglec-E, the murine functional homolog of Siglec-9, together with single-cell RNA sequencing, we demonstrated that Siglec-E inhibits phagocytosis by these cells, thereby promoting immune evasion. Loss of Siglec-E on monocyte-derived cells further enhanced antigen cross-presentation and production of pro-inflammatory cytokines, which resulted in more efficient T cell priming. This bridging of innate and adaptive responses delayed tumor growth and resulted in prolonged survival in murine models of GBM. Furthermore, we showed the combinatorial activity of Siglec-E blockade and other immunotherapies demonstrating the potential for targeting Siglec-9 as a treatment for patients with GBM.

Tumor cell-intrinsic and tumor microenvironmental conditions co-determine signaling by the glycoimmune checkpoint receptor Siglec-7

Tumors create an immunosuppressive tumor microenvironment by altering protein expression, but also by changing their glycosylation status, like altered expression of sialoglycans. Sialoglycans are capped with sialic acid sugar residues and are recognized by Siglec immune receptors. Siglec-7 is an inhibitory immune receptor similar to PD-1, and is emerging as glycoimmune checkpoint exploited by cancer cells to evade the immune system. However, the exact cellular and molecular conditions required for Siglec-7-mediated immune cell inhibition remain largely unknown. Here, we report on the development of a chimeric Siglec-7 cell system that enables dissection of Siglec-7 signaling, rather than Siglec-7 binding. Antibody-induced clustering, sialic acid-containing polymers, and highly sialylated erythrocytes effectively induced Siglec-7 signaling, thereby validating functionality of this reporter system. Moreover, the system reveals tumor cell-dependent Siglec-7 signaling. Tumor-associated conditions important for Siglec-7 signaling were defined, such as Siglec-7 ligand expression levels, presence of the known Siglec-7 ligand CD43, and sialic acid availability for sialylation of glycans. Importantly, therapeutic targeting of the Siglec-7/sialic acid axis using a sialyltransferase inhibitor resulted in strong reduction of Siglec-7 signaling. In conclusion, using a newly established cellular tool, we defined a set of tumor-associated conditions that influence Siglec-7 signaling. Moreover, the system allows to assess the efficacy of novel cancer drugs interfering with the Siglec-7/sialic acid axis as immunotherapy to treat cancer.

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.

Varicella Zoster Virus infects mucosal associated Invariant T cells

Introduction

Mucosal Associated Invariant T (MAIT) cells are innate-like T cells that respond to conserved pathogen-derived vitamin B metabolites presented by the MHC class I related-1 molecule (MR1) antigen presentation pathway. Whilst viruses do not synthesize these metabolites, we have reported that varicella zoster virus (VZV) profoundly suppresses MR1 expression, implicating this virus in manipulation of the MR1:MAIT cell axis. During primary infection, the lymphotropism of VZV is likely to be instrumental in hematogenous dissemination of virus to gain access to cutaneous sites where it clinically manifests as varicella (chickenpox). However, MAIT cells, which are found in the blood and at mucosal and other organ sites, have yet to be examined in the context of VZV infection. The goal of this study was to examine any direct impact of VZV on MAIT cells.

Methods

Using flow cytometry, we interrogated whether primary blood derived MAIT cells are permissive to infection by VZV whilst further analysing differential levels of infection between various MAIT cell subpopulations. Changes in cell surface extravasation, skin homing, activation and proliferation markers after VZV infection of MAIT cells was also assessed via flow cytometry. Finally the capacity of MAIT cells to transfer infectious virus was tested through an infectious center assay and imaged via fluorescence microscopy.

Results

We identify primary blood-derived MAIT cells as being permissive to VZV infection. A consequence of VZV infection of MAIT cells was their capacity to transfer infectious virus to other permissive cells, consistent with MAIT cells supporting productive infection. When subgrouping MAIT cells by their co- expression of a variety cell surface markers, there was a higher proportion of VZV infected MAIT cells co-expressing CD4+ and CD4+/CD8+ MAIT cells compared to the more phenotypically dominant CD8+ MAIT cells, whereas infection was not associated with differences in co-expression of CD56 (MAIT cell subset with enhanced responsiveness to innate cytokine stimulation), CD27 (co-stimulatory) or PD-1 (immune checkpoint). Infected MAIT cells retained high expression of CCR2, CCR5, CCR6, CLA and CCR4, indicating a potentially intact capacity for transendothelial migration, extravasation and trafficking to skin sites. Infected MAIT cells also displayed increased expression of CD69 (early activation) and CD71 (proliferation) markers.

Discussion

These data identify MAIT cells as being permissive to VZV infection and identify impacts of such infection on co- expressed functional markers.

Sialylated IgG in epithelial cancers inhibits antitumor function of T cells via Siglec-7

Although effective, immune checkpoint blockade induces response in only a subset of cancer patients. There is an urgent need to discover new immune checkpoint targets. Recently, it was found that a class of sialic acid-binding immunoglobulin-like lectins (Siglecs) expressed on the surface of T cells in cancer patients inhibit T cell activation through their intracellular immunosuppressive motifs by recognizing sialic acid-carrying glycans, sialoglycans. However, ligands of Siglecs remain elusive. Here, we report sialylated IgG (SIA-IgG), a ligand to Siglec-7, that is highly expressed in epithelial cancer cells. SIA-IgG binds Siglec-7 directly and inhibits TCR signals. Blocking of either SIA-IgG or Siglec-7 elicited potent antitumor immunity in T cells. Our study suggests that blocking of Siglec-7/SIA-IgG offers an opportunity to enhance immune function while simultaneously sensitizing cancer cells to immune attack.

Glycoconjugates: Synthesis, Functional Studies, and Therapeutic Developments

Glycoconjugates are major constituents of mammalian cells that are formed via covalent conjugation of carbohydrates to other biomolecules like proteins and lipids and often expressed on the cell surfaces. Among the three major classes of glycoconjugates, proteoglycans and glycoproteins contain glycans linked to the protein backbone via amino acid residues such as Asn for N-linked glycans and Ser/Thr for O-linked glycans. In glycolipids, glycans are linked to a lipid component such as glycerol, polyisoprenyl pyrophosphate, fatty acid ester, or sphingolipid. Recently, glycoconjugates have become better structurally defined and biosynthetically understood, especially those associated with human diseases, and are accessible to new drug, diagnostic, and therapeutic developments. This review describes the status and new advances in the biological study and therapeutic applications of natural and synthetic glycoconjugates, including proteoglycans, glycoproteins, and glycolipids. The scope, limitations, and novel methodologies in the synthesis and clinical development of glycoconjugates including vaccines, glyco-remodeled antibodies, glycan-based adjuvants, glycan-specific receptor-mediated drug delivery platforms, etc., and their future prospectus are discussed.

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

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

Siglec-7 represents a glyco-immune checkpoint for non-exhausted effector memory CD8+ T cells with high functional and metabolic capacities

While inhibitory Siglec receptors are known to regulate myeloid cells, less is known about their expression and function in lymphocytes subsets. Here we identified Siglec-7 as a glyco-immune checkpoint expressed on non-exhausted effector memory CD8+ T cells that exhibit high functional and metabolic capacities. Seahorse analysis revealed higher basal respiration and glycolysis levels of Siglec-7+ CD8+ T cells in steady state, and particularly upon activation. Siglec-7 polarization into the T cell immune synapse was dependent on sialoglycan interactions in trans and prevented actin polarization and effective T cell responses. Siglec-7 ligands were found to be expressed on both leukemic stem cells and acute myeloid leukemia (AML) cells suggesting the occurrence of glyco-immune checkpoints for Siglec-7+ CD8+ T cells, which were found in patients’ peripheral blood and bone marrow. Our findings project Siglec-7 as a glyco-immune checkpoint and therapeutic target for T cell-driven disorders and cancer.

Aberrant Sialylation in Cancer: Therapeutic Opportunities

The surface of every eukaryotic cell is coated in a thick layer of glycans that acts as a key interface with the extracellular environment. Cancer cells have a different ‘glycan coat’ to healthy cells and aberrant glycosylation is a universal feature of cancer cells linked to all of the cancer hallmarks. This means glycans hold huge potential for the development of new diagnostic and therapeutic strategies. One key change in tumour glycosylation is increased sialylation, both on N-glycans and O-glycans, which leads to a dense forest of sialylated structures covering the cell surface. This hypersialylation has far-reaching consequences for cancer cells, and sialylated glycans are fundamental in tumour growth, metastasis, immune evasion and drug resistance. The development of strategies to inhibit aberrant sialylation in cancer represents an important opportunity to develop new therapeutics. Here, I summarise recent advances to target aberrant sialylation in cancer, including the development of sialyltransferase inhibitors and strategies to inhibit Siglecs and Selectins, and discuss opportunities for the future.

The relationship of early- and late-onset Alzheimer’s disease genes with COVID-19

Individuals with Alzheimer’s disease and other neurodegenerative diseases have been exposed to excess risk by the COVID-19 pandemic. COVID-19’s main manifestations include high body temperature, dry cough, and exhaustion. Nevertheless, some affected individuals may have an atypical presentation at diagnosis but suffer neurological signs and symptoms as the first disease manifestation. These findings collectively show the neurotropic nature of SARS-CoV-2 virus and its ability to involve the central nervous system. In addition, Alzheimer’s disease and COVID-19 has a number of common risk factors and comorbid conditions including age, sex, hypertension, diabetes, and the expression of APOE ε4. Until now, a plethora of studies have examined the COVID-19 disease but only a few studies has yet examined the relationship of COVID-19 and Alzheimer’s disease as risk factors of each other. This review emphasizes the recently published evidence on the role of the genes of early- or late-onset Alzheimer’s disease in the susceptibility of individuals currently suffering or recovered from COVID-19 to Alzheimer’s disease or in the susceptibility of individuals at risk of or with Alzheimer’s disease to COVID-19 or increased COVID-19 severity and mortality. Furthermore, the present review also draws attention to other uninvestigated early- and late-onset Alzheimer’s disease genes to elucidate the relationship between this multifactorial disease and COVID-19.

Transgenic mouse models to study the physiological and pathophysiological roles of human Siglecs

Sialic acid-binding immunoglobulin-like lectins (Siglecs) are important immunomodulatory receptors. Due to differences between human and mouse Siglecs, defining the in vivo roles for human Siglecs (hSiglecs) can be challenging. One solution is the development and use of hSiglec transgenic mice to assess the physiological roles of hSiglecs in health and disease. These transgenic mice can also serve as important models for the pre-clinical testing of immunomodulatory approaches that are based on targeting hSiglecs. Four general methods have been used to create hSiglec-expressing transgenic mice, each with associated advantages and disadvantages. To date, transgenic mouse models expressing hSiglec-2 (CD22), -3 (CD33), -7, -8, -9, -11, and -16 have been created. This review focuses on both the generation of these hSiglec transgenic mice, along with the important findings that have been made through their study. Cumulatively, hSiglec transgenic mouse models are providing a deeper understanding of the differences between human and mice orthologs/paralogs, mechanisms by which Siglecs regulate immune cell signaling, physiological roles of Siglecs in disease, and different paradigms where targeting Siglecs may be therapeutically advantageous.

Functional and Phenotypic Characterization of Siglec-6 on Human Mast Cells

Mast cells are tissue-resident cells that contribute to allergic diseases, among others, due to excessive or inappropriate cellular activation and degranulation. Therapeutic approaches to modulate mast cell activation are urgently needed. Siglec-6 is an immunoreceptor tyrosine-based inhibitory motif (ITIM)-bearing receptor selectively expressed by mast cells, making it a promising target for therapeutic intervention. However, the effects of its engagement on mast cells are poorly defined. Siglec-6 expression and endocytosis on primary human mast cells and mast cell lines were assessed by flow cytometry. SIGLEC6 mRNA expression was examined by single-cell RNAseq in esophageal tissue biopsy samples. The ability of Siglec-6 engagement or co-engagement to prevent primary mast cell activation was determined based on assessments of mediator and cytokine secretion and degranulation markers. Siglec-6 was highly expressed by all mast cells examined, and the SIGLEC6 transcript was restricted to mast cells in esophageal biopsy samples. Siglec-6 endocytosis occurred with delayed kinetics relative to the related receptor Siglec-8. Co-crosslinking of Siglec-6 with FcεRIα enhanced the inhibition of mast cell activation and diminished downstream ERK1/2 and p38 phosphorylation. The selective, stable expression and potent inhibitory capacity of Siglec-6 on human mast cells are favorable for its use as a therapeutic target in mast cell-driven diseases.

Siglec-5 is an inhibitory immune checkpoint molecule for human T cells

Sialic acid-binding immunoglobulin-type lectins (Siglecs) are a family of immunoglobulin-type lectins that mediate protein-carbohydrate interactions via sialic acids attached to glycoproteins or glycolipids. Most of the CD33-related Siglecs (CD33rSiglecs), a major subfamily of rapidly evolving Siglecs, contain a cytoplasmic signaling domain consisting of the immunoreceptor tyrosine-based inhibitory motif (ITIM) and immunoreceptor tyrosine-based switch motif (ITSM) and mediate suppressive signals for lymphoid and myeloid cells. While most CD33rSiglecs are expressed by innate immune cells, such as monocytes and neutrophils, to date, the expression of Siglecs in human T cells has not been well appreciated. In this study, we found that Siglec-5, a member of the CD33rSiglecs, is expressed by most activated T cells upon antigen receptor stimulation. Functionally, Siglec-5 suppresses T cell activation. In support of these findings, we found that Siglec-5 overexpression abrogates antigen receptor induced activation of NFAT and AP-1. Furthermore, we show that GBS β-protein, a known bacterial ligand of Siglec-5, reduces the production of cytokines and cytolytic molecules by activated primary T cells in a Siglec-5 dependent manner. Our data also show that some cancer cell lines express a putative Siglec-5 ligand(s), and that the presence of soluble Siglec-5 enhances tumor-cell specific T cell activation, suggesting that some tumor cells inhibit T cell activation via Siglec-5. Together, our data demonstrate that Siglec-5 is a previously unrecognized inhibitory T cell immune checkpoint molecule and suggest that blockade of Siglec-5 could serve as a new strategy to enhance anti-tumor T cell functions.

Enhancing CAR T function with the engineered secretion of C. perfringens neuraminidase

Prior to adoptive transfer, CAR T cells are activated, lentivirally infected with CAR transgenes, and expanded over 9 to 11 days. An unintended consequence of this process is the progressive differentiation of CAR T cells over time in culture. Differentiated T cells engraft poorly, which limits their ability to persist and provide sustained tumor control in hematologic as well as solid tumors. Solid tumors include other barriers to CAR T cell therapies, including immune and metabolic checkpoints that suppress effector function and durability. Sialic acids are ubiquitous surface molecules with known immune checkpoint functions. The enzyme C. perfringens neuraminidase (CpNA) removes sialic acid residues from target cells, with good activity at physiologic conditions. In combination with galactose oxidase (GO), NA has been found to stimulate T cell mitogenesis and cytotoxicity in vitro. Here we determine whether CpNA alone and in combination with GO promotes CAR T cell antitumor efficacy. We show that CpNA restrains CAR T cell differentiation during ex vivo culture, giving rise to progeny with enhanced therapeutic potential. CAR T cells expressing CpNA have superior effector function and cytotoxicity in vitro. In a Nalm-6 xenograft model of leukemia, CAR T cells expressing CpNA show enhanced antitumor efficacy. Arming CAR T cells with CpNA also enhanced tumor control in xenograft models of glioblastoma as well as a syngeneic model of melanoma. Given our findings, we hypothesize that charge repulsion via surface glycans is a regulatory parameter influencing differentiation. As T cells engage target cells within tumors and undergo constitutive activation through their CARs, critical thresholds of negative charge may impede cell-cell interactions underlying synapse formation and cytolysis. Removing the dense pool of negative cell-surface charge with CpNA is an effective approach to limit CAR T cell differentiation and enhance overall persistence and efficacy.

Blockade of novel immune checkpoints and new therapeutic combinations to boost antitumor immunity

Immunotherapy has emerged as a promising strategy for boosting antitumoral immunity. Blockade of immune checkpoints (ICs), which regulate the activity of cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells has proven clinical benefits. Antibodies targeting CTLA-4, PD-1, and PD-L1 are IC-blockade drugs approved for the treatment of various solid and hematological malignancies. However, a large subset of patients does not respond to current anti-IC immunotherapy. An integrative understanding of tumor-immune infiltrate, and IC expression and function in immune cell populations is fundamental to the design of effective therapies. The simultaneous blockade of newly identified ICs, as well as of previously described ICs, could improve antitumor response. We review the potential for novel combinatory blockade strategies as antitumoral therapy, and their effects on immune cells expressing the targeted ICs. Preclinical evidence and clinical trials involving the blockade of the various ICs are reported. We finally discuss the rationale of IC co-blockade strategy with respect to its downstream signaling in order to improve effective antitumoral immunity and prevent an increased risk of immune-related adverse events (irAEs).

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.

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.

Siglec-9 defines and restrains a natural killer subpopulation highly cytotoxic to HIV-infected cells

Siglec-9 is an MHC-independent inhibitory receptor expressed on a subset of natural killer (NK) cells. Siglec-9 restrains NK cytotoxicity by binding to sialoglycans (sialic acid-containing glycans) on target cells. Despite the importance of Siglec-9 interactions in tumor immune evasion, their role as an immune evasion mechanism during HIV infection has not been investigated. Using in vivo phenotypic analyses, we found that Siglec-9+ CD56dim NK cells, during HIV infection, exhibit an activated phenotype with higher expression of activating receptors and markers (NKp30, CD38, CD16, DNAM-1, perforin) and lower expression of the inhibitory receptor NKG2A, compared to Siglec-9- CD56dim NK cells. We also found that levels of Siglec-9+ CD56dim NK cells inversely correlate with viral load during viremic infection and CD4+ T cell-associated HIV DNA during suppressed infection. Using in vitro cytotoxicity assays, we confirmed that Siglec-9+ NK cells exhibit higher cytotoxicity towards HIV-infected cells compared to Siglec-9- NK cells. These data are consistent with the notion that Siglec-9+ NK cells are highly cytotoxic against HIV-infected cells. However, blocking Siglec-9 enhanced NK cells' ability to lyse HIV-infected cells, consistent with the known inhibitory function of the Siglec-9 molecule. Together, these data support a model in which the Siglec-9+ CD56dim NK subpopulation is highly cytotoxic against HIV-infected cells even whilst being restrained by the inhibitory effects of Siglec-9. To harness the cytotoxic capacity of the Siglec-9+ NK subpopulation, which is dampened by Siglec-9, we developed a proof-of-concept approach to selectively disrupt Siglec/sialoglycan interactions between NK and HIV-infected cells. We achieved this goal by conjugating Sialidase to several HIV broadly neutralizing antibodies. These conjugates selectively desialylated HIV-infected cells and enhanced NK cells' capacity to kill them. In summary, we identified a novel, glycan-based interaction that may contribute to HIV-infected cells' ability to evade NK immunosurveillance and developed an approach to break this interaction.

Installation of high-affinity Siglec-1 ligand on tumor surface for macrophage-engaged tumor suppression

Siglecs that binds cell surface sialoglycans are a family of immunomodulatory receptors, of which, Siglec-7 expressed on natural killer (NK) cells promotes tumor immunoevation while the role of Siglec-1 expressed on macrophages on tumor development remains largely unexplored. Herein, we selectively introduced high affinity sialoside ligands of Siglec-1 and Siglec-7 to tumor cell surface via in vivo Strain-promoted Azide-Alkyne cyclization of ^TCCSiaα2,3-Lactose or ^FITCSiaα2,6-Lactose with 9-azido sialic acid (^AzSia) metabolically installed on tumor cell surface. We found that ^TCCSiaα2,3-Lactose conjugated on tumor surface moderately inhibited tumor growth while ^FITCSiaα2,6-Lactose promote tumor growth. These results suggest high-affinity ligand of Siglec-1 dispalyed on tumors surface provide a new perspective for tumor immunotherapy.

Siglec-8 Signals Through a Non-Canonical Pathway to Cause Human Eosinophil Death In Vitro

Sialic acid-binding immunoglobulin-like lectin (Siglec)-8 is a glycan-binding receptor bearing immunoreceptor tyrosine-based inhibitory and switch motifs (ITIM and ITSM, respectively) that is selectively expressed on eosinophils, mast cells, and, to a lesser extent, basophils. Previous work has shown that engagement of Siglec-8 on IL-5-primed eosinophils causes cell death via CD11b/CD18 integrin-mediated adhesion and NADPH oxidase activity and identified signaling molecules linking adhesion, reactive oxygen species (ROS) production, and cell death. However, the proximal signaling cascade activated directly by Siglec-8 engagement has remained elusive. Most members of the Siglec family possess similar cytoplasmic signaling motifs and recruit the protein tyrosine phosphatases SHP-1/2, consistent with ITIM-mediated signaling, to dampen cellular activation. However, the dependence of Siglec-8 function in eosinophils on these phosphatases has not been studied. Using Siglec-8 antibody engagement and pharmacological inhibition in conjunction with assays to measure cell-surface upregulation and conformational activation of CD11b integrin, ROS production, and cell death, we sought to identify molecules involved in Siglec-8 signaling and determine the stage of the process in which each molecule plays a role. We demonstrate here that the enzymatic activities of Src family kinases (SFKs), Syk, SHIP1, PAK1, MEK1, ERK1/2, PLC, PKC, acid sphingomyelinase/ceramidase, and Btk are all necessary for Siglec-8-induced eosinophil cell death, with no apparent role for SHP-1/2, SHIP2, or c-Raf. While most of these signaling molecules are necessary for Siglec-8-induced upregulation of CD11b integrin at the eosinophil cell surface, Btk is phosphorylated and activated later in the signaling cascade and is instead necessary for CD11b activation. In contrast, SFKs and ERK1/2 are phosphorylated far earlier in the process, consistent with their role in augmenting cell-surface levels of CD11b. In addition, pretreatment of eosinophils with latrunculin B or jasplakinolide revealed that actin filament disassembly is necessary and sufficient for surface CD11b integrin upregulation and that actin polymerization is necessary for downstream ROS production. These results show that Siglec-8 signals through an unanticipated set of signaling molecules in IL-5-primed eosinophils to induce cell death and challenges the expectation that ITIM-bearing Siglecs signal through inhibitory pathways involving protein tyrosine phosphatases to achieve their downstream functions.

A new algorithm shows superior ability to discriminate liver fibrosis stages in chronic hepatitis C

Previous evidence suggests that sialic acid-binding Ig-like lectin 7 (Siglec-7) protein is significantly increased in patients with chronic hepatitis C virus (HCV) infection and directly correlates with clinical parameters of liver inflammation and fibrosis. The aim of this study was to determine the diagnostic value of Siglec-7 as a non-invasive tool to assess liver fibrosis in patients with chronic hepatitis C in a cross-sectional study. Serum levels of Siglec-7 were retrospectively tested in 1007 consecutive patients with chronic HCV infection recruited at three different European sites and data examined by the 'imperfect gold-standard' statistical analysis. Liver stiffness obtained by transient elastography (TE) was considered the standard reference. Liver fibrosis was staged according to published cut-offs of liver stiffness measurement by TE. Accuracy of detection of liver fibrosis stage was not increased by Siglec-7 alone. However, we developed a new index (SiGAP) including Siglec-7, γ-glutamyl transferase, age and platelet count which showed increased sensitivity and specificity in predicting fibrosis compared with APRI or FIB4 indices. The AUROC of SiGAP for the diagnosis of significant (≥F2) and advanced liver fibrosis (≥F3) showed significantly higher values than those of APRI and FIB-4. Siglec-7 may be useful as a complementary tool to assess liver fibrosis stage in patients with chronic hepatitis C when included in a specifically designed algorithm, which showed high level of accuracy in the detection of F2 and F3 fibrosis stage.

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

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

The evolving role of MUC16 (CA125) in the transformation of ovarian cells and the progression of neoplasia

MUC16 (the cancer antigen CA125) is the most commonly used serum biomarker in epithelial ovarian cancer, with increasing levels reflecting disease progression. It is a transmembrane glycoprotein with multiple isoforms, undergoing significant changes through the metastatic process. Aberrant glycosylation and cleavage with overexpression of a small membrane-bound fragment consist MUC16-related mechanisms that enhance malignant potential. Even MUC16 knockdown can induce an aggressive phenotype but can also increase susceptibility to chemotherapy. Variable MUC16 functions help ovarian cancer cells avoid immune cytotoxicity, survive inside ascites and form metastases. This review provides a comprehensive insight into MUC16 transformations and interactions, with description of activated oncogenic signalling pathways, and adds new elements on the role of its differential glycosylation. By following the journey of the molecule from pre-malignant states to advanced stages of disease it demonstrates its behaviour, in relation to the phenotypic shifts and progression of ovarian cancer. Additionally, it presents proposed differences of MUC16 structure in normal/benign conditions and epithelial ovarian malignancy.

Ganglioside GD3 May Suppress the Functional Activities of Benign Skin T Cells in Cutaneous T-Cell Lymphoma

In cutaneous T-cell lymphoma (CTCL), which arises from skin-tropic memory T cells, malignant T cells and benign T cells are confined in the same skin lesions. It is thus difficult to evaluate the phenotypic characteristics and functional activities of benign T cells in CTCL. Disialoganglioside with three glycosyl groups (GD3) is increasingly expressed on the surface of solid malignant tumor cells and takes part in tumor progression and suppression of tumor immunity. However, the role of GD3 in CTCL is not well-understood. In this study, the malignant and benign T cells in CTCL skin lesions were distinguished by flow cytometry and their phenotypic characteristics were compared with those of T cells from control skin specimens. In CTCL skin lesions, the benign T cells included limited resident memory T cells (T_RM), which are sessile in skin and known to exert strong antitumor function. The benign T cells showed diminished Th17 property, and the expression of GD3 was high in the malignant T cells. The expression of GD3 in the malignant T cells inversely correlated with IL-17A production from the benign CD4 T cells. GD3 from the malignant T cells was implied to be involved in suppressing the Th17 activity of the benign T cells independent of the regulation of T_RM differentiation in CTCL. Revealing the role of GD3 in inhibiting the production of IL-17A in CTCL would aid the understanding of the suppressive mechanism of the antitumor activity by malignant tumor cells.

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

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

Targeting glyco-immune checkpoints for cancer therapy

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

Sialic Acids and Their Influence on Human NK Cell Function

Sialic acids are sugars with a nine-carbon backbone, present on the surface of all cells in humans, including immune cells and their target cells, with various functions. Natural Killer (NK) cells are cells of the innate immune system, capable of killing virus-infected and tumor cells. Sialic acids can influence the interaction of NK cells with potential targets in several ways. Different NK cell receptors can bind sialic acids, leading to NK cell inhibition or activation. Moreover, NK cells have sialic acids on their surface, which can regulate receptor abundance and activity. This review is focused on how sialic acids on NK cells and their target cells are involved in NK cell function.

Modulation of immune cell reactivity with cis-binding Siglec agonists

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

Targeted glycan degradation potentiates the anticancer immune response in vivo

Currently approved immune checkpoint inhibitor therapies targeting the PD-1 and CTLA-4 receptor pathways are powerful treatment options for certain cancers; however, most patients across cancer types still fail to respond. Consequently, there is interest in discovering and blocking alternative pathways that mediate immune suppression. One such mechanism is an upregulation of sialoglycans in malignancy, which has been recently shown to inhibit immune cell activation through multiple mechanisms and therefore represents a targetable glycoimmune checkpoint. Since these glycans are not canonically druggable, we designed an αHER2 antibody-sialidase conjugate that potently and selectively strips diverse sialoglycans from breast cancer cells. In syngeneic breast cancer models, desialylation enhanced immune cell infiltration and activation and prolonged the survival of mice, an effect that was dependent on expression of the Siglec-E checkpoint receptor found on tumor-infiltrating myeloid cells. Thus, antibody-sialidase conjugates represent a promising modality for glycoimmune checkpoint therapy.

Eosinophil and mast cell Siglecs: From biology to drug target

Mast cells and eosinophils are innate immune cells involved in both acute and chronic inflammatory responses. Siglecs are a family of cell surface receptors that share sialic acid binding activity. Over the past 20 years, our knowledge of the expression and function of Siglecs on cells of the immune system and others has greatly expanded, as has our understanding of their signaling, ligands, and possible roles in disease pathophysiology. Because of this, Siglecs have garnered interest as potential drug targets using strategies ranging from biologics to ligand-directed nanoparticles. This mini-review will highlight the state of our knowledge regarding human eosinophil and mast cell Siglecs, their biology, what they recognize, tools developed for in vitro and preclinical experimentation, and the status of ongoing efforts to develop drugs that engage eosinophil and mast cell Siglecs for potential therapeutic benefit.

Identification of unique clusters of T, dendritic, and innate lymphoid cells in the peritoneal fluid of ovarian cancer patients

PROBLEM

We hypothesize that activated peritoneal immune cells can be redirected to target ovarian tumors. Here, we obtain fundamental knowledge of the peritoneal immune environment through deep immunophenotyping of T cells, dendritic cells (DC), and innate lymphoid cells (ILC) of ovarian cancer patients.

METHOD OF STUDY

T cells, DC, and ILC from ascites of ovarian cancer patients (n = 15) and peripheral blood of post-menopausal healthy donors (n = 6) were immunophenotyped on a BD Fortessa cytometer using three panels-each composed of 16 antibodies. The data were analyzed manually and by t-SNE/DensVM. CA125 levels were obtained from patient charts.

RESULTS

We observed decreased CD3⁺ T cells and a higher proportion of activated CD4⁺ and effector memory CD4⁺ /CD8⁺ T cells, plasmacytoid DC, CD1c⁺ and CD141⁺ myeloid DC and CD56^Hi NK cells in ascites. t-SNE/DensVM identified eight T cell, 17 DC, and 17 ILC clusters that were unique in the ascites compared to controls. Hierarchical clustering of cell frequency distinctly segregated the T-cell and ILC clusters from controls. Increased CA125 levels were associated with decreased CD8⁺ /CD45RA⁺ /CD45RO^- /CCR7^- T cells.

CONCLUSION

The identified immune clusters serve as the basis for interrogation of the peritoneal immune environment and the development of novel immunologic modalities against ovarian cancer.

Discovery of a new sialic acid binding region that regulates Siglec-7

Siglec-7 is a human CD33-like siglec, and is localised predominantly on human natural killer (NK) cells and monocytes. Siglec-7 is considered to function as an immunoreceptor in a sialic acid-dependent manner. However, the underlying mechanisms linking sialic acid-binding and function remain unknown. Here, to gain new insights into the ligand-binding properties of Siglec-7, we carried out in silico analysis and site-directed mutagenesis, and found a new sialic acid-binding region (site 2 containing R67) in addition to the well-known primary ligand-binding region (site 1 containing R124). This was supported by equilibrium dialysis, STD-NMR experiments, and inhibition analysis of GD3-binding toward Siglec-7 using synthetic sialoglycoconjugates and a comprehensive set of ganglioside-based glycoconjugates. Our results suggest that the two ligand-binding sites are potentially controlled by each other due to the flexible conformation of the C-C′ loop of Siglec-7.

Siglec-10 expression is up-regulated in activated human CD4+ T cells

Most sialic acid-binding immunoglobulin-like lectins (Siglecs) suppress immune cell function but are expressed at low levels on human T cells. We found that soluble CD52 inhibited T cell signalling by ligating Siglec-10, but the presence of Siglec-10 on human T cells has been questioned. To address this concern, we examined the expression of Siglec-10 at the RNA and protein level in human CD4⁺ T cells. Analysis by RNAseq, qPCR and flow cytometry demonstrated that, in contrast to other Siglecs, after activation of CD4⁺ T cells Siglec-10 was selectively upregulated in a subset of cells also high for CD52 expression. This observation is consistent with a homeostatic role for Siglec-10 in human CD4⁺ T cells.

Sialic acid-binding immunoglobulin-like lectins (Siglecs) detect self-associated molecular patterns to regulate immune responses

The mammalian immune system evolved to tightly regulate the elimination of pathogenic microbes and neoplastic transformed cells while tolerating our own healthy cells. Here, we summarize experimental evidence for the role of Siglecs-in particular CD33-related Siglecs-as self-receptors and their sialoglycan ligands in regulating this balance between recognition of self and non-self. Sialoglycans are found in the glycocalyx and extracellular fluids and matrices of all mammalian cells and can be considered as self-associated molecular patterns (SAMPs). We also provide an overview of the known interactions of Siglec receptors and sialoglycan-SAMPs. Manipulation of the Siglec-SAMP axis offers new therapeutic opportunities for the treatment of inflammatory conditions, autoimmune diseases and also cancer immunotherapy.

Siglecs as Immune Cell Checkpoints in Disease

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

Cancer-derived sialylated IgG promotes tumor immune escape by binding to Siglecs on effector T cells

To date, IgG in the tumor microenvironment (TME) has been considered a product of B cells and serves as an antitumor antibody. However, in this study, using a monoclonal antibody against cancer-derived IgG (Cancer-IgG), we found that cancer cells could secrete IgG into the TME. Furthermore, Cancer-IgG, which carries an abnormal sialic acid modification in the CH1 domain, directly inhibited effector T-cell proliferation and significantly promoted tumor growth by reducing CD4⁺ and CD8⁺ T-cell infiltration into tumor tissues. Mechanistic studies showed that the immunosuppressive effect of sialylated Cancer-IgG is dependent on its sialylation and binding to sialic acid-binding immunoglobulin-type lectins (Siglecs) on effector CD4⁺ and CD8⁺ T cells. Importantly, we show that several Siglecs are overexpressed on effector T cells from cancer patients, but not those from healthy donors. These findings suggest that sialylated Cancer-IgG may be a ligand for Siglecs, which may serve as potential checkpoint proteins and mediate tumor immune evasion.

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

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