Sialic Acid-binding Immunoglobulin-like Lectin Research Articles

Sialylated keratan sulfates on MUC5B are Siglec-8 ligands in the human esophagus.

Human sialic acid-binding immunoglobulin-like lectins (Siglecs) are expressed on subsets of immune cells. Siglec-8 is an immune inhibitory Siglec on eosinophils and mast cells, which are effectors in allergic disorders including eosinophilic esophagitis. Inhibition occurs when Siglec-8 is crosslinked by multivalent Siglec ligands in target tissues. Previously we discovered a high-affinity Siglec-8 sialoglycan ligand on human airways composed of terminally sialylated keratan sulfate chains carried on a single protein, DMBT1. Here we extend that approach to another allergic inflammatory target tissue, human esophagus. Lectin overlay histochemistry revealed that Siglec-8 ligands are expressed predominantly by esophageal submucosal glands, and are densely packed in submucosal ducts leading to the lumen. Expression is tissue-specific; esophageal glands express Siglec-8 ligand whereas nearby gastric glands do not. Extraction and resolution by gel electrophoresis revealed a single predominant human esophageal Siglec-8 ligand migrating at >2MDa. Purification by size exclusion and affinity chromatography, followed by proteomic mass spectrometry, revealed the protein carrier to be MUC5B. Whereas all human esophageal submucosal cells express MUC5B, only a portion convert it to Siglec-8 ligand by adding terminally sialylated keratan sulfate chains. We refer to this as MUC5B S8L. Material from the esophageal lumen of live subjects revealed MUC5B S8L species ranging from ~1-4MDa. We conclude that MUC5B in the human esophagus is a protein canvas on which Siglec-8 binding sialylated keratan sulfate chains are post-translationally added. These data expand understanding of Siglec-8 ligands and may help us understand their roles in allergic immune regulation.

A bioinspired supramolecular nanoprodrug for precision therapy of B-cell non-Hodgkin’s lymphoma

Fludarabine (FA) is still considered as a first-line chemotherapy drug for hematological tumors related to B lymphocytes. However, it is worth noting that the non-specific distribution and non-different cytotoxicity of FA may lead to irreversible consequences such as central nervous system damage such as blindness, coma, and even death. Therefore, it is very important to develop a system to targeting delivery FA. In preliminary studies, it was found that B lymphoma cells would specific highly expressing the sialic acid-binding immunoglobulin-like lectin 2 (known as CD22). Inspired by the specific recognition of sialic acid residues and CD22, we have developed a supramolecular prodrug based on polysialic acid, an endogenous biomacromolecule, achieving targeted-therapy of B-cell non-Hodgkin’s lymphoma (B-NHL). Specifically, the prepared hydrophobic reactive oxygen species-responsive FA dimeric prodrug (F2A) interacts with the TPSA, which polysialic acid were modified by the thymidine derivatives, through non-covalent intermolecular interactions similar to “Watson–Crick” base pairing, resulting in the formation of nanoscale supramolecular prodrug (F@TPSA). Cell experiments have confirmed that F@TPSA can be endocytosed by CD22+ B lymphoma cells including Raji and Ramos cells, and there is a significant difference of endocytosis in other leukocytes. Furthermore, in B-NHL mouse model, compared with FA, F@TPSA is determined to have a stronger tumor targeting and inhibitory effect. More importantly, the distribution of F@TPSA in vivo tends to be enriched in lymphoma tissue rather than nonspecific, thus reducing the leukopenia of FA. The targeted delivery system based on PSA provides a new prodrug modification strategy for targeted treatment of B-NHL.Graphical abstract

Glioma-Associated Sialoglycans Drive the Immune Suppressive Phenotype and Function of Myeloid Cells.

The tumor microenvironment of glioblastoma IDH-wildtype is highly immune suppressive and is characterized by a strong component of myeloid-derived suppressor cells (MDSCs). To interfere with the immune suppressive functions of MDSCs, a comprehensive understanding on how MDSCs acquire their suppressive phenotype is essential. Previously, we and others have shown a distinct Sialic acid-binding immunoglobulin-like lectin (Siglec) receptor expression profile for MDSCs in glioblastoma. Siglec receptors can transmit inhibitory signals comparable to PD-1 and are suggested to act as glyco-immune checkpoints. Here, we investigated how glioma specific Siglec-sialic acid interactions influence myeloid immune suppressive functions. Co-culturing monocytes with glioblastoma cells induced CD163 expression on the monocytes. Upon desialylation of the glioblastoma cells, this induction of CD163 was hampered, and furthermore, the monocytes were now able to secrete higher amounts of IL-6 and TNFα compared to fully sialylated glioblastoma cells. Additionally, Siglec-specific triggering using anti-Siglec-7 or Siglec-9 antibodies displayed a decreased TNFα secretion by the monocytes, validating the role of the Siglec-Sialic axis in the co-culture experiments. Together, our results demonstrate that glioblastoma cells induce a myeloid immune-suppressive phenotype that could be partly rescued by lowering the glioblastoma-associated sialic acid levels. This manuscript supports further research of the Siglec-Sialic acid axis in the context of glioblastoma and its potential to improve clinical outcome.

A Siglec3-like lectin from Nile tilapia (Oreochromis niloticus): Transcriptional profiling upon Streptococcus agalactiae infection and its potential role in immune escape

Siglec-3, also known as CD33, is a type of sialic acid binding immunoglobulin-like lectin that plays a role in regulating cellular activation in the innate immune system and subsequent inflammatory responses. In this study, we cloned the cDNA encoding OnSiglec3-like ORF sequence (1170 bp) in Nile tilapia, contained two Ig-like domains in the extracellular region and one inhibitory ITIM-like motif in the intracellular region. Through qRT-PCR analysis, we found that the mRNA of OnSiglec3-like lectin was highly expressed in the brain and head kidney. When challenged with Streptococcus agalactiae both in vivo and in vitro, we observed a significant up-regulation in the expression of OnSiglec3-like lectin in the head kidney tissues and leukocytes, respectively. Additionally, we investigated the interaction between a recombinant extracellular region protein (referred to as (r)exSiglec3-like protein) and three different phenotypes of S. agalactiae: WT (wild type), ∆cps (cps-deficient mutant), and ∆neuA (sia-deficient mutant) using ELISA. We found that the exOnSiglec3-like protein, when combined with RNA interference and bacterial infection, significantly enhanced the phagocytic ability against S. agalactiae. Furthermore, the expression of four common inflammatory factors (OnIL-6, OnIL-10, OnTNF-α, and OnTGF-β) showed a significant up-regulation. Overall, this study suggests that OnSigelc3-like lectin may play a role in host defence against bacterial infection and mediate immune escape in Nile tilapia.

Abstract 6594: 4-1BB: A double-edged sword for T cells

Abstract Background: The costimulatory receptor 4-1BB (CD137) is expressed on the surface of activated T cells, playing a pivotal role in enhancing T cell responses and emerging as a promising target for cancer immunotherapy. Its primary ligand, 4-1BBL (CD137L), is found on antigen-presenting cells and various immune cells. The interaction between 4-1BBL and 4-1BB provides crucial costimulatory signals to both CD4+ and CD8+ T cells, amplifying their activation and effector functions. However, recent studies have shed light on a distinct role of 4-1BB in T cells. Prior work has revealed that 4-1BB can distinguish intratumoral regulatory T cells (Tregs) from peripheral Tregs and other CD4 lineage T cells in cancer patients, and the overexpression of 4-1BB on Tregs has been associated with unfavorable survival outcomes. Additionally, targeting 4-1BB with IgG2a isotype antibodies has shown significant anti-tumor efficacy in preclinical models by depleting Tregs. Furthermore, agonist monoclonal antibodies targeting 4-1BB have not only shown promise in inducing tumor regressions but also demonstrated improved outcomes in mouse models of autoimmune diseases induced by autoreactive CD4+ T cells. These findings indicate the existence of specific ligands or factors that modulate 4-1BB function on T cells that can vary depending on the disease state. Methods: To identify potential ligand(s) involved in the modulation of 4-1BB function, we developed an innovative CRISPR/Cas9-based transcriptional activation (CRISPRa) screening system that integrates CRISPR/Cas9-guide mediated gene activation, high avidity bead-based selection, serial enrichment, and flow cytometry-based monitoring. This screening platform provides enhanced sensitivity for identifying extracellular interactions between receptor-ligand partners that have not been fully characterized before, even with low-affinity. Results: Through our unbiased CRISPRa screen, we discovered that Myelin-Associated Glycoprotein (MAG), also known as siglec-4, a member of the siglec (sialic acid-binding immunoglobulin-like lectins) family, interacts with 4-1BB in trans. Our study demonstrated that MAG binds to activated T cells via 4-1BB and inhibits their activation and proliferation, suggesting a potential role of the MAG-4-1BB interaction in modulating T cell function. Notably, our data revealed that trans-binding to MAG-positive cells inhibits c-Jun signaling in activated T cells. Furthermore, the presence of MAG on target cells can hinder Chimeric Antigen Receptor (CAR) T-cell activity. Conclusion: Utilizing a highly efficient cell-based CRISPRa system, we discovered a novel interactor of 4-1BB capable of modulating T cell activation through its interaction with 4-1BB. This interaction suggests an immunosuppressive role of 4-1BB, explaining the paradoxical roles of 4-1BB in T cells. These findings provide crucial insights for the development of precise therapeutics targeting 4-1BB. Citation Format: Liping Yang, Timothy P. Sheets, Yang Feng, Kuo-Sheng Hsu, Guojun Yu, Steven Seaman, Pradip Bajgain, Daeho So, Jaewon Lee, Raj Chari, Brad St. Croix. 4-1BB: A double-edged sword for T cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 6594.

Therapeutic potential to target sialylation and SIGLECs in neurodegenerative and psychiatric diseases.

Sialic acids, commonly found as the terminal carbohydrate on the glycocalyx of mammalian cells, are pivotal checkpoint inhibitors of the innate immune system, particularly within the central nervous system (CNS). Sialic acid-binding immunoglobulin-like lectins (SIGLECs) expressed on microglia are key players in maintaining microglial homeostasis by recognizing intact sialylation. The finely balanced sialic acid-SIGLEC system ensures the prevention of excessive and detrimental immune responses in the CNS. However, loss of sialylation and SIGLEC receptor dysfunctions contribute to several chronic CNS diseases. Genetic variants of SIGLEC3/CD33, SIGLEC11, and SIGLEC14 have been associated with neurodegenerative diseases such as Alzheimer's disease, while sialyltransferase ST8SIA2 and SIGLEC4/MAG have been linked to psychiatric diseases such as schizophrenia, bipolar disorders, and autism spectrum disorders. Consequently, immune-modulatory functions of polysialic acids and SIGLEC binding antibodies have been exploited experimentally in animal models of Alzheimer's disease and inflammation-induced CNS tissue damage, including retinal damage. While the potential of these therapeutic approaches is evident, only a few therapies to target either sialylation or SIGLEC receptors have been tested in patient clinical trials. Here, we provide an overview of the critical role played by the sialic acid-SIGLEC axis in shaping microglial activation and function within the context of neurodegeneration and synaptopathies and discuss the current landscape of therapies that target sialylation or SIGLECs.

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 receptorsand are highly sialylated. In murine cancer models of emergency myelopoiesis, Siglec-E knockout in myeloid cells resulted in prolonged survival and increased tumorinfiltration 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.

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

Background and Aim: Sialic acid-binding immunoglobulin-like lectin 1 (Siglec-1) is a transmembrane glycoprotein involved in the sialic acid (Sia)-dependent regulation of the immune system. Siglec-1 expression has recently been identified in the male reproductive tract (MRT) of several species, including humans, cattle, horses, and sheep, and may play a role in modulating fertility in a Sia-dependent manner. Materials and Methods: In this study, protein–protein interaction (PPI) analysis of Siglec-1 was conducted to identify associated network protein conservation, and the expression of Siglec-1 in the MRT of mice and rats, including their accessory sex glands and spermatozoa was determined by immunostaining. Results: Network analysis of proteins with Siglec-1 in mice and rats demonstrated significant similarity to human Siglec-1 networks, suggesting a similar conservation of network proteins between these species and, hence, a potential conservation role in immune modulation and function. Specific immunostaining patterns of mouse and rat testes, epididymis, ductus deferens, accessory sex gland tissues, and sperm were detected using human Siglec-1. These results confirmed that the human Siglec-1 antibody could cross-react with mouse and rat Siglec-1, suggesting that the specific expression patterns of Siglec-1 in the MRT and sperm of both mice and rats are similar to those observed in other species. Conclusion: The conservation of Siglec-1 expression patterns in sperm and within the MRT and the similarity of protein networks for Siglec-1 across species suggest that Siglec-1 may function in a similar manner across species. These results also suggest that rodents may serve as a valuable model system for exploring the function of Siglecs in the reproductive system across species and their potential role in modulating fertility in a Sia-dependent manner. Keywords: female reproductive tract, male reproductive tract, protein–protein interaction, sialic acid, sialic acid-binding immunoglobulin-like lectin-1.

Gene expression patterns of CRYM and SIGLEC10 in Alzheimer's disease: potential early diagnostic indicators.

Alzheimer's disease (AD) is a neurological condition that may lead to dementia as well as a slow and steady decline in cognitive ability. Finding early signs that may be used in the diagnosis of ADis still a difficult aim to achieve in the field of medical practice. The purpose of this research was to investigate to determineany differences in the gene expression patterns of crystallin mu (CRYM) and sialic acid-binding immunoglobulin-like lectin 10 (SIGLEC10) in whole blood samples obtained from fifty individuals who were diagnosed with ADand fifty individuals as a control group. When compared with controls, it was discovered that the expression of the CRYM gene was substantially decreased in AD patients, but the expression of the SIGLEC10 gene was significantly higher. A positive correlation between CRYM and SIGLEC10 was noticed solely in patients with AD. Furthermore, assessing the diagnostic value of these genes, CRYM and SIGLEC10 transcript levels displayed an area under the curve (AUC) of 0.74 and 0.81, respectively. These results suggest that alterations in CRYM and SIGLEC10 expression may be implicated in AD pathology and that these genes expression levels can potentially serve as biomarkers for early detection and diagnosis of AD. Nevertheless, further validation of these findings requires the inclusion of more extensive and heterogeneous cohorts. The findings derived from this study possess the capability to offer a significant contribution towards the progression of innovative diagnostic and therapeutic strategies for AD.

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

BackgroundLaryngeal squamous cell carcinoma (LSCC) is the ultimate common malignant head and neck cancer with dismal prognosis. The expression pattern and clinical significance of Siglec-15 (Sialic acid-binding immunoglobulin-like lectin 15) in LSCC are poorly understood. In order to lay the groundwork for future immune-related research on Siglec-15 in LSCC, we set out to study its expression and prognostic importance in the disease, as well as to use bioinformatics to investigate the immune features modulated by Siglec-15 in LSCC. Methods① In order to get the gene expression profile and clinical data for TCGA head and neck cancer (TCGA-HNSC), you may access the relevant data from UCSC xena and use 110 cases of laryngeal cancer as a training set. Two datasets, GSE27020 and GSE25727, were obtained from the GEO databank and utilized as validation sets. These datasets include expression profiles and clinical information. The Siglec-15 gene and immune characteristics were analyzed by bioinformatics methods. ② Retrospectively collected routine paraffin specimens from patients with pathological diagnosis of squamous cell carcinoma from December 2012 to November 2015 in Sun Yat-sen Memorial Hospital and fresh frozen tissue of patients from June 2021 to March 2022. Immunohistochemistry method, immunofluorescence technique and real-time quantitative PCR was used to examine the difference of Siglec-15 appearance in LSCC tissue and adjacent tissue, and its correlation of prognosis, clinic pathological characteristics and CD8+T lymphocyte infiltration. Using human laryngeal cancer cell line (LCC), we studied the influence of Siglec-15 in cell proliferation and invasion. ResultsWe identified Siglec-15 was upregulated in LSCC. The patients in Siglec-15 high expression group had a poor overall survival (OS) based on the clinical information from TGCA and 111 LSCC patients that hospitalized in Sun Yat-sen Memorial Hospital. The COX regression analysis indicated Siglec-15 as an independent predictor for poor prognosis of LSCC. Bioinformatic analysis suggested that the high expression of Siglec-15 shape an immune suppressive tumor microenvironment (TEM), leading to poor response to immunotherapy in LSCC. Siglec-15 enhanced cell invasion and proliferation, as we showed in vitro. ConclusionOur study support Siglec-15 as a potential predictor for LSCC prognosis and an attractive target for LSCC immunotherapy.

RNA Profile of Cell Bodies and Exosomes Released by Tumorigenic and Non-Tumorigenic Thyroid Cells.

Tumor cells release exosomes, extracellular vesicle containing various bioactive molecules such as protein, DNA and RNA. The analysis of RNA molecules packaged in exosomes may provide new potential diagnostic or prognostic tumor biomarkers. The treatment of radioiodine-refractory aggressive thyroid cancer is still an unresolved clinical challenge, and the search for biomarkers that are detectable in early phase of the disease has become a fundamental goal for thyroid cancer research. By using transcriptome analysis, this study aimed to analyze the gene expression profiles of exosomes secreted by a non-tumorigenic thyroid cell line (Nthy-ori 3.1-exo) and a papillary thyroid cancer (TPC-1-exo) cell line, comparing them with those of cell bodies (Nthy-ori 3.1-cells and TPC-1-cells). A total of 9107 transcripts were identified as differentially expressed when comparing TPC-1-exo with TPC-1-cells and 5861 when comparing Nthy-ori 3.1-exo with Nthy-ori 3.1-cells. Among them, Sialic acid-binding immunoglobulin-like lectins 10 and 11 (SIGLEC10, SIGLEC11) and Keratin-associated protein 5 (KRTAP5-3) transcripts, genes known to be involved in cancer progression, turned out to be up-regulated only in TPC-1-exo. Gene ontology analysis revealed significantly enriched pathways, and only in TPC-1-exo were the differential expressed genes associated with an up-regulation in epigenetic processes. These findings provide a proof of concept that some mRNA species are specifically packaged in tumor-cell-derived exosomes and may constitute a starting point for the identification of new biomarkers for thyroid tumors.

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.

Regulation of mast cells by overlapping but distinct protein interactions of Siglec-6 and Siglec-8.

Sialic acid-binding immunoglobulin-like lectin (Siglec)-6 and Siglec-8 are closely related mast cell (MC) receptors with broad inhibitory activity, but whose functional differences are incompletely understood. Proteomic profiling using quantitative mass spectrometry was performed on primary mouse MCs to identify proteins associated with Siglec-6 and Siglec-8. For functional characterization, each receptor was evaluated biochemically and in exvivo and invivo inhibition models of IgE and non-IgE-mediated MC activation in Siglec-6- or Siglec-8-expressing transgenic mice. Siglec-6 and Siglec-8 were found in MCs within large complexes, interacting with 66 and 86 proteins, respectively. Strikingly, Siglec-6 and Siglec-8 interacted with a large cluster of proteins involved in IgE and non-IgE-mediated MC activation, including the high affinity IgE receptor, stem cell factor (SCF) receptor KIT/CD117, IL-4 and IL-33 receptors, and intracellular kinases LYN and JAK1. Protein interaction networks revealed Siglec-6 and Siglec-8 had overlapping yet distinct MC functions, with a potentially broader regulatory role for Siglec-6. Indeed, Siglec-6 preferentially interacted with the mature form of KIT at the cell surface, and treatment with an anti-Siglec-6 antibody significantly inhibited SCF-mediated MC activation more in comparison to targeting Siglec-8. These data demonstrate a central role for Siglec-6 and Siglec-8 in controlling MC activation through interactions with multiple activating receptors and key signaling molecules. Our findings suggest that Siglec-6 has a role distinct from that of Siglec-8 in regulating MC function and represents a distinct potential therapeutic target in mast cell-driven diseases.

High expression of SIGLEC7 may promote M2-type macrophage polarization leading to adverse prognosis in glioma patients.

Gliomas are the most common primary intracranial tumors, known for their high invasiveness and destructiveness. Sialic acid-binding immunoglobulin-like lectin 7 (SIGLEC7) is present in various immune cells, especially macrophages, and significantly affects immune homeostasis and cancer cell response. However, research on the role and prognostic impact of SIGLEC7 in glioma patients is currently limited. We utilized transcriptomic data from 702 glioma patients in The Cancer Genome Atlas (TCGA) and 693 glioma patients in the Chinese Glioma Genome Atlas (CGGA), along with clinical samples we collected, to comprehensively investigate the impact of SIGLEC7 on glioma expression patterns, biological functions, and prognostic value. We focused on its role in glioma-related immune responses and immune cell infiltration and analyzed its expression at the single-cell level. Finally, we validated the role of SIGLEC7 in gliomas through tissue and cell experiments. SIGLEC7 expression was significantly increased in glioma patients with malignant characteristics. Survival analysis indicated that glioma patients with high SIGLEC7 expression had significantly lower survival rates. Gene function analysis revealed that SIGLEC7 is primarily involved in immune and inflammatory responses and is strongly negatively correlated with tumor-associated immune regulation. Additionally, the expression of most immune checkpoints was positively correlated with SIGLEC7, and immune cell infiltration analysis clearly demonstrated a significant positive correlation between SIGLEC7 expression and M2 macrophage infiltration levels. Single-cell analysis, along with tissue and cell experiments, confirmed that SIGLEC7 enhances macrophage polarization towards the M2 phenotype, thereby promoting glioma invasiveness through the immunosuppressive effects of M2 macrophages. Cox regression analysis and the establishment of survival prediction models indicated that high SIGLEC7 expression is an unfavorable prognostic factor for glioma patients. High SIGLEC7 expression predicts poor prognosis in glioma patients and is closely associated with M2 macrophages in the tumor environment. In the future, SIGLEC7 may become a promising target for glioma immunotherapy.

Uncloaking the viral glycocalyx: How do viruses exploit glycoimmune checkpoints?

The surfaces of cells and enveloped viruses alike are coated in carbohydrates that play multifarious roles in infection and immunity. Organisms across all kingdoms of life make use of a diverse set of monosaccharide subunits, glycosidic linkages, and branching patterns to encode information within glycans. Accordingly, sugar-patterning enzymes and glycan binding proteins play integral roles in cell and organismal biology, ranging from glycoprotein quality control within the endoplasmic reticulum to lymphocyte migration, coagulation, inflammation, and tissue homeostasis. Unsurprisingly, genes involved in generating and recognizing oligosaccharide patterns are playgrounds for evolutionary conflicts that abound in cross-species interactions, exemplified by the myriad plant lectins that function as toxins. In vertebrates, glycans bearing acidic nine-carbon sugars called sialic acids are key regulators of immune responses. Various bacterial and fungal pathogens adorn their cells in sialic acids that either mimic their hosts' or are stolen from them. Yet, how viruses commandeer host sugar-patterning enzymes to thwart immune responses remains poorly studied. Here, we review examples of viruses that interact with sialic acid-binding immunoglobulin-like lectins (Siglecs), a family of immune cell receptors that regulate toll-like receptor signaling and govern glycoimmune checkpoints, while highlighting knowledge gaps that merit investigation. Efforts to illuminate how viruses leverage glycan-dependent checkpoints may translate into new clinical treatments that uncloak viral antigens and infected cell surfaces by removing or masking immunosuppressive sialoglycans, or by inhibiting viral gene products that induce their biosynthesis. Such approaches may hold the potential to unleash the immune system to clear long intractable chronic viral infections.

Siglec-15 on macrophages suppress the immune microenvironment in patients with PD-L1 negative non-metastasis lung adenocarcinoma

Sialic acid-binding immunoglobulin-like lectin 15 (Siglec-15) is an immune checkpoint molecule with sequence homology to programmed cell death ligand 1 (PD-L1), which is mainly expressed on macrophages and tumor cells. However, whether Siglec-15-induced immunosuppression and poor prognosis are independent of PD-L1 remains unclear. In this study, we collected samples of 135 non-small cell lung cancers and found that Siglec-15 and PD-L1 expression were independent in non-small cell lung cancer by multiple immunofluorescence staining. Siglec-15 on macrophages (Mφ-Siglec-15) was significantly associated with DFS (p < 0.05) in PD-L1− patients with non-metastasis lung adenocarcinoma, not in PD-L1+ or lung squamous cell carcinoma patients. Moreover, stromal Siglec-15+ macrophages of Mφ-Siglec-15+PD-L1− patients were significantly more than those of Mφ-Siglec-15−PD-L1− patients (p = 0.002). We further found that Siglec-15+ macrophages polarized toward M2 and produced more IL-10, negatively associated with inflamed immunophenotype in PD-L1− patients and may inhibit CD8+T cells infiltration. In conclusion, PD-L1-independent Siglec-15+ macrophages contribute to the formation of an immunosuppressive microenvironment in non-metastasis lung adenocarcinoma patients, which may cause a higher risk of recurrence. Siglec-15 could be a potential target for normalizing cancer immunotherapy, benefiting patients who fail to respond to anti-PD-L1 therapy.

The Immune Checkpoint Siglec-15 in Promoting Immune Dysregulation in Non-Hodgkin's Lymphomas

Outcomes for patients with relapsed or refractory (r/r) hematologic malignancies remain dismal despite therapeutic advances over the last several years. This is particularly striking in patients with r/r aggressive mature B-cell lymphomas such as Diffuse Large B-cell Lymphoma (DLBCL) and Burkitt's Lymphoma (BL). With use of intensive multiagent chemo-immunotherapy, DLBCL, which accounts for approximately 10-20% of pediatric Non-Hodgkin's Lymphoma (NHL) cases and is the most common subtype in adults, is highly curable with 5-year event free survival &amp;gt;75%. However, approximately 30-40% of adults diagnosed with DLBCL will become refractory to or relapse after first-line treatment. Yet even with intensive salvage regimens, r/r DLBCL remains a clinical challenge. There remains an unmet need to identify specific defects in the immune response, among other pathways, that drive lymphomagenesis that will in turn inform the development of novel therapeutics to improve survival in this population of patients. Siglec-15 (Sig-15), a member of the sialic acid binding immunoglobulin-like lectin family of proteins, has recently been identified as a critical immune suppressor that is highly expressed in human cancers and intra-tumoral myeloid cells. Importantly, recent reports have demonstrated inhibition of Sig-15, has a restorative effect on local anti-tumor immune responses and abrogated tumor progression. While reported in solid malignancies and acute leukemias, a role for Siglec-15 in promoting disease progression in aggressive B-cell lymphomas has not yet been described. We have found higher expression of SIGLEC15 in DLBCL cell lines compared to normal B cells at the RNA level, through analyses of the publicly available ONCOMINE database. This was confirmed at the protein level by western blot demonstrating higher Sig-15 expression in various lymphoma cell lines (RAJI, SUDHL6, OCI-LY10, HBL-1) compared to healthy donor PBMCs. Further, immunohistochemistry evaluating Sig-15 expression was performed on a tumor microarray consisting of 139 cases of DLBCL from adult patients along with validation samples from 15 primary pediatric lymphoma cases (DLBCL, BL and Anaplastic Large Cell Lymphoma (ALCL)). Sig-15 was found to be highly expressed, with histologic scores ranging from weak (1+) to strong (3+) in 124 out of the 139 adult cases (Figure 1A) and in all pediatric lymphoma samples but with distinct staining patterns observed in the aggressive B-cell lymphomas. Specifically, Sig-15 appears to be highly expressed and associated with the cell membrane in most DLBCL and Burkitt's lymphoma. Expression is more variable in ALCL, primarily elevated in the cytoplasm at low levels and/or in macrophages (Figure 1A). There is heterogeneity in Sig-15 expression with preliminary analysis of RNA-seq data from a cohort of 693 DLBCL patient samples demonstrates higher levels in the germinal center B-cell (GCB) versus activated B-cell molecular (ABC) subtype of DLBCL when compared to healthy controls (Kruskal-Wallis, *p = 0.041). Finally, using a well-established murine lymphoma model, A20 cells were stably transduced with control, non-silencing shRNA (shNS) or shRNA against Sig-15 and injected into un-irradiated immune competent (wild type, WT) or immune deficient ( Rag1 -/-) BALB/C mice and monitored for signs of lymphoma development. Knockdown of Sig-15 in A20 cells abrogates disease progression in WT but not immune deficient mice (Log-rank, **p &amp;lt;0.0001, n=10 mice/group), consistent with a role for Sig-15 in immune evasion in lymphoma (Figure 1B). Together, these data implicate Sig-15 as an immune checkpoint that may be inhibited therapeutically to promote an immune response to lymphoma cells.

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

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

Siglec 15 as a biomarker or a druggable molecule for non-small cell lung cancer.

Lung cancer has been the main cause of cancer mortality worldwide. Furthermore, lung cancer rates of new cases per year evidenced a large incidence of this neoplasm in both men and women. Because there is no biomarker for early detection, it is frequently detected late, at an advanced state. The introduction of multiple lines of tyrosine kinase inhibitors in patients with EGFR, ALK, ROS1, and NTRK mutations has modified the therapy of lung cancer. Immunotherapy advances have resulted in substantial improvements in overall survival and disease-free survival, making immune checkpoint inhibitors (ICIs) a potential option for lung cancer treatment. Current PD-1/PD-L1/CTLA-4 immunotherapies have resulted in important response and survival rates. However, existing medicines only function in around 20% of unselected, advanced NSCLC patients, and primary and acquired resistance remain unsolved obstacles. Therefore, precise predictive indicators must be identified to choose the best patients for ICI treatment. Thus, Sialic acid-binding immunoglobulin-like lectin 15 (Siglec-15) stands out as a potential tumor biomarker, with distinctive expression in normal tissues, in tumor immune involvement, and a high structural similarity to PD-L1. Understanding the tumor immune response and the search for new therapeutic targets leads to the improvement of therapeutic pathways directed at the tumor microenvironment. The present review aims to analyze Siglec-15 potential as a diagnostic, prognostic, and response biomarker in lung cancer, considering its results evidenced in the current literature.

The Contribution of Serum Sialic Acid Binding Immunoglobulin-Like Lectin 1(sSIGLEC-1) as an IFN I Signature Biomarker in the Progression of Atherosclerosis in Egyptian Systemic Lupus Erythematosus (SLE) Patients.

Clinical symptoms of systemic lupus erythematosus (SLE), such as atherosclerosis and related cardiovascular diseases, are caused by inflammatory cytokines and endothelial cell damage. The serum sialic acid binding immunoglobulin-like lectin 1 (sSIGLEC-1) is thought to be an alternative biomarker of IFN signature and may have a role in the pathogenesis of atherosclerosis. The aim of the study was to measure the levels of sSIGLEC-1 in the serum of SLE patients in comparison to a control group and examine the associations between sSIGLEC-1, SLEDAI, lipid profile, oxidized low density lipoprotein (oxLDL), and carotid intima media thickness (CIMT) to investigate whether sSIGLEC-1 participates in the development of atherosclerosis. sSIGLEC-1 levels were tested in 53 patients and 20 volunteers using ELISA kit. Duplex measurements were performed on all subjects to measure CIMT. SLE patients had significantly higher values of sSIGLEC-1 (P < 0.0001), total cholesterol (P = 0.029), triglycerides (P = 0.001), low density lipoprotein (P = 0.032), oxLDL (P = 0.001), right CIMT (P = 0.0099) and a significantly lower value of high-density lipoprotein (P = 0.04) when compared to controls. sSIGLEC-1 had significant positive correlations with right CIMT (r = 0.5, P < 0.0002) and oxLDL (r = 0.67, P < 0.0001) in all SLE patients. When compared to non-dyslipidemic patients, the dyslipidemic group exhibited significantly higher levels of all previous parameters except HDL and left CIMT. Circulating form of SIGLEC-1 accelerates atherosclerosis and provides a simple way to predict the occurrence of atherosclerosis in SLE patients.