Siglec-5 (CD170) Can Mediate Inhibitory Signaling in the Absence of Immunoreceptor Tyrosine-based Inhibitory Motif Phosphorylation

Role of CRISPR-Cas systems in periodontal disease pathogenesis and potential for periodontal therapy: A review

Clustered regularly interspaced short palindromic repeats (CRISPRs) are DNA sequences capable of editing a host genome sequence. CRISPR and its specific CRISPR-associated (Cas) protein complexes have been adapted for various applications. These include activating or inhibiting specific genetic sequences or acting as molecular scissors to cut and modify the host DNA precisely. CRISPR-Cas systems are also naturally present in many oral bacteria, where they aid in nutrition, biofilm formation, inter- and intraspecies communication (quorum sensing), horizontal gene transfer, virulence, inflammation modulation, coinfection, and immune response evasion. It even functions as an adaptive immune system, defending microbes against invading viruses and foreign genetic elements from other bacteria by targeting and degrading their DNA. Recently, CRISPR-Cas systems have been tested as molecular editing tools to manipulate specific genes linked with periodontal disease (such as periodontitis) and as novel methods of delivering antimicrobial agents to overcome antimicrobial resistance. With the rapidly increasing role of CRISPR in treating inflammatory diseases, its application in periodontal disease is also becoming popular. Therefore, this review aims to discuss the different types of CRISPR-Cas in oral microbes and their role in periodontal disease pathogenesis and precision periodontal therapy.

Neisseria gonorrhoeae scavenges host sialic acid for Siglec-mediated, complement-independent suppression of neutrophil activation

Gonorrhea, caused by the bacterium Neisseria gonorrhoeae (Gc), is characterized by neutrophilic influx to infection sites. Gc has developed mechanisms to resist killing by neutrophils that include modifications to its surface lipooligosaccharide (LOS). One such LOS modification is sialylation: Gc sialylates its terminal LOS sugars with cytidine-5'-monophosphate-N-acetylneuraminic acid, which is scavenged from the host using LOS sialyltransferase (Lst) since Gc cannot make its sialic acid. Sialylation enables sensitive strains of Gc to resist complement-mediated killing in a serum-dependent manner. However, little is known about the contribution of sialylation to complement-independent, direct Gc-neutrophil interactions. In the absence of complement, we found sialylated Gc expressing opacity-associated (Opa) proteins decreased the oxidative burst and granule exocytosis from primary human neutrophils. In addition, sialylated Opa+ Gc survived better than vehicle treated or Δlst Gc when challenged with neutrophils. However, Gc sialylation did not significantly affect Opa-dependent association with or internalization of Gc by neutrophils. Previous studies have implicated sialic acid-binding immunoglobulin-type lectins (Siglecs) in modulating neutrophil interactions with sialylated Gc. Blocking neutrophil Siglecs with antibodies that bind to their extracellular domains eliminated the ability of sialylated Opa+ Gc to suppress the oxidative burst and resist neutrophil killing. These findings highlight a new role for sialylation in Gc evasion of human innate immunity, with implications for the development of vaccines and therapeutics for gonorrhea.

IMPORTANCE

Neisseria gonorrhoeae, the bacterium that causes gonorrhea, is an urgent global health concern due to increasing infection rates, widespread antibiotic resistance, and its ability to thwart protective immune responses. The mechanisms by which Gc subverts protective immune responses remain poorly characterized. One way N. gonorrhoeae evades human immunity is by adding sialic acid that is scavenged from the host onto its lipooligosaccharide, using the sialyltransferase Lst. Here, we found that sialylation enhances N. gonorrhoeae survival from neutrophil assault and inhibits neutrophil activation, independently of the complement system. Our results implicate bacterial binding of sialic acid-binding lectins (Siglecs) on the neutrophil surface, which dampens neutrophil antimicrobial responses. This work identifies a new role for sialylation in protecting N. gonorrhoeae from cellular innate immunity, which can be targeted to enhance the human immune response in gonorrhea.

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.

A genome-wide association study meta-analysis in a European sample of stage III/IV grade C periodontitis patients ≤35 years of age identifies new risk loci

AIM

Few genome-wide association studies (GWAS) have been conducted for severe forms of periodontitis (stage III/IV grade C), and the number of known risk genes is scarce. To identify further genetic risk variants to improve the understanding of the disease aetiology, a GWAS meta-analysis in cases with a diagnosis at ≤35 years of age was performed.

MATERIALS AND METHODS

Genotypes from German, Dutch and Spanish GWAS studies of III/IV-C periodontitis diagnosed at age ≤35 years were imputed using TopMed. After quality control, a meta-analysis was conducted on 8,666,460 variants in 1306 cases and 7817 controls with METAL. Variants were prioritized using FUMA for gene-based tests, functional annotation and a transcriptome-wide association study integrating eQTL data.

RESULTS

The study identified a novel genome-wide significant association in the FCER1G gene (p = 1.0 × 10-9 ), which was previously suggestively associated with III/IV-C periodontitis. Six additional genes showed suggestive association with p < 10-5 , including the known risk gene SIGLEC5. HMCN2 showed the second strongest association in this study (p = 6.1 × 10-8 ).

CONCLUSIONS

This study expands the set of known genetic loci for severe periodontitis with an age of onset ≤35 years. The putative functions ascribed to the associated genes highlight the significance of oral barrier tissue stability, wound healing and tissue regeneration in the aetiology of these periodontitis forms and suggest the importance of tissue regeneration in maintaining oral health.

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.

Neisseria gonorrhoeae scavenges host sialic acid for Siglec-mediated, complement-independent suppression of neutrophil activation

Gonorrhea, caused by the bacterium Neisseria gonorrhoeae (Gc), is characterized by neutrophil influx to infection sites. Gc has developed mechanisms to resist killing by neutrophils that include modifications to its surface lipooligosaccharide (LOS). One such LOS modification is sialylation: Gc sialylates its terminal LOS sugars with cytidine-5'-monophosphate-N-acetylneuraminic acid (CMP-NANA) scavenged from the host using LOS sialyltransferase (Lst), since Gc cannot make its own sialic acid. Sialylation enables sensitive strains of Gc to resist complement-mediated killing in a serum-dependent manner. However, little is known about the contribution of sialylation to complement-independent, direct Gc-neutrophil interactions. In the absence of complement, we found sialylated Gc expressing opacity-associated (Opa) proteins decreased the oxidative burst and granule exocytosis from primary human neutrophils. In addition, sialylated Opa+ Gc survived better than vehicle treated or Δlst Gc when challenged with neutrophils. However, Gc sialylation did not significantly affect Opa-dependent association with or internalization of Gc by neutrophils. Previous studies have implicated sialic acid-binding immunoglobulin-type lectins (Siglecs) in modulating neutrophil interactions with sialylated Gc. Blocking neutrophil Siglecs with antibodies that bind to their extracellular domains eliminated the ability of sialylated Opa+ Gc to suppress oxidative burst and resist neutrophil killing. These findings highlight a new role for sialylation in Gc evasion of human innate immunity, with implications for the development of vaccines and therapeutics for gonorrhea.

Identification of disease-associate variants of aggressive periodontitis using genome-wide association studies

Aggressive periodontitis (AgP), Stage III or IV and Grade C according to the new periodontitis classification, is characterized by the rapid destruction of periodontal tissues in the systemically healthy population and often causes premature tooth loss. The presence of familial aggregation suggests the involvement of genetic factors in the pathogenesis. However, the genes associated with the onset and progression of the disease and details of its pathogenesis have not yet been fully identified. In recent years, the genome-wide approach (GWAS), a comprehensive genome analysis method using bioinformatics, has been used to search for disease-related genes, and the results have been applied in genomic medicine for various diseases, such as cancer. In this review, we discuss GWAS in the context of AgP. First, we introduce the relationship between single-nucleotide polymorphisms (SNPs) and susceptibility to diseases and how GWAS is useful for searching disease-related SNPs. Furthermore, we summarize the recent findings of disease-related genes using GWAS on AgP inside and outside Japan and a possible mechanism of the pathogenesis of AgP based on available literature and our research findings. These findings will lead to advancements in the prevention, prognosis, and treatment of AgP.

Invariant natural killer T cells drive hepatic homeostasis in nonalcoholic fatty liver disease via sustained IL-10 expression in CD170+ Kupffer cells

Kupffer cells (KCs) are liver-resident macrophages involved in hepatic inflammatory responses, including nonalcoholic fatty liver disease (NAFLD) development. However, the contribution of KC subsets to liver inflammation remains unclear. Here, using high-dimensional single-cell RNA sequencing, we characterized murine embryo-derived KCs and identified two KC populations with different gene expression profiles: KC-1 and KC-2. KC-1 expressed CD170, exhibiting immunoreactivity and immune-regulatory abilities, while KC-2 highly expressed lipid metabolism-associated genes. In a high-fat diet-induced NAFLD model, KC-1 cells differentiated into pro-inflammatory phenotypes and initiated more frequent communications with invariant natural killer T (iNKT) cells. In KC-1, interleukin (IL)-10 expression was unaffected by the high-fat diet but impaired by iNKT cell ablation and upregulated by iNKT cell adoptive transfer in vivo. Moreover, in a cellular co-culture system, primary hepatic iNKT cells promoted IL-10 expression in RAW264.7 and primary KC-1 cells. CD206 signal blocking in KC-1 or CD206 knockdown in RAW264.7 cells significantly reduced IL-10 expression. In conclusion, we identified two embryo-derived KC subpopulations with distinct transcriptional profiles. The CD206-mediated crosstalk between iNKT and KC-1 cells maintains IL-10 expression in KC-1 cells, affecting hepatic immune balance. Therefore, KC-based therapeutic strategies must consider cellular heterogeneity and the local immune microenvironment for enhanced specificity and efficiency.

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.

Non-canonical roles of Siglecs: Beyond sialic acid-binding and immune cell modulation

Siglecs (Sialic acid-binding immunoglobulin-type lectins) are I-type lectins that bind with sialic acid ligands (Sia). Most are expressed on the surface of leukocytes and are involved in immune regulation and possess immune tyrosine-based inhibitory motif (ITIM) in the intracellular domain, thus leading to inhibition of the immune response. This signaling is instrumental in maintaining quiescence under physiological conditions and acts as a brake for inflammatory cascades. By contrast, activating Siglecs carry positively charged residues in the transmembrane domain and interact with immune tyrosine-based activating motif (ITAM)-containing proteins, a DNAX-activating protein of 10-12 kDa (DAP10/12), to activate immune cells. There are various characteristics of Siglecs that do not fit within the classification of Siglec receptors as being either inhibitory or activating in nature. This review focuses on elucidating the non-canonical functions and interactions of Siglec receptors, which include Sia-independent interactions such as protein-protein interactions and interactions with lipids or other sugars. This review also summarizes Siglec expression and function on non-immune cells, and non-classical signaling of the receptor. Thus, this review will be beneficial to researchers interested in the field of Siglecs and sialic acid biology.

Hypertension and periodontitis: A joint report by the Italian society of hypertension (SIIA) and the Italian society of periodontology and implantology (SIdP)

An accumulating body of evidence supports an independent association between high blood pressure (BP) and periodontitis, possibly mediated by low-grade inflammation. This joint report by the Italian Society of Hypertension (SIIA) and the Italian Society of Periodontology and Implantology (SIdP) working group on Hypertension and Periodontitis (Hy-Per Group) provides a review of the evidence on this topic encompassing epidemiology, biological plausibility, relevance, magnitude, and treatment management. Consensus recommendations are provided for health professionals on how to manage BP in individuals showing signs of poor oral health. In summary, (1) large epidemiological studies highlight that individuals with periodontal diseases have increased risk for high/uncontrolled BP independent of confounders; (2) mechanistically, low-grade inflammation might have a causal role in the association; (3) BP profile and control might benefit from periodontal treatment in pre-hypertensive and hypertensive individuals; (4) oral health status should be evaluated as a potential risk factor for high/uncontrolled BP, and effective oral care should be included as an adjunct lifestyle measure during hypertension management. Further research is needed to optimize BP management in individuals with poor oral health.

Replication of association at the LPP and UBASH3A loci in a UK autoimmune Addison's disease cohort

Autoimmune Addison's disease (AAD) arises from a complex interplay between multiple genetic susceptibility polymorphisms and environmental factors. The first genome wide association study (GWAS) with patients from Scandinavian Addison's registries has identified association signals at four novel loci in the genes LPP, SH2B3, SIGLEC5, and UBASH3A. To verify these novel risk loci, we performed a case-control association study in our independent cohort of 420 patients with AAD from the across the UK. We report significant association of alleles of the LPP and UBASH3A genes [odds ratio (95% confidence intervals), 1.46 (1.21-1.75)and 1.40 (1.16-1.68), respectively] with AAD in our UK cohort. In addition, we report nominal association of AAD with SH2B3 [OR 1.18 (1.02-1.35)]. We confirm that variants at the LPP and UBASH3A loci confer susceptibility to AAD in a UK population. Further studies with larger patient cohorts are required to robustly confirm the association of SH2B3 and SIGLEC5/SPACA6 alleles.

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

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

Sialic acid and food allergies: The link between nutrition and immunology

Food allergies (FA), a major public health problem recognized by the World Health Organization, affect an estimated 3%-10% of adults and 8% of children worldwide. However, effective treatments for FA are still lacking. Recent advances in glycoimmunology have demonstrated the great potential of sialic acids (SAs) in the treatment of FA. SAs are a group of nine-carbon α-ketoacids usually linked to glycoproteins and glycolipids as terminal glycans. They play an essential role in modulating immune responses and may be an effective target for FA intervention. As exogenous food components, sialylated polysaccharides have anti-FA effects. In contrast, as endogenous components, SAs on immunoglobulin E and immune cell surfaces contribute to the pathogenesis of FA. Given the lack of comprehensive information on the effects of SAs on FA, we reviewed the roles of endogenous and exogenous SAs in the pathogenesis and treatment of FA. In addition, we considered the structure-function relationship of SAs to provide a theoretical basis for the development of SA-based FA treatments.

Update on Extracellular Vesicle-Based Vaccines and Therapeutics to Combat COVID-19

The COVID-19 pandemic has had a deep impact on people worldwide since late 2019 when SARS-CoV-2 was first identified in Wuhan, China. In addition to its effect on public health, it has affected humans in various aspects of life, including social, economic, cultural, and political. It is also true that researchers have made vigorous efforts to overcome COVID-19 throughout the world, but they still have a long way to go. Accordingly, innumerable therapeutics and vaccine candidates have been studied for their efficacies and have been tried clinically in a very short span of time. For example, the versatility of extracellular vesicles, which are membrane-bound particles released from all types of cells, have recently been highlighted in terms of their effectiveness, biocompatibility, and safety in the fight against COVID-19. Thus, here, we tried to explain the use of extracellular vesicles as therapeutics and for the development of vaccines against COVID-19. Along with the mechanisms and a comprehensive background of their application in trapping the coronavirus or controlling the cytokine storm, we also discuss the obstacles to the clinical use of extracellular vesicles and how these could be resolved in the future.

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.

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-F Promotes IL-33-Induced Cytokine Release from Bone Marrow-Derived Eosinophils Independently of the ITIM and ITIM-like Motif Phosphorylation

Eosinophils are potent innate effector cells associated mainly with type 2 immune responses elicited by helminths and allergens. Their activity needs to be tightly controlled to prevent severe inflammation and tissue damage. Eosinophil degranulation and secretion of inflammatory effector molecules, including cytokines, chemokines, and lipid mediators, can be regulated by activating and inhibitory receptors on the cell surface. In this study, we investigated the modulation of proliferation, apoptosis, gene expression, and cytokine/chemokine secretion from IL-33-activated Mus musculus eosinophils on cross-linking of the transmembrane receptor Sialic acid-binding Ig-like lectin F (Siglec-F). Siglec-F contains an ITIM plus an ITIM-like motif in its intracellular tail and is mainly regarded as an inhibitory and apoptosis-inducing receptor. In vitro costimulation of bone marrow-derived eosinophils with anti-Siglec-F and IL-33 compared with treatment with either alone led to enhanced STAT6 phosphorylation, stronger induction of hypoxia/glycolysis-related proinflammatory genes, and elevated secretion of type 2 cytokines (IL-4, IL-13) and chemokines (CCL3, CCL4) with only minor effects on proliferation and apoptosis. Using a competitive mixed bone marrow chimera approach with wild-type and Siglec-F-deficient eosinophils, we observed no evidence for Siglec-F-regulated inhibition of Aspergillus fumigatus-elicited lung eosinophilia. Truncation of the Siglec-F cytoplasmic tail, but not mutation of the ITIM and ITIM-like motifs, ablated the effect of enhanced cytokine/chemokine secretion. This provides evidence for an ITIM phosphorylation-independent signaling pathway from the cytoplasmic tail of the Siglec-F receptor that enhances effector molecule release from activated eosinophils.

An integrative genomics approach identifies KDM4 as a modulator of trained immunity

Innate immune cells are able to build memory characteristics via a process termed “trained immunity.” Host factors that influence the magnitude of the individual trained immunity response remain largely unknown. Using an integrative genomics approach, our study aimed to prioritize and understand the role of specific genes in trained immunity responses. In vitro‐induced trained immunity responses were assessed in two independent population‐based cohorts of healthy individuals, the 300 Bacillus Calmette‐Guérin (300BCG; n = 267) and 200 Functional Genomics (200FG; n = 110) cohorts from the Human Functional Genomics Project. Genetic loci that influence cytokine responses upon trained immunity were identified by conducting a meta‐analysis of QTLs identified in the 300BCG and 200FG cohorts. From the identified QTL loci, we functionally validated the role of PI3K‐Akt signaling pathway and two genes that belong to the family of Siglec receptors (Siglec‐5 and Siglec‐14). Furthermore, we identified the H3K9 histone demethylases of the KDM4 family as major regulators of trained immunity responses. These data pinpoint an important role of metabolic and epigenetic processes in the regulation of trained immunity responses, and these findings may open new avenues for vaccine design and therapeutic interventions.

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-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.

Hypertension and Periodontitis: A Joint Report by the Italian Society of Hypertension (SIIA) and the Italian Society of Periodontology and Implantology (SIdP)

An accumulating body of evidence supports an independent association between high blood pressure (BP) and periodontitis, possibly mediated by low-grade inflammation. This joint report by the Italian Society of Hypertension (SIIA) and the Italian Society of Periodontology and Implantology (SIdP) working group on Hypertension and Periodontitis (Hy-Per Group) provides a review of the evidence on this topic encompassing epidemiology, biological plausibility, relevance, magnitude, and treatment management. Consensus recommendations are provided for health professionals on how to manage BP in individuals showing signs of poor oral health. In summary, (1) large epidemiological studies highlight that individuals with periodontal diseases have increased risk for high/uncontrolled BP independent of confounders; (2) mechanistically, low-grade inflammation might have a causal role in the association; (3) BP profile and control might benefit from periodontal treatment in pre-hypertensive and hypertensive individuals; (4) oral health status should be evaluated as a potential risk factor for high/uncontrolled BP, and effective oral care should be included as an adjunct lifestyle measure during hypertension management. Further research is needed to optimize BP management in individuals with poor oral health.

Different glycoforms of alpha-1-acid glycoprotein contribute to its functional alterations in platelets and neutrophils

Alpha-1-acid glycoprotein (AGP-1) is a positive acute phase glycoprotein with uncertain functions. Serum AGP-1 (sAGP-1) is primarily derived from hepatocytes and circulates as 12-20 different glycoforms. We isolated a glycoform secreted from platelet-activating factor (PAF)-stimulated human neutrophils (nAGP-1). Its peptide sequence was identical to hepatocyte-derived sAGP-1, but nAGP-1 differed from sAGP-1 in its chromatographic behavior, electrophoretic mobility, and pattern of glycosylation. The function of these 2 glycoforms also differed. sAGP-1 activated neutrophil adhesion, migration, and neutrophil extracellular traps (NETosis) involving myeloperoxidase, peptidylarginine deiminase 4, and phosphorylation of ERK in a dose-dependent fashion, whereas nAGP-1 was ineffective as an agonist for these events. Furthermore, sAGP-1, but not nAGP-1, inhibited LPS-stimulated NETosis. Interestingly, nAGP-1 inhibited sAGP-1-stimulated neutrophil NETosis. The discordant effect of the differentially glycosylated AGP-1 glycoforms was also observed in platelets where neither of the AGP-1 glycoforms alone stimulated aggregation of washed human platelets, but sAGP-1, and not nAGP-1, inhibited aggregation induced by PAF or ADP, but not by thrombin. These functional effects of sAGP-1 correlated with intracellular cAMP accumulation and phosphorylation of the protein kinase A substrate vasodilator-stimulated phosphoprotein and reduction of Akt, ERK, and p38 phosphorylation. Thus, the sAGP-1 glycoform limits platelet reactivity, whereas nAGP-1 glycoform also limits proinflammatory actions of sAGP-1. These studies identify new functions for this acute phase glycoprotein and demonstrate that the glycosylation of AGP-1 controls its effects on 2 critical cells of acute inflammation.

Novel insights into the function of CD24: A driving force in cancer

CD24 is a highly glycosylated protein with a small protein core that is linked to the plasma membrane via a glycosyl-phosphatidylinositol anchor. CD24 is primarily expressed by immune cells but is often overexpressed in human tumors. In cancer, CD24 is a regulator of cell migration, invasion and proliferation. Its expression is associated with poor prognosis and it is used as cancer stemness marker. Recently, CD24 on tumor cells was identified as a phagocytic inhibitor ("do not eat me" signal) having a suppressive role in tumor immunity via binding to Siglec-10 on macrophages. This finding is reminiscent of the demonstration that soluble CD24-Fc can dampen the immune system in autoimmune disease. In the present review, we summarize recent progress on the role of the CD24-Siglec-10 binding axis at the interface between tumor cells and the immune system, and the role of CD24 genetic polymorphisms in cancer. We describe the specific function of cytoplasmic CD24 and discuss the presence of CD24 on tumor-released extracellular vesicles. Finally, we evaluate the potential of CD24-based immunotherapy.

Control of Innate Immunity by Sialic Acids in the Nervous Tissue

Sialic acids (Sias) are the most abundant terminal sugar residues of glycoproteins and glycolipids on the surface of mammalian cells. The nervous tissue is the organ with the highest expression level of Sias. The ‘sialylation’ of glycoconjugates is performed via sialyltransferases, whereas ‘desialylation’ is done by sialidases or is a possible consequence of oxidative damage. Sialic acid residues on the neural cell surfaces inhibit complement and microglial activation, as well as phagocytosis of the underlying structures, via binding to (i) complement factor H (CFH) or (ii) sialic acid-binding immunoglobulin-like lectin (SIGLEC) receptors. In contrast, activated microglial cells show sialidase activity that desialylates both microglia and neurons, and further stimulates innate immunity via microglia and complement activation. The desialylation conveys neurons to become susceptible to phagocytosis, as well as triggers a microglial phagocytosis-associated oxidative burst and inflammation. Dysfunctions of the ‘Sia–SIGLEC’ and/or ‘Sia–complement’ axes often lead to neurological diseases. Thus, Sias on glycoconjugates of the intact glycocalyx and its desialylation are major regulators of neuroinflammation.

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.

Leukocyte Immunoglobulin-Like Receptors (LILRs) on Human Neutrophils: Modulators of Infection and Immunity

Neutrophils have a crucial role in defense against microbes. Immune receptors allow neutrophils to sense their environment, with many receptors functioning to recognize signs of infection and to promote antimicrobial effector functions. However, the neutrophil response must be tightly regulated to prevent excessive inflammation and tissue damage, and regulation is achieved by expression of inhibitory receptors that can raise activation thresholds. The leukocyte immunoglobulin-like receptor (LILR) family contain activating and inhibitory members that can up- or down-regulate immune cell activity. New ligands and functions for LILR continue to emerge. Understanding the role of LILR in neutrophil biology is of general interest as they can activate and suppress antimicrobial responses of neutrophils and because several human pathogens exploit these receptors for immune evasion. This review focuses on the role of LILR in neutrophil biology. We focus on the current knowledge of LILR expression on neutrophils, the known functions of LILR on neutrophils, and how these receptors may contribute to shaping neutrophil responses during infection.

The Orphan Immune Receptor LILRB3 Modulates Fc Receptor-Mediated Functions of Neutrophils

Neutrophils are critical to the generation of effective immune responses and for killing invading microbes. Paired immune receptors provide important mechanisms to modulate neutrophil activation thresholds and effector functions. Expression of the leukocyte Ig-like receptor (LILR)A6 (ILT8/CD85b) and LILRB3 (ILT5/CD85a) paired-receptor system on human neutrophils has remained unclear because of the lack of specific molecular tools. Additionally, there is little known of their possible functions in neutrophil biology. The objective of this study was to characterize expression of LILRA6/LILRB3 receptors during human neutrophil differentiation and activation, and to assess their roles in modulating Fc receptor-mediated effector functions. LILRB3, but not LILRA6, was detected in human neutrophil lysates following immunoprecipitation by mass spectrometry. We demonstrate high LILRB3 expression on the surface of resting neutrophils and release from the surface following neutrophil activation. Surface expression was recapitulated in a human PLB-985 cell model of neutrophil-like differentiation. Continuous ligation of LILRB3 inhibited key IgA-mediated effector functions, including production of reactive oxygen species, phagocytic uptake, and microbial killing. This suggests that LILRB3 provides an important checkpoint to control human neutrophil activation and their antimicrobial effector functions during resting and early-activation stages of the neutrophil life cycle.

Nanoparticles Equipped with α2,8-Linked Sialic Acid Chains Inhibit the Release of Neutrophil Extracellular Traps

Neutrophils can combat the invasion of pathogens by the formation of neutrophil extracellular traps (NETs). The NET mechanism is not only an effective tool for combating pathogens, but is also associated with diseases. Therefore, NETs are a potential target for combating pathologies, such as cystic fibrosis and thrombosis. We investigated the potential of nanoparticles, which were modified with α2,8-linked sialic acid chains, to modulate NET release during phorbol myristate acetate stimulation. Interestingly, when these nanoparticles were applied, the formation of reactive oxygen species was partly inhibited and the release of NET was counteracted. However, although the release of NET fibers was prevented, the nuclei still lost their characteristic segmented structure and became swollen, indicating that only the release, and not complete activation was suppressed. Intriguingly, coincubation of α2,8-sialylated particles with free sialic acid chains prevented the outlined inhibitory effects. Thus, the sialic acid chains must be attached to a linker molecule to generate an active bioconjugate that is able to inhibit the release of NET.

Genetic susceptibility of common polymorphisms in NIN and SIGLEC5 to chronic periodontitis

Chronic periodontitis (CP) is a common oral disease characterized by the slow progression of alveolar attachment loss and bone destruction. Genetic components have been reported to play an important role in the onset and development of CP. In the present study, we aimed to replicate the association signals of NIN and SIGLEC5 identified in previous genome-wide association studies (GWASs) of samples from Chinese Han individuals. Association signals between clinical severity indicators of CP and relevant single nucleotide polymorphisms (SNPs) were also examined. A total of 3,160 study subjects, including 1,076 CP patients and 2,084 healthy controls, were recruited. A total of 32 SNPs, including 22 from NIN and 10 from SIGLEC5, were selected for genotyping. SNPs rs12883458 (OR = 1.45, P = 1.22 × 10-5, NIN) and rs4284742 (OR = 0.75, P = 1.69 × 10-5, SIGLEC5) were significantly associated with CP disease status. rs4284742 was significantly associated with all 3 clinical severity indicators, including bleeding on probing (BOP), probing depth (PD) and clinical attachment loss (CAL). According to evidence from bioinformatics analyses, both significant SNPs, rs12883458 and rs4284742, are likely surrogates of underlying variants with true effects. In summary, our findings provide direct evidence for the association of NIN and SIGLEC5 with CP susceptibility.

Siglecs: A journey through the evolution of sialic acid-binding immunoglobulin-type lectins

Siglecs (sialic acid-binding immunoglobulin-type lectins) are a family of immune regulatory receptors predominantly found on the cells of the hematopoietic system. A V-set Ig-like domain mediates the recognition of different sialylated glycoconjugates, which can lead to the activation or inhibition of the immune response, depending on the involved Siglecs. Siglecs are categorized into two subgroups: one including all CD33-related Siglecs and the other consisting of Siglec-1 (Sialoadhesin), Siglec-2 (CD22), Siglec-4 (myelin-associated glycoprotein, MAG) and Siglec-15. In contrast to the members of the CD33-related Siglecs, which share ∼50-99% sequence identity, Siglecs of the other subgroup show quite low homology (approximately 25-30% sequence identity). Based on the published sequences and functions of Siglecs, we performed phylogenetic analyses and sequence alignments to reveal the conservation of Siglecs throughout evolution. Therefore, we focused on the presence of Siglecs in different classes of vertebrates (fishes, amphibians, birds, reptiles and mammals), offering a bridge between the presence of different Siglecs and the biological situations of the selected animals.

Sialic acid-binding immunoglobulin-like lectin 8 (Siglec-8) is an activating receptor mediating β2-integrin-dependent function in human eosinophils

BACKGROUND

Siglec-8 is a CD33 subfamily cell-surface receptor selectively expressed on human eosinophils. After cytokine priming, Siglec-8 mAb or glycan ligand binding causes eosinophil apoptosis associated with reactive oxygen species (ROS) production. Most CD33-related Siglecs function as inhibitory receptors, but the ability of Siglec-8 to stimulate eosinophil ROS production and apoptosis suggests that Siglec-8 might instead function as an activating receptor.

OBJECTIVE

We sought to determine the role of IL-5 priming and identify the signaling molecules involved in Siglec-8 function for human eosinophils.

METHODS

We used an mAb and/or a multimeric synthetic sulfated sialoglycan ligand recognizing Siglec-8 in combination with integrin blocking antibodies, pharmacologic inhibitors, phosphoproteomics, and Western blot analysis to define the necessity of various proteins involved in Siglec-8 function for human eosinophils.

RESULTS

Cytokine priming was required to elicit the unanticipated finding that Siglec-8 engagement promotes rapid β2-integrin-dependent eosinophil adhesion. Also novel was the finding that this adhesion was necessary for subsequent ROS production and apoptosis. Siglec-8-mediated ROS was generated through reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activation because pretreatment of eosinophils with catalase (an extracellular superoxide scavenger) or NSC 23766 (a Rac GTPase inhibitor) completely inhibited Siglec-8-mediated eosinophil apoptosis. Finally, engagement of Siglec-8 on IL-5-primed eosinophils resulted in increased phosphorylation of Akt, p38, and c-Jun N-terminal kinase 1 that was also β2-integrin dependent; pharmacologic inhibition of these kinases completely prevented Siglec-8-mediated eosinophil apoptosis.

CONCLUSIONS

These data demonstrate that Siglec-8 functions uniquely as an activating receptor on IL-5-primed eosinophils through a novel pathway involving regulation of β2-integrin-dependent adhesion, NADPH oxidase, and a subset of protein kinases.

Expression of Siglec-E Alters the Proteome of Lipopolysaccharide (LPS)-Activated Macrophages but Does Not Affect LPS-Driven Cytokine Production or Toll-Like Receptor 4 Endocytosis

Siglec-E is a murine CD33-related siglec that functions as an inhibitory receptor and is expressed mainly on neutrophils and macrophage populations. Recent studies have suggested that siglec-E is an important negative regulator of lipopolysaccharide (LPS)-toll-like receptor 4 (TLR4) signaling and one report (1) claimed that siglec-E is required for TLR4 endocytosis following uptake of Escherichia coli by macrophages and dendritic cells (DCs). Our attempts to reproduce these observations using cells from wild-type (WT) and siglec-E-deficient mice were unsuccessful. We used a variety of assays to determine if siglec-E expressed by different macrophage populations can regulate TLR4 signaling in response to LPS, but found no consistent differences in cytokine secretion in vitro and in vivo, comparing three different strains of siglec-E-deficient mice with matched WT controls. No evidence was found that the siglec-E deficiency was compensated by expression of siglecs-F and -G, the other murine inhibitory CD33-related siglecs. Quantitative proteomics was used as an unbiased approach and provided additional evidence that siglec-E does not suppress inflammatory TLR4 signaling. Interestingly, proteomics revealed a siglec-E-dependent alteration in macrophage protein composition that could be relevant to functional responses in host defense. In support of this, siglec-E-deficient mice exhibited enhanced growth of Salmonella enterica serovar Typhimurium in the liver following intravenous infection, but macrophages lacking siglec-E did not show altered uptake or killing of bacteria in vitro. Using various cell types including bone marrow-derived DCs (BMDCs), splenic DCs, and macrophages from WT and siglec-E-deficient mice, we showed that siglec-E is not required for TLR4 endocytosis following E. coli uptake or LPS challenge. We failed to see expression of siglec-E by BMDC even after LPS-induced maturation, but confirmed previous studies that splenic DCs express low levels of siglec-E. Taken together, our findings do not support a major role of siglec-E in regulation of TLR4 signaling functions or TLR4 endocytosis in macrophages or DCs. Instead, they reveal that induction of siglec-E by LPS can modulate the phenotype of macrophages, the functional significance of which is currently unclear.

A genome-wide association study identifies nucleotide variants at SIGLEC5 and DEFA1A3 as risk loci for periodontitis

Periodontitis is one of the most common inflammatory diseases, with a prevalence of 11% worldwide for the severe forms and an estimated heritability of 50%. The disease is characterized by destruction of the alveolar bone due to an aberrant host inflammatory response to a dysbiotic oral microbiome. Previous genome-wide association studies (GWAS) have reported several suggestive susceptibility loci. Here, we conducted a GWAS using a German and Dutch case-control sample of aggressive periodontitis (AgP, 896 cases, 7,104 controls), a rare but highly severe and early-onset form of periodontitis, validated the associations in a German sample of severe forms of the more moderate phenotype chronic periodontitis (CP) (993 cases, 1,419 controls). Positive findings were replicated in a Turkish sample of AgP (223 cases, 564 controls). A locus at SIGLEC5 (sialic acid binding Ig-like lectin 5) and a chromosomal region downstream of the DEFA1A3 locus (defensin alpha 1-3) showed association with both disease phenotypes and were associated with periodontitis at a genome-wide significance level in the pooled samples, with P = 1.09E-08 (rs4284742,-G; OR = 1.34, 95% CI = 1.21-1.48) and P = 5.48E-10 (rs2738058,-T; OR = 1.28, 95% CI = 1.18-1.38), respectively. SIGLEC5 is expressed in various myeloid immune cells and classified as an inhibitory receptor with the potential to mediate tyrosine phosphatases SHP-1/-2 dependent signaling. Alpha defensins are antimicrobial peptides with expression in neutrophils and mucosal surfaces and a role in phagocyte-mediated host defense. This study identifies the first shared genetic risk loci of AgP and CP with genome-wide significance and highlights the role of innate and adaptive immunity in the etiology of periodontitis.

Lectin Receptors Expressed on Myeloid Cells

Lectins recognize a diverse array of carbohydrate structures and perform numerous essential biological functions. Here we focus on only two families of lectins, the Siglecs and C-type lectins. Triggering of intracellular signaling cascades following ligand recognition by these receptors can have profound effects on the induction and modulation of immunity. In this chapter, we provide a brief overview of each family and then focus on selected examples that highlight how these lectins can influence myeloid cell functioning in health and disease. Receptors that are discussed include Sn (Siglec-1), CD33 (Siglec-3), and Siglec-5, -7, -8, -9, -10, -11, -14, -15, -E, -F, and -G as well as Dectin-1, MICL, Dectin-2, Mincle/MCL, and the macrophage mannose receptor.

Glycobiology of Eosinophilic Inflammation: Contributions of Siglecs, Glycans, and Other Glycan-Binding Proteins

The historical focus on protein-protein interactions in biological systems, at the expense of attention given to interactions between other classes of molecules, has overlooked important and clinically relevant processes and points of potential clinical intervention. For example, the significance of protein-carbohydrate interactions, especially in the regulation of immune responses, has recently received greater recognition and appreciation. This review discusses several ways by which cell-surface lectin-glycan interactions can modulate eosinophil function, particularly at the levels of eosinophil recruitment and survival, and how such interactions can be exploited therapeutically. A primary focus is on discoveries concerning Siglec-8, a glycan-binding protein selectively expressed on human eosinophils, and its closest functional paralog in the mouse, Siglec-F. Recent advances in the synthesis of polymeric ligands, the identification of physiological ligands for Siglec-8 and Siglec-F in the airway, and the determination of the basis of glycan ligand discrimination of Siglec-8 are discussed. Important similarities and differences between these siglecs are outlined. Eosinophil expression of additional glycan-binding proteins or their glycan ligands, including interactions involving members of the selectin, galectin, and siglec families, is summarized. The roles of these molecules in eosinophil recruitment, survival, and inflammation are described. Finally, the modulation of these interactions and potential therapeutic exploitation of glycan-binding proteins and their ligands to ameliorate eosinophil-associated diseases are considered.

Siglec-7 Defines a Highly Functional Natural Killer Cell Subset and Inhibits Cell-Mediated Activities

Sialic acid-binding immunoglobulin-like lectin-7 (Siglec-7) is an inhibitory receptor expressed on natural killer (NK) cells. In this study, we investigated the relationship between Siglec-7 expression and NK cell functions. Siglec-7 was highly expressed on NK cells and was preferentially expressed by mature NK cells from peripheral blood of healthy adults. Siglec-7(+) NK cells displayed higher levels of activating receptors CD38, CD16, DNAM1, NKp30 and NKp46, but lower levels of inhibitory receptors such as NKG2A and CD158b, compared with Siglec-7(-) NK cells. Functional tests showed that Siglec-7(+) NK cells displayed more CD107a degranulation and IFN-γ production than Siglec-7(-) NK cells. Siglec-7 inhibited NK cell functions when interacting with specific antibodies. These data suggest that Siglec-7 defines a highly functional NK cell subset and suppresses NK cell-mediated functions when cross-linked with specific antibodies.

Soluble Siglec-5 associates to PSGL-1 and displays anti-inflammatory activity

Interactions between endothelial selectins and the leukocyte counter-receptor PSGL1 mediates leukocyte recruitment to inflammation sites. PSGL1 is highly sialylated, making it a potential ligand for Siglec-5, a leukocyte-receptor that recognizes sialic acid structures. Binding assays using soluble Siglec-5 variants (sSiglec-5/C4BP and sSiglec-5/Fc) revealed a dose- and calcium-dependent binding to PSGL1. Pre-treatment of PSGL1 with sialidase reduced Siglec-5 binding by 79 ± 4%. In confocal immune-fluorescence assays, we observed that 50% of Peripheral Blood Mononuclear Cells (PBMCs) simultaneously express PSGL1 and Siglec-5. Duolink-proximity ligation analysis demonstrated that PSGL1 and Siglec-5 are in close proximity (<40 nm) in 31 ± 4% of PBMCs. In vitro perfusion assays revealed that leukocyte-rolling over E- and P-selectin was inhibited by sSiglec-5/Fc or sSiglec-5/C4BP, while adhesion onto VCAM1 was unaffected. When applied to healthy mice (0.8 mg/kg), sSiglec-5/C4BP significantly reduced the number of rolling leukocytes under basal conditions (10.9 ± 3.7 versus 23.5 ± 9.3 leukocytes/field/min for sSiglec-5/C4BP-treated and control mice, respectively; p = 0.0093). Moreover, leukocyte recruitment was inhibited over a 5-h observation period in an in vivo model of TNFalpha-induced inflammation following injection sSiglec-5/C4BP (0.8 mg/kg). Our data identify PSGL1 as a ligand for Siglec-5, and soluble Siglec-5 variants appear efficient in blocking PSGL1-mediated leukocyte rolling and the inflammatory response in general.

Interaction of an Iridium(III) Complex with G-Quadruplex DNA and Its Application in Luminescent Switch-On Detection of Siglec-5

Sialic acid (Sia) binding immunoglobulin (Ig)-like lectin-5 (Siglec-5) is a type-I transmembrane protein, and it has been demonstrated as a biomarker of granulocytic maturation and acute myeloid leukemia phenotype. Herein we aimed to construct a method that could sensitively detect Siglec-5 by taking advantage of the high affinity and selectivity of the K19 aptamer for its cognate target, and the selective interaction of luminescent iridium(III) transition metal complexes with G-quadruplex DNA. The iridium(III) complex 1 [Ir(tpyd)₂(2,9-dmphen)]PF₆ (where tpyd =2-(m-tolyl)pyridine; 2,9-dmphen =2,9-dimethyl-1,10-phenanthroline) was synthesized, and it displayed high luminescence for G-quadruplex DNA compared to dsDNA and ssDNA. Additionally, complex 1 exhibited a blue shift luminescence response to c-kit2 G-quadruplex, and the interaction between 1 and G-quadruplexes was discussed based on the results of G-tetrad assay, loop effect assay, and other assays. Then complex 1 was utilized to develop a G-quadruplex-based sensing platform for Siglec-5 in aqueous solution. Upon the addition of Siglec-5, the specific binding of the K19 aptamer sequence results in a conformational change that generates a split G-quadruplex structure, which is then recognized by the G-quadruplex-specific iridium(III) complex with an enhanced luminescent response. Futhermore, the use of the assay for detecting Siglec-5 in cellular debris was demonstrated.

A novel siglec (CgSiglec-1) from the Pacific oyster (Crassostrea gigas) with broad recognition spectrum and inhibitory activity to apoptosis, phagocytosis and cytokine release

Sialic acid binding immunoglobulin-type lectin (siglec) belongs to the immunoglobulin superfamily (IgSF), which acts as regulator involved in glycan recognition and signal transduction in the immune and nervous systems. In the present study, a siglec gene (designated CgSiglec-1) was characterized from the Pacific oyster, Crassostrea gigas. The cDNA of CgSiglec-1 was of 1251 bp encoding a predicted polypeptide of 416 amino acids. CgSiglec-1 was composed of two I-set immunoglobulin (Ig) domains, one transmembrane (TM) domain and two ITIM motifs, sharing a sequence similarity with vertebrate CD22 homologs. The mRNA expression of CgSiglec-1 could be detected in all the selected tissues, with the highest level in hemocytes and labial palps. The confocal analysis revealed that CgSiglec-1 mainly distributed on the cytoplasmic membrane of the oyster hemocytes. In addition, the mRNA transcripts of CgSiglec-1 in hemocytes increased significantly (4.29-fold to that of control group, p < 0.05) after Vibrio splendidus stimulation. The recombinant CgSiglec-1 protein (rCgSiglec-1) could bind to poly sialic acid (pSIAS), lipopolysaccharides (LPS) and peptidoglycan (PGN) in a dose-dependent manner. The blockade of CgSiglec-1 by specific polyclonal antibodies could enhance the LPS-induced cell apoptosis, phagocytosis towards V. splendidus and the release of cytokines, such as CgTNF-1, CgIFNLP and CgIL-17. The results collectively indicated that CgSiglec-1 could act as a bridge molecule between invader recognition and signal transduction cascade, and modulate the immune response by inhibiting various important processes of immunity in oyster.

Distinct Oral Neutrophil Subsets Define Health and Periodontal Disease States

Neutrophils exit the vasculature and swarm to sites of inflammation and infection. However, these cells are abundant in the healthy, inflammation-free human oral environment, suggesting a unique immune surveillance role within the periodontium. We hypothesize that neutrophils in the healthy oral cavity occur in an intermediary parainflammatory state that allows them to interact with and contain the oral microflora without eliciting a marked inflammatory response. Based on a high-throughput screen of neutrophil CD (cluster of differentiation) marker expression and a thorough literature review, we developed multicolor flow cytometry panels to determine the surface marker signatures of oral neutrophil subsets in periodontal health and disease. We define here 3 distinct neutrophil subsets: resting/naive circulatory neutrophils, parainflammatory neutrophils found in the healthy oral cavity, and proinflammatory neutrophils found in the oral cavity during chronic periodontal disease. Furthermore, parainflammatory neutrophils manifest as 2 distinct subpopulations-based on size, granularity, and expression of specific CD markers-and exhibit intermediate levels of activation as compared with the proinflammatory oral neutrophils. These intermediately activated parainflammatory populations occur in equal proportions in the healthy oral cavity, with a shift to one highly activated proinflammatory neutrophil population in chronic periodontal disease. This work is the first to identify and characterize oral parainflammatory neutrophils that interact with commensal biofilms without inducing an inflammatory response, thereby demonstrating that not all neutrophils trafficking through periodontal tissues are fully activated. In addition to establishing possible diagnostic and treatment monitoring biomarkers, this oral neutrophil phenotype model builds on existing literature suggesting that the healthy periodontium may be in a parainflammatory state.

Correlation between myeloid-derived suppressor cells and S100A8/A9 in tumor and autoimmune diseases

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of cells that constitute an important component of immune regulatory system. Two calcium-binding proteins S100A8 and S100A9 act as important mediators in acute and chronic inflammation. In recent years, many researchers have found that MDSCs and S100A8/A9 operated with one another through a positive feedback loop to promote tumor development and metastasis. However, the correlation between MDSCs and S100A8/A9 in autoimmune diseases (AIDs) remains unknown. In this review, we discussed the co-operation of MDSCs and S100A8/A9 in tumor environment, and also, the role of these two components in AIDs.

The T-Cell Inhibitory Molecule Butyrophilin-Like 2 Is Up-regulated in Mild Plasmodium falciparum Infection and Is Protective During Experimental Cerebral Malaria

Plasmodium falciparum infection can result in severe disease that is associated with elevated inflammation and vital organ dysfunction; however, malaria-endemic residents gain protection from lethal outcomes and manifest only mild symptoms during infection. To characterize host responses associated with this more effective antimalarial response, we characterized whole-blood transcriptional profiles in Rwandan adults during a mild malaria episode and compared them with findings from a convalescence sample. We observed transcriptional up-regulation in many pathways, including type I interferon, interferon γ, complement activation, and nitric oxide during malaria infection, which provide benchmarks of mild disease physiology. Transcripts encoding negative regulators of T-cell activation, such as programmed death ligand 1 (PD-L1), programmed death 1 ligand 2 (PD-L2), and the butyrophilin family member butyrophilin-like 2 (BTNL2) were also increased. To support an important functional role for BTNL2 during malaria infection, we studied chimeric mice reconstituted with BTNL2(-/-) or wild-type hematopoietic cells that were inoculated with Plasmodium berghei ANKA, a murine model of cerebral malaria. We found that BTNL2(-/-) chimeric mice had a significant decrease in survival compared with wild-type counterparts. Collectively these data characterize the immune responses associated with mild malaria and uncover a novel role for BTNL2 in the host response to malaria.

Regulation of natural cytotoxicity receptors by heparan sulfate proteoglycans in -cis: A lesson from NKp44

NKp44 (NCR2) is a distinct member of natural cytotoxicity receptors (NCRs) family that can induce cytokine production and cytolytic activity in human NK cells. Heparan sulfate proteoglycans (HSPGs) are differentially expressed in various normal and cancerous tissues. HSPGs were reported to serve as ligands/co-ligands for NKp44 and other NCRs. However, HSPG expression is not restricted to either group and can be found also in NK cells. Our current study reveals that NKp44 function can be modulated through interactions with HSPGs on NK cells themselves in -cis rather than on target cells in -trans. The intimate interaction of NKp44 and the NK cell-associated HSPG syndecan-4 (SDC4) in -cis can directly regulate membrane distribution of NKp44 and constitutively dampens the triggering of the receptor. We further demonstrate, that the disruption of NKp44 and SDC4 interaction releases the receptor to engage with its ligands in -trans and therefore enhances NKp44 activation potential and NK cell functional response.

Genetic and mutational characterization of the small alarmone synthetase gene relV of Vibrio cholerae

In Vibrio cholerae, the causative agent of cholera, products of three genes, relA, spoT and relV, govern nutritional stress related stringent response (SR). SR in bacteria is critically regulated by two intracellular small molecules, guanosine 3'-diphosphate 5'-triphosphate (pppGpp) and guanosine 3',5'-bis(diphosphate) (ppGpp), collectively called (p)ppGpp or alarmone. Evolution of relV is unique in V. cholerae because other Gram-negative bacteria carry only relA and spoT genes. Recent reports suggest that RelV is needed for pathogenesis. RelV carries a single (p)ppGpp synthetase or RelA-SpoT domain (SYNTH/RSD) and belongs to the small alarmone synthetase (SAS) family of proteins. Here, we report extensive functional characterizations of the relV gene by constructing several deletion and site-directed mutants followed by their controlled expression in (p)ppGpp(0) cells of Escherichia coli or V. cholerae. Substitution analysis indicated that the amino acid residues K107, D129, R132, L150 and E188 of the RSD region of RelV are essential for its activity. While K107, D129 and E188 are highly conserved in RelA and SAS proteins, L150 appears to be conserved in the latter group of enzymes, and the R132 residue was found to be unique in RelV. Extensive progressive deletion analysis indicated that the amino acid residues at positions 59 and 248 of the RelV protein are the functional N- and C-terminal boundaries, respectively. Since the minimal functional length of RelV was found to be 189 aa, which includes the 94 aa long RSD region, it seems that the flanking residues of the RSD are also important for maintaining the (p)ppGpp synthetase activity.

Molecular characterization and expression of porcine Siglec-5

In this study we describe the characterization of the porcine orthologue of Siglec-5. A cDNa clone was obtained from a porcine cDNa library derived from swine small intestine which encodes a 555 a-a type 1 transmembrane protein with sequence homology to human Siglec-5. This protein consists of four Ig-like domains, a transmembrane region, and a cytoplasmic tail with two tyrosine-based signalling motifs. When expressed as a recombinant protein fused to the Fc region of human IgG1, porcine Siglec-5 was able to bind porcine red blood cells in a sialic acid-dependent manner. Monoclonal antibodies (mAb) were developed against porcine Siglec-5 and used to analyse its expression in bone marrow and blood cells, and lymphoid tissues. Porcine Siglec-5 expression was mainly restricted to myelomonocytic cells and their precursors, being detected also, although at low levels, on plasmacytoid dendritic cells and B lymphocytes. In lymphoid tissues, ellipsoids of the spleen and subcapsular and medullar sinuses of lymph nodes were positive for Siglec-5. These mAbs were able to precipitate, from granulocyte lysates, a protein of approximately 85 kDa under non-reducing conditions, indicating that porcine Siglec-5 is expressed as a monomer in the plasma membrane.

Enhanced lentiviral vector production in 293FT cells expressing Siglec-9

Siglecs, sialic acid-recognizing Ig-superfamily lectins, regulate various aspects of immune responses, and have also been shown to induce the endocytosis of binding materials such as anti-Siglec antibodies or sialic acid-harboring bacteria. In this study, we demonstrated that the expression of Siglec-9 enhanced the transfection efficiency of several cell lines such as macrophage RAW264 and non-hematopoietic 293FT cells. We applied this finding to the production of a lentiviral vector in which cells were transfected simultaneously with multiple vectors, and achieved a twice increase in viral production levels. Furthermore, 293FT cells expressing lectin-defective Siglec-9 produced three- to seven-fold higher titer of viral vector compared with parental 293FT cells. These results suggest that Siglec-9 enhanced lentiviral vector production in a lectin-independent manner.

Siglec-E is a negative regulator of acute pulmonary neutrophil inflammation and suppresses CD11b β2-integrin-dependent signaling

Neutrophil entry into the lung tissues is a key step in host defense to bacterial and yeast infections, but if uncontrolled can lead to severe tissue damage. Here, we demonstrate for the first time that sialic acid binding Ig-like lectin E (siglec-E) functions to selectively regulate early neutrophil recruitment into the lung. In a model of acute lung inflammation induced by aerosolized lipopolysaccharide, siglec-E-deficient mice exhibited exaggerated neutrophil recruitment that was reversed by blockade of the β2 integrin, CD11b. Siglec-E suppressed CD11b "outside-in" signaling, because siglec-E-deficient neutrophils plated on the CD11b ligand fibrinogen showed exaggerated phosphorylation of Syk and p38 mitogen-activated protein kinase. Sialidase treatment of fibrinogen reversed the suppressive effect of siglec-E on CD11b signaling, suggesting that sialic acid recognition by siglec-E is required for its inhibitory function. Siglec-E in neutrophils was constitutively associated with the tyrosine phosphatase SHP-1 and may therefore function to constitutively dampen inflammatory responses of neutrophils. These data reveal that siglec-E is an important negative regulator of neutrophil recruitment to the lung and β2 integrin-dependent signaling. Our findings have implications for the human functional ortholog, siglec-9, and its potential role in regulating inflammatory lung disease.

Cell-intrinsic mechanism involving Siglec-5 associated with divergent outcomes of HIV-1 infection in human and chimpanzee CD4 T cells

Human and chimpanzee CD4+ T cells differ markedly in expression of the inhibitory receptor Siglec-5, which contributes towards differential responses to activating stimuli. While CD4+ T cells from both species are equally susceptible to HIV-1 infection, chimpanzee cells survive better, suggesting a cell-intrinsic difference. We hypothesized that Siglec-5 expression protects T cells from activation-induced and HIV-1-induced cell death. Transduction of human CEM T cells with Siglec-5 decreased cell responses to stimulation. Following HIV-1 infection, a higher percentage of Siglec-5-positive cells survived, suggesting relative resistance to virus-induced cell death. Consistent with this, we observed an increase in percentage of Siglec-5-positive cells surviving in mixed infected cultures. Siglec-5-transduced cells also showed decreased expression of apoptosis-related proteins following infection and reduced susceptibility to Fas-mediated cell death. Similar Siglec-5-dependent differences were seen when comparing infection outcomes in primary CD4+ T cells from humans and chimpanzees. A protective effect of Siglec-5 was further supported by observing greater proportions of circulating CD4+ T cells expressing Siglec-5 in acutely infected HIV-1 patients, compared to controls. Taken together, our results suggest that Siglec-5 expression protects T cells from HIV-1- and apoptosis-induced cell death and contributes to the different outcomes of HIV-1 infection in humans and chimpanzees.