Characterization of Siglec-5, a Novel Glycoprotein Expressed on Myeloid Cells Related to CD33

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.

The prognostic impact of SIGLEC5-induced impairment of CD8+ T cell activation in sepsis

BACKGROUND

Sepsis is associated with T-cell exhaustion, which significantly reduces patient outcomes. Therefore, targeting of immune checkpoints (ICs) is deemed necessary for effective sepsis management. Here, we evaluated the role of SIGLEC5 as an IC ligand and explored its potential as a biomarker for sepsis.

METHODS

In vitro and in vivo assays were conducted to both analyse SIGLEC5's role as an IC ligand, as well as assess its impact on survival in sepsis. A multicentre prospective cohort study was conducted to evaluate the plasmatic soluble SIGLEC5 (sSIGLEC5) as a mortality predictor in the first 60 days after admission in sepsis patients. Recruitment included sepsis patients (n = 346), controls with systemic inflammatory response syndrome (n = 80), aneurism (n = 11), stroke (n = 16), and healthy volunteers (HVs, n = 100).

FINDINGS

SIGLEC5 expression on monocytes was increased by HIF1α and was higher in septic patients than in healthy volunteers after ex vivo LPS challenge. Furthermore, SIGLEC5-PSGL1 interaction inhibited CD8+ T-cell proliferation. Administration of sSIGLEC5r (0.8 mg/kg) had adverse effects in mouse endotoxemia models. Additionally, plasma sSIGLEC5 levels of septic patients were higher than HVs and ROC analysis revealed it as a mortality marker with an AUC of 0.713 (95% CI, 0.656-0.769; p < 0.0001). Kaplan-Meier survival curve showed a significant decrease in survival above the calculated cut-off (HR of 3.418, 95% CI, 2.380-4.907, p < 0.0001 by log-rank test) estimated by Youden Index (523.6 ng/mL).

INTERPRETATION

SIGLEC5 displays the hallmarks of an IC ligand, and plasma levels of sSIGLEC5 have been linked with increased mortality in septic patients.

FUNDING

Instituto de Salud Carlos III (ISCIII) and "Fondos FEDER" to ELC (PIE15/00065, PI18/00148, PI14/01234, PI21/00869), CDF (PI21/01178), RLR (FI19/00334) and JAO (CD21/00059).

Immunolocalization and expression of Siglec5 protein in the male reproductive tract of dromedary camel during rutting season

Lectins are carbohydrate-binding proteins that are highly selective for sugar groups on other molecules. Siglec5 is a cell-surface lectin that belongs to the sialic acid-binding Ig-like lectins (Siglecs) and acts as a suppressor of immune responses. In this study, immunohistochemistry, western blotting and quantitative real-time polymerase chain reaction (qRT-PCR) were used to detect the expression of Siglec5 in the male reproductive tract of dromedary camels during the rutting season. Siglec5 displayed strong immunostaining in the cranial and caudal testicular regions and moderate immunostaining in the rete testis. Different parts of the epididymis showed varying immunoreactions to Siglec5. The spermatozoa in the testes and epididymis also showed positive immunostaining for Siglec5, whereas, the vas deferens showed negative immunostaining for the protein. The results obtained by western blotting confirmed the immunohistochemical detection of the protein in the testicular and epididymal tissues. The results of qRT-PCR showed that Siglec mRNA was expressed differently in each part of the testis and epididymis; the highest levels of expression were observed in the caudal part of the testis and in the head of the epididymis. In conclusion, the present study revealed that Siglec5 is mainly located in the testis and epididymis, where sperm production and maturation occur. Therefore, this protein may play an essential role in the development, maturation and protection of camel sperm.

PSGL-1: a novel immune checkpoint driving T-cell dysfunction in obstructive sleep apnea

Introduction

Although higher incidence of cancer represents a major burden for obstructive sleep apnea (OSA) patients, the molecular pathways driving this association are not completely understood. Recently, the adhesion receptor P-selectin glycoprotein-1 (PSGL 1) has been identified as a novel immune checkpoint, which are recognized major hallmarks in several types of cancer and have revolutionized cancer therapy.

Methods

The expression of PSGL-1 and its ligands VISTA and SIGLEC-5 was assessed in the leucocytes of OSA patients and control subjects exploring the role of intermittent hypoxia (IH) using in vitro models. In addition, PSGL-1 impact on T-cells function was evaluated by ex vivo models.

Results

Data showed PSGL-1 expression is upregulated in the T-lymphocytes from patients with severe OSA, indicating a relevant role of hypoxemia mediated by intermittent hypoxia. Besides, results suggest an inhibitory role of PSGL-1 on T-cell proliferation capacity. Finally, the expression of SIGLEC-5 but not VISTA was increased in monocytes from OSA patients, suggesting a regulatory role of intermittent hypoxia.

Discussion

In conclusion, PSGL-1 might constitute an additional immune checkpoint leading to T-cell dysfunction in OSA patients, contributing to the disruption of immune surveillance, which might provide biological plausibility to the higher incidence and aggressiveness of several tumors in these patients.

Role of Siglecs in viral infections: A double-edged sword interaction

Sialic-acid-binding immunoglobulin-like lectins are cell surface immune receptors known as Siglecs that play a paramount role as modulators of immunity. In recent years, research has underscored how the underlaying biology of this family of receptors influences the outcome of viral infections. While Siglecs are needed to promote effective antiviral immune responses, they can also pave the way to viral dissemination within tissues. Here, we review how recent preclinical findings focusing on the interplay between Siglecs and viruses may translate into promising broad-spectrum therapeutic interventions or key biomarkers to monitor the course of viral infections.

Discovery, classification, evolution and diversity of Siglecs

Immunoglobulin (Ig) superfamily proteins play diverse roles in vertebrates, including regulation of cellular responses by sensing endogenous or exogenous ligands. Siglecs are a family of glycan-recognizing proteins belonging to the Ig superfamily (i.e., I-type lectins). Siglecs are expressed on various leukocyte types and are involved in diverse aspects of immunity, including the regulation of inflammatory responses, leukocyte proliferation, host-microbe interaction, and cancer immunity. Sialoadhesin/Siglec-1, CD22/Siglec-2, and myelin-associated glycoprotein/Siglec-4 were among the first to be characterized as members of the Siglec family, and along with Siglec-15, they are relatively well-conserved among tetrapods. Conversely, CD33/Siglec-3-related Siglecs (CD33rSiglecs, so named as they show high sequence similarity with CD33/Siglec-3) are encoded in a gene cluster with many interspecies variations and even intraspecies variations within some lineages such as humans. The rapid evolution of CD33rSiglecs expressed on leukocytes involved in innate immunity likely reflects the selective pressure by pathogens that interact and possibly exploit these Siglecs. Human Siglecs have several additional unique and/or polymorphic properties as compared with closely related great apes, changes possibly related to the loss of the sialic acid Neu5Gc, another distinctly human event in sialobiology. Multiple changes in human CD33rSiglecs compared to great apes include many examples of human-specific expression in non-immune cells, coinciding with human-specific diseases involving such cell types. Some Siglec gene polymorphisms have dual consequences-beneficial in a situation but detrimental in another. The association of human Siglec gene polymorphisms with several infectious and non-infectious diseases likely reflects the ongoing competition between the host and microbial pathogens.

Development of Effective Siglec-9 Antibodies Against Cancer

PURPOSE OF REVIEW

This study aims to review state-of-the-art advances in Siglec-9-directed antibodies and to highlight specific aspects of Siglec-9 antibodies that are suitable to mount anti-tumor immunity.

RECENT FINDINGS

Controversies surrounding studies on Siglec-9 antibodies can confound future studies. In this review, we have highlighted some controversies, explained the distinction between Siglec-9 agonistic and antagonistic (endocytic) antibodies, and discussed their suitability in sustaining anti-tumor immunity. Siglec-9 is an immune checkpoint target and an immunoinhibitory receptor that can engage either sialic acid ligands or agonistic antibodies. Through Siglec-9 sialic acid interactions, activated immunoreceptor tyrosine-based inhibitory signaling of the immune cells can lead to unfavorable immunosuppression. To overcome tumor-related immunosuppression, different types of Siglec-9 antibody blockade need to be developed. However, whether a Siglec-9-directed antibody is agonistic or antagonistic is probably affinity-dependent and not epitope-dependent. Additionally, unlike immune-modulatory antibodies such as agonistic antibodies (OX40, CD28, ICOS, and 4-1BB) or Fc-inert antibodies (PD1 and PD-L1) directed against cancer cells, the nature of antagonistic Siglec-9 antibodies is more suitable to enhance anti-tumor immunity and will be discussed.

Colorectal Cancer Stem Cells Fuse with Monocytes to Form Tumour Hybrid Cells with the Ability to Migrate and Evade the Immune System

Simple Summary

Colorectal cancer survival rates strongly decrease from initial to more advanced stages, primarily because of the occurrence of metastatic lesions. In this line, the search for clinical markers is of critical need. The aim of our study was to examine in vitro generated colorectal tumour hybrid cells (THCs) as a fusion between colorectal cancer (CRC) stem cells and human monocytes, as well as to evaluate their presence in tissue and blood samples from CRC patients. THCs, defined as CD45⁺CD14⁺EpCAM⁺, showed enhanced migratory, proliferative and immune evasion abilities compared to their parental cells. In a retrospective cohort of 23 patients, our data showed the potential relevance of resident tissue THCs in the generation of distant metastases. In addition, in a prospective cohort of 38 patients, our data confirmed the correlation between circulating THCs and sSIGLEC5 levels, a molecule which has already been previously described as a marker of poor prognosis in CRC patients. Altogether, our findings indicate that the number of THCs could serve as a novel biomarker for metastasis prediction in colorectal cancer patients.

Abstract

Background: The cancer cell fusion theory could be one of the best explanations for the metastasis from primary tumours. Methods: Herein, we co-cultured colorectal cancer (CRC) stem cells with human monocytes and analysed the properties of the generated tumour hybrid cells (THCs). The presence of THCs in the bloodstream together with samples from primary and metastatic lesions and their clinical correlations were evaluated in CRC patients and were detected by both FACS and immunofluorescence methods. Additionally, the role of SIGLEC5 as an immune evasion molecule in colorectal cancer was evaluated. Results: Our data demonstrated the generation of THCs after the in vitro co-culture of CRC stem cells and monocytes. These cells, defined as CD45⁺CD14⁺EpCAM⁺, showed enhanced migratory and proliferative abilities. The THC-specific cell surface signature allows identification in matched primary tumour tissues and metastases as well as in the bloodstream from patients with CRC, thus functioning as a biomarker. Moreover, SIG-LEC5 expression on in vitro generated THCs has shown to be involved in the mechanism for immune evasion. Additionally, sSIGLEC5 levels correlated with THC numbers in the prospective cohort of patients. Conclusions: Our results indicate the generation of a hybrid entity after the in vitro co-culture between CRC stem cells and human monocytes. Moreover, THC numbers present in patients are related to both prognosis and the later spread of metastases in CRC patients.

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.

OUP accepted manuscript

Type 2 diabetes is a complex, systemic disease affected by both genetic and environmental factors. Previous research has identified genetic variants associated with type 2 diabetes risk; however, gene regulatory changes underlying progression to metabolic dysfunction are still largely unknown. We investigated RNA expression changes that occur during diabetes progression using a two-stage approach. In our discovery stage, we compared changes in gene expression using two longitudinally collected blood samples from subjects whose fasting blood glucose transitioned to a level consistent with type 2 diabetes diagnosis between the time points against those who did not with a novel analytical network approach. Our network methodology identified 17 networks, one of which was significantly associated with transition status. This 822-gene network harbors many genes novel to the type 2 diabetes literature but is also significantly enriched for genes previously associated with type 2 diabetes. In the validation stage, we queried associations of genetically determined expression with diabetes-related traits in a large biobank with linked electronic health records. We observed a significant enrichment of genes in our identified network whose genetically determined expression is associated with type 2 diabetes and other metabolic traits and validated 31 genes that are not near previously reported type 2 diabetes loci. Finally, we provide additional functional support, which suggests that the genes in this network are regulated by enhancers that operate in human pancreatic islet cells. We present an innovative and systematic approach that identified and validated key gene expression changes associated with type 2 diabetes transition status and demonstrated their translational relevance in a large clinical resource.

CRISPR/Cas-powered nanobiosensors for diagnostics

CRISPR diagnostics (CRISPR-Dx) offer a wide range of enhancements compared to traditional nanobiosensors by taking advantage of the excellent trans-cleavage activity of the CRISPR/Cas systems. However, the single-stranded DNA/RNA reporters of the current CRISPR-Dx suffer from poor stability and limited sensitivity, which make their application in complex biological environments difficult. In comparison, nanomaterials, especially metal nanoparticles, exhibits robust stability and desirable optical and electrocatalytical properties, which make them ideal as reporter molecules. Therefore, biosensing research is moving towards the use of the trans-cleavage activity of CRISPR/Cas effectors on metal nanoparticles and apply the new phenomenon to develop novel nanobiosensors to target various targets such as viral infections, genetic mutations and tumor biomarkers, by using different sensing methods, including, but not limited to fluorescence, luminescence resonance, colorimetric and electrochemical signal readout. In this review, we explore some of the most recent advances in the field of CRISPR-powered nanotechnological biosensors. Demonstrating high accuracy, sensitivity, selectivity and versatility, nanobiosensors along with CRISPR/Cas technology offer tremendous potential for next-generation diagnostics of multiple targets, especially at the point of care and without any target amplification.

Siglec Ligands

A dense and diverse array of glycans on glycoproteins and glycolipids decorate all cell surfaces. In vertebrates, many of these carry sialic acid, in a variety of linkages and glycan contexts, as their outermost sugar moiety. Among their functions, glycans engage complementary glycan binding proteins (lectins) to regulate cell physiology. Among the glycan binding proteins are the Siglecs, sialic acid binding immunoglobulin-like lectins. In humans, there are 14 Siglecs, most of which are expressed on overlapping subsets of immune system cells. Each Siglec engages distinct, endogenous sialylated glycans that initiate signaling programs and regulate cellular responses. Here, we explore the emerging science of Siglec ligands, including endogenous sialoglycoproteins and glycolipids and synthetic sialomimetics. Knowledge in this field promises to reveal new molecular pathways controlling cell physiology and new opportunities for therapeutic intervention.

Enhanced expression of immune checkpoint receptors during SARS-CoV-2 viral infection

The immune system is tightly regulated by the activity of stimulatory and inhibitory immune receptors. This immune homeostasis is usually disturbed during chronic viral infection. Using publicly available transcriptomic datasets, we conducted in silico analyses to evaluate the expression pattern of 38 selected immune inhibitory receptors (IRs) associated with different myeloid and lymphoid immune cells during coronavirus disease 2019 (COVID-19) infection. Our analyses revealed a pattern of overall upregulation of IR mRNA during severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. A large number of IRs expressed on both lymphoid and myeloid cells were upregulated in nasopharyngeal swabs (NPSs), while lymphoid-associated IRs were specifically upregulated in autopsies, reflecting severe, terminal stage COVID-19 disease. Eight genes (BTLA, LAG3, FCGR2B, PDCD1, CEACAM1, CTLA4, CD72, and SIGLEC7), shared by NPSs and autopsies, were more expressed in autopsies and were directly correlated with viral levels. Single-cell data from blood and bronchoalveolar samples also reflected the observed association between IR upregulation and disease severity. Moreover, compared to SARS-CoV-1, influenza, and respiratory syncytial virus infections, the number and intensities of upregulated IRs were higher in SARS-CoV-2 infections. In conclusion, the immunopathology and severity of COVID-19 could be attributed to dysregulation of different immune inhibitors. Targeting one or more of these immune inhibitors could represent an effective therapeutic approach for the treatment of COVID-19 early and late immune dysregulations.

Identification and functional characterization of a Siglec-7 counter-receptor on K562 cells

Sialic acid (Sia)-binding immunoglobulin-like lectin 7 (Siglec-7) is an inhibitory receptor primarily expressed on natural killer (NK) cells and monocytes. Siglec-7 is known to negatively regulate the innate immune system through Sia binding to distinguish self and nonself; however, a counter-receptor bearing its natural ligand remains largely unclear. Here, we identified a counter-receptor of Siglec-7 using K562 hematopoietic carcinoma cells presenting cell surface ligands for Siglec-7. We affinity-purified the ligands using Fc-ligated recombinant Siglec-7 and diSia-dextran polymer, a strong inhibitor for Siglec-7. We then confirmed the counter-receptor for Siglec-7 as leukosialin (CD43) through mass spectrometry, immunoprecipitation, and proximity labeling. Additionally, we demonstrated that the cytotoxicity of NK cells toward K562 cells was suppressed by overexpression of leukosialin in a Siglec-7-dependent manner. Taken together, our data suggest that leukosialin on K562 is a counter-receptor for Siglec-7 on NK cells and that a cluster of the Sia-containing glycan epitope on leukosialin is key as trans-ligand for unmasking the cis-ligand.

PSGL-1 Immune Checkpoint Inhibition for CD4+ T Cell Cancer Immunotherapy

Immune checkpoint inhibition targeting T cells has shown tremendous promise in the treatment of many cancer types and are now standard therapies for patients. While standard therapies have focused on PD-1 and CTLA-4 blockade, additional immune checkpoints have shown promise in promoting anti-tumor immunity. PSGL-1, primarily known for its role in cellular migration, has also been shown to function as a negative regulator of CD4+ T cells in numerous disease settings including cancer. PSGL-1 is highly expressed on T cells and can engage numerous ligands that impact signaling pathways, which may modulate CD4+ T cell differentiation and function. PSGL-1 engagement in the tumor microenvironment may promote CD4+ T cell exhaustion pathways that favor tumor growth. Here we highlight that blocking the PSGL-1 pathway on CD4+ T cells may represent a new cancer therapy approach to eradicate tumors.

Human Lectins, Their Carbohydrate Affinities and Where to Find Them

Lectins are a class of proteins responsible for several biological roles such as cell-cell interactions, signaling pathways, and several innate immune responses against pathogens. Since lectins are able to bind to carbohydrates, they can be a viable target for targeted drug delivery systems. In fact, several lectins were approved by Food and Drug Administration for that purpose. Information about specific carbohydrate recognition by lectin receptors was gathered herein, plus the specific organs where those lectins can be found within the human body.

Structural advances of Siglecs: insight into synthetic glycan ligands for immunomodulation

Sialic acid-binding immunoglobulin-like lectins (Siglecs) are transmembrane proteins of the immunoglobulin (Ig) superfamily predominantly expressed on the cells of our immune system. Siglecs recognize sialic acid via their terminal V-set domain. In mammals, sialic acid-terminated glycolipids and glycoproteins are the ligands of Siglecs, and the monomeric affinity of Siglecs for their sialic acid-containing ligands is weak. Significant efforts have been devoted toward the development of chemically modified sialoside ligands to target Siglecs with higher affinity and selectivity. In this review we discuss natural and synthetic sialoside ligands for each human Siglec, emphasizing the ligand binding determinants uncovered from recent advances in protein structural information. Potential therapeutic applications of these ligands are also discussed.

Siglecs that Associate with DAP12

Siglecs are a family of transmembrane receptor-like glycan-recognition proteins expressed primarily on leukocytes. Majority of Siglecs have an intracellular sequence motif called immunoreceptor tyrosine-based inhibitory motif (ITIM) and associate with Src homology region 2 domain-containing tyrosine phosphatase-1 (SHP-1), and negatively regulate tyrosine phosphorylation-mediated intracellular signaling events. On the other hand, some Siglecs have a positively charged amino acid residue in the transmembrane domain and associate with DNAX activation protein of 12 kDa (DAP12), which in turn recruits spleen tyrosine kinase (Syk). These DAP12-associated Siglecs play diverse functions. For example, Siglec-15 is conserved throughout vertebrate evolution and plays a role in bone homeostasis by regulating osteoclast development and function. Human Siglec-14 and -16 have inhibitory counterparts (Siglec-5 and -11, respectively), which show extremely high sequence similarity with them at the extracellular domain but interact with SHP-1. The DAP12-associated Siglec in such "paired receptor" configuration counteracts the pathogens that exploit the inhibitory counterpart. Polymorphisms (mutations) that render DAP12-associated inactive Siglecs are found in humans, and some of these appear to be associated with sensitivity or resistance of human hosts to bacterially induced conditions. Studies of mouse Siglec-H have revealed complex and intriguing functions it plays in regulating adaptive immunity. Many questions remain unanswered, and further molecular and genetic studies of DAP12-associated Siglecs will yield valuable insights with translational relevance.

Fluorescence Signal Amplification by Using β-Galactosidase for Flow Cytometry; Advantages of an Endogenous Activity-Free Enzyme

We previously proposed using a hydrolysis enzyme for fluorescent signal amplification in flow cytometric detection of antigen proteins, which was named the catalyzed reporter penetration (CARP) method. In this method, antigen proteins are labeled with enzyme-modified antibodies, and then fluorophore-modified substrates stain cells by penetrating the cell membrane upon hydrolysis of the substrate. We proved the concept by using alkaline phosphatase (AP) as the hydrolysis enzyme. However, a required prior inactivation process of endogenous AP activity on the cell surface risked disrupting recognition of antigen proteins by antibodies. In this report, the CARP method was extended to β-galactosidase (β-gal) as an amplification enzyme, which circumvented the requirement of an initial inactivation process because endogenous β-gal activity on the surface of examined cells was found to be negligible. The substrate structure for β-gal was optimized and used for the CARP method. The CARP method showed significantly higher fluorescent signals than a conventional method using fluorophore-modified antibodies. Moreover, the degree of amplification of the fluorescence signal was higher for antigens with low expression levels, showing that the CARP method is a suitable signal amplification method over current conventional approaches.

Sialic acid and biology of life: An introduction

Sialic acids are important molecule with high structural diversity. They are known to occur in higher animals such as Echinoderms, Hemichordata, Cephalochorda, and Vertebrata and also in other animals such as Platyhelminthes, Cephalopoda, and Crustaceae. Plants are known to lack sialic acid. But they are reported to occur in viruses, bacteria, protozoa, and fungi. Deaminated neuraminic acid although occurs in vertebrates and bacteria, is reported to occur in abundance in the lower vertebrates. Sialic acids are mostly located in terminal ends of glycoproteins and glycolipids, capsular and tissue polysialic acids, bacterial lipooligosaccharides/polysaccharides, and in different forms that dictate their role in biology. Sialic acid play important roles in human physiology of cell-cell interaction, communication, cell-cell signaling, carbohydrate-protein interactions, cellular aggregation, development processes, immune reactions, reproduction, and in neurobiology and human diseases in enabling the infection process by bacteria and virus, tumor growth and metastasis, microbiome biology, and pathology. It enables molecular mimicry in pathogens that allows them to escape host immune responses. Recently sialic acid has found role in therapeutics. In this chapter we have highlighted the (i) diversity of sialic acid, (ii) their occurrence in the diverse life forms, (iii) sialylation and disease, and (iv) sialic acid and therapeutics.

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

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

The female reproductive tract contains multiple innate sialic acid-binding immunoglobulin-like lectins (Siglecs) that facilitate sperm survival

A sperm that fertilizes an egg has successfully survived multiple checkpoints within the female reproductive tract, termed pre-fertilization events. The leukocytic response is a pre-fertilization event in which sperm trigger an immune response that promotes homing of circulating leukocytes to the uterine lumen to destroy most sperm. Various glycoconjugates decorate the sperm surface, including sialic acids, which are abundant at the sperm surface where they cap most glycan chains and regulate sperm migration through cervical mucus, formation of the sperm oviductal reservoir, and sperm capacitation. However, the role of sperm-associated sialic acids in the leukocytic reaction remains unknown. The cognate endogenous binding partners of sialic acids, sialic acid-binding immunoglobulin-like lectins (Siglecs) play a pivotal role in regulating many immune responses. Here we investigated whether sperm-associated sialic acids inhibit activation of neutrophils, one of the major immune cells involved in the leukocytic reaction. We used in vitro interactions between sperm and neutrophils as well as binding assays between sperm and recombinant Siglec-Fc chimeric proteins to measure interactions. Moreover, we examined whether Siglecs are expressed on human and mouse endometria, which have a role in initiating the leukocytic reaction. Surprisingly less sialylated, capacitated, sperm did not increase neutrophil activation in vitro However, we observed expression of several Siglecs on the endometrium and that these receptors interact with sialylated sperm. Our results indicate that sperm sialic acids may interact with endometrial Siglecs and that these interactions facilitate sperm survival in the face of female immunity.

Siglec expression on the surface of human, bull and ram sperm

Sialic acid (Sia) is a major constituent of both the sperm glycocalyx and female reproductive mucosal surface and is involved in regulating sperm migration, uterotubal reservoir formation and oocyte binding. Siglecs (sialic acid-binding immunoglobulin - like lectins) commonly found on immune cells, bind to Sia in a linkage- and sugar-specific manner and often mediate cell-to-cell interactions and signalling. Proteomic and transcriptomic analysis of human and bovine sperm have listed Siglecs, but to date, their presence and/or localisation on sperm has not been studied. Therefore, the aim of this study was to characterise the presence of Siglecs on the surface of bovine, human and ovine sperm using both immunostaining and Western blotting. Siglec 1, 2, 5, 6, 10 and 14 were identified and displayed both species- and regional-specific expression on sperm. Almost universal expression across Siglecs and species was evident in the sperm neck and midpiece region while variable expression among Siglecs, similar among species, was detected in the head and tail regions of the sperm. The possible role for these proteins on sperm is discussed.

Targeting Notch-1 positive acute leukemia cells by novel fucose-bound liposomes carrying daunorubicin

Complete remission by induction therapy in acute myelogenous leukemia (AML) can be achieved due to improvements in supportive and optimized therapy. However, more than 20% of patients will still need to undergo salvage therapy, and most will have a poor prognosis. Determining the specificity of drugs to leukemia cells is important since this will maximize the dose of chemotherapeutic agents that can be administered to AML patients. In turn, this would be expected to lead to reduced drug toxicity and its increased efficacy. We targeted Notch-1 positive AML cells utilizing fucose-bound liposomes, since activation of Notch-1 is required for O-fucosylation. Herein, we report that intravenously injected, L-fucose-bound liposomes containing daunorubicin can be successfully delivered to AML cells that express fucosylated antigens. This resulted in efficient tumor growth inhibition in tumor-bearing mice and decreased proliferation of AML patient-derived leukemia cells. Thus, biological targeting by fucose-bound liposomes that takes advantage of the intrinsic characteristics of AML cells could be a promising new strategy for Notch-1 positive-AML treatment.

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.

Host Immune Responses in HIV-1 Infection: The Emerging Pathogenic Role of Siglecs and Their Clinical Correlates

A better understanding of the mechanisms employed by HIV-1 to escape immune responses still represents one of the major tasks required for the development of novel therapeutic approaches targeting a disease still lacking a definitive cure. Host innate immune responses against HIV-1 are key in the early phases of the infection as they could prevent the development and the establishment of two hallmarks of the infection: chronic inflammation and viral reservoirs. Sialic acid-binding immunoglobulin-like lectins (Siglecs) belong to a family of transmembrane proteins able to dampen host immune responses and set appropriate immune activation thresholds upon ligation with their natural ligands, the sialylated carbohydrates. This immune-modulatory function is also targeted by many pathogens that have evolved to express sialic acids on their surface in order to escape host immune responses. HIV-1 envelope glycoprotein 120 (gp120) is extensively covered by carbohydrates playing active roles in life cycle of the virus. Indeed, besides forming a protecting shield from antibody recognition, this coat of N-linked glycans interferes with the folding of viral glycoproteins and enhances virus infectivity. In particular, the sialic acid residues present on gp120 can bind Siglec-7 on natural killer and monocytes/macrophages and Siglec-1 on monocytes/macrophages and dendritic cells. The interactions between these two members of the Siglec family and the sialylated glycans present on HIV-1 envelope either induce or increase HIV-1 entry in conventional and unconventional target cells, thus contributing to viral dissemination and disease progression. In this review, we address the impact of Siglecs in the pathogenesis of HIV-1 infection and discuss how they could be employed as clinic and therapeutic targets.

Studies on the Detection, Expression, Glycosylation, Dimerization, and Ligand Binding Properties of Mouse Siglec-E

CD33-related Siglecs are a family of proteins widely expressed on innate immune cells. Binding of sialylated glycans or other ligands triggers signals that inhibit or activate inflammation. Immunomodulation by Siglecs has been extensively studied, but relationships between structure and functions are poorly explored. Here we present new data relating to the structure and function of Siglec-E, the major CD33-related Siglec expressed on mouse neutrophils, monocytes, macrophages, and dendritic cells. We generated nine new rat monoclonal antibodies specific to mouse Siglec-E, with no cross-reactivity to Siglec-F. Although all antibodies detected Siglec-E on transfected human HEK-293T cells, only two reacted with mouse bone marrow neutrophils by flow cytometry and on spleen sections by immunohistochemistry. Moreover, whereas all antibodies recognized Siglec-E-Fc on immunoblots, binding was dependent on intact disulfide bonds and N-glycans, and only two antibodies recognized native Siglec-E within spleen lysates. Thus, we further investigated the impact of Siglec-E homodimerization. Homology-based structural modeling predicted a cysteine residue (Cys-298) in position to form a disulfide bridge between two Siglec-E polypeptides. Mutagenesis of Cys-298 confirmed its role in dimerization. In keeping with the high level of 9-O-acetylation found in mice, sialoglycan array studies indicate that this modification has complex effects on recognition by Siglec-E, in relationship to the underlying structures. However, we found no differences in phosphorylation or SHP-1 recruitment between dimeric and monomeric Siglec-E expressed on HEK293A cells. Phylogenomic analyses predicted that only some human and mouse Siglecs form disulfide-linked dimers. Notably, Siglec-9, the functionally equivalent human paralog of Siglec-E, occurs as a monomer.

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.

Multivalent Interactions of Human Primary Amine Oxidase with the V and C22 Domains of Sialic Acid-Binding Immunoglobulin-Like Lectin-9 Regulate Its Binding and Amine Oxidase Activity

Sialic acid-binding immunoglobulin-like lectin-9 (Siglec-9) on leukocyte surface is a counter-receptor for endothelial cell surface adhesin, human primary amine oxidase (hAOC3), a target protein for anti-inflammatory agents. This interaction can be used to detect inflammation and cancer in vivo, since the labeled peptides derived from the second C2 domain (C2₂) of Siglec-9 specifically bind to the inflammation-inducible hAOC3. As limited knowledge on the interaction between Siglec-9 and hAOC3 has hampered both hAOC3-targeted drug design and in vivo imaging applications, we have now produced and purified the extracellular region of Siglec-9 (Siglec-9-EC) consisting of the V, C2₁ and C2₂ domains, modeled its 3D structure and characterized the hAOC3–Siglec-9 interactions using biophysical methods and activity/inhibition assays. Our results assign individual, previously unknown roles for the V and C2₂ domains. The V domain is responsible for the unusually tight Siglec-9–hAOC3 interactions whereas the intact C2₂ domain of Siglec-9 is required for modulating the enzymatic activity of hAOC3, crucial for the hAOC3-mediated leukocyte trafficking. By characterizing the Siglec-9-EC mutants, we could conclude that R120 in the V domain likely interacts with the terminal sialic acids of hAOC3 attached glycans whereas residues R284 and R290 in C2₂ are involved in the interactions with the active site channel of hAOC3. Furthermore, the C2₂ domain binding enhances the enzymatic activity of hAOC3 although the sialic acid-binding capacity of the V domain of Siglec-9 is abolished by the R120S mutation. To conclude, our results prove that the V and C2₂ domains of Siglec-9-EC interact with hAOC3 in a multifaceted and unique way, forming both glycan-mediated and direct protein-protein interactions, respectively. The reported results on the mechanism of the Siglec-9–hAOC3 interaction are valuable for the development of hAOC3-targeted therapeutics and diagnostic tools.

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.

Sialic Acid Is Required for Neuronal Inhibition by Soluble MAG but not for Membrane Bound MAG

Myelin-Associated Glycoprotein (MAG), a major inhibitor of axonal growth, is a member of the immunoglobulin (Ig) super-family. Importantly, MAG (also known as Siglec-4) is a member of the Siglec family of proteins (sialic acid-binding, immunoglobulin-like lectins), MAG binds to complex gangliosides, specifically GD1a and/or GT1b. Therefore, it has been proposed as neuronal receptors for MAG inhibitory effect of axonal growth. Previously, we showed that MAG binds sialic acid through domain 1 at Arg118 and is able to inhibit axonal growth through domain 5. We developed a neurite outgrowth (NOG) assay, in which both wild type MAG and mutated MAG (MAG Arg118) are expressed on cells. In addition we also developed a soluble form NOG in which we utilized soluble MAG-Fc and mutated MAG (Arg118-Fc). Only MAG-Fc is able to inhibit NOG, but not mutated MAG (Arg118)-Fc that has been mutated at its sialic acid binding site. However, both forms of membrane bound MAG- and MAG (Arg118)- expressing cells still inhibit NOG. Here, we review various results from different groups regarding MAG’s inhibition of axonal growth. Also, we propose a model in which the sialic acid binding is not necessary for the inhibition induced by the membrane form of MAG, but it is necessary for the soluble form of MAG. This finding highlights the importance of understanding the different mechanisms by which MAG inhibits NOG in both the soluble fragmented form and the membrane-bound form in myelin debris following CNS damage.

Siglec-9 modulated IL-4 responses in the macrophage cell line RAW264

Siglecs, an immunoglobulin-like lectin family that recognizes the sialic acid moiety, regulate various aspects of immune responses. In the present study, we investigated the effects of Siglecs on the macrophage cell line RAW264, which was stimulated with interleukin-4 (IL-4). The induction of arginase-1 (Arg1) by IL-4 was stronger in Siglec-9-expressing cells than in mock cells. Mutations in the cytoplasmic tyrosine-based inhibitory motifs in Siglec-9 markedly reduced the expression of Arg1. The phosphorylation of Akt by IL-4 and extracellular signal-regulated kinase (ERK) without IL-4 was stronger in Siglec-9-expressing cells, indicating the enhanced activation of the phosphatidylinositol 3 kinase (PI-3K) and mitogen-activated protein kinase kinase (MEK)/ERK pathways, respectively. The enhanced expression of Arg1 was inhibited by MEK inhibitors, but not by PI-3K inhibitor. These results indicate that Siglec-9 affects several different signaling pathways in IL-4-stimulated macrophages, which resulted in enhanced induction of Arg1 in Siglec-9-expressing RAW264 cells.

Molecular and functional characterization of porcine Siglec-3/CD33 and analysis of its expression in blood and tissues

A cDNA clone encoding a 380 a-a type 1 transmembrane protein with homology to human Siglec-3/CD33 was obtained from a swine small intestine library. An analysis of protein sequence identified two immunoglobulin-like domains, a transmembrane region, and a carboxi-terminal tail with two tyrosine-based signalling motifs. Binding assays of Siglec-3 transfected CHO cells to polyacrylamide glycoconjugates showed a preference for α2-6-linked sialic acids. Using mAbs raised against a fragment containing the two Ig-like domains, porcine Siglec-3 was found to be expressed on monocytes and granulocytes, and their bone marrow precursors. It was also detected in lymph node, splenic and alveolar macrophages. MAbs immunoprecipitated, from granulocyte lysates, a protein of 51-60 kDa under both non-reducing and reducing conditions. MAbs were also used to analyse functional activity of Siglec-3 on bone marrow and blood cells. Engagement of Siglec-3 by mAb had no apparent effect on cell proliferation or cytokine production.

Molecular characterization of porcine Siglec-10 and analysis of its expression in blood and tissues

Siglecs are sialic acid binding Ig-like proteins involved in the control of leukocyte responses. In this study we describe the characterization of a porcine orthologue of Siglec-10. A cDNA clone was obtained from a porcine library which encodes a protein with sequence homology to human Siglec-10. This cDNA codes for a type I transmembrane protein containing four Ig-like domains, a transmembrane region, and a cytoplasmic tail with three tyrosine-based motifs, including a membrane-proximal Grb2-binding motif, and two ITIM motifs. When expressed on transfected cells, porcine Siglec-10 was able to bind red blood cells in a sialic acid-dependent manner. Monoclonal antibodies were developed against this protein and used to examine its cell and tissue distribution in the pig. Siglec-10 was found to be expressed on blood B cells and B cell areas of the spleen and lymph nodes. A weak expression was also detected on monocytes.

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.

Developing aptamer probes for acute myelogenous leukemia detection and surface protein biomarker discovery

BACKGROUND

The majority of patients with acute myelogenous leukemia (AML) still die of their disease. In order to improve survival rates in AML patients, new strategies are necessary to discover biomarkers for the detection and targeted therapy of AML. One of the advantages of the aptamer-based technology is the unique cell-based selection process, which allows us to efficiently select for cell-specific aptamers without knowing which target molecules are present on the cell surface.

METHODS

The NB4 AML cell line was used as the target cell population for selecting single stranded DNA aptamers. After determining the affinity of selected aptamers to leukocytes, the aptamers were used to phenotype human bone marrow leukocytes and AML cells in clinical specimens. Then a biotin-labelled aptamer was used to enrich and identify its target surface protein.

RESULTS

Three new aptamers were characterized from the selected aptamer pools (JH6, JH19, and K19). All of them can selectively recognize myeloid cells with Kd in the low nanomole range (2.77 to 12.37 nM). The target of the biotin-labelled K19 aptamer probe was identified as Siglec-5, a surface membrane protein in low abundance whose expression can serve as a biomarker of granulocytic maturation and be used to phenotype AML. More importantly, Siglec-5 expression can be used to detect low concentrations of AML cells in human bone marrow specimens, and functions as a potential target for leukemic therapy.

CONCLUSIONS

We have demonstrated a pipeline approach for developing single stranded DNA aptamer probes, phenotyping AML cells in clinical specimens, and then identifying the aptamer-recognized target protein. The developed aptamer probes and identified Siglec-5 protein may potentially be used for leukemic cell detection and therapy in our future clinical practice.

Dectin-1 is essential for reverse transcytosis of glycosylated SIgA-antigen complexes by intestinal M cells

This work reports the long-awaited identification of Dectin-1 and Siglec-5 as the M cell co-receptors that mediate the reverse transcytosis of secretory IgA molecules to mount a gut immune response.

Intestinal microfold (M) cells possess a high transcytosis capacity and are able to transport a broad range of materials including particulate antigens, soluble macromolecules, and pathogens from the intestinal lumen to inductive sites of the mucosal immune system. M cells are also the primary pathway for delivery of secretory IgA (SIgA) to the gut-associated lymphoid tissue. However, although the consequences of SIgA uptake by M cells are now well known and described, the mechanisms whereby SIgA is selectively bound and taken up remain poorly understood. Here we first demonstrate that both the Cα1 region and glycosylation, more particularly sialic acid residues, are involved in M cell–mediated reverse transcytosis. Second, we found that SIgA is taken up by M cells via the Dectin-1 receptor, with the possible involvement of Siglec-5 acting as a co-receptor. Third, we establish that transcytosed SIgA is taken up by mucosal CX3CR1⁺ dendritic cells (DCs) via the DC-SIGN receptor. Fourth, we show that mucosal and systemic antibody responses against the HIV p24-SIgA complexes administered orally is strictly dependent on the expression of Dectin-1. Having deciphered the mechanisms leading to specific targeting of SIgA-based Ag complexes paves the way to the use of such a vehicle for mucosal vaccination against various infectious diseases.

Secretory IgA (SIgA) antibodies are secreted into the gut lumen and are considered to be a first line of defense in protecting the intestinal epithelium from gut pathogens. SIgA patrol the mucus and are usually known to help immune tolerance via entrapping dietary antigens and microorganisms and other mechanisms. SIgA, in complex with its antigens, can also be taken back up by the intestinal epithelium in a process known as reverse transcytosis. SIgA can thereby promote the uptake and delivery of antigens from the intestinal lumen to the Gut-Associated Lymphoid Tissues (GALT), influencing inflammatory responses. This reverse transcytosis of SIgA is mediated by specialized epithelial M cells. Because M cells possess the ability to take up antigens and are therefore important to the local immune system, they are a key target for the specific delivery of novel mucosal vaccines against various diseases. M cell receptors that take up the SIgA-antigen complexes, which serve as mucosal vaccine vehicles, represent an important aspect of this vaccine strategy. The identification of SIgA receptor(s) on the surface of M cells has, however, remained elusive for more than a decade. In this study, we now identify Dectin-1 and Siglec-5 as the key receptors for M cell–mediated reverse transcytosis of SIgA complexes. We further find that the glycosylation modification, and particularly sialylation, of SIgA is required for its uptake by M cells. We show that, when administered orally in complex with SIgA, the HIV p24 antigen is taken up in a strictly Dectin-1-dependent manner to stimulate a mucosal and systemic antibody response. These findings are considered important for understanding gut immunity.

Human Siglec-5 inhibitory receptor and immunoglobulin A (IgA) have separate binding sites in streptococcal beta protein

Sialic acid-binding immunoglobulin-like lectins (Siglecs) are receptors believed to be important for regulation of cellular activation and inflammation. Several pathogenic microbes bind specific Siglecs via sialic acid-containing structures at the microbial surface, interactions that may result in modulation of host responses. Recently, it was shown that the group B Streptococcus (GBS) binds to human Siglec-5 (hSiglec-5), an inhibitory receptor expressed on macrophages and neutrophils, via the IgA-binding surface β protein, providing the first example of a protein/protein interaction between a pathogenic microbe and a Siglec. Here we show that the hSiglec-5-binding part of β resides in the N-terminal half of the protein, which also harbors the previously determined IgA-binding region. We constructed bacterial mutants expressing variants of the β protein with non-overlapping deletions in the N-terminal half of the protein. Using these mutants and recombinant β fragments, we showed that the hSiglec-5-binding site is located in the most N-terminal part of β (B6N region; amino acids 1–152) and that the hSiglec-5- and IgA-binding domains in β are completely separate. We showed with BIAcore^TM analysis that tandem variants of the hSiglec-5- and IgA-binding domains bind to their respective ligands with high affinity. Finally, we showed that the B6N region, but not the IgA-binding region of β, triggers recruitment of the tyrosine phosphatase SHP-2 to hSiglec-5 in U937 monocytes. Taken together, we have identified and isolated the first microbial non-sialic acid Siglec-binding region that can be used as a tool in studies of the β/hSiglec-5 interaction.

SIGLEC12, a human-specific segregating (pseudo)gene, encodes a signaling molecule expressed in prostate carcinomas

The primate SIGLEC12 gene encodes one of the CD33-related Siglec family of signaling molecules in immune cells. We had previously reported that this gene harbors a human-specific missense mutation of the codon for an Arg residue required for sialic acid recognition. Here we show that this R122C mutation of the Siglec-XII protein is fixed in the human population, i.e. it occurred prior to the origin of modern humans. Additional mutations have since completely inactivated the SIGLEC12 gene in some but not all humans. The most common inactivating mutation with a global allele frequency of 58% is a single nucleotide frameshift that markedly shortens the open reading frame. Unlike other CD33-related Siglecs that are primarily found on immune cells, we found that Siglec-XII protein is expressed not only on some macrophages but also on various epithelial cell surfaces in humans and chimpanzees. We also found expression on certain human prostate epithelial carcinomas and carcinoma cell lines. This expression correlates with the presence of the nonframeshifted, intact SIGLEC12 allele. Although SIGLEC12 allele status did not predict prostate carcinoma incidence, restoration of expression in a prostate carcinoma cell line homozygous for the frameshift mutation induced altered regulation of several genes associated with carcinoma progression. These stably transfected Siglec-XII-expressing prostate cancer cells also showed enhanced growth in nude mice. Finally, monoclonal antibodies against the protein were internalized by Siglec-XII-expressing prostate carcinoma cells, allowing targeting of a toxin to such cells. Polymorphic expression of Siglec-XII in humans thus has implications for prostate cancer biology and therapeutics.

Adipose tissue deficiency and chronic inflammation in diabetic Goto-Kakizaki rats

Type 2 diabetes (T2DM) is a heterogeneous group of diseases that is progressive and involves multiple tissues. Goto-Kakizaki (GK) rats are a polygenic model with elevated blood glucose, peripheral insulin resistance, a non-obese phenotype, and exhibit many degenerative changes observed in human T2DM. As part of a systems analysis of disease progression in this animal model, this study characterized the contribution of adipose tissue to pathophysiology of the disease. We sacrificed subgroups of GK rats and appropriate controls at 4, 8, 12, 16 and 20 weeks of age and carried out a gene array analysis of white adipose tissue. We expanded our physiological analysis of the animals that accompanied our initial gene array study on the livers from these animals. The expanded analysis included adipose tissue weights, HbA1c, additional hormonal profiles, lipid profiles, differential blood cell counts, and food consumption. HbA1c progressively increased in the GK animals. Altered corticosterone, leptin, and adiponectin profiles were also documented in GK animals. Gene array analysis identified 412 genes that were differentially expressed in adipose tissue of GKs relative to controls. The GK animals exhibited an age-specific failure to accumulate body fat despite their relatively higher calorie consumption which was well supported by the altered expression of genes involved in adipogenesis and lipogenesis in the white adipose tissue of these animals, including Fasn, Acly, Kklf9, and Stat3. Systemic inflammation was reflected by chronically elevated white blood cell counts. Furthermore, chronic inflammation in adipose tissue was evident from the differential expression of genes involved in inflammatory responses and activation of natural immunity, including two interferon regulated genes, Ifit and Iipg, as well as MHC class II genes. This study demonstrates an age specific failure to accumulate adipose tissue in the GK rat and the presence of chronic inflammation in adipose tissue from these animals.

Immune inhibitory receptors: essential regulators of phagocyte function

Phagocytes, including neutrophils, monocytes, and macrophages, play a crucial role in host defense by recognition and elimination of invading pathogens. Phagocytic cells produce reactive oxygen species (ROS), inflammatory cytokines, and chemokines, leading to bacterial killing and to recruitment and activation of additional immune cells. However, inflammatory mediators are potentially harmful for the host and their production is therefore tightly controlled by multiple regulatory mechanisms. One such mechanism is immune suppression by immune inhibitory receptors, which are increasingly acknowledged as potent regulators of the immune response. So far, research has focused on the role of these receptors in the regulation of NK cells, B cells, and T cells. Importantly, an accumulating number of inhibitory receptors have been identified on phagocytes. Here, we review the role of inhibitory receptors in the regulation of phagocyte cytokine production, migration, apoptosis, ROS production, and phagocytosis. Furthermore, we discuss the intracellular mechanisms utilized by distinct inhibitory receptors to regulate specific phagocyte functions. We demonstrate that inhibitory receptors are important regulators of the immune response, which bacteria can use to their advantage.

Evolution of CD33-related siglecs: regulating host immune functions and escaping pathogen exploitation?

Sialic-acid-binding immunoglobulin-like lectins, siglecs, are important immune receptors expressed widely in mammals. A unique feature of siglecs is their ability to bind sialylated glycans and transmit signals to immune cells. The CD33-related siglecs (CD33rSiglecs) form a major subfamily of the siglecs, containing a large, rapidly evolving group of genes that expanded in mammals through an inverse duplication event involving a primordial cluster of siglec genes over 180 million years ago. Humans express a much larger set of CD33rSiglecs than mice and rats, a feature that can be explained by a dramatic loss of CD33rSiglec genes in rodents. Most CD33rSiglecs have immune receptor tyrosine-based inhibitory motifs and signal negatively. Interestingly, novel DAP-12-coupled 'activating' CD33rSiglecs have been identified, such as siglec-14 and siglec-16, which are paired with the inhibitory receptors, siglec-5 and siglec-11, respectively. The evolution of these activating receptors may have been driven in part by pathogen exploitation of inhibitory siglecs, thereby providing the host with additional pathways by which to combat these pathogens. Inhibitory siglecs seem to play important and varied roles in the regulation of host immune responses. For example, several CD33rSiglecs have been implicated in the negative regulation of Toll-like receptor signalling during innate responses; siglec-G functions as a negative regulator of B1-cell expansion and appears to suppress inflammatory responses to host-derived 'danger-associated molecular patterns'. Recent work has also shown that engagement of neutrophil-expressed siglec-9 by certain strains of sialylated Group B streptococci can suppress killing responses, thereby providing experimental support for pathogen exploitation of host CD33rSiglecs.

Relative over-reactivity of human versus chimpanzee lymphocytes: implications for the human diseases associated with immune activation

Although humans and chimpanzees share >99% identity in alignable protein sequences, they differ surprisingly in the incidence and severity of some common diseases. In general, humans infected with various viruses, such as HIV and hepatitis C virus, appear to develop stronger reactions and long-term complications. Humans also appear to suffer more from other diseases associated with over-reactivity of the adaptive immune system, such as asthma, psoriasis, and rheumatoid arthritis. In this study, we show that human T cells are more reactive than chimpanzee T cells to a wide variety of stimuli, including anti-TCR Abs of multiple isotypes, l-phytohemagglutin, Staphylococcus aureus superantigen, a superagonist anti-CD28 Ab, and in MLRs. We also extend this observation to B cells, again showing a human propensity to react more strongly to stimuli. Finally, we show a relative increase in activation markers and cytokine production in human lymphocytes in response to uridine-rich (viral-like) ssRNA. Thus, humans manifest a generalized lymphocyte over-reactivity relative to chimpanzees, a finding that is correlated with decreased levels of inhibitory sialic acid-recognizing Ig-superfamily lectins (Siglecs; particularly Siglec-5) on human T and B cells. Furthermore, Siglec-5 levels are upregulated by activation in chimpanzee but not human lymphocytes, and human T cell reactivity can be downmodulated by forced expression of Siglec-5. Thus, a key difference in the immune reactivity of chimp and human lymphocytes appears to be related to the differential expression of Siglec-5. Taken together, these data may help explain human propensities for diseases associated with excessive activation of the adaptive immune system.