CD33-related sialic-acid-binding immunoglobulin-like lectins in health and disease

Sequence variety in the CC' loop of Siglec-8/9/3 determines the recognitions to sulfated oligosaccharides

Siglecs are important lectins found in different types of immune cells and function as regulatory molecules by recognizing self-associated glycans and converting extracellular interactions into signals for inhibiting immune cell functions. Although many Siglecs have been found to show broad specificities and recognize different types of sulfated oligosaccharides, Siglec-8 and Siglec-9 displayed a high degree of specificity for sialyl N-acetyllactosamine (sLacNAc) with sulfations at O6-positions of the galactose (6'-sulfation) and N-acetylglucosamine (6-sulfation), respectively. Siglec-3 was recently discovered to bind sLacNAc both sulfations. In addition to a conserved arginine residue for binding to sialic acid residue, the sequence variety in the CC' loop may provide binding specificities to sulfated oligosaccharides in Siglecs. Thus, the present study employed molecular models to study the impact of different residues in the CC' loops of Siglec-8/9/3 to the recognitions of 6-sulfations in Gal and/or GlcNAc of sLacNAc. The negatively charged residues in the CC' loop of Siglec-9 formed unfavorable electrostatic repulsions with the 6-sulfate in Gal and resulted no recognitions, in contrast to the favorable interactions formed between the positively charged residues in the CC' loop of Siglec-8 and the 6-sulfate in Gal resulting strong specificity. A two-state binding model was proposed for Siglec-3 recognizing 6-sulfations in Gal and GlcNAc of sLacNAc, as the neutral residues in the CC' loop of Siglec-3 could not form strong favorable interactions to lock the 6-sulfate in Gal within a single binding pose or strong unfavorable interactions to repel the 6-sulfate in Gal. The oligosaccharide adopted two distinctive binding poses and oriented the sulfate groups to form interactions with residues in the CC' loop and G-strand. The present study provided a structural mechanism for the sequence variety in the CC' loop of Siglec-8/9/3 determining the recognitions to the sulfated oligosaccharides and offered insights into the binding specificities for Siglecs.

Human sialoglycan ligands for immune inhibitory Siglecs

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

Siglec-6 Signaling Uses Src Kinase Tyrosine Phosphorylation and SHP-2 Recruitment

Preeclampsia is a pregnancy-specific disorder involving placental abnormalities. Elevated placental Sialic acid immunoglobulin-like lectin (Siglec)-6 expression has been correlated with preeclampsia. Siglec-6 is a transmembrane receptor, expressed predominantly by the trophoblast cells in the human placenta. It interacts with sialyl glycans such as sialyl-TN glycans as well as binds leptin. Siglec-6 overexpression has been shown to influence proliferation, apoptosis, and invasion in the trophoblast (BeWo) cell model. However, there is no direct evidence that Siglec-6 plays a role in preeclampsia pathogenesis and its signaling potential is still largely unexplored. Siglec-6 contains an immunoreceptor tyrosine-based inhibitory motif (ITIM) and an ITIM-like motif in its cytoplasmic tail suggesting a signaling function. Site-directed mutagenesis and transfection were employed to create a series of Siglec-6 expressing HTR-8/SVneo trophoblastic cell lines with mutations in specific functional residues to explore the signaling potential of Siglec-6. Co-immunoprecipitation and inhibitory assays were utilized to investigate the association of Src-kinases and SH-2 domain-containing phosphatases with Siglec-6. In this study, we show that Siglec-6 is phosphorylated at ITIM and ITIM-like domains by Src family kinases. Phosphorylation of both ITIM and ITIM-like motifs is essential for the recruitment of phosphatases like Src homology region 2 containing protein tyrosine phosphatase 2 (SHP-2), which has downstream signaling capabilities. These findings suggest Siglec-6 as a signaling molecule in human trophoblasts. Further investigation is warranted to determine which signaling pathways are activated downstream to SHP-2 recruitment and how overexpression of Siglec-6 in preeclamptic placentas impacts pathogenesis.

Immunotherapy of targeting MDSCs in tumor microenvironment

Myeloid-derived suppressor cells (MDSCs) are a group of heterogeneous cells which are abnormally accumulated during the differentiation of myeloid cells. Immunosuppression is the main functional feature of MDSCs, which inhibit T cell activity in the tumor microenvironment (TME) and promote tumoral immune escape. The main principle for immunotherapy is to modulate, restore, and remodel the plasticity and potential of immune system to have an effective anti-tumor response. In the TME, MDSCs are major obstacles to cancer immunotherapy through reducing the anti-tumor efficacy and making tumor cells more resistant to immunotherapy. Therefore, targeting MDSCs treatment becomes the priority of relevant studies and provides new immunotherapeutic strategy for cancer treatment. In this review, we mainly discuss the functions and mechanisms of MDSCs as well as their functional changes in the TME. Further, we review therapeutic effects of immunotherapy against MDSCs and potential breakthroughs regarding immunotherapy targeting MDSCs and immune checkpoint blockade (ICB) immunotherapy.

Re-Expression of Poly/Oligo-Sialylated Adhesion Molecules on the Surface of Tumor Cells Disrupts Their Interaction with Immune-Effector Cells and Contributes to Pathophysiological Immune Escape

Glycans linked to surface proteins are the most complex biological macromolecules that play an active role in various cellular mechanisms. This diversity is the basis of cell-cell interaction and communication, cell growth, cell migration, as well as co-stimulatory or inhibitory signaling. Our review describes the importance of neuraminic acid and its derivatives as recognition elements, which are located at the outermost positions of carbohydrate chains linked to specific glycoproteins or glycolipids. Tumor cells, especially from solid tumors, mask themselves by re-expression of hypersialylated neural cell adhesion molecule (NCAM), neuropilin-2 (NRP-2), or synaptic cell adhesion molecule 1 (SynCAM 1) in order to protect themselves against the cytotoxic attack of the also highly sialylated immune effector cells. More particularly, we focus on α-2,8-linked polysialic acid chains, which characterize carrier glycoproteins such as NCAM, NRP-2, or SynCam-1. This characteristic property correlates with an aggressive clinical phenotype and endows them with multiple roles in biological processes that underlie all steps of cancer progression, including regulation of cell-cell and/or cell-extracellular matrix interactions, as well as increased proliferation, migration, reduced apoptosis rate of tumor cells, angiogenesis, and metastasis. Specifically, re-expression of poly/oligo-sialylated adhesion molecules on the surface of tumor cells disrupts their interaction with immune-effector cells and contributes to pathophysiological immune escape. Further, sialylated glycoproteins induce immunoregulatory cytokines and growth factors through interactions with sialic acid-binding immunoglobulin-like lectins. We describe the processes, which modulate the interaction between sialylated carrier glycoproteins and their ligands, and illustrate that sialic acids could be targets of novel therapeutic strategies for treatment of cancer and immune diseases.

Novel CD33 antibodies unravel localization, biology and therapeutic implications of CD33 isoforms

The aim of this study was to establish the therapeutic relevance of the CD33D2 isoform by developing novel antibodies targeting the IgC domain of CD33. Two novel IgC-targeting antibodies, HL2541 and 5C11-2, were developed, and CD33 isoforms were assessed using multiple assays in cells overexpressing either CD33FL or CD33D2 isoforms, unmodified acute myeloid leukemia (AML) cell lines and primary AML specimens representing different genotypes for the CD33 splicing single nucleotide polymorphism. CD33D2 was recognized on cells overexpressing CD33D2 and unmodified AML cell lines; however, minimal/no cell surface detection of CD33D2 was observed in primary AML specimens. Both isoforms were detected intracellularly using novel antibodies. Minimal cell surface expression of CD33D2 on primary AML/progenitor cells warrants further studies on anti-CD33D2 immunotherapeutics.

Reverse expression of α2,6-sialic acid ratios on IgG, IgM, and IgG/IgM autoantibodies correlates with mouse arthritis and rheumatoid arthritis disease activity

BACKGROUND

Sialic acids (SIAs), for example, α2,6-SIAs, can link to conserved N-glycans of immunoglobulin G (IgG). In this study, we investigated the correlation between α2,6-SIA on IgG and IgM and the disease activity of arthritis and rheumatoid arthritis (RA) in mice.

METHODS

We measured α2,6-SIA levels in IgGs and IgMs in collagen-induced arthritis (CIA). Additionally, α2,6-SIA levels in rheumatoid factors (RFs) and anti-cyclic citrullinated peptide (anti-CCP) antibodies in RA patients were measured. Correlations between α2,6-SIA on Igs and CIA were analyzed and also in RA patients by utilizing the disease activity score 28 (DAS28). The ability to differentiate RA progression by Ig and autoantibody α2,6-SIA levels was examined.

RESULTS

In CIA mice, plasma IgG-α2,6-SIA/IgG ratios decreased, whereas plasma IgM-α2,6-SIA/IgM ratios increased. Moreover, arthritis was not observed in collagen-injected mice with decreased IgG-α2,6-SIA/IgG ratios and without increased IgM-α2,6-SIA/IgM ratios. Isolated IgG-α2,6-SIA/IgG ratios displayed a significant inverse correlation with DAS28 scores (r = -0.383, p = 0.037). In contrast, isolated IgM-α2,6-SIA/IgM ratios correlated positively with DAS28 (r = 0.351, p = 0.009). Isolated IgG-anti-CCP-α2,6-SIA/plasma IgG-anti-CCP ratios were differentiated into either the remission (higher ratios) or the nonremission (lower ratios) category (p = 0.061), which is similar to the pattern for C-reactive protein (CRP) (p = 0.041) but different from that for the erythrocyte sedimentation rate (ESR) (p = 0.421). Using multiple linear regression analysis, plasma IgMRF-α2,6-SIA/IgMRF ratios displayed a correlation with DAS28 (p = 0.006), which was also observed in the ESR (p = 0.005), but was different from that for CRP (p = 0.222).

CONCLUSION

Concurrent reverse expression of α2,6-SIA ratios on IgM and IgG correlated with the occurrence of CIA and RA disease activity. Thus, α2,6-SIA ratios on IgG-anti-CCP antibodies and IgMRF are potential markers for evaluating RA disease activities.

Coronaviruses: Is Sialic Acid a Gate to the Eye of Cytokine Storm? From the Entry to the Effects

Coronaviruses (CoVs) are a diverse family of the enveloped human and animal viruses reported as causative agents for respiratory and intestinal infections. The high pathogenic potential of human CoVs, including SARS-CoV, MERS-CoV and SARS-CoV-2, is closely related to the invasion mechanisms underlying the attachment and entry of viral particles to the host cells. There is increasing evidence that sialylated compounds of cellular glycocalyx can serve as an important factor in the mechanism of CoVs infection. Additionally, the sialic acid-mediated cross-reactivity with the host immune lectins is known to exert the immune response of different intensity in selected pathological stages. Here, we focus on the last findings in the field of glycobiology in the context of the role of sialic acid in tissue tropism, viral entry kinetics and immune regulation in the CoVs infections.

Sialic Acid-Siglec Axis as Molecular Checkpoints Targeting of Immune System: Smart Players in Pathology and Conventional Therapy

The sialic acid-based molecular mimicry in pathogens and malignant cells is a regulatory mechanism that leads to cross-reactivity with host antigens resulting in suppression and tolerance in the immune system. The interplay between sialoglycans and immunoregulatory Siglec receptors promotes foreign antigens hiding and immunosurveillance impairment. Therefore, molecular targeting of immune checkpoints, including sialic acid-Siglec axis, is a promising new field of inflammatory disorders and cancer therapy. However, the conventional drugs used in regular management can interfere with glycome machinery and exert a divergent effect on immune controlling systems. Here, we focus on the known effects of standard therapies on the sialoglycan-Siglec checkpoint and their importance in diagnosis, prediction, and clinical outcomes.

Identification of Nanobodies against the Acute Myeloid Leukemia Marker CD33

Nanobodies (Nbs) are the smallest antigen-binding, single domain fragments derived from heavy-chain-only antibodies from Camelidae. Among the several advantages over conventional monoclonal antibodies, their small size (12-15 kDa) allows them to extravasate rapidly, to show improved tissue penetration, and to clear rapidly from blood, which are important characteristics for cancer imaging and targeted radiotherapy. Herein, we identified Nbs against CD33, a marker for acute myeloid leukemia (AML). A total of 12 Nbs were generated against recombinant CD33 protein, out of which six bound natively CD33 protein, expressed on the surface of acute myeloid leukemia THP-1 cells. The equilibrium dissociation constants (KD) of these six Nbs and CD33 range from 4 to 270 nM, and their melting temperature (Tm) varies between 52.67 and 67.80 °C. None of these Nbs showed leukemogenicity activity in vitro. The selected six candidates were radiolabeled with 99mTc, and their biodistribution was evaluated in THP-1-tumor-bearing mice. The imaging results demonstrated the fast tumor-targeting capacity of the Nbs in vivo. Among the anti-CD33 Nbs, Nb_7 showed the highest tumor uptake (2.53 ± 0.69 % injected activity per gram (IA/g), with low background signal, except in the kidneys and bladder. Overall, Nb_7 exhibits the best characteristics to be used as an anti-CD33 targeting vehicle for future diagnostic or therapeutic applications.

MDSC targeting with Gemtuzumab ozogamicin restores T cell immunity and immunotherapy against cancers

BACKGROUND

Targeting of MDSCs is a major clinical challenge in the era of immunotherapy. Antibodies which deplete MDSCs in murine models can reactivate T cell responses. In humans such approaches have not developed due to difficulties in identifying targets amenable to clinical translation.

METHODS

RNA-sequencing of M-MDSCs and G-MDSCs from cancer patients was undertaken. Flow cytometry and immunohistochemistry of blood and tumours determined MDSC CD33 expression. MDSCs were treated with Gemtuzumab ozogamicin and internalisation kinetics, and cell death mechanisms determined by flow cytometry, confocal microscopy and electron microscopy. Effects on T cell proliferation and CAR-T cell anti-tumour cytotoxicity were identified in the presence of Gemtuzumab ozogamicin.

FINDINGS

RNA-sequencing of human M-MDSCs and G-MDSCs identified transcriptomic differences, but that CD33 is a common surface marker. Flow cytometry indicated CD33 expression is higher on M-MDSCs, and CD33+ MDSCs are found in the blood and tumours regardless of cancer subtype. Treatment of human MDSCs leads to Gemtuzumab ozogamicin internalisation, increased p-ATM, and cell death; restoring T cell proliferation. Anti-GD2-/mesothelin-/EGFRvIII-CAR-T cell activity is enhanced in combination with the anti-MDSC effects of Gemtuzumab ozogamicin.

INTERPRETATION

The study identifies that M-MDSCs and G-MDSCs are transcriptomically different but CD33 is a therapeutic target on peripheral and infiltrating MDSCs across cancer subtypes. The immunotoxin Gemtuzumab ozogamicin can deplete MDSCs providing a translational approach to reactivate T cell and CAR-T cell responses against multiple cancers. In the rare conditions of HLH/MAS gemtuzumab ozogamicin provides a novel anti-myeloid strategy. FUND: This work was supported by Cancer Research UK, CCLG, Treating Children with Cancer, and the alumni and donors to the University of Birmingham.

SRSF1 and PTBP1 Are trans-Acting Factors That Suppress the Formation of a CD33 Splicing Isoform Linked to Alzheimer's Disease Risk

A single nucleotide polymorphism (SNP) in exon 2 of the CD33 gene is associated with reduced susceptibility to late-onset Alzheimer’s disease (AD) and causal for elevated mRNA lacking exon 2. In contrast to full-length CD33, transcripts lacking exon 2 result in CD33 protein unable to suppress activation responses in myeloid cells, including microglia. Currently, little is known about the regulation of CD33 exon 2 splicing.

A single nucleotide polymorphism (SNP) in exon 2 of the CD33 gene is associated with reduced susceptibility to late-onset Alzheimer’s disease (AD) and causal for elevated mRNA lacking exon 2. In contrast to full-length CD33, transcripts lacking exon 2 result in CD33 protein unable to suppress activation responses in myeloid cells, including microglia. Currently, little is known about the regulation of CD33 exon 2 splicing. Using functional genomics and proteomic approaches, we found that SRSF1 and PTBP1 act as splicing enhancers to increase CD33 exon 2 inclusion in mRNA. Binding of PTBP1 to RNA sequences proximal to the intron 1-exon 2 splice junction is altered by the SNP and represents a potential mechanism behind the SNP-genotype dependent alternative splicing. Our studies also reveal that binding of SRSF1 to the CD33 RNA is not altered by the SNP genotype. Instead, a putative SRSF1 binding sequence at the 3′ end of exon 2 directs CD33 exon 2 inclusion into the mRNA, indicating that PTBP1 and SRSF1 promote full-length isoform expression through different mechanisms. Our findings shed light on molecular interactions that regulate CD33 exon 2 splicing, ultimately impacting receptor expression on the cell surface. These data aid in the understanding of CD33’s regulation of microglial signaling underpinning the AD genetic associations.

Next generation chimeric antigen receptor T cells: safety strategies to overcome toxicity

Chimeric antigen receptor T (CAR-T) cell therapy is an emerging and effective cancer immunotherapy. Especially in hematological malignancies, CAR-T cells have achieved exciting results. Two Anti-CD19 CAR-T therapies have been approved for the treatment of CD19-positive leukemia or lymphoma. However, the application of CAR-T cells is obviously hampered by the adverse effects, such as cytokines release syndrome and on-target off-tumor toxicity. In some clinical trials, patients quitted the treatment of CAR-T cells due to life-threatening toxicity. Seeking to alleviate these toxicities or prevent the occurrence, researchers have developed a number of safety strategies of CAR-T cells, including suicide genes, synthetic Notch receptor, on-switch CAR, combinatorial target-antigen recognition, bispecific T cell engager and inhibitory CAR. This review summarized the preclinical studies and clinical trials of the safety strategies of CAR-T cells and their respective strengths and weaknesses.

Building upon the success of CART19: chimeric antigen receptor T cells for hematologic malignancies

Chimeric antigen receptor T cell (CART) therapy has dramatically changed the therapeutic prospects for B cell malignancies. Over the last decade CD19-redirected CART have demonstrated the ability to induce deep, long-lasting remissions and possibly cure patients with relapsing B cell neoplasms. Such impressive results with CART19 fostered efforts to expand this technology to other incurable malignancies that naturally do not express CD19, such as acute myeloid leukemia (AML), Hodgkin lymphoma (HL) and multiple myeloma (MM). However, to reach this goal, several hurdles have to be overcome, in particular: (i) the apparent lack of suitable targets as effective as CD19; (ii) the immunosuppressive tumor microenvironment; (iii) intra-tumoral heterogeneity and antigen-negative relapses. Therefore, new strategies that allow safer and more potent CART platforms are under development and may provide grounds for new exciting breakthroughs in the field.

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.

In Vitro Pre-Clinical Validation of Suicide Gene Modified Anti-CD33 Redirected Chimeric Antigen Receptor T-Cells for Acute Myeloid Leukemia

Background

Approximately fifty percent of patients with acute myeloid leukemia can be cured with current therapeutic strategies which include, standard dose chemotherapy for patients at standard risk of relapse as assessed by cytogenetic and molecular analysis, or high-dose chemotherapy with allogeneic hematopoietic stem cell transplant for high-risk patients. Despite allogeneic hematopoietic stem cell transplant about 25% of patients still succumb to disease relapse, therefore, novel strategies are needed to improve the outcome of patients with acute myeloid leukemia.

Methods and findings

We developed an immunotherapeutic strategy targeting the CD33 myeloid antigen, expressed in ~ 85–90% of patients with acute myeloid leukemia, using chimeric antigen receptor redirected T-cells. Considering that administration of CAR T-cells has been associated with cytokine release syndrome and other potential off-tumor effects in patients, safety measures were here investigated and reported. We genetically modified human activated T-cells from healthy donors or patients with acute myeloid leukemia with retroviral supernatant encoding the inducible Caspase9 suicide gene, a ΔCD19 selectable marker, and a humanized third generation chimeric antigen receptor recognizing human CD33. ΔCD19 selected inducible Caspase9-CAR.CD33 T-cells had a 75±3.8% (average ± standard error of the mean) chimeric antigen receptor expression, were able to specifically lyse CD33⁺ targets in vitro, including freshly isolated leukemic blasts from patients, produce significant amount of tumor-necrosis-factor-alpha and interferon-gamma, express the CD107a degranulation marker, and proliferate upon antigen specific stimulation. Challenging ΔCD19 selected inducible Caspase9-CAR.CD33 T-cells with programmed-death-ligand-1 enriched leukemia blasts resulted in significant killing like observed for the programmed-death-ligand-1 negative leukemic blasts fraction. Since the administration of 10 nanomolar of a non-therapeutic dimerizer to activate the suicide gene resulted in the elimination of only 76.4±2.0% gene modified cells in vitro, we found that co-administration of the dimerizer with either the BCL-2 inhibitor ABT-199, the pan-BCL inhibitor ABT-737, or mafosfamide, resulted in an additive effect up to complete cell elimination.

Conclusions

This strategy could be investigated for the safety of CAR T-cell applications, and targeting CD33 could be used as a ‘bridge” therapy for patients coming to allogeneic hematopoietic stem cell transplant, as anti-leukemia activity from infusing CAR.CD33 T-cells has been demonstrated in an ongoing clinical trial. Albeit never performed in the clinical setting, our future plan is to investigate the utility of iC9-CAR.CD33 T-cells as part of the conditioning therapy for an allogeneic hematopoietic stem cell transplant for acute myeloid leukemia, together with other myelosuppressive agents, whilst the activation of the inducible Caspase9 suicide gene would grant elimination of the infused gene modified T-cells prior to stem cell infusion to reduce the risk of engraftment failure as the CD33 is also expressed on a proportion of the donor stem cell graft.

The Inflammatory Role of Platelets via Their TLRs and Siglec Receptors

Platelets are non-nucleated cells that play central roles in the processes of hemostasis, innate immunity, and inflammation; however, several reports show that these distinct functions are more closely linked than initially thought. Platelets express numerous receptors and contain hundreds of secretory products. These receptors and secretory products are instrumental to the platelet functional responses. The capacity of platelets to secrete copious amounts of cytokines, chemokines, and related molecules appears intimately related to the role of the platelet in inflammation. Platelets exhibit non-self-infectious danger detection molecules on their surfaces, including those belonging to the “toll-like receptor” family, as well as pathogen sensors of other natures (Ig- or complement receptors, etc.). These receptors permit platelets to both bind infectious agents and deliver differential signals leading to the secretion of cytokines/chemokines, under the control of specific intracellular regulatory pathways. In contrast, dysfunctional receptors or dysregulation of the intracellular pathway may increase the susceptibility to pathological inflammation. Physiological vs. pathological inflammation is tightly controlled by the sensors of danger expressed in resting, as well as in activated, platelets. These sensors, referred to as pathogen recognition receptors, primarily sense danger signals termed pathogen associated molecular patterns. As platelets are found in inflamed tissues and are involved in auto-immune disorders, it is possible that they can also be stimulated by internal pathogens. In such cases, platelets can also sense danger signals using damage associated molecular patterns (DAMPs). Some of the most significant DAMP family members are the alarmins, to which the Siglec family of molecules belongs. This review examines the role of platelets in anti-infection immunity via their TLRs and Siglec receptors.

Role of Siglec-7 in apoptosis in human platelets

Background

Platelets participate in tissue repair and innate immune responses. Sialic acid-binding immunoglobulin-like lectins (Siglecs) are well-characterized I-type lectins, which control apoptosis.

Methodology/Principal Findings

We characterized the expression of Siglec-7 in human platelets isolated from healthy volunteers using flow cytometry and confocal microscopy. Siglec-7 is primarily expressed on α granular membranes and colocalized with CD62P. Siglec-7 expression was increased upon platelet activation and correlated closely with CD62P expression. Cross-linking Siglec-7 with its ligand, ganglioside, resulted in platelet apoptosis without any significant effects on activation, aggregation, cell morphology by electron microscopy analysis or secretion. We show that ganglioside triggered four key pathways leading to apoptosis in human platelets: (i) mitochondrial inner transmembrane potential (ΔΨm) depolarization; (ii) elevated expression of pro-apoptotic Bax and Bak proteins with reduced expression of anti-apoptotic Bcl-2 protein; (iii) phosphatidylserine exposure and (iv), microparticle formation. Inhibition of NAPDH oxidase, PI3K, or PKC rescued platelets from apoptosis induced by Siglec-7 recruitment, suggesting that the platelet receptors P2Y1 and GPIIbIIIa are essential for ganglioside-induced platelet apoptosis.

Conclusions/Significance

The present work characterizes the role of Siglec-7 and platelet receptors in regulating apoptosis and death. Because some platelet pathology involves apoptosis (idiopathic thrombocytopenic purpura and possibly storage lesions), Siglec-7 might be a molecular target for therapeutic intervention/prevention.

Refolded recombinant Siglec5 for NMR investigation of complex carbohydrate binding

Sialic-acid-binding immunoglobulin-like lectin (Siglec5) is a carbohydrate-binding surface receptor expressed on neutrophils, monocytes and B cells in human lymphoid and myeloid cell lineages. Existing structural and functional data fail to define the clear ligand specificity of Siglec5, though like other Siglec family members, it binds a variety of complex carbohydrates containing a sialic acid at the non-reducing terminus. Prokaryotic expression of this protein has proven challenging due to disulfide bonds and Asn-linked glycosylation. We developed an expression and purification protocol that uses an on-column strategy to refold Escherichia coli expressed protein that produced a high yield (2 mg/L) of the single N-terminal Siglec5 carbohydrate recognition domain (CRD). A 2D heteronuclear single-quantum coherence (HSQC) nuclear magnetic resonance (NMR) spectrum showed this material was folded, and a secondary structure prediction based on the assigned chemical shifts of backbone atoms was consistent with a previously determined X-ray model. NMR chemical shift mapping of Siglec5 binding to three carbohydrate ligands revealed similarities in binding interfaces and affinities. In addition, the role of alternate protein conformations identified by NMR in ligand binding is discussed. These studies demonstrate the Siglec5 CRD alone is sufficient for binding sialylated carbohydrates and provide a foundation for further investigation of Siglec5 structure and function.

Mapping the N-glycome of human von Willebrand factor

vWF (von Willebrand factor) is a key component for maintenance of normal haemostasis, acting as the carrier protein of the coagulant Factor VIII and mediating platelet adhesion at sites of vascular injury. There is ample evidence that vWF glycan moieties are crucial determinants of its expression and function. Of particular clinical interest, ABH antigens influence vWF plasma levels according to the blood group of individuals, although the molecular mechanism underlying this phenomenon remains incompletely understood. The present paper reports analyses of the human plasma vWF N-glycan population using advanced MS. Glycomics analyses revealed approximately 100 distinct N-glycan compositions and identified a variety of structural features, including lactosaminic extensions, ABH antigens and sulfated antennae, as well as bisecting and terminal GlcNAc residues. We estimate that some 300 N-glycan structures are carried by human vWF. Glycoproteomics analyses mapped ten of the consensus sites known to carry N-glycans. Glycan populations were found to be distinct, although many structural features were shared across all sites. Notably, the H antigen is not restricted to particular N-glycosylation sites. Also, the Asn(2635) site, previously designated as unoccupied, was found to be highly glycosylated. The delineation of such varied glycan populations in conjunction with current models explaining vWF activity will facilitate research aimed at providing a better understanding of the influence of glycosylation on vWF function.

Siglec-8 as a drugable target to treat eosinophil and mast cell-associated conditions

Siglecs (sialic acid immunoglobulin-like lectins) are members of the immunoglobulin gene family that contain sialoside binding N-terminal domains. They are cell surface proteins found predominantly on cells of the immune system. Among them, Siglec-8 is uniquely expressed by human eosinophils and mast cells, as well as basophils. Engaging this structure with antibodies or glycan ligands results in apoptosis in human eosinophils and inhibition of release of preformed and newly generated mediators from human mast cells without affecting their survival. Pro-apoptotic effects are also seen when its closest functional paralog, Siglec-F, on mouse eosinophils is similarly engaged in vitro, and beneficial effects are observed after administration of Siglec-F antibody using models of eosinophilic pulmonary and gastrointestinal inflammation in vivo. Siglec-8 targeting may thus provide a means to specifically inhibit or deplete these cell types. Cell-directed therapies are increasingly sought after by the pharmaceutical industry for their potential to reduce side effects and increase safety. The challenge is to identify suitable targets on the cell type of interest, and selectively deliver a therapeutic agent. By targeting Siglec-8, monoclonal antibodies and glycan ligand-conjugated nanoparticles may be ideally suited for treatment of eosinophil and mast cell-related diseases, such as asthma, chronic rhinosinusitis, chronic urticaria, hypereosinophilic syndromes, mast cell and eosinophil malignancies and eosinophilic gastrointestinal disorders.

Siglec-7 tetramers characterize B-cell subpopulations and leukemic blasts

Cell surface glycosylation has important regulatory functions in the maturation, activation, and homeostasis of lymphocytes. The family of human sialic acid-binding immunoglobulin-like lectins (siglecs) comprises inhibitory as well as activating receptors intimately involved in the regulation of immune responses. Analyses of the interaction between siglecs and glycans are hampered by the low affinity of this interaction. Therefore, we expressed siglec-7 in eukaryotic cells, allowing for glycosylation, and oligomerized the protein in analogy to MHC tetramers. Using this tool, flow cytometric analysis of lymphocytes became possible. Sialic acid-dependent binding of siglec-7 tetramers was confirmed by glycan array analysis and loss of siglec tetramer binding after neuraminidase treatment of lymphocytes. In contrast to most lymphocyte subpopulations, which showed high siglec-7 ligand expression, B-cell subpopulations could be further subdivided according to different siglec-7 ligand expression levels. We also analyzed blasts from acute lymphoblastic leukemias of the B-cell lineage as well as the T-cell lineage, since malignant transformation is often associated with aberrant cell surface glycosylation. While pediatric T-ALL blasts highly expressed siglec-7 ligands, siglec-7 ligands were barely detectable on cALL blasts. Taken together, oligomerization of recombinant soluble siglec-7 enabled flow cytometric identification of physiologic lymphocyte subpopulations and malignant blasts.

Microbiota and dietary interactions: an update to the hygiene hypothesis?

The dramatic increase in the incidence and severity of allergy and asthma has been proposed to be linked with an altered exposure to, and colonization by, micro-organisms, particularly early in life. However, other lifestyle factors such as diet and physical activity are also thought to be important, and it is likely that multiple environmental factors with currently unrecognized interactions contribute to the atopic state. This review will focus on the potential role of microbial metabolites in immunoregulatory functions and highlights the known molecular mechanisms, which may mediate the interactions between diet, microbiota, and protection from allergy and asthma.

In Vitro and In Vivo Antitumor Effect of Anti-CD33 Chimeric Receptor-Expressing EBV-CTL against CD33 Acute Myeloid Leukemia

Genetic engineering of T cells with chimeric T-cell receptors (CARs) is an attractive strategy to treat malignancies. It extends the range of antigens for adoptive T-cell immunotherapy, and major mechanisms of tumor escape are bypassed. With this strategy we redirected immune responses towards the CD33 antigen to target acute myeloid leukemia. To improve in vivo T-cell persistence, we modified human Epstein Barr Virus-(EBV-) specific cytotoxic T cells with an anti-CD33.CAR. Genetically modified T cells displayed EBV and HLA-unrestricted CD33 bispecificity in vitro. In addition, though showing a myeloablative activity, they did not irreversibly impair the clonogenic potential of normal CD34(+) hematopoietic progenitors. Moreover, after intravenous administration into CD33(+) human acute myeloid leukemia-bearing NOD-SCID mice, anti-CD33-EBV-specific T cells reached the tumor sites exerting antitumor activity in vivo. In conclusion, targeting CD33 by CAR-modified EBV-specific T cells may provide additional therapeutic benefit to AML patients as compared to conventional chemotherapy or transplantation regimens alone.

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.

Chemoenzymatic synthesis of sialooligosaccharides on arrays for studies of cell surface adhesion

Sialooligosaccharides were generated by direct enzymatic glycosylation on arrays and the resulting surfaces were suitable for the study of carbohydrate-specific cell adhesion.

Campylobacter jejuni lipooligosaccharides modulate dendritic cell-mediated T cell polarization in a sialic acid linkage-dependent manner

Carbohydrate mimicry between Campylobacter jejuni lipooligosaccharides (LOS) and host neural gangliosides plays a crucial role in the pathogenesis of Guillain-Barré syndrome (GBS). Campylobacter jejuni LOS may mimic various gangliosides, which affects the immunogenicity and the type of neurological deficits in GBS patients. Previous studies have shown the interaction of LOS with sialic acid-specific siglec receptors, although the functional consequences remain unknown. Cells that express high levels of siglecs include dendritic cells (DCs), which are crucial for initiation and differentiation of immune responses. We confirm that α2,3-sialylated GD1a/GM1a mimic and α2,8-sialylated GD1c mimic LOS structures interact with recombinant Sn and siglec-7, respectively. Although the linkage of the terminal sialic acid of LOS did not regulate expression of DC maturation markers, it displayed clear opposite expression levels of interleukin-12 (IL-12) and OX40L, molecules involved in DC-mediated Th cell differentiation. Accordingly, targeting DC-expressed siglec-7 with α2,8-linked sialylated LOS resulted in Th1 responses, whereas Th2 responses were induced by targeting with LOS containing α2,3-linked sialic acid. Thus, our data demonstrate for the first time that depending on the sialylated composition of Campylobacter jejuni LOS, specific Th differentiation programs are initiated, possibly through targeting of distinct DC-expressed siglecs.

Epitope mapping, expression and post-translational modifications of two isoforms of CD33 (CD33M and CD33m) on lymphoid and myeloid human cells

We have tested the usefulness of several commercial anti-CD33 monoclonal antibodies (mAb) to determine the expression and localization of the two CD33 isoforms on several hematopoietic cell lines. The expression of the isoform CD33m, a CD33 transmembrane splice variant lacking the ligand-binding V immunoglobulin (Ig)-like domain, was detected by RT-polymerase chain reaction, western blot, confocal microscopy and flow cytometry on the membrane of several human cell types. CD33m was only detected by the anti-CD33 mAb HIM3-4 on the cell surface, whereas WM53, P67.6, 4D3, HIM3-4, WM54, D3HL60.251 or MY9 detected the CD33M isoform, indicating that HIM3-4 is the only mAb recognizing CD33 C(2) Ig domain. Accordingly, HIM3-4 binding to CD33 did not interfere with the binding of other antibodies against the CD33 V-domain. P67.6 mAb interfered with recognition by the rest of antibodies specific for the V domain. HIM3-4 staining could be increased after the sialidase treatment of all CD33(+) cells. However, this increase was stronger in activated T cells, suggesting a CD33 masking state in this cell population. Confocal microscopy analysis of CD33m HEK 293T-transfected cells revealed that this protein is expressed on the cell membrane and also detected in the Golgi compartment. CD33 is constitutively located outside the lipid raft domains, whereas cross-linked CD33 is highly recruited to this signaling platform. The unique ability of HIM3-4 mAb to detect the masking state of CD33 on different cell lineages makes it a good tool to improve the knowledge of the biological role of this sialic acid-binding Ig-like lectin.

Chemoenzymatic synthesis of α2-3-sialylated carbohydrate epitopes

Sialic acids are common terminal carbohydrates on cell surface. Together with internal carbohydrate structures, they play important roles in many physiological and pathological processes. In order to obtain α2-3-sialylated oligosaccharides, a highly efficient one-pot three-enzyme synthetic approach was applied. The P. multocida α2-3-sialyltransferase (PmST1) involved in the synthesis was a multifunctional enzyme with extremely flexible donor and acceptor substrate specificities. Sialyltransferase acceptors, including type 1 structure (Galβ1-3GlcNAcβProN(3)), type 2 structures (Galβ1-4GlcNAcβProN(3) and 6-sulfo-Galβ1-4GlcNAcβProN(3)), type 4 structure (Galβ1-3GalNAcβProN(3)), type 3 or core 1 structure (Galβ1-3GalNAcαProN(3)) and human milk oligosaccharide or lipooligosaccharide lacto-N-tetraose (LNT) (Galβ1-3GlcNAcβ1-3Galβ1-4GlcβProN(3)), were chemically synthesized. They were then used in one-pot three-enzyme reactions with sialic acid precursor ManNAc or ManNGc, to synthesize a library of natural occurring α2-3-linked sialosides with different internal sugar units. The sialylated oligosaccharides obtained are valuable probes for their biological studies.

Sialic acids as regulators of molecular and cellular interactions

The wide occurrence of sialic acids (Sia) in various chemical forms linked as monomers or polymers in an outstanding position in a multitude of complex carbohydrates of animals and microorganisms renders them as most versatile function modulators in cell biology and pathology. A survey is presented of recent advances in the study of the influences that Sias have as bulky hydrophilic and electronegatively charged monosaccharides on animal cells and on their interaction with microorganisms. Some highlights are: sialylation leads to increased anti-inflammatory activity of IgG antibodies, facilitates the escape of microorganisms from the host's immune system, and in polymeric form is involved in the regulation of embryogenesis and neuronal growth and function. The role of siglecs in immunoregulation, the dynamics of lymphocyte binding to selectins and the interactions of toxins, viruses, and other microorganisms with the host's Sia are now better understood. N-Glycolylneuraminic acid from food is antigenic in man and seems to have pathogenic potential. Sia O-acetylation mediated by various eukaryotic and prokaryotic O-acetyltransferases modulates the affinity of these monosaccharides to mammalian and microbial receptors and hinders apoptosis. The functionally versatile O-acetylated ganglioside GD3 is an onco-fetal antigen.

Siglec receptors and hiding plaques in Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disease. One hallmark of this disease is the continuous increase in the numbers and size of aggregating amyloid plaques. The accumulation of extracellular plaques is an immunologically interesting phenomenon since microglial cells, brain-specific macrophages, should be able to cleanse the aggregating material from the human brain. Immunotherapy targeting beta-amyloid peptides in plaques with antibodies represents a promising therapy in AD. Recent progress in pattern recognition receptors of monocytes and macrophages has revealed that the sialic acid-binding, immunoglobulin-like lectin (Siglec) family of receptors is an important recognition receptor for sialylated glycoproteins and glycolipids. Interestingly, recent studies have revealed that microglial cells contain only one type of Siglec receptors, Siglec-11, which mediates immunosuppressive signals and thus inhibits the function of other microglial pattern recognition receptors, such as TLRs, NLRs, and RAGE receptors. We will review here the recent literature which clearly indicates that aggregating amyloid plaques are masked in AD by sialylated glycoproteins and gangliosides. Sialylation and glycosylation of plaques, mimicking the cell surface glycocalyx, can activate the immunosuppressive Siglec-11 receptors, as well as hiding the neuritic plaques, allowing them to evade the immune surveillance of microglial cells. This kind of immune evasion can prevent the microglial cleansing process of aggregating amyloid plaques in AD.

Basic and clinical immunology of Siglecs

Siglecs are cell-surface proteins found primarily on hematopoietic cells. By definition, they are members of the immunoglobulin gene super-family and bind sialic acid. Most contain cytoplasmic tyrosine motifs implicated in cell signaling. This review will first summarize characteristics common and unique to Siglecs, followed by a discussion of each human Siglec in numerical order, mentioning in turn its closest murine ortholog or paralog. Each section will describe its pattern of cellular expression, latest known immune functions, ligands, and signaling pathways, with the focus being predominantly on CD33-related Siglecs. Potential clinical and therapeutic implications of each Siglec will also be covered.

Siglecs as positive and negative regulators of the immune system

Siglecs (sialic acid-binding Ig-like lectins) are mainly expressed in the immune system. Sn (sialoadhesin) (siglec-1), CD22 (siglec-2) and siglec-15 are well conserved, whereas the CD33-related siglecs are undergoing rapid evolution, as reflected in large differences in repertoires among the different mammals studied so far. In the present paper, we review recent findings on the signalling properties of the CD33-related siglecs and discuss the emergence of both inhibitory and activating forms of this family. We also discuss how Sn may function as a positive regulator of adaptive immune responses and its emerging role as an induced macrophage pattern-recognition molecule for sialylated pathogens, especially enveloped viruses.