Generation of mice deficient for macrophage galactose- and N-acetylgalactosamine-specific lectin: limited role in lymphoid and erythroid homeostasis and evidence for multiple lectins

Role of a 49 kDa Trypanosoma cruzi Mucin-Associated Surface Protein (MASP49) during the Infection Process and Identification of a Mammalian Cell Surface Receptor

Trypanosoma cruzi is the etiologic agent of Chagas disease, a parasitic disease of great medical importance on the American continent. Trypomastigote infection's initial step in a mammalian host is vital for the parasite's life cycle. A trypomastigote's surface presents many molecules, some of which have been proposed to be involved in the infection process, including a glycoprotein family called mucin-associated surface proteins (MASPs). This work describes a 49-kDa molecule (MASP49) that belongs to this family and is expressed mainly on the surfaces of amastigotes and trypomastigotes but can be found in extracts and the membrane-enriched fractions of epimastigotes. This protein is partially GPI-anchored to the surface and has a role during the internalization process, since its blockade with specific antibodies decreases parasite entry into Vero cells by 62%. This work shows that MASP49 binds to peritoneal macrophages and rat cardiomyocytes, undergoes glycosylation via galactose N-acetylgalactosamine, and can attach to the macrophage murine C-type lectin receptor (mMGL). These results suggest that MASP49 can be considered a virulence factor in T. cruzi, and a better understanding of its role in the infection process is necessary.

Toxocara canis and Toxocara cati Somatic and Excretory-Secretory Antigens Are Recognised by C-Type Lectin Receptors

Toxocara canis and Toxocara cati, the worldwide occurring intestinal roundworms of canids and felids, represent an important public health threat due to various disease manifestations in humans. Host recognition of pathogens is mediated by pattern recognition receptors (PRRs). Myeloid C-type lectin receptors (CLRs) are PRRs and recognise carbohydrate structures of various pathogens. As Toxocara excretory-secretory products (TES) are predominantly composed of glycoconjugates, they represent suitable targets for CLRs. However, the range of host-derived CLRs recognising Toxocara spp. is still unknown. Using a CLR-hFc fusion protein library, T. canis and T. cati L3 somatic antigens (TSOM) were bound by a variety of CLRs in enzyme-linked immunosorbent assay (ELISA), while their TES products interacted with macrophage galactose-type lectin-1 (MGL-1). Two prominent candidate CLRs, MGL-1 and macrophage C-type lectin (MCL), were selected for further binding studies. Immunofluorescence microscopy revealed binding of MGL-1 to the oral aperture of L3. Immunoblot experiments identified distinct protein fractions representing potential ligands for MGL-1 and MCL. To evaluate how these interactions influence the host immune response, bone marrow-derived dendritic cell (BMDC) assays were performed, showing MCL-dependent T. cati-mediated cytokine production. In conclusion, MGL-1 and MCL are promising candidates for immune modulation during Toxocara infection, deserving further investigation in the future.

Intestinal lamina propria macrophages upregulate interleukin-10 mRNA in response to signals from commensal bacteria recognized by MGL1/CD301a

Ligand-induced cellular signaling involved in interleukin 10 (IL-10) production by lamina propria macrophages (LPMs) during their interactions with commensal bacteria is not clearly understood. We previously showed, using mice lacking a C-type lectin MGL1/CD301a, that this molecule on colonic LPMs plays an important role in the induction of IL-10 upon interaction with commensal bacteria, Streptococcus sp.

In the present report, we show that the physical engagement of MGL1/CD301a on LPMs with in-situ isolated Streptococcus sp. bacteria leads to IL-10 messenger RNA (mRNA) induction. Spleen tyrosine kinase (Syk), caspase recruitment domain 9 (CARD9) and extracellular signal-regulated kinase (ERK), but not NF-κB pathway, are shown to be indispensable for IL-10 mRNA induction after stimulation with heat-killed Streptococcus sp. Guanidine hydrochloride treatment of Streptococcus sp., which is known to extract bacterial cell surface glycan-rich components, abolished bacterial binding to recombinant MGL1/CD301a. The extract contained materials which bound rMGL1 in ELISA and appeared to induce IL-10 mRNA expression in LPMs in vitro. Lectin blotting showed that the extract contained glycoproteins that are considered as putative ligands for MGL1. Some human commensal Lactobacillus species also induced IL-10 mRNA expression by colonic LPMs in vitro, which depends on the presence of MGL1/CD301a and CARD9. The present results are the first to show that MGL1/CD301a acts as a signal transducer during colonic host–microbe interactions.

MGL1 Receptor Plays a Key Role in the Control of T. cruzi Infection by Increasing Macrophage Activation through Modulation of ERK1/2, c-Jun, NF-κB and NLRP3 Pathways

Macrophage galactose-C type lectin (MGL)1 receptor is involved in the recognition of Trypanosoma cruzi (T. cruzi) parasites and is important for the modulation of the innate and adaptive immune responses. However, the mechanism by which MGL1 promotes resistance to T. cruzi remains unclear. Here, we show that MGL1 knockout macrophages (MGL1^-/- Mφ) infected in vitro with T. cruzi were heavily parasitized and showed decreased levels of reactive oxygen species (ROS), nitric oxide (NO), IL-12 and TNF-α compared to wild-type macrophages (WT Mφ). MGL1^-/- Mφ stimulated in vitro with T. cruzi antigen (TcAg) showed low expression of TLR-2, TLR-4 and MHC-II, which resulted in deficient splenic cell activation compared with similar co-cultured WT Mφ. Importantly, the activation of p-ERK1/2, p-c-Jun and p-NF-κB p65 were significantly reduced in MGL1^-/- Mφ exposed to TcAg. Similarly, procaspase 1, caspase 1 and NLRP3 inflammasome also displayed a reduced expression that was associated with low IL-β production. Our data reveal a previously unappreciated role for MGL1 in Mφ activation through the modulation of ERK1/2, c-Jun, NF-κB and NLRP3 signaling pathways, and to the development of protective innate immunity against experimental T. cruzi infection.

Clec10a regulates mite-induced dermatitis

House dust mite (HDM) is a major allergen that causes allergic diseases such as atopic dermatitis. However, the regulatory mechanisms of HDM-induced immune responses are incompletely understood. NC/Nga mice are an inbred strain that is more susceptible to HDM and develops more severe dermatitis than other strains. Using whole-exome sequencing, we found that NC/Nga mice carry a stop-gain mutation inClec10a, which encodes a C-type lectin receptor, Clec10a (MGL1/CD301a). The repair of this gene mutation using the CRISPR-Cas9 system ameliorated HDM-induced dermatitis, indicating that the Clec10a mutation is responsible for hypersensitivity to HDM in NC/Nga mice. Similarly,Clec10a−/−mice on the C57BL/6J background showed exacerbated HDM-induced dermatitis. Clec10a expressed on skin macrophages inhibits HDM-induced Toll-like receptor 4 (TLR4)–mediated inflammatory cytokine production through the inhibitory immunoreceptor tyrosine activating motif in its cytoplasmic portion. We identified asialoglycoprotein receptor 1 (Asgr1) as a functional homolog of mouse Clec10a in humans. Moreover, we found that a mucin-like molecule in HDM is a ligand for mouse Clec10a and human Asgr1. Skin application of the ligand ameliorated a TLR4 ligand-induced dermatitis in mice. Our findings suggest that Clec10a in mice and Asgr1 in humans play an important role in skin homeostasis against inflammation associated with HDM-induced dermatitis.

Macrophage galactose-type lectin (MGL) is induced on M2 microglia and participates in the resolution phase of autoimmune neuroinflammation

BACKGROUND

Multiple sclerosis (MS) involves a misdirected immune attack against myelin in the brain and spinal cord, leading to profound neuroinflammation and neurodegeneration. While the mechanisms of disease pathogenesis have been widely studied, the suppression mechanisms that lead to the resolution of the autoimmune response are still poorly understood. Here, we investigated the role of the C-type lectin receptor macrophage galactose-type lectin (MGL), usually expressed on tolerogenic antigen-presenting cells (APCs), as a negative regulator of autoimmune-driven neuroinflammation.

METHODS

We used in silico, immunohistochemical, immunofluorescence, quantitative real-time polymerase chain reaction (qRT-PCR) and flow cytometry analysis to explore the expression and functionality of MGL in human macrophages and microglia, as well as in MS post-mortem tissue. In vitro, we studied the capacity of MGL to mediate apoptosis of experimental autoimmune encephalomyelitis (EAE)-derived T cells and mouse CD4⁺ T cells. Finally, we evaluated in vivo and ex vivo the immunomodulatory potential of MGL in EAE.

RESULTS

MGL plays a critical role in the resolution phase of EAE as MGL1-deficient (Clec10a^-/-) mice showed a similar day of onset but experienced a higher clinical score to that of WT littermates. We demonstrate that the mouse ortholog MGL1 induces apoptosis of autoreactive T cells and diminishes the expression of pro-inflammatory cytokines and inflammatory autoantibodies. Moreover, we show that MGL1 but not MGL2 induces apoptosis of activated mouse CD4⁺ T cells in vitro. In human settings, we show that MGL expression is increased in active MS lesions and on alternatively activated microglia and macrophages which, in turn, induces the secretion of the immunoregulatory cytokine IL-10, underscoring the clinical relevance of this lectin.

CONCLUSIONS

Our results show a new role of MGL-expressing APCs as an anti-inflammatory mechanism in autoimmune neuroinflammation by dampening pathogenic T and B cell responses, uncovering a novel clue for neuroprotective therapeutic strategies with relevance for in MS clinical applications.

Metabolic Glycoengineering with N-Acyl Side Chain Modified Mannosamines

In metabolic glycoengineering (MGE), cells or animals are treated with unnatural derivatives of monosaccharides. After entering the cytosol, these sugar analogues are metabolized and subsequently expressed on newly synthesized glycoconjugates. The feasibility of MGE was first discovered for sialylated glycans, by using N-acyl-modified mannosamines as precursor molecules for unnatural sialic acids. Prerequisite is the promiscuity of the enzymes of the Roseman-Warren biosynthetic pathway. These enzymes were shown to tolerate specific modifications of the N-acyl side chain of mannosamine analogues, for example, elongation by one or more methylene groups (aliphatic modifications) or by insertion of reactive groups (bioorthogonal modifications). Unnatural sialic acids are incorporated into glycoconjugates of cells and organs. MGE has intriguing biological consequences for treated cells (aliphatic MGE) and offers the opportunity to visualize the topography and dynamics of sialylated glycans in vitro, ex vivo, and in vivo (bioorthogonal MGE).

Liver-targeted antiviral peptide nanocomplexes as potential anti-HCV therapeutics

Great success in HCV therapy was achieved by the development of direct-acting antivirals (DAA). However, the unsolved issues such as high cost and genotype dependency drive us to pursue additional therapeutic agents to be used instead or in combination with DAA. The cationic peptide p41 is one of such candidates displaying submicromolar anti-HCV potency. By electrostatic coupling of p41 with anionic poly(amino acid)-based block copolymers, antiviral peptide nanocomplexes (APN) platform was developed to improve peptide stability and to reduce cytotoxicity associated with positive charge. Herein, we developed a facile method to prepare galactosylated Gal-APN and tested their feasibility as liver-specific delivery system. In vitro, Gal-APN displayed specific internalization in hepatoma cell lines. Even though liver-targeted and non-targeted APN displayed comparable antiviral activity, Gal-APN offered prominent advantages to prevent HCV association with lipid droplets and suppress intracellular expression of HCV proteins. Moreover, in vivo preferential liver accumulation of Gal-APN was revealed in the biodistribution study. Altogether, this work illustrates the potential of Gal-APN as a novel liver-targeted therapy against HCV.

Determination of RBC Survival in C57BL/6 and C57BL/6-Tg(UBC-GFP) Mice

Although several methods for determining erythrocyte lifespan are used in research studies that involve mice, all involve the alteration of RBC to allow for its tracking over time, which may affect overall RBC survival. The aims of this study were to determine 1) whether sex affects RBC survival; 2) whether RBC survival differs between the biotin method and an alternative method that uses GFP; and 3) whether repeat exposure of mice to biotin results in an antibiotin antibody response or decreased RBC survival. The results suggest no difference in the RBC half-life between male and female C57BL/6 mice (22.9 ± 1.2 and 22.4 ± 0.9 d, respectively). In addition, RBC half-life did not differ between the biotin- and GFP-based methods (20.5 ± 2.1 d and 22.7 ± 2.1 d, respectively). Finally, retransfusion of mice 90 d after an initial transfusion with biotin-labeled RBC did not induce detectable antibiotin antibodies nor alter the half-life of transfused biotin-labeled RBC (initial transfusion, 22.0 ± 1.2 d; subsequent transfusion, 23.4 ± 1.4 d, respectively).

Mouse macrophage galactose-type lectin (mMGL) is critical for host resistance against Trypanosoma cruzi infection

The C-type lectin receptor mMGL is expressed exclusively by myeloid antigen presenting cells (APC) such as dendritic cells (DC) and macrophages (Mφ), and it mediates binding to glycoproteins carrying terminal galactose and α- or β-N-acetylgalactosamine (Gal/GalNAc) residues. Trypanosoma cruzi (T. cruzi) expresses large amounts of mucin (TcMUC)-like glycoproteins. Here, we show by lectin-blot that galactose moieties are also expressed on the surface of T. cruzi. Male mMGL knockout (-/-) and wild-type (WT) C57BL/6 mice were infected intraperitoneally with 10⁴T. cruzi trypomastigotes (Queretaro strain). Following T. cruzi infection, mMGL-/- mice developed higher parasitemia and higher mortality rates compared with WT mice. Although hearts from T. cruzi-infected WT mice presented few amastigote nests, mMGL-/- mice displayed higher numbers of amastigote nests. Compared with WT, Mφ from mMGL-/- mice had low production of nitric oxide (NO), interleukin (IL)-12 and tumor necrosis factor (TNF)-α in response to soluble T. cruzi antigens (TcAg). Interestingly, upon in vitro T. cruzi infection, mMGL-/- Mφ expressed lower levels of MHC-II and TLR-4 and harbored higher numbers of parasites, even when mMGL-/- Mφ were previously primed with IFN-γ or LPS/IFN-γ. These data suggest that mMGL plays an important role during T. cruzi infection, is required for optimal Mφ activation, and may synergize with TLR-4-induced pathways to produce TNF-α, IL-1β and NO during the early phase of infection.

Comprehensive functional analysis of N-linked glycans on Ebola virus GP1

Ebola virus (EBOV) entry requires the virion surface-associated glycoprotein (GP) that is composed of a trimer of heterodimers (GP1/GP2). The GP1 subunit contains two heavily glycosylated domains, the glycan cap and the mucin-like domain (MLD). The glycan cap contains only N-linked glycans, whereas the MLD contains both N- and O-linked glycans. Site-directed mutagenesis was performed on EBOV GP1 to systematically disrupt N-linked glycan sites to gain an understanding of their role in GP structure and function. All 15 N-glycosylation sites of EBOV GP1 could be removed without compromising the expression of GP. The loss of these 15 glycosylation sites significantly enhanced pseudovirion transduction in Vero cells, which correlated with an increase in protease sensitivity. Interestingly, exposing the receptor-binding domain (RBD) by removing the glycan shield did not allow interaction with the endosomal receptor, NPC1, indicating that the glycan cap/MLD domains mask RBD residues required for binding. The effects of the loss of GP1 N-linked glycans on Ca(2+)-dependent (C-type) lectin (CLEC)-dependent transduction were complex, and the effect was unique for each of the CLECs tested. Surprisingly, EBOV entry into murine peritoneal macrophages was independent of GP1 N-glycans, suggesting that CLEC-GP1 N-glycan interactions are not required for entry into this important primary cell. Finally, the removal of all GP1 N-glycans outside the MLD enhanced antiserum and antibody sensitivity. In total, our results provide evidence that the conserved N-linked glycans on the EBOV GP1 core protect GP from antibody neutralization despite the negative impact the glycans have on viral entry efficiency.

IMPORTANCE

Filovirus outbreaks occur sporadically throughout central Africa, causing high fatality rates among the general public and health care workers. These unpredictable hemorrhagic fever outbreaks are caused by multiple species of Ebola viruses, as well as Marburg virus. While filovirus vaccines and therapeutics are being developed, there are no licensed products. The sole viral envelope glycoprotein, which is a principal immunogenic target, contains a heavy shield of glycans surrounding the conserved receptor-binding domain. We find that disruption of this shield through targeted mutagenesis leads to an increase in cell entry, protease sensitivity, and antiserum/antibody sensitivity but is not sufficient to allow virion binding to the intracellular receptor NPC1. Therefore, our studies provide evidence that filoviruses maintain glycoprotein glycosylation to protect against proteases and antibody neutralization at the expense of efficient entry. Our results unveil interesting insights into the unique entry process of filoviruses and potential immune evasion tactics of the virus.

Copy-number variation of functional galectin genes: studying animal galectin-7 (p53-induced gene 1 in man) and tandem-repeat-type galectins-4 and -9

Galectins are potent adhesion/growth-regulatory effectors with characteristic expression profiles. Understanding the molecular basis of gene regulation in each case requires detailed information on copy number of genes and sequence(s) of their promoter(s). Our report reveals plasticity in this respect between galectins and species. We here describe occurrence of a two-gene constellation for human galectin (Gal)-7 and define current extent of promoter-sequence divergence. Interestingly, cross-species genome analyses also detected single-copy display. Because the regulatory potential will then be different, extrapolations of expression profiles are precluded between respective species pairs. Gal-4 coding in chromosomal vicinity was found to be confined to one gene, whereas copy-number variation also applied to Gal-9. The example of rat Gal-9 teaches the lesson that the presence of multiple bands in Southern blotting despite a single-copy gene constellation is attributable to two pseudogenes. The documented copy-number variability should thus be taken into consideration when studying regulation of galectin genes, in a species and in comparison between species.

Application of galactose-modified liposomes as a potent antigen presenting cell targeted carrier for intranasal immunization

The mucosal immune system produces secretory IgA (sIgA) as the first line of defense against invasion by foreign pathogens. Our aim was to develop a galactose-modified liposome as a targeted carrier which can be specifically recognized by macrophage, one of the most important antigen presenting cells. First, galactose was covalently conjugated with 1,2-didodecanoyl-sn-glycero-3-phosphoethanolamine (DLPE) to give a targeted ligand, a galactosyl lipid. The galactosyl lipid was then incorporated into a liposomal bilayer to form a galactosylated liposome carrier. Further, the ovalbumin (OVA) was encapsulated into the galactosylated liposome carriers and mice were intranasally immunized. Confocal laser scanning microscopy and flow cytometry analysis showed that the targeted galactosylated liposome carrier had a higher uptake rate than unmodified liposomes. The targeted galactosylated liposome induced higher levels of tumor necrosis factor-α and interleukin-6 production than unmodified liposomes (P<0.05). Furthermore, 6-week-old BALB/c female mice immunized with the OVA-encapsulated targeted galactosylated liposome had significantly higher OVA-specific s-IgA levels in the nasal and lung wash fluid (P<0.05). In addition, the targeted galactosylated liposome simultaneously augmented the serum IgG antibody response. In summary, the OVA-encapsulated targeted galactosylated liposome induced significantly higher mucosal IgA and systemic IgG antibody titers and is a potential antigen delivery carrier for further clinical applications.

Context-dependent multifunctionality of galectin-1: a challenge for defining the lectin as therapeutic target

INTRODUCTION

One route of translating the information encoded in the glycan chains of cellular glycoconjugates into physiological effects is via receptor (lectin) binding. A family of endogenous lectins, sharing folding, a distinct sequence signature and affinity for β-galactosides (thus termed galectins), does so effectively in a context-dependent manner.

AREAS COVERED

An overview is given on the multifunctional nature of galectins, with emphasis on galectin-1. The broad range of functions includes vital processes such as adhesion via glycan bridging, glycoconjugate transport or triggering signaling relevant, for example, for growth regulation. Besides distinct glycoconjugates, this lectin can also interact with certain proteins so that it can target counterreceptors at all sites of location, that is, in the cytoplasm and/or nucleus, at both sides of the membrane or extracellularly. Approaches to strategically exploit galectin activities with therapeutic intentions are outlined.

EXPERT OPINION

The wide versatility of sugar coding and the multifunctionality of galectin-1 explain why considering to turn the protein into a therapeutic target is an ambitious aim. Natural pathways shaped by physiologic master regulators (e.g., the tumor suppressor p16(INK4a)) are suggested to teach inspiring lessons as to how the lectin might be recruited to clinical service.

A phenotype survey of 36 mutant mouse strains with gene-targeted defects in glycosyltransferases or glycan-binding proteins

The consortium for functional glycomics (CFG) was a large research initiative providing networking and resources for investigators studying the role of glycans and glycan-binding proteins in health and disease. Starting in 2001, six scientific cores were established to generate data, materials and new technologies. By the end of funding in 2011, the mouse phenotype core (MPC) submitted data to a website from the phenotype screen of 36 mutant mouse strains deficient in a gene for either a glycan-binding protein (GBP) or glycosyltransferase (GT). Each mutant strain was allotted three months for analysis and screened by standard phenotype assays used in the fields of immunology, histology, hematology, coagulation, serum chemistry, metabolism and behavior. Twenty of the deficient mouse strains had been studied in other laboratories, and additional tests were performed on these strains to confirm previous observations and discover new data. The CFG constructed 16 new homozygous mutant mouse strains and completed the initial phenotype screen of the majority of these new mutant strains. In total, >300 phenotype changes were observed, but considering the over 100 assays performed on each strain, most of the phenotypes were unchanged. Phenotype differences include abnormal testis morphology in GlcNAcT9- and Siglec-H-deficient mice and lethality in Pomgnt1-deficient mice. The numerous altered phenotypes discovered, along with the consideration of the significant findings of normality, will provide a platform for future characterization to understand the important roles of glycans and GBPs in the mechanisms of health and disease.

Carbohydrate clearance receptors in transfusion medicine

BACKGROUND

Complex carbohydrates play important functions for circulation of proteins and cells. They provide protective shields and refraction from non-specific interactions with negative charges from sialic acids to enhance circulatory half-life. For recombinant protein therapeutics carbohydrates are especially important to enhance size and reduce glomerular filtration loss. Carbohydrates are, however, also ligands for a large number of carbohydrate-binding lectins exposed to the circulatory system that serve as scavenger receptors for the innate immune system, or have more specific roles in targeting of glycoproteins and cells.

SCOPE OF REVIEW

Here we provide an overview of the common lectin receptors that play roles for circulating glycoproteins and cells, and present a discussion of ways to engineer glycosylation of recombinant biologics and cells to improve therapeutic effects.

MAJOR CONCLUSIONS

While the pharmaceutical industry has learned how to exploit carbohydrates to improve pharmacokinetic properties of recombinant therapeutics, our understanding of how to improve cell-based therapies by manipulation of complex carbohydrates is still at its infancy. Progress with the latter has recently been achieved with cold-stored platelets, where exposure of uncapped glycans lead to rapid clearance from circulation by several lectin-mediated pathways.

GENERAL SIGNIFICANCE

Understanding lectin-mediated clearance pathways is essential for progress in development of biological pharmaceuticals.

C-type Lectin Receptors for Tumor Eradication: Future Directions

Dendritic cells are key regulators in directing immune responses and therefore are under extensive research for the induction of anti-tumor responses. DCs express a large array of receptors by which they scan their surroundings for recognition and uptake of pathogens. One of the receptor-families is the C-type lectins (CLR), which bind carbohydrate structures and internalize antigens upon recognition. Intracellular routing of antigen through CLR enhances loading and presentation of antigen through MHC class I and II, inducing antigen-specific CD4⁺ and CD8⁺ T-cell proliferation and skewing T-helper cells. These characteristics make CLRs very interesting targets for DC-based immunotherapy. Profound research has been done on targeting specific tumor antigens to CLR using either antibodies or the natural ligands such as glycan structures. In this review we will focus on the current data showing the potency of CLR-targeting and discuss improvements that can be achieved to enhance anti-tumor activity in the near future.

Tumour-associated glycan modifications of antigen enhance MGL2 dependent uptake and MHC class I restricted CD8 T cell responses

We recently showed that MGL2 specifically binds tumour-associated glycan N-acetylgalactosamine (GalNAc). We here demonstrate that modification of an antigen with tumour-associated glycan GalNAc, targets antigen specifically to the MGL2 on bone marrow derived (BM)-DCs and splenic DCs. Glycan-modification of antigen with GalNAc that mimics tumour-associated glycosylation, promoted antigen internalisation in DCs and presentation to CD4 T cells, as well as differentiation of IFN-γ producing CD4 T cells. Furthermore, GalNAc modified antigen enhanced cross-presentation of both BM-DCs and primary splenic DCs resulting in enhanced antigen specific CD8 T cell responses. Using MyD88-TRIFF(-/-) BM-DCs we demonstrate that the enhanced cross-presentation of the GalNAc modified antigen is TLR independent. Our data strongly suggest that tumour-associated GalNAc modification of antigen targets MGL on DCs and greatly enhances both MHC class II and class I presentation in a TLR independent manner.

Distribution and function of macrophage galactose-type C-type lectin 2 (MGL2/CD301b): efficient uptake and presentation of glycosylated antigens by dendritic cells

Dendritic cells (DCs) express cell surface lectins that are potentially involved in the recognition, uptake, and presentation of glycosylated foreign substances. A unique calcium-type (C-type) lectin, the macrophage galactose (Gal)-type C-type lectin (MGL/CD301) expressed on DCs, is thought to participate in the recognition of molecules from both altered self and pathogens due to its monosaccharide specificity for Gal and N-acetylgalactosamine (GalNAc). Although mice have two MGL genes, Mgl1 and Mgl2, their distinct roles have not been previously explored. The present report characterizes the properties of MGL2 by examining its distribution and its role in antigen presentation by DCs. We generated an MGL2-specific monoclonal antibody and examined MGL2 expression in tissues by immunohistochemistry and in isolated cells by flow cytometry. The cells reactive with this antibody were shown to be a portion of MGL1-expressing cells, mostly conventional DCs. Internalization of soluble polyacrylamide polymers (PAA) with alpha-GalNAc residues (GalNAc-PAA) by bone marrow-derived DCs (BM-DCs) was mediated by MGL2, as revealed by a comparison of Mgl1(-/-) and Mgl2(-/-) BM-DCs with wild-type BM-DCs. Biotinylated GalNAc-PAA conjugated to streptavidin (SAv) was more efficiently presented to SAv-primed T cells by BM-DCs than beta-N-acetylglucosamine-PAA conjugated to SAv or SAv alone as shown by thymidine uptake and cytokine production. This is the first report that demonstrates the involvement of GalNAc residues in antigen uptake and presentation by DCs that lead to CD4(+) T cell activation.

Endogenous ligands for C-type lectin receptors: the true regulators of immune homeostasis

C-type lectin receptors (CLRs) have long been known as pattern-recognition receptors implicated in the recognition of pathogens by the innate immune system. However, evidence is accumulating that many CLRs are also able to recognize endogenous 'self' ligands and that this recognition event often plays an important role in immune homeostasis. In the present review, we focus on the human and mouse CLRs for which endogenous ligands have been described. Special attention is given to the signaling events initiated upon recognition of the self ligand and the regulation of glycosylation as a switch modulating CLR recognition, and therefore, immune homeostasis.

MGL2 Dermal dendritic cells are sufficient to initiate contact hypersensitivity in vivo

Background

Dendritic cells (DCs) are the most potent antigen-presenting cells in the mammalian immune system. In the skin, epidermal Langerhans cells (LCs) and dermal dendritic cells (DDCs) survey for invasive pathogens and present antigens to T cells after migration to the cutaneous lymph nodes (LNs). So far, functional and phenotypic differences between these two DC subsets remain unclear due to lack of markers to identify DDCs.

Methodology/Principal Findings

In the present report, we demonstrated that macrophage galactose-type C-type lectin (MGL) 2 was exclusively expressed in the DDC subset in the skin-to-LN immune system. In the skin, MGL2 was expressed on the majority (about 88%) of MHCII⁺CD11c⁺ cells in the dermis. In the cutaneous LN, MGL2 expression was restricted to B220⁻CD8α^loCD11b⁺CD11c⁺MHCII^hi tissue-derived DC. MGL2⁺DDC migrated from the dermis into the draining LNs within 24 h after skin sensitization with FITC. Distinct from LCs, MGL2⁺DDCs localized near the high endothelial venules in the outer T cell cortex. In FITC-induced contact hypersensitivity (CHS), adoptive transfer of FITC⁺MGL2⁺DDCs, but not FITC⁺MGL2⁻DCs into naive mice resulted in the induction of FITC-specific ear swelling, indicating that DDCs played a key role in initiation of immune responses in the skin.

Conclusions/Significance

These results demonstrated the availability of MGL2 as a novel marker for DDCs and suggested the contribution of MGL2⁺ DDCs for initiating CHS.

The generation of influenza-specific humoral responses is impaired in ST6Gal I-deficient mice

Posttranslational modification of proteins, such as glycosylation, can impact cell signaling and function. ST6Gal I, a glycosyltransferase expressed by B cells, catalyzes the addition of alpha-2,6 sialic acid to galactose, a modification found on N-linked glycoproteins such as CD22, a negative regulator of B cell activation. We show that SNA lectin, which binds alpha-2,6 sialic acid linked to galactose, shows high binding on plasma blasts and germinal center B cells following viral infection, suggesting ST6Gal I expression remains high on activated B cells in vivo. To understand the relevance of this modification on the antiviral B cell immune response, we infected ST6Gal I(-/-) mice with influenza A/HKx31. We demonstrate that the loss of ST6Gal I expression results in similar influenza infectivity in the lung, but significantly reduced early influenza-specific IgM and IgG levels in the serum, as well as significantly reduced numbers of early viral-specific Ab-secreting cells. At later memory time points, ST6Gal I(-/-) mice show comparable numbers of IgG influenza-specific memory B cells and long-lived plasma cells, with similarly high antiviral IgG titers, with the exception of IgG2c. Finally, we adoptively transfer purified B cells from wild-type or ST6Gal I(-/-) mice into B cell-deficient (microMT(-/-)) mice. Recipient mice that received ST6Gal I(-/-) B cells demonstrated reduced influenza-specific IgM levels, but similar levels of influenza-specific IgG, compared with mice that received wild-type B cells. These data suggest that a B cell intrinsic defect partially contributes to the impaired antiviral humoral response.

Characterization of murine MGL1 and MGL2 C-type lectins: distinct glycan specificities and tumor binding properties

Antigen presenting cells (APC) express a variety of pattern recognition receptors, including the C-type lectin receptors (CLR) that specifically recognize carbohydrate structures expressed on self-tissue and pathogens. The CLR play an important role in antigen uptake and presentation and have been shown to mediate cellular interactions. The ligand specificity of the human macrophage galactose-type lectin (MGL) has been characterized extensively. Here, we set out to determine the glycan specificity of the murine homologues, MGL1 and MGL2, using a glycan array. Murine MGL1 was found to be highly specific for Lewis X and Lewis A structures, whereas mMGL2, more similar to the human MGL, recognized N-acetylgalactosamine (GalNAc) and galactose, including the O-linked Tn-antigen, TF-antigen and core 2. The generation of MGL1 and MGL2-Fc proteins allowed us to identify ligands in lymph nodes, and MGL1-Fc additionally recognized high endothelial venules. Strikingly, MGL2 interacted strongly to adenocarcinoma cells, suggesting a potential role in tumor immunity.

A C-type lectin MGL1/CD301a plays an anti-inflammatory role in murine experimental colitis

Inflammatory bowel disease is caused by abnormal inflammatory and immune responses to harmless substances, such as commensal bacteria, in the large bowel. Such responses appear to be suppressed under healthy conditions, although the mechanism of such suppression is currently unclear. The present study aimed to reveal whether the recognition of bacterial surface carbohydrates by the macrophage galactose-type C-type lectin-1, MGL1/CD301a, induces both the production and secretion of interleukin (IL)-10. Dextran sulfate sodium salt (DSS) was orally administrated to mice that lacked MGL1/CD301a (Mgl1(-/-) mice) and their wild-type littermates. Mgl1(-/-) mice showed significantly more severe inflammation than wild-type mice after administration of DSS. MGL1-positive cells in the colonic lamina propria corresponded to macrophage-like cells with F4/80-high, CD11b-positive, and CD11c-intermediate expression. These cells in Mgl1(-/-) mice produced a lower level of IL-10 mRNA compared with wild-type mice after the administration of DSS for 2 days. Recombinant MGL1 was found to bind both Streptococcus sp. and Lactobacillus sp. among commensal bacteria isolated from mesenteric lymph nodes of DSS-treated mice. Heat-killed Streptococcus sp. induced an increase in IL-10 secretion by MGL1-positive colonic lamina propria macrophages, but not the macrophage population from Mgl1(-/-) mice. These results strongly suggest that MGL1/CD301a plays a protective role against colitis by effectively inducing IL-10 production by colonic lamina propria macrophages in response to invading commensal bacteria.

Sweet preferences of MGL: carbohydrate specificity and function

C-type lectins play important roles in both innate and adaptive immune responses. In contrast to the mannose- or fucose-specific C-type lectins DC-SIGN and mannose receptor, the galactose-type lectins, of which only macrophage galactose-type lectin (MGL) is found within the immune system, are less well known. MGL is selectively expressed by immature dendritic cells and macrophages with elevated levels on tolerogenic or alternatively activated subsets. Human MGL has an exclusive specificity for rare terminal GalNAc structures, which are revealed on the tumor-associated mucin MUC1 and CD45 on effector T cells. These findings implicate MGL in the homeostatic control of adaptive immunity. We discuss here the functional similarities and differences between MGL orthologs and compare MGL to its closest homolog, the liver-specific asialoglycoprotein receptor (ASGP-R).

Multiple hepatic receptors cooperate to eliminate secretory mucins aberrantly entering the bloodstream: are circulating cancer mucins the "tip of the iceberg"?

Hollow organs lined by columnar epithelial cells normally secrete mucins and their proteolytic fragments vectorially into the lumen. These heterogeneously O-glycosylated molecules are known to aberrantly enter the bloodstream in the setting of epithelial carcinomas and possibly during injury or inflammation. We have recently shown that carcinoma mucin fragments can trigger the rapid formation of platelet-rich microthrombi in vivo. Thus, mechanisms to clear such aberrantly secreted mucins must exist. Indeed, we found that i.v. injected carcinoma mucin fragments had an approximately 1 minute half-life in mice, which was primarily due to rapid clearance by hepatic reticuloendothelial cells. Inhibition of known glycan-recognizing hepatic clearance receptors showed involvement of multiple partially overlapping clearance systems. Studies of genetically deficient mice and incomplete competition between different mucins confirmed this result. Thus, multiple hepatic clearance receptors cooperate to eliminate secretory mucins entering the circulation, limiting potential pathology. This may also explain why mucin-type clustered O-glycosylation is rare on plasma proteins. Notably, small subsets of injected carcinoma mucins remained unrecognized by clearance systems, had a much longer half-life, and carried highly sialylated O-glycans. Similar circulating mucins were found in tumor-bearing mice despite lack of saturation of hepatic clearance mechanisms. Thus, circulating cancer mucins currently used as clinical diagnostic markers likely represent only the clearance-resistant "tip of the iceberg." Such aberrantly circulating mucins could play pathologic roles not only in cancer but also during injury or inflammation of hollow organs and in liver disease.

Retardation of removal of radiation-induced apoptotic cells in developing neural tubes in macrophage galactose-type C-type lectin-1-deficient mouse embryos

MGL1/CD301a is a C-type lectin that recognizes galactose and N-acetylgalactosamine as monosaccharides and is expressed on limited populations of macrophages and dendritic cells at least in adult mice. In this study, pregnant mice with Mgl1+/- genotype were mated with Mgl1+/- or Mgl1-/- genotype males, and the embryos were used to assess a hypothesis that this molecule plays an important role in the clearance of apoptotic cells. After X-ray irradiation at 1 Gy of developing embryos at 10.5 days post coitus (d.p.c.), the number of Mgl1-/- pups was significantly reduced as compared with Mgl1+/+ pups. Distributions of MGL1-positive cells, MGL2-positive cells, and apoptotic cells were histologically examined in irradiated Mgl1+/+ embryos. MGL1-positive cells were detected in the neural tube in which many cells undergo apoptosis, whereas MGL2-positive cells were not observed. Biotinylated recombinant MGL1 bound a significant portion of the apoptotic cells. When Mgl1+/+ and Mgl1-/- embryos were examined for the presence of apoptotic cells, similar numbers of apoptotic cells gave rise, but the clearance of these cells was slower in Mgl1-/- embryos than in Mgl1+/+ embryos. These results strongly suggest that MGL1/CD301a is involved in the clearance of apoptotic cells. This process should be essential in the repair and normal development of X-ray-irradiated embryos.

PNA-binding glycans are expressed at high levels on horse mature and immature T lymphocytes and a subpopulation of B lymphocytes

In mammals, the binding of peanut agglutinin (PNA) on the plasma membrane defines subpopulations among lymphocytes from peripheral blood and lymphoid organs. PNA binds Galbeta 1,3GalNAc residues provided that they are not sialylated. Here, we studied the expression of PNA-binding glycans on healthy horse peripheral blood, thymus, lymph node and spleen lymphocytes. We first demonstrated the binding specificity of PNA for galactose residues by competition experiments and the inhibitory role of sialic acids in PNA binding by sialidase digestion. Unlike human and murine lymphocytes, all equine lymphocytes were found positive by flow cytometry analysis. Double-staining analyses showed that lymphocytes expressing high levels of PNA-binding glycans (PNA(high) lymphocytes) were made up of the great majority of CD5(+), CD4(+) and CD8(+) cells, and of 30 and 50% of sIg-bearing lymphocytes in peripheral blood and in lymph nodes or spleen, respectively. Lectin histochemistry suggested that lymph node germinal centres contained PNA(high) B cells. Contrary to what is found in humans and mice, PNA staining intensity on CD5(+), CD4(+) and CD8(+) cells did not differentiate immature from mature T lymphocytes in the equine thymus. The functional consequences of these differences are discussed.

Granulation tissue formation by nonspecific inflammatory agent occurs independently of macrophage galactose-type C-type lectin-1

The role of a macrophage galactose-type calcium-type lectin-1 (MGL1) in antigen-independent granulation tissue formation was investigated. Granulation tissue was induced by injection of carrageenan in an air pouch and distribution of macrophages expressing MGL1/2 was histologically examined. MGL1/2-positive cells were not observed in the granulation tissue induced by carrageenan though these cells were present in dermis. This was distinct from the fact that MGL1/2-positive cells were abundant in granulation tissue induced by antigenic stimulation. CD11b-positive cells were in dermis and carrageenan-induced granulation tissue. Because antigen-induced granulation tissue formation was previously shown to decrease in MGL1-deficient mice or after treatment with anti-MGL1 antibody, we investigated the effects of MGL1-deficient status on carrageenan-induced granulation tissue formation. The thickness of granulation tissue was almost identical between wild-type and MGL1-deficient mice. It is highly likely that MGL1-positive cells are not involved in tissue remodeling when inflammation is driven by nonspecific stimuli.

Carbohydrate profiling reveals a distinctive role for the C-type lectin MGL in the recognition of helminth parasites and tumor antigens by dendritic cells

Dendritic cells (DCs) are key to the maintenance of peripheral tolerance to self-antigens and the orchestration of an immune reaction to foreign antigens. C-type lectins, expressed by DCs, recognize carbohydrate moieties on antigens that can be internalized for processing and presentation. Little is known about the exact glycan structures on self-antigens and pathogens that are specifically recognized by the different C-type lectins and how this interaction influences DC function. We have analyzed the carbohydrate specificity of the human C-type lectin macrophage galactose-type lectin (MGL) using glycan microarray profiling and identified an exclusive specificity for terminal alpha- and beta-linked GalNAc residues that naturally occur as parts of glycoproteins or glycosphingolipids. Specific glycan structures containing terminal GalNAc moieties, expressed by the human helminth parasite Schistosoma mansoni as well as tumor antigens and a subset of gangliosides, were identified as ligands for MGL. Our results indicate an endogenous function for DC-expressed MGL in the clearance and tolerance to self-gangliosides, and in the pattern recognition of tumor antigens and foreign glycoproteins derived from helminth parasites.

Lack of antigen-specific tissue remodeling in mice deficient in the macrophage galactose-type calcium-type lectin 1/CD301a

Macrophage galactose-type C-type lectins (MGLs), which were recently named CD301, have 2 homologues in mice: MGL1 and MGL2. MGLs are expressed on macrophages and immature dendritic cells. The persistent presence of granulation tissue induced by a protein antigen was observed in wild-type mice but not in mice lacking an endogenous, macrophage-specific, galactose-type calcium-type lectin 1 (MGL1) in an air pouch model. The anti-MGL1 antibody suppressed the granulation tissue formation in wild-type mice. A large number of cells, present only in the pouch of MGL1-deficient mice, were not myeloid or lymphoid lineage cells and the number significantly declined after administration of interleukin 1 alpha (IL-1alpha) into the pouch of MGL1-deficient mice. Furthermore, granulation tissue was restored by this treatment and the cells obtained from the pouch of MGL1-deficient mice were incorporated into the granulation tissue when injected with IL-1alpha. Taken together, MGL1 expressed on a specific subpopulation of macrophages that secrete IL-1alpha was proposed to regulate specific cellular interactions crucial to granulation tissue formation.

Vascular Smooth Muscle Polyploidization as a Biomarker for Aging and Its Impact on Differential Gene Expression

Polyploidy is characterized by a greater than diploid content of DNA in a cell. Previous measurements of ploidy level in different organs of humans and rodents, including the aorta, indicated an increase in old versus young. We hypothesized that aortic vascular smooth muscle polyploidy is a biomarker for aging and that the augmented DNA dosage affects selective gene-specific transcript expression. Our results demonstrate that tetraploidy increases exponentially over the life span of the animal, serving as an indicator of age. Approximately 60% of the vascular smooth muscle cells in the thoracic aorta of 36-month-old Brown Norway rats are tetraploid compared with 8% in their 3-month-old counterparts. Microarray analysis and reverse transcriptase-PCR was performed with mRNA isolated from sorted diploid (2N) and tetraploid (4N) vascular smooth muscle cells from old rats to identify differentially expressed transcripts. For the majority of detectable transcripts, an increase in DNA content led to a proportional increase in mRNA. A select group of transcripts, however, were reduced in tetraploid compared with diploid cells. These mRNAs correspond to guanine deaminase, to the matrix proteins rat glypican 3 (OCI-5) and decorin, as well as to the inflammation-associated transcripts, insulin-like growth factor-binding protein 6, macrophage inflammatory protein 2 precursor, macrophage galactose N-acetylgalactoseamine-specific lectin, and complement component C4. Our study is the first to describe aortic ploidy level as a biomarker for aging and to indicate that changes associated with increased DNA content per cell may selectively suppress the expression of specific genes.

Molecular cloning and characterization of a novel mouse macrophage C-type lectin, mMGL2, which has a distinct carbohydrate specificity from mMGL1

A novel mouse macrophage galactose-type C-type lectin 2 (mMGL2) was identified by BLAST analysis of expressed sequence tags. The sequence of mMGL2 is highly homologous to the mMGL, which should now be called mMGL1. The open reading frame of mMGL2 contains a sequence corresponding to a type II transmembrane protein with 332 amino acids having a single extracellular C-type lectin domain. The 3'-untranslated region included long terminal repeats of mouse early transposon. The Mgl2 gene was cloned from a 129/SvJ mouse genomic library and sequenced. The gene spans 7,136 base pairs and consists of 10 exons, which is similar to the genomic organization of mMGL1. The reverse transcriptase-PCR analysis indicates that mMGL2 is expressed in cell lines and normal mouse tissues in a macrophage-restricted manner, also very similar to that of mMGL1. The mMGL2 mRNA was also detected in mMGL1-positive cells, which were sorted from thioglycollate-induced peritoneal cells with a mMGL1-specific monoclonal antibody, LOM-8.7. The soluble recombinant proteins of mMGL2 exhibited carbohydrate specificity for alpha- and beta-GalNAc-conjugated soluble polyacrylamides, whereas mMGL1 preferentially bound Lewis X-conjugated soluble polyacrylamides in solid phase assays. These two lectins may function cooperatively as recognition and endocytic molecules on macrophages and related cells.