An extended conformation of the macrophage mannose receptor

Interaction of human dendritic cell receptor DEC205/CD205 with keratins

DEC205 (CD205) is one of the major endocytic receptors on dendritic cells and has been widely used as a receptor target in immune therapies. It has been shown that DEC205 can recognize dead cells through keratins in a pH-dependent manner. However, the mechanism underlying the interaction between DEC205 and keratins remains unclear. Here we determine the crystal structures of an N-terminal fragment of human DEC205 (CysR∼CTLD3). The structural data show that DEC205 shares similar overall features with the other mannose receptor family members such as the mannose receptor and Endo180, but the individual domains of DEC205 in the crystal structure exhibit distinct structural features that may lead to specific ligand binding properties of the molecule. Among them, CTLD3 of DEC205 adopts a unique fold of CTLD, which may correlate with the binding of keratins. Furthermore, we examine the interaction of DEC205 with keratins by mutagenesis and biochemical assays based on the structural information and identify an XGGGX motif on keratins that can be recognized by DEC205, thereby providing insights into the interaction between DEC205 and keratins. Overall, these findings not only improve the understanding of the diverse ligand specificities of the mannose receptor family members at the molecular level but may also give clues for the interactions of keratins with their binding partners in the corresponding pathways.

Mannose Ligands for Mannose Receptor Targeting

The mannose receptor (MR, CD 206) is an endocytic receptor primarily expressed by macrophages and dendritic cells, which plays a critical role in both endocytosis and antigen processing and presentation. MR carbohydrate recognition domains (CRDs) exhibit a high binding affinity for branched and linear oligosaccharides. Furthermore, multivalent mannose presentation on the various templates like peptides, proteins, polymers, micelles, and dendrimers was proven to be a valuable approach for the selective and efficient delivery of various therapeutically active agents to MR. This review provides a detailed account of the most relevant and recent aspects of the synthesis and application of mannosylated bioactive formulations for MR-mediated delivery in treatments of cancer and other infectious diseases. It further highlights recent findings related to the necessary structural features of the mannose-containing ligands for successful binding to the MR.

Straight to the point: targeted mRNA-delivery to immune cells for improved vaccine design

With the deepening of our understanding of adaptive immunity at the cellular and molecular level, targeting antigens directly to immune cells has proven to be a successful strategy to develop innovative and potent vaccines. Indeed, it offers the potential to increase vaccine potency and/or modulate immune response quality while reducing off-target effects. With mRNA-vaccines establishing themselves as a versatile technology for future applications, in the last years several approaches have been explored to target nanoparticles-enabled mRNA-delivery systems to immune cells, with a focus on dendritic cells. Dendritic cells (DCs) are the most potent antigen presenting cells and key mediators of B- and T-cell immunity, and therefore considered as an ideal target for cell-specific antigen delivery. Indeed, improved potency of DC-targeted vaccines has been proved in vitro and in vivo. This review discusses the potential specific targets for immune system-directed mRNA delivery, as well as the different targeting ligand classes and delivery systems used for this purpose.

Applications of mannose-binding lectins and mannan glycoconjugates in nanomedicine

Glycosylated nanoparticles (NPs) have drawn a lot of attention in the biomedical field over the past few decades, particularly in applications like targeted drug delivery. Mannosylated NPs and mannan-binding lectins/proteins (MBL/MBP) are emerging as promising tools for delivery of drugs, medicines, and enzymes to targeted tissues and cells as nanocarriers, enhancing their therapeutic benefits while avoiding the adverse effects of the drug. The occurrence of plenty of lectin receptors and their mannan ligands on cell surfaces makes them multifaceted carriers appropriate for specific delivery of bioactive drug materials to their targeted sites. Thus, the present review describes the tethering of mannose (Man) to several nanostructures, like micelles, liposomes, and other NPs, applicable for drug delivery systems. Bioadhesion through MBL-like receptors on cells has involvements applicable to additional arenas of science, for example gene delivery, tissue engineering, biomaterials, and nanotechnology. This review also focuses on the role of various aspects of drug/antigen delivery using (i) mannosylated NPs, (ii) mannosylated lectins, (iii) amphiphilic glycopolymer NPs, and (iv) natural mannan-containing polysaccharides, with most significant applications of MBL-based NPs as multivalent scaffolds, using different strategies.

GRAPHICAL ABSTRACT

Mannosylated NPs and/or MBL/MBP are coming up as viable and versatile tools as nanocarriers to deliver drugs and enzymes precisely to their target tissues or cells. The presence of abundant number of lectin receptors and their mannan ligands on cell surfaces makes them versatile carriers suitable for the targeted delivery of bioactive drugs.

N-glycosylation of mannose receptor (CD206) regulates glycan binding by C-type lectin domains

The macrophage mannose receptor (MR, CD206) is a transmembrane endocytic lectin receptor, expressed in selected immune and endothelial cells, and is involved in immunity and maintaining homeostasis. Eight of the ten extracellular domains of the MR are C-type lectin domains (CTLDs) which mediate the binding of mannose, fucose, and GlcNAc in a calcium-dependent manner. Previous studies indicated that self-glycosylation of MR regulates its glycan binding. To further explore this structure-function relationship, we studied herein a recombinant version of mouse MR CTLD4-7 fused to human Fc-portion of IgG (MR-Fc). The construct was expressed in different glycosylation-mutant cell lines to study the influence of differential glycosylation on receptor glycan-binding properties. We conducted site-specific N- and O-glycosylation analysis and glycosylation site characterization using mass spectrometry by which several novel O-glycosylation sites were identified in mouse MR and confirmed in human full-length MR. This information guided experiments evaluating the receptor functionality by glycan microarray analysis in combination with glycan-modifying enzymes. Treatment of active MR-Fc with combinations of exoglycosidases, including neuraminidase and galactosidases, resulted in the loss of trans-binding (binding of MR CTLDs to non-MR glycans), due to unmasking of terminal, nonreducing GlcNAc in N-glycans of the MR CTLDs. Regalactosylation of N-glycans rescues mannose binding by MR-Fc. Our results indicate that glycans within the MR CTLDs act as a regulatory switch by masking and unmasking self-ligands, including terminal, nonreducing GlcNAc in N-glycans, which could control MR activity in a tissue- and cell-specific manner or which potentially affect bacterial pathogenesis in an immunomodulatory fashion.

Single-molecule imaging of glycan-lectin interactions on cells with Glyco-PAINT

Most lectins bind carbohydrate ligands with relatively low affinity, making the identification of optimal ligands challenging. Here we introduce a point accumulation in nanoscale topography (PAINT) super-resolution microscopy method to capture weak glycan-lectin interactions at the single-molecule level in living cells (Glyco-PAINT). Glyco-PAINT exploits weak and reversible sugar binding to directly achieve single-molecule detection and quantification in cells and is used to establish the relative k_on and k_off rates of a synthesized library of carbohydrate-based probes, as well as the diffusion coefficient of the receptor-sugar complex. Uptake of ligands correlates with their binding affinity and residence time to establish structure-function relations for various synthetic glycans. We reveal how sugar multivalency and presentation geometry can be optimized for binding and internalization. Overall, Glyco-PAINT represents a powerful approach to study weak glycan-lectin interactions on the surface of living cells, one that can be potentially extended to a variety of lectin-sugar interactions.

A fresh trim provides a new look at the human mannose receptor

The human mannose receptor plays an important role in scavenging a variety of glycans and glycoconjugates, which contributes to both innate and adaptive immunity. However, the fine details of its ligand specificity, and specifically that of carbohydrate-recognition domain 4, the most functionally relevant C-type lectin domain within the receptor, are not completely understood. Feinberg et al. use glycan arrays, crystallography, and a newly trimmed version of carbohydrate-recognition domain 4 to elucidate the molecular mechanisms driving binding specificity. These data contribute to our molecular understanding of Ca²⁺-mediated binding promiscuity in the human mannose receptor and the scavenging role of the receptor itself and highlight unexpected interactions that should inspire further study.

Structural analysis of carbohydrate binding by the macrophage mannose receptor CD206

The human mannose receptor expressed on macrophages and hepatic endothelial cells scavenges released lysosomal enzymes, glycopeptide fragments of collagen, and pathogenic microorganisms and thus reduces damage following tissue injury. The receptor binds mannose, fucose, or N-acetylglucosamine (GlcNAc) residues on these targets. C-type carbohydrate-recognition domain 4 (CRD4) of the receptor contains the site for Ca2+-dependent interaction with sugars. To investigate the details of CRD4 binding, glycan array screening was used to identify oligosaccharide ligands. The strongest signals were for glycans that contain either Manα1-2Man constituents or fucose in various linkages. The mechanisms of binding to monosaccharides and oligosaccharide substructures present in many of these ligands were examined in multiple crystal structures of CRD4. Binding of mannose residues to CRD4 results primarily from interaction of the equatorial 3- and 4-OH groups with a conserved principal Ca2+ common to almost all sugar-binding C-type CRDs. In the Manα1-2Man complex, supplementary interactions with the reducing mannose residue explain the enhanced affinity for this disaccharide. Bound GlcNAc also interacts with the principal Ca2+ through equatorial 3- and 4-OH groups, whereas fucose residues can bind in several orientations, through either the 2- and 3-OH groups or the 3- and 4-OH groups. Secondary contacts with additional sugars in fucose-containing oligosaccharides, such as the Lewis-a trisaccharide, provide enhanced affinity for these glycans. These results explain many of the biologically important interactions of the mannose receptor with both mammalian glycoproteins and microbes such as yeast and suggest additional classes of ligands that have not been previously identified.

The cryo-EM structure of the endocytic receptor DEC-205

DEC-205 (CD205), a member of the macrophage mannose receptor protein family, is the prototypic endocytic receptor of dendritic cells, whose ligands include phosphorothioated cytosine-guanosine oligonucleotides, a motif often seen in bacterial or viral DNA. However, despite growing biological and clinical significance, little is known about the structural arrangement of this receptor or any of its family members. Here, we describe the 3.2 Å cryo-EM structure of human DEC-205, thereby illuminating the structure of the mannose receptor protein family. The DEC-205 monomer forms a compact structure comprising two intercalated rings of C-type lectin-like domains, where the N-terminal cysteine-rich and fibronectin domains reside at the central intersection. We establish a pH-dependent oligomerization pathway forming tetrameric DEC-205 using solution-based techniques and ultimately solved the 4.9 Å cryo-EM structure of the DEC-205 tetramer to identify the unfurling of the second lectin ring which enables tetramer formation. Furthermore, we suggest the relevance of this oligomerization pathway within a cellular setting, whereby cytosine-guanosine binding appeared to disrupt this cell-surface oligomer. Accordingly, we provide insight into the structure and oligomeric assembly of the DEC-205 receptor.

Mannose Receptor 1 Restricts HIV Particle Release from Infected Macrophages

Human mannose receptor 1 (hMRC1) is expressed on the surface of most tissue macrophages, dendritic cells, and select lymphatic or liver endothelial cells. HMRC1 contributes to the binding of HIV-1 to monocyte-derived macrophages (MDMs) and is involved in the endocytic uptake of HIV-1 into these cells. Here, we identify hMRC1 as an antiviral factor that inhibits virus release through a bone marrow stromal antigen 2 (BST-2)-like mechanism. Virions produced in the presence of hMRC1 accumulated in clusters at the cell surface but were fully infectious. HIV-1 counteracted the effect by transcriptional silencing of hMRC1. The effect of hMRC1 was not virus isolate specific. Surprisingly, deletion of the Env protein, which is known to interact with hMRC1, did not relieve the hMRC1 antiviral activity, suggesting the involvement of additional cellular factor(s) in the process. Our data reveal an antiviral mechanism that is active in primary human macrophages and is counteracted by HIV-1 through downregulation of hMRC1.

Single-domain antibody fusion proteins can target and shuttle functional proteins into macrophage mannose receptor expressing macrophages

The tumor microenvironment of numerous prevalent cancer types is abundantly infiltrated with tumor-associated macrophages (TAMs). Macrophage mannose receptor (MMR or CD206) expressing TAMs have been shown to be key promoters of tumor progression and major opponents of successful cancer therapy. Therefore, depleting MMR⁺ TAMs is an interesting approach to synergize with current antitumor therapies. We studied the potential of single-domain antibodies (sdAbs) specific for MMR to target proteins to MMR⁺ TAMs. Anti-MMR sdAbs were genetically coupled to a reporter protein, mWasabi (wasabi green, WG), generating sdAb "drug" fusion proteins (SFPs), referred to as WG-SFPs. The resulting WG-SFPs were highly efficient in targeting MMR⁺ macrophages both in vitro and in vivo. As we showed that second mitochondria-derived activator of caspase (SMAC) mimetics modulate MMR⁺ macrophages, we further coupled the anti-MMR sdAb to an active form of SMAC, referred to as tSMAC. The resulting tSMAC-SFPs were able to bind and upregulate caspase3/7 activity in MMR⁺ macrophages in vitro. In conclusion, we report the proof-of-concept of an elegant approach to conjugate anti-MMR sdAbs to proteins, which opens new avenues for targeted manipulation of MMR⁺ tumor-promoting TAMs.

Structural Insights into the pH-Dependent Conformational Change and Collagen Recognition of the Human Mannose Receptor

Mannose receptor (MR, CD206) is an endocytic receptor on microphages and dendritic cells. It recognizes multiple ligands and plays important roles in regulating immune responses and maintaining glycoprotein homeostasis. However, the structure and functional mechanism of MR remain unclear. Here we determine the crystal structures of the N-terminal fragments of MR and reveal the potential binding mode of collagen on the fibronectin II domain. The SAXS and other biophysical data suggest that MR adopts an extended conformation at physiological pH and undergoes conformational changes as pH decreases, resulting in a compact conformation in an acidic environment. Moreover, biochemical data show that MR binds to collagen in a Ca²⁺-enhanced manner at physiological pH, whereas Ca²⁺ has no effect on the binding at acidic pH. These results provide a model for the dynamic mechanism of MR regarding its ligand binding and release during the recycling between cell surface and endosomes.

Poly-glutamic dendrimer-based conjugates for cancer vaccination - a computational design for targeted delivery of antigens

Computational techniques are useful to predict interaction models and molecular properties for the design of drug delivery systems, such as dendrimers. This work evaluated the impact of surface modifications of mannosamine-conjugated multifunctional poly(glutamic acid) (PG)-dendrimers as nanocarriers of the tumour associated antigens (TAA) MART-1, gp100:44 and gp100:209. Molecular dynamics simulations and docking studies were performed. Nitrobenzoxadiazole (NBD)-PG-G4-dendrimer displayed 64 carboxylic groups, however, the Frontier Molecular Orbital Theory study evidenced that only 32 of those were available to form covalent bonds. When the number of mannosamines conjugated to dendrimer was increased from 16 to 32, the dendrimer interacted with the receptor with higher affinity. However, 16 mannosamines-NBD-PG-G4-dendrimer was chosen to conjugate TAA for added functionality as no carboxylic end groups were available for further conjugation in the 32 mannosamines-dendrimer. Docking results showed that the majority of TAA-conjugated NBD-PG-G4-dendrimer demonstrated a favourable interaction with mannosamine binding site on mannose receptor, thus constituting a promising tool for TAA targeted delivery. Our in silico approach effectively narrows down the selection of the best candidates for the synthesis of functionalised PG-dendrimers with desired functionalities. These results will significantly reduce the time and efforts required to experimentally synthesise modified dendrimers for optimal antigen delivery.

Insights into Collagen Uptake by C-type Mannose Receptors from the Crystal Structure of Endo180 Domains 1-4

The C-type mannose receptor and its homolog Endo180 (or uPARAP, for urokinase plasminogen activator receptor-associated protein) mediate the endocytic uptake of collagen by macrophages and fibroblasts. This process is required for normal tissue remodeling, but also facilitates the growth and dissemination of tumors. We have determined the crystal structure at 2.5 Å resolution of the N-terminal region of Endo180, consisting of a ricin-like domain, a fibronectin type II (FN2) domain, and two C-type lectin (CTL) domains. The L-shaped arrangement of these domains creates a shallow trench spanning the FN2 and CTL1 domains, which was shown by mutagenesis to bind triple-helical and denatured collagen. Small-angle X-ray scattering showed that the L-shaped structure is maintained in solution at neutral and acidic pH, irrespective of calcium ion loading. Collagen binding was equally unaffected by acidic pH, suggesting that collagen release in endosomes is not regulated by changes within the Endo180 N-terminal region.

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Domains 1–4 of the endocytic receptor Endo180 form an L-shaped structure

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The fibronectin type II domain (domain 2) is crucial for collagen/gelatin binding

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The first C-type lectin domain (domain 3) also contributes to collagen binding

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Collagen binding to Endo180 domains 1–4 is not reduced by low pH or Ca²⁺ depletion

pH-Dependent recognition of apoptotic and necrotic cells by the human dendritic cell receptor DEC205

Dendritic cells play important roles in regulating innate and adaptive immune responses. DEC205 (CD205) is one of the major endocytotic receptors on dendritic cells and has been widely used for vaccine generation against viruses and tumors. However, little is known about its structure and functional mechanism. Here we determine the structure of the human DEC205 ectodomain by cryoelectron microscopy. The structure shows that the 12 extracellular domains form a compact double ring-shaped conformation at acidic pH and become extended at basic pH. Biochemical data indicate that the pH-dependent conformational change of DEC205 is correlated with ligand binding and release. DEC205 only binds to apoptotic and necrotic cells at acidic pH, whereas live cells cannot be recognized by DEC205 at either acidic or basic conditions. These results suggest that DEC205 is an immune receptor that recognizes apoptotic and necrotic cells specifically through a pH-dependent mechanism.

C-type lectin-like domain and fibronectin-like type II domain of phospholipase A(2) receptor 1 modulate binding and migratory responses to collagen

Phospholipase A2 receptor 1 (PLA2R) mediates collagen-dependent migration. The mechanisms by which PLA2R interacts with collagen remain unclear. We produced HEK293 cells expressing full-length wild-type PLA2R or a truncated PLA2R that lacks fibronectin-like type II (FNII) domains or several regions of C-type lectin-like domain (CTLD). We show that the CTLD1-2 as well as the FNII domain of PLA2R are responsible for binding to collagen and for collagen-dependent migration. Thus, multiple regions and domains of the extracellular portion of PLA2R participate in the responses to collagen. These data suggest a potentially new mechanism for PLA2R-mediated biological response beyond that of a receptor for secretory PLA2.

The use of anchored agonists of phagocytic receptors for cancer immunotherapy: B16-F10 murine melanoma model

The application of the phagocytic receptor agonists in cancer immunotherapy was studied. Agonists (laminarin, molecules with terminal mannose, N-Formyl-methioninyl-leucyl-phenylalanine) were firmly anchored to the tumor cell surface. When particular agonists of phagocytic receptors were used together with LPS (Toll-like receptor agonist), high synergy causing tumour shrinkage and a temporary or permanent disappearance was observed. Methods of anchoring phagocytic receptor agonists (charge interactions, anchoring based on hydrophobic chains, covalent bonds) and various regimes of phagocytic agonist/LPS mixture applications were tested to achieve maximum therapeutic effect. Combinations of mannan/LPS and f-MLF/LPS (hydrophobic anchors) in appropriate (pulse) regimes resulted in an 80% and 60% recovery for mice, respectively. We propose that substantial synergy between agonists of phagocytic and Toll-like receptors (TLR) is based on two events. The TLR ligand induces early and massive inflammatory infiltration of tumors. The effect of this cell infiltrate is directed towards tumor cells, bearing agonists of phagocytic receptors on their surface. The result of these processes was effective killing of tumor cells. This novel approach represents exploitation of innate immunity mechanisms for treating cancer.

Influence of placental mannose/n-acetyl glucosamine-binding proteins on the interaction of insulin and insulin-like growth factors with their receptors

Placenta is a source of carbohydrate-binding proteins that function as molecular scavengers, but they could also be involved in interactions that assist in metabolic control. Mannose/N-acetyl-glucosamine (Man/GlcNAc)-binding proteins from placenta were isolated and their reactivity towards placental insulin and insulin-like growth factor receptors (IR and IGF-Rs) was analyzed. The lectins reduced the binding of insulin and IGF-I in a dose-dependent manner, while almost no effect was observed on the binding of IGF-II. The shape of the inhibition curves changed, suggesting altered binding specificity. The presence of sugar could not reverse completely the effect of the lectins, implicating both lectin-sugar and protein-protein conformational recognition. Since biological molecules in our experimental system were those that are in close relation in vivo, placental Man/GlcNAc-specific lectins may be regarded as potential allosteric modulators of ligand-receptor interactions in a system of homologous ligands, selectively affecting only binding to tyrosine kinase type receptors (IR and IGF-1R).

Structure of FcRY, an avian immunoglobulin receptor related to mammalian mannose receptors, and its complex with IgY

Fc receptors transport maternal antibodies across epithelial cell barriers to passively immunize newborns. FcRY, the functional counterpart of mammalian FcRn (a major histocompatibility complex homolog), transfers IgY across the avian yolk sac, and represents a new class of Fc receptor related to the mammalian mannose receptor family. FcRY and FcRn bind immunoglobulins at pH ≤6.5, but not pH ≥7, allowing receptor-ligand association inside intracellular vesicles and release at the pH of blood. We obtained structures of monomeric and dimeric FcRY and an FcRY-IgY complex and explored FcRY's pH-dependent binding mechanism using electron cryomicroscopy (cryoEM) and small-angle X-ray scattering. The cryoEM structure of FcRY at pH 6 revealed a compact double-ring "head," in which the N-terminal cysteine-rich and fibronectin II domains were folded back to contact C-type lectin-like domains 1-6, and a "tail" comprising C-type lectin-like domains 7-8. Conformational changes at pH 8 created a more elongated structure that cannot bind IgY. CryoEM reconstruction of FcRY dimers at pH 6 and small-angle X-ray scattering analysis at both pH values confirmed both structures. The cryoEM structure of the FcRY-IgY revealed symmetric binding of two FcRY heads to the dimeric FcY, each head contacting the C(H)4 domain of one FcY chain. FcRY shares structural properties with mannose receptor family members, including a head and tail domain organization, multimerization that may regulate ligand binding, and pH-dependent conformational changes. Our results facilitate understanding of immune recognition by the structurally related mannose receptor family and comparison of diverse methods of Ig transport across evolution.

Pichia pastoris-produced mucin-type fusion proteins with multivalent O-glycan substitution as targeting molecules for mannose-specific receptors of the immune system

Mannose-binding proteins like the macrophage mannose receptor (MR), the dendritic cell-specific intercellular adhesion molecule-3 grabbing non-integrin (DC-SIGN) and mannose-binding lectin (MBL) play crucial roles in both innate and adaptive immune responses. Immunoglobulin fusion proteins of the P-selectin glycoprotein ligand-1 (PSGL-1/mIgG(2b)) carrying mostly O-glycans and, as a control, the α1-acid glycoprotein (AGP/mIgG(2b)) carrying mainly N-linked glycans were stably expressed in the yeast Pichia pastoris. Pichia pastoris-produced PSGL-1/mIgG(2b) was shown to carry O-glycans that mediated strong binding to mannose-specific lectins in a lectin array and were susceptible to cleavage by α-mannosidases including an α1,2- but not an α1,6-mannosidase. Electrospray ionization ion-trap mass spectrometry confirmed the presence of O-glycans containing up to nine hexoses with the penta- and hexasaccharides being the predominant ones. α1,2- and α1,3-linked, but not α1,6-linked, mannose residues were detected by (1)H-nuclear magnetic resonance spectroscopy confirming the results of the mannosidase cleavage. The apparent equilibrium dissociation constants for binding of PNGase F-treated mannosylated PSGL-1/mIgG(2b) to MR, DC-SIGN and MBL were shown by surface plasmon resonance to be 126, 56 and 16 nM, respectively. In conclusion, PSGL-1/mIgG(2b) expressed in P. pastoris carried O-glycans mainly comprised of α-linked mannoses and with up to nine residues. It bound mannose-specific receptors with high apparent affinity and may become a potent targeting molecule for these receptors in vivo.

Identification of the ovine mannose receptor and its possible role in Visna/Maedi virus infection

This study aims to characterize the mannose receptor (MR) gene in sheep and its role in ovine visna/maedi virus (VMV) infection. The deduced amino acid sequence of ovine MR was compatible with a transmembrane protein having a cysteine-rich ricin-type amino-terminal region, a fibronectin type II repeat, eight tandem C-type lectin carbohydrate-recognition domains (CRD), a transmembrane region, and a cytoplasmic carboxy-terminal tail. The ovine and bovine MR sequences were closer to each other compared to human or swine MR. Concanavalin A (ConA) inhibited VMV productive infection, which was restored by mannan totally in ovine skin fibroblasts (OSF) and partially in blood monocyte-derived macrophages (BMDM), suggesting the involvement of mannosylated residues of the VMV ENV protein in the process. ConA impaired also syncytium formation in OSF transfected with an ENV-encoding pN3-plasmid. MR transcripts were found in two common SRLV targets, BMDM and synovial membrane (GSM) cells, but not in OSF. Viral infection of BMDM and especially GSM cells was inhibited by mannan, strongly suggesting that in these cells the MR is an important route of infection involving VMV Env mannosylated residues. Thus, at least three patterns of viral entry into SRLV-target cells can be proposed, involving mainly MR in GSM cells (target in SRLV-induced arthritis), MR in addition to an alternative route in BMDM (target in SRLV infections), and an alternative route excluding MR in OSF (target in cell culture). Different routes of SRLV infection may thus coexist related to the involvement of MR differential expression.

Oligomerization of the macrophage mannose receptor enhances gp120-mediated binding of HIV-1

C-type lectin receptors expressed on the surface of dendritic cells and macrophages are able to bind glycoproteins of microbial pathogens via mannose, fucose, and N-acetylglucosamine. Langerin on Langerhans cells, dendritic cell-specific intercellular adhesion molecule 3-grabbing nonintegrin on dendritic cells, and mannose receptor (MR) on dendritic cells and macrophages bind the human immunodeficiency virus (HIV) envelope protein gp120 principally via high mannose oligosaccharides. These C-type lectin receptors can also oligomerize to facilitate enhanced ligand binding. This study examined the effect of oligomerization of MR on its ability to bind to mannan, monomeric gp120, native trimeric gp140, and HIV type 1 BaL. Mass spectrometry analysis of cross-linked MR showed homodimerization on the surface of primary monocyte-derived dendritic cells and macrophages. Both monomeric and dimeric MR were precipitated by mannan, but only the dimeric form was co-immunoprecipitated by gp120. These results were confirmed independently by flow cytometry analysis of soluble monomeric and trimeric HIV envelope and a cellular HIV virion capture assay. As expected, mannan bound to the carbohydrate recognition domains of MR dimers mostly in a calcium-dependent fashion. Unexpectedly, gp120-mediated binding of HIV to dimers on MR-transfected Rat-6 cells and macrophages was not calcium-dependent, was only partially blocked by mannan, and was also partially inhibited by N-acetylgalactosamine 4-sulfate. Thus gp120-mediated HIV binding occurs via the calcium-dependent, non-calcium-dependent carbohydrate recognition domains and the cysteine-rich domain at the C terminus of MR dimers, presenting a much broader target for potential inhibitors of gp120-MR binding.

Design and synthesis of novel multivalent mannosides targeting the mannose receptor

According to the characteristics of C-type lectin-like domains in the mannose receptor (MR), a novel design of multivalent mannosides targeting the MR was accomplished. Beginning with a divalent mannoside as the sugar unit, a series of multivalent mannosides with variations in both valence and space were synthesized in a convergent approach. The synthetic multivalent mannosides are to be explored to study MR-sugar binding events.

Effect of differential N-linked and O-linked mannosylation on recognition of fungal antigens by dendritic cells

BACKGROUND

An experimental approach for improving vaccine efficacy involves targeting antigens to mannose receptors (MRs) on dendritic cells (DCs) and other professional antigen presenting cells. Previously, we demonstrated that mannosylated Pichia pastoris-derived recombinant proteins exhibited increased immunogenicity compared to proteins lacking mannosylation. In order to gain insight into the mechanisms responsible for this observation, the present study examined the cellular uptake of the mannosylated and deglycosylated recombinant proteins.

METHODOLOGY/PRINCIPAL FINDINGS

Utilizing transfected cell lines, roles for the macrophage mannose receptor (MMR, CD206) and DC-SIGN (CD209) in the recognition of the mannosylated, but not deglycosylated, antigens were demonstrated. The uptake of mannosylated antigens into murine bone marrow-derived DCs (BMDCs) was inhibited by yeast mannans (YMs), suggesting a mannose-specific C-type lectin receptor-dependent process, while the uptake of deglycosylated antigens remained unaffected. In particular, antigens with both N-linked and extensive O-linked mannosylation showed the highest binding and uptake by BMDCs. Finally, confocal microscopy studies revealed that both mannosylated and deglycosylated P. pastoris-derived recombinant proteins localized in MHC class II+ compartments within BMDCs.

CONCLUSIONS/SIGNIFICANCE

Taken together with our previous results, these data suggest that increased uptake by mannose-specific C-type lectin receptors is the major mechanism responsible for the enhanced antigenicity seen with mannosylated proteins. These findings have important implications for vaccine design and contribute to our understanding of how glycosylation affects the immune response to eukaryotic pathogens.

Mammalian collagen receptors

Collagen-rich extracellular matrices are abundant and ubiquitous in the mammalian body. Collagens are not only essential for the mechanical stability of tissues, but are also intimately involved in controlling cell behaviour. The hallmark of collagens is a triple helix made up of polypeptide chains containing glycine-X-Y repeats. A structurally and functionally diverse group of cell surface receptors mediates the recognition of triple-helical collagen: integrins, discoidin domain receptors, glycoprotein VI, leukocyte-associated IG-like receptor-1, and members of the mannose receptor family. In this review, we discuss the structure and function of these receptors, focussing on the principles involved in collagen recognition.

Calcium is not required for immulectin-2 binding, but protects the protein from proteinase digestion

Mammalian C-type lectins are calcium-dependent carbohydrate-binding proteins. They serve as cell adhesion molecules in cell-cell interactions, or function as pattern-recognition receptors in innate immunity. Calcium is a direct ligand for carbohydrate binding in mammalian C-type lectins such as mannose-binding proteins and macrophage mannose receptor. In the tobacco hornworm Manduca sexta, a group of lectins named immulectins have been discovered. Each immulectin contains dual carbohydrate-recognition domains. Previously, we showed that immulectin-2 (IML-2) binds to a bacterial lipopolysaccharide, and agglutination of Escherichia coli cells by IML-2 is calcium dependent. In this study, we demonstrated that IML-2 bound to bacterial lipid A, smooth and rough mutants of lipopolysaccharide, lipoteichoic acid and peptidoglycan, as well as to fungal mannan and beta-1, 3-glucan (laminarin and curdlan). Binding of IML-2 to microbial components was calcium independent, and was increased by addition of spermine, a polyamine. In addition, plasma IML-2 bound to mannan-agarose independent of calcium. But trypsin digestion of IML-2 was inhibited in the presence of calcium. Our results suggest that calcium is not required for IML-2 binding but protects IML-2 from trypsin digestion.

Collagen binding by the mannose receptor mediated through the fibronectin type II domain

The macrophage mannose receptor is the prototype for a family of receptors each having an extracellular region consisting of an N-terminal cysteine-rich domain related to the R-type carbohydrate-recognition domain of ricin, a fibronectin type II domain and eight to ten domains related to C-type carbohydrate-recognition domains. The mannose receptor acts as a molecular scavenger, clearing harmful glycoconjugates or micro-organisms through recognition of their defining carbohydrate structures. Cell-adhesion assays, as well as collagen-binding assays, have now been used to show that the mannose receptor can also bind collagen and that the fibronectin type II domain mediates this activity. Neither of the two types of sugar-binding domain in the receptor is involved in collagen binding. Fibroblasts expressing the mannose receptor adhere to type I, type III and type IV collagens, but not to type V collagen, and the adherence is inhibited by isolated mannose receptor fibronectin type II domain. The fibronectin type II domain shows the same specificity for collagen as the whole receptor, binding to type I, type III and type IV collagens. This is the first activity assigned to the fibronectin type II domain of the mannose receptor. The results suggest additional roles for this multifunctional receptor in mediating collagen clearance or cell-matrix adhesion.

Structural Model for the Mannose Receptor Family Uncovered by Electron Microscopy of Endo180 and the Mannose Receptor

The mannose receptor family comprises four members in mammals, Endo180 (CD280), DEC-205 (CD205), phospholipase A(2) receptor (PLA(2)R) and the mannose receptor (MR, CD206), whose extracellular portion contains a similar domain arrangement: an N-terminal cysteine-rich domain (CysR) followed by a single fibronectin type II domain (FNII) and 8-10 C-type lectin-like domains (CTLDs). These proteins mediate diverse functions ranging from extracellular matrix turnover through collagen uptake to homeostasis and immunity based on sugar recognition. Endo180 and the MR are multivalent transmembrane receptors capable of interacting with multiple ligands; in both receptors FNII recognizes collagens, and a single CTLD retains lectin activity (CTLD2 in Endo180 and CTLD4 in MR). It is expected that the overall conformation of these multivalent molecules would deeply influence their function as the availability of their binding sites could be altered under different conditions. However, conflicting reports have been published on the three-dimensional arrangement of these receptors. Here, we have used single particle electron microscopy to elucidate the three-dimensional organization of the MR and Endo180. Strikingly, we have found that both receptors display distinct three-dimensional structures, which are, however, conceptually very similar: a bent and compact conformation built upon interactions of the CysR domain and the lone functional CTLD. Biochemical and electron microscopy experiments indicate that, under a low pH mimicking the endosomal environment, both MR and Endo180 experience large conformational changes. We propose a structural model for the mannose receptor family where at least two conformations exist that may serve to regulate differences in ligand selectivity.

The chicken yolk sac IgY receptor, a functional equivalent of the mammalian MHC-related Fc receptor, is a phospholipase A2 receptor homolog

In mammals, IgG is transferred from mother to young by the MHC-related receptor FcRn, which binds IgG in acidic endosomes and releases it at basic pH into blood. Maternal IgY, the avian counterpart of IgG, is transferred to embryos across yolk sac membranes. We affinity-purified the chicken yolk sac IgY receptor (FcRY) and sequenced its gene. FcRY is unrelated to MHC molecules but is a homolog of the mammalian phospholipase A(2) receptor. Analytical ultracentrifugation and truncation experiments suggest that FcRY forms a compact structure containing an IgY binding site at acidic pH but undergoes a conformational change at basic pH that disrupts the site. FcRY is thus unrelated to mammalian FcRn in both its structure and mechanism for pH-dependent binding, illustrating distinct routes utilized by evolution to transfer antibodies.

The urokinase receptor (uPAR) and the uPAR-associated protein (uPARAP/Endo180): membrane proteins engaged in matrix turnover during tissue remodeling

The breakdown of the barriers formed by extracellular matrix proteins is a pre-requisite for all processes of tissue remodeling. Matrix degradation reactions take part in specific physiological events in the healthy organism but also represent a crucial step in cancer invasion. These degradation processes involve a highly organized interplay between proteases and their cellular binding sites as well as specific substrates and internalization receptors. This review article is focused on two components, the urokinase plasminogen activator receptor (uPAR) and the uPAR-associated protein (uPARAP, also designated Endo180), that are considered crucially engaged in matrix degradation. uPAR and uPARAP have highly diverse functions, but on certain cell types they interact with each other in a process that is still incompletely understood. uPAR is a glycosyl-phosphatidylinositol-anchored glycoprotein on the surface of various cell types that serves to bind the urokinase plasminogen activator and localize the activation reactions in the proteolytic cascade system of plasminogen activation. uPARAP is an integral membrane protein with a pronounced role in the internalization of collagen for intracellular degradation. Both receptors have additional functions that are currently being unraveled. The present discussion of uPAR and uPARAP is centered on their protein structure and molecular and cellular function.

Three-dimensional interplay among the ligand-binding domains of the urokinase-plasminogen-activator-receptor-associated protein, Endo180

Endo180, also known as the urokinase plasminogen activator receptor (uPAR)-associated protein (uPARAP), is one of the four members of the mannose receptor family, and is implicated in extracellular-matrix remodelling through its interactions with collagens, sugars and uPAR. The extracellular portion of Endo180 contains an amino-terminal cysteine-rich domain, a single fibronectin type II domain and eight C-type lectin-like domains. We have purified a soluble version of Endo180 and analysed it by single-particle electron microscopy to obtain a three-dimensional structure of the N-terminal part of the protein at a resolution of 17 A and reveal, for the first time, the interactions between non-adjacent domains in the mannose receptor family. We show that for Endo180, the cysteine-rich domain contacts the second C-type lectin-like domain, thus providing structural insight into how modulation of its several ligand interactions may regulate Endo180 receptor function.

A targeted deletion in the endocytic receptor gene Endo180 results in a defect in collagen uptake

The four members of the mannose receptor family (the mannose receptor, the M-type phospholipase A(2) receptor, DEC-205 and Endo180) share a common extracellular arrangement of an amino-terminal cysteine-rich domain followed by a fibronectin type II (FNII) domain and multiple C-type lectin-like domains (CTLDs). In addition, all have a short cytoplasmic domain, which mediates their constitutive recycling between the plasma membrane and the endosomal apparatus, suggesting that these receptors function to internalize ligands for intracellular delivery. We have generated mice with a targeted deletion of Endo180 exons 2-6 and show that this mutation results in the efficient expression of a truncated Endo180 protein that lacks the cysteine-rich domain, the FNII domain and CTLD1. Analysis of embryonic fibroblasts reveals that this mutation does not disrupt the C-type lectin activity that is mediated by CTLD2, but results in cells that have a defect in collagen binding and internalization and an impaired migratory phenotype.

Oligolysine-based oligosaccharide clusters: selective recognition and endocytosis by the mannose receptor and dendritic cell-specific intercellular adhesion molecule 3 (ICAM-3)-grabbing nonintegrin

Dendritic cells are potent antigen-presenting cells that express several membrane lectins, including the mannose receptor and DC-SIGN (dendritic cell-specific ICAM-3-grabbing nonintegrin). To identify highly specific ligands for these dendritic cell receptors, oligosaccharides were converted into glycosynthons (Os1) and were used to prepare oligolysine-based glycoclusters, Os-[Lys(Os)]n-Ala-Cys-NH2. Clusters containing two to six dimannosides as well as clusters containing four or five pentasaccharides (Lewisa or Lewisx) or hexasaccharides (Lewisb) were synthesized. The thiol group of the appended cysteine residue allows easy tagging by a fluorescent probe or convenient substitution with an antigen. Surface plasmon resonance was used to determine the affinity of the different glycoclusters for purified mannose receptor and DC-SIGN, whereas flow cytometry and confocal microscopy analysis allowed assessment of cell uptake of fluoresceinyl-labeled glycoclusters. Dimannoside clusters are recognized by the mannose receptor with an affinity constant close to 106 liter.mol-1 but have a very low affinity for DC-SIGN (less than 104 liter x mol-1). Conversely, Lewis clusters have a higher affinity toward DC-SIGN than toward the mannose receptor. Dimannoside clusters are efficiently taken up by human dendritic cells as well as by rat fibroblasts expressing the mannose receptor but not by HeLa cells or rat fibroblasts expressing DC-SIGN; DC-SIGN-expressing cells take up Lewis clusters. The results suggest that ligands containing dimannoside clusters can be used specifically to target the mannose receptor, whereas ligands containing Lewis clusters will be targeted to DC-SIGN.

A mannose-receptor is possibly involved in the phagocytosis of Saccharomyces cerevisiae by seabream (Sparus aurata L.) leucocytes

In this paper the possible involvement of the mannose-receptor on the non-specific recognition and phagocytosis of heat killed yeast cells (Saccharomyces cerevisiae) by gilthead seabream (Sparus aurata L.) head-kidney leucocytes was established by studying the ability of different sugars to inhibit the uptake of the yeast cells by leucocytes. Leucocytes were preincubated for 30min with different concentrations of sugar (alpha-mannan, d-mannose, d-fucose, l-fucose, d-glucose, d-glucosamine and n-acetyl-glucosamine, all of them described as specific ligands of the vertebrate mannose-receptor) and afterwards incubated with FITC-labelled yeast cells for phagocytosis assays. The phagocytic ability (percentage of cells with one or more ingested yeast cells within the total cell population) and capacity (number of ingested yeast cells per cell) of leucocytes was analysed by flow cytometry. The results demonstrate the potential existence of a specific receptor-sugar or receptor-yeast cell binding process, which was saturable, specific and dose-dependent. More specifically, when leucocytes were preincubated with appropriate doses of d-mannose, d- or l-fucose, d-glucose or n-acetyl-glucosamine the phagocytosis of yeast cells by head-kidney leucocytes was partially blocked. Seabream leucocytes were also preincubated with chloroquine, a lysosomotropic drug which downregulates (in a nonspecific manner) the expression of mannose-receptors in mammals, before phagocytosis assays were performed. The results demonstrated that the phagocytosis of yeast was completely blocked by this substance. The overall results seem to corroborate the presence of the mannose-receptor in seabream phagocytes, which is involved in the non-specific binding and phagocytosis of yeast cells by head-kidney leucocytes.

Influence of spacer length on interaction of mannosylated liposomes with human phagocytic cells

PURPOSE

To improve target specificity and uptake of liposomes by macrophages, one can improve high-affinity receptor binding to mannose determinants with their 175-kDa mannose receptor (MR), which is mainly influenced by the length and flexibility of the spacer between the carbohydrate head group and liposome surface. Liposomes containing alkylmannosides with hydrophilic spacers 0 to 8 ethyleneoxy units (EO) long (Man0...Man8) were used to investigate systematically the effects of spacer length on liposome-cell interactions.

METHODS

Concanavalin A (ConA)-induced liposome aggregation was studied by turbidity measurement and cell uptake using PMA-induced HL-60 cells or native human macrophages by determining 6-CF after cell lysis or NBD-fluorescence with flow cytometry. Detection of MR in native cell populations was carried out by an antibody assay using flow cytometry; MR-representing cells were selected analytically.

RESULTS

Liposomes containing mannosides with more than one EO spacer length were specifically aggregated by ConA, indicating accessibility of the carbohydrate ligands of these derivatives. Increase in EO spacer units of incorporated mannosides (two or more EO) led to suppression of cellular uptake of mannosylated liposomes by phagocytes lacking MR (HL60, U937). The extent of suppression increased with spacer length. Liposome uptake by native macrophages expressing MR was, on the contrary, improved, particularly by Man6 and Man8.

CONCLUSIONS

Uptake of liposomes modified with Man6 or Man8 by native cells was enhanced but did not reach an optimum. Thus, Man6, Man8, and mannosides with even longer spacer arms are of potential use in receptor-mediated targeting.

Characterization of sugar binding by the mannose receptor family member, Endo180

Members of the mannose receptor family, the mannose receptor, the phospholipase A(2) receptor, DEC-205, and Endo180, contain multiple C-type lectin-like domains (CTLDs) within a single polypeptide. In addition, at their N termini, all four family members contain a cysteine-rich domain similar to the R-type carbohydrate recognition domains of ricin. However, despite the common presence of multiple lectin-like domains, these four endocytic receptors have divergent ligand binding activities, and it is clear that the majority of these domains do not bind sugars. Here the functions of the lectin-like domains of the most recently discovered family member, Endo180, have been investigated. Endo180 is shown to bind in a Ca(2+)-dependent manner to mannose, fucose, and N-acetylglucosamine but not to galactose. This activity is mediated by one of the eight CTLDs, CTLD2. Competition assays indicate that the monosaccharide binding specificity of Endo180 CTLD2 is similar to that of mannose receptor CTLD4. However, additional experiments indicate that, unlike the cysteine-rich domain of the mannose receptor, the cysteine-rich domain of Endo180 does not bind sulfated sugars. Thus, although Endo180 and the mannose receptor are now both known to be mannose binding lectins, each receptor is likely to have a distinct set of glycoprotein ligands in vivo.

Characterization of microglial responses to Staphylococcus aureus: effects on cytokine, costimulatory molecule, and Toll-like receptor expression

Microglia participate in innate immune responses in the central nervous system (CNS). This work demonstrates that microglia can recognize heat-inactivated Staphylococcus aureus and its cell wall product peptidoglycan (PGN) through the elaboration of numerous proinflammatory cytokines and chemokines. Microglia also exhibited S. aureus bactericidal activity. Microglia constitutively expressed low levels of Toll-like receptor 1 (TLR1), TLR2, TLR6, and CD14, all of which were enhanced following S. aureus exposure. Activation of resident microglia by S. aureus may serve to amplify the CNS antibacterial immune response through the release of cytokines, chemokines, and induction of bactericidal activity.