Glycosaminoglycans are interactants of Langerin: comparison with gp120 highlights an unexpected calcium-independent binding mode

Secondary Sites of the C-type Lectin-Like Fold

C-type lectins are a large superfamily of proteins involved in a multitude of biological processes. In particular, their involvement in immunity and homeostasis has rendered them attractive targets for diverse therapeutic interventions. They share a characteristic C-type lectin-like domain whose adaptability enables them to bind a broad spectrum of ligands beyond the originally defined canonical Ca2+-dependent carbohydrate binding. Together with variable domain architecture and high-level conformational plasticity, this enables C-type lectins to meet diverse functional demands. Secondary sites provide another layer of regulation and are often intricately linked to functional diversity. Located remote from the canonical primary binding site, secondary sites can accommodate ligands with other physicochemical properties and alter protein dynamics, thus enhancing selectivity and enabling fine-tuning of the biological response. In this review, we outline the structural determinants allowing C-type lectins to perform a large variety of tasks and to accommodate the ligands associated with it. Using the six well-characterized Ca2+-dependent and Ca2+-independent C-type lectin receptors DC-SIGN, langerin, MGL, dectin-1, CLEC-2 and NKG2D as examples, we focus on the characteristics of non-canonical interactions and secondary sites and their potential use in drug discovery endeavors.

Multivalent, calcium-independent binding of surfactant protein A and D to sulfated glycosaminoglycans of the alveolar epithelial glycocalyx

Lung surfactant collectins, surfactant protein A (SP-A) and D (SP-D), are oligomeric C-type lectins involved in lung immunity. Through their carbohydrate recognition domain, they recognize carbohydrates at pathogen surfaces and initiate lung innate immune response. Here, we propose that they may also be able to bind to other carbohydrates present in typical cell surfaces, such as the alveolar epithelial glycocalyx. To test this hypothesis, we analyzed and quantified the binding affinity of SP-A and SP-D to different sugars and glycosaminoglycans (GAGs) by microscale thermophoresis (MST). In addition, by changing the calcium concentration, we aimed to characterize any consequences on the binding behavior. Our results show that both oligomeric proteins bind with high affinity (in nanomolar range) to GAGs, such as hyaluronan (HA), heparan sulfate (HS) and chondroitin sulfate (CS). Binding to HS and CS was calcium-independent, as it was not affected by changing calcium concentration in the buffer. Quantification of GAGs in bronchoalveolar lavage (BAL) fluid from animals deficient in either SP-A or SP-D showed changes in GAG composition, and electron micrographs showed differences in alveolar glycocalyx ultrastructure in vivo. Taken together, SP-A and SP-D bind to model sulfated glycosaminoglycans of the alveolar epithelial glycocalyx in a multivalent and calcium-independent way. These findings provide a potential mechanism for SP-A and SP-D as an integral part of the alveolar epithelial glycocalyx binding and interconnecting free GAGs, proteoglycans, and other glycans in glycoproteins, which may influence glycocalyx composition and structure.NEW & NOTEWORTHY SP-A and SP-D function has been related to innate immunity of the lung based on their binding to sugar residues at pathogen surfaces. However, their function in the healthy alveolus was considered as limited to interaction with surfactant lipids. Here, we demonstrated that these proteins bind to glycosaminoglycans present at typical cell surfaces like the alveolar epithelial glycocalyx. We propose a model where these proteins play an important role in interconnecting alveolar epithelial glycocalyx components.

The unique 3D arrangement of macrophage galactose lectin enables Escherichia coli lipopolysaccharide recognition through two distinct interfaces

Lipopolysaccharides are a hallmark of gram-negative bacteria, and their presence at the cell surface is key for bacterial integrity. As surface-exposed components, they are recognized by immunity C-type lectin receptors present on antigen-presenting cells. Human macrophage galactose lectin binds Escherichia coli surface that presents a specific glycan motif. Nevertheless, this high-affinity interaction occurs regardless of the integrity of its canonical calcium-dependent glycan-binding site. NMR of macrophage galactose-type lectin (MGL) carbohydrate recognition domain and complete extracellular domain revealed a glycan-binding site opposite to the canonical site. A model of trimeric macrophage galactose lectin was determined based on a combination of small-angle X-ray scattering and AlphaFold. A disulfide bond positions the carbohydrate recognition domain perpendicular to the coiled-coil domain. This unique configuration for a C-type lectin orients the six glycan sites of MGL in an ideal position to bind lipopolysaccharides at the bacterial surface with high avidity.

Multivalent glycosystems for human lectins

Human lectins are involved in a wide variety of biological processes, both physiological and pathological, which have attracted the interest of the scientific community working in the glycoscience field. Multivalent glycosystems have been employed as useful tools to understand carbohydrate-lectin binding processes as well as for biomedical applications. The review shows the different scaffolds designed for a multivalent presentation of sugars and their corresponding binding studies to lectins and in some cases, their biological activities. We summarise this research by organizing based on lectin types to highlight the progression in this active field. The paper provides an overall picture of how these contributions have furnished relevant information on this topic to help in understanding and participate in these carbohydrate-lectin interactions.

Medicinal chemistry of the myeloid C-type lectin receptors Mincle, Langerin, and DC-SIGN

In their role as pattern-recognition receptors on cells of the innate immune system, myeloid C-type lectin receptors (CLRs) assume important biological functions related to immunity, homeostasis, and cancer. As such, this family of receptors represents an appealing target for therapeutic interventions for modulating the outcome of many pathological processes, in particular related to infectious diseases. This review summarizes the current state of research into glycomimetic or drug-like small molecule ligands for the CLRs Mincle, Langerin, and DC-SIGN, which have potential therapeutic applications in vaccine research and anti-infective therapy.

Synthesis, self-assembly and Langerin recognition studies of a resorcinarene-based glycocluster exposing a hyaluronic acid thiodisaccharide mimetic

Herein, we report the synthesis of an octavalent glycocluster exposing a thiodisaccharide mimetic of the repetitive unit of hyaluronic acid, βSGlcA(1 → 3)βSGlcNAc, constructed on a calix[4]resorcinarene scaffold by CuAAC reaction of suitable precursors. This glycocluster showed a strong tendency toward self-aggregation. DOSY-NMR and DLS experiments demonstrated the formation of spherical micelles of d ≅ 6.2 nm, in good agreement. TEM micrographs showed the presence of particles of different sizes, depending on the pH of the starting solution, thus evidencing that the negative charge on the micelle surface due to ionization of the GlcA residues plays an important role in the aggregation process. STD-NMR and DLS experiments provided evidence of the interaction between the synthetic glycocluster and Langerin, a relevant C-type lectin. This interaction was not observed in the STD-NMR experiments performed with the basic disaccharide, providing evidence of a multivalent effect.

GAG Multivalent Systems to interact with Langerin

Langerin is a C-type Lectin expressed at the surface of Langerhans cells, which play a pivotal role in protecting organisms against pathogen infections. To address this aim, Langerin presents at least two recognition sites, one Ca2+-dependent and another one independent, capable of recognizing a variety of carbohydrate ligands. In contrast to other lectins, Langerin recognizes sulfated glycosaminoglycans (GAGs), a family of complex and heterogeneous polysaccharides present in the cell membrane and the extracellular matrix at the interphase generated in the trimeric form of Langerin but absent in the monomeric form. The complexity of these oligosaccharides has impeded the development of well-defined monodisperse structures to study these interaction processes. However, in the last few decades, an improvement of synthetic developments to achieve the preparation of carbohydrate multivalent systems mimicking the GAGs has been described. Despite all these contributions, very few examples are reported where the GAG multivalent structures are used to evaluate the interaction with Langerin. These molecules should pave the way to explore these GAG-Langerin interactions.

DC/L-SIGN recognition of spike glycoprotein promotes SARS-CoV-2 trans-infection and can be inhibited by a glycomimetic antagonist

The efficient spread of SARS-CoV-2 resulted in a unique pandemic in modern history. Despite early identification of ACE2 as the receptor for viral spike protein, much remains to be understood about the molecular events behind viral dissemination. We evaluated the contribution of C-type lectin receptors (CLRS) of antigen-presenting cells, widely present in respiratory mucosa and lung tissue. DC-SIGN, L-SIGN, Langerin and MGL bind to diverse glycans of the spike using multiple interaction areas. Using pseudovirus and cells derived from monocytes or T-lymphocytes, we demonstrate that while virus capture by the CLRs examined does not allow direct cell infection, DC/L-SIGN, among these receptors, promote virus transfer to permissive ACE2+ Vero E6 cells. A glycomimetic compound designed against DC-SIGN, enable inhibition of this process. These data have been then confirmed using authentic SARS-CoV-2 virus and human respiratory cell lines. Thus, we described a mechanism potentiating viral spreading of infection.

Molecular Recognition in C-Type Lectins: The Cases of DC-SIGN, Langerin, MGL, and L-Sectin

Carbohydrates play a pivotal role in intercellular communication processes. In particular, glycan antigens are key for sustaining homeostasis, helping leukocytes to distinguish damaged tissues and invading pathogens from healthy tissues. From a structural perspective, this cross-talk is fairly complex, and multiple membrane proteins guide these recognition processes, including lectins and Toll-like receptors. Since the beginning of this century, lectins have become potential targets for therapeutics for controlling and/or avoiding the progression of pathologies derived from an incorrect immune outcome, including infectious processes, cancer, or autoimmune diseases. Therefore, a detailed knowledge of these receptors is mandatory for the development of specific treatments. In this review, we summarize the current knowledge about four key C-type lectins whose importance has been steadily growing in recent years, focusing in particular on how glycan recognition takes place at the molecular level, but also looking at recent progresses in the quest for therapeutics.

MASP-2 Is a Heparin-Binding Protease; Identification of Blocking Oligosaccharides

It is well-known that heparin and other glycosaminoglycans (GAGs) inhibit complement activation. It is however not known whether fractionation and/or modification of GAGs might deliver pathway-specific inhibition of the complement system. Therefore, we evaluated a library of GAGs and their derivatives for their functional pathway specific complement inhibition, including the MASP-specific C4 deposition assay. Interaction of human MASP-2 with heparan sulfate/heparin was evaluated by surface plasmon resonance, ELISA and in renal tissue. In vitro pathway-specific complement assays showed that highly sulfated GAGs inhibited all three pathways of complement. Small heparin- and heparan sulfate-derived oligosaccharides were selective inhibitors of the lectin pathway (LP). These small oligosaccharides showed identical inhibition of the ficolin-3 mediated LP activation, failed to inhibit the binding of MBL to mannan, but inhibited C4 cleavage by MASPs. Hexa- and pentasulfated tetrasaccharides represent the smallest MASP inhibitors both in the functional LP assay as well in the MASP-mediated C4 assay. Surface plasmon resonance showed MASP-2 binding with heparin and heparan sulfate, revealing high Kon and Koff rates resulted in a Kd of ~2 μM and confirmed inhibition by heparin-derived tetrasaccharide. In renal tissue, MASP-2 partially colocalized with agrin and heparan sulfate, but not with activated C3, suggesting docking, storage, and potential inactivation of MASP-2 by heparan sulfate in basement membranes. Our data show that highly sulfated GAGs mediated inhibition of all three complement pathways, whereas short heparin- and heparan sulfate-derived oligosaccharides selectively blocked the lectin pathway via MASP-2 inhibition. Binding of MASP-2 to immobilized heparan sulfate/heparin and partial co-localization of agrin/heparan sulfate with MASP, but not C3b, might suggest that in vivo heparan sulfate proteoglycans act as a docking platform for MASP-2 and possibly prevent the lectin pathway from activation.

Asymmetrically Branched Precision Glycooligomers Targeting Langerin

Asymmetrically branched precision glycooligomers are synthesized by solid-phase polymer synthesis for studying multivalent carbohydrate-protein interactions. Through the stepwise assembly of Fmoc-protected oligo(amidoamine) building blocks and Fmoc/Dde-protected lysine, straightforward variation of structural parameters such as the number and length of arms, as well as the number and position of carbohydrate ligands, is achieved. Binding of 1-arm and 3-arm glycooligomers toward lectin receptors langerin and concanavalin A (ConA) was evaluated where the smallest 3-arm glycooligomer shows the highest binding toward langerin, and stepwise elongation of one, two, or all three arms leads to decreased binding. When directly comparing binding toward langerin and ConA, we find that structural variation of the scaffold affects glycomimetic ligand binding differently for the different targets, indicating the potential to tune such ligands not only for their avidity but also for their selectivity toward different lectins.

Langerin-Heparin Interaction: Analysis of the Binding to the Non-Lectin Site

Langerin is a C-type lectin involved in the immune response that forms a trimer in its active form. It can interact with carbohydrates using 2 sites with different selectivity, the C-lectin site, a Ca2+-mediated binding, and the cleft between chains. Here we report the complementary analysis of the interaction between a heparin-like hexasaccharide 1 and langerin at the second site.

A Specific, Glycomimetic Langerin Ligand for Human Langerhans Cell Targeting

Langerhans cells are a subset of dendritic cells residing in the epidermis of the human skin. As such, they are key mediators of immune regulation and have emerged as prime targets for novel transcutaneous cancer vaccines. Importantly, the induction of protective T cell immunity by these vaccines requires the efficient and specific delivery of both tumor-associated antigens and adjuvants. Langerhans cells uniquely express Langerin (CD207), an endocytic C-type lectin receptor. Here, we report the discovery of a specific, glycomimetic Langerin ligand employing a heparin-inspired design strategy and structural characterization by NMR spectroscopy and molecular docking. The conjugation of this glycomimetic to liposomes enabled the specific and efficient targeting of Langerhans cells in the human skin. We further demonstrate the doxorubicin-mediated killing of a Langerin+ monocyte cell line, highlighting its therapeutic and diagnostic potential in Langerhans cell histiocytosis, caused by the abnormal proliferation of Langerin+ myeloid progenitor cells. Overall, our delivery platform provides superior versatility over antibody-based approaches and novel modalities to overcome current limitations of dendritic cell-targeted immuno- and chemotherapy.

The recognition of glycans by protein receptors. Insights from NMR spectroscopy

Carbohydrates (glycans, saccharides, sugars) are everywhere. In fact, glycan-protein interactions are involved in many essential processes of life and disease. The understanding of the key structural details at the atomic and molecular level is of paramount importance to effectively design molecules for therapeutic purposes. Different approximations may be employed to decipher these molecular recognition processes with high resolution. Advances in cryo-electron microscopy are providing exquisite details on different biological mechanisms involving sugars, while better and better protocols for structural refinement in the application of X-ray methods for protein-sugar complexes and glycoproteins are also permitting fantastic advances in the glycoscience arena. Alternatively, NMR spectroscopy remains as one of the most rewarding techniques to explore protein-carbohydrate interactions. In fact, given the intrinsic dynamic nature of saccharides, NMR can afford exquisite structural information at the atomic detail, not accessible by other techniques. However, the access to this information is sometimes intricate, and requires careful analysis and well-defined strategies. In this review, we have highlighted these issues and presented an overview of different modern NMR approaches with a focus on the latest developments and challenges.

On-Chip Screening of a Glycomimetic Library with C-Type Lectins Reveals Structural Features Responsible for Preferential Binding of Dectin-2 over DC-SIGN/R and Langerin

A library of mannose‐ and fucose‐based glycomimetics was synthesized and screened in a microarray format against a set of C‐type lectin receptors (CLRs) that included DC‐SIGN, DC‐SIGNR, langerin, and dectin‐2. Glycomimetic ligands able to interact with dectin‐2 were identified for the first time. Comparative analysis of binding profiles allowed their selectivity against other CLRs to be probed.

Implication of C-type lectin receptor langerin and keratan sulfate disaccharide in emphysema

Glycosylation is profoundly involved in various diseases, and interactions between glycan binding proteins and their sugar ligands are plausible drug targets. Keratan sulfate (KS), a glycosaminoglycan, is downregulated in lungs by cigarette smoking, suggesting that KS is involved in smoking-related diseases, such as chronic obstructive pulmonary disease (COPD). We found that a highly sulfated KS disaccharide, L4, suppresses lung inflammation and is effective against COPD and its exacerbation in mouse models. Its anti-inflammatory activity was comparable to that of a steroid. As a possible mechanism, langerin, a C-type lectin receptor (CLR) expressed in dendritic cells, was suggested to function as an L4 receptor. Oligomeric L4 derivatives were chemically designed to create new ligands with higher affinity and activity. The synthetic L4 oligomers bound to langerin with over 1000-fold higher affinity than the L4 monomer, suggesting that these compounds are effective drug candidates against COPD and inflammatory diseases.

High affinity sugar ligands of C-type lectin receptor langerin

BACKGROUND

Langerin, a C-type lectin receptor (CLR) expressed in a subset of dendritic cells (DCs), binds to glycan ligands for pathogen capture and clearance. Previous studies revealed that langerin has an unusual binding affinity toward 6-sulfated galactose (Gal), a structure primarily found in keratan sulfate (KS). However, details and biological outcomes of this interaction have not been characterized. Based on a recent discovery that the disaccharide L4, a KS component that contains 6-sulfo-Gal, exhibits anti-inflammatory activity in mouse lung, we hypothesized that L4-related compounds are useful tools for characterizing the langerin-ligand interactions and their therapeutic application.

METHODS

We performed binding analysis between purified long and short forms of langerin and a series of KS disaccharide components. We also chemically synthesized oligomeric derivatives of L4 to develop a new high-affinity ligand of langerin.

RESULTS

We show that the binding critically requires the 6-sulfation of Gal and that the long form of langerin displays higher affinity than the short form. The synthesized trimeric (also designated as triangle or Tri) and polymeric (pendant) L4 derivatives displayed over 1000-fold higher affinity toward langerin than monomeric L4. The pendant L4, but not the L4 monomer, was found to effectively transduce langerin signaling in a model cell system.

CONCLUSIONS

L4 is a specific ligand for langerin. Oligomerization of L4 unit increased the affinity toward langerin.

GENERAL SIGNIFICANCE

These results suggest that oligomeric L4 derivatives will be useful for clarifying the langerin functions and for the development of new glycan-based anti-inflammatory drugs.

Rational-Differential Design of Highly Specific Glycomimetic Ligands: Targeting DC-SIGN and Excluding Langerin Recognition

At the surface of dendritic cells, C-type lectin receptors (CLRs) allow the recognition of carbohydrate-based PAMPS or DAMPS (pathogen- or danger-associated molecular patterns, respectively) and promote immune response regulation. However, some CLRs are hijacked by viral and bacterial pathogens. Thus, the design of ligands able to target specifically one CLR, to either modulate an immune response or to inhibit a given infection mechanism, has great potential value in therapeutic design. A case study is the selective blocking of DC-SIGN, involved notably in HIV trans-infection of T lymphocytes, without interfering with langerin-mediated HIV clearance. This is a challenging task due to their overlapping carbohydrate specificity. Toward the rational design of DC-SIGN selective ligands, we performed a comparative affinity study between DC-SIGN and langerin with natural ligands. We found that GlcNAc is recognized by both CLRs; however, selective sulfation are shown to increase the selectivity in favor of langerin. With the combination of site-directed mutagenesis and X-ray structural analysis of the langerin/GlcNS6S complex, we highlighted that 6-sulfation of the carbohydrate ligand induced langerin specificity. Additionally, the K313 residue from langerin was identified as a critical feature of its binding site. Using a rational and a differential approach in the study of CLR binding sites, we designed, synthesized, and characterized a new glycomimetic, which is highly specific for DC-SIGN vs langerin. STD NMR, SPR, and ITC characterizations show that compound 7 conserved the overall binding mode of the natural disaccharide while possessing an improved affinity and a strict specificity for DC-SIGN.

Calcitonin Gene-Related Peptide Induces HIV-1 Proteasomal Degradation in Mucosal Langerhans Cells

The neuroimmune dialogue between peripheral neurons and Langerhans cells (LCs) within mucosal epithelia protects against incoming pathogens. LCs rapidly internalize human immunodeficiency virus type 1 (HIV-1) upon its sexual transmission and then trans-infect CD4⁺ T cells. We recently found that the neuropeptide calcitonin gene-related peptide (CGRP), secreted mucosally from peripheral neurons, inhibits LC-mediated HIV-1 trans-infection. In this study, we investigated the mechanism of CGRP-induced inhibition, focusing on HIV-1 degradation in LCs and its interplay with trans-infection. We first show that HIV-1 degradation occurs in endolysosomes in untreated LCs, and functionally blocking such degradation with lysosomotropic agents results in increased trans-infection. We demonstrate that CGRP acts via its cognate receptor and at a viral postentry step to induce faster HIV-1 degradation, but without affecting the kinetics of endolysosomal degradation. We reveal that unexpectedly, CGRP shifts HIV-1 degradation from endolysosomes toward the proteasome, providing the first evidence for functional HIV-1 proteasomal degradation in LCs. Such efficient proteasomal degradation significantly inhibits the first phase of trans-infection, and proteasomal, but not endolysosomal, inhibitors abrogate CGRP-induced inhibition. Together, our results establish that CGRP controls the HIV-1 degradation mode in LCs. The presence of endogenous CGRP within innervated mucosal tissues, especially during the sexual response, to which CGRP contributes, suggests that HIV-1 proteasomal degradation predominates in vivo Hence, proteasomal, rather than endolysosomal, HIV-1 degradation in LCs should be enhanced clinically to effectively restrict HIV-1 trans-infection.IMPORTANCE During sexual transmission, HIV-1 is internalized and degraded in LCs, the resident antigen-presenting cells in mucosal epithelia. Yet during trans-infection, infectious virions escaping degradation are transferred to CD4⁺ T cells, the principal HIV-1 targets. We previously found that the neuroimmune dialogue between LCs and peripheral neurons, innervating mucosal epithelia, significantly inhibits trans-infection via the action of the secreted neuropeptide CGRP on LCs. In this study, we investigated whether CGRP-induced inhibition of trans-infection is linked to CGRP-controlled HIV-1 degradation in LCs. We show that in untreated LCs, HIV-1 is functionally degraded in endolysosomes. In sharp contrast, we reveal that in CGRP-treated LCs, HIV-1 is diverted toward and degraded via another cytosolic protein degradative pathway, namely, the proteasome. These results establish that CGRP regulates HIV-1 degradation in LCs. As CGRP contributes to the sexual response and present within mucosal epithelia, HIV-1 proteasomal degradation in LCs might predominate in vivo and should be enhanced clinically.

Calcium-Independent Activation of an Allosteric Network in Langerin by Heparin Oligosaccharides

The C-type lectin receptor Langerin is a glycan-binding protein that serves as an uptake receptor on Langerhans cells and is essential for the formation of Birbeck granules. Whereas most Langerin ligands are recognized by a canonical Ca²⁺ -dependent binding site, heparins have been proposed to make additional contacts to a secondary, Ca²⁺ -independent site. Glycan array screening and biomolecular NMR spectroscopy were employed to investigate the molecular mechanism of these interactions. We observed that binding of heparin hexasaccharides to a secondary site did not require the presence of Ca²⁺ and activated a previously identified intradomain allosteric network of Langerin (thus far only associated with Ca²⁺ affinity and release). We propose a communication hub between these two binding sites, which sheds new light on modulatory functions of Langerin-heparin interactions.

Bacterial Polysaccharide Specificity of the Pattern Recognition Receptor Langerin Is Highly Species-dependent

The recognition of pathogen surface polysaccharides by glycan-binding proteins is a cornerstone of innate host defense. Many members of the C-type lectin receptor family serve as pattern recognition receptors facilitating pathogen uptake, antigen processing, and immunomodulation. Despite the high evolutionary pressure in host-pathogen interactions, it is still widely assumed that genetic homology conveys similar specificities. Here, we investigate the ligand specificities of the human and murine forms of the myeloid C-type lectin receptor langerin for simple and complex ligands augmented by structural insight into murine langerin. Although the two homologs share the same three-dimensional structure and recognize simple ligands identically, a screening of more than 300 bacterial polysaccharides revealed highly diverging avidity and selectivity for larger and more complex glycans. Structural and evolutionary conservation analysis identified a highly variable surface adjacent to the canonic binding site, potentially forming a secondary site of interaction for large glycans.

(19)F NMR-Guided Design of Glycomimetic Langerin Ligands

C-type lectin receptors (CLRs) play a pivotal role in pathogen defense and immune homeostasis. Langerin, a CLR predominantly expressed on Langerhans cells, represents a potential target receptor for the development of anti-infectives or immunomodulatory therapies. As mammalian carbohydrate binding sites typically display high solvent exposure and hydrophilicity, the recognition of natural monosaccharide ligands is characterized by low affinities. Consequently, glycomimetic ligand design poses challenges that extend to the development of suitable assays. Here, we report the first application of (19)F R2-filtered NMR to address these challenges for a CLR, i.e., Langerin. The homogeneous, monovalent assay was essential to evaluating the in silico design of 2-deoxy-2-carboxamido-α-mannoside analogs and enabled the implementation of a fragment screening against the carbohydrate binding site. With the identification of both potent monosaccharide analogs and fragment hits, this study represents an important advancement toward the design of glycomimetic Langerin ligands and highlights the importance of assay development for other CLRs.

Kinetic and Structural Studies of Interactions between Glycosaminoglycans and Langerin

Langerin, a C-type lectin, is expressed in Langerhans cells. It was reported that langerin binds sulfated glycans, which is an important initial step for its role in blocking human immunodeficiency virus (HIV) transmission by capturing HIV pathogens and mediating their internalization into Birbeck granules for their elimination. It is fundamentally important to understand these interactions at the molecular level for the design of new highly specific therapeutic agents for HIV. Surface plasmon resonance (SPR), which allows for the real-time, direct, quantitative analysis of the label-free molecular interactions, has been used successfully for biophysical characterization of glycosaminoglycan (GAG)-protein interactions. In this study, we report kinetics, structural analysis, and the effects of physiological conditions (e.g., pH, salt concentration, and Ca(2+) and Zn(2+)concentrations) on the interactions between GAGs and langerin using SPR. SPR results revealed that langerin binds to heparin with high affinity (KD ∼ 2.4 nM) and the oligosaccharide length required for the interactions is larger than a tetrasaccharide. This heparin/heparan sulfate-binding protein also interacts with other GAGs, including dermatan sulfate, chondroitin sulfates C-E and KS. In addition, liquid chromatography-mass spectrometry analysis was used to characterize the structure of sulfated glycans that bound to langerin.

"Rules of Engagement" of Protein-Glycoconjugate Interactions: A Molecular View Achievable by using NMR Spectroscopy and Molecular Modeling

Understanding the dynamics of protein-ligand interactions, which lie at the heart of host-pathogen recognition, represents a crucial step to clarify the molecular determinants implicated in binding events, as well as to optimize the design of new molecules with therapeutic aims. Over the last decade, advances in complementary biophysical and spectroscopic methods permitted us to deeply dissect the fine structural details of biologically relevant molecular recognition processes with high resolution. This Review focuses on the development and use of modern nuclear magnetic resonance (NMR) techniques to dissect binding events. These spectroscopic methods, complementing X-ray crystallography and molecular modeling methodologies, will be taken into account as indispensable tools to provide a complete picture of protein-glycoconjugate binding mechanisms related to biomedicine applications against infectious diseases.

The Ligands of C-Type Lectins

In this chapter, a comprehensive overview of the known ligands for the C-type lectins (CTLs) is provided. Emphasis has been placed on the chemical structure of the glycans that bind to the different CTLs and the amount of structural variation (or overlap) that each CTL can tolerate. In this way, both the synthetic carbohydrate chemist and the immunologist can more readily gain insight into the existing structure-activity space for the CTL ligands and, ideally, see areas of synergy that will help identify and refine the ligands for these receptors.

Alteration of the langerin oligomerization state affects Birbeck granule formation

Langerin, a trimeric C-type lectin specifically expressed in Langerhans cells, has been reported to be a pathogen receptor through the recognition of glycan motifs by its three carbohydrate recognition domains (CRD). In the context of HIV-1 (human immunodeficiency virus-1) transmission, Langerhans cells of genital mucosa play a protective role by internalizing virions in Birbeck Granules (BG) for elimination. Langerin (Lg) is directly involved in virion binding and BG formation through its CRDs. However, nothing is known regarding the mechanism of langerin assembly underlying BG formation. We investigated at the molecular level the impact of two CRD mutations, W264R and F241L, on langerin structure, function, and BG assembly using a combination of biochemical and biophysical approaches. Although the W264R mutation causes CRD global unfolding, the F241L mutation does not affect the overall structure and gp120 (surface HIV-1 glycoprotein of 120 kDa) binding capacities of isolated Lg-CRD. In contrast, this mutation induces major functional and structural alterations of the whole trimeric langerin extracellular domain (Lg-ECD). As demonstrated by small-angle x-ray scattering comparative analysis of wild-type and mutant forms, the F241L mutation perturbs the oligomerization state and the global architecture of Lg-ECD. Correlatively, despite conserved intrinsic lectin activity of the CRD, avidity property of Lg-ECD is affected as shown by a marked decrease of gp120 binding. Beyond the change of residue itself, the F241L mutation induces relocation of the K200 side chain also located within the interface between protomers of trimeric Lg-ECD, thereby explaining the defective oligomerization of mutant Lg. We conclude that not only functional CRDs but also their correct spatial presentation are critical for BG formation as well as gp120 binding.

Sugar recognition and protein-protein interaction of mammalian lectins conferring diverse functions

Recent advances in structural analyses of mammalian lectins reveal atomic-level details of their fine specificities toward diverse endogenous and exogenous glycans. Local variations on a common scaffold can enable certain lectins to recognize complex carbohydrate ligands including branched glycans and O-glycosylated peptides. Simultaneous recognition of both glycan and the aglycon moieties enhances the affinity and specificity of lectins such as CLEC-2 and PILRα. Attention has been paid to the roles of galectin and RegIII family of proteins in protein-protein interactions involved in critical biological functions including signal transduction and bactericidal pore formation.

[Pulmonary Langerhans cell histiocytosis]

Pulmonary Langerhans cell histiocytosis is regarded as a reactive proliferation of the dendritic Langerhans cell population stimulated by chronic tobacco-derived plant proteins due to incomplete combustion but can also occur in childhood as a tumor-like systemic disease. Currently, both these forms cannot be morphologically distinguished. In the lungs a nodular proliferation of Langerhans cells occurs in the bronchial mucosa and also peripherally in the alveolar septa with an accompanying infiltration by eosinophilic granulocytes and destruction of the bronchial wall. Langerhans cells can be selectively detected with antibodies against CD1a and langerin. In the reactive isolated pulmonary form, abstinence from tobacco smoking in most patients leads to regression of infiltration and improvement of symptoms. In high-resolution computed tomography (HRCT) the small star-like scars can still be detected even after complete cessation of tobacco smoking.

Current structural biology of the heparin interactome

Heparin is the best known therapeutically active carbohydrate. It can bind and regulate multiple functional proteins such as coagulation cofactors, chemokines, and growth factors. This versatility has led to the recently developed concept of the heparin interactome--a group of proteins that, as the name implies, interact with heparin. The heparin interactome is structurally and functionally diverse. Though natural ligands of this class of proteins may be any of the glycosaminoglycans however, their structural biology is generally studied using heparin as a model compound. NMR spectroscopy contributes significantly to structural investigations of the resultant complexes in solution. This review aims therefore at discussing the current status in structural biology of the molecular complexes formed between heparin and its protein partners through the current concept of the heparin interactome.

Increased density of tolerogenic dendritic cells in the small bowel mucosa of celiac patients

AIM: To investigate the densities of dendritic cells (DCs) and FOXP3⁺ regulatory T cells (Tregs) and their interrelations in the small bowel mucosa in untreated celiac disease (CD) patients with and without type 1 diabetes (T1D).

METHODS: Seventy-four patients (45 female, 29 male, mean age 11.1 ± 6.8 years) who underwent small bowel biopsy were studied. CD without T1D was diagnosed in 18 patients, and CD with T1D was diagnosed in 15 patients. Normal small bowel mucosa was found in two T1D patients. Thirty-nine patients (mean age 12.8 ± 4.9 years) with other diagnoses (functional dyspepsia, duodenal ulcer, erosive gastritis, etc.) formed the control group. All CD patients had partial or subtotal villous atrophy according to the Marsh classification: Marsh grade IIIa in 9, grade IIIb in 21 and grade IIIc in 3 cases. Thirty-nine patients without CD and 2 with T1D had normal small bowel mucosa (Marsh grade 0). The densities of CD11c⁺, IDO⁺, CD103⁺, Langerin (CD207⁺) DCs and FOXP3⁺ Tregs were investigated by immunohistochemistry (on paraffin-embedded specimens) and immunofluorescence (on cryostat sections) methods using a combination of mono- and double-staining. Sixty-six serum samples were tested for IgA-tissue transglutaminase (tTG) using a fully automated EliA™ Celikey^® IgA assay (Pharmacia Diagnostics, Freiburg, Germany).

RESULTS: The density of CD11c⁺ DCs was significantly increased in CD patients compared with patients with normal mucosa (21.67 ± 2.49 vs 13.58 ± 1.51, P = 0.007). The numbers of FOXP3⁺ cells were significantly higher in CD patients (10.66 ± 1.50 vs 1.92 ± 0.37, P = 0.0002) and in patients with CD and coexisting T1D (8.11 ± 1.64 vs 1.92 ± 0.37, P = 0.002) compared with patients with normal mucosa. The density of FOXP3⁺ cells significantly correlated with the histological grade of atrophic changes in the small bowel mucosa according to the March classification (r = 0.62; P < 0.0001) and with levels of IgA antibody (r = 0.55; P < 0.0001). The densities of IDO⁺ DCs were significantly higher in CD patients (21.6 ± 2.67 vs 6.26 ± 0.84, P = 0.00003) and in patients with CD and coexisting T1D (19.08 ± 3.61 vs 6.26 ± 0.84, P = 0.004) compared with patients with normal mucosa. A significant correlation was identified between the densities of IDO⁺ DCs and FOXP3⁺ T cells (r = 0.76; P = 0.0001). The mean values of CD103⁺ DCs were significantly higher in CD patients (10.66 ± 1.53 vs 6.34 ± 0.61, P = 0.01) and in patients with CD and associated T1D (11.13 ± 0.72 vs 6.34 ± 0.61, P = 0.00002) compared with subjects with normal small bowel mucosa. The mean value of Langerin⁺ DCs was higher in CD patients compared with persons with normal mucosa (7.4 ± 0.92 vs 5.64 ± 0.46, P = 0.04).

CONCLUSION: The participation of diverse DC subsets in the pathological processes of CD and the possible involvement of tolerogenic DCs in Tregs development to maintain intestinal immunological tolerance in CD patients are revealed.

Recent advances in employing molecular modelling to determine the specificity of glycan-binding proteins

Impressive improvements in docking performance can be achieved by applying energy bonuses to poses in which glycan hydroxyl groups occupy positions otherwise preferred by bound waters. In addition, inclusion of glycosidic conformational energies allows unlikely glycan conformations to be appropriately penalized. A method for predicting the binding specificity of glycan-binding proteins has been developed, which is based on grafting glycan branches onto a minimal binding determinant in the binding site. Grafting can be used either to screen virtual libraries of glycans, such as the known glycome, or to identify docked poses of minimal binding determinants that are consistent with specificity data. The reviewed advances allow accurate modelling of carbohydrate-protein 3D co-complexes, but challenges remain in ranking the affinity of congeners.

Inhibition of two temporal phases of HIV-1 transfer from primary Langerhans cells to T cells: the role of langerin

Epidermal Langerhans cells (eLCs) uniquely express the C-type lectin receptor langerin in addition to the HIV entry receptors CD4 and CCR5. They are among the first target cells to encounter HIV in the anogenital stratified squamous mucosa during sexual transmission. Previous reports on the mechanism of HIV transfer to T cells and the role of langerin have been contradictory. In this study, we examined HIV replication and langerin-mediated viral transfer by authentic immature eLCs and model Mutz-3 LCs. eLCs were productively infected with HIV, whereas Mutz-3 LCs were not susceptible because of a lack of CCR5 expression. Two successive phases of HIV viral transfer to T cells via cave/vesicular trafficking and de novo replication were observed with eLCs as previously described in monocyte-derived or blood dendritic cells, but only first phase transfer was observed with Mutz-3 LCs. Langerin was expressed as trimers after cross-linking on the cell surface of Mutz-3 LCs and in this form preferentially bound HIV envelope protein gp140 and whole HIV particles via the carbohydrate recognition domain (CRD). Both phases of HIV transfer from eLCs to T cells were inhibited when eLCs were pretreated with a mAb to langerin CRD or when HIV was pretreated with a soluble langerin trimeric extracellular domain or by a CRD homolog. However, the langerin homolog did not inhibit direct HIV infection of T cells. These two novel soluble langerin inhibitors could be developed to prevent HIV uptake, infection, and subsequent transfer to T cells during early stages of infection.

Common polymorphisms in human langerin change specificity for glycan ligands

Langerin, a C-type lectin on Langerhans cells, mediates carbohydrate-dependent uptake of pathogens in the first step of antigen presentation to the adaptive immune system. Langerin binds a diverse range of carbohydrates including high mannose structures, fucosylated blood group antigens, and glycans with terminal 6-sulfated galactose. Mutagenesis and quantitative binding assays indicate that salt bridges between the sulfate group and two lysine residues compensate for the nonoptimal binding of galactose at the primary Ca²⁺ site. A commonly occurring single nucleotide polymorphism (SNP) in human langerin results in change of one of these lysine residues, Lys-313, to isoleucine. Glycan array screening reveals that this amino acid change abolishes binding to oligosaccharides with terminal 6SO₄-Gal and enhances binding to oligosaccharides with terminal GlcNAc residues. Structural analysis shows that enhanced binding to GlcNAc may result from Ile-313 packing against the N-acetyl group. The K313I polymorphism is tightly linked to another SNP that results in the change N288D, which reduces affinity for glycan ligands by destabilizing the Ca²⁺-binding site. Langerin with Asp-288 and Ile-313 shows no binding to 6SO₄-Gal-terminated glycans and increased binding to GlcNAc-terminated structures, but overall decreased binding to glycans. Altered langerin function in individuals with the linked N288D and K313I polymorphisms may affect susceptibility to infection by microorganisms.

HSulf sulfatases catalyze processive and oriented 6-O-desulfation of heparan sulfate that differentially regulates fibroblast growth factor activity

Sulfs are extracellular sulfatases that have emerged recently as critical regulators of heparan sulfate (HS) activities through their ability to catalyze specific 6-O-desulfation of the polysaccharide. Consequently, Sulfs have been involved in many physiological and pathological processes, and notably for Sulf-2, in the development of cancers with poor prognosis. Despite growing interest, little is known about the structure and activity of these enzymes and the way they induce dynamic remodeling of HS 6-O-sulfation status. Here, we have combined an array of analytical approaches, including mass spectrometry, NMR, HS oligosaccharide sequencing, and FACS, to dissect HSulf-2 sulfatase activity, either on a purified octasaccharide used as a mimic of HS functional domains, or on intact cell-surface HS chains. In parallel, we have studied the functional consequences of HSulf-2 activity on fibroblast growth factor (FGF)-induced mitogenesis and found that the enzyme could differentially regulate FGF1 and FGF2 activities. Notably, these data supported the existence of precise 6-O-sulfation patterns for FGF activation and provided new insights into the saccharide structures involved. Altogether, our data bring to light an original processive enzymatic mechanism, by which HSulfs catalyze oriented alteration of HS 6-O-desulfation patterns and direct fine and differential regulation of HS functions.