Structural studies of langerin and Birbeck granule: a macromolecular organization model

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.

Melanoma tumour-derived glycans hijack dendritic cell subsets through C-type lectin receptor binding

Dendritic cell (DC) subsets play a crucial role in shaping anti-tumour immunity. Cancer escapes from the control immune system by hijacking DC functions. Yet, bases for such subversion are only partially understood. Tumour cells display aberrant glycan motifs on surface glycoproteins and glycolipids. Such carbohydrate patterns can be sensed by DCs through C-type lectin receptors (CLRs) that are critical to shape and orientate immune responses. We recently demonstrated that melanoma tumour cells harboured an aberrant 'glyco-code,' and that circulating and tumour-infiltrating DCs from melanoma patients displayed major perturbations in their CLR profiles. To decipher whether melanoma, through aberrant glycan patterns, may exploit CLR pathways to mislead DCs and evade immune control, we explored the impact of glycan motifs aberrantly found in melanoma (neoglycoproteins [NeoGP] functionalised with Gal, Man, GalNAc, s-Tn, fucose [Fuc] and GlcNAc residues) on features of human DC subsets (cDC2s, cDC1s and pDCs). We examined the ability of glycans to bind to purified DCs, and assessed their impact on DC basal properties and functional features using flow cytometry, confocal microscopy and multiplex secreted protein analysis. DC subsets differentially bound and internalised NeoGP depending on the nature of the glycan. Strikingly, Fuc directly remodelled the expression of activation markers and immune checkpoints, as well as the cytokine/chemokine secretion profile of DC subsets. NeoGP interfered with Toll like receptor (TLR)-signalling and pre-conditioned DCs to exhibit an altered response to subsequent TLR stimulation, dampening antitumor mediators while triggering pro-tumoral factors. We further demonstrated that DC subsets can bind NeoGP through CLRs, and identified GalNAc/MGL and s-Tn/ C-type lectin-like receptor 2 (CLEC2) as potential candidates. Moreover, DC dysfunction induced by tumour-associated carbohydrate molecules may be reversed by interfering with the glycan/CLR axis. These findings revealed the glycan/CLR axis as a promising checkpoint to exploit in order to reshape potent antitumor immunity while impeding immunosuppressive pathways triggered by aberrant tumour glycosylation patterns. This may rescue DCs from tumour hijacking and improve clinical success in cancer patients.

Cryo-electron tomography of Birbeck granules reveals the molecular mechanism of langerin lattice formation

Langerhans cells are specialized antigen-presenting cells localized within the epidermis and mucosal epithelium. Upon contact with Langerhans cells, pathogens are captured by the C-type lectin langerin and internalized into a structurally unique vesicle known as a Birbeck granule. Although the immunological role of Langerhans cells and Birbeck granules have been extensively studied, the mechanism by which the characteristic zippered membrane structure of Birbeck granules is formed remains elusive. In this study, we observed isolated Birbeck granules using cryo-electron tomography and reconstructed the 3D structure of the repeating unit of the honeycomb lattice of langerin at 6.4 Å resolution. We found that the interaction between the two langerin trimers was mediated by docking the flexible loop at residues 258–263 into the secondary carbohydrate-binding cleft. Mutations within the loop inhibited Birbeck granule formation and the internalization of HIV pseudovirus. These findings suggest a molecular mechanism for membrane zippering during Birbeck granule biogenesis and provide insight into the role of langerin in the defense against viral infection.

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.

Impact of Glycan Linkage to Staphylococcus aureus Wall Teichoic Acid on Langerin Recognition and Langerhans Cell Activation

Staphylococcus aureus is the leading cause of skin and soft tissue infections. It remains incompletely understood how skin-resident immune cells respond to invading S. aureus and contribute to an effective immune response. Langerhans cells (LCs), the only professional antigen-presenting cell type in the epidermis, sense S. aureus through their pattern-recognition receptor langerin, triggering a proinflammatory response. Langerin recognizes the β-1,4-linked N-acetylglucosamine (β1,4-GlcNAc) but not α-1,4-linked GlcNAc (α1,4-GlcNAc) modifications, which are added by dedicated glycosyltransferases TarS and TarM, respectively, on the cell wall glycopolymer wall teichoic acid (WTA). Recently, an alternative WTA glycosyltransferase, TarP, was identified, which also modifies WTA with β-GlcNAc but at the C-3 position (β1,3-GlcNAc) of the WTA ribitol phosphate (RboP) subunit. Here, we aimed to unravel the impact of β-GlcNAc linkage position for langerin binding and LC activation. Using genetically modified S. aureus strains, we observed that langerin similarly recognized bacteria that produce either TarS- or TarP-modified WTA, yet tarP-expressing S. aureus induced increased cytokine production and maturation of in vitro-generated LCs compared to tarS-expressing S. aureus. Chemically synthesized WTA molecules, representative of the different S. aureus WTA glycosylation patterns, were used to identify langerin-WTA binding requirements. We established that β-GlcNAc is sufficient to confer langerin binding, thereby presenting synthetic WTA molecules as a novel glycobiology tool for structure-binding studies and for elucidating S. aureus molecular pathogenesis. Overall, our data suggest that LCs are able to sense all β-GlcNAc-WTA producing S. aureus strains, likely performing an important role as first responders upon S. aureus skin invasion.

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.

Targeting of the C-Type Lectin Receptor Langerin Using Bifunctional Mannosylated Antigens

Langerhans cells (LCs) are antigen-presenting cells that reside in the skin. They uniquely express high levels of the C-type lectin receptor Langerin (CD207), which is an attractive target for antigen delivery in immunotherapeutic vaccination strategies against cancer. We here assess a library of 20 synthetic, well-defined mannoside clusters, built up from one, two, and three of six monomannosides, dimannosides, or trimannosides, appended to an oligopeptide backbone, for binding with Langerin using surface plasmon resonance and flow cytometric quantification. It is found that Langerin binding affinity increases with increasing number of mannosides. Hexavalent presentation of the mannosides resulted in binding affinities ranging from 3 to 12 μM. Trivalent presentation of the dimannosides and trimannosides led to Langerin affinity in the same range. The model melanoma gp100 antigenic peptide was subsequently equipped with a hexavalent cluster of the dimannosides and trimannosides as targeting moieties. Surprisingly, although the bifunctional conjugates were taken up in LCs in a Langerin-dependent manner, limited antigen presentation to cytotoxic T cells was observed. These results indicate that targeting glycan moieties on immunotherapeutic vaccines should not only be validated for target binding, but also on the continued effects on biology, such as antigen presentation to both CD8⁺ and CD4⁺ T cells.

C-Type Lectin Receptors in Host Defense Against Bacterial Pathogens

Antigen-presenting cells (APCs) are present throughout the human body—in tissues, at barrier sites and in the circulation. They are critical for processing external signals to instruct both local and systemic responses toward immune tolerance or immune defense. APCs express an extensive repertoire of pattern-recognition receptors (PRRs) to detect and transduce these signals. C-type lectin receptors (CLRs) comprise a subfamily of PRRs dedicated to sensing glycans, including those expressed by commensal and pathogenic bacteria. This review summarizes recent findings on the recognition of and responses to bacteria by membrane-expressed CLRs on different APC subsets, which are discussed according to the primary site of infection. Many CLR-bacterial interactions promote bacterial clearance, whereas other interactions are exploited by bacteria to enhance their pathogenic potential. The discrimination between protective and virulence-enhancing interactions is essential to understand which interactions to target with new prophylactic or treatment strategies. CLRs are also densely concentrated at APC dendrites that sample the environment across intact barrier sites. This suggests an–as yet–underappreciated role for CLR-mediated recognition of microbiota-produced glycans in maintaining tolerance at barrier sites. In addition to providing a concise overview of identified CLR-bacteria interactions, we discuss the main challenges and potential solutions for the identification of new CLR-bacterial interactions, including those with commensal bacteria, and for in-depth structure-function studies on CLR-bacterial glycan interactions. Finally, we highlight the necessity for more relevant tissue-specific in vitro, in vivo and ex vivo models to develop therapeutic applications in this area.

The Essential Role of anxA2 in Langerhans Cell Birbeck Granules Formation

Langerhans cells (LC) are the resident antigen presenting cells of the mucosal epithelium and play an essential role in initiating immune responses. LC are the only cells in the body to contain Birbeck granules (BG), which are unique cytoplasmic organelles comprised of c-type lectin langerin. Studies of BG have historically focused on morphological characterizations, but BG have also been implicated in viral antigen processing which suggests that they can serve a function in antiviral immunity. This study focused on investigating proteins that could be involved in BG formation to further characterize their structure using transmission electron microscopy (TEM). Here, we report a critical role for the protein annexin A2 (anxA2) in the proper formation of BG structures. When anxA2 expression is downregulated, langerin expression decreases, cytoplasmic BG are nearly ablated, and the presence of malformed BG-like structures increases. Furthermore, in the absence of anxA2, we found langerin was no longer localized to BG or BG-like structures. Taken together, these results indicate an essential role for anxA2 in facilitating the proper formation of BG.

Langerhans cell histiocytosis of an intra-mammary lymph node in an 18-year-old woman

We describe an 18-year-old woman with several month's history of a 12 x 7 mm palpable mammary nodule, that was hypoechoic, with regular margins and vascularization areas by ultrasound. A fibroadenoma was hypothesized (American College of Radiology BI-RADS 3). A 14 G needle biopsy was performed, showing a LC proliferation suspected for LCH of a lymph node, with florid dermatopathic lymphadenopathy in differential diagnosis. The multidisciplinary team of the breast clinic decided to perform a lumpectomy and a diagnosis of LCH involving an intra-mammary lymph node was made. Langerhans cells (LC) are dendritic cells characterized by grooved nuclei, irregular nuclear contours, and abundant cytoplasm, that normally reside in the skin and mucosal surfaces. They were positive for CD1a, langerin/CD207, and S100 by immunohistochemistry. Langerhans cell histiocytosis (LCH) is a clonal proliferation of histiocytes that is thought to be neoplastic in most cases. Reactive LC can be distinguished from LCH by cyclin D1 immunostaining, which is positive only in LCH. About 50% of cases have BRAF V600E mutations. The revised classification of histiocytes divides LCH in subtypes: LCH SS (single system), LCH lung positive, LCH Multiple System/Risk Organ negative and LCH Multiple System/Risk Organ positive. Localized disease can progress to multisystem involvement. The diagnosis of LCH is based on clinical and radiological findings in combination with histopathological, immunophenotypic or ultrastructural analyses identifying tissue infiltration by LC. It is recommended that biopsy confirmation of suspected LCH be performed in all cases. Lymph nodes may be the only site of disease or a part of multisystem involvement by LCH. The histologic differential diagnosis is discussed.

Development of C-type lectin-oriented surfaces for high avidity glycoconjugates: towards mimicking multivalent interactions on the cell surface

Here we described C-type lectin-oriented surfaces for SPR analysis. They allow the preservation of receptor topology, accessibility of binding sites, better evaluation of high avidity compounds and assessment of multivalent effect at cell surface.

Allostery in C-type lectins

C-type lectins are the largest and most diverse family of mammalian carbohydrate-binding proteins. They share a common protein fold, which provides the unifying basis for calcium-mediated carbohydrate recognition. Their involvement in a multitude of biological functions is remarkable. Here, we review the variety of tasks these lectins are involved in alongside with the structural demands on the overall protein architecture. Subtle changes of the protein structure are implemented to cope with such diverse functional requirements. The presence of a high level of structural dynamics over a broad palette of time scales is paired with the presence of secondary binding sites and allosteric coordination of remote sites and renders this lectin fold a highly adaptable scaffold.

Glyco-Dendrimers as Intradermal Anti-Tumor Vaccine Targeting Multiple Skin DC Subsets

The human skin is an attractive anti-tumor vaccination site due to the vast network of dendritic cell (DC) subsets that carry antigens to the draining lymph nodes and stimulate tumor specific CD4⁺ and CD8⁺ T cells in. Specific vaccine delivery to skin DC can be accomplished by targeting glycan coated antigens to C-type lectin receptors (CLRs) such as DC-SIGN expressed by human dermal DCs and Langerin expressed by Langerhans cells (LCs), which facilitate endocytosis and processing for antigen presentation and T cell activation. Although there are multiple human skin DC subsets, targeting individual DC subsets and receptors has been a focus in the past. However, the simultaneous targeting of multiple human skin DC subsets that mobilize the majority of the skin antigen presenting cells (APC) is preferred to accomplish more robust and efficient T cell stimulation. Dual CLR targeting using a single tumor vaccine has been difficult, as we previously showed Langerin to favor binding and uptake of monovalent glyco-peptides whereas DC-SIGN favors binding of larger multivalent glyco-particles such as glyco-liposomes. Methods: We used branched polyamidoamine (PAMAM) dendrimers as scaffold for melanoma specific gp100 synthetic long peptides and the common DC-SIGN and Langerin ligand Lewis Y (Le^Y), to create multivalent glyco-dendrimers with varying molecular weights for investigating dual DC-SIGN and Langerin targeting. Using DC-SIGN⁺ monocyte derived DC (moDC) and Langerin⁺ primary LC we investigated glyco-dendrimer CLR targeting properties and subsequent gp100 specific CD8⁺ T cell activation in vitro. In situ targeting ability to human dermal DC and LC through intradermal injection in a human skin explant model was elucidated. Results: Dual DC-SIGN and Langerin binding was achieved using glyco-dendrimers of approximately 100kD, thereby fulfilling our criteria to simultaneously target LCs and CD1a⁺ and CD14⁺ dermal DC in situ. Both DC-SIGN and Langerin targeting by glyco-dendrimers resulted in enhanced internalization and gp100 specific CD8⁺ T cell activation. Conclusion: We designed the first glyco-vaccine with dual CLR targeting properties, thereby reaching multiple human skin DC subsets in situ for improved anti-tumor CD8⁺ T cell responses.

A Common Genetic Variation in Langerin (CD207) Compromises Cellular Uptake of Staphylococcus aureus

Langerhans cells are key sentinel cells of the skin and mucosal lining. They sense microorganisms through their repertoire of pattern-recognition receptors to mount and direct appropriate immune responses. We recently demonstrated that human Langerhans cells interact with the Gram-positive pathogen Staphylococcus aureus through the Langerhans cell-specific receptor langerin (CD207). It was previously hypothesized that two linked single nucleotide polymorphisms (SNPs; N288D and K313I) in the carbohydrate recognition domain of langerin would affect interaction with microorganisms. We show that recognition of S. aureus by recombinant langerin molecules is abrogated in the co-inheriting SNP variant, which is mainly explained by the N288D SNP and further enhanced by K313I. Moreover, introduction of SNP N288D in ectopically-expressed langerin affected cellular distribution of the receptor such that langerin displayed enhanced plasma membraneexpression. Despite this increased binding of S. aureus by the langerin double SNP variant, uptake of bacteria by this langerin variant was compromised. Our findings indicate that in a proportion of the human population, the recognition and uptake of S. aureus by Langerhans cells may be affected, which could have important consequences for proper immune activation and S. aureus-associated disease.

Human Dendritic Cell Subsets, Ontogeny, and Impact on HIV Infection

Dendritic cells (DCs) play important roles in orchestrating host immunity against invading pathogens, representing one of the first responders to infection by mucosal invaders. From their discovery by Ralph Steinman in the 1970s followed shortly after with descriptions of their in vivo diversity and distribution by Derek Hart, we are still continuing to progressively elucidate the spectrum of DCs present in various anatomical compartments. With the power of high-dimensional approaches such as single-cell sequencing and multiparameter cytometry, recent studies have shed new light on the identities and functions of DC subtypes. Notable examples include the reclassification of plasmacytoid DCs as purely interferon-producing cells and re-evaluation of intestinal conventional DCs and macrophages as derived from monocyte precursors. Collectively, these observations have changed how we view these cells not only in steady-state immunity but also during disease and infection. In this review, we will discuss the current landscape of DCs and their ontogeny, and how this influences our understanding of their roles during HIV infection.

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.

HIV-1 replicates and persists in vaginal epithelial dendritic cells

HIV-1 acquisition occurs most commonly after sexual contact. To establish infection, HIV-1 must infect cells that support high-level replication, namely CD4⁺ T cells, which are absent from the outermost genital epithelium. Dendritic cells (DCs), present in mucosal epithelia, potentially facilitate HIV-1 acquisition. We show that vaginal epithelial DCs, termed CD1a⁺ VEDCs, are unlike other blood- and tissue-derived DCs because they express langerin but not DC-SIGN, and unlike skin-based langerin⁺ DC subset Langerhans cells (LCs), they do not harbor Birbeck granules. Individuals primarily acquire HIV-1 that utilizes the CCR5 receptor (termed either R5 or R5X4) during heterosexual transmission, and the mechanism for the block against variants that only use the CXCR4 receptor (classified as X4) remains unclear. We show that X4 as compared with R5 HIV-1 shows limited to no replication in CD1a⁺ VEDCs. This differential replication occurs after fusion, suggesting that receptor usage influences postentry steps in the virus life cycle. Furthermore, CD1a⁺ VEDCs isolated from HIV-1–infected virologically suppressed women harbor HIV-1 DNA. Thus, CD1a⁺ VEDCs are potentially infected early during heterosexual transmission and also retain virus during treatment. Understanding the interplay between HIV-1 and CD1a⁺ VEDCs is important for future prevention and cure strategies.

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.

Ultrastructure of Highly Ordered Granules in Alveolar Type II Cells in Several Species

Alveolar Type II cells from seven mammalian species were examined for a protein in the rough endoplasmic reticulum (RER), which showed a multilayered, repeating motif. Each motif, 100 nm in width, comprised two parallel outer dense layers, a less dense central layer, and often 1-3 faint layers on either side of the latter. Outer layers showed periodicities at 3-4 densities/100 nm of width, while layers on either side of the central layer showed 5-7 densities/100 nm of width. RER membranes were ribosome-free when parallel to these layers, but showed four ribosomes per motif at the growing ends: one ribosome at each outer dense layer, and one on either side of the less dense central layer. Granules appeared as single or as multiple motifs, stacked, curved, folded, or branching together within the same RER profile. Hexagons of around 30 nm in diameter with central densities were seen in tangential cuts of outer dense layers. Granule incidence varied: guinea pig > ferret > dog. Possible homologous structures occurred in rabbit and cat, but not in rat or mouse. Surfactant protein A (SP-A), a C-type lectin produced in Type II cells, forms trimers and bouquet-like 18-mer and can oligomerize further. Two pairs of SP-A 18-mers with carbohydrate recognition domains pointing inwardly and outwardly, stacked vertically as a column of four molecules, then repeated side by side in rows, approximated the size and layering patterns observed in these granules. Sequence analyses of SP-A from these species showed phylogenetic distances consistent with the observed occurrence and frequency of patterned granules. Anat Rec, 301:1290-1302, 2018. © 2018 Wiley Periodicals, Inc.

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.

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.

Langerhans cells and sexual transmission of HIV and HSV

Langerhans cells (LCs) situated in stratified squamous epithelium of the skin and mucosal tissue are amongst the first cells that sexually transmitted pathogens encounter during transmission. They are potent antigen presenting cells and play a key role in the host mounting an appropriate immune response. As such, viruses have evolved complex strategies to manipulate these cells to facilitate successful transmission. One of best studied examples is HIV, which manipulates the natural function of these cells to interact with CD4 T cells, which are the main target cell for HIV in which rapid replication occurs. However, there is controversy in the literature as to the role that LCs play in this process. Langerhans cells also play a key role in the way the body mounts an immune response to HSV, and there is also a complex interplay between the transmission of HSV and HIV that involves LCs. In this article, we review both past and present literatures with a particular focus on a few very recent studies that shed new light on the role that LCs play in the transmission and immune response to these 2 pathogens.

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.

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.

Langerhans Cell Histiocytosis Shows Distinct Cytoplasmic Expression of Major Histocompatibility Class II Antigens

Objectives

Langerhans cell histiocytosis (LCH) is a monoclonal proliferation of antigen presenting cells (APC). In benign APCs, antigen loading occurs in the Major Histocompatibility class II (MHCII)-lysosomal compartment of the endocytic pathway followed by transport to the cell surface upon antigen stimulation. The pattern of MHC II expression in LCH is not well characterized.

Methods

The cellular localization of MHCII was determined using immunohistochemisty (IHC). Staining pattern for the representative MHCII molecule, HLA-DR, (cell surface, cytoplasmic granular, or cytoplasmic globular) and intensity (0 to 3+) were recorded for normal tissues and 44 LCH samples along with available clinicopathologic features. Results were confirmed with a different antibody to confirm the appearance.

Results

In the normal tissue survey, strong HLA-DR cell surface expression was present on APCs, benign B cells, some T cells, and pulmonary macrophages. A granular cytoplasmic staining pattern (without surface expression) was seen in benign Langerhans cells (LCs) in the skin and histiocytes. Strikingly, all 44 LCH samples demonstrated both cytoplasmic granular and an unusual "globular" staining pattern with no surface staining.

Conclusion

This is the first report of a highly specific HLA-DR staining pattern in LCH detected by IHC. The cytoplasmic perinuclear globular localization of MHCII may possibly be useful in diagnostics and may result from an immature/antigen-naïve differentiation state of the neoplastic cell.

The C-type Lectin Langerin Functions as a Receptor for Attachment and Infectious Entry of Influenza A Virus

It is well established that influenza A virus (IAV) attachment to and infection of epithelial cells is dependent on sialic acid (SIA) at the cell surface, although the specific receptors that mediate IAV entry have not been defined and multiple receptors may exist. Lec2 Chinese hamster ovary (CHO) cells are SIA deficient and resistant to IAV infection. Here we demonstrate that the expression of the C-type lectin receptor langerin in Lec2 cells (Lec2-Lg) rendered them permissive to IAV infection, as measured by replication of the viral genome, transcription of viral mRNA, and synthesis of viral proteins. Unlike SIA-dependent infection of parental CHO cells, IAV attachment and infection of Lec2-Lg cells was mediated via lectin-mediated recognition of mannose-rich glycans expressed by the viral hemagglutinin glycoprotein. Lec2 cells expressing endocytosis-defective langerin bound IAV efficiently but remained resistant to IAV infection, confirming that internalization via langerin was essential for infectious entry. Langerin-mediated infection of Lec2-Lg cells was pH and dynamin dependent, occurred via clathrin- and caveolin-mediated endocytic pathways, and utilized early (Rab5(+)) but not late (Rab7(+)) endosomes. This study is the first to demonstrate that langerin represents an authentic receptor that binds and internalizes IAV to facilitate infection. Moreover, it describes a unique experimental system to probe specific pathways and compartments involved in infectious entry following recognition of IAV by a single cell surface receptor.

IMPORTANCE

On the surface of host cells, sialic acid (SIA) functions as the major attachment factor for influenza A viruses (IAV). However, few studies have identified specific transmembrane receptors that bind and internalize IAV to facilitate infection. Here we identify human langerin as a transmembrane glycoprotein that can act as an attachment factor and a bone fide endocytic receptor for IAV infection. Expression of langerin by an SIA-deficient cell line resistant to IAV rendered cells permissive to infection. As langerin represented the sole receptor for IAV infection in this system, we have defined the pathways and compartments involved in infectious entry of IAV into cells following recognition by langerin.

HIV-1 border patrols: Langerhans cells control antiviral responses and viral transmission

Langerhans cells (LCs) reside in the mucosal epithelia and are refractory to HIV-1 infection; HIV-1 capture by C-type lectin receptor langerin and subsequent targeting to Birbeck granules prevents infection. Furthermore, LCs restrict transmission of CXCR4-using HIV-1 variants, which underscores the role of immature LCs as gatekeepers in the selection of HIV-1 variants. Interaction of langerin on LCs with hyaluronic acid on dendritic cells facilitates cross-presentation of HIV-1 to CD8+ T cells. Activation of LCs upon inflammation bypasses the langerin-dependent barrier, which favors cross-presentation and increases susceptibility of LCs to HIV-1 infection. These recent developments not only highlight the plasticity of LCs but also define an important role for LC-dendritic cell crosstalk at the periphery in directing adaptive immune responses to viruses.

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.

Cross-presentation through langerin and DC-SIGN targeting requires different formulations of glycan-modified antigens

Dendritic cells (DCs) and Langerhans cells (LC) are professional antigen presenting cells (APCs) that initiate humoral and cellular immune responses. Targeted delivery of antigen towards DC- or LC-specific receptors enhances vaccine efficacy. In this study, we compared the efficiency of glycan-based antigen targeting to both the human DC-specific C-type lectin receptor (CLR) DC-SIGN and the LC-specific CLR langerin. Since DC-SIGN and langerin are able to recognize the difucosylated oligosaccharide Lewis Y (Le(Y)), we prepared neoglycoconjugates bearing this glycan epitope to allow targeting of both lectins. Le(Y)-modified liposomes, with an approximate diameter of 200nm, were significantly endocytosed by DC-SIGN(+) DCs and mediated efficient antigen presentation to CD4(+) and CD8(+) T cells. Surprisingly, although langerin bound to Le(Y)-modified liposomes, LCs exposed to Le(Y)-modified liposomes could not endocytose liposomes nor mediate antigen presentation to T cells. However, LCs mediated an enhanced cross-presentation when antigen was delivered through langerin using Le(Y)-modified synthetic long peptides. In contrast, Le(Y)-modified synthetic long peptides were recognized by DC-SIGN, but did not trigger antigen internalization nor antigen cross-presentation. These data demonstrate that langerin and DC-SIGN have different size requirements for antigen uptake. Although using glycans remains an interesting option in the design of anti-cancer vaccines targeting multiple CLRs, aspects such as molecule size and conformation need to be taken in consideration.

Caveolin-1 mediated uptake via langerin restricts HIV-1 infection in human Langerhans cells

Background

Human Langerhans cells (LCs) reside in foreskin and vaginal mucosa and are the first immune cells to interact with HIV-1 during sexual transmission. LCs capture HIV-1 through the C-type lectin receptor langerin, which routes the virus into Birbeck granules (BGs), thereby preventing HIV-1 infection. BGs are langerin-positive organelles exclusively present in LCs, however, their origin and function are unknown.

Results

Here, we not only show that langerin and caveolin-1 co-localize at the cell membrane and in vesicles but also that BGs are langerin/caveolin-1-positive vesicles are linked to the lysosomal degradation pathway in LCs. Moreover, inhibition of caveolar endocytosis in primary LCs abrogated HIV-1 sequestering into langerin⁺ caveolar structures. Notably, both inhibition of caveolar uptake and silencing of caveolar structure protein caveolin-1 resulted in increased HIV-1 integration and subsequent infection. In contrast, inhibition of clathrin-mediated endocytosis did not affect HIV-1 integration, even though HIV-1 uptake was decreased, suggesting that clathrin-mediated endocytosis is not involved in HIV-1 restriction in LCs.

Conclusions

Thus, our data strongly indicate that BGs belong to the caveolar endocytosis pathway and that caveolin-1 mediated HIV-1 uptake is an intrinsic restriction mechanism present in human LCs that prevents HIV-1 infection. Harnessing this particular internalization pathway has the potential to facilitate strategies to combat HIV-1 transmission.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-014-0123-7) contains supplementary material, which is available to authorized users.

Human blood BDCA-1 dendritic cells differentiate into Langerhans-like cells with thymic stromal lymphopoietin and TGF-β

Human blood BDCA-1+ DCs have precursor potential. TSLP can be implicated in LC ontogenesis during inflammation.

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.

Birbeck granule-like "organized smooth endoplasmic reticulum" resulting from the expression of a cytoplasmic YFP-tagged langerin

Langerin is required for the biogenesis of Birbeck granules (BGs), the characteristic organelles of Langerhans cells. We previously used a Langerin-YFP fusion protein having a C-terminal luminal YFP tag to dynamically decipher the molecular and cellular processes which accompany the traffic of Langerin. In order to elucidate the interactions of Langerin with its trafficking effectors and their structural impact on the biogenesis of BGs, we generated a YFP-Langerin chimera with an N-terminal, cytosolic YFP tag. This latter fusion protein induced the formation of YFP-positive large puncta. Live cell imaging coupled to a fluorescence recovery after photobleaching approach showed that this coalescence of proteins in newly formed compartments was static. In contrast, the YFP-positive structures present in the pericentriolar region of cells expressing Langerin-YFP chimera, displayed fluorescent recovery characteristics compatible with active membrane exchanges. Using correlative light-electron microscopy we showed that the coalescent structures represented highly organized stacks of membranes with a pentalaminar architecture typical of BGs. Continuities between these organelles and the rough endoplasmic reticulum allowed us to identify the stacks of membranes as a form of “Organized Smooth Endoplasmic Reticulum” (OSER), with distinct molecular and physiological properties. The involvement of homotypic interactions between cytoplasmic YFP molecules was demonstrated using an A206K variant of YFP, which restored most of the Langerin traffic and BG characteristics observed in Langerhans cells. Mutation of the carbohydrate recognition domain also blocked the formation of OSER. Hence, a “double-lock” mechanism governs the behavior of YFP-Langerin, where asymmetric homodimerization of the YFP tag and homotypic interactions between the lectin domains of Langerin molecules participate in its retention and the subsequent formation of BG-like OSER. These observations confirm that BG-like structures appear wherever Langerin accumulates and confirm that membrane trafficking effectors dictate their physiology and, illustrate the importance of molecular interactions in the architecture of intracellular membranes.

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

Langerin is a C-type lectin specifically expressed in Langerhans cells. As recently shown for HIV, Langerin is thought to capture pathogens and mediate their internalisation into Birbeck Granules for elimination. However, the precise functions of Langerin remain elusive, mostly because of the lack of information on its binding properties and physiological ligands. Based on recent reports that Langerin binds to sulfated sugars, we conducted here a comparative analysis of Langerin interaction with mannose-rich HIV glycoprotein gp120 and glycosaminoglycan (GAGs), a family of sulfated polysaccharides expressed at the surface of most mammalian cells. Our results first revealed that Langerin bound to these different glycans through very distinct mechanisms and led to the identification of a novel, GAG-specific binding mode within Langerin. In contrast to the canonical lectin domain, this new binding site showed no Ca(2+)-dependency, and could only be detected in entire, trimeric extracellular domains of Langerin. Interestingly binding to GAGs, did not simply rely on a net charge effect, but rather on more discrete saccharide features, such as 6-O-sulfation, or iduronic acid content. Using molecular modelling simulations, we proposed a model of Langerin/heparin complex, which located the GAG binding site at the interface of two of the three Carbohydrate-recognition domains of the protein, at the edge of the a-helix coiled-coil. To our knowledge, the binding properties that we have highlighted here for Langerin, have never been reported for C-type lectins before. These findings provide new insights towards the understanding of Langerin biological functions.

A glycomimetic compound inhibits DC-SIGN-mediated HIV infection in cellular and cervical explant models

OBJECTIVE

Dendritic cell-specific intercellular adhesion molecule (ICAM)-3 grabbing nonintegrin (DC-SIGN) participates in the initial stages of sexually transmitted HIV-1 infection by recognizing highly mannosylated structures presented in multiple copies on HIV-1 gp120 and promoting virus dissemination. Inhibition of HIV interaction with DC-SIGN thus represents a potential therapeutic approach for viral entry inhibition at the mucosal level.

DESIGN

Herein we evaluate the efficacy in inhibiting HIV-1 infection and the potential toxicity of a multimeric glycomimetic DC-SIGN ligand (Dendron 12).

METHODS

The ability of Dendron 12 to block HIV-1 infection was assessed in cellular and human cervical explant models. Selectivity of Dendron 12 towards DC-SIGN and langerin was evaluated by surface plasmon resonance studies. β chemokine production following stimulation with Dendron 12 was also analyzed. Toxicity of the compound was evaluated in cellular and tissue models.

RESULTS

Dendron 12 averted HIV-1 trans infection of CD4(+) T lymphocytes in presence of elevated viral loads and prevented HIV-1 infection of human cervical tissues, under conditions mimicking compromised epithelial integrity, by multiple clades of R5 and X4 tropic viruses. Treatment with Dendron 12 did not interfere with the activity of langerin and also significantly elicited the production of the β chemokines MIP-1α, MIP-1β and RANTES.

CONCLUSION

Dendron 12 thus inhibits HIV-1 infection by competition with binding of HIV to DC-SIGN and stimulation of β-chemokine production. Dendron 12 represents a promising lead compound for the development of anti-HIV topical microbicides.

DC-SIGN increases the affinity of HIV-1 envelope glycoprotein interaction with CD4

Mannose-binding C-type lectin receptors, expressed on Langerhans cells and subepithelial dendritic cells (DCs) of cervico-vaginal tissues, play an important role in HIV-1 capture and subsequent dissemination to lymph nodes. DC-SIGN has been implicated in both productive infection of DCs and the DC-mediated trans infection of CD4⁺ T cells that occurs in the absence of replication. However, the molecular events that underlie this efficient transmission have not been fully defined. In this study, we have examined the effect of the extracellular domains of DC-SIGN and Langerin on the stability of the interaction of the HIV-1 envelope glycoprotein with CD4 and also on replication in permissive cells. Surface plasmon resonance analysis showed that DC-SIGN increases the binding affinity of trimeric gp140 envelope glycoproteins to CD4. In contrast, Langerin had no effect on the stability of the gp140:CD4 complex. In vitro infection experiments to compare DC-SIGN enhancement of CD4-dependent and CD4-independent strains demonstrated significantly lower enhancement of the CD4-independent strain. In addition DC-SIGN increased the relative rate of infection of the CD4-dependent strain but had no effect on the CD4-independent strain. DC-SIGN binding to the HIV envelope protein effectively increases exposure of the CD4 binding site, which in turn contributes to enhancement of infection.

Comparative analysis reveals selective recognition of glycans by the dendritic cell receptors DC-SIGN and Langerin

DC-SIGN (dendritic cell-specific ICAM-3 grabbing non-integrin) and Langerin are homologous C-type lectins expressed as cell-surface receptors on different populations of dendritic cells (DCs). DC-SIGN interacts with glycan structures on HIV-1, facilitating virus survival, transmission and infection, whereas Langerin, which is characteristic of Langerhans cells (LCs), promotes HIV-1 uptake and degradation. Here we describe a comprehensive comparison of the glycan specificities of both proteins by probing a synthetic carbohydrate microarray comprising 275 sugar compounds using the bacterially produced and fluorescence-labeled, monomeric carbohydrate-recognition domains (CRDs) of DC-SIGN and Langerin. In this side-by-side study DC-SIGN was found to preferentially bind internal mannose residues of high-mannose-type saccharides and the fucose-containing blood-type antigens H, A, B, Le(a), Le(b) Le(x), Le(y), sialyl-Le(a) as well as sulfatated derivatives of Le(a) and Le(x). In contrast, Langerin appeared to recognize a different spectrum of compounds, especially those containing terminal mannose, terminal N-acetylglucosamine and 6-sulfogalactose residues, but also the blood-type antigens H, A and B. Of the Lewis antigens, only Le(b), Le(y), sialyl-Le(a) and the sialyl-Le(x) derivative with 6'-sulfatation at the galactose (sialyl-6SGal Le(x)) were weakly bound by Langerin. Notably, Ca(2+)-independent glycan-binding activity of Langerin could not be detected either by probing the glycan array or by isothermal titration calorimetry of the CRD with mannose and mannobiose. The precise knowledge of carbohydrate specificity of DC-SIGN and Langerin receptors resulting from our study may aid the future design of microbicides that specifically affect the DC-SIGN/HIV-1 interaction while not compromising the protective function of Langerin.

Extending the usability of the phasing power of diselenide bonds: SeCys SAD phasing of CsgC using a non-auxotrophic strain

The CsgC protein is a component of the curli system in Escherichia coli. Reported here is the successful incorporation of selenocysteine (SeCys) and selenomethionine (SeMet) into recombinant CsgC, yielding derivatized crystals suitable for structural determination. Unlike in previous reports, a standard autotrophic expression strain was used and only single-wavelength anomalous dispersion (SAD) data were required for successful phasing. The level of SeCys/SeMet incorporation was estimated by mass spectrometry to be about 80%. The native protein crystallized in two different crystal forms (form 1 belonging to space group C222(1) and form 2 belonging to space group C2), which diffracted to 2.4 and 2.0 Å resolution, respectively, whilst Se-derivatized protein crystallized in space group C2 and diffracted to 1.7 Å resolution. The Se-derivatized crystals are suitable for SAD structure determination using only the anomalous signal derived from the SeCys residues. These results extend the usability of SeCys labelling to more general and less favourable cases, rendering it a suitable alternative to traditional phasing approaches.

Structural basis for langerin recognition of diverse pathogen and mammalian glycans through a single binding site

Langerin mediates the carbohydrate-dependent uptake of pathogens by Langerhans cells in the first step of antigen presentation to the adaptive immune system. Langerin binds to an unusually diverse number of endogenous and pathogenic cell surface carbohydrates, including mannose-containing O-specific polysaccharides derived from bacterial lipopolysaccharides identified here by probing a microarray of bacterial polysaccharides. Crystal structures of the carbohydrate-recognition domain from human langerin bound to a series of oligomannose compounds, the blood group B antigen, and a fragment of β-glucan reveal binding to mannose, fucose, and glucose residues by Ca²⁺ coordination of vicinal hydroxyl groups with similar stereochemistry. Oligomannose compounds bind through a single mannose residue, with no other mannose residues contacting the protein directly. There is no evidence for a second Ca²⁺-independent binding site. Likewise, a β-glucan fragment, Glcβ1–3Glcβ1–3Glc, binds to langerin through the interaction of a single glucose residue with the Ca²⁺ site. The fucose moiety of the blood group B trisaccharide Galα1–3(Fucα1–2)Gal also binds to the Ca²⁺ site, and selective binding to this glycan compared to other fucose-containing oligosaccharides results from additional favorable interactions of the nonreducing terminal galactose, as well as of the fucose residue. Surprisingly, the equatorial 3-OH group and the axial 4-OH group of the galactose residue in 6SO₄–Galβ1–4GlcNAc also coordinate Ca²⁺, a heretofore unobserved mode of galactose binding in a C-type carbohydrate-recognition domain bearing the Glu-Pro-Asn signature motif characteristic of mannose binding sites. Salt bridges between the sulfate group and two lysine residues appear to compensate for the nonoptimal binding of galactose at this site.