Binding Sites for Acylated Trehalose Analogs of Glycolipid Ligands on an Extended Carbohydrate Recognition Domain of the Macrophage Receptor Mincle

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.

Versatile approach towards fully desymmetrized trehalose with a novel set of orthogonal protecting groups

Trehalose-containing glycans play an essential role in bacterial pathogenesis, host-pathogen interaction, and cell signaling. The investigation of trehalose uptake and metabolism in Mycobacteria using synthetic desymmetrized trehalose probes is an important approach for the development of diagnostic tools and potential therapeutics for tuberculosis. Trehalose-derived mycobacterial glycolipids activate the innate immune response through recognition by the C-type lectin Mincle, justifying efforts to develop novel trehalose-based Mincle-dependent adjuvants. The chemical synthesis of trehalose-based glycoconjugates, glycolipids, and small-molecule trehalose probes requires the challenging chemical desymmetrization of eight hydroxyl groups in a C 2-symmetric disaccharide αGlc(1↔1)αGlc. Using a novel set of orthogonal protecting groups, we developed a flexible multiscale synthetic approach to a collection of differently and variably protected fully desymmetrized trehalose derivatives, ready for final chemical modification with relevant functional or reporter groups. Using a regioselective and site-specific protecting group strategy, we performed multiple symmetry-breaking operations, resulting in a library of trehalose-derived orthogonally protected building blocks as a versatile source for the synthesis of complex trehalose-containing glycans.

Pathogenicity and virulence of Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) is the causative agent of tuberculosis, an infectious disease with one of the highest morbidity and mortality rates worldwide. Leveraging its highly evolved repertoire of non-protein and protein virulence factors, Mtb invades through the airway, subverts host immunity, establishes its survival niche, and ultimately escapes in the setting of active disease to initiate another round of infection in a naive host. In this review, we will provide a concise synopsis of the infectious life cycle of Mtb and its clinical and epidemiologic significance. We will also take stock of its virulence factors and pathogenic mechanisms that modulate host immunity and facilitate its spread. Developing a greater understanding of the interface between Mtb virulence factors and host defences will enable progress toward improved vaccines and therapeutics to prevent and treat tuberculosis.

Structural basis for plastic glycolipid recognition of the C-type lectin Mincle

Mincle (macrophage-inducible C-type lectin, CLEC4E) is a C-type lectin immune-stimulatory receptor for cord factor, trehalose dimycolate (TDM), which serves as a potent component of adjuvants. The recognition of glycolipids by Mincle, especially their lipid parts, is poorly understood. Here, we performed nuclear magnetic resonance analysis, revealing that titration of trehalose harboring a linear short acyl chain showed a chemical shift perturbation of hydrophobic residues next to the Ca-binding site. Notably, there were split signals for Tyr201 upon complex formation, indicating two binding modes for the acyl chain. In addition, most Mincle residues close to the Ca-binding site showed no observable signals, suggesting their mobility on an ∼ ms scale even after complex formation. Mutagenesis study supported two putative lipid-binding modes for branched acyl-chain TDM binding. These results provide novel insights into the plastic-binding modes of Mincle toward a wide range of glycol- and glycerol-lipids, important for rational adjuvant development.

PGL-III, a Rare Intermediate of Mycobacterium leprae Phenolic Glycolipid Biosynthesis, Is a Potent Mincle Ligand

Although leprosy (Hansen's disease) is one of the oldest known diseases, the pathogenicity of Mycobacterium leprae (M. leprae) remains enigmatic. Indeed, the cell wall components responsible for the immune response against M. leprae are as yet largely unidentified. We reveal here phenolic glycolipid-III (PGL-III) as an M. leprae-specific ligand for the immune receptor Mincle. PGL-III is a scarcely present trisaccharide intermediate in the biosynthetic pathway to PGL-I, an abundant and characteristic M. leprae glycolipid. Using activity-based purification, we identified PGL-III as a Mincle ligand that is more potent than the well-known M. tuberculosis trehalose dimycolate. The cocrystal structure of Mincle and a synthetic PGL-III analogue revealed a unique recognition mode, implying that it can engage multiple Mincle molecules. In Mincle-deficient mice infected with M. leprae, increased bacterial burden with gross pathologies were observed. These results show that PGL-III is a noncanonical ligand recognized by Mincle, triggering protective immunity.

Characterisation of the bovine C-type lectin receptor Mincle and potential evidence for an endogenous ligand

Innate immune receptors that form complexes with secondary receptors, activating multiple signalling pathways, modulate cellular activation and play essential roles in regulating homeostasis and immunity. We have previously identified a variety of bovine C-type lectin-like receptors that possess similar functionality than their human orthologues. Mincle (CLEC4E), a heavily glycosylated monomer, is involved in the recognition of the mycobacterial component Cord factor (trehalose 6,6'-dimycolate). Here we characterise the bovine homologue of Mincle (boMincle), and demonstrate that the receptor is structurally and functionally similar to the human orthologue (huMincle), although there are some notable differences. In the absence of cross-reacting antibodies, boMincle-specific antibodies were created and used to demonstrate that, like the human receptor, boMincle is predominantly expressed by myeloid cells. BoMincle surface expression increases during the maturation of monocytes to macrophages. However, boMincle mRNA transcripts were also detected in granulocytes, B cells, and T cells. Finally, we show that boMincle binds to isolated bovine CD4⁺ T cells in a specific manner, indicating the potential to recognise endogenous ligands. This suggests that the receptor might also play a role in homeostasis in cattle.

Synthesis and Biological Evaluation of Trehalose-based Bi-aryl Derivatives as C-type Lectin Ligands

The identification of Mincle as the C-type lectin receptor on innate immune cells responsible for binding TDM and the realization that this receptor could be key to productive vaccines for mycobacterial infection has raised interest in the development of synthetic Mincle ligands as novel adjuvants. We recently reported on the synthesis and evaluation of Brartemicin analog UM-1024 that demonstrated Mincle agonist activity, exhibiting potent Th1/Th17 adjuvant activity that was greater than that of trehalose dibehenate (TDB). Our pursuit to understand Mincle/ligand relationships and improve the pharmacologic properties of the ligands has expanded and continues to reveal new and exciting structure activity relationships. Herein we report the synthesis of novel bi-aryl trehalose derivatives in good to excellent yields. These compounds were evaluated for their ability to engage the human Mincle receptor and tested for the induction of cytokines from human peripheral blood mononuclear cells. A preliminary structure-activity relationship (SAR) of these novel bi-aryl derivatives revealed that bi-aryl trehalose ligand 3D showed relatively high potency in cytokine production in comparison to trehalose glycolipid adjuvant TDB and the natural ligand TDM and induced dose-dependent, Mincle selective stimulation in hMincle HEK reporter cells. Also, through computational studies, we provide an insight into the potential mode of binding of 6,6'-Biaryl trehalose compounds on human Mincle receptor.

Molecular Cloning and Characteristics of a Lectin from the Bivalve Glycymeris yessoensis

C-type lectins (CTLs) are a family of carbohydrate-binding proteins that mediate multiple biological events, including adhesion between cells, the turnover of serum glycoproteins, and innate immune system reactions to prospective invaders. Here, we describe the cDNA cloning of lectin from the bivalve Glycymeris yessoensis (GYL), which encodes 161 amino acids and the C-type carbohydrate recognition domain (CRD) with EPN and WND motifs. The deduced amino acid sequence showed similarity to other CTLs. GYL is a glycoprotein containing two N-glycosylation sites per subunit. N-glycans are made up of xylose, mannose, D-glucosamine, 3-O-methylated galactose, D-quinovoses, and 3-O-methylated 6-deoxy-D-glucose. The potential CRD tertiary structure of the GYL adopted CTL-typical long-form double-loop structure and included three disulfide bridges at the bases of the loops. Additionally, when confirming the GYL sequence, eight isoforms of this lectin were identified. This fact indicates the presence of a multigene family of GYL-like C-type lectins in the bivalve G. yessoensis. Using the glycan microarray approach, natural carbohydrate ligands were established, and the glycotope for GYL was reconstructed as "Galβ1-4GlcNAcβ obligatory containing an additional fragment", like a sulfate group or a methyl group of fucose or N-acetylgalactosamine residues.

Trehalose diesters containing a polar functional group-modified lipid moiety: Synthesis and evaluation of Mincle-mediated signaling activity

Mincle, a C-type lectin receptor (CLR), activates the innate immune system by recognizing certain complex lipid compounds. In this study, we designed and synthesized trehalose disteate (TDS) and dibehenate (TDB), containing a polar-functional group in the middle of fatty acid moieties, based on a model of the Mincle-glycolipids interaction. The modified fatty acids were prepared using hydroxy fatty acids as common intermediates, and conjugated with an appropriate trehalose moiety to synthesize the desired trehalose diesters. TDE derivatives containing the modified fatty acid have different Mincle-mediated signaling activities depending on the position of the functional group and the length of the lipids. The newly developed TDE derivatives exhibit signaling activity comparable or superior to that of TDS or TDB, and the results suggest that Mincle tolerates polar functional groups at a certain position of the lipid chain of TDE. The introduction of the polar functional groups into the lipid moiety of the glycolipids also resulted in improved solubility in polar solvents, which would be advantageous for various analyses and applications.

Water-soluble trehalose glycolipids show superior Mincle binding and signaling but impaired phagocytosis and IL-1β production

The tremendous potential of trehalose glycolipids as vaccine adjuvants has incentivized the study of how the structures of these ligands relate to their Mincle-mediated agonist activities. Despite this, structure-activity work in the field has been largely empirical, and less is known about how Mincle-independent pathways might be affected by different trehalose glycolipids, and whether Mincle binding by itself can serve as a proxy for adjuvanticity. There is also much demand for more water-soluble Mincle ligands. To address this need, we prepared polyethylene glycol modified trehalose glycolipids (PEG-TGLs) with enhanced water solubility and strong murine Mincle (mMincle) binding and signaling. However, only modest cytokine and chemokine responses were observed upon the treatment of GM-CSF treated bone-marrow cells with the PEG-TGLs. Notability, no IL-1β was observed. Using RNA-Seq analysis and a representative PEG-TGL, we determined that the more water-soluble adducts were less able to activate phagocytic pathways, and hence, failed to induce IL-1β production. Taken together, our data suggests that in addition to strong Mincle binding, which is a pre-requisite for Mincle-mediated cellular responses, the physical presentation of trehalose glycolipids in colloidal form is required for inflammasome activation, and hence, a strong inflammatory immune response.

Identification and functional characterization of C-type lectins and crustins provide new insights into the immune response of Portunus trituberculatus

A meticulous understanding of the immune characteristics of aquaculture animals is the basis for developing precise disease prevention and control strategies. In this study, four novel C-type lectins (PtCTL-5, PtCTL-6, PtCTL-7 and PtCTL-8) including a single carbohydrate-recognition domain (CRD), and four novel crustins (Ptcrustin-1, Ptcrustin-2, Ptcrustin-3 and Ptcrustin-4) with a single whey acidic protein (WAP) domain were identified from the swimming crab Portunus trituberculatus. Tissue distribution analysis indicated that most of the target genes were predominantly expressed in the hepatopancreas in all examined tissues, except for Ptcrustin-1 which were mainly expressed in the gills. Our results showed that the eight genes displayed various transcriptional profiles across different tissues. In hemocytes, the PtCTL-7 responded quickly to Vibrio alginolyticus and exhibited much more strongly up-regulation than other three PtCTLs. The Ptcrustin-1 rapidly responded to V. alginolyticus within 3 h in all the three tested tissues. Furthermore, recombinant proteins of PtCTL-5 and PtCTL-8 were successfully obtained, and both of them displayed bacterial binding activities toward V. alginolyticus, V. harveyi and Staphylococcus aureus, and only showed antibacterial activity against V. harveyi. These findings provided new insights into the diverse immune response of P. trituberculatus and laid theoretical foundations for the development of precise disease prevention and control strategies in P. trituberculatus farming. Moreover, the specific anti-V. harveyi activities exhibited by rPtCTL-5 and rPtCTL-8 suggested their promising application prospects for controlling diseases caused by V. harveyi.

From the banal to the bizarre: unravelling immune recognition and response to microbial lipids

Microbes produce a rich array of lipidic species that through their location in the cell wall and ability to mingle with host lipids represent a privileged class of immune-active molecules. Lipid-sensing immunity recognizes microbial lipids from pathogens and commensals causing immune responses. Yet microbial lipids are often heterogeneous, in limited supply and in some cases their structures are incompletely defined. Total synthesis can assist in structural determination, overcome supply issues, and provide access to high-purity, homogeneous samples and analogues. This account highlights synthetic approaches to lipidic species from pathogenic and commensal bacteria and fungi that have supported immunological studies involving lipid sensing through the pattern recognition receptor Mincle and cell-mediated immunity through the CD1-T cell axis.

Lipophilic glucose monoesters and glycosides are potent human Mincle agonists

Macrophage inducible C-type lectin (Mincle) is a pattern recognition receptor on myeloid cells that represents a promising target for Th1-stimulating adjuvants. We report on the synthesis of branched and aromatic...

Synthetic Vaccines Targeting Mincle Through Conjugation of Trehalose Dibehenate

The covalent fusion of immunostimulatory adjuvants to immunogenic antigens is a promising strategy for the development of effective synthetic vaccines for infectious diseases. Herein, we describe the conjugation of a...

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.

What is the Sugar Code?

A code is defined by the nature of the symbols, which are used to generate information‐storing combinations (e. g. oligo‐ and polymers). Like nucleic acids and proteins, oligo‐ and polysaccharides are ubiquitous, and they are a biochemical platform for establishing molecular messages. Of note, the letters of the sugar code system (third alphabet of life) excel in coding capacity by making an unsurpassed versatility for isomer (code word) formation possible by variability in anomery and linkage position of the glycosidic bond, ring size and branching. The enzymatic machinery for glycan biosynthesis (writers) realizes this enormous potential for building a large vocabulary. It includes possibilities for dynamic editing/erasing as known from nucleic acids and proteins. Matching the glycome diversity, a large panel of sugar receptors (lectins) has developed based on more than a dozen folds. Lectins ‘read’ the glycan‐encoded information. Hydrogen/coordination bonding and ionic pairing together with stacking and C−H/π‐interactions as well as modes of spatial glycan presentation underlie the selectivity and specificity of glycan‐lectin recognition. Modular design of lectins together with glycan display and the nature of the cognate glycoconjugate account for the large number of post‐binding events. They give an entry to the glycan vocabulary its functional, often context‐dependent meaning(s), hereby building the dictionary of the sugar code.

Cholesteryl glucosides signal through the carbohydrate recognition domain of the macrophage inducible C-type lectin (mincle)

Cholesteryl α-d-glucosides (αGCs) are unique metabolic products of the cancer-causing human pathogen Helicobacter pylori. Via signalling through the Macrophage inducible C-type lectin (Mincle) and the induction of a pro-inflammatory response, they are thought to play a role in the development of gastric atrophy. Herein, we prepared the first library of steryl d-glucosides and determined that they preferentially signal through the carbohydrate recognition domain of human Mincle, rather than the amino acid consensus motif. Lipidated steryl d-glucosides exhibited enhanced Mincle agonist activity, with C18 cholesteryl 6-O-acyl-α-d-glucoside (2c) being the most potent activator of human monocytes. Despite exhibiting strong Mincle signalling, sito- (5b) and stigmasterol glycosides (6b) led to a poor inflammatory response in primary cells, suggesting that Mincle is a potential therapeutic target for preventing H. pylori-mediated inflammation and cancer.

ANALYSIS OF CARBOHYDRATES AND GLYCOCONJUGATES BY MATRIX-ASSISTED LASER DESORPTION/IONIZATION MASS SPECTROMETRY: AN UPDATE FOR 2015-2016

This review is the ninth update of the original article published in 1999 on the application of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2016. Also included are papers that describe methods appropriate to analysis by MALDI, such as sample preparation techniques, even though the ionization method is not MALDI. Topics covered in the first part of the review include general aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, fragmentation and arrays. The second part of the review is devoted to applications to various structural types such as oligo- and poly-saccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals. Much of this material is presented in tabular form. The third part of the review covers medical and industrial applications of the technique, studies of enzyme reactions and applications to chemical synthesis. The reported work shows increasing use of combined new techniques such as ion mobility and the enormous impact that MALDI imaging is having. MALDI, although invented over 30 years ago is still an ideal technique for carbohydrate analysis and advancements in the technique and range of applications show no sign of deminishing. © 2020 Wiley Periodicals, Inc.

3D Structural View of Pathogen Recognition by Mammalian Lectin Receptors

Humans and other mammals resist exogenous pathogens by recognizing them as non-self. How do they do this? The answer lies in the recognition by mammalian lectin receptors of glycans usually found on the surface of pathogens and whose chemical structure is species-specific. Some glycan components, such as galactofuranose, only occur in microbes, and is the principal means by which mammalian lectin receptors recognize non-self. Several lectins may function together as pattern recognition receptors to survey the infecting pathogen before the adaptive immune system is invoked. Most lectins have primary and secondary monosaccharide-binding sites which together determine the specificity of a receptor toward microbial glycans. There may also be a hydrophobic groove alongside the sugar binding sites that increases specificity. Another elaboration is through oligomerization of lectin domains with defined spacing and arrangement that creates high-affinity binding towards multiply-presented glycans on microbes. Microbe-specific polysaccharides may arise through unique sugar linkages. Specificity can come from mammalian receptors possessing a shallow binding site and binding only internal disaccharide units, as in the recognition of mannan by Dectin-2. The accumulation of 3D structural information on lectins receptors has allowed the recognition modes of microbe glycans to be classified into several groupings. This review is an introduction to our current knowledge on the mechanisms of pathogen recognition by representative mammalian lectin receptors.

Trehalose diamide glycolipids augment antigen-specific antibody responses in a Mincle-dependent manner

Many studies have investigated how trehalose glycolipid structures can be modified to improve their Macrophage inducible C-type lectin (Mincle)-mediated adjuvanticity. However, in all instances, the ester-linkage of α,ά-trehalose to the lipid of choice remained. We investigated how changing this ester-linkage to an amide influences Mincle signalling and agonist activity and demonstrated that Mincle tolerates this functional group change. In in vivo vaccination studies in murine and ovine model systems, using OVA or Mannheimia haemolytica and Mycoplasma ovipneumoniae as vaccine antigens, respectively, it was demonstrated that a representative trehalose diamide glycolipid was able to enhance antibody-specific immune responses. Notably, IgG titres against M. ovipneumoniae were significantly greater when using trehalose dibehenamide (A-TDB) compared to trehalose dibehenate (TDB). This is particularly important as infection with M. ovipneumoniae predisposes sheep to pneumonia.

Candida albicans steryl 6-O-acyl-α-D-mannosides agonize signalling through Mincle

The C-type lectin receptor Mincle binds Candida albicans and has been implicated in its pathobiology, but the molecular effectors responsible have not been identified. We report the synthesis of cholesteryl and ergosteryl 6-O-acyl-α-d-mannosides, produced by C. albicans mycelium, and demonstrate their ability to signal through human and mouse Mincle.

Trehalose and bacterial virulence

Trehalose is a disaccharide of two D-glucose molecules linked by a glycosidic linkage, which plays both structural and functional roles in bacteria. Trehalose can be synthesized and degraded by several pathways, and induction of trehalose biosynthesis is typically associated with exposure to abiotic stress. The ability of trehalose to protect against abiotic stress has been exploited to stabilize a range of bacterial vaccines. More recently, there has been interest in the role of this molecule in microbial virulence. There is now evidence that trehalose or trehalose derivatives play important roles in virulence of a diverse range of Gram-positive and Gram-negative pathogens of animals or plants. Trehalose and/or trehalose derivatives can play important roles in host colonization and growth in the host, and can modulate the interactions with host defense mechanisms. However, the roles are typically pathogen-specific. These findings suggest that trehalose metabolism may be a target for novel pathogen-specific rather than broad spectrum interventions.

6,6'-Aryl trehalose analogs as potential Mincle ligands

6,6'-Aryl trehalose derivatives have been synthesized with a view towards identifying novel Th-17-inducing vaccine adjuvants based on the high affinity Mincle ligand Brartemicin. The initial structure-activity relationships of these novel trehalose-based compounds were investigated. All compounds have been evaluated for their ability to engage the Mincle receptor and induce a potential pro-Th17 cytokine profile from human peripheral blood mononuclear cells based on IL-6 production in human peripheral blood mononuclear cells. The preliminary biological characterization of the designed analogs presented in this paper should aid in the future design and testing of more affine ligands that may foster the discovery of novel adjuvants with improved pharmacological properties.

Formation of Lung Inducible Bronchus Associated Lymphoid Tissue Is Regulated by Mycobacterium tuberculosis Expressed Determinants

Mycobacterium tuberculosis (Mtb) is the causative agent of the infectious disease tuberculosis (TB), which is a leading cause of death worldwide. Approximately one fourth of the world's population is infected with Mtb. A major unresolved question is delineating the inducers of protective long-lasting immune response without inducing overt, lung inflammation. Previous studies have shown that the presence of inducible Bronchus-Associated Lymphoid Tissue (iBALT) correlate with protection from Mtb infection. In this study, we hypothesized that specific Mtb factors could influence the formation of iBALT, thus skewing the outcome of TB disease. We infected non-human primates (NHPs) with a transposon mutant library of Mtb, and identified specific Mtb mutants that were over-represented within iBALT-containing granulomas. A major pathway reflected in these mutants was Mtb cell wall lipid transport and metabolism. We mechanistically addressed the function of one such Mtb mutant lacking mycobacteria membrane protein large 7 (MmpL7), which transports phthiocerol dimycocerosate (PDIM) to the mycobacterial outer membrane (MOM). Accordingly, murine aerosol infection with the Mtb mutant Δmmpl7 correlated with increased iBALT-containing granulomas. Our studies showed that the Δmmpl7 mutant lacking PDIMs on the surface overexpressed diacyl trehaloses (DATs) in the cell wall, which altered the cytokine/chemokine production of epithelial and myeloid cells, thus leading to a dampened inflammatory response. Thus, this study describes an Mtb specific factor that participates in the induction of iBALT formation during TB by directly modulating cytokine and chemokine production in host cells.

Structural insight into the recognition of pathogen-derived phosphoglycolipids by C-type lectin receptor DCAR

The C-type lectin receptors (CLRs) form a family of pattern recognition receptors that recognize numerous pathogens, such as bacteria and fungi, and trigger innate immune responses. The extracellular carbohydrate-recognition domain (CRD) of CLRs forms a globular structure that can coordinate a Ca2+ ion, allowing receptor interactions with sugar-containing ligands. Although well-conserved, the CRD fold can also display differences that directly affect the specificity of the receptors for their ligands. Here, we report crystal structures at 1.8-2.3 Å resolutions of the CRD of murine dendritic cell-immunoactivating receptor (DCAR, or Clec4b1), the CLR that binds phosphoglycolipids such as acylated phosphatidyl-myo-inositol mannosides (AcPIMs) of mycobacteria. Using mutagenesis analysis, we identified critical residues, Ala136 and Gln198, on the surface surrounding the ligand-binding site of DCAR, as well as an atypical Ca2+-binding motif (Glu-Pro-Ser/EPS168-170). By chemically synthesizing a water-soluble ligand analog, inositol-monophosphate dimannose (IPM2), we confirmed the direct interaction of DCAR with the polar moiety of AcPIMs by biolayer interferometry and co-crystallization approaches. We also observed a hydrophobic groove extending from the ligand-binding site that is in a suitable position to interact with the lipid portion of whole AcPIMs. These results suggest that the hydroxyl group-binding ability and hydrophobic groove of DCAR mediate its specific binding to pathogen-derived phosphoglycolipids such as mycobacterial AcPIMs.

Design of potent Mincle signalling agonists based on an alkyl β-glucoside template

The innate immune receptor Mincle senses lipid-based molecules derived from pathogens, commensals and altered self. Based on emerging structure-activity relationships we design simple alkyl 6-O-acyl-β-d-glucosides that are effective agonists of Mincle and signal with potency on par with the prototypical ligand trehalose dimycolate.

ortho-Substituted lipidated Brartemicin derivative shows promising Mincle-mediated adjuvant activity

Structure activity relationship studies of lipidated Brartemicin analogues have revealed the potent adjuvant activity of ortho-substituted Brartemicin analogue 5a, which was better than that of p-OC₁₈ (5c) and C18dMeBrar (4).

Aryl Trehalose Derivatives as Vaccine Adjuvants for Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) continues to be a major health threat worldwide, and the development of Mtb vaccines could play a pivotal role in the prevention and control of this devastating epidemic. Th17-mediated immunity has been implicated in disease protection correlates of immune protection against Mtb. Currently, there are no approved adjuvants capable of driving a Th17 response in a vaccine setting. Recent clinical trial results using trehalose dibehenate have demonstrated a formulation-dependant proof of concept adjuvant system CAF01 capable of inducing long-lived protection. We have discovered a new class of Th17-inducing vaccine adjuvants based on the natural product Brartemicin. We synthesized and evaluated the capacity of a library of aryl trehalose derivatives to drive immunostimulatory reresponses and evaluated the structure-activity relationships in terms of the ability to engage the Mincle receptor and induce production of innate cytokines from human and murine cells. We elaborated on the structure-activity relationship of the new scaffold and demonstrated the ability of the lead entity to induce a pro-Th17 cytokine profile from primary human peripheral blood mononuclear cells and demonstrated efficacy in generating antibodies in combination with tuberculosis antigen M72 in a mouse model.

Draft Genome Sequence of Nonomuraea sp. Strain C10, a Producer of Brartemicin, Isolated from a Mud Dauber Wasp Nest in Nepal

Nonomuraea sp. strain C10 produces the cytotoxic natural product brartemicin. Here, we report its draft genome sequence to get insight into brartemicin biosynthesis and to enable genome mining for novel secondary metabolites.

A novel C-type lectin with microbiostatic and immune regulatory functions from Litopenaeus vannamei

C-type lectins (CTLs) are a group of lectins with at least one carbohydrate recognition domain (CRD), the binding of which to carbohydrates requires the presence of calcium ions. CTLs generally function as pattern recognition receptors (PRRs), essentially participating in innate immunity. In the current study, a novel CTL termed LvCTL5 was identified from Pacific white shrimp Litopenaeus vannamei, which shared sequence identities with other crustacean CTLs. LvCTL5 was highly expressed in hepatopancreas and could be activated by infection with bacteria, virus and fungi. The recombinant LvCTL5 protein purified from E. coli showed microbiostatic and agglutination activities against bacteria and fungi in vitro. Silencing of LvCTL5 in vivo could significantly affect expression of a series of immune effector genes and down-regulate the phagocytic activity of hemocytes. Compared with controls, the LvCTL5-silenced shrimp were highly susceptible to Vibrio parahaemolyticus and white spot syndrome virus (WSSV) infections. These suggest that LvCTL5 has microbiostatic and immune regulatory activities and is implicated in antiviral and antibacterial responses.

Mincle: 20 years of a versatile sensor of insults

Macrophage-inducible C-type lectin, better known as Mincle, is a member of the C-type lectin receptor family and is encoded by Clec4e. Mincle was an orphan receptor for a long time after having been discovered as a lipopolysaccharide-induced protein, yet later an adjuvant glycolipid in mycobacteria-trehalose dimycolate-was identified as a ligand. Ligands for Mincle were also found existing in bacteria, fungi and even mammals. When confronted with foreign elements, Mincle can recognize characteristic pathogen-associated molecular patterns, mostly glycolipids, from Mycobacterium tuberculosis and other pathogens, and thus induce immune responses against infection. To maintain self-homeostasis, Mincle can recognize lipid-based damage-associated molecular patterns, thereby monitoring the internal environment. The mechanism by which Mincle functions in the immune system is also becoming more clear along with the identification of its ligands. Being expressed widely on antigen-presenting cells, Mincle activation leads to the production of cytokines and chemokines, neutrophil infiltration and other inflammatory responses. Besides, Mincle can induce acquired immunity such as antigen-specific T-cell responses and antibody production as an adjuvant receptor. In this review, we will retrospectively sketch the discovery and study of Mincle, and outline some current work on this receptor.

Mammalian sugar-binding receptors: known functions and unexplored roles

Mammalian glycan-binding receptors, sometimes known as lectins, interact with glycans, the oligosaccharide portions of endogenous mammalian glycoproteins and glycolipids as well as sugars on the surfaces of microbes. These receptors guide glycoproteins out of and back into cells, facilitate communication between cells through both adhesion and signaling, and allow the innate immune system to respond quickly to viral, fungal, bacterial, and parasitic pathogens. For many of the roughly 100 glycan-binding receptors that are known in humans, there are good descriptions of what types of glycans they bind and how selectivity for these ligands is achieved at the molecular level. In some cases, there is also comprehensive evidence for the roles that the receptors play at the cellular and organismal levels. In addition to highlighting these well-understood paradigms for glycan-binding receptors, this review will suggest where gaps remain in our understanding of the physiological functions that they can serve.

A low-density lipoprotein receptor (LDLR) class A domain-containing C-type lectin from Litopenaeus vannamei plays opposite roles in antibacterial and antiviral responses

C-type lectins (CTLs) are a group of pattern recognition receptors (PRRs) that contain carbohydrate recognition domains and play important roles in innate immunity. CTLs that contain an additional low-density lipoprotein receptor (LDLR) class A domain (LdlrCTL) have been identified in many crustaceans, but their functions in immune responses are mostly unknown. In this study, a novel LdlrCTL was identified from pacific white shrimp Litopenaeus vannamei (LvLdlrCTL), which showed high homology with previously reported crustacean LdlrCTLs. LvLdlrCTL was highly expressed in hemocytes and its expression was up-regulated after immune stimulations. Silencing of LdlrCTL significantly promoted infection of shrimp by Vibrio parahaemolyticus but inhibited infection by white spot syndrome virus (WSSV), suggesting that LdlrCTL could play opposite roles in antibacterial and antiviral responses. LdlrCTL exhibited agglutination activity against bacteria and fungi and could potentiate the phagocytosis of hemocytes. Moreover, the expression of many immune effector genes and signalling pathway components was significantly changed in LdlrCTL-silenced shrimp, indicating that LdlrCTL could be involved in immune regulation.

Synthesis of glycerolipids containing simple linear acyl chains or aromatic rings and evaluation of their Mincle signaling activity

The synthesized glycerolipid derivatives possessing simple alkyl chains can stimulate a Mincle-mediated signaling assay relevant for the innate immune system.

Structural Aspects of Carbohydrate Recognition Mechanisms of C-Type Lectins

Carbohydrate recognition is an essential function occurring in all living organisms. Lectins are carbohydrate-binding proteins and are classified into several families. In mammals, Ca²⁺-dependent C-type lectins, such as β-galactoside-binding galectin and sialic acid-binding siglec, play crucial roles in the immune response and homeostasis. C-type lectins are abundant and diverse in animals. Their immunological activities include lymphocyte homing, pathogen recognition, and clearance of apoptotic bodies. C-type lectin domains are composed of 110-130 amino acid residues with highly conserved structural folds. Remarkably, individual lectins can accept a wide variety of sugar ligands and can distinguish subtle structural differences in closely related ligands. In addition, several C-type lectin-like proteins specifically bind to carbohydrate ligands in Ca²⁺-independent ways. The accumulated 3D structural evidence clarifies the unexpected structural versatility of C-type lectins underlying the variety of ligand binding modes. In this issue, we focus on the structural aspects of carbohydrate recognition mechanisms of C-type lectins and C-type lectin-like proteins.

Interaction of Glycolipids with the Macrophage Surface Receptor Mincle - a Systematic Molecular Dynamics Study

Synthetic analogues of mycobacterial trehalose-dimycolate such as trehalose acyl esters have been proposed as novel adjuvants for vaccination. They induce an immune response by binding to the macrophage C-type lectin receptor Mincle. The binding site of trehalose is known, but there is yet only very limited structural information about the binding mode of the acyl esters. Here, we performed a systematic molecular dynamics study of trehalose mono-and diesters with different chain lengths. All acyl chains investigated exhibited a high flexibility and interacted almost exclusively with a hydrophobic groove on Mincle. Despite the limited length of this hydrophobic groove, the distal parts of the longer monoesters can still form additional interactions with this surface region due to their conformational flexibility. In diesters, a certain length of the second acyl chain is required to contact the hydrophobic groove. However, a stable concomitant accommodation of both acyl chains in the groove is hampered by the conformational rigidity of Mincle. Instead, multiple dynamic interaction modes are observed, in which the second acyl chain contributes to binding. This detailed structural information is considered helpful for the future design of more affine ligands that may foster the development of novel adjuvants.

Identification and Characterization of C-type Lectins in Ostrinia furnacalis (Lepidoptera: Pyralidae)

C-type lectins (CTLs) are a large family of calcium-dependent carbohydrate-binding proteins. They function primarily in cell adhesion and immunity by recognizing various glycoconjugates. We identified 14 transcripts encoding proteins with one or two CTL domains from the transcriptome from Asian corn borer, Ostrinia furnacalis (Guenée; Lepidoptera: Pyralidae). Among them, five (OfCTL-S1 through S5) only contain one CTL domain, the remaining nine (OfIML-1 through 9) have two tandem CTL domains. Five CTL-Ss and six OfIMLs have a signal peptide are likely extracellular while another two OfIMLs might be cytoplasmic. Phylogenetic analysis indicated that OfCTL-Ss had 1:1 orthologs in Lepidoptera, Diptera, Coleoptera and Hymenoptera species, but OfIMLs only clustered with immulectins (IMLs) from Lepidopteran. Structural modeling revealed that the 22 CTL domains adopt a similar double-loop fold consisting of β-sheets and α-helices. The key residues for calcium-dependent or independent binding of specific carbohydrates by CTL domains were predicted with homology modeling. Expression profiles assay showed distinct expression pattern of 14 CTLs: the expression and induction were related to the developmental stages and infected microorganisms. Overall, our work including the gene identification, sequence alignment, phylogenetic analysis, structural modeling, and expression profile assay would provide a valuable basis for the further functional studies of O. furnacalis CTLs.

Identification and Biological Activity of Synthetic Macrophage Inducible C-Type Lectin Ligands

The macrophage inducible C-type lectin (Mincle) is a pattern recognition receptor able to recognize both damage-associated and pathogen-associated molecular patterns, and in this respect, there has been much interest in determining the scope of ligands that bind Mincle and how structural modifications to these ligands influence ensuing immune responses. In this review, we will present Mincle ligands of known chemical structure, with a focus on ligands that have been synthetically prepared, such as trehalose glycolipids, glycerol-based ligands, and 6-acylated glucose and mannose derivatives. The ability of the different classes of ligands to influence the innate, and consequently, the adaptive, immune response will be described, and where appropriate, structure-activity relationships within each class of Mincle ligands will be presented.

Sensing Lipids with Mincle: Structure and Function

Mincle is a C-type lectin receptor that has emerged as an important player in innate immunity through its capacity to recognize a wide range of lipidic species derived from damaged/altered self and foreign microorganisms. Self-ligands include sterols (e.g., cholesterol), and β-glucosylceramides, and the protein SAP130, which is released upon cell death. Foreign lipids comprise those from both microbial pathogens and commensals and include glycerol, glucose and trehalose mycolates, and glycosyl diglycerides. A large effort has focused on structural variation of these ligands to illuminate the structure–activity relationships required for the agonism of signaling though Mincle and has helped identify key differences in ligand recognition between human and rodent Mincle. These studies in turn have helped identify new Mincle ligands, further broadening our understanding of the diversity of organisms and lipidic species recognized by Mincle. Finally, progress toward the development of Mincle agonists as vaccine adjuvants providing humoral and cell-mediated immunity with reduced toxicity is discussed.

Host Responses to Malassezia spp. in the Mammalian Skin

The skin of mammalian organisms is home for a myriad of microbes. Many of these commensals are thought to have beneficial effects on the host by critically contributing to immune homeostasis. Consequently, dysbiosis can have detrimental effects for the host that may manifest with inflammatory diseases at the barrier tissue. Besides bacteria, fungi make an important contribution to the microbiota and among these, the yeast Malassezia widely dominates in most areas of the skin in healthy individuals. There is accumulating evidence that Malassezia spp. are involved in a variety of skin disorders in humans ranging from non- or mildly inflammatory conditions such as dandruff and pityriasis versicolor to more severe inflammatory skin diseases like seborrheic eczema and atopic dermatitis. In addition, Malassezia is strongly linked to the development of dermatitis and otitis externa in dogs. However, the association of Malassezia spp. with such diseases remains poorly characterized. Until now, studies on the fungus–host interaction remain sparse and they are mostly limited to experiments with isolated host cells in vitro. They suggest a multifaceted crosstalk of Malassezia spp. with the skin by direct activation of the host via conserved pattern recognition receptors and indirectly via the release of fungus-derived metabolites that can modulate the function of hematopoietic and/or non-hematopoietic cells in the barrier tissue. In this review, we discuss our current understanding of the host response to Malassezia spp. in the mammalian skin.

Contact, Collaboration, and Conflict: Signal Integration of Syk-Coupled C-Type Lectin Receptors

Several spleen tyrosine kinase-coupled C-type lectin receptors (CLRs) have emerged as important pattern recognition receptors for infectious danger. Because encounter with microbial pathogens leads to the simultaneous ligation of several CLRs and TLRs, the signals emanating from different pattern recognition receptors have to be integrated to achieve appropriate biological responses. In this review, we briefly summarize current knowledge about ligand recognition and core signaling by Syk-coupled CLRs. We then address mechanisms of synergistic and antagonistic crosstalk between different CLRs and with TLRs. Emerging evidence suggests that signal integration occurs through 1) direct interaction between receptors, 2) regulation of expression levels and localization, and 3) collaborative or conflicting signaling interference. Accordingly, we aim to provide a conceptual framework for the complex and sometimes unexpected outcome of CLR ligation in bacterial and fungal infection.

Synthetic agonists of NOD-like, RIG-I-like, and C-type lectin receptors for probing the inflammatory immune response

Synthetic agonists of innate immune cells are of interest to immunologists due to their synthesis from well-defined materials, optimized activity, and monodisperse chemical purity. These molecules are used in both prophylactic and therapeutic contexts from vaccines to cancer immunotherapies. In this review we highlight synthetic agonists that activate innate immune cells through three classes of pattern recognition receptors: NOD-like receptors, RIG-I-like receptors, and C-type lectin receptors. We classify these agonists by the receptor they activate and present them from a chemical perspective, focusing on structural components that define agonist activity. We anticipate this review will be useful to the medicinal chemist as a guide to chemical motifs that activate each receptor, ultimately illuminating a chemical space ripe for exploration.