The C-type lectin receptor MGL senses N-acetylgalactosamine on the unique Staphylococcus aureus ST395 wall teichoic acid

Classification of C-Type Lectins and Recognition of Pathogens

C-type lectins are calcium-dependent glycan-binding proteins that play key roles in the innate immune response by recognizing pathogens. Soluble C-type lectins agglutinate and neutralize pathogens, activate the complement system, and promote pathogen clearance via opsonization. Membrane-bound C-type lectins, also known as C-type lectin receptors (CLRs), internalize pathogens and induce their degradation in lysosomes, presenting pathogen-derived antigens to MHC-II molecules to activate adaptive immunity. CLRs also have signaling capabilities. Some contain the immunoreceptor tyrosine-based activation motif (ITAM), which induces inflammatory responses by activating transcription factors, such as NF-κB and NFAT. Others contain the immunoreceptor tyrosine-based inhibitory motif (ITIM), which suppresses activating signals by activating phosphatases, such as SHP-1. This creates a balance between activation and inhibition. C-type lectins are classified into 17 groups based on their structural domains, with Groups II and V members being particularly important for pathogen recognition. In this review, we present the accumulated and recent information on pathogen recognition by C-type lectins, along with their classification and basic functions.

Recognition of Staphylococcus aureus by the pattern recognition molecules langerin, mannan-binding lectin, and surfactant protein D: the influence of capsular polysaccharides and wall teichoic acid

The innate immune system plays a critical role in the rapid recognition and elimination of pathogens through pattern recognition receptors (PRRs). Among these PRRs are the C-type lectins (CTLs) langerin, mannan-binding lectin (MBL), and surfactant protein D (SP-D), which recognize carbohydrate patterns on pathogens. Each represents proteins from different compartments of the body and employs separate effector mechanisms. We have investigated their interaction with the Gram-positive opportunistic pathogen Staphylococcus aureus, a bacterium whose cell wall contains two key glycopolymers: capsular polysaccharide (CP) and wall teichoic acid (WTA). Using a langerin-expressing cell line and recombinant langerin, MBL, and SP-D, we demonstrated that langerin, MBL, and SP-D all recognize nonencapsulated S. aureus. However, the bacterium may produce CP that effectively shields S. aureus from recognition by all three CTLs. Experiments utilizing mutant S. aureus strains confirmed that WTA is a ligand for MBL, but that langerin likely interacts with an additional unknown ligand. A competition assay revealed that MBL and SP-D inhibit langerin's interaction with S. aureus, highlighting the intricate redundancy and cooperation within the innate immune system. This study highlights the dynamic interplay of langerin, MBL, and SP-D in recognizing specific surface structures on S. aureus and provides insight into how this pathogen evades innate immune recognition.

O-glycosylation of IgA1 and the pathogenesis of an autoimmune disease IgA nephropathy

IgA nephropathy is a kidney disease characterized by deposition of immune complexes containing abnormally O-glycosylated IgA1 in the glomeruli. Specifically, some O-glycans are missing galactose that is normally β1,3-linked to N-acetylgalactosamine of the core 1 glycans. These galactose-deficient IgA1 glycoforms are produced by IgA1-secreting cells due to a dysregulated expression and activity of several glycosyltransferases. Galactose-deficient IgA1 in the circulation of patients with IgA nephropathy is bound by IgG autoantibodies and the resultant immune complexes can contain additional proteins, such as complement C3. These complexes, if not removed from the circulation, can enter the glomerular mesangium, activate the resident mesangial cells, and induce glomerular injury. In this review, we briefly summarize clinical and pathological features of IgA nephropathy, review normal and aberrant IgA1 O-glycosylation pathways, and discuss the origins and potential significance of natural anti-glycan antibodies, namely those recognizing N-acetylgalactosamine. We also discuss the features of autoantibodies specific for galactose-deficient IgA1 and the characteristics of pathogenic immune complexes containing IgA1 and IgG. In IgA nephropathy, kidneys are injured by IgA1-containing immune complexes as innocent bystanders. Most patients with IgA nephropathy progress to kidney failure and require dialysis or transplantation. Moreover, most patients after transplantation experience a recurrent disease. Thus, a better understanding of the pathogenetic mechanisms is needed to develop new disease-specific treatments.

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.

Insight into the structure, biosynthesis, isolation method and biological function of teichoic acid in different gram-positive microorganisms: A review

Teichoic acid (TA) is a weakly anionic polymer present in the cell walls of Gram-positive bacteria. It can be classified into wall teichoic acid (WTA) and lipoteichoic acid (LTA) based on its localization in the cell wall. The structure and biosynthetic pathway of TAs are strain-specific and have a significant role in maintaining cell wall stability. TAs have various beneficial functions, such as immunomodulatory, anticancer and antioxidant activities. However, the purity and yield of TAs are generally not high, and different isolation methods may even affect their structural integrity, which limits the research progress on the probiotic functions of TA. This paper reviews an overview of the structure and biosynthetic pathway of TAs in different strains, as well as the research progress of the isolation and purification methods of TAs. Furthermore, this review also highlights the current research status on the biological functions of TAs. Through a comprehensive understanding of this review, it is expected to pave the way for advancements in isolating and purifying high-quality TAs and, in turn, lay a foundation for contributing to the development of targeted probiotic therapies.

Ligand Recognition by the Macrophage Galactose-Type C-Type Lectin: Self or Non-Self?-A Way to Trick the Host's Immune System

The cells and numerous macromolecules of living organisms carry an array of simple and complex carbohydrates on their surface, which may be recognized by many types of proteins, including lectins. Human macrophage galactose-type lectin (MGL, also known as hMGL/CLEC10A/CD301) is a C-type lectin receptor expressed on professional antigen-presenting cells (APCs) specific to glycans containing terminal GalNAc residue, such as Tn antigen or LacdiNAc but also sialylated Tn antigens. Macrophage galactose-type lectin (MGL) exhibits immunosuppressive properties, thus facilitating the maintenance of immune homeostasis. Hence, MGL is exploited by tumors and some pathogens to trick the host immune system and induce an immunosuppressive environment to escape immune control. The aims of this article are to discuss the immunological outcomes of human MGL ligand recognition, provide insights into the molecular aspects of these interactions, and review the MGL ligands discovered so far. Lastly, based on the human fetoembryonic defense system (Hu-FEDS) hypothesis, this paper raises the question as to whether MGL-mediated interactions may be relevant in the development of maternal tolerance toward male gametes and the fetus.

Wall Teichoic Acid Mediates Staphylococcus aureus Binding to Endothelial Cells via the Scavenger Receptor LOX-1

The success of Staphylococcus aureus as a major cause for endovascular infections depends on effective interactions with blood-vessel walls. We have previously shown that S. aureus uses its wall teichoic acid (WTA), a surface glycopolymer, to attach to endothelial cells. However, the endothelial WTA receptor remained unknown. We show here that the endothelial oxidized low-density lipoprotein receptor 1 (LOX-1) interacts with S. aureus WTA and permits effective binding of S. aureus to human endothelial cells. Purified LOX-1 bound to isolated S. aureus WTA. Ectopic LOX-1 expression led to increased binding of S. aureus wild type but not of a WTA-deficient mutant to a cell line, and LOX-1 blockage prevented S. aureus binding to endothelial cells. Moreover, WTA and LOX-1 expression levels correlated with the efficacy of the S. aureus-endothelial interaction. Thus, LOX-1 is an endothelial ligand for S. aureus, whose blockage may help to prevent or treat severe endovascular infections.

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

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

Multimodal control of dendritic cell functions by nociceptors

It is known that interactions between nociceptors and dendritic cells (DCs) can modulate immune responses in barrier tissues. However, our understanding of the underlying communication frameworks remains rudimentary. Here, we show that nociceptors control DCs in three molecularly distinct ways. First, nociceptors release the calcitonin gene-related peptide that imparts a distinct transcriptional profile on steady-state DCs characterized by expression of pro-interleukin-1β and other genes implicated in DC sentinel functions. Second, nociceptor activation induces contact-dependent calcium fluxes and membrane depolarization in DCs and enhances their production of proinflammatory cytokines when stimulated. Finally, nociceptor-derived chemokine CCL2 contributes to the orchestration of DC-dependent local inflammation and the induction of adaptive responses against skin-acquired antigens. Thus, the combined actions of nociceptor-derived chemokines, neuropeptides, and electrical activity fine-tune DC responses in barrier tissues.

The Influence of Antibiotic Resistance on Innate Immune Responses to Staphylococcus aureus Infection

Staphylococcus aureus (S. aureus) causes a broad range of infections and is associated with significant morbidity and mortality. S. aureus produces a diverse range of cellular and extracellular factors responsible for its invasiveness and ability to resist immune attack. In recent years, increasing resistance to last-line anti-staphylococcal antibiotics daptomycin and vancomycin has been observed. Resistant strains of S. aureus are highly efficient in invading a variety of professional and nonprofessional phagocytes and are able to survive inside host cells. Eliciting immune protection against antibiotic-resistant S. aureus infection is a global challenge, requiring both innate and adaptive immune effector mechanisms. Dendritic cells (DC), which sit at the interface between innate and adaptive immune responses, are central to the induction of immune protection against S. aureus. However, it has been observed that S. aureus has the capacity to develop further antibiotic resistance and acquire increased resistance to immunological recognition by the innate immune system. In this article, we review the strategies utilised by S. aureus to circumvent antibiotic and innate immune responses, especially the interaction between S. aureus and DC, focusing on how this relationship is perturbed with the development of antibiotic resistance.

Glycan-mediated molecular interactions in bacterial pathogenesis

Glycans are expressed on the surface of nearly all host and bacterial cells. Not surprisingly, glycan-mediated molecular interactions play a vital role in bacterial pathogenesis and host responses against pathogens. Glycan-mediated host-pathogen interactions can benefit the pathogen, host, or both. Here, we discuss (i) bacterial glycans that play a critical role in bacterial colonization and/or immune evasion, (ii) host glycans that are utilized by bacteria for pathogenesis, and (iii) bacterial and host glycans involved in immune responses against pathogens. We further discuss (iv) opportunities and challenges for transforming these research findings into more effective antibacterial strategies, and (v) technological advances in glycoscience that have helped to accelerate progress in research. These studies collectively offer valuable insights into new perspectives on antibacterial strategies that may effectively tackle the drug-resistant pathogens that are rapidly spreading globally.

Dual targeted delivery of statins and nucleic acids by chitosan-based nanoparticles for enhanced antiatherosclerotic efficacy

Cardiovascular disease caused by atherosclerosis is a leading cause of morbidity and mortality worldwide. Owing to the synergistic regulation of cholesterol metabolism and lesion inflammation, the simultaneous administration of statins and nucleic acids is expected to alleviate atherosclerosis. In this work, we prepared atorvastatin- and galactose-modified trimethyl chitosan nanoparticles (GTANPs) with dual targeting to hepatocytes and lesional macrophages for encapsulating Baf60a siRNA (siBaf60a) and anti-miR-33 pDNA (pAnti-miR-33), attaining the effective codelivery of statins and nucleic acids. We demonstrated that GTANPs/siBaf60a and GTANPs/pAnti-miR-33 had in vitro antiinflammatory and lipid regulating efficacy. In ApoE-knockout atherosclerotic mice, intravenously injected GTANPs/siBaf60a synergistically reduced the plasma cholesterol and atherosclerotic plaque area; more importantly, orally delivered GTANPs/pAnti-miR-33 synergistically increased the levels of plasma high-density lipoprotein cholesterol (HDL-C) and antiinflammatory cytokines, resulting in a satisfactory antiatherosclerotic outcome. Our results suggest that codelivery of statins and nucleic acids provides a promising strategy for the treatment of atherosclerosis.

Novel mechanisms of microbial crosstalk with skin innate immunity

Skin is an organ with a dynamic ecosystem that harbours pathogenic and commensal microbes, which constantly communicate amongst each other and with the host immune system. Evolutionarily, skin and its microbiota have evolved to remain in homeostasis. However, frequently this homeostatic relationship is disturbed by a variety of factors such as environmental stress, diet, genetic mutations, and the microbiome itself. Commensal microbes also play a major role in the maintenance of microbial homeostasis. In addition to their ability to limit pathogens, many skin commensals such as Staphylococcus epidermidis and Cutibacterium acnes have recently been implicated in disease pathogenesis either by directly modulating the host immune components or by supporting the expansion of other pathogenic microbes. Likewise, opportunistic skin pathogens such as Staphylococcus aureus and Staphylococcus lugdunensis are able to breach the skin and cause disease. Though much has been established about the microbiota's function in skin immunity, we are in a time where newer mechanistic insights rapidly redefine our understanding of the host/microbial interface in the skin. In this review, we provide a concise summary of recent advances in our understanding of the interplay between host defense strategies and the skin microbiota.

Carbohydrates in allergy: from disease to novel immunotherapies

Respiratory allergic disorders are a global public health problem that are responsible for substantial morbidity and healthcare expenditure. Despite the availability of allergen immunotherapy (AIT), its efficacy is suboptimal and regimens are lengthy, with a significant risk of potentially severe side effects. Studies on the recognition of allergens by immune cells through carbohydrate-lectin interactions, which play a crucial role in immune modulation and pathogenesis of allergy, have paved the way for improvements in AIT. We highlight innovative approaches for more effective and safer AIT, including the use of allergens conjugated to specific carbohydrates that bind to C-type lectins (CLRs) and sialic acid-binding immunoglobulin-type lectins (Siglecs) on immune cells to induce suppressive responses.

Wall teichoic acids: physiology and applications

Wall teichoic acids (WTAs) are charged glycopolymers containing phosphodiester-linked polyol units and represent one of the major components of Gram-positive cell envelope. WTAs have important physiological functions in cell division, gene transfer, surface adhesion, drug resistance and biofilm formation, and are critical virulence factors and vital determinants in mediating cell interaction with and tolerance to environmental factors. Here, we first briefly introduce WTA structure, biosynthesis and its regulation, and then summarize in detail four major physiological roles played by WTAs, i.e. WTA-mediated resistance to antimicrobials, virulence to mammalian cells, interaction with bacteriolytic enzymes and regulation of cell metabolism. We also review the applications of WTAs in these fields that are closely related to the human society, including antibacterial drug discovery targeting WTA biosynthesis, development of vaccines and antibodies regarding WTA-mediated pathogenicity, specific and sensitive detection of pathogens in food using WTAs as a surface epitope and regulation of WTA-related pathways for efficient microbial production of useful compounds. We also point out major problems remaining in these fields, and discuss some possible directions in the future exploration of WTA physiology and applications.

Anti-inflammatory effects of astaxanthin against fungal keratitis

AIM

To characterize effect of astaxanthin (ASX) in Aspergillus fumigatus (A. fumigatus) induced keratitis in mouse model.

METHODS

In vivo, fungal keratitis mouse model was established in C57BL/6 mice using A. fumigatus, followed by ASX or dimethyl sulfoxide (DMSO) treatment. Clinical responses were evaluated by clinical score and myeloperoxidase (MPO) assay. Inflammatory cytokines were assessed by reverse-transcription polymerase chain reaction (RT-PCR), Western blot, immunofluorescence, and enzyme-linked immuno sorbent assay (ELISA).

RESULTS

In animal model, ASX improved corneal transparency and clinical response, suppressed the expression of inflammatory cytokine like IL-1β, TNF-α, and HMGB-1. Neutrophil levels have been shown to decrease in ASX-treated cornea by immunofluorescence and MPO. TLR2 and TLR4 levels were lower in ASX-treated group than DMSO-treated.

CONCLUSION

ASX can suppress inflammatory response and reduce inflammatory cytokine production in mice model with A. fumigatus keratitis.

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.

ASGR1 and Its Enigmatic Relative, CLEC10A

The large family of C-type lectin (CLEC) receptors comprises carbohydrate-binding proteins that require Ca²⁺ to bind a ligand. The prototypic receptor is the asialoglycoprotein receptor-1 (ASGR1, CLEC4H1) that is expressed primarily by hepatocytes. The early work on ASGR1, which is highly specific for N-acetylgalactosamine (GalNAc), established the foundation for understanding the overall function of CLEC receptors. Cells of the immune system generally express more than one CLEC receptor that serve diverse functions such as pathogen-recognition, initiation of cellular signaling, cellular adhesion, glycoprotein turnover, inflammation and immune responses. The receptor CLEC10A (C-type lectin domain family 10 member A, CD301; also called the macrophage galactose-type lectin, MGL) contains a carbohydrate-recognition domain (CRD) that is homologous to the CRD of ASGR1, and thus, is also specific for GalNAc. CLEC10A is most highly expressed on immature DCs, monocyte-derived DCs, and alternatively activated macrophages (subtype M2a) as well as oocytes and progenitor cells at several stages of embryonic development. This receptor is involved in initiation of T_H1, T_H2, and T_H17 immune responses and induction of tolerance in naïve T cells. Ligand-mediated endocytosis of CLEC receptors initiates a Ca²⁺ signal that interestingly has different outcomes depending on ligand properties, concentration, and frequency of administration. This review summarizes studies that have been carried out on these receptors.

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.

Stories From the Dendritic Cell Guardhouse

Phagocytic cells [dendritic cells (DCs), macrophages, monocytes, neutrophils, and mast cells] utilize C-type (Ca²⁺-dependent) lectin-like (CLEC) receptors to identify and internalize pathogens or danger signals. As monitors of environmental imbalances, CLEC receptors are particularly important in the function of DCs. Activation of the immune system requires, in sequence, presentation of antigen to the T cell receptor (TCR) by DCs, interaction of co-stimulatory factors such as CD40/80/86 on DCs with CD40L and CD28 on T cells, and production of IL-12 and/or IFN-α/β to amplify T cell differentiation and expansion. Without this sequence of events within an inflammatory environment, or in a different order, antigen-specific T cells become unresponsive, are deleted or become regulatory T cells. Thus, the mode by which CLEC receptors on DCs are engaged can either elicit activation of T cells to achieve an immune response or induce tolerance. This minireview illustrates these aspects with Dectin-1, DEC205, the mannose receptor and CLEC10A as examples.

The C-type lectin receptor MGL senses N-acetylgalactosamine on the unique Staphylococcus aureus ST395 wall teichoic acid

Staphylococcus aureus is a common skin commensal but is also associated with various skin and soft tissue pathologies. Upon invasion, S. aureus is detected by resident innate immune cells through pattern-recognition receptors (PRRs), although a comprehensive understanding of the specific molecular interactions is lacking. Recently, we demonstrated that the PRR langerin (CD207) on epidermal Langerhans cells senses the conserved β-1,4-linked N-acetylglucosamine (GlcNAc) modification on S. aureus wall teichoic acid (WTA), thereby increasing skin inflammation. Interestingly, the S. aureus ST395 lineage as well as certain species of coagulase-negative staphylococci (CoNS) produce a structurally different WTA molecule, consisting of poly-glycerolphosphate with α-O-N-acetylgalactosamine (GalNAc) residues, which are attached by the glycosyltransferase TagN. Here, we demonstrate that S. aureus ST395 strains interact with the human Macrophage galactose-type lectin (MGL; CD301) receptor, which is expressed by dendritic cells and macrophages in the dermis. MGL bound S. aureus ST395 in a tagN- and GalNAc-dependent manner but did not interact with different tagN-positive CoNS species. However, heterologous expression of Staphylococcus lugdunensis tagN in S. aureus conferred phage infection and MGL binding, confirming the role of this CoNS enzyme as GalNAc-transferase. Functionally, the detection of GalNAc on S. aureus ST395 WTA by human monocyte-derived dendritic cells significantly enhanced cytokine production. Together, our findings highlight differential recognition of S. aureus glycoprofiles by specific human innate receptors, which may affect downstream adaptive immune responses and pathogen clearance.