Abstract
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.
Introduction
Glycans are covalently linked to proteins or lipids and are mostly expressed on the cell surface. The glycan structures are highly diverse and species-specific, in contrast to other biomolecules such as proteins and nucleic acids. Pathogens often express glycans which do not occur in humans and other mammals. The mammalian immune system has the ability to recognize pathogen-specific glycans, and in doing so an immune response is triggered. Lectin receptors are on the front line and play a direct role in detecting the glycans. Lectin-glycan interaction is generally weak and hence the specificity is low. One strategy to overcome the inherent weak affinity is to form a lectin oligomer gaining its increased affinity toward multiply presented glycans on microbes. How then do mammalian lectin receptors distinguish such exogenous glycans from the abundant endogenous glycans? The answer is provided by 3D structures and interaction modes of the mammalian lectin receptors. 3D structures of lectin receptors are deposited in the Protein Data Bank (PDB), and we could see how the glycan is recognized by lectin receptors. Knowledge on 3D structures of lectins is accumulating and summarized with curated information in UniLectin3D (). Here we focus on mammalian lectin receptors whose atomic 3D structures and binding modes toward the glycans of pathogens are available. This review is not a comprehensive list and analysis of all mammalian lectin receptors, the aim is rather to categorize the recognition modes of the glycans of pathogens, to gain an understanding of the structural basis of self/non-self-discrimination. Lectins are often categorized by their structural fold and monosaccharide-binding specificity. We here attempt to classify the recognition modes of microbe glycan into four modes: i) recognition of specific group epitope, ii) recognition of oligosaccharide with unique linkage and aglycon, iii) pattern recognition of special arrangements of sugar residues, and iv) internal recognition of polysaccharide (Figure 1). This classification is a starting point for discussion and will be revised according to new findings and inputs. Hereafter these recognition modes are explained by means of 3D structures of representative lectin receptors (Table 1).
FIGURE 1
TABLE 1
| recognition Mode | lectin | Species | PDB IDa | UniLectin3D IDb | ligand | Reference |
|---|---|---|---|---|---|---|
| Group epitope recognition | Intelectin-1 | Human | 4WMY | Q8WWA0 | Allyl-β-D-Galf | Wesener et al. (2015) |
| Intelectin-1 | Human | 6USC | Q8WWA0 | α-KO | ||
| SP-D | Human | 2RIB | P35247 | l-glycero-d-manno-heptose | Wang et al. (2008) | |
| Unique linkage /aglycon recognition | Dectin-1 | Mouse | 2BPE | Q6QLQ4 | – | |
| Mincle | Cow | 4ZRV | E1BHM0 | Trehalose | ||
| DCAR | Mouse | 6LFJ | – | Phosphoglycolipids | ||
| ZG16p | Human | 3VY7 | O60844 | αMan-O-Ser | ||
| Spatial pattern recognition | MBP-A trimer | Rat | 1KWU | P19999 | αMan-O-Me | |
| Langerin trimer | Human | 3KQG | Q9UJ71 | – | ||
| Langerin monomer | Human | 3P5D | Q9UJ71 | Manα1-3Man | ||
| Internal polysaccharide recognition | Dectin-2 | Human | 5VYB | Q6EIG7 | Manα1-2Manα1-3(Manα1-2Manα1-6)Man |
List of mammalian lectin receptors described in this review.
Unilectin3D:https://unilectin.eu/unilectin3D/
Recognition of Specific Group Epitope
How does the mammalian immune system sense microbe glycans as non-self? One possible simple and straightforward strategy is to detect microbe-specific sugar residues not found in mammals. An example is galactofuranose recognition by intelectin-1. Galactofuranose is a five-membered ring monosaccharide found in microbes but not in humans. Targeting galactofuranose as a non-self component is therefore a reasonable strategy to detect an invader. Human intelectin-1 shows unique specificity towards furanose residues, including ribofuranose and β-galactofuranose-containing disaccharide (
FIGURE 2

Galactofuranose and α-KO recognition by human intelectin-1 (A). Overall structure of trimeric intelectin-1 in complex with allyl-β-D-galactofuranose (β-Galf) ((B), PDB ID: 4WMY) and close-up view of ligand-binding site with β-Galf or α-KO ((C), PDB ID: 6USC). 3D structure of SP-D with L,D-heptose is shown for comparison ((D), PDB ID: 2RIB). Calcium ion is shown as a green sphere and the ligand in stick representation.
Recognition of Oligosaccharide with Unique Linkage and Aglycon
Lectins are typically categorized according to their folds and their monosaccharide specificity. This classification is based on the fact that lectins in a similar fold can recognize a single key monosaccharide residue using the monosaccharide-binding site. Some lectins, however, have an additional sugar binding site, adjacent to the primary binding site. The presence of the secondary binding site allows the lectin to bind certain oligosaccharides with higher specificity. For example, such binding mode is seen in crystal structure of E-selectin (
For example, β-glucan is composed of glucose residues also found in mammalian N-glycans and glycolipids. However, the Glcβ1-3Glc repeating unit is not found in mammalian glycans. The recognition of β-glucan as non-self must be because of a lectin specific to oligo/polysaccharides with this unique linkage. Dectin-1 is a mammalian β-glucan receptor whose lectin domain is responsible for Ca2+-independent β-glucan binding. The minimum length of β-glucan chain required for detectable binding to Dectin-1is a 10 or 11-mer, as determined by a glycan microarray experiment (
Another example is the recognition of the unusual mycobacterial glycolipid with a trehalose structure, Glcα1-1Glcα. This unique α1-α1 linkage is not found in mammals and the disaccharide unit is an important flag for the presence of mycobacteria. Trehalose dimycolate presented on the surface of Mycobacterium tuberculosis and the binding of a lectin undoubtedly initiates an immune response. Macrophage-inducible C-type lectin, Mincle is involved in the recognition of this key glycolipid. Mincle is a type II transmembrane lectin receptor and has been reported to bind pathogenic fungi and mycobacteria (
FIGURE 3

Trehalose (Glcα1-α1Glc) recognition by Mincle (PDB ID: 4ZRV). Overall structure of Mincle lectin domain with trehalose ligand (A) and close-up view of ligand-binding site with trehalose monobutylate (B). Calcium ion is shown as a green sphere and the ligand in stick representation. In the right panel, hydrophobic surfaces are colored in red and the putative acyl chain-binding site is indicated with a dotted circle.
Analogous to Mincle, the C-type lectin DCAR (dendritic cell immunoactivating receptor) is a receptor for acylated phosphatidyl-myo-inositol mannosides, which are expressed on the surface of mycobacteria. There is a crystal structure of DCAR lectin domain in complex with acyl chain-free, inositol-monophosphate dimannose at 1.8 Å (
ZG16p is a mammalian Jacalin-related mannose-binding lectin with a β-prism fold which also binds to phosphatidyl-myo-inositol mannosides (
Recognition of Specially Arranged Mannose Residues
Many C-type lectins show selectivity toward mannose residues and this mannose binding is utilized for microbe sensing. One of the most studied examples is mannose-binding protein, MBP, which is known to activate the complement lectin pathway. MBP belongs to a structurally homologous family of innate immune defense proteins known as collectins including surfactant protein A (SP-A) and SP-D. Collectin is named after their collagen-like lectin domains. Why does mannose recognition play this role in detecting non-self when mannose residues occur frequently in mammalian N-glycans, e.g., high-mannose glycan. The likely explanation is in the higher spatial density of mannose residues on bacteria relative to that of mammalian glycans. Couple this with the fact that mannose binding lectins are oligomers, a mechanism for enhancing the apparent affinity (avidity) presents itself. Although mannose residues are found in endogenous glycans, the low density and spatial patterns may not fit the binding sites of oligomeric lectins. There are crystal structures of a trimeric MBP-A lectin domain with an α-helical coiled coil region containing various ligands (
FIGURE 4

Oligomeric structure of lectin domains guided by the coiled coil region. Trimeric structures of MBP-A ((A), PDB ID: 1KWU) and langerin ((B), PDB ID: 3KQG) are shown. Ligand (O-methyl-mannose) are shown in stick representation for MBP-A. Calcium ions are shown as green spheres.
In addition to MBP, many other C-type lectins form oligomers possibly for the same purpose, namely, detecting multiply-presented mannose residues. Langerin is a C-type lectin with a coiled-coil region and a neck region in a trimeric structure, and has a distance of roughly 40 Å between binding sites (
Recognition of Internal Mannose Residues in Mannan
Related to the above section, certain lectin receptors bind mannan and bacterial polysaccharides. Dectin-2 is a C-type lectin expressed on macrophages and involved in the innate immune system. Dectin-2 binds to glycans containing the Manα1-2Man epitope, which is found in fungal mannans and bacterial polysaccharides but also occurs in endogenous high mannose N-glycan. How does Dectin-2 discriminate between the di-mannose structure of self and non-self? The crystal structure of Dectin-2 in complex with an oligosaccharide ligand shows that the binding site of Dectin-2 accommodates internally positioned Manα1-2Man of mannans and other polysaccharides (
FIGURE 5

Mannan polysaccharide recognition by Dectin-2 (PDB ID: 5VYB). Overall structure of Dectin-2 lectin domain (A) and close-up view of ligand-binding site (B) are shown. Manα1-2Manα1-2Man part of the whole N-glycan ligand (Man9GlcNAc2) is shown in stick representation.
Future Perspective
Mammalian lectin receptors adopt several strategies for detecting exogenous microbe glycans while tolerating endogenous glycans. Each lectin receptor prefers certain ligands and has been refined to bind to these and only these. Self/non-self-discrimination seems finely balanced, and this balance may be broken under certain disease conditions. Further study of the structural basis of glycan recognition will help gain an understanding of the maintenance of homeostasis, as well as the recognition of damaged cells, and the etiology and occurrence of autoimmune diseases. Structural knowledge is needed for the development of vaccines/drugs against infectious diseases. We know very little about the chemical structures of glycans presented on the surface of the different pathogens. For example, the lipopolysaccharide (LPS) structure is highly diverse, and probably unique for each bacterial strain (
Statements
Author contributions
Original draft preparation, YY; writing, editing and review, YY and NM.
Funding
This work was supported by grants-in-aid of KAKENHI (19H03362) and the Fugaku Trust for Medicinal Research.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Summary
Keywords
mammalian lectin receptor, microbe glycan, 3D structure, interaction, non-self
Citation
Manabe N and Yamaguchi Y (2021) 3D Structural View of Pathogen Recognition by Mammalian Lectin Receptors. Front. Mol. Biosci. 8:670780. doi: 10.3389/fmolb.2021.670780
Received
22 February 2021
Accepted
10 May 2021
Published
25 May 2021
Volume
8 - 2021
Edited by
James A. Garnett, King’s College London, United Kingdom
Reviewed by
Franck Fieschi, Université Grenoble Alpes, France
Frederique Lisacek, Swiss Institute of Bioinformatics (SIB), Switzerland
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*Correspondence: Yoshiki Yamaguchi, yyoshiki@tohoku-mpu.ac.jp
This article was submitted to Structural Biology, a section of the journal Frontiers in Molecular Biosciences
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