Maureen E. Taylor

The C‐type lectin superfamily in the immune system

Summary: Protein‐carbohydrate interactions serve multiple functions in the immune system. Many animal lectins (sugar‐binding proteins) mediate both pathogen recognition and cell‐cell interactions using structurally related Ca2+‐dependent carbohydrate‐recognition domains (C‐type CRDs). Pathogen recognition by soluble collections such as serum mannose‐binding protein and pulmonary surfactant proteins, and also the macrophage cell‐surface mannose receptor, is effected by binding of terminal monosaccharide residues characteristic of bacterial and fungal cell surfaces. The broad selectivity of the monosaccharide‐binding site and the geometrical arrangement of multiple CRDs in the intact lectins explains the ability of the proteins to mediate discrimination between self and non‐self. In contrast, the much narrower binding specificity of selectin cell adhesion molecules results from an extended binding site within a single CRD. Other proteins, particularly receptors on the surface of natural killer cells, contain C‐type lectin‐like domains (CTLDs) that are evolutionarily divergent from the C‐type lectins and which would be predicted to function through different mechanisms.

Structural basis for distinct ligand-binding and targeting properties of the receptors DC-SIGN and DC-SIGNR

Both the dendritic cell receptor DC-SIGN and the closely related endothelial cell receptor DC-SIGNR bind human immunodeficiency virus and enhance infection. However, biochemical and structural comparison of these receptors now reveals that they have very different physiological functions. By screening an extensive glycan array, we demonstrated that DC-SIGN and DC-SIGNR have distinct ligand-binding properties. Our structural and mutagenesis data explain how both receptors bind high-mannose oligosaccharides on enveloped viruses and why only DC-SIGN binds blood group antigens, including those present on microorganisms. DC-SIGN mediates endocytosis, trafficking as a recycling receptor and releasing ligand at endosomal pH, whereas DC-SIGNR does not release ligand at low pH or mediate endocytosis. Thus, whereas DC-SIGN has dual ligand-binding properties and functions both in adhesion and in endocytosis of pathogens, DC-SIGNR binds a restricted set of ligands and has only the properties of an adhesion receptor.

Evolving views of protein glycosylation

The composition, biosynthesis and known roles of oligosaccharides that are attached to glycoproteins suggest that multiple forces have driven the evolution of proteins that create and recognize these structures. The evolution of glycoprotein biosynthesis and recognition mechanisms can be best understood as a sequential development of functions associated with oligosaccharides.

Oligolysine-based oligosaccharide clusters. Selective recognition and endocytosis by the mannose receptor and dendritic cell-specific intercellular adhesion molecule 3 (ICAM-3)-grabbing nonintegrin.

Dendritic cells are potent antigen-presenting cells that express several membrane lectins, including the mannose receptor and DC-SIGN (dendritic cell-specific ICAM-3-grabbing nonintegrin). To identify highly specific ligands for these dendritic cell receptors, oligosaccharides were converted into glycosynthons (Os1) and were used to prepare oligolysine-based glycoclusters, Os-[Lys(Os)]n-Ala-Cys-NH2. Clusters containing two to six dimannosides as well as clusters containing four or five pentasaccharides (Lewisa or Lewisx) or hexasaccharides (Lewisb) were synthesized. The thiol group of the appended cysteine residue allows easy tagging by a fluorescent probe or convenient substitution with an antigen. Surface plasmon resonance was used to determine the affinity of the different glycoclusters for purified mannose receptor and DC-SIGN, whereas flow cytometry and confocal microscopy analysis allowed assessment of cell uptake of fluoresceinyl-labeled glycoclusters. Dimannoside clusters are recognized by the mannose receptor with an affinity constant close to 106 liter·mol–1 but have a very low affinity for DC-SIGN (less than 104 liter·mol–1). Conversely, Lewis clusters have a higher affinity toward DC-SIGN than toward the mannose receptor. Dimannoside clusters are efficiently taken up by human dendritic cells as well as by rat fibroblasts expressing the mannose receptor but not by HeLa cells or rat fibroblasts expressing DC-SIGN; DC-SIGN-expressing cells take up Lewis clusters. The results suggest that ligands containing dimannoside clusters can be used specifically to target the mannose receptor, whereas ligands containing Lewis clusters will be targeted to DC-SIGN.

Mechanism of Ca2+ and Monosaccharide Binding to a C-type Carbohydrate-recognition Domain of the Macrophage Mannose Receptor

Site-directed mutagenesis has been used to identify residues that ligate Ca2+ and sugar to the fourth C-type carbohydrate-recognition domain (CRD) of the macrophage mannose receptor. CRD-4 is the only one of the eight CRDs of the mannose receptor to exhibit detectable monosaccharide binding when expressed in isolation, and it is central to ligand binding by the receptor. CRD-4 requires two Ca2+ for sugar binding, like the CRD of rat serum mannose-binding protein (MBP-A). Sequence comparisons between the two CRDs suggest that the binding site for one Ca2+, which ligates directly to the bound sugar in MBP-A, is conserved in CRD-4 but that the auxiliary Ca2+ binding site is not. Mutation of the four residues at positions in CRD-4 equivalent to the auxiliary Ca2+ binding site in MBP-A indicates that only one, Asn728, is involved in ligation of Ca2+. Alanine-scanning mutagenesis was used to identify two other asparagine residues and one glutamic acid residue that are probably involved in ligation of the auxiliary Ca2+ to CRD-4. Sequence comparisons with other C-type CRDs suggest that the proposed binding site for the auxiliary Ca2+ in CRD-4 of the mannose receptor is unique. Evidence that the conserved Ca2+ in CRD-4 bridges between the protein and bound sugar in a manner analogous to MBP-A was obtained by mutation of one of the amino acid side chains at this site. Ring current shifts seen in the 1H NMR spectra of methyl glycosides of mannose, GlcNAc, and fucose in the presence of CRD-4 and site-directed mutagenesis indicate that a stacking interaction with Tyr729 is also involved in binding of sugars to CRD-4. This interaction contributes about 25% of the total free energy of binding to mannose. C-5 and C-6 of mannose interact with Tyr729, whereas C-2 of GlcNAc is closest to this residue, indicating that these two sugars bind to CRD-4 in opposite orientations. Sequence comparisons with other mannose/GlcNAc-specific C-type CRDs suggest that use of a stacking interaction in the binding of these sugars is probably unique to CRD-4 of the mannose receptor.

Selective Binding of the Scavenger Receptor C-type Lectin to Lewisx Trisaccharide and Related Glycan Ligands

The scavenger receptor C-type lectin (SRCL) is an endothelial receptor that is similar in organization to type A scavenger receptors for modified low density lipoproteins but contains a C-type carbohydrate-recognition domain (CRD). Fragments of the receptor consisting of the entire extracellular domain and the CRD have been expressed and characterized. The extracellular domain is a trimer held together by collagen-like and coiled-coil domains adjacent to the CRD. The amino acid sequence of the CRD is very similar to the CRD of the asialoglycoprotein receptor and other galactose-specific receptors, but SRCL binds selectively to asialo-orosomucoid rather than generally to asialoglycoproteins. Screening of a glycan array and further quantitative binding studies indicate that this selectivity results from high affinity binding to glycans bearing the Lewisx trisaccharide. Thus, SRCL shares with the dendritic cell receptor DC-SIGN the ability to bind the Lewisx epitope. However, it does so in a fundamentally different way, making a primary binding interaction with the galactose moiety of the glycan rather than the fucose residue. SRCL shares with the asialoglycoprotein receptor the ability to mediate endocytosis and degradation of glycoprotein ligands. These studies suggest that SRCL might be involved in selective clearance of specific desialylated glycoproteins from circulation and/or interaction of cells bearing Lewisx-type structures with the vascular endothelium.

Structure and Function of the Macrophage Mannose Receptor

The mannose receptor acts as a molecular scavenger by mediating Ca2+-dependent recognition and internalization of glycoconjugates terminating in mannose, N-acetylglucosamine or fucose. The receptor was identified when it was found that glycoproteins terminating in GlcNAc or mannose, including lysosomal enzymes, are rapidly cleared from the bloodstream by the liver (Schlesinger et al. 1976). The mannose receptor was found to be located on hepatic endothelial cells and Kupffer cells but not on hepatocytes (Schlesinger et al. 1978). The receptor has since been found on most types of tissue macrophages, including those of the placenta, but not on circulating monocytes (Shepherd et al. 1982). The retinal pigmented epithelium, a phagocytic cell layer, also expresses the mannose receptor (Shepherd et al. 1991). More recently, the mannose receptor has been identified on CD1-positive dendritic cells and Langerhan’s cells (Sallusto et al. 1995; Condaminet et al. 1998).

Complex Encounters at the Macrophage-Mycobacterium Interface: Studies on the Role of the Mannose Receptor and CD14 in Experimental Infection Models with Mycobacterium Avium

The initial interactions between mycobacterial cell wall components and receptor structures on the surface of macrophages may be critical in determining the outcome of infection. They may trigger the ingestion and digestion of microorganisms, but they may also promote the intracellular persistence and growth of mycobacteria. Using Mycobacterium avium as a model system, three approaches of different complexities were used to analyse some structural features and some functional consequences of M. avium interacting with the macrophage mannose receptor or CD14, a pattern recognition receptor. Binding specificities of a recombinant, truncated extracellular portion of the mannose receptor were assayed in a novel ELISA-formatted system using viable M. avium cells as ligands. Infection with M. avium strains differing in their virulence were performed in murine bone marrow-derived macrophages and in mice with a targeted deletion of the CD14 gene. These parallel and converging approaches not only help define the molecular basis for understanding early events in the pathogenesis of mycobacterial infections, but are also necessary to ultimately determine the relevance of in vitro findings in the context of actual manifestations of disease in vivo.

Carbohydrate-Recognition Proteins of Macrophages and Related Cells

The widespread distribution of complex carbohydrates and the variety of different naturally occurring structures form the basis for the hypothesis that carbohydrates act as recognition elements. There is experimental evidence for the involvement of carbohydrates in cell—cell adhesion, in interaction of cells with the extracellular matrix, and in specific recognition of one cell by another (Yamada, 1983; Edelman, 1985; Hook et al., 1984, Rademacher et al., 1988). For carbohydrates to act as recognition elements, there must be other molecules that selectively interact with them. Lectins, which are nonenzymatic, nonimmune proteins that bind to carbohydrates (Sharon and Lis, 1989), have the potential to fulfill this function. Many animal lectins have been isolated, from a wide variety of tissues and cell types. Before considering lectins of macrophages and related cells, the properties of animal lectins in general will be reviewed briefly.