The mouse genome sequence has been examined to identify the complete set of proteins related to the human glycanbinding receptor, DC-SIGN. In addition to five SIGNR proteins previously described, a pseudogene, encoding a hypothetical SIGNR6, and a further two expressed proteins, SIGNR7 and SIGNR8, have been identified. The ligand-binding properties of these novel proteins and of the previously described mouse SIGNs have been systematically investigated in order to define the mouse proteins that most resemble human DC-SIGN and DC-SIGNR. Results from screening of a glycan array demonstrate that only mouse SIGNR3 shares with human DC-SIGN the ability to bind both high mannose and fucose-terminated glycans in this format and to mediate endocytosis. The finding that neither SIGNR1 nor SIGNR5 binds with high affinity to specific ligands in a large panel of mammalian glycans is consistent with the suggestion that these receptors bind surface polysaccharides on bacterial and fungal pathogens in a manner analogous to serum mannosebinding protein. The data also reveal that two of the mouse SIGNs have unusual binding specificities that have not been previously described for members of the C-type lectin family; the newly identified SIGNR7 binds preferentially to the 6-sulfo-sialyl Lewis x oligosaccharide, whereas SIGNR2 binds almost exclusively to glycans that bear terminal GlcNAc residues. The results presented demonstrate that the mouse homologs of DC-SIGN have a diverse set of ligand-binding and intracellular trafficking properties, some of which are distinct from the properties of any of the human receptors.The human receptor designated as the dendritic cell-specific intercellular adhesion molecule 3 (ICAM-3)-grabbing nonintegrin (DC-SIGN) 4 has been identified both as an adhesion molecule that facilitates attachment of T cells to dendritic cells and as a potential pathogen-binding receptor (1, 2). DC-SIGNR or L-SIGN is a closely related receptor found on endothelial cells of liver, lymph nodes, and placenta (3). Both of these receptors have been of considerable interest because of their ability to bind human immunodeficiency virus and present it to CD4-positive T cells, greatly facilitating the efficiency of infection (4). Similar trans activity for other viruses has been reported, and the receptors can also directly mediate infection of cells in cis (3, 5, 6). There is also evidence that these receptors interact with bacterial pathogens and with parasites (7). Although the original names do not encompass all of the functions and expression patterns of DC-SIGN and DC-SIGNR, these acronyms have generally been retained and used as simple designations for the molecules. Both DC-SIGN and DC-SIGNR bind to surface glycoproteins of viruses by interacting with high mannose oligosaccharides. They bind with highest affinity to larger glycans that contain 8 or 9 mannose residues (8, 9). In addition, DC-SIGN, but not DC-SIGNR, binds to fucose-containing glycans, such as those present on the surfaces of nematode parasites (7, 9). T...

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The dendritic cell receptor DC-SIGN mediates pathogen recognition by binding to glycans characteristic of pathogen surfaces, including those found on HIV. Clustering of carbohydrate-binding sites in the receptor tetramer is believed to be critical for targeting of pathogen glycans, but the arrangement of these sites remains poorly understood. Surface force measurements between apposed lipid bilayers displaying the extracellular domain of DC-SIGN and a neoglycolipid bearing an oligosaccharide ligand provide evidence that the receptor is in an extended conformation and that glycan docking is associated with a conformational change that repositions the carbohydrate-recognition domains during ligand binding. The results further show that the lateral mobility of membranebound ligands enhances the engagement of multiple carbohydrate-recognition domains in the receptor oligomer with appropriately spaced ligands. These studies highlight differences between pathogen targeting by DC-SIGN and receptors in which binding sites at fixed spacing bind to simple molecular patterns.adhesion ͉ molecular recognition ͉ pathogen selectivity ͉ multivalent receptors

Langerhans cells are specialized skin dendritic cells that take up and degrade antigens for presentation to the immune system. Langerin, a cell surface C-type lectin of Langerhans cells, can be internalized and accumulates in Birbeck granules, subdomains of the endosomal recycling compartment that are specific to Langerhans cells. Langerin binds and mediates uptake and degradation of glycoconjugates containing mannose and related sugars. Analysis of the human genome has identified three single nucleotide polymorphisms that result in amino acid changes in the carbohydrate-recognition domain of langerin. The effects of the amino acid changes on the activity of langerin were examined by expressing each of the polymorphic forms. Expression of full-length versions of the four common langerin haplotypes in fibroblasts revealed that all of these forms can mediate endocytosis of neoglycoprotein ligands. However, sugar binding assays and differential scanning calorimetry performed on fragments from the extracellular domain showed that two of the amino acid changes reduce the affinity of the carbohydrate-recognition domain for mannose and decrease the stability of the extracellular domain. In addition, analysis of sugar binding by langerin containing the rare W264R mutation, previously identified in an individual lacking Birbeck granules, shows that this mutation abolishes sugar binding activity. These findings suggest that certain langerin haplotypes may differ in their binding to pathogens and thus might be associated with susceptibility to infection.Langerin (CD207) is a cell surface C-type lectin located on Langerhans cells (1, 2). Langerin is associated with Birbeck granules, subdomains of the endosomal recycling compartment specific to Langerhans cells that form where langerin accumulates following internalization (1-4). Langerin binds glycoconjugates containing mannose and related sugars and is able to mediate uptake and degradation of such ligands (5, 6). These properties may allow langerin to play a role in antigen uptake and processing, the main function of Langerhans cells (7). Evidence for such a role comes from the finding that anti-human langerin antibodies block uptake and presentation of mycobacterial nonpeptide antigens to CD1a-restricted T cells by Langerhans cells (8).Langerin is a type II transmembrane protein with a short cytoplasmic domain and an extracellular region consisting of a neck and a C-terminal C-type carbohydrate-recognition domain (CRD).3 The CRDs bind mannose and related sugars, but like other C-type CRDs they exhibit only low affinity for monosaccharides and oligomerization is required for binding of oligosaccharide ligands (6). Langerin oligomerizes to form stable trimers held together by a coiled coil of ␣-helices in the neck region (6). The cytoplasmic domain of langerin is likely to be involved in internalization, but it does not contain typical internalization motifs identified in other endocytic receptors (1).Examination of the single nucleotide polymorphism (SNP) data base (www.n...