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.

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 Ca(2+)-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.

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Background: Mincle facilitates establishment of persistent infections of macrophages by Mycobacterium tuberculosis.Results: The mechanism of mincle binding to mycobacterial glycolipids has been defined, and inhibitors have been synthesized.Conclusion: Mincle binds to both the sugar portion of the glycolipid and the hydrocarbon tail.Significance: The results suggest ways to manipulate the response to mycobacteria and to improve adjuvants that stimulate the immune system.

Langerin is a type II transmembrane cell surface receptor found on Langerhans cells. The extracellular domain of langerin consists of a neck region containing a series of heptad repeats and a C-terminal C-type carbohydrate-recognition domain (CRD). A role for langerin in processing of glycoprotein antigens has been proposed, but until now there has been little study of the langerin protein. In this study, analytical ultracentrifugation and circular dichroism spectroscopy of recombinant soluble fragments of human langerin have been used to show that the extracellular region of this receptor exists as a stable trimer held together by a coiled coil of alpha-helices formed by the neck region. The langerin CRD shows specificity for mannose, GlcNAc, and fucose, but only the trimeric extracellular domain fragment binds to glycoprotein ligands. Langerin extracellular domain binds mammalian high mannose oligosaccharides, as well mannose-containing structures on yeast invertase but does not bind complex glycan structures. Full-length langerin stably expressed in rat fibroblast transfectants mediates efficient uptake and degradation of a mannosylated neoglycoprotein ligand. pH-dependent ligand release appears to involve interactions between the CRDs or between the CRDs and the neck region in the trimer. The results are consistent with a role for langerin in internalization of both self and nonself glycoprotein antigens.