Glycosylation is the enzymatic process that produces glycosidic linkages of saccharides to other saccharides, proteins and lipids, and is probably as ancient as life itself. Unicellular and multicellular organisms depend on glycosylation to produce monomeric and multimeric glycan linkages that are essential for cell viability and normal function1���4. The resulting glycome encompasses a diverse and abundant repertoire of glycans, which are one of the four fundamental macromolecular com ponents of all cells (together with nucleic acids, proteins and lipids) (FIG. 1). Glycans have important biological functions in protein maturation and turnover, cell adhesion and trafficking, and receptor binding and activation5���8. Glycosylation is prominent in the lumen of the endo- plasmic reticulum (ER) and in the Golgi apparatus. The cellular repertoire of glycans that are produced by gly- cosylation in these organelles of the secretory pathway reflects the combinatorial expression of subsets of glyco- syltransferase and glycosidase enzymes, of which there are more than 200 in the mammalian genome. The formation and breakdown of glycans are regulated at several levels in the cell. One of the mechanisms involves transcriptional regulation of the genes that encode these enzymes, but others include access to substrates and molecular inter- actions that alter enzyme localization in the lumen of the ER and Golgi2,9,10. Changes in the glycome can occur in response to environmental and genetic stimuli, and are frequently associated with the acquisition of altered cellular pheno types1���10. Glycosylation also occurs among proteins in the cytoplasm and nucleus through the actions of the Ogt glycosyltransferase, which produces a reversible O-linked ��-N-acetylglucosamine (O-GlcNAc) post- translational modification11. O-GlcNAc linkages are important in numerous physiological processes and disease. In contrast to this intracellular glycosidic bond that is formed by Ogt, secretory glycosylation in the ER and Golgi produces a large structural repertoire, including oligomeric glycan linkages that are presented at the cell surface and in extracellular compartments. Intracellular glycosylation has been reviewed elsewhere and is not discussed further in this Review12. Glycosylation can substantially modify the structure and function of proteins by steric influences involving intermolecular and intramolecular interactions and can mediate the production of glycan ligands for lectins (FIG. 2). Lectins are proteins with glycan-binding activ- ity that were first described in plants but subsequently have been found in all cells, from microorganisms to humans13���15. Lectin���glycan binding is a means of molec- ular recognition, which organisms can use to identify and decode the biological information that is present in their own cellular glycome as well as the glycomes of other organisms. The extracellular positioning of the secretory gly- come and its phylogenetic variation between organisms (Box 1) allows glycans to be involved in pathogen���host interactions16. Glycan linkages of the mammalian host are an abundant and attractive target for pathogens to establish an infection through lectin binding. Mammals have developed a phyloglycomic recognition system, which includes a subset of Toll-like receptors (TLRs) and C-type lectins, to detect the glycans of lower organisms as a means of non-self discrimination and immunological activation17. These pattern-recognition receptors and their responses to non-vertebrate glycans are not considered further in this Review. The development and function of the mammalian immune system are associated with restructuring of the cellular glycome. The immune-cell glycome is altered during cell differentiation, activation and apoptosis, and these alterations are being linked to both homeostatic Department of Cellular and Molecular Medicine and the Howard Hughes Medical Institute, 9500 Gilman Drive- 0625, University of California San Diego, La Jolla, California 92093, USA. Correspondence to J.D.M. e-mail: jmarth@ucsd.edu doi:10.1038/nri2417 Published online 10 october 2008 Glycome The biological repertoire of glycan structures. Mammalian glycosylation in immunity Jamey D. Marth and Prabhjit K. Grewal Abstract | Glycosylation produces a diverse and abundant repertoire of glycans, which are collectively known as the glycome. Glycans are one of the four fundamental macromolecular components of all cells, and are highly regulated in the immune system. Their diversity reflects their multiple biological functions that encompass ligands for proteinaceous receptors known as lectins. Since the discovery that selectins and their glycan ligands are important for the regulation of leukocyte trafficking, it has been shown that additional features of the vertebrate immune system are also controlled by endogenous cellular glycosylation. This Review focuses on the emerging immunological roles of the mammalian glycome. REVIEWS 874 | NOvEMbER 2008 | vOLUME 8 www.nature.com/reviews/immunol

6S 4S 4S 2S 6S Secretory pathway Intracellular High mannose Bi-antennary ��2 ��2 ��6 ��4 ��2 ��2 ��2 ��3 ��3 ��3 ��3 ��3 ��3 ��3 ��3 ��3 ��3 ��3 ��6 ��6 ��6 ��4 ��4 Asn ��3 ��3 ��3 ��3 ��3 ��3 ��3 ��3 ��3 ��3 ��6 ��6 ��6 ��6 ��6 ��6 ��6 ��6 ��6 ��4 ��4 ��4 ��4 ��3 ��3 ��3 ��3 ��3 ��3 ��3 ��3 ��3 ��3 ��3 ��3 ��3 ��4 ��4 ��4 ��6 ��6 ��6 ��6 ��4 ��4 ��4 ��4 ��4 ��4 ��4 ��4 ��4 ��4 ��4 ��4 ��4 ��4 ��4 ��4 ��4 ��4 ��4 ��4 ��4 ��4 ��4 ��4 ��4 ��4 ��4 ��4 ��4 ��3 ��3 ��3 ��4 ��4 ��4 ��4 ��4 ��4 ��4 ��4 ��4 ��4 ��3 ��3 ��3 ��3 ��3 ��4 ��3 ��3 ��3 ��4 ��4 ��3 ��3 ��3 ��4 ��4 ��3 ��4 ��3 ��3 ��3 ��3 ��3 ��3 ��3 ��4 ��4 ��4 ��4 ��3 ��3 ��3 ��3 ��4 ��4 ��4 ��2 ��2 ��2 ��2 ��2 ��2 ��2 ��6 ��4 Asn Tri-antennary Tetra-antennary Asn Asn Hyaluronan N-glycans O-glycans Ser/Thr Ser/Thr Ser/Thr Ser/Thr Ser/Thr Ser/Thr Core 1 Core 2 Core 3 Core 4 Ser/Thr Ser/Thr O-mannose O-fucose O-glucose P NH2 Et NS NS 6S 6S 6S 6S 6S 6S 6S 6S 6S Nature Reviews | Immunologyaretype 6S PI Ser Ser Ser Ser Heparan Chondroitin Dermatan Keratan Glycosaminoglycans Glycolipids GPI anchor O-GIcNAc Sialic acid Galactose N-acetylglucosamine N-acetylgalactosamine Fucose Xylose Glucose Mannose Glucuronic acid Iduronic acid Ceramide Figure 1 | The types of mammalian glycan. Glycan structures of the six classes of secretory glycan (N-glycans, hyaluronan, O-glycans, glycolipids, glycosylphosphatidylinositol (GPI) anchor and glycosaminoglycans) and the single intracellular glycan O-linked ��-N-acetylglucosamine (O-GlcNAc) are shown. Representative examples of each indicated using the symbol nomenclature for monosaccharides (see key). Multiple and multi-antennary examples of the predominant mature, complex-type N-glycans are shown, including the high-mannose N-glycan that is found attached to some glycoproteins. Examples of core 1���4 O-glycans are depicted, as well as O-mannose,O-fucose and O-glucose structures. Core 5���7 O-glycans are not shown. The GPI anchor and examples of the glycosaminoglycans and glycolipids are also depicted. In all examples, glycan linkages are identified by the anomeric configuration (�� or ��) of the donor saccharide following its linkage to the ring position (1���6) of the glycan acceptor. Et���P denotes a phosphoethanolamine linkage. NS, 2S, 4S and 6S denote the sulphation positions of the glycosaminoglycan chains. Asn, asparagine Ser, serine Thr, threonine. REVIEWS NATURE REvIEws | immunology vOLUME 8 | NOvEMbER 2008 | 875