Glycosylation and the Immune System Pauline M. Rudd, 1 * Tim Elliott, 2 Peter Cresswell, 3 Ian A. Wilson, 4 Raymond A. Dwek 1 * Almost all of the key molecules involved in the innate and adaptive immune response are glycoproteins. In the cellular immune system, spe- cific glycoforms are involved in the folding, quality control, and assembly of peptide-loaded major histocompatibility complex (MHC) antigens and the T cell receptor complex. Although some glycopeptide antigens are presented by the MHC, the generation of peptide antigens from glyco- proteins may require enzymatic removal of sugars before the protein can be cleaved. Oligosaccharides attached to glycoproteins in the junction between T cells and antigen-presenting cells help to orient binding faces, provide protease protection, and restrict nonspecific lateral protein-pro- tein interactions. In the humoral immune system, all of the immunoglobu- lins and most of the complement components are glycosylated. Although a major function for sugars is to contribute to the stability of the proteins to which they are attached, specific glycoforms are involved in recognition events. For example, in rheumatoid arthritis, an autoimmune disease, agalac- tosylated glycoforms of aggregated immunoglobulin G may induce associa- tion with the mannose-binding lectin and contribute to the pathology. A current model of the immune system clas- sifies the processes by which foreign antigens are eliminated from their hosts into three interrelated systems, depending on whether the initial event takes place in the cytosol, the endocytic pathway or the extracellular space. In the cytosol, antigenic peptides generated by the proteasome are transported by mem- brane-bound transporters associated with an- tigen processing (TAP1 and 2) into the ER where they bind to MHC class I molecules (1). In the endocytic pathway, protein anti- gens are degraded in specialized acidic com- partments [MHC class II–containing com- partments (MIIC)] (2) into peptides prior to loading onto MHC class II molecules (3, 4 ). Also in the endocytic pathway, glycosylphos- phatidylinositol (GPI) anchors and glycolipid antigens associate with CD1 (2, 5–7 ). MHC class I and class II molecules, as well as CD1, are recognized by T lymphocytes. In the ex- tracellular space, intact antigens are recog- nized either by antibody molecules or by the mannose-binding lectin (MBL), both of which subsequently activate the complement system. The diversity of protein glycosylation plays an important role in the biosynthesis and bio- logical activity of the glycoproteins involved in antigen recognition. During transport of the gly- coproteins through the secretory pathway, the sugar chains undergo successive modifications which are regulated by the glycosylation pro- cessing enzymes. This process ensures that each glycoprotein displays the relevant glycan structures for its range of essential functions. The sugars play a role in protein folding and assembly, quality control, ER-associated retro- grade transport of misfolded proteins, the gen- eration and loading of antigenic peptides into MHC class I, and also influence the range of antigenic peptides generated in the endosomal pathway for presentation by MHC class II. By virtue of their size, glycans can shield large regions of the protein surfaces, providing protease protection for immune molecules in serum and secretions and in the T cell synapse, where they also limit nonspecific lateral pro- tein-protein interactions. Glycans located close to the cell membrane, or in heavily glycosylated O-linked domains, can also determine the ori- entation and location of the binding faces of the proteins to which they are attached (8). In ad- dition, the local three-dimensional structure of the individual protein directs its final glycoform pattern (9) such that a diverse repertoire of oligosaccharides will be displayed on the sur- face of a single cell. This makes it unlikely that the cell will bind lectins involved in the innate immune system, since these generally require repetitive arrays of specific sugars for function- al recognition. Some of these advantages conferred by gly- cosylation are exploited by viruses, which use the host glycosylation machinery to assemble their own envelope glycoproteins such that they help avoid immune detection (10). In autoim- mune diseases, such as rheumatoid arthritis (RA) and systemic lupus erythrematosus (11, 12), significant changes in the populations of immunoglobulin G (IgG) glycoforms have been noted. In RA, aggregated agalactosyl gly- coforms of IgG are specifically recognized by the MBL such that inappropriate activation of the innate immune system may occur (13). Similarly on tumor cells, aberrant glycosylation can expose new epitopes for recognition by the immune system, such as for MUC1 (14 ), whereas mice deficient in 1,6 N-acetylglu- cosaminyltransferase V (Mgat5) showed kid- ney autoimmune disease and enhanced de- layed-type hypersensitivity and increased sus- ceptibility to experimental autoimmune en- cephalomyelitis (15). Glycosylation and Protein Folding Almost all of the key molecules involved in antigen recognition and the orchestration of the subsequent events are glycoproteins con- sisting of two or more subunits. Some, such as the T cell receptor (TCR) and CD3, are assembled in the ER into multimolecular com- plexes. The proper folding and controlled as- sembly of many newly synthesized glycopro- teins requires them to engage in a series of coordinated interactions with chaperones and enzymes, through the attachment of a common oligosaccharide precursor, GlcNAc 2 Man 9 Glc 3 (Fig. 1A), to N-linked glycosylation sites. This sugar precursor is rapidly processed to GlcNAc 2 Man 9 Glc 1 (Fig. 1B), which can bind two chaperones, the membrane-bound cal- nexin (Clx) and soluble calreticulin (Clr) (Fig. 1B) (16, 17 ). The lectin-like interactions of Clx and/or Clr with nascent glycoproteins provide access to a folding pathway (18), allow the recruitment of the thiol oxidoreductase, ERp57, and assist the assembly of subunits. Clx and Clr are also involved in the loading of antigenic peptides onto MHC class I from complexes of TAP and tapasin. In their role as quality control factors, Clx and Clr retain unfolded glycoproteins in the ER until they are correctly folded and assem- bled, an event which is signaled by the perma- nent removal of the terminal glucose residue by glucosidase II (Fig. 2). The folded glycoprotein, or the assembled multimolecular complex, is then transferred to the Golgi apparatus where the oligomannose sugars may be further pro- cessed. Misfolded or unassembled subunits are reglucosylated by uridine 5'-diphosphate–glu- cose:glycoprotein glucosyltransferase. Reglu- cosylation of unfolded proteins allows them to rebind Clx and enter a cyclical pathway until they either achieve their correctly folded structure and are released or they are targeted 1 The Glycobiology Institute, Department of Biochem- istry, University of Oxford, South Parks Road, Oxford OX1 3QU, UK. 2 Cancer Sciences Division, University of Southampton Medical School, CRC Medical Oncol- ogy Unit, Level F, Centre Block, Southampton General Hospital, Tremona Road, Southampton SO16 6YD, UK. 3 Howard Hughes Medical Institute, Section of Immunobiology, Yale University School of Medicine, 310 Cedar Street, New Haven, CT 06510, USA. 4 De- partment of Molecular Biology and Skaggs Institute for Chemical Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, CA 92037, USA. *To whom correspondence should be addressed. E- mail: pmr@glycob.ox.ac.uk (P.M.R.) and raymond. dwek@exeter.oxford.ac.uk (R.A.D.) 23 MARCH 2001 VOL 291 SCIENCE www.sciencemag.org 2370 C ARBOHYDRATES AND G LYCOBIOLOGY REVIEW