Siglecs in the immune system

Abstract

Siglecs1 (sialic acid binding Ig-like lectins) are I-type (Ig-type) lectins2 characterized by an N-terminal V-set Ig domain that mediates sialic acid binding,3 followed by varying numbers of C2-set Ig domains (Fig. 1). The initial discovery of this lectin family came about through independent studies on sialoadhesin (Siglec-1/CD169), a macrophage lectin-like adhesion molecule,4 and CD22 (Siglec-2), a B-cell restricted member of the Ig superfamily (IgSF)5 that plays an important role in regulating B-cell activation. Both molecules were found to mediate cell–cell interactions in vitro via recognition of sialylated glycoconjugates.6-10 The cloning of sialoadhesin11 revealed striking sequence similarities to CD22 and led to the demonstration that two other related IgSF proteins, myelin-associated glycoprotein (MAG/Siglec-4) and CD33 (Siglec-3), which were not previously known to bind sialic acids, were also members of the Siglec family (Table 1).12, 13 Structural features of Siglecs. (a) Siglecs are type I membrane proteins with an extracellular region containing a sialic acid binding V-set Ig-like domain at the N-terminus and 1–16 C2-set Ig-like domains. The cytoplasmic tails of all Siglecs apart from sialoadhesin have one or more tyrosine residues within potential signalling motifs. A crystal structure of the sialoadhesin N-terminal domain complexed to 3′-sialyllactose3 revealed some key features of sialic acid recognition by Siglecs, including the presence of an essential arginine (Arg97) on the F strand (conserved in the other Siglecs) which forms a salt bridge with the carboxylate of sialic acid (b). The V-set and adjacent C2-set domains of Siglecs contain an unusual arrangement of conserved cysteines that give rise to an intrasheet disulphide (rather than the more usual intersheet disulphide) within the V-set domain and a disulphide between the domains. The conserved intrasheet disulphide in sialoadhesin results in widening of the Ig β-sandwich and exposure of two tryptophans (Trp2 and Trp106) on the A and G strands that form hydrophobic contacts with the N-acetyl and glycerol side groups of NeuAc, respectively (see 3for further details). Six additional human Siglecs (Siglecs 5–10) have been identified and characterized over the last 3 years. These previously unknown molecules share a high degree of sequence similarity with CD33 in their extracellular and intracellular regions, and are hence collectively referred to as 'CD33-related Siglecs'. A striking feature of the CD33-related Siglecs is their differential expression pattern amongst the cell lineages of the haemopoietic system (see Table 1). This, together with the presence of two conserved immunoreceptor tyrosine-based inhibition motif (ITIM)-like motifs in their cytoplasmic tails, suggests that, like CD22, CD33-related Siglecs are involved in regulating cellular activation within the immune system. Here we discuss how sialic acid recognition by Siglecs might contribute to the regulation of immune functions. Sialic acid is a generic term for a large family of 9-carbon sugars that are all derivatives of neuraminic acid (Neu) or keto-deoxynonulosonic acid (KDN). They are typically found at the exposed, non-reducing ends of oligosaccharide chains attached to a wide variety of proteins and lipids. Many different types of sialic acid exist in mammals and each can occur in a variety of glycosidic linkages. Thus, in addition to their well-known roles in preventing cell–cell interactions through charge-repulsion effects and in masking subterminal sugars, sialic acids are also well situated to act as ligands for mediating selective cell–cell interactions (reviewed in 14-16). With the exception of Siglec-6, which shows restricted specificity for the sialyl Tn antigen,17 all of the known Siglecs recognize forms and linkages of sialic acid that are commonly found at cell surfaces and in the extracellular environment. One consequence is that the Siglec binding site can be masked by cis interactions with sialic acids on the same cell surface, thereby preventing them from mediating cell–cell interactions (Fig. 2). Diagram illustrating how the interaction of Siglecs with cells or sialylated pathogens could be affected by cis interactions with sialylated glycoconjugates. (a) (i) Some Siglecs such as sialoadhesin appear to be unmasked and able to mediate cell–cell interactions constitutively whereas (ii) the majority of Siglecs appear to be naturally masked due to cis interactions with adjacent sialic acids. Cis interactions may be important for signalling functions; for example CD22 is closely associated with the B-cell receptor complex and provides inhibitory signals that raise B-cell activation thresholds. (iii) Unmasking of Siglecs can occur in some cases by cellular activation or by exposure to sialidases. The unmasked Siglec may now be capable of stronger interactions with ligands on other cells. This could result in increased cellular interactions and/or signal