Does the cellular glycome influence the binding properties and signalling functions of siglecs in the immune system?

Siglecs are a family of up to 11 transmembrane sialic acid‐binding immunoglobulin superfamily. They are mostly expressed on discrete cellular subsets of the immune system where they are thought to be involved in both adhesive and signalling functions. Each siglec has a distinct preference for both the type of sialic acid as well as its linkage to other sugars. As sialic acids are commonly found on glycoproteins and glycolipids at the cell surface, siglecs can potentially interact with these ligands both in cis (on the same cell) as well as in trans (between cells). Cis interactions with sialic acids can mask the sialic acid binding activity of siglecs and unmasking can occur after sialidase treatment or in some cases by cellular activation. We have focused on siglec‐7 as a paradigm for a new group of siglecs related to CD33 that includes siglecs‐5, 6, 7, 8, 9, 10 and 11. Siglec‐7 is expressed on mature human natural killer (NK) cells and binds preferentially to glycans containing α2,8‐linked disialic acid. Similar to all CD33‐related siglecs, siglec‐7 has two tyrosine‐based motifs that are predicted to mediate inhibitory signalling functions. When expressed on NK cells, siglec‐7 is masked by cis interactions with sialic acids, but it can be unmasked following sialidase treatment. P815 target cells were transfected with GD3 synthase to induce expression of GD3, a simple ganglioside containing α2,8‐linked disialic acid. Expression of GD3 increased their resistance to cytotoxicity mediated by sialidase‐treated NK cell killing but had little effect on killing mediated by non sialidase‐treated cells. To investigate whether NK cells might express surface α2,8‐linked disialic acids that could explain the masked status of siglec‐7, we have analysed the binding of antibodies thought to bind specifically to this structure. By FACS analysis, we observed preferential binding to NK cells compared to other blood leucocytes including B cells, T cells and monocytes. Real time PCR has been used to investigate expression of all known α2,8‐specific sialyltransferases by NK cells and other blood leucocytes. Taken together, our findings indicate that NK cells express relatively high levels of α2,8‐linked disialic acids and that the binding specificity of siglec‐7 has become adapted to bind this structure, presumably to regulate NK‐cell activation.