Sialic acid binding receptors (siglecs) expressed by macrophages

Sialic acids are structurally and topographically well-suited to function a s ligands in. cellular recognition events. Sialoadhesin (Sn) is a sialic ac id binding receptor uniquely expressed by macrophage subsets, It is a membe r of the immunoglobulin (Ig) superfamily with 17 extracellular domains, Sn is a prototypical member of the siglec family of sialic acid binding protei ns, which includes CD22, myelin-associated glycoprotein, CD33, and siglec-5 , These membrane proteins are involved in discrete functions in the hemopoi etic, immune, and nervous systems. The sialic acid binding region of siglec s is localized within the membrane-distal, amino-terminal domain and in the case of Sn, it has been characterized in atomic detail by,X-ray crystallog raphy, nuclear magnetic resonance, and site-directed mutagenesis, Our studi es on Sn indicate that this receptor is likely to function as a macrophage accessory molecule in a variety of cell-cell and cell-extracellular matrix interactions. CD33 and siglec-5 are also expressed on macrophage subsets as well as other myeloid cells. However, unlike Sn, the properties of these m olecules indicate a predominant role in signaling functions rather than in cell-cell interactions.