Sialic acid binding receptors (siglecs) expressed by macrophages

Sialic acids are structurally and topographically well‐suited to function as ligands in cellular recognition events. Sialoadhesin (Sn) is a sialic acid binding receptor uniquely expressed by macrophage subsets. It is a member of the immunoglobulin (Ig) superfamily with 17 extracellular domains. Sn is a prototypical member of the siglec family of sialic acid binding proteins, which includes CD22, myelin‐associated glycoprotein, CD33, and siglec‐5. These membrane proteins are involved in discrete functions in the hemopoietic, immune, and nervous systems. The sialic acid binding region of siglecs 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 crystallography, nuclear magnetic resonance, and site‐directed mutagenesis. Our studies 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 molecules indicate a predominant role in signaling functions rather than in cell‐cell interactions. J. Leukoc. Biol. 66: 705–711; 1999.