CHARACTERIZATION OF CD33 AS A NEW MEMBER OF THE SIALOADHESIN FAMILY OF CELLULAR INTERACTION MOLECULES

CD33 is a member of the Ig superfamily that is restricted to cells of the myelomonocytic lineage but whose functions and binding properties are unknown. It shares sequence similarity with sialoadhesin, CD22, an d the myelin-associated glycoprotein, which constitute the Sialoadhesi n family of sialic acid-dependent cell adhesion molecules, In the pres ent study, we show that CD33 is a fourth member of this family. As a m odel for sialic acid-dependent binding, human erythrocytes were deriva tized with N-acetylneuraminic acid (NeuAc) in different linkages, A re combinant soluble form of CD33, Fc-CD33, bound red blood cells with a specificity similar to that of sialoadhesin, preferring NeuAc alpha 2, 3Gal in N- and O-glycans over NeuAc alpha 2,6Gal in N-glycans. Fc-CD33 also bound selectively to the myeloid cell lines HL-60 and U937. Howe ver, CD33 was unable to mediate cell binding after transient expressio n in COS cells, despite high levels of surface expression, Pretreatmen t of the CD33-transfected cells with sialidase rendered them capable o f mediating sialic acid-dependent binding. These results show that CD3 3 can function as a sialic acid-dependent cell adhesion molecule and t hat binding can be modulated by endogenous sialoglycoconjugates when C D33 is expressed in a plasma membrane. (C) 1995 by The American Societ y of Hematology.