Myeloid specific human CD33 is an inhibitory receptor with differential ITIM function in recruiting the phosphatases SHP-1 and SHP-2

CD33 is a myeloid specific member of the sialic acid-binding receptor famil y and is expressed highly on myeloid progenitor cells but at much lower lev els in differentiated cells. Human CD33 has two tyrosine residues in its cy toplasmic domain (Y340 and Y358), When phosphorylated, these tyrosines coul d function as docking sites for the phosphatases, SHP-1 and/or SHP-2, enabl ing CD33 to function as an inhibitory receptor. Here we demonstrate that CD 33 is tyrosine phosphorylated in the presence of the phosphatase inhibitor, pervanadate, and recruits SHP-1 and SHP-2, Go-expression studies suggest t hat the Src-family kinase Lck is effective at phosphorylating Y340, but not Y358, suggesting that these residues may function in the selective recruit ment of adapter molecules and have distinct functions. Further support for overlapping, but nonredundant, roles for Y340 and Y358 comes from peptide-b inding studies that revealed the recruitment of both SHP-1 end SHP-2 to Y34 0 but only SHP-2 to Y358, Analysis using mutants of SHP-1 demonstrated that binding Y340 of CD33 was primarily to the amino Src homology-2 domain of S HP-1, The potential of CD33 to function as an inhibitory receptor was demon strated by its ability to downregulate CD64-induced calcium mobilization in U937. The dependence of this inhibition on SHP-1 was demonstrated by block ing CD33-mediated effects with dominant negative SHP-1. This result implies that CD33 is an inhibitory receptor and also that SHP-1 phosphatase has a significant role in mediating CD33 function. Further studies are essential to identify the receptor(s) that CD33 inhibits in vivo and its function in myeloid lineage development. (C) 2000 by The American Society of Hematology .