Siglec-9, a novel sialic acid binding member of the immunoglobulin superfamily expressed broadly on human blood leukocytes

Here we characterize the properties and expression pattern of Siglec-9 (sia lic acid-binding Ig-like lectin-9), a new member of the Siglec subgroup of the immunoglobulin superfamily, A full-length cDNA encoding Siglec-9 was is olated from a dibutyryl cAMP-treated HL-60 cell cDNA library. Siglec-9 is p redicted to contain three extracellular immunoglobulin-like domains that co mprise an N-terminal V-set domain and two C2-set domains, a transmembrane r egion and a cytoplasmic tail containing two putative tyrosine-based signali ng motifs, Overall, Siglec-9 is similar to 80% identical in amino acid sequ ence to Siglec-7, suggesting that the genes encoding these two proteins aro se relatively recently by gene duplication. Binding assays showed that, sim ilar to Siglec-7, Siglec-9 recognized sialic acid in either the alpha 2,3- or alpha 2,6-glycosidic linkage to galactose, Using a specific mAb, Siglec- 9 was found to be expressed at high or intermediate levels by monocytes, ne utrophils, and a minor population of CD16(+), CD56(-) cells. Weaker express ion was observed on similar to 50%, of B cells and NK cells and minor subse ts of CD8(+) T cells and CD4(+) T cells. These results show that despite th eir high degree of sequence similarity, Siglec-7 and Siglec-9 have distinct expression profiles.