Here we characterize the properties and expression pattern of Siglec-9 (sialic 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 isolated from a dibutyryl cAMP-treated HL-60 cell cDNA library. Siglec-9 is predicted to contain three extracellular immunoglobulin-like domains that comprise an N-terminal V-set domain and two C2-set domains, a transmembrane region and a cytoplasmic tail containing two putative tyrosine-based signaling motifs. Overall, Siglec-9 is ϳ80% identical in amino acid sequence to Siglec-7, suggesting that the genes encoding these two proteins arose relatively recently by gene duplication. Binding assays showed that, similar to Siglec-7, Siglec-9 recognized sialic acid in either the ␣2,3-or ␣2,6-glycosidic linkage to galactose. Using a specific mAb, Siglec-9 was found to be expressed at high or intermediate levels by monocytes, neutrophils, and a minor population of CD16 Sialic acid-binding immunoglobulin-like lectins (Siglecs) 1 are transmembrane sialic acid-binding proteins of the immunoglobulin (Ig) superfamily characterized by the presence of an N-terminal V-set Ig-like domain and variable numbers of C2 set domains (1). The first Siglecs to be characterized were sialoadhesin/Siglec-1, CD22/Siglec-2, CD33/Siglec-3 and myelin-associated glycoprotein/Siglec-4 which share ϳ25-30% sequence identity within the extracellular regions (2). Recent studies (3-8) have uncovered the existence of a cluster of genes on human chromosome 19q13.3-4 that encode novel Siglecs highly related to CD33. This CD33-related subgroup includes Siglecs-3, -5, -6, -7, and -8, each of which share ϳ50 -70% sequence identity, suggesting that the genes encoding them have arisen relatively recently by gene duplication and exon shuffling. Despite their sequence similarity, all novel Siglecs characterized to date are expressed on distinct subsets of hemopoietic cells, such as neutrophils (Siglec-5) (4), B cells (Siglec-6) (8), natural killer (NK) cells (Siglec-7) (5, 6), and eosinophils (Siglec-8) (7). Each of these Siglecs also exhibits distinct carbohydrate binding properties (4, 5, 7-10). These differences in expression and ligand binding suggest that each Siglec mediates a specific, nonredundant function in hemopoietic cell biology.The cytoplasmic tails of most CD33-related Siglecs contain two homologous tyrosine-based motifs, one of which fits the consensus for immune receptor tyrosine-based inhibitory motifs (ITIMs) (11). The presence of one or more ITIMs has been described in a growing number of other leukocyte membrane receptors, many of which are tightly linked to CD33-related Siglecs on chromosome 19q13.4, in a region known as the leukocyte receptor cluster (12). The consensus that has emerged is that receptors bearing ITIMs mediate inhibitory functions when co-cross-linked with activating receptors bearing tyrosine based activatory motifs (reviewed in Ref. 11). This has been shown to be due to tyrosine phosphorylation of t...

Murine (m) Siglec-E and mSiglec-F are recently discovered murine sialic acid-binding Ig-like lectins with tyrosine-based inhibitory signaling motifs. They are postulated to be the orthologs of human (h) siglec-7, -8 or -9 and siglec-5, respectively. We report here the first detailed characterization of mSiglec-E, and compare its expression pattern with mSiglec-F. Similar to hSiglec-7, mSiglec-E preferred § 2-8-linked disialic acid over § 2-3-and § 2-6-linked sialic acids. Using a specific Ab, FACS analysis demonstrated that mSiglec-E was expressed mainly on neutrophils in blood and their immature precursors in bone marrow. mSiglec-E was present on peritoneal cavity macrophages and on subsets of mature NK cells and splenic dendritic cells. mSiglec-E was also found on a novel population of peritoneal cavity B-1a-like cells and a subset of splenic B cells enriched in transitional T2 and marginal zone B cells. In striking contrast to mSiglec-E, mSiglec-F was expressed predominantly on eosinophils in blood and their precursors in the bone marrow. The distinct and largely nonoverlapping expression profiles of mSiglec-E and mSiglec-F suggest that they play nonredundant roles in the innate immune system. mSiglec-E is likely to modulate the functions of several types of effector cells, whereas mSiglec-F is likely to be more restricted to eosinophil biology.

The porcine reproductive and respiratory syndrome virus (PRRSV) is a major threat to swine health worldwide and is considered the most significant viral disease in the swine industry today. In past years, studies on the entry of the virus into its host cell have led to the identification of a number of essential virus receptors and entry mediators. However, viral counterparts for these molecules have remained elusive and this has made rational development of new generation vaccines impossible. The main objective of this study was to identify the viral counterparts for sialoadhesin, a crucial PRRSV receptor on macrophages. For this purpose, a soluble form of sialoadhesin was constructed and validated. The soluble sialoadhesin could bind PRRSV in a sialic acid-dependent manner and could neutralize PRRSV infection of macrophages, thereby confirming the role of sialoadhesin as an essential PRRSV receptor on macrophages. Although sialic acids are present on the GP3, GP4 and GP5 envelope glycoproteins, only the M/GP5 glycoprotein complex of PRRSV was identified as a ligand for sialoadhesin. The interaction was found to be dependent on the sialic acid binding capacity of sialoadhesin and on the presence of sialic acids on GP5. These findings not only contribute to a better understanding of PRRSV biology, but the knowledge and tools generated in this study also hold the key to the development of a new generation of PRRSV vaccines.