The Membrane-Proximal Immunoreceptor Tyrosine-Based Inhibitory Motif Is Critical for the Inhibitory Signaling Mediated by Siglecs-7 and -9, CD33-Related Siglecs Expressed on Human Monocytes and NK Cells

Division of Cell Biology and Immunology, The Wellcome Trust Biocentre, University of Dundee, Dundee, United Kingdom.
The Journal of Immunology (Impact Factor: 4.92). 01/2005; 173(11):6841-9. DOI: 10.4049/jimmunol.173.11.6841
Source: PubMed


Siglec-7 and Siglec-9 are two members of the recently characterized CD33-related Siglec family of sialic acid binding proteins and are both expressed on human monocytes and NK cells. In addition to their ability to recognize sialic acid residues, these Siglecs display two conserved tyrosine-based motifs in their cytoplasmic region similar to those found in inhibitory receptors of the immune system. In the present study, we use the rat basophilic leukemia (RBL) model to examine the potential of Siglecs-7 and -9 to function as inhibitory receptors and investigate the molecular basis for this. We first demonstrate that Siglecs-7 and -9 are able to inhibit the FcepsilonRI-mediated serotonin release from RBL cells following co-crosslinking. In addition, we show that under these conditions or after pervanadate treatment, Siglecs-7 and -9 associate with the Src homology region 2 domain-containing phosphatases (SHP), SHP-1 and SHP-2, both in immunoprecipitation and in fluorescence microscopy experiments using GFP fusion proteins. We then show by site-directed mutagenesis that the membrane-proximal tyrosine motif is essential for the inhibitory function of both Siglec-7 and -9, and is also required for tyrosine phosphorylation and recruitment of SHP-1 and SHP-2 phosphatases. Finally, mutation of the membrane-proximal motif increased the sialic acid binding activity of Siglecs-7 and -9, raising the possibility that "inside-out" signaling may occur to regulate ligand binding.

41 Reads
  • Source
    • "The postulated inhibitory activity of Siglec 9 on the immune response through host and bacterial sialoglycans recognition [16] [17] prompted us to verify its expression on monocyte subsets at baseline and after IVIg. "
    [Show abstract] [Hide abstract]
    ABSTRACT: Intravenous IgG administration induces significant modifications in the innate and adaptive compartment of the immune system including the monocyte/macrophage system. We analyzed the in vivo effect of IgG administered at replacement dosages on the frequency of monocytes subsets, on the modulation of CD11b and sialic acid-binding immunoglobulin-like lectin receptor (Siglec 9) expression and on monocytes production of reactive oxygen species. We showed that patients with Common Variable Immune Deficiency have an increased frequency pro-inflammatory intermediate CD14(++)CD16(+) monocytes and an increased expression of CD11b and Siglec 9 on monocytes. IgG administered at replacement dosages exerted an in vivo anti-inflammatory effect as shown by a reduction of circulating monocytes, of intermediate pro-inflammatory monocytes, of CD11b and Siglec 9 expression and of ex vivo monocytes oxidative burst. Nevertheless, intravenous IgG administration did not affect the monocyte functional ability to respond to a bacterial stimulation in terms of CD11b and Siglec 9 expression and reactive oxygen species production. Copyright © 2015. Published by Elsevier B.V.
    International immunopharmacology 07/2015; 28(1):596-603. DOI:10.1016/j.intimp.2015.07.017 · 2.47 Impact Factor
  • Source
    • "Siglec-9 is also expressed on monocytes and MUC16 promotes binding of monocytes to cancer cells. Upon binding to its ligand, Siglec-9 is phosphorylated on its Immunoreceptor Tyrosine-based Inhibition Motif (ITIM) tail, triggering an inhibitory signaling cascade that results in inhibition of the NK cell response [121]. Ovarian cancer cells are therefore likely to be protected from NK cell and monocyte attack due to the negative signaling induced via MUC16-Siglec-9 interaction (Figure 5). "
    [Show abstract] [Hide abstract]
    ABSTRACT: Over three decades have passed since the first report on the expression of CA125 by ovarian tumors. Since that time our understanding of ovarian cancer biology has changed significantly to the point that these tumors are now classified based on molecular phenotype and not purely on histological attributes. However, CA125 continues to be, with the recent exception of HE4, the only clinically reliable diagnostic marker for ovarian cancer. Many large-scale clinical trials have been conducted or are underway to determine potential use of serum CA125 levels as a screening modality or to distinguish between benign and malignant pelvic masses. CA125 is a peptide epitope of a 3-5 million Da mucin, MUC16. Here we provide an in-depth review of the literature to highlight the importance of CA125 as a prognostic and diagnostic marker for ovarian cancer. We focus on the increasing body of literature describing the biological role of MUC16 in the progression and metastasis of ovarian tumors. Finally, we consider previous and on-going efforts to develop therapeutic approaches to eradicate ovarian tumors by targeting MUC16. Even though CA125 is a crucial marker for ovarian cancer, the exact structural definition of this antigen continues to be elusive. The importance of MUC16/CA125 in the diagnosis, progression and therapy of ovarian cancer warrants the need for in-depth research on the biochemistry and biology of this mucin. A renewed focus on MUC16 is likely to culminate in novel and more efficient strategies for the detection and treatment of ovarian cancer.
    Molecular Cancer 05/2014; 13(1):129. DOI:10.1186/1476-4598-13-129 · 4.26 Impact Factor
  • Source
    • "Moreover, we also demonstrate that constitutive expression of Siglec-7 on MDMs markedly contributes to their HIV-1 infection, since specific Siglec-7 blockade induced a reduction of HIV-RNA in MDMs in a 4-hour entry assay. The degree of surface levels of Siglec-7 on monocytes and macrophages is still being debated, as there is one study showing that expression of this molecule on monocytes and MDMs is very low [29], while several other reports demonstrated that monocytes express high levels of Siglec-7 [9,11,12,46,52]. In our experiments, the constitutive expression of Siglec-7 on monocytes paralleled the ones observed on NK cells [13], and remained very high on MDMs even after 7 days of culture with GM-CSF although at lower levels if compared to freshly isolated monocytes. "
    [Show abstract] [Hide abstract]
    ABSTRACT: Sialic acid-binding Ig-like lectin-7 (Siglec-7) expression is strongly reduced on natural killer (NK) cells from HIV-1 infected viremic patients. To investigate the mechanism(s) underlying this phenomenon, we hypothesized that Siglec-7 could contribute to the infection of CD4pos target cells following its interaction with HIV-1 envelope (Env) glycoprotein 120 (gp120). The ability of Siglec-7 to bind gp120 Env in a sialic acid-dependent manner facilitates the infection of both T cells and monocyte-derived macrophages (MDMs). Indeed, pre-incubation of HIV-1 with soluble Siglec-7 (sSiglec-7) increases the infection rate of CD4pos T cells, which do not constitutively express Siglec-7. Conversely, selective blockade of Siglec-7 markedly reduces the degree of HIV-1 infection in Siglec-7pos MDMs. Finally, the sSiglec-7 amount is increased in the serum of AIDS patients with high levels of HIV-1 viremia and inversely correlates with CD4pos T cell counts. Our results show that Siglec-7 binds HIV-1 and contributes to enhance the susceptibility to infection of CD4pos T cells and MDMs. This phenomenon plays a role in HIV-1 pathogenesis and in disease progression, as suggested by the inverse correlation between high serum level of sSiglec-7 and the low CD4pos T cell count observed in AIDS patients in the presence of chronic viral replication.
    Retrovirology 12/2013; 10(1):154. DOI:10.1186/1742-4690-10-154 · 4.19 Impact Factor
Show more