Talk abstracts

Talk on Saturday 11:20-11:35am submitted by Payal Mehta

Differences in signaling properties of FcγRI and FcγRIIa in human monocytic cells

Payal Mehta (Ohio State Biochemistry Program, The Ohio State University), Anne-Sophie Wavreille (Department of Internal Medicine, The Ohio State University), Jonathan P Butchar (Department of Internal Medicine, The Ohio State University), Susheela Tridandapani (Department of Internal Medicine, The Ohio State University)

Abstract:
Fcγ receptors are involved in mediating a variety of effector functions such as phagocytosis of IgG-coated particles, release of pro-inflammatory cytokines and Antibody Dependent Cell-mediated Cytotoxicity. Given the importance of FcγR function in autoimmune diseases and antibody-based therapies for cancer, our goal is to understand the mechanism of action of these receptors. Two classes of Fcγ receptors exist. The activating receptors include FcγRI, FcγRIIa and FcγRIII whereas FcγRIIb is an inhibitory receptor. The activating receptors are associated with an Immunoreceptor Tyrosine based Activation Motif. FcγRIIa contains ITAM within its cytoplasmic tail while FcγRI and FcγRIII associate with the γ-subunit containing ITAM. Recent work from our lab shows that FcγRI and FcγRIIa differ in their functional responses. Activation of FcγRI but not FcγRIIa caused an increased cytokine production, even though flow cytometry data indicated that both FcγRI and FcγRIIa are expressed abundantly on the cell surface. It was further shown that blocking FcγRI activity lead to decreased killing of antibody coated tumor cells suggesting a potential role for FcγRI in cancer therapy. We speculate that these differences may be due to differential interaction of downstream signaling proteins with the γ-chain versus FcγRIIa ITAMs. To test this hypothesis we performed peptide pull-down with synthetic peptides designed to mimic the ITAMs of γ-chain associated with the FcγRI and FcγRIIa. Binding affinities were quantitated by Biacore analysis. Our data indicate that while Syk binds equally to both FcγRI and FcγRIIa ITAM, the inhibitory phosphatases SHIP-1 and SHIP-2 associate preferentially with FcγRIIa ITAM. The ITAMs of both FcγRI and FcγRIIa contains two critical tyrosine residues which become phosphorylated upon activation of Fcγ receptors. We are currently investigating which of these tyrosine residues is important for association with signaling proteins.

References:
1) Sanchez-Mejorada G, Rosales C. Signal Transduction by Immunoglobulin Fc receptors. J Leukoc Biol. 1998;63:521-533
2) Ganesan LP, Joshi T, Fang H, Kutala VK, Roda J, Trotta R, Lehman A, Kuppusamy P, Byrd JC, Carson WE, Caligiuri MA, Tridandapani S. FcγR-induces production of superoxide and inflammatory cytokines is differentially regulated by SHIP through its influence on PI3K and/or Ras/Erk pathways. Blood. 2006;108:718-725

Keywords: FcR, Antibody Dependent Cell-mediated Cytotoxicity, Immunoreceptor Tyrosine based Activation Motif