The inducible costimulator (ICOS) potentiates TCR-mediated calcium mobilization through Lck (IRM10P.749)

Abstract

Abstract It was previously shown that T cell costimulatory receptor ICOS plays an important role in inducing acute graft-versus-host disease (GVHD) in murine models. Although ICOS potentiates TCR-mediated phosphoinositide 3-kinase (PI3K) activation and intracellular calcium mobilization, relative roles of each signaling pathway in GVHD remains to be clarified. Using a knock-in strain of mice in which ICOS selectively lost the ability to activate PI3K, we have shown that T cells utilize PI3K-independent ICOS signaling pathways to induce GVHD. This led us to hypothesize that ICOS-Ca2+signaling pathways play important roles in GVHD. Through site-directed mutagenesis and flow cytometric analysis of calcium fluxing capacities of mutant ICOS proteins, we discovered a membrane proximal triple lysine motif in the ICOS cytoplasmic tail that is essential in inducing Ca2+ flux. We also have genetic and biochemical evidence that the Src family kinase Lck plays a crucial role in ICOS-mediated Ca2+signaling through the activation of ZAP-70 and PLCγ1. Moreover, ICOS appears to utilize different mechanisms than CD28 to potentiate TCR-mediated Ca2+ flux suggesting a synergy between the two related T cell costimulators. Thus our study identified ICOS-Lck-PLCγ1 signaling axis in T cell calcium signaling. Based on these, we aim to define the role of ICOS-Ca2+ signaling in GVHD by generating ICOS mutant mice with selective defect in Ca2+ signaling.