IGSF4 is a novel TCR ζ-chain-interacting protein that enhances TCR-mediated signaling (178.5)

Abstract Immunoglobulin superfamily member 4 (IGSF4) is a known ligand of CRTAM, a receptor expressed in activated NKT and CD8+ T cells, but its function in T cell immunity has not been elucidated. Here, we show that IGSF4 directly interacts with the TCR ζ-chain and enhances TCR signaling by enhancing ζ-chain phosphorylation. Ectopic overexpression of IGSF4 enhances TCR-mediated T cell activation. In contrast, IGSF4 knockdown shows a dramatic decrease in markers associated with T cell activation compared with those in control siRNA. The transmembrane domain is essential for TCR ζ-chain association and clustering to the immunological synapse, and the ectodomain is associated with T cell interaction with APCs. IGSF4-deficient mice have impaired TCR-mediated thymocyte selection and maturation. Further, these mice reveal attenuated effector T cell functions accompanied by defective TCR signaling. Collectively, the results indicate that IGSF4 plays a central role in T cell functioning by dual independent mechanisms, control of TCR signaling and control of T cell-APC interaction.