IP4 regulates FasL-mediated T lymphocyte death via inhibition of Orai1 (115.1)

Abstract Antigen receptor-mediated production of Ins(1,4,5)P3 induces the release of Ca2+ from intracellular stores, resulting in the opening of store-operated Ca2+ (SOC) channels. We previously reported a mouse line deficient in Inositol(1,4,5)P3-3 kinase B (ITPKb), which converts Ins(1,4,5)P3 (IP3) to Ins(1,3,4,5)P4 (IP4). Mice lacking ITPKb exhibit a complete block in T cell positive selection and possess impaired B cell development and activation. Interestingly, ITPKb-/- cells exhibit enhanced Ag receptor-induced SOC signaling, which can be rescued by adding cell-permeable IP4 to ITPKb-/- cells, suggesting that IP4 serves to inhibit SOC channels. Due to developmental blocks, the role of ITPKb in mature lymphocyte function is unknown. Using the ER-Cre-loxP system, tamoxifen-induced deletion of ITPKb (ITPKbfl/fl) demonstrates a crucial role for ITPKb in mature T cell function. While ITPKbfl/fl mice have normal T-independent Ab responses, T-dependent Ab responses are completely abolished. In vitro, mature ITPKbfl/fl T cells exhibit enhanced SOC entry and reduced proliferative responses to TCR signals. Furthermore, we found that ITPKbfl/fl T cells die rapidly after activation, due in part to enhanced FasL upregulation. Using a HEK293 cell line stably expressing Stim1 and Orai1, we demonstrate that IP4 is an inhibitor of open-state Orai1 channels. These data identify ITPKb and IP4 as essential mediators of lymphocyte development and T cell activation by regulating SOC entry via Orai1.