The study investigates the role of PD-L1 in the development and function of induced regulatory T (iTreg) cells. PD-L1 is shown to be essential for the induction of iTreg cells, as PD-L1−/− antigen-presenting cells (APCs) fail to convert naive CD4 T cells into iTreg cells. PD-L1-coated beads can induce iTreg cells in vitro, indicating that PD-L1 itself regulates iTreg cell development. PD-L1 enhances and sustains Foxp3 expression and suppressive function in iTreg cells. In vivo studies using PD-L1−/−PD-L2−/− Rag−/− recipients of naive CD4 T cells demonstrate a marked reduction in iTreg cell conversion and a rapid onset of fatal inflammatory disease. The mechanism involves down-regulation of phospho-Akt, mTOR, S6, and ERK2, and up-regulation of PTEN, key signaling molecules critical for iTreg cell development. These findings highlight a novel mechanism for iTreg cell development and function, suggesting PD-L1 as a potential therapeutic target for controlling Treg cell plasticity.The study investigates the role of PD-L1 in the development and function of induced regulatory T (iTreg) cells. PD-L1 is shown to be essential for the induction of iTreg cells, as PD-L1−/− antigen-presenting cells (APCs) fail to convert naive CD4 T cells into iTreg cells. PD-L1-coated beads can induce iTreg cells in vitro, indicating that PD-L1 itself regulates iTreg cell development. PD-L1 enhances and sustains Foxp3 expression and suppressive function in iTreg cells. In vivo studies using PD-L1−/−PD-L2−/− Rag−/− recipients of naive CD4 T cells demonstrate a marked reduction in iTreg cell conversion and a rapid onset of fatal inflammatory disease. The mechanism involves down-regulation of phospho-Akt, mTOR, S6, and ERK2, and up-regulation of PTEN, key signaling molecules critical for iTreg cell development. These findings highlight a novel mechanism for iTreg cell development and function, suggesting PD-L1 as a potential therapeutic target for controlling Treg cell plasticity.