PD-L1 plays a critical role in the development, maintenance, and function of induced regulatory T (iT reg) cells. The study demonstrates that PD-L1 is essential for the induction of iT reg cells and their suppressive function. PD-L1-deficient antigen-presenting cells (APCs) fail to convert naive CD4+ T cells into iT reg cells, indicating the essential role of PD-L1 in iT reg cell induction. PD-L1-coated beads induce iT reg cells in vitro, showing that PD-L1 directly regulates iT reg cell development. PD-L1 enhances and sustains Foxp3 expression and the suppressive function of iT reg cells. In vivo, PD-L1 deficiency leads to a marked reduction in iT reg cell conversion and rapid onset of a fatal inflammatory phenotype in PD-L1-/- PD-L2-/- Rag-/- recipients of naive CD4+ T cells. PD-L1 mediates iT reg cell development through the down-regulation of phospho-Akt, mTOR, S6, and ERK2, and the up-regulation of PTEN, key signaling molecules critical for iT reg cell development. PD-L1 inhibits T cell responses by promoting both the induction and maintenance of iT reg cells. These findings define a novel mechanism for iT reg cell development and function, as well as a new strategy for controlling T reg cell plasticity. PD-L1 is widely expressed on hematopoietic and nonhematopoietic cells and plays a critical role in regulating the dynamic balance between T eff and T reg cells in vivo. PD-L1 may protect tissues from self-reactive T eff cells by inhibiting their function and increasing the frequency and function of T reg cells. PD-L1 also plays a role in immune privilege, especially in environments where TGF-β is present. PD-L1 may exert inhibitory effects on anti-tumor and anti-microbial immunity by inducing T reg cell development and sustaining iT reg cell function. PD-L1 deficiency leads to impaired T reg cell conversion in vivo, resulting in severe immune-mediated tissue damage. PD-L1 antagonizes the Akt-mTOR signaling cascade during iT reg cell induction, reducing Akt and mTOR phosphorylation while increasing PTEN expression. PD-L1 also modulates the MAP kinase signaling cascade, attenuating ERK2 phosphorylation. These findings highlight the critical role of PD-L1 in regulating T reg cell development and function, and suggest that PD-L1 may be an attractive therapeutic target for controlling T reg cell plasticity.PD-L1 plays a critical role in the development, maintenance, and function of induced regulatory T (iT reg) cells. The study demonstrates that PD-L1 is essential for the induction of iT reg cells and their suppressive function. PD-L1-deficient antigen-presenting cells (APCs) fail to convert naive CD4+ T cells into iT reg cells, indicating the essential role of PD-L1 in iT reg cell induction. PD-L1-coated beads induce iT reg cells in vitro, showing that PD-L1 directly regulates iT reg cell development. PD-L1 enhances and sustains Foxp3 expression and the suppressive function of iT reg cells. In vivo, PD-L1 deficiency leads to a marked reduction in iT reg cell conversion and rapid onset of a fatal inflammatory phenotype in PD-L1-/- PD-L2-/- Rag-/- recipients of naive CD4+ T cells. PD-L1 mediates iT reg cell development through the down-regulation of phospho-Akt, mTOR, S6, and ERK2, and the up-regulation of PTEN, key signaling molecules critical for iT reg cell development. PD-L1 inhibits T cell responses by promoting both the induction and maintenance of iT reg cells. These findings define a novel mechanism for iT reg cell development and function, as well as a new strategy for controlling T reg cell plasticity. PD-L1 is widely expressed on hematopoietic and nonhematopoietic cells and plays a critical role in regulating the dynamic balance between T eff and T reg cells in vivo. PD-L1 may protect tissues from self-reactive T eff cells by inhibiting their function and increasing the frequency and function of T reg cells. PD-L1 also plays a role in immune privilege, especially in environments where TGF-β is present. PD-L1 may exert inhibitory effects on anti-tumor and anti-microbial immunity by inducing T reg cell development and sustaining iT reg cell function. PD-L1 deficiency leads to impaired T reg cell conversion in vivo, resulting in severe immune-mediated tissue damage. PD-L1 antagonizes the Akt-mTOR signaling cascade during iT reg cell induction, reducing Akt and mTOR phosphorylation while increasing PTEN expression. PD-L1 also modulates the MAP kinase signaling cascade, attenuating ERK2 phosphorylation. These findings highlight the critical role of PD-L1 in regulating T reg cell development and function, and suggest that PD-L1 may be an attractive therapeutic target for controlling T reg cell plasticity.