Key Points: IL-2 induces expression of PD-1 on Tregs, and PD-1 blockade promotes Treg differentiation and apoptosis. PD-1 regulates IL-2–induced Treg proliferation and prolongs Treg survival in murine models and in patients receiving low-dose IL-2 therapy.

Key Points: PD-1 blockade exacerbated GVHD by altering the homeostasis of Tregs and effector T cells after HSCT. PTCy ameliorated GVHD after PD-1 blockade by restoring the homeostatic balance of T-cell subsets.