Low-dose interleukin-2 therapy restores regulatory T cell homeostasis in patients with chronic graft-versus-host disease

CD4⁺Foxp3⁺ regulatory T cells (T_regs) play a central role in the maintenance of immune tolerance after allogeneic hematopoietic stem cell transplantation. We recently reported that daily administration of low-dose interleukin-2 (IL-2) induces selective expansion of functional T_regs and clinical improvement of chronic graft-versus-host disease (GVHD). To define the mechanisms of action of IL-2 therapy, we examined the immunologic effects of this treatment on homeostasis of CD4⁺ T cell subsets after transplant. We first demonstrated that chronic GVHD is characterized by constitutive phosphorylation of signal transducer and activator of transcription 5 (Stat5) in conventional CD4 ⁺ T cells (T_cons) associated with elevated amounts of IL-7 and IL-15 and relative functional deficiency of IL-2. IL-2 therapy resulted in the selective increase of Stat5 phosphorylation in T_regs and a decrease of phosphorylated Stat5 in T_cons. Over an 8-week period, IL-2 therapy induced a series of changes in T_reg homeostasis, including increased proliferation, increased thymic export, and enhanced resistance to apoptosis. Low-dose IL-2 had minimal effects on T_cons. These findings define the mechanisms whereby low-dose IL-2 therapy restores the homeostasis of CD4⁺ T cell subsets and promotes the reestablishment of immune tolerance.