Background: Regulatory T (TReg) cells expressing the transcription factor forkhead box P3 (FOXP3) are naturally present in the immune system. They are indispensable for the maintenance of dominant self-tolerance and immune-homo stasis. Their dysfunction (for example, owing to FOXP3 gene mutation) causes fatal autoimmune disease, immuno pathology and allergy1. FOXP3+ TReg cells, most of which are CD4+ T cells that express CD25 (the interleukin-2 (IL-2) receptor α-chain), can suppress the activation, proliferation and effector functions — such as cytokine production — of a wide range of immune cells, including CD4+ and CD8+ T cells, natural killer(NK) and NKT cells, B cells and antigen- presenting cells (APCs) in vitro and in vivo.