CD4 and Autoimmunity: Naturally arising CD4+ Treg cells expressing the IL-2 receptor α-chain (CD25) and the transcription factor forkhead box P3 (FoxP3) represent a subset of thymus-derived CD4+ T cells critical for the control of most immune responses, including autoimmunity, transplantation tolerance, antitumor immunity and anti-infectious reactions [6,7].