In order to control immune reactions and to prevent autoimmunity, the immune system relies on a multitude of regulatory T cells (Treg). Most work in this field has focussed on Treg of the CD4 + T cell subset because of the central role CD4 + T cells play as initiators and regulators of immune responses. One discovery of particular importance was the identification of naturally occurring CD4 + CD25 + Treg that arise in the thymus and express the transcription factor Foxp3; however, Treg can also be induced in the periphery after immune activation. A paper in this issue of the European Journal of Immunology identifies a novel type of induced murine CD8 + Treg that arise in TCR HY transgenic female mice after injection of MHC class I tetramers loaded with HY peptide. These Treg prevent subsequent rejection of male skin grafts suggesting that their manipulation may represent a new way of improving graft rejections against minor histocompatibility barriers. Humans have developed two principally different mechanisms to secure self tolerance: thymic negative selection and extrathymic control by specialized T cells with potential regulatory capacity. In the first process, developing T cell precursors that happen to express an autoreactive TCR are deleted. Thymic negative selection is also surprisingly efficient against tissue-specific antigens. The reason for this is the presence of a unique transcription factor, autoimmune regulator (AIRE), that operates in rare antigen-presenting cells in the thymic medulla [1]. AIRE liberates expression of tissue-specific antigens and, in this way, allows negative selection of autoreactive thymocytes. The importance of the AIRE gene for negative selection is illustrated by the large range of autoimmune manifestations that result from a lack of AIRE function [2,3].Even if negative selection operates normally, autoreactive T cells nevertheless escape from the thymus, circulate in the blood and seed the peripheral lymphoid organs; however, while autoreactive T cells can be detected in all individuals upon in vitro re-stimulation, not all people develop autoimmune diseases. One reason for this is the existence of peripheral tolerance mechanisms that keep autoreactive cells in check. Of these, so called "regulatory T cells" (Treg) attract much attention. The most studied Treg is the CD4 + CD25 + cell, also called naturally arising regulatory T cells [4]. These cells originate in the thymus and are identified by the activity of the transcription factor Foxp3 [5]. CD4 + CD25 + Foxp3 + Treg exit from the mouse thymus