Induced peripheral regulatory T cells: the family grows larger.

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.