Foxp3+ regulatory T cells: differentiation, specification, subphenotypes

Regulatory T cells (Treg cells) characterized by expression of the transcription factor Foxp3 play a key role in immune homeostasis. Rather than a monomorphic population strictly determined by Foxp3 as a 'master regulator', the emerging view is one of Treg cells as a population with many levels of complexity. Several regulatory factors partake in the control of their transcriptional 'signature', with Foxp3 being a key regulator but insufficient and unnecessary to specify all aspects of the lineage. Distinct subphenotypes of Foxp3+ Treg cells are found in different anatomical locations. Some subphenotypes specifically control different facets of effector T cell function and, perhaps surprisingly, share transcriptional control elements with the very cells they regulate. This review will focus on these novel aspects of Treg cell diversity.