T regulatory cells - in addition to clonal deletion and energy - are essent
ial for the downregulation of T cell responses to both foreign and self ant
igens, and for the prevention of autoimmunity. Recent progress has been mad
e in characterising the different subsets of T regulatory cells, the factor
s that drive their differentiation, and their mode of action. The resolutio
n of these mechanisms will make it possible to use T regulatory cells thera
peutically in human autoimmune diseases.