In addition, the composition of the local milieu, including the types of antigen-presenting cells and cytokines (such as tumour necrosis factor (TNF)29,111, IL-4 (REF. 112), IL-6 (REF. 29), IL-12 (REF. 113), IL-7, IL-15 (REF. 114) and IL-21 (REF. 28)), can influence TReg cell function.Multiple mechanisms by which effector T cells resist TReg cells have been proposed (reviewed in REF. 31).