The importance of IL-2 for Treg function is well established [29, 35, 36] and has been confirmed in clinical studies. In patients with either hepatitis C virus-induced vasculitis or graft versus host disease, treatment with low doses of IL-2 therapy increased Treg numbers and decreased clinical disease [38, 39] . These results are also consistent with others showing that Treg numbers were increased when cancer patients were treated with low dose IL-2 [40, 41] . Exogenously administered IL-2 also corrects IL-12mediated decreases in Foxp3 levels in Tregs (Fig. 8B) ; whether low level IL-2 treatment also increases Foxp3 levels in Tregs in patients, in addition to increasing Treg numbers, has not yet been addressed.