IL-2-and CD25-dependent immunoregulatory mechanisms in the homeostasis of T-cell subsets

IL-2 plays a pivotal role in regulating the adaptive immune system by controlling the survival and proliferation of regulatory T (Treg) cells, which are required for the maintenance of immune tolerance. Moreover, IL-2 is implicated in the differentiation and homeostasis of effector T-cell subsets, including T(H)1, T(H)2, T(H)17, and memory CD8(+) T cells. The IL-2 receptor is composed of 3 distinct subunits, namely the alpha (CD25), beta (CD122), and gamma (gamma c) chains. Of crucial importance for the delivery of IL-2 signals to Treg cells is the expression of CD25, which, along with CD122 and gamma c, confers high affinity binding to IL-2. Notably, recent findings suggest a novel role for CD25, whereby CD25 molecules on Treg cells and possibly other cells are capable of influencing T-cell homeostasis by means of IL-2 deprivation. This review explores these findings and integrates them into our current understanding of T-cell homeostasis. (J Allergy Clin Immunol 2009;123:758-62.)