The Interleukin-2 Receptor γ Chain: Its Role in the Multiple Cytokine Receptor Complexes and T Cell Development in XSCID

Interleukin 2 (IL-2), a T cell–derived cytokine, targets a variety of cells to induce their growth, differentiation, and functional activation. IL-2 inserts signals into the cells through IL-2 receptors expressed on cell surfaces to induce such actions. In humans, the functional IL-2 receptor consists of the subunit complexes of the α, β, and γ chains, or the β and γ chains. The third component, the γ chain, of IL-2 receptor plays a pivotal role in formation of the full-fledged IL-2 receptor; together with the β chain, the γ chain participates in increasing the IL-2 binding affinity and intracellular signal transduction. Moreover, the cytokine receptors for at least IL-2, IL-4, IL-7, IL-9, and IL-15 utilize the same γ chain as an essential subunit. Interestingly, mutations of the γ chain gene cause human X-linked severe combined immunodeficiency (XSCID) characterized by a complete or profound T cell defect. Among the cytokines sharing the γ chain, at least IL-7 is essentially involved in early T cell development in the mouse organ culture system. The molecular identification of the γ chain brought a grasp of the structures and functions of the cytokine receptor and an in-depth understanding of the cause of human XSCID. To investigate the mechanism of XSCID and development of gene therapy for XSCID, knockout mice for the γ chain gene were produced that showed similar but not exactly the same phenotypes as human XSCID.