Interleukin-2 (IL-2) is a O-glycosylated, four alpha -helix bundle cytokine that has potent stimulatory activity for antigen-activated T?cells. It is expressed by CD4+ and CD8+ T?cells, gamma Ѵ T?cells, B?cells, dendritic cells, and eosinophils (1-3). Mature human IL-2 shares 56%?and 66%?aa sequence identity with mouse and rat IL-2, respectively. Human and mouse IL-2 exhibit cross-species activity (4). The receptor for IL-2 consists of three subunits that are present on the cell surface in varying preformed complexes (5-7). The 55 kDa IL-2?R alpha is specific for IL-2 and binds with low affinity. The 75?kDa IL-2 R beta, which is also a component of the IL-15 receptor, binds IL-2 with intermediate affinity. The 64?kDa common gamma chain gamma c/IL-2 R gamma, which is shared with the receptors for IL-4, -7, -9, -15, and -21, does not independently interact with IL-2. Upon ligand binding, signal transduction is performed by both IL-2 R beta and gamma c. IL-2 is best known for its autocrine and paracrine?activity on T?cells. It drives resting T?cells to proliferate and induces IL-2 and IL-2 R alpha synthesis (1,?2). It contributes to T?cell homeostasis by promoting the Fas-induced death of na?ve CD4+ T?cells but not activated CD4+ memory lymphocytes (8). IL-2 plays a central role in the expansion and maintenance of regulatory T?cells, although it inhibits the development of Th17 polarized cells (9-11). Thus, IL-2 may be a key cytokine in the natural suppression of autoimmunity (12,?13).
