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 mouse IL?2 shares 56% and 73% aa sequence identity with human and rat IL?2, respectively. It shows strain?specific heterogeneity in an N?terminal region that contains a poly?glutamine stretch (4). Mouse and human IL?2 exhibit cross?species activity (5). The receptor for IL?2 consists of three subunits that are present on the cell surface in varying preformed complexes (6?8). 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 (9). IL?2 plays a central role in the expansion and maintenance of regulatory T cells, although it inhibits the development of Th17 polarized cells (10?12). Thus, IL?2 may be a key cytokine in the natural suppression of autoimmunity (13, 14).
