Interleukin 2 (IL-2), also known as T cell growth factor (TCGF), is a 15-18 kDa variably glycosylated alpha -helical polypeptide that is a member of the Common gamma Chain (gamma c) cytokine family. It exists as a monomer and has a notably short half-life (< 30 minutes). Human IL-2 is synthesized as a 153 amino acid (aa) precursor that contains a 20 aa signal sequence plus a 133 aa mature region. The mature region is alpha -helical in nature, and contains one utilized O-linked glycosylation site at Thr3 plus three cysteines, two of which form an intrachain disulfide bond that is essential for activity. Mature human IL-2 shares 73%, 66%, 78% and 97% aa identity with canine, rat, feline and rhesus monkey IL-2, respectively. Although human IL-2 shares only approximately 60% aa identity with the highly polymorphic mouse IL-2, human IL-2 is known to be active on mouse IL-2 responsive cells. Cells reported to secrete IL-2 include gamma δ T cells, activated conventional CD4+ and CD8+ T cells, neurons, microglia, and hematopoietic stem cells.
The receptor for IL-2 (IL-2 R) is composed of three subunits, the 55 kDa CD25/IL-2 R alpha chain, the 70 kDa IL-2 R beta chain, and the 65 kDa Common gamma Chain. IL-2 first binds to CD25, the binary complex then recruits IL-2 R beta and gamma c to form the quaternary signaling complex. In addition to IL-2, IL-2 R beta is used by IL-15 in its quaternary signaling complex. gamma c also serves as a signaling receptor for IL-4, -7, -9, -15, and -21.
In vitro studies have shown an important role for IL-2 in T cell activation and expansion. In vivo, IL-2 is critical for the development, maintenance and function of regulatory T cells (Treg) which provide protection against autoimmune disease. On the other hand, IL-2 can also promote autoimmune inflammation in target organs through its roles in regulating the expression of T cell trafficking genes, and production of Th2 cytokines. Within the CD8+ T cell subset, IL-2 is essential for optimal primary responses and differentiation into terminal effector cells. IL-2 also promotes the development of activated CD8+ T cells into memory cells.
