Interleukin 2

Interleukin-2 (IL-2) is an interleukin, a type of cytokine signaling molecule in the immune system. It is a 15.5–16 kDa protein that regulates the activities of white blood cells (leukocytes, often lymphocytes) that are responsible for immunity. IL-2 is part of the body's natural response to microbial infection, and in discriminating between foreign ("non-self") and "self". IL-2 mediates its effects by binding to IL-2 receptors, which are expressed by lymphocytes. The major sources of IL-2 are activated CD4+ T cells and activated CD8+ T cells. Put shortly the function of IL-2 is to stimulate the growth of helper, cytotoxic and regulatory T cells. IL-2 is a member of a cytokine family, each member of which has a four alpha helix bundle; the family also includes IL-4, IL-7, IL-9, IL-15 and IL-21. IL-2 signals through the IL-2 receptor, a complex consisting of three chains, termed alpha (CD25), beta (CD122) and gamma (CD132). The gamma chain is shared by all family members. The IL-2 receptor (IL-2R) α subunit binds IL-2 with low affinity (Kd~ 10−8 M). Interaction of IL-2 and CD25 alone does not lead to signal transduction due to its short intracellular chain but has the ability (when bound to the β and γ subunit) to increase the IL-2R affinity 100-fold. Heterodimerization of the β and γ subunits of IL-2R is essential for signalling in T cells. IL-2 can signalize either via intermediate-affinity dimeric CD122/CD132 IL-2R (Kd~ 10−9 M) or high-affinity trimeric CD25/CD122/CD132 IL-2R (Kd~ 10−11 M). Dimeric IL-2R is expressed by memory CD8+ T cells and NK cells, whereas regulatory T cells and activated T cells express high levels of trimeric IL-2R. Instructions to express proteins in response to an IL-2 signal (called IL-2 transduction) can take place via 3 different signaling pathways; they are: (1) the JAK-STAT pathway, (2) the PI3K/Akt/mTOR pathway and (3) the MAPK/ERK pathway. The signalling is commenced by IL-2 binding to its receptor, following which cytoplasmatic domains of CD122 and CD132 heterodimerize.