IL-2 receptor
The interleukin-2 receptor (IL-2R) is a heterotrimeric protein expressed on the surface of certain immune cells, such as lymphocytes, that binds and responds to a cytokine called IL-2.
Composition
IL-2 binds to the IL-2 receptor, which has three forms, generated by different combinations of three different proteins, often referred to as "chains": α (alpha) (also called IL-2Rα, CD25, or Tac antigen), β (beta) (also called IL-2Rβ, or CD122), and γ (gamma) (also called IL-2Rγ, γc, common gamma chain, or CD132); these subunits are also parts of receptors for other cytokines. The β and γ chains of the IL-2R are members of the type I cytokine receptor family.
Structure-activity relationships of the IL-2/IL-2R interaction
The three receptor chains are expressed separately and differently on various cell types and can assemble in different combinations and orders to generate low, intermediate, and high affinity IL-2 receptors.
The α chain binds IL-2 with low affinity, the combination of β and γ together form a complex that binds IL-2 with intermediate affinity, primarily on memory T cells and NK cells; and all three receptor chains form a complex that binds IL-2 with high affinity (Kd ~ 10−11 M) on activated T cells and regulatory T cells The intermediate and high affinity receptor forms are functional and cause changes in the cell when IL-2 binds to them.
CD25
CD25 is the alpha chain of the IL-2 receptor. It is a type I transmembrane protein present on activated T cells, activated B cells, some thymocytes, myeloid precursors, and oligodendrocytes that associates with CD122 to form a heterodimer that can act as a high-affinity receptor for IL-2. Though CD25 has been used as a marker to identify CD4+FoxP3+ regulatory T cells in mice, it has been found that a large proportion of resting memory T cells constitutively express CD25 in humans.
CD25 is expressed in most B-cell neoplasms, some acute nonlymphocytic leukemias, neuroblastomas, mastocytosis and tumor infiltrating lymphocytes. It functions as the receptor for HTLV-1 and is consequently expressed on neoplastic cells in adult T cell lymphoma/leukemia. Its soluble form, called sIL-2R may be elevated in these diseases and is occasionally used to track disease progression.
