The biological effects of interleukin 2 (IL-2) are mediated by its binding to a multi-chain cellular receptor complex. The IL-2 receptor (IL-2 R) complex consists of three glycoprotein chains. The alpha chain (IL-2 R alpha/CD25) and beta chain (IL-2 R beta/CD122) act together with the gamma chain (IL-2 R gamma/CD132) to form a high affinity receptor that transduces the signal upon IL-2 binding.
Alone, CD25/IL-2 R alpha binds IL-2 with low affinity, but is unable to transduce a signal. The alpha/beta combination binds IL-2 with intermediate affinity, but is still unable to transmit a signal inside the cell. The beta/gamma complex has intermediate affinity and is capable of signaling if the concentration of IL-2 is relatively high.
A soluble form of CD25/IL-2 R alpha is present in the serum in parallel with its increased cellular expression. There are reports of soluble IL-2 R beta as well. The biologic function of soluble CD25/IL-2 R alpha is unclear because, due to its low affinity, it is expected to be a poor inhibitor of IL-2. CD25/IL-2 R alpha is a precursor type I membrane protein with a signal peptide, an extracellular region, a transmembrane region, and a cytoplasmic domain. Studies suggest a correlation between the levels of CD25/IL-2 R alpha in serum with the onset of rejection in allograft recipients, with autoimmune disease activation in rheumatoid arthritis and systemic lupus erythematosis patients and with the course of some leukemias and lymphomas.