Human IL-2 receptor alpha (IL-2 R alpha ), also known as Tac antigen and as CD25, was initially identified as a 55 kDa membrane glycoprotein that is capable of binding IL‑2. The IL-2 R alpha cDNA encodes a 272 amino acid residue precursor Type I membrane protein with a 21 residue signal peptide, a 219 residue extracellular region, a 19 residue transmembrane region and a 13 residue cytoplasmic domain. IL-2 R alpha lacks structural features characteristic of members of the cytokine receptor superfamily. By itself, IL-2 R alpha binds IL-2 with low affinity. However, when IL-2 R alpha is associated with the IL-2 receptor beta and gamma chains, a high affinity heterotrimeric receptor complex that transduces IL-2 signals is formed.
Soluble forms of many cytokine receptors have been reported, and a soluble form of IL-2 R alpha (IL‑2 sR alpha ) appears in serum, concomitant with its increased expression on cells. The function of the soluble IL-2 R alpha is unclear. Increased levels of IL‑2 sR alpha in biological fluids reportedly correlate with increased T and B cell activation and immune system activation. Increased serum concentration of IL-2 sR alpha has been observed in patients with a variety of inflammatory conditions and in the course of some leukemias and lymphomas.
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