The soluble IL‐2 receptor α/CD25 as a modulator of IL‐2 function

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Abstract
The pleiotropic cytokine interleukin‐2 (IL‐2) is an integral regulator of healthy and pathological immune responses, with the most important role in regulating the homeostasis of regulatory T cells. IL‐2 signalling involves three distinct receptors: The IL‐2 receptor α (IL‐2Rα/CD25), IL‐2Rβ, and IL‐2Rγ/γc. While IL‐2Rβ and γc are essential for signal transduction, IL‐2Rα regulates the affinity of the receptor complex towards IL‐2. A soluble form of the IL‐2Rα (sIL‐2Rα) is present in the blood of healthy individuals and increased under various pathological conditions. Although it is known that the sIL‐2Rα retains its ability to bind IL‐2, it is not fully understood how this molecule affects IL‐2 function and thus immune responses. Here, we summarize the current knowledge on the generation and function of the sIL‐2Rα. We describe the molecular mechanisms leading to sIL‐2Rα generation and discuss the different IL‐2 modulating functions that have been attributed to the sIL‐2Rα. Finally, we describe attempts to utilize the sIL‐2Rα as a therapeutic tool.

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