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Soluble Interleukin-2 Receptor (sIL-2R)

Soluble interleukin-2 receptor (sIL-2R) is a protein found in the blood that is related to the immune system. Interleukin-2 (IL-2) is a cytokine, which is a type of signaling molecule that plays a crucial role in the regulation of immune responses. IL-2 is produced by activated T cells, and it functions to stimulate the growth and proliferation of T cells, which are important components of the immune system.

The IL-2 receptor exists in two forms: a membrane-bound form (mIL-2R) found on the surface of T cells and a soluble form (sIL-2R) that is released into the bloodstream. The membrane-bound form is involved in transmitting signals into the T cell when IL-2 binds to it, promoting T cell activation and proliferation.

The interleukin 2 (IL-2) receptor is a heterotrimeric protein (αβγ) complex with the γ-chain identical to interleukin 4 (IL-4) and interleukin 7 (IL-7). The α subunit of this complex, IL-2Rα (also known as Tac antigen and CD25) is a transmembrane glycoprotein with a MB of 55 kDa and with only 13 amino acids out of 351 located on the cytoplasmic side of the membrane. IL-2Rα, the IL-2Rβ chain, and the IL-2Rγ chain constitute the high-affinity IL-2 receptor. IL-2Rα homodimeric chains create a low-affinity receptor, whereas IL-2Rβ homodimeric chains create an intermediate-affinity receptor. The soluble form of IL-2Ra appears in the serum, in parallel with its increased expression in cells.

The soluble form, sIL-2R, is released when T cells are activated and express the membrane-bound IL-2 receptor. This soluble form can be detected in the blood and serves as a marker of T-cell activation. Elevated levels of sIL-2R in the blood are often associated with certain immune system disorders, such as autoimmune diseases and lymphoproliferative disorders (conditions where there is abnormal cell growth in the lymphatic system).

Measuring sIL-2R levels in the blood can be useful in diagnosing and monitoring certain diseases. For example, increased sIL-2R levels may be observed in conditions like rheumatoid arthritis, systemic lupus erythematosus, and certain types of lymphomas. It can be used as a biomarker to assess the degree of T cell activation and the intensity of the immune response.

Numerous studies have reported elevated levels of sIL-2R in sarcoidosis patients, establishing it as a recognized biomarker for the disease. Some studies suggest that measuring sIL-2R can serve as an indicator of treatment success.

The sensitivity of serum sIL-2R as a diagnostic biomarker for sarcoidosis is around 79%, and in patients with uveitis, the sensitivity of elevated sIL-2R levels in identifying underlying sarcoidosis is approximately 81–98%. Notably, patients with extrapulmonary involvement tend to have relatively high levels of serum sIL-2R, indicating its potential as a biomarker for staging and/or assessing disease severity.

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