sIL2

Soluble interleukin-2 (sIL-2) is a term used in immunology to describe soluble forms associated with interleukin-2 signaling. The phrase most commonly refers to soluble IL-2 receptor fragments, known as sIL-2R or soluble CD25 (sCD25), which are shed from activated T cells or produced by alternative splicing. IL-2 itself is a cytokine produced by activated T cells; the circulating soluble receptor can bind IL-2 and modulate signaling.

Biology and function of sIL-2–related forms are typically linked to the IL-2 receptor complex. The soluble IL-2 receptor alpha chain (CD25) can bind IL-2, acting as a decoy that reduces soluble IL-2 availability and dampens signaling, while also serving as a biomarker of T cell activation. Soluble forms therefore reflect immune activation and the turnover of IL-2 receptor–expressing cells.

Clinical significance and uses are focused on biomarker utility. Elevated levels of sIL-2R/sCD25 are a non-specific indicator of immune activation and are observed in various conditions, including autoimmune diseases (such as systemic autoimmune disorders and adult-onset Still’s disease), infections, cancers (notably lymphomas and leukemias), and transplant rejection or macrophage activation syndromes. Because elevations are not disease-specific, sIL-2R is typically interpreted in the context of clinical presentation and other laboratory data.

Measurement and interpretation involve immunoassays, commonly ELISAs, with results expressed in units that vary by assay. sIL-2R/sCD25 can aid in assessing disease activity or prognosis in certain conditions, but it is not a stand-alone diagnostic test. Therapeutic use of IL-2 itself, or IL-2–targeted therapies, is distinct from the biomarker role of sIL-2–related forms.