SFAs

Soluble Fas, abbreviated sFas, is a soluble form of the Fas receptor (CD95) that belongs to the TNF receptor superfamily. Unlike the membrane-bound Fas, sFas lacks the transmembrane and cytoplasmic death signaling domains, and it circulates in blood and other body fluids. It is produced by two main cellular processes: alternative splicing of Fas mRNA that generates a secreted isoform, and proteolytic shedding of the membrane-bound receptor by surface metalloproteases.

Functionally, sFas acts as a decoy receptor for Fas ligand (FasL). By binding FasL, sFas can prevent FasL from engaging membrane-bound Fas, thereby reducing Fas-mediated apoptosis in neighboring cells. This modulation of apoptotic signaling can influence immune responses, tissue homeostasis, and developmental processes where controlled cell death is important.

Physiological and clinical relevance of sFas is an area of ongoing research. Baseline levels occur in healthy individuals, but elevated sFas concentrations have been reported in a variety of conditions, including autoimmune diseases, infections, and certain cancers. In some diseases, higher sFas levels correlate with disease activity or prognosis, while in others the findings are inconsistent. Because assay methods for measuring sFas vary, comparisons across studies can be challenging, and the clinical utility of sFas as a standalone biomarker remains context-dependent.

See also Fas ligand and Fas receptor.