AN N-TERMINAL DOMAIN SHARED BY FAS APO-1 (CD95) SOLUBLE VARIANTS PREVENTS CELL-DEATH IN-VITRO/

Fas-Apo-1 molecule, also designated as CD95, is a member of the TNF re ceptor family. Fas cross-linking by its natural ligand or by agonistic mAbs results in rapid induction of apoptosis in susceptible cells. In addition to the Fas full-length mRNA, human activated PBMC and tumor cell lines express several mRNA Fas variants that derive from alternat ive splicing of the primary transcript. All five variants identified, two of which are newly described here, code for soluble proteins that, with the exception of FasTMDel, are truncated in the extracytoplasmic region and possess short C-terminal amino acid sequences correspondin g to a different reading frame. We have identified Abs that recognize all splicing variants and established a sandwich ELISA by which the so luble Fas molecules could be detected in culture supernatants of trans fected cell lines and in PBMC following T cell activation. Next, we ha ve studied in detail the functional role of these variants by apoptosi s inhibition studies. We found that all soluble proteins block the apo ptosis induced by either an agonistic Ab or, more importantly, by the natural Fas ligand in Fas-positive sensitive cell lines. Interestingly , this functional property can be assigned to the first 49 amino acids of the mature protein that is the only region shared by the five solu ble Fas molecules.