The Fas system, comprising the Fas receptor (Fas, CD95, APO-1) and its ligand, Fas ligand (FasL), is a central mediator of programmed cell death in various physiological and pathological processes. Recent evidence indicated that tumor cells can exploit this system to their benefit in the dialogue with the host immune system. We have shown that all human pancreatic adenocarcinoma cell lines tested by fluorescence-activated cell sorting analysis (6 of 6) and immunocytochemistry (12 of 12) were positive for Fas expression, as were normal and malignant duct cells in pancreatic tissue sections. However, despite Fas expression, pancreatic tumor cells have become largely resistant toward recombinant FasL- or anti-APO-1 agonistic antibody-induced apoptosis. This resistance correlated with high levels in pancreatic tumor cells of mRNA for FAP-1, a Fas-associated phosphatase that can block the apoptotic function of Fas. Using a variety of methodological approaches, we also present evidence for the production of FasL by pancreatic tumor cells because 6 of 6 pancreatic tumor cell lines were found to contain FasL mRNA as well as the Mr 40,000 and Mr 26,000 forms of the FasL protein. Likewise, pancreatic tissue revealed FasL-specific immunostaining in pancreatic tumor cells but not in the surrounding stroma. In coculture experiments, pancreatic tumor cells displayed a cytotoxic effect toward the Fas-sensitive Jurkat T-cell line, which could be inhibited by a FasL-specific neutralizing antibody. Together, these results support the recently proposed "counterattack model" for local deletion of tumor-reactive T-cells by tumor cell-derived FasL.