Overexpression of Fas ligand does not confer immune privilege to a pancreatic β tumor cell line (βTC-3)

Background. Fas ligand (Fas-L) is thought to provide immune privilege to specific tissues and tumors by inducing an apoptotic signal of cytotoxic T cells expressing its Fas receptor. Purpose. The purpose of this work was to evaluate whether an immortalized insulin-secreting cell line (beta TC-S) gains immune privilege by inducing overexpression of Fas-L. Methods. A lipofection technique was used to transfect a beta TC-3 tumor cell line with a plasmid (pcDNA3.1/Zee) carrying the Fas-L gene and a zeocin resistance gene. Insertion of Fas-L into PTC was characterized by reverse transcription polymerase chain reaction (RT-PCR) and the ability of transfectants (beta TC-3/Fas-L) to induce apoptosis of Fas-sensitive T cells. Transfectants and control cells were tested for insulin secretion following which 1 x 10(6) insulin-secreting beta TC-3 and beta TC-3/Fas-L cells were subcutaneously implanted into syngeneic, allogeneic, and Fas mutant (Ipr) syngeneic mice. Survival of the insulin-secreting cells was then determined by monitoring serum glucose levels in recipients. Results. Successful transfection of vector resistance gene was achieved in the transfected beta TC-3 cells, which was confirmed by zeocin resistance. BT-PCR in resistant Fas-L clones confirmed the transcription of Fas-L, which was absent in controls. Fas-L transfectants induced 20 +/- 4.2% apoptosis of Fas-sensitive T cells, while controls induced 3.47 +/- 2.3% by flow cytometry (P = 0.04, n = 3). Insulin secretion was equivalent in both beta TC-3 and beta TC-3/Fas-L cells. Syngeneic mice implanted with control beta TC-3 cells died within 3 weeks from hypoglycemia due to overgrowth of beta TC-3 tumor. Implanted Fas-L transfected beta TC-3 cells were killed and had no effect on glycemic status except in Fas mutant hosts, where tumors formed in two of three mice. Conclusions. Despite the ability of transfected beta TC-3 cells to induce apoptosis of T cells in vitro, expression of Fas-L provided no immune privilege to these cells in vivo but paradoxically induced killing of beta TC-3 cells even in syngeneic hosts. (C) 1999 Academic Press.