AGONIST ANTIBODY AND FAS LIGAND MEDIATE DIFFERENT SENSITIVITY TO DEATH IN THE SIGNALING PATHWAYS OF FAS AND CYTOPLASMIC MUTANTS

We have produced three forms of human Fas: full-length Fas, Fas with a C-terminal deletion, and a chimera between extracellular Fas and the intracellular domain of the tumor necrosis factor receptor I p55 subun it. We transfected cell lines with these constructs to compare the rel ative capacity of antibody agonists and the physiological Fas ligand ( Fast) to stimulate death. With two agonistic antibodies, the chimera i s 100- to 1000-fold more sensitive to induction of death than the full -length Fas. The C-terminal deletion mutant also shows greatly enhance d death in comparison to the wild-type receptor. In contrast, when Fas t is used to trigger the Fas pathway, wild-type Fas and the deletion m utant are similarly sensitive, whereas the chimera is 100-fold less su sceptible to ligand-mediated killing than Fas. This demonstrates that antibody agonists and natural ligand can stimulate different signaling pathways and emphasizes the limitations of defining physiologically i mportant signaling pathways solely by antibody agonists.