The Fas receptor delivers signals crucial for lymphocyte apoptosis through its cytoplasmic death domain. Several Fas cytoplasmic-associated proteins have been reported and studied in cell lines. So far, only Fas-associated death domain protein (FADD), another death domain-containing molecule has been shown to be essential for Fas signals in vivo. FADD is thought to function by recruiting caspase-8 through its death-effector domain. To test whether FADD is sufficient to deliver Fas signals, we generated transgenic mice expressing a chimera comprised of the Fas extracellular domain and FADD death-effector domain. Expression of this protein in lymphocytes of Fas-deficient MRL-lpr/lpr mice completely diminishes their T cell but not their B cell abnormalities. These results suggest that FADD alone is sufficient for initiation of Fas signaling in primary T cells, but other pathways may operate in B cells.
