Apoptosis in L929 cells expressing a CD40/Fas chimeric receptor: dissociation of stimulatory from inhibitory death signalling functions.
A chimeric transmembrane receptor was constructed by fusing the extracellular domain of human CD40 to the transmembrane/intracellular domain of human Fas. When stably overexpressed in L929, the chimera retained the ligand binding specificity of CD40 and the basic signalling properties of Fas since an apoptotic response could be induced in a dose-dependent manner upon administration of recombinant, soluble CD40 ligand trimer or immobilized antiCD40 antibodies. However, this apoptotic response was independent of actinomycin D which is in contrast to results previously reported for L929 cells stably expressing wildtype human Fas. A labile protein factor was postulated to be responsible for inhibition of Fas-induced apoptosis in these cells. The apoptotic Fas signal tranduced by the chimeric CD40/Fas receptor is not regulated by this putative inhibitor. Our data suggest that the extracellular domains of CD40 and Fas form functionally similar mono- and multimeric structures and that the extracellular domain of Fas participates in the regulation of Fas-specific signal transduction.