Fas ligand (CD95L) protects neurons against perforin-mediated T lymphocytecytotoxicity

Previous work showed that neurons of the CNS are protected against perforin -mediated T cell cytotoxicity, but are susceptible to Fas-mediated apoptosi s. In this study, we report that Fas ligand (FasL) expression by neurons is involved in protection against perforin-mediated T cell cytotoxicity. Gene transcripts for FasL were identified in single murine hippocampal neurons by RT-PCR combined with patch clamp electrophysiology, and constitutive exp ression of FasL protein was confirmed in neurons by immunohistochemistry. N eurons derived from wild-type C57BL/6 (BL6) mice and mutant BL6.gld mice la cking functional FaSL were confronted with allogeneic, CTLs and continuousl y monitored in real time for changes in levels of intracellular calcium ([C a2+](i)), an indicator of cytotoxic damage. Perforin-mediated plasma membra ne lysis, characterized by rapid, massive [Ca2+](i) influx into the target cells within 0.5 h, was not detected in wild-type neurons. In striking cont rast, FasL-deficient neurons showed rapid increase in [Ca2+](i) within 0.5 h, reflecting perforin-dependent cell lysis. FasL seems to protect neurons by blocking degranulation of CTLs, since CD3-induced release of cytotoxic g ranules was reduced by coapplication of Fas-specific Abs or rFasL.