MHC class I-restricted killing of neurons by virus-specific CD8(+) T lymphocytes is effected through the Fas/FasL, but not the perforin pathway

Induction of MHC class I genes in neurons of the central nervous system req uires signals by pro-inflammatory cytokines, in particular IFN-gamma, and t he blockade of electric activity, which is known to suppress induction of M HC related genes in a highly ordered, but unusual fashion [1, 2]. The prese nt experiments explore the immunological function of neuronal MHC class I a ntigens expressed under permissive conditions. MHC class I proteins were in duced in electrically silenced murine hippocampal neurons by treatment with the sodium channel blocker tetrodotoxin and recombinant IFN-gamma, conditi ons which also resulted in the induction of Fas molecules. The MHC class I positive neurons were challenged with CD8(+) cytotoxic T lymphocytes (CTL) specific for the H2-D-b binding peptide GP33, a dominant epitope of the lym phocytic choriomeningitis virus envelope glycoprotein, or with alloreactive CTL. Single primed neurons, attacked by GP33-specific CTL, were continuous ly monitored for changes in intracellular calcium ([Ca2+](i)), an indicator of cytotoxic damage. MHC class I-induced neurons pulsed with the GP33 pept ide, but not a control peptide, showed a gradual and sustained increase in [Ca2+](i) within 3 h following attack by GP33-specific CTL, while in astroc ytes [Ca2+](i) elevation was rapid. The slow course of the neuronal respons e was consistent with a delayed apoptotic killing mechanism rather than rap id granule-mediated plasma membrane lysis. Indeed, the attacked neurons bou nd annexin V, indicating membrane alterations preceding apoptotic cell deat h. In further support of apoptotic cell death, this sustained increase of [ Ca2+](i) levels was also observed following attack by perforin-deficient CT L, but was not detected in neurons derived from mutant Ipr mice, which lack functional Fas molecules.