Regulation of T-Cell Death Genes: Selective Inhibition of FasL- but Not Fas-Mediated Function

Activation-induced cell death (AICD) requires coexpression of Fas and FasL. Hybridoma T-cells express detectable FasL mRNA 3 to 5 hr after culture in anti-CD3-coated wells. High and steady expression of FasL mRNA was observed after 8–10 hr of activation. Expression of FasL cytotoxicity and AICD is consistent with the time-course of FasL mRNA induction. Fas–Ig was effective in inhibiting AICD when added no later than 5 hr after activation, but was ineffective when added after 8–10 hr of activation. These observations suggest that FasL gene activation is a critical step for AICD. By contrast, Fas was constitutively expressed and the time-course study did not support the idea that up-regulation of Fas is critical for AICD. The critical role of FasL gene activation for AICD was confirmed by studies using inhibitors of AICD. Dexamethasone (Dex) inhibited FasL induction and Fas up-regulation, but not basal Fas expression of hybridoma T-cells. All-transretinoic acid (RA) inhibited FasL induction, but had little effect on Fas up-regulation. Both agents inhibited FasL cytotoxicity. The Fas-mediated death pathway distal to Fas/FasL interactions remained intact in the protected hybridoma T-cells. These results demonstrate that FasL gene activation, but not Fas up-regulation, is critical for AICD and that Dex and all-transRA selectively inhibits FasL but not Fas function. The system may prove useful for the identification of critical factors regulating T-cell death genes. It may also serve as a useful system to study gene regulation in AICD-dependent phenomena.