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 Past mRNA 3 to 5 hr after culture in anti-CD3-coated wells. High and steady expression of Fast mRNA was observed after 8-10 hr of activation. Expression of Fast cytotoxicity and AICD is consistent with the time-course of Fast 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 Fast gene activation is a critical step for AICD. By contrast, Pas was constitutively expressed and the time-course study did not support the idea that up-regulation of Pas is critical for AICD. The critical role of Fast gene activation for AICD was confirmed by studies using inhibitors of AICD. Dexamethasone (Dex) inhibited Fast induction and Pas up-regulation, but not basal Pas expression of hybridoma T-cells. All-trans retinoic acid (RA) inhibited Fast induction, but had little effect on Pas up-regulation. Both agents inhibited Fast cytotoxicity. The Fas-mediated death pathway distal to Fas/FasL interactions remained intact in the protected hybridoma T-cells. These results demonstrate that Fast gene activation, but not Pas up-regulation, is critical for AICD and that Dex and all-trans RA selectively inhibits Fast but not Pas 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. (C) 1996 Academic Press, Inc.