Loss of Fas (CD95/APO-1) expression by antigen-specific cytotoxic T cells is reversed by inhibiting DNA methylation

Elimination of clonally expanded peripheral CD8 T cells was thought to invo lve apoptosis induction mediated principally by TNF, but recently Fas (CD95 /APO-1) has been shown to play a role in certain responses, Here we study F as expression and sensitivity to its ligation on murine CD8 cells specific for the CW3 antigen expressed by transfected P815 cells. Fas was progressiv ely downregulated after successive in vitro restimulations of antigen-speci fic CD8 cells, until clones became Fas negative and totally resistant to th e effects of recombinant Fas ligand. In contrast, Fas expression by in vivo restimulated antigen-specific cells did not diminish. Loss of Fas expressi on in vitro was not totally irreversible, since it could be reinduced by in hibition of DNA methylation. Understanding how Fas may be differentially re gulated in vivo and in vitro is an important issue for the optimal manipula tion of T cells for adoptive immunotherapy protocols. (C) 2000 Academic Pre ss.