AUTOCRINE AND PARACRINE APOPTOSIS ARE MEDIATED BY DIFFERENTIAL REGULATION OF FAS LIGAND ACTIVITY IN 2 DISTINCT JURKAT T-CELL POPULATIONS

Fas ligand (FasL) produced by activated T cells mediates antocrine-ind uced apoptosis to limit T cell expansion. To investigate the regulatio n of FasL activity, Jurkat cells were stably transfected with a 2,3-kb fragment of human Fast promoter that controlled the expression of a G FP reporter gene. Two populations of Jurkat cells with different level s of GFP expression were obtained. One population constitutively expre ssed high levels of GFP (GFP(+)), while the other population expressed low Bevels of GFP (GFP(-)). The level of GFP expression in the two po pulations correlated with their levels of Fast transcription and its f unctional activity, Autocrine regulation of apoptosis was demonstrated by increased FasL, activity after stimulation of GFP(-) cells with an ti-CD3, phorbyl myristyl acetate plus ionomycin, or Con A. Paracrine r egulation of apoptosis was suggested by the induction of apoptosis of GFP(-) cells after coculture with unstimulated GFP(+) cells. GFP(+) ce lls exhibited a decreased sensitivity to FasL-mediated apoptosis compa red with GFP(-) cells, Furthermore, the cell surface expression of Fas and CD4 was lower on GFP(+) cells than GFP(-) cells, whereas the expr ession of CD45RO was higher, A decreased level of IL-2 was produced by GFP(+) cells after phorbyl myristyl acetate and ionomycin stimulation , Our results indicate that a subpopulation of T cells that express lo w levels of Fast and IL-2, which are responsive to up-regulation of th ese molecules after activation, can undergo apoptosis either by suicid e after activation or by a paracrine pathway mediated by T cells that constitutively express higher levels of FasL.