Resistance of cultured peripheral T cells towards activation-induced cell death involves a lack of recruitment of FLICE (MACH/caspase 8) to the CD95 death-inducing signaling complex

Phytohemagglutinin-activated peripheral CD95⁺ T cells (day 1 T cells) are resistant to CD95-mediated apoptosis. After prolonged interleukin-2 treatment, these T cells become CD95-mediated apoptosis-sensitive (day 6T cells). To elucidate the molecular mechanism of apoptosis resistance, day 1 and day 6 T cells were tested for formation of the CD95 death-inducing signaling complex (DISC). DISC-associated active Fas-associated DD protein (FADD)-like interleukin-1β-converting enzyme-like protease (FLICE) also referred to as MACH/caspase 8 was only found in apoptosis-sensitive day 6 T cells. Further analysis of mRNA and protein expression levels of apoptosis-signaling molecules FADD, receptor interacting protein, hematopoietic cell protein tyrosine phosphatase, Fas-associated phosphatase-1, FLICE, bcl-2, bcl-x_L, and, bax-α showed that only the expression level of bcl-x_L correlated with T cell resistance to CD95-mediated apoptosis (day 1 T cells: bcl-x^hi_L; day 6 T cells: bcl-x^lo_L). In T cells activated in vitro, up-regulation of bcl-x_L has previously been correlated with general apoptosis resistance. However, the experiments presented suggest that resistance to CD95-mediated apoptosis in T cells can also be regulated at the level of recruitment of FLICE to the DISC.