The role and regulation of caspases during T cell activation and proliferation
In the present study, the effects of two caspase inhibitors, z-VAD-FMK and z-IETD-FMK, on T cell activation and proliferation were compared to the negative control, z-FA-FMK. All three compounds inhibited T cell proliferation, blast formation, CD25 expression, IL-2 signalling and nuclear translocation of the NF-kappaB subunit, p65. However, unlike z-VAD-FMK and z-IETD-FMK, z-FA-FMK inhibited IL-2 and IFN-gamma secretion and blocked cell cycle entry. Furthermore, although both z-VAD-FMK and z-IETD-FMK inhibited Fas-induced caspase activation, they had no effect on caspase-8 and -3 processing during T cell activation. In contrast, z-FA-FMK, which did not inhibit caspase processing during apoptosis, blocked caspase-8 and -3 processing in activated T cells. This suggests that peptidyl-fluoromethylketones have pleiotropic immunosuppressive properties which may not involve the processing of caspase-8 or -3. In addition, these data suggest that the processing of caspases is regulated differently during T cell activation compared to apoptosis. To further characterise the role of caspase-8 during T cell activation, an activation-induced cell death (AICD) model using caspase-8-/- Jurkat T cells was examined. In addition, the involvement of FADD, the docking protein for caspase-8, was also investigated. Caspase-8-/- Jurkat T cells were resistant to AICD induced by PMA and ionomycin, whereas FADD-/- Jurkat T cells underwent TNF-alpha-mediated necrosis. These data suggest that caspase-8 but not FADD is required for the T cell activation signalling pathway and also implicate TNF-alpha in AICD in the absence of the Fas-signalling pathway. In conclusion, the results suggest that non-specific effects of caspase inhibitors are involved in the inhibition of T cell activation and proliferation and that the regulation of caspases during T cell activation is different from that during apoptosis.