Inhibition of NF-kappa B activity in human T lymphocytes induces caspase-dependent apoptosis without detectable activation of caspase-1 and -3. 
NF-kappa B is involved in the transcriptional control of various genes that act as extrinsic and intrinsic survival factors for T cells. Our findings show that suppression of NF-kappa B activity with cell-permeable SN50 peptide, which masks the nuclear localization sequence of NF-kappa B1 dimers and prevents their nuclear localization, induces apoptosis in resting normal human PBL. Inhibition of NF-kappa B resulted in the externalization of phosphatidylserine, induction of DNA breaks, and morphological changes consistent with apoptosis. DNA fragmentation was efficiently blocked by the caspase inhibitor Z-VAD-fmk and partially blocked by Ac-DEVD-fmk, suggesting that SN50-mediated apoptosis is caspase-dependent. Interestingly, apoptosis induced by NF-kappa B suppression, in contrast to that induced by TPEN (N,N,N',N'-tetrakis [2-pyridylmethyl]ethylenediamine) or soluble Fas ligand (CD95), was observed in the absence of active death effector proteases caspase-1-like (IL-1 converting enzyme), caspase-3-like (CPP32/Yama/apopain), and caspase-6-like and without cleavage of caspase-3 substrates poly(ADP-ribose) polymerase and DNA fragmentation factor-45. These findings suggest either low level of activation is required or that different caspases are involved. Preactivation of T cells resulting in NF-kappa B nuclear translocation protected cells from SN50-induced apoptosis. Our findings demonstrate an essential role of NF-kappa B in survival of naive PBL.