Activation induces apoptosis in Herpesvirus saimiri-transformed T cells independent of CD95 (Fas,APO-1)

Signaling via the T cell receptor (TCR)/CD3 complex of pre-activated T cells induces apoptosis. Such an activation-induced cell death (AICD) is thought to play an important role in the regulation of cellular immune responses. In this study we analyzed pathways of AICD by using human T cells transformed by Herpesvirus saimiri. These growth-transformed T cells show the phenotype of activated mature T cells and continue to express a functionally intact TCR. We show that human H. saimiri-transformed T cell clones readily undergo cell death upon signaling via the TCR/CD3 complex or via phorbol 12-myristate 13-acetate (PMA) + ionomycin. The AICD in H. saimiri-transformed T cells was detectable a few hours after activation and it was not affected by the presence of interleukin (IL)-2 or by anti-CD4 cross-linking. However, AICD required tyrosine phosphorylation, since it could be blocked by herbimycin A. Cyclosporin A (CsA) did not block the development of AICD, but other consequences of activation in H. saimiri-transformed T cells like the production of interferon-γ. Surprisingly, the development of AICD was not reduced by neutralizing antibodies to tumor necrosis factor (TNF)-α or blocking antibodies directed to CD95 (Fas, APO-1), although H. saimiri-transformed T cells were sensitive to CD95 ligation. To confirm that this form of AICD is really independent of CD95, we have established an H. saimiri-transformed T cell line from a patient with a homozygous deletion in the CD95 gene. This CD95-deficient T cell line was as sensitive to AICD as other CD95-expressing H. saimiri-transformed T cells. In conclusion, we describe here a type of AICD in H. saimiri-transformed T cells that is independent of CD95 and TNF-α, not sensitive to CsA, but requires tyrosine phosphorylation. This system should be useful for the investigation of CD95-independent forms of AICD.