Two distinct human diseases have been described in association with human T cell lymphotropic virus type I (HTLV-I) infection: adult T cell leukaemia and tropical spastic paraparesis/HTLV-I-associated myelopathy. Although comprehensive understanding of specific mechanisms underlying pathogenesis of either disease has not yet been achieved, the viral regulatory protein Tax is believed to play a significant role. Previous studies demonstrated the potential of Tax to transform host cells. Here, it is shown that the Tax transactivator has in addition the potential to induce T cell death by apoptosis. Using an inducible system (Jurkat cell line JPX-9), significant apoptotic cell death upon Tax expression was observed. In an attempt to detect the cellular genes mediating this effect, it was found that induction of Tax was associated with marked upregulation of the Fas ligand (FasL) gene. Tax-induced apoptosis was inhibited when the Fas/FasL pathway was interrupted by YVAD-cmk, the inhibitor of ICE-like proteases. Transient expression experiments provided additional support for the putative role of endogenous FasL in Tax-induced apoptosis. Upon cotransfection with Tax-expressing plasmid, the transcriptional activity of the FasL promoter was found to be significantly upregulated in Jurkat cells and several other cell lines, as measured by reporter gene expression. Furthermore, cotransfection using different Tax mutants demonstrated that both CREB and NF-kappaB activation domains of Tax protein were required for the transactivation to take effect.

*Antigens, CD95/IMMUNOLOGY *Apoptosis/IMMUNOLOGY *Gene Products, tax/IMMUNOLOGY *HTLV-I *HTLV-I Infections/PATHOLOGY *Membrane Glycoproteins/IMMUNOLOGY *T-Lymphocytes/VIROLOGY

www.aegis.org