Identification of HERV-W8 LTR regions required for activation by T cell activators and by Tax. (A) Tested 5′ deletion mutants of the HERV-W8 LTR. (B) Reporter plasmids carrying the complete LTR and 5′ deletion mutants were transfected into Jurkat cells along with pRcActin-LacZ. At 24 h post-transfection, cells were stimulated with PHA, PMA, PHA/PMA, PMA/Ionomycin, bpV[pic], Forskolin, bpV[pic] /Forskolin, bpV[pic] /PMA, OKT3/9.3 and TNFα. After 8 h of stimulation, cells were harvested and assayed for luciferase activity, which was normalized against β-galactosidase activity. Results are shown as fold stimulation relative to luciferase activity of untreated cells. (C) Jurkat cells were co-transfected with a plasmid carrying the luciferase reporter gene under the control of different HERV 5′ LTRs, pHβPr.1neoTax (vs. the empty vector pHβPr.1neo) and pRcActin-LacZ. At 24 h post-transfection, cells were harvested and assayed for luciferase activity. Results are shown as normalized luciferase values and represent the mean values of three independently transfected cell samples.