Schematic representation of the vectors used in this study. All vectors are plasmid DNA constructed on the backbone of pGL2-Basic. Promoters are either the constitutive SV40 (pSV40) or tetracycline-responsive (Ptet). Encoded transgenes are the reverse tetracycline transactivator (rtTA), the improved version (rtTA2S-M2), the tetracycline targeted repressor-Kruppel-associated box (tetR-KRAB), firefly luciferase (Luc), or the dimeric tumour necrosis factor receptor II (dTNFR). Other DNA elements are the internal ribosome entry site (IRES) derived from encephalomyocarditis virus, the SV40 polyadenylation signal (black triangle), and the spacer sequence of the non-coded zeocin (Zeo) resistance gene.