Lentiviral vectors: regulated gene expression.

Lentiviral vectors can deliver and express genes in a wide variety of dividing and nondividing cells. These include terminally differentiated neurons, myotubes, hepatocytes, and hematopoietic stem cells. We now describe the generation of lentiviral vectors in which the expression of the transgene can be regulated. We have developed an inducible lentiviral vector system that contains the entire tetracycline (Tet)-regulated system developed by H. Bujard and colleagues. The novel vector expresses the GFP reporter gene and the tetracycline transactivator under the control of the tetracycline-inducible promoter and the human CMV promoter, respectively. In vitro transduction of human 293 cells resulted in a very low basal expression of GFP in the presence of the effector substance doxycyline. Withdrawal of doxycyline induced a more than 500-fold increase in transgene expression. Switching transgene expression "off and on" did not change either the kinetics or the magnitude of induction. Maximal suppression of GFP mRNA transcription was achieved within 24 h of addition of the drug; however, due to the slow turnover rate of GFP, green fluorescent cells could be detected up to 10 days following doxycyline treatment. Following transduction of rat brain with recombinant lentiviruses, doxycyline-regulated GFP expression could be observed in terminally differentiated neurons. Specifically, by adding or withdrawing doxycyline from the rats' drinking water, induction and suppression of GFP expression could be regulated in vivo. These studies show that an inducible lentiviral vector can deliver and regulate transgene expression in vivo. We believe that regulated gene expression is an essential tool for successful gene therapy approaches.