Inducible gene expression from defective herpes simplex virus vectors using the tetracycline-responsive promoter system.

Herpes simplex virus-based amplicon vectors have been used for gene transfer into cultured neurons and the adult CNS. Since constitutive expression of a foreign gene or overexpression of an endogenous gene may have deleterious effects, the ability to control temporal expression would be advantageous. To achieve inducible gene expression, we have incorporated the tetracycline-responsive promoter system into amplicon vectors and showed, both in vitro and in vivo, that expression can be modulated by tetracycline. Using the firefly luciferase as the reporter gene, maximal repression by tetracycline in hippocampal cultures was about 50-fold. Withdrawal of tetracycline derepressed gene expression, reaching maximal levels within 10-12 h. In contrast, addition of tetracycline to cultures without prior tetracycline exposure inhibited gene expression rapidly; luciferase activity was reduced to less than 8% within 24 h. In adult rat hippocampus, vectors expressing luciferase or the Escherichia coli lacZ were repressed by tetracycline 9- and 60-fold, respectively. Maximum gene expression from the vectors occurred 2-3 days post-infection and declined thereafter. Such decline impeded further induction of expression by withdrawing tetracycline. This study demonstrates the feasibility of incorporating a powerful inducible promoter system into HSV vectors. The development of such an inducible viral vector system for gene transfer into the adult CNS might prove to be of experimental and therapeutic value.