Tetracycline-regulated gene expression in replication-incompetent herpes simplex virus vectors

Although herpes simplex virus (HSV) vectors appear to have great potential as gene delivery vectors both in vitro and in vivo, the expression of foreign genes in such vectors cannot be easily regulated. Of the known eukaryotic regulatory systems, the tetracycline-inducible gene expression system is perhaps the most widely used because of its induction characteristics and because of the well-known pharmacological properties of tetracycline (Tet) and analogs such as doxycycline. Here, we describe the adaptation of the Tet-inducible system for use in replication-incompetent HSV vectors. HSV vectors were constructed that contained several types of Tet-inducible promoters for foreign gene expression. These promoters contained a tetracycline response element (TRE) linked to either a minimal cytomegalovirus (CMV) immediate-early promoter, a minimal HSV ICP0 promoter, or a truncated HSV ICP0 promoter containing one copy of the HSV TAATGARAT cis-acting immediate-early regulatory element (where R represents a prime base). All three promoter constructs were regulated appropriately by doxycycline, as shown by the expression of the marker gene lacZ in cell lines engineered to express Tet transactivators. The ICP0 promoter constructs expressed the highest and most sustained levels of lacZ, but the CMV promoter construct had the highest relative level of induction, suggesting their use in different applications. To extend the utility of Tet-regulated HSV vectors, vectors were constructed that coexpressed an inducible Tet transactivator in addition to the inducible lacZ marker gene. This modification resulted in tetracycline-inducible gene expression that was not restricted to specific cell lines, and this vector was capable of inducible expression in irreversibly differentiated NT2 cells (NT-neurons) for several days. Finally, HSV vectors were constructed that expressed modified Tet transactivators, resulting in improved induction properties and indicating the flexibility of the Tet-regulated system for regulation of foreign gene expression in HSV vector-infected cells.