Pharmacologically regulated production of targeted retrovirus from T cells for systemic antitumor gene therapy.

We aimed to use cell-based carriers to direct vector production to target sites for systemic therapy. We used T cells engineered to express a chimeric T cell receptor that can specifically recognize target cells expressing the tumor-associated carcinoembryonic antigen (CEA). These T cells were modified to produce a retrovirus under tight pharmacological control using the rapamycin-inducible transcriptional regulatory system. The retroviral vectors produced were transcriptionally targeted to CEA-expressing target cells. We found that vector production and transgene expression from these T cells in vitro was dependent on pharmacological induction and expression of CEA in target cells, respectively. Mice bearing metastatic tumors that received cell carriers delivering the HSVtk gene demonstrated a significant increase in survival, but only in response to pharmacological induction of vector production. Interestingly, the therapeutic effect required the presence of the tumor-specific chimeric receptor on T cells. Further studies demonstrated that systemic delivery of tumor-specific T cells to mice bearing metastatic tumors caused recruitment of nonspecific T cells to the tumor site. We hypothesize that this enhanced targeting to tumor sites is responsible for the efficiency of T cell-mediated retroviral gene transfer and that this principle can be used to enhance systemic therapies using immune-cell carriers.