A Heat-Inducible Gene Expression System for a Dominant Negative Version of a TGF-β-Receptor Reveals Antitumor Activity in a Rodent Tumor Model

In an attempt to design novel therapeutic strategies against solid tumors, we hypothesized that spatial and temporal control of transgene expression may allow for the delivery of immunomodulatory molecules at the site of tumor. Here, we assess the potential of the HSP70b promotor to control lentivirus-mediated gene expression in tumor-infiltrating hematopoietic cells. We generated a series of lentiviral vectors and characterized transgene expression in vitro and in vivo. Up to 20fold induction of GFP expression was seen when lentivirus-transduced cells were heated to 43°C in vitro. Only minimal leakiness was seen at 37°C. Next, we analyzed HSP-driven GFP expression in hematopoietic cells upon transplantation of lentivirus-transduced hematopoietic stem cells, using constitutively active promotors and empty vectors as controls. Mice were inoculated with the glioma cell line GL261 and GFP expression was assessed in various organs as well as in tumor-infiltrating hematopoietic cells. Interestingly, HSP70-driven transgene expression was 10–15fold higher in tumor-infiltrating cells when compared to spleen cells, suggesting that the HSP70b promoter was active in the tumor microenvironment. To assess whether confined expression of a dominant negative TGF-β-receptor within the tumor microenvironment would induce anti-tumor immunity, we transplanted mice with hematopoietic stem cells transduced with TGF-β-DNR and injected GL261 cells subcutaneously. The tumor volume was decreased up to 40 fold in mice expressing HSP70b-TGF-β-DNR, comparable to mice transplanted with hematopoietic stem cells that constitutively express TGF-β-DNR. Functional studies suggest a role for perforin, IFN-α and Granzyme B in this TNF-β-DNR-dependent immunomodulatory circuit. In summary, HSP70b-mediated transgene expression in hematopoietic cells may provide a novel experimental strategy for controlled release of immunmodulatory molecules to induce anti-tumor therapies.