Abstract
Malignant glioma is a highly invasive and neurologically destructive tumor frequently resulting in death within nine months after diagnosis. Suicide gene therapy has been explored as an alternative to chemotherapy for treating or curing the disease. While treatments utilizing gene therapeutic vectors expressing HSV1-thymidine kinase (HSV1-TK) in combination with the pro-drug ganciclovir (GCV) are 100% efficient in many preclinical glioma models, phase 1 and 2 clinical trials in human patients have not shown such dramatic results. We have developed a macroscopic preclinical model of glioma in rats and have shown that all single cytokine and cytotoxic gene therapies fail. Furthermore, combined treatments with HSV1-TK and various immuno-modulatory agents were also unsuccessful for improving survival. Only combined treatment with HSV1-TK and the potent cytokine for stimulating dendritic cells, human soluble Fms like tyrosine kinase 3 ligand (hsFlt3L), provided 80% survival in this syngeneic model of glioma. This model is a more faithful reflection of the clinical results obtained utilizing single gene therapies in human patients suffering from glioma. In the present study we have analysed the immune responses in both naïve and glioma bearing animals when treated with RAdHSV1-TK and RAdhsFlt3L alone or in combination. We found that when adenoviral vectors expressing HSV1-TK and hsFlt3L are injected into naïve brains, large increases in B, NK-T, NK and plasmacytoid dendritic cell (pDC) populations were observed. NK and NK-T cells are important mediators of antigen-independent anti-tumor cytotoxicity. pDC's may improve antigen presentation and prime an efficient adaptive immune response against the tumor. ELISA and ICC were used to assess long-term expression of HSV1-TK and hsFlt3L transgenes. In summary, our results indicate that the combined anti-tumor effect of HSV-1TK and hsFlt3L enhances both innate and adaptive branches of the immune system and improves survival in a syngeneic macroscopic glioma model.