Preclinical Studies on Viro-Immunotherapy for Brain Tumors

Retroviral replicating vectors (RRV) are capable of tumor-selective replication and stable integration into the cancer cell genome, and are currently under investigation for prodrug activator gene therapy of recurrent high grade glioma in phase II/III clinical trials. In preclinical studies evaluating RRV gene therapy in intracranial glioma models, we showed effective long-term tumor control in human glioma xenograft models as long as prodrug administration was continued. In contrast, tumor eradication was achieved even after cessation of prodrug administration in immunocompetent syngeneic glioma models. Accordingly, my research has focused on investigating the immunological mechanisms involved in RRV gene therapy, and evaluating combined viro-immunotherapy approaches to improve RRV-mediated tumor transduction and therapeutic efficacy, as well as to confer a dominant foreign antigen to be targeted by adoptively transferred T cells. RRV replication may be delayed at lower inoculation doses, if initial distribution is confined to the area surrounding an intratumoral injection site. More rapid tumor transduction may be achieved if the inoculum is distributed more evenly within the tumor. Alloreactive cytotoxic T lymphocytes (alloCTL) are migratory cells trained to react to major histocompatibility complex (MHC) that are selectively displayed on gliomas. Combining alloCTL and RRV gene therapy as individual therapies showed added benefit compared to either modality alone. We further tested the use of alloCTL as vector producer cells to deliver marker and suicide genes to tumors after their transduction with RRV (alloCTL/RRV). When compared against either therapy alone in a subcutaneous glioma model, the greatest benefit was achieved by alloCTL/RRV. This suggests that alloCTL/RRV can serve as anti-tumor effector cells targeting endogenous antigens, as well as motile ‘carriers’ that facilitate spread of RRV to tumor cells.Aside from MHC, there are few well-characterized / immunogenic tumor-associated antigens with defined epitopes in glioblastoma. The efficient spread and permanent genomic integration of RRV can be utilized to induce stable expression of viral antigens that may represent useful immunotherapeutic targets. We developed an RRV displaying the lymphocytic choriomeningitis virus glycoprotein gp33-43 (gp33) epitope fused to the viral env gene. Target cell-specific cytotoxicity by transgenic lymphocytes engineered with a T cell receptor recognizing the gp33 epitope (P14 cells) was observed in vitro and in vivo. P14 adoptive transfer in immunocompetent hosts increased survival. Thus, the use of RRV to deliver highly immunogenic viral antigens to glioma could enable more effective immunotherapy for this disease