Abstract
Glioblastoma (GBM) leads to systemic and local immunosuppression and immunotherapies have had limited clinical success. We evaluated the treatment efficacy of RLI (receptor-linker-IL-15), a superagonist of T-cell activator IL-15, delivered to tumor cells using a tumor-selective retroviral replicating vector (RRV) in the syngeneic murine SB28 and Tu2449 GBM models, which are poorly immunogenic with low-mutational burden and known resistance to immunotherapy, similar to human GBM. RRV-RLI replicated and spread in cultured SB28 murine GBM cells with robust production of functional RLI (165.4 ± 5.3 ng/mL). Stereotactic injection of RRV-RLI into pre-established intracerebral Tu2449 tumors led to complete eradication of intracerebral tumors with long term survival (median not reached) in > 85% of treated mice vs. median survival of 12.5 days in control mice (p=0.001). Treated mice demonstrated significantly increased CD8 T cell infiltration. These mice also demonstrated immunologic memory to intracranial rechallenge. In SB28, RRV-RLI delivery into intracerebral tumors reduced tumor growth on bioluminescence imaging, and increased median survival compared to controls (55 vs. 19 days, p=0.002), with long-term survival in 12% of treated mice. Combining RRV-RLI treatment with systemically administered temozolomide (TMZ) in the SB28 model provided long term survival (median not reached) in >60% of treated mice. Immunologic analysis of treated tumors revealed a significant increase in CD8 T cell (2.1% to 20.5% of CD45 cells) and Natural Killer cell (1.4% to 9.6% of CD45 cells )infiltration in RLI-RRV+TMZ mice vs. PBS+TMZ mice. RLI was not detected in the blood of treated mice, and tumor-localized RRV-RLI gene delivery showed no adverse systemic immune effects. In summary, RRV-mediated RLI immunotherapy results in immunostimulatory changes that are further potentiated by systemically administered temozolomide. This tumor-localized gene therapy has the potential to synergize with standard of care treatment to reverse the immunosuppressed GBM tumor microenvironment and provide a significant treatment benefit.