EXTH-13. LOCAL DELIVERY OF AN IL-15 SUPERAGONIST USING A REPLICATING RETROVIRUS SIGNIFICANTLY IMPROVES SURVIVAL AND LYMPHOCYTE INFILTRATION IN POORLY IMMUNOGENIC MURINE GLIOBLASTOMA MODELS

Glioblastoma (GBM) leads to severe systemic and local immunosuppression, and immunotherapies have had limited clinical success. Here, we evaluated the treatment efficacy of RLI, 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 both engineered to be poorly immunogenic with low-mutational burden and known resistance to immunotherapy, and hence more accurate biomimetic models of human GBM. RRV-RLI replicated and spread effectively in cultured murine GBM cells with robust production of functional RLI (165.4 ± 5.3 ng/mL). Stereotactic injection of RRV-RLI into pre-established intracerebral SB28 tumors significantly reduced tumor growth on bioluminescent imaging, and increased median survival compared to control mice (55 vs. 19 days, p=0.002), leading to long-term survival in 12% of treated mice. In the Tu2449 model, imaging results showed complete eradication of intracerebral tumors after RRV-RLI treatment, with long-term survival (median not reached) in > 85% of treated mice, compared to a median survival of 12.5 days in control mice (p=0.001). RRV-RLI treated tumors showed significantly increased CD8 T-cell infiltration, without altering immunosuppressive cell populations. Similarly, broad anti-tumor inflammatory changes, including increased expression of genes involved in T-cell activation and killing, were observed in the NanoString nCounter platform using a 770-gene panel representing various immune cell types. Notably, RLI was not detected in the blood of treated mice, and tumor-localized RRV-RLI gene delivery showed no adverse systemic immune effects in either model. In summary, RRV-mediated RLI immunotherapy results in immunostimulatory and pro-inflammatory changes to the tumor microenvironment and achieves a significant survival benefit in two poorly immunogenic syngeneic murine models of GBM. This tumor-localized immunomodulatory gene therapy has the potential to safely reverse the T-cell depleted immunophenotype of GBM.