Local Delivery of an IL-15 Superagonist Using a Replicating Retrovirus Improves Survival and Lymphocyte Infiltration in Poorly Immunogenic Mouse Models of Glioblastoma

Glioblastoma (GBM) leads to severe systemic and local immunosuppression, and immunotherapies have had limited success. One of the hallmarks of GBM immunosuppression is poor T cell infiltration and significant intratumoral T cell dysfunction. IL-15 is a promising immunocytokine with the ability to stimulate T cell activation and proliferation. In this study, we evaluate the treatment efficacy of RLI (an IL-15 superagonist) delivered to tumor cells using a tumor-selective retroviral replicating vector (RRV) in the poorly immunogenic syngeneic murine SB28 and Tu2449 models of GBM. RRV-RLI replicates and spreads effectively in cultured SB28 cells. Transduced SB28 cells express high levels of RLI (175 ng/mL), which sustained T cell growth in culture. Treatment with RRV-RLI by stereotactic injection into intracerebral murine SB28 and Tu2449 tumors significantly reduced tumor growth on bioluminescence imaging, and increased survival relative to control mice, leading to long-term survival in a subset of treated mice. RRV-RLI treated tumors showed significantly increased CD4 and CD8 T cell infiltration, without changes in immunosuppressive cell populations, including myeloid-derived suppressor cells or T regulatory cells. This study demonstrates that localized tumor-specific immunomodulatory gene therapy delivered in an RRV has the potential to safely reverse the T-cell depleted immunophenotype of GBM. Overall design: A retroviral replicating vector was used to deliver RLI (IL-15 superagonist) in vivo to tumor cells in the SB28 murine model of GBM. Conditions tested included a (1) PBS + vehicle control, (2) PBS + TMZ, (3) RRV RLI + vehicle, and (4) RRV RLI + TMZ. CD45+ leukocytes were then isolated from these tumors using flow cytommetry and were sent for single-cell RNA sequencing (scRNA-seq) as well as single cell immune profiling of the V(D)J repertoire of T-cells.