624. Immunovirotherapy Combined with Immune Checkpoint Inhibitors for Treating Glioblastoma

Glioblastoma, the most common primary brain tumor in adults, is invariably fatal despite all current therapies, with a median survival of about 15 months. Glioblastoma contains cancer stem cells or tumor-initiating cells, a subpopulation of cancer cells that are thought to be important in tumor initiation, progression, heterogeneity, recurrence and resistance to therapy, and thus critical targets for therapy. We have established a mouse syngeneic glioblastoma stem cell (GSC) tumor model using 005 GSCs, isolated from activated H-Ras and Akt gliomas induced in Tp53+/- C57Bl/6 mice, which recapitulates the hallmarks of human glioblastoma. Immunovirotheray takes advantage of the natural inflammatory responses to virus infection and oncolytic virus-induced cancer cell death to drive antitumor immunity. Oncolytic herpes simplex viruses (oHSVs) are genetically-engineered to selectively replicate in cancer cells without harming normal tissue. In this study, we test G47Δ-mIL12, an oHSV expressing murine IL-12, a critical cytokine involved in adaptive and innate immune responses as well as anti-angiogenesis. G47Δ-mIL12 increases the survival of immunocompetent mice bearing 005 GSC-derived intracerebral tumors, compared to non-cytokine expressing G47Δ-empty. Immune checkpoints, especially co-inhibitory molecules like PD-1 and its ligand PD-L1, play a critical role in regulating immune responses and suppressing antitumor immune effector cells. PD-1 is expressed on immune cells, as is PD-L1, which is also expressed on endothelial and tumor cells. PD-L1 is expressed on a minority of 005 GSCs in vitro, but is induced by IFNγ in almost all 005 GSCs. Blocking antibodies to these molecules have been shown to be very effective at unleashing antitumor immunity. Therefore, we hypothesized that oHSV, which induces antitumor immune responses, should synergize with checkpoint inhibitor antibodies in inhibiting glioblastoma growth. Mice with established 005 GSC-derived tumors were treated with a single intratumoral injection of G47Δ-mIL12 or PBS, followed by systemic anti-PD-1, anti-PD-L1, or control antibody. Single treatments had a small but significant impact on survival, compared to mock treatment. The combination further extended survival, compared to virus or anti-PD-1 alone. We are examining the immunologic effects of treatment in order to understand the mechanism of action and how to improve outcomes. Combining local delivery of oncolytic viruses (immunovirotherapy) with systemic blockade of immune checkpoints is a promising strategy to target cancer stem cells.