TMIC-61. The intracranial tissue environment contributes to a unique response of glioblastoma against immunotherapy

Abstract Immunotherapy, while effective for many solid tumors, has not shown benefits against glioblastoma (GBM). This is postulated to result from limited T cell infiltration and an immunosuppressive tumor microenvironment (TME). We hypothesized that the brain tissue environment, distinct from extracranial tissues, contributes to treatment resistance. To test this hypothesis, we established a mouse subcutaneous (s.c.) SB28 GBM model, representing an extracranial tissue environment, and compared it to orthotopic (intracranial) SB28. Treatment with Immune check-point blockade (ICB) increased overall survival in s.c. SB28, but not in intracranial (i.c.) SB28 model. We then analyzed ICB effects on the immune landscape in the circulation and TME in each model, using flow cytometry. ICB increased circulating CD8 and CD4 T cells subsets in both i.c. and s.c. tumor models compared to a vehicle control group. Yet, antigen-experienced CD8 and CD4 T cells increased only in the TME of the s.c. model. To enhance T cell migration into i.c. tumors, we targeted CXCL12, leveraging its synergistic effect with ICB on T cell infiltration observed in other extracranial tumors. We compared treatment-induced changes in the immune landscape in mice receiving CXCL12 inhibitor NOX-A12, ICB, the combination of NOX-A12 and ICB, or vehicle control. As expected, combination therapy significantly increased antigen-experienced CD8 T cells in i.c. and s.c. SB28 compared to ICB or NOX-A12 monotherapy. Combination treatment also increased antigen-experienced CD4 T cells in i.c. tumors, but not in s.c. tumors, indicating a tissue-dependent treatment effect. These treatment effects were not evident in the blood, indicating a local mechanism of action within the TME. However, the increase in intra-tumoral T-cells induced by combination treatment was not translated into tumor growth reduction, implying that additional factors contribute to GBM resistance to immunotherapy. Elucidating those factors is imperative for the development of innovative strategies to augment immunotherapy efficacy in GBM.