IMMU-56. Targeting skull-resident immuno-suppressors synergizes with immunotherapy in glioblastoma

Abstract INTRODUCTION The immune landscape within high-grade gliomas (HGGs) has been well characterized by the field. Recent studies in neurological diseases have identified the skull/meningeal bone marrow as an immune cell reservoir, but its role in HGGs has not been previously described. Our objective was to determine if skull-derived immune cells infiltrate cortical gliomas, and if skull-MDSCs contribute to the immunosuppressive TME. METHODS We created skull-marrow chimeras using CD45.1-donor marrow to reconstitute the skull. C57BL/6J mice received whole brain irradiation (WBI) and intravenous CD45.1 bone marrow. These mice received orthotopic KR158B-gliomas 4 days after. To determine cellular makeup, tumor immunophenotyping was conducted using flow cytometry. To test for MDSC function, T-cell suppression assays were conducted. To determine the skull-marrow’s role on treatment responses, we conducted adoptive cellular therapy (ACT) or HSC + aPD-1 therapy +/- WBI. RESULTS To determine if skull-derived MDSCs suppressed T-cell function, skull- or-femoral-derived MDSCs were co-cultured with activated T-cells. Surprisingly, we found increased suppressive capacity of skull-derived MDSCs relative to femoral-MDSCs. We then asked if skull-MDSCs infiltrate gliomas. In the chimeras, we found CD45.1 immune cell engraftment in the skull/meninges at 5 weeks, but CD45.1 cells were not found in peripheral lymphoid organs. Here, chimeras received orthotopic gliomas and immunophenotypic analysis identified that 5-15% of intratumoral MDSCs were from skull-derived CD45.1 cells. We then hypothesized that skull-MDSC ablation may decrease TME immunosuppression and increase therapeutic efficacy. Tumor-bearing mice received WBI and either ACT or HSC + aPD-1, two efficacious therapies against KR158B. We found that skull marrow ablation significantly increased survival in both immunotherapeutic strategies. CONCLUSION We determined that the TME harbors a skull/meningeal immune subset that is significantly more immunosuppressive than peripherally derived cells. WBI-mediated ablation synergized with two immunologic modalities, suggesting that the skull/meningeal immune niche is a novel therapeutic vulnerability against HGGs.