TMIC-34. SKULL-DERIVED MYELOID CELLS AUGMENT T CELL RESPONSES AND SUPPRESS GLIOBLASTOMA GROWTH

Abstract

Abstract While recent studies have implicated the skull marrow as a functional immune reservoir in CNS pathology, its contribution to malignancy remains unexplored. To that end, we transplanted fluorescently-labeled calvarial flaps into syngeneic glioblastoma-bearing mice and cytometrically characterized tumor-infiltrative immune cells, observing a robust population of skull-derived myeloid cells (SDMCs; 40.3% of all intratumoral myeloid cells) in a Balb/C mouse tumor model (CMFDA-stained flaps, 72h). These SDMCs were robust, remaining viable for 3 weeks in an alternative longitudinal model (C57BL/6 mice, GFP+ flaps). As MHCII+ leukocytes were disproportionately enriched among SDMCs (87.9% vs 67.3%, n=3, p=0.012), we interrogated the immunogenicity of these cells through targeted skull ablation, achieved via 13 Gy irradiation of body-shielded mice prior to orthotopic glioblastoma implantation. Indeed, skull-ablated mice had reduced infiltration by antigen-presenting cells than controls (cDCs: 1.1% vs 4.5%, p=0.002; macrophages: 14.4% vs 36.6%, n=10/group, p< 0.001) and 6-fold greater necrosis (67.7% vs 13.5% apoptotic, p< 0.001), suggesting an impaired ability to mount a controlled, adaptive immune response. To clarify the role of SDMCs in mediating secondary immune function, we administered the CXCR4 antagonist AMD3100 between the inner and outer tables of the skull, inhibiting marrow homing signals and promoting myeloid precursor egress (0.26% vs 0.02% of intratumoral myeloid cells, n=10/group, p< 0.001). This yielded cDC2 expansion (10.4% vs 3.0%, p=0.003) and consequent CD4+ T effector memory differentiation (92.3% vs 60.0%, p=0.007). These differences were clinically meaningful, as skull irradiation accelerated tumor growth and attenuated survival compared to controls, while AMD3100 treatment accomplished the opposite (median OS: 17d vs 22d, p< 0.001; 26.5d vs 22d, p=0.006). Concurrent αCD4/CD8/Thy1.2 administration abrogated the therapeutic effect of AMD3100 entirely, confirming the dependency of SDMC-mediated tumor suppression on T cells. Together, our work clarifies the antitumoral role of skull marrow and represents the first-ever therapeutic modulation of this space.