IMMU-71. CRANIOENCEPHALIC FUNCTIONAL LYMPHOID UNITS IN GLIOBLASTOMA

Abstract The general accessibility of the brain to immune cells has been a subject of intense debate and reinterpretation within the last decade. While glioblastoma’s brain tumor ecosystem is typically considered immunosuppressed, we report the presence of a tumor-reactive immune cell niche within the cranial bone marrow (CB) situated adjacent to treatment-naive tumors. At time of initial diagnosis, glioblastoma patients underwent clinical and anatomical imaging, which revealed an unexpected co-morbid association between the CB and the tumor. Following this observation, we conducted an extensive experimental study including multiplex immunofluorescence, immunoprofiling, functional assays, and single-cell analysis, which identified an active lymphoid niche within the patient’s CB. CD8+ T cell effector cells residing in this niche exhibited acute and durable antitumor responses, and, compared to cells from the distal bone marrow, showed increased expression of the lymphoid egress marker S1PR1. Single-cell T cell receptor sequencing revealed a rich reservoir of non-exhausted tumor-shared CD8+ T cell clonotypes within the CB lymphoid niche, suggesting re-circulation between the proximal bone and the tumor tissue. These clonotypes were characterized by a high predicted tumor-reactivity, significantly surpassing levels found in blood and distal bone marrow. We discovered the entire developmental spectrum of CD8+ T cells in the CB, indicating that anatomical proximity may play a critical role in the process of early anti-tumor response. Correspondingly, cranial enhancement of CXCR4 radiolabel may serve as a surrogate marker, indicating focal association with improved progression-free survival. Our findings support the preservation and further exploitation of these cranioencephalic units to improve clinical care in glioblastoma patients.