The blood-brain tumor barrier as a spatial immune checkpoint in glioblastoma: leukocyte entry, perivascular retention, and therapeutic implications

Glioblastoma (GBM) combines heterogeneous vascular dysfunction with a predominantly myeloid immune landscape. This Mini Review focuses on a specific problem: vascular permeability is not equivalent to productive leukocyte access. Entry requires endothelial capture, chemokine-guided arrest, transendothelial migration, release from perivascular sites, intratumoral dispersion, target recognition, and sustained effector function. Human tissue and spatial studies show regionally distinct neurovascular and myeloid states, whereas much of the causal evidence for trafficking and retention remains preclinical. We therefore distinguish published observations from a hypothesis-generating model in which the blood-brain tumor barrier (BBTB) and perivascular myeloid niche can act as coupled but separable checkpoints. The main conceptual advance is not the already recognized heterogeneity of the BBTB, but the argument that leakage-based imaging should not be used as a surrogate for immune-cell entry and that each trafficking stage requires its own pharmacodynamic readout. We summarize regional BBTB states, differential lymphoid and myeloid access, and selected strategies-vascular normalization, myeloid reprogramming, local cell delivery, and focused ultrasound-while emphasizing their evidence limits. Clinical translation will require spatially registered human sampling, explicit model provenance, and tests that can reject individual components of the framework.