IMPS-29REFUTING BRAIN IMMUNE PRIVILEGE: LONGITUDINAL MULTIPHOTON IMAGING THROUGH SKULL WINDOWS REVEALS THE IMMUNE SURVEILLANCE OF BRAIN METASTASES AND ITS REGULATION BY FRACTALKINE

INTRODUCTION: The brain has conventionally been considered to be immune-privileged, which has detracted from the pursuit of immunotherapies for brain tumors. However, whether brain malignancies remain insulated from the immune system is unclear. T cell migration in tissues is regulated in part by chemokines often in an organ-specific manner. We hypothesized that brain tumorsare accessible to T cell surveillance, and that this process is regulated by a chemokine called fractalkine, which is highly expressed in the brain. METHODS: Cranial windows for intravital multiphoton microscopy were established in novel transgenic murine models in which T cells, macrophages, microglia, and dendritic cells were distinctly gene-labeled with fluorescent tags. For comparison, multicolor reporter mice deficient in fractalkine receptor (CX3CR1gfp/gfp-KO) were used. Brain metastases (BM) were induced via carotid artery injection of fluorescent MCA fibrosarcomacells and the immune dynamics at brain tumorlesions was imaged longitudinally. RESULTS: BM regressed after initial progression in wild-type (WT) mice, but were lethal in Rag1-KO mice, and importantly also lethal in CX3CR1-KO mice. Intravital microscopyin WT reporter mice revealed hotspots of T cell migration at BM foci followed by tumor fragmentation. In striking contrast, T cells were reduced in BM in CX3CR1-KO reporter mice and exhibited increased meandering and decreased engagement with cancer cells. CONCLUSION: With live imaging, we directly show the existence of robust and effective immune surveillance within the brain contrary to the concept of "brain immune privilege". Furthermore, we identify the fractalkine/ CX3CR1chemokine-receptor axis as a key regulator of T cell immune surveillance in the malignant brain.