Brain infiltrating T cells mediate microglial dysregulation and neuronal loss following SAH

The contribution of T cells to neuroinflammation after aneurysmal subarachnoid hemorrhage (SAH) remains poorly understood. Using a murine pre-chiasmatic injection model of SAH we demonstrate that T cell infiltration into the brain modulates microglial activation and promotes neuronal death. Targeted transcriptomic profiling revealed a sustained neuroimmune response at 7 days post injury (dpi) characterized by a major involvement of T cells and microglia activation. Immunohistochemistry confirmed focal CD3+ T cell infiltration, predominantly CD4+, in the brain at the site of blood injection (BI), choroid plexus and meninges in SAH mice at 3- and 7-dpi. This temporal pattern was also observed in the CSF of a human SAH cohort. T cell presence spatially correlated with regions of microglial reactivity and neuronal loss. Notably, CD3-knockout mice exhibited reduced microglial activation and preserved neuronal viability. These findings identify T cells as key amplifiers of post-SAH neuroinflammation and neuronal damage. Targeting T cell–microglia crosstalk may represent a novel therapeutic avenue for SAH. Summary statement This study shows that brain T-cell infiltration after subarachnoid hemorrhage drives microglial activation and neuronal loss in mice, with similar patterns observed in patients. Data indicate T cells as key mediators of post-injury neuroinflammation with therapeutic implications.