The choroid plexus (CP) compartment in the ventricles of the brain comprises fenestrated vasculature and， therefore， it is permeable to blood-borne mediators of inflammation. Here， we explored whether T-cell activation in the CP plays a role in regulating central nervous system (CNS) inflammation. We show that CD4 T cells injected into the lateral ventricles adhere to the CP， transmigrate across its epithelium， and undergo antigen-specific activation and proliferation. This process is enhanced following peripheral immune stimulation and significantly impacts the immune signaling induced by the CP. studies demonstrate that T-cell harboring the CP through its apical surface is a chemokine- and adhesion molecule-dependent process. We suggest that， within the CNS， the CP serves an immunological niche， which rapidly responds to peripheral inflammation and， thereby， promotes two-way T-cell trafficking that impact adaptive immunity in the CNS.