LAUSR

The cross‐talk between blood proteins, immune cells, and brain function involves complex mechanisms. Plasma protein C1 inhibitor (C1INH) is an inhibitor of vascular inflammation that is induced by activation of the kallikrein‐kinin system (KKS) and the complement system. Knockout of C1INH was previously correlated with peripheral vascular permeability via the bradykinin pathway, yet there was no evidence of its correlation with blood–brain barrier (BBB) integrity and brain function. In order to understand the effect of plasma C1INH on brain pathology via the vascular system, we knocked down circulating C1INH in wild‐type (WT) mice using an antisense oligonucleotide (ASO), without affecting C1INH expression in peripheral immune cells or the brain, and examined brain pathology. Long‐term elimination of endogenous C1INH in the plasma induced the activation of the KKS and peritoneal macrophages but did not activate the complement system. Bradykinin pathway proteins were elevated in the periphery and the brain, resulting in hypotension. BBB permeability, extravasation of plasma proteins into the brain parenchyma, activation of glial cells, and elevation of pro‐inflammatory response mediators were detected. Furthermore, infiltrating innate immune cells were observed entering the brain through the lateral ventricle walls and the neurovascular unit. Mice showed normal locomotion function, yet cognition was impaired and depressive‐like behavior was evident. In conclusion, our results highlight the important role of regulated plasma C1INH as it acts as a gatekeeper to the brain via the neurovascular system. Thus, manipulation of C1INH in neurovascular disorders might be therapeutically beneficial.