Viral Pathogen-Associated Molecular Patterns Regulate Blood-Brain Barrier Integrity via Competing Innate Cytokine Signals

ABSTRACT Pattern recognition receptor (PRR) detection of pathogen-associated molecular patterns (PAMPs), such as viral RNA, drives innate immune responses againstWest Nile virus (WNV), an emerging neurotropic pathogen. Here we demonstrate that WNV PAMPs orchestrate endothelial responses toWNV via competing innate immune cytokine signals at the blood-brain bar-rier (BBB), a multicellular interface with highly specialized brain endothelial cells that normally prevents pathogen entry. While Th1 cytokines increase the permeability of endothelial barriers, type I interferon (IFN) promoted and stabilized BBB function. Induction of innate cytokines by pattern recognition pathways directly regulated BBB permeability and tight junction formation via balanced activation of the small GTPases Rac1 and RhoA, which in turn regulated the transendothelial trafficking ofWNV. In vivo, mice with attenuated type I IFN signaling or IFN induction (Ifnar/ Irf7/) exhibited enhanced BBB permeability and tight junction dysregulation afterWNV infection. Together, these data provide new insight into host-pathogen interactions at the BBB during neurotropic viral infection. IMPORTANCE West Nile virus (WNV) is an emerging pathogen capable of infecting the central nervous system (CNS), causing fatal encephalitis. However, the mechanisms that control the ability ofWNV to cross the blood-brain barrier (BBB) and access the CNS are unclear. In this study, we show that detection ofWNV by host tissues induces innate immune cytokine expression at the BBB, regulating BBB structure and function and impacting transendothelial trafficking ofWNV. This regulatory effect is shown to happen rapidly following exposure to virus, to occur independently of viral replication within BBB cells, and to requir