Fluorescence micrograph showing breakdown of the blood–brain barrier in WNV-infected mice. Image kindly provided by T. Town © (2004) Macmillan Magazines Ltd.

A new report in Nature Medicine has revealed that recognition of West Nile virus (WNV) by Toll-like receptor 3 (TLR3) is the main factor that allows this virus to cross the blood–brain barrier and cause lethal encephalitis.

WNV mainly infects birds and mosquitoes; however, humans and horses can also become infected. In humans, infection is generally asymptomatic, but in elderly and immunocompromised individuals, WNV infection can progress to severe neurological disease. The molecular details of the pathogenesis of severe disease are scarce. It is known that some TLRs can detect viral motifs such as single-stranded RNA. In this study, researchers investigated the role of TLR3 in the detection of WNV, using a mouse model of WNV encephalitis.

Initial investigations showed that, after intraperitoneal challenge with a lethal dose of WNV, Tlr3−/− mice were more resistant to infection than wild-type mice. Quantitative PCR assays showed that, after this challenge, the viral burden in the periphery (blood and spleen) was increased in Tlr3−/− mice compared with wild-type mice. Analysis of blood cytokine levels showed increased amounts of inflammatory cytokines in wild-type mice compared with Tlr3−/− mice early in infection. The analysis was then switched from the periphery to the brain. A comparison of the viral load in brain tissue found that, in Tlr3−/− mice, the levels of WNV RNA were significantly lower than in wild-type mice at day 6 after infection. The inflammatory-cytokine profiles indicated that the inflammatory reaction was markedly reduced in Tlr3−/− mice, and immunofluorescence experiments showed that the numbers of activated microglia (brain macrophages) and infiltrating leukocytes were reduced in Tlr3−/− mice, indicating fewer neuropathological effects.

Collectively, these results strongly indicated that TLR3 has a role in WNV entry to the brain. This was confirmed by comparing the permeability of the blood–brain barrier in wild-type and Tlr3−/− mice: after infection with WNV or stimulation with the viral mimic poly I:C (polyinosinic–polycytidylic acid), permeability was increased in wild-type mice but not in Tlr3−/− mice. A further insight into the pathogenesis of severe disease was provided by results indicating that signalling through tumour-necrosis-factor receptor 1 downstream of TLR3 promotes WNV entry to the brain.

Sporadic outbreaks of WNV infection of humans have become increasingly common in the past 5 years, particularly in North America and Europe. This new work identifying TLR3 as the receptor that allows WNV to enter the brain can hopefully be exploited for the development of new therapeutics.