The resting CNS is an immunospecialized environment, devoid of most immune processes, although substantial inflammatory responses can be initiated. The innate immune mechanisms mediating recognition of CNS infections are unknown. This study provides a comprehensive analysis of Toll-like receptor (TLR) gene expression in the resting and virus-infected murine CNS. TLR transcripts were expressed in the resting CNS with strikingly high expression of TLR 3. Extraneural infection with neuroinvasive Semliki Forest virus resulted in CNS infection followed by rapid selective upregulation of TLR gene expression. Upregulation was independent of T-cell responses. Upregulation of TLR gene expression was also observed following rabies virus infection. TLR upregulation was appropriate to the pathogen and proportional to the virus load. Upregulation of TLR 3 and 9 was dependent upon the type-I interferon response and may act to increase the threshold of sensitivity to detect virus infection in cells surrounding virally infected cells. (c) 2005 Elsevier Inc. All rights reserved.