TLR7 and TLR9 Trigger Distinct Neuroinflammatory Responses in the CNS

Toll-like receptors (TLRs) 7 and 9 recognize nucleic acid determinants from viruses and bacteria and elicit the production of type I interferons and proinflammatory cytokines. TLR7 and TLR9 are similar regarding localization and signal transduction mechanisms. However, stimulation of these receptors has differing effects in modulating viral pathogenesis and in direct toxicity in the central nervous system (CNS). In the present study, we examined the potential of the TLR7 agonist imiquimod and the TLR9 agonist cytosine-phosphate-guanosine oligodeoxynucleotide (CpG-ODN) to induce neuroinflammation after intracerebroventricular inoculation. CpG-ODN induced a more robust inflammatory response than did imiquimod after inoculation into the CNS, with higher levels of several proinflammatory cytokines and chemokines. The increase in cytokines and chemokines correlated with breakdown of the blood-cerebrospinal fluid barrier and recruitment of peripheral cells to the CNS in CpG-ODN-inoculated mice. In contrast, TLR7 agonists induced a strong interferon β response in the CNS but only low levels of other cytokines. The difference in response to these agonists was not due to differences in distribution or longevity of the agonists but rather was correlated with cytokine production by choroid plexus cells. These results indicate that despite the high similarity of TLR7 and TLR9 in binding nucleic acids and inducing similar downstream signaling, the neuroinflammation response induced by these receptors differs dramatically due, at least in part, to activation of cells in the choroid plexus.