The brain does not contain a resident dendritic cell population so resident cells such as microglia and astrocytes play an important role in initiating innate immune responses in the central nervous system (CNS) following virus infection. However, the activation of these cells following stimulation of virus-recognizing toll-like receptors (TLRs) such as TLR7 and TLR8 remains unclear. Poly T oligonucleotides (pT-ODNs) were recently shown to enhance signaling through TLR8, including murine TLR8 which was previously thought to be non-functional. In the current study, we analyzed the ability of pT-ODNs to enhance TLR7/8 activation of murine astrocytes and microglia. We found that the TLR7/8 agonist CL075 (3M002) induced the expression of glial cell activation markers and induced the production of multiple proinflammatory cytokines and chemokines. Addition of pT-ODNs enhanced the expression of proinflammatory cytokines and chemokines as well as the microglia/macrophage activation markers F4/80 and Iba1. Surprisingly, pT-ODNs enhancement of CL075-induced glial activation was abolished in TLR7 deficient mice, with the exception of Gfap (glial fibrillary acidic protein) mRNA upregulation. Thus in glial cells, pT-ODNs enhanced TLR7/8-induced proinflammatory responses through a TLR7-dependent pathway.

This research was supported by the Intramural Research Program of the NIH.