Interactions Between TLR7 and TLR9 Agonists and Receptors Regulate Innate Immune Responses by Astrocytes and Microglia

Toll-like receptors 7 (TLR7) and 9 (TLR9) are important mediators of innate immune responses. Both receptors are located in endosomal compartments, recognize nucleic acids, and signal via Myeloid differentiation factor 88 (MyD88). In the current study, we analyzed TLR7 and TLR9 induced activation of astrocytes and microglia, two cell types that contribute to innate immune responses in the CNS. TLR7 and TLR9 agonists induced similar cytokine profiles within each cell type. However, there were notable differences in the cytokine profile between astrocytes and microglia, including the production of the anti-inflammatory cytokine IL-10 and antiapoptotic cytokines G-CSF and IL-9 by microglia but not astrocytes. Costimulation studies demonstrated that the TLR7 agonist, imiquimod, could inhibit TLR9 agonist-induced innate immune responses, in both cell types, in a concentration-dependent manner. Surprisingly, this inhibition was not mediated by TLR7, as deficiency in TLR7 did not alter suppression of the TLR9 agonist-induced responses. The suppression of innate immune responses was also not due to an inhibition of TLR9 agonist uptake. This suggested that imiquimod suppression may be a direct effect, possibly by blocking CpG-ODN binding and/or signaling with TLR9, thus limiting cell activation. An antagonistic relationship was also observed between the two receptors in microglia, with TLR7 deficiency resulting in enhanced cytokine responses to CpG-ODN stimulation. Thus, both TLR7 and its agonist can have inhibitory effects on TLR9-induced cytokine responses in glial cells.