Retinal Microglial Activation Following Topical Application of Intracellular Toll-Like Receptor Ligands

We previously have reported that application of the intracellular toll-like receptor (TLR)-9 ligand CpG-ODN onto the injured corneal surface induces widespread inflammation within the eye, including the retina. We tested the hypothesis that topical application of two other intracellular TLR agonists, Poly I:C and R848, would cause retinal microglial activation and migration into the subretinal space.

C57BL/6J wild-type and Cx3cr1gfp/+ mice were anesthetized and received central corneal abrasions followed by topical application of Poly I:C (TLR3 agonist), R848 (TLR7/8 agonist), or CpG-ODN (TLR9 agonist). Eyes were imaged in vivo by using spectral-domain optical coherence tomography to assess and quantify vitreous cells and retinal edema. Tissues were processed for whole-mount immunofluorescence staining or gene expression studies. Microglial activation was determined by morphologic changes, major histocompatibility complex (MHC) class II reactivity, and migration to the subretinal space. Expression of proinflammatory cytokine gene IL-6, IL-1β, IFN-γ, and MCP-1 in retinal tissues were analyzed.

At 24 hours, topical treatment with CpG-ODN and R848, but not Poly I:C, led to altered microglial morphology. One week after CpG-ODN and R848-treatment, eyes exhibited vitritis and mild inner retinal edema, increased number of subretinal Iba-1+ cells, and an increase in MHC II+ cells in the neural retina. Proinflammatory cytokine genes were upregulated after R848 treatment, whereas in the CpG-ODN group, only IL-1β and MCP-1 were significantly upregulated. Retinal microglial activation was not observed in the Poly I:C-treated group.

Topical application of CpG-ODN and R848, but not Poly I:C, to the damaged corneal surface can cause activation and migration of retinal microglia.