TLR9 and TLR7/8 activation induces formation of keratic precipitates and giant macrophages in the mouse cornea

Abstract

Macrophage adherence to the inner corneal surface and formation of MGCs in the stroma are common signs of chronic inflammation following corneal infection. To determine whether macrophage adherence (known clinically as KPs) and giant cell formation were specific to innate immune activation via particular TLR ligands, macrophage activation was examined in a murine model of TLR-mediated corneal inflammation. The corneal epithelium was debrided and highly purified TLR ligands were topically applied once to the cornea of TLR7(-/-), TLR9(-/-), Cx3cr1(gfp/+), CD11c(eYFP), and IL-4(-/-) mice. At 1 week post-treatment macrophage activation and phenotype was evaluated in the cornea. Treatment with TLR2, TLR3, TLR4, and TLR5 ligands caused an increase in the number of activated stromal macrophages in the central cornea at 1 week post-treatment. However, treatment with TLR9 ligand CpG-ODN and the TLR7/8 ligand R848/Resiquimod led to an accumulation of macrophages on the corneal endothelium and formation of multinucleated giant macrophages in the corneal stroma. We suggest that giant cell formation, which is a characteristic feature of granuloma formation in many tissues, may be a unique feature of TLR9- and TLR7/8-mediated macrophage activation.

Longitudinal Changes to Tight Junction Expression and Endothelial Cell Integrity in a Mouse Model of Sterile Corneal Inflammation

“…The observed morphologic changes in endothelial cells at 1 week are independent of the presence of keratic precipitates and activated stromal macrophages, which are a known feature of corneal inflammation in response to CpGtreatment. 13 For all conditions, the absolute degree of endothelial cell hexagonality was unchanged, supporting the concept that polymegethism is a more sensitive index of corneal endothelial responses to inflammation/injury than the degree of hexagonality (pleomorphism). 16 We recently reported the presence of large, multinucleated giant macrophages in the mouse cornea 1 week after CpG-ODN treatment; however, this was not associated with any overt corneal endothelial dysfunction.…”

“…16 We recently reported the presence of large, multinucleated giant macrophages in the mouse cornea 1 week after CpG-ODN treatment; however, this was not associated with any overt corneal endothelial dysfunction. 13 In the present study, intact ZO-1 expression, normal CCT, and normal ECD were observed in all groups at 1 week post treatment; however, there was reduced ECD and increased CCT at 4 weeks after CpG-ODN treatment. These signs of corneal pathology in the CpG-ODN-treated group were associated with the presence of enlarged CD45 þ cells throughout the corneal stroma, indicating persistent activation of inflammatory cells.…”

“…20 In the mouse cornea, CpG induces inflammation that closely parallels the immunopathology associated with herpes simplex virus type I (HSV-1) keratitis, including stromal lesions, 21 angiogenesis, 22 chemokine production 23 and activated macrophages. 13 Our laboratory, and others, have previously reported that acute corneal inflammation induced by topical CpG is associated with corneal edema, as indicated by increased CCT at 24 hours, 11 which subsides by 1 week post treatment. 6 To explore the mechanisms underlying this CpG-induced stromal edema, we investigated the expression of the tight junction protein ZO-1 and other indices of endothelial integrity including cell density, polymorphism, and polymegethism over 4 weeks.…”

“…In light of our previous reports of the presence of giant stromal macrophages and the keratic precipitates adhering to the corneal endothelium 1 week following topical CpG-ODN, 13 we examined whether the clinically observable corneal edema (Fig. 2B) and alterations to the corneal endothelium (Table) were associated with persistence of inflammatory cells in the cornea after 4 weeks.…”

“…13 The accumulation of keratic precipitates is not associated with overt structural changes to the corneal endothelium, and corneal edema largely subsides by 1 week, iovs.arvojournals.org j ISSN: suggesting that the presence of inflammatory cells in the cornea is not associated with corneal endothelial dysfunction. To explore the mechanisms of increased corneal edema during the acute phase of CpG-induced corneal inflammation, we investigated the expression of the tight junction protein ZO-1 by corneal endothelial cells and the density of corneal endothelial cells.…”