Matrix metalloproteinases (MMPs) are zinc-dependent endopeptidases with the potential to degrade all types of extracellular matrix. The ADAM (a disintegrin and metalloproteinase) family of peptidases was recently identified as cleaving the extracellular domain of transmembrane proteins. This was termed ectodomain shedding. We investigated the MMP expression in patients with corneal diseases and the potential role of ADAMs in corneal pathophysiology. We detected upregulation of the active form of MMP-2 and MMP-9 in the tear fluid from patients with corneal melting or recurrent corneal erosion. Using human corneal epithelial cells, we observed ADAM17-dependent ectodomain shedding of soluble tumor necrosis factor receptor 1 and soluble interleukin-6 (IL-6) receptor (sIL-6R). The production of sIL-6R was also induced by messenger RNA splicing in the human corneal epithelial cells. IL-6/sIL-6R-induced signal transducer and activator of transcription 3 phosphorylation was observed in cultured human corneal fibroblasts, suggesting that IL-6 trans-signaling induced inflammatory cellular signaling in the human corneal fibroblasts. We demonstrated that MMPs are significantly upregulated in collagen-destructive disorders of the cornea. Additionally, we observed that ectodomain shedding by ADAMs in corneal epithelial cells mediated the production of soluble cytokine receptors. Trans-signaling of IL-6 can induce an inflammatory response in corneal stroma, indicating the significance of IL-6 trans-signaling in ocular surface inflammation. Thus, MMPs and ADAMs play an important role in the pathophysiology of corneal diseases.

We evaluated the role of NLR family pyrin domain containing 3 (NLRP3) inflammasome in sterile corneal inflammation caused by lipopolysaccharide (LPS) or alkali burns in C57BL6 mice or NLRP3 KO (Nlrp3^−/−) mice. Various molecules related to the NLRP3 inflammasome were upregulated in C57BL6 mice after both alkali burn injury and LPS treatment. After alkali burn injury, the corneal opacity grade was significantly reduced inNlrp3^−/−mice compared with C57BL6 mice. InNlrp3^−/−mice, Gr-1 immunoreactivity and MMP-9 mRNA expression in the corneal stroma were significantly reduced by both LPS treatment and alkali burn injury. Quantitative PCR and immunohistochemistry revealed that IL-1β and MMP-9 expression in the corneal stroma were down-regulated inNlrp3^−/−mice with both alkali burn injury and LPS treatment. These findings suggest that the NLRP3 inflammasome has a pro-inflammatory effect in the cornea by recruiting neutrophils to sites of inflammation.

Purpose.: We evaluated an anti-inflammatory effect of TNF receptor 1 (TNFR1) ectodomain shedding in ocular surface. Methods.: Human corneal epithelial cell (HCEC) was first pretreated by TNF-α. Ectodomain shedding was stimulated by uridine triphosphate (UTP) or peptidoglycan (PGN), with or without shedding inhibition using TNF-α processing inhibitor (TAPI). The phosphorylation of the NF-κB inhibitory protein, IκB, was assessed by Western blotting and concentrations of soluble TNFR1 (sTNFR1) in culture medium were analyzed by ELISA. Tear fluid from patients with Sjögren syndrome and graft-versus-host disease (GVHD) was collected and analyzed by ELISA for sTNFR1 concentration. Five dry eye patients underwent topical treatment using diquafosol sodium eye drops, a purinergic P2Y2 receptor agonist, and the tear fluid of the patients was sampled before and 4 weeks after the treatment for sTNFR1 ELISA. Results.: Phosphorylation of IκB was diminished by adding UTP or PGN, and this down-regulation of IκB phosphorylation was reversed by adding TAPI. In HCEC medium, sTNFR1 release was increased significantly by adding UTP or PGN, and inhibited significantly by adding TAPI. In the tears of the patients with Sjögren syndrome and GVHD, sTNFR1 expression was upregulated. In the tears of the patients with short breakup time (BUT) dry eye, sTNFR1 concentrations (ng/mL) in the tears were 1.30 ± 0.58 ng/mL for the pretreatment baseline, and 1.64 ± 0.70 after treatment, statistically significantly higher than those for the pretreatment (P < 0.01). Conclusions.: Ectodomain shedding of sTNFR1 blocked TNF-α–induced intracellular signaling in corneal epithelium. The upregulation of sTNFR1 in inflamed ocular surfaces suggests an anti-inflammatory role of sTNFR1 ectodomain shedding at the ocular surface.