Inflammatory Response to Lipopolysaccharide on the
Ocular Surface in a Murine Dry Eye Model
This study determined whether dry eye enhances expression of inflammatory mediators in response to topically applied TLR4 ligand.
Toll-like receptor 4 (TLR4) alerts cells to the presence of bacteria by initiating an inflammatory response. We hypothesize that disruption of the ocular surface barrier in dry eye enhances TLR4 signaling. This study determined whether dry eye enhances expression of inflammatory mediators in response to topically applied TLR4 ligand. A single dose of lipopolysaccharide (LPS) or vehicle (endotoxin-free water) was applied to the cornea of nonstressed (NS) mice or mice subjected to 5 days of desiccating stress (DS). After 4 hours, corneal epithelium and conjunctiva were extracted to analyze expression of inflammatory mediators via PCR. Protein expression was confirmed by immunobead assay and immunostaining. Topically applied LPS increased expression of inflammatory mediators IL-1b, CXCL10, IL-12a, and IFN-c in the conjunctiva, and IL-1b and CXCL10 in the cornea of NS mice compared to that in untreated controls. LPS in DS mice produced 3-fold increased expression of IL-1b in cornea and 2-fold increased expression in IL-12a in conjunctiva compared to that in LPS-treated control mice. LPS increased expression of inflammatory cytokines on the ocular surface. This expression was further increased in dry eye, which suggests that epithelial barrier disruption enhances exposure of LPS to TLR4þ cells and that the inflammatory response to endotoxin producing commensal or pathogenic bacteria may be more severe in dry eye disease.
In conclusion, this study demonstrates that not only is TLR4 capable of producing a proinflammatory response on the ocular surface but that response is enhanced during dry eye (DE) disease. TLR4 activation during DE appears to be important, especially in the initial stages of DE. In the cornea, disruption of the epithelium may expose additional TLR4 molecules, increasing susceptibility to LPS stimulation. In the conjunctiva, enhanced expression of IL-12a after TLR4 activation may promote Th1 differentiation, leading to more severe disease. Further research into amplification of inflammation by TLR4 activation in DE is needed. The role of commensal bacteria and endogenous TLR4 ligands in activation of TLR4 during DE also needs further exploration. Despite this, the proinflammatory reactions produced by TLR4 on the ocular surface illustrate that TLR4 is a potential pharmaceutical target for dry eye inflammation.