Purpose: To determine the expression and function of toll-like receptors (TLRs) in human conjunctival, limbal and corneal epithelial cells.
Methods: Expression of TLRs was examined by real-time polymerase chain reaction, immunohistochemistry, and western blot analysis in human conjunctival, corneal and limbal epithelial cells and tissues. Ligand-stimulated nuclear factor kappaB activation; interleukin 6 and interleukin 8 protein secretion was measured in the cultured conjunctival and limbal epithelial cells by ELISA analysis.
Results: Expression of TLR1, 2, 3, 5, and 6 was found in all conjunctival and limbal epithelial cell samples analyzed by real time PCR and western blot. TLR4 and TLR9 transcripts were undetectable in some samples by real-time PCR. TLR7, 8 and 10 transcripts were not detected by real time PCR in any of the samples tested. TLR1, 2, 3, 4, and 5 proteins were found in conjunctival, limbal and corneal epithelium by immunohistochemistry. Cultured conjunctival epithelial cells expressed significantly lower levels of TLRs than uncultured conjunctival cells obtained by applying nitrocellulose paper to the bulbar conjunctival surface. Cultured limbal and conjunctival cells responded to stimulation by polyriboinosinic polyribocytidylic acid (poly[I:C]), palmitoyl-3-cysteine-serine-lysine-4 (Pam3CSK) and flagellin with increased secretion of IL-6 and IL-8 and the activation of NFkappaB. Peptidoglycans (PGN) and CpG DNA caused increased NFkappaB activity; however, only conjunctival epithelial cells showed increased cytokine secretion. Lipoteichoic acid (LTA) or lipopolysacchride (LPS) did not change cytokine secretion or NFkappaB levels in either cell type.
Conclusions: The TLRs found in human conjunctival and limbal epithelial cells provide a basis for responses to many common ocular pathogens. Although the mRNA and protein for TLR4 and TLR2 was found, neither conjunctival or limbal cells in culture responded to LPS or LTA stimulation.