Abstract
In the present study we tested the responsiveness of human corneal epithelial cells (HCECs) and corneal fibroblasts to lipopolysaccharide (LPS), a Toll-like receptor (TLR) 4 ligand. Purified Pseudomonas aeruginosa LPS was used to stimulate telomerase-immortalized HCECs (HUCL) and stromal fibroblast (THK) cell lines. Exposure of cells to LPS induced a time-dependent activation of NF-kappaB in THK but not in HUCL cells, as assessed by an increase in IkappaB-alpha phosphorylation and degradation. Concomitant with NF-kappaB activation, LPS-treated THK cells, but not HUCL cells, produced a significantly larger number of cytokines than control untreated cells. A cell surface biotinylation assay revealed that HUCL cells express TLR4 intracellularly, whereas TLR5 is expressed on the cell surface. Furthermore, reverse transcriptase-PCR analysis revealed that HUCL and primary HCECs, in contrast to THK cells, do not express myeloid differentiation (MD)-2. Thus, our results demonstrate that the LPS unresponsiveness of HCECs might be due to deficient expression of MD-2, an essential component for LPS-TLR4 signaling.