Regulation of endotoxin-induced keratitis by PECAM-1, MIP-2, and toll-like receptor 4

Bacterial lipopolysaccharide (LPS, endotoxin) is a potent stimulator of inflammatory responses and is likely to contribute to microbial keratitis and to the pathogenesis of sterile corneal ulcers. The purpose of the present study was to identify specific mediators of endotoxin-induced keratitis.

The corneal epithelium of BALB/c, C3H/HeJ, and C3H/HeN mice was abraded, and Pseudomonas aeruginosa endotoxin (10 microg in 1 microL) was added. Stromal thickness and haze were measured by in vivo scanning confocal microscopy, and neutrophil recruitment determined by immunohistochemistry.

Pseudomonas endotoxin induced a significant increase in stromal thickness and haze compared with untreated control corneas at each time point examined, and the severity coincided with neutrophil infiltration into the corneal stroma. Furthermore, systemic depletion of neutrophils completely abrogated endotoxin-induced increases in stromal thickness and haze, indicating an essential role for these cells. Expression of platelet endothelial cell adhesion molecule (PECAM)-1 on vascular endothelium and production of macrophage inflammatory protein (MIP)-2 in the corneal stroma were also significantly elevated after exposure to endotoxin, and antibody blockade inhibited neutrophil recruitment to the cornea and abrogated endotoxin-induced increases in stromal thickness and haze. In LPS-hyporesponsive C3H/HeJ mice, PECAM-1 and MIP-2 were not upregulated after exposure to endotoxin, and endotoxin-induced keratitis did not develop in these mice.

These findings demonstrate that endotoxin-induced keratitis is regulated by toll-like receptor-4 (TLR4)-dependent expression of PECAM-1 and MIP-2, which are essential for recruitment of neutrophils to this site and for development of endotoxin-induced stromal disease.