Proteases in Bacterial Keratitis

Purpose. To investigate the proteases that may contribute to corneal ulceration in bacterial keratitis, especially those caused by Pseudomonas aeruginosa. Methods. Culture supernatants of several bacteria were examined for protease by using gelatin and casein zymography. A minimum amount (1 μg) of highly purified pseudomonal elastase and alkaline protease was injected into rabbit corneas and the corneal lesions were evaluated. In pseudomonal keratitis models in rats, intracorneal proteases were analyzed with gelatin and casein zymography 12 to 24 hours after infection. Proteases derived from rat corneas and peritoneal exudate cells (mostly polymorphonuclear leukocytes [PMNs]) were similarly analyzed. Results. All tested strains of P. aeruginosa and Serratia marcescens produced at least one protease, whereas Staphylococcus aureus strains did not. Pseudomonal proteases elicited liquefactive necrosis with remarkable stromal swelling similar to that caused by serratial 56K protease. Intracorneal proteases produced during infection with P. aeruginosa included matrix metalloproteinase (MMP)-2, MMP-9, the activated forms of MMP-2 and MMP-9, and gelatinases with molecular weights of >200 kd, as well as caseinase with molecular weight of 25 kd. Conclusion. Proteases that may contribute to ulceration in the early stages of corneal infection with P. aeruginosa could be of bacterial or corneal cell origin. However, in the later stages when numerous PMNs accumulate in the cornea, PMN-derived proteases predominate as the pathogenic factor in tissue destruction.