The immune system in experimental Pseudomonas keratitis. Model and early effects.

A model to study the immune system in Pseudomonas keratitis was developed using defined flora rats (WAG/RijMCW) that have not been exposed to Pseudomonas aeruginosa. One group of rats was made immunocompetent towards P. aeruginosa by intraperitoneal injection of phenol-killed P. aeruginosa while a second group remained naive to this organism. Corneas of both groups were scratched centrally with a 21-g needle, before inoculation with 2 X 10(8) P. aeruginosa organisms. Corneas of control animals were either only scratched or only inoculated with the bacterium. At 18 hr, the naive animals were killed. In naive rat corneas, light and electron microscopy showed bacteria throughout the cornea, polymorphonuclear leukocytes (PMNs) distributed from the limbus towards the center, and little stromal degradation. In contrast, massive corneal degradation was observed in the immunocompetent rats; PMNs were present, but no bacteria were observed free in the stroma. The total acid protease content was higher in the immunocompetent than in the naive rat corneas, a possible reason for the observed difference in corneal degradation. This difference was not due to increased numbers of PMNs since nearly equal numbers of PMNs were counted after enzymatic disaggregation of both types of corneas. Glycogen-induced peritoneal PMNs from both types of rats migrated equally well towards P. aeruginosa culture media and media of corneas incubated with this bacterium. The authors conclude that immune recognition is (1) involved in the corneal host response to P. aeruginosa and (2) required for efficient phagocytosis by PMNs but not their recruitment.