Effector mechanisms of protection against Pseudomonas aeruginosa keratitis in immunized rats

Pseudomonas aeruginosa is an opportunistic pathogen which causes sight-threatening corneal infections in humans. The purpose of this study was to evaluate various immunization routes that may provide protection against Pseudomonas keratitis and to define the molecular mechanisms involved in the protection. Sprague-Dawley rats (10 to 12 weeks old) were immunized using paraformaldehyde-killed P. aeruginosa (strain 6206) via oral, nasal, and intra-Peyer's patch (IPP) routes followed by an ocular topical booster dose. Scratched corneas were challenged with an infective dose of P. aeruginosa. Following clinical examination, eyes were enucleated for histology, polymorphonuclear leukocyte (PMN) quantitation, bacterial count, enzyme-linked immunosorbent assay, and RNase protection assay. PMN infiltration was higher early (4 h) during the infection in immunized rats than in nonimmunized rats. Later during the infection, the number of PMNs diminished in immunized rats while in nonimmunized animals the number of PMNs continued to increase. Bacteria were cleared much faster from immunized groups than from the nonimmunized group, and the nasally immunized group had the most efficacious response among the immunized groups. Nasal and IPP immunization groups had increased cytokine expression of interleukin-2 (IL-2) and IL-5 and differed from each other for IL-6. All three immunized groups had significantly reduced IL-1 beta levels when compared with the nonimmunized rats and a significantly altered profile for CINC-1 expression. This study has shown that the route of immunization modulates the inflammatory response to ocular P. aeruginosa infection, thus affecting the severity of keratitis and adverse pathology, with nasal immunization being the most effective.