Purpose: In Pseudomonas aeruginosa ocular infection, T-helper cell 1-responsive mouse strains are susceptible (the cornea perforates), and neutralization of IFN-gamma before infection has been shown to delay the onset of perforation. IFN-gamma is the predominant cytokine induced by IL-12, and positive regulation of IL-12 by IFN-gamma, if unchecked, leads to excessive cytokine production and toxicity. Despite its potential importance, the role of IL-12 in ocular infection with P. aeruginosa remains unexplored and was the purpose of this study.
Methods: IL-12 knockout mice, histopathology, RT/PCR and ELISA analyses, immunocytochemistry, and quantitation of viable bacteria in cornea were used to examine the role of IL-12 in IFN-gamma production and the susceptibility phenotype.
Results: To directly test the effect of IL-12 on IFN-gamma production, IL-12 knockout and wild-type C57BL/6 mice were used. Both groups of mice were susceptible to infection, with corneal perforation seen at 5 to 7 days after infection. RT-PCR and ELISA analyses confirmed that IL-12 message and protein levels were elevated after infection only in the wild-type mouse cornea. Other differences between the two groups were detected. Knockout versus wild-type mice showed a significant decrease in IFN-gamma mRNA levels in the cornea and cervical lymph nodes and decreased TNF-alpha protein levels in cornea. Corneas of knockout mice also had a significant increase in bacterial load at 5 days after infection when compared with wild-type mice.
Conclusions: These data provide evidence that IL-12 is important in IFN-gamma production and in the absence of the cytokine, both IFN-gamma and TNF-alpha levels in cornea are significantly decreased, resulting in unchecked bacterial growth and perforation.