Soluble FasL Is Critical in Triggering Protective Anti–Bacterial Inflammation in the Eye

Abstract

Abstract: : Purpose: We previously demonstrated that FasL was required to clear bacteria and resolve inflammation associated with endophthalmitis. An inoculum of 500 CFU of S. aureus induced mild inflammation in C57BL/6J mice that completely cleared the bacterial infection. By contrast, a similar infection in FasL–defective gld mice produced massive inflammation and uncontrolled bacterial growth. The disease observed in gld mice was similar to that observed in C57BL/6J mice inoculated with 5,000 CFU of S. aureus. Since we recently observed that FasL is predominantly expressed in the eye in the soluble form (sFasL), we hypothesized that sFasL is critical in the development of a successful anti–bacterial inflammatory response that eliminates bacteria without destroying normal ocular tissue. To test this hypothesis we examined sFasL in the eyes of mice that were either destroyed, or recovered from a bacterial infection of the posterior segment.Methods: C57BL/6J mice received intravitreal inoculations of either 500, or 5,000 CFU of S. aureus. Daily slit lamp examinations and histological sections were used to assess inflammation. Fas ligand expression was assessed at 0, 24, 48, and 72 hours post infection. Total protein lysates were prepared from the posterior segment of the infected eyes and processed for FasL analysis by Western Blot. Results: Groups of C57BL/6J mice received an intravitreal inoculum of either 500, or 5,000 CFU of S. aureus. Within 24 hrs post infection, both groups of mice had a similar inflammatory response and bacterial load. However, sFasL was only present in mice receiving 500 CFU. By 48 hrs post infection there was a dramatic difference between the groups of mice. Eyes destined to resolve the infection (500 CFU) displayed an increasing amount of sFasL, a mild inflammation, and lower bacterial load. Eyes destined to be destroyed by the infection (5,000 CFU) displayed no sFasL, increasing inflammation, and an increased bacterial load. By 72 hrs post inoculation, the infection was resolved in eyes receiving 500 CFU. By contrast, in eyes receiving 5,000 CFU there was uncontrolled bacterial growth, and massive inflammation. Conclusions: Soluble FasL is critical in successfully resolving bacterial infections in the posterior of the eye. We predict that sFasL triggers innate inflammatory cells resulting in phaygocytic cells that are more effective in clearing bacteria.

Keywords: immunomodulation/immunoregulation • endophthalmitis • inflammation