Impaired Fas-Fas Ligand Interactions Result in Greater Recurrent Herpetic Stromal Keratitis in Mice

Fas/FasL Contributes to HSV-1 Brain Infection and Neuroinflammation

The Fas/FasL pathway plays a key role in immune homeostasis and immune surveillance. In the central nervous system (CNS) Fas/FasL is involved in axonal outgrowth and adult neurogenesis. However, little is known about the role of the Fas/FasL pathway in herpes encephalitis. In this study, we used a neuropathogenic clinical strain of herpes simplex virus type 1 (HSV-1) to explore infection-induced inflammation and immune responses in the mouse brain and the role of Fas/FasL in antiviral CNS immunity. HSV-1 CNS infection induced the infiltration of Fas- FasL-bearing monocytes and T cells in the brain and also to an up-regulation of Fas and FasL expression on resident astrocytes and microglia within infected sites. Upon infection, Fas- and FasL-deficient mice (lpr and gld) were partially protected from encephalitis with a decreased morbidity and mortality compared to WT mice. Fas/FasL deficiency promoted cell-mediated immunity within the CNS. Fas receptor stimulation abrogated HSV-1 induced activation and inflammatory reactions in microglia from WT mice, while lack of Fas or FasL led to a more pronounced activation of monocytes and microglia and also to an enhanced differentiation of these cells into a pro-inflammatory M1 phenotype. Furthermore, the specific immune system was more efficient in Fas- and FasL-deficient mice with significantly higher numbers of infiltrating HSV-1-specific cytotoxic T cells in the brain. Our data indicate that the Fas/FasL pathway leads to excessive neuroinflammation during HSV-1 infection, which is associated with a diminished anti-viral response and an excessive neuroinflammation.

The FasLane to ocular pathology-metalloproteinase cleavage of membrane-bound FasL determines FasL function

Fas ligand (FasL) is best known for its ability to induce cell death in a wide range of Fas-expressing targets and to limit inflammation in immunoprivileged sites such as the eye. In addition, the ability of FasL to induce a much more extensive list of outcomes is being increasingly explored and accepted. These outcomes include the induction of proinflammatory cytokine production, T cell activation, and cell motility. However, the distinct and opposing functions of membrane-associated FasL (mFasL) and the C-terminal soluble FasL fragment (sFasL) released by metalloproteinase cleavage is less well documented and understood. Both mFasL and sFasL can form trimers that engage the trimeric Fas receptor, but only mFasL can form a multimeric complex in lipid rafts to trigger apoptosis and inflammation. By contrast, a number of reports have now documented the anti-apoptotic and anti-inflammatory activity of sFasL, pointing to a critical regulatory function of the soluble molecule. The immunomodulatory activity of FasL is particularly evident in ocular pathology where elimination of the metalloproteinase cleavage site and the ensuing increased expression of mFasL can severely exacerbate the extent of inflammation and cell death. By contrast, both homeostatic and increased expression of sFasL can limit inflammation and cell death. The mechanism(s) responsible for the protective activity of sFasL are discussed but remain controversial. Nevertheless, it will be important to consider therapeutic applications of sFasL for the treatment of ocular diseases such as glaucoma.

Soluble Fas ligand blocks destructive corneal inflammation in mouse models of corneal epithelial debridement and LPS induced keratitis

Neutrophil-mediated inflammation plays a critical role in corneal damage following injury or infection. Previous studies demonstrated that membrane-bound FasL (mFasL) induces neutrophil chemokine production. However, the extracellular domain of mFasL is normally cleaved by matrix metalloproteinases to release a soluble form of FasL (sFasL) and sFasL antagonizes mFasL-mediated chemokine production. Therefore, we hypothesized that sFasL could be used to prevent neutrophil-mediated corneal inflammation associated with injury and bacterial keratitis. To test this hypothesis, GFP-only, sFasL-GFP, or mFasL-GFP were expressed in the corneal stroma of C57BL/6 mice, using intra-stromal injections of plasmid DNA or adenoviral vectors (AV) and the role of mFasL and sFasL in corneal inflammation was examined in models of corneal injury and LPS-induced keratitis. Our work addresses an important area of disagreement in the field of FasL, with regard to the mechanism by which sFasL regulates ocular inflammation. Herein, we demonstrate that an intrastromal injection of GFP-only, sFasL-GFP, or mFasL-GFP plasmid DNA resulted in GFP expression throughout the corneal stroma for up to two weeks with little to no evidence of inflammation in the GFP-only and sFasL-GFP groups and mild corneal inflammation in the mFasL-GFP group. Similarly, following epithelial debridement, corneas expressing GFP-only or sFasL-GFP showed no significant signs of corneal inflammation, with clear corneas at 15 days post debridement. By contrast, epithelial debridement of corneas expressing mFasL-GFP triggered persistent corneal inflammation and the development of central corneal opacities that was blocked by sFasL. Similar to the mFasL-GFP plasmid DNA, intrastromal injection of mFasL-GFP AV triggered mild corneal inflammation, but it was transient and resolved by day 10 with corneas remaining clear out to 30 days post injection. Nevertheless, intrastromal expression of mFasL-GFP AV exacerbated LPS-induced keratitis, corneal opacity, and neovascularization, while sFasL-GFP AV expression prevented LPS-induced keratitis, resulting in a clear cornea. Histological analysis of corneas with LPS-induced keratitis revealed a robust infiltration of macrophages and neutrophils and sFasL expression specifically blocked the neutrophil influx. Overall, our data demonstrate that stromal expression of mFasL is inflammatory, while sFasL is non-inflammatory, and opposes the effects of mFasL in mouse models of epithelial debridement and LPS-induced keratitis. These data demonstrate that a delicate balance between sFasL and mFasL regulates ocular inflammation. This study further identifies sFasL as a potent inhibitor of neutrophil-mediated corneal damage, and supports the potential use of sFasL in the treatment of neutrophil-mediated keratitis. These results strongly support the hypothesis that, in the immune privileged environment of the eye, the isoform of FasL regulates immune privilege and determines the extent of inflammation: mFasL promotes inflammation and sFasL blocks inflammation.