THE FAS-FAS LIGAND SYSTEM AND OTHER MODULATORS OF APOPTOSIS IN THE CORNEA

Purpose. Previous studies have suggested chat the disappearance of ant erior keratocytes after injury to the overlying epithelium is mediated by apoptosis. The authors examined the expression of the apoptosis-re lated modulators, Fas (receptor), Fas ligand, Bar, Bcl-2, Bcl-X(L), an d interleukin-1 beta converting enzyme (ICE) in corneal cells as candi date mediators of this response and tested the effect of Fas receptor- stimulating antibody on corneal stromal fibroblast cells in vitro. Met hods. Reverse-transcription-polymerase chain reaction was used to dete ct FAS, FAS ligand, Bar, Bcl-2, Bcl-X(L), and ICE mRNA expression in p rimary cultures of human corneal epithelial, stromal fibroblast, and e ndothelial cells. Immunohistochemistry was applied to detect Fas and F as ligand proteins in fresh-frozen sections of normal human cornea. Th e effect of FAS-stimulating monoclonal antibody on first-passage strom al fibroblasts was studied using a DNA fragmentation assay, the live-d ead assay with fluorescent microscopy, toluidene blue staining with li ght microscopy, and electron microscopy. Results. FAS, Fas ligand, Bax , Bcl-2, Bcl-X(L), and ICE mRNAs are expressed in all three major cell types of the cornea. Fas protein is expressed in corneal epithelial, keratocyte, and endothelial cells in fresh-frozen human cornea. Fas li gand protein, however, was detected in corneal epithelial and endothel ial, but not keratocyte, cells. Fas-stimulating antibody induced first -passage stromal fibroblast cell death with morphologic changes and DN A fragmentation consistent with apoptosis. Conclusions. The Fas system (Fas and Fas ligand) modulators and final common pathway mediators of apoptosis are expressed in corneal cells. The distribution of Fas (ep ithelial, keratocyte, and endothelial cells) and Fas ligand (epithelia l and endothelial cells) protein expression in fresh-frozen corneal ti ssue suggests that Fas ligand expressed in corneal epithelial and endo thelial cells modulates functions in keratocyte cells and, possibly, a utocrine-juxtacrine functions in epithelium and endothelium. The Fas-F as ligand system is expressed in the cornea and could have important f unctions in normal corneal physiology and in the pathophysiology of co rneal disease, including modulation of keratocyte apoptosis after epit helial injury.