The type II transmembrane protein Fas ligand (Fasl) was first identified as a death receptor ligand that induced Fas+ target cells to undergo apoptosis. As such, its constitutive expression in the eye has historically been linked to the phenomenon of immune privilege and its ability to kill activated eye-infiltrating Fas+ effector cells, or eye-infiltrating Fas+ vascular endothelial cells. However, this notion is confounded by the fact that many non-hematopoietic cell types in the eye, including retinal ganglion cells (RGCs), constitutively express Fas. In fact, Fasl-mediated destruction of RGCs is a key factor in glaucoma pathogenesis, either by direct killing of RGCs and/or by inducing the production of proinflammatory chemokines by Fas+ glial cells (eg. astrocytes), that recruit proinflammatory cells to the retina and thereby causing neurotoxic inflammation. This apparent conundrum can be explained if one accepts our hypothesis that constitutive metalloproteinase-mediated cleavage of membraned-bound Fasl (mFasL), releases a soluble fragment (sFasL) that opposes the neurotoxic activity of mFasL. This premise is supported by preliminary data showing: (a) mice with a gene-targeted mutation of Fasl that eliminates this Fasl cleavage site (mFaslmice) develop accelerated glaucoma in spontaneous and inducible glaucoma models; {b} in healthy eyes, retinal Fasl is constitutively cleaved, but in glaucomatous eyes, retinal Fasl is membrane-bound; and (c) intravitreal injection of an AAV2-sFasL vector prior to disease onset can prevent the development of glaucoma, while injection of AAV2-sFasL after disease onset can reverse functional defects. Together, these data point to Fasl as an important therapeutic target for patients with glaucoma. However, a number of key questions remain unanswered and will be addressed by the proposed 3 specific aims: (Aim 1) When and how is Fasl cleavage suppressed during the development and progression of glaucoma and how do ADAM10 and TIMP1 in regulate Fasl cleavage ?; (Aim 2) To what extent does the direct engagement of Fas, expressed by astrocytes and/or RGCs, contribute to the development of glaucoma?; and (Aim 3) Can sFasL directly engage Fas to elicit a protective gene expression program? Our research strategy will involve both accepted and novel experimental tools, including (a) sortase-tagged-Fasl mice (provide by Dr. Ploegh}, that will greatly facilitate our ability to monitor mFasL vs sFasL protein levels in the eye, {b} Fas-flexed mice crossed to RGC- and astrocyte/muller-specific ere-deleter lines, that will allow us to identify the importance of these cells in the development of glaucoma; (c) allophenic (tetraparental) chimeric mice made by fusing Fas+ and Fasn•9 embryos, that will allow us to distinguish direct and indirect effects of Fasl engagement in the context of glaucoma, and {d} AAV2-sFasL vectors that will allow us to determine if sFasL functions independently of mFasL. The mechanistic insights gained f...