PROJECT SUMMARY Degenerative vision disorders are caused by the loss of retinal cells. In zebrafish, retinal Müller glial (MG) act as stem cells, generating MG-derived progenitor cells (MGPCs) which replace lost retinal cells. Mammalian MG/MGPCs can also produce new retinal neurons, however, proliferative capacity is limited and disease- relevant cell types (e.g., photoreceptors) are rarely generated. Our goal is to understand why the regenerative potential of MG/MGPCs are limited in mammals in comparison to a regenerative species (i.e. zebrafish). In zebrafish, widespread retinal cell loss triggers “developmental” regeneration, producing all major retinal cell classes. Conversely, selective retinal cell loss results in a “fate-biased” process where MGPCs give rise to the lost cell type. How MG sense the extent of loss, however, is unknown. As the immune system plays critical roles during retinal regeneration, we examined the impact of microglia reactivity on MG activation in zebrafish. We observed that microglia are required for MG stem cell activation and that immunosuppression can inhibit or enhance regeneration kinetics depending on the treatment timing. We hypothesize that microglia reactivity levels scale to the extent, duration, and/or specificity of retinal cell loss in order to coordinate MG activation, MGPC proliferation rates and fat decisions to the type of injury incurred. To test our hypothesis, here we propose to combine an improved targeted cell ablation system enabling titratable, sustainable, and selective retinal cell loss with resources/methods for labeling of immune cell types, intravital timelapse imaging, lineage tracing, and single cell transcriptomics. Specifically we will define how the extent (Aim 1), the duration (Aim 2) and specificity (Aim 3) of cone photoreceptor or retinal ganglion cell loss impact immune cell reactivity and retinal regeneration. These experiments will serve to define roles specific immune cell subtypes play in shaping MGPC proliferation and cell fates during regeneration in zebrafish.