PROJECT SUMMARY Microglia, the resident innate immune cells of the CNS, play essential roles in CNS development and homeostasis, and are key responders to neuronal stress, injury and disease, including in retina. While retinal microglia have been well characterized in rodent models, much less is known about the properties and responses of human retinal microglia, particularly in an in vivo context. To address this, we seek to establish a model to study human microglia in vivo in the context of the retina. Here we will introduce human induced pluripotent stem cell derived hematopoietic progenitor cells (iHPCs) into the newborn murine brain and assess engraftment of the resulting human xenotransplanted microglia (xMGs) in the retina. In the first aim, we will characterize the engraftment, including density and distribution, of xMGs into mouse retina and optic nerve, and will determine the impact on host retinal cells and tissue organization. In the second aim we will use single cell RNA sequencing to define the human xMG transcriptome, will compare gene expression to endogenous human microglia, and finally will analyze xMG responses via transcriptional changes to optic nerve crush, a well characterized axon injury model resulting in retinal ganglion cell degeneration. This study will provide critical foundational insight into the properties of human xMGs integrated into the mouse retina and test their responses to retinal ganglion cell injury, providing a potentially powerful resource for studying in vivo human retinal microglia responses to injury, disease or neurodegeneration.

Project Narrative Microglia, the resident innate immune cells of the CNS, have been implicated in multiple retinal degenerative diseases and are potential targets for treatment of ocular disorders, although the properties and responses of human retinal microglia are still being defined. The goal of this study is to develop a model to define properties of human microglia engrafted in the mouse retina and study their responses to a damaging insult in vivo as a step towards modeling their responses in retinal disease.