The Innate Immune System Regulates Stem Cell Responsiveness During Zebrafish Retinal Regeneration

Abstract

Zebrafish replace lost retinal cells via activation of a potentially conserved vertebrate retinal stem cell type, Müller glia. We hypothesize that the innate immune system plays a key role in regulating Müller glia responsiveness to retinal cell death, as occurs during degenerative disease, thereby impacting the regenerative potential of retinal stem cells. To test this, we visualized immune cell subtypes via intravital imaging following induction of selective rod photoreceptor loss. Time-lapse imaging and immunolabeling showed that macrophages and microglia showed immune cell hallmarks consistent with reactivity to rod cell death. However, whereas microglia acted within the retina directly, macrophages were restricted to the extraocular space. Microglia activation was characterized by translocation toward the rod cell layer, proliferation, and phagocytosis of dying rod cells. To test the role of microglia during regeneration, we co-ablated microglia/rod cells or applied immune suppression, and characterized the kinetics of: (1) rod cell clearance, (2) stem cell proliferation, and (3) rod cell regeneration. The data revealed that the rate of stem cell proliferation and rod cell replacement were dependent on the presence of microglia, establishing a role for this innate immune cell subtype in regulating retinal regeneration. Additionally, characterization of the retinal milieu following rod cell ablation indicated a complex inflammatory response. Determining how innate immune cells shape retinal stem cell responsiveness will help to inform therapeutic strategies—e.g., modulating cytokine signaling to promote stem cell proliferation—aimed at reversing vision loss caused by degenerative retinal conditions.