Cone photoreceptor ablation in microglia-deficient larval zebrafish retina elicits a regenerative response alongside a compensatory immune cell response

Emerging evidence implicates retinal microglia and inflammation as important components impacting the outcome of retinal regeneration, which is spontaneously achieved in zebrafish retina following acute damage but is limited or blocked in mammals. In this paper, we describe the regenerative response in the larval zebrafish retina following ablation of cone photoreceptors. To investigate the role of microglia in the regenerative response, we used both irf8st95 heterozygote (microglia-sufficient) and irf8st95 homozygous mutant (microglia-deficient) zebrafish. We compared multiple aspects of the regenerative response in irf8+/- and irf8-/- larval retinas, including entry of the Muller glia (MG) into the cell cycle, the amplification of MG-derived progenitor cell (MGPC) proliferation, inflammatory and glial reactivity-associated gene expression, and the regeneration of cones. We found only modest impacts to early and late stages of MGPC proliferation and to inflammatory gene expression in irf8 mutants, with no obvious impacts to the regeneration of cones. Notably, we detected a population of immune cells in irf8 mutants that emerged following cone ablation, which expanded in number then were reduced over time, following a trajectory similar to microglia-sufficient siblings but at markedly reduced abundance. The immune cells detected in irf8 mutants included a subset with L-plastin/4C4 antibody staining patterns different than those in microglia-sufficient siblings, suggesting distinct origins and/or phenotype compared to resident microglia in controls. Though strong conclusions about the role of microglia were limited due to the presence of such immune cell populations in irf8 mutants, our results are consistent with several reports that indicate a role for microglia and inflammation in regulating MGPC proliferation in the regenerating retina. Collectively considered with other reports, our results further indicate that compensatory responses, which may include different immune cells and/or signaling from other retinal cell types such as the Muller glia, are elicited in microglia-deficient retinas upon neuronal damage.