Abstract
Purpose :
In many models and cases of photoreceptor degeneration, the inner retina remains intact and contains an assortment of histologically normal-looking immune cells. However, it remains unclear if these immune cells are indeed functionally normal microglia. Here, we used immunohistochemistry, in vivo imaging, and single cell sequencing to investigate the identity and functional state of immune cells remaining in the inner retina after photoreceptor loss.
Methods :
All mice were handled according to ARVO, NIH, and UC Davis IACUC guidelines. We used immunohistochemistry, in vivo imaging, and single cell RNA sequencing to investigate immune cells (CD45+CD11b+) in murine retinas before and after loss of photoreceptors in an inducible model of rapid photoreceptor degeneration (Arr1-/- mice, 0 hours and 1+ weeks of ambient light exposure, respectively).
Results :
Previously, we found that during photoreceptor loss peripheral monocytes infiltrate the retina and transform into macrophages. Here, we report that after photoreceptor degeneration is complete, the inner retina is tiled with a population of immune cells consisting of both resident microglia and peripheral macrophages that have adopted a ramified, microglia-like phenotype. This new population of resident macrophages maintain a subset of gene and protein expression indicative of a macrophage lineage, such as MHCII-related genes, for at least several weeks, allowing us to distinguish them from the resident microglial lineage population. The transcriptomes of these macrophages, as examined using single cell sequencing, are distinct from macrophage transcriptomes previously investigated during the loss of photoreceptors. Instead, after photoreceptor loss they are more similar to the transcriptomes of resident microglia.
Conclusions :
Peripheral monocytes that enter the retina during photoreceptor degeneration can differentiate into long-lived macrophages that take up residence in the inner retina, joining the population of resident microglia. The peripheral macrophages that persist in the retina adopt a generally more microglia-like phenotype. However, these cells do maintain expression of some macrophage-lineage markers, and the immune population remains heterogeneous several weeks after onset of photoreceptor degeneration.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.