Background: Microglia are essential to the development of the CNS and its homeostasis. Our prior findings suggested a niche model to describe the behaviors of retinal microglia. Here, we ask whether new myeloid cells recruited to the retina are constrained to resemble endogenous microglia morphologically and functionally. Methods: Use of CD11cDTR/GFP transgenic mouse allowed identification of two niches of retinal microglia distinguished by being GFPlo or GFPhi. We also used transgenic mice in which CX3CR1+ cells expressed YFP and were depletable following tamoxifen-induced expression of diphtheria toxin subunit A. We employed several ablation and injury stimulation protocols to examine the origin and fate of myeloid cells repopulating the retina. Analysis of retinal myeloid cells was done by microscopy, flow cytometry, and qRT-PCR. Results: We found that the origin of new GFPhi and GFPlo myeloid cells in the retina of CD11cDTR/GFP mice, whether recruited or local, depended on the ablation and stimulation protocols. Regardless of origin, new GFPlo and GFPhi retinal myeloid cells were CD45medCD11b+Ly6G-Ly6CloIba1+F4/80+, similar to endogenous microglia. Following tamoxifen-induced diphtheria toxin ablation, myeloid cell repopulation differed in the retina compared to the brain and optic nerve. Stimulation of replacement GFPhi cells was substantially attenuated in repopulating retinas after tamoxifen-induced diphtheria toxin ablation compared to control or radiation-ablated mice. In radiation bone marrow chimeric mice, replacement GFPhi myeloid cells from the circulation were slow to repopulate the retina unless stimulated by an optic nerve crush injury. However, once stimulated, recruited GFPhi cells were found to concentrate on injured retinal ganglion cells and were morphologically similar to GFPhi cells in non-ablated control CD11cDTR/GFP mice. Conclusions: The results support the idea that GFPhi cells in the CD11cDTR/GFP mouse, whether recruited or from resident microglia, mark a unique niche of activated retinal myeloid cells. We conclude that the retinal environment has a potent influence on the function, morphology, and proliferative capacity of new myeloid cells regardless of their origin, compelling them to be equivalent to the endogenous microglia.
Bibliographical noteFunding Information:
The authors thank Mark J. Pierson for outstanding technical assistance, the Biomedical Image Processing Laboratory at the University of Minnesota for help with confocal microscopy, and Dr. Richard A. Lang for a helpful critique of the manuscript. This work was supported by the US National Institutes of Health (R01-EY021003 and R01-EY025209 to D.S.G., T32-EY07133-17 to U.L., and P30- EY011374), The Wallin Neuroscience Discovery Fund, Research to Prevent Blindness, Inc., and the Minnesota Lions Clubs.
This work was supported by the US National Institutes of Health (R01-EY021003 and R01-EY025209 to D.S.G., T32-EY07133-17 to U.L., and P30-EY011374), The Wallin Neuroscience Discovery Fund, Research to Prevent Blindness, Inc., and the Minnesota Lions Clubs.
© 2019 The Author(s).
- Injury response
- Myeloid cells