Abstract
Purpose :
What maintains the retinal microglia niche? Does retina substantially determine the niche, or do niche occupants regulate the niche? We studied this question by altering the retina, and altering the occupants of the niche.
Methods :
Mononuclear cells in the retinal niche were altered by 3 strategies: 1) ablation of retinal microglia (MG) using mice whose CX3CR1-YFP-creER-iDTA cells are depleted by Tamoxifen; 2) depletion of retinal dendritic cells in CD11c-DTR/GFP mice by treatment with diphtheria toxin; 3) radiation damage of retinal mononuclear cells to limit their survival and responses. The retinal environment was stimulated by: 1) an optic nerve crush injury; or 2) spontaneous retinal dystrophy (LCA2).
Results :
1) Stability of the MG niche in normal retina was demonstrated by parabiosis using B6J mice and Bactin-GFP mice. Chimerism was established in blood and bone marrow, but after 6 months the parabionts showed an average of less than one CD11b+Bactin/GFP+ cell/retina. 2) Injury or stress change the niche. Near total depletion of MG by tamoxifen treatment of CX3CR1-YFP-creER-iDTA x CD11c-DTR/GFP mice led to repopulation of retina in 47 days by CD11b mononuclear cells that were >70% GFPhiYFPhi, unlike the YFPhiGFPlo cells that are dominant in normal retina. An optic nerve crush injury 5 or 10 days after TAM-depletion promoted recruitment of retinal CD45medCD11b+GFPhiYFPhi DC that were similar to MG by conventional markers. 3) In radiation BM chimeras made with B6J recipients of CD11c-DTR/GFP BM, host retinas were 40% chimeric by 10 weeks. Conversely, an optic nerve crush 5 wks post-BM grafting led to >80% retinal chimerism 1 week later. 4) Recruited CD45medGFPhi cells were nearly indistinguishable from MG, except for stronger APC function than MG. Recruited GFPhi cells sustained retinal health, and produced neurotrophic factors associated with MG.
Conclusions :
Recruited CD45medGFPhi mononuclear cells in retina were very similar to retinal MG. Although recruited cells adopted MG-like morphology and expressed similar macrophage markers, they were not bona fide MG since their origin was adult BM, not embryonic yolk sac. Since the recruited cells are not MG, the term "pseudo-MG" may be a more accurate designation for cells recruited into retina. The similarities of pseudo-MG to resident MG suggest that the retinal environment largely determines the morphology, phenotype and function of recruited mononuclear cells.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.