Abstract
BackgroundMicroglia 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.MethodsUse 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.ResultsWe 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.ConclusionsThe 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.
Highlights
Microglia are essential to the development of the central nervous system (CNS) and its homeostasis
Given the recent recognition of non-parenchymal CNS macrophages in meninges, perivascular spaces, and choroid plexus that are distinct from microglia [38], the contaminating presence of non-parenchymal myeloid cells may confuse the analysis of either the putative parenchymal microglia or the GFPhi myeloid cells we have found in the retina, optic nerve, and brain of CD11cDTR/green fluorescent protein (GFP) mice
The GFPhi perivascular macrophages found after an optic nerve crush (ONC) might be interpreted as showing their origin in the circulation, but our previous studies with parabiosis showed that recruitment into retina post-ONC was rare [37]
Summary
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. Innate immune cells mount an early response to stress, injury, and infection in central nervous system (CNS) tissue, including the retina [1], and are important contributors to CNS development and homeostasis [2,3,4]. A substantial literature attributes a wide range of innate immune functions in the CNS to microglia, the tissueresident myeloid cells of the CNS [5,6,7,8]. Most other tissue macrophages originate in bone marrow [14]. Regardless of their origin, these macrophages were recruited to and live in tissue niches in which multiple factors, including chemokines, cytokines, and corresponding receptors, play an important role in maintaining their presence and regulating their activity [15].
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