Fate Mapping In Vivo to Distinguish Bona Fide Microglia Versus Recruited Monocyte-Derived Macrophages in Retinal Disease.

With the new understanding that adult microglia in mice have embryonic origins and are maintained in situ throughout life, it has become pertinent to now understand how these unique cells differ from monocyte-derived macrophages. The latter are recruited into the neural retina (and elsewhere in CNS) in certain diseased states, such as in various forms of retinal degeneration. However, phenotypic markers expressed by microglia and monocyte-derived macrophages largely overlap, thereby making it technically challenging to distinguish the two cell types in disease. To address this problem in mice, we have established an in vivo fate mapping system that enables distinguishing these two cell types in retinal disease models. Our approach leverages the seminal work that originally developed Cx3cr1-CreER mice and is based on commercially available mouse strains. Here, we detail our protocol and how to apply this fate mapping method paired with flow cytometry (or immunohistochemistry) to faithfully distinguish and examine microglia vs. monocyte-derived macrophages in a mutually exclusive manner. This approach will henceforth empower new efforts to identify functional specializations of these two populations in the pathobiology of retinal degenerative diseases and possibly other conditions of the retina where monocyte recruitment is observed, such as in glaucoma, diabetic retinopathy, ischemia reperfusion, retinal detachment, and so on.