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Dale S Gregerson, Neal D Heuss, Mark J Pierson, Scott W McPherson; The retinal microenvironment induces microglia-like function, morphology, and phenotype in recruited mononuclear cells. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):3555. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Evidence has suggested that large influxes of recruited mononuclear cells into retina resolve by mechanisms that retain the MG once the recruited cells are gone. We have proposed that retina contains at least two distinct niches, one occupied by MG (N1) and one (or more) occupied by recruited cells (N2), with MG preserved in N1 by receipt of a specific signal(s). Recruited cells occupying N2 are proposed to lack expression of critical receptor(s) that would retain them in retina. These studies ask if recruited cells can occupy N1 in the absence of MG.
Transgenic mice expressing YFP and a Tamoxifen (TAM) receptor linked to cre in the chemokine receptor CX3CR1 were crossed with floxed diphtheria toxin A subunit so that CX3CR1+ cells suicide in the presence of TAM. These mice were crossed to CD11c-DTR/GFP mice in which dendritic cells constitutively express the diphtheria toxin receptor and GFP, allowing recruited cells to be tracked, analyzed, depleted, and/or distinguished from resident MG. Populations of retinal mononuclear cells were also sorted for analysis by single cell RT-PCR. Mice were given an optic nerve crush (ONC) to stimulate active recruitment of circulating mononuclear cells.
Flow cytometry and RT-PCR analysis of chemokines and receptors showed that recruited cells residing in retina become microglia-like, while newly recruited cells have a distinct chemokine receptor profile. Recruited mononuclear cells produced the same neurotrophic factors associated with microglia. Total, acute depletion of microglia led to slow, but substantial, reconstitution with recruited mononuclear cells. A unilateral ONC after TAM depletion strongly recruited mononuclear cells to the ipsilateral retina, with a smaller but significant influx to the contralateral retina. Fluorescence microscopy of whole mounts showed that reconstituting cells had a microglia-like distribution in both normal and injured retina.
Recruited mononuclear cells are nearly indistinguishable from microglia in short-term assays. A subset of recruited cells also expressed GFP from the CD11c-DTR/GFP transgene, similar to studies we have done in mice not depleted of microglia. Since recruited cells cannot, by strict definition, be called MG, the term "pseudo-microglia" may be a more accurate designation for the GFP- population in CD11c-DTR/GFP mice.
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