Turnover of resident retinal microglia in the normal adult mouse

Abstract

The retina contains two distinct populations of monocyte-derived cells: perivascular cells (macrophages) and parenchymal cells (microglia), important in homeostasis, neuroinflammation, degeneration, and injury. The turnover of these cells in the retina and their repopulation in normal physiological conditions have not been clarified. Bone marrow (BM) cells from EGFP-transgenic mice were adoptively transferred into lethally irradiated normal adult C57BL/6 mice. Eight, 14, and 26 weeks later mice were sacrificed and retinal flatmounts were prepared. Retinal microglia were identified by F4/80, CD45, and Iba-1 immunostaining. BrdU was injected into normal mice for 3-14 days and cell proliferation was examined by confocal microscopy of retinal flatmounts. Few (6.15 +/- 2.02 cells/retina) BrdU(+) cells were detected and of these some coexpressed CD11b (1.67 +/- 0.62 cells/retina) or F4/80 (0.57 +/- 0.30 cells/retina). BM-derived EGFP(+) cells were detected by 8-weeks post-transplantation. By 6 months, all retinal myeloid cells were EGFP(+). Consecutively, donor BM-EGFP(+) cells were demonstrated within the: (1) peripheral and juxtapapillary retina, (2) ganglion cell layer, (3) inner and outer plexiform layers, and (4) photoreceptor layer. EGFP(+) cells within the ganglion layer were amoeboid in shape and F4/80(high)CD45(high)Iba-1(high), whereas cells in the inner and outer plexiform layers were ramified and F4/80(low) CD45(low)Iba-1(low). Perivascular macrophages expressed less F4/80, CD45, and Iba-1 compared with parenchymal microglia. Our results suggest that BM-derived monocyte precursor cells are able to migrate across the BRB and replace retinal microglia/macrophages. The complete replacement of retinal microglia/macrophages takes about 6 months. In situ proliferation was predominantly of nonhemopoetic retinal cells.

Microglia/Macrophages Migrate through Retinal Epithelium Barrier by a Transcellular Route in Diabetic Retinopathy

“…5,20 Microglia activation state was analyzed on flat mounts of the neuroretina double stained by glial fibrillary acidic protein (GFAP)/IBA-1 (Figure 7, A-H) and iNOS/IBA-1 (Figure 7, J-M). After 12 months of hyperglycemia in GK rats, activated microglial cells are identified as IBA-1-positive round amoeboid-shaped cells, located around blood vessels surrounded by GFAP-stained astrocytes ( Figure 7, B and D) compared with the control (Figure 7, A and C).…”

Neural inflammation and the microglial response in diabetic retinopathy

“…The hematopoietic origins of these cells suggest that they may be continuously in equilibrium with BM-derived precursors. A study using radiation and bone marrow transplantation (BMT) to create chimeric mice with green fluorescent protein-labeled monocytes suggested that the complete replenishment of retinal microglia by BM-derived monocytes takes approximately 6 months [15]. However, the radiation depletion process may accelerate microglial turnover, as another study suggested that replacement of retinal microglia by bloodborn monocytes is negligible unless retinal damage occurs [16].…”

Privilege revisited: an evaluation of the eye's defence mechanisms

“…In fact, the retina contains a population of bone marrowderived cells, known as microglia, which increase in number with age 51 and are constantly replenished from precursor cells. 52 In order to reach the retinal parenchyma, microglia must penetrate the BRB and clearly do so at a constant low frequency throughout life. Furthermore, recent studies have revealed a small population of professional antigen-presenting cells (dendritic cells), which express high levels of MHC Class II and reside in the retinal periphery and around the optic nerve (Figure 7).…”