Monocytes of bone marrow (BM) origin are circulating precursors that replenish dendritic cells and macrophage populations in peripheral tissues during homeostasis. The eye provides a unique range of varying tissue microenvironments in which to compare the different turnover rates of monocyte-derived cells. This was investigated in the present study using radiation chimeras, whereby BM from Cx3cr1(+/gfp) mice was used to rescue myeloablated wild-type (WT) BALB/c mice (conventional chimeras). The use of Cx3cr1(+/gfp) mice as BM donors allowed the clear visualization of newly recruited monocyte-derived cells. Following BM reconstitution, mice were killed at 2, 4, 6, and 8 weeks, and wholemount ocular tissues were processed for immunohistochemistry and confocal microscopy. "Reverse" chimeras (WT into Cx3cr1(+/gfp)) were also created to act as a further method of cross-referencing cell turnover rates. In conventional chimeras, Cx3cr1(+/gfp) cells began repopulating the uveal tract (iris, ciliary body, choroid) 2 weeks post-transplantation with close to complete replenishment by 8 weeks. By contrast, the earliest recruitment of Cx3cr1(+/gfp) cells into the host retina occurred at 4 weeks. In reverse chimeras, a steady accumulation of host Cx3cr1(+/gfp) macrophages in the subretinal space of Cx3cr1(+/gfp) adult mice suggests that these cells arise from long-term resident microglia and not newly recruited WT donor cells. In summary, chimeric mouse models, in which lineage-specific cells carry a fluorescent reporter, have been used in the present study to visualize the turnover of monocyte-derived cells in different tissue compartments of the eye. These data provide valuable insights into differential monocyte turnover rates within a single complex organ.
Kezic, J. M., & Mcmenamin, P. (2008). Differential turnover rates of monocyte-derived cells in varied ocular tissue microenvironments. Journal of Leukocyte Biology, 84(3), 721-729. https://doi.org/10.1189/jlb.0308166