Abstract
Purpose: :
The presence of distinct populations of bone marrow (BM)-derived cells in the normal cornea has been firmly established and their roles in corneal disease are rapidly evolving. The characterization of their turnover rate has, thus far, solely relied on the analysis in BM-chimera mice requiring irradiation. The purpose of this study was to study the physiologic turnover rate of corneal BM-derived cells in a model of parabiosis (surgical joining of two mice to create a shared circulation).
Methods: :
Chimeric animals obtained by parabiosis, requiring neither irradiation nor transplantation, were generated between CD45.1 wild-type (WT) and CD45.2 transgenic β-actin GFP mice. The recruitment of circulating BM-derived cells to the cornea of parabionts was compared with that observed in whole-body irradiated mice that received a whole BM transplant with or without facial lead masks. At 1, 3, 6, 8 and 12 months post-surgery, the number of GFP+ or CD45.2+ cells in WT, and CD45.1+ cells in transgenic corneal whole-mounts were determined using confocal microscopy with or without immunofluorescence. BM-chimeric animals were analyzed 3 months post-reconstitution.
Results: :
The density of BM-derived cells in normal central, peripheral cornea, and the limbus was 175, 350, and 500 cells/mm2 respectively. Surprisingly, replacement of BM-derived cells for the central, peripheral cornea, and limbus in parabionts was 0%, 3% and 33% at 1 month, 0%, 7% and 50% at both 3 and 6 months, 5%, 11% and 50% at 8 months, and 7%, 12% and 50% at 12 months respectively. Replacement of BM-derived cells in BM-chimeric animals without lead shield at 3 months demonstrated 15% and 31% in the central and peripheral cornea respectively. In contrast, the replacement rate in BM-chimeric mice with lead masks was close to that of parabionts with 0% and 5% at 3 months. The majority of replaced cells resided in the anterior stroma with few cells present in the epithelium and posterior stroma. Overall 12% were CD11c+, 32% were CD11b positive and 25% were MHC-II+. The majority of turned over cells, however, did not co-stain for any of the above markers.
Conclusions: :
The current study demonstrates that contrary to the high turnover rate that was previously shown in BM-chimeric mice, the physiologic turnover of BM-derived cells from the blood is very low for the 12 months duration studied. Our data suggests that maintenance and local expansion of corneal BM-derived cells are largely dependent on the self-renewal or persistence of resident cells.
Keywords: antigen presentation/processing • cell survival • cornea: basic science