Bone marrow-derived cells home to and regenerate retinal pigment epithelium after injury. Invest Ophthalmol Vis Sci

Abstract

PURPOSE. To determine whether hematopoietic stem and progenitor cells (HSCs/HPCs) can home to and regenerate the retinal pigment epithelium (RPE) after induced injury. METHODS. Enriched HSCs/HPCs from green fluorescent protein (gfp) transgenic mice were transplanted into irradiated recipient mice to track bone marrow-derived cells. Physical damage was induced by breaching Bruch's membrane and inducing vascular endothelial growth factor A (VEGFa) expression to promote neovascularization. RPE damage was also induced by sodium iodate injection (40 mg/kg) into wild-type or albino C57Bl/6 mice. Cell morphology, gfp expression, the presence of the Y chromosome, and the presence of melanosomes were used to determine whether the injured RPE was being repaired by the donor bone marrow. 2 Attempts to repair the RPE include transplantation of RPE cells into the subretinal space. Animal studies, RPE transplantation in humans, and macular relocation surgery have all shown that replacing diseased RPE with healthier RPE can rescue photoreceptors, prevent further visual loss, and even promote visual improvement. RESULTS. 3,4 Also, recent work on human RPE patch graft transplantation demonstrates survival and rescue of photoreceptors for a substantial time after grafting and holds some promise. Rescue of RPE and photoreceptors beyond the area of donor cell distribution suggests that diffusible factors are also involved in the rescue process. However, some problems exist, including the ability to obtain an adequate source of autologous RPE and that homologous cells have been associated with rejection. Fetal or adult transplanted RPE cells attach to Bruch's membrane with poor efficiency and do not proliferate. 5 These transplantation procedures are complex, associated with high complication rates, and often result in only short-term success. RPE integrity is an essential component for retinal function and visual health. The RPE consists of a monolayer of cuboidal cells that separates the photoreceptors and the choroid. 8 Adult stem cells of the bone marrow include hematopoietic stem cells (HSCs), which are multipotent and have been shown to transdifferentiate into multiple tissues such as endothelium, 9,10 epithelium, 9 -11 myocardium, 12 and liver. 13,14 These cells represent a renewable source of cells within our bodies and harnessing the regenerative ability of the HSCs may aid in the cure of degenerative diseases. In our model, the bone marrow transplantation was performed using CD117 (c-kit) cells. This technique enriches for HSCs and hematopoietic progenitor cells (HPCs). 15 In addition to HSC/HPC transdifferentiation, the ability for the HSCs/HPCs to home to an area of injury is a quintessential characteristic. When injury occurs, cytokines and chemokines are released into the blood, causing an inflammatory response. HSCs/HPCs home along this chemokine gradient to repair areas of injury. Stromal cell-derived factor 1 (SDF-1) has been shown to be the primary cytokine for HSC/SPC mobilization. 16 -18 SDF-1 has also been shown to be upregulated in damaged tissues thus facilitating recruitment of stem-progenitor cells to promote repair. 21 In this article we examined two models of injury. Our first model consisted of physical damage to the RPE layer by needle rupture of Bruch's membrane and then injection of recombinant adenoassociated virus vascular endothelial growth factor A (rAAV-VEGFa) into the subretinal space. This leads to choroidal neovascularization