In the present study, we report that human TMSCs preferentially localize to the TM region and maintain viability for at least 4 months after transplantation into mouse anterior chamber. The cells integrate into the TM, do not elicit inflammatory response, nor do they cause an increase in IOP. An intrinsic property of adult stem cells is to identify and localize in specific tissues where they exhibit tissue-specific differentiation.
19–22 The behavior of TMSCs in the anterior chamber is clearly distinct from that of corneal fibroblasts and has all the aspects of a classic homing response typical of adult stem cells. The ability of TMSCs to home to the TM and become differentiated TM cells even in a normal TM without damage is a novel and important observation. It opens a door to explore stem cell-based therapy for glaucoma.
Most stem cells are slow cycling, retaining DNA label over an extended pulse-chase period. This phenomenon is termed “label-retaining cells.”
15,23 Lipophilic cell membrane dyes, DiO, DiI,
24 or PKH26
25 have also been used to identify slow-cycling stem cells. We show here that cloned TMSCs retain DiO during in vitro culture and express stem cell markers Notch1, ABCG2, and MUC1. These cells also exhibit the ability to efflux DNA-binding dye Hoechst 33342, a stem cell–specific cellular function that is widely used as the basis of a technique for isolation of side population stem cells by FACS.
17 Hoechst efflux confirms TMSCs as stem cells and the slow-cycling property can be used to explore the location and function of endogenous TMSCs in vivo.
Most importantly, our data provide clear evidence that TMSCs, after anterior chamber injection, are able to home to the TM region (
Figs. 2,
5) maintaining viability for at least 4 months (
Figs. 5,
6). In contrast, injected fibroblasts attached to a variety of tissues after injection into the anterior chamber. These results support the idea that localization in the TM is not simply a function of the TMSCs being carried passively by aqueous outflow, but a result of a tissue affinity of the TMSCs. TMSCs also appeared in the iris (
Figs. 2A,
2E), a tissue with similar developmental origin to TM. Increasingly abundant evidence supports the ability of mesenchymal stem cells to localize and regenerate damaged tissue in vivo (see review articles by Kang et al.
21 and by Fong et al.
26 ). We thus believe that in glaucomatous eyes, injected TMSCs may be able to localize to pathological TM and improve aqueous outflow. In glaucomatous eyes, abnormal extracellular matrix of the TM may have effect on stem cells' homing. Our next step is to explore the homing and function of TMSCs in a mouse glaucoma model.
Mesenchymal-like stem cells have been shown with the ability to mediate immunosuppression.
13,27–30 Our current study confirms that xenotransplantation of human TMSCs to mouse anterior chamber does not elicit inflammatory response (
Fig. 4). It ensures the survival of transplanted stem cells to function in vivo. This observation provides an argument that these stem cells could be tolerated in human allogeneic transplantation. The ability of TMSCs to undergo extensive expansion in vitro makes allogeneic transplantation possible. Since glaucoma has underlying genetic components, it would not seem feasible to do autologous transplantation using the same genetically abnormal cells. The expansion ability of TMSCs provides a possibility to regenerate TM in glaucomatous eyes by allogeneic transplantation of TMSCs without glaucomatous genetic disorders.
Chitinase 3-like 1 (CHI3L1) has been identified as a marker of TM cells
10 (Liton PB, et al.
IOVS 2009;50:ARVO E-Abstract 4859). CHI3L1 can directly interact with type I collagen that plays an important role in CHI3L1 tissue-remodeling activity.
31 Our results demonstrate that a proportion of the transplanted TMSCs expressed the TM marker CHI3L1 (
Fig. 7). It clearly indicates that the transplanted TMSCs function in vivo instead of simply attaching to the tissue, which could become a barrier for outflow and cause increase of IOP.
We also show that TMSCs can be expanded in vitro and retain DiO labeling, expressing stem cell markers ABCG2, Notch1, and MUC1. After in vivo injection, the TMSCs home to the TM region and iris while fibroblasts attach to the TM, iris, corneal endothelium, and epithelium of the lens (
Fig. 2). The inner layer of the TM, the uveal meshwork, is formed by prolongations of connective tissue arising from the iris and ciliary body stromas,
32 which are derived from neural crest cells. Since the TM and the iris have the same origination, it is not surprising that injected TMSCs can home to both TM and iris. We would assume that TMSCs would home to the damaged TM tissue but not the iris if they were introduced into the eye with TM damage. The injected cells do not affect the corneal transparency and do not cause increased intraocular pressure dramatically. Xenotransplantation of TMSCs does not elicit an inflammatory response nor stimulate endogenous TM cell division. The endogenous TM cells in the eyes with the transplanted TMSCs were quiescent with no BrdU incorporation (
Fig. 8). Although there were more injected green cells in the TM at 4 weeks than at 1 week after injection, there should not be a concern because IOP was not increased and neither the injected TMSCs nor the endogenous TM cells were actively replicating. Since segmental flow exists in mouse eyes (Swaminathan SS, et al.
IOVS 2009;52:ARVO E-Abstract 6620), it is possible that more TMSCs localize in some regions of the TM than other regions. We did not compare the same region of the mouse eyes at different time points. Comparatively, the mouse TM cells in the eyes with the fibroblast-transplantation were stimulated to divide at 1 week represented by BrdU incorporation (
Fig. 8C). FACS sorted DiO+ green cells from fibroblast-injected TM tissues expressed both human and mouse TBP (
Fig. 6). This phenomenon may be related to the increased dividing of resident mouse cells after fibroblast injection and the dividing cells fused together. Fibroblast transplantation can also result in cell apoptosis as shown in
Figure 5 with a positive TUNEL result. Inflammatory response related to fibroblast injection may induce cell apoptosis. This suggests that stem cells are a more appropriate option for cell-based therapy than fibroblasts.
Our demonstration of a resident stem cell population in the TM and their slow-cycling property raises questions as to their roles in vivo. It seems possible that, like dermis, intestine, or corneal epithelium, the TM is a self-renewing tissue, supported by a resident population of stem cells. Such a hypothesis is supported by a recent study of gene expression in TM showing expression of cell cycle and proliferation related genes.
33 Future studies elucidating factors controlling TMSC proliferation in vivo might allow development of a pharmacological approach focusing on TM repopulation in eyes with increased intraocular pressure.
An exciting potential application of TMSCs is developing cell-based therapy for glaucoma. The ability of TMSCs introduced into the anterior chamber to home to the TM and adopt a TM phenotype supports the idea that the TM in eyes with high IOP may be restored via such an approach. With the ability to alter the cellular composition of the TM, it will be possible to investigate the mechanism by which aqueous outflow is controlled by the metabolic activity of the TM cells. Information revealed by such studies can point the way to design a cell based-therapy approach to regulate aqueous outflow through the TM.