Limbal stem cells: Central concepts of corneal epithelial homeostasis

A strong cohort of evidence exists that supports the localisation of corneal stem cells at the limbus. The distinguishing characteristics of limbal cells as stem cells include slow cycling properties, high proliferative potential when required, clonogenicity, absence of differentiation marker expression coupled with positive expression of progenitor markers, multipotency, centripetal migration, requirement for a distinct niche environment and the ability of transplanted limbal cells to regenerate the entire corneal epithelium. The existence of limbal stem cells supports the prevailing theory of corneal homeostasis, known as the XYZ hypothesis where X represents proliferation and stratification of limbal basal cells, Y centripetal migration of basal cells and Z desquamation of superficial cells. To maintain the mass of cornea, the sum of X and Y must equal Z and very elegant cell tracking experiments provide strong evidence in support of this theory. However, several recent stud-ies have suggested the existence of oligopotent stem cells capable of corneal maintenance outside of the limbus. This review presents a summary of data which led to the current concepts of corneal epithelial homeostasis and discusses areas of controversy surrounding the existence of a secondary stem cell reservoir on the corneal surface     

Regulation and clinical implications of corneal epithelial stem cells

The corneal epithelium is known to have a rapid self-renewing capacity. The major advance in the field of cornead epithelial cell biology in the last decade is the establishment of the location of corneal epithelial stem cells at the limbus, i.e., the junctional zone between the cornea and the conjunctiva. This concept has helped explain several experimental and clinical paradoxes, produced a number of important clinical applications, and spawned many other research studies. This unique enrichment of epithelial stem cells at a site anatomically separated from their transient amplifying cells makes the ocular surface an ideal model to study the regulation of epithelial stem cells. The present review includes data from more recent studies and lays out other areas for future investigation, especially with respect to the role of apoptosis and cytokine dialogue between limbal epithelial stem cells and their stromal microenvironment.Abbreviations EGF epidermal growth factor - EGFR epidermal growth factor receptor - bFGF basic fibroblast growth factor - HGF hepatocyte growth factor - IGF-I insulin-like growth factor type I - IL-1 interleukin 1 - K3 or K12 keratin type 3 or 12 - KGF keratinocyte growth factor - LIF leukemia inhibitory factor - PDGF platelet-derived growth factor - PKC protein kinase C - TGF- transforming growth factor- - TGF- transforming growth factor- - TPA phorbol ester tumor promoting agents     

骨髓间充质干细胞向角膜上皮细胞分化及其免疫调节作用

目的：探讨大鼠骨髓间充质干细胞（rBMMSCs）转分化为角膜上皮的潜能,并在体外共培养体系中研究rBMMSCs对促炎细胞因子干扰素-γ（IFN-γ）和肿瘤坏死因子-α（TNF-α）刺激下的人角膜上皮细胞（hCECs）的免疫调节作用。方法采用聚蔗糖梯密度离心法获得rBMMSCs,并通过上皮细胞培养微环境来诱导rBMMSCs分化为上皮样细胞。通过免疫组织化学方法鉴定CD29、CD34、CK5＆8和ZO-1等标记物在rBMMSCs及诱导的上皮样细胞中的表达。流式细胞术用来分析CD29/CD34的表达及细胞分化过程中表达量的变化。hCECs单独培养或与rBMMSCs共培养,并采用IFN-γ/TNF-α刺激24或48 h。通过流式细胞术来分析细胞间黏附分子-1（ICAM-1）于IFN-γ/TNF-α刺激前后在hCECs上的表达,并通过黏附分析实验验证rBMMSC条件培养基对单核细胞黏附于IFN-γ/TNF-α刺激后的hCECs的作用。多组间比较采用单因素方差分析（ANOVA）,两组间比较采用双侧t检验。结果成功分离rBMMSCs,细胞表达CD29,但不表达CD34。在上皮细胞培养条件中培养5 d,大约4﹪的rBMMSCs可分化为上皮样细胞。此类细胞失去了CD29的标志,转为表达CK5＆8和ZO-1。IFN-γ/TNF-α能显著上调hCECs中ICAM-1的表达,在IFN-γ/TNF-α处理24 h和48 h后,ICAM-1分别呈现10倍和8倍的升高,分别达到4524±554.2和3107±329.6（P=0.0025,0.0014）。但与MSC共同培养时,上调作用被显著抑制,ICAM-1平均值为1356±325.6（24 h）与1323±106.6（48 h）（P=0.0079,0.0024）。MSC条件培养基可显著抑制单核细胞对hCECs的黏附作用,黏附细胞数从（10.01±3.01）×10^3/ml细胞降至（2.21±0.19）×10^3/ml细胞（P=0.0271）。结论rBMMSCs可转分化为角膜上皮样细胞,并抑制由促炎细胞因子诱导的ICAM-1在hCECs上的表达,同时对促炎细胞因子诱导的单核细胞的黏附性具有抑制作用,提示BMMSCs具有在角膜炎症疾病和损伤修复中的治疗潜能。     

Isolation and therapeutic potential of human haemopoietic stem cells

The haemopoietic stem cell (HSC) has long been regarded as an archetypal, tissue specific, stem cell, capable of completely regenerating haemopoiesis after myeloablation. It has proved relatively easy to harvest HSC, from bone marrow or peripheral blood. In turn, isolation of these cells has allowed therapeutic stem cell transplantation protocols to be developed, that capitalise on their prodigious self renewal and proliferative capabilities. Ex vivo approaches have been described to isolate, genetically manipulateand expand pluripotent stem cell subsets. These techniques have been crucial to the development of gene therapy, and may allow adults to enjoy the potential advantages of cord blood transplantation. Recently, huge conceptual changes have occurred in stem cell biology. In particular, the dogma that, in adults, stem cells are exclusively tissue restricted has been questioned and there is great excitement surrounding the potential plasticity of these cells, with the profound implications that this has, for developing novel cellular therapies. Mesenchymal stem cells, multipotent adult progenitor cells and embryonic stem cells are potential sources of cells for transplantation purposes. These cells may be directed toproduce HSC, in vitro and in the future may be used for therapeutic, or drug development, purposes. This revised version was published online in July 2006 with corrections to the Cover Date.     

间充质干细胞治疗急性放射性损伤的研究

急性放射性损伤是组织损伤的一种重要类型,目前未有较理想的治疗方案。间充质干细胞（MSCs）能够多向分化、自我更新,且具有分泌多种细胞因子、抗炎、免疫调节等生物活性。其在促进组织修复的优势显而易见,而移植的时机、剂量长期以来莫衷一是。致瘤性等安全问题制约其临床研究的进一步开展。近年来,MSCs趋向于无细胞化移植取得了明显成效。这一研究新进展势必迎来急性放射性损伤治疗的新格局,本文对此研究现状及进展进行综述。     

A novel in vitro retinal differentiation model by co-culturing adult human bone marrow stem cells with retinal pigmented epithelium cells

Human retinal pigment epithelium (HRPE) cells are important in maintaining the normal physiology within the neurosensory retina and photoreceptors. Recently, transplantation of HRPE has become a possible therapeutic approach for retinal degeneration. By negative immunoselection (CD45 and glycophorin A), in this study, we have isolated and cultivated adult human bone marrow stem cells (BMSCs) with multilineage differentiation potential. After a 2- to 4-week culture under chondrogenic, osteogenic, adipogenic, and hepatogenic induction medium, these BMSCs were found to differentiate into cartilage, bone, adipocyte, and hepatocyte-like cells, respectively. We also showed that these BMSCs could differentiate into neural precursor cells (nestin-positive) and mature neurons (MAP-2 and Tuj1-positive) following treatment of neural selection and induction medium for 1 month. Furthermore, the plasticity of BMSCs was confirmed by initiating their differentiation into retinal cells and photoreceptor lineages by co-culturing with HRPE cells. The latter system provides an ex vivo expansion model of culturing photoreceptors for the treatment of retinal degeneration diseases.     

Enrichment of corneal epithelial stem/progenitor cells using cell surface markers, integrin alpha6 and CD71

Corneal epithelial stem cells are believed to reside in the basal layer of the limbal epithelium, but no definitive cell surface markers have been identified. For keratinocytes, stem/progenitor cells are known to be enriched by cell surface markers, integrin α₆ and CD71, as a minor subpopulation which shows high integrin α₆ and low CD71 expressions (α₆^bri/CD71^dim). In the present study, we investigated the possibility that corneal epithelial stem cells can be enriched by integrin α₆ and CD71. The α₆^bri/CD71^dim cells were separated by fluorescence-activated cell sorting, as a minor subpopulation of the limbal epithelial cells. They were enriched for relatively small cells, showing a higher clonogenic capacity and expression of stem cell markers, but a lower expression of differentiation markers, compared to other cell populations. The cells were localized immunohistochemically in the basal region of the limbal epithelium. These results indicate that the α₆^bri/CD71^dim subpopulation enriched corneal epithelial stem cells.     

间充质干细胞对新型冠状病毒肺炎免疫损伤潜在修复作用的研究进展

针对新型冠状病毒肆虐以及暂无特效药物治疗的情况,多地已开展间充质干细胞(MSCs)在新型冠状病毒感染重症救治方面的临床研究,在规范应用的前提下,经过严格的临床检验后,对若干重型患者进行治疗并取得了一定效果。MSCs能抑制免疫系统过度激活,通过改善微环境促进内源性修复、抑制肺部炎症的进展达到缓解呼吸窘迫症状的目的。本文就新型冠状病毒免疫损伤的发生机制、治疗现状以及MSCs在治疗新型冠状病毒感染的潜在治疗机制作一介绍。     

Staphylococcus aureus protein A induced inflammatory response in human corneal epithelial cells

In the present study, we examined the role of Staphylococcus aureus protein A (SpA) in inducing inflammatory response in human corneal epithelial cells (HCECs). Exposure of HCECs to SpA induces rapid NF-kappaB activation and secretion of proinflammatory cytokine/chemokines (TNF-alpha and IL-8) in both concentration and time-dependent manner. Challenge of HCECs with live SpA(-/-) mutant S. aureus strains resulted in significantly reduced production of the cytokines when compared to the wild-type S. aureus strain. SpA also elicited the activation of MAP Kinases P38, ERK, but not JNK, in HCECs. SpA-induced production of proinflammatory cytokine were completely blocked by the NF-kappaB and p38 inhibitors and partially inhibited by the Jnk inhibitor. Pretreatment with anti-TLR2 neutralizing antibody had no effect on SpA-induced inflammatory response in HCECs, suggesting that this response is independent of TLR2 signaling. Moreover, unlike TLR2 ligands, SpA failed to induce the expression of antimicrobial peptides (hBD2 and LL-37) in HCECs. These studies indicate that SpA is a S. aureus virulence factor that stimulates HCEC inflammatory response through a pathway distinct from TLR2 in HCECs.     

Co-culture of human bone marrow mesenchymal stem cells and macrophages attenuates lipopolysaccharide-induced inflammation in human corneal epithelial cells

Dry eye syndrome (DES) is considered as an ocular surface inflammatory disease. Previous studies have shown inflammation plays an important role in the progression and onset of DES. Co-culture of human bone marrow mesenchymal stem cells (HBMSCs) and macrophages showed immunomodulatory effects via regulation of cytokine regulation. Thus, the aim of this study was to investigate the effect of the interaction of these cells on in vitro DES model. The conditioned media (CM) from macrophages, HBMSCs, and HBMSCs + macrophages were treated to human corneal epithelial cells, which showed significant reduction in IL-1α and IL-1β expression levels in HBMSCs + macrophages group. Moreover, the IL-1 Receptor Antagonist (IL-1RA) was highly expressed in the CM from the HBMSCs + macrophages group. Wounded eyes of mice were treated with IL-1RA at 0–100 ng/mL for 16 h, the wound size was reduced. The results of this study might lead to the identification of new therapeutic targets for DES.     

In vitro differentiation of human mesenchymal stem cells to epithelial lineage

Our study examined whether human bone marrow-derived MSCs are able to differentiate, in vitro, into functional epithelial-like cells. MSCs were isolated from the sternum of 8 patients with different hematological disorders. The surface phenotype of these cells was characterized.To induce epithelial differentiation, MSCs were cultured using Epidermal Growth Factor, Keratinocyte Growth Factor, Hepatocyte Growth Factor and Insulin-like growth Factor-II. Differentiated cells were further characterized both morphologically and functionally by their capacity to express markers with specificity for epithelial lineage. The expression of cytokeratin 19 was assessed by immunocytochemistry, and cytokeratin 18 was evaluated by quantitative RT-PCR (Taq-man). The data demonstrate that human MSCs isolated from human bone marrow can differentiate into epithelial-like cells and may thus serve as a cell source for tissue engineering and cell therapy of epithelial tissue.     

Identification of novel epithelial stem cell-like cells in human deciduous dental pulp

It is well known that interactions between epithelial components and mesenchymal components are essential for tooth development. Therefore, it has been postulated that both types of stem cells might be involved in the regeneration of dental hard tissues. Recently, mesenchymal dental pulp stem cells that have odontogenic potential were identified from human dental pulp. However, the existence of epithelial cells has never been reported in human dental pulp. In the present study, we isolated and characterized epithelial cell-like cells from human deciduous dental pulp. They had characteristic epithelial morphology and expressed epithelial markers. Moreover, they expressed epithelial stem cell-related genes such as ABCG2, Bmi-1, ΔNp63, and p75. Taken together, our findings suggest that epithelial stem cell-like cells might exist in human deciduous dental pulp and might play a role as an epithelial component for the repair or regeneration of teeth.     

Reconstruction of corneal epithelium with cryopreserved corneal limbal stem cells in a goat model

We describe a procedure to construct an artificial corneal epithelium from cryopreserved limbal stem cells (LSCs) for corneal transplantation. The LSCs were separated from limbal tissue of male goats. The primary LSCs were identified by flow cytometry and were expanded. They were examined for stem cell-relevant properties and cryopreserved in liquid nitrogen. Cryopreserved LSCs were thawed and then transplanted onto human amniotic membrane, framed on a nitrocellulose sheet, to construct corneal epithelium sheets. The artificial corneal epithelium was transplanted into the right eye of pathological models of total limbal stem cell deficiency (LSCD). Then, the effects of reconstruction were evaluated by clinical observation and histological examination. Polymerase chain reaction analysis was used to detect the SRY gene. The data showed that transplantation of cryopreserved LSCs, like fresh LSCs, successfully reconstructed damaged goat corneal surface gradually, but the SRY gene expression from male goat cells could only be detected in the first 2 months after transplantation. The therapeutic effect of the transplantation may be associated with the inhibition of inflammation-related angiogenesis after transplantation of cryopreserved LSCs. This study provides the first line of evidence that cryopreserved LSCs can be used for reconstruction of damaged corneas, presenting a remarkable potential source for transplantation in the treatment of corneal disorders.     

In vitro cardiomyogenic potential of human umbilical vein-derived mesenchymal stem cells

Cardiomyocyte loss in the ischemically injured human heart often leads to irreversible defects in cardiac function. Recently, cellular cardiomyoplasty with mesenchymal stem cells, which are multipotent cells with the ability to differentiate into specialized cells under appropriate stimuli, has emerged as a new approach for repairing damaged myocardium. In the present study, the potential of human umbilical cord-derived mesenchymal stem cells to differentiate into cells with characteristics of cardiomyocyte was investigated. Mesenchymal stem cells were isolated from endothelial/subendothelial layers of the human umbilical cords using a method similar to that of human umbilical vein endothelial cell isolation. Isolated cells were characterized by transdifferentiation ability to adipocytes and osteoblasts, and also with flow cytometry analysis. After treatment with 5-azacytidine, the human umbilical cord-derived mesenchymal stem cells were morphologically transformed into cardiomyocyte-like cells and expressed cardiac differentiation markers. During the differentiation, cells were monitored by a phase contrast microscope and their morphological changes were demonstrated. Immunostaining of the differentiated cells for sarcomeric myosin (MF20), desmin, cardiac troponin I, and sarcomeric alpha-actinin was positive. RT-PCR analysis showed that these differentiated cells express cardiac-specific genes. Transmission electron microscopy revealed a cardiomyocyte-like ultrastructure and typical sarcomers. These observations confirm that human umbilical cord-derived mesenchymal stem cells can be chemically transformed into cardiomyocytes and can be considered as a source of cells for cellular cardiomyoplasty.     

Rat model of anal sphincter injury and two approaches for stem cell administration

AIM To establish a rat model of anal sphincter injury and test different systems to provide stem cells to injured area.METHODS Adipose-derived stem cells(ASCs) were isolated from BDIX rats and were transfected with green fluorescent protein(GFP) for cell tracking. Biosutures(sutures covered with ASCs) were prepared with 1.5 × 10~6 GFPASCs, and solutions of 10~6 GFP-ASCs in normal saline were prepared for injection. Anorectal normal anatomy was studied on Wistar and BDIX female rats. Then, we designed an anal sphincter injury model consisting of a 1-cm extra-mucosal miotomy beginning at the anal verge in the anterior middle line. The sphincter lesion was confirmed with conventional histology(hematoxylin and eosin) and immunofluorescence with 4', 6-diamidino-2-phenylindole(commonly known as DAPI), GFP and α-actin. Functional effect was assessed with basal anal manometry, prior to and after injury. After sphincter damage, 36 BDIX rats were randomized to three groups for:(1) Cell injection without repair;(2) biosuture repair; and(3) conventional suture repair and cell injection. Functional and safety studies were conducted on all the animals. Rats were sacrificed after 1, 4 or 7 d. Then, histological and immunofluorescence studies were performed on the surgical area.RESULTS With the described protocol, biosutures had been covered with at least 820000-860000 ASCs, with 100% viability. Our studies demonstrated that some ASCs remained adhered after suture passage through the muscle. Morphological assessment showed that the rat anal anatomy is comparable with human anatomy; two sphincters are present, but the external sphincter is poorly developed. Anal sphincter pressure data showed spontaneous, consistent, rhythmic anal contractions, taking the form of "plateaus" with multiple twitches(peaks) in each pressure wave. These basal contractions were very heterogeneous; their frequency was 0.91-4.17 per min(mean 1.6980, SD 0.57698), their mean duration was 26.67 s and mean number of peaks was 12.53. Our morphological assessment revealed that with the aforementioned surgical procedure, both sphincters were completely sectioned. In manometry, the described activity disappeared and was replaced by a gentle oscillation of basal line, without a recognizable pattern. Surprisingly, these findings appeared irrespective of injury repair or not. ASCs survived in this potentially septic area for 7 d, at least. We were able to identify them in 84% of animals, mainly in the muscular section area or in the tissue between the muscular endings. ASCs formed a kind of "conglomerate" in rats treated with injections, while in the biosuture group, they wrapped the suture. ASCs were also able to migrate to the damaged zone. No relevant adverse events or mortality could be related to the stem cells in our study. We also did not find unexpected tissue growths. CONCLUSION The proposed procedure produces a consistent sphincter lesion. Biosutures and injections are suitable for cell delivery. ASCs survive and are completely safe in this clinical setting.     

A scaffold-free in vitro model for osteogenesis of human mesenchymal stem cells

For studying cellular processes three-dimensional (3D) in vitro models are of a high importance. For tissue engineering approaches osseous differentiation is performed on 3D scaffolds, but material depending influences promote cellular processes like adhesion, proliferation and differentiation. To investigate developmental processes of mesenchymal stem cells without cell-substrate interactions, self-contained in vitro models mimicking physiological condition are required. However, with respect to scientific investigations and pharmaceutical tests, it is essential that these tissue models are well characterised and are of a high reproducibility. In order to establish an appropriate in vitro model for bone formation, different protocols are compared and optimised regarding their aggregate formation efficiency, homogeneity of the aggregates, the viability and their ability to induce differentiation into the osteogenic lineage. The protocols for the generation of 3D cell models are based on rotation culture, hanging drop technique, and the cultivation in non adhesive culture vessels (single vessels as well as 96 well plates). To conclude, the cultivation of hMSCs in 96 well non adhesive plates facilitates an easy way to cultivate homogenous cellular aggregates with high performance efficiency in parallel. The size can be controlled by the initial cell density per well and within this spheroids, bone formation has been induced.     

Defining the optimal cryoprotectant and concentration for cryopreservation of limbal stem cells

Limbal stem cell (LSC) deficiency causes progressive loss of vision but may be treated by transplant of autologous LSCs. Cryopreservation has the potential to indefinitely extend the lifespan of LSCs allowing re-transplant in case of graft failure. In this study, we aimed to identify the optimal cryoprotectant and cryoprotectant concentration for LSC cultures. Suspension cultures derived from cadaveric corneoscleral rims were cooled to 4 °C with Me₂SO, propylene glycol or ethylene glycol at a concentration of 5%, 10% or 15%. Cell tolerance was measured in terms of membrane integrity, colony-forming efficiency and alamarBlue^® reduction. Increasing cryoprotectant concentration above 5% reduced membrane integrity, metabolism and colony-forming efficiency. Cryoprotectant choice did not significantly influence these characteristics. Cells demonstrating Side Population were maintained after cryopreservation with 5% propylene glycol in vapour phase liquid nitrogen for 1 week, indicating that cryopreservation of LSCs with relatively low cryoprotectant concentration (5%) has promise in low-temperature eye banking.