Concise Review: Epidermal Grafting: The Case for Pluripotent Stem Cells

Stability and biosafety of human epidermal stem cell for wound repair: preclinical evaluation

BACKGROUND

Cell therapy is a key technology to prevent sacrificing normal skin. Although some studies have shown the promise of human epidermal stem cells (EpiSCs), the efficacy, biosafety and quality control of EpiSC therapy have not been systematically reported.

METHODS

The biosafety, stemness maintenance and wound repair of EpiSC were systematically verified by in vitro and in vivo experiments. EpiSC were prepared from the foreskin using a collagen type IV rapid adherence method. The EpiSCs were identified by flow cytometry, immunofluorescence staining and cell morphology. The well-growing passage 1 (P1) EpiSCs were used to determine the proliferation curve (counting method). EpiSC clone formation assay was performed by Giemsa staining. Nude mice were used to prepare a full-thickness skin defect wound model to detect the repair effect of EpiSCs. The biosafety of EpiSCs was double tested in vitro and in vivo.

RESULTS

The results showed that the expression of specific markers and clone formation efficiency was stable when passage 1 (P1) to P8 cells were cultured, and the stemness rate of P8 cells was close to 85.1%. EpiSCs were expanded in vitro for 25 days, the number of cells reached 2.5 × 10⁸, and the transplantable area was approximately 75% of the total body surface area (TBSA). At 45 days, the total number of cells was approximately 30 billion, and the transplantable area was approximately the size of a volleyball court. A nude mouse wound model indicated that EpiSCs could rapidly close a wound. On postinjury day 7, the wound epithelialization rate in the cell transplantation group was significantly higher than that in the NaCl group (P < 0.05). In vitro, cell senescence increased, and telomerase activity decreased in P1 to P8 EpiSCs. In vivo, there were no solid tumors or metastatic tumors after EpiSC (P8) transplantation. In addition, the quality control of cultured cells met the clinical application criteria for cell therapy.

CONCLUSION

This preclinical study showed the stability and biosafety of human EpiSC therapy for wound repair.

Clinical Grade Human Pluripotent Stem Cell-Derived Engineered Skin Substitutes Promote Keratinocytes Wound Closure In Vitro

Chronic wounds, such as leg ulcers associated with sickle cell disease, occur as a consequence of a prolonged inflammatory phase during the healing process. They are extremely hard to heal and persist as a significant health care problem due to the absence of effective treatment and the uprising number of patients. Indeed, there is a critical need to develop novel cell- and tissue-based therapies to treat these chronic wounds. Development in skin engineering leads to a small catalogue of available substitutes manufactured in Good Manufacturing Practices compliant (GMPc) conditions. Those substitutes are produced using primary cells that could limit their use due to restricted sourcing. Here, we propose GMPc protocols to produce functional populations of keratinocytes and fibroblasts derived from pluripotent stem cells to reconstruct the associated dermo-epidermal substitute with plasma-based fibrin matrix. In addition, this manufactured composite skin is biologically active and enhances in vitro wounding of keratinocytes. The proposed composite skin opens new perspectives for skin replacement using allogeneic substitute.

KLF4 inhibition promotes the expansion of keratinocyte precursors from adult human skin and of embryonic-stem-cell-derived keratinocytes

Expanded autologous skin keratinocytes are currently used in cutaneous cell therapy, and embryonic-stem-cell-derived keratinocytes could become a complementary alternative. Regardless of keratinocyte provenance, for efficient therapy it is necessary to preserve immature keratinocyte precursors during cell expansion and graft processing. Here, we show that stable and transient downregulation of the transcription factor Krüppel-like factor 4 (KLF4) in keratinocyte precursors from adult skin, using anti-KLF4 RNA interference or kenpaullone, promotes keratinocyte immaturity and keratinocyte self-renewal in vitro, and enhances the capacity for epidermal regeneration in mice. Both stable and transient KLF4 downregulation had no impact on the genomic integrity of adult keratinocytes. Moreover, transient KLF4 downregulation in human-embryonic-stem-cell-derived keratinocytes increased the efficiency of skin-orientated differentiation and of keratinocyte immaturity, and was associated with improved generation of epidermis. As a regulator of the cell fate of keratinocyte precursors, KLF4 could be used for promoting the ex vivo expansion and maintenance of functional immature keratinocyte precursors.

Differentiation of nonhuman primate pluripotent stem cells into functional keratinocytes

BACKGROUND

Epidermal grafting using cells derived from pluripotent stem cells will change the face of this side of regenerative cutaneous medicine. To date, the safety of the graft would be the major unmet deal in order to implement long-term skin grafting. In this context, experiments on large animals appear unavoidable to assess this question and possible rejection. Cellular tools for large animal models should be constructed.

METHODS

In this study, we generated monkey pluripotent stem cell-derived keratinocytes and evaluated their capacities to reconstruct an epidermis, in vitro as well as in vivo.

RESULTS

Monkey pluripotent stem cells were differentiated efficiently into keratinocytes able to reconstruct fully epidermis presenting a low level of major histocompatibility complex class-I antigens, opening the way for autologous or allogeneic epidermal long-term grafting.

CONCLUSIONS

Functional keratinocytes generated from nonhuman primate embryonic stem cells and induced pluripotent stem cells reproduce an in-vitro and in-vivo stratified epidermis. These monkey skin grafts will be considered to model autologous or allogeneic epidermal grafting using either embryonic stem cells or induced pluripotent stem cells. This graft model will allow us to further investigate the safety, efficacy and immunogenicity of nonhuman primate PSC-derived epidermis in the perspective of human skin cell therapy.

Potential applications of keratinocytes derived from human embryonic stem cells

Although skin grafting is one of the most advanced cell therapy technique, wide application of skin substitutes is hampered by the difficulty in securing sufficient amount of epidermal substitute. Additionally, in understanding the progression of skin aging and disease, and in screening the cosmetic and pharmaceutical products, there is lack of a satisfactory human skin-specific in vitro model. Recently, human embryonic stem cells (hESCs) have been proposed as an unlimited and reliable cell source to obtain almost all cell types present in the human body. This review focuses on the potential off-the-shelf use of hESC-derived keratinocytes for future clinical applications as well as a powerful in vitro skin model to study skin function and integrity, host-pathogen interactions and disease pathogenesis. Furthermore, we discuss the industrial applications of hESC-derived keratinized multi-layer epithelium which provides a human-like test platform for understanding disease pathogenesis, evaluation of new therapeutic modalities and assessment of the safety and efficacy of skin cosmetics and therapeutics. Overall, we conclude that the hESC-derived keratinocytes have great potential for clinical, research and industrial applications.

Heterelogous expression of mutated HLA-G decreases immunogenicity of human embryonic stem cells and their epidermal derivatives

Human embryonic stem cells (hESCs) are capable of extensive self-renewal and expansion and can differentiate into any somatic tissue, making them useful for regenerative medicine applications. Allogeneic transplantation of hESC-derived tissues from results in immunological rejection absent adjunctive immunosuppression. The goal of our study was to generate a universal pluripotent stem cell source by nucleofecting a mutated human leukocyte antigen G (mHLA-G) gene into hESCs using the PiggyBac transposon. We successfully generated stable mHLA-G(EF1α)-hESC lines using chEF1α promoter system that stably expressed mHLA-G protein during prolonged undifferentiated proliferation andin differentiated embryoid bodies as well as teratomas. Morphology, karyotype, and telomerase activity of mHLA-G expressing hESC were normal. Immunofluorescence staining and flow cytometry analysis revealed persistent expression of pluripotent markers, OCT-3/4 and SSEA-4, in undifferentiated mHLA-G(EF1α)-hESC. Nucleofected hESC formed teratomas and when directed to differentiate into epidermal precursors, expressed high levels of mHLA-G and keratinocyte markers K14 and CD29. Natural killer cell cytotoxicity assays demonstrated a significant decrease in lysis of mHLA-G(EF1a)-hESC targets relative to control cells. Similar results were obtained with mHLA-G(EF1α)-hESC-derived epidermal progenitors (hEEP). One way mixed T lymphocyte reactions unveiled that mHLA-G(EF1a)-hESC and -hEEP restrained the proliferative activity of mixed T lymphocytes. We conclude that heterologous expression of mHLA-G decreases immunogenicity of hESCs and their epidermal differentiated derivatives.

Epidermal stem cells and their epigenetic regulation

Stem cells play an essential role in embryonic development, cell differentiation and tissue regeneration. Tissue homeostasis in adults is maintained by adult stem cells resident in the niches of different tissues. As one kind of adult stem cell, epidermal stem cells have the potential to generate diversified types of progeny cells in the skin. Although its biology is still largely unclarified, epidermal stem cells are widely used in stem cell research and regenerative medicine given its easy accessibility and pluripotency. Despite the same genome, cells within an organism have different fates due to the epigenetic regulation of gene expression. In this review, we will briefly discuss the current understanding of epigenetic modulation in epidermal stem cells.

Differentiation of human embryonic stem cells into clinically amenable keratinocytes in an autogenic environment

Human embryonic stem cells (hESCs)-derived keratinocytes hold great clinical and research potential. However, the current techniques are hampered by the use of xenogenic components that limits their clinical application. Here we demonstrated an efficient differentiation of H9 hESCs (H9-hESCs) into keratinocytes (H9-Kert) with the minimum use of animal-derived materials. For differentiation, we established two microenvironment systems originated from H9-hESCs (autogenic microenvironment). These autogenic microenvironment systems consist of an autogenic coculture system (ACC) and an autogenic feeder-free system (AFF). In addition, we showed a stage-specific effect of Activin in promoting keratinocyte differentiation from H9-hESCs while repressing the expression of early neural markers in the ACC system. Furthermore, we also explained the effect of Activin in construction of the AFF system made up of extracellular matrix similar to basement membrane extracted from H9-hESC-derived fibroblasts. H9-Kert differentiated in both systems expressed keratinocyte markers at mRNA and protein levels. H9-Kert were also able to undergo terminal differentiation in high Ca(2+) medium. These findings support the transition toward the establishment of an animal-free microenvironment for successful differentiation of hESCs into keratinocytes for potential clinical application.

Skin graft vascular maturation and remodeling: a multifractal approach to morphological quantification

OBJECTIVE

One important contributor to tissue graft viability is angiogenic maturation of the graft tissue bed. This study uses scale-invariant microvascular morphological quantification to track vessel maturation and remodeling in a split-thickness skin-grafting model over 21 days, comparing the results to classical techniques.

METHODS

Images from a previous study of split-thickness skin grafting in rats were analyzed. Microvascular morphology (fractal and multifractal dimensions, lacunarity, and vessel density) within fibrin interfaces of samples over time was quantified using classical semi-automated methods and automated multifractal and lacunarity analyses.

RESULTS

Microvessel morphology increased in density and complexity, from three to seven days after engraftment and then regressed by 21 days. Vessel density increased from 0.07 on day 3 to 0.20 on day 7 and then decreased to 0.06 on day 21. A similar trend was seen for the fractal dimension that increased from 1.56 at three days to 1.77 at seven days then decreased to 1.57 by 21 days. Vessel diameters did not change whereas complexity and density did, signaling remodeling.

CONCLUSIONS

This new automated analysis identified design parameters for tissue engraftment and could be used in other models of graft vessel biology to track proliferation and pruning of complex vessel beds.

25 years of epidermal stem cell research

This is a chronicle of concepts in the field of epidermal stem cell biology and a historic look at their development over time. The past 25 years have seen the evolution of epidermal stem cell science, from first fundamental studies to a sophisticated science. The study of epithelial stem cell biology was aided by the ability to visualize the distribution of stem cells and their progeny through lineage analysis studies. The excellent progress we have made in understanding epidermal stem cell biology is discussed in this article. The challenges we still face in understanding epidermal stem cells include defining molecular markers for stem and progenitor sub-populations, determining the locations and contributions of the different stem cell niches, and mapping regulatory pathways of epidermal stem cell proliferation and differentiation. However, our rapidly evolving understanding of epidermal stem cells has many potential uses that promise to translate into improved patient therapy.

Engineered tissue grafts: opportunities and challenges in regenerative medicine

The human body has limited regenerative capacity in most of the major tissues and organs. This fact has spurred the field of regenerative medicine, promising to repair damage following traumatic injury or disease. Multiple therapeutic strategies are being explored including small molecules, gene delivery, and stem cells; however, tissue engineering remains a primary approach to achieving regeneration. Organ transplantation demonstrates that damaged tissues can be replaced, but technology to regenerate complex organs de novo is not yet available. Instead, tissue engineering can augment the body's own regenerative ability by replacing tissue sections and enhancing the regenerative cascade. As a consequence of these opportunities, it is timely to review the criteria and current status of engineered tissue grafts designed as patches to replace or regenerate damaged or diseased tissue and restore organ function. This topic will be explored starting from the biomaterials and cells incorporated into the engineered graft, the environment into which the graft is implanted and the integration of the engineered graft with the host. Common issues will be addressed that are relevant to regeneration in multiple tissue and organ systems. Specific examples will focus on engineered grafts for myocardial and corneal repair to illustrate the tissue-specific challenges and opportunities and highlight the innovation needed as the field moves forward.