The influence of pancreas-derived stem cells on scaffold based skin regeneration

A Short History of Skin Grafting in Burns: From the Gold Standard of Autologous Skin Grafting to the Possibilities of Allogeneic Skin Grafting with Immunomodulatory Approaches

Due to groundbreaking and pioneering developments in the last century, significant improvements in the care of burn patients have been achieved. In addition to the still valid therapeutic standard of autologous split-thickness skin grafting, various commercially available skin substitutes are currently available. Significant progress in the field of tissue engineering has led to the development of promising therapeutic approaches. However, scientific advances in the field of allografting and transplant immunology are of great importance. The achievement of various milestones over the past decades has provided thought-provoking impulses in the field of skin allotransplantation. Thus, biologically viable skin allotransplantation is still not a part of the clinical routine. The purpose of this article is to review the achievements in burn surgery with regards to skin allotransplantation in recent years.

Down-Regulation of MHC Class I Expression in Human Keratinocytes Using Viral Vectors Containing US11 Gene of Human Cytomegalovirus and Cultivation on Bovine Collagen-Elastin Matrix (Matriderm®): Potential Approach for an Immune-Privileged Skin Substitute

Skin transplantation, especially in burn patients, is still challenging because surgeons are faced with limited disposability of autologous donor side material. The in vitro culture of keratinocytes has become an important reconstructive option. However, only non-immunogenic allogenic keratinocytes offer the opportunity to develop a skin graft that can overcome rejection. The purpose of the study was to develop targeted gene modification of keratinocytes in order to reduce immunogenicity for the use as allogenic transplantable skin graft by decreasing the expression of MHC class I. To reduce MHC class I expression, viral vectors containing the US11 gene of human cytomegalovirus were generated and tested on their functionality using Western blotting, indirect immunofluorescence staining, and flow cytometry. Transfected keratinocytes were seeded on commercially available bovine collagen-elastin matrices and further cultured for histological and cell survival assays. Results showed transient down-regulation of MHC class I after 24 h post-transfection, with recovery of MHC class I expression after 48 h. Histological assessments showed long-term cell survival as well as histological patterns comparable to epidermal layers of healthy human skin. The data postulates the potential application of US11 transfected keratinocytes as an approach towards an immune-privileged skin substitute. Nevertheless, further studies and data are needed.

Current and emerging vascularization strategies in skin tissue engineering

Vascularization is a key process in skin tissue engineering, determining the biological function of artificial skin implants. Hence, efficient vascularization strategies are a major prerequisite for the safe application of these implants in clinical practice. Current approaches include (i) modification of structural and physicochemical properties of dermal scaffolds, (ii) biological scaffold activation with growth factor-releasing systems or gene vectors, and (iii) generation of prevascularized skin substitutes by seeding scaffolds with vessel-forming cells. These conventional approaches may be further supplemented by emerging strategies, such as transplantation of adipose tissue-derived microvascular fragments, 3D bioprinting and microfluidics, miRNA modulation, cell sheet engineering, and fabrication of photosynthetic scaffolds. The successful translation of these vascularization strategies from bench to bedside may pave the way for a broad clinical implementation of skin tissue engineering.

LPS-Stimulated Human Skin-Derived Stem Cells Enhance Neo-Vascularization during Dermal Regeneration

High numbers of adult stem cells are still required to improve the formation of new vessels in scaffolds to accelerate dermal regeneration. Recent data indicate a benefit for vascularization capacity by stimulating stem cells with lipopolysaccharide (LPS). In this study, stem cells derived from human skin (SDSC) were activated with LPS and seeded in a commercially available dermal substitute to examine vascularization in vivo. Besides, in vitro assays were performed to evaluate angiogenic factor release and tube formation ability. Results showed that LPS-activated SDSC significantly enhanced vascularization of the scaffolds, compared to unstimulated stem cells in vivo. Further, in vitro assays confirmed higher secretion rates of proangiogenic as well as proinflammatoric factors in the presence of LPS-activated SDSC. Our results suggest that combining activated stem cells and a dermal substitute is a promising option to enhance vascularization in scaffold-mediated dermal regeneration.

Quantum Dots Do Not Alter the Differentiation Potential of Pancreatic Stem Cells and Are Distributed Randomly among Daughter Cells

With the increasing relevance of cell-based therapies, there is a demand for cell-labeling techniques for in vitro and in vivo studies. For the reasonable tracking of transplanted stem cells in animal models, the usage of quantum dots (QDs) for sensitive cellular imaging has major advances. QDs could be delivered to the cytoplasm of the cells providing intense and stable fluorescence. Although QDs are emerging as favourable nanoparticles for bioimaging, substantial investigations are still required to consider their application for adult stem cells. Therefore, rat pancreatic stem cells (PSCs) were labeled with different concentrations of CdSe quantum dots (Qtracker 605 nanocrystals). The QD labeled PSCs showed normal proliferation and their usual spontaneous differentiation potential in vitro. The labeling of the cell population was concentration dependent, with increasing cell load from 5 nM QDs to 20 nM QDs. With time-lapse microscopy, we observed that the transmission of the QD particles during cell divisions was random, appearing as equal or unequal transmission to daughter cells. We report here that QDs offered an efficient and nontoxic way to label pancreatic stem cells without genetic modifications. In summary, QD nanocrystals are a promising tool for stem cell labeling and facilitate tracking of transplanted cells in animal models.

Cultivation of keratinocytes and fibroblasts in a three-dimensional bovine collagen-elastin matrix (Matriderm®) and application for full thickness wound coverage in vivo

New skin substitutes for burn medicine or reconstructive surgery pose an important issue in plastic surgery. Matriderm^® is a clinically approved three-dimensional bovine collagen-elastin matrix which is already used as a dermal substitute of full thickness burn wounds. The drawback of an avital matrix is the limited integration in full thickness skin defects, depending on the defect size. To further optimize this process, Matriderm^® has also been studied as a matrix for tissue engineering of skin albeit long-term cultivation of the matrix with cells has been difficult. Cells have generally been seeded onto the matrix with high cell loss and minimal time-consuming migration. Here we developed a cell seeded skin equivalent after microtransfer of cells directly into the matrix. First, cells were cultured, and microinjected into Matriderm^®. Then, cell viability in the matrix was determined by histology in vitro. As a next step, the skin substitute was applied in vivo into a full thickness rodent wound model. The wound coverage and healing was observed over a period of two weeks followed by histological examination assessing cell viability, proliferation and integration into the host. Viable and proliferating cells could be found throughout the entire matrix. The presented skin substitute resembles healthy skin in morphology and integrity. Based on this study, future investigations are planned to examine behaviour of epidermal stem cells injected into a collagen-elastin matrix under the aspects of establishment of stem cell niches and differentiation.

Cellular modulation of polymeric device surfaces: promise of adult stem cells for neuro-prosthetics

Minimizing the foreign body response is seen as one critical research strategy for implants especially when designed for immune-privileged organs like the brain. The context of this work is to improve deep brain stimulating devices used in a consistently growing spectrum of psychomotor and psychiatric diseases mainly in form of stiff electrodes. Based on the compliance match hypothesis of biocompatibility we present another step forward using flexible implant materials covered with brain cell-mimicking layers. We covered two types of flexible polyimide films with glandular stem cells derived from pancreatic acini. Using real time-PCR and fluorescent immunocytochemistry we analyzed markers representing various cell types of all three germ layers and stemness. The results demonstrate an unchanged differentiation potential of the polyimide fixated cells as measured by mRNA and protein level. Additionally we developed a fibrinous hydrogel coating to protect them against shear forces upon eventual implantation. By repeating previous analysis and additional metabolism tests for all stages we corroborate the validity of this improvement. Consequently we assume that a stem cell-containing cover may provide a native, fully and actively integrating brain-mimicking interface to the neuropil.

The role of single cell derived vascular resident endothelial progenitor cells in the enhancement of vascularization in scaffold-based skin regeneration

Increasing evidence suggests that vascular resident endothelial progenitor cells (VR-EPCs) are present in several organs, playing an important role in postnatal neovascularization. Here, we isolated and characterized VR-EPCs from cardiac tissue in vitro, evaluating their regenerative potential in vivo. VR-EPCs showed to be highly clonogenic and expressed several stem and differentiation markers. Under endothelial differentiation conditions, cells form capillary-like structures, in contrast to osteogenic or adipogenic differentiation conditions where no functional changes were observed. After seeding in scaffolds, cells were distributed homogeneously and directly attached to the scaffold. Then, cell seeded scaffolds were used to induce dermal regeneration in a nude mice full skin defect model. The presence of VR-EPCs enhanced dermal vascularization. Histological assays showed increased vessel number (p < 0.05) and cellularization (p < 0.05) in VR-EPCs group. In order to explore possible mechanisms of vascular regeneration, in vitro experiments were performed. Results showed that pro-angiogenic environments increased the migration capacity (p < 0.001) and ability to form capillary-like structures (p < 0.05) of VR-EPC. In addition, VR-EPCs secreted several pro-angiogenic molecules including VEGF and PDGF. These results indicate that a highly clonogenic population of VR-EPCs might be established in vitro, representing a new source for therapeutic vascularization in tissue engineering and regeneration.

Synergistic angiogenesis promoting effects of extracellular matrix scaffolds and adipose-derived stem cells during wound repair

Slow vascularization rate is considered one of the main drawbacks of scaffolds used in wound healing. Several efforts, including cellular and acellular technologies, have been made to induce vascular growth in scaffolds. However, thus far, there is no established technology for inducing vascular growth. The aim of this study was to promote the vascularization capacities of scaffolds by seeding adipose-derived stem cells (ADSCs) on them and to compare the vascularization capacities of different scaffolds seeded with ADSCs. Two kinds of extracellular matrix scaffolds (small intestinal submucosa [SIS] and acellular dermal matrix [ADM]) and a kind of composite scaffold (collagen-chondroitin sulfate-hyaluronic acid [Co-CS-HA]) were selected. Subcutaneous implantation analysis showed that the vascularization capacity of SIS and ADM was greater than that of Co-CS-HA. ADSCs seeded in SIS and ADM secreted greater amounts of vascular endothelial growth factor than those seeded in Co-CS-HA. In a murine skin injury model, ADSC-seeded scaffolds enhanced the angiogenesis and wound healing rate compared with the nonseeded scaffolds. Moreover, ADSC-SIS and ADSC-ADM had greater vascularization capacity than that of ADSC-Co-CS-HA. Taken together, these results suggest that ADSCs could be used as a cell source to promote the vascularization capacities of scaffolds. The vascularization capacities of ADSC-seeded scaffolds were influenced by both the vascularization capacities of the scaffolds themselves and their effects on the angiogenic potential of ADSCs; the combination of extracellular matrix scaffolds and ADSCs exhibited synergistic angiogenesis promoting effects.

Development of chitosan-collagen hydrogel incorporated with lysostaphin (CCHL) burn dressing with anti-methicillin-resistant Staphylococcus aureus and promotion wound healing properties

Methicillin-resistant Staphylococcus aureus (MRSA) have become increasingly prevalent as nosocomial pathogens, especially in burn patients, which is the leading cause of their death. A drug delivery system of chitosan-collagen hydrogel incorporated with lysostaphin (CCHL) based on the lysostaphin gauze was developed for MRSA infected burn wounds. CCHL scaffold consisted of numerous interconnected sphericles and tubular bodies with an average diameter of 100-200 µm, 20-60-fold swelling, high water retention capacity, and cell proliferation properties. The minimal inhibitory concentration of CCHL was 0.053 U/mL. By the second week after its application on MRSA infected third-degree burn wounds, no bacteria could be detected and the burn wounds had started healing. Therefore, CCHL should be studied further as a promising candidate of burn treatment dressing against MRSA infections for clinics.

Isolation and in vitro cultivation turns cells from exocrine human pancreas into multipotent stem-cells

Several research groups have reported on the existence and in vitro characterization of multipotent stem-cells from the pancreas. However, the origin of these cells remains largely unexplained. Here, we report that in vitro culturing itself can turn adult cells from human exocrine pancreas into a cell population with typical stem cell characteristics. A simple, yet reliable method enabled us to track cell fates: Combining automated continuous observation using time-lapse microscopy with immunocytochemical analyses, we found that a significant fraction of the pancreatic cells ( approximately 14%) can survive trypsination and displays a drastic change in the protein expression profile. After further cultivation, these cells give rise to a heterogeneous cell population with typical multipotent stem cell characteristics; i.e. they proliferate over long time periods and continuously give rise to specialized cells from at least two germ layers. Although we cannot exclude that a rare pre-existing stem cell-type also contributes to the final in vitro-population, the majority of cells must have been arisen from mature pancreatic cells. Our findings indicate that multipotent cells for regenerative medicine, instead of being laboriously isolated, can be generated in large amounts by in vitro de-differentiation.