Immunological challenges associated with artificial skin grafts: available solutions and stem cells in future design of synthetic skin

Hypoxic preconditioning of human urine-derived stem cell-laden small intestinal submucosa enhances wound healing potential

Background

Urine-derived stem cells (USCs) are a valuable stem cell source for tissue engineering because they can be harvested non-invasively. Small intestine submucosa (SIS) has been used as scaffolds for soft tissue repair in the clinic. However, the feasibility and efficacy of a combination of USCs and SIS for skin wound healing has not been reported. In this study, we created a tissue-engineered skin graft, termed the SIS+USC composite, and hypothesized that hypoxic preconditioning would improve its wound healing potential.

Methods

USCs were seeded on SIS membranes to fabricate the SIS+USC composites, which were then cultured in normoxia (21% O₂) or preconditioned in hypoxia (1% O₂) for 24 h, respectively. The viability and morphology of USCs, the expression of genes related to wound angiogenesis and reepithelialization, and the secretion of growth factors were determined in vitro. The wound healing ability of the SIS+USC composites was evaluated in a mouse full-thickness skin wound model.

Results

USCs showed good cell viability and morphology in both normoxia and hypoxic preconditioning groups. In vitro, hypoxic preconditioning enhanced not only the expression of genes related to wound angiogenesis (VEGF and Ang-2) and reepithelialization (bFGF and EGF) but also the secretion of growth factors (VEGF, EGF, and bFGF). In vivo, hypoxic preconditioning significantly improved the wound healing potential of the SIS+USC composites. It enhanced wound angiogenesis at the early stage of wound healing, promoted reepithelialization, and improved the deposition and remodeling of collagen fibers at the late stage of wound healing.

Conclusions

Taken together, this study shows that hypoxic preconditioning provides an easy and efficient strategy to enhance the wound healing potential of the SIS+USC composite.

Current Approaches Targeting the Wound Healing Phases to Attenuate Fibrosis and Scarring

Cutaneous fibrosis results from suboptimal wound healing following significant tissue injury such as severe burns, trauma, and major surgeries. Pathologic skin fibrosis results in scars that are disfiguring, limit normal movement, and prevent patient recovery and reintegration into society. While various therapeutic strategies have been used to accelerate wound healing and decrease the incidence of scarring, recent studies have targeted the molecular regulators of each phase of wound healing, including the inflammatory, proliferative, and remodeling phases. Here, we reviewed the most recent literature elucidating molecular pathways that can be targeted to reduce fibrosis with a particular focus on post-burn scarring. Current research targeting inflammatory mediators, the epithelial to mesenchymal transition, and regulators of myofibroblast differentiation shows promising results. However, a multimodal approach addressing all three phases of wound healing may provide the best therapeutic outcome.

Aging: A cell source limiting factor in tissue engineering

Tissue engineering has yet to reach its ideal goal, i.e. creating profitable off-the-shelf tissues and organs, designing scaffolds and three-dimensional tissue architectures that can maintain the blood supply, proper biomaterial selection, and identifying the most efficient cell source for use in cell therapy and tissue engineering. These are still the major challenges in this field. Regarding the identification of the most appropriate cell source, aging as a factor that affects both somatic and stem cells and limits their function and applications is a preventable and, at least to some extents, a reversible phenomenon. Here, we reviewed different stem cell types, namely embryonic stem cells, adult stem cells, induced pluripotent stem cells, and genetically modified stem cells, as well as their sources, i.e. autologous, allogeneic, and xenogeneic sources. Afterward, we approached aging by discussing the functional decline of aged stem cells and different intrinsic and extrinsic factors that are involved in stem cell aging including replicative senescence and Hayflick limit, autophagy, epigenetic changes, miRNAs, mTOR and AMPK pathways, and the role of mitochondria in stem cell senescence. Finally, various interventions for rejuvenation and geroprotection of stem cells are discussed. These interventions can be applied in cell therapy and tissue engineering methods to conquer aging as a limiting factor, both in original cell source and in the in vitro proliferated cells.

A three-dimensional human skin model to evaluate the inhibition of Staphylococcus aureus by antimicrobial peptide-functionalized silver carbon nanotubes

Objective

To investigate the toxicity and antibacterial application of antimicrobial peptide-functionalized silver-coated carbon nanotubes against Staphylococcus infection using a full thickness human three-dimensional skin model.

Materials and methods

The three-dimensional skin formation on the scaffolds was characterized by electron microscopy and investigation of several skin cell markers by real time–reverse transcriptase polymerase chain reaction. Functionalized silver-coated carbon nanotubes were prepared using carboxylated silver-coated carbon nanotubes with antimicrobial peptides such as TP359, TP226 and TP557. Following the characterization and toxicity evaluation, the antibacterial activity of functionalized silver-coated carbon nanotubes against Staphylococcus aureus was investigated using a bacterial enumeration assay and scanning electron microscopy. For this purpose, a scar on the human three-dimensional skin grown on Alvetex scaffold using keratinocytes and fibroblasts cells was created by taking precaution not to break the scaffold beneath, followed by incubation with 5 µg/mL of functionalized silver-coated carbon nanotubes re-suspended in minimum essential medium for 2 h. Post 2-h incubation, 200 µL of minimum essential medium containing 1 × 104 colony forming units of Staphylococcus aureus were incubated for 2 h. After incubation with bacteria, the colony forming unit/gram (cfu/g) of skin tissue were counted using the plate count assay and the samples were processed for scanning electron microscopy analysis.

Results

MTT assay revealed no toxicity of functionalized silver-coated carbon nanotubes to the skin cells such as keratinocytes and fibroblasts at 5 µg/mL with 98% cell viability. The bacterial count increased from 104 to 108 cfu/g in the non-treated skin model, whereas skin treated with functionalized silver-coated carbon nanotubes showed only a small increase from 104 to 105 cfu/g (1000-fold viable cfu difference). Scanning electron microscopy analysis showed the presence of Staphylococcus aureus on the non-treated skin as opposed to the treated skin.

Conclusion

Thus, our results showed that functionalized silver-coated carbon nanotubes are not only non-toxic, but also help reduce the infection due to their antibacterial activity. These findings will aid in the development of novel antibacterial skin substitutes.

Advances in the Biofabrication of 3D Skin in vitro: Healthy and Pathological Models

The relevance for in vitro three-dimensional (3D) tissue culture of skin has been present for almost a century. From using skin biopsies in organ culture, to vascularized organotypic full-thickness reconstructed human skin equivalents, in vitro tissue regeneration of 3D skin has reached a golden era. However, the reconstruction of 3D skin still has room to grow and develop. The need for reproducible methodology, physiological structures and tissue architecture, and perfusable vasculature are only recently becoming a reality, though the addition of more complex structures such as glands and tactile corpuscles require advanced technologies. In this review, we will discuss the current methodology for biofabrication of 3D skin models and highlight the advantages and disadvantages of the existing systems as well as emphasize how new techniques can aid in the production of a truly physiologically relevant skin construct for preclinical innovation.

Establishment of a complex skin structure via layered co-culture of keratinocytes and fibroblasts derived from induced pluripotent stem cells

Background

Skin is an organ that plays an important role as a physical barrier and has many other complex functions. Skin mimetics may be useful for studying the pathophysiology of diseases in vitro and for repairing lesions in vivo. Cord blood mononuclear cells (CBMCs) have emerged as a potential cell source for regenerative medicine. Human induced pluripotent stem cells (iPSCs) derived from CBMCs have great potential for allogenic regenerative medicine. Further study is needed on skin differentiation using CBMC-iPSCs.

Methods

Human iPSCs were generated from CBMCs by Sendai virus. CBMC-iPSCs were differentiated to fibroblasts and keratinocytes using embryonic body formation. To generate CBMC-iPSC-derived 3D skin organoid, CBMC-iPSC-derived fibroblasts were added into the insert of a Transwell plate and CBMC-iPSC-derived keratinocytes were seeded onto the fibroblast layer. Transplantation of 3D skin organoid was performed by the tie-over dressing method.

Results

Epidermal and dermal layers were developed using keratinocytes and fibroblasts differentiated from cord blood-derived human iPSCs, respectively. A complex 3D skin organoid was generated by overlaying the epidermal layer onto the dermal layer. A humanized skin model was generated by transplanting this human skin organoid into SCID mice and effectively healed skin lesions.

Conclusions

This study reveals that a human skin organoid generated using CBMC iPSCs is a novel tool for in-vitro and in-vivo dermatologic research.

Electronic supplementary material

The online version of this article (10.1186/s13287-018-0958-2) contains supplementary material, which is available to authorized users.

Tissue Engineered Skin Substitutes

The fundamental skin role is to supply a supportive barrier to protect body against harmful agents and injuries. Three layers of skin including epidermis, dermis and hypodermis form a sophisticated tissue composed of extracellular matrix (ECM) mainly made of collagens and glycosaminoglycans (GAGs) as a scaffold, different cell types such as keratinocytes, fibroblasts and functional cells embedded in the ECM. When the skin is injured, depends on its severity, the majority of mentioned components are recruited to wound regeneration. Additionally, different growth factors like fibroblast growth factor (FGF), epidermal growth factor (EGF), vascular endothelial growth factor (VEGF) are needed to orchestrated wound healing process. In case of large surface area wounds, natural wound repair seems inefficient. Inspired by nature, scientists in tissue engineering field attempt to engineered constructs mimicking natural healing process to promote skin restoration in untreatable injuries. There are three main types of commercially available engineered skin substitutes including epidermal, dermal, and dermoepidermal. Each of them could be composed of scaffold, desired cell types or growth factors. These substitutes could have autologous, allogeneic, or xenogeneic origin. Moreover, they may be cellular or acellular. They are used to accelerate wound healing and recover normal skin functions with pain relief. Although there are a wide variety of commercially available skin substitutes, almost none of them considered as an ideal equivalents required for proper wound healing.