Stem Cells Derived from Burned Skin: The Future of Burn Care

Autologous skin grafting is the gold standard for wound coverage following debridement and cleaning. However, this method creates a new wound in a healthy area, bringing about complications of additional scarring, infection and pain. Autografting is also not sufficient for large area wounds where the amount of healthy intact skin to transplant is limited. There is therefore a need for an alternative wound coverage method that is easily accessible and promotes rapid healing.?

During skin healing, a myriad of cells is recruited to the site of injury to promote tissue reconstruction, including stem cells in the epithelium which differentiate into the different cell lineages of the skin. In the dermis, fibroblasts and mesenchymal progenitor cells contribute to its regeneration, through the formation of granulation tissue which provides a scaffold for epithelialization and neovascularization. Several studies and analyses have confirmed that this granulation tissue has a mesenchymal phenotype (positive CD73 and CD105 markers, negative CD34 and CD45 markers) and therefore suggests that mesenchymal stem cells (MSCs) can be used as a potential therapy to promote skin tissue regeneration. Additional studies have tested and confirmed this notion using MSCs sourced from bone marrow, adipose and umbilical cord tissue. However, these sources have limited availability, pose the risk of immunological rejection, come with ethical concerns and complex and invasive extraction methods, and therefore are not a feasible solution, especially for large-scale burn treatment. Skin mesenchymal cells on the other hand are more easily accessible, immune friendly and might be the ideal cell type for dermal reconstruction. In this study, the researchers aimed to source viable MSCs from severely burned skin that is normally discarded, and use them to promote regeneration and wound healing in full thickness burn wounds in mice and pigs.?

To do this, burned skin was cut, blood was removed and the tissue was washed in ethanol, followed by phosphate buffered saline. Burn-derived MSCs (BD-MSCs) were extracted using two methods: enzymatic cell extraction and conventional extraction. The former involved homogenizing tissue pieces and incubating them in an enzyme solution containing collagenase 1, dispase, trypsin and DMEM culture medium. The solution was then filtered and centrifuged and the remaining cell pellet was cultured in an enriched medium. The latter involved creating small scratches on the dermal surfaces of the tissue pieces and then culturing them in tissue culture plates with supplemented growth medium. This promoted the growth of new cells from the scratches, which adhered to the culture plate and continued to grow, even after the tissue was removed. The cells were then extracted upon reaching 80% confluence and underwent one to two passages. Both methods of cell extraction from burned tissue resulted in a similar quality of isolated viable MSCs, with flow cytometry confirming similar cell markers to mesenchymal cells (positive CD73 and CD105, negative CD34 and CD45) with no tumor-forming potential.?

Ten 6-8 week old nude mice were used to study the efficacy of the BD-MSCs in wound healing, where they each received two 6mm full thickness wounds on their backs. All wounds were secured with a sutured silicone ring to prevent healing through wound contraction. 5 of the mice received the control treatment- a meshed acellular scaffold dressing, composed of bovine collagen and silicone (Integra) supplemented with 100μl Matrigel, while the other 5 mice were treated with the same amount of Matrigel containing 110,000 BD-MSCs on each wound. The mice were then monitored for 12 days to allow wounds to fully close and then sacrificed for analysis. The scar area and surrounding tissue of all wounds were excised for histological and immunohistochemistry analysis. ?

The mice treated with BD-MSCs displayed visibly faster healing and wound closure, with reduced scar formation, smaller wound sizes, and thinner keratinocyte layer. Granulation tissue formation and proliferation started at about 7 days post wounding in this treatment group, and by day 12, they had passed the peak proliferation phase and transitioned into remodeling, unlike the control group, which was still undergoing peak proliferation by day 12. BD-MSCs were still present within the newly formed tissue even after complete wound closure and re-epithelialization and did not cause any adverse side effects in the mice.?

A similar experiment was then performed on four male Yorkshire pigs of roughly equal weight, where 5 x 5 cm full thickness burns were created and then treated with either acellular Integra dressings or with Integra containing 5.000, 200.000 or 400.000 BD-MSCs per sq. cm. Photos of the wounds were taken with each dressing change over a 3-week period and the rate of re-epithelialization and neovascularization?was monitored.?The wounds treated with BD-MSC exhibited an accelerated epithelialization time and a larger re-epithelialized area compared to the control, with a much higher number of blood vessels in the dermal component of the reconstituted skin. Like in the mouse models, the pigs did not react adversely to the BD-MSCs.?

These studies demonstrate that full thickness burned skin still contains useful viable cells and should not be completely discarded. Mesenchymal stem cells extracted from these debrided tissue pieces are biologically identical to MSCs derived from healthy skin tissue and do not pose the risk of immune rejection or tumor formation. These stem cells can also be easily and readily sourced, even from the patient's own burned skin, making it a more accessible and cost-effective cell source that doesn’t require further extraction from the patient such as in adipose or bone marrow-derived MSCs. Additionally, BD-MSCs do not raise ethical issues such as in embryonic or cadaverous stem cell extraction, and almost all burn patients would be willing to donate this debrided skin to assist in their treatment. This unexploited cell source is especially beneficial for burn patients with large area wounds, where the large amount of debrided skin corresponds with the higher amount of BD-MSCs that can be extracted and used to treat their wounds. BD-MSCs were also easily integrated within other wound dressing types including Matrigel and Integra, without significant loss in cell numbers or viability. This versatility gives more hope that they can easily be implemented with various biocompatible dressings already available on the market, eliminating the need for developing new methods of stem cell delivery to the wound. ?

Burn-derived stem cells are truly a?promising new source of skin stem cells for skin tissue regeneration, sourced from a readily available and unexploited commodity, and can potentially revolutionize our approach to wound care.?

Source: Amini-Nik, S., Dolp, R., Eylert, G., Datu, A.-K., Parousis, A., Blakeley, C. and Jeschke, M.G. “Stem cells derived from burned skin - The future of burn care,” EBioMedicine, vol. 37, pp. 509–520, Nov. 2018, doi: 10.1016/j.ebiom.2018.10.014.?