Composite autologous-allogeneic skin replacement: development and clinical application

Treatment of burns with skin substitutes

More than 2 million persons sustain thermal injuries in the United States annually (Monafo and Crabtree, 1985) and more than 10,000 burn victims die (Collini and Kealey, 1989). The principal factors affecting mortality are the total area burned and the area of third degree (full thickness) burns (Tompkins et al., 1985) with wound sepsis being the leading cause of mortality. Early aggressive excision and immediate covering of the wounds improve survival (Herndon and Parks, 1986). Various biological and synthetic substrates have been employed to replace the injured skin. Most of these provide a permeability barrier which substitutes for the epidermal function of the lost skin. An ideal skin replacement should also provide a substitute for dermis, which provides both support and stability for the epidermal replacement and prevents wound contraction. The dermal and epidermal replacement should be firmly integrated by a complete basement membrane zone (BMZ).

Absence of tumorigenicity in athymic mice by normal human epidermal keratinocytes after culture in serum-free medium

The very rapid growth rate (1 population doubling/day) of normal human epidermal keratinocytes (HK) cultured in serum-free medium can be utilized for wound closure in burn treatment. However, rapid growth in vitro may present the possibility of neoplastic transformation. To investigate this possibility, HK were cultured from primary isolation to large populations in MCDB 153 medium supplemented with epidermal growth factor (EGF, 10 ng/ml), insulin (5 micrograms/ml), hydrocortisone (0.5 micrograms/ml), and Bovine Pituitary Extract (BPE, 70 micrograms/ml). HK were studied for their ability to form tumors in athymic mice after subcutaneous inoculation. Sixteen separate HK strains were inoculated from primary cultures, or from secondary cultures either before or after storage in liquid nitrogen. Transformed cell lines, SCC 13 and FL, derived from human epithelial carcinomata were used as controls for tumor formation. HK formed no tumors (0/79) after 26 weeks incubation, SCC 13 formed nodular tumors (3/5) after 20 weeks incubation, and FL formed tumors (5/5) after 4 weeks incubation. HK cells were not found by histological examination of inoculation sites of keratinocyte cultures derived from primary culture from skin. In contrast, palpable tumors from both SCC 13 and FL were returned to tissue culture and continued to proliferate. These results support the conclusion that the rapid growth rate of human epidermal keratinocytes in vitro can be attributed to permissive culture conditions, and not to neoplastic transformation.

In vivo optimization of a living dermal substitute employing cultured human fibroblasts on a biodegradable polyglycolic acid or polyglactin mesh

The design of a skin-substitute must address the need for a dermal component, as this mesenchymally-derived tissue is important in maintaining the integrity and function of skin. An in vivo study was undertaken to assess the use of two biodegradable meshes, polyglycolic acid and polyglactin-910, as carriers for cultured human fibroblasts in a living dermal replacement. The consistent vascularization and epithelialization of these grafts placed on athymic mice showed that this has potential in re-creating the dermis in a skin-substitute.

Synthetic membranes and cultured keratinocyte grafts

Significant progress has been made in skin replacement options in the past several decades. Although initially various materials have been used mainly for burn coverage, their application to dermatologic practice has increased significantly. We review the research, progress, and other aspects of wound coverage with synthetic membranes and cultured epithelial sheets in both burn and nonburn wound management.

Cultured epidermal autografts and allografts: a study of differentiation and allograft survival

Cultured epidermal sheets were examined before and at various times after grafting on skin ulcer beds. Before grafting, the sheet consisted of four to five layers of keratinocytes with incomplete differentiation. Ten days after grafting, graft recipient sites showed compact hyperkeratosis, a normal-appearing epidermis, and a flat dermoepidermal junction. At 6 months, the stratum corneum had a basket-weave appearance but the dermoepidermal junction remained flat. Monoclonal antibodies to keratins 14 and 10 showed normal basal and suprabasal localization, respectively. Electron microscopy showed a normal basement membrane with anchoring fibrils. LH7:2, a monoclonal antibody that binds to the type VII collagen molecule, stained the dermoepidermal junction in all biopsy specimens. AE-1, an antibody that stains suprabasal cells in hyperproliferative skin, was expressed suprabasally for up to 12 weeks after healing (16 weeks after grafting), but expression was confined to the basal layer at 18 weeks after healing (6 months after grafting). Anti-involucrin staining was found in the deeper layers of the epidermis up to 12 weeks after healing (16 weeks after grafting) but had receded to a normal distribution in upper spinous and granular layers at 18 weeks (6 months after grafting). Overall, the histologic patterns observed in recipient sites during the first 4 months after grafting resembled those observed for 10 to 14 days in newly healed epidermis and in hyperproliferative states such as psoriasis. In four sex-mismatched graft sites, specimens were reacted with a biotinylated probe to the Y chromosome by in situ hybridization. Lack of Y chromosome-positive cells suggested that host keratinocytes had replaced the allografts. Multilocus DNA analysis in one patient confirmed this observation. Our data suggest that an altered state of epithelial maturation persists for several months after culture grafting, with restoration of the normal pattern by 6 months. No differences were detected between autografted and allografted sites.

Characterization of "neo-dermis" formation beneath cultured human epidermal autografts transplanted on muscle fascia

Cultured human keratinocyte autografts were transplanted to burn wounds that had been completely excised down to muscle fascia such that all cutaneous elements were removed from the wounds. Healing autografts were biopsied from days 6-153 in five patients, and the "neo-dermis" beneath the autografts was examined by immunofluorescent staining using antibody probes to connective tissue molecules, by histochemical staining for elastin fibers, and by electron microscopy. We found that the neo-dermis contained most of the major connective tissue elements early in the post-transplantation period. However, regardless of the time examined, there was a paucity of elastin fibers and poor organization of linkin (microthread-like fibers) in the neo-dermis beneath autografts. The perturbations of these connective tissue components in the neo-dermis may play a role in the poor recoil and elastic properties of burn wounds treated with autografts.

In vitro effects of matrix peptides on a cultured dermal-epidermal skin substitute

Composite dermal-epidermal skin substitutes rely on a firm attachment of human keratinocytes (HK) to the dermal substrate for graft survival on the wound. An in vitro study was performed assessing whether the addition of matrix peptides to the dermal substrate effected the epithelial thickness. Cultured grafts were made by attaching HK to the external surface of a collagen-chondroitin 6-sulfate (GAG) membrane and inoculating human fibroblasts (HF) internally. If the matrix peptide (RGD Peptide) was added to the collagen-GAG membrane prior to placement of the HK and HF, the resultant epithelial layer at the end of the normal 4-day culture period was significantly thicker (19.7 +/- 0.9 microns versus 13.5 +/- 1.0 microns). Subjectively, the HF content was also greater on the peptide-treated grafts. When HF were not placed on the cultured graft, i.e., only collagen-GAG membrane, RGD peptide, and HK, the resultant epithelial thickness was even greater (28.3 +/- 1.0 microns). These data suggest that addition of matrix peptides, which increase cell attachment to the dermal substrate, may prove effective in the improvement of this cultured composite dermal-epidermal skin substitute.

Cytotoxicity to cultured human keratinocytes of topical antimicrobial agents

Cultured skin grafts administered clinically for closure of burn wounds may be contacted by topically applied antimicrobial agents. A study was performed to assess whether commonly used topical antimicrobial agents are toxic to cultured human keratinocytes (HK) in vitro. Serum-free MCDB 153 culture medium containing Neosporin G.U. irrigant (Neomycin, 40 micrograms/ml-polymyxin B sulfate, 200 units/ml) and a standard tissue culture antimicrobial agent of penicillin (10,000 units/ml)-streptomycin (10,000 micrograms/ml)-amphotericin B (25 micrograms/ml) had no effect on the keratinocyte growth rates when compared to standard MCDB 153 medium without antibiotics. Medium containing Sulfamylon (mafenide acetate, 0.85%), Polysporin (polymyxin B sulfate, 1 x 10(4) units/ml-bacitracin, 500 units/ml), gentamicin sulfate (0.1%), modified Dakins solution (25%), and acetic acid (0.25%) all showed statistically significant (P less than 0.01) decreases in keratinocyte growth rates. This data suggests that commonly applied antimicrobials may not be appropriate for cultured grafts in the concentrations that are used clinically.

Use of cultured human epidermal keratinocytes for allografting burns and conditions for temporary banking of the cultured allografts

Five children who suffered burns clinically regarded as full skin thickness loss were grafted with cultured allogeneic skin from newborn prepuce. The wounds had remained open and infected without healing for about 20 days before the patients were received in the burn unit. To avoid losing surviving deep epidermal cells the wounds were débrided but not deeply excised and, a few days before allografting, they were washed with isodine solution and sterile water, and treated with silvadene cream application. All children received 76 cultured allografts of about 60 cm2 each. After allografting, the wounds were epithelized in 7-10 days and the allogeneic grafted skin began desquamation suggesting that the allograft did not 'take' permanently but was replaced by the newly formed skin. On the other hand, since allografting is an adequate therapy to provide early temporary coverage in extensively burned patients, we developed conditions for banking cultured skin to make it available for immediate use. The conditions described allow banking of the cultured grafts for 15-20 days with retention of clonal growth ability similar to that of unstored epithelia. The results show that cultured epidermal cells obtained from human newborn foreskin, when used as allografts for coverage of full skin or deep partial skin thickness burns, allow rapid epithelization of the burn wounds.

Basement membrane and human epidermal differentiation in vitro

To test the role of basement membrane in differentiation of human epidermis reconstituted on human dermis, we prepared dermis with and without basement membrane and cultured epidermal cells on these two dermal substrata. The absence of basement membrane components was confirmed by immunofluorescence staining for laminin and type IV collagen and by electron microscopy. A high degree of differentiation of reconstituted epidermis did not require basement membrane as shown by the development of basal, spinous, and granular cell layers, and synthesis of 58 and 65-67 kDa keratins when epidermis was attached directly to dermis. On the other hand, we found that the basement membrane regulated the adhesive interaction between the epidermis and dermis. On dermis with basement membrane, attached epidermal cells formed hemidesmosomes and mechanically stable bonding. In the absence of basement membrane, the epidermal cells did not form hemidesmosomes, and bonding between the epidermis and dermis was unstable. Moreover, dermis from which the basement membrane was removed was reorganized by the epidermal cell layer.

Intermingled skin grafts with in vitro cultured keratinocytes--experiments with rats

In vitro cultured autogenic keratinocytes were used instead of autogenic skin islets in intermingled skin grafts on rats. According to the observations made with the original method, the allogenic epidermis is being rejected. The cultured keratinocytes start to grow out in circles, eventually covering the remaining allodermis in the form of a multilayered, well-differentiated epidermis. The prospects of these findings for possible future clinical applications are discussed.

Attachment of peptide growth factors to implantable collagen

Ingrowth of fibrovascular tissue from the woundbed into collagen-based dermal substitutes and survival of cultured epithelium after transplantation may be enhanced by attachment of heparin binding growth factor 2 (HBGF2) and epidermal growth factor (EGF) to collagen. Biotinylation of collagen and the growth factors allows immobilization of HBGF2 and EGF by high affinity binding of tetravalent avidin. Biotinylated HBGF2 and EGF (B-GF) were exposed to complexes of biotinylated collagen (B-COL)-avidin (A) and detected with peroxidase-labeled avidin (AP) followed by chromagen formation on nitrocellulose paper. Binding of biotinylated HBGF2 and EGF was specific (*, P less than 0.05), proportional to the concentration of biotinylated collagen, and resistant to ionic (NaCl) displacement. Data are expressed as mean percentages of maximum binding +/- SEMs: (table; see text) Growth response of cultured human epidermal keratinocytes to HBGF2 (population doubling time, PDT = 0.70 population doublings (PD)/day) confirmed the retention of mitogenic activity after biotinylation (PDT = 0.80 PD/day). Specific binding of biotinylated HBGF2, EGF, or other biologically active molecules (antibiotics, NSAIDs) to implantable collagen may provide a mechanism for positive therapeutic modulation of wound healing, including repair of full-thickness skin wounds with cultured cell-collagen composite grafts.

Ultrastructural study of grafted autologous cultured human epithelium

An electron microscopical study of grafted autologous cultured human epithelium is presented. Biopsy samples were collected from four patients with full thickness burns at 9 days, 6 weeks and 5-21 months after grafting of the cultured epithelium. By the sixth week after transplantation, grafted cultured epithelial sheets had developed to consist of 10 to 20 layers of cells and the epithelium showed distinct basal, spinous, granular and horny layers, and a patchy basement membrane had formed. Langerhans cells and melanocytes were identifiable. From 5 months onwards flat basal cells became oval, and oval keratohyalin granules in the keratinocytes also assumed a normal irregular shape. Membrane-coating granules in the keratinocytes increased in number. The fine structures of desmosomes also showed a normal mature appearance. Furthermore, complete extension of the basement membrane could be observed. The maturation of cultured human epithelium is complete by 5 months after grafting.

Reconstitution of structure and cell function in human skin grafts derived from cryopreserved allogeneic dermis and autologous cultured keratinocytes

Grafts of allogeneic dermis plus autologous epidermal cell cultures were used to replace extensively burned skin. Cryopreserved split-thickness cadaveric skin was grafted onto debrided burn wound, and autologous keratinocytes were cultured from uninjured donor sites. Several weeks later, allograft epidermis was abraded and replaced with the keratinocyte cultures. The final grafts were thus composites of autologous cultured epidermis and allogeneic dermis. In a case with 28 months follow-up, reconstitution of the dermal-epidermal (BMZ.1) and microvascular (BMZ.2) basement membrane zones was studied immunohistochemically and ultrastructurally. Immediately before grafting, thawed cryopreserved skin reacted with antibodies against laminin and type IV collagen in normal patterns. Twenty-nine days after grafting, BMZ.1 reacted weakly with both antibodies, and anticollagen type IV reactivity was absent from BMZ.2. Antilaminin reactivity of BMZ.2, however, was moderately intense, consistent with recent neovascularization. On day 29, the allograft epidermis was replaced with autologous keratinocyte cultures. Twenty-five days later (54 d after allografting), staining of both BMZs was intense with both antibodies. Ultrastructurally, at day 76 (47 d after culture placement) BMZ.1 revealed only small hemidesmosomes, few incipient anchoring fibrils, and a discontinuous lamina densa. BMZ.2, however, was fully reconstituted. By 124 d, both BMZs appeared normal. Observations in the dermis at 76 d included the presence of lymphocytes, organellar debris, and hyperactive collagen fibrillogenesis, all indicative of dermal remodelling. The microvasculature was well differentiated, but no elastic fibers or nerves were found. In the epidermis, melanocytes and evidence of melanosome transfer were seen at 5, 47, and 95 d after grafting of keratinocyte cultures. We conclude that the composite procedure reconstitutes skin with excellent textural and histologic qualities.