Genetic modification of cultured skin substitutes by transduction of human keratinocytes and fibroblasts with platelet-derived growth factor-A

Adipocytes constitutively release factors that accelerate keratinocyte proliferation in vitro

Keratinocytes grown directly on adipose tissue have greater proliferation rates than keratinocytes grown alone. It is unknown if factors released by adipose tissue into culture media could increase keratinocyte proliferation without requiring incorporation of adipose tissue into skin graft models, or serve as a substitute for the fibroblast feeder layer.Human keratinocytes were grown with and without NIH 3T3 fibroblast feeder layer in the following conditions (12 cultures per group) adipose tissue coculture (AT), cultures supplemented with medium from whole adipose tissue referred to as adipose-conditioned medium (ACM), and control. Proliferation was measured with a colorimetric proliferation assay and digital calculations of percent confluence over time. Culture morphology was assessed by light microscopy.ACM cultures without 3T3s, AT cultures with and without 3T3s, and 3T3 control cultures demonstrated a similarly significant keratinocyte proliferation increase over non-3T3 control (P < 0.05) corresponding with a 2-fold increase in percent confluence by day 7. ACM cultures with 3T3s proliferated significantly faster than all other treatment groups (P < 0.05) resulting in complete confluence by day 5. ACM cultures with and without 3T3s produced a thick keratinized layer by day 7 whereas all other cultures including AT cultures did not.Engineered tissue replacement can be accelerated and simplified by ACM without requiring the addition of adipose tissue or a fibroblast feeder layer to keratinocyte culture systems. ACM supplementation provides an additive proliferation benefit when combined with a feeder layer producing mature grafts in approximately half the time as keratinocytes alone by accelerating proliferation and increasing keratinization.

Collagen membranes loaded with collagen-binding human PDGF-BB accelerate wound healing in a rabbit dermal ischemic ulcer model

Studies have shown that exogenous platelet-derived growth factor-BB (PDGF-BB) could accelerate the ulcer healing, but the lack of efficient growth factor delivery system limits its clinical application. Our previous work has demonstrated that the native human PDGF-BB was added a collagen-binding domain (CBD), TKKTLRT, to develop a collagen-based PDGF targeting delivery system. Here, we showed that this CBD-fused PDGF-BB (CBD-PDGF) could bind to collagen membrane efficiently. We used the rabbit dermal ischemic ulcer model to study the effects of CBD-PDGF loaded on collagen membranes. Results revealed that this system maintained a higher concentration and stronger bioactivity of PDGF-BB on the collagen membranes and promoted the re-epithelialization of dermal ulcer wounds, the collagen deposition, and the formation of capillary lumens within the newly formed tissue area. It demonstrated that collagen membranes loaded with collagen-targeting human PDGF-BB could effectively promote ulcer healing.

Growth factors in the treatment of diabetic foot ulcers: new technologies, any promises?

Foot ulcers remain a common problem, leading to increased morbidity in patients with diabetes. Despite the progress that has been achieved in revascularization techniques as well as in off-loading to relieve high-pressure areas, diabetic foot wounds remain a clinical challenge. Growth factors are a major technological advance that promised to change the face of wound healing. The most important of growth factors are recombinant human platelet-derived growth factor-BB and granulocyte colony-stimulating factor. The former has been approved by the FDA for the treatment of neuropathic ulcers when there is adequate blood supply. The latter is less demonstrably useful. Advances include methods of delivering growth factors.

Preparation and evaluation of gene-transfected cultured skin as a novel drug delivery system for severely burned skin

PURPOSE

The purpose of this study is to prepare and evaluate gene-transfected cultured skin to establish a dermal patch consisting of cultured skin as a new and novel delivery system for severely burned skin.

MATERIALS AND METHODS

Plasmid DNA encoding the green fluorescent protein (GFP) gene was used as a model gene and transfected to rat and human cultured dermis models (CDMs) using the hemagglutinating virus of Japan envelope vector (HVJ-E) to prepare gene-transfected CDM and evaluate GFP expression in the CDM. Two kinds of transfection methods were evaluated. In pre-transfection, the gene was first transfected into fibroblasts and then CDM was prepared using these gene-transfected cells. In post-transfection, the gene was transfected directly into CDM.

RESULTS

GFP expression was observed in both the pre- and post-transfected CDMs. The post-transfection method showed higher GFP expression in the CDM than pre-transfection, although no statistically significant difference was observed. The cell viability of these transfected CDMs was also examined with MTT assay. Slight decrease in viability was observed in these transfected CDMs. These methods could be useful in preparing gene-transfected cultured skins with low cell damage.

CONCLUSION

Gene transfection to cultured skin may produce several dermal patches that release potent endogenous bioactive peptides.

Microarray analysis of gene expression in cultured skin substitutes compared with native human skin

Cultured skin substitutes (CSS), prepared using keratinocytes, fibroblasts, and biopolymers, can facilitate closure of massive burn wounds by increasing the availability of autologous tissue for grafting. But because they contain only two cell types, skin substitutes cannot replace all of the functions of native human skin. To better understand the physiological and molecular differences between CSS and native skin, we undertook a comprehensive analysis of gene expression in native skin, cultured keratinocytes, cultured fibroblasts, and skin substitutes using Affymetrix gene chip microarrays. Hierarchical tree clustering identified six major clusters of coordinately regulated genes, using a list of 1030 genes that were the most differentially expressed between groups. These clusters correspond to biomarker pools representing expression signatures for native skin, fibroblasts, keratinocytes, and cultured skin. The expression analysis revealed that entire clusters of genes were either up- or downregulated upon combination of fibroblasts and keratinocytes in cultured skin grafts. Further, several categories of genes were overexpressed in CSS compared with native skin, including genes associated with hyperproliferative skin or activated keratinocytes. The observed pattern of expression indicates that CSS in vitro, which display a well-differentiated epidermal layer, exhibit a hyperproliferative phenotype similar to wounded native skin.

[Regenerative medicine and plastic surgery]

Regenerative medicine recently evolved as a new medical field that includes tissue engineering, cell/system biology, nanotechnology, pharmacology, stem-cell biology, and bioengineering. Regenerative medicine targets new forms of therapy to promote and support the intrinsic, autologous, regenerative potential of human biological systems. All fields of surgery have profited from these developments, and spectacular experimental results and clinical benefits have been obtained. Plastic surgery has shown interest in regenerative medicine due to its focus on reconstructive surgery. Early on, several interdisciplinary experimental working groups were founded including plastic surgery. This overview takes a closer look at common experimental and clinical results of regenerative medicine and plastic surgery.

Gene therapy in the treatment of lower extremity wounds

This article presents a brief overview of the etiology of chronic wounds of the lower extremities and their current medical and surgical treatment. Gene therapy as a potential tool for treating therapeutically challenging wounds is described in terms of the vectors employed in gene transfer, as well as the strategies used to promote wound healing. Results from animal model studies, as well as clinical trials, are presented.

An In Vivo Model of Wound Healing in Genetically Modified Skin-Humanized Mice

Cutaneous wound-healing disorders are a major health problem that requires the development of innovative treatments. Whithin this context, the search for reliable human wound-healing models that allow us to address both mechanistic and therapeutic matters is warranted. In this study, we have developed a novel invivo wound-healing model in a genetically modified human context. Our model is based on the regeneration of human skin on the back of nude mice by transplantation of a cultured bioengineered skin equivalent previously designed in our laboratory. In this setting, human keratinocytes in the epidermal compartment were genetically modified with a retroviral vector encoding the enhanced green fluorescent protein (EGFP). After stable engraftment of the EGFP skin was achieved (9-12 wk after grafting), a small circular full thickness wound was performed on this mature human skin. A wide variety of parameters involved in wound healing were monitored, including tissue architecture, cell proliferation, epidermal differentiation, dermal remodelling, and basement membrane regeneration. Wounded gene-targeted skin-humanized mice re-capitulated native skin wound-healing features. In addition, when keratinocyte growth factor (KGF), a growth factor that has been shown to improve wound healing, was added to wounds during 3 d, the re-epithelialization was significantly accelerated. The present wound-healing model system provides a suitable in vivo tool to test gene transfer strategies for human skin repair. It also serves as a complementary platform for studies in genetically modified mice and as a model to evaluate pharmaceutical therapeutic approaches for impaired wound healing.

Gene therapy in wound healing

Gene therapy is a new and emerging technology that has been catalyzed by the progress of the Human Genome Project. It employs the process of manipulating genes to achieve a clinically beneficial alteration in gene product. Wound healing lends itself to the application of gene therapy by virtue of the vast array of proteins involved in its complex cascade. This article provides an overview of the background to gene therapy and describes current techniques in use as applied to wound healing. The authors show the potential role that many candidate genes may offer in the future for optimizing wound healing through gene therapy.

Platelet derived growth factor (PDGF) responsive epidermis formed from human keratinocytes transduced with the PDGF beta receptor gene

Platelet-derived growth factor is a major proliferative and migratory stimulus for connective tissue cells during the initiation of skin repair processes. In response to injury, locally produced platelet-derived growth factor is secreted by a diversity of cutaneous cell types whereas target activity is confined to cells of mesenchymal origin, e.g. dermal fibroblasts and smooth muscle cells. Although epidermal cells contribute to cutaneous platelet-derived growth factor activity by their ample capacity to secrete platelet-derived growth factor ligand, normal epidermal keratinocytes are not known to express any member of the platelet-derived growth factor receptor family. In order to study if epidermis may be genetically transformed to a platelet-derived growth factor sensitive compartment we aimed to introduce the gene encoding human platelet-derived growth factor receptor beta (PDGF beta R) into epidermal keratinocytes using a retrovirus-derived vector. Successful gene transfer to primary cells was confirmed by immunofluorescence staining, southern blotting, and ligand-induced receptor autophosphorylation. By culturing a mixture of PDGF beta R-transduced and unmodified keratinocytes at the air-liquid interface on devitalized dermis, we were able to establish a multilayered epithelium showing histologic similarities to that evolved from native keratinocytes or keratinocytes transduced with the reporter gene encoding enhanced green fluorescent protein. Receptor-modified epidermal tissue cultured for 6 days and examined by immunofluorescence microscopy was shown to contain PDGF beta R-expressing keratinocytes distributed in all layers of living epidermis. By continued tissue culture in serum-containing medium, the epidermis became increasingly cornified although receptor-positive cells were still observed within the viable basal compartment. Stimulation of PDGF beta R-transduced epidermis with recombinant platelet-derived growth factor BB had a mitogenic effect as reflected by an increased frequency of Ki-67 positive keratinocytes. The study demonstrates that transgene expression of human PDGF beta R can be achieved in epidermal keratinocytes by retroviral transduction, and that ligand activation of such gene-modified skin equivalent enhances cell proliferation. In perspective, viral PDGF beta R gene transfer to keratinocytes may be a useful approach in studies of receptor tyrosine kinase mediated skin repair and epithelialization.

Growth factors in the treatment of diabetic foot ulcers

BACKGROUND

Chronic foot ulceration is a major source of morbidity in diabetic patients. Despite traditional comprehensive wound management, including vascular reconstruction, there remains a cohort of patients with non-responding wounds, often resulting in amputation. These wounds may benefit from molecular manipulation of growth factors to enhance the microcirculation.

METHODS

A review of the current literature was performed using Pubmed, with secondary references obtained from key articles.

RESULTS AND CONCLUSION

There has been a generally disappointing clinical outcome from growth factor trials, although topical platelet-derived growth factor has shown significant benefit and should be considered in non-healing, well perfused ulcers after failure of conventional wound care. The modulatory role of the extracellular matrix in the cellular response to growth factors and data from regenerative-type fetal wound healing are further areas of interest. The chemical induction of microvessel formation may become a future therapeutic option.

Green fluorescent protein-adenoviral construct as a model for transient gene therapy for human cultured keratinocytes in an athymic mouse model

BACKGROUND

The goal of gene therapy for cultured keratinocyte grafts is to accelerate growth and wound healing following engraftment without producing long-term complications from the delivered gene. We studied a Green Fluorescent Protein-Adenoviral construct (GFP-ADV) to determine the characteristics of gene expression in human cultured keratinocyte grafts.

METHODS

Twelve GFP-ADV grafts and twelve control grafts were transplanted to the flanks of 24 athymic mice. Mouse flanks were monitored with fluorescence-filtered microscopy and, on Day 21, were sectioned and stained with anti-human MHC Class I with H&E counterstaining. Real-time PCR was performed on graft biopsies for adenoviral DNA.

RESULTS

Fluorescence decreased from Days 3 to 5 resulting in no difference between GFP-ADV and control grafts from days 5 to 10. All grafts were positive for human MHC Class I with an epithelial architecture by H&E. Day 21 GFP-ADV grafts were negative for adenoviral DNA.

CONCLUSION

The delivered gene was transiently expressed without the persistence of viral DNA, demonstrating the potential of adenoviral gene delivery for the improvement of wound healing without long-term adverse effects to the graft.

Topical platelet-derived growth factor enhances wound closure in the absence of wound contraction: an experimental and clinical study

The effects of platelet-derived growth factor (PDGF) on wound healing in animal and human models were investigated. Four 1-cm2 wounds were made on the dorsum of 3 rats. A 0.5-cm punch wound was made behind each ear of 4 patients. Half the wounds were treated daily with vehicle, controls, and the rest were treated with PDGF. Treated wounds closed faster than the controls (animals: 16 +/- 3.2 days vs. 17.8 +/- 2.17 days; p < 0.05) and (patients: 16 +/- 0.67 days vs. 19.5 +/- 0.33 days; p < 0.05). Biopsies were taken at day 20 for polarized light-Sirius red histological analysis. The granulation tissue of PDGF-treated wounds showed fine collagen fibers with weak birefringence, characteristic of immature granulation tissue, deposited throughout the healed wound site. Such a pattern indicates wound closure by reepithelialization and filling in with scar. Control wound biopsies showed a small area of immature granulation tissue surrounded by intact dermal thick collagen fibers with strong birefringence. Such a pattern indicates wound closure by wound contraction. This shows that PDGF enhances wound closure by reepithelialization and the prevention of wound contraction.

Tissue-engineered skin. Current status in wound healing

Tissue-engineered skin is a significant advance in the field of wound healing and was developed due to limitations associated with the use of autografts. These limitations include the creation of a donor site which is at risk of developing pain, scarring, infection and/or slow healing. A number of products are commercially available and many others are in development. Cultured epidermal autografts can provide permanent coverage of large area from a skin biopsy. However, 3 weeks are needed for graft cultivation. Cultured epidermal allografts are available immediately and no biopsy is necessary. They can be cryopreserved and banked, but are not currently commercially available. A nonliving allogeneic acellular dermal matrix with intact basement membrane complex (Alloderm) is immunologically inert. It prepares the wound bed for grafting allowing improved cultured allograft 'take' and provides an intact basement membrane. A nonliving extracellular matrix of collagen and chondroitin-6-sulfate with silicone backing (Integra) serves to generate neodermis. A collagen and glycosaminoglycan dermal matrix inoculated with autologous fibroblasts and keratinocytes has been investigated but is not commercially available. It requires 3 to 4 weeks for cultivation. Dermagraft consists of living allogeneic dermal fibroblasts grown on degradable scaffold. It has good resistance to tearing. An extracellular matrix generated by allogeneic human dermal fibroblasts (TransCyte) serves as a matrix for neodermis generation. Apligraf is a living allogeneic bilayered construct containing keratinocytes, fibroblasts and bovine type I collagen. It can be used on an outpatient basis and avoids the need for a donor site wound. Another living skin equivalent, composite cultured skin (OrCel), consists of allogeneic fibroblasts and keratinocytes seeded on opposite sides of bilayered matrix of bovine collagen. There are limited clinical data available for this product, but large clinical trials are ongoing. Limited data are also available for 2 types of dressing material derived from pigs: porcine small intestinal submucosa acellular collagen matrix (Oasis) and an acellular xenogeneic collagen matrix (E-Z-Derm). Both products have a long shelf life. Other novel skin substitutes are being investigated. The potential risks and benefits of using tissue-engineered skin need to be further evaluated in clinical trials but it is obvious that they offer a new option for the treatment of wounds.

Cultured skin substitutes reduce donor skin harvesting for closure of excised, full-thickness burns

OBJECTIVE

Comparison of cultured skin substitutes (CSS) and split-thickness skin autograft (AG) was performed to assess whether donor-site harvesting can be reduced quantitatively and whether functional and cosmetic outcome is similar qualitatively in the treatment of patients with massive cutaneous burns.

SUMMARY BACKGROUND DATA

Cultured skin substitutes consisting of collagen-glycosaminoglycan substrates populated with autologous fibroblasts and keratinocytes have been shown to close full-thickness skin wounds in preclinical and clinical studies with acceptable functional and cosmetic results.

METHODS

Qualitative outcome was compared between CSS and AG in 45 patients on an ordinal scale (0, worst; 10, best) with primary analyses at postoperative day 28 and after about 1 year for erythema, pigmentation, pliability, raised scar, epithelial blistering, and surface texture. In the latest 12 of the 45 patients, tracings were performed of donor skin biopsies and wounds treated with CSS at postoperative days 14 and 28 to calculate percentage engraftment, the ratio of closed wound:donor skin areas, and the percentage of total body surface area closed with CSS.

RESULTS

Measures of qualitative outcome of CSS or AG were not different statistically at 1 year after grafting. Engraftment at postoperative day 14 exceeded 75% in the 12 patients evaluated. The ratio of closed wound:donor skin areas for CSS at postoperative day 28 was significantly greater than for conventional 4:1 meshed autografts. The percentage of total body surface area closed with CSS at postoperative day 28 was significantly less than with AG.

CONCLUSIONS

The requirement for harvesting of donor skin for CSS was less than for conventional skin autografts. These results suggest that acute-phase recovery of patients with extensive burns is facilitated and that complications are reduced by the use of CSS together with conventional skin grafting.

Skin as a potential organ for ectopic monoclonal antibody production

The therapeutic potential of monoclonal antibodies for treating a variety of severe or life-threatening diseases is high. Although intravenous infusion appears the simplest and most obvious mode of administration, it is not applicable to many long-term treatments. It might be advantageously replaced by gene/cell therapies, however, rendering treatments cost-effective and eliminating the short- and long-term side-effects associated with injection of massive doses of antibodies. We have tested whether skin can potentially be used as an organ for production and systemic delivery of ectopic antibodies. Normal human primary keratinocytes were shown to be capable of synthesis and secretion of a model monoclonal antibody directed against human thyroglobulin upon retroviral gene transduction in vitro. Neo- epidermis reconstructed in vitro, either in cell culture inserts or on dermal substrates, from such modified keratinocytes also produced the monoclonal antibody. Interestingly, the latter could cross the epidermis basal layer and be released in culture fluids. Finally, grafting of epidermis reconstituted in vitro on dermal substrates to SCID mice permitted sustained monoclonal antibody delivery into the bloodstream to be achieved. Our data thus show that genetically engineered keratinocytes can potentially be used for genetic antibody-based immunotherapies. They also indicate that proteins as big as 150 kDa, after release by engineered keratinocytes into skin intercellular spaces, can migrate to the general circulation, which is potentially important for a number of other gene-based therapies.

Tissue-engineered skin substitutes

The last two years have seen new tissue-engineered skin substitutes come onto the market and begin to resolve the various roles to which each is best suited. It is becoming evident that some of the very expensive cell-based products have cost-benefit advantage despite their high price and are valuable within the restricted applications for which they are intended. The use of skin substitutes for testing purposes has extended from epidermal keratinocytes to other integumentary epithelia and into preparations containing multiple cell types in which reactions resulting from paracrine interactions can be examined. Challenges remain in the application of gene therapy techniques to skin substitutes, both the control of transgene expression and in the selection of suitable genes to transfect. A coming challenge is the production of tissue-engineered products without the use of animal products other than human cells. A challenge that may be diminishing is the importance of acute rejection of allogeneic tissue-engineered skin substitutes.