The derivation and characterization of pig keratinocyte cell lines that retain the ability to differentiate

Meshed skin grafts placed upside down can take if desiccation is prevented

BACKGROUND

The role of the wet environment in wound healing has been investigated in various studies. The current study explores the role of the wet wound environment in promoting healing of skin grafts. The authors hypothesized that the survival of skin grafts is dependent not only on the orientation of transplantation but also on the environment into which the skin is transplanted.

METHODS

This study included 72 full-thickness (2.5 x 2.5-cm) wounds in six Yorkshire pigs. The wounds were grafted with autologous split-thickness skin grafts (meshed or sheet), placed either regularly (dermal side down) or inverted (dermal side up), and treated in a wet or a dry environment. Behavior of the skin grafts and healing were analyzed in histologic specimens collected on days 4, 6, 9, and 12 after wounding. Wound contraction was quantified by photoplanimetry.

RESULTS

In the wet environment, not only did inverted meshed skin grafts survive, but also they proliferated to accelerate reepithelialization. In this environment, wounds transplanted with inverted and regular meshed grafts showed no significant difference in reepithelialization rate and contraction. In contrast, in the dry environment, wounds transplanted with inverted meshed grafts showed a significantly lower reepithelialization rate and a higher contraction rate than wounds transplanted with regular grafts. Inverted meshed grafts in a dry environment and inverted sheet grafts did not survive.

CONCLUSION

The wound environment has an important role in the survival and proliferation of skin grafts, as demonstrated by survival of inverted meshed grafts in the wet environment and their contribution to accelerated reepithelialization, equal to the regularly placed grafts.

Establishment and characteristics of Gottingen minipig skin in organ culture and monolayer cell culture: relevance to drug safety testing

Skin from Gottingen minipigs was used as a source of tissue for organ and cell culture and compared to human skin for growth conditions and sensitivity to irritants. Optimal organ culture conditions were determined, based on the preservation of the histological structure. These included serum-free, growth factor-free conditions with a calcium concentration of 1.5mM. Formulations in which the calcium concentration were low (0.075-0.15mM) failed to support tissue viability (even in the presence of dialyzed serum). Epidermal keratinocytes were grown from tissue explants and as single cells from enzyme-disrupted tissue. Optimal keratinocyte growth was achieved using a serum-free, growth factor-supplemented culture medium with a calcium concentration of 0.15mM. Fibroblasts were optimally grown from explant cultures using a medium with 1.5mM calcium and 10% fetal bovine serum. The conditions that were optimal for maintenance of intact pig skin, as well as for the isolated cells, are the same conditions that have been shown previously to be optimal for intact human skin and skin cells. In additional studies, pig skin keratinocytes and fibroblasts were exposed to a panel of contact irritants and contact sensitizers. Using growth inhibition as the response, the median effective dose values with each agent were very similar to the values previously determined for human epidermal keratinocytes and human dermal fibroblasts. Taken together, these data suggest that the skin from the Gottingen minipig can be used as a surrogate for human skin in ex vivo skin safety studies.

Transient non-viral cutaneous gene delivery in burn wounds

BACKGROUND

Gene transfer to burn wounds could present an alternative to conventional and often insufficient topical and systemic application of therapeutic agents to aid in wound healing. The goals of this study were to assess and optimize the potential of transient non-viral gene delivery to burn wounds.

METHODS

HaCaT cells were transfected with luciferase or beta-galactosidase transgene using either pure plasmid DNA (pDNA) or complexed with Lipofectamine 2000, FuGENE6, or DOTAP-Chol. Expression was determined by bioluminescence and fluorescence. Forty male Sprague-Dawley rats received naked pDNA, lipoplexes, or carrier control intradermally into either unburned skin, superficial, partial, or full-thickness scald burn. Animals were sacrificed after 24 h, 48 h, or 7 days, and transgene expression was assessed.

RESULTS

Gene transfer to HaCaT cells showed the overall highest expression for DOTAP/Chol (77.85 ng luciferase/mg protein), followed by Lipofectamine 2000 (33.14 ng luciferase/mg protein). pDNA-derived gene transfer to superficial burn wounds showed the highest expression among burn groups (0.77 ng luciferase/mg protein). However, lipoplex-derived gene transfer to superficial burns and unburned skin failed to show higher expression.

CONCLUSIONS

Lipofectamine 2000 and DOTAP/Chol lipoplex showed significantly enhanced gene transfer, whereas no transfection was detectable for naked DNA in vitro. In contrast to the in vitro study, naked DNA was the only agent with which gene delivery was successful in experimental burn wounds. These findings highlight the limited predictability of in vitro analysis for gene delivery as a therapeutic approach.

Uptake and trafficking of DNA in keratinocytes: evidence for DNA-binding proteins

The skin is an interesting organ for human gene therapy due to accessibility, immunologic potential and synthesis capabilities. In this study, we attempted to visualize and measure the uptake of naked FITC-labeled plasmid by FACS analysis detecting up to 15% internalization in a dose- and time-dependent manner. Cycloheximide treatment inhibited the uptake by >90%, suggesting a protein-mediated uptake. The inhibition of different internalization pathways demonstrated that blocking macropinocytosis (by amiloride and N,N-dimethylamylorid) reduced DNA uptake by >85%, while the inhibition of clathrin-coated pits (by chlorpromazine) and caveolae (by nystatin/filipin III) did not limit the uptake. Colocalization studies using confocal laser microscopy revealed a time-dependent accumulation of plasmid DNA in endosomes and lysosomes. When a green fluorescent protein (GFP) expression vector was used, specific GFP-RNA became detectable by reverse transcriptase-PCR, whereas measurable amounts of protein could not be identified in FACS experiments. To detect the potential DNA receptors on the keratinocyte surface, membrane proteins were extracted and subjected to South-Western blotting using digoxigenin-labeled calf thymus and lambda-phage DNA. Two DNA-binding proteins, ezrin and moesin, known as plasma membrane-actin linkers, were identified by one- and two-dimensional-South-Western blots and matrix-assisted laser desorption and ionization-mass spectrometry. Ezrin and moesin are functionally associated with a number of transmembrane receptors such as the EGF, CD44 or ICAM-1 receptor. Taken together, naked plasmid DNA seems to enter human keratinocytes through different pathways, mainly by macropinocytosis. Two DNA-binding proteins were identified that seemed to be involved in binding/trafficking of internalized DNA.

Establishment and characterization of a normal melanocyte cell line derived from pig skin

Several minipig strains develop spontaneous malignant melanoma. As a first step toward the analysis of genes involved in the tumoral progression of melanoma in these animal models, we developed culture conditions for pig melanocytes whereby melanocytes from normal epidermis can be isolated directly onto mitotically inactivated keratinocytes in Eagle's minimal essential medium supplemented with fetal calf serum, tetradecanoyl phorbol acetate (TPA) and cholera toxin. We also derived an immortal line of pigmented melanocytes from the epidermis of a healthy Meishan pig. This cell line, designated PigMel, retains differentiation function in culture, dependence on TPA and cholera toxin and a diploid chromosome number. PigMel melanocytes exhibit morphological and molecular characteristics common to normal mammalian skin melanocytes.

Keratinocytes and cytokines

The skin has long been recognized as a major producer of cytokines, but the keratinocyte as principal epidermal cell has received less attention as potential source and target of cytokines. Nevertheless, keratinocytes produce a plethora of cytokines including interleukin (IL)-1, -6, -7, -8, -10, -12, -15, -18, and -20, and tumor necrosis factor alpha (TNF). The production by keratinocytes of pro-inflammatory (IL)-1, -6, -8, and TNF was recognized early and is well studied. Keratinocyte-derived IL-7 and -15 are considered to be significant in T-cell trafficking, possibly even in the pathogenesis of cutaneous T-cell lymphoma. Immunomodulatory IL-10 and -12 originating from keratinocytes are considered to be responsible for systemic effects, and IL-18 perhaps has a similar action. Keratinocytes were fairly recently recognized as being source or target of other IL-10 family members like IL-20 and IL-24 and the role of these cytokines in specific diseases is under investigation. In addition, a variety of cytokine receptors are present on keratinocytes like those for IL-4, -13, and -17 and to lesser degree IL-2. The ability to study the expression of cytokines by keratinocytes in vivo and in vitro using primary cells, immortalized cells or even organotypic culture systems offers many possibilities to further investigate the role of cytokine production in keratinocyte biology and disease.

Cultured autologous fibroblasts augment epidermal repair

BACKGROUND

Autologous dermal fibroblasts may be useful in the treatment of skin wounds and for the enhancement of keratinocyte proliferation. This paper addressed the following questions: (1) can cultured fibroblasts (CF) be transplanted as suspensions to full-thickness skin wounds and do they influence wound healing; (2) will the transplanted CF be integrated into the new dermis; (3) can a transgene that encodes a secretable marker, human epidermal growth factor (hEGF), be expressed in the wound fluid by the transplanted CF; and (4) do CF cotransplanted with cultured keratinocytes (CK) influence the rate of wound healing?

METHODS

Suspensions of CF were transplanted alone or together with CK to full-thickness wounds covered with liquid-containing chambers in an established porcine model.

RESULTS

Transplantation of CF accelerated reepithelialization as determined from wound histologies and sequential measurements of protein efflux over the wound surface. CF transfected with a marker gene, beta-galactosidase, resulted in in vivo gene expression and demonstrated that transplanted CF integrated into the developing dermis. Transplantation of hEGF gene-transfected CF resulted in significant hEGF expression in wound fluid. The hEGF levels peaked at day 1 (2450 pg/ml) and then sharply decreased to low levels on day 6. CF cotransplanted with CK led to greater number of keratinocyte colonies in the wound and accelerated reepithelialization as compared with CK alone.

CONCLUSIONS

Transplanted CF integrated into the dermis, accelerated reepithelialization, and improved the outcome of CK transplantation. CF may also be used for the expression of transgenes in wound and wound fluid.

Cultivation and characterisation of primary and subcultured equine keratinocytes

We describe the establishment and characterisation of equine keratinocyte cultures with maintenance of a high proliferative capacity up to the second passage. Improved attachment and growth were obtained by seeding primary cells on equine feeder layers. Subcultured keratinocytes showed optimal growth when seeded on collagen type I. The proliferation rate of cells on this substrate exceeded that seen for cells seeded on equine feeder layers. By immunohistochemistry, epithelial origin and state of differentiation of the equine keratinocytes were determined. They expressed keratin and desmoplakin I/II, but lacked keratin 10. Electron microscopy revealed typical features of cultured keratinocytes. Purity of keratinocyte cultures was determined by vimentin staining. This is the first report on the establishment of equine keratinocytes derived from lip epithelium. It forms the basis to study equine keratinocyte biology and the pathogenesis of epidermal diseases. Since wound healing represents a severe problem in equine dermatology, our data may be essential for the establishment of new and improved therapy.

Autologous skin transplantation: comparison of minced skin to other techniques

BACKGROUND

Skin grafting may be necessary to close nonhealing skin wounds. This report describes a fast and minimally invasive method to produce minced skin suitable for transplantation to skin wounds. The technique was evaluated in an established porcine skin wound healing model and was compared to split-thickness skin grafts and suspensions of cultured and noncultured keratinocytes.

MATERIALS AND METHODS

The study included 90 wounds on 3 pigs. Fluid-treated full-thickness skin wounds were grafted with minced skin, split-thickness skin grafts, noncultured keratinocytes, or cultured keratinocytes. Controls received either fluid or dry treatment. The wound healing process was analyzed in histologies collected at Days 8 to 43 postwounding. Wound contraction was quantified by photoplanimetry.

RESULTS

Wounds transplanted with minced skin and keratinocyte suspension contained several colonies of keratinocytes in the newly formed granulation tissue. During the healing phase, the colonies progressed upward and reepithelialization was accelerated. Minced skin and split-thickness skin grafts reduced contraction as compared to keratinocyte suspensions and saline controls. Granulation tissue formation was also reduced in split-thickness skin-grafted wounds.

CONCLUSIONS

Minced skin grafting accelerates reepithelialization of fluid-treated skin wounds. The technique is faster and less expensive than split-thickness skin grafting and keratinocyte suspension transplantation. Minced skin grafting may have implications for the treatment of chronic wounds.

Autologous keratinocyte suspensions accelerate epidermal wound healing in pigs

BACKGROUND

Tissue culture techniques enable in vitro expansion of keratinocytes that can be used to treat burns and chronic wounds. These keratinocytes are commonly grafted onto the wounds as differentiated sheets of mature epithelium. Less is however known about the effects of transplanting the cells as suspensions. This study evaluated epidermal regeneration in fluid-treated skin wounds treated with suspensions of cultured and noncultured autologous keratinocytes.

MATERIALS AND METHODS

Eighty-seven full-thickness excisional skin wounds were created on the back of 6 pigs and then transplanted with either cultured or noncultured autologous keratinocytes. The wounds were enclosed with liquid-tight chambers containing saline to provide a hydrated and standardized environment.

RESULTS

Keratinocyte transplantation resulted in several cell colonies within the granulation tissue of the wound. These colonies progressively coalesced and contributed to a new epithelium. The origin of the transplanted keratinocytes was confirmed by histochemical staining of wounds transplanted with transfected keratinocytes expressing beta-galactosidase. Transplantation of 0.125 x 10(6), 0.5 x 10(6), and 2.0 x 10(6) cultured keratinocytes, and 0.5 x 10(6) and 5.0 x 10(6) noncultured keratinocytes, increased reepithelialization dose dependently over saline-treated controls. The epithelial barrier function recovered faster in transplanted wounds as demonstrated by less protein leakage over the wound surface on Days 7-10 as compared to control wounds. Wound reepithelialization and the number of keratinocyte colonies observed in granulation tissue were significantly less in wounds transplanted with noncultured keratinocytes compared to wounds seeded with cultured keratinocytes.

CONCLUSION

Our study demonstrates successful transplantation of keratinocyte suspensions and their dose-dependent acceleration of wound repair. Selection of proliferative cells during culture and higher colony-forming efficiency may explain the greater effects observed with cultured keratinocytes.

Cytokine induction as a measure of cutaneous toxicity in primary and immortalized porcine keratinocytes exposed to jet fuels, and their relationship to normal human epidermal keratinocytes

The purpose of this study was to identify biomarkers of toxicity in primary porcine keratinocytes (PKC) and an immortalized porcine keratinocyte cell line (MSK3877) exposed to jet fuels Jet A, JP-8, and JP-8+100. Cells were exposed to 0.1% jet fuels and assayed for interleukin-8 (IL-8) and tumor necrosis factor alpha (TNF-alpha) mRNA using the TaqMan real time quantitative reverse transcriptase PCR assay. IL-8 and TNF-alpha protein release was measured using an ELISA. PKC exposed to jet fuels caused a slight upregulation of TNF-alpha mRNA at early time points, but no significant differences in TNF-alpha protein production were detected. IL-8 mRNA was increased at 4 h following exposure, and IL-8 protein was increased at 8 h. In MSK 3877 cells, jet fuels were shown to increase the production and expression of TNF-alpha mRNA and protein at 30 min and 1 h following exposure, respectively. IL-8 mRNA was only slightly induced compared to control. IL-8 protein release was suppressed by jet fuel exposure. These results were compared with those of a previous study in our laboratory to evaluate the utility of using porcine cells in lieu of normal human epidermal keratinocytes (NHEK). Similarities exist between PKC and NHEK with respect to both TNF-alpha and IL-8 production. The expression profile of TNF-alpha in MSK3877 cells mimics that of NHEK. In contrast, the profile of IL-8 expression opposes that of PKC and NHEK. These results suggest that porcine keratinocytes are susceptible to jet fuel toxicity. However, the responses of immortalized cells may vary from those of PKC and NHEK necessitating cautious interpretation of such data.

Demonstration of epidermal transfer from a polymer membrane using genetically marked porcine keratinocytes

The culture of keratinocytes on flexible membranes has been proposed as a means to simplify, accelerate and improve the efficiency with which proliferating cells are delivered to full thickness or non-healing skin defects. However, there have been no studies that monitor the transfer of cells from such membranes to the wound bed. We have used a porcine model of lacZ gene marked cultured autologous keratinocyte grafting to demonstrate unambiguously the transfer of cultured cells to cutaneous wounds from the EpiGen polymer membrane developed by Smith & Nephew Group plc. Full thickness wounds enclosed within rigid chambers were first grafted with autologous de-epidermalised dermis (DED). Keratinocytes were cultured on EpiGen membranes and applied to the wound beds 7 days after the DED grafts. Epidermal remnants persist within the DED and the resultant epidermis is therefore, a mixture of wound regeneration and delivered cultured cells. Unequivocal evidence for keratinocyte transfer from the membrane was obtained through the observed macroscopic surface staining for lacZ transduced cells and lacZ positive cells detected in sections through deeper layers of epidermal tissue. This method offers a general approach for evaluating the efficiency of keratinocyte delivery using upside-down flexible membrane transfer.

Retrovirus-mediated transduction of porcine keratinocytes in organ culture

Direct transfer of new genetic information to keratinocytes in epidermis may prove effective in treating certain genodermatoses; however, current methods for in vivo gene transfer to skin do not lead to persistence of the transgene. The goal of this study was to explore direct gene transfer using retrovirus-mediated transduction. Retroviral vectors integrate a DNA copy of their genome into the host chromosome and therefore have the potential to effect a permanent gene therapy. To facilitate development of methods for in vivo transduction with retroviral vectors, a porcine skin organ culture model was constructed in which the denuded surface was repopulated with replicating keratinocytes from hair follicles and epidermal remnants. In situ transduction was carried out by topical application of two retrovirus vectors, MFGlacZ (10(7) blue forming units per ml) and LZRN pseudotyped with the G protein of vesicular stomatitis virus (VSV) (10(9) colony forming units per ml), each encoding the beta-galactosidase reporter gene and the latter encoding the neomycin phosphotransferase selectable gene. Beta-galactosidase expressing cells were observed more frequently with LZRN than with MFGlacZ; however, transduction efficiency remained low in both instances. At equivalent titers, the VSV-G pseudotyped retroviral vector was shown to transduce porcine keratinocytes more efficiently than a similar vector with the amphotropic envelope. The number of beta-gal+ cells in organ culture could be increased by selection of LZRN-transduced cells in situ with G418. To achieve transduction of epidermis in vivo, these studies point out the importance of high titer retroviral vectors, pseudotyping with VSV-G protein, and in situ selection.

Re-epithelialization of porcine skin by the sweat apparatus

The behavior of the keratinocyte during the initial stages of cutaneous wound repair has been the subject of intense investigation. Most of these studies have focused on the lateral edges of wounds as the source of activated keratinocytes. Less attention has been directed towards elucidating the role of the appendageal structures as sources of keratinocytes for re-epithelialization, particularly the sweat apparatus. Surgical wounds of specific depths were created in pig skin, above and below hair follicles, and wound healing was allowed to take place in a setting in which lateral ingrowth of keratinocytes by migration was prevented. In this manner, all re-epithelialization occurred from residual appendageal structures. In those wounds where only sweat gland elements remained, an epithelium formed that had clinical, morphologic, and protein electrophoretic features closer to palmar/plantar or mucosal-like epithelia. In contrast, wounds that retained elements of the hair follicle healed faster and the resultant epithelium clinically, morphologically, and biochemically resembled the surrounding nonwounded epidermis. These findings establish that the sweat apparatus is capable of re-epithelializing the skin surface after a major cutaneous wound, but may not be capable of mimicking the epidermis.

Retroviral gene transfer into porcine keratinocytes following improved methods of cultivation

We embarked on a program examining the application of cultured epithelial sheets to skin wounds in pigs using retroviral gene transfer as a means to follow the grafted cells. In the past similar studies have been hampered by an inability to grow porcine keratinocytes without seeding at an extremely high density. In this study we found that excellent results could be achieved with Opti-MEM-1 (Gibco BRL Life Technologies) containing 1 per cent foetal calf serum, 0.5 mM Ca2+ and no other growth factors or stimulants. Keratinocytes were plated on gamma-irradiated 3T3 feeders on surfaces which had previously been coated with rat tail collagen I. Keratinocyte cultures were established at a seeding density of 5 x 10(4) cm-2. The yield of cells from 1 cm2 of skin was sufficient to set up a 75 cm2 flask. Cultures reached 80-90 per cent confluence in 7-10 days, after which they were passaged 1:3 multiple times, taking 3-4 days to reach the same confluency. Allowing cultures to remain confluent for 1 week was sufficient to allow Dispase removal of an intact sheet. Using these techniques porcine keratinocytes were transduced at an average frequency of 25.3 per cent (+/- 14.0 SEM) with the retroviral vector MFG lacZ nls by growth on the gamma-irradiated retroviral producer line GP + envAm12.

Retroviral marking identifies grafted autologous keratinocytes in porcine wounds receiving cultured epithelium

Cultured epithelial autografts are often applied to wounds with a capacity for regeneration from dermal appendages. It is unclear in these circumstances whether the cultured autografts act merely as a biologic dressing or whether they become incorporated into the new epithelium. We have used retroviral gene transfer techniques to identify autologous keratinocytes in an established porcine model of cultured epidermal (CE) grafting. Porcine keratinocytes were transduced with an MFG-lacZ nls vector produced by the amphotropic packaging line GP+EnvAm12. Transduction rates of 15.1%, in the absence of selection, were achieved by a single passage on gamma-irradiated retroviral producers as a feeder layer. Full-thickness wounds were created on Large White pigs and isolated from the surrounding skin by a polytetrafluoroethylene chamber. Wounds were grafted initially with autologous de-epidermized dermis (DED), followed 7 d later by sheets of retrovirally marked or unmarked CE autografts. Two weeks after grafting, the mean area of epithelium was 48.4% in wounds that received CE grafts and 32.3% in wounds that were left as DED alone. The epithelium on DED represents regeneration from dermal appendages. The contribution made by the autograft cells to the new epidermis was demonstrated unequivocally, however, by lacZ-positive areas visible macroscopically on the surface of the excised wound. In cryostat sections through the lacZ-positive areas, retrovirally marked cells were present at both superficial and basal positions in the new epithelium.