Host immune responses in ex vivo approaches to cutaneous gene therapy targeted to keratinocytes

Options for tracking GFP-Labeled transplanted myoblasts using in vivo fluorescence imaging: implications for tracking stem cell fate

BACKGROUND

Green fluorescent protein (GFP) is a useful biomarker, widely used in biomedical research to track stem cells after transplantation and/or to assess therapeutic transgene expression. However, both GFP and therapeutic gene products themselves may be immunogenic to the recipient. The main aim of this study was to use animal models to evaluate potential impact of GFP on the cell engraftment and to optimize tracking strategies prior to transplantation.

RESULTS

By using a fluorescent imaging (FLI) system, we investigated the dynamic cell behavior of GFP-transduced myoblasts in tibialis anterior (TA) muscles of immunocompetent mdx mice and immuno-compromised nude mice over a period of three months. The results suggested an apparent underlying host immunorejection in the mdx mice. Dystrophin immunostaining showed that the engraftment of wild type myoblasts was much more effective than that of the GFP-labeled counterparts in the mdx mice, further confirming an antigen role of GFP in this process. We tracked the GFP-transduced myoblasts in C57BL/6 mice and found GFP to be minimally immunogenic in these animals, as indicated by the GFP signal maintaining a much stronger level than that found in mdx and BALB/c mice at parallel time points. We also compared the in vivo cell behavior differences between myoblasts from virally GFP-transduced and GFP transgenic mice. The latter displayed much better engraftment, as determined both biomaging and histological observations.

CONCLUSIONS

Our results not only demonstrated the immunogenicity of GFP in immunocompetent mice, but determined the optimized conditions for GFP-based in vivo stem cells tracking, that can potentially be extrapolated to human biomedical research.

Distinct strategies are required to suppress antigen-specific responses to genetically modified keratinocytes and fibroblasts

Keratinocytes and fibroblasts are potential targets of gene/cell therapy for genodermatoses. Immune elimination of genetically modified cells, however, presents a major impediment to effective therapy. Using ex vivo approaches to gene transfer, we have previously shown that expression of an antigen by either cell type in skin induces immune rejection of transplanted cells, although the nature of immune responses induced by these two cell types are distinct. In this study, we explore the efficacy of local immunosuppressive strategies to divert destructive immune responses from genetically modified fibroblast and keratinocytes. Expression of CTLA4Ig and, to a lesser extent, PDL1, by antigenic fibroblasts protected them from immune rejection resulting in long-term graft survival (>18 weeks). Similar treatment was not effective for antigenic keratinocytes. Long-term protection of transgenic keratinocytes was achieved through transient blockade of CD40/CD154 interactions during the first 2 weeks of cell transplantation. Although neither of these strategies induced antigen-specific tolerance, they were sufficient to prevent rejection of genetically modified cells. These results indicate that different strategies are required to protect antigenic cell types even within the same tissue. Moreover, induction of antigen-specific tolerance is not a necessary requirement for long-term survival of genetically modified skin cells.

Correction of dog dystrophic epidermolysis bullosa by transplantation of genetically modified epidermal autografts

Recessive dystrophic epidermolysis bullosa (RDEB) is a severe skin blistering condition caused by mutations in the gene coding for collagen type VII. Genetically engineered RDEB dog keratinocytes were used to generate autologous epidermal sheets subsequently grafted on two RDEB dogs carrying a homozygous missense mutation in the col7a1 gene and expressing baseline amounts of the aberrant protein. Transplanted cells regenerated a differentiated and vascularized auto-renewing epidermis progressively repopulated by dendritic cells and melanocytes. No adverse immune reaction was detected in either dog. In dog 1, the grafted epidermis firmly adhered to the dermis throughout the 24-month follow-up, which correlated with efficient transduction (100%) of highly clonogenic epithelial cells and sustained transgene expression. In dog 2, less efficient (65%) transduction of primary keratinocytes resulted in a loss of the transplanted epidermis and graft blistering 5 months after transplantation. These data provide the proof of principle for ex vivo gene therapy of RDEB patients with missense mutations in collagen type VII by engraftment of the reconstructed epidermis, and demonstrate that highly efficient transduction of epidermal stem cells is crucial for successful gene therapy of inherited skin diseases in which correction of the genetic defect confers no major selective advantage in cell culture.

Protein kinase D is implicated in the reversible commitment to differentiation in primary cultures of mouse keratinocytes

Although commitment to epidermal differentiation is generally considered to be irreversible, differentiated keratinocytes (KCs) have been shown to maintain a regenerative potential and to reform skin epithelia when placed in a suitable environment. To obtain insights into the mechanism of reinitiation of this proliferative response in differentiated KCs, we examined the reversibility of commitment to Ca(2+)-induced differentiation. Lowering Ca(2+) concentration to micromolar levels triggered culture-wide morphological and biochemical changes, as indicated by derepression of cyclin D1, reinitiation of DNA synthesis, and acquisition of basal cell-like characteristics. These responses were inhibited by Goedecke 6976, an inhibitor of protein kinase D (PKD) and PKCalpha, but not with GF109203X, a general inhibitor of PKCs, suggesting PKD activation by a PKC-independent mechanism. PKD activation followed complex kinetics with a biphasic early transient phosphorylation within the first 6 h, followed by a sustained and progressive phosphorylation beginning at 24 h. The second phase of PKD activation was followed by prolonged ERK1/2 signaling and progression to DNA synthesis in response to the low Ca(2+) switch. Specific knockdown of PKD-1 by RNA interference or expression of a dominant negative form of PKD-1 did not have a significant effect on normal KC proliferation and differentiation but did inhibit Ca(2+)-mediated reinitiation of proliferation and reversion in differentiated cultures. The present study identifies PKD as a major regulator of a proliferative response in differentiated KCs, probably through sustained activation of the ERK-MAPK pathway, and provides new insights into the process of epidermal regeneration and wound healing.

Characterization of rat hair follicle stem cells selected by vario magnetic activated cell sorting system

Hair follicle stem cells (HfSCs) play crucial roles in hair follicle morphogenesis and hair cycling. These stem cells are self-renewable and have the multi-lineage potential to generate epidermis, sebaceous glands, and hair follicle. The separation and identification of hair follicle stem cells are important for further research in stem cell biology. In this study, we report on the successful enrichment of rat hair follicle stem cells through vario magnetic activated cell sorting (Vario MACS) and the biological characteristics of the stem cells. We chose the HfSCs positive surface markers CD34, α6-integrin and the negative marker CD71 to design four isolation strategies: positive selection with single marker of CD34, positive selection with single marker of α6-integrin, CD71 depletion followed by CD34 positive selection, and CD71 depletion followed by α6-integrin positive selection. The results of flow cytometry analysis showed that all four strategies had ideal effects. Specifically, we conducted a series of researches on HfSCs characterized by their high level of CD34, termed CD34^bri cells, and low to undetectable expression of CD34, termed CD34^dim cells. CD34^bri cells had greater proliferative potential and higher colony-forming ability than CD34^dim cells. Furthermore, CD34^bri cells had some typical characteristics as progenitor cells, such as large nucleus, obvious nucleolus, large nuclear:cytoplasmic ratio and few cytoplasmic organelles. Our findings clearly demonstrated that HfSCs with high purity and viability could be successfully enriched with Vario MACS.

Regeneration of multilineage skin epithelia by differentiated keratinocytes

Although homeostasis of rapidly renewing tissues like skin epithelia is maintained by stem cells, the committed progeny of stem cells in the basal layer of epidermis retain regenerative potential and are capable of forming epidermis in response to environmental cues. It is not clear, however, at what point within the epidermal lineage keratinocytes lose this regenerative potential. In this study, we examined the extent of tissue formation by post-mitotic differentiated keratinocytes. We show that cultures of mouse keratinocytes that were, by all measures, differentiated were able to reform a self-renewing, hair-bearing skin when transplanted onto suitable sites in vivo. Genetic labeling and lineage-tracing studies in combination with an involucrin-driven Cre/lox reporter system confirmed that transplanted differentiated keratinocytes were indeed the source of the regenerated skin. More importantly, analysis of early stages of skin regeneration showed hallmarks of dedifferentiation of transplanted differentiated keratinocytes. These data indicate that commitment to differentiation does not prohibit cells from re-entering the cell cycle, de-differentiating, and acquiring "stemness". These findings suggest that epidermis can use different strategies for homeostasis and tissue regeneration.

Transgene-specific host responses in cutaneous gene therapy: the role of cells expressing the transgene

A major issue in long-term gene therapy is host immune responses to therapeutic cells when transgene encodes a potential antigen. The nature of these responses depends on several factors including the type of cell and tissue expressing the transgene. Keratinocytes and fibroblasts, which are known to display distinct immunogenic profiles, are both potential targets for transgene expression in cutaneous gene therapy. However, whether there is an immunological advantage in targeting one cell type over the other is not known. To study the effect of cell type on transgene-specific host responses independent of antigen levels or methods of gene transfer and transplantation, we used a skin transplantation model in which transgene expression can be targeted transgene to either keratinocytes or fibroblasts. Although targeting an antigen to either cell type resulted in the induction of immune responses, these responses differed significantly. Transgenic keratinocytes were rejected acutely by a dominant Th2 response, while in the majority of grafted animals transgenic fibroblasts failed to induce acute rejection despite the induction of Th1 type inflammation in the graft. In a small number of mice, transgenic fibroblasts persisted for at least 20 weeks despite elicitation of antigen-specific responses. Therefore, fibroblasts may be an immunologically preferred target over keratinocytes for cutaneous gene therapy.

Genetic modification of cells for transplantation

Progress in gene therapy has produced promising results that translate experimental research into clinical treatment. Gene modification has been extensively employed in cell transplantation. The main barrier is an effective gene delivery system. Several viral vectors were utilized in end-stage differentiated cells. Recently, successful applications were described with adenovirus-associated vectors. As an alternative, embryonic stem cell- and stem cell-like systems were established for generation of tissue-specified gene-modified cells. Owing to the feasibility for genetic manipulations and the self-renewing potency of these cells they can be used in a way enabling large-scale in vitro production. This approach offers the establishment of in vitro cell culture systems that will deliver sufficient amounts of highly purified, immunoautologous cells suitable for application in regenerative medicine. In this review, the current technology of gene delivery systems to cells is recapitulated and the latest developments for cell transplantation are discussed.

Human bullous pemphigoid antigen 2 transgenic skin elicits specific IgG in wild-type mice

Bullous pemphigoid antigen 2 (BPAG2) is targeted by autoantibodies in patients with bullous pemphigoid (BP), and absent in patients with one type of epidermolysis bullosa (OMIM #226650). A keratin 14 promoter construct was used to produce transgenic (Tg) mice appropriately expressing human BPAG2 (hBPAG2) in murine epidermal basement membrane (BM). Grafts of Tg skin placed on gender-matched, syngeneic wild type (Wt) or major histocompatibility complex I (MHC I)-/- mice elicited IgG that bound human epidermal BM and BPAG2. Production of such IgG in grafted mice was prompt (detectable within 16+/-2 days), robust (titer > or = 1,280), durable (present > or = 380 days), and correlated with the involution and loss of Tg skin grafts. MHC II-/- mice grafted with Tg skin did not develop anti-hBPAG2 IgG or graft loss indicating that MHC II:CD4+ T cell interactions were crucial for these responses. Tg skin grafts on Wt mice developed neutrophil-rich infiltrates, dermal edema, subepidermal blisters, and deposits of immunoreactants in epidermal BM. This model shows fidelity to alterations seen in patients with BP, has relevance to immune responses that may arise in patients with epidermolysis bullosa following BPAG2 gene replacement, and can be used to identify interventions that may block production of IgG against proteins in epidermal BM.

Long-term Engraftment of Single Genetically Modified Human Epidermal Holoclones Enables Safety Pre-assessment of Cutaneous Gene Therapy

Predicting the risks of permanent gene therapy approaches involving the use of integrative gene-targeting vectors has become a critical issue after the unfortunate episode of a clinical trial in children with X-linked severe combined immunodeficiency (X-SCID). Safety pre-assessment of single isolated gene-targeted stem cells or their derivative clones able to regenerate their tissue of origin would be a major asset in addressing untoward gene therapy effects in advance. Human epidermal stem cells, which have extensive proliferative potential in vitro, theoretically offer such a possibility as a method of assessment. By means of optimized organotypic culture and grafting methods, we demonstrate the long-term in vivo regenerative capacity of single gene-targeted human epidermal stem cell clones (holoclones). Both histopathological analysis of holoclone-derived grafts in immunodeficient mice and retroviral insertion site mapping performed in the holoclone in vitro and after grafting provide proof of the feasibility of pre-assessing genotoxicity risks in isolated stem cells before transplantation into patients. Our results provide an experimental basis for previously untested assumptions about the in vivo behavior of epidermal stem cells prospectively isolated in vitro and pave the way for a safer approach to cutaneous gene therapy.

Hair follicle stem cells: walking the maze

The discovery of epithelial stem cells (eSCs) in the bulge region of the outer root sheath of hair follicles in mice and man has encouraged research into utilizing the hair follicle as a therapeutic source of stem cells (SCs) for regenerative medicine, and has called attention to the hair follicle as a highly instructive model system for SC biology. Under physiological circumstances, bulge eSCs serve as cell pool for the cyclic regeneration of the anagen hair bulb, while they can also regenerate the sebaceous gland and the epidermis after injury. More recently, melanocyte SCs, nestin+, mesenchymal and additional, as yet ill-defined "stem cell" populations, have also been identified in or immediately adjacent to the hair follicle epithelium, including in the specialized hair follicle mesenchyme (connective tissue sheath), which is crucial to wound healing. Thus the hair follicle and its adjacent tissue environment contain unipotent, multipotent, and possibly even pluripotent SC populations of different developmental origin. It provides an ideal model system for the study of central issues in SC biology such as plasticity and SC niches, and for the identification of reliable, specific SC markers, which distinguish them from their immediate progeny (e.g. transient amplifying cells). The current review attempts to provide some guidance in this growing maze of hair follicle-associated SCs and their progeny, critically reviews potential or claimed hair follicle SC markers, highlights related differences between murine and human hair follicles, and defines major unanswered questions in this rapidly advancing field.

Loss of transgene following ex vivo gene transfer is associated with a dominant Th2 response: implications for cutaneous gene therapy

Host responses to therapeutic gene products are potentially serious complications in cutaneous gene therapy. Controlling immune responses to the therapeutic antigen may therefore be critical for an effective therapy. Both ex vivo and in vivo gene transfer to epidermal stem cells has been shown to induce transgene-specific immune responses; however, whether the mechanism of immune activation is the same is not clear. In this study, we have characterized transgene-specific immune responses in an ex vivo model of epidermal gene transfer using green fluorescent protein as a model antigen and retrovirus-mediated gene delivery. Contrary to T helper (Th)1-type responses induced following in vivo gene transfer to epidermis, rejection of ex vivo-transduced keratinocytes was associated with Th2/eosinophilc responses. These responses were characterized by interleukin (IL)-4 and IL-5 production by T cells, a predominance of anti-green fluorescent protein IgG1 in serum, the presence of numerous eosinophils within rejected skin, and a lack of class I-restricted cytotoxic T lymphocyte response. Pretreatment of mice receiving ex vivo transduced keratinocytes with neutralizing anti-IL-5 antibody prevented eosinophil infiltration and prolonged survival of transduced epidermis. These data indicate a role for the Th2/eosinophilic pathway in rejection of ex vivo-transduced keratinocytes, suggesting different requirements for achieving tolerance for ex vivo and in vivo approaches to gene therapy.

Cutaneous gene therapy

Possibilities of using the skin for somatic gene therapy have been investigated for more than 20 years. Strategies have included both direct gene transfer into the skin and indirect gene transfer utilizing cultured cells as an intermediate step for gene manipulation. Viral as well as nonviral vectors have been used, and both gene addition and gene editing have been performed. Although cutaneous gene therapy has now begun translating into clinical medicine (as seen by the first clinical gene therapy project of an inherited skin disorder) further developments are still required.

Gene therapy progress and prospects: the skin – easily accessible, but still far away

Significant progress has been made in corrective gene therapy of inherited skin diseases. This includes advances in vector technology, targeted gene expression, gene replacement, and the availability of appropriate animal models for a variety of candidate diseases. In addition, an increased understanding of the uptake and trafficking mechanisms inside keratinocytes has evolved. Topical application facilitates DNA vaccination through the skin, albeit clinical benefits have not yet materialized. However, the translation into clinical trials has only been partially mastered. The latter and the control of immune responses represent challenges for the research community.

A large preclinical animal model to assess ex vivo skin gene therapy applications

Because of its easy accessibility, the skin is a very attractive target for gene therapy purposes. To study potential clinical applications in a preclinical setting, appropriate animal models are needed. Pig skin is very similar to human skin, and a variety of human diseases that are potentially amenable to gene therapy applications also occur in pigs. Only a few studies have analyzed the engraftment of transduced keratinocytes (KC) in pigs, however, with limited success. We describe a porcine model in which pig KC were transduced ex vivo with a retroviral vector encoding a marker gene and subsequently grafted onto the autologous host, utilizing a relatively simple grafting technique. Enhanced transduction efficiency was achieved by an optimized transduction protocol including centrifugation of the retroviral vector at a temperature of 32 degrees C. Transduced KC were then seeded onto acellular dermis, forming a stratified epidermis. Grafting was performed by creating full thickness wounds and placing the skin graft onto the muscle fascia, covered by a protective skin flap for several days. Successful engraftment of transduced KC was demonstrated by immunohistochemistry of biopsies taken at different time points, showing transgene expression in 40-50% of grafted KC. After 4 weeks, KC expressing a foreign marker gene was lost, suggesting a transgene-specific immune response in the immunocompetent pigs and highlighting the potential problems for clinical gene therapy studies when transferring new genetic material into a patient. The model presented here may be used to examine applications of skin gene therapy, where retroviral vectors encoding endogenous pig genes will be expressed in the skin.

Sustained phenotypic reversion of junctional epidermolysis bullosa dog keratinocytes: Establishment of an immunocompetent animal model for cutaneous gene therapy

Gene transfer represents the unique therapeutic issue for a number of inherited skin disorders including junctional epidermolysis bullosa (JEB), an untreatable genodermatose caused by mutations in the adhesion ligand laminin 5 (alpha3beta3gamma2) that is secreted in the extracellular matrix by the epidermal basal keratinocytes. Because gene therapy protocols require validation in animal models, we have phenotypically reverted by oncoretroviral transfer of the curative gene the keratinocytes isolated from dogs with a spontaneous form of JEB associated with a genetic mutation in the alpha3 chain of laminin 5. We show that the transduced dog JEB keratinocytes: (1) display a sustained secretion of laminin 5 in the extracellular matrix; (2) recover the adhesion, proliferation, and clonogenic capacity of wild-type keratinocytes; (3) generate fully differentiated stratified epithelia that after grafting on immunocompromised mice produce phenotypically normal skin and sustain permanent expression of the transgene. We validate an animal model that appears particularly suitable to demonstrate feasibility, efficacy, and safety of genetic therapeutic strategies for cutaneous disorders before undertaking human clinical trials.