Biology of Langerhans cells: selective migration of Langerhans cells into allogeneic and xenogeneic grafts on nude mice

Mouse Models for Human Skin Transplantation: A Systematic Review

Immunodeficient mouse models with human skin xenografts have been developed in the past decades to study different conditions of the skin. Features such as follow-up period and size of the graft are of different relevance depending on the purpose of an investigation. The aim of this study is to analyze the different mouse models grafted with human skin. A systematic review of the literature was performed in line with the PRISMA statement using MEDLINE/PubMed databases from January 1970 to June 2020. Articles describing human skin grafted onto mice were included. Animal models other than mice, skin substitutes, bioengineered skin, postmortem or fetal skin, and duplicated studies were excluded. The mouse strain, origin of human skin, graft dimensions, follow-up of the skin graft, and goals of the study were analyzed. Ninety-one models were included in the final review. Five different applications were found: physiology of the skin (25 models, mean human skin graft size 1.43 cm2 and follow-up 72.92 days), immunology and graft rejection (17 models, mean human skin graft size 1.34 cm2 and follow-up 86 days), carcinogenesis (9 models, mean human skin graft size 1.98 cm2 and follow-up 253 days), skin diseases (25 models, mean human skin graft size 1.55 cm2 and follow-up 86.48 days), and would healing/scars (15 models, mean human skin graft size 2.54 cm2 and follow-up 129 days). The follow-up period was longer in carcinogenesis models (253 ± 233.73 days), and the skin graft size was bigger in wound healing applications (2.54 ± 3.08 cm2). Depending on the research application, different models are suggested. Careful consideration regarding graft size, follow-up, immunosuppression, and costs should be analyzed and compared before choosing any of these mouse models. To our knowledge, this is the first systematic review of mouse models with human skin transplantation.

UVB-induced depletion of donor-derived dendritic cells prevents allograft rejection of immune-privileged hair follicles in humanized mice

Dendritic cells (DCs) are key targets for immunity and tolerance induction; they present donor antigens to recipient T cells by donor- and recipient-derived pathways. Donor-derived DCs, which are critical during the acute posttransplant period, can be depleted in graft tissue by forced migration via ultraviolet B light (UVB) irradiation. Here, we investigated the tolerogenic potential of donor-derived DC depletion through in vivo and ex vivo UVB preirradiation (UV) combined with the injection of anti-CD154 antibody (Ab) into recipients in an MHC-mismatched hair follicle (HF) allograft model in humanized mice. Surprisingly, human HF allografts achieved long-term survival with newly growing pigmented hair shafts in both Ab-treated groups (Ab-only and UV plus Ab) and in the UV-only group, whereas the control mice rejected all HF allografts with no hair regrowth. Perifollicular human CD3⁺ T cell and MHC class II⁺ cell infiltration was significantly diminished in the presence of UV and/or Ab treatment. HF allografts in the UV-only group showed stable maintenance of the immune privilege in the HF epithelium without evidence of antigen-specific T cell tolerance, which is likely promoted by normal HFs in vivo. This immunomodulatory strategy targeting the donor tissue exhibited novel biological relevance for clinical allogeneic transplantation without generalized immunosuppression.

Adult zebrafish Langerhans cells arise from hematopoietic stem/progenitor cells

The origin of Langerhans cells (LCs), which are skin epidermis-resident macrophages, remains unclear. Current lineage tracing of LCs largely relies on the promoter-Cre-LoxP system, which often gives rise to contradictory conclusions with different promoters. Thus, reinvestigation with an improved tracing method is necessary. Here, using a laser-mediated temporal-spatial resolved cell labeling method, we demonstrated that most adult LCs originated from the ventral wall of the dorsal aorta (VDA), an equivalent to the mouse aorta, gonads, and mesonephros (AGM), where both hematopoietic stem cells (HSCs) and non-HSC progenitors are generated. Further fine-fate mapping analysis revealed that the appearance of LCs in adult zebrafish was correlated with the development of HSCs, but not T cell progenitors. Finally, we showed that the appearance of tissue-resident macrophages in the brain, liver, heart, and gut of adult zebrafish was also correlated with HSCs. Thus, the results of our study challenged the EMP-origin theory for LCs.

Langerhans cell: exciting developments in health and disease

Langerhans cells (LCs) have been the subject of much research since their discovery in 1868. LCs belong to the subset of leucocytes called dendritic cells. They are present in the epidermis and the pilosebaceous apparatus and monitor the cutaneous environment for changes in homeostasis. During embryogenesis, a wave of yolk sac macrophages seed the fetal skin. Then, fetal liver monocytes largely replace the yolk sac macrophages and comprise the majority of adult LCs. In the presence of skin irritation, LCs process antigen and travel to regional lymph nodes to present antigen to reactive T lymphocytes. Changes in LCs' surface markers during the journey occur under the influence of cytokines. The difference in expression of surface markers and the ability to resist radiation have allowed researchers to differentiate LCs from the murine Langerin-positive dermal dendritic cells. Exciting discoveries have been made recently regarding their role in inflammatory skin diseases, cancer and HIV. New research has shown that antibodies blocking CD1a appear to mitigate inflammation in contact hypersensitivity reactions and psoriasis. While it has been established that LCs have the potential to induce effector cells of the adaptive immune system to counter oncogenesis, recent studies have demonstrated that LCs coordinate with natural killer cells to impair development of squamous cell carcinoma caused by chemical carcinogens. However, LCs may also physiologically suppress T cells and permit keratinocyte transformation and tumorigenesis. Although long known to play a primary role in the progression of HIV infection, it is now understood that LCs also possess the ability to restrict the progression of the disease. There is a pressing need to discover more about how these cells affect various aspects of health and disease; new information gathered thus far seems promising and exciting.

Langerhans cells: an update

Langerhans cells belong to the family of dendritic cells, professional antigen-presenting cells, and populate the skin and epithelia of mammals. It was the extensive investigation of this particular dendritic cell subpopulation in earlier days, which contributed crucially to the current understanding of the regulation of antigen processing and presentation, a concept, which was termed "the Langerhans cell paradigm". Extensive research during the last decades has revealed that Langerhans cells might not only be involved in the induction of adaptive immune responses but also in the maintenance of peripheral tolerance in order to prevent auto-immunity. In addition it appeared that Langerhans cells represent a rather extravagant dendritic cell population with a unique origin and homeostatic regulation. This review highlights the most important findings about Langerhans cell ontogeny and homeostasis as well as their function in the immune system.

Ontogeny of Tissue-Resident Macrophages

The origin of tissue-resident macrophages, crucial for homeostasis and immunity, has remained controversial until recently. Originally described as part of the mononuclear phagocyte system, macrophages were long thought to derive solely from adult blood circulating monocytes. However, accumulating evidence now shows that certain macrophage populations are in fact independent from monocyte and even from adult bone marrow hematopoiesis. These tissue-resident macrophages derive from sequential seeding of tissues by two precursors during embryonic development. Primitive macrophages generated in the yolk sac (YS) from early erythro-myeloid progenitors (EMPs), independently of the transcription factor c-Myb and bypassing monocytic intermediates, first give rise to microglia. Later, fetal monocytes, generated from c-Myb⁺ EMPs that initially seed the fetal liver (FL), then give rise to the majority of other adult macrophages. Thus, hematopoietic stem cell-independent embryonic precursors transiently present in the YS and the FL give rise to long-lasting self-renewing macrophage populations.

Regulation of Dendritic Cell Function in Inflammation

Dendritic cells (DC) are professional antigen presenting cells and link the innate and adaptive immune system. During steady state immune surveillance in skin, DC act as sentinels against commensals and invading pathogens. Under pathological skin conditions, inflammatory cytokines, secreted by surrounding keratinocytes, dermal fibroblasts, and immune cells, influence the activation and maturation of different DC populations including Langerhans cells (LC) and dermal DC. In this review we address critical differences in human DC subtypes during inflammatory settings compared to steady state. We also highlight the functional characteristics of human DC subsets in inflammatory skin environments and skin diseases including psoriasis and atopic dermatitis. Understanding the complex immunoregulatory role of distinct DC subsets in inflamed human skin will be a key element in developing novel strategies in anti-inflammatory therapy.

Langerhans cells come in waves

It is unclear how the Langerhans cell (LC) network is maintained in adult epidermis. In this issue of Immunity, Seré et al. (2012) show that LCs are replenished in two waves. Monocyte-derived, short-lived LCs come first. A second wave follows, and these LCs of nonmonocytic origin are long-lived.

Human epidermal Langerhans cells replenish skin xenografts and are depleted by alloreactive T cells in vivo

Epidermal Langerhans cells (LC) are potent APCs surveying the skin. They are crucial regulators of T cell activation in the context of inflammatory skin disease and graft-versus-host disease (GVHD). In contrast to other dendritic cell subtypes, murine LC are able to reconstitute after local depletion without the need of peripheral blood-derived precursors. In this study, we introduce an experimental model of human skin grafted to NOD-SCID IL2Rγ(null) mice. In this model, we demonstrate that xenografting leads to the transient loss of LC from the human skin grafts. Despite the lack of a human hematopoietic system, human LC repopulated the xenografts 6 to 9 wk after transplantation. By staining of LC with the proliferation marker Ki67, we show that one third of the replenishing LC exhibit proliferative activity in vivo. We further used the skin xenograft as an in vivo model for human GVHD. HLA-disparate third-party T cells stimulated with skin donor-derived dendritic cells were injected intravenously into NOD-SCID IL2Rγ(null) mice that had been transplanted with human skin. The application of alloreactive T cells led to erythema and was associated with histological signs of GVHD limited to the transplanted human skin. The inflammation also led to the depletion of LC from the epidermis. In summary, we provide evidence that human LC are able to repopulate the skin independent of blood-derived precursor cells and that this at least partly relates to their proliferative capacity. Our data also propose xeno-transplantation of human skin as a model system for studying the role of skin dendritic cells in the efferent arm of GVHD.

Langerhans cells and more: langerin-expressing dendritic cell subsets in the skin

Langerhans cells (LCs) are antigen-presenting dendritic cells (DCs) that reside in epithelia. The best studied example is the LC of the epidermis. By electron microscopy, their identifying feature is the unique rod- or tennis racket-shaped Birbeck granule. The phenotypic hallmark is their expression of the C-type lectin receptor langerin/CD207. Langerin, however, is also expressed on a recently discovered population of DC in the dermis and other tissues of the body. These 'dermal langerin(+) dendritic cells' are unrelated to LCs. The complex field of langerin-negative dermal DCs is not dealt with here. In this article, we briefly review the history, ontogeny, and homeostasis of LCs. More emphasis is laid on the discussion of functional properties in vivo. Novel models using genetically engineered mice are contributing tremendously to our understanding of the role of LCs in eliciting adaptive immune responses against pathogens or tumors and in inducing and maintaining tolerance against self antigens and innocuous substances in vivo. Also, innate effector functions are increasingly being recognized. Current activities in this area are reviewed, and possibilities for future exploitation of LC in medicine, e.g. for the improvement of vaccines, are contemplated.

Ontogeny and homeostasis of Langerhans cells

Langerhans cells (LCs) refer to the dendritic cells (DCs) that populate the epidermis. Strategically located at one of the body's largest interfaces with the external environment, they form the first line of defense against pathogens that breach the skin. Although LCs share several phenotypical and functional features with lymphoid and non-lymphoid organ DCs, they also have unique properties that distinguish them from most DC populations. In this review, we will discuss the key mechanisms that regulate LC homeostasis in quiescent and inflamed skin. We will also discuss recent evidence that suggests that LCs arise from dedicated precursors during early embryonic development.

Development of an ex vivo human skin model for intradermal vaccination: tissue viability and Langerhans cell behaviour

The presence of resident Langerhans cells (LCs) in the epidermis makes the skin an attractive target for DNA vaccination. However, reliable animal models for cutaneous vaccination studies are limited. We demonstrate an ex vivo human skin model for cutaneous DNA vaccination which can potentially bridge the gap between pre-clinical in vivo animal models and clinical studies. Cutaneous transgene expression was utilised to demonstrate epidermal tissue viability in culture. LC response to the culture environment was monitored by immunohistochemistry. Full-thickness and split-thickness skin remained genetically viable in culture for at least 72 h in both phosphate-buffered saline (PBS) and full organ culture medium (OCM). The epidermis of explants cultured in OCM remained morphologically intact throughout the culture duration. LCs in full-thickness skin exhibited a delayed response (reduction in cell number and increase in cell size) to the culture conditions compared with split-thickness skin, whose response was immediate. In conclusion, excised human skin can be cultured for a minimum of 72 h for analysis of gene expression and immune cell activation. However, the use of split-thickness skin for vaccine formulation studies may not be appropriate because of the nature of the activation. Full-thickness skin explants are a more suitable model to assess cutaneous vaccination ex vivo.

Origin, homeostasis and function of Langerhans cells and other langerin-expressing dendritic cells

Langerhans cells (LCs) are a specialized subset of dendritic cells (DCs) that populate the epidermal layer of the skin. Langerin is a lectin that serves as a valuable marker for LCs in mice and humans. In recent years, new mouse models have led to the identification of other langerin(+) DC subsets that are not present in the epidermis, including a subset of DCs that is found in most non-lymphoid tissues. In this Review we describe new developments in the understanding of the biology of LCs and other langerin(+) DCs and discuss the challenges that remain in identifying the role of different DC subsets in tissue immunity.

Human epidermal Langerhans cells express the tight junction protein claudin-1 and are present in human genetic claudin-1 deficiency (NISCH syndrome)

Claudin-1 (CLDN1) is a structural tight junction (TJ) protein and is expressed in differentiating keratinocytes and Langerhans cells in the epidermis. Our objective was to identify immunoreactive CLDN1 in human epidermal Langerhans cells and to examine the pattern of epidermal Langerhans cells in genetic human CLDN1 deficiency [neonatal ichthyosis, sclerosing cholangitis (NISCH) syndrome]. Epidermal cells from healthy human skin labelled with CLDN1-specific antibodies were analysed by confocal laser immunofluorescence microscopy and flow cytometry. Skin biopsy sections of two patients with NISCH syndrome were stained with an antibody to CD1a expressed on epidermal Langerhans cells. Epidermal Langerhans cells and a subpopulation of keratinocytes from healthy skin were positive for CLDN1. The gross number and distribution of epidermal Langerhans cells of two patients with molecularly confirmed NISCH syndrome, however, was not grossly altered. Therefore, CLDN1 is unlikely to play a critical role in migration of Langerhans cells (or their precursors) to the epidermis or their positioning within the epidermis. Our findings do not exclude a role of this TJ molecule once Langerhans cells have left the epidermis for draining lymph nodes.

Long-term productive human immunodeficiency virus infection of CD1a-sorted myeloid dendritic cells

Myeloid, CD1a-sorted dendritic cells (MDC) productively replicated human immunodeficiency virus strains encoding envelope genes of either primary X4R5 or R5 strains for up to 45 days. Cell-free supernatant collected from long-term infected MDC, which had been exposed to an X4R5 virus 45 days earlier, was still infectious when placed over activated T cells. These data imply that DC can act as a persistent reservoir of infectious virus.

Fetal and neonatal murine skin harbors Langerhans cell precursors

Resident epidermal Langerhans cells (LC) in adult mice express ADPase, major histocompatibility complex (MHC) class II, and CD205 and CD207 molecules, while the first dendritic leukocytes that colonize the fetal and newborn epidermis are only ADPase(+). In this study, we tested whether dendritic epidermal leukocytes (DEL) are end-stage cells or represent LC precursors. In epidermal sheets of fetal and neonatal mice, we found no apoptotic leukocytes, suggesting that these cells do not die in situ. To address whether DEL can give rise to LC, sorted DEL from murine newborn skin were cultured with cytokines used to generate LC from human CD34(+) precursors. After 7-14 days, DEL proliferated and acquired the morphology and phenotype of cells reminiscent of LC. In concordance with this finding, we show that neonatal epidermis harbors 10-20 times the number of cycling MHC class II(+) leukocytes as adult tissue. To test whether LC can differentiate from skin precursors in vivo, we developed a transplantation model. As it was impossible to transplant fetal epidermis, whole fetal skin was grafted onto adult severe combined immunodeficient mice. As opposed to the uniform absence of donor LC at the time of transplantation, examination of the epidermis from the grafts after 2-4 weeks revealed MHC class II(+) donor cells, which had acquired CD205 and CD207, thus qualifying them as LC. Finally, we present evidence that endogenous LC persist in skin grafts for the observation period of 45 days. These studies show that hematopoietic precursors seed the skin during embryonic life and can give rise to LC.

Langerhans cells - dendritic cells of the epidermis

Langerhans cells (LC) are dendritic cells of the epidermis. They are highly specialized leukocytes that serve immunogenic and tolerogenic purposes. Here, we review some aspects of LC biology, emphasizing those areas where LC are or may turn out to be special.

Langerhans cells renew in the skin throughout life under steady-state conditions

Langerhans cells (LCs) are bone marrow (BM)-derived epidermal dendritic cells (DCs) that represent a critical immunologic barrier to the external environment, but little is known about their life cycle. Here, we show that in lethally irradiated mice that had received BM transplants, LCs of host origin remained for at least 18 months, whereas DCs in other organs were almost completely replaced by donor cells within 2 months. In parabiotic mice with separate organs, but a shared blood circulation, there was no mixing of LCs. However, in skin exposed to ultraviolet light, LCs rapidly disappeared and were replaced by circulating LC precursors within 2 weeks. The recruitment of new LCs was dependent on their expression of the CCR2 chemokine receptor and on the secretion of CCR2-binding chemokines by inflamed skin. These data indicate that under steady-state conditions, LCs are maintained locally, but inflammatory changes in the skin result in their replacement by blood-borne LC progenitors.

Effect of granulocyte-macrophage colony-stimulating factor on the generation of epidermal Langerhans cells

The role of granulocyte-macrophage colony-stimulating factor (GM-CSF) and Flt3 ligand in the in vivo development of Langerhans cells (LC) was assessed, considering both the steady-state levels of LC in the epidermis and the rate of LC recovery after depletion following lipopolysaccharide (LPS) treatment. The density of LC was determined by counting following IA-specific immunofluorescent staining of epidermal sections from mouse ears. LC levels were compared in beta common chain receptor null (beta c(-/-)) mice that fail to respond to GM-CSF interleukin-5 (IL-5), in GM-CSF transgenic mice with elevated GM-CSF levels, and in mice given daily injections of Flt3 ligand. In the steady state, LC levels were increased in GM-CSF transgenic mice and present at reduced levels in beta c(-/-) mice but unchanged in Flt3 ligand-injected mice. Application of LPS to the ears of control BL/6 mice led to an approximately 70% reduction in LC 4 days later, with recovery beginning by day 8 and a return to normal levels by 2 weeks. This recovery was significantly delayed in beta c(-/-) mice and unchanged in Flt3 ligand-injected mice. These results suggest that GM-CSF (but not Flt3 ligand) enhances recruitment/maturation of LC even though GM-CSF is not essential for their formation.

Dendritic cells lose ability to present protein antigen after stimulating antigen-specific T cell responses, despite upregulation of MHC class II expression

Immature dendritic cells (DC) take up, process and present protein antigens; mature DC are specialized for stimulating primary T cell responses with increased expression of MHC class II and co-stimulatory molecules, but are incapable of processing and presenting soluble protein. The current study examined whether maturation of DC is triggered by T cell recognition of antigens presented by immature DC. Human DC derived from CD34+ progenitor cells by culture with granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-6 (IL-6) in serum-free medium could prime naive CD4+ T cells to keyhole limpet hemocyanin (KLH) and ovalbumin (OVA). The cultured DC retained the ability to prime T cells to native protein for at least 15 days. To test for changes in DC function after participation in an immune response, DC were co-cultured with either allogeneic or autologous CD4+ T cells. DC co-cultured with autologous T cells retained the ability to prime T cells to intact protein antigens. By contrast, DC which had previously stimulated an allogeneic T cell response lost ability to prime T cells to soluble proteins. However, such <<T cell-activated DC>> induced a MLR and stimulated peptide-specific primary CD4+ T cell responses. This indicated that <<T cell-activated DC>> did not die or lose the ability to prime, but lost the ability to process and present subsequent antigens. Following participation in T cell activation, DC increased surface expression of MHC class II, co-stimulatory molecules CD40 and B7.2, and the intercellular adhesion molecule-1 (ICAM-1). In addition, our data suggest that interferon gamma (IFN-gamma) and tumor necrosis factor alpha (TNF-alpha) are involved in this T cell-mediated DC maturation.

The repopulation of murine Langerhans cells after depletion by mild heat injury

We have developed a model of focal Langerhans cell depletion by mild heat injury and used it to investigate the mechanisms of Langerhans cell repopulation in the injured epidermis. The possibility whether repopulation occurred by recruitment of precursor cells from the circulation or dermis or, alternatively, by migration from the surrounding normal epidermis into the injured area was considered. Repopulation was studied by evaluating the pattern of Langerhans cell reappearance and calculating the rate of change in the density. Heat injury followed by whole-body irradiation with shielding of the injured skin was used to assess repopulation in the absence of bone marrow precursors. Using tritiated thymidine autoradiography, we also investigated whether the newly arrived Langerhans cells (be they from circulating precursors or surrounding normal epidermis) actually divide. The results showed that heat injury completely eliminated the Langerhans cells within the area delineated by the injury. Two hours after injury, the Langerhans cells were fragmented and 2 days later, they could not be detected. Regeneration of the epidermis occurred 2 days after injury and Langerhans cells reappeared scattered somewhat sparsely in the centre of the lesion on day 3. These cells were small and slender, bearing one or two short dendrites. As the dendrites increased in number and in length, the cells became similar morphologically and phenotypically to normal Langerhans cells. The rate of repopulation increased dramatically between days 5 and 7 and reached normal density on day 11. The pattern of Langerhans cell repopulation in the injured area and the lack of repopulation in the irradiated animals indicated that repopulation occurs by immigration of precursors from the circulation or dermis. There was no indication of migration of Langerhans cells from surrounding normal epidermis. Lastly, the newly arrived Langerhans cells failed to divide at the site of injury.

Epidermal denervation and its effects on keratinocytes and Langerhans cells

Skin innervation has been considered to subserve sensory perception only, but several lines of evidence suggest that there are "effector' influences of skin innervation on the immune system and keratotinocytes. In this study, we transected the sciatic nerves of rats and examined the effects of denervation on the epidermis. In normal skin, the epidermis was densely innervated by fine axons that were immunostained with several axonal markers, including neuronal ubiquitin carboxyl terminal hydrolase (protein gene product 9.5). All of the epidermal axons in the regions innervated by sciatic nerve disappeared within 24-48 h after transection of sciatic nerve, and remained absent as long as subsequent reinnervation by regenerating axonal sprouts was prevented. Denervation produced changes in both the keratinocytes and the Langerhans cells, the bone marrow-derived antigen-presenting cells of the epidermis. The thickness of epidermis decreased within 7 days. By 48 h after transection, the Langerhans cells and their dendritic processes became intensely immunoreactive for protein gene product. Protein gene product 9.5 expression on Langerhans cells remained prominent as long as skin was denervated, but disappeared with reinnervation. By reverse transcription-polymerase chain reaction, we demonstrated the presence of the transcripts for protein gene product 9.5 in epidermis, consistent with the synthesis of the protein by the Langerhans cells. We conclude that epidermal sensory fibres have novel influences on both keratinocytes and Langerhans cells of the epidermis.

Origin and steady-state turnover of major histocompatibility complex class II-positive dendritic cells and resident-tissue macrophages in the iris of the rat eye

Recent studies have identified distinct but co-existing networks of resident tissue macrophages and MHC class II-positive DC present in tissues bordering the anterior chamber of the eye, a site classically regarded as 'immune-privileged'. As the DC network, present at approximately 500 cells/mm2, accounts for virtually all MHC class II immunostaining in these tissues and possesses potent capacity to stimulate primary allogenic responses in vitro, it is proposed that these cells may play an important role in immune surveillance of the anterior chamber. Tissue macrophage and DC population kinetics in the iris were examined by using X-irradiation exposure to interrupt the steady-state renewal of these cells by haematopoietically derived precursors. MHC class II-positive iris DC exhibited a half-life of approximately 3 days, a rapid turnover rate which closely resembled that of DC present in mucosal epithelia. In contrast, the resident tissue macrophage population displayed a considerably slower turnover (half-life of 10-12 days) comparable to that of epidermal Langerhans cells in the present study. Bone marrow transplantation studies confirmed the haematopoietic origin of the iris DC population. The present study provides the first estimates of the steady-state population kinetics of antigen-presenting cell populations in the iris and has important implications for understanding the role of these cells in immunological homeostasis of the anterior chamber.

Migration of human antigen-presenting cells in a human skin graft onto nude mice model after contact sensitization

Fluorescent contact chemical allergens provoke sensitization after application on both syngeneic and allogeneic skin grafts in mice. We attempted to determine whether the functional activity in a contact sensitization response of human skin graft was affected at the level of antigen uptake and migration. After xenogeneic skin transplantation, we examined the effect of topical exposure of the graft to rhodamine B isothiocyanate (RITC). This paper describes the migration of RITC-carrying cells and human major histocompatibility complex (MHC) class II DR (HLA-DR)+ cells, from the graft to mouse draining lymph nodes. As demonstrated by immunohistochemistry, grafting resulted in a time-dependent decrease of human HLA-DR+ and CD1a+ cells, and an increase of mouse MHC class II (Ia)+ cells within the graft. Application of RITC on a 3-week-old human skin graft showed optimal migration capability compared to 6- or 9-week-old grafts. In addition, the time-dependent increase of frequencies of RITC+ and HLA-DR+ cells in the draining lymph nodes, and the time-dependent decrease of HLA-DR+ cells in the 3-week-old human skin graft, were concurrent. Supporting these data, human cytokine interleukin-1 alpha (IL-1 alpha), IL-1 beta and tumour necrosis factor-alpha (TNF-alpha), analysis in situ revealed that cytokine production by keratinocytes, a property associated with dendritic cell migration, was preserved in the human skin graft. Thus, like dendritic cells in contact sensitization in allografted skin, dendritic cells from human xenografted skin onto nude mice are capable of migration to mouse draining lymph nodes after allergen application. Induction of contact hypersensitivity is possible in a human skin graft onto nude mice model, although the use of this ex vivo model to analyze contact sensitivity is probably limited to 3 weeks after transplantation.

Maturation and migration of cutaneous dendritic cells

Dendritic cells have been isolated from the epidermis, dermis, and lymphatics of skin. Cells from each cutaneous compartment can exhibit the distinct morphology, surface phenotype, and strong T-cell-stimulating activity of dendritic cells that are isolated from other organs. Of importance are the mechanisms by which the maturation and movement of dendritic cells are regulated within intact tissues. Epidermal dendritic cells turn over slowly in the steady state. Stimuli, including contact allergens and transplantation, perhaps by inducing a release of cytokines such as granulocyte macrophage-colony-stimulating factor, mobilize these dendritic cells into the dermis and lymph. This migration is accompanied by the maturation of dendritic cell functions; e.g., antigen-presenting major histocompatibility complex molecules and B7 costimulators increase markedly. On the other hand, there is a sizable, steady-state flux of dendritic cells in afferent lymph draining the skin, which suggests a constant traffic through the dermis that is independent of sessile epidermal dendritic cells. When explants of skin are placed in organ culture, dendritic cells emigrate into the medium for 1-3 d. The dendritic cells are mature and can bind tightly to small memory T cells that also migrate in these cultures. The emigrated mixtures of dendritic cells and T cells should be useful in the study of many clinical states. This is illustrated by recent experiments showing that migratory skin cells are readily infected with human immunodeficiency virus (HIV)-1. A strong productive infection takes place in the absence of exogenous cytokines, foreign sera, or mitogens or antigens. The dendritic cell-T-cell conjugates are the essential site for infection. This cellular milieu may model events during the sexual transmission of HIV-1, where relevant mucosal surfaces are covered by skin-like epithelia. The capture of CD4+ memory T cells by dendritic cells may explain the chronic drain of immune memory in HIV infection.

Severe combined immunodeficiency mouse and human psoriatic skin chimeras. Validation of a new animal model

Research into the cause and pathophysiological mechanisms underlying expression of psoriatric skin lesions has been hampered by lack of an appropriate animal model for this common and enigmatic cutaneous disease. These studies characterize normal skin, pre-psoriatic skin, and psoriatic plaque skin samples transplanted onto severe combined immunodeficiency mice. In this report we document that 1), normal, prepsoriatic, and psoriatic plaque keratome skin samples can be transplanted onto severe combined immunodeficiency mice reliably with high rates of graft survival (> 85%) and with reproducible changes consistently observed over prolonged periods of engraftment; 2), after transplantation, by clinical assessment and routine light microscopy, normal skin remained essentially normal whereas pre-psoriatic skin became thicker, and psoriatic plaque skin retained its characteristic plaque-type elevation and scale; 3), by using a panel of antibodies and immunohistochemical analysis, the overall phenotype of human cell types (including immunocytes) that persisted in the transplanted skin was remarkably similar to the immunophenotype of pretransplanted skin samples; 4), clearly recognized interface zones between human and murine skin within the epidermal and dermal compartments could be identified by routine microscopy and immunostaining, with focal areas of chimerism; and 5), elevated interleukin 8 cytokine levels were present in transplanted pre-psoriatic and psoriatic plaque skin samples. We conclude that there are many similarities between pre- and post-transplanted human samples of normal and psoriatic skin that are grafted onto severe combined immunodeficiency mice. Thus, we propose that this new animal model is appropriate for additional mechanistic-type studies designed to reveal the underlying genetic/etiological abnormality, as well as better illuminate the pathophysiological basis, for this important skin disease.

Antigen processing: cultured lymph-borne dendritic cells can process and present native protein antigens

Langerhans' cells (LC) cultured for 1-3 days lose their ability to process native protein antigens but acquire the ability to stimulate resting T cells as assessed in an allogeneic mixed lymphocyte response (MLR). Lymph-borne dendritic cells (L-DC) are physiologically involved in the transport of antigens to lymph nodes but it is not known whether these cells lose the ability to process antigens in culture. To investigate this, we cultured L-DC derived from the intestine for 20-72 hr and tested their ability to process and present antigens. Our results show that these L-DC are able to present antigen to primed spleen T cells as effectively as fresh cells. To exclude the possibility that commercial ovalbumin (OVA) preparations contain peptides which might bind directly to major histocompatibility complex (MHC) molecules, OVA was filtered through Sephadex G50 and the peak fractions used as antigen. The results show that cultured L-DC are also able to present G50-filtered OVA efficiently to primed spleen T cells. More importantly, these G50-OVA-pulsed L-DC are able to prime naive T cells specifically in vivo. Chloroquine inhibited the ability of both fresh and cultured L-DC to present antigen to primed T cells but did not inhibit their ability to stimulate a MLR, indicating that processing was a necessary step for antigen presentation. Taken together, these results clearly show that cultured L-DC are active in processing and presenting native antigens and the hypothesis proposed for LC does not apply to rat lymph-borne dendritic cells. The physiological significance of these observations is discussed.

Histological, biochemical, and ultrastructural studies on hyperpigmented human skin xenografts

The mechanisms for hyperpigmentation observed in human cutaneous xenografts placed on athymic nude mice was investigated. Histologic, biochemical, histochemical, and ultrastructural examinations were performed on human skin prior to grafting and at various times ranging from 2 weeks to 30 weeks post-grafting (PG). Hyperpigmentation was macroscopically visible on the graft as early as 4-6 weeks. The number of Dopa-positive melanocytes per unit area was increased at 2 weeks PG and remained elevated until 20 weeks PG. The surface area of the melanocytes, a measure of the activity of the cells, also increased significantly and remained above the pre-grafting size throughout the study. Western blot analysis using tyrosinase specific antibody (alpha Ty-SP) revealed the presence of tyrosinase exclusively in the grafted skin from 2 weeks to 12 weeks PG tested. Histological and ultrastructural observations revealed the presence of numerous dendritic melanocytes, indeterminant clear cells suggestive of Langerhans cells, and dermal melanophages. The results of this study suggest that the observed hyperpigmentation in grafted tissue is caused by an increase in the number of Dopa-positive melanocytes and probably from enhanced melanin production. Extracts of proteins from the xenografts exhibited prominent differences in low and high molecular proteins between pre- and post-grafted skin. Among them, the exclusive appearance of a protein doublet with apparent mw approximately 14 kDa was found in grafted skin, and subsequent studies indicate it has potent effects on melanocyte function.

Migration of Langerhans cells into human epidermis of "reconstructed" skin, normal skin, or healing skin, after grafting onto the nude mouse

Human skin equivalents composed of keratinocytes cultured on a lattice constituted of human fibroblasts embedded in type I collagen were grafted onto the nude mouse. It is demonstrated, by indirect immunofluorescence and electron microscopy, that, after grafting, mouse Langerhans cells migrate into the human epidermis. Human Langerhans cells are not present in this system. In split-thickness human skin grafts, at long periods (5 and 12 months) after transplantation, a progressive migration of murine Ia(+) cells in the human epidermis and the presence of human Langerhans cells were shown by indirect immunofluorescence. Creation of a wound at the center of the grafted human skin and identification of the Langerhans cell origin shows a repopulation with human Langerhans cells provided the injury was performed early (2 months) after grafting. Injury at a later stage (5 months) resulted in presence of both human and murine Langerhans cells. These observations show 1) that, after grafting of "reconstructed" human skin or of split-thickness human skin onto nude mice, mouse Langerhans cells migrate into the grafted human epidermis; and 2) that the Langerhans cells repopulating a healing grafted epidermis devoid of Langerhans cells derived from the non-injured surrounding epidermis. The present work thus shows that besides bone marrow, lymph nodes, or/and spleen, surrounding cutaneous regions can also serve as sources of Langerhans cells.

Migration of Langerhans cells into the epidermis of human skin grafted into nude mice

In a previous study, it was demonstrated that human Langerhans cells (LC) are preserved in human skin grafted onto a nude mouse. Moreover, although it was observed that mouse LC of the host invade skin grafts from allogeneic mouse or rat, they do not penetrate in human skin grafts. In most of the human skin equivalent systems produced in vitro, LC appear to be lost. The present study was designed to investigate whether the mouse LC will repopulate a human skin equivalent. For this purpose, two different systems of skin equivalent have been grafted into the nude mouse. They were composed of human keratinocytes deposited on dead human dermis, or on lattice composed of human fibroblasts embedded in type I collagen. At different times after grafting, the presence of LC in the transplants was assayed either by indirect immunofluorescence or by electron microscopy. Indirect immunofluorescence was performed on frozen sections or on epidermal sheets with anti-Ia, anti-HLA-DR, or OKT6 antibodies. It was observed that, at 2 months after grafting, Ia(+) HLA-DR(-) OKT6(-) cells are present in grafted human epidermis. Moreover, LC with typical Birbeck granules are also detected by electron microscopy. It could be concluded, from this study, that mouse LC can repopulate human epidermis devoid of human LC.

Comparative epidermal Langerhans cell migration studies in epidermal and epidermal/dermal equivalent grafts

Immigration of Langerhans cell precursors from the peripheral blood to the skin was studied in human grafts placed on severe combined immunodeficient (SCID) mice. Monocyte fractions of human blood were injected intraperitoneally to SCID bearing either reconstituted (Langerhans cell free) epidermal sheets (E) or living skin equivalents (E/D) consisting of both epidermis and dermis. A range of immunocytochemical and ultrastructural markers was employed to monitor the colonization of the grafts, i.e., CD1a/c, Birbeck granules. In situ hybridization with probes against Alu sequences of human DNA were employed together with immunostaining for MHC class I mouse and human antigens to document graft survival. Although unequivocal LC were detected within E grafts, including both human (CD1a positive) and murine (NLDC-145 positive), no migration was achieved in the E/D situations.

The role of graft-derived dendritic leukocytes in the rejection of vascularized organ allografts. Recent findings on the migration and function of dendritic leukocytes after transplantation

Dendritic cells isolated from lymphoid tissues are potent stimulators of primary allogeneic T-cell responses in vitro and in vivo. Similar major histocompatibility complex class II-bearing dendritic-shaped leukocytes are contained within transplanted organs and these are thought to be important passenger leukocytes that trigger rejection. Recent findings on the migration, phenotype, and function of cardiac dendritic leukocytes (DLs) are reviewed. After transplantation donor DLs migrate rapidly from mouse cardiac allografts into the recipients's spleens. Within the spleens donor DLs associate with recipient CD4+ T cells. Isolated cardiac DLs, like lymphoid dendritic cells, are potent stimulators of T-cell proliferation in vitro. This suggests that DLs function as passenger leukocytes by migrating from grafts into the lymphoid tissues of the recipient and that sensitization to vascularized organ allografts may occur centrally within lymphoid tissues rather than peripherally in the graft itself.

Tumor necrosis factor alpha maintains the viability of murine epidermal Langerhans cells in culture, but in contrast to granulocyte/macrophage colony-stimulating factor, without inducing their functional maturation

Freshly isolated murine epidermal Langerhans cells (LC) are weak stimulators of resting T cells but increase their stimulatory capacity 10-30-fold upon 2-3 d of culture together with other epidermal cells. This maturation of LC is mediated by two keratinocyte products. Granulocyte/macrophage colony-stimulating factor (GM-CSF) maintains viability and increases function. IL-1 alone does not keep LC alive, but when combined with GM-CSF further enhances their stimulatory activity. We have now searched for a cytokine that would keep LC in a viable, but functionally immature state. When LC (enriched to greater than 75%) were cultured in the presence of GM-CSF (2 ng/ml) or murine (TNF-alpha) (plateau effect at 62 U/ml), the recovery of viable LC after 72 h was identical. The LC cultured in murine TNF-alpha, however, were 10-30 times less active in stimulating resting T cells. A series of experiments demonstrated that this phenomenon was not due to the induction of insufficient amounts of GM-CSF, the induction of a suppressor factor, or a toxic effect of TNF-alpha. Interestingly, the observed TNF-alpha activity exhibited a species preference, as human TNF-alpha was not active at comparable doses. We have observed an unexpected effect of TNF-alpha on LC in vitro. Though we found that freshly prepared epidermal cells express TNF-alpha mRNA, further studies are needed to establish whether TNF-alpha plays a role in vivo by keeping resident LC in a viable, but functionally immature state.

Replacement of Langerhans cells in murine palate

Palates from C3H mice were implanted onto prepared graft beds in histocompatible F1 hybrid mice. Biopsies taken 1, 2, 4, 8, and 16 wk later were prepared to demonstrate Langerhans cells (LC) of C3H and F1 (host) origin. After 1 wk only occasional LC (all of C3H origin) were present. By 2 wk total LC numbers had increased to a level approximately 50% greater than in control (non-implanted) palate, with most of this increase due to C3H LC proliferation. From 4 through 16 wk total LC numbers were not significantly different from those of control palate. During weeks 2 through 16 the percentages of LC of F1 origin were 31, 70, 39, and 19% respectively. These results indicate an increased proliferation of C3H LC with an initial migration of F1 LC which stops as C3H LC numbers increase.

Effect of systemic and topical application of testosterone propionate on the density of epidermal Langerhans cells in the mouse

Epidermal Langerhans cells (LCs) are bone marrow-derived immune cells in the epidermis. Recently, we reported that adenosine triphosphatase (ATPase)-positive LC density in the hind-limb skin of male mice was lower than that of female and that orchiectomy resulted in an increase in LC density, though ovariectomy had no significant effect. To further investigate the control mechanisms of sex differences in LC density, the effect of systemic and topical application of testosterone propionate (TP) on LC density was examined in C57BL/6 mice. Subcutaneous injections of TP 5.8 X 10(-8) mol (20 micrograms)/day/mouse for 14 d resulted in a significant decrease in LC density both in orchiectomized males and normal females, and such an effect was also observed in adrenalectomized mice, suggesting that this effect of TP is not indirectly mediated by glucocorticosteroids. TP was also effective when applied as an ointment (1% or 5%) to the right hind-limb skin of both orchiectomized males and normal females for 14 d; namely, the LC density of the right hind-limb was lower than that of the left. Beta-estradiol and progesterone 5.8 X 10(-8) mol/day/mouse had no significant effect on LC density when systemically applied for 14 d to normal males and females. These results suggest that sex differences in LC density may result from higher concentrations of testosterone or its metabolites in males, and that the function of testosterone may be local.

Benzo[a]pyrene diol epoxide I modification of DNA in human skin xenografts

Human skin xenografts were established on the subscapular area of skin of nude (nu/nu NIH-Swiss background) mice. When treated with benzo[a]pyrene diol epoxide I (BPDE I), specific carcinogen-DNA adducts were formed. Separation and identification of these adducts by the 32P-postlabeling technique indicated that the 7R- and 7S-BPDE I-dpGp adducts were the major adducts. Xenografts pretreated with either allantoin or anthralin showed an increase in the major 7R- and 7S-BPDE I adducts compared to only BPDE I treatment. Likewise, we observed an increase in the quantity of different minor adducts. The ratios between the minor and major adducts in the pretreated grafts remained consistent with the ratio in the grafts treated with BPDE I only. We conclude that these modulators induce cells in the xenograft to enter S phase of the cell cycle. Moreover, we observed that these compounds altered the quantity of the minor carcinogen-DNA adducts without altering the overall ratios between the major 7R- and 7S-BPDE I-dpGp adducts and the minor carcinogen-DNA adducts.

Alterations in Langerhans cells during growth of transplantable murine tumors

To investigate the response of Langerhans cells to tumor growth, we examined the appearance and number of ATPase+ and Ia+ dendritic cells in the epidermis covering subcutaneous tumors. Mice were injected with cells from syngeneic UVB- and PUVA-induced tumors and a melanoma, and the overlying skin was examined at various times during progressive tumor growth. An increase in the number of ATPase+ and Ia+ dendritic cells was observed in skin over all three tumor types. Morphologic alterations in the cells were also noted, including a decrease in dendricity. These changes were apparent only in skin directly over growing tumor masses; contralateral and perilesional skin was unaffected. Injection of nontumorigenic cells and implantation of silicon did not induce changes in Langerhans cells. Regression of highly antigenic tumor cells and tumor regression in immunized mice were not accompanied by detectable alterations in Langerhans cells, whereas changes in Langerhans cells were apparent during tumor growth in nude mice. These results demonstrate that changes in the number and morphology of Langerhans cells occur in response to tumor growth but that the changes are not dependent on immunologic or inflammatory responses.

In situ characterization of the cutaneous immune response in Ethiopian cutaneous leishmaniasis

Cryostat sections from 10 patients with localized cutaneous leishmaniasis (LCL) and eight patients with diffuse cutaneous leishmaniasis (DCL) from Ethiopia were studied with immunofluorescence methods for the phenotypic characterization of cells in the lesions. Higher numbers of Leu 2+ and Leu 3+ cells (P less than 0.005) were found in LCL than in DCL, while the Leu 3a + b/Leu 2a ratios were the same. No differences were found in the numbers of transferrin receptor, HLA-DR, and HLA-DQ expressing cells in the granulomas. Significantly (P less than 0.0001) lower numbers of IL-2 receptors expressing (Tac+) cells were found in DCL than LCL lesions, suggesting interference in the activation of the T cells. IL-2-containing cells were absent in DCL and were found in LCL lesions. Epidermal keratinocytes above the LCL but not the DCL lesions expressed HLA-DR (but not HLA-DQ) antigen, suggesting a lower gamma-interferon production in the DCL granulomas. The number of Langerhans' cells (Leu 6+) was higher in the epidermis of DCL (P less than 0.005) than in LCL, while a lower number of Leu 6+ cells were seen in the dermal lesions (P less than 0.001). These observations could account for some of the mechanisms responsible for the disturbed immunostimulation and immunoregulation observed in the lesions of DCL.

Epidermal Langerhans cells undergo mitosis during the early recovery phase after ultraviolet-B irradiation

We studied the recovery phase of immune response-associated (Ia)-positive or ATPase-positive epidermal Langerhans cells (ELCs) after ultraviolet B (UVB)-induced depletion by using mouse ear epidermal sheets. An area 3 mm in diameter was irradiated with 300 nm UVB light (40 mJ/cm2). A time sequence study was carried out to 56 days. During this period the Ia-positive ELC population increased stepwise, i.e., first a rapid increase between day 7 and day 14, which we called the early recovery phase, and next a gradual increase between day 42 and day 56, which we called the late recovery phase. During the early recovery phase, we found polymorphous ELCs in the irradiated area which were giant or normal in size, dendritic or round in shape, and single or paired in distribution. Electron microscopy revealed some of round and some of paired ATPase-positive ELCs to be in metaphase or telophase of mitotic division. Within the entire observation period of our study, there was no evidence suggesting migration of ELCs from hair follicles or from the nonirradiated epidermis. These findings indicated that mitosis of ELCs contribute to their repopulation during the early recovery phase.

Further evidence for the self-reproducing capacity of Langerhans cells in human skin

The limited number of Langerhans cells (LC) in the epidermis is one of the main reasons for the technical difficulties in resolving the question of LC kinetics. In the present paper, we describe a method to evaluate the LC replication potential in epidermis. The procedure is based on the specific incorporation of bromodeoxyuridine (BrdU), a thymidine analogue, into the DNA during the S-phase of the cell cycle. Mice, bearing human skin grafts, were injected s.c. every 6 h for up to 17 days with BrdU. At different times, the incorporated BrdU as well as the human epidermal LC were revealed on skin sections using anti-BrdU and OKT-6 monoclonal antibodies, respectively. After 6 h, 4.9% of the LC were labeled with BrdU. Then, the number of OKT-6(+) BrdU(+) cells increased in a linear manner and achieved 34% at 120 h, 67% at 240 h, and 94% at 400 h during the course of continuous labeling procedures. Based on this result we calculated a total cell cycle time of 392 h (16.3 days) and 12 h for the S-phase for human epidermal LC. Applying this technique, we were able to show also that 48 h after local treatment with 12-O-tetradecanoylphorbol-13-acetate or after stripping, the number of BrdU-labeled LC was considerably increased. Furthermore, after i.p. injection of colchicine in the nude mouse, human epidermal LC undergoing mitosis were evidenced by electron microscopy in the graft. From these results we conclude that the LC are actively cycling--therewith a self-reproducing cell population in human epidermis.

Occurrence of donor Langerhans cells in mouse and rat chimeras and their replacement in skin grafts

Evidence is presented that some endogenous Langerhans cells (LCs) may persist indefinitely in skin grafts. This evidence is based on the observation that although 2 weeks after grafting F1 hybrid mice and rats with genetically compatible skin, most of the LCs in the grafts were replaced with those of the host, some LCs of graft origin persisted for as long as the grafts were followed (154 days in mice and 249 days in rats). It has also been demonstrated that the spleen may be as good a source of LCs as the marrow. Thus, 6 weeks after lethally irradiated mice were restored with F1 hybrid spleen cells, most of the LCs in the epidermis of their pinnae were of donor origin. LCs of donor origin also were found in the epidermis of the pinnae of animals that had been inoculated at birth with spleen and lymph node cells (mice) or bone marrow cells (rats). Hence the occurrence of these cells provides another means of confirming that tolerance (chimerism) has been induced.

Detection of distinct subpopulations of Langerhans cells by flow cytometry and sorting

Flow cytometry was found to be a very appropriate tool for the study of Langerhans cells (LC), which represent a minor cell population (2-3%) of human epidermis, and allowed us to obtain new phenotypic, functional, and cell cycle data on these rare cells. The phenotypic analysis of cell surface antigens demonstrates the existence of two subpopulations of LC: the former is HLA-DR+ and OKT 6+ (about 90% of total HLA-DR+ cells) and the latter is HLA-DR+ and OKT 6- (about 10% of total HLA-DR+ cells). These subpopulations of LC are both able to stimulate the proliferation of peripheral blood lymphocytes (PBL) in the presence of keratinocytes i.e., in mixed skin lymphocyte reaction (MSLR). Analysis of the cell cycle could be performed on OKT 6+ LC. Results show that they can be found in the various phases of the cell cycle, suggesting that the large majority of Langerhans cells are able to proliferate in situ in normal human epidermis.