Melanosomes in phagocytic vacuoles in Langerhans cells. Electron microscopy of keratin-stripped human epidermis

Lrig1- and Wnt-dependent niches dictate segregation of resident immune cells and melanocytes in murine tail epidermis

The barrier-forming, self-renewing mammalian epidermis comprises keratinocytes, pigment-producing melanocytes and resident immune cells as first-line host defense. In murine tail skin, interfollicular epidermis patterns into pigmented ‘scale’ and hypopigmented ‘interscale’ epidermis. Why and how mature melanocytes accumulate in scale epidermis is unresolved. Here, we delineate a cellular hierarchy among epidermal cell types that determines skin patterning. Already during postnatal development, melanocytes co-segregate with newly forming scale compartments. Intriguingly, this process coincides with partitioning of both Langerhans cells and dendritic epidermal T cells to interscale epidermis, suggesting functional segregation of pigmentation and immune surveillance. Analysis of non-pigmented mice and of mice lacking melanocytes or resident immune cells revealed that immunocyte patterning is melanocyte and melanin independent and, vice versa, immune cells do not control melanocyte localization. Instead, genetically enforced progressive scale fusion upon Lrig1 deletion showed that melanocytes and immune cells dynamically follow epithelial scale:interscale patterns. Importantly, disrupting Wnt-Lef1 function in keratinocytes caused melanocyte mislocalization to interscale epidermis, implicating canonical Wnt signaling in organizing the pigmentation pattern. Together, this work uncovers cellular and molecular principles underlying the compartmentalization of tissue functions in skin.

Summary: Pigmentation and immune surveillance functions in murine tail skin are spatially segregated by Lrig1- and Wnt-Lef1-dependent keratinocyte lineages that control the partitioning of melanocytes and tissue-resident immune cells into distinct epidermal niches.

Real-time visualization of macromolecule uptake by epidermal Langerhans cells in living animals

As a skin-resident member of the dendritic cell family, Langerhans cells (LCs) are generally regarded to function as professional antigen-presenting cells. Here we report a simple method to visualize the endocytotic activity of LCs in living animals. BALB/c mice received subcutaneous injection of FITC-conjugated dextran (DX) probes into the ear skin and were then examined under confocal microscopy. Large numbers of FITC(+) epidermal cells became detectable 12-24 hours after injection as background fluorescence signals began to disappear. Most (>90%) of the FITC(+) epidermal cells expressed Langerin, and >95% of Langerin(+) epidermal cells exhibited significant FITC signals. To assess intracellular localization, Alexa Fluor 546-conjugated DX probes were locally injected into IAβ-enhanced green fluorescent protein (EGFP) knock-in mice and Langerin-EGFP-diphtheria toxin receptor mice--three dimensional rotation images showed close association of most of the internalized DX probes with major histocompatibility complex (MHC) class II molecules, but not with Langerin molecules. These observations support the current view that LCs constantly sample surrounding materials, including harmful and innocuous antigens, at the environmental interface. Our data also validate the potential utility of the newly developed imaging approach to monitor LC function in wild-type animals.

Dendritic cell control of tolerogenic responses

One of the most fundamental problems in immunology is the seemingly schizophrenic ability of the immune system to launch robust immunity against pathogens, while acquiring and maintaining a state of tolerance to the body's own tissues and the trillions of commensal microorganisms and food antigens that confront it every day. A fundamental role for the innate immune system, particularly dendritic cells (DCs), in orchestrating immunological tolerance has been appreciated, but emerging studies have highlighted the nature of the innate receptors and the signaling pathways that program DCs to a tolerogenic state. Furthermore, several studies have emphasized the major role played by cellular interactions and the microenvironment in programming tolerogenic DCs. Here, we review these studies and suggest that the innate control of tolerogenic responses can be viewed as different hierarchies of organization, in which DCs, their innate receptors and signaling networks, and their interactions with other cells and local microenvironments represent different levels of the hierarchy.

Langerhans cells and melanocytes share similar morphologic features under in vivo reflectance confocal microscopy: a challenge for melanoma diagnosis

BACKGROUND

Intraepidermal Langerhans cells (ILC) are difficult to differentiate from melanocytes under reflectance confocal microscopy (RCM) and their presence may simulate pagetoid spread of melanocytes on RCM images.

OBJECTIVE

We sought to correlate bright round and dendritic cells in a pagetoid pattern identified on RCM with findings of conventional histopathology and immunohistochemistry for lesions that were falsely diagnosed as melanoma by RCM.

METHODS

This retrospective study included histopathologically proven nevi, imaged by RCM, which displayed bright cells in a pagetoid pattern (BCPP) under RCM, resulting in the incorrect RCM diagnosis of melanoma. Morphological comparisons were made between RCM images of nevi showing BCPP, histopathologically proven melanomas displaying BCPP, and biopsy-proven nevi without BCPP.

RESULTS

We identified 24 nevi that were falsely diagnosed as melanoma by RCM because of the presence of BCPP. These pagetoid cells on RCM corresponded on histopathology to ILC with a high density in 23 of the 24 nevi (95%) and to melanocytes in 7 of the 24 nevi (29%). Among 6 melanomas displaying BCPP on RCM, ILC with high density were observed histopathologically in 5 of the 6 cases (83%) and pagetoid melanocytes were seen in all 6 cases (100%).

LIMITATIONS

The results cannot be generalized to clinically banal-appearing nevi.

CONCLUSIONS

Although the finding of BCPP is a useful RCM feature for the diagnosis of melanoma, it does not always imply the presence of pagetoid melanocytes but may at times represent ILC.

Apoptotic cell-based therapies against transplant rejection: role of recipient's dendritic cells

One of the ultimate goals in transplantation is to develop novel therapeutic methods for induction of donor-specific tolerance to reduce the side effects caused by the generalized immunosuppression associated to the currently used pharmacologic regimens. Interaction or phagocytosis of cells in early apoptosis exerts potent anti-inflammatory and immunosuppressive effects on antigen (Ag)-presenting cells (APC) like dendritic cells (DC) and macrophages. This observation led to the idea that apoptotic cell-based therapies could be employed to deliver donor-Ag in combination with regulatory signals to recipient’s APC as therapeutic approach to restrain the anti-donor response. This review describes the multiple mechanisms by which apoptotic cells down-modulate the immuno-stimulatory and pro-inflammatory functions of DC and macrophages, and the role of the interaction between apoptotic cells and APC in self-tolerance and in apoptotic cell-based therapies to prevent/treat allograft rejection and graft-versus-host disease in murine experimental systems and in humans. It also explores the role that in vivo-generated apoptotic cells could have in the beneficial effects of extracorporeal photopheresis, donor-specific transfusion, and tolerogenic DC-based therapies in transplantation.

Revisiting the tolerogenicity of epidermal Langerhans cells

Langerhans cells (LC) are unique members of the dendritic cell (DC) family residing in the epidermis of skin and mucosa. Specific autocrine and environmental factors shape the biology of LC, such as TGF-beta1, IL-10, vitamin D(3), UV light or neuropeptides, which are required for LC development but also influence their capacity to induce immunity or tolerance. Both, immunogenic and tolerogenic functions require antigen transport from the skin to the draining lymph nodes, but the LC maturation grade directs the differential outcome. In this review, we recapitulate early indications for LC tolerogenicity and oppose them to more recent findings with gene-targeted mice, which dramatically challenged some of the early results. The newly discovered Langerin(+) dermal DC subset (DDC) seems to be responsible also for many tolerogenic effects that were initially attributed to steady state migratory LC. Transfer of antigens from LC to other DC subsets as well as transport of HIV are discussed as part of the complex interactions between LC and other cells or as mechanisms of immune evasion. Finally, the first clinical trials on allergy therapies targeting skin DC in the steady state are mentioned as they may open the door to curative tolerogenic therapies.

Neurocutaneous syndrome with mental delay, autism, blockage in intracellular vescicular trafficking and melanosome defects

We evaluated a 11-year-old male patient with mental delay, autism and brownish and whitish skin spots. The former resembled those of neurofibromatosis, the latter those of tuberous sclerosis. The patient received a complete clinical work-up to exclude neurofibromatosis, tuberous sclerosis, or any other known neurocutaneous disease, with biochemistry, chromosome analysis and analysis of skin specimens. Being all the other tests not significant, two main ultrastructural defects were observed. The first was a blockage in intracellular vescicular trafficking with sparing of the mitochondria; the second an aberrant presence of melanosomes in vacuoles of several cell lines and abnormal transfer of these organelles to keratinocytes. This patient presented with a unique clinical picture distinct from neurofibromatosis or tuberous sclerosis or any other known neurocutaneous disease. The ultrastructural abnormalities suggested a defect in cell trafficking involving several cell lines and compartments.

Behavioral responses of epidermal Langerhans cells in situ to local pathological stimuli

Pathological stimuli provoke coordinated changes in gene expression, surface phenotype, and function of dendritic cells (DCs), thereby facilitating the induction of adaptive immune responses. This concept of DC maturation was established mainly by studying epidermal Langerhans cells (LCs), a prototypic immature DC subset at the environmental interface. Taking advantage of I-Abeta-enhanced green fluorescent protein (EGFP) knock-in mice in which LCs can be visualized in intact skin, we recorded the dynamic movement of EGFP+ LCs by time-lapse confocal microscopy. LCs exhibited a unique behavior, termed dendrite surveillance extension and retraction cycling habitude (dSEARCH), characterized by rhythmic extension and retraction of dendrites through intercellular spaces between keratinocytes. When monitored after skin organ culture or subcutaneous injection of tumor necrosis factor alpha, LCs showed amplified dSEARCH and amoeba-like lateral migration between keratinocytes. Intravital imaging experiments further revealed steady-state dSEARCH motion in 5-10% of LCs. Topical application of a reactive hapten, DNFB, augmented dSEARCH and triggered lateral migration of LC in vivo. These observations introduce a new concept that in situ maturation of LCs is further accompanied by coordinated reprogramming of motile activities.

Simultaneous Induction of CD4 T Cell Tolerance and CD8 T Cell Immunity by Semimature Dendritic Cells

Previous studies suggested that depending on their maturation state, dendritic cells (DC) could either induce T cell tolerance (immature and semimature DC) or T cell activation (mature DC). Pretreatment of C57BL/6 mice with encephalitogenic myelin oligodendrocyte glycoprotein (MOG)(35-55) peptide-loaded semimature DC protected from MOG-induced autoimmune encephalomyelitis. This protection was mediated by IL-10-producing CD4 T cells specific for the self Ag. Here we show that semimature DC loaded with the MHC class II-restricted nonself peptide Ag (OVA) induce an identical regulatory T cell cytokine pattern. However, semimature DC loaded simultaneously with MHC class II- and MHC class I-restricted peptides, could efficiently initiate CD8 T cell responses leading to autoimmune diabetes in a TCR-transgenic adoptive transfer model. Double-peptide-loaded semimature DC also induced simultaneously in the same animal partially activated CD8 T cells with cytolytic function as well as protection from MOG-induced autoimmune encephalomyelitis. Our study suggests that the decision between tolerance and immunity not only depends on the DC, but also on the type and activation requirements of the responding T cell.

Dendritic cells: regulators of alloimmunity and opportunities for tolerance induction

Dendritic cells (DCs) are uniquely well-equipped antigen-presenting cells (APCs) regarded classically as sentinels of the immune response, which induce and regulate T-cell reactivity. They play critical roles in central tolerance and in the maintenance of peripheral tolerance in the normal steady state. Following cell or organ transplantation, DCs present antigen to T cells via the direct or indirect pathways of allorecognition. These functions of DCs set in train the rejection response, but they also serve as potential targets for suppression of alloimmune reactivity and promotion of tolerance induction. Much evidence from various model systems now indicates that DCs can induce specific T-cell tolerance. Although underlying mechanisms have not been fully elucidated, the capacity to induce T-regulatory cells may be an important property of tolerogenic or regulatory DCs. Efforts to generate "designer" DCs with tolerogenic properties in the laboratory using specific cytokines, immunologic or pharmacologic reagents, or genetic engineering approaches have already met with some success. Alternatively, targeting of DCs in vivo (e.g. by infusion of apoptotic allogeneic cells) to take advantage of their inherent tolerogenicity has also demonstrated exciting potential. The remarkable heterogeneity and plasticity of these important APCs present additional challenges to optimizing DC-based therapies that may lead to improved tolerance-enhancing strategies in the clinic.

Dendritic cells in peripheral tolerance and immunity

Dendritic cells capable of influencing immunity exist as functionally distinct subsets, T cell-tolerizing and T cell-immunizing subsets. The present paper reviews how these subsets of DCs develop, differentiate and function in vivo and in vitro at the cellular and molecular level. In particular, the role of DCs in the generation of regulatory T cells is highlighted.

Immature, semi-mature and fully mature dendritic cells: which signals induce tolerance or immunity?

Dendritic cells (DCs) are currently divided into tolerogenic immature and immunogenic mature differentiation stages. However, recent findings challenge this model by reporting mature DCs as inducers of regulatory CD4+ T cells in vivo. This implies that decisive tolerogenic and immunogenic maturation signals for DCs might exist. Closer inspection reveals that tolerance is observed when partial- or semi-maturation of DCs occurs, whereas only full DC maturation is immunogenic. The decisive immunogenic signal seems to be the release of proinflammatory cytokines from the DCs. Moreover, the semi-mature DC phenotype is comparable to steady-state migratory veiled DCs within the lymphatics, which seem to continuously tolerize lymph node T cells against tissue-derived self-antigens or apoptotic cells.

Repetitive injections of dendritic cells matured with tumor necrosis factor alpha induce antigen-specific protection of mice from autoimmunity

Mature dendritic cells (DCs) are believed to induce T cell immunity, whereas immature DCs induce T cell tolerance. Here we describe that injections of DCs matured with tumor necrosis factor (TNF)-alpha (TNF/DCs) induce antigen-specific protection from experimental autoimmune encephalomyelitis (EAE) in mice. Maturation by TNF-alpha induced high levels of major histocompatibility complex class II and costimulatory molecules on DCs, but they remained weak producers of proinflammatory cytokines. One injection of such TNF/DCs pulsed with auto-antigenic peptide ameliorated the disease score of EAE. This could not be observed with immature DCs or DCs matured with lipopolysaccharide (LPS) plus anti-CD40. Three consecutive injections of peptide-pulsed TNF/DCs derived from wild-type led to the induction of peptide-specific predominantly interleukin (IL)-10-producing CD4(+) T cells and complete protection from EAE. Blocking of IL-10 in vivo could only partially restore the susceptibility to EAE, suggesting an important but not exclusive role of IL-10 for EAE prevention. Notably, the protection was peptide specific, as TNF/DCs pulsed with unrelated peptide could not prevent EAE. In conclusion, this study describes that stimulation by TNF-alpha results in incompletely matured DCs (semi-mature DCs) which induce peptide-specific IL-10-producing T cells in vivo and prevent EAE.

The immunohistochemical identification of human oral mucosal melanocytes

With the aim of identifying melanocytes, 60 samples of wax-embedded palatal and buccal mucosa front 30 autopsies were analysed for expression of S100, HMB-45 and NKI/C3 in combination with a Masson Fontana stain. The counts of positive cells were compared with those derived from 3,4 dihydroxyphenylalanine (DOPA) staining in fresh samples of gingiva. Polyclonal antisera to S100 stained dendritic cells throughout the epithelium, but monoclonal antibodies to S100, HMB-45 and NKI/C3 labelled cells that were restricted to the basal layer, often in clusters with some sections negative. The mean counts of positive basal dendritic cells per millimetre basement-membrane length were 4.3 (polyclonal S100+. range 0-19). 0.8 (monoclonal S100+. range 0-7.3), 2.0 (HMB-45, range 0-17.8) and 0.9 (NKI/C3, range 0-14.9). Of the last three, HMB-45 produced the clearest and most specific staining. Counts of basal dendritic cells were significantly higher (p < 0.0005) with polyclonal antisera to S100 than with all other markers except DOPA but HLA-DR staining showed that Langerhans cells may also be located in the basal layer. The highest values were always in buccal mucosa, but statistically significantly so (p < 0.0005) only with polyclonal anti-S100. Basally located DOPA+ cells had a mean density of 4.6 per millimetre basement-membrane length, but in five cases DOPA also labelled suprabasal dendritic cells, presumably Langerhans cells. The mean basal DOPA+ values were significantly higher than those with monoclonal antibodies to S100. HMB-45 and NKI/C3 (p < 0.001). The variable cell counts suggest that S100, HMB-45 and NKI/C3 are markers of melanocytes in normal oral mucosa, but are not constitutively expressed and may be induced during function.

Oral mucosal Langerhans' cells

Langerhans' cells (LC) are dendritic, antigen-presenting cells present within the epithelium of skin and mucosa, including that of the oral cavity. This article reviews the literature on the phenotypic and functional features of oral mucosal Langerhans' cells, and speculates on other aspects by extrapolating from data on their epidermal counterparts.

Accumulation of membrane-bound melanosomes occurs in Langerhans cells of patients with the Leopard syndrome

The Langerhans cells in the lentigines of four patients with the Leopard syndrome contained large membrane bound accumulations of melanin granules. Giant melanosomes were only seen in two patients. The patients had no immune-based symptoms relating to their lentigines. The Leopard Syndrome, also known as multiple lentigines syndrome, progressive cardiomyopathic lentiginosis, lentiginosis profusa syndrome and the cardiocutaneous syndrome, refers to an inherited abnormality of the skin, often associated with cardiomyopathy. The aetiology of the condition is so far unknown and the penetrance is variable. Here we describe electron microscopical findings of large accumulations of melanin within Langerhans cells.

Congenital dyschromia with erythrocyte, platelet, and tryptophan metabolism abnormalities

The case of a female child with a unique generalized congenital dyschromia is reported. She had hypopimented skin, with hypomelanosis and hypomelanocytosis, and many pigmented macules, which consisted of epidermal and dermal hypermelanosis without hypermelanocytosis. Biochemical investigations revealed normal catecholamine metabolism but abnormal tryptophan metabolism, including a decrease in blood serotonin and melatonin. A slight platelet storage pool disease was demonstrated, and a recurrent megaloblastic folate-related anemia occurred. The possible relationship between the pigmentary disease and the biochemical abnormalities is discussed. We suggest that this case represents a previously undescribed association of dyschromia, erythrocyte, platelet, and tryptophan metabolism abnormalities.

Melanophore death and disappearance produces color metamorphosis in the polychromatic Midas cichlid (Cichlasoma citrinellum)

We describe the histological basis of color metamorphosis in the polychromatic Midas cichlid, Cichlasoma citrinellum. Eight percent of the individuals in a natural population transform from gray with black markings to orange, simultaneously losing their ability to adjust coloration in response to background and social context. This trait is inherited. Light- and electron microscopy revealed that this transformation is a two-step process. First, the melanophores die, then macrophage-like cells remove the debris. As a result of this initial process, the underlying xanthophores become visible, producing the orange coloration. A similar process may occur in individuals that further transform to white, or go directly from gray to white.

The epidermal melanocyte system in individuals of Scandinavian origin, determined by DOPA-staining and TEM

The quantitative evaluation of DOPA-positive epidermal melanocytes in 16 patients of Scandinavian origin showed both individual and regional differences in the melanocyte count. Our data is in agreement with other published studies. The distribution in the number of melanocytes varies significantly in some specimens. This is due partly to the preparation procedure and partly to normal biological variations. We believe that we have demonstrated a cyclic function of the melanocyte in the epidermis. The varying density of cells in epidermal sheets as well as their varying morphology support the theory concerning the presence of the epidermal melanin unit.

Cells containing Langerhans cell granules in human lymph nodes of "atypical hyperplasia" with fatal outcome and leukemic reticuloendotheliosis

Langerhans cell granules could be found in atypical histiocytes in lymph nodes of three patients with "atypical hyperplasia" with fatal outcome and one patient with leukemic reticuloendotheliosis. These atypical histiocytes might be derived from immature mesenchymal cells in lymph nodes and the Langerhans cell granules might be induced in these cells by a particular condition. Only one of the Langerhans cell granules seemingly associated with the plasma membrane could be observed in these atypical histiocytes and all of the granules were seen within the cytoplasm. Quite a number of Langerhans cell granules were located near the Golgi apparatus. Several atypical granules very similar to the Langerhans cell granules could also be observed in these atypical histiocytes. These Langerhans cell granules were assumed to be directly derived from the Golgi apparatus and/or derived from the atypical granules which were secreted from the Golgi apparatus. The relationship between the Langerhans cell granules and the microtubules must also be considered, because Langerhans cell granules were found near the centrioles and microtubules.

A model implicating the Langerhans cell in keratinocyte proliferation control

A tentative model is presented which is based on existing data and our own cell kinetic and morphological observations in mice. The model suggests that the epidermal Langerhans cell plays a role in proliferation control of keratinocytes and may also act as an epidermal stem cell.

Dense bodies and Langerhans granules after application of podophyllum resin

The application of an alcoholic solution of podophyllum resin at concentrations above 10% for seven hours on guinea-pig udders increases the number of dense intracytoplasmic bodies in the Langerhans cells localized in the epidermis. A close relationship can be established between these inclusions and the typical Langerhans granules in view of the numerous intermediate forms with features of one or the other. Serial sections of these structures confirm the irregularity not only of the lysosome-like bodies but also of the Langerhans granules, which may be fusiform or oblong or even present two vesicular dilatations, one at each end of the rod.

Serial reconstruction of the characteristic granule of the Langerhans cell

Three-dimensional models of individual granules in the same Langerhans cell were made after analyzing serial sections of human epidermis in the electron microscope. These models revealed that the granule is made up of a flattened or curved orthogonal net of particles which is bounded externally by a limiting membrane and which may be disc-shaped, cup-shaped, or combinations of both shapes. This variety of shapes accounts for the many configurations of the granule seen in individual electron micrographs. Usually, the granule has a vesicular portion at, or near one margin. This demonstration of the three-dimensional structure of the granule establishes the inaccuracy of previously used descriptive terms, the granule should be called simply the "Langerhans cell granule."