HLA class II expression on human epidermal Langerhans cells in situ: upregulation during the elicitation of allergic contact dermatitis

Neurofibromatosis type 1 protein and amyloid precursor protein interact in normal human melanocytes and colocalize with melanosomes

The neurofibromatosis type 1 (NF1) gene product, neurofibromin, is known to interact with Ras, thereby negatively regulating its growth-promoting function. Although this is a well-established interaction, the discovery of other neurofibromin interacting partners could reveal new functional properties of this large protein. Using yeast two-hybrid analysis against a brain cDNA library, we identified a novel interaction between the amyloid precursor protein and the GTPase activating protein-related domain of neurofibromin. This interaction was further analyzed in human melanocytes and confirmed by immunoprecipitation and colocalization studies. In addition, we observed a colocalization of amyloid precursor protein and neurofibromin with melanosomes. Amyloid precursor protein has been proposed to function as a vesicle cargo receptor for the motor protein kinesin-1 in neurons. This colocalization of amyloid precursor protein and neurofibromin with melanosomes was lost in melanocytes obtained from normal skin of a NF1 patient. We suggest that a complex between amyloid precursor protein, neurofibromin, and melanosomes might be important in melanosome transport, which could shed a new light on the etiopathogenesis of pigment-cell-related manifestations in NF1.

The vesicular monoamine transporter 2 (VMAT2) is expressed by normal and tumor cutaneous mast cells and Langerhans cells of the skin but is absent from Langerhans cell histiocytosis

Monoamine storage in secretory granules is mediated by the vesicular monoamine transporters 1 and 2 (VMAT1 and VMAT2). The aim of our study was to identify monoamine-handling normal and neoplastic inflammatory cells in the skin by their expression of VMAT1 and VMAT2. Normal skin from various parts of the body, as well as 21 cases of cutaneous mastocytosis and 10 cases of cutaneous Langerhans cell histiocytosis were analyzed by immunohistochemistry, radioactive in situ hybridization, and double-fluorescence confocal microscopy. VMAT2-positive cells in the subepidermal layer were identified as mast cells by their expression of tryptase. Neoplastic mast cells in all cases of cutaneous mastocytosis retained their VMAT2 positivity. The intraepidermal VMAT2-expressing cells were identified as Langerhans cells by their CD1a positivity. VMAT2 was absent from Langerhans cell histiocytosis. VMAT2 is an excellent marker for normal and neoplastic mast cells. The expression of VMAT2 demonstrates the capacity of mast cells for monoamine storage and handling. The presence of VMAT2 in epidermal Langerhans cells revealed a previously unrecognized monoamine-handling phenotype of these cells and indicates possible involvement of amine storage and release associated with antigen presentation. Absence of VMAT2 in neoplastic Langerhans cells indicates a loss of monoamine handling capacity of these cells during tumorigenesis.

The tolerogenic molecule CD95-L is expressed on the plasma membrane of human activated, but not resting, Langerhans' cells

Although dendritic cells (DC) are well known for their immunogenic capacities, they may even induce peripheral T-cell tolerance, and such a tolerogenic potential can be exerted in mouse through the expression on the DC plasma membrane of the CD95-ligand (CD95-L) molecule, which is able to trigger apoptosis of CD95-expressing antigen-specific T cells. We therefore asked whether epidermal DC, namely Langerhans' cells (LC), either resting (i.e. within the epidermis, 'in situ') or activated (i.e. suspended from the epidermis) or both, could express the CD95-L molecule on the plasma membrane. For such a purpose, two colloidal gold-immunoelectron microscopy (IEM) double-step procedures were carried out: an 'in situ' method, able to investigate resting LC, was performed on ultrathin frozen sections obtained by ultracryomicrotomy (UCMT) of normal skin biopsies; a pre-embedding (P-E) method, able to investigate suspended LC, was performed on epidermal cells (EC) suspended from normal skin specimens. In UCMT/IEM sections, resting LC showed gold particles within the cytoplasm but very rarely within organelles and never along the plasma membrane: resting LC are therefore capable of synthesizing CD95-L but not of expressing it in a functional location, thus autoreactive phenomena against CD95-expressing EC being avoided in normal epidermis. On the other hand, in P-E/IEM preparations, suspended LC showed several gold particles along the plasma membrane: activated LC are therefore capable of expressing CD95-L in a functional location, thus bearing the potential to exert tolerogenic capabilities against CD95-expressing T cells, e.g. to prevent inflammatory/autoimmune cutaneous disorders and/or favor the resolution thereof.

Keratinocytes constitutively express the CD95 ligand molecule on the plasma membrane: an in situ immunoelectron microscopy study on ultracryosections of normal human skin

BACKGROUND

Tissue homeostasis is mainly preserved by cytolytic functions. Cytolytic cells, when expressing the CD95 ligand (Fas-L) molecule on the cell membrane, are able to kill CD95 (Fas)-expressing target cells. Although cultured epidermal keratinocytes (KC) have been shown to express Fas-L, and normal skin has been shown to bear Fas-L mRNA, efforts so far to find possible constitutive Fas-L expression on the cell membrane by resting KC in normal human epidermis (i.e. in a functionally active location) have been inconclusive.

OBJECTIVES

The aim of the present study was therefore to show the constitutive expression of Fas-L on the plasma membrane of KC.

METHODS

Gold immunoelectronmicroscopy, a highly specific and sensitive immunodetection system, was performed in situ on skin sections obtained by ultracryomicrotomy, without previous embedding (i.e. in conditions strictly similar to the in vivo situation).

RESULTS

Relatively few (51.55 +/- 28.61), 10-nm colloidal gold particles were observed at the cell surface of KC in the basal layer of the epidermis and an even smaller (P < 0.005) number of gold granules was detected in the KC of the spinous layer.

CONCLUSIONS

Although scanty, the constitutive Fas-L expressed on the surface of KC can bind Fas expressed by possible occasional inflammatory cells entering the epidermis, and kill them, so preventing inflammation. Fas-L-expressing KC could moreover induce apoptosis of epidermal cells bearing viral or neoplastic antigens. Thus, the expression of Fas-L by KC may contribute to the preservation of epidermal homeostasis in vivo.

Expression and function of the mannose receptor CD206 on epidermal dendritic cells in inflammatory skin diseases

The capability to take up mannosylated protein antigens is important for the biologic function of dendritic cells, as many glycoproteins derived from bacteria and fungi, e.g., Malassezia furfur, are mannosylated. The expression of the mannose receptor CD206 has been regarded a differentiation hallmark of immature dendritic cells, whereas monocytes and mature dendritic cells as well as epidermal Langerhans cells do not express CD206. This study describes some epidermal dendritic cells that may express CD206 under inflammatory skin conditions: Immunohistochemical and flow cytometric analysis with the CD206-specific D547 antibody confirmed that Langerhans cells from normal human skin do not express CD206. Epidermal cell suspensions from atopic dermatitis and psoriasis revealed two distinct subsets of epidermal dendritic cells: a CD1a(+++)/CD206(-) cell population (i.e., Langerhans cells) and a CD1a(+)/CD206(++) cell population, corresponding to the previously described inflammatory dendritic epidermal cells. CD206-mediated endocytosis, assessed by dextran-fluorescein isothiocyanate uptake, was demonstrated in inflammatory dendritic epidermal cells but not in Langerhans cells. CD206-independent uptake of the fluorescent dye Lucifer yellow, a pinocytosis marker, was demonstrated in both Langerhans cells and inflammatory dendritic epidermal cells. Electron microscopic examination, known to distinguish Langerhans cells from inflammatory dendritic epidermal cells by their Birbeck granules, revealed Langerhans cells with Birbeck granules and inflammatory dendritic epidermal cells without Birbeck granules. Inflammatory dendritic epidermal cells exhibited numerous coated pits and vesicles, the latter fusing with large endosome-like structures, thus suggesting a high endocytotic activity. Immunogold staining with D547 monoclonal antibody confirmed that inflammatory dendritic epidermal cells were positive for CD206. In conclusion, inflammatory dendritic epidermal cells but not Langerhans cells are expressing CD206 in situ and use it for receptor-mediated endocytosis.

Birbeck granules are subdomains of endosomal recycling compartment in human epidermal Langerhans cells, which form where Langerin accumulates

Birbeck granules are unusual rod-shaped structures specific to epidermal Langerhans cells, whose origin and function remain undetermined. We investigated the intracellular location and fate of Langerin, a protein implicated in Birbeck granule biogenesis, in human epidermal Langerhans cells. In the steady state, Langerin is predominantly found in the endosomal recycling compartment and in Birbeck granules. Langerin internalizes by classical receptor-mediated endocytosis and the first Birbeck granules accessible to endocytosed Langerin are those connected to recycling endosomes in the pericentriolar area, where Langerin accumulates. Drug-induced inhibition of endocytosis results in the appearance of abundant open-ended Birbeck granule-like structures appended to the plasma membrane, whereas inhibition of recycling induces Birbeck granules to merge with a tubular endosomal network. In mature Langerhans cells, Langerin traffic is abolished and the loss of internal Langerin is associated with a concomitant depletion of Birbeck granules. Our results demonstrate an exchange of Langerin between early endosomal compartments and the plasma membrane, with dynamic retention in the endosomal recycling compartment. They show that Birbeck granules are not endocytotic structures, rather they are subdomains of the endosomal recycling compartment that form where Langerin accumulates. Finally, our results implicate ADP-ribosylation factor proteins in Langerin trafficking and the exchange between Birbeck granules and other endosomal membranes.

Refined characterization of corneodesmosin proteolysis during terminal differentiation of human epidermis and its relationship to desquamation

Corneodesmosin is a putative adhesion glycoprotein located in the extracellular part of the desmosomes in the upper layers of the epidermis. Synthesized by granular keratinocytes as a 52-56-kDa protein, corneodesmosin is progressively proteolysed during corneocyte maturation. This processing is a prerequisite for desquamation. Two glycine- and serine-rich domains of the protein might take on the conformation of adhesive secondary structures similar to glycine loops. Corneodesmosin proteolysis was further characterized. Deglycosylation experiments and reactivity with lectins demonstrated that the corneodesmosin carbohydrate moiety does not prevent the proteolysis. Immunoblotting, immunohistochemistry, and immunoelectron microscopy experiments using affinity-purified anti-peptide antibodies raised to four of the five structural domains of corneodesmosin and a monoclonal antibody against its fifth central domain showed that the first step in corneodesmosin processing is the cleavage of its extremities and probably occurs before its incorporation into desmosomes. Then the glycine loop-related domains are cleaved, first the N-terminal and then part of the C-terminal domain. At the epidermis surface, the multistep proteolytic cleavage leaves intact only the central domain, which was detected on exfoliated corneocytes and probably lacks adhesive properties. Importantly, corneodesmosin was demonstrated to be a preferred substrate of two serine proteases involved in desquamation, the stratum corneum tryptic and chymotryptic enzymes.

CD1a molecules traffic through the early recycling endosomal pathway in human Langerhans cells

In this work, we studied the localization and traffic of CD1a molecules in human epidermal Langerhans cells and the ability of these cells to stimulate CD1a-restricted T cell clones. We found that CD1a was spontaneously internalized into freshly isolated Langerhans cells, where it was rapidly distributed to the early/sorting endosomes and then to the early/recycling endosomes. In the latter compartments, CD1a colocalized with Rab11, a small GTPase known to be involved in the recycling of transmembrane proteins from early endosomes to the cell surface. In the steady state, intracellular CD1a was mainly located in Rab11+ recycling endosomal compartments. When endocytosis was blocked, intracellular CD1a moved rapidly from the early/recycling endosomes to the cell surface where it accumulated. The resultant increase in the cell surface expression of CD1a enhanced the capacity of Langerhans cells to stimulate a CD1a-restricted T cell clone. These findings are consistent with a dynamic exchange of CD1a between recycling compartments and the plasma membrane and suggest that the antigen-presenting function of CD1a depends on its traffic through the early/recycling endosomal pathway.

Cytoplasmic dynein colocalizes with melanosomes in normal human melanocytes

BACKGROUND

Melanocytic dendrites consist of a central core of microtubules and a subcortical actin network. Several studies provide arguments supporting the hypothesis that actin-based and microtubule-based motor proteins co-operate in melanosome transport towards the dendrite tips. Melanosomes can move bidirectionally along microtubules in vitro, and in murine melanocytes, they move towards the cell periphery and back again. Microtubules have a fast-growing plus end and a slow-growing minus end. Microtubule-associated motor proteins move unidirectionally either towards the plus or towards the minus end. However, it is not known which motor protein is responsible for minus end-directed movement of melanosomes.

OBJECTIVES

We aimed to investigate the in vitro expression of the minus end-directed motor protein cytoplasmic dynein in normal human epidermal melanocytes, keratinocytes and dermal fibroblasts.

METHODS

Reverse transcription-polymerase chain reaction and Northern blot analysis were used. In addition, an attempt to obtain insight into the subcellular localization of cytoplasmic dynein, immunofluorescence studies and immunogold electron microscopic studies were performed.

RESULTS

The three different forms of cytoplasmic dynein heavy chain were expressed in all studied skin cells. Immunofluorescence staining showed similar punctate distributions for dynein heavy chain 1 and dynein heavy chain 2 in melanocytes, with accentuation in the perinuclear area and dendrite tips. Double labelling with a melanosome marker showed apparent co-localization of both dynein heavy chains 1 and 2 with melanosomes in the perinuclear area and dendrite tips. For the dynein intermediate chain of 74 kDa, again a punctate staining pattern was seen with intense centrosomal staining. A close association of dynein intermediate chain 74 and alpha-tubulin with the melanosome surface was detected using immunogold electron microscopy.

CONCLUSIONS

The colocalization of different subunits of the cytoplasmic dynein complex with melanosomes is consistent with the hypothesis that this motor protein supports minus end-directed melanosome movement along microtubules.

CIIV, MIIC and other compartments for MHC class II loading

Virtually every endocytic compartment has been claimed to be an MIIC, a site where class II molecules accumulate. Here, it is argued that the definition of MIIC is not accurate and often pointless. MIIC can better be used as a working title for a collection of late endocytic compartments that contain the goodies necessary for efficient peptide loading of class II molecules.

Human epidermal Langerhans cells lack functional mannose receptors and a fully developed endosomal/lysosomal compartment for loading of HLA class II molecules

Langerhans cells (LC) represent the dendritic cell (DC) lineage in the epidermis. They capture and process antigens in the skin and subsequently migrate to the draining lymph nodes to activate naive T cells. Efficient uptake and processing of protein antigens by LC would, therefore, seem a prerequisite. We have now compared the capacity of human epidermal LC, blood-derived DC and peripheral blood mononuclear cells to endocytose and present (mannosylated) antigens to antigen-specific T cells. Moreover, we have determined the expression of mannose receptors, and the composition of the intracellular endosomal/lysosomal MHC class II-positive compartment. The results indicate that LC have poor endocytic capacity and do not exploit mannose receptor-mediated endocytosis pathways. Furthermore, the composition of the class II compartment in LC is distinct from that in other antigen-presenting cells and is characterized by the presence of relatively low levels of lysosomal markers. These results underscore the unique properties of LC and indicate that LC are relatively inefficient in antigen uptake, processing and presentation. This may serve to avoid hyper-responsiveness to harmless protein antigens that are likely to be frequently encountered in the skin due to (mechanical) skin damage.

Myosin V colocalizes with melanosomes and subcortical actin bundles not associated with stress fibers in human epidermal melanocytes

Mutations of the gene encoding myosin V can produce a dilute or silvery hair color and various neurologic defects in mice and patients with Griscelli syndrome, leading to speculations that the myosin V motor protein plays a critical role in transporting melanosomes within melanocytes and neurosecretory vesicles within neurons. Therefore, we investigated the in vitro expression of myosin V in cultured normal human melanocytes, keratinocytes, and dermal fibroblasts using reverse transcriptase-polymerase chain reaction and northern blot analysis. Subcellular distribution of myosin V and proximity to actin bundles and melanosomes were determined by double indirect immunofluorescence labeling and immunogold electron microscopy. In all studied cells myosin V is expressed and treatment of melanocytes with the cyclic AMP-inducer 3-isobutyl-1-methylxanthine causes an induction of the myosin V message. In all cells myosin V colocalizes with actin bundles, concentrating in the subcortical cell zone. In the melanocyte it is closely associated with melanosomes. Quantitative analysis of myosin V labeling in melanocytes reveals a significantly higher (p < 0.005) presence of myosin V in the periphery of dendrites. These results suggest that myosin V is important in melanosome transport in human melanocytes. Possible roles in the other skin cells remain to be elucidated.

Differential effects of sunscreens on UVB-induced immunomodulation in humans

Ultraviolet radiation has been shown to suppress the (skin) immune system both in animal species and in humans. Whether sunscreens can prevent immunosuppression is a matter of debate. This study investigated the protective capacity of a commercial sunscreen lotion in humans. Part of the right arm of healthy volunteers was exposed to erythemagenic ultraviolet B doses of 160 mJ per cm2 for four consecutive days. Before irradiation, sunscreen was applied either directly onto the skin or onto a piece of quartz fixed to the skin (to avoid penetration of the sunscreen in the epidermis where it cannot block the photoisomerization of trans-urocanic acid in cis-urocanic acid in the stratum corneum). The control group was irradiated without prior application of sunscreen. Four h after the last irradiation, epidermal sheets were obtained by the suction-blister method from both arms and epidermal cells were used as stimulator cells in the mixed epidermal cell lymphocyte reaction. Responses directed to epidermal cells derived from irradiated skin were expressed as percentages of responses directed to epidermal cells derived from the nonirradiated left arm. The mixed epidermal cell lymphocyte reaction responses in the control group were found to be significantly increased (205%). This enhancement of the mixed epidermal cell lymphocyte reaction responses was associated with an influx of CD36+DR+ macrophages in the irradiated skin. Application of the sunscreen, either onto a piece of quartz or directly onto the skin, prevented the increase of the mixed epidermal cell lymphocyte reaction responses and the influx of CD36+DR+ cells. In an earlier study, volunteers were exposed three times weekly to suberythemagenic doses of ultraviolet B over 4 wk, resulting in mixed epidermal cell lymphocyte reaction responses that were decreased to 20%. The same sunscreen was not able to prevent this suppression. These contradicting results indicate that the protective effect of sunscreens with respect to ultraviolet-induced immunomodulation is critically dependent on the choice of ultraviolet treatment.

Mannose receptor-mediated uptake of antigens strongly enhances HLA class II-restricted antigen presentation by cultured dendritic cells

Dendritic cells (DC) efficiently take up antigens by macropinocytosis and mannose receptor-mediated endocytosis. Here we show that endocytosis of mannose receptor-antigen complexes takes place via small coated vesicles, while non-mannosylated antigens were mainly present in larger vesicles. Shortly after internalization the mannose receptor and its ligand appeared in the larger vesicles. Within 10 min, the mannosylated and non-mannosylated antigens co-localized with typical markers for major histocompatibility complex class II-enriched compartments and lysosomes. In contrast, the mannose receptor appeared not to reach these compartments, suggesting that it releases its ligand in an earlier endosomal structure. Moreover, we demonstrate that mannosylation of protein antigen and peptides resulted in a 200-10,000-fold enhanced potency to stimulate HLA class II-restricted peptide-specific T cell clones compared to non-mannosylated peptides. Our results indicate that mannosylation of antigen leads to selective targeting and subsequent superior presentation by DC which may be applicable in vaccine design.

Mannose receptor mediated uptake of antigens strongly enhances HLA-class II restricted antigen presentation by cultured dendritic cells

Dendritic cells (DCs) use macropinocytosis and mannose receptor mediated endocytosis for the uptake of exogenous antigens. Here we show that the endocytosis of the mannose receptor and mannosylated antigen is distinct from that of a non-mannosylated antigen. Shortly after internalization, however, both mannosylated and non-mannosylated antigen are found in an MIIC like compartment. The mannose receptor itself does not reach this compartment, and probably releases its ligand in an earlier endosomal structure. Finally, we found that mannosylation of peptides strongly enhanced their potency to stimulate HLA class II-restricted peptide-specific T cell clones. Our results indicate that mannosylation of antigen leads to selective targeting and subsequent superior presentation by DCs which may be useful for vaccine design.

Effects of topical testosterone propionate on the positive nickel patch test

In a group of nickel sensitized women, we investigated the effects of topical application of testosterone propionate on the epidermic density of CD1 + dendritic cells and on the response to patch tests performed with scaled nickel concentrations. In a significant number of examined subjects, treatment with testosterone propionate induced an increase of the minimum eliciting dose of nickel and an evident reduction of CD1 + dendritic cell epidermic density. In those subjects in which the minimum eliciting dose resulted unmodified, the epidermic density of CD1 + dendritic cells also did not undergo significant variations following treatment with testosterone propionate. This parallelism between the behaviour of the responses to patch tests and the epidermic density of CD1 + dendritic cells induces us to think it possible that testosterone propionate is able to increase the tolerance to contact with allergen by interfering with the activity of Langerhans cells. The possible mechanisms of testosterone action on Langerhans cells remain to be elucidated.

Functional expression of minor histocompatibility antigens on human peripheral blood dendritic cells and epidermal Langerhans cells

Adequate presentation and cell surface expression of foreign minor histocompatibility antigens (mHag) to allogeneic T cells can lead to graft versus-host disease (GvHD) after HLA matched bone marrow transplantation (BMT). Cells of the dendritic cell (DC) lineage, including epidermal Langerhans cells (LC), are the most potent inducers of primary alloreactive T cell responses in vivo and in vitro. To explore the possible role of peripheral blood DC and of skin derived LC in the induction of alloimmune responses against mHag, we analysed the functional expression of mHag on these professional antigen-presenting cells (APC). To this end, cytotoxic T cell (CTL) clones specific for mHag H-Y and HA-1 to HA-4 were used to demonstrate the presence of these antigens on highly purified DC and LC. Our results demonstrate that, like other cells of the hematopoietic lineage, DC and LC express all the mHag tested for. The functional expression of mHag on these potent APC suggests their involvement in the induction of mHag specific GvH directed T cell responses after allogeneic BMT.

Antigen capture and major histocompatibility class II compartments of freshly isolated and cultured human blood dendritic cells

Dendritic cells (DC) represent potent antigen-presenting cells for the induction of T cell-dependent immune responses. Previous work on antigen uptake and presentation by human DC is based largely on studies of blood DC that have been cultured for various periods of time before analysis. These cultured cells may therefore have undergone a maturation process from precursors that have different capacities for antigen capture and presentation. We have now used immunoelectron microscopy and antigen presentation assays to compare freshly isolated DC (f-DC) and cultured DC (c-DC). f-DC display a round appearance, whereas c-DC display characteristic long processes. c-DC express much more cell surface major histocompatibility complex (MHC) class II than f-DC. The uptake of colloidal gold-labeled bovine serum albumin (BSA), however, is greater in f-DC, as is the presentation of 65-kD heat shock protein to T cell clones. The most striking discovery is that the majority of MHC class II molecules in both f-DC and c-DC occur in intracellular vacuoles with a complex shape (multivesicular and multilaminar). These MHC class II enriched compartments (MIIC) represent the site to which BSA is transported within 30 min. Although MIIC appear as more dense structures with less MHC class II molecules in f-DC than c-DC, the marker characteristics are very similar. The MIIC in both types of DC are acidic, contain invariant chain, and express the recently described HLA-DM molecule that can contribute to antigen presentation. CD19+ peripheral blood B cells have fewer MIIC and surface MHC class II expression than DCs, while monocytes had low levels of MIIC and surface MHC class II. These results demonstrate in dendritic cells the elaborate development of MIIC expressing several of the components that are required for efficient antigen presentation.

Distribution of HLA class II molecules in epidermal Langerhans cells in situ

We performed immunoelectron microscopic studies to investigate the expression of HLA class II molecules in Langerhans cells (LC). In the epidermis, LC expressed class II molecules on the plasma membrane of the dendrites, resulting in a class II positive reticulo-epithelial network, but not on the surface of the cell body. In contrast, isolated LC as well as activated LC in situ displayed an even and strong expression of class II molecules on their entire cell surface. Therefore, isolation and/or activation results in redistribution and up-regulation of class II molecules on the cell surface. In addition, double-labeling experiments were carried out with monoclonal antibodies to class II antigens and to LAMP-1, CD63 and alpha-glucosidase, specific markers for organelles of the endosomal/lysosomal system. The results show expression of class II molecules on intracellular, electron-dense vesicular structures, and co-localization of class II molecules and the markers for late endosomes and early lysosomes in human LC in situ. Expression of these markers was not found on Birbeck granules, an LC-specific organelle.

In cutaneous T-cell lymphoma, class II MHC molecules on CD1+ antigen-presenting cells are upregulated in involved compared with uninvolved epidermis

CD1+ antigen-presenting cells in involved epidermis of patients with cutaneous T-cell lymphoma exhibit and enhanced functional capacity to activate autologous CD4+ T cells compared with CD1+ antigen-presenting cells from uninvolved and normal epidermis. Class II major histocompatibility complex molecules are involved in antigen presentation, and their expression on CD1+ Langerhans cells is known to vary. The expression of all three class II (HLA-DR, -DQ, -DP) molecules was therefore determined on CD1+ epidermal cells from both involved and uninvolved epidermis, using flow cytometry. The involved CD1+ epidermal cells exhibited a 1.5-1.6-fold, statistically significant increase in fluorescence intensity after staining of the class II molecules (HLA-DR, -DQ, -DP) compared with CD1+ epidermal cells from uninvolved epidermis. The autologous CD4+ T cells, activation was almost completely blocked by anti-HLA-DR, and partly by anti-HLA-DQ and anti-HLA-DP. In contrast, an antibody against class I, and an irrelevant control antibody, had no blocking effect. In a pokeweed mitogen assay it was demonstrated that autologous CD4+ T cells, activated by involved epidermal cells, demonstrated suppressor activity rather than helper activity. The suppressor activity was dependent on the presence of HLA-DR-positive epidermal cells. Thus, in cutaneous T-cell lymphoma, class II molecules on the individual CD1+ antigen-presenting cell are upregulated in clinically involved compared with uninvolved epidermis, and these molecules are crucially involved in activation of CD4+ T cells.

Functional human epidermal Langerhans cells that lack Birbeck granules

Birbeck granules (BG) are cytoplasmic organelles that are only found in Langerhans cells (LC). The function of BG is still unclear, although it has been claimed that they are actively involved in receptor-mediated endocytosis and participate in the antigen-processing/presenting function of LC. We have identified a healthy white 29-year-old man whose LC completely lack the presence of BG as determined by electronmicroscopic studies. This was observed repeatedly using skin biopsy specimens taken from several places on the body during a period of 2.5 years. The absence of BG in these LG was documented further by the lack of staining with a BG-specific monoclonal antibody. Despite the complete lack of BG, LC were present in normal numbers, had all the usual morphologic characteristics, and were CD1a and human leukocyte antigen (HLA) class II positive. Two observations indicate that these BG-negative LC display normal antigen-presenting capacity. First, the individual could be sensitized by the hapten diphenylcyclopropenone. This was accompanied by a strong increase in the cell surface expression of HLA class II antigens on his LC, suggesting LC activation. Second, his epidermal cells elicited a normal positive response in an allogeneic mixed epidermal cell lymphocyte reaction. Together these observations strongly suggest that BG are not a prerequisite for normal LC function in vivo and in vitro.

Human resident langerhans cells display a lysosomal compartment enriched in MHC class II

Langerhans cells are the antigen-presenting cells of the skin, belonging to the family of dendritic cells, which present exogenous antigens in the context of major histocompatibility complex class II (MHC-II) molecules to CD4+ T lymphocytes. Langerhans cells are potent stimulators of different T-cell responses including primary immune responses. Culturing of Langerhans cells leads to modulation of their phenotype and function, as they seem more capable of activating T cells, whereas freshly isolated Langerhans cells are specialized in the endocytosing and processing of antigen. We studied the intracellular distribution of MHC-II molecules and invariant chain (I-chain) in resident Langerhans cells using immunogold labeling of ultrathin cryosections of human epidermis and found the majority of intracellular MHC-II molecules present on membranes of rough endoplasmic reticulum and in so-called MHC-II-enriched compartments (MIIC). The MIIC appeared to be negative for the cation-independent mannose 6-phosphate receptor and positive for the lysosomal enzyme beta-hexosaminidase and acquired the endocytotic tracer, cationized horseradish peroxidase, only after 60 min of internalization. Taken together, these data show that MIIC in Langerhans cells share characteristics with lysosomes. I-chain, which is associated with MHC-II molecules in early biosynthetic compartments, was found in the rough endoplasmic reticulum and Golgi complex, but was detected only occasionally in MIIC and at the plasma membrane. MIIC with internal membrane vesicles showed some I-chain labeling, suggesting that these are newly formed MIIC in which degradation of the I-chain is not yet complete.

Increases in human epidermal DR+CD1+, DR+CD1-CD36+, and DR-CD3+ cells in allergic versus irritant patch test responses

In an attempt to differentiate an allergic patch test response from an irritant response, we evaluated by flow cytometry the percentages of various epidermal cell populations isolated from allergen and irritant-treated patch test sites. Nine allergic individuals were patch tested with various allergens (Rhus, dinitrochlorobenzene [DNCB], or nickel chloride) and a vehicle control for 48 h. Eight additional individuals were patch tested with irritating chemicals (sodium lauryl sulfate or nonanoic acid) and with a vehicle control for 48 h. Epidermal cells, isolated from suction blisters, were double labeled for CD1/HLA-DR, CD3/HLA-DR, or CD36/HLA-DR cell surface markers and analyzed by flow cytometry to determine the percentage of various cell populations. A mean increase of 0.91 +/- 0.3 in the percentage of DR+CD1+ Langerhans cells over the vehicle control patch test site was detected in allergen-positive patch test sites in allergic individuals, whereas a decrease of 0.19 +/- 0.2 in the percentage of DR+CD1+ Langerhans cells from the vehicle control patch test site was detected in irritant-treated patch test sites. Epidermal cells from allergen-positive patch test sites also exhibited an increase of 5.2 +/- 1.8 in percentage of DR+CD1- cells over the vehicle control patch test site compared to an increase change of 0.8 +/- 0.4 in epidermal cells isolated from irritant-treated patch test sites. We also found that DR+ cells that lacked the CD1 determinant expressed the macrophage/monocyte antigen CD36 (OKM5). Finally, a 2.3 +/- 0.8 increase in the percentage of DR-CD3+ cells over the vehicle control patch test site was observed in allergen-positive patch test sites compared to an increase of 0.2 +/- 0.2 observed in irritant-treated patch test sites. These results demonstrate a significant increase in DR+CD1+, DR+CD1-CD36+, and DR-CD3+ epidermal cells in allergen-positive patch test sites compared to irritant patch test sites.

Long-term ultraviolet B-induced impairment of Langerhans cell function: an immunoelectron microscopic study

The influence of low-dose, long-term ultraviolet B (UVB) light exposure on HLA class II-positive human epidermal Langerhans cells (LC) was studied using a sensitive immunoelectron microscopic technique for the ultrastructural assessment of HLA class II expression on LC and for quantification of these cells in situ. Six healthy Caucasian volunteers participated in the experiments and received thrice weekly UVB treatments for 4 weeks. The initial dose ranged from 30 to 50 mJ/cm2 and the total dose from 600 to 3500 mJ/cm2, depending on skin type. Suction blisters and biopsies were obtained before the start of the UVB protocol and 48 h after the last UVB irradiation, and processed for the mixed epidermal cell-lymphocyte reaction (MECLR) and electronmicroscopy, respectively. The MECLR was used as a measure of the immune response. The distribution of HLA class II molecules on LC was studied by incubating ultrathin cryosections of human skin tissue with an anti-HLA class II MoAb that was conjugated to 10 nm colloidal gold. Furthermore, the number of LC was assessed ultrastructurally, when they could be recognized by their unique cytoplasmic organelle, the Birbeck granule (BG). The UVB protocol that was employed caused a marked suppression of the MECLR responses. This UVB-induced reduction of the immune response was not paralleled by changes in HLA class II expression on LC, nor in the number of epidermal LC. These findings are further support for our hypothesis that UVB-induced immune suppression in the skin is not due to a depletion of local LC.

Ultrastructural studies bearing on the mechanism of UVB-impaired induction of contact hypersensitivity to DNCB in man

In both murine and human experimental systems, acute, low dose exposure of skin to ultraviolet B light (UVB) impairs the induction of allergic contact dermatitis (ACD) by haptens such as dinitrochlorobenzene (DNCB) in a significant proportion of individuals. By light microscopy, epidermal Langerhans cells (LC) have been reported to be depleted by UVB exposure as well as by epicutaneous hapten application, implying that LC may be the locus of action of the effects of both UVB and DNCB. However, light microscopy can not readily distinguish cell density changes secondary to LC necrosis from changes resulting from down-modulation of expression of LC surface molecules. Using a highly sensitive immunogold electron microscopic approach, we have evaluated the differential effects of UVB and/or DNCB on human epidermal LC. The results reveal that DNCB alone caused significant up-regulation of cell surface HLA class II expression on a very small number of LC, the major fraction of LC expressing normal levels of HLA class II. Furthermore, DNCB alone caused a modest reduction in the density of LC at the treated sites without evidence of cell necrosis. Treatment with UVB alone or UVB exposure followed by DNCB resulted in a reduction in the density of LC, with widespread evidence of LC necrosis. However, the few remaining intact LC were all intensely HLA class II-positive after UVB exposure followed by DNCB, whereas treatment with UVB alone did not result in changes in LC HLA class II expression. The findings that after DNCB painting only a small proportion of the LC were strongly HLA class II-positive, but after UVB exposure followed by DNCB all intact LC displayed significant up-regulation of cell surface HLA class II expression, imply that UVB exposure inhibits the migration of epidermal LC. This is consistent with the view that DNCB fails to induce ACD when hapten is painted on UVB-exposed skin because insufficient LC are available to initiate T cell activation in the draining lymph node.