Langerhans Cells Differentiation Research Articles

IL-10 induces CCR6 expression during Langerhans cell development while IL-4 and IFN-gamma suppress it.

Immune responses are initiated by dendritic cells (DC) that form a network comprising different populations. In particular, Langerhans cells (LC) appear as a unique population of cells colonizing epithelial surfaces. We have recently shown that macrophage-inflammatory protein-3alpha/CCL20, a chemokine secreted by epithelial cells, induces the selective migration of LC among DC populations. In this study, we investigated the effects of cytokines on the expression of the CCL20 receptor, CCR6, during differentiation of LC. We found that both IL-4 and IFN-gamma blocked the expression of CCR6 and CCL20 responsiveness at different stages of LC development. The effect of IL-4 was reversible and most likely due to the transient blockade of LC differentiation. In contrast, IFN-gamma-induced CCR6 loss was irreversible and was concomitant to the induction of DC maturation. When other cytokines involved in DC and T cell differentiation were tested, we found that IL-10, unlike IL-4 and IFN-gamma, maintained CCR6 expression. The effect of IL-10 was reversible and upon IL-10 withdrawn, CCR6 was lost concomitantly to final LC differentiation. In addition, IL-10 induced the expression of CCR6 and responsiveness to CCL20 in differentiated monocytes that preserve their ability to differentiate into mature DC. Finally, TGF-beta, which induces LC differentiation, did not alter early CCR6 expression, but triggered its irreversible down-regulation, in parallel to terminal LC differentiation. Taken together, these results suggest that the recruitment of LC at epithelial surface might be suppressed during Th1 and Th2 immune responses, and amplified during regulatory immune responses involving IL-10 and TGF-beta.

Inhibited differentiation of Langerhans cells in the rat epidermis upon systemic treatment with cyclosporin A.

The immunosuppressant drug cyclosporin A (CsA) is known to cause reduction in number, DNA synthesis and function of Langerhans cells (LC). Since also the differentiation of LC is known to be hampered in conditions of acquired immunodeficiency not due to drugs, we investigated whether this occurs with CsA. Rats were injected subcutaneously with CsA (5, 10 and 50 mgxkg(-1) x d(-1)) for three weeks; the skin was analyzed by Ia immunohistochemistry and by electron microscopy. Epidermal immunolabeled cells were 15+/-3.5 (mean +/- SEM) per 100 basal keratinocytes in untreated controls and 8.75+/-1.3, 4.75+/-1.0 and 1.7+/-1.2 upon increasing doses of CsA (p<0.01). By electron microscopy, monocytoid cells with deep invaginations of the plasma membrane and roundish LC poor in Birbeck granules appeared in the epidermis upon treatment. The results suggest that CsA inhibits the differentiation of LC precursors in the epidermis and that this can in part explain the selective increase in the risk of skin viral disease and cancer in chronically treated patients.

Functional involvement of E-cadherin in TGF-beta 1-induced cell cluster formation of in vitro developing human Langerhans-type dendritic cells.

Epithelial Langerhans cells (LC) represent immature dendritic cells that require TGF-beta 1 stimulation for their development. Little is known about the mechanisms regulating LC generation from their precursor cells. We demonstrate here that LC development from human CD34+ hemopoietic progenitor cells in response to TGF-beta 1 costimulation (basic cytokine combination GM-CSF plus TNF-alpha, stem cell factor, and Flt3 ligand) is associated with pronounced cell cluster formation of developing LC precursor cells. This cell-clustering phenomenon requires hemopoietic progenitor cell differentiation, since it is first seen on day 4 after culture initiation of CD34+ cells. Cell cluster formation morphologically indicates progenitor cell development along the LC pathway, because parallel cultures set up in the absence of exogenous TGF-beta 1 fail to form cell clusters and predominantly give rise to monocyte, but not LC, development (CD1a-, lysozyme+, CD14+). TGF-beta 1 costimulation of CD34+ cells induces neoexpression of the homophilic adhesion molecule E-cadherin in the absence of the E-cadherin heteroligand CD103. Addition of anti-E-cadherin mAb or mAbs to any of the constitutively expressed adhesion molecule (CD99, CD31, LFA-1, or CD18) to TGF-beta 1-supplemented progenitor cell cultures inhibits LC precursor cell cluster formation, and this effect is, with the exception of anti-E-cadherin mAb, associated with inhibition of LC generation. Addition of anti-E-cadherin mAb to the culture allows cell cluster-independent generation of LC from CD34+ cells. Thus, functional E-cadherin expression and homotypic cell cluster formation represent a regular response of LC precursor cells to TGF-beta 1 stimulation, and cytoadhesive interactions may modulate LC differentiation from hemopoietic progenitor cells.

Evidence That Langerhans Cells in Adult Pulmonary Langerhans Cell Histiocytosis Are Mature Dendritic Cells: Importance of the Cytokine Microenvironment

Because Langerhans cells (LC) in peripheral tissues are generally "immature" cells with poor lymphostimulatory activity, the contribution of immune responses initiated by LC to the pathogenesis of pulmonary LC histiocytosis (LCH) has been uncertain. In this study we demonstrate that LC accumulating in LCH granulomas are phenotypically similar to mature lymphostimulatory dendritic cells present in lymphoid organs. LC in LCH granulomas intensely expressed B7-1 and B7-2 molecules, whereas normal pulmonary LC and LC accumulating in other pathologic lung disorders did not express these costimulatory molecules. The presence of B7+ LC in LCH granulomas was associated with the expression in these lesions, but not at other sites in the lung, of a unique profile of cytokines (presence of GM-CSF, TNF-alpha, and IL-1beta and the absence of IL-10) that is known to promote the in vitro differentiation of LC into cells expressing a lymphostimulatory phenotype. Finally, LCH granulomas were the only site where CD154-positive T cells could be identified in close contact with LC intensely expressing CD40 Ags. Taken together, these results strongly support the idea that an abnormal immune response initiated by LC may participate in the pathogenesis of pulmonary LCH, and suggest that therapeutic strategies aimed at modifying the lymphostimulatory phenotype of LC may be useful in the treatment of this disorder.

The monoclonal antibody DCGM4 recognizes Langerin, a protein specific of Langerhans cells, and is rapidly internalized from the cell surface.

We generated monoclonal antibody (mAb) DCGM4 by immunization with human dendritic cells (DC) from CD34+ progenitors cultured with granulocyte-macrophage colony-stimulating factor and TNF-alpha. mAb DCGM4 was selected for its reactivity with a cell surface epitope present only on a subset of DC. Reactivity was strongly enhanced by the Langerhans cell (LC) differentiation factor TGF-beta and down-regulated by CD40 ligation. mAb DCGM4 selectively stained LC, hence we propose that the antigen be termed Langerin. mAb DCGM4 also stained intracytoplasmically, but neither colocalized with MHC class II nor with lysosomal LAMP-1 markers. Notably, mAb DCGM4 was rapidly internalized at 37 degrees C, but did not gain access to MHC class II compartments. Finally, Langerin was immunoprecipitated as a 40-kDa protein with a pI of 5.2 - 5.5. mAb DCGM4 will be useful to further characterize Langerin, an LC-restricted molecule involved in routing of cell surface material in immature DC.

TGF-β1 Prevents the Noncognate Maturation of Human Dendritic Langerhans Cells

Abstract TGF-β1 is critical for differentiation of epithelial-associated dendritic Langerhans cells (LC). In accordance with the characteristics of in vivo LC, we show that LC obtained from human monocytes in vitro in the presence of TGF-β1 1) express almost exclusively intracellular class II Ags, low CD80, and no CD83 and CD86 Ags and 2) down-regulate TNF-RI (p55) and do not produce IL-10 after stimulation, in contrast to dermal dendritic cells and monocyte-derived dendritic cells. Surprisingly, while LC exhibit E-cadherin down-regulation upon exposure to TNF-α and IL-1, TGF-β1 prevents the final LC maturation in response to TNF-α, IL-1, and LPS with respect to Class II CD80, CD86, and CD83 Ag expression, loss of FITC-dextran uptake, production of IL-12, and Ag presentation. In sharp contrast, CD40 ligand cognate signal induces full maturation of LC and is not inhibited by TGF-β1. The presence of emigrated immature LCs in human reactive skin-draining lymph nodes provides in vivo evidence that LC migration and final maturation may be differentially regulated. Therefore, due to the effects of TGF-β1, inflammatory stimuli may not be sufficient to induce full maturation of LC, thus avoiding potentially harmful immune responses. We conclude that TGF-β1 appears to be responsible for both the acquisition of LC phenotype, cytokine production pattern, and prevention of noncognate maturation.

Langerhans Cell Histiocytosis of the Vulva: An Ultrastructural Study

Langerhans cells histiocytosis (LCH) is a proliferative disorder of Langerhans cells. The lesions are normally characterized by infiltration of eosinophils, neutrophils, lymphocytes, plasma cells, and Langerhans cells. The specific cells of LCH contain Birbeck granules, express the phenotype of Langerhans cells but with markers fixed at an early stage of activation, and are functionally defective in antigen-presenting ability. The disease most often affects children; when it occurs in older patients, anal and groin involvement is quite common and vulvar lesions can be found in older females. The authors report a case of a 64-year-old woman with LCH of the vulva and diabetes insipidus. An immunohistochemical and ultrastructural study of the vulvar lesions showed an infiltrate in which antigenically and morphologically mature Langerhans cells, monocytoid cells, and cells with an intermediate phenotype between monocytes and Langerhans cells were concurrently observed. Although the clinical and histological aspects of LCH are well established, the pathogenetic mechanism of lesions is not yet known. The finding of an infiltrate composed by Langerhans cells and many putative precursors of these cells suggests the hypothesis of an in situ differentiation of Langerhans cells from immature monocytoid precursors.

Regulation of the High Affinity Receptor for IgE on Human Epidermal Langerhans Cells

Human epidermal Langerhans cells (LC) express variable amounts of the high affinity receptor for IgE (Fc epsilonRI); the strongest expression is characteristic of atopic dermatitis. The receptor is suggested to take part in the pathophysiology of this disease by acting as a link between aeroallergens and Ag-specific T cells in an IgE-mediated, delayed-type hypersensitivity reaction. In the present study we show that even in the absence of surface expression, normal LC maintain an intracellular pool of the alpha-chain of Fc epsilonRI (Fc epsilonRIalpha) of the same m.w. as the surface-bound Fc epsilonRIalpha that is able to bind significant amounts of IgE. The lack of surface expression is linked to the absence or very low expression of the gamma-chain (Fc epsilonRIgamma). Moreover, the amount of Fc epsilonRIalpha expressed at the cell surface significantly correlates with the amount of Fc epsilonRIgamma. LC differentiation toward lymphoid dendritic cells is accompanied by the disappearance of transcripts for Fc epsilonRIalpha, but not for Fc epsilonRIgamma. This leads to a rapid decrease in the intracellular and surface levels of Fc epsilonRIalpha, which cannot be influenced by IL-4, IgE, or other agents. Overall, our findings suggest that these mechanisms enable LC to be highly versatile APCs by rapidly adapting the surface level of Fc epsilonRI to distinct inflammatory environments.

Transforming Growth Factor β1, in the Presence of Granulocyte/Macrophage Colony-stimulating Factor and Interleukin 4, Induces Differentiation of Human Peripheral Blood Monocytes into Dendritic Langerhans Cells

Langerhans cells (LCs) are dendritic cells (DCs) that are present in the epidermis, bronchi, and mucosae. Although LCs originate in bone marrow, little is known about their lineage of origin. In this study, we demonstrate that in vitro LCs may originate from monocytes. Adult peripheral blood CD14+ monocytes differentiate into LCs (CD1a+, E-cadherin+, cutaneous lymphocyte-associated antigen+, Birbeck granules+, Lag+) in the presence of granulocyte/macrophage colony-stimulating factor, interleukin 4, and transforming growth factor beta1 (TGF-beta1). This process occurs with virtually no cell proliferation and is not impaired by 30 Gy irradiation. Selection of monocyte subpopulations is ruled out since monocyte-derived DCs can further differentiate into LCs. Our data suggest that in vivo LC differentiation may be induced peripherally, from a nonproliferating myeloid precursor, i.e., the monocyte, in response to a TGF-beta1-rich microenvironment, as found in the skin and epithelia. Therefore, the monocyte may represent a circulating precursor critical to the immune response in vivo.

Epidermal Langerhans Cell Development and Differentiation

Epidermal Langerhans cells (LC) play a critical role in host defense. Still we know rather little about the development and functional specialization of these bone marrow-derived dendritic cells (DC) located in the most peripheral ectodermal tissue of the mammalian organism. How LC develop from their primitive progenitors in bone marrow and to what extent LC are related in their development to other lineages of the hemopoietic system is still under debate. There are currently 3 major areas of debate: 1) which are the signals required for LC development and differentiation to occur, 2) what are the (molecular) characteristics of the intermediate stages of LC differentiation, and 3) how are LC related in their development and/or function to other cells of the hemopoietic system? A better understanding of LC development and answers to these questions can be expected from recently developed technologies which allow the in vitro generation of DC with the typical molecular, morphological and functional features of LC from purified CD34 + progenitor cells under defined serum-free culture conditions. TGF-α1 was found to be an absolute requirement for in vitro LC development under serum-free conditions upon stimulation with the classical DC growth and differentiation factors GM-CSF, TNF-α and SCF. The recently identified cytokine FLT3 ligand further dramatically enhanced in vitro LC development and even allowed efficient in vitro generation of LC colonies from serum-free single cell cultures of CD34 + hemopoietic progenitor cells.

Fibronectin Upregulates In Vitro Generation of Dendritic Langerhans Cells from Human Cord Blood CD34+ Progenitors

Several studies have demonstrated that dendritic cells can be generated in vitro from CD34+ hematopoietic progenitor cells. In vivo, dendritic cells are found in many tissues and reside in direct proximity to extracellular matrix proteins. Because extracellular matrix proteins affect differentiation and location of cells in tissues, this study was designed to investigate potential effects of extracellular matrix proteins on differentiation of dendritic cells. Dendritic cells were generated from CD34+ human cord blood cells in the presence of granulocyte-macrophage colony-stimulating factor and tumor necrosis factor-alpha for 6 d and subsequently cultured for an additional 6-d period on tissue culture plates coated with various extracellular matrix proteins. Among the extracellular matrix proteins tested, exposure to fibronectin stimulated dendritic cell/Langerhans cell differentiation as indicated by the 50% increase of the number of cells expressing the Birbeck granule-associated marker Lag and displaying numerous Birbeck granules. Adhesion on fibronectin was shown to be specifically mediated by the integrin alpha5beta1. Because laminin and collagen were unable to cause similar changes in Langerhans cell development, these results suggest that fibronectin may cause changes affecting cellular differentiation of progenitors. Hematopoietic progenitors may exhibit maturational regulated differences in response to both matrix molecules and cytokines. The influence of combined signals emanating from a supportive microenvironment, specific integrins, and particular cytokines in the differentiation of Langerhans cells is discussed.

Synthesis, stability, and subcellular distribution of major histocompatibility complex class II molecules in Langerhans cells infected with Leishmania major.

Protozoan parasites of the genus Leishmania exist as obligatory intracellular amastigotes and invade macrophages and Langerhans cells, the dendritic cells of the skin. Langerhans cells are much more efficient in presenting Leishmania major antigen to T cells than macrophages are and have the unique ability to retain parasite antigen in immunogenic form for prolonged periods. To analyze the mechanisms that are responsible for this potency, we defined the synthesis, turnover, conformation, and localization of major histocompatibility complex (MHC) class II molecules in Langerhans cells. Hence, Langerhans cells were pulse-labeled; immunoprecipitation of MHC class II molecules and gel electrophoresis followed. In addition, the subcellular distribution of MHC class II molecules in L. major-infected Langerhans cells was analyzed by confocal microscopy. The results show that (i) newly synthesized MHC class II molecules are required for L. major antigen presentation by Langerhans cells, (ii) MHC class II-peptide complexes in Langerhans cells are long-lived, (iii) phagocytosis of L. major modulates MHC class II biosynthesis by reducing its downregulation during Langerhans cell differentiation, and (iv) newly synthesized MHC class II molecules are associated with the parasitophorous vacuole of infected Langerhans cells. These findings support the conclusion that the traits of MHC class II expression correspond to the highly specialized functions of Langerhans cells in the immunoregulation of cutaneous leishmaniasis.

Functional expression and modulation of C5a receptor (CD88) on skin dendritic cells.

The process of differentiation of resident epidermal Langerhans cells (LC) generates: i) changes in the level of expression and repertoire of cell surface molecules; ii) a decrease in the antigen processing capacity; iii) a significant increase in the ability to present peptides to T cells; and iv) the mobilisation of dendritic cells (DC) from the epidermis towards the draining lymph nodes1. There is little information about the factors involved in DC migration from non-lymphoid peripheral tissues as part of the process of skin DC differentiation. “In vivo” studies demonstrated that the administration of lipopolysaccharide (LPS), Tumour Necrosis Factor (TNF) α or Interleukin 1 (IL-1) can trigger the trafficking of DC from non-lymphoid tissues (i.e. skin, lung, gut, heart and kidney) towards secondary lymphoid organs2–7. A similar situation is observed in the inflammatory reaction that takes place during the first three days after skin transplantation; or after short term culture of skin organ explants8. In both cases it is likely that the skin injury by itself releases inflammatory mediators, sufficient to trigger both phenotypic and functional DC changes and migration of completely or partially mature skin DC.

Expression of GM-CSF receptor by Langerhans' cell histiocytosis cells.

Langerhans' cell histiocytosis (LCH) is characterized by the proliferation of large mononucleated cells containing Birbeck granules and expressing CD1a. Recent studies have demonstrated that LCH is a clonal proliferation; however, its aetiology is still unknown. Growth and differentiation of bone-marrow-derived cells are controlled by cytokines. The proliferation, differentiation and activation of normal Langerhans cells are controlled by granulocyte/macrophage colony-stimulating factor (GM-CSF) in vitro. Therefore, GM-CSF could be implicated in the pathogenesis of LCH. Indeed, LCH cells contain GM-CSF, and children with disseminated LCH have an elevated GM-CSF serum level. As a cytokine only acts on cells expressing a specific receptor, we investigated the presence of GM-CSF receptor on LCH cells. Fourteen frozen tissue samples from children with LCH were studied by in situ immunohistochemistry with two mouse monoclonal antibodies specific for the alpha chain of the GM-CSF receptor (CDw116). LCH cells of all the samples were positively stained with both antibodies. This study suggests that GM-CSF may be a growth factor for LCH cells.

Differentiation of Langerhans cells from interdigitating cells using CD1a and S-100 protein antibodies.

The present study shows that Langerhans cells can be differentiated from interdigitating cells at the light microscopic level. Superficial lymph nodes and skin taken from necropsies and the lymph nodes of dermatopathic lymphadenopathy (DPL) were used for this experiment. Sections of lymph node and skin were embedded using the acetone, methyl benzoate and xylene (AMeX) method and dendritic cells were immunostained with anti S-100 protein antibody (S-100, and OKT-6 (CD1a) using the restaining method. Langerhans cells in the skin were positive for both CD1a and S-100. Dendritic cells positive for both CD1a and S-100, and dendritic cells positive for S-100, but not for CD1a were observed in superficial lymph nodes. In normal superficial lymph nodes, there were more interdigitating cells than Langerhans cells. The majority of the dendritic cells in the DPL were Langerhans cells. We conclude that the S-100 and CD1a positive cells are Langerhans cells, and the S-100 positive-CD1a negative cells are interdigitating cells.

Migration of murine epidermal Langerhans cells to regional lymph nodes: engagement of major histocompatibility complex class II antigens induces migration of Langerhans cells.

Langerhans cells are resident dendritic cells in the epidermis. Once they are loaded with epicutaneously-delivered antigens, they leave the epidermis and migrate to the regional lymph nodes where they initiate primary T cell responses as antigen-presenting cells. However, the stimulus that initiates such migration remains unknown. Because major histocompatibility complex class II (Ia) antigens on B lymphocytes or monocytic cells have been shown to function as signal transducers, we evaluated the effect of the engagement of Ia antigens on the migration of murine epidermal Langerhans cells. The intradermal injection of an anti-Ia monoclonal antibody (mAb) reduced the density of Langerhans cells in epidermis and produced a dose- and time-dependent increase in the frequency of cells reactive with NLDC145 (Langerhans cell- and dendritic cell-specific mAb) within the regional lymph nodes. Injection of a control mAb had no effect. The NLDC145+ cells that were induced to accumulate in the regional lymph nodes were Ia+, large dendritic cells, some of which were positive for both NLDC145 and F4/80, a phenotype corresponding to that of murine epidermal Langerhans cells. Thus, the engagement of Ia antigens on Langerhans cells by mAb induces the migration of Langerhans cells from the epidermis to the regional lymph nodes. Analysis of these changes in Langerhans cells in vitro may help to reveal the biochemical sequence of events involved in the activation and differentiation of Langerhans cells.

Differentiation of epidermal Langerhans cells in macrophage colony-stimulating-factor-deficient mice homozygous for the osteopetrosis (op) mutation.

The osteopetrosis (op) mutation is within the gene for macrophage colony-stimulating factor (M-CSF). Homozygotes (op/op) lack M-CSF activity and show abnormalities in the differentiation of osteoclasts and other cells within the macrophage lineage. The effect of the op mutation on the development of Langerhans cells (LC) was determined in order to assess differentiation of such cells in vivo in the absence of M-CSF. (C57BL/6J X C3HeB/FeJ)F2-op/op and +/? Littermate control mice were raised from +/? breeders obtained from the Jackson Laboratory. The mice were killed with ether anesthesia at 4 weeks after birth and skin specimens were excised and examined by immunohistochemistry using anti-mouse pan macrophage monoclonal antibodies (MoAb), F4/80 and BM8; anti-mouse LC MoAb, NLDC-145, M1-8, and MIDC8; and anti-mouse Ia MoAb, M5/114. In epidermal sheets, numbers of LC were counted. Histochemical staining of adenosine diphosphatase (ADPase) localization was also performed as a marker of LC. Epidermal LC from op/op mice showed reactivity with all these MoAb. Numbers of LC were slightly reduced, but the reduction was not significant statistically. The presence of Birbeck granules in LC of op/op mice was confirmed by electron microscopy but the cytoplasmic projection of LC was not prominent. From these results, it appears obvious that the development and differentiation of LC do not require M-CSF.

Development, differentiation, and proliferation of epidermal Langerhans cells in rat ontogeny studied by a novel monoclonal antibody against epidermal Langerhans cells, RED-1.

An antirat monoclonal antibody (mAb) against nonlymphoid dendritic cells, RED-1, was produced using epidermal Langerhans cells (LCs) as the immunogen. This mAb reacted mainly with the LCs and indeterminate dendritic cells (ICs), interdigitating cells in the T cell areas of lymphoid tissues, and monocyte/macrophages in various organs and tissues of adult rats. In the epidermal sheets prepared from adult rats, it specifically recognized the cell surface antigen(s) present on LCs and ICs. In the fetal rat skin, primitive or fetal macrophages migrated into the epidermis and expressed RED-1 at fetal day 17. With advance of gestation, RED-1-positive cells increased, started expressing Ia antigens at fetal day 18, and subsequently differentiated into dendritic cells. Most of them showed Ia expression by fetal day 20 and differentiated into LCs within a few days after birth. The labeling index of 5-bromo-2'-deoxyuridine in RED-1-positive cells was 18% at fetal day 17 and decreased to 5 to 6% in the postnatal period. These results imply that proliferative capacity of RED-1-positive cells is important for the formation and expansion of the IC population in the fetal stage and for the survival of LCs in the postnatal period.

Differentiation of interdigitating reticulum cells and Langerhans cells in the human skin with T-lymphoid infiltrate. An immunocytochemical and ultrastructural study.

The differentiation of two types of T-lymphocyte accessory cells, i.e., interdigitating reticulum cells and Langerhans cells, was studied immunocytochemically and ultrastructurally on cutaneous lesions from patients with mycosis fungoides, a neoplasm of mature T-lymphocytes. In such a condition the lymphoid infiltrate creates, adjacent to the epidermis, a microenvironment in the dermis similar to that of T-cell areas of lymphoid organs. Immunocytochemistry revealed that CD11c+ CD1a- putative monocytic cells co-exist with CD11c+ CD1a+ putative mature accessory cells. By electron microscopy, large numbers of interdigitating reticulum cells in the dermal infiltrate and Langerhans cells in the epidermis were found. Furthermore, monocytes were frequently observed, at times with cells showing intermediate features between monocytes and interdigitating reticulum cells on the one hand and Langerhans cells on the other. In the absence of proliferative phenomena of the above cells, it is conceivable that both interdigitating reticulum cells and Langerhans cells originate from locally migrated monocytes. A possible role of the local tissue micro-environment--namely the T-lymphoid microenvironment for interdigitating reticulum cells and the epidermal microenvironment for Langerhans cells--in inducing the differentiation of monocytes into the two kinds of accessory cells is proposed.

Then Phenotypic Spectrum of Histiocytosis X Cells

Proliferating cells in histiocytosis X (histiocytosis X cells) share many structural and immunophenotypic features with Langerhans cells, leading to the assumption that histiocytosis X represents a proliferative disorder of Langerhans cells. Because, depending on their state of activation and/or differentiation, Langerhans cells exhibit a varying immunophenotype, we investigated whether histiocytosis X cells display a similar phenotypic heterogeneity and, if so, whether the heterogenous biological behavior of histiocytosis X is reflected by differences in the immunophenotype of the proliferating cells. In 21 patients suffering from different clinical manifestations of histiocytosis X, proliferating cells uniformly expressed class I and II alloantigens, T200, CD1, CD4, and S100 protein. In 12 of 21 cases, histiocytosis X cells additionally exhibited immunocytochemically detectable amounts of C3b and C3bi receptors and certain monocyte/macrophage antigens (CDw14, Ki-M1, Ki-M6). This immunophenotypic heterogeneity of histiocytosis X cells could not be correlated with clinical course, prognosis, and final outcome of the disease in a given patient. The capacity of histiocytosis X cells to immunophenotypically mimic various states of Langerhans cell activation and/or differentiation, however, underscores the concept of histiocytosis X as a proliferative disorder of Langerhans cell origin.