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Indeterminate cell tumor (ICT) is a rare neoplastic dendritic cell disorder that has been poorly defined due to its rarity and poorly understood histogenesis and pathogenesis. It is characterized by a proliferation of dendritic cells, which mimic Langerhans cells immunophenotypically (positive for CD1a and S-100 protein), but lack Birbeck granules characteristic of Langerhans cells. The clinical, morphologic, immunophenotypic, and ultrastructural features of 5 ICT cases are reported in an attempt to further define ICT and to examine the postulated relationship between indeterminate cells and Langerhans cells. Four of 5 patients were females, and 4 of 5 were older than 68 years. Three of 5 patients had cutaneous lesions, whereas 2 presented with cervical lymph node involvement. Two patients had a possible association with lymphoma: first patient had a history of progressive follicular lymphoma that led to patient's demise and the second patient had unexplained systemic lymphadenopathy and died 1 week after the biopsy. All 5 ICT cases expressed CD1a and S-100 protein, but lacked Langerin expression and Birbeck granules ultrastructurally. Interestingly, a t(14;18) was detected by fluorescence in situ hybridization in the ICT cells of the patient with previous follicular lymphoma and a monoclonal kappa light chain gene rearrangement was detected by polymerase chain reaction in the patient with systemic lymphadenopathy. In both cases, there was no morphologic or immunophenotypic evidence of a concurrent B-cell lymphoma. In conclusion, ICT is a rare neoplasm that can occur de novo or in association with a B-cell lymphoma, possibly as a result of B-cell dedifferentiation caused by relatively unknown mechanisms. Finally, Langerin immunostaining may be used as a surrogate marker for the ultrastructural demonstration of Birbeck granules, the absence of which represents a strong diagnostic criterion for ICT.

Indeterminate cell tumor (ICT) is a rare neoplastic dendritic cell disorder that has been poorly defined due to its rarity and poorly understood histogenesis and pathogenesis. It is characterized by a proliferation of dendritic cells, which mimic Langerhans cells immunophenotypically (positive for CD1a and S-100 protein), but lack Birbeck granules characteristic of Langerhans cells. The clinical, morphologic, immunophenotypic, and ultrastructural features of 5 ICT cases are reported in an attempt to further define ICT and to examine the postulated relationship between indeterminate cells and Langerhans cells. Four of 5 patients were females, and 4 of 5 were older than 68 years. Three of 5 patients had cutaneous lesions, whereas 2 presented with cervical lymph node involvement. Two patients had a possible association with lymphoma: first patient had a history of progressive follicular lymphoma that led to patient's demise and the second patient had unexplained systemic lymphadenopathy and died 1 week after the biopsy. All 5 ICT cases expressed CD1a and S-100 protein, but lacked Langerin expression and Birbeck granules ultrastructurally. Interestingly, a t(14;18) was detected by fluorescence in situ hybridization in the ICT cells of the patient with previous follicular lymphoma and a monoclonal kappa light chain gene rearrangement was detected by polymerase chain reaction in the patient with systemic lymphadenopathy. In both cases, there was no morphologic or immunophenotypic evidence of a concurrent B-cell lymphoma. In conclusion, ICT is a rare neoplasm that can occur de novo or in association with a B-cell lymphoma, possibly as a result of B-cell dedifferentiation caused by relatively unknown mechanisms. Finally, Langerin immunostaining may be used as a surrogate marker for the ultrastructural demonstration of Birbeck granules, the absence of which represents a strong diagnostic criterion for ICT.

Background Langerhans cell histiocytosis (LCH) is a multisystemic dendritic cell proliferation that is relatively uncommon in adults. Central nervous system LCH outside the pituitary gland is even more uncommon. Case Description We report the case of a 42-year-old man who had complained of right-side hemicranial pain and left arm minor paresis. The symptoms were due to a right insular lobe heterogeneous-enhancing lesion associated with extensive vasogenic edema. The first diagnostic impression suggested glioblastoma multiforme or localized metastasis. The thoracic, abdominal, pelvic computed tomography scan only detected small upper lung inactive nodules suggesting silent focal LCH. A very hard lesion was almost completely removed through a pterional craniotomy approach, with no fluorescence after aminolevulinic acid infusion. The intraoperative biopsy findings ruled out glioma but could not confirm lymphoma. The definitive cerebral biopsy findings showed lymphocytes and histiocytes (CD1a+, S-1001+), with a diagnosis of intracerebral parenchymal LCH. Fractioned radiotherapy resulted in clinical and radiological remission. Conclusions The present case is so rare it should not be used as a guide. We probably will never see a single intraparenchymal supratentorial central nervous system LCH lesion. However, we hope our report will help colleagues in the future with the thought process. Highlights • LCH is a rare heterogeneous disease characterized by an accumulation of dendritic cells in various organs. • CNS involvement that excludes the pituitary gland is extremely uncommon. • A rare intraparenchymal radiological presentation can be misinterpreted as high-grade glioma. • A multispecialty approach should be mandatory when CNS LGH is suspected. [ABSTRACT FROM AUTHOR]

Langerhans cells (LC) are the resident antigen presenting cells of the mucosal epithelium and play an essential role in initiating immune responses. LC are the only cells in the body to contain Birbeck granules (BG), which are unique cytoplasmic organelles comprised of c-type lectin langerin. Studies of BG have historically focused on morphological characterizations, but BG have also been implicated in viral antigen processing which suggests that they can serve a function in antiviral immunity. This study focused on investigating proteins that could be involved in BG formation to further characterize their structure using transmission electron microscopy (TEM). Here, we report a critical role for the protein annexin A2 (anxA2) in the proper formation of BG structures. When anxA2 expression is downregulated, langerin expression decreases, cytoplasmic BG are nearly ablated, and the presence of malformed BG-like structures increases. Furthermore, in the absence of anxA2, we found langerin was no longer localized to BG or BG-like structures. Taken together, these results indicate an essential role for anxA2 in facilitating the proper formation of BG.

Telocytes (TCs) are cells with long, thin and moniliform processes called telopodes. These cells have been found in numerous tissues, including the eye choroid and sclera. Lamina fusca (LF), an anatomical structure located at the sclera-choroid junction, has outer fibroblastic lamellae containing cells with long telopodes. The purpose of this study was to evaluate, via transmission electron microscopy, the LF for the presence of endothelial-specific ultrastructural features, such as Weibel–Palade bodies (WPBs), in the residing TCs. We found that the outer fibroblastic layer of LF lacked pigmented cells but contained numerous cells with telopodes. These cells had incomplete or absent basal laminae, were united by focal adhesions and close contacts, and displayed scarce caveolae and shedding vesicles. Within the stromal cells of LF, numerous WPBs in various stages of maturation and vesicular structures, as secretory pods that ensure the exocytosis of WPBs content, were observed. The WPBs content of the cells with telopodes in the LF could indicate either their involvement in vasculogenesis and/or lymphangiogenesis or that they are the P-selectin- and CD63-containing pools that play roles in scleral or choroidal inflammation. [ABSTRACT FROM AUTHOR]

Langerhans cell histiocytosis (LCH) is a rare multi-system disease. It presents infrequently as a childhood orbital tumor, and can mimic more common inflammatory orbital disease processes. We report the clinical, histopathological, and electron microscopic findings of orbital LCH in an 18-month-old child, along with a review of the recent literature regarding molecular pathogenetic analysis of LCH. The child presented with a two-week history of progressive left periorbital edema and redness. He was initially diagnosed and treated empirically for bacterial periorbital cellulitis, but subsequently underwent ophthalmological consultation after he failed to improve. Histopathological examination of an orbital biopsy specimen revealed numerous Langerhans-type cells, which stain positive for CD1A and CD207 (langerin). Electron microscopic examination demonstrated characteristic Birbeck granules within the Langerhans-type cells. Three year follow-up did not demonstrate recurrence or disease progression. [ABSTRACT FROM AUTHOR]

Background Indeterminate dendritic cell tumor (IDCT) is an extremely rare hematologic disorder with poorly understood pathogenesis. Occasionally encountered by hematologists, unusual presentations of IDCT have not been reported in the spine literature. Methods We report a 51-year-old man who presented with a 3-month history of progressively worsening axial thoracic back pain radiating to his sides. Magnetic resonance imaging revealed a 3-cm enhancing mass at the T9 vertebral body with an exophytic component causing significant canal stenosis. Initial percutaneous biopsy revealed histiocytic sarcoma. Results The patient underwent exploratory thoracotomy and en bloc resection of the lesion with T8-10 fusion. Final pathology results revealed IDCT with fibrosis. IDCT immunostaining was partially positive for Langerhans cell marker (positive for S100 and CD1a, but lacked Birbeck granules and Langerin stain) and partially positive for blastic plasmacytoid dendritic cell neoplasm. Additionally, it was positive for CD45, CD68, and CD163. Lymphadenopathy was absent in this patient. Conclusions Although first reported in the 1980s, IDCT has been omitted from most classifications owing to its rarity. Hematologists have debated the cell of origin; it is believed to comprise pre–Langerhans cells, as Birbeck granules are acquired after migration to the epidermis. IDCT remains of indeterminate origin. We report the first case of spinal IDCT. Familiarity with the histologic features is warranted to ensure accurate diagnosis and appropriate treatment. [ABSTRACT FROM AUTHOR]

Dendritic cells (DCs) are the most powerful antigen presenting cells (APCs) that have a critical role in bridging innate and adaptive immune responses in vertebrates. Dendritic cells have been characterized morphologically and functionally in the teleost fish models such as rainbow trout, salmonids, medaka, and zebrafish. The presence of DCs with remarkable similarities to human Langerhans cells (LCs) has been described in the spleen and anterior kidney of salmonids and rainbow trout. However, there is no evidence of the presence of DCs and their role in channel catfish immunity. In this study, we assessed DC-like cells in the immunocompetent tissues of channel catfish by immunohistochemistry (IHC), flow cytometry and transmission electron microscopy (TEM). We identified Langerin/CD207 + (L/CD207 + ) cells in the channel catfish anterior kidney, spleen and gill by IHC. Moreover, we described the cells that resembled mammal LC DCs containing Birbeck-like (BL) granules in channel catfish spleen, anterior and posterior kidneys and gill by TEM. Our data suggest that cells with DC-like morphology in the immune related organs of catfish may share morphological and functional properties with previously reported DCs in teleost fish and mammals. More detailed knowledge of the phenotype and the function of catfish DCs will not only help gain insight into the evolution of the vertebrate adaptive immune system but will also provide valuable information for development and optimization of immunotherapies and vaccination protocols for aquaculture use. [ABSTRACT FROM AUTHOR]

Langerhans cell histiocytosis (LCH), mainly affects the skull, vertebrae, ribs and mandible in children and the long bones of adults. Symptoms range from none to pain, swelling and tenderness over the site of the lesion. This disease presents oral manifestations which can sometimes be the first expression of the condition. It occurs in three forms namely eosinophilic granuloma in which isolated or multiple bones are involved, But has a good prognosis whereas other variants Hand-Shuller-Christian disease (chronic dessiminated variant) and Letterer-Siwe disease (acute dessiminated form) have poor prognosis. Occasionally only soft tissues are affected without bony involvement. Males are more commonly affected than females. This article describes a rare variant of eosinophilic granuloma of labial mucosa without bony involvement.

Background Human Langerhans cells (LCs) reside in foreskin and vaginal mucosa and are the first immune cells to interact with HIV-1 during sexual transmission. LCs capture HIV-1 through the C-type lectin receptor langerin, which routes the virus into Birbeck granules (BGs), thereby preventing HIV-1 infection. BGs are langerin-positive organelles exclusively present in LCs, however, their origin and function are unknown. Results Here, we not only show that langerin and caveolin-1 co-localize at the cell membrane and in vesicles but also that BGs are langerin/caveolin-1-positive vesicles that are linked to the lysosomal degradation pathway in LCs. Moreover, inhibition of caveolar endocytosis in primary LCs abrogated HIV-1 sequestering into langerin+ caveolar structures. Notably, both inhibition of caveolar uptake and silencing of caveolar structure protein caveolin-1 resulted in increased HIV-1 integration and subsequent infection. In contrast, inhibition of clathrinmediated endocytosis did not affect HIV-1 integration, even though HIV-1 uptake was decreased, suggesting that clathrin-mediated endocytosis is not involved in HIV-1 restriction in LCs. Conclusions Thus, our data strongly indicate that BGs belong to the caveolar endocytosis pathway and that caveolin-1 mediated HIV-1 uptake is an intrinsic restriction mechanism present in human LCs that prevents HIV-1 infection. Harnessing this particular internalization pathway has the potential to facilitate strategies to combat HIV-1 transmission. [ABSTRACT FROM AUTHOR]

Congenital self-healing Langerhans cell histiocytosis or Hashimoto-Pritzker disease is a rare condition present at birth or in the neonatal period characterized by small reddish-brown crusted papulonodular lesions. In most cases these lesions are not accompanied by systemic findings and tend to involute spontaneously within weeks or months, but in other cases there may be extracutaneous involvement and/or recurrence of the disease. This emphasizes that the clinical course is variable and a long-term follow-up is mandatory in order to reveal possible systemic involvement. We describe two cases of congenital self-healing Langerhans cell histiocytosis with widespread and very similar cutaneous manifestations but different clinical course. The first patient had multisystemic disease (with lymph nodes, bones, liver and lungs affected) that required systemic therapy. The second patient had cutaneous and bony lesions that resolved spontaneously. We think that the adjective 'self-healing' is misleading and should be abandoned. We stress the importance of a complete systemic evaluation and the necessity of a long-term follow-up. [ABSTRACT FROM AUTHOR]

Langerhans cell histiocytosis (LCH) is a rare disease of unknown etiology with a heterogeneous and unspecific clinical presentation. Any organ or system may be involved but the most frequent is the skeleton. The diagnostic gold standard is done through histopathology combined with immunohistochemistry in the correct clinical setting. Treatment is still controversial. The objective of this study is to present a series of pediatric patients in order to contribute to the diagnosis and treatment of LCH. A longitudinal, descriptive and retrospective study was done in 20 pediatric patients histologically diagnosed of LCH int the last 10 years, in a tertiary center. A review of the medical history was done and data collection included: epidemiological data, clinical manifestations and classification, lesion characteristics, treatment employed, follow-up and evolution. The mean age presentation was 6.5 years (2–12). The most frequent disease presentation was a palpable skull tumor followed by headache. Other otorhinolaryngologic presenting symptoms were bloody otorrhea and post auricular swelling. Single-systems LCH with skull tumors was the most frequent classification. Treatment options (surgery, chemotherapy and corticosteroid infiltration) were indicated according to disease classification. LCH should be suspected in pediatric patients with a palpable cranial vault tumor, as well as in patients with chronic otorrhea with granulation tissue at the external ear canal. Management of LCH fundamentally includes chemotherapy and surgery, or the combination of both. High-risk organs lesions, including temporal bone lesions, and multi-system LCH are predictors of recurrence and should have a long-term follow-up. [ABSTRACT FROM AUTHOR]

Primary cutaneous neoplasms of histiocytes and dendritic cells are rare. Langerhans cells are a subset of antigen-presenting dendritic cells. Neoplasms of Langerhans cells are classified into cytologically benign Langerhans cell histiocytosis and cytologically malignant Langerhans cell sarcoma. Langerhans cell sarcoma is a rare entity characterized by multiorgan involvement and an aggressive clinical course. To date, only 30 cases of Langerhans cell sarcoma, including the present case, have been reported. We report a new case of Langerhans cell sarcoma that presented with multifocal cutaneous involvement. Diagnosis was done based on histopathological, immunohistochemical evaluation, as well as ultrastructural analysis identifying the presence of Birbeck granules. Our case represents a new case of this extremely rare, overtly aggressive neoplasm of Langerhans cells. Within 2 years of diagnosis, the patient developed metastatic disease and consequently died. Early recognition is important because of the tendency of Langerhans cell sarcoma to recur and metastasize. Therefore, ancillary techniques such as immunohistochemical and ultrastructural studies to confirm the diagnosis are very advantageous. [ABSTRACT FROM AUTHOR]

Competing Interests: The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Summary: Indeterminate cell tumor is an extremely rare neoplasm that mainly occurs in the skin. We report a case of indeterminate cell tumor arising from the spleen, a previously unreported site for indeterminate cell tumor. Histologically, the tumor showed nests, nodules, and sheets of large polygonal cells with mostly oval nuclei; open chromatin; variable nucleoli; and abundant, eosinophilic cytoplasm. Some cells possessed irregularly convoluted nuclei with nuclear grooves and granular cytoplasm, suggestive of Langerhans cells. Immunohistochemically, the tumor cells were diffusely positive for S-100 and CD1a and negative for Langerin. No Birbeck granules were found by electron microscopy. Clinical and radiologic examination showed no other organomegaly or lymphadenopathy. A diagnosis of primary indeterminate cell tumor of the spleen was rendered. To the best of our knowledge, this is the first indeterminate cell tumor reported in the spleen. Biologic insights into dendritic cells in the spleen and the pertinent literature on these entities are reviewed. [Copyright &y& Elsevier]

The ontogeny and disease responses of Langerhans-like cells within lymphoid tissues of Atlantic salmon, Salmo salar, and rainbow trout, Oncorhynchus mykiss, were investigated. These cells were studied in situ with the use of two markers: the ultrastructural presence of Birbeck-like granules and immunohistochemistry with an antibody against human langerin/CD207 that cross-reacts with salmonid tissues. The appearance of Birbeck-like granules was observed in rainbow trout at 2 weeks post-hatch (PH) in the thymus and anterior kidney prior to the development of the spleen. Spleen first appeared at 3 weeks PH in both Atlantic salmon and rainbow trout, and Birbeck-like granules were observed within cells of the newly developed spleens. The cross-reactivity of langerin as seen by immunohistochemistry was not clearly observed in kidney and spleen until 9 weeks PH, when a strong cytoplasmic reaction was observed. To study langerin-positive cells in spleen and kidney during disease, microsporidial gill disease (MGD) in rainbow trout was used as a known disease model inducing a strong cell-mediated adaptive immune response. Langerin-positive cells in healthy fish were seen predominantly in the spleen, and only low numbers were present in the anterior kidney. During MGD, langerin-positive cell numbers were elevated in the anterior kidney and were significantly higher during 5, 6, and 10 weeks post-exposure (PE) compared with healthy control tissue. During MGD, the distribution of langerin-positive cells in the spleen and anterior kidney shifted from having significantly higher numbers of cells in the spleen than in the kidney in controls and at 1 and 4 weeks PE to having a similar distribution of the cells in the two organs at 2, 3, 5, and 6 weeks PE. By 10 weeks PE, significantly higher numbers of langerin-positive cells occurred in the anterior kidney compared with the spleen. [ABSTRACT FROM AUTHOR]

Background: Langerhans cell histiocytosis (LCH) is a proliferative disorder predominantly found in children. It often presents with pain in calvarium or spine and may cause neuroendocrine symptoms. The gold standard for diagnosing LCH is the detection of Birbeck Granules by EM. Here, we describe two unique presentations of LCH and we review current treatment guidelines. Case Description: The first patient was a 23-year-old man who presented with progressive swelling and redness of the left eye. MRI revealed a left retrobulbar lesion extending into the middle cranial fossa with no signal abnormality in the brain parenchyma. The lesion was resected and pathological analysis revealed LCH. Bone scans were negative and the patient was discharged soon after. He later underwent fractionated radiotherapy (cumulative dose 26 Gy). Follow-up MRIs show no disease at 24 months post-op. The second patient was a 56-year-old man with left frontal skull pain for 5 months. Imaging showed a solitary osteolytic lesion extending into both dura and scalp with no signal abnormality of the parenchyma. Excisional biopsy revealed LCH. Surgery was well tolerated and follow-up imaging shows no recurrence at 24 months post-op. Conclusion: We demonstrate that LCH, though uncommon, must remain on the differential when osteolytic lesions present in the adult. Although LCH often has the clinical and radiographical presentation of an abscess, pathology analysis can successfully diagnose LCH based on markers and morphological characteristics. LCH has an excellent prognosis when treated aggressively with surgical resection and radiotherapy as both of our patients were and are now disease free at 2 year follow-up. [ABSTRACT FROM AUTHOR]

Langerhans' cell histiocytosis (LCH) is a neoplastic disease due to uncontrolled proliferation of Langerhans' cells (LCs); damage to organs involved appears to be due to the various cytokines secreted. The true cause of LCH remains a mystery. LCH can present at any age; there is female predominance and the skin is often the first site to be involved. Scaly papules, vesicles, purpuric nodules, plaques and ulcers are the main diverse presentations typically found on the scalp, flexures (including the perineum), crease below the breast, palms, soles and nails. The patient described here is a 35-year-old Saudi Arabian woman with a vulval ulcer that had been misdiagnosed as an infective lesion 11 months before the correct diagnosis of LCH was made by means of biopsy examination. Diffuse infiltration of LCs with large pale reniform/convoluted nuclei and abundant amphophilic cytoplasm in the company of neutrophils, eosinophils, lymphocytes and plasma cells were observed on light microscopy. Immunohistochemical staining, indicating S100 protein positivity in LCs and the demonstration of Birbeck granules by means of electron microscopy, confirmed the diagnosis.The `skin-only' LCH may later progress to involve other organs; alternatively skin lesions may be part of the multisystem, disseminated disease. There is need for careful follow-up of all LCH patients, as the course of the disease cannot be reliably predicted from either the clinical presentation or histology of the lesion. In many cases the prognosis is good, there being limited involvement that does not progress. Surgery or local steroids might suffice as therapy. However, for severely symptomatic disease, in which there are complications associated with dysfunction (or failure) of vital organs such as the liver, lungs, and central nervous system, the use of a variety of immunosuppressive treatments — and the recently described 2-chloro-2-deoxyadenosine (2CdA) (cladribine) — have been suggested; however, the type and extent of disease that will respond to this therapy has not yet been clarified. Solid laboratory research and rigorous clinical trials are required for more effective and less toxic therapies for patients with progressive LCH. [ABSTRACT FROM PUBLISHER]

Abstract: A novel pathway of CD8+ T-cell activation by a previously established human Langerhans cell (LC)-like cell line, ELD-1 [Dendritic Cells 9 (1999) 41] is reported. ELD-1 cells possess LC-specific and dendritic cell (DC) lineage-specific markers including Birbeck granules. Intriguingly, ELD-1 cells stimulated interferon (IFN)-γ production by purified allogeneic CD8+ T cells in an IL-2- but not IL-12-dependent manner, but failed to stimulate CD4+ T cells due to their lack of HLA-DR, CD40, CD80, and CD86 expression. Comparing active and inactive subclones of ELD-1 cells revealed that CD70 was a key molecule determining stimulatory ability. This was confirmed by the ability of transfected CD70-encoding cDNA to confer stimulatory capacity on inactive subclones of ELD-1. Therefore, it is concluded that CD70 expressed on ELD-1 cells has a crucial role in stimulating IFN-γ production by CD8+ T cells through an alternative pathway which does not require CD4+ T-cell help or CD28–B7 interactions. [Copyright &y& Elsevier]

Unlabelled: Congenital self-healing reticulohistiocytosis (CSHRH), a rare disorder initially seen at birth or in the newborn period, is characterized by spontaneously involuting skin lesions. We present the case of a 12-d-old female who was born with many red-brownish nodules scattered over her entire body, and which regressed spontaneously.Conclusion: Although there are no clues to differentiate CSHRH from Letterer-Siwe disease, some signs and symptoms might enable us to reach an accurate diagnosis. A long-term follow-up to detect evidence of relapse or progression of the disease would be well advised. [ABSTRACT FROM AUTHOR]

Abstract: We have characterized a stereospecific transport mechanism for L-dopa into human epidermal Langerhans cells (LCs). It is different from any other amino acid transport system. It is highly concentrative, largely pH-independent, and independent of exogenous Na[sup +] , glucose and oxygen, and fuelled by a renewable intracellular energy source inhibited by iodoacetate but not by arsenate. We propose that the mechanism is a unidirectional L-dopa/proton counter-transport system. We have recently demonstrated anaerobic glycolysis in human epidermis, which substantiates the need of proton pumps for resident LCs. The findings prompt a re-evaluation of the profound changes LCs undergo when exposed to oxygen in aerobic culture. L-dopa is not metabolized by LCs but can rapidly be dislocated to the intercellular space by certain extracellular amino acids, i.e. LCs can profit by L-dopa in a dualistic way, altogether a remarkable biological phenomenon. [ABSTRACT FROM AUTHOR]

The influence of repeated low-dose ultraviolet B (UVB) radiation, to which we are exposed in daily life, has not been fully clarified, although the damage caused by exposure to high-dose UVB radiation has been well-studied in recent years. To investigate skin damage caused by repeated low-dose exposure, we evaluated the extent of injury to the Langerhans cells which are known to be involved in the cutaneous immune system. The backs of hairless mice were exposed to the following doses of UVB radiation: 100 mJ/cm2 once, 50 mJ/cm2 twice, 25 mJ/cm2 four times or 10 mJ/cm2 ten times. Skin specimens were taken for histochemical and electron microscopic examination 24 h after the final irradiation. Epidermis exposed to UVB radiation demonstrated a decrease in the number of Langerhans cells which showed less dendricity. The population of these cells in specimens exposed to repeated suberythemal doses was reduced to 40%, whereas exposure to a single high dose of UVB with the same energy resulted in a reduction of only 33%. These results indicate that repeated suberythemal doses injure Langerhans cells more than a single high-dose exposure. Furthermore, Birbeck granules in Langerhans cells of UVB-irradiated epidermis were reduced and tended to show shortening of their rod portion. The present study suggested that repeated challenge with suberythemal UVB radiation, to which we are all exposed in daily life, can cause substantial damage to Langerhans cells. [ABSTRACT FROM AUTHOR]