Epidermal Langerhans cells are not required for UV-induced immunosuppression

Langerhans cells orchestrate apoptosis of DNA-damaged keratinocytes upon high-dose UVB skin exposure

Ultraviolet (UV) irradiation of the skin causes mutations that can promote the development of melanoma and nonmelanoma skin cancer. High-dose UVB exposure triggers a vigorous skin reaction characterized by inflammation resulting in acute sunburn. This response includes the formation of sunburn cells and keratinocytes (KC) undergoing programmed cell death (apoptosis) when repair mechanisms of DNA damage are inadequate. The primary objective of this research was to clarify the involvement of Langerhans cells (LC) in the development of acute sunburn following intense UVB skin irradiation. To address this, we subjected the dorsal skin of mice to a single high-dose UVB exposure and analyzed the immediate immune response occurring within the skin tissue. Acute sunburn triggered an activation of LC, coinciding with a rapid influx of neutrophils that produced TNF-α. Furthermore, our investigation unveiled a marked increase in DNA-damaged KC and the subsequent induction of apoptosis in these cells. Importantly, we demonstrate a crucial link between the inflammatory cascade, the initiation of apoptosis in DNA-damaged KC, and the presence of LC in the skin. LC were observed to modulate the chemokine response in the skin following exposure to UVB, thereby affecting the trafficking of neutrophils. Skin lacking LC revealed diminished inflammation, contained fewer TNF-α-producing neutrophils, and due to the prevention of apoptosis induction, a lingering population of DNA-damaged KC, presumably carrying the risk of enduring genomic alterations. In summary, our results underscore the pivotal role of LC in preserving the homeostasis of UVB-irradiated skin. These findings contribute to a deeper understanding of the intricate mechanisms underlying acute sunburn responses and their implications for UV-induced skin cancer.

The novel role of lymphatic vessels in the pathogenesis of ocular diseases

Historically, the eye has been considered as an organ free of lymphatic vessels. In recent years, however, it became evident, that lymphatic vessels or lymphatic-like vessels contribute to several ocular pathologies at various peri- and intraocular locations. The aim of this review is to outline the pathogenetic role of ocular lymphatics, the respective molecular mechanisms and to discuss current and future therapeutic options based thereon. We will give an overview on the vascular anatomy of the healthy ocular surface and the molecular mechanisms contributing to corneal (lymph)angiogenic privilege. In addition, we present (i) current insights into the cellular and molecular mechanisms occurring during pathological neovascularization of the cornea triggered e.g. by inflammation or trauma, (ii) the role of lymphatic vessels in different ocular surface pathologies such as dry eye disease, corneal graft rejection, ocular graft versus host disease, allergy, and pterygium, (iii) the involvement of lymphatic vessels in ocular tumors and metastasis, and (iv) the novel role of the lymphatic-like structure of Schlemm's canal in glaucoma. Identification of the underlying molecular mechanisms and of novel modulators of lymphangiogenesis will contribute to the development of new therapeutic targets for the treatment of ocular diseases associated with pathological lymphangiogenesis in the future. The preclinical data presented here outline novel therapeutic concepts for promoting transplant survival, inhibiting metastasis of ocular tumors, reducing inflammation of the ocular surface, and treating glaucoma. Initial data from clinical trials suggest first success of novel treatment strategies to promote transplant survival based on pretransplant corneal lymphangioregression.

The Roles of Skin Langerhans Cells in Immune Tolerance and Cancer Immunity

Langerhans cells (LC) are a unique population of tissue-resident macrophages with dendritic cell (DC) functionality that form a network of cells across the epidermis of the skin. Their location at the skin barrier suggests an important role for LC as immune sentinels at the skin surface. The classification of LC as DC over the past few decades has driven the scientific community to extensively study how LC function as DC-like cells that prime T cell immunity. However, LC are a unique type of tissue-resident macrophages, and recent evidence also supports an immunoregulatory role of LC at steady state and during specific inflammatory conditions, highlighting the impact of cutaneous environment in shaping LC functionality. In this mini review, we discuss the recent literature on the immune tolerance function of LC in homeostasis and disease conditions, including malignant transformation and progression; as well as LC functional plasticity for adaption to microenvironmental cues and the potential connection between LC population heterogeneity and functional diversity. Future investigation into the molecular mechanisms that LC use to integrate different microenvironment cues and adapt immunological responses for controlling LC functional plasticity is needed for future breakthroughs in tumor immunology, vaccine development, and treatments for inflammatory skin diseases.

A decade of checkpoint blockade immunotherapy in melanoma: understanding the molecular basis for immune sensitivity and resistance

Ten years since the immune checkpoint inhibitor ipilimumab was approved for advanced melanoma, it is time to reflect on the lessons learned regarding modulation of the immune system to treat cancer and on novel approaches to further extend the efficacy of current and emerging immunotherapies. Here, we review the studies that led to our current understanding of the melanoma immune microenvironment in humans and the mechanistic work supporting these observations. We discuss how this information is guiding more precise analyses of the mechanisms of action of immune checkpoint blockade and novel immunotherapeutic approaches. Lastly, we review emerging evidence supporting the negative impact of melanoma metabolic adaptation on anti-tumor immunity and discuss how to counteract such mechanisms for more successful use of immunotherapy.

Ultraviolet diagnostic and treatment modalities in the coronavirus disease 2019 pandemic

Ultraviolet (UV) therapy is an effective and well-tolerated therapeutic method for various dermatologic conditions due to its antiproliferative and immunosuppressive effects. Contemporary phototherapy includes broadband UVB, narrowband UVB, UVA1, PUVA, and excimer laser therapy. The coronavirus disease 2019 pandemic has resulted in the closure of many patient care facilities, including phototherapy units worldwide. Home phototherapy, thalassotherapy, and other UV therapy modalities are an alternative for many patients with chronic dermatoses. We highlighted possible interactions of UV therapy effects and the coronavirus disease 2019 pandemic, and focused on organization and measures against transmission of infections in phototherapy units. Dermatology departments have reopened their units, assessing the risks and benefits for patients, optimizing safety regulations, and adhering to the rules for disinfection.

Noncanonical autophagy in dermal dendritic cells mediates immunosuppressive effects of UV exposure

BACKGROUND

Control of the inflammatory response is critical to maintaining homeostasis, and failure to do so contributes to the burden of chronic inflammation associated with several disease states. The mechanisms that underlie immunosuppression, however, remain largely unknown. Although defects in autophagy machinery have been associated with inflammatory pathologic conditions, we now appreciate that autophagic components participate in noncanonical pathways distinct from classical autophagy. We have previously demonstrated that LC3-associated phagocytosis (LAP), a noncanonical autophagic process dependent on Rubicon (rubicon autophagy regulator [RUBCN]), contributes to immunosuppression.

OBJECTIVE

We used Rubcn^-/- mice to examine the role of the LAP pathway in mediating the UV-induced immunotolerant program in a model of contact hypersensitivity (CHS).

METHODS

Flow cytometry and transcriptional analysis were used to measure immune cell infiltration and activation in the skin of Rubcn^+/+ and Rubcn^-/- mice during the CHS response.

RESULTS

Here, we demonstrate that LAP is required for UV-induced immunosuppression and that UV exposure induces a broadly anti-inflammatory transcriptional program dependent on Rubicon. Rubcn^-/- mice are resistant to UV-induced immunosuppression and instead display exaggerated inflammation in a model of CHS. Specifically, RUBCN deficiency in CD301b⁺ dermal dendritic cells results in their increased antigen presentation capacity and subsequent hyperactivation of the CD8⁺ T-cell response.

CONCLUSIONS

LAP functions to limit the immune response and is critical in maintaining the balance between homeostasis and inflammation.

Macroenvironment-gene-microenvironment interactions in ultraviolet radiation-induced melanomagenesis

Cutaneous malignant melanoma is one of the few major cancers that continue to exhibit a positive rate of increase in the developed world. A wealth of epidemiological data has undisputedly implicated ultraviolet radiation (UVR) from sunlight and artificial sources as the major risk factor for melanomagenesis. However, the molecular mechanisms of this cause-and-effect relationship remain murky and understudied. Recent efforts on multiple fronts have brought unprecedented expansion of our knowledge base on this subject and it is now clear that melanoma is caused by a complex interaction between genetic predisposition and environmental exposure, primarily to UVR. Here we provide an overview of the effects of the macroenvironment (UVR) on the skin microenvironment and melanocyte-specific intrinsic (mostly genetic) landscape, which conspire to produce one of the deadliest malignancies.

Human skin explants an in vitro approach for assessing UVB induced damage

Lifestyle changes involving frequent outdoor activities are contributing to higher exposure to harmful ultraviolet light (UVB). The acute effects of UVB irradiation on human skin was evaluated in this study using freshly excised human skin from elective surgery subjected to UVB doses (0-3.76 J/cm²). The assessment of UVB induced cellular and skin damages was undertaken at two time points immediately and 24 h post exposure using in vitro, and immunohistochemical staining techniques. The results indicated no significant loss of skin integrity or significant acute mitochondrial cellular damages in UVB exposed skin sections as measured by the MTS cytotoxicity assay. The other key markers of damage showed significant extracellular LDH membrane leakages and upregulation of inflammatory cytokines such as IL-1β. Skin integrity analysis was also undertaken using H&E, HLADR, and anti-cytokeratin antibodies. The results showed significant epidermal changes, basal cell activation and Langerhans cells depletion. The research proved the usefulness of freshly excised human skin explant model in measuring UVB damage. Furthermore, freshly excised human skin maintains the natural layering and therefore does not pose the same challenges faced by commercially available reconstructed skin in terms of higher costs and accurate mimicking of all the complex interactions observed in human skin.

Uncovering the Mysteries of Langerhans Cells, Inflammatory Dendritic Epidermal Cells, and Monocyte-Derived Langerhans Cell-Like Cells in the Epidermis

The identity of Langerhans cells (LCs) has been called into question of late due to the increasing evidence that LCs originate from macrophage lineage instead of dendritic cell (DC) lineage as previously thought. For many years, LCs have been assumed to be DCs due to its migratory capabilities. However, recent studies have demonstrated that LCs are from macrophage lineage of the adult fetal liver (FL) progenitor. Bona fide LCs are now considered tissue-resident macrophages as they originate from the FL as shown by fate mapping models. In recent years, studies have shown that there are three types of antigen-presenting cells present in the epidermis, such as LCs, monocyte-derived LC-like cells, and inflammatory dendritic epidermal cells (IDECs). Of these, LC-like cells have been characterized in both human and mouse studies, while IDECs have only been described in human studies. This has shed a new light on the area of epidermal macrophages, suggesting that there might be a misidentification and misclassification of LCs. IDECs and LC-like cells have been shown to be present in both steady state and inflammatory state, but they are present in more significant amounts under inflammatory conditions such as atopic dermatitis, ultra violet injury, and psoriasis. In this review, we discuss what is already known and discuss the possible roles of LCs, LC-like cells, and IDECs during inflammation. Most intriguingly, we discuss the possibility of LCs having a dual identity as both a macrophage and a DC. This is shown as LCs are the only tissue-resident macrophage to have shown migratory property-like DCs.

Insight into immunocytes infiltrations in polymorphous light eruption

Polymorphous light eruption (PLE) which is one of the most common photodermatoses has been demonstrated to be immune-mediated disorder. Resistance to UV-induced immunosuppression resulting from differential immune cells infiltration and cytokines secretion has been highlighted in the pathogenesis of PLE. In this study, we reviewed differential patterns of immune cells infiltrations and cytokines secretion that may contribute to PLE occurrence and development.

Crosstalk Among UV-Induced Inflammatory Mediators, DNA Damage and Epigenetic Regulators Facilitates Suppression of the Immune System

The suppression of the immune system by overexposure to ultraviolet (UV) radiation has been implicated in the initiation and progression of photocarcinogenesis. Numerous changes occur in the skin on UVB exposure, including the generation of inflammatory mediators, DNA damage, epigenetic modifications, and migration and functional alterations in the antigen-presenting dendritic cells. Although each of these alterations can elicit a cascade of events that have the potential to modulate immune sensitivity alone, there is emerging evidence that there is considerable crosstalk between these cascades. The development of an understanding of UV-induced changes in the skin that culminate in UV-induced immunosuppression, which has been implicated in the risk of nonmelanoma skin cancer, as a network of events has implications for the development of more effective chemopreventive strategies. In the current review article, we discuss the evidence of interactions between the various molecular targets and signaling mechanisms associated with UV-induced immunosuppression.

Sunlight Effects on Immune System: Is There Something Else in addition to UV-Induced Immunosuppression?

Sunlight, composed of different types of radiation, including ultraviolet wavelengths, is an essential source of light and warmth for life on earth but has strong negative effects on human health, such as promoting the malignant transformation of skin cells and suppressing the ability of the human immune system to efficiently detect and attack malignant cells. UV-induced immunosuppression has been extensively studied since it was first described by Dr. Kripke and Dr. Fisher in the late 1970s. However, skin exposure to sunlight has not only this and other unfavorable effects, for example, mutagenesis and carcinogenesis, but also a positive one: the induction of Vitamin D synthesis, which performs several roles within the immune system in addition to favoring bone homeostasis. The impact of low levels of UV exposure on the immune system has not been fully reported yet, but it bears interesting differences with the suppressive effect of high levels of UV radiation, as shown by some recent studies. The aim of this article is to put some ideas in perspective and pose some questions within the field of photoimmunology based on established and new information, which may lead to new experimental approaches and, eventually, to a better understanding of the effects of sunlight on the human immune system.

Langerhans Cells Facilitate UVB-Induced Epidermal Carcinogenesis

UVB light is considered the major environmental inducer of human keratinocyte (KC) DNA mutations, including within the tumor-suppressor gene p53, and chronic exposure is associated with cutaneous squamous cell carcinoma formation. Langerhans cells (LCs) comprise a dendritic network within the suprabasilar epidermis, yet the role of LCs in UVB-induced carcinogenesis is largely unknown. Herein we show that LC-intact epidermis develops UVB-induced tumors more readily than LC-deficient epidermis. Although levels of epidermal cyclopyrimidine dimers following acute UVB exposure are equivalent in the presence or absence of LCs, chronic UVB-induced p53 mutant clonal islands expand more readily in association with LCs, which remain largely intact and are preferentially found in proximity to the expanding mutant KC populations. The observed LC facilitation of mutant p53 clonal expansion is completely αβ and γδ T-cell independent and is associated with increased intraepidermal expression of IL-22 and the presence of group 3 innate lymphoid cells. These data demonstrate that LCs have a key role in UVB-induced cutaneous carcinogenesis and suggest that LCs locally stimulate KC proliferation and innate immune cells that provoke tumor outgrowth.

Interleukin-17 mediated inflammatory responses are required for ultraviolet radiation-induced immune suppression

Ultraviolet radiation (UVR) induces immunosuppression and is a major factor for development of skin cancer. Numerous efforts have been made to determine mechanisms for UVR-induced immunosuppression and to develop strategies for prevention and treatment of UVR-induced cancers. In the current study, we use IL-17 receptor (IL-17R) deficient mice to examine whether IL-17 mediated responses have a role in UVB (290-320)-induced immunosuppression of contact hypersensitivity responses. Results demonstrate that IL-17 mediated responses are required for UVB-induced immunosuppression of contact hypersensitivity responses. The systemic immune suppression and development of regulatory T cells are inhibited in UVB-treated IL-17R deficient mice compared to wild-type animals. The deficiency in IL-17R inhibits the infiltration and development of a tolerogenic myeloid cell population in UVB-treated skin, which expresses CD11b and Gr-1 and produces reactive oxygen species. We speculate that the development of the tolerogenic myeloid cells is dependent on IL-17-induced chemokines and inflammatory mediators in UVB-treated skin. The inhibition of the tolerogenic myeloid cells may be attributed to the suppression of regulatory T cells in UVR-treated IL-17R(-/-) mice. The findings may be exploited to new strategies for prevention and treatment of UVR-induced skin diseases and cancers.

Molecular Mechanisms of UV-Induced Apoptosis and Its Effects on Skin Residential Cells: The Implication in UV-Based Phototherapy

The human skin is an integral system that acts as a physical and immunological barrier to outside pathogens, toxicants, and harmful irradiations. Environmental ultraviolet rays (UV) from the sun might potentially play a more active role in regulating several important biological responses in the context of global warming. UV rays first encounter the uppermost epidermal keratinocytes causing apoptosis. The molecular mechanisms of UV-induced apoptosis of keratinocytes include direct DNA damage (intrinsic), clustering of death receptors on the cell surface (extrinsic), and generation of ROS. When apoptotic keratinocytes are processed by adjacent immature Langerhans cells (LCs), the inappropriately activated Langerhans cells could result in immunosuppression. Furthermore, UV can deplete LCs in the epidermis and impair their migratory capacity, leading to their accumulation in the dermis. Intriguingly, receptor activator of NF-κB (RANK) activation of LCs by UV can induce the pro-survival and anti-apoptotic signals due to the upregulation of Bcl-xL, leading to the generation of regulatory T cells. Meanwhile, a physiological dosage of UV can also enhance melanocyte survival and melanogenesis. Analogous to its effect in keratinocytes, a therapeutic dosage of UV can induce cell cycle arrest, activate antioxidant and DNA repair enzymes, and induce apoptosis through translocation of the Bcl-2 family proteins in melanocytes to ensure genomic integrity and survival of melanocytes. Furthermore, UV can elicit the synthesis of vitamin D, an important molecule in calcium homeostasis of various types of skin cells contributing to DNA repair and immunomodulation. Taken together, the above-mentioned effects of UV on apoptosis and its related biological effects such as proliferation inhibition, melanin synthesis, and immunomodulations on skin residential cells have provided an integrated biochemical and molecular biological basis for phototherapy that has been widely used in the treatment of many dermatological diseases.

Platelet-activating factor does not mediate UVB-induced local immune suppression

The lipid mediator Platelet-activating factor (PAF) and oxidized glycerophosphocholine PAF agonists produced by UVB have been demonstrated to play a pivotal role in UVB-mediated systemic immunosuppression. Importantly, employing the ability of distant UVB irradiation to inhibit contact hypersensitivity (CHS) responses to the chemical antigen dinitrofluorobenzene (DNFB) to an area of unirradiated murine skin, we and others have demonstrated that UVB-mediated systemic immunosuppression was only observed in PAF-R expressing wild type (WT) mice and not in PAF-R-knockout (Pafr-/-) mice. As it is not known if PAF is involved in UVB-mediated local immunosuppression, these studies compared local UVB on CHS responses in WT versus Pafr-/- mice. We demonstrate that the application of DNFB onto UVB-exposed (locally) area of mouse skin resulted in a similar significant inhibition of subsequent CHS responses in both WT and Pafr-/- mice compared to sham-irradiated control mice. Furthermore, the expression of langerin, a marker for the presence of Langerhans cells was substantially reduced equally in the epidermal ears of UVB-irradiated WT and Pafr-/- mice compared to their respective sham control groups. These findings indicate that the PAF-R is not involved UVB-induced local immunosuppression.

Conditional deletion of TGF-βR1 using Langerin-Cre mice results in Langerhans cell deficiency and reduced contact hypersensitivity

The critical role of Langerhans cells (LC) in contact hypersensitivity (CHS) was recently questioned in studies using different LC-depletion mouse models. On one hand, inducible ablation of LC led to diminished ear swelling, suggesting functional redundancy between LC and (Langerin(+)) dermal dendritic cells (DC). On the other hand, constitutive or acute depletion of LC resulted in an enhanced reaction, supporting a regulatory role of LC in CHS. To address this controversy by conditional gene targeting, we generated Langerin-Cre knockin mice. Breeding these mice to a Cre-reporter strain demonstrated robust and specific DNA recombination in LC, as well as other Langerin(+) tissue DC. In agreement with the vital requirement of TGF-β signaling for LC development, crossing Langerin-Cre to mice homozygous for a loxP-flanked TGF-βR1 allele resulted in permanent LC deficiency, whereas the homeostasis of dermal Langerin(+) DC was unaffected. In the absence of LC, induction of CHS in these Langerin(+) DC-specific TGF-βR1-deficient mice elicited decreased ear swelling compared with controls. This novel approach provided further evidence against a regulatory function of LC in CHS. Moreover, these Langerin-Cre mice represent a unique and powerful tool to dissect the role and molecular control of Langerin(+) DC populations beyond LC.

Systemic low-dose UVB inhibits CD8 T cells and skin inflammation by alternative and novel mechanisms

Exposure to UVB radiation before antigen delivery at an unirradiated site inhibits functional immunological responses. Mice treated dorsally with suberythemal low-dose UVB and immunized with ova in abdominal skin generated ova-specific CD8 T cells with a significantly decreased activation, expansion, and cytotoxic activity compared with unirradiated mice. UVB also impaired the delayed-type hypersensitivity (DTH) reaction to ova. Transfer of CD4⁺CD25⁺cells from UVB-exposed mice did not suppress the ova-specific CD8 T-cell response or DTH reaction in unexposed mice, confirming that systemic low-dose UVB does not induce long-lived functional regulatory CD4⁺CD25⁺ T cells. Repairing cyclobutane pyrimidine dimer-type DNA damage and blocking aryl hydrocarbon receptor signaling also did not reverse the immunosuppressive effect of UVB on ova-specific CD8 T cells and DTH, suggesting that cyclobutane pyrimidine dimers and the aryl hydrocarbon receptor are not required in systemic low-dose UVB-induced immunosuppression. The known UVB chromophore, cis-urocanic acid, and reactive oxygen species triggered the inhibition of DTH caused by UVB, but they were not involved in the modulation of CD8 T cells. These findings indicate that systemic low-dose UVB impedes the primary response of antigen-specific CD8 T cells by a novel mechanism that is independent of pathways known to be involved in systemic suppression of DTH.

Regulatory T cells in cutaneous immune responses

Regulatory T cells (Treg) are a subset of T cells with strong immunosuppressive activity. In the skin, it has recently been revealed that Treg play important roles not only in the maintenance of skin homeostasis but also in the regulation of the immune responses, such as contact hypersensitivity and atopic dermatitis. Furthermore, the skin plays important roles in the induction of Treg in the periphery. In this review, we will provide an overview of the mechanism of Treg-mediated immunosuppression and discuss the role of Treg in the skin.

Leishmaniasis, contact hypersensitivity and graft-versus-host disease: understanding the role of dendritic cell subsets in balancing skin immunity and tolerance

Dendritic cells (DC) are key elements of the immune system. In peripheral tissues, they function as sentinels taking up and processing antigens. After migration to the draining lymph nodes, the DC either present antigenic peptides by themselves or transfer them to lymph node-resident DC. The skin is the primary interface between the body and the environment and host's various DC subsets, including dermal DC (dDC) and Langerhans cells (LC). Because of their anatomical position in the epidermis, LC are believed to be responsible for induction of adaptive cutaneous immune responses. The functions of LC and dDC in the skin immune system in vivo are manifold, and it is still discussed controversially whether the differentiation of T-cell subtypes (e.g. effector T cells and regulatory T cells) may be initiated by distinct DC subtypes. As skin DC are able to promote or downmodulate immune responses, we chose different skin diseases (cutaneous leishmaniasis, contact hypersensitivity, UV radiation-induced suppression, and graft-versus-host disease) to describe the biological interactions between different DC subtypes and T cells that lead to the development of efficient or unwanted immune responses. A detailed knowledge about the immune modulatory capacity of different cutaneous DC subsets might be helpful to specifically target these cells through the skin during therapeutic interventions.

The contribution of Langerhans cells to cutaneous malignancy

The skin is at the forefront of environmental exposures, such as ultraviolet radiation and a myriad of chemicals, and is at risk for malignant transformation. The skin is a highly responsive immunological organ that contains a unique population of immature intraepidermal dendritic cells (DCs) called Langerhans cells (LCs). Although LCs show morphological and migratory changes in response to epidermal perturbation, and can function as antigen-presenting cells to activate T cells, their role in carcinogenesis is unknown. Here we review recent studies that have provided clues to the potential roles that LCs might play in the pathogenesis of skin cancer, beyond their stimulation or regulation of adaptive immunity. Understanding this role of LCs might provide new perspectives on the relevance of DC populations that are resident within other epithelial tissues for cancer.

Molecular mechanisms of ultraviolet radiation-induced immunosuppression

Solar ultraviolet radiation (UVR) is well known for its immunosuppressive properties. UVR can suppress immune reactions both in a local and a systemic fashion. One of the major molecular mediators of photoimmunosuppression is UVR-induced DNA damage. In contrast to immunosuppressive drugs, UVR does not act in a general but antigen-specific fashion. This is due to the induction of regulatory T cells. Epidermal Langerhans cells harboring UVR-induced DNA damage appear to be essentially involved in the induction of these cells. Cytokines including interleukin (IL)-12, -18 and -23 exert the capacity to reduce UVR-induced DNA damage via induction of DNA repair. Accordingly, these cytokines prevent UVR-mediated immunosuppression. In contrast to IL-18, IL-12 and IL-23 can also inhibit the suppressive activity of regulatory T cells by a mechanism which still needs to be determined. Clarification of the molecular mechanisms underlying UVR-induced immunosuppression will help to develop new immunosuppressive therapeutic strategies by utilizing UVR-induced regulatory T cells for the treatment of immune-mediated diseases. In addition, these insights will contribute to a better understanding of photocarcinogenesis since suppression of the immune system by UVR essentially contributes to the induction of skin cancer.

Elevated epidermal ornithine decarboxylase activity suppresses contact hypersensitivity

Previous reports have shown that elevated polyamine biosynthesis is sufficient to promote skin tumorigenesis in susceptible mouse strains. We hypothesized that increased activity of epidermal ornithine decarboxylase (ODC), a key regulatory enzyme in polyamine biosynthesis, may suppress the cutaneous immune response in addition to stimulating proliferation. Using an ODCER transgenic mouse model in which ODC is targeted to the epidermis, we examined the effect of ODC overexpression in keratinocytes on a classic contact hypersensitivity (CHS) response. Compared with normal littermate mice, ODCER transgenic mice showed reduced ear swelling, reduced neutrophil infiltration, and decreased migration of fluorescein isothiocyanate-loaded dendritic cells (DCs) to draining lymph nodes following hapten elicitation of CHS. In addition, elevated epidermal ODC activity suppressed the levels of cytokines keratinocyte-derived chemokine, monocyte chemoattractant protein-1, interleukin-6 (IL-6), and IL-10. Adoptive transfer of lymphocytes from sensitized ODCER transgenic or normal littermate mice to naive ODCER transgenic or wild-type mice indicated that elevated epidermal ODC activity suppresses both the sensitization and elicitation phases of CHS. The specific ODC inhibitor, α-difluoromethylornithine, abrogated all suppressive effects of ODC in CHS reactions. Collectively, these data suggest that the immunosuppression promoted by increased epidermal ODC is mediated by a reduction in cytokine levels, which suppresses DC migration and reduces immune cell infiltration to the site of hapten application.

Langerhans cells serve as immunoregulatory cells by activating NKT cells

Ultraviolet exposure alters the morphology and function of epidermal Langerhans cells (LCs), which play a role in UV-induced immune suppression. It is generally believed that UV exposure triggers the migration of immature LCs from the skin to the draining lymph nodes (LNs), where they induce tolerance. However, because most of the previous studies employed in vitro UV-irradiated LCs, the data generated may not adequately reflect what is happening in vivo. In this study, we isolated migrating LCs from the LNs of UV-irradiated mice and studied their function. We found prolonged LC survival in the LNs of UV-irradiated mice. LCs were necessary for UV-induced immune suppression because no immune suppression was observed in LC-deficient mice. Transferring LCs from UV-irradiated mice into normal recipient animals transferred immune suppression and induced tolerance. We found that LCs colocalized with LN NKT cells. No immune suppression was observed when LCs were transferred from UV-irradiated mice into NKT cell-deficient mice. NKT cells isolated from the LNs of UV-irradiated mice secreted significantly more IL-4 than NKT cells isolated from nonirradiated controls. Injecting the wild-type mice with anti-IL-4 blocked the induction of immune suppression. Our findings indicate that UV exposure activates the migration of mature LC to the skin draining LNs, where they induce immune regulation in vivo by activating NKT cells.

[Recent advances in the roles of dendritic cells on contact dermatitis]

Dendritic cells are one of antigen presenting cells and are thought to play an important role in the establishment of immune responses. However, recent advances in immunology have revealed that there exist several dendritic cell subsets in the skin. In addition, a certain dendritic cell subset is able to regulate cutaneous immune responses. We used contact hypersensitivity as a cutaneous immune response model and review the recent advances in the roles of dendritic cells in contact hypersensitivity.

Possible implication of local immune response in Darier's disease: an immunohistochemical characterization of lesional inflammatory infiltrate

Cell-mediated immunity is considered to be normal in Darier's Disease (DD), an inherited skin disorder complicated by skin infections. To date, there are no investigations on the local inflammatory infiltrate in DD skin lesions. In this immunohistochemical study we characterized and quantified it, making comparisons with two other inflammatory skin disorders, that is, pemphigus vulgaris (PV) and lichen ruber planus (LRP), and with the normal skin (NSk). We found a significant (P < .05) decrease of CD1a+ Langerhans cells (LCs) in DD, compared to PV, LRP, and NSk, and of CD123+ plasmacytoid dendritic cells (pDCs), compared to PV and LRP. We hypothesize that the genetic damage of keratinocytes might result in a loss of some subsets of dendritic cells and, consequently, in an impaired local immune response, which might worsen the infections that inevitably occur in this disease.

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

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

The evolving function of Langerhans cells in adaptive skin immunity

Langerhans cells (LC) are dendritic cell that resides in the epidermis of skin. Paul Langerhans originally observed and named this epinonymous cell more than 140 years ago. Their network-like distribution and dendritic processes that extended up into the stratum corneum convinced him that they represented peripheral nerve cells. It was not determined until almost 100 years later that LC are, in fact, bone marrow-derived and function as skin-resident antigen presenting cells. Many studies have shown that LC are highly immunostimulatory. Recently, data have begun to accumulate suggesting LC have immunoregulatory properties. This review will focus on the participation of LC in the development and regulation of adaptive immune responses.

The mandatory role of IL-10-producing and OX40 ligand-expressing mature Langerhans cells in local UVB-induced immunosuppression

The mechanism underlying the local UVB-induced immunosuppression is a central issue to be clarified in photoimmunology. There have been reported a considerable number of cells and factors that participate in the sensitization phase-dependent suppression, including Langerhans cells (LCs), regulatory T cells, IL-10, and TNF-alpha. The recent important finding that LC-depleted mice rather exhibit enhanced contact hypersensitivity responses urged us to re-evaluate the role of LCs along with dermal dendritic cells (dDCs) in the mechanism of UVB-induced immunosuppression. We studied the surface expression of OX40 ligand (OX40L) and the intracellular expression of IL-10 in LCs and dDCs from UVB-irradiated (300 mJ/cm(2)) skin of BALB/c mice and those migrating to the regional lymph nodes from UVB-irradiated, hapten-painted mice. In epidermal and dermal cell suspensions prepared from the UVB-irradiated skin, LCs expressed OX40L as well as CD86 and produced IL-10 at a higher level than Langerin(-) dDCs. The UVB-induced immunosuppression was attenuated by the administration of IL-10-neutralizing or OX40L-blocking Abs. In mice whose UVB-irradiated, hapten-painted skin was dissected 1 d after hapten application, the contact hypersensitivity response was restored, because this treatment allowed dDCs but not LCs to migrate to the draining lymph nodes. Moreover, LC-depleted mice by using Langerin-diphtheria toxin receptor-knocked-in mice showed impaired UVB-induced immunosuppression. These results suggest that IL-10-producing and OX40L-expressing LCs in the UVB-exposed skin are mandatory for the induction of Ag-specific regulatory T cells.

Langerhans cells: critical regulators of skin immunity?

Langerhans cells (LC) are members of the heterogenous family of professional antigen presenting dendritic cells (DC). They are identified by the C-type lectin receptor Langerin and form a contiguous network in the epidermis. Consequently, LC are an integral part of the skin barrier to the environment and were considered to be critical inducers of skin immunity, whereas dermal DC were largely overlooked. However, with the identification of a distinct subset of Langerin expressing dermal DC, the situation in the skin has become more complex and the relative contribution of the different cutaneous DC populations in balancing immunity and tolerance has become a matter of active debate. Here, we briefly review the classical paradigm and recent challenges of LC function, before focusing on advances concerning their role in contact hypersensitivity and ultraviolet radiation-induced immunosuppression obtained with in vivo LC ablation models. We then discuss novel LC/DC-specific gene targeting approaches currently used to dissect the role of the regulatory cytokines transforming growth factor-beta and interleukin-10 to govern LC and DC function in vivo. This second generation of LC-specific genetically engineered mice will considerably extend our understanding of the molecular control of LC function in regulating skin immunity and tolerance in the near future.

Revisiting the tolerogenicity of epidermal Langerhans cells

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

Langerhans cells are required for UVR-induced immunosuppression

Painting of haptens onto UVR-exposed skin does not result in sensitization but induces regulatory T cells (Treg). This was explained by UVR-mediated depletion of Langerhans cells (LCs). Furthermore, migration of UVR-damaged but still viable LCs into lymph nodes appears to be essential to induce Treg. Accordingly, the steroid mometasone, which kills LCs, inhibited sensitization but did not induce Treg. In Langerin-diphtheria toxin receptor knock-in (DTR) mice, LCs can be depleted by injection of diphtheria toxin (DT). LC-depleted mice could be sensitized though less pronounced than wild-type mice, but sensitization was not suppressed by UVR. Similarly, Treg did not develop. Langerin is not only expressed in LCs but also in some dermal dendritic cells (dDCs). Langerin-positive dDCs repopulate within 10 days after depletion, whereas LCs are still absent. Langerin-DTR mice treated with DT 10 days before UVR and sensitization were still resistant to UVR-induced inhibition of contact hypersensitivity (CHS). Similarly, Treg did not arise. As in this setting only LCs but not Langerin-positive dDCs are absent, LCs appear to be essential for both the suppression of CHS and the induction of Treg by UVR. This supports the concept that LCs are more important for the downregulation than the induction of immune responses in the skin.