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RTN1 is an endoplasmic reticulum-associated protein that was initially identified in neuronal tissues. Here we show that the main isoform RTN1A is a marker for dendritic cells. In the skin, HLA-DR+CD1ahighCD207+CD11cweak Langerhans cells were the only cells in the epidermis, and HLA-DR+CD11c+ dendritic cells were the main cells in the dermis, expressing this protein. RTN1A+ dendritic cells were also found in gingiva, trachea, tonsil, thymus, and peripheral blood. During differentiation of MUTZ-3 cells into Langerhans cells, expression of RTN1A mRNA and protein preceded established Langerhans cell markers CD1a and CD207, and RTN1A protein partially co-localized with the endoplasmic reticulum marker protein disulfide isomerase. In line with this observation, we found that RTN1A was expressed by around 80% of Langerhans cell precursors in human embryonic skin. Our findings show that RTN1A is a marker for cells of the dendritic lineage, including Langerhans cells and dermal dendritic cells. This unexpected finding will serve as a starting point for the elucidation of the, until now, elusive functional roles of RTN1A in both the immune and the nervous system.

Background CARD18 contains a caspase recruitment domain (CARD) via which it binds to caspase-1 and thereby inhibits caspase-1-mediated activation of the pro-inflammatory cytokine interleukin (IL)-1β. Objectives To determine the expression profile and the role of CARD18 during differentiation of keratinocytes and to compare the expression of CARD18 in normal skin and in inflammatory skin diseases. Methods Human keratinocytes were induced to differentiate in monolayer and in 3D skin equivalent cultures. In some experiments, CARD18-specific siRNAs were used to knock down expression of CARD18. CARD18 mRNA levels were determined by quantitative real-time PCR, and CARD18 protein was detected by Western blot and immunofluorescence analyses. In situ expression was analyzed in skin biopsies obtained from healthy donors and patients with psoriasis and lichen planus. Results CARD18 mRNA was expressed in the epidermis at more than 100-fold higher levels than in any other human tissue. Within the epidermis, CARD18 was specifically expressed in the granular layer. In vitro CARD18 was strongly upregulated at both mRNA and protein levels in keratinocytes undergoing terminal differentiation. In skin equivalent cultures the expression of CARD18 was efficiently suppressed by siRNAs without impairing stratum corneum formation. Epidermal expression of CARD18 was increased after ultraviolet (UV)B irradiation of skin explants. In skin biopsies of patients with psoriasis no consistent regulation of CARD18 expression was observed, however, in lesional epidermis of patients with lichen planus, CARD18 expression was either greatly diminished or entirely absent whereas in non-lesional areas expression was comparable to normal skin. Conclusions Our results identify CARD18 as a differentiation-associated keratinocyte protein that is altered in abundance by UV stress. Its downregulation in lichen planus indicates a potential role in inflammatory reactions of the epidermis in this disease.

Fish oil rich in docosahexaenoic acid (DHA) has beneficial effects on human health. Omega-3 polyunsaturated fatty acids are precursors of eicosanoids and docosanoids, signaling molecules that control inflammation and immunity, and their dietary uptake improves a range of disorders including cardiovascular diseases, ulcerative colitis, rheumatoid arthritis, and psoriasis. The unsaturated nature of these fatty acids, however, makes them prone to oxidation, especially when they are incorporated into (membrane) phospholipids. The skin is an organ strongly exposed to oxidative stress, mainly due to solar ultraviolet radiation. Thus, increased levels of PUFA in combination with oxidative stress could cause increased local generation of oxidized lipids, whose action spectrum reaches from signaling molecules to reactive carbonyl compounds that can crosslink biomolecules. Here, we investigated whether PUFA supplements to fibroblasts are incorporated into membrane phospholipids and whether an increase of PUFA within phospholipids affects the responses of the cells to UV exposure. The redox-sensitive transcription factor Nrf2 is the major regulator of the fibroblast stress response to ultraviolet radiation or exposure to oxidized lipids. Here we addressed how Nrf2 signaling would be affected in PUFA-supplemented human dermal fibroblasts and mouse dermal fibroblasts from Nrf2-deficient and wild type mice. We found, using HPLC-tandem MS, that DHA supplements to culture media of human and murine fibroblasts were readily incorporated into phospholipids and that subsequent irradiation of the supplemented cells with UVA resulted in an increase in 1-palmitoyl-2-(epoxyisoprostane-E2)-sn-glycero-3-phosphorylcholine and Oxo-DHA esterified to phospholipid, both of which are Nrf2 agonists. Also, induction of Nrf2 target genes was enhanced in the DHA-supplemented fibroblasts after UVA irradiation. In Nrf2-deficient murine fibroblasts, the expression of the target genes was, as expected, decreased, but surprisingly, expression of TNFα and MMP13 was strongly induced in DHA-supplemented, UVA-irradiated cells. Also, Nrf2-deficient cells had increased levels of oxidized phospholipids relative to the unoxidized precursors after UVA irradiation. Our data suggest that under ultraviolet stress a functioning Nrf2 system is required to prevent DHA-induced inflammation and matrix degradation in dermal fibroblasts.

Background The human lung is considered a nonsterile organ, and surgical interventions therefore take place in a more or less contaminated operating field. Nevertheless, infectious complications of the pleural cavity are low after major lung resections. Antimicrobial peptides (AMPs) are part of the innate immunity and display a broad capacity to kill pathogens. We hypothesized that the pleural space must have a high natural antimicrobial barrier and that AMPs might effectively protect the pleural cavity. Methods Pleural effusions were collected after lung operations. Antimicrobial activity of the fluids against gram-positive and gram-negative pathogens was analyzed by microdilution assays. AMPs were determined by enzyme-linked immunosorbent assay (ELISA), polymerase chain reaction (PCR), and immunohistochemical analysis. The impact of proinflammatory triggers on AMP release from pleural mesothelial cells was evaluated. Results Antimicrobial activity assays revealed high bactericidal properties of postoperative pleural drainage fluids. They effectively killed gram-negative pathogens ( Escherichia coli , Pseudomonas aeruginosa ) as well as gram-positive pathogens ( Staphylococcus aureus , Streptococcus pneumoniae, and Streptococcus pyogenes ). A variety of AMPs was detected at constantly high concentrations in the pleural fluids. They mainly derived from leukocytes and pleural epithelium. Although proinflammatory cytokine levels were elevated in the postoperative pleural fluids, AMP expression could not be augmented by Toll-like receptor (TLR) triggering or by the proinflammatory cytokines interleukin (IL)-1β and tumor necrosis factor (TNF)α. Conclusions We provide the first evidence of a high abundance of AMPs in postoperative pleural fluids. Our findings might explain the broad protection against infectious complications of the pleural space after major lung operations.

Antimicrobial peptides (AMPs), an important part of the innate immune system, are crucial for defense against invading microorganisms. Whereas AMPs have been extensively studied in adult skin, little is known about the impact of AMPs in the developing human skin. We therefore compared the expression and regulation of AMPs in fetal, neonatal, and adult keratinocytes (KCs) in vitro. The constitutive expression of human β-defensin-2 (HBD-2), HBD-3, S100 protein family members, and cathelicidin was significantly higher in KCs from fetal skin than in KCs from postnatal skin. The capacity to further increase AMP production was comparable between prenatal and postnatal KCs. Analysis of skin equivalents (SEs) revealed a strong constitutive expression of S100 proteins in fetal but not in neonatal and adult SEs. The elevated AMP levels correlated with reduced H3K27me3 (tri-methyl-lysine 27 on histone H3) levels and increased expression of the histone demethylase JMJD3. Knockdown of JMJD3 in fetal KCs elevated H3K27me3 levels and significantly downregulated the expression of HBD-3, S100A7, S100A8, S100A9, and cathelicidin. Our data indicate a crucial contribution of histone modifications in the regulation of AMP expression in the skin during ontogeny. The elevated AMP expression in prenatal skin might represent an essential defense strategy of the unborn.

Cornifelin (CNFN) has been identified as a protein component of epidermal corneocytes. Here, we investigated the tissue distribution of CNFN and potential consequences of CNFN deficiency on epithelial function in in vitro models of human skin and oral mucosa. Our detailed bioinformatics and immunostaining analysis revealed that CNFN is not only expressed in human epidermis but also in noncornifying oral mucosa. In normal epidermis, CNFN was confined to the upper granular layer and the stratum corneum. By contrast, in both partly cornifying and noncornifying oral mucosa, CNFN was expressed in a cell membrane-associated pattern over several suprabasal layers. Small interfering RNA-mediated knockdown of CNFN in epidermal keratinocytes (KCs) was associated with only subtle alterations of the overall epidermal architecture in skin models in vitro but led to altered morphology of corneodesmosomes, as detected by electron microscopy. Using dispase treatment followed by mechanical stress, epithelial sheets of CNFN-deficient epidermal KCs were easily disrupted, whereas their CNFN-competent counterparts remained intact. In contrast to the epidermal KCs, CNFN knockdown in oral KCs had a more severe effect and caused pronounced acantholysis in organotypic models of oral mucosa. Together, these findings indicate that CNFN is a structural component of the cell adhesion system of differentiated KCs in both epidermis and oral mucosa.

Background Neutrophils and allergen-specific T cells accumulate in patients with allergic late-phase reactions (LPRs). Their presence is associated with severe inflammation. Cytokines, such as GM-CSF, IFN-γ, and IL-3, which are typically found in patients with allergic LPRs, have been proposed to convert neutrophils into antigen-presenting cells (APCs). Objective We sought to assess the antigen-processing and antigen-presenting capacities of neutrophils from allergic patients. Methods Neutrophils were isolated from peripheral blood of donors with birch pollen allergy and stimulated with GM-CSF, IFN-γ, and IL-3. The viability and expression of HLA-DR, CD80, and CD86 were assessed by using flow cytometry. HLA-DM expression was analyzed by means of immunoblotting. Allergen uptake was studied after fluorescence labeling of the major birch pollen allergen Bet v 1. Bet v 1 was digested with neutrophilic endolysosomal extracts, and the resulting fragments were sequenced by using mass spectrometry. Neutrophils were used as APCs in coculture experiments with autologous HLA-DR–restricted and Bet v 1–specific T-cell clones reactive with epitopes in different regions of the allergen. In all experiments monocytes were used for comparison. Fluids from suction blisters formed on top of LPRs induced by using intradermal allergen injection were assessed for HLA-DR+ neutrophils by using flow cytometry. Results The cytokines significantly enhanced the survival, allergen uptake, and expression of HLA-DM and HLA-DR on neutrophils. Neutrophils rapidly degraded Bet v 1 into fragments containing all relevant T-cell epitopes. Cytokine-activated, allergen-pulsed neutrophils induced proliferative and cytokine responses of Bet v 1–specific T cells irrespective of epitope specificity, confirming that they fully processed and presented the allergen. HLA-DR+ neutrophils were detected in patients with cutaneous allergic LPRs. Conclusion Neutrophils can serve as APCs for local allergen-specific effector T cells in patients with allergic LPRs.

Systemic antagonists of the histamine type 1 and 2 receptors (H1/2r) are widely used as anti-pruritics and central sedatives, but demonstrate only modest anti-inflammatory activity. Because many inflammatory dermatoses result from defects in cutaneous barrier function, and because keratinocytes express both Hr1 and Hr2, we hypothesized that H1/2r antagonists might be more effective if they were used topically to treat inflammatory dermatoses. Topical H1/2r antagonists additively enhanced permeability barrier homeostasis in normal mouse skin by the following mechanisms: (i) stimulation of epidermal differentiation, leading to thickened cornified envelopes; and (ii) enhanced epidermal lipid synthesis and secretion. As barrier homeostasis was enhanced to a comparable extent in mast cell-deficient mice, with no further improvement following application of topical H1/2r antagonists, H1/2r antagonists likely oppose mast cell-derived histamines. In four immunologically diverse, murine disease models, characterized by either inflammation alone (acute irritant contact dermatitis, acute allergic contact dermatitis) or by prominent barrier abnormalities (subacute allergic contact dermatitis, atopic dermatitis), topical H1/2r agonists aggravated, whereas H1/2r antagonists improved, inflammation and/or barrier function. The apparent ability of topical H1r/2r antagonists to target epidermal H1/2r could translate into increased efficacy in the treatment of inflammatory dermatoses, likely due to decreased inflammation and enhanced barrier function. These results could shift current paradigms of antihistamine utilization from a predominantly systemic to a topical approach.

The histamine H4 receptor is functionally expressed on CD4(+) T cells and in particular on human CD4(+) Th2-polarized T cells. Interleukin (IL)-17-producing T cells (Th17 cells) represent a newly defined major CD4(+) T-cell subset, having been identified in psoriatic plaques and in acute skin lesions of atopic dermatitis where histamine is also present in high concentrations. To elucidate the role of the histamine H4 receptor (H4R) on these effector T cells, we polarized human memory T cells into Th17 cells. Further, we investigated H4R expression and assessed its function by real-time PCR, by a cytokine secretion assay of IL-17, and by electrophoretic mobility shift assay of activating protein-1 (AP-1). We show that Th17 cells polarized by IL-1β together with IL-23 express the H4R on mRNA and protein level. Additionally, we identified IL-17-positive cells in psoriatic skin lesions. The IL-17-positive lymphocytes were all positive also for functional H4R. Stimulation with histamine or a H4R agonist increased the production of IL-17 and induced activating protein-1 in Th17 cells. In inflammatory skin diseases with enhanced histamine release, such as psoriasis and atopic dermatitis, histamine might foster the immunomodulatory potency of skin-infiltrating Th17 cells.

Skin inflammation and the migration of cells at the site of the immune response play an important role in allergic skin diseases. It has already been described that matrix metalloproteinase 9 (MMP-9) influences tissue remodeling and facilitates cell migration by proteolytic degradation of basal membrane components. The aim of this study was to investigate MMP-9 expression on human primary keratinocytes (KCs) upon stimulation with histamine, a potent mediator in allergic responses. With ELISA and zymography, we could show that histamine induced dose-dependent upregulation of MMP-9 in cultured KCs and in punch biopsies of human skin. The histamine H(1) receptor (H(1)R) agonist beta-histine-but not agonists for H(2)R, H(3)R, and H(4)R-induced MMP-9, whereas the H(1)R antagonist clemastine blocked the effect in a dose-dependent manner. Immunohistological staining showed that histamine-induced MMP-9 led to destruction of type IV collagen at the basement membrane in healthy skin. In a coculture system of KCs and T cells, migration of T cells through an artificial basement membrane was increased after histamine stimulation of KCs. Our findings demonstrate enhanced MMP-9 production and cell migration after histamine stimulation and may represent a new mechanism by which KCs contribute to the pathology of skin diseases.