Mechanism of pathogenesis of imiquimod‐induced skin inflammation in the mouse: A role for interferon‐alpha in dendritic cell activation by imiquimod

Topical application of imiquimod (IMQ), a Toll‐like receptor (TLR)7 ligand, can induce and exacerbate psoriasis, a chronic inflammatory skin disorder. In a mouse model of IMQ‐induced psoriasis‐like skin inflammation, T‐helper (Th)17 cells and interleukin (IL)‐17/IL‐22‐producing γδ‐T cells have been shown to play a pivotal role. However, the mechanisms of induction of the Th17 pathway and development of psoriasis‐like skin inflammation by IMQ treatment remain unclear. In this study, we investigated pathogenic mechanisms of IMQ‐induced psoriasis‐like skin inflammation in mice. We first confirmed that, together with an increase in IL‐17 and IL‐22 production, application of IMQ to mouse skin induced the expression of cytokines required for activation of the Th17 pathway, and pro‐inflammatory mediators involved in the pathology of psoriasis. Analysis of Tlr7^?/? mice demonstrated that most of the in vivo effects of IMQ were mediated via TLR7. In an in vitro study using plasmacytoid dendritic cells (DCs), IMQ induced production of interferon (IFN)‐α, IL‐23, IL‐6 and tumor necrosis factor (TNF)‐α. Furthermore, when we analyzed in vitro‐generated bone marrow‐derived DCs with features similar to TNF‐α and inducible nitric oxide synthase (iNOS)‐producing DCs, IL‐23, IL‐6, IL‐1β, TNF‐α and iNOS/NO production was weakly induced by IMQ alone and further enhanced after co‐stimulation with IMQ and IFN‐α. These in vitro effects of IMQ were also mediated via TLR7 and the synergistic effect of IMQ, and IFN‐α was suggested to be caused by upregulation of TLR7 expression by IFN‐α. These results demonstrate part of the mechanism by which the Th17 pathway and psoriasis‐like skin inflammation are induced by IMQ and IFN‐α in a mouse model.     

Depletion of antigen-presenting cells by clodronate liposomes reverses the psoriatic skin phenotype in KC-Tie2 mice

Background There is ongoing debate regarding the initiation of psoriatic plaque as primarily arising from an anomaly in epidermal keratinocytes (KCs) or from abnormalities in immunocytes that secondarily activate otherwise normal KCs. In mice engineered to overexpress the angiopoietin receptor Tie2 in KCs, skin spontaneously develops the characteristic clinical, histological and immune cell phenotypes of psoriasis which can be reversed with either transgene repression or ciclosporin administration, suggesting key roles for both KCs and T cells in mediating the skin disease in this murine model. Objectives To determine if antigen‐presenting cells (APCs) and macrophages alone are sufficient to sustain psoriasiform inflammation in the KC‐Tie2 murine model of psoriasis. Methods Clodronate liposomes were intradermally injected into involved dorsal skin of KC‐Tie2 or control animals once a week for 6 weeks and acanthosis, angiogenesis, immune cell infiltration and cytokine production were quantitated using immunohistochemistry and interactive image analyses, enzyme‐linked immunosorbent assay (ELISAs) and quantitative real‐time polymerase chain reaction (qRT‐PCR). Results Clodronate liposome injection eliminated CD11c+, F4/80+ and CD11b+ cells in the skin and returned CD8+ T‐cell numbers to control mouse levels. APC depletion in KC‐Tie2 mouse skin resulted in resolution of the acanthotic skin phenotype, decreased dermal angiogenesis, and a return to control mouse levels for interleukin (IL)‐1α, IL‐6, IL‐23 and tumour necrosis factor (TNF)‐α expression and modest reductions in interferon‐γ and IL‐17. Conclusions These findings suggest a critical role for APCs and myeloid cell‐derived IL‐23 and TNF‐α and underscore the importance of Th1 and Th17 T cells in maintaining the psoriasiform skin phenotype in the KC‐Tie2 mouse model.     

High‐fat diet exacerbates imiquimod‐induced psoriasis‐like dermatitis in mice

Psoriasis, a chronic inflammatory skin disease, is closely related to systemic metabolism. An elevated body mass index (BMI) is a risk factor for psoriasis; inflammasomes are activated by adipose tissue macrophages in obese subjects. We hypothesized that hyperlipidaemia is involved in the pathogenesis of psoriasis and examined the role of a high‐fat diet (HFD) in the development of psoriasis in imiquimod (IMQ)‐treated mice. The body weight and serum level of cholesterol were significantly higher in mice fed an HFD than in a regular diet (RD). HFD mice had higher psoriasis skin scores, and the number of neutrophils infiltrating into the lesional skin was elevated. IL‐17A mRNA expression was significantly increased in the skin of IMQ‐treated HFD mice; the expression of IL‐22, IL‐23 and TNF‐α mRNA was not enhanced. Caspase‐1 and IL‐1β were activated in the skin of IMQ‐treated HFD mice, and their serum level of IL‐17A, TNF‐α and IL‐1β was significantly upregulated. Our findings strongly suggest that hyperlipidaemia is involved in the development and progression of psoriasis via systemic inflammation and inflammasome activation.     

Reduction of inflammatory slan (6‐sulfo LacNAc) dendritic cells in psoriatic skin of patients treated with etanercept

Dermal dendritic cells (DCs) play a central role in the immunopathology of psoriasis. We previously identified slanDCs as pro‐inflammatory TNF‐α, IL‐23‐ and IL‐12‐producing DCs in human blood and as prominent inflammatory dermal TNF‐α secreting and CD11c‐positive DC subset in psoriasis. Here, we ask for the effects of TNF‐α‐inhibition on inflammatory slanDCs in skin and blood of 10 patients with psoriasis during 24 weeks of treatment with etanercept. Treatment with etanercept reduced the frequency of dermal slanDCs but did not induce apoptosis as determined by lack of increased active caspase‐3‐expression. In parallel, we found increased frequencies of slanDCs in blood which expressed lower levels of HLA‐DR. Stimulating slanDCs isolated from the blood of healthy donors in vitro induced a strong production of IL‐1β, IL‐6, IL‐23 and IL‐12p70. This capacity was efficiently reduced in the presence of etanercept, thereby indicating that TNF‐α is an autocrine stimulus for maturation and pro‐inflammatory cytokine production of slanDCs. In vivo, we noticed that treatment with etanercept did reduce the number of dermal slanDCs in parallel to the overall expression of TNF‐α and IL‐23p19. However, successful treatment did not down‐regulated the percentage of dermal slanDCs that stained positive for TNF‐α and IL‐23p19 indicating that remaining slanDCs kept their pro‐inflammatory capacity. This study provides novel insights into the immune regulatory properties of etanercept at the level of inflammatory slanDCs in vivo in skin and blood as well as in vitro.     

Etanercept suppresses regenerative hyperplasia in psoriasis by acutely downregulating epidermal expression of interleukin (IL)-19, IL-20 and IL-24

Background  Psoriasis is a Th17/Th1‐mediated skin disease that often responds to antitumour necrosis factor (TNF)‐α therapies, such as etanercept. Objectives  To better define mechanisms by which etanercept improves psoriasis and to gain insight into disease pathogenesis. Methods  We investigated the early biochemical and cellular effects of etanercept on skin lesions in responder patients prior to substantial clinical improvement (≤ 4 weeks). Results  By 1 week, etanercept acutely suppressed gene expression of the interleukin (IL)‐20 subfamily of cytokines (IL‐19, IL‐20, IL‐24), which were found to be predominantly epidermis‐derived and which are implicated in stimulating epidermal hyperplasia. Additionally, by 1 week of therapy, suppression of other keratinocyte‐derived products (chemokines, antimicrobial proteins) occurred, while suppression of epidermal regenerative hyperplasia occurred within 1–3 weeks. Th17 elements (IL‐23p19, IL‐12p40, IL‐17A, IL‐22) were suppressed by 3–4 weeks. In vitro, TNF‐α and IL‐17A coordinately stimulated the expression of the IL‐20 subfamily in normal keratinocytes. Conclusions  Based on the rapid suppression of regenerative hyperplasia, chemokines and other keratinocyte‐derived products, including the IL‐20 subfamily, we propose that epidermal activation is a very early target of etanercept. As many of these keratinocyte markers are stimulated by TNF‐α, their rapid downregulation is likely to reflect etanercept’s antagonism of TNF‐α. Additionally, decreased epidermal hyperplasia might result specifically from acute suppression of the IL‐20 subfamily, which is also a likely consequence of etanercept’s antagonism of TNF‐α. Thus, the IL‐20 subfamily has potential importance in the pathogenesis of psoriasis and therapeutic response to etanercept.     

Research in practice: IL‐22 and IL‐20: significance for epithelial homeostasis and psoriasis pathogenesis

Psoriasis vulgaris is a frequent, chronically relapsing, immune‐mediated, systemic disease with characteristic skin changes. Despite the importance of this disease there are currently limited therapeutic options indicating a need for effective, long‐lasting treatment strategies with few side effects. The most recent discoveries regarding psoriasis pathogenesis, particularly our results regarding two cytokines – IL‐22 and IL‐20 – could prove to be the foundation for such therapies. Whereas IL‐22 is mainly produced by activated T‐cell sub‐populations (Th22, Th1, Th17), monocytes, dendritic cells and keratinocytes produce IL‐20. Blood and lesional skin samples from psoriasis patients demonstrate high levels of IL‐22 and IL‐20. Interestingly, both cytokines act principally on keratinocytes and do not impact the immune system. Similar to the changes in the psoriasis epidermis these cytokines inhibit the terminal differentiation of keratinocytes although they simultaneously increase their innate defense, mobility, and the production of some chemokines. Some IL‐22 effects are amplified by TNF‐α, IL‐17, and IFN‐α. IL‐22/IL‐20 lends the reconstructed epidermis a psoriasis‐like appearance with acanthosis, hypogranularity, and hyperkeratosis. In addition, mice that constitutively express high levels of IL‐22 or IL‐20 demonstrate a psoriasis‐like appearance. A therapy counteracting IL‐22 and IL‐20 would be an innovative treatment with the potential for few side effects that would act on the final phase of psoriasis pathogenesis.     

Interleukin (IL)‐22, IL‐17, IL‐23, IL‐8, vascular endothelial growth factor and tumour necrosis factor‐α levels in patients with psoriasis before,during and after psoralen–ultraviolet A and narrowband ultraviolet B therapy

Background Several cross‐sectional studies have shown that different cytokines and growth factors are enhanced in psoriasis. Objectives We aimed to understand the role/relation of interleukin (IL)‐22, IL‐17, IL‐23, IL‐8, vascular endothelial growth factor (VEGF) and tumour necrosis factor (TNF)‐α in psoriasis vulgaris, addressing their levels and changes before, during and after psoralen–ultraviolet A (PUVA) and narrowband ultraviolet B (NB‐UVB) treatment. Methods A cross‐sectional and a longitudinal study (n = 34) – before (T0) and at 3 (T3), 6 (T6) and 12 (T12) weeks of NB‐UVB and PUVA therapy – were performed; 17 patients started NB‐UVB and 17 PUVA, and IL‐22, IL‐17, IL‐23, IL‐8, TNF‐α and VEGF levels were evaluated. Results At T0, compared with controls (n = 20), all the parameters were significantly higher in patients, except for TNF‐α. Both NB‐UVB and PUVA treatment gave, at T3, a significant decrease in TNF‐α and IL‐23; IL‐22 and IL‐17 decreased significantly at T6; all parameters and Psoriasis Area and Severity Index decreased significantly at T12. However, in both groups, at T12, VEGF was still significantly higher than control. Conclusions Psoriasis seems to be a complex disease in which the cytokine network is disturbed, namely in levels of IL‐22, IL‐17, IL‐23, IL‐8, TNF‐α and VEGF. NB‐UVB and PUVA follow‐up studies suggested that the reduction in the IL‐23/Th17 axis might be important in the pathogenic mechanisms of psoriasis. Further follow‐up studies of patients with psoriasis treated with these and other therapies could be very helpful for the understanding of the disturbance in the cytokine network in psoriasis and indirectly in its pathogenesis.     

S‐allyl cysteine inhibits TNF‐α‐induced inflammation in HaCaT keratinocytes by inhibition of NF‐ κB‐dependent gene expression via sustained ERK activation

Tumor necrosis factor‐α (TNF‐α)‐induced keratinocyte inflammation plays a key role in the pathogenesis of multiple inflammatory skin diseases. Here we investigated the anti‐inflammatory effect of S‐allyl cysteine (SAC) on TNF‐α‐induced HaCaT keratinocyte cells and the mechanism behind its anti‐inflammatory potential. SAC was found to inhibit TNF‐α‐stimulated cytokine expression. Further, SAC was found to inhibit TNF‐α‐induced activation of p38, JNK and NF‐κB pathways. Interestingly, SAC was found to differentially regulate ERK MAP kinase in cells. TNF‐α‐induced transient ERK activation and SAC treatment resulted in sustained ERK activation both in the presence and absence of TNF‐α. Additionally, SAC failed to inhibit the TNF‐α‐induced expression of the pro‐inflammatory cytokines TNF‐α and IL‐1β when cells were treated with the MEK inhibitor PD98059, suggesting that the anti‐inflammatory effect of SAC is via sustained activation of the ERK pathway. Since ERK activation has been reported to negatively regulate NF‐κB‐driven gene expression and we find that SAC activates ERK and negatively regulates NF‐κB, we investigated whether there existed any crosstalk between the ERK and the NF‐κB pathways. NF‐κB‐dependent reporter assay, visualization of the nuclear translocation of NF‐κB‐p65 subunit and determination of the cellular levels of I‐κB, the inhibitor of NF‐κB, revealed that SAC inhibited TNF‐α‐induced NF‐κB activation, and PD98059 treatment reversed this effect. These results collectively suggest that SAC inhibits TNF‐α‐induced inflammation in HaCaT cells via a combined effect entailing the inhibition of the p38 and the JNK pathways and NF‐κB pathway via the sustained activation of ERK.     

Granzyme A potentiates chemokine production in IL‐17‐stimulated keratinocytes

Plaque psoriasis presents with focal skin inflammation, partially maintained by IL‐17‐mediated interactions between infiltrating epidermal T cells and activated keratinocytes. Here we show that the majority of lesional epidermal CD8 T cells express granzyme A, alone or in combination with IL‐17. To assess proinflammatory properties of granzyme A in psoriasis, primary human keratinocytes were stimulated with granzyme A in the presence or absence of IL‐17. Out of 33 analysed keratinocyte‐derived inflammatory mediators, granzyme A potentiated IL‐17‐induced secretion of CXCL 1, CXCL 12 and CCL 4. Intriguingly, all three chemokines are implicated in psoriasis pathogenesis and are involved in recruitment of T cells, neutrophils and pDCs into inflamed tissues. Our results indicate that granzyme A produced by lesional CD8 T cells specifically increase the chemokine production from inflamed keratinocytes, thereby amplifying a chemotactic inflammatory loop that sustains psoriasis lesions.     

IL‐17F regulates psoriasis‐associated genes through IκBζ

Psoriasis is a common chronic inflammatory and immune‐mediated skin disease. Antagonists of TNF‐α and, recently, IL‐17 have proven to be highly effective in the treatment for psoriasis; however, the molecular mechanisms involved in the pathogenesis of psoriasis are poorly understood. Recently, we presented evidence that IκBζ is a key regulator in the development of psoriasis through its role in mediating IL‐17A‐driven effects. Like IL‐17A, IL‐17F is produced by a variety of immune cells, and the expression of IL‐17F is increased in psoriatic skin. The purpose of this study was to characterize the role of IL‐17F in the regulation of IκBζ expression and to investigate whether IL‐17F regulates psoriasis‐associated genes in human keratinocytes through IκBζ. Here, we demonstrate that IL‐17F stimulation induces IκBζ expression at both the mRNA and the protein levels in normal human keratinocytes. Moreover, silencing IκBζ by siRNA revealed that IκBζ is a key regulator of specific IL‐17F‐inducible psoriasis‐associated genes and proteins, including DEFB4/hBD2, S100A7, CCL20, IL‐8 and CHI3L1. In addition, IL‐17F‐induced IκBζ expression is mediated by a mechanism involving the p38 MAPK and NF‐κB signalling pathways, as shown by the clear reduction in IL‐17F‐mediated expression of IκBζ during chemical inhibition of these two signalling pathways. In summary, we present IκBζ as a novel key regulator of IL‐17F‐driven effects in psoriasis. Thus, antagonists to IκBζ could potentially provide a more targeted approach for treating psoriasis as well as for treating the other inflammatory and immune‐mediated diseases for which IL‐17‐targeting drugs have recently been approved.     

CCL7 contributes to the TNF‐alpha‐dependent inflammation of lesional psoriatic skin

Chemokines are small chemotactic proteins that have a crucial role in leukocyte recruitment into tissue. Targeting these mediators has been suggested as a potential therapeutic option in inflammatory skin diseases such as psoriasis. Using quantitative RT‐PCR, we found CCL7, a chemokine ligand known to interact with multiple C‐C chemokine receptors, to be markedly increased in lesional psoriasis as opposed to atopic dermatitis, lichen planus, non‐lesional psoriatic and normal control skin. Surprisingly, this increase in CCL7 mRNA expression exceeded that of all other chemokines investigated, and keratinocytes and dermal blood endothelial cells were identified as its likely cellular sources. In an imiquimod‐induced psoriasis‐like mouse model, CCL7 had a profound impact on myeloid cell inflammation as well as on the upregulation of key pro‐psoriatic cytokines such as CCL20, IL‐12p40 and IL‐17C, while its blockade led to an increase in the antipsoriatic cytokine IL‐4. In humans receiving the TNF‐α‐blocker infliximab, CCL7 was downregulated in lesional psoriatic skin already within 16 hours after a single intravenous infusion. These data suggest that CCL7 acts as a driver of TNF‐α‐dependent Th1/Th17‐mediated inflammation in lesional psoriatic skin.     

Nicotinamide downregulates gene expression of interleukin‐6, interleukin‐10, monocyte chemoattractant protein‐1, and tumour necrosis factor‐α gene expression in HaCaT keratinocytes after ultraviolet B irradiation

Ultraviolet (UV) radiation has profound effects on human skin, causing sunburn, inflammation, cellular‐tissue injury, cell death, and skin cancer. Most of these effects are mediated by a number of cytokines produced by keratinocytes. In this study we investigated whether nicotinamide (NCT), the amide form of vitamin B3, might have a protective function in reducing the expression of interleukin (IL)‐1β, IL‐6, IL‐8, IL‐10, monocyte chemoattractant protein (MCP)‐1 and tumour necrosis factor (TNF)‐α in UV‐irradiated keratinocytes. HaCaT cells were treated with UVB in the presence or absence of NCT, and cytokine mRNA levels were examined by quantitative real‐time PCR. NCT significantly downregulated IL‐6, IL‐10, MCP‐1 and TNF‐α mRNA expression, whereas it did not exert any significant effect on IL‐1β or IL‐8 expression. Because of its ability to decrease these cytokine mediators after UV exposure, NCT is a possible therapy to improve or prevent conditions induced or aggravated by UV light.     

Lipocalin‐2 exacerbates psoriasiform skin inflammation by augmenting T‐helper 17 response

Lipocalin‐2 (LCN2) is an antimicrobial protein and adipokine associated with insulin resistance, obesity and atherosclerotic disease. Psoriasis is a T‐helper (Th)1/Th17‐mediated, chronic inflammatory dermatosis related to metabolic syndromes and serum LCN2 levels are elevated in psoriatic patients. We examined the in vivo effects of LCN2 on topical imiquimod (IMQ)‐induced psoriasiform skin in BALB/c mice and in vitro on human keratinocytes (KC). Clinically, i.p. injected LCN2 exacerbated erythema and scaling in IMQ‐treated murine skin compared with phosphate‐buffered saline injection alone, and it augmented interleukin (IL)‐17A, IL‐17F, IL‐22, IL‐23p19, IL‐12p40, CCL20, tumor necrosis factor‐α, chemokine (C‐X‐C motif) ligand (CXCL)1, CXCL2, DEFB4, DEFB14, LCN2 and S100A7 mRNA levels of IMQ‐treated murine skin while it did not increase the mRNA levels of interferon‐γ, IL‐12p35 or CXCL10. LCN2 in synergy with IL‐17 increased mRNA levels of CCL20, LCN2 and DEFB4A but not of CXCL10 in human KC in vitro. These results suggest that LCN2 enhances the expression of Th17 cytokines/chemokines and antimicrobial peptides in murine IMQ‐treated psoriatic skin and KC. LCN2 may potentiate the development of psoriasis via the enhancement of Th17‐ and antimicrobial peptide‐mediated inflammation.     

Interleukin‐21 receptor signalling is not critically required for imiquimod‐induced psoriasiform dermatitis in mice

Psoriasis is largely mediated by interleukin (IL)‐23/T helper (Th) 17 axis, and IL‐21 is a pleiotropic cytokine expressed by Th17 cells. Despite previously reported possible pathogenic roles of IL‐21 in human psoriasis, we found that IL‐21 receptor (IL‐21R) signalling was not crucial for imiquimod‐induced psoriatic inflammation, using IL‐21R^?/? mice. The severity of imiquimod‐induced psoriatic manifestation and pro‐inflammatory Th17 cytokine levels, IL‐17A‐producing γδ T cells and CD4+ T cells, and in vitro IL‐17A production by γδ T cells after IL‐23 stimulation was comparable between wild‐type and IL‐21R^?/? mice. Collectively, IL‐21R signalling was not critically involved in IMQ‐induced psoriatic inflammation despite an increased IL‐21 expression in the IMQ‐treated mouse skin. Our data may represent the significant differences between human psoriasis and murine psoriasis model, and further studies using other models will be required to elucidate the role of IL‐21 in psoriasis pathogenesis.     

TNF‐α: a treatment target or cause of sarcoidosis?

Sarcoidosis is a systemic granulomatous disease that affects numerous organs, commonly manifesting at the lungs and skin. While corticosteroids remain the first line of treatment, tumour necrosis factor alpha (TNF‐α) inhibitors have been investigated as one potential steroid sparing treatment for sarcoidosis. TNF‐α is one of many components involved in the formation of granulomas in sarcoidosis. While there have been larger scale studies of biologic TNF‐α inhibition in systemic sarcoidosis, studies in cutaneous disease are limited. Paradoxically, in some patients treated with biologic TNF‐α inhibitors for other diseases, treatment can induce the development of sarcoidosis. In the light of this complexity, we discuss the role of TNF‐α in granuloma formation, the therapeutic role of TNF‐α inhibition and immunologic abnormalities following treatment with these TNF‐α inhibitors including drug‐specific alterations involving interferon‐γ, lymphotoxin‐α, TNF receptor 2 (TNFR2) and T‐regulatory cells.     

Increased expression of glucagon‐like peptide‐1 receptors in psoriasis plaques

Recent case reports suggest that treatment with glucagon‐like peptide‐1 (GLP‐1) agonists results in clinical improvement of psoriasis. The purpose of this study was to determine whether GLP‐1 receptors (GLP‐1Rs) are found in the skin of healthy volunteers and psoriasis patients and if so, whether GLP‐1Rs are located on keratinocytes or immune cells. Three mm‐punch skin biopsies were taken for gene expression analysis from six healthy volunteers and from affected and unaffected skin of six psoriasis patients. In addition, a blood sample was obtained from all participants. Cultured human keratinocytes were either untreated or incubated with tumor necrosis factor‐ α (TNF‐α), interferon‐γ (IFN‐γ) or a combination of TNF‐α and IFN‐γ for 48 h. Total RNA was extracted from all the samples, reversely transcribed and analysed for the expression of GLP‐1R using real‐time PCR. Gene expression analysis showed expression of GLP‐1Rs in five of six skin biopsies from psoriasis plaques, in one of six biopsies from unaffected psoriatic skin and in one of six biopsies from healthy skin. GLP‐1R expression was found in the blood of both healthy volunteers and psoriasis patients. No GLP‐1R expression was found in either stimulated or unstimulated cultured human keratinocytes. Our results show increased presence of GLP‐1Rs in psoriasis plaques and that this most likely is due to infiltration with immune cells. This offers a possible explanation for the positive effect of treatment with GLP‐1R agonists in patients with psoriasis.     

UVA‐1 exposure in vivo leads to an IL‐6 surge within the skin

UVA‐1 is a known promotor of skin ageing. Cytokines like IL‐1α, Il‐1β or TNF‐α, VEGF and IL‐6 orchestrate UV effects, and IL‐6 is furthermore an effector of UVA‐induced photoageing. We investigated how fractionated UVA‐1 doses influence the cytokine milieu and especially the IL‐6 levels in the skin in vivo. In a study with 35 participants, we exposed previously unirradiated human skin to three UVA‐1 irradiation regimes. Cytokine levels in interstitial skin fluid were measured up to 48 hours postexposure and compared to unirradiated control skin fluid. Our results show that IL‐6 levels increased significantly after UVA‐1 exposure at selected time points. The other candidates IL‐1α, Il‐1β or TNF‐α and VEGF show no significant response after UVA‐1 exposure in vivo. UVA‐1 thus raises selectively IL‐6 levels in vivo, a fact that underlines its role in photoageing and has potential implications for its modulatory effect on photoageing pathology.     

TNFα‐ and IL‐17A‐mediated S100A8 expression is regulated by p38 MAPK

The antimicrobial peptide S100A8 is known to be upregulated in lesional psoriatic skin compared with non‐lesional psoriatic skin and is believed to play a role in the pathogenesis of psoriasis. However, little is known about the signalling pathways involved in the regulation of S100A8 expression. Using quantitative real‐time RT‐PCR analysis, we demonstrated that stimulation with TNFα and IL‐17A in combination resulted in a significant and synergistic induction of S100A8 mRNA in human keratinocytes. TNFα and IL‐17A also induced the S100A8 promoter activity synergistically. This was demonstrated by a gene reporter assay in cells transfected with a luciferase plasmid construct, consisting of 3502 base pairs of the human S100A8 promoter. The TNFα‐ and IL‐17A‐mediated induction of S100A8 mRNA and protein was mediated by a p38 MAPK‐dependent mechanism, as demonstrated by the use of a p38 MAPK inhibitor. Finally, adalimumab treatment for patients with psoriasis significantly decreased S100A8 mRNA at day fourteen after start of treatment, but not at day four. Taken together, this study demonstrates that the p38 MAPK signalling pathway plays a key role in the TNFα‐ and IL‐17A‐induced expression of S100A8 in cultured human keratinocytes.