The epidermal phenotype as observed in psoriatic skin results from inflammation and abnormal proliferation and terminal differentiation of keratinocytes. Mice deficient for interferon regulatory factor-2, a repressor of interferon signaling, display psoriasis-like skin inflammation. The development of this phenotype is strictly dependent on type I interferon (interferon-alpha/beta) signaling. The aim of this study was to assess the involvement of interferon-alpha/beta in the pathogenesis of human psoriasis. In psoriatic skin, we measured an increased expression of components that play central and crucial roles in interferon-alpha/beta signal transduction. Culturing keratinocytes or healthy skin biopsies with recombinant interferon-alpha stimulated this signaling pathway; however, this did not induce the expression of markers that are generally used to define the psoriasis phenotype. Furthermore, skin from psoriasis patients responded identically to interferon-alpha stimulation, demonstrating that psoriatic skin does not have an aberrant sensitivity to type I interferon. We conclude that in psoriatic lesional skin the type I interferon signaling pathway is activated, despite an unaltered interferon-alpha sensitivity. Our data furthermore show that type I interferon, in contrast to interferon-gamma, does not act directly on keratinocytes to induce a psoriatic phenotype. Thus, if the observed activated type I interferon signaling is indeed functionally involved in the pathogenesis of psoriasis, its contribution might be indirect, putatively involving other cell types besides keratinocytes.

Keratinocytes play a key role in innate immune responses of the skin to bacterial and viral pathogens. Viral double-stranded RNA and its synthetic analogue polyriboinosinic-polyribocytidylic acid (poly-IC) are recognized via multiple pathways involving the receptors Toll-like receptor 3 (TLR3), protein kinase R (PKR), and the recently described cytosolic RNA helicases retinoic acid-inducible gene-I (RIG-I) and melanoma differentiation-associated gene 5 (MDA5). We show that preincubation of human keratinocytes with IFN-alpha enhances the proinflammatory responses to poly-IC. Kinetic studies suggest that this is mediated via upregulation of the receptors TLR3, PKR, RIG-I, and MDA5. Interestingly, expression of RIG-I, MDA5, and PKR was significantly increased in lesional skin from patients with psoriasis, a chronic inflammatory skin disease that is characterized by high IFN-alpha levels. These results suggest that psoriatic keratinocytes show increased sensitivity to viral RNA intermediates, thereby leading to excessive proinflammatory responses and maintenance of the inflammatory skin phenotype. Here, we provide early evidence that point toward a role for the recently described cytosolic innate RNA receptors in non-viral chronic inflammatory diseases.

To investigate the immunological function of cells in normal and diseased skin under conditions approximating the in vivo situation, it is necessary to maintain the structural integrity of the tissue. To achieve this, freshly isolated skin has to be cultured ex vivo, or an in vitro-constructed complete skin equivalent may be used. Different skin organ culture systems have been described. Basically two systems prevail: submerged or air-exposed skin organ cultures. The former model has been used for measuring cytokine secretion by skin cells in the medium, and the latter to study the expression of cell membrane proteins in situ and the kinetics of epidermal Langerhans cells. Here we present a modified ex vivo skin organ culture system which approaches the in vivo situation by maintaining the normal skin architecture without spontaneous induction of the regenerative maturation markers. This method allowed the expression of cell membrane proteins in situ to be measured, and the cytokine secretion by skin cells in the culture medium to be quantitated in the same experiment. In this system, both general and specific stimuli (LPS and IL-1beta) upregulated the expression of skin-derived cytokines IL-8 and IL-6 in the medium and different markers (ICAM-1, CD40 and CD86) on cells in the tissue in a 24-hour culture-formed. Elevation of both cytokine and cell marker expression could be blocked by dexamethasone and by IL-1ra which acts specifically on IL-1beta-mediated responses. The system presented here is both quick and simple and can be used as a model to study the behaviour of skin cells in their natural microenvironment.

Psoriasis is a T-cell-mediated inflammatory skin disease. A Th1 cytokine profile with increased levels of interferon-gamma (IFN-gamma) is predominant in skin and peripheral blood mononuclear cells (PBMCs) from psoriasis patients. Furthermore, psoriatic keratinocytes exhibit an aberrant sensitivity and response to IFN-gamma. The transcriptional activator interferon regulatory factor-1 (IRF-1) plays a crucial role in the activation of IFN-gamma-induced gene expression. Recently it was shown that mice deficient in IRF-2, a transcriptional repressor of IFN signalling and thereby acting as an IRF-1 antagonist, display psoriasis-like skin abnormalities. It was therefore hypothesized that a dysbalance between IRF-1 and IRF-2, the activator and repressor of IFN responses, respectively, contributes to the altered IFN-gamma signalling observed in patients with psoriasis. In the epidermis of patients with psoriasis and healthy controls, similar IRF-1 and IRF-2 mRNA expression levels were observed. Furthermore, it was not possible to detect any differences in IRF-1 and IRF-2 protein levels in nuclear extracts from the epidermis of controls and psoriasis patients by electrophoretic mobility shift assay and western blot analysis. Using double immunofluorescence labelling, it was observed that in normal skin IRF-1 was expressed in keratinocytes throughout the epidermis, whereas IRF-2 was restricted to the basal cell layer. In psoriatic skin, IRF-1 expression was comparable to normal skin, whereas IRF-2 was expressed in both basal and suprabasal cell layers. This altered IRF-2 expression in suprabasal cell layers may therefore result in a dysbalance between the activator and repressor of IFN responses in these cell layers, putatively contributing to aberrant responses to IFN-gamma and eventually to the psoriatic skin phenotype.

CD40 ligation by CD40 ligand(+) CD4(+) T cells has been claimed to be involved in inflammatory responses in human skin. However, these data are derived from in vitro cell culture systems and immunohistochemistry, and the mechanisms involved have not been fully elucidated. We previously observed that cells in intact normal human skin secrete high levels of IL-6 and IL-8 upon stimulation with IL-1 beta. In vitro studies have shown that CD40 ligation on human keratinocytes results in the production of IL-6 and IL-8 as well. We used a novel tissue culture system with intact normal human skin, and show that antibody ligation of CD40 results in the induction of several pro- and anti-inflammatory cytokines. IL-6, IL-8, tumor necrosis factor (TNF)-alpha, IL-12 and IL-1 beta were induced upon CD40 ligation and IFN-gamma stimulation, while IL-10 could be induced by CD40 ligation alone and was reduced again by the addition of IFN-gamma. Since CD40 ligation on monocytes and dendritic cells in vitro results in the secretion of IL-1, which is pre-stored in high concentrations in normal human keratinocytes, we subsequently investigated whether CD40 induced IL-6 and IL-8 production in skin is mediated via IL-1. Indeed IL-1 receptor antagonist inhibited the CD40 ligation-induced IL-6 and IL-8 production, while TNF-alpha and IL-10 production were not affected. These data show that CD40 ligation-induced secretion of IL-6 and IL-8, but not TNF-alpha and IL-10, is partially mediated via IL-1 and that IL-1 plays a prominent role in the inflammatory response initiated by CD40 ligation in intact human skin.