Keratinocytes regain momentum as instigators of cutaneous inflammation

The primary role of skin is to serve as a protective coat and epidermal keratinocytes are responsible for this barrier function. Besides providing structural support, keratinocytes can initiate inflammatory reactions, thereby enhancing healing of skin that follows barrier perturbation. In complex diseases such as psoriasis, in which both barrier function and cutaneous inflammation are dysregulated, it is unclear whether the primary pathogenic disturbance resides in keratinocytes or in immunocytes, which are commingled in psoriatic plaques. Researchers have turned to animal models of cutaneous inflammation to gain insights into the pathogenesis of psoriasis. A recent report in which the inducible epidermal deletion of Jun proteins in adult mice triggered inflammatory skin lesions and destructive arthritis has shifted momentum towards the keratinocyte as a key instigator of cutaneous inflammation. However, because this transgenic mouse model mimics only some features of psoriasis, further studies are required before the prevailing view of psoriasis as a fundamentally immunocyte-driven disease can be replaced by the notion that keratinocytes are the primary pathogenic cells in psoriasis.     

Cytokines in psoriasis

Psoriasis is a common inflammatory skin disease with an incompletely understood etiology. The disease is characterized by red, scaly and well-demarcated skin lesions formed by the hyperproliferation of epidermal keratinocytes. This hyperproliferation is driven by cytokines secreted by activated resident immune cells, an infiltrate of T cells, dendritic cells and cells of the innate immune system, as well as the keratinocytes themselves. Psoriasis has a strong hereditary character and has a complex genetic background. Genome-wide association studies have identified polymorphisms within or near a number of genes encoding cytokines, cytokine receptors or elements of their signal transduction pathways, further implicating these cytokines in the psoriasis pathomechanism. A considerable number of inflammatory cytokines have been shown to be elevated in lesional psoriasis skin, and the serum concentrations of a subset of these also correlate with psoriasis disease severity. The combined effects of the cytokines found in psoriasis lesions likely explain most of the clinical features of psoriasis, such as the hyperproliferation of keratinocytes, increased neovascularization and skin inflammation. Thus, understanding which cytokines play a pivotal role in the disease process can suggest potential therapeutic targets. A number of cytokines have been therapeutically targeted with success, revolutionizing treatment of this disease. Here we review a number of key cytokines implicated in the pathogenesis of psoriasis.     

EGFR-driven up-regulation of decoy receptor 3 in keratinocytes contributes to the pathogenesis of psoriasis

Decoy receptor 3 (DcR3) is a soluble receptor of Fas ligand (FasL), LIGHT (TNFSF14) and TNF-like molecule 1A (TL1A) and plays pleiotropic roles in many inflammatory and autoimmune disorders and malignant diseases. In cutaneous biology, DcR3 is expressed in primary human epidermal keratinocytes and is upregulated in skin lesions in psoriasis, which is characterized by chronic inflammation and angiogenesis. However, the regulatory mechanisms of DcR3 over-expression in skin lesions of psoriasis are unknown. Here, we demonstrate that DcR3 can be detected in both dermal blood vessels and epidermal layers of psoriatic skin lesions. Analysis of serum samples showed that DcR3 was elevated, but FasL was downregulated in psoriatic patients compared with normal individuals. Additional cell studies revealed a central role of epidermal growth factor receptor (EGFR) in controlling the basal expression of DcR3 in keratinocytes. Activation of EGFR by epidermal growth factor (EGF) and transforming growth factor (TGF)-α strikingly upregulated DcR3 production. TNF-α?enhanced DcR3 expression in both keratinocytes and endothelial cells compared with various inflammatory cytokines involved in psoriasis. Additionally, TNF-α-enhanced DcR3 expression in keratinocytes was inhibited when EGFR was knocked down or EGFR inhibitor was used. The NF-κB pathway was critically involved in the molecular mechanisms underlying the action of EGFR and inflammatory cytokines. Collectively, the novel regulatory mechanisms of DcR3 expression in psoriasis, particularly in keratinocytes and endothelial cells, provides new insight into the pathogenesis of psoriasis and may also contribute to the understanding of other diseases that involve DcR3 overexpression.     

Interplay between keratinocytes and immune cells—Recent insights into psoriasis pathogenesis

Psoriasis is one of the most common human inflammatory skin diseases characterised by hyperproliferation and aberrant differentiation of keratinocytes. The trigger of the typical epidermal changes seen in psoriasis was considered to be a dysregulated immune response with Th-1/Tc1 cells playing a central role. Recent studies have provided new insights into psoriasis pathogenesis in defining intraepidermal α₁β₁+ T cells as key effectors driving keratinocyte changes. Critical roles for IFN-α secreted by plasmacytoid dendritic cells and the IL-23/Th-17 axis were postulated. Initially, these subsequent stages are at least partially driven by the endogenous antimicrobial peptide LL37 that converts inert self-DNA into a potent trigger of interferon production by binding and delivering the DNA into plasmacytoid dendritic cells to trigger toll-like receptor 9. As LL37 is expressed by keratinocytes upon various stimuli, keratinocytes might regain momentum as instigators of an aberrant immune response which then precedes the characteristic changes in the epidermis. Data from these new studies indicate a complex interplay between keratinocytes overexpressing antimicrobial peptides and immune cells driving epidermal hyperproliferation and aberrant keratinocyte differentiation in the pathogenesis of psoriasis.     

Overexpression of CD1d by keratinocytes in psoriasis and CD1d-dependent IFN-gamma production by NK-T cells

The MHC class I-like protein CD1d is a nonpolymorphic molecule that plays a central role in development and activation of a subset of T cells that coexpress receptors used by NK cells (NK-T cells). Recently, T cells bearing NK receptors were identified in acute and chronic lesions of psoriasis. To determine whether NK-T cells could interact with epidermal cells, we examined the pattern of expression of CD1d in normal skin, psoriasis, and related skin disorders, using a panel of CD1d-specific mAbs. CD1d was expressed by keratinocytes in normal skin, although expression was at a relatively low level and was generally confined to upper level keratinocytes immediately beneath the lipid-rich stratum corneum. In contrast, there was overexpression of CD1d in chronic, active psoriatic plaques. CD1d could be rapidly induced on keratinocytes in normal skin by physical trauma that disrupted barrier function or by application of a potent contact-sensitizing agent. Keratinocytes displayed enhanced CD1d following exposure to IFN-gamma. Combining CD1d-positive keratinocytes with human NK-T cell clones resulted in clustering of NK-T cells, and while no significant proliferation ensued, NK-T cells became activated to produce large amounts of IFN-gamma. We conclude that CD1d can be expressed in a functionally active form by keratinocytes and is up-regulated in psoriasis and other inflammatory dermatoses. The ability of IFN-gamma to enhance keratinocyte CD1d expression and the subsequent ability of CD1d-positive keratinocytes to activate NK-T cells to produce IFN-gamma, could provide a mechanism that contributes to the pathogenesis of psoriasis and other skin disorders.     

The Crosstalk between IL-22 Signaling and miR-197 in Human Keratinocytes

The interaction between the immune system and epithelial cells is tightly regulated. Aberrations of this balance may result in inflammatory diseases such as psoriasis, inflammatory bowel disease and rheumatoid arthritis. IL-22 is produced by Th17, Th22 and Th1 cells. Putative targets for IL-22 are cells in the skin, kidney, digestive and respiratory systems. The highest expression of IL-22 receptor is found in the skin. IL-22 plays an important role in the pathogenesis of T cell-mediated inflammatory diseases such as psoriasis, inflammatory bowel disease and rheumatoid arthritis. Recently, we found that miR-197 is down regulated in psoriatic lesions. In the present work we show that miR-197 over expression inhibits keratinocytes proliferation induced by IL-22 and keratinocytes migration. In addition, we found that IL-22 activates miR-197 expression through the binding of phosphorylated STAT3 to sequences in the putative promoter of miR-197. Finally we found that IL-22 receptor subunit IL22RA1 is a direct target of miR-197. Hence, we identified a novel feedback loop controlling IL-22 signaling, in which IL-22 induces miR-197, which in turn, negatively regulates IL-22 receptor and attenuates the biological outcome of such signaling. Regulation of this pathway may be important in inflammatory skin disorders such a psoriasis and in wound healing.     

Genetically programmed differences in epidermal host defense between psoriasis and atopic dermatitis patients

In the past decades, chronic inflammatory diseases such as psoriasis, atopic dermatitis, asthma, Crohn's disease and celiac disease were generally regarded as immune-mediated conditions involving activated T-cells and proinflammatory cytokines produced by these cells. This paradigm has recently been challenged by the finding that mutations and polymorphisms in epithelium-expressed genes involved in physical barrier function or innate immunity, are risk factors of these conditions. We used a functional genomics approach to analyze cultured keratinocytes from patients with psoriasis or atopic dermatitis and healthy controls. First passage primary cells derived from non-lesional skin were stimulated with pro-inflammatory cytokines, and expression of a panel of 55 genes associated with epidermal differentiation and cutaneous inflammation was measured by quantitative PCR. A subset of these genes was analyzed at the protein level. Using cluster analysis and multivariate analysis of variance we identified groups of genes that were differentially expressed, and could, depending on the stimulus, provide a disease-specific gene expression signature. We found particularly large differences in expression levels of innate immunity genes between keratinocytes from psoriasis patients and atopic dermatitis patients. Our findings indicate that cell-autonomous differences exist between cultured keratinocytes of psoriasis and atopic dermatitis patients, which we interpret to be genetically determined. We hypothesize that polymorphisms of innate immunity genes both with signaling and effector functions are coadapted, each with balancing advantages and disadvantages. In the case of psoriasis, high expression levels of antimicrobial proteins genes putatively confer increased protection against microbial infection, but the biological cost could be a beneficial system gone awry, leading to overt inflammatory disease.     

Inhibition of keratinocyte ferroptosis suppresses psoriatic inflammation

Psoriasis is a common, chronic, and recurrent inflammatory disease. It is characterized by hyperproliferation and abnormal differentiation of keratinocytes. Keratinocyte death is also involved in many pathophysiological conditions and amplifies the inflammatory cascade. As a newly recognized form of cell death, ferroptosis is involved in several inflammatory diseases. In this study, we aimed to investigate a previously unrecognized role for ferroptosis in psoriasis. Ferroptosis is mediated by lipid peroxidation and iron overload. Compared with normal lesions, the mRNA expression of acyl-CoA synthetase long-chain family member 4 (ACSL4), prostaglandin-endoperoxide synthase 2 (PTGS2), and transferrin receptor (TFRC) were highly expressed in psoriatic lesions, with decreased levels of glutathione peroxidase 4 (GPX4), ferritin light chain (FTL), and ferritin heavy chain 1 (FTH1). The protein levels of ACSL4 and GPX4 were consistent with their mRNA levels. A similar tendency of ferroptosis was also observed in erastin-treated human primary keratinocytes and the Imiquimod (IMQ)-induced model of psoriasis. To investigate the correlation between inflammation and peroxidation, we analyzed single-cell RNA-sequencing data and identified 15 cell types. There was a high correlation between the activity of the lipid oxidation and the Th22/Th17 response in keratinocytes at a single-cell level. Moreover, ferrostatin-1 (Fer-1), a potent inhibitor of lipid peroxidation, suppressed ferroptosis-related changes in erastin-treated keratinocytes and alleviated psoriasiform dermatitis of IMQ-induced models. Additionally, Fer-1 blocked inflammatory responses in vitro and in vivo, reducing the production of cytokines including TNF-α, IL-6, IL-1α, IL-1β, IL-17, IL-22, and IL-23. This study revealed an expression pattern of ferroptosis in which specific molecules enhance inflammatory reactions in psoriasis.Subject terms: Cell death, Diseases     

Activation of the STING‐IRF3 pathway involved in psoriasis with diabetes mellitus

Psoriasis and type 2 diabetes mellitus (T2DM) share similar inflammatory pathways in their pathogenesis. The stimulator of interferon genes (STING)‐interferon regulatory factor 3 (IRF3) pathway has recently been shown to play an important role in immune and metabolic diseases. In this study, we investigated the activation of the STING‐IRF3 pathway in human immortalized keratinocytes (HaCaT) cells treated with palmitic acid (PA) and imiquimod (IMQ). Additionally, we detected the STING‐IRF3 pathway in diabetic mice with imiquimod (IMQ)‐induced psoriasis and assessed the potential of STING inhibitor C‐176. Furthermore, skin samples from patients with psoriasis and diabetes were collected for immunohistochemical analysis. The results indicated that the STING‐IRF3 pathway was activated in HaCaT cells. Moreover, the STING pathway was also found to be induced in the skin tissue of diabetic mice with psoriasis; the inflammatory responses were ameliorated by treatment with C‐176. In the skin tissue samples of patients with psoriasis and diabetes, immunohistochemistry showed that the expression levels of STING and phosphorylated IRF3 were also significantly increased. Thus, we conclude that the STING‐IRF3 pathway is involved in the inflammatory response in the manifestation of psoriasis with T2DM. Inhibition of the activation of the STING pathway can ameliorate the development of psoriasis in diabetes and could be targeted for the development of therapeutic agents for these conditions.     

Therapeutic Efficacy and Immunological Response of CCL5 Antagonists in Models of Contact Skin Reaction

Skin-infiltrating T-cells play a predominant role in allergic and inflammatory skin diseases such as atopic dermatitis, psoriasis and allergic contact dermatitis. These T-cells are attracted by several chemotactic factors including the chemokine CCL5/RANTES, a CC chemokine inducing both the migration and activation of specific leukocyte subsets. CCL5 has been found to be associated with various cell-mediated hypersensitive disorders such as psoriasis, atopic dermatitis and irritant contact dermatitis. We have used two antagonists, the first, Met-CCL5, a dual CCR1/CCR5 antagonist and the second, a variant in which GAG binding is abrogated, ⁴⁴AANA⁴⁷-CCL5, which acts as a dominant negative inhibitor of CCL5. The antagonists were tested in two models of contact skin reaction. The first, irritant contact dermatitis (ICD) is a pathological non-specific inflammatory skin condition arising from the release of pro-inflammatory cytokines by keratinocytes in response to haptens, usually chemicals. The second, contact hypersensitivity (CHS) is a T-cell dependent model, mimicking in part the T-cell-mediated skin diseases such as psoriasis. In both models, the CCL5 antagonists showed therapeutic efficacy by reducing swelling by 50% as well as the reduction of soluble mediators in homogenates derived from challenged ears. These results demonstrate that blocking the receptor or the ligand are both effective strategies to inhibit skin inflammation.     

Gardiquimod inhibits the expression of calcium-induced differentiation markers in HaCaT cells

Toll-like receptor 7 (TLR7) is an important member in pattern recognition receptors families. TLR7 signal pathway is involved in the physiological process in many type cells, but the impact of TRL7 on differentiation in the human keratinocytes is still unknown. In this study, we investigated the expression of TLR7 in keratinocytes, and the effect of TLR7 agonist gardiquimod on the expression of calcium (Ca²⁺)-induced keratinocytes differentiation markers in HaCaT cells. Immunohistochemistry and western-blotting analysis showed that TLR7 is expressed in basal keratinocytes of normal skin and in the human keratinocyte cell line HaCaT, but not expressed in the keratinocytes of psoriasis lesions. Pretreatment with gardiquimod could down-regulate Ca²⁺-induced differentiation marker expression and activate Raf-MEK-ERK and PI3K-AKT signal pathways in HaCaT cells. However, specific inhibitors studies showed that the down-regulation of the differentiation markers expression by gardiquimod was not dependent on the activation of these two pathways. TLR7 may play an important role in the pathogenesis of psoriasis through regulating the differentiation of the keratinocytes, and will give a new insight into the psoriasis.