Distinct molecular signatures of mild extrinsic and intrinsic atopic dermatitis

Clinical phenotypes and endophenotypes of atopic dermatitis: Where are we, and where should we go?

Atopic dermatitis (AD) is a paradigmatic chronic inflammatory skin disease characterized by a complex pathophysiology and a wide spectrum of the clinical phenotype. Despite this high degree of heterogeneity, AD is still considered a single disease and usually treated according to the "one-size-fits-all" approach. Thus more tailored prevention and therapeutic strategies are still lacking. As for other disciplines, such as oncology or rheumatology, we have to approach AD in a more differentiated way (ie, to dissect and stratify the complex clinical phenotype into more homogeneous subgroups based on the endophenotype [panel of biomarkers]) with the aim to refine the management of this condition. Because we are now entering the era of personalized medicine, a systems biology approach merging the numerous clinical phenotypes with robust (ie, relevant and validated) biomarkers will be needed to best exploit their potential significance for the future molecular taxonomy of AD. This approach will not only allow an optimized prevention and treatment with the available drugs but also hopefully help assign newly developed medicinal products to those patients who will have the best benefit/risk ratio.

The immunoglobulin superfamily member CD200R identifies cells involved in type 2 immune responses

BACKGROUND

The pathology of allergic diseases involves type 2 immune cells, such as Th2, ILC2, and basophils exerting their effect by production of IL-4, IL-5, and IL-13. However, surface receptors that are specifically expressed on type 2 immune cells are less well documented. The aim of this investigation was to identify surface markers associated with type 2 inflammation.

METHODS

Naïve human CD4⁺ T cells were short-term activated in the presence or absence of IL-4 and analyzed for expression of >300 cell-surface proteins. Ex vivo-isolated peripheral blood mononuclear cells (PBMCs) from peanut-allergic (PA) and nonallergic subjects were stimulated (14-16 h) with peanut extract to detect peanut-specific CD4⁺ CD154⁺ T cells. Biopsies were obtained for transcriptomic analysis from healthy controls and patients with extrinsic or intrinsic atopic dermatitis (AD) and psoriasis.

RESULTS

Expression analysis of >300 surface proteins enabled identification of IL-4-upregulated surface proteins, such as CD90, CD108, CD109, and CD200R (CD200R1). Additional analysis of in vitro-differentiated Th0, Th1, and Th2 cultures identified CD200R as upregulated on Th2 cells. From ex vivo-isolated PBMCs, we found high expression of CD200R on Th2 and ILC2 cells and basophils. In PA subjects, the peanut-specific Th2 (CD154⁺ CRTh2⁺ ) cells expressed more CD200R than the non-allergen-specific Th2 (CD154^- CRTh2⁺ ) cells. Moreover, costaining of CD161 and CD200R identified peanut-specific highly differentiated IL-4⁺ IL-5⁺ Th2 cells. Finally, transcriptomic analysis revealed upregulation of CD200R in lesional skin from subjects with an extrinsic AD phenotype compared to healthy skin.

CONCLUSION

These results indicate that CD200R expression strongly correlates with Th2 pathology; though, the mechanism is as yet elusive.

Atopic dermatitis: immune deviation, barrier dysfunction, IgE autoreactivity and new therapies

Atopic dermatitis (AD) is a chronic or chronically relapsing, eczematous, severely pruritic skin disorder mostly associated with IgE elevation and skin barrier dysfunction due to decreased filaggrin expression. The lesional skin of AD exhibits Th2- and Th22-deviated immune reactions that are progressive during disease chronicity. Th2 and Th22 cytokines further deteriorate the skin barrier by inhibiting filaggrin expression. Some IgEs are reactive to self-antigens. The IgE autoreactivity may precipitate the chronicity of AD. Upon activation of the ORAI1 calcium channel, atopic epidermis releases large amounts of thymic stromal lymphopoietin (TSLP), which initiates the Th2 and Th22 immune response. Th2-derived interleukin-31 and TSLP induce an itch sensation. Taken together, TSLP/Th2/Th22 pathway is a promising target for developing new therapeutics for AD. Enhancing filaggrin expression using ligands for the aryl hydrocarbon receptor may also be an adjunctive measure to restore the disrupted barrier function specifically for AD.

TWEAK mediates inflammation in experimental atopic dermatitis and psoriasis

Atopic dermatitis (AD) and psoriasis are driven by alternate type 2 and type 17 immune responses, but some proteins might be critical to both diseases. Here we show that a deficiency of the TNF superfamily molecule TWEAK (TNFSF12) in mice results in defective maintenance of AD-specific T helper type 2 (Th2) and psoriasis-specific Th17 cells in the skin, and impaired expression of disease-characteristic chemokines and cytokines, such as CCL17 and TSLP in AD, and CCL20 and IL-19 in psoriasis. The TWEAK receptor, Fn14, is upregulated in keratinocytes and dermal fibroblasts, and TWEAK induces these cytokines and chemokines alone and in synergy with the signature T helper cytokines of either disease, IL-13 and IL-17. Furthermore, subcutaneous injection of recombinant TWEAK into naive mice induces cutaneous inflammation with histological and molecular signs of both diseases. TWEAK is therefore a critical contributor to skin inflammation and a possible therapeutic target in AD and psoriasis.

How to Define Atopic Dermatitis?

Although atopic dermatitis (AD) is the most common skin disorder, there remains an ongoing debate on this denomination, its definition and the binary view based on immunoglobulin E measurement. The wide spectrum of the clinical phenotype of AD reflects the complex genetic and pathophysiologic mechanisms underlying the disease. The diagnostic criteria have to be reconsidered and adapted to different ethnic populations. There is an urgent need for biomarker discovery further supporting the clinical diagnostic criteria as well as the precision medicine approach on a global level.

Early Activation of Th2/Th22 Inflammatory and Pruritogenic Pathways in Acute Canine Atopic Dermatitis Skin Lesions

Determining inflammation and itch pathway activation in patients with atopic dermatitis (AD) is fraught with the inability to precisely assess the age of skin lesions, thus affecting the analysis of time-dependent mediators. To characterize inflammatory events occurring during early experimental acute AD lesions, biopsy samples were collected 6, 24, and 48 hours after epicutaneous application of Dermatophagoides farinae house dust mites to sensitized atopic dogs. The skin transcriptome was assessed using a dog-specific microarray and quantitative PCR. Acute canine AD skin lesions had a significant up-regulation of genes encoding T helper (Th) 2 (e.g., IL4, IL5, IL13, IL31, and IL33), Th9 (IL9), and Th22 (IL22) cytokines as well as Th2-promoting chemokines such as CCL5 and CCL17. Proinflammatory (e.g., IL6, LTB, and IL18) cytokines were also up-regulated. Other known pruritogenic pathways were also activated: there was significant up-regulation of genes encoding proteases cathepsin S (CTSS), mast cell chymase (CMA1), tryptase (TPS1) and mastin, neuromedin-B (NMB), nerve growth factor (NGF), and leukotriene-synthesis enzymes (ALOX5, ALOX5AP, and LTA4H). Experimental acute canine house dust mite-induced AD lesions exhibit an activation of innate and adaptive immune responses and pruritogenic pathways similar to those seen in humans with acute AD, thereby validating this model to test innovative therapeutics modalities for this disease.