Atopic dermatitis (AD) is the most common chronic inflammatory skin disease. Genetic predisposition, epidermal barrier disruption, and dysregulation of the immune system are some of the critical components of AD. An impaired skin barrier may be the initial step in the development of the atopic march as well as AD, which leads to further skin inflammation and allergic sensitization. Type 2 cytokines as well as interleukin 17 and interleukin 22 contribute to skin barrier dysfunction and the development of AD. New insights into the pathophysiology of AD have focused on epidermal lipid profiles, neuroimmune interactions, and microbial dysbiosis. Newer therapeutic strategies focus on improving skin barrier function and targeting polarized immune pathways found in AD. Further understanding of AD pathophysiology will allow us to achieve a more precision medicine approach to the prevention and the treatment of AD.

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Pathophysiology of atopic dermatitis: Clinical implications.

Background Nemolizumab targets the IL-31 receptor α subunit involved in atopic dermatitis (AD) pathogenesis. Objective We sought to evaluate a new dosing strategy of nemolizumab in patients with AD. Methods We performed a 24-week, randomized, double-blind, multicenter study of nemolizumab (10, 30, and 90 mg) subcutaneous injections every 4 weeks versus placebo, with topical corticosteroids in adults with moderate-to-severe AD, severe pruritus, and inadequate control with topical treatment (n = 226). The Eczema Area and Severity Index (EASI), the peak pruritus (PP) numeric rating scale (NRS), and the Investigator's Global Assessment (IGA) were assessed. Standard safety assessments were performed. Results Nemolizumab improved EASI, IGA, and/or NRS-itch scores, with the 30-mg dose being most effective. Nemolizumab (30 mg) reduced EASI scores versus placebo at week 24 (−68.8% vs −52.1%, P = .016); significant differences were observed by week 8 (P ≤ .01). With significant improvement (P = .028) as early as week 4, IGA 0/1 rates were higher for 30 mg of nemolizumab versus placebo at week 16 (33.3% vs 12.3%, P = .008) but not week 24 because of an increased placebo/topical corticosteroid effect (36.8% vs 21.1%, P = .06). PP-NRS scores were improved for 30 mg of nemolizumab versus placebo at week 16 (−68.6% vs −34.3%, P  Conclusions Nemolizumab resulted in rapid and sustained improvements in cutaneous signs of inflammation and pruritus in patients with AD, with maximal efficacy observed at 30 mg. Nemolizumab had an acceptable safety profile.

Phase 2B randomized study of nemolizumab in adults with moderate-to-severe atopic dermatitis and severe pruritus

The endocannabinoid system (ECS) has lately been proven to be an important, multifaceted homeostatic regulator, which influences a wide-variety of physiological processes all over the body. Its members, the endocannabinoids (eCBs; e.g., anandamide), the eCB-responsive receptors (e.g., CB1, CB2), as well as the complex enzyme and transporter apparatus involved in the metabolism of the ligands were shown to be expressed in several tissues, including the skin. Although the best studied functions over the ECS are related to the central nervous system and to immune processes, experimental efforts over the last two decades have unambiguously confirmed that cutaneous cannabinoid (“c[ut]annabinoid”) signaling is deeply involved in the maintenance of skin homeostasis, barrier formation and regeneration, and its dysregulation was implicated to contribute to several highly prevalent diseases and disorders, e.g., atopic dermatitis, psoriasis, scleroderma, acne, hair growth and pigmentation disorders, keratin diseases, various tumors, and itch. The current review aims to give an overview of the available skin-relevant endo- and phytocannabinoid literature with a special emphasis on the putative translational potential, and to highlight promising future research directions as well as existing challenges.

https://www.mdpi.com/1420-3049/24/5/918/pdf

Cannabinoid Signaling in the Skin: Therapeutic Potential of the "C(ut)annabinoid" System.

The intimate interaction between mast cells and sensory nerves can be illustrated by the wheal and surrounding flare in an urticarial reaction in human skin. This reaction is typically associated with an intense itch at the reaction site. Upon activation, cutaneous mast cells release powerful mediators, such as histamine, tryptase, cytokines, and growth factors that can directly stimulate corresponding receptors on itch-mediating sensory nerves. These include, e.g., H1- and H4-receptors, protease-activated receptor-2, IL-31 receptor, and the high-affinity receptor of nerve growth factor (TrkA). On the other hand, sensory nerves can release neuropeptides, including substance P and vasoactive intestinal peptide, that are able to stimulate mast cells to release mediators leading to potentiation of the reciprocal interaction, inflammation, and itch. Even though mast cells are well recognized for their role in allergic skin whealing and urticaria, increasing evidence supports the reciprocal function between mast cells and sensory nerves in neurogenic inflammation in chronic skin diseases, such as psoriasis and atopic dermatitis, which are often characterized by distressing itch, and exacerbated by psychological stress. Increased morphological contacts between mast cells and sensory nerves in the lesional skin in psoriasis and atopic dermatitis as well as experimental models in mice and rats support the essential role for mast cell-sensory nerve communication in consequent pruritus. Therefore, we summarize here the present literature pointing to a close association between mast cells and sensory nerves in pruritic skin diseases as well as review the essential supporting findings on pruritic models in mice and rats.

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Mast Cells and Sensory Nerves Contribute to Neurogenic Inflammation and Pruritus in Chronic Skin Inflammation

Bullous pemphigoid (BP) is an autoimmune blistering disease which carries a significant mortality and morbidity. While historically BP has been characterized as an IgG driven disease mediated by anti-BP180 and BP230 IgG autoantibodies, developments in recent years have further elucidated the role of eosinophils and IgE autoantibodies. In fact, eosinophil infiltration and eosinophilic spongiosis are prominent features in BP. Several observations support a pathogenic role of eosinophils in BP: IL-5, eotaxin, and eosinophil-colony stimulating factor are present in blister fluid; eosinophils line the dermo-epidermal junction (DEJ) in the presence of BP serum, metalloprotease-9 is released by eosinophils at the site of blisters; eosinophil degranulation proteins are found on the affected basement membrane zone as well as in serum corresponding with clinical disease; eosinophil extracellular DNA traps directed against the basement membrane zone are present, IL-5 activated eosinophils cause separation of the DEJ in the presence of BP serum; and eosinophils are the necessary cell required to drive anti-BP180 IgE mediated skin blistering. Still, it is likely that eosinophils contribute to the pathogenesis of BP in numerous other ways that have yet to be explored based on the known biology of eosinophils. We herein will review the role of eosinophils in BP and provide a framework for understanding eosinophil pathogenic mechanisms in mucocutaneous disease.

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The Role of Eosinophils in Bullous Pemphigoid: A Developing Model of Eosinophil Pathogenicity in Mucocutaneous Disease.

Background T H 2 cell–released IL-31 is a critical mediator in patients with atopic dermatitis (AD), a prevalent and debilitating chronic skin disorder Brain-derived natriuretic peptide (BNP) has been described as a central itch mediator The importance of BNP in peripheral (skin-derived) itch and its functional link to IL-31 within the neuroimmune axis of the skin is unknown Objective We sought to investigate the function of BNP in the peripheral sensory system and skin in IL-31–induced itch and neuroepidermal communication in patients with AD Methods Ca 2+ imaging, immunohistochemistry, quantitative real-time PCR, RNA sequencing, knockdown, cytokine/phosphokinase arrays, enzyme immune assay, and pharmacologic inhibition were performed to examine the cellular basis of the IL-31–stimulated, BNP-related itch signaling in dorsal root ganglionic neurons (DRGs) and skin cells, transgenic AD-like mouse models, and human skin of patients with AD and healthy subjects Results In human DRGs we confirmed expression and co-occurrence of oncostatin M receptor β subunit and IL-31 receptor A in a small subset of the neuronal population Furthermore, IL-31 activated approximately 50% of endothelin-1–responsive neurons, and half of the latter also responded to histamine In murine DRGs IL-31 upregulated Nppb and induced soluble N-ethylmaleimide–sensitive factor activating protein receptor–dependent BNP release In Grhl3PAR2 /+ mice house dust mite–induced severe AD-like dermatitis was associated with Nppb upregulation Lesional IL-31 transgenic mice also exhibited increased Nppb transcripts in DRGs and the skin; accordingly, skin BNP receptor levels were increased Importantly, expression of BNP and its receptor were increased in the skin of patients with AD In human skin cells BNP stimulated a proinflammatory and itch-promoting phenotype Conclusion For the first time, our findings show that BNP is implicated in AD and that IL-31 regulates BNP in both DRGs and the skin IL-31 enhances BNP release and synthesis and orchestrates cytokine and chemokine release from skin cells, thereby coordinating the signaling pathways involved in itch Inhibiting peripheral BNP function might be a novel therapeutic strategy for AD and pruritic conditions

Masaki Moriyama

Papers

New mechanism underlying IL-31-induced atopic dermatitis.