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Background: The contribution of Staphylococcus aureus to the exacerbation of atopic dermatitis (AD) is widely documented, but its role as a primary trigger of AD skin symptoms remains poorly explored., Objectives: This study sought to reappraise the main bacterial factors and underlying immune mechanisms by which S aureus triggers AD-like inflammation., Methods: This study capitalized on a preclinical model, in which different clinical isolates were applied in the absence of any prior experimental skin injury., Results: The development of S aureus-induced dermatitis depended on the nature of the S aureus strain, its viability, the concentration of the applied bacterial suspension, the production of secreted and nonsecreted factors, as well as the activation of accessory gene regulatory quorum sensing system. In addition, the rising dermatitis, which exhibited the well-documented AD cytokine signature, was significantly inhibited in inflammasome adaptor apoptosis-associated speck-like protein containing a CARD domain- and monocyte/macrophage-deficient animals, but not in T- and B-cell-deficient mice, suggesting a major role for the innate response in the induction of skin inflammation. However, bacterial exposure generated a robust adaptive immune response against S aureus, and an accumulation of S aureus-specific γδ and CD4 + tissue resident memory T cells at the site of previous dermatitis. The latter both contributed to worsen the flares of AD-like dermatitis on new bacteria exposures, but also, protected the mice from persistent bacterial colonization., Conclusions: These data highlight the induction of unique AD-like inflammation, with the generation of proinflammatory but protective tissue resident memory T cells in a context of natural exposure to pathogenic S aureus strains., Competing Interests: Disclosure statement This study was supported by institutional grants from the Institut National de la Santé et de la Recherche Médicale (INSERM) and from Pfizer. Disclosure of potential conflict of interest: The authors declare that they have no relevant conflicts of interest., (Copyright © 2024 American Academy of Allergy, Asthma & Immunology. Published by Elsevier Inc. All rights reserved.)

Background: Tissue-resident memory T (T RM ) cells are detrimental in allergic contact dermatitis (ACD), in which they contribute to the chronicity and severity of the disease., Methods: We assessed the impact of a standard topical corticosteroid (TCS) treatment, triamcinolone acetonide (TA), on the formation, maintenance and reactivation of epidermal T RM cells in a preclinical model of ACD to 2,4-dinitrofluorobenzene. TA 0.01% was applied at different time points of ACD response and we monitored skin inflammation and tracked CD8+ CD69+ CD103+ T RM by flow cytometry and RNA sequencing., Results: The impact of TA on T RM formation depended on treatment regimen: (i) in a preventive mode, that is, in sensitized mice before challenge, TA transiently inhibited the infiltration of effector T cells and the accumulation of T RM upon hapten challenge. In contrast, (ii) in a curative mode, that is, at the peak of the ACD response, TA blocked skin inflammation but failed to prevent the formation of T RM . Finally, (iii) in a proactive mode, that is, on previous eczema lesions, TA had no effect on the survival of skin T RM , but transiently inhibited their reactivation program upon allergen reexposure. Indeed, specific T RM progressively regained proliferative functions upon TA discontinuation and expanded in the tissue, leading to exaggerated iterative responses. Interestingly, T RM re-expansion correlated with the decreased clearance of hapten moieties from the skin induced by repeated TA applications., Conclusions: Our results demonstrate that TCS successfully treat ACD inflammation, but are mostly ineffective in impeding the formation and expansion of allergen-specific T RM , which certainly restricts the induction of lasting tolerance in patients with chronic dermatitis., (© 2023 European Academy of Allergy and Clinical Immunology and John Wiley & Sons Ltd.)

Atopic dermatitis (AD) is a common chronic inflammatory skin disease that significantly affects the patient's quality of life. A disrupted skin barrier, type 2 cytokine-dominated inflammation, and microbial dysbiosis with increased Staphylococcus aureus colonization are critical components of AD pathogenesis. Patients with AD exhibit decreased expression of antimicrobial peptides (AMPs) which is linked to increased colonization by Staphylococcus aureus . The skin microbiome itself is a source of several AMPs. These host- and microbiome-derived AMPs define the microbial landscape of the skin based on their differential antimicrobial activity against a range of skin microbes or their quorum sensing inhibitory properties. These are particularly important in preventing and limiting dysbiotic colonization with Staphylococcus aureus . In addition, AMPs are critical for immune homeostasis. In this article, we share our perspectives about the implications of microbial derived AMPs in AD patients and their potential effects on overlapping factors involved in AD. We argue and discuss the potential of bacterial AMPs as therapeutics in AD., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2023 Joshi, Vocanson, Nicolas, Wolf and Patra.)

The cutaneous microbiota contributes to skin barrier function, ensuring effective protection against pathogens and contributing to the maintenance of epidermal integrity. Dysbiosis is frequently present in atopic dermatitis (AD), a chronic inflammatory disease associated with skin barrier defects. Dysbiosis is associated with reduced bacterial diversity and marked Staphylococcus aureus colonization, which is favoured in the case of certain local AD-specific properties such as reduced skin acidity, eased bacterial adhesion and decreased antimicrobial peptide production. Furthermore, S. aureus-associated skin dysbiosis, via the production of staphylococcal virulence factors, may also participate in the immunopathology of AD by altering the epidermal barrier and inducing an inflammatory response. However, there are currently no arguments for recommending screening for, and treatment of S. aureus-associated dysbiosis outside the setting of cutaneous superinfection. Nonetheless, modulation of the skin microbiota may hold promise for AD management. Here, we describe the relationships that exist between the skin microbiota and AD.

Atopy is defined by the propensity to develop an exaggerated type-2 inflammatory response to environmental molecules. Clinically, atopy is diagnosed when atopic disease occurs: atopic dermatitis, food allergy, atopic asthma and allergic rhinitis and conjunctivitis. Whereas the classical "atopic march" is increasingly challenged through epidemiological studies, type-2 cellular inflammation is a characteristic shared by the atopic diseases. This inflammation can be innate (non-specific: eosinophils, mast cells, dendritic cells, innate lymphoid cells [ILC]), or adaptive (antigen-specific, involving T cells). Interleukins (IL-)4, 5 and 13 are major actors of type-2 inflammation and are mainly produced by ILC and T cells. The efficacy of treatments targeting these type-2 cytokines highlight the importance of type-2 inflammation in atopic diseases. However, several patients do not respond to type-2 targeting treatments, highlighting the presence of other actors in pathophysiology of atopic diseases: alteration of epithelial barrier, IgE-mediated allergic responses, type-17 inflammation. Thus, the term "endotype" can illustrate this diversity in pathophysiology. Finally, a global approach of atopic diseases, as type-2 inflammatory diseases, is fundamental, but not sufficient. An approach by endotype is advisable, in a personalized medicine perspective. © 2020 Elsevier Masson SAS. All rights reserved., (Copyright © 2020 Elsevier Masson SAS.)

Background: Ivermectin (IVM) is widely used in both human and veterinary medicine to treat parasitic infections. Recent reports have suggested that IVM could also have anti-inflammatory properties., Methods: Here, we investigated the activity of IVM in a murine model of atopic dermatitis (AD) induced by repeated exposure to the allergen Dermatophagoides farinae, and in standard cellular immunological assays., Results: Our results show that topical IVM improved allergic skin inflammation by reducing the priming and activation of allergen-specific T cells, as well as the production of inflammatory cytokines. While IVM had no major impact on the functions of dendritic cells in vivo and in vitro, IVM impaired T-cell activation, proliferation, and cytokine production following polyclonal and antigen-specific stimulation., Conclusion: Altogether, our results show that IVM is endowed with topical anti-inflammatory properties that could have important applications for the treatment of T-cell-mediated skin inflammatory diseases., (© 2017 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

Atopic dermatitis (AD) is a common skin inflammatory disease characterized by the production of thymic stromal lymphopoietin (TSLP) and marked T H 2 polarization. Recent studies suggest that IL-1β contributes to the development of AD skin inflammation. Here, we have investigated the impact of IL-1β signalling on the epidermal homeostasis of both healthy subjects and AD patients [with functional filaggrin (FLG) alleles], with particular attention to TSLP production and keratinocyte differentiation. In healthy reconstructed human epidermis (RHE), IL-1β promoted (i) robust secretion of TSLP in an NF-κB-dependent manner and (ii) a significant decrease in the expression of filaggrin and other proteins of the epidermal differentiation complex. These effects were prevented by treatment of RHE with the anti-IL-1β mAb canakinumab and by the IL-1 receptor antagonist anakinra. Interestingly, RHE generated from AD donors behaved like that of healthy individuals and showed comparable responses to IL-1β signals. Collectively, our results suggest that IL-1β may be an early key mediator for the acquisition of an AD phenotype through induction of TSLP and alteration of the epidermal homeostasis. Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd., (Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.)

Background: The diverse effects of histamine on immune regulation are a result of the differential expression and regulation of 4 histamine receptors. Many of the immediate allergic and inflammatory actions of histamine are mediated via the type 1 receptor (H1R)., Objectives: We hypothesized that H1R was involved in the fine-tuning of the initiation of T cell-mediated skin pathology-that is, dermatitis., Methods: The impact of the H1R invalidation on the development of skin inflammation was analyzed in a mouse model of atopic dermatitis., Results: We show that H1r(-)/(-) mice developed reduced allergen-specific skin lesions. Lack of H1R expression on dendritic cells (DCs) led to diminished IL-12, upregulated IL-23, and IL-6 production upon allergen stimulation. H1R engagement on dendritic cells was necessary for DC activation and subsequent priming of effector IFN-γ(+)CD8(+) T cells. We demonstrate here that H1R blockade on DCs promotes generation of noneffector IL-17(+)CD8(+) T cells that are unable to initiate the skin inflammation., Conclusion: Our data identify that histamine signaling through the H1R on DCs is an important early event conditioning the quality of the skin effector immune response., (Copyright © 2011 American Academy of Allergy, Asthma & Immunology. Published by Mosby, Inc. All rights reserved.)

Exposure of atopic dermatitis (AD) patients to aeroallergens or food allergens can exacerbate or maintain the disease. Atopy patch tests (APTs) are able to identify these triggering factors and consist of the epicutaneous application of allergens for 48hours with evaluation of the resulting eczematous lesions after 48 and 72hours, according to the reading criteria of the European Task Force on Atopic Dermatitis (ETFAD). APTs show a higher specificity than skin prick and specific IgE tests, since the pathophysiological mechanism of the reaction induced is very similar to what occurs in AD lesions. The standardization of APTs to aeroallergens has brought a certain degree of reliability to this method, which is not the case for food APTs, where the positive predictive value must be improved in order to avoid any unnecessary dietary restrictions. Thus, optimization of APTs and furtherance of knowledge of the pathophysiology of eczemas could help to develop new immunobiological diagnostic methods and AD-specific immunotherapy.

Skin lesions in the allergic form of atopic dermatitis (AD) are induced by allergen-specific T cells that infiltrate the skin at the site of allergen exposure. Although Th2-type CD4+ T cells appear to be crucial in AD pathophysiology, little is known about the contribution of CD8+ T cells in the development of the allergic skin inflammation. In the present study, we have analyzed the respective role of CD8+ and CD4+ T cells in the development of AD skin lesions in a mouse model of allergen-induced AD. In sensitized mice, CD8+ T cells are rapidly and transiently recruited to the allergen-exposed site and initiate the inflammatory process leading to skin infiltration with eosinophils and Th1/Th2-producing cells. CD8+ T cell-depleted mice show no inflammation, demonstrating that these cells are mandatory for the development of AD. In contrast, CD4+ T cell-depleted mice develop a severe form of eczema. Furthermore, adoptive transfer of CD8+ T cells from sensitized mice into naive recipient mice leads to skin inflammation soon after allergen exposure. These data indicate that allergen-primed CD8+ T cells are required for the development of AD-like lesions in mice.