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The skin microbiome is an extensive community of bacteria, fungi, mites, viruses and archaea colonizing the skin. Fluctuations in the composition of the skin microbiome have been observed in atopic dermatitis (AD) and food allergy (FA), particularly in early life, established disease, and associated with therapeutics. However, AD is a multifactorial disease characterized by skin barrier aberrations modulated by genetics, immunology, and environmental influences, thus the skin microbiome is not the sole feature of this disease. Future research should focus on mechanistic understanding of how early-life skin microbial shifts may influence AD and FA onset, to guide potential early intervention strategies or as microbial biomarkers to identify high-risk infants who may benefit from possible microbiome-based biotherapeutic strategies. Harnessing skin microbes as AD biotherapeutics is an emerging field, but more work is needed to investigate whether this approach can lead to sustained clinical responses., (© 2024 The Authors. Allergy published by European Academy of Allergy and Clinical Immunology and John Wiley & Sons Ltd. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.)

The ZAKα-driven ribotoxic stress response (RSR) is activated by ribosome stalling and/or collisions. Recent work demonstrates that RSR also plays a role in innate immunity by activating the human NLRP1 inflammasome. Here, we report that ZAKα and NLRP1 sense bacterial exotoxins that target ribosome elongation factors. One such toxin, diphtheria toxin (DT), the causative agent for human diphtheria, triggers RSR-dependent inflammasome activation in primary human keratinocytes. This process requires iron-mediated DT production in the bacteria, as well as diphthamide synthesis and ZAKα/p38-driven NLRP1 phosphorylation in host cells. NLRP1 deletion abrogates IL-1β and IL-18 secretion by DT-intoxicated keratinocytes, while ZAKα deletion or inhibition additionally limits both pyroptotic and inflammasome-independent non-pyroptotic cell death. Consequently, pharmacologic inhibition of ZAKα is more effective than caspase-1 inhibition at protecting the epidermal barrier in a 3D skin model of cutaneous diphtheria. In summary, these findings implicate ZAKα-driven RSR and the NLRP1 inflammasome in antibacterial immunity and might explain certain aspects of diphtheria pathogenesis., (© 2023 Robinson et al.)

Background: Atopic dermatitis (AD) is a common chronic skin condition in children (15-20%) that can significantly impair their quality of life. As a result of its relapsing nature and enrichment of Staphylococcus aureus during flares, clinical management can include eradicating S aureus from the skin of children; however, this does not extend to their healthy caregivers, who are potential reservoirs., Objective: Our aim was to understand skin microbiome sharing and microbial features in children with AD and their healthy adult caregivers., Methods: We utilized whole-metagenome profiling at 4 body sites (volar forearm, antecubital fossae, cheeks, and lesions) in combination with sequencing of S aureus isolates to characterize a cohort of children with AD and their healthy caregivers (n = 30 families) compared to matched pairs from control households (n = 30 families)., Results: Metagenomic analysis revealed distinct microbiome configurations in the nonlesional skin of AD children and their healthy caregivers versus controls, which were sufficient to accurately predict case-control status (area under the receiver operating characteristic curve > 0.8). These differences were accompanied by significant microbiome similarity between children and their caregivers, indicating that microbiome sharing may play a role in recurrent disease flares. Whole-genome comparisons with high-quality S aureus isolate genomes (n = 55) confirmed significant strain sharing between AD children and their caregivers and AD-specific enrichment of strains expressing enterotoxins Q and K/K2., Conclusion: Our results highlight the distinctive skin microbiome features of healthy caregivers for children with AD and support their inclusion in strategies for the treatment of recurrent pediatric AD., (Copyright © 2022 American Academy of Allergy, Asthma & Immunology. Published by Elsevier Inc. All rights reserved.)

Background: The ichthyoses are rare genetic keratinizing disorders that share the characteristics of an impaired epidermal barrier and increased risk of microbial infections. Although ichthyotic diseases share a T helper (Th) 17 cell immune signature, including increased expression of antimicrobial peptides, the skin microbiota of ichthyoses is virtually unexplored., Objectives: To analyse the metagenome profile of skin microbiome for major congenital ichthyosis subtypes., Methods: Body site-matched skin surface samples were collected from the scalp, upper arm and upper buttocks of 16 healthy control participants and 22 adult patients with congenital forms of ichthyosis for whole metagenomics sequencing analysis., Results: Taxonomic profiling showed significant shifts in bacteria and fungi abundance and sporadic viral increases across ichthyosis subtypes. Cutibacterium acnes and Malassezia were significantly reduced across body sites, consistent with skin barrier disruption and depletion of lipids. Microbial richness was reduced, with specific increases in Staphylococcus and Corynebacterium genera, as well as shifts in fungal species, including Malassezia. Malassezia globosa was reduced at all body sites, whereas M. sympodialis was reduced in the ichthyotic upper arm and upper buttocks. Malassezia slooffiae, by contrast, was strikingly increased at all body sites in participants with congenital ichthyosiform erythroderma (CIE) and lamellar ichthyosis (LI). A previously undescribed Trichophyton species was also detected as sporadically colonizing the skin of patients with CIE, LI and epidermolytic ichthyosis subtypes., Conclusions: The ichthyosis skin microbiome is significantly altered from healthy skin with specific changes predominating among ichthyosis subtypes. Skewing towards the Th17 pathway may represent a response to the altered microbial colonization in ichthyosis. What is already known about this topic? The skin microbiome of congenital ichthyoses is largely unexplored. Microbes play an important role in pathogenesis, as infections are common. The relative abundances of staphylococci and corynebacteria is increased in the cutaneous microbiome of patients with Netherton syndrome, but extension of these abundances to all congenital ichthyoses is unexplored. What does this study add? A common skin microbiome signature was observed across congenital ichthyoses. Distinct microbiome features were associated with ichthyosis subtypes. Changes in microbiome may contribute to T helper 17 cell immune polarization. What is the translational message? These data provide the basis for comparison of the microbiome with lipidomic and transcriptomic alterations in these forms of ichthyosis and consideration of correcting the dysbiosis as a therapeutic intervention., (© 2022 British Association of Dermatologists.)

Atopic dermatitis (AD) is a skin inflammatory disease in which the opportunistic pathogen Staphylococcus aureus is prevalent and abundant. S. aureus harbors several secreted virulence factors that have well-studied functions in infection models, but it is unclear whether these extracellular microbial factors are relevant in the context of AD. To address this question, we designed a culture-independent method to detect and quantify S. aureus virulence factors expressed at the skin sites. We utilized RNase-H‒dependent multiplex PCR for preamplification of reverse-transcribed RNA extracted from tape strips of patients with AD sampled at skin sites with differing severity and assessed the expression of a panel of S. aureus virulence factors using qPCR. We observed an increase in viable S. aureus abundance on sites with increased severity of disease, and many virulence factors were expressed at the AD skin sites. Surprisingly, we did not observe any significant upregulation of the virulence factors at the lesional sites compared with those at the nonlesional control. Overall, we utilized a robust assay to directly detect and quantify viable S. aureus and its associated virulence factors at the site of AD skin lesions. This method can be extended to study the expression of skin microbial genes at the sites of various dermatological conditions., (© 2022 The Authors.)

Background: Observational studies suggest an increased risk of eczema in children living in hard versus soft water areas, and there is, therefore, an interest in knowing whether softening water may prevent eczema. We evaluated the feasibility of a parallel-group assessor-blinded pilot randomized controlled trial to test whether installing a domestic ion-exchange water softener before birth in hard water areas reduces the risk of eczema in infants with a family history of atopy., Methods: Pregnant women living in hard water areas (>250 mg/L calcium carbonate) in and around London UK, were randomized 1:1 antenatally to either have an ion-exchange water softener installed in their home or not (ie to continue to receive usual domestic hard water). Infants were assessed at birth and followed up for 6 months. The main end-points were around feasibility, the primary end-point being the proportion of eligible families screened who were willing and able to be randomized. Clinical end-points were evaluated including frequency of parent-reported doctor-diagnosed eczema and visible eczema on skin examination. Descriptive analyses were conducted, and no statistical testing was performed as this was a pilot study., Results: One hundred and forty-nine families screened were eligible antenatally and 28% (41/149) could not have a water softener installed due to technical reasons or lack of landlord approval. Eighty of 149 (54%) were randomized, the primary end-point. Two participants withdrew immediately after randomization, leaving 39 participants in each arm (78 total). Attrition was 15% (12/78) by 6 months postpartum. All respondents (n = 69) to the study acceptability questionnaire reported that the study was acceptable. Fifty-six of 708 (7.9%) water samples in the water softener arm were above the hard water threshold of 20 mg/L CaCO 3 . At 6 months of age 27/67 infants (40%) developed visible eczema, 12/36 (33%) vs. 15/31 (48%) in the water softener and control groups, respectively, difference -15% (95% CI -38, 8.3%), with most assessments (≥96%) remaining blinded. Similarly, a lower proportion of infants in the water softener arm had parent-reported, doctor-diagnosed eczema by 6 months compared to the control arm, 6/17 (35%) versus 9/19 (47%), difference -12% (95% CI -44, 20%)., Conclusion: A randomized controlled trial of water softeners for the prevention of atopic eczema in high-risk infants is feasible and acceptable., Trial Registration: NCT03270566 (clinicaltrials.gov)., (© 2021 John Wiley & Sons Ltd.)

In the skin fragility disorder epidermolysis bullosa simplex (EBS), mutations in keratin 14 (K14, also known as KRT14) or keratin 5 (K5, also known as KRT5) lead to keratinocyte rupture and skin blistering. Severe forms of EBS are associated with cytoplasmic protein aggregates, with elevated kinase activation of ERK1 and ERK2 (ERK1/2; also known as MAPK3 and MAPK1, respectively), suggesting intrinsic stress caused by misfolded keratin protein. Human keratinocyte EBS reporter cells stably expressing GFP-tagged EBS-mimetic mutant K14 were used to optimize a semi-automated system to quantify the effects of test compounds on keratin aggregates. Screening of a protein kinase inhibitor library identified several candidates that reduced aggregates and impacted on epidermal growth factor receptor (EGFR) signalling. EGF ligand exposure induced keratin aggregates in EBS reporter keratinocytes, which was reversible by EGFR inhibition. EBS keratinocytes treated with a known EGFR inhibitor, afatinib, were driven out of activation and towards quiescence with minimal cell death. Aggregate reduction was accompanied by denser keratin filament networks with enhanced intercellular cohesion and resilience, which when extrapolated to a whole tissue context would predict reduced epidermal fragility in EBS patients. This assay system provides a powerful tool for discovery and development of new pathway intervention therapeutic avenues for EBS., Competing Interests: Competing interests The authors declare no competing or financial interests., (© 2021. Published by The Company of Biologists Ltd.)

Maintaining tissue homeostasis depends on a balance between cell proliferation, differentiation, and apoptosis. Within the epidermis, the levels of the polyamines putrescine, spermidine, and spermine are altered in many different skin conditions, yet their role in epidermal tissue homeostasis is poorly understood. We identify the polyamine regulator, Adenosylmethionine decarboxylase 1 (AMD1), as a crucial regulator of keratinocyte (KC) differentiation. AMD1 protein is upregulated on differentiation and is highly expressed in the suprabasal layers of the human epidermis. During KC differentiation, elevated AMD1 promotes decreased putrescine and increased spermine levels. Knockdown or inhibition of AMD1 results in reduced spermine levels and inhibition of KC differentiation. Supplementing AMD1-knockdown KCs with exogenous spermidine or spermine rescued aberrant differentiation. We show that the polyamine shift is critical for the regulation of key transcription factors and signaling proteins that drive KC differentiation, including KLF4 and ZNF750. These findings show that human KCs use controlled changes in polyamine levels to modulate gene expression to drive cellular behavior changes. Modulation of polyamine levels during epidermal differentiation could impact skin barrier formation or can be used in the treatment of hyperproliferative skin disorders., (Copyright © 2021 The Authors. Published by Elsevier Inc. All rights reserved.)

Background: Neutrophilic folliculitis is an inflammatory condition of hair follicles. In some neutrophilic folliculitis, such as in patients with acne and hidradenitis suppurativa, follicular hyperkeratosis is also observed. Neutrophilic folliculitis is often induced and/or exacerbated by a high-fat diet (HFD). However, the molecular mechanisms by which an HFD affects neutrophilic folliculitis are not fully understood., Objective: Our aim was to elucidate how an HFD promotes the development of neutrophilic folliculitis., Methods: Mice were fed an HFD, and their skin was subjected to histologic, RNA sequencing, and imaging mass spectrometry analyses. To examine the effect of an HFD on neutrophil accumulation around the hair follicles, phorbol 12-myristate 13-acetate (PMA) was used as an irritant to the skin., Results: Histologic analysis revealed follicular hyperkeratosis in the skin of HFD-fed mice. RNA sequencing analysis showed that genes related to keratinization, especially in upper hair follicular keratinocytes, were significantly upregulated in HFD-fed mice. Application of PMA to the skin induced neutrophilic folliculitis in HFD-fed mice but not in mice fed a normal diet. Accumulation of neutrophils in the skin and around hair follicles was dependent on CXCR2 signaling, and CXCL1 (a CXCR2 ligand) was produced mainly by hair follicular keratinocytes. Imaging mass spectrometry analysis revealed an increase in fatty acids in the skin of HFD-fed mice. Application of these fatty acids to the skin induced follicular hyperkeratosis and caused PMA-induced neutrophilic folliculitis even in mice fed a normal diet., Conclusion: An HFD can facilitate the development of neutrophilic folliculitis with the induction of hyperkeratosis of hair follicles and increased neutrophil infiltration around the hair follicles via CXCR2 signaling., (Copyright © 2021 American Academy of Allergy, Asthma & Immunology. Published by Elsevier Inc. All rights reserved.)

Background: Atopic dermatitis (AD) is a common skin disease affecting up to 20% of the global population, with significant clinical heterogeneity and limited information about molecular subtypes and actionable biomarkers. Although alterations in the skin microbiome have been described in subjects with AD during progression to flare state, the prognostic value of baseline microbiome configurations has not been explored., Objective: Our aim was to identify microbial signatures on AD skin that are predictive of disease fate., Methods: Nonlesional skin of patients with AD and healthy control subjects were sampled at 2 time points separated by at least 4 weeks. Using whole metagenome analysis of skin microbiomes of patients with AD and control subjects (n = 49 and 189 samples), we identified distinct microbiome configurations (dermotypes A and B). Blood was collected for immunophenotyping, and skin surface samples were analyzed for correlations with natural moisturizing factors and antimicrobial peptides., Results: Dermotypes were robust and validated across 2 additional cohorts (63 individuals), with strong enrichment of subjects with AD in dermotype B. Dermotype B was characterized by reduced microbial richness, depletion of Cutibacterium acnes, Dermacoccus and Methylobacterium species, individual-specific outlier abundance of Staphylococcus species (eg, S epidermidis, S capitis, S aureus), and enrichment in metabolic pathways (eg, branched chain amino acids and arginine biosynthesis) and virulence genes (eg, β-toxin, δ-toxin) that defined a pathogenic ecology. Skin surface and circulating host biomarkers exhibited a distinct microbial-associated signature that was further reflected in more severe itching, frequent flares, and increased disease severity in patients harboring the dermotype B microbiome., Conclusion: We report distinct clusters of microbial profiles that delineate the role of microbiome configurations in AD heterogeneity, highlight a mechanism for ongoing inflammation, and provide prognostic utility toward microbiome-based disease stratification., (Copyright © 2020 American Academy of Allergy, Asthma & Immunology. Published by Elsevier Inc. All rights reserved.)

The skin microbiome plays a central role in inflammatory skin disorders such as atopic dermatitis (AD). In AD patients, an imbalance between pathogenic Staphylococcus aureus (S. aureus) and resident skin symbionts creates a state of dysbiosis which induces immune dysregulation and impairs skin barrier function. There are now exciting new prospects for microbiome-based interventions for AD prevention. In the hopes of achieving sustained control and management of disease in AD patients, current emerging biotherapeutic strategies aim to harness the skin microbiome associated with health by restoring a more diverse symbiotic skin microbiome, while selectively removing pathogenic S. aureus. Examples of such strategies are demonstrated in skin microbiome transplants, phage-derived anti-S. aureus endolysins, monoclonal antibodies, and quorum sensing (QS) inhibitors. However, further understanding of the skin microbiome and its role in AD pathogenesis is still needed to understand how these biotherapeutics alter the dynamics of the microbiome community; to optimize patient selection, drug delivery, and treatment duration; overcome rapid recolonization upon treatment cessation; and improve efficacy to allow these therapeutic options to eventually reach routine clinical practice., (© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Background: Atopic dermatitis (AD) is a common chronic inflammatory skin disease. Skin barrier defects contribute to disease initiation and development; however, underlying mechanisms remain elusive., Objective: To understand the underlying cause of barrier defect, we investigated aberrant expression of specific microRNAs (miRNAs) in AD. Delineating the molecular mechanism of dysregulated miRNA network, we focused on identification of specific drugs that can modulate miRNA expression and repair the defective barrier in AD., Methods: A screen for differentially expressed miRNAs between healthy skin and AD lesional skin resulted in the identification of miR-335 as the most consistently downregulated miRNA in AD. Using in silico prediction combined with experimental validation, we characterized downstream miR-335 targets and elucidated the molecular pathways by which this microRNA maintains epidermal homeostasis in healthy skin., Results: miR-335 was identified as a potent inducer of keratinocyte differentiation; it exerts this effect by directly repressing SOX6. By recruiting SMARCA complex components, SOX6 suppresses epidermal differentiation and epigenetically silences critical genes involved in keratinocyte differentiation. In AD lesional skin, miR-335 expression is aberrantly lost. SOX6 is abnormally expressed throughout the epidermis, where it impairs skin barrier development. We demonstrate that miR-335 is epigenetically regulated by histone deacetylases; a screen for suitable histone deacetylase inhibitors identified belinostat as a candidate drug that can restore epidermal miR-335 expression and rescue the defective skin barrier in AD., Conclusion: Belinostat is of clinical significance not only as a candidate drug for AD treatment, but also as a potential means of stopping the atopic march and further progression of this systemic allergic disease., (Copyright © 2020 The Authors. Published by Elsevier Inc. All rights reserved.)

We have generated MLi003-A, a new induced pluripotent stem cell (iPSC) line derived from hair follicle keratinocytes of a healthy male characterized with a maximum number of filaggrin tandem repeats, making this iPSC line the best control for studies on skin barrier function. The characterization of the MLi003-A cell line consisted of molecular karyotyping, high-throughput array-based sequencing composed of Fluidigm microfluidics technology and next-generation sequencing of the filaggrin alleles, and pluripotency and differentiation potentials testing by immunofluorescence of associated markers both in vitro and in vivo. The MLi-003A line has been also tested for ability to differentiate into keratinocytes., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 The Authors. Published by Elsevier B.V. All rights reserved.)

Background: Acne vulgaris is a highly prevalent inflammatory skin disorder with a complex pathogenesis, characterized by comedones, papules, pustules and nodules. Familial preponderance clearly indicates a genetic basis for acne vulgaris, but until recently solid genetic associations were lacking., Results: The advent of high-resolution genotyping array technologies has allowed for large-scale studies with both family-based and cross-sectional designs. These studies have revealed genetic loci encompassing genes that could be active in biological pathways and processes underlying acne vulgaris. However, specific functional consequences of those variants remain elusive. In parallel, investigations into rare disorders and syndromes that incorporate features of acne or acne-like lesions have recently accelerated our understanding of disease pathogenesis. The genes revealed by these rare disorders highlight mechanisms cardinal for pilosebaceous biology and therefore anchor our insights from genetic association studies for acne vulgaris., Conclusions: The next phase of research will require more in-depth mechanistic investigations of loci and genes implicated in acne phenotypes to define the key molecular players driving the disorder. Concurrently, new treatments for acne vulgaris could be developed by dissecting the candidate molecular pathways to identify druggable targets., (© 2019 British Association of Dermatologists.)

Introduction: Atopic eczema affects 20% of UK children, and environmental factors are important in its aetiology. Several observational studies suggest an increased risk of atopic eczema in children living in hard water areas. The Softened Water for Eczema Prevention pilot trial tests the feasibility of installing domestic ion-exchange water softeners around the time of birth to reduce the risk of atopic eczema in children with a family history of atopy. A further aim is to explore the pathophysiological mechanisms for this in an embedded mechanistic study., Methods and Analysis: Multicentre parallel group assessor-blinded randomised controlled pilot trial. Participants are newborn babies (n=80) living in a hard water (>250 mg/L calcium carbonate) area at risk of developing atopic eczema because of a family history of atopy. Participants will be randomised prior to birth in a 1:1 ratio. The intervention group will have an ion-exchange water softener installed prior to birth. The control group will receive their usual domestic hard water supply. Follow-up will be until 6 months of age. Data will be collected at birth (baseline), 1, 3 and 6 months of age. The main outcome is the proportion of eligible families screened who are willing and able to be randomised. Several secondary feasibility and clinical endpoints will also be evaluated, alongside mechanistic outcomes. Data will be analysed on an intention-to-treat basis. There will be no hypothesis testing for the clinical outcomes. Study acceptability will be evaluated through semistructured interviews., Ethics and Dissemination: This study has been reviewed and given a favourable opinion by the North West-Liverpool East Research Ethics Committee (Ref: 17/NW/0661). The results of the study will be reported at international conferences and in peer-reviewed scientific journals. We will send participating families a summary of the pilot trial results., Trial Registration Number: NCT03270566., Competing Interests: Competing interests: CF has received investigator-led research funding from Sanofi. His department has received clinical trial funding from Sanofi and AbbVie to test novel therapeutics in patients with paediatric atopic eczema. MJC is an investigator and consultant for Regeneron, Sanofi Genzyme, Pfizer, Leo, Galapagos, Novartis, Boots, L’Oreal, Dermavant, Menlo, Reckitt Benckiser, Oxagen, Johnson & Johnson, Hyphens, Astellas, Amlar, AbbVie, Galderma and Procter & Gamble. Harvey Water Softeners (HWS) contributed to the design and operational running of the study (supply and installation of water softeners, testing of water samples). Final decisions around design and conduct were made independently by investigators. HWS will not be involved in the analysis or interpretation of the results., (© Author(s) (or their employer(s)) 2019. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ.)

We have generated an induced pluripotent stem cell (iPSC) line KCLi003-A (iOP101) from epidermal keratinocytes of a female donor, heterozygous for the loss-of-function mutation p.R501X in the filaggrin gene (FLG), using non-integrating Sendai virus vectors. Derivation and expansion of iPSCs were performed under xeno-free culture conditions. Characterization and validation of KCLi003-A line included molecular karyotyping, mutation screening using restriction enzyme digestion, next generation sequencing (NGS), while pluripotency and differentiation potential were confirmed by expression of associated markers in vitro and by in vivo teratoma assay., (Copyright © 2019 King's College London. Published by Elsevier B.V. All rights reserved.)