Filaggrin and beyond: New insights into the skin barrier in atopic dermatitis and allergic diseases, from genetics to therapeutic perspectives

An Assessment of Administration Route on MSC-sEV Therapeutic Efficacy

Mesenchymal stem/stromal cell-derived small extracellular vesicles (MSC-sEVs) are promising therapeutic agents. In this study, we investigated how the administration route of MSC-sEVs affects their therapeutic efficacy in a mouse model of bleomycin (BLM)-induced skin scleroderma (SSc). We evaluated the impact of topical (TOP), subcutaneous (SC), and intraperitoneal (IP) administration of MSC-sEVs on dermal fibrosis, collagen density, and thickness. All three routes of administration significantly reduced BLM-induced fibrosis in the skin, as determined by Masson's Trichrome staining. However, only TOP administration reduced BLM-induced dermal collagen density, with no effect on dermal thickness observed for all administration routes. Moreover, SC, but not TOP or IP administration, increased anti-inflammatory profibrotic CD163+ M2 macrophages. These findings indicate that the administration route influences the therapeutic efficacy of MSC-sEVs in alleviating dermal fibrosis, with TOP administration being the most effective, and this efficacy is not mediated by M2 macrophages. Since both TOP and SC administration target the skin, the difference in their efficacy likely stems from variations in MSC-sEV delivery in the skin. Fluorescence-labelled TOP, but not SC MSC-sEVs when applied to skin explant cultures, localized in the stratum corneum. Hence, the superior efficacy of TOP over SC MSC-sEVs could be attributed to this localization. A comparison of the proteomes of stratum corneum and MSC-sEVs revealed the presence of >100 common proteins. Most of these proteins, such as filaggrin, were known to be crucial for maintaining skin barrier function against irritants and toxins, thereby mitigating inflammation-induced fibrosis. Therefore, the superior efficacy of TOP MSC-sEVs over SC and IP MSC-sEVs against SSc is mediated by the delivery of proteins to the stratum corneum to reinforce the skin barrier.

Skin Barrier in Atopic Dermatitis

A compromised permeability barrier is a hallmark of atopic dermatitis (AD). Localized to the outermost skin layer, the stratum corneum (SC) is critically dependent on terminal differentiation of epidermal keratinocytes, which transform into protein-rich corneocytes surrounded by extracellular lamellae of unique epidermal lipids, conferring permeability barrier function. These structures are disrupted in AD. A leaky barrier is prone to environmental insult, which in AD elicits type 2-dominant inflammation, in turn resulting in a vicious cycle further impairing the SC structure. Therapies directed at enforcing SC structure and anti-inflammatory strategies administered by topical and systemic route as well as UV therapy have differential effects on the permeability barrier. The expanding armamentarium of therapeutic modalities for AD treatment warrants optimization of their effects on permeability barrier function.

A Novel Recombinant Human Filaggrin Segment (rhFLA-10) Alleviated a Skin Lesion of Atopic Dermatitis

Atopic dermatitis (AD), a prevalent chronic inflammatory skin disorder, is marked by impaired skin barrier function and persistent pruritus. It significantly deteriorates patients' quality of life, making it one of the most burdensome non-lethal skin disorders. Filaggrin plays a crucial role in the pathophysiology of barrier disruption in AD, interacting with inflammatory mediators. It is an integral part of the extracellular matrix architecture, serving to protect the skin barrier and attenuate the inflammatory cascade. In this study, we engineered a novel recombinant human filaggrin (rhFLA-10) expression vector, which was subsequently synthesized and purified. In vitro and ex vivo efficacy experiments were conducted for AD. rhFLA-10, at low concentrations (5 to 20 μg/mL), was non-toxic to HACaT cells, significantly inhibited the degranulation of P815 mast cells, and was readily absorbed by cells, thereby exerting a soothing therapeutic effect. Furthermore, rhFLA-10 demonstrated anti-inflammatory properties (p < 0.05). In vivo, efficacy experiments further substantiated that rhFLA-10 could effectively ameliorate AD in mice and facilitate the repair of damaged skin (p < 0.001). These findings underscore the considerable potential of rhFLA-10 in the treatment of AD.

Filaggrin loss-of-function variants are associated with atopic dermatitis phenotypes in a diverse, early-life prospective cohort

Loss-of-function (LoF) variants in the filaggrin (FLG) gene are the strongest known genetic risk factor for atopic dermatitis (AD), but the impact of these variants on AD outcomes is poorly understood. We comprehensively identified genetic variants through targeted region sequencing of FLG in children participating in the Mechanisms of Progression of Atopic Dermatitis to Asthma in Children cohort. Twenty FLG LoF variants were identified, including 1 novel variant and 9 variants not previously associated with AD. FLG LoF variants were found in the cohort. Among these children, the presence of 1 or more FLG LoF variants was associated with moderate/severe AD compared with those with mild AD. Children with FLG LoF variants had a higher SCORing for Atopic Dermatitis (SCORAD) and higher likelihood of food allergy within the first 2.5 years of life. LoF variants were associated with higher transepidermal water loss (TEWL) in both lesional and nonlesional skin. Collectively, our study identifies established and potentially novel AD-associated FLG LoF variants and associates FLG LoF variants with higher TEWL in lesional and nonlesional skin.